129Xe nuclear magnetic resonance investigation of carbon black aggregate morphology

Carbon 37 (1999) 1443–1448

129

Xe nuclear magnetic resonance investigation of carbon black aggregate morphology Kenneth J. McGrath Code 6122, Naval Research Laboratory, Washington, DC 20375 -5342, USA Received 17 June 1998; accepted 23 December 1998

Abstract The morphology of three ‘normal’ ASTM carbon black reinforcement fillers (N110, N347, and N472) is investigated for the first time using one- and two-dimensional 129 Xe nuclear magnetic resonance (NMR) spectroscopy. A wide range in 129 Xe NMR chemical shift is observed among samples, and is attributed to variations in the physical nature of the unoccupied volume formed by the subunit particles of the aggregate. 129 Xe NMR chemical shift and integrated intensity parameters are extracted from a single one-dimensional experiment, and interpreted in terms of aggregate void size, volume, and density. The rate of exchange of 129 Xe atoms between different aggregates in a three component blend is approximated using two-dimensional exchange NMR spectroscopy, and found to occur on a timescale |6 ms#tex #|100 ms. No evidence of xenon exchange between samples N110 and N472 was observed on timescales less than 100 ms, an apparent consequence of the small aggregate void size in these samples. The 129 Xe NMR chemical shift assignments for samples N110, N347, and N472 are shown to be not significantly shifted from their non-exchanging values.  1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon black; C. Nuclear magnetic resonance

1. Introduction A wide variety of standard ASTM techniques are available for characterizing carbon black fillers used in the manufacture of rubber products [1]. Most of these methods quantify the various microscopic physical properties (e.g. surface area, surface activity, ‘structure’, etc.) that are known to influence mechanical behavior in compounded product. For example, ASTM [D2414 dibutyl phthalate (DBP) absorption is used in quantifying aggregate ‘structure’, and ASTM [D3037 nitrogen adsorption is used to quantify aggregate surface area. Less empirical methods, such as atomic force microscopy imaging (three dimensional aggregate morphology [2]) and Raman spectroscopic analysis (carbon black bonding chemistry [3]) have also been recently applied. 129 Xe nuclear magnetic resonance (NMR) spectroscopy has not been previously used in characterizing carbon black filler materials, although it has been applied in studying the chemical and microscopic physical properties of clathrates [4], zeolites [5], organic homopolymers and blends [6], and a variety of other materials [7]. Nonetheless, the morphology of carbon black fillers makes them an

ideal candidate for 129 Xe NMR analysis. The variation of reinforcement properties in filled elastomers is largely attributable to the subtle microscopic physical properties associated with the filler, and the principal attribute of 129 Xe NMR analysis is its extreme sensitivity to the microscopic environment that the xenon atom occupies. Xenon atoms are highly polarizable (due to their large number of electrons), and collisions of the atom result in magnetic field fluctuations at the nucleus. In microporous structures, the time average of this field, and resulting chemical shift, is governed largely by the number of collisions per unit time [8]; xenon chemical shifts also depend upon the chemistry of the host environment, but generally to a much lesser extent than the collision frequency [9]. The high polarizability of xenon can result in an NMR chemical shift range of over 5000 ppm [10], although most chemical shifts are in the range of about 500 ppm. Since xenon is a noble gas, it is not chemically reactive, and can be nondestructively introduced into a wide variety of samples. The purpose of the present study is to investigate carbon black aggregate morphology using 129 Xe NMR spectroscopic analysis. We find a large variation in the 129 Xe

0008-6223 / 99 / $ – see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 99 )00006-8

K. J. McGrath / Carbon 37 (1999) 1443 – 1448

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NMR chemical shift among differently (ASTM) designated samples, and this allows us to extract various NMR parameters specific to each. Their significance is interpreted in the context of aggregate morphology using oneand two-dimensional NMR methods.

2. Experimental Three carbon black samples were selected for this investigation (N110, N347, and N472), all of which are characterized as having ‘normal’ influence on the cure rate of rubber. Thus, the surface chemistry of these samples has not been modified, so their 129 Xe NMR response can be interpreted primarily from a physical perspective. The samples were selected in order to obtain a range of particle size, density, surface area, and structure that is typical among reinforcement blacks currently in use. The nominal properties of each sample, according to their ASTM classification, are listed in Table 1, along with their room 129 temperature Xe NMR chemical shifts. All samples were obtained from Cabot Corporation, Boston, MA, and used without further physical or chemical modification. 129 Xe NMR experiments were conducted on each of the three neat carbon black samples, and also a physical blend composed of 1.89 g of N110, 3.00 g of N347, and 1.00 g of N472. The blend quantities represent the amounts necessary to yield equal surface areas of each component, according to their nitrogen adsorption (ASTM D3037 [1]) values (see Table 1). The blend was prepared by adding the three components to a 25 mm glass vial, and mixing by slow rotation (|15 Hz) for 10 min. Samples (|1 g) were added to a 10 mm NMR tube and evacuated for 24 h at room temperature, then allowed to soak under a regulated 2 2.4310 kPa (25 psi) xenon gas (with natural abundance 129 of Xe) atmosphere overnight. The NMR sample tube 2 was sealed at 2.4310 kPa prior to collecting the spectra. All experiments were conducted at room temperature in a static magnetic field of 7 Tesla, corresponding to a 129 Xe Larmor frequency of 83.01 MHz. The exchange rate of

