Research
What is Polymer physics?
Polymers or chain molecules are very versatile materials. Their special mechanical, optical or electrical properties are at the basis of many different applications in which we encounter polymers in daily life. Also biological matter consists to a large part of polymers. Due to their macromolecular character polymers display a number of typical and special properties like e.g. viscoelasticity, rubber elasticity, semicrystallinity. Also self assembly is a common feature.
On which research topics do we work?
1. Interface Induced Crystallization
https://pubs.acs.org/doi/10.1021/acs.jpclett.9b00608
It is well known that crystallization of liquids most often starts at an interface to a solid surface. In general, a solid surface can induce crystallization by either heterogeneous nucleation or prefreezing, whereas these two processes are very different from a thermodynamic point of view. While heterogeneous nucleation is a non-equilibrium process taking place below bulk melting point Tm, prefreezing is an equilibrium phenomenon that occurs above Tm. However, a detailed description of the interface induced crystallization of liquids on a microscopic level is limited. By combining experimental and theoretical approaches, our aim is to understand the material parameters that affect thermodynamics of prefreezing and kinetics of heterogeneous nucleation. For this, we investigate crystallization of thin films and isolated droplets of semicrystalline polymers on solid substrates. We use in-situ atomic force microscopy (AFM), optical microscopy, and wide angle X-ray scattering (WAXS, GI-WAXS) as our experimental methods. Furthermore, we develop the phenomenological theory of prefreezing and describe the equilibrium properties of the prefrozen phase.
2. Crystallization and Molecular Dynamics of Polymers
https://pubs.acs.org/doi/10.1021/acs.macromol.8b01102
Polymers typically crystallize in a semicrystalline structure. There are ordered/crystalline areas (lamellae) and disordered amorphous areas. The resulting nanostructure can be formed in many different variations and significantly influences the macroscopic material properties. It is therefore very important to have a precise understanding of the crystallization process. We see this as a complex interplay of the polmer molecular dynamics in the melt, the growth kinetics of the lamellae, and additional polymer-specific effects. We use SAXS, DSC, Flash DSC, and rheology for investigation.
3. Structure Formation in Semiconducting Polymers
Reprinted with permission from https://pubs.acs.org/doi/10.1021/acs.macromol.8b01301. Copyright 2018 American Chemical Society
We investigate the parameters, which influence the structure formation in semiconducting polymers primarily for organic photovoltaic. A nanostructured morphology is necessary for an efficient charge separation in this material class, which we realize by diblock copolymers. The demixing tendency of the polymers leads to the formation of an ordered structure of some 10 nm (microphase separation). Since semiconducting polymers usually have an additional crystalline or liquid crystalline order, the structure formation in these materials is complex. The morphology is determined by the competition of microphase separation and crystallization. We investigate how different influencing factors such as molecular weight, chemical composition, interfaces, and molecular dynamics affect the structure formation and thus the efficiency of solar cell materials.
4. Supramolecular Polymer Networks
https://www.nature.com/articles/srep32356
Polymer networks containing short polymer chains bonded reversibly, i.e. by hydrogen bonds, metal complexes, are called supramolecular networks. These networks show new material properties like self-healing due to their ability to reassemble after application of stress and fracture. Furthermore, the reassembly can be triggered by external stimuli, i.e. change in temperature or pH. In our group, we utilize rheological measurements and Small-Angle-X ray scattering methods to separate the equilibrium molecular dynamics from self-healing kinetics.
5. Crystallization of Biodegradable Polyesters
Examples of polyesters with different numbers of methylene groups (CH2)
Biodegradable polymers can be alternatives to standard plastics in some applications, but they have not yet been fully explored. We examine the crystallization behavior of biodegradable polyesters using calorimetry (DSC, Flash-DSC), X-ray scattering (SAXS), and nuclear magnetic resonance (NMR) and would like to answer the following scientific questions:
1) What influence does the number of methylene groups between ester groups have on the crystallization behavior of polyesters?
2) Is there a polyester with intracrystalline molecular dynamics?
6. Structure and mechanical properties
Structure and mechanical properties
The majority of polymer materials used in various applications are semi-crystalline. During crystallisation, these form a complex nanostructure of thin crystallites lying on top of each other and amorphous areas in between. This structure is the basis for the advantageous mechanical properties of these materials. The aim of our current project is to fundamentally and quantitatively understand this relationship between microscopic structure and molecular dynamics and the mechanical properties. An important role here is played by the entanglements between polymer chains.