Thermal frontal polymerization

Some other works that we would like to mention studied the kinetics effects in FP [85], the influence of the gel effect on the propagation of thermal frontal polymerization waves [28], the use of complex initiators as a means to increase the degree of conversion of the monomer [30], and FP of metal-containing monomers [3]. 

Thermal frontal polymerization involves the coupling of thermal diffusion and Arrhenius reaction kinetics of an exothermic polymerization (55). Thermal frontal polymerization has promise for making specialized materials in which the rapid reaction is valuable 56-58) or for which a special gradient is needed... 

Thermal frontal polymerization is a mode of converting monomer into polymer via a localized exothermic reaction zone that propagates through the coupling of thermal diffusion and the Arrhenius reaction kinetics of an exothermic polymerization. We review the range of nonlinear phenomena that have been observed in frontal polymerization systems and report new results on the role of gravity in spin modes and the development of spherically-propagating fronts. 

Thermal frontal polymerization exhibits the full range of nonlinear dynamics phenomena, including those driven by hydrodynamics as well as driven by intrinsic feedbacks in the chemistry. Features unique to polymerization kinetics and properties allow the study of convection in fronts and novel spin modes. 

Nonlinear phenomena in any system require some type of feedback. The most obvious source of feedback in polymerization reactions is thermal autocatalysis, often called thermal runaway in the engineering literature. The heat released by the reaction increases the rate of reaction, which increases the rate of heat release, and so on. This phenomenon can occur in almost any reaction and will be important when we consider thermal frontal polymerization. 

Pojman discusses thermal frontal polymerization in Chapter 4. He focuses on thermal frontal polymerization in which a locahzed reaction zone propagates through the coupling of thermal diffusion and the Arrhenius dependence of the kinetics of an exothermic polymerization. Frontal polymerization is close to commercial apphcation for cure-on-demand appHcations and is also showing value as a way to make some materials that are superior to those prepared by traditional methods. It also manifests many types of instabihties, including buoyancy-driven convection, surface-tension-driven convection, and spin modes. 

D., and Gross, L. (2007) Snell s law of refraction observed in thermal frontal polymerization. Chaos, 17, 033125. 

Frontal polymerization (FP) is a class of polymerization reactions that polymerize directionally, usually from one end of their container to another, instead of polymerizing in bulk (i.e., uniform polymerization throughout the container) as traditionally many polymerization reactions do. Because many more people are familiar with thermal frontal polymerization (TFP), we begin this chapter by comparing isothermal frontal polymerization (IFP) to TFP including differences in their mechanisms, reaction properties, and finished products. In addition, we review all previous work on IFP (experimental and mathematical) and present a brief summary of this information. 

Ivanov, V.V., Stegno, E.V., and Pushchaeva, L.M. (1997) Non-thermal frontal polymerization of methyl methacrylate with a polymeric seed modified with cobalt-porphyrin complex additives. Chem. Phys. Rep., 16, 947-951. 

Photofrontal polymerization produces fronts whose positions depend logarithmically on time if the initiator continues to absorb light and linearly on time if the initiator is photo-bleached. It is limited in application to unfilled systems. IFP propagates on the order of 1 cm day" and only for total distances of about 1 cm. Thermal frontal polymerization has the widest range of velocities and types of chemistries that can be used. 

Thermal frontal polymerization is a process in which a localized reaction zone propagates from the coupling of thermal diffusion and the Arrhenius dependence of reaction rate of an exothermic polymerization. Thermal frontal polymerization was discovered at the Institute of Chemical Physics in Chernogolovka, Russia by Chechilo and Enikolopyan. They studied methyl methacrylate polymerization under 3500 atm pressure. (We will consider later why these extreme conditions were used.) The literature from that Institute was reviewed in 1984. Pojman rediscovered what he called traveling fronts of polymerization in 1991. Pojman et a . reviewed the field in 1996. There have been other focused reviews. ... 

Thermal frontal polymerization is by far the most commonly studied form of FP, so we will henceforth refer to it as FP. We will first consider the necessary conditions for FP and give an overview of the types of systems that have been studied. 

Thermal frontal polymerization can be applied to the widest range of materials. Any polymerization that follows... 

