Polymer Degradation and Stability

An amphiphilic macromolecule (AM) was exposed to ionizing radiation (both electron beam and gamma) at doses of 25 kGy and 50 kGy to study the impact of these sterilization methods on the physicochemical properties and bioactivity of the AM. Proton nuclear magnetic resonance and gel permeation chromatography were used to determine the chemical structure and molecular weight, respectively. Size and zeta potential of the micelles formed from AMs in aqueous media were evaluated by dynamic light scattering. Bioactivity of irradiated AMs was evaluated by measuring inhibition of oxidized low-density lipoprotein uptake in macrophages. From these studies, no significant changes in the physicochemical properties or bioactivity were observed after the irradiation, demonstrating that the AMs can withstand typical radiation doses used to sterilize materials.

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.

Storage stability was evaluated on a biodegradable salicylate-based poly(anhydride-ester) to elucidate the effects of storage conditions over time. The hydrolytically labile polymer samples were stored in powdered form at five relevant storage temperatures (-12 °C, 4 °C, 27 °C, 37 °C, 50 °C) and monitored over four weeks for changes in color, glass transition temperature, molecular weight, and extent of hydrolysis. Samples stored at lower temperatures remained relatively constant with respect to bond hydrolysis and molecular weight. Whereas, samples stored at higher temperatures displayed significant hydrolysis. For hydrolytically degradable polymers, such as these poly(anhydride-esters), samples are best stored at low temperatures under an inert atmosphere.

The role of hydration in degradation and erosion of materials, especially biomaterials used in scaffolds and implants, was investigated by studying the distribution of water at length scales from 0.1 nm to 0.1 mm using Raman spectroscopy, small-angle neutron scattering (SANS), Raman confocal imaging, and scanning electron microscopy (SEM). The measurements were demonstrated using L-tyrosine derived polyarylates. Bound- and free- water were characterized using their respective signatures in the Raman spectra. In the presence of deuterium oxide (D(2)O), H-D exchange occurred at the amide carbonyl but was not detected at the ester carbonyl. Water appeared to be present in the polymer even in regions where there was little evidence for N-H to N-D exchange. SANS showed that water is not uniformly dispersed in the polymer matrix. The distribution of water can be described as mass fractals in polymers with low water content (~5 wt%), and surface fractals in polymers with larger water content (15 to 60 wt%). These fluctuations in the density of water distribution are presumed to be the precursors of the ~ 20 μm water pockets seen by Raman confocal imaging, and also give rise to 10-50 μm porous network seen in SEM. The surfaces of these polymers appeared to resist erosion while the core of the films continued to erode to form a porous structure. This could be due to differences in either the density of the polymer or the solvent environment in the bulk vs. the surface, or a combination of these two factors. There was no correlation between the rate of degradation and the amount of water uptake in these polymers, and this suggests that it is the bound-water and not the total amount of water that contributes to hydrolytic degradation.

Fourteen explanted Dynesys® spinal devices were analyzed for biostability and compared with a reference, never implanted, control. Both poly(carbonate-urethane) (PCU) spacers and polyethylene-terephthalate (PET) cords were analyzed. The effect of implantation was evaluated through the observation of physical alterations of the device surfaces, evaluation of the chemical degradation and fluids absorption on the devices and examination of the morphological and mechanical features. PCU spacers exhibited a variety of surface damage mechanisms, the most significant being abrasion and localized, microscopic surface cracks. Evidence of oxidation and chain scission were detected on PCU spacers ATR-FTIR. ATR-FTIR, DSC and hardness measurements also showed a slight heterogeneity in the composition of PCU. The extraction carried out on the PCU spacers revealed the presence of extractable polycarbonate segments. One spacer and all PET cords visually exhibited the presence of adherent biological material (proteins), confirmed by the ATR-FTIR results. GC/MS analyses of the extracts from PET cords revealed the presence of biological fluids residues, mainly cholesterol derivatives and fatty acids, probably trapped into the fiber network. No further chemical alterations were observed on the PET cords. Although the observed physical and chemical damage can be considered superficial, greater attention must be paid to the chemical degradation mechanisms of PCU and to the effect of byproducts on the body.

