Photochemistry of polymeric systems. II. Photocrosslinking of polymers and copolymers containing various aromatic N-oxide groups

2-Methyl-5-vinylpyridine-N-oxide, 4-vinylquinoline-N-oxide. 9-vinylacridine-N-oxide, p-N,N-dimethylaminostyrene-N-oxide units were introduced in polymeric chains as homopolymers or/and as styrene copolymers to study their photocrosslinking. The method used for characterization of photocrosslinked films was a “photoresist test” described in Part I of this series. The photosensitivity of the different chromophores bound to the different polymer has also been studied by UV, IR, and fluorescence spectrophotometries. The use of aromatic amine N-oxide groups in polymers seems to be a general means to produce their photocrosslinking by radical reactions. Among the different polymeric materials prepared, 4-vinylpyridine-N-oxide and 4-vinylquinoline-N-oxide are the most photosensitive.     

SBS光交联反应及其性能研究

利用测定凝胶含量的方法 ,对SBS热塑性弹性体在紫外光辐照下的交联反应进行了研究。研究表明 ,SBS经光交联剂的引发能很好地发生交联 ,在所采用的光引发剂蒽醌、苯芴酮、三硝基芴酮、安息香乙醚中 ,安息香乙醚的交联效果最佳。另外 ,通过红外光谱对交联反应动力学进行了研究 ,结果发现SBS中乙烯双键含量的增加对交联反应影响不大 ,因为乙烯双键含量的增加 ,只会加速分子内反应 ,对分子间的交联反应没有贡献。辐照膜的差示扫描量热及热失重分析表明 ,交联后的SBS膜热稳定性增大 ,且随交联程度的增加 ,热稳定性增大 ,并用扫描电镜对交联膜的微观结构进行了表征     

光交联壳聚糖膜的制备及其性能研究

成功制备了光交联壳聚糖膜,并用傅立叶红外光谱(FTIR)和扫描电子显微镜(SEM)方法对其结构进行了表征,并测试了其力学性能。结果表明:光交联作用明显提高了膜的抗张强度和抗水性,并有效地降低了溶菌酶对其降解速率。该光交联膜有望用作可控降解生物医用材料。     

Wavelength effect on the photoinduced reaction of polymethylmethacrylate

Polymethylmethacrylate (PMMA) containing benzophenone (BP) was photo-irradiated with monochromatic radiation of wavelength 260–360 nm using the Okazaki Large Spec-trograph (OLS). On irradiation of PMMA films containing BP in air, the sensitized main-chain scission and photocrosslinking of PMMA took place simultaneously. These reactions are dependent on irradiation wavelength. The threshold wavelength for both reactions is found to be ca. 380 nm. The number of main-chain scission and amount of gel increased with the increase of BP concentration in PMMA. Photosensitized main-chain scission favors the irradiation of radiation at ca. 280 nm and photocrosslinking takes place efficiently with the exposure of 340 nm radiation. A possible mechanism for photosensitized reaction is proposed. © 1995 John Wiley & Sons, Inc.     

RECENT DEVELOPMENTS IN PHOTOINITIATED CROSSLINKING OF POLYETHYLENE AND ITS INDUSTRIAL APPLICATIONS

The recent developments in the photoinitiated cross-linking of polyethlene and the significant breakthrough of itsindustrial application are reviewed, The enhanced photo-initiation system, the dynamics of photoinitiated crosslinking, theoptimum conditions, the crystal morphological structures and related properties, and the photo- and thermo-oxidationstability of photocrosslinked polyethylene (XLPE) materials have been elucidated systematically. A new technique forproducing photocrosslinked XLPE-insulated wire and cable is described in detail. It can be expected that the fiture applications of photocrosslinking technique of polyolefins will be very promising.     

