Investigation of Biodegradable and Non-Biodegradable Solvents for the Fabrication of Polylactic Acid Membranes via Nonsolvent Induced Phase Separation (NIPS) for Air Filtration Applications

Abstract

The substitution of hazardous, environmentally persistent solvents (NMP and DMAc) with more sustainable alternatives (ETAc and GBL) in fabricating flat sheet polyactic acid (PLA) membranes via nonsolvent-induced phase separation for air filtration applications was the focus of this study. The polymer-solvent affinity was first evaluated using Hansen solubility parameters, confirming suitable Relative Energy Difference (RED) values (<1) for all solvent candidates. Dope solutions prepared with biodegradable solvents demonstrated higher viscosity compared to those prepared with environmentally persistent solvents. These biodegradable solvent systems also exhibited slower precipitation rates during membrane formation. This resulted in spongelike cross-sectional morphologies, contrasting with the combined fingerlike and spongelike structures observed in membranes fabricated with environmentally persistent NMP and DMAc. Thermal analysis revealed that membranes fabricated with biodegradable solvents exhibited superior thermal stability with higher glass transition temperatures (Tg = 54.39–55.34 °C) compared to those made with environmentally persistent solvents (Tg = 49.97–50.71 °C). Membranes fabricated with ethyl acetate (ETAc) showed the highest hydrophobicity (contact angle = 115.1 ± 9°), airflow rate (12.7 ± 0.28 LPM at 0.4 bar) and maintained filtration efficiency at values greater than 95% for 0.3-μm aerosols.

1. Introduction

The rapid pace of global industrialization has led to significant air quality degradation [1]. As a result, the adverse health effects of air pollution have been studied extensively since exposure to fine particulate matter typically less than 2.5 μm in diameter (PM_2.5) [2,3,4] can cause respiratory, cardiovascular, and other diseases [5,6]. Recent global studies continue to quantify the substantial mortality burden associated with PM_2.5 exposure [7,8] as toxic substances, such as organic compounds and microorganisms, adhere to these fine particles, facilitating subsequent transportation into the body and resulting in deleterious health outcomes [3,9]. Air filtration has proven to be a highly effective technology for purifying and improving air quality across various industries [10,11,12,13]. For critical applications such as healthcare, high-efficiency particulate air (HEPA) filters capable of capturing >99% of particulate matter 0.3 μm or smaller (PM_0.3), which is the most penetrating particle size (MPPS), have been recommended by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) and by the Center for Disease Control and Prevention (CDC) [14,15]. Conventional HEPA filters, which are usually non-woven polymeric materials, achieve this high filtration efficiency via a combination of mechanical (impaction, interception, size exclusion) and electrostatic filtration mechanisms [16]. Non-woven filters achieve high filtration efficiency for larger particle sizes (>0.5 µm) by mechanical filtration mechanisms but rely on electrostatic interaction to trap particles lower than their pore sizes [16,17,18,19]. Charges responsible for filtration by electrostatic interaction can deplete over time or after common cleaning and decontamination treatment, resulting in decreased filtration efficiency, especially of smaller-sized particles, such as PM_0.3 [20]. This reduction in performance can allow penetration of hazardous air pollutants, posing health risks [21]. While these filters can be easily disposed, and replaced at a relatively cost-effective manner, the waste generated causes environmental concerns as materials used in fabricating these conventional air such as fiberglass and polypropylene, are nonbiodegradable and can contribute to the already existing global plastic waste crisis at end-of-life [22,23].

Recent studies have explored utilizing flat sheet polymeric membranes fabricated via phase inversion as potential alternatives for sustainable air filters, owing to their high PM_0.3 capture efficiency and effective reusability since they have uncompromised filtration efficiency over time and post decontamination [15,20,24,25,26,27]. The main challenge of using polymeric membranes fabricated via phase inversion is the formation of a thick selective layer that increases pressure drop [15,26]. The formation of the thick selective layer can be controlled by altering the temperature of the coagulation bath, choice of solvent, and solvent evaporation time [26,28,29,30]. Van Goethem et al. successfully fabricated highly porous PVDF membranes via NIPS using dimethylformamide (DMF) as the solvent, which exhibited high air permeation comparable to commercially available non-woven filters [26]. Similarly, Ogbuoji et al. demonstrated that polysulfone membranes fabricated via NIPS could achieve high filtration efficiency for fine particulate matter with good reusability [15]. However, these studies utilized conventional non-biodegradable polymers (PVDF, polysulfone) and environmentally persistent solvents (DMF, NMP, DMAc) that pose environmental and health concerns. While Dong et al. reviewed sustainable solvents for membrane fabrication, the focus was primarily on water treatment applications rather than air filtration, and practical demonstrations of biodegradable solvent use for PLA air filtration membranes via NIPS remain absent from the literature [27]. Large pore sizes (typically > 1 µm) are required to achieve high air permeation through flat sheet membranes. Although particles smaller than these pores may penetrate the membrane surface, the tortuous pore path within the membrane structure intercepts the particles, enabling filtration. Unlike conventional non-woven air filters, these membranes can be reused multiple times without compromising performance; however, they still cause plastic and microplastic pollution on disposal, like conventional air filters, since common polymers used in fabricating membranes are usually nonbiodegradable [2,31]. In addition, widely used organic solvents for dissolving polymers in membrane fabrication, such as n-methylpyrrolidone (NMP), dimethylacetamide (DMAc), and DMF, are deleterious to human health and the environment, which is why these solvents have been tagged as substances of very high concern by the environmental protection agency (EPA) and the registration, evaluation, authorization and restriction of chemicals (REACH) [27,32,33]. Hence, there is a need to fabricate flat sheet polymeric membrane air filters using sustainable polymers and solvents, as recent studies have emphasized the growing importance of bioplastic certifications and regulatory frameworks for sustainable material adoption [34].

