Trans-vinylene Unsaturation Research Articles

Effects of gamma ray and electron beam irradiation on the mechanical, thermal, structural and physicochemical properties of poly (ether-block-amide) thermoplastic elastomers

Both gamma ray and electron beam irradiation are widely used as a means of medical device sterilisation. However, it is known that the radiation produced by both processes can lead to undesirable changes within biomedical polymers. The main objective of this research was to conduct a comparative study on the two key radiosterilisation methods (gamma ray and electron beam) in order to identify the more detrimental process in terms of the mechanical, structural, chemical and thermal properties of a common biomedical grade polymer. Poly (ether-block-amide) (PEBA) was prepared by injection moulding ASTM testing specimens and these were exposed to an extensive range of irradiation doses (5–200kGy) in an air atmosphere. The effect of varying the irradiation dose concentration on the resultant PEBA properties was apparent. For instance, the tensile strength, percentage elongation at break and shore D hardness can be increased/decreased by controlling the aforementioned criteria. In addition, it was observed that the stiffness of the material increased with incremental irradiation doses as anticipated. Melt flow index demonstrated a dramatic increase in the melting strength of the material indicating a sharp increase in molecular weight. Conversely, modulated differential scanning calorimetry established that there were no significant alterations to the thermal transitions. Noteworthy trends were observed for the dynamic frequency sweeps of the material, where the crosslink density increased according to an increase in electron beam irradiation dose. Trans-vinylene unsaturations and the carbonyl group concentration increased with an increment in irradiation dose for both processes when observed by FTIR. The relationship between the irradiation dose rate, mechanical properties and the subsequent surface properties of PEBA material is further elucidated throughout this paper. This study revealed that the gamma irradiation process produced more adverse effects in the PEBA material in contrast to the electron beam irradiation process.

Dual Site Ethene/1-Hexene Copolymerization with MAO Activated (1,2,4-Me3Cp)2ZrCl2 and (Me5Cp)2ZrCl2 Catalysts. Possible Transfer of Polymer Chains between the Sites

Ethene homopolymerization and copolymerization in toluene solution with varying amounts of 1-hexene were catalyzed by the methylaluminoxane (MAO) activated zirconocene catalysts (1,2,4-Me3Cp)2ZrCl2 and (Me5Cp)2ZrCl2 and a mixture of these two in a 1:1 molar ratio. The dual site catalyst (1:1 mixture) has a homo- and copolymerization activity intermediate between the individual catalysts. Deviations from the expected average behavior taking into account the ca. 6-fold higher 1 h yield of (1,2,4-Me3Cp)2ZrCl2 are observed. The dual site system results in a copolymer with ca. 30% too low comonomer incorporation. The melting endotherm attributed to (Me5Cp)2ZrCl2 is found to be ca. 30−70% too high, and the observed molecular weights for the copolymers are higher than expected and even higher than for the two individual catalysts. Chain transfer to trimethylaluminum (TMA) is found to become less important at high comonomer concentrations concurrent with an increase of the concentration of vinyl and trans-vinylene unsaturations. Further, it is noteworthy that with the dual site system it is possible to produce ethene/1-hexene copolymers with narrow molecular weight distribution as a mixture of chains containing low and high comonomer contents. The observed discrepancies in the mixed system from the average of the (1,2,4-Me3Cp)2ZrCl2 and (Me5Cp)2ZrCl2 catalysts are provisionally explained by chain transfer between the sites coupled with an activity increase of the latter of these sites.

The use of trans-vinylene formation in quantifying the spatial distribution of electron beam penetration in polyethylene. Single-sided, double-sided and shielded irradiation

The use of the quantification of trans-vinylene unsaturations in irradiated ultrahigh molecular weight polyethylene (UHMWPE) in determining the spatial distribution of electron beam penetration was investigated. UHMWPE was irradiated with a 10 MeV linear electron beam accelerator to 100 kGy at both room temperature and 125°C in air. The irradiation was carried out in shielded and unshielded single-sided and unshielded double-sided irradiation modes. Dose–depth profiles were measured by quantifying the yields of trans-vinylene unsaturations as a function of depth away from the e-beam incidence surfaces using infrared spectroscopy. The extent of beam penetration increased with increasing irradiation temperature as was manifested by the increase in the iso-dose penetration with the unshielded single-sided irradiation and increased dose overlap with the unshielded double-sided irradiation. The optimum thickness with maximum uniformity in dose–depth distribution for double-sided irradiation was 85 and 90 mm for 25°C and 125°C irradiation temperatures, respectively.

