Multiple Dynamic Processes Contribute to the Complex Steady Shear Behavior of Cross-Linked Supramolecular Networks of Semidilute Entangled Polymer Solutions

Molecular theories of shear thickening and shear thinning in associative polymer networks are typically united in that they involve a single kinetic parameter that describes the network  a relaxation time that is related to the lifetime of the associative bonds. Here we report the steady-shear behavior of two structurally identical metallo-supramolecular polymer networks, for which single-relaxation parameter models break down in dramatic fashion. The networks are formed by the addition of reversible cross-linkers to semidilute entangled solutions of poly(4-vinylpyridine) (PVP) in dimethylsulfoxide (DMSO), and they differ only in the lifetime of the reversible cross-links. Shear thickening is observed for cross-linkers that have a slower dissociation rate (17 s⁻¹), while shear thinning is observed for samples that have a faster dissociation rate (ca. 1400 s⁻¹). The difference in the steady shear behavior of the unentangled versus entangled regime reveals an unexpected, additional competing relaxation, ascribed to topological disentanglement in the semidilute entangled regime that contributes to the rheological properties

Synthesis of Structure-Controlled Polyborosiloxanes and Investigation on Their Viscoelastic Response to Molecular Mass of Polydimethylsiloxane Triggered by Both Chemical and Physical Interactions

A series of polyborosiloxanes (PBSs) was synthesized by mixing hydroxy-terminated polydimethylsiloxanes (PDMS) and boric acid (BA) in toluene at 120 °C. The molecular masses of selected PDMS precursors were in a wide range, covering from below up to far above the critical entanglement molecular mass of PDMS. The reaction kinetics was followed by using Fourier transform infrared (FTIR) spectroscopy. Unreacted BA was removed from raw PBSs after the reactions. The influence of molecular mass of PDMS precursors on the rheological property of PBSs was explored by dynamic oscillatory frequency sweeps. The results showed that the plateau elastic moduli of PBSs were highly dependent on the molecular mass of PDMS precursors. The plateau elastic moduli of PBSs decreased at first and then increased with increasing molecular mass of PDMS precursors. PBS1 and PBS2 prepared from unentangled PDMS precursors showed sufficient fits by using the two-mode Maxwell model, whereas PBS3 to PBS6 prepared from highly entangled PDMS precursors showed obvious deviations from the two-mode Maxwell model. It could be concluded that the changing trend of plateau elastic modulus of PBSs versus molecular mass of PDMS precursors was determined by the number density of supramolecular interactions (Si–O:B weak bonding and hydrogen-bonding of the end groups Si–O–B­(OH)₂) and the number density of topological entanglements

Dependence of Melt Behavior of Star Polystyrene/POSS Composites on the Molecular Weight of Arm Chains

Rheological behavior of three-arm and six-arm star polystyrene (SPS) with a small amount of polyhedral oligosilsesquioxane (POSS) was studied. Both linear oscillatory frequency sweep and steady state shear results of SPS/POSS composites showed the reduction of melt viscosity in the unentangled SPS matrix and the increase of viscosity in the entangled SPS matrix. In particular, when molecular weight of the arm (<i>M</i>_a) of SPS was smaller than the critical molecular weight for entanglement (<i>M</i>_c) of PS, the melt viscosity of SPS/POSS composites with low content of POSS was lower than that of pure SPS. The abnormal phenomenon of reduced melt viscosity in SPS/POSS composites was in coincidence with the melt viscosity behavior of SPS/C₆₀ composites reported in our previous work (Soft Matter 2013, 9, 6282−6290), although the diameters of two nanoparticles and their interaction with SPS matrix were different. A possible mechanism behind the melt viscosity behavior was discussed. Furthermore, the time–temperature superposition principle (TTS) was applied in SPS and SPS/POSS composites. The Cox–Merz empirical relationship was verified to be valid for SPS/POSS composites when the content of POSS was low (1 wt %)

Synthesis, Structure, and Photoluminescence of Color-Tunable and White-Light-Emitting Lanthanide Metal–Organic Open Frameworks Composed of AlMo₆(OH)₆O₁₈^3– Polyanion and Nicotinate

