High Frequency Viscoelastic Characterization of Thermally Aged Silicone Elastomers using Quartz Crystal Microbalance Joshua Yeh1 , Randy Schmidt2 and Kenneth R. Shull1 1 Department of Materials Science and Engineering, Northwestern University, Evanston, IL, 60201, USA; 2 Dow Corning Corporation, Midland, MI, 48686, USA [email protected] 1 Introduction was introduced at the midpoint of one of the edges of the sample and the midsection of the sample was colored in with a permanent marker in order to increase the contrast of the propagating crack edge relative to the background. Increasing the contrast of the crack edge enabled an accurate measurement of the crack length from an image correlation program in MATLAB. The sample was placed between two metal grips and pulled at a constant strain rate until the onset of crack propagation was visibly observed. A constant grip displacement was then maintained as the crack propagated. The applied force and applied strain were measured, and were correlated with the crack length determined from video images of the sample collected throughout the experiment. This information was used to obtain the energy release rate, using standard methods. Prolonged exposure of elastomeric materials to high temperatures can cause material embrittlement leading to crack propagation and premature material failure. Thus, a mechanistic understanding of the aging properties of elastomers is of high importance in improving fracture toughness of soft materials in high temperature applications. The high frequency visocelastic properties (in the MHz frequency range) that accompany aging events can be quantified with a relatively new technique utilizing the quartz crystal microbalance (QCM). The QCM provides an ideal non-destructive characterization method that provides real-time measurements of mechanical properties of thin films. These characteristics provide an attractive alternative to conventional destructive, fracture testing of materials, which require a relatively large sample volume and number of tests. Also, the simplicity of QCM measurements allows for high throughput measurements that are not easily achieved with conventional fracture testing methods. In this study, the viscoelastic properties of a model silicone material system undergoing thermal aging is measured with a QCM and correlated with values of the modulus and fracture toughness obtained from pure shear fracture tests. 2 2.1 2.2 Quartz crystal microbalance measurements A two-part model silicone material system, OE6630, provided by Dow Corning, was synthesized by combining two monomer mixtures, parts A and B, with a stoichiometric ratio of 1:4 wt. ratio, respectively. The combined monomer mixture was dissolved in toluene and spuncast onto a 5 MHz QCM crystal (Inficon, East Syracuse, NY). The solvent content was determined based on the desired film thickness. The deposited thin silicone film was cured at an elevated temperature for a set amount of time, both parameters chosen based on the type of QCM measurement. For the in situ curing experiment the properties were monitored continuously while the sample was cured at 80 ◦ C, a temperature near the upper range that was compatible with the QCM electronics. For the thermal aging experiments, the film was cured at 150 ◦ C for one hour and subsequently aged Experimental Pure shear fracture testing Cured 1.5 mm thick slabs of the model silicone elastomer, OE6630, were provided by Dow Corning (Midland, MI). The cured slabs were cut into 38 x 38 mm square samples and exposed to a high temperature environment, 200 ◦ C, for a designated amount of time. Once aged, a notch with a length of 2 mm 1 for an extended period of time at 200 ◦ C. Measurements made on the thermally aged thin film were performed at room temperature. All temperature treatments and QCM measurments were done in the ambient atmosphere. An analysis developed by DeNolf et al. [1] was utilized to extract out viscoelastic property information from the raw QCM data. This analysis is conceptually similar to the previous analysis of Lucklum et al. [2, 3], but provides additional information by considering data from multiple resonant harmonics of the quartz crystal. 