Web Release Date: March 17,
Thermodynamics of the Liquid Expanded to Condensed Phase Transition of Poly(L-lactic acid) in Langmuir Monolayers
Department of Chemistry (0212), Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
Received January 9, 2006
In Final Form: February 22, 2006
Abstract:
Surface pressure-area per monomer (
-A) isotherms show that poly(L-lactic acid) (PLLA) Langmuir monolayers
exhibit a liquid expanded-to-condensed (LE/LC) phase transition at low surface pressure. Brewster angle microscopy
images show circular domains where the LC phase is surrounded by the LE phase during phase coexistence. Morphology
studies via atomic force microscopy show that well-ordered patterns are only observed for Langmuir-Blodgett films
prepared in the LC phase, while no ordered features are observed in the LE phase. The morphological differences
confirm that during the LE/LC phase transition PLLA molecules form well-ordered structures at the air/water interface.
Analysis by the two-dimensional Clausius-Clapeyron equation is used to predict the critical parameters (Xc). Both
critical parameters, the critical temperature (Tc) and the critical pressure (c), increase with increasing number average
molar mass (Mn) as Xc = Xc,
- KMn-1, where Xc, is the value of the critical parameter at infinite molar mass and
K is a constant. For PLLA Tc, = 36.2 ± 0.3 
C and c, = 4.53 ± 0.06 mN·m-1. This study provides a model polymer
system for examining critical behavior in two dimensions.
Download the full text: PDF | HTML
