Configuration Of Grafted Polystyrene Chains In The Melt: Temperature And Concentration Dependence

The concentration profiles of carboxy-terminated polystyrene chains in the melt grafted onto oxide-covered silicon substrates were measured using secondary-ion mass spectroscopy. The grafting density increased with temperature and an enthalpy of +7.4 kcal/mole was deduced for the grafting reaction, SiOH + R(COOH) ⇄ R(COOSi) + H2O. Relatively high grafting densities (σ∼6.6·mg/m2) were achieved with minimal chain distortion or displacement of long chains by shorter ones. Significant stretching of the grafted chains occurred for σ > 10 mg/m2. An equilibrium constant for the grafting reaction incorporating entropy is discussed.69577677

Growth, microstructure, and failure of crazes in glassy polymers

We report on an extensive study of craze formation in glassy polymers. Molecular dynamics simulations of a coarse-grained bead-spring model were employed to investigate the molecular level processes during craze nucleation, widening, and breakdown for a wide range of temperature, polymer chain length $N$, entanglement length $N_e$ and strength of adhesive interactions between polymer chains. Craze widening proceeds via a fibril-drawing process at constant drawing stress. The extension ratio is determined by the entanglement length, and the characteristic length of stretched chain segments in the polymer craze is $N_e/3$. In the craze, tension is mostly carried by the covalent backbone bonds, and the force distribution develops an exponential tail at large tensile forces. The failure mode of crazes changes from disentanglement to scission for $N/N_e\sim 10$, and breakdown through scission is governed by large stress fluctuations. The simulations also reveal inconsistencies with previous theoretical models of craze widening that were based on continuum level hydrodynamics

Characteristic Ratios of Model Polydienes and Polyolefins

Homogeneous anionic polymerization offers several unique advantages in the polymerization of diene monomers. Polymers prepared by this method possess extremely narrow molecular weight distributions and are free of branching. The microstructure of the polymer may be controlled by the choice of polymerization conditions. Amorphous polyolefins can be obtained by hydrogenation of diene polymers. Polyolefins prepared in this manner retain the desirable characteristics of the parent material, i.e., narrow molecular weight distributions and an absence of branching. Hence these polymers are ideal models for testing molecular theories. Several series of model polydienes and polyolefins have been prepared by the aforementioned techniques. These polymers have been carefully characterized, with particular emphasis on determining the effect of molecular architecture on chain stiffness as defined by the characteristic ratio. Glass transition temperatures are also reported. © 1984, American Chemical Society. All rights reserved

Solution Properties and Chain Flexibility of Poly (thiolmethacrylates). 2. Poly (cyclohexyl thiolmethacrylate)

The dilute-solution properties of eight poly(cyclohexyl thiolmethacrylate) (PTCy) fractions covering the molecular weight (Mw) range 4.8 X 105 to 5.9 X 105 (Mw/Mn m 1.4) were studied in tetrahydrofuran, toluene, and cyclohexane. The unperturbed dimensions were calculated by using light scattering and intrinsic viscosity data. The results indicate that PTCy is a less extended chain than poly(cyclohexyl methacrylate). © 1981, American Chemical Society. All rights reserved

Solvent and Temperature Influences on Polystyrene Unperturbed Dimensions

The unperturbed chain dimensions of near-monodisperse atactic polystyrenes were evaluated from intrinsic viscosity measurements in 12 different solvents under θ or near-θ conditions over the temperature range 8.5-75.0 °C. In consonance with the indications of other workers, larger values of unperturbed dimensions were found in cyclic hydrocarbon solvents than in other ideal solvents at similar temperatures. Negative values for the temperature coefficient of chain dimensions were found within the three solvent series examined. Under conditions where specific solvent effects are eliminated or minimized, measurements yielded results in excellent agreement with the theoretical predictions for atactic polystyrene, i.e., d In (r2)0/dt = -1.1 X 10-3 deg-1. © 1985, American Chemical Society. All rights reserved

Solvent and Temperature Influences on Polystyrene Unperturbed Dimensions

The unperturbed chain dimensions of near-monodisperse atactic polystyrenes were evaluated from intrinsic viscosity measurements in 12 different solvents under θ or near-θ conditions over the temperature range 8.5-75.0 °C. In consonance with the indications of other workers, larger values of unperturbed dimensions were found in cyclic hydrocarbon solvents than in other ideal solvents at similar temperatures. Negative values for the temperature coefficient of chain dimensions were found within the three solvent series examined. Under conditions where specific solvent effects are eliminated or minimized, measurements yielded results in excellent agreement with the theoretical predictions for atactic polystyrene, i.e., d In (r2)0/dt = -1.1 X 10-3 deg-1. © 1985, American Chemical Society. All rights reserved