Phase behavior of SEBS triblock copolymer gels

Dynamic density functional theory calculations were performed for thermoplastic elastomer gels composed of an ABA triblock copolymer immersed in a B‐attractive solvent. The triblock copolymer model was parameterized for poly[styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene] (SEBS), while the solvent model was parameterized for the hydrocarbon oil tetradecane. The effect of the solvent concentration and S‐EB interaction on the morphology was investigated, where complementary experimental data was used to validate results at χ_ABN ≈ 100. Agreement was observed at solvent volume fractions of 0.2, 0.4, and 0.6, which correspond to the cylindrical, spherical, and spherical phases, respectively. Qualitative agreement was observed for 0.8 volume fraction solvent, where a core‐shell spherical micelle morphology was found. For a 50/50 vol % mixture of polymer/solvent, the effect of solvent molecular weight on the morphology was considered, where a transition between micro and macrophase separation was predicted at a critical solvent molecular weight. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1479–1491, 2011     

Synthesis and self-assembly behavior of thermoresponsive triblock copolymer

Block copolymers comprising thermosensitive poly(N-isopropylacrylamide) (PNIPAM) and hydrophobic poly(n-butyl acrylate) (PBA) blocks, were synthesized using the reversible addition-fragmentation chain transfer polymerization (RAFT), their thermosensitive behavior was studied by ultraviolet spectrophotometer (UV) and dynamic light scattering (DLS). The lower critical solution temperature (LCST) was strongly correlated to the hydrophobic/hydrophilic ratio of the copolymers. Their micellization and self-assembly behavior in dilute aqueous solution were studied by surface tension (SFT), DLS and TEM. The resulting block copolymers reversibly formed or deformed micellar assemblies during their LCSTs. The critical micelle concentration (CMC) was controlled by the composition of PBA and PNIPAM, indicating the successful formation of the block copolymers.     

Self-assembly of ABC star triblock copolymer thin films confined with a preferential surface: a self-consistent mean field theory

The microphase separation and morphology of a nearly symmetric A(0.3)B(0.3)C(0.4) star triblock copolymer thin film confined between two parallel, homogeneous hard walls have been investigated by self-consistent mean field theory (SCMFT) with a pseudospectral method. Our simulation experiments reveal that under surface confinement, in addition to the typically parallel, perpendicular, and tilted cylinders, other phases such as lamellae, perforated lamellae, and complex hybrid phases have been found to be stable, which is attributed to block-substrate interactions, especially for those hybrid phases in which A and B blocks disperse as spheres and alternately arrange as cubic CsCl structures, with a network preferred structure of C block. The results show that these hybrid phases are also stable within a broad hybrid region (H region) under a suitable film thickness and a broad field strength of substrates because their free energies are too similar to being distinguished. Phase diagrams have been evaluated by purposefully and systematically varying the film thickness and field strength for three different cases of Flory-Huggins interaction parameters between species in the star polymer. We also compare the phase diagrams for weak and strong preferential substrates, each with a couple of opposite quality, and discuss the influence of confinement, substrate preference, and the nature of the star polymer on the stability of relatively thinner and thick film phases in this work.     

Lateral structures of buried interfaces in ABA-type triblock copolymer films

The lateral structure of an ABA-type triblock copolymer polyparamethylstyrene- block-polystyrene- block-polyparamethylstyrene at the buried silicon substrate interface is studied as a function of different substrate surface treatments. With grazing incidence small-angle neutron scattering (GISANS), high interface sensitivity is reached. With GISANS, the orientation and degree of order of the morphology are probed. The powderlike oriented lamellar structure in the bulk orients along the surface normal in the vicinity of the substrate. A modification of the short-ranged interface potential of the substrate introduces a lateral stretching of this lamellar structure of up to 8% as compared to the bulk. The decay in stretching toward the volume structure is probed with depth profiling. It extends at least up to a distance of 51 nm from the solid surface.     

