Session K25: Advanced Characterization of Polymers: Morphology, Interfaces, and Dynamics

Solving the spatial distributions of multicomponent polymer thin films with resonant soft x-ray scattering

The self-assembly of block copolymers into periodic nanostructures at length scales between 5-50 nm makes them ideal tools for nanofabrication. Systems with more than two components offer increased morphological flexibility over binary systems but are significantly more challenging to characterize due to poor contrast between components without additional labeling or staining. Resonant soft X-ray scattering (RSoXS) can provide tunable contrast based on the functional groups in the system, providing label-free access to rich, hyperspectral scattering data. However, interpretation of that data is complex and time intensive because it requires solving a non-trivial inverse problem. To address this issue, we have developed a contrast variation approach which deconvolutes the hyperspectral data into partial scattering functions (PSFs), analogous to neutron scattering methods. The resulting PSFs describe the spatial distribution of the individual components in the system that can then be compared against forward simulations, significantly reducing the difficulty of the analysis. We demonstrate the utility of this RSoXS technique through characterization of a model triblock copolymer thin film, poly(isoprene-block-styrene-2-vinyl pyridine), which exhibits rich morphological variations depending on its processing history. This contrast variation RSoXS technique is applicable to characterizing a broad range of multicomponent systems that are currently intractable through scattering or other conventional methods.   

Quantifying Ion Distribution Profiles in Block Copolymers via Resonant Soft X-ray Scattering

Ion-doped block copolymers (BCPs) are a versatile, easy-to-process, and inexpensive class of materials with highly tunable structures. These attributes are promising for various applications, including nanolithography and ion/electron transport. The distribution of ions throughout a BCP phase is important to characterize, as it will affect the local polymer conformations, polymer/ion dynamics, and material performance. In this work, ionic liquid (IL) distributions in PS-b-PMMA were studied using resonant soft X-ray scattering (RSoXS). RSoXS harnesses the energy-dependent absorption of soft X-rays by organic functional groups to enhance chemical contrast in thin films, enabling interrogation of multicomponent systems without labeling. The ion distributions were spatially resolved via a data fusion approach, in which scattering patterns were generated from a newly developed simulation framework (https://github.com/usnistgov/NRSS) using real-space composition models mapped onto microscope images of the sample. Preliminary analyses suggested that the IL becomes more enriched near the center of the PMMA phase as total IL loading in the BCP is increased. These results provided a quantitative means to link the physics to material performance.   

Label-free characterization of aqueous micelle nanostructures via novel liquid in-situ Resonant Soft X-ray Scattering (RSoXS)

Micelles are fundamental to nanocarrier applications from drug delivery to environmental remediation. Their structure and dynamics are critical to their properties and functions but are challenging to measure. Here, we demonstrate a novel technique capable of such measurements based on resonant soft X-ray scattering (RSoXS). It uniquely probes organic materials using their intrinsic chemical bonds rather than laborious and disruptive labeling techniques. Our customized microfluidic cell enables RSoXS to be performed in liquid environments, allowing structure and dynamics to be measured in-situ. Using this technique in a multimodal approach, we investigated an amphiphilic polyelectrolyte copolymer micelle of three different molecular weights (Mw). Despite no measurable critical micelle concentration, structural analyses point towards multimeric structures for most Mw’s. The sizes of the micelle substructures are independent of both concentration and Mw. Combining these results with a Mw-invariant surface charge and zeta potential strengthens the link between nanoparticle size and ionic charge in solution that governs polysoap micelle structure. Such control would be critical for nanocarrier applications such as drug delivery and water remediation.   

