• Das, Abhijna

Abstract

Upon compression, i.e., reducing the mean area per molecule (A), quasi-two-dimensional Langmuir films can undergo a sequence of several phase transitions. Typically, these phase transitions are interpreted from a thermodynamic perspective. However, properties of the compressed films often show a rate-dependent dynamic behavior, which represents the focus of our work. In this thesis, we explored the rate-dependent behavior of Langmuir films of poly L-lactide (PLLA -24) related to phase transitions. Starting from randomly dispersed molecules on a water surface, we observed the formation of a condensed two-dimensional film upon compression up to a critical area (AM). Compressing beyond AM led to the emergence of three-dimensional structures (mesoscopic clusters formed via a nucleation and growth process) within this two-dimensional film. Upon further compression, the amount of 3D structures increased at the expense of the area fraction covered by a 2D film. In this coexistence region, the surface pressure only slightly increased upon decreasing A, i.e., surface pressure exhibited an “apparent” plateau at a value, which, however, was significantly larger than the value of surface pressure measured at AM. Thus, we concluded that the mesoscopic clusters grew within an over-compressed two-dimensional film. The nucleation density of these mesoscopic clusters could be controlled precisely by changing the rate of compression. For a given surface density and temperature, increasing the rate of compression by two orders of magnitude allowed to increase the number of clusters also by two orders of magnitude, i.e., following a linear relation between nucleation density and compression rate. This observation clearly shows that the kinetics of changing A in Langmuir polymer films has an obvious and distinct influence on the nucleation probability. The rate of compression had also a significant impact on the morphologies, which were obtained during the compression of Langmuir films of PLLA -24. Compressing Langmuir films slowly ...

Topics

• density
• impedance spectroscopy
• surface
• cluster
• polymer
• phase
• phase transition
• two-dimensional
• aging
• additive manufacturing
• aging