Session K9: Glass Formation and Dynamics in Nanostructured Polymers and Glasses II

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

Nanoconfinement is known to influence the glass transition temperature (T$_{\mathrm{g}})$. In the case of polymer ultrathin films supported on neutral surfaces, T$_{\mathrm{g}}$ 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 PMMA and polystyrene (PS) nanorods over five decades of cooling rates, from 0.1 -- 1000 K/s, using Flash differential scanning calorimetry. The supported PS nanorods are prepared by the vacuum infiltration of 2000 kg/mol PS into AAO templates having a thickness of 5 $\mu $m and a pore diameter of 55 nm. Preliminary results indicate that T$_{\mathrm{g}}$ increases for native AAO pores.   

Cloaking the Effects of Strongly Interacting Substrates in Thin Polymer Films

It is widely appreciated that the glass transition of thin polymer films can be strongly altered compared to bulk polymers. Furthermore, it is generally agreed that attractive substrate interactions slow relaxation and increase the glass-transition temperature $T_g$. However, there is evidence that the magnitude of $T_g$ changes for strong substrate interactions are much smaller are expected, a phenomenon that also occurs in polymer-nanoparticle composites. Here, we use molecular simulations to reproduce this effect in supported polymer thin films; we explain the lack of sensitivity of $T_g$ to strong substrate interactions as a result of the emergence of a layer of ``bound'' polymer at the substrate. This bound polymer effectively cloaks the remainder of the film from the strong interfacial interactions. This bound layer manifests itself by an additional relaxation process in self-intermediate scattering function. We further characterize the temperature and substrate interaction strength dependence of the bound layer relaxation time from the overall relaxation, as well as the relaxation from polymer chains near the film center.   

Local Glass Transition Temperature Tg(z) Profiles of Polystyrene Next to End-Grafted Substrates

Modifying polymer-substrate or polymer-nanoparticle interactions using grafted chains is frequently investigated as a possible means of altering and reinforcing the neighboring polymer matrix. The mechanism by which this occurs and how it can be optimized is not well understood because there are numerous parameters, such as grafting density, chain length, surface coverage, grafting strength, and matrix interpenetration, that are all interdependent factors controlling the behavior. Studies of such grafted-polymer-substrate effects in thin films are frequently complicated by the additional presence of a free surface. Here, we end-graft polystyrene (PS) to silica substrates and measured the local glass transition temperature Tg(z) as a function of distance z from the substrate interface. We observe local Tg(z) increases next to the chain-grafted substrate in excess of 45 K depending on grafting density. The length-scale over which this Tg(z) perturbation persists from the interface is large, z ≈ 80 nm before bulk Tg is recovered. Comparing with our group’s recent work on polymer-polymer interfaces, this length scale is slightly smaller, but comparable to that observed for PS next to a higher-Tg polymer such as polysulfone (another form of so-called “hard confinement”).   

Theory of Cooperative Activated Structural Relaxation in Polymer Nanocomposites Composed of Small and Sticky Particles

Recently, Cheng, Sokolov and coworkers have discovered qualitatively new dynamic behavior (exceptionally large Tg and fragility increases, unusual thermal and viscoelastic responses) in polymer nanocomposites composed of nanoparticles comparable in size to a polymer segment which form physical bonds with both themselves and segments. We generalize the Elastically Collective Nonlinear Langevin Equation theory of deeply supercooled molecular and polymer liquids to study the cooperative activated hopping dynamics of this system based on the dynamic free energy surface concept. The theoretical calculations are consistent with segmental relaxation time measurements as a function of temperature and nanoparticle volume fraction, and also the nearly linear growth of Tg with NP loading; predictions are made for the influence of nonuniversal chemical effects. The theory suggests the alpha process involves strongly coupled activated motion of segments and nanoparticles, consistent with the observed negligible change of the heat capacity jump with filler loading. Based on cohesive energy calculations and transient network ideas, full structural relaxation is suggested to involve a second, slower bond dissociation process with distinctive features and implications.   

Impact of substrate roughness on the segmental mobility of thin polymer films.

We have investigated the impact of substrate roughness on the segmental mobility of spincasted thin films of different polymers. In neat disagreement with predictions of molecular dynamics simulations of the structural relaxation of Lennard-Jones liquids [1], segmental dynamics does not slow-down upon increase of the roughness of the substrate on which polymer layer is deposited. Our results indicate, on the contrary, an enhancement in segmental mobility with interfacial roughness, proportional to the sensitivity of the polymer to a perturbation in volume. In line with current literature, we deduce that the reduction in relaxation time with increasing roughness implies an excess in interfacial free volume, probably caused by partial wetting at the polymer/substrate interface. [1] P Scheidler, W. Kob and K. Binder, Europhys. Lett. 59, 701 (2002)   

Interfacial Energy and Glass Temperature of Polymers Confined to Nanoporous Alumina

