STRUCTURE AND PROPERTIES OF SILK

Module 1: Physical Structure of Textile Fibres

● Physical structure of Silk, Cotton, Wool, PET, Nylon, and Acrylic

fibres.

Silk

Silk is a natural fiber produced by the silkworm, specifically the Bombyx mori. The
structure of silk fibers is characterized by:

● Composition: Silk is primarily made up of proteins, particularly fibroin, which

forms the core of the fiber. The outer layer, sericin, acts as a protective coating.
● Morphology: Under a microscope, silk fibers appear smooth and lustrous, with a
triangular cross-section that contributes to their sheen. The fibers are long and
continuous, typically measuring several hundred meters in length.
● Mechanical Properties: Silk exhibits high tensile strength (up to 740 MPa) due to
its beta-sheet crystalline structure, which provides excellent mechanical
properties and resilience .

Cotton
Cotton is a natural fiber derived from the seed bolls of the cotton plant. Its physical
structure includes:

● Composition: Cotton fibers are primarily composed of cellulose, a polysaccharide

that provides strength and absorbency.
● Morphology: Cotton fibers are short, typically 1 to 2 cm in length, and have a
flattened, twisted ribbon-like appearance under a microscope. This structure
enhances their ability to absorb moisture.
● Mechanical Properties: Cotton is known for its softness and breathability,
making it comfortable to wear. However, it wrinkles easily and has lower tensile
strength compared to synthetic fibers .

Wool

Wool is an animal fiber obtained from sheep and other animals like goats (cashmere,
mohair) and rabbits (angora). Its structure is characterized by:

● Composition: Wool fibers are made of keratin, a protein that provides elasticity
and resilience.
● Morphology: Wool fibers are crimped and have a scaly surface, which helps in
trapping air and providing insulation. The fibers are typically 5 to 30 cm long.
● Mechanical Properties: Wool has good elasticity and can absorb moisture
without feeling wet, making it ideal for warm clothing. It also has natural flame
resistance .

PET (Polyethylene Terephthalate)

PET is a synthetic fiber commonly used in textiles and packaging. Its structure includes:

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 ● Composition: PET is a polymer made from the reaction of ethylene glycol and
terephthalic acid. It is a type of polyester.
● Morphology: PET fibers are long and continuous, with a smooth surface. They
can be produced in various thicknesses and can be engineered to have different
properties.
● Mechanical Properties: PET fibers are strong, resistant to stretching and
shrinking, and have good dimensional stability. They are also resistant to
wrinkling and are often blended with natural fibers to enhance performance 4.

Nylon

Nylon is a synthetic fiber known for its strength and versatility. Its structure is
characterized by:

● Composition: Nylon is a polyamide, created through the polymerization of

amines and carboxylic acids.
● Morphology: Nylon fibers are smooth and have a round cross-section. They can
be produced as continuous filaments or staple fibers.
● Mechanical Properties: Nylon is known for its high tensile strength, elasticity, and
resistance to abrasion. It is often used in applications requiring durability, such as
clothing, ropes, and industrial fabrics.

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Acrylic

Acrylic is a synthetic fiber that mimics the properties of wool. Its structure includes:

● Composition: Acrylic fibers are made from polyacrylonitrile, a polymer that is

lightweight and soft.
● Morphology: Acrylic fibers are typically produced as continuous filaments and
have a smooth, cylindrical shape. They can be dyed easily and retain color well.
● Mechanical Properties: Acrylic is known for its warmth and softness, making it a
popular choice for sweaters and blankets. It is also resistant to moths and
mildew, but it has lower tensile strength compared to nylon .

● Study of two-phase and one-phase models of fibre physical

structure.

One-Phase Models

One-phase models consider fibers as homogeneous materials, where the entire fiber is
treated as a single phase. This approach simplifies the analysis and is
useful for understanding basic properties of fibers.

● Homogeneity: In this model, the fiber is assumed to have uniform properties

throughout its structure. This means that mechanical, thermal, and optical
properties are consistent across the fiber.
● Crystalline and Amorphous Regions: Even in one-phase models, fibers can
exhibit crystalline and amorphous regions. The crystalline regions provide

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 strength and stability, while the amorphous regions contribute to flexibility and
processability. However, these distinctions are not emphasized in one-phase
models.
● Mechanical Properties: The mechanical behavior of fibers, such as tensile
strength and elasticity, can be analyzed using simplified equations derived from
continuum mechanics. The focus is on average properties rather than local
variations.

Two-Phase Models

Two-phase models provide a more detailed representation of fiber structure by

considering the coexistence of different phases within the fiber. This
approach is crucial for understanding the complex interactions that
influence fiber properties.

● Phases: In two-phase models, fibers are typically composed of a crystalline

phase and an amorphous phase. The crystalline phase consists of ordered
molecular arrangements, while the amorphous phase is disordered. This dual
structure significantly affects the fiber's mechanical and thermal properties.
● Supramolecular Structures: Fibers often possess supramolecular structures,
which are influenced by molecular orientation and the distribution of crystalline
and amorphous regions. These structures can be formed during processing and
are critical for determining the thermomechanical properties of the fibers .
● Mechanical Behavior: The mechanical properties of fibers in two-phase models
can be analyzed using more complex theories that account for the interactions
between the two phases. For example, the strength of a fiber can be influenced
by the proportion of crystalline to amorphous regions, as well as the interface
between these phases. This model allows for a better understanding of
phenomena such as stress transfer and failure mechanisms.

● Introduction to textile fibre structure analysis using:

1. Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry (DSC) is a thermal analysis technique that measures

the heat flow associated with phase transitions in materials as a
function of temperature.

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 ● Principle: DSC compares the heat flow to a sample and a reference material as
they are subjected to a controlled temperature program. The differences in heat
flow are recorded, allowing for the determination of thermal properties such as
melting points, glass transition temperatures, and crystallization temperatures.
● Applications: In textile fiber analysis, DSC is used to assess thermal stability,
phase transitions, and the degree of crystallinity in fibers. This information is
crucial for understanding how fibers will perform under heat exposure and in
various processing conditions .

2. Dynamic Mechanical Analysis (DMA)

Dynamic Mechanical Analysis (DMA) is a technique used to study the mechanical

properties of materials as a function of temperature, time, and
frequency.

● Principle: DMA applies a sinusoidal stress to a material and measures the

resulting strain, allowing for the calculation of storage modulus (elastic
response) and loss modulus (viscous response). This provides insights into the
viscoelastic behavior of fibers.
● Applications: DMA is particularly useful for evaluating the mechanical
performance of textile fibers under dynamic conditions, such as during stretching
or bending. It helps in understanding how fibers will behave in real-world
applications, including their durability and comfort .

3. X-ray Diffraction (XRD)

X-ray Diffraction (XRD) is a powerful technique used to determine the crystalline

structure of materials.

● Principle: When X-rays are directed at a crystalline material, they are scattered in
specific directions based on the arrangement of atoms within the crystal lattice.
The resulting diffraction pattern provides information about the crystal structure,
including lattice parameters and crystallinity.
● Applications: In textile fibers, XRD is used to analyze the degree of crystallinity
and the orientation of molecular chains. This information is essential for
predicting the mechanical properties and thermal behavior of fibers.

4. Infrared Spectroscopy (IRS)

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Infrared Spectroscopy (IRS), particularly Fourier Transform Infrared Spectroscopy
(FTIR), is used to identify chemical bonds and functional groups in
materials.

● Principle: IRS measures the absorption of infrared light by a sample, which

causes molecular vibrations. The resulting spectrum provides a fingerprint of the
material's chemical composition.
● Applications: In textile fiber analysis, IRS is used to identify the chemical
structure of fibers, assess the presence of additives or contaminants, and
monitor chemical changes during processing.

5. Nuclear Magnetic Resonance (NMR)

Nuclear Magnetic Resonance (NMR) is a technique that provides detailed information

about the molecular structure and dynamics of materials.

● Principle: NMR exploits the magnetic properties of certain nuclei (e.g., hydrogen)
in a magnetic field. By applying radiofrequency radiation, it induces transitions
between energy levels, allowing for the analysis of molecular environments.
● Applications: In textile fibers, NMR can be used to study the molecular mobility,
crystallinity, and interactions between different components within the fibers.

