Exploring the Loading Capacity of Generation Six to Eight Dendronized Polymers in Aqueous Solution (original) (raw)
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Solvent induced phenomena in a dendronized linear polymer
Colloid and Polymer Science, 2013
The properties of a dendronized linear polymer (DP) in dilute solutions depending on solvent quality and temperature are described. The polymer has a contour length of L c =1,060 nm. The sample of the fourth generation (PG4) was analyzed in the thermodynamically good solvents dioxane, chloroform, and methanol. The wormlike macromolecule has a persistence length l p =7 nm in dioxane and a cross-section radius determined by small angle X-ray scattering (SAXS) of R c (SAXS) =2.8 nm. The bulk density of PG4 determined by SAXS was compared with solution density. Evidence for substantial swelling of the cross-section was found. Toluene acts as a thermodynamically poor solvent (θ solvent). Above the θ temperature T θ , a strong temperature dependence of the size and the Young's modulus E was observed. Following Odijk, E/k B T ∼1 was found. Below T θ , a regime characterized by unswelling of the wormlike chains was observed. The results suggest that DPs can be described as soft colloid filaments, which are subject to commonly observed interactions in colloidal systems. A phase diagram indicates a regime below T θ in which fluctuations of osmotic pressure inside the filaments result in periodic undulation of the chains. In summary, introducing a dense dendritic shell around the backbone converts conventional polymers into molecular colloids.
Rsc Advances, 2013
Atomistic molecular dynamics simulations in chloroform and solvent-free environments are used to build and study a homologous series of neutral dendronized linear polymers (DPs), whose repeat units are regularly branched dendrons of generations g = 1-7, excluding g = 5. We find that a DP with g ¡ 4 displays an elongated conformation, while a DP with g = 6 exhibits a helical backbone. The conformations essentially differ in their alternating (elongated) or regular (helical) twist with respect to the macromolecular axis, at similar average distance between repeat units (2.1-2.3 Å). With increasing g the dendrons tend to induce an increasing strain, stiffness and overall cylindrical shape onto the DP; the existence of DPs with g ¢ 7 is excluded. The fractal dimensionality of the backbone appears similar for DPs with g ¡ 4, while a discontinuous fractal behavior found for g = 6 is consistent with its helical backbone.
Exact geometric theory of dendronized polymer dynamics
Advances in Applied Mathematics, 2012
Dendronized polymers consist of an elastic backbone with a set of iterated branch structures (dendrimers) attached at every base point of the backbone. The conformations of such molecules depend on the elastic deformation of the backbone and the branches, as well as on nonlocal (e.g., electrostatic, or Lennard-Jones) interactions between the elementary molecular units comprising the dendrimers and/or backbone. We develop a geometrically exact theory for the dynamics of such polymers, taking into account both local (elastic) and nonlocal interactions. The theory is based on applying symmetry reduction of Hamilton's principle for a Lagrangian defined on the tangent bundle of iterated semidirect products of the rotation groups that represent the relative orientations of the dendritic branches of the polymer. The resulting symmetry-reduced equations of motion are written in conservative form.
Dendronized Polymers: Molecular Objects between Conventional Linear Polymers and Colloidal Particles
ACS Macro Letters, 2014
The term molecular object (MO) is introduced to describe single, shape persistent macromolecules that retain their form and mesoscopic dimensions irrespective of solvent quality and adsorption onto a surface. The concept is illustrated with results concerning homologous series of dendronized polymers (DP). In particular, we discuss imaging experiments quantifying deformation upon adsorption, defect characterization, and atomistic molecular dynamics simulations of DP structure. We argue that MOs such as high generation DP, with their large dimensions and high internal density, provide an opportunity to address fundamental questions regarding the onset of bulk-like behavior in single molecules. Illustrative examples of such questions concern the smallest MO exhibiting a glass transition, glassy behavior or a constant bulk density. The characteristics of DP MO are highlighted by comparison to polymer beads, polymeric micelles, globular proteins, and carbon nanotubes. We discuss future research directions and speculate on possibilities involving multiarmed and toroid DP and the effect of DP on friction and rheology, as well as their utilization for nanoconstruction.
