Microstructures of a highly short-chain branched polyethylene

Microstructures of a metallocene-based polyethylene (mPE1) with an ethyl br anching content of 10.4 mol% have been systematically investigated here. Cr ystallization and melting behavior are studied using differential scanning calorimetry (DSC). A broad distribution of crystal perfection, revealed by the melting endotherm with a plateau-like shape, is observed due to a dense and equal population distribution of short chain branching. Based on the D SC stepwise fractionation method, the length of the crystallizable ethylene sequence is estimated to be insufficiently long to develop a fold as norma lly observed in the lamellar crystals, which is consistent with results obt ained by the temperature-rising elution fractionation (TREF) technique. Thu s, characteristics of fringed-micelle-like crystals of this particular mPE1 are expected owing to the high level of butene comonomer content. When sam ples are crystallized for a prolonged time, thickening of less perfect crys tals takes place but the crystals with more perfection remain intact. A lin ear relation with a slope of unity between the apparent melting peak temper ature, T-m, of the less perfect crystals and the crystallization temperatur e, T-c, is found, i.e. T-m (degreesC) = T-c + 5.1, at an extremely low leve l of crystallinity. The determination of equilibrium melting temperature of this unique mPE1, based on the Hoffman-Weeks approach, becomes unfeasible due to the absence of a feature attributable to lamellar microstructures. To characterize the dimensions of fringed-micelle-like crystals, the long p eriod and the crystalline thickness of mPE1 crystallized slowly from the mo lten state to room temperature have been determined by small-angle X-ray sc attering. Although the two-phase model does not seem appropriate for this h ighly branched mPE1, one-dimensional correlation function approach has tent atively been applied. The deduced thickness of the crystallites is signific antly small, ca. 2.6 nm, which is in good agreement with results obtained f rom DSC fractionation and TREF. Based on the measured elastic modulus of mP E1 and the Guth theory for composites, the aspect ratio of the fringed-mice lle-like crystals is estimated as well to be ca. 30 which is relatively sma ll, compared to that for lamellar crystals, ca. 100-1000. (C) 2000 Elsevier Science Ltd. All rights reserved.