Langmuir 1996, 12, 3779-3782
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Amphiphilic Derivatives of Propylene Glycol Alginate: A Revisit of Their Physicochemical Behavior in Dilute Aqueous Solution A. Sinquin, M. C. Houzelle, P. Hubert,* L. Choplin,† M. L. Viriot,‡ and E. Dellacherie LCPM-URA CNRS 494, GEMICO, DCPR (GRAPP)-URA CNRS 328, ENSIC, 1 rue Grandville, BP 451, 54001 Nancy Cedex, France Received November 14, 1995. In Final Form: May 28, 1996X The physicochemical behavior in aqueous solution of propylene glycol alginate and of its hydrophobicallymodified derivatives is governed, especially in the dilute regime, by the occurrence or the absence of both long range electrostatic repulsions and hydrophobic attractions. According to the salt concentration in the aqueous polymer solution under concern, the resulting conformations may be fully expanded or, on the contrary, shrunken and tight. Such a difference results in some discrepancies in the viscometric data, depending on whether the measurements are performed by a high shear capillary technique or with a low shear rotatory instrument. Further indications of the behavior of these derivatives in dilute aqueous solution are drawn from fluorescence and surface tension correlations.
Introduction In solution, the fractional increase in viscosity due to the presence of macromolecules (solute) is defined as the specific viscosity:
η - ηs ηsp ) ηs
(1)
with η the solution viscosity and ηs the solvent viscosity. Because the degree of viscosity enhancement depends on the amount of dissolved material as well as on molecular size, a more fundamental quantity, the reduced viscosity, is defined as
ηred )
ηsp c
(2)
In the limit of infinite dilution, ηred characterizes the fractional viscosity increase due to each isolated macromolecule and is known as the limiting viscosity number or the intrinsic viscosity:
[η] ) lim(ηred)
(3)
cf0
The intrinsic viscosity has units of volume per unit mass. It is then thought of as the effective hydrodynamic volume of the macromolecule in solution or, according to Wolff et al.,1 as a measure of the shape and size of the macromolecule. Intrinsic viscosity depends not only on the molecular weight, the structure, and the conformation of the macromolecule in solution but also on the temperature and the solvation state. Furthermore, the intrinsic viscosity depends on shear rate in much the same way as the viscosity of the macromolecular solution. In the particular case of relatively high shear rate, the macromolecules may be deformed in the direction of flow, and the resistance to flow, or in other words the viscosity measurement, will be affected. To be a typical dynamic global or physical property, the intrinsic viscosity of a * To whom correspondence should be addressed. † GEMICO. ‡ DCPR (GRAPP)-URA CNRS 328. X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) Wolff, C.; Silberberg, A.; Priel, Z.; Layec-Raphalen, M. N. Polymer 1979, 20, 281.
