Rate of Spherulitic Crystallization with Chain Folds in Polychlorotrifluoroethylene

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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.

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