Lutz Willner - Academia.edu (original) (raw)

Papers by Lutz Willner

Langmuir

We investigated the influence of an n-alkyl-PEO polymer on the structure and dynamics of phosphol... more We investigated the influence of an n-alkyl-PEO polymer on the structure and dynamics of phospholipid vesicles. Multilayer formation and about a 9% increase in the size in vesicles were observed by cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), and small-angle neutron/X-ray scattering (SANS/SAXS). The results indicate a change in the lamellar structure of the vesicles by a partial disruption caused by polymer chains, which seems to correlate with about a 30% reduction in bending rigidity per unit bilayer, as revealed by neutron spin echo (NSE) spectroscopy. Also, a strong change in lipid tail relaxation was observed. Our results point to opportunities using synthetic polymers to control the structure and dynamics of membranes, with possible applications in technical materials and also in drug and nutraceutical delivery.

Langmuir

Mixtures of amphiphilic polymers and surfactants are used in a wide range of applications, e.g., ... more Mixtures of amphiphilic polymers and surfactants are used in a wide range of applications, e.g., pharmaceuticals, detergents, cosmetics, and drug delivery systems. Still, many questions remain on how the structure and, in particular, the kinetics of block copolymer micelles are affected in the presence of surfactants and what controls the solubilization kinetics. In this work, we have studied the stability and solubilization kinetics of block copolymer micelles upon the addition of the surfactant sodium dodecyl sulfate (SDS) using small-angle X-ray/neutron scattering. The ability of the surfactant to dissolve polymer micelles or form mixed micelles has been investigated using two types of amphiphilic polymers, poly(ethylene-alt-propylene)−poly-(ethylene oxide) (PEP1-PEO20) and n-alkyl-functionalized PEO (C 28-PEO5). The exchange kinetics of C 28-PEO5 micelles are in the order of hours, while PEP1-PEO20 micelles are known to be frozen on a practical timescale. In this work, we show that the addition of SDS to PEP1-PEO20 provides virtually no solubilization, even after an extended period of time. However, upon adding SDS to C 28-PEO5 micelles, we observe micellar dissolution and formation of mixed micelles occurring on the timescale of hours. Using a coexistence model of mixed and neat micelles, the SAXS data were analyzed to provide detailed structural parameters over time. First, we observe a fast fragmentation/fission step followed by a slow reorganization process. The latter process is essentially independent of concentration at low volume fraction but is greatly accelerated at larger concentrations. This might indicate a crossover from a predominance of molecular exchange to fusion/fission processes.

Physical Review Letters

Telechelic polymers contain two chain ends that are able to promote self-assembly into "flowerlik... more Telechelic polymers contain two chain ends that are able to promote self-assembly into "flowerlike" or interconnected micellar structures. Here, we investigate the molecular exchange kinetics of such micelles using time-resolved small-angle neutron scattering. We show that the activation energies of monofunctional and telechelic chain exchange are identical. This demonstrates that the two chain ends are not simultaneously released in a single event. Instead, the results show that, contrary to regular micelles, the kinetics occurs in a multistep process involving a collision-induced single-molecule exchange mechanism where the exchange rate is directly proportional to the polymer concentration. We show that this novel mechanism can be quantitatively explained by a simple kinetic model.

Physical Review Letters

Molecular exchange processes are important equilibration and transport mechanisms in both synthet... more Molecular exchange processes are important equilibration and transport mechanisms in both synthetic and biological self-assembled systems such as micelles, vesicles, and membranes. Still, these processes are not entirely understood, in particular the effect of crystallinity and the interplay between cooperative melting processes and chain exchange. Here we focus on a set of simple polymer micelles formed by binary mixtures of poly(ethylene oxide)-mono-n-alkyl-ethers (C n-PEO5) which allows the melting point to be tuned over a wide range. We show that the melting transition is cooperative in the confined 4-5 nm micellar core, whereas the exchange process is widely decoupled and unimeric in nature. As confirmed by differential scanning calorimetry, the total activation energy for ejecting a molecule out of the micellar core below the melting point is the sum of the enthalpy of fusion and the corresponding activation energy in the melt state. This suggests that a "local, single-chain melting process" preludes the molecular diffusion out of the micelle during chain exchange.

