Plenary Speakers
Conformational Mechanics of Tethered Stimulus-Responsive
Stefan Zauscher
Duke University
The design and fabrication of surfaces with patterned, conformationally switchable (bio)polymeric structures is important for sensing and actuation applications on the micro and nanoscales. We recently demonstrated that stimulus-responsive, elastin-like polypeptides (ELPs) and their hierarchical assemblies on surfaces show great promise for actuation, sensing and biotechnological applications. ELPs offer a tunable means to transduce and amplify changes in the solvent environment, such as changes in pH, temperature or ionic strength by a large change in their molecular conformation and surface energy. While interfacial applications of ELPs have been prototypically demonstrated, a systematic investigation of the conformational mechanics at the solid-liquid interface is lacking. We will present results from experiments ranging from the ensemble to the single molecule level that employ and explore the conformational and surface energetic behavior of surface-grafted ELPs. We show that hydrophobic interactions between ELPs grafted to a surface and ELPs free in solution are responsible for their reversible association above the lower critical solution temperature. Furthermore, we show results from force-spectroscopy measurements on the single molecule level that suggest that ELPs are well described by a random coil polymer model, without significant secondary structure and, importantly, that force spectroscopy is able to distinguish differences in the hydrophobic hydration of single ELP molecules. Our results contribute significantly to the understanding of the conformational and mechanical consequences of an environmentally triggered phase transition in stimulus-responsive macromolecules.
Molecular Size-Dependent Flow Switching in Nanocapillary Array
Membranes Mediated by Free Volume Transitions in Polymer Brushes Grafted by Atom Transfer Radical Polymerization
Paul W. Bohn
University of Notre Dame
Exploiting external stimuli to operate a size-selective switchable molecular gate to control transport has a variety of applications in microfluidic systems. For example, the cylindrical nanochannels of the polymer-grafted membrane can be implemented as an actively-switchable nanofluidic interconnect to establish controllable fluidic communication between micrometer-scale channels operating in different planes of an integrated microfluidic device. This capability is enhanced even further if the switching can be intelligent in the sense that the on-off transition can be tuned to a given molecular size threshold. Thus, we are pursuing the construction and characterization of nanometer-scale analogs of fluidic valves, ratchets and diodes by combining free-volume transitions in network polymers prepared by grafting-from atom-transfer radical polymerization with the nanopores found on the surface of nanocapillary array membranes (NCAMs). We have demonstrated actively controlled transport that is thermally-switchable and size-selective prepared by grafting poly(N-isopropylacrylamide) (PNIPAAm) brushes onto the exterior surface of a Au-coated polycarbonate track-etched membrane. Molecular transport through the PNIPAAm polymer brush-modified NCAMs has been investigated by real-time fluorescence measurements using fluorescein isothiocyanate (FITC)-labeled dextrans, and we find that manipulating the temperature of the NCAM through the PNIPAAm lower critical solution temperature (LCST) causes large, size-dependent changes in the transport rates. Over specific ranges of probe size, transport is completely blocked below the LCST but strongly allowed above the LCST. The combination of the highly uniform PNIPAAm brush and the monodisperse pore size distribution is critical in producing highly reproducible switching behavior.
Aqueous RAFT Polymerization to Produce Stimuli Responsive Micelles and Vesicles for Pharmaceutical Applications
Charles McCormick
The University of Southern Mississippi
Reversible addition fragmentation chain transfer (RAFT) polymerization is a versatile and robust technique that allows for the polymerization of a wide range of functional monomers under facile conditions, including the use of water as a solvent. An inherent advantage of RAFT is its ability to form complex polymer architectures including well-defined stimuli-responsive block copolymers with α, ω chain-end functionality. Recent work in our group has focused on the synthesis and solution properties of a number of thermally responsive block copolymers containing N-isopropyl acrylamide (NIPAM). These copolymers are capable of self assembling in aqueous solution when the temperature is raised above the lower critical solution temperature (LCST) and form either micelle or vesicle structures that can serve as nano-carriers for targeted delivery applications. Additional research has focused on the ability to “lock” the self-assembled structures of these block copolymers. We have recently communicated several facile cross-linking strategies including the chemical cross-linking of copolymers containing the active monomer N-acryloxy succinimide with a diamine or fully-reversible cross-linking of the copolymers utilizing the disulfide cystamine. We have also utilized polyelectrolyte complexation to “lock” our micelle or vesicle morphologies which is reversible in the presence of added electrolyte. In addition, our group has utilized RAFT to produce block copolymers containing the hydrophilic, non-immunogenic monomer N-(2-hydroxypropyl) methacrylamide (HPMA) which are capable of complexing small interfering ribonucleic acid (siRNA) for therapeutic stabilization and targeted delivery applications.
