Quantitative infrared methods for the measurement of crystallinity and its temperature dependence: polyethylene

Abstract

Vibrational spectroscopic methods are widely used to characterize semicrystalline polymers in terms of crystallinity. The temperature coefficient of crystallinity, an important and fundamental quantity, is seldom determined for lack of a sensitive method. In this paper, we describe an infrared approach to the measurement of the temperature coefficient of crystallinity. We start from the well-known observation that the integrated intensities of the bands in the spectrum of a semicrystalline polymer change with temperature. It is also known, though less appreciated, that only part of the change is due to changes in crystallinity, the remaining part being due to changes in the intrinsic intensity of the bands. We outline a method for separating these overlapping effects. The method has been applied to a variety of semicrystalline polyethylene samples. The temperature coefficients are found to be highly dependent both on the temperature and on the morphology of the sample. In addition we report crystallinity measurements on a solution crystallized low molecular weight (A& = 13 600) sample, discuss the origin of an apparent anomalous temperature dependence of band intensity cited in the literature, and offer quantitative evidence that the temperature dependence of specific volume is, at temperatures above 0 C, largely determined by partial melting.