Electron-beam treatment of poly(lactic acid) to control degradation profiles

Bioresorbable polymers such as polylactide (PLA) and polylactide-co-glycolide (PLGA) have been used successfully as biomaterials in a wide range of medical applications. However, their slow degradation rates and propensity to lose strength before mass have caused problems. A central challenge for the development of these materials is the assurance of consistent and predictable in vivo degradation. Previous work has illustrated the potential to influence polymer degradation using electron beam (e-beam) radiation. The work addressed in this paper investigates further the utilisation of e-beam radiation in order to achieve a more surface specific effect. Variation of e-beam energy was studied as a means to control the effective penetrative depth in poly-l-lactide (PLLA). PLLA samples were exposed to e-beam radiation at individual energies of 0.5 MeV, 0.75 MeV and 1.5 MeV. The near-surface region of the PLLA samples was shown to be affected by e-beam irradiation with induced changes in molecular weight, morphology, flexural strength and degradation profile. Moreover, the depth to which the physical properties of the polymer were affected is dependent on the beam energy used. Computer modelling of the transmission of each e-beam energy level used corresponded well with these findings.     

Radiation effects on polymer materials in radiation sterilization of medical supplies

Radiation degradations of polypropylene were studied by measuring chemiluminescence of the irradiated samples. The chemiluminescence emitted by recombination of peroxy radicals were found to increase with increasing dose, reflecting degree of oxidation of polymers. The degradation of PP may be attributed mainly to the oxidation, since the degradation of PP irradiated in air were markedly larger than that in vacuo. It was found that degree of oxidation of EB-irradiated PP was only one-half that for γ-ray-irradiated one. And also the degree of oxidation was found to be very high at the surface area of the film where oxygen can diffuse during irradiation and decrease sharply with increasing depth from the surface.The degradation during storage after irradiation were estimated by the decay curves of chemiluminescence. The degradation during storage occurred at earlier stage up to three months, while in later stage, it hardly proceeded. The degradation of EB-irradiated samples during storage was much smaller than that found in gamma rays irradiation. Irradiation of 2.5 Mrad with EB caused degradation only during storage. It was suggested that the degradation during storage depends on the degree of radiation oxidation. Both degradations, during irradiation and during storage after irradiation, caused by EB irradiation were also found to be smaller than those by γ-rays. The relationship between radiation degradation and degree of crystallinity of polypropylene was also discussed in this paper.     

Steady-state radiolysis and product analysis of aqueous diphenyloxide in the presence of air and N2O

The radiation-induced decomposition and product analysis of diphenyloxide (DPO; diphenylether), selected as a representative of the toxic polyaromatic, volatile, hydrocarbons produced by combustion of coal, was investigated in aqueous solution. In the presence of air an initial degradation yield, G_i(-DPO)0=4.6 was obtained. Phenol (G_i=1.3) appeared to be the major decomposition product in addition to a number of hydroxy-compounds as well as a mixture of aldehydes and carboxylic acids. DPO as well as the resulting products can be decomposed at an absorbed radiation dose of about 5 kGy.By performing analogous studies in solution saturated with N₂O an initial yield of G_i(-DPO)=5.0 and similar radiolytic products were found. Again phenol (G_i=1.5) is observed as the main product. In this case a much lower radiation dose (∼2.5 kGy) is found to be sufficient for the decomposition of DPO and its radiolytic products. Most likely reaction mechanisms are presented for explanation of the observed products.     

γ-Radiolysis of aqueous 2-chloroanisole

The radiation-induced degradation of 2-chloroanisole (2-ClAn) is investigated under various experimental conditions in neutral aqueous media as a function of absorbed radiation dose. The initial yields (G_i-values) of substrate degradation as well as those of the resulting major products were determined by HPLC analysis. Probable reaction mechanisms are suggested.     

