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Article http://pubs.acs.org/journal/acsodf
Dependences of Rheological and Compression Mechanical Properties on Cellular Structures for Impact-Protective Materials Miao Tang,†,‡,# Gang Huang,‡,§,# Huanhuan Zhang,‡,§ Yuling Liu,‡,∥ Haijian Chang,⊥ Hongzan Song,∥ Donghua Xu,*,‡ and Zhigang Wang*,† †
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui Province 230026, P. R. China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin Province 130022, P. R. China § University of Chinese Academy of Sciences, Beijing 100049, P. R. China ∥ College of Chemistry and Environmental Science, Hebei University, Baoding, Hebei Province 071002, P. R. China ⊥ College of Materials Science and Engineering, Jilin University, Changchun, Jilin Province 130025, P. R. China S Supporting Information *
ABSTRACT: 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. shear rate is beyond a critical value.10,12,13 Because shearthickening occurs under some specific conditions and fluids might easily flow out once the fluid foam composites are destroyed, their practical applications are still limited. Impact-hardening polymers can significantly achieve impact protection because of their particular and excellent smart softness−stiffness switch performance.14 Several commercial impact-hardening polymers are practically applied as impactprotective materials. As an impact-protective material, D3O invented by Dr. Palmer in 2004 is being hailed as one of the top ten future materials. D3O behaves soft and flexible in its normal state; however, when impacted by a force it locks itself, disperses energy, and then returns to its normal state.15,16 D3O molecule keeps as a commercial secret, belonging to “the
1. INTRODUCTION Impact-protective materials are a type of foam, which can absorb and distribute the force of direct impacts to exposed areas as a result of fall, blow, or other sudden impact.1−3 Traditional impact-protective materials have drawbacks such as rigidity, inflexibility, and heaviness, which restrict their wide applications in the modern era, whereas, impact-protective materials with comfort, flexibility, and light weight are widely used in military equipment,4 sporting goods,5,6 medical equipment,7 household appliances, and so on. Shear-thickening fluids, impact-hardening polymers, and materials with negative Poisson’s ratio are thought to be potential candidates for impact-protective materials.8−18 Shear-thickening as a typical non-Newtonian flowing behavior results from dilation and jamming of suspended particles in dense suspensions8,9 or arises from the formation of particle clusters in moderate concentrated suspensions induced by lubrication hydrodynamics,10,11 in which the viscosity steeply increases once the applied © 2017 American Chemical Society
Received: March 1, 2017 Accepted: May 10, 2017 Published: May 22, 2017 2214
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expanding foam” material. D3O is also a kind of “smart molecule” composed of a single material with impact resistance.17 Another impact-protective material, PORON XRD (Rogers Corporation, USA), shows excellent resistance to compression deformation, absorbing 90% of the energy even at a high-frequency impact. PORON XRD appears to be an open-cell urethane foam. PORON XRD shows softness and flexibility when at rest because the glass transition temperature of the urethane molecules is low; whereas PORON XRD shows stiffness when impacted quickly because the glass transition temperature drops.15 The third impact-protective material, DEFLEXION (Dow Corning, USA), is worthy of a great emphasis, showing a strong energy-absorption ability. As a flexible silicone sheet, DEFLEXION is made of silicon. To a hard impact like a bullet shot, the molecules in the shot area in DEFLEXION gather to make it instantly turn into a rocklike solid. DEFLEXION disperses and absorbs the impact energy and can be used in a bulletproof vest.18 The abovementioned three impact-protective materials, D3O, PORON XRD, and DEFLEXION, have received a great deal of attention. However, the relationship between the internal cell structures and the rheological and mechanical properties of these impact-protective materials is not wellunderstood yet. In this work, the internal cell structures of D3O, PORON XRD, and DEFLEXION were first examined using scanning electron microscopy (SEM); second, their rheological properties were examined using a rheometer in the dynamic oscillatory mode, and third, their mechanical properties in the compression mode were examined using an Instron universal tensile testing machine. The influences of the internal cell structures, such as cell type, cell size, size distribution, and cell volume fraction, on the rheological and mechanical properties of D3O, PORON XRD, and DEFLEXION are discussed.
