Thermodilatometric analysis of hypercrosslinked polymers

All networks of the third group (Fig. 7.44c) based on the copolymer with 2.7% DVB possess a relatively high density and, correspondingly, the lowest mobhity. The samples with 43, 66, and 100% crossfinks experience the usual thermal expansion up to the temperature of chemical degradation ( 300—320°C). The polymers with the highest degrees of 

In order to rmderstand the dramatically strong shrinking, nearly collapse, of the networks at high temperatures, it is necessary to consider the results given in Fig. 7.45. When heated in air the product of a styrene—2.7% DVB copolymer post-crosslinked to 200% starts to shrink at a temperature slightly above 200°C. However, when heated in an inert atmosphere, the 

The chemical modification of hypercrosshnked polymers in air was found to be accompanied by the appearance of carbonyl-containing groups, which may reveal themselves at 1680, and 1726 cm in the infrared (IR) spectrum of the network 0.6 X 100 exposed to air for 30 min at 280°C. 

we can conclude that the shrinking of the hypercrosslinked polymers at temperatures in the range of250-300°C is caused by oxidation, with the rupture of a certain portion of the stressed carbon—carbon bonds. Under the permanent tendency for reducing the free volume of the material, the scission of the most stressed bonds leads unavoidably to the formation of a more compact structure and to the decrease of the beads volume and their inner specific surface area. The rupture of bonds can then be followed by their partial recombination or termination with oxygen. The latter process seems to proceed to a lower extent, since no noticeable weight change of the air-exposed samples was observed. 

---