Influence of the macromolecular architecture on the self-assembly of amphiphilic copolymers based on poly(N,N-dimethylamino-2-ethyl methacrylate) and poly(epsilon-caprolactone)

The self-assembly of amphiphilic copolymers consisting of poly( N, N-dimethylamino-2-ethyl methacrylate) (PDMAEMA) and poly(-caprolactone) (PCL) segments arranged in graft and linear diblock architectures was investigated in this work by means of dynamic light scattering (DLS) in aqueous solution and by atomic force microscopy (AFM) on thin deposits. The solid-state deposits of the micelles were generated by a "freeze-drying" technique that preserves the initial micelle morphology in solution. A comparison between the morphological properties of graft copolymers with corresponding diblock copolymers was established to demonstrate the effect of the copolymer architecture on the micelle structure and organization.     

Drug release and biocompatibility of self‐assembled micelles prepared from poly (ɛ‐caprolactone/glycolide)‐poly (ethylene glycol) block copolymers

A series of poly(?‐caprolactone/glycolide)‐poly(ethylene glycol) (P(CL/GA)‐PEG) diblock copolymers were prepared by ring opening polymerization of a mixture of ?‐caprolactone and glycolide using mPEG as macro‐initiator and stannous octoate as catalyst. Self‐assembled micelles were prepared from the copolymers using nanoprecipitation method. The micelles were spherical in shape. The micelle size was larger for copolymers with longer PEG blocks. In contrast, the critical micelle concentration of copolymers increased with decreasing the overall hydrophobic block length. Drug loading and drug release studies were performed under in vitro conditions, using paclitaxel as a hydrophobic model drug. Higher drug loading was obtained for micelles with longer poly(ε‐caprolactone) blocks. Faster drug release was obtained for micelles of mPEG2000 initiated copolymers than those of mPEG5000 initiated ones. Higher GA content in the copolymers led to faster drug release. Moreover, drug release rate was enhanced in the presence of lipase from Pseudomonas sp., indicating that drug release is facilitated by copolymer degradation. The biocompatibility of copolymers was evaluated from hemolysis, dynamic clotting time, and plasma recalcification time tests, as well as MTT assay and agar diffusion test. Data showed that copolymer micelles present outstanding hemocompatibility and cytocompatibility, thus suggesting that P(CL/GA)‐PEG micelles are promising for prolonged release of hydrophobic drugs.     

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.     

Effects of block architecture and composition on the association properties of poly(oxyalkylene) copolymers in aqueous solution

A block copolymer of hydrophilic poly(ethylene oxide) and a hydrophobic poly(alkylene oxide) can associate in dilute aqueous solution to form micelles. The results of recent investigations of the micellisation behaviour and micelle properties of such copolymers are described. Copolymers of ethylene oxide with propylene oxide, 1,2‐butylene oxide or styrene oxide are considered, including aspects of their preparation. Experimental methods for determination of critical conditions for micellisation, micelle association number and spherical‐micelle radius are summarised. Effects of temperature, composition, block length and block architecture (diblock, triblock and cyclic‐diblock) are described and, where possible, related to the predictions of theory. Brief consideration is given to the dynamics of micelle formation/dissociation, to cylindrical micelles, and to effects of added salts.     

多分散性对两亲性两嵌段共聚物在选择性溶剂中自组装行为影响的Monte Carlo模拟

采用Monte Carlo模拟方法研究了多分散性AB两亲性两嵌段共聚物在选择性溶剂中的自组装行为.模拟结果表明,嵌段共聚物的多分散性对体系在选择性溶剂中自组装所形成的胶束形貌结构有很大影响.当AB两嵌段共聚物的多分散系数由1.0增加至1.4时,体系中自组装所形成的胶束将会发生由囊泡到片层直至球状的一系列形态转变.通过统...     

