Elsevier

Polymer

Volume 41, Issue 7, March 2000, Pages 2711-2716
Polymer

Polymer Communication
Identification of the formation of aggregates in PEO solutions

https://doi.org/10.1016/S0032-3861(99)00657-6Get rights and content

Abstract

The properties of poly(ethylene oxide) PEO chains (molecular weight Mw: 6500) have been examined in dilute solutions in methanol using static (SLS) and dynamic (DLS) light scattering. The ability of PEO chains to aggregate depends on the history of the sample. Thus two relaxation times have been observed in the correlation functions of the intensity of the light scattered by PEO in methanol when the sample was previously dissolved in hot water (t≥60°C). The fast mode is attributed to the well-solvated monomolecular species and the slow mode is because of the formation of aggregates that are generated by hydrophobic interactions. These aggregates are well defined, relatively monodisperse and very stable. No aggregation is observed for PEO in methanol when the sample has been previously dissolved in a lot of solvents such as water (t=30°C), chloroform, dioxane, dimethylsulfoxyde (DMSO), dimethylformamide (DMF) or acetonitrile.

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 distilledwater (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)

  • W. Brown

    Polymer

    (1985)
  • H.G. Elias et al.

    Makromol Chem

    (1966)
  • K. Devanand et al.

    Nature

    (1990)
  • K. Devanand et al.

    Macromolecules

    (1991)
  • S. Kinugasa et al.

    Macromolecules

    (1994)
  • S. Kinugasa et al.

    J Polym Sci, Polym Phys Ed

    (1996)
  • W.F. Polik et al.

    Macromolecules

    (1983)
  • Y. Layec et al.

    J Phys Lett (Paris)

    (1983)
  • W. Brown et al.

    J Polym Sci, Polym Phys Ed

    (1983)
There are more references available in the full text version of this article.

Cited by (0)

View full text