In situ grazing incidence small‐angle X‐ray scattering study of solvent vapor annealing in lamellae‐forming block copolymer thin films: Trade‐off of defects in deswelling

Solvent vapor annealing (SVA) is one route to prepare block copolymer (BCP) thin films with long‐range lateral ordering. The lattice defects in the spin‐coated BCP thin film can be effectively and rapidly reduced using SVA. The solvent evaporation after annealing was shown to have a significant impact on the in‐plane ordering of BCP microdomains. However, the effect of solvent evaporation on the out‐of‐plane defects in BCPs has not been considered. Using grazing‐incidence x‐ray scattering, the morphology evolution of lamellae‐forming poly(2‐vinlypyridine)‐b‐polystyrene‐b‐poly(2vinylpyridine) triblock copolymers, having lamellar microdomains oriented normal to substrate surface during SVA, was studied in this work. A micelle to lamellae transformation was observed during solvent uptake. The influence of solvent swelling ratio and solvent removal rate on both the in‐plane and out‐of‐plane defect density was studied. It shows that there is a trade‐off between the in‐plane and out‐of‐plane defect densities during solvent evaporation. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 980–989     

Controlling domain orientation of liquid crystalline block copolymer in thin films through tuning mesogenic chemical structures

Controlling the macroscopic orientation of nanoscale periodic structures of amphiphilic liquid crystalline block copolymers (LC BCPs) is important to a variety of technical applications (e.g., lithium conducting polymer electrolytes). To study LC BCP domain orientation, a series of LC BCPs containing a poly(ethylene oxide) (PEO) block as a conventional hydrophilic coil block and LC blocks containing azobenzene mesogens is designed and synthesized. LC ordering in thin films of the BCP leads to the formation of highly ordered, microphase‐separated nanostructures, with hexagonally arranged PEO cylinders. Substitution on the tail of the azobenzene mesogen is shown to control the orientation of the PEO cylinders. When the substitution on the mesogenic tails is an alkyl chain, the PEO cylinders have a perpendicular orientation to the substrate surface, provided the thin film is above a critical thickness value. In contrast, when the substitution on the mesogenic tails has an ether group the PEO cylinders assemble parallel to the substrate surface regardless of the film thickness value. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 532–541     

Synthesis and phase‐separation behavior of α,ω‐difunctionalized diblock copolymers

A series of diblock copolymers of n‐pentyl methacrylate and methyl methacrylate (PPMA/PMMA BCP) with one or two terminal functional groups was prepared by sequential anionic polymerization of PMA and MMA using an allyl‐functionalized initiator and/or and end‐capping with allyl bromide. Allyl functional groups were successfully converted into OH groups by hydroboration. The morphology in bulk was examined by temperature‐dependent small‐angle X‐ray measurements (T‐SAXS) and transmission electron microscopy (TEM) showing that functional groups induced a weak change in d‐spacings L₀ as well as in the thermal expansion behavior. T‐SAXS proved that the lamellar morphologies were stable over multiple heating/cooling cycles without order‐disorder transition (ODT) until 300 °C. While non‐functionalized BCP formed parallel lamellae morphologies, additional OH‐termination at the PMMA block forced in very thin films (ratio between film thickness and lamellar d‐spacing below 1) the generation of perpendicular lamellae morphology through the whole film thickness, as shown by Grazing‐incidence small‐angle X‐ray scattering experiments (GISAXS) measurements. Functionalized BCP were successfully used in thin films as templates for silica nanoparticles in an in‐situ sol–gel process. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Substrate interaction effects on order to disorder transition behavior in block copolymer films

We present an overview of the recent progress on the phase transition in the block copolymer (BCP) films in terms of the interfacial interactions effects of the substrates and the χ (Flory-Huggins segmental interaction parameter) effects between the two blocks. For the BCP films thinner than a critical thickness (L_c) above which the transition is independent of film thickness, the order-to-disorder transition (ODT) increased or decreased with decreasing film thickness depending on the interfacial interaction types. The rapid and slow changes in the ODT were attributed to the relative magnitude of enthalpic contribution to χ between two blocks. Interestingly, a periodic amplification in the block composition for the BCP films suppressed the compositional fluctuation in the film geometry, resulting in the ODT shifts from the bulk ODTs above L_c. This effect of the BCP films was more illustrated by the ODT shift effects depending on the strength of the preferential interactions on the substrates. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

