Session L18: Focus Session: John H. Dillon Medal Symposium

John H. Dillon Medal Talk: Polymer Droplets

The simplicity of a liquid droplet, say a dew drop on spider silk, is both esthetically beautiful and scientifically intriguing. The interplay of surface energies, thermal motion, and confinement of the liquid, especially on small length scales can reveal interesting physics. Droplets are an ideal confining geometry because the length scales can be easily controlled and it is possible to arrange the system such that each droplet acts as an independent experiment. The talk will focus on some recent examples where we have used the droplet geometry to learn about material properties. It will become apparent in the presentation that the deviations from the ``expected'' behaviour in confined systems are far from subtle!   

Surface relaxation in glassy polymers

We have used nanohole relaxation to measure the viscoelastic properties of the first 1-4 nm of the surface of glassy polymer PS and i-PMMA. In both cases we find evidence for complete relaxation of the surface below the bulk glass transition temperature $T_g$. For the case of PS, the temperature dependence of the relaxation was measured in the range $240K [Preview Abstract]

Tg and Cure of a Polycyanurate at the Nanoscale

Nanoscale constraint is known to have a significant impact on the thermal properties of materials. Although thermosetting resins have been cured in the presence of nanoparticles and nanotubes, cure of thermosetting resins under the well defined nanoscale constraints imposed by controlled pore glass (CPG) or similar matrices has not been previously documented. In this work, we investigate the isothermal curing under nanoscale constraint of a thermosetting resin, bisphenol M dicyanate ester (BMDC), which trimerizes to form a polycyanurate network material. Differential scanning calorimeter is used to monitor the evolution of the glass transition temperature (Tg) and the conversion during cure as a function of the diameter of the silanized control pore glass matrix which is used for confinement. A Tg depression is observed for both the bisphenol M dicyanate ester monomer and the polycyanurate networks; the depression is only a few degrees for the monomer, whereas a 56 K depression is observed for the ``fully-cured'' network in 11.5 nm pores. The nanoscale constraint is also found to accelerate the cure of the bisphenol M dicyanate ester, but it does not affect the normalized Tg versus conversion relationship. The appearance of a secondary Tg above the primary Tg in the smaller pores and the associated length scale are discussed.   

Comparison of surface mobility of polymeric and low molecular weight glass-formers.

The last ten years has seen considerable effort to understand dynamics at the surface of polymer melts and glasses. For comparison, we present data on two low molecular weight glass formers: trisnaphthylbenzene (TNB) and indomethacin (IMC). Neutron reflectivity provides direct information about mobility in the top several nanometers of TNB glasses. Two other measurements (surface crystal growth rates and the enthalpy of glasses prepared by vapor deposition) offer indirect information on surface mobility for IMC and TNB. These measurements indicate that surface dynamics at Tg are 2-5 orders of magnitude faster than bulk dynamics. The temperature dependence of the surface relaxation process is weak below the bulk Tg, in qualitative agreement with recent measurements on polymer surfaces in this regime.   

Modeling Solvent Evaporation from Glass-Forming Polymer Films by MD Simulations

By means of molecular-dynamics simulations we study solvent evaporation from glass-forming, free-standing and supported polymer films. Polymers are represented by a commonly employed bead-spring model, solvent molecules are modeled as Lennard-Jones particles, and polymer-solvent interactions are tuned such that good-solvent conditions are realized. We start the simulations from a dense solution with a solvent content of 20\% and explore the evaporation process for temperatures $T$ above and below the glass transition temperature $T_{\mathrm{g}}$ of the pure polymer film. At all $T$ we observe the formation of a polymer-rich crust at the free surface upon solvent evaporation. For $T > T_{\mathrm{g}}$ we can reproduce the simulation results (reduction of film thickness with time, solvent and polymer density profiles, etc.) by a Fickian diffusion model with a constant diffusion coefficient. For $T < T_{\mathrm{g}}$ deviations from Fickian diffusion are observed. We suggest that these deviations may be rationalized by a diffusion coefficient of the solvent, which depends on film composition and distance from the free surface. We attempt to compare our results to recent experiments.   

Studies of Glassy Colloidal Systems Under Shear

In analogy with the glass transition of polymer (and other molecular) liquids, colloidal suspensions can undergo dynamic arrest to form a glassy solid, when the system is concentrated beyond a critical volume fraction. However, in contrast to their molecular counterparts, studies of the glass transition in colloidal systems are facilitated by their natural length- and time-scales, which make it possible to directly visualize the behaviour of the individual constituent particles. Using confocal microscopy, we follow the dynamics of colloidal suspensions near the glass transition, and in particular, their reaction to an imposed deformation. We investigate the evolution from a quiescent solid to a shear melted liquid, to elucidate the nature of the structural rearrangements that govern the properties of glassy materials.   

Growth and Stability of Polymer Surface Wrinkles

For certain materials and geometries, the surface of a polymer film will wrinkle upon the application of a critical in-plane stress due to the onset of an elastic instability. The resulting morphology minimizes the in-plane strains and the system energy by locally bending the surface plane. This process and morphology have been studied at an increasing rate over the last decade and demonstrated in applications ranging from materials metrology to adhesion control. In general, the knowledge and use of this phenomenon has been developed for conditions that far exceed the point of initiation, under static equilibrium. In this presentation, we highlight recent experiments that explore the growth and morphological transitions of surface wrinkles. We quantify the growth mechanism under different conditions of mechanical constraint and demonstrate a unique ability to stabilize a wide variety of unique, non-predicted, surface wrinkle morphologies.   

