JOURNAL OF MATERIALS SCIENCE LETTERS 14 (1995) 1638-1640 Investigation of axial fibre stresses in transcrystalline regions of single- fibre aramid/polypropylene composites using Raman spectroscopy M. HEPPENSTALL-BUTLER, R. J. YOUNG Manchester Materials Science Centre, UMIST/University of Manchester, Grosvenor Street, Manchester, M1 7HS, UK Transcrystallinity (TC) is a phenomenon that can occur in a wide range of thermoplastic-matrix polymer composites. Nucleated from reinforcing fibres, this structure grows into the matrix and in high volume fraction fibre-reinforced composites can dominate the matrix morphology [1]. It is essential, therefore, to understand the properties of both the transcrystallinity and the interface between TC and the nucleating fibres. Despite all the literature produced on this subject, the causes and effects of TC are not understood fully. Transcrystalline regions are thought to have a higher modulus than the spherulitic matrix [2], but their effects on the interfacial adhesion are still unclear. The different techniques used to characterize different morphological types have yielded a selection of results and views on how TC is caused and whether or not it is useful for an improvement in composite properties [1]. Investigating cotton cellulose in isotactic polypropylene (iPP), Felix and Gatenholm found an increase in interfacial shear stress (ISS) with increasingly thick TC [3]. Yue and Cheung [4], reported that nuclei adhering to glass fibres after pull- out from iPP indicated that adhesion between fibre and matrix was stronger at the point of nucleation than in the outer lamellae of spherulites. From this they deduced that the interfacial bond between a fibre and TC would be stronger than that between fibre and a spherulitic matrix. Folkes and Wong [5], found that surface-treatment-nucleated transcrystallinity re- duced the interfacial bond strength between glass fibres and iPP. Others report, however, (e.g. Moon [6]), that the effect of TC upon interfacial strength varies with other matrix variables, or with fibre volume fraction, but whatever the effect TC has upon interfacial properties it is only minor [7-9]. There has also been experimentation to determine the levels of thermal residual stresses induced in the fibres due to the presence of TC regions. Incardona et al. [10] concluded that increasing the thickness of the transcrystalline region lowers the residual thermal stresses present in the fibre. In this communication we present measurements of the point-to-point thermal residual stress distributions along Twaron fibres in various transcrystalline and non-transcrystalline isotactic polypropylene (iPP) matrix morphologies as measured using Raman spectroscopy. This is the first time that such measurements have been reported. 1638 The materials used were Twaron D1056 aramid fibres, and SY6100 iPP, as described in references [11-13] by Thomason and Van Rooyen. Long single- fibres were placed between small slivers of iPP, melted to 200 °C~ and pressed between glass slides to reduce the thickness. After cooling, these samples were placed in a Linkam THMS600 hot-stage between glass cover slips in an argon atmosphere and blank melted at 200°C for 5min before receiving one of five thermal histories. Four of the thermal histories involved a period of constant temperature crystallization (T~), the fifth was fast- cooled directly from the melt to room temperature at -2.5 Ks -1. The constant temperature samples were cooled at -10Kmin -1 to Tc=140°C, 135°C, 130°C, or 120°C for 300, 120, 70, or 10min, respectively. Following this, they were then fast- cooled at about -2.5 K s -1 to room temperature. The thermal histories will be referred to as FC (fast cooled), 140, 135, 130, or 120. The 135, 130, and 120 histories produce well-defined TC. The increas- ing nucleation rate with decreasing T~ results in finer TC with lower T~. The radii of spherulites and TC are also affected, decreasing from -100gm in 135 samples to about -80~tm in 120 samples. FC produces very fine TC of radius -20 girl. The 140 treatment produces a spherulitic matrix with no transcrystallinity. The samples were 60 gm _+ 15 gm thick, with fibres of diameter 11.7 gm _+ 0.4 gm passing straight through the iPP films. Raman spectroscopy can be used to measure the axial stresses in the outer surface of aramid fibres. The residual axial stress distribution along each embedded fibre was measured using a series of Raman spectra taken at intervals along the fibre in the matrix. The Raman spectra were obtained as follows. The 632.8 nm red line of a 15 mW helium- neon laser was focused to a point about 5 gm diameter on the fibre through a modified Olympus optical microscope so that the polarized laser light was oriented parallel to the axis of the fibre. The scattered radiation from the fibre was passed through a set of a collection optics to a SPEX 1000M monochromator. The spectrum was collected using a Wright Instruments Charge-Coupled Device (CCD) camera cooled with liquid nitrogen [14]. Raman spectra were taken along the fibre at 10, 20 or 50 gm intervals from the region of fibre outside the matrix through up to 2000 gm into the matrix. The region 0261-8028 © 1995 Chapman & Hall