Roles of ‘solute’ and heat flow in the development of polymer microstructure

Abstract

During the crystallization of polymers, both uncrystallizable material (solute') and the heat of fusion are released at the growing interface. Both must diffuse away rapidly enough to permit interface propagation at a velocity (the ‘natural’ velocity) determined by the thermodynamic driving force. In general, the final microstructure of the solid is determined by the degree to which the flux of solute and heat are compatible with the natural velocity. When the diffusion length δ = D V ( D = diffusivity, V = interface velocity) is equivalent to or smaller than a dimension of the growing body, diffusional processes control the transformation and the ultimate microstructure. Except for cases of high orientation and relatively large effective undercooling, only solute flow is important. Diffusion solutions for solute flow predict a critical radius, beyond which fibrillated spherulites with solute incorporated between the fibril arms must form. Using a eutectic model, the inter-arm spacing is predicted, with crystallization temperature and diffusivity as governing parameters. Under extreme strain, it is possible for a non-diffusive transformation to take place. In this case, all solute is captured within the growing crystal and the microstructure is governed by the dissipation of the heat of fusion. Very fine, defective fibrillar crystals are predicted. In fibre spinlines, fibrillar crystals grow into a stationary thermal gradient. Modelling of the situation is based on the growth of a thermal dendrite. At each spinline temperature, there is a critical spinline velocity above which crystal growth, in thermal dendrite form, is not possible. This critical velocity dictates the dendrite tip radius. Under these conditions, the fibril diameter must be in the range of 10–100 nm.