Specific heat studies of transition and relaxation behavior in polymers

Specific heat studies of polymers are reviewed within the framework of Buyer's classification: ( a ) melting transition ( T m ); ( b ) glass transition ( T g ); ( c ) subglass relaxation ( T < T g ); and ( d ) intermediate relaxation ( T g < T < T m ). The melting transition is readily apparent in specific heat measurements because of the heat of fusion resulting from the transition. Thermodynamic parameters obtained from calorimetric measurements confirm that melting is a first‐order transition and, in general, the phenomena are well understood. The change in C p at T g probably provides the least ambiguous assignment of T g by precision calorimetric measurements. Specific heat studies focus sharply on the thermodynamic aspects of glass formation but still show the relaxational properties. Although complicated, the specific heat behavior of semi crystal line polymers is less complex than other techniques. There are no abrupt changes in C p from near 0°K. to Tt similar to the mechanical or dielectric relaxations observed below T g . Although the molecular processes responsible for the low temperature relaxations contribute to C p , the processes are not sufficiently cooperative to occur over a narrow temperature range. Some examples of molecular processes such as methyl or other group rotation and con‐formational isomerism, which contribute to C p , are discussed. The intermediate relaxations ( T g < T < T m ) which occur in semi crystalline polymers are complex, and many mechanisms, such as premelting, crystal‐crystal transitions, defects and crystal‐amorphous interactions have been proposed. The few specific heat results available are not conclusive and considerable work is needed in this area.