Nucleation‐controlled growth. Observations on (110) twinned polyethylene crystals

Abstract

AbstractThe growth of (110) twinned crystals of a sharp fraction of linear polyethylene (Mw/Mn = 1.10) of moderate molecular weight (Mw = 17,000) is followed during crystallization by the isochronous decoration method. New morphological features are observed. The fast‐growing tip of our laths presents, in addition to the two (100) facets usually observed, a possibly stable small reentrant (110) corner. This is a situation intermediate between the facies described by Dawson and Keller. Moreover, the slow tip of our laths presents various degrees of asymmetry with respect to the junction plane. A new characteristic length Ln = j/i is introduced to explain our morphological observations on (110) twinned crystals: j is the nucleation rate at a reentrant corner and i the nucleation rate on a smooth facet. Three linear growth rates Ghkl are calculated as a function of the length L of the face (hkl): Ghkl and Gĥkl or G′hkl are respectively the growth rates of a face bordered by two salient corners and by a reentrant corner. A distinction between Gĥkl and G′hkl is introduced to take into account the relative sizes of the two faces of the reentrant dihedral angle. The major points of the discussion concern (i) the stability of the (110) reentrant corner of the fast tip of the lath, (ii) the nearly constant shape of the twinned crystals, (iii) the effects of dislocations incorporated in the fast edge of the laths, and (iv) the various asymmetries observed in the slow tip of our laths. Theories of surface nucleation‐controlled growth explain our various morphological observations on (110) twinned PE crystals, and growth usually proceeds in regime II.