Aqueous solution blow spinning of poly(vinyl alcohol) micro- and nanofibers
Adillys M. C. Santos a, Eudes L. G. Medeirosa, Jonny J. Blakerb, Eliton S. Medeirosa*
a
Materials and Biosystems Laboratory (LAMAB), Department of Materials Engineering
(DEMat), Federal University of Paraiba (UFPB), CEP58051-900, João Pessoa- PB,
Brazil.
b
Bio-/Active Materials Group, School of Materials, MSS Tower, Manchester
University, Manchester, M13 9PL, UK
E-mail: [email protected]
Abstract
This work addresses the challenge to produce fibers from the water-soluble
polymer poly(vinyl alcohol) (PVA), using solution blow spinning (SBS) with forced
solvent evaporation at the point of fiber formation. PVA at two different molecular
weights, with different degrees of acetylation, were successfully blow spun into nanoand micro-fiber membranes, across a range of concentrations in water (12-20 w/v%).
Fiber spinnability, morphology and size are correlated to precursor solution viscosity
and PVA type. PVA with degree of polymerisation 1100 and degree of hydrolysis 9899% was relatively facile to spin into fibers. Difficulties in spinning high molecular
weight PVA were overcome by introducing hot air at the point of fiber formation to
force water evaporation. The procedure developed here opens SBS to other aqueousbased polymer and composite systems for environmentally benign fiber and membrane
formation.
Graphical abstract
Keywords: Solution blow spinning, poly(vinyl alcohol), nanofibers membranes
1. Introduction
Solution Blow Spinning (SBS) is a simple rapid technique to produce nanofibers,
using pressurized air-stream as driving force for fiber formation. SBS overcomes some
drawbacks associated with electrospinning such as the use of electric field. The high
velocity airflow used in SBS aids solvent evaporation for dry fiber production [1]. Most
published work on solution blow spinning involves the use of highly volatile organic
solvents (or solvent mixtures)[2,3]. Spinning from low volatile solvents like water is
challenging since the fibers do not fully dry before reaching the collector, causing the
fibrillar morphology to be destroyed by coiling of polymer molecules.
Poly (vinyl alcohol), is a water soluble and semi-crystalline polymer obtained
from controlled hydrolysis of poly(vinyl acetate) (PVAc). PVA has good chemical and
thermal stability, is nontoxic and biocompatible with a wide spectrum of
applications[4,5]. Electrospun fibers from PVA have been studied with respect to
production parameters including molecular weight [6,7], concentration [7–9], and with
nanofillers, including cellulose nanocrystals [10], carbon nanotubes [11] , silica [12],
and hydroxyapatite [13]. PVA biofibers have been processed by SBS [14].
Here, PVA aqueous solutions are spun into fibers applying a modified SBS
heads and forced evaporation using heat applied at the air exit. Fiber spinnability,
morphology and size are correlated to precursor solution viscosity and PVA type.
2. Experimental
2.1 Preparation and characterization of the polymer solutions
PVA-110 (degree of polymerization 1100 (Mw 49,000 g.mol-1); degree of
hydrolysis 98-99%) and PVA-224 (Mw 118,000 g.mol-1; degree of hydrolysis 87-89%)
were kindly donated from Kuraray Ltd, Brazil, in powder form. These dissimilar
physicochemical properties were chosen to study their influence on the solvent
evaporation. PVA was dissolved in distilled water at 12, 16 and 20 w/v%, under
constant stirring, at 90 °C for 2h. Rheological properties were assessed using a coneand-plate rheometer (AR2000, TA instruments, USA), disc diameter of 40 mm, by
varying the angular frequency from 1 to 100 rad.s-1and applying the Cox-Merz rule
[15].
2.2 Fiber spinning methodology and characterization
The SBS apparatus and detail of the concentric nozzles is shown in Figure 1.
Aqueous PVA solutions were injected into the inner nozzle at 120 µL.min-1, with the
outer nozzle supplying pressurized air at 0.55 MPa. The inner nozzle tip (diameter 0.7
mm) protruded the outer one by 15 mm. When necessary, a Bunsen-burner with the air
vent and gas flux adjusted to get the coolest output flame, which was suitable yelloworange in color, was positioned below the SBS nozzle (see Figure 1). This acted to
increase the temperature of the air exiting the annulus at high speed, which draws the
heat to the point of fiber formation. Interestingly, the temperature at the vicinity of air
exit was measured at 30°C and at the point of fiber formation about 45 °C, both
assessed with a digital thermocouple, and arrowed in Figure 1.
The morphology of the resultant fibers were assessed on gold sputter coated
samples using scanning electron microscopy (SEM, FEI Quanta 450, Czech Republic,
at 8 kV). Fiber diameters were measured using ImageJ (Version 1.48, NIH, USA), with
50 individual fiber measurements per sample at two different sites.
