Article pubs.acs.org/Macromolecules
Multiresponsive Hydrogels Formed by Interpenetrated SelfAssembled Polymer Networks A. Klymenko, T. Nicolai,* L. Benyahia, C. Chassenieux, O. Colombani, and E. Nicol †
LUNAM Université, Université du Maine, IMMM − UMR CNRS 6283, Université du Maine, 72085 Le Mans cedex 9, France S Supporting Information *
ABSTRACT: It is shown how interpenetrated polymer networks can be formed by self-assembly in water of two different amphiphilic triblock copolymers, which preserve and combine the properties and stimuli-responsiveness of each polymer. Aqueous solutions of a pH and a UV-sensitive triblock copolymer spontaneously formed a self-assembled interpenetrated network when mixed. The polymers were based on poly(acrylic acid) and poly(ethylene oxide), respectively, and had different hydrophobic end blocks. The structure of the mixed networks was studied with light scattering and the dynamic mechanical properties were studied by oscillatory shear measurements. Synergy led to reduction of the percolation concentration of the individual polymer networks within the interpenetrated network.
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INTRODUCTION Amphiphilic BAB triblock copolymers consisting of a hydrophilic central A block and hydrophobic lateral B blocks selfassemble in water leading to the formation of viscoelastic liquids or hydrogels, with applications in many areas.1−6 The hydrophobic end blocks associate into multiplets forming mainly flower-like aggregates in dilute solution. At higher concentrations the two end blocks of the same chain can enter different multiplets so that the central block acts as a connecting bridge between the multiplets. In this way, aggregates are formed that increase in size with increasing concentration, until at a critical concentration (Cp) a percolating network is formed. Relaxation of the network is determined by the exchange dynamics of hydrophobic blocks between multiplets.7,8 A range of single self-assembled networks have been documented with a variety of properties and sensitivity to different environmental conditions such as temperature, pH, and UV irradiation.9−12 In order to form single networks that respond to multiple stimuli, one needs to incorporate chemically different responsive units into the polymers13 or to select multiresponsive monomer units.14 Here we propose an alternative method to obtain multiresponsive networks by mixing different triblock copolymers that each respond to a different stimulus in order to form a multiresponsive interpenetrated self-assembled polymer network (IPSAN). In this way the range of properties and responses to multiple stimuli could be extended much more easily than by synthesizing new types of block copolymers. An additional potential advantage of IPSANs over single networks is that synergy between the two networks may lead to enhanced mechanical properties that are not simply the addition of the individual networks. As far as we © XXXX American Chemical Society
are aware, no attempts have been made so far to combine the properties of two different triblock copolymers by forming an IPSAN. To demonstrate the viability of this approach, we present here an investigation of an IPSAN formed by combining pH-sensitive (tPAA) and UV-sensitive (tPEO) triblock copolymers. Figure 1 shows a schematic representation of this system.
