Effect of Crystallinity on Tg

The previous discussion centered on amorphous polymers, with atactic polystyrene being the most frequently studied polymer. Semicrystalline polymers such as polyethylene or polypropylene or of the polyamide and polyester types also exhibit glass transitions, though only in the amorphous portions of Uiese polymers. The Tg is often increased in temperature by the molecular-motion restricting crystallites. Sometimes Tg appears to be masked, especially for highly crystalline polymers. 

Boyer (9) points out that many semicrystalline polymers appear to possess two glass temperatures (a) a lower one, Tg(L), that refers to the completely amorphous state and that should be used in all correlations with chemical structure (this transition correlates with the molecular phenomena discussed in previous sections), and (b) an upper value, Tg(U), that occurs in the semi-crystaUine material and varies with extent of crystallinity and morphology. 

Linear polyethylene, frequently referred to as polymethylene, offers a complete contrast with polystyrene in that it has no side groups and has a high degree of crystallinity, usually in excess of 80%. Because of the high degree of crystallinity, molecular motions associated with Tg are partly masked, leading 

Davis and Eby support the -30°C value on the basis of volume-time measurements Stehling and Mandelkern (127) favor the -128°C value based on mechanical measurements, lllers (128) and Boyer (9) support the value of -80°C based on extrapolations of completely amorphous ethylene-vinyl acetate copolymer data with copolymer-T relationships. Boyer (9) supports the position that -80° is Tg L) and -30°C represents Tg(U). Tlie transition at -128°C is thought to be related to the Schatzki crankshaft motion (Section 8.4.1), although the situation apparently is more complicated (128). Tobolsky (129) obtained -81 °C for amorphous polyethylene based on a Fox plot [see equation (8.73)] of statistical copolymers of ethylene and propylene, it-polypropylene having a Tg of -18°C. 

The usually stated Tg range is Tg — +40°C for polyamide 612 to Tg — 60°C for polyamide 6 (9) however, Tg depends on the crystallinity of the particular sample. A -methylated polyamides, with a lower hydrogen bonding, have lower Tg % (130). As X and y increase in equations (8.76) and (8.77), the structure becomes more polyethylene-like, and Tg gradually decreases. Interestingly, when jc 4, there is a characteristic mechanical loss peak at about -130°C, again suggestive of the Schatzki motion (Section 8.4.1). 

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