Chain Scission Yield Research Articles

Radiation induced degradation of xanthan gum in aqueous solution

In our previous study, we have investigated the effect of gamma rays on xanthan gum in the solid state and it was determined that dose rate was an important factor effecting the radiation degradation of xanthan gum. In the present study, in order to provide a better understanding of how ionizing radiation effect xanthan gum, we have investigated the effects of ionizing radiation on aqueous solutions of xanthan at various concentrations (0.5–4%). Xanthan solutions were irradiated with gamma rays in air, at ambient temperature, at different dose rates (0.1–3.3–7.0kGy/h) and doses (2.5–50kGy). Change in their molecular weights was followed by size exclusion chromatography (SEC). Chain scission yield (G(S)), and degradation rate constants (k) were calculated. It was determined that, solution concentration was a factor effecting the degradation chemical yield and degradation rate of xanthan gum. Chain scission reactions were more effective for lower solution concentrations.

Photostabilizing Efficiency of PVC in the Presence of Schiff Bases as Photostabilizers.

The photostabilization of polyvinyl chloride (PVC) films by Schiff bases was investigated. Polyvinyl chloride films containing 0.5 wt % Schiff bases were produced using the same casting method as that used for additive-free PVC films from tetrahydrofuran (THF) solvent. The photostabilization activities of these compounds were determined by monitoring the carbonyl, polyene and hydroxyl indices with irradiation time. The changes in viscosity average molecular weight of PVC with irradiation time were also monitored using THF as a solvent. The quantum yield of chain scission (Φcs) for the studied complexes in PVC was estimated to range between 4.72 and 8.99 × 10−8. According to the experimental results, several mechanisms were suggested, depending on the structure of the additive. Ultra violet (UV) absorption, peroxide decomposition and radical scavenging were suggested as the photostabilizing mechanisms.

Radiation induced degradation of xanthan gum in the solid state

In this study, the effect of ionizing radiation on xanthan gum was investigated. Xanthan samples were irradiated with gamma rays in air at ambient temperature in the solid state at different dose rates and doses. Change in their molecular weights was followed by size exclusion chromatography (SEC). Chain scission yield (G(S)), and degradation rate constants (k) were calculated. The calculated G(S) values are 0.0151±0.0015, 0.0144±0.0020, 0.0098±0.0010µmol/J and k values are 1.4×10−8±1.4×10−9, 1.3×10−8±2.0×10−9, 8.7×10−9±1.0×10−9Gy−1 for 0.1, 3.3 and 7.0kGy/h dose rates, respectively. It was observed that the dose rate was an important factor controlling the G(S) and degradation rate of xanthan gum.Considering its use in food industry, the effect of irradiation on rheological properties of xanthan gum solutions was also investigated and flow model parameters were determined for all dose rates and doses. Rheological analysis showed that xanthan solution showed non-Newtonian shear thinning behaviour and ionizing radiation does not change the non-Newtonian and shear thinning flow behaviour of xanthan gum solutions in concentration ranges of this work. It was determined that, Power Law model well described the flow behaviour of unirradiated and irradiated xanthan solutions.

Synthesis of New Thiophene Derivatives and Their Use as Photostabilizers for Rigid Poly(vinyl chloride)

