New insights into the characteristics of early stage crystallization of a polyethylene

Small angle light scattering has been used to probe structure formation during isothermal crystallization of an ethylene-1-hexene copolymer (EH064, Mw = 70,000 g/mol, ρ = 0.900 g/cm3, Mw/Mn ∼ 2, 6.4 mol% hexene). It is shown that clear structural information on size scales ranging from hundreds of nanometers to several micrometers during early stage crystallization can be obtained by this method when crystallizing the polyethylenes at the high temperatures (above the peak melting temperature of a rapidly crystallized polymer sample) required for resolving early stage crystallization without the influence of the crystal growth. The results show that the early stage crystallization is characterized by large scale orientation fluctuations that precede the formation of local crystalline order manifest in X-ray scattering and the initial collapse of these large scale anisotropic/ordered domains. The scattering intensity increases exponentially with time initially, and the wave vector dependence of the growth rate of fluctuations is consistent with predictions for initial stages of a phase transformation process. However, the detailed mechanism cannot be described by existing models. The implications of our results are discussed within the context of proposed models for early stage crystallization.