Mechanisms of Directional Polymer Crystallization

Abstract

Zone annealing, a directional crystallization technique originally used for the purification of semiconductors, is applied here to crystalline polymers. Tight control over the final lamellar orientation and thickness of semicrystalline polymers can be obtained by directionally solidifying the material under optimal conditions. It has previously been postulated by Lovinger and Gryte that, at steady state, the crystal growth rate of a polymer undergoing zone annealing is equal to the velocity at which the sample is drawn through the temperature gradient. These researchers further implied that directional crystallization only occurs below a critical velocity, when crystal growth rate dominates over nucleation. Here, we perform an analysis of small-angle X-ray scattering, differential scanning calorimetry, and cross-polarized optical microscopy of zone-annealed poly(ethylene oxide) to examine these conjectures. Our long period data validate the steady-state ansatz, while an analysis of Herman’s orientation function confirms the existence of a transitional region around a critical velocity, v crit , where there is a coexistence of oriented and isotropic domains. Below v crit , directional crystallization is achieved, while above v crit , the mechanism more closely resembles that of conventional isotropic isothermal crystallization.

Modeling polymer crystallisation induced by a moving heat sink

“…4c supports the ansatz of Lovinger and Gryte 1 that there exists an equivalent isothermal crystallisation temperature for each ZA velocity, and that this temperature decreases with increasing ZA velocity. These results also validate our experimental results, 4 namely that the long period of a ZA polymer tracks the long period of an isothermally crystallised polymer, when the ZA velocity is converted to an effective isothermal crystallisation temperature through the Lauritzen-Hoffman crystal growth theory.…”

“…A comparison between the two approaches is provided in Appendix B. 4, where we describe a similar scenario, the solidification of simple fluids. The stationary heat sink in Case I and the moving heat sink in Case II take away the latent heat released during the crystallisation of the solid as well as the specific heat required to bring down the temperature in the material.…”

“…For the simulations we have considered the material properties of poly(ethylene oxide) (PEO), the polymer employed by Lovinger and Gryte 1 as well as in our followup investigation 4 . The material is considered as homogeneous with well-defined, temperature-and crystallinityindependent material properties.…”

“…Our interest is towards obtaining a theoretical understanding of polymer zone annealing results first obtained by Lovinger and Gryte 1 nearly fifty years ago, and elaborated on by us recently 4 . In contrast to small molecules, e.g.…”

“…Zone annealing (ZA), a technique which applies directional solidification for the purification of semiconductors, 2 has been explored as a potential processing technique for semicrystalline polymers 1,3,4 and block copolymers. [5][6][7] A relatively large body of theoretical work has been done in the metallurgical field focusing on the segregation of impurities during ZA of semiconductors.…”