Poly(sodium styrene sulfonate)-b-poly(methyl methacrylate) diblock copolymers through direct atom transfer radical polymerization: Influence of hydrophilic–hydrophobic balance on self-organization in aqueous solution

A series of poly(sodium styrene sulfonate)-b-poly(methyl methacrylate), PSSNa-b-PMMA, amphiphilic diblock copolymers have been synthesized through atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) in N,N-dimethylformamide/water mixtures, starting from a PSSNa macroinitiator. The kinetics of the polymerization was followed by ¹H NMR, while the chemical composition of the copolymers was verified by a variety of techniques, such as ¹H NMR, FTIR and TGA. The MMA content of the copolymers ranges from 0 up to 60 mol%, while the number–average molecular weight of the PSSNa macroinitiator was 9000 g/mol. The self-association of the diblock copolymers in aqueous solution was compared to the respective behavior of similar random P(SSNa-co-MMA) copolymers through optical density measurements, pyrene fluorescence probing, dynamic light scattering and surface tension measurements. It is shown that the diblock copolymers form micellar structures in water, characterized by an increasing hydrophobic character and a decreasing size as the length of the PMMA block increases. These micelle-like structures turn from surface inactive to surface active as the length of the PMMA block increases. Moreover, contrary to the MMA-rich random copolymers, the respective diblock copolymers form water insoluble polymer/surfactant complexes with cationic surfactants such as hexadecyltrimethyl ammonium bromide (HTAB), leading to materials with antimicrobial activity.     

Synthesis and characterization of polycholesteryl methacrylate–polyhydroxyethyl methyacrylate block copolymers

We report the synthesis and characterization of a series of novel diblock copolymers, poly(cholesteryl methacrylate‐b‐2‐hydroxyethyl methacrylate) (PCMA‐b‐PHEMA). Monomers, cholesteryl methacrylate (CMA) and 2‐(trimethylsiloxy)ethyl methacrylate (HEMA‐TMS), were prepared from methyacryloyl chloride and 2‐hydroxyethyl methacrylate, respectively. Homopolymers of CMA, PCMA, with well‐defined molecular weights and polydispersity indices (PDI), were prepared by reversible addition fragmentation and chain transfer (RAFT) method. Precursor diblock copolymers, PCMA‐b‐P(HEMA‐TMS), were synthesized using PCMA as macromolecular chain transfer agent and monomer, HEMA‐TMS. Product diblock copolymers, PCMA‐b‐PHEMA, were prepared by deprotecting trimethylsilyl units in the precursor diblock copolymers using acid catalysts. Detailed molecular characterization of the precursor diblock copolymers, PCMA‐b‐P(HEMA‐TMS), and the product diblock copolymers, PCMA‐b‐PHEMA, confirmed the composition and structure of these polymers. This versatile synthetic strategy can be used to prepare new amphiphilic block copolymers with cholesterol in one block and hydrogen‐bonding moieties in the second block. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6801–6809, 2008     

Tem- and neutron reflection studies on surfaces and interfaces with block copolymers

The behavior of block copolymers at various interfaces is studied by transmission electron microscopy and neutron reflection. A thin film of a symmetric diblock copolymer of styrene and methyl methacrylate forms layer structures when in contact with air and a random copolymer of styrene and acrylonitrile containing 35 wt% acrylonitrile. When the random copolymer has an acrylonitrile content of 25 wt%, a competition between layer formation and diffusion of disordered micelles takes place. Driving force for these processes are different interfacial tensions and a changing miscibility behavior as a function of acrylonitrile contents of the random copolymers. The ordering behavior of a symmetric diblock copolymer of deuterated styrene and isoprene in contact with poly(3,5-dimethyl phenylene ether) is studied by neutron reflection. Polystyrene-block-poly(ethene-co-but-1-ene)-block-polystyrene with cylindrical PS microdomains shows an interfacial phase transition to lamellae near to the interface with different polymers. The morphological studies are in agreement with adhesion data obtained by peel tests on different bilayer specimens.     

