Cooperative Dynamics of Highly Entangled Linear Polymers within the Entanglement Tube.

We present a quantitative comparison of the dynamic structure factors from unentangled and strongly entangled poly(butylene oxide) (PBO) melts. As expected, the low molecular weight PBO displays Rouse dynamics, however, with very significant subdiffusive center-of-mass diffusion. The spectra from high molecular weight entangled PBO can be very well described by the dynamic structure factor based on the concept of local reptation, including the Rouse dynamics within the tube and allowing for non-Gaussian corrections. Comparing quantitatively the spectra from both polymers leads to the surprising result that their spectra differ only by the contribution of classical Rouse diffusion for the low molecular weight melt. The subdiffusive component is common for both the low and high molecular weight PBO melts, indicating that in both melts the same interchain potential is active, thereby supporting the validity of the Generalized Langevin Equation approach.

Interchain Hydrodynamic Interaction and Internal Friction of Polyelectrolytes.

Polyelectrolytes (PE) are polymeric macromolecules in aqueous solutions characterized by their chain topology and intrinsic charge in a neutralizing fluid. Structure and dynamics are related to several characteristic screening length scales determined by electrostatic, excluded volume, and hydrodynamic interactions. We examine PE dynamics in dilute to semidilute conditions using dynamic light scattering, neutron spinecho spectroscopy, and pulse field gradient NMR spectroscopy. We connect macroscopic diffusion to segmental chain dynamics, revealing a decoupling of local chain dynamics from interchain interactions. Collective diffusion is described within a colloidal picture, including electrostatic and hydrodynamic interactions. Chain dynamics is characterized by the classical Zimm model of a neutral chain retarded by internal friction. We observe that hydrodynamic interactions are not fully screened between chains and that the internal friction within the chain increases with an increase in ion condensation on the chain.

Dynamic structure factors of polymer melts as observed by neutron spin echo: Direct comparison and reevaluation.

In this work, we compare the single chain dynamic structure factors for five different polymers: polyolefins (PE and PEP), poly-dienes (PB and PI), and a polyether (PEO). For this purpose, we have extended the De Gennes approximation for the dynamic structure factor. We describe the single chain dynamic structure factor in multiplying the coherent scattering functions for local reptation and Rouse motion within the Rouse blob. Important results are (i) the simple De Gennes structure factor S(Q, t)DG approximates within a few Å the outcome for the tube diameter of the more elaborate structure factor (exception PI); (ii) the extended De Gennes structure factor together with the Rouse blob describes the neutron spin echo spectra from the different polymers over the complete momentum transfer range and the full time regime from early Rouse motion to local reptation; and (iii) the representation of the scattering functions could significantly be improved by introducing non-Gaussianity corrections to the Rouse-blob dynamics. (iv) The microscopic tube step length in all cases is significantly larger than the rheological one; further tweaking the relation between tube length and entanglement blob size may indicate a possible trend toward an anisotropic lean tube with a step-length larger than the lateral extension. (v) All considered polymer data coincide after proper (Q, t) scaling to a universal behavior according to the length scale of the tube, while the relevant time scale is the entanglement time τe. (vi) In terms of the packing model, the required number of chains spanning the entanglement volume consistently is about 40% larger than that obtained from rheology.

Chain Confinement and Anomalous Diffusion in the Cross over Regime between Rouse and Reptation.

By neutron spin echo (NSE) and pulsed field gradient (PFG) NMR, we study the dynamics of a polyethylene-oxide melt (PEO) with a molecular weight in the transition regime between Rouse and reptation dynamics. We analyze the data with a Rouse mode analysis allowing for reduced long wavelength Rouse modes amplitudes. For short times, subdiffusive center-of-mass mean square displacement ⟨rcom2(t)⟩ was allowed. This approach captures the NSE data well and provides accurate information on the topological constraints in a chain length regime, where the tube model is inapplicable. As predicted by reptation for the polymer ⟨rcom2(t)⟩, we experimentally found the subdiffusive regime with an exponent close to µ=12, which, however, crosses over to Fickian diffusion not at the Rouse time, but at a later time, when the ⟨rcom2(t)⟩ has covered a distance related to the tube diameter.

Passive Macromolecular Translocation Mechanism through Lipid Membranes.

