Phase behaviour and morphology of high hard block content polyurethanes

 

Thermoplastic Polyurethanes (TPU) are linear block copolymers typically constructed of statistically alternating soft (SS) and hard (HS) segments. Due to their numerous industrial applications these materials have received considerable attention. Many characterisation techniques have been used to try to understand the relationship between chemical architecture, morphology and mechanical properties of TPUs. Their versatile physical properties are usually attributed to their microphase separated structure deriving from the thermodynamic incompatibility between the soft and hard segments

      

TEM micrographs of sample annealed at 120 ºC for 72 hours without not sheared (left and centre) and sheared on cooling (right)

In this project we have investigated the morphology and phase behaviour of a set of linear model polyurethanes (PU) with a high content of hard segments: from 65 to 100% (wt.). The investigation was carried out using mainly differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Through this work we are able to elucidate the origin on the multiple melting endotherms observed via DSC as well as propose a new morphological model for high hard block content TPUs.

 

Proposed morphological models for the melt-quenched samples (top) and annealed samples (bottom)

The investigation of the melt-quenched samples suggested that these samples have after melt-quenching a two phase morphology one "pure" hard segment phase co-existing with a mixed phase with a hard segment content of 65 % (wt.). When annealed at 120 ºC the mixed phase undergoes microphase separation resulting in a phase separated morphology with the same interdomain distance (di ~ 15 nm) for all the samples. The two high temperature melting endotherms observed via DSC were assigned to the melting of an ordered structure appearing in the hard segment phase and to the microphase mixing of the soft and hard segments.

 

Publications:

Origin of multiple melting endotherms in a high hard block content polyurethane: Effect of annealing temperature; A. Saiani, A. Novak, L. Rodier, G. Eeckhaut, J.-W. Leenslag, J.S. Higgins; Macromolecules, 40, 7252-7262 (2007)

Origin of multiple melting endotherms in a high hard block content polyurethane: 2. Structural investigation, A. Saiani, W.A. Daunch, G. Eeckhaut, J.-W. Leenslag, J.S. Higgins  Macromolecules, 37, 1411-1421 (2004)

Origin of multiple melting endotherms in a high hard block content polyurethane: 1. Thermodynamic investigation; A. Saiani, W.A. Daunch, H. Verbeke, J.-W. Leenslag, J.S. Higgins; Macromolecules, 34, 9059-9068 (2001)

 

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