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Physico-chemical studies of block copolymers in aqueous solution

Physico-chemical studies of block copolymers in aqueous solution

Armstrong, Jonathan Keith (1997) Physico-chemical studies of block copolymers in aqueous solution. PhD thesis, University of Greenwich.

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The dilute aqueous solution behaviour of oxyethylene/oxypropylene copolymers has been investigated as a function of temperature (275-370K), copolymer concentration (0.1-5% w/v) and copolymer composition [polyoxypropylene (POP) and polyoxyethylene (POE) homopolymers (750-4000gmol-1 ) and diblock copolymers (3450-13300gmol-1 ), poloxamers (POE-POP-POE triblock copolymers, 1100-14000gmol-1 ) and poloxamines (ethylene diamine alkoxylates, 1650-26000gmol-1 )] using the macroscopic techniques of high sensitivity differential scanning calorimetry (HSDSC), and differential scanning densitometry (DSD) and also using the technique of 1H and 13C-NMR. The observed phase transitions from HSDSC data are indicative of an aggregation process and are consequent upon changes associated with POP involving dehydration, a conformational change and aggregation. For POP homopolymers, the phase transition results in phase separation of the polymer (cloud point) but for the diblock copolymers, poloxamers and poloxamines these copolymers remain in solution due to the effects of the POE portion of the copolymer. The phase transition temperature (Tm ) decreases and the calorimetric enthalpy (AHcai) increases with increasing molecular mass of the POP block and show no relationship to the POE content. DSD data of poloxamers in water at a concentration of 1 % (w/v) show a sharp increase in partial specific volume (v) with increasing temperature, the mid-point of the transition in agreement with the Tm observed by HSDSC. The partial specific volume change (Av) approach zero as the POE:POP ratio approaches 1:0 indicative that the phase transition is associated with changes in the POP portion of the copolymer. TI relaxation NMR data for poloxamer 237 in D20 as a function of temperature shows a gradual increase in relaxation times for -CH2- and -CH(Me)-resonances with increasing temperature due to increased molecular motion, but a sharp decrease of the -CH(Me)- relaxation time was observed at the Tm relating to a change in conformation of the POP portion of the copolymer. The effects of cosolutes (NaCl, Na2HPO4/NaH2PO4, urea and guanidinium chloride) and cosolvents (methanol, ethanol, n-propanol, n-butanol and formamide) on the observed phase transition of poloxamers have been investigated using HSDSC. Sodium chloride, phosphate buffer, n-propanol and n-butanol favour aggregation of the copolymer reflected in a lowering the Tm and an increase in AHcal]. Conversely, urea, guanidinium chloride, methanol, ethanol and formamide prevent the onset of aggregation reflected in an increase in Tm and a lowering of AHcai. The effects of cosolutes and cosolvents on the aggregation behaviour of poloxamers are explained in terms of enhancing or breaking water structure or by possibly replacing water molecules in the solvation sphere of the POP portion of the copolymer. The calorimetric output has been analysed using a model fitting procedure based upon a mass action description to obtain estimates for thermodynamic parameters which characterise the aggregation process. These important parameters include T½, the temperature at which the aggregation process is half completed, AHcal, AHvH . the van't Hoff enthalpy and n the aggregation number. The modelled excess heat capacity data are in good agreement with the experimental calorimetric outputs. An enthalpy-entropy compensation plot for all data obtained indicate that the solvent-solute interactions that are responsible for the phase transitions observed by HSDSC are the same for all of the copolymers investigated regardless of the copolymer composition and concentration.

Item Type: Thesis (PhD)
Additional Information: The work described in this thesis was carried out in the laboratories of the School of Biological and Chemical Sciences, University of Greenwich, between 1990 and 1994.
Uncontrolled Keywords: copolymers, chemistry
Subjects: Q Science > QD Chemistry
Pre-2014 Departments: School of Chemical and Life Sciences
Last Modified: 16 Aug 2018 15:20

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