Ultrathin Copolymer Films Research Articles

Morphology and Mechanics of Star Copolymer Ultrathin Films Probed by Atomic Force Microscopy in the Air and in Liquid.

Star copolymer films were produced by using spin-coating, drop-casting, and casting deposition techniques, thus obtaining ultrathin and thick films, respectively. The morphology is generally flat, but it becomes substrate-dependent for ultrathin films where the planarization effect of films is not efficient. The indentation hardness of films was investigated by Force Volume Maps in both the air and liquid. In the air, ultrathin films are in the substrate-dominated zone and, thus, the elastic modulus E is overestimated, while E reaches its bulk value for drop-casted ultrathin and thick films. In liquid (water), E follows an exponential decay for all films with a minimum soaked time t0 of 0.37 and 2.65 h for ultrathin and drop-casted ultrathin and thick films, respectively. After this time, E saturates to a value on average 92% smaller than that measured in the air due to film swelling. Such results support the role of film morphology in the antimicrobial activity envisaged in the literature, suggesting also an additional role of film hardness.

Morphological Transitions in Micelles of Amphiphilic Bottlebrushes upon the Adsorption and Compression at the Liquid Interface.

Densely grafted comb-like macromolecules (bottlebrushes) with alternating solvophobic and solvophilic side chains were studied in a selective solvent and at the liquid interface using mesoscopic computer simulations. The effects of backbone length and copolymer composition were considered. While self-assembly in solution revealed only spherical aggregates for all ar-chitectures studied, adsorption onto the liquid interface in particular cases resulted in morpho-logical changes, with worm-like aggregates or a continuous monolayer observed. In turn, the compression of macromolecules at the interface also leads to morphological transitions, includ-ing the formation of a mesh-like percolated structure. The obtained results may be useful for the preparation of solid nanoparticles of anisotropic shape or nanostructured ultra-thin copolymer films.

Film thickness and strain rate dependences of the mechanical properties of polystyrene-b-polyisoprene-b-polystyrene block copolymer ultrathin films forming a spherical domain

While the mechanical properties of ultrathin polymer films have been extensively studied, only few studies have focused on the ultrathin block copolymer films with complex topologies. The structure-property relationship of ultrathin films (thickness = 35–1000 nm) of polystyrene- b -polyisoprene- b -polystyrene (SIS) containing 14 wt% polystyrene (PS) is established in this study. Grazing-incidence small-angle X-ray scattering, dynamic secondary ion mass spectrometry, and atomic force microscopy were used to characterize the film structure. The difference in surface energy and interfacial energy of the domains resulted in the SIS thin films to primarily consist of three layers: a PS-rich subsurface layer (30 wt%), a spherical PS bulk layer (14 wt%), and a PS-rich interface layer (20 wt%). The film-on-water tensile test was used to investigate both the film thickness and strain rate dependences of mechanical properties. We found that the Young's modulus increased dramatically as film thickness decreased, although only few PS-rich layers consisting of disperse cylindrical and spherical PS domains existed at the surface and interface. Furthermore, the strain rate dependence of the modulus and stress relaxation was more significant for thinner films than for thicker films. The increase in modulus and unusual relaxation behavior are hypothesized to be the result of a combination of different factors, including PS domain alignment, confinement effects, and surface and interface interactions. • The structure-property relationship of ultrathin SIS films is analyzed. • Young's modulus increases upon decreasing thickness. • The extent of stress relaxation is larger for thinner films. • The stiffening may result from PS domain alignment.

Controlling the ordered transition of PS-b-P4VP block copolymer ultrathin films by solvent annealing

A change in morphology of ultrathin films of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer was induced by annealing under chloroform and toluene vapor and was investigated by atomic force microscopy (AFM). The microphase separation morphology with spherical structures were formed when PS-b-P4VP was dissolved in toluene, which is a selective solvent for polystyrene (PS) segments. After chloroform annealing, a series of surface morphologies of PS-b-P4VP were observed, including spherical and network structures such as gyroid structures. During the continuous annealing treatment for the films with network structures by toluene, a reversible structure conversion from network to sea-island was observed. In addition, we have demonstrated that Au-NPs can be easily adhered to the PS-b-P4VP films with network structure on surface.

