Effects of water freezing on the mechanical properties of nafion membranes

Most of the research efforts on Nafion have been devoted to the study of the perfluorinated ionomer membranes at optimal conditions for the desired applications, such as high temperature and low relative humidity for polymer electrolyte membrane fuel cells (PEM FC). In view of the possible changes induced by the freezing of water in the structure of Nafion and considering that in cold start conditions of a PEM FC device, Nafion needs to work also below 273 K, we measured the Young's modulus (Y) and the elastic energy dissipation (tan δ) in the temperature range between 90 and 470 K and the stress–strain curves between 300 and 173 K. The measurements reported here indicate that the mechanical properties of wet Nafion membrane change dramatically with temperature, that is, from a rubber‐like behavior at room temperature to a brittle behavior below 180 K. Moreover, we observed that the freezing of the nanoconfined water is complete only below 180 K, as indicated by a large increase of the Young's modulus. Between 180 and 300 K, the large values of tan δ suggest the occurrence of friction between the liquid water bound to the walls of the hydrophilic domains and the solid ice residing in the center of channels. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

A spectroscopic ellipsometry study of PET membranes from IR to Vis-FUV

A detailed study of Spectroscopic Ellipsometry (SE) spectra of uniaxially stressed, 12μm thick poly(ethylene terephthalate) (PET) membranes were realized in both IR (900-3500 cm⁻¹) and Vis-FUV (1.5 to 6.5 eV) spectral regions and in various orientation angles ∅︁ between the plane of incidence and the machine direction (MD) of the PET membranes. In the IR spectra, the three main modes of the ester group at ∼1126, 1256 and 1721 cm⁻¹ are observed. The first two of these peaks show identical polarization dependence, with preferential orientation ∅︁≈0° (i.e. the plane of reflection parallel to the MD), while the peak at ∼1721 cm⁻¹ is excited in the perpendicular direction, ∅︁≈90°. In the Vis-FUV SE spectra, the dependence of the characteristic electronic absorption bands on the orientation angle ∅︁ is studied. Besides the well known benzene band centered at ∼5 eV, a doublet of less intense peaks appear just above the absorption edge between 4.1 and 4.3 eV with the most striking characteristic that they show polarization selection rules opposing to that of the benzene band. The above characteristics of the electronic Vis-FUV spectra of PET are discussed in a manner comparative to the corresponding IR spectra. The discussion becomes in the context of determining the molecular orientation of PET using a non-destructive technique such as SE.     

Recent advances in the formation of ultrathin polymeric membranes for gas separations

During the past 20 years membrane systems have been applied to a limited number of commercial gas separations. To further advance membrane-based gas separations, current research efforts focus on optimization of (i) membrane materials, (ii) membrane structures and (iii) membrane system design. In this overview, recent developments in the formation of high-performance gas separation membranes are discussed. The gas separation properties of state-of-the-art integrally skinned asymmetric membranes and thin-film composite membranes are summarized. Future directions for the preparation of advanced gas separation membranes are highlighted.     

Separation of liquids by pervaporation through polymeric membranes

Fundamental and applied aspects of liquid separation by means of pervaporation through polymeric membranes are considered. The review gives the state of the art as well as prospects of development of this branch of membrane science and technology.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 208–219, February, 1994.     

Young's modulus and loss tangent measurement of polydimethylsiloxane using an optical lever

In this article, the Young's modulus and the loss tangent of polydimethylsiloxane (PDMS) is obtained in the frequency range between 10 and 1500 Hz using an optical technique. The first three mechanical modes of vibrating PDMS beams are detected by measuring the tip rotational displacement using an optical lever. The experiments are carried out for 10:1 and 20:1 volume mixing ratios between the polymer base and the curing agent. The experimental results show that the Young's modulus varies between 1 and 2.6 MPa for 10:1 while its values are between 0.6 and 1.1 MPa for 20:1. The loss tangent is between 0.2 and 0.4 for 10:1 and 0.2 and 0.5 for 20:1. However, the measured values of the loss tangent are greater than the values reported in ref. 13. We also found that if the PDMS is not cured properly, its mechanical properties are time dependent. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 747–751     

