Melt crystallization/dewetting of ultrathin PEO films via carbon dioxide annealing: the effects of polymer adsorbed layers

Abstract

The effects of CO2 annealing on the melting and subsequent melt crystallization processes of spin-cast poly(ethylene oxide) (PEO) ultrathin films (20-100 nm in thickness) prepared on Si substrates were investigated. By using in situ neutron reflectivity, we found that all the PEO thin films show melting at a pressure as low as P = 2.9 MPa and at T = 48 °C which is below the bulk melting temperature (Tm). The films were then subjected to quick depressurization to atmospheric pressure, resulting in the non-equilibrium swollen state, and the melt crystallization (and/or dewetting) process was carried out in air via subsequent annealing at given temperatures below Tm. Detailed structural characterization using grazing incidence X-ray diffraction, atomic force microscopy, and polarized optical microscopy revealed two unique aspects of the CO2-treated PEO films: (i) a flat-on lamellar orientation, where the molecular chains stand normal to the film surface, is formed within the entire film regardless of the original film thickness and the annealing temperature; and (ii) the dewetting kinetics for the 20 nm thick film is much slower than that for the thicker films. The key to these phenomena is the formation of irreversibly adsorbed layers on the substrates during the CO2 annealing: the limited plasticization effect of CO2 at the polymer-substrate interface promotes polymer adsorption rather than melting. Here we explain the mechanisms of the melt crystallization and dewetting processes where the adsorbed layers play vital roles.

Stability of Adsorbed Polystyrene Nanolayers on Silicon Substrates

“…The choice of the two‐layer model was determined by the corresponding FT profile of the XR profile, and the details of the data analysis have been described elsewhere . The densification of the polymer chains in the vicinity of the substrate surface is in line with previous experimental and simulation findings . The presence of the two‐layer formation indicates that the irreversible adsorption of polymer chains on a solid surface is kinetically controlled: the later arriving polymer chains “reel‐in” the empty spaces of the flattened layer, forming the outer loosely adsorbed polymer chains on the solid surface.…”

“…Since irreversible polymer adsorption takes place even when a polymer–solid interaction is weak, the formation of an adsorbed layer at the interface is almost unavoidable when a thin polymer film is annealed at temperatures far above the bulk glass transition temperature ( T >> T g ). Recent studies have shown that the irreversible adsorption of polymer chains at the polymer–solid interface leads to significant deviations in the physical and mechanical properties of thin polymer films, including glass transition, thermal expansivity, local viscosity, interdiffusion, thermal stability, and crystallization processes, relative to their bulk counterparts. Moreover, although the adsorbed layer is typically much thinner than the unperturbed radius of polymer gyration ( R g ), the effect can propagate several tens of nanometers into the film interior …”

“…We further advanced the leaching conditions (type of a solvent, leaching time, leaching temperature, etc. ), allowing us to remove the outer loosely adsorbed chains preferentially and to uncover the inner flattened chains. This is possible by utilizing the large difference in desorption energy between the two layers which is proportional to the number of segment–surface contacts .…”

Crystallization of polymer chains confined in nanodomains

“…In fact, it is found from many studies on the crystallization behavior that the crystallinity of block chains confined in nanolamellae increases sigmoidally with a finite induction time, indicating that the crystallization is virtually driven by a combined mechanism of heterogeneous nucleation and subsequent crystal growth. Because a comprehensive understanding of the detailed crystallization mechanism of homopolymers confined in nanolamellae is directly applicable to the surface crystallization of homopolymers [88][89][90][91][92] or crystallization within thin films [93][94][95][96][97][98], it is essential to examine the crystallization of homopolymers (and also block chains) confined in various nanolamellae.…”

Cyclic Topology Enhancing Structural Ordering and Stability of Comb-Shaped Polypeptoid Thin Films against Melt-Induced Dewetting

“… 20 − 31 Depending on the polymer characteristics and thermal processing history, polymer chains that directly interact with the substrate surface via physisorption can possess different chain conformation, adsorbed amount, surface coverage, and density on the substrate. 20 , 21 , 24 , 26 − 28 , 30 , 32 Recent studies have revealed a strong correlation between autophobic dewetting of thin polymer films and the nanoarchitectures/conformations of adsorbed chains at the interface, where the conformational difference between the adsorbed chains and free chains plays a significant role. 25 , 29 − 31 Interestingly, by using the top-down solvent leaching method (also known as Guiselin’s approach), 33 it has been found that cyclic PS melt forms thicker irreversibly adsorbed layers on solid substrates than the linear PS melt, which can dramatically alter the interfacial effect on chain dynamics and dewetting behaviors of polymer thin films.…”