Polylactic acid (PLA) is a biodegradable thermoplastic polyester synthesized from lactic acid, a chiral molecule that exists as L- and D-lactic acid, and is a leading biopolymer due to its biocompatibility, mechanical properties, and processability [3]. It is worth noting that comprehensive reviews highlight PLA among leading biopolymers with promising sustainability profiles [35,36]. This has motivated investigations into PLA for fabricating membranes for air filtration applications [2,3,37,38,39]. PLA can be produced through various pathways, i.e., direct polycondensation, which yields low molecular weight PLA, and azeotropic polycondensation, which results in high molecular weight polymer [40,41]. PLA can be highly crystalline, semi-crystalline, or amorphous, depending on the D-lactide content [40]. PLA is soluble in polar organic solvents, making it suitable for membrane fabrication [42]. Flat sheet membranes of PLA have been fabricated in recent times for air filtration, mainly by electrospinning, because of the excellent filtration efficiency at low pressure drop [2,43]. However, electrospun membrane filters achieve high air filtration for PM_0.3 by electrostatic interaction, like commercially available non-woven air filters [37,38,39]. PLA has a low dielectric constant (poor ability to trap charges) and requires modifications, such as additives, to match the dielectric constants of polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyethylene (PE), and polypropylene (PP) [3,44]. In addition, the electrostatic charges on the electrospun PLA membranes can dissipate at high humidity and on exposure to common treatment methods, resulting in a significant drop in filtration performance [2,20]. These limitations of electrospun PLA membranes underscore a critical distinction between fabrication methods: electrospinning produces fibrous structures that rely heavily on electrostatic interactions for fine particle capture, while phase inversion techniques produce membranes with controlled pore structures that achieve filtration through purely mechanical mechanisms, including size exclusion, interception, and tortuous path entrapment. This mechanical filtration approach offers inherent advantages in long-term performance stability and reusability since it does not depend on charge retention. Hence, it is important to fabricate flat sheet PLA membranes using techniques such as phase inversion, which achieves high filtration efficiency of particulate matter (PM₁₀, PM_2.5, PM_0.3) solely by mechanical mechanisms [20,45]. Several studies have reported fabricating high-performing flat sheet PLA membranes by phase inversion for water purification [46,47,48,49,50]. However, NMP, a known teratogen, was used as a solvent for dissolving the PLA during the fabrication process and should be replaced with less toxic alternatives.

Environmentally persistent solvents can be replaced with eco-friendly alternatives to attain a more sustainable PLA membrane fabrication process. One biodegradable solvent that can be used for this purpose is ethyl acetate (ETAc) [51]. Ethyl acetate (ETAc) can be produced by microorganisms such as bacteria, mold, and yeast [52,53,54,55]; however, it is produced in commercial quantities by the esterification of ethanol and acetic acid [54]. The biodegradability of ETAc is attributed to the presence of ester linkages in its structure, which are very susceptible to enzymes present in bacteria and fungi, such as esterases and lipases [56]. A study reported complete biodegradation of ETAc by a fungus (P.esterophilus) at room temperature due to the presence of the esterase enzyme [57]. ETAc released into the environment can easily undergo hydrolysis through hydroxyl radical attack on its ester bonds, converting it into ethanol and acetic acid, which are readily metabolized by the standard biochemical pathway [54]. Theoretically, ETAc can dissolve PLA since both have significant polar regions (ester groups) in their molecular structure, which interact to cause dissolution [58]. Another potential green solvent for PLA is γ-butyrolactone (GBL), a polar organic biodegradable solvent, which can also biodegrade in the presence of esterases and hydrolases due to the presence of a susceptible functional group [59].

This study investigated the effect of substituting commonly used environmentally persistent solvents (NMP and DMAc) with more sustainable alternatives (ETAc and GBL) in fabricating biodegradable flat sheet PLA membranes via phase inversion for air filtration applications. While previous studies have explored biodegradable PLA membranes for air filtration primarily through electrospinning techniques [2,3,37,39], these approaches still rely on environmentally persistent solvents and electrostatic filtration mechanisms that can degrade over time. This study addresses the gap in the literature by systematically replacing environmentally persistent solvents (NMP, DMAc) with biodegradable alternatives (ETAc, GBL) in PLA membrane fabrication via NIPS for air filtration applications, a solvent substitution that has not been previously reported for this polymer-application combination. Hansen solubility parameters were utilized to predict and validate biodegradable solvent-PLA compatibility. A comprehensive comparison of membrane morphology, thermal stability, and filtration performance was conducted between membranes fabricated with ETAc and GBL and those fabricated with NMP and DMAc. This approach demonstrates a fully biodegradable membrane system combining biodegradable polymer, biodegradable solvent, and biodegradable pore former for mechanical filtration of fine particulate matter through size exclusion mechanisms rather than electrostatic interaction. NIPS was chosen in fabricating the PLA membranes over other phase separation techniques due to its ease of controlling membrane morphology and process scalability [20,60]. To ensure a sustainable fabrication process, water was used as the non-solvent bath while polyethylene glycol (PEG), a biodegradable polymer, was employed as a pore former to increase pore size and improve airflow through the resulting membranes. The solvents were selected based on solubility in the non-solvent bath and the capability to dissolve PLA, as determined by calculating the relative energy density (RED) values using the Hansen solubility parameters. The effect of different solvents on the membrane surface, pore morphology, filtration, and performance of the fabricated membranes was analyzed through comprehensive characterization.

