Session W32: Polymers with Special Architectures: From Molecular Design to Physical Properties II

Molecular conformation of rigid cyclic and branched polymers in solution

Local conformational properties of rigid cyclic and branched polymers are not always the same as those for the corresponding linear chain unlike flexible polymers. We recently investigated rigid cyclic amylose derivatives and 3-arm star poly(quinoxaline-2,3-diyl) to determine their molecular conformation and intermolecular interactions in dilute solution. We found that local helical structure for the cyclic polymers are appreciably extended and therefore the main chain is more flexible than that for the corresponding linear chain. Regarding with this, the chiral separation columns made of the cyclic amylose derivative are substantially different separation behavior from those for the linear chain, indicating that the local conformation plays an important role for the molecular recognition ability. Both the linear and star chains have lyotropic liquid crystallinity. The phase diagram of isotropic and nematic phases are explained by a modified scaled particle theory when we assume some specific conformation in the nematic phase.   

Scattering from Melts of Combs and Bottlebrushes: Molecular Dynamics Simulations and Theoretical Study

We use coarse-grained molecular dynamics simulations to establish correlations between peak position in the static structure factor S(q) of combs and bottlebrushes in a melt and their architectural parameters such as degree of polymerization of the side chains n_sc and number of backbone bonds n_g between side chain grafting points. Analysis of the scattering function derived in the framework of the Random Phase Approximation and one obtained in simulations shows that in the comb regime with dilute side chains, n_g > n_sc, the wave number q* corresponding to the peak position in function S(q) scales with macromolecular parameters as q* ∝ (n_scn_g)^-1/4. A new scaling law q* ∝ n_sc^-3/8 emerges in the bottlebrush regime, where interactions between side chains stiffen the backbone at short length scales causing the effective backbone Kuhn length to increase with the degree of polymerization of the side chains as b_K ∝ n_sc^1/2. The established correlation between the peak position q* and bottlebrush Kuhn length provides foundation for a new method of obtaining the Kuhn length from scattering data. This approach does not require labeling of bottlebrush backbone and relays on natural contrast between side chains and backbones in the bottlebrush melts.
  

Impact of Architectural Asymmetry on Frank-Kasper Phase Formation in Block Polymer Melts

Block polymer self-assembly reflects a multiscale competition between enthalpic and entropic factors, producing periodic nanostructures with rich potential to tune the composition, geometry, and length scales. Until recently, the body-centered cubic lattice was believed to be the universal equilibrium state in the sphere-forming region. However, the recent discoveries of equilibrium Frank-Kasper σ and A15 phases in diblock polymer melts have revealed remarkable new complexity in these ostensibly simple materials. These complex low-symmetry phases have been experimentally identified in two types of neat block polymers: (1) linear AB diblocks and (2) linear ABA'C tetrablocks. To the best of our knowledge, non-linear architectures remain unexplored. In this talk, we will discuss the connections between molecular architecture and symmetry selection in block polymers. We have systematically tuned the architectural asymmetry in block polymers via living polymerization, from the linear-b-linear to linear-b-bottlebrush limits. Analysis by small-angle X-ray scattering provides insight into the impact of architectural asymmetry on block polymer self-assembly. Increasing the asymmetry between blocks skews the phase diagram and introduces new potential for block polymer design.   

Molecular architecture directs linear-bottlebrush-linear triblock copolymers self-assemble to soft, reprocessable elastomers

Linear-bottlebrush-linear (LBBL) triblock copolymers represent an emerging system for creating multifunctional nanostructures. Their self-assembly depends on molecular architecture but remains poorly explored. We synthesize polystyrene-block-bottlebrush polydimethylsiloxane-block-polystyrene triblock copolymers with controlled molecular architecture, and use them as a model system to study the self-assembly of LBBL polymers. Unlike the classical stiff rod-flexible linear block copolymers that are prone to form highly ordered nanostructures such as lamellae, at small weight fractions of the linear blocks, LBBL polymers self-assemble to a disordered sphere phase regardless of the bottlebrush stiffness. Microscopically, the characteristic lengths of the self-assembled nanostructures increase with the stiffness of bottlebrush block by a power law, which is captured by a scaling analysis. Macroscopically, the formed nanostructures are ultrasoft, reprocessable elastomers with shear moduli of about 1 kPa, two orders of magnitude lower than that of conventional polydimethylsiloxane elastomers. Our results provide insights on exploiting the self-assembly of LBBL polymers to create soft functional nanostructures.   

