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Research Report 2009
Polymer Research 2009: innovative | interdisciplinary | international
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Structure and Dynamics
The experimental and theoretical understanding of the structural and dynamical properties of macromolecular systems is a central objective of the institute’s work. On the way to achieving complex self-assemblies and functional materials the understanding of basic structural and dynamical, or relaxational aspects and mechanisms is an indispensable part of rational materials design. Many pro-jects in this field of research are currently being pursued, of which a small subset is described below. These activities range from the analytic theory of single macromolecules to multiscale simulations of complex aggregates, and from the structure of hydrogels or ionic aggregates to instabilities in colloidal flow. The latter project illustrates that non-equilibrium phenomena are attracting increasing attention.
Though the theory of single chains might seem rather well-known, there are still many unsolved problems which are of direct relevance to either numerical modeling or experimentation. A typical problem of this class is a single chain under strong tension. The dynamics can no longer be described within the Rouse scheme and the nonlinearity, originating from bond length constraints, dominates the behavior leading to shock waves propagating through the molecule. While such complicated situations on the single polymer molecule level allow for analytical treatment, complicated amorphous and structurally organized macromolecular systems are investigated by systematic multiscale modeling strategies.
These methods have been developed in the institute for more than ten years and are being applied to a number of problems, among them amorphous polymer melts with and without low molecular weight additives and polymers under constraints. In a recent extension liquid crystalline macromolecules and peptides close to a (metal) surface have been studied. Along another line, by systematically linking all atom force field simulations, it was shown how we could determine the time-scaling within a multiscale approach. This was then used to predict diffusion constants of i.e. a polystyrene melt of up to 50 kDa molecular weight in excellent agreement to the experiment without any adjustable parameter.
Structure formation and controlled assembly are the focus of joint simulations and various experiments and cover both ionic systems and hydrogen bond dominated hydrogels. The formation of DNA, or charged dendrimer assemblies, can be linked to co- and counterions, but also to pH effects, and are studied by UV spectroscopy. Details of interaction between the macroions and counterions can directly be studied by ESR spectroscopy. This method allows us to determine whether the counterions are metastably bound or whether the localization is transient with only a short time effect that has direct influence on the overall polyelectrolyte conformations. These conformations and the assembly of polyelectrolytes due to counter- and salt ion condensation are also studied extensively by computer simulations. For a model, which is adjusted in its parameters to correspond to polyparaphenylenes (PPP), the typical size of such aggregates as a function of salt ion valence was determined by computer simulations. Though related, the situation in hydrogels can be somewhat different. Photo crosslinkable systems of PNiPAAm can be used as actuators and are sometimes termed as smart materials. To characterize their structure and function a whole set of experimental techniques is employed including various spectroscopic methods and surface plasmon resonance (SPR).
Theoretically structure formation in block copolymer systems quite often is studied by self consistent field methods. Extensive simulations now have been employed to carefully study their possibilities and limitations. In an extension another option to steer structure formation is through spatial confinement. Block copolymers, where the attraction of the wall to one component is varied, are perfect candidates for such an investigation.
Of course, not only structural aspects are being investigated, but to an equal extent those of dynamical properties. A special situation occurs in semi-crystalline polymers as the amorphous and crystalline regimes are governed by significantly different mobilities of the chain segments. This is of immediate relevance to processing properties. Here, NMR spectroscopy offers unique opportunities for linking structural information to diffusion constant measurements. It has been established that the interface between the crystalline and non-crystalline regions plays a vital role in determining not only chain diffusion between the two regions, but also processability. While the previous example dealt with polymer diffusion in a varying polymeric surrounding, the dynamics of confined macromolecules is another topic of high interest. Especially when confinement occurs on a nanoscale, mobility is significantly influenced by conformational constraints. To study the mobility, the diffusion of poly(isoprene) (PI) in nanoporous aluminium oxide was investigated by fluorescence correlation spectroscopy. Due to the special structure of the nanoporous matrix material this can be extended to observe even single molecules.
Transport properties are not only considered for polymeric systems. Polymer-based biodegradable nanoparticles as drug carriers can be designed to reach target tissue very effectively and may even pass the blood brain barrier. Such particles produced by miniemulsion polymerization can be fluorescently labeled and followed in in vivo or in vitro experiments to demonstrate the high potential of such a strategy. These objects are typically rather fluffy and deformable. Hollow silica microcapsules, however, are much more rigid and can also be employed as carrier materials. These capsules, with a diameter of about 1µm, have been synthesized and their elasticity measured via an AFM and/or Brillouin scattering.
So far equilibrium properties have mostly been discussed. A better understanding of these properties is what we are basing our research on with an extension of our activities to non-equilibrium situations, which certainly occur more frequently in nature. In many cases externally driven systems are of high interest. Almost all biological systems are permanently driven by a continuous supply of energy. In a typical experimental situation the driving force can originate from fields (electric, shear etc.) or temperature (energy). Given this, it is useful and appropriate to begin with rather idealized theoretical models or experimental systems.
