We study the properties of a polymer brush exposed to a mixture of two solvents where one component, called the co-nonsolvent (CNS), has a stronger preference with respect to the polymer. The concept of preferential adsorption of CNS onto the polymer, as recently proposed by Mukherhi, Kremer, and Marques [Nat. Commun. 2014, 5, 4882], is combined with the mean-field Alexander−de Gennes approach for the polymer brush. The key assumption is that CNS can form bridges between two monomers which is associated with a further gain in free energy, thus leading to an effective monomer−monomer attraction. The adsorption equilibrium of CNS at a given value of the monomer concentration results in a concentration-dependent χ-function for the polymer brush which describes the effective interactions between the monomers in the mixed solvent. This in turn can lead to a discontinuous collapse transition and to a corresponding reentry transition at higher CNS concentrations. The problem can be analytically treated for a minimal model where the increase of self-volume of the monomers due to adsorption of CNS is neglected. In this case the collapse and the reswelling transition have the same signature. For low brush densities in the noncollapsed state we give an analytic approximation for the spinodal of the collapse. This also allows to define two scaling variables instead of the three control parameters which are the grafting density, the CNS−monomer selectivity, and the volume faction of CNS. The proposed effective free energy contribution resulting from the CNS adsorption equilibrium can be transferred to other systems such as to gels or dendrimers.

Co-nonsolvency occurs if a mixture of two good solvents causes the collapse or demixing of polymers into a polymer-rich phase in a certain range of compositions of these two solvents. In this work, we systematically study the co-nonsolvency behavior of poly­(N-isopropylacrylamide) brushes of different grafting densities in a series of alcohol–water binary mixtures with increasing hydrophobic parts ranging from methanol to 1-butanol by using ellipsometry. We report a strong collapse transition by increasing the alcohol concentration in the water-rich phase, which is enhanced for longer-chain alcohols. The analysis of the thermodynamic properties of the alcohol–water series displays that an increasing demixing tendency between alcohol and water is correlated with an enhancement of the collapse transition of the brush. The increase of grafting density weakens the transition behavior but does not shift the solvent composition point of maximum brush collapse, which is in agreement with the predictions of a recently proposed mean-field model based on the preferential adsorption concept. Among the fully miscible solvents, the most sensitive switching behavior of the brush is found for 1-propanol while 1-butanol already displays a miscibility gap at higher volume fractions.

research has revealed that, together with the provision of morphogens and the presentation of adhesion ligands, [ 2 ] the mechanical characteristics of extracellular matrices have a decisive infl uence on cell fate, provoking the development of materials with effective physical properties. [ 3 ] This interplay of biomolecular and biophysical signals thus defi nes an obvious, but until now unmet, need for a new generation of biomaterials that can be selectively and independently tuned for biomolecular properties and physical material parameters. A conceptual basis to address this need is currently missing. As such, we have developed a rational design approach relying on mean fi eld concepts to guide the design of biofunctional matrices. Considering the decisive role of electrostatic interactions in functional assemblies of living matter we selected a system that allows for a far-reaching modulation of structure-determining forces: crosslinking a hydrophilic and fl exible, multi-armed polymer (with four-armed, amino-terminated poly(ethylene glycol) (starPEG) as an example system known for its anti-adhesive characteristics towards proteins, [ 4 ] with a multifunctional, highly charged crosslinker (such as heparin (HEP) or a similarly charged glycosaminoglycan), which can function as a multivalent binding site capable of complexing a plethora of important bioactive molecules. [ 5 ] We explored whether and how the combination of the particular gel components permits varying the physical and biomolecular characteristics of the swollen materials independently.Based on the successful experimental verifi cation of the theoretical predictions and the functionalization of starPEG-heparin gels with adhesive ligand peptides (such as the integrinbinding arginine-glycine-aspartic acid sequence (RGD))) and morphogens (vascular endothelial growth factor (VEGF), bone morphogenetic protein-2 (BMP-2)) through covalent and noncovalent conjugation schemes we were able to illustrate the resulting options for two selected example systems: studying the interplay of matrix elasticity and growth factor presentation in inducing the pro-angiogenic state of human endothelial cells and promoting osteogenic differentiation of human mesenchymal stem cells we identifi ed effective combinations of matrix parameters and demonstrated exciting options for the fully matrix controlled direction of the cells, i.e., removed the Using Mean Field Theory to Guide Biofunctional Materials DesignCell-instructive characteristics of extracellular matrices (ECM) resulting from a subtle balance of biomolecular and biophysical signals must be recapitulated in engineered biomaterials to facilitate regenerative therapies. However, no material explored so far allows the independent tuning of the involved molecular and physical cues due to the inherent correlation between biopolymer concentration and material properties. Addressing the resulting challenge, a rational design strategy for ECM-inspired biohybrid hydrogels based on multi-armed poly(ethylene glycol) and he...