Polyamide 11 (PA 11) is a crystallizable bio-based polymer used in high-performance engineering applications as it has excellent resistance to chemicals, good thermal stability, good impact resistance at low temperature, light weight property and low moisture uptake. Its properties are affected by the semicrystalline morphology including the fraction and structure of crystals, their size, shape and perfection, and their organization in a superstructure, with all these parameters controlled by the crystallization/processing conditions. Injection molding is a common processing route due to its efficiency to produce both simple and intricate parts with high production output. During injection molding, the structure forms under shear and thermal gradients, often leading to the development of variable morphologies between the surface and inner parts called skin/core morphologies, with implications on the property profile. Crystallization of PA 11 under shear conditions has not yet been investigated. The lack of knowledge of critical shear parameters required to enhance crystallization during injection molding of polyamides is a main motivation of present research activities since such knowledge is required to tailor processing routes (by variation of molding conditions) for the purpose of controlling structure and property gradients.
PhD student: Katalee Jariyavidyanont
Supervisor: apl. Prof. Dr. René Androsch
Poly(lactic acid) (PLA) and Poly(lactic-co-glycolic acid) (PLGA) are the gold standards of pharmaceutically used biodegradable polymers. These play a key role as drug carriers in drug delivery systems (DDS´s) and have many advantages like full biodegradability and safety. Nonetheless, there are some disadvantages such as autocatalytic polymer degradation and the formation of highly acidic microenvironments which make it necessary to look for alternatives. A possible alternative is fatty acid-modified Poly(glycerol adipate) (FA-PGA) as a new biodegradable polymer. The esterification of the free hydroxyl group of the polymer backbone with a fatty acid (i.e. stearic or behenic acid) enables the synthesis of versatile FA-PGAs with tunable properties. This study aims to characterize various FA-PGAs regarding their suitability to form different DDS’s for parenteral controlled drug release. Furthermore, this project focuses on the incorporation of a lipophilic model drug and the drug release in vitro and in vivo.
In rubber blends, a difference in reactivity of the constituent rubbers in the blends may cause diffusion of curatives between the blend phases, leading to different extents of crosslinking of the phases, thereby affecting the properties of the blends. As a result, measurement of the phase-specific crosslink density (CLD) becomes important. Widely popular techniques based on equilibrium swelling of the vulcanizate, rubber elasticity theory, and AFM lack phase-resolved determination of CLDs. NMR has so far been successfully used for phase-resolved determination, albeit qualitatively. In this work, the effect of thermo-oxidative (T-O) ageing, weathering, and the influence of mixing on the distribution of CLDs in sulfur-vulcanized blends of different diene rubbers will be studied by 1H multiple quantum magic-angle spinning (MAS) NMR spectroscopy. Residual dipolar coupling (), arising due to an anisotropy in molecular motions upon crosslinking can be readily obtained by fitting the double quantum build-up curves. A correlation to microstructural information for a range of blend compositions subjected to the above conditions is thus made. In unfilled natural rubber (NR) and styrene-butadiene rubber (SBR) systems, unaged single elastomers of NR and SBR were found to crosslink to similar extents, whereas a distribution in CLD is observed in their blend due to a slower crosslinking of the SBR phase. Upon 1000 hours T-O ageing, a modification of sulfur bonds is observed as SBR phase undergoes further crosslinking, and NR phase undergoes chain scission and oxidative hardening leading to molecular rigidity, and thus to a further distribution in Dres.
Traumata and degeneration of skeletal tissues like bone, cartilage, tendon and ligament belong to the most frequent clinical problems and require often the use of implants or grafts to restore their function. Therefore, various tissue engineering approaches, which include the programming of cells’ differentiation, signaling molecules and scaffolds, have been developed. Programming of mesenchymal stem cells (MSCs) differentiation can be achieved by controlling the mechanical properties of substrata, their molecular composition and the presence of growth factors (GFs) like GDF-5 and BMP-2 that drive MSCs differentiation chondrocytes and osteocytes, respectively. The layer-by-layer (LbL) technique, which is based on the alternating adsorption of polyelectrolytes on charged substrata; permits the formation of nanostructured surface coatings useful for delivery of bioactive drugs and proteins, like GFs. Therefore, in this project, glycosaminoglycans (GAGs) will be used as polyanions together with collagen I or chitosan as polycations to fabricate biomimetic multilayer coatings using LbL technique. The functionalization of GAGs with an aldehyde group will generate an imine bond (intrinsic cross-linking) between the aldehyde groups on oxidized GAGs (oGAGs) and amino group of the polycations, which allow controlling the mechanical properties of the substrate, as well as, the release of the loaded growth factors, and thus, the subsequent cell differentiation into the desired lineages. Physiochemical characterization of the multilayers made of native and oxidized GAGs will be performed. In addition, biological studies including cell adhesion and differentiation into osteoblasts and chondrocytes will be checked. Release of growth factors from the fabricated multilayer systems will be also detected.