The Journal of Chemical Physics

A recent model for unentangled polymer chains in confinement [M. Dolgushev and M. Krutyeva, Macro... more A recent model for unentangled polymer chains in confinement [M. Dolgushev and M. Krutyeva, Macromol. Theory Simul. 21, 565 (2012)] is scrutinized by small-angle neutron scattering (SANS) with respect to its static prediction, the single-chain structure factor. We find a remarkable agreement although the model simplifies the effect of the confinement to a harmonic potential. The effective confinement size from fits of SANS data with the model agrees well with the actual pore size. Starting from this result we discuss the possibility of an experiment on the dynamic structure factor predicted by the model. It turns out that such an experiment would need a large ratio polymer dimension/pore size which is difficult but not impossible to achieve.

Europhysics Letters, 1992

In this paper we present the experimental evidence of incipient ordering of star polymers in solu... more In this paper we present the experimental evidence of incipient ordering of star polymers in solution near the overlap concentration ϕ* as predicted by scaling laws. We have studied 8- and 18-arm polyisoprene stars in the dilute and semi-dilute regime. A peak in the interparticle structure factor S(Q) due to a liquidlike ordering of spheres was found by small-angle neutron

Macromolecules, Jun 1, 1998

In order to elucidate the structure of intermediate species present during the early stages of “l... more In order to elucidate the structure of intermediate species present during the early stages of “living” anionic polymerization, the aggregation behavior of styryllithium head groups [RCH2CH-(C6H5) Li+] in benzene was investigated by small angle neutron scattering ( ...

Macromolecules, 2009

ABSTRACT This work reports a systematic study of the aggregation behavior and structural properti... more ABSTRACT This work reports a systematic study of the aggregation behavior and structural properties of micelles formed by a selectively deuterated h-polystyrene-d-polybutadiene (h-PS10-d-PB10, where 10 denotes the approximate block molecular weight in kilograms per mole) block copolymer in a series of n-alkanes, CnH2n+2, where n = 7, 10, 12, 14, 16. By applying small-angle neutron scattering (SANS) in combination with contrast variation, very detailed aspects of the micellar structure are resolved. The results show that the micelles are rather poorly segregated with a large quantity of solvent (approximately 35−55%) penetrating the core and a smaller compact corona. Interestingly, the solvent fraction in the core decreases, and the overall aggregation number increases with the size of the solvent molecules. At first sight, this is not intuitively expected because the Flory−Huggins interaction parameter, χ, or the interfacial tension, γ, stay constant or are even slightly decreasing, thus suggesting that the enthalpic interactions between the core-forming PS segments and the n-alkanes remain unchanged or decrease. However, this behavior can be understood semiquantitatively by applying a modified mean field theory where the solvent entropy effects in both core and corona are properly taken into account. This work thus directly demonstrates that contributions of the solvent entropy to the overall free energy are important whenever the interfacial tension is low.

The phase diagram, the Flory-Huggins parameter Γ, and the Ginzburg parameter Gi of a polybutadien... more The phase diagram, the Flory-Huggins parameter Γ, and the Ginzburg parameter Gi of a polybutadiene/polystyrene blend and diblock copolymer were determined by small-angle neutron scattering in the pressure range of 0.1 and 200MPa. Both systems show qualitatively the same pressure dependence. The enthalpic and entropic terms of Γ are larger in blends. In the diblock copolymer a constant order-disorder temperature was observed which could be explained by a compensation effect between Γ and Gi.

The Journal of Physical Chemistry B, 2008

Thermal copolymer fluctuations were explored in a three-component blend consisting of a critical ... more Thermal copolymer fluctuations were explored in a three-component blend consisting of a critical (A/B) homopolymer blend and a symmetric A-B diblock copolymer using the technique of neutron small angle scattering. The copolymer has the function of an external nonordering field and thereby determines phase behavior as well as the regimes of 3d-Ising, isotropic Lifshitz, and Brasovskiî critical universality. It was found that the random phase approximation (RPA) does not correctly describe the copolymer structure function because of strong thermal fluctuations. On the other hand a weak coupling of copolymer and homopolymer was confirmed, consistent with predictions from RPA. Self-assembly of the copolymers was observed prior to the ordering of the "total" blend, e.g. inclusive of the homopolymers, into bicontinuous and lamellar ordered phases.

