Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B55: Confined Polymer Glasses II: Mobility GradientsFocus
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Sponsoring Units: DPOLY GSNP Chair: Simone Napolitano, Univ Libre De Brussels Room: LACC 515A |
Monday, March 5, 2018 11:15AM - 11:27AM |
B55.00001: Average and Local Tg Shifts in Plasticized PVC from Simulations Yuxing Zhou, Scott Milner The effects of small molecules on the dynamics of polymer/plasticizer blends are of high engineering importance and theoretical interests. Although it is well known that plasticisers can alter the glass transition of polymers, the interplay between polymers and plasticizers on the molecular scale and the resulting Tg shifts are not fully understood. In this work, we perform simulations on di-(2-ethylhexyl) phthalate (DEHP) plasticized polyvinyl chloride (PVC) using OPLS-AA force field. After validating the model by comparing simulated thermodynamic and mechanical properties with experimental values, we obtain both average and local Tg shifts from short-time segmental dynamics by collapsing the data. We find the average Tg determined from fast dynamics decreases substantially with increasing plasticizer concentration, consistent with volumetric Tg referred from the temperature dependence of density. Moreover, the local Tg reduction of PVC decays exponentially as a function of the distance from the nearest DEHP. Interestingly, for all the concentrations studied, the local Tg reduction can be described by a simple additive rule—it is approximately the sum of the distance-dependent local Tg shifts arising from surrounding plasticizers in the dilute limit. |
Monday, March 5, 2018 11:27AM - 11:39AM |
B55.00002: Impact of Changing PDMS Modulus on the Local Glass Transition Temperature Tg(z) of Neighboring Polystyrene Yannic Gagnon, Connie Roth Our group recently characterized the local glass transition temperature profile Tg(z) of polystyrene (PS) vs. distance from the interface of PS with a higher Tg polymer (PSF and PMMA) or a lower Tg polymer (PnBMA and PiBMA) [J Chem Phys 2017, 146, 203307]. For PS next to an interface with a lower Tg material ("soft confinement") a distance of z ≈ 225-250 nm away from the interface was needed for PS to recover its bulk Tg, while z ≈ 100-125 nm for an interface with a higher Tg material ("hard confinement"). This difference between hard and soft interfaces, along with recent theoretical efforts in the field, motivated us to investigate a system where the modulus could be varied without also changing chemical structure. Here we present measurements of Tg(z) in PS next to cross-linked polydimethylsiloxane (PDMS). By varying the ratio of cross-linker to base monomer, we varied the cross-link density and hence modulus of PDMS. We find that the Tg(z) in PS at a distance of z = 50 nm away from the PS/PDMS interface can vary by more than 40 K when the PDMS modulus varies from ~1 to 3 MPa. Interestingly, the length scale z ≈ 70-90 nm at which bulk Tg(z) of PS is recovered is significantly shorter than what would be expected for this soft neighboring domain based on our previous results. |
Monday, March 5, 2018 11:39AM - 11:51AM |
B55.00003: Influence of Rough and End-Tethered Substrates on the Local Glass Transition Temperature Tg(z) of Polystyrene Xinru Huang, Connie Roth Modifying interfacial interactions are an efficient way to tune polymer properties, however, the mechanism of how a given interface perturbs local material properties is poorly understood, with efforts often complicated by competing factors. We have recently studied the local glass transition temperature profile Tg(z) in polystyrene (PS) near end-tethered substrates, finding a maximum local Tg increase of 50 K next to the substrate at an optimum grafting density of σ = 0.011 chains/nm2, coinciding with the "mushroom-to-brush" crossover regime. This optimum grafting density also corresponds to when the Tg(z) perturbation extends the furthest from the interface, comparable to our group's recent work on PS next to a higher-Tg polymer such as polysulfone. More recently, we have investigated the local Tg in PS next to silica substrates with varying roughness, using hydrogen fluoride vapor to etch silica without changing the surface chemistry. We find a local Tg increase of ~10 K with increasing RMS roughness from 0.5 to 28 nm, consistent with predictions from computer simulations. Interface roughness has been heavily studied in computer simulations where the specific details of the substrate must be constructed at its most basic level, but virtually unexplored experimentally. |
