Structure and dynamics of carbon black filled elastomers II, IMS and IR.

Heterogeneous filler addition is a common practice in the rubber industry to stiffen and strengthen amorphous rubbers. In addition to the reinforcement, fine and particulate fillers, most notably carbon black (CB), suppress elasticity of gum rubbers and render better processability, such as less die swell, less shrinkage, less melt fracture and less nerve (ref. 1). Interactions between CB and rubber are physical and strong, analogous to that within polymer crystals, which are resistant to solvent, but can yield upon stressing. As particle size decreases with a corresponding increase in specific surface area, strength of CB filled vulcanized rubbers, in general, increases (ref. 2). This could be attributed to an increase in volume fraction of the rubber phase with restricted mobility due to particle spacing or greater surface area per unit volume of the filler (ref. 3). At the same time, particle spacing considerations could be used to explain the difficulties in dispersing fillers of high concentrations. Uniform dispersion requires a particle-to-particle spacing less than the rubber coil size. The most finely divided CB, N110, has a diameter of 20 nm and has the nearest particle spacing of 5.6 nm in a rubber containing 20 vol. % of N110 (ref. 4). Tiffs value is significantly less than the coil dimension of 13.4 nm for a rubber with 500,000 molecular weight. Instead of distorting polymer coil to fit in between particles with entropy penalty, poor dispersion results with regions of filler agglomerates with a minimum separation distance that is equal to the coil dimension. In addition to poor dispersion, a high filler loading could reduce tensile strength and elongation of filled rubbers (ref. 5).

Brominated poly(isobutylene-co-para-methylstyrene), BIMS by ISO 1629 abbreviation, is an excellent rubber component in CB-filled rubber compounds for its impermeability, high damping, good ozone and aging resistance. Applying complementary viscoelastic and tapping mode AFM (atomic force microscopy) measurements to elucidate the structure-property relationship in carbon black filled BIMS has been reported previously (ref. 6). The linear and non-linear viscoelastic behavior of these CB-filled BIMS suggests a sharp transition in flow properties at around 9 vol. % filler, the same composition at which the nearest neighbor distances as inferred from AFM measurements also exhibit a sudden change. These changes in structural and dynamic properties were suggested as an indication of the formation of a percolated filler network for composites containing more than 9 vol. % carbon black. The percolation threshold is considerably lower than that expected from the theoretical calculations of overlapping spheres and those based on the electrical conductivity measurements of conducting and insulating spheres. AFM measurements clearly indicate that the CB particles are not agglomerated and also suggest that their shape is only slightly anisotropic, thereby eliminating the possibility that the presence of highly anisotropic filler particles or aggregates is primarily responsible for the decreased percolation threshold. With only a few benzylic bromine groups along a given BIMS chain that are interacting with CB particles, it was proposed that either a single BIMS chain is absorbed onto several filler aggregates or long loops are formed for absorbed chains. The strong mediation of the CB filler network structure by BIMS could be the result of either filler connectivity by BIMS or entanglements of loops from absorbed chains.

In addition to the incorporation of a filler in a single elastomer, two or more polymers are commonly used in rubber or elastomer blends for various reasons. For example, BIMS elastomer, despite its excellent impermeability, high damping, good ozone and aging resistance, is prone to wear and relatively costly. Therefore, BIMS is commonly blended with other low-cost high diene rubbers, such as poly(butadiene) (BR, butadiene rubber), poly(styrene-co-butadiene), (SBR, styrene-butadiene rubber), natural rubber (NR), or poly(isoprene) (IR, isoprene rubber), for cost saving, for improved adhesion (refs. 7 and 8), and for better wear and abrasion resistance (ref. 9). In these BIMS and high diene rubber blends, filler phase distribution is expected due to phase partition. Filler depletion or crowding in one polymer phase could have significant impacts on the final blend properties. Low CB presence in one rubber phase may lead to lower strength and stiffness of that rubber phase in addition to affecting the cure rate of sulfur-cured rubbers (ref. 2). Higher filler loading in the other rubber phase, instead, could lead to poor filler dispersion in that phase and the over-stiffening and embrittlement. Therefore, it is essential to understand and to control the filler phase partition in rubber blends.

