CP/MAS ¹³C NMR Analyses of the Chain Conformation and Hydrogen Bonding for Frozen Poly(vinyl alcohol) Solutions

Abstract

CP/MAS 13C NMR analyses have been made for different frozen solutions of poly(vinyl alcohol) (PVA) samples with different tacticities to obtain further information about the intramolecular hydrogen bonding of PVA. Aqueous or deuterated dimethyl sulfoxide (DMSO-d 6 ) solutions (3 wt %) have been frozen in a rotating MAS rotor by cooling to −50 °C. The CH resonance lines of the frozen PVA solutions split in different ways, significantly depending on the tacticities and the solvents. These CH resonance lines can be well resolved into six to eight constituent lines by the least-squares method, whose lines have different chemical shifts evaluated by assuming the downfield shifts due to the intramolecular hydrogen bonding and the upfield shifts induced by the γ-gauche effect. Furthermore, the relative intensities of the lines thus resolved are successfully interpreted for the frozen atactic PVA solutions by the statistical treatment assuming the statistical formation of the intramolecular hydrogen bonds and the random distribution of the trans and gauche conformations along the PVA chains. The probabilities of the trans conformation and the intramolecular hydrogen bond are also determined through this analysis for atactic PVA samples. In contrast, such calculations based on Bernoullian statistics cannot be applied to the cases of the frozen isotactic PVA solutions, suggesting that certain specific conformations are preferably formed for the isotactic PVA samples.

Solid-State 13C NMR Studies of the Structure and Chain Conformation of Thermotropic Liquid Crystalline Polyether Crystallized from the Liquid Crystalline Glassy Phase

“…To confirm such a suggestion, 13 C spin-lattice relaxation behavior has been examined in detail and the spectra of the crystalline and noncrystalline components have been separately obtained by using their difference in T 1C . (2) The evaluation of the 13 C chemical shift values of the spacer CH 2 carbons in the respective spectra mainly by the -gauche effect reveals that the CH 2 sequences adopt the t 0 xxxtxxxt 0 conformation in the crystalline region while their preferable conformation is t 0 xxxxxxxt 0 in the noncrystalline region.…”

“…We have characterized in detail the structure and chain conformation for the form sample of a new type of thermotropic LC polyether (EDMB-10), which was crystallized from the LC glassy phase, by solidstate 13 C NMR spectroscopy and obtained the following conclusions: (1) CP/MAS 13 C NMR spectrum of the form sample suggests that the chain conformation of the spacer CH 2 sequences and mesogen groups in this sample may be significantly different from those previously reported for the form sample that was crystallized by slowly cooling from the melt through the nematic phase. To confirm such a suggestion, 13 C spin-lattice relaxation behavior has been examined in detail and the spectra of the crystalline and noncrystalline components have been separately obtained by using their difference in T 1C .…”

“…13 C NMR Measurements Solid-state 13 C NMR measurements were mainly performed at room temperature on a Chemagnetics CMX-200 spectrometer equipped with a JEOL solidstate NMR system operating at 50.0 MHz under a static magnetic field of 4.7 T. 1,7,10-13 1 H and 13 C radio-frequency fields B 1 =2 were 62.5 kHz in the cross polarization (CP) process and the 1 H decoupling field strength was reduced by about 10%. The contact time for the CP process was 1 ms and the recycle time after the acquisition of a free induction decay (FID) was 5 s throughout this work.…”

“…However, this is not the case shown in Figure 3a. The assignment, 13 C chemical shift values, and the spacer conformation thus determined for the crystalline component are shown in Figure 3a, Table II, and Figure 4, respectively. For comparison, the spacer conformation of each component in the form sample is also shown in Figure 4.…”

“…However, there is no significant difference in molecular mobility in the noncrystalline region between the form and form samples, although the mesogen groups are subjected to much more enhanced motion in this region compared to those in the crystalline region. To examine the molecular motion of the mesogen units in more detail, 13 C CSA spectra of the respective carbons have been measured for the form sample in a similar way to the case of the form sample 7 by 2D MAT 13 C NMR spectroscopy.…”