Formation and Viscoelastic Properties of Cellulose Gels in Aqueous Alkali

Abstract

Detailed gelling conditions of cellulose-9 wt.% aqueous NaOH solution system were investigated by kinematic viscosity and light scattering methods. The system has two gelation temperatures in the room temperature region. The gel generated at a higher temperature region (HTG) is thermally irreversible, while the gel formed at a lower temperature shows thermal reversibility. Sol-gel transition cannot be explained in terms of liquid-liquid phase separation. The viscoelastic behavior accompanied with sol-gel transition, and the thermal properties of the HTG, such as swelling in the solvent medium and syneresis, were also examined. Storage modulus in the transition increases monotonically with time over 100 hr and more, and the syneresis continues for at least one month when kept above 30°C. From these findings, HTG is concluded to be in a pseudo-equilibrium state in the temperature range of 30-50°C.Recently polysaccharide gel has been receiving considerable attention because of its wide potential applicability to industrial fields such as medicine and civil engineering. Gelation phenomena of cellulose derivative solutions, for example, the methylcellulose-water system on cooling (1), and the cellulose nitrate-organic solvent system on warming (2), have been studied so far, and especially for the former system, detailed conditions of gelation and some thermal properties of these gels have emerged. With respect to cellulose solutions, it has been well known for a long time that the gel arises by vaporization of ammonia from a concentrated cellulose cuprammonium solution. However, contrary to cellulose derivatives, a systematic investigation on the gelation behavior of cellulose solutions has never before been performed. The fact that a stable and simple solvent, in which cellulose

NMR Study on the Dissolved State of Polyacrylonitrile in Various Solvents

“…Therefore, the larger (w + e 1 ) value corresponds to the larger degree of direct interaction and the value is lowest for aq HN0 3 . Keeping the above discussion in mind the result shown in Table II are summarized as follows: (1) The order of or is DMAc> DMSO»DMF>HN0 3 , hence the degree of the shielding effect from the interacted solvent (double bond part) is also in the above order, (2) the larger is the apparent 6 1 effect on CH carbon centered at mr triad sequence, unexpectedly the smaller is the w + e 1 effect, indicating the specific interaction between polymer and solvents, that is, this fact implies that -CN loses a nature of triple bond but does'nt make the n electron conjugate system, (3) the order of magnitude of b' is similar to band being HN0 3 »DMSO>DMF»DMAc, suggesting that the strength of interaction of solvent to -C = N is in the above order, (4) the order of magnitude of w + e', which is a measure for the degree of direct interaction to -CH proton in polymer, is DMAc>DMF» DMSO>HN0 3 , (5) 0' 2 -0' 1 evaluated by three methods is constant in most solvents (DMSO ( = 0.078 ppm)> DMF = DMAc ( = 0.073 ppm)» HN0 3 (0 ppm) (NaSCN (0.046---0.030 ppm) is positioned just between DMAc and HN0 3 )), (6) for all solvents 0' 2 -0' 1 =e holds (therefore, e=Oppm for HN0 3 ) when evaluated from the chemical shift difference between mmrr and rmrm peaks, and interestingly for organic solvents the evaluated 0' 2 -0' 1 (=e) values fall almost on 0.073-0.078, (7) some exceptions appear among the e values evaluated by four other methods depending on solvent nature (DMF, e ( = 0.097 ppm) evaluated from the chemical shift difference between mrrm and mrrr; DMSO, e ( = 0.058 ppm) evaluated from mmrm-mmrr); DMAc (0.024, 0.097 ppm) evaluated from mmrm-mmrr and mrrm-mrrr (or mrrr-rrrr), (8) Facts (5), (8), and (9) indicate that the interaction of HN0 3 with -C = N in mm and mr sequence of polymer is quite different from that for other organic solvents, which basically form n electron conjugate system with -C = N more or less, and might correspond to the facts reported by Yamazaki 16 (see, 4) and 5) in the present text). Facts (3) and ( 4) seem to correlate with so-called solvent power against highly isotactic PAN.…”

“…8 On the one hand, Kamide et a!. 9 reported that cellulose solutions obtained by dissolving cellulose with different crystal forms into a same solvent (9 wt% aq sodium hydroxide or cadoxen) exhibited quite different flow birefringence phenomena. Authors 10 also have pointed out that cellulose acetates dissolved in different polar solvents showed the specific solvation form depending on solvent employed.…”