Effect of temperature and nanoparticle type on hydrolytic degradation of poly(lactic acid) nanocomposites

PLA and its nanocomposite films based on modified montmorillonite (CLO30B) or fluorohectorite (SOM MEE) and unmodified sepiolite (SEPS9) were processed at a clay loading of 5 wt% and hydrolytically degraded at 37 and 58 °C in a pH 7.0 phosphate-buffered solution. An effective hydrolytic degradation for neat PLA and nanocomposites was obtained at both temperatures of degradation, with higher extent at 58 °C due to more extensive micro-structural changes and molecular rearrangements, allowing a higher water absorption into the polymer matrix.The addition of CLO30B and SEPS9 delayed the degradation of PLA at 37 °C due to their inducing PLA crystallization effect and/or to their high water uptake reducing the amount of water available for polymer matrix hydrolysis. The presence of SOM MEE also induced polymer crystallization, but it was also found to catalyze hydrolysis of PLA. Concerning hydrolysis at 58 °C, the presence of any nanoparticle did not significantly affect the degradation trend of PLA, achieving similar molecular weight decreases for all the studied materials. This was related to the easy access of water molecules to the bulk material at this temperature, minimizing the effect of polymer crystallinity clay nature and aspect ratio on the polymer degradation.     

Thermal and weathering degradation of poly(propylene carbonate)

High molecular-weight poly(propylene carbonate) (PPC) can remain intact upon storage in ambient air or in water for 8 months once the catalyst is completely removed. Catalyst-free pure PPC is also thermally stable below 180 °C. At 200 °C, degradation occurs, mainly due to attack of the chain-ended hydroxyl group onto a carbonate linkage, through which the molecular weight distribution is broadened by simultaneous formation of low and high molecular weight fractions. Incomplete removal of hydrogen peroxide generated during the catalyst preparation results in a prepared polymer that contains a substantial amount of polymer chains grown biaxially from hydrogen peroxide, which gives rise to more severe thermal degradation. Experiments conducted in a weathering chamber at high temperature (63 °C) and high humidity (50%) revealed another degradation process involving chain scission through an attack of water molecules onto the carbonate linkage, which progressively and temporally lowers molecular weight.     

Investigation of the influence of different commercial softeners on the stability of poly(lactic acid) fabrics during storage

We have studied the potential degradation of poly(lactic acid)-based fabrics treated with commercial softeners and stored under two sets of conditions for one year. Initial wet-processing caused a fall in molecular weight of about 28%, irrespective of after-treatment. Storage at 40 °C and 80% RH produced further degradation which, with few exceptions, was aggravated by the presence of softeners. Ultimately, all samples degraded beyond the point of commercial usefulness. No clear distinction could be made between the effects of softeners having differing compositions. In contrast, fabrics stored under milder conditions of 23 °C and 50% RH showed no significant time-dependent polymer degradation, irrespective of the treatment applied. There were slight changes in tensile properties and some evidence of physical structural effects having occurred, which we attribute to physical aging. However, we do not believe these to be so serious as to call into question the long-term viability of PLA-based textile products.     

Low-temperature oxidative degradation of PBX 9501 and its components determined via molecular weight analysis of the Poly[ester urethane] binder

The results of following the oxidative degradation of a plastic-bonded explosive (PBX 9501) are reported. Into over 1100 sealed containers were placed samples of PBX 9501 and combinations of its components and aged at relatively low temperatures to induce oxidative degradation of the samples. One of the components of the explosive is a poly(ester urethane) polymer and the oxidative degradation of the samples were following by measuring the molecular weight change of the polymer by gel permeation chromatography (coupled with both differential refractive index and multiangle laser light scattering detectors). Multiple temperatures between 40 and 64 °C were used to accelerate the aging of the samples. Interesting induction period behavior, along with both molecular weight increasing (crosslinking) and decreasing (chain scissioning) processes, were found at these relatively mild conditions. The molecular weight growth rates were fit to a random crosslinking model for all the combinations of components. The fit rate coefficients show Arrhenius behavior and activation energies and frequency factors were obtained. The kinetics of molecular weight growth shows a compensatory effect between the Arrhenius prefactors and activation energies, suggesting a common degradation process between PBX 9501 and the various combinations of its constituents. An oxidative chemical mechanism of the polymer is postulated, consistent with previous experimental results, that involves a competition between urethane radical crosslinking and carbonyl formation.     

