Session Z26: Polymers and Block Copolymers at Interfaces II

A novel shear and dilatational interfacial rheometer for the study of complex interfaces applied to polymers at the air-water interface.

The radial trough has been introduced to facilitate the study of dilatational interfacial rheology of complex fluid interfaces by overcoming the mixed interfacial flow fields encountered in the standard rectangular Langmuir trough or the pedant drop. For the measurement of interfacial shear rheology, it has been determined that control of the surface pressure during measurements as carried out in the Double Wall Ring device mounted on a Langmuir trough (DWR-T¹) or the ISR, is required for obtaining reliable reproducible data.  In this contribution, we utilize a new trough instrument, the Quadrotrough   (Y.S. Tein et al. Review of Scientific Instruments, 93, 093903 (2022)), with the capability of performing on the same device (and sample) independent shear and dilatational deformations at the air/liquid interface under strain and surface pressure control. This enables unique control of the shear history of the interface as well as providing a method for shear annealing of the interface, enabling determination of the true equilibrium interfacial surface pressure and rheology.  Furthermore, dynamic oscillatory deformations enable determining the frequency dependent bulk and shear moduli, enabling calculation of the Poisson ratio for the interface.  Brewster angle microscopy imaging, particle tracking, and neutron reflectometry can be carried out in situ simultaneously with the rheological measurements to provide microstructural characterization from the molecular to mesoscale for determining rheological structure-property relationships at interfaces. Instrument, performance is validated by measuring the interfacial rheology of poly (tert-butyl methacrylate, PtBMA) at the air/water interface. Shear and dilatational deformation measurements were carried out over a wide range of surface concentrations. BAM images were obtained during shear and compression. The results obtained by the Quadrotrough are shown to be in agreement with measurements performed previously with the DWR-T and the radial trough. Measurements of the shear and dilatational moduli both in nonequilibrium equilibrium states as well as at finite frequencies and amplitudes are presented for several polyacrylates with increasing glass transition temperature and compared with prevailing theories for polymers at the interface.   

Encoding Latent Domain Orientation in Spray Deposited Block Copolymer Thin Films

The self-assembly of block copolymer (BCP) thin films is a powerful approach to nanopatterning, yet the majority of BCP film research relies on spin coating, which is limited to small, planar substrates (i.e., wafers). Here we report the self-assembly of asymmetric, cylinder-assembling polystyrene-block-poly(4-vinylpyridine) thin films deposited by ultrasonic spraying. "Wet films" are deposited from dilute solutions and film uniformity is regulated by solvent evaporation. Surprisingly, the evaporation time also encodes the latent orientation of cylinders within the films, which fully emerges after domain ordering by solvent vapor annealing. Specifically, short and long drying times facilitate assembly of vertical and horizontal cylinders, respectively. Characterization by grazing-incidence small-angle X-ray scattering (GISAXS) identifies the timescale for polymer vitrification that dictates latent cylinder orientation. This phenomenon can be controlled by substrate heating, forced convection, and solvent formulation. Moreover, it enables encoding of domain orientation on both planar and high-curvature substrates and can be used to deposit films with selective-area orientation control and well-ordered horizontal/vertical cylinder bilayers.   

Finite-size effects on capacitive energy storage in ultra-thin polymer and block copolymer films

Ultrathin polymer films present unique opportunities to understand the physics and properties of polymers at the nanoscale when the film thicknesses become comparable to the polymer dimensions. In this work, we demonstrate that ultrathin glassy polymer films (~100 nm) show an order of magnitude higher dielectric strength (E_BD) and capacitive energy density (U_max ∝ E_BD²) of ~27 J/cm³ as compared to the bulk polymer films when used as dielectric capacitors. We believe that the enhancement of the dielectric strength and capacitive energy density is due to the tighter chain packing of polymers in ultrathin films. We test the density of thin polymer films by optical measurements and observe that the ultrathin films show higher densities as compared to their bulk counterparts, which might be a governing factor for the enhancement of the dielectric strength. Furthermore, the ultrathin films of polymers having sub-room temperature glass transition don’t show ultra-high dielectric strength and capacitive energy density. More recently, we have shown that the introduction of 2D sheet-like nanoparticles can greatly enhance the dielectric constant in addition to the thin film effect for capacitive energy density (U_max ∝ E_BD²) of ~76 J/cm³ which is significantly higher than the highest energy density reported to date of ~36 J/ cm³ using a blending approach. Thus, multilayered hybrid inorganic/organic polymer thin films have much potential for applications as high-energy density storage media. Work funded by NSF-DMR.   

