(177n) Self-Healing Poly(?-methyl-?-caprolactone) Elastomer with Crosslinking through Hydrogen Bonding

Self-healing materials are widely recognized materials for a sustainable society to realize the reduction of waste and the extension of the lifetime of the products. For self-healing elastomers, made by e.g. silicone or polyurethane, the self-healing mechanism relies on the interdiffusion of the polymer chains as well as the reorganization of reversible bonding in the polymers when damaged especially at the interface. The intrinsic self-healing ability may also lead to the reproducibility of the materials. Most of such self-healing polymers that have been recently developed, however, are regarded as unsustainable due to their feedstock composed of petroleum-based polymers. Therefore, renewable and eco-friendly biomass materials have recently gained a lot of attention to deal with the shortage of fossil resources and thereby solving the deterioration of the environment.

Poly(γ-methyl-ε-caprolactone) (PMCL) was used and investigated in this work, synthesized from p-cresol, which was a component of bio-oils produced by depolymerization of lignin. Lignin has been viewed as an enticing starting material owing to its abundant supply as a byproduct obtained through common industrial processes such as paper-pulp refining and lignocellulose-ethanol processing.

PMCL is in a liquid state at room temperature as its glass transition temperature is T_g = -62ºC and its amorphous structures can derive from the random coexistence of enantiomeric monomers in its molecular chains. In addition, its low entanglement molar mass is suitable for the development of tough elastomers.

In this work, we have developed a self-healing elastomer by polyurethanization of PMCL with crosslinkings through hydrogen bonding formed by urethane groups. We discussed the relationship between the self-healing ability and the viscoelastic property of polyurethanized PMCL (PU/MCL) by varying the concentration of urethane groups with the control of the molecular weight of PMCL prepolymers.

The bulk polymerization of γ-methyl-ε-caprolactone (MCL) using 1,4-benzenedimethanol (BDM), a difunctional initiator, and Sn(Oct)₂, a traditional catalyst for ring-opening transesterification polymerization, produced an α,ω-hydroxy telechelic polymer. The molecular weight of PMCL was changed and controlled to become 2k, 3k, 5k, 8k, and 10k, by adjusting the ratio of monomers to initiators. The polyurethanization of the synthesized telechelic polymers with hexamethylene diisocyanate (HDI) was carried out to obtain PMCL-based polyurethanes (PU/MCL). Each PU/MCL was named 2U and 3U, depending on the molecular weight of the PMCL prepolymers.

The concentration of the urethane groups was studied using FTIR. The peak of the N-H stretching vibrations was observed from 3385 to 3450 cm^-1. The peak area increased with the decrease in the molecular weight of PMCL prepolymers, suggesting that the concentration of the urethane groups could be controlled by changing the molecular weight of the PMCL prepolymers.

For the self-healing analysis, film specimens were cut in half using a razor blade. The cut specimens were then put in contact immediately again, and subsequently left at room temperature for 24 h. The tensile testing was conducted for the self-healed PU/MCL films, and the self-healing rate, defined as the ratio of the tensile strengths of the healed film and the original film, was calculated and analyzed. It was found that all PU/MCLs presented self-healing ability, and that especially 2U, 3U, and 10U reached 100% or more in the self-healing rate. It was also found that the self-healing rate increased with the decrease in the molecular weight of PU/MCLs.

The master curves through G’ and G’’ were constructed, revealing the longest relaxation time and M_p, the molecular weight between physical crosslinking points caused by the entanglement of polymer chains and hydrogen-bonding points. It was found that PU/MCLs with a high concentration of urethane groups exhibited smaller M_p as compared with pure PMCL. Also, the longest relaxation time was negatively correlated with the self-healing rate: as the relaxation time decreased, the self-healing with the interdiffusion of the polymer chains increased.