Initiator-free photocrosslinking of electrospun biodegradable polyester fiber based tubular scaffolds and their cell affinity for vascular tissue engineering

<正>Random copolyester of poly(ε-caprolactone-co-L-lactide)(PCLA) with a 50:50 feeding molar ratio was synthesized via the ring-opening polymerization and functionalized by the end-capping reaction with acryloyl chloride.The resulting acrylated PCLA was then fabricated into small diameter tubular scaffolds by electrospinning technique and the formed scaffolds were followed by photocrosslinking under UV irradiation in the absence of photoinitiator.The mechanical strengths including tensile,suture retention and burst pressure were greatly enhanced after the photocrosslinking.The in vitro degradation data clearly revealed that the mechanical properties of the crosslinked scaffolds still remained after one month degradation in PBS solution,while those of the non-crosslinked ones lost heavily.The cytotoxicity assay on the mouse fibroblast L929 cells was conducted via MTT measurement.Furthermore,the observation on endothelial and fibroblast cell adhesion and proliferation was also made by using scanning electron microscopy(SEM).The initiator-free photocrosslinked tubular scaffolds show the potential to be used in vascular tissue engineering.     

Photocrosslinked methacrylated chitosan-based nanofibrous scaffolds as potential skin substitute

Nanofibers based on natural polymers have recently been attracting research interest as promising materials for use as skin substitutes. Here, we prepared photocrosslinked nanofibrous scaffolds based on methacrylated chitosan (MACS) by photocrosslinking electrospun methacrylated chitosan/poly (vinyl alcohol) (PVA) mats and subsequently removing PVA from the nanofibers. We comprehensively investigated the solution properties of MACS/PVA precursors, the intermolecular action between MACS and PVA components, and the morphology of MACS/PVA nanofibers. Results indicated that the fiber diameter and morphology of the photocrosslinked methacrylated chitosan-based nanofibrous scaffolds were controlled by the MACS/PVA mass ratio and showed highly micro-porous structures with many fibrils. In vitro cytotoxicity evaluation and cell culture experiments confirmed that MACS-based mats with micro-pore structure were biocompatible with L929 cells and facilitated cellular migration into the 3D matrix, demonstrating their potential application as skin replacements for wound repair.     

A convenient photochemical method to immobilize enzymes

A method to immobilize enzymes photochemically with use of photocrosslinkable poly(vinylalcohol) bearing stilbazolium groups is described. Irradiation of a film of the photosensitive polymer containing enzyme gave water-insoluble, cellophane-like transparent film demonstrating high enzyme activity. The dissolution of enzymes from the film was dependent upon the molecular weight of the proteins and the content of the photocrosslinking unit. It was found that various enzymes are sufficiently entrapped in the photocrosslinked polymer matrix when the content of the photosensitive group is more than about 1.0 mol%.     

Photocrosslinking of poly(2,3-epithiopropyl methacrylate) by the oxidation of pendant episulfide groups

In the photocrosslinking of poly(2,3-epithiopropyl methacrylate) (PETMA) films the effect of the pendant episulfide group's oxidation on the crosslinking of PETMA was investigated. Thermal crosslinking of PETMA is promoted by peroxides such as benzoyl peroxide and hydrogen peroxide. IR spectrum of the crosslinked PETMA showed that the reaction proceeded through the oxidation of episulfide groups by the peroxides. The anthracene (An) sensitized photocrosslinking of PETMA films also proceeded via the oxidation of episulfide groups by singlet oxygen. It was found that residual tetrahydrofuran (THF) in the films remarkably increased the rate of the photocrosslinking and/or reduced the induction period. From the further investigation concerning casting solvents it was found that residual CS₂, CCl₄, and CHCl₃ in films increased the rate of the photocrosslinking and/or reduced the induction period of the photocrosslinking. The disappearance rate of An in the films was also increased by the presence of residual CS₂, CCl₄, and CHCl₃, differring from the result of THF. These results were explained by a difference in lifetime of singlet oxygen in the films. From the results were explained by a difference in lifetime of singlet oxygen in the films. From the results concerning the effects of hydroperoxides such as THF hydroperoxide and t-butyl hydroperoxide on the photocrosslinking of PETMA films the acceleration effect of the residual THF was deduced to be due to the promotion of singlet oxygen-oxidation of sulfide groups by protic compounds such as THF hydroperoxide and H₂O in the THF.     

