Relaxation behavior of polyurethane networks with different composition and crosslinking density

Martin Zajac, Heike Kahl, Bernd Schade, Thomas Rödel, Madalena Dionisio, Mario Beiner

Research output: Contribution to journalArticle

10 Citations (Scopus)

Abstract

The relaxation behavior of a series of solvent free polyurethane model networks with variable cross-link density prepared based on different commercial diols and a diisocyanate containing component are studied by differential scanning calorimetry (DSC), dynamic-mechanical analysis (DMA) and dielectric relaxation spectroscopy (DRS). A systematic decrease of the calorimetric glass temperature Tg as well as of the softening temperatures Tα DMA and Tα DRS from relaxation methods is observed with increasing length of the diol sequences between neighbored diisocyanate units acting as cross-linker. This trend is explained based on an internal plasticization of the polymeric network by long, highly mobile diol units containing an increasing fraction of methylene sequences. Cold crystallization effects are only indicated for the longest diol sequence under investigation. This is understood as a consequence of a large fraction of methylene sequences in combination with weaker geometrical constraints. Two secondary relaxations, β and γ, are observed in the glassy state for all amorphous samples at low temperatures by dielectric spectroscopy indicating the existence of localized motions in the polyurethane networks. Below Tg these relaxation processes are practically unaffected by changes in the length of the diol units and the softening behavior of the polymeric model networks. Interrelations between secondary β relaxation and cooperative α dynamics are indicated. An onset of the dielectric α relaxation strength Δεα is observed for all amorphous polyurethane networks. A linear extrapolation of Δεα vs. 1/T gives onset temperatures Ton which are in good agreement with αβ crossover temperatures Tαβ being the temperature where the difference between α and β relaxation times τα−τβ approaches a minimum. This finding supports an onset of the cooperative α motions in the αβ crossover region as reported in the previous literature for many other glass forming materials.

Original languageEnglish
Pages (from-to)83-90
Number of pages8
JournalPolymer (United Kingdom)
Volume111
DOIs
Publication statusPublished - 24 Feb 2017

Fingerprint

Polyurethanes
Crosslinking
Dielectric relaxation
Chemical analysis
Dynamic mechanical analysis
Temperature
Spectroscopy
Glass
Dielectric spectroscopy
Relaxation processes
Crystallization
Extrapolation
Relaxation time
Differential scanning calorimetry

Keywords

  • Dielectric spectroscopy
  • Glass transition
  • Polyurethanes
  • Relaxation behavior

Cite this

Zajac, Martin ; Kahl, Heike ; Schade, Bernd ; Rödel, Thomas ; Dionisio, Madalena ; Beiner, Mario. / Relaxation behavior of polyurethane networks with different composition and crosslinking density. In: Polymer (United Kingdom). 2017 ; Vol. 111. pp. 83-90.
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Relaxation behavior of polyurethane networks with different composition and crosslinking density. / Zajac, Martin; Kahl, Heike; Schade, Bernd; Rödel, Thomas; Dionisio, Madalena; Beiner, Mario.

In: Polymer (United Kingdom), Vol. 111, 24.02.2017, p. 83-90.

Research output: Contribution to journalArticle

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T1 - Relaxation behavior of polyurethane networks with different composition and crosslinking density

AU - Zajac, Martin

AU - Kahl, Heike

AU - Schade, Bernd

AU - Rödel, Thomas

AU - Dionisio, Madalena

AU - Beiner, Mario

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N2 - The relaxation behavior of a series of solvent free polyurethane model networks with variable cross-link density prepared based on different commercial diols and a diisocyanate containing component are studied by differential scanning calorimetry (DSC), dynamic-mechanical analysis (DMA) and dielectric relaxation spectroscopy (DRS). A systematic decrease of the calorimetric glass temperature Tg as well as of the softening temperatures Tα DMA and Tα DRS from relaxation methods is observed with increasing length of the diol sequences between neighbored diisocyanate units acting as cross-linker. This trend is explained based on an internal plasticization of the polymeric network by long, highly mobile diol units containing an increasing fraction of methylene sequences. Cold crystallization effects are only indicated for the longest diol sequence under investigation. This is understood as a consequence of a large fraction of methylene sequences in combination with weaker geometrical constraints. Two secondary relaxations, β and γ, are observed in the glassy state for all amorphous samples at low temperatures by dielectric spectroscopy indicating the existence of localized motions in the polyurethane networks. Below Tg these relaxation processes are practically unaffected by changes in the length of the diol units and the softening behavior of the polymeric model networks. Interrelations between secondary β relaxation and cooperative α dynamics are indicated. An onset of the dielectric α relaxation strength Δεα is observed for all amorphous polyurethane networks. A linear extrapolation of Δεα vs. 1/T gives onset temperatures Ton which are in good agreement with αβ crossover temperatures Tαβ being the temperature where the difference between α and β relaxation times τα−τβ approaches a minimum. This finding supports an onset of the cooperative α motions in the αβ crossover region as reported in the previous literature for many other glass forming materials.

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