arXiv subjects

Kevin Hagmann

Publications and source records attributed to Kevin Hagmann.

4 recordsLinked to original sources

arXiv2026

Exploring structure-property relationship on a nanoscale for tailoring films of amphiphilic polymer co-networks

Amphiphilic polymer co-networks (APCNs) provide a large toolbox for tuning coatings important for applications such as bio-interfaces. Therefore, we investigate the influence of network composition and environmental conditions on the structure and mechanical and adhesive properties of thin films composed of hydrophobic tetra-PCL and hydrophilic tetra-PEG stars of varying sizes. State-of-the-art atomic force microscopy (AFM) techniques, including phase imaging, fast quantitative static indentation and dynamic indentation, provide insights into the structure-property-relationship on various length scales. PEG-rich networks exhibit amorphous morphologies with spherical nanodomains and elastic moduli of a few MPa, while PCL-rich networks form semicrystalline cylindrical arrangements with moduli up to several hundred MPa in water. Temperature-dependent measurements in water revealed a strong hysteresis of elastic moduli while shifting the melting/crystallization transitions or preventing crystallization in PEG-rich networks. All networks displayed predominantly elastic behavior. Co-networks in non-selective solvent conditions are overall softer, less adhesive and structurally more homogeneous. These results establish a predictable correlation of network composition, physical and chemical environment, structure and properties, which makes them suitable for a rational design of amphiphilic systems for various applications.

cond-mat.soft↗

arXiv2025

Magnetic Correlation Spectroscopy in CrSBr

CrSBr is an air-stable magnetic van der Waals semiconductor with strong magnetic anisotropy, where the interaction of excitons with the magnetic order enables the optical identification of different magnetic phases. Here, we study the magnetic anisotropy of multi-layer CrSBr inside a three-axis vector magnet and correlate magnetic order and optical transitions in emission and absorption. We identify layer by layer switching of the magnetization through drastic changes of the optical emission and absorption energy and strength as a function of the applied magnetic field. We correlate optical transitions in reflection spectra with photoluminescence (PL) emission using a transfer-matrix analysis and find that ferromagnetic and antiferromagnetic order between layers can coexist in the same crystal. In the multi-peak PL emission the intensity of energetically lower lying transitions reduces monotonously with increasing field strength whereas energetically higher lying transitions around the bright exciton $X_B$ brighten close to the saturation field. Using this contrasting behavior we can therefore correlate transitions with each other.

cond-mat.mtrl-sci↗

arXiv2024

Raman Polarization Switching in CrSBr

Semiconducting CrSBr is a layered A-type antiferromagnet, with individual layers antiferromagnetically coupled along the stacking direction. Due to its unique orthorhombic crystal structure, CrSBr exhibits highly anisotropic mechanical and optoelectronic properties acting itself as a quasi-1D material. CrSBr demonstrates complex coupling phenomena involving phonons, excitons, magnons, and polaritons. Here we show through polarization-resolved resonant Raman scattering the intricate interaction between the vibrational and electronic properties of CrSBr. For samples spanning from few-layer to bulk thickness, we observe that the polarization of the A$_g^2$ Raman mode can be rotated by 90 degrees, shifting from alignment with the crystallographic a (intermediate magnetic) axis to the b (easy magnetic) axis, depending on the excitation energy. In contrast, the A$_g^1$ and A$_g^3$ modes consistently remain polarized along the b axis, regardless of the laser energy used. We access real and imaginary parts of the Raman tensor in our analysis, uncovering resonant electron-phonon coupling.

cond-mat.mtrl-sci↗

arXiv2022

Amphiphilic co-polymer network gel films based on tetra-poly(ethylene glycol) and tetra-poly({\epsilon}-caprolactone)

In order to allow the simultaneous transport of hydrophilic and hydrophobic substances, polymeric networks with finely distributed hydrophilic and hydrophobic components are very suitable. When designing new soft materials like coatings, in addition to the structure in the volume phase, the structure at the interface plays a critical role. In this study, two alternating tetra-arm star polymers poly({\epsilon}-caprolactone) (tetra-PCL-Ox) and amino-terminated poly(ethylene glycol) (tetra-PEG-NH2) form an amphiphilic co-polymer network. They serve as a model system for controlling and understanding the structure-property relationship at the interface of ACN gels. The correlation between different synthesis strategies for gel films and their resulting properties will be described. Through various spin coating techniques, control over film thickness and roughness is achievable and highlights differences to macroscopic gel samples. Atomic force microscopy (AFM) measurements reveal the effect of solvents of different polarities on the swelling ability and surface structure. This correlates with AFM investigations of the mechanical properties on ACN gel films, demonstrating a strong effect on the resulting elastic modulus E, depending on the presence or absence of a good solvent during synthesis. Furthermore, a higher E modulus is obtained in the presence of the selective solvent water, compared to the non-selective solvent toluene. This observation is explained through selective swelling of the tetra-arm star polymers displaying a different hydrophobicity.

cond-mat.soft↗