Soft Elastomers via Introduction of Poly(butyl acrylate) “Diluent” to Poly(hydroxyethyl acrylate)-Based Gel Networks

We report a new strategy for the synthesis of stable and well-defined supersoft elastomers. First, four-arm star-like polymers, poly­(trimethylsilyloxyethyl acrylate), were synthesized and cross-linked to form a relatively uniform polymer gel network. Second, short poly­(<i>n</i>-butyl acrylate) side chains were grown from the initiating sites along the network backbone via atom transfer radical polymerization. These soft side chains act as low molecular weight “diluent” that “swells” the cross-linked polyHEA network, but cannot be leached from it. Using this strategy, materials with shear modulus less than 5 kPa were prepared and the effect of grafted side chain fraction on their mechanical properties was explored

Branch-Induced Heterogeneous Chain Motion in Precision Polyolefins

The effect of branching on the chain dynamics of model branched polyethylene was studied by solid state ²H and ¹³C NMR. Methyl branched polyethylene models with branches at every 15th (PE15) and 21st (PE21) chain carbon were synthesized with deuterons placed either at the carbon alpha to the branching point or in the middle of the chain between branches. Line shape analysis of the temperature-dependent ²H spectra revealed that the distribution of motional amplitudes in the solid state is homogeneous along the crystalline chain but heterogeneous in the less constrained amorphous phase. The ¹³C and ²H longitudinal spin relaxation data obtained at different positions along the chain are explained in terms of the segmental dynamics as a function of branch incorporation into the crystals. A detailed analysis of the chain dynamics in the conformationally disordered phase exhibited in PE15 is also provided

Dissolution and Crystallization of Polyamides in Superheated Water and Concentrated Ionic Solutions

The dissolution and recrystallization of Polyamide 46 (PA46) from “superheated” (i) water and (ii) concentrated ionic solutions of strong solubilizing mono- and divalent Hofmeister ions are studied, utilizing <i>in situ</i> high-resolution magic-angle spinning (HR-MAS) nuclear magnetic resonance (NMR) supported by wide-angle X-ray diffraction (WAXD), attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR), and gel permeation chromatography (GPC). The samples are sealed in glass capillaries, and employing variable-temperature ¹H HR-MAS NMR spectroscopy, the dissolution process of PA46 as a function of temperature and pressure can be followed <i>in situ</i>. The purpose of such a study is to obtain molecular insight into the dissolution process of these hydrogen-bonded synthetic polymers in the different aqueous solutions. In pure water, at temperatures close to the dissolution of PA46, two distinct ¹H resonances from water are observed. These resonances are associated with water molecules in the vicinity of PA46 and water in the bulk state. On further heating, the signal from water molecules in the vicinity of PA46 dominates. This sudden change in the environment suggests that water molecules, which have escaped the dense hydrogen-bonded network of bulk water, can diffuse into the structure of PA46, triggering dissolution of the polymer. This happens at a temperature that is more than 100 °C below the melting temperature of the polymer, notably without hydrolysis as verified by GPC performed prior to and after the dissolution experiments. On cooling, recrystallization of PA46 from aqueous solution is observed where water molecules are incorporated in the crystal structure. In the presence of salts, such as LiI and Cal₂, weakening of the hydrogen-bonding network of the water molecules occurs. However, above room temperature, independent of the choice of salt, depopulation of the hydrogen bonding between the water molecules occurs, observed as a decrease in the ¹H chemical shift value. The reduced hydrogen bonding in the presence of ions facilitates the dissolution of PA46 at much lower temperatures compared to pure water and ultimately results in the complete suppression of crystallization from solution even at room temperature. Depending on the valency of the cation a more mobile or frozen amorphous state of PA46 is obtained at low temperatures as verified by ¹³C HR-MAS NMR, ATR-FTIR, and WAXD

Synthesis and Selective Loading of Polyhydroxyethyl Methacrylate‑<i>l</i>‑Polysulfone Amphiphilic Polymer Conetworks

Polyhydroxyethyl methacrylate-<i>linked by</i>-polysulfone amphiphilic polymer conetworks of two types of segments with <i>T</i>_g above room temperature are presented. The conetworks are prepared by free radical copolymerization of methacryloyl-terminated PSU macromers with 2-ethyl methacrylate, followed by removal of the TMS protecting groups by acidic hydrolysis. Phase separation in the nanometer range due to the immiscibility of the two covalently linked segments is observed using transmission electron and scanning force microscopy. The swelling of the conetworks in water and methanol as polar solvents and chloroform as nonpolar solvent are studied gravimetrically and then in a more detailed fashion by solid-state NMR spectroscopy. Selective swelling and also targeted loading of a small organic model compound specifically to one of the two phases are demonstrated

The Cushion Method: A New Technique for the Recovery of Hydrophilic Nanocarriers

Microencapsulation of hydrophilic therapeutic agents such as proteins or nucleotides into a nanocarrier is frequently accomplished in inverse (water-in-oil) emulsions. However, the redispersion of the nanocarriers in aqueous media often involves a complicated purification process, and the redispersion usually requires additional surfactants for its colloidal stability, which is not favored for biological applications. We propose a simple, fast, and mild method to recover hydrophilic nanocarriers prepared in inverse emulsions by temporary coating of the nanocarriers with biocompatible small molecules, so that the final aqueous dispersion of the nanocarriers can be dispersed with high recovery rate, minimal aggregation, and no additional surfactants. Such a method is termed the “cushion method” and was adopted in the preparation of chitosan nanocarriers. The nanocarriers recovered with the cushion method release encapsulated peptides in a pH-responsive manner and do not require surfactants for colloidal stabilization

