Interplay Between Kinetics and Apparent Thermodynamics in Thin Polymer Blend Films
Open Access
- Author:
- Indrakanti, Ananth
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 21, 2004
- Committee Members:
- Prof Sanat K Kumar, Committee Chair/Co-Chair
Margaret K Duda, Committee Member
Kristen Ann Fichthorn, Committee Member
Janna Kay Maranas, Committee Member
Ralph H Colby, Committee Member - Keywords:
- single chain mean field theory
neutron scattering
polymer blends
phase behavior - Abstract:
- Mean-field theories predict significant film thickness (D) dependence of the phase behavior in confined polymer blends, especially as D approaches molecular dimensions, i.e., D~Rg, where Rg is the bulk radius of gyration of chains. Pioneering light scattering measurements on thin films of a blend of polystyrene/poly (vinyl methyl ether) (PS/PVME) corroborated the theoretical predictions. In stark contrast to these results, recent Small Angle Neutron Scattering (SANS) experiments on thin films of a blend of deuterated polybutadiene and polyisoprene (dPB/hPI) demonstrate the invariance of the phase boundary, represented by the binodal curve, with decrease in D, for D>4Rg. Motivated by the glaring inconsistencies, we performed SANS on thin films of two blends, that exhibit an Upper Critical Solution Temperature (UCST) in the bulk. Our data demonstrate that there is no finite-size effect on the thin film binodal temperatures, for 3Rg < D < 20Rg, in qualitative agreement with the SANS results from dPB/hPI system. In this work, the bulk-like phase behavior in highly confined polymer blend films is attributed to non-equilibrium effects at surfaces, due to strong binding energies between silicon substrates and monomers of the hydrocarbon-based polymers used in experiment. Failure of the mean-field theory to reproduce the composition profiles in dPB/hPI films, obtained from independent Neutron Reflectivity (NR) measurements, is interpreted as an evidence for non-equilibrium conformations of chains at surfaces, that have an impact on the thermodynamic behavior in thin polymer blend films. Slowing down of chain kinetics near surfaces is studied using theory and simulations, and its effect on the apparent blend thermodynamics is elucidated using a detailed SCMF (Single Chain Mean-Field) model for confined polymer blends. The first direct connection between kinetics and the observed thermodynamics in thin polymer film blends is made in this work.