Morphological transitions in aggregates of thermosensitive poly(ethylene oxide)‐b‐poly(N‐isopropylacrylamide) block copolymers prepared via RAFT polymerization

Abstract

Double hydrophilic poly(ethylene oxide)‐b‐poly(N‐isopropylacrylamide) (PEO‐b‐PNIPAM) block copolymers were synthesized via reversible addition‐fragmentation chain transfer (RAFT) polymerization, using a PEO‐based chain transfer agent (PEO‐CTA). The molecular structures of the copolymers were designed to be asymmetric with a short PEO block and long PNIPAM blocks. Temperature‐induced aggregation behavior of the block copolymers in dilute aqueous solutions was systematically investigated by a combination of static and dynamic light scattering. The effects of copolymer composition, concentration (Cp), and heating rate on the size, aggregation number, and morphology of the aggregates formed at temperatures above the LCST were studied. In slow heating processes, the aggregates formed by the copolymer having the longest PNIPAM block, were found to have the same morphology (spherical “crew‐cut” micelles) within the full range of Cp. Nevertheless, for the copolymer having the shortest PNIPAM block, the morphology of the aggregates showed a great dependence on Cp. Elongation of the aggregates from spherical to ellipsoidal or even cylindrical was observed. Moreover, vesicles were observed at the highest Cp investigated. Fast heating leads to different characteristics of the aggregates, including lower sizes and aggregation numbers, higher densities, and different morphologies. Thermodynamic and kinetic mechanisms were proposed to interpret these observations, including the competition between PNIPAM intrachain collapse and interchain aggregation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4099–4110, 2009

Thermoresponsive behavior of micellar aggregates from end-functionalized PnBA-b-PNIPAM-COOH block copolymers and their complexes with lysozyme

“…Monitoring the scattered light intensity at a fixed angle for different temperatures is a sensitive probe of transitions in solutions 24 . PNIPAM's coil-to globule transition in water causes an abrupt increase of the scattered intensity around 40 °C (figure 1).…”

“…The increased 8 hydrophilicity of the system because of the presence of fully charged end carboxylic units at pH 7 is a possible cause for the LCST appearing at temperatures higher than 32 ºC 24 . Nevertheless a more complicated mechanism may be involved since the transition observed by our scattering methods is dominated by morphological changes at higher aggregation states.…”

Thermo-responsive block copolymers with multiple phase transition temperatures in aqueous solutions

“…The effect of the neighboring block is easiest to observe in systems consisting only of a single thermo-responsive block. For example, the cloud point-increasing effect of the more hydrophilic PEG on the block and graft copolymers of PiPA has been well demonstrated[114][115][116][117][118][119], although the demixing temperature and kinetics have been reported to strongly depend on the polymer architecture and concentration, as T dem decreases significantly at high concentrations[120]. The effect of the relative block lengths on the LCSTs has also been demonstrated for other thermo-responsive block copolymers with a single LCST[121][122][123].Laschewsky and coworkers showed a shift in the CPs of dually thermo-responsive PnPA-b-PEA diblock copolymers according to the relative block lengths[124].When the relative length of the more hydrophilic PEA block increased upon changing the nPA:EA ratio from 2:1 to 1:1 and further to 1:3, the CP of the more hydrophobic PnPA block increased from 17 to 23 and then to 27 °C, respectively.The less pronounced CPs of the PEA block were in the range of 60−70 °C.…”

Kinetics of temperature response of PEO-b-PNIPAM-b-PAA triblock terpolymer aggregates and of their complexes with lysozyme

“…The tri-block terpolymer poly(ethylene oxide)-block-poly(Nisopropylacrylamide)-block-poly(acrylic acid) (PEO-b-PNIPAM-b-PAA) was synthesized using reversible additionefragmentation chain transfer (RAFT) polymerization [10,11]. The resulting M w is M ¼ 56$10 3 g/mol with respective weight contents: 7% for PEO, 68% for PNIPAM and 25% for PAA (giving the following numbereaverage degrees of polymerization: PEO 89 -PNIPAM 337 -PAA 194 ) and a polydispersity index M w /M n ¼ 1.28 from SEC in DMF (see Supplementary Material).…”