Polyamide 6/Silica Nanocomposites Prepared

by In Situ Polymerization

FENG YANG, YUCHUN OU, ZHONGZHEN YU

State Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100080, People’s Republic of China

Received 3 June 1997; accepted 2 November 1997

ABSTRACT: A polyamide 6 (PA 6)/silica nanocomposite was obtained through a novel

method, in situ polymerization, by first suspending silica particles in e-caproamide
under stirring and then polymerizing this mixture at high temperature under a nitro-
gen atmosphere. The silicas were premodified with aminobutyric acid prior to the
polymerization. The effects of the addition of unmodified and modified silicas on the
dispersion, interfacial adhesion, isothermal crystallization, and mechanical properties
of PA 6 nanocomposites were investigated by using scanning electron microscopy, dy-
namic mechanical analysis, differential scanning calorimetry, and mechanical tests,
respectively. The results show that the silicas dispersed homogeneously in the PA 6
matrix. The addition of silicas increases the glass transition temperature and crystalli-
zation rate of PA 6. The mechanical properties such as impact strength, tensile strength,
and elongation at break of the PA 6/modified silica nanocomposites showed a tendency
to increase and decrease with increase of the silica content and have maximum values
at 5% silica content, whereas those of the PA 6/unmodified silica system decreased
gradually. q 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 355–361, 1998

Key words: in situ polymerization; nanocomposites; mechanical properties; polyam-

ide 6; silica

INTRODUCTION the properties of traditional composites, but also

they exhibit unique optical, electric, and magnetic
Particle-filled polymer composites have attracted properties.4 – 11
strong interest for a long time because of their At present, organic/inorganic nanocomposites
widespread applications in the automobile, house- are prepared mainly via three methods:
hold, and electrical industries.1 The employment (I) Sol–gel processing, 12 – 18 which includes
of inorganic particle-filled composites not only can two approaches: hydrolysis of the metal
improve the physical properties of the materials alkoxides and then polycondensation of
such as the mechanical properties, thermal resis- the hydrolyzed intermediates. Most of the
tance, and chemical reagent resistance, 2,3 but also interest in this method is concentrated on
can provide high-performance materials at a metal organic alkoxides, especially silicon
lower cost.1 Recently, according to the develop- oxide (silica) since they can form an oxide
ment of the nanotechnique, there has been a grow- network in organic matrices. The hydro-
ing interest in the field of nanocomposites due to lysis approach can be described as formu-
their special properties: Not only can they provide lation (1):
H/
Correspondence to: Y. Ou. Si{OR / H2O r0
OH
Journal of Applied Polymer Science, Vol. 69, 355 – 361 ( 1998 )
q 1998 John Wiley & Sons, Inc. CCC 0021-8995/98 / 020355-07 Si{OH / R{OH (1)
355

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356 YANG, OU, AND YU

According to the difference of final filler interfaces.28,32,33 Inorganic particles

products, there are two reaction types of may disperse homogeneously in the poly-
polycondesation, which are shown as for- mer matrices when they are premodified
mulations (2) and (3): by a coupling agent. Furthermore, the re-
sulting materials obtained by this method
H/ also can be easily processed since they
Si{OH / HO{Si r0 have good flowing properties.
OH