Theoretically non-equilibrium phenomena are tackled in two ways. On the one hand, we employ macroscopic hydrodynamic theory to study driven complex fluids and the instabilities therein, leading to phenomena such as convective flow. This in turn influences the structure. Typical examples studied by this approach are liquid crystals or ferrofluids. While in the first case shear and temperature gradients drive the systems, magnetic fields are used for ferrofluids. On the other hand, when it comes to conformational distortions of a macromolecule in a flow field, or the deformation of the charge distribution around a polyelectrolyte, or a charged colloid, mesoscopic computer simulations are the method of choice. Due to the coupling of different methodologies such complex problems can currently be approached. Recently, the combination of lattice Boltzmann and molecular dynamics simulations allowed us to work on the first steps towards the study of turbulent drag reduction by solvated polymers.
From the experimental point of view, colloidal suspensions offer many ways to directly observe non-equilibrium problems. Soft colloid-like systems and stiff cylindrical bottle brush polymers are especially susceptible to shear fields. By varying shear and density they show lyotropic behavior. The variation of density in addition allows manipulation of the shape of the interaction potential. Another important property of colloidal suspensions is the strong dielectric contrast between the colloid and the surrounding water. In non-uniform electric fields this leads to a motion of the colloids due to dielectrophoretic forces. This effect can also be used to control the motion of individual colloids or to study transient effects due to the switching on and off of the electric field. While colloidal systems are important, model systems allowing a deeper insight into non-equilibrium phenomena, polymers, such as polystyrene in cyclohexane, or low molecular weight mixtures, display fascinating phenomena as a result of the competition of the time derivative of the temperature (i. e. cooling rate) and phase transition kinetics. This competition creates very interesting intermediate structures of oscillatory behavior and a variety of characteristic droplet sizes upon phase separation.
Related Topics:
Polymer Theory ...read more
Polymer Spectroscopy ...read more
Surfaces and Interfaces
Characterizing interfaces, studying processes which are dominated by interfaces, and making surfaces with a specific structure and function are key activities of the Max Planck Institute for Polymer Research. Surfaces and interfaces of soft materials are not only interesting per se. They are essential when it comes to small objects. Objects, which at least in one dimension extend only over microns or nanometers, have a high surface-to-volume ratio. Their properties are dominated by interfaces rather than gravitation or inertia.
A whole range of techniques for the characterization of surfaces is available at the institute. For structural analysis scanning probe microscopes such as atomic force microscopes, a scanning tunneling microscope, and a near field optical microscope are used. They are complemented by scanning electron and several light microscopes. X-ray reflectometry gives detailed information on film thickness, roughness and mean composition of thin polymer films. Adsorption processes are monitored by surface plasmon resonance, ellipsometry, or by the quartz crystal microbalance. With the colloid probe technique, surface forces between particles and planar surfaces are measured.
We not only use existing techniques, but constantly improve devices, develop new methods or modify techniques to apply them to special problems encountered during a project. One example is the new technique developed to measure adhesion on a fast time scale. Usually adhesion between particles is measured on the 0.1 s time scale and longer. To understand the adhesion process better we developed a new force microscope, which is able to measure adhesion for contact times down to 10 µs. This instrument allowed us to distinguish between different binding processes.
Measuring surface forces is not only interesting in itself, but also helps us to understand the aggregation process of particles. The aggregation of nanoparticles is used in several projects, e.g. to obtain phononic crystals.
Processes, rather than a system in equilibrium, become increasingly the focus of our research. A typical example is the project “evaporation of microdrops”. So far only the evaporation of macroscopic drops could be measured, mainly by optical techniques. To measure the evaporation process of small drops a special technique was developed, which is based on atomic force microscopy. The question is, how do liquid droplets of dia-meters significantly below 0.1 mm evaporate from a solid surface and how do the surface properties influence the evaporation process?
At a fundamental level one of the unanswered questions concerns the structure of polymers at a solid surface. Polymer-solid contacts are important for adhesion, coating, and (nano)composites.
Several projects are devoted to the polymer-solid interface, e.g. “polymers at surfaces and interfaces”, “modelling and interactions of macromolecules and biomolecules with surfaces” and “ordering of polydisperse systems in the presence of interfaces”.
An understanding of interfaces is also a prerequisite for making micro- or nanostructures because their behavior is dominated by surface effects. Structuring polymer surfaces by selective deposition, by etching and by methods with no net mass change, is one of the main topics of the institute. A focused ion beam allows us to structure surfaces on the 10 nm scale. In some projects the focus is to coat surfaces with a homogeneous, thin layer rather than lateral structuring. This is for example achieved by plasma polymerization. Using plasma polymerization, solid surfaces can be coated with a wide variety of polymers. Making surfaces with properties for a specific function was the aim of several projects. For example, complex architectures have been designed for biomolecule detection by optical techniques. Surfaces made of hybrid materials are applied in photoactive layers.
