Frontal polymerization studies

A typical example of frontal polymerization is the polymerization of methyl methacrylate (or an oligomer), placed inside a long aluminum tube 249 these tubes continuously dip into a bath with a liquid heated up to temperature of 70 - 80°C. The part at the tubes above the bath are cooled so that the reactive material does not polymerize. Polymerization shrinkage is compensated by continuous injection of a monomer or oligomer into the reaction zone. The appropriate combination of injection rate, velocity of tube movement through the reaction zone, and tube diameter are chosen according to experimental studies of the process. 

Results of studies on the FP of oxirane and oxetanes demonstrated that some monofunctional and difunctional 3,3-disubstituted oxetane, arylalkyl, and alkyl glycidyl ether monomers display frontal polymerization characteristics [157]. 

Mathematical models of frontal polymerization have also been developed. They are discussed below. These works study the velocity of the reaction front... 

We now return to the base model (2,17)-(2.21) of frontal polymerization. We want to find uiuformly propagating FP waves and perform linear and nonlinear stability analyses, as we did in the case of the gasless combustion model. Before we study the model, we would like to reformulate it using the reaction front approximation. 

Mathematical models of the frontal copolymerization process were developed, studied and compared with experimental data in [67, 90]. An interesting observation was that the propagation speed of the copolymerization wave was not necessarily related to the propagation speeds in the two homopolymerization processes, in which the same two monomers were polymerized separately. For example, the propagation speeds in the homopolymerization processes could be 1 cm/min in each, but in the copolymerization process, the speed could be 0.5 cm/min. Mathematical models of free-radical binary frontal polymerization were presented and studied in [66, 91]. Another model in which two different monomers were present in the system (thiol-ene polymerization) was discussed in [21]. A mathematical model that describes both free-radical binary frontal polymerization and frontal copolymerization was presented in [65]. The paper was devoted to the linear stability analysis of polymerization waves in two monomer systems. It turned out that the dispersion relation for two monomer systems was the same as the dispersion relation for homopolymerization. In fact, this dispersion relation held true for W-monomer systems provided that there is only one reaction front, and the final concentrations of the monomers could be written as a function of the reaction front temperature. 

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]. 

Frontal polymerization can be used to synthesize valuable products, e.g. nanocomposites and liquid crystals, and theoretical studies represent an important tool for understanding the process. 

There are several modes for the frontal polymerization of metal-containing monomers. One such mode is polymerization in a high-temperature burning regime. This method is followed by thermolysis of the products to obtain metal-containing composites that include nano-sized materials. The kinetic peculiarities of high-temperature pyrolysis of the Co(II) acrylamide complex have been studied [84], The rate of the process is approximately satisfied by a first-order equation of autocatalysis (Eq. 4-26), where k = 4.2-10 exp[-24,000/(7 )] (s ), 0 l.910 ... 

Frontal free-radical polymerization is fairly well understood. Studies on the velocity dependence on temperature and initiator concentration have been performed (7,11,23), Frontal polymerization in solution was performed (70), and initiators that do not produce gas were developed (24), The velocity can be affected by the initiator type and concentration but is on the order of a cm/min for monofunctional acrylates and as high as 20 cm/min for multifunctional acrylates (24). 

Shult and Volpert performed the linear stability analysis for the same model and confirmed this result (48), Spade and Volpert studied linear stability for nonadiabatic systems (49), Gross and Volpert performed a nonlinear stability for the one-dimensional case (50), Commissiong et al. extended the nonlinear analysis to two dimensions (this volume). In the former analysis, they confirmed that, unlike in SHS (57), uniform pulsations are difficult to observe in frontal polymerization. In fact, no such one-dimensional pulsating modes have been observed. 

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. 

In 2000, we have begun our research on this topic paying particular attention to the possibility of finding new chemical systems able to frontally polymerize and new FP applications. Specifically, we have studied polyurethanes (19, 20), polyester tyrene resins (21), polydicylopentadiene (22) and its BPNs with polyacrylates (23), Furthermore, we were able to prepare films (24) and to apply FP to the consolidation of porous materials (i.e. stones, woods, flaxes, papers), in particular -but not only- those having a historical-artistic interest (24), In this chapter we present a brief overview of these recent findings. 

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). 

Frontal polymerization can be used to study interesting modes not observable in other systems. For example, spherically propagating fronts can be studied. 

Motivation for Studying Nonlinear Dynamics with Frontal Polymerization 51... 

Frontal polymerization can be achieved with a variety of monomers and has been studied with thermosets and thermoplastics. Examples include n-butyl acrylate, benzyl acrylate, styrene, dodecyl acrylate and hexyl acrylate. If the front is ascending, the monomer inmiediately above the front is lower in density because of the temperature gradient than the bulk monomer and so simple convection can occur for a thermoset (/P) or a thermoplastic. 20) A descending front with a thermoset is stable but a thermoplastic is unstable because even though the polymer is very hot, it is more dense than the unreacted monomer. This leads to the Rayleigh-Taylor instability. 16,21)... 

Bubble interactions in frontal polymerization reactions were studied by Pojman et al. on the Conquest I sounding rocket with n-butyl acrylate 21) and were followed by a series of KC-135 parabolic flights studying thermosets. 22-24) They reported evidence of unusual bubble interactions in reduced gravity. In the Conquest I sample, they observed a periodic pattern of bubbles connected like a necklace with a region of polymer followed by a large bubble, followed by another region of polymer. 

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. 

Arrhenius kinetics and is highly exothermic can support localized polymerizations that propagate. Frontal polymerization has been studied with many different polymerization mechanisms but free-radical polymerization is the most studied. Most of the work has focused on the dynamics of the process, but recently applications have been studied. Hydrogels have been prepared frontally, which have superior properties to those prepared by conventional methods. 

Figure 5. Reactor for the study of frontal polymerization under controlled pressure and temperature.

Chechilo et al. studied frontal polymerization of methyl methacrylate with benzoyl peroxide as the initiator. By placing several thermocouples along the length of the metal reaction tube, they could infer the front velocity and found a 0.36 power dependence for the velocity on the benzoyl peroxide concentration 18). More detailed studies for several initiators showed 0,223 for t-butyl peroxide, 0.324 for BPO and 0.339 for cyclohexylperoxide carbonate 4). Figure 8 shows the bubble-free velocities for n-butyl acrylate fronts as a function of initiator concentration, with power function dependencies similar to those of Chechilo et al. 

The most pernicious convective instability occurs with monomers that produce a molten polymer at the front, such as n-butyl acrylate, styrene and methyl methacrylate. A Rayleigh-Taylor instability (2, 25 ), which also appears as fingers as the more dense molten polymer streams down from the reaction zone and destroys the front (Figure 16). The only currently available methods to study frontal polymerization with thermoplastics are to add a crosslinking monomer to produce a thermoset or to increase the viscosity with a viscosifier such as ultrafine silica gel (CAB-O-SBL). To prepare pure poly(n-butyl acrylate) frontally, Pojman et al resorted to performing the reaction under weightless conditions of a sounding rocket (26 ). 

V. Ivanov, C. Decker, Kinetic study of photoinitiated frontal polymerization , Polym. Intern., 2001, 50, 113-118. 

---