A study of the surface morphology of poly(p-phenylene terephthalamide) chars using scanning probe microscopy

The objective of this work was to investigate the changes in surface morphology associated with thermal degradation of poly(p-phenylene terephthalamide) (PPTA) into chars. To this end, PPTA samples decomposed at several temperatures up to 800 °C were studied on a local scale using atomic force microscopy (AFM) and scanning tunnelling microscopy (STM). Domains with a diameter of 40-50 nm started appearing among PPTA nanofibrils at about 500 °C. At this temperature and above, a film coating the fibre developed. This layer was much less rigid than PPTA, and remained deposited on the fibres, even at high temperatures. At 800 °C, the STM images showed a surface distribution typical of a carbonaceous material, isotropic although somewhat heterogeneous. When an intermediate isothermal step (500 °C, 200 min) was introduced along with heat treatment of PPTA under a constant rate, the material obtained at the end of this step was conductive enough to be studied by STM. Although the coating over the fibres also remained after the isothermal step, it was less homogeneous than in the absence of isothermal treatment. On further heating, the residue exhibited a surface morphology typical of a carbonaceous material, but much more homogeneous and isotropic than in the absence of the isothermal step.     

Studies of the hydrolytic stability of poly(urethane-urea) elastomers synthesized from oligocarbonate diols

Polyurethane and poly(urethane-urea) elastomers synthesized from oligocarbonate diols are characterized by very good mechanical properties, biocompatibility and excellent resistance to both oxidation and hydrolysis and therefore are widely used in medical applications. In this paper the results of studies on hydrolytic stability of poly(urethane-urea) elastomers (PURC) obtained by moisture-curing of corresponding urethane prepolymers synthesized from isophorone diisocyanate (IPDI) and four different oligocarbonate diols (OCD) are presented. OCD were synthesized from dimethyl carbonate and 1,6-hexanediol, from cyclic ethylene carbonate and 1,6-hexanediol as well as from trimethylene carbonate. The changes of the sample weight, mechanical properties and surface properties after immersion in a standard phosphate buffer solution (pH = 7.4) for up to 3 months at 70 °C were monitored. It was shown that neither sample weight nor mechanical properties changed significantly for PURC obtained from OCDs synthesized from 1,6-hexanediol and dimethyl carbonate or ethylene carbonate what confirms good resistance to hydrolysis of those PURC. Also SEM studies of those samples before and after immersion did not reveal any surface degradation effects. However, PURC sample obtained from OCD synthesized from trimethylene carbonate showed significant changes in mechanical properties and distinct change of appearance and surface erosion after 3 months immersion. The initial decrease and later increase of stress at break observed for PURC samples during immersion, was explained by the reaction of residual NCO groups present in PURC with water leading to molecular weight increase which proceeded during immersion period in parallel to hydrolysis of carbonate bond.     

Solid state polymerization of poly(lactic acid): Some fundamental parameters

Poly(lactic acid) (PLA) was submitted to solid state polymerization (SSP) in a fixed bed reactor under nitrogen flow, so as to examine technique efficiency for increasing the molecular weight and hence permitting the reduction of the melt polymerization residence times. In order to use a suitable starting material, SSP prepolymers of low and medium molecular weight were first prepared through solid state hydrolysis of commercial PLA grade under acidic and alkaline conditions. During these degradation runs, hydrolysis involved the random scission of ester groups in the polymer backbone, while the relevant kinetics and the resulting thermal properties were also examined. In a subsequent step, the prepolymers obtained were subjected to SSP at three temperatures, approximately 2.5–25.0 °C below their melting point. The process achieved an increase of up to 1.7 times the initial molecular weight, however, with different trends depending on the prepolymer characteristics, reaction temperature and time, as well as the pH of the hydrolysis medium. In addition to molecular weight build up, the effect of the SSP process on end product thermal properties was also investigated.     

