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