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ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Perspective experimental times are usually necessary to obtain meaningful results. Since mass loss measurements alone are not easy to interpret or extrapolate, it is desirable to combine this method with some of the other analytical techniques described below.80 CO2 Evolution. CO2 is the ultimate fate of carbon under aerobic polymer degradation conditions81−84 (although polyesters can produce some CO2 under anaerobic con- ditions).85,86 Its formation is frequently used as an indicator of biological degradation. In anaerobic conditions, soluble carbon compounds are metabolized by methanogens or sulfate reducers, producing CH4 and CO2, respectively.3,4 The polymer degradation rate is inferred by measuring the CO2 liberated during abiotic or biotic mineralization in a controlled environment.87−89 CO2 can be quantified by trapping and titration methods90 or by analytical techniques such as gas chromatography with thermal conductivity detection (GC- TCD)84 and IR spectroscopy.91 The CO2 yield is defined as in eq 1: degradation, cause changes in the surface energy, which are reflected in the contact angle with liquids.108,109 Hydrophilic surfaces, with their high wettability, have higher surface energies and give lower contact angles with water. Thus, formation of polar functional groups in polymers due to environmental weathering effects (e.g., UV exposure) results in a decrease in contact angle. Increased hydrophilicity promotes the attachment of microorganisms to the polymer surface, further accelerating the degradation rate.110 Assessing Changes in Materials Properties. Dynamic Mechanical Analysis (DMA). This technique is typically used to characterize polymer strength. Changes in the tensile strength and elongation at break are also indicators of physical deterioration during polymer degradation.111,112 Changes in these mechanical properties are associated with the formation of cracks and pores at the surface, as well as a reduction in molecular weight.113 Thermal Analysis. This method generally involves heating or cooling a sample at a controlled rate while monitoring its physical characteristics.114−116 Differential scanning calorim- etry (DSC) measures heat capacity (Cp), melting temperature (Tm), and glass transition temperature (Tg).117 A decrease in Tg during polymer degradation results from a decrease in the average chain length, due to the higher motility of shorter chains.118 Thermal gravimetric analysis (TGA) records mass changes that occur upon heating. When coupled with product analysis, this method can provide information on the nature of the decomposition, such as oxidation or loss of volatiles, during thermal degradation.119 Surface Analysis. Surface modification of polymers during degradation can be detected with scanning electron micros- copy (SEM) and atomic force microscopy (AFM).120−122 These methods can directly image topographical changes at the polymer surface, such as the formation of holes and cracks, increases in roughness, or even attachment of microbes to the surface.123 Typical changes in polymer morphology are visible as cracks and cavities; surface degradation and deterioration of HDPE films can be seen after 6 months in the marine e■nvironment.124 RATES OF PLASTICS DEGRADATION AND EXTRAPOLATED LIFETIMES Describing Degradation Rates. In the literature, the term degradation may include depolymerization, chemical modification, alteration of physical properties, overall mass loss by any and all mechanisms, or complete mineralization to CO2 and H2O. For the purposes of this study, we limit the definition of degradation to overall mass loss from the initial polymer piece. However, it must be noted that this definition is most appropriate for large plastic pieces. Recent studies suggest that surface ablation may be important for small plastic pieces in the marine environment.125 Loss of microplastic or nanoplastic fragments reduces the initial mass, without changing the total amount of plastic present. The polymer degradation rate, rd, is the differential mass loss per unit time n−n CO (%) = CO2,test CO2,control 2 nCO2,theoretical × 100% where nCO2,test is the total accumulated amount of CO2 product from polymer degradation, nCO2,control is the amount of CO2 released in a blank experiment, and nCO2,theoretical is the total amount of CO2 that would be liberated by complete mineralization of the polymer sample.92 However, at short time scales, it is unlikely that polymer carbon is fully oxidized to CO2. Therefore, the use of CO2 evolution as a probe to measure the kinetics of polymer degradation must be applied with caution. Gel Permeation Chromatography (GPC). This method reveals changes in the molecular weight, an important parameter in polymer degradation, by size exclusion.89,93,94 Reduction in the molecular weight of partially degraded polymers has been observed during both biotic and abiotic degradation processes, which increase the concentration of chain ends and can lead to mineralization of the smaller polymer chains.27 GPC requires the polymer to be dissolved in a carrier solvent, which for polyolefins requires high temper- atures. Care must be taken to ensure that dissolution of the polymer or the high-temperature measurement conditions do not cause further degradation.95 Assessing Changes in Chemical Functionality. Chem- ical Analysis. Nuclear magnetic resonance (NMR) and infrared (IR) spectroscopies can readily detect the presence and concentration of certain functional groups in polymers, even at low concentrations.96−99 For example, 13C CP/MAS NMR reveals the formation of cross-linked polymer chains (via a peak at 39.7 ppm) upon γ-irradiation of HDPE.100 IR is particularly valuable for detecting polar functional groups, such as ketones and ester carbonyls (intense peaks at ca. 1715 and 1735 cm−1, respectively), which are typical of oxidative degradation pathways.101−104 The extent of oxidation during degradation can be quantified from the change in absorbance for the carbonyl stretch relative to the C−H stretching modes,105,106 although a recent study suggests that this Carbonyl Index may be less accurate than other modes, such as methyl deformation in the case of PP, to quantify the extent of oxidation.107 Contact Angle. Changes in the surface density of polar functional groups, for example, those formed during oxidative dm rd=−dt =k·SA (2) (1) 3498 Since degradation occurs principally at exposed surfaces, we assume the degradation rate to be proportional to the surface area SA and the rate constant k to have dimensions kg s−1 m−2. Therefore, the rate of degradation depends not only on the https://dx.doi.org/10.1021/acssuschemeng.9b06635 ACS Sustainable Chem. Eng. 2020, 8, 3494−3511PDF Image | Degradation Rates of Plastics in the Environment
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