Viscoelasticity of Biomaterials - American Chemical Society

Chapter 23

Solution Properties of a Hydrophobically Associating Cellulosic Polymer Downloaded by STANFORD UNIV GREEN LIBR on October 12, 2012 | http://pubs.acs.org Publication Date: May 8, 1992 | doi: 10.1021/bk-1992-0489.ch023

Electron Spin Resonance Spectroscopy P . A . Williams, J. Meadows, Glyn O. Phillips, and R . Tanaka North East Wales Institute of Higher Education, Connah's Quay, Clwyd C H 5 4BR, Wales Electron spin resonance spectroscopy has been used to investigate the solution properties of a hydrophobically associating cellulosic polymer. Nitroxide spin labels covalently attached to the cellulosic backbone have given information with regard to the segmental motion of the polymer chains, whereas nitroxide spin probes have demonstrated the formation of regions of hydrophobicity above a critical polymer concentration. The data is consistent with the formation of an extensive three-dimensional network i n which the hydrophilic cellulosic backbones are effectively cross-linked by the intermolecular association of neighbouring hydrophobic side chains. The work has been extended to study the interaction of the polymer with sodium dodecyl sulphate surfactant and the electron spin resonance data has been used to elucidate the mechanism of interaction, and to explain the unusual rheological behaviour. Hydrophobically associating polymers are finding increasing industrial use due to their ability to impart improved rheological behavior to particulate dispersions (1,2). Consequently, there have been a number of research publications concerning such polymers over recent years (3-13). Essentially, these polymers consist of a hydrophilic backbone and possess a small number of hydrophobic side chains, usually i n the range of 8 to 40 carbon atoms in length. Whilst the nature of their backbone usually renders the polymer soluble in aqueous media, intermolecular association of the hydrophobic groups leads to the formation of a weak three-dimensional network structure giving rise to solutions of very high viscosity at low shear rates (8). Addition of surfactants to solutions of hydrophobically associating polymers has also been shown to have a dramatic effect on the rheological properties 0097-6156/92/0489-0341$06.00/0 © 1992 American Chemical Society

In Viscoelasticity of Biomaterials; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

342

VISCOELASTICITY OF BIOMATERIALS

(8-11). For example, Gelman (11) found that the addition of 0.04% sodium oleate to a 0.7% aqueous solution of hydrophobically modified hydroxyethyl cellulose produced an approximate hundred-fold increase in the Brookfield viscosity of the solution. Nitroxide spin labels and spin probes are ideally suited to studying the dynamics of these systems since the nitroxide free radical is able to monitor molecular events i n the 1 0 " to 1 0 " s timescale. Spin labels, which (by definition) are covalently attached to the backbone, can give information regarding the segmental motion of the polymer chains, whilst specially selected spin probes present in solution (but not covalently attached) can monitor the formation of intermolecular hydrophobic associations. This paper reports on the use of electron spin resonance spectroscopy ( E S R ) to study the solution properties of hydrophobically modified hydroxyethyl cellulose ( H M H E C ) and its interactions with the anionic surfactant, sodium dodecyl sulphate (SDS).

Downloaded by STANFORD UNIV GREEN LIBR on October 12, 2012 | http://pubs.acs.org Publication Date: May 8, 1992 | doi: 10.1021/bk-1992-0489.ch023

