Design of Functional Polyesters for Electronic and Biological Applications

Melt polymerization and novel monomers enabled the synthesis of polyesters for electronic and biological applications. Inspiration from nature and a passion for environmental preservation instigated an emphasis on the incorporation of renewable resources into polymeric materials. Critical analysis of current research surrounding bisphenol-A replacements and ioncontaining segmented polyurethanes aided in identifying benchmark polymers, including limitations, challenges, and future needs. Structure-property-morphology relationships were investigated to evaluate the polymers for success in the proposed applications as well as to improve understanding of polyester compositions to further design and develop sophisticated polymers for emerging applications.

Aiming to utilize the reported [2 + 2] cycloaddition of the known mesogen 4,4’-dimethyltrans-stilbene dicarboxylate (SDE) to overcome ultraviolet (UV) induced degradation issues in electronic encasings, the synthesis of copolyesters containing SDE ensued. 1,6-Hexanediol (HD) and 1,4-butanediol comonomers in varying weight ratios readily copolymerized with SDE under melt transesterification conditions to afford a systematic series of copolyesters. Differential scanning calorimetry revealed all copolyesters exhibited liquid crystalline transitions and melting temperatures ranged from 196 °C – 317 °C. Additionally, melt rheology displayed shear thinning to facilitate melt processing. Compression molded films exhibited high storage moduli, a glassy plateau until the onset of flow, and tensile testing revealed a Young’s iii modulus of ~900 MPa for poly(SDE-HD). These properties enable a wide range of working temperatures and environments for electronic applications.

Adding complexity to linear liquid crystalline copolyesters, copolymerization with oligomeric hydroxyl-functionalized polyethers afforded segmented liquid crystalline copolyesters. 4,4’-Biphenyl dicarboxylate (BDE), commercially available diols containing 4, 5, 6, 8, or 10 methylene units, and introducing poly(tetramethylene oxide) or a Pluronic® triblock oligoethers in varying weight % were used to synthesize multiple series of segmented copolyesters. Comparing melting transitions as a function of methylene spacer length elucidated the expected even-odd effect and melting temperatures ranged from 150 °C to 300 °C. Furthermore, incorporating the flexible soft segment did not prevent formation of a liquid crystalline morphology. Complementary findings between differential scanning calorimetry and small-angle X-ray scattering confirmed a microphase-separated morphology. Thermomechanical analysis revealed tunable plateau moduli and temperature windows based on both soft segment content and methylene spacer length, and tensile testing showed the strain at break doubled from 75 weight % to 50 weight % hard segment content. The same compositions Young’s moduli decreased from 107 ± 12 MPa at 75 weight % hard segment to 19 ± 1 MPa with 50 weight % hard segment, demonstrating the mechanical trade-off and range of properties possible with small compositional changes. These segmented copolyesters could find use in high-performance applications including electronic and aerospace industries.

A two-step synthesis transformed caffeine into a novel caffeine-containing methacrylate (CMA). Conventional free radical copolymerization with a comonomer known to provide a low glass transition temperature (Tg), 2-ethylhexyl methacrylate (EHMA), allowed the investigation of the effect of small amounts of pendant caffeine on polymer properties. Thermal and iv thermomechanical testing indicated CMA incorporation dramatically increased the storage modulus, however, a microphase-separated morphology was not attained. Association of the pendant caffeine groups through non-covalent π-π stacking could present opportunities for novel thermoplastics and it is proposed that placing the pendant group further from the backbone, and potentially increasing the concentration, could aid in promoting microphase-separation.

Alkenes are reactive sites for placing functional groups, particularly those required for polyester synthesis. Methyl 9-decenoate (9-DAME), a plant-based fatty acid, provided a platform for novel biodegradable, renewable, polyesters. A formic acid hydration reaction generated an isomeric mixture of AB hydroxyester or AB hydroxyacid monomers for melt polymerization. Thermal analysis elucidated the plant-based polyesters exhibited a single transition, a Tg of about -60 °C. Aliphatic polyesters commonly crystallize, thus the isomeric mixture of secondary alcohols seemed to introduce enough irregularity to prevent crystallization. These polyesters offer an amorphous, biodegradable, sustainable replacement for applications currently using semi-crystalline poly(ε-caprolactone), which is not obtained from renewable monomers and also exhibits a -60 °C Tg. Additional applications requiring low-Tg polymers such as pressure sensitive adhesives or thermoplastic elastomers could also benefit from these novel polyesters. 9-DAME also was transformed into an ABB’ monomer after an epoxidation and subsequent hydrolysis. Successful gelation under melt transesterification conditions provided evidence that the multifunctional monomer could perform as a renewable, biodegradable, branching and/or crosslinking agent.

Novel copolyesters comprised of a bromomethyl imidazolium diol and adipic acid demonstrated potential as non-viral gene delivery vectors. Melt polycondensation produced water dispersible polyesters which bound deoxyribonucleic acid at low N/P ratios. The v polyplexes showed stability in water over 24 h and no cytotoxic effect on human cervical cancer cells (HeLa). A luciferase transfection assay revealed the copolyesters successfully underwent endocytosis and released the nucleic acid better than controls. The copolyesters with pendant imidazolium functionality also provided tunable Tgs, -41 °C to 40 °C, and the ability to electrospin into fibers upon blending with poly(ethylene oxide). These additional properties furthered potential applications to include pressure sensitive adhesives and biocompatible antibacterial bandages.