Concentrated Block Copolymer Solution Research Articles

Large-Grained Cylindrical Block Copolymer Morphologies by One-Step Room-Temperature Casting.

We report a facile method of ordering block copolymer (BCP) morphologies in which the conventional two-step casting and annealing steps are replaced by a single-step process where microphase separation and grain coarsening are seamlessly integrated within the casting protocol. This is achieved by slowing down solvent evaporation during casting by introducing a nonvolatile solvent into the BCP casting solution that effectively prolongs the duration of the grain-growth phase. We demonstrate the utility of this solvent evaporation annealing (SEA) method by producing well-ordered large-molecular-weight BCP thin films in a total processing time shorter than 3 min without resorting to any extra laboratory equipment other than a basic casting device, i.e., spin- or blade-coater. By analyzing the morphologies of the quenched samples, we identify a relatively narrow range of polymer concentration in the wet film, just above the order–disorder concentration, to be critical for obtaining large-grained morphologies. This finding is corroborated by the analysis of the grain-growth kinetics of horizontally oriented cylindrical domains where relatively large growth exponents (1/2) are observed, indicative of a more rapid defect-annihilation mechanism in the concentrated BCP solution than in thermally annealed BCP melts. Furthermore, the analysis of temperature-resolved kinetics data allows us to calculate the Arrhenius activation energy of the grain coarsening in this one-step BCP ordering process.

Influence of Initial Order on the Microscopic Mechanism of Electric Field Induced Alignment of Block Copolymer Microdomains

We investigate the mechanism of microdomain orientation in concentrated block copolymer solutions exposed to a dc electric field by in situ synchrotron small-angle X-ray scattering (SAXS). As a model system, we use concentrated solutions of a lamellar polystyrene-b-polyisoprene block copolymer in toluene. We find that both the microscopic mechanism of reorientation and the kinetics of the process strongly depend on the initial degree of order in the system. In a highly ordered lamellar system with the lamellae being aligned perpendicular to the electric field vector, only nucleation and growth of domains is possible as a pathway to reorientation and the process proceeds rather slowly. In less ordered samples, grain rotation becomes possible as an alternative pathway, and the process proceeds considerably faster. The interpretation of our finding is strongly corroborated by dynamic self-consistent field simulations.

Order−Disorder Transition and Critical Micelle Temperature in Concentrated Block Copolymer Solutions

The phase behavior of symmetric styrene−isoprene (SI) diblock copolymers in selective solvents in the vicinity of the order−disorder transition (ODT) was investigated by small-angle neutron scattering (SANS). Particular emphasis is placed on the region just above the ODT, where a disordered phase of micelles is observed. To understand morphological changes in more detail, one of the blocks is deuterated, i.e., PS−dPI and dPS−PI, and the scattering length density (SLD) of the solvents used were identical to the SLD of the corona chains. Two approaches for the analysis of SANS data were taken: the generalized indirect Fourier transformation (GIFT) and direct model fitting with relevant form and structure factors. With increasing temperature, the micellar aggregation number decreases, and the core radius is roughly maintained by the increased solvent swelling of the core. The critical micelle temperature (cmt) is experimentally described in the present study by an abrupt decrease in the size, aggregation number, and volume fraction of micelles. The micelles were found to dissociate into free chains approximately 20−30 °C higher than the TODT, and the cmt is found to be quite close to the mean-field spinodal temperature TS. A mean-field temperature, TMF, is located by the crossover temperature from the linear behavior of Imax-1 vs T-1 plot, and the structure factor above the experimentally determined TMF is also found to be in good agreement with the Leibler−Landau-type mean-field theory. We examined two different ODTs: body-centered cubic lattices to disorder and hexagonal cylinders to disorder, with similar results in both cases.

Electric Field Induced Alignment of Concentrated Block Copolymer Solutions

We investigate the microdomain orientation kinetics of concentrated block copolymer solutions exposed to a dc electric field by time-resolved synchrotron small-angle X-ray scattering. As a model system, we use a lamellar polystyrene-b-polyisoprene block copolymer dissolved in toluene. Our results indicate two different microscopic mechanisms, i.e., nucleation and growth of domains and grain rotation. The former dominates close to the order−disorder transition, while the latter prevails under more strongly segregated conditions. This conclusion is corroborated by computer simulations based on dynamic density functional theory. The orientation kinetics follows a single-exponential behavior with characteristic time constants varying from a few seconds to some minutes depending on polymer concentration, temperature, and electric field strength. From the experimental results we deduce optimum conditions for the preparation of highly anisotropic bulk polymer samples via solvent casting in the presence of an electric field.

