(364c) Non-Isocyanate Polyurethane Thermoplastic Elastomer: Amide-Based Chain Extender Yields Enhanced Nanophase Separation and Properties in Polyhydroxyurethane

Polyurethane (PU) represents an important class of commodity polymers with a broad range of uses such as foams, elastomers, coatings, films and adhesives. It is produced from a step-growth reaction between isocyanates and alcohols.¹ Isocyanates are facing increasing regulatory scrutiny across the world, e.g., from the United States Environmental Protection Agency and the European Union REACH regulation, necessitating the search for alternative route to PU or PU-like material without employing isocyanates.

Cyclic carbonate aminolysis is a very promising chemistry to produce polyhydroxyurethane (PHU), a PU-like material with an additional hydroxyl group adjacent to every urethane linkage.^2,3 Significant fractions of commercial PU are segmented type of copolymers, but most PHU studies to-date often focus on single-phase thermoplastics or crosslinked thermosets.^1-3 There are currently very limited studies on segmented, nanophase-separated PHUs and how to control and tune their properties.^4-7 Very recent studies by Torkelson and coworkers on segmented PHUs demonstrate that the hydroxyl groups and soft-segment choice play a critical role on the development of nanophase separation and elastomeric character in polyether-based PHUs.^4,5 Polyethylene oxide (PEO)-based PHUs are single-phase materials due to significant phase mixing whereas polytetramethylene oxide (PTMO)-based PHUs exhibit nanophase separation with broad interphase having a wide range of local compositions.⁴ Furthermore, the thermal and mechanical properties of segmented PHUs still needs improvement to match those of conventional thermoplastic polyurethane (TPU) elastomers.^4-9 Achieving competitive properties in PHU elastomer is essential to produce replacement materials for isocyanate-based TPU elastomer.

Here, we report our approach in designing non-isocyanate polyurethane (NIPU) thermoplastic elastomers with properties competitive to conventional TPU elastomer by incorporating amide-based, diamine diamide (DDA) chain extender. Above a certain hard-segment concentration, nanophase separation can be achieved in PEO-based PHUs, a system previously known to yield only single phase PHUs, by using DDA chain extender. In the PTMO-based system, we obtained PHU elastomers with wide, relatively temperature-independent, rubbery plateau regions with flow temperature up to 200 °C. These PTMO-based PHUs exhibit tunable mechanical properties with Young’s modulus ranging from 6.6 to 43.2 MPa, tensile strength ranging from 2.4 to 6.7 MPa, and elongation at break of ~300%. They also possess elastomeric recovery similar to that of conventional TPUs. The amide linkages in DDA chain extender lead to stronger hydrogen bonding in the hard segment, thereby overcoming the competing effect of intersegmental hydrogen bonding from the hydroxyl groups. The regular structure of amide-based segments in the DDA chain extender helps to drive hard-segment crystallization, thereby resulting in significantly enhanced nanophase separation and thermal properties over PHUs made without DDA chain extender. Incorporation of DDA chain extender is a powerful strategy to produce PHU thermoplastic elastomer with competitive properties rivaling those of conventional TPUs.

References

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