Session S52: Block Copolymer Thin Films I: Theory and Simulation

Self-assembly of Bottlebrush Block Polymers at Surfaces using Coarse-grained Molecular Dynamics Simulations

Self-assembly of block polymers on surfaces is used to engineer nanostructured materials for electronics, optics, sensing, and chemical separation applications. Past experimental, theoretical, and computational work has extensively focused on the self-assembly of block copolymers with linear architecture, and relatively fewer studies have established the polymer physics underlying self-assembly of non-linear architectures such as bottlebrushes. In this talk, I will present our coarse-grained molecular dynamics simulation study aimed at understanding structure (i.e., chain conformations, assembled morphologies) and thermodynamics (i.e., interactions between monomers vs. interactions with surface) of solutions of amphiphilic bottlebrush block polymers near surfaces. We establish the role of bottlebrush side chain length and side chain grafting density on the backbone on the behavior of amphiphilic bottlebrush block polymers near surfaces.   

Program the self-assembly of block copolymers for desired mesocrystals

The packing of particles in soft matter as a fundamental problem has been paid a lot of attension. As the enthalpic and entropic contributions to the free energy is comparable and could be readily tuned, the packing of soft particles leads to rich crystalline structures (i.e. mesocrystals). To understand the packing mechanism of various soft particles into different crystalline structures and thus to modulate the packing of soft particles for desired mesocrystals is an important problem to be solved. Block copolymer provides a perfect model for solving this problem due to two main reasons. The first reason is that the self-assembly of block copolymers can be programmed by tuning the molecular architectures and modern synthesis techniques enable the precise control of the architectures. The second one is that the self-assembly of block copolymers can be accurately predicted by the well-established self-consistent field theory (SCFT). In this talk, I will talk about programming the self-assembly of block copolymers for desired mesocrystals or even mesoscale quasicrystals based on SCFT calculations.   

Dynamics at the Interface of Structured Block Co-Polymer Thin Films with Polar Solvents: Molecular Dynamics Simulations Insights

Structured ionic co-polymers consist of multiple blocks whose dynamics facilitate their uses. Here, the dynamics at the interface of an ABCBA pentablock co-polymer with polar solvents is studied by molecular dynamics simulations. The ionic block C is a sulfonated polystyrene (sPS) with sulfonation fraction f=0.55, B is polyethylene-propylene (PEP), and A is poly(t-butyl styrene) (tBS). Simulations are performed using LAMMPS with the polymer and propanol modeled with OPLS-AA force field. The interface evolution with exposure time to propanol is followed and the results are compared with those for the polymer-water interface. The air interface of the neat polymer is dominated by the PEP block. With exposure to propanol, more ionic groups migrate to the interface, similar to the polymer-water interface. However, in contrast to water exposed films, the interface remains dominated by the PEP blocks. Remarkably, the interfacial width of the membrane-propanol system grows with time while the width of the membrane-water interface decreases. Further, water molecules associate predominantly with the ionic blocks and break the ionic clusters while propanol associates with both the ionic and non-ionic polymer segments.   

Microphase Separation in Dipolar Diblock Copolymer Melts

Understanding microphase separation in polar-non-polar diblock copolymer melts is a topic of great interest due to its fundamental and technological importance. In this talk, we will present our work related to modeling the effects of dipolar interactions in affecting microphase separation in linear diblock copolymer melts. Results obtained for freely rotating dipoles, applicable to high temperature disordered melts, will be presented. These results will be compared with field theory-based and coarse-grained molecular dynamics simulations for lamellar morphology. Advantages and disadvantages of the field theory-based simulations will be discussed and importance of correlations in orientations of dipoles will be highlighted.   

