Page 81 - Carrahers_Polymer_Chemistry,_Eighth_Edition

44 Carraher’s Polymer Chemistry

TABLE 2.5
Selected Property–Structure Relationships
Glass Transition Temperature
Increases with the presence of
Bulky pendant groups
Stiffening groups as 1,4-phenylene
Chain symmetry
Polar groups
Cross-linking
Decreases with the presence of
Additives like plasticizers
Flexible main chain groups
Nonpolar groups
Dissymmetry
Solubility
Favored by
Longer chain lengths
Low interchain forces
Disorder and dissymmetry
Increased temperature
Compatible solvent
Crystallinity
Favored by
High interchain forces
Regular structure; high symmetry
Decrease in volume
Increased stress
Slow cooling from melt
Homogeneous chain length

Selected property–structure relationships are summarized in Tables 2.5 and 2.6. As noted before,
some polymers can be classifi ed into two categories, with properties being greatly varied by vary-
ing molecular weight, end group, processing, cross-linking, plasticizer, and so on. Nylon-66 in its

more crystalline form behaves as a fiber, whereas less crystalline forms of nylon-66 are generally

classified as plastics.
There are some general guidelines with respect to a material’s T and T , its general amorphous/
g m
crystalline structure, and the potential use area. Elastomers are cross-linked, amorphous polymers
where the use temperature is above its T . An adhesive is a linear or branched amorphous polymer
g
that is used above its T . Coatings are generally near their T when used so that some fl exibility is
g g
present allowing the coatings material to withstand temperature changes without cracking and so
coalescing occurs on drying. Plastics can be either amorphous or partially amorphous. Amorphous
(or partially crystalline) plastics such as PP and PE should have a use temperature below the T
m
but above the T . Fibers are composed of crystalline polymers where the use temperature is below
g
the T .
m
2.6 CROSS-LINKING
Cross-linking is important because this is a major mechanism for retaining shape which, in turn,

influences the physical properties, such as solubility, of polymers. There are three types of cross-
linking present in synthetic and natural polymer. Two of the three types are physical and the third
is chemical.

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