Isothermal frontal polymerization

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. 

A Comparison between TFP and IFP Their Mechanisms and Front Properties 

Nonlinear Dynamics with Polymers Fundamentals, Methods and Applications. Edited by John A. Pojman and Qui Tran-Cong-Miyata Copyright 2010 WILEY-VCH Verlag GmbH 8c Co. KGaA, Weinheim ISBN 978-3-527-32529-0 

Newly formed polymer and remaining polymer seed 

FP systems must have certain conditions for the front to autocatalytically occur. The necessary and sufficient conditions for TFP include a monomer that will polymerize via free-radical polymerization and a thermal initiator (a photoinitiator may be used to start the reaction, but a thermal initiator is necessary to sustain the reaction) [9,10]. The necessary and sufficient conditions for IFP include a monomer that will dissolve the polymer seed, polymerize via free-radical polymerization, and exhibit the gel effect a thermal initiator and a viscous region in which the gel effect can occur (i.e., the seed dissolving) [6, 11]. Ideally, another necessary IFP condition is a monomer-polymer system that produces an optically clear product because most IFP products are used in optical applications. 

---

Diffusion of small molecules, usually solvents, into glassy polymers exhibits anomalous or non-Fickian behavior 34). As the solvent penetrates, the diffusion coefficient increases because the glass transition temperature is lowered. The solvent acts as a plasticizer, increasing the free volume and the mobility of the solvent. Thus we have an autocatalytic diffusion process. This can be relevant in Isothermal Frontal Polymerization, which we discuss below. 

Isothermal Frontal Polymerization (IFP), also called Interfacial Gel Polymerization, is a slow process in which polymerization occurs at a constant temperature and a localized reaction zone propagates because of the gel effect (64,65). Using IFP (27), one can control the gradient of an added material like a dye, to generate materials useful, for example, in optical applications (66,67). Lewis et al. provide experimental and theoretical results in chapter 14. 

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. 

Lewis and Volpert continue the discussion of the isothermal form of frontal polymerization in Chapter 5. Isothermal frontal polymerization is also a localized reaction zone that propagates but because of the autoacceleration of the rate of free-radical polymerization with conversion. A seed of poly(methyl methacrylate) is placed in contact with a solution of a peroxide or nitrile initiator, and a front propagates from the seed. The monomer diffuses into the seed, creating a viscous zone in which the rate of polymerization is faster than in the bulk solution. The result is a front that propagates but not with a constant velocity because the reaction is proceeding in the bulk solution at a slower rate. This process is used to create gradient refractive index materials by adding the appropriate dopant. 

Lewis, LL. (2003) The development and characterization of an optical monitoring technique and a mathematical algorithm for isothermal frontal polymerization. Dissertation, The University of Southern Mississippi. 

J. A., and Volpert, VA. (2005) Isothermal frontal polymerization confirmation of the mechanism and determination of factors affecting Ifont velocity, front shape, and propagation distance with comparison to mathematical modeling. 

V.A. (2002) Dopant distribution in isothermal frontal polymerization. Appl. 

Evstratova, S.I., Antrim, D., Fillingane, C., and Pojman, J.A. (2006) Isothermal frontal polymerization confirmation of the isothermal nature of the process and the effect of oxygen and polymer seed molecular weight on front propagation. 

A. (2001) Optical gradient materials produced via low-temperature isothermal frontal polymerization. J. Appl. Polym. Sci., 80, 686-691. 

---