Diblock Copolymer Solutions Research Articles

Junction-Controlled Elasticity of Single-Walled Carbon Nanotube Dispersions in Acrylic Copolymer Gels and Solutions

Oscillatory shear rheometry is used to study the mechanical response of single-walled carbon nanotubes dispersed in solutions of acrylic diblock or triblock copolymers in 2-ethyl-1-hexanol. Thermal transitions in the copolymer solutions provide a route for the easy processing of these composite materials, with excellent dispersion of the nanotubes as verified by near-infrared photoluminescence spectroscopy. The nanotube dispersions form elastic networks with properties that are controlled by the junction points between nanotubes, featuring a temperature-dependent elastic response that is controlled by the dynamic properties of the matrix copolymer solution. The data are consistent with the formation of micelle-like aggregates around the nanotubes. At low temperatures the core-forming poly(methyl methacrylate) blocks are glassy, and the overall mechanical response of the composite does not evolve with time. At higher temperatures the enhanced mobility of the core-forming blocks enables the junctions to ach...

Mixed versus Ordinary Micelles in the Dilute Solution of AB and BC Diblock Copolymers

We predict the thermodynamic stability of mixed micelles of various morphologies (spheres, cylinders, lamellae) in the solution of AB and BC diblock copolymers in a selective solvent. The B blocks are assumed to be insoluble and form the core of the micelle, whereas soluble, incompatible (with each other) A and C blocks form the corona. A homogeneous mixing of the A and C blocks in the corona (mixed micelles) is possible if their incompatibility is relatively weak. Otherwise, a mixture of “pure” (ordinary) micelles consisting of either AB or BC copolymers is stable. Phase behavior of the solution, which depends on the relative and overall lengths of the blocks, and on the surface tension coefficients, is analyzed.

A Rheo-optical Study of Stress−Fluctuations Coupling in a Disordered and Entangled Diblock Copolymer Solution

We report on rheo-optical experiments performed with an entangled asymmetric polystyrene−polyisoprene diblock copolymer solution in dioctyl phthalate (DOP), in the regime of pretransitional fluctua...

Solubilisation of drugs in micellar solutions of diblock copolymers of ethylene oxide and styrene oxide

The solubilisation of two poorly soluble drugs, furosemide and nabumetone, in micellar solutions of diblock copolymers of ethylene oxide and styrene oxide has been studied at 25 and 37 °C and solubilisation capacities compared with published values for griseofulvin and docetaxel. Solubilisation in the micelle core, corrected for the different proportions of poly(styrene oxide) in the copolymers, was similar for all four drugs. The highest solubilisation capacities were found for a copolymer with worm-like micelles.

Cononsolvency-Induced Micellization of Pyrene End-Labeled Diblock Copolymers of N-Isopropylacrylamide and Oligo(ethylene glycol) Methyl Ether Methacrylate

Pyrene end-labeled double hydrophilic diblock copolymers, poly(N-isopropylacrylamide)-b-poly(oligo(ethylene glycol) methyl ether methacrylate) (Py-PNIPAM-b-POEGMA), were synthesized via consecutive reversible addition-fragmentation chain transfer polymerization using a pyrene-containing dithioester as the chain transfer agent. These diblock copolymers molecularly dissolve in pure methanol and water, but form well-defined and nearly monodisperse PNIPAM-core micelles in an appropriate mixture of them due to the cononsolvency of PNIPAM block. 1H NMR, laser light scattering, fluorescence spectroscopy, and transmission electron microscopy were employed to characterize the cononsolvency-induced PNIPAM-core micelles. When the volume fraction of water, phi water, in the methanol/water mixture is in the range of 0.5-0.8, the sizes of micelles are in the range of 20-30 nm in radius for Py-PNIPAM50-b- POEGMA18. At phi water = 0.5, the formed micelles possess the highest overall micelle density and the largest molar mass. The effects of varying the block lengths of Py-PNIPAM-b-POEGMA diblock copolymers on the structural parameters of PNIPAM-core micelles have also been explored. Although we can observe the immediate appearance of bluish tinge upon mixing the diblock copolymer solution in methanol with equal volume of water (phi water = 0.5), which is characteristic of the formation of micellar aggregates, the whole micellization process apparently takes a relatively long time to complete, as revealed by monitoring the time dependence of fluorescence emission spectra. The excimer/monomer fluorescence intensity ratios, IE/IM, continuously decrease with time and then reach a plateau value after approximately 20 min. The decrease of IE/IM after the initial formation of pseudo-equilibrium micelles should be ascribed to the structural rearrangement and further packing of PNIPAM segments within the micelle core, restricting the mobility of pyrene end groups and decreasing the probability of contact between them. Compared to the conventional cosolvent approach employed for the micellization of block copolymers in selective solvents, the reported cononsolvency-induced unimer-micelle-unimer transition of Py-PNIPAM-b-POEGMA in methanol/water mixtures has been unprecedented.

