TIT# Melting, Solidification and Crystallization of a Thermoplastic Polyurethane
     as a Function of Hard Segment Content
AUT# Stribeck, Almut; Eling, Berend; Pöselt, Elmar; Malfois, Marc; Schander, Edgar;
SOU# Macromol. Chem. Phys. (2019), 220, 1900074
DOI# 10.1002/macp.201900074
LOC# xv162 @ xv162_online.pdf
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ABS# Thermoplastic polyurethanes (TPU) with varying hard segment contents (HSC)
     are monitored during melting and solidifying ($\left|\dot{T}\right|=20 K/min$,
     $T_{max}=220 °C$) by small-angle and wide-angle X-ray scattering (WAXS and
     SAXS). Hard segments: MDI/BD. Soft segments: PTHF1000®. The neat materials
     are injection-molded having small amorphous hard domains (chord length
     $\bar{d}_{h}<6 nm$). Results indicate complexity of morphology changes.
     E.g. crystals are predominantly produced in freshly segregating,
     sufficiently big hard domains, while the temperature decreases slowly enough.
     After the thermal treatment, only the materials with HSC~$\gtrsim$35\%
     show sharp Bragg peaks and bigger hard domains ($\bar{d}_{h}>7 nm$).
     When heated, small domains melt, but crystallization in the remaining
     big domains is not detected. Upon cooling, first big agglomerates
     segregate, which crystallize immediately. Segregation starts for HSC=42%
     at 160 °C and for HSC=75% at 210 °C. When HSC \ensuremath{\le}30%, the
     morphologies before and after are similar, but afterwards many hard
     blocks are dissolved in the soft phase on expense of the hard domain
     fraction. In heating and cooling the melts, we observe multiple
     homogenization and segregation processes, which are explained by
     agglomeration of hard blocks of different lengths in the colloidal fluid.