Metal-organic frameworks (MOFs) combine metal nodes and organic linkers (ligands) to obtain desired frameworks as one-, two-, or three-dimensional coordination networks. In most cases, MOFs have geometrically and crystallographically well-defined porous framework structures able to exert a large variety of catalytic processes and acts as storage materials for gases and energy. The length and functionality of the used scaffolding ligand determine the size of the resulting pores and can be easily tuned resulting in structures with high surface area and adjustable chemical properties. One prominent example for such linkers are imidazolium-containing ligands, which are precursors of N-heterocyclic carbenes (NHCs). The introduction of NHCs as structural elements into MOFs is a promising aspect in view of their catalytic performance, for e.g. triggered copper(I)-catalyzed azide/alkyne cycloaddition (CuAAC) “click” reactions. We plan to (see Graphical Abstract) synthesize imidazolium-containing ligands (2) from precursors displaying hard and soft segments (R) (1), which in turn is used to synthesize MOFs (3). These MOFs can be used to produce bis(NHC) copper(I) containing MOFs (4). A balance between rigid and soft segments in the MOFs will enable to adjust their brittleness to be activated during mechanochemical activation procedures (via ultrasonication, compression or tensile testing (5)) while remaining a complex network under ambient conditions. The activated bis(NHC) copper(I) MOFs can then be used to promote self-healing in polymeric materials via the CuAAC of multivalent alkynes and azides (6).
PhD student: Kshitij S. Shinde
Supervisor: Prof. Dr. Wolfgang H. Binder
In this PhD research, we study the effects of both nanoscopic confinement and curvature on the dynamics and properties of a 1,4-polybutadiene (PBD) melt. We use the GROMACS package to perform chemically realistic molecular dynamics(MD) simulations to study the relaxation processes of PBD at alumina surfaces. This serves as a possible model of a composite material where nano-meter sized spheres are embedded into a melt. The length and time scales occurring in such composites exceed those of chemically realistic MD simulations thus we do not attempt to model a realistic filler network. However, we would like to keep the nano-size of the filler and the surface curvature effects on the structure and dynamics of the adsorbed polymer. Therefore, we study a model system of a melt infiltrated in alumina pores of 10 nm diameter size, as well as of a melt surrounding an alumina cylinder at temperatures above the glass transition. The former system has also been extensively investigated by different experimental techniques, allowing for a comparison of our simulations to the experimental data.
PhD student: Lama Tannoury
Supervisor: Prof. Dr. Wolfgang Paul
In this work, the release behavior of the 16-doxyl stearic acid as a model drug from hydrogels made from bovine serum albumin is characterized. Two procedures of preparing hydrogels such as heat induced technique at temperatures above and below albumin’s denaturation temperature and neutral pH, and pH-induced method at low pH and body temperature are used. The properties of these hydrogels at the macroscopic level, the viscoelastic behavior via rheological characterization and changes in the secondary structure of the protein during gelation through ATR-IR spectroscopy are explored. We focus on studying the combined effects of fatty acid concentration, incubation time (the time at which hydrogel formation is processing) and gelation procedures on release behavior via CW EPR spectroscopy and DLS measurements which provide deep insight on the interaction of 16-DSA with BSA hydrogels and the nature and size of the released components, respectively. We find that the release rate of the fatty acid from BSA hydrogels is dependent on its loading percentage, duration of hydrogel formation and the mentioned gelation methods. The final results confirm the potential of these gels for drug delivery applications allowing the sustained release of drug.