Photochromic indolylfulgimides covalently attached to polymers have beneficial properties for optical switching. A 3-indolylfulgide and two 3-indolylfulgimides with one or two polymerizable styrene groups attached on the nitrogen atom(s) were synthesized. Copolymerization with methyl methacrylate (MMA) provided linear copolymers (one styrene group) or a cross-linked copolymer (two styrene groups). The properties of the monomers and copolymers in toluene or as thin films were characterized. The new copolymers were photochromic (reversible Z-to-C isomerization), absorbed visible light, and revealed good thermal and photochemical stability. At room temperature, all copolymer films showed no loss of absorbance after 5 weeks. At 80 °C in either toluene or as films, the Z-forms copolymers were less stable than the C-form copolymers, which showed little or no degradation after 400 h. The degradation rate due to repeated ring-closing - ring opening cycles was less than 3% per 100 cycles. The cross-linked copolymer showed photochemical stability comparable to monomeric fulgides in toluene, <1% per 100 cycles. In general, the properties of the linear and cross-linked copolymers were similar to the corresponding monomers in toluene. In films, the conformations of the Z-form were restricted due to the matrix indicating that the preparation of films from the C-form is advantageous.

In many cases of accelerated thermal aging of polymeric materials of electrical interest, for various Arrhenius lifelines corresponding to various properties of a given material, to various materials with the same endlife criterion, and to various endlife criteria for a given material, a linear correlation exists between the logarithm of the preexponential factor and the activation energy. An attempt is made to demonstrate that, in many important cases, the existence of such a correlation, called the compensation effect, results from the non-Arrhenian character of the kinetics. An experimental and a numerical example show that, for an aging process resulting from two successive steps, the non-Arrhenian kinetics leads to an apparent compensation effect. Another experimental and numerical example shows that, for a sequential process, an apparent compensation effect can also be observed in a comparative study of various materials of close composition

The products of pyrolysis, in the temperature range up to 440°C both in vacuum and in nitrogen, from an unsaturated polyester based on HET-acid (1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid), maleic anhydride and 1,2-propanediol have been analysed. The major products are HET-anhydride, hexachlorocyclopentadiene, pentachlorocyclopentadiene, tetrachlorocyclopentadiene, maleic anhydride and isomers of dimethyldioxane. An especially remarkable product is 1,2,3,4,7,7-hexachlorobicyclo[2.2.1]hepta-2,5-diene, the Diels-Alder adduct of hexachlorocyclopentadiene with acetylene. Further chlorinated maleic esters, esters containing the HET-acid unit, 5-hydroxy-α-pyrone, and an alkyl-substituted 5-hydroxy-α-pyrone are identified among the minor products of pyrolysis. Detailed mechanisms are proposed to explain the formation of the various products. For example, the formation of two dimethyldioxane isomers is explained via intermediate polyether formation.

Degradable bioelastomers represent a useful class of biomaterials. In this paper, a novel biodegradable network of elastomeric polyesters, poly((1,2-propanediol-sebacate)-citrate) (PPSC), was synthesized by condensation of 1,2-propanediol, sebacic acid and citric acid without any catalyst. An oligomeric diol of 1,2-propanediol-sebacate was first synthesized by carrying out a controlled condensation reaction between 1,2-propanediol and sebacic acid, and then a pre-polymer was synthesized by condensation of the diol and citric acid, whereat the pre-polymer was post-polymerized and simultaneously crosslinked in mold at 120 °C. A series of PPSC polymers were prepared at different post-polymerization times and different monomers' ratio. Tg confirms that PPSC is totally amorphous at 37 °C. The mechanical properties of PPSC testified that the new polymers are typical elastomers with low hardness and large elongation. The different post-polymerization times and monomers' ratio had strong influence on the degradation rates and mechanical performances. The material was expected to be useful for drug controlled delivery, tissue engineering scaffold and other biomedical applications.

The important polymer stabilizer, 1,2-bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyl)hydrazine, which serves a dual role as a metal deactivator and antioxidant, is shown to have crystal polymorphism. Although the published melting range is 225–232 °C, which is well above the processing temperature of many polymers in which it is used, existence of a second polymorph that transforms below 205 °C is demonstrated. This α polymorph, which is thermodynamically stable at room temperature, is thermodynamically un-favored at temperatures above about 176 °C. It is shown that under some conditions the α polymorph can endothermically pass directly into the melt state at temperatures below 205 °C, while under other conditions it undergoes a direct endothermic solid–solid transition to the higher melting β polymorph.The results highlight the potential importance of polymorphs for controlling polymer additive behavior and elucidate important phenomena relevant to dispersion of this additive in polymer compounds.

