Session P49: Tribology of Polymers and Soft Materials II: Friction and Slip

Shear-driven polymerization: critical roles of chemisorption and molecular deformation

Shear force is used to drive chemical reactions in many natural and engineering processes. However, despite the important role of these so-called tribochemical reactions, their fundamental mechanisms are still poorly understood due to challenges associated with directly measuring processes that occur inside a moving contact. To address this, we investigate shear-driven polymerization of molecules adsorbed on silica surfaces, a model system that enables identification of shear-driven reaction pathways. This system is studied using reactive molecular dynamics simulations complemented by pre- and post-sliding surface characterization. The results show that interfacial shear not only accelerates polymerization reactions but opens reaction pathways that are not accessible thermally. Specifically, the simulations reveal that chemisorption and shear-induced deformation of the reactants are critical steps in tribochemical processes. In general, these findings may form the basis for design of systems where shear force can be leveraged to tune or lower the energy cost of chemical reactions.   

Slippage of polymers at interfaces

The slippage of polymer melts is now well understood. In particular, Hénot et al. have investigated the dependence of the slip length on the viscosity of the sheared polymer melt [1]. They proved that the friction of polymer melt is due to the friction of monomers on the surface.
More recently, we studied the effect of temperature on slippage of polymer melts far above glass transition temperature [2]. We showed that friction is a thermodynamically activated process.

With solutions, the concentration can change the interdigitation between the chains anchored to a surface and the solution. Furthermore, depletion layer may appear. The disentanglement and depletion are competing.

We will present the results of experiments of friction of polystyrene in diethyl phthalate on different substrates. We investigate the dependence of the slip length on concentration of the solutions in the Newtonian and the shear-thinning regimes.

[1] Hénot, Marceau, Drockenmuller, Eric, Léger, Liliane, et al. Friction of polymers: from PDMS melts to PDMS elastomers. ACS Macro Letters, 2018, vol. 7, no 1, p. 112-115

[2] Hénot, Marceau, Grzelka, Marion, Zhang, Jian, et al. Temperature-controlled slip of polymer melts on ideal substrates. In Press   

An Effect of Interfacial Aggregation States on Frictional Properties of Hydrogel Thin Films

Frictional Properties, which are closely related to aggregation states at the outermost surface, of hydrogels are pivotal importance for many applications. Recently, we synthesized poly(2-methoxyethyl vinyl ether) (PMOVE) containing photo-cross-linkable 2-(vinyloxy)ethyl methacrylate (VEM) units by living cationic polymerization. Here, interfacial density profiles and frictional properties of cross-linked PMOVE (c-MrV) thin films in water were examined by neutron reflectivity (NR) and lateral force microscopic (LFM) measurements, respectively. The density profile of the c-MrV films in close proximity to the water interface was well described by a parabolic function, which was generally used for polymer brushes in a liquid. The lateral force (F_L) for the hydrogel films swollen in water was a function of normal force (F_N) applied on to a probe tip. For all films, F_L first increased with F_N and then remained constant at an F_N. The critical depth, in which F_L reached a constant, well corresponded to the thickness of the interfacial swollen layer determined by the NR measurement.   

Microgel systems containing phospholipid: Role of component interactions on rheology and tribology

Microgel systems containing phospholipids are often used in personal care products due to their gelling and emulsifying properties. Herein, we investigate how the type of microgel (hydrophobically modified versus hydrophilic) and its interaction with phospholipid manifest itself on the tribology and rheological behavior of the system. We examine two different polymers one forming a microgel with hydrophobic moieties on the surface (Pemulen) and the other being a hydrophilic microgel (Carbopol). While both polymers exhibit gel-like features rheologically, their mode of interaction with phospholipid is different. Phospholipid addition to the Pemulen leads to an increase in elastic modulus because of the interaction of the hydrophobic moieties with the phospholipid tails. Such active participation is verified using heat of interaction measured through isothermal calorimetry. Tribological behavior, measured with a soft model polydimethylsiloxane (PDMS) contact, reveals lower friction coefficients in the boundary regime for the hydrophobic Pemulen with and without phospholipid. Adsorption of phospholipids and Pemulen on the PDMS substrate are atrributed to the decreased friction coefficient, which we verify through optical microscopy and with Quartz Crytal Microbalance measurements.
  

