Isothermal frontal polymerization: Confirmation of the mechanism and determination of factors affecting the front velocity, front shape, and propagation distance with comparison to mathematical modeling

Abstract

Isothermal frontal polymerization (IFP) is a directional polymerization that uses the Trommsdorff, or gel, effect to produce gradient materials for optical applications. When a solution of methyl methacrylate and a thermal initiator contacts a polymer seed (a small piece of polymer), a viscous region is formed in which the polymerization rate is faster because of the Trommsdorff effect. Using the optical techniques of laser line deflection (Weiner's method) and shadowgraphy along with controls, we obtained definitive experimental evidence of IFP. Moreover, we were able to measure accurately and precisely the front position and front concentration profile as a function of time by monitoring IFP systems and controls of various initiator concentrations and cure temperatures. The experimental data were compared with theoretical predictions from a model using mass‐diffusion and radical polymerization kinetics. The model reproduced the decrease of the propagation time and showed an increase in the propagation velocity for an increase in the initiator concentration and/or cure temperature. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5774–5786, 2005

Isothermal Frontal Polymerization

“…IFP differs from TFP in that the entire process is contained in an isothermal environment and the mechanism requires a ''polymer seed'' (a piece of preformed polymer) [7,8]. Koike discovered IFP and proposed its mechanism in 1988 with additional definitions in 1990 ( Figure 5.1b).…”

“…As a new polymer is produced in the viscous region, the process of the monomer solution diffusing into this new polymer region, swelling it, and producing additional new polymer continues. The process goes on autocatalytically ( Figure 5.1b, time = after front starts) until the monomer solution polymerizes sufficiently (i.e., high molecular weight chains) so that the monomer solution is no longer able to diffuse into the newly formed polymer [7,8]. In 1988, Koike termed the process interfacial-gel polymerization (IGP) [7].…”

“…The process goes on autocatalytically ( Figure 5.1b, time = after front starts) until the monomer solution polymerizes sufficiently (i.e., high molecular weight chains) so that the monomer solution is no longer able to diffuse into the newly formed polymer [7,8]. In 1988, Koike termed the process interfacial-gel polymerization (IGP) [7]. This term, as well as the terms IFP and nonthermal polymerization, are used throughout the literature.…”

“…Typically, TFP is faster than IFP [2][3][4][5][6], and, therefore, different free-radical polymerization chemical systems are used for each. A typical TFP system is methacrylic acid (MA) [2][3][4][5], whereas a typical IFP system is methyl methacrylate (MMA) [6,7,11,12]. MA has less steric hindrance than MMA, allowing FP to proceed faster: Typical TFP times are on the order of minutes [2][3][4][5], while typical IFP times are on the order of hours [6,7,12,13].…”

“…A typical TFP system is methacrylic acid (MA) [2][3][4][5], whereas a typical IFP system is methyl methacrylate (MMA) [6,7,11,12]. MA has less steric hindrance than MMA, allowing FP to proceed faster: Typical TFP times are on the order of minutes [2][3][4][5], while typical IFP times are on the order of hours [6,7,12,13]. One use for TFP products is coatings, which do not have to be optically clear, whereas IFP products are used for gradient refractive index materials (GRINs), which do require optical clarity [8,[14][15][16][17][18][19].…”