Session P38: Glasses Altered by Interfaces II

\textbf{In Situ Analysis of the Glass Transition Temperature of Irreversibly Adsorbed Polymer Nanolayers}

When a polymer thin film is annealed in the melt state, individual chains can strongly adsorb to the supporting substrate in a manner that appears to be irreversible. This irreversible adsorption of polymer chains results from the large number of individual contacts made between monomer units and the substrate that stabilize the polymer from desorption. The formation and development of irreversibly adsorbed layers with increased annealing time has been shown to correlate with changes in their structure and dynamic properties and may impact the properties of thin polymer films. Here we investigate the link between deviations in the glass transition temperature ($T_{g})$ of polymer thin films from the bulk value and the growth of irreversibly adsorbed nanolayers. Through the use of fluorescence spectroscopy, we are able to directly measure $T_{g}$ of polystyrene adsorbed nanolayers in an exposed geometry and in situ. The results allow us to examine the influences of interfaces and molecular weight on the $T_{g}$ of adsorbed nanolayers throughout their development. By undertaking these studies, critical information is obtained that assists in the design and processing of technologies in which thin polymer films are placed in contact with solid interfaces.   

Tailoring Glassy Dynamics on the Nanoscale: Covalent Bonding versus Physical Adsorption in Polymer-based Nanocomposites

Polymer grafted nanoparticles (PGNs) offer improved miscibility, stability, and most importantly dispersibility in a polymer matrix over traditional nanoparticle fillers. However, despite the large interest in the miscibility and dispersibility of PGNs within polymer melts, few attempts have been made to study the material properties of solvent-free PGNs and specifically how they compare to traditional polymer nanocomposites (PNCs). In this experiment, the structure and dynamical properties of PGNs of three different molecular weights with identical grafting densities are directly compared to PNCs with similar polymer volume fraction. We find that the segmental dynamics in the interfacial layer are more strongly influenced by the covalent bonding than in the composite system (chain adsorption) and ascribe this is to highly stretched chain conformations near the interface, similar to a polymer brush. These results show that the chain conformations in a polymer melt near an interface, especially on the nanoscale, can strongly influence segmental dynamics and offers an explanation to many of the disputed results in the polymer nanocomposite literature.   

Polymer Dynamics Effects on Solute Transport in Hairy Nanoparticle Membranes

Molecular transport measurements in matrix-free grafted nanoparticle (MFGNP) films have shown remarkable enhancement of permeability and ideal selectivity of small condensable molecules and simple gases over the neat polymer melts and conventional, dispersed nanoparticle composites. Films comprised of covalently-attached poly(methyl acrylate) PMA chains to the surface of 14nm silica particles self-assemble into ordered arrays, and we postulate this structure plays an important role in regulating solute transport. This self-assembly creates interstitial spaces between the nanoparticle cores, which the polymer chains can only fill by stretching. Here we use small-angle neutron scattering (SANS), broadband dielectric spectroscopy (BDS), rheology and temperature-modulated differential scanning calorimetry (TMDSC) to probe polymer chain and segmental dynamics and investigate this hypothesis of chain stretching in MFGNP materials. We found that grafting slows both chain and segmental relaxation, and increases fragility, indicating that the chains are more ``frustrated'' in the grafted systems. We propose that the effects of the chain/surface interactions on chain dynamics leads to an increase in available free volume and thus enhances transport properties in MFGNP systems.   

\textbf{Effect of Molecular Architecture on Polymer Melt Surface Dynamics}

The dynamics of the thermally stimulated surface height fluctuations in a polymer melt dictate wetting, adhesion, and tribology at that surface. These surface fluctuations can be profoundly altered by tethering of the chains. One type of tethering is the tethering of one part of a molecule to another part of the same molecule. This tethering is found in both long chain branched polymers and in macrocycles. We have studied the surface fluctuations with X-ray Photon Correlation Spectroscopy for melts of well-defined, anionically polymerized polystyrenes of various architectures, including linear, 6 arm star, pom-pom, comb and cyclic architectures. For linear chains, the variation of surface relaxation time with in-plane scattering vector can be fit using a hydrodynamic continuum theory (HCT) of thermally stimulated capillary waves that knows nothing of the chain architecture. Assuming the theory is applicable, apparent viscosities of the films may then be inferred from the XPCS data. For unentangled linear chains, the viscosity inferred from XPCS data in this manner is the same as that measured by conventional bulk rheometry. The HCT does a reasonable job of describing the variation of relaxation time with scattering vector for long branched chains also, but only if a viscosity much larger than that of the bulk is assumed. The discrepancy between the viscosity inferred from surface relaxation times using the HCT and that derived from conventional rheometry grows larger as the bulk Tg is approached and is different for each long chain branched architecture. However, for densely branched combs and cyclic chains different behaviors are found. Acknowledgement: Thanks to NSF (CBET 0730692) and the Advanced Photon Source, supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Science, under contract No. W-31-109-ENG-38.   

