Session K38: Glasses Altered by Interfaces I

Nanorheology of confined polymer films

Liquid films with a non-uniform thickness flatten in order minimize surface energy, a process driven by surface tension and mediated by viscosity. For a viscous thin film, the time evolution of the film height profile is accurately described with lubrication theory by the capillary-driven thin film equation. Previous experiments have successfully applied the thin film equation to measure the rheological properties of polymeric liquids. Here we probe confinement effects in thin polymer films. We measure the viscosity by tracking the levelling of surface perturbations with AFM. For films with thicknesses thinner than the end-to-end distance of the molecule we observe deviations from a thin film model with bulk viscosity.   

Understanding the relationship between different measures of nanoconfinement effects on segmental dynamics and the glass transition.

Several decades of research have indicated that confinement of a polymer or other glass-forming material to a nanoscale domain can significantly alter its glass transition temperature and segmental dynamics. Such effects have been reported in thin films, nanolayered and block copolymers, ionomers, and semicrystalline polymers. These alterations in glass formation behavior have implications for applications ranging from microelectronics to water purification. Many of the major open questions in this field center on apparent differences in nanoconfinement effects as probed by different methods. Recent studies have reported substantial differences in the apparent magnitude, direction, and onset temperature of these effects as probed via ellipsometry, calorimetry, fluorescence, dielectric spectroscopy, neutron scattering, various measures of viscous relaxation, and simulation. Here we employ molecular dynamics simulations to elucidate relationships between different measures of nanoconfinement effects. Particular emphasis is placed on differences in the manner in which different metrologies average over interfacial gradients in dynamics and pseudothermodynamic properties.   

Length Scales of Local Glass Transition Temperature Gradients Near Soft and Hard Polymer-Polymer Interfaces

Polymer-polymer interfaces are ubiquitous in polymer blends and block copolymers, while opening up another avenue for the study of interfacial perturbations to the local glass transition temperature Tg(z).~ We have previously reported the full local Tg(z) profile across a glassy-rubbery polymer interface between polystyrene (PS) and poly(n-butyl methacrylate) (PnBMA), an 80 K difference in bulk Tg [Baglay {\&} Roth, J Chem Phys 2015, 143, 111101].~ By using local fluorescence measurements, we revealed how the Tg(z) profile extends hundreds of nanometers away from the interface showing an asymmetric behavior penetrating deeper into the glassy PS side relative to the composition profile.~ Here, we extend these measurements to investigate how the local Tg profile in PS varies when in contact with a variety of immiscible polymers whose Tgs vary between $+$90 K and --80 K relative to the bulk Tg of PS, so-called hard vs. soft confinement.~ The data reveal that the onset of local Tg deviation from bulk in PS occurs at two distinct length scales, which depend on whether PS is the low Tg component (hard confinement) or the high Tg component (soft confinement).~ In addition, we explore the influence of finite system size on the range of dynamics by the introduction of periodic boundary conditions, as is commonly encountered in computer simulations or block copolymer systems.   

Glassy Dynamics Altered by a Free Surface

Studies of polymer dynamics in thin films showed that a highly mobile region exists at the free surface of most if not all polymers. In this talk, I shall review some of these observations, with highlights given to the recent findings that chain flexibility and connectivity may on occasions be necessary for the free surface to exercise its influence. Afterward, I shall ponder on how the influence of the free surface may penetrate as far as several polymer radii of gyration into the inner region, as found both in experiments and simulations. Near the glass transition temperature, our MD simulations showed that the dynamics consist mainly of string-like particle hopping motions, as found by others. Importantly, as the temperature decreases, the hopping motions become increasingly repetitive and back-and-forth, contributing no structural relaxations. We propose that structural relaxations are then brought about by pair-interactions between strings. Near the free surface, however, similar repetitive hopping motions are only observed sufficiently far removed from the free surface. We propose that the free surface induces a penetrating surface mobile region by breaking the memory in the particle dynamics. A possible mechanism based on string interactions will be discussed.   

Understanding and characterizing the effect of nanoscale confinement on glass transition temperature and film dewetting of macrocylic polystyrene

There is a growing interest in the dynamics of different polymer topologies when confined to nanoscopic length scales. Macrocyclic polymers have attracted research interest because their lack of chain ends and cyclic topology has led to a range of unique physical properties. Cyclic polystyrene ($c$-PS) of well-defined molecular weight (MW) ranging from 2,300 to 8,700 g/mol was synthesized via click cyclization of dilute solutions of linear PS ($l$-PS) with azide and alkyne end functionalities. The click reaction enables nearly quantitative cyclization of $l$-PS. Differential scanning calorimetry was used to measure bulk glass transition temperature ($T_{g})$ and fragility of $c$-PS, both of which exhibit less MW dependence compared to $l$-PS. Compared to thin $l$-PS films, thin $c$-PS films exhibited extraordinary stability against dewetting. 22-nm-thick $c$-PS films were nearly stable for up to 4 hr at bulk $T_{g} \quad +$ 45 \textdegree C; in contrast, 22-nm-thick $l$-PS films underwent severe dewetting. Nanoconfinement effects on $T_{g\, }$and fragility of $c$-PS are investigated using ellipsometry and/or fluorescence spectroscopy and compared to effects for $l$-PS precursors as well as commercial anionic $l$-PS standards.   