Monday, March 5, 2018 11:51AM - 12:27PM |
B55.00004: Think globally, act locally: How interfaces can alter mobility and glassiness Invited Speaker: Jane E Lipson This talk will focus on links between changes in local mobility and free volume as interfaces are introduced, and how those changes translate into altering dynamic relaxation and the glass transition. We have recently shown that the Cooperative Free Volume (CFV) theory can resolve bulk dynamic relaxation data over a broad temperature and pressure range, including the span from Arrhenius to non-Arrhenius behaviour. An extension to thin films will be introduced and applied to experimental dielectric relaxation data. Turning to simulation, we have previously demonstrated that the Limited Mobility (LM) model successfully predicts the impact of one or two free surfaces on the glass transition of a thin film, as well as the asymmetric effect on local mobility of an interface between two materials having different glass transition temperatures. We now turn to the modeling of interfacial effects caused by introducing additives into a bulk sample. Results will be presented linking the un/favourability of very short-range polymer-additive interactions to sample-wide changes in glassiness and local mobility. Close connections between LM predictions and experimental results will also be discussed. |
Monday, March 5, 2018 12:27PM - 12:39PM |
B55.00005: Effect of Polymer-Polymer Interface on Polymer Chain Dynamics Sriramvignesh Mani, Rafikul Islam, Rajesh Khare Polymer membrane based separation techniques offer promise as energy efficient alternatives to distillation. In practice, thin membranes of polymers of interest are supported on another polymeric substrate for mechanical stability. It is thus important to decipher the effects of the polymeric support on the chain dynamics of the supported polymer. |
Monday, March 5, 2018 12:39PM - 12:51PM |
B55.00006: Influence of Segregation Strength on the Local Glass Transition of Lamellar Diblock Copolymers Dane Christie, Richard Register, Rodney Priestley By selectively incorporating a pyrene-labeled monomer into the hard block of a model rubbery/hard block copolymer, forming a self-assembled microdomain structure, local measurements of the glass transition temperature (Tg) are performed as a function of segregation strength. The temperature dependence of the fluorescence emission of pyrene enables the determination of the location-specific Tg. The average position of a pyrene segment relative to the microdomain interface is estimated via fluctuation-corrected self-consistent field theory (SCFT) composition profiles. At weak and intermediate segregation strengths, lamellar poly(butyl methacrylate -b- methyl methacrylate) diblock copolymers, PBMA-PMMA, show gradients in Tg vs. this average label position of 22 K/nm and 14 K/nm, respectively, from the microdomain interface into the PMMA-rich domain. The role of the relative softness of the confining and confined blocks is demonstrated by a comparison of the observed Tg to values calculated based the Lodge-McLeish model of self-concentration. At the domain interface, labeled PMMA segments at both segregation strengths exhibit a 30 K depression in Tg relative to the calculated value. |
Monday, March 5, 2018 12:51PM - 1:03PM |
B55.00007: Stiffness Gradients in Supported Thin Polymer Films and in Polymer Model Nanocomposites: Evaluation via a Novel Fluorescence Method with Comparison to AFM Results John Torkelson, Shadid Askar, Min Zhang, L. Catherine Brinson The issues of how significantly and over what length scales stiffness or modulus is modified by a substrate or nanoparticle interface is important in the performance of polymer nanocomposites and thin films. We provide the first comparison of stiffness gradient length scales in polymers characterized by two methods: fluorescence and AFM. Bilayer film studies show that stiffness is enhanced near a substrate and reduced near a free