Journal of Applied Crystallography, 2015

Following demand from the user community regarding the possibility of improving the experimental ... more Following demand from the user community regarding the possibility of improving the experimental resolution, the dedicated high-intensity/extendedQ-range SANS diffractometer KWS-2 of the Jülich Centre for Neutron Science at the Heinz Maier-Leibnitz Center in Garching was equipped with a double-disc chopper with a variable opening slit window and time-of-flight (TOF) data acquisition option. The chopper used in concert with a dedicated high-intensity velocity selector enables the tuning at will of the wavelength resolution Δλ/λ within a broad range, from 20% (standard) down to 2%, in a convenient and safe manner following pre-planned or spontaneous decisions during the experiment. The new working mode is described in detail, and its efficiency is demonstrated on several standard samples with known properties and on a completely new crystallizable copolymer system, which were investigated using both the conventional (static) and TOF modes.

Applied Physics a Materials Science Processing, Nov 30, 2002

ABSTRACT With small-angle neutron scattering (SANS) we measured thermal composition fluctuations ... more ABSTRACT With small-angle neutron scattering (SANS) we measured thermal composition fluctuations in a homopolymer blend, PB/PS, of critical composition mixed with different amounts of the symmetric diblockcopolymer PB-PS (PB and PS being polybutadiene (1,4) and polystyrene, respectively) in the disordered state along the isopleths. The diblockcopolymer acts as a surfactant enhancing the compatibility of both homopolymers up to the Lifshitz line and simultaneously changes the critical university class from 3D-Ising to isotropic Lifshitz critical behavior. Above &#75&#440.3 diblock content the samples show diblock-like behavior with a first-order ordering transition. The intermediate regime between &#75&#440.075 and 0.3 can be characterized by a micro-emulsion phase. The temperature dependence of the Lifshitz line, the disorder line and the critical line is non-monotonous.

Advances in Polymer Science, 2013

ABSTRACT This article reviews recent progress in studying the kinetics of block copolymer micella... more ABSTRACT This article reviews recent progress in studying the kinetics of block copolymer micellar systems by time-resolved small angle scattering techniques. The review includes an overview of the theoretical background concerning block copolymer micellar structure and kinetics, with a clear distinction between equilibrium and non-equilibrium processes. Basic principles of both static and time-resolved small-angle X-ray and neutron scattering (TR-SAXS and TR-SANS) techniques are summarized, with a special emphasis on the characterization of block copolymer micellar systems. In particular, the principle of SANS in combination with hydrogen/deuterium (H/D) contrast variation for the determination of chain exchange under equilibrium conditions is highlighted. In the experimental part, we first review results on equilibrium kinetics obtained within the last decade by the TR-SANS/H/D labeling technique. In general, the experimental results strongly indicate that the component exchange between different micelles proceeds via the exchange of single unimers. In agreement with the theoretical prediction, chain expulsion is the rate-determining step. The corresponding activation energy is mainly governed by the interfacial tension and the length of the insoluble block, which determine the exchange rate with a double exponential dependence. Thus, due to this extremely strong dependence, even synthetic polymers with modest chain length distribution show a logarithmic time dependence instead of the theoretically expected single exponential decay. In the second part, the kinetic results obtained under non-equilibrium conditions, i.e., relaxation processes obtained after perturbations from equilibrium, are reviewed. This part covers formation kinetics as well as reorganization and morphological transition kinetics. We present, as a special highlight, TR-SAXS measurements with millisecond resolution on the formation of star-like micelles after stopped-flow mixing of molecularly dissolved block copolymers with a selective solvent. The micellization process could be modelled as a nucleation & growth process with unimer exchange as the elemental mechanism. The resulting scenario could be described as a three step process that includes a fast nucleation event, a region of micellar growth, and a final equilibration to thermodynamically stable micelles. In summary, this review demonstrates the importance of small angle scattering techniques for studying fundamental aspects of kinetics in block copolymer micelles and in soft matter materials in general.