Effect of Copolymer Architecture on the Micellization and Gelation of Aqueous Solutions of Copolymers of Ethylene Oxide and Styrene Oxide

The micellar properties and solubilization capacity of poorly water soluble drugs of several micellar and gel solutions of diblock and triblock copolymers of styrene oxide/ethylene oxide have been measured and compared with block copolymers of butylene oxide/ethylene oxide, showing that the solubilization capacity of the styrene oxide block is approximately four times that of a butylenes oxide block for dilute solutions. To continue establishing the correlation between micellar characteristics and solubilization capacity, we have found it interesting to compare the micellar and gelation properties of the diblock and triblock copolymers PSO10PEO135 and PEO69PSO8PEO69 (subindexes are the number-average block lengths), with different architecture but similar average block lengths. Surface tension measurements allowed the determination of the critical micelle concentrations at several temperatures and, so, to calculate standard enthalpies of micellization. Static and dynamic light scattering data permitted us to determine micellar parameters and to obtain qualitatively the extent of hydration of the copolymer micelle. A tube inversion method was used to define the mobile-immobile (soft-hard gel) phase boundary. To refine the phase diagram and observe the existence of additional phases, rheological measurements were done. The results are in good agreement with previous values published for PSOnPEOm and PEOmPSOnPEOm copolymers.

Reverse Thermal Organogel

Poly(ethylene glycol)-polypeptide diblock copolymer solutions undergo a clear-sol-to-clear-gel transition (reverse thermal gelation) as the temperature increases at as low a concentration as 1.0 wt % in chloroform (see figure). The micellar aggregation model involving desolvation of the poly(ethylene glycol) is suggested for the unique behavior of the polymer solution.

Multiple morphologies of self-assembled star aggregates of amphiphilic PEO-b-PNPMA diblock copolymers in solution, synthesis and micellization

Novel amphiphilic block copolymers, poly(ethylene oxide)-b-poly(p-nitrophenyl methacrylate) (PEO-b-PNPMA) with controlled molecular weights and narrow molecular weight distribution were successively synthesized by ATRP of NPMA using PEO–Br as initiator. Self-assembling of the diblock copolymer PEO113-b-PNPMA28 in the different solvent mixtures yielded various morphologies of star micelle-like aggregates, such as spheres, vesicles, cauliflower-like aggregates and rod-like aggregates, which are determined by the nature of the common solvents and the selective solvents. Thus the critical selective solvent contents and the solvent contents in PNPMA-rich phase were measured, and they have the following order: ethanol>methanol>water, and THF>CH3NO2>DMSO. The probable self-assembling mechanism is discussed. This method is convenient for preparation of multiple morphological star micelle-like aggregates in solution, especially from the amphiphilic block copolymers with relatively longer block shell.

Isotropic-nematic phase transition in athermal solutions of rod-coil diblock copolymers

We present a hybrid method to investigate the isotropic-nematic (I-N) transition in athermal solutions of rod-coil copolymers. This method incorporates the scaled-particle theory for semiflexible chains with two-chain Monte Carlo simulation for the osmotic second virial coefficient and for the angle-dependent excluded volume. We compare the theoretical prediction with Monte Carlo simulations for fused rod-coil copolymers and find good agreement for both the equation of state and the orientational order parameter. The theory is also used to examine the effects of the bond length, the chain length, and the chain composition on orientational ordering in athermal solutions of rod-coil block copolymers. It predicts I-N transition in rod-coil copolymers with fixed rod length but a variable flexible tail in good agreement with experiments.

