Thermal behavior of some polyarylene ethers: A comparative study of the kinetics of degradation

The degradation of four aromatic thermoplastics containing carbonyl, ether and sulfone linkages was performed in a thermogravimetric analyzer in both nitrogen and air environments, in isothermal and dynamic heating conditions. The obtained results suggest that degradations started by random chain scission under all the experimental conditions. Under N₂ flow branching and crosslinking were superimposed on the initial process, while in air complete oxidative degradation occurred. The apparent activation energies associated with the first degradation stage were evaluated and correlated with the linkages present in the polymer chains. The obtained values indicate that the chemical reactions occurring under nitrogen are different from those in air. In addition, the ratio of the chain scission rate to the branching and crosslinking rates in nitrogen is dependent on temperature in isothermal experiments and on heating rate in the dynamic ones. A comparison with poly(ether ether ketone) is also reported.     

Characterization of UHMWPE hip cups run on joint simulators

The density and crystallinity of UHMWPE-hip cups were investigated as a function of thickness from the inner stressed surface to the unstressed outer surface. The effects of mechanical strain and chemical reactions during simulation tests, and damage of the material due to pretreatments and storage, resulted in changes of the structure, as indicated by variations in the crystallinity. Independent of either the batch of UHMWPE supplied or the manufacturer and the type of simulator used, the individual sample-sets showed a similar characteristic curve of density versus wall thickness. Infrared spectroscopic evaluations indicated the presence of oxidative degradation, and answers the question as to which areas of the polymers are changed by aging and which compounds are newly formed. The characteristic carbonyl groups were also determined. The concentration trend of carbonyl groups versus wall thickness obtained agrees surprisingly well with the locally determined density and crystallinity trend. As these compounds are formed by reactions which produce stable oxidative degradation products and also crosslinking, we have determined the degree of cross-linking. The determination of the soluble constituents after extraction showed lower degree of crosslinking on the surface than in the middle of the material. Hence it follows that on the surfaces oxidative chain scission is prevailing, whereas in the interior mainly crosslinking is developed. These results indicate that the samples used for the simulation tests had distinct differences in characteristics. Generally the results show that wear tests in joint simulators lead to property changes in UHMWPE which differ considerably from test results previously obtained on retrieved hip cups.     

Effect of soft-segment chemistry on polyurethane biostability during in vitro fatigue loading

The effect of soft-segment chemistry on biostability of polyurethane elastomers was studied with a diaphragm-type film specimen under conditions of static and dynamic loading. During testing, the films were exposed to an H(2)O(2)/CoCl(2) solution, which simulated the oxidative component of the in vivo environment. Films treated for up to 24 days were evaluated by IR spectroscopy and by optical and scanning electron microscopy. Biostability of a poly(ether urethane) (PEU), which is known to undergo oxidative degradation, was compared with biostability of a poly(carbonate urethane) (PCU), which is thought to be more resistant to oxidation than PEU. Materials similar to PEU and PCU, in which the polyether or polycarbonate soft segment was partially replaced with poly(dimethylsiloxane) (PDMS), were also tested with the expectation that PDMS would improve soft-segment biostability. Oxidative degradation of the polyether soft segment of PEU was manifest chemically as chain scission and cross-linking and physically as surface pitting. Biaxial fatigue accelerated chemical degradation of PEU and eventually caused brittle stress cracking. In comparison, the polycarbonate soft segment was more stable to oxidation; there was minimal chemical or physical degradation of PCU, even in biaxial fatigue. Partial substitution of the polyether soft segment with PDMS enhanced oxidative stability of PEU. Although both strategies for modifying soft-segment chemistry improved the resistance to oxidative degradation, the outstanding mechanical properties of PEU were compromised to some extent.     

The role of oxidation and enzymatic hydrolysis on the in vivo degradation of trimethylene carbonate based photocrosslinkable elastomers

The in vivo degradation of trimethylene carbonate (TMC) containing elastomers was investigated, and the mechanism of degradation explored through in vitro degradation under enzymatic and oxidative conditions. The elastomers were prepared via UV initiated crosslinking of prepolymers of TMC and equimolar amounts of TMC and epsilon-caprolactone (CL). The degradation process was followed by investigating the changes in the mechanical properties, mass loss, water uptake, sol content, differential scanning calorimetry, and surface chemistry through attenuated total reflectance infrared (ATR-FTIR) spectroscopy. During in vivo degradation, TMC and TMCCL elastomers exhibited surface erosion. The tissue response was of greater intensity in the case of the TMC elastomer. Both elastomers exhibited degradation in cholesterol esterase containing solutions in vitro, but no parallels were found between the rate of in vivo degradation and the rate of in vitro degradation. Only the TMCCL elastomer degraded in lipase. Degradation in a stable superoxide anion in vitro medium was consistent with the observed in vivo degradation results, indicating a dominant role of oxidation through the secretion of this reactive oxygen species by adherent phagocytic cells in the degradation of these elastomers.     