XRD was taken as the sample. For DEFLEXION (Dow Corning, USA), the internal gray color DEFLEXION was taken as the sample. Figure S1 shows the Fourier transform infrared (FTIR) spectra of D3O, PORON XRD, and DEFLEXION. It can be seen that the FTIR spectra of D3O and PORON XRD are more close to each other. For D3O and PORON XRD, the absorption band located at 3325 cm−1 is attributed to the stretching vibration of N−H bonds that form intermolecular hydrogen bonding. The absorption band at 1725 cm−1 is attributed to the stretching vibration of CO bonds. The absorption band at 1530 cm−1 corresponds to the N−H deformation. The absorption band at 1217 cm−1 is attributed to the asymmetric stretching vibration of C−O−C bonds of ester groups. The above result indicates that D3O and PORON XRD are polyurethane foams. The absorption band at 1085 cm−1 for DEFLEXION is attributed to the symmetric stretching vibration of Si−O−Si bonds. The absorption bands at 2958 and 2869 cm−1 are indicative of the asymmetric and symmetric stretching vibrations of the methyl group, respectively. The FTIR spectrum of DEFLEXION is highly consistent with that of polydimethylsiloxane. Moreover, SEM with energy dispersive spectroscopy (EDS) was used to obtain the SEM−EDS spectrum of DEFLEXION, and the SEM−EDS spectrum shown in Figure S2 indicates that DEFLEXION contains silicon. Therefore, the material in DEFLEXION is a kind of silicone rubber. It is interesting to find that the SEM− EDS spectrum of PORON XRD indicates that PORON XRD also contains silicon. Figure S3 shows the thermogravimetric analysis (TGA) curves for D3O, PORON XRD, and DEFLEXION taken under nitrogen and air atmosphere. It can be observed from Figure S3a that the thermal degradation for D3O and PORON XRD under a nitrogen atmosphere is consistent with that of polyurethane and very possible polyurethane/SiO2 composite, respectively. DEFLEXION shows a single-step thermal decomposition, demonstrating a single component in the sample, whereas D3O and PORON XRD show a multiple-step decomposition. As shown in Figure S3b, D3O can fully burn out in air at 600 °C, whereas 17 wt % of mass remains for DEFLEXION in air and 11 wt % of mass remains for PORON XRD in air. These results indicate that the Si contents are different in PORON XRD and DEFLEXION, which is consistent with the SEM−EDS spectra shown in Figure S2. Overall, it can be concluded that the materials in D3O and PORON XRD are polyurethane-based foams, whereas the material in DEFLEXION is silicone rubber. The difference between PORON XRD and D3O is that, PORON XRD contains possibly SiO2 particles as fillers. To examine the cross-linking degrees, the solubility of the three samples in the respective good solvents according to their chemical compositions was conducted, and the results are shown in Figure S4. D3O and PORON XRD cannot be dissolved in tetrahydrofuran, a good solvent for polyurethanes, indicating that D3O and PORON XRD are relatively highly cross-linked materials. The volume expansion ratios for D3O and PORON XRD are approximately 11 and 9, respectively. The dissolved mass portions for D3O and PORON XRD in tetrahydrofuran are 2.1 and 4.2 wt %, respectively. The dissolved mass portion for DEFLEXION in n-hexane is 77.3 wt %, which is so much high that DEFLEXION cannot keep intact in the solvent. The above result indicates that the crosslinking degree for DEFLEXION is much lower than those for D3O and PORON XRD.
