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 (<i>T</i>_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

Reversible Shape-Memory Effect in Cross-Linked Linear Poly(ε-caprolactone) under Stress and Stress-Free Conditions

The effect of cross-link density on the reversible shape-memory effect (SME) under constant load was systematically studied in cross-linked linear poly­(ε-caprolactone) (PCL). A remarkable reversible SME under stress-free conditions was observed in PCL with the highest achieved cross-link density. Thermal properties as well as morphology, size, and orientation of the nanocrystalline structure formed in covalent networks of PCL under load were compared with those in PCL crystallized under stress-free conditions. As shown, the oriented growth of crystals is the origin of both the reversible SME under and without load. Furthermore, a significant rise of crystallinity and crystal thickness was detected in PCL crystallized under constant load. The fitting curves of the temperature-dependent strain as well as the quantities of crystallinity, type of crystalline structure, size, and orientation of the crystals got by modeling the reversible SME in PCL under stress well correspond to their values obtained experimentally