Effect of PET support membrane thickness on water permeation behavior of thermally responsive PNIPAM-g-PET membranes

Using Excimer Laser for Manufacturing Stimuli Responsive Membranes

A 248 nm KrF excimer laser can be used to manufacture temperature and pH-responsive polymer-based membranes for controlled transport applications. This is done by a two-step approach. In the first step, well-defined/shaped and orderly pores are created on commercially available polymer films by ablation by using an excimer laser. The same laser is used subsequently for energetic grafting and polymerization of a responsive hydrogel polymer inside the pores fabricated during the first step. Thus, these smart membranes allow controllable solute transport. In this paper, determination of appropriate laser parameters and grafting solution characteristics are illustrated to obtain the desired membrane performance. Fabrication of membranes with 600 nm to 25 μm pore sizes by using the laser through different metal mesh templates is discussed first. Laser fluence and the number of pulses need to be optimized to obtain the desired pore size. Mesh size and film thickness primarily control the pore sizes. Typically, pore size increases with increasing fluence and the number of pulses. Larger pores can be created by using higher fluence at a given laser energy. The vertical cross-section of the pores turns out to be inherently tapered due to the ablative action of the laser beam. The pores created by laser ablation can be grafted with PNIPAM hydrogel by using the same laser to perform a bottom-up grafting-from type pulsed laser polymerization (PLP) in order to achieve the desired transport function controlled by temperature. For this purpose, a set of laser frequencies and pulse numbers need to be determined to obtain the desired hydrogel grafting density and the extent of cross-linking, which ultimately provide controlled transport by smart gating. In other words, on-demand switchable solute release rates can be achieved by controlling the cross-linking level of the microporous PNIPAM network. The PLP process is extremely fast (few seconds) and provides higher water permeability above the lower critical solution temperature (LCST) of the hydrogel. Experiments have shown high mechanical integrity for these pore-filled membranes, which can sustain pressures up to 0.31 MPa. The monomer (NIPAM) and cross-linker (mBAAm) concentrations in the grafting solution need to be optimized in order to control the network growth inside the support membrane pores. The cross-linker concentration typically has a stronger effect on the temperature responsiveness. The pulsed laser polymerization process described can be extended to different unsaturated monomers, which can be polymerized by the free radical process. For example, poly(acrylic acid) can be the grafted to provide pH responsiveness to membranes. As for the effects of thickness, a decreasing trend is observed in the permeability coefficient with increasing thickness. Furthermore, the film thickness has little or no effect on PLP kinetics. The experimental results have shown that membranes manufactured by excimer laser are excellent choices for applications where flow uniformity is the prime requirement, as they possess uniform pore sizes and distribution.

Recent Developments and Perspectives of Recycled Poly(ethylene terephthalate)-Based Membranes: A Review

Post-consumer poly(ethylene terephthalate) (PET) waste disposal is an important task of modern industry, and the development of new PET-based value added products and methods for their production is one of the ways to solve it. Membranes for various purposes, in this regard are such products. The aim of the review, on the one hand, is to systematize the known methods of processing PET and copolyesters, highlighting their advantages and disadvantages and, on the other hand, to show what valuable membrane products could be obtained, and in what areas of the economy they can be used. Among the various approaches to the processing of PET waste, we single out chemical methods as having the greatest promise. They are divided into two large categories: (1) aimed at obtaining polyethylene terephthalate, similar in properties to the primary one, and (2) aimed at obtaining copolyesters. It is shown that among the former, glycolysis has the greatest potential, and among the latter, destruction followed by copolycondensation and interchain exchange with other polyesters, have the greatest prospects. Next, the key technologies for obtaining membranes, based on polyethylene terephthalate and copolyesters are considered: (1) ion track technology, (2) electrospinning, and (3) non-solvent induced phase separation. The methods for the additional modification of membranes to impart hydrophobicity, hydrophilicity, selective transmission of various substances, and other properties are also given. In each case, examples of the use are considered, including gas purification, water filtration, medical and food industry use, analytical and others. Promising directions for further research are highlighted, both in obtaining recycled PET-based materials, and in post-processing and modification methods.

The role of lateral size of MXene nanosheets in membrane filtration of dyeing wastewater: Membrane characteristic and performance

New two-dimensional (2D) material MXene based lamellar membranes constructed from 2D MXene nanosheets have shown promising potential for water treatment with excellent selective property and high water flux. However, the effect of lateral size of MXene nanosheets on the membrane property and performance was rarely considered. Herein, the MXene nanosheets with different lateral size (552.3 nm, 397.5 nm and 281.8 nm) segregated via adjusting centrifugation conditions were used to prepare MXene membranes. XRD and cross-sectional SEM images confirmed that the resulting MXene membranes had the similar d-spacing and thickness. The MXene membrane with the smallest lateral size, MXene_(S)-M, owned the largest surface roughness with reduced surface hydrophilicity. Lateral size determined mass transfer pathway and transfer resistance, which consequently influenced the water permeance and rejection of MXene membranes for dyeing wastewater treatment. MXene_(S)-M with the shortest mass transfer pathway had the high water permeance while the MXene membrane with larger lateral size (MXene_(L)-M and MXene_(M)-M), possessing longer mass transport pathway, promoted high dye rejection.