Related Topics:
Physics at Interfaces ...read more
Supramolecular Architectures
Molecular chemistry is mainly focussing on the understanding of the behavior of individual molecules and their construction from constituent atoms; strong association forces such as covalent and ionic bonds between atoms are used to assemble the atoms as building blocks.
However, it is obvious that nature not only uses only covalent bonds for structure formation, but many different reversible non-covalent interactions including metal coordination, π-π interactions, hydrophobic forces, van der Waals forces, and hydrogen bonding in order to obtain the remarkable properties and functional capabilities of biological systems. A discrete number of molecules organize themselves in a collective manner into longer-range order and higher-order functional structures by comparatively weak forces; such complex systems are considered as hierarchical. The weak interactions play an essential role for the function of the system, one example is a membrane which is built from low lipids.
The idea to imitate nature's supramolecular design motifs to generate robust nanostructured materials in a facile manner has inspired chemists for quite a while to design new architectures as supramolecular assemblies by using not only covalent, but also non-covalent interactions to build new functional, and technologically important materials. Such synthesis approaches of “bottom-up” materials based on supramolecular chemistry can provide a solution to the size limitations of “top-down” approaches which are currently used e.g. in photolithography. Therefore, supramolecular architectures can also be used to assemble active components of miniaturized electronic devices.
A deeper understanding of structure property relationships at a molecular and supramolecular level as well as a combination of molecular and supramolecular design principles allows a great possibility of the design of a large variety of molecules from biological to synthesized, from inorganic to organic, and from monomeric to macromolecular in origin. Different concepts like molecular self-assembly, molecular recognition, guest-host chemistry, mechanically interlocked architectures, and dynamic covalent chemistry are just a few examples of possibilities in current research. They promise to create systems which serve as biocompatible materials, biomimetic catalysts, effective electronic devices, excellent bio-recognition, chemical sensing, and so on.
Each supramolecular assembly consists of molecular building blocks which can be either small molecules such as molecular liquid crystals, hydrogen bonding molecules, catenanes and rotaxanes, or macromolecules such as block copolymers with incompatible blocks and dendrimers. It is now the interest of current research to design molecules which are suitable for supramolecular architectures. Additionally, a detailed study and characterization of the structure formation and their connected function using the different non-covalent interactions of supramolecular architectures are crucial for the understanding of many processes in materials science. It is the strength of the institute to cover the entire range from the synthesis of the building units, the study of the kinetics and dynamics of the structure formation to the characterization of the systems.
The synthesis and investigation of liquid crystals have been key activities of the institute for years, large efforts have been devoted to the investigation of the dynamics by using NMR, dielectric spectroscopy, scattering, and computer simulation. However, for the application as electronic materials, the intrinsic disk mobility can also influence the charge carrier significantly.
However, the assignment of the slower dynamics was not known. Dipole-functionalized hexabenzocoronenes have now been employed for detailed investigations of the self-assembly behavior, the thermodynamics and pathways of structure formation.
A special interest was in the study of the slow dynamics by a combination of dielectric spectroscopy and site-specific NMR techniques.
For the study of non-covalent hydrogen bonds and hydrogen-bonded aggregates, ultrafast magic-angle spinning proton solid-state NMR is shown to be a useful and sensitive tool. The method allows the investigation of assemblies by different molecules.
There is no doubt that extended π-electron systems belong to the key specialties of the institute. Beside the synthetic approaches which are required to build new molecular architectures (see also synthetic approaches), the arrangement of the systems including the morphologies and electronic states of extended π-electron systems is important for further application as electronic materials.
Multifunctional dendritic molecules can be used in combination with metal particles in order to build hybrid nano-assemblies. The idea is to place the multifuctional molecules with chromophores in the nanosized gap between the metal particle and the metal surface in order to strongly alter the photophysical properties, e.g. to increase significantly the stability and the brightness.
Typcial representatives of supramolecular structures are membranes. Biological membranes around a biological cell consist of a lipid layer with many components as transporters or receptors embedded. Since natural membranes are quite complex due to multiple interactions of various components systematic investigations are difficult.
Building units of much higher complexity than lipids in membranes are biological molecules such as peptides and proteins. The secondary structures of the peptides and proteins which play a key role in determining the biological activity describes the arrangement of the monomeric units. It is defined by hydrogen bonds between elements of defined topology and by the primary structure. Tailoring of the chemical structure would allow one to affect the supramolecular architecture and connected with this the activity.
Model membranes can be constructed e.g. with functional ion channel incorporation which allows modeling of the ion transport. The immobilization of proteins in the membrane layer can be investigated by using electrochemically controlled surface-enhanced infrared resonance Raman spectroscopy.
