Structure and properties of EVM reinforced by in situ prepared sodium methacrylate.

In general, elastomers are reinforced by conventional reinforcing agents such as carbon black and silica. Seeking new reinforcing agents for rubbers is an important subject for the sake of rubber products with good processability and special properties, in recent years, considerable research has been focused on the metal salts of unsaturated carboxylic acid for their effective reinforcement for rubbers (refs. 1 and 2). The metal salts of unsaturated carboxylic aids such as zinc methacrylate (Zn[(MAA).sub.2]) have been used to snake super strong hydrogenated acrylonitrile-butadiene rubber (HNBR) vulcanizates with higher than 55 MPa tensile strength and over 500% elongation at break (ref. 3). The metal salts of unsaturated carboxylic acid can be added to rubbers directly or prepared in situ in rubbers through the neutralization of metal oxides or hydroxides and acids. For the sake of good mechanical properties, the in situ preparation is better than the direct addition of the metal salts (refs. 4 and 5). Japanese patents (refs. 6 and 7) reported that styrene-butadiene rubber (SBR) and isoprene rubber (IR) vulcanizates reinforced by in situ prepared Zn[(MAA).sub.2] had excellent mechanical properties. Our previous research on the in situ reinforcement of acrylonitrile-butadiene rubber (NBR) with Zn[(MAA).sub.2] (ref. 8), the reinforcement of ethylene-propylene-diene rubber (EPDM) with aluminum methacrylate (Al[(MAA).sub.3]) (ref. 9) and the reinforcement of SBR with magnesium methacrylate (Mg[(MAA).sub.2]) (ref. 10) showed that the resulting vulcanizates had excellent mechanical properties.

Up to now, most of the research about in situ prepared unsaturated carboxylates focuses on multivalent metallic salts, such as zinc and magnesium salts of unsaturated carboxylic acid. To our knowledge, monovalent metallic salts of unsatusated carboxylic acid have been less studied. In this article, the reinforcement of ethylene-vinyl acetate rubber (EVM) by in situ prepared sodium methacrylate (NaMAA) was investigated. Sodium hydroxide (NaOH) and methacrylic acid (MAA) were used to react with each other in the rubber matrix during mixing. The properties and structure of EVM vulcanizates were investigated in terms of the mechanical properties, optical properties, crosslink density and morphology.

Experimental

Materials Raw materials used in the study are shown in table l.

Sample preparation

The mixtures of EVM and additives were prepared in the mixing chamber of a Rheometer at 50[degrees]C and at a rotor speed of 32 rpm. First, a gum rubber was allowed to plasticate for two minutes, then NaOH was added to mix with EVM lot two minutes, followed by the addition of MAA and mixed for another six minutes. Finally, DCP was added and mixed for two minutes. The compounding process lasted about 12 minutes. Then, the compound was sheeted out on a two-roll mill and press-cured at 170[degrees]C for 10 minutes to form a 2 mm thick sheet, and the sheet was cut into specimens for measurement.

Mechanical properties

The tensile strength was measured with dumbbell specimens according to ASTM D412-97. The values for each sample were taken from the median values of five specimens. Tear strength was measured on unnotched 90[degrees] angle test specimens according to ASTM D624-98. The tests were performed with an automated materials tester at a crosshead speed of 500 mm/min. Hardness was determined using a handheld A-scale durometer according to ASTM D2240-97.

Optical properties

The light transmission and haze of EVM vulcanizates were measured with 1 mm sheet on a spherical hazemeter according to Chinese Standard GB2410-80.

Determination of crosslink density

The crosslink density was determined by equilibrium swelling. Molded samples were swollen in xylene at 25[degrees]C for 72 hours in order to achieve the equilibrium swelling condition. The weight of the samples was measured under swollen conditions. Then, the samples were dried in a vacuum oven for 36 hours at 90[degrees]C to remove all the solvent, and reweighed. The volume fraction of rubber in swollen gel (Vr), which was used to represent the relative crosslink density of the vulcanizate, was determined by the following equation (ref. 8):

(1) [V.sub.r] = [m.sub.0][empty set](1-[alpha]/[[rho].sub.r])/[m.sub.0][empty set](1-[alpha]/[[rho].sub.r] + ([m.sub.1]-[m.sub.2])[[rho].sub.s]

Where [m.sub.0] is the sample mass before swelling, [m.sub.1] and [m.sub.2] are sample masses before and after drying, [phi] is the mass fraction of rubber in the vulcanizate, [alpha] is the mass loss of the gum EVM vulcanizate during swelling, and [[rho].sub.r] and [[rho].sub.s] are the rubber and solvent density, respectively.

