Investigation of the poly(L-lactide)/poly(D-lactide) stereocomplex at the air-water interface by polarization modulation infrared reflection absorption spectroscopy

The polarization modulation infrared reflection-absorption spectroscopy (PM -IRRAS) technique has been used in situ to determine the orientation and mo lecular structure of an equimolar mixture of poly(L-lactide) (PLLA) and pol y(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 fe atures in the PM-IRRAS spectra of the stereocomplex is the derivative shape of the band due to the C=O stretching vibration, providing a spectral sign ature of the presence of polylactide helices oriented parallel to the water surface. The positive and the negative components of the C=O band observed at 1749 and 1765 cm(-1) are assigned to the A and E modes of the helical s tructure, respectively. This assigment was confirmed by recording transmiss ion spectra of the transferred stereocomplex at normal and oblique incidenc e. Compression of the monolayer past 17 Angstrom (2)/repeat unit results in the formation of a bilayer structure. The surface pressure-area isotherm a nd 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 th e PLLA/PDLA stereocomplex. In the bulk crystalline structure, the molecules adopt a 3(1)-helix conformation, and a segment of a PLLA molecule is paire d 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 struc ture of these polymeric Langmuir films and definitely shows that helical po lymeric structures can be directly observed at the air-water interface.