Effect of CO2 on a Mobility Gradient of Polymer Chains near an Impenetrable Solid

Production and Application of Polymer Foams Employing Supercritical Carbon Dioxide

Polymeric foams have characteristics that make them attractive for different applications. However, some foaming methods rely on chemicals that are not environmentally friendly. One of the possibilities to tackle the environmental issue is to utilize supercritical carbon dioxide ScCO2 since it is a “green” solvent, thus facilitating a sustainable method of producing foams. ScCO2 is nontoxic, chemically inert, and soluble in molten plastic. It can act as a plasticizer, decreasing the viscosity of polymers according to temperature and pressure. Most foam processes can benefit from ScCO2 since the methods rely on nucleation, growth, and expansion mechanisms. Process considerations such as pretreatment, temperature, pressure, pressure drop, and diffusion time are relevant parameters for foaming. Other variables such as additives, fillers, and chain extenders also play a role in the foaming process. This review highlights the morphology, performance, and features of the foam produced with ScCO2, considering relevant aspects of replacing or introducing a novel foam. Recent findings related to foaming assisted by ScCO2 and how processing parameters influence the foam product are addressed. In addition, we discuss possible applications where foams have significant benefits. This review shows the recent progress and possibilities of ScCO2 in processing polymer foams.

Free Space Makes the Polymer "Dead Layer" Alive

The effect of free space on molecular motion inside the polymer "dead layer" or adsorbed nanolayers on solid surfaces is investigated. Free space is introduced into the nanolayer by choosing a polymer with a relatively big side group, poly n-butyl methacrylate (PnBMA), and polarization-resolved single-molecule fluorescence microscopy is adopted as the method. The rotational motion of the doped fluorescent probes is found to be considerably excited at moderate temperatures, attributed to the free space brought by the side group of the PnBMA. The development of the adsorbed nanolayer by the prolonged annealing of the parent film is carefully monitored, together with the evolution of the molecular motion and the glass transition temperature (T_g). The T_g values of the exposed nanolayers are considerably lower than that of the bulk system, while they become higher than those in the bulk situation when the nanolayer is covered with a polymer top layer. The experimental evidence has demonstrated that the free space made available by the side group and the air-polymer interface has considerably promoted the molecular motion inside the adsorbed nanolayers, even under the situation of overwhelming surface attraction.

Polymer Nanocomposite Dielectrics: Understanding the Matrix/Particle Interface

Polymer nanocomposite dielectrics possess exceptional electric properties that are absent in the pristine dielectric polymers. The matrix/particle interface in polymer nanocomposite dielectrics is suggested to play decisive roles on the bulk material performance. Herein, we present a critical overview of recent research advances and important insights in understanding the matrix/particle interfacial characteristics in polymer nanocomposite dielectrics. The primary experimental strategies and state-of-the-art characterization techniques for resolving the local property-structure correlation of the matrix/particle interface are dissected in depth, with a focus on the characterization capabilities of each strategy or technique that other approaches cannot compete with. Limitations to each of the experimental strategy are evaluated as well. In the last section of this Review, we summarize and compare the three experimental strategies from multiple aspects and point out their advantages and disadvantages, critical issues, and possible experimental schemes to be established. Finally, the authors' personal viewpoints regarding the challenges of the existing experimental strategies are presented, and potential directions for the interface study are proposed for future research.

The role of the molecular weight of the adsorbed layer on a substrate in the suppressed dynamics of supported thin polystyrene films

