Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F54: Confined Polymer Glasses I: Modeling, Aging, and Local ConnectivityFocus
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Sponsoring Units: DPOLY GSOFT GSNP Chair: Reika Katsumata, University of Massachusetts, Amherst Room: BCEC 254A |
Tuesday, March 5, 2019 11:15AM - 11:27AM |
F54.00001: Relaxation-Function-Dependent Two-Barrier Model for Nanoconfinement Effects on the Glass Transition David Simmons, Daniel Mauricio Diaz Vela One of the central challenges in the study of nanoconfined systems has been the fact that distinct measures of these systems can report quantitative and qualitative differences in perturbations from bulk. This scenario has been further complicated by the recent observation in experiment of an apparent onset condition of nanoconfinement effects at temperatures only modestly above Tg. This observation has raised additional questions regarding the interpretation of large observed nanoconfinement effects in simulated systems at temperatures relatively much further above Tg. Here, using simulations and scaling theory, we show that this rich phenomenology can be unified via the combination of a two-barrier model of the glass transition with a barrier-truncation scenario for nanoconfinement effects. We discuss strong implications of these results for the viability of existing theories of the glass transition. |
Tuesday, March 5, 2019 11:27AM - 11:39AM |
F54.00002: Modeling the Depth-dependent Dynamics of Confined Systems Jane E Lipson, Ronald White This talk will focus on our recent results modeling dynamics and glassiness in confined systems. One technique is the Limited Mobility model (LM), a coarse-grained kinetic simulation approach where mobility at a site depends on facilitation through mobility at nearby sites. LM results include mobile layer depths in freestanding films, as well as the effect of nanoparticles on the surrounding matrix. Our Cooperative Free Volume model (CFV) is a different approach. This is a rate model in which the system's free volume (Vfree), predicted through our Locally Correlated Lattice (LCL) equation of state analysis of bulk thermodynamic data, plays the key role in determining the local molecular cooperativity. The CFV model describes independent T- and Vfree - contributions to the pressure-dependent segmental relaxation times, tau(T,V). This talk will focus on our ability to describe the dynamics of confined systems, where the local density (and thus the Vfree-contribution) is altered by the presence of an interface. For example, our recent CFV work reveals the point at which film samples lose the contribution from regions of bulk-like dynamics, which will lead to insight on the position-dependent mobility across an interfacial region. |
Tuesday, March 5, 2019 11:39AM - 11:51AM |
F54.00003: The Important Roles of Topological Constraints on Interfacial Effects of Nanoconfined Polymers Biao Zuo, Rodney Priestley, Xinping Wang Interfaces play a significant role in determining the dynamics of nanoconfined polymers. Recent experiments and simulations have revealed the long-range nature over which interfacial effects originating at the polymer-substrate interface can propagate within a thin film. Here, we show that topological constraints, that is, the interpenetration and entanglement between chains directly adsorbed to the substrate and neighboring non-adsorbed chains, play a major role in the suppression of thin film dynamics. This is because such constraints increase motional coupling of free and adsorbed chains, thus giving rise to a mechanism for the propagation of substrate effects. In addition, we demonstrate that the propagation of the suppressed interfacial dynamics can be tuned by controlling the local conformation of chains adsorbed to the substrate surface. |
Tuesday, March 5, 2019 11:51AM - 12:27PM |
F54.00004: Surprising Impact of Chain Connectivity in Altering Local Glass Transition Temperature Near Interfaces Invited Speaker: Connie Roth Our group has recently investigated how the local glass transition temperature Tg(z) changes across interfaces between two polymers with widely different bulk glass transition temperatures Tgbulk. Starting with a single interface between two semi-infinite domains (ΔTgbulk ≈ 80 K), we used a localized fluorescence method to demonstrate broad profiles in Tg(z) across dissimilar polymer-polymer interfaces spanning hundreds of nanometers and observed to be asymmetric relative to the composition profile. A key finding of these results is the observation that the broad coupling of dynamics across the dissimilar polymer-polymer interface only occurs if this interface is annealed to equilibrium. Efforts to understand what factors during polymer interface formation cause these broad profiles in Tg(z) find that chain connectivity appears to be surprisingly important. This is confirmed by measurements near silica substrates with tethered chains where low grafting densities (~10 vol% tethered chains) are observed to cause large +50 K increases in local Tg. We now explore what property changes take place in multilayer systems during interface annealing using different experimental techniques with the goal of understanding how interfaces mediate dynamical coupling across dissimilar polymer domains. Advanced materials design is headed towards increasing amounts of interfaces with progressively smaller domain sizes where the geometrical arrangement of these interfaces can be utilized to alter local material properties for desired applications. |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F54.00005: Comparison of Physical Aging and Local Glass Transition in Glassy-Rubbery Bilayer Films Jennifer A McGuire, Michael Thees, Connie Roth Block copolymers consisting of glassy and rubbery