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.Topics: Distribution function; Materials science; Mechanical properties; Solubility; Thermal properties;

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 (feas) and Deborah number (De = ωτ, τ is the relaxation time of sample). For the samples with Type I behavior (both storage modulus G′ and loss modulus G″ increase with strain amplitude γ, i.e., intercycle strain hardening), the critical value of strain amplitude (γcrit) at the onset of intercycle strain hardening is almost the same when De > ∼2 (Region 3), while the value of Weissenberg number (Wi = γDe) at γcrit is similar when De < ∼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 Wi > 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 < De < ∼2), two involved mechanisms both contribute to intercycle strain hardening. Furthermore, by analyzing the influence of characteristic value of De 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 De < 1, while strain-induced non-Gaussian stretching dominates when De > 1.

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.

The cure kinetics for two-component silicone rubber formed by addition reaction was studied by the rheological method. The influence of reaction temperature (T) on the cure kinetics was explored in detail. It was observed that the data of gel time (tgel, i.e. the time when the reaction reaches the gel point) or a specific reaction time (tnc) (defined as the reaction time before which time the influence of confinement of network on the diffusion of reaction components can be neglected) versus T obey certain functional relationship, which was well explained by the cure kinetics model of thermoset network. The cure kinetics for the two-component silicone rubber can be well fitted by the Kamal-Sourour(autocatalyst) reaction model rather than Kissinger model. When the reaction time was before or equal to tnc, the reaction order obtained by the Kamal-Sourour reaction model was 2, which was consistent with the reaction order inferred from the two components chemical reaction when the diffusion of reaction components was not influenced by the formed cross-linked polymer network. When the reaction time was larger than tnc, such as to the end of reaction (te), the influence of confinement of network on the diffusion of reaction components cannot be neglected, and the reaction order obtained by the Kamal-Sourour reaction model was larger than 2. It was concluded that the confinement effect of network had a greater influence on the cure kinetics of the silicone rubber. The reaction rate constants (kr) under different temperatures were also determined by Kamal-Sourour reaction model. The activation energy (E) for the two-component silicone rubber was also calculated from the results of lntgel, lntnc, and lnkr versus 1/T, respectively. The three values of E were close, which indicated that above analyses were self-consistent.

The structure and rheological properties of graphene-based particle (GP-x)/polydimethylsiloxane (PDMS) composites are investigated as the surface oxygen content of graphene-based particle is varied, i.e., from 6.6% (GP-1) to 15.3% (GP-2), 25.5% (GP-3) and 43.1% (GP-4). Interestingly, the dispersion state of graphene-based particles in PDMS does not change monotonically with increasing surface oxygen content. The size of layered stacks and aggregates first decreases from GP-1 to GP-3 and then increases from GP-3 to GP-4 with increasing surface oxygen content. The larger size of layered stacks and aggregates in GP-1 and GP-4 suspensions results from strong inter-particle π–π and hydrogen bonding interactions. Under weak shear, GP-1 and GP-4 form larger aggregates in PDMS, which align along the vorticity direction, inducing negative normal stress differences (ΔN) in the composites. However, GP-2 and GP-3 do not further aggregate under weak shear and the ΔN is almost zero. It is further inferred that the strong inter-particle attractive interaction leads to the vorticity alignment of aggregates under weak shear.

The structure and rheological properties of carbon-based particle suspensions, i.e., carbon black (CB), multi-wall carbon nanotube (MWNT), graphene and hollow carbon sphere (HCS) suspended in polydimethylsiloxane (PDMS), are investigated. In order to study the effect of particle shape on the structure and rheological properties of suspensions, the content of surface oxygen-containing functional groups of carbon-based particles is controlled to be similar. Original spherical-like CB (fractal filler), rod-like MWNT and sheet-like graphene form large agglomerates in PDMS, while spherical HCS particles disperse relatively well in PDMS. The dispersion state of carbon-based particles affects the critical concentration of forming a rheological percolation network. Under weak shear, negative normal stress differences (ΔN) are observed in CB, MWNT and graphene suspensions, while ΔN is nearly zero for HCS suspensions. It is concluded that the vorticity alignment of CB, MWNT and graphene agglomerates under shear results in the negative ΔN. However, no obvious structural change is observed in HCS suspension under weak shear, and accordingly, the ΔN is almost zero.

•Comb-like PSVS-g-PS with high branching density has been successfully synthesized.•PSVS-g-PS shows hierarchical relaxation process during linear rheological testing.•The melt behavior of PSVS-g-PS is different from both bottlebrush and entangled linear polymer chains.A series of model comb-like polymers (PSVS-g-PS) with high branching degree were synthesized by nucleophilic substitution reaction between iodinated copolymer of styrene (St) with 4-(vinylphenyl)-1-butene (VSt) (PSVSI, backbone) and living polystyrene lithium (PSLi, side chain). This method is a facile way for modular synthesis of graft copolymers based on living anionic polymerization and hydrozirconation reaction. The branching density of PSVS-g-PS (the number of branch chain per 100 repeat backbone units) ranged from 2.4 to 20.7%. The backbone length of PSVS-g-PS was larger than the entanglement molecular weight (Me) of PS, while the molecular weight of branch chain was designed to be below or above the Me of PS. The results from the linear rheological properties of the PSVS-g-PS showed hierarchical relaxation process. One appeared in the intermediate angular frequency region, corresponding to the relaxation of the side chain. The other is in the terminal region, which is related to the movement of the whole comb polymer. From scaling law of zero shear viscosity (η0) of PSVS-g-PS with versus molecular weight, it could be seen that the dynamical behavior of PSVS-g-PS was different from those of both poly(macromonomers) and entangled linear polymer chains, indicating that there is certain entanglement for PSVS-g-PS with long branch chain.Figure optionsDownload full-size imageDownload as PowerPoint slide

