Heat treatment of Electrospun Membrane

Figure 1. Typical SEM micrographs of the PVA electrospun nanofibers: (Left) Heat treated at  (85°C, which is ~0.5 Tm ); and (Right) heat treated at ~1.65  (140°C, which is ~0.7 Tm). [Mahir Es-saheb and Ahmed Elzatahry, International Journal of Polymer Science, vol. 2014, Article ID 605938, 6 pages, 2014. doi:10.1155/2014/605938 This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Heat treatment can be used to improve several properties of electrospun membrane. Some of the advantages of heat treatment are,

1. Improve membrane compactness (eliminate 'fluffiness' or stray fibers)
2. Improve mechanical property
3. Improve chemical stability
4. Reduce intra-membrane layer delamination

Presence of residual charges on the membrane may reduce the speed of the electrospinning jet as it nears the collector. Thus the later fibers may be laid down at reduced speed resulting in a thicker but compressible membrane. In application such as water filtration, such physical characteristic is not desirable as the membrane will be compressed during usage [Homaeigohar 2011]. Heat treatment will also allow movement in the molecular chains which may result in changes to its property.

Heat treatment is usually carried out at temperature between the glass transition temperature (Tg) and melting temperature of the material. For electrospun fibers, these temperatures may be different from bulk material due to difference in the crystallinity and molecular orientation following the electrospinning process [Liu et al 2009]. For example the glass transition temperature of poly[(lactic acid)-co-(glycolic acid)] nanofibers was found to reduce to 45.3 °C, an 8 °C drop from its raw material [Liu et al 2009] and the glass transition temperature of polycarbonate nanofiber was reported to increase to 110 °C from 101 °C of its granules [Dhakate et al 2010]. Differential scanning calorimetry (DSC) may be used to determine the glass transition temperature and the melting point of the electrospun material prior to heat treatment.

The heat treatment configuration may be varied as follows,

1. No force application on the membrane during treatment
2. Membrane perimeters restrained during treatment
3. Treatment under pressure (Hot pressing)

Figure 2. Schematic of heat treatment configuration for electrospun membrane

When no force is applied on the membrane during heat treatment, the membrane may shrink or warp due to stress relaxation. Thus the perimeters of the membrane may be restrained during the treatment to maintained dimensional stability. Comparing the diameter of the fiber, heating of free standing membrane saw a slight increase in fiber diameter while restrained membrane showed a significant reduction in fiber diameter. The reduction in fiber diameter is due to molecular relaxation and rearrangement along the length of the fiber as it is prevented from shrinking [Zhang et al 2012].

Material	Heat treatment condition	Reference
Cellulose acetate membrane, fiber diameter range from 200 nm to 1 micron	208 °C for 1 h	Ma et al 2005
Polyvinyl alcohol, fiber diameter range from 150 to 210 nm	85 °C	Es-saheb et al 2014
Polyvinylidene fluoride, fiber diameter of about 380 nm	145 °C for 18 h	Gopal et al 2006
Polysulfone, fiber diameter 1.75 microns	190 °C for 3 h	Zhang et al 2012

Several studies have shown that heat treatment is able to increase the mechanical strength of the electrospun membrane [Zhang et al 2012, Es-saheb et al 2014, Ma et al 2005, Chen et al 2013]. This is probably expected as the heat treatment encourages fusion at interfiber contact points which provides a greater strengthening effect. Examination of the fracture characteristic after heat treatment of the membrane showed little fiber slippage [Na et al 2009]. Chavoshnejad et al (2020) used mathematical modeling to show that with inter-fiber bonding, stiffness of the membrane increases by 60% regardless of the mechanical properties of the individual fibers. Increasing elastic modulus of individual fibers led to a linear increase in membrane stiffness while increasing fiber diameter led to a parabolic increase in membrane stiffness. Comparing the stiffness ratio of 100% bonding over unbonded membrane, membranes with lower porosity or greater fiber density showed greater stiffening effect with inter-fiber bonding. For other membrane characteristics such as fiber elastic modulus and diameter, the stiffness ratio did not change significantly. However, excessive heating temperature and/or duration may results in a net lowering of mechanical strength. Zhang et al (2012) carried out detailed studies to find the optimal condition for the heat treatment of electrospun polysulfone membrane. Their results showed that polysulfone membrane heated with dimensional constrain at 190 °C (glass transition temperature of polysulfone) for 3 hours having the best mechanical property and excellent chemical stability. Increasing the temperature or heating duration will lead to deterioration of the mechanical property.

