Mechanical Properties of Ultrathin Polystyrene-b-Polybutadiene-b-Polystyrene Block Copolymer Films: Film Thickness-Dependent Young’s Modulus

Dewetting of thin polymer films reveals non-equilibrium conformational difference between substrate and air sides

The anomalous dynamics of thin polymer films, often attributed to geometrical confinement and interfacial interaction, have aroused considerable interest, particularly with regard to the inherent and processing-induced chain conformation changes. Here, the capillary peeling method is employed to obtain reattached thin polystyrene films with either the substrate or the air side beneath. Compared to traditional dewetting experiments solely conducted on the substrate side of as-cast films, the difference in dewetting behavior between the two sides of films is demonstrated, with the air side showing a faster dewetting velocity in the early stage and a larger apparent residual stress. Through this method, the non-equilibrium conformation of polymer chains that used to be located on the air side due to spin-casting can be thoroughly investigated without any interference from free surface effects. Our findings offer evidence of the difference in interfacial chain entanglement density, which determines variations in the performance on relaxation of residual stress within the thin films. This work is an improved method to study polymer dynamics through dewetting and gives insight into chain conformation at interfaces during processing.

Advancements in Novel Mechano-Rheological Probes for Studying Glassy Dynamics in Nanoconfined Thin Polymer Films

The nanoconfinement effects of glassy polymer thin films on their thermal and mechanical properties have been investigated thoroughly, especially with an emphasis on its altered glass transition behavior compared to bulk polymer, which has been known for almost three decades. While research in this direction is still evolving, reaching new heights to unravel the underlying physics of phenomena observed in confined thin polymer films, we have a much clearer picture now. This, in turn, has promoted their application in miniaturized and functional applications. To extract the full potential of such confined films, starting from their fabrication, function, and various applications, we must realize the necessity to have an understanding and availability of robust characterization protocols that specifically target thin film thermo-mechanical stability. Being nanometer-sized in thickness, often atop a solid substrate, direct mechanical testing on such films becomes extremely challenging and often encounters serious complexity from the dominating effect of the substrate. In this review, we have compiled together a few important novel and promising techniques for mechano-rheological characterization of glassy polymer thin films. The conceptual background involved in each technique, constitutive equations, methodology, and current status of research are touched upon following a pedagogical tutorial approach. Further, we discussed each technique's success and limitations, carefully covering the puzzling or contradicting observations reported within the broad nexus of glass transition temperature-viscosity-modulus-molecular mobility (including diffusion and relaxation).

Unveiling Strong Thin Film Confinement Effects on Semirigid Conjugated Polymers

Nanoconfinement has been recognized to induce significant changes in the physical properties of polymeric films when their thickness is less than 100 nm. Despite extensive research on the effect of nanoconfinement on nonconjugated polymers, studies focusing on the confinement effects on dynamics and associated electronic and mechanical properties for semiconductive and semirigid conjugated polymers remain limited. In this study, we conducted a comprehensive investigation into the nanoconfinement effects on both p- and n-type conjugated polymers having varying chain rigidity under different degrees of confinement. Using the flash differential scanning calorimetry technique, it was found that the increased molecular mobility with decreasing film thickness, as indicated by the depression of glass transition temperature (T g) from its bulk values, was directly proportional to chain rigidity. This relationship between chain rigidity and enhanced segmental mobility was further corroborated through molecular dynamics simulations. Thinner films exhibited a higher degree of crystallinity for all conjugated polymers, and a significant reduction of more than 50% in elastic modulus was observed for films with approximately 20 nm thickness compared to those of 105 nm thickness, particularly for highly rigid conjugated polymers. Interestingly, we found that the charge mobility remained independent of film thickness, with all samples demonstrating good charge mobility regardless of the different film thicknesses for devices measured here. Nanoconfined conjugated polymer thin films exhibited a combination of mechanical compliance and good charge carrier mobility properties, making them promising candidates for the next generation of flexible and portable organic electronics. From an engineering standpoint, confinement could be an effective strategy to tailor the dynamics and mechanical properties without significant loss of electronic property.

Revealing the Kinetic Phase Behavior of Block Copolymer Complexes Using Solvent Vapor Absorption-Desorption Isotherms

Controlling the self-assembled morphologies in block copolymers heavily depends on their molecular architecture and processing conditions. Solvent vapor annealing is a versatile processive pathway to obtain highly periodic self-assemblies from high chi (χ) block copolymers (BCPs) and supramolecular BCP complexes. Despite the importance of navigating the energy landscape, controlled solvent vapor annealing (SVA) has not been investigated in BCP complexes, partly due to its intricate multicomponent nature. We introduce characteristic absorption-desorption solvent vapor isotherms as an effective way to understand swelling behavior and follow the morphological evolution of the polystyrene-block-poly(4-vinylpyridine) block copolymer complexed with pentadecylphenol (PS-b-P4VP(PDP)). Using the sorption isotherms, we identify the glass transition points, polymer-solvent interaction parameters, and bulk modulus. These parameters indicate that complexation completely screens the polymer interchain interactions. Furthermore, we established that the sorption isotherm of the homopolymer blocks serves to deconvolute the intricacy of BCP complexes. We applied our findings by developing annealing pathways for grain coarsening while preventing macroscopic film dewetting under SVA. Here, grain coarsening obeyed a power law and the growth exponent revealed a kinetic transition point for rapid self-assembly. Overall, SVA-based sorption isotherms have emerged as a critical method for understanding and developing annealing pathways for BCP complexes.

