Session S19: Focus Session: Thin Films of Block Copolymers and Hybrid Materials I - Solvent Vapor Annealing

Self-assembly kinetics in Symmetric Diblock Copolymer Thin Films during solvent assisted thermal treatments

Block copolymer (BCP) microphase separation and ordering by thermal annealing is often a challenge because of its slow kinetic. Towards the objective of rapid processing and accessing desired nanostructures, in this study we propose and discuss an alternative approach based on the use of a Rapid Thermal Processing (RTP) system that allows self-organizing symmetric polystyrene-b- poly(methyl methacrylate) (PS-b-PMMA) thin films in few seconds, taking advantage of the residual amount of solvent present in the film after the spinning process. Distinct ordered morphologies, coexisting along the sample thickness, can be obtained in PS-b-PMMA samples with the formation of lamellae laying over a hexagonal pattern of PMMA cylinders embedded in the PS matrix and perpendicularly oriented with respect to the substrate. The thermal evolution of the entrapped solvent and the dynamics and morphological stabilization of the coexisting phases are described and the intimate mechanism of the self-assembly process are discussed and fully elucidated.   

Thin Film Morphologies of Bulk-Gyroid Polystyrene-block-Polydimethylsiloxane under Solvent Vapor Annealing

Thin film morphologies of a 75.5 kg/mol polystyrene-block-polydimethylsiloxane (PS-PDMS) diblock copolymer (SD75) subject to solvent vapor annealing are described. Thin films were spin-cast from 1{\%} solution of SD75 in cyclohexane and annealed in cosolvent vapors consisting of mixed toluene and heptane vapors. The PS-PDMS has a double-gyroid morphology in bulk, but as a thin film the morphology consists of spheres, cylinders, perforated lamellae or gyroids, depending on the film thickness, its commensurability with the microdomain period, and the ratio of toluene:heptane vapors used for the solvent annealing process. The morphologies are described by self-consistent field theory simulations. Thin film structures with excellent long-range order were produced, which are promising for nano patterning applications.   

Design, construction, and testing a purpose-built climate-controlled solvent vapor annealing chamber for guided self-assembly of block polymer thin films

Despite its efficacy to produce well-ordered, periodic nanostructures, the intricate role multiple parameters play in solvent vapor annealing has not been fully established. In solvent vapor annealing a thin polymer film is exposed to the vapors of a solvent(s) thus forming a swollen and mobile layer to direct the self-assembly process at the nanoscale. Recent developments in both theory and experiment have directly identified critical parameters, but controlling them in any systematic way has proven non-trivial. These identified parameters include vapor pressure, solvent concentration in the film, and, critically, the solvent evaporation rate. To explore their role, a purpose-built solvent vapor annealing chamber was designed and constructed. The all-metal chamber is inert to solvent exposure and pneumatically actuated valves allow for precision timing in the introduction and withdrawal of solvent vapor. Furthermore, the mass flow controlled inlet, chamber pressure gauges, \textit{in situ} spectral reflectance-based thickness monitoring, and high precision micrometer relief valve, give real-time monitoring and control during the annealing and evaporation phases. Using atomic force microscopy to image the annealed films, we are able to map out the parameter space for a series of polystyrene-$b$-polylactide ($M_{n} \quad =$ 75 kg/mol and $f_{PLA} \quad =$ 0.28) block polymer thin films with an intrinsic cylindrical morphology and identify their role in directed assembly. Funded by Creighton University Summer Research Grant.   

Block copolymer alignment by shear induced during solvent vapor annealing with a crosslinked elastomer capping layer

The long range alignment of block copolymers (BCPs) is generally accomplished through application of a gradient shear force or by topographical or chemical cues patterned into the substrate. These techniques require lithographic patterning, specialty substrates or custom built equipment to achieve the alignment, which limits the broad academic application of aligned BCPs. One technique to improve the large range ordering of BCPs is solvent vapor annealing (SVA), which exposes the BCP film to a controlled atmosphere of solvent vapor to swell the BCP and provide significant enhancements in the chain mobility. Here, we discuss a minor modification of the SVA process; a thin piece of crosslinked poly(dimethyl siloxane) (PDMS) is placed on top of the BCP film before SVA. Exposure to organic solvent vapors causes the PDMS to swell, while the solvent also plasticizes the BCP film. Removal of the solvent induces a shear to the BCP film as the PDMS shrinks back to its initial dimensions. The shape of the PDMS cap determines the anisotropy in the stress applied on deswelling that aligns and orients the BCP domains. Polystyrene-block-polyisoprene-block-polystyrene (SIS) is utilized as a model system to illustrate how the processing parameters impact the orientation as determined by both grazing incidence small angle x-ray scattering (GISAXS) and atomic force microscopy (AFM). Quantification of the alignment by Herman's orientational parameter (S) illustrates high degree of alignment (S$=$0.95) is possible through appropriate selection of processing conditions. This SVA-based alignment method provides a relatively simple method to orient BCP films within general SVA processing protocols.   