三嵌段共聚物和两嵌段共聚物在水溶液中共混自组装成多室胶束结构的耗散粒子动力学模拟(英文)

利用耗散粒子动力学模拟研究了在水溶液中混合不同的线形三嵌段共聚物AxByCz和线形两嵌段共聚物AmBn对多室胶束的形貌多样性的影响.通过改变线形的三嵌段共聚物和两嵌段共聚物的链长来寻找多室胶束的形成条件.由线形三嵌段共聚物和线形两嵌段共聚物的不同混合形成的多室胶束结构是多种多样的,例如"蠕虫状"胶束、"汉堡包"胶束、"球上球"胶束、"核-壳-壳"胶束等等.多室胶束的整体形貌和内部结构的控制都可以从线形三嵌段共聚物和两嵌段共聚物的二元共混得到.为了表征获得的多室胶束结构,我们计算了密度图和成对分布函数图.在此工作中,可以获得和观察到复杂的多室胶束.结果表明,简单地混合线形的三嵌段共聚物和线形的两嵌段共聚物是一个控制多室胶束形貌和结构的有效方法,在工程实验中可以更简单更经济地形成多室胶束结构.因此,在设计新的多室胶束方面,聚合物共混仍然是未来值得更加关注的一个话题.     

Polymer micelles with crystalline cores for thermally triggered release

Interest in the use of poly(ethylene glycol)-b-polycaprolactone diblock copolymers in a targeted, magnetically triggered drug delivery system has led to this study of the phase behavior of the polycaprolactone core. Four different diblock copolymers were prepared by the ring-opening polymerization of caprolactone from the alcohol terminus of poly(ethylene glycol) monomethylether, M(n) ≈ 2000. The critical micelle concentration depended on the degree of polymerization for the polycaprolactone block and was in the range of 2.9 to 41 mg/L. Differential scanning calorimetry curves for polymer solutions with a concentration above the critical micelle concentration showed a melting endotherm in the range of 40 to 45 °C, indicating the polycaprolactone core was semicrystalline. Pyrene was entrapped in the micelle core without interfering with the ability of the polycaprolactone to crystallize. When the polymer solution was heated above the melting point of the micelle core, the pyrene was free to leave the core. Temperature-dependent measurements of the critical micelle concentration and temperature-dependent dynamic light scattering showed that the micelles remain intact at temperatures above the melting point of the polycaprolactone core.     

Isothermal titration calorimetry and dynamic light scattering studies of interactions between gemini surfactants of different structure and Pluronic block copolymers

The interactions between triblock copolymers of poly(ethylene oxide) and poly(propylene oxide), P103 and F108, EO(n)PO(m)EO(n), m=56 and n=17 and 132, respectively, and m-s-m type gemini surfactants, m=8, 10, 12, and 18, and s = 3, 6, 12, and 16, have been studied in aqueous solution using isothermal titration calorimetry and dynamic light scattering techniques. The enthalpograms of F108 as a function of surfactant concentration show one broad peak at polymer concentrations C(p) < or = 0.50 wt%, below the cmc of the copolymer at 25 degrees C. It is attributed to interactions between the surfactant and the triblock copolymer monomer. DLS results show hydrodynamic radii (R(h)) initially consistent with copolymer monomers that change to values consistent with gemini surfactant micelles as the surfactant concentration is increased. In P103 solutions at C(p) > or = 0.05 wt%, two peaks appear in the enthalpograms, and they are attributed to the interactions between the gemini surfactant and the micelle or monomer forms of the copolymer. An origin-based nonlinear fitting program was employed to deconvolute the two peaks and to obtain estimates of peak properties. An estimate of the fraction of copolymer in aggregated form was also obtained. The enthalpy change due to interactions between the surfactants and P103 aggregates is very large compared to values obtained for traditional surfactants. This suggests that extensive reorganization of copolymer aggregates and surrounding solvent occurs during the interaction. DLS results for the P103 systems containing C(p) > or = 0.05% show evidence of very large aggregates in solution, likely P103 micelle clusters. The transitions observed in the hydrodynamic radii are consistent with a breakdown of micelle clusters with addition of gemini surfactant, followed by mixed micelle formation and/or deaggregation into monomer P103. This is followed by interactions similar to those typically observed in surfactant-nonionic polymer systems. Mechanisms for the interaction and the observed structural changes are discussed.     

pH-responsive diblock copolymer micelles at the silica/aqueous solution interface: Adsorption kinetics and equilibrium studies

