Methods of studying macromolecular entanglements

Abstract

AbstractMacromolecular entanglements in polyacrylonitrile (PAN) and polyacrylonitrile copolymer (PAC) fibers have been systematically studied by the methods of swelling thermal analysis, i.e., swelling differential scanning calorimetry (SDSC), swelling shrinking stress (SSS), swelling shrinkage (SS), and terminal swelling modulus (TSM). When a PAC fiber sample was immersed in a swelling agent, e.g., 80% aqueous dimethylformamide (DMF) and subjected to a programmed heating, the fiber swells, with the dissociable intermolecular couplings gradually broken until finally the fiber was completely dissolved. If this process was done in a DSC analyzer, a small endothermic peak appears on the latter part of the SDSC thermogram. This represents the disentanglement energy of the entangled bonds in the sample. If done in a thermomechanical analyzer, the SSS, SS, and TSM, measured just before incipient dissolution, i.e., the point that intermolecular couplings were broken and only the entangled loopings junctions were left, should reflect the number of the entanglement loops. Thus, the long argued problem whether an entanglement is topological, or due to loci of chain coupling, or is of dual nature may be experimentally elucidated. By using these methods, problems were detailed concerning entanglements, such as the different textural changes of PAC fiber during ambient storage to below 0°C, the effects of heating at 125°C for various times on the PAC fiber texture, and the textural characteristics of gel‐spun PAN fibers with different molecular weight and different draw ratios.