A series of isostructural compounds Na­(HL)­(CH₃COO)­Ln­(Al­(OH)₆Mo₆O₁₈)­(H₂O)₆·10H₂O [L = nicotinate; Ln = Eu (<b>1</b>), Tb (<b>2</b>)] and Na­(HL)­(CH₃COO)­Eu_<i>m</i>Tb_<i>n</i>La_{1<i>–m–n</i>}(AlMo₆(OH)₆O₁₈)­(H₂O)₆·10H₂O (<b>3</b>–<b>8</b>, L = nicotinate), wherein Anderson-type polyanions AlMo₆(OH)₆O₁₈^3– as basic inorganic building blocks are connected by Eu­(CH₃COO)­(HL)­(H₂O)₃]₂⁴⁺ and [Na₂(H₂O)₈]²⁺ cations, resulting in formation of three-dimensional lanthanide metal–organic open frameworks, were synthesized successfully with AlCl₃·6H₂O, Na₂MoO₄·2H₂O, nicotinic acid, and lanthanide nitrates as starting materials. The compounds were characterized by UV–vis, IR, elemental analysis, powder XRD, and TG–DTA measurements. The single-crystal structures of compounds <b>1</b> and <b>2</b> show that the two compounds display three-dimensional open frameworks with 1D channels along the <i>b</i> and <i>c</i> axes. Investigation of the energy transfer mechanism indicated that the organic nicotinate ligand can transfer energy efficiently to Tb³⁺ rather than Eu³⁺. The influence of the POM moiety on the fluorescence of the compounds is also studied. Compounds <b>1</b>–<b>8</b> exhibit tunable luminescence color, and emitting of white light was realized through adjusting the molar ratio of Eu:Tb:La within the compounds

Dependences of Rheological and Compression Mechanical Properties on Cellular Structures for Impact-Protective Materials

In this study, three typical impact-protective materials, D3O, PORON XRD, and DEFLEXION were chosen to explore the dependences of rheological and compression mechanical properties on the internal cellular structures with polymer matrix characteristics, which were examined using Fourier transform infrared spectroscopy, thermogravimetric analyses, and scanning electron microscopy with energy dispersive spectroscopy. The rheological property of these three foaming materials were examined using a rheometer, and the mechanical property in a compression mode was further examined using an Instron universal tensile testing machine. The dependences of rheological parameters, such as dynamic moduli, normalized moduli, and loss tangent, on angular frequency, and the dependences of mechanical properties in compression, such as the degree of strain-hardening, hysteresis, and elastic recovery, on the strain rate for D3O, PORON XRD, and DEFLEXION can be well-correlated with their internal cellular structural parameters, revealing, for example, that D3O and PORON XRD exhibit simultaneously high strength and great energy loss in a high-frequency impact, making them suitable for use as soft, close-fitting materials; however, DEFLEXION dissipates much energy whether it suffers a large strain rate or not, making it suitable for use as a high-risk impact-protective material. The rheometry and compression tests used in this study can provide the basic references for selecting and characterizing certain impact-protective materials for applications

Strain Hardening Behavior of Poly(vinyl alcohol)/Borate Hydrogels

The large-amplitude oscillatory shear (LAOS) behavior of poly­(vinyl alcohol) (PVA)/borate hydrogels was investigated with the change of scanning frequency (ω) as well as concentrations of borate and PVA. The different types (Types I–IV) of LAOS behavior are successfully classified by the mean number of elastically active subchains per PVA chain (<i>f</i>_eas) and Deborah number (<i>D</i>_e = ωτ, τ is the relaxation time of sample). For the samples with Type I behavior (both storage modulus <i>G</i>′ and loss modulus <i>G</i>″ increase with strain amplitude γ, i.e., intercycle strain hardening), the critical value of strain amplitude (γ_crit) at the onset of intercycle strain hardening is almost the same when <i>D</i>_e > ∼2 (Region 3), while the value of Weissenberg number (<i>Wi</i> = γ<i>D</i>_e) at γ_crit is similar when <i>D</i>_e < ∼0.2 (Region 1). For intracycle behavior in the Lissajous curve, intracycle strain hardening is only observed in viscous Lissajous curve of Region 1 or in the elastic Lissajous curve of Region 3. In Region 1, both intercycle and intracycle strain hardening are mainly caused by the strain rate-induced increase in the number of elastically active chains, while non-Gaussian stretching of polymer chains starts to contribute as <i>Wi</i> > 1. In Region 3, strain-induced non-Gaussian stretching of polymer chains results in both intercycle and intracycle strain hardening. In Region 2 (∼0.2 < <i>D</i>_e < ∼2), two involved mechanisms both contribute to intercycle strain hardening. Furthermore, by analyzing the influence of characteristic value of <i>D</i>_e as 1 on the rheological behavior of PVA/borate hydrogels, it is concluded that intercycle strain hardening is dominated by strain-rate-induced increase in the number of elastically active chains when <i>D</i>_e < 1, while strain-induced non-Gaussian stretching dominates when <i>D</i>_e > 1

