Effect of molecular structure and topology on network formation in peroxide crosslinked polyethylene

The present study compared the crosslinking performance of single site linear low density polyethylenes (LLDPE) with high pressure, free radical polymerised, low density polyethylenes (LDPE). The difference in crosslinking performance is not fully explained by different structural parameters such as molar mass distribution (MMD), M_n, MFR₂ value and vinyl groups but is related more to the phenomenon of a long chain branched LDPE macromolecule being smaller in size in the molten state than a macromolecule of a linear LLDPE sample of the same molar mass. The result of the difference in size is that the LDPE will contain a larger number of intramolecular crosslinking points than the LLDPE, which, on the other hand, will contain a larger fraction of intermolecular crosslinking points. The crosslinking points mentioned are of either a physical or chemical nature. From the perspective of the network build-up, the intermolecular crosslinking points are the most efficient. To compensate for the larger fraction of intramolecular crosslinking points in LDPE, more peroxide can be added or vinyl groups can be introduced.     

Synthesis and properties of hydrophilic copolypeptide membranes containing L-aspartic acid as one component

Three-component random copolypeptide consisting of N-hydroxypropyl-L -glutamine, L -aspartic acid, and L -lysine, related two component random copolypeptides and homopolypeptides, were prepared by carrying out aminolysis reaction with 3-amino-1-propanol, followed by crosslinking reaction with 1,8-octamethylenediamine on starting polymer membranes consisting of γ-methyl-L -glutamate, L -aspartic acid, and L -lysine. The effective crosslink density was shown to be proportional to the content of the crosslinker in the reaction mixture. The tensile properties of these hydrophilic membranes were highly dependent on the degree of swelling in the pseudoextracellular fluid, hydrophobicity of the side chains, and the effective charge density of membranes, and their behavior was typical of an elastomer. A higher rate of water permeability was obtained with charged membranes than noncharged and/or compensated charged membranes with the same order of the degree of swelling in the pseudo-extracellular fluid. Biodegradation of the samples in vitro by papain indicated that the degradation could be regarded as a bulk rather than a surface phenomenon. The rate of degradation was also highly dependent on the degree of swelling of membranes, as well as on the hydrophobicity and the effective charge density of side chains of sample membranes.     

Hydration,porosity and water dynamics in the polyamide layer of reverse osmosis membranes: A molecular dynamics study

This study employs molecular dynamics to gain insight into the mechanisms underlying the formation, hydration and functioning of aromatic polyamide membrane used for water purification by reverse osmosis. A polyamide membrane structure was successfully generated with a chemical composition and degree of crosslinking that closely match recent experimental data. The density of dry polymer was found to be slightly lower than in previous studies but closer to recent experimental measurements. The volume expansion and mass gain upon hydration show marked variations due to the presence of a significant fraction of permanent voids, revealed by dynamically averaged water concentration maps. The mass hydration and porosity were also larger than previously obtained, but water self-diffusivity was similar to previous simulations, presumably, since the larger porosity was offset by the higher cross-linking. Using a radial distribution function of water within polyamide, strong next-neighbor correlations and random long-distance water–water correlations could be differentiated, which could be assigned to small network and large aggregate pores. Unfortunately, this dual porosity picture could not be linked to the experimentally found bimodal distribution of the pKa values of the COOH groups, since the distribution of the local water content around COOH was found to be unimodal.     

Pervaporation separation of aqueous organic mixtures through sulfated zirconia‐poly(vinyl alcohol) membrane

Sulfated zirconia‐poly(vinyl alcohol) membranes were prepared, and pervaporation performances for aqueous organic mixtures were investigated. These hydrophilic membranes were formed by crosslinking poly(vinyl alcohol) (PVA) with the solid acid of sulfated zirconia by an acid‐catalyzed reaction. The pervaporation performances were measured as a function of the content ratio of sulfated zirconia to PVA, which affected the degree of swelling for water and the crosslinking density of the membrane. The membrane selectivity in pervaporation of aqueous organic mixtures increased in order of acetic acid < ethanol < 2‐ethoxyethanol without sacrificing the permeation rate, depending on their feed compositions. The effects of feed temperature and concentration on the membrane performance were also significant. It was found that sulfated zirconia in the membrane preparation played an important role as a filler material as well as an effective crosslinking or insolubilization agent in improving and controlling the membrane performance, i.e., permeation rate and selectivity. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1450–1455, 2001     

