Molecular structure of electrospun polymer fibers

There are numerous studies on the molecular orientation and crystallinity of the polymer fiber after electrospinning. It is often thought that electrospun fibers exhibits low crystallinity but higher molecular orientation compared to neat pallets or cast film due to rapid vaporization and stretching of the solution. However, this is not always the case as it has been shown that molecular orientation and crystallinity of electrospun fibers are also affected by many other factors such as concentration, voltage applied, solvents used, polymer molecular properties and others. Electrospun fibers does not always exhibit lower crystallinity compared to cast film for all types of polymers. Comparison of the crystallinity of electrospun polyethylene oxide [Wang et al 2008, Oliveira et al 2013] and Nylon 11 [Dhanalakshmi and Jog 2008] with its cast film counterpart showed lower crystallinity in electrospun fibers. However electrospun polylactic acid fibers showed higher crystallinity than its cast film [Oliveira et al 2013].

Electrospun polycaprolactone fibers exhibit lower crystallinity than its raw pallet form. This has been attributed to rapid vaporization of the solvent during the electrospinning process which does not allow sufficient time for ordering of the molecules. Kolbuk et al (2012) found that the birefringence of electrospun polycaprolactone fibers changes from positive to negative values depending on the spatial positioning which indicates variation in the molecular orientation along the fibers.

The existence of difference phases of crystals in nylon depends on its electrospinning parameters. Electrospun Nylon 11 fibers exhibited γ-phase form while melt pressed and solvent cast films exhibited α-phase crystals [Dhanalakshmi and Jog 2008]. The formation of γ form has been attributed to the high rate of elongation acting on the molecules during fiber formation. Solvent cast Nylon 11 film showed much higher crystallinity than melt press and electrospun fibers. The crystallinity of as-electrospun Nylon 6 is only 22 wt% and a significant amount of amorphous and γ-form Nylon 6 chain was found to orientate in alignment with the fiber axes [Liu et al 2007]. Such molecular alignment has been attributed to stretching during electrospinning. However, α-form crystals growth in the nanofibers during annealing had a preferred crystal orientation with the chain axes perpendicular to the fiber direction. With polyimide coating, which confined the Nylon 6 nanofibers, the nano-confinement effect causes the Brill transition to be about 20°C higher at 180-190°C compared to unconfined Nylon 6. It is hypothesized that this increment in temperature is due to restriction in volume expansion during melt-recrystallization and Brill transition in Nylon 6 nanofibers [Liu et al 2007].Solution concentration for electrospinning was found to influence the crystalline phases found in the resultant fibers. At lower concentration (12 wt%), electrospun Nylon-6 showed higher γ-form crystals [Wang et al 2012]. However as the concentration increases α-form starts to dominate until at 22 wt% where only α-form exists. At lower concentration, the diameter of the fabricated fibers are smaller and this may lead to greater shear force on the electrospinning jet which gives rise to γ-form crystals. During annealing, γ-form of the crystal in the fibers where found to be stable with no transition to the α phase until γ crystals melts at 200 ° C [Wang et al 2012]. This differs from the results reported by Liu et al (2007) where the meta-stable γ-form transformed into thermodynamically stable α-form upon annealing. Wang et al (2012) attributed this differences to the solvent used in their solution for electrospinning. Liu et al (2007) used 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) to prepare the nylon solution which is more volatile than formic acid which Wang et al (2012) used. Faster solvent evaporation may have led to the formation of less stable γ crystals that are more susceptible to transformation. Pardo et al (2014) did a comparison of various parameters (solution concentration, flow rate, voltage applied and distance from the tip of the needle to collector) and their influence on the crystallinity of electrospun polyamide (Nylon) 6,6 fibers. Increasing voltage was found to increase the crystallinity of electrospun polyamide (Nylon) 6,6 fibers while other parameters did not show a consistent effect.

