The development and characterization of an optical monitoring technique and of a mathematical algorithm for isothermal frontal polymerization
Isothermal frontal polymerization (IFP) is a method of converting monomer to polymer in a directional, self-sustaining reaction and occurs when a monomer solution with a thermal initiator diffuses into a polymer seed dissolving the seed to create a viscous region. Polymerization occurs in both the viscous region and in the monomer solution but occurs faster in the viscous region due to the gel effect. IFP is used industrially to manufacture gradient refractive index materials, or GRINs, such as fiber-optic cables for local-area networks. GRINs are produced by adding a dopant material, e.g. a second monomer, to the IFP monomer solution. While extensive research has been published on GRINs, there has been no systematic study reported on IFP systems without dopants due largely to the lack of a non-invasive monitoring technique to observe IFP without disturbing the propagating fronts. This study coupled two optics techniques, laser line deflection (LLD) and shadowgraphy, to monitor IFP and controls for the initiator/monomer/polymer seed system of 2,2' -azobisisobutyronitrile/methyl methacrylate/poly(methyl methacrylate) (AIBN/MMA/PMMA) at the reaction conditions of 0.03, 0.06, and 0.15% AIBN in MMA isothermally cured at the temperatures ranges of 42-47, 47-52, and 66-68°C. In addition, a mathematical algorithm was developed to analyze the images obtained from LLD and shadowgraphy to determine the front propagations, the front velocities, and the gradient profiles (a measure of the change in conversion). The IFP samples and controls were characterized via differential scanning calorimetry and gel permeation chromatography to determine relative percent conversions and molecular weights. The IFP characteristics of front propagation, front velocities, and gradient profiles were generated by the mathematical model of Spade and Volpert and compared to the experimental IFP characteristics of this study to gauge the accuracy of the model.