Glass transition temperature, potential energy

Composite-based PTC thermistors are potentially more economical. These devices are based on a combination of a conductor in a semicrystalline polymer—for example, carbon black in polyethylene. Other fillers include copper, iron, and silver. Important filler parameters in addition to conductivity include particle size, distribution, morphology, surface energy, oxidation state, and thermal expansion coefficient. Important polymer matrix characteristics in addition to conductivity include the glass transition temperature, Tg, and thermal expansion coefficient. Interfacial effects are extremely important in these materials and can influence the ultimate electrical properties of the composite. 

The polydihalophosphazenes are examined by conformational analysis using nonbonding intramolecular interactions based on a 6-12 Lennard-Jones potential and a Coulombic term. The results provide an insight into the reasons for the low glass transition temperatures, the high chain flexibilities, and the conformational preferences of these molecules. Minimum energy conformations are discussed. 

The conformational entropies of copolymer chains are calculated through utilization of semiempirical potential energy functions and adoption of the RIS model of polymers. It is assumed that the glass transition temperature, Tg, is inversely related to the intramolecular, equilibrium flexibility of a copolymer chain as manifested by its conformational entropy. This approach is applied to the vinyl copolymers of vinyl chloride and vinylidene chloride with methyl acrylate, where the stereoregularity of each copolymer is explicitly considered, and correctly predicts the observed deviations from the Fox relation when they occur. It therefore appears that the sequence distribution - Tg effects observed in many copolymers may have an intramolecular origin in the form of specific molecular interactions between adjacent monomer units, which can be characterized by estimating the resultant conformational entropy. 

Siloxane elastomers present an attractive alternative to the butadiene acrylonitrile elastomers most often used for epoxy modification. Poly(dimethyl siloxanes) exhibit glass transition temperatures well below those of butadiene acrylonitrile modifiers (minimum —123 °C vs. about —40 °C) and also display very good thermal stability13, 14). Other favorable and potentially useful attributes include good weatherability, oxidative stability, and moisture resistance. Finally, the non-polar nature and low surface energy of poly(dimethyl siloxanes) constitute a thermodynamic driving force... 

X HE IMPORTANCE AND UTILITY of multiphase copolymer systems have been well documented in the literature (1-4), with emphasis on their unique combination of properties and their potential material applications. Or-ganosiloxane block polymers are a particularly interesting type of multiphase copolymer system because of the unusual characteristics of poly siloxanes, such as their stability to heat and UV radiation, low glass transition temperature, high gas permeability, and low surface energy (i, 2, 5). The incorporation of polysiloxanes into various engineering polymers offers an opportunity for many improvements, such as lower temperatures for the ductile-to-brittle transitions and improved impact strength. 

Classical methods are designed to obtain thermodynamic and transport information, for example molar volume, density, viscosity, and surface tension. The effects of pressure and temperature on these properties can also be evaluated, and thus phase transition information such as melting points and glass transition temperatures. If molecular dynamics (in contrast to Monte Carlo) is used, data relating to reorientation of molecules, self-diffusion and residence times are all available. Information can also be obtained from the simulation equations on the contribution made by kinetic, coulombic, intramolecular and dispersion energies to the total potential energy. However, because the charges are fixed and there is no explicit wavefunction included in the classical methods, no electronic information can be obtained. 

This chapter discusses the form and parameterization of the potential energy terms that are used for the atomistic simulation of polymers. The sum of potential terms constitutes a molecular force field that can be used in molecular mechanics, molecular dynamics, and Monte Carlo simulations of polymeric systems. Molecular simulation methods can be used to determine such properties as PVT data, selfdiffusion coefficients, modulus, phase equilibrium, x-ray and neutron diffraction spectra, small molecule solubility, and glass transition temperatures with considerable accuracy and reliability using current force fields. Included in the coverage of Chapter 4 is a review of the fundamentals of molecular mechanics and a survey of the most widely used force fields for the simulation of polymer systems. In addition, references to the use of specific force fields in the study of important polymer groups are given. 

Tbd Temperature of brittle-ductile transition Tg Temperature of glass transition Uch Bond energy of polymer chain Up Potential energy of the rubber particle a Coefficient of thermal expansion 8 Cohesive energy density Volume strain... 

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