Release Profile of BSA or BDNF from Temperature Sensitive Hydrogels

Methoxy poly(ethylene glycol) (MPEG)-b-poly(ε-caprolactone) (PCL) diblock copolymer was synthesized by ring-opening of ε-caprolactone (ε-CL) in the presence of a monomer activator with the terminal alcohol of MPEG as an initiator. The temperature sensitive behavior of the prepared MPEG-PCL diblock copolymer solution was examined. The polymer solution formed translucent sol at the room temperature. As the temperature increased from room temperature, the sol became gel, indicating that the diblock copolymer solution at room temperature can form gel at body temperature. Brain-derived neurotrophic factor (BDNF) loaded MPEG-PCL diblock copolymer solution and Pluronic solution for comparison were prepared to examine the release behavior of BDNF. Pluronic gel exhibited nearly complete release of BDNF even at 2 day, while the release of BDNF in MPEG-PCL gel showed the prolonged release profile for 21 days. In this study, we confirmed that thermosensitive MPEG-PCL diblock copolymer in this work could utilize as a potential carrier of BDNF.

Fluctuation Effects on Viscoelastic Properties of Diblock Copolymer Solutions in Disordered State

Viscoelastic properties of three poly(styrene-d8-block-isoprene)s in dioctyl phthalate are measured under oscillatory and steady shear flows in the disordered state, to examine the fluctuation effects on zero shear viscosity η0 and steady state compliance Je based on the effective interaction parameter (χN)eff. Flow-induced microphase separation is not observed when (χN)eff is lower than 9.9. Enhancement of η0 and Je due to the fluctuation effects, which is more obvious for Je, are observed in oscillatory flow, while suppression of the fluctuation effects are observed in steady shear flow, which is confirmed by small angle neutron scattering measurements. Comparing these results with those for poly(styrene-block-2-vinylpyridine)s (SP), it is concluded that the viscosity enhancement due to the fluctuation effects slightly depend on component viscosities. When viscosity ratio of components is close to 1 (i.e., SP), the viscosity enhancement is negligible. On the other hand, enhancement of reduced steady state compliance JeR seems to be insensitive to the viscosity ratio of components.

Aggregates Formed by Poly(Ethylene Oxide)-b-Poly(1H, 1H-Perfluorooctyl Methacrylate) Diblock Copolymers in Solutions

Well-defined block copolymers consisting of a hydrophilic poly (ethylene oxide) (PEO) and lipophobic poly (1H,1H-perfluorooctyl methacrylate) (PFOMA) were synthesized with controlled molecular weight via atom transfer radical polymerization (ATRP). The block copolymers formed stable micelles where PEO is the shell and PFOMA is the core in chloroform. The micellar morphology of PEO5K-b-PFOMA5K was found to be spherical with average diameter of ca. 12 nm, while meandering cylindrical with average diameter of ca. 13 nm was observed for PEO5K-b-PFOMA13K by TEM analysis. The self-assembled structures were reorganized to thermodynamically stable morphologies upon annealing above glass transition temperature (Tg). For example, spherical shape of PEO5K-b-PFOMA5K and cylindrical domain of PFOMA block in PEO5K-b-PFOMA5K became ordered cylindrical and continuous phase, respectively.

In Vitro and in Vivo Release of Albumin Using a Biodegradable MPEG-PCL Diblock Copolymer as an in Situ Gel-Forming Carrier

