Stimuli-responsive Materials and Structures with Electrically Tunable Mechanical ...d- 2017-04-11آ 

Stimuli-responsive Materials and Structures with Electrically Tunable Mechanical

Properties

by

Jeffrey Thomas Auletta

B.S. Chemistry, University of North Florida, 2009

Submitted to the Graduate Faculty of

the Kenneth P. Dietrich School of Arts and Sciences in partial fulfillment

of the requirements for the degree of

Doctor of Philosophy

University of Pittsburgh

2017

ii

UNIVERSITY OF PITTSBURGH

DIETRICH SCHOOL OF ARTS AND SCIENCES

This dissertation was presented

by

Jeffrey Thomas Auletta

It was defended on

March 30, 2017

and approved by

Tara Y. Meyer, Associate Professor, Chemistry

David H. Waldeck, Professor, Chemistry

Alexander Star, Professor, Chemistry

William W. Clark, Professor, Mechanical Engineering & Materials Science

Dissertation Advisor: Tara Y. Meyer, Associate Professor, Chemistry

iii

Copyright © by Jeffrey Thomas Auletta

2017

Stimuli-responsive Materials and Structures with Electrically Tunable Mechanical Properties

Jeffrey Thomas Auletta, PhD

University of Pittsburgh, 2017

iv

Electricity, a convenient stimulus, was used to manipulate the mechanical properties of two classes

of materials, each with a different mechanism. In the first system, macroscale electroplastic

elastomer hydrogels (EPEs) were reversibly cycled through soft and hard states by sequential

application of oxidative and reductive potentials. Electrochemically reversible crosslinks were

switched between strongly binding Fe3+ and weak to non-binding Fe2+, as determined by

potentiometric titration. With the incorporation of graphene oxide (GO) into the EPE, a significant

enhancement in modulus and toughness was observed, allowing for the preparation of thinner EPE

samples, which could be reversibly cycled between soft and hard states over 30 minutes. Further

characterization of this EPE by magnetic susceptibility measurements suggested the formation of

multinuclear iron clusters within the gel.

Copper-derived EPEs which exploited the same redox-controlled mechanism for switching

between hard and soft states were also prepared. Here, the density of temporary crosslinks and the

mechanical properties were controlled by reversibly switching between the +1 and +2 oxidation

states, using a combination of electrochemical/air oxidation and chemical reduction. In addition to

undergoing redox-controlled changes in modulus, these EPEs exhibited shape memory.

In the second system, electroadhesion between ionomer layers was exploited to create

laminate structures whose rigidity depended on the reversible polarization of the dielectric

polymers. The role of the counter-ion in determining the intrinsic and electroadhesive properties

Stimuli-responsive Materials and Structures with Electrically Tunable Mechanical

Properties

Jeffrey Thomas Auletta, PhD

University of Pittsburgh, 2017

v

of poly(ethylene-co-acrylic acid) ionomers in bi- and tri-layered laminate structures was examined.

PEAA ionomers were prepared with three tetraalkylammonium cations (NR4+, R = methyl, TMA+;

ethyl, TEA+; and propyl, TPA+). Reflecting the increasing hydrophobicity of the longer alkyl

chains, water uptake changed as a function of counterion with TMA+ > TEA+ > TPA+. The glass

transition temperatures, electrical resistivities, elastic moduli, and coefficients of friction were

measured and found to depend on the cation identity. Overall, the cation-influenced mechanical

properties of the ionomer determined the flexural rigidity range, but not the magnitude of the

rigidity change, between the on and off states.

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TABLE OF CONTENTS

PREFACE ............................................................................................................................. XXVII

1.0 INTRODUCTION ........................................................................................................ 1

1.1 OVERVIEW ........................................................................................................ 1

1.2 STIMULI-RESPONSIVE MATERIALS.......................................................... 2

1.3 HYDROGELS AND MATERIALS WITH REDOX-ACTIVE

CROSSLINKS ...................................................................................................................... 6

1.3.1 Redox-responsive materials with tunable mechanical properties ............... 6

1.3.1.1 Metal-ion based materials with changes in primary

coordination sphere ..............................................................................................6

1.3.1.2 Materials with intact complexes that undergo changes in oxidation

state without changes in primary coordination sphere .....................................7

1.3.1.3 Other redox-based mechanisms which do not utilize metal ions or

coordination complexes ........................................................................................8

1.3.2 Electroplastic elastomers ................................................................................ 8

1.3.3 Hydrogels .......................................................................................................... 9

1.3.3.1 Theory of rubber elasticity ..................................................................10

1.4 CLAY AND GRAPHENE OXIDE NANOCOMPOSITES ........................... 13

1.5 ELECTROAHESIVE LAMINATES WITH REVERSIBLE CHANGES IN

FLEXURAL RIGIDITY .................................................................................................... 16

1.6 THESIS OVERVIEW ....................................................................................... 17

2.0 MANIPULATING MECHANICAL PROPERTIES WITH ELECTRICITY:

ELECTROPLASTIC ELASTOMER HYDROGELS ............................................................ 19

2.1 OVERVIEW ...................................................................................................... 19

2.2 RESULTS AND DISCUSSION ........................................................................ 23

2.2.1 EPE synthesis ................................................................................................. 23

2.2.2 Iron content .................................................................................................... 24

2.2.3 Electrochemical transitioning of EPE and change in mechanical

properties .................................................................................................................... 24

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2.2.4 Reversible electrochemical oxidation and reduction .................................. 26

2.3 CONCLUSIONS ................................................................................................ 30

2.4 MATERIALS AND METHODS ...................................................................... 30

2.4.1 Typical hydrogel preparation ....................................................................... 31

2.4.2 Iron doping ..................................................................................................... 31

2.4.3 Incorporation of vinyl-functionalized MWNTs .......................................... 31

2.4.4 Mössbauer spectroscopy ............................................................................... 32

2.4.5 Mechanical measurements ............................................................................ 33

2.4.6 Electrochemical methods .............................................................................. 33

2.4.7 Control experiments ...................................................................................... 34

2.4.8 Chronoamperometry and chronocoulometry for redox cycling of Fe3+

hydrogel ....................................................................................................................... 35

2.4.9 Quantification of iron .................................................................................... 35

2.4.10 Mechanical properties of Fe2+ and Fe3+ doped hydrogels and

Fe:carboxylate ratio ................................................................................................... 36

3.0 CHEMICAL AND ELECTROCHEMICAL MANIPULATION OF

MECHANICAL PROPERTIES IN STIMULI-RESPONSIVE COPPER-CROSSLINKED

HYDROGELS ............................................................................................................................. 37

3.1 INTRODUCTION ............................................................................................. 37

3.2 RESULTS AND DISCUSSION ........................................................................ 38

3.3 CONCLUSIONS ................................................................................................ 49

3.4 MATERIALS AND METHODS ...................................................................... 49

3.4.1 Typical hydrogel preparation ....................................................................... 50

3.4.1.1 P