Not only molecules, but also colloidal particles can be used for supramolecular architectures. Using colloidal particles with a high homogeneity in size leads to a defined three dimensional arrangement of the spheres resulting in photonic crystals. The photonic band gap can be tuned in such hierarchical colloid systems, which are also called colloidal crystals.
Supramolecular structures are also utilized in the formation of multiple stable emulsions. Polymerizations in such organized systems can be used for the formation of nanocapsules which can serve as (drug) delivery systems.
In summary, tailoring structure and function of extended assemblies ranging from small molecules to macromolecular systems and colloidal particles is of special interest in our institute. Here an interplay of chemistry, physics, and biology is important and is demonstrated by the different activities in the institute encompassing significant activities of all groups. The design of functional building blocks to new synthetic systems will lead to materials with more useful ensemble properties originating directly from an interplay of nanoscale and microscale ordering.
Related Topics:
Physical Chemistry of Polymers ...read more
Synthetic Chemistry ...read more
Functional Materials and Devices
The aim of the institute is to understand the structure and dynamics of matter at different length and time scales. Ideally, “understanding” implies that from the chemical structure and the process chosen the properties of a material could be predicted. With this knowledge materials with desired functions can be designed. Even though we are still far away form such an understanding the expertise is applied to make functional materials. In this chapter we focus on three fields in which the MPI-P has made substantial contributions: polymer electronics, energy, and drug delivery. These three fields not only demonstrate our awareness of long-term technological developments, but they also show how topics are investigated in an interdisciplinary approach by project leaders with different backgrounds. irrespective of the particular group to which they belong.
In the field of organic electronics, the goal is to create processable organic or polymeric materials with an enhanced life duration and an improved performance. This requires a better understanding of the physical processes which lead to the deterioration of device performance. Research is not concentrated on isolated materials, but on the function and performance of a material in its complex device configuration. In addition, a spectrum of processes to be used in device fabrication needs to be developed and improved. This involves the design of molecular and supramolecular structures of organic material. Existing organic or polymeric materials need to be further developed to enhance their usefulness in fast and industrially acceptable processes, e.g. ink-jet printing, roll-to-roll manufacturing. Moreover, their properties in adhesion to metal and/or semiconductor electrodes, corrosion of contacts, undesired migration of dopants etc., need to be considered. For the future ecological use of energy, three different core technologies, to which polymer science can significantly contribute, are developed: Collection of energy, storage of energy, and delivery of energy. For the collection of energy in an environmentally-friendly way, solar cells will play a decisive role. Photovoltaics serves to transform solar light into other types of energy or to generate hydrogen for energy storage. The control of supramolecular ordering in a photovoltaic device is essential for successful development. New technologies are being developed to generate nanostructures with perfect phase separation between the donor and acceptor structures and with a maximized interface for the charge separation. For storage, lithium-ion batteries are commercially available. However, due to the low specific capacity of the electrode materials, batteries small in size and with a high power supply are presently difficult to fabricate. Metal oxides (MOs) yield high specific capacity during reversible redox reactions with lithium, and thus as anode materials are expected to miniaturize the battery and at the same time significantly improve the battery performance. One major problem with the MO-based batteries is the poor cycling stability. To solve this problem, we developed a concept based on the extensive study of nanographenes. Graphene layers are used for covering the surface of MO nanoparticles to form MO-core graphene-shell structures. The graphene-covered MO displays significantly improved cycling performance. The use of solar energy stored as hydrogen requires the replacement of combustion engines. Therefore, fuel cells will be a core technology in the development of “zero emission” energy production. To rise to this challenge we are developing new proton exchange membranes. Although numerous membranes exist, the demanding criteria for automotive applications have not been satisfied. Long-term temperature stability at a desired working temperature of 130 °C is still not guaranteed. More importantly, due to the production of water during the operation of the fuel cells, the electrolytes in the membranes are diluted. Both processes result in a slow decrease of the performance of the membranes. To solve this problem hybrid materials based on phosphonic acids are developed, which are stable even at high temperatures. Developing drug carriers for directed delivery is another active research area at the MPI-P. In contrast to passive diffusion, directed delivery allows to reduce the amount of drugs that has to be applied. Thus, the total dose and negative side effects can be reduced.
One of our major research interests includes the design of novel bio-hybrid materials for the visualization and initiation of directed drug delivery. Our platform consists of multifunctional oligolysines as well as proteins such as bovine serum albumin (BSA) or human serum albumin (HSA). Up to three different types of functionalities could be attached to both carrier molecules in a spatially organized way.
Related Topics:
Physical Chemistry of Polymers ...read more
Polymer Physics ...read more
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Approaches to Synthesis
We are living in a material world...” (quoted from Madonna). Indeed, there can be no technological or modern-day progress without new materials – which must be synthesized. Many materials are of a polymeric nature which indicates the key role of polymer synthesis. Competence in synthesis is therefore a cornerstone of our institute.
