Exponential vs Gaussian Correlation Functions in the Characterization of Block Copolymer Grain Structure by Depolarized Light Scattering.

Block copolymer (BCP) grain structure affects the mechanical, optical, and electrical properties of BCP materials, making the accurate characterization of this grain structure an important goal. In this study, improved BCP grain parameters were obtained by employing an exponentially decaying correlation function within the ellipsoidal grain model, instead of the Gaussian correlation function that was used in previous work. The exponential correlation function provides a better fit to the experimental depolarized light scattering data, which outweighs the disadvantage that it requires numerical integration to obtain the model scattered intensity.

Chimney-Shaped Phase Diagram in a Polymer Blend Electrolyte.

The phase behavior of polymer blend electrolytes comprising poly(ethylene oxide) (PEO)/poly(methyl methacrylate) (PMMA)/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was determined using a combination of light and small angle neutron scattering (SANS) experiments. The results at a fixed temperature (110 °C) are presented on a PEO concentration versus salt (LiTFSI) concentration plot. The blends are miscible at all PEO concentrations in the absence of salt. With added salt, a region of immiscibility is obtained in PEO-lean polymer blend electrolytes; blends rich in PEO remain miscible at most salt concentrations. A narrow region of immiscibility juts into the miscible region, giving the phase diagram a chimney-like appearance. The data are qualitatively consistent with a simple extension of Flory-Huggins theory with a composition-dependent Flory-Huggins interaction parameter, χ, that was determined independently from SANS data from homogeneous blend electrolytes. Phase diagrams like the one we obtained were anticipated by self-consistent field theory calculations that account for correlations between ions. The relationship between these theories and measured χ remains to be established.

Dynamic Light Scattering Study of a Laser-Induced Phase-Separated Droplet of Aqueous Glycine.

Tightly focusing a continuous-wave, near-infrared laser beam at the air/solution interface of a millimeter-thick layer of glycine in D2O forms a crystal through a polymorphically and spatially controlled nucleation process known as gradient-force laser-induced nucleation or optical-tweezer laser-induced nucleation. However, when this same beam is focused at the glass/solution interface of a film of aqueous glycine, a highly concentrated laser-induced phase-separated (LIPS) solution droplet is formed that does not nucleate while the focusing beam remains on. Two competing theories have emerged about the nature of the LIPS droplet: one proposes that it is a merger of prenucleation metastable nanodroplets and clusters into one large homogeneous "dense liquid droplet", and the other stipulates that it is the result of the partitioning of larger droplets into the new phase, but not a merging of droplets, around the focal point of the beam. In order to determine the nature of the LIPS droplet, dynamic light scattering was used to detect the presence of nanodroplets undergoing Brownian motion within the droplet and to measure their relative size following a range of laser exposure times. The observation of nanodroplets in motion in the center of the LIPS droplet revealed that the application of optical tweezers at the glass/solution interface forms a relatively monodisperse collection of large nanodroplets (>700 nm) concentrated around the focal point of the beam with smaller particles (<100 nm) depleted within the first 2 min of laser exposure. The LIPS droplet quickly reaches a steady state and is not affected by increasing focusing times. These findings allow for a better understanding of the interactions of optical tweezers with aqueous glycine nanodroplets. This understanding will help in studying the fundamental nature of metastable nanodroplets. More practically, laser-induced phase separation makes possible the nucleation-free separation of large nanodroplets from small clusters, facilitating materials technologies such as high purity, polymorphically selective nucleation of crystals and co-crystals used for pharmaceuticals, dyes, and photovoltaics.

Miscible Polyether/Poly(ether-acetal) Electrolyte Blends.

This study shows that it is possible to obtain homogeneous mixtures of two chemically distinct polymers with a lithium salt for electrolytic applications. This approach is motivated by the success of using mixtures of organic solvents in modern lithium-ion batteries. The properties of mixtures of a polyether, poly(ethylene oxide) (PEO), a poly(ether-acetal), poly(1,3,6-trioxocane) (P(2EO-MO)), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt were studied by small-angle neutron scattering (SANS) and electrochemical characterization in symmetric cells. The SANS data are used to determine the miscibility window and quantify the effect of added salt on the thermodynamic interactions between the polymers. In the absence of salt, PEO/P(2EO-MO) blends are homogeneous and characterized by attractive interactions, i.e., a negative Flory-Huggins interaction parameter, χ. The addition of small amounts of salt results in a positive effective Flory-Huggins interaction parameter, χ eff, and macrophase separation. Surprisingly, miscible blends and negative χ eff parameters are obtained when the salt concentration is increased beyond a critical value. The electrochemical properties of PEO/P(2EO-MO)/LiTFSI blends at a given salt concentration were close to those obtained in PEO/LiTFSI electrolytes at the same salt concentration. This suggests that in the presence of PEO the electrochemical properties exhibited by P(2EO-MO) chains are similar to those of PEO chains. This work opens the door to a new direction for creating new and improved polymer electrolytes either by combining existing polymers and salt or by synthesizing new polymers with the specific aim of including them in miscible polymer blend electrolytes.

Phase Behavior of Mixtures of Block Copolymers and a Lithium Salt.

We present experimental results on the phase behavior of block copolymer/salt mixtures over a wide range of copolymer compositions, molecular weights, and salt concentrations. The experimental system comprises polystyrene- block-poly(ethylene oxide) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. It is well established that LiTFSI interacts favorably with poly(ethylene oxide) relative to polystyrene. The relationship between chain length and copolymer composition at fixed temperature is U-shaped, as seen in experiments on conventional block copolymers and as anticipated from the standard self-consistent field theory (SCFT) of block copolymer melts. The phase behavior can be explained in terms of an effective Flory-Huggins interaction parameter between the polystyrene monomers and poly(ethylene oxide) monomers complexed with the salt, χeff, which increases linearly with salt concentration. The phase behavior of salt-containing block copolymers, plotted on a segregation strength versus copolymer composition plot, is similar to that of conventional (uncharged) block copolymer melts, when the parameter χeff replaces χ in segregation strength.

