Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J19: Focus Session: Theory and Simulations of Macromolecules VI - Block Copolymers |
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Sponsoring Units: DPOLY Chair: Sean Paradiso, University of California, Santa Barbara Room: 404 |
Tuesday, March 4, 2014 2:30PM - 3:06PM |
J19.00001: DILLON MEDAL SYMPOSIUM BREAK Invited Speaker: |
Tuesday, March 4, 2014 3:06PM - 3:42PM |
J19.00002: Simulations of self-assembly in polymeric systems Invited Speaker: Alfredo Alexander-Katz |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J19.00003: Nucleation of ordered microphases in fluctuation-induced first-order phase transitions Michael Carilli, Glenn Fredrickson, Kris Delaney The Landau-Brazovskii model is a field-based Hamiltonian describing a variety of systems which exhibit ordered microphases defined by characteristic periodicity and symmetries (e.g., lamellar, hexagonal, body-centered cubic). Interestingly, this model can undergo a fluctuation-induced first-order phase transition: for the symmetric model, the disorder-to-lamellar transition is second-order at the mean-field level but takes on first-order character when fluctuations are added. A disordered phase supercooled to within the resulting metastable region will then transition to the stable lamellar phase via nucleation. We demonstrate it is possible to discover the critical nucleus' size and geometry by applying the numerical string method\footnote{Weinan E et al, J. Chem. Phys. \textbf{126}, 164103 (2007)} to a renormalized Landau-Brazovskii Hamiltonian which incorporates the effects of fluctuations. We find good agreement with predicted nucleus size and shape obtained by analytic approximation. Hohenberg and Swift\footnote{P. C. Hohenberg and J. B. Swift, Phys. Rev. E \textbf{52}, 1828 (1995)} predict that for this transition, certain defect structures in the critical nucleus might act to lower the nucleation free energy barrier; we present a search for these structures. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J19.00004: Universal phenomenology of the order-disorder transition in symmetric diblock copolymers Pavani Medapuram, Jens Glase, David Morse The order-disorder transition (ODT) in melts of symmetric diblock copolymers has been precisely identified by a free-energy based technique in several simulation models over a wide range of experimentally relevant values of the invariant degree of polymerization $\overline{N}$. To compare results of different models, we determine the parameter dependence of $\chi$ for each model from a fit of disordered phase data for the structure function $S(q)$ to the renormalized one-loop theory. The value of $\chi N$ at the transition obtained using this estimate of $\chi$ is found to exhibit a universal dependence on $\overline{N}$. Simulation results for the both the ODT and strength of order appear to slowly converge above a crossover value of $\overline{N}$ of order 10,000. This value corresponds to a crossover between strongly segregated low-N regime, in which the center of each domain is nearly pure even at the ODT, and the beginning of the more weakly segregated regime for which the FH theory was originally designed. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J19.00005: Molecular Interaction Control in Diblock Copolymer Blends and Multiblock Copolymers with Opposite Phase Behaviors Junhan Cho Here we show how to control molecular interactions via mixing AB and AC diblock copolymers, where one copolymer exhibits upper order-disorder transition and the other does lower disorder-order transition. Linear ABC triblock copolymers possessing both barotropic and baroplastic pairs are also taken into account. A recently developed random-phase approximation (RPA) theory and the self-consistent field theory (SCFT) for general compressible mixtures are used to analyze stability criteria and morphologies for the given systems. It is demonstrated that the copolymer systems can yield a variety of phase behaviors in their temperature and pressure dependence upon proper mixing conditions and compositions, which is caused by the delicate force fields generated in the systems. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J19.00006: Computational study of solvated block-copolymer microphases Wei Li, Kris Delaney, Glenn Fredrickson Using field-theoretic simulations, we study the equilibrium self-assembly of solvated block copolymer microphases with different chain architectures and block selectivities. Initially within the mean-field approximation (self-consistent field theory), we employ unit-cell calculations to determine the phase diagram by comparing the free energy of candidate phases. We find good agreement with prior computational and experimental reports in the literature. We subsequently move beyond the mean-field approximation using Complex Langevin sampling to investigate the effect of fluctuations on relative phase stabilities. