Triblock Copolymers in a Selective Solvent. 1. Aggregation Process in Dilute Solution

Asymmetric polymer mixtures in good solvents: A light scattering study

Dilute solutions of asymmetric polystyrene/poly(methyl methacrylate) (PS/PMMA) mixtures in toluene and tetrahydrofuran (THF) are investigated by static and dynamic light scattering (SLS and DLS). Both solvents are good solvents for each of the two polymers. In toluene, the PMMA refractive index increment is close to zero and SLS provides a direct measurement of the PS static scattering functions. DLS correlatively leads to a single relaxation mode connected with these PS chains. Contrarily, two modes well separated in time are observed for identical PS/PMMA mixtures in THF. Scattering from the PMMA chains is no longer negligible and partial static scattering functions can only be obtained through SLS-DLS combination. For identical polymer concentration and PS/PMMA mixture composition, PS scattering functions are then found to be different in both solvents. The difference increases with concentration and is only partly due to distinct thermodynamic parameters according to the solvent. PMMA scattering functions lead to similar conclusions. The Random Phase Approximation (RPA) describes all scattering functions. However, the SLS-DLS combination affords a reasonable approximation for extracting the partial static scattering functions only for the lowest concentrations, i.e. provided the cross polymer correlation term remains negligible.

Viscosity of Plasma as a Key Factor in Assessment of Extracellular Vesicles by Light Scattering

Extracellular vesicles (EVs) isolated from biological samples are a promising material for use in medicine and technology. However, the assessment methods that would yield repeatable concentrations, sizes and compositions of the harvested material are missing. A plausible model for the description of EV isolates has not been developed. Furthermore, the identity and genesis of EVs are still obscure and the relevant parameters have not yet been identified. The purpose of this work is to better understand the mechanisms taking place during harvesting of EVs, in particular the role of viscosity of EV suspension. The EVs were harvested from blood plasma by repeated centrifugation and washing of samples. Their size and shape were assessed by using a combination of static and dynamic light scattering. The average shape parameter of the assessed particles was found to be ρ ~ 1 (0.94–1.1 in exosome standards and 0.7–1.2 in blood plasma and EV isolates), pertaining to spherical shells (spherical vesicles). This study has estimated the value of the viscosity coefficient of the medium in blood plasma to be 1.2 mPa/s. It can be concluded that light scattering could be a plausible method for the assessment of EVs upon considering that EVs are a dynamic material with a transient identity.

Enthalpy-driven micellization of oligocarbonate-fluorene end-functionalized Poly(ethylene glycol)☆

A fluorescent pyrene probe method was applied to measure the critical micelle concentration (CMC) of oligocarbonate-fluorene end-functionalized poly(ethylene glycol) (FmE445Fm) triblock copolymers in water. The CMC decreases with lower temperature and higher values of the hydrophobic block length, m. When analyzed by a closed-assembly micelle model, the estimated energetic parameters find a negative ΔH°mic and small positive ΔS°mic suggestive of enthalpy-driven micellization, which differs from entropy-driven oxyethylene/oxybutylene triblock copolymers and octaethylene glycol-n-alkyl ethers. The enthalpy-driven micellization of FmE445Fm may result from the limited hydration of individual hydrophobic F blocks that leads to few hydrogen-bonded waters released during F block association. The π-π stacking oligocarbonate-fluorene system also observed enthalpy-entropy compensation when compared to a series of published data on diblock and triblock copolymer systems. An anomalously low partition equilibrium constant for m = 15.3 implies a tightly-packed core that excludes pyrene intercalation into the fluorene core. This is discussed along with the possible limited applicability to estimate the CMC and potential model drug molecule insertions into the intercalated micelle core.

