Temperature-composition phase diagrams of binary blends of block copolymer and homopolymer

The temperature-composition phase diagrams for six pairs of diblock copolymer and homopolymer are presented, putting emphasis on the effects of block copolymer composition and the molecular weight of added homopolymers. For the study, two polystyrene-block-polyisoprene (SI diblock) copolymers having lamellar or spherical microdomains, a polystyrene-block-polybutadiene (SB diblock) copolymer having lamellar microdomains, and a series of polystyrene (PS), polyisoprene (PI), and polybutadiene (PB) were used to prepare SI/PS, SI/PI, SB/PS, and SB/PB binary blends, via solvent casting, over a wide range of compositions. The shape of temperature-composition phase diagram of block copolymer/homopolymer blend is greatly affected by a small change in the ratio of the molecular weight of added homopolymer to the molecular weight of corresponding block (M_H,A/M_C,A or M_H,B/M_C,B) when the block copolymer is highly asymmetric in composition but only moderately even for a large change in M_H,A/M_C,A ratio when the block copolymer is symmetric or nearly symmetric in composition. The boundary between the mesophase (M₁) of block copolymer and the homogeneous phase (H) of block copolymer/homopolymer blend was determined using oscillatory shear rheometry, and the boundary between the homogeneous phase (H) and two-phase liquid mixture (L₁+L₂) with L₁ being disordered block copolymer and L₂ being macrophase-separated homopolymer was determined using cloud point measurement. It is found that the addition of PI to a lamella-forming SI diblock copolymer or the addition of PB to a lamella-forming SB diblock copolymer gives rise to disordered micelles (DM) having no long-range order, while the addition of PS to a lamella-forming SB diblock copolymer retains lamellar microdomain structure until microdomains disappear completely. Thus, the phase diagram of SI/PI or SB/PB blends looks more complicated than that of SI/PS or SB/PS blends.     

Dissipative particle dynamics study on the multicompartment micelles self-assembled from the mixture of diblock copolymer poly(ethyl ethylene)-block-poly(ethylene oxide) and homopolymer poly(propylene oxide) in aqueous solution

Dissipative particle dynamics (DPD) method is applied to model the self-assembly of diblock copolymer poly(ethyl ethylene)-block-poly(ethylene oxide) (PEE-b-PEO) and homopolymer poly(propylene oxide) (PPO) in aqueous solution. In this study, several segments are coarse-grained into a single simulation bead based on the experimental density. For the self-assembly of pure diblock copolymer PEE-b-PEO in dilute solution, the DPD simulation results are in good agreement with experimental data of micelle morphologies and sizes. The chain lengths of the block copolymers and the volume ratios between PPO and PEE-b-PEO are varied to find the conditions of forming multicompartment micelles. The micelles with core-shell-corona structure and the micelles with two compartments are both formed from the mixture of PEE-b-PEO and PPO in aqueous solution.     

Doubly thermo-responsive brush-linear diblock copolymers and formation of core-shell-corona micelles

Doubly thermo-responsive brush-linear diblock copolymer of poly[poly(ethylene glycol) methyl ether vinylphenyl]-block-poly(N-isopropylacrylamide) (PmPEGV-b-PNIPAM) is prepared by RAFT polymerization. The obtained brush-linear diblock copolymer exhibits two lower critical solution temperatures (LCSTs) corresponding to the linear poly(N-isopropylacrylamide) (PNIPAM) block and the brush poly[poly(ethylene glycol) methyl ether vinylphenyl] (PmPEGV) block in water. This brush-linear diblock copolymer undergoes a two-step temperature sensitive micellization. At temperature above the first LCST, the brush-linear diblock copolymer self-assembles into core-corona micelles with the dehydrated PNIPAM block forming the core and the solvated brush PmPEGV block forming the corona. When temperature increases above the second LCST, the polystyrene backbone in the brush PmPEGV block collapses onto the dehydrated PNIPAM core to form core-shell-corona micelles, in which the dehydrated PNIPAM block forms the core, the collapsed polystyrene backbone in the brush PmPEGV block forms the shell and the solvated poly(ethylene glycol) side-chains forms the corona. The effect of the length of the PNIPAM block and the length of the poly(ethylene glycol) side-chains on the thermo-responsive micellization and the size of core-shell-corona micelles is investigated.     

