Session P9: Architectural Design of Polymers I

Universality between Experiment and Simulation of a Diblock Copolymer Melt

The equivalent behavior among analogous block copolymer systems involving chemically distinct molecules or mathematically different models has long hinted at an underlying universality, but only recently has it been rigorously demonstrated by matching results from different simulations. The profound implication of universality is that simple coarse-grained models can be calibrated so as to provide quantitatively accurate predictions to experiment. Here, we provide the first compelling demonstration of this by simulating a polyisoprene-polylactide diblock copolymer melt using a previously calibrated lattice model. The simulation successfully predicts the peak in the disordered-state structure function, the position of the order-disorder transition and the latent heat of the transition in excellent quantitative agreement with experiment. This could mark a new era of precision in the field of block copolymer research.   

Dynamics in Model Microphase Separated Tapered Copolymers

Microphase separating copolymer systems are of interest in transport and separation applications because one can take advantage of the different properties of two different monomer types. For typical AB diblock copolymers, the main control parameters that set the structure and properties are the monomer-monomer interactions and the fraction of A monomers; one can further control both structure and dynamics by modifying the sequence of A and B along the chain. Here we consider how adding a tapered midblock between pure A and B blocks impacts the dynamics of the chains and of added monomer-sized penetrants that are selectively solvated by the A microphase. Specifically, we perform coarse-grained molecular dynamics simulations of linear, fully flexible, symmetric polymers, and the tapered region has a linear gradient in composition from pure A to pure B (or from pure B to pure A for an inverse taper); we also consider systems with a random midblock for comparison. As the length of the normal taper increases, the diffusion constant of both polymers and penetrants parallel to the lamellae increases. In contrast, the diffusion constant of inverse tapered chains is non-monotonic as a function of taper length. We show how different types of polymer conformations contribute to this trend.   

Stabilizing Various Bicontinuous Morphologies via Polydispersity of Diblock Copolymers

Diblock copolymers are macromolecules composed of two chemically distinct homopolymers covalently bound end-to-end. The ability to self-assembly into a wide variety of ordered periodic structures, as means of minimizing the free energy, is their most well-studied property. There are many factors affecting the observed equilibrium morphology, one of which is polydispersity. The phase behaviour of polydispersed diblock copolymers is more rich, and diverse when compared to their monodispersed counterpart. The rich behaviour of polydispersed diblock copolymers provides an opportunity to engineer novel morphologies which are not available in monodispersed systems. Using the self-consistent field theory (SCFT), we explore the possibility of exploiting polydispersity of diblock copolymers in binary mixtures to stabilize the various bicontinuous phases, such as the double-diamond morphology. Specifically, solutions of the SCFT equations corresponding to different bicontinuous phases are obtained numerically for binary mixtures of diblock copolymers. The relative stability of the different ordered phases is examined by comparing their free energy. From the study, we determine optimal sets of parameters that stabilize the double-diamond or other exotic morphologies.   

Elastomer genome: Reverse tissue engineering.

Soft elastic materials enable the creation of implants, substrates, and haptic robotic digits with mechanical properties matching those of biological tissues. Currently, polymer gels are the only viable class of synthetic materials with a Young's modulus below 100 kPa. However, the liquid fraction in the gels causes practical troubles including phase separation and solvent leakage on deformation. Herein, we have created bottlebrush and comb-like networks that are superelastic ($\lambda =$1-12) and ultrasoft (G$=$10$^{\mathrm{2}}$ -- 10$^{\mathrm{5}}$ Pa), even in the absence of solvent [1]. The brush-like architecture causes an increase in the diameter of individual polymer molecules, but unlike typical filaments, the molecules remain flexible. This enables a significant decrease in the entanglement density, which reduces the limit of stiffness in dry polymer materials by 1000 times and has opened up new applications not available to stiffer materials or materials with liquid fractions [2]. The comb-like architecture offers three independently controlled parameters -- side-chain length, grafting density, and crosslink density - that allow for combinatorial variations of elastomer mechanical properties impossible for conventional linear chain elastomers, e.g. simultaneously increasing rigidity and elasticity. Based on this materials design platform, we have prepared elastomers that closely match the mechanical behaviour of biological tissue. Furthermore, this architecture affords many chain-ends that are amendable for chemical modifications and enhance molecular mobility, which directly affects vital physical properties ranging from glass transition and crystallization temperatures to adhesion and permeability. [1] Daniel, W.F.M.; Burdy\'{n}ska,J.; Vatankhah-Varnoosfaderani, M.V.; Matyjaszewski, K.; Paturej, J.; Rubinstein, M.; Dobrynin, A.D.; Sheiko, S.S. Nature Materials 2016, 15, 183-189. [2]Vatankhah-Varnosfaderani, M.; Daniel, W.F.M.; Zhushma, A.P.; Li, Q., Morgan, B.J.; Matyjaszewski, K.; Armstrong, D.P.; Spontak, R.J.; Dobrynin, A.V.; Sheiko, S.S. Advanced Materials 2016, DOI: 10.1002/adma.201604209   

