Block copolymers of incompatible segments form microphase-separated structures in the bulk or in solution. The observed morphologies include spheres, rods, vesicles, lamellae, worm-like micelles, and several others.1, 2 Block copolymer micelles that are thermodynamically stable under a given set of conditions have many potential applications. They have, for example, been used as templates to produce metal nanoparticle arrays3 or drug delivery vehicles.4
The morphological characteristics of a given system depend on the conditions of preparation such as, for instance, the thermodynamic quality of the solvent, temperature, and concentration. For this reason, it is impossible to use these systems (e.g., spherical micelles) in specific applications under different conditions, for example, in a different solvent, at a different temperature or at other concentrations. In the recent years, block copolymer nanostructures have been chemically processed to yield permanent nanostructures5 including hollow nanospheres,6 vesicles,7 and nanotubes.8
Crosslinking has been shown to be important to the preservation of mesostructural ordering. Without crosslinks, the block copolymer nanostructures rearrange extensively upon solvent swelling, destroying the spatial patterning imposed by preprocessing. The preparation of fixed nanometer-sized structures based on block copolymers has attracted much attention mainly due to the fact that they span the size range from 10 to 500 nm and fill the gap between structures obtained by lithographic techniques and those from small molecule self-assembly.
Various strategies have been used to construct stabilized supramolecular assemblies. One strategy is based on the self-assembly of block copolymers in the bulk.9, 10 Depending on their composition, they can form various morphologies in the solid state. Selective crosslinking of one of the blocks and redissolution leads to nanoscopic particles. Concerning the preparation of one-dimensional polymer nanofibers, Liu et al.9 stabilized polystyrene-b-poly(cinnamoylethyl methacrylate) chains by performing photo crosslinking reaction in the bulk. In a different approach, Müller and coworkers10 have chemically crosslinked the polybutadiene chains of a triblock terpolymer by treating with S2Cl2 reagent. The as-prepared nanostructures were found to be dispersed in organic media.
Another strategy to form stabilized polymeric nano-objects is based on the self assembly of block copolymers in solution. In the recent years, there have been several reports studying the stabilization of morphologies in solution.11–16 In all the aforementioned studies, diblock copolymers and occasionally triblock terpolymers have been used to produce polymer-based nano-objects. On the contrary, to the best of our knowledge, polymeric nano-objects consisting of heteroarm star copolymers of the type AnBn (comprising pure A and B arms) have not been prepared yet.
Previous reports on self-assembly studies of the AnBn star copolymers have demonstrated their ability to form reversible nanostructures in selective solvents,17 at surfaces18 and in the bulk.19 In this work, we report on the first attempt to fabricate well-defined irreversible “hairy” nano-objects using as building elements PS5P2VP5 heteroarm star copolymers after chemical crosslinking of their self-assemblies using 1,4-dibromobutane.15, 20, 21 To obtain nano-objects of diverse morphology from the same star copolymer (symmetric in heteroarm lengths), different self-assembly and casting conditions were attempted.
The PS5P2VP5 heteroarm star copolymer was prepared via a three-step sequential “living” anionic polymerization procedure (“In–Out” method) using divinyl benzene (DVB) linkage as described elsewhere.22 In the first step, the polystyrene arms were prepared using sec-butyl lithium as the initiator at −40 °C in THF. After the consumption of the styrene monomer and sampling out, a small amount of divinylbenzene was added to the reaction medium. Star-shaped polystyrene (PSn) was thus formed, part of which was deactivated and sampled out for the purpose of characterization. The rest of the “living” star polymer was used to initiate the polymerization of a chosen amount of 2-vinyl pyridine that was added to the reaction medium at −78 °C. After complete polymerization of 2-vinyl pyridine, the reaction mixture was deactivated with degassed methanol, and the final product was recovered by precipitation in cold heptane, dried, redissolved in benzene, and freeze-dried. The molecular characteristics of the heteroarm star copolymer are given in Table 1.
Table 1. Molecular Characteristics of the PSnP2VPn Heteroarm Star Copolymer
Crosslinking of Block Copolymer Films
A star copolymer film 0.1-mm thick was cast from 0.5 wt % toluene or chloroform solutions on a Teflon sheet. After spontaneous solvent evaporation at room temperature, the as-prepared polymer film was dried under vacuum at 35 °C overnight. The film obtained after evaporation of chloroform was annealed for 15 h at 150 °C, well above the glass transition temperatures of PS and P2VP. Four milligrams of the polymeric film was put into a small desiccator while 0.5 mL of 1,4-dibromobutane was added also in a separate small beaker. The whole system was under vacuum and for the first 5 min of the process some mild heating was used to produce the vapors of the dibromide. After this short period, the system remained in a reduced pressure for 3, 17, 26, 40, and 67 h, respectively. After the crosslinking reaction, the liquid dibromide was taken away from the dessicator and the film was dried under vacuum at 35 °C. The dried film was added to THF and heated in a closed glass tube at 55 °C for 48 h for better dissolution.
