Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S32: Dynamics of Glassy Polymers Under Nanoscale Confinement IIIFocus
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Sponsoring Units: DPOLY DSOFT DCP Chair: Jane Lipson, Dartmouth College Room: 504 |
Thursday, March 5, 2020 11:15AM - 11:27AM |
S32.00001: A Novel Technique for the Characterization of Freestanding Ultrathin Film Mechanics Luke Galuska, Song Zhang, Eric Muckley, Dakota Ehlenberg, Ilia Ivanov, Xiaodan Gu The contribution of nanoconfinement to ultrathin film mechanics has challenged the field for many years. Our previous work utilized a pseudo-free-standing tensile test which relies on water as a support and has successfully been employed throughout many systems. However, the effect of water on the mechanics of nanoconfined films is unclear. Here, we developed a novel truly free-standing (in air) tensile platform to study sub-100 nm thick films. Tensile characterization of stiff glassy polymer such as polystyrene and soft semicrystalline poly(3-hexylthiophene-2,5-diyl) films has been achieved at sub-80 nm thicknesses. We report negligible differences between the moduli obtained from on water and in air measurements, leading us to conclude that water plays an insignificant role despite other factors such as confinement, which is evident in the reduced modulus of PS with decreasing thickness. Our current work is to utilize this newly developed free-standing tensile platform to provide in-situ characterization of ultrathin film deformation mechanics across multiple size scales. |
Thursday, March 5, 2020 11:27AM - 11:39AM |
S32.00002: Chains entanglements and flow within the mobile surface layers of glassy polymers Biao Zuo, Zhiwei Hao, Xinping Wang, Rodney Priestley The existence of a mobile surface layer of several nanometers thick has been established as critical to dynamical deviations in nanostructured polymers. Several questions arise, such as i) How are segments in this region entangled? and ii) How do polymers in this region flow especially when polymer size is larger than thickness of the surface layer? Herein, we design a creep experiment to investigate rheology in this layer. In this experiment, a micro-droplet was placed atop polymer surfaces to induce a nanometer deformation, called a wetting ridge, by the vertical component of surface tension of the droplet. The time evolution of the height of the wetting ridge, reflecting polymer relaxation at different length scales, was monitored. Results show that segments in the very thin surface layer are readily entangled. The terminal relaxation time was scaled by molecular weight (Mw) with a power law of 1.5. We proposed a 2D entanglement and limited flow to explain the phenomena. Within the surface layer, segments were entangled in direction parallel to the surface, but free of entanglements in film thickness direction. Flow of polymer in surface layer was constrained by the glassy underlying bulk, resulting in lower Mw dependence. |
Thursday, March 5, 2020 11:39AM - 11:51AM |
S32.00003: Entanglement Effect on Mechanical Properties of Ultrathin Glassy Polymer Films Cynthia Bukowski, Alfred J Crosby Entanglement density is known to influence the large strain and failure responses of glassy polymer films. For ultrathin films, recent extension measurements show that failure strength decreases severely as film thickness decreases below the average size of an unconfined chain. The hypothesized cause for this effect is the loss of interchain entanglements as polymers statistically interact with themselves more than neighbors when confined. To provide more insight into how entanglements control the failure mechanisms of glassy polymers, especially in ultrathin films, we introduce polymer chains shorter than the entanglement molecular weight to control the entanglement density by effectively swelling the entanglement network. Specifically, we blend short (10.5 kDa or 61.8 kDa) and long (151.5 kDa) polystyrene chains and measure the changes in mechanical properties using The Uniaxial Tensile Tester for UltraThin films (TUTTUT). For 100 nm films, we observe a decrease in yield stress and work to failure with increasing diluent concentration that is diluent length dependent. These results establish the framework of how entanglement density affects mechanical properties of ultrathin polymer films. |
Thursday, March 5, 2020 11:51AM - 12:27PM |
S32.00004: Effect of Confinement on Modulus and Fracture of Thin Conjugated Polymer Films For Organic Electronics Invited Speaker: Xiaodan Gu In this talk, I will discuss our research on understanding the glass transition phenomena and thin-film mechanical property for those rigid conjugated polymers. This class polymers are electronically active and find great potential in future solution-processed electronics. Since those polymers are predominantly used in thin-film devices ( e.g. <100nm), thus confinement effect plays a key role in those thin films. I will discuss our research journey using a thin-film tensile tester to study those novel class of polymers down to 20nm supported on the water surface. Specifically, we found the confinement effect plays a critical role in the modulus and fracture behavior of polymeric materials. I will also discuss the effect of the backbone rigidity on the thin film mechanical property studied by neutron scattering. |
Thursday, March 5, 2020 12:27PM - 12:39PM |
