Efficient Determination of Slip-Link Parameters from Broadly Polydisperse Linear Melts.

We investigate the ability of a coarse-grained slip-link model and a simple double reptation model to describe the linear rheology of polydisperse linear polymer melts. Our slip-link model is a well-defined mathematical object that can describe the equilibrium dynamics and non-linear rheology of flexible polymer melts with arbitrary polydispersity and architecture with a minimum of inputs: the molecular weight of a Kuhn step, the entanglement activity, and Kuhn step friction. However, this detailed model is computationally expensive, so we also examine predictions of the cheaper double reptation model, which is restricted to only linear rheology near the terminal zone. We report the storage and loss moduli for polydisperse polymer melts and compare with experimental measurements from small amplitude oscillatory shear. We examine three chemistries: polybutadiene, polypropylene and polyethylene. We also use a simple double reptation model to estimate parameters for the slip-link model and examine under which circumstances this simplified model works. The computational implementation of the slip-link model is accelerated with the help of graphics processing units, which allow us to simulate in parallel large ensembles made of up to 50,000 chains. We show that our simulation can predict the dynamic moduli for highly entangled polymer melts over nine decades of frequency. Although the double reptation model performs well only near the terminal zone, it does provide a convenient and inexpensive way to estimate the entanglement parameter for the slip-link model from polydisperse data.

Radical Cationic Pathway for the Decay of Ionized Glyme Molecules in Liquid Solution.

Chemical stability of primary radical cations (RCs) generated in irradiated matter determines substantially the radiation resistance of organic materials. Transformations of the RCs of the glyme molecules, R(-O-CH2-CH2-)nO-R (R = CH3, n = 1-4) has been studied on the nanosecond time scale by measuring the magnetic field effects in the recombination fluorescence from irradiated liquid solutions of the glymes. In all cases, the RCs observed were different from that expected for the primary ones and revealed very similar hyperfine couplings independent of the poly(ethylene oxide) chain length and of the substitution of terminal methyl groups by C2H5 or CH2CH2Cl, as has been shown with diglyme as an example. Quantum chemical analysis of possible chemical transformations for the monoglyme RC as a model system allowed us to discover the reaction pathway yielding the methyl vinyl ether RC. The pathway involves intramolecular proton transfer followed by C-O bond cleavage. Only one (-O-CH2-CH2-O-) fragment is involved in this transformation, which is nearly barrierless due to the catalytic effect of adjacent glyme molecules. The rapid formation of the methyl vinyl ether RC in the irradiated monoglyme was confirmed by the numerical simulation of the experimental curves of the time-resolved magnetic field effect. These findings suggest that the R'-O-CH═CH2(•+) formation is a typical decay pathway for the primary RCs in irradiated liquid glymes.