High‐throughput characterization of pattern formation in symmetric diblock copolymer films

Surface‐pattern formation in thin block copolymer films was investigated by utilizing a high‐throughput methodology to validate the combinatorial measurement approach and to demonstrate the value of the combinatorial method for scientific investigation. We constructed measurement libraries from images of subregions of block copolymer films having gradients in film thickness and a range of molecular mass, M. A single gradient film covers a wide range of film morphologies and contains information equivalent to a large number of measurements of films having a fixed thickness, h. Notably, the scale of the surface patterns is generally much larger than the molecular dimensions so that the interpretation of the patterns is more subtle than ordering in bulk block copolymer materials, and there is no predictive theory of this type of surface‐pattern formation. We observed a succession of surface patterns that repeat across the film with increasing h [extended smooth regions, regions containing circular islands, labyrinthine (“spinodal”) patterns, holes, and smooth regions again]. The extended smooth regions and the labyrinthine patterns appear to be novel features revealed by our combinatorial study, and these patterns occurred as bands of h that were quantized by integral multiples of the bulk lamellar period, L_o. The magnitude of the height gradient influenced the width of the bands, and the smooth regions occupied an increasing fraction of the film‐surface area with an increasing film gradient. The average size of the spinodal patterns, λ, was found to scale as λ ～ L or λ ～ M^?1.65 and reached a limiting size at long annealing times. The hole and island features had a size comparable to λ, and their size likewise decreased with increasing M. The smooth regions were attributed to an increase in the surface‐chain density in the outer brushlike block copolymer layer with increasing h, and the scaling of λ with M was interpreted in terms of the increasing surface elasticity with M. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2141–2158, 2001     

Ordering poly(trimethylsilyl styrene‐block‐D,L‐lactide) block copolymers in thin films by solvent annealing using a mixture of domain‐selective solvents

Controlling the morphology, domain orientation, and domain size of block copolymer (BCP) thin films is desirable for many applications in nanotechnology. These properties can be tuned during solvent annealing by varying the solvent choice and degree of swelling which affect the effective miscibility and volume fraction of the BCP domains. In this work, we demonstrate with a bulk lamellae‐forming BCP, poly(4‐trimethylsilylstyrene‐block‐D ,L ‐lactide) (PTMSS‐b‐PLA), that varying the composition of a mixture of solvent vapors containing cyclohexane (PTMSS‐selective) and acetone (PLA‐selective), enables formation of perpendicularly oriented lamellae with sub‐20‐nm pitch lines. The BCP domain periodicity was also observed to increase by 30%, compared to bulk, following solvent annealing. Furthermore, solvent annealing alone is shown to induce a transition from a disordered to an ordered BCP. We rationalize our observations by hypothesizing that the use of a combination of domain selective solvent mixtures serves to increase the effective repulsion between the blocks of the copolymer. We furnish results from self‐consistent field theory calculations to support the proposed mechanism. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014 , 52, 36–45     

Physical mechanisms responsible for the water-induced degradation of PC61BM P3HT photovoltaic thin films

We show that [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) at the surface of thin film blends of poly(3-hexylthiophene) (P3HT):PC₆₁BM can be patterned by water. Using a series of heating and cooling steps, water droplets condense onto the blend film surface. This is possible due to the liquid-like, water swollen layer of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. Breath pattern water deformation and subsequent drying on the film surface results in isolated PC₆₁BM structures, showing that migration of PC₆₁BM takes place. This was confirmed by selective wavelength illumination to spatially map the photoluminescence from the P3HT and PC₆₁BM. Within a device, redistribution of the surface PC₆₁BM into aggregates would be catastrophic, as it would markedly alter device performance. We also postulate that repeated volume change of the poly(3,4-ethylenedioxythiophene) polystyrene sulfonate layer by water swelling may be, in part, responsible for the delamination failure mechanism in thin film solar cells devices. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 141–146     

Microdomain morphology of lamella‐forming diblock copolymer confined in a thin film

