Random copolymers consisting of n-butyl acrylate backbones with quadruple hydrogen-bonding side chains based on 2-ureido-4[1H]-pyrimidinone (UPy) have been synthesized via controlled radical polymerization and postpolymerization functionalization. Through this synthetic strategy high UPy monomer content (15 mol %) can be reached while maintaining low polydispersity and excellent control over molecular weight, providing model reversible networks with well-defined molecular architecture. Despite low T g s and a lack of entanglements or crystallinity, these materials behave as thermoplastic elastomers through the strong but reversible association of UPy groups. Bulk properties such as the plateau modulus, tensile modulus, and relaxation time scale are primarily determined by the average distance between UPy’s along the chain. Starting from a difunctional initiator, triblock copolymers can also be synthesized containing a homopolymer midblock and random copolymer end blocks, effectively concentrating the hydrogen-bonding groups near the chain ends. By controlling both the average composition and distribution of UPy’s along the polymer chain, macroscopic material properties such as stiffness and resistance to creep can be independently tuned.

A modular strategy for hydrogel formation based on the self‐organization of well‐defined ABA triblock copolyelectrolytes through ionic interactions in water is reported. The nature of the ionic domains, which constitute the physical crosslinks, provides for robust, yet highly tunable materials. These materials represent a diverse platform for hydrogel formation with enhanced mechanical properties and ease of synthesis while retaining a dynamic responsive nature.

A new strategy for synthesizing well-defined, chain-end-functionalized polymers containing multiple hydrogen-bonding (MHB) groups capable of heterodimerization in both solution and the melt has been developed. Two complementary MHB systems were chosen for initial studies: 2-ureido-4[1H]-pyrimidinone (UPy) and 2,7-diamido-1,8-naphthyridine (Napy) and ATRP initiators containing either UPy or Napy were prepared and shown to produce well-defined (meth)acrylic polymers with the desired MHB functionality present at the chain end. To characterize the effectiveness of the MHB interaction in the melt, blends of chain-end-functionalized linear polymers were cast into films, annealed at various temperatures above T g , and then quenched, and their structures were analyzed by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). It was shown that the nature of the hydrogen-bonding group(s) present in the blend has a significant effect on bulk microstructure and thermal behavior, in particular for blends of UPy-and Napy-functional chains.

The surface properties and self‐adhesion mechanism of self‐healing poly(butyl acrylate) (PBA) copolymers containing comonomers with 2‐ureido‐4[1H]‐pyrimidinone quadruple hydrogen bonding groups (UPy) are investigated using a surface forces apparatus (SFA) coupled with a top‐view optical microscope. The surface energies of PBA–UPy4.0 and PBA–UPy7.2 (with mole percentages of UPy 4.0% and 7.2%, respectively) are estimated to be 45–56 mJ m−2 under dry condition by contact angle measurements using a three probe liquid method and also by contact and adhesion mechanics tests, as compared to the reported literature value of 31–34 mJ m−2 for PBA, an increase that is attributed to the strong UPy–UPy H‐bonding interactions. The adhesion strengths of PBA–UPy polymers depend on the UPy content, contact time, temperature and humidity level. Fractured PBA–UPy films can fully recover their self‐adhesion strength to 40, 81, and 100% in 10 s, 3 h, and 50 h, respectively, under almost zero external load. The fracture patterns (i.e., viscous fingers and highly “self‐organized” parallel stripe patterns) have implications for fabricating patterned surfaces in materials science and nanotechnology. These results provide new insights into the fundamental understanding of adhesive mechanisms of multiple hydrogen‐bonding polymers and development of novel self‐healing and stimuli‐responsive materials.

Blends of diamidonaphthyridine (Napy) end-functional poly(n-butyl acrylate) (PnBA) and ureidopyrimidinone (UPy) end-functional poly(benzyl methacrylate) (PbnMA) were studied as a function of the component molecular weights to compare with prior theoretical predictions.1 Macroscopic phase separation was observed to be prevented by the reversible association of end-functional polymers to form supramolecular diblock copolymers, resulting in stabilization of the interface between the polymers. At low molecular weights homogeneous microstructures were observed, in contrast to nonfunctional homopolymer blends of the same molecular lengths, which rapidly phase separate over macroscopic length scales. At higher molecular weights, the blend structure was reminiscent of compatibilized homopolymer blends, with the phase-separated domain size rapidly increasing with temperature. To compare with theoretical phase diagrams, the temperature-dependent Flory-Huggins χ parameter was measured, and it was found that PnBA/PbnMA covalent diblock copolymers show unusual lower critical ordering (LCOT) behavior with χ slightly increasing with temperature (χ(T) = 0.036 -0.56/T).

