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.     

Phase behavior of a blend of polymer-tethered nanoparticles with diblock copolymers

Using the self-consistent field theory (SCFT), we investigate the phase behavior of a mixture of diblock copolymers and nanoparticles with monodisperse polymer chains tethered to their surfaces. We assume the size of the nanoparticles to be much smaller than that of the attached polymer chains and therefore model the particles with their grafted polymer "shell" as star polymers. The polymer chains attached to the particles are of the same species as one of the blocks of the symmetric diblock copolymer. Of primary interest is how to tune the shell of the particle by changing both the length and number of tethered polymers in order to achieve higher loading of nanoparticles within an ordered structure without macrophase separation occurring. We find that the phase behavior of the system is very sensitive to the size of the particle including its tethered shell. The region of microphase separation is increased upon decreasing the star polymer size, which may be achieved by shortening and/or removing tethered polymer chains. To explore the possible structures in these systems we employ SCFT simulations that provide insight into the arrangement of the different species in these complex composites.     

Micellar aggregation in blends of linear and cyclic poly(styrene-b-isoprene) diblock copolymers

The morphology of micelles formed from blends of linear and cyclic poly(styrene-b-isoprene) (PS-b-PI) block copolymers has been investigated in solution using dynamic light scattering (DLS) and in thin solid deposits by atomic force microscopy (AFM) and transmission electron microscopy under cryogenic conditions (cryo-TEM). Micelles of the pure cyclic PS(290)-b-PI(110) copolymers are wormlike cylindrical objects built by unidirectional aggregation of 33 nm wide sunflower micelles, while the linear block copolymer having the same volume fraction and molar mass forms spherical micelles 40 nm in diameter. The DLS, AFM, and cryo-TEM results consistently show that the addition of the linear copolymer (even for amounts as low as 5% w/w) to the cyclic copolymer rather favors the formation of spherical micelles at the expense of the cylindrical aggregates. Those results clearly show that the linear block copolymer chains can be used to stabilize the thermodynamically unstable elementary sunflower micelle. The thermal stability of the micelles (from the pure copolymers and from the blends) has been examined in solid deposits with in situ AFM measurements. Coalescence starts at about 70 degrees C, and the surface roughness shows a two-step decrease toward a fully homogeneous and flat structure.     

Orientation fluctuations in diblock copolymers

Depolarized light scattering and dielectric relaxation spectroscopy reveal pertinent composition fluctuations effects on the orientation dynamics in diblock copolymers near the ordering transition (ODT). The main evidence stems from the broadening of the block relaxation function and collective chain orientation in the disordered state near ODT as well as a slow relaxation process below ODT.     

Supramolecular self-assembly of conjugated diblock copolymers

This paper describes the synthesis and characterization of a novel series of copolymers with different lengths of oligo(phenylene vinylene) (OPV) as the rod block, and poly(propylene oxide) as the coil block. Detailed characterization by means of transmission electron microscopy (TEM), atomic force microscopy (AFM), and small-angle neutron scattering (SANS) revealed the strong tendency of these copolymers to self-assemble into cylindrical micelles in solution and as-casted films on a nanometer scale. These micelles have a cylindrical OPV core surrounded by a poly(propylene glycol) (PPG) corona and readily align with each other to form parallel packed structures when mica is used as the substrate. A packing model has been proposed for these cylindrical micelles.     

Soft confinement-induced morphologies of diblock copolymers

The self-assembly of diblock copolymers under soft confinement is studied systematically using a simulated annealing method applied to a lattice model of polymers. The soft confinement is realized by the formation of polymer droplets in a poor solvent environment. Multiple sequences of soft confinement-induced copolymer aggregates with different shapes and self-assembled internal morphologies are predicted as functions of solvent-polymer interaction and the monomer concentration. It is discovered that the self-assembled internal morphology of the aggregates is largely controlled by a competition between the bulk morphology of the copolymer and the solvent-polymer interaction, and the shape of the aggregates can be non-spherical when the internal morphology is anisotropic and the solvent-polymer interaction is weak. These results demonstrate that droplets of diblock copolymers formed in poor solvents can be used as a model system to study the self-assembly of copolymers under soft confinement.     