Gradient crosslinking of UHMWPE using irradiation in molten state for total joint arthroplasty

Increased crosslink density increases the wear resistance of ultra-high molecular weight polyethylene (UHMWPE) acetabular components used in total hip reconstructions. However, increasing crosslink density can reduce the mechanical properties of UHMWPE. Other researchers have tried to limit the crosslinking to a superficial layer on the articulating surfaces to retain the properties of the virgin polymer. We present here a method of producing a gradient of crosslink density across the acetabular component. Acetabular liners with 26 mm inner diameter were machined from the UHMWPE. The liners were then irradiated at 140°C in the molten state of the polymer using a 2 MeV electron beam with limited penetration of the effects of radiation into polyethylene. The liners were then machined to an inner diameter of 32 mm to remove the radiation induced oxidized surface layer. The limited penetration of the e-beam resulted in a gradient of crosslink density with a crosslink density of 0.15±0.01 mol/dm 3 near the articulating surfaces and 0.12±0.01 mol/dm 3 near the backside. The concentration of the trans-vinylene unsaturations decreased gradually as a function of depth from the articulating surfaces to the backside of the liners. The wear resistance of the melt-irradiated liners was contrasted with those of conventional liners using the Boston hip simulator. The gravimetric wear rate was 27±5 mg/million cycles with the conventional liners, while the melt-irradiated acetabular liners did not show any weight loss.

Identification and quantification of irradiation in UHMWPE through trans-vinylene yield.

trans-Vinylene unsaturations generated in ultrahigh-molecular-weight polyethylene (UHMWPE) after radiation treatment scales linearly with the absorbed dose level, and can serve as an internal dosimeter for determining the radiation dose level for sterilizing or crosslinking UHMWPE. We measured the trans-vinylene concentration using infrared spectroscopy and generated calibration curves for ultrahigh-molecular-weight polyethylene (ram-extruded GUR 1050) after cold e-beam, cold gamma, and warm e-beam irradiations. The trans-vinylene content increased linearly with the absorbed dose level in all cases except with cold gamma irradiation at dose levels higher than approximately 70 kGy. At that dose level the trans-vinylene content of the warm irradiated samples was higher than those of the cold irradiated samples. Following postirradiation melting, the trans-vinylene content of the gamma irradiated samples increased. Such trans-vinylene quantification can be an internal dosimeter for crosslinked UHMWPE when radiation is used for improving its wear resistance or sterilizing the device made thereof. It can also be utilized to assess the spatial uniformity of the absorbed radiation dose level.

High-density polyethylene dosimetry by transvinylene FTIR analysis

Abstract The formation of transvinylene unsaturation, CHCH, due to free-radical or cationic-initiated dehydrogenation by irradiation, is a basic reaction in polyethylene and is useful for dosimetry at high absorbed doses. The radiation-enhanced infrared absorption having a maximum at ν =965 cm −1 ( λ =10.36 μm) is stable in air and can be measured by Fourier-transform infrared (FTIR) spectrophotometry. The quantitative analysis is a useful means of product end-point dosimetry for radiation processing with gamma rays and electrons, where polyethylene is a component of the processed product. The transvinylene response in air to gamma radiation is linear with dose and has relatively low yield compared with the response to electrons, whereas the response in deaerated polyethylene samples is also linear, but is more sensitive, and has negligible dose-rate dependence in its response to gamma rays and electrons. The useful dose range of 0.053 cm thick high-density polyethylene film ( ρ =0.961 g cm −3 ; melt index=0.8 dg min −1 ), for irradiations by 60 Co gamma radiation and 2.0 and 0.4 MeV electron beams in deaerated atmosphere (N 2 gas), is about 50–10 3 kGy for FTIR transvinylene spectrophotometric analysis. The measured product of linear molar extinction coefficient and radiation chemical yield for transvinylene formation in this case is 3.22×10 −6 m 2 J −1 , and G(CHCH)=0.116 μmol J −1 =1.12 (100 eV) −1 . The estimated uncertainty for deriving these values by this method is about ±5% at 67% confidence level.

