𝔖 Bobbio Scriptorium
✦   LIBER   ✦

Structure and molecular dynamics of hyperbranched polymeric systems with urethane and urea linkages

✍ Scribed by A. Kyritsis; K. Raftopoulos; M. Abdel Rehim; Sh. Said Shabaan; A. Ghoneim; G. Turky


Publisher
Elsevier Science
Year
2009
Tongue
English
Weight
574 KB
Volume
50
Category
Article
ISSN
0032-3861

No coin nor oath required. For personal study only.

✦ Synopsis


Two series of hyperbranched polymers (HP), polyurethanes and polyureas, with aromatic and aliphatic structures, are synthesized in one-pot method using commercially available monomers. The obtained HP samples were characterized by 1 H Nuclear Magnetic Resonance (NMR) spectroscopy, Gel Permeation Chromatography (GPC), Differential Scanning Calorimetry (DSC) and Thermal Gravimetric Analysis (TGA) measurements. Molecular dynamics in these systems were investigated by combining Thermally Stimulated Depolarization Currents (TSDC) and broadband Dielectric Relaxation Spectroscopy (DRS) techniques. High conductivity contribution in dielectric loss does not allow the study of the segmental a relaxation associated with the glass transition. In the glassy state two secondary relaxation mechanisms have been investigated, the g and the b mechanism. The g relaxation mechanism, at low temperatures/high frequencies, is attributed to motions of the end groups (-OH for polyurethanes and -NH 2 for polyureas), and has been found faster in the hyperbranched polyureas. In addition, our results reveal that g relaxation mechanism in both series depends on the chemical structure, being faster for aliphatic structures. The b relaxation mechanism, at higher temperatures/lower frequencies, is attributed to the motions of branched ends with polar groups. Our study suggests that this mechanism may be a typical relaxation process for hyperbranched polyurethanes structures, not existed in the linear counterparts. All the systems exhibit dc conductivity at temperatures higher than T g which shows Arrhenius-like temperature dependence and is characterized by rather high activation energies (in the order of 200 kJ/mol). At temperatures lower than T g all the systems studied exhibit remarkably high charge mobility. In particular, aliphatic hyperbranched polyureas exhibit dc conductivity which has been found to be of VTF type concerning the temperature dependence. This result implies that the conduction mechanism is coupled with molecular motions in the glassy state of the polymer.


πŸ“œ SIMILAR VOLUMES


Studies of the molecular dynamics in pol
✍ Przemyslaw Czech; Lidia Okrasa; Jacek Ulanski; Gisele Boiteux; Francoise Mechin; πŸ“‚ Article πŸ“… 2007 πŸ› John Wiley and Sons 🌐 English βš– 657 KB

## Abstract Several new polyurethane networks based on hyperbranched polyesters (trade name Boltorn) were synthesized to investigate the influence of the hyperbranched crosslinking agent on the molecular dynamics of the linear segments containing urethane groups. For comparison, linear polyurethane

Mikto-Arm Star Copolymers with Hyperbran
✍ Koji Ishizu; Yuya Furuta; Nobuyuki Okamoto; Satoshi Uchida; Masaaki Ozawa πŸ“‚ Article πŸ“… 2010 πŸ› John Wiley and Sons 🌐 English βš– 214 KB

## Abstract Living free‐radical copolymerization of the inimer 4‐vinylbenzyl __N,N‐__diethyldithiocarbamate with maleic anhydride in the presence of methyl methacrylate produces highly branched poly(methyl methacrylate) star polymers. Subsequently, mikto‐arm stars can be synthesized by condensation

Molecular dynamics of 18-crown-6 complex
✍ E.T.H. Leuwerink; S. Harkema; W.J. Briels; D. Feil πŸ“‚ Article πŸ“… 1993 πŸ› John Wiley and Sons 🌐 English βš– 642 KB

## Abstract Complexes of 18‐crown‐6 with alkali–metal cations (Na^+^, K^+^, and Rb^+^), urea, and the uncomplexed crown ether were studied __in vacuo__ with the molecular dynamics method. Conformational data from these calculations (simulation times in the range from 6–15 ns) was compared with info