2. Materials and Methods

2.1. Materials

Extrusion-grade granular PLA (MW~193 kDa) was purchased from Goodfellow Corporation (Pittsburgh, PA, USA), and PEG (600 Da) was obtained from Alfa Aesar (Haverhill, MA, USA). ETAc (≥99.5%) and NMP (~99%) were obtained from VWR Chemicals (Radnor, PA, USA), while GBL and DMAc were purchased from Millipore sigma (St. Louis, MO, USA). Sodium chloride (NaCl) used for aerosol generation was purchased from VWR Chemicals (Radnor, PA, USA). Proteinase K (molecular biology grade) was purchased from New England Biolabs (Boston, MA, USA). Isopropanol (IPA) was purchased from EMD Millipore Corporation (Billerica, MA, USA). Tris (hydroxymethyl) aminomethane (≥99.8%) and Tris HCl (≥99%) were purchased from Millipore sigma (St. Louis, MO, USA) and ThermoFisher Scientific (Waltham, MA, USA), respectively.

2.2. Thermodynamic Study

Polymer-solvent affinity was estimated by first calculating a solubility parameter distance (R_a), using their respective Hansen Solubility Parameters (HSPs), as described by Equation (1):

$$R_a=\sqrt{4({{\delta }_{d2}-{\delta }_{d1})}^2+({{\delta }_{p2}-{\delta }_{p1})}^2+({{\delta }_{h2}-{\delta }_{h1})}^2},$$

(1)

where ${\delta }_d$ is the non-polar dispersive forces, ${\delta }_p$ polar permanent dipole–dipole forces, ${\delta }_h$ polar hydrogen bonding [60,61]. The HSPs were calculated by the group contribution method using Equations (2)–(4):

$${\delta }_{di}={\displaystyle \frac{\sum F_{di}}{V}},$$

(2)

$${\delta }_{pi}={\displaystyle \frac{\sqrt{\sum F_{pi}^2}}{V}},$$

(3)

$${\delta }_{hi}=\sqrt{{\displaystyle \frac{\sum E_{hi}}{V}}},$$

(4)

where $F_{di}$, $F_{pi}$, and $E_{hi}$ are functional group contributions of dispersive forces, polar forces, and hydrogen bonding forces, respectively, of species i; $V$ is the molar volume [62].

These HSPs are the center coordinates for a Hansen solubility sphere [60]. Solvents with high affinity for the solute fall within this sphere, and the radius of this sphere is known as the interaction radius $R_0$. Theoretically, solubility should be achievable when $R_0>R_a$. The Relative Energy Difference (RED) relates the $R_a$ with $R_0$ as shown in Equation (5):

$$RED={\displaystyle \frac{R_a}{R_0}},$$

(5)

RED < 1 indicates a strong solute-solvent affinity, RED > 1 indicates low affinity, RED = 1 reflects a boundary condition, and RED = 0 means no energy difference [60].

2.3. PLA Membrane Fabrication Process

Dope solutions used for membrane fabrication consisted of PLA, PEG 600, and the solvent in a (13/2/85) wt.% ratio. The solutions were tightly sealed, heated to 75 °C, and stirred at 300 rpm until dissolution was complete. When the solutions were fully dissolved, they were kept to cool at room temperature for approximately 20 min before casting on a glass substrate (Gardner Glass Products, North Wilkesboro, NC, USA) using a doctor’s blade (Micrometer Adjustable Film Applicator–250 mm, MTI Corp, Richmond, CA, USA) on an automatic benchtop casting machine (Model: BTFS-TC, PMI, Ithaca, NY, USA) at a casting speed of 500 cm/min. The glass substrate with the thinly spread solution was then immersed and allowed to stay in a static water bath at room temperature until complete phase separation occurred, with the fully formed membrane detaching from the glass substrate unaided (between 4 and 8 min). The membranes were then stored in DI water for at least 24 h to remove any excess solvent before drying at 30 °C in a gravity convection oven (Binder ED56, Binder incorporated, Bohemia, NY, USA) for 24 h. Fabricated membranes were then cut to appropriate sizes for characterizations and tests. The nomenclature for the different membrane samples is summarized in

Table 1. Membrane sample nomenclature: all samples consisted of PLA/PEG/solvent in a 13/2/85 wt.% ratio.

Sample ID	Solvents
MNMP	NMP
MDMAc	DMAc
METAc	ETAc
MGBL	GBL

.

