Search for: All records

Abstract The intrinsic reversibility of dynamic covalent bonding, such as the furan‐maleimide Diels‐Alder (DA) cycloaddition reactions, enables reprocessable, self‐healing polymer materials that can be reconfigured via the mechanism of solid‐state plasticity. In this work, the temperature‐dependent exchange rates of stereochemically distinctendoandexoDA bonds are leveraged to achieve tunable, temperature‐ and stress‐activated shape morphing in Diels‐Alder polymer (DAP) networks. Through thermal annealing, ≈35% ofendoDA isomers are converted in neat DAP networks to the thermodynamically favoredexoform, achieving ≈97%exoafter complete annealing at 60 °C. This conversion results in a ≈1.7 fold increase in elastic modulus, from 1.7 to 3.0 MPa, and significantly alters the stress relaxation and shape recovery behavior. Spatially resolved annealing, is further showcased enabling the precise control of spatial distributions ofendoandexoDA bonds across planar geometries. The locally distinct concentrations ofendo/exoisomers, achieved by temperature‐induced conversion ofendoDA isomers to the thermodynamically stableexoDA isomers, gave rise to the spatial distributions of stress relaxation rates and elastic strain recovery mismatch to enable controlled stereochemical shape morphing. This approach provides a simplified, thermally driven method for shape morphing, with potential applications in soft robotics and flexible electronics. 

This work explored solution properties of linear and star poly(methacrylic acids) with four, six, and eight arms (LPMAA, 4PMAA, PMAA, and 8PMAA, respectively) of matched molecular weights in a wide range of pH, salt, and polymer concentrations. Experimental measurements of self-diffusion were performed by fluorescence correlation spectroscopy (FCS), and the results were interpreted using the scaling theory of polyelectrolyte solutions. While all PMAAs were pH sensitive and showed an increase in hydrodynamic radius (Rh) with pH in the dilute regime, the Rh of star polymers (measured at basic pH values) was significantly smaller for the star polyacids due to their more compact structure. Fully ionized star PMAAs were also found to be less sensitive to changes in salt concentration and type of the counterion compared to linear PMAA. While Rh of fully ionized linear PMAA decreased in the series Li+ > Na+ > K+ > Cs+ in agreement with the Hofmeister series, Rh of star PMAAs was virtually independent of type of the counterion for eight-arm PMAA. However, molecular architecture strongly affected interactions of counterions with PMAAs. In particular, 7Li NMR revealed that the spin−lattice relaxation time T1 of Li+ ions in low-salt solutions of eight-arm PMAA was ∼2-fold smaller than that in the solution of linear PMAA, suggesting slower Li+-ion dynamics within star polymers. An increase in concentration of monovalent chloride salts, cs, above that of the PMAA monomer unit concentration (cm) resulted in shrinking of both linear and star molecules, with the hydrodynamic size Rh scaling as Rh ∝ cs −0.11±0.01. Self-diffusion of linear and star polyelectrolytes was then studied in a wide range of polyelectrolyte concentrations (10−3 mol/L < cm < 0.5 mol/L) in low-salt (<10−4 mol/L of added salt) and high-salt (1 mol/L) solutions. In both the low-salt and high-salt regimes, diffusion coefficient D was lower for PMAAs with a larger number of arms at a fixed cm. In addition, in both cases, D plateaued at low polymer concentrations and decreased at higher polymer concentrations. However, while in the high-salt conditions, the concentration dependence of D reflected transitions between the dilute to semidilute solution regimes as expected for neutral chains in good and theta solvents, analysis of the diffusion data in the low-salt conditions using the scaling theory revealed a different origin of the concentration dependence of D. Specifically, in the low-salt solutions, both linear and star PMAAs exhibited unentangled (Rouse-like) dynamics in the entire range of polyelectrolyte concentrations. 

Despite the potential use of polyelectrolyte multilayers for biomedical, separation, and energy applications, their dynamic properties are not sufficiently understood. In this work, center-of-mass diffusion of a weak polyacid—poly(methacrylic acid) (PMAA) of linear and 8-arm architecture (L-PMAA and 8-PMAA, respectively) and matched molecular weight—was studied in layer-by-layer (LbL) assemblies with poly(diallyldimethylammonium) chloride (PDADMAC) of varied molecular weight. The film deposition at low-salt, acidic conditions when PMAA was only partially ionized yielded thicker, more diffused layers with shorter PDADMAC chains, and bilayer thickness decreased for multilayers constructed with longer PDADMAC. The molecular architecture of PMAA had a weak effect on film growth, with bilayer thickness being ∼20% larger for L-PMAA for the films constructed with the shortest PDADMAC (35 kDa) and identical film growth for L-PMAA and 8-PMAA with the longest PDADMAC (300 kDa). The exposure of the multilayer films to 0.2M NaCl triggered a reduction in PMAA ionization and significant lateral diffusivity of fluorescently labeled PMAA molecules (PMAA*), with diffusion coefficients D ranging from 10−13 to 10−12 cm2/s, as determined by the fluorescence recovery after photobleaching technique. For all the films, polymer mobility was higher for star polyacids as compared to their linear counterparts, and the dependence of PMAA diffusion coefficient D on PDADMAC molecular weight (D ∼ M−n) was relatively weak (n < 0.6). However, 8-PMAA demonstrated an approximately doubled power exponent compared to the L-PMAA chains, suggesting a stronger effect of the molecular connectivity of the partner polycation molecules on the diffusion of star polyelectrolytes. 

We report synthesis of temperature-responsive linear and star poly(2-ureido aminoethyl methacrylates) (PUEMs) of matched molecular weights, their phase transitions in aqueous solutions and interactions with hydrogen bonding and hydrophobic small molecules. PUEMs with number of arms up to 8 were synthesized via the activator regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) technique using the core-first approach. The degrees of branching were determined using gel permeation chromatography (GPC) equipped with the multi-angle laser light scattering and viscometry detectors. The polymer molecular architecture had a neglectable effect on the upper critical solution temperature (UCST) behavior in aqueous solutions, while the presence of a strong hydrogen-bonded acceptor – dimethyl sulfoxide (DMSO) – suppressed the transition temperature for both linear and star UCST polymers. Importantly, star PUEMs showed an enhanced ability of trapping model drug molecules – proflavine and pyrene. In particular, an increase in polymer branching led to 4.5-fold more efficient proflavine trapping and stronger binding of pyrene molecules within the hydrophobic domains of star polymers below their UCST. The trapped molecules could be then fully released from the star polymers upon temperature increase, demonstrating potential for controlled delivery applications.