[acs symposium series] nmr spectroscopy of polymers: innovative strategies for complex...

Chapter 29

Latex State and Solid-State NMR Spectroscopyof Elastomers

Seiichi Kawahara*

Department of Materials Science and Technology, Faculty of Engineering,Nagaoka University of Technology, Nagaoka, Niigata 940-2188, Japan

*E-mail: [email protected]

Latex state NMR spectroscopy and field gradient fast magicangle spinning solid-state NMR spectroscopy were utilized forstructural characterization of rubbery polymers and crosslinkedrubbery materials. High resolution NMR spectra for elastomerswere achieved either through Brownian motion of a latexdispersion or fast magic angle spinning for a solid. Sequencedistribution of cis-1,4-, trans-1,4- and 1,2-butadiene unitswas investigated for polybutadiene through latex state NMRspectroscopy because the polybutadiene used in this work wasprepared by emulsion polymerization. Crosslinking junctionsof vulcanized natural rubber were analyzed by field gradientfast magic angle spinning solid-state NMR spectroscopy. Thevulcanized natural rubber with quaternary carbons linking tosulfur as the crosslinking junction was found to be superior inmechanical properties compared to the rubber samples withtertiary carbons linking to sulfur.

1. Introduction

Rubbery polymers and polymer gels hold a unique place in polymer sciencebecause they exhibit both liquid and solid behavior in terms of molecularmotion. They are usually crosslinked in practical use to maintain their physicaldimension. This crosslinking makes it difficult to apply solution NMR forstructural characterization. Moreover, the molecular motions present in them alsopose difficulties for solid-state NMR spectroscopy of the crosslinked rubbers;e.g., two-dimensional NMR measurements cannot be easily performed on them

© 2011 American Chemical Society

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In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

in the solid state. This is explained to be due to very low concentration of thecrosslinking junctions and their unsatisfied molecular motions, compared withrepeating units. It is necessary to develop a special NMR technique for therubbery polymers and polymer gels in order to perform two-dimensional NMRmeasurements for their structural characterization.

Two-dimensional NMRmeasurements for the rubbery polymers and polymergels may be performed by eliminating dipole-dipole interactions. However, in thecase of the crosslinked rubbers, which consist of the mobile main chain and theconstrained crosslinking junctions, motionary inhomogeneity prevent the efficientelimination of the dipole-dipole interactions. One way to eliminate the interactionsis a superposition of another motions onto molecular motions. Thus, the rotationalmovement of latex particles in a dispersion may be superposed onto molecularmotions. An alternative way is to carry out fast magic angle spinning (FMAS) insolid-state NMR (1, 2); in this case, at least 20 kHz spinning is required to obtaina high resolution NMR spectrum (3), and a large amount of sample is needed foran analysis of crosslinking junctions. In order to carry out the analysis of thecrosslinking junctions as extremely small amount of building block through two-dimensional NMR, it is necessary to develop latex state NMR spectroscopy (1, 2)and use a solid-state NMR spectrometer equipped with a field gradient-fast magicangle spinning (FG-FMAS) probe (4). Using the FG-FMAS probe, particulary,we may achieve large increase in sensitivity for the measurement.

Thus far, only several preliminary studies of the latex state NMR spectroscopyhave been reported: an observation of hydrolysis on the surface of PMMAdispersoid in latex by Tarcha (5) and determination of dried rubber content fornatural rubber latex by Ang (6) and Gambhir (7). In my opinion, latex stateNMR spectroscopy has not been studied systematically in the previous articles,and the conditions for getting a high resolution spectrum via latex state NMRspectroscopy have not been determined. Moreover, fast MAS (more than 20kHz) has not been achieved for solid-state NMR measurement equipped withFG-FMAS probe, even though some results of structural characterizations of softmaterials have been reported earlier (8–10).

In a series of earlier studies, we have conducted two-dimensional NMRmeasurements of rubbery polymers and polymer gels through latex state NMRspectroscopy, using a solid-state NMR spectrometer equipped with a FG-FMASprobe. In this article, I shall attempt to summarize the work we have done so far.