Light-responsive Hydrophobic Binding of Colloids and Azobenzene-modified Polymers in Solutions
Christophe Tribet
CNRS, Physico-chimie des Polymeres et Milieux Disperses
Azobenzene-modified polyacrylic acids (AMPs) belonging to the class of amphiphilic macromolecules form transient hydrophobic associations in water with small particles such as surfactant micelles, liposomes or proteins. Exposures to UV-visible light of AMPs lead to reversible switches of polarity of their azobenzene chromophore groups, which in turn affect the composition and structure of complexes formed with other particles. Responses of high magnitude, up to macroscopic scale were achieved at optimal AMP structures, in dilute solutions (binding/release [1]), at interfaces (permeabilization of lipid membranes, emulsion stability[2]) and in semi-dilute solutions (sol-gel [1] or phase transitions). From a practical standpoint, an efficient amplification affords the control in a few seconds of centimeter-thick samples, even highly viscous ones, using a clean trigger with no additives. The presentation will focus on the origin of the responses, due to the macromolecular nature of AMPs. In dilute regime, binding isotherms obtained on AMPs with varying degree of modification exemplify how protein or micelle can “recognize” the isomerization of side-groups on the chains. In semi-dilute regime, the rheological properties of mixture show that inter-AMP connectivity also displays important sensitivity to light. Due to their low chromophore content (<1 mol% of the monomers), and the number of applications involving amphiphilic polymers, AMP-colloid mixtures should hold some future in cosmetics, agriculture and formulation. They provide an interesting way to cycle or trigger for instance interfacial properties, or viscosity swings.
Bistable Electrical Switching and Memory Performance in Electroactive Polymer Thin Films
En-Tang Kang
National University of Singapore
Memories based on polymeric and organic materials can exhibit simplicity in structure, drive-free read and write capability, good scalability, 3-D stacking ability, low-cost potential, and a large capacity for data storage. Rather than encoding “0” and “1” from the amount of charges stored in a silicon cell, a polymer memory stores data, for instance, based on the high and low conductivity response to an applied voltage. The molecular design-cum-synthesis approach has allowed several polymer memories, including flash (rewritable) memory, write-once-read-many-times (WORM) memory and dynamic random access memory (DRAM) to be realized. A series of polymer systems were designed and studied in an attempt to elucidate the memory property-molecular structure relationship. These polymers systems have included: (1) conjugated and non-conjugated polymers and copolymers with or without lanthanide complexes, (2) polymers exhibiting regioregularity and conformational states, (3) functional polyimides containing both electron donor (D) and acceptor (A) moieties and (4) polymers containing hetero-molecules and polymer nanocomposites. All the polymer memory devices exhibited high ON/OFF current ratios of 104 to 107, with ON and OFF states which were stable under a constant voltage stress, and endured up to 108 read cycles under ambient conditions.
Designer Biomaterials: Engineering Biointerfaces with Controlled Properties
Joerg Lahann
University of Michigan
Future biomaterials will use advanced surface engineering technologies to actively modulate cellular microenvironments. Three technological examples towards materials with controllable biointerfaces will be presented. Towards this goal, the dynamic modulation of materials-mitigated cues will be critical. Our recent work on dynamically switchable surfaces based on low-density monolayers will be reported. The ability to control dynamic events on a surface, then switch it from one state to the other, has been a longtime challenge for surface scientists. Such “smart surfaces” can reversibly switch properties in response to an external stimulus. To demonstrate these findings, a surface design was developed that can be changed from water-attracting to water-repelling with the application of a weak electric field. Designed as a switch, single-layered molecular-level machines are aligned on a surface using self-assembly and then are flipped between defined microscopic states. To create stable surfaces, vapor-based polymer coatings have been interesting candidates for the coating of biomedical or microfluidic devices, because of their advanced processibility. A diverse class of polymer coatings (functionalized poly-p-xylylenes) has been prepared by chemical vapor deposition (CVD) polymerization that provides chemically reactive groups for the immobilization of biomolecules. In addition, bioinertness (Suppression of unspecific protein adsorption and cell adhesion) has been identified as a critical design criterion and efforts towards bioinert coatings will be outlined. The design and synthesis of polymer-based particles with two distinct phases will be reported. The biphasic geometry of these Janus particles is induced by the simultaneous electrohydrodynamic jetting of parallel polymer solutions under the influence of an electrical field. The high electrical potentials (typically several thousand volts) applied between the jetting liquids that are fed through a capillary and a collecting substrate will induce jetting of the charged liquid. The final morphologies of the resulting nano-objects are mainly determined by the properties of the jetting liquids and the process parameters.