An MPEG-PCL diblock copolymer was synthesized as an in situ gel carrier, and its phase transition behavior in aqueous solutions was examined. For comparison, aqueous solutions of Pluronic F-127, a widely used injectable gel-forming solution, were also studied. Both MPEG-PCL copolymer and Pluronic aqueous solutions were sols at room temperature. As the temperature was increased above room temperature, the diblock copolymer and Pluronic solutions underwent a sol-to-gel phase transition, which manifested as an increase in viscosity indicative of the formation of a gel. All of the copolymer solutions became gels at body temperature, although the gel viscosity increased with the increasing concentration of the MPEG-PCL diblock copolymer in the solution. In in vitro experiments, in which the gels were exposed to PBS, the MPEG-PCL gels maintained their structural integrity for more than 28 days, whereas the Pluronic gel disappeared within 2 days. The same results were observed when the polymer solutions were subcutaneously injected into rats. The MPEG-PCL gels maintained their structural integrity longer than 30 days, while the Pluronic gel could not be observed after 2 days. The ability of the gels as drug carriers was studied by measuring the release of fluorescein isothiocyanate-labeled bovine serum albumin (BSA-FITC) from MPEG-PCL diblock copolymer gels in vitro as well as in vivo. In vitro, BSA release was sustained above 20 days, with a greater release at lower diblock copolymer concentration; by contrast, Pluronic gels exhibited almost complete release of BSA-FITC within 1 day. When the BSA-FITC-loaded diblock copolymer and Pluronic solutions were subcutaneously injected into rats, they immediately transformed into a gel. In vivo, sustained release of BSA-FITC over 30 days was observed from the MPEG-PCL gel, whereas BSA-FITC release from the Pluronic gel ceased within 3 days. Collectively, the present findings show that MPEG-PCL diblock copolymer solutions are thermo-responsive and maintain their structural integrity under physiological conditions, indicating that they are suitable for use as injectable drug carriers.

A Dynamic Light Scattering Study of Fast Relaxations in Polymer Solutions

We have analyzed the fine structure of the distribution of relaxation times obtained from dynamic light scattering on solutions of homopolymers and diblock copolymers. We have shown that in a carefully designed experiment two additional fast modes can be identified in the distribution of relaxation times, besides the usual cooperative mode, heterogeneity mode, and cluster mode. We have shown that the faster of these two modes, well observable only at small angles, is due to the thermal diffusion coefficient of the solvent. For the slower of these two modes, several experimental findings lead to the conclusion that this mode is representative of the motion of the solvent molecules in the solution.

Self-Assembly of Amphiphilic ABC Star Triblock Copolymers and Their Blends with AB Diblock Copolymers in Solution: Self-Consistent Field Theory Simulations

The self-assembled morphologies of amphiphilic ABC star triblock copolymers consisting of hydrophilic A blocks and hydrophobic B and C blocks and the blends with their counterpart linear AB diblock copolymers in solution are investigated by 2D real-space implementation of self-consistent field theory (SCFT) simulation. The star triblock copolymers self-assemble in solution to form various micellar structures from hamburger, to segmented wormlike, to toroidal segmented micelles, and finally to vesicles with simultaneously increasing hydrophobic lengths of blocks B and C. When the length of hydrophobic blocks B and C is asymmetric, specific bead-on-string worm micelles are found. Particularly, when the star ABC triblock copolymer is in a strong segregation regime and both B and C blocks are strongly hydrophobic, quite long segmented wormlike micelles are obtained, which had not been found in previously investigated diblock and linear ABC triblock copolymers solution. Additionally, raspberry micelles with beads dispersed on the core also occur in the strong segregation regime of bulk star ABC triblock copolymers. Furthermore, the aggregate morphology of ABC star triblock copolymers is strongly influenced by the addition of linear AB diblock copolymers. The most significant feature is that the long segmented worms will become shorter, to form hamburger micelles with the addition of AB diblock copolymers. These simulations are in good agreement with the experimental findings by Lodge's group.

Three-Dimensional Analysis of Dynamic Light Scattering Data: Application to Self-Organized Polymer Solutions

Semi-dilute solutions of diblock copolymers in selective partially miscible solvent mixtures have been studied by dynamic light scattering and pulsed-field gradient NMR. Several dynamic modes have been identified as cooperative diffusion, polymer self-diffusion, and cluster diffusion. Their temperature dependences changed dramatically at a certain temperature, below which solutions underwent self-organization. Interpretation of the complex behavior of the dynamic processes observed has been made. The importance of a 3-D representation of the distributions of relaxation times is demonstrated.

Athermal models for diblock copolymer self-assembly

Recent work on coarse-grained representations of interacting homopolymers and diblock copolymers in solution is briefly reviewed. Long homopolymer chains are modelled as ‘ultrasoft colloids’, with the effective interaction between their centres of mass (CM) determined by averaging over polymer conformations for fixed CM–CM distances. Diblock copolymers are modelled as ‘ultrasoft dumbells’, with the CMs of the two strands tethered by an anharmonic intramolecular potential. Monte Carlo simulations of these coarse-grained models allow the self-assembly of model diblock copolymers into lamellar and micellar microphases to be investigated. We show that the onset of clustering of two athermal models of diblock copolymers can be predicted within the ‘reference interaction site model’ (RISM) integral equation theory for the pair structure, using the simple random phase closure assumption. This analytic formulation highlights the natural link between bulk phase separation of binary polymer mixtures and microphase separation of the corresponding diblock copolymer in the same selective solvent.

