Bimodal elastomers

Some viscoelasticity results have been reported for bimodal PDMS [120], using a Rheovibron (an instrument for measuring the dynamic tensile moduli of polymers). Also, measurements have been made on permanent set for PDMS networks in compressive cyclic deformations [121]. There appeared to be less permanent set or "creep" in the case of the bimodal elastomers. This is consistent in a general way with some early results for polyurethane elastomers [122], Specifically, cyclic elongation measurements on unimodal and bimodal networks indicated that the bimodal ones survived many more cycles before the occurrence of fatigue failure. The number of cycles to failure was found to be approximately an order of magnitude higher for the bimodal networks, at the same modulus at 10% deformation [5] ... 

Birefringence measurements have been shown to be very sensitive to bimodality, and have therefore also been used to characterize non-Gaussian effects resulting from it in PDMS bimodal elastomers [5,123]. The freezing points of solvents absorbed into bimodal networks are also of interest since solvent molecules constrained to small volumes form only relatively small crystallites upon crystallization, and therefore exhibit lower crystallization temperatures [124—126]. Some differential scanning calorimetry (DSC) measurements on... 

Additional insights into the dynamics and structure of bimodal elastomers have been obtained by dynamic light-scattering experiments [129], neutron scattering experiments [130] and calculations [131], dual cross-linking system experiments [132], non-affine swelling [133], and the computer simulations already mentioned. 

Bimodal elastomers prepared by end linking have very good ultimate properties, which underlies recent interest in such materials. ... 

The distribution of network chain lengths in a bimodal elastomer can be much different from the usual unimodal distribution obtained in less-controlled methods of cross linking. Figure 7.15 shows a schematic... 

Ultimate strength, as represented by the modulus at rupture, shown as a function of the molecular weight between cross links for a unimodal and bimodal elastomer compared at two temperatures. The improvement is larger at the lower temperature, presumably due to enhanced strain-induced crystallization. 

Representative stress-strain isotherm for a bimodal elastomer in both uniaxial extension (left side) and biaxial extension (right side). 

Tearing energies for a unimodal and bimodal elastomer as a function of tearing rate... 

Maximum tensile strengths in tearing for bimodal elastomers as a function of the ratio of the molecular weights of the short and long chains. 

Birefringence can be used to characterize non-Gaussian behavior in PDMS bimodal elastomers. - A large decrease the stress-optical coefficient (ratio of birefringence to stress) was observed over a relatively small range in elongation, presumably due to limited extensibility of the short chains. 

Von Lockette, P. R., Examination of the Effects of Computationally Determined Network Topology on an Analytical Constitutive Model for Bimodal Elastomers. Polymer 2008,49,5158-5168. 

Viers, B. D. Mark, J. E., Elastomeric Properties of Polysiloxane Networks. Bimodal Elastomers That Are Spatially Inhomogeneous and Others That Are Very Broadly Multimodal. J. Macro. Sci., PureAppl. Chem. 2007,44,131-138. 

Birefringence measurements are very sensitive to himodality and have therefore also been used to chararterize non-Gaussian effects resulting from it in PDMS bimodal elastomers. ... 

Bimodal elastomers prepared by these end-linking techniques have very good ultimate properties (5,13,83,84,86). In addition to this experimental work, there are now theoretical studies addressing the novel mechanical properties of bimodal elastomers (13). 

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