Session H24: Polymers at Interfaces

Adsorption-induced fracture of branched macromolecules.

Recently, we have discovered the remarkable phenomenon that brush-like macromolecules with long side chains undergo scission of the backbone bonds as a result of adsorption onto a substrate. The macromolecule's self-destruction occurs because its side chains stretch the polymer backbone as the macromolecule struggles to reconfigure and maximize the number of contacts with the substrate. We show that the tension imposed by the surface attraction is unevenly distributed over the covalent bonds of the molecular skeleton. Along the brush axis, a major fraction of the tensile force is carried by the backbone, while in the perpendicular direction the tension is distributed over many side chains. Using molecular visualization and computer simulation, we confirmed the first order kinetics and measured the corresponding rate constant, which revealed strong dependence on the attraction to the substrate.   

The swelling and dissolution kinetics of polymer thin films

We consider a theoretical analysis of the swelling and dissolution phenomena in polymer thin films. To determine the overall kinetics, we first consider the swelling process in a two-component system(solvent and polymer) and then apply this theory in conjunction with the theory of diffusive penetration of the solvent into the thin film. We also consider the dissolution of a pre-formed swelled gel of cross-linked polymers. Both analyses involve multiple moving boundaries. A comparison of our theoretical results with experimental data will be presented.   

Sum-Frequency Vibrational Spectroscopy on Rubbed Poly(vinyl cinnamate) films for Liquid Crystal Alignment..

Rubbing or uv-irradiation of poly(vinyl cinnamate) (PVCi)-coated substrates can effectively align liquid crystal (LC) films deposited on them. We report here the use of surface-specific sum-frequency vibrational spectroscopy (SFVS) to investigate how rubbing affects the interfacial molecular structure of PVCi.~ The stretch modes of the pendant side-chain phenyl groups dominate the spectra, from which the average orientation of the phenyl groups is deduced. The results show that rubbing aligns the main chains of PVCi along the rubbing direction.~ While hard rubbing also aligns the phenyl rings toward the rubbing direction, soft rubbing preferentially aligns them perpendicular to the rubbing direction. The observations are correlated with the fact that hard-rubbed substrates induce LC alignment along the rubbing direction, and soft-rubbed substrates induce LC alignment perpendicular to the rubbing direction. This work was supported by NSF.   

Structure of Confined Fluid between an Elastomer and a Flat Surface

The behavior of simple fluids under spatial restrictions is of considerable technological and fundamental interest. Experiments using force measurements have indicated that viscosity of confined fluids is different from that in the bulk. We have studied the confinement of linear alkanes between a poly (dimethyl siloxane) and a sapphire surface using surface sensitive infrared sum frequency generation spectroscopy (SFG). The results indicate ordering of the alkane molecules upon confinement above their bulk melting temperature. The SFG spectra of confined alkanes cooled below the bulk melting temperature shows a structure that is very different from the crystal structure of unconfined alkanes. A simple model will be presented that shows the alkane molecules crystallizing with the chains lying flat next to the sapphire surface.   

Resonant Soft X-ray Reflectivity of Polymer Thin Films

Resonant Soft x-ray reflectivity [1], a new method for low Z materials that combines aspects of neutron reflectivity and x-ray reflectivity, is presented and its usefulness delineated. Resonant reflectivity provides enhanced and selective sensitivity to specific chemical moieties near the absorption edges of constituent elements and was demonstrated through the characterization of a bilayer polymer thin film. The relative reflectivity of a particular interface could be tuned by adjusting the incident photon energy. Using photons in the 270-320 eV energy range, measurements were performed on thin film bilayers of polystyrene on top of poly(methyl methacrylate). Large changes in reflectivity at an interface and the resulting interference in thin films and multilayers were observed as the complex index of refraction, n = 1-$\delta $-i$\beta $, changed rapidly as a function of photon energy. Through the use of specific energies, the sensitivity to the polymer-polymer or polymer-vacuum interface can be selectively enhanced. The resulting chemical specificity is analogous to using deuteration as a marker in neutron reflectivity, but without requiring special sample preparation. The interfacial width determined with resonant reflectivity is virtually identical to that measured previously by XR and NR confirming that RXR is an excellent, complementary tool for the study of low-Z material thin films. [1]C. Wang, T. Araki, and H. Ade, Appl. Phys. Lett. 87, 214109 (2005).   

