Intellectual Merit: Propagation mechanisms that are driven by free-radical active centers are plagued by oxygen inhibition. Cationic polymerization schemes suffer from moisture effects and slower cure rates. To address the limitations of these mechanisms, hybrid resin systems have been designed to photopolymerize using a combination of cationic and free-radical mechanisms. (The advantages of using light to initiate polymerization rather than heat include significant savings in energy costs, processing space, and time; solvent-free systems; and increased control over the production of initiating species.) These hybrid systems exhibit lower sensitivity to oxygen and moisture and offer advantages such as increased cure speed and improved film-forming properties. To date, most studies have focused on the development of these systems; thus, there is a need for in-depth studies in order to create a fundamental base of knowledge that allows the impact and utility of these systems to be optimized.
This research will investigate hybrid resin systems based on formulations that contain both an epoxide moiety, which undergoes cationic ring-opening photopolymerization, and an acrylate moiety, which undergoes free-radical photopolymerization. The goal of this research is to acquire a better understanding of how experimental variables affect the atmospheric sensitivity of these systems (both hybrid monomers and multi-monomer systems), which in turn will facilitate the optimal incorporation of hybrid monomers or systems in practical formulations. This goal will be accomplished by: - Determining the most influential factors in altering the oxygen-diffusion-affected region in hybrid monomers/systems. - Characterizing the interplay between photoinitiator systems in hybrid monomers/systems and their effects on kinetics and physical properties. - Clarifying the effect of water on the kinetics and physical properties of hybrid polymers. - Evaluating improvements made in acrylate and epoxide formulations by the incorporation of hybrid monomers/systems.
Conversion, rate of polymerization, and composition will be obtained using Raman and near-infrared spectroscopies and Raman confocal microscopy. Dynamic mechanical analysis and surface hardness testing will be used to correlate polymer composition with physical properties. Comparisons of this kinetic and physical information will aid in reaction design for these polymers and will address important issues on the interactions between the two photopolymerization systems. Results will provide insight for tailoring resin formulations to specific end-use applications, especially in films, coatings, and adhesives.
Broader Impacts: This research could impact the photopolymerization industries by decreasing costs associated with combating oxygen inhibition, facilitating faster reactions, decreasing the dependence of product quality upon humidity conditions, providing polymers with better surface properties, and enabling the production of thin films at ambient conditions. This characterization will also provide opportunities for growth of new applications in the fields of biomedicine, telecommunications, and aerospace engineering. The project will also have a direct impact on the educational experience of students at various levels. Students in engineering and science will benefit from a polymer science course series that will incorporate these research results. Several undergraduate students and high-school teachers and/or students will be invited to participate in portions of this research. Project Lead the Way Iowa will introduce secondary students and teachers to engineering concepts through a summer training program and academic-year classes.
Intellectual Merit. Tacky coatings. Adhesives that do not stick. Short-lived dental fillings. Optical fiber claddings that pull away from their cores. What do they all have in common? Product failures caused by atmospheric sensitivity of polymerization processes. Propagation mechanisms that are driven by free-radical active centers are plagued by oxygen inhibition. Cationic polymerization schemes suffer from moisture effects and slower cure rates. To address the limitations of these mechanisms, hybrid resin systems have been designed to photopolymerize using a combination of cationic and free-radical mechanisms. (The advantages of using light to initiate polymerization rather than heat include significant savings in energy costs, processing space, and time; solvent-free systems; and increased control over the production of initiating species.) These hybrid systems exhibit lower sensitivity to oxygen and moisture and offer advantages such as increased cure speed and improved film-forming properties. This research investigated hybrid resin systems based on formulations that contain both an epoxide moiety, which undergoes cationic ring-opening photopolymerization, and an acrylate moiety, which undergoes free-radical photopolymerization. As a result of this research, a better understanding of how experimental variables affect the atmospheric sensitivity of these systems (both hybrid monomers and multi-monomer systems was acquired, including: The effects of epoxide concentration, photoinitiator system, and light intensity on the oxygen sensitivity of the acrylate free-radical reaction were quantified. The effects of acrylate concentration, structure and functionality on the epoxide polymerization rate, conversion and polymer properties were demonstrated. The effects of water and alcohol concentration, structure and functionality on the epoxide polymerization rate, conversion and polymer properties were systematically evaluated. The sequence of epoxide and acrylate reactions was controlled via photoinitiator system selection and illumination schemes. Phase separation between the acrylate and epoxide polymer networks was controlled via the formulation and acrylate secondary functionality. These results and others obtained in this research provide insight for tailoring resin formulations to specific end-use applications, especially in films, coatings, and adhesives. Broader Impacts. Results from this systematic and fundamental characterization of the kinetics and physical properties of epoxy-acrylate hybrid resins thus far have been disseminated in 7 conference proceedings papers and over 20 presentations at national and international societal meetings. Five peer-reviewed journal articles and a book chapter directly stemming from this research have been published, and 5 additional articles are currently submitted or in preparation. These results can be used in the photopolymerization industries to decrease costs associated with combating oxygen inhibition, facilitate faster reactions, decrease the dependence of product quality upon humidity conditions, provide polymers with better surface properties, and enable the production of thin films at ambient conditions. Finally, this research program has had a direct impact on the educational experience of students at various levels. Through lab tours and hands-on polymer demonstrations, the PI has reached over 100 K-12 students from eastern Iowa and western Illinois each year. As an affiliate professor for Project Lead the Way Iowa, the PI has also introduced secondary students and teachers to engineering concepts through summer training programs and academic-year classes. University of Iowa students in engineering and science have benefited from a polymer science course series that has incorporated research results from this project. Eight undergraduate students (5 males, 3 females) and 7 high-school students (3 males, 4 females) participated in portions of this research, and 3 graduate students (3 males, 1 minority) received advance degrees based on their investigations of these hybrid systems. Two female graduate students continue aspects of this project as part of an on-going research program in this area.
