Session P34: Characterization of Non-Equilibrium or Exotic Structures of Polymers

Non-equilibrium laser annealing derived mesostructured silicon templated in mesoporous thin film network structures from block copolymer self-assembly

Block copolymer self-assembly derived network structures are combined with non-equilibrium transient laser heating to serve as nanotemplates for directing structures of inorganic materials, which otherwise are difficult to achieve mesoscale order. Amphiphilic block copolymers swelled by carbon precursors self-assemble into equilibrium bicontinuous gyroid phases via solvent vapor annealing. After pyrolysis in nitrogen, the resulting mesoporous gyroidal carbon thin films are backfilled with amorphous Si (a-Si). The poor tolerance of carbon toward high temperature in air and the high melting points of inorganic materials present an enormous challenge in preparing crystalline mesostructured Si in the carbon templates. The carbon templates' stability is significantly enhanced, however, using nanosecond laser heating. Melting and recrystallization of Si (a-Si melts at ~1250 °C) during this ultrafast, highly non-equilibrium process allow conformal template backfilling so that crystallized Si inherits the templates' mesostructural order. Such non-equilibrium nanostructure formation approaches in conjunction with equilibrium self-assembly process open up opportunities for novel materials in catalysis and photonics.   

Probing Block-Copolymer Self-Assembly Kinetics with In-Situ Spectroscopic Ellipsometry

Understanding the assembly of block copolymers during various processing procedures is critical for the development of functional materials, as well as providing information on the underlying polymer physics. However, direct observation of the assembly process in-situ is exceedingly challenging and time-consuming. In this work, we develop and implement a novel characterization method to monitor the assembly of cylinder- and lamellar- forming block copolymers using spectroscopic ellipsometry, which tracks the optical anisotropy that results from microphase separation of the assembled domains. Measurements of the birefringence are well-correlated with surface AFM images and are in good agreement with the birefringence expected from simple geometric arguments. Furthermore, the fast sampling rate (1 sec) permits in-situ measurements of the polymer self-assembly during both thermal and solvent vapor annealing in a single, rapid measurement, and correlate with the . Ellipsometry thus presents a means to not only obtain information to inform future experimental parameters for obtaining ordered phases, but also may be utilized as a novel technique to study the assembly kinetics of soft materials.   

A fluorescence-based method for determining order-disorder transition temperatures in block copolymers

We present a technique for observing the order-disorder transition (ODT) in block copolymers through changes in the temperature-dependent behavior of an optically fluorescent probe. This technique provides potential advantages over existing strategies such as scattering and rheology, as it is non-invasive and can be directly applied to thin films. Pyrene molecules were covalently attached to the styrene block of a PS-b-PMMA copolymer through addition of trace levels of functionalized monomer during synthesis by controlled radical polymerization. We observed a discontinuous change in the intensity ratio of the pyrene vibronic emission bands as the polymer was heated through its ODT. As this intensity ratio is sensitive to the chemical environment surrounding these molecules, we hypothesized that this change occurs as the pyrenes are in contact with only PS when the polymer is ordered, but experiences an averaged effect of both PS and PMMA in the disordered state. We then applied this ODT measurement technique to thin films, where an increase in T_ODT is observed when the substrate preferentially interacts with the PMMA and the film thickness is sufficiently small. Further experiments tuned this effect by modulating the surface energy of the substrate.   

Evolution of disordered hyperuniformity in block copolymers thin films by homopolymer dilution

Disordered hyperuniform materials display long range order despite their seemingly random structure – long range density fluctuations are suppressed as reflected in a vanishing structure factor at zero wavevector. Close-packed micelles of block copolymers (BCPs) display such behavior. Here, we explore the degree of structural order in BCP thin films and manipulation of such order, through the lens of disordered hyperuniformity. We explore the effect of gradually increasing disorder by increasing the nearest-neighbor distance between micelles by homopolymer addition. The hyperuniformity of the system is evaluated as a function of BCP concentration via numerical image analysis of AFM or SEM images of the BCP films. We observe a continuous transition from a DH state to randomly disordered, as characterized by the change in scaling of particle number density with increasing BCP dilution. These results offer a new perspective on structural order in BCP thin films and add to the understanding of the evolution and emergence of hyperuniform states in disordered materials. The findings are also relevant for improved engineering of nanoscale arrays based on BCP templates towards ‘designer made’ DH materials.
  