2. Latex State NMR Spectroscopy

2.1. Effect of Crosslinking

Aliphatic carbon signals obtained for polybutadiene (PB) by latex state 13C-NMR spectroscopy are shown in Figures 1 and 2, and they are comparedwith thoseobtained for solution and solid samples, in which the solid-state measurement wasperformed with gated high power decoupling at magic angle spnning of 5 kHz.Characteristics of the PB latices are tabulated in Table 1.The chemical shifts ofthese signals were shown in Table 2 together with the assignment reported thus far

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(11–13), where the number of the carbon atoms in 1,4 and 1,2 units of PB belongsto IUPAC nomenclature.

Figure 1. 13C-NMR spectra for PBD-S.

Figure 2. 13C-NMR spectra for PBD-G.

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Table 1. Glass transition temperature, melting temperature, gel content,molecular weight, polydispersity, volume mean particle diameter of PBD

latex

Specimen Tg/oC

Tm/oC

gel content/w/w%

Mw/10 6

Mw/Mn <D>1/µm

PBD-S -75.8 -6.7 0 3.2 4.9 0.111

PBD-G -76.0 80 0.1061 <D> : volume mean particle diameter

Table 2. 13C-NMR chemical shifts for solution, latex and solid samples ofPBD-S and PBD-G

Chemical Shift / ppm

Satoet.al.9

PBD-S PBD-G

Sequence C Solution Latex Solid Solu-tion

Latex Solid

C-v 4 24.98-25.10

24.93 25.06 24.95 24.93 25.00 24.85

C-1,41,4-C

41

27.42-27.57

27.47 27.64 27.47 27.45 27.58 27.47

T-v 4 30.16 30.10 30.76 30.19 30.14 30.22 30.19

v-v-C (m) 1 31.60-32.13

32.19 32.12

1,4-v-CT-1,41,4-T

141

32.72 32.72 32.89 32.72 32.73 32.79 32.72

v-v-C (r) 1 33.35-33.53

33.60 33.21

1,4-V-1,4 1 33.99-34.16

34.05 34.17 34.06 34.05 34.13 33.98

1,4-V-v(m)

1 34.31

1,4-V-v(r) 1 35.63-36.00

35.74 35.90 36.00 35.72 35.84 35.67

v-v-T(m) 1 37.24-37.48

37.60 37.62 37.58 37.53 37.71 37.40

1,4-v-T 1 38.18 38.13 38.39 38.26 38.17 38.30 38.26

Continued on next page.

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Table 2. (Continued). 13C-NMR chemical shifts for solution, latex and solidsamples of PBD-S and PBD-G

Chemical Shift / ppm

Satoet.al.9

PBD-S PBD-G

Sequence C Solution Latex Solid Solu-tion

Latex Solid

v-V-v 2 38.57-39.13

39.05 39.30 39.13 39.06 39.23

v-v-T(r) 1 38.96-39.13

39.58 39.59 39.62 39.44 39.52 39.61

v-V 1 39.43-41.72

40.60 40.75 40.59 40.68 40.75

1,4-V-v 2 40.55-41.00

41.08 41.22 41.08 41.11 41.21 41.08

v-V-1,4 2 41.13-41.73

41.57 41.70 41.56 41.59 41.66

1,4-V-1,4 2 43.47-43.70

43.45 43.60 43.48 43.53 43.51 43.50

C, cis-1,4 unit; T, trans-1,4 unit; V, 1,2 unit; 1,4, cis- and trans-1,4 units m, meso; r,racemic

As shown in Figure 1, for PB-S solution sample containing no gel fraction,all of the expected signals appeared in the spectrum, but several signals wereinvisible due to line broadening for latex and solid samples. The invisible signalsare explained by the constraint of molecular movement of the polymer in the solidphase. The data demonstrate the important role that active molecular movementof polymer plays in the latex state 13C-NMR spectroscopy. For latex, solution andsolid samples of PB-G, some diad and triad signals were invisible in the spectrum.Since the gel content of PB-G was more than 80 w/w%while PB-S did not containany gels, the invisible signals in PB-G was due to inhomogeneity in the molecularmovement due to the presence of crosslinking junctions.

