research papers 62 Jan van der Elsken et al.  PVP±water mixtures J. Appl. Cryst. (2001). 34, 62±64 Journal of Applied Crystallography ISSN 0021-8898 Received 13 July 2000 Accepted 27 November 2000 # 2001 International Union of Crystallography Printed in Great Britain ± all rights reserved Small-angle X-ray scattering by PVP±water mixtures Jan van der Elsken,* Wim Bras and Jan Michielsen FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands. Correspondence e-mail: library@amolf.nl Small-angle X-ray scattering experiments reveal the formation of large-scale structures when a 60 wt% poly(vinylpyrrolidone) (PVP)±water mixture is cooled to 260 K. The formation of these structures leads to an enhancement of continuous small-angle scattering with decreasing temperature. This is accompanied by the appearance of sharp Bragg peaks that have a very short lifetime. The scattering angles of these peaks are in accordance with a hexagonal columnar structure. It appears that such structures occasionally live long enough to undergo rotational Brownian motion. 1. Introduction Poly(vinylpyrrolidone)±water solutions show a remarkable behaviour at temperatures below the normal freezing point of water, in particular at concentrations above about 57 wt% PVP, where the solutions show a peculiarity in the phase diagram known as `unfreezable water'. This term expresses the fact that the liquidus of the temperature versus composition (T±x) binary phase diagram, which starts at low concentration in a perfectly normal way, bends sharply downwards and seems to become vertical at about 60 wt% polymer. Cooling of such high-concentration solutions does not result in segrega- tion of ice; there is just a rapid increase in viscosity. Such a diverging slope of the liquidus in a T±x phase diagram cannot be understood in terms of a system of two components in thermodynamic equilibrium. Evidence has been given for the existence of a glass transition at temperatures ranging from about 230 to 280 K for concentrations of about 60 to 75 wt% PVP (M = 30 000 g mol 1 ) (Franks, 1982). These data on states and transitions are based on vapour-pressure and calorimetry measurements (MacKenzie & Rasmussen, 1972). More recent dielectric studies (Shinyashiki et al., 1994) on M = 10 000 g mol 1 solutions con®rm the phase diagram, but esti- mate the glass transition temperatures to be much lower, namely 158±165 K. We report here the results of scattering measurements on PVP (Fluka, K15, M = 10 000 g mol 1 ) solutions in water of concentration 60 wt% PVP. 2. Experimental Temperature recordings during the X-ray scattering experi- ments showed that small amounts of heat are released inter- mittently, during cooling as well as during periods when the temperature is held constant, down to 250 K. Similar obser- vations were made for a 65 wt% and a 70 wt% PVP sample. The estimated amount of heat released is of the order of 10 J g 1 in all cases. For the small-angle X-ray scattering (SAXS) experiment, a small drop of the solution was held between two shallow brass cups, which were cooled with the aid of Peltier elements. The distance between the cups could be varied in such a way that an approximately cylindrical drop, with 5 mm diameter and 5 mm height, of free surface was obtained. Scattering from cell walls was avoided in this way. Most of the X-ray scattering experiments were performed at the Synchrotron Radiation Source at Daresbury, UK. Scattering was measured as a function of the scattering angle  from the direct beam, de®ning a scattering wavevector modulus q = (4/)sin , with  = 0.15 nm, the wavelength of the X-rays. A quadrant detector was used in conjunction with a curved delay-line detector. This allows us to measure both the small- (SAXS) and the wide-angle X-ray scattering (WAXS) intensity simultaneously. Diffractograms could be collected every 20 s. 3. Results Two key observations were made in the X-ray scattering experiments at temperatures below approximately 263 K. First, pronounced continuous scattering at small angles is observed (Fig. 1), indicating the presence of large-scale structures. The typical length scale associated with the scat- tering is 2/q = 30 nm. This length scale does not change when the temperature is lowered to 250 K; the amplitude, however, increases considerably (Fig. 1, inset). The second observation is that in roughly 10% of the 20 s frames, sharp scattering peaks appear, at a wavevector value of 0.1 nm 1 and higher, on top of the rising scattering curve. Such peaks appear mostly in one or two 20 s frames only, and have a varying ratio between the width and the value of the scattering vector, Áq/q. It is signi®cant that the narrower and more intense peaks often appear in combinations, while the broader peaks are mostly solitary. In Bragg scattering, the Áq/q ratio is proportional to the ratio of the unit length in a periodic array, to the total magnitude of that array, i.e. the number of units. Moreover, an