AMPHIPHILIC GRADIENT COPOLYMERS: SYNTHESIS AND
3.3 SELF-ASSEMBLY .1 Gradient Copolymers
The association of poly(acrylic acid-grad-styrene) (P(AA-grad-S)) gradient copoly- mers with ∼70% of AA in aqueous solution has been studied by combination of static and dynamic light scattering methods [36]. It was found that, in contrast to homologous diblock PAA-b-PS copolymers, the gradient copolymers do not undergo cooperative association into micelles and that the solution contains unimers coexist- ing with large and loose aggregates. At the same time it was demonstrated that the gradient copolymers exhibit sufficiently high surface activity.
A comprehensive theoretical study of self-assembly of symmetric gradient copoly- mers with variable shape of compositional gradient was performed using Monte Carlo simulations [37]. It was demonstrated that selective solvent copolymers with sharp gradient undergo self-assembly following the closed association mechanism. The micelle-like core-corona structures with a narrow size distribution that result are sim- ilar to conventional diblock copolymer micelles that arise in a solution of gradient copolymers beyond the CMC threshold. However, copolymers with smooth gradi- ents associate and form aggregates of ill-defined shape and wide size distribution, which is a monotonically decreasing function of the number of chains in the aggre- gate. This feature is typical for the open (noncooperative) association mechanism. At high copolymer concentrations, ordered arrays of micelles are observed in a solution of copolymers with sharp gradients of composition whereas copolymers with smooth gradients form gel-like structures.
k k 3.3.2 Diblock-Gradient Copolymers
In contrast to symmetric gradient copolymers, the diblock-gradient copolymers form a gradient block related to a pure solvophilic block and exhibit cooperative associa- tions in selective solvents that give rise to well-defined nano-aggregates. In this case the poor solubility of the gradient block is the driving force behind the association of the block-gradient copolymers. Moreover, the presence of a (small) fraction of solvophilic monomer units in the gradient block brings to the self-assembly process a feature that ensures both its reversibility and stability.
The pH-controlled self-assembly of amphiphilic ionic block-gradient copolymers of styrene and acrylic acid, P(AA-grad-S)-b-AA, in dilute aqueous solutions was studied in reference [36] by combination of DLS, SANS, and TEM. The molar fraction of hydrophobic S monomers in the gradient blocks was varied in the range 40–50%, whereas the molar fraction of hydrophilic AA monomers in the whole block-gradient copolymer was∼70%.
The DLS titration experiments indicated that the apparent hydrodynamic radius of the solute particles remains virtually constant (∼2 nm) in the alkaline conditions (pH>7–7.5), which corresponds to molecular solution of block-gradient copolymers with mostly deprotonated (ionized) AA functions. A decrease in pH below pH= 7 provokes a monotonous increase in the hydrodynamic radius of the particles that can be interpreted to result from the formation of finite size (7–8 nm) aggregates followed by their coalescence into larger aggregates and eventually, below pH=4, by macrophase separation. Interestingly, the onset of association caused by partial protonation (deionization) of AA functions in both P(AA-grad-S) and PAA blocks occurs at pH higher than pKafor the AA monomers. This is a consequence of shift of local pH inside the aggregates toward lower values compared to the pH in the surrounding solution. The coupling between association and ionization is an inherent feature for amphiphilic ionic pH-sensitive block copolymers [178, 179].
The most remarkable feature of the pH-controlled assembly of P(AA-grad-S)- b-PAA copolymers is its perfect reversibility in cycles of pH variation (with the accu- racy of effects caused by increasing ionic strength of the medium). This behavior is strikingly different from irreversible association of PAA-b-PS block copolymers with the same net composition: once formed (e.g., upon dialysis from the common for both blocks solvent against water), the micelles of the block-copolymers cannot be re-arranged upon action of such external stimuli as pH or ionic strength due to the glassy state of the solvent-free PS core. On the contrary, presence of 50–60%
of AA units in the gradient blocks does not prevent their association at neutral or slightly acidic pH but ensures the persistence of the dynamic character of this associ- ation, possibly due to larger residual amount of water in the styrene-rich core domains formed by the gradient blocks.
The SANS experiments performed in solutions of P(AA-grad-S)-b-PAA in D2O confirm the effect of pH-triggered reversible association. The typical SANS spectra for solution of P(AA-grad-S)-b-PAA copolymer obtained at different pH are pre- sented in Figure 3.11.
k k
0,02 0
2 4 6 8 10 12 14 16
0,04 0,06
q, A–1
I(q), cm–1
0,08 0,10
pH 5.3 pH 5.9 pH 6.4 pH 7 pH 7.6 pH 8
Figure 3.11 SANS spectra from 10 w% solution of P(AA24-grad-S46)-b-PAA100copolymer at varied pH.
As can be seen in Figure 3.11, the scattering curves exhibit a pronounced cor- relation peak at pH < 7.5. Upon lowering of the pH the magnitude of the peak increases, whereas the peak position is displaced to lowerq. This behavior should be interpreted in terms of increasing repulsion between aggregates of growing size (increasing aggregation number) and concomitant increase of the average interparti- cle distance, which is in a line with the DLS results on pH-controlled association.
