In polymer films supported by a substrate, it is established that the free surface enhances the nearby polymer dynamics while the substrate surface diminishes it. However, it remains elusive how the two effects may interact when the two surfaces are suffic iently close by. To address this question, we used time-of-flight secondary ion mass spectrometry (Tof-SIMS) and contact angle (CA) measurements to measure the surface glass transition temperature ( T_g^surf) and width (4w) within ~1 nm of the free surface of polystyrene (PS) supported by different substrates with varying PS thicknesses (h).

For PS supported by silica (PS-SiOx) or polydimethylsiloxane (PS-PDMS), 4w broadened noticeably when h was decreased below a characteristic thickness (h₀) equaling the polymer radius of gyration (R...[ Read more ]

In polymer films supported by a substrate, it is established that the free surface enhances the nearby polymer dynamics while the substrate surface diminishes it. However, it remains elusive how the two effects may interact when the two surfaces are suffic iently close by. To address this question, we used time-of-flight secondary ion mass spectrometry (Tof-SIMS) and contact angle (CA) measurements to measure the surface glass transition temperature ( T_g^surf) and width (4w) within ~1 nm of the free surface of polystyrene (PS) supported by different substrates with varying PS thicknesses (h).

For PS supported by silica (PS-SiOx) or polydimethylsiloxane (PS-PDMS), 4w broadened noticeably when h was decreased below a characteristic thickness (h₀) equaling the polymer radius of gyration (R_g); however, T_g^surf decreased with decreasing h in approximately the same manner for all R_g below 100 nm. This result suggests that the connectivity between the free and substrate surfaces made by individual polymer chains promotes heterogeneity of the surface dynamics but affects the T_g^surf mildly.

For PS supported by a PS brush or adsorbed PS, T_g^surf was a constant, equal to that of the 100 nm PS-SiOx, while 4w no longer broadened when h < R_g. Because PS chains can slide on SiOx or PDMS but not on the other surfaces, we tentatively interpret the broadening (4w(h)) found in PS-SiOx and PS-PDMS to be caused by (chain) sliding-facilitated mobility transfer discussed by de Gennes. To explore if the different T_g^surf(h) found in different films were caused by the different polymer-substrate interfaces involved, we pre-annealed the films under different conditions prior to the T_g^surf measurement, which is known to diminish excessive free volume holes (FVHs), if present, at the polymer-substrate interface. The result of a comprehensive study supports the existence of such FVHs and their contribution to the observed T_g^surf(h).