Concurrent application of pressure and heat may shorten the treatment duration and improve the connectivity between fiber intersections. Heating of electrospun membrane under pressure can be achieved easily using a hot press machine. It is important to note that the temperature to cause fusion at inter-fiber contact points will be lower under pressure. Na et al (2008) showed that with a hot-pressed temperature of 145 °C, polyvinylidene fluoride fibers have merged together to form a network of large pores. Under normal heating at that temperature, the fibers would just fused together at the junction while retaining individual fiber morphology [Gopal et al 2006 ]. Where electrospun nanofibers are deposited on a backing material hot pressing has been shown to be effective in preventing delamination between the two materials [Kaur 2011] and improve adhesion [Viet 2010]. Hot pressing also eliminates loose strands of fibers that may be present on the top surface of electrospun membrane [Anwar et al 2013]. In the construction of carbon paper using electrospun polyacrylonitrile (PAN), hot pressing prior to carbonization was found to improve the electrical conductivity. This was attributed to the formation of junctions between fibers which reduces the charge traveling distance [Yang et al 2012]. Remarkable increase in the mechanical strength of hot pressed PAN electrospun nanofiber membrane has also been reported. Ali et al (2009) was able to obtain PAN electrospun nanofiber membrane with tensile strength up to 63 MPa after hotpressing at 220 °C compared to just 8.73 MPa of as-spun membrane.

Application temperature below the Tg of the polymer has also been shown to be useful in forming bonds between fibers. Homaeigohar (2011) found that heating of electrospun polyethersulfone fibers (PES) below its Tg but higher than the boiling point of its solvent improves adhesion between electrospun fibers and the underlying substrate. This was attributed to diffusion of the solvent to the surface of the nanofibers causing local re-dissolution at the contact point between the fiber and substrate.

Heat treatment is often used to improve the mechanical stability of the electrospun membrane. However, it has also been employed for other non-conventional purpose. Poologasundarampillai et al (2011) used heat treatment to partially melt poly(lactic-glycolic acid) (PLGA) within a blended electrospun membrane of siloxane-containing vaterite (SiV)/poly(L-lactic acid) (PLLA). Both the PLGA and the composite material were electrospun simultaneously so that the resultant mat contained a mixture of both. Heating the temperature to 110 °C causes the PLGA fibers to melt and flow into the matrix. This fusion of PLGA with the SiV/poly(L-lactic acid) (PLLA) fibers improves the overall mechanical properties of the mixture. Electrospinning two materials of different melting points for the purpose of bonding has also been used by Borisova et al (2020). Two polymer solutions, poly(3-hydroxybutyrate) (PHB, Tm = 160 ° C) and poly(ε-caprolactone) (PCL, Tm = 60 ° C), were simultaneously electrospun. In their setup, a rotating drum was used to get aligned fibers. The electrospinning emitters were placed on either side of the drum collector. This allows both nanofibers to be dispersed evenly throughout the membrane. A heat treatment temperature of 80 °C was used to bond PCL to PHB fibers. The heat treated membrane showed enhanced tensile strength due to bonding between the fibers. Polytetrafluoroethylene (PTFE) is known to be highly chemical resistant and exhibit good hydrophobicity characteristic. However, due to its chemical stability, it is difficult to dissolve this material for electrospinning. Its high melting temperature also make sit difficult to use melt electrospinning. To overcome these limitations, Zhou et al (2014) used a suspension of polytetrafluoroethylene (PTFE) fine particles in water for blending with water soluble polyvinyl alcohol (PVA). The PVA with PTFE particles were electrospun and the PVA component removed through sintering up to 380°C for 30 minutes. At this temperature the PTFE particles melted and fused together to form an interconnected nanofibrous network of PTFE.

Heat treatment of electrospun fibers is also use to affect its crystallinity. By heating the material above its glass transition temperature, 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.

[+]

[-] comments powered by Disqus

Content [Heat Treatment]