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Scientific highlights in polymer science

Here you will find selected scientific highlights of the collaborative research center in the fields of polymer physics, polymer chemistry and biophysics. Our transregional research center is a joint initiative of Martin Luther University Halle-Wittenberg and Leipzig University.

Aggregation of biological macromolecules

Highlights 2018

Impact of nanoparticles on amyloid peptide and protein aggregation

The effect of surfaces on amyloid formation is investigated in project B01. By a combination of experimental investigations of fibrilliation kinetics and molecular dynamics simulations it was possible to elucidate the effects of metallic nano particles on fibril formation of selected amyloid-forming sequences. The results indicate that a thin protein layer adsorbed on the metal surface, the so-called corona, is the key to understand the observed effects. It depends on the internal structure of the corona, if fibrillation is nucleated at the surface and therefore enhanced.

See also

Highlights 2017

Perturbation of the F19-L34 Contact in Amyloid β (1-40) Fibrils Induces Only Local Structural Changes but Abolishes Cytotoxicity

The effects of specific local mutations on the local and global structure of Aβ (1-40) peptides are in the focus of the investigations in project A06. NMR experiments and computer simulations were combined in a collaboration together with A09 to explore structural details of fibrils formed by one specifically selected mutation of an Amyloid β (1-40) peptide. Although morphologically robust, local perturbations of the sequence led to moderate structural alterations on a local scale with large impact on the physiological properties, as the modified system showed no toxicity. Similarly strong effects on toxicity accompanied by changes in fibrillation kinetics were found for other local mutations.

Foundation of the Outstanding Toughness in Biomimetic and Natural Spider Silk

Spider dragline silk exhibits the highest toughness among all known materials. Even though their morphology has been thoroughly studied, it was not possible to recreate comparable materials until recently. On the basis of fibers from the recombinant protein (AQ)12NR3 –which are so far the only threads able to compete in toughness with natural silk– the molecular origin of the fibers’ high toughness is elucidated. It is demonstrated that in addition to the sample’s morphology its functional structure is essential for the material’s performance; post-spinning strain orients protein chains and induces a microscopic non-equilibrium. As a consequence macroscopic stress is directly transduced to crystalline fiber segments dissipating mechanical load. This mechanism known from natural spider silk is for the first time revealed for artificial silk.

Highlights 2016

Coupling and Decoupling of Rotational and Translational Diffusion of Proteins under Crowding Conditions

Cataract, that is, the loss of vision due to growing opacity of the eye lens, is one of the most frequent deseases with a correspondingly huge social (and economic) impact on humankind. It is associated with either phase separation or denaturation and ensuing aggregation in the concentrated solution of various lens proteins, the so-called crystallins. Our understanding of the molecular underpinnings of these processes is as yet incomplete, and it is one of the goals of project A08 to test in how far amyloid formation may be relevant in this context. As a step towards this goal, the constrained diffusive motion of proteins in such a highly crowded environment should be characterized first. It is well established that the large-scale translational diffusion is governed by the viscosity of the system, in agreement with the Stokes-Einstein relation. However, the applicability of the coresponding Stokes-Einstein-Debye relation for rotational diffusion, which is more localized short-time process, is still poorly understood. Using a combination of NMR and fluorescence spectroscopy techniques, we investigate the (de)coupling of the rotational from the translational diffusion in a series of crowded model proteins, revealing that proteins can span the whole range of phenomena in dependence of their complex mutual interactions.

Highlights 2015

Single Molecules Trapped by Dynamic Inhomogeneous Temperature Fields

Holding single molecules in liquids is still a challenge as Brownian motion fueled by thermal energy is randomizing molecular position quickly and common forces to counteract this erratic motion commonly scale with the volume. The benefits of hold and manipulating a single molecule are however huge as it would allow for long time observations of molecular conformation detecting rare events, biomolecular reactions and true bi-molecular interaction studies. Here we report on a method which employs the fuel of Brownian motion - thermal energy - to confine Brownian motion of single molecules in liquid. By generating time-dependent feedback controlled inhomogeneous temperature fields with a laser heated gold structure, we create thermophoretic drive fields which allow the confinement of single molecules in solution. The feedback control and the inhomogeneous character of the temperature field even allow for a trapping a well controlled number of multiple molecules.  We expect that this simple method and an extension to large arrays of traps will pave the way for controlled molecular interactions studies.

