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Controlling the mechanical behavior of novel supramolecular materials is of the utmost importance and requires a fundamental understanding of the underlying physical processes. We present a multimethods approach to the dynamics of entangled transient polyisoprene networks. Small-angle neutron scattering (SANS) on randomly functionalized chains shows homogeneous supramolecular melts with Gaussian chain conformations. The H-bond lifetimes (dielectric α*-process) and the rheological response in terms of the loss modulus G″ differ by 2 orders of magnitude in time. Within the concept of a compact random walk (RW), where the random walker (urazole group acting as a sticker) undergoes multiple returns to its starting point and following the concept of theoretical proposed renormalized sticky bond lifetimes, we quantitatively solve this longstanding and unexplained large discrepancy: While the bond opening gives rise to the dielectric response, for rheological relaxation the association with a new partner is rele...

In this study, the evolution of the scattering function of silica-filled styrene-butadiene copolymer rubbers (SBRs) under a continuous uniaxial strain cycle is investigated in situ with a complemen...

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.

We present a detailed small-angle neutron scattering investigation of mixtures of poly(ethylene oxide) (PEO)/polyhedral oligomeric silsesquioxane (POSS) hybrids, a popular nanofiller, addressing the conformation of the model-grafted nanoparticles as well as their influence on the linear polymer matrix in which they are embedded. The nanofiller has a well-defined graft arm length and is monomolecular. Because of the identical chemical nature of the grafted arm and matrix chains, it can be considered as an ideal athermal mixture. We have introduced a random phase approximation approach with general application possibilities for other grafted nanofillers, combining the star-like and particle-like characteristics of the POSS nanofiller. The size of the nanofiller remained unaffected and Gaussian upon mixing with linear chains of various lengths, and no aggregation of the nanofiller was observed. This turns the POSS-based nanocomposite into an interesting model compound. The mixtures of PEO chains with the oct...

We present results from very long (on the order of several microseconds) atomistic molecular dynamics (MD) simulations for the density, microscopic structure, conformation, and local and segmental dynamics of pure, strictly monodisperse ring and linear poly(ethylene oxide) (PEO) melts, ranging in molar mass from ∼5300 to ∼20 000 g/mol. The MD results are compared with recent experimental data for the chain center-of-mass self-diffusion coefficient and the normalized single-chain dynamic structure factor obtained from small-angle neutron scattering, neutron spin echo, and pulse-field gradient NMR, and remarkable qualitative and quantitative agreement is observed, despite certain subtle disagreements in important details regarding mainly internal ring motion (loop dynamics). A detailed normal-mode analysis allowed us to check the degree of consistency of ring PEO melt dynamics with the ring Rouse model and indicated a strong reduction of the normalized mode amplitudes for the smaller mode numbers (compared ...

We review some of our latest contributions in the field of neutron scattering of polymers with linear and non-linear architecture. On the one hand, the reviewed Small Angle Neutron Scattering investigations cover dilute mixtures of dendritic polymers and linear polymers that were investigated during relaxation after a fast step strain or in situ during a strain ramping. The focus was on the otherwise not accessible long time dynamics of the inner branches. The parameters of the model are those of the tube model and include the evolution of liberated chain ends, the size and time dependence of the confining tube diameter and branch-point related dynamics. The case of deformed tube diameters could be obtained from a permanently-linked network state as the limiting case of a deformed melt. On the other hand, recently, cyclic polymers as a special class of branched architectures showed to be of high significance to detect similar features directly from equilibrium measurements by means of quasi-elastic Neutron Spin Echo spectroscopy. We show that by means of polymer rings as a probe different aspects of the tube concept can be distinguished in a unique way.

We present a small angle neutron scattering (SANS) investigation of a blend composed of a dendritic polymer and a linear matrix with comparable viscosity in start-up of an elongational flow at T-g + 50. The two-generation dendritic polymer is diluted to 10% by weight in a matrix of a long well-entangled linear chains. Both components consist of mainly 1,4-cis-polyisoprene but differ in isotopic composition. The resulting scattering contrast is sufficiently high to permit time-resolved measurements of the system structure factor during the start-up phase and to follow the retraction processes involving the inner sections of the branched polymer in the nonlinear deformation response. The outer branches and the linear matrix, on the contrary, are in the linear deformation regime. The linear matrix dominates the rheological signature of the blend and the influence of the branched component can barely be detected. However, the neutron scattering intensity is predominantly that of the (branched) minority component so that its dynamics is clearly evident. In the present paper, we use the neutron scattering data to validate the branch point withdrawal process, which could not be unambiguously discerned from rheological measurements in this blend. The maximal tube stretch that the inner branches experience, before the relaxed outer arm material is incorporated into the tube is determined. The in situ scattering experiments demonstrate for the first time the leveling-off of the strain as the result of branch point withdrawal and chain retraction directly on the molecular level. We conclude that branch point motion in the mixture of architecturally complex polymers occurs earlier than would be expected in a purely branched system, presumably due to the different topological, environment that the linear matrix,presents to the hierarchically deep-buried tube sections.