The adsorption behavior of two examples of a weakly basic diblock copolymer, poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA), at the silica/aqueous solution interface has been investigated using a quartz crystal microbalance with dissipation monitoring and an optical reflectometer. Dynamic and static light scattering measurements have also been carried out to assess aqueous solution properties of such pH-responsive copolymers. In alkaline solution, core-shell micelles are formed above the critical micelle concentration (cmc) by both copolymers, whereas the chains are molecularly dissolved (as unimers) at all concentrations in acidic solution. As a result, the adsorption behavior of PDMA-PDEA diblock copolymers on silica is strongly dependent on both the copolymer concentration and the solution pH. Below the cmc at pH 9, the cationic PDMA-PDEA copolymers adsorb as unimers and the conformation of the adsorbed polymer is essentially flat. At concentrations just above the cmc, the initial adsorption of copolymer onto the silica is dominated by the unimers due to their faster diffusion compared to the much larger micelles. Rearrangement of the adsorbed unimers and/or their subsequent displacement by micelles from solution is then observed during an equilibration period, and the final adsorbed mass is greater than that observed below the cmc. At concentrations well above the cmc, the much higher proportion of micelles in solution facilitates more effective competition for the surface at all stages of the adsorption process and no replacement of initially adsorbed unimers by micelles is evident. However, the adsorbed layer undergoes gradual rearrangement after initial adsorption. This relaxation is believed to result from a combination of further copolymer adsorption and swelling of the adsorbed layer.     

Non-surface activity and micellization of ionic amphiphilic diblock copolymers in water. Hydrophobic chain length dependence and salt effect on surface activity and the critical micelle concentration

We reported previously (Macromolecules 2003, 36, 5321; Langmuir, 2004, 20, 7412) that amphiphilic diblock copolymers having polyelectrolytes as a hydrophilic segment show almost no surface activity but form micelles in water. In this study, to further investigate this curious and novel phenomenon in surface and interface science, we synthesized another water-soluble ionic amphiphilic diblock copolymer poly(hydrogenated isoprene)-b-sodium poly(styrenesulfonate) PIp-h2-b-PSSNa by living anionic polymerization. Several diblock copolymers with different hydrophobic chain lengths were synthesized and the adsorption behavior at the air/water interface was investigated using surface tension measurement and X-ray reflectivity. A dye-solubilization experiment was carried out to detect the micelle formation. We found that the polymers used in this study also formed micelles above a certain polymer concentration (cmc) without adsorption at the air-water interface under a no-salt condition. Hence, we further confirmed that this phenomenon is universal for amphiphilic ionic block copolymer although it is hard to believe from current surface and interface science. For polymers with long hydrophobic chains (more than three times in length to hydrophilic chain), and at a high salt concentration, a slight adsorption of polymer was observed at the air-water interface. Long hydrophobic chain polymers showed behavior "normal" for low molecular weight ionic surfactants with increasing salt concentration. Hence, the origin of this curious phenomenon might be the macroionic nature of the hydrophilic part. Dynamic light scattering analysis revealed that the hydrodynamic radius of the block copolymer micelle was not largely affected by the addition of salt. The hydrophobic chain length-cmc relationship was found to be unusual; some kind of transition point was found. Furthermore, very interestingly, the cmc of the block copolymer did not decrease with the increase in salt concentration, which is in clear contrast to the fact that cmc of usual ionic small surfactants decreases with increasing salt concentration (Corrin-Harkins law). These behaviors are thought to be the special, but universal, characteristics of ionic amphiphilic diblock copolymers, and the key factor is thought to be a balance between the repulsive force from the water surface by the image charge effect and the hydrophobic adsorption.     

