Networks, bimodal Gaussian

For chains having fewer than 50 bonds, such as the short chains in a bimodal network, for example, the distribution departs markedly from the Gaussian limit. 

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... 

Non-Gaussian EflFects and Intermolecular Correlations in Bimodal Networks of Poly(dimethylsiloxane)... 

Values of the correlational contribution are listed in Table I. The correlational contributions decrease with an increase in the mole percent of short chains in the network. This trend is also consistent with the conclusion that intermolecular interactions have little to do with the non-Gaussian behavior exhibited by bimodal PDMS networks. 

C. Menduina, C. McBride, and C. Vega (2001) Correctly averaged Non-Gaussian theory of rubber-like elasticity - application to the description of the behavior of poly(dimethylsiloxane) bimodal networks. Phys. Chem,. Chem. Phys. 3, p. 1289... 

The long and short chains in the bimodal PDMS networks were assumed to have values of n of 20 and 250, respectively (9-16). The long chains were modeled as Gaussian chains, whereas the distribution function for the short chains was determined from Monte Carlo calculations as already described. The entropy of the bimodal network was then taken to be the sum of... 

For chains having fewer than 50 bonds, such as the short chains in a bimodal network for example, the distribution departs markedly from the Gaussian limit. Among various representations of w r) for short chains are the Hermite series (Flory and Yoon (1974, 1974)), the Fixman-Alben distribution, and Monte Carlo simulations (Erman and Mark, 1988). The Fixman-Alben distribution is given by... 

Numerical simulations of solvation dynamics in polar molecular solvents have been carried out on many models of molecular systems during the last decade. The study described in sections 4.3.4-4.3.5 focused on a generic model for a simple polar solvent, a structureless Stockmayer fluid. It is found that solvation dynamics in this model solvent is qualitatively similar to that observed in more realistic models of more structured simple solvents, including solvents like water whose energetics is strongly influenced by the H-bond network. In particular, the bimodal nature of the dynamics and the existing of a prominent fast Gaussian relaxation component are common to all models studied. 

More quantitative characterization of limited chain extensibility requires a non-Gaussian distribution function for the end-to-end separation, r, of the short network chains. The Fixman-Alben distribution was used to calculate stress-strain isotherms in elongation for bimodal PDMS networks. Good agreement was found between theory and experiment. Other non-Gaussian distribution functions have also been successfully used. The experimental isotherms can also be interpreted... 

Menduina, C. Freire, J. J. Uorente, M. A. Vdgis, T., Correctly Averaged Non-Gaussian Theory of Ruhherlike Elasticity. Application to the Description of the Behavior of Poly(dimethylsiloxane) Bimodal Networks. Macromolecules 1986,19,1212-1217. 

Kloczkowski, A. Mark, J. E. Erman, B., Fluctuations, Correlations and Small Angle Neutron Scattering from Endlinked Gaussian Chains in Regular Bimodal Networks. Macromolecules 1991, 24, 3266-3275. 

Curro, J. G. Mark, J. E., A Non-Gaussian Theory of Rubberlike Elasticity Based on Rotational Isomeric State Simulations of Network Chain Configurations. II. Bimodal Poly(dimethylsiloxane) Networks. J. Chem. Phys. 1984, 80, 4521-4525. 

Galiatsatos, V Mark, J. E., Non-Gaussian Effects and Intermolecular Correlations in Bimodal Networks of Poly(dimeth siloxane). In Advances in Silicon-Based Polymer Science. A Comprehensive Resource, Zeigler, J. M. Fearon, F. W. 

Kloczkowski A, Mark JE, Erman B. Fluctuations, correlations and small angle neutron scattering from end-linked Gaussian chains in regular bimodal networks. Macromolecules 1991 24 3266-75. 

Elastomers cured with two crosslinking systems such as sulphur and the polymerisation products of p-benzoquinone are shown to have much improved overall mechanical properties. Non-Gaussian behaviour of quinone polymer crosslinked elastomers viewed as bimodal networks was studied. The study focused on the effect of ageing time on the reduced stress values of the networks in relation to the elongation of the samples. The study is also extended to cover the possible effect of these bound antioxidants on the onset of the vulcanisation process and the hardness values of the elastomeric networks. 15 refs. 

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