Heat and entropy of fusion of isotactic polypropylene

Abstract

Heats and entropies of fusion of polymers are customarily evaluated through melting point depression measurements with the assumption that the observed and thermodynamic melting points are nearly identical. However, it has recently become evident that the crystal surface free energy is not negligible, so that apparent melting points measured dilatometrically or calorimetrically are low by perhaps 5–15°C. the status of the accumulated body of thermodynamic data is rendered still more uncertain by the possibility that the surface free energy may vary in a systematic manner with diluent concentration. To investigate this question, dilatometric data were obtained by two procedures for tetralin solutions of isotactic polypropylene. The first set of measurements was performed upon samples which had been slowly cooled from the melt, while in the second procedure the apparent melting point was measured as a function of the isothermal crystallization temperature. Extrapolation of the latter data gave tM° = 186 ± 2°C., a value of 12°C. higher than the apparent melting temperature observed according to the first procedure; however, the heats of fusion obtained by the two methods agreed to within the experimental error. A possible explanation for this behavior may be offered by the observations that both the parameter β of Hoffman and Weeks, and the ratio of the temperature of maximum crystallization rate to the melting temperature, are independent of diluent concentration over the range investigated. This suggests that the addition of diluent does not affect the distribution of fold lengths. If this behavior proves to be general, dilatometric heats of fusion will be reinstated as valid thermodynamic data for polymers.

The degree of crystallinity of monoclinic isotactic poly(propylene)

“…The results of several different investigations, using a variety of diluents, has given the value of 2100 cal/mol of repeating unit, or 50 cal/g for ⌬H u of isotactic poly(propylene). 43,44 This value was used in this work to calculate (1Ϫ) ⌬H by means of eq. (2).…”

Silica‐reinforced dynamically vulcanized ethylene–propylene–diene monomer/polypropylene thermoplastic elastomers: Morphology, rheology, and dynamic mechanical properties

“…where ⌬H f and ⌬H* represent the heat of fusion of the blend sample obtained from the DSC diagram and 100% crystalline PP (209 J/g), 12 respectively; and is the weight fraction of polypropylene in the blend sample.…”

Ultradrawing of Isotactic Polypropylene Films Produced by Gelation/Crystallization from Solutions

“…The second peak at A.= 100 attains 178°C which is close to the equilibrium melting point (186±2°C). 20 However, the measurable melting point is lower than the equilibrium value. This indicates that ultradrawn polypropylene is independent of superheating phenomenon associated with that polymer chains in the melt retammg the extended chain arrangement and therefore the entropy of fushion would obviously be smaller than the value obtained from random coils in the melt.…”