Stability of a salicylate-based poly(anhydride-ester) to electron beam and gamma radiation

The effect of electron beam and gamma radiation on the physicochemical properties of a salicylate-based poly(anhydride-ester) was studied by exposing polymers to 0 (control), 25 and 50 kGy. After radiation exposure, salicylic acid release in vitro was monitored to assess any changes in drug release profiles. Molecular weight, glass transition temperature and decomposition temperature were evaluated for polymer chain scission and/or crosslinking as well as changes in thermal properties. Proton nuclear magnetic resonance and infrared spectroscopies were also used to determine polymer degradation and/or chain scission. In vitro cell studies were performed to identify cytocompatibility following radiation exposure. These studies demonstrate that the physicochemical properties of the polymer are not substantially affected by exposure to electron beam and gamma radiation.     

Effect of gamma radiation on dilute aqueous solutions and thin films of N-succinyl chitosan

N-Succinyl chitosan (N-SC) products with various degrees of substitution were synthesized by a direct reaction between chitosan and succinic anhydride. The susceptibility of the as-synthesized polymers to degradation upon their exposure to γ-ray radiation was investigated. The results were compared with the as-received chitosan. The size exclusion chromatographic results showed that chitosan and N-SC products in their dilute aqueous solution state were more subservient to degradation by γ-ray radiation than in their solid film state, despite the much less exposure to the radiation (i.e., 5-30 kGy for the solutions versus 20-100 kGy for the films). Increasing the radiation dose resulted in the rather monotonous decrease in the molecular weights of the polymers. Structural analyses of the irradiated polymers by Fourier-transformed infrared spectroscopy (FT-IR) and UV-visible spectrophotometry indicated the increase in the amount of carbonyl groups with the radiation dose. The formation of the carbonyl groups suggested that the radiolysis of chitosan and N-SC products occurred at the glycosidic linkages. In addition, FT-IR, elemental analysis and proton nuclear magnetic resonance spectroscopy (¹H NMR) results suggested that γ-ray radiation affected both the N-acetyl and N-substituted groups on the polymer chains.     

Monitoring the influence of different weathering conditions on polyethylene pipes by IR-microscopy

Plastic pipes are often exposed to a range of environmental conditions which may lead to their degradation. The most important influence factors are UV radiation, humidity and temperature. These can cause leaching of long-term and light stabilisers and finally oxidation of the polymer. In this study we demonstrate how the elemental steps of the photooxidative degradation of polyethylene pipes can be monitored by IR-microscopy. In detail the influence of UV radiation leads to a depletion of the phenolic long-term stabiliser, Irganox 1010. Calibration of the spectroscopic data enables IR-microscopy to be carried out in a quantitative manner and the rate constants for the stabiliser loss to be calculated for the first time. The results obtained from IR-microscopy are well in agreement with those obtained by mechanical sample preparation and measurement of the oxidative induction time (OIT) as well as extraction coupled with chromatographic analysis (HPLC). A mechanism based on Norrish type cleavage is proposed. Also the formation of trans-vinylidene groups as unsaturated degradation products of the PE can be observed. In summary it can be shown that IR-microscopy is highly superior to the conventional approach of mechanical sample preparation with regard to spatial resolution and offers the advantage of being less labour intensive.     

Radiation-induced crosslinking of polyacrylonitrile fibers and the subsequent regulative effect on the preoxidation process

To investigate the radiation effect on polyacrylonitrile (PAN) fibers as well as on the preoxidation process, PAN fibers were irradiated by γ-rays at room temperature at 50–500 kGy in vacuum and then were thermally oxidized in air. Gel fraction determination indicated that γ irradiation led to the predominant crosslinking of PAN fibers, with G values (the number of event per 100 eV absorbed) of G(X)=0.28 and G(S)=0.16 for chain crosslinking and scission, respectively. It was found that irradiation caused a slight change in the crystal structure and tensile strength at low dose. Radiation led to a reduction of the onset temperature of cyclization reaction and moderated the exothermic behavior. The density of the PAN fibers after thermal oxidation was used to evaluate the preoxidation extent. It was proven that radiation could significantly accelerate the preoxidation process and consequently shortened the preoxidation time. Radiation crosslinking may have potential application in the production of PAN-based carbon fibers.     