3 |G∗ | ρ, and φ, where d, ρ, |G∗ |, and φ represent the deposited film thickness, density, magnitude of the complex shear modulus, and viscoelastic phase angle, respectively. Values of | G ∗ | and φ correspond to the fundamental resonant frequency of 5 MHz. The density of the silicone material was estimated to be ∼1 g/cm3 , a value that was used to convert measured values of |G∗ | ρ to | G ∗ |. The simultaneous measurement of the QCM φ allows for the determination of the storage modulus (G 0 = |G∗ | cos φ) and loss modulus (G 00 = |G∗ | sin φ) as well. Table 1: Calculated shear modulus of aged silicone samples from fracture testings and QCM measurements. Results and Discussion 3.1 Pure shear fracture testing results Day 0 1 2 13 14 2 Energy release rate (J/m ) 8000 6000 4000 Day 13 2000 Day 1 0 −3 10 −2 −1 0 1 2 10 Figure 1: Relationship between energy release rate and crack velocity for aged and unaged samples. From the recorded load-displacement data and video data, the energy release rate and crack velocity were calculated for each sample, with the results shown in Figure 1. Data for unaged samples for a range of crack velocities are included, in addition to data for samples aged for different times at 200 ◦ C. As the material thermally aged, the modulus increased substantially, and crack propagation occurred at much lower applied strains. The crack velocity for a given energy release rate increased as the aging time increased as well. 3.2 G 0 (5 MHz) (MPa) G 00 (5 MHz) (MPa) 738 744 749 810 815 857 854 849 793 787 From the in situ results shown in Figure 2, the total mass remained relatively constant. The increase in the shear modulus and decrease in the phase angle is due to the curing process of the silicone at 80 ◦ C. The thermal aging results displayed a similar trend in the changes of viscoelastic properties. The slight decrease in mass at the longest times is likely caused by oxidative processes that can occur at 200 ◦ C in this case. Comparing Figures 2 and 3, the higher shear modulus and lower viscoelastic phase angle at t = 0 min from Figure 3 indicate that the thermally aged sample was stiffer than the 80 ◦ C cured sample. This can be explained by the differences in sample preparation. Specifically, the thermally aged sample was previously cured at 150 ◦ C for 1 hour and the sample used for the in situ experiment was curing at 80 ◦ C. Day 2 Day 14 10 10 10 10 Crack velocity (mm/sec) G (MPa) tensile test 0.49 1.9 2.1 17 31 3.3 Comparison between QCM and pure shear fracture tests A comparison between the calculated shear moduli from the two measurements is shown in Table 1. Both methods displayed an increase in shear modulus as a function of the aging time at 200 ◦ C. However, the calculated shear moduli between the two techniques differed significantly. This difference was primarily due to the very different time scales of the two measurements. The QCM measurement gives Quartz crystal microbalance results As described by DeNolf et al. [1], information is extracted from the QCM experiments by measuring both the resonant frequency and bandwidth of the crystal response at multiple resonant frequencies. From the measured quantities, we extract dρ, 2 7 11 1.915 1.91 1.905 0 10 t (hours) x 10 31 10 φ (deg.) |G*|ρ (Pa−g/cm3) dρ (g/m2) 1.92 9 8 0 20 10 t (hours) 20 30 29 28 0 10 t (hours) 20 5 t (days) 10 Figure 2: Measured film properties during curing at 80◦ C. 8 8 4.65 4.6 0 5 t (days) 10 x 10 8 7.5 7.5 φ (deg.) |G*|ρ (Pa−g/cm3) dρ (g/m2) 4.7 7 6.5 0 7 6.5 5 t (days) 10 6 0 Figure 3: Properties of a cured silicone film measured at room temperature as a function of the aging time at 200◦ C. the viscoelastic properties at a 5 MHz frequency. The inverse of this frequency corresponds to a time scale that is being probed by the QCM device, a time scale that is about 7 orders of magnitude smaller than the relevant time scale for the pure shear test. We are currently in the process of using traditional dynamic mechanic testing at different temperatures to verify that these differences are quantitatively consistent with the frequency dependence of the linear viscoelastic properties of these materials. 4 terials used in these experiments, and our current efforts are focused on correlating these changes to the changes in the observed fracture toughness and stress-strain behavior at low strain rates. References [1] DeNolf, G. C. et al. High Frequency Rheometry of Viscoelastic Coatings with the Quartz Crystal Microbalance. Langmuir 27, 9873–9879 (2011). URL http://dx.doi.org/10.1021/la200646h, PMID: 21766810. Conclusions The results presented here are preliminary measure- [2] Lucklum, R. & Hauptmann, P. Determination of polymer shear modulus with quartz crystal resments of the fracture toughness and high frequency onators. Faraday Discuss. 123 (1997). viscoelastic properties of silicones after thermal aging at elevated temperature. The most significant [3] Lucklum, R., Behling, C., Cernosek, R. W. & Marchange in the fracture properties after aging of these tin, S. J. Determination of complex shear modumaterials at 200 ◦ C is that the crack propagation oclus with thickness shear mode resonators. Jourcurs for lower values of the far-field strain, consisnal of Physics D: Applied Physics 30, 346 (1997). tent with the observed mechanical stiffening of these materials. The QCM measurements of the viscoelastic properties in the MHz frequency range are sensitive to the cure and aging state of the silicone ma3