See also

Highlights 2012

Solid-state NMR Reveals a Close Structural Relationship between Amyloid-β Protofibrils and Oligomers

Proteins, essential components of every living being, have been posing riddles to researchers worldwide for decades. The function of these long-chain biomolecules is determined by the spatial configuration. Strictly speaking a protein folds into a specific three-dimensional structure. Properly folded, the protein can fulfill the specific biological function in our body, e.g., the control of cell growth.
However, if anything is wrong with the folding - known as the misfolding of proteins - toxic substances can arise that might lead to diseases such as Alzheimer's disease and diabetes. Through intermediate stages, a gradual aggregation of misfolded proteins results in the so-called mature fibrils. This final stage possesses the form of elongated rods which are composed of helically wound, zipper-like interconnected strips.

Holger Scheidt, from the group of Daniel Huster at University of Leipzig, and his colleagues have now succeeded in finding a greater similarity of the internal three-dimensional structure of the two intermediates for the first time. Mature fibrils, however, show an altered spatial arrangement. In detail, the protein amyloid-beta (1-40) was examined by using high-resolution nuclear magnetic resonance (NMR) spectroscopy within the CRC project A06.

Crystallization of synthetic polymers

Highlights 2019

Phenomenological Theory of First-Order Prefreezing.

The microscopic ordering process that a liquid undergoes during crystallization is often initiated at an interface to a solid. Different processes have been suggested by theory to occur at this interface. A particularly interesting process is prefreezing—the formation of a thin crystalline layer at the interface already at temperatures above the melting temperature. A direct experimental observation of this process was only recently achieved. We here now present a phenomenological theory of prefreezing and analyze the thermodynamic properties of the prefrozen crystalline layer. Specifically our theory describes the first order nature of the transition and allows a quantitative analysis of previously obtained experimental data for poly(ε-caprolactone) crystallized on graphite via prefreezing. The validity of our results is not restricted to polymer systems but contributes to our general understanding of crystallization.

Highlights 2018

Crystallization in melts of short, semiflexible hard polymer chains: An interplay of entropies and dimensions.

What is the driving force for the crystallization of a melt of semiflexible polymers? This fundamental question has been addressed in project A07. Different from the case of crystallization of small molecules, which can be reduced in a most rudimentary model to the translational ordering of hard spheres, polymer crystallization always combines translational and orientational (conformational) ordering. Stochastic approximation Monte-Carlo simulations that give complete thermodynamic information were used to study an ensemble of short chains, for which kinetics do not yet play a role. The authors were able to show that in fact the orientational interaction sets the thermodynamic driving force whereas translational ordering follows downstream.

Crystallization of Poly(ethylene oxide) on the Surface of Aqueous Salt Solutions Studied by Grazing Incidence Wide-Angle X‐ray Scattering

Investigation of the dendritic shape of the crystals is observed, which are floating 
free on the solution surface. Cover Letter of Langmuir.

Investigation of the dendritic shape of the crystals is observed, which are floating free on the solution surface. Cover Letter of Langmuir.

Polymers under constraints in thin films have been studied intensively in recent years caused by their fundamental role for such sophisticated applications as lithography for electronics, surface engineering for biomedical devices, and sensor technology. During solvent evaporation, spin coated polymer films are frozen in a nonequilibrium state which leads to relaxation processes when semicrystalline polymers are kept above the melting temperature. Thus, after film formation of semicrystalline poly(ethylene oxide) (PEO) on silicon wafers, a pseudo dewetting mechanism of 50−100 nm thin polymer layers in the melt is followed by crystallization upon cooling. An approach to investigate thin layer crystallization is the use of Langmuir experiments. A polymer solution (e.g., in chloroform) is spread on an aqueous surface of a Langmuir trough between movable barriers. After solvent evaporation the barriers are compressed to decrease the available surface, that is, to continuously increase the polymer surface concentration. By this method, the polymer film can be varied from separated chains (i.e., gaseous phase) to a continuous layer with several nanometer thicknesses (i.e., liquid or crystalline phase), as long as the polymer does not dissolve in the subphase. Recent results from B07 demonstrated that PEO crystallizes on K2CO3 aqueous solution during compression on a Langmuir trough. PEO is usually considered as a water-soluble polymer because of strong interactions between ether oxygen and water, but it has also an amphiphilic character. By addition of kosmotropic salts like K2CO3 to the water subphase, dissolution of PEO is reduced and completely prevented at higher salt concentrations because of the Hofmeister effect. Using Brewster angle microscopy, the dendritic shape of the crystals is observed, which are floating free on the solution surface. With grazing-incidence wide-angle X-ray scattering (GIWAXS), the orientation of the crystallites has been be identified.

