Introduction to amorphous polymers

Let us consider a polymer which is in its molten state. What may happen when it cools down The two possibilities are shown in Fig. 5.1. The polymer may either crystallize (route a) or cool down to its glassy, amorphous state (route b). The temperature at which the slope in the specific volume-temperature graph (route b) changes is referred to as the glass transition temperature, Tg. Rigid-rod polymers, i.e. polymers with very inflexible groups in the backbone chain or in side chains, may form liquid-crystalline states, an issue dealt with in Chapter 6. 

What molecular factors determine whether a polymer will crystallize or not The regularity of the polymer is the key factor isotactic polypropylene crystallizes, whereas atactic polypropylene does not. Atactic polymers generally do not crystallize with two exceptions  

The X group forms a long regular side chain. Side-chain crystallization may occur provided that the pendant groups are of sufficient length. 

Random copolymers are incapable of crystallizing except when one of the constituents is at a significantly higher concentration than the other constituent. Linear low-density polyethylene with a crystallinity of about 50% contains 98.5 mol% of methylene units and 1.5 mol% of CHX units, where X is -CH2CH3 or a longer homologue. Polymers which are potentially crystallizable may be quenched to a 

Cooling past the glass transition temperature is accompanied by a dramatic change in the mechanical properties. The elastic modulus increases by a factor of 1000 when the polymeric liquid is cooled below Tg and the modulus of the glassy polymers is relatively insensitive to changes in molar mass and repeating unit structure. The actual value of Tg is, however, very dependent on the repeating unit, the molecular architecture and the presence of low molar mass species, as is shown in section 5.2. 

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