Dispersion of modified fumed silica in elastomeric nanocomposites

Abstract

In polymer nanocomposites, surface modification of silica aggregates can shield Coulombic interactions that inhibit agglomeration and formation of a network of agglomerates. Surface modification is usually achieved with silane coupling agents although carbon-coating during pyrolytic silica production is also possible. Pyrogenic silica with varying surface carbon contents were dispersed in styrene-butadiene (SBR) rubber to explore the impact on hierarchical dispersion, the emergence of meso-scale structures, and the rheological response. Pristine pyrogenic silica aggregates at concentrations above a critical value (related to the Debye screening length) display correlated meso-scale structures and poor filler network formation in rubber nanocomposites due to the presence of silanol groups on the surface. In the present study, flame synthesized silica with sufficient surface carbon monolayers can mitigate the charge repulsion thereby impacting network structural emergence. The impact of the surface carbon on the van der Waals enthalpic attraction, $a*$ , is determined. The van der Waals model for polymer nanocomposites is drawn through an analogy between thermal energy, $k_{B}T$ , and the accumulated strain, $\gamma$ . The rheological response of the emergent meso-scale structures depends on the surface density of both carbon and silanol groups.

Fig. 1.

(a) Silica aggregates with charge dispersion and specific interactions. (b) Clusters of aggregates that display mean field interactions between aggregates on the nanoscale. $\xi$ indicates the correlation length. Nanoscale Coulombic repulsion prevents formation or agglomerates of aggregates and the resulting agglomerate network formation at macroscopic scales required for improvement in performance.

Fig. 2.

Illustrations of the different chemical species on the surface of silica. (a) Isolated silanol groups on as-produced silica. (b) Hexamethyldisilazane treated silica. (c) Carbon/soot coated silica.

Fig. 3.

(left) Schematic of the flame synthesis setup used to generate carbon/soot coated silica particles with an exploded view of the actual flame wherein the pristine silica is coated with carbon downstream (yellow flame emission) Reprinted with minor changes from AIChE Journal, Vol 47, H.K. Kammler, R. Mueller, O. Senn, S.E. Pratsinis, Synthesis of silica-carbon particles in a turbulent H2-air flame aerosol reactor, Pages 1533–1543, Copyright © 2001 American Institute of Chemical Engineers (AIChE) with permission from John Wiley and Sons [43]; (right) Flame synthesized particles with varying surface carbon content used in this study. The surface carbon content determined via elemental analysis is mentioned on each fumed silica vial. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4.

(a) A cartoon of the modified fumed silica surface depicting the surface silanols and the coated carbon/graphitic monolayers. (b) Percentage of surface carbon content from elemental analysis compared with the carbon content from TGA measurements (listed in Table T1 in Appendix A in the Supplementary Information) for the modified fumed silica powders.

Fig. 5.

Log-log plot of the reduced scattered intensity, $I_0(q)/{\varphi }_0$, vs. $q$ (scattering vector) for the flame-synthesized silica nanofiller coated with 0.74 wt% of carbon in the SBR polymer matrix at a dilute concentration, ${\varphi }_0\approx 0.0043$. The inset shows a simulated aggregate structure [60] whose topology agrees with the aggregate topological parameters based on the Unified Fit [44,45], eq. (1). The reduced scattered intensity, $I_0(q)/{\varphi }_0$, vs. $q$ plots for the remaining silicas coated with 0 wt% (Aerosil 200), 0.37 wt%, 0.86 wt% and 2.03 wt% carbon shown in Figs. S2, S3, S4, and S5, respectively in Appendix B in the Supplementary Information.

Fig. 6.

(a) A comparison of the specific surface area of the flame-synthesized carbon-coated silica nanofillers from USAXS and BET gas adsorption. Note that the BET specific surface area for the commercial fumed silica grade was obtained as the average value of the range specified in the product specifications [64], whereas the values for the synthesized carbon-coated silica are listed in Table T1 in Appendix A in the Supplementary Information. The specific surface area from USAXS is larger since the X-rays can measure both open and closed pores. (b) A plot showing the dependence of the degree of aggregation on the silica nanofiller primary particle size. The plot indicates that as the primary particle size reduces, the degree of aggregation increases proportional to the specific surface area.

Fig. 7.

Log-log plot of the reduced scattered intensities, $I_O(q)/{\varphi }_O$ and $I(q)/\varphi$ (read from the left ordinate) and the inter-particle structure factor, $S(q)$ (read from the right ordinate) as a function of the scattering vector $q$ for pristine, (a) 0 wt%, and modified silica nanofillers with (b) 2.03 wt% surface carbon content in SBR. The plots for 0.37 wt%, 0.74 wt%, and 0.86 wt% surface carbons are shown in Appendix B in the Supplementary Information. Note that ${\varphi }_0$ and $\varphi$ represent the dilute and semi-dilute filler concentrations, respectively, as listed in the plots. For (a), a broad peak at intermediate $q$ in the $S(q)$ plots, indicates the emergence of correlated aggregates. For (b), an absence of a peak in the aggregate region at intermediate $q$ in the $S(q)$ plots indicates that the nano-aggregates overlap and are randomly distributed.

Fig. 8.

Plot of the particle interaction parameter, $a\mathrm{*}$, expressed in cm³/aggregate as a function of the linear sum of the surface carbon content $\left(N_{\mathrm{C}}\right)$ and surface hydroxyl content $\left(N_{\mathrm{O}\mathrm{H}}\right)$ weighted differently. $N_{\mathrm{C}}$ and $N_{\mathrm{O}\mathrm{H}}$ are reported in Table 1 $a\mathrm{*}$ is an attractive potential so negative values indicate relative repulsion between aggregates that increases with surface carbon content. That is, surface carbon enhances aggregate/polymer attraction relative to aggregate/aggregate attraction. The fit parameters, $A^{\prime },{K\mathrm{’}}_{\mathrm{C}}$, and ${K\mathrm{’}}_{\mathrm{O}\mathrm{H}}$ were obtained through least squares minimization.

Fig. 9.

$\mathrm{t}\mathrm{a}\mathrm{n}\delta$ for rolling resistance and wet grip as a function of the surface content of the hydroxyl groups ($N_{\mathrm{O}\mathrm{H}}$) and carbon coating ($N_{\mathrm{C}}$) weighted differently. The fit parameters, $A,K_{\mathrm{C}}$, and $K_{\mathrm{O}\mathrm{H}}$ were obtained through least squares minimization.