MORPHOLOGY OF BLENDS OF LINEAR AND LONG-CHAIN-BRANCHED POLYETHYLENES IN THE SOLID-STATE - A STUDY BY SANS, SAXS, AND DSC

Differential scanning calorimetry (DSC), small-angle neutron scatterin g (SANS), and X-ray scattering (SAXS) have been used to investigate th e solid-state morphology of blends of linear (high density) and long-c hain-branched (low-density) polyethylenes (HDPE/LDPE). The blends are homogeneous in the melt, as previously demonstrated by SANS using the contrast obtained by deuterating the linear polymer. However, due to t he structural and melting point differences (similar to 20 degrees C) between HDPE and LDPE, the components may phase segregate on slow cool ing (0.75 degrees C/min). For high concentrations (phi greater than or equal to 0.5) of HDPE, relatively high rates of crystallization of th e linear component lead to the formation of separate stacks of HDPE an d LDPE lamellae, as indicated by two-peak SAXS curves. For predominant ly branched blends, the difference in crystallization rate of the comp onents becomes smaller and only one SAXS peak is observed, indicating that the two species are in the same lamellar stack. Moreover, the pha ses no longer consist of the pure components and the HDPE lamellae con tain up to 15-20% LDPE (and vice versa). Rapid quenching into dry ice/ 2-propanol (-78 degrees C) produces only one SAXS peak (and hence one lamellar stack) over the whole concentration range. The blends show ex tensive cocrystallization, along with a tendency for the branched mate rial to be preferentially located in the amorphous interlamellar regio ns. For high concentrations (phi > 0.5) of HDPE-D, the overall scatter ing length density (SLD) is high and the excess concentration of LDPE between the lamellae enhances the SLD contrast between the crystalline and amorphous phases. Thus, the interlamellar spacing (long period) i s clearly visible in the SANS pattern. The blend morphology is a stron g function of the quenching rate, and samples quenched less rapidly (e .g., into water at 23 degrees C) are similar to slowly cot!ed blends. The combination of SANS, SAXS, and DSC techniques allows us to interpr et morphological differences in the solid state of these blends.