2.4. Characterization

The characterization techniques implemented in this work included viscosity measurement of the dope solution, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), porosity, contact angle measurement, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The dope solutions were characterized for viscosity to compare the fluid behavior and the effect on phase inversion [63]. The viscosity of the dope solutions was obtained at 25 °C and at a shear rate (20 1/s) using a rheometer (TA instrument AG-G2, New Castle, DE, USA) equipped with a 40 mm parallel plate geometry (Peltier plate stainless steel). Prior to fabricated membrane characterization, all samples were dried in a vacuum at 50 °C to remove residual solvents in the microstructure. The presence of various functional groups and bonds on the fabricated membranes was analyzed using an attenuated total reflectance (ATR) FTIR (Nicolet^TM iS50, ThermoFisher Scientific (Waltham, MA, USA). Sample spectra were collected using the diamond ATR mode across a broad wavenumber range of 800–4000 cm⁻¹. Surface and cross-sectional images of the fabricated membranes were obtained using FEI Helios Nanolab 660 SEM equipment (Thermo Fischer Scientific, Waltham, MA, USA). Prior to imaging, samples were coated with platinum using a high vacuum sputter coater (Leica EM ACE600, Deerfield, IL, USA) to 3 nm thickness to ensure high-quality images. The porosity of the membranes was determined through a gravimetric technique, which involved immersing dry membrane samples (0.49 cm²) in isopropanol for 24 h. The weight of the wet membranes was recorded afterwards. Subsequently, the membranes were dried at 50 °C for 24 h and then reweighed. The porosity percentages were then calculated with an equation described in the literature [15]. The hydrophobicity of the membrane surfaces was analyzed through contact angle measurements using the sessile drop technique with contact angle goniometer equipment (Kruss DSA100, Matthews, NC, USA). Twenty microliter (20 µL) droplets of deionized water were deposited on the sample, which was fixed on a glass substrate held horizontally on the device stage. To ensure experimental consistency and statistical validity of the results, the contact angle reported corresponded to the average of sessile drop angles measured at three different locations on each sample. Thermal stability, measured as weight loss of the fabricated membranes as a function of temperature increase, was evaluated by TGA (TA instruments, TGA 550, New Castle, DE, USA). Properly dried samples were heated from room temperature to 800 °C at a heating rate of 20 °C/min in an inert environment. DSC (TA instruments, DSC 250, New Castle, DE, USA) was used to further analyze the thermal properties of the fabricated membranes. Dried membranes were heated in a tightly capped platinum pan (TA instruments, TGA 550, New Castle, DE, USA) from 50 °C to 250 °C at a heating rate of 10 °C/min.

2.5. Aerosol Filtration Efficiency and Airflow Test

An aerosol generation setup was developed for testing filtration efficiency and airflow rate of the membranes, as extensively described in the literature [20] and shown in Supplementary Materials Figure S2. A constant output atomizer (TSI incorporated, model 3076, Shoreview, MN, USA) connected to a NaCl solution was used to generate aerosols of varying size distribution following the American Society for Testing and Materials (ASTM) standards for testing air filters [64]. The generated aerosols were dried and transported by dry compressed air through a sample-holding cell containing a mesh support to prevent sample damage. Filtration efficiency testing was conducted at a constant pressure of 0.4 bar. At this pressure, each membrane exhibited different flow rates based on its permeability. For each membrane, a baseline measurement was first obtained by counting aerosol particles passing through the empty sample-holding cell at 0.4 bar (E_c). The membrane was then installed in the cell, and the pressure was again set to 0.4 bar while aerosol particles passing through the membrane were counted (F_c). This approach ensured that each membrane’s filtration efficiency was evaluated against its corresponding baseline at the same pressure and flow conditions. Airflow rate measurements were performed at multiple pressures ranging from approximately 0.1 to 0.8 bar to characterize membrane permeability across different driving forces. A MET One particle counter (GT-526S, Grant Pass, OR, USA) was used to quantify the permeate aerosol; aerosol counts were conducted to estimate the aerosol permeation as shown in Equation (6):

$$P_M= {\displaystyle \frac{{E_c-F}_c}{E_c}}\times 100,$$

(6)

where $P_M$ is the filtration efficiency, $E_c$ is aerosol permeation count through an empty cell and $F_c$ are permeated by aerosols through a cell holding the membrane.

3. Results and Discussion

3.1. Polymer–Solvent Affinity

The polymer-solvent affinity was estimated by calculating the RED values using the HSPs as previously discussed in Section 3.2 (RED < 1) indicates good polymer-solvent affinity) [60]. The dispersive, polar, and hydrogen bonding HSPs for all solvents and the polymer were provided in

Table 2. RED calculated using HSPs obtained from the literature (61).

Dope Composition	${\mathsf{\delta }}_{\mathbf{d}}$	${\mathsf{\delta }}_{\mathbf{p}}$	${\mathsf{\delta }}_{\mathbf{h}}$	${\mathbf{R}}_{\mathbf{a}}$	${\mathbf{R}}_0$	RED
PLA (polymer)	15.56	8.57	11.06	-	11.1	-
NMP	18.00	12.30	7.20	3.43	-	0.31
DMAc	16.80	11.50	10.20	6.37	-	0.57
ETAc	15.80	5.30	7.20	8.28	-	0.75
GBL	19.00	16.60	7.40	6.72	-	0.60

. As shown in Table 2, all solvents exhibited RED values below 1, indicating that all solvents, including the biodegradable ETAc and GBL, can theoretically dissolve PLA. This was expected since all the solvents are polar and can interact with the polar groups in PLA, which can facilitate the disruption of intermolecular forces between the polymer chain molecules, leading to dissolution.

