Glass Transition and Self-Diffusion of Unentangled Polymer Melts Nanoconfined by Different Interfaces

Abstract

In nanoconfined thin films, numerous studies have revealed the thickness dependencies of different thermophysical properties, including the glass transition temperature (T g) and self-diffusion coefficient (D). While quantitative relationships between these properties are well-known for bulk polymers, analogous relationships for nanoconfined polymers are still not clear. Herein, T g−D relationships are studied under nanoconfinement using spectroscopic ellipsometry for measuring T g and fluorescence recovery after photobleaching for measuring D. Poly­(isobutyl methacrylate) (PiBMA) was selected as a model unentangled polymer, and it was nanoconfined to 14–300 nm thick films. Multilayered geometries incorporating PiBMA were constructed to systematically study the influence of free surfaces (i.e., polymer surfaces exposed directly to air, also called uncapped) and surfaces that were in contact with a secondary polymer (also called capped). This multilayer approach additionally allowed investigation of both relatively weak and strong interactions between the polymer and substrate, depending on the existence of hydrogen bonding. The T g–D relationship observed in nanoconfined thin films deviated from that in the bulk state (e.g., as described by Williams–Landel–Ferry and Stokes–Einstein, or similar relationships). A model was employed that considered the effects of molecular friction between the different confining interfaces and PiBMA, and it successfully described the deviation from bulk behavior.

Thickness Dependence of Contact Angles in Multilayered Ultrathin Polymer Films

“…As film thickness decreases, a wide range of polymer material properties often deviate from the bulk. Examples include glass-transition temperature ( T g ), − density, , refractive index, , wetting/dewetting behavior, − and many others. , Among these polymer material properties, the wettability of ultrathin films is less explored, perhaps because of the difficulty of conducting a systematic study on homogeneous ultrathin polymer films, but no less important. The wetting behavior of ultrathin polymer films is critical knowledge for the design of tailored wettability in performance coatings from flexible electronics, water collection, antifouling, to self-healing …”

Decoupling Role of Film Thickness and Interfacial Effect on Polymer Thin Film Dynamics

“…Thin polymer films have been used in a wide variety of applications, such as nanoprinting, electronics, and sensors. Experiments and simulations show that molecular dynamics and associated physical properties, such as the glass transition temperature ( T g ), − physical aging, , diffusion, viscosity, , and crystallization, , all deviate from the corresponding bulk properties …”

Enhanced Dynamics of Confined Polymers near the Immiscible Polymer–Polymer Interface: Neutron Reflectivity Studies

“…In addition to polymer–solid and polymer–air interfaces, studies have recently focused on polymer structure and properties, such as T g and dynamics of nanoconfined polymers, near polymer–polymer interfaces. − The contribution of the region near the polymer–polymer interface in polymer blend systems is not ignorable if the domain size is sub-100 nm, and the interface predominantly influences the overall blend structure and properties. , In this respect, understanding the local dynamics is clearly necessary. We have previously investigated the diffusion dynamics of PMMA/ d PMMA confined between graphene oxide (GO) sheets (GO/PMMA/ d PMMA/GO) using the neutron reflectivity (NR) technique .…”