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In recent years, surface patterning of membranes has been explored as a new strategy to modify surface properties of polymeric membranes. A variety of methods including template-based micromolding and direct printing have been developed for effective fabrication of surface-patterned membranes. In this review, we compare the underlying pattern replication mechanisms and the advantages and challenges associated with the range of different fabrication methods. The presence of the surface patterns, when properly created, can enlarge the active surface/interfacial area, create more effective conduction pathways, and enhance the hydrodynamic effects. These effects can be harnessed for improving membrane performance for a wide range of applications. [ABSTRACT FROM AUTHOR]

The influence of substrate topography and interfacial polymerization (IP) conditions were investigated during the fabrication of patterned thin-film composite (TFC) membranes. Aromatic and semi-aromatic polyamide layers were formed atop patterned ultrafiltration (UF) membrane supports by IP using different concentrations of m-phenylenediamine (MPD) or piperazine (PIP) in water of 0.01–2% w/v with a fixed concentration of trimesoyl chloride in hexane of 0.1% w/v. For all the conditions evaluated, TFC membranes with regular surface patterns were achieved by maintaining amine soaking time and IP reaction time within 120 s. Importantly, the surface topography of the patterned TFC membranes was determined to be independent of IP reaction time. Characterization of the morphological details suggests non-conformal growth of the barrier layer on the patterned UF substrates. Results indicate that the extent of such non-conformal growth can be reduced by decreasing the amine concentration as well as by choosing an amine monomer such as PIP that produces a thinner semi-aromatic barrier layer. The overall findings of this study provide a means for achieving desired surface features for specific membrane applications. [ABSTRACT FROM AUTHOR]

The effect of pressure on fouling layers formed by diluted activated sludge was studied in real time with ultrasonic reflectometry by fouling microfiltration membranes for 300 min at constant pressure (0.15 bar (15 kPa) and 0.25 bar (25 kPa)), as well as by performing a series of pressure-step experiments in which the pressure was instantaneously increased from 0.15 to 0.25 bar. For the constant pressure experiments, the change in ultrasonic signal amplitude was inversely proportional to the degree of fouling as represented by changes in permeability and the amount of deposited material. This finding was verified by a series of replicated filtration experiments of 15, 30, and 60-min duration at 0.15 bar, which indicated a statistically significant correlation between the degree of fouling (quantified by permeability loss and post-mortem characterization metrics) and ultrasonic amplitude change. Diluted activated sludge filtrations at varying pressures revealed that there is less ultrasonic amplitude reduction at higher pressure where the resistance of the fouling layer is higher. This finding reflects the formation of a hydrated fouling deposit that serves as an impedance matching layer with the water-filled membrane that produces lower reflection at lower pressure. Thus, the fouling layer is thought to be less hydrated and denser at higher pressures, which confirms that the fouling layer is a compressible structure. Given that fouling layer mechanical behavior may well influence membrane filtration performance, it may be possible to improve membrane bioreactor filtration by engineering fouling layer compressibility. [ABSTRACT FROM PUBLISHER]

Previous work has suggested that membrane patterning is a promising approach to fouling mitigation. In this systematic study, we describe performance metrics for the cross-flow filtration of colloidal suspensions through ultrafiltration membranes with lithographically patterned surfaces. The effects of particle size, cross-flow velocity, pattern size, and flow configuration on membrane performance are specifically considered. Using an unpatterned membrane as a reference, results show that the presence of surface patterns increases the critical flux associated with filtration of colloidal feed solutions. For the patterned membranes, the critical flux increases with particle size, cross-flow velocity, the angle between the feed flow and the pattern lines, and the pattern height. These experimental findings can be correlated with a back-transport mechanism such as shear-induced diffusion, which ultimately increases the threshold permeation flux associated with the onset of particle deposition. [ABSTRACT FROM AUTHOR]

Abstract: Thin film composite (TFC) membranes are critical components for reverse osmosis (RO) and nanofiltration (NF) processes. Similar to other liquid-based filtration membranes, TFC membranes are susceptible to concentration polarization and fouling/scaling. Recently, surface topography modification has been shown as a potential approach for fouling mitigation. However, for TFC membranes, tailoring the surface topography remains a challenge. Here, we demonstrate for the first time, successful fabrication of a patterned TFC membrane. A two-step fabrication process was carried out by (1) nanoimprinting a polyethersulfone (PES) support, and (2) forming a thin dense film atop the PES support via interfacial polymerization (IP) with trimesoyl chloride and 1,3-phenylenediamine solutions. Chemical, topographic, and permeation characterization was performed on the imprinted IP membranes, and their permselectivity was compared with that of a flat (non-imprinted) TFC membrane prepared using the same IP procedure. [Copyright &y& Elsevier]

