Partial Vitrification

As observed in the previous sections, relaxation phenomena are superimposed as local (downward) extremes in the heat flow phase. Thus, the heat flow phase gives an indication of a vitrification or devitrification process during the thermal treatment. In non-isothermal experiments, conditions of partial vitrification—a zone where the material is in between the liquid/ rubbery and the glassy state—can occur depending on the heating rate and the reactivity of the curing system (section 5.4). 

More recently, Montserrat and Cima [85] presented similar epoxy cure experiments, but their interpretation is somewhat misleading because they did not account for the partial vitrification phenomenon. The way the mobility factor, DF, was calculated in [85] is in contradiction with the 

Taken together, the experimental observations reported in the previous sections suggest that the formation of colloidally stable particles heated above their phase-transition temperature may be a universal phenomenon, taking place not only in aqueous polymer solutions, but also in solutions of polymers that can undergo a coil-globule transition in organic solvents. 

Interactions of such glassy polymeric particles should resemble the collisions of hard spheres. Phase diagrams of the type shown in Fig. 36 have been obtained for various polymer-organic solvent mixtures [85,94,345-353]. 

Tg (-22 °C) of a homogeneous 70/30 PNIPAM-water mixture. Observation of samples by scanning electron microscopy and optical microscopy revealed that the morphology of the polymer-rich phase is preserved only if the polymer solutions are brought to zone C. Polymer solutions heated to zone B undergo demixing upon quench-cooling [160]. Aqueous solutions of PVCL, PNIPMAM, and PNIPMA exhibit similar behaviour [157,158,369,370]. 

Can vitrification of PVCL, PNIPAM, and PNIPMA also take place within mesoglobules generated upon heating aqueous solutions of the respective polymers above their LCST A mesoglobule formed by a large number of short chains is expected to have a lower Tg than a single chain of the same size, since as a rule the Tg of a polymer increases with molar mass [334]. The physical state of 

Partial vitrification may affect kinetic processes during the coil-globule transition. Thus, at very high dilution, macroscopic phase separation well above the LCST might be stopped by partial vitrification of the polymer-rich phase. At this point we can only speculate whether vitrification interferes with the coil-globule transition or not. This problem is open for discussion and needs experimental confirmation. 

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Partial vitrification is also observed in isothermally cured epoxy systems. However, the effect is less pronounced since the glass transition domain at goo is narrower for these networks [80]. An example is given in Figure 2.14 for the system DGEBA-MDA T oo = 102°C). At 80°C, a stepwise decrease in Cp and a relaxation peak are observed. At 100°C, the system is partially vitrifjdng and the phase angle remains in the relaxation regime at the end of cure. At 120°C, no vitrification effect is noticed anymore, neither in Cp, nor in heat flow phase. 

A mobility factor based on heat capacity, DF, was proposed in our work. The points for which DF equals 0.5 can be used to quantify the times and temperatures of vitrification and devitrification (for the organic systems studied). Moreover, the DF curve gives information on the degree of vitrification while the reaction occurs in mobility-restricted conditions. If an isothermal cure experiment is performed close to the glass transition of the fully-cured resin, partial vitrification occurs and the fiilly glassy state will never be reached at that temperature. 

Fig. 1 Possible phase diagrams for polymers showing either (a) UCST (e.g. PS in cyclohexane) or (b) LCST type 11 (e.g. PiPAAm in aqueous medium) phase separation behaviour. Tdem is the demixing temperature, Tq is the theta temperature, and Tbp is the temperature corresponding to the Berghmans point [76], For both polymers, Tg in their solid state is weU above Tdsm- For this UCST-type polymer, Tg cannot be lower than Tbp. At temperatures below Tbp, the polymer is frozen in, and phase morphology is preserved [77]. For the LCST-type polymer shown, partial vitrification takes place at Tbp < T < To [78]...

When the cure temperature is below the full cure glass transition of the epoxy-amine (Tg = 95 C), vitrification of the epoxy-amine-rich phase also takes place. This clearly occurs in Hg. 2.116 for the reaction at 80°C.The heat flow phase angle is important in this respect, showing a second relaxation peak. Partial vitrification is seen at 90 C, and it is again most obvious in the heat flow phase or phase angle. 

It is apparent from Figure 17 that in the process of initial fast eooling ( 20 K/min) of the sample from room temperature up to 80 K the saturated solution of water in starch vitrificated, whereas the phase of the water surplus partially vitrificated and partially crystallized. Heat capacity of the system in the aforementioned state monotonously increases within the range of 80-180 K (seetion AB). Then the anomalous heat eapacity increase (section BC), attributed to devitrifieation of the saturated solution of water in amorphous (Tgi=188 K) and ordered (Tgi=250 K) microregions of stareh and vitreous fraction of the free water phase is observed on the Cp=XT) curve. A slight exothermal effect due to crystallization of the water surplus from the supereooled fluid state is observed at 260 K. 

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