Research focus and management
The IUCRC researchers will conduct a comprehensive study of the kinetics and mechanisms of photopolymerizations and their impact on the structure and properties of photopolymerized materials. The kinetic and mechanistic contributions to photopolymerization processes each play a critical role in determining the macroscopic structure and material properties of the product. Within the center both radical and cationic photopolymerizations will be examined with state-of-the-art experimental techniques to elucidate the complex chemical and physical mechanisms which control the initiation, propagation and termination of the active centers. Additionally, the macroscopic structure and molecular architecture will be characterized so that they may be correlated with the material properties. These fundamental studies will elucidate the underlying reaction-structure-property relationships which, we believe, will lead to the development of new applications and profound improvements in existing photopolymerizations processes.
Research program
The faculty of the center is conducting a comprehensive study of the kinetics and mechanisms of photopolymerizations and their impact on the structure and properties of photopolymerized materials. The kinetic and mechanistic contributions to photopolymerizations each play a critical role in determining the macroscopic structure and material properties of the product. The Center unites a productive research team with expertise that spans a variety of scientific arenas including fundamental photophysical and photochemical processes, polymerization reaction engineering, spectroscopic characterization methods, network formation modeling, and advanced high resolution microscopies. Both radical and cationic photopolymerizations are being examined with state-of-the-art experimental techniques to elucidate the complex chemical and physical mechanisms that control the initiation, propagation and termination of the active centers. These fundamental studies are helping to establish the underlying reaction-structure-property relationships that will lead to the development of new applications and to profound improvements in existing photopolymerization processes.
Research projects
January 1, 2000
Christopher N. Bowman
The kinetics of photopolymerizing systems are generally complicated by a variety of phenomena including autoacceleration, autodeceleration, primary cyclization, oxygen inhibition, reaction diffusion controlled termination, chain length dependent termination, and incomplete reaction of the double bonds. Each of these phenomena tends to confound the photopolymerization kinetics, making it difficult to predict the kinetics or ascertain exactly how changes in the reaction conditions will impact the polymerization kinetics and, ultimately, the polymer properties. The overall objective of this work is the thorough experimental characterization of these phenomena and the developments of a comprehensive theoretical model for describing the photopolymerization kinetics under a wide range of circumstances.
Thus, this work focuses on experimental characterization of certain aspects of the kinetics and the development of an advanced, comprehensive model for predicting photopolymerization kinetics.
Project focus summary:
- determining the effects of kinetic chain length and chain transfer reactions on the termination kinetic constant in crosslinked acrylic systems;
- determining the relative impacts of reactivity and dilution in copolymerization reactions by polymerizing in reactive diluents that have been hydrogenated; and
- developing and improving the model for photopolymerization kinetics to include the effects of temperature, heat transfer, and oxygen inhibition as well as all of the various kinetic features such as diffusion limited reactions, chain length dependent termination, and primary cyclization.
January 2000
Vishal Sipani and Alec Scranton
Objective: to provide a fundamental characterization of the kinetics of cationic photopolymerizations of epoxide monomers as well as hybrid cationic/radical photopolymerizations of epoxides with acrylate monomers. Research in the past year has focused on characterization of cationic photopolymerizations of epoxides. UV-initiated cationic photopolymerizations have received much less attention than free-radical photopolymerizations even through they offer several advantages since they are not inhibited by oxygen, can exhibit significant post-cure, and may be used to polymerize monomers such as epoxides and vinyl ethers.
Project focus:
Research in this project has focused on two aspects of cationic polymerizations:
- Characterization of the general effects of a variety of reaction variables: type and concentration of photoinitiator, type and concentration of photosensitizer, monomer functionality, temperature and initiating light intensity; on the kinetics of the cationic photopolymerizations.
- Dark cure studies to characterize the active center lifetime and investigate the effective propagation rate constant in cationic photopolymerizations.
The work done so far has provided the foundation for the future research, which will focus on hybrid radical/cationic polymerizations of acrylates and epoxides using three component initiators that produce both radicals and cations. These studies will include dark cure studies that can distinguish between the radical and cationic polymerizations due to the large difference in the active center lifetime. Studies will also investigate the effect of oxygen (which will inhibit the radicals but not the cations) and the effect of moisture (which will inhibit the cations but not the radical).
January 2000
Dongkwan Kim and Alec Scranton
Three-component photoinitiator systems typically contain a light absorbing molecule (the dye), and electron donor (typically an amine) and a third component (usually an iodonium salt). These systems have consistently been found to be faster, more efficient, and more sensitive than the traditional single-component a-cleavable initiators or the two-component electron transfer systems. Moreover, since a wide variety of dyes may be used, the three-component initiator systems are extremely flexible in selection of the initiating wavelength. Finally, with proper selection of the components the same initiating systems may be effective for initiation of cationic polymerizations as well as radical polymerizations.
