Macromolecules in the Amorphous Glassy State

When polymer melts, mbbers, or elastomers are cooled down below Tg, they may freeze to glasses (noncrystalline amorphous phases). The rotational motions of the chain segments (micro-Brownian motions) are almost stopped now, and the transparent materials become stiff and (in most cases) brittle. 

Polymer glasses are formed best when the macromolecular chains are irregular in stmcture (atactic, branched, crosslinked) so that crystallization is prevented. 

Regular (isotactic, syndiotactic unbranched) polymer chains form glasses only if they are cooled down so fast that crystallization is prevented such a quenching procedure freezes the material in the glassy state even if the polymer is able to crystallize. 

Amorphous polymers are characterized by the following properties They are transparent and very often soluble in common organic solvents at room temperature. The following amorphous polymers have gained industrial importance as thermoplastic materials poly(vinyl chloride), polystyrene, poly(methyl methacrylate), ABS-polymers, polycarbonate, cycloolefine copolymers, polysulfone, poly (ether sulfone), poly(ether imide). 

In most cases, however, polymers crystallize neither completely nor perfectly. Instead, they give semicrystalline materials, containing crystalline regions separated by adjacent amorphous phases. Moreover, the ordered crystalline regions may be disturbed to some extent by lattice defects. The crystalline regions thus embedded in an amorphous matrix typically extend over average distances of 10-40 nm. The fraction of crystalline material is termed the degree of crystallinity. This is an important parameter of semicrystalline materials. 

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