Linear and non-linear rheology of heterogeneous polymeric systems
Authors: Dessi Claudia 
Published: September 2021

Since more than one hundred years ago the discovery of polymerization mechanisms that allowed obtaining polymeric systems with precise and complex molecular structures has fascinated scientists from all over the world and dramatically changed the concept of industry production and our daily life. The reason why industrial polymers have got such great attention in many different scientific and technological applications is the existence of an intimate relationship between their molecular structure and rheological properties with final processing and mechanical properties. As a result, industrial standard methods were intensively developed which most of them allow to obtain easily and quickly important structural and mechanical information for the final application to which the material is intended. However, some technical issues may not be properly taken into account given a wrong characterization of the material. One of the most common industrial dynamic mechanical test employed in order to characterize the rheological response involve torsion deformations. Torsion measurements are in general performed on stiff and elastic materials, such as non-filled and filled vulcanized rubbers, in order to overcome instrument compliance issues and/or wall slip phenomena between the sample and the testing geometry that may importantly affect the measurement itself. In the first part of this work we clearly show that large departures of the rubbery modulus for industrial elastomers may occur when dynamic mechanical measurements are performed in torsion according to industrial standards. The testing sample was chosen in such a way that a comparison with the most common and reliable rheological protocol, small amplitude oscillatory shear, was possible. Once compliance issue and other possible sources of errors were addressed, experimental results in dynamic torsion measurements were found to depend on specimen geometry and its aspect ratios, and the sample loading by clamping. However, so far the empirical corrections proposed do not completely fulfilled the experimental artefact. This observation made this work necessary to establish a better guideline that allows more reliable torsion dynamic mechanical measurements in industrial standard protocols. One of the class of polymer materials that obtain extensive application in the technology field (in particular rubber technology) is represented by the category of filled polymers. It is well-known that the addition of fillers into a polymer matrix generates mechanical reinforcement in the resulting polymer compound. However, the reinforcement mechanism still remains in debate since it does strongly depend on the specific chemistry and properties resulting from polymer-particle interactions. More specifically, the role of the presence of a bound polymer layer on particle surface is not completely addressed. One strategy in order to investigate the reinforcement mechanism is to study the rheological response of favorably interacting filled polymer systems. The second part of this work, hence, is focused on a better understanding of the polymer-particle flow dynamics in nanofilled model polymers for which the presence of bound polymer layers was not always possible to assess. Surprisingly, rheological results show a clear transition from polymer dominated dynamics into “glassy” network dynamics leading to a percolated behavior. These findings encouraged us to further investigate the nature of the polymer-particle interactions in the percolated structure that was found to be most likely related to the presence of hydrogen bonds. Another peculiar feature of the mechanical behavior shown from polymer systems is the capability to be deformed to large amplitude deformations without losing their macroscopical shape. Often large deformations are oscillatory and industrial rubbers widely fulfill this characteristic. Despite mechanical properties in large deformations of polymer network have been observed and investigated for more than six decades, addressing a proper physical model description to the material response remains a challenge. The last part of this work took up the chance to develop a new continuum model that for the first time can describe, at least qualitatively and partially quantitatively, the experimental asymmetric hysteresis response of filled industrial rubbers when they are subjected to large amplitude oscillatory deformations in uniaxial extension superimposed to a steady one.