This work reveals the influence of pendant hydrogen bonding strength and distribution on self-assembly and the resulting thermomechanical properties of A-AB-A triblock copolymers. Reversible addition-fragmentation chain transfer polymerization afforded a library of A-AB-A acrylic triblock copolymers, wherein the A unit contained cytosine acrylate (CyA) or post-functionalized ureido cytosine acrylate (UCyA) and the B unit consisted of n-butyl acrylate (nBA). Differential scanning calorimetry revealed two glass transition temperatures, suggesting microphase-separation in the A-AB-A triblock copolymers. Thermomechanical and morphological analysis revealed the effects of hydrogen bonding distribution and strength on the self-assembly and microphase-separated morphology. Dynamic mechanical analysis showed multiple tan delta (δ) transitions that correlated to chain relaxation and hydrogen bonding dissociation, further confirming the microphase-separated structure. In addition, UCyA triblock copolymers possessed an extended modulus plateau versus temperature compared to the CyA analogs due to the stronger association of quadruple hydrogen bonding. CyA triblock copolymers exhibited a cylindrical microphase-separated morphology according to small-angle X-ray scattering. In contrast, UCyA triblock copolymers lacked long-range ordering due to hydrogen bonding induced phase mixing. The incorporation of UCyA into the soft central block resulted in improved tensile strength, extensibility, and toughness compared to the AB random copolymer and A-B-A triblock copolymer comparisons. This study provides insight into the structure-property relationships of A-AB-A supramolecular triblock copolymers that result from tunable association strengths.
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Melt acidolysis polymerization of hydroquinone with a kinked monomer, biphenyl 3,4′-bibenzoate, afforded a novel liquid crystalline polymer (LCP), poly( p -phenylene 3,4′-bibenzoate) (poly(HQ-3,4′BB)). Selection of hydroquinone diacetate (HQ a ) or hydroquinone dipivilate (HQ p ) facilitated either a tan or white final polymer, respectively. 1 H NMR spectroscopy confirmed consistent polymer backbone structure for polymers synthesized with either derivative of hydroquinone. Poly(HQ-3,4′BB) exhibited the onset of weight loss at about 480 °C, similar to commercially available Vectra® LCP. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) revealed a glass transition temperature ( T g ) of 190 °C and an isotropic temperature ( T i ) near 330 °C. The observation of a melting temperature ( T m ) depended upon the thermal history of the polymer. Wide-angle X-ray scattering (WAXS) and polarized optical microscopy (POM) confirmed the formation of a nematic glass morphology after quench-cooling from the isotropic state. Subsequent annealing at 280 °C or mechanical deformation induced crystallization of the polymer. Rheological studies demonstrated similar shear thinning behavior for poly(HQ-3,4′BB) and Vectra® RD501 in the power law region at 340 °C. Zero-shear viscosity measurements indicated that HQ a afforded higher melt viscosities after identical polymerization conditions relative to HQ p , suggesting higher molecular weights.more » « less