Recycling of waste poly(ethylene terephthalate) with castor oil using microwave heating

The chemical recycling of waste poly(ethylene terephthalate) (PET) using castor oil (CO) as a reagent is reported. CO presents a renewable alternative to petrochemical based reagents, e.g. glycols, and enables also substantial modification of final physico-chemical properties of a received product. Advantageously, microwave irradiation was used to accelerate the depolymerization of PET. A composition of obtained product was strongly influenced by the reaction temperature. When the decomposition of PET was performed at temperature higher than 240 °C, then a significant extent of side products based on PET oligomers and transesterified CO was observed due to dehydration and hydrolysis of CO. Contrary to that, PET decomposition took place at slow rate below 230 °C and the optimal reaction temperature lies in the relatively narrow interval from 230 °C to 240 °C. The product prepared in the optimal temperature range did not contain any high molecular weight PET oligomers. MALDI-TOF mass spectrometry enabled to identify the structures included in the obtained polyol product. The maximum number of six repeating monomeric unit of PET was found in the product, which confirmed practically the complete depolymerization of PET chain and good reactivity of the acylester hydroxyl groups of CO.     

The effect of different storage temperatures on the physical properties of pectin solutions and gels

The stability (in terms of viscosity and gel strength) of pectin solutions and gels potentially plays an important role in their behaviour and functional properties in a wide range of applications and therefore any changes over time must be understood. The gel strength of pectin gels and intrinsic viscosity of pectin solutions at different temperatures (4 °C, 25 °C and 40 °C) have been investigated using a “rolling ball” viscometer and a texture analyser respectively. Both the intrinsic viscosity ([η]) and gel strength decrease with increased storage time, although this more pronounced at elevated temperatures. The changes in intrinsic viscosity with storage time and temperature were used to determine the depolymerisation constant (k). Pectin storage conditions and particularly temperature have an influence on depolymerisation, particularly elevated storage temperatures, but whether or not this will be detrimental to its intended application will depend on the functional significance of the changes that occur. In this case based on the previous diffusion studies on a model drug (paracetamol) we conclude that the decreases in viscosity and gel strength within the range observed have no detrimental effect on the drug release properties.     

The influence of biotic and abiotic factors on the rate of degradation of poly(lactic) acid (PLA) coupons buried in compost and soil

Poly(lactic) acid (PLA) is a compostable biopolymer and has been commercialised for the for the manufacture of short-shelf life products. As a result, increasing amounts of PLA are entering waste management systems and the environment; however, the degradation mechanism is unclear. While hydrolysis of the polymer occurs abiotically at elevated temperature in the presence of water, potential catalytic role for microbes in this process is yet to be established. In this study, we examined the degradation of PLA coupons from commercial packaging at a range of temperatures (25°, 37°, 45°, 50° and 55 °C) in soil and compost and compared with the degradation rates in sterile aqueous conditions by measuring loss of tensile strength and molecular weight (Mw). In addition, in order to assess the possible influence of abiotic soluble factors in compost and soil on degradation of PLA, degradation rates in microorganism-rich compost and soil were compared with sterile compost and soil extract at 50 °C. Temperature was determined to be the key parameter in PLA degradation and degradation rates in microorganism-rich compost and soil were faster than in sterile water at temperatures 45° and 50 °C determined by tensile strength and Mw loss. Furthermore, all tensile strength was lost faster after 30 and 36 days in microorganism-rich compost and soil, respectively, than in sterile compost and soil extract, 57 and 54 days, respectively at 50 °C. Significantly more Mw, 68% and 64%, was lost in compost and soil, respectively than in compost extract, Mw, 53%; and in soil extract, 57%. Therefore, degradation rates were faster in microorganism-rich compost and soil than in sterile compost and soil extract, which contained the abiotic soluble factors of compost and soil at 50 °C. These comparative studies support a direct role for microorganisms in PLA degradation at elevated temperatures in humid environments. No change in tensile strength or Mw was observed either 25° or 37 °C after 1 year suggesting that accumulation of PLA in the environment may cause future pollution issues.     

