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

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  • 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.

  • vi

    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

  • vii

    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

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