交联聚乙烯的结晶行为研究

本论文采用熔融共混法制备了高密度聚乙烯(HDPE)/光引发剂(BDK)/交联剂 (TAIC) 共混物,再用高压汞灯辐照制备一系列不同交联程度的样品。通过广角X射线衍射(XRD)与差示扫描量热法(DSC)等手段研究了紫外光交联对HDPE的晶体结构及非等温结晶动力学行为的影响。研究结果表明：紫外光交联不改变HDPE晶型;随着光交联时间的增加,结晶峰的强度下降;晶粒尺寸变小。随着降温速率的增加,HDPE与交联HDPE (XLPE)的结晶峰变宽,结晶温度(Tp)向低温方向移动,结晶速率变快,结晶度(Xc)下降。相比较而言,XLPE的Tp较低,F(T)较大,Xc较小,结晶活化能(?E)较高,表明紫外光交联抑制了HDPE的结晶。     

Synthesis of polymers having photocrosslinkable moieties for second-order nonlinear optics

Second-order non-linear optical polymers having photocrosslinkable moieties were synthesized by cationic polymerization of monomer (I) and monomer (II). The polymerization proceeded rapidly to give linear polymers in high yields. Monomer reactivity ratios were calculated to be r₁ = 0.90 and r₂ = 0.96 (r₁r₂ = 0.86), indicating that these monomers copolymerized through the almost ideal copolymerization mechanism. The photocrosslinking reaction of an equimolar copolymer film underwent the conversion of up to ca. 70% upon irradiation with a 500 W high-presure mercury lamp for 5 min. The electric field induced polar orientation of the chromophores (pendant 4-nitrophenyloxy groups) in a photocrosslinked polymer was stable for more than 10 days. This polymer exhibits a nonlinear coefficient d₃₃ of 5.6 × 10^-10 esu measured at a pumping wavelength of 1064 nm.     

Dynamically Photocrosslinking of Polypropylene／EPDM Blends as a Thermoplastic Elastomer

The mechanical properties and the crystal morphological structures of the dynamically photocrosslinked polypropylene (PP)/ethylene-propylene-diene terpolymer (EPDM) blends have been studied by means of mechanical tests, wide-angle X-ray diffraction(WAXD), and differential scanning calorimetry(DSC). The dynamically photocrosslinking of the PP/EPDM blends can improve the mechanical properties considerably, especially the notched Izod impact strength at low temperatures. The data obtained from the mechanical tests show that the notched Izod impact strength of the dynamically photocrosslinked sample with 30% EPDM at -20℃ is about six times that of the uncrosslinked sample with the same EPDM component. The results from the gel content, the results of WAXD, and the DSC measurements reveal the enhanced mechanism of the impact strength for the dynamically photocrosslinked PP/EPDM blends as follows: (1) There exists the crosslinking of the EPDM phase in the photocrosslinked PP/EPDM blends ; (2) The β-type crystal structureof PP is formed and the content of α-type crystal decreases with increasing the EPDM component; (3) The graft copolymer of PP-g-EPDM is formed at the interface between the PP and EPDM components. All the above changes of the crystal morphological structures are favorable for increasing the compatibility and enhancing the toughness of the PP/EPDM blends at low temperatures.     

Photocrosslinking of thin polymer films – materials for sensors and actuators

Polymers with temperature dependent degrees of swelling, especially polymers which exhibit lower critical solution temperature (LCST) behaviour in aqueous solutions, are of interest for applications in microsystems. For these applications it is necessary to form and pattern thin films in the μm-range. This has been achieved through photocrosslinking of linear prepolymers. Copolymers based on N-isopropylacrylamide (NIPAAm) were modified with a stilbazolium salt chromophore to yield photocrosslinkable temperature sensitive polymers. The chromophore reacts via a [2+2]-cycloaddition under irradiation, this can be used to crosslink the polymer. The photocrosslinking properties were studied by UV irradiation of thin films and measuring the changes in UV absorption spectra. Through irradiation of thin films through a mask it was possible to obtain patterned networks in the μm-range (20 μm space width and > 50 μm line width). The polymers exhibited LCST behaviour, which was measured using DSC. The resulting patterned networks had temperature dependent swelling properties in aqueous media.     