Strain Hardening Behavior of Poly(vinyl alcohol)/Borate Hydrogels

The large-amplitude oscillatory shear (LAOS) behavior of poly­(vinyl alcohol) (PVA)/borate hydrogels was investigated with the change of scanning frequency (ω) as well as concentrations of borate and PVA. The different types (Types I–IV) of LAOS behavior are successfully classified by the mean number of elastically active subchains per PVA chain (<i>f</i>_eas) and Deborah number (<i>D</i>_e = ωτ, τ is the relaxation time of sample). For the samples with Type I behavior (both storage modulus <i>G</i>′ and loss modulus <i>G</i>″ increase with strain amplitude γ, i.e., intercycle strain hardening), the critical value of strain amplitude (γ_crit) at the onset of intercycle strain hardening is almost the same when <i>D</i>_e > ∼2 (Region 3), while the value of Weissenberg number (<i>Wi</i> = γ<i>D</i>_e) at γ_crit is similar when <i>D</i>_e < ∼0.2 (Region 1). For intracycle behavior in the Lissajous curve, intracycle strain hardening is only observed in viscous Lissajous curve of Region 1 or in the elastic Lissajous curve of Region 3. In Region 1, both intercycle and intracycle strain hardening are mainly caused by the strain rate-induced increase in the number of elastically active chains, while non-Gaussian stretching of polymer chains starts to contribute as <i>Wi</i> > 1. In Region 3, strain-induced non-Gaussian stretching of polymer chains results in both intercycle and intracycle strain hardening. In Region 2 (∼0.2 < <i>D</i>_e < ∼2), two involved mechanisms both contribute to intercycle strain hardening. Furthermore, by analyzing the influence of characteristic value of <i>D</i>_e as 1 on the rheological behavior of PVA/borate hydrogels, it is concluded that intercycle strain hardening is dominated by strain-rate-induced increase in the number of elastically active chains when <i>D</i>_e < 1, while strain-induced non-Gaussian stretching dominates when <i>D</i>_e > 1

Nanostructure and Linear Rheological Response of Comb-like Copolymer PSVS‑<i>g</i>‑PE Melts: Influences of Branching Densities and Branching Chain Length

Comb-like poly­(styrene-<i>co</i>-4-(vinylphenyl)-1-butene)-<i>g</i>-polyethylene copolymers (PSVS-<i>g</i>-PE) with various branching parameters were synthesized to study the influence of branch chains on morphology (at melt state) and linear rheological response of the copolymers. The results showed that both the branching density and branch chain length of PSVS-<i>g</i>-PE copolymers strongly affected linear rheological behavior of the copolymers, resulting from the formation of different microphase separation structure in the melt state. PSVS-<i>g</i>-PE copolymers with low branching density (2.3–3.5 branch chains per 100 repeating units of the backbone) showed a microphase-separated structure at the melt state, and a typical rheological characteristic for network-like structure was observed. Furthermore, the type of microphase-separated structure at the melt state strongly influences the applicability of the time–temperature superposition (TTS) principle. As a result, the TTS failure was observed in the modulus curves for PSVS52.7-3.5-PE4.9 (poor-order lamellar structure) and PSVS54.4-2.7-PE10.7 (long tubular structure). In contrast, the PSVS-<i>g</i>-PE sample with high branching density (16.6–24.5 branch chains per 100 repeating units of the backbone) showed homogeneous phase structure and normal rheological behavior, similar to linear or comb-like homopolymers. The gel-like state appeared in a limited frequency regime (a plateau regime of tan δ versus ω) during decreasing the frequency from the high frequency regime in these comb-like copolymers