Uncovering the relation between fatigue resistance of polymers and their molecular weight

Significance 

Plastics are moldable, synthetic materials derived mostly from fossil fuels. These materials mainly consist of long molecules giving them most of their unique properties. Polystyrene is among the most abundant plastics therefore making it quite important to predict its mechanical behavior under different stress-strain conditions. A plethora of research literature exist about its behavior under static loading conditions. However, little has been done with regard to its mechanical failure when subject to cyclic loading. Recent publications have highlighted that two parameters (harmonic intensity ratios normalized by the fundamental one; i.e. I2/1 and I3/1) can serve as criteria for determining the behavior of the materials. Therefore, using these parameters, there is need to determine the fatigue state at any given point in time and to detect failure onset for materials such as polystyrene.

To this effect, Professor Denis Rodrigue at Laval University in Canada in collaboration with Professor Manfred Wilhelm at the Karlsruhe Institute of Technology in Germany and their team (Valerian Hirschberg, Lukas Schwab, Miriam Cziep) investigated the influence of molecular properties (molecular weight, polydispersity and molecular weight distribution (MWD)) on the mechanical fatigue of model systems of anionic synthesized polystyrene, their blends and commercial polystyrene under strain-controlled oscillatory shear. Their work is currently published in the research journal, Polymer.

Briefly, their research was earmarked with the synthesis of model systems of low polydispersity polystyrene via anionic polymerization (well defined Mw) after which they were compared with the broad commercial MWD polystyrene and bimodal MWD polystyrene. Next, the researchers then proceeded to investigate the influence of the molecular weight distribution using the Wöhler curves. These parameters were calculated via Fourier transform of the stress response in dynamic tests. Lastly, the well-defined molecular properties of the polystyrene systems were studied so as to allow the results obtained to be related to their fatigue properties.

The authors observed that the fatigue life was proportional to the strain amplitude, independent of the molecular weight and polydispersity. Moreover, for polystyrene with a broad polydispersity and a bimodal molecular weight distribution, the number average molecular weight (Mn) was determined to be the most important parameter to correlate the fatigue resistance compared to the weight average molecular weight (Mw). The researchers also noted that the macroscopic crack followed a similar trend to that of Mw. Eventually, they recorded that the storage modulus G’ decreased while the loss modulus G’’ increased, thereby presenting a new type of behavior representative of solids which was different from that of (polymer) melts and solutions.

In a nutshell, the study presented thorough investigation on the dynamic mechanical fatigue of model polystyrene systems with respect to molecular properties such as the weight average molecular weight, the number average molecular weight and polydispersity. Generally, a power-law relation between lifetime and strain amplitude was found, with the exponent independent, but the pre-factor highly dependent, on molecular characteristics. Altogether, the conclusions obtained in this work can be applied to other polymers and testing conditions and this work is currently under investigation.

relation between fatigue resistance of polymers and molecular weight, Advances in Engineering

About the author

Denis Rodrigue obtained a B.Sc. (1991) and a Ph.D. (1996) in chemical engineering from Université de Sherbrooke (Sherbrooke, Canada) with a specialization in non-Newtonian fluid mechanics. In 1996 he moved to Université Laval (Quebec City, Canada) where he is now full professor.

His main research areas are in the characterization and the modelling of the morphological / mechanical / thermal / rheological properties of polymer foams and composites based on thermoplastics and elastomers. His main focus is related to polymer recycling and rheology.

He is the co-editor of Current Applied Polymer Science and Journal of Cellular Plastics and a member of the editorial board of Cellular Polymers and Elastomery.

About the author

Valerian Hirschberg studied Chemistry at the Karlsruhe Institute of Technology (Germany) and Université Laval in Quebec City (Canada, Prof. Denis Rodrigue). His master’s thesis at the Institute of Technical chemistry and polymer chemistry (KIT) was focused on the synthesis (anionic polymerization) of polymer model systems and the investigation of their mechanical fatigue in the group of Prof. Manfred Wilhelm (2015). Since 2016 he does his Ph.D. in Chemical Engineering under the supervision of Prof. Denis Rodrigue and Prof. Manfred Wilhelm (co-supervisor).

The topic of his Ph.D. is the development of characterization tools for mechanical fatigue of polymer systems, based on the decomposition of the material’s stress response via Fourier transform during ongoing loading/fatigue. Oscillatory torsion is applied in the non-linear regime of the materials, so higher mechanical harmonics can be detected at the beginning of the test, and their evolution can be analyzed meaningful. In summer 2018 he spent time working at the CERMEL (Université Tours, France) with Prof. Florian Lacroix, working on the application of the developed concepts in uniaxial tension and multiaxial fatigue.

About the author

Prof. Manfred Wilhelm
Chair Polymeric Materials, University of Karlsruhe, Germany
phone: +49 721 608 – 43150
fax: +49 721 608 – 994004
Manfred Wilhelm∂kit edu

Research interests:
– Method development (non-linear methods for material characterisation)
– Rheology, especially FT-Rheology
– Combinations of Rheology with molecular spectroscopy
– Chemical detectors for SEC
– Solid-State-NMR

Short Bio:

1986-1992 Study of Chemistry at the University of Mainz partly supported with a stipend of the Friedrich-Ebert-society .

09/1989-03/1990 exchange student at the University of Toronto (Canada) in the group of Prof. M.A. Winnik (with a scholarship of the DAAD). Topic: detection of CMC in block copolymers via fluorescence.

Diploma 10/1992 Title: Order and mobility in polymers:
1H and 13C Solid-State NMR in the group of Prof. H.W. Spiess at the Max-Planck-Institute for Polymer Research in Mainz.

Ph.D 07/1995 with the dissertation
“Development and application of multidimensional NMR-methods to detect orientation and dynamics in inorganic and organic polymers” in the group of Prof. H.W. Spiess.

10/1993-04/1994 visit at the UCSB , Santa Barbara (USA), via a further DAAD stipend. Prof. B.F. Chmelka on the topic: multidimensional 13C-exchange NMR applied to zeolites.

Postdoc from 09/1995 to 4/1997 at the Weizmann-Institute (Israel) in the group of Prof. J. Klein (currently at the University of Oxford, UK) using dendrimers under confinement and shear.
This postdoctoral stay was founded via a MINERVA stipend.

Staff scientist from 05/1997 to 10/2004 at the Max-Planck-Institute for Polymer Research in Mainz in the group of Prof. H.W. Spiess where a habilitation was conducted. The habilitation is the German equivalent of an assistant professorship.
The topic of the habilitation (1997 – 2001) was the development and application of non-linear mechanical methods for the characterization of materials . This work was rewarded by the Reimund-Stadler prize of the Fachgruppe Makromolekulare Chemie (GDCh) in 1999.

11/2004-09/2006 Professorship at Technical University Darmstadt in joint cooperation with Max-Planck-Institut for Polymer Research. Head of a Max-Planck research group “Mechanics of Polymers” in the engineering mechanics department.

From 10/2006 Professorship “Polymeric Materials“ at University of Karlsruhe, Institute for chemical technology and polymer chemistry.

Reference

Valerian Hirschberg, Lukas Schwab, Miriam Cziep, Manfred Wilhelm, Denis Rodrigue. Influence of molecular properties on the mechanical fatigue of polystyrene (PS) analyzed via Wohler curves and Fourier Transform rheology. Polymer, volume 138 (2018) page 1-7.

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