Reidar Lund

Starlike micelles with starlike interactions: a quantitative evaluation of structure factors and phase diagram.

Starlike PEP-PEO block copolymer micelles offer the possibility to investigate the phase behavior and interactions of regular star polymers (ultrasoft colloids). Micellar functionality f can be smoothly varied by changing solvent composition (interfacial tension). Structure factors obtained by small-angle neutron-scattering can be quantitatively described in terms of an effective potential for star polymers. The experimental phase diagram reproduces to a high level of accuracy the predicted liquid-solid transition. Whereas for intermediate f a bcc phase is observed, for high f the formation of a fcc phase is preempted by glass formation.

Direct observation of the formation of surfactant micelles under nonisothermal conditions by synchrotron SAXS.

Self-assembly of amphiphilic molecules into micelles occurs on very short times scales of typically some milliseconds, and the structural evolution is therefore very challenging to observe experimentally. While rate constants of surfactant micelle kinetics have been accessed by spectroscopic techniques for decades, so far no experiments providing detailed information on the structural evolution of surfactant micelles during their formation process have been reported. In this work we show that by applying synchrotron small-angle X-ray scattering (SAXS) in combination with the stopped-flow mixing technique, the entire micelle formation process from single surfactants to equilibrium micelles can be followed in situ. Using a sugar-based surfactant system of dodecyl maltoside (DDM) in dimethylformamide (DMF), micelle formation can be induced simply by adding water, and this can be followed in situ by SAXS. Mixing of water and DMF is an exothermic process where the micelle formation process occurs under nonisothermal conditions with a temperature gradient relaxing from about 40 to 20 °C. A kinetic nucleation and growth mechanism model describing micelle formation by insertion/expulsion of single molecules under nonisothermal conditions was developed and shown to describe the data very well.

Equilibrium exchange kinetics in n-alkyl–PEO polymeric micelles: single exponential relaxation and chain length dependence

In this communication we present first results on the chain exchange kinetics of n-alkyl–PEO polymeric micelles by time-resolved small angle neutron scattering. We found that the rate strongly depends on the alkyl-chain length and that the relaxation function almost perfectly follows the single exponential decay predicted by theory. The key achievement of this study is the experimental verification that core block polydispersity accounts for the almost logarithmic time decay in block copolymer micelles as recently suggested by Choi et al. The results thus directly show that unimer exchange is the main mechanism for molecular exchange in block copolymer micelles.

Hydration and Dynamic State of Nanoconfined Polymer Layers Govern Toughness in Nacre-mimetic Nanocomposites

Biological high-performance composites inspire to create new tough, strong, and stiff structural materials. We show a brittle-to-ductile transition in a self-assembled nacre-inspired poly(vinyl alcohol)/nanoclay composite based on a hydration-induced glass-to-rubber transition in the 2D-nanoconfined poly(vinyl alcohol) layers. The findings open routes to design dissipative toughening mechanisms to combine stiffness and strength in nanocomposites.

Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods

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.

Solution structural characterization of coiled-coil peptide-polymer side-conjugates.

Detailed structural characterization of protein-polymer conjugates and understanding of the interactions between covalently attached polymers and biomolecules will build a foundation to design and synthesize hybrid biomaterials. Conjugates based on simple protein structures are ideal model system to achieve these ends. Here we present a systematic structural study of coiled-coil peptide-poly(ethylene glycol) (PEG) side-conjugates in solution, using circular dichroism, dynamic light scattering, and small-angle X-ray scattering, to determine the conformation of conjugated PEG chains. The overall size and shape of side-conjugates were determined using a cylindrical form factor model. Detailed structural information of the covalently attached PEG chains was extracted using a newly developed model where each peptide-PEG conjugate was modeled as a Gaussian chain attached to a cylinder, which was further arranged in a bundle-like configuration of three or four cylinders. The peptide-polymer side-conjugates were found to retain helix bundle structure, with the polymers slightly compressed in comparison with the conformation of free polymers in solution. Such detailed structural characterization of the peptide-polymer conjugates, which elucidates the conformation of conjugated PEG around the peptide and assesses the effect of PEG on peptide structure, will contribute to the rational design of this new family of soft materials.

Self-assembly of crystalline nanotubes from monodisperse amphiphilic diblock copolypeptoid tiles

Significance A fundamental challenge in materials science is to create synthetic nanoarchitectures that rival the structural complexity found in nature. A promising bioinspired approach is to synthesize sequence-defined polymer chains that fold into precise protein-like structures. Here, we discovered a family of sequence-defined, amphiphilic diblock copolypeptoids that spontaneously form crystalline, hollow nanotubes of uniform diameter, without a central hydrophobic core in aqueous solution. These low–molecular-weight polymers have two chemically distinct domains that are congruent in size and shape, which allows them to behave like molecular tile units that can form brick-like supramolecular lattices. This demonstrates the remarkable structure-directing influence of n-alkane and ethyleneoxy side chains, and that flexible polymers can serve as tile units that can assemble into precision supramolecular architectures. The folding and assembly of sequence-defined polymers into precisely ordered nanostructures promises a class of well-defined biomimetic architectures with specific function. Amphiphilic diblock copolymers are known to self-assemble in water to form a variety of nanostructured morphologies including spheres, disks, cylinders, and vesicles. In all of these cases, the predominant driving force for assembly is the formation of a hydrophobic core that excludes water, whereas the hydrophilic blocks are solvated and extend into the aqueous phase. However, such polymer systems typically have broad molar mass distributions and lack the purity and sequence-defined structure often associated with biologically derived polymers. Here, we demonstrate that purified, monodisperse amphiphilic diblock copolypeptoids, with chemically distinct domains that are congruent in size and shape, can behave like molecular tile units that spontaneously assemble into hollow, crystalline nanotubes in water. The nanotubes consist of stacked, porous crystalline rings, and are held together primarily by side-chain van der Waals interactions. The peptoid nanotubes form without a central hydrophobic core, chirality, a hydrogen bond network, and electrostatic or π–π interactions. These results demonstrate the remarkable structure-directing influence of n-alkane and ethyleneoxy side chains in polymer self-assembly. More broadly, this work suggests that flexible, low–molecular-weight sequence-defined polymers can serve as molecular tile units that can assemble into precision supramolecular architectures.