Diffusive permeability of ionic solutes in charged chitosan membrane

The permeabilities of three kinds of solutes with similar sizes such as anionic benzenesulfonic acid, neutral styrene glycol, and cationic theophylline in the chitosan membrane were investigated. The chitosan membrane becomes cationic below its pKa value. Benzenesulfonic acid showed the highest permeability, whereas theophylline showed the lowest, although these solutes have almost the same size. This can be explained by the electrostatic attraction or repulsion between the solute and the membrane instead of the size exclusion effect. The permeabilities of benzenesulfonic acid and theophylline increased and decreased, respectively, with the decrease of pH from 7.4 to 4.0 because of the increase of the charge density of the membrane. Thus, the selectivity of benzenesulfonic acid to theophylline increased and reached about 30 at pH 4.0, in contrast to the selectivity of nearly unity in poly(vinyl alcohol) (PVA) membrane. The partition coefficients for three solutes showed the similar tendencies to those in the permeabilities. Contrary to the results of the permeabilities, the partition coefficients increased with the increase of the degree of the crosslinking in the membrane. This is probably due to the increase of the electrostatic interaction between the solute and the membrane brought about by the smaller mesh size and also due to the increase of the bound water fraction in the membrane. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 397–404, 1999     

Mechanical and thermal insulation properties of isocyanate crosslinked resorcinol formaldehyde aerogel: Effect of isocyanate structure

In this article, the mechanical and thermal properties of resorcinol formaldehyde (RF) aerogels were improved using two different crosslinkers including hexamethylene diisocyanate (HDI) and methylene diphenyl diisocyanate (MDI). The crosslinking was performed after the aerogel drying process, with two concentrations of crosslinking agent. The formation of urethane linkages was investigated by Fourier transform infrared spectroscopy. The effect of crosslinking process and crosslinking agent type on the mechanical properties was analyzed by compression, bending, and impact tests. The improvement of mechanical strength was attributed to the neck thickness, which was studied by atomic force microscopic test. The results revealed that a higher improvement was obtained by increasing the crosslinking agent value. HDI crosslinked aerogel represented the highest strain-at-break and MDI-reinforced sample showed the highest strength. By crosslinking, the samples density was increased <59% and the reduced compressive strength was enhanced upto eight times. The effect of crosslinking on the thermal conductivity was explored. The increment of neck size and density enhanced the solid conductivity and, consequently, raised the thermal conductivity. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48196.     

Surface Modification of Commercial Aromatic Polyamide Reverse Osmosis Membranes by Crosslinking Treatments

Crosslinking treatments for a commercially available aromatic polyamide reverse osmosis membrane were carried out to improve its chlorine resistance. The crosslinking agents including 1,6-hexanediol diglycidyl ether, adipoyl dichloride and hexamethylene diisocyanate ester with long flexible aliphatic chains and high reactivity with N-H groups were used in the experiments. Attenuated total reflective Fourier transform infrared spectra verified the successful preparation of highly crosslinked membranes by crosslinking treatments. It was suggested that the crosslinking agents were connected to membrane surface through the reactions with amine and amide II groups, which is confirmed by surface charge measurements. Based on contact angle measurements, crosslinking treatments decreased membrane hydrophilicity by introducing methylene groups to membrane surface. With increasing amount of crosslinking agent molecules connected to membrane surface, the hydrolysis of unconnected functional groups of crosslinking agent produced polar groups and increased membrane hydrophilicity. The highly crosslinked membranes showed higher salt rejections and lower water fluxes as compared with the raw membrane. Since the active sites (N-H groups) vulnerable to free chlorine on membrane surface were eliminated by crosslinking treatments, the chlorine resistances of the highly crosslinked membranes were significantly improved by slighter changes in both water fluxes and salt rejections after chlorination.     

Effect of crosslinking on the properties of sodium caseinate films

Protein films are used as effective lipid, oxygen, and aroma barriers at moderate relative humidity conditions. However, they perform poorly as moisture barriers. The introduction of crosslinks within or between protein chains by enzymatic or chemical modification has been proposed as an alternative means to achieving a stronger polymeric matrix structure, which would result in better functional film properties. In this article, we report the preparation and characterization of sodium caseinate (SC) films crosslinked by glutaraldehyde (GTA) or heat. The crosslinking density increased with GTA content. The thermal stability and tensile modulus and strength increased with GTA content, although films with a low crosslinking density exhibited lower properties than the uncrosslinked sample. Unexpectedly, water vapor permeability and absorption also increased with crosslinking density. The crosslinking of SC was also induced by simple heating. The resulting films showed enhanced thermal, mechanical, and barrier properties compared to the unmodified SC films and even the GTA‐crosslinked samples. GTA crosslinking was unable to reduce the high hydrophilicity of the SC films. Thermally induced crosslinking was revealed to be a valid alternative for improving the properties of SC films, without the inherent complications associated with the use of a chemical crosslinking agent. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Theoretical studies on structural and electrical properties of PES/SPEEK blend nanofiltration membrane

