Measuring Tie Chains and Trapped Entanglements in Semicrystalline Polymers

A label-free method for quantifying stress transmitter (or elastically effective molecule) content (p) in semicrystalline polymers, including both tie molecules and bridging entanglements, is developed and demonstrated based on swelling with deuterated vapor and characterization with small-angle neutron scattering. The p results are compared with the predictions of recent semiempirical, statistical values for tie molecule content and structural characterization parameters, including strain hardening modulus and an infrared-spectroscopy-derived parameter (β) that describes the degree of difficulty for the amorphous content to align and reshape over a distance with the applied load. A strong correspondence is observed, suggesting that the initial network of elastically active molecules, dictated by the molecular architecture and crystallization conditions, can be directly correlated to the postyield tensile values irrespective of the subsequent morphological changes that result during the tensile deformation. These comparisons are also consistent with simulations, indicating that polyethylene homopolymers have more bridging entanglements than copolymers and that the average tie molecule has a larger impact on the mechanical properties than the average bridging entanglement. Contrary to high-temperature bulk swelling measurements, it is found that the Michaels–Hausslein vapor swelling theory cannot fit the experimental data, while our modified Flory–Rehner theory can fit the data.