In-situ Tensile Test in NMR
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In-situ tensile test in NMR
Development of a hybrid testing method for coupling NMR spectroscopy and mechanical tensile testing for the investigation of elastomers
Objective
The aim of the work presented here is to develop a hybrid method for polymer diagnostics that does not yet exist in this form [1−3]. In this hybrid method, conventional tensile testing and, in future, stress relaxation testing under tensile stress are to be coupled with the nuclear magnetic resonance (NMR) method known from chemical analysis (see Fig. 1) in order to establish a direct link to structural changes during the deformation of an elastomer material [4−8]. This makes it possible to obtain stress–strain diagrams recorded in the tensile test with information about the strength and deformation properties of the material being tested and, at the same time, to obtain information about the molecular structure and mobility of polymer chains using the nuclear magnetic resonance method [9–11] (see: nuclear magnetic resonance spectroscopy). The structural parameters for polymeric materials obtained in this way are to serve as the basis for establishing structure-property relationships.
| Fig. 1: | Schematic presentation of the test setup for performing tensile tests in an NMR device system |
Development of the hybrid test method
The innovative in-situ tensile testing device was constructed on the basis of a technological concept that involves the use of a pneumatic cylinder, as shown in Fig. 2. The corresponding designs and material specifications were based on the specifications of the superconducting NMR magnet, which has an 8 cm diameter room air drilling and a 3 cm diameter “sample chamber” in the NMR sample head. The pneumatic cylinder was placed in the upper part of the ambient air bore and is connected to the tensile or compression yoke by carbon fibre rods or, in the area of the NMR sample head, by glass rods. The tensile or compression yoke and all other mountings were manufactured from acrylonitrile butadiene styrene (abbreviation: ABS) using rapid prototyping technologies. The general choice of materials was based on their performance in strong magnetic fields so as not to interfere with the subsequent high-frequency excitation by the NMR method.
| Fig. 2: | Technological concept for the development of the innovative hybrid test equipment |
The necessary software was implemented in Microsoft's Visual Studio.NET development environment using the C# programming language. Various modules were implemented, including those for recording, displaying, and evaluating stress–strain diagrams. The functionality is based on commercially available software for evaluating tensile tests (e.g., ZwickRoell GmbH & Co. KG, Ulm]). This resulted in software components that provide a corresponding user interface for entering the test specimen geometry and important experiment parameters, as well as the option for evaluation. Furthermore, software components were implemented to control the in-situ tensile testing device and thus to send control signals and read out data signals from the sensors. These components enable the control of the pressure in the pneumatic cylinder, the measurement of the crosshead path and, indirectly, via prior calibration, the measurement of the force.
Application
Figure 3 shows examples of the results obtained using the new hybrid testing method. Separate core magnetic (determination of relaxation times T2 as structural parameters) and mechanical experiments were carried out on natural rubber vulcanizates (NR) with varying cross-linking densities. Prismatic test specimens measuring 5 mm x 2 mm with a clamping length of 50 mm were used for the tensile tests, and test specimens measuring 5 mm x 5 mm x 2 mm were used for the nuclear magnetic investigations. The tensile test was used to determine the stress σ and strain ε, as well as relevant characteristic values. A correlation was found between the structure-sensitive T2 relaxation times and the strain determined at the maximum force from the tensile test. The strain at maximum force εFmax and the T2 relaxation time decrease with increasing cross-linking degree of the NR vulcanizates, which is due to the higher cross-linking and the associated greater restriction of chain mobility due to additional cross-linking points.
| Fig. 3: | Comparison of the relaxation time of free molecules and chain ends in the network with the strain at maximum force |
Outlook
The completion of the development of the novel hybrid testing method requires further comparative investigations with reference systems. A further step will be the coupled investigation of stress relaxation processes (see: relaxation plastics) and thus the establishment of structure–property correlations. This new method is intended to make a significant contribution to the targeted improvement of application-relevant properties of elastomeric materials. In addition to the use of the hybrid method in the context of material development and optimization, the structural changes associated with ageing processes are also to be directly detected by recording changes in relaxation times.
See also
- Nuclear magnetic resonance spectroscopy (NMR spectroscopy)
- Hybrid methods
- Polymer diagnostic
- Hybrids methods, examples
References
| [1] | Bierögel, C.: Hybrid Methods of Polymer Diagnostics. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2025) 4th Edition, pp. 487–499 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22) |
| [2] | Grellmann, W., Langer, B.: Methods for Polymer Diagnostics for the Automotive Industry. Materialprüfung 55 (2013) 17–22 Download as pdf |
| [3] | Grellmann, W., Bierögel, C.: Laserextensometrie anwenden. Materialprüfung 40 (1998) 452–459; https://doi.org/10.1515/mt-1998-4011-1206 |
| [4] | Döhler, S., Heuert, U., Grellmann, W.: Material Properties Imaging zur Werkstoffdiagnostik In: Forschungsbericht, Hochschule Merseburg (2011), S. 119–125 (ISBN 978-3-942703-07-9) |
| [5] | Döhler, S., Heuert, U., Grellmann, W.: Material Properties Imaging zur Aufklärung von Materialeigenschaften, 13. Tagung Problemseminar „Deformation und Bruchverhalten von Kunststoffen“, Proceedings (2011), pp. 419–422 |
| [6] | Döhler, S., Glatz, D., Heuert, U., Grellmann, W.: Entwicklung einer neuartigen in-situ-Zugeinrichtung für kernmagnetische Untersuchungen an Elastomeren In: Honekamp, W., Schindler, P. (Eds.), 13. Nachwuchswissenschaftlerkonferenz Proceedings (2012), pp. 185–190 (ISBN 978-3-86870-436-5) |
| [7] | Döhler, S., Reincke, K., Heuert, U., Grellmann, W.: Entwicklung eines hybriden Prüfverfahrens zur strukturellen Aufklärung von Deformationseigenschaften elastomerer Werkstoffe, 14. Problemseminar "Deformation und Bruchverhalten von Kunststoffen", 25.–27. Juni 2014, Merseburg, Proceedings pp. 657–663 (ISBN 978-3-942703-30-7) |
| [8] | Döhler, S., Heuert, U., Reincke, K., Grellmann, W.: Entwicklung eines neuartigen hybriden Prüfverfahrens zur Kopplung von Zugversuch und strukturaufklärender NMR-Spektroskopie, Werkstoffprüfung 2015, Fortschritte in der Werkstoffprüfung für Forschung und Praxis, December 3 and 4, 2015, Bad Neuenahr, Proceedings pp. 267–272 (Eds.: Borsutzki, M., Moninger, G.) (ISBN 978-3-514-00816-8); see AMK-Library under M 36) |
| [9] | Claridge, T. D. W.: High-Resolution NMR Techniques in Organic Chemistry Third Edition, Elsevier Science, Oxford (2016) (ISBN 978-0-0809-9986-9; E-Book ISBN 978-0-0809-9993-7) |
| [10] | Friebolin, H.: Ein- und zweidimensionale NMR-Spektroskopie: Eine Einführung, 5th Edition Wiley-VCH Publishing (2013) (ISBN 978-3-527-33492-6) |
| [11] | Blümich, B.: NMR Imaging of Materials, Clarendon Press, Oxford (2000) (ISBN 9780-1985-2676-6) |