Phase behavior and local dynamics of concentrated triblock copolymer micelles

We report a neutron-scattering study to characterize the ordering and local dynamics of spherical micelles formed by the triblock copolymer polyethylene oxide (PEO)--polypropylene oxide (PPO)--polyethylene oxide (Pluronic) in aqueous solution. The study focuses on two Pluronic species, F68 and F108, that have the same weight fraction of PEO but that differ in chain length by approximately a factor of 2. At sufficiently high concentration, both species undergo a sequence of phase changes with increasing temperature from dissolved chains to micelles with liquid-like order to a cubic crystal phase and finally back to a micelle liquid phase. A comparison of the phase diagrams constructed from small-angle neutron scattering indicates that crystallization is suppressed for shorter chain micelles due to fluctuation effects. The intermediate scattering function I(Q,t)I(Q,0) determined by neutron spin echo displays a line shape with two distinct relaxations. Comparisons between I(Q,t)I(Q,0) for fully hydrogenated F68 chains in D2O and for F68 with deuterated PEO blocks reveal that the slower relaxation corresponds to Rouse modes of the PPO segments in the concentrated micelle cores. The faster relaxation is identified with longitudinal diffusive modes in the PEO corona characteristic of a polymer brush.     

Coarse-grained simulations of ABA amphiphilic triblock copolymer solutions in thin films

We study a coarse-grained model of A(10)-B(20)-A(10) amphiphilic triblock copolymers in aqueous solution under confinement. We focus on the influence of the wall interaction on the morphology of the ensuing self-assembled structures. We also study the dynamics of the polymers. All our simulations are confined between two walls. We study three different combinations of walls: hydrophobic and hydrophobic, hydrophobic and hydrophilic, hydrophilic and hydrophilic. We moreover elucidate the concentration influence. The conformation and behavior of the copolymer in strongly confined systems depend on the type of wall interaction and concentration.     

Interface-induced morphology transition in triblock copolymer films swollen with low-molecular-weight homopolymer

The morphology transition due to midblock swelling with low-molecular-weight homopolymer polystyrene of an ABA-type triblock copolymer polyparamethylstyrene-block-polystyrene-block-polyparamethylstyrene at the buried silicon substrate interface is studied as a function of different substrate surface treatments. With grazing incidence small-angle neutron scattering (GISANS), high interface sensitivity is reached. The powderlike oriented lamellar structure in the bulk becomes oriented along the surface normal in the vicinity of the substrate. A transition of the lamellar into a cylinder phase at the polymer-silicon interface is probed with GISANS. The transition is induced by the addition of the homopolymer, but the modification of the short-ranged interface potential of the substrate influences the amount of homopolymer that is necessary for this transition. Without and with 0.1 vol % added homopolymer, the lateral spacing is stretched at the interface as compared to the bulk whereas for a higher added amount of homopolymer no stretching occurs.     

Self-assembly of star ABC triblock copolymer thin films: self-consistent field theory

Microphase separation and morphology of star ABC triblock copolymers confined between two identical parallel walls (symmetric wetting or dewetting) are investigated with self-consistent field theory (SCFT) combined with the "masking" technique to describe the geometric confinement of the films. In particular, we examine the morphology of confined near-symmetric star triblock copolymers under symmetric and asymmetric interactions as a function of the film thickness and the surface field. Under the interplay between the degree of spatial confinement, characterized by the ratio of the film thickness to bulk period, and surface field, the confined star ABC triblock copolymers are found to exhibit a rich phase behavior. In the parameter space we have explored, the thin film morphologies are described by four primary classes including cylinders, perforated lamellae, lamellae, and other complex hybrid structures. Some of them involve novel structures, such as spheres in a continuous matrix and cylinders with alternating helices structure, which are observed to be stable with suitable film thickness and surface field. In particular, complex hybrid network structures in thin films of bulk cylinder-forming star triblock copolymers are found when the natural domain period is not commensurate with the film thickness. Furthermore, a strong surface field is found to be more significant than the spatial confinement on changing the morphology of star triblock copolymers in bulk. These findings provide a guide to designing novel microstructures involving star triblock copolymers via geometric confinement and surface fields.     

