Depletion-induced surface alignment of asymmetric diblock copolymer in selective solvents

Phase separation of asymmetric diblock copolymer near surfaces in selective solvents is theoretically investigated by using the real-space version of self-consistent field theory (SCFT). Several morphologies are predicted and the phase diagram is constructed by varying the distance between two parallel hard surfaces (or the film thickness) W and the block copolymer concentration f(P). Morphologies of the diblock copolymer in dilute solution are found to change significantly with different film thicknesses. In confined systems, stable morphologies found in the bulk solution become unstable due to the loss of polymer conformation entropy. The vesicle phase region contracts when the repulsive interaction between the blocks is strong (strong segregation regime). The mixture of vesicles, rodlike and spherelike micelles and the mixture of vesicles and sphere-like micelles disappear in contrast to the weakly segregating regime. The walls strongly affect the phase separation of block copolymer in selective solvent, and the depletion layer near the surface contributes much to the micelle formation of the block copolymer. Interestingly, the self-assembled morphologies stay near the walls with the distance on the order of the radius of gyration of the block copolymer. The oscillation of the polymer distribution near the walls allows the surface phase separation to be observed due to the strong repulsion between the blocks A and B.     

Surface-induced morphologies in thin films of a rod-coil diblock copolymer

A block copolymer containing a rodlike block is studied for its adsorption and formation of nanostructured thin films on the substrate surface. The block copolymer is poly(styrene-b-3-triethoxysilylpropylisocyanate) (PS-b-PIC) of which the PIC chain consists of repeating amide units with triethoxysilyl side groups. As the copolymer chains are adsorbed onto silica surfaces, the PIC blocks pack laterally on the plane in a smectic manner, and the PS chains segregate along the ordered PIC chains, resulting in stripe patterns. The width of the stripes formed on the silica surface appeared to be much larger that on the carbon surface. This was accounted for by the bilayered smectic packing of the rod blocks that is induced by rod-surface attractive interaction. The periodicity of the stripe pattern on the carbon surface indicates that interdigitated packing is preferred by the copolymers on the hydrophobic surface in a manner similar to those in the bulk state of rod-coils. Excess rod-coils on the bilayered smectic layer resulted in a terraced morphology due to large difference in the periodicity between the bilayered smectic layer at the substrate surface and the interdigitated smectic layer in the bulk.     

Crowding effect induced phase transition of amphiphilic diblock copolymer in solution

The crowding agent induced phase transition of amphiphilic block copolymers in solution was explicitly considered. The influence of the size and the volume fraction of the crowding agent on the phase separation of amphiphilic diblock copolymers is investigated by using self-consistent field theory (SCFT) method. The concentration of the disorder to order transition of the block copolymer decreases when the size of the crowding agent is larger than that of the solvent. The higher volume fraction of the crowding agent will induce the transition of the block copolymer from disorder to order state at a lower concentration. The relation between the size and the volume fraction of the crowding agent is elucidated. When the size of the crowding agent is larger, its volume fraction of the disorder to order transition of the block copolymer will be lower. The conformation of the crowding agent considered as a polymer chain is also studied and compared.     

Simulated annealing study of asymmetric diblock copolymer thin films

We report a simulated annealing study of the morphology of asymmetric diblock copolymer thin films confined between two homogeneous and identical surfaces. We have focused on copolymers that form a gyroidal morphology in the bulk. The morphological dependence of the confined films on the film thickness and the surface-polymer interaction has been systematically investigated. From the simulations it is found that much richer morphologies can form for the gyroid-forming asymmetric diblock copolymer thin films, in contrast to the lamella-forming symmetric and cylinder-forming asymmetric diblock copolymer films. Multiple morphological transitions induced by changing the film thickness and polymer-surface interactions are observed.     

