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Lecture 3

The document discusses goals of polymer lectures including understanding what polymers are, calculating average molecular weights and degrees of polymerization, and factors controlling crystallinity. It provides examples of calculating number-average molecular weight, weight-average molecular weight, number-average degree of polymerization, and weight-average degree of polymerization using data from molecular weight distributions of polypropylene and polyvinyl chloride.

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bilal sheikh
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69 views9 pages

Lecture 3

The document discusses goals of polymer lectures including understanding what polymers are, calculating average molecular weights and degrees of polymerization, and factors controlling crystallinity. It provides examples of calculating number-average molecular weight, weight-average molecular weight, number-average degree of polymerization, and weight-average degree of polymerization using data from molecular weight distributions of polypropylene and polyvinyl chloride.

Uploaded by

bilal sheikh
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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Goals of Lecture 1 and 2

1. Understand what a polymer is, and understand polymer shape, structure and
configuration, and copolymers

2. Understand and be able to calculate number- and weight-average molecular


weight, and number- and weight-average degree of polymerization.

3. Understand the factors that control the extent of crystallinity in a polymer


sample
Go to Lecture 4

Problem
Below, molecular weight data for a polypropylene material are tabulated.
Compute (a) the number-average molecular weight, (b) the weight-average

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molecular weight, (c) the number-average degree of polymerization, and (d) the
weight-average degree of polymerization.

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Solution
(a) From the tabulated data, we are asked to compute Mn, the number-
average molecular weight.

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(b) From the tabulated data, we are asked to compute Mw, the weight-average
molecular weight.

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(c) Now we are asked to compute the degree of polymerization, which is
possible using Equation 4.6. For polypropylene, the repeat unit molecular
weight is just

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Problem
Assume that the molecular weight distributions shown in Figure 4.3 are for
polyvinyl chloride. For this material, compute (a) the number-average molecu-lar
weight; (b) the number-average degree of polymerization; and (c) the weight-
average molecular weight.

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SOLUTION
(a) The data necessary for this computation, as taken from Figure 4.3 a, are
presented in Table 4.4a. According to Equation 4.3a, summation of all the xi Mi
products (from the right-hand column) yields the number-average molecular
weight, which in this case is 21,150 g/mol.

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(b) To determine the number-average degree of polymerization (Equation
4.4a), it first becomes necessary to compute the mer molecular weight. For
PVC, each mer consists of two carbon atoms, three hydrogen atoms, and a

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single chlorine atom (Table 4.3). Furthermore, the atomic weights of C, H, and
Cl are, respectively, 12.01, 1.01, and 35.45 g/mol. Thus, for PVC m= 2(12.01 g/mol)
+ 3(1.01 g/mol) + 35.45 g/mol= 62.50 g/mol
and
nn = Mn/m =21,150 g/mol/62.50 g/mol = 338
(c) Table 4.4b shows the data for the weight-average molecular weight, as
taken from Figure. The wi Mi products for the several size intervals are
tabulated in the right-hand column. The sum of these products yields a value of
23,200 g/mol for Mw

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