Effect of Thermal Conditions and Mechanical Stress During Vitrification on Physical Aging of Thin Polymer Films Pubblico
Yoon, Suk Whan (2012)
Published
Abstract
Glasses are formed by cooling, or quenching, a glass-forming
material from above the glass
transition temperature Tg to below Tg. Once formed,
glasses undergo a process named physical
aging to advance towards a stable equilibrium state because they
are thermodynamically
unstable. Thin glass-forming polymer films have previously
exhibited inconsistent physical
aging behaviors in that thin freestanding polysulfone (PSF) films
400 nm to 4000 nm thick have
previously exhibited an increased aging rate with decreasing film
thickness behavior while thin
supported polystyrene (PS) films 100 nm to 2500 nm thick have
previously exhibited no change
in aging rate with decreasing film thickness. We resolve this
inconsistency by showing that this
difference arises because the PS and PSF films were prepared under
different conditions. Various
conditions that may affect the aging rate of thin polymer films are
then examined. In particular,
effects of mechanical stress during quenching and thermal
conditions, such as annealing
temperature, quench temperature, and quench rate through Tg
were examined. It is shown that the
aging rate of the thin film increases with respect to cooling rate
at Tg, and that the quench
temperature Tquench and annealing temperature do not affect
the physical aging rate. The
increased aging rate behavior of the film with increasing cooling
rate was justified with a free
volume and a potential energy landscape argument. Specifically, it
was noted that a fast quench
could trap glasses at a higher volume and energy state than a slow
quench. Based on the
suggestion that the potential energy landscape can be altered with
stress and strain, we propose a
possibility that stress during the thermal quenching process may
trap glasses in a different
energetic state, inducing a different aging behavior than would be
expected of quenched films
without stress. We hypothesize that, since thinner freestanding
films - which undergo faster
aging - experience a larger stress than thicker freestanding films,
aging rate increases may
correlate with applied stress, but more data must be collected
before any conclusive relationship
can be drawn between stress and physical aging rate.
Table of Contents
I.
Introduction 1
Motivation 3
II. Background
4
Ellipsometry 4
Existing Literature 10
III. Open
Questions and Goals of This Thesis 15
Stiff Backbone vs. Flexible Backbone
16
Freestanding vs. Supported Quenching Method 16
IV. Experimental Procedures 17
Sample Preparation 17
Aging Measurements by Ellipsometry 20
V. Results and
Discussion 22
Chemical Structure: Stiff Backbone
vs. Flexible Backbone 22
Quench Conditions: Annealing and
Quench Temperatures 24
Quench Conditions: Quench Rate
26
Free Volume and Energy Landscape
Argument 30
Quench Conditions: Mechanical Stress
During Quenching 33
Directly Hanging Mass to PS Film
39
Directly Hanging Mass to PS Film
During Annealing: Preliminary Results 42
VI. Conclusions 43
References 45
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