Why Nanoreinforced Polymers?: Mechanics Issues

1. Why Nanoreinforced Polymers?: Mechanics Issues Cate Brinson Frank Fisher Roger Bradshaw T. Ramanathan Northwestern University

2. Qian, Dickey, et al 2000 Outline • Motivation • Why nano reinforced polymers? • What are nanotubes anyway? • Modeling • Top-down: micromechanics • Geometry effects • Experiments • Bulk time dependent behavior (DMA) • TTSP  relaxation spectra • Probes non-bulk polymer behavior • Summary and Future Directions Northwestern University

3. Motivation: why nanocomposites? • Why nanotubes? • 1TPa modulus • High tensile strains (5% experimental) • Chemical interactions • Small volume fraction  large non-bulk polymer phase • Functionality of NT-matrix tailorable • Geometrical constraints • High surface to volume ratio • Interparticle distance decreases • Entanglement leads to strength? Northwestern University

4. Overlapping interphases 10 mm fiber Nano tube • Overlap interphases 10% vf (not 60%+) Motivation: Why the interphase? Surface Area/Volume • Nanotube Reinforced • Surface area/volume 103 to 104 higher than micron sized fibers rf Vi Vf Northwestern University

5. traditional nanocomposite Stress polymer Strain Motivation: goals Expected behavior • The holy grail: high stiffness, high strength, high toughness, low weight • Understand mobility changes in polymer due to NTs • Understand effects of NT geometry • Bridging of mechanics models at several length scales Northwestern University

6. (scale bar = 5 nm) What are Carbon Nanotubes • Hexagonal sheet of carbon atoms rolled into 1D cylinder • Different forms of nanotubes: SWNTs, MWNTs, and NT ropes or bundles (Harris 1999) • Nanotube diameters from 1 to 50+ nm, and lengths on the order of µm (aspect ratios of 1000+) Northwestern University

7. Comparison of reinforcement fillers • Expected mechanical properties of carbon nanotubes compare quite favorably with other types of structural reinforcement • NT fracture strains of 15% (numerical) and 5%+ (experimental) Northwestern University

8. Exploit extraordinary mechanical properties for high stiffness, high strength Multifunctionality: electrical percolation at <0.1%, tune conductive to semi-conductive with chirality Increase temperature range of polymer Possible to use standard polymer processing methods High cost for high purity SWNT ($100/g) Poorly understood NT-polymer interface effects Difficult to achieve uniform dispersion of NTs Lack of control of NT geometry within composite Existing contradictory data Motivation: Benefits and Obstacles Journet, et. al., 1997 SWNT bundles formed viaarc discharge method. Northwestern University

9. Factors influencing NRP effective properties • Properties of NTs • Method of fabrication, SWNT vs MWNT vs bundle • Matrix-nanotube bonding / load transfer • NT dispersion • NT geometry • Alignment • Curvature/waviness • Influence of NTs on viscoelastic behavior of NRP Northwestern University

10. Modeling: Nanotube geometric effects • Current: top-down approach • Use micromechanics tools at nanomechanics level • Account for NT geometry and moduli • Predict elastic response • Future: bottom-up approach • Use MD simulations • Calculate NT impact on polymer locally • Bring key response parameters upscale Modeling Mini-outline • Mori-Tanaka (MT) method • Moduli predictions - alignment • NT waviness - hybrid FE/MT approach Northwestern University

11. Modeling: Micromechanics • Mori-Tanaka method • Uses Eshelby’s classic inclusion analysis • Random to aligned inclusions • Quick analytic technique • Extendable for viscoelastic behavior • Average Fields • Strain Concentration Matrix A2 for dilute soln Strain in fibere2 Strain farfield e0 Northwestern University

12. Modeling: Micromechanics Strain on boundarye1 • Mori-Tanaka theory • Each inclusion “sees” boundarystrain equal to the averagestrain in the matrixe1 • Particle interaction • Stiffness C* in terms of the dilute concentration matrix A2 • Eshelby dilute solution for A2 Strain in fibere2 Strain farfield e0 Eshelby Tensor (inclusion shape, moduli) Northwestern University

13. Effective modulus of NRPs • Qian et al, 2000 • 1% (wt) MWNT’s in polystyrene • Nanotube diameter of ~30 nm • ~35% increase in modulus • ~25% increase in ultimate stress • Schadler et al, 1998 • 5 wt% MWNTs in an epoxy matrix • NTs were poorly distributed, but well dispersed (individual tubes) • “NTs remained curved and interwoven in the epoxy” • 20-25% increase in modulus Northwestern University

