Sensing Polymer Chain Dynamics through Ring Topology: A Neutron Spin Echo Study.

Using neutron spin echo spectroscopy, we show that the segmental dynamics of polymer rings immersed in linear chains is completely controlled by the host. This transforms rings into ideal probes for studying the entanglement dynamics of the embedding matrix. As a consequence of the unique ring topology, in long chain matrices the entanglement spacing is directly revealed, unaffected by local reptation of the host molecules beyond this distance. In shorter entangled matrices, where in the time frame of the experiment secondary effects such as contour length fluctuations or constraint release could play a role, the ring motion reveals that the contour length fluctuation is weaker than assumed in state-of-the-art rheology and that the constraint release is negligible. We expect that rings, as topological probes, will also grant direct access to molecular aspects of polymer motion which have been inaccessible until now within chains adhering to more complex architectures.

The segmental and chain relaxation modes in high-cis-polyisoprene as studied by thermally stimulated currents.

The technique of Thermally Stimulated Currents is used to study the slow molecular mobility in a series of poly (1,4-cis-isoprene) samples with different molecular weights, Mw, and low polydispersity. The technique revealed a high resolution power, particularly useful in the study of the lower molecular weight samples where the chain and the segmental relaxations strongly overlap. The dynamic crossover that is reported for the normal mode by varying the molecular weight is clearly revealed by the thermally stimulated depolarization currents results through the temperature location, TMn, of the normal mode peak, the values of the relaxation time at TMn, τ(TMn), and the value of the fragility index of the normal mode, mn. The kinetic features of the glass transition relaxation of polyisoprene have also been determined.

Monitoring the internal structure of poly(N-vinylcaprolactam) microgels with variable cross-link concentration.

The combination of a set of complementary techniques allows us to construct an unprecedented and comprehensive picture of the internal structure, temperature dependent swelling behavior, and the dependence of these properties on the cross-linker concentration of microgel particles based on N-vinylcaprolactam (VCL). The microgels were synthesized by precipitation polymerization using different amounts of cross-linking agent. Characterization was performed by small-angle neutron scattering (SANS) using two complementary neutron instruments to cover a uniquely broad Q-range with one probe. Additionally we used dynamic light scattering (DLS), atomic force microscopy (AFM), and differential scanning calorimetry (DSC). Previously obtained nuclear magnetic resonance spectroscopy (NMR) results on the same PVCL particles are utilized to round the picture off. Our study shows that both the particle radius and the cross-link density and therefore also the stiffness of the microgels rises with increasing cross-linker content. Hence, more cross-linker reduces the swelling capability distinctly. These findings are supported by SANS and AFM measurements. Independent DLS experiments also found the increase in particle size but suggest an unchanged cross-link density. The reason for the apparent contradiction is the indirect extraction of the parameters via a model in the evaluation of DLS measurements. The more direct approach in AFM by evaluating the cross section profiles of observed microgel particles gives evidence of significantly softer and more deformable particles at lower cross-linker concentrations and therefore verifies the change in cross-link density. DSC data indicate a minor but unexpected shift of the volume phase transition temperature (VPTT) to higher temperatures and exposes a more heterogeneous internal structure of the microgels with increasing cross-link density. Moreover, a change in the total energy transfer during the VPT gives evidence that the strength of hydrogen bonds is significantly affected by the cross-link density. A strong and reproducible deviation of the material density of the cross-linked microgel polymer chains toward a higher value compared to the respective linear chains has yet to be explained.

Compact structure and non-Gaussian dynamics of ring polymer melts.

We present a neutron scattering analysis of the structure and dynamics of PEO polymer rings with a molecular weight 2.5 times higher than the entanglement mass. The melt structure was found to be more compact than a Gaussian model would suggest. With increasing time the center of mass (c.o.m.) diffusion undergoes a transition from sub-diffusive to diffusive behavior. The transition time agrees well with the decorrelation time predicted by a mode coupling approach. As a novel feature well pronounced non-Gaussian behavior of the c.o.m. diffusion was found that shows surprising analogies to the cage effect known from glassy systems. Finally, the longest wavelength Rouse modes are suppressed possibly as a consequence of an onset of lattice animal features as hypothesized in theoretical approaches.

