Characterizing polymers with the Kerr effect
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- Tez No: 402687
- Danışmanlar: DR. ALAN E. TONELLI
- Tez Türü: Doktora
- Konular: Polimer Bilim ve Teknolojisi, Polymer Science and Technology
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2015
- Dil: İngilizce
- Üniversite: North Carolina State University
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 134
Özet
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Özet (Çeviri)
Advanced synthetic techniques have been recently developed in to tailor the architectures of polymers. While polymer chemists are discovering new ways to control polymerizations to design specific polymer micro- and macrostructures, techniques to characterize these elaborate polymer architectures/macrostructures have not similarly advanced.1 Polymer macrostructures, i.e., the positions or locations of constituent microstructures along their chains, are essentially the complete molecular architectures of their chains. A complete characterization of polymer macrostructures should be detailed enough to identify, count/quantify, and locate all microstructures along the polymer chains2. This is necessary to establish a relevant structure-property relation and to predict their processability. Currently the best characterization method is the observation of polymer solutions with high field, high resolution 13C-NMR spectroscopy. It is possible to characterize microstructures as long as monomer tetrads using new methodologies, such as heteronuclear 2D-NMR. However, because of its local sensitivity it is impossible to determine the exact locations or the distribution of these microstructural elements along or among the polymer backbones due to NMRs 3,4,5. Here we suggest a non-traditional method for characterizing polymer macrostructures, i.e., coupling of NMR spectroscopy, sensitive to short-range microstructures, with the Kerr effect, a macroscopic experimental probe which is sensitive enough to distinguish polymer macrostructures6. The Kerr effect of polymer solutes is the birefringence contributed to their dilute solutions when subjected to strong electric fields7. We focused on examining the sensitivity of the Kerr effect and molar Kerr constants, mKs, of dilute polymer solutions to their microstructures and determine if the same microstructures located at different positions in polymers are reflected in distinct mKs. In the first and second study, styrene (S)/para-Br-styrene (p-BrS) tri-blocks pBrS90-S120-pBrS90 (I) and S60-pBrS180-S60 (II) were analyzed and distinguished by the signs of their Kerr constants, B. A series of random and gradient S/p-BrS copolymers synthesized by RAFT polymerization were characterized. While NMR spectra could reveal only the local structures of copolymers, a comparison of observed and calculated mKs of dilute copolymer solutes indicated both the difference between the copolymers and an additional gradient in stereosequence that parallels their comonomer gradient in the gradient S/p-BrS copolymers. We investigated the surface stability of S/pBrS copolymer thin films on silica surface by observing their dewetting mechanisms. Two random S/pBrS copolymers, both with 50:50 S:pBrS composition, synthesized by RAFT and the other by free-radical initiated polymerization, were annealed above their glass transition temperature (Tg). While the random S/pBrS film prepared by uncontrolled free-radical polymerization did dewet from silica surface, RAFT film was stable. This verified our Kerr effect observations and revealed the importance of polymer stereosequence on thermal stability of polymer thin films. Later, we synthesized S/pBrS copolymers with random, gradient, diblock comonomer sequences using ATRP and measure their mKs. These were compared to those already measured RAFT synthesized random and gradient S/pBrS copolymers. The experimental results confirmed that ATRP produce atactic stereosequence as expected in contrast with our previously performed RAFT polymerizations. Precise and random, regioregular or irregular, and atactic and isotactic ethylene/vinyl acetate E/VAc copolymers and regularly alternating styrene/butadiene multi-block copolymers which were kindly sent to us from other collaborators were measured. The Kerr effect observations of these appeared to be sensitive to their carefully synthesized macrostructures. Single concentration measurements on precise, regioregular, isotactic and atactic E-VAcs yielded, respectively, large negative and moderate positive B. On the other hand, their 13C-NMR spectra are identical. In the last study, the potential heterogeneity of polymer micro-/macrostructures and the ability of the Kerr effect to distinguish diluted samples of atactic S/p-BrS copolymers (50:50, 20:80, 80:20 S: pBrS ) with homogeneous and heterogeneous populations of micro- and macrostructures were demonstrated.
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