Investigation of structure‐performance properties of a special type of polysulfone blended membranes

Membranes require superior mechanical strength due to applied harsh conditions. The mechanical properties of membranes decrease with increasing hydrophilicity of its elements. In this study, mechanical properties were investigated for two special blended membranes which were made by blending polysulfone with (polysulfone‐g‐poly (n‐butylacrylate) and polysulfone‐g‐poly (tert‐butylacrylate) as components. All of the prepared membranes were characterized by differential scanning calorimeter, thermal gravimetric analysis, field emission scanning electron microscope, and atomic electron microscope and were investigated in terms of pure water flux, water contact angle, molecular weight cut off, and morphology. It was found that water contact angle decreased from 73.6° which belongs to neat membrane decreased to 46° for blended membranes containing higher amounts of copolymers; however, the pure water flux increased with increasing copolymer content considerably compared with the neat membrane. Also, molecular weight cut off increased aggressively. Furthermore, mechanical properties including tensile strength, Young modulus, and elongation at break were measured and compared with the neat polysulfone membrane. Results showed that the tensile strength and modulus decreased with an increase in the copolymers content, despite the increase in the elongation at break. The effect of applied pressure on the membrane structure and also bursting strength were studied, and it has been proved that not only the structure of the membranes but also their performance is strongly affected by the composition of the membranes.     

Reduced stress‐optical coefficient of polycarbonate by antiplasticization

We have investigated the effect of antiplasticization on the stress‐optical behavior of polycarbonate (PC) containing terphenyls (tPh) and di(2‐ethylhexyl)adipate (DEHA). Addition of the three tPhs (p‐, o‐, and m‐tPh) and DEHA at contents of 5–10 wt % increases the tensile storage modulus (E' ) of PC owing to the antiplasticization effect. In particular, p‐tPh increases E' more than the other additives, suggesting that the rod‐like shape matches the free volume of PC in the glassy state. The three tPh isomers improve the glassy birefringence of PC while DEHA does not change the glassy birefringence, which corresponds to the polarizability anisotropy. The stress‐optical coefficient, a ratio of stress and birefringence, of PC decreases with increasing additive content in order of p‐tPh ? o‐tPh > m‐tPh = DEHA. This result is agreement with a restricted rotational motion of additive molecule in PC, which is observed in dynamic mechanical and birefringence data. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 1837–1842     

Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes

A streaming potential analyzer has been used to investigate the effect of solution chemistry on the surface charge of four commercial reverse osmosis and nanofiltration membranes. Zeta potentials of these membranes were analyzed for aqueous solutions of various chemical compositions over a pH range of 2 to 9. In the presence of an indifferent electrolyte (NaCl), the isoelectric points of these membranes range from 3.0 to 5.2. The curves of zeta potential versus solution pH for all membranes display a shape characteristic of amphoteric surfaces with acidic and basic functional groups. Results with salts containing divalent ions (CaCl₂, Na₂SO₄, and MgSO₄) indicate that divalent cations more readily adsorb to the membrane surface than divalent anions, especially in the higher pH range. Three sources of humic acid, Suwannee River humic acid, peat humic acid, and Aldrich humic acid, were used to investigate the effect of dissolved natural organic matter on membrane surface charge. Other solution chemistries involved in this investigation include an anionic surfactant (sodium dodecyl sulfate) and a cationic surfactant (dodecyltrimethylammonium bromide). Results show that humic substances and surfactants readily adsorb to the membrane surface and markedly influence the membrane surface charge.     

