Structural and morphological inhomogeneity of short-chain branched polyethylenes in multiple-step crystallization

Three commercial metallocene-catalyst synthesized short-chain branched poly ethylene (SCBPE) samples with similar molecular weights and molecular weigh t distributions were investigated in terms of their molecular structural (c omonomer sequences and compositions) inhomogeneity and crystal morphology. Two of these samples [SCBPE(BI) and SCBPE(B2)] contained different ratios o f a butene comonomer (20.7 and 26.8 SCB/1000 carbons, respectively), while the third sample, SCBPE(HI. contained a branched hexane comonomer (7.8 SCB/ 1000 carbons). A linear PE fraction was also investigated for comparison. D ifferential scanning calorimetry (DSC) results indicate that multiple-step isothermal (thermal segregation) experiments lead to multiple endothermic m elting processes in these SCBPEs during heating, a phenomenon that was not observed In the linear PE. Wide-angle X-ray diffraction (WAXD) experiments show that all of these SCBPEs possess an orthorhombic crystal lattice with different crystallinities. Linear coefficients of thermal expansion along b oth the a- and b-axes of the PE crystals were also determined using WAXD at different temperatures. This lateral lattice expansion is critically assoc iated with the comonomer size and composition ratio of the SCB series. Smal l-angle X-ray (SAXS) scattering results of the thermal segregated samples o btained during heating required a differential scattering data treatment. I t was found that the long period for each isothermal crystallization step w as different and increased with increasing temperature. By increasing the c omonomer composition at a constant temperature, the long period was also in creased, although the thickness of the crystal lamellae decreased. This was due to an increase of the noncrystalline layer thickness between two neigh boring lamellae. The crystalline morphology was observed under transmission electron microscopy (TEM). During multiple-step isothermal crystallization . the crystalline morphology exhibited a clearly separated lamellar domain texture. All of the experimental results presented suggest that a phase sep aration occurs during multiple-step crystallization due to the inhomogeneit y that exists in there SCBPE molecular structures. These samples may thus p ossess an intermolecular heterogeneity in comonomer composition and sequenc e.