A Model Study of the Influence of the Natural Rubber (NR)- Endogenous Gel Fraction on the Rheological Performance of NR Using Synthetic Polyisoprene Rubber (IR) Blends with Different Ratios of Gel

Nonrubber components (NRCs) contribute to the natural networks and endow natural rubber (NR) with exceptional qualities. It is known that the previously proposed NR microstructure has two types of terminal groups, the ω-terminal, which binds to proteins by intermolecular hydrogen bonds, and the α-terminal, which is linked with phospholipids. These connections are believed to be the basis for the formation of naturally occurring networks or gels, which is attributable to the distinctive properties of NR. Using a model of synthetic polyisoprene rubber (IR), the contribution of the gel structure was scrutinized by removing the endogenous gel component derived from NR and adding it in varying quantities to the IR. Through the use of Fourier transform infrared spectroscopy (FT-IR), gel permeation chromatography (GPC), and super-resolution laser scanning confocal microscopy (SR-LSCM), the microstructure of the prepared materials was analyzed. The SR-LSCM analysis revealed that phospholipids exclusively dominated the structure of the sol fraction, whereas both of proteins and phospholipids made up the majority of the gel fraction in NR. The gel microstructure was verified by the GPC analysis, which revealed that short NR chains were networked together. The viscoelastic and mechanical properties of rubber were assessed using an oscillatory shear measurements and tensile testing. The viscoelastic and mechanical performance of the mixed IR with the NR-endogenous gel fraction were strengthened when a substantial proportion of the gel or branch points was incorporated

Dynamic Viscoelasticity and Birefringence of Poly(ionic liquids) in the Vicinity of Glass Transition Zone

Dynamic viscoelasticity and birefringence of two poly­(ionic liquid)­s, PC₄VITfO and PC₄VITFSI, are investigated to clarify the molecular origin of viscoelastic response of PILs. According to a previous study, PC₄VITFSI having larger counterions shows a broader viscoelastic relaxation spectra around the glass-to-rubber transition zone than PC₄VITfO. The rheo-optical data were analyzed with the modified stress-optical rule: The complex modulus for PC₄VITfO was separated into two components, the rubbery and the glassy component, similarly to the ordinary amorphous polymers while for PC₄VITFSI system an additional component was necessary in addition to the two ordinary components for a reasonable separation. From the frequency dependence, the additional component was attributed to the sub-Rouse mode of chain which is enhanced by the counterions decreasing the interchain interactions

Dynamic Segment Size of the Cellulose Chain in an Ionic Liquid

Viscoelasticity and strain-induced birefringence under oscillatory shear flow of cellulose/1-buthyl-3-methylimidazolium chloride (BmimCl) solutions were measured at various temperatures covering a wide frequency zone from the terminal flow to the glassy zone for dilute (2 wt %) and semiconcentrated solution (10 wt %) to clarify the dynamical segment size of the cellulose chain. The estimated dynamical segment size, <i>M</i>_S, obtained from viscoelasticity is much smaller than that from flow birefringence. <i>M</i>_S estimated from dynamic birefringence was 2300 corresponding to 14 repeating glucose residues from 2–10 wt %, showing weak concentration dependence. This value is comparable to the reported value of Kuhn segment size, <i>M</i>_K. This relationship, <i>M</i>_S ≈ <i>M</i>_K, holding even at dilute solution, is in contrast with the large difference (<i>M</i>_S ≈ 5<i>M</i>_K) for polystyrene in dilute solution, indicating that the chain rigidity affects the relationship between <i>M</i>_S and <i>M</i>_K

BCC Grain Formation Triggered by Miscibility Jump on Temperature Drop

Formation of a body-centered cubic (BCC) structure and grain growth process triggered by segregation-power jump on a temperature drop was studied by small-angle X-ray scattering (SAXS), rheology, and differential scanning calorimetry (DSC). Polystyrene-<i>block</i>-poly­(ethylene-<i>alt</i>-propylene) (SEP) was dissolved in tridecyl-2,2,4-trimethyl hexanoate (salacos913) that was a practically neutral good solvent for both of polystyrene (PS) and PEP block chains at temperature above 84 °C (<i>T</i>_sol), while it was highly selective (good for PEP) below <i>T</i>_sol. Spherical microdomains in a short-range liquid-like order were formed above <i>T</i>_sol; the system was in the so-called “lattice disordered state”, designated as disordered sphere. The solution was annealed at a temperature (130 °C) above <i>T</i>_sol for 10 min and successively subjected to a temperature drop across <i>T</i>_sol. The system stayed in the lattice-disordered state for a certain induction period. During this induction period, stronger segregation power at the lower temperature increased the domain spacing, whereas a storage shear modulus (<i>G</i>′) showed liquid-like behavior (<i>G</i>′<i>∝ ω</i>²) at low frequencies (ω < 0.2 s^–1) in a terminal zone and a shoulder at ω ∼ 1 s^–1. The shoulder shifted toward the smaller ω region, arising from dissociation of the PS block from the solvent. Once BCC lattice structures of spherical microdomains formed, grains eventually grew in size up to ca. 2.5 μm with a large size-distribution as revealed by the 2d-SAXS with spot-like scatterings, whereas <i>G</i>′ in the terminal region increased, arising from the increase in correlation length of the spherical microdomains. Eventually, the <i>G</i>′ showed plateau at lower frequencies at ω < 0.2 s^–1, indicating that the BCC lattice of spheres with long-range order (grain stuructures) was percolated throughout the solution. The number of the grains still continued to increase at the cost of spherical microdomains in the lattice disordered state, which caused the further increase in <i>G</i>′ at the plateau until the end of the ordering process of the BCC structure

