Polyelectrolyte brushes are monolayers of charged polymer chains attached on one side of the polymer to a surface while the other side of the chain remains free.[1-5] A remarkable characteristic of polyelectrolyte brushes is their capacity to respond to changes to ionic strength in the environment. Conformational changes of single chains translate into variations of the thickness of the entire brush monolayer. An increase in the ionic strength usually results in an abrupt reduction of the thickness of the entire brush monolayer called collapse.[6, 7] Collapse of a polyelectrolyte brush can be explained with electrostatic arguments, as the increase in the ionic strength in bulk reduces internal repulsion in the polyelectrolyte chains, which can then coil. It can be also explained by osmotic arguments since in conditions of low ionic strength the high density of monomers in a brush and their counterions generate a positive osmotic difference with bulk making the brush swell. When the ionic strength is increased, this osmotic difference is reduced and even overcome by the ions in bulk preventing the brush from swelling and triggering collapse.
Associated with this reversible variation in conformation and thickness, other properties of the brushes such as lubrication, hydrophobicity, and mechanical properties also change in a reversible manner in response to variations in the ionic strength.[8-12]
A variety of techniques have been applied to study the collapse: Atomic Force Microscopy (AFM), Ellipsometry, Quartz Crystal Microbalance with Dissipation (QCM-D), X-Ray, Neutron Reflectivity, and so forth.[9, 13-21]
Among the mentioned techniques, the QCM-D technique provides highly valuable information on the water content, thickness and mechanical properties of polyelectrolyte brushes during collapse as well as the brush response in real time. The resonance frequency f and the energy dissipation D of the shear oscillatory motion of a piezoelectric quartz crystal sensor, change upon adsorption or desorption of material on the surface. The measured parameters are highly sensitive to the mass and the mechanical properties of the surface-bound layer. Mass resolution is on the order of a few ng cm−2. The QCM-D technique is not only sensitive to the adsorbed molecules, but also to the solvent retained within or hydrodynamically coupled to the surface-bound film. It is hence often difficult to extract the adsorbed molecular mass from the QCM frequency response alone, that is, to separate the contribution of the adsorbate from the contribution of the solvent that is coupled to it. Moya et al. showed that QCM-D could be applied to quantify the loss of water during collapse from positively charged poly(2-methacryloyloxy ethyl trimethyl ammonium chloride) (PMETAC) brushes synthesized from thiol initiators forming a monolayer on the gold coated QCM-D resonators. The collapse of the brush with the ionic strength results in a reversible increase of the frequency, meaning that mass is being lost from the brush. As the change is reversible and the polymer is anchored to the gold surface the mass lost when the ionic strength corresponds to water being liberated. Decreasing ionic strength the brush mass increases again. Moreover, a reversible decrease in dissipation could be observed when the brush collapsed hinting a change from a viscoelastic to a more solid behavior. In the same article, the authors show that the brush behaves as an ionomer when the collapse is induced by divalent salts that act as crosslinkers for the monovalently charged monomers in PMETAC. Once the brush is exposed to the divalent salt at a high concentration the frequency response is similar to a brush in the collapsed state even if the bulk solution is water. The observations from QCM-D were corroborated with AFM. By means of QCM-D the same authors also showed how the exchange of Cl− counterions by ClO4− in PMETAC can induce a sort of collapse, a so called “hydrophobic collapse.”[24, 25] QCM-D shows that the exchange of the counter ions for ClO4− changes the water content of the brush and makes the brush lose viscoelastic characteristics. The brush with the new counterions collapses more pronouncedly and at lower ionic strengths compared to Cl−. The microbalance was also used to show ion retention and slow ion release when the brush collapses in these conditions. The technique showed as well that in the presence of bulky organic ions the brushes do not collapse.
The QCM-D technique provides significant insight into the behavior of polyelectrolyte brushes during collapse. It allows the determination of the amount of water lost during the collapse, and to follow the behavior of the brush in the presence of different ions. It is also fast and simple. However, proper data interpretation requires that the QCM-D results be validated with other techniques such as AFM as in the cited papers or by means of elipsometry and contact angle, IR, and so forth.
In this article, we want to address the latest work dealing with the use of QCM-D and brush collapse. In particular, we will show how the combination of QCM-D with ellipsometry in a single device increases the capabilities of the technique to provide a better description of the collapse process.
Quantitative Evaluation of QCM-D Data
The Sauerbrey equation links frequency shifts and adsorbed mass per unit area in a very simple manner:
with the mass sensitivity constant, C = 18.06 ± 0.15 ng cm−2 Hz−1 for sensors with a resonance frequency of 4.95 ± 0.02 MHz, and the overtone number i. This acoustic mass accounts for the mass of the adsorbed polymer and the mass of the solvent that is trapped inside or hydrodynamically coupled to the polymer film. The applicability of eq. (1) is limited to sufficiently rigid films.
Equation (1) can be applied to calculate the percentage of mass released as water from the total brush mass, as a function of the ionic strength but it is not possible to know how much the hydration of the brush has been affected by the ionic strength since the mass of the polymer chains is unknown.
These issues has been addressed in Iturri and Moya applying QCM-D and spectroscopic ellipsometry in a combined set up to measure the polymer mass and water content of charged polyelectrolyte brushes of poly(potassium sulfo propyl methacrylate) (PSPM) and PMETAC.
