Abstract
Due to the thermodynamic incompatibility between blocks, diblock copolymers can self‐assemble in a wide variety of nanostructures, covalent linkage among blocks preventing the phase separation at macroscopic scale. Those nanostructures depend on copolymer composition (f), Flory‐Huggins interaction parameter among both blocks (χ), and polymerization degree of the copolymer (N). Thin films of block copolymers can show different equilibrium morphologies such as spheres, cylinders, gyroids, and lamellas. Besides mentioned parameters, film preparation process (substrate, annealing process if any) and used solvent will determine self‐assembled morphology. In the present review, the most important morphologies or microstructures obtained for different diblock copolymer films are presented, as well as the most important phase transitions among them. Different microstructures and the way in which they can be obtained become of great importance, as they could be used as templates for nanoparticle deposition, nanolithography, or nanopatterned materials with several potential applications in different fields such as nanoelectronics or nanomedicine.
Keywords
- self‐assembly
- nanostructure
- morphology
- lamellar
- cylinders
- gyroids
1. Introduction
Block copolymers consist in macromolecules produced by joining two or more chemically distinct polymer blocks, that may be thermodynamically incompatible. Segregation of these blocks on the molecular scale (5–100 nm) can produce many different complex nanostructures. More than three decades of theoretical development have culminated in remarkably predictive statistical theories that can account for the domain shapes, dimensions, connectivity and ordered symmetry of many types of block copolymers. Nowadays, the possibility to join blocks in novel molecular architectures can produce a really huge number of structured materials endowed with tailored mechanical, optical, electrical, barrier, and other physical properties [1, 2].
Many different block configurations can be constructed with different synthetic chemistry techniques. Based on the number of chemically distinct blocks, and their linear or branched sequencing linear and branched diblock copolymers, linear ABC triblock copolymers and ABC star or heteroarm triblock copolymers can be distinguished. Those would be the most important ones, even if block copolymers with more than three blocks can be also found.
The simplest and most studied architecture is the linear AB diblock, consisting of a long sequence of A type monomers covalently bonded to a chain of B type monomers. (AB)n multiblocks are formed by coupling additional A and B blocks. Morphologies obtained by self‐assembly of diblock copolymers in films will be shown and reviewed in the present chapter.
Due to the thermodynamic incompatibility between blocks and the covalent bond among them that avoids macrophase separation, block copolymers can result in many different nanostructures [1, 3, 4]. For this reason, many researchers have shown interest in block copolymers during the last decade [5–7]. Block copolymers can self‐assemble into different nanostructures that depend on several parameters such as copolymer composition (
In the present chapter, a deep review on the generation of nanostructures by self‐assembly of diblock copolymers in thin films is presented. Besides classical lamellar, cylindrical or spherical morphologies obtained for many different diblock copolymers, more complex nanostructures such as perforated lamellae or bicontinuous gyroids will be shown as obtained by several authors for many different diblock copolymer types.
2. Morphology of diblock copolymer thin films
Matsen and Schick [14] calculated the phase diagram for AB diblock copolymers. For
2.1. Lamellar morphology
It constitutes the stable phase for symmetric diblock copolymers. It has been commonly found for many diblock copolymer thin films, both without any further treatment or after thermal or solvent vapor annealing. Both thermal and solvent annealing methods have been used to bring the samples into their equilibrium states and to reduce the number of defects [15]. Lamellar morphology can be used to create line and space patterns on substrates by controlling their orientation. Lamellar orientation depends on several parameters and/or factors such as the substrate used for film preparation [16, 17], thermal or solvent vapor annealing treatments [15, 17, 18], chain length or molar mass [16, 17, 19], pressure application during annealing [20] or the use of graphoepitaxial techniques for film preparation [21–23]. Thus, different orientations have been obtained for different block copolymers by controlling those parameters.