Although such a synthesis must be practical and provide sufficient quantities, the limitations of the synthetic method with respect to the occurrence of side products and structural defects must be carefully investigated. Polymer synthesis thus plays a dual role: on the one hand the researcher can rely upon established synthetic methods and focus on known polymer structures; on the other hand, he can design novel methods of synthesis and aim for unprecedented targets. The novel methods approach also has a dual character, because the research concept can be significantly different, depending on whether the work is more method- or structure-oriented. Not long ago, the search for new polymers was regarded as obsolete, since most of the important goals of polymer research were supposed to be achievable using classes of compounds then available. Interest in polymer synthesis has since been revived, however, by several events including:
- - the utilization of new synthetic methods developed in organic and organometallic chemistry, and this interdisciplinary approach has greatly increased the efficiency and scope of polymer synthesis,
- - the introduction of novel polymer topologies made from biopolymers and synthetic polymers or equally important synthetic polymers and inorganic materials,
- - the discovery that macromolecules have the potential to form supramolecular assemblies with unique properties.
The focus of polymer research cannot be restricted to molecular structures, but we also have to consider supramolecular ordering at different length scales. While the relevant intermolecular forces may well depend on the conditions of processing, there is the additional challenge for polymer synthesis to prearrange supramolecular effects, e.g. by incorporating units able to self-assemble and thus encode factors for supramolecular structure formation.
Research directed towards electronic materials and devices relies heavily on the synthesis of new structures. Classical chain structures such as polyphenylenes and poly(phenylene vinylene)s do not cover the full scope. New two- and three-dimensional conjugated polymers which we have introduced allow careful tuning of electrical, optical and even magnetic properties. Carbon-rich polymers are of particular interest because of their unique chemical properties and their role as graphite-related electronic materials.
Creative structure design and the use of methods from organic chemistry have produced remarkable results including shape-persistent dendritic polyphenylenes and giant polycyclic aromatic hydrocarbons as processable graphenes. Graphenes are presently one of the hottest topics of solid-state physics due to their outstanding properties such as ballistic charge transport and quantum Hall effect. These single-sheet materials are obtained by exfoliation from graphite. Such methods define an urgent need for a synthetic “bottom-up”-approach toward perfect graphenes, and this becomes possible when using our well-defined polyphenylene precursors.
The so-called nanosciences have recently attracted great attention, since they offer opportunities for visualizing and manipulating single molecules or nano-sized aggregates made from a few molecules.
While sophisticated physical methods are required, synthesis plays an indispensible role: it provides designer-made molecular objects of well-defined size, shape and dimensionality, and it deals with the aspects of processing and pattern formation, which are the ingredients of single molecule detection. The above mentioned 2D and 3D nanoparticles can exhibit complex electronic functions and, thus, may serve as kind of miniaturized devices.
Statements such as ”learning from nature” are often used by chemists when selling their results to a broader public. A counter-argument is that we do not construct aeroplanes from feathers. Let us assign the question of “natural” or “synthetic” to the world of propaganda slogans, because it is clear that the technical use of polymeric biomaterials requires synthetic methods for their transformation and that polymer research must include biopolymers as key materials of life. But are there synthetic challenges? While biodegradation is not a research issue in our institute, new hybrid architectures, e.g. di- and multiblock copolymers, dendrimers, core-shell systems, and bottle-brush macromolecules comprised of a biological and a synthetic part attract great attention. Hybrids containing polypeptide entities are particularly relevant for studying biological functions, including cell adhesion, enzyme mimicking and tissue engineering. The generation of peptide segments does not only rely on ring opening polymerizations of suited amino acid monomers and automated solid phase synthesis, but also on recombinant protein expression, especially when larger peptide units are required.
Progress in polymer synthesis does not only come from the invention of new reactions or new structures, but also from better solutions for “old” processes. While anionic polymerization has recently produced impressive results in the synthesis of, for example, block copolymers or star topologies, controlled radical polymerization, has afforded not only unprecedented amphiphilic block copolymers, but also unique core-shell structures. These have a strong influence on related projects, among which are emulsion polymerization using amphiphilic block copolymers as emulsifiers, and controlled mineralization using block, comb or core-shell copolymers as additives in crystallization from an aqueous solution – so called mineralization. Unique morphologies of titanium dioxide which have long been elusive are now available. This work receives a special twist by new core-shell systems. Firstly, a fluorophore as a signalling site is encapsulated in its own dendrimer shell and, secondly, functions on the surface are used as initiators for the construction of polyelectrolyte shells by controlled radical polymerization. Depending on the nature and number of charges, drosophila larval tissue can be shown by confocal microscopy to undergo cell-uptake with high specificity.