In order to distinguish ionic crosslinks from covalent crosslinks, samples were swollen in the mixture of xylene and chloroacetic acid for five days to destroy ionic crosslinks, followed by swelling in xylene for two days, weighed, then vacuum dried and reweighed. [Vr.sub.1] was calculated from equation 1, which represented the covalent crosslink density. [Vr.sub.2], which is calculated by subtracting [Vr.sub.1] from Vr, was used to represent the ionic crosslink density.

FTIR study

Infrared transmission spectra were obtained using a Fourier transform infrared (FTIR) spectrometer. The sample of MAA was prepared by casting MAA onto a KBr disc. The samples of EVM and NaMAA filled EVM compound were prepared by compression molding them to thin films at room temperature. The sample of NaMAA filled EVM vulcanizate was prepared by compression molding at 170[degrees]C for 10 minutes.

Dynamic mechanical properties

Dynamic mechanical thermal analysis (DMTA) was carried out in a dynamic mechanical thermal analyzer. Measurements were performed in the rectangular tension mode with a frequency of 10 Hz over a temperature range of -80[degrees] to +80[degrees]C, and at a heating rate of 3[degrees]C [min.sup.-1]. Specimen dimensions were 20 x 4 x [(mm). The strain amplitude remained at 0.01%.

Small-angle X-ray scattering (SAXS) analysis

SAXS diagrams were taken on a diffractometer. The test was operated under the following experimental conditions: Cu[K.sub.[alpha]] radiation, 40kV; generator current, 30 mA; speed of scanning, 0.3[degrees]/min.; type of scanning, step. The dimension of vulcanizate sample for testing is 20 x 20 x 1(mm). Background diagrams were made in all cases, and only background corrected scattering values were used for calculation.

SPM study

Surface morphology was observed using a scan probe microscopy (SPM). The specimens were fractured under cryogenic condition of liquid nitrogen, then the fractured surfaces were observed using SPM with tapping mode. Height and phase images were recorded simultaneously. The height image gives a topographic mapping of the surface, and the phase image provides compositional mapping of the same surface portion.

Results and discussion

Mechanical properties

The effect of DCP content on the mechanical properties of EVM vulcanizates is shown in figure 1. The mol ratio of NaOH/MAA is 1.0 (equivalent) for all the formulations in figure 1. The amount of NaMAA represents the theoretical value obtained from the neutralization of NaOH and MAA. As can be seen from figure 1(a) and (b), the modulus at 300% of the EVM vulcanizates increased gradually with increasing the DCP content, while the elongation at break decreased rapidly. However, even when the DCP content is as high as seven phr, the elongation at break of the vulcanizate is still over 300%. The tensile strength of EVM vulcanizate is over 30 MPa when the DCP content is about 3-5 phr. The tear strength is as high as 69.4 kN/m with the DCP content at 3 phr. Considering the mechanical properties of EVM/NaMAA vulcanizates, 3 phr of DCP was chosen as the base for the following study.

[FIGURE 1 OMITTED]

The mol ratio of metal oxide or metal hydroxide to methacrylic acid influences the mechanical properties of vulcanizates. The research of Zeon Chemicals showed that, in highly saturated nitrile elastomers (HNBR), the best reinforcement was obtained when ZnO was in excess, and the optimum mol ratio of ZnO to MAA was about 0.75 (ref. 3). Our previous research showed that in a magnesium methacrylate reinforced SBR vulcanizate, the optimum mol ratio of MgO to MAA was about 0.5-0.75 (ref. 10). The effect of the mol ratio of NaOH to MAA at various levels (0.5 to 2.0 mol ratio) on the mechanical properties of EVM vulcanizate was studied. Based on 30 phr of NaMAA prepared from neutralization, the mol ratio of NaOH/MAA was varied from 0.5 to 2.0 (the corresponding weight ratio varied from 11.1/47.8 to 22.2/23.9), and the results are listed in table 2. As the mol ratio of NaOH to MAA increases, the hardness increased at a slower rate and tension set increased greatly. The tensile strength and 300% modulus vary with similar trends and exhibit a maximum when NaOH and MAA are equivalent in mol mass (mol ratio = 1). While for the case of NaOH in excess, the tensile strength decreases dramatically with increase of mol ratio. The mol ratio of NaOH to MAA has little effect on the tear strength of vulcanizates, but the excess of NaOH increases the teat strength slightly. For the general consideration of the mechanical properties of EVM vulcanizates, the optimum mol ratio of NaOH to MAA is 1.0 (equivalent).