Correlation between Functionated Structure and Nanomechanical Property Profiles of Silsesquioxane Films and Bulks

Three silsesquioxane (SSO) building blocks based on [3-(methacryloxy)propyl]trimethoxysilane (MPMS), [(3-glycidoxy)propyl] trimethoxysilane (GPMS), (vinyl)trimethoxysilane (VMS) and modified with 15 wt-% tetraethoxysilane (TEOS) were prepared using hydrolytic condensation. The hardness, elastic modulus and harmonic contact stiffness for three films [f-MTSSO (film-MPMS-TEOS-SSO), f-GTSSO and f-VTSSO] and bulks (b-MTSSO, b-GTSSO and b-VTSSO) derived from the three SSOs were tested by instrument-indentation testing. Different profiles of nanomechanical properties of the films can be attributed to a different functionality and the functionated structure of the different SSOs. The f-MTSSO shows better mechanical properties which are higher than the other two films containing more OH or OCH₃ groups. For the f-GTSSO with a bulky organic substituent, the unsatisfactory properties resulting from ring opening during hydrolytic condensation and an incomplete crosslinking network are discussed. However, the nanomechanical properties of the three bulks are very different from the corresponding nanomechanical properties of their films: b-VTSSO possesses the best nanomechanical properties (note: not b-MTSSO) among three bulks due to the differences in reaction conditions and chain less flexibility which are also discussed.     

Synthesis and proton conductivity studies of polystyrene‐based triazole functional polymer membranes

In this study, poly(vinylbenzylchloride) (PVBC) was produced by free‐radical polymerization of 4‐vinylbenzylchloride, and then it was functionalized with 3‐amino‐1,2,4‐triazole (ATri) and 1H‐1,2,4‐triazole (Tri). The composition of the polymers was verified by elemental analysis, and the structure was characterized by Fourier transform infrared and ¹³C‐nuclear magnetic resonance spectra. PVBC was modified by ATri with 68% and Tri with 50% yield. The polymers were doped with trifluoromethanesulfonic acid (TA) at various molar ratios, X = 0.5, 1, 2, and 3 with respect to aminotriazole and triazole units. Proton transfer from TA to the triazole rings was proved with Fourier transform infrared spectroscopy. Thermogravimetric analysis showed that the samples are thermally stable up to approximately 200 °C. Differential scanning calorimetry results illustrated the homogeneity of the materials. Under anhydrous conditions, PVBCATri3TA and PVBCTri3TA showed highest proton conductivity of 0.086 and 0.042 S/cm, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Brillouin scattering studies of polymeric nanostructures *

For a range of applications, polymers are now being patterned into nanometer‐sized features. In these applications, the robust mechanical properties of the nanostructures are critical for performance and stability. Brillouin light scattering is presented as a nondestructive, noncontact tool used to quantify the elastic constants in such nanostructures. We demonstrate this through a series of thin films and parallel ridges and spacings (gratings) with ridge widths ranging from 180 to 80 nm. For the set of films and structures presented here, the room‐temperature elastic moduli did not change with decreasing film thickness or grating ridge width, and this implied that one‐dimensional and two‐dimensional confinement‐induced changes of the mechanical properties were not significant down to feature sizes of 80 nm. Additionally, Brillouin spectra of submicrometer gratings revealed new modes not present in the spectra of thin films. The origin of these new modes remains unclear. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1106–1113, 2004     

Elastic excitations in polycarbonate membranes

Brillouin light scattering was used to probe acoustic waves propagating with both longitudinal and transverse polarizations in the surface and the bulk of self‐supported particle track‐etched polycarbonate membranes with 15‐ and 80‐nm nanopores. The recorded scattering line shape at gigahertz frequencies reveals changes in the surface waves of the membranes which are more pronounced for the 80‐nm nanopores despite the low porosity (0.7 and 0.05%). Because the measured elastic constants (1.2 and 6.2 GPa) were found to compare very well with the values for thick polycarbonate film, modifications of the elasto‐optical coefficients and/or the transparency might be the reason for the different scattering line shapes. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3311–3317, 2004     

Modification of porous poly(ether sulfone) membranes by low‐temperature CO2‐plasma treatment