Improving the performance of desalination membranes requires better measurements of salt permeability in the polyamide separating layer to elucidate the thermodynamic and kinetic components of membrane permselectivity. In this work, electrochemical impedance spectroscopy (EIS) is introduced as a technique to measure the salt permeability and estimate the salt partition coefficient in thin polyamide films created using molecular layer-by-layer deposition. The impedance of supported polyamide films ranging in thickness from 3.5 nm to 28.5 nm were measured in different electrolyte solutions. Impedance spectra were modeled with equivalent circuits containing resistive and capacitive elements associated with the EIS measurement system as well as characteristic low-frequency parallel resistive and capacitive elements that are associated with the polyamide film. The characteristic polyamide membrane resistance increases with film thickness, decreases with solution concentration, and is an order of magnitude greater for a divalent cationic solution than for a monovalent cationic solution. For each polyamide film, salt permeability is calculated from the membrane resistance, and a salt partition coefficient is estimated. At the highest solution concentration measured, which is representative of brackish water desalination conditions, the calculated salt permeabilities range from Ps = 1.3 × 10−16 m s−1 to 3.9 × 10−16 m s−1, and the estimated salt partition coefficients range from Ks = 0.008 to 0.016. These measurements demonstrate that EIS is a powerful tool for studying membrane permselectivity through the measurement of salt permeability in thin polyamide films.

Herein, we investigate the influence of spacer length on the homoassociation and heteroassociation of endfunctionalized hydrogen-bonding polymers based on poly(nbutyl acrylate). Two monofunctional ureido-pyrimidinone (UPy) end-functionalized polymers were prepared by atom transfer radical polymerization using self-complementary UPyfunctional initiators that differ in the spacer length between the multiple-hydrogen-bonding group and the chain initiation site. The self-complementary binding strength (K dim ) of these end-functionalized polymers was shown to depend critically on the spacer length as evident from 1 H NMR and diffusionordered spectroscopy. In addition, the heteroassociation strength of the end-functionalized UPy polymers with endfunctionalized polymers containing the complementary 2,7-diamido-1,8-naphthyridine (NaPy) hydrogen-bond motif is also affected when the aliphatic spacer length is too short. KEYWORDS: atom transfer radical polymerization (ATRP); diffusion-ordered spectroscopy; hydrogen bonding; supramolecular diblock copolymers INTRODUCTION The combination of supramolecular chemistry and the controlled phase separation of diblock copolymers can result in a wealth of nanoscale morphologies with applications ranging from semiconductor integrated circuit design to the development of subnanometer porous films for separation processes. [1][2][3][4] Theoretical work has shown that the domain size and morphology of the phase-separated structures critically depend on the strength of the interpolymer noncovalent interactions.5-10 To achieve these attractive enthalpic interpolymer interactions, end-functionalized homopolymers have been prepared with functional groups capable of noncovalent assembly such as hydrogen bonding, [11][12][13][14][15][16][17][18][19][20] ionic, 21 transition-metal, 22,23 host-guest, 24-26 and fluorophilic 27 interactions. Typically, these functional groups are attached to a homopolymer via a short, aliphatic spacer. Recent reports, however, have shown that the association strength of noncovalent assemblies can be decreased by competitive noncovalent interactions with functional groups present in the side-chains. For example, we have recently shown that the ureido-pyrimidinone (UPy) dimerization con-

Random copolymers consisting of n-butyl acrylate backbones with quadruple hydrogenbonding side chains based on 2-ureido-4[1H]-pyrimidinone (UPy) have been synthesized via controlled radical polymerization and postpolymerization functionalization. Through this synthetic strategy high UPy monomer content (15 mol %) can be reached while maintaining low polydispersity and excellent control over molecular weight, providing model reversible networks with well-defined molecular architecture. Despite low T g s and a lack of entanglements or crystallinity, these materials behave as thermoplastic elastomers through the strong but reversible association of UPy groups. Bulk properties such as the plateau modulus, tensile modulus, and relaxation time scale are primarily determined by the average distance between UPy's along the chain. Starting from a difunctional initiator, triblock copolymers can also be synthesized containing a homopolymer midblock and random copolymer end blocks, effectively concentrating the hydrogen-bonding groups near the chain ends. By controlling both the average composition and distribution of UPy's along the polymer chain, macroscopic material properties such as stiffness and resistance to creep can be independently tuned.