Figure 1. Schematic representation of an interpenetrated selfassembled polymer network obtained by mixing a UV-responsive network formed by amphiphilic triblock copolymers based on PEO with a pH-responsive network formed by amphiphilic triblock copolymers based on PAA with different hydrophobic blocks. Received: September 27, 2014 Revised: November 3, 2014
A
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The properties of single self-assembled networks formed by tPEO15,16 and tPAA17,18 individually have already been reported in detail elsewhere. tPEO consists of a poly(ethylene oxide) central block (number-average degree of polymerization, DP = 270) and two short poly(methacryloyloxyethyl acrylate) end blocks (DP = 8). It was shown that tPEO forms a percolating network for CPEO ≥ 17 g/L. The exchange dynamics of the tPEO network is relatively fast and independent of the pH. However, the hydrophobic blocks of tPEO contain polymerizable units that irreversibly cross-link the cores when irradiated with UV-light in the presence of a photoinitiator. This means that the transient network can be transformed in situ into a covalently bound network with a UV stimulus. The central block of the pH-sensitive triblock copolymer tPAA is poly(acrylic acid) (DP = 200) and the two end blocks consist of random block copolymers of 50 nbutyl acrylate units and 50 acrylic acid units. The percolation concentration of tPAA increases with increasing degree of ionization of the acrylic acid units (α) from Cp ≈ 4 g/L at α = 0.3 to Cp ≈ 60 g/L at α = 0.7. The exchange dynamics of the tPAA networks slow down strongly with decreasing α below α = 0.7, because the end blocks become increasingly more hydrophobic. This means that the viscoelastic properties of the tPAA network can be finely tuned by the pH from a low viscosity liquid at α = 0.7 (pH 7) to a self-supporting soft solid at α = 0.3 (pH 5). Visually homogeneous and transparent IPSAN could be obtained by simply mixing the two polymers in aqueous solution. We will show that the IPSAN preserves and combines the response of the tPEO network to UV-irradiation and that of the tPAA network to the pH and that it has interesting synergistic dynamic mechanical properties. The structure of the IPSAN was investigated using scattering techniques.
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Figure 2. Chemical structure: (a) Poly(2-hydroxyethyl acrylate)-bpoly(ethylene oxide)-b-poly(2-hydroxyethyl acrylate) and (b) poly(2methacryloyloxyethyl acrylate)-b-poly(ethylene oxide)-b-poly(2-methacryloyloxyethyl acrylate) (tPEO) triblock copolymers. detections. Mn = 4.5 × 104 g/mol, Mw = 4.8 × 104 g/mol, Đ =1.07. The tert-butyl acrylate (tBA) units were finally selectively and quantitatively transformed into AA units by acidolysis with trifluoroacetic acid to yield the targeted tPAA. Sample Preparation. To prepare tPAA solutions, the polymer powder was dissolved in deionized water (MilliPore) containing 70% of the NaOH necessary to obtain an ionization degree (α) close to 1. The solutions were stored in these conditions prior to use in order to avoid risks of hydrolysis which could occur in the presence of an excess of NaOH. Just before mixing with the tPEO solution, the tPAA solution was set to α = 1 by adding the required quantity of NaOH while stirring. Subsequently, the degree of ionization was reduced by adding aliquots of a (D-glucono-δ-lactone) (GDL) solution (1 M) in the required amount, calculated according to the chemical structure of the copolymer. See ref 21 for further details. The tPEO solution was prepared by dissolving the polymer powder in deionized water (MilliPore). For photo-cross-linking experiments a solution of 2,2-dimethoxy-2-phenylacetophenone (DMPA) photoinitiator (0.01M) was prepared in THF. The required amount of solution of DMPA was placed on the walls of a glass vial, and THF was evaporated under a gentle flow of argon. Then, the polymer dissolved in deionized Milli-Q water was introduced into the vial that was sealed with a rubber septum. The vial was rotated overnight on a roller stirrer. The quantity of DMPA was fixed to around 10 molecules per micelle considering an average aggregation number of 40 hydrophobic blocks per micelle. Finally, the IPSANs were prepared by mixing the same volumes of the tPAA solution and the tPEO solution at the required concentrations. GDL was added to the tPAA solution just before mixing the tPEO and tPAA solutions. The samples were analyzed after reaching the equilibrium state, which took the same time for the IPSAN as for tPAA alone, namely ∼24 h after GDL addition. No significant change in the rheological properties of the IPSAN was observed over 1 week for any of the systems implying that hydrolysis was negligible with the above-mentioned precautions. For light scattering measurements the solutions were filtered immediately after mixing through 0.20 μm pore size filters (Anatope).
