Preparation and Characterization of Mechanical Stimuli-Responsive Conjugated Microporous Polymers

Abstract

Mechanical stimuli are among simplest and easiest stimuli that can be applied to materials. Therefore, there have been extensive studies on the relationships between mechanical stimuli and the responses of materials from ancient time to the modern era in the fields of architecture, mechanics, electronics, and others. Although a great amount of knowledge has been accumulated over a long time, there is still strong demand for materials showing novel responses to mechanical stimuli, and studies of such materials are widely conducted. In this study, conjugated microporous polymers with mechanical stimuli-responsive properties are prepared and characterized. First, compressible microporous polymers (MPs) are investigated. The MPs were prepared as monoliths via a Sonogashira?Hagihara coupling reaction of 1,3,5-triethynylbenzene (TEB) with the bis(bromothiophene) monomer (PBT-Br). The polymers were reversibly compressible and were easily cut into any form using a knife. Microscopy studies of the MPs found that the polymers had tubular microstructures, resembling those often found in marine sponges. Under compression, elastic buckling of the tube bundles was observed by an optical microscope. MP­0.8, which was synthesized using a 0.8:1 molar ratio of PBT-Br to TEB, showed microporosity with a BET surface area as high as 463 m2g?1. The polymer was very hydrophobic, with a water contact angle of 145˚, and it absorbed 7?17 times its own weight of organic liquids. The absorbates were released by simple compression, allowing recyclable use of the polymer. MPs are potential precursors of structured carbon materials

for example, a partially graphitic material was obtained by the pyrolysis of MP-0.8, which showed a tubular structure similar to that of MP-0.8. Secondly, a microporous polymer sponge composite showing enhanced tensile properties was studied. In the previous study, MP-0.8 exhibited stable compressibility but had poor mechanical stability under tensile stress. To offset this weakness, a composite MP-PDMS sample consisting of MP-0.8 and polydimethylsiloxane (PDMS) was prepared. MP-PDMS was prepared by synthesizing MP-0.8 in the presence of PDMS and a curing agent. The homogeneous distribution of PDMS in MP-PDMS was confirmed by solid-state 13C NMR and EDS analyses. The microstructures measured by SEM and TEM showed shapes nearly identical to those of MP-0.8, indicating that no phase separation occurred between MP-0.8 and PDMS. The decreased BET surface area implied that some micropores were filled with PDMS during the reaction. However, elongation of the polymer was increased with the formation of the composite. Lastly, the piezochromic behaviors of C3-symmetric molecules having p-bromophenyl side groups connected to a phenyl ring core through cyano-vinylene bridges were studied. Upon grinding, α-BPAN-Br with cyano groups at the α-position relative to the phenyl ring core showed substantial quenching of a bluish green emission (on?off switching) and its constitutional isomer, β-BPAN-Br, exhibited an emission color change from bluish green to deep blue (color tuning). When the molecules were exposed to an organic vapor, the initial emission of each molecule was recovered. Powder X-ray diffraction and DSC studies revealed that the as-synthesized and vapor-annealed samples had the same crystalline structures, while the ground samples had amorphous structures. Their structural analogues, α-BPAN-H and β-BPAN-H which had no bromo groups, did not show any piezochromic or vapochromic behavior. Poly(β-BPAN) consisting of covalently linked β-BPAN units was synthesized by an Ullmann reaction of β-BPAN-Br. The polymer showed luminescence similar to that of crystalline β-BPAN-Br, but its initial emission was not changed by grinding.