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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.
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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