Self-assembled three-dimensional structure of epoxy/ polyethersulphone/silver adhesives with electrical conductivity Hajime Kishi * , Sanae Tanaka, Yasuaki Nakashima, Takashi Saruwatari Graduate School of Engineering, University of Hyogo, 2167, Shosha, Himeji, Hyogo, 671-2201, Japan article info Article history: Received 22 September 2015 Received in revised form 19 November 2015 Accepted 21 November 2015 Available online 24 November 2015 Keywords: Functional adhesives Epoxy Electrical conductivity abstract Diglycidyl ethers of bisphenol-A (DGEBA)/4,4 0 -diaminodiphenylmethane (DDM)/polyethersulphone (PES) blends were prepared as matrix resins for electrically conductive adhesives containing silver (Ag) fillers. The epoxy/PES blends formed co-continuous phase structures from initial homogeneous solutions via reaction-induced phase separation during the curing process. The Ag fillers were selectively localized in the epoxy-rich phases that had high affinity toward the surface of the Ag fillers. The co-continuous phase structures of the epoxy/PES blends acted as templates for the three-dimensional continuous structures of the Ag fillers. The self-assembled structures of the Ag fillers were connected in a continuous phase that possessed high electrical conductivity using a relatively small amount of Ag filler. In addition, the epoxy/PES/Ag adhesives had excellent shear adhesive strength. The fracture toughness of the epoxy/ PES blends was the source of the high adhesive strength. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Epoxy resins are widely used as thermoset adhesives in the electronic, automotive, and construction industries. Effort has been made to develop new conductive epoxy adhesives as alternative materials for lead-containing solder in electronics applications [1e6]. Electrically conductive epoxy adhesives have the advantages of low temperature processing, low residual stress, and low envi- ronmental impact. The conductive adhesives consist of metallic conductive fillers dispersed in an epoxy matrix. Typical metallic fillers are silver (Ag) particles or flakes, such as 80 wt% (30 vol%) Ag filler blended into an epoxy polymeric matrix. However, the addi- tion of a large amount of Ag filler produces a highly viscous material that is difficult to work and has low adhesive strength. Many studies on epoxy conductive adhesives have investigated the shapes and sizes of Ag fillers to determine their effects on con- ductivity [7e11]. The present study focuses on polymeric matrix systems as an alternative approach for producing electrically conductive epoxy adhesives using a small amount of Ag filler. When the conductive channels of the Ag fillers are assembled spontaneously using the phase structures produced during curing of the epoxy polymer blends, the threshold amount of Ag for achieving high conductivity can be smaller than that of conventional conductive epoxy adhesives. The electrical conductivity of thermoplastic polymer composites with carbon-black or carbon-nanotubes has been studied in high- density polyethylene (HDPE)/poly(methyl methacrylate) (PMMA) blends, PE/polystyrene (PS) blends, HDPE/polypropylene (PP) blends, PMMA/PP blends, poly(vinylidene fluoride) (PVDF)/PMMA blends, and PMMA/PS blends [12e15]. Studies focused on the dis- persibility of the carbonaceous fillers and the concentration of the fillers in the polymeric matrices in relation to the conductivity of the thermoplastic polymer composites. In these studies, the carbonaceous fillers were added to the thermoplastic polymer blends, which were phase separated (i.e., the phase structures of the thermoplastic polymer blends existed before the addition of the fillers). However, in the present study, the mixture of epoxy (thermoset) oligomers and thermoplastic polymers produced a homogeneous solution. The epoxy oligomers acted as the solvent for the thermoplastic polymers, and the fillers were added into the homogeneous epoxy/thermoplastic polymer solution. Then, phase separation of the epoxy/thermoplastic polymer and self-assembly of the fillers in the polymer matrices occurred simultaneously from the homogeneous mixture of the epoxy/thermoplastic poly- mer/fillers during the curing process. The novelty of these ther- moset (epoxy)/thermoplastic conductive adhesives lies in the simultaneous formation of the micro-phase structures of the * Corresponding author. E-mail address: kishi@eng.u-hyogo.ac.jp (H. Kishi). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer http://dx.doi.org/10.1016/j.polymer.2015.11.043 0032-3861/© 2015 Elsevier Ltd. All rights reserved. Polymer 82 (2016) 93e99