Fast Monitoring of the Molecular Orientation in Drawn Polymers Using Micro-Raman Spectroscopy

Abstract

A new method is proposed for the estimation of the segmental orientation of vinyl or vinylidene polymers of moderate crystallinity by acquisition of only one Raman spectrum at a specific polarization geometry. A highly anisotropic rigid rod or/and resonant Raman-active guest-agent incorporated into a host-polymer can be easily detected at low concentration levels and can be used as an indicator of the molecular orientation of the processed host-polymer. A suitable polarized Raman measurement and the automatic calculation of the ratio of two Raman bands, one sensitive and another roughly nonsensitive to the drawing process, may allow, via an appropriate calibration curve, a continuous on-line molecular orientation monitoring of drawn vinyl and vinylidene polymers during an industrial process. Polymers of common use, like poly(vinyl chloride) (PVC), isotactic polypropylene (iPP) and poly(vinylidene fluoride) (PVF₂), were selected for the application of the methodology in question. Promising results were derived utilizing a new custom-made flexible laser Raman instrumentation bearing a specially designed polarized Raman micro-probe able to accumulate fast-polarized Raman spectra in all backscattering polarization geometries.