Glass transition temperature of single-chain polystyrene particles end-grafted to oxide-coated silicon.

A method based on the PeakForce QNM atomic force microscopic (AFM) adhesion measurement is employed to investigate the glassy dynamics of polystyrene (PS) single-chain particles end-grafted to SiO-Si substrates with different diameters, D, of 3.4 nm-8.8 nm and molar masses, M, of 8-123 kg/mol. As temperature was increased, the adhesion force, F, experienced by the AFM tip on pulling off the single chains after loading demonstrated a stepwise increase at an elevated temperature, which we identified to be T based on previous works. Our result shows that T of our grafted single chains increases with M in a manner consistent with the Fox-Flory equation, but the coefficient quantifying the M dependence of T is only (36 ± 6)% the value of bulk PS. In addition, the value of T in the M → ∞ limit is about 25 °C below the bulk T but more than 15 °C above that of (untethered) PS nanoparticles with D ≈ 100 nm suspended in a solution. Our findings are consistent with T of our single chains being dominated by simultaneous effects of the interfaces, which depress T, and end-grafting, which enhances T. The latter is believed to exert its influence on the glass transition dynamics by a mechanism reliant on chain connectivity and does not vary with chain length.