Polyethersulfone (PES) nanofiltration membranes were prepared using a simple dry‐jet wet spinning technique with different contents of sulfonated poly(ether ether ketone) (SPEEK) ranging from 0 to 4 wt %. The structural parameters (r_p and A_k/Δx) and electrostatic properties (ξ and X) of the blend membranes were deduced by employing the combination of irreversible thermodynamic model, steric hindrance pore (SHP) model, and Teorell‐Meyer‐Sievers (TMS) model. The modeling results obtained have been analyzed and discussed. The mean pore radius and pore size distribution of the blends were also determined based on the theoretical models. The results showed that pore radius increased with increasing the concentration of SPEEK from 0 to 2 wt % but decreased with a further increase in SPEEK content. The water flux, however, showed a systematically increase with increasing SPEEK content. The SPEEK also showed significant effect on membrane electrical properties. Both effective charge density and ratio of effective charge density to electrolyte solution increased with increasing concentration of SPEEK in the dope solution, reaching a value of ?21.02 and ?2.29, respectively. The pore radius which was determined by using different transport models has also been analyzed and discussed. It is found that the addition of SPEEK into dope solution is one of the paramount parameters in developing the negatively charged nanofiltration membrane with enhanced water flux while retaining the pore radius in the nanometer range. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Characterization and gas‐permeation properties of crosslinked diacetylene‐containing polymer membranes from ferulic acid

Thermally and UV crosslinked poly[propargyl(3‐methoxy‐4‐propargyloxy) cinnamate] (PPOF) were investigated in terms of their physical, thermal, optical, and gas‐permeation properties. The crosslinked membranes had high gel contents because of the formation of a diacetylene network. The wide‐angle X‐ray diffraction patterns showed that all of the membranes were amorphous in structure, regardless of the type of crosslinking reaction. The membrane density increased after the crosslinking reaction; this suggested that the free volume of the crosslinked membrane was lower than that of the untreated membranes. Drastic color changes in the membranes were also observed because of the highly conjugated crosslinked network of diacetylene. In addition, the conjugation caused by diacetylene crosslinking led to visible absorption within the range 400–600 nm. The gas permeation of the crosslinked membrane was reduced compared with that of the untreated membranes. In particular, the gas permeability of the thermally crosslinked membrane was lower than that of UV‐irradiated membrane. On the basis of this result, the degree of crosslinking by thermal treatment was higher than that of UV irradiation. Hence, the crosslinked PPOF membranes showed improved gas‐barrier properties due to the high conjugation of the crosslinked diacetylene network induced by thermal treatment and UV irradiation. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Chemorheological study of vulcanized rubbers under mechanical stress

At lower than the glass transition temperature, the cleavage of the molecular chains of extended vulcanized rubbers will occur. The crosslinking density of vulcanized rubbers, which were placed at room temperature for about 4 hr in nitrogen atmosphere, increased about 17% after extension of 200% at ?76°C. This may be due to the recombination of cleft chains, and this can be shown by the Maxwellian type of curves between crosslinking density and forced strain. Considering the decreasing proportion of crosslinking density by the radical acceptors included in the rubbers and the fact that crosslinking density increases because of recombination of cleft chains, this was attributed to the recombination of unstable chains produced by the added mechanical stimulus.     

Pervaporation separation of pentane-alcohol mixtures through ionically crosslinked blended membranes. I. Thermal measurements,electron microscopy and tensile studies

Ionically crosslinked blended membranes were prepared from blends of nylon 6 and poly(acrylic acid) (PAA) with the proportion of PAA ranging from 25 to 45 wt.-%. The technique consists of casting a film of the blend, followed by drying and immersion in an aluminum salt crosslinking solution for a predetermined time. The glass transition temperature (T_g) of the membranes was determined by differential scanning calorimetry (DSC). All the samples exhibited a single T_g, which is higher than that of either polymer. There is also a shift in the T_g with increasing scanning time. This phenomenon is attributed to the elimination of water molecules and the formation of anhydrides in the non-crosslinked PAA portion of the membrane. The membrane morphology was studied using scanning electron microscopy (SEM) and shows a dense structure without any pores. No phase separation is observed by scanning cross-sections of the samples, indicating that nylon 6 and PAA are completely miscible in the ranges studied. The membrane material strength lies in the range of 5 to 26 MPa and varies with the amount of PAA in the membrane. Samples with higher PAA content show lower tensile strength in spite of increased crosslinking density. This is due to the inherent low strength of PAA, coupled with increasing swelling of the membrane with increasing PAA content. The latter is confirmed by the measurement of water uptake into a dry membrane which increases from 23.6% to 76.3% with the membrane PAA content increasing from 25 to 45 wt.%.     

