Evolution of order in thin block copolymer films

Depth-Resolved Characterization of Perylenediimide Side-Chain Polymer Thin Film Structure Using Grazing-Incidence Wide-Angle X-ray Diffraction with Tender X-rays

Polymers with a perylenediimide (PDI) side chain (PAc12PDI) consist of two kinds of crystalline structures with various types of orientations in a thin film. Understanding the population of the microcrystalline structure and its orientation along the thickness is strongly desired. Grazing-incidence wide-angle X-ray diffraction (GIWAXD) measurements with hard X-rays, which are generally chosen as λ = 0.1 nm, are a powerful tool to evaluate the molecular aggregation structure in thin films. A depth-resolved analysis for the outermost surface of the polymeric materials using conventional GIWAXD measurements, however, has limitations on depth resolution because the X-ray penetration depth dramatically increases above the critical angle. Meanwhile, tender X-rays (λ = 0.5 nm) have the potential advantage that the penetration depth gradually increases above the critical angle, leading to precise characterization for the population of crystallite distribution along the thickness. The population of the microcrystalline states in the PAc12PDI thin film was precisely characterized utilizing GIWAXD measurements using tender X-rays. The outermost surface of the PAc12PDI thin film is occupied by a monoclinic lattice with a = 2.38 nm, b = 0.74 nm, c = 5.98 nm, and β = 108.13°, while maintaining the c-axis perpendicular to the substrate surface. Additionally, the presence of solid substrate controls the formation of the crystallite with unidirectional orientation.

Effect of Preferential Orientation of Lamellae in the Interfacial Region between a Block Copolymer-based Pressure-Sensitive Adhesive and a Solid Substrate on the Peel Strength

We have investigated the relationship between the peel strength of a block copolymer-based pressure-sensitive adhesive comprising of poly(methyl methacrylate) (PMMA) and poly(n-butyl acrylate) (PnBA) components from the substrate and the microdomain orientations in the interfacial region between the adhesive and the substrate. For the PMMA substrate, the PMMA component in the adhesive with a strong affinity for the substrate is attached to the surface of the substrate during an aging process of the sample at 140 °C. Next, the PMMA layer adjacent to the substrate surface is overlaid with a PnBA layer, which gets covalently connected, resulting in the horizontal alignment of the lamellae in the interfacial region. The peel strength of the adhesive substantially increases during aging at 140 °C, which takes the same time as the completion of the horizontally oriented lamellar structure. However, in the case of the polystyrene (PS) substrate, both the components in the adhesive repel the substrate, leading to the formation of the vertically oriented lamellar structure. As a result, the peel strength of the adhesive with respect to its PS substrate does not entirely increase on aging. It is suggested that the peel strength of the adhesive is highly correlated with the interfacial energy between the adhesive and substrate, which can be estimated from the microdomain orientation in the interfacial region.

Consequences of surface neutralization in diblock copolymer thin films

Two high-χ block copolymers, lamella-forming poly(styrene-block-[isoprene-random-epoxyisoprene]) (PS-PEI78, with 78 mol % epoxidation) and lamella-forming poly(4-trimethylsilylstyrene-block-d,l-lactide) (PTMSS-PLA), were used to study three combinations of interfacial neutrality involving at least one neutral interface. PS-PEI78 annealed on a nonpreferential polymer mat (SMG) produced perpendicular lamellae independent of film thickness, indicating a neutral substrate and neutral free surface. In contrast, the presence of only one neutral interface results in the formation of surface topography ("islands" and "holes") with 0.5L0 step heights. PS-PEI78 (neutral free surface) annealed on PS brush (PS block preferential) forms "half" islands and holes. The inverse experiment, PTMSS-PLA (with a PTMSS preferential free surface) annealed on a neutral (or near neutral) substrate surface, also generates 0.5L0 topography. These "half" island and hole structures are stable to extended thermal annealing. PS-PEI78 exposes both blocks at the free surface in contrast to PTMSS-PLA, which exposes just one. All three combinations of interfacial neutrality are explained by the precise balancing of the wetting tendencies of the two blocks. Evolution of the 0.5L0 motifs appears to be facilitated by a preference to form half-period thick nuclei in the initial stages of morphological development.

A facile route to reassemble titania nanoparticles into ordered chain-like networks on substrate

A facile route to reassemble titania nanoparticles within the titania-block copolymer composite films has been developed. The titania nanoparticles templated by the amphiphilic block copolymer of poly(styrene)-block-poly (ethylene oxide) (PS-b-PEO) were frozen in the continuous PS matrix. Upon UV exposure, the PS matrix was partially degraded, allowing the titania nanoparticles to rearrange into chain-like networks exhibiting a closer packing. The local structures of the Titania chain-like networks were investigated by both AFM and SEM; the lateral structures and vertical structures of the films were studied by GISAXS and X-ray reflectivity respectively. Both the image analysis and X-ray scattering characterization prove the reassembly of the titania nanoparticles after UV exposure. The mechanism of the nanoparticle assembly is discussed.

