Increasing crystallinity of electrospun fibers

Crystallinity of polymer fibers have significant influence on its properties. Mechanical and piezoelectric depending on the polymers, are some properties that are important to their applications. Most electrospun fibers exhibits relatively lower level of crystallinity due to rapid evaporation and solidification during the spinning process. Inai et al (2005) showed that electrospun poly(L-lactic acid) (PLLA) did not exhibit any crystalline phase while as-received pallets and solvent cast films of PLLA does contain crystalline phase. However, there are several ways of increasing the crystallinity of electrospun fibers either by modifying the electrospinning process, using additives or through post processing treatment.

Annealing is probably the most common and direct method of increasing the crystallinity of electrospun fibers. This is carried out by heating the material above its glass transition temperature such that the molecular chains are able to re-organize to its stable form. With PLLA, Inai et al (2005) found that their electrospun fibers does not exhibit any crystalline phase diffraction peaks using XRD. However, when the samples were annealed at a temperature of 80 °C, the crystalline phase diffraction peaks were clearly visible. Ribeiro et al (2011) did a more detailed study on using annealing to control the crystallinity of electrospun PLLA. Rate of crystallization in electrospun PLLA fibers were found to be much faster than its film form. Annealing at 90 °C for 1 hour for electrospun PLLA fibers were able to yield 27% crystallinity but for film it is only 9%. Annealing electrospun PLLA fibers for 2 mins at 140 °C was able to give rise to 31% crystallinity. Silva et al (2011) reported that piezoelectric PVDF thin film will lose its piezoelectric properties as the temperature increases above 80 °. However, they attributed this to depoling of the material rather than the degree of crystallinity or β-phase content. Nevertheless, any change in crystal phase will affect its piezoelectric performance. Parker et al (2018) tested the stability of electrospun polyvinylidene fluoride (PVDF) membrane in particular, its crystal phase. Their study showed that thermal annealing of the electrospun PVDF membrane at temperatures up to 100 ° resulted in more β phase conversion either at the expense of α phase or amorphous phase. The increase in β phase may give rise to greater piezoelectricity and hence its utility as sensor at elevated temperature. Nevertheless, it will be interesting to investigate the effect of elevated temperature on piezoelectric property in electrospun PVDF membranes.

DSC normalized thermograms of a PLLA sample electrospun at a traveling distance of 15 cm, a needle diameter of 0.50 mm, an applied voltage of 20 kV and a flow rate of 2?ml h?1. Scan 1 is the first heating scan, scan 2 was recorded after heating the sample to 65 °C followed by cooling at 40 °C min?1 to 20 °C, and scan 3 was performed after cooling the sample at 10 °C min?1 from 200 °C to 20 °C [Ribeiro et al 2011].

For some polymers, improving the crystallinity of the electrospun fibers does not require heat. Lee et al (2017) showed that a freeze/thawing process can be used to enhance the crystallinity in cross-linked poly(vinyl alcohol) (PVA) nanofibers. The freeze/thawing cycle involves dipping PVA nanofibers in water and freezing at -20 °C followed by thawing at 15 °C. Improvement in crystallinity has been attributed to intrachain or interchain hydrogen bondings formed after freezing/thawing cycles. 5 freezing/thawing cycles and the corresponding increase in its crystallinity increases the tensile strength of the membrane by about 165%. Another method of post-electrospinning treatment to increase crystallinity is by dipping the material in appropriate solvent. Lu et al (2021) showed that with PLLA, acetone was found to cause swelling of the electrospun fibers. Pure electrospun PLLA fibers without any treatment exhibited low crystallization. However, when immersed in ethanol/acetone mixture, the crystallinity of the PLLA fibers increased with increasing acetone concentration with four distinct peaks in XRD diagrams at 50% acetone mixture. However, to induce rapid crystallinity rate, pure acetone was used. Pure acetone causes the PLLA fibers to swell quickly which gives free spaces for molecules to rearrange and form crystalline phases. With greater packing of the polymer chains, pores are formed both inside and on the surface of the fibers. The electrospun PLLA fibers were only dipped in acetone for 5 min for pores formation.

