Carbon Nanotube Polyester Blends Ester Exchange Reaction

Transesterification in the polyester blends is an important subject which has gained increasing attention in recent years. The block or random copolyesters could be in-situ formed via transesterification between the components during melt mixing, which not only provides a new route to fabricating novel copolymers with designed composition and sequential order, but also provides a good template to explore the reaction between macromolecules.Therefore, understanding the factors on the transesterification is vital to design the chain structure and the physical morphology of the blends, and to control the final properties of produced materials.Poly(trimethylene terephthalate) (PTT) is a new member of linear saturated polyester family. It combines high mechanical strength and good heat resistance of poly(ethylene terephthalate) (PET) and nice flexibility and processing features of poly(butylene terephthalate) (PBT), and hence, is a promising candidate in the field of engineering thermoplastics.Moreover, the monomer of PTT, namely propanediol, can be derived from corn sugar, a renewable resource, and hence PTT has also drawn considerable attention for having a biobased origin yet having the properties of an engineering plastic. Comparing with those already on the traditional polyester PET and PBT, however, the modification work on the PTT is not sufficient and the transesterification in the blends based on PTT and other polyesters have not yet been fully studied, which is very important to fabricate PTT based materials with high performance and to extend their applications.Thus, in this work, the poly(trimethylene terephthalate)/poly(butylenes terephthalate) blend (PTT/PBT) and poly(trimethylene terephthalate)/polycarbonate blend (PTT/PC) were prepared by melt mixing. Then, the transesterifications in these two blends were studied by nuclear magnetic resonance spectroscopy (NMR) and differential scanning calorimetry (DSC).The effects of blending time, addition of the traditional catalyst (tetrabutyl orthotitanate) and a new type catalyst (surface modified carbon nanotube) on the transesterification level and the morphologies of these two blends were deeply explored, aiming at relating number-average length, degree of randomness to the transesterification level among the component polymers. The preliminary results are as follows.(1)Miscible PTT/PBT BlendFor the PTT/PBT blend, the morphological characterization results show that the system is thermodynamically miscible. Prolonging the blending time has few effects on the transesterification level.But the tetrabutyl orthotitanate (Ti(OBu)4) shows high catalytic activity to the transesterification, which yields the PTT-PBT copolymers.The transesterification level increases monotonically with the increase of Ti(OBu)4 contents, and the number-average length of PTT and PBT in the copolymers tends to be reduced and the degree of randomness of blends increases as a result, accompanied by the decrease of melt temperature (Tm) and crystallization temperature (Tc) of blends.The presence of surface-modified carbon nanotube could also catalyze the transesterification between two component polymers.Small addition of carbon nanotubes (CNT) could promote transesterification remarkably while excessive addition of CNT depressed the transesterification slightly becauses of increasing viscosity of the blends.Compared with hydroxy carbon nanotubes (OH-CNT), carboxylic carbon nanotubes (COOH-CNT) can better promote the transesterification. But the presence of COOH-CNT leads to the crystalline separation between PTT and PBT.(2) Immiscible PTT/PC BlendFor the PTT/PC blend,two typical immiscible morphologies, i.e.,spherical droplet and co-continuous structures can be observed at various compositions. The Ti(OBu)4 also has high catalytic activity to the transesterification between PTT and PC,yielding the PTT-PC copolymers.The transesterification level increases monotonically with increasing Ti(OBu)4 contents, which improved the compatibility of the PTT/PC blend evidently. After transesterification, the original two glass transition temperatures (Tg) of the blends were found to shift to each other and finally merge into one single Tg, and the transesterificated system exhibits a homogeneous phase morphology. With increasing transesterification level, the melting behavior of semicrystalline PTT disappears and the PTT component becomes amorphous.Small addition of COOH-CNT also promotes the transesterification remarkably, while superfluous COOH-CNT reduces reaction levels slightly. However, the presence of COOH-CNT could increase Tg of PTT/PC blends.