Physical Review Letters, 2014

It is well known that liquids confined to small nanoscopic pores and droplets exhibit thermal beh... more It is well known that liquids confined to small nanoscopic pores and droplets exhibit thermal behavior very different from bulk samples. Less is known about liquids spontaneously confined through selfassembly into micellar structures. Here we demonstrate, using a very well-defined n-alkyl-poly(ethylene oxide) polymer system with a tunable structure, that n-alkane(s) forming 2-3 nm small micellar cores are affected considerably by confinement in the form of melting point depressions. Moreover, comparing the reduction in melting points, ΔT m , determined through volumetric and calorimetric methods with the micellar core radius, R c , obtained from small-angle x-ray scattering, we find excellent agreement with the well-known Gibbs-Thomson equation, ΔT m ∼ R −1 c. This demonstrates that the reduced size, i.e., the Laplace pressure, is the dominant parameter governing the melting point depression in micellar systems.

Physical Review Letters, 2015

We investigate the dynamics of kinetically frozen block copolymer micelles of different softness ... more We investigate the dynamics of kinetically frozen block copolymer micelles of different softness across a wide range of particle concentrations, from the fluid to the onset of glassy behavior, through a combination of rheology, dynamic light scattering, and pulsed field gradient NMR spectroscopy. We additionally perform Brownian dynamics simulations based on an ultrasoft coarse-grained potential, which are found to be in quantitative agreement with experiments, capturing even the very details of dynamic structure factors S(Q,t) on approaching the glass transition. We provide evidence that for these systems the Stokes-Einstein relation holds up to the glass transition; given that it is violated for dense suspensions of hard colloids, our findings suggest that its validity is an intriguing signature of ultrasoft interactions.

Thermal composition fluctuations in a three component mixture of the two homopolymers PB/PS of cr... more Thermal composition fluctuations in a three component mixture of the two homopolymers PB/PS of critical composition mixed with different amounts of a symmetric diblock copolymer PB-PS were explored with small angle neutron scattering (SANS)[PB: poly(butadiene); PS: poly(styrene)]. The diblock copolymer was of about 6.3 times larger molar volume than the homopolymers. Samples with two different scattering contrasts, namely, dPB/PS/dPB-PS and dPB/dPS/dPB-PS ("d" means deuterated) were prepared for better characterization the phase diagram. Phase boundaries of various ordered phases including two critical universality classes and the Lifshitz line (LL) were determined. Strong thermal fluctuations lead to a non-monotonic LL, to droplet and bicontinuous micro emulsion phases, and to a reentrant two-phase region near the LL with an upper and lower critical solution temperature and a double critical point (DCP). The expected doubling of the Lifshitz critical exponent of the susce...

ACS Macro Letters, 2013

ABSTRACT Here we present an in situ study of the nonequilibrium cylinder-to-sphere morphological ... more ABSTRACT Here we present an in situ study of the nonequilibrium cylinder-to-sphere morphological transition kinetics on the millisecond range in a model block copolymer micelle system revealing the underlying mechanism and pathways of the process. By employing the stopped-flow mixing technique, the system was rapidly brought (≈100 μs) deep into the instability region, and the kinetics was followed on the time scale of milliseconds using both time-resolved small-angle neutron and X-ray scattering (TR-SANS and TR-SAXS, respectively). Due to the difference in contrast and resolution, SAXS and SANS provide unique complementary information. Our analysis shows that the morphological transition is characterized by a single rate constant indicating a two-state model where the transition proceeds through direct decomposition (fragmentation) of the cylinders without any transient intermediate structures. The cylindrical segments formed in the disintegration process subsequently grow into spherical micelles possibly through the molecular exchange mechanism until near equilibrium micelles are formed. The observation of a two-step kinetic mechanism, fluctuation-induced fragmentation and ″ripening″ processes, provides unique insight into the nonequilibrium behavior of block copolymer micelles in dilute solutions.

