SOFT POLYELECTROLYTE HYDROGEL AS VERSATILE MATERIAL IN DIFFERENT APPLICATION By YUJEN WANG Submitted in partial fulfillment of the requirements for the degree of Master of Science Thesis Advisors: Dr. Gary Wnek, Dr. Joao Maia Department of Macromolecular Science and Engineering CASE WESTERN RESERVE UNIVERSITY August 2016 CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES We hereby approve the thesis/dissertation of _______________YU-JEN WANG ____________________________ candidate for the ________Master of Science__________degree *. (signed)______ Gary E Wnek_________________(chair of the committee) _____________ Joao M Maia ________________________ ______________Alexander M Jamieson__________________________ ______________________________________________________ ______________________________________________________ (date) ____ 06/30/2016________________ *We also certify that written approval has been obtained for any proprietary material contained therein. 2 Table of Contexts Committee Approval Sheet…………………………………………………… 2 Table of Contents……………………………………………………………… 3 List of Tables………………………………………………………………….. 5 List of Figures…………………………………………………………………. 6 Acknowledgements …………………………………………………………… 7 Abstract…………………………………………………………………………8 Chapter 1 Introduction 1.1 Polyelectrolytes……………………………………………………………….10 1.2 Poly(acrylic acid)…………………………………………………….……….14 1.3 Previous Work from the Wnek Laboratory.....………………………………...15 1.4 Calcium Ions…………………………………………………………………..16 1.5 Adenosine Triphosphate………………………………………………………17 1.6 Chelators………………………………………………………………………18 1.7 Elactroactive Polymer and Hydrogel Rheology………………………………20 1.8 Poly(acrylic acid) hydrogel as a osmotic pressure-induced artificial muscle...22 1.9 Glycerol as an additive for PAA hydrogel……………………………………23 1.10 Summary…………………………………………………………………….24 Chapter 2 Methodology 2.1 Hydrogel Synthesis……………………………………………………………25 2.2 Calcium Titration and Chelation…….………………………………………..26 2.3 PAA thread Lengths as a Function of pH……………………………………..27 2.4 Electrospun PAA Tube…….………………………………………………….27 2.5 Differential Scanning Calorimetry……………………………………………29 3 2.6 Thermogravimetric Analysis………………………………………………….30 2.7 Rheology………………………………………………………………………30 2.8 Mechanical(Tensile) Testing………………………………………………….32 Chapter 3 Results and Discussion 3.1 PAA Hydrogel with Shrinkage and Expansion Cycle………………………...33 3.2 PAA Shrinkage Ability………………………………………………………..39 3.3 Chelation Abilities…………………………………………………………….40 3.4 Rheology………………………………………………………………………43 4. Conclusion ………………………………………………………………………..46 5. Future Work ………………………………………………………………………47 6. Bibliography..…………………………………………………………………….48 4 List of Tables Table 1, Configuration of PAA gel………………………………………………… 17 Table 2, Ingredient of HBS…………………………………………………………..26 Table 3, Configuration of PAA with different percentage glycerol…………………31 5 List of Figures Fig 1. Collection of Chelators………………………………………………………..18 Fig 2.CaCl2 and EDTA Reversible Titration Curve………………………………….33 Fig 3. PAA Hydrogel Thread Length variation vs Ph………………………………..37 Fig 4. The two stages of PAA gels………………………………………………...…38 Fig 5. The CaCl2 Titration Curve…………………………………………………….39 Fig 6, Chelators Titration Plot……………………………………………………..…41 Fig 7-1, PAA hydrogel frequency sweep storage modulus. …………………………44 Fig 7-2, PAA hydrogel frequency sweep loss modulus……………………………...44 6 Acknowledgement I would like to thank my lab mates in different groups, Maia’s group and Wnek’s group. For graduate students in Wnek group, they shared their experience with me and helped me setting up my experiments. Especially Anne Walker, she had a good experience in PAA hydrogel. Without her, I could spend at least half year more. For undergraduate students with me, I really appreciated for your diligence and curiosity. I was happy to share my perspective and experiment with you all. Chioma Onukwuire, Sara Rudolph, Anita Venkataswamy were the three main people helping me complete the titration part. Stephanie Kresic was really helpful for the rheology part. Jae Young Choi was a freshman. Jae and I discovered some fundamental behavior of PAA hydrogels. With Maia’s group, Jesse Gadley and Parker Lee provided me with information about gel rheology. Arman Boromand was a very knowledgeable senior graduate student, who was willing to guide me through the theoretical parts. 