Switching dynamics enhancement in P(VDF-TrFE) copolymer ultrathin films with symmetric organic film electrodes

Abstract The polarization switching behaviors of ultrathin films of ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) capacitors with different thicknesses of top PEDOT-PSSH layer have been studied. The polarization switching results are related to an effective internal electric field, and a large, permanent, internal electric field can be ensured by introducing a symmetric PEDOT-PSSH layer into the capacitors. The switching speed is potentially influenced by the PEDOT-PSSH layer and the asymmetric PEDOT-PSSH layer leads to a different density of surface charge, resulting in a build-in field. As a result, these enhanced switching dynamics in spin-coating 25 nm P(VDF-TrFE) copolymer films are achieved by introduced a symmetric electroactive layer to optimize morphology and the internal electric field. At high temperature (60 °C), we also show that the enhanced switching speed is two times as fast as what is measured at room temperature (25 °C) because the temperature depended active field is related to the switching kinetic energy. Switching time of P(VDF-TrFE) films with an electroactive layer as a function of the number of switching cycles show the excellent switching characteristics of ferroelectric capacitors.

Solvent-Free Deposition of Ultrathin Copolymer Films with Tunable Viscoelasticity for Application to Pressure-Sensitive Adhesives.

A new fabrication method for an ultrathin (500 nm thick) pressure-sensitive adhesive (PSA) was demonstrated by utilizing a series of in situ cross-linked viscoelastic copolymer films. Viscoelastic films composed of poly(2-hydroxyethyl acrylate- co-2-ethylhexyl acrylate) were synthesized successfully in a one-step manner by an initiated chemical vapor deposition (iCVD) process, where free-radical polymerization is triggered in the vapor phase either by heat or UV, or a combination of both. In particular, the photoinitiated chemical vapor deposition method generated a highly cross-linked polymer film, whereas cross-linking of the copolymer film was suppressed greatly in the conventional thermal iCVD method. A combination of thermal and photoinitiated chemical vapor deposition could regulate the cross-linking density of the copolymer films. We controlled the cross-linking density of the copolymer films to exhibit a viscoelastic property so that they would readily adhere to various kinds of substrates with only 500 nm thick copolymer PSA. The adhesion performance of the PSA was systematically optimized by tuning the copolymer composition as well as the cross-linking density, and consequently a high shear strength of more than 85.2 ± 5 N/cm2 was achieved despite the 500 nm thickness. In addition, the PSA was completely transparent. We expect that the ultrathin PSAs developed in this work will be utilized widely for the realization of various soft electronic devices, which usually require strong adhesion, tunable viscoelastic properties, and optical transparency.

Modeling of Structure Effect for Ferroelectric Capacitor Based on Poly(vinylidene fluoride-trifluoroethylene) Ultrathin Films.

The characteristics of ferroelectric capacitors with poly(vinylidene fluoride-trifluoroethlene) (P(VDF-TrFE)) films have been studied at different structures of cell electrodes. It is suggested that the effect of electrode structures could induce changes of performance. Remarkably, cells with line electrodes display a better polarization and fatigue resistance than those with flat electrodes. For P(VDF-TrFE) ultrathin films with different electrode structures, the models of charge compensation mechanism for depolarization field and domain fatigue decomposition are used to explain the effect of electrode structure. Furthermore, the driving voltage based on normal speed-functionality is designed, and the testing results show that the line electrode structure could induce a robust switching, which is determined by the free charges concentration in active layer. These findings provide an effective route to design the optimum structure for a ferroelectric capacitor based on P(VDF-TrFE) copolymer ultrathin film.

Conformation mediated preferential swelling of amphiphilic block copolymer ultrathin films

Steady-state and dynamic swelling characteristics of PEO-PPO-PEO copolymer ultrathin films comprising 2-layered structure, where the bottom layer (L1) is made of one PEO (hydrophilic)-PPO (hydrophobic) bilayer and the top layer (L2) is made of two alternating PEO-PPO bilayers, have been studied at room temperature in precisely controlled environment of 10–93% relative humidity (RH). Swelling, which is monitored by measuring the thickness of the films using X-ray reflectivity (XRR), is found to be largely dissimilar in differently thick films prepared by varying the concentration of mother solutions from 1 g/L to 4 g/L. This is well explained considering a preferential-swelling of amphiphilic block copolymer, where only the hydrophilic PEO blocks contribute in swelling and PPO acts a barrier in the diffusion of water molecules. The swelling kinetics is governed by the thickness/width and coverage of outer PPO block layer.

