Photopolymerization is considered an attractive alternative in many industries to traditional polymerization processes. The advantages of photopolymerization over other types of polymerization include elimination of heat sources, faster cure times, and reduction in the use of volatile organic solvents. Despite these environmental and cost-saving advantages, photopolymerizations have several limitations. Light attenuation can be a problem for systems containing pigments or fillers. The radiation source penetrates only to a shallow depth beneath the surface, limiting the thickness of strongly pigmented or filled coatings and films. Photopolymerization is also generally limited to systems with simple geometries that can be uniformly illuminated. Coatings on three-dimensional substrates, or other systems with complex geometries, are difficult to uniformly cure. These problems can be solved by "shadow cure," which is defined as the reactive diffusion of photoinitiated active centers into regions of a polymer that are unilluminated. In this contribution, the generation and subsequent spatial and temporal evolution of the active center concentrations during illumination are analyzed using the differential equations that govern the light intensity gradient and photoinitiator concentration gradient for polychromatic illumination. Reactive diffusion of the active centers during the post-illumination period is shown to result in cure of unilluminated regions. A kinetic analysis is performed by coupling the active center concentration profiles with the propagation rate equation, yielding predicted cure times for a variety of applications. This analysis is used for the evaluation of cationic shadow cure in pigmented photopolymerization systems, and systems with complex geometries. The extensive characterization of cationic systems is then applied to free-radical photopolymerization to examine the potential of shadow cure for active centers with much shorter lifetimes. An example of a free-radical photopolymerization system is characterized in which the dimensional scales are small enough to utilize the short lifetimes of the active centers. The results presented for both free-radical and cationic shadow cure indicate that the reactive diffusion of photoinitiated active centers may be used for effective cure in unilluminated regions of a photopolymer. This research will potentially allow photopolymerization to be applied in industries where it has never before been utilized.

Radiation polymerization is a rapid, sustainable process, requiring no environmentally damaging solvents and less energy than thermal polymerization methods. This process is used extensively each year to produce millions of tons of films, coatings, inks, and adhesives. In this work, kinetic- and property-control strategies were developed for three underdeveloped areas of radiation polymerization: free-radical electron beam (EB) polymerization, free-radical/cationic hybrid photopolymerization, and cationic shadow cure. Raman spectroscopy, an analytical technique for studying photopolymerization kinetics, was established as a method of determining the conversion of EB-initiated polymer films. This technique, in conjunction with dynamic mechanical analysis (DMA), was used to investigate the impact of chemical structure on the magnitude of EB dose rate effects (DREs). A strong correlation was determined between the DRE magnitude and monomer size, which may be attributed to chain transfer opportunities. A preliminary predictive relationship was developed to estimate the magnitude of the DRE using the property shift caused by changes in dose, enabling scale-up of process variables for polymers prone to dose rate effects. In addition, a protocol was developed to produce films with equivalent energy deposition for both EB and photopolymerizations, allowing the effect of the initiating radiation to be studied. Distinct kinetic and physical property differences were shown in the resulting EB- and photo-initiated films, despite equivalent initiation energies and energy rates. Monomer chemistry was determined to be an important factor in the magnitude of these differences. In order to control the phase separation that can occur in free-radical/cationic hybrid systems, the cationic AM mechanism was promoted through a hydroxyl group located on the (meth)acrylate, covalently bonding the (meth)acrylate and epoxide networks. The impact of the AM mechanism on the reaction kinetics and physical properties was studied using real-time Raman spectroscopy and DMA to compare a hydroxyl-containing acrylate and methacrylate to non-hydroxyl-containing controls. The promotion of the AM mechanism improved epoxide conversion and network homogeneity. The affect on the (meth)acrylate kinetics correlated to the propagation rate of the neat (meth)acrylate. It was also demonstrated that the glass transition temperature of the hybrid system could be controlled by varying the ratio of (meth)acrylate to epoxide. Cationic shadow cure, which offers a means of circumventing the light penetration limitations in photopolymerization, was modeled using a central composite design. This model was shown to be predictive of both shadow cure length and gel fraction while varying effective irradiance, exposure time, exposure area, and sample depth. Moreover, the model helped ascertain the impact of each variable and its interactions: shadow cure length was most influenced by sample depth, but the gel fraction was reliant on the other three variables. Active center mobility was also qualitatively tracked, and it was established that the section of solid polymer formed during illumination was restricting the movement of the active centers, preventing complete cure. Through this discovery, a new method of shadow cure was developed, termed transferable shadow cure (TSC). This new method separates the initiation and propagation mechanisms, and, as the name suggests, allows for the active-center-containing monomer to be transferred to areas unreachable by light before solidifying. Conversion of the TSC, as determined via Raman spectroscopy, was also modeled using a central composite design. The model predicts TSC conversion is equally dependent on effective irradiance, sample depth, and exposure time, but independent of exposure area. Through the development of control strategies in these three areas, this work provides a better fundamental understanding of radiation polymerization, as well as guidelines that aid in product design and technology expansion.

