Nano-matrix structure formed for NR.
Natural rubber is well known to be superior in tensile strength (ref. 4), tear strength (ref. 5), green strength (ref. 6) and so forth, compared with synthetic rubbers. However, it is inferior in oil resistance (ref. 7), ozone resistance (ref. 8), weather resistance (ref. 9) and so forth. Therefore, if we improve the inferior properties without sacrificing the superior properties, we will meet the most excellent material that we have ever seen. For instance, not only outstanding mechanical properties, but also oil resistance, may be achieved by forming a nano-matrix structure.
The nano-matrix structure for natural rubber may be formed in principle by graft-copolymerization of any monomer on a surface of the rubber particle in the latex stage (ref. 1). However, the graft-copolymerization is prevented by proteins that cover the rubber particle. Therefore, we have to remove the proteins from natural rubber to form the nano-matrix structure. As long as we use deproteinized natural rubber latex as a source, the side reaction may be suppressed to attain high conversion and high grafting efficiency for the graft-copolymerization (ref. 10).
In previous works, we proposed two methods to remove the proteins from natural rubber in latex stage, that is, enzymatic deproteinization (ref. 11) and urea-deproteinization (ref. 12). We prepared highly deproteinized natural rubber (ref. 13) and hyper-deproteinized natural rubber (ref. 14), respectively. Using these deproteinized natural rubbers, we may form the nano-matrix structure for natural rubber.
In the present work, an attempt to prepare nano-matrix-dispersed natural rubber from deproteinized natural rubber (DPNR) is performed with styrene or acrylonitrile as a monomer. The morphology of the products is observed by transmission electron microscopy to confirm the formation of the nano-matrix structure. The oil resistance and tensile strength of the products were measured to emphasize the advantage of the nano-matrix structure.
Natural rubber latex used in this study was commercial high ammonia latex. The incubation of natural rubber latex diluted to 30 wt. % dry rubber content (DRC) was made with 0.1 wt. % urea in the presence of 1 wt. % sodium dodecyl sulfate (SDS) at 300[degrees]C. The cream fraction was re-dispersed in 1 wt. % SDS to make 30% DRC latex, and was washed twice by centrifugation to prepare DPNR latex containing 0.1 wt. % SDS. The natural rubber latex, which was diluted to 30% DRC, was also deproteinized by incubation of the latex with 0.04 wt. % proteolytic enzyme (Kao, KP-3939) in the presence of 1 wt. % SDS for 12 hours at 32[degrees]C, followed by centrifugation. The cream fraction was re-dispersed in 1 wt. % SDS to make 30% DRC latex and was washed twice by centrifugation to prepare DPNR latex containing 0.1 wt. % SDS.
Measurement of the nitrogen content of the rubbers was made by the Kjeldahl method, as described in RRIM (Rubber Research Institute of Malaysia) Test Method B7 (ref. 12).
Graft-copolymerization of the DPNR latex was carried out with styrene or acrylonitrile as a monomer, using tert-butyl hydroperoxide with tetraethylenepentamine as an initiator. The DPNR latex was charged with [N.sub.2] gas for one hour at 30[degrees]C. The initiator of 3.3 x [10.sup.-2] mol/kg rubber and monomer were added to the latex, respectively. The reaction was carried out by stirring the latex at about 400 rpm for two hours at 30[degrees]C. The unreacted monomer was removed by using a rotary evaporator under reduced pressure. The as-prepared graft copolymer (gross polymer) was obtained by dipping the glass tube into the reacted latex and dried under reduced pressure at ambient temperature for more than a week. A feed of monomer for the graft-copolymerization is summarized in table 1.
The gross polymer was extracted with acetone/2-butanone (3/1) mixture in a Soxhlet apparatus for 24 hours under nitrogen atmosphere in the dark and dried under reduced pressure for about one week, in which the removal of almost all free-polystyrene, isolated from natural rubber, was completed by the extraction for 24 hours.
NMR measurements were carried out using a JEOL EX400 NMR spectrometer operating at 399.65 MHz for [sup.1]H. The polymer was dissolved into chloroform-d without TMS. Chemical shifts were referred to chloroform in chloroform-d. [sup.1]H-NMR measurement was carded out at 50[degrees]C at the pulse repetition time of 7 sec.
Ozonization was carded out by blowing an equimolar amount of ozone in ozonated oxygen through a 0.4 w/v% methylene chloride solution of the extracted graft-copolymer at 243K (-30[degrees]C). Reductive degradation of the resulting ozonide was performed by reaction with lithium aluminum hydride (LiA1[H.sub.4]) in diethyl ether, followed by decomposition of residual LiA1[H.sub.4] with water. After reductive degradation, grafted polystyrene, thus isolated from the graft copolymer, was dissolved in a small amount of chloroform. The chloroform solution was centrifuged, and the polymer was precipitated with methanol.
