Strain-induced structural and dynamic changes in
segmented polyurethane elastomers
Wang, FF (Wang, Fenfen)[ 1 ] ; Chen, SL (Chen, Shengli)[ 1 ] ; Wu, Q (Wu, Qiang)[ 1 ] ; Zhang, RC (Zhang, Rongchun)[ 2,3 ] ; Sun, PC (Sun, Pingchuan)[ 1,2 ]
POLYMER, 2019, 163: 154-161
DOI: 10.1016/j.polymer.2018.12.062
Abstract
Polyurethane
elastomers have been widely used in the industry and daily life due to their
versatile physical and chemical properties. Therefore, a fundamental
understanding of the structures and dynamics at a molecular level will provide
piercing insights into the precise design and application of new polyurethane
materials. In this study, we mainly focused on investigating the strain-induced
structural and dynamic changes in a typical polyurethane elastomer composed of
poly(epsilon-caprolactone) (PCL) and 4,4'-diphenylmethylene diisocyanate (MDI)
as the soft and hard segments, respectively. Obvious strain-hardening
phenomenon was observed during the mechanical tensile test, and a systematic
comparison was performed on the fractured and pristine samples. DSC results
revealed that the crystallization of PCL chains was still going on after the
sample was fractured, and the crystallite structures became stable after
physical aging at 25 degrees C for two days. H-1 solid-state nuclear magnetic
resonance (NMR) experiment was further employed to determine the fraction of
mobile PCL chains that were converted to crystallites during stretching.
Besides, the microphase separation was also significantly enhanced in the
fractured sample. The mobility of amorphous PCL chains was largely reduced due
to the strain-induced crystallization of nearby PCL segments, as revealed by
the H-1 magic-sandwich echo (MSE) NMR experiments. H-1 multiple quantum (MQ)
NMR experiments also quantitatively revealed the strain-induced orientation of
amorphous PCL chains in the fractured sample, indicating that the PCL
crystallites were acting as the physical cross-linkages to prevent the
contraction of the elongated PCL chains when the sample is fractured.
