Carboxyl-functionalized multiwalled carbon nanotube (c-MWCNT) was used to enhance the properties of poly(butylene terephthalate)/polycarbonate (PBT/PC) blends by melt blending. The c-MWCNT inhibited transesterification reactions between PBT and PC. SEM result showed that most of the c-MWCNT dispersed in the PBT matrix and partial c-MWCNT lied between the interface of PBT and PC phases. c-MWCNT promoted the crystallization of PBT due to the decreased transesterification and the heterogeneous nucleating effect, which led to a 20 ℃ increase of the crystallization temperature. DMA test indicated that the miscibility between PBT and PC decreased with the c-MWCNT loading since the inhibited transesterification. When the c-MWCNT content was 4 wt%, the yield strength of 65.9 MPa and tensile modulus of 2116MPa were achieved, which corresponded to 23.9 and 19.5% increase relative to the pure PBT/PC blend. The conductivity and dielectric properties of PBT/PC were largely enhanced by the c-MWCNT. The σ_dc values of PBT/PC improved greatly by several orders of magnitude from 10^-18 to 10^-7 when the c-MWCNT content was 2 wt%. The dielectric constant at 100 Hz for the nanocomposites increased from 2.8 to 146 when the c-MWCNT loading varied from 0.1 to 4 wt%, which improved more than 50 times.

Keywords: poly(butylene terephthalate), polycarbonate, carbon nanotube, nanocomposites

Polymer blending has become an important method to prepare novel materials with much better processability, chemical resistance, toughness and stiffness, electrical properties, and so forth.^1-7 For the polymer blends, aromatic polyester blends represent a major kind and have been investigated widely. poly(butylene terephthalate) (PBT) and polycarbonate (PC) blends belong to one important polyester pair, which combine the good balance of chemical and impact resistance, excellent flow characteristics and dimensional stability. PBT/PC blends have been used in automotive industry, outdoor power, appliance housings and so on.^8-14
For the PBT/PC blends, most researches have been focused on the transesterification reactions, thermal properties, miscibility, phase morphology and impact modification.^15-19 Recently, nanocomposites based on PBT/PC blends attracted much attention. For example, Depolo and Baird investigated the effect of particle size of talc on dimensional stability and mechanical properties of PBT/PC blends. They found that using nanotalc instead of micro-size talc reduced the level of talc reinforcement from 6 to 1 wt% without sacrificing the coefficient of linear thermal expansion and shrinkage of injection-molded plaques. Furthermore, 14 and 120% improvement in the flexural strength and tensile toughness were achieved. Nanoclay has also been used to modify the PBT/PC blends. Compared to nano talc, nanoclay particles show much higher aspect ratio and lower density. It was found that by using only 1 wt% nanocaly as opposed to nano-talc, the flexural strength and tensile toughness of the nanocomposites increased by 12 and 27%, respectively, and maintained a flexural modulus of 2.5 GPa.²⁰
Among the nanofillers, carbon nanotube has been widely used to enhance the properties of polymers due to its unique and extraordinary thermal, mechanical and electrical properties. ^21,22 The blends of carbon nanotube with PBT or PC have been prepared and investigated by some researchers.^23-25 As for the PBT nanocomposites, carbon nanotube improved the crystallization rate and degree. At the same time, the PBT blends showed much higher stiffness, thermal stability and conductivity. Similar with the PBT nanocomposites, carbon nanotube modified PC blends also showed improved modulus and strength, enhanced chemical resistance and thermal properties.
Not only the carbon nanotube properties but also the preparation method can affect the electrical properties of PC. The PC/c-MWCNT nanocomposites with different percolation threshold (Pc) have been reported by some researchers. Though PBT/PC blends have been studied for many years and used widely in many engineering applications, few reports on the carbon nanotube modified PBT/PC blends could been found from the literatures. S. Maiti et al. used PBT to decrease the percolation threshold of PC/multiwalled carbon nanotubes. It was found that only 0.35 wt% multiwalled carbon nanotubes were enough to improve the conductivity of the nanocomposites. 26 Yu et al. used carbon nanotube to improve the conductivity of PBT/PC blends. They found that the carbon nanotube distributed in the PBT phase and the conductive materials were obtained with 1 wt% carbon nanotube addition. ²⁷ Elastomer particles and carbon nanotubes were utilized by Wang et al. to improve the toughness of PBT/PC blends. They pointed out that the elastomer particles (SEBS-g-MA) enhanced the impact strength of PBT/PC blend. Furthermore, the introduction of carbon nanotube improved the toughening efficiency of SEBS-g-MA for PBT/PC blends.²⁸ Different with these researches, carboxyl-functionalized multiwalled carbon nanotube (c-MWCNT) was used to improve the properties of PBT/PC blends. The carboxyl groups on the carbon nanotube increase the polarity which should be beneficial to enhance the miscibility between carbon nanotube and the polyester matrix. Furthermore, the inhibition of transesterification in PBT/PC blends due to the c-MWCNT was firstly reported and the dielectric properties were studied in detail, which have not been reported in the above papers. In the present paper, the effect of carboxyl-functionalized multiwalled carbon nanotube on the mechanical properties, morphology, crystallization, conductive and dielectric properties of PBT/PC blends was investigated in detail.