The effect of dibenzylidene sorbitol on the crystallization behaviour of polyethylene

The influence of dibenzylidene sorbitol (DBS) on the optical clarity, crystallization behaviour and morphology of polyethylene has been studied by UV/visible spectroscopy, differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). By combining crystal growth rates with Avrami parameters derived from DSC crystallization exotherms, nucleation densities were determined as a function of crystallization temperature. In the blend without DBS, the nucleation density decreased progressively with increasing crystallization temperature whereas, in the system containing DBS, the nucleating efficiency was found to remain constant and then decrease dramatically above 120 C. Throughout the crystallization range studied, the DBS increased the nucleation density substantially, often to a value comparable with that seen in a commercial sorbitol-clarified propylene/ethylene copolymer system. SEM revealed a change from a continuous lamellar texture at low temperatures to one based upon isolated sheaf-like objects at 124 C. At high crystallization temperatures, both the DSC and the SEM also provided evidence of a significant rejected linear fraction, which appeared to decorate the sorbitol network on quenching. Possible explanations for these effects are discussed