Filler phase partition is related to polymer-filler interactions. In this study, as a continuation of our previous work on BIMS (ref. 6), filler percolation thresholds of N234 CB in unbrominated BIMS or IMS (poly(isobutylene-co-paramethylstyrene), no bromine) and in IR were determined based on changes in rheological properties and changes in particle spacing, as measured from tapping phase AFM morphologies. By comparing the filler percolation threshold values reported previously in BIMS (ref. 6) and observed in IMS in this study, the effect of bromine on BIMS-CB interactions is established and further confirmed by bound rubber results. However, the filler percolation threshold of N234 CB in IR as determined from rheology is different from that calculated from inter-particle distances. Degradation of IR during mixing and testing may have contributed to this discrepancy. Phase and straight mixing methods were applied to prepare CB-filled blends of BIMS with various diene rubbers. Preferential filler partition into the BIMS phase was observed in these blends. Based on the temperature dependent results of the bound rubber measurement, this preferential partition is attributed to the strong and relative temperature insensitive BIMS-CB interactions versus those for CB and diene rubbers.

Experimental

Filler percolation threshold

BIMS (1.2 mol. % Br, 7.5 wt. % PMS, 38 ML), IMS (7.5 wt. % PMS, 45ML) and IR containing various concentrations of N234 CB (~24 nm in diameter, ~126 [m.sup.2]/g [N.sub.2] surface area) up to about 20 vol. % were prepared using an internal mixer. These filled rubbers without curatives were cryofaced at -150[degrees]C with a cryo-microtome and evaluated by tapping phase AFM. Tapping phase images were subsequently processed with an image processing tool kit. Shape factors of fillers were measured to ensure good filler dispersion without significant agglomeration. Coordinates of fillers were also determined and were applied to calculate particle spacing using an internal developed program based on Matlab. Detailed procedures for experimental determination of the filler percolation threshold in N234 filled BIMS using morphological methods can be found elsewhere (ref. 6).

The linear viscoelastic properties of the particle filled and unfilled samples were characterized via small amplitude oscillatory shear testing on an advanced rheometric expansion system. Parallel plate test fixtures with 25 mm diameter were utilized for data acquisition. Polymer specimens having the same diameter as the fixtures were cut from compression molded 50 x 50 x 2 mm pads using a sharpened circular die. These pads were prepared by compression molding the sample between heated platens of a press for approximately 20 minutes at a temperature and force equal to 120[degrees]C and 6,800 kg, respectively.

Prior to loading a specimen between the fixtures of the rheometer, the gap between the test fixtures was zeroed after the fixtures had been heated to the desired test temperature for approximately 20 minutes. Alter loading the specimen, the upper fixture of the rheometer was brought down until it sufficiently sandwiched the specimen between the plates. The oven was then closed and the specimen was heated for about 15 minutes to the test temperature. The edge of the specimen was then trimmed to match the diameter of the fixtures, followed by closing in the gap by an additional 100 [micro]m. The sample was allowed to sufficiently relax out any elastic stresses that were introduced during loading. The time required for sample relaxation varied from approximately one hour at 60[degrees]C to about 10 minutes at 180[degrees]C.

Small amplitude oscillatory shear testing was performed on each sample at temperatures of 60, 90, 120, 150 and 180[degrees]C. For most tests, a frequency range of 0.1-100 rad/s was applied. The test strain was varied for each sample. Optimal linear viscoelastic test strain was determined for each sample by performing a strain sweep on a separate specimen at a frequency of 10 rad/s. Overall, the applied strains varied from 0.05-3%, in which the smaller values were applied to specimens at lower temperatures and higher particulate loadings. Final master curves and activation energies for each filled and unfilled sample were determined via time-temperature superposition, which was performed utilizing the IRIS software package.

Bound rubber

The procedure for bound rubber determination has been described previously (ref. 10). Bound rubber is the amount of rubber unextractable from the unvulcanized polymer-filler blend after immersion in a solvent such as cyclohexane at room temperature for one week. Of course, before the bound rubber measurements, one wants to make sure that the rubber itself is totally soluble in the solvent. Bound rubber ([R.sub.B]) was then calculated according to the following equation:

[R.sub.B] = [(wt, of specimen after immersion - wt. of filler in specimen) x 100 %1 / (wt. of polymer in specimen)

A stainless steel thimble was used to contain the polymer/filler blend for solvent extraction. Bound rubbers at high temperatures were also measured by extracting the polymer-filler blend in the steel thimble using cyclohexane reflux (80[degrees]C) and toluene reflux (110[degrees]C) for 48 hours.

