We
report that hot stretching of poly(ethylene oxide) (PEO)-based
solid polymer electrolytes (SPEs) can lead to a preferred orientation
of PEO crystalline lamellae, thereby reducing the tortuosity of the
ion-conduction pathway along the thickness direction of the SPE film,
causing improved ionic conductivity. The hot stretching method is
implemented by stretching SPE films above the melting point of PEO
in an inert environment followed by crystallization at room temperature
while maintaining the applied strain. The effect of hot stretching
on the crystalline orientation, crystallinity, morphology, and ion
transport in PEO with two types of salts, lithium bis(trifluoromethanesulfonyl)imide
(LiTFSI) and lithium triflate (LiCF3SO3), is
investigated in detail. Wide-angle X-ray scattering (WAXS) and small-angle
X-ray scattering (SAXS) show that the orientation of PEO crystalline
lamellae induces the formation of a short ion-conduction pathway along
the through-plane direction of the SPE films, leading to 1.4- to 3.5-fold
enhancement in the through-plane ionic conductivity.
In the present study,
we focused on the intermolecular H-bonding
interactions of poly[(R)-3-hydroxybutyrate] (PHB) with an inorganic
material, pseudoboehmite (PB), and their effect on PHB crystallization.
Noncrystallizable atactic PHB and crystallizable isotactic PHB (a-PHB
and i-PHB) ultrathin films were spin-coated on a PB substrate, as
well as an aluminum oxide (AO) and a gold substrate for comparison.
Infrared reflection–absorption spectroscopy (IRRAS) data show
an absorption peak in the carbonyl region located at 1724 cm–1 for a 2.8 nm a-PHB film deposited on PB. A peak at this frequency,
often observed for thick bulk crystalline i-PHB films, was not observed
for a 1.4 nm a-PHB film deposited on a gold or AO substrate, indicating
that the 1724 cm–1 peak observed for a-PHB on PB
is not due to a geometric confinement effect or crystallization but
due to the existence of intermolecular H-bonding (H-bondinter) between −CO of a-PHB and −OH from PB. Supercooled,
amorphous i-PHB was also found to exhibit the same H-bondinter with PB. It was found that a PB surface significantly modified the
crystal orientation and morphologies of the films. Grazing incident
wide-angle X-ray diffraction (GIWAXD) data show that the crystallites
in i-PHB on PB are randomly oriented, whereas those on AO are predominantly
edge-on oriented. Polarized optical microscopy (POM) images show spherulites
for i-PHB on AO, whereas no spherulites were observed for i-PHB on
PB. This study demonstrates a novel method of using PB to modulate
the crystallization behavior of PHB thin films.
Ultrathin films of
biodegradable poly[(R)-3-hydroxybutyrate]
(PHB) and its random copolymer poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (PHBHx) were
prepared by spin-coating onto aluminum substrates with a naturally
oxidized aluminum oxide (AO) surface layer or, alternatively, on gold
substrates. The opposite surface of the film was in contact with ambient
air. Isothermal crystallization kinetics of these films at room temperature
were studied using infrared reflection absorption spectroscopy. The
overall crystallization rate for all the polymers when crystallizing
on AO is significantly retarded compared with the same polymer crystallizing
on gold. It was found that the retardation effect was not due to a
confinement effect. The crystallization retardation effect was especially
enhanced for PHBHx with a higher (R)-3-hydroxyhexanoate
content. Avrami analysis showed that the crystallization rate constant k (min–1) for all of the polymers on AO
is approximately 3 to 4 orders of magnitude less than that found for
the same polymer on gold. Grazing incident wide-angle X-ray diffraction
showed that polymers on gold have both flat-on and edge-on crystallite
orientations, whereas polymers on AO have a dominating edge-on crystallite
orientation. Infrared studies on a quasi-monolayer film revealed no
detectable H-bonding between PHB/PHBHx and the AO surface. The crystallization
retardation mechanism was explained as being a sum of the dipole–dipole
interactions of −CO of PHB or PHBHx and the −O–Al–O–
groups of AO coupled with the rigid disordered amorphous nature of
the AO surface.
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