Five new thiophenes, namely,N-[(3-bromo-2-methylthiophen-5-yl)methylene]-4-methoxyaniline (4a),N-[(3-bromo-2-methylthiophen-5-yl)methylene]-3,4-dimethoxyaniline (4b),N-[(3-bromo-2-methylthiophen-5-yl)methylene]-3,4-dimethylaniline (4c), 3-[(3-bromo-2-methylthiophen-5-yl)methyleneamino]-2-methylquinazolin-4(3H)-one (4d), and 3-[(3-bromo-2-methylthiophen-5-yl)methyleneamino]-2-isopropylquinazolin-4(3H)-one (4e), have been synthesized. All of these materials brought about a reduction in the level of photodegradation of poly(vinyl chloride) (PVC) films containing the synthesized thiophenes (0.5%; by weight). The results obtained showed that the extent of photostabilization of PVC in the presence of an additive was in the order4e > 4d > 4b > 4a > 4c. For the most favorable additive (4e), the rate of appearance of infrared absorption bands of degradation products was reduced by around two-thirds, while the quantum yield of chain scission was calculated to be reduced by a factor of more than one thousand. It is suggested that the additives may help stabilize PVC by direct absorption of UV radiation and dissipation of the energy as heat or that electrostatic attraction between the additives and PVC may assist transfer of energy from excited state PVC to the additive, from where it can be dissipated.

The effect of environmental humidity on radiation-induced degradation of carrageenans

Better understanding of the chemistry of radiation-induced degradation is becoming of increasing importance on account of the utilization of polymeric materials in a variety of radiation environments as well as beneficial uses of degraded polymers. In this report the importance of environmental humidity on the degrading effect of radiation has been considered from the point of view of controlling the molecular weights of kappa- and iota-carrageenans. These two polysaccharides were irradiated in solid form under strictly controlled environmental humidity conditions by incubating and later irradiating the samples over saturated aqueous salt solutions of NaCl, NaNO3 and MgCl2. The degradation was followed in detail by a careful gel permeation chromatographic analysis of their respective molecular weights before and after irradiation. The chain scission yield values G(S) were found to decrease with the water adsorbed from environment at every absorbed dose in the range of 5–100kGy. On the other hand at very high water uptakes the yield of chain scission again increases especially at low doses. The decrease in degradation yield was attributed to the plastifying effect of water trapped in between the polymer chains facilitating the macroradical recombinations thus reducing the extent of chain scission. This study showed that although carrageenans were irradiated in solid form, the difference in their water uptake from changing environmental humidity has a profound effect in controlling their molecular weights by irradiation with ionizing radiation.

“Green” molecular weight degradation of chitosan using microwave irradiation

Chitosan degradation in 0.1 M acetic acid solution by microwave radiation alone (without any additives and initiators) is reported. At the condition of reduced thermal effects of microwave heating degradation manifested most of typical features associated with mechanical (and or mechanochemical) degradations induced by shear forces, in this particular case oscillating molecules. At the absorbed energy dose of 9 MJ kg−1 chitosan in 1% w/v solution degrades within 20 min to the weight average molecular weight of 30 kDa.Comparative evaluation of degradation yield of chain scission (Gs) with other high energy radiations such as ultraviolet- and gamma radiation showed microwave to be the least efficient, but considering the environmental impact and low running costs it presents a viable alternative to obtain targeted molecular weights of charged polysaccharides.

Mechanical degradation of elastomers in the presence of silica and inhibitors using a new design of mechano reactor

