Sub-micron Scale Mechanical Properties of Polypropylene Fibers Exposed to Ultraviolet and Thermal Degradation Nandula Wanasekara1, Vijaya Chalivendra2* and Paul Calvert1 1Materials & Textiles Department 2Department of Mechanical Engineering University of Massachusetts Dartmouth, MA 02747 *Corresponding author, vchalivendra@umassd.edu, 508-910-6572 ABSTRACT Nanoindentation studies using atomic force microscopy (AFM) was conducted to investigate the effect of accelerated ultra violet (UV) and thermal degradation on mechanical properties of Polypropylene textile fibers. The Polypropylene fibers with initial stabilizers were exposed to UV degradation using Q-UV Panel Weatherometer. The effect of degradation on gradation of Young’s modulus values across fiber cross-section was investigated by doing progressive nanoindentation from the surface to the center of the fiber. It was identified that UV degradation initially increases the Young’s modulus values from center to surface of the fibers till 120 hours of exposure and the values show decreasing trend at 144 hours of exposure. The Youngs modulus values at 144 hours exposure are less than those of unexposed fibers. To investigate thermal degradation effect on Polypropylene fibers, the fibers were exposed to 125oC as a function of number of weeks in increments of one week for four weeks. Results indicate that the thermal exposure did not have much impact on variation of Young’s modulus values for the first three weeks and showed increase in Young’s modulus values at the surface when they are exposed to four weeks. The increase in Young’s modulus values of Polypropylene fibers exposed to UV will be correlated with oxidation chemistry and micro-structural changes using Wide Angle X-ray spectroscopy and infra red spectroscopy techniques. 1. INTRODUCTION Photo-oxidative degradation of polypropylene has been a major concern since its invention. This has drawn the attention of many researchers to investigate the mechanisms involved in degradation using various experimental techniques. The polypropylene melt is spun to fibers using a melt spinning process followed by drawing. Initial work was focused on finding out how these process parameters affect the degradation. The work conducted on weatherability of polypropylene involved laboratory extrusion of monofilaments and photo oxidation using a xenon arc lamp weatherometer. Oxidation is more likely to result from free radicals produced by bond rupture on drawing the polymer melt to fibers. It has been found that filament sensitivity to photo-oxidation is significantly affected by the extrusion and draw conditions and photosensitivity increases with increasing draw speed and decreasing draw temperature [1]. Microstructural changes that occur in polypropylene upon photo-oxidation were explained using established models of microstructure. These significant morphological and structural changes take place in crystalline or fibrillar regions showing much more resistance to scission than the amorphous regions [2]. It is well established that oxidation is confined to amorphous regions between crystallites since oxygen is insoluble in the crystal phase of polyolefins [3, 4]. Various types of analytical techniques were also used by researchers to characterize the non-uniformity of oxidation. These techniques include Fourier transform infrared (FTIR) Spectroscopy [5, 6] and photoacoustic spectroscopy [7]. A detailed study has been carried out using infrared absorption spectroscopy on the effect of molecular structure of polypropylene fibers that revealed the extent of changes on molecular structure of fibers under UV radiation depended on initial fiber structure, the added modifiers and macroscopic features [8]. Thermal degradation of polypropylene is also explained by similar mechanisms. The molecular weight distributions of polypropylene samples subjected to various degrees of thermal degradation were experimentally determined and compared with those expected theoretically for random scission of the polymer chains. The comparison demonstrated that the chain scission is completely random [9]. It has been found that the rate of oxidation increases with the increase in the partial pressure of oxygen and also with temperature. The most significant Proceedings of the SEM Annual Conference June 7-10, 2010 Indianapolis, Indiana USA ©2010 Society for Experimental Mechanics Inc. 275 T. Proulx (ed.), MEMS and Nanotechnology, Volume 2, Conference Proceedings of the Society for Experimental Mechanics Series 2, DOI 10.1007/978-1-4419-8825-6_40, © The Society for Experimental Mechanics, Inc. 2011
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