聚丙烯的交联

本文论述了聚丙烯的辐射交联、化学交联和二步法水交联的研究进展,特别对近年来获得成功的二步法水交联法中各种影响因素进行了介绍.     

Thermally reversible crosslinking of polystyrene via the furan–maleimide Diels–Alder reaction

Mixtures of maleimidomethylated polystyrene with difurfuryl adipate or with furfuryl alcohol-esterified poly[styrene-co-(maleic anhydride)] underwent thermally reversible Diels–Alder crosslinking. Reversal of crosslinking occurred rapidly at 150°C. The instability of the furfuryl group, however, limits the practicability of the reaction. © 1992 John Wiley & Sons, Inc.     

医用高分子水凝胶的设计与合成

作为一类重要的医用功能材料,高分子水凝胶可望在药物控释、软骨支架构建、活性细胞封装等方面获得广泛应用。综述了基于化学交联和物理交联的有关水凝胶的设计与合成方法,重点介绍了通过自由基共聚反应、结构互补基团间化学反应形成的化学交联水凝胶以及通过荷电相反离子问相互作用、两亲性嵌段或接枝共聚物疏水缔合、结晶与氢键相互作用形成的物理交联水凝胶。     

多官能团乙烯基单体原子转移自由基交联聚合机理

乙烯基单体;原子转移自由基交联聚合;聚合机理;交联网络     

明胶膜的制备及其交联性能的研究

探讨了溶剂、温度及pH值对明胶膜性能的影响，并以甲醛和戊二醛为交联剂，采用溶液交联和蒸汽交联两种方法对明胶膜进行交联改性。研究结果表明：相对于溶液交联，甲醛蒸汽交联所得膜的拉伸强度从25MPa上升到42MPa，戊二醛交联的膜的拉伸强度从15MPa上升到40MPa，而溶胀率和溶出率均有所下降，蒸汽交联的膜的性能优于溶液交联的膜。     

柠檬酸铝/HPAM凝胶的制备与DSC表征

利用DSC可以精确判断柠檬酸铝／HPAM体系在反应过程中是发生分子内交联还是发生分子间交联，以及体系的反应进行程度，分子间交联的体系产物在较高的温度下有分解，分子内交联的体系产物在较低的温度分解，在本体系和该测量条件下，分子间交联的产物和分子内交联的产物的分解温度界限为70℃，反应是否进行，可以通过体系产物的分解温度来判断，如果分解温度一直上升，说明体系还在反应，反之可认为反应终止。     

Design and Fabrication of Highly Stretchable and Tough Hydrogels

Abstract

Stretchable and tough hydrogels have attracted a lot of attention due to their great potential in applications such as wound healing, drug delivery, tissue culture, etc. They can also be paired with electronic components to create artificial skin, wearable electronics, and patches. To promote the development of more hydrogels, we will summarize methods and materials that have been used to develop these gels, and then we will compare the performance of these gels in an aim to guide the future development of gels for superior performance, especially for specific applications.     

Reversible gelatin-based hydrogels: Finetuning of material properties

The present work reports on the synthesis and evaluation of a crosslinkable thiolated gelatin derivative. The effect of varying two parameters including the pH of the reaction buffer and the thiolating agent applied (i.e. N-acetylhomocysteine thiolactone versus Traut’s reagent) on the obtained modification degree was studied in a first part. The gelatin derivatives synthesized starting from N-acetylhomocysteine thiolactone and Traut’s reagent were characterized in depth using size exclusion chromatography and UV–VIS spectrophotometry. In a subsequent part of the present work, hydrogel films were prepared starting from the thiolated gelatin derivative developed using N-acetylhomocysteine thiolactone. The contributions of both the chemical and the physical crosslinking of the hydrogels developed were studied in depth using rheology, swelling experiments and texturometry. The results indicate that the physical structuring, inherent to gelatin, contributes to a large extent to the mechanical properties. However, the chemical crosslinking mostly determines the final hydrogel properties and can be controlled to a large extent. The gelatin-based gels are flexible, strong and transparent. A major advantage of disulfide-crosslinked hydrogels is the fact that the crosslinking is reversible. The latter could be interesting in view of future applications as cell carriers for tissue engineering.     