SEM images of porous PLLA fibres with different polymer concentrations after dipping in acetone. (a) 1.4 wt% (PLLA-1.4); (b) 1.6 wt% (PLLA-1.6); (c) 1.8 wt% (PLLA-1.8); (d) 2.0 wt% (PLLA-2.0); (e) 2.2 wt% (PLLA-2.2); (f) 2.4 wt% (PLLA-2.4) [Lu et al 2021].

While post-spinning process may be used to improve the crystallinity of electrospun fibers, modification of the electrospinning process has also been shown to influence electrospun fibers crystallinity. Lin et al (2009) showed that the modification of electrospinning into a gas-jet electrospinning process was able to increase the crystallinity and β-phase content in poly(vinylidene fluoride) (PVDF) fibers. The gas-jet electrospinning process involves having an outer orifice surrounding the inner electrospinning nozzle where gas was ejected at high speed through the outer orifice and facilitated in the drawing of the electrospinning jet. Greater crystallinity of fibers produced by gas-jet electrospinning has been attributed to the higher stretch ratio which may have facilitated the nucleation of β-phase.

Interestingly, the positioning of the electrospinning nozzle to the collector may also have an effect on the crystallinity of the resultant fibers. Al-Hazeem et al (2021) reported that for electrospinning polyvinylpyrrolidone (PVP)/TiO₂ fibers, having the nozzle at the bottom and the collector above yields well-crystalline structure compared to other orientation with eight 2θ peaks compared to only five peaks found on the nozzle top to collector bottom setup with four peaks barely visible. The grain size in the nanofiber was also the largest at a bottom-up setting (68 nm) and smallest (11 nm) at a top-down setting. Such differences may be attributed to the effect of gravity on the electrospinning jet and Taylor cone. When the electrospinning jet and gravity are pointing in the same direction (top-down setting), the acceleration of the jet and impact on the collector may reduce the time needed for molecular organization. However, in a bottom-up setting, the electrospinning jet traveling against the force of gravity may slow down the jet hence giving more time for molecular organization prior to deposition on the collector.

Solubility of polymer in solvent has been shown to influence the crystallinity of electrospun fibers. Chen et al (2019) electrospun core-shell poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(methyl methacrylate) (PMMA) fibers with chloroform (CF), chlorobenzene (CB) and 1,2,4-trichlorobenzene (TCB) as solvents for P3HT. Of these 3 solvents, TCB is a poor solvent for P3HT and showed the greatest amount of crystalline aggregates in the solution. Following electrospinning, the resultant fibers from P3HT dissolved in TCB showed high crystallinity. Such high crystallinity probably comes from crystalline aggregates in the solution. P3HT dissolved in CF which is a better solvent showed low crystallinity after it is electrospun into fibers. Despite the smaller crystal size in P3HT electrospun nanofibers from CF and CB solutions, the polymer chains and crystalline grains are highly oriented. P3HT electrospun nanofibers from TCB solution showed poor orientation between the crystalline grains although the crystalline grains are larger.

Additives may also be used to improve the crystallinity of electrospun fibers. Wang et al (2014) showed that the addition of multi-walled carbon nanotube (MWCNT) in PVDF solution for electrospinning increases in the β crystalline phase of PVDF in the resultant composite fiber. While it is not clear how the addition of MWCNT have helped improve the crystallinity of the resultant composite PVDF fibers, it may be due to faster stretching from the presence of MWCNT in the solution or the increased conductivity facilitating the polarization of PVDF molecules. Tyubaeva et al (201) showed that the addition of Meso-tetraphenylporphyrin iron (III) chloride complex (FeCl(TPP)) into biopolymer (poly(3-hydroxybutyrate)) (PHB) solution for electrospinning was able to yield PHB nanofibers with greater crystallinity. They hypothesized that the addition of FeCl(TPP) had a plasticizing effect and this led to increase intermolecular distance and mobility of the chains during electrospinning which facilitates orientation and organization of the molecules into crystalline structure.

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