End-linking polymer chains by means of a multifunctional cross-linking agent provides an ideal way for obtaining elastomeric networks of any desired distribution of chain lengths. In the present investigation, this technique was employed to give polydimethylsiloxane networks consisting of various proportions of relatively long and very short network chains. The stress–strain isotherms of these networks generally showed an anomalous increase in the modulus at high elongation, and the increase persisted at high temperatures and high degrees of swelling. This non-Gaussian effect was quantitatively correlated with the limited extensibility of the network chains; specifcally, the increase in modulus was found to begin at approximately 60%–70% of the maximum extensibility of the network chains, and network rupture at 80%–90%. The elongation at which the increase becomes evident increases with decrease in the proportion of the very short chains, thus verifying the nonaffine nature of the deformation at high elongation. In addition, the elasticity constants characterizing the Gaussian regions of the isotherms, and the values of the degree of equilibrium swelling were used to evaluate the most recent molecular theories of rubberlike elasticity, particularly with regard to the structure factor relating the modulus of an elastomer to the average length or molecular weight of the network chains.

Mechanical properties of polyacrylamide gels covering a wide range of polymer concentrations have been studied. Gels were synthesized by using N,N-methylenebis(acrylamide) as cross-linking agent whose weight percentage, with respect to the total weight of comonomers, ranged from 0.663% to 14.50%. The resulting gels were analyzed by means of their stress-strain isotherms in elongation at 30 °C. Mooney-Rivlin type plots of the data show a large increase of the modulus or upturn, particularly on gels with high percentage of cross-linking monomer and at high polymer concentrations, due to non-Gaussian effects arising from the very heterogeneous molecular network structure. The elastic modulus was found to increase exponentially with total comonomer concentration, keeping constant the percentage of bisacrylamide comonomer.On the other hand, the modulus passes through a maximum as the amount of cross-linking agent is increased. The ultimate properties found and the comparison of the cross-linking densities obtained from the elastic results with the theoretical ones, determined from the initial comonomer compositions, confirm the very high heterogeneity of polyacrylamide gels.

Elastomeric networks were prepared by tetrafunctionally end‐linking mixtures of various proportions of relatively long and very short polydimethylsiloxane (PDMS) chains. The former had a number‐average molecular weight of 18,500 and the latter either 660 or 220 g mole−1. The series of (unfilled) bimodal networks thus prepared were studied in elongation to the rupture point at 25°C, and in swelling equilibrium in benzene at room temperature. Elasticity constants characterizing the Gaussian regions of the stress–strain isotherms, and values of the degree of equilibrium swelling were used to evaluate the most recent molecular theories of rubberlike elasticity. The isotherms also gave values of the elongation at which the modulus begins to increase anomalously because of limited chain extensibility, and values of the elongation and nominal stress at the point of rupture. These results were interpreted in terms of the known configurational characteristics of the constituent PDMS chains. Values of the energy or work required for rupture were used as an overall measure of the “toughness” of the networks. The very short chains were found to give a marked increase in toughness, through an increase in ultimate strength without the usual corresponding decrease in maximum extensibility. A variety of additional experiments will be required in order to elucidate the molecular origins of this important effect.

The crosslinking copolymerization of acrylamide (AA) and N,N′-methylene-bis-acrylamide (BA) has been studied by high resolution 1 H-NMR with copolymerization in situ. This procedure allows calculation of the copolymer composition at zero degree of conversion and as a function of the polymerization time. Monomer reactivity ratios were calculated by the KelenTüdös method in the pre-gel state and during gelation. Sequence distributions of both comonomers were then characterized.

The weight conversion at the gel point (oc 0 ) was determined by the gas bubble method in two sets of polyacrylamide (PAA) gels crosslinked with' N,N'-methylene-bisacrylamide (BA). oc 0 remained practically constant when the crosslinker ratio was C = 10% (w/w) and the total concentration of comonomers in the feed ranged from C T = 5 to 10 g/100 mi. When C T was kept constant at 5 g/100 mi and C changed from O to 25% (w/w), oc 0 showed a minimum at C = 7%. By comparing the experimental values of oc 0 with those calculated for an ideal network, it was concluded that the crosslinking efficiency of BA is very low. The dependence on C of oc 0 and other properties like the elastic phantom modulus, 1/ ;h l, and the polymer volume fraction at swelling equilibrium, v 2 ,, was explained in terms of the formation of network defects: the formation of BA sequences longer than unity and that of intramolecular cycles both have an increasing contribution at C > 7% but can be neglected at lower values of C.