Properties and hydrolysis of PLGA and PLLA cross-linked with electron beam radiation

Radiation has been used as a processing tool to modify the properties of polymers. The aim of this study is to understand how electron beam radiation, together with pentaerythritol tetraacrylate (PTTA) as a tetra-functional monomer, can alter the properties (i.e. thermal and mechanical) and hydrolysis rates of PLGA and PLLA. The effects of radiation dose and PFM concentration on the physical properties of the polymers were investigated. The results showed that upon irradiation PLGA and PLLA cross-linked, and an increased in gel content was observed. Glass transition temperature (T_g) and mechanical properties of the polymers also increased. Cross-linked PLGA and PLLA samples were found to retard hydrolytic degradation. The mechanical properties of these polymers were also unaffected by hydrolysis. In summary, PLGA and PLLA cross-linked with PTTA were found to have enhanced mechanical properties and were able to retard hydrolytic degradation.     

Responses in the morphology,physiology and biochemistry of Taxus chinensis var. mairei grown under supplementary UV-B radiation

The effects of supplemental UV-B radiation on Taxus chinensis var. mairei were studied. Leaf traits, gas exchange parameters and the concentrations of photosynthetic pigments, cellular defense system products, secondary metabolites and ultrastructure were determined. UV-B radiation significantly decreased leaf area (p < 0.05). Leaf number, secondary branch number, leaf weight per plant and leaf moisture all increased dramatically (p < 0.05). Neither the leaf weight nor the specific leaf weight (SLW) exhibited significant differences between ambient and enhanced UV-B radiation. Gas exchange parameters were all dramatically reduced by enhanced UV-B radiation (p < 0.05). The contents of chlorophyll and the chlorophyll a/b ratio were not distinctly affected by UV-B radiation, while carotenoids content significantly decreased (p < 0.05). Supplemental UV-B treatment induced significant flavonoid accumulation (p < 0.05), which was able to protect plant from radiation damage. Meanwhile, the appendage content, abaxial stomatal density, papilla density and particulate matter content in substomatic chambers increased noticeably by supplemental UV-B radiation, whereas the aperture size of single stomata was diminished. The number and area of plastoglobuli were apparently reduced by UV-B radiation, but stroma and grana lamellae were not destroyed. Our results demonstrated that T. chinensis var. mairei can activate several defense mechanisms against oxidative stress injury caused by supplemental UV-B radiation.     

Studies of high linear energy transfer dosimetry by 10B(n,α)7Li reactions in aqueous and organic solvents

Advanced chemical treatment processes such as aqueous separation techniques can be used to separate the reusable materials from used nuclear fuel, reducing the radiotoxicity and storage time of the remaining waste. The degradation of chemicals in these processes has been studied utilizing gamma radiation. However, radiolytic degradation by internal alpha emitters has not been as widely investigated due to the difficulty of finding appropriate internal sources. This work presents results using a method to produce alpha particles in situ in aqueous and organic solvents representative of liquid–liquid extraction systems. The method is based on the widely studied ¹⁰B(n,α)⁷Li reaction which has previously been studied in aqueous solutions. Neutrons were supplied from the UCI TRIGA^® nuclear reactor. Comparisons were also made to gamma radiation from a ¹³⁷Cs source. We report that the method is useful for inducing high linear energy transfer (LET) doses in aqueous and organic solutions. We used the classic iron sulfate-based Fricke dosimeter for dosimetry in aqueous solutions and methyl red (2-[(4-dimethylaminophenyl)diazenyl]benzoic acid) dissolved in n-dodecane for organic solvents. High LET doses in both aqueous and organic solvents are well described and a simple linear relationship was found based on the neutron flux and total boron concentration. We have established, using spectroscopic determination, that methyl red degrades in a linear fashion with absorbed dose up to 80 kGy and G-values for the methyl red degradation in n-dodecane were found to be 4.66 × 10^?4 μmol/J for external ¹³⁷Cs gamma radiation and 3.0 × 10^?5 μmol/J for ¹⁰B(n,α)⁷Li induced high LET radiation.     

