Growth and self-assembly of macromolecular systems/

In the first part of this thesis, we consider the physical phenomena accompanying the growth of highly branched polymers through computer simulation. The resultant dendritic molecules have unique properties arising from their modified Cayley tree branching pattern. We first consider growth of starburst molecules with flexible spacers separating tri-functional branch points. A self-avoiding walk algorithm is employed to kinetically grow the molecules. From the intramolecular density profiles of these structures, we find that the branch ends traverse the molecule throughout growth and are not confined to its surface. Further we observe that the branches are highly folded at all stages of growth. We observe power law relationships correlating the radius of gyration of the molecule to its molecular weight, M, and spacer length, P, finding in general: R\sb{g}\sim M\sp\rho P\sp\nu with $\rho=0.22\pm 0.05$ and $\nu=0.50\pm 0.05$ at high molecular weights. From this we predict the hydrodynamic characteristics of the molecule. We then explore generalizations of the starburst structure by considering first the effect of branch stiffening, and second, the effect of changes in dendrimer connectivity by considering a related structure, the comb-burst. We repeat our study described above for these structures. In general we observe similar behavior to that described above, however slightly modified due to the structural modifications employed. The second part of this thesis addresses polymeric systems exhibiting the phenomenon of self-assembly. The specific problem under consideration is the characterization of phase transitions in diblock copolymer systems using density functional theory. We present a comprehensive, general scheme which allows the characterization of microphase separation of A-B diblock copolymer systems in terms of observed physical phenomena at all degrees of segregation. Our method is based on the density functional theory of Melenkevitz and Muthukumar and uses the technique of density profile parameterization to greatly reduce the technical complexity of the solution. We find that the microphase separated systems pass through three stages of ordering as the system is quenched. These are the weak, intermediate, and strong segregation regimes. We have calculated the phase diagram for three ordered morphologies: lamellae, hexagonally-packed cylinders, and body-centered-cubic spheres. We also characterize these microphases by the dependence of the lattice constant, D, and the interfacial width, \sigma\sb{o}, on the quench parameter $\chi N.$ We correctly reproduce the behavior predicted by previous theories describing the weak and strong segregation regimes. Through investigation of D\sim N\sp\alpha, we find that $\alpha$ depends on both block length and morphology in the intermediate segregation regime. We attribute this behavior to chain stretching arising from the phenomenon of localization

Soft Interaction Between Dissolved Dendrimers: Theory and Experiment

Using small-angle neutron scattering and liquid integral equation theory, we relate the structure factor of flexible dendrimers of 4th generation to their average shape. The shape is measured as a radial density profile of monomers belonging to a single dendrimer. From that, we derive an effective interaction of Gaussian form between pairs of dendrimers and compute the structure factor using the hypernetted chain approximation. Excellent agreement with the corresponding experimental results is obtained, without the use of adjustable parameters. The present analysis thus strongly supports the previous finding that flexible dendrimers of low generation present fluctuating structures akin to star polymers.Comment: 20 pages, 4 figures, submitted to Macromolecules on July 24, 200

Interfaces in Diblocks: A Study of Miktoarm Star Copolymers

We study AB$_n$ miktoarm star block copolymers in the strong segregation limit, focussing on the role that the AB interface plays in determining the phase behavior. We develop an extension of the kinked-path approach which allows us to explore the energetic dependence on interfacial shape. We consider a one-parameter family of interfaces to study the columnar to lamellar transition in asymmetric stars. We compare with recent experimental results. We discuss the stability of the A15 lattice of sphere-like micelles in the context of interfacial energy minimization. We corroborate our theory by implementing a numerically exact self-consistent field theory to probe the phase diagram and the shape of the AB interface.Comment: 12 pages, 11 included figure

Exploiting dense shell/packing principles to invoke stereoselectivity in a reaction accelerated by a chiral dendrimer

As dendrimers approach their dense shell or dense packed limit, a certain amount of conformational organization exists. Any substrate binding within the dendrimer’s external layerwill experience the same organizational effects. This paper describes how these effects can be exploited towards stereocontrol with respect to binding and reactivity

Deformability of poly(amidoamine) dendrimers

Experimental data indicates that poly(amidoamine) (PAMAM) dendrimers flatten when in contact with a substrate, i.e. they are no longer spherical, but resemble flat disks. In order to better understand the deformation behavior of these branched polymers, a series of atomistic molecular dynamics simulations is performed. The resulting flattened dendrimer conformations are compared to atomic force microscopy (AFM) images of individual dendrimers at air/mica and water/mica interfaces. The ability of the polymers to deform is investigated as a function of dendrimer generation (2-5) and the required energies are calculated. Our modeling results show good agreement with the experimental AFM images, namely that dendrimers are highly flexible and capable of forming multiple interaction sites between most of their branch ends and the substrate. The deformation energy scales with dendrimer generation and does not indicate an increase in stiffness between generations 2 and 5 due to steric effects.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45831/1/10189_2003_Article_10087.pd

Architecturally-induced tricontinuous cubic morphology in compositionally symmetric miktoarm starblock copolymers

We report the synthesis and morphological characterization of a miktoarm block copolymer architecture: (PSαM-PIM)n-(PSM-PI αM)n, where M ∼ 20 000, n = 1, 2, and the arm asymmetry parameter α = 1, 2, or 4 (α is the ratio of the outer block molecular weight to that of the inner block). These block copolymers are symmetric in overall composition and exhibit n- and α-dependent microdomain morphologies. Alternating lamellae are observed for linear tetrablocks (n = 1), α = 1, 2, 4, and for inverse starblock (n = 2), α = 1, 2. An architecturally-induced morphological transition from lamellae to a tricontinuous cubic structure is observed with n = 2 and α = 4. The formation of the tricontinuous cubic microdomain structure in this compositionally symmetric system is thought to relieve the overcrowding of the four peripheral PS-PI junctions by providing a curved intermaterial dividing surface with a triply periodic microdomain structure, allowing some bridging by the interior blocks of the miktoarm star