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How Composition Determines the Properties of Isodimorphic Poly(butylene succinate-ran-butylene azelate) Random Biobased Copolymers: From Single to Double Crystalline Random Copolymers
journal contribution
posted on 2015-01-13, 00:00 authored by Idoia Arandia, Agurtzane Mugica, Manuela Zubitur, Arantxa Arbe, Guoming Liu, Dujin Wang, Rosica Mincheva, Philippe Dubois, Alejandro J. MüllerThe
structure, morphology, and thermal properties of biobased and
biodegradable poly(butylene succinate-ran-butylene
azelate) random copolyesters with a wide composition range were studied.
These PBS-ran-PBAz copolymers can crystallize in
the entire composition range despite being random, displaying a eutectic
point when their melting point is plotted as a function of composition.
Wide angle X-ray scattering (WAXS) studies confirmed isodimorphic
behavior where PBS-rich copolymers crystallize with PBS type unit
cells with some PBAz repeating unit inclusions and vice versa for
PBAz-rich copolymers. Away from eutectic compositions the copolymers
exhibit only one crystalline phase (PBS-rich or PBAz-rich crystalline
phases) while at the eutectic point both phases can crystallize. The
only double crystalline copolymer among those prepared had a composition
close to the eutectic point of 45 mol % PBS (and 55 mol % PBAz). The
crystallization of the two phases occurred in the same temperature
range upon cooling from the melt at 10 °C/min in a DSC (i.e.,
coincident crystallization). Self-nucleation (SN) studies of the PBS-rich
phase were performed. After SN, the separate crystallization of each
phase was clearly observed during cooling from the self-nucleation
temperature (i.e., PBS and PBAz-rich phases). Small angle X-ray scattering
(SAXS) experiments were performed for the first time for these types
of isodimorphic copolymers. The results show that the lamellar long
period is a strong function of composition. While limited inclusions
of PBAz units within the crystal lattice only cause a slight expansion
of the PBS component unit cell, the increase of comonomer content
produces an unexpected synergistic increase in long periods and lamellar
thickness values. In the case of the only double crystalline copolymer
examined, the PBS-rich phase forms space filling spherulites (observed
by polarized light optical microscopy, PLOM) at higher temperatures
that template the superstructural morphology of the copolymer. These
PBS-rich phase spherulites contain radial lamellar stacks whose long
period was determined by SAXS. Upon further cooling, the PBAz-rich
phase crystallizes in the intraspherulitic amorphous regions with
newly formed lamellae that have their own distinct long period according
to SAXS results. AFM observations of the PBS-rich crystalline lamellae
confirmed the lamellar thickness and long spacings determined by SAXS.
A schematic morphological model of the mixed spherulites produced
by this double crystalline diblock copolymer is proposed based on
the experimental evidence collected by SAXS, PLOM, and AFM.