Crosslinking of Block Copolymer Aggregates in Solution
The crosslinking of the star copolymer was carried out in toluene at room temperature by using liquid DBB. The copolymer concentration was fixed at 0.5 or 2.5 wt %, while DBB was added at a desired ratio with 2-vinyl pyridine moieties (2VP:DBB = 2:3). The resulting reaction mixture was equilibrated for 4 days and was diluted with THF before casting onto a mica surface for scanning electron microscopy (SEM) imaging.
Imaging of the Polymer Nanostructures
The samples were prepared by two different methods: solution casting or combination of bath sonication and solution casting onto a mica surface and dried at room temperature. SEM was performed using a LEO SUPRA 35 VP scanning electron microscope. All specimens were sputtered with gold before imaging.
RESULTS AND DISCUSSION
The goal of this work was to fabricate nano-objects taking advantage of the self-association ability of star-shaped copolymers to form reversible nanostructures, which can be permanently stabilized by covalent bonding. Prerequisite for such a process is that one of the set of arms of the copolymer to bear crosslinkable moieties, which are met in P2VP-based star copolymers, and that they can form discernible nanodomains. After crosslinking the P2VP domains, the geometric shape of the stabilized nano-objects should be retained.
The influence of the architecture on the phase behavior of symmetric PSnP2VPn heteroarm star copolymers was extensively investigated previously.17, 19 Transmission electron microscopy (TEM) images of thin films of PSnP2VPn samples obtained from dilute polymer solutions in a selective medium, such as toluene (0.01 wt %), yielded micellar nanospheres irrespective of the P2VP fraction.17(b) The internal structure of these “hairy” soft spheres is that of a core–shell type, where the P2VP arms constitute the core and the PS chains the outer protecting shell.
In the initial attempt of this study, thin films cast from semidilute polymer solutions (0.5 wt %) in toluene were prepared, followed by selective crosslinking of the P2VP domains by using DBB vapors. From the seminal work of Ishizu and coworkers,20, 23 it was clear that P2VP domains in block copolymer films having various shapes (spherical, lamellar and cylindrical) were quaternized with DBB in such reaction conditions. In these studies, the authors used copolymers with low poly(vinyl pyridine) content (less than 35 wt %). After incubation with DBB vapors, the solubility of the crosslinked polymeric nanostructures in organic media, such as THF, was appreciably decreased at room temperature.
To observe the result of the chemical stabilization of the polymeric nanostructures in the bulk and to determine their morphology, the crosslinked films were disrupted and the nano-objects were dissociated by heating the film in a common good solvent for both block-arms before being imaged by SEM. By this procedure, the formation of reversible self-assemblies could be avoided. The crosslinked nanostructures were dispersed in THF by heating at 55 °C in closed vials for several hours under stirring to aid good dispersion. The solubility of the nanostructures was largely dependent on the incubation time between the polymer film and the DBB vapors. It is obvious that the nanostructures, obtained after the longest reaction time (67 h), showed the lowest dispersability in THF, due to the increased size of the stabilized nanostructures. To investigate the morphology of the crosslinked nanostructures, SEM imaging was carried out on nano-objects supported on a mica substrate.
Figure 1 shows typical images of crosslinked nanostructures after incubating a film in DBB atmosphere for 3, 40, and 67 h, respectively. In general, after 1 day of incubation, the nanostructures still seem to exhibit an amorphous shape [see Fig. 1(A)]. This demonstrates clearly that the quaternization of pyridine moieties by DBB molecules within the polymer films is a relatively slow process (rate constant <10−4 lt/mol min).24 After a 40 h incubation time, almost cylindrical nano-objects were observed [Fig. 1(B)], whereas at 67 h, perfect cylindrical nanostructures were obtained [Fig. 1(C)]. The average diameter of these nanostructures was estimated to be about 60 nm, whereas the length ranges in the μm scale.
