Neutron Diffraction by Crystalline Polymers - ACS Publications

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 16, 2014 | http://pubs.acs.org Publication Date: July 30, 1999 | doi: 10.1021/bk-2000-0739.ch005

Chapter 5

Neutron Diffraction by Crystalline Polymers Yasuhiro Takahashi Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560, Japan

Neutron diffraction has the several advantages in comparison with X ray diffraction. Accordingly, new informations about the crystal structure could be obtained differing from the X-ray work. In the present study, two crystalline polymers, poly(vinyl alcohol) and polyethylene-d were studied by neutron diffraction. Different crystal structure models have been proposed for atactic poly(vinyl alcohol) by Bunn and Sakurada et al. The models differ principally in the azimuthal angle of the planar zigzag backbone and the hydrogen bonding network. In the present study, the reexamination of the crystal structure analysis was carried out by using both the X-ray and neutron diffraction methods. The crystal structure model proposed by Bunn is found to be correct. The ( F - F ) synthesis was made for the neutron data (100K), in which the hydrogen atoms to be associated with the hydrogen bonds are not incorporated into F calculation. On the map, three peaks were found, which may be attributed to the hydrogen atoms to be associated with the intramolecular hydrogen bonds between OH groups in an isotactic sequence and the two kinds of intermolecular hydrogen bonds. Neutron structure analysis of polyethylene-d was carried out for the data (equator) measured at 10K, 100K, 200K, and 300K by using the rigid body least-squares method, where the translational and librational displacements of the molecular chain were estimated by using the rigid body temperature factor reported by Pawley. The temperature 4

0

c

c

4

© 2000 American Chemical Society

In Scattering from Polymers; Cebe, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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dependences of the lattice parameters, a and b, were also estimated by the least-squares method. The φ value, the azimuthal angle of the molecular plane with respect to the b-axis, was estimated as 45° within the accuracy of the standard deviation 1° independent of temperature. From the translational and librational displacements at 10K, the static disorder of polyethylene was concluded to be mainly of the translational displacements.

INTRODUCTION Neutron diffraction has the several advantages in comparison with X-ray diffraction. (i> 2) Neutron is diffracted by atomic nucleus, while X-ray is diffracted by electron. Accordingly, the scattering length of an atom by neutron is independent of the atomic number. Hydrogen and deuterium have the same atomic number, i . e., the same chemical properties, although they have somewhat different physical properties,(3, 4) and have the same scattering length for X-ray diffraction, but they have the different scattering lengths for neutron (Table I). Table I. Scattering lengths of atoms by Neutron and X-ray. a

Neutron X-ray Coherent Incoherent 1 Η -3.74 25.22 D 6.67 1 4.03 6 C 0 6.65 7 Ν 9.37 1.98 8 Ο 5.80 0 aThe values at θ = 0°.

The scattering length by neutron is further independent of the scattering angle 0. The intensities of the reflections with large θ values can be observed strongly and can be measured accurately. This is especially the advantage for crystalline polymers, in which the intensities become weak with the Bragg angle θ because of the disorder contained in the crystalline region and low degree of orientation. Furthermore, absorption of neutron by most elements, for example, A l , is very small. Therefore, the apparatus for low- and high-temperature measurements can be designed easily and the measurements at low and high temperature are easy. Energy of neutron is small because of de Broglie wave. Therefore, it interacts with phonon, i . e., molecular



76 vibration (inelastic scattering) and also interacts with molecular motion (quasielastic scattering). Examples of the crystal structure analyses of crystalline polymers by neutron diffraction are limited so far.(5,6) This may be attributed to the incoherent scattering by hydrogen atom and to the weak scattering power of crystalline polymer. Incoherent scattering length by hydrogen atom is very large in comparison with the coherent scattering length (Table I). Therefore, it has been considered that the deuterated derivatives of polymers needed for neutron diffraction measurements. Crystalline polymers consist of both crystalline and amorphous regions, and the crystalline region contains a considerable amount of disorder. Diffraction intensity by crystalline polymer are, generally said, so weak and the power of neutron source is not so strong. Therefore, it has been considered to be difficult to measure the sufficient number of reflections accurately. In the present paper, neutron structure analyses of atactic polyvinyl alcohol) and polyethylene^ are successfully carried out. Especially, the former is the protonated polymer and includes statistical disorder. This suggests that neutron diffraction can be applied to a wider range of crystalline polymers.

N E U T R O N S T R U C T U R E ANALYSIS OF POLY (VINYL ALCOHOL)( ) ?

In 1948, C. W. Bunn(8) briefly reported the crystal structure of atactic polyvinyl

Figure 1. Crystal structure models of atactic polyvinyl alcohol) proposed by (a) Bunni ) and (b) Sakurada et al (9) Broken lines show hydrogen bonds. (Reprodu with permissionfromreference 7. Copyright 1997, Wiley-Inter Science) 8



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Figure 2. Neutron intensity distributions on the equator of poly (vinyl alcohol) (a)before and (b) after smoothing, which are measured at 100K. Al denotes the reflections due to aluminum foil. (Reproduced with permission from reference 7. Copyright 1997, Wiley-InterScience)



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alcohol) in which OH groups are statistically located on both sides of the molecular plane. Thereafter, Sakurada et al.(9 - π ) briefly reported the crystal structure model, which is different in the azimuthal angle of the molecular plane and hydrogen bonding network from Bunn's model. Figure 1 shows the two crystal structure models. The detailed X-ray structure analysis (Fourier method) of atactic polyvinyl alcohol) was made and briefly reported by Nitta et al., (12) where they supported Bunn's model. Neutron diffraction has an advantage that the hydrogen atoms contribute to the diffraction intensity more than X-ray diffraction. Therefore, it may be possible to determine the azimuthal angle more accurately than X-ray diffraction and to clarify the position of the hydrogen atoms to be associated with hydrogen bonds. In the present analysis, the structure analysis was carried out by using both the X-ray data by Nitta et al.(i2) and the neutron diffraction data newly collected.

Experimental The commercially supplied atactic polyvinyl alcohol)fiber(Unitika Co., Ltd.) was used as the sample. The specimen for X-ray diffraction measurements was made by arranging the fibers in a cylindrical bundle about 0.5 mm in diameter. X-ray measurements were made by CuKa radiation monochromatized by pyrolite graphite. Neutron diffraction experiments were carried out by high resolution powder diffractometer (HRPD) equipped on JRR-3M installed on Japan Atomic Energy Research Institute (JAERI) using λ = 1.8232 A. The specimen was made by arranging the fibers in a cylindrical bundle about 10 mm in diameter, covered by aluminum foil, and set into the aluminum sample tube with 10 cm diameter. Intensity distribution on the equator was measured every 0.05° from 2Θ = 5° to 165° at 100K, 200K, and room temperature. Due to the limited long range order of the sample, 1(29) data were recorded over the internal 5°

Neutron Diffraction by Crystalline Polymers - ACS Publications

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