Análise experimental dos efeitos da humidade no envelhecimento e na durabilidade de polímeros termoendurecíveis
Thermosetting polymers are an important class of materials for structural applications, exhibiting an ever-expanding use in areas such as transportation, civil infrastructures and biomedical implants. This fact brings out the requirement of experimental data and theoretical models to predict the long-term behavior of those materials. In the actual service conditions of structures, the mechanical loads act in combination with environmental loads (e. g., temperature, moisture and radiation). These coupled actions give rise to a complex phenomena at the molecular level of polymeric materials, which induces changes in their macroscopic properties. The understanding of these phenomena and the modelling of their influence on the long-term mechanical behaviour remains an open problem.
It is a crucial task to solve this problem in order to take full advantage of polymeric materials for highly demanding structural applications. Before trying to overcome the complexity of the synergistic effects of mechanical and environmental loads on the durability of polymeric materials, it is necessary to analyse the influence of each type of load alone. The moisture effects are less studied, although it is the most critical issue of long-term behaviour of those materials. These effects can be grouped in two main categories: chemical and physical effects. The present research project is focused on the physical effects: plasticization, swelling, and physical ageing (evolution towards the equilibrium state of the supermolecular structure of glassy polymers, after cooling below the glass transition temperature). The main objectives of the present research project are: (1) to develop an experimental methodology to characterize the physical effects of moisture sorption on the mechanical behaviour of thermosetting polymers, (2) to provide a reliable data base which can be used to check theoretical models, and (3) to integrate the moisture effects into the constitutive equations. A special attention will be given to the physical ageing promoted by moisture sorption, a subject that was not addressed by any published work. The key feature of the experimental methodology to be developed is the use of different techniques and measurements to assess the molecular mechanisms of moisture/polymer interaction. The infrared spectroscopy will be used to ascertain the existence of chemical effects. Diffusion tests will be performed in order to characterize the kinetics of moisture sorption. These tests, conjointly with swelling tests differential scanning calorimetry, and dynamic thermo-mechanical analysis will contribute to enlighten the moisture/polymer interaction. The effects of physical ageing on the mechanical behaviour will be examined by means of dynamic thermo-mechanical tests and tensile tests. The surface inspection of failed tensile specimens by scanning electron microscopy will allow the identification of possible micro-structural damage events promoted by moisture sorption. The free volume theory will be the basis to model the physical effects associated to moisture sorption. A particular emphasis will be put on the modelling of history dependent effects and on the integration of physical ageing into the constitutive equations of viscoelasticity, through the effective time theory.