None are left to hold a pendant group. Some polymers are flexible. Others are very stiff. Just think of the many types of plastics: The material in a flexible soda bottle is very different from that in a rigid pipe made from polyvinyl chloride PVC.
Sometimes materials scientists add other things to their polymers to make them flexible. Known as plasticizers PLAA-stih-sy-zurs , these take up space between individual polymer chains.
Think of them as acting like a molecular-scale lubricant. They let the individual chains slide across each other more easily. As many polymers age, they may lose plasticizers to the environment. Or, aging polymers may react with other chemicals in the environment. Such changes help explain why some plastics start out flexible but later become stiff or brittle.
Instead, plastics and other materials made from polymers tend to soften gradually as they heat up. By Sid Perkins October 13, at am.
The anatomy of a polymer Polymer structures can have two different components. Polymers are made by chemically linking up many copies of simpler groups called monomers.
For example, polyvinyl chloride PVC is made by linking long chains of monomers shown in the bracket. And finally, polymer gels are driving exciting breakthroughs in medicine. Originally published by Cosmos as Explainer: What is a polymer?
Ellen Phiddian is a science journalist at Cosmos. Cosmos is published by The Royal Institution of Australia, a charity dedicated to connecting people with the world of science. Financial contributions, however big or small, help us provide access to trusted science information at a time when the world needs it most. Please support us by making a donation or purchasing a subscription today. Share Tweet. Model of a Teflon, or polytetrafluoroethylene, molecule.
More on:. A monomer of polyethylene. A monomer of PET. A branched polymer chain. Amylose: made of monomers of glucose. Amylopectin: similar to amylose, but containing branches. Precisely for this reason, we are now receiving a huge load of information on plastic materials with less impact to the environment.
And much of this information is contradictory, not bringing acceptable scientific references on the assertions made. Even the norms for biodegradation tests have been developed under influence of the manufacturers of biodegradable products as a tool to ward off competitors. Despite the somewhat confusing situation we are currently experiencing, the products on the market are being tested by consumers and the trend is that the most suitable materials in every situation be known over time.
An important aspect to consider is to know where the polymer material will be disposed, to evaluate the conditions for biodegradation. Some polymers that biodegrade well in industrial composting conditions high temperatures, high levels of moisture and oxygen biodegrade much more slowly in the soil at ambient temperatures, PLA or polylactic acid being an example.
In landfills, all polymers biodegrade very slowly, due to restrictions of oxygen and moisture in the layers below the surface. Biodegradation under anaerobic conditions deprived of oxygen produces CH 4 methane , CO 2 carbon dioxide , water and biomass living cells. Methane is a much more potent gas than CO 2 to the greenhouse effect. Biodegradation under aerobic conditions oxygen abundance , otherwise, does not produces methane, producing mainly CO 2 , water and biomass [ 4 ]. Biodegradability is a feature that has been highly valued in polymers from the environmental standpoint, but is not the only important one.
Sooner or later, all components in a polymer material will be returned to the environment, with the degradation, so it is very important to use pigments, fillers and additives that are not toxic in nature. Furthermore, the environmental impacts should be studied from birth to death of the polymer or "from cradle to grave". The use of raw materials from renewable resources plants has also been highlighted. However, one point to be considered here is the use of arable land for monocultures in farms that could be producing food and are instead producing raw materials for commodities like plastics.
Likewise, one must consider the possibility of using fertilizers and pesticides in excess, what could impact in eutrophication, acidification, global warming, poisoning of the environment, etc.
Another point is to know if the farming practices are conservationists or not, ie, if they seek to preserve the soil or not. For example, the practice of burning crop residues after harvest should be avoided. Another interesting aspect is the one of polymer production. Very complex processes with many steps, which consume much energy and generate much waste tend to be disadvantaged. The need for transportation of raw materials to the factory or of finished products to the consumer market must also be considered.
All the characteristics above are usually considered in the life cycle assessment of the product, which, being somewhat complex, can be aided by regulatory standards e. During the s percipient environmentalists became aware that the increase in volume of synthetic polymers, particularly in the form of one-trip packaging, presented a potential threat to the environment, what became evident in the appearance of plastics packaging litter in the streets, in the countryside and in the seas [ 8 ].
