Polyurethane (PU) is one of the most important commodity polymers in the world accounting for 5% of the global polymer market. Its total production was estimated to be 14 million tons in 2010 and is projected to reach 18 million tons by 2016. It is produced from step growth reaction of polyisocyanates and polyol. PU’s attractiveness stems from its fast reaction kinetics and the presence of micro/nanophase separation between hard and soft segments as well as hydrogen bonding between urethane linkages in the hard segment. The availability of a diverse range of precursors gives rise to a large permutation of commercial products with a wide range of properties. Consequently, PU can be found in many applications including flexible and rigid foams, coatings, adhesives, fibers, elastomers and biomedical implants. However, in recent years, there have been increasing regulatory constraints on the use, transport and handling of isocyanates. This has led to significant research into alternative chemistries for PU material formation.
Among several alternatives, cyclic carbonate aminolysis has emerged as a very promising route to make PU-like material. Reaction between five-membered ring carbonates with amines affords polyhydroxyurethane (PHU), which possesses urethane bonds with additional primary or secondary hydroxyl groups located adjacent to the urethane bonds. As reviewed in several reports (Guan et al., Ind. Eng. Chem. Res. 2011, 50, 6517-6527; Kathalewar et al., RSC Adv. 2013, 3, 4110-4129; Blattmann et al. Macromol. Rapid Comm., 2014, 35, 1238-1254), there are numerous studies on the synthesis of PHUs as non-isocyanate polyurethane (NIPU) material by various groups in the last two decades. These studies revolve around the production of functional cyclic carbonate monomers, catalyst evaluation, the production of single-phase material and crosslinked network from carbonated form of epoxy monomer such as Bisphenol A diglycidyl ether (BADGE) or epoxidized renewable resources such as soybean oil. To the best of our knowledge, no research has been published on the production of segmented, multicomponent and nanophase-separated PHU as a potential replacement to thermoplastic PU elastomer.
In this work, we present the first successful synthesis and characterization of thermoplastic PHU elastomer. We found that the selection of soft segment molecule has a profound impact on the development of nanophase-separated structures and on the final material properties. This is inherently different from the common wisdom established in traditional PU formulation because the hydroxyl groups in PHU can participate in hydrogen bonding to the soft segment. When a poly(ethylene oxide) (PEO)-based soft segment is used to synthesize PHU, hydrogen bonding between hydroxyl units in the hard segment and oxygen atoms in the soft segment backbone leads to phase mixing rather than nanophase separation resulting in material with poor mechanical properties. This phase mixing can be overcome by diluting the oxygen atoms by using poly(propylene oxide) (PPO)- and poly(tetramethylene oxide) (PTMO)-based soft segments.
The suppression of hydrogen bonding leads to nanophase-separated morphology as confirmed by atomic force microscopy (AFM) phase imaging and small angle X-ray scattering (SAXS) measurement. The nanophase separation results in robust thermoplastic elastomeric behavior with tunable mechanical properties. The PPO-based PHUs exhibited Young’s modulus ranging from 47 to 230 MPa, ultimate strength ranging from 0.5 to 4.3 MPa and elongation at break ranging from 230-1800 %. The PTMO-based PHUs exhibited Young’s modulus ranging from 52 to 89 MPa, ultimate strength ranging from 0.4 to 4.3 MPa and elongation at break ranging from 270 to greater than 2000%.
Thermal property evaluations by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) show two thermal transitions attributed to the soft segment glass transition well below room temperature and the hard segment glass transition as indicated by flow temperature above room temperature. Our results also suggest that thermoplastic PHU elastomer manifests broad hard segment glass transition response due to slight interphase mixing contributed by the hydroxyl groups. The DMA response also differs significantly from that of thermoplastic PU elastomer. This can potentially lead to a new application previously not attainable by conventional PU.