Si{O{Si / H2O (2) In this article, we employed in situ polymeriza-

tion to prepare PA 6/silica nanocomposites. The
H/
Si{OH / RO{Si r0 interfacial interaction between PA 6 and silica
OH was investigated to explain the relationship of the
structure and properties of this composite. Other
Si{O{Si / R2O (3) properties such as crystallization behavior and
mechanical properties were also characterized to
The sol–gel process provides a method show the efficiency of this method.
for the preparation of inorganic metal ox-
ides under mild conditions starting from
organic metal alkoxides. This permits EXPERIMENTAL
structural variation without composi-
tional alteration. However, the formation Materials
of a crosslinking network of organic metal
oxides makes it difficult to process, and e-Caproamide and aminocapric acid were used to
it is a disadvantage that circumscribed prepare PA 6. The inorganic filler used was silica
the application of this method. (AEROSIL, R972).
(II) In situ intercalative polymerization, which
is a good method for the preparation of Preparation of Composites
polymer/clay mineral hybrid or nanocom-
posites, 19–22 such as the PA 6/montmoril- The surface of the silica was premodified by
lonite hybrid.22 Usuki et al.22–24 reported aminobutyric acid first; then, the modified silica
that v-amino acid can intercalate between was dried in oven for the preparation of PA 6/
the n-montmorillonite layer and so the silica nanocomposites. The preparation of PA 6/
montmorillonite can be swollen. Many silica nanocomposites included two processes:
other experiments reported on the synthe- The modified silicas were dispersed in e-caproa-
ses of polymers in the interlayer space of mide at 907C first, and aminocapric acid as the
clay.25–27 The thickness of montmorillonite initiator was added at the same time; then, this
in the resulting composite ranged from 1 mixture was polymerized at high temperature un-
to 3 nm while the diameter of them is ap- der a nitrogen atmosphere. The PA 6/unmodified
proximately 100 nm. It is an effective silica composite can also be prepared by this
method to prepare a polymer/clay compos- method.
ite which can provide high-performance
materials at a relative low cost, but this
Measurements
method only adapts to clay minerals, which
is also a significant disadvantage for its ap- The dynamic mechanical loss (tan d ) and glass
plication. transition temperature (Tg ) of the samples were
(III) In situ polymerization, 28 – 31 which is a tested by a Perkin–Elmer DMA-7 tester. The
method where nanometer scale inorganic mode of force loading was three-point bending.
fillers or reinforcements are dispersed in The testing frequency, heating rate, and tempera-
the monomer first; then, this mixture is ture scanning range were 1 Hz, 107C/min, and
polymerized using a technique similar to 050–2007C, respectively. The surface of the sam-
bulk polymerization. It is obvious that ple was covered with aluminum foil and then a
the most important factors that affect the dielectric analysis tester was used to measure the
properties of composites are the disper- dielectric dissipation and dielectric constant of the
sion and the adhesion at the polymer and sample at room temperature. The testing fre-

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 POLYAMIDE 6/SILICA NANOCOMPOSITES 357

Figure 1 SEM micrographs of (a) PA 6/10% unmodified silica nanocomposite and

(b) PA 6/10% modified silica nanocomposite.

quency range was 500–9000 Hz. Isothermal and composite is more significant than that of the PA
nonisothermal crystallization behaviors of the 6/unmodified silica system, indicating that the in-
sample were measured by a Perkin–Elmer DSC- terfacial interaction between silica and PA 6 can
7 tester in the scanning range of 60–2507C and be improved by premodifying the silica surface. It
the scanning rate of 107C/min. Scanning electron also can be seen that the premodification of the
micrographs were obtained with a Hitachi S-530 silicas reduces the areas of the dynamic loss peak
scanning electronic microscope. Tension testing of PA 6 nanocomposites in comparison with the
was examined by a CS-183 mechanical tester with PA 6/unmodified silica system, which means that
the tension rate at 5 rpm. good adhesion between the modified silica parti-
cles and the nylon 6 matrix can limit the motion
of the PA 6 molecular chain.
RESULTS AND DISCUSSION Furthermore, to obtain more information about
the interfacial interaction of the PA 6/silica nano-
SEM Observation composite, dynamic dielectric tests were per-
formed. As shown in Figure 3, the addition of the
To investigate the dispersion quality of silica par-
unmodified silica improves the dielectric constant
ticles, the morphology of PA 6/silica composites
of PA 6, while the modification of silica reduces
were studied and the results are shown in Figure
slightly the dielectric constant of the PA 6 nano-
1. It is evident that the modified silicas have a
composite. Based on the theory of interfacial po-
more homogeneous dispersion in the PA 6 matrix
compared with the unmodified silicas. It can also
be seen from Figure 1 that the sizes of both the
modified and unmodified silica particles are
mostly in the range of 50–110 nm except the ag-
gregated silicas, suggesting that in situ polymer-
ization is a good method for preparing inorganic/
organic nanocomposites.

