Polymers, small impurity molecule

Diffusion of small impurity molecules is an important issue of polymers particularly with respect to their mechanical, dielectric, and optical properties as well as ageing, and has stimulated extensive theoretical and experimental work within the past years [90,91], Diffusion in amorphous polymers is often random on a macroscopic scale, this must not necessarily... 

There have been numerous studies of the electrical and emission properties of conjugated polymer-, small molecule-, and molecularly doped polymer-based OLEDs. The current-voltage and radiance-voltage characteristics have been nica sured as a function of thickness of the organic layer, temperature, different metal electrodes, etc. in an attempt to understand the device physics. A major factor in hibiting progress is the purity of the organic impurities that are incorporated dur-... 

Previous authors [19, 21-23, 34] also reported that the polymer concentration is nearly constant in the region 250-420 °C although their absolute values in some cases differ from more recent results because of deficiencies in the quenching procedure or the impact of impurities including the water used for chilling the melt. Generally, it is assumed that the polymer is present as an ideal solution in a melt of small ring molecules. 

Consider a linear polymer with molecular weight Mq and a small impurity of a similar polymer with a higher molecular weight M. We shall assume that the amount of the high-molecular-weight impurity is so small that its molecules do not interact with each other, so that the medium in which the molecules move is a system consisting of a linear polymer of molecular weight Mo, which is characterised by the modulus... 

The molecules used in the study described in Fig. 2.15 were model compounds characterized by a high degree of uniformity. When branching is encountered, it is generally in a far less uniform way. As a matter of fact, traces of impurities or random chain transfer during polymer preparation may result in a small amount of unsuspected branching in samples of ostensibly linear molecules. Such adventitious branched molecules can have an effect on viscosity which far exceeds their numerical abundance. It is quite possible that anomalous experimental results may be due to such effects. 

As Skinner has pointed out [7], there is no evidence for the existence of BFyH20 in the gas phase at ordinary temperatures, and the solid monohydrate of BF3 owes its stability to the lattice energy thus D(BF3 - OH2) must be very small. The calculation of AH2 shows that even if BFyH20 could exist in solution as isolated molecules at low temperatures, reaction (3) would not take place. We conclude therefore that proton transfer to the complex anion cannot occur in this system and that there is probably no true termination except by impurities. The only termination reactions which have been definitely established in cationic polymerisations have been described before [2, 8], and cannot at present be discussed profitably in terms of their energetics. It should be noted, however, that in systems such as styrene-S C/4 the smaller proton affinity of the dead (unsaturated or cyclised) polymer, coupled, with the greater size of the anion and smaller size of the cation may make AHX much less positive so that reaction (2) may then be possible because AG° 0. This would mean that the equilibrium between initiation and termination is in an intermediate position. 

The processes of photochemistry are the same for polymers and small molecules. The Grotthus-Draper law sfafes fhat no photochemical reactions can occur unless a photon of lighf is absorbed. This means, for example, thaf many commercial plastics transparent in the near UV can undergo photodegradation only as a result of the absorption of light by impurities. 

Sometimes the isolation of individual members of the series is not an issue, as in polymer synthesis. Pure neat propellane polymerizes spontaneously in a matter of hours at room temperature. The process can be suppressed by dilution with a solvent or addition of a small amount of a radical inhibitor. Although a possible catalytic role of impurities and Teflon-coated container walls has not been ruled out completely rigorously, it appears likely that this may be a genuine example of a process in which two closed-shell molecules react to produce a biradical which then triggers oligomerization and polymerization. A SINDOl computational study has led to the proposal that the reaction proceeds through a [2]staffane-3,3 -diyl triplet formed by the interaction of two monomers followed by intersystem crossing . ... 

Reucroft and Takahashi (1975) reported room temperature hole mobilities of PVK. The mobilities were field dependent and described by a power-law relationship. The studies of Reucroft and Takahashi and Mort and Emerald (1974) are two of few references in the literature to a field dependence of log t °< PE. The presence of deep traps at 5 to 10 x 1012 cm-3 was reported. It was suggested that the traps were due to either small molecule impurities, impurity monomer units incorporated into the polymer chain, or localized regions of compression introduced by polymer chain entanglements. The lower limit of the trap cross section was reported as 10 A2. It was suggested that the traps were primarily located in surface regions. 

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