Effect of Block Copolymer Adsorption on Thin Film Dewetting Kinetics

Evolution of homopolymer thin-film instability on surface-anchored diblock copolymers varying in composition

The stability of molecularly thin polymer films deposited on various material substrates is of critical importance to many contemporary nanotechnologies involving functional coatings and nano/micropatterned surfaces, in which case the causes responsible for film destabilization must be fully understood. Previous experimental studies report that factors such as film thickness and polymer molecular weight play significant roles in governing the rate, as well as mechanism, of destabilization. Complementary theoretical predictions reveal that surface heterogeneities can likewise induce (and regulate the process of) destabilization. In this study, we investigate the destabilization rate and mechanism of homopolystyrene (PS) films differing in thickness on top of poly(styrene-b-methyl methacrylate) (SM) diblock copolymer monolayers varying in chemical composition anchored to flat silica-like substrates to examine the effect of surface constitution on PS stability. Copolymers with a long M block consistently promote PS dewetting by nucleation and growth, wherein the linear dewetting rate decreases monotonically with increasing PS molecular weight, film thickness, and S fraction in the SM copolymer. In analogous studies involving a copolymer with a relatively short M block, however, PS dewetting proceeds instead by spinodal dewetting that evolves gradually into nucleation and growth as the film thickness is increased.

Control of dispersion state of silsesquioxane nanofillers for stabilization of polystyrene thin films

The influence of the dispersion states of the nanofillers on the dewetting behavior of the polymer thin film was investigated. Polyhedral oligomeric silsesquioxanes (POSS) with various substituents were added into polystyrene (PS) thin films as the nanofillers. The dewetting rate of the films drastically changed with the surface substituents of POSS additives. Neutron reflectivity measurements indicated that the difference of the dewetting rate was associated with the dispersion state of POSS additives in the films. POSS with phenethyl groups (PhPOSS), which homogeneously dispersed into the films, resulted in the decrease of the glass transition temperature of PS and the enhancement of the dewetting of the films. POSS with a fluoroalkyl group (CpPOSS-R f) segregated to the film surface and showed the retardation of the dewetting by the decrease of the surface energy of the film. POSS with hydroxyl groups (CpPOSS-2OH) segregated to the film surface and film-substrate interface and led to the elimination of the dewetting, suggesting the importance of the interfacial segregation for the inhibition of dewetting. These results revealed the strong relationship between the dispersion state of the nanofillers and the dewetting of the nanofilled films.

Adsorption of Block Copolymers from Selective Solvents on Curved Surfaces

We have investigated the adsorption of asymmetric poly(styrene-b-methyl methacrylate) block copolymers (PS-PMMA) from a selective solvent onto alumina (Al(2)O(3)) particles having variable and controllable radii. The solvent used was a bad solvent for the PS block (block A) and a good solvent for the PMMA block (block B), which has a higher affinity of the surface. Such a case represents a new class of adsorption, where both blocks compete for the adsorption sites of the metallic surface. Two theoretical models, the modified drops model and the perforated film model, have been evaluated as appropriate representation of such an adsorption scenario. The experimental results indicated that the adsorption of the PS-PMMA block copolymer generated a patterned surface comprised of a homogeneous melt layer of the PS block perforated with holes having a variable PMMA structure, depending on the distance from the bottom of the hole (alumina surface) and the distance from walls of the hole. The density gradient of the PMMA moiety in the hole reverted to the classical brush morphology at a critical distance from the surface of the hole.

Influence of progressive cross-linking on dewetting of polystyrene thin films

We present dewetting experiments on thin polymer films as a function of cross-linking density. Covalent cross-links were obtained in the glassy state on the basis of azide photochemistry of linear random copolymers of styrene and p-(azidomethyl)styrene, i.e., 106 and 2500 kg/mol with 7% and 1% azide functionality among the polymer backbone, respectively. Upon ultraviolet radiation, azides generate highly unstable nitrene radicals which react with the surrounding polymer backbone, yielding covalent cross-links. We determined the probability for film rupture, defined by the number of holes formed per unit area, and the relaxation time (tauw) of residual stresses which resulted from the film preparation process. For the lower molar mass polymer studied and for azide conversion rates lower than 60%, only partial cross-linking occurred. The effective molar mass of the polymer increased, and consequently, an increase in tauw was observed. The increase in tauw was accompanied by a decrease in hole density, indicating that the still present residual stresses in the films were not able anymore to rupture the films at the high probability of un-cross-linked polymers. For high conversion (>60%), cross-linking was significant enough to lead to the formation of a three-dimensional rubbery network which, in turn, generated an elastic force that counteracted the driving forces. This elastic force eventually inhibited dewetting and the relaxation of residual stresses. Thus, at high conversions, the relaxation time tauw grew exponentially and the number of holes tended toward zero. For the higher molar mass polymer, no changes in the relaxation time tauw were observed for low conversion (<30%). However, at a higher conversion rate, tauw increased drastically, suggesting an almost infinitely long relaxation time at 100% conversion. Consequently, to successfully stabilize thin polymer films by cross-linking, it is preferable to use long polymer chains.