The photochemistry of ring substituted polystyrenes, 5. Photolysis of poly(p-amino) and poly(p-nitrostyrenes)

The short-wave (λ = 254 nm) photochemistry of poly(p-aminostyrene) (PPAS), and poly(p-nitrostyrene) (PPNS) films was studied under high vacuum conditions (≈ 10^?6 mbar) at 25°C. The principal reaction product in both cases was hydrogen, but both polymers also underwent coloration, crosslinking, and chain scission. Rates of chain scission and hydrogen formation were found to be considerably greater for PPAS, and these was attributed to the stabilizing effect of the p-amino group (relative to that of the p-nitro group) on the intermediate free radicals. The very high rate of crosslinking of PPAS would suggest the participation of the p-amino group in crosslinking reactions.     

Versuche zum thermischen abbau von copolyamiden aliphatischer und aromatischer aminocarbonsäuren

It was demonstrated that amide exchange reactions in mixtures of low molecular weight amides, as well as in mixtures of polyamides with simple amides or other polyamides occur slowly and only to a slight extent at 200°C, whereas they occur rapidly at 250°C. The thermal degradation of copolyamides formed from aliphatic or aromatic amino acids occurs almost exclusively through scission of bonds in the aliphatic amino acids. In this reaction 4-aminobutyryl and 5-aminovaleryl moieties are preferentially splitoff as lactames, while α-, β- and ε-aminoacids are split off through various degradation reactions. Formation of lactams from the center of a chain can occur starting at about 250°C. This, however, does not lead to chain scission, since the neighbouring amino acids are linked together after formation of the lactam. The thermal behaviour of copolyamides is compared with the properties of low molecular weight model substances. The degradation reactions were examined kinetically and their mechanism is discussed.     

Gpc study of thermo-oxidative degradation of poly(2,6-dimethyl-1,4-phenylene oxide)

The method of gel permeation chromatography was used to study the structural changes of poly(2,6-dimethyl-1,4-phenylene oxide) in the course of thermooxidative degradation. During the degradation two principal processes occur: random scission and crosslinking. The latter process leads to the formation of insoluble fractions. The reaction rates and activation energies of both processes have been determined. The mechanism of degradation as well as the influence of some additives is discussed on the basis of experimental data.     

Caractérisation de réactions primaires de dégradation oxydante au cours de l'autoxydation des poly(oxyéthylène)s à 25°C: Étude en solution aqueuse avec amorçage par radiolyse du solvant, 8. Étude cinétique en fonction du ph compris entre 1 et 13

Radiation-induced experiments showed that primary oxidative degradation reactions leading to chain scission occur during the autoxidation of poly(ethylene oxide) in aqueous solution at 25°C. The same scheme proposed to explain the results obtained for the reactions at natural pH is suitable for the pH range from 1 to 13. Up to pH 10, the rate of initiation varies like the G_HO. value of water radiolysis. At pH > 10, the rate decreases and becomes equal to zero near pH 13.     

Biostability and macrophage-mediated foreign body reaction of silicone-modified polyurethanes