三偏磷酸钠交联壳聚糖/聚乙烯醇共混膜的制备及其性能研究

将充分溶胀的壳聚糖/聚乙烯醇共混膜置于一定浓度的三偏磷酸钠溶液中进行交联反应制备出交联壳聚糖膜。用红外光谱(FTIR-ATR)、X-射线衍射(XRD)和扫描电镜(SEM)表征了膜的结构,并测试了其吸水率、力学性能、酶降解性能。结果表明,交联作用明显提高了膜的抗张强度和抗水性,并有效地降低了溶菌酶对其降解速率。     

Photocrosslinkable second-order nonlinear optical polymers synthesized by cationic copolymerization

Photocrosslinkable second-order nonlinear optical (NLO) polymers were synthesized from cationic copolymerization of a vinyl ether monomer bearing 4′-nitrobiphenyl-4-oxy group as the NLO chromophore with a vinyl ether monomer bearing cinnamoyl group as the photoreactive moiety. To obtain a suitable poling method involving photocrosslinking, which is capable of inducing a higher and more stable second-order nonlinear coefficient, d₃₃, for NLO polymer films, some poling procedures were investigated. An optimized poling method was as follows. Ultraviolet (UV) irradiation is performed for 90 sec during poling at 50°C for 20 min, followed by poling at 150°C for 20 min. By using this poling method NLO polymer films exhibited a higher and considerably stable d₃₃ value at room temperature, even though they had rather lower glass transition temperatures before photocrosslinking. Some photocrosslinking mechanism for NLO polymers investigated here were considered.     

Photosensitization of bioinspired thymine-containing polymers

Here, we report a sensitization study on a family of water-soluble photopolymers based on thymine. The goal of this study was to determine whether the presence of sensitizer molecules would promote photocrosslinking/immobilization of the polymers using low-energy irradiation (520 nm) as compared to the UV irradiation (approximately 280 nm) necessary for the standard photoinduced process to take place. With the aid of Eosin Y Spirit Soluble (EY) as a sensitizer, water-soluble polystyrene copolymers of vinylbenzylthymine-vinylbenzyltriethylammonium chloride (VBT-VBA) were immobilized after exposure to visible irradiation. By exciting the sensitizer molecule in the presence of VBT copolymers at a wavelength where absorption by the latter does not occur, the triplet state of the sensitizer is generated in high yields, and consequently, polymer photocross-linking takes place. UV-vis spectroscopy has been used to study the effect of irradiation dose, copolymer composition, and sensitizer concentration on the photoreactivity of VBT polymers. These studies demonstrate the feasibility of using Eosin Y as a sensitizer to achieve the thymine photodimer formation, resulting in immobilization of VBT-VBA-EY films on PET substrate. This provides complementary information on photoinduced immobilization of VBT-VBA films that are crucial for developing new classes of environmentally benign materials and new energy-saving methods.     

Synthesis and characterization of photocrosslinked poly(ε‐caprolactone)s showing shape‐memory properties

Poly(ε‐caprolactone) (PCL) with a pendent coumarin group was prepared by solution polycondensation from 7‐(3,5‐dicarboxyphenyl) carbonylmethoxycoumarin dichloride and α, ω‐dihydroxy terminated poly(ε‐caprolactone) with molecular weights of 1250, 3000, and 10,000 g/mol. These photosensitive polymers underwent a rapid reversible photocrosslinking upon exposure to irradiation with alternating wavelengths (>280/254 nm) without a photoinitiator. The thermal and mechanical properties of the photocrosslinked films were examined by means of differential scanning calorimetry and stress–strain measurements. The crosslinked films exhibited elastic properties above the melting temperature of the PCL segment along with significant decrease in the ultimate tensile strength and Young's modulus. Shape‐memory properties such as strain fixity ratio (R_f) and strain recovery ratio (R_r) were determined by means of a cyclic thermomechanical tensile experiments under varying maximum strains (ε_m = 100, 300, and 500%). The crosslinked ICM/PCL‐3000 and ‐10,000 films exhibited the excellent shape‐memory properties in which both R_f and R_r values were 88–100% for tensile strain of 100–500%; after the deformation, the films recovered their permanent shapes instantaneously. In vitro degradation was performed in a phosphate buffer saline (pH 7.2) at 37 °C with or without the presence of Pseudomonas cepacia lipase. The presence of the pendent coumarin group and the crosslinking of the polymers pronouncedly decreased the degradation rate. The crosslinked biodegradable PCL showing a good shape‐memory property is promising as a new material for biomedical applications. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2422–2433, 2009     

