Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J32: Dynamics of Glassy Polymers Under Nanoscale Confinement IFocus
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Sponsoring Units: DPOLY DSOFT DCP Chair: Robert Riggleman, University of Pennsylvania Room: 504 |
Tuesday, March 3, 2020 2:30PM - 3:06PM |
J32.00001: BREAK
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Tuesday, March 3, 2020 3:06PM - 3:42PM |
J32.00002: Mobility Gradient of Polymer Chains near a Solid Interface Invited Speaker: Keiji Tanaka Polymer composites have been widely used in a variety of engineering fields. The performance and functionality of the composites are closely related to the quality of the interface between polymer and filler. Thus, it is important to study polymer behavior at the filler interface. The local conformation of rubbery chains in direct contact with a quartz substrate was here examined by interface-sensitive sum-frequency generation (SFG) spectroscopy. SFG signals, which could be obtained from functional groups only oriented at the interface, were clearly observed for the rubber polymer in a film at room temperature which was much higher than the bulk glass transition temperature (Tg). When the film was thermally annealed, rubbery chains at the quartz interface changed their conformation to one with a lower energy state, accompanied by the randomization of both the main and side chain parts. The characteristic temperature, at which interfacial chains started to lose their orientations, was much higher than the bulk Tg. Also, the extent found to be more remarkable for the spin-coated film than for the solvent-cast one. This implies that the stress accumulated at the interface, which resulted from the centrifugal force during the spin-coating process, accelerates the mobility of chains there. Then, the relaxation dynamics of rubbery chains at the quartz interface were directly probed as a function of distance from the quartz surface using time-resolved evanescent wave-induced fluorescence anisotropy (TRFA), dielectric relaxation spectroscopy (DRS) and SFG spectroscopy. We found the presence of the dynamics gradient of chains in the interfacial region with the quartz surface. The segmental relaxation of chains in the strongly adsorbed layer at the interface could be slower than that of bulk chains by more than 10 orders. |
Tuesday, March 3, 2020 3:42PM - 3:54PM |
J32.00003: Direct observation of mobility of thin polymer layers via asymmetric interdiffusion using neutron reflectivity measurements Koji Fukao, Megumi Ooe, Kairi Miyata, Jun Yoshioka, Norifumi L. Yamada In this study, we investigated the diffusion dynamics at the interface between deuterated poly(methyl methacrylate) (d-PMMA) and protonated PMMA (h-PMMA) in two-layered thin films of d- and h-PMMA layers via neutron reflectivity (NR) measurements during annealing above the glass transition temperature Tg. When Tg of d-PMMA was higher than that of h-PMMA, the d-PMMA layer thickness increased with annealing time ta, and simultaneously the h-PMMA layer thickness decreased. However, the opposite ta dependence of the layer thicknesses was observed, if the Tg of d-PMMA was decreased. This change in the ta dependence of the layer thickness was related to the change in the mobility of the d-PMMA layer. With the decrease in the d-PMMA layer thickness when the h-PMMA layer thickness was maintained, the ta dependence of the layer thickness changed, and the mobility of the d-PMMA layer dramatically increased. Hence, the Tg of the d-PMMA layer decreased with the decrease in the d-PMMA layer thickness. These results suggest that the mobility and Tg of thin polymer films can be determined by interfacial dynamics via NR measurements. |
Tuesday, March 3, 2020 3:54PM - 4:06PM |