When NaOH and MAA are at equivalent mol ratio, the effect of NaMAA (NaOH/MAA) content on the mechanical properties of EVM vulcanizate is shown in figure 2. It may be noted that both 300% modulus and hardness increase greatly with increasing the amount of NaMAA. The tensile strength increases rapidly between 10 and 30 phr NaMAA content, and at a slower rate thereafter. The tensile strength of the EVM vulcanizate with 50 phr NaMAA content reaches as high as 34.8 MPa. The tear strength increases monotonically and increases by more than 200% when the NaMAA content increases from 10 to 50 phr (figure 2a). The modulus at 300% and the hardness of vulcanizate almost linearly increase with the increase of NaMAA content (figure 2b). The elongation at break almost has no variation between 10 and 30 phr NaMAA content, and decreased rapidly thereafter, but it is still over 350%, even when NaMAA content is 50 phr (figure 2c). The tension set increases obviously with increasing the NaMAA content (figure 2c). It appears that in situ prepared NaMAA is effective for the reinforcement of EVM and can maintain high elongation at break while increasing the modulus and tensile strength of the EVM vulcanizate.

[FIGURE 2 OMITTED]

Optical properties

The effect of NaOH/MAA mol ratios on the light transmission and haze of EVM vulcanizates is shown in table 3. The EVM vulcanizate reinforced by in situ prepared NaMAA has high transparency, especially at low mol ratio of NaOH to MAA. When the mol ratio was between about 0.5-1.25, all the EVM vulcanizates were transparent, and the light transmission remained over 72%. With further increase of NaOH content, the EVM vulcanizate changed from transparent to opaque. With the increase of mol ratio of NaOH to MAA, the haze of vulcanizate also increased.

Table 4 shows the effect of NaMAA content on the light transmission of EVM vulcanizates. With the increase of NaMAA content, the haze of vulcanizates gradually increased and light transmission slightly decreased. Even if NaMAA content was 50 phr, the EVM vulcanizate still had high transparency and the light transmission was 76.3%. After the addition of MAA to EVM/NaOH, the compound changed from transparent to white; while after vulcanization, it turned to transparent. Saito et al (refs. 11-13). investigated the polymeric behavior of Zn[(MAA).sub.2] in the HNBR/Zn[(MAA).sub.2] compound during the curing process and found that Zn[(MAA).sub.2] underwent homo polymerization and graft polymerization. Their study showed that 20-30 nm poly(Zn[(MAA).sub.2]) particles were produced as the vulcanization proceeded. The chemical reaction of Mg[(MAA).sub.2] in SBR during vulcanization was reported in our early publication (ref. 14). So it is supposed that in situ prepared NaMAA undergoes chemical reaction in the peroxide curing of EVM vulcanizate. In the process of vulcanization, NaMAA partly dissolves in EVM at first, then polymerizes under the initiation of DCP and forms polysodium methacrylate (P-NaMAA), which distributes in EVM matrix as nano scale particles. So the EVM vulcanizate has high transparency.

Crosslink density

It is well known that DCP can initiate the crosslink of rubber in the peroxide curing system and at the stone time it initiates the polymerization of the metallic methacrylate, including both homopolymerization and graft polymerization (ref. 12). In order to investigate the effect of NaMAA on the crosslink structure of the peroxide cured EVM vulcanizate, it is necessary to measure the crosslink densities of ionic and covalent bonds. In the monovalent metallic salt reinforced vulcanizates, the ionic crosslink bonds result from the electrostatic forces between the bound ions on the polymer chain (ref. 15). The relative crosslink densities were determined by the equilibrium swelling method. The values of Vr (representing the gross crosslink density of the original vulcanizates) and [Vr.sub.1] (representing covalent crosslink density of the vulcanizates treated with acid) were calculated according to equation 1. [Vr.sub.2] (representing ionic crosslink density of the vulcanizates) was calculated by subtracting the covalent crosslink density from the gross crosslink density.

