Synthesis and characterization of polyurethanes crosslinked by polyrotaxanes consisting of half-methylated cyclodextrins and PEGs with different chain lengths
Graphical abstract
Introduction
Polyurethanes (PUs) are among the most important macromolecules because their applications are widely spread industrially and commercially. The applications are based on their characteristic properties originated from the variation of starting materials, preparation conditions, and etc. [1], [1]a), [1]b), [1]c), [1]d), [1]e), [1]f), [1]g), [1]h), [1]i). The type of crosslink point also plays a critical role for the appearance of elastic properties. Therefore, novel design of the crosslink point should enable us to develop unique PU elastomers.
Recently, supramolecular materials have attracted much attention because of their distinctive features [2], [2]a), [2]b), [2]c), [2]d), [2]e), [2]f). A rotaxane is a molecule where a ring molecule is interlocked by an axle molecule with bulky terminal cap groups that prevent the ring molecule from release [3]. Among the interesting properties of the rotaxane based on the interlocked structure, free sliding of the ring molecule along the axle molecule is remarkable. The use of the sliding makes it possible to construct molecular machines such as molecular shuttles and switches [4], [4]a), [4]b), [4]c).
Since the first discovery of polyrotaxanes (PRXs) consisting of α-cyclodextrin (CyD) and poly(ethylene glycol) (PEG) by Harada et al. [5], a lot of PRXs have been reported [6], [6]a), [6]b), [6]c), [6]d), [6]e), [6]f), [6]g). Ito et al. reported an elegant sliding network gel based on a PRX consisting of α-CyD and PEGs with long chain lengths and their physical properties in detail [7], [7]a), [7]b), [7]c), [7]d), [7]e), [7]f). Hadziioannou et al. also reported similar sliding gels [8], [8]a), [8]b), [8]c). PRXs for medical use were strenuously investigated by Yui et al. [9], [9]a), [9]b), [9]c), [9]d). Takata et al. also synthesized a PRX crosslinked by 4,4′-Diphenylmethanediisocyanate [10], [11]a), [11]b). Advanced functional polymers crosslinked by rotaxanes has been summarized in reviews [11]a), [11]b), [11].
PRXs modified with a variety of polymers have also been published. Molecular rope curtains based on PRXs reported by Araki et al. shows unique and excellent liquid crystal behaviors [12]b), [12]c), [12], [12]a). Takeoka et al. synthesized a PRX bearing poly(N-isopropylacrylamide) and characterized its sliding-gel property [13]. Ito et al. has published a sliding graft copolymer, in which many linear poly(ε-caprolactone) side chains are bound to cyclodextrin rings of PRX [12a].
As described above, studies of various kinds of PRXs have so far been published. Especially, the unique sliding property inspired us with an idea of incorporation of the polyrotaxane as crosslink points instead of the hard segment domains of PUs. Polyurethanes crosslinked by CyDs have been published [14]d), [14]e), [14], [14]a), [14]b), [14]c). However, no report of the polyurethane properties based on the structure of polyrotaxanes as the crosslink points has been published in our best knowledge except our latest report [15]. Therefore, we describe herein the synthesis of three PUs (PRX1500Me-PU, PRX4000Me-PU, and PRX6000Me-PU) crosslinked by PRXs, which consist of half-methylated α-cyclodextrins (CyDs) and poly(oxyethylene)glycols with different chain lengths (PEG1500, PEG4000, and PEG6000), and their characteristics using the results of the measurements of gel fraction, attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy, differential scanning calorimetry (DSC), dynamic viscoelastic test and tensile test.
Section snippets
Materials
α,ω-Bis(aminopropyl)poly(oxyethylene)#1500 (PEG1500; Mn = ca. 1500) was provided by NOF Co., Ltd. and used as received. Poly(oxyethylene)#4000 (PEG4000; Mn = ca. 3100) and poly(oxyethylene)#6000 (PEG6000; Mn = ca. 7500) were purchased from Nacalai Tesque Inc. and dried in vacuo prior to use. Polyrotaxanes consisting of the PEGs and α-cyclodextrin were synthesized according to the literature [5], [16]. Form the results of 1H NMR measurements, the numbers of CyDs for PRX1500, PRX4000 and PRX6000
Gel fraction and degree of swelling
Gel fractions and degrees of swelling of the PUs were summarized in Table 1. All of the PUs showed similar gel fraction values, indicating that the concentration of urethane linkage between the prepolymer and PRX in the PUs is at the same level. In addition, all of the PUs left their gel components even in DMSO. In the PU synthesis, the isocyanate index was 2. If ideal reaction occurred, no crosslinks between the PRXs be formed. Consequently, the gel fraction results suggest that the crosslinks
Conclusion
We have demonstrated the synthesis and characterization of three PUs crosslinked by PRXs consisting of half-methylated CyDs and PEGs with different chain lengths. The results are follows. PRX1500Me with the higher filling ratio of CyD and shorter length of the axle enhances the reorganized-crystallization of the soft segment chains because of the formation of the pure domain for the soft segment chains. The CyDs of PRX6000Me with the lower filling ratio of CyD and longer length of the axle act
Acknowledgment
This work was supported by the Eno Science and Technology Foundation, and Scientific Research C (No. 20550191) of a Grant-in-Aid for Scientific Research from The Ministry of Education, Culture, Sports, Science and Technology, Japan. We would like to thank Dr. Kojio for the useful discussion for the results of the DSC, dynamic viscoelastic measurement, and tension test.
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