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  • STIMULI-RESPONSIVE POLYMERS

    IN SOLUTION AND ON GRAFTED SURFACES

    A Dissertation

    by

    HUI FU

    Submitted to the Office of Graduate Studies of Texas A&M University

    in partial fulfillment of the requirements for the degree of

    DOCTOR OF PHILOSOPHY

    May 2010

    Major Subject: Chemistry

  • STIMULI-RESPONSIVE POLYMERS

    IN SOLUTION AND ON GRAFTED SURFACES

    A Dissertation

    by

    HUI FU

    Submitted to the Office of Graduate Studies of Texas A&M University

    in partial fulfillment of the requirements for the degree of

    DOCTOR OF PHILOSOPHY

    Approved by:

    Chair of Committee, David E. Bergbreiter Committee Members, Daniel A. Singleton James D. Batteas Melissa A. Grunlan Head of Department, David H. Russell

    May 2010

    Major Subject: Chemistry

  • iii

    ABSTRACT

    Stimuli-responsive Polymers in Solution and on Grafted Surfaces. (May 2010)

    Hui Fu, B.S., Sichuan University; M.S., Tsinghua University; M.S., University of

    Southern California

    Chair of Advisory Committee: Dr. David E. Bergbreiter

    Thermoresponsive polymers such as poly(N-isopropylacrylamide) (PNIPAM)

    have lower critical solution temperature (LCST) in aqueous solutions. Below the LCST,

    these polymers are hydrophilic with an extended coil conformation. Above the LCST,

    they undergo a sharp phase transition to form a collapsed hydrophobic conformation.

    The LCSTs are also affected by cosolutes and the effects of anions on LCSTs follow the

    Hofmeister series.

    We successfully used a simple digital melting point apparatus to study the effects

    of heating rates, solvent compositions, cosolutes, and redox state, on the LCSTs of

    thermoresponsive polymers. Moreover, the temperature range of the apparatus allowed

    for analyses at much higher temperatures and provides a simple way to examine

    irregular clouding behavior in more complex systems.

    Meanwhile, stimuli-responsive surfaces grafted with thermoresponsive polymers

    can switch from hydrophilic to hydrophobic thermally. As the LCST can be

    subsequently changed with the addition of salts, the salt effects on the wettability of

    these thermoresponsive surfaces will dramatically impact the surface performance. In

  • iv

    this dissertation, I prepared PNIPAM/SiO2 nanocomposite surfaces by a covalent layer-

    by-layer assembly procedure and such surfaces were then used in studies of salts effects

    on surface wettability.

    Both the effects of anions and cations on the changes of advancing angles (ΔΘa)

    of the PNIPAM/SiO2 nanocomposite surfaces were significant (ΔΘa up to 90°). The

    anion effects on the surface wettability followed the Hofmeister effect as expected.

    Parallel studies on solution showed that variation of cations had a large effect on the

    LCST of PNIPAM too. Moreover, analyses of the Θa and LCST data using activity

    instead of using concentration showed different orders for the cation effects which were

    readily grouped by the cation charge numbers. No difference was seen for the anion

    effects in similar studies. AFM studies showed that surface morphology changes were

    correlated with the ΔΘa.

  • v

    DEDICATION

    To my husband, Yiqin for his encouragement, his unconditional support, and for

    his strong belief in my capability. To my children, Farrah and Andrew for bringing

    happiness and hope into my daily life, and for their understanding and support.

  • vi

    ACKNOWLEDGEMENTS

    I would like to thank Dr. David Bergbreiter for giving me the opportunity to

    work in his lab as a research associate when I moved in College Station in 2005. This

    work experience turned out to be a smooth transition for my return to graduate school to

    finish my Ph.D. I would also like to thank him for being a great mentor, for his guidance,

    and for his support, during my graduate study.

    I would like to express my gratitude to the members of Bergbreiter’s group for

    giving me suggestions in the preparation of my student seminar, my preliminary

    presentation, and my final defense. I am glad to have had the chance to know and to

    work with all the former and current group members, especially Jianhua, Shawn, Film,

    Su, Chris, and Jeff.

    I would also like to thank Dr. James Batteas, Mr. Albert Wan and Dr. Xiaoting

    Hong, for their collaborations and discussions on most of my research projects. I would

    like to thank Dr. Singleton, Dr. Batteas, Dr. Grunlan, and Dr. Connell for serving as my

    committee members. My gratitude also goes to Jill for helping everyone in the lab

    dealing with trivial business issues.

    Finally, I would like to thank all the undergraduates I had worked with - David

    Darmon and Danielle Policarpio, who were sponsored by National Science Foundation

    (NSF) Research Experiences for Undergraduates (REU) program, for letting me guide

    them on their research projects; Alex Mijalis, for letting me teach him about chemistry,

    and for teaching me English in return.

  • vii

    TABLE OF CONTENTS

    Page

    ABSTRACT ..................................................................................................................... iii 

    DEDICATION ................................................................................................................... v 

    ACKNOWLEDGEMENTS .............................................................................................. vi 

    TABLE OF CONTENTS ................................................................................................. vii 

    LIST OF SCHEMES .......................................................................................................... x 

    LIST OF FIGURES ........................................................................................................... xi 

    LIST OF TABLES .......................................................................................................... xvi 

    CHAPTER

    I INTRODUCTION .............................................................................................. 1 

    Stimuli-responsive Polymers .......................................................................... 1  Analyses of Stimuli-responsive Behavior ....................................................... 4  Additive Effects on the Responsiveness of Thermoresponsive Polymers .......................................................................................................... 9  Design of Stimuli-responsive Polymers ........................................................ 17  Grafting Stimuli-responsive Polymers on Solid Surfaces ............................. 22 

    II THERMODYNAMIC CLOUD POINT ASSAYS ........................................... 33 

    Introduction ................................................................................................... 33  Melting Point Apparatus and LCS Measurements ........................................ 35  Studies of LCSTs by an Automatic Melting Point Apparatus ...................... 36  Conclusion .................................................................................................... 52 

  • viii

    CHAPTER Page 

    III DESIGNING SURFACES WITH WETTABILITY THAT VARIED IN RESPONSE TO SOLUTE IDENTITY AND CONCENTRATION ................ 53 

    Introduction ................................................................................................... 53  Preparing the Solute-responsive PE/(PNIPAM/SiO2)6/PNIPAM Nanocomposite Surfaces by Covalent Layer-by-layer Self-assembly Method .......................................................................................................... 55  Studies of the Solute-Responsive Wettability ............................................... 58 Atomic Force Microscopy Studies ................................................................ 63 Conclusion .................................................................................................... 64 

    IV PARALLEL EFFECTS OF CATIONS ON PNIPAM GRAFT WETTABILITY AND PNIPAM SOLUBILITY ............................................. 66 

    Introduction ................................................................................................... 66  Studies of Cation Effects on PE/(PNIPAM/SiO2)6/PNIPAM Nanocomposite Surfaces ............................................................................... 68  Atomic Force Microscopy Studies ................................................................ 76  Studies of Cation Effects on LCST of PNIPAM Solution ............................ 78  Conclusion .................................................................................................... 82 

    V COVALENT LAYER-BY-LAYER ASSEMBLY OF SOLUTE- RESPONSIVE SURFACES ON SILICON WAFER ....................................... 84 

    Introduction ................................................................................................... 84  Preparing the PNIPAM/SiO2 Nanocomposite on Silicon Wafer Surfaces by Covalent Layer-by

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