Synthesis and characterization of fluorinated cellulose derivatives
Frazier, Charles Edward
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The synthesis of fluorinated cellulose derivatives was pursued for the purpose of investigating the effects of fluorine on the interfacial properties of cellulose derivatives, in multiphase materials. Synthetic approaches included: 1) the replacement of hydroxyl groups on cellulose with fluorine using fluoride displacement chemistry, or direct, one step fluorination 2) the replacement of hydroxyl groups on hydroxypropyl cellulose, HPC, with direct one step fluorinating agents, or 3) the conventional derivatization of cellulose with prefluorinated agents. Fluoride displacement of cellulose sulfonate esters was plagued low yields, and was therefore ineffective. Direct cellulose fluorination with aminofluorosulfur, and aminofluorocarbon reagents was effective for the surface fluorination of cellulose. However, the bulk, homogeneous fluorination of cellulose was complicated by side reactions that often precluded fluorination. Cellulose dissolved in DMAC/LiCI underwent simultaneous chlorination, and branching reactions when treated with dialkylaminosulfur trifluoride, DAST. Branching resulted from an anhydrous HF catalyzed trans-glycosidation reaction, which produced mainly (3-1,6 branching. Serendipitously, this discovery allows for the first known synthesis of long chain branched cellulose derivatives, with increased molar mass, and increased polydispersity. Treatment of HPC with DAST and FAR gave good levels of fluorination; however, the HF catalyzed gelation was always a complicating factor. The lower reactivity of FAR allowed for partial control of gelation with the use of nonnucleophilic bases. The easiest way to introduce fluorine was to perform conventional derivatization using prefluorinated reagents such as for the synthesis of fluorobenzyl cellulose. Mixed benzyl ethers of controlled fluorine content were made by altering the ratio of pfluorobenzyl chloride to benzyl chloride during reaction. 2-Dimensional NMR techniques were used to identify most proton and carbon resonances of cellulose and amylose fluorobenzyl ethers. Thermal analysis, solution analysis, and l3C spin-lattice relaxation experiments were used to compare the rod-like and coil-like behavior of the fluorobenzyl cellulose and amylose. Polycaprolactone, PCL, was blended with tri-O-benzylated cellulose with different degrees of fluorination. PCL was found to be immiscible with all derivatives, of fluorine contents from 0 % to as high as 11.7%. PCL did display some degree of mechanical compatibility with all derivatives. The greatest compatibility was found with benzylated cellulose having fluorine contents slightly below the maximum. It was found that no specific interactions were occurring between peL and fluorobenzyl cellulose. The enhanced compatibility of the mixed fluorobenzyl/benzyl cellulose ethers was postulated to arise from intramolecular interactions, which served to enhance the mixing of the copolymeric mixed benzyl ethers with PCL.
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