Vitrification point

Fig. 44. Dependence of the 13C Tte relaxation times (yBj = 60 kHz) for PTEGDMA on the extent of cure inner CH20 ( ), end CH20 (O), CH2 (4-) and CH3 (A) for quaternary ( ), represents Tle/10. Standard errors do not exceed the bounds of the symbols. The dots represent the compression modulus, E (MPa). V indicates the vitrification point (reprinted from ref.2481 with permission)...

Fig. 29. Change of the specific volume (Vsp) of a system during cure. Tjure < T < T UIC O Vitrification points at Tjute and TPure (conversion The start of cure ...

The higher uncured Tg of the quinoxaline resin requires a lower extent of reaction to reach the vitrification point during the scan. The cumulative effect at vitrification may not be sufficient to show different vitrification temperatures. 

Arrelano et al. (1989) evaluated the gel points of various DGEBA-based epoxy-resin systems via the crossover method. They also defined the vitrification point as the maximum in G" in an isothermal dynamic time test. In all these gel-point measurements the frequency must be chosen such that the relaxation of the network is enabled during data sampling. This is represented as... 

FIGURE 3.2 (a) Schematic representation of the polymerization rate vs. conversion, (b) Schematic representation of the vitrification points for a bulk polymerization of poly(methyl methacrylate) at various polymerization temperatures. The solid line represents the volume fraction of polymer at which polymerization stops, as a function of polymerization temperature. 

On analyzing the M5 reactions involving insoluble polymers, two types of restrictions ould be considered, first, diffusional (transfer of the functional gro ps after the system has crossed a vitrification point) and topological ones (a virtually complete absence of the translational diffusion in these groi grafted onto the polymer backbone). The diffusional process results primarily in the reaction of the surface functional groups therefore, the penetration of the MX into the polymer block is restricted. 

Furthermore, the change in the mechanical properties of the materials should be noted. Generally their elasticity drops with decreasing temperature. Especially organic materials at cryogenic temperatures are already far below the temperature where they have reached their vitrification point at which they may break easily under mechanical load. 

For the calculation of Tg, measured in cooling or heating experiments by DMTA, TMA or DETA, similar calculation procedures as for the vitrification point are valid [58]. 

Fig. 8. Superposition of the Tg versus In(time) data to form a master curve at 140°C by shifting each curve in Figure 7 by a constant factor [In (ot) = ln(ti4ooc) — Initi)] (see eq. 13) along the In(time) axis so that its beginning section (Tg < 90° C) coincides with the curve for Tcure = 140°C. Isothermal vitrification points at different cure temperatures are marked by arrows. Note that vitrification points at all cure temperatures lie on the master curve, ie vitrification occurs during chemical control of the reaction 100°C, 120°C, 140°C, 150°C,1160°C, 180°C. From Ref. 40.

Careful inspection of the qualitative plots in Fig. 6.30 reveals that the relaxation frequency of the dipole motions moves to lower values as cure proceeds, indicative of the loss of free space and the increase of the effective Tg of the matrix. Also, r decreases and Ae gets smaller, somewhat like ACp. The curing time at which the frequency of the a-relaxation loss peak is approximately 0.1 Hz can be taken as indicative of the vitrification point of the matrix. 

Thus the Arrhenius type of temperature dependence applies, but here the prediction that AHa is the same for all systems irrespective of molecular constitution is certainly incorrect. Among liquids far above their freezing (or vitrification) points, where the proportion of free volume is far higher (perhaps 0.3 instead of 0.03) the apparent activation energies vary widely and can be correlated with chemical structure. We must expect, then, the WLF equation to become inapplicable at high temperatures and the temperature dependence of relaxation processes to be governed by more specific features. 

---