A new dual network elastomer is developed that consists of polybutadiene crosslinked with both permanent and transient bonds. The transient network is formed by the association of urazole groups that are randomly attached to the polymer backbone. Subsequent orthogonal covalent crosslinking through hydrosilylation in the melt state leads to a dual network with variable permanent and transient crosslinking density. Small angle neutron scattering (SANS) investigations show a homogeneous distribution of the transient bonds in both the functionalized polymers and the dual network products. The enhanced mechanical properties of the dual networks compared to conventional polybutadiene elastomers are characterized by stress–strain measurements. The increased toughness can be explained by a protective mechanism where the weaker supramolecular bonds act as sacrificial bonds that dissipate energy.

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 a 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 ring 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 < Z < 20, η0,linear/η0,ring Z1.2±0.3, is weaker than the scaling prediction (η0,linear/η0,ring Z1.6±0.3) and the simulations (η0,linear/η0,ring Z2.0±0.3). Nevertheless, the present collection of state-of-the-art experimental data unambiguously demonstrates that rings exhibit a universal trend clearly departing from that of their linear counterparts, and hence it represents a major step toward resolving a 30-year-old problem.

In this work we present a combined analysis of small-angle neutron scattering, linear rheology and pulsed field gradient nuclear magnetic resonance spectroscopy experiments on the supramolecular association and chain structure of well-defined telechelically modified poly(ethylene glycol) (PEG) in the bulk. Oligomeric PEG was functionalized with directed heterocomplementary hydrogen-bonding end-groups, thymine (Thy) and diaminotriazine (DAT). The polarity of the backbone polymer is comparable to the end groups and avoids clustering of the groups basing on energetic arguments. Their linear association behavior in the ideal melt state was investigated on the microscopic/molecular level as a function of temperature. By means of a selective labeling scheme, which should ideally lead to the formation of alternating hydrogeneous-deuterated building block sequences if the hydrogen bonding reaction is exclusively heterocomplementary, we showed that the Thy–DAT association is dominant and a Thy–Thy homoassociation ...

We revisit the nonuniversal aspect of polymer dynamics by considering both new and existing data on the zero-shear viscosity and linear viscoelastic response of various polymers, each with a wide range of molecular weights. Analysis of the zero-shear viscosity data in terms of the packing length p, whose role in entanglements has been discussed previously by Fetters and co-workers, reveals a behavior that is irreconcilable with our current understanding based on the tube model. Specifically, we find that the transition regime between Rouse and pure reptation dynamics, currently understood as the regime where contour length fluctuations are active, systematically shrinks as the packing length of the polymer increases. Further, we find that the slope of the loss moduli in the high-frequency wing of the terminal peak of well-entangled systems also decreases from the common −0.25 to −0.125 with increasing p. This is contrary to the single expected value of −0.25 from tube models which include contour length f...

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.

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.

The structure and linear viscoelastic behavior of four different model star polymer melts were investigated experimentally. The star polymers were prepared via different synthetic routes based on atom transfer radical polymerization (ATRP). Stars with small elongated (linear backbone) cores exhibited slight differences in the asymmetry of the core, which however did not affect the rheological properties significantly. The relaxation behavior of these stars with an asymmetric core was well-described by available tube models. On the other hand, stars with large cross-linked cores exhibited a core–shell morphology and their stress relaxation was dominated by a power-law decay over about 8 decades, akin to gel-like soft systems. This behavior reflected their liquid-like ordering and small intercore distances, and bares analogueies to that of interpenetrating soft colloids and microgels.

Viscoelastic solids such as rubber exhibit a complex elastic modulus E(ω), which depends on the frequency ω of the applied stress or strain. The modulus E(ω) can often be determined in a wide frequency range by performing measurements in a limited frequency range for many different temperatures, and then shift the frequency segments horizontally along the frequency axis to obtain a continuous master curve. We show that one can use the spectral representation of E(ω) (or 1/E(ω)), which obeys causality, to determine the optimal shifting procedure, and to test the accuracy of the measured data and the calculated master curve.

