Molecular glasses experimental findings

It may be appropriate to discuss the NMR findings on the /1-process in the context of results from other experimental methods. Unlike 2H NMR, the vast majority of experimental techniques are not capable of resolving slow reorientation about very small angles. In particular, several studies on the /1-process of molecular glasses may have overlooked the small-angle contribution of the majority of molecules and concluded that a small fraction of molecules is involved in the secondary relaxation process. In contrast, straightforward analysis of 2H NMR solid echo (cf. Section 3.2.1) and spin-lattice relaxation data (cf. Section 3.2.4 and in particular Ref. 115) clearly shows that essentially all molecules participate in the /1-process. However, the amplitude of the reorientation differs among the molecules. The mean... 

The idea that the fractional free-volume at glass temperature as found experimentally depends on the mode of molecular motions was put forward in 196746 47 as a result of calculating/g from data obtained from isothermal volume relaxation for some polymer systems. By estimating average relaxation time at different temperatures it was possible to find the fractional free-volume/g at Te according to WLF theory. If we accept the validity of the theory as regards the universal dependence of the reduction factor aT on (T - Tg), then on the basis of data on Aa and theoretical values aT calculated from universal values of the coefficients C and C, it is possible to make an estimate of/g. In this case the value found corresponds to the universal one. If, however, we use the experimental values aT, the fractional free-... 

The study of static and dynamic properties of polymer liquids and glasses is one of the central topics of research in polymer science. A variety of experimental and theoretical techniqi s have been mobilized for this purpose. In recent years the techniques of computer simulations are increasingly finding application toward this goal. AU the various methods of simulating polymer molecules on computers, such as Monte Carlo, molecular mechanics. Brownian dynamics, and molecular dynamics simulation techniques have been utilized. Early works relied mostly on Monte Carlo techniques apfdied to schematic models of polymer Uquid built on a lattice. With increased capabilities of computers available in recent years, the use of more computationally intensive methods has become feasible, allowing simulations of more realistic, off-latdce models of bulk polymers by Brownian and molecular dynamics simulation techniques as well as by Monte Carlo methods. 

In addition to the value of ViF, one can also obtain an estimate for two other terms. The value of Of is small, because in the absence of an adsorption film, the contact can be regarded as direct, almost an ideal one. Although Of is small, it is still nonzero, as it would have been in the case of a complete cohesion of two surfaces, such as in the coalescence of two liquid droplets. There are two issues that introduce some uncertainty. First, the screening of the glass with methyl groups may be incomplete. Second, the surface of particles (fused glass) may not necessarily be a molecularly smooth spherical surface. For this reason, the value of Of assumes finite positive values on the order of tenths and even units on mJ/m. This is further confirmed by the observed spread in the 14F values for various specimens from 18 to 22 mJ/m. The sum X Acjf( ) = X + X = 12 now yields an approximate estimate for the surface free energy, 0,2. The reliability of this estimate is biased by the mentioned uncertainty in Cf and the 10% variation in the experimental values of p, and F. For the nonpolar solid phase, we therefore find a quantitative estimate of 012 = ( sg 20 mJ/m with the precision down to a few mJ/m. ... 

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