Design And Control Of Co-continuous Networks In Immiscible-Polymer-Blend-Based Composites

In recent years, much effort has been devoted to polymer blends with a co-continuous structure due to their substantially improved properties, including elastic modulus, heat resistance, electrical and thermal conductivity, and chemical resistance to solvents. For immiscible polymer blends, co-continuous morphologies are mainly formed close to the phase inversion point. The remaining challenge for the design of low-cost and high performance materials is how to reduce the critical content of the minor polymer. In this work, carbon black (CB) and fibers were used as modifiers to mediate the co-continuity of immiscible polymer blends. The self-networking of CB and the competitive encapsulation between polymer components (or metal alloy) to fibers were studied, and a series of polymeric composites with continuous filler-polymer (or metal alloy) network were prepared.First, we studied the effect of CB on the morphology evolution of acrylonitrile-butadiene-styrene (ABS)/polyamide 6 (PA6) blends. It was found that the CB particles were selectively located in the PA6 phase. The addition of CB leads to a phase transition from a sea-island structure to a co-continuous one. With an increase in CB loading (ΦCB), the PA6 content (ΦPA6) required for the formation of the co-continuous structure decreases, and a smaller domain size is observed. Further experimental results verified that the formation of co-continuity in the immiscible blends was driven by an intrinsic cooperation effect between the CB and the CB-localized PA6 phase. The self-networking of CB plays a key role in extending the phase co-continuity over a much larger composition range. There exists a quantitative relation betweenΦCB andΦPA6.The product ofΦPA6 andΦCB, n, remains constant for a given system, and decreases with an increase in the DBP adsorption number of CB. This relation appears universal for nanoparticle-filled immiscible polymer blends and may be practically used as guide for designing and controlling their co-continuous morphologies. The CB self-assembly induced co-continuous structure could significantly improve the mechanical properties of ABS/PA6 blends in the high-temperature range, providing an important approach to fabricate a polymer blend with a desired heat resistance. Meanwhile, due to the heterogeneous distribution of CB and the double percolation effect, the electrical percolation concentration decreased remarkably.Second, we studied the effect of the addition of glass fiber (GF) with high aspect ratio on the co-continuity of polystyrene (PS)/PA6 blends. Due to the strong interaction between surface treated GF and PA6, many fibers are welded together by the minor PA6 phase, and a continuous GF-PA6 network is formed throughout the PS matrix. As a result, the elastic modulus is enhanced remarkably over a wide temperature region from the Tg of PS to the Tm of PA6, and the heat distortion temperature (HDT) of the composites increases significantly up to 201℃. The encapsulation kinetics of PA6 on GF during their melt compounding with PS was studied to correlate the encapsulation with the mechanical strength of the ternary PS/PA6/GF composites at temperatures higher than the Tg of the PS matrix. We verified that the bulk strength of the GF-PA6 network depends on the encapsulation ratio, NPA6, a parameter denoting the percentage of the PA6 phase encapsulating on the fibers. As mixing time increases, NPA6 increases gradually and then remains constant. The PA6 with a lower viscosity shows a rapid increase in NPA6, but a larger difference in viscosity between PA6 and PS results in a higher saturating value. A remarkable increase in NPA6 and the ratio of the efficiency of PA6 (obtained from the fitting of DMA curves) to NPA6 was observed for samples after isothermal post-treatments. It was concluded that the encapsulation of polymers on the GF surface and the strength of the GF-PA6 networks are kinetically determined by the migration of the dispersed PA6 domains to the GF surface and the preferential segregation of these PA6 domains to the junction point of fibers under the driving force of capillarity.Furthermore, we studied the effect of the thermodynamic factor (the difference of the interfacial interaction between GF and polymer components) on the constructing of the GF-PA6 network. The work of adhesion between GF and polymers was calculated based on the data of the contact angle and the interfacial shear strength (IFSS), and was used to estimate the interfacial interaction. The relationship between the relative interfacial affinity (ΔWa*, i.e. the difference of the work of adhesion between GF and polymers) and the encapsulation ratio NPA6 was studied. It was found that the encapsulation of GF by PA6 was suppressed when AWA* was minus, and GFs were mainly encapsulated by the matrix polymer. On the contrary, the encapsulation of GF by PA6 is thermodynamic favorable, and a high value ofΔWA* was beneficial for the constructing of a strong GF-PA6 network in the matrix. It was found thatΔWa is a necessary criterion for preferential segregation of PA6 on GF, however, the NPA6 value depends greatly on the wettability between GF and the matrix polymers. The preferential segregation of PA6 onto the surface of GF becomes difficult in the case when a polymer matrix owns good wettability to the GF surface.As demonstrated before, the key to further reduce the critical content of PA6 while retain the strong GF-PA6 network is how to increase NPA6. It was found that a sequential procedure was beneficial for achieving a higher saturating value of NPA6 within a fixed mixing time. Increasing the GF loading results in a large increase of the number of junction points, while the strength of the junction point remains constant. Therefore, a high GF loading is also favorable. Accordingly, high temperature resistant PS/PA6/GF composites with extremely low PA6 loading were prepared. It should be noted that the addition of only 3 wt% PA6 is enough for the constructing of a continuous and strong GF-PA6 network, and the HDT of the composites combined with 3 wt% PA6 was significantly enhanced up to 170℃. This can be practically used as guide for designing low-cost but high temperature resistant materials.At last, another kind of fiber-solder network was constructed in copper fiber (CuF) filled ABS composites via the incorporation of small amount of tin-lead alloy (Sn). Morphology observation indicated that CuF is soldered together by Sn, and a continuous CuF/Sn network is formed throughout the ABS matrix due to the good solderability between CuF and Sn. As a result, the percolation concentration of ABS/CuF composites decreases from 4 vol% to 3 vol% by the addition of 1 vol% Sn, and the addition of Sn to the ABS composites containing 5 vol% CuF leads to a further decrease of electrical resistivity, which is one order of magnitude lower than for ABS/CuF composites with corresponding filler contents. Therefore, a high EMI shielding effectiveness was achieved for ABS/CuF/Sn (90/5/5 in volume ratio) composite. The processability of the composites is also enhanced. Different form the traditional conductive polymer composites, the electrical resistivity of ABS/CuF/Sn composite showed no temperature dependence. Furthermore, the thermal stability of electrical resistivity for ABS/CuF composite is greatly enhanced by the incorporation of Sn, which constructs the CuF/Sn network and thereby prevents the CuF surface from oxidation at the junction points.