Interfacial Interaction
Dynamic mechanical analysis (DMA) is an effi-
cient method to investigate the interfacial interac-
tion of a composite. Figure 2 shows the DMA
results of the resulting composites prepared
through in situ polymerization. The Tg of the PA
6/silica nanocomposites shifts to a high tempera- Figure 2 Dynamic mechanical spectra of (1) PA 6,
ture whether the silicas were premodified or not, (2) PA 6/modified silica nanocomposite, and (3) PA 6/
while the shift of the PA 6/modified silica nano- unmodified silica nanocomposite.

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358 YANG, OU, AND YU

Ìa /Ìt Å K 1 / n (T ) f ( a ) (2)
ln K(T ) Å ln K0 0 Ea /RT (3)
1
ln(Ìa /Ìt) Å ln K(T ) / ln n / ln(1 0 a )
n
n01
/ ln[0 ln(1 0 a )] (4)
n
1
ln(Ìa /Ìt) 0 ln(1 0 a ) Å ln K(T ) / ln n
n
n01
/ ln[0 ln(1 0 a )] (5)
n
Figure 3 Dielectric constant versus frequency of ( l)
PA 6, ( j) PA 6/10% modified silica nanocomposite, and In eqs. (1) and (2), Ìa /Ìt, K(T ), and n represent
(h) PA 6/10% unmodified silica nanocomposite. the crystallizing rate, the constant of crystallizing
rate, and the Avrami index, respectively. K is a
function of temperature which obeys the theory
larization, the existence of faults in the interface of Arrhenius shown in eq. (3), in which Ea is the
between the silica particles and the PA 6 matrix crystallizing activation energy. Equation (4) is de-
such as lattice vacancy and electron vacancy can rived from eqs. (1) and (2), and eq. (5) is the
lead to the variation of the dissipation of positive transformation of eq. (4), in which K(T ) and n
and negative electric charges. When the sample are independent of time. If we let ln[ 0 ln(1 0 a )]
was exposed to an electric field, positive and nega- be the abscissa and ln(Ìa /Ìt) 0 ln(1 0 a ) be
tive electric charges move to the cathode and the ordinate, we get n and ln K(T ). Furthermore,
anode, respectively, and as a result, these electric according to eq. (3), Ea can also be obtained, and
charges accumulate at the faults in the interface the results are listed in Table I.
of the composite and formed an electric dipole mo- Based on the Avrami thoery, n depends on the
ment, resulting in the polarization of space elec- geometry of the crystal growth and on the type of
tric charges. Due to the good interfacial adhesion nucleation. Athermal nucleation and spherulitic
and dispersion quality of the PA 6 nanocomposite growth lead to an Avrami exponent of 3, whereas
containing the modified silicas, its faults men- thermal nucleation and spherulitic growth, to a
tioned above are much fewer than those in the value of 4. According to the experimental data
composite containing the unmodified silicas, and listed in Table I, it can be seen that the n of pure
this may be the principal reason why the dielectric PA 6 is approximately 4, while those of PA 6 in the
constant of the PA 6 nanocomposite decreases other two composites are nearly 3. The decrease of
when the silica surface was modified. n is due to that the silicas in the PA 6 nanocom-
posites act as heterogeneous nucleation sites, fol-
lowed by three-dimensional spherulitic growth.
Crystallization Behavior of PA 6/Silica Moreover, the data of Ea indicate that the addition
Nanocomposite of silica particles enhances the crystallization
of PA 6.
It is generally known that the crystallization of According to eq. (6), 25 the half-time of crystalli-
crystalline polymers can be divided into two pro- zation can be calculated. Figure 4 gives the plots
cesses: nucleation and crystal growth. The most of half-time versus temperature of PA 6 and its
successful description of the crystallizing rate of composites. The unmodified silicas indeed in-
a polymer is of the kinetics equations of Avrami crease the crystallization rate of PA 6. On the
published in 1939, 34,35 which have been amended other hand, the modification of the silica surface
many times 36,37 in the following years and gained causes a slight decrease in the crystallization rate
worldwide acceptance. The equations are listed in of the crystallization of PA 6. There are two effects
the following: of the silicas on the crystallization of PA 6: First,
the silicas serving as nucleation sites accelerate
f ( a ) Å n(1 0 a )[0 ln(1 0 a )] ( n0 1 / n ) (1) the process of PA 6 crystallization; on the other

