Photocycloaddition at excimer sites in a solid polyester of p-phenylenediacrylic acid

Cascading Retro-Diels-Alder Cycloreversion and Sydnone-Maleimide Based Double 1,3-Dipolar Cycloaddition for Quantitative Thermal Cross-Linking of an Amorphous Polymer Solid

A new approach to polymer cross-linking is investigated using a cascading cycloreversion of a maleimide-furan adduct and a double 1,3-dipolar cycloaddition between a sydnone and maleimide. The cross-linking proceeds quantitatively above 63 °C, despite the polymer possessing no observable glass transition temperature. The resulting polymer network possesses a high thermal stability (>300 °C) due to the irreversibility of the sydnone-maleimide cycloaddition, which releases CO₂ during the cross-linking, driving the reaction. The rigid three-dimensional structure of the bis-maleimide-sydnone cycloadduct produced local free volumes in films, decreasing the dielectric constant of the material.

Thermoresponsive semicrystalline poly(ε-caprolactone) networks: exploiting cross-linking with cinnamoyl moieties to design polymers with tunable shape memory

The overall goal of this study was to synthesize semicrystalline poly(ε-caprolactone) (PCL) copolymer networks with stimuli-responsive shape memory behavior. Herein, we investigate the influence of a cinnamoyl moiety to design shape memory polymer networks with tunable transition temperatures. The effect of various copolymer architectures (random or ABA triblock), the molecular weight of the crystalline domains, PCL diol, (M(w) 1250 or 2000 g mol(-1)) and its composition in the triblock (50 or 80 mol %) were also investigated. The polymer microstructures were confirmed by NMR, DSC, WAXS and UV-vis spectroscopic techniques. The thermal and mechanical properties and the cross-linking density of the networks were characterized by DSC, tensile testing and solvent swelling, respectively. Detailed thermomechanical investigations conducted using DMA showed that shape memory behavior was obtained only in the ABA triblock copolymers. The best shape memory fixity, R(f) of ~99% and shape recovery, R(r) of ~99% was obtained when PCL diol with M(w) 2000 g mol(-1) was incorporated in the triblock copolymer at a concentration of 50 mol %. The series of triblock copolymers with PCL at 50 mol % also showed mechanical properties with tunable shape memory transition temperatures, ranging from 54 °C to close to body temperature. Our work establishes a general design concept for inducing a shape memory effect into any semicrystalline polyester network. More specifically, it can be applied to systems which have the highest transition temperature closest to the application temperature. An advantage of our novel copolymers is their ability to be cross-linked with UV radiation without any initiator or chemical cross-linker. Possible applications are envisioned in the area of endovascular treatment of ischemic stroke and cerebrovascular aneurysms, and for femoral stents.

Two-dimensional correlation analysis study of the photo-induced molecular reorientations of photosensitive polyester film containing 1,4-phenylenediacryloyl units in the backbone

The photochemical reaction and molecular reorientation of a novel photosensitive polyester, poly[oxy(4-n-butyl-3,5-benzoate)oxy-1,4-phenylenediacryloyl] (PPDA-C4BZ), which contains n-butyl side groups and 1,4-phenylenediacryloyl units (PDA chromophores) in the main chain, are reported in detail. We applied two-dimensional (2D) correlation analysis for the infrared (IR) and ultraviolet (UV) absorption spectra of nanoscaled films of PPDA-C4BZ to establish the sequence of the photo-induced segmental reorientations that result from UV irradiation. The photochemical reaction was found to have a greater effect on the polymer's main chains than on its side groups and to induce the reorientation of the polymer molecules. In particular, a cycloaddition process occurs first in the PDA chromophore units and then the local reorientation of the polymer molecules is induced. Namely, such photodimerization of the PDA chromophores induces the molecular reorientations of the PDA chromophores and the benzoate units in the main chain. The photo-induced molecular reorientations occur in the following sequence: photodimerization --> benzoate units --> PDA chromophores --> n-butyl side groups. In addition, a two-dimensional map of the first derivatives of the UV absorption spectra with respect to the exposure energy provided evidence of the formation of head-to-head aggregates (i.e., H-aggregates) of PPDA-C4BZ molecules.