Conference Agenda

Residual and applied stresses that act on polymer materials can result in their deformation, whitening and cracking. Understanding the distribution of these stresses is important for ensuring material quality. Raman micro-spectroscopy is a non-destructive technique that provides microscopic resolution, which has been used to analyze the stress distribution in semiconductor devices, ceramics and carbon materials. However, there are few examples of its application to polymer materials, mainly because of the difficulty in interpreting spectra associated with polymorphism. In this study, we investigated the feasibility and effectiveness of Raman spectroscopy to determine stress and strain distribution in polycarbonate (PC) molding. PC is a widely used industrial material that exhibits excellent transparency and impact resistance, however, can easily lead to cracks when exposed to solvents under stress.

Firstly, a 2 mm thick PC plate was subjected to four-point bending, and Raman spectral shifts near 1600 cm⁻¹ were analyzed. The results showed a linear relationship between the peak shift and the applied strain within the elastic range (~0.2 cm^-1/% strain), and non-linear behavior in the plastic region. This validated the measurement accuracy as 0.2% strain, corresponding to a stress detection limit of approx. 4 MPa. Next, three-dimensional mapping measurements were performed at different depths in the material. The results aligned with theoretical strain profiles, confirming the accuracy of the measurement in the thickness direction. Lastly, the effectiveness of the technique was demonstrated through analysis of solvent-induced cracks, created by applying acetone to a strained surface. The findings were as follows:

1. The maps revealed that the cracks extended to the depths of approximately 500 µm and that strain was sharply concentrated near the crack tips.

2. Finite element simulations replicated these experimental findings and enabled the quantification of strain that was undetectable by Raman method, by back-calculating from localized strain concentrations, exemplifying the synergy between the experiment and the simulation.

3. The application of polarized Raman micro-spectroscopy revealed that the solvent exposure relaxed the polymer chain orientation.

4. Interestingly, when the same plate was heated above the glass transition temperature, acetone did not induce cracks under strain, emphasizing the unique role of solvent action, such as the reduction of the intermolecular resistance.

In conclusion, Raman micro-spectroscopy enables precise, non-destructive three-dimensional visualization of stress and strain, and affords better understandings on polymer failures.

THz (low frequency) Raman spectroscopy extends the capabilities of conventional Raman spectroscopy, probing the range bellow 200 cm^-1. Signals in this region are mainly attributed to intermolecular vibrations of higher mass and phonon modes that are strongly correlated to the crystal structure of a material. They give information about the degree of crystallinity, molecular orientation and can be used to classify different allotropes and polymorphs. Therefore, THz Raman Spectroscopy is a powerful technique for obtaining both chemical and structural information in one measurement. Herein, we first introduce THz Raman spectroscopy as a fast and reliable technique for MOF-polymorph distinction, as shown with Fe-terephthalic acid based MOF examples. In addition, THz Raman can be used for in-situ phase transition analysis. Here, we probe the solvent-induced phase transitions in MOF systems and investigate the swelling behavior of MIL88 by THz Raman spectroscopy. Furthermore, we demonstrate the non-linear evolution of lattice modes as a function of temperature via low-frequency vibrational spectroscopy. We delve into the balance between conventional positive thermal expansion and phonon-driven negative thermal expansion in a mixed-linker solid solution of a frustrated metal-organic framework.

Digital microfluidics (DMF) describes a technique to manipulate micro- to nanoliter-sized droplets on a 2D-array of insulated and hydrophobic electrodes [1,2]. Research in this field has recently gained momentum due to the versatile sample handling of discrete droplets DMF offers, and the possibility to carry out chemical processes in an unrestricted and automated manner [3]. To monitor chemical processes, it is necessary to couple digital microfluidics with analytical techniques e.g. the hyphenation of DMF with mass spectroscopy [4] or surface-enhanced Raman spectroscopy [5]. However, coupling DMF with absorption-based spectroscopic techniques such as Infrared (IR), is a challenging task. With this work, we present the expansion of the current analytical toolkit for DMF, were we have now achieved the coupling of DMF with IR spectroscopy.

To enable the first DMF-IR coupling, we developed an IR transparent DMF-chip. The novel DMF chip used calcium fluoride as the substrate, enabling real-time IR imaging not only in the reflection but also in the transmission, which enhances the measurement sensitivity. To combine conductive and insulating materials with suitable IR absorption properties, a thin film sputtering-based fabrication method was developed. In order to minimize evaporation during long-term measurements, a remodelled electrode array with evaporation control functionality was designed.

A quantum cascade laser (QCL) for the mid-IR range was utilized to perform the experiments. The developed chip was evaluated by screening various IR-compatible solvents and analytes in a proof-of-concept study. The QCL-source allowed for fast imaging of entire droplets inside the chip. A long-term IR reaction monitoring of an imine synthesis reaction was performed to demonstrate the potential use of the DMF-IR device. The presented work provides an opportunity to study chemical processes in DMF more detailed using mid-IR spectroscopy and allows for real-time observation of droplets by in-situ IR imaging.

[1] M. G. Pollack, Appl. Phys. Lett., 2000, 77, 1725–1726.
[2] J. Lee, Sens. Actuators A, 2002, 95, 259–268.
[3] M. Abdelgawad, Adv. Mater., 2009, 21, 920–925.
[4] A. Das, J. Am. Chem. Soc., 2022, 144, 10353-10360.
[5] S. Fehse, Chem. Com., 2024, 60, 8252-8255.