Article pubs.acs.org/Macromolecules
Charge Dependent Dynamics of Transient Networks and Hydrogels Formed by Self-Assembled pH-Sensitive Triblock Copolyelectrolytes Aarti Shedge, Olivier Colombani,* Taco Nicolai, and Christophe Chassenieux* LUNAM Université, Université du Maine, IMMM-UMR CNRS 6283, Département Polymères, Colloïdes et Interfaces, av. O. Messiaen, 72085 Le Mans cedex 9, France
ABSTRACT: Triblock copolymers of BAB type were synthesized by ATRP with a poly(acrylic acid) central block and random copolymer end blocks containing both acrylic acid (AA) and hydrophobic n-butyl acrylate (nBA) units with varied composition of the B-blocks. Visco-elastic properties of these amphiphilic polymers were investigated by oscillatory shear measurements as a function of composition, concentration, temperature and pH. Whatever the composition for each polymer the terminal relaxation time and thus the viscosity increased sharply with decreasing degree of ionization of the AA units (α) and at low values selfsupporting hydrogels were formed. However, the α- (respectively pH-) dependence of the terminal relaxation time as well as the critical range of α (respectively pH) where a transition between visco-elastic solutions and self-supporting hydrogels occurs strongly depend on the composition of the B-block.
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interfacial tension between the B-block and the solvent.11,12 Only when the blocks are very short8,13 or γ is small14,15 is the association of block copolymers dynamic. An efficient way to decrease γ in a controlled manner is to incorporate hydrophilic units into the hydrophobic blocks.16−22 In recent years, we reported a detailed investigation of the selfassembly of triblock copolyelectrolytes with a poly(acrylic acid) (PAA) A-block and poly(n-butyl acrylate) (PnBA) B-blocks into which 50% AA units were incorporated randomly.16,23−28 Association of these systems was found to be dynamic over a wide range of ionization degrees, whereas the association of pure nBA B-blocks is kinetically frozen.14,29−31 In water, these polymers form above the percolation concentration viscoelastic liquids with a terminal relaxation time that increases sharply with decreasing α for α Cp with a frequency independent storage modulus at high frequencies (G′ > G″) and a liquid-like behavior at low frequencies (G′ ∝ f−2, G″ ∝ f−1). Results obtained at different temperatures could be superimposed by frequency−temperature superposition. In addition, master curves obtained at different ionization degrees could be superimposed by frequency-α superposition. In this way, the dependence of G′ and G″ on the frequency could be represented by a single master curve implying that the relaxation process of the transient networks is independent of the temperature and α. However, the characteristic relaxation time increased with decreasing temperature and decreasing α. The influence of the latter parameter was particularly strong as the relaxation time could be increased by more than 10 orders of magnitude by decreasing α from 0.7 to 0.2. In order to investigate the effect of the composition of the hydrophobic blocks on the rheological behavior we did the same measurements for TH40 and TH60. For these systems we could also obtain a unique master curve by combining frequency−temperature and frequency−α superposition (data not shown). In Figure 5, the master curves obtained for the three systems are compared. It is clear that they superimpose well implying that the relaxation mechanism does not depend strongly on the composition of the hydrophobic blocks. We define a characteristic terminal relaxation time as τ = (2πfc)−1 where fc is the frequency at which G′ = G″. The relaxation time at 20 °C is shown in Figure 6 as a function of α at different polymer concentrations for C > Cp. For each system, τ increased steeply with decreasing α over many orders of magnitude, but the onset of the increase shifted to lower α when the AA content was larger. It is clear that for a given value of α, the exchange dynamics is extremely sensitive to the fraction of AA units (x) in the end blocks. For instance at α = 0.5 and 20 °C the relaxation time increases from less than a millisecond at x = 60% to about a second at x = 50% and more than a day at x = 40%. We also observed an increase of τ with
Figure 6. Dependence of the relaxation time on the degree of ionization for TH40 (circles), TH50 (triangles), and TH60 (squares). Different concentrations are indicated by different colors: black (15g/ L), white (30g/L) and gray (60g/L). The crossed triangles for TH50 correspond to other concentrations. Lines are eye-guides for the reader. The data for TH50 were taken from refs 23 and 24.
increasing concentration, but this effect was relatively small compared to the huge effects of α and x. For TH50 it was shown that the temperature dependence of τ yields a straight line in the Arrhenius representation, i.e. when it is plotted as a function T−1.23 From the slope the activation energy was determined and was found to be independent of α and the polymer concentration: Ea ≈ 110 kJ/mol. In Figure 7, the temperature dependence of τ with respect to the value at 20 °C is shown for TH40 and TH60 at different α and different polymer concentrations. The temperature dependence was the same within the experimental error and was characterized by the same activation energy as for TH50 indicated by the solid line.
