Polymer CommunicationIdentification of the formation of aggregates in PEO solutions
Introduction
For about 30 years, poly(ethylene oxide) (PEO) in solution has been the subject of many studies involving a lot of laboratories and a lot of techniques. These studies have led to somewhat contradictory results concerning the ability of PEO to be perfectly soluble in dispersed species in some solvents or to form aggregates in these same solvents. For example one of the first work by Elias et al. [1] on the couple PEO/water shows that PEO is completely soluble in aqueous solution. This result was confirmed recently by Devanand et al. [2], [3] and Kinugasa et al. [4], [5]. However, the formation of aggregates on the same system has been reported in many other studies [6], [7], [8], [9], [10], [11]. Strazielle [12] has shown, by light scattering measurements on samples of various molecular weights, that the ability of PEO to form aggregates in aqueous solutions depends on the molecular weight of the sample and on the method of preparation of the solutions. The same contradictory results have been obtained for PEO in methanol that has been found to be partially associated [13] or well solvated [1], [2], [3], [4], [5], [7], [14]. Even when the results tend to ascertain the presence of aggregates, the factors that control their formation are not well identified. Thus, at low molecular weight, one can expect that crystallisation or aggregation occur under certain condition [12]. Many hypothesis have been expressed to explain the formation of aggregates including the presence of impurities, the formation of hydrogen bonds [15], the formation of complex entities associated with the presence of residual water molecules and with hydrophobic interactions [6], [16].
The purpose of this study is to clarify the situation with regard to the formation of aggregates in the solutions of PEO. In order to do that, a PEO sample has been submitted to different manipulations. It is shown that the aggregation is dependent on the history of the samples. It depends especially on the fact that one of the manipulations is the dissolution of the sample in water in a given range of temperature.
In this way the many contradictory results which are reported in the literature can be attributed to two different causes. Firstly the authors do not always know what exactly is the history of the samples they use. Secondly the techniques of investigation have not the same sensibility with regard to the formation of large particles. For example it is well known that the light scattering technique is much more sensitive to the presence of a small amount of large species than the viscosity.
Section snippets
Sample E0
The sample, which is used as reference, is a commercial sample PEO-6000 from Hoechst (Frankfurt, Germany). It is named E0 in the remainder of the paper. Gel Permeation Chromatography (GPC) in (0.1 N NaCl) gave a polydispersity of 1.04 (number-average molecular weight: Mn=6850; weight-average molecular weight: Mw=7100) while Static light scattering (SLS) in methanol gave a value of Mw=6500 that agrees with the GPC value.
Samples E1→E8
In order to characterise the influence of the history of the
Direct measurements on E0 in water, water 0.1 N NaCl and methanol
At the outset SLS and dynamic light scattering (DLS) measurements were carried out on E0 dissolved in water, water 0.1 N NaCl and methanol. The inverse of the scattered intensities varies linearly with the square of the scattering wave vector showing no downwards curvature characteristic of the presence of large molecular species (aggregates) beside small ones. A typical correlation function and the distribution function of the diffusion coefficient is shown in Fig. 1. In these three solvents
Conclusion
We are able to reproduce faithfully the formation of aggregates in the PEO solutions which is connected with some peculiar situations that are investigated in this study. It is the association of the temperature effect (t>30°C) and the nature of the solvent (water) which is the main factor governing this formation. Once they are created these clusters are difficult to brake and are still present when the sample is dissolved in very good solvent of the polymer. The many contradictions that
References (19)
Polymer
(1985)- et al.
Makromol Chem
(1966) - et al.
Nature
(1990) - et al.
Macromolecules
(1991) - et al.
Macromolecules
(1994) - et al.
J Polym Sci, Polym Phys Ed
(1996) - et al.
Macromolecules
(1983) - et al.
J Phys Lett (Paris)
(1983) - et al.
J Polym Sci, Polym Phys Ed
(1983)