Polymorphic Self-Poisoning in the Isothermal Crystallization of Thermoplastic Polyurethanes

TL;DR

This paper studies how different crystal structures form in thermoplastic polyurethanes during isothermal crystallization and how they affect material properties.

Contribution

The study reveals a nonmonotonic crystallization rate linked to polymorphic self-poisoning in TPUs with high hard segment content.

Findings

TPUs with low hard segment content crystallize exclusively in Form I with decreasing crystallization rates at higher temperatures.

TPUs with ≥50 wt% hard segments show nonmonotonic crystallization rates due to competition between two polymorphs.

Polymorphic self-poisoning is identified as a mechanism where Form I temporarily hinders the formation of Form II.

Abstract

Thermoplastic polyurethanes
(TPUs) are multiblock copolymers whose
properties are strongly influenced by the crystallization of the hard
segments (HS). Crystallized HSs based on 4,4′-methylenediphenyl
diisocyanate/1,4-butanediol can develop two distinct polymorphs: the
thermodynamically stable triclinic Form II or the kinetically favored
paracrystalline Form I, each associated with different mechanical
responses. While the effect of cooling rate on polymorphic crystallization
has been studied, the isothermal crystallization kinetics of TPUs
with varying HS content are less explored. Here, we investigate the
isothermal crystallization of TPUs containing 29–80 wt % HS
using differential scanning calorimetry (DSC), wide-angle X-ray diffraction
(WAXD), polarized light optical microscopy (PLOM), and fast scanning
calorimetry (FSC). TPUs with low HS content (29 and 33 wt %) crystallize
exclusively in Form I, and the overall crystallization rate decreases
monotonically with increasing temperature at low supercooling. In
contrast, TPUs with ≥50 wt % HS display a nonmonotonic temperature
dependence: the overall crystallization rate first increases with
supercooling, then passes through a relative minimum, and rises again
at larger supercooling. Structural analyses confirm that this inversion
of the temperature coefficient of the crystallization rate originates
from the competition between the formation of the two polymorphs.
In agreement with previous literature, the rate minimum is tentatively
attributed to polymorphic self-poisoning, in which Form I temporarily
hinders the crystallization of Form II. These findings establish a
direct link between polymorphic competition and crystallization kinetics
in TPUs, providing new insights into structure formation and strategies
for tailoring their properties.

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