Novel UV-sensitive bis-chalcone derivatives: synthesis and photocrosslinking properties in solution and solid PMMA film

A series of chalcone derivatives in which two chalcone groups are attached by alkyldioxy chains were synthesized and characterized by ¹H NMR, UV–Vis, and Fourier transform infrared (FTIR) spectral analysis. Upon the irradiation of 365-nm UV light, the chalcone groups in the molecules underwent [2π + 2π] photodimerization. The photocrosslinking properties were investigated both in solution and in polymethyl methacrylate (PMMA) solid films on quartz plate. It has been found that the photocrosslinking rates of the compounds depended on the different flexibility of the functional groups, which was determined by the length of the spacer chain between the two chalcone moieties. The longest soft chain containing derivatives has a faster photocrosslinking rate both in solution and in solid film. Irradiated by polarized ultraviolet light (PUV), all of the films doped with bis-chalcone derivatives showed an anisotropic absorption property, which may give a promising application as LC-alignment materials.     

Photocrosslinking of elastomer systems. II. Ultraviolet light-induced reactions in an EPDM modified by grafting of benzoin derivatives

Photocrosslinking experiments were performed on elastomeric polymer films (EPDM) previously modified by grafting a photosensitive initiator (benzoin derivative) onto the macromolecular backbone. This modification was performed by a series of Friedel–Crafts reactions and the resulting materials were characterized by infrared (IR) spectroscopy in conjunction with model reactions. After ultraviolet (UV) irradiation the photocrosslinked samples were studied by IR spectroscopy in conjunction with the measurement of physical properties. It apperared that grafting the photosensitizer onto the elastomer considerably enhances the crosslinking reaction even with a limited quantity of grafted benzoin derivative. Moreover, the results obtained suggest that the structure of the resulting networks is strongly dependent on the crosslinking process used; that is, curing the elastomer with peroxide or UV irradiation either of a blend of elastomer and photosensitizer or of the same elastomer modified by grafting the chromophore.     

Photocrosslinking and related properties of intumescent flame‐retardant LLDPE/EVA/IFR blends

The photoinitiated crosslinking of halogen‐free flame retarded linear low density polyethylene/poly(ethylene‐co‐vinyl acetate) blends (LLDPE/EVA) with the intumescent flame retardant (IFR) of phosphorous‐nitrogen compound (NP) in the presence of photoinitiator and crosslinker and their characterization of related properties have been investigated by gel determination, heat extension test, cone calorimeter test (CCT), thermogravimetric analysis (TGA), Fourier transfer infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), mechanical properties measurements, limiting oxygen index (LOI), UL‐94, and water resistance test. The data from the gel content and heat extension rate (HER) show that the LLDPE/EVA/IFR blends filled with NP are readily crosslinked to a gel content of above 75% and the HER values reach about 50% by UV‐irradiation of 5 sec under suitable amount of photoinitiator and crosslinker. The data obtained from the CCT and LOI indicate that photocrosslinking can considerably decrease the heat release rates (HRR) by 10–15%, prolongate the combustion time, and increase two LOI values for the LLDPE/EVA/NP blends UV irradiated for 5 sec. The results from TGA and the dynamic FTIR spectra give the evidence that the photocrosslinked LLDPE/EVA/NP samples show slower thermal degradation rate and higher thermo‐oxidative degradation temperature than the uncrosslinked LLDPE/EVA/NP samples. The morphological structures of charred residues observed by SEM give the positive evidence that the compact charred layers formed from the photocrosslinked LLDPE/EVA/NP samples play an important role in the enhancement of flame retardant and thermal properties. The data from the mechanical tests and water‐resistant measurements show that photocrosslinking can considerably improve the mechanical and water‐resistant properties of LLDPE/EVA/NP samples. Copyright © 2011 John Wiley & Sons, Ltd.