Session A35: Directed Self-Assembly of Copolymers in Confined Geometry I

Fabrication and characterization of freestanding phononic thermocrystal membranes via block-copolymer directed-self assembly

Block-copolymer (BCP) directed self-assembly (DSA) is a valuable technique that enables formation of defect-free, single crystal nanostructures over large areas. This is accomplished via a chemical template patterned via ebeam lithography. For perpendicularly oriented cylinder-forming BCPs, which spontaneously form a polycrystalline hexagonally close-packed lattice, controlling the orientation of the DSA pattern enables direct control of the in-plane lattice orientation, total pattern area, and number of periods. For BCPs such as PS-PMMA, the nanostructures can be transferred into an inorganic substrate, such as Si, and integrated into fabrication process flows for complex devices. Here, we present a methodology for integrating cylinder forming PS-PMMA DSA with a novel fabrication process to produce freestanding nanoporous Si membranes that scatter heat-carrying phonons. By controlling the orientation of the hexagonal lattice, a line-of-sight heat transport pathway can be opened or closed, which combined with nanometer scale of the BCP pores, provides the needed device length-scales to probe heat-carrying phonons. In this work, we report fabrication of and measurements on such membranes as a function of porosity, neck size, self-assembled vs DSA pores, and total pattern area.   

Engineering Block Copolymers To Achieve Equal Surface Free Energy and Tunable χN For Directed Self-Assembly Applications

Directed Self-Assembly (DSA) of block copolymers (BCPs) is a promising technique for creating well-defined nanoscale features. The suitable BCPs for this application demand both blocks have equal surface free energies (SFE), a high enough Flory-Huggins parameter (χ) for moderate phase separation but not so high as to prohibit defect annihilation, and high etch contrast. To date, the BCPs that meet these requirements are still very limited. Here we demonstrate a high throughput approach to create a series of BCPs with full pitch sizes from 8- 15 nm that meet all these requirements. Furthermore, the capability of incorporation of etch resistant elements is shown to enable potential pattern transfer. We believe the work here will broaden the scope for DSA suitable materials and promote the next generation of nanolithography.   

Enhancing the Scale of Block Copolymer Lamellae Alignment using Ionic Liquid (IL) on a Planar Supporting Substrate

Symmetric Block Copolymers (BCP) such as PS-b-PMMA when cast on Silicon wafer and ordered using thermal annealing assume a parallel lamellar structure in thin films. However, as molecular weight and film thickness increases, attaining complete parallel lamellar structure becomes elusive. Previous study has shown that increasing the thermodynamic driving force for microphase separation, cN, where c is the Flory-Huggins interaction parameter between the polymeric blocks and N is the number of segments in the BCP, enhances the degree of BCP ordering and alignment parallel to substrate. In order to control the microstructure of ordering, increasing N or reducing temperature T could be applied at the expense of either slower kinetics, or higher defect formation due to entanglements, higher glass transition temperature (T_g) and surface tension. In this work we present a method for parallel alignment of lamellar PS-b-PMMA over notably higher Mw and film thickness regime using IL to enhance c, thereby propagating substrate driven parallel layering, while lowering T_g for enhanced molecular mobility for fast kinetics. Such films may be useful in in applications barrier materials and batteries, solid state dielectric capacitors.
  

Combining polymer synthesis with self-assembly of block copolymers

Polymer self-assembly is one of the most promising nanopatterning techniques due to its advantages of low cost and high versatility. The key challenge of conventional strategy is its very limited ability to control over chemistry during or after self-assembly in order to alter the nanostructures from its thermodynamic equilibrium state. In this talk, we will demonstrate our recent development of in-film photopolymerization technique for synthesizing polymers within an self-assembled film. The vapor phase monomer is first introduced to swell a photoinitiator-containing block copolymer film, which can be subsequently converted to polymers upon UV irradiation. The synthesized homopolymers blend with BCP films, which alter the thin film nanostructures by changing the underlying polymer composition. As these altered nanostructures are locally near equilibrium, common annealing techniques such as shear aligning can be combined to improve the degree of ordering. With successful integrating polymer chemistry with assembly physics, the in-film polymerization method provides an exciting platform for on-demand manipulation of polymer functionality as well as opening up a new area for radical polymerizations within such geometrically confined, swollen films.   

