Jürgen Allgaier

Effect of amphiphilic block copolymers on the structure and phase behavior of oil–water-surfactant mixtures

The effect of amphiphilic diblock copolymers of several molecular weights on the structure and phase behavior of ternary amphiphilic systems (water, oil, and nonionic surfactant) is investigated. Small amounts of amphiphilic block copolymer polyethyleneoxide–polyethylpropylene lead to a dramatic decrease of the amount of total surfactant needed to solubilize given equal volumes of water and oil in a bicontinuous microemulsion. Neutron scattering experiments employing a high-precision two-dimensional contrast variation technique demonstrate that the polymer is distributed uniformly on the surfactant membrane. Based on these observations, we propose a mechanism for the enhancement of swelling behavior, which is due to the variation of the membrane curvature elasticity by polymer mushrooms anchored to the interface.

Structure and dynamics of polymer rings by neutron scattering: breakdown of the Rouse model

We present a static and quasi-elastic neutron scattering study on both the structure and dynamics of a ring polymer in a ring and linear polymer melt, respectively. In the first case, the ring structure proved to be significantly more compact compared to the linear chain with the same molecular weight. In the mixture, the ring molecules swell as was confirmed by small angle neutron scattering (SANS) in accordance with both theory and simulation work. The dynamical behavior of both systems, which for the first time has been explored by neutron spin echo spectroscopy (NSE), shows a surprisingly fast center of mass diffusion as compared to the linear polymer. These results agree qualitatively with the presented atomistic MD simulations. The fast diffusion turned out to be an explicit violation of the Rouse model.

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.

Phase separation in star-polymer–colloid mixtures

We examine the demixing transition in star-polymer-colloid mixtures for star arm numbers f=2,6,16,32 and different star-polymer-colloid size ratios 0.18< or =q< or =0.50. Theoretically, we solve the thermodynamically self-consistent Rogers-Young integral equations for binary mixtures using three effective pair potentials obtained from direct molecular computer simulations. The numerical results show a spinodal instability. The demixing binodals are approximately calculated and found to be consistent with experimental observations.

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.

Dynamic properties of microemulsions modified with homopolymers and diblock copolymers: The determination of bending moduli and renormalization effects

The properties of bicontinuous microemulsions, consisting of water, oil, and a surfactant, can be modified by the addition of diblock copolymers (boosting effect) and homopolymers (inverse boosting effect) or a combination of both. Here, the influence of the addition of homopolymers (PEP(X) and PEO(X), X=5k or 10k molecular weight) on the dynamics of the surfactant layer is studied with neutron spin echo spectroscopy (NSE). Combining the results with the previous findings for diblock copolymers allows for a better separation of viscosity and bending modulus effects. With the addition of homopolymers, a significant increase of the relaxation rate compared to the pure microemulsion has been observed. The influence on the bending rigidity kappa is measured with NSE experiments. Homopolymer addition reduces kappa by up to Deltakappa approximately -0.5k(B)T, whereas the diblock copolymer yields an increase of kappa by approximately 0.3k(B)T. Comparison of the bending moduli that are obtained by analysis of the dynamics to those obtained from small angle neutron scattering (SANS) sheds light on the different renormalization length scales for NSE and SANS. Variation of the surfactant concentration at otherwise constant conditions of homopolymer or diblock-copolymer concentration shows that NSE results are leading to the pure bending rigidity, while the renormalized one is measured with SANS.

Neither Gaussian chains nor hard spheres - star polymers seen as ultrasoft colloids

In dense solution high functionality star polymers show ordering phenomena which give rise to a well-pronounced peak in the static structure factor, S exp(Q), observed by small-angle neutron scattering. The concentration dependence of S exp (Q) gives evidence for unusual phase behaviour as predicted by theory. In addition, the dynamics of the star polymer solutions is dominated by an increasing amount of structural arrest with increasing concentration. The mean square displacement obtained from neutron spin-echo spectroscopy is compared to the blob size obtained from dynamic light scattering. Thermal energy enables each star core to perform restricted motion over a spatial extent equal to the blob size of the surrounding dense star polymer solution.