3.2. Membrane Surface Chemical Composition

FTIR analysis was conducted to investigate the impact of various solvents used in the preparation of dope solutions on the surface chemical composition of fabricated membranes. Solvents that alter the polymer chemical composition are not suitable solvents for the membrane fabrication process via NIPS. The FTIR spectra of all membranes exhibited absorbance peaks that were consistent with pristine PLA (as shown in Figure 1). Characteristic peaks observed at 2995 cm⁻¹ and 2946 cm⁻¹ correspond to the symmetric and asymmetric C―H stretching vibrations of CH₃ in PLA, respectively [65]. Additionally, the absorbance peaks at 1752 cm⁻¹ and 1084 cm⁻¹ were attributed to stretching vibrations of C = O and C―O bonds, respectively [66]. Bending vibrations of the CH₃ group in PLA were evident at 1454 cm⁻¹ (asymmetric) and 1362 cm⁻¹ (symmetric) [67]. The absence of absorbance peaks inconsistent with pristine PLA indicates that no reaction occurred between the polymer and solvent that could alter the polymer’s chemical composition or cause degradation. Hence, these solvents were found to be suitable for dissolving PLA for membrane fabrication using NIPS. Additionally, the IR spectra reveal the absence of characteristic peaks of PEG 600, specifically the OH stretch around 3454 cm⁻¹, despite its incorporation as an additive in the membrane fabrication process [68]. This observation suggests that the relatively short chain PEG (i.e., MW 600) diffused into the nonsolvent bath during the phase separation process, leading to an increased void fraction in the resulting membrane. This phenomenon has been reported in the literature for short-chain PEG used as pore formers during membrane fabrication [69].

3.3. Thermal Behavior of Fabricated Membranes

The thermal behavior of the fabricated membranes was analyzed using DSC. The DSC thermograms (Figure 2) and detailed thermal data (

Table 3. Transition temperatures using DSC and TGA of fabricated membranes and pristine PLA granules.

Sample	${\mathbf{T}}_{\mathbf{g}}$ (°C) *	${\mathbf{T}}_{\mathbf{m}\mathbf{o}}$ (°C) *	${\mathbf{T}}_{\mathbf{m}}$ (°C) *	${\mathbf{T}}_{\mathbf{c}\mathbf{c}}$ (°C) *	${\mathbf{T}}_{\mathbf{d},1\mathbf{\%}}$ (°C) *	${\mathbf{T}}_{\mathbf{d},5\mathbf{\%}}$ (° C) *	${\mathit{X}}_{\mathit{c}}$ (%)
PLA	55.41	144.61	150.94	129.62	324.24	342.11	0.455
MNMP	49.97	135.14	149.46	110.78	266.56	294.15	0.064
MDMAc	50.71	136.91	142.56	116.96	279.36	312.17	0.311
METAc	54.39	140.70	147.55	120.56	296.81	323.94	0.448
MGBL	55.34	143.96	150.20	118.61	296.22	320.31	0.447

* T_g = glass transition, T_mo = onset melting, T_m = melting, T_cc = cold crystallization, T_d,1% = 1% degradation, T_d,5% = 5% degradation, $X_c=$ crystallinity.

) revealed the presence of glass transition temperatures (T_g) from 49.97 to 55.41 °C and melting peaks between 142.56 and 150.94 °C for all membranes and the pristine PLA fine granules. The presence of both T_g and melting transitions indicates that all samples are semicrystalline [70]. The low T_g of the PLA membranes can be a concern, particularly for applications in hot climates. All PLA membranes exhibited T_g values below that of pristine PLA, with M_DMAc and M_NMP having the lowest T_g, making them the least favorable for use in high-temperature regions. The pristine PLA fine granules showed a higher T_g than the flat sheet membranes, following the trend: pristine PLA > M_GBL > M_ETAc > M_DMAc > M_NMP. This decrease in T_g can be attributed to the plasticizing effects of the solvents, with NMP and DMAc exhibiting stronger plasticization than ETAc and GBL due to their lower RED values (0.31 and 0.57 versus 0.75 and 0.60, respectively, Table 2). Lower RED values indicate stronger polymer-solvent interactions, which disrupt polymer-polymer interactions and increase chain mobility [71,72]. Analysis of membrane crystallinity revealed significant differences between samples (Table 3), following the trend: M_ETAc > M_GBL > pristine PLA > M_DMAc > M_NMP. The stronger polymer-solvent interactions in NMP and DMAc solutions resulted in greater disruption of polymer chain packing during membrane formation, leading to reduced crystallinity. The lower crystallinity of M_NMP and M_DMAc membranes is consistent with their lower T_g values, as enhanced chain mobility inhibits ordered crystalline domain formation. Conversely, the higher crystallinity in M_ETAc and M_GBL membranes reflects weaker solvation effects, allowing more ordered chain packing during the slower phase separation process. Interestingly, M_ETAc and M_GBL membranes surpassed pristine PLA in crystallinity, likely due to the extended time for chain rearrangement during slow precipitation. Plasticization also affects the onset melting (T_mo), melting, and cold crystallization (T_cc) temperatures (Figure 2 and Table 3), as samples with higher chain mobility require less kinetic energy to overcome intermolecular forces, resulting in reduced T_mo and T_cc for plasticized membranes.

