Polymer degradation: a short review

Introduction

Degradation is the act of reducing something or someone to a less respected state. In polymer science, degradation generally refers to a complex process by which a polymeric material exposed to the environment and workload loses its original properties (Horie et al., 2004). Degradation is usually an unwanted process. In certain cases, however, controlled polymer degradation is useful. For example, it can improve the processability of the polymer or it can be used in the recycling or natural decomposition of waste polymer (Jellinek, 1978). In the vast majority of cases, the cleavage of macromolecules is an integral part of the polymer degradation. Therefore, in a narrower sense, the very process of cleaving macromolecules into fragments of various structures and sizes is often referred to as polymer degradation (Jellinek, 1978). However, if monomer(s) are the final product, the cleavage is depolymerization, since it is the opposite of polymerization (Horie et al., 2004; Penczek et al., 2008) (Scheme 1).

Scheme 1:

Relation among polymerization, depolymerization and degradation.

Degradation in which the cleavage of macromolecules is not the dominant process is commonly referred to as polymer aging or corrosion (Jellinek, 1978). Degradation induced by organisms or isolated enzymes or by a concerted action of natural environment is called biodegradation (Vert et al., 2020; Webb, Arnott, Crawford, & Ivanova, 2013).

Depolymerization

Like any other chemical reaction, depolymerization takes place only if it leads to a decrease in the Gibbs energy of the system. If the polymer is formed by polymerization without a side product and polymerization entropy ΔS_p < 0, it depolymerizes only at temperatures above ceiling temperatureT_c ≅ ΔH_p/ΔS_p (ΔH_p < 0 is polymerization enthalpy). Rarely, such as in polymerization of sulfur S₈, ΔS_p > 0 and also ΔH_p > 0, and thus depolymerization prevails below so-called floor temperatureT_f ≅ ΔH_p/ΔS_p (Penczek et al., 2008). If the polymer is formed by polycondensation such as polyesterification and polyamidation, the byproduct is involved in the polymer-monomer(s) equilibrium and, if present (added), it promotes depolymerization. However, the temperature is of key importance even in these cases.

The ideal chain depolymerization can proceed in a living polymerization system where each macromolecule has the end-group with an active center (radical, ionic). As the activation Gibbs energy of propagation as well as depropagation is low, such system responses to the increase in temperature by the gradual release of monomer molecules from active ends of living chains, until the new equilibrium between the polymer and monomer molecules is reached. This is so-called chain-end cleavage mode frequently dubbed unzipping. Eventual subsequent temperature return results in reintegration of monomer molecules into macromolecules. The almost ideal stepwise depolymerization takes place in the hydrolysis of the water-soluble polyester in a dilute aqueous solution (reverse of polyesterification). In this case, each ester bond can be cleaved in the next time interval with the same probability, which is the random cleavage mode (Jellinek, 1978; Montroll & Simha, 1940; Vohlídal, 1994).

Initiated cleavage (degradation) of macromolecules

A polymer without active centers is currently stable at temperatures significantly above T_c, because of high activation Gibbs energy of the centers formation. It is thermodynamically unstable, but kinetically stable. However, the polymer starts to degrade when active centers are formed in it: after initiation. The dominant active centers of degradation are radicals that are created in a polymer thermally, photochemically, mechanochemically or by an oxidation process. The type of initiation is used as adjective denoting the type of polymer degradation.

Oxidative degradation

Oxidative degradation is the branched radical chain reaction that always participates in corrosion of polymers exposed to air. It can be initiated intentionally by processes indicated above, or it can occur spontaneously - then it is called autoxidation. The latter is typical of conjugated polymers that contain delocalized free radicals (Bondarev, Zednik, Plutnarova, Vohlídal, & Sedláček, 2010; Vohlídal, Rédrová, Pacovská, & Sedláček, 1993). Although these radicals are stabilized by delocalization to such an extent that they do not react with molecules, they easily react with the atmospheric oxygen, each molecule of which contains two unpaired electrons in the ground state. Because this ground state is the triplet state, the current “low-reactive” form of oxygen is called triplet oxygen (Guillet, 1985).

A typical oxidative degradation shows a long lasting induction period in which the hydroperoxyl side groups are gradually formed and accumulated in the polymer. The –OOH groups only negligibly contribute to oxidation in this reaction stage, since the first-order rate constant of a hydroperoxide dissociation at 30 °C is of the order of 10⁻⁸ s⁻¹. However, if concentration of –OOH groups is increased enough, their more energy-efficient bimolecular decomposition begins. This strongly accelerates the overall degradation rate and, as a result, the polymer rapidly loses its original properties. Therefore, one can usually see almost no change in the polymer quality for a few or many years and then a rapid decrease in the quality within a few weeks or months. In contrast, conjugated polymers of monosubstituted acetylenes degrade without a visible induction period when exposed to air though they are long-time stable in vacuum or under inert atmosphere (Kabátek, Gas, & Vohlidal, 1997; Vohlídal, Kabatek, Pacovska, Sedláček, & Grubisic-Gallot, 1996).