Phase and straight mixings

To evaluate the effects of polymer-filler interactions on filler phase partition in BIMS blends and compounds, CB filled BIMS (1.2 mol. % Br, 7.5 wt. % PMS, 45 ML) blends were prepared by both phase and straight mixings in an internal mixer with a temperature variation ranging from 70-150[degrees]C. A four-pass phase mixing was applied to mix BIMS binary blends (50/50 w/w) with BR (98% cis), NR (SMR 20) and four solution SBRs with 75 phr of N234 CB. The four solution SBRs used were 12% styrene, 15% styrene, 20% styrene and 25% styrene. Thirty phr of processing oil was also added to all blends for improved mixing homogeneity. The four-pass phase mixing involves mixing half of the fillers with BIMS and half of the fillers with a given diene rubber first to prepare two masterbatches from an internal mixer. These two hatches were then mixed together, followed with the addition of curative. Hence, four passes were required. Since the fillers are pre-partitioned between the two polymer phases prior to their mixing together, the four-pass phase mixing could provide information about filler transfer/migration resulting from differences in polymer-filler interactions between blend components. Detailed procedures for preparation of these blends could be found elsewhere (ref. 11 ). Straight mixing of BIMS (0.85 mol. % Br, 12 wt. % PMS) with BR with varying blend ratios were prepared using an internal mixer. Fifty phr of N330 CB was added 30 seconds after the polymers were mixed. No processing oil was applied. Blend ratios prepared were 20/80, 40/60, 50/50, 60/40 and 80/20 in BIMS-BR. Curatives used were zinc oxide and stearic acid for BIMS and sulfur and MBTS (sulfur accelerator, 2-benzothiazyl disulfide) for BR.

Filler phase partition

Tapping phase AFM, instead of TEM, was applied to evaluate filler phase distribution in BIMS blends prepared by phase and straight mixings. As shown in figure 1, filler over-count results from TEM due to the 3D to 2D projection nature of the transmission imaging of TEM. Since filler size is smaller than the thickness of cryo-sections used in TEM, fillers buried beneath the surface would appear in the final micrograph for their strong electron scattering. In addition, electron density contrast in BIMS blends in TEM arises from the removal of BIMS under the electron beam through chain scissions. However. the presence of fillers and their interactions with BIMS affect the chain scission kinetics of BIMS. Gray domains (due to the residual BIMS) appear around the filler agglomerates in a CB-filled BIMS, as shown in figure 2. These gray domains could be mistakenly assigned to diene rubber domains in TEM micrographs obtained from CB-filled BIMS blends.

[FIGURES 1-2 OMITTED]

Tapping phase AFM micrographs were processed to separate fillers and polymer phases for the determination of filler phase distributions in BIMS blends. Holes within the polymer phase after filler removal were back-filled. Polymer area percent was measured to ensure correct procedures followed in removing fillers and back-filling holes. Both polymer phase sizes and filler phase distribution were obtained.

Results and discussion

Filler percolation threshold

G', G" and tan [delta] master curves for the linear viscoelastic response of IR-N234 composites by varying CB loading are shown in figures 3-5. Overall, the viscoelastic responses for all CB concentrations obey the principle of time-temperature superposition. Not surprisingly, G' and G" increase monotonically with carbon black loading at all frequencies, as expected for filler reinforced systems (refs. 12-14). A loss in molecular weight was observed in CB-filled IR samples after internal mixing and in neat IR after Brabender mixing. The GPC molecular weights of the original IR are: [M.sub.n] = 371,000, [M.sub.w] = 1 965.000, [M.sub.z] = 4,626,000 and [M.sub.p] = 1,487,000, whereas those of the Brabender-masticated IR are: [M.sub.n] = 281,000, [M.sub.w] = 1 098,000, [M.sub.z] = 2,207,000 and [M.sub.p] = 1,068,000. The loss in molecular weight in CB-filled IR during internal mixing could be worse than that mixed in a Brabender mixer because of the higher residence time and mixing inhomogeneity present in an internal mixer. In addition, both degradation and crosslinking could occur in CB-filled IR during mixing and the extent of these could depend on CB filler content.

[FIGURES 3-5 OMITTED]

As shown in figure 3, the progression towards the pseudo solid like behavior is apparent as the power law dependence of G' at low frequencies decreases with increasing amounts of filler in IR. Also, a peak in tan [delta] is observed at low frequencies for composites with CB loading at ~11 vol. % (figure 5). The near independence of G' with frequency at low frequencies and the presence of a peak in tan [delta] at low frequencies (from the increasing dominance of G' over G") suggest that the composites with carbon black at 11 vol. % and higher display pseudo-solid behavior. Hence, considering the uncertainties regarding the IR polymer degradation and crosslinking during mixing and testing, a percolation threshold of 11 vol. % is suggested for CB in IR based on rheological data. However, based on the change in inter-particle distance as measured from the tapping phase AFM images of these CB-filled IR samples (figure 6), no percolation threshold could be identified up to 15 vol. % of CB in IR. This difference in filler percolation threshold result between the rheological measurements and morphological evaluations may arise from the change in IR polymer state during mixing. Since the tapping phase AFM only examines the distance between particles without considering the state of the polymer matrix, it may be a better measure of the percolation threshold in this case. It is known that rheological measurements are sensitive to polymer degradation. However, it is suggested that IR may have strong interactions with CB due to the small near-neighbor distance in between the CB aggregates in IR-CB composites based on the AFM results.