Photopolymerizations offer many advantages (such as temporal and spatial control of initiation, cost efficiency, and solvent-free systems) over traditional thermopolymerization. While they are now well-established as the preferred option for a variety of films and coating applications, they are limited from many applications due to problems such as oxygen inhibition, light attenuation, additive interference, or the creation of shadow regions and oxygen pockets due to complex shapes. These problems can be solved by using an underutilized form of photopolymerization--cationic photopolymerization. Cationic photopolymerizations have unique active centers which are essentially non-terminating causing extremely long active center lifetimes. In this contribution, the unique characteristics of cationic active centers are explored for their ability to be used in many new applications where previous photopolymerization techniques failed. It was found that the long lifetimes of the active centers permitted them to be very mobile, allowing them to migrate into and polymerize regions that were never illuminated in a process termed shadow cure. The mobility of cationic active centers provides a very efficient means of photopolymerizing of thick and pigmented systems. The long lifetimes of the cationic active centers can be used in the creation of a sequential stage curable polymer system and in the development of novel methods to cure complex shapes, two applications previously unattainable by photopolymerization. The termination of the cationic active centers was found to be reversible and can be used as a technique for external temporal control of the photopolymerization after the illumination has ceased. These abilities have great potential and will allow cationic photopolymerization to be used in many new applications where previous photopolymerization techniques failed, expanding their influence and benefits.

Photopolymerization has become the standard for many coating and printing applications that require rapid curing at room temperature due to its potential to reduce volatile organic compound (VOC) emissions while providing a means for efficient manufacturing processes. These advantages could be useful in a variety of emerging applications, such as anisotropic conductive films (ACF) if photopolymerization could extend into relatively narrow shadow regions which are not directly illuminated, and if visible wavelengths that are not absorbed by polyimide films could be used to trigger the reaction. The broad objectives of this research are i) to examine the factors that determine the attainable extent of shadow cure in free radical polymerizations, and ii) to develop initiator systems effective for polymerization using visible light and light emitting diode (LED) lamps. Project I: Shadow Cure in Free Radical Photopolymerizations In this project, the extent of shadow cure in visible-light-induced free radical photopolymerization is investigated. A number of effective methods such as adding additives, utilizing a reflective stage, and increasing the light intensity are introduced. In addition, the use of fluorescent dyes in multi-component photoinitiator systems proved to be very effective for shadow cure because the fluorescent light emitted from the dye could irradiate the shadow region. When considering practical resins, mixtures of oligomers and monomers, the viscosity is the major barrier that must be overcome in order to achieve high conversion in the shadow regions using visible-light-induced multi-component photoinitiator systems. Hence, instead of using multi-component systems, a commercial visible-light-induced single-component photoinitiator is investigated. As a result, a high conversion in shadow regions of the viscous oligomer containing resin is achieved. Project II: Experimental and Modeling Studies of Photoinitiator Systems for Effective Polymerizations with LEDs In this project, various LED photocuring systems are investigated and characterized. The light intensities of LEDs become weaker as their peak emission wavelengths decrease. Therefore, to design the practical process of LED curing, the effect of both the light intensity and the emission spectrum of the lamp must be considered. Photopolymerization for four representative UV photoinitiators with different LEDs are investigated experimentally and theoretically. The effective light source is dependent on the photoinitiators and several LEDs demonstrate high thin cure ability. The calculated results from a model display good qualitative correspondence with the experimental results. Various interesting suggestions are obtained using this model. For example, the commercialization of 355 nm LEDs might be able to superior photopolymerization compared to other currently available LED lamps.