We present a small angle neutron scattering (SANS) study of the association of heterocomplementary telechelic polypropylene glycol (PPG) polymers, bearing either diaminotriazine (DAT) or thymine (Thy) stickers as end-groups, both in the melt and in dilute solution. The SANS data are critically examined for the architecture and morphology as well as relative extent of linear assembly in the apolar solvent toluene. A random phase approximation (RPA) approach, adapted for a supramolecularly assembled multiblock copolymer is presented, which allows to extract the interaction parameters between the constituents and the medium. From the proposed approach, which describes very well heterocomplementary hydrogen-bonding telechelic polymers in both diluted toluene solution and in the melt, we conclude that linear association prevails.

The relaxation time spectrum in blends of architecturally different polymers with strongly disperse time scales has been investigated by their time-dependent small angle neutron scattering signal a...

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.

The rheological behavior of poly(ethylene-alt-propylene) (PEP), polydimethylsiloxane (PDMS) and polyisoprene (PI, two molecular weights: 70k and 200k) melts containing polyhedral oligomeric silsesquioxane (POSS) molecules was investigated by means of small angle scattering (SAXS and SANS) and oscillatory shear rheology. The dependence of the nanocomposite viscosity on the polymer–particle solubility and polymer molecular weight was studied. At high filler fractions all polymers exhibited hydrodynamic reinforcement of the plateau modulus quantified by a Guth Gold relation. Additionally, the PEP and PI70k systems showed a transition from liquid-like to solid-like rheological behavior. SAXS results enabled us to relate this behavior to the formation of a POSS network or POSS particle jamming, respectively. At low filler degrees, the zero shear viscosity of the nanocomposites was strongly dependent on the polymer solubility and entanglement number. We observed a viscosity decrease in the filled PDMS and PI200...

Using small-angle neutron scattering, the unperturbed chain dimensions of a series of poly(alkylene oxide)s (PAO’s) were studied as a function of side-chain length. The PAO’s were obtained using anionic ring-opening polymerization methods. The deuterated monomers were synthesized from commercially available precomponents. A systematic decrease of the chain dimensions with increasing length of the side chains was found. We also compare the PAO’s with the chemically very similar poly(olefin)s with respect to the characteristic ratio C∞, the chain dimensions, and the packing length as a function of the molecular weight per backbone bond. In doing so, we found significant differences between the static properties of the two systems.

By the use of small angle neutron scattering, the chain conformation in a polymer nanocomposite was studied as a function of the nanoparticle fraction for two different molecular weights. A repulsive system was realized in mixing poly(ethylene−propylene) and hydrophobically modified silica. All nanocomposite SANS data exhibit a pronounced scattering peak at intermediate momentum transfers, which is closely connected with the particle structure. Even under contrast matching conditions, the hydrophobic surface layer of the nanoparticles was found to contribute significantly to the scattering signal. In particular, in the short chain matrix the peak origin is exclusively related to direct particle scattering. In the long chain matrix, an additional peak contribution is present. Possible origins include void correlation scattering or polymer correlation scattering. We show unambiguously that the conformation of short chains with a molecular weight of 3000 g/mol is not visibly disturbed by the presence of the ...

We report a new “grafting to” technique for the functionalization of silica particles with anionically produced polymers. It is based on a two-step procedure. In the first step the silica nanoparticles were modified with multifunctional chlorosilanes. This procedure allows replacing the original Si−OH surface groups by Si−Cl groups. In the second step the anionically synthesized polymers were linked to the Si−Cl functionalized nanoparticle surface. Both the chlorosilane functionalization of the nanoparticles and the subsequent reaction with living polymer can be carried out without irreversible particle aggregation. This was proved by examining the reaction products with static and dynamic light scattering as well as small-angle X-ray scattering. The polymer linking event is accompanied by termination reactions, most likely due to residual Si−OH groups. Therefore, the raw products were purified by a simple fractionation procedure. The examination of the products by size exclusion chromatography showed tha...

Using neutron spin echo (NSE) spectroscopy and a labeling scheme unique to neutron scattering the motion of a labeled branch point of a three-arm polyethylene star has been observed on a molecular level. The measured dynamic structure factor shows a clear transition to a plateau, signifying the stronger confinement of the star center in comparison to a corresponding center labeled linear chain. A shortening of one star arm to about only one entanglement length leads to the same topological confinement as for the symmetric star within the accessible time range of NSE. This reflects a stronger effect of such a small branch than expected and is consistent with rheological measurements on the same system.

