Photodegradation of poly(vinyl alcohol) under UV and pulsed‐laser irradiation in aqueous solution

The photodegradation of poly(vinyl alcohol) (PVA) in aqueous solution was investigated under UV exposure and pulsed‐laser irradiation. The degradation under UV exposure was studied at different pH values and with the addition of potassium chloride and potassium dichromate. The pulsed‐laser degradation of PVA was investigated with a Nd : YAG laser, operating at a wavelength of 266 nm with about 6‐ns pulses. The pulsed‐laser degradation was studied at different polymer concentrations and light intensities. Samples were analyzed by gel permeation chromatography. The degradation rate coefficients were determined by the application of a continuous distribution model. The photodegradation rates under UV exposure were highest at extremes of pH and were greatly enhanced by the addition of potassium chloride and potassium dichromate. The pulsed‐laser degradation of the polymer decreased with increasing polymer concentration, although a threshold light intensity was required to initiate the degradation process. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 958–966, 2006     

Degradation of poly(L‐lactide) films under ultraviolet‐induced photografting and sterilization conditions

The degradation of polymers under ultraviolet (UV) irradiation has been a great concern for biomaterial and agricultural applications. The major objective of this research was to study the effect of UV irradiation on the representative bulk and surface properties of poly (L ‐lactide) (PLA) films. Two UV sources with different spectral outputs and intensities were chosen so that one of them could be used for surface modification and the other could be used for UV sterilization of the PLA films. The results established that the molecular weight of PLA decreased significantly during irradiation from the photografting lamp under atmospheric conditions. Irradiation through a Pyrex container was shown to minimize polymer degradation during UV exposure from the photografting lamp. The PLA films UV‐irradiated under the sterilization lamp for 12 h revealed a similar reduction in the molecular weight and no change in the surface hydrophilicity. However, significantly less photodegradation was observed under the sterilization lamp when the samples were held in a Pyrex container. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

UV Radiation-Induced Degradation of Poly(n-Propylmethylsilane-Co-Isopropylmethylsilane) in Solution

The photodegradation of poly(n-propylmethylsilane-co-isopropylmethylsilane) (NPIPSC) in solution was investigated. The results obtained indicate that a low-pressure Hg lamp photoinduces degradation. Therefore, this light source can be used as an acceptable radiation source for the photobleaching of NPIPSC. Gel permeation chromatography/light scattering (GPC/LS) measurements indicate that two photodegradation stages exist in this process. The first occurs at less than 6 s of exposure, where scission is very rapid. The second occurs between 6 and 12 s of exposure. The latter does not result in a significant loss of molecular weight. A change of slope in the plot of molecular weight as a function of elution volume is observed in the degradation process. This result indicates the presence of at least two species with different conformations. The ratio of scissions to cross-linking related to photodegradation was calculated and revealed that the scission process predominates over cross-linking. The presence of isopropylmethylsilane (IPMS) units in the copolymer appears to induce a decrease in the NPIPSC degradation efficiency. The linear plot of degradation degree as a function of exposure time suggests that there are no weak links in the copolymer. This result can be correlated with the chemical composition of the copolymer, which was nearly the same between 0 and 10 s of UV exposure. After 12 s of exposure, the IPMS was removed from the NPIPSC.     

UV Radiation-Induced Degradation of Poly(n-Propylmethylsilane-Co-Isopropylmethylsilane) in Solution

The photodegradation of poly(n-propylmethylsilane-co-isopropylmethylsilane) (NPIPSC) in solution was investigated. The results obtained indicate that a low-pressure Hg lamp photoinduces degradation. Therefore, this light source can be used as an acceptable radiation source for the photobleaching of NPIPSC. Gel permeation chromatography/light scattering (GPC/LS) measurements indicate that two photodegradation stages exist in this process. The first occurs at less than 6 s of exposure, where scission is very rapid. The second occurs between 6 and 12 s of exposure. The latter does not result in a significant loss of molecular weight. A change of slope in the plot of molecular weight as a function of elution volume is observed in the degradation process. This result indicates the presence of at least two species with different conformations. The ratio of scissions to cross-linking related to photodegradation was calculated and revealed that the scission process predominates over cross-linking. The presence of isopropylmethylsilane (IPMS) units in the copolymer appears to induce a decrease in the NPIPSC degradation efficiency. The linear plot of degradation degree as a function of exposure time suggests that there are no weak links in the copolymer. This result can be correlated with the chemical composition of the copolymer, which was nearly the same between 0 and 10 s of UV exposure. After 12 s of exposure, the IPMS was removed from the NPIPSC.     