Hexagonal pattern coarsening in cylinder-forming PS-b-PMMA block copolymer thin films

The grain coarsening in thin films (~30 nm) of cylinder forming PS-b-PMMA BCP (N=379-1281) was accomplished by tuning of the annealing temperature and time in a RTP machine. The order of the hexagonal pattern was quantified by measuring the correlation length.
The weakly dependence on T of χ_S-MMA allows decoupling the control of χN by means of N, and that of the thermally activated kinetic barriers for ordering kinetics modulating T, evidencing the order-disorder transition (ODT) and glass transition (GT) involved in BCP ordering.
Thermodynamically, weak and strong segregation limits are not distinguishable. In weak segregation limit the ordering process switches from a diffusion limited to a curvature driven mechanism. Both single chain and collective motions comply with the extensively evidenced kinetic limited coarsening and the reduced diffusivity, increasing N and moving away from ODT, and with the thermodynamic limited coarsening and the enhanced diffusivity, decreasing N and moving towards ODT.
The collective dynamic is further investigated discriminating the behavior of the penta-hepta defects within the hexagonal pattern to give a comprehensive description of thermodynamic, kinetic and topological characteristics of the hexagonal pattern coarsening.   

Selective Modification from PS-b-PMMA-b-PtBA Triblock Copolymer for Ultrafiltration Membranes

We demonstrate a feasible approach to fabricating nanoporous structures and their functionality using a triblock copolymer of polystyrene-b-poly(methyl methacrylate)-b-poly(tert-butyl acrylate) (PS-b-PMMA-b-PtBA). With casting the samples in the thin films, the continuous-type morphologies were formed as PS matrix consisting cylinders of PMMA and minor PtBA blocks. Perpendicular orientation of cylinder morphologies was exploited near two interfaces of air/polymer and polymer/neutral substrate, sandwiching the random orientation of cylinders in the interior of the film. Nondegradable, selective swelling–deswelling process of cylindrical (PMMA-b-PtBA) blocks generated nanopores with tunable pore sizes. Moreover, a simple hydrolysis of minor tBA blocks functionalized the nanopore surfaces and walls into poly(acrylic acid) layers. The pH-responsive water permeability of nanoporous membranes and their active switching with respect to biomolecules such as bovine serum albumin (BSA) were performed. These results suggest a platform to fabricate a stimuli-responsive ultrafiltration membrane using a tunable multiblock copolymer.   

Irreversible Physisorption of PS-b-PMMA for Neutral Layer

Polymer chains are irreversibly physisorbed (physically adsorbed) onto impenetrable substrates by intermolecular forces (i.e. H-bonding, vdW force or dipole moment), as the chains favor the conformations which maximizes the segmental contact to compensate the conformational entropy loss. In this study, we succeeded in guiding perpendicular microdomains of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) film with the aid of the irreversibly-adsorbed neutral layer made of PS-b-PMMA and evaluated its compositional randomness in terms of the correlation length (ξ) between the two phases of PS and PMMA. This method is widely applicable to various substrates without any necessity of random copolymer brushes or mats other than PS-b-PMMA itself.   

Failure and mechanical properties of block copolymer thin films

The physical properties of glassy polymer films can change drastically under
nanoscale confinement. These changes are often attributed to increased
average molecular mobility and reduction in entanglement density, and both
are known to alter mechanical behavior. While most previous work has focused
on the properties of homopolymers under confinement, and the mechanical
response of block copolymer thin films is still comparatively unexplored. We
have used molecular dynamics simulations to investigate the mechanical
response of free standing, lamellar-forming block copolymer thin films made
from polymer chains that span from slightly entangled to highly entangled at
different film thicknesses. We noticed that the failure and mechanical
properties become substantially distinct from bulk as the thickness is
decreased. The dynamic response of the thinnest films studied show a
different local response compared to thicker films. For all the block
copolymer films, we find that the plastic rearrangements initially
concentrate at the boundary between the two phases of the lamellae until
close to failure, when the plasticity moves to the center of a domain.   