Depth-dependent orientation in thin films of bottlebrush block copolymers

We synthesized a series of symmetric bottlebrush block copolymers (BBCPs) with deuterated-polystyrene (dPS) as core block and poly(solkeltal acrylate) (PSA) as corona block using ring-opening metathesis polymerization (ROMP), where backbone length (N_BB) and grafting density (GD) was varied. Solid state conversion through acid hydrolysis of PSA block into poly(glycerol acrylate) (PGA) block converts BBCPs from hydrophobic-hydrophobic to hydrophilic-hydrophobic, bringing BBCPs from disordered state to strongly phase separated lamellae. Atomic force microscopy (AFM) results showed that linear BBCP (N_BB=1) had mixed orientation at surface, while star-like BBCP(N_BB=6), rod-like BBCP (N_BB=50) and cylindrical-like BBCP (N_BB=100) exhibited vertical orientation. Grazing-incidence small-angle neutron scattering (GISANS) showed that star-like BBCP were oriented normal to the air and substrate interfaces and in the bulk. With increasing N_BB, the preferential orientation was lost. Variation of GD for N_BB=50 had limited impact on the orientation of BBCPs thin films. By modifying the substrate from hydrophilic to hydrophobic, the dPS block (core block) selectively interacts with the substrate, which causes a parallel orientation of the microdomains, underscoring the importance of architectural control on BBCP thin film orientation.   

Single-Molecule Dynamics of Confined Branched Polymers

Branched biopolymers such as lubricin and mucin play crucial roles in controlling biolubrication behavior. Investigating the behavior of branched biopolymers at the single-molecule level is critical to understanding their ensemble biolubrication properties. Because branched biopolymers often exist at interfaces, confined environments are good model environments within which to study single-molecule behavior. In this study, we investigate the dynamics of DNA-based branched biopolymers in 1D slit-like confinement. We find that stronger electric fields are required to introduce branched polymers into confined environments than linear polymers of equivalent backbone length, indicating a larger energy penalty to confine branched polymers. Using single-molecule fluorescence microscopy, we measure the center-of-mass diffusion and molecular relaxation times of single branched polymers in slits ranging from 75 nm to 150 nm in height. Confined branched polymers adopt larger conformations and exhibit slower diffusion coefficients than linear polymers of equivalent backbone length. Broadly, understanding of molecular-scale contributions to biolubrication will enable the design of novel materials to treat lubrication-deficient diseases.   

Nanoparticle Diffusion Coefficients at Liquid Interfaces Measured by SEM Single Particle Tracking

 Nearly monodisperse nanoparticle spheres attached as a Gibbs monolayer to a nonvolatile ionic liquid surface were tracked by a recently developed scanning electron microscopy technique to obtain their two-dimensional tracer diffusion coefficient as a function of areal fraction. To allow tracking from dilute to almost jammed, ligand-coated gold tracer particles were sparsely dispersed among silica background particles of the same diameter and surface chemistry. The in-situ technique spatially resolved both tracer and background particles for a period of ~1-2 minutes, highlighting mechanisms of diffusion, which depended strongly on areal fraction. Irrespective of diameter, beyond onset of crystallization but before jamming the diffusion coefficient decreased by over four orders of magnitude.  The normalized diffusion coefficient across this range was unexpectedly large and diameter dependent, manifesting a significant lubrication of dense particle motions by the surface-attached ligands, 5,000 g/mol poly(ethylene glycol).  More lubrication was found for a longer ligand.   

Depth-Dependent Local Shear Modulus Profile Attained from Analyzing QCM Data with a New Transfer-Matrix Model Across a Polymer-Polymer Interface

We present a model for analyzing quartz crystal microbalance (QCM) data that enables the determination of a local depth-dependent shear modulus G(z) in polymer films. This work builds on our group's recently published approach leveraging a continuum mechanics model to analyze QCM resonance shifts by matching boundary conditions between modeled layers. Here, we apply a transfer-matrix approach that can expand the analysis to an arbitrary number of modeled layers, allowing for a near-continuous G(z) gradient with arbitrary functional form. We demonstrate how this can be used to fit QCM data of glassy-rubbery bilayer films of polystyrene (PS) and polybutadiene (PB) to determine G(z) in that system. Despite a compositional interface of only ~5 nm between PS and PB, this new QCM analysis technique finds strong evidence for a long-ranged gradient in local modulus of ~100+ nm that corroborates previous local glass transition temperature Tg(z) measurements in this system. The ability to probe local modulus in systems such as this provides a new method to interrogate how interfaces impact the viscoelastic properties of polymer films, aiding in fundamental understanding and related industrial applications.   

