1. Introduction
The issue of plastic pollution is widespread, endangering animal and human health as well as the natural ecosystem[1]. According to MacLeod et al.[2], the annual release of plastic garbage into the world's water reservoirs (oceans rivers, and lakes) is estimated to be between 9 and 23 million metric tons. The usage of conventional, petroleum-based plastics is linked to recyclability, waste management, and overproduction issues, all of which have significantly increased the issue of plastic pollution. Bio-based and biodegradable polymers such as poly (L-lactic acid) or PLA have been offered as potential solutions to these challenges. PLA is an aliphatic that can be produced by fermentation from eco-friendly or natural sources like wheat, corn starch, cassava, sugarcane, and other renewable resources[ 3]. In membrane formation, PLA presents a green alternative to conventional plastics and possesses several outstanding qualities such as minimal toxicity, compatibility, and the ability to degrade organically into compounds over time[4]. Moreover, PLA is highly versatile due to its high transparency and remarkable processability[5]. However, PLA suffers from serious drawbacks that limit its use in various applications. PLA's low melt strength and naturally slow crystallization rate limit its ability to be foamed, formed, and processed[6]. Despite its superior strength and modulus, PLA has a relatively poor toughness and is brittle. Due to its slow crystallization rate and low glass transition temperature (55~60°C), PLA has a low heat deflection temperature (HDT) or low service temperature. This prevents it from being used at too-low service temperatures[3,7]. Owing to its fabrication flexibility, PLA membranes are widely prepared using various techniques such as electrospinning, solvent casting, phase separation, freeze-drying, and particle leaching. These membranes have been extensively applied in membrane applications such as water and wastewater treatment, gas separation, pervaporation, hemodialysis, adsorption, and ion exchange membranes. In this review, we discuss the fabrication of PLA membranes via the phase separation method.
2. PLA Membrane Preparation via Phase Separation
Phase separation is the widely employed method for the preparation of PLA membranes due to its simplicity in forming a porous membrane. It is a controlled demixing process whereby the initially thermodynamically stable polymer solution is transformed from a liquid to a solid state[8]. Controlling the operating conditions is crucial for obtaining membranes with desirable morphology. The velocity of phase separation and solvent removal rate are the primary determinants of the membrane’s morphology and properties. This reaction occurs in a polymer solution and splits the thermodynamically unstable polymer-rich phase from the polymer-poor phase[9]. This transformation can be accomplished via immersion precipitation or nonsolvent induced phase separation (NIPS), precipitation by vapor or evaporation induced phase separation (EIPS), and thermally induced phase separation (TIPS). Among the aforementioned, NIPS, EIPS, and TIPS are frequently employed for the fabrication of PLA membranes.
2.1. Nonsolvent induced phase separation (NIPS)
NIPS is the simplest method and most widely used method for preparing PLA membranes. This process requires a strong solvent and a polymer that is soluble in solvent mixture. After casting the polymer solution onto a glass plate or other appropriate substrate, it is submerged in a coagulation bath containing nonsolvent or solvent-nonsolvent mixture. While the nonsolvent penetrates in the polymer solution, the solvent penetrates into the nonsolvent. This exchange process continues until demixing takes place. Thus, two phases emerge from a homogeneous polymer solution: the liquid- rich phase and the polymer-rich phase, which produces a membrane with an asymmetric structure[10]. The nascent membrane structure is affected by both phase separation and mass transfer. The exchange between the solvent and nonsolvent takes place on the PLA surface, forming a non-uniform porous structure with a wide PSD and poor mechanical properties [11,12]. One drawback of NIPS is the massive amount of wastewater that is contaminated with solvents. The preparation of PLA membranes using NIPs has been the subject of several studies. For instance, Ouda et al. prepared a PLA-based membrane using PVP as the pore-forming agent[13]. The PLA membrane showed a uniform surface that was well-dispersed with nano-sized pores. By introducing amino-activated carbon (AC) modified mangrove particles (MAC) into PLA solution, Khamis et al. also prepared a green pH-responsive adsorptive membrane (Fig. 1)[14]. With the introduction of 3 wt% of MAC, the porosity slightly increased from 87.5% to 89.6%. The pure and modified PLA membranes demonstrated a porous sublayer with finger-like or macrovoid morphology. The low viscosity of the PLA solution caused the transformation of the fingerlike sublayer in PLA to a macrovoid sublayer in 1.5% MAC-PLA.
