Session K04: Physical Phenomena at Polymer/2D Material Interfaces

Revisiting 2D materials fabrication with a fully mechanized platform

Since manual mechanical exfoliation, or “tape exfoliation,” produced mono and few-layered graphene in 2004, researchers have made tremendous progress in two-dimensional (2D) material research. In particular, 2D materials are one of the most versatile motifs for generating new quantum information science materials since one layer in arbitrary stacks forms heterostructures unconstrained by epitaxy. However, while researchers have discovered many novel heterostructures with unique properties, it has been challenging to study and utilize 2D materials because of the time- and labor-consuming preparation processes involved. Here, I will introduce the Quantum Material Press (QPress), a unique, automated user facility for 2D material heterostructure fabrication. 

  First, a fully mechanized exfoliation platform, or "exfoliator," employing pressure-sensitive adhesive (PSA) tape in a roller assembly can control the entire exfoliation process instead of human hands. This roller assembly can independently regulate various rolling and peeling conditions and allows us to study physical mechanisms in tape exfoliation. Second, a mechanized dry transfer platform, "stacker," can manipulate substrate and stamp movement in 3- and 6-axis, respectively. It enables us to develop a transfer technique to fabricate high-quality 2D heterostructures with clean interfaces. In this talk, I will discuss the underlying mechanisms of tape exfoliation and dry transfer studied with the QPress facility.   

Zwitterists: Photo-patternable Polymer Zwitterions for Interfacial Dipole Doping of Monolayer Graphene

Electronic modulation of 2D materials with dipole-rich zwitterionic polymer coatings holds promise for enhancing electronic device performance. Although significant shifts in the work function (WF) of graphene have been induced by a variety of zwitterionic polymers, the influence of zwitterion structure is not well understood. We built the polymeric "zwitterist" (a portmanteau from zwitterion and photoresist) platform to understand how steric footprint impacts the WF modulation of graphene with polymer zwitterions via dipole doping. A series of zwitterionic sulfobetaine-based random copolymers with variable substituents and photo-crosslinkable benzophenone moieties was synthesized and utilized to prepare negative-tone resists for spatial WF engineering of graphene. Of the polymers investigated, the piperidinyl-substituted version possessing the bulkiest steric group induced the largest WF reduction as observed from spectroscopic measurements and theoretical calculations. The "zwitterist" design was extended to fluorinated polymer zwitterions to understand the individual contribution of two types of dipoles within the side chains: choline phosphate (zwitterion) and fluorinated alkyl groups. This platform will inspire the discovery of new and simple WF engineering approaches.   

Defect Healing in Graphene via Rapid Thermal Annealing with Polymeric "Nanobandage"

It is crucial to establish a high-throughput healing method of defects in graphene for post-silicon device fabrication, as defect introduction during synthesis and characterization processes is unavoidable. Current healing methods, such as conventional thermal annealing, are either time-consuming, highly specialized, or tedious. To this end, we have developed a new time- and energy-efficient healing approach for graphene, utilizing polymer-assisted rapid thermal annealing (RTA). In this method, a nitrogen-rich, polymeric “nanobandage” is coated directly onto graphene and processed via RTA at 800 ^oC for 15 seconds. In this process, the polymer matrix is designed to be cleanly degraded, while nitrogen released from the nanobandage diffuses into graphene, forming nitrogen-doped healed graphene. X-ray photoelectron spectroscopy (XPS) confirms that 1~3 atomic % nitrogen relative to carbon was successfully doped into the graphene, an amount comparable to other conventional methods. Furthermore, electrical transport measurements indicate that the nanobandage treatment recovers the conductivity of electron beam-treated defective graphene at ~85 %. The nanobandage approach shows great promise for use with other 2D materials with a variety of dopants by simply changing the polymer formulations.   

Crumpling and Mechanical Behaviors of Polymer-Grafted Nanosheets

Understanding the crumpling behavior and mechanical response of polymer-grafted nanosheet materials is of fundamental importance in engineering and technological applications. Here, we report the results of a systematic coarse-grained molecular dynamics simulation study of the crumpling process and compressive properties of polymer-grafted nanosheets at various polymer grafting densities. We find that the degree of crumpling and wrinkling of the nanosheets at equilibrium increases and then decreases with increasing polymer grafting density. By evaluating the evolution of the potential energy of the sheet during the crumpling process, our results show that the grafted polymer chains significantly reduce the adhesion properties of the nanosheets. Notably, the shape descriptor analysis shows less self-folding and self-adhering upon crumpling for sheets with larger grafting density. In addition, the evaluation of the compressive properties of the crumpled system further reveals the enhancement of the mechanical response of the grafted polymers to the crumpled nanosheets. Our findings highlight the critical role of grafted polymer in the crumpling process and mechanical response of nanosheets, which has significant implications for the tailored design of crumpled polymer functionalized nanosheets.

    

Polymer-graphene thin film composites exhibit exceptionally strong mechanical strength

Laminate thin-film composites that couple polymers with single-layer graphene (SLG) are lightweight and have shown superb electromechanical strength. The mechanical strengths of the composites can be described by elastic micromechanical models such as the Voigt mixing rule. However, the reinforcement behavior between polymer and SLG has raised questions about the validity of such models in laminate composites at the nanoscale. Herein, we have fabricated laminate thin films of poly(ether imide) and SLG (PEI/SLG) with varying volume fractions of SLG (?g) as a model system to evaluate the effective reinforcement using the mixing rule. Linear regression analysis of the Young’s modulus of the composite (Ec) versus ?grevealed an unexpectedly high-effective Young’s modulus of large-area, polycrystalline SLG, Eg = 1.12 ± 0.05 TPa. Further analysis of theoretical and experimental Ec using the Voigt–Poisson model showed a lower maximum value of Eg ∼ 0.9 TPa for films with ?g ≥ 0.11 vol %. Our results show that an ideal mixing rule is followed only beyond a critical value of ?g for laminate thin-film composites, which explains the wide inconsistency of Eg reported in the literature. This knowledge will guide the fabrication of laminate polymer–graphene thin films with near-ideal mechanical reinforcement.   