Termination, Isomerization, and Propagation Reactions during Ethene Polymerization Catalyzed by Cp2Zr−R+ and Cp*2Zr−R+. An Experimental and Theoretical Investigation

Ethene polymerization in toluene has been studied in the temperature range −7 to +97 °C and pressure range 0.28 to 9 bar, using two different L2ZrCl2/methylaluminoxane (MAO) catalyst systems. With bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2, L = Cp), the average activity over 1 h increases with temperature between 10 and 97 °C. With bis(pentamethylcyclopentadienyl)zirconium dichloride (Cp*2ZrCl2, L = Cp*), a maximum average activity over 1 h is observed at 45 °C. If propagation and deactivation effects are separated through kinetic modeling, the activity corresponding to chain propagation is found to increase in the whole temperature range for both catalysts. The molecular weight is higher with L = Cp* than with L = Cp below 80 °C. Above 80 °C, the opposite is observed. With L = Cp*, the molecular weight increases with increasing ethene pressure up to about 2 bar, where it levels off. With L = Cp, the molecular weight is independent of pressure between 0.28 and 9 bar. The ratio between vinyl and trans-vinylene unsaturation is approximately 6:1 with L = Cp and 1:1 with L = Cp*, both slightly increasing with increasing ethene pressure. As the temperature is increased, the relative vinyl content decreases with L = Cp and increases with L = Cp*. On the basis of density-functional calculations, we present a reaction scheme consistent with most of the experimental results. This reaction scheme, in which different agostic interactions play a crucial role, assumes a Cossee-like mechanism for chain propagation, chain termination via hydrogen transfer to a coordinated monomer (for both catalysts) or to the metal (for L = Cp*), and chain isomerization via partial hydrogen transfer to the metal, relative rotation of the olefin and the hydride, and reinsertion of the coordinated olefin. The calculated activation energy for propagation is 25−35 kJ/mol for L = Cp*, in fair agreement with the experimental value of 17 kJ/mol. For L = Cp, we calculate an activation energy of 10−20 kJ/mol, whereas the experimentally derived value is 61 kJ/mol. The poor agreement for L = Cp may indicate that the polymerization is influenced by the surrounding solvent and MAO. The calculated difference in activation energy between chain propagation and termination is larger for L = Cp* than for L = Cp, in qualitative agreement with the stronger temperature dependence of the molecular weight observed with L = Cp*. Chain isomerization is found to be easier, relative to termination, with L = Cp* than with L = Cp. This may account for the large amount of trans-vinylene unsaturation observed when Cp*2ZrCl2 is used as catalyst.

Radiolytic formation and decay of trans-vinylene unsaturation in polyethylene: Fourier transform infra-red measurements

In Part I of this series (Lyons and Johnson, 1993) we provided a theoretical basis for a binomial expression (developed empirically by WCJ) and showed that it represents the changes in trans-vinylene unsaturation concentration with radiolytic dose in polyethylene as well as, and usually better than, other published relations (Dole et al. 1958; Lyons and Crooke 1963). In this paper, which will be published in fuller form elsewhere, we present measurements, using Fourier Transform Infra Red (FT-IR) absorption spectroscopy, of the concentration of trans-vinylene unsaturation at various radiation doses and dicuss the use of such measurements in dosimetry.

Radiation-induced degradation of polyethylene: Role of amorphous region in the formation of oxygenated products and the mechanical properties

Quenched samples of linear low density, medium density and two kinds of high density polyethylene films were irradiated with γ-rays from a 60Co source in vacuum and in air at room temperature with irradiation doses ranging from 0 to 100 Mrad. On irradiation in vacuum the extent of crosslinking was about one-and-a-half times greater in the linear low density polyethylene (LLDPE) than in the high density polyethylene (HDPE). On the other hand, irradiation in air produced more crosslinking in high density polyethylene (HDPE). Growth of trans-vinylene unsaturation was found around 10 Mrad in all the samples. Initial increase in elongation and breaking strength (below 5 Mrad) occurred, which was followed by a decrease with increasing dose. LLDPE showed some elongation even at 50 Mrad, while the other samples became brittle and broke at doses far below this value. The mechanism of oxidative degradation is discussed.

Effects of electron beam preirradiation on the γ radiolysis of polyethylene

AbstractThe γ radiolysis of polyethylene preirradiated with electron beams to 3 Mrad was carried out at 30–100°C in vacuo with a dose rate of 6.35 × 105 rad/hr. The hydrogen formation in the γ radiolysis was little affected by the preirradiation of electron beams, whereas the formation of trans‐vinylene unsaturation and gel was somewhat retarded. The rates of the formation of hydrogen and trans‐vinylene unsaturation were described by the zero‐order formation kinetics with respect to each concentration combined with the first‐order disappearance. The apparent rate constants and activation energies for the formation and disappearance of hydrogen and trans‐vinylene unsaturation were almost independent of the preirradiation. The gel fraction was analyzed by using the Charlesby–Pinner equation. The G values of crosslinking and main chain scission were increased by the preirradiation, whereas their activation energies remained unaltered. On the basis of these results the effects of preirradiation on the reactions induced by γ rays in polyethylene were discussed.