In oxidative degradation, macromolecules are mostly cleaved by the β-fission processes (see Scheme 2) to fragments with aldehyde, ketone and vinyl end-groups (Dan & Guillet, 1973; Norrish & Bamford, 1937, 1938). Other groups abundant in the degradation products are –OH groups formed by radical-transfer reactions such as P–O* + P–H → P–OH + P*. The molar absorption coefficients of the IR bands characteristic of carbonyl groups (at around 1 700 cm⁻¹) are mostly very high. Therefore, the IR spectra are widely used for monitoring the oxidative polymer degradations. Another phenomenon accompanying these degradations is chemiluminescence. The fact that its intensity is proportional to the square of the concentration of peroxyl radicals indicates that it originates from disproportionation of P–OO* radicals (Scheme 3). The energy released in this reaction (>400 kJ/mol) fairly exceeds the energy needed to electronically excite keto groups (ca 300 kJ/mol). The excited keto groups then emit radiation during their transitions to the ground state. Moreover, a significant portion of the oxygen formed in the disproportionation of –OOH groups is in the electronically excited singlet state that is extremely reactive. It is so-called singlet oxygen which immediately enters reactions giving new peroxides and peroxyl radicals, so accelerating the overall polymer degradation (Jellinek, 1978).

Scheme 2:

Main reactions participating in the polymer oxidative degradation.

Scheme 3:

Disproportionation of hydroperoxyl radicals – source of singlet oxygen and luminescence.

Oxidative polymer degradation is generally monitored by various spectroscopic methods differing in the type of information provided and in the detection limit. The kinetics of this process is often studied by the “oxygen uptake” method. The surface polymer degradation can be effectively studied by using Raman, infrared or special electronic spectroscopy. However, it should be stressed that all those methods provide only indirect evidences of shortening of the polymer molecules. Such information is mostly obtained by the size exclusion chromatography (SEC) method which is very effective (sensitive) in detecting the early stage of degradation. Consider for example that the number-average degree of polymerization <X>_n of a polymer drops from 2000 to 1000 exclusively by β-scission processes whose each gives two fragments: one with and one without carbonyl end-group. There is no doubt that SEC will reliably detect this drop in <X>_n. However, the mole fraction of monomeric units with carbonyl group in the final polymer (0.0005) will mostly be significantly below the detection limit of spectroscopic methods. The SEC method also provides information how the molar-mass distribution of the polymer changes during the degradation, which helps to understand the overall process (Bondarev, Trhlíková, Sedláček, & Vohlídal, 2014; Trhlíková, Hladyš, Sedláček, & Bondarev, 2016).

Polymer burning

Polymer burning is the extreme (limiting) case of the polymer oxidative degradation. One might think that polyethylene, as a high molecular weight alkane, will burn similarly to gasoline or diesel, whose vapors form homogeneous mixtures with air, in which they can burn out almost perfectly. However, the gaseous state is unattainable for polymers since their boiling temperature fairly exceeds the temperature at which they decompose. Therefore, the burning of the polymer is preceded by its thermal degradation in the absence of oxygen (pyrolysis, thermal cracking) inside the polymer. This gives gasifiable fragments of polymer molecules that penetrate into zones above the polymer surface, where they mix with oxygen and burn. It is surface burning, in which the pyrolysis products are burned, not alkanes! And the products are unsaturated and aromatic hydrocarbons, heterocycles and their derivatives, mostly carcinogenic compounds. Because these compounds can partly escape from the flame zone and pollute the environment, the safe polymer combustion must be fast, which requires furnaces with operating temperatures of at least 1200–1400 °C or higher (Scheme 4).

Scheme 4:

Scheme of the polymer burning.

The fire risk of a polymeric material is assessed by the so-called limiting oxygen index (LOI), which indicates the minimum molar fraction of oxygen in its mixture with nitrogen required for the material to burn continuously without an external flame or heat source. It is measured by means of passing the oxygen + nitrogen mixture of the gradually decreasing oxygen content over a burning specimen until a flame is quenched (For LOI values see e.g. https://omnexus.specialchem.com/polymer-properties/properties/fire-resistance-loi#values). However, the fire risk of a polymeric material is given not only by its flammability in contact with the open flame and the flame stability (LOI), but also by the possibility of its self-ignition in an atmosphere sufficiently rich in oxygen. The auto-ignition of the polymer depends not only on its temperature, but pronouncedly also on the surrounding atmosphere pressure, because the oxygen concentration at the polymer surface is proportional to this pressure. It might be useful to know that also poly(tetrafluoroethylene) burns in a pure oxygen atmosphere (LOI = 95) and even explosively at room temperature when the oxygen pressure is high, such as for example the current pressure in oxygen steel-bottle containers (150 at).

