ABSENCE OF ISOTHERMAL THICKENING FOR A BLEND OF LINEAR AND BRANCHED POLYETHYLENE

In this paper we investigate the effect on melting stability of blendi ng linear with branched polyethylene. Linear polyethylene (LPE) was bl ended in solution with branched polyethylene (BPE) at a concentration of 10% LPE. Both homopolymers and the blend were held at 126-degrees-C , for 80 h. At this temperature solid-liquid phase separation takes pl ace in pure LPE and in the blend; pure BPE remains molten. The melting point of the isothermally crystallized blend lamellae was lower than the melting point of pure LPE lamellae crystallized under the same con ditions. This depression in melting temperature has previously been ex plained in two ways: by the inclusion of BPE into the LPE crystals, an d because of the different environment of the crystals. We assess thes e two factors and find them insufficient to explain the decrease in me lting temperature observed. We discuss another important factor, the d imension of the crystals in the chain direction. By transmission elect ron microscopy we show that crystals isothermally grown in the blend a re thinner than LPE crystals grown under the same conditions. This dif ference in crystal thickness is sufficient to explain the observed dep ression in melting temperature. We believe that the lamellae in the bl end fail to thicken during the initial stages of crystallization. We e nvisage three mechanisms that explain the suppression of thickening pr esent in the blend, and one of these is eliminated.