Effect of Metal-Ligand Coordination Complexes on Molecular Dynamics and Structure of Cross-Linked Poly(dimethylosiloxane)

Abstract

Poly(dimethylosiloxane) (PDMS) cross-linked by metal-ligand coordination has a potential functionality for electronic devices applications. In this work, the molecular dynamics of bipyridine (bpy)–PDMS-MeCl2 (Me: Mn2+, Fe2+, Ni2+, and Zn2+) are investigated by means of broadband dielectric spectroscopy and supported by differential scanning calorimetry and density functional theory calculations. The study of molecular motions covered a broad range of temperatures and frequencies and was performed for the first time for metal-ligand cross-linked PDMS. It was found that the incorporation of bpy moieties into PDMS chain prevents its crystallization. The dielectric permittivity of studied organometallic systems was elevated and almost two times higher (ε′ ~4 at 1 MHz) than in neat PDMS. BpyPDMS-MeCl2 complexes exhibit slightly higher glass transition temperature and fragility as compared to a neat PDMS. Two segmental type relaxations (α and αac) were observed in dielectric studies, and their origin was discussed in relation to the molecular structure of investigated complexes. The αac relaxation was observed for the first time in amorphous metal-ligand complexes. It originates from the lower mobility of PDMS polymer chains, which are immobilized by metal-ligand coordination centers via bipyridine moieties.

Effects of Counter Anions on AC and DC Electrical Conductivity in Poly(Dimethylsiloxane) Crosslinked by Metal-Ligand Coordination

“…All bpyPDMS-MeX 2 systems displayed elevated dielectric permittivity, depending on the type of cation originating from the metal salt; the dielectric permittivity increased in this order: neat PDMS < Ni 2+ < Mn 2+ < Zn 2+ . The highest dielectric permittivity, almost twice as high as for neat PDMS, was observed for bpyPDMS-ZnCl 2 , which is in accordance with our previous findings [10].…”

“…Their chemical structures are presented in Figure 1 . All complexes were synthesized from detailed descriptions of their synthesis reported elsewhere [ 10 ].…”

“…Nevertheless, this approach leads to large increases in the glass transition temperatures ( T g ) of the resulting materials, making them brittle [ 3 , 6 ]. An alternative approach for increasing the complex dielectric permittivity of a polymer without an undesirable increase in its stiffness is based on the introduction of a dynamic non-covalent metal-ligand coordination bond into the polymer matrix [ 8 , 9 , 10 ]. Since 2011, when the first paper on self-healing polymers based on metal-ligand interactions was publish by the group of Weder [ 11 ], great progress in these materials has been achieved [ 12 ].…”

“…In our previous work [ 10 ], we reported syntheses of a series of metallopolymers based on polydimethylsiloxane (PDMS) crosslinked by metal (Ni 2+ , Mn 2+ , Zn 2+ )–bipyridine (bpy) coordination with Cl − as a counter anion and investigated their dielectric properties and molecular dynamics. These materials have almost two times higher dielectric permittivity than neat PDMS.…”

“…The nature of the cation used directly affected the dielectric properties of the materials obtained due to the dipolar nature of the coordination bonds [ 13 , 14 ]. Nevertheless, this approach requires the introduction of metal cations into the polymer systems in the form of metal salts, for example, halides, tetrafluoroborates, or perchlorates [ 8 , 10 , 15 ]. For this reason, free remnant counter anions can noticeably affect the conductivity of such materials.…”