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Biodegradable polymers and environmental interaction.


A biodegradable polymer is a high molecular weight polymer that, owing to the action of micro- and/or macroorganisms or enzymes, degrades to lower molecular weight compounds (1). Natural polymers are by definition those which are biosynthesized by various routes in the biosphere. Proteins, polysaccharides, nucleic acids, lipids, natural rubber, and lignin, among others, are all biodegradable polymers, but the rate of this biodegradation may vary from hours to years depending on the nature of the functional group and degree of complexity. Biopolymers are organized in different ways at different scales. This hierarchical architecture of natural polymers allows the use of relatively few starting molecules (i.e. monomers), which are varied in sequences and conformations at molecular-, nano-, micro- and macroscale, forming truly environmentally adaptable polymers (2-4). On the other hand, the repetitive units of synthetic polymers are hydrolyzable, oxidizable, thermally degradable, or degradable by other means. Nature also uses these degradation modes, e.g., oxidation or hydrolysis, so in that sense there is no distinction between natural or synthetic polymers. The catalysts promoting the degradations in nature (catabolisms) are the enzymes, which are grouped in six different classes according to the reaction catalyzed (5).


Biodegradation of oxidizable polymers is generally slower than biodegradation of hydrolyzable ones. Even polyethylene, which is rather inert to direct biodegradation, has been shown to biodegrade after initial photo-oxidation (6, 7). Many inert polymers are, however, more susceptible also to biodegradation if modifications are done. Low molecular weight compounds present in polymers render the polymer more susceptible to biodegradation because they become inherently more accessible to chemical reactions. An oxidized polymer is more brittle and hydrophilic than a nonoxidized polymer, which also usually gives a material with increased biodegradability. Means to accelerate the oxidation of polyolefins have been presented. Scott and Gilead used sulfur complexed transition metal ions, in particular dithiocarbamate, which do not promote thermal degradation (8). By combining a nickel dithiocarbamate (photo antioxidant) with an iron dithiocarbamte (photo proxidant), a wide range of embrittlement times may be obtained (9).

Increased susceptibility to biodegradation of inert polymers may use the addition of a biopolymer. In this way a polymer blend is obtained that is more susceptible to biodegradation. Currently, the most commonly used biopolymer for incorporation into polyethylene is starch. Granular starch mixed with PE together with an unsaturated polymer, a thermal stabilizer, and a transition metal give a material with increased susceptibility to photo-oxidation, thermolysis, and biodegradation (10, 11). This particular material has also a induction time before degradation may be initiated. It was demonstrated that a thermolysis for 5 days at 100 [degrees] C overcame this induction period (12, 13). By liquid scintillation counting (LSC) of evolved 14C[O.sub.2], this starch/PE composite was shown to have a degradation rate about ten times higher than pure PE (14). The use of starch alone in polyethylene, for example, requires, however, rather large amounts in order to really create an increase in the biodegradation rate. Clearly a filler, which is biodegradable, always gives a matrix that is more easily accessible to, e.g., abiotic degradation.

The biodegradation mechanism of polyethylene showed that initial oxidation is followed by the removal of two-carbon fragments from the chain (7). This mechanism is confirmed by, e.g., analysis of carbonyl group formation in the PE surface, which decreases with time in biotic environments (7). During degradation of polyethylene, carboxylic acids are generally formed. In addition, several other compounds are formed that differ in type and amount depending on the type of degradation mechanisms active during the degradations (e.g. 11). We could show that the amount of carboxylic acid decreases with degradation time in biotic environments, opposite to the case in abiotic environments. This is in line with the proposed biodegradation mechanism where carboxylic groups are preferentially attacked and degraded by microorganisms (7). Thus microorganisms assimilate the degradation products of PE at the same time as the polymer chains gradually biodegrade (15). It was recently concluded that abiotic iron catalyzed photo- or thermooxidation is the rate-limiting step in the bioassimilation of photodegradable polyethylenes (16).

In addition to the release of small molecules from the polymer chains, the morphology is changed during the degradation. It is generally known that the amorphous part of a polymer is more easily degradable; in particular this is the case for hydrolyzable polymers. This gives a gradual increase in the crystallinity of the remaining polymer because of the rearrangement of the chains. We could, however, demonstrate a gradual decrease in the crystallinity of PE during biotic degradation (15). Parallel samples in abiotic degradation (photo-and thermo-oxidation) instead showed the expected increase.