polymers in direct contact with each other are common in many industrial applications, with the stability of the glassy domains critical to the material’s function. Recent fluorescence measurements by our group have shown large changes in local glass transition temperature Tg(z) across glassy-rubbery polystyrene (PS) / poly(n-butyl methacrylate) (PnBMA) multilayer films. Previously we have demonstrated a correlation between reduced physical aging rates in thin single layer PS films with local Tg reductions near the free surface. Here we explore if a similar correlation between physical aging and local Tg exists near glassy-rubbery polymer interfaces. Physical aging of PS films capped with rubbery PnBMA layers are measured using ellipsometry, avoiding the competing effects of the free surface and allowing us to isolate the impact of a single glassy-rubbery interface. Reductions in physical aging rate with decreasing PS layer thickness are observed, but the reduced layer thicknesses needed to make the ellipsometry measurements viable induce finite size effects altering the local Tg(z) profile relative to previous profiles on semi-infinite bilayers, necessitating direct comparisons on equivalent systems with both techniques. |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F54.00006: Role of hydrophilic support on the physical aging and stress development in thin polynorbornene films Elizabeth Lewis, Christopher M Stafford, Bryan Vogt Thin film properties can be impacted by their local environment, especially when in contact with other polymers. Here physical aging of poly(butylnorbornene-ran-hydroxyhexafluoroisopropyl norbornene), BuNB-r-HFANB, thin films coated on ultra-thin (2nm) layers of poly(acrylic acid), PAA, or poly(styrene sulfonate), PSS, is examined along with residual stress evolution in BuNB-r-HFANB during aging. Decreasing BuNB-r-HFANB film thickness tends to decrease the physical aging rate(β), but this quantitatively is altered by PSS or PAA. Even for thick films (>500nm), aging rate at 100○C is accelerated on PAA compared to silicon, while thickness dependence of BuNB-r-HFANB aging is enhanced (compared to silicon) when in contact with PSS with apparent aging rate decreasing by a factor of 2 as film thickness decreases from 1000 nm to 250 nm. Removal of BuNB-r-HFANB from substrate enables mechanical properties (elastic modulus and residual stress, σR) to be resolved via wrinkling. Elastic modulus is effectively invariant of substrate (polymer layer), while there is a small effect of physical aging on the modulus. The stress in the films is compressive and evolves non-monotonically during aging. β can be mostly collapsed when scaled as hσR-1/3. |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F54.00007: Effect of Substrate on the Stability of Ultra-thin Vapor Deposited Molecular Glasses Yi Jin, Yue Zhang, Connor Woods, Zahra Fakhraai We have recently demonstrated the existence of long-range correlation between the free surface and substrate dynamics in ultrathin supported molecular glass films such that when the substrate interactions are changed from weakly interacting to neutral, the extent of Tg reduction and enhanced film dynamics is reduced and the stability against dewetting is increased for films as thick as 30 nm. The stability of vapor-deposited glasses is strongly influenced by the surface mobility and its gradients. Since the surface dynamics are affected by the substrate for ultra-thin films one would expect the stability to follow. Here, we show that ultra-thin films of TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine) vapor-deposited on a neutral substrate have higher stability (higher density change and lower fictive temperatures) compared to those deposited on a weakly-interacting substrate. The density change is also found to be higher in ultrathin films (<50 nm) compared to the bulk films (>200 nm, steady state condition). These results suggest a possibility that film/substrate interactions could speed up effective aging rates, or deter dewetting during deposition, giving rise to more efficient amorphous packings. |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F54.00008: Role of Solvent Washing Conditions on the Creation of Adsorbed Layers from Melt Films Michael Thees, Jennifer A McGuire, Xinru Huang, Connie Roth Recent reports have proposed the presence of adsorbed layers may strongly affect dynamics in thin supported films. Often assumptions are made about the presence of adsorbed layers within melt films based on annealing time and temperature, where subsequent washing of films with solvent is interpreted as “revealing” the structure of the adsorbed layer within the melt (so-called Guiselin experiment). However, this interpretation is contrary to extensive literature studies from the 1980-90s on displacement experiments of adsorbed chains in solution measuring exchange kinetics with a third component called the “displacer”, either another polymer or second solvent, that displaces the initially adsorbed polymer due to its preference for the substrate interface. These studies, along with direct surface diffusion measurements by Granick’s group in the regime of strong adsorption, suggest adsorbed polymer chains are extremely mobile during solvent washing. We test this interpretation by demonstrating the measured residual adsorbed layer strongly depends on solvent washing conditions. Consistent with work by Cohen Stuart’s group, we show acetone as a displacer removes nearly all of the initially adsorbed chains, even after melt films were extensively annealed at elevated temperatures. |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F54.00009: Substrate Effect on Tg of Random Copolymers of 4-tert-butylstyrene and 4-acetoxystyrene Tong Wang, Jinsong YAN, Hailin Yuan, Ho Yi Lam, Chao Lv, Binyang Du, Ophelia Tsui Confinement effect on the dynamics of polymers in nanoscale films is known to cause variations in the glass transition temperature, Tg, of the films with film thickness. We studied the Tg of random copolymers of 4-tert-butylstyrene (4TBS) and 4-acetoxystyrene (4AS). Our result indicates that the component with a stronger substrate confinement effect dominates the Tg of the copolymer films. We have also explored the influence of pre-annealing time on the Tg’s. According to a previous study, by shortening the pre-annealing time, the Tg reductions of thin 4TBS films were enhanced. In our copolymer films, we found that that adjusting the pre-annealing time only affected the Tg of the copolymer films in which 4TBS is the majority component. |
