Session K42: Polymer Dynamics - Insight from In-Situ Scattering

Local Dynamics of Acid- and Ion-containing Copolymer Melts.

Interest in acid- and ion-containing polymers arises in part from applications as single-ion conductors for selectively transporting a counter ion for battery applications. Structurally, the low dielectric constant of organic polymers and strong ionic interactions leads to ionic aggregation. Here the polymer backbone motion was investigated through quasi-elastic neutron scattering measurements (QENS) and compared with fully atomistic molecular dynamic simulations of precise poly(ethylene-acrylic acid) copolymers and their ionomers (pxAA-y{\%}Li). The effect of carbon spacer length (x$=$9, 15, 21) between the acid groups and the degree of neutralization (y) with Li on PE backbone dynamics were considered. Systematic slowing in chain dynamics were observed with increasing neutralization where polymer dynamics appear constrained due to anchoring effects. Simulations provide complementary viewpoints indicating a gradient in chain dynamics as a distance away from acid groups. These results indicate that the addition of pendant acid groups inhibit typical PE backbone motion and the neutralized forms strongly suppress the fraction of mobile PE chain.   

Effect of Increasing Molecular Weight on the A and B blocks of a Single-ion-conducting Block Copolymer Electrolyte for Lithium Batteries

Single-ion-conducting block copolymer electrolytes are desirable for lithium metal batteries due to their ability to eliminate salt concentration gradients across the electrolyte; i.e., the lithium ion transference number is approximately unity. A series of poly(ethylene oxide)-$b$-poly(styrenesulfonyllithium(trifluoromethylsulfonyl)imide) (PEO-$b$-PSLiTFSI) copolymers was studied wherein the molecular weights of both blocks were varied. Small angle x-ray scattering and ac impedance spectroscopy were used to probe the dependence of ionic conductivity on morphology. Preliminary work suggests that increasing the molecular weights of the blocks results in increased disorder and lower conductivity.   

SUPERCOOLED WATER IN SUPRAMOLECULAR HYDROGELS

The suppression of water crystallization with appreciable water supercooling is challenging due to its large enthalpy of fusion. A common theme to supercool water is to confine the water in the pores of microporous/mesoporous solids where mechanical confinement prevents water crystallization. Nature takes a different approach with crystallization suppression through a combination of preferential adsorption on ice nuclei and confinement between hydrophobic residues using organic components only. Here, we demonstrate that mechanically robust confinement within a hard material is not necessary to significantly supercool water. In this case, a supramolecular hydrogel, based on a random amphiphilic copolymer, is used to provide soft confinement of water between the hydrophobic aggregates with an interdomain spacing \textless 8 nm. Small angle neutron scattering (SANS) provides insight into the structural evolution of the supramolecular structure of the hydrogel on supercooling. The structural changes are sensitive to the composition of the copolymer as determined by contrast variation SANS. Similarly, the dynamics of both the copolymer and water are probed using quasielastic neutron scattering (QENS). Using QENS, a highly mobile water phase (tau $\approx $23 ps) is identified to be present even when slowly cooling to as low as 220K.   

Segmental chain dynamics of ABA triblock copolymer micelles in aqueous solution

The polymer physics of hierarchical, aqueous self-assembled ABA block copolymers is an active area of research for both advanced materials and biomaterial applications [1]. Scattering-based techniques provide a direct measure of the correlations and structure across multiple length and time scales. Hierarchical clusters of micelles are formed by well-defined poly(ethylene glycol) triblock copolymers with oligo-fluorene hydrophobic end-groups in aqueous solutions. The structure and dynamics of this system was studied by small-angle neutron scattering (SANS), and static and dynamic light scattering [2]. We will present new neutron spin-echo spectroscopy (NSE) results that provides direct insight into the segmental chain dynamics constrained by the pi-pi stacking of the oligo-fluorene end groups. The dilute cluster regime within the temperature-composition phase diagram is of current interest. [1] S. Venkataraman, A.L. Lee, H.T. Maune, J.L. Hedrick, V.M. Prabhu, and Y.Y. Yang, Macromolecules 46, 4839 (2013). [2] V.M. Prabhu et al. ``Equilibrium self-assembly, structure and dynamics of clusters of star-like micelles,'' ACS Macro Letters, 4, 1128 (2015).   