Table 1 Carbon black

129

adsorbed 129 Xe gas atoms among carbon blacks was estimated using a two dimensional NMR experiment [11] with mixing times 6 ms#tm #100 ms. Both one and two dimensional xenon NMR spectra were acquired using radiofrequency fields of 25 kHz, 12.2 kHz sweep widths, and acquisition times of 3.2 ms (128 data points). A relaxation delay of 10 s was used between scans, based upon our experimental estimates of the carbon black adsorbed 129 Xe spin lattice relaxation times (T 1 #|2 s). No apodization of the free induction decay was used in processing the one-dimensional spectra; two-dimensional spectra were apodized using 300 Hz Lorentzian linebroadening in both F1 and F2 dimensions.

3. Results and discussion One-dimensional 129 Xe NMR spectra were obtained on each of the three neat carbon black samples and the three-component blend. The one-dimensional spectra of the neat samples (not shown) were used to identify the adsorbed xenon chemical shifts associated with each filler. Fig. 1 shows the 129 Xe NMR spectrum of the blend, composed of N472, N110, and N347 fillers in the ratios described. The resonance at d ¯0 ppm corresponds to xenon gas atoms which are very weakly adsorbed. The chemical shift of free xenon gas (2.4310 2 kPa) was observed using a separate experiment with a 60 s relaxation delay, and found to nearly coincide with the peak identified at d ¯0 ppm in Fig. 1. The free xenon gas peak is not observed in Fig. 1 because of its relatively long relaxation time. The farthest upfield peak (d ¯53ppm) corresponds to N347 adsorbed 129 Xe atoms, followed by N110 (d ¯68 ppm) and N472 (d ¯94 ppm) adsorbed xenon. It is unlikely that the observed chemical shift variation among the carbon blacks is due to xenon that has diffused into the carbon black subunit particles. The accepted van ˚ but the der Waals’ diameter of the xenon atom is 4.4 A, quasi-graphitic layer spacing of the paracrystalline do-

Xe NMR chemical shift and ASTM physical data N110

N347

N472

68 11–19

53 26–30

94 31–39

143

90

270

126

88

145

145

90

250

113

124

178

129

Xe NMR chemical shift (ppm) Diameter (nm) Nitrogen adsorption (D3037, m 2 / g) CTAB adsorption (D3765, m 2 / g) Iodine adsorption (D1510, g / kg) DPB adsorption (D2414, cm 3 / 100 g)

K. J. McGrath / Carbon 37 (1999) 1443 – 1448

Fig. 1. 129 Xe NMR spectrum of carbon black blend composed of N347 (d ¯53 ppm), N110 (d ¯68 ppm), and N472 (d ¯94 ppm). The quantities of each component in the blend were adjusted to yield equal surface areas, according to their ASTM D3037 N 2 adsorption values.

mains of the subunit particle is substantially smaller (|3.5 ˚ [12]), providing negligible space for xenon occupation. A, In contrast, the aggregate particle is typically composed of irregular chains of many fused nodular subunits (|10–40 nm diameter in present case), and forms a complex structure that will easily accommodate xenon. Our interpretation is that the xenon gas within the confines of the aggregate structure most likely contributes to the observed variation in chemical shifts (Fig. 1) among carbon black samples. Near and outside the periphery of this aggregate, the mean-free-path of xenon increases further, with the chemical shift approaching that of the free gas. Xenon exchange between the free gas and aggregate periphery will be relatively unimpeded and rapid, and explains the relatively broad (compared with neat Xe gas) resonance at d ¯0 ppm (Fig. 1). As discussed, it is reasonable to attribute the observed variations in 129 Xe NMR chemical shift to differences in the collision rate of adsorbed xenon atoms within the aggregate structure. Thus, the xenon atoms in sample N472 are shifted farthest downfield (d ¯94 ppm), and are interpreted as having the highest collision frequency among the three samples. The next highest xenon collision frequency would be in sample N110 (d ¯68 ppm), and the lowest in sample N347 (d ¯53 ppm). Based on these assignments, we conclude that the size of the voids within the aggregates decreases in the order N347.N110.N472. The ASTM surface area measurements (see Table 1) support this ordering of relative void size. Within each sample, the ASTM surface area measurements ([3037 N 2 adsorption, [D1510 I 2 adsorption, and [D3765 CTAB adsorption) are very consistent, except for sample N472. In this case the N 2 and I 2 surface areas are consistent, but the CTAB surface area is about 50% lower. This indicates that