6. Thermogravimetric Analysis (TGA)

Thermogravimetric Analysis (TGA) measures the change in mass of a material as it is

heated or cooled.

● Principle: TGA records the weight of a sample as a function of temperature or

time, providing insights into thermal stability, decomposition temperatures, and
moisture content.
● Applications: In textile fiber analysis, TGA is used to evaluate the thermal stability
of fibers and to determine the composition of blends by analyzing weight loss at
specific temperatures .

7. Thermomechanical Analysis (TMA)

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Thermomechanical Analysis (TMA) measures the dimensional changes of a material
as a function of temperature.

● Principle: TMA applies a constant force to a sample and measures the resulting
deformation as the temperature changes. This provides information about
thermal expansion and softening behavior.
● Applications: TMA is useful for assessing the thermal expansion properties of
textile fibers, which is important for understanding how fibers will behave in
different thermal environments.

8. Atomic Force Microscopy (AFM)

Atomic Force Microscopy (AFM) is a high-resolution imaging technique that provides

topographical information about surfaces at the nanoscale.

● Principle: AFM uses a cantilever with a sharp tip that scans the surface of a
sample. The interaction between the tip and the surface generates a
topographical map of the material.
● Applications: In textile fiber analysis, AFM can be used to study the surface
morphology and roughness of fibers, which can influence properties such as dye
uptake and friction .

9. Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy (SEM) is a technique that provides detailed images of

the surface morphology of materials.

● Principle: SEM uses a focused beam of electrons to scan the surface of a

sample. The interaction between the electrons and the sample produces
secondary electrons that are collected to form high-resolution images.
● Applications: SEM is widely used in textile fiber analysis to examine the surface
structure, diameter, and cross-sectional shape of fibers, which are critical for
understanding their performance.

10. Transmission Electron Microscopy (TEM)

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Transmission Electron Microscopy (TEM) is a technique that provides high-resolution
images of the internal structure of materials.

● Principle: TEM transmits electrons through a thin sample, allowing for the
observation of internal features at the atomic level. The resulting images provide
information about crystal structure and defects.
● Applications: In textile fibers, TEM can be used to study the internal morphology
and crystallinity, providing insights into the fiber's mechanical properties and
behavior under stress.

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Module 2: Structure of Mulberry Silk

● Composition: fibroin, sericin, amino acids.

Fibroin
● Definition: Fibroin is the structural protein that forms the core of the silk
fiber. It is responsible for the tensile strength and durability of the silk.
● Structure: Fibroin consists of long chains of amino acids arranged in a
highly ordered structure, primarily featuring antiparallel beta-sheets. This
arrangement allows for tight packing of the protein chains, contributing to
the rigidity and strength of the silk fiber .
● Amino Acid Composition: The primary amino acid sequence of fibroin
includes a repetitive pattern of glycine (Gly), serine (Ser), and alanine (Ala),
specifically the sequence (Gly-Ser-Gly-Ala-Gly-Ala)n. The high content of
glycine and alanine facilitates crystallization, enhancing the fiber's
mechanical properties .

Sericin
● Definition: Sericin is a water-soluble glycoprotein that surrounds and
protects the fibroin fibers in the silk cocoon. It acts as a natural adhesive,
holding the fibroin filaments together .
● Structure: Sericin has a more amorphous structure compared to fibroin,
with a significant presence of random coils and some beta-sheet
structures. The secondary structure of sericin is influenced by its
hydrophilic nature, which is attributed to the abundance of polar amino
acids.
● Amino Acid Composition: Sericin is composed of 18 different amino
acids, with serine, aspartic acid, and glycine being the most abundant. The
presence of hydroxyl, carboxyl, and amino groups in sericin contributes to
its hydrophilicity and adhesive properties 4.
The content of all these substances is not constant and varies within wide limits,
depending on the species of silkworm and on the location and conditions of
rearing. Silk filament contains the following (by total weight):
• 72–81% fi broin;
• 19–28% sericin;
• 0.8–1.0% fat and wax and
• 1.0–1.4% colouring matter and ash.

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 ● Microstructure and appearance.

Microstructure of Mulberry Silk

1. Fibroin Structure:
○ Crystalline and Amorphous Regions: The primary component of mulberry
silk, fibroin, exhibits a complex microstructure consisting of both
crystalline and amorphous regions. The crystalline regions are formed by
tightly packed beta-sheet structures, which provide strength and stability.
In contrast, the amorphous regions contribute to the flexibility and
elasticity of the fiber.
○ Beta-Sheet Configuration: The beta-sheet configuration of fibroin allows
for a high degree of molecular alignment, which enhances the tensile
strength of the silk. This ordered structure is crucial for the fiber's
mechanical properties, making it one of the strongest natural fibers
available.
2. Sericin Coating:
○ Protective Layer: Surrounding the fibroin core is sericin, a water-soluble
glycoprotein that acts as a protective coating. Sericin has a more
amorphous structure compared to fibroin, which helps in binding the
fibroin filaments together and providing additional moisture management
properties.
○ Surface Texture: The presence of sericin contributes to the smooth
surface texture of mulberry silk, enhancing its aesthetic appeal and
comfort against the skin.
3. Cross-Sectional Shape:
○ Triangular Cross-Section: Under microscopic examination, mulberry silk
fibers typically exhibit a triangular cross-section. This shape not only
contributes to the fiber's luster but also affects how light interacts with the
fiber, giving mulberry silk its characteristic sheen.
4. Diameter and Length:
○ Fine and Long Fibers: Mulberry silk fibers are generally fine, with
diameters ranging from 10 to 15 micrometers. The fibers can be several
hundred meters long, which is advantageous for spinning and weaving
processes. The length and fineness of the fibers contribute to the
smoothness and drape of silk fabrics.

Appearance of Mulberry Silk

● Luster and Sheen:

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 ● Natural Shine: Mulberry silk is known for its natural luster, which is a result
of its smooth surface and triangular cross-section. The fiber reflects light
beautifully, making it highly desirable for luxury textiles and garments.
● Color:
● Natural and Dyed Variants: While natural mulberry silk is typically off-white
or cream, it can be dyed in a wide range of colors. The dye uptake is
influenced by the presence of sericin, which can enhance the affinity of the
fiber for various dyes.
● Texture:
● Soft and Smooth: The combination of fibroin and sericin results in a soft,
smooth texture that feels luxurious against the skin. This tactile quality is
one of the primary reasons for mulberry silk's popularity in high-end
fashion and home textiles.
● Drape:
● Fluidity: Mulberry silk has an excellent drape, allowing it to flow gracefully
when made into garments. This property is particularly valued in evening
wear and formal attire, where the movement of the fabric enhances the
overall aesthetic.

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 ● Longitudinal and cross-sectional views.

Longitudinal View of Mulberry Silk

In the longitudinal view of mulberry silk, the fiber appears as a smooth, elongated
structure that showcases its remarkable length and fine diameter. The
surface exhibits a lustrous sheen, attributed to the triangular
cross-section, which allows light to reflect beautifully along the fiber’s
length. The longitudinal view reveals the organized arrangement of fibroin
protein chains within the fiber, highlighting the presence of crystalline
regions that contribute to its tensile strength and durability. Additionally,
the smoothness and flexibility of the fiber are evident, making it suitable
for various textile applications, including luxurious garments and intricate
weaves.

Cross-Sectional View of Mulberry Silk

In the cross-sectional view of mulberry silk, the fiber displays a distinct triangular shape,
which is characteristic of this natural fiber. This unique cross-section not
only impacts the fiber's mechanical properties but also enhances its
aesthetic appeal by creating a specific light-reflecting quality. The interior
reveals a combination of crystalline and amorphous regions, with the
crystalline areas providing strength and rigidity, while the amorphous
regions contribute to flexibility and softness. Surrounding the central
fibroin core is the sericin coating, which appears as a more amorphous
layer, influencing the fiber's moisture management and adhesion
properties. This cross-sectional structure is crucial for understanding how
mulberry silk behaves in various textile applications.

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 ● Fibre density comparison with other textile fibres.