We report the self‐consistent field theory (SCFT) of the morphology of lamella‐forming diblock copolymer thin films confined in two horizontal symmetrical/asymmetrical surfaces. The morphological dependences of thin films on the polymer‐surface interactions and confinement, such as film thickness and confinement spatial structure, have been systematically investigated. Mechanisms of the morphological transitions can be understood mainly through the polymer‐surface interactions and confinement entropy, in which the plat confinement surface provides a surface‐induced effect. The confinement is expressed in the form of the ratio D/L₀, here D is film thickness, and L₀ is the period of bulk lamellar‐structure. Much richer morphologies and multiple surface‐induced morphological transitions for the lamella‐forming diblock copolymer thin films are observed, which have not been reported before. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1–10, 2009     

Thickness dependence of thermally induced changes in surface and bulk properties of Nafion® nanofilms

Thermally induced changes in surface wettability, dewetting behavior, and proton transport of “self‐assembled” nanothin Nafion^® films (4–300 nm) on SiO₂ substrate is reported. Thermal annealing induces switching of the surface wettability of 55 nm and thinner films from hydrophilic to super‐hydrophobic. Thickness dependence of this behavior is observed with higher annealing temperature required for lower thickness films, indicating highly restrictive mobility of Nafion^® ionomer as film thickness decreases. Dewetting is only observed for 4‐nm thin film. Significant suppression in proton conductivity upon thermal annealing was noted. Similarly, two other bulk properties, water uptake and swelling, were found to decrease upon annealing. This work reports a systematic examination of the thickness dependence of thermally induced changes in both surface and bulk properties of ultra‐thin Nafion^®. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1267–1277     

Critical properties of thin films of polymer solutions

The critical properties of polymer solutions confined in thin‐film environments is studied with simple scaling arguments and a molecular theory. For purely repulsive surfaces, the critical volume fraction is a universal function of x = N^1/2/L, where N is the chain length and L is the film thickness. The critical volume fraction is nonmonotonic in x and shows a deep minimum at a film thickness several times larger than the chain's radius of gyration. This nonmonotonic behavior results from the interplay between the surface–polymer entropic repulsion and the tendency of the film to avoid large density gradients. The critical temperature is a monotonically increasing function of L, as L goes from the two‐dimensional limit to the three‐dimensional limit. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1849–1853, 2005     

Surface characterization of ion-implanted polyethylene

Polyethylene (PE) film was implanted with 1000-keV Ar⁺ ions to a fluence of 5 × 10¹⁴ ions/cm² under high vacuum conditions (2.5 × 10⁻⁶ torr) and the film surface was investigated by means of microhardness and microwear measurements, and FTIR/ATR, Raman, and XPS techniques. Ion implantation significantly increased the subsurface hardness and also significantly improved the microwear resistance of the polymer. The implanted surface region of the film was found to consist of two distinct layers. One was the outermost carbon layer with a thickness of the order of 10 nm. In this layer, ca. 75% of carbon atoms were combined by graphitic sp² and diamond-like sp³ bonds, and the remaining 25% had chemical links with oxygen atoms. Spectroscopic data suggested that the sp²-bonded carbons segregated in graphite-like clusters containing imbedded oxygen atoms, interconnected by the sp³-bonded carbons. The other was the subsurface layer resulting from PE oxidation after ion-beam treatment. This layer was characterized by high contents of O H and CO groups as well as ester and double bonds. The chemical composition of the layer was uniform and did not vary over the layer thickness of about 1.4 μm. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 715–725, 1998     

Nanopatterning and micropatterning of cobalt containing block copolymers via phase‐separation and lithographic techniques

Reversible addition fragmentation transfer (RAFT) agent functionalized polydimethylsiloxane (PDMS‐RAFT) was used as a macro‐RAFT agent to polymerize a mixed sandwich cobaltocene containing monomer featuring η⁵‐cyclopentadienyl‐cobalt‐η⁴‐cyclobutadiene. High molecular weight block copolymers (BCP) consisting of a metallic block and a PDMS block with excellent control over molecular weight and polydispersity were prepared. Solid‐state self‐assembly of this BCP resulted in hexagonal domains of metallopolymer phase‐separated from PDMS. In solution, spherical micelles with a metallic core, stabilized by a PDMS corona were prepared. Pyrolysis of the BCP resulted in magnetic nanoparticles with 30% char yield. The BCP was used as an ink material for microcontact printing (μCP) to transfer long‐ranged patterns. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2747–2754     

Manipulating morphology and orientation in thermally responsive block copolymer thin films

