Rate of Spherulitic Crystallization with Chain Folds in Polychlorotrifluoroethylene

摘要

(I) The requirements for making a critical test of whether twohyphen; or threehyphen;dimensional surface nucleation controls the radial growth of lamellar spherulites in bulk are discussed. Radialhyphen;growthhyphen;rate data were obtained on spherulites of polychlorotrifluroethylene (PCTFE) for a wide range of supercooling Dgr;T,and found to agree with a growthhyphen;rate law based on coherent twohyphen;dimensional surface nucleation, viz.,G=G0exp(mdash;Dgr;F*/RT) expmdash;Kg/T2(Dgr;T). Approximations for Dgr;F*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 constantKgis analyzed to obtain sgr;sgr;e=184 erg2/cm4sgr; is the lateral surface free energy, sgr;eis the end (chainhyphen;folded) surface free energy. A similar value of sgr;sgr;eis obtained from bulkhyphen;crystallization studies. The homogeneous nucleation process was identified at Dgr;Th=70deg;C, and a value of sgr;2sgr;e=950 erg3/cm6calculated fromKhinI=(NkT/h) exp(mdash;Dgr;F*)expmdash;Kh/T3(Dgr;T)2. Combination of sgr;2sgr;eand sgr;sgr;egives sgr;=5.2 erg/cm2and sgr;e=36 erg/cm2, the latter corresponding to a work on chain folding,q,of 3.8 kcal/mole of folds. A value of sgr;=5.4 erg/cm2is obtained independently using an empirical method. Further, an independent estimate of sgr;esim;35 erg/cm2is obtained for PCTFE from electronhyphen;microscope studies of the lamellar thickness, after accounting for the increase of thickness resulting from segmental mobility that occurs subsequent to initial growth using meltinghyphen;point data. The overhyphen;all role ofqin homogeneous nucleation and growth in linear polymers is discussed in terms of a reduced variable treatment using PCTFE and polyethylene as examples. The ``kinetic'' chainhyphen;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 bulkhyphen;crystallization isotherms associated with spherulitic growth, including stagehyphen;1 and stagehyphen;2 crystallization, and the presence of amorphous material in the spherulites.