IR-change and yellowing of polyurethane as a result of UV irradiation

In this investigation IR-change and yellowing of polyurethane as a result of UV radiation were studied. In the presence of UV radiation (200 h, λ > 300 nm), the synthesized aromatic polyurethane undergoes photodegradation with gradual change of its colour. The photochemical degradation of the polyurethane is associated with the scission of the urethane group and photooxidation of the central CH₂ group between the aromatic rings. These reactions are combined with the yellowing of the polyurethane surface. Analysis of the colour changes in PU surface during photodegradation was carried out by measuring CIEL^∗a^∗b^∗ colour components (L^∗,a^∗,b^∗ and ΔE_a,b^∗). FT-IR spectroscopy was used to study the chemical changes caused by UV irradiation. The colour difference of yellowing Δ_a,b^∗ exhibits a systematic tendency to higher values with increasing irradiation time. Overall, ΔE_a,b^∗ colour change correlates well with photodegradation of polyurethane by relative increase of the concentration of carbonyl group. Our results are in agreement with the quinone (yellow colour) formation as the chromophoric reaction product of polyurethane degradation.     

Degradation behavior of poly (l-lactide-co-glycolide) films through gamma-ray irradiation

Gamma-ray irradiation is a very useful tool to improve the physicochemical properties of various biodegradable polymers without the use of a heating and crosslinking agent. The purpose of this study was to investigate the degradation behavior of poly (l-lactide-co-glycolide) (PLGA) depending on the applied gamma-ray irradiation doses. PLGA films prepared through a solvent casting method were irradiated with gamma radiation at various irradiation doses. The irradiation was performed using 60Co gamma-ray doses of 25–500 kGy at a dose rate of 10 kGy/h.The degradation of irradiated films was observed through the main chain scission. Exposure to gamma radiation dropped the average molecular weight (M_n and M_w), and weakened the mechanical strength. Thermograms of irradiated film show various changes of thermal properties in accordance with gamma-ray irradiation doses. Gamma-ray irradiation changes the morphology of the surface, and improves the wettability. In conclusion, gamma-ray irradiation will be a useful tool to control the rate of hydrolytic degradation of these PLGA films.     

Mechanism of electron excitation energy degradation in solutions

The inductive-resonant mechanism of electronic energy degradation is proposed and proved for rare earth ions, transition metal ions and simple molecules (NO^?₂) in solutions. The interaction of two oscillators is considered, that is the vibronic interaction corresponding to the radiation spectrum of an excited ion or molecular and the vibrational spectrum corresponding to the excitation of high frequency vibrations of the solvent. The calculation of the energy degradation rare constant (k_degr.) by Förster's formula is shown to give k_degr. values of the same order as the experiment. Such a treatment can quantitatively explain all experimental regularities of the degradation process, for instance, the dependence of k_degr. on the distance from the electronic excitation centre to the nearest high frequency vibration gravity centre. It is shown that the suggested mechanism corretly explains the deuteration effect, the dependence of k_degr. on ΔE (energy gap) and the variation of this dependence for differenct classes of compounds. The possibility of proving the validity of the suggested model for the case of complex organic molecules is discussed.     

Controlling the radiation degradation of carboxymethylcellulose solution

In this study, the effects of an irradiation on the viscosity of the carboxymethylcellulose (CMC) solution were investigated, and the methods to control the degradation of the CMC caused by an irradiation were developed. The viscosity of the CMC solution was decreased with an increase in the irradiation dose, but the extent of the degradation by an irradiation was found to decrease with an increase in the CMC concentration in the solution. The dependency of the irradiation sources showed that an electron beam radiation had degraded the CMC less severely than a gamma ray radiation. An addition of vitamin C as a radical scavenger to the solution was shown to be effective in preventing the decrease of the viscosity of the solution. Also, in the case of an irradiation at −70 °C, the decrease of its viscosity was efficiently inhibited. The degradation of CMC in the solution was confirmed by the molecular weight distribution.     

