Development of Polymeric Gas Sensors

Date

2023-6-21

Author

Batır, Özge

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Detection of volatile organic compounds (VOCs) is an important issue due to their harmful impact on human health. In the first part of the study, we aimed at enhancing the sensitivity of the anisotropic polymeric films templated from cholesteric liquid crystals (CLCs) in the identification of VOCs at concentration in the order of 100 ppm. To increase sensitivity, we introduced strain to the films in the direction parallel to the helical axis and evaluated its effect on the sensitivity. Specifically, we used LC mixtures of reactive [4-(3-acryloyoxypropyloxy) benzoic acid 2-methyl-1,4-phenylene ester (RM257)], nonreactive E7 mesogen and chiral dopant [4-(1-methylheptyloxycarbonyl)phenyl-4-hexyloxybenzoate)) (S-811)] to synthesize CLC-templated polymeric films with programmed strain profiles using a wedge cell, and measured their response against a range of toluene vapor concentrations. Based on the obtained results, we demonstrated a relationship between the strain in the cholesteric pitch and the sensitivity of the sensor based on spacial responses evaluated from the change in coloring of the film. Our results showed that strain helps to increase the sensitivity of the sensors up to 15 times compared to their unstrained counterparts. Moreover, 90% of the equilibrium response is achieved in less than one minute of exposure which offers rapid diagnosis of VOCs. Our tests for the reversibility of the sensors showed that the CLC-templated polymeric films can be used multiple times without a significant loss of sensitivity. In order to increase the specificity and reusability of these structures together with sensitivity to achieve their successful integration into useful applications, in the second part of the study, we developed an interpenetrating polymeric network (IPN) based on poly(RM257) structure of CLC-templated polymeric films and polydimethylsiloxane (PDMS). Contrary to the generic CLC-hosted IPN sensors that use CLC structures to optically report the changes in the polymeric matrix, we employed the collaborative responsiveness of the constituents of the IPN (both CLC-templated and penetrated polymeric structures) to improve the sensor performance. The elastic swelling behavior of PDMS structure increased the durability and reusability of the poly(RM257) polymeric sensor upon exposure to even high VOC vapor concentrations up to their saturation. The sensor tests of both poly(RM257) and IPN films were carried out against four different VOC vapors namely toluene, hexane, acetone, ethanol and also water vapor. The results showed that the synergistic swelling behavior of PDMS and poly(RM257) polymeric structures against vapors directly affected the sensor performance in terms of earned specificity and enhanced sensitivity. Moreover, the real time experiments showed that the measurement accuracy, response, and recovery time of the polymeric sensors were also improved significantly by the development of IPN as compared to their non-IPN counterparts. This study shows a promise towards further engineering of sensors against various applications through detailed design of the constituent polymers against targeted species. In the last part of the study, we aimed to synthesize and characterize optically active chiral metallomesogens. In the light of the studies presented in literature, a synthesis pathway was developed where the resulting molecule has reactive acrylate end groups, chiral part introduced by aminoalcohols and metal cores namely as either Pd, Cu or Zn. We firstly aimed to study effect of metal core type on chiral behavior of chiral reactive metallomesogens. Therefore, (R)- or (S)-2-amino-1-butanol aminoalcohols were used to create chiral groups and complexed with three different metal acetates namely; Pd, Cu and Zn acetate. Chiral behavior of Pd and Cu complexes were confirmed by optical characterization and no significant impact of metal core type was observed on helical twisting power (HTP) of the chiral molecule. The complexes of Zn did not possess chiral symmetry due to unstable tetrahedral structure of Zn. We also aimed to examine effect of amino alcohol chiral group on behavior of the chiral reactive metallomesogens. Therefore, we used four more different amino alcohols with varying length of alkyl chains with or without phenyl groups and complexed them with palladium (II) acetate. Optical characterization results revealed that the complexes synthesized from the amino alcohols which has phenyl in their chiral group does not exhibit chiral behavior. When we compared chiral feature of the four different Pd complexes, we observed that increase of the chiral group chain length increases pitch size and decrease of HTP of the chiral reactive metallomesogens. Moreover, chiral reactive metallomesogens were photo-polymerized under UV light which was confirmed by optical characterizations and UV-visible spectrophotometry.

Subject Keywords

Optical Sensor, Cholesteric Liquid Crystal, Interpenetrating Network, Volatile Organic Compound, Metallomesogen

URI

https://hdl.handle.net/11511/104761

Collections

Graduate School of Natural and Applied Sciences, Thesis