Effect of tethering on the surface dynamics of a thin polymer melt layer

Enhanced Glass Transition Temperature of Thin Polystyrene Films Having an Underneath Cross-Linked Layer

Due to the importance of the interface in the segmental dynamics of supported macromolecule ultrathin films, the glass transition temperature (T_g) of polystyrene (PS) ultrathin films upon solid substrates modified with a cross-linked PS (CLPS) layer has been investigated. The results showed that the T_g of the thin PS films on a silica surface with a ∼5 nm cross-linked layer increased with reducing film thickness. Meanwhile, the increase in T_g of the thin PS films became more pronounced with increasing the cross-linking density of the layer. For example, a 20 nm thick PS film supported on CLPS with 1.8 kDa of cross-linking degree exhibited a ∼35 and ∼50 K increase in T_g compared to its bulk and that on neat SiO₂ substrate, respectively. Such a large T_g elevation for the ultrathin PS films was attributed to the interfacial aggregation states in which chains diffused through nanolevel voids formed in the cross-linked layer to the SiO₂-Si surface. In such a situation, the chains were topologically constrained in the cross-linked layer with less mobility. These results offer us the opportunity to tailor interfacial effects by changing the degree of cross-linking, which has great potential application in many polymer nanocomposites.

Suppressed Surface Reorganization in a High-Density Poly(methyl methacrylate) Brush

A high-density poly(methyl methacrylate) (PMMA) brush (σ = 0.77 chain/nm²) with a lower molecular weight distribution was prepared onto a silicon wafer by surface-initiated atom transfer radical polymerization. The surface of the PMMA brush chains was characterized upon the process of the environmental change, from air to water, using contact angle measurements in conjunction with sum-frequency generation spectroscopy. The surface structure and properties altered less with the changing environment from air to water for the PMMA brush than for a spin-coated film; that is, the extent of surface reorganization could be suppressed by grafting densely-packed chains onto a substrate. Also, the water penetration into the brush surface was inhibited because of the densely packed chain structure.

Role of Chain Connectivity across an Interface on the Dynamics of a Nanostructured Block Copolymer

Fluorescence labeling enables component- and location-specific measurements of the glass transition temperature (T_{g}) in complex polymer systems. Here we characterize the T_{g} of fluorescently labeled poly(methyl methacrylate) homopolymers (PMMA-py) blended at low concentrations into an unlabeled lamellar poly(n-butyl methacrylate-b-methyl methacrylate) diblock copolymer (PBMA-PMMA). In this system, the PMMA-py homopolymer is sequestered within the PMMA domains of the diblock copolymer and subject to soft confinement by the domains of the lower-T_{g} PBMA block, which lowers the homopolymer T_{g} by ∼5  K beyond the contribution of segmental mixing. In contrast to the PMMA block in the diblock copolymer, the PMMA-py homopolymer is not covalently bound to the interdomain interface. A comparison of T_{g} for the homopolymers in the blends to T_{g} for diblock copolymers with equivalent labeled segment density profiles reveals that the homopolymer's T_{g} is consistently ∼10  K higher than for diblock segments at the same location within the domain structure, highlighting the dominant contribution of a covalent bond across the interface to the perturbation of the chain dynamics in the block copolymer.

Optimizing the Grafting Density of Tethered Chains to Alter the Local Glass Transition Temperature of Polystyrene near Silica Substrates: The Advantage of Mushrooms over Brushes

We measured the local glass transition temperature T_g(z) of polystyrene (PS) as a function of distance z from a silica substrate with end-grafted chains using fluorescence, where competing effects from the free surface have been avoided to focus only on the influence of the tethered interface. The local T_g(z) increase next to the chain-grafted substrate is found to exhibit a maximum increase of 49 ± 2 K relative to bulk at an optimum grafting density that corresponds to the mushroom-to-brush transition regime. This perturbation to the local T_g(z) dynamics of the matrix is observed to persist out to a distance z ≈ 100-125 nm for this optimum grafting density before bulk T_g is recovered, a distance comparable to that previously observed by Baglay and Roth [J. Chem. Phys. 2017, 146, 203307] for PS next to the higher-T_g polymer polysulfone.

Altering surface fluctuations by blending tethered and untethered chains

"Partially tethering" a thin film of a polymer melt by covalently attaching to the substrate a fraction of the chains in an unentangled melt dramatically increases the relaxation time of the surface height fluctuations. This phenomenon is observed even when the film thickness, h, is 20 times the unperturbed chain radius, R_g,tethered, of the tethered chains, indicating that partial tethering is more influential than any physical attraction with the substrate. Furthermore, a partially tethered layer of a low average molecular weight of 5k showed much slower surface fluctuations than did a reference layer of pure untethered chains of much greater molecular weight (48k), so the partial tethering effect is stronger than the effects of entanglement and increase in glass transition temperature, T_g, with molecular weight. Partial tethering offers a means of tailoring these fluctuations which influence wetting, adhesion, and tribology of the surface.

Modifying Surface Fluctuations of Polymer Melt Films with Substrate Modification

Deposition of a plasma polymerized film on a silicon substrate substantially changes the fluctuations on the surface of a sufficiently thin melt polystyrene (PS) film atop the substrate. Surface fluctuation relaxation times measured with X-ray photon correlation spectroscopy (XPCS) for ca. 4R_g thick melt films of 131 kg/mol linear PS on hydrogen-passivated silicon (H-Si) and on a plasma polymer modified silicon wafer can both be described using a hydrodynamic continuum theory (HCT) that assumes the film is characterized throughout its depth by the bulk viscosity. However, when the film thickness is reduced to ∼3R_g, confinement effects are evident. The surface fluctuations are slower than predicted using the HCT, and the confinement effect for the PS on H-Si is larger than that for the PS on the plasma polymerized film. This deviation is due to a difference in the thicknesses of the strongly adsorbed layers at the substrate which are impacted by the substrate surface energy.