14 research outputs found

    Formation And Growth Of Mesoglobules In Aqueous Poly(N-Isopropylacrylamide) Solutions Revealed With Kinetic Small-Angle Neutron Scattering And Fast Pressure Jumps

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    The phase transition from swollen chains to polymer mesoglobules of an aqueous solution of poly(N-isopropylacrylamide) is investigated with kinetic small-angle neutron scattering with 50 ms time resolution in conjunction with millisecond pressure jumps across the coexistence line. The time-resolved study evidenced three distinct regimes: fractal clusters form during the first second and transform into compact mesoglobules. During the following ∟20 s, these grow by diffusion-limited coalescence. The final step consists of a slow growth characterized by an energy barrier of several kBT. The method opens opportunities for kinetic structural studies of multicomponent systems over wide length and time scales and gives a structural picture spanning from the chain collapse to mesoscopic phase separation

    Solvent Dynamics in Solutions of PNIPAM in Water/Methanol MixturesA Quasi-Elastic Neutron Scattering Study

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    The solvent dynamics of concentrated solutions of poly­(<i>N</i>-isopropylacrylamide) (PNIPAM, 25 wt %) in water/methanol mixtures (85:15 v/v) are measured with the aim of shedding light onto the cononsolvency effect. Quasi-elastic neutron scattering (QENS) with contrast variation has been carried out at temperatures below and above the cloud point by using in the first set of experiments the mixture H<sub>2</sub>O:<i>d</i>-MeOD (<i>d</i>-MeOD denotes fully deuterated methanol) as a solvent and in the second set of experiments the mixture D<sub>2</sub>O:MeOH (MeOH denotes methanol). As a reference, bulk H<sub>2</sub>O, bulk MeOH and the mixtures H<sub>2</sub>O:<i>d</i>-MeOD and D<sub>2</sub>O:MeOH (both 85:15 v/v) have been investigated as well. In the PNIPAM solution in H<sub>2</sub>O:<i>d</i>-MeOD, two water populations are identified, namely strongly and less strongly arrested water. At the cloud point, the former is partially released from PNIPAM. The diffusion coefficient of the latter one is similar to the one in the water/methanol mixture, and its residence time decreases at the cloud point. The PNIPAM solution in D<sub>2</sub>O:MeOH reveals similar dynamics to the one in H<sub>2</sub>O:<i>d</i>-MeOD which may reflect that the dynamics of MeOH near the PNIPAM chain is similar to the one of H<sub>2</sub>O. The similarity may, however, partially be due to H/D exchange between D<sub>2</sub>O and MeOH. In both PNIPAM solutions, the mean-square displacement of the PNIPAM chain decreases gradually above the cloud point

    Thermoresponsive Hydrogels from Symmetrical Triblock Copolymers Poly(styrene-<i>block</i>-(methoxy diethylene glycol acrylate)-<i>block</i>-styrene)

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    A series of symmetrical, thermo-responsive triblock copolymers was prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization, and studied in aqueous solution with respect to their ability to form hydrogels. Triblock copolymers were composed of two identical, permanently hydrophobic outer blocks, made of low molar mass polystyrene, and of a hydrophilic inner block of variable length, consisting of poly­(methoxy diethylene glycol acrylate) PMDEGA. The polymers exhibited a LCST-type phase transition in the range of 20–40 °C, which markedly depended on molar mass and concentration. Accordingly, the triblock copolymers behaved as amphiphiles at low temperatures, but became water-insoluble at high temperatures. The temperature dependent self-assembly of the amphiphilic block copolymers in aqueous solution was studied by turbidimetry and rheology at concentrations up to 30 wt %, to elucidate the impact of the inner thermoresponsive block on the gel properties. Additionally, small-angle X-ray scattering (SAXS) was performed to access the structural changes in the gel with temperature. For all polymers a gel phase was obtained at low temperatures, which underwent a gel–sol transition at intermediate temperatures, well below the cloud point where phase separation occurred. With increasing length of the PMDEGA inner block, the gel–sol transition shifts to markedly lower concentrations, as well as to higher transition temperatures. For the longest PMDEGA block studied (DP<sub><i>n</i></sub> about 450), gels had already formed at 3.5 wt % at low temperatures. The gel–sol transition of the hydrogels and the LCST-type phase transition of the hydrophilic inner block were found to be independent of each other

    Binding of HSA to Macromolecular <i>p</i>HPMA Based Nanoparticles for Drug Delivery: An Investigation Using Fluorescence Methods

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    Amphiphilic poly­(<i>N</i>-(2-hydroxypropyl)­methacrylamide) copolymers (<i>p</i>HPMA) bearing cholesterol side groups in phosphate buffer saline self-assemble into nanoparticles (NPs) which can be used as tumor-targeted drug carriers. It was previously shown by us that human serum albumin (HSA) interacts weakly with the NPs. However, the mechanism of this binding could not be resolved due to overlapping of signals from the complex system. Here, we use fluorescence labeling to distinguish the components and to characterize the binding: On the one hand, a fluorescent dye was attached to <i>p</i>HPMA, so that the diffusion behavior of the NPs could be studied in the presence of HSA using fluorescence lifetime correlation spectroscopy. On the other hand, quenching of the intrinsic fluorescence of HSA revealed the origin of the binding, which is mainly the complexation between HSA and cholesterol side groups. Furthermore, a binding constant was obtained