The values of T1 for most carbon atoms of latex sample were smaller thanthose of the solution sample but were actually similar to those of the solid sample.Thus, the movement of PB molecules in the latex dispersion is similar to that inthe solid sample. Since a value of glass transition temperature, Tg, for PB is about–76 oC, segmental motions are expected to occur at the observed temperature forthe polymers even in the solid state.

2.2. Effect of Dry Rubber Content (DRC)

The signal-to-noise (S/N) ratio and half-width are well-defined parameters,which express the resolution of NMR spectrum, where high resolution are

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characterized by high S/N ratio or narrow half-width. Thus, the resolution oflatex state 13C-NMR measurement was expressed via S/N ratio and half-width.

Figure 3. Dependence of S/N ratio (○) and half-width (●) on dried rubbercontent of PBD-G latex.

Figure 4. S/N ratio (○) and half-width (●) of PBD-G latex versus surfactantconcentration.

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Table 3. S/N ratio and half-width of PBD-S and PBD-G

Experimental condition S/N ratio / 102 Half-width / Hz

PBD-S

Complete decoupling

Solution b 6.6 11

Latex c 2.2 10

Gated dcoupling a

Solution b 3.5 11

Latex c 3.0 11

Solid 2.1 32

PBD-G

Complete decoupling

Solution b 0.7 26

Latex c 1.0 11

Gated dcoupling a

Solution b 0.5 26

Latex c 0.7 12

Solid 0.7 63a Gated decoupling without nuclear Overhauser effect . b 10w/v% CDCl3 solution ofPBD. c Dried rubber content of the latex was 10%.

As shown in Figure 3, the S/N ratio for PB-G latex sample increased linearlyup to 60 w/w% in DRC, while the half-width increased abruptly at about 20 w/w%in DRC. This implies that an appropriate DRC necessary for high resolution latexstate 13C-NMR measurement hovered around 10 w/w%. Yet, the S/N ratio andhalf-width were independent of surfactant concentration as shown in Figure 4.Thus, the surfactant concentration was kept at 1 w/v% for future experiments.

Table 3 shows the values of S/N ratio and half-width determined for thesignal at 32.7 ppm, for latex, solution and solid samples of PB-S and PB-G,obtained under the experimental conditions of complete decoupling (COM) andgated decoupling without nuclear Overhauser effect (NNE). In the absence of gelfraction, the S/N ratio for PB-S solution sample, measured with a COM pulsesequence, was higher than that measured with a NNE pulse sequence by a factorof about 2. This is due to the influence of nuclear Overhauser effect, as reportedin the previous work (11). For PB-S latex sample, the S/N ratio estimated byCOM measurement was similar to that estimated by NNE measurement, beingcomparable to the S/N ratio determined by the solid-state 13C-NMRmeasurement.This may be due to either the absence of nuclear Overhause effect or other

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unknown effects in the latex state 13C-NMR spectroscopy. The value of half-widthfor PB-S latex sample was nearly equal to that of the solution sample, and smallerthan the solid sample by a factor of about three. These results demonstrate thatthe resolution of 13C-NMRmeasurement for PB latex without the gel fraction wasidentical to that for the solution sample, even though the latex is heterogeneous,consisting of a polymer dispersion in water.

