The Influence of Dendrimers' Structure on the Molecular Dynamics
The comparison of the NMR relaxometry results on several types of LC dendrimers presented before provides a significant insight on the relation between the molecular architectures, the resulting mesophases' structural properties and the way they influence the molecular movements in a wide range of timescales (from milliseconds to picoseconds).
The most general and evident conclusion from the investigations on the side-chain dendrimers presented in this work is the absence of the contribution of molecular self-diffusion movements to the proton NMR spin-lattice relaxation rate. This is particularly interesting, considering the variety of dendrimers' molecular structures, generations and mesophases studied. The investigations include generations G0 (organosiloxane tetrapodes),
G1 (organosiloxane octopodes and PAMAM derivatives), and G3 (PAMAM LC codendrimer), and mesophases: nematics (Nu, Nb), smectics (SmA, SmAd, SmC, SmCd), and columnar (Colh, Colr). As referred before, this effect is due to the molecular interdigitation between mesogenic units belonging to neighboring dendrimers. The resulting entanglement within the dendrimers' mesophases strongly restrict diffusive movements of the whole dendrimers. Nevertheless, the diffusion of dendritic molecules, mediated by successive displacement of the dendritic arms is not excluded, while probably too slow to be experimentally observed by the method presented herein. This conclusion is particularly evident when the mesophases of the G0 dendrimers (tetrapodes) are compared with the similar phases of the corresponding monomers. It is important to remind that the relaxation results on the nematic phase of the monomers are similar to those observed in other low- molecular-weight liquid crystals, including the typical observation of the molecular self-diffusion process. However, the single (but determinant) difference between the monomers and the tetrapodes phases is the linking of the monomers to the central silicon atom (dendritic core), which determines a completely distinct NMR relaxation behavior, remarkably evident at the frequency range associated to self-diffusion. This is a clear indication that the dendritic structure is indeed responsible for the strong restriction of diffusive movements.
Also as a general conclusion, it was observed that in the high- Larmor-frequency domain (typically for ш/(2п) > 10 MHz) the proton spin-lattice relaxation rate is dominated by contributions compatible with BPP models. This is consistent with the occurrence of molecular rotations/reorientations of the dendritic terminal mesogenic units. In particular, the order of magnitude of the characteristic correlation times and the prefactors associated to interproton distances are similar with those of low-molecular-weight liquid crystals. Moreover, in several cases, it was observed that the temperature dependences of those BPP mechanisms are compatible with Arrhenius-type laws, with values of activation energies close to those found in low molecular mass LCs. Those Arrhenius- type dependences are, in many cases, independent of the phase transition temperatures, meaning that the same Arrhenius law is valid through a temperature range including different mesophases. This is clearly an indication that the molecular rotation/reorientation mechanism is essentially defined by local molecular conditions, independent of the long distance structure of the mesophase.
Contrary to what is observed with respect to the high-frequency regime, the low-frequency dependence of the relaxation rate, which is associated with collective movements, is normally dependent of the long-distance phase organization. Typically, dependences of the type T-1 & o-1/2 and T-1 & o-1, are observed in nematic and smectic mesophases, respectively. In columnar phases a different type of frequency dependence, corresponding to a mechanism described as columnar elastic deformations ECD, is found (see Eq. 9.20).
The effect of generations was also studied by comparing the SmA phases exhibited by G1 and G3 PAMAM dendrimers. As could be expected, the contribution of local rotations/reorientations of mesogenic units is practically independent of the generation, confirming the determinant influence of the local molecular conditions on this type of movements. The collective movements are described as LUs in both 1 and 3 generation dendrimers. The low cutoff frequencies detected in the two generations indicate that the number of dendrimers contributing to the layers undulations is the roughly the same independently of the dendrimers' generation.
However, the relation between the low-frequency dependence of the relaxation rate and the long-range molecular organization is not completely strict. Actually an interesting exception becomes evident from the comparison between G0 dendrimers with terminally (end- on) and laterally attached (side-on) mesogenic units. In the case of the organosiloxane end-on tetrapodes with a strong terminal dipole referred before, partial bilayer SmAd and SmCd are formed and the NMR relaxation data reveal an evident contribution of the LU mechanism typically observed in low-molecular-weight liquid crystals. However, in the case of the organosiloxane tetrapodes with laterally attached mesogens, the collective movements observed in the SmC phase are similar to those detected in the nematic phase (ODF). As discussed before, this effect is also extended to the isotropic phase and is. determined by the very particular phase structure of organosiloxane tetrapodes with a peculiar temperature- persistent cybotactic structure.
Based on the NMR investigations referred in this chapter, it is possible to summarize the main conclusions on the molecular dynamics of LC dendrimers in a single sentence, by stating that molecular self-diffusion is not detected, the dendrimers molecular structure determines with particular incidence the collective movements associated with the mesophase long-range organization, and the high-frequency regime is dominated by local rotational reorientation movements of the functional mesogenic units.
Taylor & Francis
Taylor Si Francis Croup