The translocation of biologically active macromolecules through cell membranes is of vital importance for cells and is a key process for drug delivery. Proteins exploit specific conformational changes in their secondary structure to facilitate membrane translocation. For the large class of biological and synthetic macromolecules, where such conformational adaptions are not possible, guidelines to tailor the structure of monomers and macromolecules to aid membrane translocation and cross-membrane drug delivery would be highly desirable. Here, we use alternating amphiphilic macromolecules to systematically investigate the relation between polarity, polymer chain length, lipid chain length, polymer concentration, and temperature on membrane partition and translocation rate. We employed pulse field gradient NMR and confocal fluorescence microscopy to determine membrane adsorption and desorption rate constants and partitioning coefficients. We find that translocation is a two-step process involving a fast adsorption and membrane insertion process and a slower desorption process. Membrane insertion is a key step that determines the molecular weight, concentration, and temperature dependences. Passive translocation is possible on time scales from minutes to hours. Macromolecules with different adapted hydrophilic/hydrophobic comonomer sequences show the same translocation rate, indicating that common optimized translocation conditions can be realized with a variety of monomer chemical structures. The investigated copolymers are biocompatible, biodegradable, and capable of transporting a hydrophobic payload through the lipid membrane. This detailed understanding of the macromolecular translocation mechanism enables to better tailor the delivery of active agents using macromolecular carriers.

Quasielastic neutron scattering reveals the temperature dependent rotational dynamics of densely grafted oleic acid.

We study the dynamics of pure oleic acid and grafted oleic acid synthesized by decomposing iron oleate into oleic acid grafted iron oxide nanoparticles. Our quasielastic neutron scattering study shows that oleic acid dominantly performs translational diffusion at room temperature. On the other hand, in nanocomposites, constraints imposed by grafting and crowding of neighboring chains restrict the grafted oleic acid to uniaxial rotation. Interestingly, it also manifests mobility in grafted oleic acid below the crystallization temperature of pure oleic acid. The data from grafted oleic acid could be effectively described using a uniaxial rotational diffusion model with an additional elastic scattering contribution. This kind of elastic scattering arises due to the restricted bond mobility and increases with decreasing temperature. The radius of rotation obtained from the fitted data agrees very well with the geometry of the molecule and grafting density. These results open possibilities of research on the confined surfactant systems, which could be analyzed using the approach described here.

Understanding the Drying Behavior of Regenerated Cellulose Gel Beads: The Effects of Concentration and Nonsolvents.

The drying behavior of regenerated cellulose gel beads swollen with different nonsolvents (e.g., water, ethanol, water/ethanol mixtures) is studied in situ on the macroscopic scale with an optical microscope as well as on nanoscale using small-angle/wide-angle X-ray scattering (SAXS/WAXS) techniques. Depending on the cellulose concentration, the structural evolution of beads during drying follows one of three distinct regimes. First, when the cellulose concentration is lower than 0.5 wt %, the drying process comprises three steps and, regardless of the water/ethanol mixture composition, a sharp structural transition corresponding to the formation of a cellulose II crystalline structure is observed. Second, when the cellulose concentration is higher than 5.0 wt %, a two-step drying process is observed and no structural transition occurs for any of the beads studied. Third, when the cellulose concentration is between 0.5 and 5.0 wt %, the drying process is dependent on the nonsolvent composition. A three-step drying process takes place for beads swollen with water/ethanol mixtures with a water content higher than 20%, while a two-step drying process is observed when the water content is lower than 20%. To describe the drying behavior governed by the cellulose concentration and nonsolvent composition, a simplified phase diagram is proposed.

Evidence of supercoolable nanoscale water clusters in an amorphous ionic liquid matrix.

Nanoscale water clusters in an ionic liquid matrix, also called "water pockets," were previously found in some mixtures of water with ionic liquids containing hydrophilic anions. However, in these systems, at least partial crystallization occurs upon supercooling. In this work, we show for mixtures of 1-butyl-3-methylimidazolium dicyanamide with water that none of the components crystallizes up to a water content of 72 mol. %. The dynamics of the ionic liquid matrix is monitored from above room temperature down to the glass transition by combining depolarized dynamic light scattering with broadband dielectric and nuclear magnetic resonance spectroscopy, revealing that the matrix behaves like a common glass former and stays amorphous in the whole temperature range. Moreover, we demonstrate by a combination of Raman spectroscopy, small angle neutron scattering, and molecular dynamics simulation that, indeed, nanoscale water clusters exist in this mixture.

Strain- and field-induced anisotropy in hybrid elastomers with elongated filler nanoparticles.