    Structure and Dynamics of Asymmetric Poly(styrene‑<i>b</i>‑1,4-isoprene) Diblock Copolymer under 1D and 2D Nanoconfinement

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    The impact of 1- and 2-dimensional (2D) confinement on the structure and dynamics of poly­(styrene-<i>b</i>-1,4-isoprene) P­(S-<i>b</i>-I) diblock copolymer is investigated by a combination of Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Grazing-Incidence Small-Angle X-ray Scattering (GISAXS), and Broadband Dielectric Spectroscopy (BDS). 1D confinement is achieved by spin coating the P­(S-<i>b</i>-I) to form nanometric thin films on silicon substrates, while in the 2D confinement, the copolymer is infiltrated into cylindrical anodized aluminum oxide (AAO) nanopores. After dissolving the AAO matrix having mean pore diameter of 150 nm, the SEM images of the exposed P­(S-<i>b</i>-I) show straight nanorods. For the thin films, GISAXS and AFM reveal hexagonally packed cylinders of PS in a PI matrix. Three dielectrically active relaxation modes assigned to the two segmental modes of the styrene and isoprene blocks and the normal mode of the latter are studied selectively by BDS. The dynamic glass transition, related to the segmental modes of the styrene and isoprene blocks, is independent of the dimensionality and the finite sizes (down to 18 nm) of confinement, but the normal mode is influenced by both factors with 2D geometrical constraints exerting greater impact. This reflects the considerable difference in the length scales on which the two kinds of fluctuations take place

    Stimuli-Responsive Amphiphilic Polyelectrolyte Heptablock Copolymer Physical Hydrogels: An Unusual pH-Response

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    An amphiphilic cationic polyelectrolyte based on poly­[2-(dimethylamino)­ethyl methacrylate] (polyDMA) and poly­(<i>n</i>-butyl methacrylate) (polyBuMA) with a BuMA–DMA–BuMA–DMA–BuMA–DMA–BuMA heptablock copolymer architecture was studied in aqueous media. This copolymer was found to form a physical hydrogel via the intermolecular hydrophobic association (physical cross-linking) of the BuMA blocks. The rheological properties of the heptablock hydrogels were investigated as a function of copolymer concentration, and pH. The results showed a peculiar pH-dependence of the rheological properties, remarkably different from those observed with associative telechelic polyelectrolytes. Aqueous solutions of this copolymer were free-flowing sols at low pH (below 2) and high pH (above 8), whereas they turned into gels at intermediate pH values. The rheological properties studied as a function of pH showed two additional stiff–soft–stiff gel transitions at pH 4.5 and 6.5. Small-angle neutron scattering revealed the formation of a 3D transient network of bridged flower-like micelles whose structural characteristics, i.e., micellar radius, hard-sphere radius and hard-sphere volume fraction, were smoothly evolving with the pD

    From Molecular Dehydration to Excess Volumes of Phase-Separating PNIPAM Solutions

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    For aqueous poly­(<i>N</i>-isopropyl acrylamide) (PNIPAM) solutions, a structural instability leads to the collapse and aggregation of the macromolecules at the temperature-induced demixing transition. The accompanying cooperative dehydration of the PNIPAM chains is known to play a crucial role in this phase separation. We elucidate the impact of partial dehydration of PNIPAM on the volume changes related to the phase separation of dilute to concentrated PNIPAM solutions. Quasi-elastic neutron scattering enables us to directly follow the isotropic jump diffusion behavior of the hydration water and the almost freely diffusing water. As the hydration number decreases from 8 to 2 for the demixing 25 mass % PNIPAM solution, only a partial dehydration of the PNIPAM chains occurs. Dilatation studies reveal that the transition-induced volume changes depend in a remarkable manner on the PNIPAM concentration of the solutions. The excess volume per mole of H<sub>2</sub>O molecules expelled from the solvation layers of PNIPAM during phase separation probably strongly increases from dilute to concentrated PNIPAM solutions. This finding is qualitatively related to the immense strain-softening previously observed for demixing PNIPAM solutions

    Structure and Crystallization Behavior of Poly(ethylene oxide) (PEO) Chains in Core–Shell Brush Copolymers with Poly(propylene oxide)-<i>block</i>-poly(ethylene oxide) Side Chains

