Local Thermal Analysis: Study of viscoelastic properties and time dependence in Surlyn
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0898-L05-08.1
Local Thermal Analysis: Study of viscoelastic properties and time dependence in Surlyn Harsha P. Kulkarnia, Gregory Mogilevskya, William Mullinsb, Alfred Kleinhammesa, and Yue Wua a Department of Physics and Astronomy & Curriculum in Applied and Materials Sciences b Department of Mathematics, University of North Carolina, Chapel Hill, NC 27599-3255 ABSTRACT Self-healing ability is a fascinating property of some materials. Structural materials with selfhealing ability require a subtle combination of flow and stiffness characteristics. A vivid demonstration of self-healing is the automatic sealing of the bullet hole after bullet penetration of the target material. The ionomer Surlyn® satisfies the needs of both structural requirements and self-healing to a certain extent. The viscosity of Surlyn is very high at room temperature, a favorable property for structural applications but unfavorable for self-healing. However, when it was heated to 100°C and then cooled back down to room temperature, its viscosity remains temporarily low at room temperature for a few minutes, a favorable condition for healing. Here we report the measurement of such short-time relaxation effects on flow properties in Surlyn using an atomic force microscope (AFM)-based local thermal mechanical analyzer (LTA). INTRODUCTION Ionomers offer interesting mechanical properties due to the presence of ionic aggregates. These ionic aggregates act as cross-linking (reversible) for polymer chains and play an important role in determining the mechanical and flow properties. The ionomer under consideration here is Surlyn 8920 made by DuPont. It is a co-polymer of ethylene and methacrylic acid (5.4 mol wt.%) which is 60% neutralized by sodium. Two melting peaks were observed in Surlyn by differential scanning calorimetry (DSC). One at ~55°C corresponds to the melting of structures around ionic aggregates and the other at ~85°C corresponds to the melting of polyethylene crystallites. The lower temperature peak relates to ordered structures around ionic aggregates which govern the stiffening behavior of the ionomer upon neutralization1, 2. Structural relaxation around such aggregates took place over a period of 30 days at room temperature after cooling from the melt3. Here we report relaxation effect on mechanical properties of Surlyn over a much shorter timescale (10 to 400 minutes) of structural relaxation after cooling from the melt. EXPERIMENTAL DETAILS
Figure 1 SEM image of the local thermal mechanical analysis probe (left) and tip (right).
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The measurements were carried out using an AFM-based µTA 2990 local thermalmechanical analyzer (LTA). The system was built on an Explorer AFM platform with added local thermal analysis features. The LTA uses a Wollaston (a Pt/Rh alloy coated with silver) wire as its AFM tip as shown in Fig. 1. It has a tip diameter of several microns with a nominal spring constant of 5 N/m. In addition to sensing the force, electric current flowing through the Wollaston wire provides local heating. In ad
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