Solid-State Structure of Copolyesters Containing a Mesogenic Monomer

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This study examined the solid-state structure of copolyesters based on smectic poly(diethylene glycol 4,4‘-bibenzoate) (PDEGBB) and non-liquid crystalline poly(diethylene glycol isophthalate) (PDEGI). The isophthalate content was varied from 10 to 50 mol %. Differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, wide-angle X-ray diffraction (WAXD), dynamic mechanical thermal analysis (DMTA), and tapping mode atomic force microscopy (AFM) were employed to characterize the polymers. The combined observations suggest that the PDEGBB-I copolymers fall into two classes depending upon isophthalate content. Copolymers with up to 20% isophthalate content possess some amount of liquid crystalline (LC) character. They exhibit a smectic-to-isotropic transition in DSC, smectic layer reflections in WAXD, and lamellar structures in AFM. It appears that isophthalate groups are excluded from the smectic layers and therefore are very effective in disrupting long-range LC order. Copolymers with 30% or more isophthalate are amorphous. These materials do not exhibit crystalline or LC character after exposure to the thermal histories used in this study. The Fox equation describes the dependence of glass transition temperature ( T g ) from loss modulus on copolymer composition for non-LC glasses and predicts that the T g of glassy PDEGBB is 14 °C higher than the observed T g of the LC glass.