Abstract: Synthetic membranes are used in applications such as controlled drug release, artificial organs, smart sensors, and water treatment that require knowing the pore-size distribution (PSD). Current PSD characterization methods can require dedicated and/or expensive equipment as well as special expertise to implement. Evapoporometry (EP) is a novel characterization technique based on vapor pressure depression that can accurately determine the pore size for ultrafiltration membranes using a simple diffusion cell and a microbalance. A straightforward characterization procedure and comprehensive error analysis are described. EP characterization of the PSD of a 100nm bilayer Anopore™ membrane used as a standard is shown to compare favorably with the nominal pore diameter, FESEM analysis, and analyses reported by prior investigators. In addition, EP is utilized for PSD characterization of commercially fabricated 20 and 50nm polyvinylidene fluoride (PVDF) membranes for which the results are compared with those obtained via liquid displacement porometry (LDP). The mean pore diameters determined by EP and LDP are in good agreement for the 50nm PVDF membranes, but the lower values obtained via LDP for the 20nm PVDF membranes may reflect the effects of high pressures that are not required in EP. The advantages and limitations of EP as well as other potential applications of this technique are discussed. [Copyright &y& Elsevier]

Ultrasonic time-domain reflectometry (UTDR) involves passing an acoustic wave through a medium and analyzing the reflected waveform. In this study, UTDR is used to track the waveform peaks reflected from the outer and inner membranes in the outermost feed channel in a Koch 2521 spiral-wound module. The UTDR amplitude is shown to be more sensitive to fouling than the transit (arrival) time. The local (point) measurement provided by UTDR is shown to be advantageous since its location can be optimized for early fouling detection. Concentration polarization is shown not to compromise UTDR. This is an advantage relative to flux decline that responds to both fouling and concentration polarization. The UTDR amplitude is found to increase with increasing temperature and decreasing pressure. This is explained by the effect these parameters have on the crystallization rate that changes the fouling layer morphology and thereby the reflected UTDR waveform. This study underscores the importance of establishing a UTDR reference surface suitable for unambiguous detection of fouling. It also emphasizes the necessity for good temperature and pressure control during UTDR measurements as well as a more comprehensive understanding of the effects of operating condition changes on the ultrasound waveforms. [ABSTRACT FROM AUTHOR]

Abstract: A model for the dry-casting process for polymeric membrane formation is generalized to include a broader spectrum of boundary conditions. This model is used to conduct a sensitivity analysis to assess which properties and process parameters are most influential and hence need to be determined most accurately. The model predictions are relatively insensitive to changes in the nonsolvent/polymer and nonsolvent/solvent friction coefficients but moderately sensitive to changes in the solvent/polymer friction coefficient. However, the predictions are very sensitive to changes in the gas-phase mass-transfer coefficient, which needs to be well-characterized and carefully controlled during the dry-casting process. Experimental studies for the water/acetone/cellulose acetate system demonstrate that changes in the boundary conditions can result in dramatically different membrane morphologies. The results of the modeling, sensitivity analysis and experimental studies are used to advance a simple phenomenological model for the dry-casting process that explains all the observed trends. This is used to develop a set of design heuristics for dry-casting based on the effect of changes in the properties and parameters on the casting solution and gas-phase mass-transfer resistances, the solvent concentration at the interface and the mass-transfer rate. [Copyright &y& Elsevier]

Abstract: The summative and formative fouling potential of industrial fermentation broth engineered to produce valuable industrial enzymes was investigated. A resistance-in-series approach was adapted to estimate the relative fouling potential of broth components at two different cross-flow conditions under high-loading scenarios. Bench-scale tests were executed in cross-flow cell modules fitted with polysulfone microfiltration (MF) membranes (0.2μm nominal pore size) where fermentation broth and its isolated components, i.e., bacterial cells, particle-free media, surrogate proteins, or virgin media, were separately introduced. Setting and fouling behavior were assessed in situ, using continuous ultrasonic and permeate flow observations. Fouled membranes were characterized using scanning electron microscopy (SEM) and scanning acoustic microscopy (SAM). The largest flux declines were observed when membranes were challenged with fresh industrial broth. Flux was least affected by virgin media that had been clarified by centrifugation and by a solution of washed bacterial cells. Acoustic spectra showed significant increases in reflected power immediately following broth challenges. Above a threshold of 14μgcm−2, SAM showed that acoustic reflection patterns from fouled membranes were statistically different than their otherwise clean-condition counterparts. SEM observations were consistent with acoustic spectra and biochemical autopsy revealing rapid, progressive biomass build-up from dynamic conglomeration of “patchy”, heterogeneous biomass deposits that are loosely anchored to the membrane surface. Results were interpreted via modeling approaches using resistance-in-series as well as combined pore-blockage and cake filtration. [Copyright &y& Elsevier]