Effect of the texture of Langmuir copolymer films on structural phase transitions

An atomic-force microscopy study has surprisingly revealed that ultrathin copolymer films fabricated according to Langmuir technology constitute a network of tangled filaments. A long-range order is absent in the samples. The observed filaments consist of several parallel-packed copolymer chains. The shape of ferroelectric domains in a film is independent of the direction of copolymer chains. Each domain includes a large number of filaments. The nature of high-temperature (ferroelectric) and low-temperature phase transitions has been discussed in terms of structural data.

High temperature-dependent imprint and switching mechanism of poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin films with electroactive interlayers

The imprint and switching behaviors at high temperature have been systematically investigated through the study on the poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) films without and with in-situ synthesized poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonic) acid, poly(vinyl phosphonic) acid (PVPA), and copper-phthalocyanine as interlayers. The lower imprint rate and the faster switching speed are observed for the sample with PVPA as interlayer even after 107 cycles at 60 °C. Combined with the results for the imprint and switching behaviors at room temperature, the temperature-dependent imprint and switching mechanisms for different electroactive interlayers in this system are proposed. Therefore, the optimum protocol could be designed for FeRAM based on P(VDF-TrFE) film, which would be in favor of the performance and the service life of the related ferroelectric devices even at high temperature.

Polarization induced resistance switching effect in ferroelectric vinylidene-fluoride/trifluoroethylene copolymer ultrathin films

We observed a clear polarization reversal-induced resistance switching effect in ferroelectric Vinylidene-fluoride (VDF)/Trifluoroethylene (TrFE) copolymer thin films. Pt and Au were used as the bottom and top electrodes, respectively, and the thickness of the VDF/TrFE copolymer film was adjusted to be 10 nm. The conduction current was 100 times higher in the case of the spontaneous polarization of the VDF/TrFE film towards the Au electrode than that in the case of the opposite direction. This resistance switching was confirmed to be reproducible after 10 successive polarization reversals.

Concentration mediated structural transition of triblock copolymer ultrathin films.

X-ray reflectivity, atomic force microscopy, X-ray photoelectron spectroscopy, and contact angle measurement techniques are used to study the structural changeover as a function of concentration of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer diluted in toluene spin-coated as ultrathin films on hydrophilic Si substrate. A lamellar structure made of three alternating incomplete bilayers is observed until the concentration of copolymer solution attains a threshold value of about 3.6-4 g/L. Around this concentration and beyond, the entanglement of polymer chains takes place during drying and the growth of a homogeneous film made of complete bilayers on Si substrate is observed. The strong hydrophilic nature of the Si substrate dictates the growth of this amphiphilic copolymer. We evidence that the lower part of the films is made of hydrophilic PEO blocks attached to the substrate while the hydrophobic PPO blocks are directed toward air.

Temperature dependence of imprint mechanism in poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin films

The temperature dependence of imprint in ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer thin film capacitors has been investigated. The descriptions of imprint versus different temperatures and annealing process are given. It is found that the temperature-induced shift of the imprint rate is limited for all the investigated temperature conditions, and this low shift is mainly associated with the competition between the trap states increasing and detrapping process induced by the temperature rise. Also, the annealing temperature-dependent imprint rates in the polymer chains have been analyzed, and the annealed cell shows low imprint rate after 104 switches at an annealing temperature above 100 °C. Re-annealing recovery process of “inherent” imprint in the ferroelectric thin film capacitor shows improved imprint behaviors, which may be a wake-up of the polarization and detrapping from imprint traps. The internal electric fields for these processes are well analyzed and it is thought that the effective fields as well as charge trap states in ferroelectric layers are strongly responsible for the switching behaviors. This result may be useful for memory device design if extended to cover the temperature dependence of the polarization reversal based on P(VDF-TrFE) copolymer thin film.

Unipolar poling-induced high switching speed and improved imprint behaviors for poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin films

The polarization switching and imprint behaviors with different pulse and unipolar poling processes for ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer ultrathin films have been studied. Compared with results for samples only through the certain protocol we studied previously, the much faster switching speed and lower voltage shift are observed for films with certain protocol as well as certain unipolar poling. The analyses show that these properties are strongly dependent on the directions of the switching and unipolar poling field. The results provide another effective route to design the optimum protocol for ferroelectric random access memory based on P(VDF-TrFE) copolymer ultrathin film.