S32.00005: Mechanical Relaxations of Free-standing Polymer Films Hailin YUAN, Ophelia Tsui Young’s modulus of free-standing polystyrene films with thicknesses, h, from 48 nm to 100 μm was studied as a function of temperature and relaxation time. For the h ≤ 115 nm or h ≥ ~3.5 mm films, data could be fit to a single Kohlrausch-Williams-Watts (KWW) function, with the former showing relaxation times shorter than the bulk (denoted ts) and the latter similar to the bulk (tbulk). For the films with thicknesses in between, data could be described by a double KWW function with relaxation times of ts and tbulk, respectively. We found that ts is similar to the fast relaxation time observed in previous optical anisotropy relaxation experiment. However, the apparent thickness found here of the film portion where fast relaxations take place is ~100 times larger. Our result shows that mechanical relaxations are much more susceptible to perturbations by the free surface than molecular relaxations are. Confinement effects may have far bigger impact on the properties of polymers than previously contemplated. |
Thursday, March 5, 2020 12:39PM - 12:51PM |
S32.00006: Effects of Nanoparticles Motion on a Bound Layer in Strongly Attractive Polymer Nanocomposites Hamed Emamy, Francis Starr, Sanat Kumar The attractive interactions between polymers and nanoparticles (NPs) in polymer composites can lead to the formation of a "bound" layer around the nanoparticle (NP) with very slow dynamics. Previous simulations show that this bound layer has a significantly larger relaxation time compared to polymer matrix. However, these studies have been done for a NP that is fixed at its initial position and does not move during simulation (infinitely massive NP). Here, we perform molecular dynamics simulations for polymer composites, where the NP can move in the polymer matrix. We explore the degree to which the NP diffusion affects nanocomposite dynamics. To explain the effects of NP diffusion, we study the dynamical properties of polymer composites, e.g., scattering function, relaxation time. We find that the motion of the NP reduces the bound layer relaxation time significantly. We show that the bound layer diffuses with NPs, and that results in shorter relaxation time. We also study the effects of NP motion on the glass transition temperature. |
Thursday, March 5, 2020 12:51PM - 1:03PM |
S32.00007: Growth of Irreversibly Adsorbed Layers and Corresponding Local Tg Perturbances in Polymer Nanocomposites Katelyn Randazzo, Biao Zuo, Rodney Priestley Annealing a polymer system at temperatures above its Tg can induce the growth of an irreversibly adsorbed layer at the polymer-substrate interface which can impact the system’s bulk properties. Several efforts have been made to explore the implications of irreversibly adsorbed layer growth in thin films, however consideration has not yet been given to polymer nanocomposites, whose high processing temperatures and significant interfacial area may especially favor irreversible adsorption. Herein, we describe the local effects of irreversible adsorption in polystyrene-silica nanocomposites. By combining direct, local techniques such as fluorescence spectroscopy and TEM imaging, we elucidate the relationship between irreversibly adsorbed layer thickness and the corresponding local Tg perturbances, as well as how these parameters evolve with annealing time. The insights provided by this characterization of local interfacial dynamics may inform future endeavors towards the engineering of new and improved polymer nanocomposite materials. |
Thursday, March 5, 2020 1:03PM - 1:15PM |
S32.00008: Comparison of Solution Grown and Film Washed Adsorbed Layers and Their Corresponding Impact on Film Dynamics Connie Roth, Michael F Thees, Jennifer A McGuire, Xinru Huang How the conformations and dynamics of surface bound chains influence neighboring chains are fundamental to adhesion and reinforcement mechanisms in polymer films and nanocomposites. Efforts to investigate adsorbed chains in melt films rely on solvent washing conditions to expose such near-surface chains. However, we find that the adsorbed layer thickness remaining from this protocol is entirely determined by the solvent washing conditions used, with only the time in solution needed to reach this final adsorbed amount affected by prior treatment of the melt film. These observations are consistent with surface diffusion and exchange measurements of adsorbed chains in solution demonstrating that surface bound chains are highly mobile, even for the case of strong adsorption. By directly comparing solution grown adsorbed layers with those formed by solvent washing melt films, we leverage the wealth of information on polymer adsorption in solution developed over several decades to inform us about polymer conformations in the melt. We investigate how these different populations of surface bound chains alter local glass transition properties of neighboring homopolymer chains, and compare them to end-tethered grafted chains. |
Thursday, March 5, 2020 1:15PM - 1:27PM |
S32.00009: Property enhancements and dynamic gradients in P2VP-silica nanocomopsites Tong Wei, John Torkelson A fluorescence technique is used to probe the glass transition temperature (Tg) associated with α-relaxation dynamics at different locations in poly(2-vinylprydine) (P2VP)-silica nanocomposites. The interfacial layer Tg (Tg,inter) is determined using pyrene dye molecule covalently labeled to filler surface. The average matrix Tg (Tg,matrix) is determined using evenly dispersed, free pyrene dye in the polymer matrix. For the first time, we decouple the interfacial dynamics from the the rest of polymer matrix in nanocomposites. In 2.1 kg/mol P2VP nanocomposites containing 0.5 vol% nanoparticle, Tg,inter is 21 °C higher than in neat P2VP Tg, indicating that the α-relaxation dynamics are significantly slowed down due to H-bonding between P2VP and silanol groups on the filler surface. The Tg,matrix is only 3 °C higher than neat P2VP due to the low fraction of interfacial regimes at low filler loading. Both Tg,inter and Tg,matrix increase with increasing filler loading. At same filler dispersion and filler loading, 110 kg/mol P2VP nanocomposites exhibit a much reduced Tg enhancement relative to 2.1 kg/mol P2VP nanocomposites. We hypothesize that 2.1 kg/mol P2VP chains can orderly pack and align at the filler surface, leading to higher H-bonding density and stronger Tg enhancement. |
