Glass Transition and Transformation Diagrams

Two main transitions may take place during the formation of a polymer network gelation, a critical transition defined by the conversion at which the mass-average molar mass becomes infinite (Chapter 3) glass transition, or vitrification, characterized by the conversion at which the polymer begins to exhibit the typical properties of a glass. 

The polymerization temperature, often called the cure temperature, affects both transitions in different ways. In Chapter 3 it was shown that the gel conversion does not depend on temperature for ideal stepwise polymerizations but may show a small dependence on temperature for the case where unequal reactivity of functional groups or substitution effects vary 

The situation is completely different for the glass transition. The pos-sibihty of producing cooperative movements of fragments of the thermosetting polymer must increase with temperature. So, the conversion at which vitrification takes place increases with the cure temperature. 

In the glassy state relaxation times become very large, making the continuation of the polymerization reaction difficult. Moreover, once in the glassy state, small advances in the conversion of functional groups produce a further increase in the already large relaxation times. Thus, the reaction becomes autoretarded and rapidly ceases for practical purposes. Only an increase in the cure temperature can restart the polymerization reaction. 

Other transitions such as degradation and phase separation may be also observed during the formation of the polymer network. Degradation is usually present when high temperatures are needed to get the maximum possible conversion. Phase separation may take place when the monomers are blended with a rubber or a thermoplastic, to generate rubber-modified or thermoplastic-modified polymer networks. In these cases, formulations are initially homogeneous but phase-separate during the polymerization reaction. This process is discussed in Chapter 8. 

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In this chapter the main characteristics of the glass transition are analyzed and equations relating the glass transition temperature with the conversion of the thermosetting polymer are discussed. Then, CTT and TTT transformation diagrams are derived and examples of their practical use provided. 

Besides the amorphous structure, both inorganic and organic glasses exhibit another characteristic property, the so-called transformation (glass transition). Transformation of glass may be demonstrated on the diagram of temperature dependence of some physical property, for instance, specific volume (Jones, 1956),... 

Boey, F., Lee. T. H., and Sullivan-Lee, P High pressure autoclave curing of composites effect of high pressure on glass transition temperature, J. Mater. Sci., 29, 5985-5989 (1994). Gilham, J. K., Time temperature transformation (TTT) state diagram and cure, in The Role of the Polymeric Matrix in the Structural Properties of Composite Materials (J. C. Seferis and L. Nicolais, eds.). Plenum Press, New York, 1983, pp. 1127 145. 

Fig. 4 Schematic phase diagrams of a polymer solution showing LL phase separation with UCST behavior. Curve s is the spinodal, curve b is the binodal, and curve g is the glass transition temperature as a function of polymer concentration. BP indicates the Berghmans point, (a) LL phase separation is the only thermodynamic transformation of the system [17,25, 36]. (b) Curve c shows the crystallization temperature of a polymer fully miscible in a solvent as a function of concentration in the solution [17, 25], The LL phase coexistence curve (combined with vitrification) is a (classical) metastable process that lies beneath the crystallization curve c. In route 1, a polymer solution is supercooled at ALj, and the only active process is polymer crystallization. In route 2, the initially homogeneous solution is supercooled to a larger undercooling than namely AL2. Crystallization may compete either with LL phase separation when reaching point C, or LL phase separation coupled with vitrification when reaching point D. At C, crystallization may take place in the polymer-rich phase. At D, both LL phase separation and crystallization may become arrested by vitrification...

Fig. 3 presents the state diagram of the system PEO-water. It is of an eutectic type exhibiting mixed crystal formation and glass transition phenomena like the system mentioned. The graph of the experimentally determined eutectic enthalpy of transformation versus mass fraction of PEO verifies the eutectic composition. 

Fig. 2.5 Annotated time-temperature transformation diagram, showing the minimum cooling rate to avoid crystallization. is the melting temperature, Tg is the glass transition temperature, and 71, and t are the temperature and time point at the locus, respectively. (Adapted from Karmwar et al. 2011a)...

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