TGA provided further insights into the thermal decomposition behavior of the membranes by determining the temperatures at 1% and 5% weight loss (T_d,1% and T_d,5%) for all samples (Table 3). The TGA thermograms (Figure 3) revealed a single degradation step for all membranes, with an onset degradation temperature (T_do) of 228.23 °C (M_NMP), 246.76 °C (M_DMAc), 259.06 °C (M_GBL), 269.21 °C (M_ETAc), 301.17 °C (PLA) and a final pristine PLA degradation temperature (T_d,100%) of 403.45 °C, consistent with typical PLA decomposition temperature [73]. In addition, significant differences were evident in the T_d,1% and T_d,5% values of the fabricated membranes. The M_NMP membrane exhibited the lowest T_d,1% and T_d,5% at 266.56 °C, and 294.15 °C, respectively (Table 3). The degradation temperatures followed the trend: pristine PLA > M_ETAc > M_GBL > M_DMAc > M_NMP. This trend aligns with the DSC observations, confirming the plasticization effect of the solvents. Plasticization enhances chain mobility, which facilitates thermal energy distribution and easier covalent bond cleavage during degradation. Hence, the MNMP membrane with the greatest plasticization exhibited the lowest T_do, T_d,1%, and T_d,5% values. In contrast, membranes fabricated with biodegradable solvents demonstrated superior thermal stability (higher transition temperatures, Table 3 and Figure 3) compared to those fabricated with environmentally persistent solvents, making them more suitable for applications in hotter climates. The plasticization effect can be mitigated by thoroughly removing residual solvents from the membrane micropores [74].

3.4. Contact Angle/Wettability Studies

Surface wettability was estimated using contact angle measurements (>90°: hydrophobic; <90°: hydrophilic) [75,76], shown in Figure 4. Hydrophobic materials are preferred for air filtration applications since they prevent water accumulation required for microbial growth [20]. All the fabricated membranes except M_ETAc showed contact angles less than 90°, which indicates a hydrophilic nature. M_ETAc displayed an average contact angle of 115.1 ± 9°, which suggests it is hydrophobic and can be used more effectively as a membrane air filter compared to the other membranes. The contrasting wettability between M_ETAc (hydrophobic) and M_GBL (hydrophilic), despite both being fabricated with biodegradable solvents, can be attributed to differences in solvent polarity and polymer-solvent interactions. GBL possesses significantly higher polarity than ETAc due to its cyclic lactone structure, which exhibits a higher dipole moment and stronger hydrogen bonding capability [77,78]. Additionally, GBL has a lower RED value (0.60) compared to ETAc (0.75), as seen in Table 2, indicating stronger polymer-solvent affinity and more extensive PLA chain solvation during membrane formation. This enhanced solvation by GBL promotes preferential orientation of PLA’s hydrophilic carbonyl groups toward the membrane surface during phase separation, increasing surface energy and resulting in hydrophilic character [79,80]. In contrast, ETAc’s lower polarity and weaker solvation allow hydrophobic methyl groups to preferentially orient toward the membrane-air interface to minimize interfacial energy, resulting in a more hydrophobic surface. The hydrophilic nature of M_GBL impacts its application feasibility for air filtration, as hydrophilic surfaces are more prone to moisture condensation in humid environments, which can increase flow resistance and pressure drop due to water accumulation in membrane pores. This makes M_GBL less suitable for applications in high-humidity environments, while M_ETAc’s hydrophobicity provides resistance to moisture-related performance degradation. The difference in surface wettability can be attributed to the surface roughness of the different membranes [81]. Surface SEM images in Figure 5 show a visible difference in roughness of the M_ETAc compared to the other membranes. The M_ETAc has a rougher surface compared to other membranes, hence its higher contact angle (Figure 4 and Figure 5). The increased contact angle with higher surface roughness can be attributed to the formation of small air pockets on the membrane surface microstructure. The air interacts with water and enhances the cohesive forces between water molecules, leading to the formation of spherical water droplets on the membrane surface and contact angle [82].

3.5. Surface and Cross-Section Analysis

SEM was used to analyze the structural morphology of the surface and cross-section of the fabricated membranes. Surface SEM images showed no significant surface defects on all membranes (Figure 5a,b). Although all the membranes were defect-free, it can be observed that the M_ETAc membrane appears significantly rougher than the other membranes (Figure 5a).

