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The aim of this study is to compare the mechanical properties of aged polymers measured at nano and macro scales in order to develop a fast characterization tools to predict polymer durability. Commercial thick PP/PE copolymer samples were aged under artificial aging conditions. The evolution of the mechanical properties with aging time was studied either at nano scale using a nanoindenter and at macro scale using a Dynamic Mechanical Analysis aparatus, nano and macro scale results were compared. Chemical degradation was monitored by infrared spectrometry and the surface topography was studied by Scanning Electron Microcopy. A Voigt model was used to calculate the Young's modulus of the bulk sample using the nano mechanical data and the results were compared to the measured macro Young's modulus. A good correlation was found between macro and nano scales as long as macroscopic defects such as cracks does not appeared. This work shows that nano indentation is a suitable technique to detect early stage degradation of polymers and therefore a powerful tool to predict durability of materials.

International audience

International audience; This study is part of a project initiated by the French Agency, ARS (Agence Regional de la Santé) in Guadeloupe to fight against the spread of the mosquito Aedes aegypti in the past years. These mosquitoes are responsible for several major tropical diseases outbreaks such as the dengue fever, Chikungkunya and more recently Zika. The finality of this project is to design and manufacture a mosquito-proof shield product to prevent larvaes of mosquitos to develop in rain recuperation reservoir and thus to reduce the mosquito population and the infection risk.

In prior work we reported results from torsion in stress relaxation experiments in which we measured simultaneously, torque and normal force for two types of amorphous polymers. In one set the materials, poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA), have large sub-vitreous β relaxations, while in the second set, polycarbonate (PC) and polysulfone (PSF), have weak β relaxations. It was found that the prominent β process influences the magnitude of the normal force modulus of PMMA and PEMA while the shear modulus is primarily affected by the α process. In the present work we have extended our study on the effect of the β and α relaxations on the normal force and torque in amorphous polymers by examining the aging behavior of both torque and normal forces of PMMA, PEMA and PC measured at various temperatures between the β and α transitions and at a given deformation (γ=0.04). It is found that for the PMMA the normal force shows aging behavior that is different from the torque response for the PMMA but ages in the same fashion as the torque for both the PEMA and the PC. These results are only partially consistent with the hypothesis that the β process is the cause of the differing behaviors.

In previous work, we found that the normal force responses in poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA) subjected to torsional deformations were strongly influenced by their prominent β-relaxation mechanisms which are related to the n -alkyl methacrylate side group motions. Using the concept of an elastic stress relaxing material we were able to extract the derivatives of the strain energy density function with respect to the first and second invariants of the deformation tensor ( W 1 and W 2 ). In the present work, we report results from two materials, polysulfone (PSF) and polycarbonate (PC), with weak β-relaxations that are due to main chain motions. We find that for the two materials, the difference from neo-Hookean behavior is real though very small, contrary to the large deviations from neo-Hookean behavior observed for PMMA and PEMA. The results support the contention that the large normal force responses and the consequent strongly non neo-Hookean behavior of the glassy n -alkyl methacrylates is due to the side chain.

Results from torsional stress relaxation experiments in which we measure the torque and normal force responses of two glassy poly(alkyl methacrylate)s are presented. Tests were performed at temperatures ranging from the dynamic glass transition (α-relaxation) to the first sub-glass transition relaxation (β-relaxation) and in the sub-yield range of deformations (strains ranging from 0.02 to 0.045) and at a fixed aging time of 4980 s. We found a significant difference between the relaxation behavior of the normal force response and the torque response of poly(methyl methacrylate) (PMMA). The results provide evidence that the normal force of PMMA is influenced by the β-relaxation. For specimens having a longer side chain length, i.e., poly(ethyl methacrylate) (PEMA), the difference between the normal force and torque relaxation behaviors is less. This is consistent with the less pronounced β-relaxation mechanism in PEMA. Furthermore, an examination of the effect of temperature on the ratio of the normal forc...

The material response after the application of a constant strain rate ramp, followed, at time t 1, by a constant strain differs from the response to an ideal (instantaneous) step of strain at short test times. Due to experimental limitations, the ideal step-strain cannot be achieved. As a result, short time stress relaxation data have to be corrected in order to obtain reliable estimates of, for example, the modulus G(t) at times shorter than approximately ten times the ramp time. Here we compare two methods of correction to the stress relaxation data obtained after a linear ramp, assuming a relaxation modulus of the form G(t) =G 0 e−(t/τ)β. The Lee and Knauss correction uses an iterative scheme based on Boltzmann superposition. We compare this method with the Zapas–Craft approach in which the `true' relaxation time becomes t−t 1/2 (t is the experimental time and t 1 is the finite time to apply the step in strain). Our numerical computations show that when the relaxation time is short, there is a substantial error in the Lee–Knauss correction. Although the Zapas–Craft approach provides a better correction for times just slightly greater than t 1/2, it is limited in that it cannot be used for times shorter than t 1/2. We also investigate the case for which the ramp-step is replaced with a more realistic nonlinear function of time. Finally, it is often desirable to have a similar correction for large deformation responses. The Lee–Knauss method is valid only for linear viscoelastic systems whereas the Zapas–Craft approach has not been rigorously evaluated for large deformations. We evaluate the use of the latter for large deformations within the context of the Bernstein, Kearsley and Zapas single integral model.