Apparent molecular weights and molecular weight distribution, [M.sub.w]/[M.sub.n], of the polymers were determined by a GPC system from Tosoh, with a CCPD pump, a RI-8012 differential refractive index detector, a UV-8011 ultraviolet spectroscopic detector and a series of three G4000HXL columns (bead size is 5 [micro]m, pore size is [10.sub.4] angstrom) with 300 mm length and 7.8 mm i.d. each. THF was used as an eluent, and the flow rate was 0.5 ml/min., at room temperature. Standard polystyrenes were used for a calibration.
Observation of the morphology for the graft-copolymer was made with a transmission electron microscope (TEM), Hitachi H-800 at accelerating voltage of 120 kV. The ultra-thin sections of the graft-copolymer were prepared by a Sovall Instruments MT6000 ultra-microtome at a temperature lower than the Tg of NR. The thin sections were stained by Os[O.sub.4] after annealing the blends at 80[degrees]C for 30 minutes.
Tensile strength was measured according to the JISK6251 test method.
Oil resistance was measured with a dilatometer composed of glass capillary and reservoir by using linoleic acid.
Results and discussion
Removal of proteins
Removal of proteins from natural rubber may be essentially concerned with methods of how to control interactions between the rubber and proteins in the latex stage, that is, chemical and physical interactions. The former is cleaved with proteolytic enzyme such as alkaline protease, and the latter varied with denaturant such as urea. Thus, we investigated a change in the amount of proteins present in natural rubber after deproteinization under various conditions. Table 2 shows total nitrogen content, X, of natural rubber and rubbers coagulated from the latex after deproteinization with alkaline protease at 32[degrees]C for a day (E-DPNR) or incubation with 0.1 wt. % urea at 30[degrees]C for an hour (U-DPNR). The total nitrogen content of natural rubber was significantly reduced by both the methods to about 1/20, reflecting that not only alkaline protease, but also urea, were effective to remove the proteins from the rubber. Since urea is well-known to change only conformation of the proteins, but not cleave any chemical linkages, the removal of almost all proteins from natural rubber with urea may suggest that the proteins present in natural rubber are attached just on the surface of the rubber particles through physical interactions without any chemical linkages.
To prove the interactions between the rubber and proteins, temperature and time necessary to remove the proteins from natural rubber were investigated. Figure 1 shows the total nitrogen content of natural rubber treated with urea, as a function of time, t. At 30[degrees] C, the total nitrogen content decreased from 0.38 wt. % to 0.022 wt. % after incubation with urea for 10 minutes. Further decrease in the content progressed to 0.020 wt. % for 60 minutes at 32[degrees]C. On the other hand, as the temperature of the incubation rose, the nitrogen content of natural rubber increased to about 0.025 wt. %. The slight increase in the nitrogen content may be attributed to less ability of urea to form hydrogen bonds between urea and the proteins at higher temperature. This demonstrates that most proteins, attached on the surface of the rubber particles, undergo a conformational change to detach themselves from the rubber particles with urea. Consequently, the temperature and time necessary to remove proteins from natural rubber with urea were determined to be room temperature and an hour, respectively.
Graft-copolymer was characterized through [sup.1]H-NMR spectroscopy. Figure 2 shows a typical [sup.1]H-NMR spectrum of the graft-copolymer, i.e., U-DPNR-PS3. Signals characteristic of cis-1,4-isoprene units appeared at 1.76, 2.10 and 5.13 ppm, which were assigned to methyl, methylene and unsaturated methyne protons of isoprene units, respectively. Broad signals around 6-7 ppm were assigned to phenyl protons of styrene units, whose intensity was dependent upon the feed of styrene as a monomer. Thus, we estimated a content of styrene units in the graft-copolymer and conversion of styrene from a ratio of the signal intensities of the phenyl to methyl protons and the feed of styrene. The estimated content of styrene units and conversion of styrene for U-DPNR-PS are shown in figure 3. The conversion of styrene was dependent upon the feed of styrene, in which a maximum was shown at styrene-feed of 1.5 mol/kg rubber. In contrast, the content of styrene units of U-DPNR-PS increased monotonically, as the feed of styrene increased. These may suggest that a suitable feed of styrene is 1.5 mol/kg rubber for the graft-copolymerization of styrene onto U-DPNR with tert-butyl hydroperoxide of 3.3 x [10.sup.-5] mol/g rubber and tetraethylenepentamine at 30[degrees]C. In figure 3 is also shown the content of styrene units and conversion of styrene for E-DPNR-PS. The content of styrene units and conversion of styrene for E-DPNR-PS were dependent upon the feed of styrene, as in the case of U-DPNR-PS. Thus, it is possible to expect that the graft-copolymerization for UDPNR may proceed in a similar way to E-DPNR.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
We estimated a grafting efficiency for U-DPNR-PS and EDPNR-PS. To estimate the grafting efficiency, free polystyrene, which was a mixture present in the graft-copolymer, was removed by extraction with acetone/2-butanone 3:1 mixture. The grafting efficiency, [upsilon], was estimated as follows:
[upsilon] = Mole of polymer linked to natural rubber / Mole of polymer produced during graft-copolymerization
The estimated value of grafting efficiency of styrene for UDPNR-PS and E-DPNR-PS is shown in figure 4. The grafting efficiency was dependent upon the feed of styrene, and a maximum was shown at 1.5 mol/kg rubber feed of styrene. The feed of styrene at the maximum was similar to that for the conversion of styrene. This may be attributed to a deactivation and chain transfer of the radicals due to less and large amount of styrene, respectively. At 1.5 mol/kg rubber feed of styrene for U-DPNR and E-DPNR, almost all polystyrene, thus produced, was proved to link up to the rubber molecule. This demonstrates that the reactivity of U-DPNR for the graft-copolymerization with free-radical-initiator is the same as that of E-DPNR.