14. Micromechanical Predictions of Effective Moduli • Start with simple micromechanics to estimate the NRP effective modulus • ENT = 450 GPa (CVD MWNTs, Rodney Andrews, U. Kentucky) • Mori-Tanaka method, 2D and 3D random orientation of inclusions • Perfect bonding between the phases • Cylindrical inclusion (defines S) • Significant modulus increase, but less than simple micromechanics predictions Schadler, et. al. (1998) 5 wt% MWNTs in epoxy Northwestern University

15. Micromechanics Prediction for NRP • Simple MT results overpredicting significantly even with low ENT • Important considerations for Nanoreinforced Polymers: • NT alignment • Accurate NT modulus • SWNT vs MWNT vs bundles • Matrix-nanotube bonding / load transfer • NT dispersion • Non-bulk polymer behavior • NT Curvature/Waviness Northwestern University

16. Strain on boundarye1 Strain in fibere2 Micromechanics: Modeling waviness • How account for NT curvature? • Use hybrid Finite Element - Analytic approach • FE unit cell with wavy NT • Fiber shape – infinitely long sinusoid • Numerically determine A2 • Use A2 in MT y = a cos (2 π z / L) L/2 Northwestern University

17. Micromechanics: Modeling waviness • Wavy Inclusion Analysis Method • Volumetrically averaged fiber strain • Applied farfield strains • Calculate resulting average fiber strain • Element strain eij is at element centroid • Calculate A2: • Problem reduced to 3 variables: ENT / Em, a / L, L / d Northwestern University

18. Wavelength L Amplitude a Micromechanics: modeling waviness • Various assumptions • Treating NT as continuum • Solid cross-section for NT • Single shape for NT • Effective reinforcing modulus concept • EERM Effective modulus of wavy NT if it were straight a / L Northwestern University

19. Multiphase composite analysis • Variable waviness within the NRP: waviness distribution (as discrete phases) • Each phase has a characteristic A2 based on the waviness of the phase • Proceed with an appropriate multiphase composite analysis for the effective properties Northwestern University

20. Effective modulus predictions for NRP • Schadler, et. al., 1998 • 5 wt% MWNTs in epoxy • Waviness distribution 2 (from table) • Andrews, et. al., 2002 • MWNTs in polystyrene Fisher, Bradshaw, Brinson: Applied Physics Letters, 2002; Composites Science & Tech., in press Northwestern University

21. From Elastic Modeling to VE Experiments • Elastic, micromechanics modeling indicates geometry of NT is a reinforcement limiting mechanism • Non-straight geometry may be important for strength, however…. (future work) • Beyond elasticity: intriguing impact on polymer time dependent response • Experiments for bulk Viscoelastic (VE) response • VE response ideal to probe non-bulk polymer behavior • Evidence of significant reduced polymer mobility with low volume fractions Northwestern University

22. Viscoelasticity of NRPs Gong et al (2000) Shaffer and Windle (1999) • NTs may drastically alter the viscoelastic behavior of the polymer • Tg shift of 35 C with NTs and surfactant as a processing aid • Broadening of the high temperature end of the tan d peak • Suggest that the NTs impact the mobility of the polymer chains PVOH epoxy Northwestern University

23. Impact of molecular mobility on VE behavior Odegard, et. al., 2002 Lordi and Yao, 2000 • Polymer chemistry • long sequences of atoms linked via primary (covalent) bonds • Polymer chains are highly entangled, networked, have side chains • Viscoelastic response - initial elastic response, followed by long-range coordination and chain rearrangement • Mobility results in time- and temperature-dependent properties, which can be investigated via • Measurement of the Tg • Frequency response • Time dependent response • Physical aging Northwestern University

24. Dynamic Mechanical Analysis • TA Instruments DMA 2980 • -150 to 600 C • 0.001 to 200 Hz • Film tension clamp (t < 2 mm) • Polycarbonate-based NRPs (blank, 1 wt%, 2 wt% MWNTs) • Tg measurements (T sweep at constant w) • Frequency response (scan w at multiple T, time-temperature superposition) • Physical Aging creep testing (time domain) • Storage modulus (E’) - measure of the elastic (in-phase) response • Loss modulus (E’’) - measure of the viscous (out-of-phase) response • Loss tangent (tan d) - ratio of storage to loss modulus Northwestern University

25. PC-MWNT Samples • Solution based processing • Evidence of good dispersion • Evidence of interphase on NT Northwestern University