Abstract: A major concern with the use of polyamide thin film composite (TFC) membranes is their loss of performance when exposed to oxidizing agents such as aqueous chlorine that is used in membrane cleaning and water disinfection. A better understanding of this oxidative degradation process would facilitate the development of improved TFC membranes. This study utilizes pendant drop mechanical analysis (PDMA) to measure chlorine-exposure-induced changes in the behavior of polyamide barrier-layers formed by the interfacial polymerization (IP) of m-phenylenediamine (MPD) and trimesoyl chloride (TMC). PDMA enables the measurement of mechanical and transport properties of unsupported MPD–TMC thin films. Based upon the PDMA results as well as those obtained from experiments conducted on commercial TFC RO membranes, a hypothesis is presented describing a two-pathway mechanism for the chlorine degradation of polyamide TFC membranes. At a constant pH, a morphology change, not polymer chain scission, occurs leading to separation of the barrier layer from the support. A second degradation pathway is observed when chlorine exposure under acidic conditions is followed by exposure of the polyamide thin-film to caustic (pH >11) conditions. The results of this investigation provide a strong rationale for further refinement of the PDMA methodology for use in basic studies as well as for screening applications that could aid in the development of chlorine-resistant IP thin films. [Copyright &y& Elsevier]

This paper describes the development of an ultrasonic technique for the characterization of membrane morphology. Ultrasonic frequency-domain reflectometry (UFDR) using a 90 MHz focused immersion transducer has been employed to obtain characteristic acoustic responses from microporous polymeric membranes with nominal pore sizes in the range of 0.1–0.6 μm. Systematic and statistically significant differences in signal amplitude are observed in the frequency domain for reflections from the back surface of poly(vinylidene fluoride) (PVDF) and mixed cellulose ester (MCE) membranes. As the pore-size of the membranes increases, the amplitude of the reflected signal at high frequencies decreases due to increased scattering of the ultrasonic wave. These UFDR differences correspond well with those obtained from independent measurements including scanning electron microscopy (SEM) and gas–liquid porometry. A simple phenomenological artificial neural network (ANN) model has been developed to predict membrane pore-size based upon the signal amplitude at five frequencies. Such predictive capability suggests that the combination of UFDR and ANN may have significant utility for membrane quality control applications. [Copyright &y& Elsevier]

Poly(vinylidene fluoride) (PVDF) membranes fabricated from conventional processes have a limited range of microstructures. For example, PVDF membranes with spherulitic and isotropic microstructures containing macrovoids are extremely difficult to avoid using standard casting techniques such as wet-casting and thermally induced phase-separation (TIPS) casting. In order to create novel PVDF membrane microstructures such as a spongy, open-celled structure, a new membrane-formation process with six independent process parameters has been developed. Because this new process, thermally assisted evaporative phase separation (TAEPS), has a large number of parameters, the membrane microstructures are easily changed. The TAEPS process can fabricate a PVDF membrane with a spongy, anisotropic microstructure without macrovoids using a simple ternary system with no additives. In addition to studying the novel and unique microstructures available with the TAEPS process, the relationship between final morphology, performance, and processing conditions has been investigated in this paper. [Copyright &y& Elsevier]

Poly (ethylene chlorotrifluoroethylene) (ECTFE) is a 1:1 alternating copolymer of ethylene and chlorotrifluoroethylene with an excellent combination of chemical, thermal and mechanical properties. Although the lack of a suitable solvent for ECTFE precludes membrane casting by conventional room-temperature processes of immersion precipitation or evaporative casting, thermally induced phase separation provides an attractive pathway for membrane fabrication. We have successfully identified three possible latent solvents for ECTFE and have studied the ECTFE-dibutyl phthalate (DBP) polymer-diluent system. ECTFE membranes were fabricated using a TIPS-casting apparatus that allowed control of several important process parameters. ECTFE membranes were characterized via scanning electron microscopy (SEM), porometry and permeation techniques. Results indicated that membranes with continuous pores having diameters in the microfiltration range (0.1–0.5 μm) could be reliably fabricated. [Copyright &y& Elsevier]

