Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session P04: Mechanics and Dynamics of Polymers Under ConfinementFocus Live
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Sponsoring Units: DPOLY DSOFT Chair: Robert Riggleman, University of Pennsylvania |
Wednesday, March 17, 2021 3:00PM - 3:12PM Live |
P04.00001: What controls failure in ultrathin glassy polymer films? R. Konane Bay, Alfred Crosby As polymer glasses are processed into ultrathin films, neighboring polymer chains become less entangled, and surface-bound chains with altered states of mobility play an increasingly important role. Such changes in physical properties have long been studied, but changes in mechanical strength and deformation processes have remained difficult to quantify. We have developed a method to directly measure the uniaxial stress-strain response of ultrathin glassy polymer films. Here, we quantify the influence of thickness (10nm-360nm), and molecular weight across a broad range (61kDa-2135kDa) on the deformation and failure response of ultrathin polystyrene films. We observe a molecular weight independent thickness-transition in strain localization and a molecular weight dependent decrease in maximum stress. We develop a model that provides new fundamental insights into how polymer behavior is altered due to changes in the entanglements and mobility in a polymer network upon dimensional confinement. |
Wednesday, March 17, 2021 3:12PM - 3:24PM Live |
P04.00002: Strain Rate and Thickness Dependences of Elastic Modulus of Free-standing Polymer Nanometer Films Hailin YUAN, Pak Man Yiu, Qiao Gu, Ping Gao, Ophelia K.C Tsui Elastic moduli, E, of free-standing polystyrene (PS) single-layers and polystyrene-polydimethylsiloxane (PS-PDMS) bilayers are measured by uniaxial tensile testing at room temperature under different strain rates, ý, and for PS thicknesses, h, from 8 to 130 nm. As ý increases, E increases initially, then approaches the bulk value, Ebulk, when ý exceeds a characteristic value (τ-1) that decreases with increasing h. The noted variation of E with ý shows that stress relaxation occurs in the films during measurement when ýτ << 1, while the noted variation of τ-1 with h shows that thinner films relax faster. Consequently, E decreases with decreasing h if ý is small, but displays independence of h if ý is large. Visually, the crossover takes place at around ý = 0.0015 s-1, whereat ýτ > 1 for all films. |
Wednesday, March 17, 2021 3:24PM - 4:00PM Live |
P04.00003: Alteration of mechanical properties in polymer films under nanoconfinement Invited Speaker: Yunlong Guo Mastering mechanical properties of nanoconfined polymers is highly demanded yet remains challenging. Pioneering advances have determined Young's modulus (E) of thin polymer films, however, the results showed diverse trends, i.e., E can increase or decrease with reducing film thickness. In this talk, we show that nanoconfinement-induced chain conformations, and entropy-driven (above Tg) or energetic-driven deformations (below Tg) may account for the controversial results. We also show stiffening of PDMS films when lowering film thickness, and alteration of the ratio between elastic and viscous response in ultrathin PDMS films when changing external mechanical loading. |
Wednesday, March 17, 2021 4:00PM - 4:12PM Live |
P04.00004: Direct measurement of stress relaxation in polymeric thin films Guorong Ma, Luke Galuska, Song Zhang, Xiaodan Gu Understanding the stress relaxation in polymeric thin films is critical for the field of deformable and wearable electronics. In the past decade, numerous high-performance semiconducting polymers have been synthesized, while their fundamental relaxation behaviors under external stress are less concerned. This is mainly due to the lack of materials and limited testing methods. In this work, we report the direct measurement of stress relaxation in polymer thin films through both film-on-water and free-standing tensile tests. The effects of film thickness and experimental temperature are investigated on polymers with different glass transition temperatures (Tg). We found that thin-film relaxation follows an Arrhenius-type temperature dependence and accelerates as film thickness decreases. Significant depression of compensation temperature is observed in thin-films due to the acceleration effect of water on thin glassy film dynamics. Meanwhile, the water is found to depress the chain dynamics for films floated on water versus free-standing ones. |
Wednesday, March 17, 2021 4:12PM - 4:24PM Live |