2. RESULTS AND DISCUSSION 2.1. Chemical Compositions and Cross-Linking Degrees. Three impact-protective materials used for this study are illustrated in Figure 1. For D3O (D3O, UK), the internal orange color D3O-molded knee cup was taken as the sample. For PORON XRD (Rogers, USA), the yellow color PORON
Figure 1. Illustrative images showing the locations of D3O, PORON XRD, and DEFLEXION in the commercial impact-protective products. 2215
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2.2. Surface and Internal Cell Structures from SEM Observation. The mechanical property of impact−protective materials depends strongly on the internal cell structures of these foam materials, such as cell type, cell fraction, cell size, cell size distribution, and so forth. Hence, the surface and internal cell structures of D3O, PORON XRD, and DEFLEXION were examined using SEM, and the results are shown in Figure 2. The internal cell parameters are summarized
reaches approximately 94.7%. The surface and internal cell structures of DEFLEXION look much different from those of D3O and PORON XRD. The surface of DEFLEXION looks rough with no pores, and the internal cell structure of DEFLEXION consists of loosely dispersed cells in the polymer matrix. The cell walls of the dispersed cells of DEFLEXION look smooth with no micropores ( f m = 0). To obtain the cell volume fraction, cell sizes (diameters), and size distributions for the internal cell structures of these three samples, the SEM micrographs in the right panel of Figure 2 were analyzed in detail using ImageJ software. The parameter β in eq 1 is used to evaluate the cell volume fraction in the above samples β (%) = A p /A t × 100%
(1)
where Ap is the integrated area occupied by all cells and At is the total area of the sample. The β values are 62, 60, and 48% for D3O, PORON XRD, and DEFLEXION, respectively, indicating that DEFLEXION has a much less cell fraction than D3O and PORON XRD, whereas D3O and PORON XRD have close cell fractions. Figure 3 shows the cell size distributions fitted using the Lorentzian functions. The cells
Figure 2. SEM micrographs taken on the surface (A, left panel) and internal section (B, right panel) for D3O, PORON XRD, and DEFLEXION. The scale bar in the left bottom micrograph represents 200 μm and is applied to other micrographs as well.
Table 1. Internal Cell Structural Parameters for D3O, PORON XRD, and DEFLEXIONa sample code
f m (%)
β (%)
D (μm)
αu
ρ (g/cm3)
D3O PORON XRD DEFLEXION
27.5 94.7 0
62 60 48
77 56 78
0.37 0.44 0.41
0.41 0.33 0.44
Note that f m means the number fraction of cells with micropores, β is the cell volume fraction, D is the mean cell size, αu is the ratio of the standard deviation of the cell sizes to the mean cell size, and ρ is the apparent density. a
in Table 1 for clarity of comparison. The surface of D3O shows discrete open pores. The internal cell structure of D3O looks much different from that on the surface. The SEM micrograph of the internal cell structure of D3O shows a continuous cellular structure with compact cells. Certain cells have multiple micropores on cell walls to allow a tunnellike connection between adjacent cells. The number fraction of the cells with micropores, f m, is approximately 27.5%. PORON XRD exhibits very similar cell structural patterns for both the surface and the internal structure. Similar to D3O, a continuous cellular structure with compact cells exists for PORON XRD. The difference between D3O and PORON XRD is that most of the cells in PORON XRD have multiple micropores on cell walls to allow a tunnellike connection between adjacent cells, and eventually, the number fraction of the cells with micropores, f m,
Figure 3. Cell size distributions for the internal structures of (a) D3O, (b) PORON XRD, and (c) DEFLEXION. 2216
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fall in the size ranges of 24−187, 15−122, and 23−172 μm for D3O, PORON XRD, and DEFLEXION, respectively. The mean cell sizes D for D3O, PORON XRD, and DEFLEXION are 77, 56, and 78 μm, respectively, as listed in Table 1. To elucidate the uniformity of the cell size distribution in the above samples, the ratio of the standard deviation of the cell sizes to the mean cell size αu is defined in eq 2 as follows n
∑i (Di − D)2
αu =
n
D
(2)