MATERIALS AND METHODS
Synthesis. tPEO was synthesized according to already published procedures.15,19,20 Briefly, a triple hydrophilic poly(2-hydroxyethyl acrylate)-b-poly(ethylene oxide)-b-poly(2-hydroxyethyl acrylate), PHEA-b-PEO-b-PHEA, (Figure 2a) was synthesized by growing two PHEA blocks from a dibrominated PEO (Br-PEO-Br) macroinitiator using Cu(0)-mediated SET−LRP (single electron transfer−living radical polymerization). The synthesis was performed in DMSO with a molar ratio [PEO-Br]/[HEA]/[Me6TREN]/[CuBr2]: 1/8/0.2/0.085. Copper wire was used as a catalyst. Prior to use the copper surface was activated with sulfuric acid. The targeted degree of polymerization of 8 for poly(2-hydroxyethyl acrylate) blocks was confirmed by 1H NMR. Polymerizable methacrylate functions were then grafted onto the PHEA-blocks leading to the UV-cross-linkable hydrophobic poly(methacryloyloxyethyl acrylate) (PMEA) blocks (Figure 2b). The number (Mn) and weight (Mw) average molar mass and dispersity (Đ = Mw/Mn) were determined by size exclusion chromatography (SEC) in THF using light scattering and refractometric detection. Mn = 1.8 × 104 g/mol, Mw = 2.3 × 104 g/mol, Đ=1.3. 1H NMR analysis of the final PMEA-b-PEO-b-PMEA (tPEO) triblock copolymer indicated a quantitative functionalization of the HEA units into methacryloyloxyethyl acrylate ones. The synthesis of the triblock copolymer P(nBA50%-stat-AA50%)-bPAA-b-P(nBA50%-stat-AA50%) (tPAA) (Figure 3) was reported in detail by Charbonneau et al.17 Briefly, a P(nBA50%-stat-tBA50%)-b-PtBA-bP(nBA50%-stat-tBA50%) triblock copolymer was first synthesized using a macroinitiator technique by atom transfer radical polymerization. The degree of polymerization of each outer P(nBA50%-stat-tBA50%) block is 100 with a statistical distribution of monomer units within these blocks, and that of the central PtBA block is 200. The number (Mn) and weight (Mw) average molar mass and dispersity (Đ = Mw/Mn) were determined by SEC using light scattering and refractive index B
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Figure 3. Chemical structure of the P(nBA50%-stat-AA50%)-b-PAA-b-P(nBA50%-stat-AA50%) (tPAA) triblock copolymer.
Figure 4. (a) Frequency dependence of the storage (filled symbols) and loss (open symbols) moduli of IPSAN containing 20 g/L tPAA and 20 g/L tPEO at different ionization degrees (α): α = 1 (o); α = 0.7 (□); α = 0.6 (∇); α = 0.5 (◊). (b) Master curves obtained by superposition of the data at different α.
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RESULTS AND DISCUSSION We will first discuss the dynamic mechanical properties and the structure of transient self-assembled interpenetrated networks of tPAA and tPEO and then discuss the effect of covalently cross-linking the tPEO network by UV-irradiation.