A newly designed mechanochemical reactor has been home built and used to investigate mechanical degradation of SBR rubber and SBR/SiO2 blends under N2 and O2 atmosphere and in the presence of aromatic amines (DTPD) and tetramethyl piperidine (HALS) radical inhibitors. Continuous on line determination of torque, temperature and O2 uptake have been made together with solution viscosity and mechano-radical yields at definite time intervals. The fraction of mechanical energy resulting in chemical effects is determined to be less than 0.01%. Chain scission and crosslinking are found to be major mechanochemical reactions with crosslinking acquiring importance with increasing the processing time. According to the results of COGEF type DFT UB3LYP 6-311++G (d,p) relaxed scan computations, the favoured chain scissions sites are predicted to be the ones leading to allylic (prominent) and benzylic radicals. The kinetics of mechano-radicals formation is coherent with torque and viscosity decay kinetics being expressed by the equation [R•](mol/kg)=−0.0775×10–2t2+0.6755×10–2t.The mechano-radical yield is greater than expected from the chain scission yield; furthermore, oxygen and radical inhibitors have minor effects on torque and viscosity decay kinetics in the first part of the process where chain scissions are dominant, whilst they do inhibit crosslinking. It is inferred from these results that a significant part of permanent chain scissions takes place via caged-radical disproportionation and that only scavengeable mechano-radicals escaping cage reactions are responsible for the initiation of crosslinking.From free radical chemistry analysis supported by DFT calculations it is inferred that H abstraction and 1-3 double bond addition by chain scission allyl radicals can be major paths for crosslinking initiation, the addition mechanism acquiring importance with increasing reaction time. The oxygen uptake is approximately linear with process time and consequently does not fit the formation and decay kinetics of scavengeable mechano-radicals; it is suggested that the contribution by other mechanisms involving mechanical excitation coupled with thermal activation may be important. The addition of SiO2 filler causes an increase of total absorbed mechanical energy and a decrease of the equilibrium molecular weight limit (enhanced chain scissions). In contrast, the scavengeable mechano-radicals yield is found to be less than 50% of that determined in the absence of SiO2. These observations are related to the mechanical degradation of bound rubber in conditions of enhanced cage effect and build up of frictional energy by silica-rubber Van der Waals interaction.

Effect of molecular weight on radiation chemical degradation yield of chain scission of γ-irradiated chitosan in solid state and in aqueous solution

Chitosan A1, A2 and A3 with molecular weight of 471, 207 and 100kDa respectively, produced from squid pen chitin was degraded by gamma rays in the solid state and in aqueous solution with various doses in air at ambient temperature. Effect of molecular weight on radiation chemical degradation yield of chain scission and degradation rate constants of γ-irradiated chitosan in solid state and in aqueous solution was investigated. The radiation chemical degradation yield G(s) and degradation rate values were calculated. The molecular weight changes were monitored by capillary viscometry method and the chemical structure changes were followed by UV analysis. The results showed that, the degradation of chitosan was faster in solution, than in solid state. The values of G(s) in solid state and in aqueous solution were respectively 1.1×10−8mol/J and 0.074×10−7mol/J for A1, 4.42×10−8mol/J and 0.28×10−7mol/J for A2 and 6.08×10−8mol/J and 0.38×10−7mol/J for A3. Degradation rate constants values ranged from 0.41×10−5 to 2.1×10−5kGy−1 in solid state, whereas in solution they ranged from 13×10−5 to 68×10−5kGy−1. The chitosan A3 was more sensitive to radiolysis than A1 and A2. The chain scission yield, G(s) and degradation rate constants seems to be greatly influenced by the initial molecular weight of the chitosan. Structural changes in irradiated chitosan are revealed by the apparition of absorption peaks at 261 and 295nm, which could be attributed to the formation of carbonyl groups. In both conditions the peak intensity was higher in chitosan A3 than in A1 and A2, the oxidative products decreased with increasing molecular weight of chitosan.

Effect of G/M ratio on the radiation-induced degradation of sodium alginate

Radiation-induced degradation of sodium alginate (NaAlg) having different G/M ratios was investigated. NaAlg samples were irradiated with gamma rays in air at ambient temperature in the solid state at low dose rate. Change in their molecular weights was followed by size exclusion chromatography (SEC). Changes in their rheological properties and viscosity values as a function of temperature, shear rate and irradiation dose were also determined. Chain scission yields, G( S), and degradation rates were calculated. It was observed that G/M ratio was an important factor controlling the G( S) and degradation rate of sodium alginate.

Radiation-induced degradation of galactomannan polysaccharides

In this study, guar gum, tara gum and locust bean gum were irradiated in a gamma cell in the solid state. The change in their molecular weights were determined by size exclusion chromatography analysis and the change in their viscosity values with change of temperature and irradiation dose were determined. Chain scission yield, G( s), and degradation rate values were calculated. The calculated G( s) values is 1.09 ± 0.16, 1.07 ± 0.06, 0.85 ± 0.10 for GG, TG and LBG, respectively. The effect of mannose–galactose ratio and initial molecular weight of these gums on the degradation behavior were discussed.