7

1 1

Materials H M H E C was kindly supplied by Aqualon ( U K ) L t d . , Warrington, U K , under the trade name Natrosol Plus Grade 330. The manufacturer reports the polymer to have a molecular mass of approximately 250,000, a molar substitution of 3.3, and to contain approximately 1-2% of chemically grafted C12-C24 alkyl side chains. A portion of the H M H E C was spin labelled with 4-amino Tempo (Sigma Chemicals L t d . , Poole, U K ) as previously described (9). The spin label attaches to hydroxyl groups of the glucose residues and it was estimated that there was 1 spin label per 7,000 residues. This low degree of labelling ensures minimum perturbation of the polymer characteristics. The spin probe used was 5-doxyl stearic acid (5-DSA; Sigma Chemicals Ltd.), and was used as supplied. Methods ESR Spectroscopy. The nitroxide free radical gives rise to a well characterized three-lined E S R spectrum and the relative shapes and intensities of the lines are a reflection of the mobility of the nitroxide moiety. If the motion of the nitroxide radical is unrestricted, then the three lines are relatively narrow and are of similar intensities. However, as the mobility of the free radical is reduced, line broadening occurs due to anisotropic effects. In spin label experiments where the nitroxide moiety is covalently attached to the polymer chain, it is argued that the shape of the E S R spectrum closely reflects the segmental motion of the polymer (14). This is illustrated in F i g ure 1, which shows the E S R spectra for spin labelled hydroxyethyl cellulose in 80% v / v aqueous glycerol as a function of temperature. A t high temperatures (spectrum a) where the solution viscosity is lowest, the motion of the polymer segments is relatively unrestricted, resulting in a mot ion ally narrowed isotropic spectrum. As the temperature is reduced and the solution viscosity increases, the segmental motion of the polymer is reduced


Hydrophobically Associating Cellulosic Polymer 343


23. W I L L I A M S E T A L .

Figure 1. E S R spectra of a 2% solution of spin labelled H M H E C i n 80% glycerol at (a) 80°C; (b) 60°C; (c) 40°C; (d) 3 0 C ; and (e) 5°C. e


344


resulting in line broadening (spectra b-e). For isotropic spectra the rota tional correlation time r of the label can be calculated using the equation of Stone et al. (15), which shows that: c

where Λ_ι, Λ and Λ+ι are the heights of the high field, central and low field lines respectively, and Wo is the line width of the central line. Zhao et ai (16) calculated the constant, K, to be 6.08 x 1 0 ~ , assuming the hyperfine coupling tensor to have values A = 32G, A and A = 5G. The spin probe experiments were carried out using 5-DSA which is amphiphilic i n nature and thus can be expected to preferentially reside close to any regions of hydrophobicity present within an aqueous environment. This is illustrated in Figure 2, which shows the E S R spectra for 5-DSA (a) in water and (b) in the presence of SDS micelles. In the former environment, the probe undergoes rapid tumbling, giving rise to an isotropic spectrum of typical correlation time r = ~ 2.3 x 1 0 " s. In the latter, the spin probe prefers to reside within the hydrophobic SDS micelles and this results in a reduction in its molecular motion, giving rise to some line broadening. The value for r in this case is calculated to be ~ 1.5 x 1 0 " s. In the spin probe experiments, all solutions were prepared by dissolving the appropriate amount of polymer and/or SDS in a slightly alkaline (pH 9) aqueous solution of 5 x 10~ mol d m " 5-DSA. H M H E C solutions were prepared by stirring continuously for at least 18 hours before use to ensure complete dissolution. In experiments performed in the presence of SDS, the polymers were completely solubilized before the addition of the appropriate amount of surfactant. The E S R spectra were recorded at 20°C on a J E O L J E S M E I X X band spectrometer ( J E O L , Japan) using a quartz cell suitable for aqueous solutions.


0

10

zz

xx

yy

1 0

c

9

c

6

3

Shear-Flow Viscosities. The viscosities of aqueous solutions of H M H E C of various concentrations were determined over the shear rate range 0-10 s " using a C a m m e d CS100 controlled stress rheometer ( C a m m e d Instruments L t d . , Dorking, U K ) . Measurements were performed at 20°C using either a 4 cm 2° or a 2 cm 2° cone and plate attachment. Each measurement was performed in duplicate. 1

Oscillatory Measurements. The storage and loss moduli ( G and G " re spectively) of 2% aqueous solutions of H M H E C at 20°C containing various amounts of added SDS were recorded at an amplitude of 6 χ 10 radians over the frequency range 1 0 " — 10 Hz using a C a m m e d CS100 controlled stress rheometer fitted with either a 4 cm 2° or a 2 cm 2° cone and plate attachment. ;