Experiments were conducted onboard a NASA KC-135 aircraft in order to assess the validity of two hypotheses proposed for the growth of macrovoid (MV) pores formed during the dry-casting of cellulose acetate (CA)/acetone/water casting solutions. The KC-135 aircraft provides the capability for greatly reducing the effective gravitational body forces that influence the buoyancy force on MVs. Buoyancy should have no effect on MV growth as proposed in the purely diffusive growth hypothesis but should influence MV growth via the solutocapillary convection hypothesis since the latter involves a balance between Marangoni, viscous drag, and buoyancy forces. CA membranes were cast in low-gravity (low-g) (KC-135) and normal-gravity (1-g) (ground-based control) from CA/acetone/water solutions as a function of the solvent/non-solvent (S/NS) ratio. Morphological analysis indicated that MV growth was enhanced in low-g only for the case in which the S/NS ratio=2.0; no effect was observed for higher values of the S/NS ratio. These studies provide support for the solutocapillary convection hypothesis; however, the present data do not unambiguously demonstrate the occurrence of solutocapillary convection. Further research is required to provide such proof. [Copyright &y& Elsevier]

The permeation resistance of the porous support in a thin film composite (TFC) membrane is believed to be negligible compared with that of the thin polyamide film. Indeed, the resistance of a porous support, as determined from direct water permeation experiments as a function of transmembrane pressure (TMP), is only a fraction of the total resistance of a typical TFC membrane. However, the true resistance of the porous support in a TFC membrane, especially during high-pressure reverse osmosis (RO), could be significantly different from the value obtained via direct permeation measurement. Although the porous support is subjected to mechanical loading equivalent to the overall TMP, the effective TMP across the porous support that generates deformation can be significantly lower due to the internal pressure of the water within the pores. In this study we utilized a serial configuration, consisting of a TFC membrane on top of a representative porous support, to quantify the true resistance of the porous support under conditions of high mechanical loading but low permeation. Unsurprisingly, the results confirm that the porous support resistance appears negligible as compared with that of the TFC membrane under low TMP. However, after exposure to high TMP in the serial configuration, the porous support evidences up to 15X larger plastic strain and 28X higher resistance as compared to the same support tested separately under low TMP. At TMP = 8.3 MPa, the additional porous support beneath the TFC membrane increases the overall resistance by 31%. Using a resistance in-series model, the data suggest that the true resistance of the porous support could be as high as ~ 45% that of the thin film under the high-TMP but low-permeation condition encountered during high-pressure RO desalination. • A serial configuration is used to determine the true permeation resistance of the porous support under RO condition. • The plastic strain of the porous support under the serial configuration is determined. • The porous support experience significantly more deformation under RO condition than under direct permeation measurements. • True resistance of the porous support is significant for a TFC membrane under high-pressure RO. [ABSTRACT FROM AUTHOR]

This review describes the use of ultrasonic reflectometry (UR) for characterizing membranes and membrane processes. A growing body of literature documents the capabilities of UR as a versatile nondestructive, noninvasive, real-time, and low-cost methodology that can provide important information about a wide range of membrane-based separations. A compact but thorough explanation of the ultrasonic measurement concepts relevant for use of the methodology for membrane applications is first presented. This section is followed by a description of the many studies in which UR has been employed for characterization of membrane structure, formation, compaction, and inorganic and organic membrane fouling, the latter in both real-time and post-mortem modes. Examples of recent work that incorporates the innovative use of UR for scaling in nanofiltration and reverse osmosis applications as well as results that suggest the potential of the ultrasonic slow wave to monitor pore closure in early-stage fouling are then highlighted. UR is then compared with other techniques for fouling detection so that the advantages and limitations of UR can be placed in proper perspective. Finally, valuable future directions for the incorporation of UR in membrane research, development, and practice are considered. [ABSTRACT FROM AUTHOR]

Molecular layer deposition (MLD) is a gas-phase deposition technique that can create ultra-thin films with precisely controlled chemical composition and thickness by depositing one monolayer at a time. This makes MLD an attractive technology for desalination membranes among other applications. Given its relatively recent development, little information has been reported regarding the properties of MLD thin films. We present the results of an initial mechanical property study of MLD films with thicknesses ranging from ∼50 to 2000 nm. MLD was utilized to create crosslinked polyamide films grown using either m -phenylenediamine (MPD) and trimesoyl chloride (TMC) reactants or piperazine (PIP) and TMC reactants. The elastic modulus of the films was determined using atomic force microscopy (AFM). The results show that the modulus was independent of film thickness with values of 4.36 ± 1.19 GPa and 5.24 ± 1.06 GPa for the MLD films grown using the MPD-TMC and PIP-TMC chemistries, respectively. These values are of the same order of magnitude as those reported for much thicker polyamide films, but higher than the modulus of polyamide films fabricated using interfacial polymerization. [Display omitted] • All-organic polyamide thin films were fabricated using molecular layer deposition. • The elastic modulus of the films was measured using atomic force microscopy. • Two film chemistries with thicknesses between ∼50 and 2000 nm were evaluated. • Modulus was independent of chemistry and film thickness. • Modulus values of ∼3–6 GPa were similar to those of bulk polyamides. [ABSTRACT FROM AUTHOR]