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DISCUSSION The results presented here show that varying the fraction of AA units in the hydrophobic end blocks has a dramatic effect both on the concentration at which a transient network is formed and on the terminal relaxation time of the network at a given 2442
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controlling the exchange dynamics. However, for the same charge density the exchange dynamics are still slower if the Bblock contains a larger fraction of nBA units. This is not surprising, because AAH units are still hydrophilic, while nBA units are hydrophobic. The effect of exchanging AAH units for nBA units is especially important at larger x since at x = 80% we did not observe significant association of the B-blocks except at α close to zero.34 In the discussion so far, we have ignored the effect of the charge density of the corona on the exchange dynamics. If the effect of the charge density of the PAA central block on τ is important, one might expect a strong effect of screening electrostatic interactions by adding salt. However, we showed elsewhere that the effect of adding NaCl on τ was very weak up to 0.5 M.25 Therefore we consider that the exchange dynamics are for the most part controlled by the charge density and the hydrophobicity of the end blocks. The latter observation is in agreement with Halperin’s theory12 and with experimental results on neutral poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymers.37 The effect of the AA content on the percolation threshold also appears to be related to differences in charge density of the B-blocks, because when Cp is plotted versus the fraction of charged monomers in the B-blocks, the results are very close, see Figure 9. Notice, however, that the relationship between α
Figure 7. Arrhenius representation of the temperature dependence of the relaxation time normalized by the value at 20 °C for TH40 (circles) and TH60 (squares). The solid line represents the temperature dependence that was reported for TH50.23
pH. However, it does not influence significantly the relaxation mechanism as the frequency dependence of the shear moduli is close for the three compositions. In addition, the temperature dependence is not significantly influenced by the composition. In fact, the observed effect of decreasing the AA content for a given α appears similar to that of decreasing α for a given AA content. The difference between decreasing α and decreasing the AA content is that in the former case charged AA units (AA−) are transformed into neutral ones (AAH), while in the latter case both charged and neutral AA units are transformed into nBA units. One possibility is therefore that the exchange dynamics is determined primarily by the number of charged monomers in the hydrophobic block. The fraction of charged units in the end blocks can be calculated as a function of α for each system using the relationship between α and the ionization degree of the Bblock as shown in Figure 2. In Figure 8, the terminal relaxation time is plotted as a function of the fraction of charged monomers in the end blocks. In this representation the evolution of the relaxation time of the three systems is much closer showing that indeed the charge density of the hydrophobic block is a key parameter for
Figure 9. Dependence of the percolation concentration on the fraction of charged monomers in the B-blocks for TH40, TH50, and TH60 (same symbol key as in Figure 1).
and the fraction of charged units is not correct at lower polymer concentrations in the absence of salt, as was mentioned above. For C < 10 g/L the fraction of charged units is in reality somewhat smaller than calculated from the titration curves. As we discussed elsewhere, the association of the micelles at dynamic equilibrium can be modeled as reversible aggregation of sticky spheres.26 Cp is determined by the volume fraction of the micelles and the probability that a bond is formed between two micelles. Notice, however, that the probability to form a bond is not necessarily correlated to the lifetime of a bond. For TH50 the aggregation number of the micelles was found to decrease with increasing α causing the volume fraction of the micelles to increase.26 The latter would lead to a decrease of Cp with increasing α if the probability to form a bond were independent of α. Clearly, the opposite is observed implying that the probability to form bonds decreases strongly with increasing α. This can to some extent be explained by the fact
Figure 8. Dependence of the relaxation time on the fraction of charged units in the hydrophobic blocks for polymers with different compositions at different concentrations (same symbol key as in Figure 6). 2443
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that the micelles contain fewer arms at higher α so that they have less possibility to bridge with another micelle.
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CONCLUSION The present study revealed that the key parameter controlling the exchange dynamics is the number of ionized hydrophilic units within the B blocks. As a consequence, it is possible to control the pH-range within which the systems transform from low viscosity solutions to free-standing hydrogels or the percolation concentration of the gels by fine-tuning the chemical structure of the polymers. Hydrophobic B-blocks of BAB triblock copolyelectrolytes associate in aqueous solution and form polymeric micelles. The micelles can aggregate by association of the two end blocks in different micelles leading to the formation of a transient network above a critical percolation concentration. The exchange rate of the end blocks between micelles depends on their charge density and their hydrophobicity. The latter can be decreased by incorporating hydrophilic units into the hydrophobic blocks. Exchange of hydrophobic monomers by hydrophilic AA groups in the end blocks increases the exchange dynamics and ionization of the AA groups further increases the exchange rate. The terminal visco-elastic relaxation time of the transient network is determined by the exchange rate and therefore decreases with increasing fraction of hydrophilic monomers and increasing degree of ionization. However, the frequency dependence of the shear modulus is independent of these parameters and is characterized by a broad relaxation distribution. Increasing the fraction of AA groups in the end blocks renders their ionization easier. As a consequence the slow-down of the exchange rate occurs at lower pH when more AA is incorporated in the end blocks and for a given degree of ionization, the percolation concentration increases when more AA groups are incorporated in the end blocks.
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AUTHOR INFORMATION
Corresponding Authors
*(O.C.) E-mail:
[email protected]. *(C.C.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.S. thanks the “Région Pays de la Loire” for funding her postdoctoral grant. REFERENCES
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