What is a “nanocomposite”?

1. 2.5 mm a) PDMS matrix i) Compressive preload ii) Dynamic stress 1 mm Dynamic Strain Hardening in Nanotube/Polymer Composites Brent J. Carey1, Lijie Ci1, Prabir K. Patra1,2, Glaura G. Silva3, Pulickel M. Ajayan1 1. Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas, USA 2. Department of Mechanical Engineering, University of Bridgeport, Bridgeport, Connecticut, USA 3. Department of Chemistry, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil Aligned multi-walled carbon nanotubes 1 mm b) c) Abstract In the case of muscles, bones, and other biomechanical tissues, active use triggers metabolic responses which lead to increased strength and resiliency. Such is not the case for engineered materials, where repeated stress typically results in fatigue and, eventually, failure. The cyclic compressive loading of vertically-aligned carbon nanotube/ poly(dimethylsiloxane) composites has revealed a seemingly-limitless active strengthening mechanism not seen for the neat polymer, resulting in a significant increase in stiffness which continues even after a million cycles. Subsequent stress relaxation testing showed no stiffening, indicating that this phenomenon is a response to dynamic stress similar to the well-known attribute of localized self-strengthening in bone tissue. The nanotube alignment in samples allowed for orientation-specific measurement, and the behavior was amplified under compression transverse to the nanotube surfaces, implying that interfacial pressure catalyzes the change. These results in conjunction with thermal analysis suggest stress-induced polymer chain alignment and curing at the interface as probable mechanisms. The dynamic-strain-induced hardening mechanism observed in these nanocomposites has significant implications in the development of self-strengthening materials and active load-bearing artificial tissues. In addition, control over the mechanism at play here and its effects on interfacial load transfer could potentially lead to the advancement of interfacial engineering in polymer nanocomposites. • Continuous Multi-Walled CNT/PDMS Composites • Filler: MWNT forests grown by chemical vapor deposition3 • Large (~100 nm) diameter • ~5% volume • Matrix: PDMS, an amorphous, cross-linked silicone elastomer • Will impregnate CNT forests due to excellent wetting4 • Produces a compliant, yet resilient anisotropic composite • CNTs span the entirety of the composite • Shown to exhibit impressive modulus and damping5 • Compressive Fatigue Testing • Dynamic Mechanical Analysis using a TA Q800 (Fig. 1) • Tested both transverse and longitudinal to CNT alignment • Can resolve evolution of damping (tan δ), storage (E’), and loss (E’’) • Dynamic-Stress-Induced Chain Alignment • Dynamic stress catalyzes the strengthening • Stress relaxation testing showed no response (Fig. 4a) • Localized order around CNTs improves load transfer • Crystallinity around nanoparticles in PDMS has been reported7 • Proposed as a mechanism of improved load transfer8 • These composites show higher thermal stability than PDMS (Fig. 4b) • Indicative of a greater degree of polymer crystallinity a) b) Figure 4 | Characterizing the potential for dynamic chain alignment. a, Stress relaxation testing at 10% strain showed no stiffening, confirming that this behavior is a result of dynamic stress. b, The composites display resilience to degradation at high temperatures. This observation indicates increased crystallinity , in agreement with recent work7,8. • Cross-linking and Chain Mobility • The peak of the tan δ defines the glass transition (Tg) (Fig. 5a) • A lower Tg and sharper tan δ peak indicate a lower cross-link density • CNTs are suggested to interrupt the cross-linking mechanism9 • Heat treatment isolates any chemical changes from dynamic behavior • The heat-treated sample does not show the same stiffening (Fig. 5b) • The confinement of polymer chains does not allow for realignment • What is a “nanocomposite”? • A material which consists of a matrix reinforced by a nanoscale filler • Both phases work together synergistically for enhanced strength • What is fatigue? • Irreversible microstructural damage due to repeated loading • Can lead to accelerated deformation or premature failure • Why don’t our bodies fatigue? • They do, but our muscles and bones have adapted the ability to repair • Repeated stressing will even lead to strengthening • Unfortunately, these effects are reversible after extended disuse • The Role of Carbon Nanotubes (CNTs) in Composites • Primary focus: improving upon traditional carbon fiber composites • CNT dispersion is a big challenge • Efficient load transfer is difficult without compromising CNT integrity • Have not yet achieved the composite strength expected • Have displayed viscoelastic behavior • Composite epoxy films show high