Optically transparent self-reinforced poly(ethylene terephthalate) composites: molecular orientation and mechanical properties

Self-reinforced composites have been fabricated by compaction of oriented polyethylene terephthalate (PET) fibers under pressure at temperatures near, but below, their melting point. The originally white fiber bundles, which were about 40% crystalline, show increased crystallinity (55%) but optical translucency after processing. Differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) were used to study the crystallization and orientation of the fibers, revealing that the degree of crystallinity was somewhat insensitive to compaction conditions while the melting point increased substantially with increasing compaction temperature. Crystalline orientation, gauged using the Hermans orientation parameter from WAXD data, indicated that no significant loss in orientation of the crystalline fraction occurs due to compaction. Mechanical characterization revealed a stepwise decrease in flexural modulus (9.4-8.1 GPa) and concomitant increase in transverse modulus and strength on increasing the compaction temperature from 255 to 259 °C. This transition in behavior was accompanied by a loss of optical transparency and a change in the distribution of amorphous fraction from fine intrafibrillar domains to coarse interfibrillar domains as seen with electron microscopy. We argue then that the mechanical properties of PET compactions are influenced more by orientation of the amorphous phase than that of the crystalline phase. The impact properties of compacted materials, characterized using an unnotched Charpy test method, showed remarkable impact resistance after compaction, with impact toughness decreasing as compaction temperature was increased.