Stilbene-containing Thermotropic Liquid Crystalline Polyesters for Aerospace Applications

Materials used in space travel must maintain thermal and dimensional stability in extreme conditions, while remaining melt processable for ease of industrial production. The rigid chemical structure of stilbene often correlates to polymers exhibiting a liquid crystalline morphology and the olefinic bond undergoes a [2 + 2] cycloaddition in the presence of UV light. Based on the combined benefits of a non-destructive photocrosslinking reaction and liquid crystallinity, we propose the use of stilbenecontaining (co)polyesters for various aerospace applications. These copolyesters include random copolyesters, and segmented copolyesters achieved using functional polyols. The synthesis and characterization of (co)polyesters containing 4,4’-dimethyl-transstilbene dicarboxylate (SDE), 1,6-hexanediol (HD), and 1,4-butanediol (BD) was performed. These (co)polyesters demonstrated thermal stability of greater than 360 °C and liquid crystalline morphologies, confirmed using polarized optical microscopy, with melting temperatures ranging from 253 – 196 °C. Dynamic mechanical analysis afforded moduli ~1000 MPa and tensile data resulted in a Young’s modulus of the same magnitude. Melt rheology was also performed and showed the characteristic shear thinning expected of liquid crystalline polyesters. To investigate the effect of comonomer composition on the thermal transitions and liquid crystallinity, polyesters containing diols of varying alkyl spacer length and an aliphatic diol with stereochemistry were also synthesized. Introduction Thermotropic liquid crystalline polyesters are a unique class of high performance polymers that exhibit order during melt processing above the crystalline melting point. 1 Mesogenic units, extended aromatic sequences, within the main chain promote the formation of this ordered morphology, often resulting in anisotropic, high modulus structures. These mesogenic units are typically either present in the backbone of the polymer or as pendant groups resulting in main-chain and side-chain liquid crystalline polymers respectively. Liquid crystalline polymers demonstrate outstanding dimensional stability at high temperatures, resistance to chemicals and corrosion, exhibit low dielectric constants, and high moduli over a wide temperature window. 2 Liquid crystalline polymers also exhibit shear thinning during melt processing, which facilitates melt extrusion and injection molding for product formation. Based on these synergistic attributes, liquid crystalline polymers offer versatility in aerospace coatings, adhesives, displays, and structures. Previous literature indicates that the incorporation of stilbene units into the polymer backbone results in liquid crystalline polymers, classifying stilbenes as mesogens. 3 It is also established that stilbenes undergo a [2 + 2] cycloaddition reaction in the presence of ultraviolet (UV) light. 4 This reaction results in photo-induced crosslinking; a nondegradative process with the ability to provide enhanced thermomechanical properties. 5 Utilizing both of these important characteristics allows the tailoring of polymers