Hierarchical Cylindrical Microdomains in an A1BA2C Tetrablock Terpolymer

Hierarchical cylindrical nanostructures with different diameters (or shapes) have received much attention because of potential applications to next-generation lithography or advanced optical devices. Here, we observed, via small-angle X-ray scattering and transmission electron microscopy, tetragonally and rectangularly packed hierarchical cylindrical nanostructures by tailoring the volume fraction of polystyrene mid-block in polystyrene-b-polyisoprene-b-polystyrene-b-poly(2-vinylpyridine) tetrablock terpolymer (S₁IS₂V). P2VP becomes the main cylinder, while PI forms satellite cylinders surrounding the main P2VP cylinder. When the length of S₂ block is relatively short, tetragonal arrangement of cylinders is observed. But, a rectangular arrangement of cylinders is formed for larger S₂ block. The experimentally observed hierarchical cylindrical nanostructures are in good agreement with the prediction by the self-consistent field theory.   

High-Density Spherical Packing Phases from A(AB3)3 Dendron-Shaped Miktoarm Star Copolymer

Block copolymers show various nanostructures depending on the volume fraction (f) of the one block. Among them, spherical microdomains are important in understanding various crystalline orders. However, the expansion of the spherical region beyond f = 0.5 remains a great challenge due to their large interfacial curvature. Miktoarm star copolymer offers an efficient strategy for shifting the phase boundaries towards a higher volume fraction since the curved interface towards a major block is energetically more preferred despite of the volume ratio.

In this study, we synthesized A(AB₃)₃ miktoarm star copolymer using polystyrene (A) and poly(2-vinylpyridine) (B) through a combination of anionic polymerization, atom transfer radical polymerization (ATRP), and click reaction. We observed the body-centered cubic (BCC) phase even at f_PS = 0.51, where PS blocks became spheres rather than a matrix. This remarkable expansion of the spherical region is attributed to the unique chain architecture of A(AB₃)₃. In this structure, three generations of arms provide a gradual radial distribution towards the A domain, thereby stabilizing the large interfacial curvature of the spherical phase. The experimental results are consistent with the predictions based on self-consistent field theory (SCFT).   

Morphology of Block Copolymer and Bottlebrush Blends in Thin Films and Bulk

The structure, properties, and assembly kinetics of block copolymers (BCPs) can be tuned through the use of additives. A variety of additives have been explored, including homopolymers, nanoparticles, salts, and other small molecules. When homopolymers are added to a BCP their distribution within the BCP depends on the relative molecular weight between the homopolymer and BCP, with high molecular weights generally leading to macrophase separation. Bottlebrush polymers, consisting of short sidechains arranged along a linear backbone, present an interesting case where the overall molecular weight is large, but the sidechains can be short relative to the BCP. Small angle X-ray scattering measurements are used to explore the bulk structure of BCP/bottlebrush blends, and the results are placed in context with well known examples of BCP/homopolymer blends. The self-assembly of thin films are interrogated with a combination of AFM, GISAXS and soft X-ray scattering. The thin films show a thickness dependence of the morphology over a critical volume fraction of the bottlebrush additive, and the structure suggests the bottlebrush distributes differently in the thin film compared to the bulk. The role of the bottlebrush on the assembly kinetics is also investigated.   

Large Surface Area Non-equilibrium Morphologies Produced via Sequential Thermal and Solvent Immersion Annealing of Block Copolymer Thin Films

Processing conditions like thermal annealing (TA), solvent vapor annealing (SVA), and direct solvent immersion annealing (DIA) expose the block copolymers (BCP) to very different swelling and diffusion environments (vacuum vs. vapor vs. liquid). These conditions significantly affect the type and orientation of microstructure achieved by the BCP. So, it can be fairly expected that adding more complexity to the processing environment, say, by combining two or more of these annealing techniques, can alter the BCP thin film's natural interactions, ordering kinetics, and morphology evolution. We explore such methods by combining DIA (liquid processed) and TA (heat processed) methods in a sequential manner. While TA slowly (hours) orders a disordered symmetric-BCP film into parallel lamellar layers on silicon substrates with large domain size (L_o,TA) owing to slow diffusion in the melt, DIA rapidly induces the parallel lamellar morphology by means of direct swelling in good solvents and with highly reduced domain size (e.g., L_o,DIA ≈ 0.5 L_o,TA), providing a way to distinguish between the two. Sequential DIA on TA-ordered lamellar structure produces highly irregular intermediate morphologies that can be trapped on solvent removal by stopping the process. These intermediate structures feature large surface areas with potential applications in filtration membranes, scaffolds for catalysis, and biomass interaction and generation. Controlling the degree of stratification for the first TA step and the duration of the second DIA step allows us to tune the magnitude and the quality of the surface roughness (peaks vs. terraces). In limits of long-time DIA, the rough transient morphology heals back into a regular lamellar morphology with L_o,DIA size domains. Considering the structural crossovers observed, a chain rearrangement mechanism for transition between the two distinct morphologies is proposed, and the underlying dynamics of this reversibility process are analyzed in terms of polymer chain length, layer swelling, diffusion, and in-plane vs. out-of-plane interfacial evolution. The various morphologies observed are plotted on an energy diagram to establish the relation between interfacial energy changes with processing time and conditions.   