2


Hydrophobically Associating Cellulosic Polymer 345


23. WILLIAMS ET AL.


346


Results The viscosities of aqueous solutions of H M H E C at a shear rate of 1 s " are given as a function of polymer concentration i n Figure 3. The viscosities of the solutions are seen to increase exponentially over the concentration range studied. The E S R spectra of spin labelled H M H E C i n aqueous solution over a similar concentration range are given i n Figure 4. The letters on the spectra correspond to the letters on the viscosity/concentration curve given in Figure 3. The spectra obtained were all motionally narrowed indicating a high degree of segmental motion. r for all the spectra was calculated to be 1.2 ± 0.2 χ 10~ s, irrespective of the polymer concentration. Figure 5 gives the E S R spectra of 5-DSA in aqueous solutions of various concentrations of H M H E C . A t relatively low polymer concentrations the observed spectra (spectrum a) indicates the spin probe has a very high degree of mobility with r having a value of ~ 2.3 x 1 0 " s. However, above a polymer concentration of approximately 0.2% (9), the spectra is seen to contain both isotropic and anisotropic components, with the proportion of the latter increasing with increasing polymer concentration. Computer analysis indicates that at a polymer concentration of 1.5% the anisotropic component corresponds to approximately 70% of the signal. The composite spectra are believed to arise from the partitioning of the spin probe into hydrophobic regions created through the intermolecular association of the polymer molecules. The effect of added SDS on G ' (at a frequency of 1 Hz) of 2% aqueous solutions of H M H E C is given in Figure 6. The value of G ' is seen to increase markedly with increasing surfactant addition up to an SDS concentration of approximately 8 x 10~ mol d m " . Above this value, however, further additions of surfactant produce a progressive decrease in the values of G ' . A t sufficiently high concentrations of added SDS, the storage modulus of the polymer/surfactant system is actually lower than that of 2% H M H E C in the absence of any surfactant. The variation of G ' and G " of a 2% H M H E C solution alone, and in the presence of 8 χ 1 0 " mol d m " SDS are given as a function of frequency of oscillation in Figure 7. The values of both G ' and G " are considerably increased in the presence of surfactant and furthermore G ' is greater than G " over a wider frequency range. This closely reflects the difference in the appearance of the samples. In the absence of SDS, the sample is fluid, whereas in the presence of this concentration of SDS, the sample appears gel-like. However, the E S R spectrum of a 2% aqueous solution of spin labelled H M H E C in the presence of 8 x 1 0 " mol d m " SDS is given in Figure 8 and is almost identical to that for the polymer in the absence of SDS indicating that the segmental motion of the polymer is virtually unchanged. The E S R spectra of 5-DSA in the presence of 0.2% H M H E C and var ious amounts of SDS are given in Figure 9. This polymer concentration corresponds to the maximum concentration which will not itself affect the observed E S R spectrum of 5-DSA (9). It is seen that the mobility of the spin probe is progressively reduced, indicating the formation of regions of 1

c


9

1 0

e

3

3

3

3

3

3



23.

WILLIAMS ET A U

Hydrophobically Associating CeUulosic Polymer 347

Polymer Concentration / % Figure 3. The effect of polymer concentration on the viscosities of aqueous solutions of H M H E C at a shear rate of 1 s " . 1

Amertcao Chemtea! Socfeftf

Library 1155 Wth St.. HM» In Viscoelasticity of Biomaterials; Washington, D C Glasser, 20036W., et al.;

ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


348


J ί!/

! j II ί ! V

i

ι

A f

m

Λ

IV

'

/

! ί

V

/I

b

/

Figure 4. E S R spectra of spin labelled H M H E C in aqueous solution at concentrations of (a) 0.5%; (b) 1.0%; and (c) 2.0%.



23. W I L L I A M S E T A L

Hydrophobicalfy Associating Cellulosic Polymer 349

c » . . À

Π Ί ί M !ί /

|ij u

(J

H

Figure 5. E S R spectra of 5 x 10~ mol d m 5-DSA in aqueous solutions of (a) 0.1% H M H E C ; (b) 0.4% H M H E C ; (c) 0.75% H M H E C ; and (d) 1.5% HMHEC. 6

- 3


350


500

4001

Viscoelasticity of Biomaterials - American Chemical Society

Recommend Documents