含固体颗粒嵌段共聚高分子熔体微相结构的Monte Carlo模拟

采用改进的键长涨落空穴扩散算法,在立方格子上对含固体颗粒的两嵌段共聚高分子熔体的微相结构进行了Monte Carlo(MC)模拟. 重点考察了固体颗粒的大小、固体颗粒与嵌段共聚高分子的选择性作用、共聚高分子链的组成fA等因素对熔体微相结构的影响. 模拟结果表明,固体颗粒与高分子链节有选择性吸附作用时不利于形成层状相,而倾向于形成柱状或网络状结构;适当大小的惰性固体颗粒(与高分子链嵌段的长度相当)有利于系统形成层状相结构;无论系统是否含有固体颗粒,嵌段共聚高分子的对称程度增加都有利于形成层状结构.     

A new approach in the study of tethered diblock copolymer surface morphology and its tethering density dependence

A poly(l-lactic acid)-block-polystyrene-block-poly(methyl methacrylate) (PLLA-b-PS-b-PMMA) triblock copolymer was synthesized with a crystalline PLLA end block. Single crystals of this triblock copolymer grown in dilute solution could generate uniformly tethered diblock copolymer brushes, PS-b-PMMA, on the PLLA single crystal substrate. The diblock copolymer brushes exhibited responsive, characteristic surface structures after solvent treatment depending upon the quality of the solvent in relation to each block. The chemical compositions of these surface structures were detected via the surface enhanced Raman scattering technique. Using atomic force microscopy, the physical morphologies of these surface structures were identified as micelles in cyclohexane and “onion”-like morphologies in 2-methoxyethanol, especially when the PS-b-PMMA tethered chains were at low tethering density.     

Poly[N-(2-hydroxypropyl)methacrylamide-block-n-butyl acrylate] micelles in water/DMF mixed solvents

Three diblock copolymers of poly[N-(2-hydroxypropyl)methacrylamide] (poly(HPMA)) and poly(n-butyl acrylate) (poly(BA)) with varying lengths of blocks were prepared by atom transfer radical polymerization. All copolymers were found to be soluble in dimethylformamide (DMF) and poorly soluble or insoluble in water. In water and mixed DMF/H₂O solvents, the copolymers were dispersed in micellar form by controlled addition of water to DMF solutions of copolymers under continuous intensive stirring. The micellar solutions in water were prepared by dialysis of solutions in DMF/H₂O (95 vol% of H₂O) against water. Solution properties of diblock copolymers of poly(HPMA) and poly(BA) were studied using static and dynamic laser light scattering to characterize the behavior of the copolymers at the supramolecular level. The effects of preparation mode, organic solvent (DMF) and copolymer chemical composition on the formation of micelles were studied. While a slower mixing procedure was optimal for copolymers with short poly(HPMA) blocks, a faster mixing was more suitable for copolymers having longer poly(HPMA) blocks. Finally, the dimensions of micelles in water were evaluated. The most compact micelles were prepared from copolymers having short hydrophilic poly(HPMA) blocks. On the other hand, the copolymer with the longest poly(HPMA) block formed micelles with the smallest size and the lowest density.     

Injectable hydrogels of poly(?-caprolactone-co-glycolide)-poly(ethylene glycol)-poly(?-caprolactone-co-glycolide) triblock copolymer aqueous solutions

Thermogelling triblock copolymers of poly(?-caprolactone-co-glycolide)-poly(ethylene glycol)-poly(?-caprolactone-co-glycolide) [P(CL-GA)-PEG-P(CL-GA)] were successfully prepared by control of the hydrophilicity/hydrophobicity balance and chemical compositions of the copolymers. The aqueous solutions of the copolymers underwent sol-gel transition as the temperature was increased from 20 to 60 °C. The amphiphilic copolymer formed micelles in water and a gel was formed by aggregation of micelles. The structure parameters played a critical role in determining sol-gel transition behavior. Either increasing [GA]/[CL] ratio or decreasing P(CL-GA) block length could induce the increase of the lower sol-gel transition temperature. Glycolide (GA) was incorporated into the polymer chain to increase the polymer degradation rate. Sustained release of rifampicin for approximately 32 days was obtained from the gel. It is believed to have potential applications in drug delivery and tissue engineering.     

