Chapter 4: ”Mechanoresponsive Polyelectrolyte Brushes” [3] introduces a promising technique for local detection of stress distributions with outstanding resolution. There-fore stress is translated by a mechanoresponsive polyelectrolyte brush into an optical output.
Accurate knowledge of stress distribution in the contact area is crucial for understand-ing soft matter contact situations. The key challenge in the experimental studies of stress distributions in soft matter contacts is the demand of combining high stress sen-sitivity (on the order of kPa) with high lateral resolution (below micrometer). Classical solutions, such as stress sensors (often called pressure sensors) using the deflection of mechanical elements like membranes as a means for quantifying stresses are reaching fundamental limits in terms of the lateral dimensions. Even most sophisticated micro-electromechanical system approaches (MEMS) have so far only reached the pressure sensitivity for lateral dimensions of >> 10µm. Mechanoresponsive materials even in their early stages of developments, overcome these fundamental limitations. In these systems, a mechanical stimulus directly affects the electrical, chemical or optical prop-erty of a material sensor. For these material based approaches, the limiting factor in terms of lateral resolution is how locally the material responds to external pressure and how accurately these changes can be read out.
Polymer brushes are particularly interesting in this respect, since they consist of in-dividual, surface grafted, but not laterally crosslinked polymers. The weak lateral coupling, indeed, is a necessary condition for high lateral resolution. At the same time, polymer brushes are themselves soft matter systems and thus match the typical range of elastic properties and deformability, allowing for suitable sensitivity. The key chal-lenge however is to modify the polymer brushes such that their compression state can
be read out in a simple fashion with high lateral resolution.
We developed promising mechanical addressable surfaces, that report stress fields by translating a mechanical stimulus (stress) into an optically detectable response in aque-ous solution (Figure 1.2A). These surfaces were realized on the basis of cationic, flu-orescently labeled polyelectrolyte brushes: Poly[2-(Methacryloyloxy)Ethyl] Trimethyl Ammonium Chloride (PMETAC) copolymer brushes labeled with carboxyfluorescein dye (CF). The dye molecules are covalently immobilized on the brush. Such sur-faces report stress by a change in fluorescence due to dye quenching. Polymer brush compression leads to an association of CF with the quaternary ammonium groups of METAC, while local stretching of the chains causes a decrease in quenching. Quanti-tative characterization of the mechanoresponsive properties of polyelectrolyte brushes were performed using soft colloidal probe AFM introduced in Ch. 3.
Pressure was applied to the brushes using an atomic force microscope (AFM) equipped with a cantilever functionalized with an elastomeric probe made of PDMS. Due to me-chanical deformation of the soft colloidal probe, the contact area of the system is large enough to be monitored with a confocal laser scanning microscope (CLSM) in situ.
Upon contact of the SCP with the surface, a dark spot surrounded by a bright rim occurs (Figure 1.2A). In order to understand the behavior of the observed response, the contact situation underneath the PDMS bead is modeled using the contact me-chanics theory of Johnson, Kendall, and Roberts (JKR). The JKR model describes the contact as interplay between elastic deformation and adhesion (Figure 1.2B). The resulting stress distribution underneath the bead remains compressive at the center, while stresses are tensile at the edge of the contact area. We can assign the decrease in fluorescence intensity (as compared to the background intensity) to areas of compres-sion and the slight increase at the rim of fluorescence to areas of tencompres-sion. With this observation, a response function I(p) which correlates local fluorescence intensity (I) to local (calculated) stress (p) was established (Figure 1.2C) . We demonstrated that stress distributions could be translated into local fluorescence signals with a lateral resolution limited by the optical read-out (1 micron) and a stress sensitivity of at least 10 kPa.
Also, the response of the sensor stabilized well before the acquisition time (1−2 s) and it is constant over several minutes and completely reversible.
Further, brush compression and quenching can be induced by the addition of salts. We could show that the dependency of the relative intensity on pis only weakly changing with salt concetration of the solution.
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Figure 1.2: Mechanoresponsive surfaces: 1.2A Experimental design to measure the fluorescence-based readout. PMETAC copolymer brush with covalently immobilized CF (brush height H ≈ 100 nm) compressed with a SCP (Radius R ≈ 15µm) and observed fluorescence signal, 1.2B Detected intensity and calculated stress profile, 1.2C Determined response function
Phototunable Surface Interactions
Chapter 5: ”Phototunable Surface Interactions” [4] reports on a novel approach to tune surface interactions gradually with light.
Gradual tuning of surface properties, in particular wettability , adhesion, and friction is important for a large number of applications and allows matching of surface properties for the desired application. For example surface gradients can be used for manipulation of the motion of liquids or to prepare water harvesting surfaces. Another application is controlled attachment or detachment of chemical compounds. This can be used for instance in drug delivery systems or lab on the chip devices.
Instead of tuning the surface properties by variation of the synthesis protocol (as vary-ing the molecular architecture) this chapter inserts a simple alternative to tune sur-face properties via light. Light responsive polymer brushes were obtained by sursur-face initiated atom transfer radical polymerization (ATRP) of a methacrylate monomer containing ionizable -COOH side groups caged with photo-removable 4,5-dimethoxy-2-nitrobenzyl (NVOC) (Figure 1.3A). Photo-response was possible using photo labile caged compounds. The neutral polymer brush (PNVOCMA) transforms to a charged, hydrophilic poly(metacrylic acid) polymer (PMAA) brush upon exposure with ultravio-let light (λ= 365 nm) due to removal of the o-nitrobenzyl groups. The light-dependent compositional change can be controlled by exposure time, intensity and allows to define intermediate interfacial states (instead of variation of brush length or grafting density).
