Scholarly article on topic 'Polymer-blend microparticles: Anew approach to nanoscale composites with tunable properties'

Polymer-blend microparticles: Anew approach to nanoscale composites with tunable properties Academic research paper on "Materials engineering"

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Academic research paper on topic "Polymer-blend microparticles: Anew approach to nanoscale composites with tunable properties"

Polymer-blend microparticles: A new approach to nanoscale composites with tunable properties

M. D. Barnes, K. C. Ng, K. Fukui, B. G. Sumpter, and D. W. Noid, Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 -6142, e-mail:barnesmd1 ©

Recently enormous commercial and scientific attention lias been focused on multi-component polymer systems as a means for producing new materials on the micron and nanometer scale with specifically tailored material, electrical and optical properties. Composite polymer particles, or polymer alloys, with specifically tailored properties could find many novel uses in such fields as electro-optic and luminescent devices.] 1] conducting materials, [2| and hybrid inorganic-organic polymer alloys, [3] However, the problem of phase separation from bulk-immiscible components in solution often poses a significant barrier to producing mail) commercially and scientifically relevant homogeneous polymer blends. [4.S.6] The route typically taken in trying to form homogeneous blends of immiscible polymers is to use compatibilizers to reduce interfacial tension. Recently, a number of different groups have examined phaseseparation in copolymer systems to fabricate fascinating and intricate meso- and micro-phase separated structures with a rich variety of morphologies. P, 8]

Our interest focuses on trying to suppress phase-separation in mixed polymer systems by very rapid solvent evaporation from small (<10 pm diameter) droplets of dilute polymer solution. Using instrumentation developed in our laboratory for probing single fluorescent molecules in 1- 10 pm diam. droplet streams, [9] we have been exploring use of microdroplets to form homogeneous polymer composites without compatibilizers as a possible route to new materials with tunable

properties. [10] Figure 1 shows an molecular dynamics simulation of polymer particle formation from a tiny droplet of solution. The primary condition for suppression of phase separation in these systems is that solvent evaporation must occur on a time scale that is fast compared to self-organization times of the polymers.This implies time scales for particle drying on the order of a few milliseconds implying droplet sizes <10 pm (depending on solvent, droplet environment, etc). In addition to a new route to forming nanoscale polymer composites, a microparticle format offers a new tool for studying multi-component polymer blend systems in confined to femtoliter and attoliter volumes where high surface area-to-volume ratios play a significant role in phase separation dynamics.

Our primary experimental tool for probing phase-separation behavior and material homogeneity in polymer composites, is essentially an interferomet-ric technique that has been used for a number of years as a method for sizing liquid droplets. Recently, this measurement technique has been used to recover information on drying kinetics, inter-polymer dynamics, and material properties such as dielectric constant. The basis of the technique involves illumination of a dielectric sphere with a plane-polarized laser to produce an inhomogeneous electric field intensity distribution, or grating, within the particle that results from interference between refracted and totally-internally-reflected waves within the particle. The angular spacing between intensity maxima, as well as the intensity envelope is a highly

sensitive function of particle size, and refractive index (both real and imaginary parts). Unlike conventional microscopy approaches with diffraction-limited X/2) spatial resolution, two-dimensional diffraction (or, angle-resolved scattering) is sensitive to material homogeneity on a length scale of = X/20 or about 20 - 30 nm for optical wavelengths J.V Ford, B.G. Sumper, D.W Nopid, M.D. Barnes, S.C. Hill, and D.B. Hillis, J. Phys. Chem. B. submitted. This dimension is comparable to single-molecule radii of gyration for relative large molecular weight (> 100 k) polymers.

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Figure 1. Molecular dynamics simulation of polymer particle formation from a nanodroplet.

In our experimental configuration, two-dimensional far-field (Fraunhofer) diffraction data are obtained using (f/1.5) collimating optics and a cooled CCD camera. [11] One-dimensional (0° polar angle) scattering data is compared quantitatively with Mie calculations for high-precision size and refractive index determination. Figure 2 illustrates qualitatively the effect of phaseseparation on fringe contrast and definition for two polymer-blend particles prepared from different sized droplets of co-dissolved polymers (poly vinyl chloride and polystyrene) in tetrahy-drofuran (THF).The particle on the top is homogeneous as evidenced by uniform fringe intensity, and high-fringe contrast and definition. Moreover, the scattering data can be matched quantitatively to Mie theory calculations that assume a homogeneous particle. The particle on the bottom has no discernible fringe structure, but does display interesting periodic 'island' structure implying some order and uniformity of phase-separated domains.