Preparation and self-assembly of double hydrophilic poly(ethylethylene phosphate)-block-poly[2-(succinyloxy)ethyl methacrylate] diblock copolymers for drug delivery

This work focuses on the synthesis and self-assembly of biodegradable and anionic double hydrophilic diblock copolymers (DHBCs) poly(ethylethylene phosphate)-block-poly[2-(succinyloxy)ethyl methacrylate] (PEEP-b-PSEMA) with different molecular weights and compositions, which were prepared via a combination of ring opening polymerization (ROP), atom transfer radical polymerization (ATRP) and polymer reaction. The chemical structures of these well-defined diblock copolymers were confirmed by ¹H NMR and FT-IR analyses. GPC results indicated that the copolymers showed symmetric peak and relatively narrow polydispersities. Subsequently, pH-responsive micellization behaviors of PEEP-b-PSEMA diblock copolymers were investigated by fluorescence probe method, dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements. The results demonstrated that these diblock copolymers were able to self-assemble into micelles with various sizes depending on the variation of pH values. Naproxen (NAP), a poorly water-soluble drug, was selected as the model drug and encapsulated into the core of micelles via dialysis method. The in vitro release behavior of NAP from these micelles was pH-dependent and could be accelerated in the presence of phosphodiesterase I which could promote the degradation of polyphosphoesters. Cytotoxicity tests by MTT assay showed that these block copolymers possessed favorable biocompatibility against HeLa cells, revealing that this kind of biodegradable, biocompatible and pH-responsive block copolymer would be served as a promising material for drug delivery.     

Detection of the rate of exchange of chains between micelles formed by diblock copolymers in aqueous solution

Summary A fluorescence method is described for the measurement of the rate of exchange of chains between micelles formed by diblock copolymers in aqueous solution. The method requires two samples of the diblock copolymer. One sample is labelled with a Förster donor, the other sample is labelled with a Förster acceptor. Successful application of the method is demonstrated with diblock copolymers composed of polystyrene and poly(ethylene oxide). The donor and acceptor are naphthalene and pyrene, respectively. The label is covalently attached to the copolymers at the junction points between the two blocks. Solutions with micelles are formed independently by the two labelled samples. At the time of mixing of the two solutions, no micelle contains both a donor and an acceptor. Micelles containing both types of labels may be formed at later times as a consequence of the exchange of labelled chains. The efficiency of nonradiative singlet energy transfer from naphthalene to pyrene is measured as a function of time after mixing of the two solutions. At 60° C the rate constant deduced from the time dependence of the fluorescence is on the order 10^-5 s^-1. At ambient temperature, however, no exchange can be detected, presumably because of the difficulty in extraction of a polystyrene block from the glassy core.     

Block copolymer micelles as carriers of transition metal ions Y(III) and Cu(II) and gelation thereof

Poly(ethylene oxide)-block-poly(2-(diethylamino)ethyl methacrylate) (PEO-b-PDEAEMA) diblock copolymer was synthesized by anionic polymerization, whose molecular structure was characterized by ¹H NMR and size exclusion chromatography (SEC). The diblock copolymer self-assembled into micelles in nonacid aqueous solution with PEO and PDEAEMA as corona and core respectively. By virtue of the coordinating property of PDEAEMA block to metal ions, the resultant micelles were then used as carriers to load metal ions Y(III) and Cu(II) in the micellar core. The morphology and stability of the metal loaded micelles were characterized by dynamic light scattering (DLS), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The metal loading amounts were determined by elemental analyses, UV spectrometry and titrimetric analysis. In addition, the Y(III) loaded micelles were demonstrated to complex with α-cyclodextrins and form supramolecular hydrogels in-situ. The metal loaded micelles and the resultant supramolecular hydrogels will have potential application for cancer internal radiotherapy.     

Regular and irregular micelles formed by A LEL triblock copolymer in aqueous solution

Laser light scattering (LLS) techniques were used to characterize the micellization of poly(d,l-lactide)-poly(ethylene glycol)-poly(d,l-lactide) (LEL) triblock copolymer (MW 1K-2K-1K) in aqueous solution. We observed the existence of both thermodynamically stable flower-like micelles (regular micelles) and large, less soluble nanoparticles (irregular micelles) in dilute aqueous solutions with the same preparation procedure. Both kinds of micelles were found to co-exist with single copolymer chains. The initial copolymer concentration determines the nature of the micelles. The regular core-shell micelle formation follows a closed association mechanism, resulting in flower-like micelles. The hydrophobicity of a L unit is estimated as ∼0.5-0.6 B (polyoxybutylene) units from the micellization parameters, which is quite consistent with earlier estimations obtained from EL diblock copolymers.     