In order to study the relationship between structure and properties, multiblock copolymers composed of poly(butylene succinate) (PBS) and poly (1,2-propylene succinate) (PPSu) have been synthesized by chain-extension at various molar ratios of hexamethylene diisocyanate (HDI) to polyester-diols, which have been abbreviated as R-values in this paper. Molecular weights of soluble fractions, gel fractions and crosslink densities have been determined. Thermal properties, mechanical properties and biodegradability have been studied and correlated with R-values. Crystallization of copolymers becomes difficult with increasing R-value. Tensile strength, flexural strength and flexural modulus tend to increase with increasing R-value up to 1.2, and vary little when R-value increases from 1.2 to 1.3, then decrease with further increase in R-value. Impact strength achieves a maximum value at R-value of 1.3. Biodegradation rate reaches a minimum value when R-value is 1.1. Biodegradation has been studied systematically by attenuated total reflectance Fourier transform infrared (ATR-FTIR), 1H NMR and SEM.

This study synthesizes a series of cyclohexanedimethanol (CHDM)-based alicyclic/aliphatic copolyesters (PBSCs) using succinic acid, 1,4-butanediol and 1,3/1,4-CHDM at various molar ratios to investigate the effects of these compositions on crystallinity, biodegradability and the mechanical properties of PBSCs. The PBSCs were characterized using proton nuclear magnetic resonance, gel permeation chromatography, wide-angle X-ray diffraction, differential scanning calorimeter and thermogravimetric analysis. Biodegradability was evaluated by enzymatic hydrolysis with a lipase from Pseudomonas cepacia. The mechanical properties of PBSCs were determined using a tensile testing machine.Experimental results reveal that the PBSCs containing 1,3/1,4-CHDM in total diol with less than 50 mol% are crystallizable, while those containing 1,3/1,4-CHDM with more than 50 mol% are amorphous. The biodegradability test results suggest that PBSCs can be classified as surface-eroding polymers with a random endo-type scission. Surface hydrophilicity of PBSCs was the predominant effect on enzymatic hydrolysis, not crystallinity.

The radiation chemistry of the copolymer of tetrafluoroethylene and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (Teflon AF®) was investigated using γ-irradiation under vacuum. Two types of resin were studied which differed in dioxole content; AF1600 65 mol% dioxole and AF2400 87 mol% dioxole. The cyclic fluoroplastic was found to undergo predominant main chain scission upon radiolysis, both above and below the glass transition temperature, which was characterised by a decrease in the glass transition temperature. FTIR analysis showed the formation of new carboxylate end groups as well as terminal unsaturation. Both CF· and CF2·, radicals were identified using Electron Spin Resonance upon γ-radiolysis and subsequent thermal annealing. The G-value for radical production at 77 K, G(R), was 1.6 for both resins.

N-methylol compounds are used as a wrinkle-resistant finish in the textile industry. They are expected to enhance the resistance of wood to weathering because they can cross-link the cell wall and dimensionally stabilise wood. Scots pine veneers were modified with 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) to weight percent gains (WPG) of 10%, 27% or 48% and exposed to artificial weathering. Initially, weight losses of unmodified veneers were significantly greater than those of DMDHEU treated specimens even though DMDHEU was leached from wood at a higher rate than loss of wood substance. The weight losses of all treated veneers after 144 h of weathering, however, were similar to those of the unmodified controls. Therefore we conclude that in the short term DMDHEU treatment can restrict weight losses of wood during weathering, which occur due to degradation of lignin and hemicellulose and loss of degraded wood fragments from wood.Infrared spectroscopy suggested that treatment of wood veneers with DMDHEU to high WPG (48%) stabilised lignin to some extent. Tensile strength losses of DMDHEU treated veneers during weathering were lower than those of untreated veneers. DMDHEU treatment, however, had a deleterious effect on the tensile strength of the veneers, possibly associated with the presence of magnesium chloride catalyst in the treatment solution. Scanning electron microscopy revealed that DMDHEU treatment was highly effective at preventing the degradation of the wood cell wall during weathering. Tracheids in unmodified veneers became distorted within 48 h of weathering exposure, whereas cells in modified veneers, especially those reacted to higher weight percent gains, retained their shape even after 144 h weathering.

The accelerated ultraviolet aging behavior of poly(1,3,4-oxadiazole) fibers (POD fibers) exposed to artificial environment for different durations were studied. The influence of ultraviolet light on the intrinsic viscosity, structure, appearance and morphology, mechanical properties of POD fibers were investigated during aging by ATR-FTIR and UV-spectra, XPS, WXRD, SEM and tensile strength tester. The results revealed that the structure and properties of POD fibers were affected by UV light. Tensile strength and breaking elongation of POD fibers were severely decreased after 48 h UV light irradiation, and the change of intrinsic viscosity indicated that only degradation but not crosslink occurred. Disruption of oxadiazole rings and formation of carbonyl and amide were observed. UV aging process in nitrogen atmosphere suggested that the oxygen was indispensable and the essence of POD UV aging was photo-oxidation process. POD was amorphous and the recrystallization on surface was present after UV aging due to degradation. Morphology of POD fiber surface was damaged after UV aging.