surface. Fluorescence/trilayer experiments and AFM agree that stiffness gradients in thin polystyrene (PS) model nancomposites, with PS sandwiched between two substrates, depend on whether the PS layer is bulk or nanoconfined. With bulk layers, room-temperature stiffness-confinement effects extend about ~80 nm from a substrate interface. With 266 nm thick PS layers, perturbations to stiffness extend ~200 nm from each interface in a nonlinear compound effect. A small stiffness enhancement is observed at the midpoint of a 266 nm model nanocomposite; in a 60 nm thick model nanocomposite, the midpoint modulus is 50% higher than bulk. Thermal history and the presence of plasticizer are also important factors in tuning the magnitudes and length scales of stiffness-confinement effects. |
Monday, March 5, 2018 1:03PM - 1:15PM |
B55.00008: Theory of spatially heterogeneous activated glassy relaxation in supported thin films Anh Phan, Kenneth Schweizer Glassy dynamics in supported thin polymer films is investigated using the Elastically Collective Nonlinear Langevin Equation theory under neutral confinement and sharp interface conditions where there is no change of equilibrium properties. In this theoretical framework, the alpha process is a mixed local-nonlocal event determined by large amplitude cage scale barrier hopping coupled with facilitating spontaneous elastic fluctuations of the surrounding particles. Both aspects are strongly modified under confinement in distinctive manners which depend on temperature, density, film thickness and the nature of the interfaces or substrates. Large enhancements of relaxation at the vapor interface and well into the film are predicted to coexist with substrate-stiffness-dependent suppression of motion as the solid surface is approached resulting in massive mobility gradients across the film. Bilayers with one interface have also been studied. Asymmetric mobility gradients and Tg profiles are predicted which depend on the absolute and relative stiffness of the two materials. Calculations of relaxation time and Tg gradients for model hard sphere and polymeric systems, and comparison with experimental measurements, will be presented. |
Monday, March 5, 2018 1:15PM - 1:27PM |
B55.00009: Glass Transition Temperature of Nanometer Ultra-thin Polymer Films Gerardo Mendoza, Xiaoyu Wei, Dmitry Bedrov, Arlette Baljon, Alexey Lyulin United-atom molecular-dynamic simulations were performed to study the nature of the glass transition(Tg) in free-standing polymeric films. Experimentally it has been found that the glass transition dependence on film thickness varies considerably with the polymeric chemistry. We are investigating linear and cyclic polystyrene(PS) as well as poly(alpha-methylstyrene). These are polymers with very similar monomers, yet they behave differently when cooled towards Tg. Both static and dynamic data will be presented. We find that for linear PS a larger number of end groups inhabit the interfacial layer rather than the middle of the film. This increases mobility and yet it does not explain the Tg dependence on film thickness observed for cyclic polymers. Hence, we hypothesized* that the observed deficit of phenyl groups in the interfacial film layers, which weakens the phenyl-phenyl aromatic(π-π) interactions, contributes to the lower Tg for thinner films of PS. In addition, local orientation mobility of the phenyl bonds is studied with the help of Legendre polynomials of the second order. |
Monday, March 5, 2018 1:27PM - 1:39PM |
B55.00010: Glassy dynamics in one- and two-dimensional nanometric confinement - a comparision Friedrich Kremer, Wycliffe K. Kipnusu Glassy dynamics of polymers (e.g. poly(methylphenylsiloxane) (PMPS) or poly(cis1,4-isoprene) (PI) ) is studied by broadband dielectric spectroscopy (BDS) in one-(1D)- and two-(2D)-dimensional nanometric confinement; the former is realized in thin layers having thicknesses down to 5 nm, the latter in unidirectional (length 50 m) nanopores with diameters varying between 4–8 nm. Based on dielectric measurements carried out in a broad spectral range at widely varying temperatures glassy dynamics is analyzed in detail in 1D and in 2D confinement. All the findings can be comprehended by considering the density of the polymer; in 1D it is assumed to be the same as in the bulk, hence the dynamic glass transition is not altered, in 2D it is reduced due to a frustration of packaging resulting in a higher free volume, as proven by ortho-positronium annihilation lifetime spectroscopy. |