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    Core–shell brush copolymers featuring a poly­(<i>p</i>-hydroxystyrene) (PHOS) backbone and PPO-<i>b</i>-PEO (PPO and PEO stand for poly­(propylene oxide) and poly­(ethylene oxide)) side chains with different molecular compositions and exhibiting two inverse molecular architectures in regard to the side chains were investigated. Differential scanning calorimetry (DSC) and temperature-resolved wide- and small-angle X-ray scattering (WAXS/SAXS) were used to characterize the thermal and structural behavior. For the sample with the crystallizable PEO block linked directly to the backbone and a high PEO fraction (84.9 wt %), our results reveal a PEO crystallization/melting behavior similar to the one of bulk PEO. Surprisingly, the crystalline order, as determined by WAXS, persists up to 30 K above the melting point determined by DSC (<i>T</i><sub>m</sub> = 54 °C). For the samples where the PPO block is directly linked to PHOS backbone and the PEO chains are dangling, our results indicate that the side arm architecture has remarkable effects on the thermal and structural behavior. With decreasing PEO fraction in the side arms, the calorimetric crystallization temperature, <i>T</i><sub>c</sub>, and the melting point, <i>T</i><sub>m</sub>, of the PEO domains are strongly suppressed, reaching values as low as −45 °C and −8 °C, respectively. Furthermore, PEO crystallizes in an asymmetric lamellar phase with a distorted PEO crystalline phase. Above <i>T</i><sub>m</sub> the morphology changes from microphase-separated symmetric lamellae to hexagonally perforated lamellae with PEO domains immersed within a PHOS/PPO matrix with decreasing PEO fraction. Our results suggest that this specific brush copolymer architecture allows for tuning the ability of PEO blocks to crystallize

    “Schizophrenic” Micelles from Doubly Thermoresponsive Polysulfobetaine‑<i>b</i>‑poly(<i>N</i>‑isopropylmethacrylamide) Diblock Copolymers

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    The 2-fold thermoresponsive diblock copolymer PSPP<sub>498</sub>-<i>b</i>-PNIPMAM<sub>144</sub>, which consists of a zwitterionic polysulfobetaine (PSPP) block and a nonionic poly­(<i>N</i>-isopropylmethacrylamide) (PNIPMAM) block, is prepared by consecutive RAFT polymerizations. It combines the upper and lower critical solution temperature (UCST and LCST) behaviors, respectively, of the constitutive homopolymers in aqueous solution. We investigate the temperature-dependent phase behavior and the self-assembled structures of the block copolymer in D<sub>2</sub>O by turbidimetry and by small-angle neutron scattering (SANS) in salt-free solution and in the presence of small amounts of NaCl and NaBr. For comparison, solutions of PNIPMAM homopolymer in D<sub>2</sub>O are studied as well. Turbidimetry indicates thermally induced “schizophrenic” aggregation behavior for PSPP<sub>498</sub>-<i>b</i>-PNIPMAM<sub>144</sub>. SANS reveals that conventional star-like core–shell micellar structures are formed above the LCST transition, whereas below the UCST-transition, structure formation is much less pronounced. This is attributed to the different types of interactions, namely hydrophobic and ionic ones, dominating in the different regimes. Despite the increased polarity contrast between the zwitterionic and the nonionic blocks, and the much wider separation of the UCST- and LCST-based cloud points, CP<sub>UCST</sub> and CP<sub>LCST</sub>, the structural features of the new PSPP<sub>498</sub>-<i>b</i>-PNIPMAM<sub>144</sub> resemble the ones found previously for the also 2-fold thermoresponsive analogue PSPP<sub>432</sub>-<i>b</i>-PNIPAM<sub>200,</sub> for which both phase transition temperatures nearly coincide. Remarkably, the addition of small amounts of NaBr or NaCl to the solution of PSPP<sub>498</sub>-<i>b</i>-PNIPMAM<sub>144</sub> causes a significant increase of CP<sub>UCST</sub>, as well as minor but notable changes in the self–assembled structures, but no gross alterations of the phase behavior

    Aggregation Behavior of Doubly Thermoresponsive Polysulfobetaine‑<i>b</i>‑poly(<i>N</i>‑isopropyl­acrylamide) Diblock Copolymers

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    A 2-fold thermoresponsive diblock copolymer PSPP<sub>430</sub>-<i>b</i>-PNIPAM<sub>200</sub> consisting of a zwitterionic polysulfobetaine (PSPP) block and a nonionic poly­(<i>N</i>-isopropyl­acrylamide) (PNIPAM) block is prepared by successive RAFT polymerizations. In aqueous solution, the corresponding homopolymers PSPP and PNIPAM feature both upper and lower critical solution temperature (UCST and LCST) behavior, respectively. The diblock copolymer exhibits thermally induced “schizophrenic” aggregation behavior in aqueous solutions. Moreover, the ion sensitivity of the cloud point of the zwitterionic PSPP block to both the ionic strength and the nature of the salt offers the possibility to create switchable systems which respond sensitively to changes of the temperature and of the electrolyte type and concentration. The diblock copolymer solutions in D<sub>2</sub>O are investigated by means of turbidimetry and small-angle neutron scattering (SANS) with respect to the phase behavior and the self-assembled structures in dependence on temperature and electrolyte content. Marked differences of the aggregation below the UCST-type and above the LCST-type transition are observed. The addition of a small amount of NaBr (0.004 M) does not affect the overall behavior, and only the UCST-type transition and aggregate structures are slightly altered, reflecting the well-known ion sensitivity of the zwitterionic PSPP block
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