Surface-induced pretransitional order in the isotropic phase near the isotropic-nematic phase transition

We have studied the formation of a nematic layer in the isotropic phase of members of the homologous series of alkylcyanobiphenyl liquid crystals (nCB, n = 5-12) near a polyamide-coated glass surface. From the temperature dependence near the isotropic-nematic transition of the standard ellipsometric quantity δ, which is directly related to the residual birefringence, the wetting behaviour of the nematic layer was investigated using a high precision rotating analyser ellipsometer. In contrast to the results of Chen et al., who observed a wetting transition for nCB-DMOAP-glass systems, our results indicate that for the nCB-polyamide-glass configurations the wetting is always partial.     

Aggregation behavior of cyclic rod-coil diblock copolymers in selective solvents

The aggregation behavior of cyclic rod-coil (RC) diblock copolymers in dilute solutions is investigated through dissipative particle dynamics simulation. By varying the rod length and coil length, cyclic RC copolymers in selective solvents exhibit various morphologies, including spherical micelle, vesicle, bilayer disc, and ribbon bundle structure. Compared with the equivalent linear RC copolymer, only spherical micelle and barrel bundle phase are observed. Rod length is the major factor that controls the liquid-crystalline behavior of RC copolymer systems, while the coil length has a secondary effect on the aggregate morphology. The size of rod bundle varies with the coil length, especially for the end-toend ribbon bundle and side-by-side barrel bundle, which are assembled by cyclic and linear RC copolymer solutions. This finding indicates that the ribbon bundle or nanofiber-like structure in cyclic RC copolymers can be obtained by controlling the rod length and coil length, and thus the optical and electrical properties of RC copolymer would be further controlled and optimized. Results illustrate that cyclization of a linear RC copolymer induces remarkable differences in the rod arrangement and aggregation behavior, thereby indicating the competition between interfacial energy, rod orientational entropy, coil stretching entropy, and packing constraints.     

The directed self-assembly of amphiphilic diblock copolymers in selective solvents

The directed self-assembly of diblock copolymers in solvents is studied systematically using a simulated annealing method. Effects of the shape, scale, and adsorption capacity of the induced surface on the morphology of the aggregates are examined. A variety of morphologies are predicted. By increasing the scale of induced surface, the micellar shape transforms from cylinder to sheet with a tail and finally to thin sheet without tail. The shape of induced surface determines the sheet’s shape, such as rounded and square. Configurations of hydrophobic blocks and interfacial energies are investigated by calculating the mean square end-to-end distances and the contact numbers between hydrophobic monomer and other species, respectively.     

Hydrogen bond mediated supramolecular micellization of diblock copolymer mixture in common solvents

We report a new approach toward preparing self-assembled hydrogen-bonded complexes having vesicle and patched spherical structures from two species of block copolymers in nonselective solvents. Two diblock copolymers, poly(styrene-b-vinyl phenol) (PS-b-PVPh) and poly(methyl methacrylate-b-4-vinylpyridine) (PMMA-b-P4VP), were synthesized through anionic polymerization. The assembly of vesicles from the intermolecular complex formed after mixing PS-b-PVPH with PMMA-b-P4VP in THF was driven by strong hydrogen bonding between the complementary binding sites on the PVPH and P4VP blocks. In contrast, well-defined patched spherical micelles formed after blending PS-b-PVPh with PMMA-b-P4VP in DMF: the weaker hydrogen bonds formed between the PVPh and P4VP blocks in DMF, relative to those in THF, resulted in the formation of spherical micelles having compartmentalized coronas consisting of PS and PMMA blocks.     

Monte Carlo phase diagram for diblock copolymer melts

A partial phase diagram is constructed for diblock copolymer melts using lattice-based Monte Carlo simulations. This is done by locating the order-disorder transition (ODT) with the aid of a recently proposed order parameter and identifying the ordered phase over a wide range of copolymer compositions (0.2相似文献   

Atomic force microscopy investigation of asymmetric diblock copolymer morphologies in thin films

Microphase separation and the resulting morphology of asymmetric diblock copolymers of poly(ε-caprolactone) (PCL) in thin films have been investigated by atomic force microscopy. Copolymers consisted of a short block of PCL (M_n∼2500-4500 g/mole) and a longer second block of poly(methyl methacrylate) (PMMA), poly(styrene) (PS) or poly(cyclohexene oxide) (PCHO). Tendency for microphase separation above the glass transition temperature of the second block (PMMA, PS or PCHO) resulted in a pitted morphology on the surface of the thin films. This tendency was strongest for PMMA and weakest for PCHO. The presence of up to 54% PMMA homopolymer in PCL-PMMA block copolymer did not prevent the formation of such pitted morphology on the surface. The effect of the chemical structure of the second block and the possible orientations of the block copolymer molecules in thin films are discussed.     