Homogeneous Nucleation of Ice Confined in Hollow Silica Spheres

Ice nucleation is studied in hollow silica (HS) spheres. These hierarchical materials comprise ∼3 nm pores within the silica network, which are confined to a ∼20 nm shell of a hollow sphere (with diameters in the range ∼190–640 nm). The multiple length scales involved in HS spheres affect the ice nucleation mechanism. We find homogeneous nucleation inside the water filled capsules, whereas heterogeneous nucleation prevails in the surrounding dispersion medium. We validate our findings for a series of hollow sphere sizes and demonstrate the absence of homogeneous nucleation in the case of polystyrene–silica core–shell particles. The present findings shed new light on the interplay between homogeneous and heterogeneous nucleation of ice with possible implications in undercooled reactions and the storage of reactive or biologically active substances

Interplay between Structure and Dynamics in Chitosan Films Investigated with Solid-State NMR, Dynamic Mechanical Analysis, and X-ray Diffraction

Modern solid-state NMR techniques, combined with X-ray diffraction, revealed the molecular origin of the difference in mechanical properties of self-associated chitosan films. Films cast from acidic aqueous solutions were compared before and after neutralization, and the role of the counterion (acetate vs Cl⁻) was investigated. There is a competition between local structure and long-range order. Hydrogen bonding gives good mechanical strength to neutralized films, which lack long-range organization. The long-range structure is better defined in films cast from acidic solutions in which strong electrostatic interactions cause rotational distortion around the chitosan chains. Plasticization by acetate counterions enhances long-range molecular organization and film flexibility. In contrast, Cl⁻ counterions act as a defect and impair the long-range organization by immobilizing hydration water. Molecular motion and proton exchange are restricted, resulting in brittle films despite the high moisture content

Dipolar Relaxation in Functionalized Poly‑<i>p</i>‑phenylenes Bearing Ultrastrong Dipoles Perpendicular to the Backbone

Local polymer dynamics are studied in polymers bearing dipoles rigidly attached to the backbone. The compounds are based on cyano-substituted dihydro­benzimidazoles bearing ultrastrong dipole moments (∼12 D per repeat unit) incorporated in a poly-<i>p</i>-phenylene backbone, giving rise to polymers with <i>rigid</i> dipoles <i>perpendicular</i> to the chain. They belong to type B polymers according to the Stockmayer classification. They are ideal model systems for studying rotational isomers in the gas phase and the self-assembly and local dynamics in the solid state. Gas phase calculations (DFT) provided the dipole moments, the energetic barriers, and the backbone conformation as a function of the dipole strength and dipole separation. Calculated dipole moments show an odd–even effect as a function of dipole separation. Specific rotational isomers that maximize the dipole moment are obtained. In the solid state, dielectric spectroscopy and site-specific NMR techniques revealed that packing through intermolecular forces such as van der Waals, π–π, and dipole–dipole interactions dictates the dynamics. Dielectric spectroscopy further identified two modes, both with Arrhenius temperature dependence and activation energies of 20–23 kcal/mol. Combined results attribute the faster process to a libration motion of the highly polar group reorienting only the nonplanar five-membered ring and the slower process to larger amplitude and/or correlated motions of the polar groups. These dynamic results are in agreement with structural investigations (X-ray diffraction) demonstrating that type B polymers in the bulk have rigid backbones

Dynamic Heterogeneity in Random Copolymers of Polymethacrylates Bearing Different Polyhedral Oligomeric Silsesquioxane Moieties (POSS)

Random copolymersusually considered as dynamically homogeneous with a single glass temperaturecan show dynamic heterogeneity by appropriate choice of their repeat units. Random copolymers based on polymethacrylates with polyhedral oligomeric silsesquioxane (POSS) moieties as side groups substituted with seven isobutyl ((i-Bu)₇POSS-OSiMe₂(CH₂)₃-MA) and 2,4,4-trimethylpentyl ((i-C₈H₁₇)₇POSS-OSiMe₂(CH₂)₃-MA) groups were synthesized and studied by dielectric spectroscopy, density functional theory, differential scanning calorimetry, and rheology. Two distinct relaxation processes were found by dielectric spectroscopy giving rise to two glass temperatures. In addition, rheology revealed significant broadening of the viscoelastic functions leading eventually to the failure of time–temperature superposition for all copolymer compositions. These results provide signatures of dynamic heterogeneity in a random copolymer

Tunable and Switchable Dielectric Constant in an Amphidynamic Crystal

The inclusion compound [(CH₃)₂­NH₂]₂­[KCo­(CN)₆] exhibits a marked temperature-dependent dielectric constant and can be considered as a model of tunable and switchable dielectric materials. Crystal structure and solid-state NMR studies reveal a switchable property between low and high dielectric states around 245 K. This originates from an order–disorder phase transition of the system, changing the dynamics of the polar dimethylammonium (DMA) cation. Furthermore, the tuning of the dielectric constant at temperatures below the phase transition point is related to increasing angular pretransitional fluctuations of the dipole moment of DMA