Fibre Type Density (g/cm³)

Mulberry Silk 1.30-1.35

Cotton 1.50-1.55

Wool 1.28-1.32

Polyester 1.38

Nylon 1.14

Rayon 1.52

Linen 1.50
● Mulberry Silk: Known for its relatively high density among silk fibers, mulberry
silk's density contributes to its strength and luxurious feel.
● Cotton: A widely used natural fiber, cotton has a higher density than mulberry silk,
which affects its weight and drape.
● Wool: Similar in density to mulberry silk, wool fibers provide excellent insulation
and moisture-wicking properties.
● Polyester and Nylon: These synthetic fibers have densities that vary slightly but
are generally higher than that of mulberry silk, contributing to their durability and
strength.
● Linen: With the highest density among the fibers listed, linen is known for its
strength and breathability.
● Acrylic: This synthetic fiber is lightweight and has a lower density, making it
suitable for various applications, including knitwear.

● Microstructure analysis:

Crystal Structure

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Mulberry silk is primarily composed of fibroin, which exhibits a beta-sheet crystal
structure. This arrangement is highly ordered and contributes to the fiber's
exceptional strength and durability. In contrast, cotton is made of
cellulose, which has a more amorphous structure with crystalline regions
that are less organized than those in silk. Wool, another natural fiber,
consists of keratin proteins that form a helical structure, providing
elasticity but not the same level of tensile strength as silk. Synthetic fibers
like polyester and nylon have varied crystal structures depending on their
manufacturing processes, often resulting in a more uniform and less
complex arrangement compared to the natural fibers.

Crystallinity

The crystallinity of mulberry silk typically ranges from 30% to 70%, indicating a
significant presence of ordered regions that enhance its mechanical
properties. This high crystallinity is crucial for the silk's strength and
stability. In comparison, cotton has a crystallinity of about 60%, which
contributes to its strength but lacks the same level of durability as silk.
Wool has a lower crystallinity, around 20-30%, which allows for greater
flexibility but results in lower tensile strength. Polyester and nylon can

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 achieve high crystallinity levels (up to 80%), which enhances their strength
and durability, making them suitable for a wide range of applications, but
they often lack the luxurious feel of silk.

Orientation

The orientation of the molecular chains in mulberry silk fibers is highly aligned along the
fiber axis due to the spinning process. This alignment enhances the
tensile strength and stiffness of the silk. In contrast, cotton fibers have a
less pronounced orientation, which contributes to their softness but
reduces their overall strength. Wool fibers exhibit a crimped structure that
provides elasticity but does not offer the same directional strength as silk.
Synthetic fibers like polyester and nylon can be engineered for high
orientation, resulting in fibers that are strong and durable, but they may not
replicate the natural feel and drape of silk.

Crystal Size

The crystal size in mulberry silk can range from a few nanometers to several
micrometers, influencing the fiber's flexibility and strength. Smaller
crystals tend to enhance flexibility, while larger crystals contribute to
rigidity. In comparison, cotton fibers have larger crystalline regions, which
can lead to a stiffer fabric. Wool fibers have a more variable crystal size
due to their complex protein structure, affecting their overall elasticity.
Synthetic fibers often have uniform crystal sizes, which can be controlled
during production to achieve desired properties, but this uniformity can
result in a lack of the unique characteristics found in natural fibers.

Sonic Modulus

The sonic modulus of mulberry silk is indicative of its stiffness and is influenced by its
crystallinity and orientation. Higher sonic modulus values suggest greater
stiffness and strength, making mulberry silk suitable for applications
requiring durability. In comparison, cotton has a lower sonic modulus,
which contributes to its softness and comfort but limits its use in
high-stress applications. Wool also exhibits a lower sonic modulus due to
its elastic properties, making it ideal for garments that require stretch.
Synthetic fibers like polyester and nylon typically have higher sonic
modulus values, indicating greater stiffness and strength, which makes

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 them suitable for a variety of industrial applications, but they often lack
the luxurious feel of silk.

● Importance of X-ray diffraction and IR Spectroscopy studies on

mulberry silk.

X-ray Diffraction (XRD)

X-ray diffraction is a powerful tool for analyzing the crystalline structure of mulberry silk.
It allows researchers to determine the crystallinity and crystal size of the
silk fibers, which are critical factors influencing their mechanical
properties. The diffraction patterns obtained from XRD can reveal the
presence of beta-sheet structures, which are predominant in silk fibroin. By
analyzing the intensity and position of the diffraction peaks, researchers
can quantify the degree of crystallinity and assess how it correlates with
the silk's strength and elasticity. For instance, higher crystallinity typically

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 indicates greater tensile strength, making XRD essential for optimizing silk
for various applications, from fashion to industrial uses.

Moreover, XRD can also be used to study the aging process of silk, providing insights
into how its structural integrity changes over time. This is particularly
important for conservation efforts and understanding the longevity of silk
products. By monitoring changes in crystallinity and crystal size,
researchers can develop strategies to enhance the durability of silk
fabrics.

Infrared (IR) Spectroscopy

Infrared spectroscopy, particularly techniques like attenuated total reflection (ATR-IR), is

invaluable for studying the molecular composition and conformational
changes in mulberry silk. IR spectroscopy provides information about the
functional groups present in the silk proteins, allowing for the
identification of specific molecular structures, such as the amide bonds
that are crucial for the silk's properties. The absorption bands in the IR

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 spectrum can indicate the presence of different secondary structures,
such as alpha-helices and beta-sheets, and their relative proportions.

This technique is particularly useful for understanding the molecular orientation of silk
fibers. The orientation of the protein chains affects the mechanical
properties of the silk, and IR spectroscopy can help quantify this
orientation. By analyzing the spectral data, researchers can gain insights
into how the processing methods, such as spinning and weaving,
influence the silk's structure and performance.

Additionally, IR spectroscopy can be employed to study the interaction of silk with

various treatments, such as dyeing or chemical modifications. This is
important for developing new silk products with enhanced properties,
such as improved dye uptake or resistance to environmental factors.

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 Module 3: Textile Fibre-Moisture Relations

● Concepts:

Moisture Equilibrium

Moisture equilibrium refers to the state at which a textile fiber has absorbed as much
moisture as the surrounding environment can provide at a specific
relative humidity (RH) level. At this point, the moisture content of
the fiber remains relatively stable, meaning that the rate of moisture
absorption equals the rate of moisture loss. The equilibrium
moisture content (EMC) is influenced by factors such as fiber type,
structure, and environmental conditions. For instance, natural fibers
like cotton and wool typically have higher moisture regain values
compared to synthetic fibers, making them more suitable for
applications where moisture management is crucial, such as in
activewear or home textiles.

Moisture Hysteresis

Moisture hysteresis describes the phenomenon where the moisture regain of a fiber
differs depending on whether it is being absorbed or desorbed.
When a fiber is exposed to varying humidity levels, the moisture
content does not follow the same path during absorption and
desorption, leading to two distinct curves on a moisture sorption
isotherm. This effect can be attributed to the structural changes
within the fiber during moisture uptake and release. For example, a
fiber that has been previously wet may exhibit a higher equilibrium
regain when re-exposed to the same humidity level compared to a
dry fiber. Understanding hysteresis is important for predicting how
fibers will behave in real-world conditions, especially in applications
where moisture cycling occurs frequently.

Moisture Regain

Moisture regain is defined as the percentage of water contained in a textile fiber relative
to its oven-dry weight. This property is crucial for assessing the
performance of fibers in various applications. For instance, cotton

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 has a moisture regain of about 8-10%, while wool can absorb even
more moisture, making it highly hygroscopic. The ability of fibers to
absorb moisture affects their comfort, thermal properties, and even
flammability. Higher moisture regain can lead to increased comfort
in clothing, as it helps regulate temperature and wick away sweat.
Conversely, fibers with low moisture regain, such as certain
synthetic materials, may feel less comfortable against the skin.

Heat of Absorption

The heat of absorption refers to the thermal energy change that occurs when moisture
is absorbed by a fiber. This process is typically exothermic,
meaning that heat is released when water vapor is absorbed into
the fiber structure. This release of heat can influence the
temperature of the surrounding environment, which is particularly
relevant in clothing applications. For example, hygroscopic fibers
like wool can absorb moisture from the air, releasing latent heat and
creating a microclimate that can enhance comfort in humid
conditions. Understanding the heat of absorption is essential for
designing textiles that provide thermal comfort and manage
moisture effectively.