Orientation of nanodomains of star-shaped (PMMA-b-PS)6 in thin films with different molecular weights

To apply block copolymer (BCP) to nanolithography, vertically oriented nanodomains in thin film are needed. When the chain architecture of BCPs was changed from linear to star shape, vertically oriented cylinders and lamellae were obtained by using only thermal annealing without any special treatment. This is because the vertical orientation is entropically favored over parallel oriented one.
Here, we study the effect of the molecular weight of (PMMA-b-PS)₆ star shaped block copolymer on the orientation of nanostructres on a silicon oxide changed. When the molecular weight is small, but still larger than the critical molecular weight corresponding to the order-disorder transition (ODT), parallel orientation is observed. This is because the entropic loss arising from the formation of vertical oriented nanostructures is not large; thus the orientation of nanostructures is greatly affected by surface treatment of a substrate. On the other hand, at large molecular weights, the entropic penalty for parallel orientation becomes huge, resulting in vertical orientation.   

Concentration-Dependent Long-Range Repulsive Interactions of Adsorbed Associative Polymers

Colloidal atomic force microscopy is used to study the quasi-static and velocity-dependent normal force-separation response of adsorbed layers of associative and non-associative olefin copolymers (OCP). OCP containing a small fraction of a polar group forms transient bonds, leading to a stronger concentration-dependent viscosity than native OCP. This functional group also imparts a strong surface affinity to OCP, leading to strong adsorption. Quasi-static measurements show that the surfaces with native OCP have minimal interaction on approach, and strong adhesion on retraction due to bridging. In contrast, surfaces with associative OCPs exhibit repulsive interactions on approach with the magnitude and onset increasing with the polymer concentration. The interaction distances are long range, approaching 10x the unperturbed polymer dimension. Adhesion on retraction is greatly reduced and occurs over a range close to the contour length of the polymer. Increasing the approach velocity leads to long range repulsion for both polymers due to hindered solvent drainage. The magnitude of repulsion is highest for the associating OCP, but the onset of interaction is comparable for both. These results correlate well with impact of these polymers on friction in the boundary regime.   

Beyond the lubrication approximation: capillary levelling of holes in freestanding polymer films

Capillary levelling experiments have been used previously to study nano-scale flow in thin viscous films with a variety of hydrodynamic boundary conditions. Theoretical models developed to understand these experiments have primarily used the lubrication approximation, which omits flow normal to the film. In this work, thin bilayer polystyrene films were prepared freestanding in air, with one of the two films containing micrometer scale cylindrical holes. The viscoelastic relaxation of the holes was studied using atomic force microscopy. In order to equilibrate internal Laplace pressure, a hole will undergo a viscous symmetrization process, resulting in identical holes at the two interfaces. A novel 3D axisymmetric hydrodynamic model, which includes vertical flow, was developed to understand the dynamics of this symmetrization process, and is shown to be in excellent agreement with experiments.   

Understanding Polymerization-Induced Nanostructural Transitions Using in situ Characterization Methods

Polymerization-induced structural transitions have been used to create nanostructured in a host of applications ranging from energy to separation technologies. Recently, our group has achieved order-order and disorder-order structural transitions by polymer grafting from linear block polymers mixed with monomer. In our approach, we are able to induce either a lamellar-to-hexagonally-packed cylinder (LAM-HEX) or a disordered-to-HEX transition via the polymerization of styrene, which initially acts as a neutral solvent for the lamellar-forming diblock copolymer, poly(styrene)-block-poly(butadiene) (PS-PBD). In situ small-angle X-ray scattering (SAXS) and rheological experiments during the polymerization process reveal a complex phase path in which we initially disorder the lamellar morphology at elevated temperatures. During the progression of the polymerization, the disordered phase transitions first to an ordered state, and then converts into the double gyroid phase. We are able to recover the HEX phase after polymerization when the sample is cooled to room temperature. The work presented here highlights how the chemical process of converting standard linear diblock copolymers to grafted-block polymers drives interesting and controllable morphology transitions.   

Lubricated friction on microtextured soft substrates

Natural cartilage is durable and elastic, providing a low friction coefficient to moving joints under frequent applications of heavy loads. However, the physical mechanisms contributing to this low friction coefficient is not well understood. We hypothesize that the non-ideal surface geometry of cartilage gives rise to its low friction coefficient in certain directions of motion. We design soft poly(dimethyl siloxane) (PDMS) substrates with controlled dimensions and spacings to study the influence of surface geometry on their frictional and lubrication properties. Tribological tests performed with a thin layer of aqueous glycerol solution between the PDMS substrates show that the frictional behavior does not follow the type of Stribeck curve that is typically observed with flat surfaces. This major difference can be explained by a scaling theory we developed, in which the friction force and the normal force are expressed with a combination of lubricant properties, experimental conditions and surface geometries. Our study establishes a design framework for the friction of elastomers based on their surface microtextures, and paves the way for engineering soft materials in technological applications such as wearable electronics, antifouling coatings, and synthetic implants.