J32.00004: Reconciling Computational and Experimental Trends in the Temperature Dependence of Interfacial Mobility in Polymer Films Jack Douglas, Wengang Zhang, Francis Starr Many measurements have indicated that thin supported polymer films in their glass state exhibit an interfacial layer of enhanced mobility whose thickness grows upon heating, as found also in crystalline materials approaching their melting temperature,Tm, while simulations and limited measurements of such films above their glass transition tempearture Tg instead exhibit a region of enhanced mobility whose thickness ξ grows upon cooling. To better understand these contradictory trends, we performed MD simulations over a T range over which our simulated polymer films enters a glassy state, and found that the relaxation time τα within the film interior, relative to the polymer-air interfacial region, exhibits a maximum near the observed computational glass transition temperature, Tg,c, reconciling previous measurements and simulations of supported polymer films in their glass and liquid states. Correspondingly, we also observe that the interfacial mobility scale exhibits a maximum near Tg,c, but the scale of collective polymer segment exchange motion increases monotonically upon cooling below Tg,c so that the interfacial mobility scale is no longer linked to the scale of collective motion in the non-equilibrium glass state. |
Tuesday, March 3, 2020 4:06PM - 4:18PM |
J32.00005: Ellipsometry Modeling with Gradient in Refractive Index Resolves Unrealistic Density Increases in Thin Polymer Films and Demonstrates Inhomogeneous Film Structure Decoupled from Dynamics Yixuan Han, Connie Roth We show polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2-vinylpyridine) (P2VP) all exhibit similar large increases in refractive index with decreasing film thickness for ellipsometry data modeled by a homogeneous Cauchy layer. Such increases in refractive index have been interpreted by others recently as large increases in density (~25%) via the Lorentz-Lorenz relation, despite being physically unrealistic. We demonstrate that an ellipsometric layer model with a gradient in refractive index provides more physically realistic parameters for very thin films, retaining a bulk-like refractive index for much of the film’s interior, while exhibiting a sharp gradient in index near the free surface whose breadth grows from 4 to 9 nm with decreasing film thickness from 50 to 30 nm. Such an inhomogeneous film structure would invalidate the use of homogeneous Cauchy layers and the Lorentz-Lorenz relation whose derivation is based on assuming an isotropic distribution of dipoles. Surprisingly the refractive index gradient has an opposite trend to that based on a simple density correlation to dynamics. A higher refractive index near the free surface could reflect more optimized molecular packing from surface mobility, similar to vapor deposited stable molecular glasses. |
Tuesday, March 3, 2020 4:18PM - 4:30PM |
J32.00006: Density Measurements of Thin Polymeric Films using Magnetic Levitation Samuel Root, Rui Gao, Shencheng Ge, George M. Whitesides When a polymeric film—supported by a sacrificial release layer—is placed in a paramagnetic solution between two magnets, it lifts off and levitates to a height that is linearly dependent upon the mass density. Using the principle of magnetic levitation, we construct a simple metrology for directly measuring the mass density of polymeric films down to 3 nm thick--an important quantity for understanding the glassy dynamics of such films. We validate the technique with several polymer films with a range of densities (1-1.4 g cm-3) and by measuring density variations as a function of thickness in polystyrene and polymethylmethacrylate films. Our results are in agreement with existing data obtained by reflectometry experiments and confirm the trends previously observed in PS and PMMA, with important implications for understanding the dynamics of such glassy systems. Moreover, we demonstrate the potential utility of our technique for studying crystallization in ultrathin films of polymers using polyethylene as a model system. MagLev of thin polymeric films provides a simple, accessible metrology that will be useful for polymer scientists and engineers to study and optimize the properties of thin films for electronics, barriers, and separation membranes. |
Tuesday, March 3, 2020 4:30PM - 4:42PM |
J32.00007: Polymeric Liquid Layer Densified by Surface Acoustic Wave Tianhao Hou, Jingfa Yang, Wen Wang, Jiang Zhao With the application of surface acoustic wave (SAW) of 39.5 MHz to a model polymer liquid film, polyisobutylene, deposited on the solid substrates, the liquid film is densified, proved by the decrease of film thickness and the increase of refractive index, measured by ellipsometry. Rotational motion of fluorescent probes doped inside the liquid film, measured by polarization-resolved single molecule fluorescence microscopy, is retarded and the dynamical heterogeneity is reduced. It is demonstrated that the application of SAW of high frequency makes the thin polymeric liquid film densified and more dynamically homogeneous. |