Methods. Oscillatory shear measurements were done with a stresscontrolled rheometer MCR-301 (Anton Paar) equipped with a cone and plate geometry (gap 0.103 mm, diameter 25 mm). For in situ cross-linking, the samples were degassed in advance and introduced into the rheometer under gentle flow of argon. In order to prevent evaporation, the geometry was covered with silicon oil. The samples were irradiated by a UV light (Dymax Blueware-200 lamp) at 365 nm with intensity 0.17 W·cm−1 during 60 s. The measurements were done in the linear response regime unless otherwise specified. Static and dynamic light scattering measurements were done using a commercial static and dynamic light scattering equipment (ALVLangen, Germany and LS-Instruments, Switzerland) operating with a vertically polarized laser with wavelength λ = 632 nm. The measured scattered light intensity was corrected for that of the solvent and normalized by that of toluene. The relative excess scattering intensity (Ir) was multiplied with the Rayleigh factor of toluene and expressed in units of cm−1. The measurements were done at 20 °C using a thermostated bath over a range of scattering wave vectors (q = 4πn sin(θ/2)/λ, with θ the angle of observation and n the refractive index of the solvent). The electric field autocorrelation function (g1(t)) was calculated from the normalized intensity autocorrelation functions (g2(t)): g2(t) = 1 + g1(t)2 and was analyzed in terms of a relaxation time distribution: g1(t ) =
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DYNAMIC MECHANICAL BEHAVIOR OF TRANSIENT IPSAN Figure 4a shows the radial frequency (ω) dependence of the storage and loss shear moduli for a transient IPSAN containing 20 g/L tPAA at different ionization degrees and 20 g/L tPEO. The response to oscillatory shear was characterized by behavior typical of an elastic solid at high frequencies and of a liquid at low frequencies. At C=20 g/L the tPEO network behaves as a low viscosity Newtonian liquid (η = 10−2 Pa·s).16 Therefore, the contribution of the tPEO network to the mechanical properties of the IPSAN can be neglected with respect to the contribution of the tPAA network. The high frequency elastic modulus (Ghf) is attributed to the rubber elasticity of the tPAA network and depends on the concentration of elastically active chains forming the network. The relaxation time of the system represents the escape time of a hydrophobic block from the core. It is clear that the strong effect of α on the dynamics of the tPAA network observed for individual tPAA networks21 is maintained in the IPSAN. Decreasing the degree of ionization renders the B-blocks more hydrophobic and slows down the relaxation process. The results obtained at different α could be superimposed using horizontal and vertical shifts, see Figure 4b. The horizontal shifts, corresponding to changes of the relaxation time, were important, but the vertical shifts corresponding to changes in the moduli were small (less than a factor of 2). The fact that the moduli were not much influenced by the ionization degree implies that the network structure and therefore the number of elastically active chains was independent of α. The master curve shows clearly that the relaxation process is
∫ A(log τ) exp(−t /τ) d[log τ]
At low polymer concentration a single relaxation mode was observed, but at higher concentrations a second slower and broader relaxation mode was observed. All correlation functions could be welldescribed using a log-normal distribution for the fast mode and a generalized exponential for the slow mode:
A(log τ ) = kτ p exp[− (τ /τ*)s ] The average relaxation rates (Γ = ⟨τ−1⟩) of the single mode at low concentrations and of the fast mode at high concentrations were found to be q2-dependent and the cooperative diffusion coefficient was calculated as
Dc = Γq−2 C
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It was already shown elsewhere17 for single tPAA networks that the relaxation time increases sharply with increasing polymer concentration above Cp and stabilizes at higher concentrations. The reason is that, close to Cp, the network contains so-called superchains consisting of several binary connected multiplets.8,23 A superchain can be broken at anyone of the constituting bridges and therefore relaxes in less time than the exchange time of a B-block. The amount of superbridges decreases with increasing concentration as more elastically active chains are incorporated into the network until a fully formed network is obtained with only single bridges. As a consequence the relaxation time increases until it is controlled by breakage of single bridges and corresponds