Polymer-matrix composites with discontinuous fillers (particles) are widely used in electronics [1
]. The terms “filler” and “functional filler” refer mostly to short, discontinuous fibers, flakes, platelets, or particles. Inorganic reinforcing fillers are stiffer than the matrix and deform less, causing an overall reduction in the matrix strain, especially in the vicinity of the particle as a result of the particle/matrix interface [2
]. In the case of a conducting composite, the greater the volume fraction of the conducting filler, the higher the conductivity of the composite will be, since the polymer matrix is usually insulating. The matrix used in making a polymer-matrix composite can be in liquid or solid form during the mixing of the matrix and the filler. In the resulting composite, the percolation attained after mixing the matrix and the filler through subsequent composite fabrication means they are involved in the flow of the thermoplastic under heat and pressure. In percolation, the filler units touch one another to form continuous paths; however, there is considerable contact resistance at the interface between the touching filler units [3
]. An effective way to decrease this contact resistance is to bond the filler units together at their junctions by using a solid that melts and wets the surface of the filler during the fabrication of the composite. The low-melting-point solid can be in the form of particles that are added to the composite mix, or it can be in the form of a coating on the filler units.
It is important to note that the use of a non-linear, conducting filler will result in interesting and useful effects on the polymer composite [4
]. This functional filler, which is made of doped zinc oxide (ZnO), disperses throughout the polymer matrix and responds to the electrical, non-linear behavior at room temperature [10
]. Due to the small sizes of the nano particles relative to the micron-sized fillers, the nano particles have a much higher interfacial area per unit volume. The reduction in particle size improves the overall properties of a varistor. Nanoparticles yield a narrow grain-size distribution and withstand relatively high energies [14
]. In order to take advantage of these effects, the nano fillers must be well dispersed in the polymer matrix.
Extensive research has been done on the multi-functional, inorganic nanoparticle, nano ZnO, due to its many significant physical and chemical properties. Tang and co-workers studied the UV-shielding properties of nano ZnO/Polymethyl methacrylate composites in 2005 [15
]. They found that increasing the amount of nano ZnO improved the UV-shielding capability of the polymer composite. Ultraviolet absorption, thermal behavior, and the visco-elastic properties of nano ZnO in Polyvinyl alcohol/Polyethylene oxide was investigated by Lee et al.
in 2008 [16
]. Photo-degradation of low density polyethylene (LDPE) containing ZnO nano particles was studied by Yang et al.
in 2010 [17
]. Raju et al.
investigated the improved and best tribological behaviors of polyester filled with ZnO nano particles [18
]. It was demonstrated that the improved tribo-performance of the nano composite was attributable to the mechanical properties of the nano particles. In addition, ceramic fillers, such as ZnO, have well-known, non-linear behaviors, which were studied by Wang et al.
]. ZnO/EPDM (ethylene propylene diene monomer) nanocomposites were processed by melt blending in their research. As a result, the geometry, as well as the intrinsic, non-linear behavior of the filler, impacted the non linearity of the composites.
ZnO varistor was announced in 1969 by Matsuoka [20
], although some such development was conducted in Russia in the early 1950s [21
]. A more-detailed paper provided by Matsuoka in 1971 described many of the essential features of varistors as we know them today. The details included ZnO semiconductors with the addition of substituted ions, densification by liquid-phase sintering with a Bi2
-rich liquid phase, and segregation of large ions to the grain boundaries. ZnO-based varistors, as one type of metal-oxide varistors, are produced by a ceramic sintering process that produces a structure of conductive grains that are composed of the matrix oxide surrounded by electrically-insulating barriers. These electrical barriers are derived from trap states at the grain boundaries that are induced by the additive elements [22
]. The milling and homogenization stages of the powders are conducted mainly in a ball mill in an aqueous mixture. Pure oxides are used as dopants in the zinc oxide ceramic varistor, including Bi2
, MnO, and others. The density of traditional ZnO-based polycrystalline ceramics is generally increased by the presence of Bi2
, which forms a liquid phase during the sintering stages. Other dopants, such as Co3
and MnO, are added in order to increase the value of the nonlinear coefficient (alpha) and the resistance against degradation [26
]. High requirements are imposed on the initial materials, both with respect to purity and dispersion composition, so as to obtain a homogenous microstructure of the sintered body. It was found that the dopants that were added to ZnO affected the formation of the microstructure and, consequently, affected the electrical and other performance properties of the varistors differently [27
]. Dopants are divided into three basic groups according to the functional applications, as follows:
Those that participate in the formation of the basic microstructure of ZnO varistors in sintering provide for the formation of inter-granular layers; Bi2O3 is one such dopant.