Chain scission of hyaluronan by carbonate and dichloride radical anions: Potential reactive oxidative species in inflammation?

The reactions of the carbonate and dichloride radical anions, CO 3 − and Cl 2 − , with the extracellular matrix glycosaminoglycan hyaluronan (HA) have been studied using the kinetic technique of pulse radiolysis and also by steady-state irradiation combined with gel permeation chromatography/multiangle laser light scattering(gpc/MALLS) to measure the rates of reaction with HA and the yield of HA chain scission, respectively. For comparison, the same measurements were made for the reactions of the free radicals OH, Br 2 − , and N 3 . The carbonate and dichloride radical anions were found to react relatively quickly with HA (7.0 × 10 5 and 6.9 × 10 6 dm 3 mol −1 s −1, respectively) although they are much less reactive than the hydroxyl radical, OH. Significant yields (20 and 38%, respectively) of chain scission of HA by these radical anions were also determined from the gpc/MALLS experiments, providing some support for their potential participation in the depolymerization of HA in vivo. These results are compared with data obtained for the other free radicals (hydroxyl, azide radicals, and dibromide radical anions) investigated in this study in order to gain an insight into their mechanism of reaction with HA. Earlier chain scission yields of HA by hydroxyl radicals determined by the authors have also been revised using the gpc/MALLS technique employed in the current study. The yields of 52% (absence of air) and 44% (in air) are much lower than the previous values. In the current study, the effect of oxygen on the yields of HA chain breaks is discussed in terms of the reactivity of HA peroxyl radicals in the presence of superoxide radical anions. The relevance of the results of this study to mechanisms of inflammation is discussed.

Degradation of chitosan and starch by 360-kHz ultrasound

Effects of 360-kHz ultrasound on aqueous solutions of chitosan and starch were studied. This treatment is an efficient procedure for reduction of molecular weight of both polysaccharides. It has been demonstrated that at the applied ultrasound frequency degradation is caused both by OH radicals and mechanochemical effects. In Ar-saturated 2×10 −2 mol dm −3 chitosan solutions, pH 3, at an ultrasound dose rate of 170 W kg −1, the average sonochemical chain scission yield in the sonication time range of 0–90 min is ca. 8×10 −11 mol J −1. This yield has been shown to depend on polymer concentration, ultrasound power and gas used to saturate the solution. Scission is accompanied by side reactions leading to the formation of carbonyl groups. Ultrasound-induced chain scission of starch proceeds with lower yield, one of the reasons being probably the different chain conformation when compared with rod-like chitosan macromolecules.

The Photodegradation of a Polyamidehydroxyurethane Type Polymer in Aqueous Solution. The Influence of Metal Ions on the Process of Degradation

The photodegradation of a polyamidehydroxyurethane type polymer in aqueous solution in the presence of various metal ions has been studied. It has been ascertained, that Fe3 + ions accelerate the process of photodegradation; Mn2 +, Co2+, Cu2+, Cr3+ ions retard the process of degradation of the polymer. With increasing Fe3 + concentration, there increase the rate of the degradation and the values of the quantum yield of chain scission, φcs. At greater Fe3 + concentrations, there is observed a stabilization of the degradation by the end of the process, that fact means that in this case the number of macroradicals recombinations is greater than that, at less Fe3+ concentrations. The photocatalytic role of Fe3+ ions has been related to the formaion of the OH radicals.

