Studies On The Preparation And Properties Of Polyester Nanofibers And Nanofiber Aggregation

Because of specific properties of large surface area, good biocompatibility, and low fluid resistance, nanofibers have found applications in many areas, such as improving filtration efficiency of membrane devices, increasing protective and comfort performance of chemical and biological protective clothing, and enhancing the sensitivity of nanosensors. Development of high throughput production processes for making thermoplastic nanofiber and nanofiber yarns are urgently needed. In this study, Poly(ethylene terephthalate) (PET), poly (trimethylene terephthalate) (PTT), and poly(butylene terephthalate) (PBT) nanofibers were prepared from PET/ cellulose acetate butyrate (CAB), PTT/CAB, PBT/CAB immiscible polymer blends by in situ microfibrillar formation during the melt extruding process. The diameter distribution and crystallization properties of PET, PTT, and PBT nanofibers were analyzed. Viscoelasticity, blend ratio, shear rate, and draw ratio were three important parameters, which could affect the shape and size of nanofibers. By varying the process conditions, the average diameter of obtained nanofibers could be controlled in the range of 10-500 nm. PTT nanofibers can be obtained at the blend ratio of PTT/CAB 10/90,20/80, and 30/70. At the blend ratio of PTT/CAB 40/60, the dispersed component began to coalescence, but fibers can still be obtained after removal of the CAB matrix. When the blend ratio increased to PTT/CAB 50/50, co-continuous structures were found. For the effect of draw ratio, less than draw ratio value of 15, the diameter of nanofibers decrease remarkably, after the draw ratio lager than 15, this decrease became less significant. The increase of shear rate was found to have no obvious effect on the final diameter of PTT nanofibers. After removing the CAB matrix phase, the nanofibers could be collected in the forms of random or aligned nanofibers and nanofiber bundles or yarns.Understanding the mechanism of the morphology development of polymer blends in a time scale or in a length scale of extruders should be helpful to control the final morphology and to design processing equipment. To understand the formation mechanism of polyester nanofibers in the CAB matrix, the morphology development of dispersed phase in PTT/CAB blends was studied by collecting samples at different stages in the extruder. Fibers began to form even under the shear flow of the twin-screw extruder. The morphology developmental mechanism of the dispersed phase involved the formation of sheets, holes and network structures, then the size reduction and formation of nanofibers. The effect of viscosity ratio, blend ratio, and shear rate on the morphology evolution was also studied by analyzing the shape and size distribution of the samples. It was found that the holistic developmental trends of dispersed phase with different parameters were nearly the same. The diameter distribution of the dispersed phase could be affected by viscosity ratio, blend ratio, and shear rate. In the initial and metaphase development of the dispersed phase, with the increase of blend ratio and shear rate, the transformation of the dispersed phase became slower. By the reason of the interference of breakup patterns of multiple, neighboring fibers, elasticity and coalescence of dispersed phase, the final diameters of nanofibers unstable with the increase of viscosity ratio in the later development of dispersed phase. Because of the interference and coalescence between micelles, the final diameters of nanofibers increased with the increase of blend ratio. The increase of shear rate was found to have no obvious effect on the final diameter of nanofibers.To control the morphology and size of the dispersed phase, it is important to know the factors, which could affect the morphology of dispersed phase and the final s ize of the dispersed particles. Therefore, the average diameter of the polyester nanofibers was predicted by two modified theoretical models. The results indicated that modified Wu and Ghodgaonkar equations could be used to predict the diameters of nanofibers produced by this process, and the modified Ghodgaonkar model provided results closer to experimental ones than the model one. Furthermore, the modified Wu model provided correct responses to the changes of viscoelasticity ratio, blend ratio, shear rate and draw ratio of the polymer systems, while the modified Ghodgaonkar model responded well to the change of blend ratio, shear rate and drawing ratio, except viscoelasticity. Besides, small angle X-ray scattering (SAXS) was also used to analyze the structure of the PTT/CAB immiscible blends prepared in different processing conditions. With the increase of blend ratio and shear rate, the developmental trends of acand L were same with the trends of experimental results. However, with the increase of drawing ratio, the SAXS results were different with the experimental results. In a heated process, the structure of polymer blends changed when the temperature was higher than 200℃, while in a cooling process, the structural change of polymer blends was observed when the temperature was lower than 110℃.Nanofiber membranes have huge potential applications in many areas due to their unique properties. In this study, PTT nanofibers were fabricated from PTT/CAB immiscible polymer blends. After removing the CAB matrix phase, PTT nanofiber membranes were prepared by a wet-laid process. Two methods were used to increase the tensile strength of the membranes, heat-treatment of the membranes and addition of PP agglutinate nanofibers. The properties of three kinds of nanofibrous membranes including morphology, apparent density, porosity, contact-angle, pore size distribution, water flux and filtration properties were investigated. After the heat-treatment, the porosity, pore size and water flux decreased slightly. After addition of the PP agglutinate nanofibers, the porosity, pore size and water flux decreased remarkably, which was unfavorable for the applications of the nanofiber membranes. Therefore, considering all the properties of the nanofiber membrane, we chose the heat-treated PTT nanofiber membrane as the most favorite one, which could be used in filtration field. The rejection rate of TiO2 suspension by the membranes was above 99%. The membrane resistance increased with the increase of permeation time, and most of the membrane resistance came from the adsorption and pore blocking. The results revealed that this method could be an efficient one to make thermoplastic polymer nanofiber membranes, and they would have a brilliant potential application for water filtration and biomaterials.Carbon nanotubes (CNTs) have special structures and excellent mechanical and electrical properties, which is considered to be an ideal filling agent for polymer materials. In this paper, the CNTs/PTT nanofibers were prepared from CNTs/PTT/CAB immiscible blends. Firstly, CNTs were embedded into PTT matrix with contents ranging from 0 to 10wt% via melt blending, and then CNTs/PTT extrudates were mixed with CAB through a twin screw extruder. The CNTs/PTT nanofibers could be prepared after removal of CAB matrix. The morphology and properties of CNTs/PTT nanofibers were analysed by SEM, FESEM, DSC, TQ XRD and so on The average diameter of CNTs/PTT nanofbers were 140nm, and the diameter of CNTs/PTT nanofiber were not affected by the amount of CNTs added into PTT. The crystallization and melting temperature changed with different contents of CNT in CNTs/PTT. The thermal stability of CNTs/PTT nanofibers was improved with the increase of CNTs.