Fabrication of pH-Responsive Polyimide Polyacrylic Acid Smart Gating Membranes: Ultrafast Method Using 248 nm Krypton Fluoride Excimer Laser

pH-responsive smart gating membranes were developed using a two-step fabricating process. In the first step, a porous polyimide (PI) support membrane with ordered, regular, and well-defined pores was obtained with a 248 nm KrF excimer laser using a lithography technique. The porous membranes were then grafted with poly(acrylic acid) (PAAc) hydrogel by free radical polymerization using the same excimer laser. The number of pulses and frequency could be varied to obtain a range of water permeabilities. Permeability of membrane changed significantly due to swelling and deswelling of PAAc inside the pores at pH 7 and pH 3, respectively. These hydrogel networks were firmly grafted inside pores and remained mechanically intact even after using high pressure during permeability studies. PAAc grafting was confirmed using ATR-FTIR. PAAc hydrogel distribution inside membrane pores was analyzed using SEM and fluorescence microscopy. To quantify the amount of polymer grafted, TGA studies were carried out. Diffusion studies were also carried out using caffeine as a drug molecule to evaluate the application of membrane in drug delivery devices. The linear drug release profile obtained from the study confirmed the potential application of membrane for drug delivery purposes. Results obtained also suggest that the fabrication method developed is fast, efficient, solvent-free, and economical.

Membrane Fouling in Algal Separation Processes: A Review of Influencing Factors and Mechanisms

The use of algal biotechnologies in the production of biofuels, food, and valuable products has gained momentum in recent years, owing to its distinctive rapid growth and compatibility to be coupled to wastewater treatment in membrane photobioreactors. However, membrane fouling is considered a main drawback that offsets the benefits of algal applications by heavily impacting the operation cost. Several fouling control strategies have been proposed, addressing aspects related to characteristics in the feed water and membranes, operational conditions, and biomass properties. However, the lack of understanding of the mechanisms behind algal biofouling and control challenges the development of cost-effective strategies needed for the long-term operation of membrane photobioreactors. This paper reviews the progress on algal membrane fouling and control strategies. Herein, we summarize information in the composition and characteristics of algal foulants, namely algal organic matter, cells, and transparent exopolymer particles; and review their dynamic responses to modifications in the feedwater, membrane surface, hydrodynamics, and cleaning methods. This review comparatively analyzes (i) efficiency in fouling control or mitigation, (ii) advantages and drawbacks, (iii) technological performance, and (iv) challenges and knowledge gaps. Ultimately, the article provides a primary reference of algal biofouling in membrane-based applications.

Intelligent Polymers, Fibers and Applications

Intelligent materials, also known as smart materials, are capable of reacting to various external stimuli or environmental changes by rearranging their structure at a molecular level and adapting functionality accordingly. The initial concept of the intelligence of a material originated from the natural biological system, following the sensing–reacting–learning mechanism. The dynamic and adaptive nature, along with the immediate responsiveness, of the polymer- and fiber-based smart materials have increased their global demand in both academia and industry. In this manuscript, the most recent progress in smart materials with various features is reviewed with a focus on their applications in diverse fields. Moreover, their performance and working mechanisms, based on different physical, chemical and biological stimuli, such as temperature, electric and magnetic field, deformation, pH and enzymes, are summarized. Finally, the study is concluded by highlighting the existing challenges and future opportunities in the field of intelligent materials.

Smart membranes by electron beam cross-linking of copolymer microgels

Poly(N-isopropylacrylamide) (pNIPAM) based copolymer microgels were used to create free-standing, transferable, thermoresponsive membranes. The microgels were synthesized by copolymerization of NIPAM with N-benzylhydrylacrylamide (NBHAM). Monolayers of these colloidal gels were subsequently cross-linked using an electron gun leading to the formation of a connected monolayer. Furthermore, the cross-linked microgel layer is detached from the supporting material by dissolving the substrate. These unique systems can be used as transferable, thermoresponsive coatings and as thermoresponsive membranes. As a proof of principle for the use of such membranes we studied the ion transport through them at different temperatures revealing drastic changes when the lower critical solution temperature of the copolymer microgels is reached.

UV cross-linked smart microgel membranes as free-standing diffusion barriers and nanoparticle bearing catalytic films

In this study we use poly(N-isopropylacrylamide) (PNIPAM) based copolymer microgels to create free-standing, transferable, thermoresponsive membranes. The microgels are synthesized by copolymerization of NIPAM with 2-hydroxy-4-(methacryloyloxy)–benzophenone (HMABP) and spin-coated on Si wafers. After subsequent cross-linking by UV-irradiation, the formed layers easily detach from the supporting material. We obtain free standing microgel membranes with lateral extensions of several millimetres and an average layer thickness of a few hundred nanometres. They can be transferred to other substrates. As one example for potential applications we investigate the temperature dependent ion transport through the membranes via resistance measurements revealing a sharp reversible increase in resistance when the lower critical solution temperature of the copolymer microgels is reached. In addition, prior to cross-linking, the microgels can be decorated with silver nanoparticles and cross-linked afterwards. Such free-standing nanoparticle hybrid membranes are then used as catalytic systems for the reduction of 4-nitrophenol, which is monitored by UV/Vis spectroscopy.

Cross-linkable microgels are synthesized by copolymerization of NIPAM with 2-hydroxy-4-(methacryloyloxy)–benzophenone (HMABP) and are subsequently UV-cross-linked to obtain smart membranes exhibiting switchable resistance.