where Di is the size of each cell and n is the cell number. Table 1 lists the αu values of 0.37, 0.44, and 0.41 for D3O, PORON XRD, and DEFLEXION, respectively. This result indicates that D3O has a relatively more uniform cell size distribution than the other two samples. The closed cells and cells with micropores for a tunnellike connection may play different important roles on the applicable mechanical properties of the impact-protective materials. In particular, for the foams with cells containing micropores (PORON XRD), the fluid flow through the interconnections affects the stress response due to fluid viscosity. In fact, the fluid inside of the cells, generally the air, can be compressed during deformation and be expelled out of the cells or absorbed back into the cells, which dissipates energy by a viscous flow. On the other hand, for the foams with closed cells (DEFLEXION), the fluid (air) contribution is dependent on the strain because a volume reduction of closed cells increases the internal pressure. Meanwhile, the fluid (air) contribution can be neglected if the foam stiffness is sufficiently high.19 2.3. Rheological Properties of Impact-Protective Materials. The rheological properties of these impactprotective materials were studied. Figure 4 shows the changes in the storage modulus G′ and the loss modulus G″ versus angular frequency ω for D3O, PORON XRD, and DEFLEXION as measured at 25 °C with different axial forces. Evidently, the axial force shows a significant influence on the moduli, with both G′ and G″ increasing with increasing axial force over the entire experimental frequency range for all of these three samples. The effect of the axial force on the modulus increase is thought to result from the compression of the samples because of their foam structures. The compression study on the samples will be presented in the next section. As can be seen from Figure 4a, D3O shows a typical elastic behavior at low frequencies, where G′ > G″ is obviously seen,20−22 and a viscous behavior at intermediate frequencies, where G′ < G″ is seen. The G′ and G″ curves show a cross-over frequency at 7.53 rad/s, at which D3O experiences a transition from an elastic state to a viscous state. It can be further predicted from Figure 4a that for D3O, G′ becomes higher than G″ at even higher frequencies above 200 rad/s, indicating that D3O might enter into another elastic state at much higher frequencies (out of the measurement limitation). As apparently shown in Figure 4b, PORON XRD exhibits a rheological behavior similar to that of D3O. However, the G′ and G″ curves for PORON XRD do not have a cross-over at any intermediate frequencies because G′ values are always higher than G″ values over the entire frequency range; nevertheless, eventually a cross-over of the G′ and G″ curves at the high-frequency end is predicted. As can be seen from Figure 4c, DEFLEXION displays a rheological behavior obviously different from D3O and PORON XRD. For DEFLEXION, the G′ values are much higher than G″ values in the entire frequency range; therefore, there is no cross-over
Figure 4. Changes in the storage modulus G′ and loss modulus G″ vs angular frequency ω for (a) D3O, (b) PORON XRD, and (c) DEFLEXION as measured at 25 °C with different axial forces.
between the G′ and G″ curves, indicating a solidlike viscoelastic behavior.14,23 On the other hand, the dynamic moduli at low and intermediate frequencies are much higher for DEFLEXION than for D3O and PORON XRD, possibly because DEFLEXION is a silicone rubber-based material. The working mechanism for impact-protective materials is to disperse the concentrated impact and transform the energy from the impact to other forms as much as possible. It is wellknown that the loss modulus G″ is a measure of energy loss, which describes that energy is transformed to heat when the material is deformed.24 For D3O and PORON XRD, the polyurethane-based foams, the G′ values are close to the G″ values at high frequencies, indicating that these two foams simultaneously exhibit high strength and great energy loss in a high-frequency impact. This type of property is suited for soft, close-fitting materials. However, for DEFLEXION, the silicone rubber-based foam, the change of G′ with frequency is modest; whereas the change of G″ with frequency is much more significant at high frequencies, indicating that DEFLEXION can dissipate much energy whether it suffers a large strain rate or not. This type of property is suited for high-risk impactprotective materials. 2217
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D3O and PORON XRD; however, for DEFLEXION, normalized G″ increases much rapidly as compared with G′ in the high-frequency region, which infers a relatively weak softness−stiffness switch characteristic. Therefore, DEFLEXION can be applied as an impact-protective material under both low and high strain rates. It is well-known that the loss tangent tan δ is sensitive to the structural change of the materials.25 Energy dissipation depends on tan δ (at the frequency of impact).26 Figure 6 shows the
Xu et al. have taken the ratio of the maximum moduli at a high frequency to the minimum moduli at a low frequency to evaluate the softness−stiffness switch characteristic of impactprotective materials.14 Referring to this definition, Figure 5
Figure 5. Changes in the normalized storage modulus G′ and loss modulus G″ by the moduli at 0.1 rad/s vs angular frequency ω for (a) D3O, (b) PORON XRD, and (c) DEFLEXION as measured at 25 °C with different axial forces. Figure 6. Changes in tan δ vs angular frequency ω for (a) D3O, (b) PORON XRD, and (c) DEFLEXION as measured at 25 °C with different axial forces.