A general method of modifying the entire cross section of porous poly(ether sulfone) membranes with a low‐temperature CO₂‐plasma treatment is reported. Both surfaces of the membranes are highly hydrophilic, with a water drop on the surface disappearing in less than 1 s, even 6 months after plasma treatment. This high hydrophilicity of both membrane surfaces results from the incorporation of hydrophilic functionalities, as evidenced by Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy. The incorporation of these hydrophilic functionalities takes place primarily during plasma treatment, with some incorporation of atmospheric oxygen and nitrogen immediately upon exposure to air. Scanning electron microscopy shows that the membrane surface is covered by a thin, white layer that is likely the result of etching and redeposition of sputtered surface fragments. An increase in the water bubble point and glass‐transition temperature is also observed for CO₂‐plasma‐treated membranes. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2473–2488, 2002     

Lowering the detection limit of calcium selective ISFETs with polymeric membranes

Detection limits of Ca sensitive ISFETs with photocured polyurethane membranes with a neutral carrier and an ion-exchanger as ionophores are studied. It is shown that conditioning of ISFETs in solutions containing only interfering ions results in reduction of the detection limit. Zero-current ion fluxes approach elaborated for polymeric membrane ion-selective electrodes (ISEs) is used to explain the obtained results.     

Investigation on the stability,electronic, optical,and mechanical properties of novel calcium carbonate hydrates via first-principles calculations

Calcium carbonate (CaCO₃) is an inorganic compound which is widely used in industry, chemistry, construction, ocean acidification, and biomineralization due to its rich constituent on earth and excellent performance, in which calcium carbonate hydrates are important systems. In Zou et al's work (Science, 2019, 363, 396-400), they found a novel calcium carbonate hemihydrate phase, but the structural stability, optical, and mechanical properties have not been studied. In this work, the stability, electronic, optical, and mechanical properties of novel calcium carbonate hydrates were investigated by using the first-principles calculations using density functional theory. CaCO₃·xH₂O (x = 1/2, 1 and 6) are determined dynamically stable phases by phonon spectrum, but the Gibbs energy of reaction of CaCO₃·1/2H₂O is higher than other calcium carbonate hydrates. That is why CaCO₃·1/2H₂O is hard to synthesize in the experiments. In addition, the optical and mechanical properties of CaCO₃·xH₂O (x = 1/2, 1 and 6) are expounded in detail. It shows that the CaCO₃·1/2H₂O has the largest bulk modulus, shear modulus, and Young's modulus with the values 60.51 GPa, 36.56 GPa, and 91.28 GPa. This work will provide guidance for experiments and its applications, such as biomineralization, geology, and industrial processes.     

Functionalization of polymeric membranes by impregnation and in situ cross-linking of a PDMS/β-cyclodextrin network

In this paper a new method for the functionalization of porous membranes with β-CD is reported. Porous polypropylene (PP) hollow fibres have been impregnated with a mixture composed by a partially cross-linked polydimethylsiloxane (PDMS) and β-cyclodextrin (β-CD). The prepolymerization of the PDMS components was necessary to avoid their inclusion in the β-CD cavity. The firm heterogenization of the β-CD was obtained by in situ cross-linking of the PDMS/β-CD network in the porous membranes. The presence of the PDMS/β-CD network in the membranes was confirmed by FT-IR-ATR (on the outer and inner surfaces) and EDX analyses (on the cross-section).The effect of the impregnation times on membrane morphology, loading and porosity has been investigated. The binding capacity of the heterogenized β-CDs has been tested using the phenolphthalein as guest molecule.     