Solution scattering studies of the hierarchical assembly of porphyrin trimers based on benzene triscarboxamide.

The self-assembly of achiral and chiral porphyrin trimers based on benzene triscarboxamide in solution is studied with the help of NMR, FT-IR, UV-vis, and CD spectroscopy. These studies revealed that in apolar solvents the porphyrin trimers self-assembled in columnar stacks via a combination of hydrogen bonding and π-π stacking interactions. While the critical aggregation constant is about 0.2 mM in chloroform, aggregation already occurs at micromolar concentrations in n-hexane. Small angle neutron scattering (SANS) studies in chloroform, toluene, and n-hexane confirmed aggregation of the trimers into columnar stacks. In chloroform and n-hexane, but not in toluene, the trimers gelated the solvent. In chloroform the stacks of the achiral trimer were found to contain on average about 70 molecules, while in toluene the stacks were much smaller and contained on average 7-9 molecules. In n-hexane the SANS studies revealed that the chiral trimer formed a gel with an average mesh size of the transient network of chains of approximately 90 nm, with chains being built up from effective cylindrical aggregates with an average length of 20 nm.

Interactions between block copolymers and single-walled carbon nanotubes in aqueous solutions: a small-angle neutron scattering study.

The amphiphilic copolymers of the Pluronic family are known to be excellent dispersants for single-walled carbon nanotubes (SWCNT) in water, especially F108 and F127, which have rather long end-blocks of poly(ethylene oxide) (PEO). In this study, the structure of the CNT/polymer hybrid formed in water is evaluated by measurements of small-angle neutron scattering (SANS) with contrast variation, as supported by cryo-transmission electron microscopy (cryo-TEM) imaging. The homogeneous, stable, inklike dispersions exhibited very small isolated bundles of carbon nanotubes in cryo-TEM images. SANS experiments were conducted at different D(2)O/H(2)O content of the dispersing solvent. The data for both systems showed surprisingly minimal intensity values at 70% D(2)O solvent composition, which is much higher than the expected value of 17% D(2)O that is based on the scattering length density (SLD) of PEO. At this near match point, the data exhibited a q(-1) power law relation of intensity to the scattering vector (q), indicating rodlike entities. Two models are evaluated, as extensions to Pederson's block copolymer micelles models. One is loosely adsorbed polymer chains on a rodlike CNT bundle. In the other, the hydrophobic block is considered to form a continuous hydrated shell on the CNT surface, whereas the hydrophilic blocks emanate into the solvent. Both models were found to fit the experimental data reasonably well. The model fit required special considerations of the tight association of water molecules around PEO chains and slight isotopic selectivity.

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.

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.

Decoupling the Contributions of ZnO and Silica in the Characterization of Industrially-Mixed Filled Rubbers by Combining Small Angle Neutron and X-Ray Scattering.

Scattering techniques with neutrons and X-rays are powerful methods for the investigation of the hierarchical structure of reinforcing fillers in rubber matrices. However, when using only X-ray scattering, the independent determination of the filler response itself sometimes remains an issue because of a strong parasitic contribution of the ZnO catalyst and activator in the vulcanization process. Microscopic characterization of filler-rubber mixtures even with only catalytic amounts of ZnO is, therefore, inevitably complex. Here, we present a study of silica aggregates dispersed in an SBR rubber in the presence of the catalyst and show that accurate partial structure factors of both components can be determined separately from the combination of the two scattering probes, neutrons, and X-rays. A unique separation of the silica filler scattering function devoid of parasitic catalyst scattering becomes possible. From the combined analysis, the catalyst contribution is determined as well and results to be prominent in the correction scheme. The experimental nano-structure of the ZnO after the mixing process as the by-product of the scattering decomposition was found also to be affected by the presence or absence of silica in the rubber mixture, correlated with the shear forces in the mixing and milling processes during sample preparation. The presented method is well suited for studies of novel dual filler systems.

