Physical characterization of suspension-crosslinked polystyrene particles and their sulphonated products: 1. Nonionic networks

Suspension-crosslinked particles of polystyrene have been prepared via a novel Friedel-Crafts reaction. Their structure and physical behaviour have been investigated using mercury porosimetry and swelling studies. The crosslinking agent used was 1,4-dichloromethyl-2,5-dimethyl benzene which exhibits limited polycondensation under the crosslinking reaction conditions. The degree of polymerization of the crosslinking agent between two crosslinks has been estimated for the crosslinked systems. The average size of the network structure has been estimated also with reference to potential g.p.c. applications.     

A mechanism for flame retardation of poly(ethylene terephthalate)

The thermolysis of vinyl methylterephthalate has been studied as a model of poly(ethylene terephthalate) (PET) thermolysis. Based upon this reaction, a potential mechanism for PET flame retardation is proposed. The assumption is made, based upon firm experimental evidence, that the initial step in PET degradation is formation of a vinyl ester and a carboxylic acid. The essentials of the mechanism are that this vinyl ester participates in a vinyl polymerization reaction and produces a linear polymer. This linear polymer then is involved in a chain-stripping reaction with the loss of methylterephthalate and the concomitant formation of a polyene. In the last step, this polyene undergoes a cyclization reaction and yields a crosslinked product. There are two stages at which PET undergoes crosslinking: In the second step the linear vinyl polymer is produced; in the last step ultimate crosslinking with the formation of cyclic structure is observed.     

Oxygen transmission through highly crosslinked polymers

Highly crosslinked polymers of varying structure were produced by the reaction of poly(glycidyl acrylate) (PGA) and methacrylate (PGM) with a variety of anhydride crosslinking agents: chlorendic (CA), glutaric (GA), maleic (MeA), succinic (SA), and polyrnalonic (pMnA) anhydrides. Topology was also varied by the use of a diluent and a comonomer in the backbone chain. Oxygen permeation measurements were made on these polymers coated onto a polypropylene film substrate before crosslinking. The crosslinking process greatly reduced the O₂ permeability which, however, was dependent not only on the degree of crosslinking (yield of the crosslinking reaction), but also on the crosslink density, the chemical nature of the structural elements, and the topology of the polymer network. Thus the most impermeable coating (XPGA/CA) was made not from the stiffest and bulkiest components (PGM and CA), but by the reaction of the bulkiest anhydride (CA) with the more flexible polymer backbone chain (PGA). This is explained in terms of the need for chain flexibility to produce a crosslinked structure of optimum space filling character and network tightness.     

Molecular design and synthesis of crosslinked polyimides using radical isomerization of vinylcyclopropane with thiols

Reactive polyimides bearing a vinylcyclopropane (VCP) moiety in the main chain were successfully synthesized from the corresponding diamine with the VCP moiety. Their radical crosslinking using a dithiol proceeded with the radical ring-opening reaction (RROR) of the VCP moiety to afford the corresponding crosslinked polyimides with the CC bonds in crosslinking moieties. Thermal properties of those crosslinked polyimides were evaluated by thermal gravimetry and differential scanning calorimetry. As a result, the increase of the crosslinking degree in the crosslinked polymer exhibited great residual weight at 600°C. In contrast, the tendency of the glass transition temperature was inverse because the increase in the amount of dithiol unit as a crosslinker would enhance the mitigation of the polymer packing structure and activates the mobility of polymer chains.     

Characteristics of Crosslinking Polymers Play Major Roles in Improving the Stability and Catalytic Properties of Immobilized Thermomyces lanuginosus Lipase

This study aimed to improve the stability and catalytic properties of Thermomyces lanuginosus lipase (TLL) adsorbed on a hydrophobic support. At the optimized conditions (pH 5 and 25 °C without any additions), the Sips isotherm model effectively fitted the equilibrium adsorption data, indicating a monolayer and the homogenous distribution of immobilized lipase molecules. To preserve the high specific activity of adsorbed lipase, the immobilized lipase (IL) with a moderate loading amount (approximately 40% surface coverage) was selected. Polyethylenimine (PEI) and chitosan (CS) were successfully applied as bridging units to in situ crosslink the immobilized lipase molecules in IL. At the low polymer concentration (0.5%, w/w) and with 1 h incubation, insignificant changes in average pore size were detected. Short-chain PEI and CS (MW ≤ 2 kDa) efficiently improved the lipase stability, i.e., the lipase loss decreased from 40% to <2%. Notably, CS performed much better than PEI in maintaining lipase activity. IL crosslinked with CS-2 kDa showed a two- to three-fold higher rate when hydrolyzing p-nitrophenyl butyrate and a two-fold increase in the catalytic efficiency in the esterification of hexanoic acid with butanol. These in situ crosslinking strategies offer good potential for modulating the catalytic properties of TLL for a specific reaction.     

