Transformation of polyethylene crystals by high‐pressure annealing

Abstract

AbstractWe investigated the possibility of obtaining high‐density polyethylene (PE) samples with crystals in a broad range of thicknesses. PE samples solidified by slow cooling were subjected to hydrostatic pressure at room temperature and then heated to the region of mobile pseudohexagonal form, without being passed through melting. The chains were allowed to rearrange for a period of time to a more extended chain conformation, forming larger crystals. During cooling under pressure, the transformation to orthorhombic crystals of larger thickness occurred. The thickness of the crystals was then controlled by pressure, temperature, and annealing time at high temperatures. The temperature and the pressure in the high‐pressure cell was maintained at constant level within 230–295°C and 260–630 MPa, respectively. In this way, we transformed under pressure chain‐folded PE crystals to partly chain‐extended crystals with peak melting temperatures up to 144.85°C. A range of samples with crystals 20–160 nm thick were produced and characterized with several techniques, including wide‐ and small‐angle X‐ray scattering, conventional and temperature‐modulated differential scanning calorimetry, dynamic mechanical analysis, and scanning electron, atomic force, and light microscopy as well as low‐frequency Raman spectroscopy. Changes in crystallographic unit cell dimensions were observed for crystals with different thicknesses. Crystallinity degree increased with increases in the lamellar thickness up to some level. The initial spherulitic structure deteriorated due to a multifold increase in the lamellar thickness. Oriented samples subjected to annealing in the pseudohexagonal region did not lose the crystal orientation while crystals grew thicker and the crystallinity degree increased; however, no drastic change to noncrystalline phase topology and structure was noticed. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1337–1350, 2002