The adsorbed layer on a solid surface plays a crucial role in the dynamics of nanoconfinement polymer materials. However, the influence of the adsorbed layer is complex, and clarifying this influence on the dynamics of confined polymers remains a major challenge. In this paper, SiO₂-Si substrates with various thicknesses and adsorbed layers of PS with various molecular weights were used to reveal the effect of the adsorbed layer on the corresponding segmental dynamics of the supported thin PS films. Strongly suppressed segmental dynamics of thin PS films were observed for the films supported on thicker adsorbed layers or prepared using higher molecular weight. Neutron reflectivity revealed that the overlap region thickness between the adsorbed layer and the top overlayer increased with increasing thickness and molecular weight of the adsorbed layer, both of which correlate well with the distance over which the polystyrene dynamics were depressed by the adsorbed layer. The results show that the influencing distance of the adsorbed layer is related to the overlap zone formed between the adsorption layer and the upper thin film. The effect of the adsorbed layer molecular weight can be ascribed to the fact that large loops and long tails in the adsorbed layer result in stronger interpenetrations and entanglements between polymer chains in the adsorbed layer and in the overlayer, causing a stronger substrate effect and suppression of the segment dynamics of the supported thin PS films.

Interphase in Polymer Nanocomposites

The lightweight and high-strength functional nanocomposites are important in many practical applications. Natural biomaterials with excellent mechanical properties provide inspiration for improving the performance of composite materials. Previous studies have usually focused on the bionic design of the material's microstructure, sometimes overlooking the importance of the interphase in the nanocomposite system. In this Perspective, we will focus on the construction and control of the interphase in confined space and the connection between the interphase and the macroscopic properties of the materials. We shall survey the current understanding of the critical size of the interphase and discuss the general rules of interphase formation. We hope to raise awareness of the interphase concept and encourage more experimental and simulation studies on this subject, with the aim of an optimal design and controllable preparation of polymer nanocomposite materials.

Solid-state polymer adsorption for surface modification: The role of molecular weight

HYPOTHESIS

Solid-state polymer adsorption offers a distinct approach for surface modification. These ultrathin, so-called Guiselin layers can easily be obtained by placing a polymer melt in contact with an interface, followed by a removal of the non-adsorbed layer with a good solvent. While the mechanism of formation has been well established for Guiselin layers, their stability, crucial from the perspective of materials applications, is not. The stability is a trade-off in the entropic penalty between cooperative detachment of the number of segments directly adsorbed on the substrate and consecutively pinned monomers.

EXPERIMENTS

Experimental model systems of Guiselin layers of polystyrene (PS) on silicon wafers with native oxide layer on top were employed. The stability of the adsorbed layers was studied as a function of PS molecular weight and polydispersibility by various microscopic and spectroscopic tools as well as quasi-static contact angle measurements.

FINDINGS

Adsorbed layers from low molecular weight PS were disrupted with typical spinodal decomposition patterns whereas high molecular weight (>500 kDa) PS resulted in stable, continuous layers. Moreover, we show that Guiselin layers offer an enticing way to modify a surface, as demonstrated by adsorbed PS that imparts a hydrophobic character to initially hydrophilic silicon wafers.

Glass transition temperature of single-chain polystyrene particles end-grafted to oxide-coated silicon

A method based on the PeakForce QNM atomic force microscopic (AFM) adhesion measurement is employed to investigate the glassy dynamics of polystyrene (PS) single-chain particles end-grafted to SiO₂-Si substrates with different diameters, D₀, of 3.4 nm-8.8 nm and molar masses, M_n, of 8-123 kg/mol. As temperature was increased, the adhesion force, F_ad, experienced by the AFM tip on pulling off the single chains after loading demonstrated a stepwise increase at an elevated temperature, which we identified to be T_g based on previous works. Our result shows that T_g of our grafted single chains increases with M_n in a manner consistent with the Fox-Flory equation, but the coefficient quantifying the M_n dependence of T_g is only (36 ± 6)% the value of bulk PS. In addition, the value of T_g in the M_n → ∞ limit is about 25 °C below the bulk T_g but more than 15 °C above that of (untethered) PS nanoparticles with D₀ ≈ 100 nm suspended in a solution. Our findings are consistent with T_g of our single chains being dominated by simultaneous effects of the interfaces, which depress T_g, and end-grafting, which enhances T_g. The latter is believed to exert its influence on the glass transition dynamics by a mechanism reliant on chain connectivity and does not vary with chain length.