to the exchange time of the B-blocks. In the presence of tPEO, τ increased sharply at much lower tPAA concentrations and reached the value characteristic for the fully formed network at about 5 g/L. The high frequency elastic modulus also increased steeply above Cp as an increasing fraction of the polymers became elastically active chains of the network. For a flexible polymer network, the elastic modulus is of the same order of magnitude as νRT, with ν the molar concentration of elastically active chains. ν = C/M for a fully formed defect free network. The high frequency elastic modulus of the IPSAN approached the expected value for the ideal fully formed network at much lower concentrations than the single tPAA network, see Figure 6b. It is clear that the presence of tPEO in the IPSAN induced a displacement of the percolation concentration (Cp) of tPAA at α = 0.5 from about 15 g/L in the absence of tPEO to about 3 g/L when 20 g/L tPEO was added. It appears that the presence of tPEO favors bridging between tPAA multiplets over loop formation. As a consequence of the decrease of Cp, both τ and Ghf are much larger for the IPSAN than for the corresponding single tPAA network at lower polymer concentrations. However, the values of τ and Ghf for the fully formed network at C ≫ Cp were the same within the experimental error. This means that the properties of the fully formed tPAA network were not significantly influenced by the presence of tPEO. We also compared the dependence of τ and Ghf on the tPEO concentration for IPSAN containing 20 g/L tPAA at α = 0.5; see Figure 7. The results of pure tPEO networks are included for comparison. At all tPEO concentrations investigated here, τ and Ghf of the single tPEO network were much smaller than for the single tPAA network, i.e., at CtPEO = 0 g/L, and the dynamic
characterized by a broad distribution of relaxation times. We showed recently that small variations of the composition of the B-blocks have a dramatic effect on the relaxation time of the network,22 which means that the broad relaxation time distribution can be explained by the polydispersity of the composition of the B-blocks even if it is very small. We define a characteristic relaxation time (τ) as the inverse of the radial frequency (ω) at which G′ and G″ cross. Figure 5 shows that τ increased exponentially with decreasing α in the same manner as was observed for individual tPAA networks.21
Figure 5. Characteristic relaxation time as a function of the ionization degree for transient IPSAN containing 20 g/L tPEO and 20 g/L tPAA. The solid line is a guide to the eye. Photographs of the samples at α =0.8 and α =0.4 shown in the figure were taken just after tilting.
The effect of the polymer concentration was explored for α = 0.5 by varying the tPAA concentration at a fixed tPEO concentration of 20 g/L and by varying the tPEO concentration at a fixed tPAA concentration of 20 g/L. Master curves obtained at different polymer concentrations were similar to those shown in Figure 4b; see the Supporting Information. The dependence of τ and the high frequency elastic modulus (Ghf) on the tPAA concentration is shown in Figure 6 for a fixed tPEO concentration of 20 g/L, where Ghf was defined arbitrarily as the value of G′ at ω = 100/τ. For comparison the results of single tPAA networks at the same ionization degree are also plotted in Figure 6.
Figure 6. Dependence of the characteristic relaxation time (a) and the high frequency elastic modulus Ghf (b) on the tPAA concentration for IPSAN containing 20 g/L tPEO at α = 0.5 (filled symbols) as well as for single tPAA network at the same ionization degree (open symbols). The solid lines are guides to the eye. The dashed line represents RTC/M corresponding to the elastic modulus of an ideal fully formed network where all chains are elastically active. Notice that the results are plotted in a lin−log representation. D
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Figure 7. (a) Dependence of the characteristic relaxation time (a) and the high frequency elastic modulus (b) on the concentration of tPEO for IPSAN containing 20 g/L tPAA at α = 0.5 (closed symbols) as well as for single tPEO networks (open symbols). The solid lines are guides for the eye.
Figure 8. Excess relative scattered light intensity for pure tPAA, and tPEO solutions and mixtures containing equal concentrations of each polymer at α = 1 (a) and α = 0.5 (b). The relative intensities are plotted as a function of the concentration of each component. For comparison the sum of the relative intensities scattered by the two pure solutions is also shown in the figure. Lines are guides to the eye.
