Thin films of complexed block copolymers

Due to their ability to microphase separate into well ordered structures with periodicities on the nanometre scale, block copolymers have received widespread attention as building blocks for the fabrication of nanomaterials. In particular, thin films of block copolymers promise new technological breakthroughs in e.g. computer memory applications. This Review gives a short overview of progress that has been made in preparing suitable thin films of conventional coil–coil diblock copolymer systems, while the advantages as well as the complexities of using more unconventional systems such as triblock copolymers and supramolecular systems are emphasized.

Hierarchical Terrace Formation in PS-b-P4VP(PDP) Supramolecular Thin Films

The terrace formation behavior of chloroform vapor annealed thin films of asymmetric, low molecular weight comb-shaped supramolecules consisting of a short polystyrene (PS) block and a long supramolecular block of poly(4-vinylpyridine) (P4VP) hydrogen bonded with pentadecylphenol (PDP) on silicon oxide (SiO2) was examined with atomic force microscopy. During annealing, PS microphase separated from the disordered P4VP(PDP) comb, resulting in the formation of terraces of parallelly oriented microdomains of PS in a matrix of P4VP. Upon evaporation of the solvent, the P4VP(PDP) combs dropped below their order-disorder transition, and formed alternating layers of P4VP and PDP, which for high P4VP(PDP) fractions were also oriented parallel to the substrate. This resulted in terraces of the short P4VP(PDP) length scale within terraces of the PS-P4VP long length scale. Washing away PDP from the thin films with ethanol provided an effective means of studying the morphology of the lowest terrace of the thin films and, for a particular system, also resulted in a uniform monolayer of cylinders with a PS core and a P4VP corona.

Incorporation of PPE in Lamellar Self-Assembled PS-b-P4VP(PDP) Supramolecules and PS-b-P4VP Diblock Copolymers

Self-assembled blends of PS-b-P4VP(PDP) supramolecules, obtained by hydrogen bonding of pentadecylphenol (PDP) side chains to poly(4-vinylpyridine), and poly(2,6-dimethyl-1,4-diphenyl oxide) (PPE) were investigated by thermal analysis and small-angle X-ray scattering (SAXS) and compared with blends of PS-b-P4VP and PPE. Differential scanning calorimetry (DSC) measurements showed a single composition dependent Tg of the PPE/PS layers for both systems, demonstrating that PPE is distributed throughout the PS layers. Furthermore, DSC showed that for the PPE/PS-b-P4VP(PDP) blends the presence of PDP is not restricted to the P4VP layers. Its partial presence in the PS-containing domains was confirmed by nuclear magnetic resonance (NMR) spectroscopy on PS-P4VP core-corona nanorods prepared from hexagonally self-assembled PS-b-P4VP(PDP) supramolecules. The results of the SAXS study on the dependence of the lamellar period of PPE/PS-b-P4VP blends on the amount of PPE were in excellent agreement with a theoretical model based on the Alexander-De Gennes approximation assuming a uniform distribution of PPE throughout the PS layers. For PPE/PS-b-P4VP(PDP) blends the dependence of the long period on the amount of PPE turned out to be somewhat stronger, which may be related to the supramolecular comb-shaped nature of the P4VP(PDP) blocks.

Phase Behavior of Solvent Vapor Annealed Thin Films of PS-b-P4VP(PDP) Supramolecules

The phase behavior and terrace formation of solvent (chloroform) vapor annealed thin films of asymmetric comb-shaped supramolecules consisting of a polystyrene (PS) block and a supramolecular block of poly(4-vinylpyridine) (P4VP) hydrogen bonded with pentadecylphenol (PDP) on silicon oxide (SiO2) were examined. P4VP(PDP) was found to be present at the SiO2 interface as well as the air interface, implying symmetric boundary conditions. Because of the inherent change in composition by swelling in a selective solvent, the morphology of a lamellar film could be changed to cylindrical by swelling at different vapor pressures of chloroform vapor. Swelling at a specific vapor pressure at the phase boundary between lamellar and cylindrical resulted in noncommon terrace formation behavior. The lowest terrace consisted of two wetting layers forming one lamella, the second terrace contained perpendicular lamellae, and the highest terrace consisted of parallel P4VP(PDP) cylinders. The results are presented in a morphology diagram as a function of film thickness and composition.