pH-responsive nanoaggregation of diblock phosphorylcholine copolymers

We have characterized three diblock copolymers bearing zwitterionic phosphorylcholine and weak tertiary amine groups, namely, poly[((2-(methacryloyloxy)ethyl)phosphorylcholine)30- block-(2-(dimethylamino)ethyl methacrylate)60] (denoted as MPC30-DMA60, Mn=18,000), poly[((2-(methacryloyloxy)ethyl)phosphorylcholine)30- block-(2-(diethylamino)ethyl methacrylate)60) (denoted as MPC30-DEA60, Mn=20,000), and poly[((2-(methacryloyloxy)ethyl)phosphorylcholine)30- block-(2-(diisopropylamino)ethyl methacrylate)60) (denoted as MPC30-DPA60, Mn=21,000), by studying their surface tension and solution aggregation through a combined approach of surface tension measurement, dynamic light scattering, and small-angle neutron scattering. Our results show that larger tertiary amine substituents lead to an increasing tendency to form micellar aggregates, which is consistent with the increasing copolymer hydrophobicity. Thus, MPC30-DMA60 did not aggregate under the experimental conditions studied. The free chains exist in the form of thin cylinders, whose length decreases with copolymer concentration and solution temperature but increases with solution pH. The diameters of the MPC30-DMA60 cylinders remained almost constant at around 30 A under all the conditions studied. At the lower copolymer concentration of 0.5 wt %, the cylindrical lengths correspond to the persistence length of the copolymer backbone and are close to its full length, indicating a rather high rigidity. Further data analysis showed that, at the two higher concentrations of 2 and 4 wt %, the phosphorylcholine and amine blocks associate, inducing bending of the copolymer backbone. One backbone kink was required to satisfy all the constraints, including the dry volume of the copolymer. MPC30-DEA60 showed a similar trend of pH- and concentration-dependent conformational responses for the free copolymer, but in addition micellar aggregation occurred at pH 9. In contrast, MPC30-DPA60 exhibited significantly reduced solubility associated with strong aggregation, which is consistent with it being the most hydrophobic copolymer in the series.     

Second virial coefficients of diblock copolymers

A perturbation theory for the second virial coefficient of A-B type diblock copolymers in solution, which can account for any branching present in the blocks, is presented. Results have been obtained for the special case of infinitely long linear blocks and it is found that the second virial coefficient is a function of five parameters, three characterizing the various interactions and two characterizing the unperturbed sizes of the two blocks, in addition to the molecular weight of the polymer chain.     

Nucleo-copolymers: oligonucleotide-based amphiphilic diblock copolymers

For the first time, poly(butadiene) has been covalently linked to an oligonucleotide sequence and the resulting nucleo-copolymer exhibits amphiphilic properties in dilute aqueous solution, self-assembling into nanometer-sized vesicular structures.     

Probe diffusion in homogeneous diblock copolymers

Forced Rayleigh scattering was used to investigate the diffusion of a photoreactive dye molecule in two homogeneous poly(styrene-b-isoprene) (SI) diblock copolymers with overall molecular weights of approximately 2000. Although diffusion rates were intermediate to TTI transport in homopolymer polystyrene (PS) and polyisoprene (PI), system dynamics appear to be largely dictated in each case by the PI block. The size of the polymer jumping unit, on the other hand, is evaluated from a free-volume analysis of the data, and is found to be governed predominantly by the PS component of the copolymer. The mechanism for tracer diffusion in low-molecular-weight block copolymers appears analogous to transport in a high molecular weight SI diblock copolymer (M_n = 13,600) that has been solvated sufficiently in toluene to be microstructurally disordered. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1739–1746, 1998     

Phase behavior of diblock copolymers; pressure and temperature dependence studied by small-angle neutron scattering

The structure factor of a poly(ethylene-propylene)poly(dimethylsiloxane) diblock copolymer has been measured by SANS as a function of temperature and pressure. In contradiction to the random phase approximation the conformational compressibility exhibits a pronounced maximum at the order-disorder phase transition. The phase boundary shows an unusual shape: with increasing pressure it first decreases and then increases. Its origin is an increase in, respectively, the entropic and the enthalpic part of the Flory-Huggins interaction parameter. The Ginzburg parameter describing the limit of the mean-field approximation is not influenced by pressure.     

