(5az) Controlling Nanoparticle Location in Block Copolymers Using External Fields: Simulations and Experiments

Spatially patterning nanoparticles using block copolymer self assembly has been demonstrated as an effective way to achieve synergistic effects of novel properties such as optical, magnetic and mechanical. My research goals are to combine the functionalities of block copolymer nanocomposites with the advantages of nanofiber mats (e. g. high surface area, open pore structure, ability to be woven into a fabric) to greatly widen the scope of potential applications.

My Ph.D. work with Prof. Yong Joo at Cornell University focused on integration of nanomaterial fabrication, nano-characterization and molecular simulations to develop self assembled nanocomposite nanofiber-based materials. These nanofibers with nanostructures hold great technological promise for applications in separations, catalysis, sensing and photonic smart fabrics, to name a few. In this work, I designed the first ever tailored nanofiber mats with internal confined assembly used as template to disperse functional nanoparticles.

Self Assembly in Block Copolymer Nanofibers [1,2,6]

Nanofiber mats with desired microphase-separated poly(styrene-block-isoprene) (PS-b-PI) structures have been fabricated. The fibers are obtained using coaxial electrospinning with self-assembling block copolymer (BCP) as the core and silica (synthesized using sol gel technique) as the shell. Silica shell provides thermal stability to the fibers and helps to anneal the materials at temperatures higher than polymer glass transition to obtain equilibrium confined assembly without destroying the fiber morphology. It also serves as a process aid for polymers that are difficult or impossible to electropsin. The shell is later etched using HF solution to obtain continuous nanofibers (100-400 nm diameters) with periodic internal structures. A systematic study was conducted on the effect of cylindrical confinement on internal self assembly for various block volume ratios with comparisons to the corresponding bulk phase diagram (see figure 1 for selected results). All materials were characterized using scanning electron microscopy (SEM), small angle x-ray scattering (SAXS) and transmission electron microscopy (TEM). To view the internal structures, the fibers were microtomed into 60 nm thick sections both along the fiber axis and perpendicular to the axis and then analyzed under TEM. These materials serve as the first step towards controlling the location of nanoparticles in electrospun fibers for many potential applications. For example, alternate concentric layers, formed by a symmetric diblock copolymer under cylindrical confinement, can create photonic band gaps at certain wavelengths when dielectric contrast between layers is enhanced by addition of high refractive index nanoparticles in one block (figure1). Such materials can be woven into fabrics to produce high performance radiation shields and filters.

Figure1. . TEM images of cylindrically confined self sssembly in PS-b-PI nanofibers with styrene:isoprene volume ratio of (left) 50:50 (right) 69:31. Dark regions are isoprene and light regions are styrene. Scale bars are 200 nm. Click image to see full view

Patterning Nanoparticles using Cylindrically Confined Self-assembly in Nanofibers [5,6]

To demonstrate an example of the technological applications of such self assembled materials and understand and utilize the effects of processing conditions during electrospinning on nanoparticle (NP) location, I have incorporated magnetite NPs into the PS-b-PI core material. Monodisperse magnetite NPs (4 nm) were synthesized and surface coated with oleic acid to provide slight selectivity towards isoprene domain while preserving their magnetic moment. Magnetite nanoparticles have been shown to display superparamagnetic behavior due to their small size making them ideal for magnetic field driven transport of drugs and bioseparations. However, controlling the location of NPs and prevention of aggregates has been the biggest challenge in fabrication of nanocomposites where inter-particle interactions (magnetic in this work) are dominant.

In this work, I showed that strong deformation (~10000 s ^-1) and fast solvent evaporation (200 nl/s) during electrospinning enables selective dispersion of magnetite NPs in the isoprene domain for as high as 20wt% NPs with respect to isoprene. The effect of different nanoparticle fractions on nanocomposite self assembly was studied (fig. 2). No aggregates or loss of domain selectivity was found in spite of strong magnetic interactions between nanoparticles. Magnetic properties were measured using superconducting quantum interference device (SQUID) magnetometer and all fiber samples exhibit superparamagnetic behavior (fig. 3).

Good control over location of magnetic nanoparticles at such high loading, opens up a plethora of possible future applications including electromagnetic shielding, high density memory, magnetic recording, drug delivery and separations. With the freedom to vary nanoparticle surface coating, BCP self assembly, processing conditions (e.g. deformation rate) and nanoparticle size, among other parameters, these results demonstrate the possibility of developing an entire spectrum of multifunctional materials with precisely controlled properties. My work provides a robust methodology to fabricate high surface area multifunctional nanocomposites including cases where particle-particle interactions are dominant.