Anomalous transition in aqueous solutions of a thermoresponsive amphiphilic diblock copolymer

The influence of shear flow on aggregation and disaggregation in aqueous solutions of the thermoresponsive methoxy-poly(ethylene glycol)-block-poly(N-isopropylacrylamide) (MPEG53-b-PNIPAAM113) copolymer that exhibits a lower critical solution temperature was investigated with the aid of turbidity, shear viscosity, and rheo small angle light scattering (rheo-SALS) methods. The turbidity results at quiescent conditions revealed a novel transition peak in the turbidity curve at intermediate temperatures, which reflects the delicate interplay between temperature-induced aggregation and shrinking of the species. A similar anomalous transition peak (located at the same temperature) was observed in the steady shear viscosity measurements at intermediate temperatures, and the amplitude of the peak was reduced with increasing shear rate as a consequence of breakup of interaggregate chains. At low temperatures (low sticking probability), enhanced shear rate generated interpolymer aggregates; whereas in the high-temperature domain (high sticking probability) association structures were broken up as the shear rate was increased. The rheo-SALS experiments disclosed growth of aggregates at low temperatures and destruction of association complexes at high temperatures. An increase of the cloud point temperature with rising shear rate is reported, which is interpreted as being a disruption of clusters under the influence of shear stresses.     

A theory of swollen hollow micelles of diblock copolymers in selective solvents

Micellization behavior of diblock copolymers in selective solvent near the critical micelle temperature (c.m.t.) is theoretically studied focusing our attention to that the core must be swollen with the solvent near c.m.t. Supposing a micellar solution of core–corona type spherical micelles with swollen hollow cores, we calculate the association number distribution and the micelle structure at equilibrium in terms of controlling parameters, i.e. intrinsic interfacial tensions (γ_eff0 and γ_AS0) for core/corona and core/solvent interfaces, and core-segment/solvent interaction parameter χ_AS. Infinitely-large micelle region (ILM-Region), where the micelle size is divergent, is found at smaller values of γ_AS0 and χ_AS other than micelle and unimer regions. In the micelle region near ILM-Region, the micelle size and the degree of swelling become extremely large as ILM-Region is approached, while the micelles become very compact far away from ILM-Region. By investigating the micellar behavior with increasing the association strength on a particular trace in the χ_AS−γ_eff0−γ_AS0 space, it is demonstrated that large swollen hollow micelles are easily formed near c.m.t., and then sharply change to be more compact micelles with decreasing solvent quality to core blocks. This is exactly similar to the so-called anomalous micellization experimentally observed near c.m.t.     

Effects of confinement on the order-disorder transition of diblock copolymer melts

The effects of confinement on the order-disorder transition of diblock copolymer melts are studied theoretically. Confinements are realized by restricting diblock copolymers in finite spaces with different geometries (slabs, cylinders, and spheres). Within the random phase approximation, the correlation functions are calculated using the eigenvalues and eigenfunctions of the Laplacian operator inverted Delta(2) in the appropriate geometries. This leads to a size-dependent scattering function, and the minimum of the inverse scattering function determines the spinodal point of the homogeneous phase. For diblock copolymers confined in a slab or in a cylindrical nanopore, the spinodal point of the homogeneous phase (chiN)(s) is found to be independent of the confinement. On the other hand, for diblock copolymers confined in a spherical nanopore, (chiN)(s) depends on the confinement and it oscillates as a function of the radius of the sphere. Further understanding of the finite-size effects is provided by examining the fluctuation modes using the Landau-Brazovskii model.     

