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Polymer–substrate interfaces are significant in determining the dynamics of nanoconfined polymers. In this study, an improved understanding of the interfacial effect was gained from the critical distance over which the suppression of interfacial dynamics originating from an interacting substrate propagates. We investigated the effective viscosity of poly­(styrene) (PS) thin films with various molecular weights (M w) on phenyl group-modified substrates and characterized the utmost distance (h c *) of the suppression of chain diffusional dynamics by the substrate. The viscosity of the PS films on the phenyl-modified substrates increased when the film thickness was decreased below a threshold thickness because of the dominant interfacial effect arising from favorable π–π polymer–substrate interactions. The extent of viscosity enhancement is apparently larger for PS with high M w on the same substrate. Most importantly, we found that the interface-induced viscosity increases for all samples with various M w values and substrates can be linearly correlated with the distance over which the suppression of interfacial dynamics extends, which makes it possible to qualitatively evaluate the effectiveness of the interfacial effect.

The red emission through upconversion (UC) upon 980 nm excitation based on Er3+/Yb3+ combination is very attractive for bioimaging applications. The intensity of the red emission is observed to be strongly dependent on the host materials. However, the origin of the behavior and the quantitative dependence remain unclear. Here, the effectiveness of the second step UC excitation from the Er3+ intermediate state 4I13/2 to the 4F9/2 level by energy transfer from Yb3+ is studied for three popular hosts (β-NaYF4, Ba5Gd8Zn4O21, and Y2O3) that have different phonon energies. Their emission efficiencies of the red emitting state are calculated, and the radiative lifetime of the 4F9/2 level in Ba5Gd8Zn4O21 is reported for the first time. We present a spectroscopic method to evaluate the relative energy transfer coefficients for the three hosts and find the coefficient increases markedly with the increase of phonon energy, reflecting the nature of phonon-assisted energy transfer. The coefficient for β-NaYF4 is 89 and 408 times smaller than that for the other two oxide hosts, well revealing the origin of the green emission governed UC in β-NaYF4 and the red emission in the other two. Accordingly, a comprehensive analysis of the luminescence dynamical processes shows that selecting material with appropriate phonon energy is essential for both effective excitation and efficient emission of the red level.

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