Morphological effect on the Raman frequency shift induced by tensile stress applied to crystalline polyoxymethylene and polyethylene: spectroscopic support for the idea of an inhomogeneous stress distribution in polymer material

Abstract

A tensile-stress-induced low-frequency shift was measured for the Raman bands of crystalline polyoxymethylene (POM) and polyethylene (PE) with different draw ratios λ. The apparent frequency-shift factor p (= Δ v ̃ Δσ bulk ) for the 539 cm −1 skeletal bending mode δ (COC) was found to become larger for the POM sample with lower λ: p = −24 cm −1/GPa ( λ = 7), −19 ( λ = 17) and −11 ( λ = 34). Such a difference in p was too small to detect for the skeletal stretching modes of PE samples with λ = 7 to 100: p = −6.0 cm −1/GPa for v as (CC) at 1064 cm −1 and −4.5 for v s (CC) at 1131 cm −1. This experimental fact, i.e. the dependence of the frequency-shift factor p on sample morphology, cannot be reasonably explained in terms of a homogeneous stress distribution or the simple mechanical series model of crystalline and amorphous phases. Based on the complex mechanical model (i.e. the parallel-series and series-parallel models), the Raman shift could be understood quantitatively, and the intrinsic shift factor α of the crystalline region was estimated as −11 cm −1/GPa for the δ (COC) mode of POM and as −5.0 cm −1/GPa for v as (CC) and −3.8 cm −1/GPa for v s (CC) of PE. A change in the Raman band profile under tensile stress was simulated and compared with the observed data.