Tilt grain boundaries in a diblock copolymer ordered nanocomposite lamellar phase

A hybrid self-consistent field theory/density functional theory method is applied to predict tilt (kink) grain boundary structures between lamellar domains of a symmetric diblock copolymer with added spherical nanoparticles. Structures consistent with experimental observations are found and theoretical evidence is provided in support of a hypothesis regarding the positioning of nanoparticles. Some particle distributions are predicted for situations not yet examined by experiment.     

Isotropic-nematic phase transition in athermal solutions of rod-coil diblock copolymers

We present a hybrid method to investigate the isotropic-nematic (I-N) transition in athermal solutions of rod-coil copolymers. This method incorporates the scaled-particle theory for semiflexible chains with two-chain Monte Carlo simulation for the osmotic second virial coefficient and for the angle-dependent excluded volume. We compare the theoretical prediction with Monte Carlo simulations for fused rod-coil copolymers and find good agreement for both the equation of state and the orientational order parameter. The theory is also used to examine the effects of the bond length, the chain length, and the chain composition on orientational ordering in athermal solutions of rod-coil block copolymers. It predicts I-N transition in rod-coil copolymers with fixed rod length but a variable flexible tail in good agreement with experiments.     

Structural evolution of cylindrical‐phase diblock copolymer thin films

We have measured the time evolution of the self‐assembly process in perpendicular‐oriented cylindrical‐phase diblock copolymer thin films using statistical analysis of high‐resolution scanning electron microscope (SEM) images. Within minutes of annealing above the polymer glass‐transition temperature, microphase separation between polymer blocks results in formation of uniform nanometer‐scale domains whose relative position is initially largely uncorrelated. On further annealing, the cylindrical polymer domains organize into a two‐dimensional hexagonal lattice whose characteristic grain size increases slowly with time (～t^1/4). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1970–1975, 2004     

Simulation of the gyroid phase in off-lattice models of pure diblock copolymer melts

Particle-based molecular simulations of pure diblock copolymer (DBC) systems were performed in continuum space via dissipative particle dynamics and Monte Carlo methods for a bead-spring chain model. This model consisted of chains of soft repulsive particles often used with dissipative particle dynamics. The gyroid phase was successfully simulated in DBC melts at selected conditions provided that the simulation box size was commensurate with the gyroid lattice spacing. Simulations were concentrated at conditions where the gyroid phase is expected to be stable which allowed us to outline approximate phase boundaries. When more than one phase was observed by varying simulation box size, thermodynamic stability was discerned by comparing the Helmholtz free energy of the competing phases. For this purpose, chemical potentials were efficiently simulated via an expanded ensemble that gradually inserts/deletes a target chain to/from the system. These simulations employed a novel combination of Bennett's [J. Comput. Phys. 22, 245 (1976)] acceptance-ratio method to estimate free-energy differences and a recently proposed method to get biasing weights that maximize the number of times that the target chain is regrown. The analysis of the gyroid nodes revealed clear evidence of packing frustration in the form of an (entropically) unfavorably overstretching of chains, a phenomenon that has been suggested to provide the structural basis for the limited region of stability of the gyroid phase in the DBC phase diagram. Finally, the G phase and nodal chain stretching were also found in simulations with a completely different DBC particle-based model.     

Chain diffusion in the melt of an asymmetric diblock copolymer in the disordered and ordered state

 The self-diffusion in a polystyrene-b-polyisoprene diblock copolymer with a strongly asymmetric composition was investigated with dependence on temperature by pulsed field gradient (PFG) NMR. The diblock shows with decreasing temperature a disorder-to-order-transition at T _ODT=393 K from a micellar liquid-like to a bcc ordered state which was recently measured by SAXS [M. Schwab and B. Stühn, Phys. Rev. Lett. (1996) 76: 924]. Two diffusivities were observed, one of the free diblock chains and one of the diblock chains fixed in micelles. The volume fraction of free chains decreases with decreasing temperature. The diffusivity of the free chains must be related to chain stretching. The experiments show that within the time of the NMR experiment (300 ms) there is no exchange between the diblock chains in the free state and those fixed in the core of the micelles. Received: 25 March 1997 Accepted: 25 April 1997