Transient effects in the crystallization of polyethylene

A specimen of linear polyethylene was subjected to isothermal secondary crystallization at a series of temperatures below the primary isothermal crystallization temperature, the melting and primary crystallization stages being held constant throughout the investigation. Dilatometric measurements exhibit an S‐character at low values of undercooling Tp – Ts, where Tp and Ts are, respectively, the primary and secondary crystallization temperatures, whereas at larger undercooling, an initial very rapid crystallization is followed by a very slow stage. When corrected for thermal contraction of the polymer, the net degree of secondary transformation is seen to peak at a temperature about 5°C below Tp. The S‐character of the isotherms and the peaked temperature variation of degree of transformation lead to the conclusion that a large portion of the secondary crystallization consists of the nucleation and growth of the new crystallites. Johnson‐Mehl‐Avrami analysis leads to a model of heterogeneous nucleation within the remaining amorphous zones, followed by one‐dimensional, diffusion‐controlled growth.