Free Copolymer Chains Research Articles

Thermodynamic stability of ibuprofen loaded poloxamer micelles

Thermodynamic stability of poloxamer micelles excluding and encapsulating ibuprofen drug at different concentrations is studied by means of free energies. Free energies are computed by a partitioning of the total free energy in order to consider the different parts of the co-polymers together with free co-polymer chains in solvent. Moreover, extra terms to the free energy are added to account for the drug-micelle and drug-solvent interactions. The thermodynamic stability of the system is found to be positively correlated with the drug loading for the same system. The RDFs computed between water and the drug molecules signify the decreasing neighboring between these pairs as the co-polymer and drug concentrations are increased in the mixtures as the system leans towards a higher thermodynamic stability. In this work, not only the thermodynamic stability but also its relation to the structure are discussed for a drug-laden system demonstrating the utility of the proposed procedure.

Thermosensitive polymer brush‐supported 4‐N,N‐dialkylaminopyridine on silica particles as catalyst for hydrolysis of an activated ester in aqueous buffers: Comparison of activity with linear polymer‐supported version and effect of LCST transition

AbstractWe report on the synthesis of a thermosensitive polymer brush‐supported 4‐N,N‐dialkylaminopyridine catalyst and the comparison of its catalytic activity with the corresponding linear polymer‐supported version in the hydrolysis of p‐nitrophenyl acetate (NPA) as well as the effect of lower critical solution temperature (LCST) transition on catalytic activity. The polymer brushes were synthesized from initiator‐functionalized silica particles by surface‐initiated atom transfer radical polymerization of methoxytri(ethylene glycol) methacrylate and 2‐(N‐methyl‐N‐(4‐pyridyl)amino)ethyl methacrylate in the presence of a free initiator. Dynamic light scattering studies showed that the onset temperatures of the LCST transition of polymer brushes in pH 7.52 and 7.82 buffers were 42 and 38 °C, respectively. Under the same reaction conditions, the net initial rate of the hydrolysis of NPA catalyzed by hairy particles was 70–80% of that catalyzed by the free copolymer at the temperature below the LCST of polymer brushes. With further increasing the temperature above the LCST, the plot of logarithm of net initial rate versus inverse temperature exhibited a shift for the reactions catalyzed by hairy particles and leveled off or decreased slightly in the case of using the free copolymer as catalyst, presumably because the structures of the aggregates of hairy particles and free copolymer chains were different. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2853–2870, 2009

Adsorption of Poly(ethylene oxide)-b-poly(ε-caprolactone) Copolymers at the Silica−Water Interface

The adsorption of amphiphilic poly(ethylene oxide)-b-poly(epsilon-caprolactone) and poly(ethylene oxide)-b-poly(gamma-methyl-epsilon-caprolactone) copolymers in aqueous solution on silica and glass surfaces has been investigated by flow microcalorimetry, small-angle neutron scattering (SANS), surface forces, and complementary techniques. The studied copolymers consist of a poly(ethylene oxide) (PEO) block of M(n) = 5000 and a hydrophobic polyester block of poly(epsilon-caprolactone) (PCL) or poly(gamma-methyl-epsilon-caprolactone) (PMCL) of M(n) in the 950-2200 range. Compared to homoPEO, the adsorption of the copolymers is significantly increased by the connection of PEO to an aliphatic polyester block. According to calorimetric experiments, the copolymers interact with the surface mainly through the hydrophilic block. At low surface coverage, the PEO block interacts with the surface such that both PEO and PCL chains are exposed to the aqueous solution. At high surface coverage, a dense copolymer layer is observed with the PEO blocks oriented toward the solution. The structure of the copolymer layer has been analyzed by neutron scattering using the contrast matching technique and by tapping mode atomic force microscopy. The experimental observations agree with the coadsorption of micelles and free copolymer chains at the interface.

Effectiveness of block copolymers as stabilizers for aqueous titanium dioxide dispersions of a high solid content

Copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and sodium methacrylate (MANa) have been synthesized and tested as dispersants for the aqueous dispersion of titanium dioxide (80 wt.% solid). The molecular composition of the block copolymers has an effect on the dispersion stability. Triblock copolymers consisting of PMANa outer blocks are unable to stabilize the dispersion. Triblock copolymers of the reverse structure form poorly stable dispersions due to at least partial particle bridging. In the case of non-blocky distribution of the DMAEMA and MANa comonomers in the chains, the stabilization capability is lost. A decrease in the anchoring strength of the diblocks has a deleterious effect on the dispersion stability. Finally, the surface coverage depends on the availability of free copolymer chains (unimers) coexisting with micelles. These trends have been confirmed in a fully formulated paint containing Cu-phthalocyanine pigment.

Thermal Behavior and Association Properties of Polystyrene-b-Poly(ethylene/butylene)-b-Polystyrene Triblock Copolymer in N-Octane/4-Methyl-2-pentanone Solutions