The major advantage of thermal frontal polymerization is the high rate of conversion. Cure-on-demand applications appear to be the most promising use for this approach. 

Frontal polymerization carried out as described above can be turned into a continuous process. In order to do this, it is necessary to move the newly formed polymer and the reactive mixture in the direction opposite to the direction of spreading of a thermal front at a velocity equal to the velocity of the front development to feed the reactor with a fresh reactive mass.254 Control of the process, choice of process parameters and proper design of the equipment require solving the system of equations modelling the main physical and chemical processes characteristic of frontal reactions. 

C. Spade and V. Volpert, On the steady state approximation in thermal free radical frontal polymerization, Chem. Eng. Sci., 55 (2000), pp. 641-654. 

Pojman and his co-workers demonstrated the feasibility of traveling fronts in solutions of thermal free-radical initiators in a variety of neat monomers at ambient pressure using liquid monomers with high boiling points (5-7) and with a solid monomer, acrylamide (8,9), Fronts in solution have also been developed (10). The macrokinetics and dynamics of frontal polymerization have been examined in detail (//). A patented process has been developed for producing functionally-gradient materials (12,13). 

Garbey et al. also predicted that for a descending liquid/liquid front, an instability could arise even though the configuration would be stable for unreactive fluids (29-31). This prediction has yet to be experimentally verified because liquid/liquid frontal polymerization exhibits the Rayleigh-Taylor instability. A thermal frontal system with a product that is less dense than the reactant is required. 

We note a significant difference between the liquid/liquid and the liquid/solid cases. For the liquid/solid case, convection in ascending fronts increases the front velocity but in the liquid/liquid case, convection slows the front. Convection increases the velocity of pH fronts and BZ waves. Why the difference between liquid/liquid frontal polymerization and other frontal systems In liquid/liquid systems the convection also mixes cold monomer into the reaction zone, which lowers the front temperature. The front velocity depends more strongly on the front temperature than on the effective transport coefficient of the autocatalyst. Convection cannot mix monomer into the reaction zone of a front with a solid product but only increases thermal transport so the velocity is increased. 

Frontal polymerization discovered in 1972 (5) could be realized in free-radical polymerization because of its nonlinear behavior. If the top of a mixture of monomer and initiator in a tube is attached to an external heat source, die initiators are locally decomposed to generate radicals. The polymerization locally initiated is autoaccelerated by the c(xnbinatithermal autocatalysis exclusively at the top of the reaction systmn. An interface between reacted and unreacted regions, called propagating front, is thus formed. Pojman et al. extensively studied the dynamics of frontal polymerization (d-P) and its applicatim in matoials syndiesis (I -I3). 

Thermal runaway should usually be avoided in polymer industrial plants, since it lowers the quality of the polymers and damages the reactor. It seemed that our trial to produce radial functionally gradient polymers by frontal polymerization failed. 

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. 

A number of radical polymerization reactions are highly exothermic and are able to support frontal polymerization. Free-radical polymerization with a thermal initiator can be approximately represented by a three-step mechanism. First, an... 

Pojman, J. A. Ilyashenko, V. M. Khan, A. M. 1996b. Free-Radical Frontal Polymerization Self-Propagating Thermal Reaction Waves, J. Chem. Soc. Faraday Trans. 92, 2825-2837. 

Savostyanov, V. S. Kritskaya, D. A. Ponomarev, A. N. Pomogailo, A. D. 1994. Thermally Initiated Frontal Polymerization of Transition Metal Nitrate Acrylamide Complexes, J. Poly. Sci. Part A Poly. Chem. 32, 1201-1212. 

Frontal polymerization (FP) is a polymerization process in which polymerization occurs directionally. There are three types of FP. The first is isothermal FP, which is discussed in Chapter 5. The second is photofrontal polymerization in which the front is driven by the continuous flux of radiation, usually UV light [1-7]. The last type is thermal FP, which we will henceforth refer to ns frontal polymerization, and it results from the coupling of thermal transport and the Arrhenius dependence of the reaction rate of an exothermic polymerization. 

Pojman, J.A., llyashenko, V.M., and Khan, A.M. (1996) Free-radical frontal polymerization self-propagating thermal reaction waves. J. Chem. Soc., Faraday Trans., 92, 2825-2837. 

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