Manufacturing and Characterisation of Electrospun Nanostructured Mats from Poly(lactic acid)

Abstract

Nanofibres and textiles made of nanofibres are relatively recent additions to the world of advanced materials. Nanofibres are attractive for their nanoscale diameters, their porous structure and light weight. When the diameters of polymer fibre materials are shrunk from micrometers to submicrons or nanometers, there appear several appealing characteristics, including very large surface area to volume ratio. These permit flexibility in surface functionalities and provide superior mechanical performances compared to the normally used material structures. Non-woven textiles composed of nanofibres have small pore sizes compared to those in commercial textiles, making them excellent candidates for filtration and membrane applications. Nanofibres have marked differences in their physical/morphological, thermal and mechanical properties (biological properties as well in the case of biodegradable nanofibres), compared to those of regular fibres and bulk polymers. Electrospinning is a simple and relatively inexpensive method of producing nanofibres by solidification of a polymer solution, stretched by an electric field. When viscosity and surface tension of a polymer solution are appropriately tuned with the applied electrical forces, the break-up of the polymer drop at the needle tip is avoided and a stable jet of micro/nanofibres is formed. The primary requirement of the process is to obtain nanofibres in continuous form with fine diameters and minimum variations. Secondly, the fibre network has to have minimum area occupied by beads to enhance the network's porosity and strength. Beads are the main demerits of electrospun fibres. These two important characteristics, when achieved, render the nanofibre mats acceptable for countless advanced applications. Although apparently, electrospinning appears to be easy and straightforward, it is a complex structure-forming process of a liquid strand. It depends on a multitude of molecular, process related, and technical parameters. It is vital to produce electrospun nanotextiles in a controlled manner so that the process gives high quality fibres with precise fibre morphology. In this doctoral research, the primary objective has been to carry out a systematic investigation of the effects of varying manufacturing parameters on the electrospinning of nanotextiles and analyse them to develop a fundamental understanding for facilitating the usage of the process. Poly(L-lactic acid) (PLLA) has been selected as the electrospinnable polymer. A particular product, PLA(3051D) of NatureWorks LLC, has been used in the entire project as the variety of products made with this grade continues to grow rapidly. PLA is a promising polymer and electrospinning is the platform technology for generating nanofibrous mat for various purposes. Taguchi method has been used to establish the desirable set of four control parameters, namely the concentration and feed rate of polymer solution, applied voltage, and the distance between the collector and the needle. On the basis of this parametric analysis, a statistical study with a broader range of parameters with extended levels has also been done using regression analysis, to get a clearer idea of the effects of different invariables of electrospinning. To achieve a better understanding of the relationship between electrospinning parameters and the morphology of the electrospun product, predictive models have been developed using multiple regression analysis. The governing parameters investigated are the concentration and feed rate of polymer solution, applied voltage (considering effective voltage 1 kV per centimetre of workable distance between collector and needle), and the relative humidity of the enclosed area. The results show that polymer concentration and feed rate have significant and controlled impacts on producing fibres with diameters in the nano-range. Voltage and humidity also have considerable effects although their contributions to fibre stretching cannot be wellcontrolled. It is evident that the relationship derived from the two major factors, polymer concentration and feed rate, can predict the diameter of the fibre produced more accurately compared, to that derived considering all the factors.