Glass transition relationships with molecular structure

Relationship Between Molecular Structure and Composition of Poly(ethylene-co-p-MS) Copolymers. From above discussion, we have shown that poly(ethylene-co-p-MS) copolymers with a wide range of compositions can be achieved by varying the p-MS monomer concentration in the feed. To fully understand the properties of this new class of materials, it is very important to know the correlation between the copolymer compositions and their molecular structures. In this section, we focus on the effect of p-MS concentration on molecular weight, molecular weight distribution, melting point (Tm), crystallinity and glass transition temperature (Tg) of the copolymer. A series of copolymers with various p-MS concentrations were analyzed by GPC and DSC. 

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. 

A fundamental property that determines the state of a reacting system is its extent of cure or chemical conversion (a). Several papers have shown that there is a unique relationship between the glass-transition temperature (Tg) and a that is independent of cure temperature and thermal history. This may imply that molecular structures of materials cured with different histories are the same or that the changes in molecular structure do not affect Tg. There are generally accepted to be two approaches to modelling glass-transition-conversion relationships, namely thermodynamic and viscoelastic approaches. These are summarized in Table 3.8. 

In this paper, the BPDA-PFMB/PEI molecular composites were oriented by means of zone annealing/drawing slightly above the glass transition temperatures of the respective molecular composites (280 - 400 °C). The dependence of draw ratio on tensile modulus, crystal orientation, and birefringence was determined as a function of composition. The relationship between structure (crystal orientation) and tensile property (modulus) of drawn films has been examined by comparing crystal chain orientations with the prediction of affine deformation (12-16). 

More recently, Class and Chu" extended the use of dynamic mechanical measurements to a systematic study of resin-elastomer blends which revealed the relationship between the structure, concentration and molecular weight of resins and their effect on the viscoelastic properties of elastomers. Dynamic mechanical data typical of that obtained from an elastomer or elastomer-resin blend is shown in Fig. 4. G is the elastic or storage modulus, G" is the viscous or loss modulus, and the ratio of G jG gives the tan 6 curve. The temperature at which the tan 6 curve shows a maximum corresponds to a dynamic glass transition temperature. Class and Chu showed that with these types of measurements, the effect of modifying resins on the viscoelastic properties of elastomers can be readily determined. Resins which are compatible with an elastomer will cause a decrease in the elastic modulus G at room temperature and an increase in the tan delta peak or glass transition temperature. Resins which are incompatible with an elastomer will cause an increase in the elastic modulus G at room temperature and will show two distinct maxima in the tan delta curve. 

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