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.

Glassy films of n-CzlH,, n-C36H74a, nd low molecular weight polyethylene were prepared by vacuum sublimation onto a CsI window held at 7 K and were studied by infrared spectroscopy. The conformational disorder achieved for the glass was comparable to that of the liquid near the sublimation temperature. The chain-organizing processes were monitored for Czl and polyethylene as the sample was warmed to 300 K. A number of separate steps are involved. Each step occurs over a more or less broad temperature region, and sometimes the steps overlap. In the case of CZ1, the first step involves a conformational ordering of the chains to their extended all-trans form. The extended chains initially pack in a monoclinic subcell, but they are not in longitudinal register; that is, the end methyl groups do not lie in parallel planes. At a higher temperature, the monoclinic subcell is converted to an orthorhombic subcell, but the chains are still not in register. In the last annealing step, which occurs over a narrow temperature range, the chains come into register so that the Czl finally assumes its stable orthorhombic crystal structure. The annealing of the polyethylene glass proceeds in exactly the same way except that there is no chain-registering step. In general, the transition temperatures are higher and the temperature range over which the transitions occur is broader for the polyethylene sample.