Comparative low-frequency Raman study of straight-chain-segment length distributions in gel-derived and melt-crystallized polyethylenes

The Raman longitudinal acoustic mode (LAM) has been examined for two ultra-high-molecular-weight reactor powders and melt-crystallized (MC) and gel-derived (GD) films and hot-drawn fibers prepared from them. The analysis of the straight-chain-segment (SCS) length distributions obtained from the LAM spectra demonstrates complete decomposition of the initial crystalline structure of the powders in the GD samples, while in MC films one could distinguish some features reminiscent of the powder structure. In addition, a certain amount of all-trans bridges linking adjacent crystallites in unoriented MC films was detected. In drawn MC samples, the average SCS length grows gradually with the draw-ratio increase, thus indicating the molecular ordering in the interface zones between the crystallite cores and the amorphous phase. A similar effect in GD fibers is accompanied by the formation of all-trans sequences whose length is much more than the crystallite core thickness, i.e., taut-tie molecules. The calculated SCS length distributions manifest clearly the presence of population of the taut-tie molecules entering a series of a few crystallites consecutively.