Ulrich Mansfeld

Orthogonal self-assembly of stimuli-responsive supramolecular polymers using one-step prepared heterotelechelic building blocks

The one-step preparation of heterodifunctional telechelic polymers containing 2,2′:6′,2′′-terpyridine (tpy) and 2-ureido-4[1H]-pyrimidinone (UPy) end-groups, as orthogonal supramolecular moieties, is reported. The utilization of an appropriately functionalized alkoxyamine, as an initiator for the nitroxide-mediated radical polymerization (NMP), directly constitutes the end-groups of the resultant polymers. The targeted alkoxyamines are based on the nitroxide structure of 2,2,5-trimethyl-3-(1-phenylethoxy)-4-phenyl-3-azahexane (TIPNO) and were obtained via stepwise functionalization of a heterodifunctional alkoxyamine skeleton. Controlled radical polymerization of styrenics using the alkoxyamine tpy–TIPNO–UPy, as an initiator, is demonstrated to generate well-defined telechelic polymers in one step. These telechelics represent promising building blocks for supramolecular architectures via self-assembly processes, yielding linear chain-extended polymers of high molar masses. Due to the orthogonality of the metal ion complexation and hydrogen bonding, the system can be addressed selectively by external stimuli. Besides for various applications, e.g. as self-healing materials, the strategy is highly attractive for tailoring the materials properties of supramolecular polymers, since the nature and the length of the polymer chain between the terminal supramolecular motifs can be controlled in a facile way.

Understanding and tuning the self-assembly of polyether-based triblock terpolymers in aqueous solution

The synthesis and self-assembly of well-defined poly(ethylene oxide)-block-poly(allyl glycidyl ether)-block-poly(tert-butyl glycidyl ether) (PEO-b-PAGE-b-PtBGE) triblock terpolymers with varying block lengths of PAGE and PtBGE are reported. The materials were synthesized using sequential living anionic ring-opening polymerization (AROP). The middle block, PAGE, was further modified by post-polymerization addition of 2,3,4,6-tetra-O-acetyl-1-thio-β-D-galactopyranose via thiol–ene chemistry, resulting in PEO-b-PAGEGal-b-PtBGE. Self-assembly of the terpolymers in aqueous media resulted in the predominant formation of core–shell–corona architectures and the aggregates featured a PtBGE core, a PAGE shell, and a PEO corona. The structures were investigated using dynamic light scattering (DLS) and cryogenic transmission electron microscopy (cryo-TEM) measurements. In addition, the presence of a PEO corona rendered the formed micellar structures thermo-responsive, as demonstrated using turbidimetry. Depending on the ratio of hydrophilic to hydrophobic segments and on the thermal history of the samples, several micellar morphologies could be identified, including spheres of different size, worm-like structures, and vesicles. More important, both reversible and irreversible structural rearrangements could be identified during the heating–cooling cycles.

Self-assembly of chiral block and gradient copolymers

Chiral micelles have a high potential for targeted drug delivery or chiral separation applications. In this contribution the self-assembly of chiral amphiphilic copolymers into chiral structures was investigated. Gradient copolymers could be obtained by statistically copolymerizing the hydrophilic 2-ethyl-2-oxazoline (EtOx) with the hydrophobic chiral R-2-butyl-4-ethyl-2-oxazoline (R-BuEtOx) or racemic RS-BuEtOx monomers. Self-assembly of the gradient enantiopure copolymers was studied by both cryogenic transmission electron spectroscopy (cryo-TEM) and dynamic light scattering (DLS) revealing the formation of spherical micelles in aqueous solution. Additionally, amphiphilic block copolymers were synthesized in a 1-pot-2-step manner. The type of self-assembled structure could be controlled by varying the hydrophobic to hydrophilic ratio within the block copolymer from spherical and cylindrical micelles to sheets and vesicles. When the enantiopure block was replaced by the corresponding racemic block, only spherical micelles could be observed, while the chiral block copolymers with similar hydrophobic content revealed cylindrical micelles.

Tuning the morphology of triblock terpoly(2-oxazoline)s containing a 2-phenyl-2-oxazoline block with varying fluorine content

The formation of nanostructures in triblock terpolymers consisting of poly[2-ethyl-2-oxazoline-block-2-(1-ethylpentyl)-2-oxazoline-block-2-(Xfluorophenyl)-2-oxazoline] (X = di, tri, tetra and penta) was investigated in water. For this purpose a gradually increasing degree of fluorination was introduced in the molecular structures and its influence on the self-assembly was studied. It can be demonstrated that the basic form of aggregation of these systems resembles rod-like micelles, which tend, upon introduction of fluorinated blocks, to aggregate first into 2-dimensional and later into 3-dimensional super-aggregates. In the case of di- and pentafluorinated terpolymers well-defined structures were observed, which represent likely intermediate, stable transient structures formed during an assumed rod-to-vesicle transition. DLS and cryo-TEM were utilized to analyze the structural features of these nanostructures and a model for their further assembly into super-structures was developed.

Small but powerful: co-assembly of polyether-based triblock terpolymers into sub-30 nm micelles and synergistic effects on cellular interactions.