Interface-Induced Crystallization of Polycaprolactone on Graphite via First-Order Prewetting of the Crystalline Phase

The crystallization of liquids is often initiated at the interface to a solid. Due to the difficult accessibility of a buried liquid-solid interface, direct experimental observations of interface induced crystallization have been scarce and only a recently crystallization induced by prefreezing, which had been theoretically predicted beforehand was observed in experiment. In prefreezing, a thin crystalline film forms on a solid substrate in a finite temperature interval above the bulk melting temperature of the liquid. The thickness of this prefrozen films increases on approaching the melting temperature and continues to grow into the bulk upon further cooling.
The phenomenon can be understood in the general framework of wetting theory, which generally describes the behavior of wetting layers forming on surfaces near phase transitions. Wetting transitions are theoretically well understood, and an important criterion for classification is the question if the transition is continuous or discontinuous. The corresponding experimental question here is, if the prefrozen film forms continuously or appears suddenly with a finite thickness. We here present an experiment on a polymeric model system, which allows a temperature dependent measurement of the prefreezing layer and shows that the transition is of first order.

In our experiment we use in-situ Atomic Force Microscopy measurements on an ultrathin film of a crystallizable polymer on graphite, which acts as a substrate inducing crystallization. Due to the general instability of the chosen sample system, it is possible for this case to directly measure the thickness of the prefrozen layer as a function of temperature.

Highlights 2017

Opposing Phase Segregation and Hydrogen-Bonding Forces in Supramolecular Polymers

Structure-formation in a wide range of materials and organisms is guided by self-assembly processes, leading to a high level of molecular organization, such as in folded macromolecules. Phase segregation between different macromolecules and specific weak interactions are the basis of molecular organization in many biological systems, held together by attractive hydrogen bonds (H-bonds),and distracted by phase segregation. We are reporting on significant changes in the association behaviour of the same, covalently attached H-bonds by the phase of attached polymer chains, revealed by a combination of synthetic methodologies, X-ray scattering and solid-state NMR-spectroscopy. Depending on the aggregation state we observe either closed H-bonds despite segregation of the phases, or macrophase separation with a larger amount of H-bonding dissociation. Thus the strength of a H-bond is significantly changed by the phase structure of the attached molecules such as polymers, displaying a phase segregating tendency.

Temperature-dependent IR-transition moment orientational analysis applied to thin supported films of poly-ε-caprolactone

When polymers crystallize from the melt, a huge variety of micro-structures can be obtained depending on the details of the production protocol. Despite about 100 years of application-driven and basic scientific research, the interplay of external parameters and microscopic mechanisms determining the micro-structure is still not unraveled. In this work, we analyze 7 µm thick, supported films of semi-crystalline Poly-ε-caprolactone using IR-Transition Moment Orientational Analysis. Based on the three-dimensional order of the crystalline and amorphous moieties, the films micro-structure is found to consist of extended flat on crystalline layers (~30%) and strongly confined spherulites. The former are commonly only found in nanometer-sized films, whereas the latter are usually dominant in such macroscopic samples. In the course of heating, the flat layers melt preferentially, whereas the spherulitic structures remain intact up to the equilibrium melting point. The findings are explained by the confined volume, accessible for crystal growth, nucleation kinetics, and the related thermodynamic stability. The work aids in understanding the interplay between kinetics and thermodynamics during the formation of polymer crystallites and their super-structures, and therefore helps to tailor polymeric materials for application.

See also

Induction of Chirality in β-Turn Mimetic Polymer Conjugates via Postpolymerization “Click” Coupling

Chirality is one of the most determining principles in chemistry and biology, dominating the assembly of large synthetic and biological macromolecules. Polymers with artificial (geometrical) constraints can - similar to alpha-helices or proteins - fold into well defined helical structures, especially those containing a rigid backbone with limited conformational flexibility. We here report on the transfer of chirality, induced by an artificial beta-turn-mimetic element, which is directly linked to a synthetic (helical) polymer. The distance over which chirality is transferred from the geometrical constraint onto the helicity of the polymers is studied, revealing that transfer of chirality is possible up to three chemical bonds between the helix and the chiral center on the beta-turn element.

