Temperature profiles, frontal polymerization

Isothermal frontal polymerization (IFP) is a self-sustaining, directional polymerization that can be used to produce gradient refractive index materials. Accurate detection of frontal properties has been difficult due to the concentration gradient that forms from the diffusion and subsequent polymerization of the monomer solution into the polymer seed. A laser technique that detects tiny differences in refractive indices has been modified to detect the various regions in propagating fronts. Propagation distances and gradient profiles have been determined both mathematically and experimentally at various initiator concentrations and cure temperatures for IFP systems of methyl methacrylate with poly(methyl methacrylate) seeds and wilh the thermal initiator 2,2 -azobisisobutryonitrile. 

Figure 11.7 shows the temperature history at a fixed point in the reaction tube as a front passes. The temperature at this point is ambient when the front is far away and rises rapidly as the front approaches. Hence, a polymerization front has a very sharp temperature profile (Pojman et al., 1995b). Figure 11.7 shows five temperature profiles measured during frontal free-radical polymerization of methacrylic acid with various concentrations of BPO initiator. Temperature maxima increase with increasing initiator concentration. For an adiabatic system, the conversion is directly proportional to the difference between the initial temperature of the unreacted medium and the maximum temperature attained by the front. The conversion depends not only on the type of initiator and its concentration but also on the thermodynamic characteristics of the polymer (Pojman et al., 1996b). 

The defining features of frontal polymerization are the sharp temperature and concentration gradients present in the front. Figure 1 shows a temperature profile for benzyl acrylate polymerization. Notice that the temperature jumps about 100 °C over 1 cm. The concentration gradient is two orders of magnitude larger. 

Figure 12 Temperature profiles for the frontal polymerization of undiluted acrylamide, of acrylamide diluted with commercial polyacrylamide, of acrylamide diluted with barium carbonate, and of acrylamide diluted with frontally polymerized acrylamide. Adapted from Fortenberry, D. I. Pojman, J. A. J. Polym. Sci. Part A Polym. Chem. 2000, 38,1129-1135. ...

Figure 15 The temperature profile of frontal polymerization of acrylamide immersed in liquid nitrogen. Adapted from Pojman, J. A. Ilyashenko, V. M. Khan, A. M. J. Chem. Soc. Faraday Trans. 1996, 92, 2825-2837. ...

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