To characterize the grafting polymer, size-exclusionchromatography after ozonolysis of E-DPNR-PS was performed, since all carbon-carbon double bonds were cleaved by ozonization followed by reduction with LiA1H4. Chromatograms for the products are shown in figure 5. A unimodal, symmetrical peak was shown in the chromatograms for the grafting polymers. From elution volume and height of the peak, number average molecular weight, Mn, weight average molecular weight, Mw, and polydispersity, Mw/Mn, were estimated, using a calibration curve drawn for polystyrene as a standard. Table 3 shows values of Mn, Mw and Mw/Mn. The Mn and Mw were dependent upon the feed of styrene, and they were the highest at 1.5 mol/kg rubber feed of styrene, while Mw/Mn was independent. The highest molecular weight for the grafting polymer, extracted from E-DPNR-PS3, may be explained to be due to the most effective graft-copolymerization of styrene onto DPNR.
[FIGURE 4 OMITTED]
Content of acrylonitrile units and conversion of acrylonitrile for U-DPNR-PAN are shown in figure 6. The conversion of acrylonitrile was dependent upon the feed of acrylonitrile, in which a maximum was shown at acrylonitrile-feed of 3.0 to 4.0 mol/kg rubber. In contrast, the content of acrylonitrile units of U-DPNR-PAN increased monotonically up to 4.0 mol/kg rubber, as the feed of acrylonitrile increased. These may suggest that a suitable feed of acrylonitrile is 4.0 mol/kg rubber for the graft-copolymerization of acrylonitrile onto U-DPNR with tert-butyl hydroperoxide of 3.3 x [10.sup.-5] mol/g rubber and tetraethylenepentamine at 30[degrees]C.
[FIGURE 5 OMITTED]
TEM photographs for E-DPNR-PS 1, E-DPNR-PS2, E-DPNRPS3 and E-DPNR-PS4 are shown in figure 7, in which a gloomy domain is natural rubber and a bright domain is polystyrene. As for E-DPNR-PS 1, little bright domain was scattered in the gloomy matrix. As the feed of polystyrene increased, the bright domain segregated together to cover the gloomy spheres. To characterize the gloomy spheres, we estimated an average diameter of gloomy sphere for E-DPNRPS3 and E-DPNR-PS4, and these are tabulated in table 4. The average diameter of gloomy sphere was similar to the volume mean particle diameter of the dispersoid present in natural rubber latex, which is about 0.7 jam in diameter. Since the film specimens were prepared by coagulation of the latex, the gloomy sphere, observed in the photograph, may be attributed to the dispersoid present in the latex. The thickness of the bright domain is also shown in table 4. The thickness was about 15 nm for E-DPNR-PS3, while it was about 25 nm for E-DPNR-PS4. This bright domain would be a matrix, that is, a continuous phase. To confirm the fact that polystyrene was the matrix, a volume fraction of polystyrene for E-DPNR-PS3 was estimated by image-analysis of the photograph. The estimated volume fraction of polystyrene was about 10v/v% for E-DPNR-PS3, which corresponded to 11v/v%, estimated from styrene unit content shown in figure 3. This demonstrates that the natural rubber particle of about 0.5 jam in diameter is dispersed in a polystyrene matrix of about 15 nm in thickness. Since the grafting efficiency of styrene for E-DPNR-PS3 is more than 90 mol %, as shown in figure 4, almost all polystyrene may link up to E-DPNR. Figure 8 shows a TEM photograph for U-DPNR-PS3. The nano-matrix structure was also found for U-DPNR-PS3, as in the case of E-DPNR-PS3. The thickness of bright domain was about 15 nm. Since the styrene content and grafting efficiency of styrene for U-DPNR-PS3 were about 12w/w% and 90%, respectively, the nano-matrix structure proved to be formed for not only E-DPNR, but also U-DPNR.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
Morphology of U-DPNR-PAN4 was also observed by TEM. Figure 9 shows TEM photographs for U-DPNR-PAN4, in which a gloomy domain is natural rubber and a bright domain is polyacrylonitrile. The natural rubber particle was covered with polyacrylonitrile to form a nano-matrix structure in which the thickness of polyacrylonitrile was less than 15 nm. This demonstrates that the nano-matrix structure is formed with not only styrene as a non-polar monomer, but also acrylonitrile as a polar monomer.