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 POLYAMIDE 6/SILICA NANOCOMPOSITES 359

Table I Data of Isothermal Crystallization Kinetics Parameters by DSC

Sample T (7C) n 1/t1/2 (S01) Ea (kJ/mol)

PA 6 195 4.23 3.306 1 1002 1064.129

194 4.00 2.087 1 1002
193 4.19 1.083 1 1002
192 4.20 2.776 1 1003
PA 6/10%SiO2 (modified) 195 3.15 4.858 1 1002 518.98
194 2.18 4.251 1 1002
193 2.05 2.776 1 1002
192 2.66 2.93 1 1003
PA 6/10%SiO2 (unmodified) 195 2.91 5.223 1 1002 448.956
194 2.93 4.569 1 1002
193 2.88 3.770 1 1002
192 2.59 5.945 1 1003

hand, the good adhesion between PA 6 and the strength, elongation at break, and impact
modified silicas impedes the motion of the PA 6 strength of the PA 6/modified silica nanocompos-
molecular chains, hampering the crystallization ites increase remarkably with the content of the
of the PA 6 matrix. The latter effect is much filler and possess maximum values at 5 wt % filler
smaller compared with the former in our experi- content, whereas those of the PA 6/unmodified
ment, so the addition of the modified silicas also silica system decrease gradually with increasing
improves the crystallization rate of the PA 6 ma- filler content. According to the analysis above, the
trix: composites containing the modified silicas have
good dispersion and interfacial adhesion, so when
t1 / 2 Å (ln 2/K) 1 / n (6) under tensile stress, the force is transferred to
silica particles through the interphase and the
silica particles become the receptor of the tensile
Mechanical Properties of Nylon 6/Silica
force. When the tensile stress added on the com-
Nanocomposite
posites is beyond a critical value, the damage to
Figure 5 shows the mechanical properties of PA the composite results from the destruction of the
6 nanocomposites filled with unmodified or modi- interphase between PA 6 and the silicas. Due to
fied silicas. It can be seen that the tensile the difference in the interfacial adhesion, the
modified and unmodified silicas show different ef-
fects on the toughness of PA 6. The unique me-
chanical behavior of a silica-modified nanocom-
posite is mainly because of the agglomeration of
silica particles beyond the silica contents of 5 wt
% as we described in another article.38 The deteri-
oration of the toughness of unmodified composites
appears to be related mainly to the bad interfacial
adhesion between unmodified silica and nylon 6,
which led to many defects and flaws in the areas
of the interphase and, consequently, made the
damage to the composites easier. In addition, the
addition of silicas improved the modulus of the
resulting composites whether the silicas were
modified or not.
Figure 4 Reciprocal of half-time of crystallization (1/ CONCLUSION
t1 / 2 ) versus temperature for (s) PA 6, (.) PA 6/10%
modified silica nanocomposite, and ( ,) PA 6/10% un- In situ polymerization is a good method for the
modified silica nanocomposite. preparation of inorganic/organic nanocomposites

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360 YANG, OU, AND YU

Figure 5 (a) Impact strength, (b) elongation at break, (c) tensile strength, and (d)
modulus versus silica content for ( l) unmodified and ( j) modified silica-filled PA 6
nanocomposites.

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