Self-organization of hydrophobic-capped triblock copolymers with a polyelectrolyte midblock: a coarse-grained molecular dynamics simulation study

We present the results of a Langevin dynamics simulation study of micellar organization and hydrogel formation in the solutions of coarse-grained ABA copolymer chains. Polymer chains are modeled as bead-spring chains of Lennard-Jones particles by explicit treatment of ionic species in implicit solvent. The studied copolymer is composed of a polyelectrolyte midblock flanked by two hydrophobic endblocks. We explore the self-assembly of copolymer solutions at a fixed polymer concentration and temperature upon systematic variation of the midblock charge fraction, valency of neutralizing counterions, and the stiffness and length of hydrophobic endblocks. Minimization of the surface energy, conformational entropy of the midblock chains, electrostatic repulsion of midblock charges, and the translational entropy of counterions are found to play central roles in controlling the self-organization features of copolymer solutions. Flower-like micelles with A-blocks forming the core of spherical aggregates and B-blocks constituting the micelle corona are established for the neutral midblocks. Increasing the charge content of B chains lowers the fraction of loop conformations and yields a spanning hydrogel network with midblocks bridging the hydrophobic clusters. Counterion valence is shown to exert a strong effect on the micelle size and network structure. The increase in the rigidity of terminal A-blocks increases the fraction of bridging chains and results in the formation of a hydrogel network with bundle-like hydrophobic domains. Longer endblocks are shown to increase the hydrophobic cluster size and enhance the bridged midblock fraction. The qualitative agreement between the experimental and theoretical studies is also discussed. The comprehensive molecular picture provides a framework for the future studies of stimuli-responsive copolymer systems.

Entangled triblock copolymer gel: morphological and mechanical properties

The morphological and mechanical properties of entangled ABA triblock copolymer gels, where solvent were selective to the midblock, were studied as a function of polymer concentration using a novel dissipative particle dynamics model which includes a modified segmental repulsive potential that restricts chain crossing. Morphological properties, such as micelle size, distance between micelles, and the bridge fraction, were calculated as a function of concentration. Although the micelle size was shown to have a strong dependence on concentration, the bridge fraction and distance between micelles were shown to plateau at moderate concentrations. Deformation under uni-axial tension was also performed to extract the cross-link and entanglement contribution to the elastic modulus. Scaling results qualitatively agree with other theoretical predications.

Phase behavior of reverse poloxamers and poloxamines in water

The phase behavior of two types of poly(ethylene oxide)/poly(propylene oxide) (PEO/PPO) copolymers in aqueous solutions was studied by light scattering, viscometry, and infrared spectroscopy. Both the reverse poloxamer (Pluronic 10R5) and the star type poloxamine (Tetronic 90R4) have practically the same PEO/PPO ratio with their hydrophobic blocks (PPO) located in the outer part. The temperature-composition phase diagrams show that both 10R5 and 90R4 tend to form aggregates in water. Up to four different phases can be detected in the case of Tetronic 90R4 for each temperature: unimers, random networks, micellar networks, and macrophase separation. Viscometric and infrared measurements complemented the results obtained by light scattering and visual inspection.

Controlled release of linear-dendritic hybrids of carbosiloxane dendrimer: the effect of hybrid's amphiphilicity on drug-incorporation; hybrid-drug interactions and hydrolytic behavior of nanocarriers

Dendritic micelles formed from amphiphilic dendritic ABA triblock copolymers based on organic linear poly(ethylene oxide) and inorganic dendritic block containing silicon atoms (OSC-D(Gn)-PEO-D(Gn)-CSO, n=1-3)(1) were evaluated as drug delivery vehicles for a drug in both lipophilic and hydrophilic forms. The physical parameters of the drug-incorporated carriers including the influences of drug:carrier ratio, the release kinetics of the drugs from the micellar solution were measured. The apparent partition constant of drug between the carriers and the external medium was studied as well. It was observed that the loading efficiency and hydrolytic behavior of the hybrids depend on several factors, such as type of interaction between host and guest molecules, generation of the dendritic copolymers and pH. The release profiles of the drugs from the micelle solution were found to be a slow steady release at pH 1, 7.4 and 10. Investigation of the drug release dynamics in buffered media at pH 7.4 showed that the drug released through the carriers with slight deviation follow Fickian and Case II diffusion mechanisms for drugs in lipophilic and hydrophilic forms, respectively.

Complexation of oppositely charged polyelectrolytes and surfactants in aqueous solutions. A review

Addition of surfactants to aqueous solutions of polyelectrolytes causes the spontaneous formation of complexes in a certain range of concentrations. In some conditions, compact monodisperse multichain complexes are obtained (short surfactant chain length and polymer rigid enough). The size of the complexes can be varied in controlled way from nanometers up to micrometers, but depends on the mixing procedure, whereas the shape of the complexes depends on the polymer backbone rigidity. These complexes exhibit microstructures analogue to that of the precipitates formed at higher concentrations. In most cases, however, the complexes are large, soft and polydisperse.