damping under shear stress1 • Improved fatigue lifetime vs. traditional composites • Exhibit crack bridging, slowing fatigue progress2 • Will still fail after sufficient stress or number of cycles, though • We report an active strengthening mechanism observed during the dynamic stressing of CNT/poly(dimethylsiloxane) (PDMS) composites Figure 2 | Evidence of dynamic strain hardening. Radial testing shows a greater strengthening (0.72 MPa) vs. axial testing (0.46 MPa) under dynamic stress, with no significant change for neat PDMS . There also does not appear to be a strengthening limit within the number of cycles typically used for the determination of a material’s fatigue lifetime. • Fatigue Strengthening • Seemingly limitless strengthening (Fig. 2) • Up to 6% increase in stiffness/storage after 500,000 cycles • Negligible change for neat PDMS • Experimentally observed to continue even after 2,000,000 cycles • PDMS biocompatibility: potential for active artificial tissues6 • Also observed with other CNT composites • 2-year-old composite: randomly-aligned MWNTs in PDMS • Double-walled CNTs in PDMS • Randomly-aligned CNTs in a fluoroelastomer matrix • Resolving the Strengthening Mechanisms • Stiffening is enhanced under transverse compression • Suggests that PDMS/CNT interfacial pressure is necessary • Majority of stiffness increase is permanent, but there is relaxation • Allowed to relax, then dynamically stressed again (Fig. 3) • Relaxation diminishes for subsequent tests • Indicates that there is more than one mechanism at play Figure 5 | The implications of reduced cross-linking. a, A lower Tg and sharper tan δ peak indicate a lesser degree of cross-linking. This allows for enhanced interfacial chain mobility and alignment under dynamic stress. b, Compared to a sample given three days of 100 °C heat treatment before testing, the “as cured “ specimen shows much greater strengthening. • Summary • We report permanent strengthening during compressive fatigue testing • Two proposed mechanisms: • Limited curing at interface allows for chain alignment/crystallinity • Curing in this arrangement improves load transfer permanently • Potential for active load-bearing artificial tissues • Occurrence in other composites suggests ubiquity with polymers • Control of this mechanism could lead to stronger nanocomposites d) • Acknowledgments • NASA Graduate Student Researchers Program • Air Force Research Laboratory • References • Suhr, J., Koratkar, N., Keblinski, P. & Ajayan, P. M., Viscoelasticity in carbon nanotube composites. Nature Materials 4, 134-137 (2005). • Zhang, W., Picu, R. C. & Koratkar, N., Suppression of fatigue crack growth in carbon nanotube composites. Applied Physics Letters 91 (193109) (2007). • Andrews, R. et al., Continuous production of aligned carbon nanotubes: a step closer to commercial realization. Chemical Physical Letters 303, 467-474 (1999). • Barber, A. H., Cohen, S. R. & Wagner, H. D., Static and Dynamic Wetting Measurements of Single Carbon Nanotubes. Physical Review Letters 92 (18), 186103 (2004). • Ci, L. et al., Continuous Carbon Nanotube Reinforced Composites. Nano Letters 8 (9), 2762-2766 (2008). • McDonald, J. C. & Whitesides, G. M., Poly(dimethylsiloxane) as a Material or Fabricating Microfluidic Devices. Accounts of Chemical Research 35 (7) (2002). • Dollase, T. et al., Effect of Interfaces on the Crystallization Behavior of PDMS. Interface Science 11 (2), (2003). • Coleman, J. N. et al., Reinforcement of polymers with carbon nanotubes. The role of an ordered polymer interfacial region: Experiment and modeling. Polymer 47, 8556-8561 (2006). • Putz, K. W. et al., Effect of Cross-Link Density on Interphase Creation in Polymer Nanocomposites. Macromolecules 41 (18), 6752-6756 (2008). • National Science Foundation • Army Research Laboratory Figure 1 | Schematic of composite samples for compressive fatigue characterization. Continuously-reinforced composites are prepared by infiltrating freestanding CNT forests with PDMS monomer and curing in situ. Orientation-specific fatigue responses were resolved by cyclically compressing the resulting composites along the a, axial (longitudinal) or b, radial (transverse) direction of CNT alignment and were compared to c, a neat PDMS control. d, The dynamic viscoelastic properties were tracked as the samples were stressed at a 5% strain amplitude at 5 Hz and 45 °C for 500,000 cycles. Figure 3 | Discretizing the strengthening mechanisms. At the offset of dynamic stress, the composites relax somewhat. As testing is resumed, they display a retained stiffness increase and will continue along the previous trend for as long as the dynamic stress is applied. The disparity in relaxation after tests #1 and #2 indicates a transformation from temporary to permanent, strongly suggesting that there is more than one mechanism occurring.