[FIGURE 10 OMITTED]
[FIGURE 11 OMITTED]
Mechanical properties and oil resistance
Figure 10 shows stress-strain curves for unvulcanized graft-copolymers, that is, E-DPNR-PS 1, E-DPNR-PS2, E-DPNRPS3 and E-DPNR-PS4. A stress at strain of 1 increased as the styrene content increased, reflecting an increase in Young's modulus. On the other hand, the highest stress at break was shown for E-DPNR-PS3. This may be attributed to not only the nano-matfix structure, but also the highest grafting efficiency. The results prove that the nano-matrix structure was formed for E-DPNR-PS3.
Oil resistance of U-DPNR-PAN is shown in figure 11. The oil resistance for U-DPNR-PAN1 and U-DPNR-PAN2 was identical to that for U-DPNR, itself. However, as the PAN content increased, i.e., for U-DPNR-PAN3, U-DPNR-PAN4 and U-DPNR-PAN5, the oil resistance increased significantly. The abrupt increase in the oil resistance of U-DPNR-PAN may be attributed to the formation of nano-matrix structure. This may be distinguished from the oil resistance of a UDPNR/PAN blend, which consists of ordinary island-matrix morphology.
Graft-copolymerization of deproteinized natural rubber was carried out with tert-butyl hydroperoxide/tetraethylenepentamine as an initiator. The highest conversion and grafting efficiency were achieved at a styrene feed of 1.5 mol/kg-rubber and were 98% and 93%, respectively. For E-DPNR-PS3, the natural rubber particles of about 0.5 [micro]m in diameter were dispersed in a polystyrene-matrix of about 15 nm in thickness. The stress at break for E-DPNR-PS3 was found to be the highest, due to not only the nano-matrix of polystyrene, but also the highest grafting efficiency. It is concluded that the nano-matrix structure, which results in outstanding mechanical properties, is formed by graft-copolymerization of styrene onto DPNR in the latex stage. On the other hand, graft-copolymerization of acrylonitrile onto DPNR was also effective to increase the oil resistance at an acrylonitrile feed of more than 3.0 mol/kg rubber.
This article is based on a paper presented at a meeting of the Rubber Division, ACS (www.rubber.org).
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by Seiichi Kawahara and Yoshimasa Yamamoto, Nagaoka University of Technology
Table 1--feed of monomer for the graft-copolymerization Rubber Monomer Feed of monomer Sample (mol/kg rubber) name Enzymatic Styrene 0.5 E-DPNR-PS1 deproteinized 1.0 E-DPNR-PS2 natural rubber 1.5 E-DPNR-PS3 2.0 E-DPNR-PS4 Urea Styrene 0.5 U-DPNR-PS1 deproteinized 1.0 U-DPNR-PS2 natural rubber 1.5 U-DPNR-PS3 2.0 U-DPNR-PS4 Urea Acrylonitrile 1.0 U-DPNR-PAN1 deproteinized 2.0 U-DPNR-PAN2 natural rubber 3.0 U-DPNR-PAN3 4.0 U-DPNR-PAN4 5.0 U-DPNR-PANS Table 2--nitrogen content of natural rubber Incubation Nitrogen Specimens time content (minute) (wt. %) Natural rubber -- 0.38 U-DPNR 60 0.020 E-DPNR 720 0.017 Table 3--[M.sub.n], [M.sub.w] and [M.sub.w]/[M.sub.n], for polystyrene linking up to E-DPNR Specimen Feed of monomer [M.sub.n]/ [M.sub.w]/ [M.sub.n]/ (mol/kg rubber) [10.sup.3] [10.sup.4] [M.sub.w] E-DPNR-PS1 0.5 8.4 11.9 1.4 E-DPNR-PS2 1.0 10.8 15.4 1.4 E-DPNR-PS3 1.5 15.3 24.7 1.6 E-DPNR-PS4 2.0 9.7 14.8 1.5 Table 4--average diameter and average thickness for E-DPNR-PS3 and E-DPNR-PS4 Specimen D ([micro]m) t (nm) E-DPNR-PS3 0.5 15 E-DPNR-PS4 0.6 25 D: average diameter t: average thickness
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|Author:||Kawahara, Seiichi; Yamamoto, Yoshimasa|
|Date:||Nov 1, 2007|
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