Gao et al. prepared a PLA membrane using PEG and NMP as pore agents and solvents, respectively[15]. PLA membranes show a smooth and compact surface with finger-like morphology. Abuhantash et al. reported a PLA/f-MWCNTs membrane at different f-MWCNTs content for oily wastewater treatment[16]. The introduction of f-MWCNTs caused the membrane's finger-like pores, pore size, and pore density to increase. This was due to the interaction between the kinetic and thermodynamic factors during the nonsolvents and solvents exchange. Due to this delay in nonsolvent and solvent exchange rate caused by the increased viscosity of the dope solution, the size of the pores increases. Khalil et al. reported a PLA membranes with porosity of around 63%[17,18]. The interconnectivity of membrane pores was impacted by an increase in polymer content, which resulted in the formation of smaller membrane pores. Nasser et al. also fabricated PLA ultrafiltration (UF) membrane via NIPS[19]. An increase in PLA content brought on by increase in dope viscosity resulted in the formation of denser surface pores. The diffusion rate of solvent (DMAC) out in the coagulation bath was slowed down by the nonsolvent (water) in the diffusion phase, which inhibits the growth of macrovoids. Xix-rodriguez et al. prepared hemocompatible membranes by blending PLA with different amphiphilic block copolymers; poly(methyl methacrylate)-b-poly(2-dimethylamino ethyl methacrylate) (PD), poly(methyl methacrylate)-b-poly (2-hydroxyethyl methacrylate) (PH) and poly(methyl methacrylate)-b-poly (2-acryloamido-2-methyl-1-propanesulfonic acid) (PA)[20]. All membranes exhibited nonhemolyzing and hemocompatibility properties. The prepared membranes demonstrated a good trade-off between the desirable retention of proteins and the convective transport of middle-sized molecules. The pristine PLA show an asymmetric membrane finger-like structure which had a detrimental effect on its mechanical properties. However, the polymeric blend containing amphiphilic co-polymer present a sponge-like surface morphology. With the addition of amphiphilic copolymers, the pore size and surface porosity were significantly reduced. Xiong et al. reported a PLA stereocomplex UF membrane containing PDLA and PLA with both high rejection and high permeability[21]. Due to the rapid liquid-liquid demixing process, the pure PLA membrane exhibit a smooth top surface, a dense skin layer with a complete finger-like morphology. Upon incorporation of PDLA, the finger-like pores was gradually confined and spherulites while sponge-like pores began to prevail in the cross-section.
In the field of membrane fabrication, the phase separation induced by water droplets has attracted attention as a potential NIPS method for preparing porous membranes. In this process, water droplets are used during the PLA membrane preparation to induce phase separation in order to reduce the exchange rate between the solvent and nonsolvent while producing membranes with different morphologies[12]. Temperature, polymer-solvent concentration, and the size of the H2O droplet are important variables that can be altered to tune the structure of PLA membranes. For example, Wang et al. prepared PLA porous membranes via H2O droplets induced phase separation at varying temperature[ 22]. The schematic illustration of the synthetic route of the porous PLA membrane is shown in Fig. 2. Their result shows that the rate of nonsolvent to solvent exchange was slower than the rate of solvent volatilization. The formation of polymer-poor and polymer- rich phases at higher temperatures was found to be responsible for the improvement in water droplet vitality. On the other hand, an increase in the rate of solvent volatilization resulted in a shorter solidification time. As a result, the volume of the polymer-poor phase decline as the solidification time drops. When the temperature rose from 25 to 75 degrees Celsius, the average pore size also drops. This is similar to study by[23]. In their study, the pore size decreased and became densely packed when the temperature rose from 25 to 100 degrees Celsius. Less water was deposited on the membrane surface at high temperatures because the water droplets move more quickly and evaporate at a faster pace. Consequently, the phase separation speed, porosity and pore size decreases. In another study by Chinyerenwa et al.[24], the phase separation process was induced by heating water droplets to a high temperature using an electric heating steam generator. This method was used to shorten the preparation time of PLA membranes produced via water droplet induced phase separation. The schematic illustration of PLA membrane preparation via the hot water droplets induced phase separation is shown in Fig. 3. They found that the hot water droplets induced phase separation was faster than the room-temperature water droplets phase separation. Regular circular pores were formed on the surface of the membrane when water droplets at room-temperature was used as the coagulant. The average pore size rose from 55.12 to 64.76 μm when hot water droplets were substituted for room temperature droplets.
2.2. Thermally induced phase separation (TIPS)
TIPS is a method in which polymers like PLA that cannot be dissolved at room temperature is dissolved in a solvent at high temperatures and the polymer solution is then cast onto a support or glass plate. Then, the temperature is then lowered to allow for demixing. Liquid-liquid phase separation occurs at lower temperatures, resulting in the formation of two new phases: a polymer-poor phase and a polymer-rich[11]. Removing the solvents requires methods such as freeze-drying, extraction, and evaporation. Finally, a porous polymer structure is obtained following the removal of the solvent. The critical parameters that influence the kinetics and thermodynamics of microphase separation and crystallization are the solvent composition, quench temperature and polymer concentration. The advantages of TIPS include high porosity, process simplicity, the ability to create micro-structures with narrow PSD, minimal tendency to produce defeats, and high reproducibility[ 10]. For example, Tanaka et al. prepared microfiltration PLA membranes via TIPS[25]. The surface has pores of ~ 10 μm, which was smaller than the internal and bottom pores of ~20 μm. Furthermore, uneven macroscopic structures were formed when the membrane was dried for 5 min or longer. In another study, asymmetric porous membranes were prepared by combining nonsolvent-induced and thermally induced phase separation (N-TIPS)[26]. This led to the formation of an asymmetric membrane structure with smooth and rough sides. It was assumed that the membrane structure at the smooth side was formed via nonsolvent phase separation. This is because the polymer solution came in contact with water, inducing the phase separation at 60 to 80 degrees Celsius. However, the structure of rough side was thought to have been formed via TIPS. This is because the side of the glass plate shielded the polymer solution from of H2O influx while the polymer solution was cooled by the H2O bath (25°C) through the plate. When the membrane was utilized as a depth filter, a rapid filtration rate was achieved.
