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Title:Bond exchange reactions in high temperature thermoset polymers
Author(s):Meyer, Jacob Lee
Director of Research:Jasiuk, Iwona
Doctoral Committee Chair(s):Jasiuk, Iwona
Doctoral Committee Member(s):Economy, James; Ewoldt, Randy; Hutchens, Shelby; Panerai, Francesco
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
in-space assembly
bond exchange
Abstract:In this work, we examine the use of exchangeable bonds within an all-aromatic thermoset to enable reversible bonding towards potential utility as an adhesive for in-space assembly. Interchain transesterification (ITR) is a type of bond exchange reaction accessible by polymers possessing ester bonds. Thermoset polymers that have a high density of exchangeable bonds are referred to in the literature as vitrimers. Vitrimers have enhanced processability via topology changes within their covalent network enabled and mediated by the exchangeable bonds. Aromatic thermosetting copolyesters (ATSP) are a family of all-aromatic thermosets synthesized through crosslinkable aromatic copolyester oligomers and they possess a high density of ester bonds while also having extremely high thermal stability, thermomechanical properties, and wear resistance. Prior literature has shown that adherends of ATSP can be bonded while in the fully cured (thermoset) state. Here, we examine the potential utility of ATSP as a reversible adhesive for in-space assembly and as a mechanism for assembling multi-material composite laminates. In this work, bonding processes enabled by the ITR mechanism are used for the bonding of composite laminates incorporating metallic sheets. This potentially enables multiple structural concepts to be available for the purposes of in-space assembly. ATSP oligomers were infiltrated into unidirectional carbon fiber laminae and cured in a vacuum-enclosed hot press at 340℃. Similarly, finely ground ATSP oligomers are deposited via electrostatic powder spray onto roughened 7075 Al substrates and cured to form a smooth coating. Laminates were bonded to plies of coated Al 7075. The bonded laminates were mechanically characterized via lap shear, 4-point bend, short beam shear, and Mode I fracture designed to examine strength and failure at the metal/composite bondline. Results showed that the failure mode was uniformly cohesive and not adhesive (delamination of polymer from aluminum substrate). A reversible bonding scheme based on the ITR mechanism is demonstrated between aluminum coupons coated with an ATSP resin. This work served as proof of principle for in-space assembly of reconfigurable structures. A custom-built reversible bonding tool kit was used to optimize bonding parameters via an orthogonal design of the experiment. This process found a bonding method that reliably produced 0% adhesive failure in debond in less than 30 minutes of process time. The optimized bonding parameters were applied for 50 cycles over which no loss in adhesive strength was observed. Following the 51st cycle, a total cumulative adhesive failure rate of 5% was observed. These results were significantly in excess of the highest cycle count, highest strength, and highest operating temperature of any reversible adhesive bonding scheme yet presented in the literature. Localized heating methods which are potentially applicable to vacuum environments were explored. Methods for inductive heating of substrates to temperature relevant for the ITR bonding scheme as well as resistive (Joule) heating through the coating were examined. The current was then passed through abutted coated layers that were filled with conductive additives and the layers were bonded through the imposed Joule heating within 6 seconds. This process though was not reversible due to arcing within the coating. A fabrication process for embedding induction-heatable substrates within ATSP/carbon fiber composites following from earlier work on multi-material laminates was developed using stainless steel as the susceptor. Reversibility over multiple cycles was shown via dynamic mechanical analyzer with the limiting constraint found to be the adhesive failure, if any, of the bonded coupons. Bonded coupons were also subjected to a thermal cycle ranging the representative temperatures in low earth orbit (LEO) under tension with no failure. A key requirement for any polymer to be used on-orbit and especially for the bonding scheme discussed herein is its stability versus the hazards present in LEO and beyond. Coupons of ATSP resin coated onto 7075 Al substrates as well as free-standing films of ATSP were exposed to two radiation species present in LEO and resultingly two damage modes: atomic oxygen present in air plasmas produce a surface (<10 nm) damage mode and 2 MeV protons which are capable of damaging the coated ATSP polymers through their entire thickness. Results showed no recession after atomic oxygen and proton fluxes equivalent to 1, 10, and 50 years in LEO as well as no interference in the reversible bonding performance. Further, activation energies for bond exchange had only minor increases after proton exposure. The influence on the activation energy for bond exchange in all-aromatic thermosets was examined with respect to polymer design parameters such as the crosslink density as well as the mesogenicity and meta-substitution fraction as well as ester bond density. This was conducted through a method based on dynamic mechanical analysis. A strong reduction in activation energy was found via meta-substitution and a significant increase in activation energy was found with an increase in mesogenicity (and concomitant decrease in ester bond density) via biphenylic-substitution. A relationship between an increase in crosslink density and an increased activation energy was also found. A meta-substituted composition allowed ITR bonding at significantly reduced temperatures. This work has the potential for impact across numerous fields. Space exploration and cislunar development efforts may have significant benefits from having access to an adhesive process that can be used in orbit. Aircraft and automotive fabrication can benefit from a rapid bonding process that provides structural adhesive strength through high temperatures. Future additive manufacturing and adhesive processes may benefit from this work through the identification of the influence of critical polymer design parameters on bond exchange reaction rates.
Issue Date:2021-04-23
Rights Information:Copyright 2021 Jacob Meyer
Date Available in IDEALS:2021-09-17
Date Deposited:2021-05

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