SCFT Study of Diblock Copolymer Melts in Electric Fields: Selective Stabilization of Orthorhombic Fddd Network Phase

Nonequilibrium Processes in Polymer Membrane Formation: Theory and Experiment

Porous polymer and copolymer membranes are useful for ultrafiltration of functional macromolecules, colloids, and water purification. In particular, block copolymer membranes offer a bottom-up approach to form isoporous membranes. To optimize permeability, selectivity, longevity, and cost, and to rationally design fabrication processes, direct insights into the spatiotemporal structure evolution are necessary. Because of a multitude of nonequilibrium processes in polymer membrane formation, theoretical predictions via continuum models and particle simulations remain a challenge. We compiled experimental observations and theoretical approaches for homo- and block copolymer membranes prepared by nonsolvent-induced phase separation and highlight the interplay of multiple nonequilibrium processes─evaporation, solvent-nonsolvent exchange, diffusion, hydrodynamic flow, viscoelasticity, macro- and microphase separation, and dynamic arrest─that dictates the complex structure of the membrane on different scales.

Vertical Cylinder-to-Lamella Transition in Thin Block Copolymer Films Induced by In-Plane Electric Field

Morphological transition between hexagonal and lamellar patterns in thin polystyrene-block-poly(4-vinyl pyridine) films simultaneously exposed to a strong in-plane electric field and saturated solvent vapor is studied with atomic force and scanning electron microscopy. In these conditions, standing cylinders made of 4-vinyl pyridine blocks arrange into threads up to tens of microns long along the field direction and then partially merge into standing lamellas. In the course of rearrangement, the copolymer remains strongly segregated, with the minor component domains keeping connectivity between the film surfaces. The ordering tendency becomes more pronounced if the cylinders are doped with Au nanorods, which can increase their dielectric permittivity. Non-selective chloroform vapor works particularly well, though it causes partial etching of the indium tin oxide cathode. On the contrary, 1,4-dioxane vapor selective to polystyrene matrix does not allow for any morphological changes.

Thickness-Dependent Photo-Aligned Thin-Film Morphologies of a Block Copolymer Containing an Azobenzene-Based Liquid Crystalline Polymer and a Poly(ionic liquid)

Photo-induced alignment of the thin-film morphologies of azobenzene-containing block copolymers (BCPs) is an effective method to obtain a uniaxial pattern of nanocylinders. Although film thickness is an important factor affecting the self-assembly of BCP thin films, the influence of film thickness on the photo-induced alignment of BCP thin-film morphology has never been systematically studied. Herein, we report the thickness-dependent photo-aligned film morphologies of the BCP containing an azobenzene-based liquid crystalline polymer and a poly(ionic liquid) (PIL), with a perfect uniaxial pattern of PIL nanocylinders. For films aligned with the unpolarized light (UPL), the out-of-plane PIL nanocylinders can be obtained in the film with a thickness of only 1L0 (∼30 nm, where L0 is the layer spacing of the hexagonally packed cylinder array), which is far lower than the thickness (more than 4L0) of the thermally annealed film needed to obtain the same morphology. This change is attributed to the orientation effect of UPL on azobenzene mesogens that suppresses the excluded volume effect. For the films aligned with linearly polarized light (LPL), to take advantage of the excluded volume effect to obtain the planar orientation of azobenzene mesogens, the thickness should be controlled to be no more than 3L0 to achieve an in-plane uniaxial alignment of PIL nanocylinders. The above relationship between the morphology and thickness of photo-aligned film eliminates the obstacles encountered in preparing films with well-ordered photo-aligned morphologies.

Field-theoretic simulations beyond δ-interactions: Overcoming the inverse potential problem in auxiliary field models

Modern field-theoretic simulations of complex fluids and polymers are constructed around a particle-to-field transformation that brings an inverse potential u^-1 in the model equations. This has restricted the application of the framework to systems characterized by relatively simple pairwise interatomic interactions; for example, excluded volume effects are treated through the use of δ-function interactions. In this study, we first review available nonbonded pair interactions in field-theoretic models and propose a classification. Then, we outline the inverse potential problem and present an alternative approach on the basis of a saddle-point approximation, enabling the use of a richer set of pair interaction functions. We test our approach by using as an example the Morse potential, which finds extensive applications in particle-based simulations, and we calibrate u^-1 with results from a molecular dynamics simulation. The u^-1 thus obtained is consistent with the field-theoretic model equations, and when used in stand-alone self-consistent field simulations, it produces the correct fluid structure starting from a random initial state of the density field.

Reorientation of Polymers in an Applied Electric Field for Electrochemical Sensors

This mini review investigates the relationship and interactions of polymers under an applied electric field (AEF) for sensor applications. Understanding how and why polymers are reoriented and manipulated by under an AEF is essential for future growth in polymer-based electrochemical sensors. Examples of polymers that can be manipulated in an AEF for sensor applications are provided. Current methods of monitoring polymer reorientation will be described, but new techniques are needed characterize polymer response to various AEF stimuli. The unique and reproducible stimuli response of polymers elicited by an AEF has significant potential for growth in the sensing community.

Jumping In and Out of the Phase Diagram Using Electric Fields: Time Scale for Remixing of Polystyrene/Poly(vinyl methyl ether) Blends

We demonstrate it is possible to repeatedly jump polystyrene (PS)/poly(vinyl methyl ether) (PVME) blends from the one-phase to two-phase region by simply turning on and off an electric field at a fixed temperature near the phase boundary. This builds on our previous work that established electric fields enhance the miscibility of PS/PVME blends by shifting the phase separation temperature T_S(E) of 50/50 blends up by 13.5 ± 1.4 K when field strengths of E = 1.7 × 10⁷ V/m are applied (J. Chem. Phys. 2014, 141, 134908). Monitoring the early stages of phase separation and remixing by fluorescence, we measure the remixing time scale τ(T) with and without electric fields, finding τ(T) is unchanged by the presence of the field and well fit by a Vogel-Fulcher-Tammann expression. These observations are consistent with a mobility-limited process several degrees from the phase boundary where electric fields have shifted the miscibility transition.

Nanoscale Resolution of Electric-field Induced Motion in Ionic Diblock Copolymer Thin Films

Understanding the responses of ionic block copolymers to applied electric fields is crucial when targeting applications in areas such as energy storage, microelectronics, and transducers. This work shows that the identity of counterions in ionic diblock copolymers substantially affects their responses to electric fields, demonstrating the importance of ionic species for materials design. In situ neutron reflectometry measurements revealed that thin films containing imidazolium based cationic diblock copolymers, tetrafluoroborate counteranions led to film contraction under applied electric fields, while bromide counteranions drove expansion under similar field strengths. Coarse-grained molecular dynamics simulations were used to develop a fundamental understanding of these responses, uncovering a nonmonotonic trend in thickness change as a function of field strength. This behavior was attributed to elastic responses of microphase separated diblock copolymer chains resulting from variations in interfacial tension of polymer-polymer interfaces due to the redistribution of counteranions in the presence of electric fields.