In this study, the effect of soft segment chemistry on the phase morphology and in vivo response of commercial-grade poly(ether urethane) (PEU), silicone-modified PEU (PEU-S), poly(carbonate urethane) (PCU), and silicone-modified PCU (PCU-S) elastomers were examined. Silicone-modified polyurethanes were developed to combine the biostability of silicone with the mechanical properties of PEUs. Results from the infrared spectroscopy confirmed the presence of silicone at the surface of the PEU-S and PCU-S films. Atomic force microscopy phase imaging indicated that the overall two-phase morphology of PEUs, necessary for its thermoplastic elastomeric properties, was not disrupted by the silicone modification. After material characterization, the in vivo foreign body response and biostability of the polyurethanes were studied using a subcutaneous cage implant protocol. The results from the cage implant study indicated that monocytes adhere, differentiate to macrophages which fuse to form foreign body giant cells on all of the polyurethanes. However, the silicone-modified surfaces promoted apoptosis of adherent macrophages at 4 days and high levels of macrophage fusion after 21 days. These results confirm that the surface of a biomaterial may influence the induction of apoptosis of adherent macrophages in vivo and are consistent with previous cell culture studies of these materials. This study validates the use of our standard cell culture protocol to predict in vivo behavior and further supports the hypothesis that interleukin-4 is the primary mediator of macrophage fusion and foreign body giant cell formation in vivo. The impact of these findings on the biostability of polyurethanes is the subject of current investigations. Attenuated total reflectance-Fourier transform infrared analysis of explanted specimens provided evidence of chain scission and crosslinking at the surface of all of the polyurethanes. The silicone modification did not fully inhibit the oxidative biodegradation of the polyether or polycarbonate soft segments; however, the rate of chain scission of PEU-S and PCU-S seemed to be slower than the control polyurethanes. To verify this finding and to quantify the rate of chain scission in order to predict long-term biostability, an in vitro environment that simulated the microenvironment at the adherent cell-material interface was used to accelerate the biodegradation of the polyurethanes. Polyurethane films were treated in vitro for up to 36 days in 20% hydrogen peroxide/0.1M cobalt chloride solution at 37 degrees Celsius. Characterization with attenuated total reflectance-Fourier transform infrared and scanning electron microscopy showed soft segment and hard segment degradation consistent with the chemical changes observed after long-term in vivo treatment. The biostability ranking of these four materials based on rate of chain scission and surface pitting was as follows: PEU < PEU-S PCU < PCU-S. The silicone modification increased the biostability of the PEU and PCU elastomers while maintaining the thermoplastic elastomeric properties.     

Thermal Stability of Polysulphone

Polysulphone, prepared from 2.2-bis(4-hydroxyphenyl)propane and 4.4′-dichlorodiphenyl sulphone, degrades when heated at 380°C i. vac. The mol. wt. increases with time of heating until after about 3 hrs gelation occurs. In addition to crosslinking scission processes have been shown to be important. Degradation is also accompanied by the evolution of gas – mainly SO₂ and CH₄. Other products include a number of substituted diphenyl ethers, phenol, the major non gaseous product, and a variety of other phenols. Solutions of the degraded polymers react with DPPH and suggest the accumulation of hydroxyl groups with time of heating. The range of products indicates that the ether, isopropylidene and sulphone groups linking the phenylene rings are involved in the degradation. The rates of the crosslinking, scission, hydroxyl accumulation and gas evolution processes have been evaluated and the possible reactions underlying these processes discussed.     

Antioxidant inhibition of poly(carbonate urethane) in vivo biodegradation

This study compared the effect of an antioxidant on the in vivo biodegradation of a poly(carbonate urethane) (PCU) and a poly(ether urethane) (PEU). Unstrained PEU and PCU films with and without Santowhite were implanted subcutaneously into 3-month-old Sprague-Dawley rats for 3, 6, and 12 months. Characterization of unstabilized PEU and PCU with ATR-FTIR and SEM showed soft-segment and hard-segment degradation consistent with previous studies. In particular, evidence of chain scission and crosslinking of the surface was present in the ATR-FTIR spectra of explanted specimens. Addition of 2.2 wt % antioxidant inhibited the in vivo degradation of both PCU and PEU. Although the antioxidant probably improved polyurethane biostability by decreasing the susceptibility of the polymer to degradation, modulation of the cellular response to prevent the release of degradative agents was also possible. To differentiate the effects, the foreign-body response was investigated with the use of a standard cage implant protocol. Polyurethane films were implanted in wire mesh cages subcutaneously in rats for 4, 7, and 21 days. There were no statistical differences among materials in the inflammatory exudate cell counts, adherent cell densities, or percent fusion of macrophages into foreign-body giant cells (FBGCs). Therefore, it was concluded that the antioxidant inhibited degradation by capturing oxygen radicals that would otherwise cause polyurethane chain scission and crosslinking.     