Human pluripotent stem cells (hPSC) having self-renewal properties and high differentiation ability into three germ layers being predominant cell sources for tissue engineering and regenerative medicine. Culturing and passaging approaches are crucial for obtaining extensive cells for the transplantation. The stimuli-thermoresponsive polymer, Poly (N-isopropylacrylamide) (PNIPAM) exhibits a low critical solution temperature (LCST) at 32°C allowing cell adhesion at body temperature of 37°C, while it becomes more hydrophilic below LCST due to the conformational changes of the polymer chains. Cells can be harvest as an intact continuous cell sheet that maintains surface proteins, extracellular matrix and cell-cell junction receptors by simply reducing the temperature below LCST. Using layer-by-layer (LbL) techniques to achieve the fixation of the thermoresponsive polymer as a culture substrate to generate cell sheets is an effective method based on oppositely charged polyelectrolytes absorbed onto an charged surface. The multilayers formed by the charged glycosaminoglycan (GAG) possess bioactivity that can promote the interaction between proteins and cells. Hence, a combination of thermoresponsive polyelectrolytes that control adsorption of adhesion- and growth-promoting proteins as well as uploading of growth factors may have enormous potential as a new tool for culture of hPSC. This research will help to pave the way for cell sheet generation for tissue engineering and transplantation.
Implantable drug-delivery systems are being developed to release drugs to the bloodstream continuously and moreover, for patients being free from hospitalized to receive intravenous infusions or frequent injections. Many of the drugs with low oral bioavailability can be loaded in the implants. However, most of the commercially available implants in pharmaceutical market are based on Poly(lactic acid) (PLA) and Poly(lactic-co-glycolic acid) (PLGA) polymers. Degradation of these polymers in the body produces acidic monomers that may cause drug inactivation and autocatalysis of the implant. Hence, finding alternative polymers or materials will be beneficial for the formulation of future implants, which can be used for parenteral controlled drug release applications. Starch is a natural polymer that is cheap and its degradation in the body produces nontoxic and nonacidic monomers. The aim of this project is to use different derivatives of starch in combination with different lipids to find an appropriate alternative to PLA and PLGA polymers. During this project the interaction between the lipid and starch will be evaluated and also the designed system will be characterized by in vitro and in vivo studies.
Myelin basic protein (MBP) is a membrane protein of the myelin sheath that is partially folded in myelin but has no secondary or tertiary structure in aqueous solutions. It can be referred as an intrinsically disordered protein (IDP) both in myelin and in solution. Ionic liquids (IL) are salts with a melting point below 100°C. Their positive properties include thermal stability, flame resistance and environmental compatibility. Thus, these came in the focus of fundamental scientific research as well as of targeted applications in recent years. The aim of this work is to examine the properties, structure and dynamics of IDPs in aqueous IL mixtures. Aqueous IL mixtures provide their own structure only depending on the properties of the IL. We try to achieve a positive effect on different proteins by the structure being provided by the ILs. As model proteins we use MBP and HPRG, which both are highly dynamic proteins that do not crystallize and do not fold into tertiary structures but fulfill many functions in vivo. For this investigation we use all kinds of spectroscopic methods like EPR, NMR and IR-spectroscopy aided by scattering methods like DLS or X-ray diffraction.
Drug delivery scaffolds are smart alternatives to conventional formulations and allow the controlled release of active compounds in vitro or in vivo. The polymer/solvent system poly (lactic acid)/ethyl butylacetylaminopropionate (PLA/IR3535®) is considered as a drug-delivery system in vitro. In this system a biodegradable and biocompatible polymer hosts a liquid with a functionality as insect repellent to prevent insect bites, infections and diseases. Such devices are typically formed by thermally induced phase separation (TIPS) involving crystallization-induced solid-liquid (S-L) phase separation in the course of cooling a homogeneous polymer solution formed at elevated temperature. Controlled evaporation of the repellent is reached by crystallization-controlled formation of a poly (l-lactic acid) (PLLA) scaffold with a pore size, which depends on the PLLA/repellent ratio, the molecular architecture of the polymer, and the conditions of solution-crystallization. In addition, non-crystallizable poly (d,l-lactic acid) (PDLLA) is used to gain information about the thermodynamic miscibility of the system components. The quantitative relations between the conditions of solution-crystallization of PLLA, the structure of the PLLA scaffold, and the drug-release rate to the environment will be established. Thus, further application of solution-crystallization-tailored PLLA scaffolds are a usable as advantageous drug delivery device.
PhD student: Fanfan Du
Supervisor: apl. Prof. Dr. René Androsch
Although highly filled rubber compounds have been widely used in the tire industry, the molecular causes of dissipation and reinforcement in such complex material systems are currently not completely understood. In consequence the application-related optimization and development of rubber compounds for tire treads occurs largely empirical. For efficient optimization and usage of new components a desirable understanding of molecular causes and application-relevant parameters such as dissipation and reinforcement are necessary. These parameters determine the tire performance and further characteristics such as rolling resistance, wet grip or abrasion resistance, which have a significant impact on the ecological fingerprint and the safety of vehicles. Main goal of this research project is to understand the how and in which extend (i) the chemical composition of the glassy rubber bridges and (ii) the topology of the filler network effect dissipation and reinforcement. For this purpose, various series of silica-filled SBR-BR model compounds with different filler content and variable blend composition are systematically analysed using dynamic-mechanical methods as well as imaging methods for quantifying the filler dispersion on different length scales. We expect to understand if theoretic ideas about the molecular causes of dissipation and reinforcement can also be used for rubber compounds with complex compositions. Moreover, this study should provide insights into which approaches can contribute to the efficient optimization of rubber compounds for application.