shows the changes in the normalized storage modulus G′ and loss modulus G″ by respective moduli at 0.1 rad/s with angular frequency ω for D3O, PORON XRD, and DEFLEXION. It can be observed that in general, the normalized G″ increases much faster than the normalized G′ in the experimental frequency range for all three samples. In particular, the normalized G′ increases more rapidly in the high frequency range for D3O than for PORON XRD and DEFLEXION, indicating that D3O has a better softness−stiffness switch characteristic and can be applied as an excellent impact-hardening material. Figure 5b shows that PORON XRD has a rheological behavior relatively similar to that of D3O, but the normalized G′ increases modestly with frequency for PORON XRD as compared with D3O. Figure 5c shows that for DEFLEXION, the change in normalized G′ with frequency is modest as compared with
changes of tan δ versus angular frequency ω with different axial forces for D3O, PORON XRD, and DEFLEXION. It can be found that the tan δ−ω curves do not depend on the axial force values for all three samples. For D3O, tan δ increases with increasing frequency, reaches a maximum value, and then slightly decreases with further increasing frequency. For PORON XRD, tan δ keeps increasing with increasing frequency and seems to reach a plateau value in the experimental frequency range. For DEFLEXION, tan δ remains almost constant at low frequencies and then keeps increasing with increasing frequency. Among the three impact-protective 2218
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materials, D3O shows the highest tan δ values (higher than 1) at frequencies above 10 rad/s, implying that D3O can dissipate most of the energy at high frequencies. PORON XRD shows tan δ values close to 1 at high frequencies, implying that PORON XRD can dissipate most of the energy at high frequencies as well but with a relatively less degree as compared with D3O. DEFLEXION shows much low tan δ values that hardly increase at low frequencies and then start to increase with a limitation at high frequencies. On the basis of the above rheological results, it is concluded that D3O and PORON XRD exhibit remarkable softness−stiffness switch characteristic under an excitation of the strain rate. The rheological characterization can be considered to be a valuable methodology for evaluating the performance of impact-protective materials. 2.4. Mechanical Property of Impact-Protective Materials. The mechanical property regarding the response to compression was measured at a fixed strain amplitude of 75%. Various compression rates were applied for the measurements. Figure 7 shows the stress−strain curves for D3O, PORON XRD, and DEFLEXION during compressive tests with various compression rates. It can be seen from Figure 7a and the inset that the stress−strain curves exhibit three distinct regions: an initial linear elasticity region (illustrated by the first dash-dot orange line in the inset of Figure 7a), a second linear stress-increasing region (illustrated by the second dash-dot orange line in the inset of Figure 7a), and a densification region, as reported also for other polymer foaming materials.1,27 Beginning with low strains, usually up to 5% (see the inset in each plot in Figure 7), the compressive mechanical performance exhibits linear elasticity, for which the cell framework in the microstructure bends elastically.28 Compressing with further increasing strain (up to 25−50%), the cells in the foam materials might collapse through elastic buckling, plastic yielding, or brittle crushing, depending on the mechanical properties of the cell walls. The collapse process undergoes a linearly increasing stress with a reduced slope as compared with that in the initial linear elasticity region. Finally, at high strains above 25−50%, a densification region can be seen, for which the bulk frameworks of the foam materials are compressed, and the porosities are predicted to decrease significantly.1,27 The densification region shows a strain-hardening behavior. When the strains are low, the energy dissipation is more obvious for DEFLEXION than for the other two samples because of the lower cell fraction and the lower cross-linking degree for DEFLEXION than for the other two samples. Note that the cell volume fraction