Experimental Test for Viscoelastic Relaxation of Polyisoprene Undergoing Monofunctional Head-to-Head Association and Dissociation

A viscoelastic test was made for end-carboxylated polyisoprene (PI-COOH) of the molecular weight <i>M</i> = 30.₅ × 10³ that underwent the interchain association and dissociation through hydrogen bonding of the COOH groups at the chain end. As a reference, the test was made also for neat PI unimer (with no COOH group at the chain end) and for PI₂ dimer (with <i>M</i> = 61.0 × 10³), the latter being synthesized through end-coupling of PI^– anions (precursor of the PI-COOH sample). The PI-COOH, neat unimer, and dimer samples were diluted in oligomeric butadiene (oB) to a concentration of 10 wt %. The neat unimer and dimer exhibited nonentangled Rouse behavior at this concentration, as expected from their molecular weights. At low temperatures (<i>T</i> ≤ 0 °C) the PI-COOH sample relaxed slower than the reference unimer but faster than the dimer, whereas the relaxation of PI-COOH approached that of the unimer with increasing <i>T</i> > 0 °C, and this change of the relaxation time of PI-COOH was associated with changes in the angular frequency (ω) dependence of the dynamic modulus. This behavior of PI-COOH was well described by a recently proposed theory considering motional coupling between the end-associating unimer and its dimer at chemical equilibrium. On the basis of this result, an effect of the polymeric character of PI-COOH chain on the viscoelastically detected association/dissociation of the hydrogen bonding of the COOH groups was discussed

BCC Grain Formation Triggered by Miscibility Jump on Temperature Drop

Formation of a body-centered cubic (BCC) structure and grain growth process triggered by segregation-power jump on a temperature drop was studied by small-angle X-ray scattering (SAXS), rheology, and differential scanning calorimetry (DSC). Polystyrene-<i>block</i>-poly­(ethylene-<i>alt</i>-propylene) (SEP) was dissolved in tridecyl-2,2,4-trimethyl hexanoate (salacos913) that was a practically neutral good solvent for both of polystyrene (PS) and PEP block chains at temperature above 84 °C (<i>T</i>_sol), while it was highly selective (good for PEP) below <i>T</i>_sol. Spherical microdomains in a short-range liquid-like order were formed above <i>T</i>_sol; the system was in the so-called “lattice disordered state”, designated as disordered sphere. The solution was annealed at a temperature (130 °C) above <i>T</i>_sol for 10 min and successively subjected to a temperature drop across <i>T</i>_sol. The system stayed in the lattice-disordered state for a certain induction period. During this induction period, stronger segregation power at the lower temperature increased the domain spacing, whereas a storage shear modulus (<i>G</i>′) showed liquid-like behavior (<i>G</i>′<i>∝ ω</i>²) at low frequencies (ω < 0.2 s^–1) in a terminal zone and a shoulder at ω ∼ 1 s^–1. The shoulder shifted toward the smaller ω region, arising from dissociation of the PS block from the solvent. Once BCC lattice structures of spherical microdomains formed, grains eventually grew in size up to ca. 2.5 μm with a large size-distribution as revealed by the 2d-SAXS with spot-like scatterings, whereas <i>G</i>′ in the terminal region increased, arising from the increase in correlation length of the spherical microdomains. Eventually, the <i>G</i>′ showed plateau at lower frequencies at ω < 0.2 s^–1, indicating that the BCC lattice of spheres with long-range order (grain stuructures) was percolated throughout the solution. The number of the grains still continued to increase at the cost of spherical microdomains in the lattice disordered state, which caused the further increase in <i>G</i>′ at the plateau until the end of the ordering process of the BCC structure

A Rheo-Optical Study on Reinforcement Effect of Silica Particle Filled Rubber

In order to clarify the filler effect on rubbers, rheo-optical measurements were performed for unfilled and filled samples of a styrene–butadiene rubber/silica system. Refractive index matching was performed to increase transparency of samples and to reduce form birefringence of particles. The component stress of matrix rubber and the filler was successfully determined by using the strain-induced birefringence data. Our results were consistent with the composite model in micromechanics if the vitrified rubber layer is consider on the surface of fillers. On the basis of the experimental results, we proposed a phenomenological equation for large tensile properties for ideal filled rubbers

