Derivation of quasi-crystalline enthalpy on melting of copolymers from solubility parameter.

For RIS models of ethylene/propylene and propylene/ethylene random copolymers, the composition dependence of Tm, a and hx was investigated. Where Tm is the melting temperature of copolymer, a is the coefficient in equation (1) and hx is the transition enthalpy per molar structural unit due to quasi-crystal in the amorphous region. The value of a changed with increasing minor component units, differing from the experimental results for nylon-6, 66 and polyethylene-terephthalate copolymers. Whereas hx under the assumption of equation (2) was 454 cal/mol (extrapolated value at 1-XA=0) for ethylene/propylene copolymer and the reasonable value was not obtained for propylene/ethylene copolymer. For latter copolymer, it was presumed that in the range of 1-XA≤0.20 the quasi-crystal is not contained in the amorphous region. XA is the mole fraction of major component units.In the next stage, the derivation method of hx from the solubility parameter δp of homopolymers was discussed and for polyethylene-terephthalate and nylon-66, the good agreement between hxs from δp, and equation (1) was obtained. In this case, the transition temperature of the quasi-crystal in the amorphous region should be near the melting temperature of crystal.