Control of pH- and temperature-responsive behavior of mPEG-b-PDMAEMA copolymers through polymer composition

A series of poly(ethylene glycol) monomethyl ether-block-poly(2-(dimethylamino)ethyl methacrylate) (mPEG-b-PDMAEMA) diblock copolymers were synthesized using atom transfer radical polymerization to achieve controlled polymer molecular weight and narrow molecular weight distribution. The thermoresponsive properties of the mPEG-b-PDMAEMA diblock copolymers in aqueous buffered solutions were determined using UV-Visible spectroscopy and dynamic light scattering. The cloud point, a soluble-to-insoluble transition, was observed for all mPEG-b-PDMAEMA diblock copolymer solutions. Increasing either the mPEG or PDMAEMA molecular weight resulted in a decrease in observed cloud points as a function of pH and polymer concentration. Changing the mPEG molecular weight determined whether a second, higher temperature, thermal transition was observed as a function of pH and polymer concentration. Controlling the thermoresponsive properties of mPEG-b-PDMAEMA diblock copolymers through polymer composition, concentration, and pH enables the tailoring of these copolymers for applications ranging from non-viral gene delivery to use as a strengthening agent in paper.     

Novel fluoroalkyl end-capped amphiphilic diblock copolymers with pH/temperature response and self-assembly behavior

A series of fluoroalkyl end-capped diblock copolymers of poly[2-(N,N-dimethylamino)ethyl methacrylate] (PDMAEMA or PDMA) and poly[2-(N,N-diethylamino)ethyl methacrylate] (PDEAEMA or PDEA) have been synthesized via oxyanion-initiated polymerization, in which a potassium alcoholate of 4,4,5,5,6,6,7,7,7-nonafluoro-1-heptanol (NFHOK) was used as an initiator. The chemical structures of the NFHO-PDMA-b-PDEA and NFHO-PDEA-b-PDMA depended on the addition sequence of the two monomers and the feeding molar ratios of [DMA] to [DEA] during the polymerization process. These copolymers have been characterized by (1)H NMR and (19)F NMR spectroscopy and gel permeation chromatography (GPC). The aggregation behavior of these copolymers in aqueous solutions at different pH media was studied using a combination of surface tension, fluorescence probe, and transmission electron microscopy (TEM). Both diblock copolymers exhibited distinct pH/temperature-responsive properties. The critical aggregation concentrations (cacs) of these copolymers have been investigated, and the results showed that these copolymers possess excellent surface activity. Besides, these fluoroalkyl end-capped diblock copolymers showed pH-induced lower critical solution temperatures (LCSTs) in water. TEM analysis indicated that the NFHO-PDMA(30)-b-PDEA(10) diblock copolymers can self-assemble into the multicompartment micelles in aqueous solutions under basic conditions, in which the pH value is higher than the pKa values of both PDMA and PDEA homopolymers, while the NFHO-PDEA(10)-b-PDMA(30) diblock copolymers can form flowerlike micelles in basic aqueous solution.     

Properties of polystyrene-poly(methylmethacrylate) random and diblock copolymers in dilute and semidilute solutions

