Thermal Analysis on Unorientated Film of Isotactic Polypropylene

Abstract

Thermal analysis by differential scanning calorimeter (DSC) was carried out on unorientated film of isotactic polypropylene in order to study the change in fine structure during heat-treatment. Fractions and whole polymers ranging in molecular weight from 4×10³ to 4.7×10⁵ are moulded into unorientated film by using the hot press method. It was acertained from NMR measurement on o-dichlorobenzene solution at 150°C that microtacticity was independent of molecular weight. Procedure of heat-treatment in detail was taken into consideration. From DSC thermogram, three characteristic temperatures are defined; T_m1, temperature of onset of melting, T_m2, temperature at which melting velocity reaches maximum, T_m3, temperature at which the trace of crystallites melts. T_m3 from DSC diagram is in very good agreement with the peak temperature of DTA (differential thermal analysis) curve. By comparing T_m3 with melting temperature obtained from dilatometry on welldeveloped crystals, T_m3 is proved to be the apparent melting temperature. Melting point is very s1 ightly influenced by annealing temperature except the narrow temperature range of heat-treatment, usually 150-170°C for polymer having molecular weight higher than 4×10⁴, for which 160-165°C treatment gives maximum melting temperature. Temperature range of melting for fractions is narrower than that for whole polymers when temperature above 150°C is used as annealing temperature. Both T_m2 and T_m3 change little by heat-treatment below 150°C, but slightly increases to the saturated value at temperature above 155°C.
Small peak or shoulder appears in DSC diagram corresponding to the heat-treatment in the range 70-150°C not only for annealed film but also for annealed solution grown crystals. Theappearance of the small peak is independent of annealing method, heating media, duration of heat-treatment, molecular weight and it's distribution. The peak temperature is several degrees higher than the annealing temperature. It is experimentally shown that the appearance of small peak can not be ascribed to melting of portion of lower degree of micretacticity, partial melting of β-form, transition of smectic form to monoclinic one, degradation, disorientation of crystallites before melting, and that small peak may be due to the melting of thermally unstable crystallites having monoclinic form of highly isotactic chains, which are formed during heat-treatment.