Original Research PaperPhase-transition induced gelation of ZnO suspensions containing thermosensitive poly(acrylic acid)-graft-poly(N-isopropylacrylamide)
Graphical abstract
Introduction
Colloidal processing has been widely used to fabricate near-net-shape ceramic parts by controlling the interparticle forces in suspensions including van der Waals force, electrostatic force, steric force, and depletion force [1]. Compared with dry pressing, colloidal processing can effectively control particle agglomeration and obtain high-quality complex-shaped ceramic parts with uniform microstructure [2]. Many near-net-shape colloidal processing methods have drawn more and more attention in the research and industry fields, such as gelcasting (GC) [3], [4], temperature induced gelation (TIG) [5], [6], and direct coagulation casting (DCC) [7], [8]. Controlling interparticle forces and tailoring rheological behavior are significant issues for the above colloidal processing methods. Gelcasting achieves solidification by adding initiator to prompt the chemical crosslinking of organic monomers and create a three-dimensional network [9], [10]. The particle motion in temperature induced gelation is controlled by changing the solubility of polymer in solvent through heating or colling [11], [12], [13]. In direct coagulation casting, particles are immobilized by shifting pH value towards the isoelectric point (IEP) [14], [15] or increasing ionic strength to compress the electric double layer [16], [17]. Recently, a novel colloidal processing method using thermosensitive polymer as coagulating agent and low-melting-point paraffin as mold has been developed to prepare complex-shaped ceramics directly from suspension by a stepwise increase in temperature [18], [19]. In this method, the rheological behavior of suspensions depends on the phase transition of thermosensitive poly(N-isopropylacrylamide) (PNIPAM) in response to temperature. Compared with other colloidal processing methods, the solidifying and demolding processes of this method are simple and easy to control, and the crack and deformation caused by demolding can be avoided. However, there are still some small pores in the microstructure of ceramic parts.
Herein, graft polymer PAA-g-PNIPAM, characterized by pH-sensitivity of PAA and temperature-sensitivity of PNIPAM simultaneously, is used as both dispersant and coagulant in suspensions. The gelation of suspensions is induced by the phase transition of PAA-g-PNIPAM above the critical transition temperature, and the porosity of ceramic parts is effectively reduced. In this work, the properties of PAA-g-PNIPAM aqueous solution, the interaction between PAA-g-PNIPAM and ZnO particles, and the rheological behavior and colloidal processing of thermosensitive ZnO suspensions were investigated.
Section snippets
Materials
High-purity ZnO powder (>99.9 wt.%, Jiangxi Huarun Co., Jiangxi, China) with an average particle size of 0.1 μm and a surface area of 4.47 m2/g was used as the ceramic phase. Thermosensitive PAA-g-PNIPAM polymer served as both dispersant and coagulant, and its synthesis process was reported in previous researches [20], [21]. The pH values of solutions and suspensions were adjusted with nitric acid (HNO3) (Xilong Chemical Co., Guangzhou, China) or sodium hydroxide (NaOH) (Xilong Chemical Co.,
Properties of PAA-g-PNIPAM aqueous solution
The ionization degree (α) of PAA-g-PNIPAM is obtained from the net uptake curves for hydroxyl ions and is shown as a function of pH value at room temperature in Fig. 1(a). The ionization behavior of PAA-g-PNIPAM is similar to that of PAA and strongly depends on the pH value of solution [22]. It is clearly observed that PAA-g-PNIPAM is essentially uncharged below pH 3 (α = 0) and fully ionized above pH 10 (α = 1). When the pH value is higher than 3, the α value of PAA-g-PNIPAM first slowly
Conclusions
The complex-shaped ceramic parts were prepared by phase-transition induced gelation using ZnO suspensions containing thermosensitive PAA-g-PNIPAM graft polymer in this work. The properties of PAA-g-PNIPAM aqueous solution, the interaction between PAA-g-PNIPAM and ZnO particles, and the rheological behavior and colloidal processing of thermosensitive ZnO suspensions were investigated. The conclusions are as follows:
- (1)
PAA-g-PNIPAM is a pH- and temperature-sensitive graft polymer. PAA-g-PNIPAM in
Acknowledgements
This work was supported by the Fundamental Research Funds for the Central Universities of Central South University (2017zzts005) and the Open-End Fund for the Valuable and Precision Instruments of Central South University (CSUZC201814).
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