Features of the thermal and thermo-oxidative degradation of some irradiated polyolefins

A study was made of features of the thermal and thermo-oxidative degradation of high and low density polyolefins and ethylene-propylene copolymers irradiated with from 1 to 300 Mrad. It was found that structural changes in the polymers under irradiation, while increasing the thermal stability of the latter, lead to reduced thermo-oxidative stability It is suggested that trans-vinylene unsaturation promotes the development of thermal degradation of the irradiated polymers, while vinylidene unsaturation inhibits this process. It is shown that crosslinks, tertiary carbon atoms and α-carbon atoms relative to the trans-vinylene double bond are the most vulnerable points during oxidation of the irradiated polymers. Crosslinking involving the formation of ether bonds takes place simultaneously with degradation occurring during oxidation of the irradiated polymers.

Accelerating Effect of Acetylene on the γ-Radiation-Induced Cross-Linking of Polyethylene

The γ-radiation-induced cross-linking of polyethylene was investigated in the presence of acetylene under an absolute pressure of 3 kg/cm2 at 30—200°C with a dose rate of 1.1×105 rad/hr. The weight of the polyethylene increased almost proportionally to the radiation dose, and the rate of the increase was larger at 30 and 90°C than at 150 and 200°C. The gel fraction increased with rising irradiation temperature, passed through a maximum at around 105°C, and thereafter decreased sharply to become almost constant above 150°C. At temperatures below the melting temperature of polyethylene, the gel fraction was much higher than in vacuo. As a characteristic structural change, the formation of terminal vinyl unsaturation was observed, and the rate of the formation increased with the temperature. The formation of hydrogen and trans-vinylene unsaturation was little affected by acetylene. On the basis of these results, the role of acetylene in the γ-radiation-induced cross-linking of polyethylene was discussed from the view-point of the reaction mechanism.

Radiation-Induced Cross-Linking of Polyethylene in the Presence of Various Acetylenic Compounds

Abstract A study was made on the radiation-induced cross-linking reaction of polyethylene by exposing film samples to [ddot]Co γ-rays in vacuum or in the presence of various acetylenic compounds such as acetylene, its C3 to C5 higher derivatives, and their mixture with tetrafluoroethylene (TFE). The samples (0.2 mm thick) were found to decrease in weight when irradiated in vacuum, but to increase in the presence of the additives. The IR spectrum measurements of the resultant showed that the trans-vinylene unsaturation increased by the irradiation, especially in the presence of the C3 to C5 derivatives, and terminal vinyl increased markedly in acetylene. However, the formation of these unsaturations were depressed when the film was irradiated in the mixed gas with TFE. The rate of the cross-linking reaction was accelerated by the addition of acetylene-TFE, acetylene, trifluoromethyl-acetylene, perfluorobutyne-2, perfluorobutyne-2-TFE, and methylacetylene-TFE [G(cross-linking) = 4.4-1.2]. It was reduced wit...

γ-Radiation-Induced Cross-Linking of Polyethylene

The γ-radiation-induced cross-linking of polyethylene was studied in vacuo at 30—200°C with a dose rate of 1.1×105 rad/hr. The gel fraction increased with rising irradiation temperature, and become almost constant above 150°C. The degree of increase was extremely large at near the melting temperature of polyethylene. The dosage to produce gelation decreased with the temperature, but was almost constant above 150°C. The gel fraction was increased by irradiation beyond the gel-point dosage at each temperature, and the relative increment decreased with the radiation dose. The amount of hydrogen formation was found to increase to some extent with temperature. The content of trans-vinylene unsaturation increased almost linearly with the dose, and the extent of the increment was little affected by the temperature. At 150 and 200°C, the methyl-group content also increased almost linearly with the dose, and the rate of increase was somewhat higher at 200°C than at 150°C. On the basis of these results, the mechanism of the γ-radiation-induced cross-linking of polyethylene was discussed.