Regarding the effect of substrate used for preparing the films, different orientations of lamellar microdomains have been found for poly(styrene‐
Both thermal and solvent vapor annealing have also been used for the generation of lamellar morphologies with different orientations. In this way, the lamellar orientation of poly(styrene‐
Thermal annealing also affects the formation and orientation of lamellar domains. For poly(styrene‐
To perform the annealing process under vacuum constitutes another way to control the formation and orientation of lamellar microdomains [17, 20]. Highly oriented films can be obtained by annealing copolymer films under pressure. In this way, for a lamella‐forming poly(styrene‐
Chain length and molar mass of copolymers also are key factors in order to obtain lamellar structures with different orientations [16, 17, 19]. For films of PS‐
Block copolymers have attracted considerable attention as fabrication method for nanopatterning, in which nanometer‐sized domains are used as lithography templates. For dense line pattern formation, lateral lamellar domains are more preferable because in their case, an aspect ratio at least greater than 2 can be obtained. Epitaxial self‐assembly by chemically or topographically nanopatterned substrates or resist patterns as guide have been demonstrated to be effective for lateral lamellar orientation [21–23]. In this way, lamellar domains of PS‐
The lamellar microdomains of PS‐
2.2. Perforated lamellar morphology
Even if in the bulk state, the perforated lamellar morphology of block copolymers has been reported to be a metastable state of the gyroid (G) phase, under thin film confinement conditions, one‐dimensional confinement provides stability to the phase [26, 27] and has been found for different diblock copolymer thin films under several conditions [26–35]. For thin films of poly(styrene‐
At around 80 nm thickness, two layers of interconnected HPL were formed. Other solvent ratios produced mixed morphology structures such as comb‐like patterns from coexisting lamellae and cylinders. For thin film composites based on PS‐
For poly(methyl methacrylate‐
At a composition of 35 vol% of poly(methyl acrilate) (PMA), the formation of a HPL morphology was observed for a polydisperse poly(styrene‐
For poly(styrene‐
For the same copolymer but with 65% of PS [33], it was concluded that order‐order transitions between the L, HPL, and G phases proceeded through nucleation and growth. Maintaining the continuity of microphase‐separated interfaces across the resulting grain boundaries involved considerable local distortion of both morphologies. The different geometrical characteristics of the phases and the imbalance in the spacings of epitaxially related lattice planes constituted the reason for that. There was an increase of the surface energy of the grains, which strongly restricted nucleation, avoiding direct L to gyroid transitions. The formation of the metastable HPL structure under such conditions reflected the ease with which the L to PL transition could occur compared to L to G one. Similar effects dominate the G to L transition. Very similar conclusions were obtained by the same authors [33] for poly((ethylene‐co‐propylene)‐
For PS‐
Finally, for a poly(ethylene oxide‐
2.3. Hexagonally packed cylinders
Considered as one of the stable morphologies for asymmetric diblock copolymers, it has been found for many different copolymers [36–52] as a result of their composition or by different annealing processes. In that way, for amphiphilic poly(4‐di(9,9‐dihexylfluoren‐2‐yl)styrene)‐
HPC morphology has also been observed for thin films of poly(styrene‐
Nonequilibrium morphologies were also found for poly(styrene‐
For PS‐
For a poly(styrene‐
Liquid crystal (LC) block copolymers or copolymers containing a liquid crystal block has also been found to assemble into HPC structures [38, 39, 46, 49]. By working with a novel class of nanophase-separated and flexible double liquid crystalline EOBC‐
Electric field–induced alignment of microphase‐separated block copolymer domains has also been carried out for different copolymers such as PS‐
HPC has also been found for several copolymers synthesized for potential biological applications [40, 41]. In that way, uracil‐functionalized poly(3‐caprolactone)‐
Finally, HPC has also been found for synthesized novel PEGylated polypeptide block copolymers of poly. The hierarchical self‐assembly of their films led to the formation of L structures as a result of microphase separation of the diblock copolymers; HPC nanostructure featured α‐helical conformations of PEGylated polypeptide segments, which were oriented perpendicularly to the director of the L structure formed by the diblock copolymers. These kinds of rod‐coil block copolymers comprise an important class of nanomaterials because of their potential uses in biological and optoelectronic applications.
2.4. Gyroid morphology
The L and HPC morphologies cover a large range of compositions and molecular weights. In contrast, the gyroid (G) morphology corresponds to a narrow composition window at the weak segregation regime [2]. G morphology and transitions from lamellar or HPC morphologies to G ones have been found for different copolymers [32, 35, 44, 53–57], showing to have several potential applications.
For the production of devices as solar cells or special membranes, it results very interesting to obtain nanomaterials with connected pores. Thus, a way to prepare nanostructured porous materials would suppose a great advance for the design of such devices. As it seems to be difficult by common lithograpy, block copolymer lithography can be used as a “bottom up” approach, especially using DG structure. This morphology consists of two connected continuous networks of both blocks.
In that way, by using diblock copolymer‐based PS‐
The stability of G morphology at large segregation values was examined for poly(isoprene‐
A successful application of an ordered bicontinuous gyroid semiconducting network obtained from block copolymer template in a hybrid bulk heterojunction solar cell has been reported [56]. The freestanding G network was fabricated by electrochemical deposition into the 10 nm wide voided channels of a self‐assembled, poly(4‐fluorostyrene‐
The epitaxial relations of phase transitions between the L, HPL and G morphologies were investigated for PS‐
Finally, the ordered bicontinuous double diamond (OBDD) structure, that has long been believed to be an unstable ordered network nanostructure, relative to the ordered bicontinuous double gyroid (OBDG) structure for diblock copolymers, has been found for a copolymer composed of a stereoregular block, syndiotactic poly(propylene‐
2.5. Spheres
Spherical morphology can consist mainly on body‐centered cubic spheres (BCC), hexagonally close packed spheres (HP) or face‐centered cubic spheres (FCC). Compared with the L and C nanostructures, the spheres of the BCC structure do not have anisotropy, and thus, the orientation of the BCC structure in thin films is in general less complicated than that of the HPC or L structures [58]. Morphology consisting on spheres has been found for several copolymers [36, 58–63]. Regarding to BCC morphology, it has been found for thin films of asymmetric poly(dimethylsiloxane‐
The mechanism and process of the thermally induced OOT of PS‐
BCC‐sphere morphology has also been found for highly asymmetric poly(styrene‐
Apart from BCC, other sphere symmetries have also been found in diblock copolymers [61–63]. For thin films of amphiphilic poly(4‐di(9,9‐dihexylfluoren‐2‐yl)styrene)‐
Acknowledgments
Financial support from the Basque Country Government (Grupos Consolidados, IT‐776–13) and the Ministry of Economy and Competitiveness (MAT2015–66149‐P)‐Spain is gratefully acknowledged.
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