We report on the effect of interfacial energy on the glass temperature, $T_{g}$, of several amorphous polymers with various glass temperatures and polymer/substrate interactions confined within self-ordered nanoporous alumina (AAO). The polymers studied include: poly(phenyl methyl siloxane) (PMPS), poly(vinyl acetate) (PVAc), 1,4 polybutadiene (PB), oligostyrene (PS) and poly(dimethyl siloxane) (PDMS). The segmental dynamics and associated $T_{g}$'s are studied by means of dielectric spectroscopy. The interfacial energy for the polymer/substrate interface, $\gamma_{SL}$, is calculated with Young's equation whereas the AAO membrane surface energy is obtained by measuring contact angles for several reference liquids. We find that interfacial energy play a significant role on the segmental dynamics of polymers under confinement within AAO. There is a trend for a \textit{decreasing} glass temperature relative to the bulk with \textit{increasing }interfacial energy. PDMS exhibits the highest interfacial energy and the highest reduction in glass temperature within AAO. Other effects that may also contribute to changes in $T_{g\, }$are discussed.   

Thin films of linear polymer exhibiting no Tg-confinement effect: Correlation with low bulk fragility

Recently, two studies have demonstrated a strong connection between the magnitude of the Tg-confinement effect in thin, supported films of linear polymer that lack significant attractive interactions with the substrate and the value of bulk polymer fragility (See Macromolecules 2013, 46, 6091 and Macromolecules 2016, 49, 5092.) Here, we discuss experimental work which extends these results to polymers with very low bulk fragility values, about 70. With decreasing bulk fragility, we observe a reduction in the Tg-confinement effect until, at sufficiently low values of bulk fragility, Tg is unchanged from its bulk value at thicknesses of about 20 nm. That is, the Tg-confinement effect is eliminated down to thicknesses of 20 nm by virtue of the fact that the fragility, which reflects chain segment packing frustration, cannot be reduced by confinement as the bulk value is already very low. With these new results, a total of twelve linear polymers are shown to exhibit a one-to-one correlation of the strength of the Tg-confinement effect and the bulk fragility value. We also note that an exception to this correlation has been observed with films of linear PMMA that lack attractive interactions with its substrate. Possible reasons for the exceptional behavior of PMMA will be discussed.   

Compositional Heterogeneity and the Role of Polystyrene Molecular Weight on the Dynamic Response of Poly(vinyl methyl ether) in Thin Film Polystyrene/Poly(vinyl methyl ether) Blends

The segmental dynamics of poly(vinyl methyl ether) (PVME) chains in miscible blends of PVME and polystyrene (PS) exhibits a strong dependence on blend composition as well as dissimilar temperature dependences; this behavior manifests the fact that each component experiences different intramolecular and intermolecular interactions associated with differing local compositional environments. A similar study of thin films of these blends confined between aluminum (Al) substrates reveals a dependence of the PVME segmental relaxations on the host PS molecular weight, M. The dynamics of the PVME chains increase with increasing PS molecular weight in the blend. This enhancement of dynamics is associated with a corresponding decrease in both the dynamic and thermal glass transition temperatures (T$_{\mathrm{g}}$s) in the same sample geometry. Depth profile measurements indicate that not only is the PVME preferentially segregated at the polymer-Al interfaces, but this segregated layer increases in thickness with increasing molecular weight of the PS component. This behavior is consistent with the overall enhancement of the PVME dynamics with increasing M.   

Prediction of the Local Glass Transition Temperature of Polystyrene and Poly(Methyl Methacrylate) Bilayer Thin Films

The local glass transition ($T_{g})$-nanoconfinement effect is characterized at nanometer resolution at polymer-substrate, polymer-vacuum, and polymer-polymer interfaces for freestanding, supported, and bilayer films. The interphase $T_{g}$-profile, is size-independent above a critical thickness and can be approximated by exponential functions. Below the critical thickness where interphase regions overlap, the $T_{g}$ -profile follows the superposition of exponentials. For an 18 nm PS thin film overlayer with a PMMA underlayer supported by an attractive substrate, the free surface effect is found to be effectively eliminated for all underlayer thicknesses due to the enhanced local $T_{g}$ near the PMMA-PS interface which cancels out the $T_{g}$ depression effect near the free surface. At very low PMMA thicknesses, the PMMA-substrate effect is able to penetrate through the soft polymer-polymer interface and causes the PS layer $T_{g}$ to appreciate. Local analytical functions are then applied to a freestanding PMMA-PS bilayer nanocomposite system with a cylindrical nanorod. The predicted spatial $T_{g}$ shows relatively good comparison with simulated results, verifying the universality of the superposition principle.   