乙醇和水在聚酰亚胺膜中的溶解和传递现象研究(Ⅱ)——乙醇和水在膜中的溶解和传递性质的关联

用Flory-Huggins溶液模型加Langmuir吸附模型的双重模式理论关联了前文测得的乙醇和水在聚酰亚胺膜中的平衡溶解度,根据吸收/解吸数据求取了扩散系数,溶解度系数及渗透率;用无限稀释理想分离因子评价渗透蒸发膜的分离性能.结果表明,聚酰亚胺中加入交联剂后,乙醇和水的扩散系数和渗透率都有明显的减小,但其无限稀释理想分离因子有所提高,分离性能有较大的改善.     

乙醇和水在聚酰亚胺膜中的溶解和传递现象研究(Ⅱ)——乙醇和水在膜中的溶解和传递性质的关联

用Flory-Huggins溶液模型加Langmuir吸附模型的双重模式理论关联了前文测得的乙醇和水在聚酰亚胺膜中的平衡溶解度,根据吸收/解吸数据求取了扩散系数,溶解度系数及渗透率;用无限稀释理想分离因子评价渗透蒸发膜的分离性能.结果表明,聚酰亚胺中加入交联剂后,乙醇和水的扩散系数和渗透率都有明显的减小,但其无限稀释理想分离因子有所提高,分离性能有较大的改善.     

Characterization of reverse osmosis membranes prepared from the oxime of poly(4-vinylpyridine-co-methyl vinyl ketone)

The copolymer (4VK) of 4-vinylpyridine (4VP) and methyl vinyl ketone (MVK) was nearly quantitatively converted into an oxime (4VKX) which could be complexed with various metal salts. 4VKX was compared with 4VK in their membrane performance. When crosslinked with divinyl sulfone (DVS), the 4VKX membrane showed a higher strength than cellulose acetate (CA) membrane and a higher performance than 4VK membrane crosslinked with dihydrazine derivatives. The most adequate DVS content and copolymer composition were found to be 25 mol % based on MVK oxime and 1:1, respectively. A metal salt taken up by the membrane in a much larger amount tended to be highly rejected. The crosslinking quaternization of 4VKX with diisobutane (DIB) very effectively improved water permeability, as did 4VK, and gave a membrane performance superior to that of 4VK. The most appropriate charge ratio of DIB was 15–20 mol % based on 4VP. Some transport characteristics and compaction in a long-term operation were investigated and discussed.     

Comparison of liquid-phase and methanol-swelling crosslinking processes of polyimide dense membrane for CO2/CH4 separation

To overcome the plasticization effect in polyimide membranes, many researchers have proposed crosslinking method. This can reduce an inter-segmental mobility by tightening and rigidifying the polymer chains. However, it is difficult to modify the whole polymer chains throughout the membrane because the reaction can be hindered by the diffusion rate of the crosslinker. In particular, it is hard for bulky crosslinker to penetrate a dense membrane with a small d-spacing. This study investigated the effect of crosslinking a dense Matrimid membrane with p-phenylenediamine (p-PDA) via two different crosslinking methods (i.e., methanol-swelling crosslinking process [M-SCP] and liquid-phase crosslinking process [L-PCP]). Most of the crosslinking reaction in M-SCP occurs on the membrane surface due to difficulty in penetration of the bulky p-PDA into the Matrimid dense membrane. In contrast, the L-PCP allows uniform crosslinking across the membrane. The membranes crosslinked using L-PCP showed excellent chemical resistance. Furthermore, the plasticization phenomenon was not observed in the membranes crosslinked using L-PCP with p-PDA more than 15%. Meanwhile, the membrane crosslinked using M-SCP exhibited poor plasticization and chemical resistance properties. These results showed that the L-PCP method can be more effective for the crosslinking of dense membrane to deliver both high plasticization and chemical resistance.     