Swelling of Textile Fibres

The swelling of textile fibers occurs when they absorb moisture, leading to changes in
their dimensions. This phenomenon is particularly pronounced in
natural fibers, which can swell significantly in the transverse
direction due to their hydrophilic nature. The swelling affects the
size, shape, stiffness, and permeability of yarns and fabrics. For
instance, when cotton absorbs moisture, it swells, which can
enhance its softness and drape but may also lead to dimensional
instability if not properly managed. In contrast, synthetic fibers
typically exhibit less swelling due to their hydrophobic
characteristics, which can be advantageous in maintaining the
integrity of the fabric during washing and wearing.

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 ● Effect of moisture on fibre properties.

Moisture plays a significant role in influencing the properties of textile fibers, affecting
their performance, comfort, and durability. The interaction between
moisture and fibers can lead to various physical and mechanical changes,
which are crucial for both natural and synthetic fibers. Here’s a detailed
exploration of how moisture affects fiber properties.

1. Mechanical Properties

Moisture can significantly alter the mechanical properties of fibers, including tensile
strength, elasticity, and stiffness. For instance, natural fibers such as
cotton and wool are hygroscopic, meaning they absorb moisture from the
environment. This absorption can enhance their flexibility and comfort but
may also lead to a reduction in tensile strength when the fibers become
overly saturated.

In contrast, synthetic fibers like polyester and nylon typically exhibit less moisture
absorption, which helps maintain their mechanical integrity under varying
humidity conditions. However, when these fibers do absorb moisture, it
can lead to a decrease in interfacial bonding in composites, affecting their
overall strength and durability 1.

2. Moisture Regain

Moisture regain refers to the amount of moisture a fiber can absorb relative to its dry
weight. This property is crucial for determining the comfort and
performance of textiles. For example, cotton has a moisture regain of
about 8-10%, which allows it to wick moisture away from the skin,
enhancing comfort in clothing. Wool can absorb even more moisture,
making it suitable for thermal insulation. On the other hand, synthetic
fibers generally have lower moisture regain, which can lead to a less
comfortable wearing experience in humid conditions 2.

3. Swelling and Dimensional Stability

When fibers absorb moisture, they often undergo swelling, which can affect their
dimensions and structural integrity. Natural fibers, particularly those with a
porous structure, can swell significantly when wet. This swelling can lead
to changes in fabric dimensions, potentially causing issues such as

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 shrinkage or distortion during washing and wearing. For example, cotton
fibers swell in the presence of moisture, which can enhance their softness
but may also lead to dimensional instability if not properly managed 2.

Synthetic fibers, while less prone to swelling, can still experience changes in their
properties when exposed to moisture. The degree of swelling and its
effects on dimensional stability depend on the fiber type and the
environmental conditions 3.

4. Frictional Properties

Moisture can also influence the frictional properties of fibers. For instance, studies have
shown that the coefficient of friction for fibers like jute increases with
relative humidity. This increase in friction can affect the handling and
processing of fibers during manufacturing and can also influence the wear
characteristics of fabrics 4. Higher moisture levels can lead to increased
friction, which may affect the comfort and performance of textiles in
applications where movement is involved.

5. Thermal Properties

The presence of moisture in fibers can impact their thermal properties. When fibers
absorb moisture, they can release heat during the absorption process,
which can create a microclimate around the fabric. This phenomenon is
particularly relevant for hygroscopic fibers like wool, which can help
regulate temperature and enhance comfort in varying environmental
conditions 5. Conversely, synthetic fibers may not provide the same
thermal comfort due to their lower moisture absorption capabilities.

6. Chemical Interactions

Moisture can also facilitate chemical interactions within fibers, particularly in natural
fibers. For example, the presence of moisture can lead to hydrolysis
reactions that may weaken the fiber structure over time. This is
particularly important in the context of natural fiber composites, where
moisture absorption can affect the durability and longevity of the material.

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 ● Calculations of moisture regain and moisture content based on
invoice weight.
● Moisture Regain: The percentage of moisture absorbed by a textile material
compared to its dry weight.
● Moisture Content: The percentage of moisture present in a textile material
compared to its total weight.

Calculations:

● Invoice Weight or Wet Weight or Total Weight: The total weight of the textile
material, including moisture.
● Oven-Dried Weight or Dry Weight: The weight of the textile material after
drying in an oven until all moisture has been removed.
● Weight of Moisture: Invoice Weight - Dry Weight.

Formula for Moisture Regain:

Moisture Regain = (Weight of Moisture / Dry Weight) * 100

Formula for Moisture Content:

Moisture Content = (Weight of Moisture / Total Weight) * 100

Examples :

Moisture Regain

Example 1:
● Invoice Weight: 120 grams
● Dry Weight: 110 grams
● Weight of Moisture: 120 - 110 = 10 grams
● Moisture Regain: (10 / 110) * 100 = 9.09%

Example 2:
● Invoice Weight: 85 grams
● Dry Weight: 78 grams
● Weight of Moisture: 85 - 78 = 7 grams
● Moisture Regain: (7 / 78) * 100 = 8.97%

Example 3:
● Invoice Weight: 150 grams
● Dry Weight: 142 grams

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 ● Weight of Moisture: 150 - 142 = 8 grams
● Moisture Regain: (8 / 142) * 100 = 5.63%

Example 4:
● Invoice Weight: 200 grams
● Dry Weight: 190 grams
● Weight of Moisture: 200 - 190 = 10 grams
● Moisture Regain: (10 / 190) * 100 = 5.26%

Moisture Content

Example 1:
● Invoice Weight: 120 grams
● Dry Weight: 110 grams
● Weight of Moisture: 120 - 110 = 10 grams
● Moisture Content: (10 / 120) * 100 = 8.33%

Example 2:
● Invoice Weight: 85 grams
● Dry Weight: 78 grams
● Weight of Moisture: 85 - 78 = 7 grams
● Moisture Content: (7 / 85) * 100 = 8.24%

Example 3:
● Invoice Weight: 150 grams
● Dry Weight: 142 grams
● Weight of Moisture: 150 - 142 = 8 grams
● Moisture Content: (8 / 150) * 100 = 5.33%

Example 4:
● Invoice Weight: 200 grams
● Dry Weight: 190 grams
● Weight of Moisture: 200 - 190 = 10 grams
● Moisture Content: (10 / 200) * 100 = 5.00%

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 ● Thermal properties and characterization of silk using:

1. Thermogravimetric Analysis (TGA)

TGA involves measuring the mass of a sample as it is heated, cooled, or held at

constant temperature. The main goal is to determine the thermal stability
and composition of materials.

● Decomposition Temperature: TGA provides critical data about the temperatures

at which silk begins to decompose. For silk, decomposition typically occurs
between 250°C and 300°C. During TGA, a significant drop in mass indicates the
breakdown of the silk's protein structure, primarily due to the denaturation of
fibroin and sericin proteins.
● Weight Loss Profiles: The weight loss curve obtained from TGA allows for the
identification of different stages of thermal decomposition. For silk, the thermal
degradation process may be multi-step, reflecting the breakdown of various
components. Understanding these stages is essential for applications where
high temperatures are involved, such as dyeing or finishing processes.
● Thermal Stability and Residual Mass: TGA can also assess the thermal stability
by examining how much weight is lost at specific temperatures. A stable fiber will
exhibit minimal degradation. The residual mass after the analysis can indicate
the presence of inorganic materials or char formation, which may be beneficial or
detrimental depending on the application.

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2. Differential Thermal Analysis (DTA)

DTA measures the temperature difference between a sample and a reference material
as they are both heated or cooled. This technique is valuable for
identifying thermal transitions in silk.

● Endothermic and Exothermic Processes: DTA can detect endothermic transitions

(heat absorption) and exothermic transitions (heat release). In silk, key
transitions include the glass transition temperature (Tg) and melting points. The
Tg for silk typically ranges between 70°C and 80°C, marking the temperature at
which the fiber transitions from a hard and brittle state to a more rubbery state,
influencing its flexibility and comfort.
● Phase Transitions: DTA allows engineers to evaluate phase transitions, such as
crystallization and melting. For instance, understanding the melting behavior of
silk is critical for processes such as dyeing, where high temperatures are applied.

