Linear Viscoelasticity and Birefringence of Poly-γ-Benzyl‑l‑Glutamate Solutions

The complex modulus and the strain-optical coefficient for two samples of poly-γ-benzyl-l-glutamate in m-cresol solution were measured over a wide concentration regime covering from a dilute to tightly entangled region. The obtained experimental data in the dilute region were compared with the theory by Morse and coworkers for dilute solutions, which predicts three relaxation modes, tension, curvature, and orientation. We found that the theory well described the complex modulus of dilute solutions. With increasing the concentration, the three relaxation modes retarded, due to the hydrodynamic interaction. In addition, the topological interaction also retarded the orientation mode. The complex modulus of the tightly entangled region was separated into the component moduli using the strain-optical coefficient and the molecular theory. The curvature stress contributed to the rubbery plateau modulus in the tightly entangled region, as predicted by the Morse theory for the entangled systems. However, the relaxation time of curvature stress was shorter than the theoretical prediction, which is based on the fixed tube model. We speculate that the constraint release accelerates the relaxation of curvature stress but has a little effect on orientational stress

Synthesis of Dibenz[<i>b,f</i>]oxepins via Manganese(III)-Based Oxidative 1,2-Radical Rearrangement

The oxidation of monoalkyl 2-(9H-xanthenyl)malonates 1 with Mn(OAc)3 gave the 9- or 10-dibenz[b,f]oxepincarboxylates 2 in good yields. The reaction proceeds with high regioselectivity except for the case of (1-methoxyxanthenyl)malonate 1 (R1 = Me, R2 = 1-MeO), which gave two regioisomers. It was proposed that the process for the formation of 2 must include the 1,2-aryl radical rearrangement followed by oxidative decarboxylation

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

Effect of Head-to-Head Association/Dissociation on Viscoelastic and Dielectric Relaxation of Entangled Linear Polyisoprene: An Experimental Test

For linear high-cis polyisoprene having a monofunctionally associative carboxyl group at the chain head (PI30-COOH; M = 30.5 × 103), linear viscoelastic and dielectric behavior was examined in its entangled bulk system. The PI30-COOH unimer chain had type-A dipoles aligned from the tail to head so that its large-scale motion (over the tail-to-head distance) activated not only viscoelastic but also dielectric relaxation. Consequently, the head-associated dimer of PI30-COOH had symmetrically inverted type-A dipoles, and its large-scale motion also activated both viscoelastic and dielectric relaxation. These unimer and dimer were coexisting in the system at equilibrium because of the association/dissociation reaction at the carboxyl group, and this reaction strongly affected the viscoelastic and dielectric behavior. Experimentally, the reaction effect was examined by utilizing two reference polyisoprenes undergoing no reaction, PI30 and (PI30)2: PI30 was a prepolymer of PI30-COOH before introducing the carboxyl group at the head, and (PI30)2 was a head-to-head dimer of PI30 prepared by coupling of the PI30 anion. The viscoelastic and dielectric relaxation was found to be faster for the PI30-COOH system than for a reference PI30/(PI30)2 blend having the unimer/dimer composition identical to that in the PI30-COOH system (determined from Fourier transform infrared measurement), and this difference between the PI30-COOH system and the blend was more significant for the viscoelastic relaxation than for the dielectric relaxation. This experimental fact unequivocally indicates that the reaction induces motional coupling between the unimer and dimer to significantly affect the relaxation behavior of these chains. This result lends support to a recent model analyzing this coupling for entangled (reptating) unimer and dimer. In fact, the model described low-frequency asymptotes of the viscoelastic and dielectric losses of PI30-COOH surprisingly well, given that the viscoelastic and dielectric asymptotes of the reference PI30 bulk system were separately fitted by the model to determine the terminal relaxation times in the model calculation in the absence of reaction. This success of the model strongly suggests that the difference of the reaction effects on the dielectric and viscoelastic relaxation reflects the vectorial and tensorial nature of the respective relaxation processes. The dielectric relaxation reflects the (vectorial) first-moment average of bond vectors u of the chain so that the dipole inversion of the dimer leads to pairwise coupling of the dielectric modes of the unimer and dimer under the reaction, thereby allowing the reaction to affect the dielectric relaxation just moderately. In contrast, the viscoelastic relaxation detects the (tensorial) second-moment average of u so that the reaction results in multiple coupling of the viscoelastic modes of the unimer and dimer to strongly affect the viscoelastic relaxation

Dielectric Relaxation of Monodisperse Linear Polyisoprene: Contribution of Constraint Release

<i>cis</i>-Polyisoprene (PI) has the type A dipole parallel along the chain backbone so that the end-to-end fluctuation of PI chains results in slow dielectric relaxation. Comparison of dielectric and viscoelastic data of PI has revealed several interesting features related to the entanglement dynamics, for example, success and failure of the full dynamic tube dilation (DTD) picture for monodisperse linear and star PI, respectively [see a review: Watanabe, H. <i>Polym. J.</i><b>2009</b>, <i>41</i>, 929, for example]. For monodisperse <i>linear</i> PI, recent modeling [Glomann et al. <i>Macromolecules</i> <b>2011</b>, <i>44</i>, 7430] and single-chain slip-link simulation [Pilyugina et al. <i>Macromolecules</i> <b>2012</b>, <i>45</i>, 5728] suggest that the constraint release (CR) mechanism has negligible influence on the dielectric relaxation time τ_ε in the entangled regime, which appears to disagree with the previous data. Thus, we revisited the classical problem: CR contribution to the dielectric relaxation of PI. Specifically, we made dielectric and viscoelastic measurements for PI/PI blends in a wide range of the molecular weights of long and short components, <i>M</i>₂ = 1.1M and <i>M</i>₁ = 21K–179K, and with a small volume fraction of the short component, υ₁ = 0.1 and/or 0.2, to examine the CR contribution in the experimentally clearest way. It turned out that τ_ε of the short component was longer in the blends than in respective monodisperse bulk even for <i>M</i>₁ = 179K. Furthermore, the viscoelastic and dielectric data of the short components (<i>M</i>₁ ≤ 43K) in the blend exhibited identical mode distribution and relaxation time, which confirmed that the CR mechanism was fully suppressed for these components in the blends. These results demonstrate that the CR mechanism <i>does</i> contribute/accelerate the dielectric relaxation in monodisperse bulk PI systems even in the highly entangled regime (<i>M</i>₁/<i>M</i>_e = 36 for <i>M</i>₁ = 179K). This CR-induced acceleration was found to be consistent with the empirical equations for the terminal relaxation time and CR time of monodisperse PI available in the literature, as noted from a simple DTD analysis of the terminal relaxation process (reptation along a partially dilated tube that wriggles in a fully dilated tube)

Rapid Stretching Vibration at the Polymer Chain End

Stretching vibrations of aliphatic C–D bonds at the chain end and midchain site of partially deuterated polystyrene (PS) were determined by Fourier transform infrared (FT-IR) spectroscopy. It was first found that the stretching vibration at the chain end is more rapid compared to that at the midchain site in the glassy bulk state. The difference in the frequencies of the stretching vibrations at the chain end and midchain site changed little even when the PS was dissolved in toluene. Moreover, the DFT vibrational frequency calculations showed that the C–D bonds at the end site intrinsically have higher vibrational frequency because of lower intramolecular interactions at the chain end. From these results, it was concluded that the main origin for the local rapid stretching vibration at the chain end is not intermolecular effects but reduced intramolecular interactions induced by the discontinuity of the repeating unit at the chain end

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]