Session K03: Confined Polymer Glasses III: Influence of Interfaces on Material Properties

Effective Viscosity of Polymer Films on a Strongly Slipping Surface

We have studied the thickness (h₀) dependence of effective viscosity (η_eff) of entangled polystyrene (PS) supported by a polydimethylsiloxane (PDMS) layer grafted on silica -- a representative of polymer film systems exhibiting strong slippage. Our result shows that η_eff was independence of the polymer molecular weight, consistent with plug flow. However, a ~h₀² scaling was found instead of the ~h₀ dependence expected. A theoretical model based on flow dynamics dominated by low-density elastic pinning sites and strong slippage is described that explains the observation.   

Tuning the Effective Viscosity of Polymer Films by Chemical Modification

We report adjustment of the dynamics of polystyrene (PS) films supported by oxide-covered silicon (SiOx) by a combination of ultraviolet ozone (UVO) modification and substrate surface modification. For all the specimens studied, greatly enhanced effective viscosity (η_eff) was found with the UVO modification when film thickness (h₀) was less than the radius of gyration (R_g) of the polymer. After etching the substrate with hydrogen fluoride, which removes the oxide layer, η_eff was much less enhanced. Surface analysis revealed that oxygenated groups were produced in the polymer near the film surface upon the UVO treatment. These groups diffused into the films during the dynamic measurement. If h₀ < R_g, the groups could interact with the hydroxyl groups on the SiOx substrate and caused strong enhancement in η_eff. When h₀ > R_g, the η_eff of the MW > 13.7 kg/mol films was bulk-like, suggesting bulk-like surface mobility of oxygenated PS. In contrast, the η_eff of the MW = 13.7 kg/mol films with h₀ > R_g was depressed in the same way as the pristine films resulting from enhancement in the surface mobility, which indicates that the thickness of the surface mobile layer is bigger than the R_g of those films.   

Quantitative Nanorheometry of Ultrathin Polymer Films

Direct mechanical property measurement of ultrathin polymer films is relevant for applications in lithography, micro-fluidic devices, and organic electronics. Traditional mechanical tests are generally unable to provide quantitative information at these length scales. While there has been extensive work measuring the Tg of these thin films using ellipsometry and other methods, there has been less work directly measuring other mechanical properties. In this work, the quartz crystal microbalance as a high frequency rheometer to measure the mechanical response of an ultrathin polymer film confined between a gold surface and a ridged over-layer. In this way, the shear compliance values of ultrathin films with thickness as low as 10 nm were measured directly.   

Mechanical Properties of Thin Nafion Films via Surface Wrinkling and Cantilever Bending

Our research program has investigated the effect of confinement on the structure and transport properties of Nafion, the most widely used ion exchange membrane material for fuel cell applications. Here, we will present recent results on the mechanical properties of thin Nafion films (h<300 nm) as measured by surface wrinkling and cantilever bending. Key parameters affecting the mechanical properties of thin Nafion are processing conditions, thickness, substrate chemistry, and humidity. We explore the roles of each of these parameters in defining the mechanical response of Nafion thin films, as well as correlate the mechanical response to the diffusion and uptake of water in these membranes.   

Mechanical Properties of Ultra-thin Polycarbonate Films

Polycarbonate (PC) is a transparent, ductile polymer used in applications varying from water bottles to optical media to aircraft canopies. The mechanical properties of PC have been extensively studied; however, understanding how these properties change as sample dimensions approach the size scale of an individual polymer chain is severely limited. Recently, we have developed TUTTUT (The Uniaxial Tensile Tester for Ultra-Thin films), enabling the direct measurement of the complete stress-strain relationship for the extension of ultra-thin polymer films. We report measurements on the effect of thickness and annealing history on the tensile properties of PC ultra-thin films. All PC thin films, regardless of thickness, showed clear shear banding deformations near a well-defined yield point; however, the Young’s modulus and maximum tensile strength decreased dramatically as the thickness decreased below the average configurational size scale for the polymer chains. In addition, we observed that the annealing history could be used to control the crystallization of PC thin films, leading to significant changes to the measured tensile properties. We propose an initial hypothesis to explain these observed changes in the mechanical properties for ultra-thin PC thin films.   

Unexpected Effect of Small Nanoparticles: A Paradigm Shift for Polymer Nanocomposites

The addition of nanoscale fillers with strong attraction to polymers causes significant changes to the polymers’ dynamics and mechanical properties. The challenge of polymer nanocomposite (PNC) research is to understand the critical microscopic parameters (e.g. chain rigidity, molecular weight, nanoparticle geometry) that control the emergent macroscopic properties of PNCs. In this work, we investigated the effects of adding very small (~1.8nm) POSS nanoparticles to a polymer (P2VP) melt. We found unexpectedly large increases in Tg (~35K) and fragility for the POSS-PNCs compared to PNCs with conventional 10-50nm silica particles. Further, the POSS-PNCs at high temperatures show essentially no viscosity increase in comparison to the neat polymer, making them highly attractive for practical applications. We ascribe this unusual behavior to two unique properties of PNCs with small nanoparticles: (i) fast mobility of small nanoparticles, and (ii) relatively short chain – nanoparticle desorption time. These results reveal a new approach for the design of PNCs by exploiting the unique properties of small nanoparticles, i.e. by tuning their mobility and chain desorption time.   