Nowadays, poly(vinyl alcohol) (PVA) has caused serious pollution in the natural environment. To eliminate PVA pollution, PVA-degrading enzymes (PVADE) were studied. Previously our group has detected PVADE in a mixed microbial culture. In this study, it was found that 1,4-butanediol could enhance PVADE production. High PVADE activity (3.43 U ml−1), which was 4.6 folds of the control (0.75 U ml−1), was achieved with 1,4-butanediol as carbon source. Concomitantly, the average PVA-degrading rate improved 2.0 folds compared to the control. Specifically, diauxic growth coupled with increased PVA-degrading rate was observed. Based on this phenomenon, two-stage fermentation by adding another carbon source at a proper time was designed. By applying this strategy, high PVADE productivity (60.8 U l−1 h−1) was achieved. Further, the two-stage fermentation was extended to three-stage fermentation by adding PVA to improve PVADE production. The PVADE activity per unit biomass (YPVADE/x) was significantly enhanced over two-stage fermentation and the maximum increment was 418 U g−1.

A series of biodegradable aliphatic/aromatic copolyesters, poly(butylene terephthalate)-co-poly(butylene cyclohexanedicarboxylate)-b-poly(ethylene glycol) (PTCG), were prepared by a two-step melt polycondensation method and characterized by means of GPC, FTIR, NMR, DSC, TGA, etc. The effects of aliphatic ester content on the physical, mechanical and thermal properties, as well as in vitro and in vivo degradation behaviors were investigated. The decrease in mechanical strength was observed with an increase in poly(butylene cyclohexanedicarboxylate) (PBC) molar fraction. DSC results showed one melting point and two glass transition temperatures in all samples, and the melting temperature was found to go down gradually as more cyclohexanedicarboxylic acid (CHDA) was added. During the in vitro and in vivo degradation processes, erosion of the surface was dominant as evidenced by scanning electron microscopic observations. The copolyesters containing many CHDA units were featured by the higher water uptake and faster degradation due to much richer amorphous phase within them.

In order to improve our understanding of the reactions occurring during the photodegradation of poly(2,6-dimethyl-1,4-phenylene oxide) we analyzed the formed products which were eluted from the irradiated solid polymer material by three different chromatographic techniques, namely GC/MS, HPLC/MS and HPLC in combination with a diode array detector. We found that there are only a few types of chromophores, like xanthones, lactones and benzoquinones which are responsible for the yellowing. These coloured species and several types of colourless species each appear in a series of homologous compounds differing by one monomeric unit. In most cases it is only one monomeric unit which is modified, e.g. by oxidation of the methyl groups. In combination with the results obtained by time-resolved experiments (Schneider S, Richter F, Brem B, 1998, Polym Degrad and Stab 1998;61:453) several new conclusions can be drawn with respect to the mechanism of photodegradation in this polymer. The by far, most important primary photochemical process is the cleavage of the hydroxyl end group of the polymer. Secondary reactions lead to the scission of a nearby ether bond thus giving rise to the formation of homologous series of photoproducts. Since a variation of the spectral composition of the photolysis light did not result in a different product distribution, we conclude that the most important primary photochemical reaction steps are independent of excitation wavelength.

To evaluate the feasibility of poly(1,4-dioxan-2-one) (PPDO) as a feed stock recycling material, the pyrolysis kinetics of PPDO were investigated. The pyrolysis of PPDO exclusively resulted in the distillation of 1,4-dioxan-2-one (PDO). From thermogravimetric measurements conducted at different heating rates, the kinetic parameters of the pyrolysis: activation energy, Ea=127 kJ mol−1; order of reaction, n=0; and pre-exponential factor, A=2.3×109 s−1, were estimated by plural analytical methods. The estimates show that the decomposition of PPDO proceeds by unzipping depolymerization as main reaction and random degradation process with lower Ea and A values. Equivalent isothermal degradation curves calculated from the thermogravimetric curves were supported by experimental isothermal degradation data. The calculation that PPDO is converted smoothly into PDO at 270°C agrees with the reported ceiling temperature of PPDO.