Monday, March 5, 2018 1:39PM - 1:51PM |
B55.00011: Characterization of PS and PMMA Chain Mobility Confined in AAO Xiaoliang Wang, Chen Zhang, Chao Teng, Gi Xue Understanding and controlling the glass transition temperature (Tg) and dynamics of polymers in confined geometries are of significance in both academia and industry. Polystyrene (PS) and poly(methyl methacrylate) (PMMA) nanofibers confined in AAO templates are investigated. At fast cooling rates, the imparted thermal stress would overcome the yield stress of polymer and peel chains off the pore walls, while at slow cooling rates, chains are kept in contact with the pore walls due to timely dissipation of the produced thermal stress during vitrification. Results of differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS) clearly identified two glass transition temperatures during an ultraslow cooling process (0.1K/min) across the Tg. A dramatic change in the glass transition behaviors is detected when the cooling rate is changed from 40 to 0.1 K/min, which reflects the inherent evolution between the shell and the core through a non-equilibrium interlayer. Our findings provide insight into achieving stable glassy polymer structures confined in nanopores by balancing the geometric curvature, interfacial interactions and cooling rate. |
Monday, March 5, 2018 1:51PM - 2:03PM |
B55.00012: Suppressed Tg-Confinement Effects in Thin Films of Comb and Branched Polystyrene Relative to Linear Polystyrene Lingqiao Li, Xi Chen, Kailong Jin, John Torkelson The effects of nanoscale confinement on the glass transition temperature, Tg, of comb and branched polystyrene (PS) are compared to those of linear PS. Comb and branched PS are synthesized using alkoxyamine dynamic chemistry. Upon appropriate stimulus, this dynamic chemistry allows the comb and branched PS to revert to its linear components, thus allowing for direct comparison. Using thin films of PS on silica substrates, Tg reductions are characterized via ellipsometry measurements of both comb and branched PS films, with confinement effects being suppressed in non-linear PS compared to their linear components, respectively. The reduced magnitude of Tg reduction observed in confined comb and branched PS films may be attributed to the effect of chain architecture (on the underlying fragility-confinement effect) and possibly to polymer-substrate interaction. Direct comparison of fragility-confinement effects in thin films of comb and branched PS thin films is also being made to effects in thin films of linear PS. |
Monday, March 5, 2018 2:03PM - 2:15PM |
B55.00013: The Effect of Extreme Spatial Confinement and Interfacial Interactions on the Glass Transition of Polymers in Polymer-infiltrated Nanoparticle Packings Haonan Wang, Jyo Lyn Hor, Daeyeon Lee, Zahra Fakhraai Chain and segmental confinement of polymer and polymer-particle interfacial interactions in polymer nanocomposites have been known to cause deviation in polymer properties relative to the bulk. Capillary Rise Infiltration (CaRI) enables polymer infiltration into nanoparticle (NP) packings, confining polymer within the small pores and increasing the interfacial area. In this study, we investigate the influence of spatial confinement and interfacial interactions on the glass transition temperature (Tg) of polymers in SiO2 NP packings. The degree of confinement is tuned by varying the polymer molecular weight as well as the size of NPs (11~100 nm), producing 3~30 nm pore diameter. Interfacial interaction is controlled by comparing polystyrene (PS) and poly(2-vinylpyridine) (P2VP), which have different interactions with SiO2. We show that small pore size is a major factor that significantly increases Tg of polymers in these NP packings. Stronger interfacial interaction further increases Tg in highly confined conditions. For example, for 8 kg/mol polymers, in 11 nm NP packings, Tg of PS and P2VP increases by 50 K and 93 K, respectively, while Tg is close to bulk in 100 nm NP packings for both polymers. |
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