Aggregation behaviour of well defined amphiphilic diblock copolymers with poly(N-isopropylacrylamide) and hydrophobic blocks (original) (raw)
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Macromolecular Chemistry and Physics, 2006
RAFT polymerization was used to prepare PMMA-b-PNIPAM copolymers. Two different chain transfer agents, tBDB and MCPDB, were used to mediate the sequential polymerizations. Micellar solutions and gels were prepared from the resulting copolymers in aqueous solution. When heated above T c of PNIPAM (about 31 8C), DLS revealed that PNIPAM coronas collapsed, resulting in aggre-gation of the original micelles. The micellar gels underwent syneresis above T c as water was expelled from the ordered gel structure, the lattice periodicity of which was determined by SANS. A large decrease in lattice spacing was observed above T c. The gel became more viscoelastic at high temperature, as revealed by shear rheometry which showed a large increase in G 00 .
Macromolecules, 2005
This investigation focused on the self-assembly of poly(N-isopropylacrylamide)-block-poly-(ethylene glycol) (PNIPA-block-PEG) in water. A quasi-living radical polymerization technique including a Ce(IV) ion redox system enabled us to prepare block copolymers with relatively narrow molecular weight distributions. We distinguish five regions in the phase diagram: a transparent sol, opaque sol, transparent gel, opaque gel, and syneresis. By examining the extent of changes in the spectroscopic properties of a fluorescence probe, pyrene, as a function of block polymer concentration and/or temperature, we determined the critical association concentration as well as the partition coefficient K v for pyrene. The spectroscopic properties indicate that the hydrophobicity around the probe starts to increase far below the demixing line of the PNIPA-block-PEG, a remarkable finding which suggests that even in the temperature region below the LCST temperature of a PNIPA block (∼32°C), this block copolymer provides more space for a preferential transfer of pyrene molecules than a bulk water medium at a higher temperature. This result may be attributed to the action of water, which starts to behave as a selective solvent for PEG blocks; the PEG chains are more swollen with water than are the PNIPA chains. Dynamic light scattering measurements also indicate that contraction of the PNIPA block starts to occur around 18°C, which is consistent with results obtained by fluorescence measurements. By employing small-angle neutron scattering, it is also confirmed that microphase separation occurs above 17°C to form disordered micelles, which includes a range of states from (i) asymmetric swelling to (ii) micelle formation with only shortrange liquidlike order. Above 30°C, network domains are formed as a result of macrophase separation due to dehydration of PNIPA blocks. As the temperature increased up to 40°C, the network domain is collapsed along a direction parallel to PNIPA-block-PEG interface, leading to increase in interfacial thickness and to macroscopic syneresis.
Journal of Polymer Science Part A-polymer Chemistry, 2008
A series of ABA amphiphilic triblock copolymers possessing polystyrene (PS) central hydrophobic blocks, one group with “short” PS blocks (DP = 54–86) and one with “long” PS blocks (DP = 183–204) were synthesized by atom transfer radical polymerization. The outer hydrophilic blocks were various lengths of poly(oligoethylene glycol methyl ether) methacrylate, a comb-like polymer. The critical aggregation concentrations were recorded for certain block copolymer samples and were found to be in the range circa 10−9 mol L−1 for short PS blocks and circa 10−12 mol L−1 for long PS blocks. Dilute aqueous solutions were analyzed by transmission electron microscopy (TEM) and demonstrated that the short PS block copolymers formed spherical micelles and the long PS block copolymers formed predominantly spherical micelles with smaller proportions of cylindrical and Y-branched cylindrical micelles. Dynamic light scattering analysis results agreed with the TEM observations demonstrating variations in micelle size with PS and POEGMA chain length: the hydrodynamic diameters (DH) of the shorter PS block copolymer micelles increased with increasing POEGMA block lengths while maintaining similar PS micellar core diameters (DC); in contrast the values of DH and DC for the longer PS block copolymer micelles decreased. Surface-pressure isotherms were recorded for two of the samples and these indicated close packing of a short PS block copolymer at the air–water interface. The aggregate solutions were demonstrated to be stable over a 38-day period with no change in aggregate size or noticeable precipitation. The cloud point temperatures of certain block copolymer aggregate solutions were measured and found to be in the range 76–93 °C; significantly these were ∼11 °C higher in temperature than those of POEGMA homopolymer samples with similar chain lengths. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7739–7756, 2008
Macromolecules, 2003
Amphiphilic AB, A 1 A 2 B, and A2B block copolymers, where A ) polyisobutylene, B ) poly-(methyl vinyl ether), and the superscripts denote molecular weight asymmetry, with constant molecular weight and composition have been synthesized by living cationic polymerization. The influence of architecture on aqueous micellar properties of these block copolymers were investigated in the temperature range 20-30°C by fluorescence spectroscopy and static and dynamic light scattering (SLS and DLS). The critical micelle concentration (cmc) measured at 23°C increased in the order A 2B < A 1 A 2 B < AB. The partition equilibrium constants, Kv of pyrene, characteristic of hydrophobicity, increased in the opposite order of cmc. The hydrodynamic radii (Rh) and aggregation numbers (Nagg) of micelles remained approximately constant in the whole temperature range for A 1 A 2 B and A2B and below 25°C for AB. At ∼25°C, however, there was a sudden increase in both Rh and Nagg for AB. Below 25°C both Rh and Nagg increased in the order AB < A 1 A 2 B < A2B. The particle size distribution for all block copolymers remained narrow in the whole temperature range. The results are discussed in terms of possible morphologies.
Langmuir, 2005
We investigate by small-angle scattering the structural behavior in water of a family of asymmetric poly(styrene-stat-(acrylic acid))-block-poly(acrylic acid), i.e., P(S-stat-AA)-b-PAA, diblock copolymers. These diblocks are of constant block ratio and increasing molar fraction, AA, ranging from 0 to 1, of acrylic acid in the first P(S-stat-AA) statistical block. We identify three types of structural behavior in water: (i) for AA e 0.25, the structures found in water are out-of-equilibrium micelle-like objects, reminiscent of the macrophase separation in the solid state, with no reorganization upon dispersion; (ii) for AA g 0.50, the diblocks dispersions in water are at equilibrium. For high AA, the diblocks are soluble in water, demonstrating that a transition from colloid-like objects to soluble macromolecules is achieved. Close to the transition, (AA ≈ 0.50), the diblocks form objects interpreted as comprising a water-swollen core formed by the P(S-stat-AA) block, surrounded by a swollen brush composed of the majority PAA block, above a apparent critical micelle concentration. However, these diblocks do not behave as macrosurfactants, and their self-association behavior is rather interpreted as a microphase separation which can arise from the incompatibility between two polymer blocks P(S-stat-AA) and PAA placed in a common solvent water.
European Polymer Journal, 2014
We report on the synthesis of poly(N-isopropyl acrylamide)-block-poly(n-butyl acrylate) (PNIPAm-b-PnBA) amphiphilic block copolymers and their temperature-responsive selfassembly behavior in aqueous solution. Well-defined PNIPAm-b-PnBA copolymers have been synthesized by a two-step RAFT polymerization scheme. The self-assembly behavior was studied by means of static and dynamic light scattering, 1 H NMR and fluorescence spectroscopy and transmission electron microscopy. The results show that already below the lower critical solution temperature (LCST) of PNIPAm, association of the PNIPAm blocks with hydrophobic dodecyl end groups of the charge transfer agent leads to the formation of loose aggregates of PNIPAm-b-PnBA micelles, the size and density of which increase with the increasing length of the PNIPAm block. The collapse of the PNIPAm blocks above the LCST leads to the decrease of the aggregates' size and the increase of their density, but the collapsed PNIPAm chains do not allow for interpenetration of the micellar shells and no further aggregation occurs.