Advanced metrology and molecular dynamics simulations for quantifying counterion condensation in block copolymer electrolyte thin films

Ionic conductivity is an important property of polymer electrolyte membranes and electrode binders. The impact of counterion condensation, a proxy for the extent and strength of ionic group dissociation in polymer electrolytes, on ionic conductivity has received less attention and is not clearly known. This talk highlights our effort to quantify counterion condensation in self-assembled block copolymer electrolyte (BCE) thin films. Advanced metrology (GI-SAXS, QCM, ICP-OES, and LC-MS) were applied to quantify counterion condensation in BCEs. These techniques identified the Donnan concentration during ion-partitioning experiments and determined the activity coefficients of these ions in thin films. Experimental results were compared against the Gibbs-Donnan Model and Manning’s Theory of Counterion Condensation, as well as classical molecular dynamics simulations. The agreements and differences between the experimental and modeling techniques will be presented and discussed.   

Modeling Surface Interactions for Block Copolymers and Polymer Brushes under Soft Confinement

The surface interaction is one of the most important factors that control the nanostructures formed by block copolymers/polymer brushes under soft confinement. In this talk, we introduce a simplified model for surface interactions where the role of soft substrates is decomposed into two independent contributions: the surface preference and the surface softness. Soft substrates are modeled by polymers grated onto hard walls. Their structures on both repulsive and attractive walls are studied by SCFT.^[1] Based on this model, we perform a numerical analysis of the stability competition between perpendicular and parallel lamellae of symmetric diblock copolymers on substrates modified by homopolymers.^[2] The effects of the surface preference and the surface softness on the alignment of lamellar domains are carefully examined. Applications of this model on understanding the defect removal process in DSA of block copolymer thin films are also demonstrated.^[3]
[1] YX Liu; HD Zhang; Chinese J. Polym. Sci., 2018, 36, 1047.
[2] JQ Song; YX Liu; HD Zhang; J. Chem. Phys., 2016, 145, 214902.
[3] JQ Song; YX Liu; HD Zhang; Acta Polym. Sin., 2018, 12, 1548.   

Degradation of Block Copolymer Films

We use coarse-grained molecular dynamics simulations to study degradation of films of multiblock copolymers in a solvent. Our simulations were designed to mimic degradation dynamics of glycine, valine, and phenylalanine based poly(ester urea)s in vitro. Simulations show that the rate of copolymer degradation is a result of a fine interplay between chain breaking kinetics, solvent diffusion, and swelling of the domains made of solvophilic blocks. The evolution of the film structure during the degradation process was monitored by calculating the scattering function S(q) of the copolymer film. The solvent diffusion into the films results in a monotonic shift of the peak position to smaller q. This shift is also accompanied by the increase in scattering intensity at q << 1 in such a way that the peak completely disappears at the later stages of the film degradation. This results in the scattering function S(q) to have two characteristic power law regimes with S(q)~1/q² and S(q)~1/q⁴, represented by interconnected vesicles with thin shells. The number average degree of polymerization of the copolymer fragments monotonically decreases with time. However, the polydispersity index of the copolymer fragments first increases as a function of time and then decreases.   

New insight into self-assembly of block copolymers at the solid-polymer melt interface

I present new pieces of experimental findings on the self-assembling process of block copolymers (BCPs) near nonneutral silicon (Si) substrate surfaces. The key is concurrent physisorption of preferred blocks and non-preferred blocks on the surface. Using an optimized solvent-rinsing approach, we successfully uncovered two different kinds of adsorbed BCP chains: one is the inner strongly adsorbed BCP chains where all constituent blocks lie flat and form a two-dimensional percolating network structure regardless of their chain architectures, microdomain structures, and interfacial energetics. The other is outer "loosely adsorbed chains" which form a poorly packed perpendicularly oriented microdomain structure. I will show that the inner microdomain structures and orientations of BCP thin films are negatively impacted by the loosely adsorbed BCP chains. Interestingly, this undesirable substrate-field effect propagates into the film interior up to the distance of ~ 70 nm. Finally, a new surface modification approach to prevent the substrate-field effect is proposed. I will demonstrate that homopolymer chains composed of one of the constituent blocks adsorbed on the Si substrates act as a “structurally neutral” surface coating for directed self-assembly of block copolymer thin films.