We investigate the time-dependent linear viscoclastic behavior of amine and alkaline metal-neutralized carboxy-telechelic 1,4-polybutadiene, and compare it with the acid precursor as well as an ester end-modified version. The ionic groups form aggregates with a aggregate spacing period around 85 A, containing about 90 chain-ends. Rheological tests highlight the existence of a strongly time-dependent terminal relaxation and an intermediate frequency relaxation, which can be assimilated to a glass transition of the ionic aggregates and their immediate environment. The terminal relaxation time after neutralization with alkali metal increases by up to 7 decades. Contrary to previously published results, this time is found to be a strongly increasing function of metal atomic mass for the equilibrated structures. The discrepancy can be rationalized by analyzing the annealing time dependence of the phenomena. Earlier studies did not report on truly equilibrated systems. While the main phase glass transition is hardly affected by neutralization, the temporary network formed by the ionic associations shows a very high and ion-dependent temperature sensitivity. Moreover, a failure of the time-temperature superposition principle is found at intermediate temperatures or frequencies, because the terminal and intermediate frequency relaxations do not follow the same temperature dependence.

Poly(bismethoxyphosphazene) (PBMP) was synthesized, giving polymer melts with Tg = −70.9 °C. The chain dynamics of melts and DMF solutions of poly(bismethoxyphosphazene) (PBMP) was investigated by Small-Angle Neutron Scattering (SANS). The radius of gyration Rg was found as 144.7 ± 0.2 A, and the average MW from SANS was 95000 Da. DMF revealed to be a good solvent for this polymer. From the scattering intensity of polymer solutions for high q-data, the slope was −1.647 in almost exact agreement with the expected excluded volume exponent by Flory which is 5/3 affirming the good solvent property of DMF. GPC measurements of THF solutions were evaluated based on a universal calibration of polystyrene standards resulting in a rather similar value of 1.05 × 105 Da. Another series of SANS experiments was done with solutions of LiSO3CF3 (LiTf) in the title polymer. They also showed low Tg values down to −50 °C at 15 wt %. The SANS results (5 and 10 mol % LiTf as referred to the monomer units) showed almost no eff...

In this work, an investigation of the hydrogen-bonding mechanism in a transiently branched comb-like polymer system in the melt is reported. The system under investigation consists of a polybutylene oxide (PBO)-based backbone, randomly functionalized with thymine (thy) groups, in combination with shorter PBO graft chains, end-functionalized with diaminotriazine (DAT) groups. The functional groups are able to associate through hydrogen bonding. The heterocomplementary association of these groups leads to the formation of a transiently branched comb-like polymer system. Since recently virtually exclusive heterocomplementary association could be observed in the supramolecular association of telechelically modified oligomeric PEG chains, here we aim to extend the supramolecular assembly mechanism toward branched structures. The present work combines small angle neutron scattering (SANS) experiments on a selectively labeled system with macroscopic dynamics measured in linear rheology response. The association of thy- and DAT-modified components was studied as a function of temperature and composition. The scattering function reveals the formation of a block copolymer and can be exclusively attributed to heterocomplementary association of the hydrogen-bonding groups. Scattering functions of nonfunctionalized blends are also reported as references and evidence the change in the microstructure induced by the heterocomplementary association. All scattering profiles were described by means of the random phase approximation (RPA) formalism from which the average aggregation number, i.e., comb arm functionality and the equilibrium association constant could be determined directly in the melt state as a function of temperature. On the other hand, rheological measurements were performed in the melt state to study the influence of the reversible bonds on the macroscopic dynamics of the polymer system. The rheology data are in good agreement with the SANS results and confirm the transient comb-like branched architecture. The supramolecular association exhibits characteristic bonding times of the groups in the order of 1 s at −25 °C and therefore makes the thy–DAT pair an ideal candidate for the development of responsive materials that combine permanent and transient linkages for novel applications and self-healing properties.

