Mechanism of Forming Organic/Inorganic Network Structures during In-situ Free-Radical Polymerization in PNIPA−Clay Nanocomposite Hydrogels

Abstract

The process of forming the unique organic/inorganic network structure of nanocomposite hydrogels (NC gels) was studied through changes in viscosity, optical transparency, X-ray diffraction, and mechanical properties. It was concluded that, during the preparation of the initial reaction solutions, a specific solution structure was formed from monomer (NIPA) and clay, where NIPA prevents gel formation of clay itself, and initiator (KPS) is located near the clay surface through ionic interactions. In subsequent in-situ free-radical polymerization, it was observed that the viscosity increased markedly during NC gel syntheses and in a manner similar to that in OR gel syntheses. Also, NC gels with different polymer contents exhibit characteristic two-step changes in the stress−strain curves, which correspond to the primary network formation and subsequent increase of cross-link density. These are because the polymerization proceeds on the clay particles which are relatively immobile, and clay platelets act as effective multifunctional cross-linking agents (plane cross-link). Then, it was proposed that clay−brush particles, consisting of exfoliated clay platelets with numbers of polymer chains grafted to their surfaces, were formed in the very early stage of polymerization, at around 7% of monomer conversion. Novel decreases in transparency were observed corresponding to the formation of clay−brush particles, but transparency recovered on further polymerization. Clay−brush particle formation was confirmed by XRD measurements on dried NC gels prepared using small amounts of monomer. Thus, a mechanism for forming the unique organic/inorganic network structure, including the formation of clay−brush particles in the synthetic pathway, is proposed. Furthermore, it was found that NC gels with excellent mechanical properties and structural homogeneity could not be prepared using other methods such as mixing clay and polymer solutions or by in-situ polymerization in the presence of the other inorganic nanoparticles instead of clay. These results indicate that the formation of organic/inorganic network structures in NC gels is highly specific and only realized by in-situ free-radical polymerization in the presence of clay.

Synthesis and properties of soft nanocomposite materials with novel organic/inorganic network structures

“…In dynanic mechanical measurements at 221C, 8 it was observed for D-NC5 gel that the storage modulus (G¢) is always greater than the loss modulus (G 00 ) in the frequency range 10 À1 B10 2 rad s À1 , and that G¢ and G 00 change Figure 2 (a) Structure of hectorite, (b) exfoliation in aqueous media, (c) house-of-card structure. 31 Soft nanocomposite materials K Haraguchi little with frequency. The constantly high G¢ (4G¢¢) indicates that viscoelastic relaxation does not occur on this time scale.…”

“…Polymer/clay network structure Various analytical studies (transmission electron microscopy, thermogravimetry, X-ray diffraction (XRD), differential-scanning calorimetry, Fourier-transform infrared spectroscopy for dried NC gels; dynamic light scattering and small-angle neutron scattering (SANS) for NC gels) revealed the following structural aspects of NC gels: 6,20,26,[31][32][33][34][35][36] (1) Disk-like inorganic clay nanoparticles (30 nm fÂ1 nm), resulting from exfoliation of the layered clay mineral (hectorite), are uniformly dispersed in a polymer matrix (XRD, transmission electron microscopy: Figure 4c (dried N-NC5 gel)). (2) Flexible polymer chains with the same T g as that of the LR exist in NC gels (differential-scanning calorimetry).…”

“…On the basis of the analytical data obtained and the excellent optical, mechanical and swelling/deswelling properties of NC gels, it was concluded that NC gels possess a unique organic (polymer)/ inoroganic (clay) network structure (Figure 4a) 6,20,26,31,34,36 in which exfoliated clay nanoparticles (uniformly dispersed in an aqueous medium) are interlinked by a number of flexible polymer chains. Here, D ic is the interparticle distance between exfoliated clay sheets.…”

“…6,20,26,31 In other words, the exfoliated clay platelets function as multifunctional crosslinkers, and hence the polymer chains in NC gels are crosslinked by planar series of crosslinks. These results were confirmed by the molecular characteristics of PNIPA separated from Soft nanocomposite materials K Haraguchi NC gels and contrast-variation SANS measurements.…”

Recent advances in hydrogels in terms of fast stimuli responsiveness and superior mechanical performance

“…The clay platelets function as multifunctional crosslinkers: the ends of the polymer chains adsorb strongly on the surface of the clay platelets by ionic and coordination interactions. 52 The intercrosslinking distance is equivalent to the distance between neighboring clay particles Figure 9 (a) Schematic illustration of a model structure for a tetra-PEG gel formed at C* (the concentration defining the border between dilute and semidilute regions). Red and blue spheres represent tetra amine-terminated poly(ethylene glycol) (PEG) and tetra-succinimidyl ester-terminated PEG, respectively.…”