values β for D3O, PORON XRD, and DEFLEXION are 62, 60, and 48%, respectively. This result infers that D3O and PORON XRD stay soft at low strains, whereas DEFLEXION stays relatively stiff at low strains. Apparently the stress−strain curves for D3O, PORON XRD, and DEFLEXION are obviously different at different compression rates, implying their different sensitivities to the compression rate, consistent with those for other polymer foaming materials.29,30 The dependences of compressive mechanical parameters on the compression rate are further analyzed, and the results are presented below. The Young’s modulus for compression, Elin, is defined as the elastic modulus in the initial elasticity region. The Young’s modulus in the vicinity of the strain of 75% (before 75%) is defined as Emax, which can be used to indicate the strain-hardening modulus. For comparison purposes, the compression rates are transformed into the strain rates. Figure 8 shows the changes in Elin,
Figure 7. Stress−strain curves for (a) D3O, (b) PORON XRD, and (c) DEFLEXION in compressive tests with different compression rates. The inset in each plot shows the stress−strain curves in the low stress range. The numbers corresponding to the color lines in (a) represent the compression rates in the unit of mm/min.
Emax, and Emax/Elin with the strain rate for D3O, PORON XRD, and DEFLEXION. It can be seen from Figure 8a that DEFLEXION has the highest Elin values, PORON XRD has the lowest Elin values, and D3O has intermediate Elin values at different strain rates. The Elin values for PORON XRD and D3O are relatively close. The relatively lower Elin values for PORON XRD and D3O indicate that these two polyurethanebased foam materials are relatively soft. On the other hand, it can be seen from Figure 8b that DEFLEXION has the highest Emax values, PORON XRD has the lowest Emax values, and D3O has intermediate Emax values at low strain rates; however, the Emax values for PORON XRD increase with an increasing strain rate and eventually become significantly higher than those for D3O and DEFLEXION at the highest two strain rates as applied in the study, possibly associated with the added SiO2 fillers in PORON XRD. The Emax/Elin values can be used to evaluate the degree of strain-hardening.31,32 Figure 8c shows that PORON XRD has the largest degrees of strain-hardening at high strain rates, and both PORON XRD and D3O have 2219
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Figure 9. Changes in the (a) hysteresis energy Uhys and (b) hysteresis uhys with the strain rate for D3O, PORON XRD, and DEFLEXION.
absolute Uhys values for D3O, PORON XRD, and DEFLEXION, shown in Figure 9a, are approximately proportional to the G″ value shown in Figure 4, indicating that both parameters are well-correlated with the energy dissipation. The Uhys values for DEFLEXION are much higher than those for D3O and PORON XRD, possibly because of its much lower cross-linking degree. The uhys values for DEFLEXION range from 60 to 75% in the applied strain rate range, whereas the uhys values for D3O and PORON XRD increase from 26 to 80% as the strain rate increases. Both Uhys and uhys increase with increasing strain rate for the three samples. In particular, the hysteresis becomes much more sensitive to a high strain rate for D3O and PORON XRD than for DEFLEXION, indicating that D3O and PORON XRD can absorb much energy and turn stiff in a short time when suffering a high-speed impact in a real application, and whether this behavior is related to their specific structures containing cells with multiple micropores on cell walls or not is worthy of further investigation. Considering the comfort needs of the human body, D3O and PORON XRD with low Young’s moduli for compression under a low strain rate are suited for use as sports-protective materials. On the contrary, DEFLEXION has better use as armor fillers suitable for suffering huge stress under a high strain rate, for example, when hit by an ox horn or shot with a bullet. Elastic recovery (ER) values can be obtained according to eq 5 as follows20
Figure 8. Changes in the (a) Young’s modulus for compression, Elin, (b) Young’s modulus in the vicinity of the strain of 75%, Emax, and (c) ratio of Emax to Elin, Emax/Elin, with the strain rate for D3O, PORON XRD, and DEFLEXION.