(a) DSC scans of PLLA fibers electrospun from 5 wt% and 8 wt% solutions at 15 kV and 10 cm needle tip to collector distance (NTCD); (b) Cold crystallization region of scans shown in A. [Ero-Phillips et al Polymers 2012; 4: 1331. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Several electrospinning parameters were found to affect the crystallinity of poly(L-lactic acid) (PLLA). An increase in solution concentration has been found to reduce the crystallinity and molecular orientation of poly(L-lactic acid) (PLLA). This has been attributed to less polymer chain mobility and faster solidification rate which leads to higher degree of cold crystallization [Ero-Phillips 2012]. A greater heat of cold crystallization will reduce the degree of crystallinity. Interestingly, there is an optimum voltage for maximum degree of crystallinity for specific solution concentration. Ero-Phillips (2012) hypothesized that increasing voltage will encourage greater polymer chain alignment and degree of crystallinity. However, above the optimum voltage, the flight time of the jet is reduced due to fast jet acceleration and this leave insufficient time for the polymer chain to crystalize. This makes it possible to tailor the crystallinity of PLLA to a range from 23% to 46% while as received pallets have a crystallinity of about 37% [Ero-Phillips 2012].

Poly(vinylidenefluoride) (PVDF) is a semi-crystalline piezoelectric polymer exhibiting four different crystal structures, α, β, γ and σ. Of these crystal structures, β phase has the best piezoelectric properties. Comparing electrospun PVDF with PVDF cast film, electrospun PVDF exhibited β phase only while its cast film exhibits both β phase and α phase crystals when tested with X-ray diffraction. DSC also showed that electrospun PVDF is more crystalline than PVDF granules. Presence of more β phase crystals in electrospun PVDF nanofibers have been attributed to greater alignment of electric dipoles found in the PVDF solution under high voltage [Gheibi et al 2014]. In a modified version of the electrospinning process, a needle extruding the feed solution was rotated to exert a centrifugal force. This centrifugal force was able to dampen the bending instability of the electrospinning bisphenol A polycarbonate (BPAPC) solution. It was found that in the absence of the centrifugal force, the electrospun BPAPC nanofibers are amorphous while with the centrifugal force, partially crystalline BPAPC fibers were formed [Liao C C et al 2010]. It will be interesting to find out whether the increment in crystallinity is due to the reduction in bending instability or the presence of the centrifugal force. Liao et al (2010) also found that the choice of solvent affects the crystallinity of the resultant fibers. When CH₂Cl₂ and CHCl₃ were used as the solvents, the resultant BPAPC showed no crystallinity while crystalline fiber was produced from BPAPC/THF solution. The ratios of the trans-trans- and trans-cis-conformers of the BPAPC in the respective solvents probably contributed to the crystallization of the polymer chain during electrospinning.

Rapid vaporization of solvent in the electrospinning solution generally reduces the time needed for crystallization of the polymer chain. However, crystals may be formed in the polymer solution prior to electrospinning and these crystals may be retained following electrospinning. 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.

Crystallinity and degree of macromolecular chain orientation for a polymer may also differ significantly depending on the electrospinning setup or fiber diameters. Wen et al (2022) apply a high voltage to a ring around the tip of the nozzle instead of applying the voltage directly to the nozzle. Such a setup changes the electric field profile between the nozzle tip and the collector. Where the high voltage is connected to the nozzle, the electric field strength drops significantly at a few centimeters away from the nozzle tip towards the collector. However, when the high voltage is applied to a ring surrounding the nozzle, the drop in electric field strength is less steep and it maintains a base electric field strength away from the nozzle tip [Wen et al 2022]. Wen et al (2022) showed that the difference in the method of charging polyethylene oxide (PEO) solution affects the characteristic of the resultant fibers. Ring-charged electrospun fibers have a much smaller diameter that is almost half that of nozzle-charged fibers, a higher degree of macromolecular chain orientation but significantly lower crystallinity of 32%. Nozzle charged electrospun fibers have a crystallinity of 75%. Further studies are needed to determine the macromolecular chain orientation and reduced crystallinity in ring-charged electrospun fibers is due to reduction of fiber diameter or changes in the electrical field profile.