Langmuir

We investigated the influence of an n-alkyl-PEO polymer on the structure and dynamics of phosphol... more We investigated the influence of an n-alkyl-PEO polymer on the structure and dynamics of phospholipid vesicles. Multilayer formation and about a 9% increase in the size in vesicles were observed by cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), and small-angle neutron/X-ray scattering (SANS/SAXS). The results indicate a change in the lamellar structure of the vesicles by a partial disruption caused by polymer chains, which seems to correlate with about a 30% reduction in bending rigidity per unit bilayer, as revealed by neutron spin echo (NSE) spectroscopy. Also, a strong change in lipid tail relaxation was observed. Our results point to opportunities using synthetic polymers to control the structure and dynamics of membranes, with possible applications in technical materials and also in drug and nutraceutical delivery.

Langmuir

Mixtures of amphiphilic polymers and surfactants are used in a wide range of applications, e.g., ... more Mixtures of amphiphilic polymers and surfactants are used in a wide range of applications, e.g., pharmaceuticals, detergents, cosmetics, and drug delivery systems. Still, many questions remain on how the structure and, in particular, the kinetics of block copolymer micelles are affected in the presence of surfactants and what controls the solubilization kinetics. In this work, we have studied the stability and solubilization kinetics of block copolymer micelles upon the addition of the surfactant sodium dodecyl sulfate (SDS) using small-angle X-ray/neutron scattering. The ability of the surfactant to dissolve polymer micelles or form mixed micelles has been investigated using two types of amphiphilic polymers, poly(ethylene-alt-propylene)−poly-(ethylene oxide) (PEP1-PEO20) and n-alkyl-functionalized PEO (C 28-PEO5). The exchange kinetics of C 28-PEO5 micelles are in the order of hours, while PEP1-PEO20 micelles are known to be frozen on a practical timescale. In this work, we show that the addition of SDS to PEP1-PEO20 provides virtually no solubilization, even after an extended period of time. However, upon adding SDS to C 28-PEO5 micelles, we observe micellar dissolution and formation of mixed micelles occurring on the timescale of hours. Using a coexistence model of mixed and neat micelles, the SAXS data were analyzed to provide detailed structural parameters over time. First, we observe a fast fragmentation/fission step followed by a slow reorganization process. The latter process is essentially independent of concentration at low volume fraction but is greatly accelerated at larger concentrations. This might indicate a crossover from a predominance of molecular exchange to fusion/fission processes.

Physical Review Letters

Telechelic polymers contain two chain ends that are able to promote self-assembly into "flowerlik... more Telechelic polymers contain two chain ends that are able to promote self-assembly into "flowerlike" or interconnected micellar structures. Here, we investigate the molecular exchange kinetics of such micelles using time-resolved small-angle neutron scattering. We show that the activation energies of monofunctional and telechelic chain exchange are identical. This demonstrates that the two chain ends are not simultaneously released in a single event. Instead, the results show that, contrary to regular micelles, the kinetics occurs in a multistep process involving a collision-induced single-molecule exchange mechanism where the exchange rate is directly proportional to the polymer concentration. We show that this novel mechanism can be quantitatively explained by a simple kinetic model.

Physical Review Letters

Molecular exchange processes are important equilibration and transport mechanisms in both synthet... more Molecular exchange processes are important equilibration and transport mechanisms in both synthetic and biological self-assembled systems such as micelles, vesicles, and membranes. Still, these processes are not entirely understood, in particular the effect of crystallinity and the interplay between cooperative melting processes and chain exchange. Here we focus on a set of simple polymer micelles formed by binary mixtures of poly(ethylene oxide)-mono-n-alkyl-ethers (C n-PEO5) which allows the melting point to be tuned over a wide range. We show that the melting transition is cooperative in the confined 4-5 nm micellar core, whereas the exchange process is widely decoupled and unimeric in nature. As confirmed by differential scanning calorimetry, the total activation energy for ejecting a molecule out of the micellar core below the melting point is the sum of the enthalpy of fusion and the corresponding activation energy in the melt state. This suggests that a "local, single-chain melting process" preludes the molecular diffusion out of the micelle during chain exchange.