The implementation of anisotropy to functional materials is a key step towards future smart materials. In this work, we evaluate the influence of preorientation and sample architecture on the strain-induced anisotropy in hybrid elastomers containing covalently attached elongated magnetic filler particles. Accordingly, silica coated spindle-type hematite nanoparticles are incorporated into poly(dimethylsiloxane)-based elastomers, and two types of composite architectures are compared: on the one hand a conventional architecture of filled, covalently crosslinked elastomers, and on the other hybrid elastomers that are crosslinked exclusively by covalent attachment of the polymer chains to the particle surface. By the application of external strain and with magnetic fields, the orientational order of the elongated nanoparticles can be manipulated, and we investigate the interplay between strain, magnetic order, and orientational order of the particles by combining 2D small angle X-ray scattering experiments under strain and fields with Mössbauer spectroscopy under similar conditions, and supplementary angular-dependent magnetization experiments. The converging information is used to quantify the order in these interesting materials, while establishing a direct link between the magnetic properties and the spatial orientation of the embedded magnetic nanoparticles.

Chain-End Effects on Supramolecular Poly(ethylene glycol) Polymers.

In this work we present a fundamental analysis based on small-angle scattering, linear rheology and differential scanning calorimetry (DSC) experiments of the role of different hydrogen bonding (H-bonding) types on the structure and dynamics of chain-end modified poly(ethylene glycol) (PEG) in bulk. As such bifunctional PEG with a molar mass below the entanglement mass Me is symmetrically end-functionalized with three different hydrogen bonding (H-bonding) groups: thymine-1-acetic acid (thy), diamino-triazine (dat) and 2-ureido-4[1H]-pyrimidinone (upy). A linear block copolymer structure and a Newtonian-like dynamics is observed for PEG-thy/dat while results for PEG-upy structure and dynamics reveal a sphere and a network-like behavior, respectively. These observations are concomitant with an increase of the Flory-Huggins interaction parameter from PEG-thy/dat to PEG-upy that is used to quantify the difference between the H-bonding types. The upy association into spherical clusters is established by the Percus-Yevick approximation that models the inter-particle structure factor for PEG-upy. Moreover, the viscosity study reveals for PEG-upy a shear thickening behavior interpreted in terms of the free path model and related to the time for PEG-upy to dissociate from the upy clusters, seen as virtual crosslinks of the formed network. Moreover, a second relaxation time of different nature is also obtained from the complex shear modulus measurements of PEG-upy by the inverse of the angular frequency where G' and G'' crosses from the network-like to glass-like transition relaxation time, which is related to the segmental friction of PEG-upy polymeric network strands. In fact, not only do PEG-thy/dat and PEG-upy have different viscoelastic properties, but the relaxation times found for PEG-upy are much slower than the ones for PEG-thy/dat. However, the activation energy related to the association dynamics is very similar for both PEG-thy/dat and PEG-upy. Concerning the segmental dynamics, the glass transition temperature obtained from both rheological and calorimetric analysis is similar and increases for PEG-upy while for PEG-thy/dat is almost independent of association behavior. Our results show how supramolecular PEG properties vary by modifying the H-bonding association type and changing the molecular Flory-Huggins interaction parameter, which can be further explored for possible applications.

Small Groups, Big Impact: Eliminating Li+ Traps in Single-Ion Conducting Polymer Electrolytes.

Single-ion conducting polymer electrolytes exhibit great potential for next-generation high-energy-density Li metal batteries, although the lack of sufficient molecular-scale insights into lithium transport mechanisms and reliable understanding of key correlations often limit the scope of modification and design of new materials. Moreover, the sensitivity to small variations of polymer chemical structures (e.g., selection of specific linkages or chemical groups) is often overlooked as potential design parameter. In this study, combined molecular dynamics simulations and experimental investigations reveal molecular-scale correlations among variations in polymer structures and Li+ transport capabilities. Based on polyamide-based single-ion conducting quasi-solid polymer electrolytes, it is demonstrated that small modifications of the polymer backbone significantly enhance the Li+ transport while governing the resulting membrane morphology. Based on the obtained insights, tailored materials with significantly improved ionic conductivity and excellent electrochemical performance are achieved and their applicability in LFP||Li and NMC||Li cells is successfully demonstrated.

Macro- and Microstructural Evolution during Drying of Regenerated Cellulose Beads.