While it is traditionally accepted that the chain interactions responsible for the elastic response in an elastomeric network are ideally permanent and instantaneously active, the ongoing investigation of self-healing materials reveals that the introduction of self-healing properties into elastomers requires high mechanical integrity under dynamic load conditions, while on long timescales (or at extended temperatures), the chain and bond dynamics must allow for an intrinsic repair of micro cracks occurring during operation and aging. Based on an acrylate-based amorphous ionomer model system with pendant carboxylate groups allowing the systematic variation of the composition and the nature of the counter ion, we demonstrate the interrelation between the morphological, thermal, and mechanical properties, and identify the prerequisites and tools for property adjustment and optimization of self-healing efficiency. While the ion fraction is directly related to the effective network density and elastic performance, the crossover frequency between viscous and elastic behavior is influenced by the nature of the counter ion. In order to achieve reliable elastic response and optimal damage repair, the ion fraction in these systems should be in the range of 5 mol% and the chain dynamics should be appropriate to allow for excellent self-healing behavior at moderate healing temperatures.

A new model is presented which can describe quantitatively the small angle neutron scattering from montmorillonite-type clay dispersions and polymer-clay nanocomposites. The model is shown to be able to describe well the data from a series of dilute montmorillonite in water dispersions in which the deuterium content of the water phase is varied. The fits combined with information from other techniques suggest strongly that H–D exchange occurs in the montmorillonite-water dispersions. Deviations from Q−2 behaviour often observed experimentally for clay dispersions are convincingly explained by the presence of a small proportion of tactoids.

The dynamics of randomly charged polystyrene caesium–sulfonate ionomers in semi-dilute solutions were studied using a combination of dynamic light scattering (DLS), small angle neutron scattering (SANS), and bulk rheology. The samples were studied in toluene solutions where the aggregation of the dipolar groups is favoured. Evidence of aggregation in dilute solution is found using DLS and SANS with both the hydrodynamic and static radius of gyration indicating that there is a contraction of the chains due to intra-chain attractive forces. SANS experiments demonstrate the evolution of the aggregates into a network structure as a function of polymer concentration. The association process is caused by the dipolar attraction between the charged groups and introduces two static correlation lengths in the mesh structure of the network; the standard semi-dilute mesh size (ξ=1.12c−0.72±0.03) and an inhomogeneity length (Ξ=24c0.58±0.05) due to micro-phase separation. The scaling of the amplitudes of the correlation lengths I1(0)∼c−0.33±0.07 and I2(0)∼c2.0±0.4 are consistent with good solvent conditions and micro-phase separation, respectively. An imposed shear causes the break up of the micro-phase separated micellar system with a characteristic yield stress for the Bingham step-like shear thinning.

The dynamic viscoelastic properties of anionically polymerized model 1,4-polyisoprene (PI) with 6-8% 3,4 microstructure and in the range of molecular weights between 1.5 x 10(3) and 3.2 x 10(6) g/mol were reanalyzed using linear rheology. Viscosities obtained in the zero shear-rate limit, eta(o), up to sufficiently high molecular weight PI samples indicated a clear transition to the pure reptation regime. All characteristic molecular weights, dividing the [eta(o),M]-interval in three different regimes could be obtained in high accuracy and a new "triangle" is proposed. The experimental data were analyzed in terms of the empirical Winter-relaxation BSW-model description for both dynamic moduli. The BSW parameters are determined and interpreted in terms of the packing model where possible, Limitations of the packing model are detected and discussed. Additionally, the parameters of the assumed distribution of relaxation times are compared to predictions of microscopic dynamic theories and useful correlations made. The publication deals, in part, with a misconception in the literature for estimates of the constraint release contribution. The monomeric friction coefficient xi(o), determined in this study for the first time rheologically for anionic PI itself agrees with the natural rubber analogue.

We present scattering data from a melt of H-shaped polybutadiene polymers, in which the central crossbar of the polymers is wholly labeled with deuterium. The melt was uniaxially stretched by a factor of 2, and using careful temperature control of the sample, the neutron scattering pattern was measured after several different times of relaxation. The scattering data are compared against the predictions of a new theory, based upon the tube model but which includes the additional effect of elastic inhomogeneities in the entanglement network. We find that with this new theory one can obtain a good representation of the data, using parameters that are consistent with relaxation times obtained from a tube model description of the linear rheological data. The most important aspect of these data is an increasing anisotropy of the correlation hole peak, and this appears to be directly related to the elastic inhomogeneities. One further aspect of the comparison to theory is that the best fits are obtained using a Warner-Edwards tube diameter that deforms in a manner consistent with recent predictions for polymer networks.