Influence of a strong polyelectrolyte block on the formation and properties of polymer micelles with a mixed corona

Joint micellization of two amphiphilic diblock copolymers is studied by velocity sedimentation, transmission electron microscopy, electrophoretic mobility measurements, and static light scattering. One of the diblock copolymers is a strong polyelectrolyte (polystyrene-block-poly(N-ethyl-4-vinylpyridinium bromide)), while the second one is a weakly charged or uncharged copolymer (polystyrene-block-poly(acrylic acid) or polystyrene-block-poly(4-vinylpyridine)). It is shown that the mixing of the diblock copolymers in a selective aqueous-organic solvent (DMF-methanol-water) leads to the formation of joint (hybrid) micelles and that the composition of these micelles is close to the composition of the polymer mixture. Micelles consist of an insoluble polystyrene core and a mixed corona composed of blocks of a strong polyelectrolyte and a weakly charged or uncharged copolymer. Aqueous dispersions of mixed micelles are obtained with the use of the dialysis technique, the spherical morphology of the micelles is ascertained, and their three-layered structure is proposed. The nonlinear dependence of the molecular mass of micelles on their composition is found. The decisive effect of electrostatic repulsion between strong polyelectrolyte units on the thermodynamics of micellization and the dispersion stability and molecular-mass characteristics of the mixed micelles is demonstrated.     

Non-surface activity and micellization behavior of cationic amphiphilic block copolymer synthesized by reversible addition-fragmentation chain transfer process

Cationic amphiphilic diblock copolymers of poly(n-butylacrylate)-b-poly(3-(methacryloylamino)propyl)trimethylammonium chloride) (PBA-b-PMAPTAC) with various hydrophobic and hydrophilic chain lengths were synthesized by a reversible addition-fragmentation chain transfer (RAFT) process. Their molecular characteristics such as surface activity/nonactivity were investigated by surface tension measurements and foam formation observation. Their micelle formation behavior and micelle structure were investigated by fluorescence probe technique, static and dynamic light scattering (SLS and DLS), etc., as a function of hydrophilic and hydrophobic chain lengths. The block copolymers were found to be non-surface active because the surface tension of the aqueous solutions did not change with increasing polymer concentration. Critical micelle concentration (cmc) of the polymers could be determined by fluorescence and SLS measurements, which means that these polymers form micelles in bulk solution, although they were non-surface active. Above the cmc, the large blue shift of the emission maximum of N-phenyl-1-naphthylamine (NPN) probe and the low micropolarity value of the pyrene probe in polymer solution indicate the core of the micelle is nonpolar in nature. Also, the high value of the relative intensity of the NPN probe and the fluorescence anisotropy of the 1,6-diphenyl-1,3,5-hexatriene (DPH) probe indicated that the core of the micelle is highly viscous in nature. DLS was used to measure the average hydrodynamic radii and size distribution of the copolymer micelles. The copolymer with the longest PBA block had the poorest water solubility and consequently formed micelles with larger size while having a lower cmc. The "non-surface activity" was confirmed for cationic amphiphilic diblock copolymers in addition to anionic ones studied previously, indicating the universality of non-surface activity nature.     

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.     

Aggregation behavior of MPEG‐PCL diblock copolymers in aqueous solutions and morphologies of the aggregates

Poly(ethylene glycol)‐b‐polycaprolactone (MPEG‐PCL) diblock copolymers were synthesized via a ring‐opening polymerization of ε‐CL monomers with MPEG as an initiator. Their solubilities and apparent critical micelle concentrations (CMC) in aqueous solution were investigated as well as the determination of the micellar hydrodynamic diameter using dynamic light scattering (DLS). As PCL block length increased, the solubility and CMC decreased while diameters of micelles increased. The gel–sol transition behaviors were investigated using a vial tilting method. Aqueous solutions of copolymers undergo a gel to sol transition with increase in temperature when their polymer concentrations are above a critical gel concentration (CGC). The CGC of the copolymers and gel–sol transition temperature are influenced by the PCL chain length. The tapping mode AFM was performed by imaging the freeze‐dried deposits from the copolymer solutions on mica to investigate a process from free chains to micelles and to gel. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3406–3417, 2006     

Sphere-to-rod transition of non-surface-active amphiphilic diblock copolymer micelles: a small-angle neutron scattering study

