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Abstract:The changes in viscosity and normal stress difference during the ageing of concentrated aqueous solutions of poly(vinyl alcohol) prepared at 80 ~ and the effect of the procedure used in the preparation of solutions on the course of these changes are described. The results are interpreted by means of shear compliance ]e and relaxation time 3~Key words: Polyvinylalcohol, ageing, concentrated solutions, non-Newtonian viscosity, normal stress.Aqueous solutions ofpoly(vinyl alcohol) (PVA) undergo the process of ageing, reflected predominantly in a rise in their viscosity with time which in concentrated solutions is followed by gel formation. This effect has been known for a long time and is the cause underlying the formation of aggregated supermolecular structures [1][2][3]. The formation of such structures is affected by the tacticity of the polymer and by the content of residual acetate groups [4]. These structures are supposedly formed also in dilute solutions [5] and contain amorphous and paracrystalline parts [5][6][7], the mutual ratio of which depends on experimental conditions during the preparation of solutions, and predominantly on the dissolution temperature. On the other hand, however, the process of ageing is affected not only by the dissolution temperature, but also by the temperature at which the ageing takes place, by the temperature prehistory of solutions, by the polymer concentration in solution and by the kind of the solvent [8][9][10][11]. In the existing studies dealing with the / problem of ageing of PVA solutions partial results are mostly presented in a descriptive manner, which does not lead to a more general and deeper characterization of theological changes in the supermolecular structures which arise during ageing. The only attempt at a quantitative explanation of the rheological behaviour of concentrated PVA solutions has been made by Nakagaki and Nishii [12]. Their interpretation is based on Rouse's theory of elasticity which holds for linear K 955 aggregates and intermolecular aggregation. In addition to a rise of viscosity with time, the formation of supermolecular structures during the ageing of concentrated PVA solutions is also reflected in their non-Newtonian behaviour [13]. So far, this effect has not been studied in greater detail, which made us use it, along with the measurement of normal stress, in a detailed investigation of the process of ageing.The objective of our investigations of concentrated aqueous PVA solutions consists in a systematic study of rheological properties during their ageing as a function of temperature, the experimental conditions during dissolution of the polymer and of the polymer concentration. In the present study we have combined viscosity and normal stress measurements. The quantitative interpretation of the results of our measurements was based on the values of shear compliance calculated from the measured values of the first normal stress difference, and of the relaxation time To calculated from the experimental dependence of r//r/o on ...

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