Polyurethanes and polyurethane cationomers bearing nitroaromatic groups and their photobehavior

Elastomer-type polyether urethanes and polyether urethane cationomers with nitroaromatic groups were prepared and the effect of UV irradiation on their characteristics was evaluated comparatively with a model compound. The photobehavior by UV irradiation with a high-pressure Hg lamp was studied in solution, following the change of absorption bands characteristic for nitro structures as a function of irradiation time. UV irradiation causes diminution of the intensity of characteristic UV absorption bands and a decrease of molecular weight.     

紫外光辐照对硬质聚氯乙烯结构及性能的影响

采用衰减全反射傅立叶变换红外光谱、紫外光谱、扫描电镜、凝胶渗透色谱、拉伸应力一应变、黄色指数、与水接触角及失重率等测试研究了紫外光(UV)辐照对硬质聚氯乙烯(PVC)结构及性能的影响。结果表明:随着UV辐照时间的延长,平均相对分子质量增大至最大值后,进一步延长辐照时间平均相对分子质量减小,多分散系数由1.771增加到1.919,相对分子质量分布变宽。分子链中引人了多种揣基官能团和多种共扼多烯结构。试样表面经历了粗糙、裂纹形成及其扩散等形态破坏过程,试样黄色指数由40.7%逐渐增加到90%以上,表面颜色逐渐变深,失重率逐渐增大到0.22%,与水接触角先由63.3迅速下降到55,然后下降程度趋于平缓,逐渐下降到平衡值50UV辐照的初期试样的拉伸强度由未经辐照时的55MPa增加到57MPa,进一步延长辐照时间后拉伸强度降低,而断裂伸长率由未辐照时的35.5%迅速降低到11.8%。     

In Vitro Cytological Responses against Laser Photobiomodulation for Periodontal Regeneration

Periodontal disease is a chronic inflammatory disease caused by periodontal bacteria. Recently, periodontal phototherapy, treatment using various types of lasers, has attracted attention. Photobiomodulation, the biological effect of low-power laser irradiation, has been widely studied. Although many types of lasers are applied in periodontal phototherapy, molecular biological effects of laser irradiation on cells in periodontal tissues are unclear. Here, we have summarized the molecular biological effects of diode, Nd:YAG, Er:YAG, Er,Cr:YSGG, and CO₂ lasers irradiation on cells in periodontal tissues. Photobiomodulation by laser irradiation enhanced cell proliferation and calcification in osteoblasts with altering gene expression. Positive effects were observed in fibroblasts on the proliferation, migration, and secretion of chemokines/cytokines. Laser irradiation suppressed gene expression related to inflammation in osteoblasts, fibroblasts, human periodontal ligament cells (hPDLCs), and endothelial cells. Furthermore, recent studies have revealed that laser irradiation affects cell differentiation in hPDLCs and stem cells. Additionally, some studies have also investigated the effects of laser irradiation on endothelial cells, cementoblasts, epithelial cells, osteoclasts, and osteocytes. The appropriate irradiation power was different for each laser apparatus and targeted cells. Thus, through this review, we tried to shed light on basic research that would ultimately lead to clinical application of periodontal phototherapy in the future.     

Effect of Irradiation on Structural Changes of Levan

Levan, as a biocompatible and renewable biopolymer with anticancer properties, is a promising candidate for a wide range of applications in various fields of industry. However, in the literature, there is a lack of information about its behavior under the influence of UV irradiation, which may limit its potential application, including medical science. Therefore, this study describes the effects of irradiation on the structural properties of levan. This type of fructan was subjected to stability tests under radiation conditions using LED and polychromatic lamps. The results showed that the photodegradation of levan irradiated with a polychromatic light occurs faster and more efficiently than the photodegradation of levan irradiated with an LED lamp. Furthermore, AFM analysis showed that the surface became smoother after irradiation, as evidenced by decreasing values of roughness parameters. Moreover, UV irradiation causes the decrease of total surface free energy and both its components in levan; however, more significant changes occur during irradiation of the sample with a polychromatic lamp.     

