Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Polyphosphazenes and Cyclotriphosphazenes with Propeller-like Tetraphenylethyleneoxy Side Groups: Tuning Mechanical and Optoelectronic Properties Yi Ren,†,‡ Kai Yang,† Dingying Shan,§ Cuiyan Tong,‡,∥ and Harry R. Allcock*,‡ †
School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China Department of Chemistry and §Department of Biomedical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ∥ Institute of Chemistry, Northeast Normal University, Changchun 130024, China
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ABSTRACT: A new class of polyphosphazene elastomers has been synthesized that combines a −PN− backbone with both 2,2,2trifluoroethoxy and tetraphenylethyleneoxy (TPEO) side groups. The polymer syntheses and photoproperties were first modeled using small molecule cyclic phosphazenes, and these were then applied to the high polymers. The TPEO groups confer useful mechanical and photophysical properties on the polymers. Thus, those polyphosphazenes with the more rigid bidentate TPEO side groups have higher fluorescence quantum yields than counterparts with monodentate TPEO groups. Different TPEO substituents also allow tuning of the side-group interactions to modify the elastomeric properties of the polymers. For example, the two-dimensional TPEO unit is a stronger physical cross-linker than the comparable cross-linkers used in earlier polyphosphazene elastomers. Thus, the elastomeric polymers containing the two-dimensional TPEO exhibit greatly improved mechanical properties compared to the previous ones. Moreover, hybrid materials that combine the new elastomers with singlewalled carbon nanotubes (SWNTs) allow the fabrication of stretchable electronics with mechanically responsive properties.
1. INTRODUCTION Elastomeric behavior is one of the most useful properties of polymeric materials, and this has been utilized for applications that range from membranes, seals, and surface coatings to automotive or aerospace devices and biomedical materials.1−3 When elasticity is combined with photonic behavior, access is obtained to devices that allow physical stress and strain to be monitored by changes in optical behavior. Moreover, such elastomers provide access to stretchable optoelectronic devices. Classical organic elastomers usually possess flexible polymer chains that are cross-linked to provide a retractive force that causes the material to return to its original shape when the stress is removed. In classical elastomers, the cross-linking is accomplished by the formation of covalent bonds. This is a permanent change that makes the elastomer insoluble and prevents recycling by solution or thermal techniques. Alternatively, in phase-separated block or graft copolymers, the rigid domains can serve as noncovalent or meltable crosslink sites that induce elastomeric characteristics in block or graft copolymers.4−6 Compared to covalently cross-linked elastomers, these polymers can be redissolved in solvents or remolded into different shapes, which is an appealing applications characteristic. Recently, noncovalent elastomers have become excellent platforms for building advanced electronics that have self-healable and stretchable characteristics.7−10 © XXXX American Chemical Society
Polyphosphazenes constitute a macromolecular system with a backbone of alternating phosphorus and nitrogen atoms and with two (usually organic) side groups attached to each phosphorus. A number of these polymers developed during recent decades provide property combinations that are difficult or impossible to find in classical all-organic counterparts. In particular, the development of elastomeric properties,11−13 optoelectronic characteristics,14−17 or bioerodible behavior18−21 is well-developed. Furthermore, the unique torsional properties of the phosphorus−nitrogen bond endow some polyphosphazenes with very low glass transition temperatures (Tg) compared to typical organic polymers. This makes them promising candidates for low-temperature elastomers. Recently, we reported that the introduction of bulky cosubstituent side groups, such as cyclotriphosphazenes or oligo-pphenyleneoxy units, into trifluoroethoxy-substituted polyphosphazenes lowers the symmetry, eliminates the crystallinity, and allows low-Tg elastomeric properties to emerge.22−24 It was also found that rigid oligo-p-phenyleneoxy side groups behave as strong physical cross-linkers via steric and intermolecular π−π interactions, which enhance the mechanical properties of polyphosphazene elastomers.22−24 However, these elastomers Received: September 21, 2018 Revised: November 8, 2018
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DOI: 10.1021/acs.macromol.8b02022 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Scheme 1. Synthesis of (a) TPEO-Substituted Trimers and (b) TPEO/Trifluoroethoxy-Substituted Polyphosphazenes
generally suffer from low Young’s moduli (
