Unexpected Temperature Behavior of Polyethylene Glycol Spacers in Copolymer Dendrimers in Chloroform

We have studied copolymer dendrimer structure: carbosilane dendrimers with terminal phenylbenzoatemesogenic groups attached by poly(ethylene) glycol (PEG) spacers. In this system PEG spacers areadditional tuning to usual copolymer structure: dendrimer with terminal mesogenic groups. Thedendrimer macromolecules were investigated in a dilute chloroform solution by 1H NMR methods(spectra and relaxations). It was found that the PEG layer in G = 5 generations dendrimer is “frozen”at high temperatures (above 260 K), but it unexpectedly becomes “unfrozen” at temperatures below250 K (i.e., melting when cooling). The transition between these two states occurs within a smalltemperature range (~10 K). Such a behavior is not observed for smaller dendrimer generations (G = 1and 3). This effect is likely related to the low critical solution temperature (LCST) of PEG and is caused bydendrimer conformations, in which the PEG group concentration in the layer increases with growing G.We suppose that the unusual behavior of PEG fragments in dendrimers will be interesting for practicalapplications such as nanocontainers or nanoreactors.</p

Lysine Dendrigraft Nanocontainers. Influence of Topology on Their Size and Internal Structure

Poly-l-ysine dendrigrafts are promising systems for biomedical applications due to their biodegradability, biocompatibility, and similarity to dendrimers. There are many papers about the use of dendrigrafts as nanocontainers for drug delivery. At the same time, the number of studies about their physical properties is limited, and computer simulations of dendrigrafts are almost absent. This paper presents the results of a systematic molecular dynamics simulation study of third-generation lysine dendrigrafts with different topologies. The size and internal structures of the dendrigrafts were calculated. We discovered that the size of dendrigrafts of the same molecular weight depends on their topology. The shape of all studied dendrigrafts is close to spherical. Density profile of dendrigrafts depends on their topology

Mechanical relaxation of functionalized carbosilane dendrimer melts

International audienceFunctionalizing the internal structure of classical dendrimers is a new way of tailoring their properties. Using atomistic molecular dynamics simulations, we investigate the rheological behavior of functionalized dendrimers (FD) melts obtained by modifying the branching of carbosilane dendrimers (CSD). The time (relaxation modulus G(t)) and frequency (storage G' and loss G" moduli) dependencies of the dynamic modulus are obtained. Fourth generation FD melts present a region where G' > G". In contrast, their non-functionalized counterparts (i.e., classical dendrimers with a regular branching) do not show such a region. The comparative analysis of FD and CSD suggests that the internal densification due to functionalization prevents the penetration of branches and causes FD to behave like colloidal particles in a crowded environment. Since CSD have no special interactions, we expect that this effect will be common for other dendrimer macromolecules

Influence of the Chemical Structure on the Mechanical Relaxation of Dendrimers

The rheological properties of macromolecules represent one of the fundamental features of polymer systems which expand the possibilities of using and developing new materials based on them. In this work, we studied the shear-stress relaxation of the second generation PAMAM and PPI dendrimer melts by atomistic molecular dynamics simulation. The time dependences of relaxation modulus G(t) and the frequency dependences of the storage G′(ω) and loss G″(ω) moduli were obtained. The results were compared with the similar dependences for the polycarbosilane (PCS) dendrimer of the same generation. The chemical structure of the dendrimer segments has been found to strongly influence their mechanical relaxation. In particular, it has been shown that hydrogen bonding in PAMAM dendrimers leads to an entanglement of macromolecules and the region is observed where G′(ω) > G″(ω). This slows down the mechanical relaxation and rotational diffusion of macromolecules. We believe that our comprehensive research contributes to the systematization of knowledge about the rheological properties of dendrimers