3. Differential Scanning Calorimetry (DSC)

DSC is a widely used technique that measures the heat flow into or out of a sample as it
is subjected to controlled temperature changes. This method provides
comprehensive insights into the thermal characteristics of silk.

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 ● Melting and Crystallization: DSC can identify specific melting points of silk,
typically observed around 200-220°C. This information is crucial for processing
methods such as heat setting, dyeing, or coating applications. The ability to
analyze the crystallization behavior of silk is also significant, as it affects the
mechanical properties of the fiber.
● Specific Heat Capacity: The specific heat capacity obtained from DSC indicates
how much heat is required to raise the temperature of silk by one degree. This
property is essential for understanding how silk will respond to temperature
fluctuations, impacting its comfort in clothing and performance in various
applications.
● Thermal History and Stability: DSC can provide insights into the thermal history
of silk, revealing how previous thermal treatments may have affected its
properties. This knowledge is vital for optimizing processing conditions to
achieve desired performance characteristics.

4. Thermomechanical Analysis (TMA)

TMA measures the dimensional changes of a material as a function of temperature,

which provides insights into its mechanical properties under thermal
stress.

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 ● Coefficient of Thermal Expansion (CTE): TMA can determine the CTE of silk,
which quantifies how much the fiber expands or contracts with temperature
changes. Understanding the CTE is crucial for ensuring dimensional stability in
textile applications, particularly when silk is blended with other fibers that may
have different thermal expansion properties.
● Softening Point and Mechanical Properties: TMA helps identify the softening
point of silk, the temperature at which the fiber begins to lose its rigidity. This is
important for processing techniques such as spinning and weaving, where
maintaining fiber integrity is crucial. Additionally, TMA can evaluate the impact of
moisture content on the mechanical properties of silk, providing insights into
how its performance might change with humidity variations.

● Optical properties:

1. Lustre

Lustre refers to the quality of light reflection from the surface of a material, contributing
to its visual appeal. In mulberry silk, lustre is a defining characteristic that
enhances its desirability in the textile industry.

● Surface Morphology: The lustrous appearance of mulberry silk is attributed to its

smooth, triangular cross-section. This unique morphology allows for efficient
light refraction and reflection, resulting in a high degree of shine. The
smoothness of the silk fibers minimizes surface irregularities, which reduces
light scattering and enhances the overall brilliance of the fabric.
● Optical Effects: The lustre of mulberry silk can exhibit iridescence, where the
color and intensity of the shine change with the angle of light incidence and the
observer's viewpoint. This optical phenomenon is due to the fiber's ability to
refract light at various angles, creating a dynamic visual effect that is particularly
valued in luxury textiles.
● Applications: The lustrous quality of mulberry silk makes it ideal for high-end
applications such as evening wear, bridal gowns, and luxury home textiles. The
aesthetic appeal of lustrous silk contributes to the overall elegance and
sophistication of these products.

2. Birefringence

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● Refractive Indices: Mulberry silk exhibits birefringence with a refractive index
typically ranging from 1.53 to 1.54 along the fiber axis and approximately 1.66 in
the transverse direction. This difference in refractive indices results in a
birefringence value of approximately 0.03. The molecular arrangement of fibroin,
the primary protein in silk, contributes to this anisotropic behavior.
● Light Interaction: When light enters a birefringent material like mulberry silk, it
splits into two rays, each traveling at different speeds and refracted at different
angles. This property can enhance the depth and richness of color in silk fabrics,
providing a unique visual experience. The birefringent nature of silk also
contributes to its mechanical strength and durability, as it allows for better light
management and reduces the risk of UV damage.
● Applications in Technology: Beyond traditional textile applications, the
birefringent properties of mulberry silk have potential uses in optical devices and
sensors. The ability to manipulate light through birefringence can be harnessed
in advanced technologies, including photonics and biomedical applications,
where precise control of light is essential.

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 Module 4: Mechanical and Dynamic Properties of Mulberry Silk

● Tensile properties:

Mulberry silk, known for its exceptional mechanical properties, exhibits unique tensile
characteristics that are crucial for its applications in textiles and
advanced materials. This section delves into the stress-strain
characteristics, viscoelastic behavior, creep and stress relaxation,
and inverse stress relaxation of mulberry silk.

1. Stress-Strain Characteristics

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The stress-strain curve of mulberry silk is a fundamental representation of its
mechanical behavior under tensile loading.

● Elastic Region: Initially, mulberry silk displays a linear elastic region where stress
is directly proportional to strain, following Hooke's Law. In this region, the
material can return to its original shape upon the removal of the load. The
Young's modulus of mulberry silk typically ranges from 8 to 12 GPa, indicating its
stiffness.
● Yield Point: As the stress increases, the silk reaches a yield point, beyond which
it begins to exhibit plastic deformation. The tensile strength of mulberry silk can

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 reach values between 300 and 500 MPa, allowing it to withstand significant
loads before failure.
● Ultimate Tensile Strength: The ultimate tensile strength (UTS) is the maximum
stress that mulberry silk can endure before breaking. The elongation at break,
which ranges from 20% to 30%, indicates that mulberry silk can stretch
considerably before failure, showcasing its ductility.

2. Viscoelastic Behavior

Viscoelastic behavior refers to the time-dependent strain response of materials that

exhibit both elastic and viscous characteristics. Mulberry silk
demonstrates significant viscoelastic properties, which are
essential for understanding its performance under varying loading
conditions.

● Time-Dependent Strain: When subjected to a constant stress, mulberry silk

exhibits a gradual increase in strain over time, known as creep. This behavior is
indicative of the material's viscous nature, where the silk fibers deform under
sustained load. The creep compliance can be modeled using viscoelastic
models, such as the Maxwell or Voigt models, which describe the relationship
between stress and strain over time.
● Stress Relaxation: Conversely, when mulberry silk is held at a constant strain, the
stress within the material decreases over time, a phenomenon known as stress
relaxation. This behavior is crucial for applications where silk is subjected to
prolonged deformation, as it allows the material to relieve internal stresses,
enhancing comfort and wearability in textiles. The relaxation modulus, which
quantifies how stress decreases with time, is a critical parameter for predicting
the long-term performance of silk.

3. Creep and Stress-Relaxation

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 ● Creep: Creep in mulberry silk occurs when a constant load is applied, leading to a
gradual increase in strain. This phenomenon can be characterized by a linear
viscoelastic model, which describes how the material continues to deform under
constant stress until it reaches a steady-state strain. The creep rate is influenced
by factors such as temperature, humidity, and the rate of loading, making it
essential to consider these variables in practical applications.
● Stress Relaxation: Stress relaxation is characterized by the reduction of stress in
a material held at a constant strain. For mulberry silk, this behavior can be
quantified by measuring the decrease in stress over time under constant
deformation. The relaxation modulus, which describes how stress decreases
with time, is essential for modeling the long-term behavior of silk in applications
such as clothing and upholstery.

4. Inverse Stress-Relaxation

Inverse stress-relaxation is a phenomenon observed in certain materials, including

mulberry silk, where the material exhibits an increase in stress over
time while held at a constant strain.

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 ● Mechanism: Inverse stress-relaxation occurs when the silk fibers, after being
subjected to a constant strain, undergo a reorganization of their molecular
structure. When the strain is maintained below a critical threshold, the internal
structure of the silk may recover, leading to an increase in internal stress. This
behavior contrasts with typical stress-relaxation, where stress decreases over
time.
● Implications: Understanding inverse stress-relaxation is important for
applications where silk is subjected to cyclic loading or varying strain conditions.
This behavior can influence the durability and longevity of silk products,
particularly in dynamic environments, and is crucial for designing textiles that
maintain their integrity under repeated stress.

● Dynamic mechanical behavior.

The dynamic mechanical behavior of silk fibers, particularly mulberry and tasar silk, has
been extensively studied to understand their mechanical properties in
relation to temperature changes. Key parameters in this analysis include
the dynamic storage modulus (E′), loss modulus (E″), and the loss tangent
(tan δ), which provide insights into the viscoelastic properties of these
materials.