The miscibility, morphology and biodegradability of binary blends of poly(3-hydroxy butyrate) (PHB) with poly(ε-caprolactone) (PCL), poly(1,4-butylene adipate) (PBA) and poly(vinyl acetate) (PVAc) have been studied by analysis of differential scanning calorimetry, mechanical properties, scanning electron micrography (SEM) and enzymatic degradation. The glass-transition temperature (Tg) data indicated that the PHB/PCL and PHB/PBA blends were immiscible in the amorphous state, while the PHV/PVAc blend was miscible. On the basis of the mechanical properties and SEM of the immiscible blends, it was suggested that the PHB/PCL blend has a macro-phase separated structure and the PHB/PBA blend has a modulated structure with a micro-phase separation. The enzymatic degradation of the PHB-based blend films was carried out at 37°C and pH 7·4 in an aqueous solution of an extracellular PHB depolymerase from Alcaligenes faecalis T1. The profiles of enzymatic degradation of PHB-based blends were strongly dependent upon the polymer component blended with PHB. In the case of PHB/PCL blend film, a complicated dependence of PCL weight fraction on the rate of enzymatic degradation was observed. In contrast, the weight loss of PHB/PBA or PHB/PVAc blend film by the PHB depolymerase decreased monotonously with increase in the weight fraction of PBA or PVAc. The enzymatic degradation data have been discussed in connection with the morphologies of PHB-based blend films.

Poly(2,6-dimethyl-1,4-phenylene oxide) (PPE) films were exposed to monochromatic radiation (254 nm, 365 nm) and to polychromatic radiation (λ300 nm, outdoor exposure) in the presence of atmospheric oxygen. Thermooxidation of PPE samples was carried out in the temperature range 100 °C–170 °C. Products formed in irradiated and thermooxidized films were identified by coupling FTIR spectroscopy to chemical derivatization reactions, physical treatments and gas phase analysis. The nature and the distribution of oxidation products are shown to be different in the two conditions of degradation.

The ratio of kinetic constants has been determined for the reaction of peroxy radicals with inhibitors of the 3-aniline-1,5-diphenylpyrazole series in the oxidation of cumene and ethylbenzene at 60 and 65°C. The chemiluminescence method and the solution calorimetry method yielded comparable results. The dependence of the ratio on the initial concentration of inhibitor is discussed.

Cellulose, the basic material of all plant substance, is also produced by green algae (Valonia) and some bacteria, principally of the generaAcetobacter, Sarcina andAgrobacterium. Special attention has been given to strains fromAcetobacter, speciallyAcetobacter xylinum.Acetobacter strains are well known for oxidizing alcohols to acids and ketones, especially for the production of vinegars using ethanol, wine or cider as carbon sources. The formation of the cellulose pellicle occurs on the upper surface of the supernatant film. Cellulose production was reported to be stimulated by addition of lactic acid, methionine, tea infusion and corn steep liquor. Although for the production process new non-conventional bioreactors have been developed, static cultures are still preferred. A large surface area is important for a good productivity. Relatively low glucose concentrations also gave better productivity and yields than higher ones. Bacterial cellulose can be applied in areas where plant cellulose can hardly be used. New applications were described as thickener to maintain viscosity in food, cosmetics, etc., as nonwoven fabric or paper for old document repair, as food additives and others. We could make use of cellulose films as a temporary substitute for human skin in the case of burns, ulcers, decubitus and others. Biofill®, Bioprocess® and Gengiflex® are products of microbial cellulose that now have wide applications in surgery and dental implants.

Chemiluminescence under isothermal and non-isothermal conditions has been used for a more detailed study of the oxidation of isotactic polypropylene stabilised with Irganox 1010 and Irganox 1076. Annealing of the stabilised films of polypropylene at lower temperatures prolongs the induction time of oxidation measured at higher temperatures. Arrhenius plots of induction time over a wide temperature interval have been derived from non-isothermal chemiluminescence runs.

The 3-dimensional structure of the Paucimonas lemoignei poly(3-hydroxybutyrate) (PHB) depolymerase PhaZ7 has significant similarity to Bacillus subtilis lipase LipA but differs from the latter by the presence of an additional domain. Analysis of this lid-like domain revealed the presence of many hydrophobic amino acid residues including Tyr105. In this study we constructed His-tag fusions of PhaZ7 for simplified purification and investigated the effect of amino acid exchange of eight tyrosine codons of the lid-like domain. Exchanges of Tyr103, Tyr172, Tyr173, Tyr203 or Tyr204 to alanine or serine had no phenotype but muteins with substitution of Tyr189, Tyr190 and Tyr105 to alanine showed a lag phase of the in vitro PHB depolymerase reaction. Replacement of Tyr105 by glutamate further increased the lag phase. Binding assays of the purified PHB depolymerase proteins with the natural substrate, native PHB granules, revealed a significantly reduced binding ability of the Tyr105Glu mutant compared to the wild type protein and confirmed that Tyr105 is involved in interaction with the polymeric substrate.