Nanorod engineering by reinforcing hexagonally self-assembled PS–b–P4VP(DDP) with PPE

Nanorods consisting of a polystyrene core and a poly(4-vinylpyridine) shell produced via the self-assembly route of comb-shaped supramolecules exhibit very poor mechanical properties. Adding a sufficient amount of poly(2,6-dimethyl-1,4-diphenyl oxide) introduces entanglements to the PS-core resulting in nanorods with much better properties, which can be used as templates for e.g. transition metal oxide tubes.

New insight into the formation of structural defects in poly(vinyl chloride)

The monomer conversion dependence of the formation of the various types of defect structures in radical suspension polymerization of vinyl chloride was examined via both 1H and 13C NMR spectrometry. The rate coefficients for model propagation and intra- and intermolecular hydrogen abstraction reactions were obtained via high-level ab initio molecular orbital calculations. An enormous increase in the formation of both branched and internal unsaturated structures was observed at conversions above 85%, and this is mirrored by a sudden decrease in stability of the resulting PVC polymer. Above this threshold-conversion, the monomer is depleted from the polymer-rich phase, and the propagation rate is thus substantially reduced, thereby allowing the chain-transfer processes to compete more effectively. In contrast to the other defects, the chloroallylic end groups were found to decrease at high conversions. On the basis of the theoretical and experimental data obtained in this study, this decrease was attributed to copolymerization and abstraction reactions that are expected to be favored at high monomer conversions. Finally, a surprising increase in the concentration of the methyl branches was reported. Although a definitive explanation for this behavior is yet to be obtained, the involvement of transfer reactions of an intra- or intermolecular nature seems likely, and (in the latter case) these could lead to the presence of tertiary chlorine in these defects

Ordered Arrays of Ferroelectric Nanoparticles by Pulsed Laser Deposition on PS-b-P4VP(PDP) Supramolecule-Based Templates

Thin films of comb-shaped supramolecules have been used to create arrays of spatially separated ordered nanorods with a polystyrene core and a poly(4-vinyl pyridine) corona. Room temperature pulsed laser deposition of a uniform layer of lead titanate on top of these nanorod arrays and subsequent heating to 565 °C, far above the degradation temperature of the block copolymer nanorods, resulted in ordered arrays of ferroelectric lead titanate nanoparticles, due to the evaporation of the polymer rods and the SrTiO3 substrate-nucleated crystallization of the lead titanate. The spacing in between the ordered cylinders of the template and the nonselective nature of the coating procedure sets the method apart from conventional block copolymer templating techniques involving parallel cylindrical structures. Given the nonselective nature of the coating method, this template procedure is applicable for a large variety of inorganics.

New Insight into the Formation of Structural Defects in Poly(Vinyl Chloride)

The monomer conversion dependence of the formation of the various types of defect structures in radical suspension polymerization of vinyl chloride was examined via both 1H and 13C NMR spectrometry. The rate coefficients for model propagation and intra- and intermolecular hydrogen abstraction reactions were obtained via high-level ab initio molecular orbital calculations. An enormous increase in the formation of both branched and internal unsaturated structures was observed at conversions above 85%, and this is mirrored by a sudden decrease in stability of the resulting PVC polymer. Above this threshold-conversion, the monomer is depleted from the polymer-rich phase, and the propagation rate is thus substantially reduced, thereby allowing the chain-transfer processes to compete more effectively. In contrast to the other defects, the chloroallylic end groups were found to decrease at high conversions. On the basis of the theoretical and experimental data obtained in this study, this decrease was attributed to copolymerization and abstraction reactions that are expected to be favored at high monomer conversions. Finally, a surprising increase in the concentration of the methyl branches was reported. Although a definitive explanation for this behavior is yet to be obtained, the involvement of transfer reactions of an intra- or intermolecular nature seems likely, and (in the latter case) these could lead to the presence of tertiary chlorine in these defects.