Optimization of long-range order in solvent-annealed polystyrene-$b$-polylactide block polymer thin films for nanolithography

We demonstrate long-range order in solvent-annealed polystyrene-$b$-polylactide block polymer thin films for nanolithographic applications. This is accomplished \textit{via} climate-controlled solvent vapor annealing, \textit{in situ} solvent concentration measurements, and small angle x-ray scattering. By connecting the properties of swollen and dried films, we identify ``best practices'' for solvent-annealing, including that exposing block polymer films to a neutral solvent concentration just below the identified (\textit{via }x-ray scattering) order-disorder transition, at low pressures, with fast solvent evaporation rates, will consistently yield large lateral correlation lengths (\textgreater 6.9 $\mu $m) of hexagonally-packed cylinders that span the entire thickness of the film with center-to-center spacing ranging from 43 -- 59 nm. The resultant films have sufficient fidelity for pattern transfer to an inorganic material, as evidenced by patterning of Ni metal nanodots using a damascene-type approach. We argue that our results can be qualitatively understood by analogy to thermal annealing of a single-component solid, where annealing just below the melting point leads to optimal recrystallization. Such reliability, combined with recently developed pattern-transfer techniques, places this cheap and rapid method of nanolithography in competition with conventional lithography schemes. Funded by NSF MRSEC and Creighton University Summer Research Award.   

Unidirectional alignment of block copolymer templated porous films using solvent vapor annealing with soft shear

Porous films templated by block copolymers (BCPs) have been extensively investigated due to their potential numerous applications such as sorbents and nanolithography. However, in many cases, their performance critically depends on their nanostructural alignment and orientation. Achieving unidirectional alignment of these nanostructures over macroscopic dimensions is still challenging especially for BCPs with very high $\chi $ and Tg. Here, we illustrate a new method based on solvent vapor annealing with soft shear (SVA-SS), where a crosslinked poly(dimethylsiloxane) (PDMS) cap is simply adhered to the polymer films during SVA, to fabricate macroscopically aligned cylindrical structured mesoporous films using poly(styrene-block-N,N,-dimethyl-n-octadecylamine p-styrenesulfonate) (PS-b-PSS-DMODA) as the soft-template and phenolic resin as the precursor. The evolution of structures through the SVA-SS, thermal annealing and carbonization is determined by grazing incidence small angle x-ray scattering (GISAXS) and atomic force microscopy (AFM). Highly ordered mesoporous carbon films with S\textgreater 0.8 can be obtained by this method. Potential applicability of this method to nanostructures besides cylinders, such as spheres and gyroid will be discussed.   

Highly enhanced dynamics of microdomains ordering by solvent vapor annealing of thin block copolymer films on polymer network supports

We studied the solvent driven ordering dynamics of block copolymer films supported by a densely cross-linked organic hard mask (HM) designed for lithographic fabrication. We found that the ordering of microphase separated domains on the HM layer proceeds significantly faster as compared to similar films on silicon wafers. This leads to a pronounced enhancement of the dynamics of both the terrace-formation as well as the long-range lateral ordering of the microdomains. The effect is independent on the chemical structure and volume composition of the studied block copolymers (cylinder-/ lamella-forming). Importantly, enhanced ordering is achieved even at a reduced degree of swelling corresponding to an intermediate to strong segregation regime, when similar films on conventional substrate show very limited ordering. In-situ ellipsometric measurements of the swollen films revealed an insignificant increase by 1-3 vol. \% in the solvent up-take by HM-supported films. Therefore we attribute the enhanced dynamics to reduced interactions at the block copolymer/HM-support interface. Apart from immediate technological impact in block copolymer-assisted nanolithography, our findings convey novel insight into effects of molecular architecture on polymer-solvent interactions.   

Direct Immersion Annealing (DIA) of Block Copolymer Thin Film

Solvent Vapor Annealing (SVA) methodologies of block copolymer (BCP) films have demonstrated excellent potential for control of nanostructures and morphologies. However, SVA designs require sophisticated instrumentation, and fine control of system parameters in batch processing mode which is relatively complex and limits its feasibility. We developed a faster and robust solvent immersion strategy for microphase separation and nanostructure control of as-cast BCP thin films with minimal sophistication. Our Direct Immersion Annealing (DIA) method requires immersion in a mixture of non-solvent and good solvent (for BCP) for annealing. A non-solvent component prevents dissolution of the film resting on substrate while a good solvent percolates through the film, plasticizes it, and shifts glass-transition below room temperature leading to microphase separation and ordering. Our study of PS-PMMA system demonstrates that a robust control over thin film ordering and transient swelling could be achieved through a fine control of solubility parameter of solvent mixture and temperature with no dead-time. Further, we exhibit the utility of DIA for alignment of BCP domains on topographically patterned substrates.   