Resonant Tender X-ray Scattering of Semiconducting Polymers

Semiconducting polymers are being studied for application in a wide range of optoelectronic devices including solar cells, LEDs and transistors. Being polymeric materials, they offer advantages over traditional semiconductors including ease of processing and mechanical flexibility. Most semiconducting polymers are semicrystalline, and the way in which polymer chains pack strongly affects their optoelectronic performance. Unlike small molecule crystals whose structure can be directly solved using established crystallographic methods, semiconducting polymers are more disordered meaning that there are not enough diffraction peaks available to analyze. To squeeze more information from the diffraction peaks that are present, we have turned to resonant tender X-ray diffraction: By varying the X-ray energy across an elemental absorption edge, variations in diffraction intensity are observed that can provide additional information about molecular packing. Also known as anomalous diffraction, this technique has been applied in other fields such as protein crystallography. As many semiconducting polymers utilise sulfur as heteroatoms, we have studied resonant diffraction effects at the sulfur K-edge in the tender X-ray regime. By performing high resolution energy scans across the sulfur K-edge, we show that spectroscopic information relating to specific bonds and molecular orientation can be discerned in the resonant X-ray diffraction profiles. Indeed, by understanding the anisotropic X-ray absorption properties of these materials we are able to interpret this data allowing us to distinguish between different crystalline polymorphs and resolve the tilting of the polymer backbone with respect to the unit cell axes. Polarization-dependent contrast furthermore is shown to be able generate scattering contrast between amorphous and crystalline domains in small-angle X-ray scattering experiments. This work in general highlights how the fields of crystallography and spectroscopy can be combined to provide new insights into the microstructure of weakly ordered soft materials.   

Polarized resonant soft X-ray scattering for organic photovoltaics

The materials science of polymeric semiconductors is complex because their intrinsic semicrystallinity always results in structural heterogeneity. This is especially true in bulk heterojunction (BHJ) organic photovoltaics (OPVs), in which two or more organic semiconductors are solidified from the same solution to create a bicontinuous percolative network with length scales of (10 to 100) nm. Heterogeneities in composition, order, and molecular orientation strongly influence the performance of polymeric semiconductors in OPVs, diodes, transistors, and sensors. Our ability to measure and correlate these heterogeneities to performance has grown enormously over the past two decades. For example, X-ray diffraction and transmission electron microscopy provide composition maps and orientation distributions of well-ordered regions.

A powerful technique for measuring the structure of polymer OPVs is Polarized Resonant Soft X-ray Scattering (P-RSoXS). In fact, RSoXS of BHJs is by far the most published application of RSoXS despite its potential to measure almost any soft matter. Typical aspects of BHJ structure that are said to be measured by RSoXS include relative phase purity and the sizes or length scales of domains or phases.

 

Using a GPU-accelerated computational framework for the forward simulation of P-RSoXS from real-space representations, I will consider a realistic BHJ “toy” model built from a blend of polymer and small molecule that routinely delivers photovoltaic power conversion efficiencies of >16%. By considering reasonable hypothetical compositional heterogeneity, sample roughness, and orientational behaviors, I will demonstrate that the most common RSoXS data analysis workflow is very likely to misrepresent – both qualitatively and quantitatively – many aspects of structure that it is relied upon to measure. Happily, the rich chemical contrast afforded by the technique provides an accesible improved analysis workflow. I will describe model-free analysis protocols to verify that the technique is measuring the aspects of structure that it is expected to measure. I will then highlight opportunities for the extraction of unique structural information from P-RSoXS data via model-based analysis to accelerate the further development of OPVs and other functional soft matter technologies.   

Combing DFT based optical models with resonant X-ray reflectivity to measure orientation at buried interfaces

Organic electronic devices have gained prominence due to their cost-effectiveness, environmental sustainability, and ease of fabrication. Controlling molecular orientation at buried interfaces in these devices is critical in optimizing device performance. However, there are no established techniques that can robustly measure molecular orientation at buried interfaces. Emerging probes such as polarized resonant soft X-ray reflectivity (pRSoXR) are promising in their ability to sample orientation depth profiles but have so far only been applied to amorphous molecular glasses. Optical models used in this analysis are based on spectroscopy that are fairly imprecise. Here we combine density functional theory (DFT) with spectroscopy to refine a robust optical model for pRSoXR to measure buried orientation. In this study, we consider crystalline thin films of Zinc Phthalocyanine (ZnPc), where we control orientation with deposition parameters. By combining these methods, we develop an optical model that can be used across all X-ray experiments enabling routine characterization and control of molecular orientation for device optimization.   

