Mechanical and Optical Behavior of Double Network Rubbers

Abstract

Stress, strain, and optical birefringence were measured for a series of peroxide-cured natural rubbers, having both isotropic and double network structures. The residual stretch (permanent set) for the latter ranged from 2.0 to 4.5, with elastic moduli that were an increasing function of this residual strain, as found in previous works. A small birefringence, ca. 10 -5 , was observed for the unstressed double networks, and its magnitude increased with increasing residual strain. The sign of this birefringence corresponded to extension of the (undeformed) double networks. Under stress, the birefringence followed the stress optical law. The double network properties were interpreted using the constrained-chain model of rubber elasticity, with the assumption of independent, additive contributions from the two component networks. The calculated results differed from the experimental findings, in particular underestimating the residual strain. This failure is a consequence of the overprediction of the stresses during compression, a limitation common to molecular theories of rubber elasticity. The modeling of the double networks does account qualitatively for the sign of their unstressed birefringence, which is due to the stressoptical coefficient being larger in tension than in compression. This particular deviation from the stressoptical law is known from both theory and experiment.

Unconventional Rubber Networks: Circumventing the Compromise Between Stiffness and Strength

“…The principal components of the refractive index, n x , n y , and n z , become unequal upon orientation, causing birefringence. As illustrated in Figure 7, double networks are birefringent in the absence of external stress, 47,49 contrary to Eq. 2.…”

“…-Engineering stress versus stretch ratio for double networks of polyisoprene having the indicated strain during the second cross-linking. 49 The total cross-link density was the same for all samples, but there is a systematic increase in stiffness with cross-linking strain, which corresponds to increasing residual strain. The stress at 100% strain is shown in the inset.…”

“…[43][44][45][46][47][48] Generally, the modulus increases parallel to the strain during cure, with negligible changes in the transverse direction. This modulus enhancement is illustrated in Figure 5 for a series of elastomers having the same total cross-link density 49 but with different strains during the second cross-linking. For double networks reinforced with carbon black, lower electrical conductivity 50 and suppression of the Payne effect 51 have been observed, both consistent with deflocculation of the filler particles by the strain imposed during curing.…”

Role of Strain Crystallization in the Fatigue Resistance of Double Network Elastomers

“…This intrinsic orientation is seen in the birefringence of double networks in an relaxed (zero stress) state. 10,25 This orientation stabilizes any crystallinity at the front of growing cracks, thereby inhibiting their propagation. Thus, the origin of the enhanced fatigue life for double networks of strain-crystallizing rubbers is analogous to the mechanism giving rise to better fatigue resistance under non-relaxing deformations of single networks of such materials.…”

Strength Enhancement From Heterogeneous Networks of Ethylene–propylene/Ethylene–propylene–diene

“…Since the properties of an elastomeric network depend on the orientation of the chains, double networks exhibit mechanical behavior distinct from the corresponding single (unoriented) networks. [6][7][8] Better mechanical properties have also been achieved with bimodal networks, in which a portion of the chains between cross-links is short, and the remaining network strands are very long. Bimodal networks are prepared by end-linking a large number of low Mw precursor chains with a large weight fraction of high Mw chains, yielding elastomers with good toughness.…”