In the presence of the gel fraction, as shown in Figures 1 and 2, the S/N ratiofor PB-G swollen with CDCl3 was smaller than that for the latex sample by aboutone-half under observation conditions of both COM and NNE, despite the factthat the polymer content in solution was the same as that in the latex. Here, thesolution measurement of swollen PB-G containing about 80 w/w% gel fractionwas carried out for the sake of comparison with the latex state measurement. Thehalf-width for latex sample was about one-half of that for the swollen sample andone-sixth of that for the solid sample. Several signals for the solution and solidsamples of PB-G were influenced by drifting baseline due to the crosslinks presentin the gel fraction, as shown in Figures 1 and 2. Thus, highly crosslinked rubbersamplesmay be quantitatively characterized by 13C-NMR spectroscopy, when theyare dispersed in water as small particles.

2.3. Effect of Particle Size

Deproteinized natural rubber (DPNR) latex was fractionated by centrifugationinto four fractions with narrow, Gaussian, and nearly unimodal distributions inparticle size. The volume mean particle diameter distribution of the four isolatedfractions of DPNR is shown in Figure 5. At least three fractions were completelyseparated from each other, and two larger fractions were somewhat overlapped.

Figure 5. Volume weighted particle size distribution for DPNR fractionated bycentrifugation: (A) DPNR-1, (B) DPNR-2, (C) DPNR-3 and (D) DPNR-4.

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Figure 6. Latex state 13C-NMR spectra for DPNR-3 obtained (A) with spinningof the sample tube and (B) without spinning.

The latex state 13C-NMRmeasurements weremade under two conditions: onewith the sample tube spinning at a rate of 15 Hz and the other without spinning.A typical 13C-NMR spectrum for DPNR-3 latex with a volume mean particlediameter of 0.901 µm is shown in Figure 6. The spectrum with spinning wasdistinguishable from that without spinning. The S/N ratio decreased to one-halfand half-width increased a little, as the sample tube was spun at 15 Hz. This isexplained to be due to segregation of the dispersed natural rubber particles (ρ=0.91g/cm3) onto the upper layer of the latex as a supernatant after spinning the NMRtube. The concentration of natural rubber at the pulse-irradiated region may, thus,be too low to obtain a sufficient S/N ratio. In Table 6, the values of S/N ratio andhalf-width for the other latex fractions are also shown. For DPNR-1 and DPNR-2latexes with the volumemean particle diameters of 0.12 and 0.36 µm, respectively,the S/N ratio and half-width were independent of sample spinning. For thesetwo fractions with the small particle diameter, the particles were homogeneouslydispersed in water after NMRmeasurement. In contrast, as for DPNR-4, no signalswere seen in the NMR spectrum when the sample tube was spun at 15 Hz. Thisdemonstrates that the latex state 13C-NMRmeasurement is significantly dependentupon the homogeneity in the latex particle distribution.

The diffusion coefficient of the Brownian motion (14) was measured by lightscattering technique, since the diffusion coefficient is proportional to the rotationalmotion of the particles, which may perhaps be related to the rotational correlationtime. We postulate that the molecular order rotational correlation time is interferedby the rotational motion of the particles of about 1 µm in the average diameter.

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The motions of the particles in the latex were expected to play an important rolein eliminating the effect of dipole-dipole interactions.

Figure 7. Dependence of half-width on diffusion coefficient of Brownianmotion for methyl carbon (solid line) and methine carbon (dotted line) :

DPNR-1–DPNR-4 latex at 50 oC (○), DPNR1 at 30–70 oC (●) and DPNR4 at30–70 oC (▵).

In order to confirm the relationship between Brownian motion and resolutionof 13C-NMR spectroscopy that is associated with dipole-dipole interactions, thehalf widthwas plotted against diffusion coefficient of the Brownianmotion. Figure7 shows a typical plot of the half width of methyl signal at 24 ppm and methine at124 ppm versus the diffusion coefficient. The half width for the latex decreasedwith increasing diffusion coefficient and was close to that for the correspondingsolution. This is strong evidence showing that the Brownian motion of the latexparticles (or its related motions) eliminates the effect of dipole-dipole interactionson the latex-state 13C-NMR measurement.

