In spite of the associated time and cost, solid state NMR remains the only generally accepted method for absolute measurement of the crystallinity index in polyolefins. As an alternative, wide angle x-ray diffraction (WAXRD) has long been recognized to have the potential to extract quantifiable information about the absolute degree of crystallinity, the extent of crystal lattice disorder, and the types of crystalline phases in semicrystalline polymers. However, the exacting level of analysis required to do this has remained a challenge for conventional x-ray diffractometers and data analysis systems.<br/>We have developed a method for analysis of WAXRD patterns from polyolefin materials that uses an efficient 2-D x-ray detector system combined with a sample preparation and sample motions in order to obtain data that is better suited for percent crystallinity calculations. We then apply raw data corrections for air scattering, polarization, Compton scattering, thickness and Lorentz type corrections. A Reitveld refinement of the crystalline part of the diffraction pattern is performed simultaneous with peak profile analysis methods for the amorphous fraction. Measurements of percent crystallinity are obtained by ratioing the integrated intensities of the separated crystalline diffraction peaks to the sum of the crystalline intensities plus the integrated amorphous contributions. Accurate measurements of the type and relative amounts of different crystalline phases, e.g. the relative ratio of gamma to alpha phase crystallinity (Wg/Wa), can also be determined. Peak halfwidths reflect the amount of disorder within the crystallites, as peaks can broaden as a result of either crystallite size reduction or lattice distortion. Consequently, our measurement of full width at half maximum of the (110) peak serves as an indicator of crystallite perfection. We attempt to separate the two components of peak broadening by collecting data up to very high scattering angles.<br/>The general method is demonstrated on three “powder” i-PP samples. Oriented samples require much more data, collected at many scattering angles to approach absolute quantification. This is thoroughly demonstrated here for the case of LLDPE blown films.<br/>

Characterization of crystallinity in polyolefins by wide angle X-ray diffraction

249th National Meeting (2015)

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