Micellization behavior of amphiphilic diblock copolymers with strong acid groups, poly(hydrogenated isoprene)-block-poly(styrenesulfonate), was investigated by small-angle neutron scattering (SANS). We have reported previously (Kaewsaiha, P.; Matsumoto, K.; Matsuoka, H. Langmuir 2005, 21, 9938) that this strongly ionic amphiphilic diblock copolymer shows almost no surface activity but forms micelles in water. In this study, the size, shape, and internal structures of the micelles formed by these unique copolymers in aqueous solution were duly investigated. The SANS data were well described by the theoretical form factor of a core-shell model and the Pedersen core-corona model. The micellar shape strongly depends on the hydrophobic chain length of the block copolymer. The polymer with the shortest hydrophobic chain was suggested to form spherical micelles, whereas the scattering curves of the longer hydrophobic chain polymers showed a q-1 dependence, reflecting the formation of rodlike micelles. Furthermore, the addition of salt at high concentration also induced the sphere-to-rod transition in micellar shape as a result of the shielding effect of electrostatic repulsion. The corona thickness was almost constant up to the critical salt concentration (around 0.2 M) and then decreased with further increases in salt concentration, which is in qualitatively agreement with existing theories. The spherical/rodlike micelle ratio was also constant up to the critical salt concentration and then decreased. The micelle size and shape of this unique polymer could be described by the common concept of the packing parameter, but the anomalously stable nature of the micelle (up to 1 M NaCl) is a special characteristic.     

Metalated diblock and triblock poly(ethylene oxide)-block-poly(4-vinylpyridine) copolymers: understanding of micelle and bulk structure

The paper provides new insights into the structure of Pt-containing diblock and triblock copolymers based on poly(ethylene oxide) (PEO) and poly(4-vinylpyridine) (P4VP), using a combination of atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and anomalous small-angle X-ray scattering (ASAXS). Parallel studies using methods contributing supplemental structural information allowed us to comprehensively characterize sophisticated polymer systems during metalation and to exclude possible ambiguity of the data interpretation of each of the methods. AFM and TEM make available the determination of sizes of the micelles and of the Pt-containing micelle cores, respectively, while a combination of XRD, TEM, and ASAXS reveals Pt-nanoparticle size distributions and locations along with the structural information about the polymer matrix. In addition, for the first time, ASAXS revealed the organization of Pt-nanoparticle-filled diblock and triblock copolymers in the bulk. The nanoparticle characteristics are mainly determined by the type of block copolymer system in which they are found: larger particles (2.0-3.0 nm) are formed in triblock copolymer micelles, while smaller ones (1.5-2.5 nm) are found in diblock copolymer micelles. This can be explained by facilitated intermicellar exchange in triblock copolymer systems. For both systems, Pt nanoparticles have narrow particle size distributions as a result of a strong interaction between the nanoparticle surface and the P4VP units inside the micelle cores. The pH of the medium mainly influences the particle location rather than the particle size. A structural model of Pt-nanoparticle clustering in the diblock PEO-b-P4VP and triblock P4VP-b-PEO-b-P4VP copolymers in the bulk was constructed ab initio from the ASAXS data. This model reveals that nearly spherical micellar cores of about 10 nm in diameter (filled with Pt nanoparticles) aggregate forming slightly oblate hollow bodies with an outer diameter of about 40 nm.     

Mixed micelles based on cationic and anionic amphiphilic diblock copolymers containing identical hydrophobic blocks

Through the use of the methods of turbidimetry, UV spectrophotometry, fluorescence spectroscopy, dynamic light scattering, and ultracentrifugation, micelle formation is studied for cationic (polysty-rene-poly-N-ethyl-4-vinylpyridium bromide) and anionic (polystyrene-sodium polyacrylate) diblock copolymers containing identical polystyrene blocks in dilute aqueous saline solutions. Mixing of aqueous dispersions of individual micelles is accompanied by the formation of only insoluble products, which likely are intermicellar interpolyelectrolyte complexes. At the same time, mixing of diblock copolymers in a nonselective solvent and its subsequent gradient replacement with water during suppressed interpolyelectrolyte interactions yields mixed diblock copolymer micelles, which are found to be dispersionally stable in an excess of charged units of any polymer component. The micelles are composed of an insoluble polystyrene core and a mixed interpolyelectrolyte corona, and their hydrodynamic characteristics are controlled by the ratio of charged units in the mixed diblock copolymers. The mixed micelles are found to be able to interact with the macromolecules of a homopolyelectrolyte, sodium poly(styrene sulfonate), in aqueous solutions and form ternary complexes. In this case, depending on the composition of the mixed micelles, ternary complexes can be dispersionally stable or can aggregate and precipitate.     