Release properties of chemical and enzymatic crosslinked gelatin-gum Arabic microparticles containing a fluorescent probe plus vetiver essential oil

Oil-containing gelatin-gum Arabic microparticles were prepared by complex coacervation followed by crosslinking with glutaraldehyde or transglutaminase. A fluorescent mixture, khusimyl dansylate (KD) as the fluorescent compound mixed to the vetiver essential oil, was used as oil model. The effect of the type of crosslinking of the coacervated gelatin-gum Arabic membrane, the physical state of microparticles, wet or freeze-dried and the type of release media, aqueous with surfactants, Sodium Dodecyl Sulphate (sds) or Tween 80 (tw) and anhydrous ethanol as organic media on the release rate of the KD from the microparticles, was experimentally investigated.It was shown that the oil was dispersed uniformly throughout the microparticles and the chemical crosslinked microparticles were more resistant to swelling, presenting smaller sizes after hydration. Also the crosslinking effect, transglutaminase or glutaraldehyde, could be confirmed by the integrity of the crosslinked gelatin-gum Arabic microparticles after incubation in the aqueous sds media, compared to complete dissolution of the uncrosslinked microparticles in this media.The cumulative fluorescent KD release from the gelatin-gum Arabic microparticles decreased in the following order of dissolution media: anhydrous ethanol > tw > sds and the wet microparticles have shown a faster KD release than freeze-dried ones. A mathematical model was used to estimate the diffusion coefficient (D). The chemically crosslinked gelatin-gum Arabic microparticles ensured a pronounced retard effect in the KD diffusion, presenting a D varying from 0.02 to 0.6 × 10⁻¹¹ cm²/s, mainly in an aqueous media, against D varying from 1.05 to 13.9 × 10⁻¹¹ cm²/s from the enzymatic crosslinked microparticles.     

Photomediated crosslinking of cinnamated PDMS for in situ direct photopatterning

Polydimethylsiloxane (PDMS) polymers incorporating pendant cinnamate groups have been synthesized and evaluated for their ability to form patterned thin films via photo‐crosslinking. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3482–3487, 2008     

Fracture mechanism of liquid‐crystalline epoxy resin systems with different phase structures

A liquid‐crystalline epoxy resin was cured at two different temperatures. The phases of the cured systems clearly showed isotropic and nematic polydomain structures, which depended on the curing temperature. The fracture toughness of the systems was measured, and the fracture mechanism was investigated with polarized IR measurements. The nematic polydomain structure system showed considerably higher fracture toughness than the isotropic structure. Moreover, both systems exhibited a reorientation of the network chains near the fracture surface during the fracture process, and the region of the network reorientation in the nematic polydomain structure system was larger than that in the isotropic structure system. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4044–4052, 2004     

Influence of the loading rate on liquid‐crystalline epoxy resin systems

The fracture toughness of liquid‐crystalline epoxy systems, which had a nematic polydomain structure (domain size about 40 μm), with an increasing loading rate was evaluated. In this system, the fracture toughness dramatically decreased from 1.96 to 0.22 MN/m^3/2 with an increasing loading rate (0.1–5 mm/min). The network orientation near the fracture surface of different loading rate systems was investigated with polarized optical microscopy and polarized infrared spectroscopy. As a result, a large oriented region of mesogenic groups was observed near the fracture surface in the relatively low loading rate (0.1 and 0.5 mm/min) systems, but such a phenomenon was not observed in the high loading rate (2 and 5 mm/min) systems. These results showed that the high fracture toughness of the system at the low loading rate was due to the magnitude and region of the reorientation of the mesogenic groups in the fracture process and that high toughness could not be achieved at a high loading rate because the loading rate was too fast to allow orientation of the networks containing the mesogenic groups. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1296–1302, 2005     

Effect of the crosslink density on the morphology and properties of reaction‐injection‐molding poly(urethane urea) elastomers

The effect of the crosslink density on the morphology and properties of reaction‐injection‐molding poly(urethane urea) (PUU) elastomers was investigated. Fourier transform infrared spectroscopy data showed that the linear and crosslinked PUU had entirely different hard‐domain sizes and hard‐segment ordering. A study of the morphology indicated that an increase in the crosslink density increased microphase mixing. Differential scanning calorimetry studies indicated that the hard‐segment initial glass‐transition temperature was independent of the crosslink density. The glass‐transition temperature of the soft segment was highest when the network was perfect. The tensile‐strength behavior showed that the mechanical properties of PUU reached a maximum when the network was perfect. The increase in the resilience of the crosslinked PUU elastomer was higher than that of the linear PUU elastomer with an increase in temperature, and the reduction of the hardness of the former was also higher than that of the latter. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1126–1131, 2004     

