No abstract

The polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) technique has been used in situ to determine the orientation and molecular structure of an equimolar mixture of poly-(L-lactide) (PLLA) and poly(D-lactide) (PDLA) spread at the air-water interface. The characteristics of the compression isotherm and of the PM-IRRAS spectra give clear evidence for the presence of a PLLA/PDLA stereocomplex. One of the most striking features in the PM-IRRAS spectra of the stereocomplex is the derivative shape of the band due to the CdO stretching vibration, providing a spectral signature of the presence of polylactide helices oriented parallel to the water surface. The positive and the negative components of the CdO band observed at 1749 and 1765 cm -1 are assigned to the A and E modes of the helical structure, respectively. This assigment was confirmed by recording transmission spectra of the transferred stereocomplex at normal and oblique incidence. Compression of the monolayer past 17 Å 2 / repeat unit results in the formation of a bilayer structure. The surface pressure-area isotherm and the PM-IRRAS features suggest that the structure of the film at the air-water interface is similar to the three-dimensional crystal structure of the PLLA/PDLA stereocomplex. In the bulk crystalline structure, the molecules adopt a 31-helix conformation, and a segment of a PLLA molecule is paired with a segment of a PDLA molecule, resulting in a racemic unit cell. The PM-IRRAS technique is thus shown to provide detailed insight into the structure of these polymeric Langmuir films and definitely shows that helical polymeric structures can be directly observed at the air-water interface. † This paper is dedicated to the memory of Prof. G. Ronald Brown.

He ´le `ne Bourque and Michel Pe ´zolet* Centre de recherche en sciences et inge ´nierie des macromole ´cules, De ´partement de chimie, Universite ´Laval, Cite ´Universitaire, Que ´bec G1K 7P4, Canada

Ultrathin films of behenic acid methyl ester (BAME) deposited by spin coating on solid substrates or spread at the air/water interface were studied by infrared spectroscopy and Brewster angle microscopy. Anisotropic optical constants of BAME were determined from a transmittance spectrum at normal incidence and a parallel-polarized reflectance spectrum at grazing incidence. A transmittance spectrum recorded at oblique incidence (60°) was used to validate these optical constants. The anisotropic extinction coefficients and the polarized ATR spectra were used to calculate the tilt angle of several transition moments and also the molecular tilt angle of BAME molecules assuming an all-trans conformation of the alkyl chain. The results indicate that the alkyl chain of the fatty acid ester is tilted and that the molecular tilt angle is close to 30°. PM-IRRAS spectra of BAME at the air/water interface were also recorded as a function of the surface pressure. The splitting of the methylene bending mode shows that the all-tans alkyl chains are packed in a quasi-crystalline structure at the air/water interface, even at low surface pressure. This finding was further confirmed by Brewster angle microscopy. Furthermore, the band progression due to the methylene wagging modes was observed for the first time in infrared spectra of a Langmuir monolayer. The simulation of the PM-IRRAS spectrum of a BAME monolayer recorded at 30 mN/m using the optical constants obtained from films prepared by spin coating indicate that, as opposed to the orientation on solid substrates, the alkyl chain of BAME is nearly perpendicular to the air/water interface in Langmuir films.