Relaxation processes and glass transition in confined 1,4-polybutadiene films: A Molecular Dynamics study

We will present results from Molecular Dynamics simulations of a chemically realistic model of 1,4-polybutadiene (PB) chains confined by graphite walls. Relaxation processes in this system are heterogeneous and anisotropic. We will present evidence for a slow additional relaxation process related to chain desorption from the walls. We also study the structural relaxation resolved with respect to the distance from the graphite walls and show the influence of structural changes on the relaxation behavior. The temperature dependence of the dielectric relaxation in layers of different thickness near the walls shows no indication of a shift of $T_g$ as a function of thickness when analyzed with a Vogel-Fulcher fit. We explain this by the importance of intramolecular dihedral barriers for the glass transition in PB which dominate over the density changes next to a wall except for a 1 nm thick layer directly at the wall.   

Unusual Molecular Weight Dependence to the Physical Aging of Thin Polystyrene Films

Physical aging and the glass transition are intimately related, with the physical aging rate providing a measure of the stability of the glassy state formed.~ Previously, we have investigated the physical aging rate in thin supported polystyrene (PS) films finding that the local aging rate is correlated with the local glass transition temperature [Pye et al., Macromolecules 43, 8296 (2010)].~ These studies were able to provide a measure of the depth to which bulk glassy dynamics are perturbed by the free surface interface, a distance much further than similar measures of liquid-like dynamics.~ Here, we present physical aging measurements of thin PS films using ellipsometry.~ Surprisingly, we observe a distinctive molecular weight dependence to the physical aging behavior of thin (30 nm thick) films not present in bulk (1000 nm thick) films for very high molecular weights (Mw \textgreater 3000 kg/mol).~ These results indicate that chain connectivity plays a subtle, but important role in how gradients of glassy dynamics are propagated between the free surface and substrate interfaces   

Effects of molecular weight and tacticity on the Tg of poly(methyl methacrylate) films supported by silica

The glass transition temperature ($T_{\mathrm{g}})$ of poly(methyl methacrylate) (PMMA) films supported by silica is studied as a function of film thickness at different molecular weights ($M_{\mathrm{w}})$ for different polymer tacticities. The $T_{\mathrm{g}}$ confinement effect is found to depend on the $M_{\mathrm{w}}$ and tacticity. For the films with a low $M_{\mathrm{w}}$ of 2.5 kg/mol,$ T_{\mathrm{g}}$ is depressed for the atactic films, consistent with previous results. In contrast, the films with a higher syndiotactic content exhibit $T_{\mathrm{g}}$ enlargement as thickness decreases. We tentatively suggest this to be caused the influence of chain stiffness on the $T_{\mathrm{g}}$ that dominates at low $M_{\mathrm{w}}$ and varies with tacticity. For sufficiently high $M_{\mathrm{w}}$, the effect of chain stiffness is expected to be small. At $M_{\mathrm{w}} \quad =$ 50 kg/mol, the $T_{\mathrm{g}}$ confinement effect of the atatic and more syndiotactic films reverses from that observed of the low-$M_{\mathrm{w}}$ counterpart films. We suggest the chain stiffness effect to be negligible at this $M_{\mathrm{w}}$, and attribute the opposite $T_{\mathrm{g}}$ confinement effect to be caused by a competition between the surface $T_{\mathrm{g}}$ and the substrate $T_{\mathrm{g}}$ in these films. The $T_{\mathrm{g}}$ found of bilayers made of the atatic and more syndiotatic PMMAs with this $M_{\mathrm{w}}$ supports our attribute.   

Chain conformation near the substrate interface in nanoparticle stabilized polymer thin films

When nanoparticles (NPs) are added to polymer thin films, they often migrate to the film-substrate interface and form a ``diffused immobile interfacial layer'', which serves to screen the polymer-substrate interaction and suppress dewetting. The fundamental, but unsolved question is how the conformations of the polymer chains in the layer are affected by the NPs and how that impacts the enhancement of film stability. To address the question, we used dodecane thiol-functionalized gold NPs (2.4 nm diameter) and polystyrene (PS, Mw$=$30kDa). We found that the critical concentration of the Au NPs to induce complete dewetting suppression of 20 nm-thick PS/Au thin films on cleaned Si substrates is 5 wt{\%} (wt of particle/wt of polymer). To investigate the interfacial structures at the polymer-solid interface, we rinsed the annealed PS/Au thin films with toluene and characterized the residual interfacial layers by using various x-ray and neutron scattering techniques. The results indicate that the conformation of the polymer chains closer to the substrate becomes less flattened with the addition of gold NPs, allowing chains at the substrate to entangle more effectively with free chains comprising the bulk film. The detailed mechanism will be discussed.   