Incorporation and release behavior of amitriptylene in core-shell-corona type triblock copolymer micelles

A cationic antidepressant drug, amitriptylene (AMT), was successfully incorporated into core-shell-corona micelles of poly[styrene-b-sodium 2-(acrylamido)-2-methyl-1-propanesulfonate-b-ethylene oxide] (PS-b-PAMPS-b-PEO). Zeta-potential measurements revealed that both electrostatic and hydrophobic interactions contributed to the binding of the drug to the polymer. The AMT/PS-b-PAMPS-b-PEO nanocomplexes were characterized by dynamic light scattering, scanning electron microscopy, and transmission electron microscopy. The hydrodynamic diameter of the AMT loaded nanocomplexes decreased from 80 to 40nm depending on the amount of the drug loaded on the polymer. This is attributed to the cancellation of the negative charge of the PAMPS group by the cationic drug. The AMT/PS-b-PAMPS-b-PEO nanocomplexes were stable in aqueous solution exhibiting no aggregation or no precipitation for several months. Release of the AMT from the nanocomplexes was investigated in vitro in salt-free and 0.1M NaCl solutions. The drug was released faster in the 0.1M NaCl solution than in the salt-free solution. This is due to the shielding effect of the salt on the electrostatic interaction. However, in both cases, the drug release mainly occurs by the Fickian diffusion mechanism.     

Equilibrium phase behavior of the membrane forming water-DMSO-EVAL copolymer system

The equilibrium phase behavior of ethylene vinyl alcohol (EVAL) copolymer in mixtures of DMSO (dimethylsulfoxide, solvent) and water (nonsolvent) was studied for different temperatures. Both crystallization-induced gelation and liquid-liquid demixing were observed. From the determined phase diagram of this system at 25°C, three regions may be identified, i.e., a homogeneous region, a gel region, and binodal region in which both types of phase transition take place. At higher temperatures, crystallization isotherm was found to intersect the binodal phase boundary, which is analogous to the phase behavior reported by Stokes and Berghmans for several binary systems.     

Phase behavior in thin films of confined colloid-polymer mixtures

Using self-consistent-field and density-functional theories, we first investigate colloidal self-assembly of colloid-polymer films confined between two soft surfaces grafted by polymers. With increasing colloidal concentrations, the film undergoes a series of transitions from disordered liquid --> sparse square --> hexagonal (or mixed square-hexagonal) --> dense square --> cylindrical structures in a plane, which results from the competition between the entropic elasticity of polymer brushes and the steric packing effect of colloidal particles. A phase diagram displays the stable regions of different in-layer ordering structures as the colloidal concentration is varied and layering transitions as the polymer-grafted density is decreased. Our results show a new control mechanism to stabilize the ordering of structures within the films.     

Studies on confined crystallization behavior of polycaprolactone thin films

The confined crystallization behavior of polycaprolactone (PCL) in thin and ultrathin films was studied by AFM (atomic force microscopy). It was found that the crystalline morphology of PCL depended on the film’s thickness. When the thickness is d > 2 Rg (radius of gyration), the polymer can crystallize into spherulites; when Rg < d < 2 Rg, a dense-branch morphology and dendrites could be found; when d < Rg, an “islands” structure could be obtained. Moreover, the effects of the crystallization temperature and the substrate and the molecular weight on the crystalline morphology were discussed. It was shown that the crystallization of PCL in thin films is a diffusion-controlled process, and it can be explained by diffusion-limited aggregation. __________ Translated from Acta Polymerica Sinica, 2006, (8): 964–969 [译自: 高分子学报]     