Figure 3 shows the effects of DCP content on the relative crosslink densities of the EVM vulcanizates. In the case of a given NaMAA content, with increasing the DCP content, the gross crosslink density (Vr) of EVM vulcanizates increased and the covalent crosslink density ([Vr.sub.1]) also increased obviously, while ionic crosslink density ([Vr.sub.2]) showed little change. Figure 4 shows the effect of NaMAA content on the relative crosslink density of the EVM vulcanizates. At a constant peroxide content, both the gross crosslink density (Vr) and ionic crosslink density ([Vr.sub.2]) increased with increasing NaMAA content, while covalent crosslink density ([Vr.sub.1]) showed a slight decrease. It is reasonable to suppose that the higher NaMAA concentration is favorable to the formation of grafted NaMAA onto the backbone of EVM and leads to more ionic crosslinks.

[FIGURES 3-4 OMITTED]

FTIR study

NaMAA is a salt of acrylic acid, which has a double bond in the chain. It is highly reactive in the presence of free radicals, and readily reacts to form crosslinks with both saturated and unsaturated elastomers. The chemical structure of EVM, MAA and NaMAA are shown in figure 5.

[FIGURE 5 OMITTED]

Figure 6 shows the FTIR spectra of EVM, MAA and EVM/NaOH/MAA compound and vulcanizate. Curve (a) in figure 6 is the spectrum of pure EVM gum with characteristic absorbing peaks at 2,925, 2,865, 1,465 and 1,380 [cm.sup.-1] generated from C-H stretching and deformation vibration of methyl group. The characteristic peaks at 1,740 and 1,240 [cm.sup.-1] are due to the stretching vibration of C=O and C-O. The peak at 722 [cm.sup.-1] is ascribed to the bending vibration of methylene group. MAA exists as dimer due to the strong hydrogen bond between molecules (ref. 16) (figure 5(b)). The dimer has characteristic absorption bands at 1,698 and 1,636 [cm.sup.-1] attributed to C=O and C=C stretching vibrations, respectively. When the H of the

[ILLUSTRATION OMITTED]

group was substituted by a cation, the C=O and C-O bonds attached to the same C were equalized to

[ILLUSTRATION OMITTED]

(figure 5c) (ref. 17). The carboxylate anion has two strongly coupled carbon-to-oxygen bonds that give rise to the separately asymmetric and symmetric stretching vibration. The strong asymmetrc stretching band is from 1,610 to 1,560 [cm.sup.-1]. The weak and broad peaks at 1,440 to 1,360 [cm.sup.-1] are due to the symmetric band.

[FIGURE 6 OMITTED]

Curves (c) and (d) in figure 6 are the spectra of the EVM compound and vulcanizate, respectively. When the MAA and NaOH were mixed at the equivalent ratio in the compounding process (the mass ratio was 11.1/23.9 when the resulting NaMAA was 30 phr), the characteristic absorption bands of AA disappeared. Curve (c) shows the characteristic absorption bands of salt of methacrylic acid. The band at 830 [cm.sup.-1] is ascribed to C-H out-of-plane bending vibration from =CH2. Compared with the spectrum of the EVM compound, the spectrum of vulcanizate has a significant change in the peak of 830 [cm.sup.-1]. The relative intensity of =C[H.sub.2] absorption at 830 [cm.sup.-1] becomes weak obviously. At the same time, the characteristic absorption band of C=C at 1,640 [cm.sup.-1] disappeared completely, indicating that the carbon-carbon double bonds have reacted to a great extent in the process of vulcanization.

From the analysis of the FTIR spectra, the conclusion can be drawn that NaMAA can react under the initiation of DCP during the curing process by opening its double bonds.

