Microstructures of a highly short-chain branched polyethylene

Microstructures of a metallocene-based polyethylene (mPE1) with an ethyl branching content of 10.4 mol% have been systematically investigated here. Crystallization 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 DSC stepwise fractionation method, the length of the crystallizable ethylene sequence is estimated to be insufficiently long to develop a fold as normally observed in the lamellar crystals, which is consistent with results obtained by the temperature-rising elution fractionation (TREF) technique. Thus, characteristics of fringed-micelle-like crystals of this particular mPEl are expected owing to the high level of butene comonomer content. When samples are crystallized for a prolonged time, thickening of less perfect crystals takes place but the crystals with more perfection remain intact. A linear relation with a slope of unity between the apparent melting peak temperature, T_m, of the less perfect crystals and the crystallization temperature, T_c, is found, i.e. T_m (°C) = T_c + 5.1, at an extremely low level 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 period and the crystalline thickness of mPE1 crystallized slowly from the molten state to room temperature have been determined by small-angle X-ray scattering. Although the two-phase model does not seem appropriate for this highly branched mPE1, one-dimensional correlation function approach has tentatively been applied. The deduced thickness of the crystallites is significantly small, ca. 2.6 nm, which is in good agreement with results obtained from DSC fractionation and TREF. Based on the measured elastic modulus of mPE1 and the Guth theory for composites, the aspect ratio of the fringed-micelle-like crystals is estimated as well to be ca. 30 which is relatively small, compared to that for lamellar crystals, ca. 100-1000.