Vitrification, half

The appearance of the surface of 16th century bricks in Venice, Italy, exposed to rain water run-off is shown in Figure 1. Surface erosion, as can be seen, is fairly appreciable, and especially noticeable is the uneveness in the erosion due to difference in vitrification. These bricks, and even older ones (12th century) have been studied because they have resisted rain water run-off for seven centuries and high air pollution for the past half century, the latter due to the proximity of Venice to the refinery at Porto Marghera. 

The time for half-decrease in Cp is shown and, in a separate experiment, the resin at this point was found to have a Tg of 80 C, confirming that the change in Cp arises predominantly due to vitrification of the resin during cure. If cure occurs at a temperature above then the change in Cp is small. Figure 3.7 shows the change in Cp as a function of the time of cure of an epoxy resin with an aromatic amine at three different tempemtures in a MDSC (Lange et al, 2000). 

From this equation one sees that the large-amplitude motion contributes fully in the liquid state (y = 0) and not at all in the glassy state (y = 7i/2). The glass transition temperature, defined at Ae temperature of half-vitrification or devitrification, occurs at y - tc/3. An example of the data treatment as indicated in Equation 8 is shown in Figure 1. 

The increase in heat capacity ACp always occurs over a temperature range of 5 to 20 K, and the jump is often 111 K mol of mobile units in the liquid. This means, for a monatomic liquid the decrease in heat capacity at the glass transition is 11 J K mol Macromolecules, such as polyethylene, (CHj-jx, change in heat capacity by approximately 111 K" (mol of chain atom) (see Appendix 1). To describe the glass transition, the temperature of half-vitrification, T, should be... 

In comparing glass transition temperatures, one must, furthermore, consider the differences in definition for the glass transition temperature for the different techniques. In Fig. 6.20, for example, the glass transition temperatures are compared to data obtained from DMA (using the maximum in tan 8) and TMDSC (using the temperature of half-vitrification) when measured at similar frequencies. 

The imaginary part of the complex heat capacity is fitted by a Gaussian function in the temperature region not influenced by vitrification (between the vertical bars) (see Reference 82 for details). The maximum of c"p coincides, as usual, with the half-step temperature of c p. As for other compliances, the... 

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