Reversible Adsorption of Proteins on Poly (N-Isopropyl-Acrylamide) and Implications to Biotechnology
Dale L. Huber
Sandia National Laboratories
It has been known for some time that the phase transition that aqueous gels of Poly(N-isopropyl acrylamide) (polyNIPAM) exhibit causes a change in surface properties. We have taken advantage of these changing surface properties to design surfaces and devices that can adsorb and release proteins on demand. The adsorbing species is a 4 nm-thick polyNIPAM film that can be thermally switched between an anti-fouling hydrophilic state and a hydrophobic adsorbing state. There is a great need for compact sensors that can selectively detect low concentrations of a wide range of potential biological agents, and we have harnessed polyNIPAM in a number of applications in this respect. Appropriately - designed systems can take advantage of the reversible adsorption of proteins on polyNIPAM surfaces to aid in the concentration and separation of proteins, as well as the selective adsorption of biological species. Examples of devices we have produced include a device capable of switching in milliseconds to rapidly capture and release proteins in a microfluidic environment, a porous device capable of concentrating large quantities of protein and controllably releasing them into a fluid stream. We have also produced a device capable of using adsorbed antibodies to detect biological species, then releasing the antibody - antigen complex and reusing the device. The active films can be integrated into optical or mass sensing devices to create rapid, selective arrays of detectors that can be used repeatedly without degradation of sensitivity. Preliminary results of the integration of these technologies, as well as future prospects will be discussed.
Self-directed Self-assembly of Block Copolymer/Nanoparticle Thin Film as a Stimuli-Responsive Material
Kung-Hwa Wei
National Chiao-Tung University
Nanoparticles such as gold or semiconductor nanoparticles have distinctive optoelectronic properties, but manipulations of these nanoparticles remain a great challenge. Block copolymer can self-assemble into ordered nanostructures between 10 and 100 nm and therefore can be used as templates for controlling the spatial location of nanoparticles. Self-directed self-assembly of block copolymer and nanoparticle thin film can provide a hybrid material with highly ordered pattern and useful electronic, optical and magnetic properties. In this study, we have used a diblock copolymer that incorporates two different types of surface-coated nanoparticles, cadmium selenium and gold, into different blocks of polystyrene-block-poly(4-vinyl pyridine). Due to micro-phase separation, the diblock copolymer hybrid self assembled into monolayered film with spherical morphology after spin-coating and annealing. These hybrid thin films can have different photocurrent enhancement under illuminating at different wavelength, and therefore can be used as sensors or stimuli-responsive material.
Stimuli-Responsive Colloidal Systems from Smart Nanoparticles
Sergiy Minko
Clarkson University
Nanoparticles and their assembles have attracted a great deal of attention due to their intriguing electronic, optical, magnetic, transport, mechanical and catalytic properties, which are generally associated with their nano or quantum-scale dimensions and extremely high surface to volume ratio. Here we address the problem of nanoparticles at interfaces. Amphiphilic nanoparticles are used to populate the liquid-liquid interface and stabilize emulsions (often referred to as Pickering emulsions). Amphiphilic properties can be easily approached with an organic shell grafted to nanoparticles. Organic shells in hybrid nanoparticles are used for the regulation of interparticle distance and their interactions, which allows for tuning physical properties of the colloidal dispersion. Here we report on the first experimental attempts to apply mixed brushes for the reversible in situ switching colloidal systems. We used grafting of responsive polymer brushes to fabricate smart nanoparticles and employed the smart particles to prepare responsive colloids, which demonstrated drastic transformation and switching of material properties upon external stimuli. We show that the interaction between the particles themselves and the particles and their environment can be precisely tuned by a change of solvent and pH. We show that this behavior can be used for a reversible formation of particle aggregates, stabilization and switching between w/o and o/w emulsions. We demonstrate an example of the application of the responsive colloids for the fabrication of smart coatings with textured surfaces.