  

Effective Orientation Control of Block Copolymer Nanostructures in Thin Films by Surface Modification using Self-assembled Copolymer Adsorption Layer

Block copolymers (BCPs) have been extensively studied due to their ability to self-assemble into well-defined nanostructures. In particular, self-assembly of BCPs in a two-dimensionally confined state of thin films has been widely exploited for a bottom-up nanofabrication in the use of many applications. In order to achieve meaningful nanopatterns in BCP thin films, a perpendicular orientation of BCP morphology is required. Here, we demonstrate that self-assembled copolymer adsorption layers (SCALs) can effectively control the morphology orientation of BCPs in thin films as modifying a surface energy of substrates. SCALs were derived from an interfacial self-assembly (ISA) of BCPs based on our finding that the self-assembled BCPs at the air/water interface can be transferred and irreversibly adsorbed onto a solid substrate. Precise modification of the surface energy was possible by controlling nanostructures of SCALs as a function of a surface pressure for ISA of BCPs. Moreover, we found irreversible adsorption for ISA of BCPs is universal for any substrates. Therefore, SCALs can be readily prepared on wherever ISA of BCPs is transferred to, thus enabling much more effective surface modification of various substrates, such as metals, ceramics, flexible or curved substrates.   

Contact and slip mechanics between crosslinked hydrogel surfaces using in situ microscopy

The contact mechanics of soft-soft interfaces provide a backdrop for the slip mechanics due to the conformal contact at the interface, in which any surface asperities are fully compressed, and the apparent area of contact is the real area of contact. However, assumptions regarding the nature of the soft material as a thermal, semi-dilute mesh network, poroelastic solid, or other, will control the contact mechanics, especially over time. In this work we show detailed measurements of contact areas with polyacrylamide hydrogels during microindentation and slip which are revealed through particle inclusion and/or exclusion microscopy. The particles are green fluorescent polystyrene spheres of 0.5 or 1 µm in diameter. We identify time-dependent contact mechanics in migrating, stationary, and self-mated “Gemini” contact. Our data suggest that for long times, Gemini contact approaches a constant-pressure contact model, which depends upon the equilibrium osmotic pressure of the sample. Finally, we present for the first time asymmetric contact areas as visualized by in situ particle exclusion which manifests as a flow field around the probe. The results of this work begin to connect hydrogel material properties with surface mechanics.   

Wall slip of complex fluids: Interfacial friction versus slip length

If the slip length is an useful notion notion to describe the friction of simple fluids, we will show that the slip length is not appropriate for viscoelastic liquids. Rather, the appropriate description lies in the original Navier's partial slip boundary condition, formulated in terms of an interfacial friction coefficient. We establish an exact analytical expression to extract the interfacial friction coefficient from oscillatory drainage forces between a sphere and a plane, suitable for dynamic SFA or atomic force microscopy noncontact measurements. We use this model to investigate the boundary friction of viscoelastic polymer solutions over 5 decades of film thicknesses and 1 decade in frequency. The proper use of the original Navier's condition describes accurately the complex hydrodynamic force up to scales of tens of micrometers, with a simple Newtonian-like friction coefficient that is not frequency dependent and does reflect closely the dynamics of an interfacial depletion layer at the solution-solid interface.

REF: Cross, B., Barraud, C., Picard, C., Léger, L., Restagno, F., & Charlaix, É. (2018). Wall slip of complex fluids: Interfacial friction versus slip length. Physical Review Fluids, 3(6), 062001.   

Mechanical and thermodynamic properties of Aβ42 , Aβ40 and α-synuclein fibrils from molecular-scale simulation

Atomic force microscopy (AFM) is a versatile tool to characterise the mechanical properties of biological systems. However, AFM deformations are tiny, which makes impossible the analysis of the mechanical response by experiment. Here, we have employed a simulation protocol to determine the elastic properties of several biopolymers (i.e. biological fibrils) . For these systems, the simulation approach is sufficient to provide reliable values for three different types of elastic deformation, i.e. tensile (Y_L), shear (S), and indentation (Y_T). Our results enable the comparison of the mechanical properties of these fibrils. In particular, we find a significant elastic anisotropy between axial and transverse directions for all systems. In addition, our methodology is sensitive to molecular packing of the fibrils . Interestingly, our results suggest a significant correlation between mechanical stability and aggregation propensity (rate) in amyloid systems, that is, the higher the mechanical stability the faster the fibril formation takes place.