Temperature and Time Dependence of Copolymer Mixtures on Interfacial Adhesion at PS/PMMA Interfaces

The effect of copolymer mixtures on the interfacial adhesion between slabs of PS and PMMA was investigated as a function of composition, time and temperature using the asymmetric double cantilever beam (ADCB) method. The nature of the interface was further probed using atomic force microscopy (AFM) and dynamic secondary ion mass spectroscopy (D-SIMS). The results show that mixtures of graft and block copolymers are much more effective than pure block copolymers in enhancing the interfacial adhesion. The most effective mixture consisted of a block copolymer of molecular weight 70K and a copolymer with two PS grafts of molecular weight 30K. This mixture yielded an interfacial fracture toughness of G_c = 127.5 J/m² as compared with G_c = 38.2 J/m² and G_c = 3.5 J/m² for the pure block and graft copolymer, respectively.

G_c at the PS/PMMA interface reinforced only with block copolymer was maximal after an annealing temperature of 150°C for 1 hr. It decreased by an order of magnitude when the temperature was increased to 180°C or the joining time was increased from 1 to 10 hours. G_c at the interface reinforced with a graft/diblock copolymer mixture was also maximum at an annealing temperature of 150°C but it decreased only by a factor of 2 with increasing joining time or temperature. Dynamic Secondary Ion Mass Spectroscopy (DSIMS) data show that this effect may be due to decrease in the diffusion of the copolymer from the interface when the mixture is present, i.e, the diblock copolymer is trapped within the graft copolymer.     

Chemo‐enzymatic synthesis of poly(2,2,2‐trichloroethyl 10‐hydroxydecanate)‐block‐polystyrene combining enzymatic self‐condensation polymerization and ATRP

Enzymatic polymerization in a non‐natural environment is of interest as an environmentally friendly methodology as an alternative to the use of conventional chemical organometallic catalysts. Chemo‐enzymatic synthesis of the AB‐type diblock copolymer poly(2,2,2‐trichloroethyl 10‐hydroxydecanate)‐block‐polystyrene (PHD‐b‐PSt) was carried out by combining enzymatic self‐condensation polymerization (eSCP) and atom‐transfer radical polymerization (ATRP). Biocatalyst Novozyme 435 was successful in catalyzing the eSCP of a novel ω‐hydroxyester, i.e. 2,2,2‐trichloroethyl 10‐hydroxydecanate. The resulting ? CCl₃‐terminated PHD initiated the ATRP of styrene, a ‘living’/controlled radical polymerization. The analysis of the hydrolysate from the copolymer proved the presence of a block copolymer structure. In addition, the well‐defined diblock copolymer PHD‐b‐PSt self‐assembled into nanoscale micelles in aqueous solution. The chemo‐enzymatic synthesis of diblock copolymer PHD‐b‐PSt was achieved by the combination of eSCP and ATRP. The structures and composition of the block copolymer were characterized by means of NMR, infrared and gel permeation chromatography measurements. Differential scanning calorimetry analysis showed that a microphase‐separation structure was formed in the copolymer, which was caused by the crystallization of the PHD segments. As investigated with atomic force microscopy and dynamic light scattering, these micelles had a mean diameter and a spherical shape. To our knowledge, this is the first example of a chemo‐enzymatic synthesis based on eSCP and ATRP. Copyright © 2007 Society of Chemical Industry     

Functionalization of amphiphilic coil-rod-coil triblock copolymer poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate)-b-poly(2-vinylpyridine) with florescence moiety and C60

We recently achieved quantitative synthesis of an amphiphilic coil-rod-coil triblock copolymer, poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate)-b-poly(2-vinylpyridine), by coupling in situ living diblock copolymer poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate) (P2VP-b-PHIC) using malonyl chloride in the presence of pyridine. This led to the introduction of an active methylene group that is a site for further functionalization in the rod block. The Michael addition reaction of the triblock copolymer with 7-(4-trifluoromethyl) coumarin acrylamide led to copolymer bearing a fluorescent pendent in the rod block. The fluorescent labeled copolymers were isolated in ∼94% yields. Similarly C₆₀ pendent was introduced to the rod block by the Bingel reaction. The yields of C₆₀ functionalized copolymers were ∼54%. The precursor and functionalized amphiphilic coil-rod-coil copolymer show diverse morphologies, such as micelles and vesicles by simply changing the solvent. For the C₆₀ functionalized block copolymer, structural constraints in micelles and vesicles prevented C₆₀ pendents to aggregate.     