Environmentally Responsive Gels with Tailored Periodic Structures
Zhibing Hu
University of North Texas
Polymer gels usually consist of a randomly crosslinked polymer chains and contain a large amount of water. Recently considerable efforts have been directed to creating gels with periodic structures. In this talk, the formation of such gels by self-assembling thermally responsive gel nanoparticles will be briefly reviewed. The colloidal particles made of poly-N-isopropylacrylamide (PNIPAM) will be used as a model system to discuss self-assembling processes as a function of temperature and polymer concentration. Specially, the growth of columnar gel colloidal crystals along the gravity direction will be discussed. These columnar crystals have been obtained by mixing an aqueous suspension of PNIPAM nanoparticles with organic solvent. Coalescence of micelles consisting of organic oil droplets coated by many gel nanoparticles may play a key role for the directional crystallization. A phase diagram has been determined and it can be used as a guide to selectively grow different crystals. The crystalline structures of PNIPAM nanoparticles can be stabilized by crosslinking neighboring particles into a bulk gel. Polymer gels with tailored periodic structures under mild synthesis conditions could open an avenue for
new applications.
Tailoring the Ionic Domains in Perfluorosulfonate Inonmers for Controlled Actuation Behavior in Artificial Muscles
Robert B. Moore
The University of Southern Mississippi
Over the last several decades, perfluorosulfonate ionomers (e.g., Nafion®) have become the “benchmark” membrane materials for proton exchange membrane fuel cells and more recently for ionic polymer metal composite (IPMC) artificial muscles. While this unique ionomer has yielded significant commercial success as an ionic conductor, the fundamental morphology-property relationships that govern the chemical and physical properties of this complex, nanostructured polymer have been poorly understood. In this presentation, the molecular and morphological origins of the dynamic behavior of Nafion® will be probed using a wide spectrum of analytical methods including dynamic mechanical analysis (DMA), small-angle (synchrotron) x-ray and neutron scattering (SAXS and SANS), solid-state 19F NMR spectroscopy, and dielectric spectroscopy. Based on this fundamental morphology-property information, the organization of the ionic domains may be tailored to exhibit fibrilliar morphologies, in a bio-mimetic fashion, in order to stimulate anisotropic actuation behavior in the resulting artificial muscles.
Stimuli Responsive Polymer Solubility
David Bergbreiter
Texas A&M University
The effects of polymer microstructure, size and polydispersity on lower critical solution temperatures (LCSTs) of polyacrylamides will be discussed. Synthetic approaches to libraries of very similar polymers including structurally isomeric polymers and polymers differing only in end group structure will be described. Companion studies will show how salts and solvent isotope composition affect these LCSTs. Simple new analytical approaches that provide a method to determine cloud point temperatures at any point in the clouding process independent of heating rate will be described along with earier results of studies using a temperature gradient apparatus.
Photo Responsive Polyarylate Powder Coatings
Glen Merfeld
GE Global Research
A polyarylate powder coating resin has been designed to respond to outdoor exposure by forming a skin of ultraviolet light absorbing chemistry. A photo-Fries rearrangement within the polymer backbone is the chemical basis for the formation of highly efficient UV absorbing moieties analogous to hydroxybenzophenones. This intrinsic response allows the coatings to be extremely stable to exterior conditions while affording a high degree of UV protection to underlying coatings or substrates. In a self-sacrificial manner, the coating’s top surface undergoes controlled and highly uniform erosion which allows for extremely stable long-term gloss retention. At the same time, the protective UV absorbing chemistry is regenerated continuously at the newly exposed surface. Beyond exterior durability, this polyarylate chemistry offers benefits in coating performance with an exceptional combination of toughness and chemical resistance.
Nanohybrid LBL-Films for the pH-Triggered Disintegration of a Sacrificial Film Segment Leading to the Release of Freely-Suspended Polyelectrolyte Multilayer Membranes
Shoko Sugiyama Ono
Mitsui Chemicals, Inc.