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Temperature Dependence of Dynamic Moduli

Research indicates that the dynamic storage modulus (E′) of mulberry silk shows a
slight decrease in the temperature range from −10 °C to 10 °C. As the
temperature increases up to 150 °C, the E′ value remains relatively stable.
However, significant changes occur at temperatures above 150 °C, where
the thermal movement of fibroin molecules becomes pronounced. At
around 170 °C, the E′ value decreases rapidly, indicating a transition in the
material's mechanical behavior due to increased molecular mobility.

In contrast, the loss modulus (E″) of mulberry silk does not exhibit significant changes
until it reaches 170 °C, at which point a broad peak begins to appear,

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 peaking at 220 °C. This peak is indicative of the material undergoing
maximum energy dissipation, which corresponds to the glass transition
temperature (Tg) of the silk fibers. The behavior of E″ reflects the
material's ability to absorb and dissipate energy, which is crucial for
applications requiring flexibility and resilience.

Comparison with Tasar Silk

Tasar silk fibers display a different dynamic mechanical behavior compared to mulberry
silk. Notably, the E′ value for tasar silk shows a slight increase from 30 °C
to 70 °C, which is attributed to molecular contraction. This suggests that
there are rearrangements of the tasar fibroin molecules in the amorphous
regions during heating, leading to enhanced inter-chain interactions. Such
behavior is not observed in mulberry silk, which maintains its thermal
stability below 100 °C. This stability can be partly attributed to the higher
degree of molecular orientation in the amorphous regions and the more
compact fibrous structure of mulberry silk.

The E′ peak for tasar silk appears at 230 °C, which is associated with molecular motion
in the crystalline regions. As the temperature exceeds 190 °C, the
inter-sheet distance within the crystalline structure begins to expand,
leading to a rubber-like behavior characterized by sharp changes in both E′
and E″ values. This transition indicates that the material is undergoing
significant molecular rearrangements, including the breaking and
reforming of inter- and intra-molecular hydrogen bonds.

Implications of Dynamic Mechanical Behavior

The dynamic mechanical properties of silk fibers are crucial for their applications in
various fields. For instance, the viscoelastic behavior of mulberry silk, with
its stable E′ at lower temperatures and rapid changes at higher
temperatures, makes it suitable for high-performance textiles that require
durability and flexibility. The ability of tasar silk to undergo molecular
rearrangements at elevated temperatures suggests potential applications
in environments where thermal stability and adaptability are essential.

Furthermore, the loss tangent (tan δ), which is the ratio of the loss modulus to the
storage modulus, serves as a measure of the material's damping capacity.
A higher tan δ indicates greater energy dissipation, which is beneficial in
applications such as protective clothing and cushioning materials.

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 Understanding these dynamic mechanical behaviors allows for the
optimization of silk fibers in both traditional textile applications and
advanced engineering fields.

● Structure-property correlations in mulberry silk.

Mulberry silk, produced by the Bombyx mori silkworm, is celebrated for its exceptional
mechanical properties, lustrous appearance, and biocompatibility.
Understanding the intricate relationship between its structure and
remarkable properties is essential for various applications in textiles,
biomedical engineering, and material science. This overview explores the
hierarchical structure of mulberry silk and how it correlates with its unique
properties.

1. Hierarchical Structure of Mulberry Silk

The structure of mulberry silk can be understood at multiple hierarchical levels:

1.1. Molecular Level

● Amino Acid Composition: Silk fibroin, the primary protein in mulberry silk,
consists mainly of repetitive sequences of amino acids, particularly glycine (Gly),
alanine (Ala), and serine (Ser). These small amino acids facilitate tight packing
and the formation of crystalline structures.
● Primary Structure: The repetitive sequence typically follows the pattern
(Gly-Ala-Gly-Ala-Gly-Ser), which contributes to the formation of β-sheet
structures critical for silk's strength and stability.
1.2. Secondary Structure
● β-Sheet Crystalline Regions: The organized arrangement of polypeptide chains
into antiparallel β-sheets forms highly ordered crystalline domains. These
domains are responsible for the high tensile strength and stiffness of silk.
● Amorphous Regions: Between the crystalline domains lie less ordered,
amorphous regions composed of irregular amino acid sequences. These regions
impart elasticity and flexibility to the silk fiber.

1.3. Tertiary and Quaternary Structures

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 ● Nanofibrils Formation: The combination of crystalline and amorphous regions
assembles into nanofibrils, which further bundle into microfibrils, creating a
layered and fibrous structure.
● Fiber Assembly: Multiple microfibrils aggregate to form the final silk fiber,
exhibiting a complex and hierarchical organization that enhances mechanical
performance.

2. Correlation Between Structure and Properties

The unique combination and organization of different structural levels in mulberry silk
result in a balance of properties that are difficult to replicate synthetically.

2.1. Mechanical Properties

● Tensile Strength: The β-sheet crystalline regions provide high tensile strength,
enabling silk to withstand significant loads without breaking. The tight hydrogen
bonding and dense packing of chains in these regions are crucial for this
property.
● Elasticity and Toughness: The amorphous regions allow for chain mobility under
stress, contributing to silk's elasticity and ability to absorb energy, making it
tough and resilient.
● Ductility: The interplay between crystalline and amorphous regions allows silk to
deform plastically, providing ductility and preventing brittle failure.
2.2. Thermal Properties
● Thermal Stability: The strong intermolecular interactions in crystalline regions
confer thermal stability, allowing silk to maintain structural integrity over a wide
temperature range.
● Heat Resistance: Silk exhibits moderate heat resistance, with degradation
temperatures higher than many synthetic polymers, attributed to its stable
β-sheet structures.
2.3. Optical Properties
● Luster and Shine: The smooth surface and regular arrangement of fibrils reflect
light uniformly, giving silk its characteristic luster and aesthetic appeal.
● Transparency and Opacity: Variations in fiber thickness and structure can
influence silk's transparency, useful in applications like optics and photonics.
2.4. Biocompatibility and Degradability

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● Biocompatibility: The natural protein composition and structural similarity to
human tissues make silk highly biocompatible, suitable for biomedical
applications such as sutures, scaffolds, and drug delivery systems.
● Biodegradability: Silk can be biodegraded by proteolytic enzymes, with
degradation rates influenced by the degree of crystallinity; higher crystalline
content slows degradation, allowing for controlled application lifespans.

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Module 5: Wild Silks and Spider Silk

● Structure and composition of wild silks (Tasar, Muga, Eri):

● Longitudinal and cross-sectional views

Longitudinal view of degummed silk fibres: (a) mulberry; (b)

tasar; (c) muga; (d) eri.

Cross-sectional view of degummed silk fibres: (a) mulberry;

(b) tasar; (c) muga; (d) eri.

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1. Tasar Silk

Longitudinal View: In the longitudinal view, tasar silk fibers appear relatively coarse and
have a slightly irregular surface texture. The fibers are typically
longer than those of mulberry silk, contributing to their unique
drape and feel. The longitudinal structure showcases the natural
variations in thickness and the presence of occasional nodes or
irregularities, which are characteristic of wild silk.

Cross-Sectional View: The cross-section of tasar silk is generally described as an

elongated triangle or needle-shaped. This shape contributes to its
strength and stiffness, making it suitable for applications such as
sarees and other traditional garments. The elongated triangular
cross-section allows for effective light reflection, enhancing the
silk's natural luster.

2. Muga Silk

Longitudinal View: Muga silk fibers are known for their lustrous golden hue and smooth
texture. In the longitudinal view, these fibers appear slightly thicker
and more robust compared to tasar silk. The fibers exhibit a
consistent diameter, which contributes to the overall uniformity of
the fabric produced from muga silk. The longitudinal structure
reflects the silk's durability and resilience.

Cross-Sectional View: The cross-section of muga silk is also elongated triangular,

similar to tasar silk. This shape enhances the fiber's strength and
contributes to its unique aesthetic qualities. The smooth surface
and regular arrangement of fibrils in muga silk fibers allow for
excellent light reflection, resulting in a rich, lustrous appearance
that is highly valued in traditional textiles.

3. Eri Silk

Longitudinal View: Eri silk fibers are distinctively different from tasar and muga. In the
longitudinal view, eri silk appears softer and more flexible, with a
slightly irregular surface. The fibers are shorter and have a more

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 staple-like quality, which contributes to a different texture and drape
compared to the filamentous nature of tasar and muga silk.