This study directly compares the mechanical behavior of novel molecular layer deposition (MLD) and analogous interfacial polymerization (IP) polyamide thin films in environments relevant to reverse osmosis (RO) membrane operation. The elastic modulus of the films was determined using atomic force microscopy (AFM) in dry, hydrated, and chlorinated states. Surface roughness characteristics were also obtained given their potential influence on AFM modulus measurements. The much smoother MLD films demonstrated a statistically higher modulus in all states as compared to their IP counterparts. The MLD films maintained a modulus ∼3X and ∼5X greater than that of IP films after hydration and chlorination, respectively. Such differences in behavior may be due to the higher density and correspondingly lower void content of the MLD films. Results from this study provide a rationale for future development of MLD for fabrication of polyamide films for incorporation in RO membranes. [Display omitted] • Novel polyamide thin films were fabricated by molecular layer deposition (MLD). • Physical and mechanical characteristics of MLD films were compared to interfacially polymerized (IP) films of similar thickness. • Elastic modulus and RMS roughness were measured using atomic force microscopy. • Modulus was measured in dry, hydrated, and chlorinated conditions. • MLD films were much smoother and had a statistically greater modulus as compared to IP films in all conditions. [ABSTRACT FROM AUTHOR]

Abstract: Flux decline due to membrane fouling prevails in almost all pressure-driven liquid separations. The factors controlling fouling, ranging from surface chemistry to topographic roughness, have been extensively investigated. However, the role of surface patterns, particularly at a submicron scale, on membrane fouling remains unclear. Herein, we demonstrate that submicron patterns can be successfully imprinted onto a commercial polysulfone ultrafiltration membrane surface using nanoimprint lithography (NIL) without sacrificing its permeability properties. The presence of these patterns led to significantly improved deposition resistance during filtration of colloidal silica particle suspensions, as evidenced by a 19–45% increase in the apparent critical flux, with the magnitude dependent on particle size. Post-filtration visualization reveals an intriguing anisotropy in the particle deposition, whereby the degree of anisotropy depends on the orientation angle between the surface pattern and the flow direction of the feed. These results suggest a chemical-free route to post-formation, membrane surface modification. [Copyright &y& Elsevier]

Abstract: Thin film composite (TFC) reverse osmosis (RO) membranes enjoy widespread use in desalination, but their sensitivity to oxidizing agents such as chlorine remains a continuing challenge. In contrast to many reports on the chemical aspects associated with decreased membrane performance after chlorine exposure, studies on the fundamental physical properties of the polyamide barrier layer (PBL) of TFC membranes are scarce. This omission is mostly due to the lack of techniques capable of characterizing such interfacially polymerized PBLs, which are ultrathin and insoluble. The focus of this study is the development of an AFM-based nano-thermal analysis technique that provides the first-ever result for the direct measurement of the glass transition temperature (Tg ) of the PBL on several commercial TFC RO membranes. Moreover, the technique is utilized to study the changes in Tg of the PBL after exposure to chlorine solutions as a function of concentration and duration at constant pH. Results indicate significant and systematic reduction in Tg of the PBL with increasing chlorine concentration and exposure time. [Copyright &y& Elsevier]

Abstract: The major objective of this study was to develop a mathematical model that describes the solid–liquid (S/L) thermally induced phase separation (TIPS) membrane-formation process and captures the fundamental features of the evolving structure. This is the first mathematical model to describe S/L TIPS process from the initiation of casting to the development of the final membrane structure. The model predictions were validated with experimental measurements for the isotactic polypropylene/dotriacontane system using both scanning electron and optical microscopy to determine the final spherulitic size. These experiments encompassed two different process (boundary) conditions: controlled cooling rate and constant cooling-block temperature. The final spherulite diameter was in statistical agreement with the model predictions for the controlled cooling rate condition and the constant cooling-block temperature condition when the latter was equal to or higher than 25°C. The predicted spherulite diameter was larger than observed for lower block temperatures most likely due to diluent solidification, which was not accounted for in the model. This model can be used to determine the operating conditions required to produce desired membrane morphologies in the S/L TIPS process. Moreover, the formalism used to incorporate nucleation and growth of the dispersed phase in this model can be extended to other phase-inversion processes utilized for membrane manufacture. [Copyright &y& Elsevier]