Viscoelastic hydrodynamic interactions and anomalous CM diffusion in polymer melts

We have recently discovered that anomalous center-of-mass (CM) diffusion occurring on intermediate time scales in polymer melts can be explained by the interplay of viscoelastic and hydrodynamic interactions (VHI). The theory has been solved for unentangled melts in 3D [1] and 2D [2] and excellent agreement between theory and simulation is found, also for alkanes with a force field optimized from neutron scattering [3]. The physical mechanism considers that hydrodynamic interactions are not screened: they are time dependent because of increasing viscosity before the terminal relaxation time. The VHI are generally active in melts of any topology. They are most important at early times well before the terminal relaxation time and thus affect the nanosecond time range typically observable in dynamic neutron scattering experiments [1,3]. We illustrate the effects with recent molecular dynamics simulations of linear, ring and star polymers. [1] PRL 107, 178301 (2011); PRE 85, 051807 (2012). [2] PRL 109, 248304 (2012); Soft Matter 9, 4249 (2013). [3] PRL 111, 173003 (2013).   

Nanostructures and dynamics \quad of macromolecules bound to attractive filler surfaces

We report in-situ nanostructures and dynamics of polybutadiene (PB) chains bound to carbon black (CB) fillers (the so-called ``bound polymer layer (BPL)'') in a good solvent. The BPL on the CB fillers were extracted by solvent leaching of a CB-filled PB compound and subsequently dispersed in deuterated toluene to label the BPL for small-angle neutron scattering and neutron spin echo techniques. Intriguingly, the results demonstrate that the BPL is composed of two regions regardless of molecular weights of PB: the inner unswollen region of $\approx $ 0.5 nm thick and outer swollen region where the polymer chains display a parabolic profile with a diffuse tail. This two-layer formation on the filler surface is similar to that reported for polymer chains adsorbed on planar substrates from melts [1]. In addition, the results show that the dynamics of the swollen bound chains can be explained by the so-called ``breathing mode'' and is generalized with the thickness of the swollen BPL. Furthermore, we will discuss how the breathing collective dynamics is affected by the presence of polymer chains in a matrix solution. [1] Gin et al., Phys. Rev. Lett., (2012), 109, 265501.   

SAXS/WAXS studies of shear-induced crystallization of poly(1-butene)

Flow-induced crystallization of poly(1-butene) was studied in shear flow. Flow was produced using a Linkam shear cell that has been modified to allow x-ray access for in situ studies of polymer structure using synchrotron x-ray scattering techniques. After loading in the shear cell, samples were first heated well into the melt, and then cooled to a crystallization temperature selected such that negligible quiescent crystallization would occur on reasonable time scales. A short burst of shear flow was then applied at various rates, after which simultaneous wide- and small-angle x-ray scattering (WAXS and SAXS, respectively) data were collected to study the impact of both deformation rate and total applied strain on accelerated crystallization kinetics as well as the morphology of the resulting crystallites (e.g. degree of crystallite orientation). SAXS and WAXS data generally showed qualitative agreement in measures of the extent of crystallization and the degree of crystallite orientation. Average crystallite orientation was found to decrease over the course of crystallization. The crystalline volume fraction in the sample was calculated from the (i) SAXS invariant and (ii) integrated WAXS intensity profile to quantify the extent to which the sample crystallized at various flow c   

Leveraging intrinsic chain anisotropy to align coil-coil block copolymers with magnetic fields

Magnetic field alignment of block copolymers (BCPs) has typically relied on the presence of liquid crystalline or crystalline assemblies to provide sufficient magnetic anisotropy to drive alignment. Recent experiments however show that alignment is also possible in simple coil-coil BCPs. In particular, alignment of lamellae was observed in poly(styrene-b-4-vinylpyridine) (PS-P4VP) on cooling across the order-disorder transition at field strengths as low as 1 T, with alignment improving markedly with increasing field strength and decreasing cooling rate. Here we discuss the intrinsic chain anisotropy which drives the observed alignment, and its display as a net microdomain anisotropy due to chain tethering at the block interface. We use in-situ X-ray scattering to study the phase behavior and temperature-, time-, and field- dependent dynamics of magnetic alignment in coil-coil BCPs, highlighting the important roles of chain anisotropy and grain size in alignment. For the right combination of field strength and grain size, we can leverage intrinsic chain anisotropy to magnetically direct self-assembly in other coil-coil systems, including cylinder-forming poly(styrene-b-dimethylsiloxane). Field alignment of PS-P4VP with PEO and other blends provides a route to form functional materials such as nanoporous films and ion conducting polymers.   