Intracrystalline Jump Motion in Poly(ethylene oxide) Lamellae of Variable Thickness and the Underestimated Effect of Intracrystalline Chain Dynamics on the Morphology and Stability of Semicrystalline Polymers

It has long been known that the degree of crystallinity of linear polymers depends on the presence or absence of chain motion within and through the crystalline lamellae. While this fact has not yet found widespread appreciation or even explicit consideration in theoretical accounts of polymer crystallization, recent results from project A01 demonstrate that even the morphology is qualitatively different in the two cases. That is, polymers with intracrystalline mobility display lamellar stacks with well-defined amorphous-phase thickness but a wider distribution of crystallite thickness, in some contradiction to conventional wisdom. Polymers without αlphac-process on the other hand show a much better defined thickness of the crystalline regions. It is thus of high interest to develop an understanding of the relation between the intra-crystalline chain motion and morphological parameters, such as the crystallite thickness. Here, we report on a comprehensive NMR study of intracrystalline chain dynamics in poly(ethylene oxide) and a new approach for the quantitative analysis of small-angle X-ray scattering data. In the latter one, we compare the structural characteristics of fully crystallized samples for two model polymers with and without chain motion in the crystallites.

See also

Highlights 2016

Spatial Orientation and Order of Structure-Defining Subunits in Thin Films of a High Mobility n–Type Copolymer

Orientation and order of distinct molecular subunits in solid layers of the high mobility n‑type copolymer P(NDI2OD‑T2) are investigated by means of infrared transition moment orientational analysis (IR‑TMOA). This novel spectroscopic technique based on concurrent absorbance measurements of structure-specific bands in dependence on inclination and polarization of the incoming IR light enables to determine the complete tensor of absorption independently for each IR-active transition moment. As a result, for nm‑thin films pronounced in‑plane anisotropy arising from self‑aggregated order is detected, which, however, is no longer discernable for mm-thick samples. In contrast, the out of plane orientation (inclination of molecular subunits) is retained irrespective of the widely varying layer thicknesses (150 nm vs. 1.4 mm). Thus, the conception of the sample morphology occurs as stratification of slightly misaligned layers of oriented polymers; with increasing film thickness the macroscopic in plain order diminishes, whereas the out of plain orientation is preserved.

See also

Dynamic Ordering and Phase Segregation in Hydrogen-Bonding Polymers

Hydrogen bonds constitute highly relevant structural units of molecular self-assembly. They bridge biological and synthetic sciences, implementing dynamic properties into materials and molecules. Phase segregation and crystallization on the other hand represent important assembly-principle, responsible for eg. cell compartimentation, membrane-formation and microphase segregation in polymers. We here discuss the phase segregation of H-bonding polymers in both, the solution and solid state, focusing on the specific aggregation of different H-bonding polymers in competition to phase segregation and crystallization. We illustrate that a rational architectural design within H-bonding polymer systems in interplay with phase segregation in both, the amorphous and crystalline state, opens perspectives to develop artificial supramolecular systems approaching the level of complexities and properties present in nature’s biomaterials.

Highlights 2014

Direct Observation of Prefreezing at the Interface Melt-Solid in Polymer Crystallization

The microscopic ordering process that a liquid undergoes during crystallization is often initiated at an interface to a solid. Different processes have been suggested by theory to occur at this interface. Of special interest is prefreezing—the formation of a thin crystalline layer at the interface already at temperatures above the melting temperature. Because of the difficult accessibility of the buried interface, experimental proof of crystallization by prefreezing has been elusive in molecular systems. We here present direct in situ observations of such a process in a polymeric model system. The results not only contribute to our fundamental understanding of crystallization but might also be useful for the preparation of well-ordered oriented thin films of crystalline organic materials.

Kinetic mechanism of chain folding in polymer crystallization

Polymer crystallization, which was discovered more than 50 years ago, is a particular case of solidification of matter which occurs by supercooling a polymer melt. The specific feature of polymer crystallization is the back-folding of chains in a polymer crystal leading to lamellae with a thickness of around 10-15 nm consisting of straight chain segments. The conventional description, which is based on phenomenological nucleation theory, ignores the polymeric structure and conformational interactions, and has been questioned in some recent experiments and numerical work. We develop a description of the crystallization kinetics that explains chain folding in polymer crystallization in terms of conformational interactions and the coil shape of polymer chains in the melt. The fundamental relation between lamellar thickness and supercooling is derived from the interplay between the formation time of rod shaped stems, which are induced by supercooling, and the relaxation time of coiled polymer parts of the same length. Our work suggests the existence of two separate time scales in polymer crystallization: the chain dynamics time scale at which the fold length is selected, and the much larger time scale associated with the crystal growth.