3. Solid-State NMR with FG-FMAS Probe

3.1. Effect of Crosslinking

Solid-state 13C-NMR spectrum for the vulcanized natural rubber, measuredwith a FG-FMAS probe, is shown in Figure 8, together with a solution 13C-NMRspectrum for unvulcanized natural rubber. Five major signals in the spectrumwereassigned to C nuclei of cis-1,4-isoprene units, according to the previous paper(15). Half width of the signals and signal to noise (S/N) ratio of the solid-state13C-NMR spectrum for the vulcanized natural rubber were quite similar to those

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in the solution 13C-NMR spectrum for unvulcanized natural rubber. The narrowhalf width and the ample S/N ratio of the solid-state 13C-NMR spectrum for thevulcanized natural rubber may imply that high resolution was maintained for thesolid-state 13C-NMR spectroscopy, even after vulcanization, since heteronucleardipole-dipole interactions was eliminated by FMAS. This is distinguished fromlow resolution solid-state NMR spectrum for the vulcanized natural rubberreported by Klüppel (16) and Köenig (17–22).

3.2. Spectral Assignments

In Figure 8, small signals appeared at 40, 44, 50 and 58 ppm in the solid-state13C-NMR spectrum for the vulcanized natural rubber, which were not shown inthe solution 13C-NMR spectrum for the unvulcanized natural rubber. To assignthe signals, we applied DEPT measurement at 45 o (DEPT45), 90 o (DEPT90)and 135 o (DEPT135) pulses and APT measurement to the solid-state 13C-NMRspectroscopy. Figure 9 shows DEPT45, DEPT90 and DEPT135 spectra for thevulcanized natural rubber. The signals at 24, 26 and 32 ppm characteristic ofmethyl, methylene and methine carbons of cis-1,4-isoprene unit were shown to beup, up, and up in the DEPT45 spectrum and almost null in the DEPT90 spectrum.On the other hand, in the DEPT135 spectrum, the signals were up, down, anddown. Thus, the pulse width determined for DEPT measurements was confirmedto be correct.

The small signals at 40 and 44 ppm were shown to be up in the DEPT45spectrum, null in the DEPT90 spectrum, and down in the DEPT135 spectrum;hence, they were assigned to secondary carbons. The signal at 50.5 ppm wasassigned to quaternary carbon due to null signal in the spectra, while the signalat 50 ppm was assigned to tertiary carbon due to up signals in the spectra. Incontrast, the signal at 58 ppm was assigned to tertiary and quaternary carbons dueto the very small up signals in the DEPT45, DEPT90 and DEPT135 spectra andnull signals; in fact, almost all signals disappeared. In Figure 9, the APT spectrumis also shown for the vulcanized natural rubber. The APT spectrum showed upsignals at 40 and 44 ppm, up and down signals at 50 ppm, and up and down signalsat about 58 ppm. Thus, we assigned the signals at 40 and 44 ppm to the secondarycarbons and the signals at 50 and 58 ppm to the tertiary and quaternary carbons.

Figure 10 shows solid-state 1H-NMR spectrum for the vulcanized naturalrubber and solution 1H-NMR spectrum for the unvulcanized natural rubber. Majorsignals at 1.7, 2.1 and 5.1 ppm in the spectra were assigned to methyl, methyleneand unsaturated methine protons of cis-1,4-isoprene units, respectively. Values ofhalf width and signal to noise (S/N) ratio of the signals in the solid-state 1H-NMRspectrum for the vulcanized natural rubber were a little bit larger and smaller,respectively, than the values of the half width and the S/N ratio of the signals inthe solution 1H-NMR spectrum for the unvulcanized natural rubber. For instance,the value of half width of the signals in the solid-state spectrum was about 1.5times as large as that in the solution spectrum. This may be explained to be dueto a reduced effect of the homonuclear dipole-dipole interactions in the solid-state 1H-NMR spectrum; i.e., a major portion of the homonuclear dipole-dipoleinteractions are eliminated by FMAS. (The homonuclear dipole-dipole interaction

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may be completely eliminated by extremely fast MAS with smaller sample tube,but this is not the focus of this work.) In Figure 10, small signals at 3.4 and 4.2 ppmappeared in the solid-state 1H-NMR spectrum for the vulcanized natural rubber,but not in the solution 1H-NMR spectrum for the unvulcanized natural rubber.The signal at 3.4 ppm was assigned to aliphatic 1H linking to –C-CH-Sx groupand the signal at 4.2 ppm to unsaturated aliphatic 1H linking to =C-CH-Sx group,according to literatures (23), where x represents the number of S atoms.

