Elsevier

Polymer

Volume 85, 24 February 2016, Pages 47-54
Polymer

Molecularweight effects on tensile properties of blend hydrogels composed of clay and polymers

https://doi.org/10.1016/j.polymer.2016.01.008Get rights and content

Highlights

  • We examine structures and mechanical properties of clay/PAAS/TSPP and clay/PAM/TSPP gels.

  • The elastic modulus of clay/PAAS/TSPP gel was much higher than that of clay/PAM/TSPP gel.

  • Extensibility of clay/PAM/TSPP gel was much higher than that of clay/PAAS/TSPP gel.

  • Three factors are necessary to be fulfilled for acquisition of the mechanical toughness of the hydrogels.

Abstract

Hydrogels with mechanical toughness have attracted the attention of many researchers but still the mechanism remains poorly understood. In this study, we investigated molecularweight effects on the tensile properties for two kinds of blend hydrogels such as clay/sodium polyacrylate and clay/polyacrylamide, which were prepared by simple mixing, in order to clarify key factors of the mechanical toughness. The elastic modulus of the former hydrogel was much higher than that of the latter, while the latter extensibility was much higher than that of the former. It has been found that use of polymers with molecularweights higher than a few millions is necessary to obtain high extensibility for the blend hydrogels. It has been clarified from this study that all of the following three factors, (i) use of high molecularweight polymers, (ii) dispersion of clay particles and (iii) favorable interactions of clay and polymers are necessary to be fulfilled for fabrication of the blend hydrogels with mechanical toughness.

Introduction

In recent years, hydrogels with mechanically tough properties have attracted many researchers from both scientific and industrial points of view. Clay/polymer nanocomposite hydrogels prepared by in-situ free-radical polymerization of alkylacrylamide in the presence of clay particles, which have been pioneered by Haraguchi et al., are well-known as one of hydrogels with excellent material properties, e.g., they have remarkable mechanical properties such as high extensibility more than 1500%, extraordinary mechanical toughness and self-healing [1], [2], [3], [4]. Clay particles act as a multiple cross-linker in the nanocomposite hydrogels, e.g. the number of polymer chains attached to one clay particle is in the range from about 20 to 130, depending on the clay concentration [1], [5]. Moreover, it has been shown that the nanocomposite hydrogels have high transparency due to clay particles homogeneously dispersed in water [3]. The multiplicity of cross-link points and the homogeneously dispersed clay particles may be origins of the amazing mechanical toughness, but the underlying mechanism that brings it is less well understood at the present stage. It has been believed that tough clay/polymer composite hydrogels cannot be produced by simple blending [2]. For understanding of the mechanism, it is important to find out key factors for mechanical toughness.

Recently we found that hydrogels prepared by simply blending a polyelectrolyte, sodium polyacrylate (PAAS) with clay particles including a dispersant, tetrasodium pyrophosphate (TSPP) are mechanically tough, e.g., the blend hydrogels prepared at optimum compositions were not fractured by 90% compression, and almost recovered their initial shape after the compression test [6]. Simple blending has an advantage in an approach to the mechanism of the mechanical toughness in the following reason. The advantage is that composition and characteristics of the constituents in the gel are well-defined, and as a consequence it enables us to definitely control the composition and easily vary molecularweights and kinds of constituent polymers. In a previous study we showed that there exists an optimum concentration of the dispersant and high molecular weight PAAS more than ∼106 is necessary to fabricate blend hydrogels tough for uniaxial compression [7].

In this work, we investigated the tensile and structural properties of the blend hydrogels composed of polymers and clay, and the molecularweight effects on them using two kinds of polymers, an anionic polymer (PAAS) and nonionic polymer (polyacrylamide) with molecular weights ranging from ∼105 to ∼107. Our particular intention in doing this is to clarify effects of the molecularweight and the kinds of constituent polymers on the mechanical toughness of the blend hydrogels.

Section snippets

Sample and sample preparation

In this study, we used Laponite RD (Na0.7[(Si8Mg5.5Li0.3)O20(OH)4]) as a clay sample, which was kindly supplied by RockWood Ltd, Japan. We used PAASs with different molecularweights ranging from 2.25 × 105 to 2.11 × 107 and PAMs with molecularweights of 4 × 105 to 1.8 × 107. The molecularweights of polymers used in this study are summarized in Table 1. We used tetrasodium pyrophosphate (TSPP) decahydrate as a dispersant, which was obtained from Kanto Chemicals Co., in order to prevent

Mechanical properties of clay/PAAS/TSPP hydrogels

We show pictures of 5 wt% Laponite RD/1 wt% PAAS21100K/0.5 wt% TSPP blend hydrogels in Fig. 1. These pictures show that the gel is mechanically tough and transparent. The latter's result suggests that clay particles are homogeneously dispersed in the gel. Fig. 2 depicts stress–strain curves for Laponite RD/1 wt% PAAS21100K/0.5 wt% TSPP hydrogels (a) and for Laponite RD/1 wt% PAAS564K/0.5 wt% TSPP hydrogels (b) at different clay concentrations. The blend hydrogels with lower molecular weight

Summary

We extensively studied molecularweight effects on the tensile properties for blend hydrogels composed of clay and polymers using two kinds of polymers, PAAS and PAM. The tensile properties such as Young's modulus, fracture stress and fracture strain for clay/PAAS gels were almost unchanged in the range of the molecular weights higher than a few millions, while they significantly decreased in the range of the molecular weights smaller than them. The modulus for clay/PAAS gel was much larger than

Acknowledgment

We gratefully acknowledge Mr. Katsushi Sasa from Otsuka Electronics Co. Ltd. for determination of the molecular weights and radii of gyration of PAAS564K, PAAS3500K, PAAS13800K, and PAAS21100K with light scattering measurements. The synchrotron SAXS measurements were performed under the approval of Photon Factory Program Advisory Committee (2014G160). A part of this work was supported by JSPS KAKENHI Grant Number 15K05242.

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