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PREPARATION OF NITRILE CONTAINING SILOXANE TRIBLOCK COPOLYMERS AND THEIR APPLICATION AS STABILIZERS FOR SILOXANE MAGNETIC FLUIDS

by

Chenghong Li

Master's Thesis submitted to the Faculty of the Virginia Tech in partial fulfillment of the requirements for the degree of

Master of Science

in

Chemistry Department

Approved

Judy S. Riffle, Chair
James E. McGrath
Harry W. Gibson
James P. Wightman
John G. Dillard

Dec. 11, 1996
Blacksburg, Virginia


Abstract

Nitrile containing siloxane block copolymers were developed as stabilizers for siloxane magnetic fluids. The siloxane magnetic fluids have been recently proposed as internal tamponades for retinal detachment surgery. PDMS-b-PCPMS-b-PDMSs (PDMS = polydimethylsiloxane, PCPMS = poly(3-cyanopropylmethylsiloxane) were successfully prepared through kinetically controlled polymerization of hexamethylcyclotrisiloxane initiated by lithium silanolate endcapped PCPMS macroinitiators. The macroinitiators were prepared by equilibrating mixtures of 3- cyanopropylmethylcyclosiloxanes (DxCN) and dilithium diphenylsilanediolate (DLDPS). DxCNs were synthesized by hydrolysis of 3-cyanopropylmethyldichlorosilane, followed by cyclization and equilibration of the resultant hydrolysates. DLDPS was prepared by deprotonation of diphenylsilanediol with diphenylmethyllithium. It was found that mixtures of DxCN and DLDPS could be equilibrated at 100C within 5-10 hours. By controlling the DxCN-to-DLDPS ratio, macroinitiators of different molecular weights could be obtained. The major cyclics in the macroinitiator equilibrate are tetramer (8.6 0.7 wt%), pentamer (6.3 0.8 wt%) and hexamer (2.1 0.5 wt%). 2.5k-2.5k-2.5k, 4k-4k-4k, and 8k-8k-8k triblock copolymers were prepared and characterized. These triblock copolymers are transparent, microphase separated and highly viscous liquids. It was found that these triblock copolymers can stabilize nanometer gamma-Fe2O3 and cobalt particles in octamethylcyclotetrasiloxane or hexane. Hence PDMS-b-PCPMS -b-PDMSs represent a class of promising steric stabilizers for silicone magnetic fluids.

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