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Micellar and rheological behaviour of aqueous solutions of block copolymers of thylene oxide / alkylene oxide

By Αντώνιος Κελαράκης

Abstract

In this work the effect of block composition and architecture on the self association properties in aqueous solution of block copolymers E/A have been investigated. In the general notation E/A, E denotes an oxyethylene group and A an oxyalkylene qroup which are the repeating units of the hydrophilic and the hydrophobic block of the copolymers respectively. The copolymers studied include diblocks (ΕmΑn) and triblocks (EmAnEm or AnEmAn) ; the block A consists of either oxybutylene units OCH2CH(CH2CH3), either oxypropylene units OCH2CH(CH3), or oxyphenylethylene units OCH2CH(C6H5). Α wide range of block copolymers have been synthesised by anionic polymerization by use of high vacuum technique. Characterisation of the copolymers by gel permeation chromatography [GPC, calibrated by poly(oxyethylene) standards] served to confirm the narrow chain length distributions, i.e. Mw/Mn where Mw are the mass-average and number-average molar mass respectively. Absolute values of Mn of the copolymers were obtained by 13C NNR spectroscopy based on the assignments published. The block copolyethers studied in dilute aqueous solution readily form micelles and their concentrated solutions form liquid crystal mesophases (gels) comprising packed micelles. Surface tensiometry was used to determine critical micelle concentrations (cmc) at several temperatures and thereby values of the enthalpy of micellisation (ΔmicΗο). Cmc’s values are decreased as temperature or hydrophobe block length are increased. The low values found for the E/S (and the E/B) copolymers have been explained by collapse of the S (B) blocks of copolymers in the dispersed molecular standard state and, as a consequence, a small thermal effect accompanying their incorporation in the micellar core. The drop in ΔmicΗο/n with n observed for the E/P copolymers is assigned to a similar, though much less marked effect. Comparison between the cmc values of E/P, E/B, E/S copolymers indicates hydrophobicities in ratio P:B:S = 1:6:12. Dynamic and static light scattering were used to characterise the micelles in solution, yielding micellar association number (N), hydrodynamic (rh) and thermodynamic (rt) ratio. The correlation functions from dynamic light scattering were analysed by the constrained regularised CONTIN method to obtain distributions of decay rates and ,finally, plots of intensity fraction against the logarithm of apparent hydrodynamic radius (rh,app). There were found single peaks centred on the region 10-30 nm which are typical of the closed association process characteristic of this type of copolymers. The values of are independent of temperature within the error of determination. This effect is attributed to the compensative effects of an increase in N and a decrease in swelling of the micelles fringe as temperature is increased. .........................................................................................................................................................

Topics: Μικυλλιοποίηση, Δημιουργία gel, Ρεολογία, Συμπολυμερή κατά συστάδες, Μέτρο αποθήκευσης / απωλειών, Χρόνος χαλάρωσης, Σύνθεση πολυμερών, Χαρακτηρισμός πολυμερών, Micellisation, Gelation, Hard gel, Soft gel, Sol-gel, Rheology, Block copolymers, Storage / loss modulus, Relaxation times, Synthesis of polymers, Characterization of polymers
Publisher: National and Kapodistrian University of Athens
Year: 2001
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