Membrane surface roughness is influenced by the demixing kinetics of solvent and non-solvent, which can be estimated by the precipitation rate of the membrane within the non-solvent bath [63]. These are studied as shown in Figure 6. Ethyl acetate appeared to have the slowest diffusion rate in the non-solvent bath (water) since M_ETAc showed the slowest precipitation rate (Figure 6); hence, the highest surface roughness. As mentioned earlier, this roughness, which results in a higher contact angle, can be attributed to the presence of air trapped in and on the microstructure of the flat sheet membrane, resulting in an increased contact angle due to favored water-water interaction [82]. The SEM images also showed differences in pore cross-sectional morphology for membranes manufactured using different solvents (Figure 5c). Membranes fabricated using conventional non-biodegradable solvents (M_NMP and M_DMAc) showed a combination of fingerlike and spongelike cross-sectional morphology, which can be attributed to the fast solvent-nonsolvent demixing kinetics (Figure 5c and Figure 6). However, membranes fabricated by biodegradable solvents (M_ETAc and M_GBL) displayed only a spongelike structure characteristic of membranes formed by slower demixing kinetics [15,83]. The demixing kinetics, which are the dominant factor for determining pore cross-sectional morphology, are also affected by the viscosity of the dope solutions (Figure 7). As can be seen in Figure 6 and Figure 7, membranes fabricated with biodegradable solvents exhibited the highest viscosities and slowest precipitation times compared to those fabricated with environmentally persistent solvents. Particularly, the M_ETAc dope solution showed the highest viscosity and the slowest membrane precipitation rate. This was expected since increased viscosity impedes the transport of the solvent into the non-solvent bath, thereby prolonging the precipitation time and resulting in membranes with spongelike cross-sectional morphology (Figure 5c, Figure 6 and Figure 7). Interestingly, the M_NMP dope solution displayed a higher viscosity than the M_DMAC solution but exhibited a slightly faster precipitation time (Figure 6 and Figure 7). This discrepancy can be attributed to NMP’s higher affinity for PLA compared to DMAc, as observed from the calculated RED values (Table 2). The stronger polymer-solvent interaction in the NMP solution increases the resistance to solvent-nonsolvent exchange, thus requiring more time for the phase separation to occur and resulting in a slower precipitation process. This suggests that while viscosity is a significant factor in determining mixing and demixing kinetics, other factors such as the polymer-solvent and solvent-nonsolvent interactions also play a critical role.

3.6. Aerosol Filtration Efficiency and Air Permeation Test

Aerosol filtration efficiency and airflow tests (Figure 8), as well as the porosity (Figure 9), were conducted to assess the feasibility of the fabricated membranes for air filtration applications and to evaluate the effect of different solvents on membrane performance. All membranes demonstrated greater than 95% filtration efficiency for 0.3 µm aerosols, indicating their effectiveness in filtering out small particulate matter from the air (Figure 8). Additionally, all membranes, except those fabricated with GBL (0.66 ± 0.04 LPM at 0.4 bar), exhibited significant airflow rates between 15.9 ± 0.8–24.7 ± 0.28 LPM at 0.4 bar, with M_ETAc showing the highest airflow rate. The higher airflow rate observed in M_ETAc was attributed to its slightly higher pore size, as seen in the SEM surface image (Figure 5). The significantly lower airflow rate of M_GBL membranes represents a limitation for practical air filtration applications, attributed to lower overall porosity (Figure 9) and smaller pore sizes (Figure 5) resulting from the specific precipitation kinetics and viscosity of the GBL dope solution during NIPS. Potential optimization strategies to improve M_GBL membrane permeability include adjusting fabrication parameters such as coagulation bath temperature, solvent evaporation time, or pore former concentration to alter precipitation kinetics and favor more porous structures. However, the fundamental trade-off between achieving high porosity and maintaining structural integrity in GBL-based systems may limit performance enhancement. While GBL demonstrates viability as a biodegradable solvent with excellent filtration efficiency, ETAc shows superiority for applications where air permeability is critical. Therefore, M_ETAc appeared to be the most favorable for air filtration applications due to its hydrophobicity, air permeability, and high aerosol filtration efficiency.

Beyond the performance characteristics demonstrated, the substitution of environmentally persistent solvents (NMP and DMAc) with biodegradable alternatives (ETAc and GBL) offers significant environmental and health benefits. NMP and DMAc are classified as Category 1B reproductive toxicants by REACH and listed as substances of very high concern by the EPA due to their teratogenic effects, whereas ETAc exhibits significantly lower toxicity and is approved for food applications and cosmetics in many jurisdictions [33,51]. The critical environmental advantage lies in biodegradability: ETAc undergoes complete biodegradation by microorganisms within days to weeks through enzymatic cleavage of ester bonds by esterases and lipases and readily undergoes abiotic hydrolysis to ethanol and acetic acid [54,56,57]. Similarly, GBL biodegrades in the presence of esterases and hydrolases [59]. In contrast, NMP and DMAc are highly persistent in the environment with much slower degradation rates, leading to accumulation in wastewater systems. Membrane fabrication via NIPS generates substantial solvent-containing waste streams from the coagulation bath and membrane washing steps, making the use of biodegradable solvents particularly advantageous for reducing long-term environmental impact and enabling biological wastewater treatment. The combination of biodegradable polymer (PLA), biodegradable solvents, and biodegradable pore former (PEG) creates a fully sustainable membrane system where any residual solvents would not inhibit biodegradation or generate toxic degradation products. While comprehensive life cycle assessment remains an important direction for future research, the mentioned advantages in toxicity reduction, biodegradability, and environmental persistence, combined with maintained filtration performance, provide strong justification for transitioning to biodegradable solvents in membrane fabrication. To contextualize the performance of these sustainable membranes within existing air filtration technologies, a comparative benchmarking analysis was conducted.