Figure 8. Typical 13C-NMR spectra: (A) solid-state 13C-NMR spectrum for thevulcanized natural rubber, (B) solution-state 13C-NMR spectrum for unvulcanizednatural rubber. The solid-state 13C-NMR measurement was performed with a 4mm FG-FMAS probe at 18 kHz in spinning rate. The solution state 13C-NMRmeasurement was performed with a NM-40TH5AT/FG2SL probe at 12 Hz in

spinning rate.

3.3. 2D NMR Measurement

Figure 11 shows HSQC spectra obtained through solid-state NMRspectroscopy equipped with the FG-FMAS probe. The signals at 1.7, 2.1 and 5.1ppm in the 1H-NMR spectrum were correlated with the signals in the 13C-NMRspectrum; e.g., the signal at 1.7 ppm was correlated with the signal at 23 ppm, thesignal at 2.1 ppm with the signals at 26 and 32 ppm, and the signal at 5.1 ppmwith the signal at 135 ppm. However, no heteronuclear correlation between thesmall signals was detected in HSQC spectra due to inadequate accumulation timefor HSQC measurement.

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Figure 9. Solid-state 13C-NMR spectra with pulse sequences of distortionlessenhancement by polarization transfer (DEPT) and attached proton test for thevulcanized natural rubber: (A) DEPT45, (B) DEPT90, (C) DEPT135, (D) APT.

To detect heteronuclear correlation between the small signals, HMQCmeasurement was performed in a selective region of chemical shift: 2–4 ppmin 1H domain and 35–70 ppm in 13C domain. Figure 12 shows HMQC spectraobtained by the selective experiment. The 13C-signals at 40, 44, 50 and 58 ppmwere well correlated to the 1H-signals at 2.1, 1.7, 2.8 and >3.6 ppm. In theprevious works (15, 24), the signals at 40, 44 and 58 ppm of the vulcanizedliquid cis-1,4-polyisoprene as a model were assigned through solution NMRspectroscopy with various pulse sequences, viz., DEPT, APT, HETCOR, HSQCand HMBC. The signal at 40 ppm was assigned to C4 of trans-1,4-isoprene units,obtained by isomerization of cis-1,4-isoprene units. In contrast, the signals at 44ppm in the 13C-NMR spectrum were assigned to the secondary carbons adjacentto carbons linking to S atoms. The signals at 58 ppm in 13C-NMR spectrum wereassigned to the tertiary and quaternary carbons linking to S atoms. In the presentwork, the same results were obtained by the solid-state NMR spectroscopyequipped with the FG-FMAS probe. Furthermore, the 13C-signals at 58 ppmwere correlated to the 1H-signals at 3.4 and 4.2 ppm, which were assigned to the–C-CH-Sx group and =C-CH-Sx group.

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Figure 10. Typical 1H-NMR spectra: (A) solid-state 1H-NMR spectrum forthe vulcanized natural rubber, (B) solution-state 1H-NMR spectrum for theunvulcanized natural rubber. The solid-state 1H-NMR measurement was

performed with a 4 mm FG-FMAS probe at 18 kHz in spinning rate. The solutionstate 1H-NMR measurement was performed with a NM-40TH5AT/FG2SL probe

at 12Hz in spinning rate.

Figure 11. HSQC spectra for the vulcanized natural rubber, obtained throughsolid-state NMR spectroscopy equipped with a 4 mm FG-FMAS probe at 18 kHz

in spinning rate for 1 h 2 min.