[FIGURES 6 OMITTED]

Although inconsistent percolation threshold values were obtained for CB in IR from rheology and from morphology, a good agreement was achieved between them for determining the CB filler percolation threshold in IMS (unbrominated BIMS). IMS does not degrade or crosslink during mixing and rheological testing. G', G" and tan [delta] master curves for the linear viscoelastic response of the IMS-N234 composites by varying CB loading are shown in figures 7-9. Again, viscoelastic responses for all CB-filled IMS samples obeyed the principle of time-temperature superposition. The progression towards the pseudo solid like behavior is apparent as the power law dependence of G' at low frequencies decreases with increasing amounts of filler, until it is almost frequency-independent beyond a loading of about 15 vol. % (figure 7). Also, a peak in tan [delta] is observed at low frequencies for composites with CB loading in excess of 15 vol. %. The near independence of G' with frequency at low frequencies and the presence of a peak in tan [delta] at low frequencies (from the increasing dominance of G' over G") suggest that the composites with more than 15 vol. % CB display pseudo-solid behavior.

[FIGURES 7-9 OMITTED]

The phase contrast image for each IMS-N234 composite was obtained from tapping-mode AFM. In these images, the nearest neighbor distance for each CB particle was calculated from the particle center locations, determined by digitizing the thresholded AFM images. The average of these values for each 10 x 10 [micro]m AFM image was calculated and shown in figure 10. The nearest neighbor center-to-center distance decreases sharply beyond 15 vol. % and is fairly constant above this sharp decrease. This critical volume fraction for percolation, in the neighborhood of 15 vol. %, agrees closely with the result from the rheological measurements.

[FIGURE 10 OMITTED]

From a previous work (ref. 6) and as noted above, the filler percolation threshold of N234 CB in BIMS and IMS rubbers has the following relationship: BIMS < IMS, where the percolation threshold of N234 CB in IR is not clear. A lower threshold implies a stronger polymer-filler interaction, which leads to particles with increased effective radius due to the immobilized chains surrounding the particle (figure 11). Therefore, polymer-filler interaction ranking based on percolation threshold is: BIMS > IMS.

[FIGURE 11 OMITTED]

Bound rubber

Bound rubber results of BIMS and IMS elastomers filled with N234 are in table 1. It is clear from the table that a CB-filled BIMS elastomer has not only higher, but also more temperature-resistant bound rubber than that of a CB-filled IMS. This is in agreement with the finding based oil filler percolation threshold values measured from rheology and morphology. Since there are no bromine groups present in IMS, the strong interactions between BIMS and CB could be attributed to the chemical interactions between the benzylic bromine in BIMS and the surface functional groups of CB particles.

The crosslinking and degradation experienced by diene rubbers at mixing temperatures could complicate the bound rubber analyses of CB-filled diene rubbers. Since crosslinking of diene rubbers during mixing could render them insoluble in the solvent, bound rubber measurement results could be compromised in diene rubbers that were crosslinked in mixing. Hence, it is difficult to analyze bound rubber results in CB-filled diene rubbers without quantitative characterization of rubber crosslinking or degradation during mixing. As an example, for BR loaded with 15 vol. % N234, bound rubber increases with increasing extraction temperature, suggesting the occurrence of crosslinking in BR (table 1). However, bound rubber measurements at various extraction temperatures for saturated polymers, such as BIMS and IMS, could still provide a relative measurement of the temperature stability of rubber-filler interactions. As shown in table 1, bound rubber contents remain relatively unchanged or drop only slightly with temperature in CB-filled BIMS, whereas those decrease with increasing temperature in CB-filled IR. This suggests that during mixing, where the temperature rises from 80 to 150[degrees]C due to viscous heating, BIMS could hold onto CB fillers more than that of IR, regardless of whether BIMS interacts more or less strongly with CB fillers at room temperature. The resistance of polymer-filler attachments to temperature can also be measured by the absolute value of the temperature coefficient of bound rubber, [absolute value of d[R.sub.B]/dT], as shown in table 1, where T is the temperature and [absolute value of R] is the correlation coefficient for linear regression of [R.sub.B] versus T. Values of [absolute value of d[R.sub.B]/dT] of IR-CB are about twice those of BIMS-CB. In other words, about twice the amount of bound rubber will be detached from the CB filler for IR versus BIMS with a unit increase in extraction temperature. Of course, it is understood that the more rapid drop in [R.sub.B] for the IR-CB composites with increasing extraction temperature could be due to the scission of the dangling chains of IR attached to the CB filler. Due to these experimental difficulties in assessing BIMS-CB interactions with reference to BR-CB and IR-CB interactions, phase and straight mixings of CB-filled BIMS blended with BR or IR were performed, as discussed in the next section. AFM and image processing were employed subsequently to quantify filler phase distributions in these blends for a practical ranking of polymer-filler interactions.