larger degrees of strain-hardening than DEFLEXION. Overall, it can be concluded that the strain rate shows obvious effects on the degree of strain-hardening for D3O and PORON XRD, and PORON XRD possesses the highest degree of strain-hardening when subjected to the highest strain rate as applied in this study. For impact-protective materials, energy dissipation Uhys and hysteresis uhys are important parameters in practical applications, which can be calculated from the cyclic compression curves.33,34 The Uhys and uhys values can be calculated according to eqs 3 and 4 as follows Uhys = Aloading − A unloading uhys =
Aloading − A unloading Aloading
(3)
ER (%) =
× 100% (4)
εmax − ε(0, εmax ) × 100% εmax
(5)
where εmax is the maximum strain and ε(0, εmax) is the residual strain at stress = 0 during recovery after experiencing the maximum strain εmax. Figure 10 shows the changes in the ER with the strain rate for the three impact-protective materials, which compares the ER abilities after experiencing a large compression deformation.35,36 The ER values for DEFLEXION
where Aloading and Aunloading are the areas under the compressive loading and unloading stress−strain curves, respectively. Hysteresis uhys is considered to be the percentage of energy dissipated. Figure 9 displays the changes in Uhys and uhys, with the strain rate for D3O, PORON XRD, and DEFLEXION. The 2220
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suffering a high-speed impact in a real application, whereas DEFLEXION can serve as armor fillers suitable for a huge stress under a low strain rate, for example, when hit by an ox horn or shot with a bullet. Combining the rheological property and the compression mechanical property, the relationship between the internal cellular structures associated with the polymer matrix characteristics and the mechanical performances of the impact-protective materials can be understood. The rheological characterization proves to be a valuable methodology for evaluating the performance of impactprotective materials. The results of this study can provide some detailed references on further research work on the human body-protective materials.
Figure 10. Changes in the ER with the strain rate for D3O, PORON XRD, and DEFLEXION.
4. EXPERIMENTAL SECTION 4.1. Materials. Three impact-protective materials were used for this study. D3O (D3O, UK) was obtained from a SixSixOne knee pad (USA). PORON XRD (Rogers, USA) was obtained from a G-Form knee pad (USA). DEFLEXION (Dow Corning, USA) was obtained from a Rodeo Tech Bullfighting Vest (Rodeo Tech, USA). The apparent densities ρ for D3O, PORON XRD, and DEFLEXION used in this work were 0.41, 0.33, and 0.44 g/cm3, respectively. 4.2. Test of Solubility. According to FTIR results and the solubility parameters, tetrahydrofuran was chosen to dissolve D3O and PORON XRD, and hot n-hexane was chosen to dissolve DEFLEXION. The dissolved masses from these samples were weighed. 4.3. FTIR Measurements. FTIR spectra in the wavenumber range of 4000−500 cm−1 for the three samples were recorded on a Thermo Nicolet 6700 spectrometer using the attenuated total reflectance (ATR) mode. 4.4. Thermal Property Measurements. Q5000 IR and Discovery thermogravimetric analyzers (TA Instruments, USA) were used for the TGA of the samples in the temperature range of 100−800 °C under nitrogen and air atmosphere, respectively. 4.5. Surface and Internal Cell Structures and Chemical Elements by SEM Observation. The surface and internal cell structures of these three samples were examined by using a Philips XL-30 field emission scanning electron microscope at an accelerating voltage of 10.0 or 20.0 kV. For the internal cell structural observation, the samples were cut with a blade, and the cross-section surfaces were examined using SEM. EDS associated with SEM was used to determine the chemical compositions of the samples. 4.6. Rheological Property Measurement. Rheological measurements were performed using an ARES G2 rheometer (TA Instruments, USA). Disklike specimens were cut from the original impact-protective materials. For D3O and PORON XRD, the diameter and thickness of the specimens were 25 and 2 mm, respectively. For DEFLEXION, the diameter and thickness of specimens were 8 and 2 mm, respectively. Once the specimens were loaded into the rheometer, fixed axial forces of 1−5 N were applied to prevent the sample from slipping. Strain sweep measurements were carried out to determine the linear viscoelastic regime. Dynamic oscillatory frequency sweeps from 0.1 to 200 rad/s were performed with appropriate strains in the linear viscoelastic regime. 4.7. Compression and Recovery Test. The tests were performed at quasi-static strain rates using an Instron universal tensile testing machine, model 5982. Disklike specimens were cut from the original impact-protective materials. The diameter