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Figure 12. HMQC spectra for the vulcanized natural rubber, obtained throughsolid-state NMR spectroscopy equipped with a 4 mm FG-FMAS probe at 18 kHzin spinning rate for 71.5 h. HMQC measurement was performed in a selectiveregion of chemical shift: 2–4 ppm in 1H domain and 35–70 ppm in 13C domain.

3.4. Application of FG-FMAS Solid-State NMR Spectroscopy

The crosslinking junctions of three rubbers (CV, EV and SemiEV crosslinkedrubbers) were analyzed by solid-state 1H-NMR and 13C-NMR spectroscopy. Table4 shows crosslink density of these three rubbers. Values of crosslink densityof the crosslinked rubbers were similar, implying that the crosslink density isadjustable, as long as we prepare the samples under the optimum condition forcrosslinking. The slight difference in the value of crosslink density may be due tothe comparatively short time for crosslinking and it is difficult to use exactly thesame optimum time for crosslinking.

Figure 13 shows solid-state 1H-NMR spectra for the three rubbers. Majorsignals at 1.7, 2.1 and 5.1 ppm in the spectra were assigned to methyl, methyleneand unsaturated methine protons of cis-1,4-isoprene units, respectively. In Figure13, small signals at 3.4 ppm and 4.2 ppm were found for the three crosslinkedrubbers. The signal at 3.4 ppm was assigned to aliphatic 1H linking to –C-CH-Sxgroup and the signal at 4.2 ppm to unsaturated aliphatic 1H linking to =C-CH-Sxgroup, according to our work (4). The intensity of the aliphatic 1H linking to–C-CH-Sx group (3.4 ppm) and the unsaturated aliphatic 1H linking to =C-CH-Sxgroup (4.2 ppm) was estimated from intensity ratio of the signals to the methylproton signal at 1.7 ppm. The estimated value of the intensity ratio is tabulated inTable 5. It was found that the intensity of the aliphatic 1H linking to –C-CH-Sxgroup at 3.4 ppmwas 0.02% for CV, EV and SemiEV crosslinked rubbers, whereasthat of the unsaturated aliphatic 1H linking to =C-CH-Sx group at 4.2 ppm was0.03 % for CV and SemiEV crosslinked rubbers and 0.02 % for EV crosslinkedrubber. From these results, it is concluded that the content of the small signals ofthe aliphatic 1H linking to –C-CH-Sx group and unsaturated aliphatic 1H linking to

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=C-CH-Sx group for CV, EV and SemiEV crosslinked rubbers was almost similarto each other.

Table 4. Vulcanization characteristics of HANR and DPNR compounds

Crosslink density (x10-4, mol/cm3)Sample

CV system EV system SemiEV system

HANR 1.86 1.45 1.67

Table 5. Intensity ratio of the signal at 3.4 ppm (aliphatic 1H linking to–C-CH-Sx group signal) and the signal at 4.2 ppm (unsaturated aliphatic 1H

linking to =C-CH-Sx group signal) to the signal at 1.7 ppm

Intensity of signals (%)Crosslinkedrubber 3.4 ppm (aliphatic 1H linking

to –C-CH-Sx group)4.2 ppm (unsaturated aliphatic 1Hlinking to =C-CH-Sx group)

CV 0.02 0.03

EV 0.02 0.02

SemiEV 0.02 0.03

Table 6. Intensity ratio of the signals at 40, 44 and 58 ppm to the signal at24 ppm

Intensity of crosslinking junction signals (%)Crosslinked rubber

40 ppm 44 ppm 58 ppm

CV 0.010 0.005 0.005

EV 0.010 0.000 0.000

SemiEV 0.009 0.001 0.001

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Figure 13. Solid-state 1H-NMR spectra for the crosslinked natural rubbersprepare by (A) CV, (B) EV and (C) SemiEV.