Figure 2. TEM images of sections of nanocomposite nanofibers. PS-b-PI with 50:50 styrene:isoprene volume ratio is used. (Left) 8 wt% magnetite NPs with respect to isoprene are used. Lamellar self assembly of block copolymer is preserved. (Right) 20 wt% magnetite NPs with respect to isoprene are used. Self assembly changes to bicontinuous phase due to swelling of isoprene domain. Scale bars are 200 nm. Click image to see full view.

Figure 3. Magnetic properties of above samples (20 wt% magnetite) measured using SQUID showing superparamagnetic behavior (no hysterisis).

Study of Polymer Nanocomposites under deformation using Molecular Simulations [3,4,7,8]

 I have integrated my experimental work with coarse grained molecular dynamics (MD) simulations to elucidate experimental findings and to serve as a guide for discovery of new nanocomposite based materials. First, I modeled and simulated the behavior of symmetric diblock copolymer/nanoparticle systems under simple shear flow. I considered three systems such that; a) NPs have selective interactions with one block, b) NPs have equal interactions with both blocks and c) NP-NP interactions are dominant. The aim of this study was to address the following questions: 1) How does shear flow affect the spatial distribution of NPs?; 2) How does the presence of NPs affect shear induced BCP self-assembly?; 3) How is the rheology of BCPs affected by inclusion of NPs?; and 4) How does the effect of shear vary for different NP diameters, interaction potentials and polymer chain lengths?

One of my key findings was that shear has a significant effect on prevention of NP aggregation in cases where NP-NP attraction is dominant and this effect of shear is a strong function of both nanoparticle size and polymer chain length. These results qualitatively match with my experimental results on P S-b-PI/magnetite NP nanofibers. In addition, this work provided the first ever systematic study on the effect of deformation on block copolymer nanocomposites and established the possibility of beneficial use of flow to further control nanoparticle location.

Figure 4. Effect of shear on prevention of aggregation in nanocomposite systems where NP-NP attractions are dominant. The above cartoon depicts that particles are strongly attracted to each other and slightly attracted to A domain (blue). The rightmost image shows the concentration profile of nanoparticles and A and B domains for system under shear flow. The NPs exhibit a maxima in the center of preferred domain A where most chain ends lie to minimize the entropic penalty of chain stretching.

Although shear flow does not represent dominant elongational deformation, it serves as a first step towards my study on nanocomposite behavior during electrospinning using computational modeling and simulation. Moreover, this work would enable a comparison between the effects of different flow types and discovery of more optimized methodology for fabrication of high-end nanocomposites. To draw a closer comparison with my present electrospinning experiments, I have implemented the algorithm for molecular dynamics simulations of elongational flow. Unlike the conventional MD simulations of elongational flow, this algorithm is temporally periodic and allows the simulation to continue indefinitely, which is a necessary condition to study slow relaxing systems like polymers under strong deformation. Study on the effect of elongational deformation on nanocomposite behavior is currently underway.

References

1. V. Kalra, P. Kakad, S. Mendez, T. Ivannikov, M. Kamperman, and Y. Joo, “Self Assembled Structures in Electrospun Poly (styrene-block-isoprene) Fibers”, Macromolecules, 2006, 39, 5453.

2. V. Kalra , S. Mendez, J. Lee, H. Nguyen, M. Marquez, and Y. Joo, “Confined Assembly in Coaxially Electrospun Block Copolymer Fibers” Advanced Materials, 2006, 18, 3299.

3. V. Kalra, S. Mendez, F. Escobedo, and Y. Joo, “Coarse-grained Molecular Dynamics Simulation on the Placement of Nanoparticles within Symmetric Diblock Copolymers under Shear Flow” J. Chem. Phys., 2008, 128, 164909.

4. V. Kalra, Y. Joo, “Using External Fields to Control the Location of Nanoparticles in Polymers: Simulations and Experiments” Proceedings of XVth International Congress on Rheology, 2008, in press.

5. V. Kalra, J. Lee, J. H. Lee, M. Marquez, U. Wiesner, and Y. Joo, “Controlling Nanoparticle Location via Confined Assembly in Electrospun Block Copolymer Nanofibers” Small, 2008, submitted.

6. V. Kalra, J. Panels, and Y. Joo, “Utilization of Silica Sol-gel Synthesis in Multi-axial Jet Electrospinning” Advanced materials, 2008, to be submitted.

7. V. Kalra, F. Escobedo, and Y. Joo, “Effect of shear on Nanoparticle dispersion in Polymer Melts: A Molecular Dynamics Study”, J. Chem. Phys., 2008, to be submitted.

8. V. Kalra, F. Escobedo and Y. Joo, “Effect of Elongational Flow on Nanoparticle Placement in Polymer Matrices using Molecular Dynamics Simulations”, in preparation.