| Abstract
| - In-situ structural changes at the lamellar level during uniaxial deformation and subsequentrelaxation of a semicrystalline ethylene−propylene (EP) copolymer containing 78 wt % ethylene werestudied by time-resolved synchrotron small-angle X-ray scattering (SAXS) at room temperature (25 °C).During the initial stage of deformation, the long period was found to increase in the stretching directionand decrease in the transverse direction. In addition to the elastic change of the lamellar structure, furtheranalysis suggested that there was an additional contribution of strain-induced “melting”, which beganat a relatively low strain (about 10%) and was directionally dependent. At 75% strain, new crystals,possibly with extended-chain conformation and needlelike microvoids, started to form, which coexistedwith the original crystals (dominated by folded-chain conformation). Because of the destruction of originalcrystals, the applied stress was found to decrease slightly after the yield point. Above 120% strain, thestress increased linearly with strain partially due to the formation of new crystals. Long periods of newcrystals decreased with strain upon further stretching. During relaxation, a large amount of well-orientednew crystals remained in the sample. As a result, only 55% of the original sample length was recovered.The long period of the new crystals after relaxation was about 15 nm, much smaller than that of theoriginal crystals (24 nm). Strain-induced microvoids were also found to remain in the sample afterrelaxation.
|
