Glass Transition, Structure-Property Relationships

The glass transition phenomenon has been presented in Chapter 4. Here, only structure-property relationships will be briefly examined. 

There is a relatively abundant literature on the relationships between Tg and the network structure, but it is difficult to make a coherent synthesis of the published data owing to the great diversity of empirical, semiempi-rical, and physical approaches to the problem. 

It must be first remarked that structure-Tg relationships can be reasonably established on the basis of experimental data obtained on networks of well-defined structure. Thus, it seemed interesting to us to distinguish three cases  

Some important trends of the structure-property relationships in this field are well illustrated by a comparison of some stoichiometric, fully cured epoxide-amine networks (Table 10.6). 

Three structural parameters have been calculated from the theoretical CRU for these networks  

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SW Yin, Z Shuai, and Y Wang, A quantitative structure-property relationship study of the glass transition temperature of OLED materials, J. Chem. Inf. Comput. Sci., 43 970-977, 2003. 

Katritzky AR, Rachwal P, Law KW et al. (1996) Prediction of polymer glass transition temperatures using a general quantitative structure-property relationship treatment. J Chem Inf Comput Sci 36 879-884... 

Katritzky AR, SUd S, Lobanov V et al. (1998) Quantitative structure-property relationship (QSPR) correlation of glass transition temperatures of high molecular weight polymers. J Chem Inf Comput Sci 38 300-304... 

Liu A, Wang X, Wang L et al. (2007) Prediction of dielectric constants and glass transition temperatures of polymers by quantitative structure-property relationships. Eur Polym J 43 989-995... 

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

In addition to the Bisphenol-A backbone epoxy resins, epoxies with substituted aromatic backbones and in the tri- and tetra- functional forms have been produced. Structure-property relationships exist so that an epoxy backbone chemistry can be selected for the desired end product property. Properties such as oxygen permeability, moisture vapor transmission and glass transition temperature have been related to the backbone structure of epoxy resins5). Whatever the backbone structure, resins containing only the pure monomeric form can be produced but usually a mixture of different molecular weight species are present with their distribution being dictated by the end-use of the resin. 

Relatively few processible polyimides, particularly at a reasonable cost and in reliable supply, are available commercially. Users of polyimides may have to produce intractable polyimides by themselves in situ according to methods discussed earlier, or synthesize polyimides of unique compositions in order to meet property requirements such as thermal and therm oxidative stabilities, mechanical and electrical properties, physical properties such as glass-transition temperature, crystalline melting temperature, density, solubility, optical properties, etc. It is, therefore, essential to thoroughly understand the structure—property relationships of polyimide systems, and excellent review articles are available (1—5,92). 

Other amorphous solids such as polymers, being rigid and brittle below. Tg, and elastic above it, also exhibit this behavior. Table 2.1 lists the glass transition temperatures of common solid materials. Although most solid-state textbooks deal almost exclusively with crystalline materials, this text will attempt to address both the crystalline and amorphous states, describing the structure/property relationships of major amorphous classes such as polymers and glasses. 

Katritzky, A.R., Sild, S., Lobanov, V.S. and Karelson, M. (1998c). Quantitative Structure-Property Relationship (QSPR) Correlation of Glass Transition Temperatures of High Molecular Weight Polymers. J.Chem.lnf.Comput.Sci., 38,300-304. 

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