Determination of crystallinity of olefin copolymers and polyolefin powders by inverse gas chromatography

Studies of olefin copolymers by inverse gas chromatography show that the method is suitable for determining the crystallinity of compositions where a linear extrapolation of density determinations is not applicable. Satisfactory estimates of crystallinity were obtained on ethylene copolymers with carbon monoxide and vinyl acetate and on an emulsifiable polyethylene wax. Studies were also made in which the columns were packed with physical mixtures of GC support and polyolefin powders, either obtained direct or ground from film samples. It was shown that good estimates of crystallinity could be obtained if suitable allowance is made for variance in peak retention time with flow rate, due to the relatively slow rate of diffusion of the probe molecule in polymer particles of large diameter (>100 μm). If smaller particles are used it is usually unnecessary to correct for flow rate effects. This extends the usefulness of the “molecular probe” technique to any polyolefin sample which can be ground to a fine particle size and eliminates the need for solution coating of the column packing.