Shear ordered diblock copolymers with tuneable optical properties

A series of high molecular weight poly(styrene-b-isoprene) block copolymers with optical properties defined by composition in a non-selective solvent were studied using simultaneous ultra small angle X-ray scattering (USAXS) and optical spectrometry. A small magnitude shear produces ordered and oriented states in the copolymer solutions that persist for extended periods of time, and also have superior optical properties that are directly attributable to the mesoscopic block copolymer (BCP) morphology. We have demonstrated that the optical transmission of these materials can be tuned by the addition of low molecular weight poly(isoprene) and poly(styrene) to swell their respective domains within the diblock copolymer. The optical transmission peak for the diblocks could be tuned; from 380 nm-440 nm for the 670k diblock, 425 nm-540 nm for the 850k diblock and 541 nm-625 nm for the 1 million diblock by altering the solution concentration and composition. The full width at half maximum that can be achieved for the optical transmission peaks is as small as 15 nm at 473 nm with a Δλ/λ of 0.03, highlighting the high quality ordering in these systems. Also a small shift in the transmission peak wavelength was observed across a wide angle of view (15 nm at 30°) suggesting that these materials could be used for large area narrow band optical filters.     

Micellisation and gelation of diblock copolymers of ethylene oxide and propylene oxide in aqueous solution, the effect of P-block length

The aqueous solution properties of five diblock copolymers prepared by sequential anionic copolymerisation (i.e. E₁₀₂P₃₇, E₁₀₄P₅₂, E₉₂P₅₅, E₁₀₄P₆₀ and E₉₈P₇₃ where E denotes oxyethylene and P denotes oxypropylene) were studied across a wide range of concentration. The techniques used to study micellisation and micellar properties in dilute solution were static and dynamic light scattering, surface tension, and eluent gel-permeation chromatography. The gelation of concentrated solutions was also investigated. As expected, the critical micelle concentration (CMC) was lowered and the association number of the micelles was increased by an increase in P-block length. In contrast, the critical gel concentration was unchanged, consistent with the constant E-block length leading to micelles with essentially identical E-block fringes. Comparison of the CMCs of the diblock copolymers with those of triblock E_mP_nE_m copolymers with the same P-block length shows the diblock copolymers to micellise more efficiently. A similar comparison of the CMCs of the diblock copolymers with those of E_mB_n copolymer (B denotes oxybutylene) shows the hydrophobicity of a P unit to be one-sixth that of a B unit. The possibility is explored of correlating the limiting association number of a spherical micelle with the hydrophobe block length of its constituent copolymer. Of the five copolymers, only dilute solutions of E₉₈P₇₃ were predominantly micellar at both room temperature and body temperature, and this copolymer must be a prime candidate in any consideration of the potential application of E_mP_n copolymers in the solubilisation and controlled release of drugs.     

Micellar Behavior of Polystyrene-Poly(Ethylene Oxide) Diblock Copolymers in Aqueous Media: Effect of Copolymer Composition,Temperature, Salt,and Surfactants

Formation and structure of micelles from two amphiphilic polystyrene-block-poly(ethylene oxide) (PS-PEO) diblock copolymers (PS mol.wt. 1000; PEO mol.wt. 3000 and 5000) were examined by surface tension, viscosity, steady state fluorescence, dynamic light scattering (DLS), small angle neutron scattering (SANS), and cryo-transmission electron microscopy (cryo-TEM). The critical micelle concentration (CMC) of the copolymers in aqueous solution was ca. 0.05%; micelle hydrodynamic diameter was 30–35 nm with a narrow size distribution. SANS studies show that the copolymers form ellipsoidal micelles with semi major axis ～23 nm and semi minor axis ～8 nm. No significant change in the structure was found with temperature and presence of salt. The copolymer micelles interaction with the ionic surfactants sodium dodecyl sulphate (SDS) and dodecyltrimethylammonium bromide (DTAB) was also examined by DLS and SANS.