Local dynamics of glass-forming polystyrene thin films from atomistic simulation

Despite a wide technological application ranging from protective coatings to organic solar cells, there still no consensus on the mechanism for the glass transition in polymer thin films – a manifestation of the infamous glass problem under confinement. Many experimental and computational studies have observed a large deviation of nanoscale dynamical properties in thin films from the corresponding properties in bulk. In this work, we perform extensive united-atom simulations on atactic polystyrene free-standing thin films near the glass transition temperature and focus on the effect of free surface on the local dynamics. We study the segmental dynamics as a function of distance from the surface for different temperatures, from which relaxation time and thereby local $T_\text{g}$ is obtained for each layer. We find the dynamics near free surface is not only enhanced but becomes less strongly temperature dependent as $T_\text{g}$ is approached compared to the bulk. We find an increasing length scale associated with mobility propagation from the free surface as temperature decreases, but no correlation between local structure and enhanced relaxation rates near the surface, consistent with studies on bead-spring chains.   

Influence of Finite System Size, Cooling Rate, and Chain Interpenetration on the Local Glass Transition Temperature Tg(z) Profile in Polystyrene Next to Different Polymers

We have recently shown very broad 350-400 nm and asymmetric local glass transition temperature Tg(z) profiles across glassy-rubbery polymer-polymer interfaces that have important implications for our understanding of local properties in polymer blends and block copolymers, as well as the study of interfacial perturbations. Our previous work focused on a single interface with semi-infinite domains, allowing the Tg(z) disturbance to propagate unhindered by the presence of other interfaces, allowing bulk Tg to be recovered on either side of the interface. In these systems, the penetration distance of the Tg(z) disturbance propagated 225-250 nm into polystyrene (PS) next to lower-Tg polymers such as PnBMA and PiBMA (so-called soft confinement), while penetrating 100-125 nm next to higher-Tg polymers such as PSF and PMMA (hard confinement). Here we explore factors that can make it easier to compare our experimental data with computer simulations that frequently use periodic boundary conditions creating additional interfaces and are limited to short time scales, equivalent to faster cooling rates. We find that the addition of a second PnBMA interface (finite system size) truncates the Tg(z) profile preventing bulk dynamics from being recovered even within a PS domain as large as 300 nm. In addition, we examine the influence of faster cooling rates and the implications of limiting chain interpenetration between the two domains.   

Location-Specific Measurements of The Glass Transition Temperature in Fluorescently Labeled Diblock Copolymers

Block copolymers can self-assemble into periodic structures containing a high internal surface area, nanoscale domain periods, and periodically varying composition profiles. Depending on their components, block copolymers may also exhibit variations in their dynamic properties e.g., glass transition temperature (T$_{\mathrm{g}})$ across the domain period. Measuring the variation of T$_{\mathrm{g}}$ across the domain period of block copolymers has remained a significant challenge due to the nanometer length scale of the domain period. Here we use fluorescence spectroscopy and the selective incorporation of a pyrene-containing methacrylate monomer at various positions along the chain to characterize the distribution of glass transition temperatures across the domain period of an amorphous block copolymer. The pyrene-containing monomer location is determined from the monomer segment distribution calculated using self-consistent field theory. Our model system is a lamella-forming diblock copolymer of poly(butyl methacrylate - $b $- methyl methacrylate). We show that T$_{\mathrm{g}}$ is asymmetrically distributed across the interface; as the interface is approached, larger gradients in T$_{\mathrm{g}}$ exist in the hard PMMA-rich domain than in the soft PBMA-rich domain. By characterizing T$_{\mathrm{g}}$ of PBMA or PMMA interfacial segments, we show that polymer dynamics at the interface are heterogeneous; there is a 15 K difference in T$_{\mathrm{g}}$ measured between PBMA interfacial segments and PMMA interfacial segments.   

Theory of Spatially Heterogeneous Activated Relaxation, Vitrification and Elasticity in Confined Polymer and Molecular Liquids

Building on the Elastically Collective Nonlinear Langevin Equation theory of structural relaxation in deeply supercooled bulk liquids, a force-level description is constructed of how confinement and interfaces in free standing thin films introduces spatial mobility and shear modulus gradients as a function of temperature, film thickness and location in the film. The key idea is that relaxation speeds up due to the reduction of both the local cage barrier near the surface due to loss of neighbors, and the dynamical softening and cutoff of the longer range collective elastic barrier near the vapor interface. Quantitative predictions under isothermal conditions are made for the apparent glass transition temperature as a function of film thickness, the emergence of a two-step decay and mobile layers in time domain measurements, film-averaged relaxation times, surface diffusivity, role of a nonzero liquid-vapor interfacial width, and the relationship between kinetic and pseudo-thermodynamic measurements. Dynamical behavior under non-isothermal conditions can be qualitatively different. For example, mobile layers near the vapor surface can coexist with strongly vitrified regions in the film interior, resulting in a glassy shear modulus that increases as the film thins (despite a Tg reduction) and possible large implications for chain scale viscoelasticity. Extension of the theoretical approach to the droplet geometry and systems confined by condensed phase boundaries (liquids, solids) of variable mechanical stiffness has been achieved, and initial results will be presented. The new approach can also address activated glassy dynamics in quenched porous media, polymer nanocomposites and phase-separated polymer blends.