Solution properties for random and diblock copolymers of polystyrene (PS) and poly(methyl methacrylate) (PMMA) have been measured by dynamic and total intensity light scattering in solvents of differing quality. The results are compared with the corresponding properties for PS and PMMA homopolymers of similar molecular weight, in order to determine if interactions between unlike monomers are significant. The hydrodynamic radius (R_h) and diffusion second virial coefficient (k_d) for the random copolymer are found to be larger than the corresponding values for the homopolymers in a solvent which is near-theta for the two homopolymers, whereas no such effect is observed for the block copolymer. This suggests that most intrachain interactions occur a relatively short distance along the chain backbone. In a mutual good solvent R_h and k_d of the random copolymer are comparable to the average of the values for the homopolymers, indicating that in a good solvent monomer/solvent interactions dominate over monomer/monomer interactions. For an isolated diblock copolymer in a mutual good solvent, there is no evidence that interactions between unlike monomers lead to additional expansion of the entire molecule, as measured by R_h, nor expansion of the individual blocks as probed by light scattering with one block optically masked. However, at low but finite concentration there is evidence (the coefficients of the binary interaction terms in the viscosity and the mutual diffusion coefficient, and the second and third virial coefficients) that a weak ordering effect may exist in block copolymer solutions, far from the conditions where microphase separation occurs. Finally, measurements of ternary polymer-polymer-solvent solutions show no dependence on monomer composition or monomer distribution for the tracer diffusion of probe PS-PMMA copolymers in a PMMA/toluene matrix. This indicate that the frictional interaction is largely unaffected by interactions between unlike monomers. However, there is evidence that the thermodynamic interaction is more unfavorable between a random copolymer and the homopolymer matrix than between a diblock and the matrix. © 1994 John Wiley & Sons, Inc.     

Computer simulation of architectural and molecular weight effects on the assembly of amphiphilic linear-dendritic block copolymers in solution

Langevin dynamics simulations are performed on linear-dendritic diblock copolymers containing bead-spring, freely jointed chains composed of hydrophobic linear monomers and hydrophilic dendritic monomers. The critical micelle concentration (CMC), micelle size distribution, and shape are examined as a function of dendron generation and architecture. For diblock copolymers with a linear block of fixed length, it is found that the CMC increases with increasing dendron generation. This trend qualitatively agrees with experiments on linear-dendritic diblock and triblock copolymers with hydrophilic dendritic blocks and hydrophobic linear blocks. The flexibility of the dendritic block is altered by varying the number of spacer monomers between branch points in the dendron. When comparing linear-dendritic diblock copolymers with similar molecular weights, it is shown that increasing the number of spacer monomers in the dendron lowers the CMC due to an increase in flexibility of the dendritic block. Analysis on the micellar structure shows that linear-dendritic diblock copolymers pack more densely than what would be expected for a linear-linear diblock copolymer of the same molecular weight.     

Amphiphilic polymethacrylate derivatives as antimicrobial agents

We have investigated the structure-activity relationship of cationic amphiphilic polymethacrylate derivatives in antimicrobial and hemolytic assays. The polymers were prepared by radical copolymerizations of N-(tert-butoxycarbonyl)aminoethyl methacrylate and butyl methacrylate in the presence of methyl 3-mercaptopropionate as a chain transfer agent to give precursor polymers protected with a tert-butoxycarbonyl (Boc) group. Subsequent treatment of the Boc-protected polymers with TFA affords the desired cationic random copolymers. We examined antimicrobial and hemolytic activities of a series of polymers having a wide range of mole percentage of butyl groups (0-60%) in three different molecular weight (MW) ranges. The smallest polymers (MW < 2000) showed the lowest MIC and reduced hemolytic activity compared to that of the higher MW ones. In addition, polymers containing a high percentage of butyl groups are less selective for bacterial cells than their less hydrophobic counterparts.     

Synthesis of pH-responsive amphiphilic diblock copolymers containing polyisobutylene via oxyanion-initiated polymerization and their multiple self-assembly morphologies

