Kasiraman Krishnan

Static and dynamic scattering from ternary polymer blends: Bicontinuous microemulsions, Lifshitz lines, and amphiphilicity

Ternary polymer blends, comprising two homopolymers and the corresponding diblock copolymer, have been examined by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). Two chemical systems have been employed: one consisting of polyethylethylene, polydimethylsiloxane, and poly(ethylethylene-b-dimethylsiloxane), and another containing polyethylenepropylene, polyethyleneoxide, and poly(ethylenepropylene-b-ethylene oxide). The molecular weights and compositions were chosen to emphasize the region of the phase prism dominated by the bicontinuous microemulsion (BμE) phase; the homopolymer molecular weights and volume fractions were approximately equal. The SANS intensity was compared quantitatively with the Teubner–Strey structure factor, and interpreted via the amphiphilicity factor fa. The transition from a fully disordered mixture at higher temperatures to a well-developed BμE upon cooling did not correlate well with either the disorder line (fa=+1) or the total monomer Lifshitz line (fa...

Theory and practice of ubiquitous quantitative chemical analysis using conventional computer optical disk drives.

We demonstrate a new attractive approach for ubiquitous quantitative chemical or biological sensing when analog signals are acquired from conventional optical disk drives, and these signals are used for quantitative detection of optical changes of sensing films deposited on conventional CD and DVD optical disks. Our developed analytical model of the operation of this Lab-on-DVD system describes the optical response of sensing films deposited onto the read surface of optical disks by taking into account the practical aspects of system performance that include possible reagent leaching effects, water sampling (delivering) efficiency, and possible changes of the film morphology after water removal. By applying a screen-printing process, we demonstrated a laboratory-scale automated production of sensing films with an average thickness of approximately 10 microm and a thickness relative standard deviation of <3% across multiple films. Finally, we developed a system for delivery of water-sample volumes to sensing films on the disk that utilized a multifunctional jewel case assembly.

Effects of shear flow on a polymeric bicontinuous microemulsion: Equilibrium and steady state behavior

We have investigated the effects of shear flow on a polymeric bicontinuous microemulsion using neutron scattering, light scattering, optical microscopy, and rheology. The microemulsion consists of a ternary blend of poly(ethyl ethylene) (PEE), poly(dimethyl siloxane) (PDMS), and a PEE–PDMS diblock copolymer. At equilibrium, the microemulsion contains two percolating microphases, one PEE rich and the other PDMS rich, separated by a copolymer-laden interface; the characteristic length scale of this structure is 80 nm. Low strain amplitude oscillatory shear measurements reveal behavior similar to that of block copolymer lamellar phases just above the order–disorder transition. Steady shear experiments expose four distinct regimes of response as a function of the shear rate. At low shear rates (regime I) Newtonian behavior is observed, whereas at intermediate shear rates (regime II) development of anisotropy in the morphology leads to shear thinning. When the shear rate is further increased, there is an abrup...

Epitaxial growth of 20 nm InAs and GaAs quantum dots on GaAs through block copolymer templated SiO2 masks

We report on selective area growth of InAs and GaAs quantum dots (QDs) on GaAs through ∼20 nm SiO2 windows prepared by block copolymer lithography. We discuss the mechanisms of growth through these masks, highlighting the variation of the resulting morphology (dot size, spacing, uniformity, and areal density) as a function of growth parameters. We have obtained highly uniform arrays of InAs and GaAs QDs with mean diameters and areal densities of 20.6 nm and 1×1011 cm−2, respectively. We have also investigated the optical characteristics of these QDs as a function of temperature and drawn correlations between the optical response and their crystalline quality.

Critical phenomena in binary and ternary polymer blends

We have studied the dynamic and static properties of binary and ternary polymer blends in temperature and concentration domains that include the critical temperature. The ternary blends consisted of A and B homopolymers and an A–B diblock copolymer. In binary blends and in ternary blends with lower content of the diblock copolymer, we have observed critical divergence of dynamic and static correlation lengths and of static susceptibility, as measured by dynamic light scattering and small-angle neutron scattering. The critical divergence for the binary blends could be analyzed in terms of a crossover between a mean-field and a nonclassical, Ising-type critical behavior. For ternary blends, the addition of a block copolymer stabilizes the system by decreasing the extent of composition fluctuations and by lowering the critical temperature. At a particular composition of the ternary blend, the appearance of a bicontinuous microemulsion was established by small-angle neutron scattering. It was shown that a maximum in the dynamic correlation length derived from dynamic light scattering and observed under these conditions is a characteristic signature of the microemulsion structure.

Shear alignment of a swollen lamellar phase in a ternary polymer blend

The shear-induced structural changes in the swollen lamellar phase of a ternary polymer blend are investigated, and comparisons are made with the bicontinuous microemulsion (BμE) phase. The BμE, having 10 vol % copolymer, exhibits flow-induced phase separation under steady shear, whereas the swollen lamellar phase, with only 20 vol % copolymer and 80% homopolymers, goes into a perpendicular orientation at all accessible shear rates. This unexpected behavior, seen with small angle neutron scattering, is further characterized by rheological measurements. Oscillatory shear with large amplitude, on the other hand, induces perpendicular alignments at low frequencies, and a flipping of orientation to parallel alignment at higher frequencies. The lamellar phase also exhibits a shear-induced isotropic-lamellar-isotropic transition just above the order-disorder transition. The rheological properties show marked changes as the copolymer concentration is systematically increased from the BμE into the swollen lamella...