The reinforcing phenomenon by blending soft elastomeric materials with rigid colloidal filler particles is, despite of its technological relevance, still poorly understood on the microscopic level although several macroscopic approaches have dealt with some of the mechanisms successfully. In order to contribute to bridge this gap, a study of a composite made up of a commercially silica-based hard phase and of a PI polymer matrix has been performed. The undeformed ‘reference’ state for such studies has been investigated by means of the small angle scattering technique on partially labeled samples. The single chain behaviour was isolated from neutron scattering although the condition of composition matching was not met, whereas the precipitated fractal filler structure was characterized by synchrotron X-ray scattering. The effective 3-component system has been successfully described through a combined approach of neutron and X-ray data and relationships dealing with 4-component structures have been presented. This work deals for the first time directly with the soft matrix in a reinforced 2-phase composite and is the basis for the treatment of anisotropic data in terms of both filler and chain dependence on uniaxial strain.

Structural studies of filled polymer networks under periodic deformation are of great importance to the industries concerned with rubber. The degree of reinforcement provided by the filler depends on a number of variables: the size of the polymer-filler interface, the size of primary particles, the size and shape of filler clusters. In the tyre industry it is necessary to optimize mechanical properties such as low rolling resistance, good wet grip and long lifetime. There have been studies of unfilled polymer networks and filler model-systems (block copolymers), [1,2] but because their complexity, industrial rubber samples have escaped investigations. We performed SAXS, USAXS and SANS measurements on carbon black filled natural rubber (poly-isoprene) under uniaxial deformation at ESRF ID-02 and Forschungszentrum Julich KWS-2. The scattering patterns of the studied filler particles show power-law decay over many decades in scattering vector q. At the scale of primary particles we detected surface fractal behavior and for the characteristic size of the primary particles we calculated lengths of the order of 10 nm. X-ray scattering was used for time resolved experiments (about 10 frames per second) and neutron scattering was used to distinguish between scattering from the studied fillers and the additives (especially zinc oxide) used in industrial materials.

This work is the first part of a study devoted to the understanding and the determination of the molecular mechanisms that are at the origin of the specific properties shown by reinforced elastomers. Different model filled elastomers composed of cross-linked polyethylacrylate chains reinforced with grafted silica nanoparticles were prepared varying the reactivity of the coupling agent with the ethylacrylate monomers. They were synthetized applying and adapting the method developed by Ford et al. [11] which consists to polymerize a colloidal suspension of grafted silica particles in acrylate monomers. In this paper we will present how filled elastomers having different dispersion states can be prepared whilst keeping the same interactions between the particles and the polymer chains. The dispersion states were characterized by Small Angle Neutron Scattering. We found that there are two opposite effects which control the final dispersion state of these filled elastomers during the polymerization. The first one is a depletion mechanism favoring the formation of aggregates. The second one is a repulsive steric interaction due to the growth of polymer chains from the particle surfaces avoiding contacts between the silica inclusions. Using these results we can prepare sets of samples having the same particle/matrix interface but different dispersions states. By comparing their mechanical properties we should to able to estimate the relative weight of the dispersion state quality and the one of the particle/matrix interface on the mechanical behavior of these filled elastomers.

A micelle forming polystyrene (PS)-polybutadiene (PB) block copolymer was mixed with a short (relative to the PB block of the copolymer) PB homopolymer to investigate the structural properties of the micelles by small-angle neutron scattering. Samples with varying block copolymer concentration were measured at 90 °C in both core and shell contrast. The obtained intensities were analyzed using a core/shell model with a constant core density and a variable density profile for the corona. A reasonable analysis of the data was only possible by assuming that in the measured concentration range the unimer concentration is not constant but increases with increasing polymer concentration, while the aggregation number of the micelles stays constant. The data analysis further revealed that the corona of micelles shows almost constant density which stays in contrast to micelles of block copolymers with comparable block length ratio in low molecular solvents. These could be interpreted in terms of the starlike model of Halperin, where the outer shell has a fast decaying density profile.

A small-angle neutron scattering (SANS) investigation of matrix chain deformation in an especially designed composite model system for a filled rubber−elastic network is presented. SANS experiments on the structure of polyisoprene homopolymer chains introduced into a well-microphase-separated triblock polyisoprene−polystyrene−polyisoprene (PI−PS−PI) system of the ABA type are reported. Use is made of the phase- and composition-matching techniques. The data are described in terms of the scattering behavior of the corresponding pure polyisoprene network at larger microscopic than macroscopic strains. The results confirm the model for the strain enhancement in reinforced networks and give the first direct microscopic insights into the mechanisms of reinforcement. A complementary small-angle X-ray scattering (SAXS) study yields all relevant geometrical parameters of the model filler and its detailed behavior under strain.

