Abstract
The effect of polymer concentration on the temperature-induced self-association of a block copolymer comprising a poly(2-ethyl-2-oxazoline) block and a random copolymer block consisting of 2-ethyl-2-oxazoline and 2-n-propyl-2-oxazoline (PEtO80-block-P(EtOxx-stat-PropO40-x) with x = 0, 4, or 8 were investigated by dynamic light scattering (DLS) and transmittance measurements (turbidimetry). The polymers reveal a complex aggregation behavior with up to three relaxation modes in the DLS data and with a transmittance that first goes through a minimum before it declines at high temperatures. At low temperatures, unassociated polymer chains were found to co-exist with larger aggregates. As the temperature is increased, enhanced association and contraction of the aggregates results in a drop of the transmittance values. The aggregates fragment into smaller micellar-like clusters when the temperature is raised further, causing the samples to become optically clear again. At high temperatures, the polymers aggregate into large compact clusters, and the samples become turbid. Interestingly, very large aggregates were observed at low temperatures when the polymer concentrations were low. The formation of these aggregates was also promoted by a more hydrophilic copolymer structure. The formation of large aggregates with an open structure at conditions where the solvent conditions are improved is probably caused by depletion flocculation of the smaller aggregates.
Highlights
Synthetic amphiphilic macromolecules are able to form self-assembled structures in aqueous media
We have previously studied the behavior of PEtOx80 -block-P(EtOxx -stat-PropOx40-x ) with x = 0, 4 or 8 at a fixed concentration of 5 mg/mL in water [61]
The correlation functions from dynamic light scattering reveal the presence of four different kinds of structures, of which one to three may be present at the same time
Summary
Synthetic amphiphilic macromolecules are able to form self-assembled structures in aqueous media. These polymers are well known as emulsifiers and viscosity modifiers for industrial applications [1,2,3,4,5]. Stimuli-responsive amphiphilic copolymers are “smart” materials that have the ability to respond to external stimuli such as temperature, pH, electric or magnetic fields, light, and mechanical stress. Temperature and pH responsive mechanisms have been considerably investigated during the past decades due to their potential for biomedical applications such as bio-sensors, implants, and drug-delivery devices [6]. Alteration of molecular interactions as a result of temperature changes provides systems that are useful in bio-related applications [7,8,9,10,11,12]. Thermoresponsive polymers in water may exhibit a lower critical solution temperature (LCST), i.e., an increased hydrophobicity at elevated temperatures, or an upper critical
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