Fluctuation/Correlation Effects in Symmetric Diblock Copolymers: On the Disordered Phase

While the polymer self-consistent field theory has gained great success in describing various inhomogeneous polymeric systems, particularly the self- assembled morphologies of block copolymers, for spatially homogeneous systems it reduces to the Flory-Huggins theory and gives the simplest, yet often qualitatively incorrect, predictions. We compare, without any parameter-fitting, the thermodynamic and structural properties of the disordered phase of symmetric diblock copolymers obtained from fast off lattice Monte Carlo (FOMC) simulations [Q.Wang and Y.Yin, J.Chem.Phys.130,104903(2009)], reference interaction site model (RISM) and polymer reference interaction site model (PRISM) theories, and Gaussian-fluctuation theory for the same model system of discrete Gaussian chains interacting with soft, finite-range repulsion used in the dissipative particle dynamic simulations. We compared the internal energy, entropy, Helmholtz free energy, pressure, constant-volume heat capacity, chain/block dimensions, and various structure factors in the systems, which unambiguously and quantitatively reveal the consequences of various theoretical approximations and validity of these theories in describing the fluctuations/correlations in disordered diblock copolymers.   

Multiscale Modeling of Multicompartment Micelle Consisting of Block Copolymers

In this work, we present a procedure for estimating the Flory-Huggins χ-parameter for use in atomistic and mesoscale molecular simulations for multicompartment micelle consisting of multiblock copolymers. In particular, we propose improvements upon traditional Flory-Huggins theory by considering coordination number correction, dielectric screening, and molecular volume normalization. We apply this technique to several test systems. Our results demonstrate that the newly developed procedure offers high precision and consistency in predicting the Flory-Huggins χ-parameter for miscibility analysis. In addition, we discuss the application of multicompartment micelle for nano-reactor. For this, 1) we characterize the transport of small molecules through the multicompartment micelle; 2) we introduce photo-responsive chemical structure to transform the structure of micelle. From this study, we conclude that a set of well-determined χ-parameters enables the molecular design of multiblock copolymer for variety of applications utilizing molecular self-assembly.   

Direct Observation of Block Copolymer Micelle Fragmentation in Ionic Liquids

Recently, attention has been directed to quantifying the relaxation kinetics of block copolymer micelles, such as micelle fragmentation and fusion. Studies on block copolymer micelle fragmentation typically involve indirect techniques such as small angle scattering and ex-situ transmission electron microscopy. In this work, the direct observation of fragmentation in 1,2-polybutadiene-block-poly(ethylene oxide) (PB-PEO) micelles in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide was achieved using high-temperature liquid-phase transmission electron microscopy. The kinetics of PB-PEO fragmentation were also studied for various molecular weights in one ionic liquid using time-resolved small-angle X-ray scattering and dynamic light scattering. By combining these experimental techniques, a detailed analysis of the effect of molecular weight on micelle fragmentation kinetics, along with the observation of intermediate structures during fragmentation events, was conducted. These experiments provide further insights into the equilibration kinetics of block copolymer micelles via fragmentation and the mechanism of fragmentation itself.   

The Effects of the Size of Crystal Domains to the Polymorphism of Close-Packed Micelles

Polymorphism is technologically important, but its mechanistic origin is not well understood. We recently investigated the transitional structures of strongly-segregated block copolymer micelles in water in the liquid-to-solid and solid-to-solid transitions. In both transition processes, we found that the polytypes of close-packed block copolymer micelles are regulated by the size of crystal domains. In the liquid-to-solid phase transition, the crystallization of micelles proceeds by forming metastable hexagonal close-packed (HCP) structures that transform into stable face-centered cubic (FCC) structures via intermediate randomly stacked two-dimensional hexagonal close-packed (RHCP) structures. In the solid-to-solid transformation, the martensitic shear transformation of metastable FCC structures could be initiated by heating that reduces the size of metastable FCC crystal domains. These observations suggest that the polymorphism of crystalline solids is likely regulated by the size of crystal domains.   