Structural changes of UHMWPE after e-beam irradiation and thermal treatment

Ultra-high molecular weight polyethylene (UHMWPE) was irradiated with accelerated electrons (1 MeV in air) using high dose rates (> 25 kGy/min) and thin specimens (thickness 1 mm). Parts of the specimens were remelted (200 degrees C for 10 min; 150 degrees C for 0, 2, 10, 30, 60 min). All specimens were stored in nitrogen in the dark at 5 degrees C. Supermolecular structure, extent of crosslinking, oxidative degradation, and macroradical content were studied by a number of methods (SAXS, WAXS, SEM, DSC, FTIR, ESR, TGA, solubility experiments, image analysis). The results obtained with irradiated samples were compared with those obtained with irradiated and remelted samples. It was confirmed that crosslinking predominates over chain scission at very high dose rates, even if the irradiation is performed in air. Discrepancies concerning supermolecular structure changes in UHMWPE after irradiation and thermal treatment, found in various studies in the literature, are discussed. A simple model, which describes and explains all supermolecular structure changes, is introduced. An effective way of eliminating residual macroradicals in UHMWPE is proposed.     

The thermal degradation of poly(methyl acrylate). Part II. The mechanism of chain breaking

The rate curves for the conversion of poly(methyl acrylate) to volatiles at 286–310°C show maxima in the region 10–20% conversion, consistent with random degradation with transfer. The molecular weight changes in the early stages of the reaction also suggest random breakdown. A reaction mechanism involving initiation by random homolytic back-bone bond scission followed by a chain of free radical transfer reactions, inter- and intramolecular, is proposed. Chain-end initiation appears to be unlikely. The free radical inhibitor 1.4-diaminoanthraquinone was found to retard both volatile formation and random chain scission thus supporting the proposed mechanism. The energies of activation for volatile formation and random chain scission are 35 and 33 kcal/mole respectively.     

Poly(carbonate urethane) and poly(ether urethane) biodegradation: in vivo studies

Several strategies have been used to increase the biostability of medical-grade polyurethanes while maintaining biocompatibility and mechanical properties. One approach is to chemically modify or replace the susceptible soft segment. Currently, poly(carbonate urethanes) (PCUs) are being evaluated as a replacement of poly(ether urethanes) (PEUs) in medical devices because of the increased oxidative stability of the polycarbonate soft segment. Preliminary in vivo and in vitro studies have reported improved biostability of PCUs over PEUs. Although several studies have reported evidence of in vitro degradation of these new polyurethanes, there has been no evidence of significant in vivo degradation that validates a degradation mechanism. In this study, the effect of soft segment chemistry on the phase morphology, mechanical properties, and in vivo response of commercial-grade PEU and PCU elastomers was examined. Results from dynamic mechanical testing and infrared spectroscopy suggested that the phase separation was better in PCU as compared with PEU. In addition, the higher modulus and reduced ultimate elongation of PCU was attributed to the reduced flexibility of the polycarbonate soft segment. Following material characterization, the in vivo biostability and biocompatibility of PEU and PCU were studied using a subcutaneous cage implant protocol. The results from the cage implant study and cell culture experiments indicated that monocytes adhere, differentiate, and fuse to form foreign body giant cells on both polyurethanes. It is now generally accepted that the reactive oxygen species released by these adherent macrophages and foreign body giant cells initiate PEU biodegradation. Attenuated total reflectance-Fourier transform infrared analysis of explanted samples provided evidence of chain scission and crosslinking in both polyurethanes. This indicated that the PCU was also susceptible to biodegradation by agents released from adherent cells. These results reinforce the need to evaluate and understand the biodegradation mechanisms of PCUs.     

Liquid crystal elastomers: Influence of the orientational distribution of the crosslinks on the phase behaviour and reorientation processes

The influence of the crosslinking conditions on the phase behaviour and reorientation processes of nematic elastomers is investigated. Two series of elastomers with various network anisotropies crosslinked either in the nematic or in the isotropic state were investigated by means of IR-dichroism and stress-strain measurements. The experimental results clearly demonstrate that for nematic elastomers not only the coupling between the network anisotropy and the state of order has to be considered. It is shown that the influence of the crosslinks and their orientational distribution on the phase behaviour, the state of order as well as on director reorientation processes cannot be neglected.     