P04.00005: Connecting Morphology to Mechanical Response in Thin Block Copolymer Films Cynthia Bukowski, Tianren Zhang, Robert Riggleman, Alfred J. Crosby Understanding the mechanics of polymer thin films is critical to enhancing technologies including filtration membranes and multilayer packaging. While it is known that the mechanical response of thin glassy films is defined by polymer entanglements and molecular mobility, there remains a need to connect entanglement structure, mobility changes, and morphology to mechanical strength near surfaces. To investigate the connection, poly(styrene-b-2-vinylpyridine) is used as a model polymer where each block has similar properties and the processing methods for desired self-assembly are known. With this chosen system, we can offer direct insight into the structure-property relationships. Using a newly developed method, TUFF (Tensile tester for Ultrathin Freestanding Films), we measure the full uniaxial stress-strain response of freestanding poly(styrene-b-2-vinylpyridine) films below 100 nm in thickness. We characterize the nanoscale morphology, quantify the elastic modulus, maximum stress, and failure strain, and compare these results to homopolymer films of polystyrene. Our study connects the film morphology with the mechanical response and relates these behaviors to the fundamental polymer physics at the film’s interface. |
Wednesday, March 17, 2021 4:24PM - 4:36PM Live |
P04.00006: Role of Water on Correlated Structural and Mechanical Response of Water-Supported and Free-Standing Ultrathin Films Luke Galuska, Eric Muckley, Zhiqiang Cao, Dakota Ehlenberg, Zhiyuan Qian, Song Zhang, simon Rondeau-Gagne, Minh Phan, John F Ankner, Ilia Ivanov, Xiaodan Gu Glass transition phenomena of confined polymer films have been heavily investigated for the last two decades, yet the mechanical properties of such sub-100 nm thin films have remained elusive until much more recently. The film on water (FOW) tensile test has enabled characterization of thin polymer films, however, the influence of water on the mechanics of confined films remains unclear. Here, we developed a free-standing (in air) tensile platform to study sub-100 nm thin films and compare the mechanical properties obtained to that of the FOW technique thereby elucidating the influence of the interface on the mechanics of these thin films. Characterization of stiff glassy polystyrene and soft semicrystalline poly(3-hexylthiophene) films has been achieved down to 15 and 80 nm respectively. We observed minimal difference in moduli obtained from FOW and in air measurements, while yield stress and crack-onset-strain were greater for FOW. The influence of water was further explored through quartz crystal microbalance and neutron reflectometry, which indicate a water content as high as 13.6 %. Overall, this work establishes a novel technique for measuring the mechanical properties of free-standing thin films and provides an in-depth analysis of the role of the polymer-water interface. |
Wednesday, March 17, 2021 4:36PM - 4:48PM Live |
P04.00007: Chain Entropy and Polymerization Thermodynamics: Quantifying Nanoconfinement Effects Qian Tian, Chunhao Zhai, Sindee L Simon Polymer chains lose entropy upon nanoconfinement. Here, we exploit the changes in the limiting conversion for nanoconfined free radical polymerization compared to the bulk case in order to determine the chain confinement entropy. In particular, we investigate the free radical polymerization of butyl- and benzyl-methacrylate in bulk and in controlled pore glass using differential scanning calorimetry, as well as characterizing the molecular weight of the synthesized polymer using gel permeation chromatography. We find that the entropy loss on confined polymerization can be nearly fifty percent greater than the entropy change on bulk polymerization, leading us to conclude that the entropy loss by confining the chains is as high as 50 J/mol/K (compared to 130 J/mol/K for the entropy change on propagation). Furthermore, the chain confinement entropy is found to scale with chain length N to the first power and to scale with pore size D to a higher power. Implications and comparison to theoretical predictions will be discussed. |
Wednesday, March 17, 2021 4:48PM - 5:00PM Live |
P04.00008: Examining Correlations Between Density and Dynamics in Thin Polymer Films: Modeling Refractive Index Gradients with Ellipsometry Yixuan Han, Connie Roth A long standing issue with glassy dynamics in thin polymer films has been the question of whether density and dynamics may be correlated near interfaces. Several recent works have reported large, physically unrealistic density increases (~25%) with decreasing film thickness. Using ellipsometry, we have shown that polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2-vinyl pyridine) (P2VP) all exhibit similar large increases in refractive index with decreasing thickness, also suggestive of large apparent increases in density. Results were found to be independent of molecular weight, chain connectivity, and substrate surface chemistry [Han et al., J Chem Phys 2020, 153, 044902]. Our investigation concluded these artificially large increases in film properties are likely associated with film inhomogeneities invalidating the common use of homogeneous approximations within layer model analyses. Here we demonstrate that an ellipsometric optical layer model with a depth-dependent gradient in refractive index appears to resolve most of these large, unphysical apparent increases. Modeling thin films with a linear gradient in refractive index n(z) surprisingly finds that PS and PMMA films have opposite trends in n(z), with PMMA films behaving counter to known dynamical gradients. |