Early-stage scaling detection is a crucial component for optimizing proactive anti-fouling measures that enhance membrane lifetime and decrease operation costs. In this work, we utilize Raman spectroscopy to detect and chemically quantify multiple scalants during bench-scale reverse osmosis desalination. The experiments were conducted with a commercial brackish water thin-film composite membrane at a feed pressure of 1.2 MPa (175 psi) and a feed flow velocity of 4.2 cm/s. Raman measurements were performed in real-time at a laser excitation wavelength of 785 nm. Experimental results from single-feed solutions of CaSO 4 and CaCO 3 show consistent detection of the corresponding scalants with less than a 15% permeate flux decrease at detection inclusive of the permeate decline due to compaction. Experiments utilizing a mixed-feed solution containing CaSO 4 and CaCO 3 were also conducted. Results for the mixed-feed experiments showed detection of CaSO 4 scaling only. Subsequent analysis indicated that a modified sampling strategy was required for successful real-time detection of both CaSO 4 and CaCO 3 scaling. • Real-time early-stage scaling detection via Raman spectroscopy. • Local scaling detection with chemical identification. • Demonstration of sensitive scaling detection in CaSO 4 and CaCO 3 feeds. • Modified sampling strategy for multiple scalant detection in mixed feed solutions. [ABSTRACT FROM AUTHOR]

Abstract It is commonly believed that the overall permeation resistance of thin film composite (TFC) membranes is dictated by the crosslinked, ultrathin polyamide barrier layer, while the porous support merely serves as the mechanical support. Although this assumption might be the case under low transmembrane pressure, it becomes questionable under high transmembrane pressure. A highly porous support normally yields under a pressure of a few MPa, which can result in a significant level of compressive strain that may significantly increase the resistance to permeation. However, quantifying the influence of porous support deformation on the overall resistance of the TFC membrane is challenging. In particular, it is difficult to determine the deformation/strain of the membrane during active separation. In this study, we use nanoimprint lithography (NIL) to achieve precise compressive deformation in commercial TFC membranes. By adjusting the NIL conditions, membranes were compressed to strain levels up to 60%. SEM and AFM measurements showed that the compression had minimal impact on the barrier-layer surface morphology and total surface area with most of the deformation occurring in the support layer. DI water permeation measurements revealed that the water flux reduction decreases with an increase of strain level. Most significantly, the intrinsic membrane resistance showed negligible changes at strain levels lower than 30%–40%, but increased exponentially at higher strain levels, reaching 250%–500% of pristine (unstrained) membrane values. Using a resistance-in-series model, the strain dependency of the TFC membrane resistance can be described. Highlights • Different levels of compressive strain were imposed on TFC membranes using Nanoimprint Lithography. • The intrinsic membrane resistance of the compressed TFC membranes was determined. • The intrinsic membrane resistance increases significantly when compressive strain in the support layer is above 40%. • The influence of support layer deformation on the TFC membrane resistance can be described using a resistance-in-series model. [ABSTRACT FROM AUTHOR]

Imparting sub-micron periodic patterns on the surface of ultrafiltration (UF) membranes has been shown to improve their antifouling characteristics. However, the deformation mechanism underlying membrane surface-patterning with nanoimprint lithography (NIL) is currently unclear. In response to this need, this study addresses the influence of nanoimprinting on the structural and performance characteristics of a commercial polyethersulfone (PES) UF membrane. The work utilized a flat (no pattern) as well as a pattern-containing mold such that local surface deformation associated with pattern formation could be isolated from the overall mechanical deformation in compression. The mechanical properties of the UF membrane as a function of temperature and deformation rate were characterized, and the overall thickness, molecular weight cutoff, and DI water permeance were measured for membranes imprinted using the flat mold. The data show that the influence of the NIL process on the membrane structure is length-scale dependent. Furthermore, imprinting of the UF membrane with the pattern-containing mold was examined experimentally as well as with finite element simulation. Results indicate that the non-uniform contact deformation at the membrane–mold interface provides the potential to optimize the NIL conditions for achieving desired pattern geometries without significantly increasing the membrane resistance. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the effects of sub-micron surface patterns on the fouling of a model protein solution (bovine serum albumin (BSA)), was investigated during active filtration and simple adsorption conditions. Surface patterns were created directly onto a commercial polyethersulfone ultrafiltration membrane using nanoimprint lithography, which resulted in a moderately reduced molecular mass cut-off (MWCO) but similar permeance as its non-imprinted counterpart as well as significantly improved resistance to BSA fouling. Staged filtration experiments, with backwash cleaning, revealed that the permeate flux of the imprinted membrane was considerably higher than that of the non-imprinted membrane, consistent with UV–vis measurements showing less protein deposition on the imprinted membranes. This improvement in anti-fouling characteristics for the imprinted membrane was universally observed for varying feed-solution chemistry including changes in both pH and ionic strength. From controlled protein-adsorption experiments under non-filtration conditions, it appears that the observed decrease in protein adsorption on the imprinted membranes is likely associated with altered hydrodynamics due to the presence of the sub-micron patterns. [Copyright &y& Elsevier]