Highlights 2013

Determination of the Crystallinity of Semicrystalline Poly(3-Hexylthiophene) by Means of Wide-Angle X-Ray Scattering

Poly(3-hexylthiophene) (P3HT) is often studied as a model system for the more general class of semiconducting polymers. It has been shown in the past that the electronic transport properties of this material strongly correlate with crystallinity, which can depend on the details of chemical structure, molecular weight and processing. In most cases, crystallinity is measured by Differential Scanning Calorimetry (DSC), which requires calibration in order to deliver absolute values. Unfortunately, the available calibration value is uncertain and under debate. We therefore here present small-angle and wide-angle X-ray scattering measurements on a series of chemically well-defined poly(3-hexylthiophenes), which were analyzed to determine absolute values of the crystallinities. The resulting values for the crystallinity are substantially higher than assumed in the past. An extrapolated reference melting enthalpy for a 100% crystalline material was determined by comparison with DSC measurements. The observed decrease of the crystallinity for higher molecular weights can be explained by the onset of chain folding. Additionally we show that the crystalline regions of P3HT exhibit a large amount of internal disorder.

Crystallization of supramolecular pseudo block copolymers

Due to the presence of supramolecular bonds the crystallization process of supramolecular pseudo block copolymers (SPBCP) is more complex in comparison to conventional covalently bonded block copolymers (BCP). Thus supramolecular binding motives included on the polymer chain-ends display additional dynamic effects as well as possible nuclei for the crystallization. In this article we systematically study non-isothermal crystallization processes in SPBCP’s consisting of a crystallizable poly(ε-caprolactone) (PCL) connected via triple hydrogen bonds to either a short alkyl-modified 2,4-diaminotriazine, or bound to a large block of amorphous poly(isobutylene) (PIB). The crystallization of the PCL is studied with both groups acting as supramolecular barriers for the crystallization process, either during nucleation or during crystal growth. A strong influence of the short alkyl-modified 2,4-diaminotriazine barrier on the crystallization temperature of the PCL compared to the control sample devoid of this compound is observed. In contrast, the large polymer block (PIB) acting as a barrier causes a strong decrease of the crystallization temperature and fractionated crystallization of SPBCP consisting of smaller PCL-chains is observed.

Hybrid systems

Highlights 2019

Multisegmented hybrid-polymer based on oligo-amino acids: synthesis and secondary structure in solution and the solid state

Multisegmented Hybrid Polymer Based on Oligo-Amino Acids (Reprinted with permission from J. Freudenberg et al., Macromolecules (2019). Copyright 2019 American Chemical Society.

Multisegmented Hybrid Polymer Based on Oligo-Amino Acids (Reprinted with permission from J. Freudenberg et al., Macromolecules (2019). Copyright 2019 American Chemical Society.

Proteins are among the most abundant macromolecules in nature, often forming complex architectures by folding (alpha-helices/beta-sheets) and/or aggregation into fibrillary assemblies. Refolding between different segments during assembly is often poorly understood, also guided by cooperative assembly phenomena. We here present a study on multisegmented hybrid-polymers to understand folding and assembly. Precisely engineered oligo-amino acids are repetitively imbedded into a non-interacting alkyl-chain, allowing to study conformational changes upon and during assembly in both, the solid state and in solution. The here reported observations prove that beta-sheets are thermodynamically favored, however strongly dependent on the nature of the amino-acid along the chain.

See also

Probing Polymer Chain Conformation and Fibril Formation of Peptide Conjugates

A collaborative study (projects A03/A06) investigated the conformation of hybrid molecules consisting of a thermo-responsive synthetic polymer and an amyloid-forming parathyroid hormone. Using NMR as a probe an attractive interaction between the two blocks leading to an intertwined shroud-like conformation of the synthetic polymer was found. Nevertheless, the synthetic block did not affect the secondary structure of the peptide and fibrillation was even accelerated in comparison to the pure peptide.

Highlights 2017

One-Pot Synthesis of Thermoresponsive Amyloidogenic Peptide-Polymer Conjugates via Thio-Bromo “Click” Reaction of RAFT Polymers

Conjugation to fibrillating proteins and peptides represents a methodological challenge, as potential binding sites are blocked due to the ongoing fiber-formation, blocking the sites for attachments. A synthetic strategy to efficiently prepare main-chain peptide-polymer conjugates developing an in situ tandem reaction based on the aminolysis/thio-bromo “click” reaction. This method allows to tether an amyloidogenic peptide fragment Aβ17-20(LVFF) to the ω-chain end of poly(diethylene glycol methyl ether acrylate) (PDEGA), prepared via reversible addition fragmentation chain transfer polymerization (RAFT). Tuning of the lower critical solution temperature of the polymer allows a modulation of the aggregation into micellar structures.