Rate of Spherulitic Crystallization with Chain Folds in Polychlorotrifluoroethylene

Abstract

(I) The requirements for making a critical test of whether two- or three-dimensional surface nucleation controls the radial growth of lamellar spherulites in bulk are discussed. Radial-growth-rate data were obtained on spherulites of polychlorotrifluroethylene (PCTFE) for a wide range of supercooling ΔT, and found to agree with a growth-rate law based on coherent two-dimensional surface nucleation, viz., G=G0 exp(—ΔF*/RT) exp[—Kg/T2(ΔT)]. Approximations for ΔF* are given. (II) Parameters related to the recently proposed ``kinetic'' viewpoint of homogeneous nucleation and growth of lamellar polymer crystals with chain folds are obtained. The nucleation constant Kg is analyzed to obtain σσe=184 erg2/cm4[σ is the lateral surface free energy, σe is the end (chain-folded) surface free energy]. A similar value of σσe is obtained from bulk-crystallization studies. The homogeneous nucleation process was identified at ΔTh=70°C, and a value of σ2σe=950 erg3/cm6 calculated from Kh in I=(NkT/h) exp(—ΔF*)exp[—Kh/T3(ΔT)2]. Combination of σ2σe and σσe gives σ=5.2 erg/cm2 and σe=36 erg/cm2, the latter corresponding to a work on chain folding, q, of 3.8 kcal/mole of folds. A value of σ=5.4 erg/cm2 is obtained independently using an empirical method. Further, an independent estimate of σe∼35 erg/cm2 is obtained for PCTFE from electron-microscope studies of the lamellar thickness, after accounting for the increase of thickness resulting from segmental mobility that occurs subsequent to initial growth using melting-point data. The over-all role of q in homogeneous nucleation and growth in linear polymers is discussed in terms of a reduced variable treatment using PCTFE and polyethylene as examples. The ``kinetic'' chain-fold theory gives a consistent picture of surface free energies, rates of homogeneous nucleation and growth, melting behavior, and the variation of initial step height with temperature. (III) A discussion is given concerning the shape of the bulk-crystallization isotherms associated with spherulitic growth, including stage-1 and stage-2 crystallization, and the presence of amorphous material in the spherulites.