Unraveling the equilibrium chain exchange kinetics of polymeric micelles using small-angle neutron scattering – architectural and topological effects

In this paper, we present a study of micellar structures formed by poly(styrene)-poly(butadiene) (PS10-PB10; the numbers indicate the molecular weight in kg mol−1) diblock copolymers and PB10-PS20-PB10 triblock copolymers in different n-alkane solvents. Particular emphasis is placed on the dynamic properties of these micelles under equilibrium which are studied using a novel time-resolved small-angle neutron scattering technique. The results show that the structures of the micelles are very similar for both the diblock and triblock copolymers, which allows a direct comparison of the dynamic properties. A novel logarithmic relaxation is found for both the triblock and the diblock micelles which is not consistent with theoretical expectations. However, for the diblock micelles, the relaxation kinetics seem to approach the rate and the single exponential decay predicted by Halperin & Alexander [Macromolecules, (1989), 22, 2403–2412] when the micellar cores are strongly swollen with solvent. For the triblock micelles a logarithmic relaxation is found for all cases as an effect of additional topological knots present even in highly swollen micellar cores. This behavior is assigned to an increased coupling of chain motion within the dense confined core – an effect which seems to vanish in diblock micelles when the core is sufficiently swollen.

Effect of Alkyl Length of Peptide–Polymer Amphiphile on Cargo Encapsulation Stability and Pharmacokinetics of 3-Helix Micelles

3-Helix micelles have demonstrated excellent in vitro and in vivo stability. Previous studies showed that the unique design of the peptide–polymer conjugate based on protein tertiary structure as the headgroup is the main design factor to achieve high kinetic stability. In this contribution, using amphiphiles with different alkyl tails, namely, C16 and C18, we quantified the effect of alkyl length on the stability of 3-helix micelles to delineate the contribution of the micellar core and shell on the micelle stability. Both amphiphiles form well-defined micelles, <20 nm in size, and show good stability, which can be attributed to the headgroup design. C18-micelles exhibit slightly higher kinetic stability in the presence of serum proteins at 37 °C, where the rate constant of subunit exchange is 0.20 h–1 for C18-micelles vs 0.22 h–1 for C16-micelles. The diffusion constant for drug release from C18-micelles is approximately half of that for C16-micelles. The differences between the two micelles are significantly more pronounced in terms of in vivo stability and extent of tumor accumulation. C18-micelles exhibit significantly longer blood circulation time of 29.5 h, whereas C16-micelles have a circulation time of 16.1 h. The extent of tumor accumulation at 48 h after injection is ∼43% higher for C18-micelles. The present studies underscore the importance of core composition on the biological behavior of 3-helix micelles. The quantification of the effect of this key design parameter on the stability of 3-helix micelles provides important guidelines for carrier selection and use in complex environment.

Polymer dynamics under soft confinement in a self-assembled system

Block copolymers that self-assemble into nano-structured melts provide an interesting template for soft confinement. An important question that arises is how the chain dynamics is affected in such self-assembled systems. Here we consider a system composed of poly(isoprene)-b-poly(dimethyl siloxane) (PI6-PDMS30, where the numbers indicate the molecular weight in kg mole−1) having a large asymmetry where the minor PI component block constitutes 19% of the total molecular weight. As shown by small angle neutron scattering (SANS), the system forms hexagonally ordered cylindrical structures composed of a PI phase (with a cylinder radius of about 6.4 nm) in a continuous PDMS matrix. Here we demonstrate using neutron spin-echo spectroscopy and careful H/D contrast matching schemes how the dynamics of single polymer chains (single chain contrast) and the interfacial dynamics of the domains can be resolved independently. This is achieved in the former “single chain contrast” case by matching out the net domain scattering leaving only the intra chain coherent structure factor visible. In the latter “bulk contrast” case, the protonated (h) PI and deuterated (d) PDMS domains are visible and most of the contrast comes from the small interfacial layer where h-PI/d-PDMS segments are intermixed and give rise to a significant contrast by which the fluctuations can be observed. Combining the results from the two contrast conditions, we show that the dynamics can be consistently described in terms of Rouse motion modified by the surface fluctuation and laterally restricted to 2D surface diffusion. Surprisingly, the fluctuations of the interface significantly contribute to the dynamics and the interfacial tension drives the fluctuations of a sizeable portion of the polymer chains while the dynamics is similar to Rouse motion.