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.     

Orientational ordering of nanorods in diblock copolymers

ABSTRACT

Orientational ordering of rod-like nanoparticles in the lamellae phase of diblock copolymers has been considered theoretically using the model of a nanoparticle with two interaction centres. It has been shown that strongly anisotropic nanoparticles order spontaneously in the boundary region between the blocks where the orientational order is induced by the interface and by the interaction with monomer units in different blocks. The nematic order parameter possesses opposite signs in adjacent blocks which means that the nanorods are aligned parallel or perpendicular to the boundary between the blocks on different sides of their interface. Concentration and nematic order parameter profiles have been calculated numerically for different values of the nanoparticle length and compared with the results of recent computer simulations and with the results of the previous molecular theory based on nanoparticles of spherical shape.     

Interfacial properties of semifluorinated alkane diblock copolymers

The liquid-vapor interfacial properties of semifluorinated linear alkane diblock copolymers of the form F(3)C(CF(2))(n-1)(CH(2))(m-1)CH(3) are studied by fully atomistic molecular dynamics simulations. The chemical composition and the conformation of the molecules at the interface are identified and correlated with the interfacial energies. A modified form of the Optimized Parameter for Liquid Simulation All-Atom (OPLS-AA) force field of Jorgensen and co-workers [J. Am. Chem. Soc. 106, 6638 (1984); 118, 11225 (1996); J. Phys. Chem. A 105, 4118 (2001)], which includes specific dihedral terms for H-F blocks-and corrections to the H-F nonbonded interaction, is used together with a new version of the exp-6 force field developed in this work. Both force fields yield good agreement with the available experimental liquid density and surface tension data as well as each other over significant temperature ranges and for a variety of chain lengths and compositions. The interfacial regions of semifluorinated alkanes are found to be rich in fluorinated groups compared to hydrogenated groups, an effect that decreases with increasing temperature but is independent of the fractional length of the fluorinated segments. The proliferation of fluorine at the surface substantially lowers the surface tension of the diblock copolymers, yielding values near those of perfluorinated alkanes and distinct from those of protonated alkanes of the same chain length. With decreasing temperatures within the liquid state, chains are found to preferentially align perpendicular to the interface, as previously seen.     

Morphologies of microphase-separated conformationally asymmetric diblock copolymers

The equilibrium morphological behavior of a series of conformationally asymmetric linear diblock copolymers is characterized via small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The linear diblock molecules of polyisoprene and poly(t-butylmethacrylate) (PtBMA) are prepared anionically over a range of PtBMA volume fractions 0.17 to 0.85. Solution light-scattering experiments are performed on PtBMA homopolymer at theta conditions, and the results were compared with PI data in the literature in order to characterize the degree of conformational asymmetry between the respective blocks. This conformational asymmetry is quantified by an ε of 0.75. The experimental results are compared with morphological behavior calculated utilizing self-consistent mean field theory for a diblock system with ε = 0.75. At middle to high PtBMA volume fractions, ϕ_PtBMA > 0.30, the experimental morphological behavior agrees well with the calculated behavior; the microphase boundaries are slightly shifted to higher volume fractions of the PtBMA block due to its larger Kuhn length. At ϕ_PtBMA < 0.30, however, discrepancies are found in the volume fraction dependence of experimentally determined morphological behavior and that calculated theoretically. Interestingly, extremely well-ordered cylindrical microstructures were observed for PI cylinder domains embedded in PtBMA matrices; these samples were prepared by solvent casting with no treatment, such as shearing, to enhance long-range order. These well-ordered cylinder structures contrast with PtBMA cylinders in a PI matrix on the opposite side of the phase diagram that have very poor long-range order. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2629–2643, 1997     

