Crystallization of the polyethylene block in polystyrene-b-polyethylene-b-polycaprolactone triblock copolymers, 1 Self-nucleation behavior

Abstract

The crystallisation and spherulitic morphology of poly(p-dioxanone) (PPDX), a thermoplastic polyester-ether used for medical applications have been studied. Polarised Optical Microscopy (PM) revealed that isothermally crystallised PPDX exhibits large double banded spherulites with a mean banding spacing that increases as the crystallisation temperature increases. The lack of resolution of optical microscopy prevents the double banding from being clearly observed at crystallisation temperatures below 75°C. The spherulitic morphology changed somewhat as a function of crystallisation temperature from well developed structures with a very clear Maltese cross to spherulites with a more granular texture. A Regime III to Regime II transition in growth was detected according to the Lauritzen and Hoffman analysis of spherulitic growth rate data even though there were no apparent major morphological changes in spherulitic structure as crystallisation temperature was varied apart from that described above. The equilibrium melting point was determined using the Hoffman-Weeks extrapolation procedure and found to be 127°C. Avrami theory was applied to calorimetric overall crystalline conversion data and good fits were obtained with exponents ranging from 3 to 4 as isothermal crystallisation temperature was increased from 50 to 100°C indicating a change from instantaneous to sporadic spherulitic nucleation that was consistent with PM observations. PPDX exhibited a marked tendency to undergo partial fusion and recrystallisation during DSC heating scans. Self-nucleation results evidenced the existence of the usual three self-nucleation domains depending on the self-nucleation temperature (Ts) employed. Additionally, the complex behaviour upon cooling from Ts and subsequent melting was studied in detail.