Depth-Dependent Indentation Microhardness Studies of Different Polymer Nanocomposites

Continuous depth sensing indentation microhardness measurements were performed to investigate the effect of filler content and dimensionality on the mechanical behaviour of different polymer nanocomposites. In 1D filler reinforced nanocomposites (such as PP/MWCNT system), both the hardness and the indentation modulus were found to appreciably increase up to a filler weight fraction of 1.6 wt.-%. Further addition of the filler changed the properties only insignificantly. In the nanocomposites with 2D filler (such as in PA6/LS) both the hardness and the indentation modulus increase notably with the addition of the filler and showed intense plasticity. In the investigated systems and composition range, the 3D filler (such as PP/OS2) showed no reinforcing effect at all. In was concluded that the 1D and 2D nanofillers play much more effective reinforcing role to improve the mechanical properties than the 3D fillers.     

Characterization and filtration performance of coating-modified polymeric membranes used in membrane bioreactors

One of the critical issues for membrane application in wastewater treatment is membrane fouling majorly caused by dissolved organic matters. The aim of the present study was to lower membrane fouling by adsorption of polyelectrolytes. In the paper, the feasibility of coating for diverse ultrafiltration membrane materials was investigated and their filtration performance was compared to that of the unmodified ones. Different ultrafiltration flat-sheet membranes, polyvinylidene fluoride, polyethersulfone, polysulfone and cellulose acetate were coated by branched poly(ethyleneimine) (PEI), poly(diallyldimethylammonium chloride) (PDADMAC) and poly(allylamine chloride) (PAH) and filtrated with sludge supernatant. Short term experiments showed a substantial drop of permeability: almost 40 % for PEI, 23 % for PDADMAC, and about 19 % for PAH coating. This deterioration resulted from the additional resistance of the deposited layers. On the other hand, coating led to lower fouling rates during filtration. In the stable state of filtration, coated membranes showed higher permeabilities compared to the uncoated ones. For the polyethersulfone membrane, the average permeability enhancement was 11 %. For polysulfone and cellulose acetate membranes, the permeability improved by 28 % and 15 % respectively. For polyvinylidene fluoride membranes only coating with PDADMAC enhanced the permeability, by 13 %. PEI and PAH modified membranes featured lower permeabilities than the uncoated ones. Presented at the 35th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 26–30 May 2008.     

Theoretical studies on the electronic and optical properties of two new alternating fluorene/carbazole copolymers

Poly(fluorene)-type materials are widely used in polymer-based emitting devices. During operation there appears, however, an additional emission peak at around 2.3 eV, leading to both a color instability and reduced efficiency. The incorporation of the carbazole units has been proven to efficiently suppress the keto defect emission. In this contribution, we apply quantum-chemical techniques to investigate two series of alternating fluorene/carbazole oligomers and copolymers poly[2,7-(N-(2-methyl)-carbazole)-co-alt-2,7-m(9,9-dimethylfluorene)], namely, PFmCz (m = 1,2) and gain a detailed understanding of the influence of carbazole units on the electronic and optical properties of fluorene derivatives. The electronic properties of the neutral molecules, HOMO-LUMO gaps (Delta(H-L)), in addition to the positive and negative ions, are studied using B3LYP functional. The lowest excitation energies (E(g)s) and the maximal absorption wavelength lambda(abs) of PFmCz (m = 1,2) are studied, employing the time-dependent density functional theory (TD-DFT). The properties of the two copolymers, such as Delta(H-L), E(g), IPs, and EAs were obtained by extrapolating those of the oligomers to the inverse chain length equal to zero (1/n = 0). The outcomes showed that the carbazole unit is a good electron-donating moiety for electronic materials, and the incorporation of carbazole into the polyfluorene (PF) backbone resulted in a broadened energy gap and a blue shift of both the absorption and photoluminescence emission peaks. Most importantly, the HOMO energies of PF1Cz and PF2Cz are both a higher average (0.4 eV) than polyfluorene (PF), which directly results in the decreasing of IPs of about 0.2 eV more than PF, indicating that the carbazole units have significantly improved the hole injection properties of the copolymers. In addition, the energy gap tends to broaden and the absorption and emission peaks are gradually blue-shifted to shorter wavelengths with an increase in the carbazole content in the copolymers. This is due to the interruption of the longer conjugation length of the backbone in the (F1Cz)(n) series.