Brownian dynamics simulations of associating diblock copolymers

A novel coarse-grained computational model for associating polymers is proposed that is based on a Gaussian "blob" representation of the polymer chains. The model allows a large number of model polymers to be simulated at moderate computational cost over a wide packing fraction range using the Brownian dynamics, BD, technique. The attraction of the hydrophobic part of the polymer to those on other molecules can lead to strong aggregation of the polymer molecules in real systems, and this is included in the model by an attractive potential felt by the Gaussian blobs to a common "nodal" point that represents the center of the micelle. Attention here is confined to model AB diblock copolymers in which the hydrophilic block, A, has a much higher mass than the hydrophobic moiety, B, which leads to relatively small aggregation numbers, Nagg, of approximately 8. The aggregation number at low packing fractions is found to increase with packing fraction, as observed in experiments, with a functional form that closely follows a simple theory derived here that is based on entropy-derived mean-field terms for the free-energy change associated with the incorporation of the polymer molecule into the micelle. The computational model exhibits an extremely low critical micelle concentration (cmc), and micelles with Nagg approximately 5 are observed at the lowest packing fractions, phi, simulated ( approximately 10-4), which is consistent with experiment. The long-time self-diffusion coefficient of the polymers (and hence micelles) decreases logarithmically with packing fraction, and the viscosity increased with concentration according to the Huggins equation. The spherical blob coarse graining results in the simulable time scales being longer than the Rouse time of the chain, and hence for the nonassociating polymers the intrinsic viscosity is an input parameter in the model. The introduction of association leads to the partial inclusion of the intrinsic viscosity in the simulation and has an effect on the computed Huggins coefficient, kH, which is found to be approximately 6 in those cases.     

Thin film order of symmetric diblock copolymers

The ordering in thin films of symmetric diblock copolymers of polystyrene and poly(methyl methacrylate) has been investigated by neutron reflectivity as a function of film thickness and temperature. The order-disorder transition in the thin films was found to lose its first order character in that the transition occurs in a continuous manner without the correlation length becoming infinite. In addition, a transition from a partially to fully ordered state was observed which was fully reversible. This transition depended in a power law manner on the film thickness and extrapolates to the bulk order-disorder transition temperature for thick films.     

Synthesis and characterization of conjugated diblock copolymers

This paper reports the synthesis and characterization of a new class of diblock copolymers (co-oligomers), rod-rod conjugated diblock copolymers. A general synthetic strategy is outlined, and the structures of the copolymers (oligothiophene-co-oligophenylenevinylene) are fully characterized. It was found that these copolymers exhibit efficient intramolecular energy transfer. An interesting self-assembly ability of these rod--rod copolymers was also revealed.     

Adsorption of diblock copolymers on stripe-patterned surfaces

We present the results of extensive Monte Carlo simulations of diblock copolymers adsorbed on stripe-patterned surfaces of various widths. We have found that the width of the stripe pattern is an important parameter which dictates favorable recognition on the surface. For certain stripe widths, the adsorption of diblock copolymers to striped surfaces exhibits two transitions. The process involves recognition of the surface pattern by the diblock copolymer which follows a two step process in which the first block getting adsorbed to the appropriate pattern on the surface, without any recognition of the surface pattern, followed by the adsorption of the second block, where a reorganization process happens. For small widths and also for higher widths, the chain behaves just like a homopolymer where the twofold adsorbing process changes to the typical homopolymer adsorption. We have also found that there exists an optimal width of the stripes, independent of the chain length, where the recognition on the surface pattern is most favored. The characteristic temperature of the adsorption of the second block with weaker interactions is found to be independent of the chain length at this optimal width, proving that only local rearrangements take place after the first step. Some of our results describing the thermodynamics compare very well with the recent semianalytical approach of Kriksin et al. [J. Chem. Phys. 122, 114703 (2005)] on multiblock copolymers on heterogeneous surfaces. We also present some interesting conformational properties of the copolymer chain near the stripe-patterned surface.