DPD simulation of multilayer self-assembly of block-copolymer

Self-assembly of block copolymers provides a powerful tool for patterning at small length scale. The chemical incompatibility of the two blocks, which are covalently linked together, leads to microphase separation upon annealing. Depending on the volume ratio of the two block, different morphologies, such as spheres, cylinders and lamellae structure can be obtained. Most researchers use self-consistent field theory (SCFT) as the simulation tool to predict self-assembled structure. Recently, a particle-based simulation method - dissipative particle dynamics (DPD) has been implemented to provide different insights into the self-assembly process of BCP. Here we use DPD to investigate the structural information propagation from layer to layer in a multilayer film. The self-assembly process of the first layer is directed by either graphoepitaxy or chemoepitaxy. After that, by depositing a second BCP layer on top, the structural information of the first layer will propagate and affect the structure of the second layer. By carefully designing the structure, we could achieve different bilayer structure such as dots-on-cylinders, parallel cylinders and nanomeshes.
  

Is Simple Cubic Spherical Phase Possible in Block Copolymers?

The self-assembly of amphiphilic macromolecules into various mesocrystals has attracted abiding interest. Although many interesting mesocrystals have been achieved, the simple cubic mesocrystal is rarely reported. Here we purposely design an AB-type multiblock copolymer composed of an AB diblock copolymer with A-/B-block tethered by an extra B-/A-block at the middle point to target the simple cubic spherical phase based on the self-consistent field theory. Specifically, two sophisticated mechanisms are realized synergistically in the copolymer, i.e. packing frustration release and stretched bridge block, leading to the formation of the simple cubic mesocrystal in a significant parameter space. Moreover, more other unusual phase behaviors are predicted.
  

Simulation studies reflecting the importance of kinetics on block copolymer self-assembly

The self-assembly in polymeric system is one of core principles to many of advanced nanotechnologies. Success of most of these applications utilizing the self-assembly relies on how well one can adjust, and switch the shape, size and arrangement direction of self-assembled structures. Hence, there have been active research efforts on understanding the underlying physical principles controlling the self-assembly, augmented by theoretical and numerical modeling. Complicated interactions and the wide range of length and time scales related with self-assembled structures make theoretical modeling very challenging. Moreover, many systems are in metastable states, thus kinetics, not just thermodynamics, plays a key role for the ability of a polymeric material to self-assemble into a desired state. In this work, we present our efforts on developing powerful simulation approaches capable to describe experimentally-observed microstructures, to predict new mesophases, as well as to provide the kinetic routes between various microphases in block copolymer films. Special efforts are placed on understanding the motion of defects in block copolymer thin films and their interactions, pursuing fully ordered lamellae for its successful application in nanolithography.   

Single chain in mean field (SCMF) simulation of flexible and semiflexible block copolymers.

Self-consistent field theory (SCFT) has been a popular tool for the study of equilibrium properties of block copolymer nanostructures. It is a mean field theory, and thus it has limitations in that fluctuation effects are ignored. One suggestion to incorporate fluctuations into the field theoretical calculation is the single chain in mean field (SCMF) simulation which performs explicit Monte Carlo simulation of polymer chains under quasi-instantaneously updated self-consistent field. In this research, we perform SCFT calculation and SCMF simulation of block copolymers in thin film morphology. For the symmetric block copolymers, the phase transition is suppressed when confined by two neutral walls, and the surface-perpendicular lamellar phase becomes slightly preferable to the surface-parallel one. We also perform SCMF simulation of semiflexible polymer chains by adopting angle dependent potential in both bead-spring and freely-jointed chain model, and our results reveal that stiffer chains exhibit higher tendency to self-assemble into ordered structures. For the confined system, it turns out that the surface-perpendicular lamellar phase becomes more stable as the stiffness of the chain increases.   

Effect of free surface and substrate topography on the self-assembly of block copolymer films

Theoretical and numerical studies have provided valuable insights in understanding and controlling the self-assembly of block copolymer systems, providing powerful guidelines to experimentalists. However, complicated geometric surface effects on the self-assembly are often ignored due to the limitation of previous simulation models to capture free surface effects, thus constraining systems to thin films with ad-hoc flat interfaces. In this work, we present our efforts on developing efficient simulation approaches to explore the self-assembly of complex polymeric systems with ability to develop, and interact, with non-flat interfaces. Effects of different substrate topographies and initial polymer volume shapes on the self-assembly kinetics are numerically investigated and compared with experimental data.   