Large Mechanical Response of Single Dendronized Polymers Induced by Ionic Strength
Angewandte Chemie International Edition, 2010
For about a decade, researchers have been unraveling the mechanical properties of single polymer chains. DNA helices were found to respond to stretching forces in a nontrivial fashion depending on their twisting and zipping states. Conformational transitions were identified to determine the characteristic mechanical response of elastic proteins (e.g., titin, tenascin, ankytrin) and of polysaccharides. Mechanical properties of simple neutral and charged polymers of known structure were investigated in detail and compared with ab initio calculations. To use single polymers as building blocks in molecular machines and devices, one must be able to control their mechanical response. For elastic proteins, impressive changes in their elongation by factors of 2-3 were reported in the force range 0.1-0.3 nN as a result of unfolding of individual domains. Amylose shows changes in its elongation of 10-20 % in a similar force range. These changes were interpreted in terms of conformational transitions of the pyranose rings. Although synthetic polymers can be tuned in a reversible fashion by optical excitation, redox properties, and the nature of the solvent, the changes in the elongation that were obtained are only 5-15 % and were observed for forces below 0.1 nN. Synthetic polymers featuring a more significant response, especially at larger forces, have not been described so far.
Bulk properties of dendronized polymers with tailored end-groups emanating from the same backbone
Journal of Polymer Science Part A: Polymer Chemistry, 2005
Dendronized polymers with a methacrylate backbone bearing pendant aliphatic polyester dendrons based on 2,2-bis(methylol)propionic acid have been investigated by rheological measurements, differential scanning calorimetry (DSC), size exclusion chromatography (SEC), and 1 H NMR self-diffusion techniques. The change in material properties due to the attachment of larger dendrons and/or different endgroups to a backbone of the same length is investigated. Dendronized polymers of the second to fourth generation with hydroxyl, acetonide, or hexadecyl end-group functionalities have been studied. DSC revealed that the glass transition temperature of the amorphous polymers increases with increasing size of the dendrons, and that the ability for the hexadecyl functional polymers to crystallize decreases with increasing size of dendrons. 1 H NMR self-diffusion and longitudinal relaxation data are consistent with an elongated rod-like model of the polymers in solution. Larger dendrons lead to a larger rod diameter that approximately double when increasing the generation of dendronized polymer from two to four. Rheological measurements demonstrated that the complex viscosity at low frequency increased with dendron size. Independently of the functionality, the second and third generation samples initially showed a Newtonian plateau, followed by a shear thinning region at higher frequencies. The fourth generation samples only showed shear thinning over the whole frequency region.
Tumbling of polymers in semidilute solution under shear flow
EPL (Europhysics Letters), 2011
The tumbling dynamics of individual polymers in semidilute solution is studied by large-scale non-equilibrium mesoscale hydrodynamic simulations. We find that the tumbling time is equal to the non-equilibrium relaxation time of the polymer end-to-end distance along the flow direction and strongly depends on concentration. In addition, the normalized tumbling frequency as well as the widths of the alignment distribution functions for a given concentration dependent Weissenberg number exhibit a weak concentration dependence in the cross-over regime from a dilute to a semidilute solution. For semidilute solutions a universal behavior is obtained. This is a consequence of screening of hydrodynamic interactions at polymer concentrations exceeding the overlap concentration.