S0743-7463(95)01036-5 CCC: $12.00
macromolecule in solution must be obtained via viscosity measurements performed in the limit of zero shear rate. Generally, in the dilute regime, the variation of viscosity with concentration follows the Huggins equation2,3
ηsp η - ηs ) ) [η] + kH[η]2c c ηsc
(4)
or the Kraemer equation2,4
1 1 η lnηrel ) ln ) [η] - kK[η]2c c c ηs
(5)
where kH and kK are the Huggins and Kraemer coefficients, respectively. Extrapolation to zero concentration of the ηsp/c or (1/c) lnηrel versus concentration curve gives the intrinsic viscosity, and the slopes allow the calculation of the Huggins and Kraemer parameters. The Huggins coefficient can be interpreted as a measure of the pairwise hydrodynamic interaction between macromolecules and gives an indication on the quality of the solvent. Intrinsic viscosity measurements of dilute polymer solutions are usually carried out with capillary viscometers, because of their accuracy, almost one order of magnitude higher than those obtained with most rotatory instruments.5 However, the flow field in a capillary is not linear but parabolic. The shear rate and the shear stress are consequently zero along the capillary axis and maximum at the capillary wall. If polymer solutions are non-Newtonian, the influence of the shear stress on the viscosity data is then not to be neglected, especially if high shear rates at the capillary wall are reached. In conventional Ostwald capillary viscometers it is not surprising to find values on the order of 1500 s-1. In a previous article6 we reported the viscometric behaviors of various hydrophobically-associating propylene glycol alginate (PGA) derivatives, in the dilute regime (polymer concentration, Cp, in the range (0.05-0.2) × 10-2 g/mL) at different ionic strengths. For this study, viscometric measurements had been carried out with an (2) Bohdanecky, M.; Kovar, J. In Viscosity of Polymer Solutions; Jenkins, A. D., Ed.; Elsevier Scientific Publishing Co.: New York, 1982; p 177. (3) Huggins, M. L. J. Am. Chem. Soc. 1942, 64, 2716. (4) Kraemer, E. O. Ind. Eng. Chem. 1938, 30, 1200. (5) Van Wazer, J. R.; Lyons, J. W.; Kim, K. I.; Colwell, R. E. Viscosity and Flow Measurements; Interscience, New York, 1963. (6) Sinquin, A.; Hubert, P.; Dellacherie, E. Langmuir 1993, 9, 3334.
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Ostwald-type capillary viscometer, assuming that the polymer solutions under investigation were Newtonian and that no corrections related to the shear stress had to be used. Actually, propylene glycol alginates are linear, (1 f 4) linked copolymers of β-D-mannuronic and R-L-guluronic saccharide units. The structure and the sequence of these two monomers, as well as the presence of ester groups on the macromolecule, confer a relatively high stiffness to the backbones of these polysaccharides and, hence, a high hydrodynamic volume. Under the effect of high shear forces, chains exhibiting low flexibility tend to align in the direction of flow, therefore leading to some apparent hydrodynamic volume (or viscosity) decrease. Furthermore, generally speaking, owing to the presence of both ionic charges and hydrophobic segments in the same structure, the hydrodynamic volume of hydrophobically-modified polyelectrolytes is extremely influenced by the ionic strength. For these two main reasons, some important discrepancies may therefore appear in the viscosity values, depending both on the concentration of the added salt in the polymer solution under investigation and whether or not the chains are subjected to high shear rates in the considered viscosity instrument. In the present paper, the viscometric behavior of these PGA derivatives is revisited, in the same range of concentration ((0.02-0.15) × 10-2 g/mL), using a LowShear rotatory viscometer, and comparisons will be made with results previously obtained with an Ostwald-type capillary viscometer.6 In addition, further information on the conformational arrangement of these hydrophobically-modified derivatives in the dilute regime is drawn from both surface tension and fluorescence spectroscopy measurements. Experimental Section The precursor polymer, propylene glycol alginate (PGA, batch L25A) was a generous gift by Protan (Drammen, Norway). In this derivative, prepared at the industrial scale by treatment of alginic acid with propylene oxide, approximately 70% of the original uronic acids are in the form of propylene glycol esters. PGA was transformed into amphiphilic derivatives (PGA-C12 and PGA-C14) by nucleophilic displacement of some of its ester groups by the corresponding long chain dodecyl- and tetradecylamines, according to a procedure previously described6 and derived from the initial article by Yalpani and Hall.7 Under the experimental conditions used, about 9% (mol/mol) of the total saccharide units were substituted in the resulting PGA-Cn, as determined by nitrogen analysis. In order to check the absence of any secondary reaction during the overall treatment, a control sample (PGAo) was prepared under the same conditions as the PGA-Cn derivatives, except that no alkylamine was added to the reaction mixture. Polymer solutions were prepared from ultrapure water (Milli-Q water purification system, Millipore) under vigorous stirring for 24 h. When necessary, solutions of salt at adequate concentrations were added to the previously prepared aqueous polymer solutions. The resulting mixtures were stirred for a further 12 h and then allowed to stand for at least 12 h at 4 °C before measurements were performed. Viscosity measurements were carried out at 25 ( 0.05 °C, either with an automatic (Viscologic TI.1, Sematech, France) Ostwald-type capillary viscometer (0.36 mm diameter) or with a Low-Shear LS 30 (shear rates γ˘ in the range 0.017-120 s-1, to be sure that no shear thinning (orientation effect) is exhibited) rotatory viscometer from Contraves (now Mettler, France), using the coaxial cylinders measuring system 1-1. For both techniques, calculations of the intrinsic viscosities at various salt concentrations were made, using the Huggins3 and Kraemer4 equations. Surface tension was measured, at 25 ( 0.1 °C, using the Wilhelmy plate method with a Kru¨ss tensiometer. (7) Yalpani, M.; Hall, L. D. Can. J. Chem. 1981, 59, 3105.