Further, these polyectrolytes serve as carriers for efficient DNA transfection. This is monitored not only by suitable assays, but also by AFM, light scattering, and calorimetry. An even more elegant approach towards functional polyelectrolytes comprises cationization of the bovine serum albumin. Efficiencies in DNA transfection are superior to those of the golden standard which is, again, analyzed in terms of the energetics of polyelectrolyte-polyelectrolyte complexes. Polymer topologies, which have been long sought after, now become available by innovative methods. A powerful tool is templated and geometry-confined synthesis. Thus, a porphyrin molecule serves to pre-assemble carbazole moieties for the synthesis of giant macrocycles with extended π-conjugation as a model for conjugated polymers of an infinite length. A related process is polymer synthesis in micelles leading to polymer latices. While often considered as a conventional method, it reveals an undiminished vivacity from both a practical and fundamental viewpoint. A wide breadth of functions, from optoelectronic to biomedical, can thus be obtained.
In many instances, progress in polymer synthesis is fueled by new experimental techniques such as the use of supercritical fluids, high pressure or catalytic processes with specially designed carriers. Not surprisingly, therefore, the relevant research is conducted jointly by chemists and physicists. This holds, in particular, for polymer synthesis in confined geometries, such as surface controlled reactions or latex formation (see above) and the fabrication of organic-inorganic hybrids. New polymer resins have been introduced which serve as remarkably powerful, “responsive” carriers for metallocene catalysts in polyolefin synthesis. Highly dispersed carbon-metal or carbon-metaloxide nanocomposites open up new avenues for lithium storage and catalysis.
The approach described here is comprehensive in that it covers a broad range of polymer molecules and particles with different degrees of complexity. In the first place, established polymer structures are produced more or less routinely for various purposes. Polymer scientists who prefer established chemical methods might consider dendrimers, macrocycles, or chiral polyphenylenes as a somewhat unorthodox species requiring overly sophisticated methods of synthesis. It must be stressed, however, that such synthetic efforts provide access to fundamental questions of polymer science and, as a most welcome spin-off, assist in the solution of long-standing problems of polymer synthesis. Whatever degree of complexity or simplicity is intended within polymer synthesis, structural precision, control of multiple chemical functions as well as combined approaches toward synthesis and processing are indispensible criteria.
Related Topics:
Physical Chemistry of Polymers ...read more
Synthetic Chemistry ...read more
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Development of Methods
The relationship between the macroscopic properties of polymers and their structure, dynamics, and supramolecular organization is complex. It involves enormous length- and timescales from the molecular, via mesoscopic to macroscopic dimensions and from picoseconds to years. Bulk-behavior should be compared with surface effects, and electronic, as well as mechanical and transport properties are of interest. Consequently, a considerable variety of techniques is needed in order to unravel the different aspects. In fact, numerous forms of computer simulation, spectroscopy, scattering, and microscopy are used. Advances and modifications of these techniques are needed to cope with new questions associated with new materials, for instance biomimetic systems. Therefore, from its beginning, an important aspect of the institute’s work is the development of advanced methods, including the adaptation of techniques for specific polymer problems. Likewise, versatile methods are developed which, by their very nature, can be applied to numerous problems of polymer science. This chapter describes some of these activities.
At the outset of a theoretical description traditionally a model, which is treated in terms of an analytic approach, is studied. This ansatz in the past has been very successful for rather idealized problems, elucidating the very basic principles of polymer physics. These studies then were complemented by computer simulations of rather simple, but already more complex models. This leads to a rather good understanding of scaling properties of macromolecular systems, where the above mentioned many time and length scales were well separated. In order however to take this interplay of scales properly into account and to arrive at truly quantitative predictions of specific chemical systems scale bridging methods have to be developed. Methodology along these lines has been developed in the theory group for more than ten years and nowadays is very fashionable under the name “multiscale modelling”. In order to progress here method development has to be pursued not only at the link between methods devoted to specific scales but also on the methods themselves. On the quantum mechanical level mostly ab initio density functional methods (DFT), i.e. the Car Parrinello method (CPMD), are employed. The electron density functionals, which are based on electronic orbitals, are computationally very demanding and thus mark a severe limitation when one wants to study more complicated or larger systems. To improve this situation we contribute to the development of so called orbital free DFT, which eventually should significantly speed up the ab initio DFT. Established quantum mechanical DFT methods are either used for rather small systems or in a hybrid approach, where they are coupled to a classical environment. In this quantum mechanical/molecular modelling (QM/MM) approach a small subsystem is treated by quantum DFT while the surrounding is treated by a classical force field simulation (MM part). A central aspect of improved methodology is the coupling between the quantum and the classical regime.
