Glass transition structural effects

In most cases when the temperature of measurement is above the glass transition, the effect of temperature leads to very complicated spectral effects since structural changes and temperature-induced spectroscopic changes are occuring simultaneously 201,322) jn some the structural changes are well defined as in the case of polystyrene 322). 

Ellison CJ, Mundra MK, Torkelson JM (2005) Impacts of polystyrene molecular weight and modification to the repeat unit structure on the glass transition-nanoconfinement effect and the cooperativity length scale. Macromolecules 38 1767-1778... 

Whereas the glass transition of a copolymer is usually intermediate between those of the corresponding homopolymers this is not commonly the case with the melting points. Figure 4.12 shows the effect of copolymerising hexamethylenesebacamide with hexamethyleneterephthalamide. Only when the monomer units are isomorphous, so that the molecules can take up the same structure, is there a linear relationship between melting point and composition (as with hexamethyleneadipamide and hexamethyleneterephthalamide). 

Polymers below the glass transition temperature are usually rather brittle unless modified by fibre reinforcement or by addition of rubbery additives. In some polymers where there is a small degree of crystallisation it appears that the crystallines act as knots and toughen up the mass of material, as in the case of the polycarbonates. Where, however, there are large spherulite structures this effect is more or less offset by high strains set up at the spherulite boundaries and as in the case of P4MP1 the product is rather brittle. 

In addition to the normal methylene linkage formation involved in polymerization with both resoles and novolaes, other, usually less desirable, eondensation by-products are also seen in novolac synthesis. Among these are benzodioxanes and dibenzyl ethers. The reaction pH has significant effect on the relative amounts produced. Fig. 15 shows typical structures for these by-products. When such byproducts are present, the meaning of the molar ratio changes and variability with respect to molecular weight development, glass transition point, and solubility may be seen. They also lead to poor raw material utilization. 

These general effects are summarized in Table 5.1. As can be seen in the table, unsymmetrical structure and flexible links adversely affect the glass transition. However, a combination of flexible and polar substituents can increase solubility without deleterious effect on Ts. 

TABLE 5.1 Effect of Chemical Structure on Solubility and Glass Transition... 

Features of chemical structure that affect the degree of molecular freedom influence both the crystalline melting point and the glass transition temperature. Moreover, such features have roughly similar effects on both properties, so that the empirical rule has been found that for many polymers ... 

Temperature dependence (related to the temperature dependence of the conformational structure and the morphology of polymers) of the radiation effect on various fluoropolymers e.g., poly (tetrafluoroethylene-co-hexafluoropropylene), poly(tetrafluoroethylene-co-perfluoroalkylvinylether), and poly(tetrafluoroethylene-co-ethylene) copolymers has been reported by Tabata [419]. Hill et al. [420] have investigated the effect of environment and temperature on the radiolysis of FEP. While the irradiation is carried out at temperatures above the glass transition temperature of FEP, cross-linking reactions predominate over chain scission or degradation. Forsythe et al. [421]... 

Figure 22. Shown in panel (a) is the relation between the bare energy difference e between frozen-in structural states in a glass and the effective splitting e that is smaller due the level repulsion in the tunnehng center. Panel (b) depicts schematically the derivative of e with respect to e, which is used to compute the new effective distribution P(e) of the transition energies.

We have carrried out an analysis of the multilevel structure of the tunneling centers that goes beyond a semiclassical picture of the formation of those centers at the glass transition, which was primarily employed in this chapter. These effects exhibit themselves in a deviation of the heat capacity and conductivity from the nearly linear and quadratic laws, respectively, that are predicted by the semiclassical theory. 

The TEOS content is changed from 50 to 70 wt% to study its effect on the final structure of the materials. Besides an increase of the stiffness and glass transition temperature, a speculated change in structure is observed at 70 wt% TEOS. This change is in line with that anticipated from simple theoretical consideration of composition. 

The self-similar spectrum is not valid at short times, X < X0, where the details of chemical structure become important (glass transition, entanglements, etc.). The cross-over to the glass transition at short times is typical for all polymeric materials, for both liquids and solids. The critical gel is no exception in that respect. X0 could be used as a characteristic time in the CW spectrum since it somehow characterizes the molecular building block of the critical gel however, it has no direct relation to the LST. At times shorter than X0, the LST has no immediate effect on the rheology. Indirect effects might be seen as a shift in the glass transition, for instance, but these will not be studied here. 

Monomers Molecular structure Glass transition Order-crystallinity Type of material Solubility Effect of T°... 

The formation of niclosamide hydrates, and the effect of relative humidity on the solvatomorphs obtained from acetone and ethyl acetate has been studied [79], The acetone and ethyl acetate solvatomorphs could be desolvated, and exposure to elevated humidity resulted in the formation of two hydrate structures. Each hydrate could be dehydrated into a different anhydrate phase, but only the hydrate formed from the acetone desolvate could be rehydrated to form a hydrate phase. Dynamic vapor sorption has been used to develop a method for determining the onset relative humidity of a glass transition and associated crystallization process [80]. 

Several factors related to chemical structure are known to affect the glass transition tempera lure. The most important factor is chain stiffness or flexibility of the polymer. Main-chain aliphatic groups, ether linkages, and dimethylsiloxane groups build flexibility into a polymer and lower Tg Aliphatic side chains also lower Tg, (he effect of the length of aliphatic groups is illustrated by the methacrylate series (4,38) ... 

Figure 11. The effect of reflow nearby the glass transition temperature for a PMMA-DR1 waveguide (a) device structure, (b) SEM picture of waveguide before reflow, (c) idem after reflow.

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