Abstract: Reverse osmosis (RO) desalination is a well-established process that removes salt from sea water or brackish water sources. However, scaling by sparingly soluble salts on the membrane surface has been shown to significantly degrade membrane performance. We report the results from a comprehensive study evaluating the effectiveness of flow reversal (FR) controlled by ultrasonic sensors in mitigating scaling during RO desalination. The work utilized a sophisticated multi-port bench-scale flat-sheet cross-flow module with the ability to operate in FR mode with externally mounted ultrasonic sensors. A novel signal-analysis methodology was developed to utilize the ultrasonic waveforms to control a switch in flow direction at the onset of local scaling. Experiments were conducted under controlled conditions with repeated forward-flow (FF) and reverse-flow (RF) cycles. Data from the experiments confirmed that FR controlled by ultrasonic sensors can effectively mitigate scaling on the membrane surface and avoid the expected level of permeate-flow decrease. These results were validated by post-mortem membrane analysis including scalant gravimetric and membrane area-coverage measurements. Overall, the work demonstrates the successful adaptation of ultrasonic sensors for active process control in which the timing of the change in flow direction is critical. [Copyright &y& Elsevier]

Abstract: The use of ultrasonic time-domain reflectometry (UTDR) in combination with statistically based analysis has been utilized to provide timely warning of the initial onset of local calcium carbonate scaling. The methodology was able to detect the onset of scaling in real time before any significant decrease in permeate flux was observed. Ultrasonic detection of scaling was confirmed via post-mortem examination of the membrane that showed the presence of aragonite crystals that were as small as 20μm and covered less than 10% of the membrane surface. The ultrasonic methodology was then employed to trigger a change in flow direction (reversal of the concentrate and feed ends of the flow channel) when scaling was detected. When in forward flow, the upstream, midstream and downstream sections of the membranes had aragonite supersaturation values of SI∼2.0, ∼5.3 and∼7.65, respectively; when the flow direction was changed, the SI values at the ends were switched but the SI at the midstream remained at SI∼5.3. Results indicated that flow reversal effectively mitigated scaling in downstream sections of the membrane even though they were periodically exposed to the highest supersaturation conditions. It was demonstrated that prompt intervention after an ultrasonic scaling signal improved the efficacy of flow reversal. The results reflect a significant advance in the use of UTDR in which the methodology is applied for active control of scaling miti ation rather than limited to passive detection of scale formation. [Copyright &y& Elsevier]

Abstract: In this study we report changes in the glass transition temperature (T g ) of the polyamide barrier layer (PBL) of a commercial RO membrane as a function of chlorine treatment in which concentration and pH were systematically varied. The results indicate a monotonic decrease of T g with increasing exposure time at a given chlorine concentration. Further, both the degree and the rate of T g decrease vary significantly with both the chlorine concentration and pH value of the aqueous chlorine solution. Moreover, the decrease in PBL T g correlates well with the reduction in the salt rejection of the membrane, but the magnitude of the correlation is specific to chlorine concentration and pH values. [Copyright &y& Elsevier]

Abstract: Poly(ethylene chlorotrifluoroethylene) (ECTFE) is a 1:1 alternating copolymer of ethylene and chlorotrifluoroethylene that offers excellent resistance in chemically and thermally challenging environments. ECTFE membranes with a variety of microstructures have been fabricated via thermally induced phase separation (TIPS) with dibutyl phthalate (DBP) as the diluent. A continuous flat sheet extrusion apparatus with a double rotating drum was used that permitted controlling both the casting solution thickness and axial tension on the nascent membrane. Initial compositions of ECTFE/DBP solutions in the liquid–liquid region of the binary phase diagram were chosen, resulting in membranes with an interconnected pore structure. The effects of several important process parameters were studied to determine their effect on the structure and properties of the membrane. The parameters evaluated included the initial ECTFE concentration, cooling rate, membrane thickness, co-extrusion of diluent, and stretching of the nascent membrane. The resulting membranes were characterized using SEM, porometry, and permeation measurements. For the range of process parameters studied, ECTFE membranes exhibited a decrease in surface porosity with increasing initial polymer concentration and cooling rate. The effect of membrane thickness on the permeation flux was not significant. Co-extrusion of diluent increased the surface porosity and eliminated the dense skin that was otherwise present under rapid cooling conditions. Subsequent stretching of the nascent membrane resulted in a more open structure and a significant increase in the permeation flux. [ABSTRACT FROM AUTHOR]