Effect of radiation processing on nutritional and sensory quality of minimally processed green gram and garden pea sprouts

In the present study, radiation processing of minimally processed green gram and garden pea sprouts was carried out at doses 1 and 2 kGy. The effect of this treatment on different quality parameters like vitamin C content, total carotenoids content, sensory quality, texture, and color was determined over a storage period of 12 days at two different temperatures, a 4 and 8 °C. It was observed that treatment of irradiation (1 and 2 kGy) and storage period did not have any significant effect on vitamin C content of control as well as irradiated sprout samples stored at 4 and 8 °C. Total carotenoids content of sprouts stored at 4, as well as at 8 °C, for 12 days remained almost unchanged after irradiation as well as during storage. Sensory evaluation studies showed that irradiation had no significant effect (p>0.05) on the ratings of any of the sensory attributes in green gram as well as garden pea sprouts and, thus, did not alter the overall acceptability of the irradiated sprouts. Textural studies revealed that there was no significant change (p>0.05) in the firmness of irradiated sprouts (1 and 2 kGy) as compared to control samples at both the temperatures. Storage period of 12 days also did not affect the firmness of sprouts significantly. Color measurement results indicated no drastic change in the color coordinates of the green gram samples except greenness of controls stored at both the temperatures, which showed insignificant decrease in the a^* values. Thus, the nutritional as well as sensory quality of minimally processed green gram and garden pea sprouts did not alter significantly after gamma irradiation with a dose of 1 and 2 kGy.     

Heat-resistant and soluble fluorinated poly(amide imide)s based on non-coplanar ortho-linked diimide-dicarboxylic acid

A series of novel fluorinated heat-resistant poly(amide imide)s (PAIS) based on non-coplanar diimide-diacid monomer (DIDA) were synthesized and characterized. The poly(amide imide)s were obtained in high yields and possessed inherent viscosities in the range of 0.47-0.91 dL g⁻¹. All of the polymers were amorphous in nature, showed outstanding solubility and could be readily soluble in common organic solvents such as N,N-dimethylacetamide, N-methyl-2-pyrrolidinoned, dimethyl sulfoxide, N,N-dimethylformamide, pyridine and tetrahydrofuran. Glass transition temperatures were in the range of 221-263 °C, as determined by differential scanning calorimetry. Degradation temperatures for 10% weight loss occurred all above 520 °C and char yields was more than 50% at 700 °C in nitrogen atmosphere. Moreover, these PAIs possessed low refractive indexes (n = 1.58-1.59), birefringence (Δn = 0.008-0.013), and dielectric constants (? ≈ 2.5) due to the trifluoromethyl pendent groups and ortho-catenated aromatic rings that interrupt chain packing and increase free volume.     

Loss of selenium in water samples at natural levels during storage

Selenium losses in river, ground, snow-melt and tap water samples, and the recovery of selenite, selenate and selenomethionine added to purified water have been studied. In 1-litre high-density polyethylene bottles, tap, river and snow-melt water samples (at Se concentrations of 44.5–138 ng/l) could be stored at 4 °C for up to 15 days without Se losses. In similar samples stored at room temperature Se losses of 13–25% after 15 days were found, except for groundwater, which showed no Se losses during storage for 13 months at room temperature or at 4 °C. Selenite and selenate added to purified water were recovered without losses after 15 days at 4 °C, while 7.5% of selenomethionine was lost. The stability of different chemical forms of Se during storage followed the order: selenate > selenomethionine > selenite. It is recommended that unacidified water samples should not be kept in polyethylene bottles at room temperature for more than 1 week, nor stored at 4 °C for more than 2 weeks, before analysis for Se.     