The Journal of Chemical Physics

A recent model for unentangled polymer chains in confinement [M. Dolgushev and M. Krutyeva, Macro... more A recent model for unentangled polymer chains in confinement [M. Dolgushev and M. Krutyeva, Macromol. Theory Simul. 21, 565 (2012)] is scrutinized by small-angle neutron scattering (SANS) with respect to its static prediction, the single-chain structure factor. We find a remarkable agreement although the model simplifies the effect of the confinement to a harmonic potential. The effective confinement size from fits of SANS data with the model agrees well with the actual pore size. Starting from this result we discuss the possibility of an experiment on the dynamic structure factor predicted by the model. It turns out that such an experiment would need a large ratio polymer dimension/pore size which is difficult but not impossible to achieve.

Europhysics Letters, 1992

In this paper we present the experimental evidence of incipient ordering of star polymers in solu... more In this paper we present the experimental evidence of incipient ordering of star polymers in solution near the overlap concentration ϕ* as predicted by scaling laws. We have studied 8- and 18-arm polyisoprene stars in the dilute and semi-dilute regime. A peak in the interparticle structure factor S(Q) due to a liquidlike ordering of spheres was found by small-angle neutron

Macromolecules, Jun 1, 1998

In order to elucidate the structure of intermediate species present during the early stages of “l... more In order to elucidate the structure of intermediate species present during the early stages of “living” anionic polymerization, the aggregation behavior of styryllithium head groups [RCH2CH-(C6H5) Li+] in benzene was investigated by small angle neutron scattering ( ...

Macromolecules, 2009

ABSTRACT This work reports a systematic study of the aggregation behavior and structural properti... more ABSTRACT This work reports a systematic study of the aggregation behavior and structural properties of micelles formed by a selectively deuterated h-polystyrene-d-polybutadiene (h-PS10-d-PB10, where 10 denotes the approximate block molecular weight in kilograms per mole) block copolymer in a series of n-alkanes, CnH2n+2, where n = 7, 10, 12, 14, 16. By applying small-angle neutron scattering (SANS) in combination with contrast variation, very detailed aspects of the micellar structure are resolved. The results show that the micelles are rather poorly segregated with a large quantity of solvent (approximately 35−55%) penetrating the core and a smaller compact corona. Interestingly, the solvent fraction in the core decreases, and the overall aggregation number increases with the size of the solvent molecules. At first sight, this is not intuitively expected because the Flory−Huggins interaction parameter, χ, or the interfacial tension, γ, stay constant or are even slightly decreasing, thus suggesting that the enthalpic interactions between the core-forming PS segments and the n-alkanes remain unchanged or decrease. However, this behavior can be understood semiquantitatively by applying a modified mean field theory where the solvent entropy effects in both core and corona are properly taken into account. This work thus directly demonstrates that contributions of the solvent entropy to the overall free energy are important whenever the interfacial tension is low.

The phase diagram, the Flory-Huggins parameter Γ, and the Ginzburg parameter Gi of a polybutadien... more The phase diagram, the Flory-Huggins parameter Γ, and the Ginzburg parameter Gi of a polybutadiene/polystyrene blend and diblock copolymer were determined by small-angle neutron scattering in the pressure range of 0.1 and 200MPa. Both systems show qualitatively the same pressure dependence. The enthalpic and entropic terms of Γ are larger in blends. In the diblock copolymer a constant order-disorder temperature was observed which could be explained by a compensation effect between Γ and Gi.

The Journal of Physical Chemistry B, 2008

Thermal copolymer fluctuations were explored in a three-component blend consisting of a critical ... more Thermal copolymer fluctuations were explored in a three-component blend consisting of a critical (A/B) homopolymer blend and a symmetric A-B diblock copolymer using the technique of neutron small angle scattering. The copolymer has the function of an external nonordering field and thereby determines phase behavior as well as the regimes of 3d-Ising, isotropic Lifshitz, and Brasovskiî critical universality. It was found that the random phase approximation (RPA) does not correctly describe the copolymer structure function because of strong thermal fluctuations. On the other hand a weak coupling of copolymer and homopolymer was confirmed, consistent with predictions from RPA. Self-assembly of the copolymers was observed prior to the ordering of the "total" blend, e.g. inclusive of the homopolymers, into bicontinuous and lamellar ordered phases.