Comb-like poly(styrene-co-4-(vinylphenyl)-1-butene)-g-polyethylene copolymers (PSVS-g-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-g-PE copolymers strongly affected linear rheological behavior of the copolymers, resulting from the formation of different microphase separation structure in the melt state. PSVS-g-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-g-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.

The rheological properties of two specific waterborne polyurethane (PU) paints were studied by both macrorheological and microrheological methods. During the macrorheological measurement on a rotary rheometer, evaporation of solvent cannot be totally excluded, which has an influence on the reliability of rheological results. So, the linear oscillatory frequency sweep results (storage and loss modulus versus frequency) and steady shear results (viscosity versus shear rate) got from the rotary rheometer measurement are only used for qualitative analysis. As the evaporation of solvent can be neglected during microrheological measurements on a diffusing wave spectroscope (DWS), the results of storage modulus (G′) and loss modulus (G″) versus frequency are more credible than the results obtained from the rotary rheometer measurement. Thus, the results of G′ and G″ versus frequency from DWS measurements are used for quantitative analysis in this work. The G′ for both of the waterborne PU paints are larger than G″ at low frequency and that is opposite at high frequency in the experimental angular frequency range. The values of modulus at same frequency and viscosity at low shear rate for the two PU paints have apparent difference, which determines the difference of their application.

The structure and rheological properties of multiwall carbon nanotube (MWNT)/polydimethylsiloxane (PDMS) composites under shear are investigated, as the molecular weight of PDMS, aspect ratio and concentration of MWNT are systematically varied. Negative normal stress differences (ΔN) are observed at low shear rates for samples with low molecular weight (Mw) of PDMS (lower than the critical entanglement molecular weight (Mc)), whereas positive ΔN is found in samples with high molecular weight of PDMS (Mw > Mc). More interestingly, negative ΔN is also observed for some samples under confinement when the molecular weight of PDMS is higher than the critical value (Mw > Mc). Moreover, the aspect ratio and concentration of MWNT show negligible influence on the sign of ΔN. Based on the results of optical-flow experiments, a phase diagram for the structures of samples under shear is obtained. It is concluded that the vorticity banding of MWNT aggregates results in the negative ΔN under shear through relating the evolution of structure and the rheological properties of samples under shear.

•Percolation threshold of GC/silicon oil suspensions was investigated.•Negative normal stress was found in pseudo-steady and transient shear measurements.•Strain hardening of G″ and strain softening of G′ in oscillatory shear were observed.•The rheological properties of different carbon materials were compared.The rheological properties of amorphous carbon suspensions have attracted great attention, but they are still not fully understood due to the non-equilibrium nature of the structure. In this work, the linear and nonlinear rheological properties of ginger-like amorphous carbon (GC) filled silicon oil suspensions are investigated. The percolation threshold of GC/silicon oil suspensions is 1.2 wt.% based on linear viscoelasticity and percolation theory. Moreover, a viscosity plateau and apparent negative normal stress differences are observed during pseudo-steady shear experiments for samples with GC concentration above 6 wt.%. Furthermore, constant-rate shear flow confirms the evolution of structure as functions of shear rate and time. Additionally, strain softening of storage modulus and strain hardening of loss modulus are observed during strain sweep experiments for samples with GC concentration above the percolation threshold. Relating the rheological results with the structure observed by an in situ optical shearing cell, the change of viscosity and negative normal stress differences under pseudo-steady shear is supposed to result from the structure reorganization of GC networks.

•Graphene oxide disperses better in higher molecular weight of PDMS.•Negative normal stress differences are observed in low Mw PDMS composites.•Positive normal stress differences are observed in high Mw PDMS composites.•Vorticity alignment of GO clusters is observed in low Mw PDMS composites.The structure and rheological properties of graphene oxide (GO)/polydimethylsiloxane (PDMS) composites are examined as the molecular weight of PDMS and concentration of GO are varied. Clusters formed by GO sheets get smaller and disperse better with increasing molecular weight of PDMS, which results in the higher critical concentration to form network (Ccr). Moreover, at GO concentration just above Ccr, the plateau modulus of samples decreases with the molecular weight of PDMS. During shear experiments, negative normal stress differences (ΔN) are observed in composites with PDMS molecular weight lower than critical entanglement molecular weight (Mc). However, positive ΔN is found in samples with PDMS molecular weight above Mc. It can be concluded that the vorticity alignment of GO clusters induces the negative ΔN based on the optical shear experiments. The possible mechanism for the positive ΔN is also proposed.