Dynamic mechanical properties

Fillers, when added to polymer systems, are known to cause a considerable change in dynamic mechanical properties, not only the dynamic modulus, but also loss factor (tan [delta]). As shown in figure 7, the E' and tan [delta] are functions of temperature for EVM reinforced by in situ prepared NaMAA over a loading range from 0 to 50 phr. In the EVM vulcanizates we studied, with the increase of NaMAA content, the E' increased obviously. At the same time, the NaMAA content also influences the tan [delta]. There is a decrease in the magnitude of the tan [delta] peak and an obvious decrease in the temperature at which the peaks occur with increasing NaMAA content. The decrease in the magnitude of tan [delta] peak in the transition zone can be interpreted in terms of a reduction in EVM fraction in the EVM vulcanizates. EVM is responsible for the high portion of energy dissipation, and the NaMAA as a filler in the EVM matrix may not absorb energy significantly (ref. 18). The changes of [T.sub.g] are shown in table 5. In the gum vulcanizate, the [T.sub.g] of EVM vulcanizate is -20.2[degrees]C. In the NaMAA filled EVM vulcanizates, the [T.sub.g] of EVM drops from -21.6[degrees]C at 10 phr NaMAA to -24.9[degrees]C at 50 phr NaMAA content. This phenomenon is in contradiction with convention. According to the literature (refs. 19 and 20), the [T.sub.g] of the grafted rubber is lower than that of the ungrafted rubber. The variation of [T.sub.g] of NaMAA reinforced EVM vulcanizates is in agreement with our previous study with Mg[(MAA).sub.2] filled EVM vulcanizates (ref. 21). So it is considered that the same reason causes the change as that of our previous study.

SAXS characterization

It is considered that the polymerization of sodium methacrylate should take place in the solid state, leading to the formation of amorphous separate aggregates of salt polymeric chains (ref. 22). It was suggested that the solid polysalt improves the mechanical properties of the vulcanizates (refs. 23-25). The method of SAXS has been successfully applied to investigate the internal structure of the vulcanizate. Dontsov el al (refs. 22-24) also used SAXS to characterize the distribution of Mg[(MAA).sub.2] in polybutadiene.

We have examined EVM vulcanizates with different amounts of NaMAA. The SAXS diagrams for EVM vulcanizates filled with 10, 30 and 50 phr NaMAA are reported in figure 8. As would be expected, the relative intensity of the X-ray scattering rises with an increase of NaMAA content; however, this growth is nonuniform in the range of scattering. The higher increase is observed at the smaller angles. When the scattering angles grow, the curves are drawn together and finally become a single curve.

[FIGURE 8 OMITTED]

For the resolution of the SAXS diagrams, we applied the method of Guinier approximation to take into account the relationship between scattering intensity and scattering angle. The experimental curves, after the background had been subtracted, were reported in terms of log I versus [[epsilon].sup.2], as shown in figure 9, where I is the corrected relative intensity. At first, the gyroradius was calculated from the slope of the linear portion of this curve. After subtracting the intensity of the first linear component, it was possible to plot the remaining intensity in the form of another straight line, which gives a second diameter.

[FIGURE 9 OMITTED]

The relationship of log I versus [[epsilon].sup.2] is shown as:

(2) log I = log [K.sub.0] - (4[[pi].sup.2][R.sub.0.sup.2]/3[[lambda].sup.2] log 3) [[epsilon].sup.2]

I is the scattering intensity; [lambda] is the wave length of X-ray ([lambda] = 1.542 [Angstrom] for the radiation source of Cu[K.sub.[alpha]]; [R.sub.0] is the gyroradius of the scattering system.

[R.sub.0] can be obtained from the slope ([alpha]) of the log I versus [[epsilon].sup.2].

(3) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The results obtained by the above-mentioned calculation are shown in table 6. They seem to indicate that on the NaMAA filled EVM vulcanizates, there is a distribution of rigid particles. As reported in table 6, the radii of such particles increase with the increase of NaMAA content. According to the previous FTIR analysis, we can assume that polymerization of NaMAA occurs during vulcanization, and the polysalt generated is distributed in the form of rigid particles.

Morphology of vulcanizates

SPM photographs of NaMAA reinforced EVM vulcanizates are shown in figure 10. Figure 10(a) is the phase image and figure 10(b) is the height image of 10 phr NaMAA filled EVM vulcanizate. As can be seen, most of the particles with the diameter of about 20 nm dispersed evenly in the EVM vulcanizate. It is in good agreement with the results from the SAXS analysis. Figures 10(c) and 10(d) show the phase and height image, respectively, of the 30 phr NaMAA filled EVM vulcanizate. Figures 10(e) and 10(f) show the phase and height image, respectively, of the 50 phr NaMAA filled EVM vulcanizate. As seen from figure 10, the dimension of particles gradually increases with the amount of NaMAA in EVM vulcanizate. The NaMAA powders have a great tendency to congregate to larger particles than their primary particles with the increase of NaMAA content in EVM vulcanizate. The trend is similar to that of the SAXS analysis.