Interplay between the amphipathic polyoxometalate interactions in solution and at solid-liquid interfaces: a toolbox for the technical application.

The far-reaching interplay between the speciation of polyoxometalates (POMs) in the liquid phase and the POM adsorption characteristics on substrate surfaces yet remains to be understood. The significance of this interplay is however paramount because it indicates the degree of technical applicability of solvent-processable POM molecules. Herein, we target this fundamentally important issue, shedding light on the "POM-counterion-solvent" and "POM-counterion-solvent-substrate" processes. We effectively combine the results from small-angle X-ray scattering in solution with surface sensitive scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements and present on this basis a semi-quantitative analysis which provides an excellent correlation between both approaches. The MeCN-solution speciation of a tris(alkoxo)-ligated Wells-Dawson-type polyoxoanion - explored as a representative of commonly negatively charged POM-based inorganic-organic nanostructures - is strikingly connected with the growth of porous two-dimensional molecular layers on highly oriented pyrolytic graphite (HOPG). Low water amounts dramatically transform intermolecular relationships toward hierarchical agglomeration that inhibits the layer formation on HOPG. The obtained findings lay the groundwork for a mechanistic study of controlled nucleation and growth of POM nanostructures on weakly interacting surfaces.

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).

Chemically defined, ultrasoft PDMS elastomers with selectable elasticity for mechanobiology.

Living animal cells are strongly influenced by the mechanical properties of their environment. To model physiological conditions ultrasoft cell culture substrates, in some instances with elasticity (Young's modulus) of only 1 kPa, are mandatory. Due to their long shelf life PDMS-based elastomers are a popular choice. However, uncertainty about additives in commercial formulations and difficulties to reach very soft materials limit their use. Here, we produced silicone elastomers from few, chemically defined and commercially available substances. Elastomers exhibited elasticities in the range from 1 kPa to 55 kPa. In detail, a high molecular weight (155 kg/mol), vinyl-terminated linear silicone was crosslinked with a multifunctional (f = 51) crosslinker (a copolymer of dimethyl siloxane and hydrosilane) by a platinum catalyst. The following different strategies towards ultrasoft materials were explored: sparse crosslinking, swelling with inert silicone polymers, and, finally, deliberate introduction of dangling ends into the network (inhibition). Rheological experiments with very low frequencies led to precise viscoelastic characterizations. All strategies enabled tuning of stiffness with the lowest stiffness of ~1 kPa reached by inhibition. This system was also most practical to use. Biocompatibility of materials was tested using primary cortical neurons from rats. Even after several days of cultivation no adverse effects were found.

Self-assembly of porphyrin hexamers via bidentate metal-ligand coordination.

The supramolecular assembly of metal-porphyrin hexamers with bidentate ligands in chloroform solutions is demonstrated by UV/Vis and 1H NMR-titrations, and Small Angle Neutron Scattering (SANS) experiments. Titrations of zinc porphyrin hexamer Zn1 with 1,4-diazabicyclo[2,2,2]octane (DABCO) revealed that at a DABCO/Zn1 molar ratio of 3, intermolecular sandwich complexes are formed, which can be considered as "circular-shaped porphyrin ladders". These supramolecular complexes further aggregate into larger polymeric stacks, as a result of a combination of cooperativity effects, π-π stacking interactions, and chelate effects. The presence of rodlike assemblies in solution, formed by assembly of Zn1 and DABCO, is confirmed by SANS-experiments. Using a model for cylindrical assemblies, curve fitting calculations reveal that rods with an average length of 26 nm and a radius of 30-35 Å were formed, corresponding to columnar stacks of approximately 30 hexamer molecules. In contrast, the metal-free hexamer H21 did not form extended assemblies due to the absence of coordinative intermolecular interactions.

Structure of interacting aggregates of silica nanoparticles in a polymer matrix: small-angle scattering and reverse Monte Carlo simulations.

