Micelles and Interpolyelectrolyte Complexes formed by Polyisobutylene-block-Poly([meth]acrylic acid) - Synthesis of Polymers and Characterization in Aqueous Solutions
Markus Burkhardt (08/2003-06/2007)
Support: Axel H. E. Müller
In this work PIB-b-PMAA copolymers with low PDI were studied, which self-assemble in aqueous solutions. A wide range of hydrophobic and hydrophilic block lengths were synthesized via combination of cationic an anionic polymerization.
The data we have obtained by means of SANS and DLS point to an interesting dynamic behaviour of such micelles reacting on external stimulus of changes in pH from 10 to 7 and 5 respectively. The response is not only related to a change of the degree of neutralization of the PMAA block in the corona. Quantitative evaluation of SANS curves also shows a change of the size of the hydrophobic core formed by the PIB blocks, due to a change of the aggregation numbers.
From cryo-TEM, a spherical shape of the micelles is clearly seen. This allows us to evaluate the SANS data using a model of a spherical particle with protruding arms. Evaluation of the SANS curves evidences about changes in Nagg with pH and with ionic strength. The higher the pH is, the more the arms of the micelle repel their neighbours and the higher the area at the core-corona interface of the micelle is. This leads to decreasing values of Nagg with rising pH. An increase in ionic strength has an opposite effect, resulting in higher Nagg upon improving screening of the charges of the PMAA.
DLS measurements also show the response of the corona of the micelle on external stimuli. In principle, the PMAA block is more stretched the higher the number of charges on the arms of the micelles are. This also leads to an increasing Rh. Here the hydrophilic block dominates the response of the aggregate. For DLS, the influence of the PIB core and therefore the changes in Nagg can be neglected due to the longer PMAA block compared to the PIB block of the copolymers used in this work.
Potentiometric titrations also show an effect of the ionic strength on the apparent pKa value, shifting it to lower values with increasing cNaCl, while the length of the hydrophilic block seems to play a minor role.
For the evaluation of the cmc for different diblock copolymers the PIB block determines the properties of the micellar assemblies as well. The cmc clearly depends on the length of the hydrophobic PIB block. The longer the block is, the lower the cmc is found to be.
Additionally a detailed investigation of IPECs formed by PIB-b-PMAA with P4VPQ is presented. The PIB-b-PMAA described above, are used to form water-soluble complexes with core-shell-corona structure. From cryo-TEM images, a spherical shape of the IPECs can be concluded. Slight differences in the overall shape of the complexed micelle give a hint on the proposed structure.
The process of formation of complexes can be subdivided in a kinetically driven and a thermodynamically driven process. Upon addition of the polycation to the micellar PIB-b-PMAA solution, first an increase in turbidity of the solution can be observed. In this kinetically driven regime, large assemblies of micelles are formed. With time, these aggregates are equilibrating toward the thermodynamically more stable species of a single micelle with a complex species formed around the hydrophobic PIB core. The formation process can also be seen by means of SANS, leading to higher scattering intensity with increasing Z.
SANS was used to follow the salt-induced dissociation of the complex as well. Increasing ionic strength of the IPEC solution leads to a release of the polycation, starting from about 0.2 M NaCl. Beyond 0.6 M NaCl, almost no difference in scattering behaviour of the IPEC solution compared to pure micelles can be stated. This suggests a total dissociation of the IPEC. By means of titration with a sodium selective electrode, the decrease of the activity of the Na ions can be explained by substitution of the polycation due to Manning condensation.
Additionally the influence of point of time of addition of salt to an aqueous solution of a new diblock copolymer, PIB-b-PAA, is presented. By means of cryo-TEM, DLS and SANS an effect on the shape of the particles formed in solution could be obtained, whether the salt was added before dissolution (BD) or after dissolution of the polymer (PD). For the BD samples, a high PD of the particles can be seen from in cryo-TEM. Additionally sedimentation of a certain part of the polymer is another hint on larger aggregates. For the PD samples, spherical micelles with a core-corona structure are visible. According to cryo-TEM, their PDI is quite low. This suggests that interparticle exchange of unimers between the micelles is possible, at least before addition of the NaCl.
Furthermore, it was shown, that changing the counterion to the "softer" Cs still allows the formation of equilibrium structures for BD samples, as seen from the spherical structure in cryo-TEM images. The influence of solvent for the SANS samples leads to a similar scattering behaviour for all measured samples.