Microstructure characterization of polymers by modern NMR techniques
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The purpose of this study is to utilize diverse nuclear magnetic resonance (NMR) techniques to characterize the microstructures in polymers, including monomer distributions along the backbone, chain end, and branches. In addition, quantitative NMR allows conducting component analysis and confirming the resonance assignments.
In one project, a variety of complementary 1D- and 2D-NMR techniques were used together to study silver complexes in nanoparticles. Silver carbene complexes (SCCs), which have antimicrobial activity, were incorporated into poly(lactide-co-glycolide)/ poly(lactide-co-glycolide)-b-polyethylene glycol (PLGA/PLGA-PEG-NH2) nanoparticles to provide extended release of the silver. Firstly, the structure variations in the copolymer PLGA-PEG-NH2, including monomer sequences in the PLGA chain, and the chain end / main chain units in both PLGA and PEG chain, were characterized by 2D-NMR experiments. This study indicates that the PLGA used in this work is an alternating polymer, and that the degradation of PLGA is via cleavage of GG linkages dominantly. Then, the splitting pattern in the 109Ag NMR was used to determine the forms of SCC. Lastly, quantitative 1H NMR permitted the calculation of the percentage of silver drug in the nanoparticles. In another project, the combined use of 2D-NMR experiments allow unambiguous resonance assignments for poly(vinylidene fluoride-co-tetrafluoroethylene) (PVDF-TFE). For the backbone units, the monomer sequence distributions were established step by step from 3-carbon, then 5-carbon and 9-carbon sequences. The 19F DOSY and T1 experiments were conducted to distinguish the resonances from chain end/branches from those of the main chain units. Unfortunately, the regular 2D-NMR experiments used to study the backbone units do not provide enough resolution or sensitivity for the chain end resonances. This study indicates that both sensitivity and resolution were significantly improved in 19F-19F selective COSY 2D-NMR experiments, which allow unambiguous structural determination of these low-occurrence units. After assigning the resonances, quantitative NMR study was conducted to calculate the percentages of reverse VDF, TFE and each chain end units. Based on the quantitative NMR results, the possible copolymerization mechanism was discussed.
Document number: akron1353000762
Permalink: http://rave.ohiolink.edu/etdc/view?acc_num=akron1353000762
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