Suppression of the Fragility-Confinement Effect via Low Molecular Weight Cyclic or Ring Polymer Topology

Abstract

We used differential scanning calorimetry and spectroscopic ellipsometry to measure the molecular weight (MW) dependence of bulk fragility (m bulk) and spectroscopic ellipsometry to measure the thickness dependences of the glass transition temperature (T g) and fragility (m) in supported thin films of low MW cyclic or ring polymer. The effects of confinement on T g and m of thin polymer films are important in a range of advanced technology applications, including nanoimprinting. It has previously been shown that nanoconfined films of high MW linear polystyrene (PS) exhibit major T g- and m-confinement effects whereas films of low MW cyclic PS (c-PS) show at most a very weak T g-confinement effect. In the absence of chain ends, c-PS exhibits very weak T g,bulk– and m bulk–MW dependences compared to linear PS. Despite low MW c-PS having m bulk values similar to that of high MW linear PS, we found that low MW c-PS films show a very weak m-confinement effect because of a weak free-surface effect; e.g., m for a 27 nm thick film of 3.4 kg/mol c-PS is the same as m bulk within error. Overall, these results support a strong correlation between the susceptibility of fragility perturbation and the susceptibility of T g perturbation caused by MW reduction, chain topology, and/or confinement.

Impact of bottlebrush chain architecture on Tg‐confinement and fragility‐confinement effects enabled by thermo‐cleavable bottlebrush polymers synthesized by radical coupling and atom transfer radical polymerization

“…These observations of the suppression of the fragility‐confinement effect associated with bottlebrush PS can also be explained by the constrained and aligned molecular configuration of bottlebrushes. A qualitatively similar but quantitatively stronger suppression of m ‐confinement effects was reported by Zhang et al [ 23 ] in low MW cyclic or ring PS; Lan and Torkelson [ 26 ] reported an elimination of m ‐confinement effects within error in dense PS brushes grafted from a substrate.…”

“…In comparison with bulk films of the same polymer, thin films of linear, freely deposited polymers that are a few tens of nanometers thick may exhibit significantly different properties, for example, glass transition temperature ( T g ), fragility, and stiffness among others. [ 1–48 ] For instance, in comparison with a bulk polystyrene (PS) film and as measured by ellipsometry, a 21‐nm‐thick, substrate‐supported high molecular weight (MW) PS film exhibits a T g reduction of 10°C. [ 18 ] Understanding such nanoscale T g ‐confinement effects in polymers is important for many advanced technological applications, for example, thin films used in nanolithography processes for microchip manufacturing.…”

“…Moreover, a reduction in T g with confinement has been observed for star or branched PS in other studies. [ 39,41 ] T g ‐confinement effects of thin polymer films with other chain architectures have been reported, for example, loose combs, [ 41 ] branched polymers, [ 39–42 ] dense brushes, [ 25,47 ] low MW cyclic chains or ring polymers, [ 22,23 ] and polymer networks, [ 24,46 ] among others. [ 47,48 ] Studies by Torkelson and coworkers [ 22–26 ] on films of several nonlinear or not‐freely deposited‐polymer systems have shown a one‐to‐one correspondence between the strengths of the m ‐confinement and T g ‐confinement effects.…”

“…[ 39,41 ] T g ‐confinement effects of thin polymer films with other chain architectures have been reported, for example, loose combs, [ 41 ] branched polymers, [ 39–42 ] dense brushes, [ 25,47 ] low MW cyclic chains or ring polymers, [ 22,23 ] and polymer networks, [ 24,46 ] among others. [ 47,48 ] Studies by Torkelson and coworkers [ 22–26 ] on films of several nonlinear or not‐freely deposited‐polymer systems have shown a one‐to‐one correspondence between the strengths of the m ‐confinement and T g ‐confinement effects. In comparison with the strengths of the corresponding effects for linear PS films, the strengths of both effects are greater for films of cross‐linked PS [ 24 ] (and greater yet with increasing crosslink density, [ 24 ] ) are reduced for films of low MW cyclic or ring PS, [ 22,23 ] and are negligible for dense PS brushes synthesized on flat substrates.…”

“…[ 47,48 ] Studies by Torkelson and coworkers [ 22–26 ] on films of several nonlinear or not‐freely deposited‐polymer systems have shown a one‐to‐one correspondence between the strengths of the m ‐confinement and T g ‐confinement effects. In comparison with the strengths of the corresponding effects for linear PS films, the strengths of both effects are greater for films of cross‐linked PS [ 24 ] (and greater yet with increasing crosslink density, [ 24 ] ) are reduced for films of low MW cyclic or ring PS, [ 22,23 ] and are negligible for dense PS brushes synthesized on flat substrates. [ 25,26 ] Notably, in their study on cross‐linked PS, Jin and Torkelson provided a comparison between cross‐linked polymers and their direct linear precursors.…”