We introduce a versatile ABC triblock terpoly- mer platform based on poly(ethylene oxide)-block-poly(allyl glycidyl ether)-block-poly(tert-butyl glycidyl ether) (PEO-b-PAGE-b-PtBGE) and subsequent functionalization of the PAGE segment with thiogalactose (hydroxyl), cysteamine (amino), and 2-mercaptopropionic acid (carboxy) by thiol-ene chemistry. These materials are used to prepare core-shell-corona micelles with a PtBGE core, a PAGE shell, and a PEO corona and sizes below 30 nm in aqueous media. We investigate the influence of different functional groups on micelle formation and cellular uptake. Moreover, co-assembly of differently functionalized materials allows to create micelles with a mixed shell and adjustable charge and, in that way, important characteristics such as cell uptake or cytotoxicity can be controlled. Furthermore, we demonstrate that even the uptake mechanism depends on the substitution pattern of the underlying triblock terpolymer.

Poly(2‐vinyl pyridine)‐block‐Poly(ethylene oxide) Featuring a Furan Group at the Block Junction—Synthesis and Functionalization

Furfuryl glycidyl ether (FGE) represents a highly versatile monomer for the preparation of reversibly cross-linkable nanostructured materials via Diels-Alder reactions. Here, the use of FGE for the mid-chain functionalization of a P2VP-b-PEO diblock copolymer is reported. The material features one furan moiety at the block junction, P2VP68 -FGE-b-PEO390 , which can be subsequently addressed in Diels-Alder reactions using maleimide-functionalized counterparts. The presence of the FGE moiety enables the introduction of dyes as model labels or the formation of hetero-grafted brushes as shell on hybrid Au@Polymer nanoparticles. This renders P2VP68 -FGE-b-PEO390 , a powerful tool for selective functionalization reactions, including the modification of surfaces.

Investigating the Motion of Diblock Copolymer Assemblies in Ionic Liquids by In Situ Electron Microscopy

The movement of individual block copolymer micelles in free-standing films of ionic liquids is investigated by transmission electron microscopy with the aim of providing an easily accessible high-resolution imaging tool for the in situ observation of particle movement in a liquid environment. A proof of concept and first studies on the behavior of individual particles in the fluid are demonstrated.

Amphiphilic supramolecular A(B)2A quasi-triblock copolymers

The efficient synthesis of a responsive amphiphilic supramolecular triblock copolymer A(B)2A is described, where the blocks are held together by self-complementary hydrogen bonding at the homojunction (B–B) and by heteroleptic metal complexes at the heterojunction (A–B). The supramolecular copolymer was prepared in a minimum of steps by using heterotelechelic building blocks with orthogonal binding sites polymerized in one step.

Incorporation of core–shell particles into methacrylate based composites for improvement of the mechanical properties

The fracture toughness of polymeric materials and composites can be enhanced by the incorporation of polymer nanoparticles. The combination of a soft core and a hard shell leads to an improvement of the fracture toughness of the polymeric composites. Thereby, the mechanical resistance of the materials is commonly decreased. In our approach, core–shell nanoparticles consisting of an ethylene glycol dimethacrylate (EGDMA) crosslinked poly(butyl acrylate) (PBA) core and a poly(methyl methacrylate) (PMMA) shell were synthesized. The polymer particles were incorporated into triethylene glycol dimethacrylate (TEGDMA)/urethane dimethacrylate (UDMA) based composites in order to tune the mechanical properties. Different core–shell ratios were applied to study the influence on the fracture toughness and E-modulus. An examination of shell-crosslinking with a TEGDMA content of up to 8% was performed to improve particle stability and dispersibility. The particle sizes and morphologies were characterized by dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM) and analytical ultracentrifugation (AUC). Latex particle sizes of 70 to 220 nm were obtained. The mechanical properties (flexural strength, E-modulus and K1c) of polymer composites were investigated in three-point bending tests. Core/shell ratios of 50/50 showed a decreasing effect on flexural strength, E-modulus and K1c. Polymer particles with core/shell ratios of 30/70 led to a significant increase of the mechanical properties with maxima of 1.206 MPa m1/2 (K1c) (increase of 65%), E-modulus of 1.90 GPa (increase of 18%) and flexural strength of 79 MPa (increase of 18%). This study represents the first report of a simultaneous improvement of fracture toughness and E-modulus (at the same time) of additive filled polymer composites. The improvement of mechanical properties makes these materials interesting as tougheners for hard tissue applications like bone cements or dental replacement materials.

Plasmonic nanoparticle clusters with tunable plasmonic resonances in the visible spectral region

The seeded growth of poly(ethylene imine) – gold nanoparticle clusters enables the formation of particle assemblies with tunable optical properties. Clusters with increasing particle sizes, filling factors and assemblies consisting of PEI–gold–silver core shell particles can be synthesized in this way. Profound structural characterization is carried out via TEM imaging and FIB milling which allows visualizing the cross-section of the clusters. Determination of the optical properties was performed via UV-Vis spectroscopy and spectral dark field microscopy of individual particles. Additionally, numerical calculations were carried out based on the Mie theory. The results are in good agreement with the experimental findings and reveal the contribution of different multipoles to the spectra which cannot be resolved by UV-Vis spectroscopy in solution. The isotropic nature and adjustable properties of these clusters could render them versatile building blocks for metamaterials.