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

Abstract

<p>We have studied copolymer dendrimer structure: carbosilane dendrimers with terminal phenylbenzoate<br />mesogenic groups attached by poly(ethylene) glycol (PEG) spacers. In this system PEG spacers are<br />additional tuning to usual copolymer structure: dendrimer with terminal mesogenic groups. The<br />dendrimer macromolecules were investigated in a dilute chloroform solution by 1H NMR methods<br />(spectra and relaxations). It was found that the PEG layer in G = 5 generations dendrimer is “frozen”<br />at high temperatures (above 260 K), but it unexpectedly becomes “unfrozen” at temperatures below<br />250 K (i.e., melting when cooling). The transition between these two states occurs within a small<br />temperature range (~10 K). Such a behavior is not observed for smaller dendrimer generations (G = 1<br />and 3). This effect is likely related to the low critical solution temperature (LCST) of PEG and is caused by<br />dendrimer conformations, in which the PEG group concentration in the layer increases with growing G.<br />We suppose that the unusual behavior of PEG fragments in dendrimers will be interesting for practical<br />applications such as nanocontainers or nanoreactors.<br /><br /></p>