Investigation of Melts of Polybutylcarbosilane Dendrimers by 1H NMR Spectroscopy

Abstract

Melts of polybutylcarbosilane (PBC) dendrimers from third (G3) up to sixth (G6) generations are investigated by 1H NMR spectroscopy in a wide temperature range up to 493 K. At room temperature, NMR spectra of G3-G5 dendrimers exhibit resolved, solution-like spectra (“liquid” phase). In contrast, the spectrum of the G6 dendrimer is characterized by a single unresolved broad line at whole temperature range, which supports the presence of an anomalous phase state of G6 at temperatures higher than glass transition temperature. For the first time, an unexpected transition of G5 dendrimer from a molecular liquid state to an anomalous state/phase upon temperature increase has been detected using NMR data. Specifically, an additional wide background line appears in the G5 spectrum above 473 K, and this line corresponds to a G5 state characterized by restricted molecular mobility, i.e., a state similar to the “anomalous” phase of G6 melt. The fraction of the G5 dendrimers in “anomalous” phase at 493 K is approximately 40%. Analysis of the spectral shapes suggests that changes in the G5 dendrimers are reversible with temperature.

NMR studies of excluded volume interactions in peptide dendrimers

“…However, the semiflexibility of dendrimers results in suppression of EV effects on the orientational mobility in the dendrimer 38 . We would also like to note recent studies of orientation mobility in dendrimer melts 46 – 49 .…”

A Comparative Study of Intramolecular Mobility of Single Siloxane and Carbosilane Dendrimers via Molecular Dynamics Simulations

“…Along with single dendrimers in dilute solutions [14,15,16,17,18,19,20,21,22,23], much attention is attracted nowadays to concentrated solutions and melts [24,25,26,27,28,29,30,31,32,33,34], where intermolecular interactions of dendrimer molecules are significant. It is expected that the tree-like molecular morphology of dendrimers would distinguish their behavior from that of conventional linear polymers.…”

“…The transition between these states is very sharp; it is realized within one generation step (from the fifth to the sixth generation), and it is accompanied by a six-orders-of-magnitude jump of the melt viscosity [31]. The dendrimer specificity also manifests itself in NMR spectra of melts of polybutylcarbosilane dendrimers from the fourth to the sixth generation [32]; in particular, the spectrum of the generation-6 (G6) dendrimers is characterized by a single unresolved broad line in the whole temperature range above the glass transition temperature, which suggests the presence of an anomalous phase state of G6 dendrimers with restricted molecular mobility. A similar anomalous phase state has been detected in generation-5 (G5) dendrimers at enhanced temperatures above 473 K [32].…”

“…The dendrimer specificity also manifests itself in NMR spectra of melts of polybutylcarbosilane dendrimers from the fourth to the sixth generation [32]; in particular, the spectrum of the generation-6 (G6) dendrimers is characterized by a single unresolved broad line in the whole temperature range above the glass transition temperature, which suggests the presence of an anomalous phase state of G6 dendrimers with restricted molecular mobility. A similar anomalous phase state has been detected in generation-5 (G5) dendrimers at enhanced temperatures above 473 K [32]. The physical reason for this behavior is still unclear, and further studies are needed.…”

Linear Viscoelasticity of Carbosilane Dendrimer Melts

“…This indicates the growing contribution of the interactions between the macromolecules. Interestingly, very recent experimental works , demonstrate that CSD melts of higher molecular masses can show a network formation. Here, at G4, however, we are still far from such effects neither from the entanglements, by which the moduli show for hyperbranched polymers a qualitatively distinct behavior .…”