The Role of Position-Dependent Decoupling of Translational and Reorientational Dynamics in Thin Film Metrological ‘Discrepancies’

Many of the central challenges in the study of glass formation and dynamics in nanoconfined systems center around apparent inconsistencies between results from distinct metrological simulations and from simulation. Key examples include frequently weaker effects as observed by dielectric spectroscopy than by pseudothermodynamic methods, an extrapolated disappearance of nanoconfinement effects at temperatures modestly above Tg that is not typically reported in simulation, and a seeming nonuniversality regarding the presence of smooth surface dynamic gradients vs distinct surface relaxation processes. Here we employ molecular dynamics simulations to examine these discrepancies within the context of decoupling between translational and reorientational dynamics near the interface. Results suggest that differential interfacial effects on translational and reorientational dynamics over a range of molecular stiffness play an important and largely unrecognized role in yielding differential measures of nanoconfinement by different methods.   

Nano-Glasses Formed by Ensembles of Confined Conjugated Polymers: Molecular Dynamic Simulation Study

Confined conjugated polymers form long-lived far from equilibrium highly luminescent nano particles (NPs or polydots) with an immense potential for nanomedicine. Their photophysics strongly depends on chain conformation and the number of chains within the assemblies. Single molecule NPs are characterized by glass-like dynamics with time constants of micro seconds. Here fully atomistic molecular dynamics simulations was used to study the effects of the number of polymer molecules confined, and their relative orientation on ensemble structure and stability, using dialkyl poly (para-phenylene ethynylene)s (PPEs) as a model system. Increasing number PPE molecules were confined into one particle in implicit poor solvent while their initial orientation in the confining cavity was randomly varied, maintaining the total number off monomers per particle constant ( N= 240). We find that independent of the number of chains and their relative orientation, at early times the particles assume a close to spherical configuration. However, the initial orientation and the number of chains affect the internal structure and the unwinding process the polymers assume as the temperature is increased.   

Fast Scanning Calorimetry Studies of Superheated Polycrystalline Films: Insights into the Glass Transition in the Limit of High Heating Rates

The behavior of rapidly heated thin-film materials is relevant for a broad range of existing and emerging technologies. Experimental investigations of superheated crystalline phases are important because they may speed the development of generalized theoretical approaches to studying a variety of other non-equilibrium processes, such as glass softening. Using a novel Fast Scanning Calorimetry technique (FSC), capable of heating rates in excess of 100,000 K/s, we investigated the non-equilibrium melting mechanism for micrometer-scale polycrystalline films of several molecular compounds. In all cases, the phase transition occurs heterogeneously, i.e. melting originates on a sample’s surfaces and progress into the sample’s bulk via a transformation front interface. Remarkably, every compound’s melting transformation during rapid heating is characterized by an “anomalously” high activation energy barrier. These barriers indicate that, when confined by a propagating melting front, constituent diffusion occurs in a high defect density, glass-like amorphous phase. We will discuss the fundamental significance of this recent discovery on the glass transition phenomena and the potential of exploiting the interfacial effect of FSC to probe the properties of glassy films.   

Fast Scanning Calorimetry Studies of the Interface-facilitated Devitrification Mechanism of Model Molecular Glasses

Using Fast Scanning Calorimetry capable of rapid heating rates in excess of 100,000 K/s, we have investigated the interface-facilitated devitrification kinetics of micrometer-scale, ordinary and vapor-deposited amorphous films of several organic compounds. Under conditions of rapid heat loading, glass softening follows zero-order kinetic rate law with an Arrhenius-like temperature dependence during transformation. Similar to the devitrification mechanism of stable vapor-deposited glasses, the devitrification of rapidly heated ordinary glassy films begins at the surface and progresses into the sample bulk via a propagating devitrification front. Confinement of constituents at the devitrification front interface is indicated by the very high apparent activation energy barriers for front propagation in all of glassy films. This implies that the softening mechanism is controlled by self-diffusion of constituents in a transient glassy phase that is nearly identical in structure to the initially prepared glassy film. We will discuss the observed devitrification kinetics in the framework of an extended Wilson-Frenkel model of non-equilibrium, heterogeneous phase transitions.   

