A method for the preparation of polydimethylsiloxane-urea (A-B)n segmented or multiblock copolymers has been investigated. These copolymers were synthesized from 4,4¹-diphenylmethane diisocyanate (MDI) and a,ω-bis(aminopropyl)polydimethylsiloxane (PSX) of different molecular weights by solution polymerization.
The conditions needed to synthesize well-defined and high molecular weight siloxane-urea copolymers in high yields were investigated. 2-ethoxyethyl ether (2-EEE) was a better reaction solvent than tetrahydrofuran (THF) because the polymerization kinetics appear to be faster and the molecular weights obtained were higher. The mechanism postulated was based on the fact that THF is a stronger base than 2-EEE and the former should complex, more tightly with the isocyanate groups present in the system, competing with the amine nucleophile and resulting in the decrease in the polymerization kinetics.
Penetration studies as a function of temperature using TMA showed the copolymers were multiphase and retained their integrity up to 160-180°C, a temperature range at which softening and consequently thermoplastic flow was observed. Apparently, the urea hard segment domains act as physical or pseudo crosslinks in the rubbery phase and also clearly serve as reinforcing filler materials.
Efforts to chain extend siloxane-urea copolymers with 1,6-diaminohexane (HMDA) using several different techniques in the presence of cosolvents such as DMAC or NMP, only resulted in low molecular weight products due to the premature precipitation of the polymer.
A polyurethane prepolymer formation study done in either DMAC or o-butyrolactone (BLO) at about 105°C, showed BLO to be the better solvent. Side reactions involving isocyanate (-NCO) groups are less prevalent in BLO.
ESCA was used to probe the surface chemical composition of polyether-urethane (Estane C-5), polydimethylsiloxane-urethane (PUS) copolymers and their blends. The topmost surface of the neat Estane is mostly covered with the polyether (PTMO) soft segments mixed with small amounts of the urethane hard segments. The surface of the neat polydimethylsiloxane-urethane (PUS) is almost an overlayer of the polydimethylsiloxane (PSX) segments. ESCA results strongly suggest the PSX layer to be at least 50Å thick. In the hlends studied, the trend towards having greater percentages of the silicon and hence siloxane on the surface starts dramatically at a level of around 1-2 wt. % of siloxane in the bulk. The location of such a transition in a particular blend system is important because it allows one to in a sense "tailor" the surface concentration of siloxane in these blends. / M.S.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/101230 |
| Date | January 1983 |
| Creators | Sha'aban, Ahmad Khairuddin |
| Contributors | Chemistry |
| Publisher | Virginia Polytechnic Institute and State University |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis, Text |
| Format | ix, 151 leaves, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 09960071 |
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