(750f) Influence of Carbonate Molecular Structures on the Morphology and Properties of Non-Isocyanate, Segmented Polyhydroxyurethane Copolymers

Polyurethane (PU) is an important class of commodity polymer with a total global production estimated to reach 18 million tons in 2016. It has wide ranging applications such as in adhesives, coatings, elastomers, foams, implants etc. It is produced from step growth reaction between isocyanates and alcohols with relatively fast reaction kinetics.¹ In recent years, regulatory bodies across the world have tightened the scrutiny on the use, transport and handling of isocyanates in consumer products. These moves have fueled significant investigations into alternative chemistries to produce non-isocyanate PU (NIPU) or PU-like materials.

Polyhydroxyurethane (PHU) derived from cyclic carbonate aminolysis has emerged as a very promising candidate as non-isocyanate PU-like materials. This polymer contains urethane linkages and additional primary or secondary hydroxyl groups adjacent to the urethane bonds. In the last few decades, significant investigations have been devoted toward PHU synthesis with major focus on the production of cyclic carbonate monomers, reaction catalysis, synthesis of single-phase PHUs and crosslinked PHU networks from renewable resources.^1,2 Only two studies have successfully synthesized and investigated segmented, nanophase-separated PHU.^3,4 Segmented PUs are composed of alternating sequences of hard and soft blocks; the soft segment is typically a long, flexible molecule with a glass transition temperature (T_g) below room temperature while the hard segment consists of diisocyanate condensed with small molecule diol with T_g above room temperature.

Recently, Torkelson and coworkers³ reported the synthesis and characterization of nanophase-separated, thermoplastic PHU elastomer from several polyether-based soft segments. Their study indicated that the hydroxyl groups in the PHU hard segments play a critical role in the development of nanophase separation due to their ability to form hydrogen bonding with soft segment ether oxygen. Employing the appropriate soft segment molecules that reduce or minimize the intersegmental hydrogen bonding is critical to obtain materials with robust elastomeric responses. This study only investigated one carbonate molecule as the hard segment. To date, the influence of isocyanate structure on the final properties of segmented PUs has been studied in numerous reports.⁵ In contrast, no study has yet investigated the influence of carbonate molecular architecture in the hard segment on the final properties of segmented PHU copolymers. Such an investigation is needed to advance the understanding of PHU material properties in light of the rising needs of competitive NIPU materials. Structure-property relationship studies enable the design and development of PHU materials with improved properties which might compete with commercially available PU products in the market.

Here, we investigated the influence of hard-segment carbonate structure on the morphology and properties of segmented PHU copolymers. Three carbonates prepared from commercially available epoxy sources are investigated namely divinylbenzene dicyclocarbonate (DVBDCC), bisphenol A dicarbonate (BPADC) and cyclohexane dimethanol dicarbonate (CHDMDC). These carbonates were formulated with two different soft-segment compositions: a polytetramethylene oxide (PTMO)-based soft segment as system where intersegmental hydrogen bonding is permitted and polybutadiene-co-acrylonitrile (PBN)-based soft segment as system where hydrogen bonding is restricted to occur only within the hard segment. Characterizations via small-angle X-ray scattering, dynamic mechanical analysis, infrared spectroscopy, and tensile testing reveal interesting interplays between carbonate structure and soft-segment chemical make-up on the resulting morphologies and properties of segmented PHUs.

In PTMO-based formulation, SAXS shows that only DVBDCC-based PHU possess nanophase-separated morphology with interdomain spacing of 13.8 nm while CHMDC- and BPADC-based PHUs are single phase materials. Tensile testing shows that DVBDCC-based PHU have Youngâ??s modulus of 60 MPa, tensile strength of 5.1 MPa and elongation at break 640 % while other PHUs are not mechanically robust to be tested. DMA results also show that only DVBDCC-PHU is nanophase-separated with soft segment glass transition temperature (T_g) of -65 °C and flow temperature of ~75 °C while other PHUs are single phase with single T_gs. The single phase structure of BPADC-PHU and CHDMD-PHU is likely due to a combination of the molecular flexibility of BPADC and CHDMDC relative to DVBDCC and hydrogen bonding of hydroxyl groups to oxygen atoms in the soft segment.

In contrast with PTMO-based formulation, all PHUs formulated with PBN-based soft segment are nanophase-separated with 10-16 nm interdomain spacings measured by SAXS regardless of carbonate structures employed. Tensile testing shows that DVBDCC-PHU possesses the best tensile properties with tensile strength of 3.4 MPa while BPADC-PHU has tensile strength of 1.1 MPa and CHMDC-PHU is too soft too test as the material softens near room temperature. DMA results of PHUs in this series show that they are nanophase separated with highest flow temperature obtained by DVBDCC-PHU at ~80 °C followed by BPADC-PHU and CHMDC-PHU at 48 °C and 28 °C. These results show that the switch to soft segment with no potential of intersegmental hydrogen bonding is necessary in formulating PHUs using more flexible carbonate molecules. This study highlights the important role of hard-segment carbonate structure and its interaction with soft segment on the resulting morphology and properties of segmented PHU copolymers.

 

References:

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

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

3) 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.

4) Nanclares, J.; Petrovic, Z. S.; Javni, I.; Ionescu, M.; Jaramillo, F. J. Appl. Polym. Sci. 2015, 132, 42492.

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