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56 Carraher’s Polymer Chemistry
Relatively
narrow
Relative fraction Relatively
broad
Bimodal
Molecular weight
FIGURE 3.1 Relative differential weight distribution curves.
Commercial polymer range
Melt viscosity
Tensile strength
Impact strength
Property
Molecular weight
FIGURE 3.2 Relationship between molecular weight and polymer physical properties.
The value of TMWV is dependent on the cohesive energy density (CED) of amorphous polymers,
the extent of crystallinity in crystalline polymers, and the effect of reinforcements in polymeric
composites. Thus, while a low molecular weight amorphous polymer may be satisfactory for use as
a coating or adhesive, chain lengths generally above 100 are often required if the polymer is to be
used as an elastomer or plastic.
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