Dewetting dynamics in miscible polymer-polymer thin film mixtures

Thin polystyrene films supported by oxidized silicon (SiOx/Si) substrates may be unstable or metastable, depending on the film thickness, h, and can ultimately dewet the substrate when heated above their glass transition. In the metastable regime, holes nucleate throughout the film and subsequently grow due to capillary driving forces. Recent studies have shown that the addition of a second component, such as a copolymer or miscible polymer, can suppress the dewetting process and stabilize the film. We examined the hole growth dynamics and the hole morphology in thin film mixtures composed of polystyrene and tetramethyl bisphenol-A polycarbonate (TMPC) supported by SiOx/Si substrates. The hole growth velocity decreased with increasing TMPC content beyond that expected from changes in the bulk viscosity. The authors show that the suppression of the dewetting velocity is primarily due to reductions in the capillary driving force for dewetting and to increased friction at the substrate-polymer interface. The viscosity, as determined from the hole growth dynamics, decreases with decreasing film thickness, and is connected to a depression of the glass transition of the film.

Influences of chain heterogeneity on instability of polymeric thin films: Dewetting of polystyrenes, polychloromethylstyrenes and its copolymers

This study compares the stability of various polymeric thin films supported on SiO(x)/Si substrate. Dewetting behaviors of polystyrenes (PS), polychloromethylstyrenes, and random poly(styrene-co-chloromethylstyrene)s are investigated by utilizing atomic force microscopy. A systematic addition of the chloromethylstyrene (ClMS) unit into PS chain causes the increase of segment polarity, affecting interfacial and interchain interactions in thin films. It is found that stability of the polymeric films depends on two major parameters, ratio of the ClMS unit and film thickness. For approximately 5 nm thick film, the addition of only 5 mol% ClMS unit causes a drastic increase of its stability, attributed to the enhanced interfacial interactions between ClMS group and SiO(x) layer. Further increasing the ClMS mole ratio to 20, 45, and 100% is accompanied by a systematic increase of the film stability. Thicker films (thicknesses approximately 22 and approximately 45 nm) of the copolymer with 5 mol% ClMS unit exhibit rather different behavior. They are found to be less stable compared to the PS films. However, the films of copolymers with ClMS unit of 20, 45, and 100% are still much more stable than the PS films. These dewetting behaviors of the copolymers are correlated to the interfacial interactions, interchain interactions and segmental segregation in thin films.

Dewetting of thin polystyrene films under confinement

The dewetting behavior of thin polystyrene (PS) film has been investigated by placing an upper plate with a ca. 140 nm gap from the underlying substrate with the spin-coated thin polymer films. Three different kinds of dewetting behaviors of thin PS film have been observed after annealing according to the relative position of the PS film to the upper plate. Since the upper plate is smaller than the underlying substrate, a part of the polymer film is not covered by the plate. In this region (I), thin PS film dewetting occurs in a conventional manner, as previously reported. While in the region covered by the upper plate (III), the PS film exhibits unusual dewetted patterns. Meanwhile, in the area right under the edge of the plate (II) (i.e., the area between region I and region III), highly ordered arrays of PS droplets are formed. Formation mechanisms of different dewetted patterns are discussed in detail. This study may offer an effective way to improve the understanding of various dewetting behaviors and facilitate the ongoing exploration of utilizing dewetting as a patterning technique.

Dewetting behavior of a block copolymer/homopolymer thin film on an immiscible homopolymer substrate

Numerous previous studies have established that the addition of a microphase-ordered AB diblock copolymer to a thin homopolymer A (hA) film can slow, if not altogether prevent, film rupture and subsequent film dewetting on a hard substrate such as silica. However, only a few reports have examined comparable phenomena when the hA/AB blend resides on a soft B-selective surface, such as homopolymer B (hB). In this work, the dewetting kinetics of thin films composed of polystyrene (PS) and a symmetric poly(styrene-b-methyl methacrylate) (SM) diblock copolymer on a poly(methyl methacrylate) substrate is investigated by hot-stage light microscopy. Without the SM copolymer, the dewetting rate of the PS layer is constant under isothermal conditions and exhibits Arrhenius behavior with an apparent activation energy of approximately 180 kJ/mol. Addition of the copolymer promotes a crossover from early- to late-stage dewetting kinetics, as evidenced by measurably different dewetting rates. Transmission electron microscopy reveals the morphological characteristics of dewetted PS/SM films as functions of film thickness and SM concentration.