Dendrimer structure SANS

Structural analysis of dissolved dendrimers with SANS and SAXS... 

The intramolecular stmctural organization of a dendritic macromolecule in solution remained a matter of debate for a long time. The first model proposed by de Gennes and Hervet predicted a hollov/ dendrimer structure, wherein both the local polymer density and the degree of stretching of the spacers increased in the radial direction from the center to the periphery of the dendrimer. The hollow dendrimer model suggested to use dendrimers as molecular nanocontainers that may be loaded by active substances in dmg-delivery applications. However, SANS and SAXS unambiguously indicated a tmiform distribution of polymer density in the intramolecular volume of dendritic macromolecules. These experimental results were supported by simulations and numerical SCF theory. 

SANS is a valuable research technique, since it provides information about the size and shape of the molecular scale structures in the size range 1-10 nm found in polymer or dendrimer systems. Neutrons interact with atomic nuclei, hence are sensitive not only to the details of the way such nuclei are arranged but also to the presence of different isotopes. 

We have presented a new method of using SANS to elucidate the radial structure of dissolved dendrimers [23, 24]. It has been demonstrated that SANS in conjunction with contrast variation [32-37] is a valid tool to determine the internal structure of dendrimers. The main result of [23,24] is the clear proof that the density distribution has its maximum at the center of the molecule. Hence this corroborates the general deductions of theory as discussed in the preceding section. 

The discussion in Sect. 3.3 has suggested that the intensity measured in SANS-experiments contains an appreciable contribution not related to the structure of the dissolved dendrimers. Only the part Isiq) (see Eq. 4) is connected to the average spatial structure as expressed through the shape function T(r). Figure 6... 

Most studies performed partly on molecular models [33] but also on real POPAM and PAMAM dendrimers support the latter model concept [34]. Careful studies on the three-dimensional structure of flexible dendrimers in solution were performed by Ballauff et al. by means of SANS (Small Angle Neutron Scattering) [35] (see Section 7.6). 

One particular strength of SANS over SAXS is the differing scattering behaviour of isotopes towards neutrons, which is particularly pronounced for the hydrogen isotopes H and D. The true value of the SANS technique for structure analysis of dendrimers is due to this phenomenon. Thus, by deuterium label-... 

Controversial discussions have surrounded the three-dimensional structure of flexible dendrimers in solution during the past two decades. Now, on the basis of numerous SANS experiments using the contrast variation technique, the idea that isolated flexible dendrimers in good solvents do not take on the originally predicted dense shell structure but instead assume a dense core structure [47] appears to be gaining general acceptance. This means that in such dendrimers the segment density reaches a maximum at the centre of the molecule and decreases towards the periphery (cf. Fig. 7.6). 

The information gained by SANS and SAXS - frequently also in combination with simulations and theoretical analyses - can contribute to a more fundamental understanding of the three-dimensional structure, dynamics, and interactions of flexible dendrimers in solution and affords important information about the relation between size, shape, and internal composition of such dendrimers. The knowledge acquired is also valuable for the postulation of structure-reactivity relations. Such knowledge can help in better predicting the behaviour of flexible dendrimers in solution and their suitability for certain applications,... 

Some data have also appeared for the first five generations of the smaller DAB(CN)X and DAB(NH2)X dendrimers (see Scheme 1 for structure). For DAB(CN)X in acetone-d6 SANS yields v=0.31 (generations 2-5). For DAB(NH2)X in D20 the low MW dendrimers follow Rg M0-30 [71]. In general, it is difficult to determine Rg of low MW dendrimers accurately [70]. Furthermore, all dendrimers studied have ionizable groups and may act like polyelectrolytes. Therefore solvent conditions need to be carefully controlled and specified. 

Fig. 15. Schematic representation of the model nanomolecular composite structure from SANS and SAXS measurements. Left, individual dendrimer molecule with CuS trapped at its periphery. Right, multimolecular cluster of individual dendrimer/CuS particles. Reprinted with permission from [49], Copyright 2002 Elsevier Science...

By using dendrimers as templates, dendrimer-metal nanoparticles are synthesized in aqueous and non-aqueous media. Particle sizes of metals are significantly affected by many factors including concentration of dendrimers and generation of dendrimers. In particular, structure of dendrimer-metal nanoparticles is characterized by TEM, SANS, and SAXS. 

A pronounced influence of pH and added electrolyte on the charge interactions between dendrimers with ionizable groups is expected in aqueous solution. Briber et al. ° investigated the intermolecular interactions between PAMAM dendrimers in concentrated solutions using SANS and SAXS techniques. They showed that the molecules develop large scale interactions which can be screened by the addition of excess acid or electrolyte. SANS studies of the structural... 

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