Stephen Michielsen
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Research Abstracts -Stephen Michielsen |
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Last Updated: June 27, 2000 |
1. "Raman Spectroscopy of Polymeric Fibers and Films," by Stephen Michielsen, in "Handbook of Raman Spectroscopy," edited by Ian Lewis and H. G. M. Edwards. Publisher is Marcel Dekker. In press. ABSTRACT Raman spectroscopy of polymeric fibers and films has become a powerful analytical tool for the identification of the polymer type, additives and degradation products. Current research efforts are extending the analytical tools for process control of the orientation and crystallinity of the polymers within fibers and films. The orientation averages P2 and P4 can readily be determined as well as structure gradients within fibers and films. Raman microspectroscopy allows these characterizations on single filaments or even within individual fibers or films. Raman spectroscopy also can determine the tensile and compressive stress on individual fibers within a composite nondestructively. Stress transfer from a matrix material to the reinforcing fiber and stress distributions within composites can be determined. The author is not aware of any other direct measure of the stresses within a composite material on the size scale needed. Raman spectroscopy has become a mainstream technique for characterizing and controlling the morphology of polymeric fibers, films and fiber reinforced composites. Its use in archeological and forensic science has only begun, but it shows great promise in these areas. The author believes the use of Raman spectroscopy will continue to expand as Raman instrumentation improves further and as the wealth of information available becomes more widely recognized. 2. "Determination of Density and Birefringence of Poly(Ethylene Terephthalate) Fibers Using Raman Microscopy," by Subashree Natarajan and Stephen Michielsen, J. of Appl. Polym. Sci., 73, 943-952 (1999). ABSTRACT Multiple regression analysis has been used to calibrate polarized Raman spectra of poly(ethylene terephthalate) fibers in terms of density and birefringence. The calibration spans PET fibers having a wide range of density and birefringence values. The calibration required the Raman spectrum in only one polarization direction, that is, with the polarization directions of the incident and scattered light parallel to each other and to the fiber axis. The peak at 631 cm-1, which has been used previously as an internal standard band, could be used for the prediction of density, but not for the prediction of birefringence. The peak at 702 cm-1 was found to be a good internal standard band for both density and birefringence. Density could be predicted with a standard error of prediction of 0.003g/cc using only the ratio of the intensity of the band at 996cm-1 to that of 702 cm-1 and the full width at half maximum of the 1725 cm-1 band. Birefringence was predicted with a standard error of 0.01 using the ratios of the intensities of the bands at 996 cm-1 and 1616 cm-1 to that of the 702 cm-1 band. 3. "The Use of Confocal Raman Microscopy for Determining the Structure and Orientation of the PET Interior of PET/PP Core/Shell Fibers," by Subashree Natarajan and Stephen Michielsen, Textile Research Journal, 69, 903-907 (1999). ABSTRACT Confocal Raman microspectroscopy is shown to be capable of measuring the density and the birefringence of the poly(ethylene terephthalate) core of a poly(ethylene terephthalate)/polypropylene core/shell fiber. Sample preparation is simple and data collection times are short. This technique should prove valuable in furthering the development of bicomponent fibers. 4. "The Effect of Grafted Polymeric Lubricant Molecular Weight on the Frictional Characteristics of Nylon 6,6 Fibers," by S. Michielsen, J. Appl. Polym. Sci. 73, 129-136 (1999). ABSTRACT Surface properties of fibers play a critical role in many end-use applications. Since the surface properties for a polymer often do not meet the required surface properties, there have been many attempts to change the surface properties with mixed success. One such property is boundary friction. This article describe efforts to permanently modify the boundary friction by grafting polymer chains to the fiber surface. It is shown that the boundary friction of these samples can be explained by modifying the adhesion model of friction to accomodate multiple materials on the surface where the fraction of the surface covered by the grafted chain is proportional to pRG2 divided by the surface area per graft site, where RG is the radius of gyration. However, if the grafted chain is too large, then it will block multiple graft sites on the substrate. In this case, the appropriate surface area fraction depends on pRG2 divided by 3x the surface area per graft site since the next nearest graft site is used rather than the nearest graft site. Using this analysis it is shown that a friction coefficient of zero may be attainable if the grafted chain size exactly matches the area per graft site. 5. "Fracture Energy Release Rate in Nylon Fibers," by S. Michielsen, J. Appl. Polym. Sci., 67, 1541-1544 (1998). SYNOPSIS The effect of relative humidity, RH, on the fracture energy release rate, GIc, for single nylon 6,6 fibers has been determined previously3. In the present study, it is shown that GIc is independent of RH for moisture contents of greater than 2.3% once the plastic zone correction is made. GIc is compared to various proposed mechanisms to account for the fracture energy. It is shown that the energy required to disrupt or "melt" the crystals in the plastic zone accounts for the majority of the energy required to break the specimen and should be considered explicitly in future analyses of fracture in semicrystalline polymers. |