Kinetic Characteristics of IPN Formation

One of the first investigations of the kinetics of IPN formation was done [165] for the IPNs made of epoxy and PnBA cross-linked by diethylene glycol dimethacrylate. The kinetics of formation of the components was described within the framework of traditional approaches. 

The kinetics of IPN formation was studied for the system PU/cross-linked PMMA [260]. PU was formed from POPG MM 2000 and from aromatic triisocyanate adducts in the presence of catalyst (Sn octoate). The PMMA network was synthesized by radical copolymerization of MMA and trimethy-lolpropane trimethacrylate using AIBN as initiator. First, the PU network was formed at room temperature and then at 223 K the second network was polymerized. The effect of all components of one of the networks was studied on the gelation of the other. The effect of the catalyst Sn octoate on the PMMA gel formation consisted in a decrease in the rate of gelation from 180 to 30 min. At the same time, the PMMA network components do not influence the PU network formation. The first effect was explained by the increase of the viscosity of the reaction medium, leading to an increased rate of PMMA formation in the presence PU components due to a smaller termination constant according to the equation [261]  

It was established [262] that the catalyst Sn octoate does not exert any effect on the initiator decomposition rate, but enhances the polymerization rate of monomer and comonomer due to polarization of double bonds, making addition of radicals easier as a result, the rate of PMMA formation increases. The possibility of chemical bond formation between PU and the PMMA network was also discussed. One should never forget such a possibility because of the possible chain transfer to another polymer. Two components of the PU network—POPG and triisocyanate adduct—both containing mobile hydrogen atoms, are present in the reaction medium. Because this atom may be shipped off by a radical, the possible reaction may be presented as consisting of two stages  

The reaction products were extracted by a solvent and analyzed. It was found that more than 90% of the PU component could be extracted. For the system containing 20% of PU and 80% of PMMA, after extraction there is about 1.8% of PU left in the PMMA network. This low value shows that even if the grafting really takes place, its effect on the macroscopic properties of the IPN is negligible. In the cited work it was emphasized for the first time that the analysis of the reaction kinetics should account for both the effect of one network on the formation of the second, and vice versa, and the possibifity of secondary reactions that may prevent or distort the formation of one of the networks. 

To study the kinetics of semi- and full IPN formation from PU and PMMA, FTIR spectroscopy was used which allowed simultaneously investigation of the consumption of NCO groups (absorption band at 2275 cm ) and C = C bonds (1639 cm ). The effects of various factors were established, including the temperature, cross-finking agent concentration, and PU concentration in solution, etc. It was found that the shape of the kinetic cmves of PU and PMMA formation in IPNs does not change as compared to pure component, this being the basis for the conclusion about the absence of any chemical interaction between two networks. IPNs made of the same components were 

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