Session J44: Focus Session: Dynamics of Polymers: Phenomena Due to Confinement I

A closer look at the two-layer model

Previously we have measured the viscosity of short-chain, unentangled polystyrene films coated on silicon covered with a 100 nm thick oxide layer. We found that a two-layer model, portraying the films as being made of a 3 nm thick mobile layer sitting on top of a bulk-like inner layer, was able to describe our data very well. More recently, we extended similar measurements to entangled films with a higher molecular weight of 212 kg/mol. We found that the same two-layer model was able to describe the viscosity measurements, but would imply an unphysically large stretching of the polymer chains in the films or else the near-surface chains would have to be oblate spheroids with an aspect ratio of about 4. In this talk, I will discuss new results we obtain from films with higher molecular weights of up to 2316 kg/mol and also films that are deposited on a different kind of substrate, namely hydrogen terminated silicon. Implications of these results on the two-layer model will be discussed.   

Inter- and intra-chain Proximity in PS and PMMA free-standing thin films Studied by Fluorescence NRET

The reduction in glass transition temperature (Tg) for free-standing thin polymer film has attracted a long term discussion. We found that the inter- and intra-chain coupling/constraint restricts the molecular motion of polymer chains and thus results in the deviation in glass transition dynamics. To observe inter-chain proximity and control the inter-chain interaction, we attached carbazolyl probe (donor) and anthryl probe (accepter) to some side groups of different chains respectively. Meanwhile, we also attached both donor and accepter to one chain to characterize the intra-chain constraint. A close proximity of the donor to the accepter results in a higher NRET efficiency. So the NRET results can provide information about chain proximity and packing density in polymer thin film. With decreasing film thickness h, the density of the films was reduced. The magnitude of the reduction in packing density for PS free-standing films was much larger than that in PMMA, resulted a larger reduction in Tg for PS film.   

Surface Dynamics of Partially Tethered Polymer Films

The surface dynamics of thin polystyrene (PS) melt films can be tailored by tethering some of the chains to a supporting substrate. The relaxation of surface height fluctuations for these films depends on the density of grafting, molecular weight of tethered chains, and extent to which tethered chains extend into the layer. The surface dynamics of ``partially tethered'' thin films have been studied using X-ray photon correlation spectroscopy (XPCS). PS chains have been grafted to substrates with low grafting densities untethered deuterated PS (dPS) chains spun cast on the tethered chains, and the films annealed to create layers containing both tethered and untethered chains. The degree to which the tethered PS extends into the untethered dPS chains has been measured by neutron reflectivity.   

Structural characterization of irreversibly adsorbed polymer layers at the polymer/solid interface - In-situ grazing incidence angle x-ray scattering studies

In recent years, great attention has been paid to irreversibly adsorbed polymer layers formed on solid substrates since they can modify various properties of polymeric materials confined at the nanometer scale. In this talk, by the combined use of in-situ grazing incidence small angle x-ray scattering and x-ray reflectivity techniques, we aim to characterize the detailed structures of the adsorbed layers composed of different homopolymers (polystyrene, polybutadiene, poly (ethylene oxide), and poly (methyl methacrylate)) prepared on silicon substrates. We will highlight the generality/differences in the structures, leading to a better understanding of the formation process of the adsorbed layers at the impenetrable solid interfaces.   

Relaxation of Polyisoprene in Nanoscale Confinement

This talk introduces a simple model of confined polymers in polymer nanoparticle composites. Most studies about confined polymer dynamics have been investigated from the system of polymers in porous media or the polymer thin film structure. This new class of polymer nanoparticle hybrid materials, termed Nanoscale Ionic Materials (NIMs), is synthesized in bulk scale with convenient controllability of diverse properties to create the confined polymers in nanoscale. cis-Polyisoprene (PI), type A polymer whose dipole moments are parallel along the chain backbone, are synthesized by anionic polymerization and then tethered to spherical silica nanoparticles. Broadband Dielectric Spectroscopy measures responses to the applied electric field which are normal mode relaxation indicative of whole chain relaxation, and also segmental relaxation. We show that relaxations of PI are slower when simultaneously confined and tethered. We also show that molecular weight and grafting density have a profound effect on dynamics of the twice-confined PI chains.   