Structure of bottlebrush polymers end-grafted to a planar surface

Polymer brush is a hybrid material composed of a solid substrate coated with end-grafted polymers. We conducted coarse-grained molecular dynamics simulations and scaling theory of the equilibrium structure of planar brushes formed by bottlebrush polymers. Bottlebrushes are branched macromolecules consisting of densely spaced linear side chains grafted along a central (linear) backbone. We elucidate the relationship between bottlebrush architecture, surface coverage σ and polymer brush thickness H. We study the impact of three length scales on the brush height H: D₀ , the cross-section radius of bottlebrushes determined by the degree of polymerization of side chains N_sc , R₀ the (overall) size of bottlebrushes controlled by the degree of polymerization of backbone N_bb and d the distance between nearest-neighbor tethering sites. The latter quantity provides a measure of molecular coverage σ of a substrate defined as the number of bottlebrush polymers per unit surface area σ ∝ 1/d² . Our theoretical analysis identifies three conformational regimes for the height H, which gradually establish upon increasing substrate coverage and stem from interplay between relevant length scales: d, D₀ and R₀.   

Self-assembly of Bottlebrush Amphiphilic Polymers Near/On Surfaces: Coarse-grained Molecular Dynamics Simulation Study

Nanostructured materials for a wide range of applications such as electronics, optics, and sensing are engineered by the self-assembly of block polymers on surfaces. Significant number of past experimental, theoretical, and computational studies have tackled the self-assembly of linear block polymers, both in bulk and on/near surfaces. Relatively fewer studies have been focused on self-assembly of non-linear architectures (e.g., bottlebrush, star, comb) near/on surfaces. In this talk, we will present our recent work using coarse-grained molecular dynamics simulations to elucidate how branched polymer architecture affects the assembly in solutions of amphiphilic block copolymers near/on surfaces. We establish the role of side chain length versus backbone length, side chain grafting density on the backbone of amphiphilic block polymers as well as surface attraction strength on the chain conformations and the shapes, sizes and patterns of domains assembled on attractive surfaces.   

Effects of Ionic Groups on Dynamics of Linear-Star Polymer Blends: A Molecular Dynamics Simulation Study

Tethering ionic groups to polymers influence their phase behavior and dynamics, and in turn affect their blending with other macromolecules. Here, using MD simulations we probe the effects of ionic groups on the dynamics of linear-star polymer blends, where both the ionic groups and the topology affect the behavior. The polymer chains are modeled by the beads-spring model and the ionic groups are incorporated in the form of stickers (0-5%) on either the linear or star polymers or on both. The fraction f_star of star polymer is varied from 0 to 1. We find that for non-charged blends the short linear and star polymer chains are miscible. Introducing ionic groups on either the linear or the star polymers results in ionic clustering and phase separation. We find that the mobility of the linear ionizable melt is slower than the non-ionic polymers. In contrast, stickers increase the mobility of star polymer melts. In the presence of small amounts of stickers, the mobility of the chains is not affected by chain architecture.   

Complete photonic band gaps with nonfrustrated ABC bottlebrush copolymers

Block polymers are a promising platform for photonic materials, yet progress has been limited due to the scarcity of suitable morphologies with complete photonic band gaps and the large domain sizes necessary to manipulate visible light. Here we show that nonfrustrated ABC bottlebrush copolymers can surmount both of these limitations and can be used to form materials with complete photonic band gaps. To achieve this, we have developed a computational tool that couples self-consistent field theory (SCFT) simulations to Maxwell's equations, thereby permitting a direct link between molecular design, self-assembly and the resulting photonic band structure. Using this approach, we calculate the phase diagram of ABC bottlebrush copolymers, and show that complete photonic band gaps can open in the alternating gyroid and alternating diamond phases for modest dielectric contrast between the A, B, and C components. We show that the gap width increases with segregation strength, and depends strongly of the volume fractions of the A and C networks. Notably, the maximum band gap for the alternating gyroid corresponds to a region of the bottlebrush phase diagram where SCFT predicts this phase to be stable, suggesting that these photonic materials could be realized experimentally.   