the sum of intensities of the individual binary solutions at the same concentrations as in the mixture. If the interaction is attractive the mixture will scatter more light than the sum of the individual solutions. We investigated mixtures with equal concentrations (C) of tPAA and tPEO at different ionization degrees: 1, 0.5, and 0.35. The relative excess scattering intensity (Ir) of the mixtures was compared with that of pure solutions of each polymer at the same concentration as in the mixture. The results for α = 1 and α = 0.5 are shown in Figure 8. For both ionization degrees, the scattering intensity of pure tPAA solutions increased with increasing concentration up to about 2−3 g/L. At higher concentrations it stabilized for α = 1 and decreased for α = 0.5, which is caused by strong repulsion between the chains. Ir was lower when the ionization degree was higher, because the electrostatic repulsion between tPAA chains was stronger. For pure tPEO solutions, repulsive interaction was less important so that Ir continued to increase up to the highest concentration tested (10 g/L). At low polymer concentrations the scattering intensity of the mixtures was equal to the sum of the individual polymer solutions, implying that each polymer associated in the same way as in pure solutions and that complexes were not formed between the two polymers. At higher concentrations the mixtures scattered significantly less light than the sum of the corresponding individual solutions. In fact, the intensity scattered by the mixture was even less than that of pure tPEO at the same concentration. This means that there is net
mechanical properties of the IPSAN were governed by the latter. Both τ and Ghf increased with increasing tPEO concentration. This can be explained by the decrease of the percolation concentration of tPAA when tPEO was present as was discussed above. In the absence of tPEO the tPAA network at 20 g/L still contains many defects. When tPEO is added the tPAA network becomes more densely cross-linked with fewer superbridges and other defects causing both τ and Ghf to increase.
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STRUCTURE OF TRANSIENT IPSAN The structure of the IPSAN was investigated using light scattering techniques. In general in binary solutions the relative excess scattered light intensity Ir is proportional to the osmotic compressibility and the structure factor (S(q)): Ir ∝ C(dC /dπ )S(q)
S(q) describes the dependence of Ir on the scattering wave vector and is unity if the system is homogeneous on the length scale of q−1. For the solutions studied here, S(q) was close to unity in the q-range covered in the experiment (0.0132−0.0256 nm−1). Net repulsive interaction causes a decrease of the osmotic compressibility and thus a decrease of Ir, while net attractive interaction leads to an increase of Ir. In ternary solutions one needs to consider also the interaction between two different kinds of solute molecules. If this interaction is repulsive the scattering intensity of the mixture is smaller than E
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turning on the UV-lamp; see the Supporting Information. A similar behavior was reported elsewhere for single tPEO networks,16 and it was shown that G′ became independent of the frequency after irradiation and was larger than G″, implying that a permanent network had been formed.16 It is clear that the response of the tPEO network to UV-irradiation is conserved in the IPSAN. The effect of irradiation on the frequency dependence of the shear moduli is illustrated in Figure 10 for an IPSAN containing 20 g/L tPEO and 20 g/L
repulsive interaction between tPEO and tPAA, which is at least in part due to excluded volume interaction. However, at α = 0.35 the scattered light intensity of the mixture was significantly higher than that of the sum of the two pure solutions over the whole concentration range; see the Supporting Information, indicating that self-assembly of each polymer was influenced by the presence of the other polymer. This observation can be related to the net attractive interaction between PEO and PAA homopolymers at low ionization degree (or pH) that has been reported in the literature.24−26 It was shown that H-bonds are formed between ethers of PEO and nonionized carboxylic groups of PAA at low pH. The present investigation was restricted to tPAA at higher ionization degrees for which no significant attractive interaction with tPEO was observed. Dynamic light scattering experiments allow one to measure the cooperative diffusion coefficient (Dc), see Materials and Methods, from which an apparent hydrodynamic radius (Rha) can be calculated using the Stokes−Einstein relation: Rha = kT/ (6πηDc), with η the solvent viscosity, k Boltzmann’s constant, and T the absolute temperature. In dilute solution Rha corresponds to the average hydrodynamic radius of the solute and in semidilute polymer solutions or polymer networks it corresponds to the dynamic correlation length. A second slower relaxation mode appeared at higher polymer concentrations; see the Supporting Information. The fast mode was attributed to cooperative diffusion and was used to calculate Rha. The slow mode was not observed in pure tPEO solutions in this concentration range. It was also observed for pure tPAA solutions,27 but with a smaller amplitude than in the mixtures. The slow mode is possibly caused by relaxation of heterogeneities within the transient network or interdiffusion of the two polymers.28−30 A more detailed investigation is needed to resolve this issue, which was outside the scope of the present investigation. Figure 9 shows the concentration dependence of Rha at α = 1 and α = 0.5. The hydrodynamic radius measured in dilute
Figure 10. Frequency dependence of the storage (G′, filled symbols) and loss (G″, open symbols) shear moduli of an IPSAN network containing 20 g/L tPEO and 20 g/L TH50 at α = 0.5 before (●) and after UV-irradiation (▲). The storage modulus of the pure tPEO gel after UV-irradiation is shown for comparison (◆).
