(361d) Tuning Nanophase Separation Behavior in Segmented Polyhydroxyurethanes Via Judicious Choice of Soft Segment

Segmented polyurethanes (PUs) are composed of alternating sequences of soft and hard blocks. Depending on the composition of these blocks, they can exhibit properties ranging from soft elastomers to hard plastics while retaining the processing characteristics of thermoplastics. The soft blocks are typically long, flexible molecules with a glass transition temperature (T_g) below room temperature while the hard blocks have a T_g above room temperature and are composed of diisocyanate condensed with small molecule diol. Segmented PUs typically exhibit excellent nanophase separation due to the incompatibility between the soft and hard domains as well as the hydrogen bonding between polar urethane units in the hard domain. Varying degrees of nanophase separation are expected depending upon the structures of diisocyanates, soft segments, and chain extenders as well as overall material compositions.^1,2

The increasing regulatory scrutiny on isocyanates in recent years has driven significant investigations for alternative pathways to PU or PU-like materials with cyclic carbonate aminolysis to produce polyhydroxyurethane (PHU) being one of the most promising candidates.³ Polyhydroxyurethane is analogous to PU with the exception of additional primary and secondary hydroxyl groups adjacent to the urethane bonds. Unlike segmented PUs, the nanophase separation behavior in segmented PHUs is very different due to the presence of additional hydroxyl groups in the hard segment.^3,4 Torkelson and coworkers⁵ recently reported the synthesis and characterization of segmented PHUs using several polyether-based soft segments. They showed that the hydroxyl groups caused significant phase mixing in PHUs with polyethylene oxide (PEO)-based soft segment due to a high degree of intersegmental hydrogen bonding to ether oxygen in the soft segment. This hydrogen bonding can be suppressed with polypropylene glycol (PPG)-based soft segments with sterically hindered oxygen atoms and polytetramethylene oxide (PTMO)-based soft segments with dilution of oxygen atom content. The suppression of hydrogen bonding to the soft segment produces nanophase-separated PHUs. In PTMO-based PHUs, the hydrogen bonding is suppressed but not completely eliminated leading to the formation of nanophase-separated systems with broad interphases having a wide range of local compositions with potential application as broad-temperature-range damping materials. To date, no study has yet demonstrated the ability to tune nanophase separation behavior in segmented, phase-separated PHUs to the level obtained in segmented PUs.

In this study, we demonstrate that nanophase separation behavior in segmented PHU copolymers can be significantly tuned via a judicious choice of soft segment. A series of PHUs were synthesized using two different soft segments namely PTMO and polybutadiene-co-acrylonitrile (PBN) and characterized with small-angle X-ray scattering (SAXS), dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), atomic force microscopy (AFM) and infrared spectroscopy (IR).

SAXS demonstrates that these PHUs possess nanophase-separated morphology with interdomain spacings of 10-16 nm independent of their soft segment compositions. DMA results also support the presence of nanophase-segregated structure in all PHUs with soft-segment glass transition temperature (T_g) well below room temperature and flow temperature above room temperature indicative of the hard segment undergoing glass transition. However, DMA reveals very significant difference between PTMO- and PBN-based PHUs. In PTMO-based PHUs, the storage modulus (Eâ??) show a very gradual temperature-dependent decrease and high tan δ (â?¥ 0.3) values over a broad temperature range (~70 °C) characteristic of nanophase-separated system with broad interphases having a wide range of local composition. In sharp contrast, DMA results of PBN-based PHUs display two distinct, step-wise transitions in Eâ?? with the corresponding sharp, well-defined peaks in tan δ indicative of nanophase-separated systems with a much sharper interphase similar to the case of isocyanate-based, segmented PUs and nanostructured block copolymer. DSC results also showed distinct soft segment and hard segment thermal transitions in the case of PBN-based PHUs while no discernible transition was observed for PTMO-based PHUs due to their broad interphases.

AFM phase imaging provides strong evidence of distinct soft and hard domains with sharp interphases for PBN-based PHUs whereas no distinct domain is observed in AFM images of PTMO-based PHUs due to the nature of their nanophase-separated structures despite evidences of nanophase separation from SAXS and DMA. IR characterization of the carbonyl region shows that PTMO-based PHUs has a comparable level of free, non-hydrogen bonded carbonyl (~1730 cm^â??1) and hydrogen bonded carbonyl (~1700 cm^-1) while hydrogen-bonded carbonyl is the dominant absorbance in PBN-based PHUs.

The improved nanophase separation in PBN-based PHUs also leads to improved tensile properties. PBN-based PHUs exhibit tensile strength values from 0.8 to 3.4 MPa, which are higher than those obtained by PTMO-based PHUs from 0.4 to 0.8 MPa. Using soft segment with no possibility of intersegmental hydrogen bonding (as in the case of PBN-based soft segment) significantly improves nanophase separation in segmented PHUs relative to PTMO-based soft segment. This study demonstrates the importance of judicious soft segment choice to significantly tune nanophase separation behavior in segmented PHUs to yield systems with a much sharper interphase similar to isocyanate-based, segmented PUs.

References:

1) Yilgor, I.; Yilgor, E.; Wilkes, G. L. Polymer 2015, 58, A1-A36.

2) Holden, G.; Kricheldorf, H. R.; Quirk, R. P., Thermoplastic Elastomers. (Chapter 2: Thermoplastic Polyurethane Elastomers) 3rd ed.; Hanser: 2004.

3) Guan, J.; Song, Y.; Lin, Y.; Yin, X.; Zuo, M.; Zhao, Y.; Tao, X.; Zheng, Q. Ind. Eng. Chem. Res. 2011, 50, 6517-6527.

4) Blattmann, H.; Fleischer, M.; Bähr, M.; Mülthaupt, R. Macromol. Rapid Commun. 2014, 35, 1238-1254.

5) Leitsch, E. K.; Beniah, G.; Liu, K.; Lan, T.; Heath, W. H.; Scheidt, K. A.; Torkelson, J. M. ACS Macro Letters 2016, 5, 424-429.