Thursday, March 5, 2020 1:27PM - 1:39PM |
S32.00010: Polymer conformations and dynamics in polymer nanoparticle composite with high nanoparticle loading Emily Lin, Robert Riggleman Polymer nanoparticle composites (PNC) with high loadings of nanoparticles (>50%) are an interesting class of material for a wide range of applications. Recent experiments have shown that these highly loaded PNC exhibit simultaneously improved strength and stiffness without compromising, sometimes even improving, the toughness compared to neat systems. These PNCs also show a large increase in the polymer glass transition temperature and viscosity, suggesting a slowdown in both segmental and chain-scale dynamics due to confinement. The goal of this work is to understand how confining polymers to the cavities of a random close packed nanoparticle solid affects polymer conformations and dynamics from the segmental to the chain scale. We performed molecular dynamics simulation of both entangled and unentangled coarse-grained polymer equilibrated in the voids of a nanoparticle packing. We varied the polymer fill fraction and compared the dynamics of the polymer in PNC to the pure polymer system, finding that the changes in the dynamics depend on the number of nanoparticles in contact with a polymer. Finally, we deformed the PNC uniaxially to track rearrangement behavior of nanoparticles and polymer segments to provide a molecular view of the toughening mechanism in these materials. |
Thursday, March 5, 2020 1:39PM - 1:51PM |
S32.00011: Direct probing of the fracture behavior for pseudo-free-standing polymeric ultra-thin films Song Zhang, Masato Koizumi, Lihua Jin, Xiaodan Gu The understanding of fracture mechanics of free-standing ultra-thin polymeric films is crucial for modern technology that heavily relies on thin films, including the semiconductor industry and coating. Past decades have witnessed improved understanding of several physical properties of the ultra-thin film, however, their fracture behavior is rarely explored due to limited testing methods on hard-to-handle samples. In this work, we reported a new testing methodology that not only can directly measure the fracture energy of ultra-thin films floated on the water, but also visualize the local stress field during the deformation through the wrinkling pattern of the film. Using polystyrene as a model system, we demonstrated for the first time the existence of a critical molecular weight/thickness, below which the fracture energy increases consistently, and above which levels up. Later, this method was applied to other polymeric thin films for further demonstration of its broad applicability to both stiff and soft materials. |
Thursday, March 5, 2020 1:51PM - 2:03PM |
S32.00012: Glassy, Conjugated Polymer Nanoparticles Formed by a Semiflexible Polymer: A Molecular Dynamic Study Supun Samindra Kamkanam Mohottalalage, Gary Grest, Dvora Perahia Conjugated polymers confined into nano dimensions form long-lived luminescent particles (polydots), with potential as bio markers, where the chain dynamics and conformation determine the photophysics in the confined state. Here we probe the structure and dynamics of semiflexible polymers MEH-PPV confined to their nano dimension, using fully atomistic molecular dynamics (MD) simulations as the temperature is varied. We find that the polymer swells below Tg and Rg increases linearly with temperature. A transition takes place at 605-610 K. This transition is attributed to transformation of glass-like particle to a more dynamic state. The auto correlation function of the backbones of the confined chains, were calculated and analyzed in terms of KWW stretched exponentials, yielding correlation times of ~3.5 ms. This semiflexible polymer remains in its confined state over a broad temperature range. In comparison with polydots formed by rigid polymers, backbone flexibility results in a tighter packing and formation of internal correlations of the side chains. |
Thursday, March 5, 2020 2:03PM - 2:15PM |
S32.00013: On the stability of initiators for surface-initiated controlled radical polymerization Christian Pester The covalent attachment of polymers has emerged as a powerful strategy for the preparation of multi-functional surfaces. Patterned, surface-grafted polymer brushes provide spatial control over a variety of physical properties and allow for fabrication of ‘intelligent’ substrates which selectively adapt to their environment. However, the route towards such patterned polymer brush surfaces often remains challenging, creating a demand for more efficient and less complicated fabrication strategies. Here, we describe recent advances in our group in reduction photolithography to produce topographically and chemically pattern polymer brushes by using light-mediated controlled radical polymerization. We highlight recent work on expanding our technique towards photoinduced electron/energy transfer (PET) reversible addition–fragmentation chain transfer (RAFT) polymerization. We present findings regarding the long-term stability of the surface-grafted initiating sites and discuss reproducibility of SI-PET-RAFT and other polymerization techniques. |
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