Reinforcement of elastomers by colloidal nanoparticles is an important application where microstructure needs to be understood-and, if possible, controlled-if one wishes to tune macroscopic mechanical properties. Here, the three-dimensional structure of large aggregates of nanometric silica particles embedded in a soft polymeric matrix is determined by small angle neutron scattering. Experimentally, the crowded environment leading to strong reinforcement induces a strong interaction between aggregates, which generates a prominent interaction peak in the scattering. We propose to analyze the total signal by means of a decomposition in a classical colloidal structure factor describing aggregate interaction and an aggregate form factor determined by a reverse Monte Carlo technique. The result gives new insight to the shape of aggregates and their complex interaction in elastomers. For comparison, fractal models for aggregate scattering are also discussed.

Tough Supramolecular Hydrogel Based on Strong Hydrophobic Interactions in a Multiblock Segmented Copolymer.

We report the preparation and structural and mechanical characterization of a tough supramolecular hydrogel, based exclusively on hydrophobic association. The system consists of a multiblock, segmented copolymer of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic dimer fatty acid (DFA) building blocks. A series of copolymers containing 2K, 4K, and 8K PEG were prepared. Upon swelling in water, a network is formed by self-assembly of hydrophobic DFA units in micellar domains, which act as stable physical cross-link points. The resulting hydrogels are noneroding and contain 75-92 wt % of water at swelling equilibrium. Small-angle neutron scattering (SANS) measurements showed that the aggregation number of micelles ranges from 2 × 102 to 6 × 102 DFA units, increasing with PEG molecular weight. Mechanical characterization indicated that the hydrogel containing PEG 2000 is mechanically very stable and tough, possessing a tensile toughness of 4.12 MJ/m3. The high toughness, processability, and ease of preparation make these hydrogels very attractive for applications where mechanical stability and load bearing features of soft materials are required.

Nanocomposites of Highly Monodisperse Encapsulated Superparamagnetic Iron Oxide Nanocrystals Homogeneously Dispersed in a Poly(ethylene Oxide) Melt.

Nanocomposite materials based on highly stable encapsulated superparamagnetic iron oxide nanocrystals (SPIONs) were synthesized and characterized by scattering methods and transmission electron microscopy (TEM). The combination of advanced synthesis and encapsulation techniques using different diblock copolymers and the thiol-ene click reaction for cross-linking the polymeric shell results in uniform hybrid SPIONs homogeneously dispersed in a poly(ethylene oxide) matrix. Small-angle X-ray scattering and TEM investigations demonstrate the presence of mostly single particles and a negligible amount of dyads. Consequently, an efficient control over the encapsulation and synthetic conditions is of paramount importance to minimize the fraction of agglomerates and to obtain uniform hybrid nanomaterials.

Influence of the Solvent Quality on Ring Polymer Dimensions.

We present a systematic investigation of well-characterized, experimentally pure polystyrene (PS) rings with molar mass of 161 000 g/mol in dilute solutions. We measure the ring form factor at θ- and good-solvent conditions as well as in a polymeric solvent (linear PS of roughly comparable molar mass) by means of small-angle neutron scattering (SANS). Additional dynamic light scattering (DLS) measurements support the SANS data and help elucidate the role of solvent quality and solution preparation. The results indicate the increase of ring dimensions as the solvent quality improves. Furthermore, the experimental form factors in both θ-solvent and linear matrix behave as ideal rings and are fully superimposable. The nearly Gaussian conformations of rings in a melt of linear chains provide evidence of threading of linear chains through rings. The latter result has implications for the dynamics of ring-linear polymer mixtures.

Viscosity of ring polymer melts.

