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Surgical Contamination Control. Diagnostic Testing. Indicator Strips. Test Kits. Urine Collection. Body Bags. Patient Care. Digestive Care. Respiratory Care. PPE Garments. Given the need for fabricating desalination membranes with increased selectivity 8 , 30 , 31 , it is critical to understand the role of polymer structure in desalination performance to guide membrane design. Here, we demonstrate a facile method to tune the transport properties of a polymeric desalination membrane via a plasticizer-induced swelling and subsequent deswelling process, which results in modification of the internal structure of the polymer material.
Investigations of material characteristics reveal that the plasticizer, p-nitrophenol, penetrates the polymeric matrix and transforms crystalline regions to amorphous regions. Subsequent extraction of the plasticizer from the polymer matrix by water rinsing induces the rearrangement of polymer chains followed by recrystallization.
Moreover, structural characterization by optical microscopy and X-ray diffraction suggests a size reduction of the crystallites distributed in the amorphous matrix where the latter primarily governs the mass transport properties. Our findings demonstrate that the plasticizing—extracting process can be harnessed to improve the desalination performance of asymmetric polymeric membranes. We developed a simple procedure to tune the performance of polymeric desalination membranes, as schematically illustrated in Fig.
Briefly, pristine cellulose triacetate CTA membranes were soaked in a p-nitrophenol PNP solution, followed by water rinsing to obtain modified membranes. The membranes were subsequently characterized in a forward osmosis FO setup to evaluate the effect of PNP treatment on desalination performance, namely water flux, J w , and reverse salt flux, J s. Additionally, we calculated the ratio of J w over J s , which is expressed as 32 , A pristine CTA membrane is soaked in a PNP solution, followed by a thorough rinse with water to obtain a modified membrane.
Membrane performance results were normalized to that of the corresponding pristine membranes. Error bars represent standard deviation from duplicate experiments. The dashed line is an eye guide showing the change of transport properties with an increase of PNP concentration. However, the salt flux of the modified membrane showed a sharp decrease of Notably, the modified membranes displayed an obvious tendency to curl toward the active layer Supplementary Fig. This structural change strongly suggests that PNP treatment induced an asymmetric effect on the active and the support layers of the membrane, thereby eventually influencing the water and salt transport in the polymeric CTA membrane.
We also investigated the influence of PNP species protonated vs. Moreover, under the same effective concentration of the protonated PNP i. Taken together, these results suggest that only the protonated form of PNP was responsible for the changes in membrane structure and desalination performance. The ineffectiveness of the deprotonated PNP in affecting membrane properties is likely ascribed to the electrostatic repulsion and the relatively large hydrated size of the anionic deprotonated species compared to the neutral protonated form, thereby lowering its modification effect by hindering adsorption and penetration into the polymer matrix 35 , Additionally, the unlikely formation of hydrogen bond between the deprotonated PNP and the CTA polymer is also a critical factor weakening their interactions detailed analysis provided in the Supplementary Discussion and Supplementary Fig.
Notably, the decrease of salt permeability is more pronounced than that of water permeability Supplementary Fig. To circumvent the possible influence of additives and impurities in the commercial membrane, we fabricated a pure CTA film as a model system via an identical phase separation process for mechanistic analysis.
The as-prepared CTA film denoted as pristine contained crystalline micro-domains coexisting with amorphous regions in the polymer, endowing the film with remarkable opaqueness upper panel in Fig. This partially crystalline nature of the CTA film can be demonstrated by the birefringence displayed in the polarized optical microscopy POM image upper panel in Fig. Soaking in a PNP solution, however, significantly changed the optical property of the film denoted as swelled , as reflected by the observed high transparency of the CTA film middle panel in Fig.
Moreover, the soaked CTA film became soft compared to the pristine one. Such observations suggest a remarkable penetration of PNP into the CTA film that suppressed the crystallinity of the polymer, as soaking the film in pure water did not lead to the observed change. The loss of crystallinity and birefringence implied that PNP acted as a plasticizer in CTA that homogeneously swelled the polymeric chains 37 , 38 , Since the desalination performance was evaluated when the membrane was thoroughly rinsed by water to release the PNP molecules from the polymer matrix, for consistency, we also rinsed the swelled CTA film with pure water.