This problem is addressed by developing new effective potentials to terminate the dangling chemical bond of the QM region with the special focus to optimize the electronic density and spectroscopic properties. Currently this is used to study proton conducting membranes as they are used in fuel cells. On a larger scale the penetration and solvation of polymers by whole molecules or salt ions is of great interest. Such systems are far beyond the range of applicability of the above described QM/MM ansatz. In a combined approach of a multiscale simulation of the polymeric matrix with a fast growth thermodynamic integration procedure to obtain the excess chemical potential of an inserted particle/molecule one is able to study solvation properties of rather complicated polymeric solvent solute systems. All these methods however use rather different levels of description in a fixed or sequential way.
Eventually one would like to perform a simulation of a complex macromolecular system on different levels of resolution in full equilibrium between the different regimes, meaning that we also have a free exchange of particles/molecules between the different levels of resolution.
The AdResS (Adaptive Resolution Simulation) method was developed to achieve this goal on the basis of particle based simulations. In a recent extension also the coupling to hydrodynamic continuum was shown to work properly opening the way to open system simulations. However to study hydrodynamic properties of large systems still requires different methods. To treat macromolecular objects in solution and to properly take hydrodynamic interactions into account a full blown particle based simulations for nontrivial problems can only be performed in very exceptional cases and with the massive use of newest supercomputer technology. One way out is given by a discrete lattice description of the hydrodynamics solvent (Lattice Boltzmann method) coupled frictionally to a molecular dynamics simulation of an embedded macromolecule or colloidal particle. In a recent theoretical analysis of this method it was shown how to implement thermodynamically consistently thermal noise via a thermostat and to properly describe Brownian motion. All these activities are needed and often linked together in order to maintain cutting edge modelling activities for soft matter.
On the experimental side, the current trends of science and technology towards further miniaturization calls for ever increasing sensitivity in detection and spatial resolution. Therefore, most of the recent advances in methodology described here deal with these issues. Magnetic resonance (NMR and EPR) are the least sensitive methods, yet provide unprecedented details on structure and dynamics because of the unmatched site-selectivity. Recently, several ways of improving the sensitivity have been implemented in the institute. These include the use of the highest available magnetic fields for solid-state NMR, which now operates at 850 MHz and magic angle spinning at 70 kHz. This also offers unique spectral resolution, improving the possibilities of unravelling complex hydrogen bonding and pi-stacking in supramolecular functional systems such as proton- or photoconductors. High field electron paramagnetic resonance (EPR) now can be applied to sample volumes as small as 2 μL, which makes this technique applicable to a significantly larger group of problems, in particular structure of biological macromolecules and the detection of nanheterogeneities in synthetic systems. Combining NMR and EPR by the technique of Dynamic Nuclear Polarization (DNP) can increase the NMR signals by factors of several hundred and NMR with non-thermal spin polarization by laser irradiation or by spin-conserving chemical reactions with e.g. para-hydrogen, signal enhacements of up to 4 orders of magnitude can be achieved. This offers new applications of magnetic resonance imaging in medicine as well as porous materials.
The sensitivity in optical spectroscopy and imaging is inherently much higher. This offers the possibility to observe single objects and molecules. Indeed, single-molecule fluorescence microscopy has become an established analytical technique during the last ten years or so. The spectroscopic parameters of single molecules, however, also depend on their proximity to metal surfaces, which can be exploited to increase spatial resolution or distinguish different objects in complex superstructures of organic-inorganic hybrids.
Optical plasmon resonances in nanoscopic metal particles offer new routes to manipulation of light on length scales lower than its wave length. This has been successfully exploited to generate highly ordered nanostructures with defined size and spacing. Indeed, evanescent wave optics, exploiting surface plasmon resonance remained an active area of method development. Long range surface plasmon modes enhancing fluorescence intensity were exploited to generate highly sensitive biosensors, and first experiments performing dynamic light scattering close to an interface with surface plasmon polaritons as a source of the scattered light were realized to study motions of particles as small as 10 nm. In a similar fashion, total internal reflection was used to increase the spatial resolution of fluorescence correlation spectroscopy. This allows investigating the diffusion of polymers in an observation volume of order 500 nm in diameter and a height tuneable in the range 70-250 nm.
Laser light can, of course, also be used to manipulate molecules. Along these lines Matrix Assisted Laser Desorption/Ionization - Soft Landing (MALDI-SL) and Pulsed Laser Deposition (PLD) have been applied to deposit thin films of polycyclic aromatic hydrocarbons (PAHs) with up to 222 carbon atoms. PAHs can be regarded as well defined segments of graphite, thus rendering them interesting candidates for electronic applications especially with increasing size. Another unconventional use of light exploits the strong temperature dependences of triplet-triplet annihilation and delayed fluorescence emission in organic phosphorescent molecules dispersed in a polymer matrix to sense temperature with a spatial resolution of 250 nm.