Those used in ensuring the non-linearity of the varistor ceramic promote the creation of deep charge carrier traps and cause the formation of the surface potential of the grains; Co3O4 and MnO are such dopants.
Those that stabilize inter-granular layers under electrical loads and external environmental factors (temperature and humidity) and increase the stability of the electrical characteristics and reliability of the varistors; Sb2
is one such dopant [29
Polycaprolactone (PCL) is a hydrophobic, semicrystalline polymer whose crystallinity tends to decrease with increasing molecular weight. The good solubility of PCL and its exceptional blend-compatibility have stimulated extensive research into the potential application in the biomedical field [30
]. Its numerous advantages over other polymers include tailorable degradation kinetics and mechanical properties, ease of shaping and manufacture enabling appropriate pore sizes conductive to tissue in-growth, and the controlled delivery of drugs contained within their matrix. Moreover, functional groups can also be added to render the polymer to become more hydrophilic, adhesive or biocompatible which enables favorable cell responses due to the fact that PCL degrades at a slower rate (i.e.
, up to 3–4 years).
Although it initially attracted some research interest, PCL was soon overwhelmed by the popularity of other resorbable polymers, such as polylactides and polyglycolides. Furthermore, both the medical-device and drug-delivery communities considered that faster resorbable polymers also have fewer perceived disadvantages associated with long-term degradation and intracellular resorption pathways. This consequently caused PCL to become almost forgotten for most of the last two decades. In the 1970s, it was already recognized that PCL was particularly amenable to blending and polymer blends based on PCL were categorized with three types of compatibility; these were exhibiting only a single Tg
, as mechanically compatible while exhibiting the Tg
values of each component, but with superior mechanical properties and as incompatible, i.e.
, exhibiting the enhanced properties of phase-separated materials [35
]. The compatibility of PCL with other polymers depends on the ratios employed, and PCL is generally used to produce better control over the permeability of delivery systems. Copolymers of PCL can be formed using many monomers, such as ethylene oxide, polyvinylchloride, chloroprene, polyethylene glycol, polystyrene, diisocyanates, tetrohydrofuran, diglycolide, dilactide, substituted caprolactone, methyl methacrylate and vinyl acetate [30
], which dictate the crystalline nature of PCL that enable its easy formability at relatively low temperatures.
Meanwhile, the physico-mechanical properties of several degradable polymers of PCL were studied and compared by Engelberg and Kohn who investigated thermal and tensile properties, including Young’s modulus, tensile strength, and elongation at yield and break [36
PCL is prepared by the ring-opening polymerization of the cyclic monomer, e-caprolactone, and it has been studied since the 1930s [37
]. Various catalysts, such as stannous octoate, have been used to catalyze the polymerization, whereas low-molecular-weight alcohols can be used to control the molecular weight of the polymer [38
]. There are various mechanisms that affect the polymerization of PCL, i.e.
, anionic, cationic, co-ordination, and radical mechanisms. In particular, each method affects the resulting molecular weight, molecular weight distribution, end-group composition, and the chemical structure of the copolymers [34
]. The average molecular weights of PCL samples generally vary from 3000 to 80,000 g/mol and can be graded according to their molecular weights [39
]. Based on the degradation studies presented in the literature, it can be concluded that PCL undergoes a two-stage degradation process, namely, the non-enzymatic hydrolytic cleavage of ester groups and, when the polymer is more highly crystalline and has a low molecular weight (i.e.
, less than 3000), the polymer has been shown to undergo intracellular degradation as evidenced by the observation of the uptake of PCL fragments by the phagosomes of macrophages and giant cells and within fibroblasts [40
]. This supports the theory that PCL may be resorbed completely and degraded via an intercellular mechanism once the molecular weight is reduced to 3000 or less. It is also important to note that, in the first stage, the degradation rate of PCL is essentially identical to the in vitro
hydrolysis at 40 °C and obeys first-order kinetics. Thus, it was concluded that the mechanism of PCL degradation could be attributed to the random hydrolysis chain scission of ester linkages, causing a decrease in the molecular weight. Flexibility, biodegradability, low Tg
(−61 °C) and a fairly long biodegradability cycle have been noted as particular features of PCL [41
], so that, when it is mixed with an inorganic filler, the overall properties of the composite are enhanced.
This study introduced an electrically-resistive, polymer composite with varistor-like behavior, devoted to the increased interest in the varistor-based composite. Here, we determined the morphology and the current-voltage characteristics of composites that are made up of various weight percentages of varistor powder as nano filler in the PCL.