The washed CTA film denoted as deswelled, lower panels of Fig.
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This finding suggests that the average size of the crystallites in the polymer became smaller after PNP treatment. Structural characteristics of CTA films. The pristine film displays whiteness and opaqueness upper panel in a , indicative of the presence of crystalline regions as confirmed by the birefringence upper panel in b. The crystalline structure was destroyed in the swelled film due to the penetration of PNP, which can be demonstrated by the observed transparency middle panel in a and the loss of the birefringence middle panel in b.
Further rinsing with water resulted in the recovery of the white and opaque properties and the birefringence of the deswelled film lower panels in a , b , suggesting the recrystallization of the CTA polymer chains. After soaking in PNP, the modified membrane without water rinsing i. As water rinsing proceeded, both T g and crystalline and melting peaks were gradually recovered to values comparable to those of the pristine films, implying that the gradual leaking of PNP induces rearrangement of polymeric chains.
Insets are the corresponding 2-D wide-angle X-ray diffraction patterns. The open circles are the measured data and the solid curves are the Gaussian fit of the amorphous hump centered at 1.
Generally, the pristine film displayed two strong diffraction peaks black circles. The diffraction peaks almost vanished after PNP soaking for the swelled sample red circles. The extraction of PNP from the polymer matrix leads to the recovery of the diffraction peaks of the deswelled sample blue circles , but the peak widths were broader than those of the pristine film. Note that the films were completely dehydrated before measurements to exclude the possible effects of water. The pristine film had a glass transition temperature T g at The swelled film without water rinsing i.
Release of PNP from the polymer film by water rinsing corresponded to the PNP extracting process, as evidenced by a gradual recovery of T g upon increasing the rinsing time in water. Additionally, the 8-h rinsing sample also displayed values of both crystalline enthalpy H c and melting enthalpy H m comparable to those of the pristine sample. Taken together, the essentially unchanged DSC data found in the pristine and the rinsed CTA films suggests that the PNP treatment did not lead to a remarkable alteration of the thermal properties of the polymer.
To elucidate quantitatively why such a PNP plasticizing—extracting process affects the membrane properties, we employed wide-angle X-ray diffraction WAXD to characterize the pristine, swelled, and deswelled CTA films, whose 2-D patterns are the insets of Fig. As seen from the integrated 1-D X-ray diffraction patterns Fig.
These two peaks almost vanished after the penetration of PNP into the film swelled sample , indicating the destruction of the crystallized polymer structures by PNP, which is consistent with the loss of the optical birefringence observed in the POM image. Rinsing off the PNP from the films with water deswelled sample led to the re-emergence of the two diffraction peaks characteristic of the crystal structure.
The WAXD patterns qualitatively show that the widths of the Bragg scattering peaks in the deswelled film were much broader compared to those of the pristine film. To compare the difference quantitatively, it is necessary to deconvolute the observed diffraction intensity into amorphous and crystalline components The crystalline peak profile was then obtained by subtracting the amorphous component from the original diffraction curve data processing details appear in the Supplementary Methods.
We fitted these peaks using Gaussian functions and found that the peak centered at 1. If we ignore any contribution from the instrumental line and microstrains, the broadening of the diffraction peaks indicates the reduction of the crystallite size in the deswelled films. However, the overall crystallinity of the polymer in the deswelled film did not change, consistent with the indiscernible DSC data of the pristine and the deswelled samples.
Therefore, the majority of the polymeric chains are quenched in the polymer matrix, forming a skin layer comprising large crystallites embedded in amorphous regions Fig. Schematic illustration of a proposed mechanism for the role of p-nitrophenol PNP treatment in tuning transport properties of the polymeric desalination membrane. In this step, PNP acts as a plasticizer to increase the chain mobility of the polymer matrix. This equilibrium process does not affect the overall crystallinity but induces the formation of smaller crystallites, thereby enlarging the interfacial area between the amorphous and crystalline regions.
