Substrate Effects on Glass Transition and Free Surface Viscoelasticity of Ultrathin Polystyrene Films

Abstract

We describe results from experiments in which spontaneous embedment of 20 nm silica particles was used to probe under-layer effects on the free surface dynamics and the glass transition temperature (T g ) of polystyrene. Both 13 and 20 nm thick polystyrene (PS) films were prepared and placed on different under-layer substrates, which in turn were supported on a silicon wafer (Si) substrate. The under-layer substrates used were PS, poly(2-vinylpyridine) (P2VP), and poly(methyl methacrylate) (PMMA) with thicknesses ranging from 13 to 350 nm. The particle height change during the embedment was monitored using an atomic force microscope. For the PS films supported on the PS and P2VP under-layers, experimental temperatures varied from T g − 10 K to T g + 5 K. In the case of the PMMA under-layer, experimental temperatures varied from T g − 10 K to T g + 10 K. The Hutcheson and McKenna model [Phys. Rev. Lett. 2005, 94 (7)] was applied to the particle embedment depth to obtain the surface rheological temperatures [Eur. Phys. J. E 2007, 22 (4), 281−286] and the T g . It is found that the dynamics of the top-layer PS films were faster than the bulk material below the macroscopic T g and slower above it for all under-layers considered. The T g of both the 20 and 13 nm top-layer PS films were found to be essentially independent of under-layer thickness and reduced by less than 7 °C. Upon replacing PS under-layers with the same thickness of P2VP and PMMA as the under-layers, the T g of the 20 nm PS top-layer films changed by less than 5 K.

Glass Transition Phenomenon for Conjugated Polymers

“…The T g of thin films, defined by a simple nanoconfined physical model, has been characterized by different types of measurements, including dynamic, thermodynamic, and pseudothermodynamic (see subsequently for the definitions). In addition, it can also be obtained by investigating the thin film rheological behavior …”

“…In addition, it can also be obtained by investigating the thin film rheological behavior. [101][102][103][104][105][106][107][108][109][110][111] In general, there are two main factors that contribute to the nanoconfinement of free-standing films, the finite-size effect of a polymer chain and the interface or free surface effect. The finite size effect is related to the diminishing degree of entanglements as a polymer film approaches a thickness of similar size scale to the polymer chain end-to-end distance.…”

Local Glass Transition Temperature Tg(z) Profile in Polystyrene next to Polybutadiene with and without Plasticization Effects

“…Over two decades of study on such so-called confined systems have demonstrated that interfaces perturb the local properties not only at the interface, but that the disturbance propagates into the material away from the interface for some distance. [8][9][10][17][18][19][20][21][22][23][24][25] Different types of material properties, glass transition temperature (T g ), [17,[26][27][28] modulus, [13,[29][30][31][32] viscous flow, [33][34][35][36] and physical aging, [18,19,37] have all been reported to change near interfaces and in thin films. Additives distance z ≈ 225-250 nm before bulk PS T g is recovered, while rubbery PS next to glassy polysulfone (PSF) or poly(methyl methacrylate) (PMMA), or rubbery PnBMA next to glassy PS, requires z ≈ 100-125 nm before T g bulk is recovered.…”

In Situ Probing the Relaxation Properties of Ultrathin Polystyrene Films by Using Electric Force Microscopy

“…Decreasing the thickness of thin films below 10 nm, the physical and chemical properties of ultrathin films are significantly different in comparison with thin films due to the thickness confinement effect [13–15]. To explore such a thickness-dependent effect, a great deal of effort has been devoted to investigating the molecular mobility and relaxation dynamics within ultrathin polymer films [14, 16–18]. Previous studies have revealed that the surface relaxation dynamics in ultrathin films were apparently different from that in bulk materials and normal thin films [19, 20].…”