Thermoresponsive Poly(2-oxazoline) Block Copolymers Exhibiting Two Cloud Points: Complex Multistep Assembly Behavior

Abstract

Aqueous solutions of poly(2-oxazoline) block copolymers consisting of a 2-ethyl-2-oxazoline block and a block consisting of a random copolymer of 2-ethyl-2-oxazoline and 2-n-propyl-2-oxazoline (PEtOx-block-P(EtOx-stat-PropOx)) have been studied by dynamic light scattering (DLS), static light scattering (SLS), and turbidimetry. Even at temperatures significantly below the lower critical solution temperature (LCST), polymer unimers are found to coexist with a few large aggregates with an open structure. When heated, the systems exhibit an intricate transmittance behavior whereby the samples becomes visually clear again after an initial cloud point and then exhibit a second cloud point at even higher temperatures. The DLS data indicate that the aggregates formed around the first cloud point restructure and fragment into smaller micelle-like structures ascribed to further dehydration of the more hydrophobic PPropOx containing block, causing the samples to become optically clear again. The observed fragmentation is confirmed by the SLS experiments. At even higher temperatures, both blocks become hydrophobic, causing the formation of large, compact aggregates, resulting in a second cloud point.

Characterization of temperature induced changes in liposomes coated with poly( N -isopropylacrylamide- co -methacrylic acid)

“…A size maximum when the temperature is increased has been observed several times before for thermoresponsive polymers, and is commonly attributed to the interplay between size increase caused by aggregation, and contraction of the aggregates due to enhanced intra-aggregate interactions [49][50][51][52]. Even though the size decreased at high temperatures, both the count rate and turbidity continued to rise, confirming that the declining size is caused by contraction of the aggregates [48].…”

“…4c). The combined increase in the size and turbidity suggests aggregation of the polymer [48], probably caused by enhanced intermolecular associations of PNIPAAM at elevated temperatures [23,49,50]. The increase in turbidity occurs at a higher temperature than the increase in size.…”

Study of the temperature-induced aggregation of polyoxometalate-poly(N,N-diethylacrylamide) hybrids in water

“…The preceding DLS measurements were performed in standard conditions at 5 mg mL -1 . 28 The thermally-induced aggregation of PDEAAm chains due to a coil-to-globule transition is a kinetically driven process, and depends not only on heating/cooling kinetics but also on the polymer concentration. As summarized in Table 4, DLS measurements at 70°C at different concentrations (1, 5 and 10 mg mL Similar results were obtained for the lower molar mass compound PP30-N (Table 4).…”

Design Strategies for Functionalized Poly(2-oxazoline)s and Derived Materials

“…Pulsed field gradient NMR spectroscopy confirmed the presence of aggregates, despite the block copolymer's double-hydrophilic character. Hoogenboom, Kjoniksen et al investigated the aggregation behavior of pEtOx 80 -block-p(EtOx x -statn PrOx 40−x ) with x = 0, 4, or 8 [170]. Upon heating, the block copolymers exhibit two cloud points.…”