Influence of desensitizing procedures on adhesion of resin cements to dentin

This study evaluated the effect of two desensitizer agents with different contents and Nd:YAG laser irradiation on the shear bond strength (SBS) of adhesive resin cements to dentin. New treatment options of Nd:YAG laser irradiation and tetracalcium phosphate-containing agent applications were compared with routinely used glutaraldehyde-containing agents. One hundred and twenty human, caries-free premolars were embedded in acrylic resin blocks 2 mm below the cementoenamel junction. Buccal surfaces of the teeth were ground to expose dentin. The specimens were randomly assigned into three different surface treatments (desensitizing agents, Nd:YAG laser) and the control, then into three different adhesive resin cement applications (n = 10). Resin cements (Panavia SA cement (PA), Panavia SA cement with Clearfil Universal Bond (PACU), and Multilink N (MN)) were applied to the conditioned teeth surfaces using Teflon tubes. The specimens were thermocycled (5000 cycles, 5–55 ± 1 °C, dwell time 30 s). The SBS test was performed in all groups. The results were submitted to two-way ANOVA and Tukey HSD tests (p < .05). Further, SEM analysis was performed on the dentin surfaces. SBS values were significantly difference among the surface treatment groups and also among adhesive resin cement groups (p < .05). The specimen cemented with PA showed lower SBS values than PACU- and MN-applied specimens. The highest SBS value was obtained in the Nd:YAG laser group which was cemented with PACU cement. The lowest SBS value was obtained in the control group which was cemented with PA cement. In addition, SEM evaluation revealed that desensitizing agents and Nd:YAG laser occluded dentin tubules.     

Functionality distribution of hydroxyl-terminated polybutadienes using gel permeation chromatography. I. The method and calibration procedure

A method for simultaneously determining the molecular weight averages, molecular weight distribution, hydroxyl equivalent weight, functionality averages, and functionality distribution of hydroxyl-terminated polybutadienes using GPC has been developed. The method is based on the preparation of a UV absorbing derivative of the hydroxyl group and GPC analysis using dual differential refractive index and UV detectors. In order to determine the hydroxyl equivalent weight, quantitative derivatization of the hydroxyl group is required. If the equivalent weight can be determined by an independent method, the derivatization reaction can be less than quantitative if sufficient UV absorptivity can be obtained. The procedure used for calibrating the GPC columns and UV detector are presented. Equations for calculating number- and weight-average functionalities from GPC data were also developed. The number-average functionality is independent of the functionality distribution, but the weight-average functionality is dependent upon the distribution of functionality. The ratio of the weight-average to number-average functionality is indicative of how the functionality is skewed as a function of molecular weight.     

Preparation of narrow-distribution polyvinylpyrrolidone by multi-stage high osmotic pressure chromatography

Separation of polyvinylpyrrolidone, a water-soluble polymer of neutral charge, by high osmotic pressure chromatography (HOPC) is presented. HOPC, suitable for preparative separation of polymer by molecular weight, has been applied to various organic-soluble polymers. We demonstrate here that HOPC can also separate water-soluble polymers and is capable of separating fractions of narrower polydispersity compared to preparative gel permeation chromatography (GPC). Typically fractions with a polydispersity index (PDI) of 1.4–2 were generated from broad-distribution polymers (PDI5) in significant quantities from a single separation. By multi-stage separation, standard-grade fractions were obtained. Single- and multi-pass fractions were characterized by both GPC with narrow-standard calibration and GPC in tandem with a multi-angle laser light scattering detector.     

Spherulite crystallization in bisphenol-A-polycarbonate of varying molecular weight distribution

The spherulitic structure of films of fractionated bisphenol-A-polycarbonate having a range of different average molecular weight and molecular weight distribution has been studied using the polarization microscope. Spherulitic crystallization was induced in specimens by the action of solvent or solvent vapor at room temperature or by isothermal heat treatment at 180°C. These phenomena were all shown to be a function of the average molecular weight and polydispersity of the material. The glassy amorphous and spherulite polymer phases were investigated using a microscopic etching technique and gel permeation chromatography (GPC). Results of this investigation have established that considerable segregation by molecular weight occurs during the crystallization process. Spherulites produced have been shown to exhibit variation in morphologic texture depending on conditions of induction and polydispersity of the polymer. Examples of unusual and previously unreported spherulites have been observed.     

