19 research outputs found
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>-dimethylÂacrylamide
(DMA), 2-(<i>N</i>-ethylÂperfluoroÂoctaneÂsulfonamido)Âethyl
methacrylate (FOSM) and cinnamoylÂoxyethyl 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 polyÂ(2-(<i>N</i>-ethylperfluorooctaneÂsulfonamido)Âethylmethyl
acrylate)
and polyÂ(<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 ∼10<sup>3</sup> to 4 × 10<sup>4</sup> 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<sub>s</sub>, 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<sub>s</sub> and the zero shear viscosity, η<sub>0</sub>, 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<sub>s</sub> and η<sub>0</sub> 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>-bisÂ(2-hydroxyÂethyl)-2-aminoÂethaneÂsulfonic
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 <sup>1</sup>H 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
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
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