Ultrasonics Sonochemistry Assisted Preparation of Polysiloxane Main‐Chain Liquid‐Crystalline Elastomers

For the first time, an ultrasonics sonochemistry method is developed to promote the one‐pot hydrosilylation polyaddition polymerization and crosslinking reaction in the preparation of polysiloxane main‐chain liquid‐crystalline elastomers (MC‐LCEs). Due to the extraordinary effect of acoustic cavitation, the polyaddition polymerization and crosslinking reaction can be successfully carried out in an ordinary laboratory ultrasonic cleaner at room temperature, and generates the LCE matrix network in about 30 min. The prepared MC‐LCEs demonstrate good quality, good properties, and stimuli‐actuation performances. Compared to the traditional thermal processing methods for preparing polysiloxane MC‐LCEs, this method exhibits superior properties rapidly, with high convenience and efficiency, and can be a path for batch fabrication at low cost. The work also demonstrates that the ultrasonics sonochemistry method is effective in generating linear main‐chain liquid crystal polymers through hydrosilylation polyaddition and polysiloxane side‐chain LCE matrix through hydrosilylation crosslinking reaction, thus confirming the high availability of ultrasonics sonochemistry in the processes of hydrosilylation polymerization, crosslinking reactions, or synchronous polymerization and crosslinking reactions.     

Abbau und vernetzung von polydimethylsiloxan in lösung unter dem einfluß von 60Co-γ-strahlen

The radiation chemical yields for main chain scission and for crosslinking have been determined for pure poly (dimethyl siloxane) as well as for poly (dimethyl siloxane) irradiated in toluene solutions at various concentrations. In the case of pure poly (dimethyl siloxane), the ratio of the specific densities of degradation and crosslinking amounts to P₀/q₀ = 0.015. In the case of the toluene solution, P₀/q₀ steadily increases with decreasing concentration and approaches infinity at a base mole fraction of the polymer of x_P = 0.03. At the critical concentration (x_Perit = 0.15) P₀/q₀ is equal to 2. The 100 eV-yield of crosslinking decreases with decreasing polymer concentration and becomes zero in very dilute solutions. On the contrary, the 100 eV-yield for main chain scission slightly increases with decreasing polymer concentration over a broad range of x_P; a more significant increase occurs at low concentrations. The results agree with the previously developed theory of simultaneous degradation and crosslinking of polymers in solution under the influence of ionizing radiation.     

Liquid-crystalline elastomers with cholesteric and chiral smectic C* phases

A series of 9 new chiral and crosslinkable liquid-crystalline copolymers were synthesized and transformed into liquid-crystalline elastomers by crosslinking using a hydrosilylation reaction. Both the soluble copolymers and the elastomers show cholesteric, smectic A and chiral smectic C^* phases, which are not affected by the crosslinking procedure. First measurements show that the helical superstructure of elastomers with cholesteric and chiral smectic C^* phases can be reversibly untwisted by stretching. Thereby a cholesteric structure can be transformed into a nematic structure and a chiral smectic C^* structure into a smectic C one.     

Réactions primaires de dégradation oxydante au cours de l'autoxydation du poly(oxytétraméthylène) à 25°C, 3 Cinétique de la production des coupures statistiques et caractérisation de la réaction bimoléculaire non terminante des radicaux peroxyles secondaires

The kinetics of the production of random chain scissions by oxidative degradation of poly(oxytetramethylene) was determined during the radiation-induced autoxidation of solutions of the polymer in chloroform at 25°C. Ethylene oxide resulting from autoxidation of ethylene produced by unimolecular decomposition of alkoxy-1 alkylperoxy radicals partly inhibits the chain scission. This property allowed the characterization of the existence of a second primary mechanism of oxidative degradation of the polymer as non-terminating bimolecular reaction of these peroxy radicals followed by a ß-scission of the produced alkoxy radicals.     

Conductivity relaxation of polyaniline

Measurements on polyaniline using dielectric analysis (DEA, ?50°C to 240°C, 0,4 Hz to 10⁵ Hz), differential scanning calorimetry (DSC, 30°C to 240°C) and infrared spectroscopy (IR) after heat treatment from 25°C to 300°C are carried out. After conversion of DEA results to complex electric modulus, the conductivity relaxation time distribution can be obtained, which remains unchanged for temperatures below 160°C and becomes narrower in the temperature range from 180°C to 240°C due to the occurrence of crosslinking reactions, as is confirmed by the exothermic behavior above 160°C and by the decrease of the relative intensity of quinoid absorption with respect to that of the aromatic ring. The crosslinking reaction causes a decrease in dc conductivity, a narrowing in conductivity relaxation time distribution and an increase in activation energies for conductivity and conductivity relaxation.