聚乙烯醇降解特性研究

采用凝胶色谱法（GPC）和紫外分光光度法（UV）研究了聚乙烯醇（PVA）微生物降解和化学氧化降解效果。GPC实验表明，PVA经过本实验室筛选的微生物菌株降解后，保留9min左右产生了一个小相对分子质量的峰，其峰面积约占总面积的一半；UV实验也表明了PVA在降解菌株的作用下，其浓度呈下降趋势；采用Fenton试剂（FR）对PVA进行预先氧化处理，然后再进行微生物降解。结果表明FR预处理有利于PVA的快速有效降解。     

Effect of erosive challenge and Nd:YAG laser irradiation on bond strength of adhesive systems to dentin

PurposeTo evaluate the effect of Nd:YAG laser irradiation and erosive challenge on bond strength of two adhesive systems to dentin.MethodsTwenty bovine incisors were cut and grounded to obtain eighty slabs of flat dentin. Specimens were allocated into eight groups, based on: adhesive system—a two-step etch-and-rinse and a two-step self-etch; laser irradiation—Nd:YAG (1 W/10 Hz) or control (no laser irradiation); and erosive challenge after restorative procedure—presence or absence of erosive challenge. Nd:YAG laser groups were submitted to laser irradiation before the restorative procedure. Blocks of composite resin were built up on the bonded surfaces with a Southern Dental Industries device to perform shear bond strength (SBS) test. After, each specimen of erosive challenge, groups were subjected to immersion in Sprite Zero^® (20 ml/2 h/24 °C/under agitation). The SBS test (0.5 mm/min) was performed after 24 h of water storage at 37 °C. Failure mode was evaluated with a stereomicroscope (X400). Data were analyzed with three-way ANOVA and Tukey’s post hoc tests (α=0.05).ResultsThe etch-and-rinse adhesive system presented higher bond strength values than self-etch adhesive. Laser irradiation increased the bond strengths values when erosive challenge was present. The predominant failure mode observed was adhesive.ConclusionsThe irradiation of Nd:YAG laser positively influences the bond strength values when erosive challenges are present. Moreover, the etch-and-rinse adhesive system is a better option to be used in dentin in this clinical condition.     

Exciton quantum confinement in nanocones formed on a surface of CdZnTe solid solution by laser radiation

The investigation of surface morphology using atomic force microscope has shown self-organizing of the nanocones on the surface of CdZnTe crystal after irradiation by strongly absorbed Nd:YAG laser irradiation at an intensity of 12.0 MW/cm². The formation of nanocones is explained by the presence of a thermogradient effect in the semiconductor. The appearance of a new exciton band has been observed after irradiation by the laser which is explained by the exciton quantum confinement effect in nanocones.     

Influence of UV absorber on photodegradation processes of poly(vinyl chloride) with different average degrees of polymerization

The influence of UV absorber (Chimassorb81) on the photodegradation mechanism of different average degrees of polymerization (DP ) of poly (vinyl chloride) (PVC) with UV‐irradiation time was investigated by viscosity‐average molecular weight determination, UV‐vis spectroscopy, FTIR, contact angle measurement, and scanning electron microscopy (SEM). The PVC films with different DP (1000 and 3000), which contained 0.3 or 0.5 phr Chimassorb81, were prepared by solution casting. It was carried out by exposing specimens to xenon‐arc light source with a spectral irradiance of 0.68 W/(m² nm) at 63°C. It is found that the Chimassorb81 is efficient photostabilizer for PVC with different DP . Although the Chimassorb81 delays the photodegradation of PVC, it does not influence the photodegradation mechanisms of PVC with different DP . The main photodegradation reaction for the lower DP of PVC is dehydrochlorination in the initial stage of UV‐irradiation, and then the crosslinking and chain scission reactions occurred after long irradiation. However, the main reaction of the higher DP of PVC is not dehydrochlorination but crosslinking and chain scission in the initial stage of UV‐irradiation. The results of carbonyl index, C? Cl index, contact angle measurement, and SEM also show that the photostability of Chimassorb81 is more effective for the higher DP of PVC, especially in the presence of higher concentration of Chimassorb81. POLYM. ENG. SCI., 47:1480–1490, 2007. © 2007 Society of Plastics Engineers     

