Macromolecular 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.

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SYNOPSISIn a swelling state at a temperature of incipient dissolution, an acrylic fiber is highly elastic just like a rubber. A system of entangled macromolecular chains is considered analogous to a cross-linked rubber network and from the theory of rubber elasticity, the measured extension modulus of an acrylic fiber at the terminal swelling state may be used to calculate the number of entanglements. By means of a capillary rheometer, a series of model acrylic fibers with varied degrees of entanglements were prepared. The entanglements present in these model fibers were then measured by the other two thermal methods (SDSC and SSS/ SS methods) already established in our laboratory. The die swell ratios were also measured microscopically at the exit of the capillary rheometer. The experimental results of entanglements measured by the various methods were well correlated, at least qualitatively.