Self-polarization in ultrathin Langmuir–Blodgett polymer films

Ultrathin copolymer films of vinylidene fluoride and trifluoroethylene were deposited on Al-coated polyimide substrates, by the Langmuir–Blodgett method. A top Al electrode was evaporated onto the polymer film, to form an Al/polymer/Al structured infrared detector. The pyroelectric voltage response of the detector under various polarizing processes was characterized. The detector with only one transferred polymer layer exhibited a preferential polarization direction. This was considered to result from the self-polarization of the ultrathin polymer film. It was due to the preferred alignment of the dipoles on the Al substrates. This process can be applied for designing stable fast-response infrared detectors.

Stable Dropwise Condensation for Enhancing Heat Transfer via the Initiated Chemical Vapor Deposition (iCVD) of Grafted Polymer Films

Ultra-thin copolymer films are deposited by initiated chemical deposition (iCVD) to investigate their performance under the condensation of water vapor. By forming a grafted interface between the coating and the substrate, the films exhibit stable dropwise condensation even when subjected to 100 °C steam. The applicability of the iCVD to complex substrate geometries is demonstrated on a copper condenser coil.

Influence of Temperature and Type of Solvents on the Microdomain Orientation ofPS‐b‐P2VPUltrathin Films by Solvent Annealing

SummaryThe effect of temperature and type of solvent on the microdomain orientation during solvent‐annealing of block copolymer ultrathin films on silicon substrates was investigated using asymmetric polystyrene‐block‐poly(2‐vinylpyridine) (PS‐b‐P2VP) diblock copolymer. When the solvent‐annealing was performed with THF at 20°C, the ultrathin film exhibited a mixed orientation of cylinders parallel and perpendicular to the substrate while at 25°C only parallel cylinders were observed by AFM. The solvent annealing with acetone gave a dot pattern at 20°C and a line pattern at 25°C. On the other hand, in the case of solvent‐annealing with toluene perpendicular cylinders were observed at 25°C while at 20°C the morphology was not clear. The orientation and morphology of microdomains as well as the kinetics in PS‐b‐P2VP ultrathin films on solvent‐annealing is significantly affected by temperature and type of solvent. By controlling temperature and selecting the solvent, perpendicular cylinders can be formed predominantly on the substrates.

Fast switching protocol for ferroelectric random access memory based on poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin films

The polarization switching behaviors with different pulse processes for ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer ultrathin films have been studied. The faster switching speed with imprint time is observed for the films with the certain switching and imprint directions. The internal electric fields for these processes are well analyzed, and it is found that the effective fields as well as charge trap states in ferroelectric layers are strongly responsible for the switching behaviors. This result provides an effective route to design the optimum protocol for ferroelectric random access memory based on P(VDF-TrFE) copolymer ultrathin film.

The improved imprint effect in poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin films

The imprint behaviors for poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer ultrathin films with electroactive interlayers have been studied. The different electroactive polymers are used as the interlayer between P(VDF-TrFE) and metal electrodes. It is found that the smaller voltage shift is observed for the samples with electroactive interlayers and the imprint properties are improved substantially after the electroactive interlayers are introduced. The charge trap accumulation in P(VDF-TrFE) films without and with interlayers is analyzed and it is thought that the traps would be partially compensated by the free charges from the electroactive interlayers, illustrating the importance of the appropriate interlayer materials.

The effect of electroactive interlayer on the ferroelectric properties in poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin films

A series of ferroelectric polarization reversal measurements have been conducted on spin-coated ultrathin poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer films sandwiched with different electroactive materials between P(VDF-TrFE) and metal electrodes. Three types of electroactive interlayers, including highly polarizable material copper-phthalocyanine (CuPc), electron conductive polyethylene dioxythiophene/poly(styrene sulfonate acid), and ion (proton) conductive poly (vinyl phosphonic) acid, have been employed in our experiment. In addition, the ferroelectric film with CuPc exhibits larger remnant polarization and the enhancement of fatigue resistance although higher drive voltage and more switching time can be observed, which differs from the samples with electron and proton conductors as interlayers. The further analysis shows that this may be attributed to higher dielectric constant of CuPc than that of P(VDF-TrFE) layer. These results suggest that three types of interlayers can lead to a large improvement of ferroelectric switching, which is promising for potential applications.