 Two pH-responsive amphiphilic diblock copolymers, namely polyisobutylene-block-poly[2-(N,N-dimethylamino)ethyl methacrylate] (PIB-b-PDMAEMA) and polyisobutylene-block-poly(metharylic acid) (PIB-b-PMAA), were synthesized via oxyanion-initiated polymerization, and their multiple self-assembly behaviors have been studied. An exo-olefin-terminated highly reactive polyisobutylene (HRPIB) was first changed to hydroxyl-terminated PIB (PIB-OH) via hydroboration-oxidation of C＝C double bond in the chain end, and then reacted with KH to yield a potassium alcoholate of PIB (PIB-O^-K⁺). PIB-O^-K⁺ was immediately used as a macroinitiator to polymerize DMAEMA monomer, resulting in a cationic diblock copolymer PIB-b-PDMAEMA. With the similar synthesis procedure, the anionic diblock copolymer PIB-b-PMAA could be prepared via a combination of oxyanion-initiated polymerization of tert-butyl methacrylate (tBMA) and subsequent hydrolysis of tert-butyl ester groups in PtBMA block. The functional PIB and block copolymers have been fully characterized by ¹H-NMR, FT-IR spectroscopy, and gel permeation chromatography (GPC). These samples allowed us to systematically investigate the effects of block composition on the pH responsivity and various self-assembled morphologies of the copolymers in THF/water mixed solvent. Transmission electron microscopy (TEM) images revealed that these diblock copolymers containing small amount of original PIB without exo-olefin-terminated group are able to self-assemble into micelles, vesicles with different particle sizes and cylindrical aggregates, depending on various factors including block copolymer composition, solvent polarity and pH value.     

Biofouling resistance of ultrafiltration membranes controlled by surface self-assembled coating with PEGylated copolymers

Block and random PEGylated copolymers of poly(ethylene glycol) methacrylate (PEGMA) and polystyrene (PS) were synthesized with a controlled polydispersity using an atom transfer radical polymerization method and varying molar mass ratios of PS/PEGMA. Two types of PEGylated copolymers were self-assembly coated onto the surface of poly(vinylidene fluoride) (PVDF) ultrafiltration membranes for enhancing biofouling resistance. It was found that the adsorption capacities of random copolymers on PVDF membranes were all higher than those of block copolymers. However, the specific and overall protein resistance of bovine serum albumin (BSA) on PVDF membranes coated with block copolymers was much higher than that with random copolymers. The increase in styrene content in copolymer increased the amount of polymer coating on the membrane, and the increase in PEGMA content enhanced the protein resistance of membranes. The optimum PS/PEGMA ratio was found to be close to 2 for the best resistance of protein adsorption and bacterial adhesion on the PEGylated diblock copolymer-coated membranes. The PVDF membrane coated with such a copolymer owned excellent biofouling resistance to BSA, humic acid, negatively surface charged bacteria E. coli, and positively surface charged bacteria S. maltophilia.     

Synthesis of zwitterionic block copolymers via RAFT polymerization

A series of poly [2-(dimethylamino)ethyl methacrylate (DMA)-sodium acrylate (SA)] diblock copolymers were synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization. The polymerization exhibits controlled characters: well-controlled molecular weight, narrow molecular weight distribution, molecular weight increasing with polymerization time. The zwitterionic diblock copolymers show rich solution behaviors. Dynamic light scattering (DLS) indicated the formation of micelles and reverse micelles of copolymers is affected by net charge density of copolymers. Microcalorimetry studies showed that the lower critical solution temperature (LCST) increases with incorporation of hydrophilic segments in buffer.     

Cytotoxicity of nonionic amphiphilic copolymers

The purpose of this study is to ascertain the relationship between the structure of an amphiphilic nonionic polymer and its toxicity for cells (cytotoxicity) growing in a culture. To this end, 16 polymers of different architectures and chemical structures are tested, namely, linear triblock copolymers of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronics); diblock copolymers of propylene oxide, ethylene oxide, and hyperbranched polyglycerol; alternating and diblock copolymers of ethylene oxide and dimethylsiloxane; and two surfactants containing linear (Brij-35) or branched (Triton X-100) aliphatic chains. Polymer-cell interaction is assayed in a culture medium in the absence of serum. Effective concentrations of the polymers causing 50% cell death, EC₅₀, vary within three orders of magnitude. Toxic concentrations of the alternating copolymer, Triton X-100, and Brij-35 are lower than their CMC values. In contrast, all block copolymers, regardless of their chemical structures, become toxic at concentrations above the CMC; that is, they acquire cytotoxicity only in the micellar form. The EC₅₀ values of the copolymers depend on their hydrophilic-liphophilic balance (HLB) through the following empirical formula: EC₅₀ × 10⁶ = 8.71 × HLB^2.1. This relationship makes it possible to predict the cytotoxic concentration region of a block copolymer of a known structure.     