Investigation of Crystallization, Morphology, and Mechanical Properties of Polypropylene/Polypropylene-Polyethylene Block Copolymer Blends

Polyethylene (PE)-based elastomers are the ideal choice for enhancing the compatibility of polypropylene/polyethylene (PP/PE) blends and improving the mechanical properties of PP-based materials. However, the issue of blend systems lies in the interplay between the crystallization processes. Therefore, we investigated the crystallization behavior during the cooling process of a new generation of PP/PE block copolymers (PP-b-PE) and random polypropylene (PPR, a copolymer of propylene and a small amount of ethylene or an alpha-olefin) blends using in-situ X-ray diffraction/scattering and differential scanning calorimetry (DSC) techniques. We also conducted mechanical performance tests on PPR/PP-b-PE blends at room temperature and low temperature (-5 °C). The results indicate that during the cooling process, the PP phase of PP-b-PE will follow the PPR to crystallize in advance and form a eutectic mixture, thereby enhancing the compatibility of PP/PE. Moreover, the PPR/PP-b-PE blend will form stable β-(300) crystals with excellent mechanical properties. Due to the improved compatibility of PP/PE with PP-b-PE, PE crystals are dispersed within PP crystals, providing bonding that improves the toughness of PPR under the low stiffness failure conditions of PPR/PP-b-PE blends, thereby enhancing their impact performance at low and room temperatures. This research has great significance for both recycling waste plastics and enhancing the low-temperature toughness of PPR.

Dependence of Slippery and Elastic Properties of Thin Polymer Films on the Grafted Flexible Sidechain Amount

In modern life, people face a wide number of sticky problems when adhesion is highly undesirable: water and dirt stick to clothes, useful materials stick to the walls of their containers and cannot be fully used, water sticking and freezing on airplane wings affects handling and can be dangerous, biological liquids can stick and form clots inside medical devices threatening patients' lives, etc. Slippery liquid-infused porous surfaces (SLIPSs) with pressure stable omniphobicity could help to solve these issues. Lubricant depletion from porous surface and subsequent degradation of omniphobic properties is the major problem for SLIPS. It could be resolved by attaching flexible, liquid-like sidechains to the polymer matrix. Understanding the relationship between the structure of such polymer films and wetting effects is therefore of great importance. The present work is devoted to the study of droplet pinning on crosslinked polydimethylsiloxane (PDMS) polymer films with varied amounts of attached flexible PDMS sidechains and clarification of the relationship between slippery and viscoelastic properties of the films. An one-stage approach to the synthesis of such slippery coatings on smooth and porous substrates in "eco-friendly" pressurized CO₂ solutions is proposed. Pinning force and Young's modulus (E) of the films on silicon substrates with variation of the grafted sidechains amount (x) are measured. The non-monotonic dependence of the pinning force on the amount of sidechains is obtained: the pinning force decreases at small x values (region I) and starts to increase at higher x (region II). The effects of the grafted sidechains amount, as well as matrix softening, are discussed for each case. It is demonstrated that the proposed method of film synthesis allows one to obtain thin, uniform coatings on fabrics without gluing the fibers. Such coatings with an optimal amount of PDMS sidechains demonstrate decreased sliding angles for droplets of water and aqueous alcohol solutions, as compared to PDMS coatings without grafted sidechains. The proposed technique may be of interest for deposition of coatings on porous surfaces having a complex morphology, such as textiles, aerogels, porous electrodes, etc.

Ultrathin organic membranes: Can they sustain the quest for mechanically robust device applications?

Ultrathin polymeric films have recently attracted tremendous interest as functional components of coatings, separation membranes, and sensors, with applications spanning from environment-related processes to soft robotics and wearable devices. In order to support the development of robust devices with advanced performances, it is necessary to achieve a deep comprehension of the mechanical properties of ultrathin polymeric films, which can be significantly affected by confinement effects at the nanoscale. In this review paper, we collect the most recent advances in the development of ultrathin organic membranes with emphasis on the relationship between their structure and mechanical properties. We provide the reader with a critical overview of the main approaches for the preparation of ultrathin polymeric films, the methodologies for the investigation of their mechanical properties, and models to understand the primary effects that impact their mechanical response, followed by a discussion on the current trends for designing mechanically robust organic membranes.

Dewetting-Assisted Interface Templating: Complex Emulsions to Multicavity Particles

Interfacial tension-driven formation of intricate microparticle geometries from complex emulsions is presented in this work. Emulsion-templating is a reliable platform for the generation of a diverse set of microparticles. Here, water-in-styrene-in-water complex emulsions undergo reproducible metamorphosis, i.e., from liquid state emulsions to solid structured microparticles are employed. In contrast to the traditional usage of glass-based microfluidics, polydimethylsiloxane (PDMS) swelling behavior is employed to generate complex emulsions with multiple inner cores. In the presence of block copolymer surfactant, these emulsions undergo gravity-driven dewetting of styrene, to transform into membranous structures with compartments. Further polymerization of styrene skeletal remains resulted in microparticles with interesting geometries and intact membranes. Mechanical and confocal microscopic studies prove the absence of polystyrene within these membranes. Using osmotic pressure, membrane rupture and release of encapsulated gold nanoparticles from such polymerized emulsions leading up to applications in cargo delivery and membrane transport are promoted. Even after membrane rupture, the structured microparticles have shown interesting light-scattering behavior for applications in structural coloring and biosensing. Thereby, proving PDMS-based swelling as a potential methodology for reproducible production of complex emulsions with a potential to be transformed into membranous emulsions or solid microparticles with intricate structures and multiple applications.