The local segmental dynamics of polymer thin films.

The local segmental dynamics of poly(methyl methacrylate) (PMMA) in multi-layered films with polycarbonate was investigated using dielectric spectroscopy. The segmental relaxation time decreased with layer thickness down to 4 nm. However, two measures of the cooperativity of the dynamics, the breadth of the relaxation dispersion and the dynamic correlation volume, were unaffected by the film thickness. This absence of an effect of geometric confinement on the cooperativity, even when the confinement length scale approaches the correlation length scale, requires an asymmetric correlation volume; i.e., correlating regions having a string-like nature. To further probe the effect of layering on the segmental dynamics, we measured the segmental dynamics of poly(vinylacetate) thin films in contact with variously an aluminum interface, an incompatible polymer, and air (free surface). From local dielectric relaxation measurements using an AFM tip, the dynamics were observed to be faster in all thin film configurations compared to the bulk. However, no differences were observed for the various interfaces; capping the thin films with a rigid material accelerated the segmental motions equivalently to that for an air interface. This insensitivity of the dynamics to the nature of the interface affords a means to engineer thin films while maintaining desired mechanical properties.   

Synthesis and Characterization of Fluorescently Labeled Diblock Copolymers for Location-Specific Measurements of The Glass Transition Temperature

Interfaces play a determinant role in the size dependence of the glass transition temperature (T$_{\mathrm{g}})$ of polymers confined to nanometric length scales. Interfaces are intrinsic in diblock copolymers, which, depending on their molecular weight and composition, are periodically nanostructured in the bulk. As a result diblock copolymers are model systems for characterizing the effect of interfaces on T$_{\mathrm{g}}$ in bulk nanostructured materials. Investigating the effect of intrinsic interfaces on T$_{\mathrm{g}}$ in diblock copolymers has remained unexplored due to their small periodic length scale. By selectively incorporating trace amounts of a fluorescent probe into a diblock copolymer, T$_{\mathrm{g}}$ can be characterized relative to the diblock copolymer's intrinsic interface using fluorescence spectroscopy. Here, pyrene is selectively incorporated into the poly(methyl methacrylate) (PMMA) block of lamellar forming diblock copolymers of poly(butyl-$b$-methyl methacrylate) (PBMA-PMMA). Preliminary results show a correlation of T$_{\mathrm{g}}$ as measured by fluorescence with the onset of T$_{\mathrm{g}}$ as measured by calorimetry in labeled homopolymers of PMMA. This result is consistent with previous characterizations of T$_{\mathrm{g}}$ using fluorescence spectroscopy. In selectively labeled diblock copolymers T$_{\mathrm{g}}$ is found to vary systematically depending on the distance of the probe from the PBMA-PMMA interface.   

Glass Transition of Polystyrene Thin Films on Silicon Wafer Measured by Dynamic Mechanical Analysis and Ellipsometry

Measuring the glass transition temperature, Tg, of polymer films in the thickness range of 20-500 nm is non-routine but commercially important for polymer films used in applications such as membranes and electronic circuit boards. Various specialized methods have been used or developed to determine Tg in thin films, including thermal ellipsometry and many others. Differential scanning calorimetry (DSC) is a more conventional method that has been used to measure Tg, but since the thin films must be scraped from the wafer, consolidation and annealing can occur in the pan and may negate effects due to film thickness. Here we report results for polystyrene (PS) spin coated on silicon wafers in the range of 20-500 nm using a benchtop dynamic mechanical analyzer (DMA) in the 3-point bending mode. For the DMA, the peak tan $\delta $ temperature is related to the polymer Tg and effects due to confinement as a function of film thickness are compared to literature values. We use thermal ellipsometry as a control method to measure film thickness and Tg in parallel. Low level additives present in commercial PS were observed to strongly affect the results for thin films and are described.   

Effects of Interfaces and Interactions on Stiffness-Confinement Behavior in Polymer Films: Characterization via Fluorescence and Nanoindentation

Although stiffness-confinement effects in polymers have been well studied, disagreement exists regarding even the qualitative nature of such effects. With the exception of one experimental and several simulation studies that characterize stiffness gradients, all others report average stiffness-confinement behavior in polymers. These issues demonstrate the need for comparative studies that characterize stiffness gradients in polymers from interfaces. Here, we use a fluorescence technique that utilizes the sensitivity of pyrene dye fluorescence to local caging and nanoindentation to characterize stiffness gradients in the polymer model nanocomposites. Both techniques are in qualitative agreement that stiffness gradients extend a distance exceeding 100 nm from a substrate, and that stiffness-confinement effects are tunable via surface modification of the substrate. It is observed that PMMA supported on methylated cover glass exhibits less stiffening near the substrate compared to PMMA supported on cover glass with enhanced hydroxyl groups that can hydrogen bond with PMMA. PMMA supported on PDMS shows decreasing stiffness near the interface. These findings help address some of the inconsistencies observed in literature regarding stiffness-confinement effects.   

