Self-Assembly of Block Copolymers in External Fields
Alexander Böker (01/2002-01/2002)
Support: Axel H. E. Müller
The influence of external fields on the microdomain structure of block copolymers has been studied. Both surface fields and electric fields have been considered.
The first part describes the synthesis and characterization of ABC triblock copolymers aiming towards the generation of tailor-made substances for the controlled patterning of surfaces on nanometer scale. It is demonstrated that thin and ultrathin films of polystyrene-b-poly(2‑vinyl-pyridine)‑b-poly(methyl methacrylate) and polystyrene-b-poly(2-hydroxyethyl methacrylate)-b-poly(methyl methacrylate) block copolymers on silicon wafers reveal regular surface patterns with worm‑, stripe- or island‑like morphologies. The characteristic spacings can be controlled via the molecular weight of the different blocks of the respective copolymers and the film thickness.
Thin films (~ 20 nm thickness) prepared by dip-coating from a polymer solution were found to exhibit a phase-separated worm‑like surface morphology that presumably only consists of PS and PMMA microdomains with a characteristic lateral length scale similar to the bulk period L0. The generation of such a striped surface pattern can be explained by complete coverage of the silicon oxide surface by PHEMA or P2VP, resulting in a thin film structure that consists of a homogeneous layer of the middle block adsorbed at the substrate covered with a laterally microphase-separated surface layer of PS and PMMA microdomains. The proposed model for this morphology is in agreement with recent self-consistent field calculations.
In the case of the ultrathin films (thickness < 7 nm), our results demonstrate that adsorption of a block copolymer as an ultrathin film leads to a periodic surface domain structure (stripes), where both polar blocks (B and C) adsorb to the surface. Due to significant stretching of the adsorbed blocks the spacings between the domains are large for the rather low molecular weight block copolymers. The lateral dimensions correlate well with the molecular dimensions of the A and B/C blocks according to recently derived scaling laws.
In the second part of this thesis external electric fields are used to create macroscopically oriented bulk samples. In order to circumvent limitations associated with the application of external fields to melts of high molecular weight block copolymers and multiblock copolymers of complex architecture, a new solvent-based procedure is introduced, i.e. the block copolymer microdomains are aligned by application of an electric field (E ~ 1 ‑ 2 kV/mm) during solvent casting of bulk samples.
In order to elucidate the dominating parameters and governing mechanisms, the microdomain orientation kinetics of concentrated block copolymer solutions exposed to a DC electric field is investigated by time-resolved synchrotron small-angle X-ray scattering (SAXS) at the ID02A beamline at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. As a first model system, a lamellar polystyrene-b-polyisoprene block copolymer dissolved in toluene is used. 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, electric field strength, and system size.
Furthermore, two mechanisms governing the electric field alignment of a lamellar block copolymer from concentrated solutions are identified. It is shown that depending on the segregation power (c µ fP, c µ 1/T) a single mechanism dominates the orientation process, i.e. in a weakly segregated system (low concentration or high temperature) the migration of boundaries prevails, whereas a stronger phase separated system (high concentration or low temperature) predominantly exhibits rotation of grains.
In addition, the orientation kinetics slows down with increasing polymer concentration, which can be correlated to the respective solution viscosity and the mechanism of orientation. Moreover, the influence of the electric field strength on the orientation kinetics is determined, including a threshold value below which no electric field induced orientation could be achieved on the time scale of the experiment. The time constants of the fastest processes were in the range of 0.5 sec, reaching a final orientation described by order parameters of up to P2 = -0.35. Finally, the variation of temperature yields control of the governing mechanisms at a fixed polymer concentration.
In additional studies, the dielectric contrast of the block copolymer components was varied systematically (PS-b-PI, PS-b-PMMA, PS-b-PtBMA, PS-b-PHEMA-b-PMMA, PS-b-P2VP). It is found that a high dielectric contrast leads to faster alignment kinetics (e.g. the time constants of the fastest processes for a PS-b-P2VP diblock system in THF are in the range of 0.3 sec) and reduces the threshold field strength (around 200 V/mm for PS-b-P2VP).
Furthermore, it could be shown that the interplay between degree of phase-separation, solution viscosity and dielectric contrast is crucial to decide if a given polymer/solvent system can be used for electric field-induced microdomain alignment. For example, it was found that PS-b-PtBMA shows electric field-induced orientation of the microdomains while PS-b-PMMA does not. This can be explained by the larger interaction parameter cST compared to cSM leading to a phase-separated solution at lower viscosities. In a similar way, the introduction of a high dielectric constant middle block (PHEMA) into a PS-b-PMMA, which additionally enhances phase separation, is shown to be the key to creating a well-performing methacrylate-based block copolymer system for electric field induced alignment from solution.
Finally, we could show that the even more complex lamellar and core-shell cylindrical PS-b-P2VP-b-PtBMA high molecular weight triblock copolymer systems could be oriented by virtue of an electric field from solution.
In summary, it was demonstrated that electric field alignment of block copolymer domains from solution is a powerful tool to generate highly anisotropic bulk block copolymer samples. The large variety of parameters which we can control allows us to further improve the preparation of macroscopically aligned melt samples via solvent casting in the presence of an electric field.