Journal of Applied Crystallography, 2015

Following demand from the user community regarding the possibility of improving the experimental ... more Following demand from the user community regarding the possibility of improving the experimental resolution, the dedicated high-intensity/extendedQ-range SANS diffractometer KWS-2 of the Jülich Centre for Neutron Science at the Heinz Maier-Leibnitz Center in Garching was equipped with a double-disc chopper with a variable opening slit window and time-of-flight (TOF) data acquisition option. The chopper used in concert with a dedicated high-intensity velocity selector enables the tuning at will of the wavelength resolution Δλ/λ within a broad range, from 20% (standard) down to 2%, in a convenient and safe manner following pre-planned or spontaneous decisions during the experiment. The new working mode is described in detail, and its efficiency is demonstrated on several standard samples with known properties and on a completely new crystallizable copolymer system, which were investigated using both the conventional (static) and TOF modes.

Applied Physics a Materials Science Processing, Nov 30, 2002

ABSTRACT With small-angle neutron scattering (SANS) we measured thermal composition fluctuations ... more ABSTRACT With small-angle neutron scattering (SANS) we measured thermal composition fluctuations in a homopolymer blend, PB/PS, of critical composition mixed with different amounts of the symmetric diblockcopolymer PB-PS (PB and PS being polybutadiene (1,4) and polystyrene, respectively) in the disordered state along the isopleths. The diblockcopolymer acts as a surfactant enhancing the compatibility of both homopolymers up to the Lifshitz line and simultaneously changes the critical university class from 3D-Ising to isotropic Lifshitz critical behavior. Above &#75&#440.3 diblock content the samples show diblock-like behavior with a first-order ordering transition. The intermediate regime between &#75&#440.075 and 0.3 can be characterized by a micro-emulsion phase. The temperature dependence of the Lifshitz line, the disorder line and the critical line is non-monotonous.

Advances in Polymer Science, 2013

ABSTRACT This article reviews recent progress in studying the kinetics of block copolymer micella... more ABSTRACT This article reviews recent progress in studying the kinetics of block copolymer micellar systems by time-resolved small angle scattering techniques. The review includes an overview of the theoretical background concerning block copolymer micellar structure and kinetics, with a clear distinction between equilibrium and non-equilibrium processes. Basic principles of both static and time-resolved small-angle X-ray and neutron scattering (TR-SAXS and TR-SANS) techniques are summarized, with a special emphasis on the characterization of block copolymer micellar systems. In particular, the principle of SANS in combination with hydrogen/deuterium (H/D) contrast variation for the determination of chain exchange under equilibrium conditions is highlighted. In the experimental part, we first review results on equilibrium kinetics obtained within the last decade by the TR-SANS/H/D labeling technique. In general, the experimental results strongly indicate that the component exchange between different micelles proceeds via the exchange of single unimers. In agreement with the theoretical prediction, chain expulsion is the rate-determining step. The corresponding activation energy is mainly governed by the interfacial tension and the length of the insoluble block, which determine the exchange rate with a double exponential dependence. Thus, due to this extremely strong dependence, even synthetic polymers with modest chain length distribution show a logarithmic time dependence instead of the theoretically expected single exponential decay. In the second part, the kinetic results obtained under non-equilibrium conditions, i.e., relaxation processes obtained after perturbations from equilibrium, are reviewed. This part covers formation kinetics as well as reorganization and morphological transition kinetics. We present, as a special highlight, TR-SAXS measurements with millisecond resolution on the formation of star-like micelles after stopped-flow mixing of molecularly dissolved block copolymers with a selective solvent. The micellization process could be modelled as a nucleation & growth process with unimer exchange as the elemental mechanism. The resulting scenario could be described as a three step process that includes a fast nucleation event, a region of micellar growth, and a final equilibration to thermodynamically stable micelles. In summary, this review demonstrates the importance of small angle scattering techniques for studying fundamental aspects of kinetics in block copolymer micelles and in soft matter materials in general.