tPAA at α = 0.5. In situ cross-linking of tPEO led to the appearance of a frequency independent storage modulus that remained larger than the loss modulus. Thus, the dynamic tPEO network could be transformed in situ into a covalently cross-linked network. However, relaxation of the tPAA can still be observed at higher frequencies. Preliminary small angle neutron scattering experiments showed that the local structure of the IPSAN was not significantly modified by the UVirradiation; see the Supporting Information. Figure 11a shows the frequency dependence of G′ at different α. In the presence of tPAA, we found that G′ was independent of ω over the whole accessible frequency range for α > 0.6, but for α < 0.6, we could observe the relaxation process of the tPAA network at higher frequencies. The characteristic relaxation time of the tPAA network within the permanent tPEO network increased rapidly with decreasing α. This means that the tPAA network within the IPSAN maintained its sensitivity to the pH even after the tPEO network was covalently cross-linked. At low frequencies, at which the tPAA network can completely relax, the plateau value of G′ corresponds to the elastic modulus of the tPEO network (Glf). Remarkably, the elastic modulus of the tPEO network within the IPSAN increased initially strongly with decreasing α before stabilizing at α < 0.6, see Figure 11b, although the values fluctuated significantly between different preparations. It appears therefore that self-assembly of tPAA into a network led to reinforcement of the tPEO network. The influence of tPAA on the elastic properties of the IPSAN was investigated further at α = 0.5 by varying the concentration of tPAA at a fixed tPEO concentration of 20 g/L; see Figure 12. The elastic modulus of the tPEO network increased rapidly with increasing tPAA concentration until about 10 g/L, beyond
Figure 9. Apparent hydrodynamic radius for mixtures of tPEO and tPAA containing equal concentrations (C) of each polymer at α = 1 and α = 0.5.
solutions was close to the average of the pure tPEO and tPAA solutions that were already reported elsewhere.16,31 Repulsive interaction caused a decrease of Rha at higher concentrations.
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COVALENTLY CROSS-LINKED IPSAN The B-blocks of tPEO contain units that cross-link when irradiated with UV-light. A rapid increase of the storage shear modulus at 1 Hz of the IPSAN was observed immediately after F
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Figure 11. (a) Frequency dependence of the storage shear moduli of irradiated IPSAN containing 20 g/L of each component at different ionization degrees: α = 1.0 (▽), α = 0.78 (■), α = 0.57 (◇), α = 0.50 (▲), α = 0.35 (⬡). (b) Evolution of the plateau storage modulus at low frequencies (Glf) as a function of ionization degree.
Figure 12. Concentration dependence of the plateau elastic modulus Glf at low frequencies as a function of tPAA concentration at α = 0.5 for irradiated IPSAN containing 20 g/L tPEO. The solid line is a guide to the eye.
Figure 13. Concentration dependence of the low frequency elastic modulus Glf of the tPEO network as a function of concentration for pure tPEO gels (open symbols) and for IPSAN gels at α = 0.5 formed in the presence of 20 g/L tPAA (closed symbols) after photo-crosslinking. The solid lines are guides to the eye.