Static and dynamic light scattering measurements have provided good information concerning the temperature-dependent associative behavior of a triblock copolymer polystyrene-b-poly(ethylene/butylene)-b-polystyrene dissolved in solvent mixtures of n-octane and 4-methyl-2-pentanone. Both are selective solvents, and when the solvent composition is varied, the solvent mixture shifts from a selective solvent of the copolymer end block to a selective solvent of the middle block. For solvent mixtures with a high 4-methyl-2-pentanone percentage (selective solvent for polystyrene blocks), two kinds of particles have been detected in solution. The smallest ones had a hydrodynamic radius close to 14 nm and were considered as free copolymer chains because they were only detected at temperatures higher than the critical micelle temperature (CMT). Below this temperature, a second kind of particle was detected with a hydrodynamic radius close to 37 nm, and those particles were considered as ordinary micelles with a core formed by poly(ethylene/butylene) blocks. For solvent mixtures with a high n-octane percentage (selective solvent for poly(ethylene/butylene) blocks) three kind of particles were detected. The smallest ones had a hydrodynamic radius close to 14 nm and corresponded to free copolymer chains. These particles were only detected at temperatures higher than CMTs. Below the critical micelle temperatures a second kind of particle of Rh ≈ 47 nm was detected, and those particles were considered flowerlike micelles with a core formed by polystyrene blocks. A third kind was detected at any temperature. The hydrodynamic radii of these particles were around 200 nm. Those were considered to be aggregates with a loose structure different from that of micelles. n-Octane/4-methyl-2-pentanone mixtures with similar solvent content behaved as a nonselective solvent, and no micelles were detected at 25 °C. Finally, the self-association process is enthalpy-driven, yielding closed association structures in the form of flowerlike micelles in solvent mixtures with a high n-octane content and ordinary micelles in solvent mixtures with a high 4-methyl-2-pentanone content.

Associative Behavior of a Triblock Copolymer in Mixed Selective Solvents

The associative behavior of a triblock copolymer polystyrene-b-poly(ethylene/butylene)-b-polystyrene dissolved in solvent mixtures of n-octane and 4-methyl-2-pentanone has been studied. When the solvent composition was varied, the solvent mixture shifted from a selective solvent of the end copolymer blocks (polystyrene) to a selective solvent of the middle block (poly(ethylene/butylene)). For solvent mixtures with a high 4-methyl-2-pentanone content (selective solvent of polystyrene), only one kind of particles has been detected in solution. These particles have a high molar mass and compactness and a small size (Rh ≈ 32 nm). However, for solvent mixtures with a high n-octane content (selective solvent of poly(ethylene/butylene)), two types of particles of different size were detected. The smaller ones (Rh ≈ 22 nm) were considered as free copolymer chains and they were the more numerous in the solution. The larger ones (Rh ≈ 210 nm) were assigned to aggregates with a loose structure different to ordinary ...

Adsorption of Block-Copolymer Micelles from a Selective Solvent

We have studied the adsorption of poly(styrene-b-2-vinylpyridine) block copolymers from a selective solvent onto a flat solid substrate, resulting in the formation of laterally ordered microdomains. We address the question whether the lateral domain structure is due to adsorption of entire micelles from the solution. Dynamic light scattering, atomic force microscopy, surface plasmon spectroscopy, and transmission electron microscopy were used to characterize the polymer solution, the adsorption process, and the resulting polymer layer, respectively. A quantitative comparison of the results obtained with different techniques strongly indicates direct adsorption of whole micelles onto a brush formed from free copolymer chains. Furthermore, the adsorption process is found to depend strongly on the velocity at which the samples are withdrawn from the solution.

Two-dimensional micelle formation of polystyrene-poly(vinylpyridine) diblock copolymers on mice surfaces

We have used atomic force microscopy to study the adsorption of PolyStyrene-Poly(VinylPyridine) (PS-PVP) block copolymers from a selective solvent onto atomically smooth mica surfaces. At certain copolymer concentrations, we observe a highly regular array of spherical surface micelles covering macroscopic areas of the substrate surface. Evidence is given for a thin homogeneous layer underneath the micelles which is probably due to adsorption of free copolymer chains and brush formation prior to the formation of the micellar structures. We discuss the quality of the self-assembled structures regarding different types of defects and try to identify means for improving the long range periodicity of the structures.

Size‐exclusion chromatography of solutions of block copolymers in selective solvents

AbstractSolutions of polystyrene‐block‐poly(ethylene/propylene) and polystyrene‐block‐poly(ethylene/butylene)‐block‐polystyrene copolymers in 5‐methyl‐2‐hexanone and 4‐methyl‐2‐pentanone were studied by size exclusion chromatography. The association equilibrium was studied shifting from micelles to free copolymer chains by increasing the temperature. A remarkable tailing was observed in diblock copolymer micelle peaks that may be explained by the disturbance and restablishment of the association equilibrium during separation in the chromatographic column. This phenomenon was very small for triblock copolymers. In this case, micelle hydrodynamic volumes obtained from SEC curves were compared to those calculated from light scattering and viscosity measurements. The data agreement was very good. Comicellization was analysed by mixing two different micelle solutions and injecting the solution mixture at different elapsed times. The SEC curves showed an intermediate peak corresponding to mixed micelles. The results show that three micelle species coexist in solution during the first period after mixing, which suggests that comicellization proceeds via dissociation of pure micelles and association of both copolymer free chains to form uniform mixed micelles.

Investigation of the size distribution of non-ionic micelles formed from a polystyrene–polyisoprene block copolymer in N,N-dimethylacetamide

Solutions of a polystyrene–polyisoprene block copolymer in N,N-dimethylacetamide were studied by gel permeation chromatography. The effect of temperature on the equilibrium between free copolymer chains and micelles was studied qualitatively from 26 to 110°C. Micelles studied in detail at T= 26°C were monodisperse in size, as judged by the reverse flow method. This result was supported by electron microscopy of micelles isolated from solution. Comparison with light scattering results showed that the universal calibration method of calibration was not applicable to the micelles.The application of gel permeation chromatography in a relatively bad solvent to the analysis of a mixture of a block copolymer with a component homopolymer was demonstrated.