Crosslinkable surfactants based on linoleic acid-functionalized block copolymers of ethylene oxide and ε-caprolactone for the preparation of stable PMMA latices

Amphiphilic diblock and triblock copolymers consisting of poly(ethylene oxide) (PEO) as (central) hydrophilic segment and poly(ε-caprolactone) (PCL) as hydrophobic segment(s) were prepared by ring opening polymerization. The length of the PEO segment was kept constant , whereas the length of the PCL block(s) was either 6 or 10 units for diblock copolymers and 3 or 5 units at each end for the triblock copolymers. These block copolymers were end-functionalized by esterification with linoleic acid (LA), which contains reactive double bonds. The autoxidative behavior of PEO₄₅-(CL₃-LA)₂ functionalized triblock copolymers was investigated by exposure of films to air at ambient conditions. Ninety percent of the double bonds had disappeared in 15 d and a crosslinked structure was obtained after 30 d. Critical micelle concentrations (CMC) of the crosslinkable surfactants were in the range of 0.08-0.19 mmol/l for the diblock copolymer and of 0.19-0.26 mmol/l for the triblock copolymer. The surface tension of aqueous surfactant solutions at the CMC (γ_CMC) (25 °C) varied from 47.1 to 51.4 mN/m for the diblock and from 45.6 to 48.1 mN/m for the triblock systems. For both systems CMC and γ_CMC increase with increasing HLB values. These surfactants were used in PMMA latex preparations. The latices of PMMA prepared with LA-functionalized diblock and triblock copolymers yielded narrow particle size distributions and particle sizes of 180 and 370 nm, respectively, whereas latices prepared with SDS had a particle size of 90 nm. After extraction of the latex particles with methanol, the amounts of the unextractable (either buried or copolymerized) LA-functionalized diblock and triblock copolymers found in extracted PMMA latex particles were 10 and 24% of the initial amount of surfactant added respectively. Control experiments with a stearic acid (SA) containing diblock copolymer showed that the amount of buried surfactant in PMMA latices was 6.5%. By comparing the overall latex characteristics and stability (shelf stability, freeze-thaw testing and addition of electrolyte solutions and ethanol) it was concluded that an LA-functionalized diblock copolymer (MPEO₄₅-CL₁₀-LA) gave better stabilization of PMMA latices than an LA-functionalized triblock copolymer of comparable composition and HLB value.     

Temperature and Time Dependence of Copolymer Mixtures on Interfacial Adhesion at PS/PMMA Interfaces

The effect of copolymer mixtures on the interfacial adhesion between slabs of PS and PMMA was investigated as a function of composition, time and temperature using the asymmetric double cantilever beam (ADCB) method. The nature of the interface was further probed using atomic force microscopy (AFM) and dynamic secondary ion mass spectroscopy (D-SIMS). The results show that mixtures of graft and block copolymers are much more effective than pure block copolymers in enhancing the interfacial adhesion. The most effective mixture consisted of a block copolymer of molecular weight 70K and a copolymer with two PS grafts of molecular weight 30K. This mixture yielded an interfacial fracture toughness of G_c = 127.5 J/m² as compared with G_c = 38.2 J/m² and G_c = 3.5 J/m² for the pure block and graft copolymer, respectively.

G_c at the PS/PMMA interface reinforced only with block copolymer was maximal after an annealing temperature of 150°C for 1 hr. It decreased by an order of magnitude when the temperature was increased to 180°C or the joining time was increased from 1 to 10 hours. G_c at the interface reinforced with a graft/diblock copolymer mixture was also maximum at an annealing temperature of 150°C but it decreased only by a factor of 2 with increasing joining time or temperature. Dynamic Secondary Ion Mass Spectroscopy (DSIMS) data show that this effect may be due to decrease in the diffusion of the copolymer from the interface when the mixture is present, i.e, the diblock copolymer is trapped within the graft copolymer.     

