Yi-Fu Chen

Morphological regulation improved electrical conductivity and electromagnetic interference shielding in poly(L-lactide)/poly(ε-caprolactone)/carbon nanotube nanocomposites via constructing stereocomplex crystallites

Morphological control of conductive networks in conductive polymer composites has been demonstrated to efficiently improve their electrical performance. Here, morphological regulation used for the formation of conductive networks occurs in poly(L-lactide)/poly(e-caprolactone) (PLLA/PCL) blends when stereocomplex crystallites (SCs) are formed in the PLLA phase. The SCs formed during the melt-processing increase the viscosity and elasticity of the PLLA phase, which makes the PLLA domains shrink and the PCL phase becomes continuous from the previously dispersed phase. As a result, for PLLA/PCL/multi-walled carbon nanotube (MWCNT) nanocomposites, the MWCNTs prefer to disperse in the PCL phase via morphological regulation. The electrical conductivity and the electromagnetic interference (EMI) shielding effectiveness (SE) of the PLLA/PCL/MWCNT nanocomposites can be abruptly increased and attributed to the simultaneous organization of conductive paths when the continuous PCL phase develops. For example, the electrical conductivity and the EMI SE of the PLLA/PCL/MWCNT nanocomposites increased from 2.1 × 10−12 S m−1 and 5.3–8.6 dB to 0.012 S m−1 and ∼17 dB, respectively, with 0.8 wt% MWCNTs when adding 20 wt% poly(D-lactide) (PDLA) to the PLLA phase. Furthermore, the percolation threshold of the nanocomposites was reduced from 0.13 to 0.017 vol% by adding 20 wt% poly(D-lactide) (PDLA) to the PLLA phase.

Control of the Crystalline Morphology of Poly(l-lactide) by Addition of High-Melting-Point Poly(l-lactide) and Its Effect on the Distribution of Multiwalled Carbon Nanotubes

The key to fabricating conductive polymer/carbon nanotube (CNT) nanocomposites is controlling the distribution of CNTs in the polymer matrix. Here, an effective and simple approach for controlling the distribution of multiwalled CNTs (MWCNTs) is reported to largely improve the electrical conductivity of biodegradable poly(l-lactide) (PLLA) through crystalline morphology development by addition of high-melting-point PLLA (hPLLA) crystallites. hPLLA crystallites are efficient nucleating agents, increasing the crystallinity and crystallization rate of PLLA/MWCNT nanocomposites. Furthermore, the diameter of spherulites decreases from 9.7 to 1.0 μm with an increase in the concentration of hPLLA from 0.03 to 3.0 wt %. The electrical conductivity of PLLA/MWCNT nanocomposites with 0.3 wt % MWCNTs greatly increases from 1.89 × 10(-15) to 1.56 × 10(-8) S/cm with an increase in the matrix crystallinity from 2.4 to 46.8% on introducing trace amounts of hPLLA (0.07 wt %). The percolation threshold of PLLA/MWCNT nanocomposites is reduced from 0.51 to 0.21 wt % on addition of 0.07 wt % hPLLA. The high electrical conductivity and low percolation threshold of PLLA/MWCNT nanocomposites incorporated with hPLLA are related to the high crystallinity and crystalline morphologies of the PLLA matrix. Big spherulites lock a lot of MWCNTs at the intervals in the spherulites, which is harmful to the electrical conductivity. Small spherulites, with large surface areas, also need more MWCNTs to form conductive networks in the amorphous regions. Most MWCNTs that are bundled together to form conductive paths are found in samples with mid-sized spherulites of ∼6.7 μm. More interestingly, the high crystallinity and reconstructed MWCNT network also enhanced the Young modulus, elongation at break, and elastic modulus at high temperature of PLLA/MWCNT nanocomposites with small amounts of hPLLA.

Formation of thermally conductive networks in isotactic polypropylene/hexagonal boron nitride composites via “Bridge Effect” of multi-wall carbon nanotubes and graphene nanoplatelets

Fabrication of thermally conductive networks in polymer matrices is thought to be an efficient way to improve the thermal conductivity of polymer composites. Here we show a new approach to form thermally conductive networks in isotactic polypropylene (iPP)/hexagonal boron nitride (h-BN) composites via “bridge effect” of multi-wall carbon nanotubes (MWCNTs) or graphene nanoplatelets (GNPs). The isolated h-BN particles can be connected by MWCNTs or GNPs to form three-dimensional thermally conductive networks. It is found that the thermal conductivity of the iPP/h-BN composites is obviously enhanced but maintaining the electrical insulation by adding small amount of MWCNTs or GNPs. Because of the large content area of GNPs, the “bridge effect” of GNPs is more obvious than that of MWCNTs. The thermal conductivity of the iPP/h-BN composites with 10 wt% and 30 wt% h-BN particles show 14% and 23% enhancement by incorporation of 5.0 phr MWCNTs, respectively. Meanwhile, the thermal conductivity of the iPP/h-BN composites with 10 wt% and 30 wt% h-BN particles are enhanced by 59% and 70% when adding 5.0 phr GNPs, respectively. The electrical conductivities of the iPP/h-BN composites with MWCNTs and GNPs were maintained below 2.5 × 10−13 and 2.6 × 10−15 S cm−1, respectively.