Atactic poly(vinyl alcohols) (a-PVAs) having number-average degrees of polymerization [(P n )s] of 1700 and 4000 were prepared by the solution polymerization of vinyl acetate, which was followed by the saponification of poly(vinyl acetate) to investigate the effects of molecular weights of a-PVA on the characteristics of electrospun a-PVA nanofabrics. A-PVA nanofabrics were prepared by electrospinning with controlling the process parameters including the electrical field, conductivity, tip-to-collector distance, and solution concentration. Through a series of characterization experiments, we identified that the molecular weight of a-PVA had a marked influence on the structure and properties of nanofabrics produced. That is, the higher the molecular weight of PVA, the superior the physical properties of PVA nanofabric.
High molecular weight specimens of syndiotacticity-rich
poly(vinyl alcohol) (PVA) have been
prepared by saponification of copolymers of vinyl pivalate (VPi) and
vinyl acetate (VAc). VPi and VAc
were copolymerized using the low-temperature initiator
2,2‘-azobis(2,4-dimethylvaleronitrile) and saponified in KOH/methanol/water with vigorous stirring, i.e., under shear.
The PVAs obtained had number-average degrees of polymerization in the range (5.6−16.5) ×
103 and syndiotactic diad (S-diad) contents
of 52.8−61.5%. The highest molecular weight and S-diad content
were obtained for specimens derived
from homopoly(VPi) prepared at low initiator concentration, and
these parameters declined progressively
with increasing VAc content in the parent copolymers. The
structures of these PVAs show a consistent
sharp transition between 55 and 56% S-diad content. Specimens
with lower S-diad contents have
shapeless morphologies, but at 56% S-diad content the polymers were
fibrous, with a higher degree of
crystallinity and orientation of the crystallites. The order
improves steadily with the S-diad content
above the transition and correlates with the observed changes in the
thermal and mechanical properties.
The results suggest that the small increase in S-diad content from
55 to 56% facilitates a major change
in the crystallization process, such that the crystallinity improves,
and leads at the same time to the
development of a fibrous morphology when the saponification is
performed under shear.
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