Polydispersity Stabilized Complex Morphologies in Poly(styrene-b-methyl methacrylate) Diblock Copolymers

Block copolymers have the unique property to self-assemble into ordered microstructures with well-defined periodicity. Among the various attainable microdomain morphologies, complex bicontinuous morphologies are highly desirable due to their inherent structural connectivity. While such complex structures are difficult to obtain in monodisperse block copolymers, they may be stabilized by increasing block dispersity. In this study, we synthesized a series of poly(styrene-b-methyl methacrylate) (PS-PMMA) diblocks where dispersity of both blocks were systematically varied. The domain morphologies of the resulting diblocks were examined by combination of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). At fixed volume fraction of the PS block, dispersity dictated phase transition was observed. In particular, in the case where dispersity of both blocks were high, the polymers were found to exhibit perforated lamellae (PL) and disordered bicontinuous (BIC) morphologies. These results showed that dispersity control can be used to widen the accessibility window for complex morphologies in block copolymers.   

Molecular Design of Post-functionalizing Block Copolymers for Versatile Morphologies

One of the challenges in block copolymer study is to produce bicontinuous structures with various types of polymers with high reproducibility. Conventionally, two methods are known: a method of synthesizing a polymer having a target composition ratio by precise polymerization, and a polymer blend in which fine adjustment of the composition ratio is performed while accurately blending one polymer. However, there are some problems such as lack of reproducibility, limited polymers that can be used, and macrophase separation.
In this study, we demonstrated a new approach for creating bicontinuous microphase-separated structures using the post-functionalization of side-chain type block copolymers. The molecular design of the block copolymers having a vinyl group or glycidyl group in the side chain group was carried out, and the composition ratio was precisely controlled based on the high-yield reactions in the post-functionalization. It was found that the bicontinuous gyroid structure can be obtained with good reproducibility by precisely controlling the amount of side chains introduced.   

Poly(Styrene-block-Glyicdyl Methacrylate-block-Methyl Methacrylate) as a Versatile Platform for Exploring the Phase Diagram of Linear Triblock Copolymers

Block copolymers have been of great interest because of their intrinsic ability to self-assemble into myriad ordered structures with characteristic dimensions on the order of nanometers. In particular, diblock copolymers have been the primary focus of most research because of their simple architecture leading to a comprehensive understanding of their phase diagram. In contrast, adding a third block greatly increases the number of parameters affecting the self-assembly behavior which in turn greatly increases the number of phases accessible.
We have designed a new linear triblock consisting of poly(styrene-b-glycidyl methacrylate-b-methyl methacrylate) as a new platform for exploring the phase diagram of linear triblock copolymers. The reactive glycidyl methacrylate block can be functionalized with small molecules to modify the Flory-Huggins χ parameter between the middle block and the outer blocks. Thus, we can observe the effect of the changing χ values directly since they are all derived from the same batch.   

Helical sense of wrapped belts in star terpolymers via slice and view scanning electron microscopy

We study the self-assembly behavior of well-defined 3-miktoarm star terpolymers consisting of polyisoprene (PI), polystyrene (PS), and poly(2-vinylpyridine) (P2VP), (PI)₂-b-PS-b-P2VP. By adjusting the volume fractions of each block (40 vol % for PI, 27 vol % for PS and 33 vol % for P2VP), a unique structure was formed, in which hexagonally packed P2VP cylinders are wrapped by helically twisting PS belts in a matrix of PI. This structure was initially characterized by small angle X-ray scattering of bulk samples and bright field transmission electron microscopy (BF-TEM) of thin microtomed sections. By using I₂ to selectively stain P2VP and OsO₄ for PI, we can distinguish all 3 types of microdomains. In order to study the helical sense and coherent packing between adjacent cylinders, slice and view scanning electron microscopy (SVSEM) tomography is employed. The 3D tomograms of large regions provide information on the detailed nature and packing of the helical PS belts.
  

Molecular dynamics simulations and neutron scattering provide an atomistic level understanding of the self-assembly of poloxamines

By integrating molecular dynamics simulations and neutron scattering, we have studied different poloxamines (Tetronics), a family of star-shaped block copolymers that show promise as controlled release drug delivery agents. Their amphiphilicity and the basicity of their central diamine unit control their pH-sensitive self-assembly into supramolecular complexes.

We have studied Tetronic 904 (T904), a medium-sized poloxamine, and Tetronic 304 (T304), a smaller analogue, by all-atom molecular dynamics simulations and neutron scattering. While it is well-known that T904 readily forms micelles over a range of temperatures and concentrations, T304 has previously been found to have a more limited ability to self-assemble. Together, our simulations and experiments provide a detailed microscopic picture of the structure, dynamics and hydration of the self-assembled structures formed by both T904 and T304, and for the first time have uncovered the mechanisms that typically hinder T304 self-assembly.