Journal of Applied Polymer Science, 2017
The micellization of three tailor-made triblock copolymers, such as PB 100-P2VP 100-PEO 104 , PB 185-P2VP 108-PEO 154 , and PB 37-P2VP 115-PEO 241 , having similar total molecular weights and constant poly(2-vinylpyridine) (P2VP) sequence lengths, was investigated as a function of pH and sodium dodecyl sulfate (SDS) concentration. At pH 7 the formation of intermicellar aggregates was observed, especially for copolymers of low poly(ethylene oxide) (PEO) content. A pH decrease from 7 to 3 leads to a particle size increase due to the electrostatic repulsion of the protonated P2VP chains. The influence of the PEO sequence length was also observed for zeta potential values. At pH 3, in the absence of SDS, core-shell-corona micelles are formed whereas in the presence of small amount of SDS (degree of neutralization DN 5 0%-50%), a complex is formed between SDS and the protonated P2VP which leads to the shrinkage of the shell and thus to a decrease of the micellar sizes. For higher DN values, the micellar sizes increase due to the formation of large agglomerates and a transition occurs from a monomodal to a bimodal size distribution. Furthermore, it turned out that secondary aggregation, such as intermicellar aggregation, can completely be avoided if the degree of polymerization (DPn) of the water-soluble block is significantly higher than the DPn of the water-insoluble sequence. V
Journal of Polymer Science Part B-polymer Physics, 2006
Poly(ethylene glycol)-b-polycaprolactone (MPEG-PCL) diblock copolymers were synthesized via a ring-opening polymerization of ε-CL monomers with MPEG as an initiator. Their solubilities and apparent critical micelle concentrations (CMC) in aqueous solution were investigated as well as the determination of the micellar hydrodynamic diameter using dynamic light scattering (DLS). As PCL block length increased, the solubility and CMC decreased while diameters of micelles increased. The gel–sol transition behaviors were investigated using a vial tilting method. Aqueous solutions of copolymers undergo a gel to sol transition with increase in temperature when their polymer concentrations are above a critical gel concentration (CGC). The CGC of the copolymers and gel–sol transition temperature are influenced by the PCL chain length. The tapping mode AFM was performed by imaging the freeze-dried deposits from the copolymer solutions on mica to investigate a process from free chains to micelles and to gel. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3406–3417, 2006
Polymer, 2008
Thermosensitive triblock copolymers with two hydrophilic poly(N-isopropylacrylamide) blocks flanking a central hydrophobic poly(3-caprolactone) block were synthesized by atom transfer radical polymerization. Core-shell micellization of the triblock copolymers was inferred from the 1 H NMR spectra derived in two different solvent environments (CDCl 3 and D 2 O). The micellar characteristics of these amphiphilic triblock copolymers were studied by pyrene fluorescence techniques, dynamic light scattering and transmission electron microscopy. The critical micelle concentrations of the triblock copolymers were in the range of 4-16 mg/L and the partition coefficients were in the range of 3.10 Â 10 4 to 2.46 Â 10 5. The mean diameters of the micelles, measured by light scattering, were between 90 and 120 nm. The temperature sensitivity of the triblock copolymers was demonstrated by the phase transition of a 250 mg/L aqueous polymer solution at the lower critical solution temperature (LCST). The enthalpy of the phase transition was determined by differential scanning calorimetry. PM3 quantum mechanical calculation method was used to understand the intermolecular interactions between the copolymer and the water molecules. A modular approach was used to simulate the phase transition observed at the LCST.
Self-assembling of star-like amphiphilic block copolymers with polyelectrolyte blocks. Effect of pH
Polymer, 2007
The self-assembling behaviour of a four-arm amphiphilic star block copolymer, (PMMA 73 -b-PAA 143 ) 4 , with poly(methyl methacrylate) inner blocks and poly(acrylic acid) outer blocks in ratio 1:2 (PMMA:PAA) has been investigated in aqueous solutions as a function of pH by dynamic light scattering and cryo-transmission electron microscopy. At low pH (pH 5) the amphiphile forms in the presence of salt both spherical and worm-like micellar aggregates that coexist in solution. At high pH (pH > 12) the solution contains mainly spherical micelles and a small number of larger aggregates that have 'pearl-necklace' structure, indicating the disintegration of the worm-like species. In addition to the experiments, computer simulations of the four-arm amphiphilic star block copolymer with the same ratio of the blocks as above were conducted using a coarse-grained model. The simulations predict the formation of the worm-like micellar aggregates at low pH and the spherical ones at high pH. The changes in the morphology of the aggregates are related to the higher degree of ionization of poly(acrylic acid) blocks at high pH and to the swelling of the corona of the micelles by the higher osmotic pressure due to trapped counterions.