Sidechain Dynamics Explain Dissimilar Strength of Nanoconfinement Effect in Polystyrene and Poly(methyl methacrylate) Free Standing Thin Films

Despite substantially similar bulk glass transition temperature ($T_{g}$) and other bulk properties, polystyrene (PS) and poly(methyl-methacrylate) (PMMA) exhibit characteristically different $T_{g}$ depression in the free-standing ultrathin film configuration. The mechanism for this disparate $T_{g}$-nanoconfinement effect due to the free surfaces has not been fully explained. Here we utilize recently established chemically specific coarse grain (CG) models to qualitatively reproduce contrasting thickness dependent $T_{g}$ and length scale of enhanced chain relaxation gradient in the interfacial layer. Vibrational mode analysis is utilized to uncover a relationship between the amplitude and frequency of sidechain fluctuations of reduced order models and the degree of $T_{g}$-nanoconfinement. By systematic variation of the distribution of mass in the sidechain versus the backbone, we demonstrate inertia-driven differences in sidechain fluctuations and $T_{g}$-nanoconfinement in the CG model. We hypothesize that mass distribution and sidechain flexibility are governing factors causing PS and PMMA free surface effect differences and also provide insight into broader nanoconfinement phenomena from past experiments.   

Tg-Confinement Effects in Polymer Thin Films, Nanotubes, and Nanospheres as Measured by DSC, Ellipsometry and Fluorescence

The effect of nanoscale confinement on the glass transition temperature (Tg) of supported and free-standing polymer films has been studied for two decades by various techniques. However, conventional DSC, which is the most common method for measuring Tg of bulk polymers, is not well suited for such measurements. Here, we demonstrate that Tg-confinement effects measured by conventional DSC in nanotubes of polymer supported in anodic aluminum oxide (AAO) templates compare well with with Tg-confinement effects measured in supported polymer films by ellipsometry and fluorescence. We further show that Tg-confinement effect data for nanotubes obtained by fluorescence agree well with data obtained by DSC. Finally, we draw comparisons between the Tg-confinement behavior of nanoprecipitated polymer nanospheres as measured by fluorescence to Tg-confinement effects for both supported and free-standing polymer films. The roles, if any, of confinement dimensionality (1-D vs 2-D vs 3-D) and measurement technique on the observed Tg-confinement effect will be discussed.   

Mechanophore activation in a crosslinked polymer matrix via instrumented indentation

Recent advances in mechanically-activated fluorophores will enable a host of unique scientific challenges and opportunities to be addressed. Several mechanophores (MPs) in polymers have been reported, yet the specific deformation required to activate these molecules in a bulk polymer network has not been sufficiently specified. In an effort to develop the mechano-activation/deformation relationship of a spirolactam-based MP, scratches were applied to a MP-functionalized glassy crosslinked material at varying normal loads and lateral displacement rates. This experimental design allowed strain and strain rate effects to be decoupled. The fluorescence activation was then observed with a laser scanning confocal microscope. Areas of elastic and plastic deformation as well as brittle fracture were observed within each scratch as the normal loading of the indenter increased. The fluorescence intensity increased with increasing strain. Contact mechanics models are employed to demonstrate that relatively high degrees of strain are required to initiate the ring-opening activation transition within the spirolactam-based MP. These self-reporting damage sensors can be incorporated within polymeric coatings to allow real time structural health monitoring for a myriad of applications.   

Why the Mechanical Properties of Cross-linked Polydimethylsiloxane Surface Enhance? -- A First Principles Study

Polydimethylsiloxane (PDMS) has been widely used in various areas due to its high flexibility, controllable mechanical properties, brilliant biocompatibility and low cost. Now more work on PDMS focus on tuning its surface physical and chemical properties. Our experimental group has shown that the top surface stiffness of PDMS is significantly enhanced after a surface treatment of hyperthermal hydrogen induced cross-linking (HHIC), without losing its inherent hydrophobicity. To understand why this, we investigated how the HHIC treatment changes the structure of PDMS molecules and chains, by using density functional theory (DFT) calculations with the nonlocal van der Waals interaction. The elastic and hydrophobic properties of PDMS, before and after the surface treatment, will be discussed then, providing deep understandings on the experimental observations. Our theoretical studies could give insights in the new development of HHIC tuning technology.