Combined QCM-D and Ellipsometry
Spectroscopic ellipsometry is an optical technique where the change in the polarization state of an incident light beam upon reflection at an interface is measured. The polarization change is measured in terms of the ellipsometric angles ψ and Δ as a function of the wavelength λ. The simultaneous in situ determination of ψ and Δ can provide, by proper data treatment, quantitative information on the refractive index, thickness, and mass of thin films at planar interfaces.[29, 30] Ellipsometry is sensitive to differences in the optical density between adsorbate and bulk solution, it essentially senses the adsorbate mass. The set up is sketched in Figure 1.
The adsorbed mass per unit area was determined from the ellipsometry using de Fejter's equation:
Polyelectrolyte brush was treated as a single layer assumed to be transparent and homogeneous (Cauchy medium), with a given thickness, dopt, a wavelength-dependent refractive index nbrush(λ)= Abrush + Bbrush (λ μm−1)−2, and a negligible extinction coefficient (kbrush= 0). Abrush, Bbrush, and dopt were fitted simultaneously. The semi-infinite bulk solution was also treated as a transparent Cauchy medium, with a refractive index of nsol(λ) = Asol + Bsol (λ μm−1)−2. For water, Asol = 1.323 and Bsol = 0.00322 were estimated from the literature. The optical properties of the sensor's coating, that is, gold layer were fixed to the values established during calibration.
To calculate mopt, it was considered the refractive indices at λ = 632.5 nm, and used a refractive index increment of dn/dc = 0.150 cm3 g−1.
The mass determined by QCM-D and ellipsometry respectively, can be employed to calculate the solvent content of the film. To this end, we define the hydration as the percentage of solvent contributing to the total film mass:
Then, it is possible to relate the changes in water content in terms of hydration.
Quantification of Water Lost During Collapse
In Iturri and Moya, the authors show how the hydration of the brush changes with the ionic strength, that the ratio of PMETAC and PSPM brushes is affected by the ionic strength, which was varied from 0.1 to 1 M NaCl.
In Figure 2, we can observe the changes in mass during the collapse of PMETAC and PSPM and the relative percentages of water lost during the collapse.
At 0.1 and 1 M NaCl, the water mass in the PMETAC film reduces by 26% of the initial aqueous content in 1 M NaCl, which was 67% of the total brush mass. For the PSPM brush synthesized with a co-catalizator to control polymer growth, (PSPM 4:1), there is a negligible loss of mass at 0.1 M NaCl but the brush releases, 32% of its content at 1 M NaCl from a total 45% of water. In the case of freely grown PSPM brushes, without co-catalizator, exposure to 0.1 M NaCl and lower ionic strengths (not shown) leads, surprisingly, to a mass increase in the brush. The lack of change in the mass of the PSPM grown with co-catalizator is an unexpected result and could be explained as a conformational change of PSPM chains protruding from the brush towards solution. PSPM brushes freely synthesized show long chains extending into bulk that form a loose corona over the dense brush arrangement. If several of these chains coil on top of the dense brush region, the distance between different protruding segments of chains will become smaller and the QCM-D could sense a sort of extra layer, including both the chains and the water entrapped in the coiled structure.
An immediate conclusion from the QCM-D/ellipsometry studies is that the increase in ionic strength does not cause complete dehydration, consequently the monomers remain solvated in higher ionic strength conditions. In the same article, by Iturri and Moya, it is shown that replacement of NaCl by LiClO4 in PMETAC has a stronger impact on the water content resulting in a more profound dehydration of the brushes as the ClO4− replaces the Cl− counterions generating a more hydrophobic environment.
Following the exposure of PMETAC brushes to a 0.05 M LiClO4 solution, a release of 1.65 µg cm−2 of water takes place, corresponding to 54% of the initial water content, equivalent to exposing the brush to 1 M NaCl.
The combination of QCMD and elipsometry was also very useful to study the behavior of polyelectrolyte brushes with different chain densities with ionic strength. Iturri and Moya studied the water content of PMETAC brushes synthesized from mixed monolayers of thiol initiators, ω-mercaptoundecyl bromo isobutyrate, and non initiating blank thiols, 1-undecanethiol 98%. The density of the initial thiol solution was varied by diluting them in a blank thiol. The percentage of initiator varied from 1 to 100% in the monolayer.
Once the water content of the brush is obtained from the combined QCM-D/ellipsometry, the water lost during the collapse with the ionic strength can be calculated in relation to the total water content and the hydration of the brush.
The authors observed that the water content remained between 70 and 80% for all the brush densities even for the 1% initiator with the exception of the 50% initiator sample that is significantly lower.
The differences in water content among the different initiator densities correspond to the free water and represent 10% of the total hydration. This means that most of the water in the brushes is water associated with the monomers and not freely entrapped water.
When the brushes were exposed to NaCl solutions with concentrations, ranging from 0.05 to 1 M, it was observed that the water mass variations were bigger as the percentage of initiator was higher, Figure 3. The percentage of water lost remains relatively static at approximately 30% for all the initiator percentages except for 25 and 50% where the maximum lost is reached with 40%. Again a full dehydration of the brush cannot be achieved even under exposure to 1 M NaCl.