Highly protein-resistant coatings from well-defined diblock copolymers containing sulfobetaines

Three well-defined diblock copolymers of poly(sulfobetaine methacrylate) [poly(SBMA)] and poly(propylene oxide) (PPO) were synthesized by the sequential addition of SBMA monomer to fixed amounts of PPO using an atom transfer radical polymerization method and varying poly(SBMA) lengths. These copolymers were characterized by 1H NMR and aqueous gel permeation chromatography. These copolymers were physically adsorbed onto a surface plasmon resonance (SPR) sensor surface covered by methyl-terminated self-assembled monolayers, followed by the in situ evaluation of protein adsorption on the adsorbed copolymers. It is found that the behavior of the protein adsorption depends on the molecular weight of the copolymers. Results show that the diblock copolymers containing poly(SBMA) can be highly protein resistant when surface SBMA densities are well controlled. Thus, copolymers containing zwitterionic groups are ideal for resisting protein adsorption when the surface density of zwitterionic groups is controlled.     

Solution pH-regulated interfacial adsorption of diblock phosphorylcholine copolymers

Spectroscopic ellipsometry has been used to examine the pH-responsive interfacial adsorption of a series of biocompatible diblock copolymers incorporating 2-methacryloyloxyethyl phosphorylcholine-based (MPC) residues and 2-(dialkylamino)ethyl methacrylate residues, with a specific focus on 2-(diethylamino)ethyl groups (referred to as MPCm-DEAn, where m and n refer to the mean degrees of polymerization of each block) at the hydrophilic silicon oxide/water interface. For all the copolymers studied the surface excess shows only weak concentration dependence. Increasing the length of the DEA block has little effect on the dynamic or equilibrated adsorption at pH 7, indicating that the DEA block adopts a flat conformation on the silicon oxide surface at this pH. With increasing pH, however, the surface excess shows a dramatic increase, followed by a subsequent decline. The observed maximum in surface excess represents a balance between charge over-compensation of the copolymer with the oppositely charged surface and the subsequently reduced charge density of the copolymer. Variations in the observed maxima for various MPCm-DEAn diblock copolymers indicate different surface conformations at high pH. Salt addition does not affect copolymer adsorption. This behavior is attractive for biomedical applications in which the ionic strength is variable. It was also found that the preadsorbed diblock copolymers immobilized DNA from solution to an extent that is proportional to the relative charge ratio between the anionic DNA and the cationic DEA block of the copolymer.     

Brush-type amphiphilic diblock copolymers from "living"/controlled radical polymerizations and their aggregation behavior

Two brush-type amphiphilic diblock copolymers, poly(poly(ethylene glycol)methyl ether methacrylate-block-polystyrene) (P(PEGMA)-b-PS) and poly(glycidyl methacrylate)-block-poly(poly(ethylene glycol)methyl ether methacrylate) (P(GMA)-b-P(PEGMA)) were synthesized, respectively, via consecutive atom-transfer radical polymerizations (ATRPs) and reversible addition-fragmentation chain-transfer (RAFT) polymerizations. The diblock copolymers were characterized by gel permeation chromatography (GPC), (1)H nuclear magnetic resonance (NMR) spectroscopy, and FT-IR spectroscopy. The aggregation behavior of the two amphiphilic diblock copolymers in water was also studied. Scanning electron and transmission electron microscopic images revealed that spherical micelles (40-80 nm in diameter) from self-assembly of the P(PEGMA)-b-PS copolymers and wormlike micelles (60-120 nm in length and 20-30 nm in diameter) from self-assembly of the P(GMA)-b-P(PEGMA) copolymers were prevalent. The spherical P(PEGMA)-b-PS micelles could self-assemble gradually into giant aggregates of several micrometers in diameter.     