Potentiometric Determination of the First Hydrolysis Constant of Gallium(III) in NaCl Solution to 100°C

The hydrolysis equilibrum of gallium (III) solutions in aqueous 1 mol-kg^–1 NaCl over a range of low pH was measured potentiometrically with a hydrogen ion concentration cell at temperatures from 25 to 100°C at 25°C intervals. Potentials at temperatures above 100°C increased gradually because of further hydrolysis of the gallium(III) ion, followed by precipitation. The results were treated with a nonlinear least-squares computer program to determine the equilibrium constants for gallium(III)–hydroxo complexes using the Debye–Hückel equation. The log K (mol-kg^–1) values of the first hydrolysis constant for the reaction, Ga³⁺ + H₂O GaOH²⁺ + H⁺ were –2.85 ± 0.03 at 25°C, –2.36 ± 0.03 at 50°C, –1.98 ± 0.01 at 75°C, and –1.45 ± 0.02 at 100°C. The computed standard enthalpy and entropy changes for the hydrolysis reaction are presented over the range of experimental temperatures.     

Unusual change in molecular weight of polyhydroxyalkanoate (PHA) during cultivation of PHA-accumulating Escherichia coli

This study aimed to investigate the factors affecting molecular weight of poly[(R)-3-hydroxybutyrate] [P(3HB)] when polyhydroxyalkanoate (PHA) synthase (PhaRC_Bsp) from Bacillus sp. INT005 was used for P(3HB) synthesis in Escherichia coli JM109. It was found that the molecular weight of P(3HB) decreased with time in mid- and late-phase of culture and was strongly affected by culture temperature. At 37 °C culture temperature, the molecular weight of P(3HB) rapidly decreased from 4.4 × 10⁵ to 4.8 × 10⁴ with culture time, whereas it was almost unchanged at 25 °C. Kinetic analysis suggested that the decrease in molecular weight of P(3HB) was due to random scission of the polymer chain. The decrease in molecular weight of P(3HB) was not observed when PHA synthases other than PhaRC_Bsp were expressed. This study sheds light on the unique behaviour in molecular weight change of P(3HB) that is synthesized by E. coli expressing PhaRC_Bsp.     

Shelf-life extension of convenience meat products sold in Indian supermarkets by radiation processing

A variety of ready-to-cook meat products available in Indian supermarkets (mutton mince, chicken mince, chicken chunks, and chicken legs) were studied. The samples were irradiated (2.5 kGy), or left untreated as control, and stored at 0–3 °C for up to 21 days. The effect of irradiation on the microbiological, chemical, and sensory properties was evaluated at intervals during the storage period. Irradiated samples had a longer shelf-life at 0–3 °C compared with the corresponding non-irradiated samples. Fecal coliforms were eliminated by irradiation treatment. Radiation processed samples had lower counts of Staphylococcus spp. There were no significant organoleptic changes in irradiated samples stored at chilled temperatures.     

Preservation of As(III) and As(V) in some water samples

This paper reports on the behavior of arsenite [As(III)] and arsenate [As(V)] in some water samples at storage under several conditions (pH=2/natural pH, 4°C/20°C). The investigation was carried out using⁷³As as a radiotracer for both forms and with the aid of earlier developed simple speciation methods for differentiation between arsenite and arsenate. Although arsenate is the thermodynamically stable arsenic form, it was observed that arsenate in deionized water is completely converted to the trivalent state; this phenomenon took place in about one week. By monitoring the radioactive As(III) and As(V) over a period of one month in two natural water samples, a fresh water and a sea water sample, it could be concluded that no adsorption occurs on the surface of polyethylene containers, independent of storage conditions. During that period, storage at natural pH values results for both water samples in a gradual oxidation of As(III); the oxidation rate is higher for storage at 20°C. At pH=2 As(III) is fairly stable in fresh water at both storage temperatures. However, in sea water a fast oxidation of As(III) is observed (complete oxidation within 3 d at both temperatures). As(V) is stable at all storage conditions studied.     