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roughly half of the surface area of the N472 filler is not accessible to the relatively large CTAB molecule, but is accessible to smaller volume N 2 and I 2 molecules (and 129 Xe atoms). Thus, the ASTM surface area measurements confirm that the void size in sample N472 is smallest among the three samples, in accord with the prediction based on 129 Xe NMR chemical shift. We also find an excellent qualitative correlation between N 2 (or I 2 ) surface area and 129 Xe NMR chemical shift for all three carbon black samples (see Fig. 2). Since the ASTM N 2 and I 2 techniques are a direct measure of aggregate surface area, and the chemical shifts depend largely upon collision frequency, the correlation is not a physical requirement. An increase in surface area accompanied by a decrease in void size suggests that the volume of the voids is not diminishing proportionally. Later, using direct 129 Xe NMR measurement of aggregate density, we confirm that this is the case. As noted, the ability to resolve 129 Xe NMR chemical shifts for each different carbon black sample provides sample-specific information. The integrated intensity of each resonance yields the relative number of xenon atoms that are adsorbed within the aggregate, and corresponds to 1.0:1.2:2.2 for components N472:N110:N347 in the blend. Since this blend was prepared so that each component is equal in surface area (according to ASTM N 2 adsorption), this ratio represents the relative number of xenon atoms adsorbed per unit surface area of aggregate. If the reasonable assumption is made that the quantity of adsorbed xenon (e.g. integrated intensity) is proportional to aggregate void volume, then the volume:surface area ratio is 1.0:1.2:2.2 for N472:N110:N347. For voids having similar shape, this ratio should vary roughly linearly with the (linear) dimensions of the void, and thereby provide an

Fig. 2. 129 Xe NMR chemical shifts of carbon black samples N347, N110, and N472 versus their ASTM D3037 N 2 adsorption surface areas. The solid line is a least squares fit to the data.

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approximation of relative void dimensions in each sample. In particular, the ordering of void size using this method is consistent with our conclusions that were based upon (independent) 129 Xe chemical shift / collision frequency arguments. A plot of chemical shift versus 129 Xe intensity:ASTM N 2 surface area is shown in Fig. 3. The 129 Xe integrated intensities may also be useful in characterizing aggregate structure, which is one of the most important parameters affecting filler performance. One of the standard methods used in characterizing structure is ASTM D2414 dibutyl phthalate (DBP) absorption, and is expressed in terms of the volume of DBP (per unit mass of filler) that is required to reach a titration endpoint, where internal and external voids of the aggregate particle are filled. Thus, the amount of DBP depends upon the void volume within the aggregate, and the manner in which the aggregate particles pack together. If the latter does not vary too much from one carbon black sample to another, the DBP measurement gives a good estimate of structure [12]. As shown in Table 1, the ASTM DBP values (in cm 3 DBP per 100 g filler) for N472, N110, and N347 are 178, 113, and 124, respectively. These values indicate relatively high structure in N472, and roughly equivalent (and lower) structure in N110 and N347. Using the 129 Xe NMR integrated peak intensities for each sample, along with their relative masses in the blend, we calculate the (relative) aggregate void volume per unit mass for N472, N110, and N347 as 1.7, 1.0, and 1.2, respectively. As shown in Fig. 4, we find an excellent

Fig. 3. 129 Xe NMR chemical shifts of carbon black samples N347, N110, and N472 versus their unoccupied volume:surface area ratio. The volumes for each component were determined from 129 Xe NMR integrated intensities, and the surface areas from ASTM D3037 N 2 adsorption. The relative void sizes represented by the ratios are consistent with our chemical shift interpretation of the xenon collision rates. The solid line connecting the points is a visual aid and does not represent any fit to the data.