The macro- and microstructural evolution of water swollen and ethanol swollen regenerated cellulose gel beads have been determined during drying by optical microscopy combined with analytical balance measurements, small-angle X-ray scattering (SAXS), and wide-angle X-ray scattering (WAXS). Two characteristic length scales, which are related to the molecular dimension of cellulose monomer and elongated aggregates of these monomers, could be identified for both types of beads by SAXS. For ethanol swollen beads, only small changes to the structures were detected in both the SAXS and WAXS measurements during the entire drying process. However, the drying of cellulose from water follows a more complex process when compared to drying from ethanol. As water swollen beads dried, they went through a structural transition where elongated structures changed to spherical structures and their dimensions increased from 3.6 to 13.5 nm. After complete drying from water, the nanostructures were characterized as a combination of rodlike structures with an approximate size of cellulose monomers (0.5 nm), and spherical aggregates (13.5 nm) without any indication of heterogeneous meso- or microporosity. In addition, WAXS shows that cellulose II hydrate structure appears and transforms to cellulose II during water evaporation, however it is not possible to determine the degree of crystallinity of the beads from the present measurements. This work sheds lights on the structural changes that occur within regenerated cellulose materials during drying and can aid in the design and application of cellulosic materials as fibers, adhesives, and membranes.

Supramolecular Dimerization in a Polymer Melt from Small-Angle X-Ray Scattering and Rheology: A Miscible Model System.

We present a structural and dynamic study on the simplest supramolecular hetero-association, recently investigated by the authors to prepare architectural homogeneous structures in the melt state, based on the bio-inspired hydrogen-bonding of thymine/diaminotriazine (thy-DAT) base-pairs. In the combination with an amorphous low Tg poly(butylene oxide) (PBO), no micellar structures are formed, which is expected for nonpolar polymers because of noncompatibility with the highly polar supramolecular groups. Instead, a clear polymer-like transient architecture is retrieved. This makes the heterocomplementary thy-DAT association an ideal candidate for further exploitation of the hydrogen-bonding ability in the bulk for self-healing purposes, damage management in rubbers or even the development of easily processable branched polymers with built-in plasticizer. In the present work, we investigate the temperature range from Tg + 20 °C to Tg + 150 °C of an oligomeric PBO using small-angle X-ray scattering (SAXS) and linear rheology on the pure thy and pure DAT monofunctionals and on an equimolar mixture of thy/DAT oligomers. The linear rheology performed at low temperature is found to correspond to fully closed-state dimeric configurations. At intermediate temperatures, SAXS probes the equilibrium between open and closed states of the thy-DAT mixtures. The temperature-dependent association constant in the full range between open and closed H-bonds and an enhancement of the monomeric friction coefficient due to the groups is obtained. The thy-DAT association in the melt is more stable than the DAT-DAT, whereas the thy-thy association seems to involve additional long-lived interactions.

Self-Similar Polymer Ring Conformations Based on Elementary Loops: A Direct Observation by SANS.

We report small angle neutron scattering (SANS) results on very large polyethylene-oxide (PEO) rings in the melt. Major findings are (i) the observation of a cross over in the SANS pattern from a strong Q-dependence at intermediate Q to a Q-2 dependence at higher Q that is independent of the ring size. Summing up scattering amplitudes in a minimal model that contains the ring closure and a cross over from Gaussian statistics at short distances to more compact structures at larger distances, we identify the cross over to occur at a distance along the ring of Ne,0 = 45 ± 2.5. We consider this finding as a clear signature of the theoretically predicted elementary loops that build up the ring conformation. Their size is in the range of an entanglement strand for linear PEO melts and they are characterized by Gaussian statistics. (ii) The chain length dependence of the radius of gyration Rg follows rather closely the prediction of Obukhov's decorated ring model. (iii) Other than extracted from numerous simulations that are interpreted in terms of a cross over to mass fractal behavior around N ≅ 10Ne,0 with a fractal dimension df = 3 and exponent ν = 1/3, we do not observe such a cross over, but Rg(N) ∼ Nν=0.39 holds over the entire size range.

Direct Observation of Dynamic Tube Dilation in Entangled Polymer Blends: A Combination of Neutron Scattering and Dielectric Techniques.

We report a microscopic observation of the time-dependent dynamic tube dilation process on isofrictional bidisperse melts. By applying neutron spin echo (NSE) and dielectric techniques on blends of long polyisoprene (PI) chains with short PI additives with different topology, we access the dynamics of the tube dilation process on a molecular scale. The time-dependent tube dilation is directly revealed by NSE as an additional time dependence of the dynamic structure factor in the local reptation regime. We identify the characteristic time of tube dilation as the terminal time of the additive.

Polymer dynamics under confinement.