Quasiperiodic Ordering of Minimally Hydrated Ionic Surfactant Micelles

Minimally hydrated surfactant micelles, by analogy to hard spheres, pack into high-symmetry body-centered cubic, face-centered cubic and hexagonally closest-packed structures. Recently, hydrated dianionic surfactant micelles were shown to self-assemble into low-symmetry Frank-Kasper (FK) σ and A15 morphologies. These FK phases are well-known periodic approximants of dodecagonal quasicrystals (DDQC), which possess 12-fold rotational symmetry yet lack translational symmetry. We report the formation of a well-ordered DDQC formed by oil-swollen aqueous dispersions of alkylphosphonate surfactants. Our studies show that the DDQC formation is strongly path dependent, and that different processing approaches to the same final composition instead yield periodic σ and A15 approximants. These findings coupled with the observations of irreversible thermal transformation of the DDQC into a σ phase suggest the metastability of these soft quasicrystals. This discovery illustrates avenues to tune the subtle free-energy balance prevalent among complex micellar phases and exemplifies the universality of quasiperiodic order in soft matter self-assembly.   

Processing Path-Dependent Complex Micelle Packings of Hydrated Diblock Polymer Amphiphiles

Water drives the self-assembly of short diblock polymer amphiphiles into spatially periodic lyotropic liquid crystalline (LLC) mesophases, including lamellae, polycontinuous networks, hexagonally-packed cylinders (HEX), and 3D sphere packings. Beyond high symmetry body-centered cubic (BCC) and cubic close-packed micellar phases, diblock oligomers also form tetrahedrally closest-packed Frank-Kasper (FK) A15 phases. In spite of the low molecular weight of the constituent amphiphile, we recently demonstrated that judicious thermal processing of an A15 LLC enables formation of a surprisingly long-lived, non-equilibrium state. Specifically, heating an A15 phase drives transitions to BCC and HEX phases at elevated temperatures. Quenching these LLCs unexpectedly drives formation of a remarkably well-ordered, tetragonal FK sigma phase comprising 30 quasispherical micelles per unit cell as a metastable state, which takes ~150 days at 22 °C to revert to the original A15 structure. The formation and metastability of the sigma phase is contingent on sample quench rate, quench depth, and annealing temperature. These slow order-order phase transformation kinetics stem from a complex interplay of temperature-dependent phase nucleation and growth rates, which are coupled to the molecular-level rate of particle size reconfiguration and mesoscale rate of spatial rearrangement of the micelles. These findings highlight the importance of processing path-dependence on the observed mesophases of self-assembled soft materials.   

Expanding spherical regions of block copolymers via designed chain architectures

Recently, the self-assembly of block copolymers into spherical phases has attracted renewed interest due to the discovery of Frank-Kasper spherical phases. Experiment and theory unravel that the formation of Frank-Kasper phases mainly results from the expansion of spherical region. However, it is generally believed that the spherical domains self-assembled from AB-type block copolymers are composed of the minority blocks. Breaking this generic rule so that the spherical domains are formed by the majority blocks requires new mechanisms to drastically expand the stable region of spherical packing phases. Herein, we propose a useful mechanism, and thus we design a series of dendron-like AB-type block copolymers. Our self-consistent field theory calculations predict that all these dendron-like copolymers exhibit a spherical region extending to f>0.5. In particular, the maximal spherical region even approaches f∼0.7. Interestingly, such a phase diagram exhibits multiple reentrant transitions of Frank-Kasper phases.
  

Abstract Withdrawn

Block copolymers can self-assemble into a variety of ordered nanostructures, and thus have attracted abiding interest. One of the most interesting ordered phases is the spherical phase that can be regarded as a typicl packing problem of soft particles. In particular, the spherical phase of AB-type block copolymers has attracted renewed interest due to the discovery of complex Frank-Kasper phases. Previous theoretical work reveals that ABn miktoarm star copolymer is a proper system for the formation of stable Frank-Kasper phases. However, our work unravels that the size of the overall spherical region approaches a limit as the arm number n increases. In this work, aiming to expand the spherical region further, we propose to design a new AB-type block copolymer, which is composed of an A-block connected with n arms of AB diblocks. This copolymer can be reduced to be ABn miktoarm or (AB)n star copolymer. Interestingly, we find that the spherical region changes nonmonotonically when the architecture changes from ABn to (AB)n.