Validated Stability-Indicating GC-MS Method for Characterization of Forced Degradation Products of Trans-Caffeic Acid and Trans-Ferulic Acid

When dealing with simple phenols such as caffeic acid (CA) and ferulic acid (FA), found in a variety of plants, it is very important to have control over the most important factors that accelerate their degradation reactions. This is the first report in which the stabilities of these two compounds have been systematically tested by exposure to various different factors. Forced degradation studies were performed on pure standards (trans-CA and trans-FA), dissolved in different solvents and exposed to different oxidative, photolytic and thermal stress conditions. Additionally, a rapid, sensitive, and selective stability-indicating gas chromatographic-mass spectrometric method was developed and validated for determination of trans-CA and trans-FA in the presence of their degradation products. Cis-CA and cis-FA were confirmed as the only degradation products in all the experiments performed. All the compounds were perfectly separated by gas chromatography (GC) and identified using mass spectrometry (MS), a method that additionally elucidated their structures. In general, more protic solvents, higher temperatures, UV radiation and longer storage times led to more significant degradation (isomerization) of both trans-isomers. The most progressive isomerization of both compounds (up to 43%) was observed when the polar solutions were exposed to daylight at room temperature for 1 month. The method was validated for linearity, precision as repeatability, limit of detection (LOD) and limit of quantitation (LOQ). The method was confirmed as linear over tested concentration ranges from 1−100 mg L⁻¹ (r²s were above 0.999). The LOD and LOQ for trans-FA were 0.15 mg L⁻¹ and 0.50 mg L⁻¹, respectively. The LOD and LOQ for trans-CA were 0.23 mg L⁻¹ and 0.77 mg L⁻¹, respectively.     

Preparation of low-molecular-weight carboxymethyl chitosan and their superoxide anion scavenging activity

Low-molecular-weight carboxymethyl chitosans (CMCTSs) were prepared by oxidative degradation method involving hydrogen peroxide (H₂O₂) without or with microwave radiation. Viscosity determination and end group analysis were applied to measure molecular weights of CMCTSs. Effects of concentration of H₂O₂ and degradation time on molecular weights of CMCTSs were studied. The degradation process of CMCTSs will be accelerated with the aid of microwave radiation and degradation time may be reduced greatly. The superoxide anion scavenging activity of CMCTSs was evaluated by application of flow injection chemiluminescence technology. The 50% inhibition concentrations (IC₅₀s) of CMCTSs A, B, and C (1130, 2430 and 4350 Da) were 10.36, 17.57, and 23.38 mg/mL, respectively. The above results showed that CMCTSs with lower molecular weight had better superoxide anion scavenging activity.     

Microwave-assisted photo-Fenton decomposition of chlorfenvinphos and cypermethrin in residual water

The coupling of microwave radiation with photo-Fenton reaction for wastewater treatment was evaluated. This strategy was performed in a focused microwave digestion oven with open vessels. Ultraviolet radiation was obtained from microwave lamps (MWL) activated by microwave radiation. The operational conditions were established considering the extent of degradation of chlorfenviphos and cypermethrin used for bovine ticks (Boophilus michoplus) control. The results were based on residual carbon content, always comparing the procedures in the presence or absence of UV radiation. The combination of MWL and Fenton reaction degraded pesticide residues efficiently (>98%) in about 4 min.     

Properties of crosslinked polylactides (PLLA & PDLA) by radiation and its biodegradability

Crosslinked materials derived from poly(lactide) (PLA) have been produced by radiation modification in the presence of a suitable crosslinker (triallyl isocyanurate) (TAIC). The crosslinking structure introduced in PLA films has not only much improved the heat stability but also their mechanical properties. The properties of crosslinked samples are governed by crosslinking density and these improvement seemed to increase with radiation dose. This implied that the three dimensional networks have been introduced in material by radiation and the crosslinking density depended on the structure and length of PLA chains. Biodegradability of PLA was also determined by an enzymatic degradation test and burying in compost at 55 °C. Differing with PLLA, PDLA was insignificantly degraded by proteinase K. The degradation rate of PLA in compost was postponed with the introduction of crosslinks.