Tuesday, March 3, 2020 4:42PM - 4:54PM |
J32.00008: Investigating molecular origins of mechanical stress during deformation of polymer glasses: in-situ birefringence measurements DA HUANG, Masoud Razavi, Shiqing Wang In absence of microscopic experimental tools to investigate molecular deformation due to macroscopic deformation of glassy polymers, we monitor in real time the emergence of birefringence at different temperatures during ongoing uniaxial tensile deformation, stress relaxation and creep. By comparing the stress-optical behavior among PS, PMMA and bisphenol A PC where the optical anisotropy arises from either side group (PS) or backbone (PMMA and PC), we can learn about the intermolecular and intramolecular contributions to mechanical stress and obtain more insights into the stress-optical relationship identified in the mechanical deformation of these glassy polymers. |
Tuesday, March 3, 2020 4:54PM - 5:06PM |
J32.00009: Light-facilitated dewetting in amorphous selenium thin films Aixi Zhang, Danixa Rodriguez, Richard B Stephens, Zahra Fakhraai Properties of glass thin films deviate from bulk for various materials, such as polymers, molecular glasses, and metallic glasses. When the film thickness is below a certain threshold, fast surface dynamics plays a strong role. Here, we study dewetting and glass transition temperature (Tg) of amorphous selenium (a-Se) thin films. A unique property of a-Se is that it forms random chain length and exhibits structural flexibility through bond breakage and formation of new bonds or over-coordinated sites upon above bandgap light irradiation. We investigate light-induced fluidity in a-Se and its effect on the length scale and the magnitude of enhancement in surface dynamics. Isothermal dewetting experiments are performed on a-Se films of different thicknesses below and above Tg. The hole-growth rate satisfies non-slip dewetting conditions. Thus, the growth velocity is directly related to the film's viscosity. Both the viscosity and activation energy in thin films are enhanced compared to the bulk. Films that start to deviate from bulk can be as thick as 100 nm, while this length scale turned out to be about 30 nm in most organic glasses. By applying various illumination condition, we study the relationship between the film structure and enhanced surface dynamics. |
Tuesday, March 3, 2020 5:06PM - 5:18PM |
J32.00010: Geometry-Dictated Wrinkle Patterns in Vapor-Deposited Thin Films on Liquid Substrates Robert Enright, Laura Bradley Surface patterns and wrinkles in thin films are of fundamental and technological interest for their use in tuning morphological, mechanical, optical, and surface properties. Here, we use initiated chemical vapor deposition (iCVD) to generate polymer films with controlled wrinkle patterns and geometries. The iCVD technique is a solvent-free method to produce functional polymer coatings by free-radical polymerization of surface adsorbed monomers. Previously, iCVD has been used to produce parallel and herringbone wrinkles by deposition onto pre-strained elastomers and random wrinkles by deposition onto compliant amorphous substrates. We examine wrinkle formation in thin films on liquid substrates. The resulting wrinkle patterns are dictated by the topography and boundaries of the liquid surface pinned in micropost arrays with various post sizes, spacings, and shapes. Properties of the wrinkled films also depend on the iCVD deposition conditions. This presentation will demonstrate control of the wrinkle patterns and elucidate the mechanism of wrinkle formation in thin films grown on liquid surfaces. |
Tuesday, March 3, 2020 5:18PM - 5:30PM |
J32.00011: In situ Molecular Aggregation Structure Analyses on Glassy Polymers during Mechanical Deformation Ken Kojio, Aya Fujimoto, Tomoko Kajiwara, Chao-Hung Cheng, Shiori Masuda, Nattanee Dechnarong, Kento Fukada, Kazutaka Kamitani, Atsushi Takahara Poly (methyl methacrylate) (PMMA) and polycarbonate (PC) were chosen as a glassy polymer. Molecular aggregation structure and stretching properties of these glassy films were investigated by polarized high-speed camera observation and synchrotron X-ray wide-angle and small-angle scattering techniques during uniaxial and bulge deformation processes. |
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