2.3. Evaporation induced phase separation (EIPS)
EIPS, a polymer is dissolved in a mixture of solvent and nonsolvent to prepare a homogeneous solution, where the nonsolvent is less volatile than the solvent. Phase separation and the demixing of the polymer-solvent- nonsolvent system occurs as a result of the solvent evaporation, producing a porous film. Altering the polymer-solvent-nonsolvent solutions' composition allows for the control over the pore structures. For instance, Iulianelli et al. prepared dense PLA membranes with thicknesses between 23 and 30 μm by EIP for the treatment of gaseous streams rich in CH4 and CO2[27]. They used PLA at a different thickness to fabricate symmetric dense flat sheet membranes for the treatment of gaseous streams rich in CH4 and CO2[28]. They also used a modified PLA-based polymer (PLA Easy FilTM– White) to synthesize dense membranes for CO2 separation[29]. Wu et al. combined PIM-1 and PLA to prepare a PLA/PIM-1 membranes with gas good selectivity and permeability for fruit storage application[ 30]. The preparation process of PLA/PIM-1 membranes is shown in Fig. 4. A high-temperature method was first used to prepare the PIM-1 polymer. At higher PIM-1 content ( > 0.25%), the mechanical properties of PLA/PIM-1 membrane were slightly lower than that of the neat PLA membrane. They also combined chitosan (CA) and alizarin (AL) into PLA solution to produce a pH responsive PLA membrane (PLA/CA) for food packaging[31]. Fig. 5 shows the preparation process and phase morphology of PLA/CA films. The pure PLA and PLA composite films present flat and smooth surfaces. Moreover, the thickness of PLA/AL and PLA/CS composite films increases upon adding CS and AL. Zereshki et al. prepared PLA/PVP membranes containing different contents of PVP for ethanol/ethyl tert-butyl ether azeotropic separation[32]. The membrane showed a porous structure at higher PVP concentrations in its cross-sectional micrographs. They also prepared microfiltration PLA membranes with varying PVP content[33]. While pure PLA membrane shows a completely dense surface, PLA membranes containing PVP show a closed cellular structure. Galiano et al. reported PLA green membranes using ethyl lactate as solvent via combining EIPS with NIPS (E-NIPS)[34]. The membrane prepared at the shortest evaporation time (ET) of 0.5 min presents a macrovoid structure due to the instantaneous demixing rate, which occurred in water. As ET increases, the macrovoid structure gradually disappears. At higher ET, a fully or rigid dense skill layer structure was formed due to the complete solvent evaporation. Msahel et al. also prepared PLA membranes via E-NIPS[35]. The pristine membrane presents a dense and compact surface morphology. The introduction of the Fe-MOF was accompanied by the formation of small agglomerated particles on the membrane surface. Karakoca et al. investigate the potential of the Halloysite-PLA nanocomposite membrane for desalination[36]. The membrane cross-section image shows a dense skin layer with a complete honeycomb morphology at the bottom layer.
3. Applications
Due to its biodegradability and biocompatibility, PLA membrane prepared via the phase separation method has been widely applied in numerous membrane- based applications such as water purification, wastewater treatment, gas separation, food and fruit packaging, hemodialysis pervaporation, and desalination. For instance, PLA membranes can offer excellent selectivity and permeability for gas application and filtration performance for liquid separation. The applications of PLA membranes prepared via the phase separation method are shown in Table 1.
4. Conclusion
PLA is one of the commonly used bio-based polymers for the preparation of polymeric membranes. It has received significant attention due to its numerous properties, including biodegradability, biocompatibility, easy processability, low cost, and safe waste disposal. Numerous works have been conducted on the preparation of PLA membranes via the phase separation method and its applicability for diverse applications. However, there is a major challenge in its dissolution in various organic solvents. The solubility of PLA in nonpolar organic solvents holds great potential for the preparation of membranes via phase separation such as NIPS. However, these limitations can be overcome by blending with various fillers or polymers. On the other hand, the common solvents used to fabricate PLA membranes are mostly toxic polar aprotic solvents such as chloroform, dichloromethane, 1, 4-dioxane, N-methylpyrrolidone, dimethylformamide, and dimethylacetamide. Hence, future works should replace these toxic materials with environmentally friendly alternatives such as Polarclean, Tamisolve, Cyrene, and γ-butyrolactone in order to maintain the sustainable development of membrane technology.