It is widely known that the dispersability of nanoparticles in solution depends strongly on the processing parameters (vigorous stirring and sonication). In all the aforementioned images, the preparation of the samples was carried out by drop casting THF solutions onto mica. Because of the extremely long cylinders in the bulk morphology and morphological defects connecting different polymeric cylinders to a loose network after crosslinking, the crosslinked films were dissociated and disrupted by bath sonication of a THF solution before being measured by SEM (Fig. 2). The SEM images clearly show good dispersion of nano-cylinders with a relative long persistence length (of the order of μm). This shape should be related to the intrinsically stiff, crosslinked P2VP cores, demonstrating that the crosslinking approach is very effective. Some cylinders show branch points, which either reflect morphological defects of the previous bulk structure or result from the deposition on the substrate.18(d)
The question arisen at this point is why the stabilized structure is of cylindrical shape as the micellar entities resulted from self-association of these kind of heteroarm star copolymers in diluted toluene solutions (0.01 wt %) adopt spherical morphology.17 It is well known that the copolymer concentration in a selective medium influences the morphologies of the resulting self-assemblies.25 Therefore, passing from a dilute solution to the dry state (film), the gradual increase in concentration should account for the transformation of the micellar morphology from spheres to cylinders. The fact that the predominant morphology of PSnPEOn heteroarm star copolymers in highly concentrated solutions is that of cylindrical assemblies organized in a two-dimensional hexagonal lattice corroborates the aforementioned assumption.26 Indeed, as the spherical micelles approach each other upon densification, P2VP-arm intermicellar (from adjacent cores) attractive interactions are possible by retraction of the PS arms leading to the formation of micelles of cylindrical morphology. This model has been applied to account for the interstar coupling reaction that leads to nanofibers.27 Prerequisite for this process was the low number of arms which permit their retraction. This seems to be valid in our case as the aggregation numbers for such heteroarm star copolymers are about 15 ± 5, forming spherical micelles.17 It should also be mentioned that, although the bulk equilibrium morphology is lamellae,19 the cylindrical structure is preserved without annealing due to the glassy character of both block arms. In fact, it is a non equilibrium structure.
In an alternative processing method, the chemical crosslinking of the self-assembled PSnP2VPn nanostructures was carried out in toluene solutions using liquid 1,4-dibromobutane. To determine the morphology of the polymeric-stabilized nanostructures, the toluene solution was highly diluted with THF and was cast onto mica surface. Figure 3 shows images of crosslinked nanostructures after incubating a PS5P2VP5 solution in toluene (0.5 wt %) with slight molar excess of liquid DBB for a period of 4 days.
The predominant morphology observed in Figure 3 is that of nano-spheres, which arise from the stabilization of the formed spherical core–shell micelles as expected.17 However, some sphere interconnections are visible in the micrograph of Figure 3, which should be attributed to concentration effects as discussed earlier.
To explore the ability to obtain nano-objects of lamellar morphology, we replaced the PS-selective toluene with chloroform. The chloroform-cast films were annealed for 15 h at 150 °C (well above Tg of both arms) before crosslinking with DBB vapors for a period of 67 h. Accordingly, hot THF was used to disperse the stabilized self-assemblies and deposit them on mica surface. Grayer et al.19 have investigated the microphase separation of PSnP2VPn star copolymers in the bulk with variable volume fractions of PS. As it was shown, casting from chloroform solutions films resulted in a lamellar morphology for the polymer sample having symmetric PS and P2VP arms. In Figure 4, SEM images illustrate the results of the stabilization of the “chloroform-cast” nanostructures; the morphologies of which are far from our expectations. However, some interesting observations deserve to be discussed, as nano-objects resulted from lamellae stabilization by crosslinking in the bulk and depicted by direct observation (e.g., SEM or TEM) has never been reported even for diblock copolymers. The image of Figure 4(A) shows that even in chloroform (common good solvent) and at elevated concentrations, spherical nano-objects may be formed which are transformed to ribbons toward to lamellar morphology [Fig. 4(C)]. In the images of Figure 4(B,D) one can observe interconnected ribbons emanated from lamellar fragments. In the bulk conditions, the driving force of the assembly process is the incompatibility of the different arms which may lead even to spherical nanostructures at the initial stage of microphase separation of the heteroarm stars probably due to the star polymer topology which favor spherical symmetry. The results depicted in Figure 4 concern morphologies from the same experiment, which may have resulted from either deficient crosslinking or nonequilibrium microphase separation. It is reasonable to assume that the main origin of the undefined morphologies obtained from chloroform should be attributed to the slow and hindering crosslinking quaternization reaction of 2VP moieties in the lamellar structure.
In this communication, we demonstrated the first attempt to fabricate nano-objects using as building elements star-shaped copolymers with crosslinkable arms. The process followed, comprises self-organization of PS5P2VP5 heteroarm copolymers under various conditions, stabilization of the self-assemblies by crosslinking quaternization reaction of the P2VP nanodomains, and dispersion of the resulted nano-objects in solution. Well defined “hairy” nanocylinders of about 60 nm in diameter and lengths of the order of few microns were obtained in good yield by film casting from toluene which is a selective solvent of PS arms. These nanocylinders, consisted of crosslinked P2VP cores (swellable in polar media)28 and PS grafted chains, may find applications in various fields of nanotechnology. For instance, they could be used as the filler component in multifunctional nanostructured nanocomposites.
The authors thank Mrs. Teodora Maria Popescu for her contribution on the preparation of thin films and Dr Vassilios Dracopoulos for technical assistance in SEM imaging. This work was carried out in the framework of NoE Nanofun-Poly project.