Dissertation/Thesis Abstract

Role of Fragility and Neighboring Domains on the Tg and Surface Wave Dynamics of Nanoconfined Polymers
by Evans, Christopher Michael, Ph.D., Northwestern University, 2013, 264; 3563716
Abstract (Summary)

Although the glass transition temperature (Tg) and dynamics of polymers confined to the nanoscale have been studied for twenty years, a physical understanding is still lacking. The reason for a polymer species dependent Tg-confinement effect and the role of neighboring polymer domains in perturbing the Tg of a confined species are areas with a need for greater study as they will inform many of the decisions regarding the use of polymers in nanomaterials.

In this work, fluorescence spectroscopy is used as the primary tool to characterize Tg in a number of systems. First, micelle core Tg and critical micelle temperatures can be determined via pyrenyl label fluorescence for block copolymers in organic solvent at polymer contents which cannot be reliably characterized by other standard methods. Next, measurements were extended to miscible polymer-polymer blend systems where two component Tgs can be determined via a single pyrene-labeled component. Fluorescence can characterize systems with small component Tg differences and near-infinitely dilute blend components unlike scanning calorimetry.

Studies of the near-infinitely dilute blend components reveal that a 0.1 wt% polystyrene component can have its Tg tuned over a 150 °C range depending on the blend partner. Analogous tunability of Tg is also reported in multilayer film systems with an ultrathin PS layer surrounding by bulk neighboring domains. The same limiting Tg is reported by PS for a given neighbor indicating a common physical origin of perturbations in both systems. The perturbations are correlated with fragility which also tracks with the magnitude of Tg-confinement effects in single layer polymer films. Thus, fragility provides a unifying explanation of confinement effects in multilayer films, blends, and single layer films (in the absence of attractive interactions).

Surface wave dynamics are also examined in ultrathin polystyrene layers on various substrates. It is demonstrated that surface dynamics become much slower than anticipated by capillary wave theory as the film thickness decreases. Additionally, surface wave dynamics become orders of magnitude faster as the modulus of the supporting substrate decrease.

Indexing (document details)
Advisor: Torkelson, John M.
Commitee: Broadbelt, Linda J., Burghardt, Wesley R., Shull, Kenneth R.
School: Northwestern University
Department: Chemical and Biological Engineering
School Location: United States -- Illinois
Source: DAI-B 74/09(E), Dissertation Abstracts International
Source Type: DISSERTATION
Subjects: Polymer chemistry, High Temperature Physics, Plastics
Keywords: Glass transition temperature, Nanoconfinement, Polymer blends, Surface dynamics, Thin films
Publication Number: 3563716
ISBN: 9781303122064
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