Today’s high brilliance synchrotron sources allow focussing X-ray beams down to the μm-scale and thus using them to investigate structure of coatings on microcantilever arrays as the essential parts of scanning probe microscopy, a method widely used in the institute. Decorating the cantilevers with polymers of different nature allows analyzing, e.g., the coil-to-globule transition of macromolecules containing hydrophobic and hydrophilic groups in the repeat unit. Combining transmission electron microscopy (TEM) with nano-area electron diffraction now allows the detection of individual nanoparticles in functional systems to investigate the mechanism of ion storage.
Sensitivity is also a major issue in the study of the mechanical behavior of soft matter, where often only small quantities of materials are available and the samples can be extremely fragile. In a piezorheometer both problems are solved as these devices are extremely sensitive and work with very small deformations in the range of 10-3 to 10-5.
Last, but not least, isothermal titration calorimety (ITC), a method often used to determine interaction constants, complex stoichiometry or conformational changes of biological macromolecules during assembly, was established at the institute. First results shed light on processes involved in gene transfection, a very complex and still barely understood biological process of high therapeutic relevance. In the future, the method will also be applied to synthetic polymers to study, e.g., polymerization reactions.
Related Topics:
Polymer Theory ...read more
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Research Reports for the Yearbook published by the Max Planck Society
Reports only available in German
Yearbook 2004
Bilder von Gasen
Authors: Blümler, Peter; Spiess, Hans-Wolfgang
[more]
Yearbook 2004
Polymertheorie: Von spezifischen Eigenschaften nahe einer Metallgrenzfläche bis zum Chromatin
Author: Kremer, Kurt
[more]
Yearbook 2005
Organische Nanopartikel als Trägermaterialien für Metallocenkatalysatoren
Authors: Klapper, Markus; Müllen, Klaus
[more]
Yearbook 2005
Dynamik weicher Materie
Author: Fytas, George
[more]
Yearbook 2006
Spinsondentechniken für weiche Nanostrukturen
Author: Jeschke, Gunnar
[more]
Yearbook 2006
Annihilation-Upconversion-Fluoreszenz bei nicht kohärenter Anregung mit Sonnenlicht
Authors: Baluschev, Stanislav; Laquai, Frédéric; Wegner, Gerhard
[more]
Yearbook 2007
Neue nicht-konventionelle Methoden zur Mikro- und Nanostrukturierung von Polymeroberflächen
Author: Bonaccurso, Elmar
[more]
Yearbook 2007
Die molekulare Nase
Author: Sinner, Eva-Kathrin
[more]
Yearbook 2008
Krumme Membranen machen Proteine attraktiv
Author: Deserno, Markus
[more]
Yearbook 2008
Wasserstoff-Brennstoffzellen: Die quantenmechanische Suche nach der optimalen Membran
Author: Sebastiani, Daniel
[more]
Yearbook 2009
Femtosekundenspektroskopie – Mit ultrakurzen Laserpulsen den angeregten Zuständen auf der Spur
Authors: Laquai, Frédéric; Baluschev, Stanislav
[more]
Yearbook 2009
Organische Halbleiter für die Elektronik
Authors: Baumgarten, Martin; Li, Chen; Feng, Xinliang; Müllen, Klaus
[more]
Yearbook 2010
Dynamische Oberflächen mit steuerbaren Funktionsebenen
Author: del Campo, Aránzazu
[more]
Yearbook 2010
Tropfen mögen’s weich – Kondensation auf weichen Unterlagen
Author: Auernhammer, Günter K.
[more]
Yearbook 2011
Physik, Chemie, Biologie & Medizin: Der Spin macht den Unterschied
Authors: Münnemann, Kerstin; Hinderberger, Dariush
[more]
Yearbook 2011
Organische Elektronik: umweltverträglicher, kostengünstiger und bald auch effizienter
Authors: Baumeier, Bjoern; Andrienko, Denis
[more]
Yearbook 2012
From stable droplets to functional nanocapsules
Author: Landfester, Katharina
[more]
Yearbook 2012
From stable droplets to functional nanocapsules
Author: Bonn, Mischa
[more]
Articles in the Max Planck Society's research magazine MaxPlanckResearch
Articles only available in German
Zauberkugeln aus Öl und Wasser 4/2009
Hellhörige Software und feinfühlige Prothesen 1/2009
Kunststoffe mit leuchtender Zukunft 1/2009
Bauanleitung für eine molekulare Nase 1/2007
Partnervermittlung für Photonen 4/2006
Mit magnetischen Gasen zur gläsernen Lunge 3/2005
Der Chip lebt 4/2004
Von Hard Science und Haute Cuisine 4/2003
Superrechner spielt mit Perlenkette und Kristallen 1/2002
Elektronik aus dem Nanozylinder 4/2002
Gesucht: Die Traum-Membran 1/2001
Forschung in der Nanowelt 2/2001
Harte Arbeit an weicher Materie 1/2001