Another remarkable feature of the SANS spectra presented in Figure 3.11 is the presence of only single (correlation) peak, whereas no signatures of the form-factor can be observed. This points to a wide size and/or shape distribution of the scat- tering particles. In reference [36], the SANS curves were successfully fitted by using superposition of the form-factor of polydisperse spheres and Percus–Yevick structure factor.
Similar to DLS-titration curves, the SANS spectra exhibit complete reversibility in cycles of pH variation, thus confirming the dynamic (equilibrium) nature of the block-gradient copolymers assembly.
Finally, the TEM images (Figure 3.12) provide an unambiguous evidence of self-assembly of the block gradient copolymers P(AA-grad-S)-b-PAA into spherical micelles with a wide size distribution, and this explains both relative smoothness of the DLS titration curves and absence of any characteristic features in the SANS spectra in theq>qmaxrange.
This broad size distribution of the aggregates points to relatively low cooperativity of the association of block-gradient copolymers that may be explained by low sur- face tension at the diffuse interface between the P(AA-grad-S) core and PAA corona domains.
k k
100 nm
Figure 3.12 TEM image of micelles formed P(AA24-grad-S46)-b-PAA100at pH=6. The light gray spots correspond to contrasted styrene-rich domains.
3.3.3 Triblock-Gradient Copolymers
Triblock gradient copolymers P(AA-grad-S)-b-PAA-b-P(AA-grad-S) comprise a central hydrophilic PAA block terminated at both ends with amphiphilic gradient P(AA-grad-S) blocks. The pH-triggered association of the terminal blocks governs the assembly of the triblock-gradient copolymers in aqueous solutions that was studied in reference [35]. At low polymer concentration, this assembly leads to formation of flower-like spherical micelles. In such micelles both gradient blocks are incorporated in the styrene-rich core domains, whereas the central PAA blocks form loops that constitute the hydrated corona of the micelles. These flower-like micelles resemble the star-like micelles formed by the P(AA-grad-S)-b-PAA diblock gradient copolymers. However, at the higher copolymer concentration, the second association mechanism comes into play: The bridging attraction emerges due to the possibility of exchange of terminal association blocks between neighboring hydrophobic domains, namely the conversion of some loops into bridges that is accompanied by the entropy gain on the order of thermal energy per bridge [180].
Therefore concentrated solution of triblock-gradient copolymers can be envi- sioned as an array of partially interpenetrating flower-like micells connected by bridges, namely representing a physical gel. Hence one can expect that, upon variation in pH, a reversible sol-to-gel transition can occur in semi-dilute solution of triblock gradient P(AA-grad-S)-b-PAA-b-P(AA-grad-S) copolymers. This transition is manifested in specific pH-dependent rheological properties of solution of the triblock-gradient copolymers as compared to those of homologous diblock-gradient copolymers. Indeed, it was demonstrated [35] that at high pH both behave as Newto- nian liquids with zero-shear viscosity, which is (at 10 w% copolymer concentration) 5 to 7 larger than that of pure water. However, a decrease in pH leads to a sharp increase of viscosity (by more than 4 orders of magnitude) in a narrow range of the pH-variation in the case of triblock-gradient copolymer. Remarkably, only a
k k
1E-3 0,01 0,1 1 10 100
1 10
w, rad/s
G', G'' , Pa
G'' pH 8 G'' pH 7–5 G' pH 7–5 G' pH 7–2 G'' pH 6–8 G' pH 6–8 G'' pH 6–5 G' pH 6–5 G' pH 8
Figure 3.13 The frequency dependences of the storage and loss moduli in aqueous solution of P(AA4-grad-S24)-b-PAA140-b-P(AA4-grad-S24) copolymer at different pH-values.
moderate increase in viscosity was observed in the same pH-range in the solution of a homologous diblock gradient copolymer. This finding points unambiguously that the much stronger increase in viscosity in the solution of triblock-gradient copolymers is due to formation of bridges interconnecting the hydrophobic domain.
Clearly, such bridging does not occur in the solution of homologous diblock-gradient copolymers.
The reversible pH-triggered sol-to-gel transition is also manifested in the vis- coelastic properties of the solution of triblock-gradient copolymers. The analysis of the frequency dependences of the storage and loss moduli in Figure 3.13 shows that at high pH both G′and G′′ are increasing functions of frequency, G′′ >G′and G′′
<G′at low and high frequency, respectively. This kind of the frequency dependence of the dynamic moduli is typical for viscous liquids. A decrease in pH results in a pronounced increase in the magnitude of both moduli, and they become indepen- dent of frequency, G′is larger than G′′ by the order of magnitude. This behavior is characteristic for strong gels.
Hence solutions of triblock-gradient copolymer undergo a reversible pH-triggered sol-to-gel transition, which is manifested in dramatic changes in their viscoelastic behavior.