Characterization of Viscoelastic Properties of Polymeric Materials through Nanoindentation

Nanoindentation is used to determine the dynamic viscoelastic properties of six polymer materials. It is shown that varying the harmonic frequency of the nanoindentation does not have any significant effect on the measured storage and loss moduli of the polymers. Agreement is found between these results and data from DMA testing of the same materials. Varying the harmonic amplitude of the nanoindentation does not have a significant effect on the measured properties of the highperformance resins, however, the storage modulus of the polyethylene decreases as the harmonic amplitude increases. Measured storage and loss moduli are also shown to depend on the density of the polyethylene. INTRODUCTION Nanoindentation is a promising method of measuring the mechanical properties of materials at smaller length and load scales than allowed by other testing methods, thus allowing individual constituents and local regions of heterogeneous materials to be characterized individually. The ability to measure properties on the nanometer length scale is particularly important for the development of polymer composite materials in which the localized material structure can have a significant impact on the overall or bulk behavior. The basic methods for measuring elastic stiffness and strength properties [1] and quasi-static viscoelastic properties [2-5] of polymers through nanoindentation have been developed, however, the dynamic viscoelastic characterization of polymer materials through nanoindentation has not been studied in detail [6]. For time-dependent behavior, dynamic viscoelastic testing offers the advantage of significantly decreased testing time by examining properties over a range of frequencies rather than extended time. Consistent and accurate methods for obtaining the dynamic viscoelastic properties of polymers and their composites need to be established in order to facilitate the use and development of these materials. The objective of the current paper is to establish experimental methods for dynamic nanoindentation and to investigate the ability of these nanoindentation methods to determine the dynamic viscoelastic response of bulk polymer materials. To accomplish these objectives, a series of nanoindentation tests were performed on six polymer systems, including two highperformance resins intended for elevated-temperature conditions, and four resins of high-density polyethylene (HDPE). The dynamic nanoindentation tests were conducted using a range of harmonic amplitudes and harmonic frequencies. Results from these tests, in the form of storage and loss modulus, are quantified and compared. The measured data is also compared with results obtained from standard Dynamical Mechanical Analysis (DMA) test data for the same polymer resins tested under similar conditions. MATERIALS AND SPECIMENS In this study, six polymer materials were used for viscoelastic characterization. The first material, designated 5260, is a modified bismaleimide polymer manufactured by BASF Corp. The second material, designated 8320, is a thermoplastic polymer manufactured by Amoco. The remaining four materials were high-density polyethylene (HDPE) at four different densities: 0.946, 0.947, 0.952, and 0.962 g/cc; and were manufactured by Aldrich Corp. in pellet form. All six of the materials were tested in solid form with the HDPE test specimens cut from plaques that were fabricated from the pellets via mold-press at NASA Langley Research Center. For the nanoindentation specimens, small coupons were cut from the test materials with approximate dimensions 10 mm 10 mm with a thickness at least or greater than 3 mm. The specimens were mounted onto the nanoindentation fixture using a Cyanoacrylate-based adhesive. A Buehler polishing wheel and 3 m alumina polishing solution were used to prepare the testing surface of each nanoindentation specimen. Three DMA test specimens, per material type, were also cut with approximate dimensions of 50 mm 12 mm and a thickness equal nanoindenter test specimen thickness. DYNAMIC NANOINDENTA The nanoindentation tests Berkovich indenter tip. The for determining viscoelastic shown in Fig. 1. Referring material. The displacement method was used, which al using a low magnitude oscil measured at the same freq stiffness of the material. Force Specimen Contact depth Indenter tip Original surface profile Loaded surface profile Load frame Indenter column Displacement gauge