The correlation between interfacial healing and microstructure in highly branched and hydrogen-bonded segmented polyurethanes

Thermoplastic polyurethane elastomers (TPUs) are appealing to the industry world because they combine a broad range of properties and ease of production. By introducing supramolecular moieties in the polymer architecture, a dynamic reversible network is formed and self-healing ability is imparted to the material. This is undoubtedly beneficial since it extends the lifetime of components and consequently reduces resources consumption. Despite many reports of efficiently healing polyurethanes, no research has focused on understanding the relationship between interfacial healing and TPUs microstructure. To this end we synthesized by one-shot method a set of segmented polyurethanes by reacting a highly branched polyol, 2-ethylhexane-1,3-diol (EHD) and three commercially available diisocyanates. The aromatic ones, 4,4′-Methylenebis(phenyl isocyanate) (MDI) and 1,4-Phenylene diisocyanate (PPDI), resulted in polymers with a marked elastomeric character that was demonstrated by tensile testing. On the contrary, the aliphatic Dicyclohexylmethane-4,4'-Diisocyanate (HMDI) produced a typical thermoplastic polymer with much lower mechanical properties. Dynamic Mechanical Analysis revealed a similar Tg in all cases which was attributed to the polyol soft segment mobility. The experimental curves also indicated the presence of microphase separation in the aromatic polymers and of extensive phase mixing in the aliphatic one instead. The morphological difference justified thus the contrast observed in the mechanical behavior. Attenuated Total Reflectance-imaging clearly showed the separation in a soft matrix and dispersed hard domains in the aromatic polymers and also provided quantitative measurement of the microphase separation degree. Additionally, Fourier Transform Infrared Spectroscopy demonstrated the presence of extensive hydrogen bonding in all the TPUs. Rheological measurements were conducted on the MDI containing polymer and its relaxation spectrum was calculated. A deconvolution protocol was applied to separate the main relaxation processes, which were successfully correlated with both soft and hard phase dynamics. The short time relaxation was attributed to the highly reversible supramolecular network of the soft matrix, while processes at longer times were associated with the stronger hydrogen bonds and aromatic interactions of the hard domains. Fracture mechanics was used to assess intrinsic healing properties. At room temperature the phase separated materials showed a limited but immediate recovery of properties, validating the hypothesis of a rapidly relaxing soft phase that allows immediate hydrogen bond reformation across the interface. This was confirmed by the absence of healing in the polymer characterized by a mixed phase morphology, which unavoidably reduces the soft segments mobility and prevents bond reformation. At higher healing temperatures and longer times, healing efficiencies up to 60% were obtained for the aromatic polymers and explained by the occurrence of additional molecular processes promoting the recovery of properties. The importance of isocyanate selection and resulting microphase separation in the production of self-healing polymers were thus demonstrated. Additionally, the main molecular relaxation processes were identified and correlated with the underlying healing mechanisms. This study represents thus a step forward in the development of self-healing TPUs and their commercial availability.