Figure 1. Variation of reduced viscosity (ηsp/Cp) versus polymer concentration (Cp) for (0) PGA, ([) PGAo, and (*) PGA-C12 in 0.05% NaCl (a) and 5% NaCl (b). Fluorescence emission spectra of pyrene (1.1 × 10-6 M) added to the considered polymer solutions were recorded in the 350500 nm range on a SPEX Fluorolog 2 spectrometer. The excitation wavelength was 335 nm.
Results and Discussion Owing to the presence of sodium carboxylates along the PGA backbone (less than 30% of the saccharide units), the addition of some electrolyte is required to obtain a linear plot between reduced viscosity (ηsp/Cp) and polymer concentration (Cp). For all the samples, precursors as well as hydrophobically-modified derivatives, a NaCl concentration as low as 0.01% is sufficient to screen the electrostatic long range repulsions and to reach linearity. Figure 1 shows the variation of reduced viscosity, measured by the LS 30 instrument, versus polymer concentration, for the various samples, respectively at low (0.05% NaCl) (Figure 1a) and at high (5% NaCl) (Figure 1b) salt concentrations. The corresponding intrinsic viscosities and Huggins coefficients are reported in Table 1. The two unmodified samples, PGA and PGAo, exhibit the typical behavior of nonassociative polyelectrolytes. When the salt concentration increases from 0.05% to 5% NaCl, the long range electrostatic repulsions are screened, the macromolecules adopt a more compact conformation, and, accordingly, their intrinsic viscosity decreases. In addition, the Huggins coefficients of both samples increase slightly with the ionic strength (0.38-0.59 for PGA and 0.41-0.63 for PGAo), in good agreement with the fact
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Table 1. Intrinsic Viscosity and Huggins Coefficient for PGA, PGAo, and PGA-C12 at Salt Concentrations Cs ) 0.05% and 5% NaCl 0.05 % NaCl sample PGA PGAo PGA-C12
[η]a
(mL/g)
594 ( 15 585 ( 11 618 ( 13
5.00 % NaCl kH
0.38 0.41 0.51
[η]a
(mL/g)
367 ( 10 376 ( 7 268 ( 11
kH 0.59 0.63 3.0
a Each [η] is the average of values determined from both the Huggins3 and Kraemer4 equations.
Figure 3. Variation of intrinsic viscosity [η] versus salt concentration (Cs, % NaCl) for (9, capillary; b, Low Shear 30) PGA and (0, capillary; O, Low Shear 30) PGA-C12.
Figure 2. Variation of the viscosity of PGA-C12 with shear (γ˘ ) (0) in 0.05% NaCl solution and ([) in 5% NaCl solution.
