Amorphous structure heat: Molecular structure from solution heats of polymethylmethacrylate in orthodichlorobenzene

The heats of solution in orthodichlorobenzene were measured as a function of temperature for both as-reprecipitated and quenched atactic polymethylmethacrylate. The as-reprecipitated curve decreases linearly with a slope of 0.096 cal/g°C from room temperature to 90°C, and above 110°C has a constant value of +1.5 cal/g. The quenched material data decreases linearly with a slope of 0.12 cal/g°C from room temperature to 55°C, jumps +2 cal/g at 55°C, and continues the linear decrease with a slope of 0.096 cal/g°C from 55 to 90°C. The data suggests that isotactic stereochemical impurities in the atactic material are able to crystallize at 55°C which is the Tg of i-PMMA, and therefore must be associated as i-PMMA islands in a rigid atactic-syndiotactic PMMA matrix. This also suggests that the β transition in PMMA may be associated with the isotactic impurities