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.

Figure 1

(A) Zone-annealing setup (AutoDesk Inventor rendering). (B) Temperature gradient profile on a sample holder. (C) Cross-polarized optical microscopy of zone-annealed PEO and a schematic of the proposed mechanism of crystallization. Scale bars are 100 μm. (D) Isothermal crystal growth rate data from polarized light optical microscopy and Lauritzen–Hoffman analysis.

Figure 2

SAXS data obtained on crystallized PEO samples employing the ZA method described in Figure 1. (A) Representative 2D scattering pattern (v_ZA = 0.05 μm/s) showing wedge integration of 25° centered on the angle of maximum intensity and (B) 25° centered orthogonal to the angle of maximum intensity. (C) Representative Lorentz-corrected SAXS profiles for wedge integration around angle of maximum intensity and (D) orthogonal to the angle of maximum intensity. (E) Corresponding correlation functions for angle of maximum intensity and (F) orthogonal to angle of maximum intensity.

Figure 3

(A) Long period, with , from Lorentz-corrected profiles for zone annealing (blue) and isothermally crystallized samples (orange); the position of the first maximum for the correlation function derived from the zone annealing samples (red) is also plotted. (B) Bulk crystalline volume fraction φ_c,v from DSC (blue) and linear crystallinity from the correlation function for zone annealing (red). r₀ is the position where the correlation function first crosses zero. (C) Lamellar thickness, calculated as L × φ_c,v, from the Lorentz-corrected profiles and DSC (blue) and L × w_c from the correlation function for zone annealing (red).

Figure 4

(A) Representative intensity profiles as a function of azimuthal angle, ϕ, for all probed v_ZA. ϕ = 0° corresponds to the pulling direction. (B) Average Herman’s orientation function for zone-annealed PEO as a function of T_c,eff. (C) Normalized distributions of Herman’s orientation function measured across the area of the sample.