Cross-Sectional View: The cross-section of eri silk is often described as oval or

rounded, which contrasts with the elongated triangular shapes of
tasar and muga. This rounded cross-section contributes to the silk's
softness and comfort, making it suitable for garments that require
a gentle touch against the skin. Eri silk's unique structure also
allows for better insulation properties, making it a popular choice
for warmer clothing.

● Composition: fibroin, sericin, amino acids

1. Tasar Silk
● Fibroin: The fibroin in tasar silk is characterized by a high proportion of glycine
and alanine, which facilitates the formation of strong β-sheet structures. This
contributes to the silk's tensile strength and durability. Tasar silk fibers are
generally coarser compared to mulberry silk, which affects their texture and
applications.
● Sericin: Tasar silk contains a significant amount of sericin, which acts as a
protective coating around the fibroin fibers. This protein helps maintain the
integrity of the silk and provides some moisture resistance. However, tasar silk is
less lustrous than mulberry silk due to the higher sericin content and the rougher
surface texture.
● Amino Acids: The amino acid composition of tasar silk includes a variety of
essential and non-essential amino acids, with a notable presence of serine and
threonine. This diverse composition contributes to the silk's unique properties,
such as its ability to absorb moisture and its overall resilience.

2. Muga Silk
● Fibroin: Muga silk fibroin is known for its high strength and elasticity, attributed
to its unique amino acid sequence that promotes the formation of stable β-sheet
structures. Muga silk fibers are thicker and more robust than those of tasar silk,
which enhances their durability and makes them suitable for high-quality textiles.

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 ● Sericin: The sericin content in muga silk is lower than in tasar silk, contributing to
its smoother surface and higher luster. This lower sericin content allows for
better light reflection, giving muga silk its characteristic golden sheen.
● Amino Acids: Muga silk has a rich amino acid profile, with a high concentration
of glycine, alanine, and serine. This composition not only enhances the silk's
mechanical properties but also contributes to its luxurious feel and aesthetic
appeal.

3. Eri Silk
● Fibroin: Eri silk fibroin is distinct from tasar and muga due to its staple fiber
nature, as opposed to the filamentous structure of the other silks. The fibroin in
eri silk is softer and more flexible, which contributes to its unique texture and
comfort. The β-sheet structures in eri silk are less densely packed, resulting in a
softer feel.
● Sericin: Eri silk contains a moderate amount of sericin, which provides some
protective qualities while allowing for a softer texture. The processing of eri silk
is unique in that it does not require killing the pupa, leading to a more sustainable
production method.
● Amino Acids: The amino acid composition of eri silk includes a variety of amino
acids, with a significant presence of glycine, serine, and proline. This diverse
amino acid profile contributes to the silk's warmth and durability, making it
suitable for various applications, including clothing and blankets.

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 ● Introduction to spider silk:

Spider silk is a remarkable natural material produced by spiders through specialized

glands known as spinnerets. This protein-based fiber is renowned for its exceptional
mechanical properties, including high tensile strength, elasticity, and toughness, making
it one of the strongest materials known, often compared to steel and Kevlar on a
weight-to-weight basis.

● Types of spider silk fibres

Spider silk is a versatile and complex material produced by various species of spiders,
each capable of spinning multiple types of silk fibers for different
purposes. The most common type of spider web is the orb web,
constructed primarily by orb-weaving spiders from families such as
Araneidae and Nephilidae. Below are the main types of spider silk
fibers and their functions:

1. Dragline Silk

Dragline silk is produced by the ampullate glands and is the strongest and toughest silk
a spider can produce. It serves as the structural framework of the
spider's web and is used for various purposes, including:

● Web Construction: Dragline silk forms the radial lines of the orb web, providing
stability and support.
● Safety Lines: Spiders use dragline silk to create safety lines that allow them to
escape from predators or fall safely from heights.
● Mobility: It enables spiders to swing from place to place, facilitating movement
across their environment.

2. Capture Silk

Capture silk is produced by the flagelliform glands and is specifically designed for
trapping prey. This silk is characterized by its high elasticity and
stickiness, which are crucial for its function:

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 ● Sticky Threads: Capture silk is coated with an aqueous solution that forms sticky
droplets, enhancing its ability to catch flying insects.
● Energy Absorption: The silk's viscoelastic properties allow it to absorb and
dissipate the kinetic energy of captured prey, preventing them from bouncing off
the web.

3. Spiral Silk

Spiral silk is used in the construction of the spiral sections of orb webs. This silk is less
strong than dragline silk but is essential for the web's functionality:

● Web Structure: Spiral silk is laid down in a spiral pattern, creating the sticky
capture zones of the web.
● Flexibility: It provides the necessary flexibility to the web, allowing it to stretch
and absorb impacts from flying insects.

4. Tangle Silk

Tangle silk is produced by spiders that create irregular, tangled webs. This type of silk is
less organized than orb webs and serves different purposes:

● Disorientation of Prey: Tangle webs are designed to confuse and entangle flying
insects, making them easier to capture.
● Protection: The irregular structure can also provide some protection for the
spider from predators.

5. Sheet Silk

Sheet silk is used to create flat, horizontal webs that can catch insects falling from
above. This type of silk is often found in the webs of sheet-web
spiders:

● Trapping Mechanism: Sheet webs can effectively trap insects that land on them,
providing a different strategy for prey capture.

6. Funnel Silk

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Funnel silk is produced by funnel-web spiders and is characterized by its funnel-shaped
structure:

● Ambush Strategy: This type of silk allows spiders to hide in the funnel while
waiting for prey to fall in, providing an effective ambush strategy.

7. Other Specialized Silks:

Some spiders produce additional specialized silks for unique functions, such as:
● Cocoon Silk: Used to protect eggs or young spiders, providing a safe environment
for development.
● Silk for Ballooning: Some spiders produce lightweight silk strands that can catch
the wind, allowing them to travel long distances through a process known as
ballooning.

● Chemical compositions

1. Crystalline Regions:
The crystalline regions of spider silk are primarily composed of β-sheets, which are the
most heavily studied secondary structures in silk. These β-sheets
are formed through the natural physical cross-linking of amino acid
sequences, predominantly consisting of repeats of:
● Alanine
● Glycine-Alanine
● Glycine-Alanine-Serine

The presence of these sequences allows for the formation of strong hydrogen bonds
between adjacent polypeptide chains, resulting in high tensile
strength and stability. The ordered arrangement of these β-sheets
contributes significantly to the overall strength of the silk fibers.

2. Semi-Amorphous Regions

In addition to the crystalline β-sheets, spider silk contains less organized,

semi-amorphous regions that provide elasticity. These regions
include:

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● β-Spirals: Similar to a β-turn, these structures are composed of GPGXX repeats,
where X is primarily glutamine. This motif contributes to the flexibility of the silk.
● Helical Structures: Composed of GGX, these helical regions also enhance the
elasticity of the silk fibers.

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The semi-amorphous regions allow the silk to stretch and absorb energy, which is
particularly important for functions such as prey capture.

3. Non-Repetitive Regions

At the amino- and carboxyl termini of the silk proteins, there are non-repetitive regions.
While the exact impact of these termini on the mechanical
properties of silk is not fully understood, they are believed to play a
role in the controlled assembly of silk proteins, influencing how the
silk fibers form and function.

4. Diversity of Silk Proteins

Each silk-producing spider synthesizes a variety of silk proteins, leading to a rich

diversity of primary sequences and secondary structures. For
example, the silk fibers produced by Araneomorphae (true spiders)
can be categorized into several common amino acid modules,
including:

● Poly-Ala: Rich in alanine, contributing to strength.

● Poly-Ala-Gly: A combination that enhances flexibility and strength.
● GPGXX: Provides elasticity.
● GGX: Contributes to flexible regions.

Recent studies have also identified novel silk proteins in Mygalomorphae (tarantulas)
that do not conform to the typical categories found in true spider
spidroins, indicating a broader diversity in silk protein structures.

5. Self-Assembly Mechanism

The self-assembly of silk proteins into functional materials is driven by the hydrophobic
nature of certain amino acids, particularly those rich in alanine and
glycine. During the spinning process, as water is lost in the silk
gland, the hydrophobic regions align, facilitating the formation of
β-sheets. This process occurs without chemical cross-linking,
allowing for stability while still permitting enzymatic digestion or
slow degradation under specific environmental conditions.