that kH usually increases when the hydrodynamic volume of the coil decreases.2 Moreover, these kH values remain in the range 0.3-0.8, generally reported for nonassociating polymers.8 Finally worth being noted, whatever the salt concentration, both viscosity and kH values are almost identical for PGA and PGAo, confirming that no structural modification (secondary reaction, undesirable depolymerization or change in the esterification ratio) takes place during the synthesis under the conditions used. Figure 2 shows the variation of viscosity with γ˘ for PGAC12 at the two considered NaCl concentrations. It provides evidence that, within the shear rate range investigated (0.017-120 s-1), the measured viscosities are shear rate independent. At Cs ) 0.05% NaCl (Figure 1a), the intrinsic viscosity of PGA-C12 is slightly higher than those of PGA and PGAo. At this low ionic strength, the long range electrostatic repulsions seem preponderant and the critical salt concentration (Ccs), at which the alkyl long chains fixed onto the saccharide backbone start displaying attractive hydrophobic interactions, is not yet reached. This is further evidenced by the fact that the Huggins coefficient (0.51) is still within the range of those of nonassociating polymers. The slight intrinsic viscosity increase of PGA-C12, compared to that of the parent PGA (or PGAo), is consistent with the theoretical molecular weight increase expected from the substitution of about 10% propylene glycol units by dodecyl segments. On the other hand, the presence of pendant alkyl chains on the polysaccharide backbone may induce an expansion of the coils. As evidenced by Dondos et al.9,10 the larger the number of heterocontacts between unlike constitutive elements of a macromolecule, the higher its expansion in solution (in our case, mutual contacts between carboxylate, propylene glycol ester, and (8) Brandrup, J.; Immergut, E. H. In Polymer Handbook; Wiley, J., Ed.; Interscience: New York, 1975; pp IV-19. (9) Dondos, A.; Benoit, H. Makromol. Chem. 1968, 118, 165. (10) Dondos, A.; Rempp, P.; Benoit, H. Makromol. Chem. 1969, 130, 233.
dodecyl chain for PGA-C12 instead of only carboxylate/ ester contacts in the precursor). A similar behavior (intrinsic viscosity of the hydrophobically-modified polymer superior to that of the unmodified parent, at Cs < Ccs) has already been reported for hydrophobically-modified polyacrylate derivatives.11 However, this effect is small (∆[η] ≈ 24) and could as well be explained merely in terms of the uncertainty limits of the measurements. At high salt concentration (Figure 1b), the intrinsic viscosity of PGA-C12 becomes much lower than those of unmodified PGAs. In addition to its effect on repulsive charges, the ionic strength plays a role on the reinforcement of hydrophobic interactions. In the range of polymer concentrations explored (dilute regime), these hydrophobic interactions are essentially of an intramolecular nature. These associations result in a limited ability of the chain to freely expand, a shrunken conformation, and, accordingly, a significant drop of the intrinsic viscosity, compared to those of the parent homologues. Furthermore, the Huggins coefficient becomes significantly higher than 1 at this high ionic strength, indicating a decrease of the solvent quality or, in other words, that polymer/polymer interactions become more favored than those between the polymer chains and the solvent, even if the amount of salt reinforces the hydrophobic intramolecular interactions. Figure 3 shows the variation of [η] versus salt concentration for PGA and PGA-C12, determined by both capillary and rotatory techniques. At low ionic strength (0.05% NaCl), the intrinsic viscosities of both polymers determined with the LS 30 are much higher than those determined by the capillary instrument at a very similar salt concentration (0.04% NaCl). Likewise, the Huggins coefficients are, for both samples, significantly lower than those observed with the capillary viscometer. These results confirm that, at low ionic strengthsi.e. under conditions where the coils are fully expanded, the intrinsic viscosities of PGA and of its amphiphilic derivative, PGA-C12, are highly influenced by the shear rate. As a matter of fact, under the effect of high shear rates at the capillary wall, expanded coils tend to deform and orient with the flow, resulting in underestimated viscosities. In opposition, at high ionic strength, almost no difference is observed between [η] and kH values determined from the LS 30 rotatory measurements and those obtained from capillary determinations. These observations indicate (11) Magny, B.; Iliopoulos, I.; Audebert, R. Polym. Commun. 1991, 32, 456.