26. Tg measurement • Temperature sweep • w = 1 Hz • DT = 2 °C/min • amplitude = 3 µm • Storage modulus • Higher glassy storage modulus • Much higher rubbery storage modulus • Loss Modulus • Slight shift in Tg to higher temperatures • Broadening of E’’ peak Northwestern University

27. Frequency response of NRP 2% MWNT in PC (RPI), Tref =150 C test range find fix Experimental data • Time-temperature superposition to evaluate over extended w range • Fit frequency response to a Prony series model of VE behavior Storage modulus Loss modulus Northwestern University

28. Relaxation spectrum • Given the Prony series, we have the time domain response • From E(t), we can find the relaxation spectrum H(t) • Alfrey’s approximation • Greater width of relaxation spectrum indicative of more modes of relaxation • Greater contribution of longer relaxation times - consistent with reduced mobility Northwestern University

29. Provide indications of required mobility changes due to NTs To determine interphase mobility and VE properties • Frequency domain response - can be modeled using micromechanics • Ideally • Molecular level simulations • Atomic scale experimental characterization • As a first approximation • Assume properties for the interphase behavior • Use micromechanical models to predict the NRP properties • Compare predictions with experimental data for NRPs, infer interphase volume fraction and properties Northwestern University

30. Prony series to model VE behavior bulk interphase fiber • Model the interphase as a simple shift in the relaxation times of the polymer (characterized by mobility parametera) • Neglect vertical shifting of the modulus response • First approximation: choose interphase relaxation times to match loss modulus experimental data for the NRP Northwestern University

31. Micromechanical Modeling • Mori-Tanaka 3D random alignment using Correspondence Principle • RPI 2% MWNTs in PC • Matrix moduli from DMA • ENT = 200 GPa - to match elastic (high w) response • Interphase volume fraction = 10% • Infer shift in relaxation times • Loss moduli qualitatively agree • No contribution from elastic nanotube to loss modulus a = 100 a = 1000 Northwestern University

32. Physical Aging Volume Temperature • Need to predict the long-term time-dependent properties • Physical Aging: • material in a non-equilibrium state below Tg • interpreted in terms of free volume • Material slowly evolves towards equilibrium (physical aging) • Standard physical aging test sequence • Rejuvenation • Isothermal quench • Aging time Northwestern University

33. pure PC Preliminary Aging Results • Rejuvenated 165°C for 15 min; aging temperature 140°C • Description of physical aging • Shift factor: shift of compliance curves in log space • Shift rate: slope of shift factor vs aging time • Shift rates decrease with addition of NTs • Consistent with reduced-mobility interphase • nanotubes “lock out” free volume Northwestern University

34. Summary of experimental results Standard VE test Most sensitive to NTs? Micromechanical analysis • NRPs have different viscoelastic behavior than bulk polymer • Attributed to the influence of NTs on molecular mobility of the polymer chains • Experimental data consistently interpreted by the presence of a reduced mobility, non-bulk polymer interphase region • Slight increase in effective Tg of the material • Broadening of the relaxation spectra • Decrease in the physical aging shift rates Northwestern University

35. Ongoing Work: NT Functionalization • Control interactions with polymer matrix • Design stiff/flexible interactions • Easier dispersion in solvents & polymer Northwestern University

36. Nanotube functionalization Base functionalization -CH2-PMMA More ductile composite? CO-NH-(CH2)2NH-PMMA Flexible bond CH2-NH-PMMA More Brittle composite? Stiff bond Northwestern University

37. Where we are headed: • Strength: not addressed yet • Geometric and chemical impacts on strength • Bottom up approach to modeling, from MD side • Real multiscale modeling : • MD  interface strength, nonbulk props  mesoscale models (FE and MT) to address strength • Real multiscale experiments: • Nanoindentation near NTs  local behavior • Nanotube pullout?  strength criterion • Couple with modeling • Make extremely, stiff, strong, lightweight composites Northwestern University

38. Future: Nanotube Pullout In collaboration with: R. Ruoff group at NU, L. Schadler @ RPI Northwestern University

39. Research Programs • Nanoreinforced Polymers • Shape Memory Alloys • Aging of Polymers & Composites • Porous Ti - Bone Implants Northwestern University

40. Acknowledgments NASA Langley Research Center Computational Materials: Nanotechnology Modeling and Simulation program The NASA URETI BIMat Center grant is also gratefully acknowledged Northwestern University

41. Northwestern University