Amish J. Patel
	Graduate Student Researcher B.S. Chemical Engineering, 2001 IIT Bombay Research Interest: • Equilibrium dynamics of block copolymer systems using X-ray Photon Correlation Spectroscopy (XPCS) • Non-equilibrium studies of a block copolymer system • Nucleation in phase separating polymer blends • Investigating the structure and dynamics of polymer systems using field-theoretic simulations Research | Resume | Publications
Amish J. Patel 201G Gilman Hall University of California  Dept. of Chemical Engineering Berkeley, CA 94720 USA Lab Phone 510-643-5037 Email Me!

Research Description:

Equilibrium dynamics using XPCS

Block copolymers present an interesting class of materials with a broad range of applications. The fact that these molecules self-assemble on length scales ~10nm makes them ideally suited to various nanotechnology applications including memory devices, photonic bandgap materials and lithium batteries to name a few. In recent years, a lot of work, both experimental as well as theoretical, has been done towards understanding the thermodynamic properties of block copolymers and extensive phase diagrams have been mapped for several systems. However, the understanding of the dynamics of these materials on a microscopic length scale is still in its infancy. In this project, we attempt to shed light on structural relaxation dynamics of a poly(styrene-block-isoprene) block copolymer using the conventional rheological techniques as well as a recently developed technique: X-ray Photon Correlation Spectroscopy (XPCS). We start by characterizing the equilibrium thermodyamic structure at various temperatures. Below the order-to-disorder transition temperature (TODT) of 70C, the polymer exists in the form of hexagonally packed cylinders. Above TODT there is an equilibrium volume fraction of disordered micelles dispersed in a homogenous matrix.(1) As the temperature is increased further, the volume fraction of the micelles decreases and at ~120C, we see no signatures of the presence of the micelles. We begin the dynamic studies in the region above TODT. Macroscopic rheological measurements in this region exhibit terminal behavior, allowing us to obtain a terminal stress relaxation time, tau_tr, commonly known as the longest relaxation time. On the other hand, XPCS measures the dynamic structure factor, which gives us the time it takes for microscopic concentration fluctuations to relax, tau_fluc.

Figure 1. Comparing the fluctuation relaxation time, ?fluc with the rheological terminal relaxation time, tau_tr. The dashed line, tau_FL, is the theoretical prediction for tau_fluc.

We observe that tau_fluc is 1-2 orders of magnitude larger than the so-called longest relaxation time, tau_tr (Fig. 1). Even though our system contains micelles in a homogenous disordered bulk, we compare it with a theory for a purely disordered system that was proposed by Fredrickson and Larson.(2) The theory captures the effect of simple (non-micellar) concentration fluctuations on tau_fluc and is called tau_FL (Fig. 1). There is qualitative agreement between theory and experiment in that tau_fluc>tau_tr. However, a lack of quantitative agreement suggests that the origin of the tau_fluc is linked to the presence of the micelles. The relaxation of a micellar phase can, in principle, be a result of either the dissolution of micelles or the diffusion of intact micelles. On comparing tau_fluc to the Stokes-Einstein diffusion time, tau_diff, we find good quantitative agreement. This suggests that the relaxation mode measured by XPCS is that of micelle diffusion and that micelle dissolution does not occur on the time scale given by tau_fluc.

[1] F. M. Abuzaina, A. J. Patel, S. Mochrie, S. Narayanan, A. Sandy, B. A. Garetz and N. P. Balsara, Macromolecules 38, 7090 (2005).
[2] G. H. Fredrickson and R. G. Larson, J. Chem. Phys. 86, 1553 (1987).

Publications:

1. "Structure and phase behavior of block copolymer melts near the sphere-cylinder boundary", F. M. Abuzaina, A. J. Patel, S. Mochrie, S. Narayanan, A. Sandy, B. A. Garetz and N. P. Balsara, vol. 38, p. 7090-7097, Macromolecules, 2005.

2. "Grain structure in block copolymer thin films studied by guided wave depolarized light scattering", B. A. Garetz, M. C. Newstein, J. D. Wilbur, A. J. Patel, D. A. Durkee, R. A. Segalman, J. A. Liddle and N. P. Balsara, vol. 38, p. 4282-4288, Macromolecules, 2005.

3. "Observation of nematic texture in a diblock copolymer melt", F. M. Abuzaina, B. A. Garetz, A. J. Patel, M. C. Newstein, S. P. Gido, X. C. Hu and N. P. Balsara, vol. 39, p. 3377-3385, Macromolecules, 2006.

4. "Relationship between Structural and Stress Relaxation in a Block-Copolymer Melt", A. J. Patel, S. Narayanan, A. Sandy, S. G. J. Mochrie, B. A. Garetz, H. Watanabe and N. P. Balsara, vol. 96, p. 257801, Physical Review Letters, 2006.

5. " Condensed Exponential Correlation Functions in Multicomponent Polymer Blends Measured by X-ray Photon Correlation Spectroscopy ", M. L. Ruegg, A. J. Patel, S. Narayanan, A. Sandy, S. G. J. Mochrie, H. Watanabe and N. P. Balsara, submitted to Macromolecules, 2006.

6. "Observing Nucleation Close to the Binodal by Perturbing Metastable Polymer Blends", A. J. Patel and N. P. Balsara, submitted to Macromolecules, 2006.