Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B34: Focus Session: Dynamics of Glassy Polymers Under Nanoscale Confinement |
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Sponsoring Units: DPOLY Chair: Zahra Fakhraai, University of Pennsylvania Room: 342 |
Monday, March 18, 2013 11:15AM - 11:27AM |
B34.00001: Self-Diffusion of Poly(isobutyl methacrylate) in Thin Films Joshua Katzenstein, Dustin Janes, Haley Hocker, Justin Chandler, Christopher Ellison While relevant to a variety of applications, such as nanocomposite intercalation and molecular transfer printing, the diffusion of polymers parallel to their confining interfaces has received limited experimental attention to date. A refinement of fluorescence recovery after patterned photobleaching (FRAPP) has been developed by our group as a versatile platform for understanding nanoconfined diffusion. Poly(isobutyl methacrylate) (PiBMA) is an ideal candidate for these studies because (in quartz or silicon wafer supported thin films) it exhibits a film thickness independent glass transition temperature (Tg). This is important because, according to the Rouse model, the diffusion coefficient does not depend simply on the absolute temperature, but on the distance from Tg. Therefore, in our systems the origin of the diffusion coefficient is possibly decoupled from Tg changes that are present in other polymer systems. In this talk, the effect of a variety of parameters, such as film thickness, diffusion temperature, and confining interfaces, will be discussed. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B34.00002: Viscosity of poly(methylmethacrylate) films on silicon Ophelia K. Tsui, Ranxing N. Li, Dongdong Peng Previously we showed that the viscosity of polystyrene films on silicon decreased noticeably with decreasing film thickness when the film thickness was decreased below about 10 nm. Moreover, the result could be explained by using a two-layer model presuming a hydrodynamic coupling between a mobile interfacial layer, located at the top, and the remaining, bulk-like layer underneath it. In this experiment, we study the viscosity of poly(methylmethacrylate) (PMMA) films supported by silicon. Contrary to the result found of the polystyrene films, the viscosity of the PMMA films increases with decreasing film thickness. The two-layer model still applies, but the interfacial layer has to be assumed to be slow and located at the substrate interface, beneath the bulk-like layer. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B34.00003: Relaxation of wrinkles: A new viscoelastic metrology Kamil Toga, Narayanan Menon, Thomas Russell The relaxation of a wrinkle pattern can be exploited as a viscoelastic metrology. We used spin-coated polystyrene (PS) films (thickness ranging from 69 to 299 nm) that were floated on the surface of water. Viscoelastic behavior is introduced to the film by depressing the glass transition of PS with a soluble plasticizer, dioctyl phthalate. Wrinkle patterns are formed by placing a small droplet (1$\mu $L) at the center of the floating disc. Due to the differential tension generated across the film, radial wrinkles form around the drop where the compressive axial force buckles the membrane. Thereafter, length of the wrinkles decays, and so does their wavelength. Stress and strain exerted by the droplet can be measured as a function of the size of the wrinkles. Hence, extensional slow-rate-viscosity is calculable. We have studied the relaxation of wrinkles as a function of confinement and plasticizer content. Unusual dynamic behavior due to confinement was observed. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B34.00004: Probing nano-rheology in thin polymer films Invited Speaker: Kari Dalnoki-Veress In this talk I will summarize our recent work on using stepped films to uncover some of the physics relevant to polymer rheology on length scales comparable to the size of polymer molecules. The work presented will focus on the efforts of a larger collaboration (Elie Raphael's theory group in Paris and James Forrest's group in Waterloo). The simple geometry of a polymer film on a substrate with a step at the free surface is unfavourable due to the excess interface induced by the step. Laplace pressure will drive flow within the film which can be studied with optical and atomic force microscopies. Because of the excellent agreement between theory and experiment when we probe ``bulk-like'' properties, these studies provide an opportunity to study how such systems transition from the bulk to confined. Starting with some of the results of levelling experiments on simple stepped films as well as the levelling of polymer droplets on thin films, I will finish with a discussion on our more recent efforts to elucidate confinement effects. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B34.00005: Structural relaxation of thin polymer films Bradley Frieberg, Emmanouil Glynos, Georgios Sakellariou, Peter Green Time-dependent structural relaxations, physical aging, of films with thicknesses in the range of 50 nm to 2 microns, of star-shaped polystyrene (SPS) macromolecules are dependent on film thickness, H. In contrast to linear chain PS (LPS) where the aging rate, R, is independent of molecular weight, M, R is dependent on the functionality, f, and on the molecular weight per arm, Marm for SPS macromolecules. For example, the aging rates decreased 15 percent, for f of 8, and 40 percent, for f of 16, in comparison to that of linear chains, for a given film thickness. The aging rates, R, of the SPS macromolecules moreover are appreciably slower than their linear chain analogs, for a given H. The aging rates of the linear chain and star-shaped polymer films may be reconciled in terms of a model that accounts for changes in the local glass transition of the polymer films as a function of distance from an interface. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B34.00006: Capillary-driven flow as a probe of enhanced surface mobility in glassy polymer films Yu Chai, Thomas Salez, Joshua D. McGraw, Elie Raphael, James A. Forrest We present the use of a novel experimental arrangement [\textit{McGraw et al. Soft Matter }\textbf{\textit{7}}\textit{, 7832 (2011)] }to directly distinguish the dynamical behavior and heterogeneity of polymer thin films above and below the glass transition temperature T$_{\mathrm{g}}$. In particular, by monitoring the capillary-driven evolution of a stepped thin polystyrene film over a temperature range encompassing the bulk T$_{\mathrm{g}}$ value, we find evidence suggesting enhanced surface mobility. Furthermore, by varying the initial aspect ratio of the sample we can examine the heterogeneity of the sample dynamics. The results of these experiments above T$_{\mathrm{g}}$ are consistent with homogenous viscous flow [\textit{McGraw et al. PRL }\textbf{\textit{109}}\textit{, 128303 (2012)],} whereas those below T$_{\mathrm{g}}$ indicate a localization of the flow over a thin surface layer only. We thus develop a linear thin film equation for superficial viscous flows, which is analogous to the surface diffusion model, and for which exact analytical solutions are known and in good agreement with the present experimental data. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B34.00007: Convergence to Self-Similar Regimes in Thin Polymer Films Michael Benzaquen, Thomas Salez, Elie Rapha\"el The surface of a thin liquid film with nonconstant curvature is unstable, as the Laplace pressure drives a flow mediated by viscosity. Recent experiments and theory applied to stepped polymer films have shown excellent agreement and provide a technique for the study of polymer confinement, the glass transition, and slip at the fluid substrate interface to name a few [1]. The thin film equation governs the evolution of the free surface profile in the lubrication approximation. Despite many efforts, this equation remains only partially solved. We present an analytical and numerical study of the thin film equation. Linearising this equation enables us to derive the Green's function of the problem and therefore obtain a complete set of solutions. We show that the solutions of the problem with equilibrium boundary conditions uniformly converge in time towards a first kind self-similar universal attractor. A numerical study enables us to extend our results to the nonlinear thin film equation.\\[4pt] [1] McGraw \textit{et al.} PRL \textbf {109} 128303 (2012). [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B34.00008: The Onset of Plasticity in thin Polymer Films Bekele J. Gurmessa, Andrew B. Croll Polymers are widely used materials because of their numerous advantageous mechanical properties, for example their high degree of toughness. Despite the fundamental importance of the onset of plastic deformation to many material processes, it is still relatively poorly defined in the literature. Here we employ a carefully designed experimental method in order to evaluate the point of onset of plasticity in thin, glassy polystyrene films. Essentially we utilize the residual stress caused by local bending in the thin film. We show that plastic failure is initiated at extremely low strains, of order 0.1\% for polystyrene. Not only is this critical strain small in comparison to bulk measurement, we also show that it is influenced by thin film confinement - leading to an increase in the critical strain for plastic failure as film thickness approaches zero. Finally, the same experimental method is used to investigate the response of confined block copolymer thin films in the ordered and disordered state. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B34.00009: Competitive effects in the dynamics of confined ultra-thin polymer films Chrysostomos Batistakis, Alexey Lyulin, Thijs Michels Fillers, as carbon black or silica, are widely used in polymer systems to improve mechanical properties. In high volume fractions, these fillers connect to each other through polymeric bridges and they create a percolation network inside the polymer matrix. The rigidity of this network depends on the filler volume fraction and is rapidly breaking down under loading. The scope of this work is to understand the polymer dynamical behavior of the interparticle polymeric bridges. For that purpose we have performed molecular-dynamics (MD) simulations on coarse-grained polymer films which are confined between two crystalline Lennard-Jones substrates for different substrate-substrate separations. Various polymer-substrate attraction strengths have been chosen. The polymer structure and segmental dynamics in different film layers has been analyzed. We found that increasing attraction strength leads to deceleration of the film dynamics due to a slowing down close to the substrates, but decrease of film thickness leads to an acceleration of these dynamics. We attribute this acceleration to finite-size scaling effects. For thick films an acceleration of dynamics in the middle takes place for sufficiently high attraction strengths due to the effective increase of the glassy layers thicknesses [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B34.00010: Insight into Polymer De-wetting: A Neutron Reflectivity Study of Three-Arm Polystyrene Stars in Polystyrene Thin films Thusitha Etampawala, Nampueng