Abstract
ABSTRACT For the application of polymer gels, it is necessary to control independently and precisely their various physical properties. However, the heterogeneity of polymer gels hinders the precise control over the structure, as well as the verification of theories. To understand the structure-property relationship of polymer gels, many researchers have tried to develop a homogeneous model network. Most of the model networks were made from polymer melts that did not have a solvent and had many entanglements in the structure. Because the contribution of entanglements is much larger than that of chemical crosslinking, it was difficult to focus on the crosslinking structure, which is the structure considered in conventional theories. To overcome such a situation, we have developed a new model network system that contains much solvent. Specifically, we fabricated the polymer gel (Tetra-PEG gel) by mixing two types of solutions of tetra-armed poly(ethylene glycol) (Tetra-PEG) with mutually reactive end groups (amine (-PA) and activated ester (-HS)). Because the existence of a solvent strongly reduces the effect of entanglements, the effect of the crosslinking structure on the physical properties can be extracted. In this review, we present the structure-property relationship of Tetra-PEG gel. First, we show the structural homogeneity of Tetra-PEG gels. Then, we explain gelation reaction, elastic modulus, fracture energy and kinetics of swelling and shrinking of Tetra-PEG gels by comparing the theories and experimental results.
Highlights
Polymer gels consist of three-dimensional polymer networks swollen in a solvent
We present the structure of Tetra-PEG gel, and discuss physical properties: gelation reaction, elastic modulus, fracture energy and kinetics of swelling and shrinking with comparing the theoretical predictions
The above discussion demonstrates that spatial heterogeneity and microscopic heterogeneities are strongly suppressed in Tetra-PEG gels and that TetraPEG gel satisfies the requirements as the homogeneous network (i), (iii), and (iv)
Summary
Polymer gels consist of three-dimensional polymer networks swollen in a solvent. The application field is expanding, such as biomaterials, biosensors, drug delivery systems, templates for fabrication of nanostructures, soil modifiers, and adjuncts to shale gas extraction [1–6]. Because the heterogeneity hinders the experimental verification of the theories, it is impossible to understand the polymer gels at the molecular level To solve this problem, many researchers attempt to develop a homogeneous model network [10–14]. The structure of polymers generally obeys the Gaussian distribution in a polymer melt [13,15] This form can satisfy the requirements (i), (ii), and (iii) for an ideal network. The polymer gel is formed from two types tetra-armed poly(ethylene glycol) (Tetra-PEG) with mutually reactive end groups (Figure 1) This molecular design includes three keys to suppress the heterogeneity. The reaction rate is tuned to allow the mutually reactive Tetra-PEGs to be mixed homogeneously This molecular design realized the high reaction conversion (≈ 90%) and extremely suppressed the heterogeneity compared to conventional hydrogels [7,8,19–21]. We present the structure of Tetra-PEG gel, and discuss physical properties: gelation reaction, elastic modulus, fracture energy and kinetics of swelling and shrinking with comparing the theoretical predictions
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