We demonstrate the use of combined thermal annealing and solvent vapor annealing (SVA) to tune the morphology of thermally responsive block copolymer (BCP) thin films. The BCP, poly(styrene‐b‐tert‐butyl acrylate) (PS‐b‐PtBA), undergoes a chemical deprotection to poly(styrene‐b‐acrylic anhydride) (PS‐b‐PAH) above a temperature threshold, giving rise to a structural and morphological transition. Our experiments systematically examine different thermal annealing and SVA protocols with two solvents (tetrahydrofuran and acetone) and map the resulting morphologies. Assessments of these processing protocols were accelerated using temperature gradients. Our results demonstrate that the final nanoscale morphologies after SVA are determined by the changes in the relative solvent/polymer interactions and surface tensions of the polymer blocks that accompany deprotection. Because of these driving forces, certain processing combinations led to irreversible morphological states, whereas others present opportunities for further manipulation. Accordingly, our study reveals that the morphology of this thermally sensitive BCP can be altered through judicious choice of annealing protocol. The protocols that combine equal numbers of SVA and thermal annealing (TA) steps are not necessarily equivalent, and the order of the SVA relative to TA is a deciding factor in the final morphology. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Microphase segregation and selective chain scission of poly(2‐methyl‐2‐oxazoline)‐block‐polystyrene

Herein, we report the design and synthesis of a block copolymer (BCP) with a high Flory–Huggins interaction parameter to access 10 nm feature sizes for potential lithographic applications. The investigated BCP is poly[(2‐methyl‐2‐oxazoline)‐block‐styrene] (PMeOx‐b‐PS), where the PMeOx segment functions as a hydrophilic segment. Two BCPs with different molecular weights were prepared using PMeOx as macroinitiator for copper(0) mediated controlled radical polymerization. The thin film self‐assembly of the obtained PMeOx‐b‐PS was performed by solvent annealing and investigated by atomic force microscopy. Both polymers formed PMeOx cylinders in a PS matrix with an average cylinder diameter of 10.5 nm. Additionally, the ability of the PMeOx domains to selectively degrade under ultraviolet irradiation was explored. It was shown that scission of the PMeOx block does occur selectively, and furthermore that the degraded domains can be removed while leaving the PS matrix intact. By combining synthetic accessibility, small feature sizes, and a selectively cleavable domain, this new BCP system holds significant promise as a lithographic mask for patterning surfaces with high precision. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1349–1357     

Morphology of Symmetric Diblock Copolymers Confined Between Two Stripe‐Patterned Surfaces – Tilted Lamellae and More

Summary: We report the first Monte Carlo simulations on the thin‐film morphology of symmetric diblock copolymers confined between either symmetrically or antisymmetrically stripe‐patterned surfaces. Under suitable surface configurations (where the lamellae can comply with the surface patterns and can have a period close to the bulk lamellar period L₀), tilted lamellae are observed for film thicknesses D ≥ 2L₀; the checkerboard morphology is obtained for smaller film thicknesses. The A‐B interfaces in the tilted lamellae are basically perpendicular to the surfaces in their immediate vicinity, and exhibit undulations away from them. In some cases, the severe frustration imposed by the two patterned surfaces leads to irregular or unexpected morphologies, which represent locally stable states. The efficient sampling of our expanded grand‐canonical Monte Carlo technique enables us to observe more than one locally stable morphologies and the flipping between them during a single simulation run.

Tilted lamellae between symmetrically patterned surfaces (perpendicular to z) with a surface pattern period of 1.5L₀ and a film thickness of 2.67L₀. L₀ is the bulk lamellar period and the black curves mark the A‐B interfaces.     

