(5cy) Effects of Nanoparticle Addition on the Surface and Bulk Properties of Polymers

Polymer?nanoparticle composite materials offer the unique opportunity to synergistically combine the properties of the polymer and filler on a nanoscale. My research during my Ph.D. and postdoctoral work has been aimed to discover and understand some of the unusual and unexpected enhancements in both the bulk and surface properties caused by the addition of nanoparticles to polymers.

My postdoctoral work with Prof. Gareth H. McKinley and Prof. Robert E. Cohen at Massachusetts Institute of Technology has been aimed at using nanoparticles to modify the surface properties of various polymers in order to develop surfaces with extreme liquid repellency. The various projects that I have been involved with as a postdoc are listed below:

Designing super-oleophobic surfaces.^{1, 2} The combination of surface chemistry and roughness' on the micron and nanoscale imbues enhanced repellency to the lotus leaf surface when in contact with a high surface tension liquid such as water (surface tension γ = 72.1 mN/m). This understanding has led to the creation of a number of artificial superhydrophobic surfaces (water contact angles greater than 150°, low hysteresis). However, researchers so far have been unsuccessful in producing super-oleophobic surfaces for liquids with much lower surface tensions; for example alkanes such as decane (γ = 23.8 mN/m) or octane (γ = 21.6 mN/m). Theoretical calculations suggest that a super-oleophobic surface would need to have a surface energy < 5 mN/m, whereas the lowest solid surface energies reported to date are in the range of ~6 mN/m. In this work, we explain how a third factor, surface curvature (apart from surface chemistry and roughness), can be used to significantly enhance liquid repellency, by studying electrospun polymer fibers containing very low surface energy perfluorinated nanoparticles (FluoroPOSS). Increasing the POSS concentration in the elecrospun fibers allows us to systematically transcend from super-hydrophilic to super-hydrophobic and finally to the first ever super-oleophobic surfaces (exhibiting low hysteresis and contact angles with decane and octane greater than 150°).

Design parameters for superhydrophobicity and superoleophobicity.³ Recent experiments have revealed that the wax on the lotus leaf surface by itself is weakly hydrophilic, and conventional understanding would suggest that such a surface should not be able to support a composite interface, leading instead to a fully wetted interface and low contact angles. Here, we show that this unexpected superhydrophobicity is related to the presence of ?re-entrant texture' on the surface of the lotus leaf. We extend this understanding to enable the development of superoleophobic surfaces (i.e. surfaces that repel extremely low surface tension liquids, such as various alkanes), where in most cases no naturally-oleophobic materials exist. We also develop general design parameters that allow us to evaluate the robustness of the composite interface on a particular surface, thereby allowing us to rank various superhydrophobic or superoleophobic substrates in the literature, with particular emphasis on surfaces developed from inherently hydrophilic or oleophilic materials.

My Ph.D. work was done in the lab of Prof. Michael E. Mackay at Michigan State University. Some of the projects that I was involved with during my Ph.D. are described below:

Novel strategies for nanoparticle dispersion.⁴ We demonstrated that the relative size of the nanoparticle with respect to the linear chain is the key factor governing miscibility of nanoparticles in polymers. Phase stability is possible in chemically dissimilar systems like polystyrene nanoparticles ⁵ in polymethylmethacralate or polyethylene nanoparticles in polystyrene (both of these are classical phase separating systems) as long as the nanoparticle radius is less than the radius of gyration of the linear chain; on the other hand, if the nanoparticles become bigger than the linear chain, even polystyrene nanoparticles phase separate from linear polystyrene.

Effects of nanoparticle addition on the viscosity of polymer melts and solutions.^{6, 7} Traditionally the addition of particles to suspensions and melts causes an increase in their viscosity (this was first predicted by Einstein about 100 years ago). We demonstrated for the first time non-Einstein-like behavior in polymer melts by establishing that the addition of nanoparticles, even at low volume fractions (less than 5%), to polymers can reduce the melt viscosity by as much as 90%, contradicting Einstein's prediction for an increase in viscosity of solutions on the addition of particles.

Diffusion of nanoparticles in polymer melts.⁸ One of our key predictions after the study on the viscosity decrease caused by the addition of nanoparticles to polymer melts was that the nanoparticles must diffuse faster than predicted by the Stokes-Einstein relation, to cause the observed viscosity decrease. Recent experiments using X-ray photon correlation spectroscopy (XPCS) have confirmed this prediction as we obtained order of magnitude faster diffusion coefficients than those predicted by the Stokes-Einstein relation.