Pangpaiboon, Dvora Perahia, Candice Halbert, Jim Browning, Nisanart Traiphol, Rakchart Traiphol While polymeric coatings are ubiquitous, de-wetting remains a challenge. As both enthalpic and entropic contributions often affect the de-wetting process, small changes either compositional or in processing conditions are sufficient to impact the stability of thin films. We have recently shown that blending small amounts of three-arm polystyrene (PS) star polymers are sufficient to inhibit de-wetting of thin polystyrene thin films. The role of the three-arm star has been investigated using neutron reflectometry. We have followed the distribution of the three-arm PS stars in a thin film of d-PS as function of time as the temperature was raised above Tg of the PS. Films of d-PS/h-three-arm star PS were cast from toluene and the polymer profiles were determined as a function of time as the temperature was varied. The result show a clear migration of the three-arm stars to both interfaces, enhancing the number of chain ends at the interface. As the molecular weights of the star arm increases, it migrates slower to the interface. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B34.00011: Effects of Solvents on Confinement of Conjugated Polymer into Soft Nanoparticle Naresh Osti, Thusitha Etampawala, Umesh Shrestha, Sidath Wijesinghe, Dvora Perahia Conjugated polymers when collapsed into nano dimension form soft nanoparticles (poly-dots) without losing their electro-optical characteristics. The brightness together with the bio-compatibility of these nanoparticles has shown significant potential in intracellular fluorescence imaging as well as building blocks for light harvesting devices. The conformations of the polymers in the poly-dots are a key to their stability and optical properties. The current work investigates the structure and stability of poly-dots of \textit{di-alkyl} \textit{para polyphenyleneethynylene} (PPE) conjugated polymers in ethylene glycol and in water. Small angle neutron scattering (SANS) studies have shown that PPEs form spherical fuzzy poly-dots both in water and in ethylene glycol. In water, the poly-dots remain fairly stable up to a temperature of 80$^{0}$C. In ethylene glycol however the poly-dots swell with increasing temperature. The structure of the confined structure obtained from SANS is compared with fluorescence spectroscopy results where the intensity of the fluorescence is inversely proportional to the degree of confinement. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B34.00012: Probe of Dynamic Heterogeneity in Freeze-dried Polymer with similarities to thin film Jie Xu, Chao Teng, Gi Xue Understanding the dynamics of polymer chains in confined states is still crucial in the field of soft condensed matter. Dynamic heterogeneity is widespread in the confined system and could strongly alter the overall dynamics, such as in the experimental case of a free surface or a held fixed region. In this work, we show the dynamic heterogeneity in the freeze-dried polystyrene system through a combination of fluorescence nonradiative energy transfer (NRET) method, TMA, and PALS. The NRET data shows that the interchain distance could be altered by other the primitive solution concentration of the freeze-dried PS or the thickness of the free-standing film. Striking similarity of interchain packing density effect on the Tg is find the freeze-dried systems and free-standing films. The application of stress can also make a glass flow. We applied a uniaxial stress on the freeze-dried PS, a shear-induced flow is curried in the region with reduced interchain packing density. The polymer chains in this region show increased segmental mobility, which prompts the shear-induced solid-to-fluid transition to happen well below the bulk glass transition temperature. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B34.00013: Entanglement Density Changes in Free-Standing Thin Polymer Films Joseph Stanzione, Richard Wool The entanglement molecular weight M$_{\mathrm{e}}$ is obtained when a random walk chain crosses a plane three times to form a loop (R.P. Wool '83) such that for polymers with structure --CH$_{\mathrm{2}}$-CHX- where X is the side group, it is found that M$_{\mathrm{e}} =$ 31 C$_{\infty}$ M$_{\mathrm{o}}$/j, where C$_{\infty}$ is the characteristic ratio, M$_{\mathrm{o}}$ is the monomer mol weight and j$=$2 is the number of bonds per monomer. In thin films of thickness d \textless\ 2R$_{\mathrm{g}}$, M$_{\mathrm{e}}$ behaves as M$_{\mathrm{e}}$ $\sim$ d and this behavior is confirmed by computer simulation of random walks in thin films with reflecting boundary conditions. Thus, the entanglement density v $\sim$ 1/M$_{\mathrm{e}}$ increases as d decreases and rheological properties such as plateau modulus change as G$_{{\mathrm{N}}^{\mathrm{o}}}$ $\sim$ 1/d and plateau creep compliance J$_{\mathrm{o}}$ $\sim$ d. The mechanical stiffening of thin films is in accord with recent experiments of McKenna et al (2012). The results are also in accord with the Packing model (Lin, Kavassil, Fetters 1983) where M$_{\mathrm{e}} =$ 354 p$^3$ in which p $=$ M$_{\mathrm{o}}$ j/[C$_{\infty}$ b$_{\mathrm{o}}^2$]. The packing model is exactly derived from the Wool entanglement model for these polymers since C$_{\infty} =$ 1.36 [M$_{\mathrm{o}}$/j]$^{\mathrm{1/2}}$. The empirical packing model with its excellent data correlation M$_{\mathrm{e}}$ $\sim$ p$^3$ has been misinterpreted by many and such suggestions that v decreases due to nanoconfinement and G$_{{\mathrm{N}}^{\mathrm{o}}}$ $\sim$ d are incorrect. [Preview Abstract] |
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