Physical Review Letters, 2014

It is well known that liquids confined to small nanoscopic pores and droplets exhibit thermal beh... more It is well known that liquids confined to small nanoscopic pores and droplets exhibit thermal behavior very different from bulk samples. Less is known about liquids spontaneously confined through selfassembly into micellar structures. Here we demonstrate, using a very well-defined n-alkyl-poly(ethylene oxide) polymer system with a tunable structure, that n-alkane(s) forming 2-3 nm small micellar cores are affected considerably by confinement in the form of melting point depressions. Moreover, comparing the reduction in melting points, ΔT m , determined through volumetric and calorimetric methods with the micellar core radius, R c , obtained from small-angle x-ray scattering, we find excellent agreement with the well-known Gibbs-Thomson equation, ΔT m ∼ R −1 c. This demonstrates that the reduced size, i.e., the Laplace pressure, is the dominant parameter governing the melting point depression in micellar systems.

Physical Review Letters, 2015

We investigate the dynamics of kinetically frozen block copolymer micelles of different softness ... more We investigate the dynamics of kinetically frozen block copolymer micelles of different softness across a wide range of particle concentrations, from the fluid to the onset of glassy behavior, through a combination of rheology, dynamic light scattering, and pulsed field gradient NMR spectroscopy. We additionally perform Brownian dynamics simulations based on an ultrasoft coarse-grained potential, which are found to be in quantitative agreement with experiments, capturing even the very details of dynamic structure factors S(Q,t) on approaching the glass transition. We provide evidence that for these systems the Stokes-Einstein relation holds up to the glass transition; given that it is violated for dense suspensions of hard colloids, our findings suggest that its validity is an intriguing signature of ultrasoft interactions.

Thermal composition fluctuations in a three component mixture of the two homopolymers PB/PS of cr... more Thermal composition fluctuations in a three component mixture of the two homopolymers PB/PS of critical composition mixed with different amounts of a symmetric diblock copolymer PB-PS were explored with small angle neutron scattering (SANS)[PB: poly(butadiene); PS: poly(styrene)]. The diblock copolymer was of about 6.3 times larger molar volume than the homopolymers. Samples with two different scattering contrasts, namely, dPB/PS/dPB-PS and dPB/dPS/dPB-PS ("d" means deuterated) were prepared for better characterization the phase diagram. Phase boundaries of various ordered phases including two critical universality classes and the Lifshitz line (LL) were determined. Strong thermal fluctuations lead to a non-monotonic LL, to droplet and bicontinuous micro emulsion phases, and to a reentrant two-phase region near the LL with an upper and lower critical solution temperature and a double critical point (DCP). The expected doubling of the Lifshitz critical exponent of the susce...

ACS Macro Letters, 2013

ABSTRACT Here we present an in situ study of the nonequilibrium cylinder-to-sphere morphological ... more ABSTRACT Here we present an in situ study of the nonequilibrium cylinder-to-sphere morphological transition kinetics on the millisecond range in a model block copolymer micelle system revealing the underlying mechanism and pathways of the process. By employing the stopped-flow mixing technique, the system was rapidly brought (≈100 μs) deep into the instability region, and the kinetics was followed on the time scale of milliseconds using both time-resolved small-angle neutron and X-ray scattering (TR-SANS and TR-SAXS, respectively). Due to the difference in contrast and resolution, SAXS and SANS provide unique complementary information. Our analysis shows that the morphological transition is characterized by a single rate constant indicating a two-state model where the transition proceeds through direct decomposition (fragmentation) of the cylinders without any transient intermediate structures. The cylindrical segments formed in the disintegration process subsequently grow into spherical micelles possibly through the molecular exchange mechanism until near equilibrium micelles are formed. The observation of a two-step kinetic mechanism, fluctuation-induced fragmentation and ″ripening″ processes, provides unique insight into the nonequilibrium behavior of block copolymer micelles in dilute solutions.