Aggregation behavior of a new series of ABA triblock copolymers bearing short outer A blocks in B-selective solvent: from free chains to bridged micelles

A combination of dynamic (DLS) and static (SLS) light scattering measurements was employed to study the self-assembly behavior of a new series of triblock copolymers bearing poly[5-(N,N-diethylamino isoprene)] (PAI) short outer blocks and polystyrene (PS) as the major middle block. Previously, it was verified that PAI outer blocks can be quaternized leading the formation of crew-cut aggregates in water (Riegel, I. C.; Eisenberg, A.; Petzhold, C. L.; Samios, D. Langmuir 2002, 18, 3358). Herein, we focus on the copolymer's ability in the nonquaternized version to undergo self-aggregation in dimethylformamide (DMF), a selective solvent for the middle block. Light scattering measurements showed that formation of well-defined flowerlike micelles is likely to occur. Aggregates with a relatively narrow distribution, small average size, and number of aggregation ranging from 21 to 39 chains/micelle were experimentally observed. The results also suggested that approximately 5-6 polymeric units per each short outer block are needed to induce aggregation. The middle block length governs the size of the micelles and influences the number of aggregation of the resultant particles as well. Furthermore, when the polystyrene middle block was particularly long (degree of polymerization DP > 600), dynamic and static light scattering measurements suggested the formation of bridged micelles in an open structure in concentrations as low as 15 mg mL-1.

Biocompatible wound dressings based on chemically degradable triblock copolymer hydrogels

The synthesis of a series of thermo-responsive ABA triblock copolymers in which the outer A blocks comprise poly(2-hydroxypropyl methacrylate) and the central B block is poly(2-(methacryloyloxy)ethyl phosphorylcholine) is achieved using atom transfer radical polymerization. These novel triblock copolymers form thermo-reversible physical gels with critical gelation temperatures and mechanical properties that are highly dependent on the copolymer composition and concentration. TEM studies on dried dilute copolymer solutions indicate the presence of colloidal aggregates, which is consistent with micellar gel structures. This hypothesis is consistent with the observation that incorporating a central disulfide bond within the B block leads to thermo-responsive gels that can be efficiently degraded using mild reductants such as dithiothreitol (DTT) over time scales of minutes at 37 degrees C. Moreover, the rate of gel dissolution increases at higher DTT/disulfide molar ratios. Finally, these copolymer gels are shown to be highly biocompatible. Only a modest reduction in proliferation was observed for monolayers of primary human dermal fibroblasts, with no evidence for cytotoxicity. Moreover, when placed directly on 3D tissue-engineered skin, these gels had no significant effect on cell viability. Thus, we suggest that these thermo-responsive biodegradable copolymer gels may have potential applications as wound dressings.

Thermoreversible hydrogels from RAFT-synthesized BAB triblock copolymers: steps toward biomimetic matrices for tissue regeneration

Narrowly dispersed, temperature-responsive BAB block copolymers capable of forming physical gels under physiological conditions were synthesized via aqueous reversible addition fragmentation chain transfer (RAFT) polymerization. The use of a difunctional trithiocarbonate facilitates the two-step synthesis of BAB copolymers with symmetrical outer blocks. The outer B blocks of the triblock copolymers consist of poly(N-isopropylacrylamide) (PNIPAM) and the inner A block consists of poly(N,N-dimethylacrylamide). The copolymers form reversible physical gels above the phase transition temperature of PNIPAM at concentrations as low as 7.5 wt % copolymer. Mechanical properties similar to collagen, a naturally occurring polypeptide used as a three-dimensional in vitro cell growth scaffold, have been achieved. Herein, we report the mechanical properties of the gels as a function of solvent, polymer concentration, and inner block length. Structural information about the gels was obtained through pulsed field gradient NMR experiments and confocal microscopy.