Stabilization of poly(lactic acid) by polycarbodiimide

Polycarbodiimide (CDI) was used to improve the thermal stability of poly(l-lactic acid) (PLA) during processing. The properties of PLA containing various amounts of CDI were characterized by GPC, DSC, rheology, and tensile tests. The results showed that an addition of CDI in an amount of 0.1-0.7 wt% with respect to PLA led to stabilization of PLA at even 210 °C for up to 30 min, as evidenced by much smaller changes in molecular weight, melt viscosity, and tensile strength and elongation compared to the blank PLA samples. In order to examine the possible stabilization mechanism, CDI was reacted with water, acetic acid, l-lactic acid, ethanol and low molecular weight PLA. The molecular structures of the reaction products were measured with FTIR. The results showed that CDI could react with the residual or newly formed moisture and lactic acid, or carboxyl and hydroxyl end groups in the PLA samples, and thus hamper the thermal degradation and hydrolysis of PLA.     

Thermal degradation behavior of poly(4-hydroxybutyric acid)

Thermal degradation behavior of poly(4-hydroxybutyric acid) (P(4HB)) was investigated by thermogravimetric and pyrolysis-gas chromatography mass spectrometric analyses under both isothermal and non-isothermal conditions. Based on the thermogravimetric analysis, it was found that two distinct processes occurred at temperatures below and above 350 °C during the non-isothermal degradation of P(4HB) samples depending on both the molecular weight and the heating rate. From ¹H NMR analysis of the residual P(4HB) molecules after isothermal degradations at different temperatures, it was confirmed that the ω-hydroxyl chain-end was remained unchanged in the residual P(4HB) molecules at temperatures below 300 °C, while the ω-chain-end of P(4HB) molecules was converted to 3-butenoyl units at temperatures above 300 °C. In contrast, the majority of the volatile products evolved during thermal degradation of P(4HB) was γ-butyrolactone regardless of the degradation temperature. From these results, it is concluded that during the thermal degradation of P(4HB), the selective formation of γ-butyrolactone via unzipping reaction from the ω-hydroxyl chain-end predominantly occurs at temperatures below 300 °C. At temperatures above 300 °C, both the cis-elimination reaction of 4HB unit and the formation of cyclic macromolecules of P(4HB) via intramolecular transesterification take place in addition to unzipping reaction from the ω-hydroxyl chain-end. Finally, the primary reaction of thermal degradation of P(4HB) at temperatures above 350 °C progresses by the cyclic rupture via intramolecular transesterification of P(4HB) molecules with a release of γ-butyrolactone as volatile product. Moreover, we carried out the thermal degradation tests for copolymer of 93 mol% of 4HB with 7 mol% of 3-hydroxybutyric acid (3HB) to examine the effect of 3HB units on thermal stability of P(4HB).     

New poly(arylene ether)s with pendant phosphonic acid groups

Soluble brominated poly(arylene ether)s containing mono‐ or dibromotetraphenylphenylene ether and octafluorobiphenylene units were synthesized. The polymers were high molecular weight (weight‐average molecular weight = 115,100–191,300; number‐average molecular weight = 32,300–34,000) and had high glass‐transition temperatures (>279 °C) and decomposition temperatures (>472 °C). The brominated polymers were phosphonated with diethylphosphite by a palladium‐catalyzed reaction. Quantitative phosphonation was possible when 50 mol % of a catalyst based on bromine was used. The diethylphosphonated polymers were dealkylated by a reaction with bromotrimethylsilane in carbon tetrachloride followed by hydrolysis with hydrochloric acid. The polymers with pendant phosphonic acid groups were soluble in polar solvents such as dimethyl sulfoxide and gave flexible and tough films via casting from solution. The polymers were hygroscopic and swelled in water. They did not decompose at temperatures of up to 260 °C under a nitrogen atmosphere. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3770–3779, 2001