Possible explanation of polymer surface diffusion anomaly

Surface diffusion of polymer is studied with molecular dymanics simulation. Our simulation result shows the non-monotonic behavior of the polymer surface-diffusion coefficient D as a function of surface coverage. D first increased with increasing polymer surface concentration, then suddenly drops at a critical value. This finding shows striking similarities with the experimental result [1]. We found that the strong reduction of D above the threshold concentration is related to the spontaneous formation of double-layered structures, whose bottom layers manage to lock into the registry of the substrate much better than the single-layer pancakes that form at smaller concentration. This increases the barriers for lateral diffusion and is thus consistent with small values of D at larger concentration. \newline \newline [1] J. Zhao and S. Granick, J. Am. Chem. Soc. 126, 6242 (2004).   

Unconventional Spinodal Surface Fluctuations on Polymer Films

We study the temporal growth pattern of surface fluctuations on a series of spinodally unstable polymer films where the instability is adjustable by the film thickness, $h_0 $. For the most unstable film studied (whose $\left| {\frac{h_0 -h_{sp} }{h_{sp} }} \right|=0.988$; $h_{sp} $ is the thickness where the second derivative of the interfacial potential of the film equals to zero), the growth rate function of the surface modes as a function of the wavevector fits well to the mean-field theory. As the film thickness is increased such that $\left| {\frac{h_0 -h_{sp} }{h_{sp} }} \right|\le 0.977$, the mean-field theory demonstrates marked disagreement with experiment, notwithstanding provision of the known corrections from high-order terms and thermal noises. We show that the deviations arise from large-amplitude fluctuations induced by homogeneous nucleation, which is not accounted for in the conventional treatments.   

Interfacial Characterization of Contact in Aqueous Environments with a Quartz Crystal Microbalance

Adhesion of a material to a surface in an aqueous environment requires the removal of a water layer coating the surface. The evolution of this water layer between a micron thick polymer membrane and a quartz crystal microbalance has been investigated. This quartz crystal microbalance is highly surface sensitive, allowing determination of the water thickness during membrane contact. To improve this sensitivity, the thin, polymer membrane is backed by an ambient air environment. The surface of the quartz crystal is coated with a gold layer that acts as an electrode. This same surface allows for surface modification through the grafting of thiol terminated poly(ethylene oxide) brushes. These brushes can also by established with a coating of poly(ethylene oxide-b-styrene) diblock copolymer micelles.   

Humidity influence on atomic force microscopy electrostatic nanolithography

The formation and sustainability of water menisci and bridges between solid dielectric surface and nano-asperity under external electrostatic potential is a mystery, which must be adequately explained. The goal of our study is twofold: (i) To address the influence of an ambient humidity through the water meniscus formation on the nanostructure formation in soften polymeric surfaces; (ii) Estimate an electric charge generation and transport inside the water meniscus in vicinity of nanoscale asperity taking into consideration an induced water ionization in strong non-uniform electric field of magnitude up to 10$^{10}$ Vm$^{-1}$. It is suspected that strong electric field inside a polymer matrix activates the hoping mechanism of conductivity. The electrons are supplied by tunneling of conductive tip, and also through water ionization. Electric current associated with these free carriers produces Jule heating of a small volume of polymer film heating it above the glass transition temperature. Nanostructures are created by mass transport of visco-elastic polymer melt enabling high structure densities on polymer film.   

Comparison of thermal and chemical treatments of ultrathin chitosan films

Chitosan is a biodegradable polysaccharide derived from seashell waste products. The high water absorbency and biocompatibility of chitosan have enabled its use as a hydrogel in specialty biomedical applications. Chitosan can be dissolved in weakly acidic solutions enabling its use in applications such as films and gels, which can be converted into chitin by a chemical process known as acetylation. We present the results of several experiments in which changes in the thickness, index of refraction and molecular environment in response to changes in relative humidity for ultrathin films of chitosan are examined as a function of exposure to temperatures above 150 degrees Celsius. Measurements made by ellipsometry and FTIR spectroscopy indicate that changes in the thickness and index of refraction of the films are accompanied by a change in the infrared absorption spectra similar to that associated with acetylation, which is typically accomplished by exposure of chitosan to acetic anhydride. We believe that these changes are responsible for reduced equilibrium water content in the films at all relative humidity values studied, and may offer a simple method for converting chitosan into a chitin-like material.   

Glucose/galactose binding protein changes its mechanical properties: Novel AFM method of detection in-situ.

