Features glass transition behavior

Polynadimides. The exact structure of the network crosslink is somewhat controversial but it could resemble the structure shown in Fig. 10.6. These polymers cumulate all the problems encountered in other polymers Is it really pertinent to consider that we are in the presence of hexa-functional crosslinks In this case, how do we take into account their copolymer effect In fact, if the black junctions in Fig. 10.6 connect one crosslink directly to another, we are in the presence of crosslink lines rather than dispersed individual crosslinks. Does this feature modify the whole glass transition behavior There is, to our knowledge, no satisfactory answer to these questions, and the research field remains largely open in this domain. 

This section will examine some of the characteristic features of IPN s from a physical and mechanical point of view. Emphasis will be on relating the glass transition behavior to corresponding aspects of morphology. The principal instrumentation employed in the studies discussed here includes a torsional tester for creep-type studies (Section 8.3.1) and a fixed-frequency vibrating unit for dynamic mechanical spectroscopy (see Section 8.3.2). In addition, stress-strain, tensile, and Charpy impact strength values will be briefly discussed. 

Our focus has been the elucidation of specific features of the model which give rise to the most important aspects of glass transition behavior. Thus time dependence arises naturally from a consideration of the molecular aspects of the overall phenomena involved. The pronounced nonlinearity and asymmetry of behavior result from the structure dependency of the individual retardation times introduced in the model. In fact, our analysis shows that, on a time scale appropriately compensated to take account of structural dependence, non-linearity and asymmetry vanish. The existence of a multiplicity of recovery times in the model leads to memory, which is observed in real systems. 

The transition behavior of a number of liquid crystals with side-chain mesogens is summarized in Table 5. The most obvious feature of macromolecular liquid crystals is the frequent absence of fully ordered crystals at low temperatures. If fully ordered crystals are observed, crystallization is incomplete, i.e. the observed phase states are to be described by an area on the right side of Fig. 3. Glass transitions, which were hard to find in low molecular weight liquid crystals (see Table 3), are now prominent. 

The curing behavior of PMDA-ODA (Figure 1) has been analyzed previously The overall process is characterized by various physical and chemical steps Decomplexation of the complex formed between NMP and polyamic acid 2 3 6 plasticization of the material by the decomplexed NMP n 12 evaporation of the solvent cycloimidization accompanied by re-formation of anhydride and amine, a side reaction which leads to chain scission 2 vitrification caused by solvent evaporation and imidization and finally molecular ordering of the polyimide near and above the glass transition n 13 14. The particular features of all these processes are heating rate dependent (e.g. compare Figure l.a and l.b) 2 n. 

At this point, we had the first four of the seven characteristic features of A-B-A thermoplastic elastomers, as shown in the box. That is, we were completely confident that we had a three-block polymer, rubbery behavior with high tensile strength in the unvulcanized state, and also complete solubility. We concluded from these properties that these polymers were two-phase systems. We then generated the essentials of the two-phase, domain theory and visualized the physical structure illustrated schematically in Figure 1. We also visualized applications in footwear, in injection-molded items, and in solution-based adhesives. Positive confirmation of the two-phase structure quickly followed, by detection of two separate glass transition temperatures, as well as observation of the thermoplasticlike reversibility of bulk- and... 

The transition from the glass to the rubberlike state is an important feature of polymer behavior, marking as it does a region where dramatic changes in the physical properties, such as hardness and elasticity, are observed. The changes are completely reversible, however, and the transition from a glass to a rubber is a function of molecular motion, not polyma- structure. In the rubberhke state or in the melt, the chains are in relatively rapid motion, but as the temperature is lowered, the movement... 

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