Photodegradation in Aqueous Solution of a Polyamidehydroxyurethane Type Polymer. Temperature Effect on the Process of Degradation

The present research shows some aspects of temperature effect on the process of photodegradation in aqueous solution of a polyamidehydroxyurethane type polymer. The process of photodegradation in aqueous solution of the studied polymer proceeds with chain scissions, what is promoted at elevated temperatures. At the temperatures 45°C and 65°C the values of the number average of chain scissions, S, and of the quantum yield of chain scission, φcs, are greater at the initial moments of the process than the correspondent values at 25°C and 35°C. At elevated temperatures there is an increase in the probability of the process of recombination of macroradicals, formed during chains scission. The values of S at 45°C and 65°C by the end of the process are smaller than those at 35°C and 25°C. It can also be noticed, that the values of φcs at 45 °C and 65°C continuously decrease after the initial moments of irradiation till the end of the process of degradation, while at 25°C the values of φcs increase practically during the whole process. At 35°C the behaviour of the system is more unusual.

Effect of a tensile stress on the radiolytic degradation of poly(methyl methacrylate)

Abstract Plaques (2 mm thick) of atactic, bulk polymerized poly(methyl methacrylate) (PMMA) were exposed to γ-rays (dose rate 2.5 kGy h −1 , doses ranging from 5 to 10 kGy), at ambient temperature in a nitrogen atmosphere. Tensile-loaded samples (stress: 47 MPa) were exposed together with unloaded ones. The molecular weight distribution was studied before and after irradiation by size exclusion chromatography calibrated by PMMA quasi homodisperse standards. The yield of chain scission G(S) was determined for six different doses and a total number of stressed/unstressed samples of 38. The results clearly reveal a small but significant effect of tensile loading on the rate of chain scission: G(S) stressed/ G(S) unstressed ≈ 1.2. Two possible mechanisms are discussed. The first involves a primary radiolysis of skeleton CC bonds. In this case, tensile loading would disfavour the cage recombination of radicals and, thus, increase G(S) . The second (and more frequent) mechanism results from a scission of the primary macroradical, a mechanism which could be stress-activated.

Swift heavy ion modification of polymers

We have studied the effects of dense electronic excitations on vinyl linear polymers, by performing swift heavy ion, SHI, irradiations. We used different ion beams, from C to Pb, provided by the GANIL accelerator at energies of a few MeV/u. We mainly studied the physico-chemical modifications induced in polyethylene (PE), but the role of chemical substitutions has also been investigated by studying the swift heavy ion induced damage in polyvinylidene fluoride (PVDF) and polystyrene (PS). The irradiated samples were analysed ex-situ by means of Fourier transform infrared spectroscopy in the transmission mode. Any contact with air was avoided: oxidation does not occur. Swift heavy ion irradiations are characterised by a significant increase of the yield of chain scission associated with unsaturated end groups. Moreover, we have evidences for specific modifications, i.e. modifications which are never observed when using low ionising particles as electrons or gamma-rays; namely the creation of alkyne and allene end groups. These specific modifications are little sensitive to chemical substitutions. For instance, alkyne formation is observed in PE, PVDF and PS. A simultaneous ionisation of the different atoms of a constituting monomer is likely needed for triggering the formation of the specific swift heavy ion induced modifications. On the other hand, if we pay attention to the physico-chemical modifications which are easily induced by low ionising particles, it is observed that increasing the electronic stopping power does not significantly modify the creation yield of, for instance in PE, trans-vinylene or trans-trans diene groups.

Main chain scission reaction of poly(methyl methacrylate) caused by two‐photon ionization of dopant

AbstractThe main chain scission reaction of poly(methyl methacrylate) (PMMA) doped with N,N,N,′,N′‐tetramethyl‐p‐phenylenediamine (TMPD) was examined by ESR spectroscopy and GPC measurement, and the scission mechanism was analyzed. The two‐photon ionization of TMPD with excimer laser excitation at 77 K produced an ester radical anion of PMMA (PMMA·m̌), which becomes the main chain tertiary radical CH2C˙(CH3)CH2after the detachment of the ester side group by annealing of the sample at room temperature. The main chain scission radical ˙C(CH3)(COOCH3)(PMMA˙) which was produced by the β‐scission fromCH2˙C(CH3)CH2showed the 13‐line ESR spectrum instead of the ordinary 9‐line, due to the fast quenching of the sample to 77 K. The change of the molecular weight distribution was measured by GPC after several irradiation‐and‐annealing operations. The simulation of the GPC curve confirmed that the scission re‐action occurs at random in the PMMA chain in the solid and the main chain scission yield from the ester radical anion, [PMMA˙]/[PMMA·m̌], is 0.30. © 1995 John Wiley & Sons, Inc.

