Effect of short‐chain branching on the morphology of LLDPE‐oriented thin films

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Abstract
High‐density polyethylene and randomly branched linear low‐density polyethylene of varying branch length and content were used to produce oriented thin films. Sample morphology was investigated using transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and dynamic mechanical thermal analysis (DMA). Gelation studies suggested that the film preparation technique may have involved gel‐drawing. DSC characterization of samples with approximately equal average branch content revealed very different melting behavior, suggesting differences in crystal size distributions. This was attributed to variations in the distribution of branches within samples. For similar branching distributions, the average melting temperature (and, similarly, crystal size) generally decreased as branch content increased. This was corroborated by TEM, with which crystal thickness was found to decrease as branch content increased. TEM further revealed that the lateral alignment of mosaic blocks and the resultant lamellar character of the thin films was obscured as branch content increased, a result of reduced crystal size, crystallinity, and possibly increased interphase content. DMA of compression‐molded material revealed the presence of a beta peak in branched samples only. Moreover, the alpha transition temperature shifted to lower temperatures as branch content increased.