The influence of comonomer (2‐hydroxyethyl acrylate) and keto group on photocrosslinking property of arylidene polymers

Two types of arylidene compounds were synthesized by reacting p‐hydroxybenzaldehyde with acetone [1,5‐bis(4‐hydroxyphenyl)penta‐1,4‐dien‐3‐one] (PBHP) and cyclohexanone [2,6‐bis(4‐hydroxybenzylidene)cyclohexanone] (HBC). 1,4‐Pentadien‐3‐one‐1‐p‐hydroxyphenyl‐5‐p‐phenyl methacrylate (PHPPMA) and 4‐{[‐3‐(4‐hydroxybenzylidene)‐2‐oxocyclohexylidene]methyl}phenyl acrylate (HBA) were prepared by reacting PBHP and HBC with methacryloyl chloride and acryloyl chloride in the presence of triethylamine, respectively. Copolymerization of different feed compositions of PHPPMA and HBA with 2‐hydroxyethyl acrylate (HEA) was carried out using a free‐radical solution polymerization technique in ethyl methyl ketone (MEK) using benzoyl peroxide (BPO). All the monomer and polymers were characterized by IR and NMR (¹H/¹³C) spectroscopic techniques. The reactivity ratio of the monomers were obtained using Fineman–Ross (FR), Kelen–Tudos (KT), and extended Kelen–Tudos (exKT) methods. The photocrosslinking properties of the polymers were done using a UV absorption spectroscopy technique. Homopolymers of both the arylidene polymers shows similar trend towards the rate of photocrosslinking. The rate of photocrosslinking was enhanced when the cyclohexanone based arylidene monomer was copolymerized with HEA. Thermal stability and molecular weights (M_w and M_n) of the polymers were determined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3433–3444, 2004     

Free radical initiated low temperature crosslinking of phenylethynyl (PE) end‐capped oligomides

A relatively low‐temperature crosslinking method for phenylethynyl (PE) end‐capped oligomides was developed. PE end‐capped oligomides are typically cured into crosslinked polyimides at 370 °C for about 1 h. The addition of a low viscosity mixed‐solvent of N‐methylpyrrolidinone (NMP)/dimethyl ether of polyethylene glycol (M = 250 g/mol), NMP/DM‐PEG‐250, or NMP/polyethylene glycol (M = 400 g/mol), NMP/PEG‐400, as film forming medium for PE‐end‐capped oligomides was investigated. Fourier transform infrared spectroscopy and ¹³C NMR showed that the mixed solvent addition was effective for achieving low‐temperature crosslinking of the ethynyl end‐caps over the temperature range 200–250 °C. The low temperature crosslinking process was explained by thermolysis of the PEG molecules over this temperature range forming free radical species such as ～CH₂CH₂O· or ～CH₂CH₂^· which initiate cure of the ethynyl groups resulting in a cross linked polyimide membrane. The PEG solvents also provide a radical source for the degradation polymerization of the solvents to a water and NMP insoluble polymer, which formed a miscible blend with the crosslinked membrane. Glass transition temperature (differential scanning calorimetry) data and thermo gravimetric analysis data provide evidence for the miscible blend. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3950–3963, 2010.     

Thermoreversible covalent crosslinking of maleated ethylene/propylene copolymers with diols

The objective of this study is the thermoreversible crosslinking of maleated ethylene/propylene copolymer (MAn‐g‐EPM) using the equilibrium reaction with diols. Covalent hemi‐ester crosslinks are formed via the reaction of anhydrides with alcohols, while an equilibrium shift at elevated temperatures may result in their removal. High conversions to hemi‐ester are obtained at low temperatures in the presence of p‐toluenesulfonic acid, whereas conversions are low at high temperatures. The presence of microphase‐separated aggregates acting as physical crosslinks was demonstrated for MAn‐g‐EPM and all crosslinked materials. The covalent crosslinks were only formed within the aggregates, resulting in stronger aggregates that persisted to higher temperatures. The tensile strength and elasticity were significantly improved upon increasing level of crosslinking, whereas the type of diol has less influence. The covalently crosslinked MAn‐g‐EPM was reprocessable via compression molding at temperatures above 175 °C. Irreversible diester formation occurred for the longer diols, but did not prevent reprocessing, while short diols evaporated. Both effects lowered the level of crosslinking, resulting in significantly changed mechanical properties. The reprocessability does not originate from an equilibrium shift, but from a dynamic exchange between crosslinked and non‐crosslinked functional groups, which allows crosslinks to disconnect and the corresponding chain segments to diffuse between aggregates. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1810–1825, 2008     