[FIGURE 10 OMITTED]

Conclusion

The EVM vulcanizates reinforced by NaMAA prepared in situ have a good reinforcing effect on the mechanical properties. The NaMAA/EVM vulcanizate has tensile strength up to 34.8 MPa, tear strength up to 93 kN/m and an elongation at break over 300%, at the same time it has high transparency. The good mechanical properties and high transparency of EVM/NaMAA vulcanizate can be correlated to the structure of EVM vulcanizate. The FTIR study confirms that NaMAA is in situ formed in the compounding process and NaMAA is a reactive filler that undergoes polymerization during peroxide curing. DMA results show that the [T.sub.g] of EVM vulcanizate shifts to low temperature with increasing the NaMAA content. The SAXS analysis and surface morphology of EVM vulcanizate indicate that the polysalt generated distributes evenly in the form of rigid particles and the diameter of particles is less than 100 nm.

Table 1--raw materials

Materials                             Characteristics

Ethylene-vinyl acetate      50 wt. % of vinyl acetate
 copolymer (EVM)
Sodium hydroxide (NaOH)                 Chemical pure
Methacrylic acid (MAA)                  Chemical pure
Dicumyl peroxide (DCP)                  Purity: 99.3%

Table 2--effect of NaOH/MAA mol ratios on mechanical properties of
EVM vulcanizates

                                        NaOH/MAA mol ratio

Property                             0.5          0.75           1.0
                         (11.1/47.8) (a)   (11.1/31.9)   (11.1/23.9)

Hardness (duro. A)                    75            80            81
Modulus at 300%                     9.96          8.92         18.19
 (MPa)
Tensile strength                   21.73         22.86         33.32
 (MPa)
Elongation at                        534           566           414
 break (%)
Tension set (%)                       38            52            48
Tear strength (kN/m)               65.60         65.59         67.13

                                        NaOH/MAA mol ratio

Property                            1.25           1.5           2.0
                            (13.9/23.9)    (16.7/23.9)   (22.2/23.9)

Hardness (duro. A)                    81            82            83
Modulus at 300%                    12.98         11.45          9.14
 (MPa)
Tensile strength                   26.69         18.35          9.30
 (MPa)
Elongation at                        477           514           379
 break (%)
Tension set (%)                       48            77           102
Tear strength (kN/m)               64.13         70.85         72.95

(a) Data in parentheses represent the exact amount of NaOH and MAA
corresponding to the mol ratio

Table 3--effect of NaOH/MAA mol ratios on
optical properties of EVM vulcanizates

NaOH/MAA (mole ratio)    Light transmission, %     Haze, %

         0.5                     72.1               18.7
         0.75                    78.1               10.2
         1.0                     78.2               17.5
         1.25                    72.6               26.5
         1.5                     46.6               22.2
         2.0                    Opaque

Table 4--effect of NaMAA content on optical
properties of EVM vulcanizate

    NaOH/MAA, phr        Light transmission, %     Haze, %

         10                      80.7               15.3
         20                      79.1               16.9
         30                      78.2               17.5
         40                      77.8               18.3
         50                      76.3               22.2

Table 5--effect of NaMAA content on the
dynamic mechanical properties

                                 Tan         E' at         E' at
   NaMAA        Tg of EVM     [delta] at   [T.sub.g]    25[degrees]C
content, phr   ([degrees]C)   [T.sub.g]       (Pa)          (Pa)

     0            -20.22        1.096        5.22E7        2.69E6
     10           -21.64        1.011        9.23E7        3.51E6
     30           -23.39        0.474        5.50E8        4.15E7
     50           -24.90        0.315        1.31E9        1.26E8

Table 6--the radii of scattering particles in
NaMAA reinforced EVM vulcanizates

                                Radii,
   NaMAA,                     [Angstrom]                  Average
    phr          Minimum,      Medium,      Maximum,     diameter,
                [Angstrom]    [Angstrom]   [Angstrom]    [micro]m

     10            40.4          96.6        156.0          17.7
     30            46.2          94.7        157.7          18.2
     50            45.2          95.7        162.8          19.8

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Aihua Du, Shanghai Jiao Tong University and Qingdao University of Science and Technology; and Yinxi Zhang, Zonglin Peng and Yong Zhang, Shanghai Jiao Tong University