Introducing Large Counteranions Enhances the Elastic Modulus of Imidazolium-Based Polymerized Ionic Liquids

Polymerized ionic liquids (PILs) are believed to be ideal solid-state polymer electrolytes, and hence experimental and computational studies have been widely undertaken to understand the relationship between the chemical structure and mechanical/dielectric properties and the ionic conductivity of PILs. However, it is still a challenge to understand the effect of counterion ionic volume on the material properties of PILs. Herein, we demonstrate the effect of the ionic volume ratio of counteranions to side-chain cations on linear viscoelastic response using three imidazolium-based PILs with different counteranions. We show that the elastic modulus is significantly enhanced at temperatures higher than glass transition temperature once the ionic volume of the counteranion exceeds that of the side-chain cation. Our results provide an additional strategy to improve mechanical properties of PILs, while maintaining relatively high ionic conductivity

Dielectric and Viscoelastic Behavior of Star-Branched Polyisoprene: Two Coarse-Grained Length Scales in Dynamic Tube Dilation

<i>cis</i>-Polyisoprene (PI) chain has the type A dipole parallel along the backbone so that its large-scale (global) motion results in not only viscoelastic but also dielectric relaxation. Utilizing this feature of PI, this paper examined dielectric and viscoelastic behavior of star PI probe chains (arm molecular weight 10^–3<i>M</i>_a = 9.5–23.5, volume fraction υ₁ = 0.1) blended in a matrix of long linear PI (<i>M</i> = 1.12 × 10⁶). The constraint release (CR)/dynamic tube dilation (DTD) mechanism was quenched for those dilute probes entangled with the much longer matrix, as evidenced from coincidence of the frequency dependence of the dielectric and viscoelastic losses of the probe in the blend. Comparison of the probe data in the blend and in monodisperse bulk revealed that the star probe relaxation is retarded and broadened on blending and the retardation/broadening is enhanced exponentially with <i>M</i>_a. This result in turn demonstrates significant CR/DTD contribution to the dynamics of star PI in bulk. The magnitude of retardation was quantitatively analyzed within the context of the tube model, with the aid of the dielectrically evaluated survival fraction of the dilated tube, φ′(<i>t</i>), and the literature data of CR time, τ_CR. In the conventional molecular picture of partial-DTD, the tube is assumed to dilate <i>laterally</i>, but not <i>coherently</i> along the chain backbone. The corresponding <i>lateral</i> partial-DTD relationship between φ′(<i>t</i>) and the normalized viscoelastic relaxation function μ­(<i>t</i>), μ­(<i>t</i>) = φ′(<i>t</i>)/β­(<i>t</i>) with β­(<i>t</i>) being the number of entanglement segments <i>per</i> laterally dilated segment (that was evaluated from the φ′(<i>t</i>) and τ_CR data), held for the μ­(<i>t</i>) and φ′(<i>t</i>) data of star PI in bulk. Nevertheless, the observed retardation of the star probe relaxation on blending was <i>less significant</i> compared to the retardation expected for the arm motion (retraction) along the laterally dilated tube in bulk PI. This result suggests that the relaxation time of the probe in bulk is governed by the <i>longitudinal</i> partial-DTD that occurs <i>coherently</i> along the chain backbone. In fact, the magnitude of retardation evaluated from the φ′(<i>t</i>) and τ_CR data on the basis of this <i>longitudinal</i> partial-DTD picture was close to the observation. These results strongly suggest that the star PI chains in monodisperse bulk have two different coarse-grained length scales: the diameter of laterally dilated tube that determines the modulus level and the diameter of longitudinally dilated tube that reflects the path length for the arm retraction and determines the relaxation time. Thus, the star PI chains in bulk appear to move along the longitudinally dilated tube that wriggles in the laterally dilated tube. This molecular scenario is consistent with the previous finding for bulk linear PI [Matsumiya et al. Macromolecules 2013, 46, 6067]

Polymerized Ionic Liquids: Correlation of Ionic Conductivity with Nanoscale Morphology and Counterion Volume

The impact of the chemical structure on ion transport, nanoscale morphology, and dynamics in polymerized imidazolium-based ionic liquids is investigated by broadband dielectric spectroscopy and X-ray scattering, complemented with atomistic molecular dynamics simulations. Anion volume is found to correlate strongly with <i>T</i>_g-independent ionic conductivities spanning more than 3 orders of magnitude. In addition, a systematic increase in alkyl side chain length results in about one decade decrease in <i>T</i>_g-independent ionic conductivity correlating with an increase in the characteristic backbone-to-backbone distances found from scattering and simulations. The quantitative comparison between ion sizes, morphology, and ionic conductivity underscores the need for polymerized ionic liquids with small counterions and short alkyl side chain length in order to obtain polymer electrolytes with higher ionic conductivity