Cover: Physics of Polymer Gels by Takamasa Sakai

Physics of Polymer Gels

 

 

Edited by

Takamasa Sakai

 

 

 

 

 

 

 

 

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Preface

Polymer gels are defined as a three-dimensional polymer network swollen with a solvent. If properly designed, polymer gel can hold 1000 times as much solvent as polymer weight, have deformability more than 10 times, and can retain/release macromolecular substances like protein. Such high swellability, deformability, and permeability of polymer gel are unique characteristics not found in other materials. Due to these characteristics, hydrogels are used for many applications such as absorbent materials for paper diapers, soft contact lenses, drug reservoir, etc. Recently, considerable attention has been paid as a material for future medical care such as tissue-replacement material and scaffold for regenerative medicine. This book explains the correlation between the physical properties and structure of polymer gels and is prepared for university students learning polymer gels for the first time.

Unique properties of polymer gels stem from their unique structure; though the significant component is solvent, a polymer gel is solid due to the 3D polymer network with a few percent by weight. Notably, the polymer network and the solvent are not separated in two phases, but they exist together as a gel phase. Thus, a polymer gel has both a solid-like nature stemming from a three-dimensional network and a polymer solution-like nature arising from a solvent dissolved and retained in a polymer network. This duality is the feature of polymer gels not found in other materials and the source of the uniqueness. At the same time, however, this duality often causes difficulty in understanding polymer gels. The solid nature of polymer gel is discussed based on the theory of rubber elasticity, while the liquid-like nature is presented based on the theory of polymer solutions. Therefore, to understand the basic concept of polymer gel, fundamental understanding of both is indispensable, and the harmony of both is essential.

Both rubber elasticity and polymer solution theory are based on statistical mechanics. Under ideal assumptions, both theories bring rigorous prediction by mathematical formulas for various physical property values. On the other hand, the 3D polymer network is inherently heterogeneous; this is obvious from the fact that even synthetic linear polymer chains have heterogeneous distribution in length. It is impossible to synthesize a polymer network with uniform mesh size and number of branching. Since it is not possible to accurately define the structure, it is difficult to formulate the distribution function and to adapt the statistical mechanics approach practically.

For this reason, it is difficult to understand the correlation between the structure and physical properties of a real polymer gel, and the contribution of theory to material design may be limited. However, we believe that understanding the fundamentals of polymer gels is still relevant, because the hurdle for practical application of polymer gels is high, and it is difficult to go beyond the difficulty relying only on experience. Material design based on fundamentals is indispensable to overcome this situation. Understanding the phenomenon in the form of a mathematical expression is extremely meaningful, even though we cannot observe quantitative agreement between the theory and the experimental result. Useful information is often obtained by comparing experimental results with theoretical values obtained under certain exact assumptions. It is also essential to change the degree of coarse-graining and watch over the rough sketch of physical properties using scaling theory. In this way, it is crucial to handle the heterogeneity of polymer gels. On the other hand, it is vital to deepen the fundamental understanding of polymer gels by experimentally verifying the theories using polymer gels with a well-defined structure.

Therefore, in this book, as an introduction of basic knowledge, we first explain the statistical mechanics and scaling of a polymer chain in Chapter 1 and that of polymer solution in Chapter 2. In Chapter 3, we introduce the structure of polymer gels and explain the rubber elasticity, which predicts the solid-like nature of polymer gels. In Chapter 4, we describe the swelling/deswelling, which can be understood by combining the rubber elasticity (solid-like nature) and the osmotic pressure of a polymer solution (liquid-like nature). We introduce the large deformation and fracture in Chapter 5 and the diffusion of substances in polymer gels in Chapter 6, which are essential for practical applications.

The last half of this book contains our experimental results using Tetra-PEG gels, which is a near-ideal polymer gel developed by us. We briefly explain Tetra-PEG gels in Chapter 7 and experimental results in Chapters 8–17. We examined the validity of the most of the theories presented in the first half using Tetra-PEG gels. This book was designed to be an introduction to understand polymer gels. By comparing the first half and the last half, readers can learn how to examine the models and how to utilize the models to experiments. I hope this book will help you know polymer gels.

Takamasa Sakai

Tokyo

December 2019

Acknowledgements

I want to express my gratitude to all the students who have promoted research together. This textbook is a culmination of 12 years that I learned about polymer gels with you. Special thanks go to Takuya Katashima, Xiang Li, Takeshi Fujiyabu, Yuki Yoshikawa, and Yuki Akagi. Also, I would like to express my gratitude to Junsei Kishi, who helped to publish the original Japanese version.

I sincerely express my most substantial gratitude to Prof. Ung-il Chung, who proposed writing this book. Almost every day, he has encouraged me, “Is writing textbooks progressing? You must write every day, even little by little.” I can say that this book has never been realized unless Ung-il has encouraged me.

I also thank my family, Kanako, Chiho, and Itsuki with all of my love.

Part I
Theories