Abstract: Driven by the concerns over the dwindling supplies of petroleum and the dire consequences of global warming, the emergence of a hydrogen economy appears inevitable. Such development will require the advancement of separation technologies with significantly improved energy and cost efficiencies. Membrane technology is the ideal choice for hydrogen purification due to its lower power usage and costs, simplicity in operation, as well as compactness and portability. Assuming hydrogen purity requirements of <99.5% and relatively modest operating temperatures, polymeric membranes are preferred among the various alternatives. Generally, polymeric membranes for hydrogen enrichment are either H2- or CO2-selective, depending on the governing kinetic or thermodynamic factors. In this review, the current state-of-the-art for polymeric membranes applicable to hydrogen purification is considered, and the different approaches for enhancing intrinsic gas-separation performance are evaluated. An assessment of the benefits and shortcomings of the respective membrane types are presented, and the future directions most promising for the development of robust and high-performance polymeric membranes for the hydrogen economy are highlighted. [Copyright &y& Elsevier]

Abstract: Shape memory polymers (SMPs) can retain a temporary shape after pre-deformation at an elevated temperature and subsequent cooling to a lower temperature. When reheated, the original shape can be recovered. Relatively little work in the literature has addressed the constitutive modeling of the unique thermomechanical coupling in SMPs. Constitutive models are critical for predicting the deformation and recovery of SMPs under a range of different constraints. In this study, the thermomechanics of shape storage and recovery of an epoxy resin is systematically investigated for small strains (within ±10%) in uniaxial tension and uniaxial compression. After initial pre-deformation at a high temperature, the strain is held constant for shape storage while the stress evolution is monitored. Three cases of heated recovery are selected: unconstrained free strain recovery, stress recovery under full constraint at the pre-deformation strain level (no low temperature unloading), and stress recovery under full constraint at a strain level fixed at a low temperature (low temperature unloading). The free strain recovery results indicate that the polymer can fully recover the original shape when reheated above its glass transition temperature (T g). Due to the high stiffness in the glassy state (T < T g), the evolution of the stress under strain constraint is strongly influenced by thermal expansion of the polymer. The relationship between the final recoverable stress and strain is governed by the stress–strain response of the polymer above T g. Based on the experimental results and the molecular mechanism of shape memory, a three-dimensional small-strain internal state variable constitutive model is developed. The model quantifies the storage and release of the entropic deformation during thermomechanical processes. The fraction of the material freezing a temporary entropy state is a function of temperature, which can be determined by fitting the free strain recovery response. A free energy function for the model is formulated and thermodynamic consistency is ensured. The model can predict the stress evolution of the uniaxial experimental results. The model captures differences in the tensile and compressive recovery responses caused by thermal expansion. The model is used to explore strain and stress recovery responses under various flexible external constraints that would be encountered in applications of SMPs. [Copyright &y& Elsevier]

Video-microscopy flow-visualization (VMFV) is adapted to study the development of macrovoid (MV) pores in the dry-casting of cellulose acetate (CA)/acetone/water solutions. Particle tracer velocities provide the first direct evidence for the presence of solutocapillary-driven convection that can enhance mass-transfer to a MV. Three phases of MV development are observed: fast initial growth, slow growth, and collapse. During the latter, MVs were observed on occasion to initiate far from the demixing front. These studies have led to a significantly modified hypothesis for MV development. Extremely rapid initial MV growth is thought to occur owing to coalescence of dispersed phase microdroplets. To ensure net mass-transfer to a growing MV, it is postulated that a homogeneous supersaturated solution layer must exist between the demixed fluid layer and the homogeneous stable solution layer. Fast growth also involves convective mass-transfer to the MV whose surface is initially entirely immersed in this homogeneous supersaturated solution layer. Slow growth involves net transport that results from both convective mass-transfer to the MV across the portion of its surface in contact with the homogeneous supersaturated solution layer, and convective mass-transfer from the portion of its surface that extends into the homogeneous stable solution layer. Active collapse is thought to occur owing to skin formation at the MV surface. Passive collapse occurs when the convective mass-transfer from the MV in the homogeneous stable solution layer exceeds that entering the MV in the homogeneous supersaturated solution layer. [Copyright &y& Elsevier]