Effect of Local Chain Conformation in Adsorbed Nanolayers on Confined Polymer Molecular Mobility

Interfaces play an important role in modifying the dynamics of polymers confined to the nanoscale. We demonstrate that the distance over which an interface suppresses molecular mobility in poly(styrene) thin films can be systematically increased by tens of nanometers by controlling the chain of conformation, i.e., the height of the loops in irreversibly adsorbed nanolayers. These effects arise from topological interaction between adsorbed and neighboring unadsorbed chains, respectively, which increase their motional coupling to facilitate the propagation of suppressed dynamics originating at the interface, thus highlighting the ability to manipulate interfacial effects by local conformation of chains in adsorbed nanolayers.

Self-Organization of Triblock Copolymer Melt Chains Physisorbed on Non-neutral Surfaces

We here report the self-organization process of poly(styrene-b-ethylene/butadiene-b-styrene) (SEBS) triblock copolymer chains physically adsorbed on a non-neutral surface. Spin-cast SEBS thin films were prepared on silicon (Si) substrates and then annealed at a high temperature far above the bulk glass transition temperatures of the two constituent blocks. To reveal the buried interfacial structure, we utilized solvent rinsing processes and a suite of surface-sensitive techniques including ellipsometry, X-ray reflectivity, atomic force microscopy, and grazing incidence small angle X-ray scattering. We revealed that the SEBS chains form two different chain structures on the substrate simultaneously: (i) "flattened chains" with the average height of 2.5 nm but without forming microdomain structures; (ii) "loosely adsorbed chains" with the average height of 11.0 nm and the formation of perpendicularly oriented cylindrical microdomains to the substrate surface. In addition, the kinetics to form the perpendicular-oriented cylinder was sluggish (∼200 h) and proceeded via multistep processes toward the equilibrium state. We also found that the lateral microdomain structures were distorted, and the characteristic lengths of the microdomains were slightly different from the bulk even after reaching "quasiequilibrium" state within the observed time window. Furthermore, we highlight the vital role of the adsorbed chains in the self-assembling process of the entire SEBS thin film: a long-range perturbation associated with the adsorbed chains propagates into the film interior, overwhelming the free surface effect associated with surface segregation of the lower surface tension of polystyrene blocks.

Competing polymer-substrate interactions mitigate random copolymer adsorption

Annealing a supported polymer film in the melt state, a common practice to relieve residual stresses and erase thermal history, can result in the development of an irreversibly adsorbed nanolayer. This layer of polymer chains physically adsorbed to the substrate interface has been shown to influence thin film properties such as viscosity and glass transition temperature. Its growth is attributed to many simultaneous interactions between individual monomer units and the substrate stabilizing chains against desorption. A better understanding of how these specific polymer-substrate interactions influence the growth of the adsorbed layer is needed, particularly given how strongly the properties of geometrically-confined polymeric systems are impacted by interfaces. Here, we use homopolymers and random copolymers of styrene and methyl methacrylate to form adsorbed layers and examine the influence of chemical composition and the resulting polymer-substrate interactions on adsorbed layer growth and structure. Ellipsometric measurements reveal a non-monotonic trend between composition and thickness of the adsorbed layers that is inconsistent with the behavior normally exhibited by random copolymers, being intermediate to their respective homopolymers. We examine this trend in terms of plateau thickness and growth kinetics at two different annealing temperatures and propose a mechanism for how different polymer-substrate interactions combine to influence adsorption when copolymer films are annealed. By introducing compositional heterogeneity, this mechanism extends the study of irreversible adsorption to complex chemistries and provides for a more general understanding of how annealing should be accounted for in the proper selection and processing of polymer thin films for applications in nanotechnology.