Physico-chemical modifications induced in polymers by swift heavy ions

We have studied the effects of dense electronic excitations on the physico-chemical modifications of polymers by performing heavy ion irradiations in the energy range of some MeV amu−1. The effects induced by swift heavy ion irradiations were compared to those induced by MeV electron irradiations performed in similar experimental conditions in polyethylene and poly(vinylidene difluoride) films. The irradiated samples were analysed ex situ by means of Fourier transform infrared spectroscopy in the transmission mode. High electronic stopping power, (dE/dx)e, irradiations are characterised by a significant increase of the yield of chain scission. Vinyl terminal unsaturations are recorded with radiochemical yields increasing markedly as (dE/dx)e increases. Moreover, characteristic bands of alkyne terminal groups were observed only above a threshold of (dE/dx)e. The overall set of results is analysed taking into account the radial dose deposition predicted by numerical calculations and the defect creation measured after electron irradiations. This allows us to elucidate the kind of chemical modifications influenced by the local dose rate; i.e. for which a specific effect due to a dense electronic excitation appears.

Radiation chemistry of poly(ethyl methacrylate)—IV. Effect of irradiation temperature

The effect of irradiation temperature on radiation induced degradation of poly(ethyl methacrylate) sample with a most probable distribution has been investigated. Chain scission yields, G( S), have been determined in the temperature interval of 0–140°C. The activation energy of chain scission was calculated to be 10.41±2.03 kJ/mol. No change in this value was observed when the glass transition temperature of 72°C was overcrossed. Low G( S) values determined at 120 and 140°C were explained to be due to unzipping reactions accompanying the main chain scissions.

Radiolytic crosslinking and chain scission in aliphatic and alkyl-aromatic polyamides—II

Regression analysis of the radiation parameters of nine aliphatic polyamides exposed to ionizing radiation leads to the conclusion that the decline in the ratio of chain scission to crosslinking in higher aliphatic polyamides is best related to the linear increase in the methylene content of, or the number of methylene groups in, the polyamide repeat unit. G(crosslink) [ G(X)] and G(chain scission) [ G(CS)] values, however, do not correlate well with either of these parameters. Rather it is found that the major determinant of yields [about 80–85% of the variation for G(X), 70% for G(CS)] is the number of hydrogen atoms or methylene groups in the amine residue. A minor determinant [15% of the variation for G(X) and 30% for G(CS)] is the number of hydrogen atoms or methylene groups in the acid residue of the repeat unit. Significantly, although there was little sign of a decrease in G(CS) in the higher aliphatic homologues (up to 11 methylene residues per amide group), G(CS) is found to be positively related to the number of methylene groups in the amine residue but negatively related to the number of methylene groups in the acid residue. Thus although the amine residue dominates the radiolytic response, the acid residue plays a critical if lesser role in determining chain scission yields in polyamides. Thus a polyamide with the repeat unit —(CH 2) 3 n CONH(CH 2) n NHCO— would be predicted to have a very low or zero G(CS) and, if n is 10 or more, a G(X) comparable to that of polyethylene. Although, logically, the yields of crosslinks and chain scissions in polyamides would be expected to tend to that of polyethylene as the number of methylene groups in the repeat unit increases, use of two models assuming an exponential trend to the G(X) value characteristic of polyethylene in the analysis did not provide better fits to the data than the simple linear model referred to above. Indeed, the assumption of a significant exponential trend factor led to a marked drop in the goodness of fit.