The proposed project deals with the modification of the properties of hydroxyethyl starch (HES) and its application for drug release systems (DRS). Two commercial HES samples with molar masses (Mw) of 70,000 and 200,000 g/mol will be used for this project kindly provided by Serumwerk Bernburg. The investigations will start with the lower molar mass HES sample. HES itself is a water-soluble polymer with a plethora of applications in the pharmacy where the most important commercial utilization is related to Blood Plasma Expander solutions. Formation of amphiphilic polymers when HES is partially esterified with fatty acids results in potential applications as DRS. Here, the organization of the hydrophobic alkyl chains of the saturated fatty acid chains is most important for the formation of polymer nanoparticles in an aqueous environment. These nanoparticles will be tested for DRS when loaded with poorly water-soluble drugs. The influence of the acyl side chain on crystallization has not yet been observed and investigated. This will be in the focus of the project. For this purpose, different fatty acids will be used for HES modification with varying the chain length and different degrees of substitution. Since the addition of fatty acids to HES leads to decreased water solubility depending on the degrees of substitution, it might be necessary to improve the hydrophilicity of HES. This will be achieved by grafting poly(ethylene glycol) (PEG) chains to HES. The present approach is indeed promising to produce a fully biodegradable amphiphilic polymer for the encapsulation and modulation of the systemic delivery of drug candidates.
PhD student: Syed Hossain
Supervisor: Prof. Dr. Michael Bron
It is well known that surfaces can induce crystallization and in this way influence crystallization kinetics, structure, morphology, and properties of semicrystalline polymers. Crystallization in polymer melts occurs typically in two principal stages, namely nucleation and growth of crystals from the formed nuclei. An interface to a solid material can initiate crystallization in liquids by either heterogeneous nucleation or pre-freezing. In the latter case, a crystalline layer appears on a solid surface at a temperature Tmax higher than the melting temperature Tm of a bulk crystal without interface. The main goal of this project is to contribute to the understanding of microscopic mechanisms of surface-induced polymer crystallization (surface phase transitions, chain conformations, packing and layering effects, etc.) by means of coarse-grained computer simulation. We will perform computer simulations in a wide film (large D) at given melt densities in the bulk (in the center of film). For a substrate we will consider different models: non-structured surface and structured (patterned) surface either with attracting surface potential or purely repulsive hard surface. We will develop a coarse-grained (CG) model which takes into account the main features of real systems reasonably well by means of computer simulation using flat-histogram Monte Carlo algorithms.
PhD student: Evgeniia Filimonova
Supervisor: Prof. Dr. Viktor Ivanov
This research mainly focuses on energy-storage devices. Mostly interested in lithium-based batteries, and, in particular, lithium-sulfur (Li-S) batteries, which are among the candidates for the next-generation energy-storage devices. The main goal of an electrochemical cell is to make large currents that power an electronic device. Improving the capability of Lithium-Sulfur (Li-S) cells depends on a good understanding of their mechanism. In general, the Li-S cell contains a Lithium metal electrode as an anode and a mixed Carbon/Sulfur (C/S) electrode as a cathode. The overall process occurring in them is the reaction of lithium and sulfur to form lithium polysulfides Li2Sx (2 ≤ x ≤ 8).
16Li + S8 ⇆ Li2Sx is divided into the following reactions at each electrode:
Anode Li+ + e– ⇆ Li(s)
Cathode S8 + 16e– ⇆ 8S2-
For these investigations, we need to use a combination of classical and quantum-mechanical simulations. Therefore, we use density functional theory (DFT). Most of the quantum chemistry code has a DFT module, in our studies, we use different DFT codes including CP2K, ORCA, NWChem, and SIESTA. The combination of DFT and MD simulations lets us have an acceptable observation of our system. We use these two with spectroscopy simulations such as Raman and NMR to study the chemical processes in energy storage systems to see their agreement with the experimental results.
