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GECKO-INSPIRED ELECTROSPUN FLEXIBLE FIBER ARRAYS for ADHESION a Dissertation Presented to the Graduate Faculty of the University

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GECKO-INSPIRED ELECTROSPUN FLEXIBLE FIBER ARRAYS for ADHESION a Dissertation Presented to the Graduate Faculty of the University GECKO-INSPIRED ELECTROSPUN FLEXIBLE FIBER ARRAYS FOR ADHESION A Dissertation Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Johnny F. Najem August, 2012 GECKO-INSPIRED ELECTROSPUN FLEXIBLE FIBER ARRAYS FOR ADHESION Johnny F. Najem Dissertation Approved: Accepted: ______________________________ ______________________________ Advisor D epartment Chair Dr. Shing-Chung Wong Dr. Celal Batur ______________________________ ______________________________ Committee Member D ean of the College Dr. Gregory N. Morscher Dr. George K. Haritos ______________________________ ______________________________ Committee Member Dean of the Graduate School Dr. Peter H. Niewiarowski Dr. George R. Newkome ______________________________ ______________________________ Committee Member D ate Dr. Darrell H. Reneker ______________________________ Committee Member Dr. Erol Sancaktar ______________________________ Committee Member Dr. Tirumalai S. Srivatsan ii ABSTRACT The ability of geckos to adhere to vertical solid surfaces comes from their remarkable feet with millions of projections terminating in nanometer spatulae. We present a simple yet robust method for fabricating directionally sensitive dry adhesives. By using electrospun nylon 6 nanofiber arrays, we create gecko-inspired dry adhesives, that are electrically insulating, and that show shear adhesion strength of 27 N/cm2 on a glass slide. This measured value is 270% that reported of gecko feet and 97-fold above normal adhesion strength of the same arrays. The data indicate a strong shear binding-on and easy normal lifting-off. This anisotropic strength distribution is attributed to an enhanced shear adhesion strength with decreasing fiber diameter (d) and an optimum performance of nanofiber arrays in the shear direction over a specific range of thicknesses. With use of electrospinning, we report the fabrication of nylon 6 nanofiber arrays that show a friction coefficient (µ) of ~11.5. These arrays possess significant shear adhesion strength and low normal adhesion strength. Increasing the applied normal load considerably enhances the shear adhesion strength and µ, irrespective of d and fiber arrays thickness (T). Fiber bending stiffness and fiber surface roughness are considerably decreased with diminishing d while fiber packing density is noticeably increased. These enhancements are proposed to considerably upsurge the shear adhesion strength between nanofiber arrays and a glass slide. The latter upsurge is mainly attributed to a sizeable iii proliferation in van der Waals (vdW) forces. These nanofiber arrays can be alternatively bound-on and lifted-off over a glass slide with a trivial decrease in the initial µ and adhesion strength. By using selective coating technique, we have also created hierarchical structures having closely packed nanofibers with d of 50 nm. We determine the effects of applied normal load, fiber surface roughness, loading angle, d, T, and repeated adhesion measurements on their corresponding adhesion strength and µ. These effects are determined with the aid of atomic force microscopy (AFM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), macroscopic adhesion testing, and wide angle x-ray diffraction (WAXD) techniques. iv ACKNOWLEDGEMENTS First and foremost, I would like to thank my advisor Dr. Shing-Chung Wong for all his guidance, friendship, support, constant availability, patience, and willingness to answer my questions during the course of this study. I am also very grateful for the constructive comments of my dissertation committee members, namely: Dr. Gregory N. Morscher, Dr. Peter H. Niewiarowski, Dr. Darrell H. Reneker, Dr. Erol Sancaktar, and Dr. Tirumalai S. Srivatsan. I would like to thank the Department of Mechanical Engineering, College of Polymer Science and Polymer Engineering at The University of Akron. I am deeply thankful to Dr. Reneker’s group for helping me with electrospinning. I am also genuinely thankful to Dr. Rong Bai and Rebecca Agapov for their assistance in atomic force microscopy analysis. I would like to thank all my friends including each member of Dr. Wong’s group for their help and friendship. I am very grateful for the National Science Foundation which pri marily supported this work by CAREER Awards to S.-C.W. (NSF-CMMI 0746703). Lastly, I would like to profoundly thank my father, Fares, mother, Rose, brothers Maroun and Elie, sister Marie-Therese, uncle Tony Owen, aunt Josephine Owen, Chuck Owen and his family for all their support throughout my years of study at the University of Akron. I love all of you. v TABLE OF CONTENTS Page LIST OF TABLES ............................................................................................................. xi LIST OF FIGURES .......................................................................................................... xii CHAPTER I. INTRODCUTION .......................................................................................................... 1 1.1 Statement of Problem ................................................................................................. 1 1.2 Importance of the Study ............................................................................................. 4 II. LITTERATURE REVIEW ........................................................................................... 6 2.1 Introduction to Nylon 6 .............................................................................................. 6 2.1.1 Nylon 6 Synthesis ............................................................................................ 8 2.1.2 Thermodynamic Properties and Relaxation Behavior of Nylon 6 ................... 9 2.1.3 Crystal Structure and Morphology of Nylon 6 .............................................. 12 2.1.4 Physical and Mechanical Properties of Nylon 6 ............................................ 14 2.2 Melt Spinning Review ............................................................................................. 16 2.2.1 High Modulus Fibers ..................................................................................... 17 2.2.2 Yielding, Orientation, and Fibrillation........................................................... 17 2.2.3 High-modulus and High-strength Polyamide 6.............................................. 18 2.3. Electrospinning Process .......................................................................................... 19 2.3.1 Fiber Diameter Control .................................................................................. 21 2.3.2 Fiber Alignment and Collection Methods...................................................... 22 vi 2.3.2.1 Rotating Drum Collector.......................................................................... 22 2.3.2.2 Rotating Disk Collector ........................................................................... 23 2.3.2.3 Static Parallel Electrodes ......................................................................... 23 2.3.3 Structural Properties of Electrospun Fibers ................................................... 24 2.3.3.1 Molecular Orientation .............................................................................. 24 2.3.3.2 Crystallinity.............................................................................................. 25 2.3.3.3 Effect of Fiber Diameter on the Structural Properties ............................. 26 2.3.3.4 Effect of Collector on Structural Properties............................................. 27 2.3.4 Mechanical Properties of Electrospun Fibers ................................................ 28 2.3.4.1 Effect of Structural Morphology on Tensile Properties ........................... 28 2.3.4.2 Effect of Collector Type on Tensile Properties ....................................... 29 2.3.5 Electrospun Fibers Applications .................................................................... 32 2.3.5.1 Tissue Engineering Application ............................................................... 32 2.3.5.2 Electrospun Fiber Reinforced Composites .............................................. 33 2.3.5.3 Electrospun Conductive Fibers ................................................................ 34 2.3.5.4 Filtration ................................................................................................... 35 2.4 Polycarbonate (PC) ................................................................................................... 35 2.4.1 PC Synthesis .................................................................................................. 36 2.4.2 Mechanical Properties of PC .......................................................................... 39 2.4.3 PVT, Specific Heat, and Thermal Transitions of PC ..................................... 42 2.4.3.1 PVT Transitions of PC ............................................................................. 42 2.4.3.2 Specific Heat and Thermal Transitions of PC ......................................... 43 vii 2.4.4 Annealing of PC ............................................................................................. 45 2.4.5 Crystallinity of PC ......................................................................................... 48 2.5 Adhesion................................................................................................................... 50 2.5.1 Adhesion of Polymers ...................................................................................
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