Ordering Kinetics and Alignment of Block Copolymer Lamellae under Shear Flow

The effect of large-amplitude oscillatory shear flow on a concentrated block copolymer solution with lamellar order was studied by in-situ small-angle neutron scattering. Microstructural changes were studied as a function of temperature, frequency of the oscillatory flow field, and thermal history prior to turning on the shear field. We find that the alignment path depends mainly on thermal history prior to turning on the shear field and is independent of frequency and temperature. At long times, the lamellae were aligned parallel to the shearing plates, regardless of frequency, temperature, and thermal history. We refer to this as the parallel orientation. Monotonic changes from the unaligned to the aligned state were found when the shear field was turned on after the sample was completely ordered. The alignment kinetics, in this case, occurs in two stages. The first stage consists of a rapid rotation of the grains so that the lamellar normals lie in the velocity gradient−vorticity plane. This is followed by a slower process wherein the lamellar normals get increasingly localized in the velocity gradient direction. We also studied ordering kinetics under shear, by turning on the shear field before significant ordering had taken place. In this case, the first stage of ordering resulted in the formation of lamellae aligned perpendicular to the shearing plates in addition to the parallel lamellae, regardless of temperature and frequency. Eventually the perpendicular lamellae were transformed to parallel lamellae via an undulation instability.

Undulations and Disorder in Block Copolymer Lamellae under Shear Flow

The effect of shear flow on a concentrated block copolymer solution with lamellar microstructure was studied by in-situ small angle neutron scattering. Microstructural changes were determined from scattering measurements in all three principal directions (flow, velocity gradient, and neutral directions). The lamellae were found to align along the velocity direction but exhibited two orientations: one parallel to the shearing surfaces and the other perpendicular to the shearing surfaces. The perfection of the perpendicular alignment was always significantly greater than that of the parallel alignment. In fact, our sample in the parallel alignment was, to a large extent, disordered (i.e., liquid-like). In some cases, we found transient, well-aligned, perpendicular lamellae that eventually degenerated to poorly ordered, parallel lamellae. In one case, a systematic increase in lamellar undulations was evident prior to disordering. These results are not in agreement with current theories, which only predict enhanced order in block copolymers under shear flow.

Dynamic Light Scattering from Dilute, Semidilute, and Concentrated Block Copolymer Solutions

The dynamic light scattering (DLS) properties of four symmetric polystyrene-polyisoprene diblock copolymers have been examined in the neutral good solvent toluene. Concentrations (c) ranged from the dilute, through the semidilute, and into the concentrated regimes. Pulsed-field-gradient NMR has been used to determine the translational diffusivities (D s ) of the same copolymers, at selected concentrations. The results are in substantial agreement with models developed by Benmouna and coworkers and by Semenov. Three modes are predicted : cooperative diffusion (Dc), corresponding to relaxation of fluctuations in polymer concentration ; an internal mode (Γ I ), reflecting relative motion of the two blocks within one molecule ; a heterogeneity mode (D H ), due to chain-to-chain fluctuations in composition and which relaxes by translational diffusion. In dilute solutions, a single, diffusive mode is seen and is attributed to a superposition of Dc and D H . In semidilute and concentrated solutions, Dc and D H are cleanly resolved. Dc shows the expected scaling with c and molecular weight (M), i.e., Dc∼ c 0.7 M 0 . Comparison with the NMR results confirms that the values of the diffusion coefficients D H and D s are very similar ; this result extends the previous work of Balsara and co-workers, who first noted the appearance of a block copolymer DLS mode resembling translational diffusion. Selected measurements in two other good solvents, THF and chloroform, establish that Dc and D H are independent of solvent refractive index (n s ), as expected. The relative amplitudes of the cooperative and heterogeneity modes also scale with c, M, and n s in agreement with theory. The predicted internal mode is generally difficult to resolve but is seen for the highest molecular weight sample examined. The concentration dependences of DH and D s are insensitive to the order-disorder transition, in agreement with previous measurements in solutions and melts. Preliminary measurements of D H with a novel Fourier transform heterodyne DLS apparatus were also in good agreement with the other techniques. Finally, the results are compared with previous DLS measurements of symmetric diblock copolymers in solution. It is proposed that the apparent discrepancies among the previous studies can be resolved, by appropriate assignment of the heterogeneity mode.

To the theory of superstructures in concentrated solutions of block copolymers

A concentrated solution of a two-block copolymer under conditions of phase separation is discussed. Besides chemical segregation (formation of domains of the minor component in the major component matrix), the segregation of matrix component blocks from various domains also occurs in such solution. A mean field theory of the equilibrium parameters of lamellar mesophases in the solution is developed, the dependence on the molecular mass of the copolymer, on its composition, and on the quality and quantity of solvent in the system. The segmental density profile and the distribution of free ends of component blocks in lamellar layers were determined.