A theoretical and experimental investigation of the SANS structure factor of labeled triblock HDH copolymer chains, randomly cross-linked into a network, is presented. We combine the RPA method for the consideration of the strong interchain correlations and the tube model with a harmonic deformation dependent constraining potential, modeling the action of cross-links and entanglements. With this approach a successful description of the scattering behavior under strain is achieved. The sensitivity of the method allows the detection of chain splittings during cross-linking with peroxides.

The form factor of a labeled chain randomly cross-linked into a network in a tubelike approach is presented. Within a harmonic approximation for the constraining potential, the action of cross-links and entanglements is modeled by an effective tube, which is deformation dependent. The only parameters of the model are the reduced microscopic deformation, following a power law in the strain, and the diameter of the tube, which is inversely proportional to the strength of the constraints. First experimental results on polybutadiene networks with varying cross-link density are discussed. The conclusions drawn confirm a less than affine deformation at the molecular level while segmental fluctuations are found to be unperturbed despite the network topology and of the same size as obtained from rheology

10 páginas, 11 figuras, 6 tablas.-- et al., A series of poly(alkylene oxide)s (PAO’s) with different lengths of the side chains and two different molecular weights were studied by means of differential scanning calorimetry, dielectric spectroscopy (BDS), and rheology. The glass transition temperatures are almost independent of molar mass and the length of the side chains, which is also reflected by the dielectric α-relaxation, which depends only slightly on the different samples. However, the corresponding maxima in the dielectric loss of the samples with longer side chains show a shoulder at high frequencies with a temperature-dependent shape. The shape of the normal mode changes slightly with the molar mass and the length of the side chain. The normal mode relaxation times show a molar-mass-dependent behavior indicating a significant increase of the entanglement molecular weight Me with increasing length of the side group. A Rouse mode analysis incorporating the molar mass distribution was performed but could not reproduce the experimental data of the normal mode process completely. The relaxation times obtained by rheology agree well with the dielectric results, and estimates of Me based on the number of entanglements and on the packing model confirm the values suggested by BDS. A comparison of the estimated chain dimensions with literature data supports the reliability of our calculations., The present work has been carried out in the frame of the Joint Programme of Activities of the SoftComp Network of Excellence (Contract NMP3-VT-2004-502235) granted under the FP6 by the European Commission. C.G. acknowledges financial support by the International Helmholtz Research School of Biophysics and SoftMatter (IHRS BioSoft). A.A. and J.C. acknowledge the financial support provided by the Donostia International Physics Center (DIPC), the Basque Country Government (Ref. IT-436-07, Depto. Educación, Universidades e Investigación), and the Spanish Ministry of Science and Innovation (Grant MAT 2007-63681).

The experimental small angle neutron scattering results were compared with a RIS calculation using the statistical bond parameters of Mark. Experimental and theoretical results show good agreement as far as low and intermediate temperatures are concerned. The temperature coefficient for chain dimensions of (-1.16±0.03)x10 -3 K -1 extracted from the data in the range of 25-180°c is in good agreement with the value of (-1±0.2)x10 -3 K -1 obtained from measurements in θ solutions over a smaller temperature range.

Aqueous dispersions of single-walled carbon nanotubes (SWCNT) with polymers were characterized by small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM). Two types of polymers were used: an alternating copolymer of styrene and sodium maleate (PSSty) and gum arabic (GA)-a highly branched polysaccharide. The resulting dispersions were stable and had a homogeneous dark inklike appearance. The dispersions consisted mainly of long isolated nanotubes and thin bundles, as revealed by cryo-TEM. The scattering patterns of both dispersions were fit by a modification of Pedersen's "block copolymer micelle" model, depicting the nanotubes-polymer complex as a thick cylinder hybrid. This hybrid is made of a thin core with radius of about 20 angstrom containing similar to 4 nanotubes surrounded by a thick corona of swollen polymer coils with a radius of gyration of 150-170 angstrom. Long-term stabilization is achieved by the steric barrier provided by the adsorbed polymer coils reinforced by electrostatic repulsion due to charged groups distributed along the polymers.

The structure of aqueous solutions of sodium maleate copolymers with comonomers of variable hydrophobicity are examined using a combination of small-angle neutron scattering, small-angle X-ray scattering, and rheometry. Semidilute solutions of the copolymers made with mildly hydrophobic comonomers exhibit scattering and rheology that are typical of flexible polyelectrolytes, with a correlation length scaling with polymer concentration as c(-alpha) with alpha = 1/2 and specific viscosity scaling as c(1/2)/c(3/2) when unentangled/entangled. In contrast, copolymers with more than eight carbons in their hydrocarbon comonomer form micelles (globules) at low concentrations and molecular weights. For these more hydrophobic copolymers, the correlation length scales as c(-alpha) with alpha < 1/3 and the viscosity remains small until a relatively high concentration at which the globules start to overlap. Once these more hydrophobic copolymers overlap, the micelles open up and allow intermolecular associations that cause the viscosity to increase rapidly with concentration, typical of associating polymer solutions.