Structure-induced switching of interpolymer adhesion at a solid-polymer melt interface

Here we report a link between the interfacial structure and adhesive property of homopolymer chains physically adsorbed (i.e., via physisorption) onto solids. Polyethylene oxide (PEO) was used as a model and two different chain conformations of the adsorbed polymer were created on silicon substrates via the well-established Guiselin's approach: "flattened chains" which lie flat on the solid and are densely packed, and "loosely adsorbed polymer chains" which form bridges jointing up nearby empty sites on the solid surface and cover the flattened chains. We investigated the adhesion properties of the two different adsorbed chains using a custom-built adhesion testing device. Bilayers of a thick PEO overlayer on top of the flattened chains or loosely adsorbed chains were subjected to the adhesion test. The results revealed that the flattened chains do not show any adhesion even with the chemically identical free polymer on top, while the loosely adsorbed chains exhibit adhesion. Neutron reflectivity experiments corroborated that the difference in the interfacial adhesion is not attributed to the interfacial brodening at the free polymer-adsorbed polymer interface. Instead, coarse-grained molecular dynamics simulation results suggest that the tail parts of the loosely adsorbed chains act as "connector molecules", bridging the free chains and substrate surface and improving the interfacial adhesion. These findings not only shed light on the structure-property relationship at the interface, but also provide a novel approach for developing sticking/anti-sticking technologies through precise control of the interfacial polymer nanostructures.

Irreversible Adsorption Erases the Free Surface Effect on the Tg of Supported Films of Poly(4-tert-butylstyrene)

When cooled at constant rate, a 25 nm thin film of poly(4-tert-butylstyrene) vitrifies 50 K lower than in bulk. This record sets the largest depression in thermal glass transition temperature (T_g) ever observed upon confinement at the nanoscale level. Same as for other supported polymer layers, this reduction in T_g has been attributed to the presence of a free surface, the ensemble of molecules at the interface with air remaining in the liquid state also at temperatures well below bulk T_g. Here, we verify that such tremendous shifts can be erased upon prolonged annealing in the liquid state, hinting at a metastable nature of confinement effects. We demonstrate that the recovery of bulk behavior and the manifestation of the free surface are enslaved to the kinetics of irreversible adsorption of chains on the supporting substrate.

Irreversible Adsorption Controls Crystallization in Vapor-Deposited Polymer Thin Films

Matrix-assisted pulsed laser evaporation (MAPLE) provides a gentle means for the quasi-vapor deposition of macromolecules. It offers a unique opportunity for the bottom-up control of polymer crystallization as film growth and crystallization occur simultaneously. Surprisingly, with increasing deposition time, it has been shown that crystallization becomes prohibited despite the availability of polymer via continuous deposition. In this Letter, we investigate the molecular origins of suppressed crystallization in poly(ethylene oxide) films deposited by MAPLE atop silicon substrates. We find that suppressed crystallization results from the formation of an irreversibly adsorbed polymer nanolayer at the substrate that forms during deposition. Substrate temperature is shown to influence the stability of the irreversibly adsorbed nanolayer and, hence, polymer thin film crystallization. Our investigation offers new insight into how temperature and interfacial interactions can serve as a new toolbox to tune polymer film morphology in bottom-up deposition.

Novel Effects of Compressed CO2 Molecules on Structural Ordering and Charge Transport in Conjugated Poly(3-hexylthiophene) Thin Films

We report the effects of compressed CO₂ molecules as a novel plasticization agent for poly(3-hexylthiophene) (P3HT)-conjugated polymer thin films. In situ neutron reflectivity experiments demonstrated the excess sorption of CO₂ molecules in the P3HT thin films (about 40 nm in thickness) at low pressure (P = 8.2 MPa) under the isothermal condition of T = 36 °C, which is far below the polymer bulk melting point. The results proved that these CO₂ molecules accelerated the crystallization process of the polymer on the basis of ex situ grazing incidence X-ray diffraction measurements after drying the films via rapid depressurization to atmospheric pressure: both the out-of-plane lamellar ordering of the backbone chains and the intraplane π-π stacking of the side chains were significantly improved, when compared with those in the control P3HT films subjected to conventional thermal annealing (at T = 170 °C). Electrical measurements elucidated that the CO₂-annealed P3HT thin films exhibited enhanced charge carrier mobility along with decreased background charge carrier concentration and trap density compared with those in the thermally annealed counterpart. This is attributed to the CO₂-induced increase in polymer chain mobility that can drive the detrapping of molecular oxygen and healing of conformational defects in the polymer thin film. Given the universality of the excess sorption of CO₂ regardless of the type of polymers, the present findings suggest that CO₂ annealing near the critical point can be useful as a robust processing strategy for improving the structural and electrical characteristics of other semiconducting conjugated polymers and related systems such as polymer:fullerene bulk heterojunction films.