Stoichiometric polyelectrolyte complexes of ionic block copolymers and oppositely charged polyions

Micellization in dilute solutions of diblock copolymers with a polyelectrolyte and a hydrophilic nonionic blocks and oppositely charged polyions is studied using mean-field theory. In aqueous solutions the micelle core consists of the polyelectrolyte complex (PEC) while the corona is formed by hydrophilic blocks of the block copolymers. Describing PEC as a globule in the framework of the Lifshitz [Zh. Eksp. Teor. Fiz. 55, 2408 (1968)] globule theory we calculate the surface tension of the micellar core/solvent interface as a function of the polyion degree of ionization, solvent quality, and concentration of low-molecular-mass salt. The equilibrium aggregation number of starlike micelles formed by block copolymers and homopolymers of opposite charge at stoichiometric mixture compositions is found as a function of the system parameters. It is shown that micelles disintegrate upon addition of salt.

Effects of solvent immiscibility on the phase behavior and microstructural length scales of a diblock copolymer in the presence of two solvents

We employ self-consistent mean-field theory to study the phase behavior and the microstructural sizes of AB diblock copolymers in the presence of a neutral solvent S1 and a slightly B-selective solvent S2. In particular, the effects of copolymer volume fraction phiC, the solvent ratio, and the immiscibility parameter between two solvents chiS1S2, are examined. We find that increasing chiS1S2 not only enlarges the ordered microphase region in the concentrated solutions, but also induces a less concentrated homogeneous solution to form an ordered structure and even undergo a macrophase separation. This is due to the fact that increasing chiS1S2 enhances the preferentially of S1 for A and S2 for B and, thereafter, the effective segregation between A and B. Hence, we observe that the structural length results obtained by varying chiS1S2 resemble a consequence of varying the solvent selectivity in the diblock copolymer solutions when only one solvent is added. For example, when chiS1S2 is small, the domain spacing decreases with decreasing phiC while at larger values of chiS1S2, it first shows a decreasing trend and then an increasing behavior with decreasing phiC.

In Vivo Osteogenic Differentiation of Rat Bone Marrow Stromal Cells in Thermosensitive MPEG–PCL Diblock Copolymer Gels

Methoxy poly(ethylene glycol)-poly(epsilon-caprolactone) (MPEG-PCL) diblock copolymers were prepared by ring-opening polymerization and their phase transition behavior characterized as a function of temperature. The MPEG-PCL solutions formed a sol at room temperature, and underwent sol-to-gel followed by gel-to-sol phase transitions as the temperature was increased. The temperature range over which the solutions were in a gel state could be extended simply by increasing the PCL chain length in the diblock copolymer. Scanning electron microscopy (SEM) images of MPEG-PCL solutions in the sol and gel states revealed near-regular and irregular porous structures, respectively. in vitro culture of rat bone marrow stromal cells (rBMSCs) on gel surfaces exhibited mostly round cells after 1 day of incubation. SEM images of the attached cells clearly showed the cell body and anchoring filopodia. Injection of room-temperature diblock copolymer solutions into Sprague-Dawley rats produced a gel at body temperature. In situ gel-forming scaffolds in vivo were successfully fabricated by simple subcutaneous injection of MPEG-PCL diblock copolymer solutions. The gel implants retained their original shape for 4 weeks without in- flammation at the injection site. Gel implants removed after 4 weeks were found to be surrounded by a thin fibrous capsule consisting of fibroblasts and blood vessels cells. Hematoxylin and eosin (H&E) and von Kossa staining revealed bone formation in gel implants containing both rBMSCs and dexamethasone, with the degree of bone formation increasing markedly with increasing dexamethasone concentration. Thus, our results show that in situ gel scaffolds fabricated from MPEG-PCL diblock copolymer solutions containing dexamethasone enable multipotent rBMSCs to produce viable bone when injected into rats.