Poly(l-lactic acid)/Boron Nitride Nanocomposites: Influence of Boron Nitride Functionalization on the Properties of Poly(l-lactic acid).

Differently functionalized boron nitride nanosheets (BNNSs) [hydroxyl (OH_BNNSs), amine (NH2_BNNSs), and poly(ethylene glycol) (PEG) (PEG_BNNSs)] were synthesized, and their effects on the structure and thermal properties of poly(l-lactic acid) (PLLA) along with those of the pristine BNNSs were studied. Highly dispersed nanocomposites were prepared using PLLA and 0.5 wt % of pristine/functionalized BNNSs via a solvent blending method. Homogeneous dispersion of BNNSs in the polymer matrix was confirmed using X-ray diffraction and scanning electron microscopy. Pristine BNNSs and OH_BNNSs accelerated the crystallization of PLLA as effective nucleating agents and favored the formation of the α form in melt-crystallized samples. On the other hand, NH2_BNNSs and PEG_BNNSs incorporated samples result in the moderate crystallization rate of PLLA and lead to the formation of a mixture of α and α' forms similar to the PLLA. It is also found that thermal stability and thermal conductivity of PLLA nanocomposites significantly depend on the type of functionalization of BNNSs. At 0.5 wt % loading, the thermal conductivity enhancement is maximum for PEG_BNNSs incorporated PLLA (∼62%), and that is only 9% for pristine BNNSs incorporated PLLA. The thermal stability of PLLA nanocomposites was significantly improved by 32-41 °C depending on the type of functionalized BNNSs compared to PLLA. It is proposed that the strong interaction between functionalized BNNSs and PLLA matrix is responsible for the improved thermal management properties.

Influence of Boron Nitride Nanosheets on the Crystallization and Polymorphism of Poly(l-lactide).

This work reports on the impact of delaminated boron nitride nanosheets (BNNSs) on the crystallization behavior and crystalline structure of melt-crystallized poly-L-lactic acid (PLLA). Wide-angle X-ray diffraction and scanning electron microscopy data revealed that the addition of lower loadings of BNNSs (∼0.5 wt %) resulted in the highly dispersed PLLA nanocomposites, whereas the higher loading of BNNSs (≥1 wt %) leads to the agglomerated nanocomposites. It is shown that the presence of lower loadings of the BNNSs (∼0.5 wt %) induces the formation of ordered α form when crystallizing from the melt at a cooling rate of 10 °C/min, but the mixture of α' and α forms is formed in the presence of higher loading of BNNSs (≥1 wt %). Polarized optical microscopy images revealed that the crystallization rate of PLLA was significantly enhanced in the presence of lower loading of BNNSs (∼0.5 wt %) as corroborated by the increasing number of tiny spherulites. The strong interaction between the highly dispersed BNNSs and PLLA chains induces the conformationally ordered α form, and the various experimental techniques revealed that crystallization of PLLA occurred rapidly with the narrow distribution of crystal size and degree of crystal perfection in highly dispersed nanocomposites. Furthermore, the thermal conductivity of PLLA/BNNSs nanocomposites was found to increase significantly with BNNSs loading.

Polypropylene/Layered Double Hydroxide Nanocomposites: Influence of LDH Intralayer Metal Constituents on the Properties of Polypropylene.

Sonication-assisted delamination of layered double hydroxides (LDHs) resulted in smaller-sized LDH nanoparticles (∼50-200 nm). Such delaminated Co-Al LDH, Zn-Al LDH, and Co-Zn-Al LDH solutions were used for the preparation of highly dispersed isotactic polypropylene (iPP) nanocomposites. Transmission electron microscopy and wide-angle X-ray diffraction results revealed that the LDH nanoparticles were well dispersed within the iPP matrix. The intention of this study is to understand the influence of the intralayer metal composition of LDH on the various properties of iPP/LDH nanocomposites. The sonicated LDH nanoparticles showed a significant increase in the crystallization rate of iPP; however, not much difference in the crystallization rate of iPP was observed in the presence of different types of LDH. The dynamic mechanical analysis results indicated that the storage modulus of iPP was increased significantly with the addition of LDH. The incorporation of different types of LDH showed no influence on the storage modulus of iPP. But considerable differences were observed in the flame retardancy and thermal stability of iPP with the type of LDH used for the preparation of nanocomposites. The thermal stability (50% weight loss temperature (T 0.5)) of the iPP nanocomposite containing three-metal LDH (Co-Zn-Al LDH) is superior to that of the nanocomposites made of two-metal LDH (Co-Al LDH and Zn-Al LDH). Preliminary studies on the flame-retardant properties of iPP/LDH nanocomposites using microscale combustion calorimetry showed that the peak heat release rate was reduced by 39% in the iPP/Co-Zn-Al LDH nanocomposite containing 6 wt % LDH, which is higher than that of the two-metal LDH containing nanocomposites, iPP/Co-Al LDH (24%) and iPP/Zn-Al LDH (31%). These results demonstrated that the nanocomposites prepared using three-metal LDH showed better thermal and flame-retardant properties compared to the nanocomposites prepared using two-metal LDH. This difference might be due to the better char formation capability of three-metal LDH compared to that of two-metal LDH.