Structure of microemulsion-ABA triblock copolymer networks

Structural equilibrium properties of transient networks formed by microemulsion droplets and ABA triblock copolymers in solution have been studied by Monte Carlo simulation. The droplets were represented by soft spheres, and the polymers were represented by junctions connected by harmonic bonds with an angular potential regulating the intrinsic chain stiffness. The interaction parameters were selected such that the end A-blocks were localized inside the droplets and the middle B-block in the continuous phase. The influence of (i) the polymer concentration, (ii) the polymer stiffness, and (iii) the contour length of the middle B-block on the formation and the structure of the microemulsion-polymer network were investigated using polymer end-to-end separation probability distribution functions, droplet radial distribution functions, droplet-droplet nearest-neighbor probability distribution functions, and network connectivity indicators. An increase of the polymer-droplet number ratio had a strong impact on the network formation. Under typical conditions and at an intermediate polymer-droplet number ratio, (i) the fraction of polymers forming bridges between droplets increased from essentially zero to unity and (ii) the fraction of polymers that were forming loops decreased as the ratio of the polymer end-to-end separation and the surface-to-surface separation between neighboring droplets for a hypothetical homogeneous droplet distribution was increased from 0.5 to 2. For long and flexible polymers, a mesoscopic segregation triggered by a depletion attraction between droplets appeared, and, furthermore, for sufficiently stiff chains, only bridge conformations occurred. The percolation probability could be represented as a function of the average droplet cluster size only, across all systems.     

Electric field versus surface alignment in confined films of a diblock copolymer melt

The dynamics of alignment of microstructure in confined films of diblock copolymer melts in the presence of an external electric field was studied numerically. We consider in detail a symmetric diblock copolymer melt, exhibiting a lamellar morphology. The method used is a dynamic mean-field density functional method, derived from the generalized time-dependent Ginzburg-Landau theory. The time evolution of concentration variables and therefore the alignment kinetics of the morphologies are described by a set of stochastic equations of a diffusion form with Gaussian noise. We investigated the effect of an electric field on block copolymers under the assumption that the long-range dipolar interaction induced by the fluctuations of composition pattern is a dominant mechanism of electric-field-induced domain alignment. The interactions with bounding electrode surfaces were taken into account as short-range interactions resulting in an additional term in the free energy of the sample. This term contributes only in the vicinity of the surfaces. The surfaces and the electric field compete with each other and align the microstructure in perpendicular directions. Depending on the ratio between electric field and interfacial interactions, parallel or perpendicular lamellar orientations were observed. The time scale of the electric-field-induced alignment is much larger than the time scale of the surface-induced alignment and microphase separation.     

Aggregation behavior of pluronic triblock copolymer in 1-butyl-3-methylimidazolium type ionic liquids

Three amphiphilic poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) ethers triblock copolymers, denoted Pluronic L61 (PEO3PPO30PEO3), Pluronic L64 (PEO13PPO30PEO13), and Pluronic F68 (PEO79PPO30PEO79) were shown to aggregate and form micelles in ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) and 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6). The surface tension measurements revealed that the dissolution of the copolymers in ILs depressed the surface tension in a manner analogous to aqueous solutions. The cmcs of three triblock copolymers increase following the order of L61, L64, F68, suggesting that micellar formation was driven by solvatophobic effect. cmc and gamma cmc decrease with increasing temperature because hydrogen bonds between ILs and hydrophilic group of copolymers decrease and accordingly enhance the solvatophobic interaction. Micellar droplets of irregular shape with average size of 50 nm were observed. The thermodynamic parameters DeltaGm0, DeltaHm0, DeltaSm0 of the micellization of block copolymers in bmimBF4 and bmimPF6 were also calculated. It was revealed that the micellization is a process of entropy driving, which was further confirmed by isothermal titration calorimetry (ITC) measurements.     