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J19.00007: Structure and Phase Behavior of Tapered Diblock Copolymers from Self-Consistent Field Theory Jonathan R. Brown, Scott W. Sides, Lisa M. Hall Tapered block copolymers are like AB diblock copolymers with a ``tapered block'' inserted between the A and B endblocks. This tapered block sequence is random with its average composition changing linearly from pure A to pure B (or B to A for inverse-tapered systems). Depending on the fraction of A monomers and the quantity $\chi N$, the blocks microphase separate to form various ordered morphologies. Increasing $N$ (such as to improve mechanical properties) simultaneously affects the microphase separated state. Tapering adds another adjustable parameter, taper length, that can be used to control the microphase separated state. We map the phase diagrams of model tapered and inverse tapered polymers using self-consistent field theory (SCFT). The ordered phases shift to higher $\chi N$ for tapered systems, and the shift increases as the taper length increases. Inverse tapers shift the phase diagram to even higher $\chi N$. Direct tapered systems' phase diagrams are like those of diblocks, but with a larger gyroid region. For large inverse tapered systems, the polymer appears like an ABAB tetrablock, and it folds across the interface or bridges between domains. In this case some of the ordered structures show reversed A and B domains where the majority phase is relatively impure. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J19.00008: Microphase Separation and Interfacial Behavior of Model Tapered Diblock Copolymers Lisa M. Hall, Youngmi Seo, Jonathan R. Brown We use a combination of theoretical and simulation methods to understand the microphase separated structure and dynamics of model copolymers. Tapered diblock copolymers, containing pure A and B blocks separated by a region with an A/B composition gradient, are of particular interest: the length of the tapered region can be adjusted to modify the system's phase and interfacial behavior. Experimentally, tapered diblocks have been found to form the bicontinuous gyroid phase, which is of interest for transport applications and can be difficult to access using typical diblocks. Phase diagrams from self-consistent field theory (SCFT) do show a larger gyroid region for certain tapered systems versus diblocks. To further understand the detailed microphase separated structure, we employ fluids density functional theory (fDFT) and molecular dynamics (MD) simulations together. These both capture monomer scale packing effects and are implemented for very similar models so that the fDFT results can be used as a guide to ensure the appropriate equilibrium state is formed in the MD simulations. Density profiles from SCFT, fDFT, and MD are in qualitative and sometimes quantitative agreement; tapers widen the interfacial region and large tapers decrease the maximum purity of the microphases. [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J19.00009: Block Copolymer Compatibilizers for Morphological Control on the Equilibrium Structural Characteristics of Polymer/Fullerene Blends Dylan Kipp, Venkat Ganesan We develop a single chain in mean field model for the equilibrium morphologies of solar cells based on the homopolymer/block copolymer/fullerene blend. Using our model, we study the ability of the block copolymer compatibilizer to provide morphological control on the domain and interfacial characteristics of the equilibrium structures. We focus our efforts on the case of a semiflexible homopolymer and a semiflexible/flexible diblock copolymer as these are emblematic of the kinds of molecules used in photovoltaic applications. Our results reveal a novel progression of morphologies in transitioning the ternary composition space, the rigidity of the semiflexible chains, and the flexible block ratio of the diblock copolymer. To elucidate the morphologies, we first present a series of ternary phase diagrams and then use a simple morphological characterization scheme to evaluate the domain sizes and interfacial quantities characterizing our equilibrium structures. [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J19.00010: Self-Consistent Field Theoretical Study on the Crossed Cylinder Morphology of Block Copolymers Jaeup Kim, So Jung Park, Yeongyoon Kim Cylindrical morphologies are commonly observed for various block copolymer systems. Both theory and experiment confirm that the stable bulk morphology is hexagonally packed parallel cylinders. However, it does not necessarily mean that other types of arrangements are impossible. In this work, we explore a few possible strategies to promote the formation of crossed cylinder geometry. Our self-consistent field theoretical calculations along with experiments of our collaborators demonstrate that such crossed cylinder morphology is obtainable if the system is well designed and prepared. One strategy is to use surface interaction energy. If a block copolymer thin film resides on a substrate with stripe shaped chemical patterns which prefer one block, cylinders parallel to the pattern is energetically favorable on the stripe. However, on the neutral region, thin film confinement promotes cylinders vertical to the substrate. Another strategy is to use locally inclined substrates. In general, cylinders vertical to the substrate have difficulty in fitting themselves on a non-flat substrate and they prefer to lie down on the substrate. These strategies are applicable for the formation of other non-traditional crossed geometries such as the crossed lamellar morphology. [Preview Abstract] |
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