Investigation of In-situ Sequential Interpenetrating Polymer Network of High-Performance Materials for Vat-Photopolymerization
Authors:   , Fridman Anh , Alvarez Nicolas J , Palmese Giuseppe Raffaello 
Published: July 2022

The emergence of additive manufacturing over the last four decades has transformed the manufacturing processes, especially in rapid prototyping and customization production. Additive manufacturing allows the production of parts without the use of molds, tools, and cutters. Vat-photopolymerization is a common technique in additive manufacturing that offers high dimensional accuracy; however, the major drawback of this process is the limited materials that can be used. Current vat-photopolymerization technologies require low viscosity photocurable materials. Acrylates, methacrylates, and epoxies are commonly used as can they polymerize under UV light, but their thermal properties are not suitable for applications in which high-temperature performance is needed. This dissertation focuses on in-situ sequential interpenetrating polymer network (IPN) approach where high-temperature materials can be incorporated into vat-photopolymerization resins and the advantages when in-situ sequential IPN is used. In order to achieve materials with high Tg and high thermal stability, multimaleimide and cyanate ester were selected as the major components in the study. High-temperature materials tend to have high viscosity and are difficult to process. In fact, the constraints on viscosity for vat-photopolymerization became one of the major factors in determining the chemical structures and quantity of materials that could be used. The selected multimaleimide, reactive diluent, and cyanate ester offered low viscosity while remaining miscible with each other. Poly(phenylmethane) maleimide provided the extra advantage of reducing depth of penetration, which can result in better print resolution. A study determining the content of secondary network that can be printed was also conducted. Although a high content of secondary network can be printed, the high print time and low fidelity makes it less feasible. The in-situ sequential IPN was formed in two steps: (1) a copolymerization reaction between multimaleimides and diluent occurs during printing, resulting in a cyanate-ester swollen network with sub room temperature glass transition temperature (Tg) and (2) the polymerization of cyanate ester during post-processing. The presence of cyanate ester during photopolymerization provides a less diffusion limited environment for the photo-curable components, allowing them to reach higher conversion. The resulting material has a Tg above 250 oC (loss modulus peak), significant decrease in cure shrinkage, and toughness GIc 100 J/m2. The printed parts have high accuracy and very low dimensional shrinkage after post-curing at a high temperature. The study successfully demonstrated that high-temperature materials with exceptional characteristics can be printed via masked stereolithography. This potentially opens more applications of additive manufacturing into aerospace and other heat shielding applications. Further improvement of thermomechanical and thermal stability of the multimaleimide/cyanate ester IPN was achieved using linked IPN where the individual networks are chemically connected through interlinker molecules. An improvement in glassy modulus, Tg, and thermal stability was observed. The Tg achieved in the linked IPN systems was higher than the values reported in literature or commercial vat-photopolymerization materials. One drawback of this system is that the competition between curing and degradation of monomers affects the materials' ability to reach its full potential. Overall, linked IPN was shown to enhance the overall properties of printed materials without affecting their processability and print accuracy. Using our understanding in the characterization and benefits of sequential IPN, the approach was further applied to other high temperature materials, such as benzoxazine. A methacrylate-functionalized benzoxazine was synthesized, and its application in vat-photopolymerization was investigated. Constraints in viscosity for processability continued to limit the material selection and properties; however, the developed materials achieved the desired properties without sacrificing the excellent thermal properties of benzoxazine. Yet, the addition of diluents and crosslinker did have slight effects on Tg and fracture toughness. This again showed that an advancement in printing technology, where high viscosity materials can be processed, could bring great benefits to expanding the materials library as well as improving properties.