Paper BI-TuP8
Highly Sensitive Probe of Crystalline Sub-Micron Sized Domain Structures in Amorphous Material

Tuesday, October 21, 2008, 6:30 pm, Room Hall D

Mixtures of solid compounds or phases occur in a wide variety of chemical processes. Nucleation, crystallization and separation of phases are generally integrative steps in the production of polymer compounds, pharmaceuticals and in many other chemical processes. Many techniques used for analyzing mixtures of medium and ingredients in-situ rely on the interaction of photons with the mixture.¹ Such a situation occurs in solid/solid dispersions, where two different phases coexist, as in the case of most polymers which are known to consist of crystalline domains embedded in an amorphous phase. For instance, Wide-Angle X-Ray Diffraction (WAXS) and Small-Angle X-Ray Scattering (SAXS) are used together to obtain crystallite structure, size and fraction, being limited only by the level of background signal relative to the signal itself. We developed a new technique to study crystalline domains dispersed in an amorphous phase through nonlinear optical Vibrational Sum-Frequency Generation (VSFG) Scattering.² VSFG is a second-order nonlinear optical process, which is forbidden in centrosymmetric media (e.g. liquids, amorphous solids). We show the higher sensitivity of VSFG scattering to microspheres (MS) consisted of crystalline and amorphous phase. We studied the scattering pattern of some chiral polymers, namely Poly(Lactic Acid) (PLA). MS made of P(DL)LA polymer (polymer chains consisting of a random distribution of enantiometers) were probed by X-Ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC) as well. XRD and DSC showed no crystallinity of the PDLLA MS, in agreement with previously published studies. However, VSFG spectra and scattering patterns demonstrate that there is still a finite number of crystallites with maximum extension of 250 nm (radii). This indicates VSFG has an increase in sensitivity that is roughly 2 orders of magnitude more than XRD.³ This opens up new possibilities in the description of early stages of nucleation and growth phenomena and possible new phases.

¹S Roke et al. Vibrational Sum Frequency Scattering from a Submicron Suspension. Phys. Rev. Lett. 91, 2003, 258302.
²AGF de Beer et al. Molecular and microscopic properties of buried microstructures. Submitted.
³HB de Aguiar and S Roke. Sum-Frequency Generation Scattering: a higher sensitivity probe for crystalline properties. In preparation.