We review recent neutron scattering work and related results from simulation and complementary techniques focusing on the microscopic dynamics of polymers under confinement. Confinement is either realized in model porous materials or in polymer nanocomposites (PNC). The dynamics of such confined polymers is affected on the local segmental level, the level of entanglements as well as on global levels: (i) at the segmental level the interaction with the surface is of key importance. At locally repulsive surfaces compared to the bulk the segmental dynamics is not altered. Attractive surfaces slow down the segmental dynamics in their neighborhood but do not give rise to dead, glassy layers. (ii) Confinement generally has little effect on the inter-chain entanglements: both for weakly as well as for marginally confined polymers the reptation tube size is not changed. Only for strongly confined polymers disentanglement takes place. Similarly, in PNC at higher NP loading disentanglement phenomena are observed; in addition, at very high loading a transition from polymer caused topological constraints to purely geometrical constraints is observed. (iii) On the more global scale NSE experiments revealed important information on the nature of the interphase between adsorbed layer and bulk polymer. (iv) Polymer grafts at NP mutually confine each other, an effect that is most pronounced for one component NP. (v) Global diffusion of entangled polymers both in weakly and strongly attractive PNC is governed by the ratio of bottle-neck to chain size that characterizes the 'entropic barrier' for global diffusion.

Inner structure and dynamics of microgels with low and medium crosslinker content prepared via surfactant-free precipitation polymerization and continuous monomer feeding approach.

The preparation of poly(N-isopropylacrylamide) microgels via classical precipitation polymerization (batch method) and a continuous monomer feeding approach (feeding method) leads to different internal crosslinker distributions, i.e., from core-shell-like to a more homogeneous one. The internal structure and dynamics of these microgels with low and medium crosslinker concentrations are studied with dynamic light scattering and small-angle neutron scattering in a wide q-range below and above the volume phase transition temperature. The influence of the preparation method, and crosslinker and initiator concentration on the internal structure of the microgels is investigated. In contrast to the classical conception where polymer microgels possess a core-shell structure with the averaged internal polymer density distribution within the core part, a detailed view of the internal inhomogeneities of the PNIPAM microgels and the presence of internal domains even above the volume phase transition temperature, when polymer microgels are in the deswollen state, are presented. The correlation between initiator concentration and the size of internal domains that appear inside the microgel with temperature increase is demonstrated. Moreover, the influence of internal inhomogeneities on the dynamics of the batch- and feeding-microgels studied with neutron spin-echo spectroscopy is reported.

Creating a synthetic platform for the encapsulation of nanocrystals with covalently bound polymer shells.

We present a platform for the encapsulation of superparamagnetic iron oxide nanocrystals (SPIONs) with a highly stable diblock copolymer shell allowing a homogeneous dispersion of the nanocrystals into a polymer matrix in the resulting nanocomposites. High polymer shell stability was achieved by crosslinking the inner polydiene shell for example in a persulfate based redox process. The advantage of this crosslinking reaction is the avoidance of heat and UV light for the initiation, making it suitable for heat or UV sensitive systems. In addition, we were able to minimize the ligand excess needed for the encapsulation and showcased a variation of molecular weight and composition as well as different ligands which lead to stable micelles. The encapsulated nanocrystals as well as the nanocomposite materials were characterized by transmission electron microscopy (TEM) and small angle scattering (SAXS and SANS).

Confinement Facilitated Protein Stabilization As Investigated by Small-Angle Neutron Scattering.

While mesoporous silicas have been shown to be a compelling candidate for drug delivery and the implementation of biotechnological applications requiring protein confinement and immobilization, the understanding of protein behavior upon physical adsorption into silica pores is limited. Many indirect methods are available to assess general adsorbed protein stability, such as Fourier-transform infrared spectroscopy and activity assays. However, the limitation of these methods is that spatial protein arrangement within the pores cannot be assessed. Mesoporous silicas pose a distinct challenge to direct methods, such as transmission electron microscopy, which lacks the contrast and resolution required to adequately observe immobilized protein structure, and nuclear magnetic resonance, which is computationally intensive and requires knowledge of the primary structure a priori. Small-angle neutron scattering can surmount these limitations and observe spatial protein arrangement within pores. Hereby, we observe the stabilization of fluid-like protein arrangement, facilitated by geometry-dependent crowding effects in cylindrical pores of ordered mesoporous silica, SBA-15. Stabilization is induced from a fluid-like structure factor, which is observed for samples at maximum protein loading in SBA-15 with pore diameters of 6.4 and 8.1 nm. Application of this effect for prevention of irreversible aggregation in high concentration environments is proposed.