The continuous further development of polymer materials associated with increasing use as construction and high-performance materials requires a reliable prediction of the service time in order to guarantee quality and safety of the products. A challenge in predicting the service time of plastics is the time-dependent (viscoelastic) material behavior (stress relaxation or strain relaxation, i.e. creep). Creep can lead to unacceptable deformation or structural failure of a component. Furthermore, local stress concentrations in the component being induced by the material, production or design can lead to the initiation and propagation of cracks. The crack propagation can generate failure before the service time has been reached [Grellmann and Seidler 2013]. Therefore, for prediction of the service time the mechanical and the fracture mechanical properties must be known with high precision in the initial phase of product development. This contradicts the fact that long-term creep tests are very time intensive, because the standard of those tests requires a wide range of stresses, times and ambient conditions. In order to predict the long-term creep behavior early and reliably, accelerated methods are requested. In addition, for assessing the crack initiation and propagation process plastics-compatible methods of fracture mechanics have to be included. The aim is to develop and further develop suitable test methods to characterize the long-term creep behavior and creep-fracture mechanics properties of polymer materials in a reasonable and effective way.
PhD student: Anja Berthold
Supervisor: Prof. Dr. Beate Langer
The use of the phthalate-based plasticizers in polymers and elastomers received tremendous amount of criticism for the last decade concerning its potential toxicity and the migration into the product which comes in contact. Sustainable and harmless bio-based plasticizers have been the focus of research recently to replace these market leader phthalate-based plasticizers. The objective of this PhD work is to analyze, characterize and compare the compatibility of bio-based sustainable plasticizers with the conventionally used plasticizers (phthalates, adipates, citrates) in the respective polymer and finally the comparison of their desired mechanical properties. Different analytical techniques including spectroscopic (FTIR, RAMAN), chromatographic (GC-MS, HPLC) and thermal analysis (TGA, DSC) will be applied to investigate the interaction behavior of these additives with the base polymeric materials. Artificial aging tests by simulating thermal and oxidative loading will also be performed and the consequent physical and chemical changes will be diagnosed. Moreover, mechanical and synthetical steps will be carried out accordingly to optimize the properties of the selected bio-based plasticizers to suppress their migration behavior. Last but not the least establishment of the interaction rules and the model development for the replacement of toxic conventional additives by harmless sustainable additives are also planned.
Industrial grade samples of high density polyethylene (HDPE) with a broad molecular weight distribution, cross-linked via irradiation below the melting point, are probed by low-field 1H time domain NMR to provide information about the chain motion in the molten state. For studying the chain motion with the double-quantum (DQ) NMR experiment, two methods are applied for analyzing the measured data. The first method used for probing rubbers and hydrogels in previous studies, extracts the averaged residual dipolar coupling (Dres), which mirrors the amount of anisotropic motions caused by entanglements and cross-links. The second novel method is based on a power-law model of the orientation autocorrelation function (OACF) C(t), which estimates the amplitude of the OACF and the time scaling exponent κ (slope in the plot of C(t)~t-κ against time in logarithmic scale). The κ values, which can vary between ~0 (a perfect network) and ~0.5 (a linear entangled polymer) in this sample, can demonstrate how the cross-linking affects the chain motions in such a complex system. Complementary rheological measurements also show that the decaying slope of storage modulus (G´) vs. angular frequency (ω) correlates with the irradiation intensity, which is similar to what κ values reflected. The results of this research can lead to establish a solvent free method for determining the crosslink density and gel content value. Also by using DQ NMR we can look at other networks like hydrogels and determine the homogeneity or defects of a network structure.
As well known, elongational flow or shear generated during polymer processing has a big influence on polymer crystallization, structure and eventually properties. In the research, shear induced formation of crystal nuclei in poly (L-lactic acid) (PLLA) of different molar mass was analyzed using a plate-plate rheometer. The relaxed melt was cooled from 220°C to 135°C at a rate of 4 K/min. At 135°C, the melt was subjected to rotational shear at rates between 0 and 5 s-1 for 10 seconds. Acceleration of crystallization at identical temperature of 135°C due to the shear induced nucleation is observed if the shear rate exceeds a critical value. The concept of specific work of flow is introduced in this work to obtain unique information about critical shearing conditions (including temperature/viscosity, shear rate and time for shear-induced generation of crystal nuclei). The higher the molar mass, the lower is the critical specific work of flow above which nuclei are formed. The longer molecules with increased relaxation time support the formation crystal nuclei by shear.
PhD student: Mengxue Du
Supervisor: apl. Prof. Dr. René Androsch