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.     

Effect of the nature and degree of crosslinking on the catalase‐like activity of polystyrene‐supported schiff base‐metal complexes

Catalase‐like activity of the metal complexes of various crosslinked polystyrene‐supported Schiff bases were carried out and correlated with the nature and degree of crosslinking in the polymer support. Polystyrenes with 2–20 mol % ethyleneglycol dimethacrylate (EGDMA), 1,4‐butanediol dimethacrylate (BDDMA) and 1,6‐hexanediol diacrylate (HDODA) were used as polymer supports. functions of diethylenetriamine and salicylaldehyde were incorporated to the chloromethylpolystyrene by polymer analogous reactions and complexed with Fe(II), Fe(III), Co(II), Ni(II), and Cu(II) ions. The metal uptake decreased in the order: Cu(II) > Co(II) > Ni(II) > Fe(III) > Fe(II), and extent of metal uptake by the various crosslinked system varied with the nature and degree of the crosslinking agent. The polymeric ligands and the metal complexes were characterized by various analytical techniques. The catalytic activities of these metal complexes were investigated towards the decomposition reaction of hydrogen peroxide. Generally among the various metal complexes, the catalytic activities decreased in the order: Co(II) > Cu(II) > Ni(II) > Fe(III) ? Fe(II). With increasing rigidity of the crosslinking agent their catalytic activity also decreased. Of the various crosslinked systems, the catalytic activity decreased in the order: HDODA‐ > BDDMA‐ > EGDMA‐crosslinked system. Also, the catalytic activity is higher for low crosslinked systems and decreased further with increasing degree of crosslinking. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1271–1278, 2004     

Swelling of porous styrene–divinylbenzene copolymers in water

The swelling capacity of porous styrene–divinylbenzene (DVB) copolymers in water was studied by displacing methanol from the swollen polymer. The copolymers with different amounts of DVB were prepared in the presence of solvents with different solvating powers as inert diluents. Using a solvating solvent or its mixture with a nonsolvent as diluent, most of the obtained copolymers increase their volume in water, and the increase in volume becomes more significant with increasing the degree of crosslinking in some range of the DVB contents. The swelling capacity in water for the same copolymers with a high degree of crosslinking is linearly dependent on the dilution degree in the initial reaction mixture, to some extent. The unusual swelling behaviors in water were explained by the inner strain, which existed mainly in the less crosslinked domains between the highly crosslinked microgel particles, which are released in the course of swelling of the copolymers. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 536–544, 2000     

Preparation and properties of metallocene ethylene copolymer crosslinked by vinyltrimethoxysilane

Silane‐grafting and water‐crosslinking of ethylene–octene copolymer (EOR) was carried out. The influences of grafting formulation, comonomer content in EOR and time of the crosslinking process on grafting efficiency, degree and rate of crosslinking were investigated. The mechanical properties of various crosslinked samples are reported together with correlations with their gel contents. The results show that the extent of grafting increased with an increase in the amounts of dicumyl peroxide (DCP) and vinyltrimethoxysilane (VTMS) used in the reaction. The degree of crosslinking depended mainly on the extent of grafting, while the rate of the crosslinking process depended on the amount of amorphous phase in the samples. In this study, the variation of gel contents in the samples (0–77 %) had no significant influence on tensile properties. A large number of uncrosslinked regions are believed to exist inhomogeneously within the crosslinked polymers even after the sample has reached maximum gel content. The content of gel did not increase further when the crosslinking time was increased beyond that of reaching maximum gel, but the polymer network became denser, resulting in changes in polymer behaviour. Copyright © 2005 Society of Chemical Industry     

Comparison in processability and mechanical and thermal properties of ethylene–octene copolymer crosslinked by different techniques