In order to design stimuli responsive materials at will, it is indispensable to understand and carefully design the local (nanoscale) structure of the material in question. The layer-by-layer (LBL) assembly method allows to easily construct hybrid nanoscale films or devices at will with reasonable positional order of individual molecules so that architecture-dependent characteristics of stimuli-responsive materials can easily be investigated. We have investigated pH responsive multilayers consisting of poly(acrylic acid) (PAA) and poly(ethylene glycol) (PEG) in order to use such films as sacrificial layers for releasing self-standing membranes fabricated on the sacrificial layer. This strategy led to the release ultrathin self-standing polyelectrolyte multilayer membranes with a thickness of 55 nm from solid substrates at physiological conditions or close to physiological conditions. 1) In this presentation, we focus on the growth mechanism of the nanohybrid LBL film consisting of a pH responsive multilayer segment (sacrificial layer) and an electrostatically-assembled film segment (released layer).We found that there are four growth regimes or film segments in the growth of such hybrid films. 2) For a good release, the second film segment must be thick enough to facilitate the pH-induced disentanglement of PAA and PEG. For the properties of released membrane, not only the fourth but also the third regime plays an important role. Further on we investigated the pH-induced disintegration of the sacrificial layer and the release, morphology and structural features of the released film as a function of the architecture of the whole system. This opens a way to analyze and optimize device structure and performance efficiently on a rational base.
Analysis of Stimuli-Responsive Materials by Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D)
Stephen Hussey
Q-Sense Inc.
The development of stimuli-responsive materials and the study of their interactions with biological substances are of critical importance in the advancement of new medical devices, food packaging materials, drug delivery systems and biosensors. There is an urgent need for new analytical tools to study responsive materials at the molecular level, and in real-time. Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D) is a nanomechanical, acoustic-based analytical technique that provides in-situ analysis of interactions and reactions taking place at the interface between numerous classes of stimuli-responsive materials and biomolecules. In addition to revealing molecular mass, as measured by changes in frequency of the quartz crystal, the dissipation parameter (D) provides novel insights regarding structural (viscoelastic) properties or assembly of many surface substrates, before, during and after the interaction. There are a wide range of surfaces compatible with QCM-D that can serve as both fundamental substrates for interactions, or as molecular scaffolds upon which subsequent responsive materials are applied. Specific stimuli-responsive materials that can be analyzed with QCM-D include hydrogels, polymer brushes, polyelectrolytes, metals, membranes and a variety of other thin films. Material responses that can be measured with QCM-D include hydrogel swelling or shrinking, polyelectrolyte layer build-up, protein adsorption or conformation change and enzyme degradation. In summary, QCM-D is an ideal analytical technique used to study stimuli-responsive materials and their interactions with a variety of analytes as a critical step in the development of new biomaterials.
Smart Polymer Films with Built-In Logic
Ryan Toomey
University of South Florida
The overall thrust of our research program is to develop responsive surfaces capable of directing adsorption behavior without the need for complex circuitry or bulky instrumentation. The approach is to use surface-attached, photo-cross-linked polymer films that be toggled between states in response to an external target. The target alters the balance of hydration forces in the responsive layer, resulting in changes to the swelling of the layer. An important distinction between surface-attached polymer networks and bulk polymer networks, however, is the effect of surface-attachment on the network properties, which confines swelling to a single dimension. This confinement alters the ability of the constituent chains to swell compared to their bulky, unconstrained counterparts and therefore influences both critical phenomena and volume-phase transitions in responsive networks. My talk will focus both on techniques for fabricating surface-attached networks, as well as the effect of surface-attachment on the response characteristics of the layers as measured by ellipsometry. Finally, I will also address tuning the transition behavior of stimuli-responsive layers with embedded peptides. To this end, we demonstrate that the transition temperature of poly-N-isopropylacrylamide (Poly-NIPAAm) layers can be governed by the conformation of embedded peptide sequences. The sequences are grown directly in a poly-NIPAAm network copolymerized with a lysine like monomer, N-(3-aminopropyl)methacrylamide hydrochloride using standard Merrifield synthesis techniques. We can build logic directly into the layer structure via the conjugated peptides, which alter the response characteristics of the layer depending on the conformation of the peptide.