We have measured the linear rheology of critically purified ring polyisoprenes, polystyrenes and polyethyleneoxides of different molar masses. The ratio of the zero-shear viscosities of linear polymer melts η0,linear to their ring counterparts η0,ring at isofrictional conditions is discussed as function of the number of entanglements Z. In the unentangled regime η0,linear/η0,ring is virtually constant, consistent with the earlier data, atomistic simulations, and the theoretical expectation η0,linear/η0,ring=2. In the entanglement regime, the Z-dependence of rings viscosity is much weaker than that of linear polymers, in qualitative agreement with predictions from scaling theory and simulations. The power-law extracted from the available experimental data in the rather limited range 1

Thermoresponsive copolymer hydrogels based on N-isopropylacrylamide and cationic surfactant monomers prepared from micellar solution and microemulsion in a one-step reaction.

Thermoresponsive hydrogels were prepared upon radiation-induced copolymerization of aqueous micellar solutions containing N-isopropylacrylamide (NiPAAm) and a cationic surfactant monomer (surfmer), and of microemulsions containing NiPAAm, surfmer, and styrene. Three surfmer compounds were used: (11-(acryloyloxy)undecyl)trimethylammonium bromide (AUTMAB), (11-(methacryloyloxy)undecyl)trimethylammonium bromide (MUTMAB), and (2-(methacryloyloxy)ethyl)dodecyldimethylammonium bromide (MEDDAB). Comonomer solutions were studied on their phase behavior and structure using small-angle neutron scattering (SANS). The presence of surfmers increased the solubility of NiPAAm in the aqueous phase. SANS studies indicate that the surfmers form spherical micelles, which in the presence of styrene are increased and in the presence of NiPAAm are decreased in size. Styrene is incorporated in the core, and NiPAAm is incorporated in the shell of the micelles. If styrene and NiPAAm are present, the effects of both compensate each other, the micelle size remains unchanged, and only small amounts of styrene are solubilized. Evaluation of scattering curves indicated remarkable changes in headgroup dissociation of surfmers in the presence of NiPAAm in the micellar solutions. If exposed to (60)Co-gamma irradiation (dose: 80 kGy), stable, transparent, and thermoresponsive hydrogels were directly obtained. The lower critical solution temperature (LCST) of gels containing surfmer in low concentration was higher than that for pure NiPAAm gels, whereas in gels with high surfmer concentration it was lower. The lowest LCST was observed if MEDDAB was present in the gel. 1 % (w/w) was already sufficient to lower the LCST from 33.2 to 28.5 degrees C. Gels with low surfmer concentration (< or = 1 wt %) exhibited a strong, rapid swelling in water at 20 degrees C and a rapid and reversible shrinking at 50 degrees C. For a gel containing 1% AUTMAB, the swelling ratio was 2.4 times higher (MUTMAB, 2.8; MEDDAB, 1.5) than that for a pure NiPAAm gel. Copolymer gels containing more than 1 wt % surfmer exhibited a strong and rapid swelling below and above the LCST, because the copolymerized ionic surfmer induced an osmotic pressure in the gel. The effects of a variation of NiPAAm and surfmer concentration were studied, and the origins of the thermoresponsive properties are discussed.

Persistence length of titin from rabbit skeletal muscles measured with scattering and microrheology techniques.

The persistence length of titin from rabbit skeletal muscles was measured using a combination of static and dynamic light scattering, and neutron small angle scattering. Values of persistence length in the range 9-16 nm were found for titin-II, which corresponds to mainly physiologically inelastic A-band part of the protein, and for a proteolytic fragment with 100-nm contour length from the physiologically elastic I-band part. The ratio of the hydrodynamic radius to the static radius of gyration indicates that the proteins obey Gaussian statistics typical of a flexible polymer in a -solvent. Furthermore, measurements of the flexibility as a function of temperature demonstrate that titin-II and the I-band titin fragment experience a similar denaturation process; unfolding begins at 318 K and proceeds in two stages: an initial gradual 50% change in persistence length is followed by a sharp unwinding transition at 338 K. Complementary microrheology (video particle tracking) measurements indicate that the viscoelasticity in dilute solution behaves according to the Flory/Fox model, providing a value of the radius of gyration for titin-II (63 +/- 1 nm) in agreement with static light scattering and small angle neutron scattering results.