Efficient synthesis of narrowly dispersed brush copolymers and study of their assemblies: the importance of side chain arrangement

Efficient, one-pot preparation of synthetically challenging, high molecular weight (MW), narrowly dispersed brush block copolymers and random copolymers in high conversions was achieved by ring-opening metathesis (co)polymerization (ROMP) of various macromonomers (MMs) using the highly active, fast-initiating ruthenium olefin metathesis catalyst (H(2)IMes)(pyr)(2)(Cl)(2)RuCHPh. A series of random and block copolymers were prepared from a pair of MMs containing polylactide (PLA) and poly(n-butyl acrylate) (PnBA) side chains at similar MWs. Their self-assembly in the melt state was studied by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). In brush random copolymers containing approximately equal volume fractions of PLA and PnBA, the side chains segregate into lamellae with domain spacing of 14 nm as measured by SAXS, which was in good agreement with the lamellar thickness measured by AFM. The domain spacings and order-disorder transition temperatures of brush random copolymers were insensitive to the backbone length. In contrast, brush block copolymers containing approximately equal volume fractions of these MMs self-assembled into highly ordered lamellae with domain spacing over 100 nm. Their assemblies suggested that the brush block copolymer backbone adopted an extended conformation in the ordered state.

Nanoscale rings fabricated using self-assembled triblock terpolymer templates

Although there has been extensive work on the use of self-assembled diblock copolymers for nanolithography, there are few reports of the use of multiblock copolymers, which can form a more diverse range of nanoscale pattern geometries. Pattern transfer from a self-assembled poly(butadiene-b-styrene-b-methyl methacrylate) (PB-b-PS-b-PMMA) triblock terpolymer thin film has been investigated. Polymers of different total molecular weight were synthesized with a predicted morphology consisting of PMMA-core/PS-shell cylinders in a PB matrix. By adjusting the solvent-annealing conditions and the film thickness, thin films with vertically oriented cylinders were formed. The PMMA cylinder cores and the PB matrix were then removed using selective etching to leave an array of PS rings, and the ring pattern was transferred into a silica film by reactive ion etching to form 19 nm high silica rings. This result illustrates the design and use of triblock terpolymers for self-assembled lithography.

Effect of temperature on the morphology and kinetics of surface pattern formation in thin block copolymer films

Hole formation and growth on the top layer of thin symmetric diblock copolymer films, forming an ordered lamellar structure parallel to the solid substrate (silicon wafer) within these films, is investigated as a function of time (t), temperature (T), and film thickness (l), using a high-throughput experimental technique. The kinetics of this surface pattern formation process is interpreted in terms of a first-order reaction model with a time-dependent rate constant determined uniquely by the short-time diffusive growth kinetics characteristic of this type of ordering process. On the basis of this model, we conclude that the average hole size, lambda(h), approaches a steady-state value, lambda(h)(t-->infinity) identical with lambda(h,infinity)(T), after long annealing times. The observed change in lambda(h,infinity)(T) with temperature is consistent with a reduction of the surface elasticity (Helfrich elastic constant) of the outer block copolymer layer with increasing temperature. We also find that the time constant, tau(T), characterizing the rate at which lambda(h)(t) approaches lambda(h,infinity)(T), first decreases and then increases with increasing temperature. This temperature variation of tau(T) is attributed to two basic competing effects that influence the rate of ordering in block copolymer materials: the reduction in molecular mobility at low temperatures associated with glass formation and a slowing of the rate of ordering due to fluctuation effects associated with an approach to the block copolymer film disordering temperature (T(d)) from below.

Two mechanisms of spontaneous curvature of strongly adsorbed (2D) double comblike copolymers

We propose a theory of spontaneous curvature of 2D comblike macromolecules with incompatible side chains of types A and B. It is expected that the side chains of both types are able to change their positions with respect to the backbone. We predict two mechanisms of curvature. In the case of strong incompatibility of the side chains, their complete segregation with respect to the backbone is responsible for the formation of the so-called "energetic" curvature that is a result of the difference in the length (or in the number) of the A and B chains. In the case of moderate incompatibility, partial mixing of the A and B side chains on the convex side of the molecule (a flip of shorter chains to the side of longer chains) can be entropically favorable. In this case, the stretching of the side chains decreases. The radius of the entropic curvature is determined by the length of the longer side chains.

Combinatorial study of surface pattern formation in thin block copolymer films

Surface pattern formation in diblock copolymer films is investigated as a function of film thickness h and molecular mass M. Smooth films are observed for certain h ranges centered about multiples of the lamellar thickness L0, and we attribute this effect to an increase in the surface chain density with h in the outer brushlike copolymer layer. We also observe apparently stable labyrinthine surface patterns for other h ranges, and the average size of these patterns is found to scale as lambda approximately L0(-2.5). Hole and island patterns occur for h ranges between those of the labyrinthine patterns and the smooth regions, and their size similarly decreases with L0 and M.

Growth of a single-domain smectic phase in a thin liquid-crystalline polymer film

The ordering process and kinetics in thin films (200-800-nm thick) of a thermotropic side-chain liquid-crystalline polymer have been investigated vertically and laterally, respectively, by x-ray reflectivity and atomic-force microscopy. The original smooth and amorphous spin-coated films initially become corrugated upon annealing in the smectic mesophase. The roughening of the surface results from the formation of randomly oriented microcrystalline domains in the film. At the same time, however, a laterally macroscopic crystal starts to grow from the substrate surface in the direction of the polymer-air interface at the expense of these domain structures. Finally, a nicely ordered single crystal with parallel-ordered bilayers is formed in the film as well as at the polymer-air interface. This one-dimensional crystallization, actually recrystallization, depends strongly on the temperature due to viscosity effects. At low temperatures, just above the glass-transition temperature, the ordering is very slow, but with increasing temperature the crystal growth is faster. An Arrhenius-type plot gives an activation energy of 122 kJ/mol, which we ascribe to the expected reorientations of the mesogenic groups during the recrystallization process.