Enantiomeric Polylactides at the Air−Water Interface: π−A Isotherms and PM-IRRAS Studies of Enantiomers and Their Blend

The surface pressure−area (π− A ) isotherms of enantiomeric polylactides and their equimolar blend reveal differences in the appearance of features and compression rate effects. Surface potential data indicate that the poly( l -lactide) monolayer interacts differently than the 50:50 poly( l -lactide)/poly( d -lactide) blend film with the water subphase. In the case of the 50:50 blend at 25 °C, compression−expansion experiments confirm that the change in the monolayer is irreversible within the time frame of the experiment. In the poly( l -lactide) case, hysteresis is also observed, but the film readily reverts to its original state. Infrared spectra of the polymer monolayers at the air−water interface were obtained using the polarization−modulation infrared reflection−absorption spectroscopy (PM-IRRAS) technique. PM-IRRAS spectra of a compressed poly( l -lactide) monolayer are consistent with the polylactide helices lying in the plane of the air−water interface. No absorbances are observed in the PM-IRRAS spectra of poly( l -lactide) prior to the plateau in the π− A isotherms, suggesting that the helix formation is linked to the increase in surface density which accompanies film compression. The PM-IRRAS spectra establish that the conformation of the poly( l -lactide) helices with respect to the air−water interface is different than the conformation of the d / l blend helices.