Grafting parameters and surface free energy components of photosensitive poly(methacrylated spiropyran) brushes

Abstract

Poly(methacrylated spiropyran) (P(MMASP)) brushes on silicon wafers were synthesized using the interface-mediated RAFT polymerization of methacrylated spiropyran (MMASP) monomers to determine their grafting density and surface free energy changes during photoinitiation. The grafting parameters of P(MMASP) layers such as the grafting density (σ), the radius of gyration (R_g) and average distance between polymer chains (D) were calculated. It is found that 1.25 P(MMASP) chains covered per nm² area of wafers, and the distance between chains was 1.0 nm which is smaller than doubled R_g, 1.1 nm. Thus, P(MMASP) chains overlapped each other and forced themselves to stretch away from the interface. We evaluated the surface free energy components of P(MMASP) brushes during photo initiation. Van Oss-Good method was used to calculate the surface free energy components of brushes. It is found that spiropyran (SP)-merocyanine (MC) system significantly influenced the surface energy components of P(MMASP) brushes. The electron acceptor and electron donor surface free energies of SP and MC form of P(MMASP) brushes were found as ${\text{ γ}}_{\text{s }}^{+}=0.47\mathrm{ }\text{mJ}\mathrm{ }{\mathrm{m}}^{-2},{\mathrm{\gamma }}_{\text{s }}^{-}=1.15\mathrm{ }\text{mJ}\mathrm{ }{\mathrm{m}}^{-2}$ and ${\text{ γ}}_{\text{s }}^{+}=0.85\mathrm{ }\text{mJ}\mathrm{ }{\mathrm{m}}^{-2}\mathrm{ }{\mathrm{\gamma }}_{\text{s }}^{-}=10.25\mathrm{ }\text{mJ}\mathrm{ }{\mathrm{m}}^{-2},$ respectively. It is concluded that MC form of P(MMASP) brushes possess high base character; whereas, SP forms have low base character.

Graphical Abstract

Keywords:

• Spiropyran
• surface free energy
• polymer brush
• smart materials
• surface characterization