Permeation of urea and water through sulfonated poly(styrene-isobutylene-styrene) membranes with counterion substitution

This study describes the influence of counterion substitution on the transport of urea and water through sulfonated poly(styrene-isobutylene-styrene) (SIBS) membranes. SIBS was sulfonated to 66.4% to allow for enough ionic domains to transport urea and water through the membrane. Counterions, including alkyl-substituted ammonium ions, influenced the ionic domains creating a selective barrier that hindered the transport of urea and water through SIBS. Permeability measurements demonstrated that counterion substitution significantly affected the transport of urea. Membranes with higher charge counterions exhibited a more pronounced reduction in permeability due to the formation of crosslinks and increased restriction on molecular diffusion. Membranes substituted with ammonium salts showed reduced permeability due to the alkyl chains' hydrophobic nature, which created a physical barrier against the passage of urea molecules. Pure water flux tests indicated that counterion substitution also reduced the water flux of the membranes. Membranes with +1 counterions showed a small reduction in the water flux, while membranes with higher charge counterions and alkyl chain substitutions exhibited larger reductions, highlighting the influence of crosslinking and hydrophobicity on water transport.     

Effects of charge density on water splitting at cation-exchange membrane surface in the over-limiting current region

To determine the correlation between surface properties and concentration polarization (CP) behaviors, cation exchange membranes with varying fixed charge densities were prepared and characterized by using several electrochemical analyses such as chronopotentiometry, zeta potential, and current-voltage measurements. Results showed that CP behavior depended mainly on surface charge density. With higher surface charge density, stronger electroconvection was observed, suggesting that an increase in the surface charge density increased the concentration of the counter ions at the membrane surface. As such, the electric field around the membrane surface was strengthened at a current over the limiting current density. Water splitting was also proportional to the surface charge density. This result was consistent with the classical electric field-enhanced water splitting theory, indicating that water splitting increased due to increases in the electric field and prepolarization of water molecules at the membrane-solution interface of the cation-exchange membrane. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Antifouling enhancement of PVDF membrane tethered with polyampholyte hydrogel layers

The preparation and property of antifouling poly(vinylidene fluoride) (PVDF) membrane tethered with polyampholyte hydrogel layers were described in this work. In fabricating these membranes, the [2‐(methacryloyloxy)ethyl] trimethylammonium chloride and 2‐acrylamide‐2‐methyl propane sulfonic acid monomers were grafted onto the alkali‐treated PVDF membrane to yield polyampholyte hydrogel layers via radical copolymerization with N,N′‐methylenebisacrylamide as crosslinking agent. The analyses of fourier transform infrared attenuated total reflection spectroscopy and X‐ray photoelectron spectroscopy confirm the covalent immobilization of polyampholyte hydrogel layer on PVDF membrane surface. The grafting density of polyampholyte hydrogel layer increases with the crosslinking agent growing. Especially for the membrane with a high grafting density, a hydrogel layer can be observed obviously, which results in the complete coverage of membrane pores. Because of the hydrophilic characteristic of grafted layer, the modified membranes show much lower protein adsorption than pristine PVDF membrane. Cycle filtration tests indicate that both the reversible and irreversible membrane fouling is alleviated after the incorporation of polyampholyte hydrogel layer into the PVDF membrane. This work provides an effective pathway of covalently tethering hydrogel onto the hydrophobic membrane surface to achieve fouling resistance. POLYM. ENG. SCI., 55:1367–1373, 2015. © 2015 Society of Plastics Engineers     

Investigation on the formation mechanisms of hydrogels made by combination of γ‐ray irradiation and freeze‐thawing

Hydrogels based on poly(vinyl alcohol) (PVA)/water‐soluble chitosan (ws‐chitosan)/glycerol were prepared by γ‐ray irradiation, freeze‐thawing, and combination of γ‐ray irradiation and freeze‐thawing, respectively. The influence of freeze‐thawing cycles, the irradiation doses, and the sequence of freeze‐thawing and irradiation processes on the rheological, swelling, and thermal properties of these hydrogels was investigated to evaluate the formation mechanisms of hydrogels made by combination of irradiation and freeze‐thawing. For hydrogels made by freeze‐thawing followed by irradiation, the physical crosslinking is destroyed partially while chemical crosslinking is formed by irradiation. However, the chemical crosslinking density reduces with the increase of freeze‐thawing cycles. Hydrogels made by irradiation followed by freeze‐thawing bear less degree of physical crosslinking with the increase of irradiation dose for the increased chemical crosslinking density. It is found that these hydrogels own larger swelling capacity and better transparent appearance than those made by freeze‐thawing followed by irradiation. Moreover, the former hydrogels have larger mechanical strength than the latter at low freeze‐thawing cycles. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008