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6. Example: Dragline Silk

The dragline silk from the spider Nephila clavipes originates from the major ampullate
gland and contains two primary proteins, known as major
ampullate silk protein 1 (MaSp1) and a second similar protein
enriched with proline. MaSp1 has a molecular weight of
approximately 275,000 Da and features shorter amino acid repeats
compared to silkworm fibroin. The polyalanine repeats in MaSp1
are crucial for forming the β-sheet crystals, while regions with GGX
repeats contribute to the silk's flexibility.

● General properties

Spider silk is a remarkable biopolymer known for its unique combination of mechanical
properties, making it one of the strongest and most versatile
natural materials. Here are the key properties that define spider silk:

1. High Tensile Strength

Spider silk exhibits exceptional tensile strength, often cited as being five times stronger
than steel on a weight-for-weight basis. This strength allows spider
silk fibers, which typically range from 0.2 to 1.0 mm in diameter, to
withstand significant forces without breaking. The tensile strength
can reach up to 1.75 GPa, making it superior to many synthetic
fibers, including Kevlar.

2. High Elasticity

In addition to its strength, spider silk is highly elastic, with a breaking elongation of over
26%. This elasticity enables the silk to stretch significantly before
breaking, allowing it to absorb energy effectively. The combination
of high tensile strength and elasticity makes spider silk particularly
effective for applications that require both durability and flexibility.

3. Toughness

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Spider silk is known for its toughness, which is defined as the ability to absorb energy
before breaking. It is approximately three times tougher than
aramid fibers and other industrial materials. This toughness is
crucial for the silk's role in capturing prey, as it allows the web to
absorb the impact energy of fast-moving insects without
rebounding them off.

4. Biodegradability

Unlike many synthetic fibers, spider silk is completely biodegradable. This property
makes it an environmentally friendly option for various applications,
as it decomposes naturally without leaving harmful residues.

5. Super-Contraction

A fascinating property of spider silk, particularly dragline silk, is its ability to

super-contract when wetted. The fibers can contract to less than
60% of their original length, resulting in a significant decrease in
elastic modulus and increased extensibility. This feature allows
spider webs to maintain their shape and tension when exposed to
moisture, such as dew.

6. Water Resistance

Spider silk is more waterproof than silkworm silk, allowing it to absorb moisture without
losing its structural integrity. This property enhances its
functionality in various environmental conditions.

7. Energy Absorption

Spider silk can absorb a substantial amount of energy prior to breaking, making it highly
effective for applications that require impact resistance. The silk's
ability to dissipate energy is crucial for its role in web construction,
where it must capture fast-moving prey without causing them to
bounce off.

8. Low Production Conditions

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Spider silk is produced under ambient temperatures and low pressure, using water as a
solvent. This environmentally friendly production process contrasts
sharply with the energy-intensive methods required for many
synthetic fibers.

● Tensile properties

Tensile Properties of Spider Silk

Spider silk is a natural material with exceptional tensile properties, characterized by its
unique combination of strength, elasticity, and toughness. These
properties vary across different spider species and silk types,
influenced by both intrinsic and extrinsic factors.

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Stress-Strain Behavior

The tensile behavior of spider silk is typically represented by a sigmoidal stress-strain

curve, indicating a balanced combination of strength and
extensibility. For example, studies on the silk of the Nephila
clavipes spider have demonstrated:

● High Initial Modulus: The initial modulus is approximately 34 GPa in the 0-5%
strain region, indicating significant stiffness.
● Pseudo Yield Point: A pseudo yield point occurs around 5% strain, marking the
transition to a strain-hardening phase.
● Strain Hardening: The modulus reaches a maximum of 22 GPa at 22%
elongation.
● Failure Strength and Elongation: The silk achieves a breaking strength of 1.75
GPa at an elongation of 36%.

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 ● Toughness: The toughness of spider silk is measured at 2.8 g/denier,
significantly surpassing that of aramid fiber (0.26 g/denier) and Nylon 6 fiber (0.9
g/denier).

Comparative Strength and Elasticity

Spider silk’s tensile properties make it superior to many synthetic fibers. It combines
high tensile strength with remarkable elasticity, allowing it to
absorb more energy before breaking. This makes spider silk
particularly effective for uses requiring both durability and flexibility.

Super-Contraction

One of the unique properties of spider silk, especially the dragline silk, is its ability to
super-contract when wetted. The fibers can contract to less than
60% of their original length, significantly reducing their elastic
modulus and increasing extensibility. This reversible process helps
maintain web tension and shape, particularly when moistened by
dew.

Factors Influencing Tensile Properties

The tensile properties of spider silk can vary based on several factors:

● Species and Individual Variability: Different spider species, such as Araneus,

Latrodectus, Nephila, and Euprosthenops, exhibit variations in modulus, strength,
and elongation.
● Daily and Environmental Variability: Silk properties can fluctuate daily based on
factors like the spider's diet, body temperature, and the speed of silk production.
For instance, increased production speed can enhance breaking stress and
Young's modulus but reduce elongation.
● Mechanical Processing: The spider's silk production involves a sophisticated
mechanical process, aligning protein molecules to optimize strength and
toughness without premature coagulation.

Structural Composition

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The molecular structure of spider silk contributes to its tensile properties:

● Crystalline and Elastic Modules: Alanine-rich crystalline regions enhance

strength, while proline-containing elasticity modules contribute to extensibility.
● Molecular Springs: The presence of long spirals and helices in the protein
configuration acts as molecular springs, contributing to the elasticity and
extensibility of the silk.

● Applications of spider silk

Spider silk, with its exceptional mechanical properties and environmentally friendly
production, has been utilized and explored for various applications
throughout history and in modern technology. Here are some
notable applications:

Historical Uses
1. Wound Care: Ancient Greeks used cobwebs to stop wounds from bleeding,
leveraging the silk's natural properties for medical purposes.
2. Fishing: Aborigines and people from the Solomon Islands have used spider silk
as fishing lines and nets due to its strength and flexibility.
3. Optical Devices: Until World War II, spider silk was used as crosshairs in optical
targeting devices like guns and telescopes, taking advantage of its fine and
durable nature.

Modern and Potential Applications

1. Composite Materials:
○ Magnetic Silk-Fiber Composites: By binding colloidal magnetite
nanoparticles to spider silk, researchers have created fibers that retain
high strength and elasticity while being orientable by magnetic fields.
These composites could be used in microelectronics and fiber optics.
○ Conductive and Semiconductive Composites: Similar techniques using
metallic or semiconductive nanoparticles can produce silk-fiber
composites with potential applications in smart fabrics and active filters.
2. Textiles and Protective Gear:
○ Bullet-Proof Clothing: Due to its high tensile strength and toughness,
spider silk is a candidate for developing lightweight, bullet-proof clothing.

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 ○ Wear-Resistant Clothing: Its durability makes it suitable for creating
wear-resistant and lightweight garments.
○ Ropes, Nets, and Parachutes: The strength and flexibility of spider silk
make it ideal for these applications, offering a lightweight alternative to
traditional materials.
3. Medical and Biotechnological Applications:
○ Biodegradable Materials: Spider silk's biodegradability makes it suitable
for creating environmentally friendly products like biodegradable bottles.
○ Medical Devices: Its biocompatibility and strength are promising for
applications such as bandages, surgical threads, and artificial tendons or
ligaments.
○ Tissue Engineering: Spider silk fibers have been used to seed human
Schwann cells, demonstrating potential for peripheral nerve injury
treatments. Additionally, hydrogels and porous scaffolds made from
spider silk proteins support tissue engineering by providing stable,
biocompatible structures for cell growth and tissue development.
4. Advanced Material Design:
○ Microcapsules: Recombinant spider silk proteins have been used to create
microcapsules with high mechanical stability, suitable for drug delivery
and microreactor design. These capsules can be engineered to degrade in
response to specific enzymes, offering controlled release capabilities.
5. Environmental and Sustainable Solutions:
○ Rust-Free Panels: Spider silk's properties could be harnessed to create
rust-free panels for vehicles and boats, contributing to longer-lasting and
more sustainable products.

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