which it remained constant at a value that was much larger than without tPAA. Nevertheless, it was still significantly below the value expected for the corresponding defect free tPEO network (νkT ≈ 2.6 × 103). Relaxation of the tPAA network was clearly visible only at higher concentrations of tPAA where its contribution to the total elasticity was significant. The influence of the tPEO concentration on the elastic modulus after UV-irradiation was investigated for IPSAN with the tPAA concentration fixed at 20 g/L (α = 0.5). The low frequency plateau modulus is compared with the elastic modulus of pure tPEO networks in Figure 13. A permanent gel was formed in the IPSAN at slightly lower concentrations (C > 15g/L) than in the pure tPEO (C > 17g/L). For both systems the low frequency plateau modulus increased sharply with increasing concentration above Cp due to rapid reduction of the defects in the covalently cross-linked tPEO network until it approached the value expected for a defect free network. The elastic modulus of the defect free tPEO network was almost the same for the IPSAN and the pure tPEO solution and the effect of adding tPAA on the elastic modulus of the tPEO network was only strong close to the percolation threshold. We made a preliminary investigation of the nonlinear mechanical properties by doing large amplitude oscillation measurements. Elsewhere we showed that pure tPEO gels shear harden before breakage16 and that the amplitude of shear hardening decreased with increasing stiffness of the gels, i.e., with increasing polymer concentration. In the present
investigation, we found that the irradiated IPSAN showed similar shear hardening and again the amplitude decreased with increasing stiffness of the tPEO network. We have seen that the stiffness of the tPEO network within the IPSAN can be increased not only by increasing the tPEO concentration, but also by increasing the tPAA concentration or by decreasing α. Interestingly, in both cases the effect of shear hardening was also reduced as the stiffness of the tPEO increased, see Supporting Information. This implies that the effect of shear hardening is related to the stiffness of the tPEO network and not simply to the tPEO concentration. We may conclude that interpenetration of the networks studied here did not lead to specific nonlinear rheological properties that cannot be obtained with the individual networks. The main synergetic effect of the IPSAN is that the tPAA network is formed at much lower tPAA concentrations in the presence of tPEO. For reasons that are not entirely clear, the presence of tPEO drives the formation of bridges between tPAA multiplets. The effect is all the more remarkable, because it was not seen when PEO homopolymer was added to a tPAA network. It seems that self-assembly of tPEO is a necessary condition to cause the effect. It may be useful in applications to be able to reduce the critical percolation concentration of a triblock copolymer by the addition of another triblock copolymer. This may be the case even if the total polymer concentration is not reduced. The G
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importance of the effect will depend on the particular system as is illustrated by the much smaller reduction of the percolation concentration of tPEO by adding tPAA than of the percolation concentration of tPAA by adding tPEO. The effect cannot be attributed to the formation of mixed micellar cores, as we have shown that the two polymers self-assemble independently. Further research is needed to elucidate this interesting phenomenon.
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CONCLUSION We have demonstrated that self-assembled interpenetrated polymer networks can be obtained by simply mixing solutions of two different triblock copolymers. This opens the perspective to exploit the wide range of existing block copolymers to develop new multiresponsive hydrogels in a straightforward way. IPSAN formed by self-assembly of tPAA and tPEO conserved the pH-sensitivity of tPAA and the UVsensitivity of tPEO. Addition of tPEO to tPAA led to a strong reduction of the percolation concentration of tPAA so that the high frequency elastic modulus of the transient IPSAN before UV-irradiation was much larger at low tPAA concentrations than the corresponding pure tPAA network. Addition of tPAA to tPEO led to a weak reduction of the percolation concentration of tPEO. As a consequence, the elastic modulus of the covalently bound tPEO network of irradiated IPSAN was much larger than the corresponding pure tPEO gels close to Cp.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Master curves, static light scattering at α = 0.35, dynamic light scattering, in situ irradiation, small angle neutron scattering experiments,32 and nonlinear rheology. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*(T.N.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS LLB and our local contact Jacques Jestin are thanked for providing us access to and help with small angle neutron scattering experiments. Lionel Lauber is thanked for synthesizing tPAA.
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REFERENCES
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dx.doi.org/10.1021/ma501990r | Macromolecules XXXX, XXX, XXX−XXX