Coacervate emulsions stabilized by synthetic comb polymers with varying chain characteristics

Complex coacervates are ubiquitous in nature, having been shown to be vital for many cell processes, and may provide insights into the origin of life scenarios too. In laboratory settings, they have found great use as bioadhesives and enzymatic reactors and stabilizers. Once stabilized, their emulsions can be instrumental in many other potential applications. Previous research in our group showed that commercial comb polyelectrolytes stabilize coacervate emulsions for extended periods of time. In this presentation, we will discuss the synthesis and characterization of a library of comb polyelectrolytes and their stabilization capabilities via high throughput synthesis instrumentation and stabilization studies. By synthesizing comb polyelectrolytes (cPEs) with varying backbone and sidechain lengths as well as different sidechain densities, an assessment of the critical chain characteristics that influence emulsion stability will be presented. Additionally, the interplay of comb polyelectrolyte characteristics with their required concentrations to achieve comparable stabilization of the emulsions will be presented to arrive at guidelines for comb polyelectrolyte design and mixing protocols to achieve robust stabilization of coacervate emulsions.   

Scaling Theory of Diblock Copolymer Surface Micelles

This work deals with the self-assembly of AB diblock copolymers into micelles with 2D corona and 3D core at the air-liquid interface. Flat and laterally swollen conformations of A blocks in the corona are provided by their strong interfacial adsorption and repulsions in 2D, while attractions between the insoluble B blocks result in the formation of non-spherical 3D core. A scaling theory of circular spherical micelles is developed to predict the dependencies of their dimensions and the aggregation number, p, on the block lengths of the diblock copolymer, N_A and N_B. The power laws are derived for the starlike (st) and crew-cut (cc) limits, which correspond to the micelles with a low and high core-to-corona size ratio, respectively. For 2D theta solvent for corona chains, scaling predictions are p_st ~ N_A⁰ N_B^0.5 and p_cc ~ N_A^-0.67 N_B^0.83 for starlike and crew-cut micelles, respectively. Theoretical exponents are in reasonable agreement with the experimental power law, p ~ N_A^-0.48 N_B^0.7. [1] The empirical exponents, which have been obtained by numerical fitting the data with the single power law, fall within the range theoretically found herein. A similar agreement takes place for the core and corona dimensions and holds in the case of athermal solvent for the corona A blocks. Theory generalization to 2D+2D micelles shows that scaling laws are weakly affected by the geometry of the core. The developed theory reveals principles of diblock copolymer self-assembly under partial confinement.

[1] Kim, D. H.; Kim, S. Y. Scaling Laws for Block Copolymer Surface Micelles. J. Phys. Chem. Lett. 2022, 13 (24), 5380–5386. https://doi.org/10.1021/acs.jpclett.2c00979.

 

    

Modeling of lubricant additives using a molecularly-informed field theory

Lubricants are complex formulations that, in addition to a base oil, contain a variety of small molecule and polymer additives that can self-assemble into an array of structures and greatly influence the functional properties of the formulation. However, the large number of components these formulations contain leads to an extensive design space that makes it difficult to optimize their composition with traditional experimental iteration. While computational approaches seem attractive, many types of lubricant additives are high-molecular-weight polymers present at significant concentrations (~5 wt%), which makes them difficult to study using traditional molecular dynamics methods due to length and time scale limitations. To overcome this obstacle, we use small-scale, atomistic simulations to parameterize field-theoretic models to probe the behavior of lubricant additives of realistic sizes while maintaining a connection to the underlying chemistry. Here we validate this approach by making de novo predictions of the phase behavior of a acrylic diblock copolymer model system where the predicted behavior is in excellent agreement with small-angle X-ray scattering (SAXS) measurements. Moreover, we show the ability to predict properties in the dilute regime, such as critical micelle concentrations, that are more relevant to lubricant formulations. Furthermore, we show how our workflow can be extended to simulate lubricant-surface interfaces and study behavior in boundary lubrication regimes.