Macromolecules, 1998
Steady shear flow properties of an extensive family of dendrimers were examined for the first time in medium to high concentration solutions. For this, the first seven generations of ethylenediamine (EDA) core-polyamidoamine (PAMAM) dendrimers, having molecular weights from about 500 to almost 60 000 in 30 to 75 wt % solutions in ethylenediamine (EDA) were used. It was found that these dendrimer solutions exhibited typical Newtonian flow behavior as manifested by direct proportionality of the shear stress to the shear rate (i.e., constant viscosity with respect to both shear stress and shear rate) over the entire range of shear stress and shear rate studied. In addition to this, there was no abrupt change in the slope of the shear viscosity vs molecular weight relationship, indicating that these dendrimers do not interpenetrate to form transient quasi-networks of the "entanglement"type typically found for long-chain linear or randomly branched macromolecules, nor do they engage in "sticking" interactions characteristic for the suspensions of idealized spherical particles. This rheological behavior is without precedence among high molecular weight synthetic polymers, and it is proposed that it is solely driven by the unique dendrimer macromolecular architecture which above a certain critical generation results in globular, nanoscopic spheroids whose outer surfaces close upon themselves and become impenetrable for other dendrimers or large molecules. The shear viscosity vs volume fraction dependencies showed that these dendrimers are draining to solvent molecules, but to a lesser extent than the corresponding random-coil type linear macromolecules of comparable molecular weights. These findings are consistent with a "dense-shell" model of dendrimer intramolecular morphology which can also explain their ability to encapsulate small molecular weight species in their "soft and spongy" interiors. From a typical Arrhenius-type temperature dependence of these dendrimer solutions viscosities and from the dependencies of their flow activation energy on molecular weight, it seems that the smallest kinetic unit involved in the dendrimer flow is the dendrimer molecule itself. Strong dependence of the dendrimer solution viscosity on concentration and temperature, as well as its independence on repeated loading, indicates substantial dendrimer flexibility and ability to deform. On the basis of these results and the supporting computer modeling calculations, it is proposed that the Newtonian flow behavior and the lack of an abrupt change of slope in the zero-shear viscosity vs molecular weight relationships represent characteristic "fingerprint" properties for dendrimers in general and that these properties distinguish these unique macromolecules from all other traditional classes of macromolecular architecture. It is also proposed that the critical degree of branching may be used as a defining structural criterion for distinguishing true dendrimers from their low molecular weight simple branched precursors.
Interactions in dendronized polymers: intramolecular dominates intermolecular
Soft Matter, 2014
In an attempt to relate atomistic information to the rheological response of a large dendritic object, interand intramolecular hydrogen bonds and p,p-interactions have been characterized in a dendronized polymer (DP) that consists of a polymethylmethacrylate backbone with tree-like branches of generation four (PG4) and contains both amide and aromatic groups. Extensive atomistic molecular dynamics simulations have been carried out on (i) an isolated PG4 chain and (ii) ten dimers formed by two PG4 chains associated with different degrees of interpenetration. Results indicate that the amount of nitrogen atoms involved in hydrogen bonding is 1111% while 1115% of aromatic groups participate in p,pinteractions. Furthermore, in both cases intramolecular interactions clearly dominate over intermolecular ones, while exhibiting markedly different behaviors. Specifically, the amount of intramolecular hydrogen bonds increases when the interpenetration of the two chains decreases, whereas intramolecular p,pinteractions remain practically insensitive to the amount of interpenetration. In contrast, the strength of the corresponding two types of intermolecular interactions decreases with interpenetration. Although the influence of complexation on the density and cross-sectional radius is relatively small, interpenetration affects significantly the molecular length of the DP. These results support the idea of treating DPs as long colloidal molecules.
The Journal of Chemical Physics, 1993
Molecular dynamics simulations have been performed for a bead-spring model chain of 30 beads immersed in 738 solvent molecules. The solvent-solute interaction energy .sbs has been varied in the range 0.1<~~~<0.8 kcal/mol to assess the role and importance of solvent type on the dynamic and equilibrium properties of the chain. Radial distribution functions for polymer beadsolvent and bead-bead pairs indicate the enhancement of more expanded chain configurations with increasing quality of the solvent. The translational diffusivity D of the chain exhibits an inverse linear dependence on &bs, thus decreasing in the presence of more favorable polymersolvent interactions. Molecular dimensions of the chain such as the mean-square end-to-end distance (3) and the radius of gyration R, are examined in various solvent environment. The ratio (3)/R: approximates the limiting value of 6 corresponding to infinitely long freely jointed chains. A linear dependence of D on l/R, is observed, in conformity with the Zimm theory of dilute polymer solutions subject to hydrodynamic interactions. The orientational motion of internal chain vectors is also found to slow down with increasing strength of intermolecular interactions, in parallel with the translational diffusivity. Characteristic orientational relaxation times T are calculated for chain segments of various sizes IZ, using the initial decay rates of the corresponding orientational autocorrelations functions. These are found to obey a scaling law of the form 7-n' for a given &bs . The exponent a therein decreases with the quality of the solvent, assuming values in the interval l.O<a< 1.5 throughout the investigated range of polymer-solvent interactions.