Confinement effects on electrical conductivity and dielectric permittivity of graphene and reduced graphene oxide dispersed polymer nanocomposite films

Many recent research efforts have studied how the incorporation of graphene oxide (GO) and reduced graphene oxide (rGO) impact the frequency-dependent dielectric properties in polymer nanocomposite materials. However, few studies explore the effect of film confinement in the micron to sub-micron range of the thin film regime. In this film thickness range, which is comparable to the lateral dimensions of the platelet GO and rGO nanomaterials, we can expect film confinement effects on platelet orientation, which also impacts its relative projection area to the applied electric field. Pristine graphene can be achieved by reducing graphene oxide, which has a similar honeycomb structure with both sp² and sp³ carbon imbuing the surface with limited oxygen-based functional groups that can interact with the polymer matrix via a nanometric interfacial zone.  In this work, we present the nanomaterial loading-frequency dependence of these two fundamental properties, in thin polyvinylidene fluoride (PVDF) film containing GO and rGO of thicknesses ranging from 1 µm to 100 µm. A controlled microwave was used for reducing graphene oxide in a laboratory environment. We will discuss the film finite size effects on the relative response of GO vs. rGO vs. pristine PVDF in terms of their dielectric constant, loss tangent, and AC conductivity data over the frequency range of 1 kHz to 1 GHz using a dielectric spectrometer. We observed that the AC electrical conductivity tends to increase with frequency, but the dielectric permittivity tends to decrease in general and that the dielectric properties are more frequency sensitive for the 1 µm film than the 100 µm films. AFM and SEM were used to ascertain the film morphology and quality of these nanocomposite films. This study may provide insight into the development and utilization of GO and rGO-based polymer thin films in future electronics.   

Effect of Mild Thermal Annealing on Structure and Properties of Graphene Oxide-Polymer Nanocomposites

Two-dimensional materials can be employed as interesting nanofillers due to their unique structure and properties based on large surface area. Graphene oxide (GO), a representative two-dimensional material, is an oxidized form of graphene. GO can easily interact with hydrophilic polymers through hydrogen bonding owing to the high specific surface area and abundant oxygen functional groups, Thus, chain structure and dynamics of polymers at the interface is known to be sensitively dependent on the nature of GO surface.

While the thermal- or chemical- reduction of GO significantly weakens the dispersibility of GO in a solvent and reduces the interaction with the hydrophilic polymers, undergoing of mild thermal annealing to the extent that reduction does not occur does not remove the functional groups but redistributes them to the edge. This weak opening will alter the interaction between GO and polymer and ultimately affect the final structure and properties of polymer-based products such as GO-polymer nanocomposites. In this regard, we investigate the change of the crystallization and melting behaviors of polymers with mild thermal annealing of GO and show the migration of the functional groups vary the GO-polymer interaction ultimately changing the nature of polymer crystallization.   

Effects of Chemical and Physical Features of 2D Nanofillers on the Mechanical and Viscoelastic Properties of Polymer Nanocomposites

2D nanofillers, such as graphene and graphene oxide (GO), have great potential to reinforce polymer matrices. Molecular dynamics (MD) simulations have offered significant insights into the mechanical properties and deformation mechanisms of 2D nanofiller-reinforced polymer nanocomposites. However, the effects of mesoscale chemical and physical features of 2D nanofillers, including the heterogeneous patches in GO sheets and the wrinkles formed in multi-layered graphene sheets, on the mechanical and viscoelastic properties of polymer nanocomposites remained largely elusive due to the spatiotemporal limitations of all-atomistic MD simulations and challenges in experimental characterizations. We recently built on the previously developed coarse-grained models to explicitly study the influences of these unique features of 2D nanofillers. We have found that the patchy structures in GO sheets are responsible for variations in interfacial and viscoelastic properties of GO-based nanocomposites, and the wrinkle level of 2D nanofillers influences the mechanical and viscoelastic properties of the nanocomposites. Our recent studies provide fundamental insights into the influence of unique features of 2D nanofillers on the physical properties of polymer nanocomposites, as well as underlying mechanisms.

    

A Study of Morphology Formation Mechanism of Graphene Oxide Nanofillers in Polyurethane Matrix under Flow

Recent mesoscale computational techniques have efficiently built the relationship between microstructure and macroscopic properties of materials. This work presents an implementation of mesoscopic DPD (Dissipative Particle Dynamics) simulation to investigate the coalescence and dispersion of GO (graphene oxide) nanofillers in TPU (thermoplastic polyurethane) matrix under various processing conditions. The exfoliated morphology of GO agglomeration was discovered by increasing the shear rate over 20 s^-1. It was eligible to match the laboratory data. Except for the shear flow, the dispersion state of nanofillers was also dominated by the oxidation density. Changes from intercalation to exfoliation of GO only existed with a 40% oxidation degree at a favorable shear rate. The higher oxidation density surface of GO nanosheets adsorbed more TPU rigid segments. They created larger gaps in the agglomerates and resulted in the exfoliation eventually, as Herman’s orientation factor of attached polymer chains increased from -0.45 to 0 through raising Flory-Huggins χ parameters between GO and TPU. These surface-covered nanofillers were speculated to be the morphological origin of experimentally observed dispersion in processing. Additionally, the machine learning technique was applied to extend this consequence to a versatile phase diagram fitting into broader nanocomposite systems. Our findings can be employed as a guidance to design and fabricate polymeric nanocomposites.