The Filler Network Index: A Viscosity‐Independent Rheological Criterion for Quantifying Silica Architecture in Unvulcanized Rubber Compounds

The processability of unvulcanized silica‐filled SBR/BR compounds is severely hindered by rigid filler–filler networks. Unmodified and TESPT‐pregrafted silica compounds (15, 30, 45 phr) were compared, with SEM/EDX confirming agglomerate elimination by pregrafting. Direct rheological interpretation, however, remains paradoxical: silane‐induced viscosity reduction accelerates relaxation across all timescales, masking the restriction expected from enhanced polymer–filler interactions. A constrained five‐element Weichert model deconvolved stress relaxation data into discrete spectra. Short relaxation times (), assigned to filler–filler network rupture via the Kraus framework, were larger in unmodified compounds, reflecting the yield resistance of the percolated network. Long relaxation times (), governed by chain desorption and fractal confinement per the Maier‐Göritz and Witten–Rubinstein–Colby frameworks, were proportionally far larger in the unmodified series, reflecting severe chain trapping within fractal clusters. The filler network index (FNI = /), a novel parameter exploiting the scalar invariance of this ratio under bulk viscosity shifts, isolated the structural fingerprint of the filler network. The FNI was threefold lower in unmodified (3.50–5.01 × 10 −4 ) than silanized compounds (9.39–13.4 × 10 −4 ), quantitatively distinguishing agglomerate‐dominated from polymer–filler interaction‐governed microstructures. Postvulcanization relaxation confirmed a fundamental viscoelastic inversion, validating TESPT's dual functionality as a processability aid and reinforcement anchor.