Biomimetic design of amphiphilic polycations and surface grafting onto polycarbonate urethane film as effective antibacterial agents with controlled hemocompatibility

The synthetic polycations are ideal candidates as antimicrobial agents, because they resemble natural antimicrobial peptides, but to render hemocompatibility to these materials is a great challenge. Herein, we used 2‐(tert‐butyl‐aminoethyl) methacrylate (TBAEMA), to synthesize its homopolymer and pegylated random and diblock copolymers with polyethyleneglycol methacrylate (PEGMA, M_n = 360 Da) by single‐electron transfer–living radical polymerization (SET‐LRP). In the second step, the secondary amino groups in the precursor polymers were quaternized with iodomethane and bromohexane, to obtain three series of quaternized polymers. The antimicrobial properties of these quaternized polymers were evaluated against Escherichia coli (E. coli), by studying the minimum inhibitory concentrations (MICs) which ranged between 32 and 200 mg L^?1 and showed higher values for the quaternized random than the diblock copolymers. In addition to, we have also demonstrated the grafting of these polycations onto polycarbonate urethane film surfaces, which showed good killing efficacy against E. coli. Furthermore, the hemolysis of these materials was investigated against human red blood cells, which indicated that except the quaternized homopolymers that showed highest hemolysis, all other amphiphilic polycations exhibited very low hemolytic activity. Therefore, our designed materials with controlled structures and functionality, synthesized from cheaply available resources could serve as useful agents in the field of biomedicines and implantable materials. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3166–3176     

Synthesis and supramolecular organization of amphiphilic diblock copolymers combining poly(N,N-dimethylamino-2-ethyl methacrylate) and poly(epsilon-caprolactone)

Well-defined poly(epsilon-caprolactone) (PCL)/poly(N,N-dimethylamino-2-ethyl methacrylate (PDMAEMA) diblock copolymers were synthesized, and their self-assembly was investigated as micelles both in aqueous solutions and in thin solid deposits. The synthetic approach combines controlled ring opening polymerization (ROP) of epsilon-caprolactone (CL) and atom transfer radical polymerization (ATRP) of N,N-dimethylamino-2-ethyl methacrylate (DMAEMA). Diblock copolymers were prepared by ROP of CL initiated by (Al(OiPr)3), followed by quantitative reaction of the PCL hydroxy end-groups with bromoisobutyryl bromide. The alpha-isopropyloxy omega-2-bromoisobutyrate poly(epsilon-caprolactone) (PCL-Br) obtained was used as a macroinitiator for the ATRP of DMAEMA. The molecular characterization of those diblock copolymers was performed by 1H NMR spectroscopy and gel permeation chromatography (GPC) analysis. The self-assembly of the copolymers into micellar aggregates in aqueous media was followed with dynamic light scattering (DLS), as a function of concentration and the pH. In parallel, the morphology of the solid deposits of those micelles was examined with atomic force microscopy (AFM).     

Diblock copolymers as emulsifying agents in polymer blends: Influence of molecular weight,architecture, and chemical composition

