John P. Swanson

Synthesis and evaluation of thermally-responsive coatings based upon Diels-Alder chemistry and renewable materials†

A soybean based coating with thermally responsive Diels–Alder linkages has been prepared following an automotive 2-component formulation. The resulting coatings displayed the capability to be healed following physical deformation by a thermal stimulus, and such a material has significant potential for end users. Various curing agents were employed, and resulted in variation of scratch resistance and re-healablity. Different thermally responsive soybean resins were synthesized to have varying amounts reversible and nonreversible linkages when incorporated into the coating. Additionally, different isocyanates were added at differing ratios of NCO : OH in search of the optimum coating. It was found through the analysis of rehealability, hardness, gloss, and adhesion that the optimal combination was an acetylated resin (no irreversible crosslinks) with 54% reversible Diels–Alder linkages at an NCO : OH ratio of 5 : 1 using isophorone diiscocyanate. Materials were evaluated via differential scanning calorimetry (DSC), scratch resistance, Koenig hardness, gloss measurements, and topographical analysis.

A coacervate-forming biodegradable polyester with elevated LCST based on bis-(2-methoxyethyl)amine

Recently, we reported a new class of biodegradable, thermoresponsive polyesters (TR-PEs) inspired by polyacrylamides and elastin-like peptides (ELPs). The polyesters exhibit tunable cloud point temperatures (Tcp) and thermoresponsive coacervation in aqueous solution as shown via UV-vis spectroscopy, 1H NMR, and DLS. However, the Tcp of all TR-PEs remained low (<15 °C), and higher thermoresponsivity would be beneficial for many applications. This study examines the synthesis, polymerization, and analysis of a new monomer bearing a more hydrophilic pendant group, bis-2-methoxyethylamine (bMoEtA). The resulting TR-PE, TR-bMoEtAPE, displays a threefold increase in Tcp (ca. 50 °C) that is affected by solution (DI water vs. phosphate buffered saline), concentration (1–40 mg mL−1) molecular weight (20–130 kDa), and cosolutes (Hofmeister salts and urea). The Tcp and Tg of random TR-bMoEtAPE copolymers can be tuned via comonomer feed. Variable temperature 1H NMR indicated a cooperative coacervation mechanism above Tcp, further reinforced by DLS measurements. As evidenced by UV-vis and SEC analysis, TR-bMoEtAPE underwent rapid degradation over a period of 7 days in DI water and PBS. Finally, cytotoxicity studies suggested that TR-bMoEtAPE is non-cytotoxic even at high concentrations (ca. 1000 μg mL−1). The increased Tcp and tunability suggests TR-bMoEtAPE as a potential candidate for future functionalized TR-PE therapeutic-delivery systems.

The effect of pendant group structure on the thermoresponsive properties of N-substituted polyesters

Synthetic polymers exhibiting reversible lower critical solution temperature (LCST), such as poly(N-isopropylacrylamide) (PNIPAM), are intriguing materials with various potential applications. We recently developed a new class of biodegradable thermoresponsive polyesters (TR-PEs) based on N-substituted diol monomers. TR-PEs exhibited reversible cloud point temperatures (Tcp) between 0–50 °C and were shown to be non-cytotoxic. The synthesis of N-substituted diols and TR-PEs is highly modular, allowing for a wide variety of possible homo- and copolyesters. In this work, we report the synthesis and characterization of 20 homopolyesters in order to better understand the structure–property relationship of TR-PEs. UV-vis spectroscopy showed that the Tcp of TR-PEs was highly dependent on pendant group structure, such as secondary or tertiary amides, cyclic and linear groups, and substitution of oxygen atoms. Structure-Tcp analysis provides a correlation between Tcp and the number of heteroatoms relative to the number of carbon atoms in the pendant group thereby providing a rationale for the design of thermoresponsive polyesters with desired Tcp values. To demonstrate the expanded tunability of the TR-PE system, copolyesters bearing covalently attached ibuprofen were synthesized and shown to exhibit LCST behavior dependent on comonomer hydrophilicity.