Structure-activity Studies of Fracture Energies in Polymer Hydrogels

A quantitative understanding of the molecular mechanisms underlying the mechanical fracture of polymer gels and elastomers might inform methods by which to design more robust polymeric materials. The basis for much of the molecular insight into molecular fracture mechanisms is found in the Lake-Thomas model and equation, but few quantitative tests of that model have been reported. As a first step toward a quantitative test of the Lake-Thomas model, we report here the relationship between volume fraction, average active subchain length (N), fracture energy in a well-defined covalent polymer gel.

A poly(ethylene glycol) network formed by thiol-norbornene “click” chemistry was was employed because of its synthetic accessibility, ease of modification, and well characterized single molecule mechanical behavior. The mechanical properties, including resilience, modulus, and fracture energy, of multiple gel formulations were determined. From the moduli of the gels, N was determined. The relationship of fracture energy to volume fraction and N to test whether the gels act in the manner predicted by Lake-Thomas. This was done for gels at varying volume fraction, stoichiometry, and molecular weight of monomer.

At constant stoichiometry and varying volume fraction, the 10 kDa gels and lower concentration 30 kDa gels acted as predicted by Lake and Thomas, where the fracture energy should scale linearly with volume fraction. Neither of the gels, however, followed the predicted scaling of GC as a function of N; while GC ~ N0.5 was expected, we saw GC ~ N-0.75 and N-1.75. This may be due to N not being corrected for the amount of polymer lost in dangling ends; correcting N may fix this problem. When reaction efficiency is taken into account using the Miller-Macosko treatment, GC scales as N0.3, in better agreement with the expected scaling exponent of 0.5.