Abstract
Polyurea-urethane containing polydimethylsiloxane(PDMS) soft segment was blended with multi-walled carbon nanotubes (MWCNTs) via sol-gel method. The Multi-Walled CNTs were modified by a free radical reaction of vinylsilane with MWCNT. Fourier transform infrared (FTIR), Raman spectra and thermalgravimetric analysis (TGA) was utilized to characterize the functionalized Multi-Walled CNTs. Results show that the optimal reaction time is 8h of CNT modification and organic content of modified CNT reaches 35.22%. 13C Nuclear Magnetic Resonance (NMR) and dynamic mechanical analysis (DMA) were utilized to investigate the molecular structure, molecular motion and glass transtion temperature of MWNTs / PUU nanocomposite. Results show that the degree of molecular motion of soft segment is in the order of silane end-capped PUU > phenyl end-capped PUU > Pristine PUU；the hard segment is in the order of Pristine PUU > silane end-capoed PUU > phenyl end-capped PUU. MWNTs end-capped PUU shows the highest molecular motion in soft segment, but the lowest in hard segment. DMA data reveals the Tg of soft segment is less affected by the MWNTs contents of nanocomposites, but was significantly influenced by covalent bonding between PUU and CNT. It causes the Tg of PUU to decrease from -117℃ to -127℃ The surface resistivity of MWNTs / PUU nanocomposites decreased with the increasing of MWCNTs contents. Surface resistivity of Aluminum-contacting side and Air-contacting side of pristine PUU decrease from 2.45×1015Ω/cm2 and 1.62×1015Ω/cm2 to 9.77×105 Ω/cm2 and 2.14×1013 Ω/cm2, respectively, when the 5phr MWNTs；The surface resistivity of Aluminum-contacting side and Air-contacting side of silane end-capped PUU decrease from 1.32×1015Ω/cm2 and 1.86×1015Ω/cm2 to 5.75×105Ω/cm2 and 1.32×1012Ω/cm2, respectively, by adding 5phr MWNTs；The surface resistivity of Aluminum-contacting side and Air-contacting side of 5phr / phenyl end-capped PUU decrease from 2.35×1015Ω/cm2 to 9.21×107Ω/cm2 and 3.52×1015Ω/cm2 to 3.24×1011Ω/cm2；The surface resistivity of Aluminum-contacting side and Air-contacting side of MWNTs end-capped PUU decreased from 1.32×1015Ω/cm2 and 1.68×1015Ω/cm2 to 1.66×108Ω/cm2 and 1.65×1011Ω/cm2。 The EMI measurements on MWNT / PUU with the frequency ranging from 30MHz to 2000MHz were s also investigated. Due to the lower content or the poor dispersion of MWNTs in the nanocomposites, the low conductivity was found. In this study, 5phr MWNTs / PUU nanocomposites exhibits the best EMI value, which is 1.42dB. The microwave absorption of pristine and silane end-capped PUU nanocompostes increased from 0.18dB to 5.15dB and from 0.37dB to 8.15dB at 16 GHz with 0.5~5phr MWNTs contents. In the other hand, MWNTs / phenyl end-capped PUU and MWNTs end-capped PUU indicated π-π interaction (physical bonding) and covalent bonding (chemical bonding) between MWNTs and PUU respectively. The performance of dispersion of MWNTs PUU nanocomposites would enhance by the physical and chemical bonding between MWNT and PUU. Therefore, The microwave absorption of phenyl end-capped and MWNTs end-capped PUU nanocompostes increased from 0.09dB to 8.5dB and from 0.99dB to 19.41dB at 16 GHz with 0.5~5phr MWNTs contents. The Young’s modulus and tensile stress of MWNTs / PUU nanocomposites increased with the increasing of MWNTs. MWNTs can improve the stiffness and hardness of nanocomposites. Young’s modulus and tensile strength of pristine PUU resin increased from 121.88MPa to 208.12MPa (70.76%) and from 6.21MPa to 21.09MPa (240%), respectively, by adding 5phr CNT；Young’s modulus and tensile strength of Silane end-capped PUU resin increased from 102.88MPa to 177.72MPa (72.27%) and from 6.64MPa to 19.66MPa (196%), respectively, by adding 5phr CNT；Young’s modulus and tensile strength of 5phr CNT / Phenyl end-capped PUU resin increased from 112.01MP to 189.12MPa (68.84% ) and from 6.98MPa to 22.52MPa (222%), respectively, by adding 5phr CNT；Moreover, Young’s modulus and tensile strength of MWNTs end-capped PUU resin increased from 102.88MPa to 226.74MPa (220%) and from 6.64MPa to 25.05MPa (277%), respectively, by adding 5phr CNT. Hence, the addition of MWNTs would improve the mechanical properties significantly. The thermalgravimetric analysis shows that CNTs have exellent thermal stability. CNT began degradating from 530℃ as arising the temperature from room temperature to 800℃. The Td10 of pristine PUU arise from 256℃ to 275℃, and then decrease to 265℃ by adding 0~5phr CNT；The Td10 of silane end-capped PUU arise from 252℃ to 283℃, and then decrease to 276℃ by adding 0~5phr CNT；The Td10 of phenyl end-capped PUU arise from 251℃ to 267℃, and then decrease to 265℃ by adding 0~5phr CNT；The Td10 of MWNTs end-capped PUU arise from 252℃ to 268℃ by adding 0~5phr CNT. It reveals that the Td10 is not propotional to CNTs contents. Well-dispersed CNTs in PUU by chemical bonding brings about large contacting surface between MWNT and PUU. It enhances the thermal stability of CNT / PUU nanocomposite. However, physical bonding, π-π interaction or Van der Waals between MWNT and PUU may not get over the arggregate of MWNTs. Therefore, it caused the poor dispersion of CNT in PUU that reducing the thermal stability of MWNTs / PUU nanocomposites.