Glass transition temperature cross-linking effects

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittie. Thus, the utility of unmodified common resins as stmctural adhesives would be very limited. Eor highly cross-linked resin systems to be usehil stmctural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin dexibiUty, and thus increases the fracture resistance of the cured adhesive. Recendy, stmctural adhesives have been modified by elastomers which are soluble within the uncured stmctural adhesive, but then phase separate during the cure to form a two-phase system. The matrix properties are mosdy retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved. 

Temperature dependence (related to the temperature dependence of the conformational structure and the morphology of polymers) of the radiation effect on various fluoropolymers e.g., poly (tetrafluoroethylene-co-hexafluoropropylene), poly(tetrafluoroethylene-co-perfluoroalkylvinylether), and poly(tetrafluoroethylene-co-ethylene) copolymers has been reported by Tabata [419]. Hill et al. [420] have investigated the effect of environment and temperature on the radiolysis of FEP. While the irradiation is carried out at temperatures above the glass transition temperature of FEP, cross-linking reactions predominate over chain scission or degradation. Forsythe et al. [421]... 

In this work, polyurethane (PU) and epoxy (EP) mixtures were selected for investigation because they are known to form partially miscible IPNs with broad glass transition temperatures. These were first prepared by Frisch et al(6) using a simultaneous polymerization technique in bulk. These materials showed the effects of cross-linking only one polymer component (pseudo-IPN) and intentional grafting between the component polymers. Klempner et al (2) also studied PU/EP IPNs for vibration attenuation. The polyurethanes in this work were chain extended and crosslinked with a 4 1 equivalent ratio of butanediol (BD) and trimethylol propane (TMP). 

Figure 2.32 Effect of cross-linking on the glass transition temperature of the system styrene with divinylbenzene. (From Ref. 34.)...

We have previously stressed that network formation rapidly increases viscosity and causes gelation. Cross linking after that point increases the apparent glass transition temperature. Figure 11 illustrates the effect of degree of cross linking on the dynamic modulus of a thermoplastic and a cross-linked polymer. The point at which modulus drops off rapidly is approximately the glass transition temperature. 

In the structosets, the glass transition temperature is particularly affected by cross linking, which has a major effect on the ease of rotation. 

They have been able to show that these two effects may be separated and that the change in the glass transition temperature is equal to a constant times the density of cross linking p, at moderate degrees of cross linking. 

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