In situ X-ray tools for characterising vacuum thermally evaporated small molecules for use in organic photovoltaics

Organic photovoltaics (OPV) provide an environmentally friendly solution to meet the ever-increasing global energy demand, therefore understanding how to maximize their efficiency is crucial. This work seeks to demonstrate a suite of in situ X-ray techniques for characterising the morphology of OPV made via vacuum thermal evaporation. The microstructure of OPV governs device performance and fabrication processes are a key element of tuning morphological properties. Strategies for tuning microstructure on this project include annealing processes and templating layers. To track how these processing conditions impact the morphology of the film growth, in situ grazing incidence wide angle X-ray scattering (GIWAXS) and resonant soft X-ray scattering (RSoXS) are employed on a set of high performing molecules used in OPV, including DCV5T and DTDCPB. In situ GIWAXS taken during film growth is used to show how the molecular orientation changes as a function of distance from a templating layer interface. In situ RSoXS will be taken with different annealing processes being performed, to show the evolution of mixing behaviour of two energetically offset molecular species when thermally or solvent vapour annealed. It has previously been shown that thermal annealing has a positive correlation with domain size in a bulk heterojunction mixture.   

Hybrid Unified Analysis of Hierarchical Scattering

Modern scattering facilities can produce data over 10-orders of size enabling, for the first time, understanding of hierarchical assembly and offering structural and thermodynamic metrics correlated with important properties and growth processes. For disordered, polydisperse hierarchical materials such as carbon black, silica, synthetic polymers, and pigments the Unified Scattering Function has been widely used [1,2]. Construction of hierarchical materials from narrow-distribution structures such as spherical, lamellar, and cylindrical primary particles and micelles/vesicles can be described using a hybrid approach where the primary (and secondary) structural level(s) is (are) described by conventional, structural functions, while the hierarchical assembly at larger scales can be modeled using Unified structural levels linked by a calculation of the primary radius of gyration. Examples are shown for worm-like micelles [3], polydisperse spherical aggregates [4], and polymeric lamellar crystal assemblies [5], accessing the dimensionality, branch number, length, degree of dispersion, network percoloation, width of lamellae and thermodynamic compatibility.

 

1) Beaucage G J. Appl. Cryst. 28 717-728 (1995).

2) Beaucage G PRE 70 031401 (2004).

3) Vogtt K,Beaucage G, Jiang HQ Langmuir 31 8228-8234 (2015).

4) Manning J, Brambila C, Rishi K, Beaucage G, Davies G-L, Patwardhan S, submitted ACS Sus. Chem. Eng.(2023).

5) Camara M, Rishi K, Beaucage G, Sukumaran SK Polymer 237 124281 (2021).   

Quantitative X‐ray scattering and reflectivity measurements of polymer thin films with 2D detectors

We describe a fully open-sourced Python package to process raw X-ray scattering data using a GANESHA SAXSLAB facility, and review in this manuscript the connection of X-ray scattering theories with the open-sourced package. This package affords researchers more flexibility in analyzing and visualizing X-ray scattering and reflectivity data from what is now a commonplace facility at many universities and research laboratories engaged in polymer research. We briefly review the applications of X-ray scattering and diffraction, followed by the scattering theories. A pedagogical introduction to processing X-ray scattering data is provided using the modules in the Python package. We compare conventions to visualize and interpret transmission and grazing-incidence scattering data using self-assembled lamellar morphology of bottlebrush copolymers as an example, then describe how area detectors measure specular and off-specular reflectivity. Examples of in-house reflectivity and grazing incidence scattering for the metrological examination of a grating are provided, where the full pitch and pitch height of a line pattern is measured easily without needing to simulate the full scattering intensity profile.