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*

aMaterials 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: eliton@ct.ufpb.br

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 nano-

and 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 98-

99% 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 aqueous-

based 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 cone-

and-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 yellow-

orange 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

shear thinning behavior. The apparent shear rate γo

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

γo

app=4 Q /πr3was 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 temperature-

assisted (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).