Objective: to provide a fundamental characterization of the kinetics and mechanisms of selected three component systems and to identify general underlying principles that will guide the design and selection of initiator systems based upon the desired initiating wavelength, light sensitivity, and polymerization rate. The knowledge gained in this research should allow the development of initiator systems which optimize the polymerization rate for a given light source by managing the absorption of light and the production of active centers through primary photochemical reactions and secondary reactions.
January 1, 2000
Christopher N. Bowman
Recently, pioneering work by Decker and coworkers explored a novel class of acrylic monomers found to have exceptional properties with respect to high polymerization rates, resistance to oxygen inhibition, and improved mechanical properties. These monomers were acrylic monomers substituted with secondary functional groups that dramatically improved their reactivity. Though chain transfer reactions are typically sighted as an explanation for the improved reactivity, little quantitative information is available to provide a detailed understanding of why these monomers are as reactive as they are or why they have such exceptional mechanical properties. Few studies have been performed to examine the structure of these polymers or to vary systematically the secondary substituents.
In this work, we are determining the underlying phenomena that lead to the dramatic improvements in properties of the novel class of monomers. This information is being developed from studies of synthesized model compounds and used to design monomers with even further improvements in properties.
Project focus summary:
- synthesizing a variety of acrylates with systematically varying functional groups that serve as model compounds for the characterization studies; and
- characterizing the polymerizations and polymers that result from these monomers with a focus on determining the underlying reasons for monomer reactivity and improved properties.
January 1, 2001
Christopher N. Bowman and Alec Scranton
One of the greatest limitations to the application of free radical photopolymerizations is the inhibitory effect that oxygen has on these types of reactions. The inability of free radical photopolymerizations to overcome oxygen inhibition has limited their applicability or forced the application of expensive inerting equipment. Oxygen interacts with photopolymerizing systems both at the initiation stage and in the propagation stage. Its presence generally leads to the formation of an inhibition layer at the top surface of the sample in which the diffusion rate of oxygen into the sample from the surrounding environment is more rapid than its consumption by the polymerization. Also, unique kinetic behavior is observed in the presence of oxygen as two rate maxima are often observed, and the presence of even a small amount of a monovinyl compound can have pronounced effects. In this project we are developing an improved understanding of the effects of oxygen on both the initiation and propagation steps of the polymerization. Mitigation of oxygen's effects will be attempted through the application of three component initiating species and the presence of highly abstractable hydrogens.
In this work, we are pursuing an approach designed to understand and mitigate the effects of oxygen on the photopolymerization reaction.
Project focus summary:
- investigating the effect of oxygen on the initiation step for commercial single component initiators as well as two and three component initiator systems;
- determining the effects of oxygen on the propagation step by examining the photopolymerization kinetics and surface polymer properties in environments with systematically varying oxygen levels, by examining the cure in thick films, by following the formation and subsequent degradation of peroxy functionalities, and by systematically adding compounds that have specific interactions with oxygen and peroxy radicals; and
- developing methodologies for mitigating the impact of oxygen on the photopolymerization kinetics and photopolymer properties.
Alec Scranton
Many photoinitiators exhibit photobleaching in which the absorbance decreases with illumination time when exposed to light of the proper wavelength. This occurs because the absorption characteristics of the photolysis products are different than the original initiator molecule. Two classes of a-cleavable photoinitiators for which photobleaching is particularly pronounced are aryl phosphine oxides in the 365 nm region of the spectrum, and substituted titanocenes in the 450 nm region. Photobleaching is particularly important for photopolymerization of thick polymer parts and pigmented coatings. In this project, a mathematical model is being developed for the photobleaching of photoinitiators, including multi-wavelength effects. Given the incident light intensity and wavelength as well as the initial photoinitiator concentration, the model predicts the variation of the photoinitiator concentration and light intensity with time and sample depth. The model is based upon the simultaneous solution of the governing differential equations, and the simulation predictions will be compared to experimental results for a number of photobleaching initiators. The importance of diffusional effects and multiple incident wavelengths on the photobleaching rate are being investigated.
January 1, 2001
Christopher Bowman and Charles E. Hoyle
Thiol/ene reactions have previously been developed as a high speed alternative to radical polymerizations. These reactions are radical polymerizations that proceed rapidly, even in the presence of oxygen; however, the mechanism is generally thought to be a step growth mechanism in which chain transfer to the thiol and propagation proceed sequentially. This mechanism leads to the formation of a network structure only when the functionality of either the thiol or the vinyl is greater than two.
Project Focus: In this work, we will study three different aspects of the thiol/ene systems related to improved fundamental understanding of these promising systems, development of novel methods for polymerizing these systems, and development of an advanced photocurable thiol/ene resin.
Specifically, we propose:
- synthesizing a variety of acrylates with systematically varying functional groups that serve as model compounds for the characterization studies; and
- characterizing the polymerizations and polymers that result from these monomers with a focus on determining the underlying reasons for monomer reactivity and improved properties.