Molecular and microdeformation characteristics of ethylene-propylene copolymers during tensile loading

Abstract

The microstructural modification during uniaxial tensile deformation of ethylene-propylene diblock copolymers (EP copolymers) at varying displacement rates was studied using atomic force microscopy (AFM) and Raman spectroscopy. Characterisation by AFM of the undeformed surface of the two copolymers (designated EP-M and EP-TC) indicated differences in arrangement (regular or irregular) of fibrils depending on their melt flow conditions. Type EP-M is a long chain copolymer manufactured with restricted flow, whereas EP-TC is a short chain copolymer obtained by controlled rheology. The microfibrils in both copolymers exhibited small kinks/nodules of ethylene-propylene. At the molecular level, long chain EP-M copolymer exhibited greater non-uniformity in the alignment of chains of molecules in comparison with short chain EP-TC copolymer. Atomic force micrographs taken at different fields of view for long chain EP-M copolymer tensile deformed at a low displacement rate indicated excessive stretching and merging of fibrils/microfibrils in the direction of the tensile axis, whereas short chain EP-TC copolymer exhibited reduced stretching of fibrils and microfibrils. With a subsequent increase in displacement rate of the tensile test, long chain EP-M copolymer continued to stretch along the tensile direction, but with reduced ability for stretching and merging of fibrils/microfibrils. On the other hand, tensile deformation of short chain EP-TC copolymer at a high displacement rate was irregular, and the fibrils/microfibrils were not preferentially stretched in the direction of tensile stress. The irregular deformation behaviour was a consequence of reorganisation of the microstructure. At the molecular level, stretching of individual molecular units occurred in both cases with an increase in displacement rate. However, the differences between the two copolymers were insignificant. Raman spectroscopy indicated that the rate of shift of frequency of Raman band per percentage strain was only marginally greater for long chain EP-M copolymer in comparison with short chain EP-TC copolymer, confirming AFM observations and suggesting only small differences in molecular deformation between the two copolymers.