Fig. 4. Relative 129 Xe NMR integrated peak intensity (per unit mass) versus ASTM 2414 DBP absorption. The solid line is a least squares fit to the data.

correlation between DBP structure measurement and 129 Xe NMR integrated intensity. We also note that the relative densities of the aggregates may be estimated from the reciprocal of these ‘structure’ measurements. For N472:N110:N347, this density ratio corresponds to 1.0:1.6:1.4, and confirms that void volume does not decrease proportionally with (diminishing) void size. Finally, two-dimensional 129 Xe NMR analysis is applied to estimate the rate at which xenon atoms exchange between different carbon black samples in the blend. We anticipate that a detailed application of this technique will be useful in characterizing filler materials, since the rate of exchange from one type to another will uniquely depend on aggregate morphology. In the present case, we use the method to demonstrate whether xenon exchange has altered the observed chemical shifts from their non-exchanging values, and assess whether the approximated xenon exchange times are consistent with our conclusions regarding aggregate void size. When xenon exchange between two sites is ‘rapid’ (as defined by their chemical shift difference), the observed chemical shift is a time-weighted average of the shifts in the absence of exchange [13]. Thus, in order to ascertain whether peak shifting has occurred, it is necessary to know how rapidly the exchange process is occurring. This is conveniently done using two-dimensional exchange NMR spectroscopy, the details of which can be found elsewhere [11]. The experiment is executed using different exchange times, and the exchange rate is calculated from the relative intensities of the off-diagonal and diagonal peaks; a detailed knowledge of the 129 Xe longitudinal relaxation behavior and occupational probabilities in each domain are required to obtain the rate constant associated with ex-

K. J. McGrath / Carbon 37 (1999) 1443 – 1448

change. Although the exact values of these parameters were not calculated in the present application, the technique can still be used to determine the approximate exchange rate between domains. This is possible because the presence of off-diagonal peaks in the two dimensional spectrum cannot occur unless exchange takes place on the timescale of the ‘exchange time’ used in the experiment. Fig. 5 shows the two-dimensional 129 Xe NMR exchange spectrum on the N110 / N347 / N472 blend using an exchange time of 100 ms. In particular, there is significant crosspeak intensity among all combinations of carbon black sample pairs except N472 and N110. This indicates that xenon is exchanging between the site pairs on a timescale that is less than approximately 100 ms. The absence of crosspeak intensity between N472 and N110 indicates that their exchange time is longer, and is consistent with their smaller void size as calculated previously. If the exchange time of the experiment is decreased to 12 ms (Fig. 6), the crosspeak intensities decrease substantially, and there is a complete absence of observable crosspeak intensity when the exchange time is reduced to 6 ms (Fig. 7). This indicates that the rate of exchange of xenon between any of the carbon black samples is less than approximately (6 ms)21 5167 Hz. Since the minimum peak separation in Fig. 1 (N347, d ¯53 ppm vs. N110, d ¯68ppm) is 15 ppm51240 Hz, we conclude that the peaks have not undergone appreciable shifting due to xenon exchange.

Fig. 5. Two-dimensional 129 Xe NMR exchange spectrum of N347 / N110 / N472 blend using a 100 ms exchange time. The presence of off-diagonal crosspeaks indicates that xenon exchange is occurring on a timescale no longer than approximately 100 ms. The absence of crosspeaks between N472 and N110 indicates that their xenon exchange time is longer, and may be due to their diminutive aggregate void size.

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Fig. 6. Two-dimensional 129 Xe NMR exchange spectrum of N347 / N110 / N472 blend using a 12 ms exchange time. The weak crosspeak intensities indicate a near absence of xenon exchange on this timescale.

4. Conclusions A large variation in 129 Xe NMR chemical shift is found among carbon black samples N472, N347, and N110 ‘soaked’ under a 2.4310 2 kPa Xe gas atmosphere. The ability to resolve the chemical shifts of different fillers

Fig. 7. Two-dimensional 129 Xe NMR exchange spectrum of N347 / N110 / N472 blend using a 6 ms exchange time. The absence of crosspeak intensity between all samples indicates that xenon exchange is slow on this timescale.

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allows a variety of sample-specific NMR parameters to be calculated, all of which are shown to ultimately depend upon carbon black aggregate morphology. Using 129 Xe NMR parameters obtained from a single experiment (conducted on a physical blend of three components), relative aggregate void size, volume, and density are determined for each material. In general, we find an excellent correlation between ASTM and 129 Xe NMR measurements. The timescale of xenon gas exchange between carbon black samples was estimated using two-dimensional 129 Xe NMR analysis, and demonstrates an apparent decrease in the rate of xenon gas exchange between samples with decreasing aggregate void size (as determined by 129 Xe NMR chemical shift and aggregate volume:aggregate surface area ratios). The observed chemical shift values in the blend are shown to be not significantly shifted from their non-exchanging values.

Acknowledgements This work was supported in part by the Office of Naval Research.

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