Figure 14. Solid-state 13C-NMR spectra for the crosslinked natural rubbersprepared by (A) CV, (B) EV and (C) SemiEV.

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Solid-state 13C-NMR spectra for the same samples are shown in Figure 14.Small signals at 40, 44 and 58 ppm appeared in the solid-state 13C-NMR spectra forthe CV and SemiEV crosslinked rubber, whereas only signal at 40 ppm appearedin the solid-state 13C-NMR spectrum for the EV crosslinked rubber. The signalsat 40 and 44 ppm were assigned to C4 of trans-1,4-isoprene units (25, 26) andsecondary carbons adjacent to carbons linking to S atom, respectively, and thesignals at 58 ppm were assigned to the tertiary and quaternary carbons linking to Satoms, according to our work (4). This may imply that the crosslinking junctionsof the CV and SemiEV crosslinked rubbers were not only secondary carbon butalso tertiary and quaternary carbons whereas that of the EV crosslinked rubber maycontain small amount of quaternary carbon. The intensity of the small signals at40 ppm, 44 ppm, and 58 ppm was estimated from intensity ratio of the signals tothe methyl carbon signal at 24 ppm. The estimated values of the intensity ratio ofthe small signals at 40, 44 and 58 ppm to the methyl carbon signal at 24 ppm weretabulated in Table 6. The intensity of the signal at 40 ppmwas similar to each other(0.010 %) for CV, EV and SemiEV crosslinked rubbers. This indicates that cis-trans isomerization occurs similarly for CV, EV and SemiEV crosslinked rubbersin spite of differences in formulation. In contrast, the intensity of the signals at 44ppm and 58 ppm in the solid-state 13C-NMR spectrum decreased in the order ofCV, SemiEV and EV crosslinked rubbers. The intensity of the secondary carbon(44 ppm), the tertiary and quaternary carbon (58 ppm) for CV crosslinked rubberwas the highest among the three. They decreased for SemiEV crosslinked rubberand disappeared for EV crosslinked rubber. This implies that the amount of thequaternary carbon of the CV crosslinked rubber is larger than that of the SemiEVcrosslinked rubber. The EV crosslinked rubber may contain a small amount ofquaternary carbon as a crosslinking junction.

Figure 15. Stress-strain curves for CV, EV and Semi-EV crosslinked rubbers.

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The stress-strain curves for the CV, EV and SemiEV crosslinked rubbersare shown in Figure 15. Modulus at 100% strain of the CV crosslinked rubberis almost similar with that of EV and SemiEV crosslinked rubbers. This isconsistent with the fact that the values of the crosslink density of the CV, EVand SemiEV crosslinked rubbers were similar to each other, as shown in Table4. In contrast, tensile strength of the three crosslinked rubbers was different; thatis, the value of stress at break of the CV crosslinked rubber (22.5 MPa) was thehighest among the three crosslinked rubbers and was reduced for the SemiEV(17.0 MPa) and EV (9.2 MPa), in that order. In the previous work (27), Suchivaand co-workers reported that better mechanical properties of the CV crosslinkedrubber was due to higher concentrations of the polysulfidic crosslinks comparedto those of the EV crosslinked rubber. Moreover, the superior mechanicalproperties of CV crosslinking system may be explained to be due to differencesin the vulcanization mechanism of the crosslinking systems. It may be noted thatthe conventional crosslinking system produces a homogeneous network. Yet,an efficient crosslinking system results in significant polymerization of doublebonds of adjacent chains leading to a network of unevenly distributed crosslinksand in impediment to NR crystallization and stress concentration that anticipatescompound failure (28). In the present work, it was found that the excellentmechanical properties of the CV crosslinked rubber could be attributed to thequaternary carbon of crosslinking junctions.

Acknowledgments

This work was supported in part by a Grant-in-Aid (21655080) forChallenging Exploratory Research and Grant-in-Aid (22350100) for ScientificResearch (B) from Japan Society for the Promotion of Science and JST-JICASATREPS.

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