Rational design of linear-dendritic block copolymer for overwhelming region of spherical phases

Various ordered structures can be formed by the self-assembly of block copolymer, while the formation of some sophisticated phases among them is usually driven by some sophisticated mechanisms. Several mechanisms have been established to expand the spherical region, thus stabilizing some new spherical phases, e.g. Frank-Kasper phases. However, the effect of these mechanisms is still limited. In order to widen the phase regions of these desired new spherical phases drastically, we start from an intuitive hypothesis, design a kind of linear-dendritic block copolymer and investigate its self-assembly behaviors with the self-consistent field theory (SCFT). Under the optimized control parameters, the spherical phase region is expanded to be overwhelmingly large in the phase diagram and at the same time the regions of Frank-Kasper phases are largely expanded. The most important conclusion is that the discrete spherical domains can be formed by the major component with volume fraction around 0.7. These results renew our insight into the intricate mechanisms of the self-assembly of block copolymer.   

Stability of Dodecagonal Quasicrystalline Tiling in ABC Star Terpolymers

In this work, the thermodynamic stability of cylindrical dodecagonal quasicrystalline (DDQC) phase in ABC star-shaped triblock terpolymers for both neat and blending systems is investigated. A systematic comparison of our theoretical results to those of experiments by Matsushita's group is performed using the self-consistent field theory (SCFT). Based on our previous work, SCFT coupled with the Stampfli self-similarity construction is adopted to accurately calculate the free energy of the periodic DDQC approximants and then a cluster model is used to predict the stability of aperiodic DDQC phase. We find that ideal tiling DDQC in these ABC star terpolymers is also metastable. Furthermore we show that the DDQC morphology observed experimentally is a kind of random tiling pattern as the mesoscopic coexistence of the (3,3,4,3,4) Archimedean tiling pattern and the 8/3 approximant. Accordingly, we conclude that the formation of random tiling DDQC structure may be possible in self-assembling block copolymer melts.   

Tiling Patterns Self-Assembled from Rod-Coil Copolymers with Hydrogen Bonds

Based on self-consistent field theory (SCFT), we demonstrate that X-shaped rod-coil molecules with hydrogen-bonding groups, can self-assemble into Archimedean tiling patterns. The rod blocks form the polygon edges with coil blocks filling in the inner spaces of polygons. The existence of hydrogen bonds further decreases the domain size. A new mechanism is proposed to guide the formation of Archimedean tiling patterns: the fabrication of tiling patterns is controlled by the relationship between the length to diameter ratio and volume fraction of rods in X-shaped supra-macromolecules. This study provides a concept of macromolecular tiling and suggests X-shaped rod–coil supra-macromolecules with hydrogen-bonding groups as an ideal platform for the fabrication of two-dimensional nanoscale patterns.   

Self-Assembly Behaviors of B1AB2CB3 MultiBlock Copolymer

The self-assembly behavior of linear B₁AB₂CB₃ multiblock copolymers is investigated using the self-consistent field theory (SCFT). It has been revealed that the relative lengths between the three B-blocks are able to tune the packing crystalline lattice of binary spherical or cylindrical phases. Here we focus on the impact of the molecular architecture on more phase behaviors in a wider parameter space. Our SCFT results indicate that the relative lengths of the three B-blocks have significant influences on not only spherical/cylindrical phases but also other phases such as the alternative gyroid and lamellar phases. In particular, the stability region of the gyroid phase varies drastically and non-monotonically, which is accompanied by the drastic change of other phase regions. Furthermore, our results reveal that the change of these phase regions is resulted in by different sophisticated mechanisms associated with the relative lengths of the three B-blocks, i.e. the effect of stretched bridging B2-block, the regulation of packing frustration of the B-blocks and the solubilization of short B-blocks.