(336i) Physically Crosslinked Structures of Polyurethanized Poly(?-methyl-?-caprolactone)

Introducing physical crosslinkings into polymers can provide interesting features including reprocessability, recyclability, and self-healing ability especially for liquid polymer materials. Hydrogen bonding is one of the most important physical crosslinkings establishing such intriguing features, structures, and dynamics in various polymers. Also, hydrogen bonding can be extensively employed for the creation of crosslinked polymeric materials possessing versatility, anisotropy and directionality, and reversibility. One of the most studied elastomers with physical crosslinking is polyurethane, which can form hydrogen bonding between urethane groups, which, in fact, is the first-developed reversible crosslinked polymer.

In this work, we analyzed the viscoelastic properties of thermoplastic polyurethane elastomers (TPU) with crosslinkings through hydrogen bonding. The viscoelastic properties were discussed at different concentrations of the hydrogen-bonded urethane groups fabricated by controlling the molecular weight of prepolymers. The master curve was also constructed to investigate the overall mechanical characteristics of the polymers.

Poly(γ-methyl-ε-caprolactone) (PMCL) was selected as prepolymers for the synthesis of TPU. Liquid PMCL possesses amorphous structures and low glass transition temperature (T_g = -61ºC), where hydrogen-bonded crosslinkings could become the trigger for the solidification of PMCL. The hydrogen-bonded crosslinkings in the amorphous TPU were carefully investigated. PMCL diols were synthesized via ring-opening polymerization (ROP) of γ-methyl-ε-caprolactone (MCL) monomers using 1,4-benzenedimethanol (BDM), a difunctional initiator, and Sn(Oct)₂, a traditional ROP catalyst. 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 PMCL diols with hexamethylene diisocyanate (HDI) was carried out to obtain PMCL-based polyurethanes (PU/MCL). Each PU/MCL was named as 2U and 3U depending on the molecular weight of the PMCL prepolymers. Neat PMCL without urethane groups was also prepared for comparison.

The glass transition temperature (T_g) of PU/MCLs and PMCL were analyzed using DSC. As the molecular weight of the prepolymer decreased, T_g increased and 2U exhibited the highest T_g = -51.5ºC. This suggested that the higher the concentration of urethane groups becomes, the smaller the free volume of the polymer chains becomes due to the cohesion of the urethane groups.

As for the viscoelastic property, master curves of the polymers were rheologically constructed using the standard Williams-Landel-Ferry (WLF) model based on the time-temperature superposition principle. Reference temperature T_s of the samples was set at a temperature at least 15ºC higher than T_g. In addition, the temperature dependence was studied on each sample and the softening temperature was defined by detecting the intersection point of G’ and G’’.

The molecular weight between physical crosslinking points M_p, consisting of the entanglement of the polymer chains with hydrogen bonding points, was calculated investigating the plateau region of G’. PU/MCLs except for 10U (M_p = 3.6k) with the lower concentration of urethane groups, presented smaller M_p (= 2.8-3.1k) than neat PMCL (M_p = 3.2k). It suggests that the high concentration of urethane groups could contribute to increase the physical crosslinking points. In addition, M_p could affect the solidification of PMCL considering the fact that only 10U had an intersection of G’ and G’’ in the frequency scan at room temperature, while the softening temperature was 9.4ºC, which was lower than room temperature. In the extremely low frequency region, it was found that the longest relaxation time and the molecular weight of PU/MCL rather than that of the PMCL prepolymer were positively correlated except for 2U. This was due to the fact that M_p of 2U was smaller than the molecular weight of prepolymers in 2U, which was 2.3k analyzed using ¹H NMR. It suggested that in 2U, the exchange of hydrogen bonding could strongly restrain the relaxation of molecular chains.