Effects of a PPO-PEO-PPO triblock copolymer on micellization and gelation of a PEO-PPO-PEO triblock copolymer in aqueous solution

The effects of a PPO-PEO-PPO triblock copolymer (25R4, PO(19)-EO(33)-PO(19)) on thermoreversible micellization and gelation properties of a PEO-PPO-PEO triblock copolymer (F108, EO(133)-PO(50)-EO(133)) in water were studied by means of micro-DSC and rheology. A complete, mirror-image like thermoreversible behavior has been observed for all of the samples with various molar ratios of 25R4 to F108. At a given concentration of F108, the addition of 25R4 results in the salt-out like effect on the primary micellization of F108; that is, the critical micellization temperature (CMT) of F108 shifts to lower temperatures with increasing the content of 25R4. The enthalpy changes for micellization are a linear function of the 25R4/F108 molar ratio at a fixed F108 concentration. Beyond the primary peak for the micellization of F108, a secondary peak or shoulder is observed in the DSC curves for the samples with the higher 25R4/F108 molar ratios, due to the formation of the hydrophobic aggregates from both the PPO blocks of F108 and those (i.e., PPO blocks) of 25R4. Furthermore, as an example, the dynamic viscoelastic properties of 18 wt % F108 solutions with various contents of 25R4 have been examined. It is found that, when the 25R4/F108 molar ratio < or =1, 25R4 does not affect the gelation of F108 notably. When the ratio is greater than 1, however, the formation of the 25R4-bridged micellar aggregates delays the gelation of F108 significantly. A schematic model has been proposed to explain the mechanism for the 25R4-influenced micellization and gelation of F108.     

Formation of internally nanostructured triblock copolymer particles

Particles with an internal structure have been found in dilute water solutions of a triblock copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), which has short hydrophilic PEO endblocks compared to the central hydrophobic PPO block (EO5PO68EO5, L121). The properties of the block copolymer particles (i.e., their structure, size, and time stability) have been investigated using cryogenic transmission electron microscopy (cryo-TEM) in combination with dynamic light scattering (DLS) and turbidity measurements. The particles were formed in dilute solutions by quenching the temperature to temperatures where the reversed hexagonal phase is in equilibrium with a solution of unaggregated L121 copolymers (L1). From the DLS measurements, a mean hydrodynamic radius of 158 nm was extracted. The time-scan turbidity measurements were found to be unchanged for about 46 h. At higher copolymer concentrations, a reversed hexagonal phase (H2) exists in the L121/water system. SAXS was used to investigate the internal structure of the dispersed L121-based particles containing 15 wt % L121. It was found that the internal structure transforms from H2 to an inverse micellar system (L2) as the temperature increases from 37 to 70 degrees C.     

Rheological studies of thermosensitive triblock copolymer hydrogels

Hydrogel formation by physical cross-linking is a developing area of research toward materials suitable for pharmaceutical and biomedical applications. Polymers exhibiting lower critical solution temperature (LCST) behavior in aqueous solution are used in this study to prepare hydrogels. Four triblock copolymers (ABA) with thermosensitive poly(N-(2-hydroxypropyl) methacrylamide lactate) A-blocks and a hydrophilic poly(ethylene glycol) B-block have been synthesized. The molecular weight of the hydrophilic PEG block was fixed at 10 kDa, whereas the molecular weight of the pHPMAm-lactate block was varied between 10 and 20 kDa. The rheological characteristics of these polymer hydrogels were studied as a function of temperature, concentration, and the length of the thermosensitive blocks. Gelation occurred rapidly upon increasing the temperature to 37 degrees C, which makes this system suitable as an injectable formulation. The gels became stronger with increasing temperature and concentration, and moreover they behaved as critical gels, which means that G' and G' ' follow power laws over the entire frequency range. Surprisingly, with increasing length of the thermosensitive blocks, weaker hydrogels were formed. This trend can be explained by the cross-link density of the physical network, which increases with decreasing length of the thermosensitive blocks.