Theoretical study of the self-assembly of Janus Bottlebrush Polymers from A-Branch-B Diblock Macromonomers

The self-assembly of block copolymers BCP has provided an impressive control over the nanoscale structure of soft matter. While the main focus of the research in the field has been directed towards simple linear diblocks, the development of advanced polymer architecture provided improved performance and access to new structures. In particular, bottlebrush BCPs (BBCPs) have interesting characteristics due to their dense functionality, high molecular weight, low levels of entanglement, and tendency to efficiently undergo rapid bulk phase separation. In this work, we are interested in theoretically studying the self-assembly of Janus-type ``A-\textit{branch}-B'' BBCPs where A and B blocks can phase separate with the bottlebrush polymer backbone serving as the interface between the two blocks. Hence, the polymer backbone adds an extra constraint on the equilibrium spacing between neighboring linear diblock chains. In this regard, the segment length of the backbone separating the AB junctions has a direct effect of the observed domain spacing and effective segregation strength of the AB blocks. We employ self-consistent field theoretic SCFT simulations to capture the effect of volume fraction of different constituents and construct a phase diagram of the accessible morphologies of these BBCPs.   

Molecular Mobility in Phase Segregated Bottlebrush Block Copolymer Melts

We investigate the linear viscoelastic behavior of poly(styrene)-block-poly(ethylene oxide) (PS-b-PEO) brush block copolymer (BBCP) materials over a range of vol. fractions and with side chain lengths below the entanglement molecular weights. The high chain mobility of the brush architecture results in rapid micro-phase segregation of the brush copolymer segments, which occurs during thermal annealing at mild temperatures. Master curves of the dynamic moduli were obtained by time-temperature superposition. The reduced degree of chain entanglements leads to a unique liquid-like rheology similar to that of bottlebrush homopolymers, even in the phase segregated state. We also explore the alignment of phase segregated domains at exceptionally low strain amplitudes ($\gamma \quad =$ 0.01) and mild processing temperatures using small angle X-ray scattering (SAXS). Domain orientation occurred readily at strains within the linear viscoelastic regime without noticeable effect on the moduli. This interplay of high molecular mobility and rapid phase segregation that are exhibited simultaneously in BBCPs is in contrast to the behavior of conventional linear block copolymer (LBCP) analogs and opens up new possibilities for processing BBCP materials for a wide range of nanotechnology applications.   

Molecular Dynamics Simulations of Fluorinated Bottlebrush Copolymers in Thin Films

We have performed multi-scale molecular dynamics (MD) simulations of bottlebrush copolymers with fluorinated side-chains. In thin films, coarse-grained MD simulations reveal that side-chains are preferentially located at interfaces and are slightly oriented perpendicular to the interface. At the molecular level, atomistic MD simulations show that fluorine atoms in the bottlebrush are preferentially located at interfaces, as well. Both simulation results indicate enhancement of fluorinated moieties at interfaces providing a probable explanation to the increase in hydrophobicity of the spin-coated thin films from these bottlebrush copolymers.   

