Polymers and polymeric materials exhibit flexibility between neighboring components of their molecular chains, permitting disordered conformations relative to the crystalline state. The disordered chain conformations lead to entanglements between neighboring chains, and the local intersections of polymer chains lead to voids in the material. Such voids or holes are named polymer free volume elements (FVEs).
In their study of how FVEs in polymeric materials are related to their intrinsic microporosity and transport of gases, Budd, McKeown, and Fritsch (1) found inspiration from Lucretius. To paraphrase Lucretius’ teachings about atomistic theories from the Greeks, “Nature …has two forms …: the bodies, and the void where the bodies are placed and travel their varied paths” (ref. 1, p. 1977). More recently, we find that FVE sizes are key descriptors in molecular theories of polymers, polymeric materials, and glasses (1⇓⇓⇓–5). FVE distributions are critically important in applications of such materials to self-assembled amphiphiles (6), polymer composites (7), and perm-selective membranes (8). Thus, a great deal of attention has been paid to characterizing FVEs; the major methods used in these measurements are discussed below.
Knowledge of FVE distributions in polymeric materials is needed for engineering of structural plastics. High-strength products require treatment to control and minimize the FVEs. One example is the processing of polyethylene chains (comprising repeats of the
group). When long polyethylene chains are extruded to maximize their alignment, the result is high-density polyethylene (HDPE), a widely used plastic material that we consume in vast quantities. Voids in HDPE result from shorter chains, poor chain alignment, and exposure to higher temperatures, so an increase in the number and size of FVEs correlates with a reduction in structural integrity. In other applications, such as using polymers to engineer perm-selective membranes for gas separations, increasing …