Enhanced Mechanical Relaxation below the Glass Transition Temperature in Partially Supramolecular Networks

Cross-linked polymers in which part of the covalent cross-links have been replaced by reversible cross-links have potential application in coatings with improved stress relaxation. Here we present a detailed study of materials based on copolymers of e-caprolactone and l-lactic acid, containing different ratios of covalent and ureidopyrimidinone (UPy) dimer cross-links. Their thermal and mechanical properties were tested by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and dielectric relaxation spectroscopy (DRS). By extrapolation of the α* relaxation process in DMA (arising from dynamics of the reversible UPy cross-links) to temperatures below the glass transition temperature (Tg), it was shown that the reversible cross-links contribute significantly to the relaxation of stress below Tg that was observed in creep measurements.