Ion-dipole interactions in the dispersion of organoclay nanocomposites based on polystyrene-block-poly(2-vinylpyridine) copolymer

The dispersion characteristics of organoclay nanocomposites based on polystyrene-block-poly(2-vinylpyridine) (S2VP diblock) copolymer were investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD), and solid-state nuclear magnetic resonance (NMR) spectroscopy. For the investigation, S2VP diblock copolymers having three different compositions were synthesized via sequential anionic polymerization. Each S2VP diblock copolymer was used to prepare nanocomposites by solution blending with natural clay (montmorillonite, MMT) or commercial organoclays (Cloisite 30B, Cloisite 10A, Cloisite 15A, and Cloisite 25A from Southern Clay Products). All four organoclays employed were treated with a surfactant having quaternary ammonium salt with N⁺ ion. It was found, via TEM and XRD, that the nanocomposites with MMT show very poor dispersion characteristics regardless of block copolymer composition. However, the block copolymer composition was found to have a profound influence on the dispersion characteristics of the nanocomposites with an organoclay. Specifically, the nanocomposites based on S2VP-5 having 5 wt% poly(2-vinylpyridine) (P2VP) block gave rise to a very high degree of dispersion, irrespective of the chemical structure of the surfactant residing at the surface of the organoclay employed, whereas the dispersion characteristics of the nanocomposites became progressively poorer as the amount of P2VP block in an S2VP diblock copolymer increased from 5 to 25 wt% and to 56 wt%. The observed dispersion characteristics were explained by hypothesizing the presence of ion-dipole interactions between the positively charged N⁺ ions in the surfactant residing at the surface of the organoclay nanoparticles and the dipoles in the P2VP block of S2VP diblock copolymers. The validity of this hypothesis was confirmed using solid-state NMR spectroscopy, by determining the dependence of the composition of S2VP diblock copolymer on the extent of ion-dipole interactions and thus on the dispersion characteristics of the nanocomposites prepared.     

Morphologies of diblock copolymer confined in a slit with patterned surfaces studied by dissipative particle dynamics

Diblock copolymers with ordered mesophase structures have been used as templates for nano-fabrication. Unfortunately, the ordered structure only exists at micromete rscale areas, which precludes its use in many advanced applications. To overcome this disadvantage, the diblock copolymer confined in a restricted system with a patterned surface is proved to be an effective means to prohibit the formation of defects and obtain perfect ordered domains. In this work, the morphologies of a thin film of diblock copolymer confined between patterned and neutral surfaces were studied by dissipative particle dynamics. It is shown that the morphology of the symmetric diblock copolymer is affected by the ratio of the pattern period on the surface to the lamellar period of the symmetric diblock copolymer and by the repulsion parameters between blocks and wall particles. To eliminate the defects in the lamellar phase, the pattern period on the surface must match the lamellar period. The difference in the interface energy of different compartments of the pattern should increase with increasing film thickness. The pattern period on the surface has a scaling relationship with the chain length, which is the same as that between the lamellar period and the chain length. The lamellar period is also affected by the polydispersity of the symmetric diblock copolymer. The total period is the average of the period of each component multiplied by the weight of its volume ratio. The morphologies of asymmetric diblock copolymers are also affected by the pattern on the surface, especially when the matching period of the asymmetric diblock copolymer is equal to the pattern period, which is approximately equal to the lamellar period of a symmetric diblock copolymer with the same chain length.     

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

Solutions 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.     

Nanostructured thermosets from self‐assembled amphiphilic block copolymer/epoxy resin mixtures: effect of copolymer content on nanostructures

Nanostructure formation in thermosets can allow the design of materials with interesting properties. The aim of this work was to obtain a nanostructured epoxy system by self‐assembly of an amphiphilic diblock copolymer in an unreacted epoxy/amine mixture followed by curing of the matrix. The copolymer employed was polystyrene‐block‐poly(methyl methacrylate) (PS‐b‐PMMA). The thermoset system, formed by a diglycidyl ether of bisphenol A‐type epoxy resin and diaminodiphenylmethane hardener, was chosen to ensure the miscibility of most of the PMMA block until matrix gelation. Transparent materials with microphase‐separated domains were obtained for copolymer contents lower than 40 wt%. In systems containing 20 and 30 wt% block copolymer, the PS block formed spherical micelles or worm‐like structures before curing, which were stabilized through curing by the more compatible PMMA block phase. Nanostructured thermoset systems were successfully synthesized for self‐assembled amphiphilic block copolymer–epoxy/amine mixtures for copolymer contents lower than 40 wt%. Copyright © 2009 Society of Chemical Industry