One of the most interesting aspects of this work comes from Mie analysis of the scattering data for homogeneous composite particles. Our observation for several different polymer blend systems is that the material dielectric con-

Figure 3. Fluorescence micrographs of dyedoped PVA./ PEG composite particles. Top left: high MW PEC, right: low MW PEG oligomer.

stant (manifested in both the real and imaginary parts of the refractive index) can be tuned by adjusting the relative weight fractions of the polymers in the mixture. Both Re(n) and Im(n) for the polymer-blend microparticles are intermediate between the values determined for pure single-component particles and can be controlled by adjusting the weight fractions of polymers. For both miscible and (bulk) immis-cble polymers that we have combined in homogeneous microparticles, we observe the measured refractive index to be very close to estimates obtained from a simple mass-weighted average of the two species.

Another interesting aspect of polymer composite formation in microparticles is the effect of polymer mobility M.D. Barnes, K.C. Ng, K. Fukui, B.G. Sumpter, and D.W. Noid, Macromolecules (in press). For modest molecular weight polymers, diffusional motion is highly restricted due to the surface energy barrier. However, once a homogeneous polymer-blend particle is formed, constituents with mobility high enough to overcome the surface energy can undergo phase-separation after solvent evaporation. In this case, the spherical symmetry and surface tension usually direct formation of so-called "Russian eggs," or sphere-within-a-sphere phase-separated structures. Figure 3 shows fluorescence micrographs of two different polymer blend particles doped with rhodamine 6G. Both particles were prepared from aqueous polyvinyl alcohol and polyethylene glycol solutions (5:1 relative weight fraction).The particle on the top was prepared using a high molecular weight PEG (3.4 K MW), while the particle on the bottom was generated using a low molecular weight oligomer (200 MW). The sphere-within-a-sphere structures may have interesting applications as three-dimensional waveguides or drug-delivery systems.

Although the microdroplet technique is well suited for producing nearly arbitrarily small particles (down to a single molecule limit), optical diffraction is obviously not suitable for probing particles smaller than a few hundred nanometers. To complement our experimental effort

Figure 2. Two-dimensional diffraction data from 50:50 w/w PVC/PS blend particles produced from an 8 ftm diameter droplet (above left), and a 35 fim diameter droplet (right).

on larger sized particles, we have also investigated various dynamical and steady-state properties of much smaller polymer and polymer blend nanoparticles (1 - 10 nm diameter) using molecular dynamics tools. These simulations allow development of some insight into the structure, and properties of polymer-blend particles, as well as aiding in interpretation of experimental results and guiding future experiments. Using classical molecular dynamics techniques, we have examined polymer nanoparticles of varying size (up to 300,000 atoms), chain-lengths (between 50-200 monomers), and intermolecular interaction energy allowing the systematic study of size-dependent physical properties and time dependence of segregation/equilibration of these particles.[12]

Work currently in progress in our laboratory is focusing primarily on diffraction-based probes of structural dynamics and phase transitions in single- and multi-component polymer particles. The extremely high sensitivity to size and refractive index (relative uncertainties of 10"4, and 5 x 10"4 respectively) should

make it straightforward to observe structural phase transitions accompanied by volume changes on the order of 1 percent. Other work is involved with synthesis and spectroscopy of polymer nanoparticles (5 - 10 nm diam.) with a small residual charge.[13] In close analogy with 2-dimen.sional electron confinement in semi-conductor nanocrystals or quantum dots, the 3-dimensional

confinement of charge-carriers to the surface of a dielectric nanos-phere (with a net charge) results in discrete electronic structure. There are a number of unique features of these species we have termed "quantum drops" (to reflect their three-dimensional nature) that suggest exciting potential applications to problems in heterogenous catalysis and quantum computing, j 14]

This research was sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences (Divisions of Chemical Sciences and Materials Science), under contract DE-AC05-960R22464 with Oak Ridge National Laboratory, and Laboratory-Directed Research and Development Seed Money Fund managed by Lockheed Martin Energy Research Corporation.

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9 C-Y, Kung, M. D. Barnes, N. I-ermer, W. B. Whitten, and J. M. Ramsey, Applied Optics 38, 1481 - 1487 (1999).

10 M. D. Barnes, C-Y. Kung, K. Fukui, B. G.Sumpter, D, W. Noid.andJ. (J. Otjigbe. Optics Utters 24, 121-123 (19991.

11 M. D. Barnes. N. Lcrmer, W. B, Whitten, J. M. Ramsey, Rev. Sci. Inslrum. 68. 2287 - 2291 (1997).

12 K. Fukui. B. G. Sumpter, M. D. Barnes, D. W. Noid, and J.U, Otaigbe, Macromol. Theory and Sim. 8 38-45 (1999).

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14 K. Runge. B. G. Sumpter, D. W. Noid and M. D. Barnes, Chem. Phys. Lett. 299,352-357 (1999).

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