3 Results and discussion
3.1 Rheological measurements
The viscosity of the solutions increased with increasing concentrations, with the
PVA-224 exhibiting higher viscosities than PVA-110 across all concentrations (Figure
2a), consistent with previous literature [7]. Newtonian behavior was observed at all
shear rates, with the exception of the 20 w/v % concentration, which exhibited some
o
shear thinning behavior. The apparent shear rate  app in the inner nozzle channel was
estimated assuming classical Newtonian flow for the 7 mm long nozzle portion, as
indicated in Figure 1. The well-known rheological relation for the capillary tube
o
 app  4Q / r 3 was used, where Q is the volumetric flow rate and r is the capillary
diameter [15] and the apparent shear rate found was 60 s-1. At this shear rate, it was
possible to assess the apparent viscosities of the solutions (Figure 2b), and ultimately
evaluate their effect on the fiber diameter, discussed below.
3.2 Fiber spinnability and morphological characterization
Fiber formation was relatively facile for the low molecular weight PVA-110, using the
protruded nozzle set up. At a low concentration (12 %w/v), unstable jets of fibers and
droplets formed, resulting in bead-on-string structures (Figure 3a). While some of these
fibers were individualized and loose, many were conjoined due to incomplete water
evaporation, consistent with previous observations of electrospun fibers [16]. The fibers
presented a circular cross-section with a mean diameter of circa 302 ± 99 nm.
Increasing PVA concentration to 16 and 20 w/v% resulted in smooth and larger
diameter fibers (450 ± 155 nm, and 1040 ± 440 nm, respectively), Figure 3b-c.
Increasing the polymer concentration in solution enhances chain entanglement, causing
the jet to resist deformation [10], resulting in thicker fibers (Figure 2b).
PVA-224 solutions presented difficulties in the spinning process employing the
setup used for PVA-110. Irrespective of the solution concentration, little fiber formation
was observed, probably due to insufficient water evaporation. Instead of fibers being
predominant, a wet, film-like membrane with few discernible fibers was observed (see
supplementary information). The mean diameters of these few fibers were 655 ± 380
nm, 1240 ± 290 nm and 1920 ± 1320 nm, for polymer concentrations at 12, 16 and 20
%w/v, respectively (Figure 2b). With the aid of the heat source, interestingly, the flame
tip was pulled towards the annulus tip due to the SBS gas stream (see graphical abstract)
and the fibers instantaneously appeared in the airstream. The heat was enough to
guarantee a balance between deformation and water evaporation. The morphology of
fibers at 12 w/v% and 16 w/v% were relatively smooth, with mean diameters 571 ± 232
nm, and 1551 ± 395 nm, respectively (Figure 3d-e). It was not possible to spin PVA-224
at 20% w/v concentration due to clogging at the inner nozzle. Heating the entire
concentric nozzles setup may lead to polymer degradation and poor final fiber
properties. Here, the protruding setup was more suitable for the spinning process and
avoided the flame tip to hit the polymer solution directly and, accordingly, the clogging
effect.
In aqueous solutions there is inter- and intra-chain bonding between the PVA
chains and the water molecules after dissolution [17], which can reform during solvent
evaporation. At a constant solution concentration, increased PVA molecular weight and
hydrolysis degree act to increase viscosity, reducing solvent evaporation. In contrast,
the rate of solvent evaporation is relatively high at a low molecular weight and the
fibers essentially dry before reaching the collector. Even though water evaporation is
enhanced with decreasing degree of hydrolysis, the high degree of polymerization of
PVA-224 increases the viscosity and reduces water diffusion and evaporation.
Accordingly, a heat source was required to achieve high yields of fiber formation and
reduced diameters.
Conclusions
Micro- and nanofibers were successfully solution blow spun from aqueous solutions of
PVA. Mean fiber diameter was found to be dependent on the solution concentration and
molecular weight. Low molecular weight PVA (Mw 49,000 g.mol-1) was relatively
facile to spin into fibers using a protruding inner nozzle, with a relatively high-pressure
air (pressure supplied at 0.55 MPa). Fiber formation from aqueous solutions of higher
molecular weight PVA at 12 and 16 w/v% concentrations was aided by a heat source
(Bunsen burner), heat of which was drawn into the point of fiber formation, acting to
locally heat the air to 45°C. Concentrations of high molecular weight PVA at 20 w/v%
were not spinnable due to clogging, whereas the lower molecular weight could be spun
at 12, 16 and 20 w/v%. This work opens the SBS technique to other water-soluble
polymers and composite systems for environmentally benign fiber and membrane
formation.
Acknowledgements
AMCS, JJB and ESM acknowledges support from CNPq (Brazil), grant number
400248/2014-0 and to Professor Sandro Torres and Meyson Nascimento for the
assistance with SEM.
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Figures
Figure 1. (a) Schematic of the SBS set-up for forced aqueous solvent evaporation,
indicating the position of the heat source used, and (b) positions of the temperature
assessed (arrowed), and nozzle detail.
Figure 2. (a) Viscosity values and (b) their influence on fiber mean diameter.
Figure 3. SEM micrographs of solution blown spun fibers of PVA-110 at different
polymer concentration (without heat applied, a-c), and for PVA-224 temperatureassisted (d, e).
Supplementary Information: Figures
Figure S. SEM micrographs of solution blown spun fibers of PVA-224 at different
polymer concentration (without heat applied, a-c).