Free radical reactions in cyclohexane irradiated at 77°K

The radical yield of cyclohexane, γ-irradiated at 77°K has been compared with the radiation chemical yields of the major products—dimer and trans-vinylene unsaturation. It was found that the radical—dose curve was not linear and consisted of a low dose linear region partially saturating to give a high dose linear region. The dimer-dose curve was found to follow the radical curve with the yield approximately half the radical yield over the whole dose range used whereas the unsaturation-dose curve remained linear over the dose range employed. The implications of these results have been discussed.

Comparison of alpha and gamma radiation effects in polyethylene

Exposure of polyethylene and paraffins to high-energy radiation produces main chain (transvinylene) unsaturation, and this rises to a maximum limiting concentration with increasing dose. On the other hand, vinylun saturation initially present diminishes on irradiation. Comparison of the effects of a- and y-irradiation of thin polyethylene films show that the initialGvalue for the formation of transvinylene unsaturation is not significantly affected by the l.e.t. of the radiation, but the maximum values reached differ by a factor of over 4. Similarly the decay of vinyl unsaturation is four times slower with a-radiation. The destruction of unsaturation is ascribed to the reaction of H atom s liberated by radiation with the unsaturated groups acting as a scavenger. The difference between a- and y-radiation is due to an l.e.t. effect, more H atoms being available from radiation of lower l.e.t. Estimates are made of the proportion of H atoms which escape from the tracks of the two forms of radiation. The difference is much smaller than in the case of irradiated aqueous solutions. Data are also presented on the formation of carbonyl with a- and y-radiation, both in the presence of air, an d during subsequent exposure.

Effect of atmosphere on radiation‐induced crosslinking of polyethylene. Part IV. Sensitizing effect of nitrous oxide and xenon

AbstractLow density polyethylene film was irradiated with gamma‐rays from Co60 in atmospheres of nitrous oxide and xenon. A plot of solubility (S + S1/2) against reciprocal of irradiation dose gives a nearly straight line in each case. The probability of crosslinking increases by 44% in N2O as compared with vacuum irradiation, and increases by 10% in Xe. On the other hand, the probability of main‐chain scission is scarcely affected by these gases. Therefore the increase in gel fraction due to these gases is not due to the protection against main‐chain scission, but is caused mainly by the sensitization against crosslinking. Furthermore, the formation of trans vinylene unsaturation is also sensitized by these gases.

Unsaturation in irradiated marlex 50 polyethylene: Dose rate and post‐irradiation effects

AbstractInfrared absorption spectroscopy has been used to demonstrate dose rate dependence of trans‐vinylene unsaturation in irradiated Marlex 50 polyethylene. When the irradiated polymer is stored in vacuum a decrease is observed in trans‐vinylene absorbance over a period of several weeks. After high dose rate irradiation the decay is preceded by an initial increase. These phenomena are ascribed to the reaction of “trapped” radicals.

The cross-linking and chain fracture of rubber by high-energy radiation

Quantitative analysis of the solubility data of irradiated rubber, using the method adopted by Charlesby and Pinner, has been applied to the study of the cross-linking reaction. The rate of cross-linking is directly proportional to the radiation intensity and corresponds to a G-value of 0·83 at 25°C. The activation energy of the cross-linking reaction is 0·76 and 0·26 kcal above and below the glass transition temperature, respectively. The ratio of dislinking to cross-linking is 0·05. It is largely independent of radiation intensity and temperature. Radiation induces the formation of some trans-vinylene unsaturation, believed to arise from a double bond shift. A considerable amount of terminal unsaturation, believed to arise as a result of dislinking, is also formed.

Estimation of Chain Fracture and Cross-Linking of Rubber by High-energy Radiation

Lightly masticated pale crepe rubber, which had been freshly extracted with acetone, was evacuated and sealed in glass tubes. After gamma irradiation from a cobalt-60 source or a linear electron accelerator, the soluble fraction was measured by extraction of the samples with benzene. In each case the mean intensity of the radiation was estimated by ferrous sulfate dosimetry. Solubility data for the gamma irradiation of rubber at two dose rates at 25 deg C were quantitatively analyzed by the method of Charlesby and Dinner. Results show that the rate of cross-linking is directly propertional to the intensity of radiation. Results indicate that the initial distribution of molecular weights is not perfectly random and some dislinking is necessary to obtain the results. The ratio of the rate of dislinking to cross-linking was calculated. Infrared spectroscopic analysis revealed a marked increase in the absorption near 1,639 cm/ sup -1/, identified as terminal unsaturation, which was presumed to arise from the dislinking reaction. There was also evidence of transvinylene unsaturation during irradiation. (C.H.)