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Unformatted text preview: weightaverage molecular weight, as
data
20,000–25,000
0.27
6075
taken from Figure 14.3b. The wi Mi products for the size intervals are tabu25,000–30,000
27,500
0.20
5500
lated in the
30,000–35,000 righthand column. The sum of these products (Equation 14.5b)
32,500
0.08
2600
yields a value
35,000–40,000 of 23,200 g/mol for Mw.
37,500
0.02
750
Mn 21,150 Table 14.4b Data Used for WeightAverage Molecular
Weight Computations in Example
(b) To determine the degree of polymerization (Equation 14.6), it is first necProblem 14.1
essary to compute the repeat unit molecular weight. For PVC, each repeat
Molecular Weight
Mean Mi
unit consists of two carbon atoms, three hydrogen atoms, and a single chloRange ( g/mol )
( g/mol )
wi
wiMi
rine atom (Table 14.3). Furthermore, the atomic weights of C, H, and Cl are,
5,000–10,000
7,500
150
respectively, 12.01, 1.01, and 35.45 g/mol.0.02 for PVC
Thus,
10,000–15,000
15,000–20,000 g/mol 2
m 2 1 12.01
20,000–25,000
62.50
25,000–30,000g/mol
30,000–35,000
and
35,000–40,000 12,500
17,500
3 1 1.01 g/mol 2
22,500
27,500
32,500
37,500 DP Mn
m 0.10
0.18
35.45 g/mol
0.29
0.26
0.13
0.02 21,150 g/mol
62.50 g/mol Mw 338 1250
3150
6525
7150
4225
750
23,200 Lecture 2021 Two Ways to Count Chains/Measure MW: Amount of Polymer ¯
NumberAverage M General Trends of the Distribution: N WeightAverage ¯
Mw Since the weightaverage puts a large
emphasis on the higher molecular weight
there is a: • Shift of the peak to the right and
broadening of the distribution • Decrease in the peak height as compared
to the numberaverage Molecular Weight → Polydispersity Index (PDI) ~ 2 50
PDI play a role in injection molding!
TP Lecture 2021 So, How Does MW Affect Properties?
Molecular
Weight n,
Polyethylene Tensile
Strength Material
State 58
86226
2541402
14029802
105 2 2
310
1830 gas 38
850
50350
>350 (MPa) liquid
solid
solid
solid Why as the MW increases do we see the change?
1. Polymer chains become increas...
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This document was uploaded on 03/09/2014 for the course MSE 230 at Purdue UniversityWest Lafayette.
 Spring '08
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