The Effect of Acid-Base Interactions on Conformation of Adsorbed Polymer Chains

Adsorption of polymer chains from solutions is of fundamental interest in polymer science. This absorption process is governed by the complex interplay between the solvent-polymer, polymer-substrate, and solvent-substrate interaction energies. In early 1970's, Fowkes and his coworkers have introduced the concept of acid base interactions in explaining why PMMA (basic) adsorption was extremely low on acidic substrates from acidic solvents. The acidic solvent molecules compete with the surface for binding with the basic polymer sites and this reduces the adsorption of PMMA. Here, by using interface-selective sum frequency generation spectroscopy (SFG) and attenuated-total-reflectance (ATR)-FTIR spectroscopy we directly measure whether the solvent or polymer molecules interact with the substrate in acidic, basic, and neutral solvents. Surprisingly, we find that the surface acidic site (hydroxyl) groups are still covered with PMMA chains in acidic solvent. The PMMA chains in acidic solvent adsorb with much higher fraction of chains as trains in comparison to loops and tails. Such differences in the static and dynamic conformations have consequences in understanding the exchange kinetics, colloidal stabilization, chromatographic separations, adhesion and friction, and stabilization of nanocomposites.   

Surfactants at Single-Walled Carbon Nanotube-Water Interface: Physics of Surfactants, Counter-Ions, and Hydration Shell

Specialized applications of single-walled carbon nanotubes (SWCNTs) require an efficient and reliable method to sort these materials into monodisperse fractions with respect to their defining metrics (chirality, length, etc.) while retaining their physical and chemical integrity. A popular method to achieve this goal is to use surfactants that individually disperse SWCNTs in water and then to separate the resulting colloidal mixture into fractions that are enriched in monodisperse SWCNTs. Recently, experiments at NIST have shown that subtle point mutations of chemical groups in bile salt surfactants have a large impact on the hydrodynamic properties of SWCNT-surfactant complexes during ultracentrifugation. These results provide strong motivation for understanding the rich physics underlying the assembly of surfactants around SWCNTs, the structure and dynamics of counter ions around the resulting complex, and propagation of these effects into the first hydration shell. Here, all-atom molecular dynamics simulations are used to investigate the thermodynamics of SWCNT-bile salt surfactant complexes in water with an emphasis on the buoyant characteristics of the SWCNT-surfactant complexes. Simulation results will be presented along with a comparison with experimental data.   

Polyethylene oxide hydration in grafted layers.

Hydration of water soluble polymers is one of the key-factors defining their conformation and properties, similar to biopolymers. Polyethylene oxide (PEO) is one of the most important biomedical-applications polymers and is known for its reverse temperature solubility due to hydrogen bonding with water. As in many practical applications PEO chains are grafted to surfaces, e.g. of nanoparticles or planar surfaces, it is important to understand PEO hydration in such grafted layers. Using atomistic molecular dynamic simulations we investigate the details of molecular conformation and hydration of PEO end-grafted to gold surfaces. We analyze polymer and water density distribution as a function of distance from the surface for different grafting densities. Based on a detailed analysis of hydrogen bonding between polymer and water in grafted PEO layers, we will discuss the extent of PEO hydration and its implication for polymer conformation, mobility and layer properties.   

The Unusual Conformational Behavior of Polyzwitterionic Brushes in Aqueous Solutions

Polyzwitterions constitute a peculiar class of polyelectrolytes, which are electrically neutral polymers containing both a positive and a negative charge on each repeating unit. Surfaces coated with polyzwitterionic brushes are resistant to the nonspecific accumulation of proteins and microorganisms, making them excellent candidates for a wide range of antifouling applications, from biocompatible medical devices to marine coatings. The surrounding environment can dramatically influence the conformational behavior of polyzwitterionic brushes. High-density polyzwitterionic brushes poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) were synthesized using surface initiated atom-transfer radical polymerization, and neutron reflectivity (NR) measurements were performed to investigate the ionic strength dependence of the conformational behaviors of PMPC brushes in monovalent salt solutions. Despite the numerous observations of normal pure polyelectrolyte brushes, NR results showed that both the densely concentrated layer near the substrate surface and the relatively swollen layer into the solution have been observed in different q range in a single neutron reflectivity profile. These results will definitely help us to better understand the relationship between the solution behaviors of zwitterionic polymer brushes and their antifouling properties.   

Homopolymer Adsorption on Hexagonal Surfaces: A Replica-Exchange Monte Carlo Study

The adsorption behavior and thermodynamic properties of a coarse-grained flexible homopolymer, grafted on a hexagonal patterned surface, are investigated by means of parallel-tempering replica-exchange Monte Carlo simulations. In this study, the strength of the polymer-surface interaction, which is based on a standard Lennard-Jones potential, is changed systematically, mimicking different hexagonally patterned substrate materials. Specific order parameters are introduced to discriminate structural phases, at different surface adsorptions strengths and temperatures, into classes of expanded, globular, droplet, and compact conformations. Finally, we provide a complete structural hyperphase diagram for a polymer with 55 monomers and discuss representative polymer structures.