Cross-linked latex particles grafted with polyisoprene as model rubber-compatible fillers

Model filler particles were obtained by grafting polyisoprene (PIP) chains onto spherical latex particles of polystyrene cross-linked with 12 mol% divinylbenzene. These particles, with a narrow size distribution and a diameter of ca. 400 nm, were synthesized by emulsifier-free starved-feed emulsion polymerization. Acetyl coupling sites were introduced randomly at either low (5 mol%) or high (30 mol%) target substitution levels on the latex particles by Friedel-Crafts acylation with acetyl chloride and AlCl₃ in nitrobenzene. 'Living' polyisoprenyllithium chains, generated from isoprene and sec-butyllithium (sec-BuLi), were then coupled with the acetylated particles. The PIP side chains had a high 1,4-polyisoprene microstructure content and a number-average molecular weight (M_n) of either 1.5 × 10³ (1.5 K), 5 × 10³ (5 K), or 3 × 10⁴ (30 K). The PIP content of the grafted particles was determined from the yield of isolated particles and by ¹H NMR spectroscopy analysis. The grafted latex particles were blended in solution with linear polyisoprene (M_n = 3.95 × 10⁵, 395 K). The influence of the filler-matrix interactions on the rheological behavior of the blends was determined by dynamic mechanical analysis for the different filler blends. Increases in complex viscosity and storage modulus, and decreased damping factors were observed in all cases relatively to the pure matrix polymer. The enhancements, decreasing in the order 30 mol% > 5 mol% acetylation, and with the grafted PIP chain length as 30 K > 5 K ≈ 1.5 K, are deemed to reflect the extent of interactions between the filler particles and the polymer matrix.