Introduction of Disorder in the Crystal Structures of Atactic Poly(vinyl Alcohol) and Its Iodine Complex To Solve a Dilemma between X-ray and Neutron Diffraction Data Analyses

Abstract

For the last one century, the crystal structure of atactic poly­(vinyl alcohol) (at-PVA) is assumed to have been established on the basis of the quantitative analysis of X-ray diffraction data. A similar situation is also seen for the crystal structure of PVA–iodine complex. However, it has been found that these crystal structures cannot reproduce the recently measured wide-angle neutron diffraction data satisfactorily. This dilemma between the X-ray and neutron diffraction data analyses was solved totally by the introduction of structure disorder into the previously proposed regular structure models. The concept of disorderliness has allowed us to interpret the characteristic structural features of these substances more concretely: (i) PVA crystals show a preferential orientation of the 110 planes along the rolled plane of the doubly oriented sample. The 110 planes are composed of hydrogen-bonded sheets of zigzag chains, which cause the disorderliness because of the easy slippages by the application of shear stress. (ii) In the formation mechanism of the PVA–iodine complex, the disordered packing structure of PVA and iodine ions is formed at first (form I), and it develops to form II with more or less regularly packed PVA–iodine layers, reaching finally the structure of form III with regularly arrayed PVA–iodine layers. The present study is one good example to show the significance of the coordinated analysis of X-ray and neutron diffraction data for erasing the ambiguities of the previously proposed crystal structure of a synthetic polymer.

Influence of Tacticity on the Crystal Structures of Hydrogenated Ring-Opened Poly(norbornene)s

“…The highly crystalline character of the atactic H-poly­(NB) sample is originated from the good packing of the relatively compact chains. The situation is similar to crystalline polymers having an atactic configuration such as atactic poly­(acrylonitrile), atactic poly­(vinyl alcohol), − and so on. Judging from the van der Waals radius of the H atom (1.2 Å), the closest intermolecular H···H distances are about 2.4 Å.…”

Hybridization of Wide-Angle X-ray and Neutron Diffraction Techniques in the Crystal Structure Analyses of Synthetic Polymers

“…Recently, we have encountered a serious problem, in which the structure model derived by the X-ray analysis at a high accuracy could not reproduce the observed WAND data at all. This is the case of the structure analysis of at-PVA and its iodine complex [ 31 ].…”

“…We needed to establish the structure. We measured the WAXD data again using a Mo-Kα X-ray beam of shorter wavelength, which was helpful to collect more diffraction data [ 31 ]. The observed diffraction data, in particular, the layer line profiles were not reproduced very well by the latter model.…”

“…Then, we tried to check these structures on the basis of the WAND data analyses. The WAND patterns were measured using the deuterated at-PVA sample and its iodine complex (form II) [ 31 ]. The WAND data were measured using the above-mentioned two types of the neutron diffractometers so that any inconsistency was not remained in the neutron data themselves.…”

Релаксационные Процессы В Нанокомпозитной Системе Поливиниловый Спирт–йод–йодид Калия

“…Установлено, что процесс формирования кристаллической структуры ПВС-йод происходит по мере обезвоживания студнеобразной системы ПВС-йод. Упорядоченные области образуются при удалении молекул воды, внедренных между полимерными цепями и препятствующих структурообразованию [5][6][7][8]. В [9] обнаружено, что на одну молекулу I 2 приходится 24 мономерных звена ПВС, и это составляет два витка спирали.…”