Chapter 24
Subnanometer Hole Properties of Cellulose Studied by Positron Annihilation Lifetime Spectroscopy 1
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H. Cao , J.-P. Yuan , Y. C. Jean , A. Pekarovicova , and R. A. Venditti 1
Department of Chemistry, University of Missouri at Kansas City, Kansas City, MO 64110 Department of Wood and Paper Science, North Carolina State University, Box 8005, Biltmore Hall, Raleigh, NC 27695
Downloaded by UNIV LAVAL on May 11, 2016 | http://pubs.acs.org Publication Date: January 28, 1999 | doi: 10.1021/bk-1998-0710.ch024
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Two series of cellulose samples, Avicel and Whatman CF11 cellulose ball-milled powders with different crystallinity are studied below T temperature by using positron annihilation lifetime spectroscopy. A good correlation is found between ortho-positronium formation probability and crystallinity as measured by Fourier transform infrared spectroscopy. Sub-nanometer hole distributions are found to be narrowed as a function of milling time. These are interpreted in terms of microstructural changes of cellulose. g
Cellulose has been known to have highly ordered morphology. It does not exist as an entirely crystalline material, but forms in different phases with different degrees of order. Irregular amorphous regions intersperse between regular crystalline phases. Crystallinity is an important structural feature for cellulose which can influence some properties critical for technological applications, for example, tensile strength and water sorption ability. Many different techniques have been applied to measure the crystallinity, including physical, chemical, and sorption methods. X-ray diffraction is commonly used as the reference evaluation. Some new techniques, such as solid-state C nuclear magnetic resonance (NMR) and Fouirer transform infrared spectroscopy (FTIR) have been shown to be very promising to determine the crystallinity of cellulose (1-3). In recent years, positron annihilation lifetime (PAL) spectroscopy has been demonstrated to be a special sub-nanometer probe to determine the free-volume hole size, fraction and distribution in a variety of polymers (4-9). In this technique, measured lifetimes and relative intensities of the positron and positronium, Ps (a bound atom which consists of an electron and a positron), are related to the size and fraction of sub-nanometer holes in polymeric materials. Because of the positivecharge nature, the positron and Ps are repelled by the ion core of polymer molecules and trapped in open spaces, such as holes, free volumes, and voids. The observed 13
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Corresponding author.
©1998 American Chemical Society
Tant and Hill; Structure and Properties of Glassy Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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lifetime of o-Ps (the triplet state of Ps) is found to be directly correlated to hole size and the corresponding intensity could be a measure of relative number of holes. For semi-crystalline materials, most work has been to correlate o-Ps intensity with crystallinity. While o-Ps lifetime is found to be nearly independent of crystallinity, oPs intensity decreases as the degree of crystallinity increases (4). These results suggest that o-Ps is preferentially formed in the free-volume holes of the amorphous phase. The Ps formation in defects or low electron density trapping sites in crystalline phase is another possible interpretation made by others (9). m this study, PAL measurements are performed on cellulose samples with degrees of crystallinity varied by controlled ball-milling. Our objective is to correlate the microstructural changes with hole properties investigated by PAL. Experimental Sample Preparation. Cellulose powders, Whatman CF11, short fibrous cellulose powder with mean particle size 50-350 pm (Whatman International, Ltd.) and microcrystalline cellulose Avicel, mean particle size 27.6-102 μπι, crystallinity index 0.60 (Avicel PH101, obtained from FMC, Ireland), were milled separately in a vibratory mill filled to 80% of volume with steel balls for 0 to 60 min. After milling, the samples were rubbed gently through a 50 pm sieve. All the treatments were performed at room temperature which is far below T temperature. The PAL temperature dependence data up to 200 °C do not show T onset. Detailed information about sample preparation was described elsewhere(iO). g
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Positron Annihilation Lifetime Spectroscopy. The positron annihilation lifetime spectra were acquired by detecting the prompt γ-ray (1.28 MeV) from the nuclear decay that accompanies the emission of a positron from the Na source and the annihilation γ-rays (0.511 MeV). A fast-fast coincidence circuit of a PAL spectrometer with the time resolution of 0.27 ns was used for PAL measurements. The positron source was NaCl sandwiched between Kapton foils. The sample powders without any packing or pressing were placed into the sample cell with the positron source sitting in the center. All the samples were measured under vacuum at 25 °C. Detailed description of PAL spectroscopy can be found elsewhere(4). 22
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Mean Free-Volume Hole Size and Fraction. The obtained PAL data were analyzed to finite term lifetimes using the PATFTT program (11). in these cellulose samples, it was found that three lifetime results give the best χ (