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Figure 4. Variation of surface tension versus polymer concentration (Cp) for (0) PGA, (*) PGA-C12, and (O) PGA-C14, in pure water.
that, at high enough salt concentration, the coils adopt sufficiently shrunken conformations not to undergo a large stretching under the influence of shear rate at the wall of the capillary. In our previous article,6 we pointed out that, at very low ionic strength (Cs ) 0.01% NaCl), the ∆[η] observed between PGA and PGA-C12 ([η]PGA > [η]PGA-C12) might be due either to some depolymerization during the introduction of dodecyl groups onto PGA or to the existence of intramolecular hydrophobic associations, already in pure water. The viscometric measurements performed at Cs ) 0.05% with the LS 30 viscometer lead to [η]PGA-C12 > [η]PGA and therefore discredit both previous hypotheses. The differences observed were only due to the technique of measurement, and it turns out, on the contrary, that the covalent binding of long alkyl chains on PGA leads to some expansion of the coil, as long as no hydrophobic aggregative interactions start overriding this effect. The formation of hydrophobic microdomains, evidenced by intrinsic viscosity measurements, usually results in surface-active effects. As a matter of fact, most amphiphilic polymers largely decrease the surface tension of water when their concentration increases.12-14 Figure 4 shows the variation of surface tension versus polymer concentration for PGA, PGA-C12, and PGA-C14, in pure water. As expected, the surface tension of water is not affected by the addition of parent PGA. In opposition, it decreases from 67 mN/m to about 45 mN/m when the concentration of PGA-C12 increases from 2.5 × 10-5 to 2 × 10-3 g/mL. This surface-active effect is even more significant for the PGA-C14 derivative, the transition occurring sooner and reaching lower values (42 mN/m at 1.5 × 10-3 g/mL). This decrease reflects the formation of hydrophobic microdomains in PGA-C12 and PGA-C14 solutions, similarly to the behavior of surfactants at their critical micellar concentrations (cmc’s). Pseudo-cmc’s can be estimated from these curves, corresponding to the polymer concentrations at which intermolecular cooperative hydrophobic associations between amphiphilic coils become preponderant. Such surface tension profiles can be paralleled to those observed during the previously reported15 fluorescence (12) Landoll, L. M. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 443. (13) Akiyoshi, S.; Sunamoto, J. Macromolecules 1993, 26, 3062. (14) Morishima, Y.; Kobayashi, T.; Nozakura, S. Polym. J. 1989, 21, 267. (15) Sinquin, A.; Hubert, P.; Dellacherie, E. Polymer 1994, 35, 3557.
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Figure 5. Ratio of the intensity of fluorescence vibrational bands (I1/I3) of pyrene (C ) 1.1 × 10-6 M) in the presence of (0) PGA, (*) PGA-C12, and (O) PGA-C14, in pure water.
Figure 6. Correlation between fluorescence and surface tension data for (+) PGA, (*) PGA-C12, and (O) PGA-C14, at various polymer concentrations.
study of PGA, PGA-C12, and PGA-C14 solutions, in the presence of free pyrene. This technique evidences the formation of hydrophobic microdomains, by the variation of the I1/I3 ratio (intensities of the first (I1) and the third (I3) bands of the fluorescence emission spectra of pyrene) versus polymer concentration, in pure water (Figure 5). For the amphiphilic derivatives, one observes, in agreement with surface tension data, that the longer the alkyl chain on PGA, the sooner the transition and the lower the I1/I3 ratio. For the parent PGA solution, the environment seen by the probe is essentially hydrophilic in the whole domain of concentration tested. A good correlation between fluorescence and surface tension data is obtained, as shown in Figure 6. A cloud of points centered around I1/I3 ≈ 1.8 and surface tension ≈ 68 mN/m is observed for all concentrations of PGA, in agreement with the hydrophilic character of this derivative. In opposition, for PGA-C12 and PGA-C14, as the polymer concentration increases, surface tension and I1/ I3 values follow the same decreasing trend, indicating that lipophilic moieties of these derivatives progressively organize to create hydrophobic microenvironments. LA951036L
