Polymorphic crystalline
structure has been a key factor determining
the mechanical property and degradation behavior of biodegradable
polymers. Herein, we report on the polymorphic crystalline structure,
phase transition, and their effects on the mechanical property and
degradation behavior of poly(p-dioxanone) (PPDO),
a typical biodegradable and bioabsorbable semicrystalline polymer
for biomedical applications. Both the polymorphic crystalline structure
and crystallization kinetics of PPDO depend on crystallization temperature
(T
c) drastically. Melting enthalpy and
degree of crystallinity of PPDO first decrease and then increase with
increasing T
c. PPDO forms a new mesomorphic
polymorph (denoted as the α′-form) during crystallization
at low T
c (≤10 °C), in contrast
to the common α crystals generated at high T
c (≥60 °C). The α′ crystals have
weaker interchain interactions and a shorter long period than the
common α crystals, indicating the looser chain packing of α′
crystals. The α′ crystals are metastable, and they transform
into the thermodynamically stable α crystals during heating.
The α′-form PPDO possesses a slower degradation rate,
higher flexibility, but lower strength and modulus than its α
counterpart. This would provide a feasible way to tailor the degradation
and mechanical properties of PPDO by varying crystal modification.
Polymer/mosquito-repellent
scaffolds exhibit increasing importance
in long-lasting human skin protection to be used as wearable devices
and allowing for controlled release of repellents. In this study,
ethyl butylacetylaminopropionate (IR3535) was
used as a human and environmental friendly active mosquito-repellent
serving as a solvent to form functional poly(l-lactic acid)
(PLLA) scaffolds by crystallization-based solid–liquid thermally
induced phase separation. Crystallization of PLLA in the presence
of IR3535 is faster than melt-crystallization of neat PLLA, and in
the investigated concentration range from 5 to 50 mass % PLLA, its
maximum crystallization rate increases with the PLLA content, by both,
increases of the maximum crystal growth rate and of the nuclei density.
By adjusting the polymer concentration and the crystallization temperature,
microporous scaffolds of different fine structures are obtained, hosting
the mosquito-repellent in intra- and interspherulitic pores for its
intended later evaporation.
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