3.3 Results and Discussion
3.3.4. Bottlebrushes with Poly(Acrylic Acid) Side Chains Results
a feature with a 9.8nm length scale. Since the side-chains are contrast matched to the solvent, this must be due to a backbone-backbone correlation length. Polymer mass fractals do not give rise to scattering peaks and this length scale is not explained by the overlap scaling shown in Figure 3.7. Thus we hypothesize that this peak is direct evidence of a lyotropic liquid crystalline phase with the 9.8nm corresponding to the characteristic intermolecular spacing. Lyotropic phases have been observed for bottlebrush polymers in solution [72-74], but to the best of our knowledge this is the first molecular level characterization of such a phase.
3.3.4. Bottlebrushes with Poly(Acrylic Acid) Side Chains
Table 3.8: Hydrodynamic radii obtained from DLS for PAA-95 based bottlebrushes in a series of buffer solutions with varying pH and salt concentration with the PtBA-95 based pre-polymer data for comparison.
The Rh in pH 5 buffer is significantly smaller for all bottlebrushes than the Rh in pH 9 buffer with the results for the PtBA in good solvent falling in between the two. The difference between the Rh for the PAA brush at pH 9 and the corresponding PtBA brush in toluene becomes smaller with increasing Nb. Adding salt serves to shield the charges of the acid groups from each other and reduce the repulsion between them. For 730-PAA- 95 at pH 5 the effect of the additional salt is minimal while at pH 9 the addition of NaCl serves to reduce the Rg by about 20%. Interestingly, it appears that at pH 9 with 1M salt, 730-PAA-95 has a smaller Rh than 730-PtBA-95 in toluene.
It has been previously established that the hydrodynamic radius is closely related to RCS, the length scale of the side-chains perpendicular to the backbone contour. SANS experiments were conducted on 0.3 wt% solutions of 140-PAA-95 and 730-PAA-95 in deuterated buffers corresponding to those used for DLS. Radially averaged, solvent subtracted scattering patterns (obtained at ORNL) are shown in Figure 3.11.
Figure 3.11: Radially averaged SANS patterns for two bottlebrushes with poly(acrylic acid) side-chains (140-PAA-95 top and 730-PAA-95 bottom) at 0.3wt% in aqueous buffers with varied pH and salt (NaCl) concentration. The red squares correspond to a basic condition (pH = 9) at the lowest accessible salt concentration (0.1M) with a large percentage of the acid groups deprotonated. The blue diamonds correspond to pH = 5 which is near the initial pKa of poly(acrylic acid). The open green circles correspond to pH = 9 with 1M salt concentration.
Scattering curves I(q) for 140-PAA-95 and 730-PAA-95 show qualitatively similar features to those of the equivalent PtBA-95 based brushes. There is a mid q plateau corresponding to the side-chain dimensions, followed by steep rise due to the surface fractal scattering and finally either a plateau region (although for 730-PAA-95 at pH 9 and 1.0M salt, the scattering intensity continues rising at low q). For the samples in pH 9 buffer at 0.1M salt concentration the scattering appears to be more complex than
that observed for the other conditions, with multiple hump features indicating a hierarchical structure. For 730-PAA-95 these are seen at q ~ 0.003Å-1, q ~ 0.006Å-1, masked by the surface fractal scattering and then reappear at q ~ 0.05Å-1 and possibly q ~ 0.1Å-1, although higher q features are difficult to resolve due to the weak signal. The q locations of these features are multiples of each other, which suggests that they are the scattering harmonics of some structural length scale.
Determination of the overall length scale of the bottlebrushes was not possible due to the concentration being above the overlap concentration as determined in section 3.3.2. A two-level Beaucage scattering function was successfully fitted to the data using nonlinear least squares fitting of all parameters and the higher q (q > 0.02Å-1) length scales are reported in Table 3.9.
Table 3.9: Cross-sectional radius determined by fitting a Beaucage I(q) to the SANS in Figure 3.11.
The RCS values shown in Table 3.9 are the averages of those obtained for 140- PAA-95 and 730-PAA-95 and are independent of the backbone length as was seen for PS and PtBA bottlebrushes in organic solvents. RCS values are strongly dependent on the pH and the salt concentration. For 0.1M salt concentrations the RCS increases by 27% going from pH5 to pH9, with the side-chains significantly more extended when in the charged state. Interestingly, adding salt leads to a significantly more compact conformation for the side-chains with RCS 10% smaller than that for PtBA-95 in toluene.
Discussion
As is the case for free linear poly(acrylic acid), the side-chains grafted to a polynorbornene backbone undergo conformational transition with pH[77]. The side chain dimension RCS is expanded by raising the pH and thereby increasing the charge of the side-chains and their repulsive interactions. Adding salt to shield the electrostatic interactions of the side-chains leads to a collapsed state in which the conformation is more compact than that for the equivalent PtBA side-chains in good solvent conditions.
This is consistent with the weak scaling of PAA Rg with Nb for aqueous solutions at high salt concentrations, indicating that such solutions are similar to theta solvents[78]. In order to facilitate more quantitative conclusions, further study of a series of PAA side chain lengths will be needed.
The overall dimensions of a limited set of bottlebrushes with polyelectrolyte side- chains have been studied by Atomic Force Microscopy[79, 80] and by dynamic light scattering[81]. It was observed that their apparent size is significantly larger at pH 9 than at pH 5 as is the measured persistence length[81]. In the AFM studies it was observed that densely grafted bottlebrushes with PAA side-chains had fully extended backbone conformations at all pH values[79]. This is not consistent with our observed side chain dimensions for PAA-95 in pH 5 buffer, which are only marginally more extended than PtBA-95 in good solvent (10%). SANS patterns show a clear plateau at low q for both 140-PAA-95 and 730-PAA-95 at 0.1M salt concentrations and do not show a power law dependence of I(q) ~ q-1 as would be expected for a fully extended cylindrical object[54].
The only sample showing power law scattering at low q is 730-PAA-95 at pH 9 with 1.0M salt concentration. We hypothesize that this is due to aggregation of the
bottlebrushes due to the hydrophobic interactions that become more dominant when electrostatic interactions are shielded[80].
We have established the feasibility of connecting the behavior of polyelectrolyte brushes to their ordinary hydrophobic analogues. We have also directly observed the change in the conformation of side-chain dimensions with pH and salt concentration.
Future experiments on this family of materials, incorporating multi angle light scattering and SANS at lower concentrations, will enable the determination of persistence length and conformation in solution.