Wednesday, March 17, 2021 5:00PM - 5:12PM Live |
P04.00009: Understanding the Failure and Mechanical Properties of Glassy Polymer Thin Films Tianren Zhang, Cynthia Bukowski, Alfred Crosby, Robert Riggleman The dynamic and mechanical properties of glassy polymers are known to change up confinement to the nanoscale. Confinement to free-standing thin films leads to an enhancement in segmental dynamics, and changes in chain conformation lead to changes in entanglement density in confined polymers. In this study, we investigate the role of both segmental dynamics and changes in entanglement density on the mechanical response of glassy polymer films under uniaxial tension using molecular dynamics (MD) simulations. We show that not all entanglements can carry significant stress at large deformation, and this leads to the development of a simple model to describe the number of effective entanglements per chain as a function of blending ratio. The film toughness and the strength measured experimentally can be characterized in terms of the effective entanglement density. In well-ordered diblock copolymer thin films, we find that failure tends to occur near the center of the block copolymer domains due to the high concentration of chain ends that are unable to support stress. Our studies of the thin film mechanics provide molecular insight into how segmental mobility and entanglements interplay with position and morphology to control the mechanics of thin polymer films. |
Wednesday, March 17, 2021 5:12PM - 5:24PM Live |
P04.00010: Interfacial Dynamics Governs the Mechanical Properties of Nanoconfined Glassy Polymers Wenjie Xia Understanding the mechanical properties of nanoconfined polymers is essential in design of nanostructured soft materials. Here, we investigate the mechanical properties of free-standing polymer thin films by employing an atomistically informed coarse-grained (CG) modeling approach. By examining three representative CG polymer models having distinct segmental structure, PS, PMMA, and PECPMA, our results show that the film elastic moduli are substantially reduced with decreasing film thickness compared to their bulk values at their glassy state. Specifically, the PS and PMMA films exhibit similar size-dependent elastic responses and their film moduli are reduced compared to bulk values at a thickness of less than 40 nm, which agrees well with previous experimental measurements. However, in a model methacrylate-based polymer useful in photolithography, PECPMA, the length scale where elastic modulus deviates from the bulk value is much larger. The local molecular stiffness within the films further reveals a gradient a softer interfacial layer having a size of only a few nanometers. Our simulations uncover the size scaling relationship that universally holds for all three polymers and highlights the importance of interfacial dynamics in the mechanical properties of polymer films. |
Wednesday, March 17, 2021 5:24PM - 5:36PM Live |
P04.00011: Probing 'nanoconfinement' effects on Tg of high-molecular-weight chains via molecular simulation Marcelle van Staden, Asieh Ghanekarade, David Simmons A number of experimental studies have reported on distinct alterations in the Tg of ultra-high molecular weight polymers in thin films as compared to polymers at lower molecular weights. These effects include much larger reductions in Tg (when compared at equal film thickness) and even observation of two Tg's. The origin of these new effects at high molecular weight have remained unresolved. In an effort to provide insight into this question, here we report on of simulations of glass formation in bead-spring polymer films comprised of high-molecular-weight entangled chains. We compare alterations in Tg as measured via density vs dynamics, as well as comparing to results in lower molecular weight chains. |
Wednesday, March 17, 2021 5:36PM - 5:48PM Not Participating |
P04.00012: 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 properties may inform future endeavors towards the engineering of new and improved polymer nanocomposite materials. |
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