Tuning the Viscoelastic Behavior of Hybrid Hydrogels Composed of a Physical and a Chemical Network by the Addition of an Organic Solvent

The influence of isopropanol (IPA) addition on the viscoelastic behavior of hybrid hydrogels which were prepared from chemically cross-linked copolymers of <i>N</i>,<i>N</i>-dimethyl­acrylamide (DMA), 2-(<i>N</i>-ethyl­perfluoro­octane­sulfonamido)­ethyl methacrylate (FOSM) and cinnamoyl­oxyethyl acrylate was investigated by dynamic oscillatory shear. The hybrid gels were composed of a supramolecular network formed by phase-separated FOSM nanodomains that served as physical cross-links and a chemical network derived from photo-cross-linking the cinnamate groups. The linear viscoelastic (LVE) behavior of the gels was tunable by changing the solvent ratio (IPA/water) and/or temperature. When the swelling solvent was pure water or pure IPA, the materials were hydrogels and organogels, respectively. When the IPA concentration increased from a molar ratio of IPA:FOSM of zero to 80:1, the cross-link density of the gels decreased due to weakening of the physical network as a result of solvation of the hydrophobic interactions by IPA. Above an IPA:FOSM ratio of 80:1, essentially only the covalent network persisted and the gels behaved as elastic solids. The design of these hydrogels/organogels provides three degrees of freedom for tuning the LVE properties: copolymer composition, temperature, and solvent. They also provide responsive behavior of the gels to changes of temperature and/or solvent

Facile Fabrication of a Shape Memory Polymer by Swelling Cross-Linked Natural Rubber with Stearic Acid

A facile method was developed for fabrication of a robust shape memory polymer by swelling cross-linked natural rubber with stearic acid. Commercial rubber bands were swollen in molten stearic acid at 75 °C (35 wt % stearic acid loading). When cooled the crystallization of the stearic acid formed a percolated network of crystalline platelets. The microscopic crystals and the cross-linked rubber produce a temporary network and a permanent network, respectively. These two networks allow thermal shape memory cycling with deformation and recovery above the melting point of stearic acid and fixation below that point. Under manual, strain-controlled, tensile deformation the shape memory rubber bands exhibited fixity and recovery of 100% ± 10%

Nonlinear Rheology of Random Sulfonated Polystyrene Ionomers: The Role of the Sol–Gel Transition

The linear and nonlinear rheological behaviors of nonentangled sulfonated polystyrene (SPS) ionomers near the sol–gel transition were studied. When the degree of sulfonation, <i>p</i>, was below the gel point, the ionomer exhibited sol-like linear viscoelastic (LVE) behavior, and shear thinning was observed for steady shear flow. For <i>p</i> close to the gel point, the ionomer showed power-law-like LVE behavior over a wide frequency range. Strain hardening and shear thickening behavior were observed, and their magnitudes depended on the temperature, molecular weight of the PS precursor, and the Coulomb energy of the ion pair. Above the gel point, a distinct rubbery plateau was observed in the dynamic modulus. Melt fracture occurred upon start-up shear, which prevented quantitative examination of the nonlinear rheology. The possible mechanisms for strain hardening and shear thickening near the gel point are discussed with respect to formation of large clusters that nearly percolate in space

Hydrophobic/Hydrophilic Triblock Copolymers: Synthesis and Properties of Physically Cross-Linked Hydrogels

Hydrophobic/hydrophilic triblock copolymers of poly­(2-(<i>N</i>-ethylperfluorooctane­sulfonamido)­ethylmethyl acrylate) and poly­(<i>N,N</i>′-dimethylacrylamide) (PD) were synthesized by sequential reversible addition–fragmentation chain transfer polymerization. Physically cross-linked hydrogels were produced by immersing compression-molded triblock copolymers into water. The copolymers and their hydrogels were characterized by differential scanning calorimetry, thermogravimetric analysis, thermal desorption-GC/MS analysis, swelling isotherms, wide- and small-angle X-ray scattering, and dynamic mechanical analysis. The equilibrium water sorption of the hydrogels depended on the length of the water-soluble polymer block (PD), and the block copolymers swelled more in water than a random copolymer of the same composition. The block copolymer hydrogels were viscoelastic, though the frequency dependence of the dynamic modulus was weak. The dynamic modulus of the block copolymer hydrogels ranged from ∼10³ to 4 × 10⁴ Pa, which was much lower than the modulus of a random copolymer hydrogel of the same composition

Rheological Behavior of Partially Neutralized Oligomeric Sulfonated Polystyrene Ionomers

The linear viscoelastic (LVE) behavior of partially neutralized oligomeric sulfonated polystyrene (SPS) ionomers with different degrees of sulfonation (<i>p</i>) and degrees of neutralization (<i>x</i>) was investigated. The ionic dissociation time, τ_s, obtained from the reversible gelation model [Chen Macromolecules 2015, 48, 1221−1230] is mainly controlled by the neutralization degree, <i>x</i>, rather than the functional group (i.e., sulfonic acid and metal sulfonate) concentration, <i>p</i>. For a fixed <i>p</i>, increasing <i>x</i> significantly increases τ_s and the zero shear viscosity, η₀, especially near complete neutralization. These results explain the observations reported by Lundberg et al. [Ions in Polymers; American Chemical Society: 1980; Vol. 187, pp 67−76] that the increase of the viscosity of SPS ionomers with neutralization undergoes a substantial increase between 90% and 100% neutralization of the sulfonic acid groups to metal salts. This rapid increase of τ_s and η₀ is probably related to the decrease of sulfonic acid groups in the ionic aggregates with increasing <i>x</i>