Tuesday, March 5, 2019 1:27PM - 1:39PM |
F54.00010: Theory of interface-nucleated changes of dynamical constraints and their spatial transfer in glass-forming films Kenneth Schweizer, Anh D. Phan We formulate a new force-based microscopic theory for how dynamic caging constraints in glass-forming liquids at an interface are modified and spatially transferred into the film interior in the context of the dynamic free energy concept of the Nonlinear Langevin Equation (NLE) approach. The caging constraints vary exponentially with distance from the interface with a correlation length of modest size and weak sensitivity to thermodynamic state. This imparts a roughly exponential spatial variation of all key features of the dynamic free energy, and a double exponential form for the alpha time gradient. Results have been obtained for vapor, rough pinned particle solid, vibrating (softened) particle solid, and smooth hard wall interfaces, with the crucial dynamical differences arising from the first layer where caging constraints can be weaken, softened or hardly changed. Comparison of the predictions for the dynamic localization length and glassy modulus with simulations and experiments for vapor interface films reveals good agreement. Using the new ideas in Elastically Collective NLE theory yields quantitative results for the relaxation time gradient, decoupling phenomena, Tg gradient and other properties of colloidal, molecular and polymeric films. |
Tuesday, March 5, 2019 1:39PM - 1:51PM |
F54.00011: Heterogenous dynamics, connectivity and domain formation in polymer glasses: a fractal dimension analysis approach Anna Lappala, Dominic Phillips, Jakob Jazbec, Turab Lookman, Karissa Sanbonmatsu Dynamical heterogeneity in glassy systems was first proposed in order to explain non-exponential relaxation patterns in single-component glassy systems, with different relaxation times contributing to produce the observed relaxation. It has been suggested that due to the influence of intramolecular interactions and chain connectivity, there is a fundamental difference between the nature of the glass transition in polymers and in standard glass-formers. Here, we study polymer collapse using fractal dimension (Df) analysis, demonstrating that dynamical arrest upon glass transition affects the evolution of Df in a non-trivial manner. We identify heterogenous dynamics both in bulk and near the free surface, showing characteristic domain patterns in local displacement and connectivity. We demonstrate that although covalent bonding promotes glass formation and caged dynamics, bonding sequentiality that defines a polymer chain is not critical in bulk: glassy dynamics is purely a result of the number of connections per particle, independently of how these connections are formed. However, bonding sequentiality does play an important role in the surface effects of the glass, highlighting a major difference between polymeric and colloidal glasses. |
Tuesday, March 5, 2019 1:51PM - 2:03PM |
F54.00012: Novel calculation method for the work of adhesion of polymer-grafted surface Masayuki Uranagase, Shuji Ogata Molecular simulations are often used to elucidate solid-liquid interfacial properites. In the problem of adhesion, several methods to calculate the work of adhesion, which enables quantitative evaluation of the strength of the adhesion between two materials, have already been proposed. However, application of these methods is restricted to the flat solid surface. It is necessary to improve existing methods for efficient calculation of the work of adhesion for the complex solid surfaces such as polymer-grafted surfaces. |
Tuesday, March 5, 2019 2:03PM - 2:15PM |
F54.00013: Slip Length in Deeply Supercooled Molecular Glass Formers Connor Woods, Gui Gao, Patrick J Walsh, Robert Riggleman, Zahra Fakhraai In polymer melts, the hydrodynamic boundary condition at the substrate interface can deviate from the normally assumed no-slip boundary condition such that fluid velocity at the substrate is non-zero in supported high-molecular weight polymer thin films. Depending on the substrate/polymer interactions, the measured slip lengths can be on the order of the film thickness or larger, greatly influencing film dynamics during dewetting. De Gennes’ original theory for slip in polymers predicts that the slip length should approach the monomer size in molecular liquids. We show that for deeply supercooled molecular glass formers near their Tg, in which chain dynamics cannot play a role in slip, the slip lengths can be surprisingly large and still on the order of the film thickness on a weakly interacting substrate. To better understand the origin of slip in these systems, we investigate the role of molecular shape and orientation near the substrate using a series of molecules that range from rod-like to nearly spherical. We also employ coarse-grained molecular dynamics simulations to gain insight into the origins of the observed slip. |
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