Aggregate formation in aqueous solutions of carboxymethylcellulose and cationic surfactants

The addition of cationic surfactants to an aqueous solution of an anionic polymer, carboxymethylcellulose (carboxyMC), causes the spontaneous formation of aggregates in a certain range of concentrations. Here we studied two surfactants, dodecyl and hexadecyl trimethylammonium bromide (DTAB and CTAB, respectively). Using different techniques (light scattering, potentiometry, viscosimetry, and zetametry), we found that a simple lengthening of the surfactant tail length by four CH2 groups drastically changes the aggregate morphology, size, and charge. We explored in detail how the surfactant and polymer concentrations act on these systems.

Self-assembly of metal-polymer analogues of amphiphilic triblock copolymers

Organized arrays of anisotropic nanoparticles show electronic and optical properties that originate from the coupling of shape-dependent properties of the individual nanorods. The organization of nanorods in a controllable and predictable way provides a route to the fabrication of new materials and functional devices. So far, significant progress has been achieved in the self-assembly of nanorod arrays, yet the realization of a range of different structures requires changing the surface chemistry of the nanoparticles. We organized metal nanorods in structures with varying geometries by using a striking analogy between amphiphilic ABA triblock copolymers and the hydrophilic nanorods tethered with hydrophobic polymer chains at both ends. The self-assembly was tuneable and reversible and it was achieved solely by changing the solvent quality for the constituent blocks. This approach provides a new route to the organization of anisotropic nanoparticles by using the strategies that are established for the self-assembly of block copolymers.

Dynamics of critical fluctuations in polymer solutions

Using dynamic light scattering we have investigated the time dependence of fluctuations near the critical point of phase separation in solutions of polystyrene in cyclohexane with polymer molecular weights ranging from 196,000 to 11.4 x 10(6) g mol(-1). At the lowest polymer molecular weight the dynamic correlation function follows a single-exponential decay with a decay rate that can be represented by the mode-coupling theory of critical dynamics but with a mesoscopic viscosity that characterizes the hydrodynamic environment of the polymers in the solution. At all higher polymer molecular weights two distinct dynamic modes are observed, a slow and a fast mode, that originate from a coupling of the critical concentration fluctuations with viscoelastic relaxation of the polymer chain in solutions. This coupling causes an additional slowing down of the fluctuations on top of the well-known critical slowing down expected in the absence of a coupling between the two modes. From an analysis of the time dependence of the experimental dynamic correlation functions in terms of a theory of coupling of dynamic modes we are able to determine the viscoelastic properties of the polymers in the solution. These viscoelastic properties diverge in the theta-point limit of infinite polymer molecular weight.

Self-association of bis-(α,β-d-glucopyranosyl)-polyisobutylene

In this paper, we report the structure and apparent molecular weights of bis-(alpha,beta-D-glucopyranosyl)-polyisobutylene (Gluc-PIB-Gluc) aggregates in CDCl(3) by NMR spectroscopy. Analysis of DOSY (diffusion-ordered NMR spectroscopy) experiments of a solution of Gluc-PIB-Gluc showed the presence of aggregates that were corroborated with dynamic light scattering. The structure of the aggregates was also studied by correlation spectroscopy (COSY) and nuclear Overhauser effect spectroscopy (NOESY) experiments.

Aggregation during coniferyl alcohol polymerization in pectin solution: a biomimetic approach of the first steps of lignification

Coniferyl alcohol was polymerized in pectin solution in order to mimic the lignification that is the final step of biosynthesis of plant cell wall. Dehydrogenated polymers (DHP = coniferyl alcohol polymers = synthetic lignin) interact with pectin to form hydrophobic clusters as monitored by pyrene fluorescence spectroscopy. The structure of these clusters was studied during the polymerization of synthetic lignin by static and quasielastic light scattering and small angle neutron scattering experiments. We show that synthetic lignin and pectin contribute to the same clusters, but the inner structure of these clusters is very heterogeneous and displays three phases. One observes a segregation between well separated pectin and lignin rich phases at length scales below approximately 30 nm. As a corollary of this segregation, clusters embody a large amount of solvent. On average, the density of the polymer rich phase (lignin plus pectin) inside clusters increases while its specific surface area decreases throughout the polymerization process. These results are discussed with respect to in vivo lignification of the plant cell wall.