Viscoelasticity of Reversible Gelation for Ionomers

Linear viscoelasticity (LVE) of low-ion-content and low-molecular-weight (nonentangled) randomly sulfonated polystyrene shows a sol–gel transition when the average number of ionic groups per chain approaches unity. This transition can be well understood by regarding the number of ionizable sites over a chain as the relevant functionality for cross-linking. For ionomers below but very close to the gel point, the LVE shows power law relaxation similar to gelation of chemical cross-linking. Nevertheless, ionomers near and beyond the gel point also show terminal relaxation not seen in chemically cross-linking systems, which is controlled by ionic dissociation. Careful analysis of the power law region of the frequency dependence of complex modulus close to the gel point shows a change in exponent from ∼1 at high frequency to ∼0.67 at low frequency, which strongly suggests a transition from mean-field to critical percolation known as the Ginzburg point. A mean-field percolation theory by Rubinstein and Semenov for gelation with effective breakup has been modified to include critical percolation close to the gel point and predicts well the observed LVE of lightly sulfonated polystyrene oligomers

Ionomers for Tunable Softening of Thermoplastic Polyurethane

Thermoplastic polyurethane (TPU) sulfonate ionomers with quaternary ammonium cations were synthesized to achieve soft TPUs without using conventional low molecular weight plasticizers. The sulfonated monomer <i>N</i>,<i>N</i>-bis­(2-hydroxy­ethyl)-2-amino­ethane­sulfonic acid (BES) neutralized with bulky ammonium counterions was incorporated as a chain extender to internally plasticize the TPU. Increasing the steric bulk of the counterion and the concentration of the ionic species produced softer TPUs with improved melt processability. The incorporation of the sulfonate species suppressed crystallinity of the TPU hard block, which was mainly responsible for the softening of the polymer. The synthetic procedure developed allows for facile tuning of the mechanical properties of the TPU by simply switching the counterion and/or increasing the feed ratio of ionic monomer. The precursors in this study were synthesized and analyzed via ¹H NMR, and the thermomechanical properties of the resulting TPU ionomers were characterized by differential scanning calorimetry, dynamic mechanical analysis, Shore A hardness, and static mechanical testing

Reversible Gelation Model Predictions of the Linear Viscoelasticity of Oligomeric Sulfonated Polystyrene Ionomer Blends

The linear viscoelastic (LVE) behavior of oligomeric sulfonated polystyrene ionomers (SPS) and binary blends of two SPS ionomers with different sulfonation levels and cations was compared to the predictions of the reversible gelation model for the rheology of ionomers [Macromolecules 2015, 48, 1221−1230]. Binary blends had the same gel point as the neat ionomer components if a linear mixing rule was used to calculate an average sulfonation level for the blend. The binary blends, however, exhibited a broader relaxation time distribution than the neat ionomers having the same number density of ions. A linear mixing rule for the ionic dissociation frequency of the blend was proposed, and when incorporated into the reversible gelation model, reasonable predictions of the terminal relaxation time of the blends were achieved

Tailor-Made Fluorinated Copolymer/Clay Nanocomposite by Cationic RAFT Assisted Pickering Miniemulsion Polymerization

Fluorinated polymers in emulsion find enormous applications in hydrophobic surface coating. Currently, lots of efforts are being made to develop specialty polymer emulsions which are free from surfactants. This investigation reports the preparation of a fluorinated copolymer via Pickering miniemulsion polymerization. In this case, 2,2,3,3,3-pentafluoropropyl acrylate (PFPA), methyl methacrylate (MMA), and <i>n</i>-butyl acrylate (nBA) were copolymerized in miniemulsion using Laponite-RDS as the stabilizer. The copolymerization was carried out via reversible addition–fragmentation chain transfer (RAFT) process. Here, a cationic RAFT agent, <i>S</i>-1-dodecyl-<i>S</i>′-(methylbenzyltriethylammonium bromide) trithiocarbonate (DMTTC), was used to promote polymer-Laponite interaction by means of ionic attraction. The polymerization was much faster when Laponite content was 30 wt % or above with 1.2 wt % RAFT agent. The stability of the miniemulsion in terms of zeta potential was found to be dependent on the amount of both Laponite and RAFT agent. The miniemulsion had particle sizes in the range of 200–300 nm. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) analyses showed the formation of Laponite armored spherical copolymer particles. The fluorinated copolymer films had improved surface properties because of polymer–Laponite interaction

Sulfonation Distribution in Sulfonated Polystyrene Ionomers Measured by MALDI-ToF MS

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-ToF MS) was used to quantify the sulfonation level and sulfonation distribution of sulfonated polystyrene ionomers prepared by homogeneous solution sulfonation. The sulfonation levels obtained by MALDI-ToF MS and acid–base titration were compared, and the sulfonate distributions determined by MALDI-ToF MS were compared with theoretical random distributions. The results indicate that the sulfonation reaction used produces a sample with a random sulfonate distribution