Toughening Mechanisms in Polymer Gels

I will describe a simple model that accounts for the very high toughness of double network gels. The model is based on the assumption that the first, stiff network will break up forming multiple cracks when the stress is above a defined value. These cracks are held together by the second network. A multiply cracked damage zone will form round any macroscopic crack in the material causing energy dissipation and shielding the second network. The toughness enhancement by this process is estimated to be about x40. Other techniques of gel toughening will then be discussed.   

Theory of polymer crystallization

We will present new conceptual arguments for the spontaneous selection of very small lamellar thicknesses and their melting behavior.   

On the effective charge of hydrophobic polyelectrolytes

In this paper we analyze the behavior of hydrophobic polyelectrolytes. It has been proposed that this system adopts a pearl-necklace structure reminiscent of the Rayleigh instability of a charged droplet. Using a Poisson-Boltzmann approach, we calculate the counterion distribution around a given pearl assuming the latter to be penetrable for the counterions. This allows us to calculate the effective electric charge of the pearl as a function of the chemical charge. Our predictions are in very good agreement with the recent experimental measurements of the effective charge by Essafi et al. (Europhys. Lett. 71, 938 (2005)). Our results allow to understand the large deviation from the Manning law observed in these experiments.   

Disentanglement in thin polymer films

Molecular dynamics simulations of thin polymer films confined between structureless walls show accelerated in-plane dynamics with decreasing film thickness. Using the primitive path analysis (PPA) introduced by Everaers et al [Science 303 (2004) 823] for chain length up to N=1024, we can show that the entanglement density decreases with decreasing film thickness. However, the effect becomes pronounced only for films thinner than the bulk radius of gyration where also the chain structure becomes modified by the confinement [1]. The PPA algorithm can be modified to estimate the contribution of self-entanglements: The latter become more important for thinner films, however, they do not counterbalance the global decrease of entanglements. [1] H. Meyer et al Eur. Phys. J. Sp.Top. 141 (2007) 167.   

Complex Structural Packing of ABC Triblock Copolymers Solvent Annealed at High Humidity

The use of ABC triblock copolymers to obtain industrially relevant morphologies for block copolymer lithography has been investigated. Nanoporous thin films of poly(ethylene oxide)-poly(methyl methacrylate)-polystyrene triblock copolymer spheres were prepared by solvent annealing under controlled high humidity followed by UV degradation and acid washing. Ordered half spheres at the surface that template ordering of spheres below the surface in thin films were formed as a result of the interaction between the highly hydrophilic PEO segments and water vapor in the chamber. The spherical block copolymer domains exhibit complex packing behavior on the surface and in the interior which is dependent on film thickness. Half sphere ``monolayer'' and half sphere plus whole sphere ``bilayer'' formed in thin films were shown to have hexagonal lattice symmetry. For half sphere plus two whole sphere ``trilayers'', coexistence of regions of hexagonal and square packing was observed by TEM, SFM, SEM and GISAXS. Square packing was consistent with a surface truncated unit cell of a body-centered cubic lattice with the (100) plane parallel to the surface.   

High resolution structure of bacterial cell sacculi

The major structural component of bacterial cell walls is the peptidoglycan sacculus, which is one of nature's strongest and largest macromolecules that allows the cell to maintain a large internal pressure while allowing the transport of molecules into and out of the cell and cell growth. The three-dimensional structure of this unique biopolymer is controversial, and two models have been proposed: the planar model, in which the glycan strands lie in the plane of the cell surface, and the scaffold model, in which the glycan strands lie perpendicular to the cell surface. In this study we have used atomic force microscopy (AFM) to investigate the high resolution structure of isolated, intact sacculi of both Gram-positive and Gram-negative bacterial cells. We have observed a sponge-like structure for both types of sacculi with pore diameters between 5 to 15 nm. Our data for Gram-positive sacculi provide evidence for the validity of the scaffold model, whereas our data for Gram-negative sacculi indicate an orientation along the short axis of the cell which is consistent with the planar model. To further elucidate the structure, we have exposed sacculi to the tAmiB enzyme which cleaves peptide-peptide bonds.   

Mechanical Response of Lipid Multibilayers From Micro- and Nano-Particle Embedment

We have used atomic force microscopy (AFM) to image micro- to nano-meter sized particles embedded into lipid multilayer films prepared by a spin coating technique. The lipid investigated was 1, 2-dipalmitoyl-Sn-glycero-3-phosphotidylcholine (DPPC). Gold, silica and polystyrene were used for the embedding particles. Particle diameters ranged from 50 to 300 nm and all tests were performed at atmospheric conditions and ambient temperature. Film thickness was approximately 87 nm based on AFM determination. We used the elastic analysis for contact between a rigid sphere and elastic substrate with the work of adhesion w$_{a}$ acting as the force on the sphere to determine the shear modulus G in terms of w$_{a}$, the particle radius and the height of the sphere that remains above the surface. From the AFM height measurements, we find that the shear modulus for the DPPC falls in the range from 3 to 35 MPa, but seems independent of particle type and particle diameter. The potential for the particle embedment method for mechanical property determination of soft materials will be discussed.