Crosslinking of polyvinyl chloride by electron beam irradiation in the presence of ethylene–vinyl acetate copolymer

Electron beam (EB) irradiation of polyvinyl chloride (PVC) was carried out in the presence of three different ethylene–vinyl acetate copolymers (EVA). The mechanical properties of the original and irradiated blends were tested. The gel content measurement, chlorine loss upon electron irradiation, and gel permeation chromatograph (GPC) were used to characterize the effect of EVA on the irradiation behavior of PVC/EVA blends. The content and the chemical structure of EVA in the blends had considerable effects on the mechanical properties and gel content of the blends. The incorporation of EVA into PVC blend can increase the gel content and reduce chlorine loss of the blends. The GPC analysis of the soluble part in the irradiated PVC samples showed that the addition of EVA into the PVC blend lowered the polydispersity of molecular weight of PVC. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1571–1575, 2004     

Coumarin end-capped polystyrene by ATRP and photodimerization reaction

Summary New coumarin-end-capped polystyrenes have been prepared using functional initiators by atom-transfer radical polymerization (ATRP). In the first case, the photosensitive initiator, 7-chloroacetoxy-4-methyl-coumarin, was prepared. Then the styrene was initiated by 7-chloroacetoxy-4-methyl-coumarin in the presence of a CuBr/bipyridine. The functionalization of the polymers was nearly quantitative, the molecular weight and structure were well controlled by adjusting the monomer/initiator molar ratio, polymerization temperature and time. At high monomer conversion, the polydispersity index of polymer was about 1.40. These coumarin-end-capped polystyrenes underwent [2+2] cycloaddition reaction under UV irradiation in film state. After irradiation the molecular weight and polydispersity index of polystyrene both increased, but the maximum molecular weight was less than the double of original one. It means that the photodimerization reaction was not complete throughly in film state for the diffusion of coumarin groups was blocked by polymeric chain entanglement.     

Laser drilling of liquid crystal polymer composite

This investigation studies the laser machining behavior of a glass fiber reinforced liquid crystalline polymer (LCP) composite by using two different types of lasers, namely; pulsed Nd:YAG and excimer. A theoretical model is established to predict the maximum depth-of-drill of the YAG laser machined holes. Moreover, the effects of the YAG laser output parameters on the geometry and the quality of the machined surface of the hole are discussed. The laser machining characteristics of the composite are compared for the two different types of lasers in terms of laser absorptivity. The results show that the glass fiber reinforced LCP has a higher absorption towards UV radiation than infrared light. Scanning electron microscopy examinations were also conducted to assess the surface quality of the machined holes. In YAG laser drilling, a high pulse energy or excessive pulses caused severe carbonization of the surface, and many cracks formed. On the other hand, excimer laser did not cause any significant carbonization of the surface; however, a large number of voids were found in the process affected zone.     

Synthesis of telechelic C60 end‐capped polymers under microwave irradiation

Bromo‐Double‐Terminated polystyrene (Br‐PSt‐Br) and poly(methyl methacrylate) (Br‐PMMA‐Br) with predesigned molecular weight and narrow polydispersity were prepared by atom transfer radical polymerization (ATRP) using the initiating system aa′‐dibromo‐p‐xylene(DBX) / CuBr/2,2′‐bipyridine(bipy). The precursor bromo‐terminated polymers were subsequently functionalized with fullerene C₆₀ using CuBr/bipy as the catalyst system under microwave irradiation (MI). The telechelic C₆₀ end‐capped products were characterized by gel permeation chromatography (GPC), UV‐vis, FT‐IR, TGA, DSC, ¹H NMR, and ¹³C NMR. The results showed that microwave irradiation could significantly increase the rate of fullerenation reaction, and the physical properties and structure of the C₆₀ end‐capped polymers are not modified by the use of the microwave. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 828–834, 2006