Molecular simulations of confined polymer glasses

For many technological applications ranging from semiconductor manufacturing to membranes for separations and flexible displays, the properties of glass-forming materials confined on the nanoscale are of critical importance. For example, common semiconductor manufacturing techniques rely on the mechanical integrity of amorphous polymer nanostructures. Experiments over the past several years have demonstrated that many of the properties of polymer glasses (e.g., the glass transition temperature, Tg, and the elastic constants) can change significantly and in unintuitive ways when confined to dimensions below approximately 100 nm. These confinement effects depend strongly on the detailed polymer chemistry. In this talk, I will describe our recent work using molecular simulations to study the effects of nanoscale confinement on both the dynamic and mechanical properties of a series of coarse-grained polymer models. Our coarse-grained models systematically explore the effects of polymer chemistry by gradually changing the polymer backbone stiffness and analyzing changes in various properties in free-standing thin films. We find that the confinement effects can vary drastically depending on the properties measured, and in some cases we observe significant ordering of our polymer chains   

Structure and Dynamics of Polymer / Single Wall Carbon Nanotube Nanocomposites

We simulate the static\footnote{Karatrantos A., Composto R.J., Winey K.I., Clarke N., Macromolecules, accepted.} and dynamic\footnote{Karatrantos A., Composto R.J., Winey K.I., Clarke N., in preparation.} properties of monodisperse polymers in the presence of a SWCNT using molecular dynamics. The SWCNT has a large aspect ratio and radius smaller than the polymer radius of gyration. We find that although the local chain structure is significantly affected, the overall configuration, as characterized by the radius of gyration, is not perturbed by either the interaction strength between the polymer and the SWCNT or by variations in the SWCNT radius. In contrast, for the dynamics of entangled polymers, we find a significant heterogeneity, with the center of mass polymer diffusion being affected by the strength of the enthalpic interaction between monomers and the SWCNT, as well as the SWCNT radii. In addition, we find that the polymer chain diffusivity perpendicular to the SWCNT is smaller than that parallel to the SWCNT in the case of entangled polymers.   

Polymer Conformations in SWCNT/Polystyrene Nanocomposites using SANS

Polymer conformations are a critical factor that affects the performance of polymer nanocomposites. Using small angle neutron scattering, we probe chain conformations and confinement of polymers in SWCNT/polystyrene nanocomposites. We established the contrast matching criteria by measuring nanocomposites with different dPS:PS ratios (79:21 to 63:37) for two different SWCNT loadings and found the best ratio of dPS:PS (72.5:27.5) which gives us a similar scattering strength as SWCNT. Therefore, the scattering signal from SWCNT are screened and contribute little to the total scattering intensity. By making contrast matched samples, we are able to focus on the scattering intensity from polymer chains and determine Rg by fitting the intensity data. We found that the Rg of the polymer chain increases weakly with 1 wt{\%} ($<$ 5{\%}) nanotube and then strongly ($>$25{\%}) with 3 wt{\%} nanotube, while the chain conformation still follows Gaussian statistics.   

Structures and Dynamics of Polymers Adsorbed on Supported Cationic Lipid Membrane

The structures and dynamics of polymers adsorbed on various surfaces have received attentions not only as a subject concerning fundamental polymer physics, but also for their important roles in many industrial and biological processes. In this talk, I will discuss our recent work utilizing single-molecule imaging to study the conformation and dynamics of fluorescent dye labeled $\lambda$-DNA molecules adsorbed on supported cationic lipid membrane. High-magnification snapshots of individual molecules, combined with statistical analyses on their apparent size and anisotropy as well as dynamic analyses on their diffusions, reveal interesting details that could have been overlooked in measurements based on ensemble averaging and theories based on polymers in 2D.   

Structure and Dynamics in Hyperbranched Nanohybrids

The structure and dynamics of a hyperbranched polyester-amide (Hybrane$^{\textregistered}$ 1200, Mn=1200, Tg=45$^{\circ}$C) polymer and its nanocomposites with natural montmorillonite (Na$^{+}$-MMT) are investigated to offer a detailed picture of its behavior in bulk and under confinement and reveal its potential use for various applications. The static properties were studied utilizing X-ray diffraction (XRD), while the dynamics using energy-resolved elastic and quasi-elastic neutron scattering (QENS). XRD reveals that the polymer chains reside within the galleries of the Na$^{+}$-MMT producing an intercalated nanocomposite. The elastically scattered intensity for the polymer exhibits two distinct relaxation steps, which are attributed to the methyl group rotation and to the segmental motion. The intensity for the nanocomposite shows the first step broader than the respective of the pure polymer indicating restricted local motion whereas it indicates frozen dynamics under confinement at temperatures higher than the bulk polymer glass transition temperature, Tg. The QENS spectra measured at temperatures covering the regimes below and above Tg are in agreement with the elastic measurements. Sponsored by the Greek GSRT ($\Sigma YNEP \Gamma A \Sigma IA$; $09\Sigma YN-42-580)$ and by the EU (CP-IP 246095-2).   

