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.

We report a new photophysical phenomenon in which 1.06 mm pulses from a Q-switched Nd:YAG laser induce crystallization in supersaturated solutions of urea in water. Because the solutions are transparent at the incident wavelength, a photochemical mechanism is unlikely. The needle-shaped crystals that initially form tend to be aligned parallel to the electric field vector of the light, suggesting a Kerr-like field-induced alignment of urea molecules that aids in organizing prenucleating clusters. The effect has application to pump-probe nucleation studies and to clean nucleation in sealed systems. [S0031-9007(96)01456-1]

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Supersaturated aqueous solutions of glycine exposed to intense pulses of plane-polarized laser light at 1.06 μm unexpectedly crystallized into the polar γ-polymorph of glycine. Control solutions not exposed to the laser always produced crystals of α-glycine, the expected and most stable form. This result suggests a new approach to polymorph control and possibly a means to produce new polymorphs.

The temperature and concentration dependence of nonphotochemical laser-induced nucleation (NPLIN) was studied in aqueous glycine solutions at wavelengths of 532 and 1064 nm, using linearly, circularly, and elliptically polarized light. We observed a narrow supersaturation window (c/c 0 ) 1.45-1.55, where c is the solution molality, and c 0 is the molality of a saturated solution) for "polarization switching", i.e., different polarizations producing different polymorphs. We also observed that, within this window, a small range of ellipticities near unity could induce the nucleation of the R-polymorph and that this range depended on supersaturation. Similar "polarization switching" behavior was observed at wavelengths of 1064 and 532 nm, although the supersaturation window became narrower at lower laser intensities at both wavelengths. Order-parameter ellipsoids and triangles based on optical Kerr alignment are presented to aid in the interpretation of the experimental results.

Ordering kinetics after quenching a block copolymer melt from the disordered state to the ordered state were studied by time-resolved, depolarized light scattering. A theoretical framework for relating the scattering patterns to the granular organization within the sample was developed. In some cases we found that ordering proceeds by the growth of individual grains. These systems were characterized by scattering profiles that are monotonic functions of scattering angle. In contrast, the scattering profiles, under certain quench conditions, contained peaks indicating the presence of intergrain correlations. We demonstrate that these correlations arise due to the presence of three-dimensional grain clusters. Estimates of the size, shape, and concentration of grains during the transformation from disorder to order were obtained. In the early stages of ordering, the grains grew quickly at the expense of the disordered phase until the entire sample was occupied by grains. This stage was followed by extremely slow grain growth (or undetectable growth, in the case of large quench depths) due to defect annihilation.

A systematic study of the dependence of ionic conductivity on the grain size of a lamellar block copolymer electrolyte was performed. A freeze-dried mixture of poly(styrene)-block-poly(ethylene oxide) and lithium bis-(trifluoromethylsulfonyl)imide salt was heated in steps from 29 to 116°C and then cooled back to 29°C with an annealing time ranging from 30 to 60 min at each temperature. Grain structure and ionic conductivity during these steps were quantified by in situ small-angle X-ray scattering and ac impedance spectroscopy, respectively. Conductivity depends both on grain structure and temperature. A normalization scheme to decouple the dependence of conductivity on temperature and grain structure is described. Ionic conductivity at a given temperature was found to decrease by a factor of 5.2 ± 0.9 as the SAXS measure of grain size increased from 13 to 88 nm. The fact that in the system studied, large, well-formed lamellar grains are less conducting than poorly defined, small grains suggests a new approach for optimizing the transport properties of block copolymer electrolytes. Further work is necessary to confirm the generality of this finding.

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