Dynamical SCFT Simulations of Solvent Annealed Thin Films

Block copolymer thin films are ideal candidates for a broad range of technologies including rejection layers for ultrafiltration membranes, proton-exchange membranes in solar cells, optically active coatings, and lithographic masks for bit patterning storage media. Optimizing the performance of these materials often hinges on tuning the orientation and long-range order of the film's internal nanostructure. In response, solvent annealing techniques have been developed for their promise to afford additional flexibility in tuning thin film morphology, but pronounced processing history dependence and a dizzying parameter space have resulted in slow progress towards developing clear design rules for solvent annealing systems. In this talk, we will report recent theoretical progress in understanding the self assembly dynamics relevant to solvent-annealed and solution-cast block copolymer films. Emphasis will be placed on evaporation-induced ordering trends in both the slow and fast drying regimes for cylinder-forming block copolymers from initially ordered and disordered films, along with the role solvent selectivity plays in the ordering dynamics.   

Solvent-Assisted Self-Assembly of Block Copolymer Films: A Simulation Approach

Solvent annealing has been shown to provide an effective means for controlling the self assembly in block copolymer films; it also provides opportunities to create structures that cannot be achieved by thermal annealing. The intrinsic non-equilibrium nature of these processes presents challenges to their theoretical understanding. We have developed an efficient simulation tool for modeling the solvent annealing of block copolymer films that enables study of the evolution of microstructure and the transformations between various microphases in response to film swelling and solvent evaporation. We study the effect of process and thermodynamical variables such as solvent pressure, molecular weight and segregation force, on the self assembled structure of block copolymer thin films. And we identify conditions that lead to a defect-free copolymer morphology. We also discuss the effects of relative time scales of solvent evaporation, diffusion of solvent and of polymer chains on the self-assembly of block copolymer thin films.   

An in-situ Study of Kinetics of Rapid Self- assembly in Lamellar Forming Poly (styrene-b- lactic acid) (PS-b-PLA) Block Copolymer during Microwave Annealing

This work exploits the effect of microwave annealing on kinetics of pattern formation for lamellar PS-$b$-PLA film. A well-ordered pattern lamellar PS-$b$-PLA is formed on UV/ozone treated Si in less than one minute upon exposure to microwave energy in presence of THF. To understand the interaction of polymers with microwave radiation, we carried out an \textit{in-situ }temperature measurement of the Si substrate during the annealing. Our \textit{in-situ} experiment shows neither Si nor PS-$b$-PLA go through dramatic temperature rise during exposure to microwave energy. We suggest the dopant level in our Si is not high enough to activate the microwave absorption. Also, the high frequency of the electromagnetic field does not allow polar substances like PLA enough time to oscillate. We believe THF which is a polar liquid contribute significantly to the rapid self-assembly of the film. The vapor pressure of THF rises from 19.8 kPa to 70 kPa (at 55 $^{\circ}$C) within few seconds. The high pressure plasticizes the polymers. The highly mobilized chains phase separate quickly due to high-$\chi $ parameter. The results are compared with conventional thermal annealing method.   

An in situ GISAXS study of BCP thin films during annealing in selective solvent vapor: Solvent removal effects in films of different initial thickness

Solvent vapor annealing is a rapid and effective means to achieve well-ordered structures in block copolymer (BCP) thin films. The underlying physical mechanisms however are ill understood and systematic studies of the annealing process are scarce. Here, we used grazing-incidence small-angle x-ray scattering (GISAXS) to investigate the ordering of BCP microdomains as solvent vapor was added or removed. We studied polystyrene-block-poly(4-vinyl pyridine) (PS-b-P4VP) BCP thin films of different initial thickness ranging from a few ten to a few hundred nanometers during annealing in THF vapor, a selective solvent for PS. While the degree of lateral order of the BCP microdomains in the swollen state was found to be exceptional for all film thicknesses, the packing of microdomains was found to depend on the initial film thickness and the amount of swelling. The effect of solvent removal on the degree of lateral order was studied by deswelling films of different thickness at different removal rates. Here, we observed a substantial deterioration of lateral order of microdomains that is significantly stronger than in comparable deswelling studies of BCP thin films in neutral solvent vapors.   

An \textit{in situ} grazing incidence x-ray scattering study of block copolymer thin films during solvent vapor annealing

Although solvent vapor annealing (SVA) has been widely applied to block copolymer (BCP) thin films to obtain well-ordered microdomains, the mechanism of enhancing lateral order is not well understood. Here, we used real time \textit{in situ} grazing-incidence small-angle x-ray scattering (\textit{in situ~}GISAXS) to study the self-assembly of PS-b-P2VP BCP BCPs with different molecular weights thin films in THF vapor, a near neutral solvent for both blocks. Both swelling and deswelling behavior of BCP thin films were examined. The extent of swelling$~$and the solvent removal rate not only affect the domain spacing of BCPs but also dictate the extent of lateral ordering of the BCP microdomains. Larger grains were observed at higher values of the swelling ratio (close to disordering). To preserve the maximal lateral ordering of the microdomains in the swollen state, the fastest solvent removal rate is required to freeze in the ordered microdomain structure of the swollen BCP film.