Interfacial agents used in the compatibilization of immiscible polymer blends often consist of block copolymers containing at least one segment compatible with each of the two phases of the blend. This work examines the influence of the molecular weight, architecture, and chemical composition of the interfacial agent on its ability to emulsify a polymer blend. The system chosen is a blend containing 80% polystyrene and 20% ethylene-propylene rubber, compatibilized by diblock copolymers of poly(styrene-hydrogenated butadiene). The emulsification curve, which relates the dispersed phase particle size to the concentration of interfacial agent added to the system, was used as a tool to characterize the efficacy of the different interfacial agents. The observed behavior is similar to that of classical emulsions: a rapid drop in phase size at low concentrations of interfacial modifier, followed by a levelling off to an equilibrium diameter value once a “critical” concentration has been reached. For systems compatibilized by symmetrical diblocks (i.e., containing approximately 50% styrene by weight), the volume average particle diameter decreased from 2.7 μm for the unmodified system to about 0.4 μm once interfacial saturation is reached. The critical concentration for emulsification decreased with increasing interfacial agent molecular weight, due to the higher interfacial area occupied by longer molecules; however, this parameter did not affect the equilibrium particle diameter. The asymmetrical diblock copolymer (30% styrene) was found to be less effective than the symmetrical ones over the entire range of concentrations studied (5 to 35% modifier, based on the volume of the minor phase). Asymmetrical diblock copolymers would tend to form micelles, whereas symmetrical copolymers are less constrained at the interface. No significant difference was observed between the emulsifying capability of tapered and pure diblocks of similar composition and molecular weight. © 1996 John Wiley & Sons, Inc.     

Self-organization behavior of methacrylate-based amphiphilic di- and triblock copolymers

Amphiphilic di- and triblock copolymers having different hydrophilic-to-hydrophobic block length ratio were synthesized using ATRP. The self-assembly behavior of these AB and ABA block copolymers consisting of poly(n-butyl methacrylate) (B) and poly(2,2-(dimethylaminoethyl methacrylate) (A) was investigated using a combination of dynamic light scattering, negative-stain transmission electron microscopy, cryoelectron microscopy, and atomic force microscopy. Two populations of self-organized structures in aqueous solution, micelles and compound micelles, were detected for diblock copolymers. Triblock copolymers assembled into vesicular structures of uniform sizes. Furthermore it was found that these vesicles tended to compensate the high curvature by additional organization of the polymer chains outside of the membrane. The chain hydrophilicity of the polymers appeared to have a critical impact on the self-assembly response toward temperature change. The self-reorganization of the polymers at different temperatures is discussed.     

Capillary electrophoresis of associative diblock copolymers.

Water soluble diblock copolymers composed of a long poly(styrene sulfonate) chain (between 200 and 400 monomers) and a short poly(ethylene propylene) or poly(tert.-butylstyrene) hydrophobic end (20-50 monomers) are highly associative and form micelles in aqueous solution. The micelles are composed of a small hydrophobic core and a polyelectrolyte corona, the dimensions of which can be estimated by neutron and light scattering. These physical techniques are, however, not amenable to discriminate easily between the free copolymer and the copolymer micelle. Capillary electrophoresis was implemented in this work as a new and effective tool to investigate the behaviour of such associative copolymer systems. Since the rate of exchange between the micellised and free states is very slow in comparison with the time scale of the electrophoretic process, the electropherograms of the diblock copolymers obtained in plain aqueous borate buffers exhibit two peaks assigned to the two states mentioned above. The identification of the two peaks was first made on the basis of the retention orders of the two peaks equally obtained in similar conditions by size-exclusion chromatography. The copolymer micelles appeared to have a smaller electrophoretic mobility than the free copolymers. This peak assignment is also consistent with the observed ratio of the time-corrected peak areas and peak dispersions. The effects of the copolymer concentration, electric field, temperature and hydroorganic composition of the medium was also studied. Such systems do not exhibit a defined concentration threshold equivalent to a classical critical micelle concentration. Adding methanol to the electrolyte resulted in the progressive loss of baseline return between the two peaks, which might be attributed to a slight increase in the rate of exchange between the two states. Finally, adding a neutral surfactant to the electrolyte at a concentration in excess of its critical micelle concentration resulted in a decrease in the electrophoretic mobility of the peak attributed to the free copoplymer, while the electrophoretic mobility of the copolymer micelle remained unperturbed.