As a consequence the surface properties change. We show how the physical properties, in particular wettability, hydrophobicity, adhesion, and lubrication of the brush can be gradually tuned with the exposure dose using quarz micro balance technique, conden-sation microscopy, atomic force microscopy (AFM), force mapping and friction force spectroscopy. For this purpose patterned brush substrates were prepared by irradiated through a structured quartz mask. In this way an internal standard was conserved in the experiment that allowes comparison between different samples and to create an internal reference for the surface properties.
We obtained a relationship between photoconversion and irradiation dose and followed the light-modulated generation of hydrophilic COOH groups using quartz crystal mi-cro balance technique. Here the water uptake of the hydrophilic polymer brush was detected that increases as a function of time (Figure 1.3B). Visualization of the wet-tability differences between the PNVOCMA and the PMAA polymer brushes was also possible by condensation microscopy. Water condensed primarily on the exposed re-gions, i.e. PMAA-rich areas that are more hydrophilic than areas covered by unexposed
PNVOCMA. This can be for example exploit for selective adsorbtion of particles and therefore for the design of hirachical structures from colloidal building blocks. Further, using imaging AFM, we could identify a topographic contrast of around3 nm between 0% und 100% conversion that can be asigned to the release of NVOC groups.
As a consequence of photoconversion, interfacial surface forces change as well. To analyze the physical properties of the defined intermediate chemical states we used force spectroscopy and friction measurements. Therefore, we determined the interfa-cial properties between the polymer brush and the cantilever probe (SiO2tip). By force spectroscopy and quantitative imaging (every pixel of a detected image contains infor-mation on adhesion and repulsion) we demonstrated that adhesion forces on irradiated areas decreased and repulsive forces increased due to electrostatic repulsion. Above 75% conversion, a clear contrast between irradiated and no irradiated areas could be observed, which was not detectable below 50%. Also, solvent forces and steric inter-actions contribute to this behavior. In case of friction measurements we could observe a continuous increase of friction force contrast between PNVOCMA and PMAA areas (Figure 1.3C).
Interactions of Spherical Polyelectrolyte Brushes
Chapter 6: ”Interactions of Spherical Polyelectrolyte Brushes” [5] covers, how surface properties, in particular interaction forces, can be tailored to adjust the adsorption behavior of spherical polyelectrolyte brushes for hierarchical particle organization.
Alternative to functionalize surfaces with PE molecules such as for example using poly-mer brushes (as shown in Ch. 5), surface modification is also possible using colloidal building blocks. This offers interesting possibilities since the colloids can carry vari-ous functionalities. Additionally, the size of colloidal particles increases the adsorption energy as compared to single (macro-) molecules while ensuring that interfacial inter-actions are dominant over inertia or other forces for the macro-scale.
Understanding the underlying interactions between the colloidal building block and the substrate of interest is fundamental for surface modification and further applications.
Examples are the design of hierarchical structures of metal colloids out of suspension that allows surface enhanced raman spectoscopy due to plasmon coupling between ad-jacent particles.
In this work, we investigated the interaction of the colloidal building blocks, i.e. anionic spherical polyelectrolyte brushes (SPB: Polystyrene (PS) core and attached polystyrene sulfonate (PSS) chains) and substrates functionalized with polyelectrolyte multilayers
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Figure 1.3: Phototunable surface interactions: 1.3A Photo-responsive system 1.3B time and water uptake vs. photo conversion and 1.3C friction contrast vs. photo conversion.
consisting of polystyrene sulfonate (PSS) and poly-(diallyl dimethyl ammonium chlo-ride) (PDA) (Figure 1.4A). For this purpose, we established a protocol for the prepa-ration of micrometer-sized polystyrene particles (orders of 10µm) decorated with PSS chains as model system for SPBs nanoparticles. Using centrifugal sedimentation and zeta potential measurements we could prove successful functionalization.
The particles were glued to an AFM cantilever and interactions between the particles and polyelectrolyte multilayers were measured using force spectroscopy. Comparison of the interactions between SPBs that were used as CPs, with oppositely charged, amino-functionalized substrates, and uncoated PS cores with amino-amino-functionalized substrates confirm the fuctionalization of the PS particles with PSS chains. Using these probes and measure the interactions between these ”micron SPBs” and PDA and PSS ter-minated multilayers we could show that the adhesive properties of the SPBs can be controlled by the ionic strength and the charge of the substrate (Figure 1.4B).
In addition, we studied the adsorption behavior of SPBs as a function of the ionic strength and the influence of the substrate charge (Figure 1.4C). For this purpose, we used nanosized SPBs (order of 100 nm) consisting of a PS core grafted with PSS chains. Covering a wide range of ionic strengths we have found a clear dependence of the surface coverage of SPBs on the substrate on the NaCl concentration and the substrate charge.
With increasing ionic strength, the coverage increased for oppositely charged surfaces up to an ionic strength of 10 mM. No SPB adsorption occurs on equally charged sur-faces. Further increase of the ionic strength of the solution results in a gradual loss of the substrate selectivity. This can be explained by the transition of the polymer brush from the osmotic to the salted brush regime. In the osmotic brush regime the release of counterions and electrostatic repulsion of SPBs and charged substrates de-termine adsorption respectively. In the salted brush regime that can be assigned to ionic strengths >10 mM, attractive secondary interactions become dominant.
We utilize this behavior for the design of hierarchical surface patterns. Therefore we prepared charge patterned substrates using micro contact printing for selective SPB adsorption.