The influence of a flat interface on the isotropic to nematic (I/N) phase transition was investigated for aqueous solutions of cylindrical micelles of polybutadiene-poly(ethylene oxide) block copolymers using specular neutron reflectivity. While the I/N transition in the bulk occurs at the solute volume fraction phi(N/I), we observed the formation of a condensed layer close to a silicon single-crystal interface at volume fractions of about phi(i) approximate to 0.85phi(I/N). The thickness of this layer does not vary if the solute fraction is changed, while its density increases drastically within a very narrow range of volume fractions. This observation is interpreted as the formation of a nematically ordered layer which is induced by the interface.

We present a small-angle neutron scattering (SANS) investigation of the relaxation process of branched polymer melts under deformation. The selected model polymer is an H-shaped polyisoprene having deuterium-labeled blocks at the dangling tips of the arms. The melt is step strained in a strain rig especially built for that purpose, where the sample temperature and its deformation conditions are precisely controlled. Uniaxial extensions to values of 2 and 3 are performed at a temperature above the polymer glass transition temperature T-g. The sample is then allowed to relax during a given time before being quenched to below its T-g in order to freeze the chains conformation to perform the SANS experiments. Very large anisotropies in S(q) develop on time scales corresponding to the relaxation of the dangling arms. The experimental structure function is then compared to theoretical ones obtained by the random phase approximation applied to the tube model. While theory describes well the scattering for short times, it seems not to be able to describe the increase of intensity in the direction parallel to the strain at higher times, even when highly nonaffine processes such as arm retraction and branch point withdrawal are taken into account.

Cis—Polybutadiene networks (98% cis), crosslinked in the bulk state by gamma irradiation were characterized in terms of their mechanical properties. Data from stress-strain analyses, swelling degrees, stress relaxation and random—linking theory are in accordance and the estimated gel points practically coincide. Trapped entanglements contributed up to 30% to the network modulus, from which also the plateau modulus was determined. The tensile data were obtained from Mooney—Rivlin representations. It was concluded that the assumption of additive contributions from chemical and entanglement crosslink densities in the Langley theory works well in the range of doses investigated here. A comparison with a tube—like approach of these networks is made.

A novel strategy to synthesize amphiphilic surface-compartmentalized nanoparticles based on linear ABC triblock copolymers is presented. These so-called Janus micelles consist of a cross-linked core and a corona with a "northern" and a "southern" hemisphere. Selectively cross-linking spherical domains of the polybutadiene middle block in a well-ordered bulk morphology of a polystyrene-block-polybutadiene-block-poly( methyl methacrylate) triblock copolymer (SBM) leads to the conservation of the compartmentalization of the outer blocks after dissolution of the material. Multi-angle laser light scattering gel permeation chromatography, fluorescence correlation spectroscopy, small-angle neutron scattering, and static and dynamic light scattering, as well as scanning force microscopy, indicate the existence of an equilibrium between molecularly dissolved Janus micelles (unimers) and aggregates (multimers), so-called supermicelles.

We present a SANS study on the length scale dependence of chain deformation patterns in dense cross-linked elastomers. Three different polyisoprene networks of long primary block-copolymer chains of the HDH-type were analyzed. The total length of the primary chains is identical, while the length of the deuterated middle block was varied in order to cover several length scale regimes of interest. The scattering data are analyzed in the frame of the tube model of rubber elasticity in combination with the random phase approximation (RPA), which is used in order to account for the interchain correlations. We show experimentally that for the longest labeled path the rubber elastic response is dominated by both cross-link and entanglement contributions, whereas it is of clear phantomlike chain behavior if the labeled middle block of the chain is shorter than the distance between two successive entanglements, i.e., the tube diameter. We further propose schemes to quantify nonaffine deformations on various length scales and chain scissioning processes during the radical cross-linking process.

The copolymerization and structural properties of the ternary o/w-microemulsions formed from water, styrene and (11-(acryloyloxy)undecyl)trimethylammonium bromide or (2-(methacryloyloxy)ethyl)dodec...