Kinetics of cleaving macromolecules when chain depolymerization is negligible

Such cleavage is common in degradations occurring well below the ceiling temperature, for example in hydrolytic, mechano-chemical or autoxidative degradations at room temperature, i.e., at conditions under which polymers are mostly applied. The cleavage is mostly monitored by SEC and the obtained data mostly in terms of scission index, B, defined as the average number of cleavable bonds (typically links between monomeric units), b, that were cleaved per one initial macromolecule (Jellinek, 1978):

N₀ is the initial number of macromolecules in the system and b₀ initial number of links.

For random cleavage, in which all links have the same probability of being cleaved in a given time interval, the cleavage is formally the first-order reaction (links decay exponentially with the rate constant ν: b = b₀ e^−νt and thus B = (b₀/N₀) ⋅ (1 − e^−νt) = B_∞⋅(1 − e^−νt)). The rate constant ν is thus obtained as the slope of the plot:

where B_∞ = <X>_n,0 − 1. Note that a linear chain of the degree of polymerization of X contains (X − 1) bonds, and that their number is the maximum number of bonds that can be cleaved per one original macromolecule. If Eq. (2) well fits the experimental data, the random scission mode can be considered but it still should be verified using additional plots, because the extent of degradation covered with experimental data is mostly not high enough and, moreover, experimental values of <X>_n (or <M>_n) suffer from low accuracy. Experimental values of the weight-average degree of polymerization <X>_w or weight-average molar mass <M>_w (Stepto et al., 2015) are in general significantly more reliable. If Eq. (2) does not fit the data, another scission mode must be considered (Vohlídal, 1994). For the mechano-chemical degradation induced by the ultrasound or a hydrodynamic force field, Eq. (2) was found to be applicable, too, though it obeys the mid-chain scission mode. However, in that case, macromolecules of molar masses up to ca 100,000 g/mol are usually not cleaved and so B_∞ must be determined experimentally (Netopilik et al., 1990; Peterson et al., 2020).

Degradation in polymer recycling

The preferable way of polymer recycling is a use of the purified and suitably pretreated polymer in production of new products. This is easy in cases where the degradation of the waste polymer is so small that it is not important for the properties of the new product. For example PET bottles are chopped into pieces that are washed and used to make PET fibers for car mats and fabrics and wadding for the upholstery of armchairs and seats. The recycled high-density polyethylene (HDPE) is currently used in production of garden furniture, buckets and similar products. Nevertheless, similar procedures are not too frequently seen.

Depolymerization of the waste polymer into monomer(s) that are then used in the synthesis of new polymers is another possibility of recycling, which might be nice mainly for polymers prepared by polycondensation, such as polyesters, polycarbonates and polyamides that are depolymerized by hydrolysis or solvolysis (Webb et al, 2013). These processes require high temperatures or the microwave assistance and efficient separation and purification of obtained products. This makes them expensive; nevertheless, they are increasingly used.

Protection of polymers against degradation

Polymer degradation is a complex process by which the polymer loses its original functional properties owing to irreversible changes in its structure. All types of polymer reactions take part in corrosion: (i) chemical transformation of functional groups, (ii) cleavage of polymer molecules by oxidation, hydrolysis, photochemical and other chemical reactions, mechano-chemical processes and by depolymerization, and (iii) crosslinking of polymer molecules as well as their fragments. Crosslinking makes the aged polymer brittle while cleavage processes makes it waxy or sticky and, in addition, yellowish or brownish owing to unsaturated chromophores formed (Jellinek, 1978; Ranby & Rabek, 1975).

Undesirable degradation processes are significantly reduced by stabilizers admixed into the polymeric material. A single stabilizer is usually not enough; a mixture of a few stabilizers mostly has to be used. According to the function, stabilizers are roughly divided to photostabilizers, antioxidants, antiozonants, radical scavengers, peroxide scavengers, acid scavengers, fire retardants and biocides. Because the stabilizer is in general only admixed into a polymer, not bound to it, its second key property is a good compatibility with the polymer. When the compatibility mediated by the intermolecular interactions is poor, the stabilizer is spontaneously displaced to the polymer surface by the ever-present thermal motion, from where it is removed by external influences. As a result, the polymer becomes unprotected. If a good compatible stabilizer is not accessible, a new one can be obtained by connecting one or more oligomeric chains good compatible with the polymer to an available stabilizer with the given basic function.