Effect of method of preparation and process variables on controlled release of insoluble drug from chitosan microspheres

An inexpensive and simple method was adopted for the preparation of chitosan microspheres, crosslinked with glutaraldehyde (GA), for the controlled release of an insoluble drug‐ibuprofen, which is a commonly used NSAID (non‐steroidal anti‐inflammatory drug). The chitosan microspheres were prepared by different methods and varying the process conditions such as rate of stirring, concentration of crosslinking agent, and drug:polymer ratio in order to optimize these process variables on microsphere size, size distribution, degree of swelling, drug entrapment efficiency, and release rates. The absence of any chemical interaction between drug, polymer, and the crosslinking agent was confirmed by Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), and thermogravimetric analyses (TGA) techniques. The microspheres were characterized by optical microscopy, which indicated that the particles were in the size range of 30–200 µm and scanning electron microscopy (SEM) studies revealed a smooth surface and spherical shape of microspheres. The microsphere size/size distributions were increased with the decreased stirring rates as well as GA concentration in the suspension medium. Decreasing the concentration of crosslinker increased the swelling ratio whereas extended crosslinking exhibited lowered entrapment efficiency. The in vitro drug release was controlled and extended up to 10 hr. Copyright © 2007 John Wiley & Sons, Ltd.     

Poly(siloxane‐urethane) crosslinked structures obtained by sol‐gel technique

Poly(siloxane‐urethane) crosslinked structures were prepared from isophorone diisocyanate, α,ω‐bis(hydroxybutyl)oligodimethylsiloxane and a new hybrid diol containing hydrolysable Si? OC₂H₅ groups besides OH groups. The latest was synthesized by the acid‐catalyzed reaction between 1,3‐bis(3‐glycidoxypropyl)tetramethyldisiloxane and 3‐aminopropyltriethoxysilane. The formations of the urethane groups along the polymer backbone as well as the formation of the silica domains were first confirmed by the presence of the specific bands in Fourier transform infrared spectra. The resulted materials were characterized using differential scanning calorimetry, thermogravimetric analysis and scanning electron microscopy. The results of the dynamic mechanical analysis (DMA) performed at various frequencies revealed shape memory capabilities for some of the obtained structures. The silica formed because of the hydrolysis‐condensation reactions proved to have reinforcing effect upon siloxane‐urethane structure also evidenced by DMA and increasing water vapor sorption capacity as was measured by dynamic vapor sorption. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Hybrid ionic/covalent polymer networks derived from functional imidazolium ionomers

A new class of ionomers is described wherein ion‐pairs bear reactive functionality, thereby facilitating further chemical modification. Halide displacement from brominated poly(isobutylene‐co‐isoprene) by 1‐vinylimidazole is used to prepare ionomer derivatives that can be crosslinked by radical oligiomerization of pendant vinyl groups. The resulting thermosets contain a labile network of imidazolium bromide ion‐pair aggregates as well as a stable covalent network. As a result of their hybrid ionic/covalent composition, these thermoset elastomers provide a unique combination of rheological, tensile and stress relaxation properties that cannot be achieved from conventional covalent or ionic networks. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2438–2444     

Functional Microgels Tailored by Droplet‐Based Microfluidics

Monodisperse polymer gel particles with micrometer‐scale dimensions serve for a variety of applications, including those as microcapsules for actives or as micrometer‐sized matrixes for mesoscopic additives. These particles can be produced with exquisite control through the use of droplet‐based microfluidic templating followed by subsequent droplet solidification. This can be achieved by two ways: One way is to use pre‐microgel solutions of low molecular weight monomers and to form microgels by polymerizing these monomers. Another way is to use pre‐polymerized, high molecular weight precursors and to gel them by polymer‐analogous crosslinking. Both approaches have their specific advantages, allowing microgels to be tailored and optimized for specific needs such as those as delivery systems or scaffolds for living cells. This article highlights some recent achievements in the development and use of these microfluidic techniques to fabricate functional microgel particles.