BIMS-CB interactions

The ability of BIMS to hold onto fillers with increasing temperature during mixing may explain the results shown in table 2, in which CB migrates into the BIMS phase in all six blends that were prepared by the four-pass phase mixing. Greater than 80% of CB ends up residing in the BIMS phase, despite the original 50/50 split of fillers between BIMS and a diene rubber. This suggests that the partition coefficient of CB between BIMS and a diene rubber (only for the diene rubbers examined in this study) is 80/20, with the assumption that an equilibrium is reached in phase mixing. Similarly, more CB fillers were incorporated into the BIMS phase in straight mixing (table 3 and figure 12). The dotted line in the graph of % filler in BIMS versus phr BIMS in figure 12 represents the line of equal CB partition in BIMS and BR phases. All the data points from table 3 are above this dotted line. This suggests preferential filler partition in the BIMS phase at various BIMS/BR blend ratios, even though the area coverage of CB in the BIMS phase, calculated by dividing the measured CB % in BIMS by BIMS % in the blend, decreases with increasing BIMS content. This is not surprising because each compound contains a constant amount of CB. A higher BIMS content will result in a lower share of CB per unit wt. of BIMS.

[FIGURE 12 OMITTED]

Conclusions

This work employed rheology, bound rubber (solvent extraction) and AFM to identify, differentiate and scale polymer-filler interactions in BIMS-CB, IMS-CB, BR-CB and IR-CB composites. Both rheology and AFM imaging measurements show that the percolation threshold of BIMS-CB is about half that of IMS-CB. A lower threshold implies a stronger polymer-filler interaction, which leads to particles with increased effective radius due to the immobilized chains surrounding the particle. Since there are no bromine groups present in IMS, the strong interactions between BIMS and CB could be attributed to the chemical interactions between the benzylic bromine in BIMS and the surface functional groups of CB particles. The presence of crosslinking or degradation of BR and IR at high temperatures during their mixing with CB and/or during bound rubber measurements compromises characterization of BR-CB and IR-CB interactions. Phase and straight mixing methods were applied to prepare CB-filled blends of BIMS with various diene rubbers. Preferential filler partition into the BIMS phase was observed in these blends. Based on the temperature dependent results of the bound rubber measurement, this preferential partition is attributed to the strong and relative temperature insensitive BIMS-CB interactions versus those for CB and diene rubbers.

Table 1--bound rubbers of N234 filled BIMS, IMS,
IR and BR

CB vol. %    [R.sub.B],     [R.sub.B],     [R.sub.B],
            25[degrees]C   80[degrees]C   120[degrees]C

BIMS
  15             28             26             23
  20             36             34             30
IMS
  15             10            2.5            0.63
  20            8.9            4.1             3.4
IR
  15             35             29             23
  20             45             40             32
BR
  15             21             21             25

CB vol. %    Id[R.sub.B]/dTI,     IRI
            [degrees][C.sup.-1]

BIMS
  15               0.05           0.98
  20               0.06           0.96
IMS
  15               0.10           0.97
  20               0.06           0.95
IR
  15               0.12           0.99
  20               0.13           0.97
BR
  15

Table 2--carbon black partitioning in BIMS after
four-pass phase mixing (each compound has
75 phr CB and 30 phr oil)

High diene rubber   Measured CB % in BIMS

BR                          89.2
sSBR, 12% styrene           87.7
sSBR, 15% styrene           85.5
sSBR, 20% styrene           91.7
sSBR, 25% styrene           77.8
NR                          89.5

Table 3--carbon black partitioning in BIMS after
straight mixing with BR (each compound has
50 phr CB)

Blend ratio   Measured CB    Area coverage
 (BIMS/BR)     % in BIMS    of CB in BIMS *

   20/80         56.9             2.8
   40/60         63.3             1.6
   50/50         72.7             1.5
   60/40         81.3             1.4
   80/20         88.1             1.1

* (Measured CB % in BIMS)/(BIMS % in blend)

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