are the lowest and remain approximately constant within the measured strain rate range, possibly because of its lowest cell fraction value in the silicone rubber matrix and its lowest crosslinking degree for the matrix itself. D3O shows sufficient ER values at low strain rates, whereas its ER values become poor at high strain rates. PORON XRD has the best ER values on the whole, with an ER of approximately 100% at low strain rates and an ER of >90% at high strain rates. In some situations where a significant reversal of loading occurs, an independence of ER on the strain rate is preferred, and for such a case, PORON XRD can be a good choice. From the rheological behaviors shown in Figures 4−6, it is observed that D3O and PORON XRD show the obvious frequency dependences for various rheological parameters, whereas DEFLEXION shows the weak frequency dependences for rheological parameters. From the mechanical properties shown in Figures 7−10, it is also apparently observed that D3O and PORON XRD exhibit apparent strain rate sensitivities for the mechanical property parameters, whereas DEFLEXION shows a much weak strain rate sensitivity in the measured strain rate. All of the above results demonstrate that the measurements from rheometry and compression tests provide consistent evidences.
3. CONCLUSIONS In this work, three foam materials, D3O, PORON XRD, and DEFLEXION, were chosen as typical representative impactprotective materials to explore the relationship between the internal cellular structures with the polymer matrix characteristics and rheological and compression mechanical properties. It is found that the internal cellular structures of polymer foam materials show significant influences on the rheological and compression mechanical properties of impact-protective materials. D3O and PORON XRD, the polyurethane-based foams with relatively high cross-linking degrees, have close cell fractions, whereas DEFLEXION, the silicone rubber-based foam with a much low cross-linking degree, has a much less cell fraction than D3O and PORON XRD. Therefore, D3O and PORON XRD display obviously different rheological behaviors from DEFLEXION. D3O and PORON XRD exhibit remarkable softness−stiffness switch characteristic under an excitation of the strain rate; however, DEFLEXION exhibits a relatively weak softness−stiffness switch characteristic. In particular, D3O and PORON XRD have hysteresis values that are much more sensitive to the high strain rates than DEFLEXION, indicating that D3O and PORON XRD can absorb much energy and turn stiff in a short time when 2221
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and thickness of the specimens were 11−13 and 6−8 mm, respectively. Force−displacement data were converted to stress−strain data using the sample dimensions. Energy losses at a constant strain amplitude of 75% at different compression rates of 0.6, 2, 6, 60, and 120 mm/min were obtained.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00242. Solubility test, FTIR spectra, SEM−EDS spectra, and TGA curves for the three foam samples (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (D.X.). *E-mail:
[email protected]. Phone: +86-0551-63607703. Fax: +86-0551-63607703 (Z.W.). ORCID
Hongzan Song: 0000-0003-3112-2361 Donghua Xu: 0000-0003-1828-1210 Zhigang Wang: 0000-0002-6090-3274 Author Contributions #
M.T. and G.H. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
Z.W. (grant nos. 51473155 and 51673183) and D.X. (grant no. 21274152) acknowledge the financial support from the National Natural Science Foundation of China. Z.W. and H.S. are thankful for the financial support from the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science.
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