Controlling phase transitions in Langmuir polymer films

Medientyp: E-Book

Titel: Controlling phase transitions in Langmuir polymer films

Beteiligte: Das, Abhijna [Verfasser]; Reiter, Günter [Akademischer Betreuer]; Reiter, Renate [Akademischer Betreuer]

Körperschaft: Albert-Ludwigs-Universität Freiburg, Fakultät für Mathematik und Physik

Erschienen: Freiburg: Universität, 2020

Umfang: Online-Ressource

Sprache: Englisch

DOI: 10.6094/UNIFR/167266

Identifikator:

Schlagwörter: Polymers ; Langmuir-Blodgett-Film ; Polymere ; Phase transitions ; Quasi two-dimensional films ; Phase co-existence ; Rod-like polymers ; Non-equillibrium ; Metastable states ; Rate-dependent behavior ; Nucleation density ; Nucleation and growth ; 2D to 3D transition ; Compression rate ; Self-seeding ; (local)doctoralThesis

Entstehung:

Hochschulschrift: Dissertation, Universität Freiburg, 2020

Anmerkungen:

Beschreibung: 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 or increasing the time, which has elapsed when the movement of the barriers was stopped at an area within the plateau region, provided possibilities for re-organization within the 3D structures. We observed a transformation of mesoscopic clusters into a thermodynamically more stable filamentary morphology. Accordingly, the mesoscopic clusters were identified as metastable structures, which were formed at fast rates of compression, i.e., when the polymers did not have sufficient time for ordering into more stable arrangements. Deviations from an equilibrium co-existence between two phases were also responsible for the observation of the “apparent” plateau in the Surface pressure −A isotherm. The measured value at the onset of the plateau was significantly higher than the expected value for an equilibrium phase coexistence, yielding an excess surface pressure in the “apparent” plateau region. For the coexistence region, we interpret the excess surface pressure as a measure of the deviation from equilibrium. Alternatively, we can quantify the difference to the boundary to the homogeneous one-phase system by the amount of over-compression of the Langmuir film expressed by excess surface pressure. We assumed that excess surface pressure was a consequence of the nucleation process. In general, in order to achieve a measurable nucleation rate, a system must be in the metastable region, sufficiently far away from the boundary to the homogeneous one-phase system. In order to bypass the nucleation process during compression of the Langmuir film, we relied on a “self-seeding approach”, which ensured that the growth of 3D structures could start from already pre-existing nuclei. Slowing down the compression process with the presence of pre-existing nuclei enabled us to obtain the thermodynamically more stable filamentary morphology. Furthermore, the value of excess surface pressure decreased significantly upon decreasing the rate at which the “self-seeded” Langmuir film was re-compressed. However, excess surface pressure = 0 was only approached approximately with the logarithm of the re-compression rate, indicating that the equilibrium of the coexistence of 3D structures within a 2D film cannot be established within experimentally accessible conditions. References: [1] Abhijna Das, Ahmed S. El-Tawargy, Emna Khechine, Sebastian Noack, Helmut Schlaad, Günter Reiter, and Renate Reiter. Controlling Nucleation in Quasi-Two-Dimensional Langmuir Poly(l-lactide) Films through Variation of the Rate of Compression. Langmuir 35 (18), 6129–6136 (2019). [2] Abhijna Das, Sebastian Noack, Helmut Schlaad, Günter Reiter, and Renate Reiter. Exploring Pathways to Equilibrate Langmuir Polymer Films. Langmuir 36 (28), 8184–8192 (2020)

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