Fractionated crystallisation of polyethylene and ethylene/α-olefin copolymers dispersed in immiscible polystyrene matrices

Abstract

In this work, several PS/HDPE (high density polyethylene), PS/LLDPE (linear low density polyethylene) and PS/ULDPE (ultra low density polyethylene) blends were prepared in a composition range were atactic polystyrene (PS) was always the matrix component. All the types of polyethylenes employed, when dispersed into droplets, exhibited fractionated crystallisation exotherms in the temperature range between 67 and 70°C. These low crystallisation temperatures are probably closer to the homogeneous nucleation temperature of linear polyethylene than any previously reported value, since they occur at higher supercoolings. The higher values of crystallisation temperatures previously reported could be explained by the crystallisation from heterogeneous nuclei of relatively low nucleation efficiency or by a weak nucleation capacity of the droplets interface. By applying a self-nucleation procedure we were able to corroborate that what causes the fractionated crystallisation is the lack of highly active heterogeneous nuclei (i. e., those normally active at low supercoolings in the bulk polymer) in every droplet. When polyethylene/α-olefin copolymers are finely dispersed in a PS matrix, a molecular segregation process can be induced during mixing facilitated by the heterogeneous distribution of short chain branches in the copolymers. The crystallisation of droplets that contained mostly these highly branched chains occurs at temperatures lower than 50°C, thereby producing another low temperature exotherm which may be the lowest present in the blend and could be mistaken by the crystallisation of homogeneously nucleated crystals. We have shown that the self-nucleation technique can help to distinguish these low temperature exotherms from those originated by differences in nucleation behaviour; therefore, a plausible interpretation of all the possible fractionated crystallisation exotherms of ethylene/α-olefin copolymer droplets was made possible.