Effect of the glass transition temperature

Reactions which require very little change in the geometry of the excited state and reactants should proceed equally well in solid glassy polymer matrices and in solution. Dissociation of free radical pairs can be relatively efficient in the solid-state if one component is a small free radical but will be significantly inhibited if both components are polymeric radicals. [874]. 

Reactions which can be considered to be associated with caged radicals, such as the photo-Fries, will require very little free volume and can be expected to be quite efficient in solid polymers below the glass transition, whereas photochemical processes such as the Norrish Type II process are substantially reduced in glassy polymers below the glass transition temperature (Tg), unless the geometry of the cyclic six-membered ring is particularly favoured by steric factors in the chain, so that the most stable conformation corresponds to that required for reaction [874]. 

Bimolecular reactions which require the diffusion of a small molecule reagent to a species in a polymer matrix will depend on both the diffusion constant and the solubility of the material in the matrix. Diffusion in solid glass matrices, particularly of small molecules, is much higher than would be predicted from the bulk viscosity of the medium. Solid polymers generally have internal viscosities only two to three orders of magnitude less than 

Above the glass transition temperature (TJ, the conformational freedom of the polymer chain can be comparable to that in solution, and consequently has equivalent chemical reactivity. However, it is not applicable to all photochemical reactions in solid systems. In the case of epoxy resins, a significant decrease of the glass transition temperature (Tg) has been observed during the photochemical oxidation [216, 220, 728]. 

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No.22, 2nd Nov. 1999, p.7617-29 EFFECT OF THE GLASS-TRANSITION TEMPERATURE ON FILM FORMATION IN 2-ETHYLHEXYLACRYLATE/METHYL METHACRYLATE EMULSION COPOLYMERS... 

What is the effect of the glass transition temperature on the Hildebrandt solubility parameter estimate of the polymer ... 

Figure 2-11. The effect of the glass-transition temperature (Tg) on the volume or length of TPs.

The advantage of the simulations compared to the experiments is that the correspondence between the tracer diffusion coefficient and the internal states of the chains can be investigated without additional assumptions. In order to perform a more complete analysis of the data one has to look at the quench-rate and chain-length dependence of the glass transition temperature for a given density [43]. A detailed discussion of these effects is far beyond the scope of this review. Here we just want to discuss a characteristic quantity which one can analyze in this context. 

Apparently, annealing was not impeded by crosslinks (Fig. 5.1). The density effects observed agree with the results of the glass transition temperature measurements (Sect. 4.2). There, the Tg of the annealed (and therefore denser) sample was consistently higher by about 2 K than the Tg of the quenched polymer. 

The melting temperatures of the even-even PAs are higher than for odier AA-BB-type PAs and A-B-type PAs.6,11,21 However, the amide order has little effect on the glass transition temperature (Fig. 3.3). 

The effects of these different factors can be seen in the Tg values of some typical polymers. A number of these values are shown in Table 3.1, together with a brief note about what feature particularly contributes to the relative level of the glass transition temperature. 

In general, the miscibility between two polymers can be predicted by thermal characterization of the blends [36], One of the most simple and effective ways to predict miscibility between two polymers is to consider the behavior of the glass transition temperature in the blend systems, which is known as the Tg method. In miscible blend systems, only a single 7 g intermediate between two components appears in the amorphous state. Therefore, we studied the change of... 

As expected, the terminal functional groups mainly determine the reactivity of these siloxane oligomers towards other reactants. The variations in the backbone composition have critical effects on the glass transition temperature, solubility parameter, thermal stability and surface behavior of the resulting oligomers(12,13). 

The nature of the chain end groups have also been shown to have a dramatic effect on the glass transition temperature of dendritic macromolecules. A variety of workers have investigated this question by the preparation of chain end... 

The time and temperature dependent properties of crosslinked polymers including epoxy resins (1-3) and rubber networks (4-7) have been studied in the past. Crosslinking has a strong effect on the glass transition temperature (Tg), on viscoelastic response, and on plastic deformation. Although experimental observations and empirical expressions have been made and proposed, respectively, progress has been slow in understanding the nonequilibrium mechanisms responsible for the time dependent behavior. 

The so called plasticization effect, i.e., the depression of the glass transition temperature is also an important feature the sorbed gas acts as a kind of lubricant , making it easier for chain molecules to slip over one another, and thus causing polymer softening. 

Though MGI units have similar effects on the glass transition temperature as CMI comonomer, the specific features of the MGIMx copolymer chemical structure yield mechanical behaviours, in particular in fracture, quite different to the CMIM series (as described in [1]). For this reason, it is interesting to investigate the effect of MGI unit content on the p transition of the copolymers and compare it to that observed in the CMIM series. 

Plasticization, whose main manifestation is the decrease of the glass transition temperature (a transition in dynamic mechanical spectra), is generally accompanied by an increase of the glassy modulus in the temperature interval between Tp and T. , an effect is known as antiplasticization. 

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