Microstructural organization of polydimethylsiloxane soft segment polyurethanes derived from a single macrodiol
Graphical abstract
Introduction
It has long been known that poly(dimethylsiloxane) (PDMS) elastomers exhibit outstanding biocompatibility and biostability, but display mechanical properties that are unsuitable for many medical device applications [1], [2], [3]. On the other hand, segmented polyurethane (PU) and poly(urethane urea) copolymers have been widely used as mechanically robust elastomers, but typical polyether and polyester soft segment chemistries are susceptible to oxidative and/or hydrolytic degradation in vivo, particularly in components involved in longer term implants [4], [5], [6], [7]. Early attempts at incorporating PDMS soft segments in PUs, to combine desirable mechanical performance (arising from hard segment microphase separation) with the biostability and biocompatibility of PDMS, did not meet with success due to the difficulty in synthesizing acceptably high molecular PU copolymers from a mixture of polar and non-polar monomers [8], [9], [10], [11], [12]. However, significant advances have been made in the past 10 years, with the discovery that high molecular weight (predominately) PDMS soft segments [α,ω-bis(6-hydroxyethoxypropyl) PDMS] could be incorporated into segmented polyurethane copolymers along with a relatively small amount of a second polyether comonomer [poly(hexamethylene oxide), typically 20% of the total soft segment], creating a family of segmented PUs with excellent and tunable mechanical properties that have shown great promise in blood-contacting biomedical applications [13], [14], [15], [16], [17], [18], [19], [20], [21]. In a broad sense, the PHMO enhances the “compatibility” between hard segments containing polar urethane moieties and the non-polar siloxane soft segments.
These ethoxypropyl PDMS/PHMO soft segment copolymers, having 4,4′-methylenediphenyl diisocyanate (MDI) and 1,4-butanediol (BDO) as the hard segments, have been found to exhibit a unique three phase microstructure: a PDMS matrix phase, hard domains composed of hard segments, and a mixed phase containing the ether end group segments (ethoxypropyl) of the macrodiol, PHMO, and some dissolved hard segments [19], [20]. The mixed phase serves to ‘compatibilize’ the non-polar PDMS and the dispersed polar hard domains.
To further enhance the biostability of PUs derived from the α,ω-bis(6-hydroxyethoxypropyl) PDMS macrodiol, reduction of PHMO in the soft segment mixture is desirable. Recently [22], a synthetic procedure (summarized in the experimental section) was uncovered that leads to high molecular weight MDI–BDO hard segment PUs using only α,ω-bis(6-hydroxyethoxypropyl) PDMS macrodiol in the reaction. In an early report on these copolymers, we demonstrated that the in vitro oxidative biostability was outstanding compared to segmented PUs with other soft segment chemistries under consideration for blood-contacting applications [23]. The phase-separated microstructure and morphology of PUs derived solely from the α,ω-bis(6-hydroxyethoxypropyl) PDMS macrodiol have not however been investigated previously. In the present study, we synthesize a series of segmented polyurethanes (with varying hard segment content) from MDI, BDO and the α,ω-bis(6-hydroxyethoxypropyl) PDMS macrodiol [24], and explore the phase-separated morphology, thermal properties, and unlike segment demixing.
Section snippets
Materials
The polyurethanes used in the present study were synthesized using 4,4′-methylenediphenyl diisocyanate and 1,4-butanediol as the hard segment, and an α,ω-bis(6-hydroxyethoxypropyl) PDMS macrodiol as the soft segment (see Fig. 1). MDI and BDO were purchased from Sigma Aldrich (98% purity) and the α,ω-hydroxy-terminated PDMS was synthesized at AorTech Biomaterials. The hydroxyterminated poly(dimethylsiloxane) was subjected to thorough fractional distillation on a thin film evaporator and the
Nanoscale segregation
Tapping mode phase images of two representative PU copolymer films (10035 and 10052 at a tapping force corresponding to rsp = 0.3) are displayed in Fig. 2. As determined in prior investigations, the brighter regions (yellow) represent hard domains and the darker regions (brown) the soft segment–rich matrix. The hard domains are dispersed and are ∼10 nm in diameter, and we do not observe any measurable differences between the morphologies of the various PDMS-based PUs under investigation here.
Summary
Segmented PUs with soft segments derived from α,ω-bis(6-hydroxyethoxypropyl) PDMS macrodiol were synthesized with varying hard segment contents, in order to investigate the microstructural organization of these unique copolymers. From the combined results from AFM, DMA and DSC experiments, these copolymers, like their mixed soft segment counterparts, were found to exhibit three microphases: a PDMS phase, hard domains, and a mixed phase containing PDMS ether end group segments and some dissolved
Acknowledgments
The authors would like to thank Amanda McDermott for her assistance with the WAXD experiments, Prof. Paul C. Painter for helpful discussions on FTIR spectra fitting, and Prof. Evangelos Manias for using his AFM.
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