Molecular Weight Dependence of the Glass Transition Temperature (Tg)-Confinement Effect in Well-Dispersed Poly(2-vinyl pyridine)–Silica Nanocomposites: Comparison of Interfacial Layer Tg and Matrix Tg

Nanocomposites (NCs) of poly(2-vinyl pyridine) (P2VP) and nanosilica were prepared by spin-coating and characterized by fluorescence to assess the alterations in matrix glass transition temperature (Tg,ₘₐₜᵣᵢₓ) and interfacial layer Tg (Tg,ᵢₙₜₑᵣ) as functions of P2VP molecular weight (MW) and silica content. Relative to neat P2VP Tg (Tg,ₙₑₐₜ), major increases in Tg,ₘₐₜᵣᵢₓ were observed with increasing silica content in low-MW, 2.1 kg/mol P2VP NCs; Tg,ₘₐₜᵣᵢₓ – Tg,ₙₑₐₜ = 15 and 35 °C at 1.0 and 10 vol % silica, respectively. High-MW, 100 kg/mol P2VP NCs exhibited much smaller increases in Tg,ₘₐₜᵣᵢₓ. To understand better the Tg,ₘₐₜᵣᵢₓ enhancements, trace levels of dye were labeled to silica to characterize the near-interface (≤4 nm from the surface) Tg (Tg,ᵢₙₜₑᵣ). Profound increases in Tg,ᵢₙₜₑᵣ were observed in low-MW P2VP NCs: Tg,ᵢₙₜₑᵣ – Tg,ₙₑₐₜ = 25 and 40 °C at 1.0 and 10 vol % silica, respectively. In contrast, in high-MW P2VP NCs, Tg,ᵢₙₜₑᵣ – Tg,ₙₑₐₜ = 8 and 16 °C at 1.0 and 10 vol % silica, respectively. Because interfacial Tg perturbations propagate tens of nanometers into a matrix, the larger increases in Tg,ᵢₙₜₑᵣ in low-MW P2VP and at increasing silica content lead to larger increases in Tg,ₘₐₜᵣᵢₓ. At a given silica content, the greater alteration in Tg,ᵢₙₜₑᵣ in low-MW P2VP NCs is explained by differences in interfacial conformations of low- and high-MW P2VPs and greater interfacial hydrogen bonding in low-MW P2VP. The stronger attractive interfacial interactions in low-MW P2VP NCs lead to greater alterations in Tg,ᵢₙₜₑᵣ, which result in greater changes in Tg,ₘₐₜᵣᵢₓ. In contrast, the increase in Tg,ᵢₙₜₑᵣ with increasing filler is due to Tg,ᵢₙₜₑᵣ perturbations propagating into the matrix. At sufficient silica loading, perturbations to Tg from one interface modify Tgs at other interfaces. At a constant silica content in the range of 3.0–10 vol %, spatial Tg gradients in the NCs and near-interface gradients have little MW dependence. At 0.5 and 1.0 vol % silica, our results suggest that near-interface gradients are larger in low-MW P2VP NCs.