This increased interfacial area could facilitate the fixation of the amorphous loops in the crystalline lattice, thereby reducing the number of nonselective pathways for mass transport. Specifically, the partitioning of PNP molecules into the polymeric matrix not only enhances the chain mobility in the amorphous regions but also swells the crystalline domains, resulting in disruption of the molecular packing. When rinsed with water, the incorporated PNP molecules tend to leach out of the polymer film. Notably, as the overall crystallinity of the deswelled sample did not show a notable change compared to that of the pristine sample, smaller crystallites result in a larger interfacial area between amorphous and crystalline regions.
While this fundamental analysis was the main goal of this study, the results provide guidelines for the fabrication of membranes with desired performance and permselectivity. PNP treatment directly influences polymeric membrane transport properties by decreasing both water and salt permeabilities. However, membranes soaked in lower PNP concentrations e.
The unchanged water flux is a result of the combined effects of a simultaneous reduction in both water and salt permeabilities. While a lower water permeability leads to a reduction in water flux, a decrease in salt permeability reduces the reverse salt flux, thus reducing internal concentration polarization ICP inside the membrane support. In this case, the suppression of ICP, driven by a reduction in reverse salt flux, compensates for the reduction in water flux caused by the reduction in water permeability details of the mathematical analysis provided in the Supplementary Discussion 43 , 44 , Moreover, the enhanced permselectivity, achieved through PNP treatment, is advantageous for improving product water quality in current desalination systems by rejecting pollutants with low molecular weight Optimization of the treatment conditions, such as soaking duration Supplementary Fig.
In conclusion, we have developed an effective approach to tune the permselectivity of polymeric desalination membranes by a plasticizer-induced swelling and deswelling process. The treated membranes exhibit decreases in both water and salt permeabilities, owing to the reduction of crystallite size in the crystalline regions and chain mobility in the amorphous regions.
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The more significant effect on salt permeability relative to water permeability led to a higher permselectivity, thereby rendering the modified membrane with enhanced desalination performance. Our findings not only provide mechanistic insights into the structure-property-performance relationship of polymeric desalination membranes, but also offer a lesson for the future design of asymmetric polymeric membranes with desired desalination performance.
Following this step, the treated membranes were thoroughly rinsed with DI water to obtain modified membranes.
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Membrane desalination performance was characterized using a custom-built forward osmosis FO setup Supplementary Fig. Water flux, J w , and reverse salt flux, J s , were obtained by measuring the change of volume and salt concentration in the feed solution using a scale and a conductivity meter, respectively. Considering the inherent variation in membrane performance among coupons, we characterized the desalination performance of every pristine membrane prior to the modification.
Results for a modified membrane were normalized to its corresponding pristine data. Based on the measured desalination performance data, we calculated the transport coefficients, including water permeability, A , and salt permeability, B , which are independent of testing conditions details in the Supplementary Methods. These pristine CTA films also underwent a modification procedure identical to that for the commercial membrane coupons to obtain modified films.
The films were air dried prior to material characterization. The calibration of the resultant 2-D WAXD patterns was done by using a silver behenate standard d-spacing of 3. The data that support the findings of this study are available from the corresponding authors upon reasonable request. United Nations General Assembly. Press Release Mauter, M. The role of nanotechnology in tackling global water challenges. Shannon, M. Science and technology for water purification in the coming decades.
Nature , — Gleick P. Water in crisis: a guide to the worlds fresh water resources Oxford University Press, Greenlee, L. Water Res. Lattemann S. Elsevier, Werber J. Materials for next-generation desalination and water purification membranes. Park, H. Maximizing the right stuff: The trade-off between membrane permeability and selectivity.
Science , eaab Loeb S. American Chemical Society, Cadotte, J. A new thin-film composite seawater reverse osmosis membrane. Desalination 32 , 25—31 Hinds, B. Aligned multiwalled carbon nanotube membranes. Science , 62—65 McGinnis R. Large-scale polymeric carbon nanotube membranes with sub—1.
Nair, R. Unimpeded permeation of water through helium-leak—tight graphene-based membranes. Science , — Surwade, S. Water desalination using nanoporous single-layer graphene. Kumar, M. Highly permeable polymeric membranes based on the incorporation of the functional water channel protein Aquaporin Z. Natl Acad. USA , — Werber, J. Permselectivity limits of biomimetic desalination membranes.