Highly enhanced thermoelectric performance of WS2 nanosheets upon embedding PEDOT:PSS

Two‐dimensional (2D) WS₂ nanosheets (NSs) as a promising thermoelectric (TE) material have gained great concern recently. The low electrical conductivity significantly limits its further development. Herein, we reported an effective method to enhance the TE performance of WS₂ NSs by combining poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS). The restacked WS₂ NSs thin film with 1T phase structure obtained by a common chemical lithium intercalation show a high Seebeck coefficient of 98 μV K^?1 and a poor electrical conductivity of 12.5 S cm^?1. The introduction of PEDOT:PSS with different contents obviously improve the electrical conductivity of WS₂ NSs thin films. Although a declining Seebeck coefficient was observed, an optimized TE power factor of 45.2 μW m^?1 k^?1 was achieved for WS₂/PEDOT:PSS composite thin film. Moreover, the as‐prepared WS₂/PEDOT:PSS thin film can be easily peeled off and transferred to other substrate leading to a more promising application. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 997–1004     

Unexpected Molecular Weight Dependence to the Physical Aging of Thin Polystyrene Films Present at Ultra‐High Molecular Weights

The physical aging behavior, time‐dependent densification, of thin polystyrene (PS) films supported on silicon are investigated using ellipsometry for a large range of molecular weights (MWs) from M_w = 97 to 10,100 kg mol^?1. We report an unexpected MW dependence to the physical aging rate of h < 80‐nm thick films not present in bulk films, where samples made from ultra‐high MWs ≥ 6500 kg mol^?1 exhibit on average a 45% faster aging response at an aging temperature of 40 °C compared with equivalent films made from (merely) high MWs ≤ 3500 kg mol^?1. This MW‐dependent difference in physical aging response indicates that the breadth of the gradient in dynamics originating from the free surface in these thin films is diminished for films of ultra‐high MW PS. In contrast, measures of the film‐average glass transition temperature T _g(h) and effective average film density (molecular packing) show no corresponding change for the same range of film thicknesses, suggesting physical aging may be more sensitive to differences in dynamical gradients. These results contribute to growing literature reports signaling that chain connectivity and entropy play a subtle, but important role in how glassy dynamics are propagated from interfaces. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1224–1238     

Highly oriented and ordered microstructures in block copolymer films

Block copolymer (BCP) films with long-range lateral ordering and orientation are crucial for many applications. Here, we report a simple, versatile strategy based on a solution casting procedure, to produce millimeter thick film of BCPs with highly oriented nanostructures. Transmission electron microscope (TEM), small angle X-ray scattering (SAXS), and Hansen solubility parameters were used to study the morphology and interactions of the system. A variety of BCP-solvent pairs were investigated. Factors including set-up geometry, BCP characteristics, solvent evaporation, surface tension, and interactions, such as solvent-BCP, solvent-substrate, and BCP-substrate were examined. A mechanism is proposed to describe the observed long-range lateral ordering and orientation in films up to 1 mm in thickness. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1369–1375     

Nanoporous Films with Sub‐10 nm in Pore Size from Acid‐Cleavable Block Copolymers

Nanoporous thin films with pore size of sub‐10 nm are fabricated using an acid‐cleavable block copolymer (BCP), a benzoic imine junction between poly(ethylene oxide) (PEO) and poly(methacrylate) (PMAAz) bearing an azobenzene side chain (denoted as PEO‐bei‐PMAAz) as the precursor. After a thermal annealing, the block copolymers are self‐assembled to form highly ordered PEO cylinders within a PMAAz matrix normal to the film, even in the case of low BCP molecular weight due to the existing of the liquid crystalline (LC) azobenzene rigid segment. Thus, PMAAz thin films with pore size of ≈7 nm and density of ≈10¹² cm⁻² are obtained after removal of the PEO minor phase by breaking the benzoic imine junction under mild acidic conditions. This work enriches the nanoporous polymer films from BCP precursors and introduces the LC property as a functionality which can further enhance the mechanical properties of the films and broaden their applications.

     

Patterned growth of oriented 2D covalent organic framework thin films on single-layer graphene

Two-dimensional covalent organic frameworks (COFs) are polymer networks that organize molecular building blocks into porous, layered structures of interest for organic optoelectronic and energy storage devices. Current synthetic methods produce these materials as either insoluble, microcrystalline powders or as oriented thin films on various substrates, including single-layer graphene (SLG). Under these conditions, COF thin films form on both the graphene-coated and bare regions of the substrate, suggesting uncontrolled nucleation processes that occur either in solution or nonselectively on different surfaces. Here, we describe modified polymerization conditions that provide COF films selectively on SLG. This finding enables COF films to be grown on lithographically patterned SLG substrates, which provide insight into the uniformity of film growth across the substrate and factors relevant to their nucleation and growth. The ability to grow COF films selectively on lithographically patterned SLG will facilitate their integration into devices. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 378–384