On the improved properties of injection-molded, carbon nanotube-filled PET/PVDF blends

Abstract

The mechanisms for improved mechanical and electrical properties of an injection molded, carbon nanotubes (CNTs) filled, polyethylene terephthalate (PET)/polyvinylidene fluoride (PVDF) blend have been investigated. It is found that the improved properties are due to the formation of a triple-continuous structure in the CNT-filled polymer blend; CNT segregates in the continuous PET phase, forming a continuous conductive path to provide the composite an electrical short circuit. The continuous PVDF phase free from CNT, on the other hand, offers crack bridging and the interface between the PET and PVDF phases provides crack deflection for the composite. As a result of such a combination, the CNT-filled PET/PVDF has better electrical conductivity, strength and elongation than the CNT-filled PET with the same CNT loading. The segregation of CNT in the PET phase of the CNT-filled PET/PVDF blend is due to the thermodynamic driving force that favors the segregation of CNT in the PET.

Introduction

Currently, bipolar plates of polymer electrolyte membrane (PEM) fuel cells are either made of Poco™ graphite or carbon-filled polymers. However, the material and manufacturing costs of these bipolar plates are very high and need to be reduced by ∼10 times before PEM fuel cells can be fully embraced for the automotive application [1]. For example, the cost of a graphite bipolar plate is currently about US$ 10 per plate if both the material and machining costs are included [2]. Such a high cost is due to the brittleness of graphite, which drives the machining cost of the flow channels on bipolar plates to a prohibitive level.

Extensive research has been conducted to search for low cost, high performance bipolar plates. These research efforts include studies of metal-based bipolar plates [3], [4], [5], [6], carbon-filled polymers [7], [8], [9], and carbon/carbon composites [2]. However, these efforts have met limited success; either the performance is not satisfactory or the cost is still high [2], [3], [4], [5], [6], [7], [8], [9]. For example, metal-based bipolar plates typically suffer from corrosion in the fuel cell environment, which results in a release of cations. The cations, in turn, lead to an increase in membrane resistance and to poisoning of the electrode catalysts [6]. In the case of carbon-filled polymers, high carbon loadings (typically > 50 vol.%) are needed in order to provide the required electrical conductivity [10], [11]. As a result of such high carbon concentrations, utilization of injection molding, which is suitable for mass manufacturing and will result in low-cost bipolar plates [1], is precluded because of the difficulty in processability; instead, compression molding, which is a slow process because one must allow the mold to cool down before the part can be taken out, often becomes the only choice of processing method [10]. An additional problem associated with high carbon concentrations in polymers is the substantial reduction in the strength and ductility of the polymer composites. It is well known that the tensile strengths of polymer composites increase initially with the addition of a small amount of fillers (∼5–20 vol.% CB), but decrease with higher filler loading [12], [13], [14]. Such phenomena have normally been attributed to the weak filler-matrix interface [12].

Recently, we have proposed a concept of making carbon-filled polymer blends containing a triple-continuous structure in 3D space [15], [16]. Fig. 1 shows the schematic of such a carbon-filled polymer blend, which consists of a binary polymer blend both phases of which (i.e., Phases A and B in Fig. 1) are continuous in 3D space. The conductive carbon is preferentially located in one phase (Phase A in Fig. 1) and its concentration is high enough to form a continuous structure (i.e. at least higher than the percolation threshold in Phase A) so that a continuous electrical conductive path is present in the polymer. Such a triple-continuous structure has the advantage of achieving conductive polymer composites at lower carbon concentrations since only the percolation threshold in one phase, rather than the entire polymer blend, needs to be exceeded [17], [18]. Such triple-continuous, carbon-filled polymer blends also offer two additional advantages, which are (i) the improved processability because of the low carbon concentration required, and (ii) minimal degradation in tensile properties because of the presence of a continuous neat polymer phase (Phase B in Fig. 1).

The feasibility of making triple continuous, carbon-filled polymer blends via injection molding has been demonstrated previously using a carbon nanotube (CNT) filled polyethylene terephthalate (PET)/polyvinylidene fluoride (PVDF) blend. The CNT-filled PET/PVDF blend exhibits 2500% improvement in electrical conductivity, 36% increase in tensile strength, and 320% improvement in elongation over the CNT-filled PET at the same carbon loading [16]. Thus, the triple continuous, carbon-filled polymer blends have great potentials for manufacture of low cost conductive polymers with superior conductivity and strength for bipolar plate applications of PEM fuel cells. To develop fundamental understanding of the improved properties of the CNT-filled PET/PVDF over the CNT-filled PET, additional experiments have been performed in this study. The results and analysis based on these new experiments are presented below.