Enhanced thermal stability in polymer-nanoparticle composites.⁹ We demonstrated for the first time that the addition of nanoparticles greatly improves (up to 10 fold) the thermal stability of polymers. This observation is particularly surprising in the case of polymeric nanoparticles (like polystyrene nanoparticles) where the nanoparticles and the linear polymer have a much faster degradation rate individually.

Chain expansion on the addition of nanoparticles.¹⁰ Recently there has been a lot of controversy in the literature debating the effects of nanoparticle addition on the radius of gyration of the linear polymer. Our extensive experiments on the ideal system of polystyrene nanoparticles in linear polystyrene using small angle neutron scattering have shown conclusively that the nanoparticle addition causes an expansion in the radius of linear chains (swelling).

Rheological characterization of intramolecularly crosslinked nanoparticles.⁵ In this work, novel polystyrene nanoparticles were synthesized by the controlled intramolecular crosslinking of linear polymer chains to produce well defined single-molecule nanoparticles of varying molecular mass, corresponding directly to the original linear precursor chain. These molecules were ideal to study the rheological/relaxation behavior of high molecular mass polymers in the absence of entanglements, as the high crosslink density present in these molecules potentially inhibits entanglements. It was found that the reptation model (or the presence of a tube for relaxation) does not explain many of the important rheological features seen for our systems. Instead, a coupling or cooperative motion between the crosslinked loops of the molecules was found to play a significant role in the relaxation process.

Multifunctionality in nanocomposites.⁹ Many of the surprising property enhancements caused by the addition of nanoparticles to polymers, as described above, can be engineered to happen for the same volume fraction of the added nanoparticles. Our studies have shown that the addition of certain nanoparticles can simultaneously lead to better processability (reduction in viscosity of up to 80%, contrary to Einstein's prediction), higher electrical conductivity (greater than Maxwell's prediction) and improved mechanical damping and enhanced thermal stability, leading to truly multifunctional nanocomposites.

References:

1. A. Tuteja, W. Choi, M. Ma, J. M. Mabry, S.A. Mazzella, G.C. Rutledge, G.H. McKinley, R.E. Cohen, ?Designing Superoleophobic Surfaces', Science, 2007, 318, 1618-1622.

2. A. Tuteja, W. Choi, J.M. Mabry, G.H. McKinley, R.E. Cohen, ?Creating Super-oleophobic Surfaces', European Polymer Journal ? Special Topics, In Press.

3. A. Tuteja, W. Choi, G. H. McKinley, R. E. Cohen, M. F. Rubner, 'Design parameters for superhydrophobicity and superoleophobicity', MRS Bulletin, in press (to appear August, 2008).

4. M. E. Mackay, A. Tuteja et al. ?General Strategies for Nanoparticle Dispersion', Science, 311, 1740-1743 (2006).

5. A. Tuteja, M. E. Mackay, C. J. Hawker, B. Van Horn, & D. L Ho, 'Molecular architecture and rheological characterization of novel intramolecularly crosslinked polystyrene nanoparticles.', Journal Of Polymer Science Part B-Polymer Physics, 44, 1930-1947 (2006).

6. M. E Mackay, T.T. Dao A. Tuteja et al., ?Nanoscale effects leading to non-Einstein-like decrease in viscosity', Nature Materials, 2, 762-766 (2003).

7. A. Tuteja, M. E. Mackay, C. J. Hawker & B. Van Horn, ?Effect of Ideal, Organic Nanoparticles on the Flow Properties of Linear Polymers: Non-Einstein-like Behavior', Macromolecules, 38, 8000-8011 (2005).

8. A. Tuteja, M. E. Mackay, S. Narayanan, S. Asokan & M. S. Wong, ?Breakdown of the continuum stokes-einstein relation for nanoparticle diffusion', Nano Letters, 7, 1276-81 (2007).

9. A. Tuteja, P. M. Duxbury, M.E. Mackay, ?Multifunctional nanocomposites with reduced viscosity', Macromolecules, 2007, 40, 9427-9434.

10. A. Tuteja, P. M. Duxbury, M. E. Mackay, 'Polymer chain swelling induced by dispersed nanoparticles', Physical Review Letters, 2008, 100, 077801.