Infrared Dichroism Evidence of Tie Molecules in Polyethylene.

While consideration of the crystalline domains have long dominated research in understanding the properties of semi-crystalline polymers, a satisfactory understanding of crack growth in these materials can be realized only by developing corresponding analytical tools to characterize the amorphous region. Since slow stable cracks in these materials preferentially form between crystalline lamellae, the role of tie molecules--the amorphous chains that bridge adjacent crystalline lamellae--are particularly important in this regard. Unfortunately, there is no method readily available for quantitative assessment of tie molecules. Through deformation and subsequent chlorination of polyethylene films, it is demonstrated that infrared dichroism can be used to determine relative tie molecule concentration. By using this technique, one can a priori predict which resin in a series having comparable densities but widely varying molecular weights or comonomer distributions exhibits better crack resistance.