The coexistence of two different conformational states in one molecule has been visualized by scanning force microscopy for a rod-globule transition of brush molecules adsorbed on a water surface. The transition, which occurred upon lateral compression of monolayers, was also examined theoretically by scaling analysis that proved its first-order character. The transition becomes less distinct with decreasing length of the side chains and finally vanishes below a certain critical value.
High molar mass polymacromonomers based on methacryloyl end-functionalized oligo methacrylates (Mn ) 2410 g/mol) adopt the conformation of wormlike cylindrical brushes. Comparison of the absolute molar mass, Mw, determined by static light scattering and the contour length, Lw, of the molecules measured by SFM in the dry state revealed the length per vinylic main chain monomer of the cylindrical structure to be less than 0.1 nm, thus being much shorter than the maximum value of 0.25 nm. In solution this shrinkage could be quantified to 0.071 nm per monomer by Holtzer analysis of the scattering curves which in addition yielded the Kuhn statistical segment length lk ) 120 nm. GPC MALLS investigations of such samples showed an anomalous elution effect: After a regular elution at small elution volumes the molar mass of the eluting molecules increased drastically with increasing elution volume. Fractionation by GPC showed that this effect is caused by a fraction of extremely high molar mass molecules which elute by an unknown mechanism rather than by size exclusion.
The synthesis and structural analysis of a library containing 13 taper- and conical-shaped self-assembling dendrons, 16 dendritic monomers, and their corresponding polymers is reported. Fifteen of these
polymers exhibit a well-defined cylindrical shape produced by the self-assembly of their dendritic side groups
that self-organizes in a hexagonal columnar two-dimensional liquid crystalline lattice. The retrosynthetic analysis
of this lattice by X-ray diffraction (XRD) showed that the diameter (60 to 41 Å) and the number of repeat
units forming the cylinder cross-section (7 to 1.9) of these polymers are determined by the structure of their
dendritic side groups. This demonstrates that, in the hexagonal columnar lattice, the conformation (from helical
to fully extended) and the stiffness of the polymer backbone penetrating through the center of the cylinder are
controlled in a systematic and predictive way by the structure of the side groups. Dynamic and static light-scattering experiments have demonstrated the same trend for the stiffness of these polymers in solution (Kuhn
segment length from 200 to 1032 Å). Single chains and monolayers of these polymers were visualized and
quantitatively analyzed by scanning force microscopy (SFM) on a graphite surface to provide the first
comparative study of the conformation, stiffness, and contour length in solution (by light scattering), in the
disordered solid state on a flat substrate (by SFM), and in the hexagonal columnar lattice (by XRD). The
elaboration of this library of visualizable cylindrical macromolecules with controlled chain conformation and
stiffness accesses the elucidation of many fundamental problems of the field of polymer science at the molecular
level and the design of multifunctional nanoscale systems based on single polymer chains.
A simple and efficient concept has been developed for the synthesis of pH‐responsive molecular nanocarriers based on commercially available hyperbranched polymers. These dendritic core‐shell architectures can encapsulate, transport, and selectively release polar guest molecules in an acidic environment (pH 3–6, see scheme). The observed release properties render these molecular nanocarriers promising candidates for controlled drug and gene delivery.
Axial contraction of cylindrical molecular brushes of polymethylmethacrylate was observed by static light scattering and scanning force microscopy. Single brush molecules were visualized as worm-like particles whose length was almost three times shorter than the contour length of the backbone. A somewhat larger length was measured by light scattering in a good solvent. A scaling approach has been used to analyze the driving forces for the axial contraction and the conformation of the molecular brushes.
Absolute-molecular-weight distribution of cylindrical brush molecules were determined using a combination of the Langmuir Blodget (LB) technique and Atomic Force Microscopy (AFM). The LB technique gives mass density of a monolayer, i.e., mass per unit area, whereas visualization of individual molecules by AFM enables accurate measurements of the molecular density, i.e., number of molecules per unit area. From the ratio of the mass density to the molecular density, one can determine the absolute value for the number average molecular weight. Assuming that the structure of brush molecules is uniform along the backbone, the length distribution should be virtually identical to the molecular weight distribution. Although we used only brush molecules for demonstration purpose, this approach can be applied for a large variety of molecular and colloidal species that can be visualized by a microscopic technique.
Scanning force microscopy on monodendron-jacketed linear polystyrenes demonstrated
unusual conformations and ordering depending on the branching density and interplay between
intramolecular and surface interactions of the dendritic coat. Single molecules were visualized as wormlike
cylinders, which could be twisted to plectonemic coils or aligned in nematic-like order on the substrate.
The attained resolution enabled identification of chain-end stacking, hairpin folds and crossovers of
individual macromolecules. The structure of the more densely branched 3,4,5-tris[3‘,4‘,5‘-tris(n-dodecan-1-yloxy)benzyloxy]benzyl was predominantly controlled by the intramolecular repulsion of the crowded
dendron substituents which enforced an extended conformation of the molecular backbone. Formation of
molecular monolayers was affected by wetting/dewetting events during casting, and the film structure
did not depend on the substrate type. In contrast, the less crowded 3,4,5-tris[4-(n-tetradecan-1-yloxy)benzyloxy]benzoate (14-ABG-PS) was demonstrated to undergo conformational transitions upon drying
and adsorption, as indicated by the observed built-up of torsional stress in the backbone. On mica, 14-ABG-PS formed a web of intertwined twisted cylinders. On graphite, lattice matching between the
crystalline surface and the all-trans conformation of the alkyl side chains caused alignment of the
macromolecules according to the 3-fold symmetry of highly oriented pyrolytic graphite. Subsequent
annealing resulted in the formation of large domains of parallel aligned macromolecules.
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