Recently a periplasmic glucose/galactose binding protein, GGRQ26C, immobilized on gold surface has been used as an active part of a glucose biosensor based on quartz microbalance technique (QCM). However the nature of the glucose detection was not clear. Here we have found that the receptor protein film immobilized on the gold surface increases its rigidity when glucose is added, which explains the unexpected detection signal. To study the rigidity change, we developed a new fast and simple method based on using atomic force microscopy (AFM) in tapping mode. The method was verified by explicit measurements of the Young's modulus of the protein film by conventional AFM methods. Since there are a host of receptors that undergo structural change when activated by ligand, AFM can play a key role in the development and/or optimization of biosensors based on rigidity changes in biomolecules. From fundamental point of view, the developed method can be used for study of mechanics of proteins in different environments. This can be compared with molecular simulations to get additional information about the protein structure.   

Modifying Surfaces with Light.

The ability to locate particular functional groups at surfaces is an enabling aspect of many important technologies. Unfortunately, high energy functional groups are not thermodynamically stable at the surface of polymers and other low surface tension materials. We show that this difficulty can be overcome through the synthesis of surface active block copolymers and self-assembled monolayers that contain photoactive functional groups. Photoactive functional groups are protected during the synthesis step, but can be deprotected upon exposure to light. To accomplish this task we borrow from photoresist technology and employ photoacid generators to deprotect carboxylic acid groups delivered to a surface upon exposure to light. Since all modifications are accomplished with light, direct micropatterning of surface functional groups is possible simply by exposure through a mask. The technique is thus a simple, direct alternative to other patterning techniques such as stamping. We also demonstrate that polymers and carbohydrates can be photografted to a surface using photoactive groups that are capable of hydrogen abstraction. Applications that are demonstrated include prevention of thin film dewetting, general surface functionalization, micropatterning of functional groups, polymers and biopolymers, and the creation of the first covalently bound carbohydrate microarrays. This work is funded by grants from the Polymer Program of the National Science Foundation Division of Materials Research and the U.S. Army Research Office.   

Dynamical simulations of rheology and phase behavior of ternary polymer blend systems.

We present the results of a multiscale simulation approach which combines Brownian dynamics simulations with polymer self-consistent field theory to study flow induced phase transitions in microemulsion phases of ternary polymer blends with homopolymers A and B with added copolymer AB. The results match qualitatively with the experimental observations and suggest flow transition of microemulsion phases into a three-phase coexistence followed by macrophase separation at stronger shears. We explore the effect of viscoelastic asymmetry (by varying polymer mobilities) and the copolymer (AB) to homopolymer length ratio ($\alpha )$ on flow induced phase transitions. Though the series of phase transitions remain unchanged, quantitative differences arise as a function of viscoelastic asymmetry and $\alpha $. These transitions are also accompanied by a strong shear-thinning behavior in the rheological response. The results suggest significant differences between ternary polymeric systems and oil-water-surfactant systems. We rationalize the above results from a molecular viewpoint.   

Orientation competition of lamellar phases in sheared block copolymers via hydrodynamic instability

We study the shear alignment of three dimensional lamellar diblock copolymers for achieving macroscopic order in defected, multidomain systems. We find that the competition between differently oriented microdomains, and thus the orientation selection between lamellar phases, is determined by the domain viscosity contrast which originates from different uniaxial domain symmetries relative to the imposed shear. This variation of domain viscosity causes hydrodynamic instability at high enough shear frequencies, breaking the degeneracy between parallel and perpendicular orientations in the system. The instability is found to obey a thin layer effect, inducing a size dependence of perpendicular domain expansion. Our calculations also suggest that the effective boundary of the experimentally observable regime with predominant perpendicular alignment follows a power law behavior between shear amplitude and frequency, with an exponent equal to 3/4.   

Defects in a Noncentrosymmetric Lamellar Block Copolymer Blend

Here we report results from a defect study on the noncentrosymmetric (NCS) lamellar blend of an ABCD tetrablock copolymer and an AD diblock copolymer. The block copolymers used were polystyrene-\textit{block}-polybutadiene-\textit{block}-polyisoprene-\textit{block}-polycyclohexadiene and polystyrene-\textit{block}-polycyclohexadiene. Coexisting tetrablock-rich mixed centrosymmetric (CS) and NCS lamellar morphologies were seen in TEM, as predicted by the mean-field theory. NCS grain boundary defects similar to those in CS lamellar systems were observed as well as new defects unique to NCS layered systems, such as chain polarity reversals and kink bands with dilation or compression of the layers. In addition to morphology observations, geometrical and energetic calculations were performed on several new NCS defects, which are in good agreement with experimental results.