In the 1960s poly(L-lactide), PLLA, was proposed as a biocompatible, biodegradable, and bioresorbable material for biomedical applications (17-19). In recent years, environmental concerns have led to an escalated interest in PLLA, as well as other biodegradable polymers, as an alternative to traditional commodity plastics (20, 21). PLLA has the advantage of being not only biodegradable but also renewable since the raw material, lactic acid, may be produced by microbial fermentation of biomass.

Polycaprolactone (PCL) is another aliphatic polyester that has attained interest because of its biodegradability, Already in 1970s it was shown that PCL is easily biodegraded and utilized as carbon source by various microbial species (22-25). Even though it is a synthetic polymer, microorganisms capable of degrading PCL are widely distributed in nature (26). The reason is that enzymes that hyclrolyze naturally occurring hydrophobic (poly)esters such as cutin and lipids may also attack PCL (27, 28). In most environments PCL has been found to biodegrade slower than other biodegradable polymers such as poly[hydroxybutyrateco-valerate] (PHB/HV), regenerated cellulose, starch and chitosan (29, 30). The low melting point of PCL, 60 [degrees] C, excludes it from many applications but could also be an advantage leading to a faster degradation in, for example, composts operating at high temperatures (20). The thermal stability of PCL may be improved by crosslinking with organic peroxides or gamma radiation (31, 32), although the barrier properties are severely deteriorated above the crystalline melting point (33).

The processing of biodegradable polymers is generally more difficult than the processing of synthetic inert polymers. A sensitivity to high temperatures combined with a marked low hydrolysis resistance may initiate the degradation already during processing or even change the morphology to such an extent that biodegradation is retarded. A 50% difference in molecular weights was obtained by varying the processing temperatures and the screw speed (34). High mechanical properties of PHB/HV was obtained at processing temperatures of 165 [degrees] C and a screw speed of 25 rpm, although initial studies indicate that this is not optimal if a very rapid degradation is needed (34). We have recently shown that melt-extrusion at processing temperatures between 147 [degrees] C and 177 [degrees] C and screw speeds between 5 and 40 rpm of PHB/HV cause a change in the atomic position of the oxygen atom in the helix structure due to torsion of the main chain bonds (35).


The natural polymers are biodegraded by a range of catabolic metabolisms catalyzed by series of enzymes. This catabolism produce both energy and building blocks for the biosynthesis of new materials. The ultimate degradation products of biopolymers are carbon dioxide, water, and to some extent ammonia. But it is very important to remember that only a very marginal amount of the biopolymers ends up as carbon dioxide and water. Series of other so-called natural metabolites accumulate and are used again and again in nature.

Biodegradable polymers are intended to degrade within certain times but it is then very important to know the type of small molecules formed during these events. The degradation products of oxidizable polymers are generally very complex in their pattern (11, 13). The hydrolyzable on the other hand form one or at the most three or four degradation products. It is then possible to correlate the change in the polymeric properties with the formation of degradation products. A new method was developed to separate and identify the hydroxyacids in hydrolyzed PLA and the copolymer of this with 1,5-dioxepan-2-one (DXO) using solid-phase extraction [SPE] (36). The method separated and derivitized the hydroxyacids in one step, which improved the detection of these products (36).

Homopolymers and copolymers of PLA and poly(glycolide)s PGA were hydrolyzed at two temperatures for periods of up to 80 days. The hydrolysis of the materials proceeds in three stages: stage one shows a rapid decrease in molecular weights with little change in weight; stage two shows instead a decrease in the molecular weight change and a severe weight loss. At the same time the formation of degradation products is initiated. During the third and final stage, almost total weight loss is seen, but the amount of lactic acid and glycolic acid account only for about 50% of the polymer (37). Eventually, all oligomers are also degraded to the smallest molecules, which are lactic acid and glycolic acid.


Biodegradable polymers are natural or synthetic materials with susceptibility to microbial and/or enzymatic degradation. The rate of this biodegradation is correlated with the type of repetitive unit, morphology (e.g. crystallinity, size of spherulites), hydrophilicity, surface area and additives. It is possible to design new biodegradable polymers with predetermined degradation times without losing important polymeric properties. Several biodegradable synthetic materials have an induction time before biodegradation is possible, avoiding degradation during processing and use.

The key question regarding biodegradable polymers is the formation of the degradation products. The type and amount of these small molecules should be such that they fit into the catabolisms of nature. It is not enough to know that the degradation product is a natural metabolite, as most natural metabolites take part in various events, e.g., regulating the anabolic and catabolic metabolisms.


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Author:Karlsson, Sigbritt; Albertson, Ann-Christine
Publication:Polymer Engineering and Science
Date:Aug 1, 1998
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