Analysis of the effects of micro-gravity on the recrystallization of polyethylene thin films

Studies have shown that thin films of polyethylene (PE) possess very different thermal, physical and optical properties compared to bulk polyethylene. Greater ratios of degrees of freedom in polymer chains at the surface of the thin films have been suggested as a possible explanation for this behavior. The objective of this study is to verify this hypothesis by further increasing the degrees of freedom by placing the sample under microgravity conditions. The effect of micro-gravity, a reduction in gravitational force allowing freedom of movement in polymer chains, crystal size and orientation in thin PE films was studied. PE solutions were cooled in two environments: standard gravity conditions (1G) and micro-gravity conditions (μG). Both sample types were allowed to set for time periods of 24 and 48 hours before analysis. Thermal analysis using differential scanning calorimetry (DSC) was used to determine the onset of melting, the melting temperature, and the enthalpy of fusion for all samples. Molecular changes were studied using Fourier transform infrared (FTIR) analysis and crystal sizes and orientation were determined by polarized light microscopy (PLM).;DSC traces show identical melting points for both film types, however, 24 hour μG films possessed an average enthalpy of 137 J/g, where as 24 hour 1G films only possess an average enthalpy of 93.9 J/g. This result suggests that a higher crystalline fraction is found under micro-gravity. FTIR analysis of the characteristic PE doublet found between 730 cm-1 to 720 cm-1, attributed to CH₂ rocking vibrations, as a function of temperature suggested an increase in the number of smaller, more randomly packed crystals in the 1G sample and fewer, larger, more oriented crystals which melted with better cooperativity in μG samples. Finally, PLM images confirmed results from DSC and FTIR by showing little or no orientation for 1G film samples while μG samples presented larger crystals growing in three dimensions with significant orientation. These results were interpreted as suggesting that micro-gravity induces less dense crystalline packing of polymer chains supporting the hypothesis that the increased degree of freedom at the surface of thin PE films may result in modified physical properties compared to bulk samples.