7 Soft Polyelectrolyte Hydrogel as Versatile Material in Different Application Abstract by YUJEN WANG The principal biochemical role of adenosine triphosphate (ATP) involves the driving of metabolic processes linked to its large, negative free energy of hydrolysis. However, less well known is ATP’s ability to act as a chelator for divalent metal ions. We recently have become very interested in poly(acrylic acid) (PAA) gels as a model system for biomimicking physiological systems such as muscles and nerves, especially the ability of PAA to switch from a compact to a swollen gel as the result of divalent/monovalent ion exchange. Swelling of calcium ion-containing gels can be accomplished using common chelators such as EDTA or citrate, but it was found in the present work that such gels efficiently re-swell using ATP as the chelator. This is of great interest because two biochemically-significant species, Ca2+ and ATP, can be used to switch PAA gels between compact and swollen states in water. A comparison of the relative abilities of EDTA, citrate and ATP to swell calcium ion-contracted gels is presented. 8 In addition polymer such as PAA are wet hydrogel materials and can serve as artificial muscles which have recently become a popular topic of research. While applying a voltage, hydrogels deform and translate electrical power to mechanical power. However, Ionic Polymer Metal Composites (IPMC) have to be wet in order to function well. PAA is a new candidate for IPMC, as it is a biocompatible polymer and can absorb a lot of water. Thus, crosslinked PAA hydrogel networks can serve as new IPMC materials. Toward that end, this research also introduces glycerol as a component of PAA gels for solving two problems at the same time, namely mechanical and rheological modulation of such hydrogels. 9 1. Introduction 1.1 Polyelectrolytes Polyelectrolytes are a group of water soluble polymers that include charges in their monomer units. They are different from ionomers, which have only a few charge groups within the polymer chains. For an understanding polyelectrolytes, a few concepts need to be considered. An electrolyte is a substance that produces an electrically conducting solution when dissolved in and at least partially ionized solvent such as water. There are strong electrolytes, which completely ionized in water, and weak electrolytes, which are ionize only incompletely. In order to describe how strongly an electrolyte is ionized, dissociation constant Ka and Kb are defined as [H+][A−] [OH−][B+] K = where HA ↔ H+ + A−, and K = where BOH ↔ B+ + a [HA] b [BOH] OH−. The dissociation constants do not change with solution condition in the same temperature, only the concentration of each species varies. Also, the ion product of water, Kw, controls the concentrations of hydrogen and hydroxyl ions in water. At − + −14 20℃, Kw = [OH ] × [H ] = 10 . Thus, pure water without any solute will have a pH value of 7, which means [H+] = 10−7 = [OH−]. Recall that the pH value equals to - log[H+]. [H+][A−] α2퐶 We may also write K = as = , where α is the degree of a [HA] 1−α ionization. Since pH = - log[H+] and [H+] = 훼퐶, then 훼 log = 푝퐻 + 푙표푔퐾 = 푝퐻 − 푝퐾 . 1−훼 푎 푎 Polyelectrolytes can also be divided into weak and strong types. Like weak electrolytes, weak polyelectrolytes are not dissociated fully. Polyelectrolyte gels are 10 known for volume changes when then imbibe solvent. Macroscopically, the morphology of polyelectrolyte network can be ionized. Microscopically, attractions and repulsions are caused by each monomer unit in polymer chains as well as small, charged particles, such as counterions. The Bjerrum length defines how close a counterion can reach to polymer chains. That is to say, the Bjerrum length equation is an energy balance, which balance the energy of individual particle with level temperature, so the energy of individual particle is the Boltzmann constant (푘퐵) times absolute temperature (Kelvin) 푇, and the Coulomb attraction force energy is Coulomb’s constant times two charges over the distance of two charges. 푒2 λ퐵 = (1) 4휋휀0휀푟푘퐵푇 Equation 1,the Bjerrum Length equation, where λ퐵 is Bjerrum Length, e is the charge of electron, 휀0 is the permittivity of free space, 휀푟 is the dielectric constant, 푘퐵 is the Boltzmann constant, and T is the absolute temperature. However, the Bjerrum length does not take salt concentration into account, so the Debye length (휄퐷) comes into play. The Debye length (휄퐷) is defined as −1 휀0휀푟푘퐵푇 휄퐷 = 휅 = √ 2 (2) 2푁퐴푒 퐼 1 where Ι = Σ퐶 푍2, e is the charge of ion, 휀 is the permittivity of free space, 휀 is 2 푖 푖 0 푟 the dielectric constant, 푘퐵 is the Boltzmann constant, T is the absolute temperature, and 푁퐴 is Avogadro’s number. The surrounding ions in the system are taken into consideration. As to equations, the ionic strength is put into the equation. With polyelectrolyte system, not only the positive and negative charged ions are taken into 11 account but also the polyelectrolyte concentration and effective charges. The general 1 2 1 2 ionic strength equation (I) is Ι = Σ퐶 푍 . For polyelectrolyte, Ι = 퐶 푍 + 2 푖 푖 2 푐푎푡푖표푛 푐푎푡푖표푛 1 퐶 푍2 + 퐶 ∥ 푍 ∥. Due to the attraction forces, counterions are going 2 푎푡푖표푛 푎푡푖표푛 푝표푙푦푚푒푟 푒푓푓 to