The architecture of the adsorbed layer at the substrate interface determines the glass transition of supported ultrathin polystyrene films

To elucidate the mechanism underlying the effect of polymer/solid interfacial interactions on the dynamics of thin polymer films, the glass transition of thin end-functionalized polystyrene films supported on SiO₂-Si, such as proton-terminated PS (PS-H), α,ω-dicarboxy-terminated PS (PS-COOH), and α,ω-dihydroxyl-terminated PS (PS-OH), was investigated. All the PS films exhibited a substantial depression in T_g with decreasing film thickness, while the extent of such depression was strongly dependent on the chemical structure of the end groups and molecular weights. It was found that T - T of the various PS films increased linearly with increasing h_ads/R_g, in which h_ads is the thickness of the interfacial adsorbed layer and R_g is the radius of gyration of PS. The h_ads/R_g is a direct reflection of the macromolecular chain conformation within the adsorbed layer which was affected by its end groups and molecular weights. These findings are in line with the work of Napolitano, and present direct experimental evidence.

Flattening Process of Polymer Chains Irreversibly Adsorbed on a Solid

We report the structural relaxation process of irreversibly adsorbed polymer chains via thermal annealing that lie flat on a solid ("flattened chains"). Amorphous polystyrene and quartz, which together constitute a weakly attractive system, was used as a model where the local chain conformations of the flattened chains were investigated by sum frequency generation spectroscopy (SFG). Two different film preparation processes (i.e., spin coating and dip coating methods) were utilized to create different initial chain conformations. The spin-coated and dip-coated PS thin films were annealed at a temperature far above the bulk glass transition temperature to reach the "quasiequilibrium" state and subsequently rinsed with chloroform to uncover the buried flattened chains. The SFG results revealed that the backbone chains (constituted of CH and CH₂ groups) of the flattened PS chains preferentially orient to the weakly interactive substrate surface via thermal annealing regardless of the initial chain conformations, while the orientation of the phenyl rings becomes randomized. We postulate that increasing the number of surface-segmental contacts (i.e., enthalpic gain) is the driving force for the flattening process of the polymer chains, even onto a weakly interactive solid to overcome the conformational entropy loss in the total free energy.

Nanoscale adsorbed structures as a robust approach for tailoring polymer film stability

The stability or wettability of thin polymer films on solids is of vital interest in traditional technologies as well as in new emerging nanotechnologies. We report here that nanoscale structures of polymer chains adsorbed onto a solid surface play a crucial role in the thermal stability of the film. In this study, polystyrene (PS) spin-cast films (20 nm in thickness) with eight different molecular weights prepared on silicon (Si) substrates were used as a model. When low molecular weight (Mw≤ 50 kDa) PS films were subjected to thermal annealing at temperatures far above the bulk glass transition temperature, dewetting occurred promptly, while high molecular weight (Mw≥ 123 kDa) PS films were stable for at least 6 weeks at 150 °C. We reveal a strong correlation between the film stability and the two different interfacial structures of the adsorbed polymer chains: their opposing wettability against chemically identical free polymer chains results in a wetting-dewetting transition at the adsorbed polymer-free polymer interface. This is a unique aspect of the stability of polymer thin films and may be generalizable to other polymer systems regardless of the magnitude of solid-polymer attractive interactions.