The crosslinking of metallocene ethylene–octene copolymer was investigated. The crosslinked polymers were prepared using two different techniques, i.e., peroxide crosslinking and silane–water crosslinking. In the former, the crosslinking reaction was conducted in a twin‐screw extruder, in the presence of dicumylperoxide. In the latter, the polymer was first grafted with vinyl trimethoxysilane in the extruder and subsequently crosslinked with water. The paper aims at investigation of the differences between these two techniques, in terms of processing and product mechanical and thermal properties. The results showed that the silane‐crosslinked polymers could be prepared with much higher gel contents than the peroxide‐crosslinked samples. The silane‐crosslinked polymers also retained the elastomeric characteristics of the pure polymer and showed remarkably higher extensibility, better thermal stability, and energy storage capacity. An explanation for the property differences between peroxide‐crosslinked and silane‐crosslinked polymers was proposed. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1179–1185, 2004     

Self‐crosslinking and cocrosslinking with nitrile rubber of poly(vinyl chloride) with pendent N,N‐diethyldithiocarbamate group

In order to realize the self‐crosslinking and cocrosslinking of poly(vinyl chloride) (PVC) with nitrile‐butadiene rubber (NBR), PVC with pendent N,N‐diethyldithiocarbamate groups (PVC‐SR) was prepared from the reaction of PVC with sodium SR in butanone. The PVC‐SR was self‐crosslinked and the PVC‐SR/NBR blend was cocrosslinked under heating at 170°C. The effect of the degree of functionality of PVC‐SR on the torque, gel content, glass‐transition temperature, and tensile properties was investigated. The results showed that the crosslinking reaction did not occur for PVC, NBR, or the PVC/NBR blend. Introducing the SR groups into PVC caused the crosslinking reaction to occur and the high gel contents of the crosslinked samples were obtained in 15 min. The degree of crosslinking increased with the degree of functionality of PVC‐SR. The mechanism of the crosslinking reaction was discussed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 634–638, 2001     

Photoinitiated crosslinking of low density polyethylene. IV: Continuous extrusion application

A new continuous method is developed to crosslink polyethylene, based on a photoinitiated radical reaction. An experimental production line for low density polyethylene (LDPE) extrudate is constructed with a Brabender^® extruder and an “Irradiation Unit” on line. Some new initiators of benzophenone derivatives are prepared to overcome the volatility of benzophenone and increase its compatibility with the polymer matrix. Extraction and gel swelling experiments are performed to characterize the crosslinking of the product and the results are quite satisfactory: under the conditions used, crosslinking can be optimized to about 65% gel content within 5 to 15 s irradiation time, corresponding to a production rate of about 1.5 to 0.5 m/min, respectively. The dynamic mechanical properties of the crosslinked polymer are measured through the melting region and the effects of crosslinking are demonstrated. Without affecting the rate of crosslinking, the stability against thermal oxidation of the crosslinked products is greatly improved by adding stabilizers to the compounds before crosslinking.     

Characterization of network morphology in anion binding hydrogels used for wastewater remediation

This work reports on morphological features of hydrogels, which have been used for the ultimate removal and recovery of nutrient and toxic anions from wastewater effluents. The sorbent used was crosslinked polyamine (PAA·HCl) polymeric hydrogels. The surface topography and morphology of these hydrogels were characterized by tapping mode atomic force microscopy. The change of the gel surface in response to the degree of crosslinking was observed via phase imaging. The crosslinker amount affects both the crosslink density and uniformity. Phase images were recorded at moderate to hard tapping conditions (A_sp/A₀=0.3-0.6) and related to surface stiffness variations associated with Young's modulus (E) change. Bright ellipse and sponge-like domains of submicrometer scale were found on irregularly crosslinked gels, while the gel topography was uniform in gels that were prepared with a more regular distribution of crosslinks. The observed AFM domain size was strongly affected by the gel's degree of crosslinking.     

A convenient crosslinking method for sulfonated poly(ether ether ketone) membranes via friedel–crafts reaction using 1,6‐dibromohexane and aluminum trichloride

In this study, sulfonated poly(ether ether ketone) (SPEEK) was very efficiently crosslinked via a Friedel–Craft reaction using 1,6‐dibromohexane and AlCl₃. The resulting crosslinked SPEEK (c‐SPEEK) membranes exhibited improved dimensional stability, thermal and chemical stability, and mechanical strength with slight reduction in the elongation. The methanol permeability was reduced by approximately two orders of magnitude by the crosslinking reaction. The proton conductivities of c‐SPEEK membranes were greater than Nafion‐212 in the temperature range of 30–90°C. Overall, this new crosslinking method can be conveniently and efficiently applicable to most aromatic hydrocarbon polymer membranes. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40695.     