Large Amplitude Shear Deformations of Nano-Filled Elastomers: Evidence of Glassy Dynamics in the Vicinity of the Fillers

Adding fillers in elastomers is known to increase the elastic modulus and the wear resistance of elastomers, but also to increase non-linear dissipation, a phenomenon known as the Payne effect. Indeed, when submitted to deformations of the order of a few per cents or more, the elastic modulus can decrease down to values much smaller than the initial one. On the other hand, when submitted to large amplitude oscillatory shear, frequency analysis shows that the contribution of higher harmonics to the response is quite small. This might appear somehow as a paradox since the non-linear behavior of filled elastomers can be strongly marked. We proposed a possible physical origin of these various features. We do it by comparing experimental results performed on model elastomers to the prediction of a model proposed recently, based on the presence of glassy bridges linking neighboring particles. The non-linear response is a consequence of a shift towards shorter time scales (as compared to the non-perturbed distribution) of the relaxation time distribution when a large amplitude periodic solicitation is applied. The non-harmonic response is the consequence of the fluctuation during one cycle of the distribution of relaxation times and is thus a much smaller effect.   

Polymer Diffusion Slows Down in Nanocomposites Containing Hard Nanospheres

Here, we present two experimental studies of tracer diffusion in polymer nanocomposites (PNCs). Tracer diffusion of deuterated polystyrene (dPS) in a PS matrix containing phenyl-grafted silica (13nm and 29nm) represents a PNC with a weak polymer/particle interaction [1]. However, tracer diffusion of deuterated poly(methyl methacrylate) (dPMMA) in a PMMA matrix containing hydroxyl-terminated silica (13 nm and 28nm) represents a PNC with a strongly attractive polymer/particle interaction. In the former case, the normalized diffusion coefficients fall on a master curve when plotted against the confinement parameter, namely the interparticle separation normalized by the probe size. However in the latter case, the reduced diffusion coefficients in the PMMA matrix with 13nm nanoparticles are less than those with 29nm nanoparticles, suggesting that enthalpic interactions play a role in slowing down diffusion. These experimental studies indicate that a successful model of polymer dynamics must include contributions due to constraints imposed by the nanoparticles as well as interactions between polymer and nanoparticles.\\[4pt] [1] Gam, S., \textit{et. al.}, \textit{Macromolecules} \textbf{44}, 3494-3501, 2011.   

Time and Temperature Dependent Surface Stiffness of Poly(alpha-methylstyrene)(PAMS) through Particle Embedment

In the present work, we have used the particle embedment technique with sub-micron particles to study the time dependence surface modulus of poly(alpha-methylstyrene)(PAMS) at different temperature ranging from room temperature to 1.1T$_{g }$of PAMS. The surface was found softer at room temperature and at 1.02T$_{g}$ compared to the bulk film while at 1.1T$_{g }$the surface was found stiffer compared to the macroscopic modulus measured for the same PAMS. The embedment of the particle is determined from atomic force microscope measurements and the modulus was determined using the elastic analysis of Johnson, Kendall and Roberts (JKR) with surface energy estimates of the work of adhesion as the driving force for embedment. REFERENCES 1. K. L. Johnson, K. Kendall and A. D. Roberts, \textit{P. Royal Society of Lonodon A, }\textbf{324}$, $301-313\textbf{\textit{ }}(1971). 2. J. H. Teichroeb and J. A. Forrest, \textit{Physical Review Letter, }\textbf{91}$, $016104 (2003).   

The Calorimetric Glass Transition of Polystyrene Ultrathin Films

Although the glass transition behavior of polystyrene ultrathin films has been widely studied, calorimetric measurements are limited due to difficulties in sample preparation. Here we report the use of a rapid scanning calorimeter based on a membrane sensor to measure the rate-dependent glass transition temperature (T$_{g})$ for single ultrathin films. Both microtomed and spin-coated films are investigated. Preliminary results suggest that the magnitude of the T$_{g}$ depression is similar to that observed for freely-standing films. The T$_{g}$ depression also depends on the cooling rate such that at the highest rates used (1000 K/s), the depression is only a few degrees. The kinetics of dewetting are followed, with T$_{g}$ values increasing as a function of time and finally reverting to the bulk values after several hours at 160 \r{ }C.   

Dynamics of anchored polymer chains in a tunable micelle confinement system

We investigated the dynamics of the polyisoprene (PI) component confined within micelles in the blends of polystyrene-$b$-polyisoprene (PS-$b$-PI) diblock copolymers and polystyrene (PS) homopolymers, using broadband dielectric spectroscopy (BDS). In these blend, the diameter of the PS-b-PI micelle cores, composed of the PI component, increased with increasing molecular weight of the PS host,~$P$, and reached a plateau at high~$P$~regime. The BDS results show that the dynamics of the PI component were fastest within the smallest micelle cores. Additionally, the relaxation intensities were weakest within smallest micelle cores. The local glass transition temperature (Tg) of the PI component was 201 K when its core diameter is $\sim $29 nm, while the Tg dropped to 193 K with a core diameter of $\sim $21 nm. Although faster dynamics with confinement has been previously observed in several systems, few have examined confinement at length scales much greater than that of the typical cooperative rearranging region, which is normally a few nanometers. Therefore, our study may shed new light on understanding the dynamical behavior of confinement, especially in domains formed in microphase separated block copolymer structures.