A microscopic view on the large scale chain dynamics in nanocomposites with attractive interactions

We use neutron spin-echo spectroscopy to investigate the large scale chain dynamics in unentangled polymer nanocomposites where stable polymer layers around nanoparticles are dynamically formed due to attractive segment–surface interactions. The work here focuses on the detailed microscopic characterization of the dynamics within these layers of bound poly(ethylene glycol) (PEO) and poly(butylene oxide) (PBO) chains at a fixed silica nanoparticle fraction of 15%. The substitution of hydroxy by methoxy terminated chains thereby clearly evidences the importance of the chain end chemistry in these systems as the layer structure and dynamics therein significantly depend on the specific interaction mechanism. The experimental data reveal a densely packed thick shell of end-attached chains in the case of hydroxy ends contrasted by a thin shell of laterally adsorbed chains with multiple attachments in the methoxy case. In all cases a consistent quantitative modeling is presented that evidences unchanged segmental dynamics within the bound layers. The obtained picture is further validated on an independent model system based on PBO polymers which shows surprisingly similar chain dynamics as for PEG in the nanocomposite pointing to a very generic dynamic scenario.

Hydrophilic Alcohol Ethoxylates as Efficiency Boosters for Microemulsions

Highly amphiphilic polyalkane-PEO diblock copolymers drastically increase the solubilization capacity of surfactants in microemulsions if they are used in small quantities as additive to the surfactant. This effect goes along with an additional reduction of the already very low interfacial tension between water and oil. Lamellar phases, which usually develop when the surfactant becomes more efficient, are suppressed to a large extent. In this work we use another type of additive, namely hydrophilic alcohol ethoxylates. These amphiphiles are identical with the previously used block copolymers with respect to the hydrophilic moiety. However, they contain only small hydrocarbon groups ranging from C8 to C18. A typical example from the hydrophilic alcohol ethoxylates is C12E100. Both additive types increase surfactant efficiency equally with respect to mass fraction in the mixture. Because the alcohol ethoxylate additives decorate the surfactant film only on the aqueous side, they influence the curvature of the surfactant membrane or, in other words, the temperature behavior of the microemulsion. Together with nonionic surfactants, however, the shift of the one-phase region to higher temperatures is only a few degrees Celsius. Just as with the polyalkane-PEO block copolymers, the hydrophilic alcohol ethoxylates suppress lamellar phases. This behavior is especially pronounced if the hydrophobic groups are small or the PEO chains are long. We found that hydrophobic units as short as C 8 are sufficient to largely anchor the PEO chains at the interface. If C12 or C18 hydrocarbon unit are used instead, the PEO chains are fully interfacially active, even if the hydrophilic chain contains up to about 500 EO units. We applied the new additives in bicontinuous and in droplet microemulsions and used nonionic, as well as ionic, surfactants, namely C10E4 and AOT. In contrast to polyalkane-PEO blockcopolymers the new additives are easy to synthesize and are commercially available. Therefore, they might be interesting in applications.

Real time SANS study on head group self-assembly for lithium based anionic polymerizations

Abstract Small angle neutron scattering was used to in situ study the aggregated structures formed in the course of the polymerization of butadiene and isoprene in deuterated n-heptane. The samples were designed to have equal degrees of polymerization. These measurements showed, at low Q, that the start of the butadiene propagation event was accompanied by the presence of highly extended large-scale structures. As propagation progressed these initial structures diminished in size and were replaced, at least in part, by star-like aggregates. At the cessation of the polymerization reaction the star micelles, mid-Q regime, exhibited a mean aggregation state of 8.4. At lower conversions (and thus lower chain molecular weights) the presence of large three-dimensional aggregates was indicated. Conversely, the isoprene system in its initial moments of propagation did not show the same extent of large-scale structures although the low Q data did indicate the formation of architectures larger than the star-like aggregates. The star shaped micelles exhibited the mean degree of aggregation of 4. These results demonstrate that the association behavior of these polar dienyllithium headgroups is more varied than permitted by the current ‘textbook’ mechanism where the solitary permissible aggregation state is four. These findings concur with those suggested from a recent semi-empirical and ab initio quantum chemistry based series of calculations.