过氧化物交联聚乙烯热收缩管成型研究

简要介绍了聚乙烯高能辐射交联、硅烷化学交联和过氧化物化学交联,给出了交联前后分子结构变化的示意图,说明3种交联方法所得到的交联聚乙烯分子结构十分接近,在性能上基本没有差别。采用最新的过氧化物交联配方,并给出详细的生产工艺,使用最新研制的具有自主知识产权的活塞式挤出机加工成型交联聚乙烯热收缩管坯,并用自制的扩管装置扩张成型热收缩管。针对试验结果,对产品的凝胶率、力学物理性能和电性能进行了讨论,并比较了辐射交联、硅烷交联和过氧化物交联3种交联生产工艺在设备投资成本、产品质量控制、能耗、交联反应时间、产品的实际交联度和产品的表面质量等方面的优劣。     

交联比和乳化剂用量对大粒径交联聚合物微球溶胀性能的影响

利用不同交联比和乳化剂用量制得孔喉尺度交联聚合物微球。通过透射电子显微镜(TEM)评价,当交联比为0.7%且乳化剂用量为10%时,初始粒径集中分布在175 nm附近。水化后,通过动态光散射(DLS)实验,发现所制备的交联聚合物微球的膨胀属于有限溶胀;当在交联比为0.7%且乳化剂用量为10%时,在一定时间内,微球溶胀后平均粒径最大,可达2 500nm,同时溶胀倍数也最大,约在10倍以上。     

Open-pore morphology of i-PP copolymer crystallized from a gel state in supercritical propane

Open-pore morphology was produced in low-density polypropylene (i-PP) by first crosslinking the i-PP and then crystallizing it from a highly swollen state in supercritical propane. The extent of crosslinking, expressed by an increasing gel fraction and an increasing compression modulus, was found to have a strong effect on pore size but less so on crystal structure. The resulting pore structure, studied using scanning electron microscopy, showed a decrease in pore size with an increase in gel fraction. Typical pore sizes are between 1 and 10 μm. The polymer morphology in the wall, analyzed using wide-angle X-ray diffraction, indicated the presence of α-PP crystals. The experimental results showed that mostly loose chains (sol fraction) contribute to the crystallinity of the polymer. For our specific i-PP, the crosslinked network devoid of sol fraction (after extraction) collapses during cooling and does not produce any open pore structure.     

Zur Kinetik der Bildung von löslichen Polymeren beim Metathese-Abbau von C—c-vernetztem 1,4-Polybutadien

1,4-Polybutadiene was crosslinked with dicumyl peroxide. Samples with different degree of crosslinking were ground to particles with a diameter of < 0,1 mm and degraded in a metathesis reaction with a small amount of 4-octene and the catalyst WCl₆/(CH₃)₄Sn at 20°C. The fraction of the soluble polymer (soluble in chlorobenzene and precipitable by methanol) was determined as a function of reaction time and investigated by gel permeation chromatography. This fraction and in particular the higher molecular part (M > 120 000 g/mol) passes a maximum, which is more evident at a lower degree of crosslinking. The kinetics show peculiarties of the metathesis degradation and crosslinking.     

Computer-Aided Design of Crosslinked Polymer Membrane Using Machine Learning and Molecular Dynamics

The formation of crosslinking network between polymer chains has significant influence on polymer properties. In particular, the crosslinked structure of ionic networks like proton exchange membrane affects the conductivity performance. To further develop in this area, a framework for polymer membrane design based on the developed quantitative prediction model of the properties of crosslinked polymer is proposed. First, polymers with different crosslinking degrees are constructed by a crosslinking algorithm. Next, molecular dynamics is used to calculate the properties of crosslinked polymers. Then, the quantitative relationship between crosslinked polymer structures and macroscopical properties is developed. Subsequently, computer-aided polymer design method is integrated with the developed quantitative predict model. The crosslinked polymer design problem is expressed as an optimization problem to obtain the optimal crosslinking degree. Bayesian optimization strategy is used to solve the established optimization model. Finally, two case studies of perfluoro sulfonic acid and perfluoro imide acid design are given to illustrate the application of the proposed polymer design framework.