An investigation into thermally induced structural modifications in isotactic polypropylene and its blends with amorphous terpolymers
Isotactic polypropylene undergoes rearrangement of its ternary structure when rapidly quenched from the molten state. This is an important area since cooling from the molten state is a fundamental aspect of a typical industrial extrusion process. Studies were carried out on a range of commercial isotactic polypropylene samples and quench induced structural changes were investigated using FTIR, DSC, x-ray diffraction, SEM and hot-stage polarised microscopy. Results showed clear evidence of induced structural disorder attributable to the fomration of conformationally disordered isotactic helices (or smectic phace) which, depending on the degree of quenching, resulted in either a biphasic or triphasic system. Results also showed that difference in the degree of quenching exist between various commercial grade polypropylenes. DSC studies on the samples quenched rapidly from the molten state revealed the presence of a double melting endotherm. Further DSC investigations using controlled quenching rates in situ showed that multiple peaks were caused by progressive reorientation of the metastable disordered phase towards a higher crystalline order, and that they were dependent on both the DSC quench rate and the heating rate subsequent to quenching. Stepwise downward annealing studies using DSC showed that each step induced crystallite cluster formation which resulted in a discontinuous multiple endothermic melting profile, whose individual peaks occurred at a consistent point above the corresponding temperature of annealing and the relative percentage of the total ΔHf for each peak in the melting profile was found to be highly dependant on molecular weight and stereoregularity. Solvent extraction of a particular commercial polypropylene sample yielded fractions which, when characterised by GPC and 13C-nmr, clearly showed that fractionation had been achieved on the basis of molecular weight and stereoregularity. Studies on these fractions using stepwise downward annealing gave insights into the role of molecular weight/stereoregularity in the overall thermal behaviour of polypropylene. Studies were also carried out on isotactic polypropylene/amorphous terpolymer blends using a similar range of techniques. Results showed a remarkable degree of compatibility in the blends and an "intrisically determined" morphology dependent on terpolymer structure and blend ratio. The effect of temperature on the infrared spectroscopy of the blends, in particular, showed an interesting relationship between the onset temperature of structural breakdown and the blend composition.
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