Benchmarking the fabricated PLA membranes against existing air filtration technologies provides important context for evaluating their practical significance.

Table 4. Performance comparison of fabricated PLA membranes with commercial HEPA filters, and other PLA-based air filters.

Filter Type	Filtration Mechanism	Efficiency (%)	Airflow Rate	Biodegradable	Ref.
METAc	Mechanical	>95	12.7 LPM @ 0.4 bar	Yes (polymer + solvent)	This study
Commercial HEPA	Mechanical + Electrostatic	>99.97	High †	No	[14,16]
Electrospun PLA	Electrostatic	99.2	9.8 cm/s @ 0.00125 bar ‡	Partially (polymer only)	[2]
Electrospun PLA-MOF	Electrostatic	99.5	Low pressure drop	Partially (polymer only)	[3]
PVDF	Mechanical	-	~180 L/m2/h @ 0.001 bar	No	[26]

† Specific values and/or process vary by commercial product and are often proprietary. ‡ Converted units.

presents a performance comparison of M_ETAc membranes with commercial HEPA filters and other PLA-based air filters reported in the literature. While commercial HEPA filters achieve higher filtration efficiency, they rely on electrostatic mechanisms that can dissipate over time or after decontamination, leading to performance degradation [14,16]. Electrospun PLA membranes achieve high filtration efficiency but face similar electrostatic charge dissipation challenges and continue to use environmentally persistent solvents in their fabrication [2]. The NIPS-fabricated membranes in this study demonstrate purely mechanical filtration through biodegradable processing routes, representing a distinct approach to sustainable air filtration.

The comparison demonstrates that M_ETAc membranes occupy a unique position among air filtration technologies by combining end-to-end sustainability with competitive performance. While filtration efficiency and airflow rates vary across different systems due to differences in testing conditions and membrane configurations, M_ETAc represents the only fully biodegradable air filtration membrane system combining biodegradable polymer, biodegradable solvent, biodegradable pore former, and purely mechanical filtration mechanisms. This sustainable approach positions these membranes as viable, environmentally friendly alternatives for air filtration applications.

4. Conclusions

Potentially biodegradable PLA membranes were fabricated via NIPS for air filtration using PEG 600 as a pore former. Solvents for dissolving PLA were selected based on RED values calculated using Hansen solubility parameters. Both environmentally persistent solvents (NMP and DMAc) and biodegradable solvents (ETAc and GBL) with RED < 1, which is required for high solvent-polymer affinity, were chosen for this study. Surface analysis conducted by Fourier transform infrared spectroscopy (FTIR) showed that all membranes exhibited spectra similar to pristine PLA, suggesting no chemical reactions occurred between the polymer and the solvents. Additionally, there were no peaks indicating the presence of the pore former (PEG 600), which suggested migration of the relatively short-chain pore former into the nonsolvent during the NIPS process. Thermal analysis via DSC and TGA indicated that PLA membranes fabricated using ETAc and GBL demonstrated superior thermal stability compared to those made with NMP and DMAc. This enhanced stability was attributed to the reduced plasticization effect of the biodegradable solvents on the PLA membranes. Furthermore, membranes fabricated using ETAc exhibited higher contact angles compared to the other membranes, making them more attractive for air filtration applications. Aerosol filtration efficiency tests showed that all membranes captured more than 95% of 0.3 µm aerosols; M_GBL did not show significant airflow rate. Based on the results obtained, M_(ETAc) membranes appeared to be the most favorable for air filtration applications due to their hydrophobicity, air permeability, and high aerosol filtration efficiency performance. It is noteworthy that the asymmetric structure of NIPS-fabricated membranes includes dense surface layers that may limit air permeability compared to the porous bulk structure. Future optimization studies could investigate controlled surface layer removal techniques (mild chemical etching or mechanical polishing) or modification of NIPS parameters (delayed immersion or controlled humidity exposure) to reduce selective layer thickness and enhance air permeability without compromising filtration efficiency or structural integrity.

This study focused on initial characterization and performance evaluation under controlled laboratory conditions. Long-term performance under real-world operating conditions remains to be investigated, including stability and filtration efficiency over extended periods of continuous or intermittent use, performance under varying environmental conditions, particularly high humidity, where moisture may affect PLA properties, temperature cycling effects on membrane structure given the relatively low glass transition temperature of PLA membranes, and regeneration capability through multiple cleaning cycles with assessment of structural integrity and performance retention after repeated decontamination treatments. Additionally, long-term biodegradability and degradation kinetics under various environmental conditions, as well as scale-up challenges in translating laboratory-scale fabrication to industrial production, warrant further investigation. Future work should address these limitations through systematic long-term stability studies, accelerated aging tests, and real-world pilot demonstrations. Investigation of PLA modifications such as blending, cross-linking, or surface treatments could potentially improve thermal stability and moisture resistance while maintaining biodegradability. Economic analysis comparing the cost of biodegradable versus conventional solvent systems, including waste disposal considerations, would also inform practical implementation decisions. Despite these limitations, this study provides a strong foundation demonstrating that biodegradable solvents can successfully replace hazardous alternatives in PLA membrane fabrication for air filtration, achieving competitive performance while offering significant environmental and health benefits. The fully biodegradable M_ETAc membrane system represents a promising pathway toward sustainable air filtration technology concerns.