Influence of side chain length and volume fraction on the morphology of bottlebrush block copolymers

A systematic study was conducted to investigate the morphology transitions that occur in polystyrene-block-poly (ethylene oxide) (PS-b-PEO) bottlebrush block copolymers (BBCP) upon varying PEO volume fraction (fPEO) from 16 {\%} to 81 {\%}. A series of PS-b-PEO BBCPs with different PEO side chain lengths were prepared using ring opening metathesis polymerization of PEO-norbornene (PEO-NB) (Mn \textasciitilde 2.0 or 5.0 kg/mol) and PS-norbornene (PS-NB) (Mn \textasciitilde 3.5 kg/mol) macromonomers. Symmetric and asymmetric lamellar morphologies were observed in the BBCPs over an exceptionally wide range of fPEO from 28 {\%} to 72 {\%}. Temperature controlled SAXS and WAXS revealed the presence of high order reflections arising from phase segregation above the PEO melting point. A progression from strong to weak phase segregation was observed over a temperature range of 150-180 Celsius degrees. The findings in this study provide insight into the rich phase behavior of this relatively new class of macromolecules, and may lay the groundwork for their use as templates directing the nanofillers with high aspect ratio.   

Effects of co-Polymer Structured Architecture on Solution Assembly

Diblock copolymers assemble into a rich variety of micelles whose shape is governed by the degree of incompatibility of the blocks and their interactions with the solvents. Tethering multiple blocks into structured architectures enhances the span of interactions that control assembly. Here we probed the assembly of a 1Wt{\%} A-B-C-B-A architecture, co-polymer of 100,000 gr/mol (C is polystyrene [PS], B is hydrogenated polyisoprene [PI] and A is poly (t-butyl styrene) in solutions using SANS. In cyclohexane though PS-PI forms star-like micelles, the pentablock associates into fractal aggregates. Increasing solvent polarity, by addition of propanol, drives the formation of elongated core-shell micelles with the PS blocks in the core and PI blocks reside in a highly swollen corona. The structured architecture enhances entropy resulting in less defined shapes that are maintained over a broad temperature range.   

Renewable Pentablock Copolymers Containing Bulky Natural Rosin for Tough Bioplastics.

Renewable polymers have received significant attention due to environmental concerns on petrochemical counterparts. One of the most abundant natural biomass is resin acids. However, most polymers derived from resin acids are low molecular weight and brittle because of the high chain entanglement molecular weight resulted from the bulky hydrophenanthrene pendant group. It is well established that the brittleness can be overcome by synthesizing multi-block copolymers with low entanglement molecular weight components. We investigated the effects of chain architecture and microdomain orientation on mechanical properties of both tri and pentablock copolymers. We synthesized rosin-containing A-B-A-B-A type pentablock and A-B-A type triblock copolymers to improve their mechanical properties. Pentablock copolymers showed higher strength and better toughness as compared to triblock copolymers, both superior to homopolymers. The greater toughness of pentablock copolymers is due to the presence of the rosin based midblock chains that act as bridging chains between two polynorbornene blocks.   

Quantifying the molecular interactions between block copolymers and lipid bilayers

Block copolymers have been widely used in cell membrane stabilization and permeabilization. Herein, we investigate the effect of polymer structure on molecular interactions between phospholipid unilamellar liposomes as model membranes and block copolymers comprising poly(ethylene oxide) and poly(propylene oxide) by varying the relative hydrophobic/hydrophilic composition, overall molecular weight and architecture of the polymer. Pulsed field gradient NMR (PFG-NMR) is employed to probe the diffusion of polymers in the presence of liposomes. The diffusion of the polymers associated with liposomes can be differentiated from that of free polymer coils based on their distinct diffusivities, thereby quantifying the association fraction of polymers. Increasing the hydrophobicity and overall molecular weight of the polymer significantly enhances the fraction of polymers associated with liposomes. These results demonstrate that PFG-NMR is a powerful tool to quantify the polymer-lipid bilayer association and bring new insights into the fundamental mechanism of the interactions between block copolymers and lipid bilayers.   

Determination of Globally Stable Block Copolymer Phases Using Particle Swarm Optimization

The unguided search for the stable phase of a block copolymer of a given composition and architecture is a problem of global optimization with important ramifications from a materials design perspective. A diverse collection of heuristic algorithms for solving global optimization problems is available to employ. In this talk, we discuss the development of a reciprocal-space Particle Swarm Optimization (PSO)-SCFT method applied to a diblock copolymer. By manipulating the Fourier components of SCFT fields near the principal shell, the dimensionality of the search space is greatly reduced compared to algorithms which work directly on the real-space field values.