Elucidating the Molecular Deformation Mechanism of Entangled Polymers in Fast Flow by Small Angle Neutron Scattering

Understanding the viscoelastic properties of polymers is of fundamental and practical importance because of the vast and ever expanding demand of polymeric materials in daily life. Our current theoretical framework for describing the nonlinear flow behavior of entangled polymers is built upon the tube model pioneered by de Gennes, Doi, and Edwards. In this work, we critically examine the central hypothesis of the tube model for nonlinear rheology using small angle neutron scattering (SANS). While the tube model envisions a unique non-affine elastic deformation mechanism for entangled polymers, our SANS measurements show that the evolution of chain conformation of a well-entangled polystyrene melt closely follows the affine deformation mechanism in uniaxial extension, even when the Rouse Weissenberg number is much smaller than unity. This result provides a key clue for understanding the molecular deformation mechanism of entangled polymers in fast flow. Several implications from our analysis will be discussed in this talk.   

\textbf{Insights into the Dynamics of Polymers and Nanocomposites via Quasi-elastic Neutron Scattering}

Neutron scattering is a powerful technique to study polymer dynamics. In particular, its angular momentum (Q) dependence provide a unique opportunity to study polymer motions at different length scales of interest. This is particularly important when examining some of the crucial aspects related with glass transitions and in particular chain behavior under confinement. Of particular interests are the examples of nanocomposites and ionomers. The interactions between polymer chain and nanoparticle in nanocomposites can cause significant heterogeneities in the polymer dynamics and strongly affect its properties. Dynamical perturbations are generally expected to be limited to interfacial polymer segments. However, composites with highly anisotropic nanoparticles usually exhibit very low percolation threshold and in such systems a complex polymer relaxation behavior can be observed that is not anticipated from dilute nanoparticle dispersions in polymer matrix. A lot of progress has been made over the last decade, however, the effect of chemical binding, physical adsorption and inclusion of nanoparticles in polymer matrix on the local and global dynamics of chain molecules still remains controversial subject. In this talk I will present some examples of our recent work where neutron scattering has been able to make some important breakthroughs in these topics.   

SAXS studies of the structure of a BCC-ordered block copolymer melt subjected to uniaxial extensional flow

We report in situ small-angle x-ray scattering (SAXS) investigations of a spherically-ordered block copolymer melt with a low styrene content (13\%) resulting in spherical polystyrene microdomains ordered in BCC lattice. Melt annealing after clearing above the ODT produces ordered samples that have a macroscopically random orientation distribution of BCC 'grains'. Melt samples are subjected to uniaxial extensional flow in a counter-rotating drum extensional flow fixture housed in an oven with synchrotron x-ray access. During flow, initially isotropic diffraction rings in SAXS patterns become deformed, reflecting distortion of the BCC lattice. Diffracted intensity also concentrates azimuthally, indicating macroscopic alignment of the BCC lattice. There is evidence that extensional flow leads to progressive disordering of the BCC structure, with loss of higher order peaks and the emergence of a diffuse 'halo' of scattering. While the primary diffraction peak is visible in directions parallel and perpendicular to the stretching direction, the deformation of the lattice d-spacing follows affine deformation. Indications of ordering persist to higher strains in samples stretched at higher extension rates, and evidence of affine lattice deformation persists to very high strains (Hencky   

Neutron Reflectivity Measurement for Polymer Dynamics near Graphene Oxide Monolayers.

We investigated the diffusion dynamics of polymer chains confined between graphene oxide layers using neutron reflectivity (NR). The bilayers of polymethylmetacrylate (PMMA)/ deuterated PMMA (d-PMMA) films and polystyrene (PS)/d-PS films with various film thickness sandwiched between Langmuir-Blodgett (LB) monolayers of graphene oxide (GO) were prepared. From the NR results, we found that PMMA diffusion dynamics was reduced near the GO surface while the PS diffusion was not significantly changed. This is due to the different strength of GO-polymer interaction. In this talk, these diffusion results will be compared with dewetting dynamics of polymer thin films on the GO monolayers. This has given us the basis for development of graphene-based nanoelectronics with high efficiency, such as heterojunction devices for polymer photovoltaic (OPV) applications.