Periodic plasmonic enhancing epitopes on a whispering gallery mode biosensor.

We propose the attachment of a periodic array of gold nanoparticles (epitopes) to the equator of a Whispering Gallery Mode Biosensor for the purpose of plasmonically enhancing nanoparticle sensing in a self-referencing manner while increasing the capture rate of analyte to antibodies attached to these plasmonic epitopes. Our approach can be applied to a variety of whispering gallery mode resonators from silicon/silica rings and disks to capillaries. The interpretation of the signals is particularly simple since the optical phase difference between the epitopes is designed to be an integer multiple of ?, allowing the wavelength shift from each binding event to add independently.

Nonphotochemical laser-induced nucleation of nematic phase and alignment of nematic director from a supercooled thermotropic liquid crystal.

A nonphotochemical laser-induced phase transition was studied in a supercooled 4;{'}-n -pentyl-4-cyanobiphenyl (5CB, also referred to as PCB and K15) liquid crystal, using linearly polarized 45 ps light pulses at a wavelength of 532 nm. The laser induced nucleation from the metastable supercooled isotropic phase to the nematic phase during slow cooling (0.001 degrees C/min) and high light intensity (3.9 MW/cm{2}) . The resulting nematic director tended to be aligned along the direction of the plane of polarization of the light. At the intensities used, there is no observable laser-induced realignment of the director once the sample is in the nematic phase, nor any permanent laser-induced ordering when the sample is illuminated only in the stable isotropic phase during slow cooling. These experimental results are consistent with a mechanism based on optical Kerr alignment.

Relationship between structural and stress relaxation in a block-copolymer melt.

The relationship between structural relaxation on molecular length scales and macroscopic stress relaxation was explored in a disordered block-copolymer melt. Experiments show that the structural relaxation time, measured by x-ray photon correlation spectroscopy is larger than the terminal stress relaxation time, measured by rheology, by factors as large as 100. We demonstrate that the structural relaxation data are dominated by the diffusion of intact micelles while the stress relaxation data are dominated by contributions due to disordered concentration fluctuations.

Strong dc electric field applied to supersaturated aqueous glycine solution induces nucleation of the gamma polymorph.

Applying a strong static electric field to supersaturated aqueous glycine solutions resulted in the nucleation of the gamma polymorph. This is the first report of a strong dc field inducing the nucleation of a neutral solute in a supersaturated solution. We attribute this effect to the electric-field-induced orientation of the highly polar glycine molecules in large preexisting solute clusters, helping them organize into a crystalline structure. This result also lends further support to our proposed optical-Kerr mechanism for nonphotochemical laser-induced nucleation.

The effect of molecular architecture on the grain growth kinetics of AnBn star block copolymers.

To investigate the effect of molecular architecture on the grain growth kinetics of star block copolymers, a series of AnBn miktoarm star block copolymers with different numbers of arms (n = 1, 2, 4 and 16) was studied. Across this entire series of materials, all the A arms are polystyrene (PS) blocks from the same anionically synthesized batch, and thus all the A arms are identical. Likewise, all the B arms are polyisoprene (PI) blocks from the same anionically synthesized batch, and thus all the B arms are identical. All the stars employed in this study are therefore composed of the same A and B arms liked together in symmetric numbers. The coarsening kinetics of grain growth was monitored in real space by transmission electron microscopy (TEM), followed by subsequent micrograph image analysis. It was found that the molecular architecture influenced the grain growth kinetics of these AnBn star copolymers dramatically. The grain coarsening kinetics was found to follow a scaling law as V approximately t(beta), where V is the characteristic grain volume and t is time. The exponent, beta, was found to be about 0.2 for the diblock copolymer (n = 1) and 0.4 for all three of the star block copolymers (n = 2, 4 and 16) in the series. It is postulated that the difference in grain growth rate between the diblock and the various stars is due to a reduction in molecular entanglements resulting from chain stretching near the junction points in the stars.

Chiral conflict. The effect of temperature on the helical sense of a polymer controlled by the competition between structurally different enantiomers: from dilute solution to the lyotropic liquid crystal state.

Helical polymers appended with paired structurally different enantiomers, which have opposing helical sense preferences, yield a new kind of relationship between optical activity and temperature, and also reveal unusual details of the nature of chiral interactions. Consistent with a statistical physical theory developed for these experiments, the proportion of the competing chiral groups, determined by synthesis, fixes the compensation temperature at which the helical senses are equally populated. The lyotropic liquid crystal state formed by these polymers yields therefore a nematic state at any chosen temperature over a very wide range, with a cholesteric state arising with tightening pitch as temperature deviates from this point. Far from the nematic temperature, the pitch reaches the nanometer scale and therefore the reflection of visible light. Before crossing zero at the nematic temperature, the optical activity becomes so large that it may be observed with the unaided eye through crossed polarizers.

Polarization switching of crystal structure in the nonphotochemical light-induced nucleation of supersaturated aqueous glycine solutions.

By switching between linear and circular polarization in the irradiation of supersaturated solutions of the amino acid glycine in water with intense nanosecond pulses of near-infrared laser light, we have obtained the gamma and alpha phases, respectively, through nonphotochemical light-induced nucleation (NPLIN). This is the first report of light polarization controlling crystal structure. The intensity dependence of NPLIN in aqueous urea is also reported.