The glass transition of supported and unsupported polystyrene nanorods using Flash differential scanning calorimetry

Nanoconfinement is known to influence the glass transition temperature (T_g). In the case of polymer ultrathin films supported on neutral surfaces, T_g generally decreases with decreasing film thickness, and the magnitude of the depression increases as cooling rates decreases. Compared to ultrathin films, glass-forming polymeric materials in nanopores have been less well studied. Here, we aim to examine the glass transition behavior of supported and unsupported polystyrene (PS) nanorods over five decades of cooling rates, from 0.1 – 1000 K/s, using Flash differential scanning calorimetry. The samples are prepared by the vacuum infiltration of 2000 kg/mol PS into AAO templates having a thickness of 5 μm and pore diameters of 20, 55 and 350 nm. Preliminary results indicate that T_g increases for supported PS nanorods of all the pore sizes studied; however, bulk behavior is observed for unsupported PS nanorods. The implications of the results are discussed within the context of current work in the field.
Keywords: Nanoconfinement, Flash DSC, nanorods, glass transition, AAO templates
  

How Do Ultra-thin Glassy Polymer Films Fail?

The physical properties of polymers confined to thin films can be vastly different from their bulk behavior. For confined polystyrene (PS) films, previous work has found a reduction in interchain entanglement density and an increase in average molecular mobility. In our work, we determine how these two effects influence the failure mechanism of PS films in the ultra-thin regime (15 nm – 60 nm) as a function of molecular weight (MW= 132 kDa, 345 kDa, 592 kDa). We directly measure the complete uniaxial stress-strain relationship by holding a film between a flexible cantilever and a movable rigid boundary, measuring force-displacement from the cantilever deflection. In addition, we use dark-field optical microscopy to observe the failure mechanisms in-situ and transmission electron microscopy to examine the strain localization morphology. We observe a dramatic decrease in yield stress and modulus for films less than 30 nm in thickness. In addition, most surprisingly, we find a sharp transition in observed strain localization mode for films with thickness changing from 30 nm to 20 nm. We present these results and our hypothesis for how average molecular mobility trades off with the decrease in entanglement density to cause these transitions.   

Molecular Dynamics of Polystyrene Films: Comparison Between Atomistic Simulations and beta-NMR Measurements

The enhancement of dynamics near the free surface of glassy polymers continues to attract great interest, as a universally accepted microscopic understanding of this effect remains elusive. To compare to beta-detected nuclear magnetic resonance (beta-NMR) measurements, we have studied the depth dependence of molecular motion in polystyrene thin films with atomistic molecular dynamics simulations. Beta-NMR is a non-destructive technique well suited to study local properties of thin films, including glassy polymers [1]. The technique allows for nanoscale depth-resolved measurements of dynamics occurring at nanosecond timescales, attainable in simulation. The spin-lattice relaxation rate of the ⁸Li⁺ probe is believed to be due to the motion of the phenyl side groups. We compare this to several measures of molecular mobility inferred from the simulated polymer films. Our study is the first side-by-side comparison of molecular relaxation rates in terms of their depth dependence as well as their Arrhenius temperature dependence.

1. McKenzie, I. et al. Enhanced high-frequency molecular dynamics in the near-surface region of polystyrene thin films observed with b-NMR. Soft Matter 11, 1755–1761 (2015).   

Aging in diblock copolymers under soft and hard confinement

Self-assembly of block copolymer formats hierarchical architectures that could be utilized as templates or scaffolds in nanotechnologies. Naturally, polymers under T_g are non-equilibrium materials，aging may dramatically impact properties of a polymeric material in its service life. On the other hand, the microphase separation in block copolymers creates mass of interfaces between macromolecular blocks in nanometer length scale, and thus, the physical properties of blocks in copolymers should deviate from the corresponding homopolymers, owing to the confinement effect. Here we show the aging behavior of copolymer blocks, with emphasis on effect of hard and soft confinements. This is achieved by investigating aging of PS blocks in PS-b-PMMA and PS-b-PBMA at the same temperature and time scale. All blocks in the two model materials have the same molecular mass. Due to the T_g difference of PMMA (120 °C) and PBMA ( 26 °C), the PS blocks at aging temperature not far below its T_g (105 °C) in these materials are considered under hard ( confined by PMMA glass) and soft (confined by PBMA liquid) confinements, respectively. Our results show the aging behaivor and associated phase evolution, and compare the effect of hard and soft confinement on physical aging of copolymers.   

Why We Need to Look Beyond the Glass Transition Temperature to Characterize the Dynamics of Thin Supported Polymer Films

There is a significant variation in the reported magnitude, and even the sign of T_g shifts in thin polymer films. Many measurements assume that methods to estimate T_g of bulk materials can be applied to ultra-thin films. We validate this assumption by examining T_g from several methods using molecular simulations. As observed in many experiments, we find T_g from the variation of thermodynamics decrease for thinner films for all polymer-substrate interaction strengths. In contrast, T_g defined dynamically from the density-density correlation function similarly shows a T_g decrease with decreasing thickness for weak polymer-substrate interactions, but a T_g increase with strong polymer-substrate interactions. We show the sensitivity of these measures to the mobility gradient across the film profile is key to resolving contradictory T_g shifts. The slow-moving substrate layer of the film dominates the overall dynamic T_g, while the thermodynamic T_g is not sensitive to the substrate behavior. Our results emphasize the limitation of using a single T_g to characterize film dynamics. Instead, it is vital to consider the mobile gradient to understand changes in dynamics.