PVC see Figure 1 is a good example. Although the density of PVC is around 1. PVC has a high concentration of chlorine atoms in an organic chemical structure that is new in nature i. On the other hand, PVC degrades easily under the action of light or heat, and its decomposition is catalyzed by the HCl released, forming a poly-unsaturated structure which is very degradable. In order to get a more flexible PVC, plasticizers based on phthalate are commonly used, many of them having chronic toxicity to animals, showing body growth problems ie, teratogenic effects and reproduction complications in humans.
Small fragments of PVC molecules can evaporate. Just as halogenated solvents, these molecules are very inert and can rise to the stratosphere, contributing to the destruction of the ozone layer [ 9 ]. In addition to the accumulation in the environment and to the possible toxicity of the additives, it was realized that the incineration of PVC generated many toxic products such as dioxins, due to the high concentration of chlorine atoms present [ 1 ].
Polycarbonate plastic and epoxy resins coating and adhesive are normally produced with bisphenol A as one of the monomers. This substance may also be used as an additive for plastics. It is an endocrine disruptor it can mimic hormones [ 1 ]. Some studies have shown toxicity, carcinogenic effects and possible neurotoxicity at low doses in animals [ 11 - 15 ].
In the case of decomposition of the resin, this toxic monomer might be released into the environment. Polycarbonate can be recycled.
Its biodegradation is very slow due to the presence of aromatic rings in the main chain. Polystyrene PS, Figure 1 has a density of about 1. The presence of aromatic rings at a short distance from the main chain increases its resistance to biodegradation ie. In addition, PS has rigid although not crystalline molecules, making difficult the enzymatic action.
Although most of the additives used with PS are not toxic, the residual amounts of free styrene, "dimers" and "trimers" obtained after polymerization must be kept very low, because they are volatile and can migrate out of the part [ 16 ]. This polymer can be recycled. Polyethylene terephtalate PET, Figure 1 has a density around 1.
PET presents aromatic rings in the main chain, which makes it highly recalcitrant, despite having hydrolysable ester groups. Additionally, catalysts residues employed in their synthesis either by esterification or transesterification are present in the polymer. Examples of catalysts are manganese, zinc and cobalt salts transesterification and compounds of antimony, germanium, titanium and tin esterification at typical concentrations of ppm [ 17 ].
Phosphorus compounds used to deactivate transesterification catalysts can cause eutrophication of ocean waters. PET has been reused and recycled on a large scale in many countries around the world. Polyamide 11 PA 11 is a biopolymer derived from vegetable oil. It is commercialized by Arkema and DSM. PA 11 belongs to the technical polymers family and is not biodegradable. Its properties are similar to those of PA 12, although emissions of greenhouse gases and consumption of nonrenewable resources are reduced during its production.
It is used in high-performance applications like automotive fuel lines, pneumatic airbrake tubing, flexible oil and gas pipes, sports shoes and electronic device components [ 18 ].
The nylon 6 and nylon 66 polyamides are considered polymers with superior thermal and mechanical properties, and are therefore referred to as engineering polymers.
Although they are in the group of the non-biodegradable polymers [ 19 ], they may be significantly degraded by white-rot lignin degrading fungi. Its surface erosion suggests that nylons are degraded to soluble monomers [ 20 ]. They can be degraded by hydrolysis and also by oxidation. In the latter case, the more reactive hydrogen atoms are those attached to the carbon atom adjacent to the nitrogen atom of the amide group [ 21 ].
Polyurethanes PURs , or carbamates are polyethers or polyesters with molecular weight of about - g mol -1 copolymerized by a polyaddition process, with monomers containing isocyanate groups, resulting the characteristic urethane groups in the main chain, which are generally in very low proportions.
In most applications, they are thermosetting or thermo-crosslinkable polymers, ie they form a three-dimensional network by chemical reactions under heating, and they do not soften under further heating [ 1 ].
The company Cargill produces polyol for polyurethane cushioning, which is soy-based BiOH polyol , designed especially for flexible foams. It is believed that most of PUR biodegradation occurs by the action of esterases, however polyester-polyurethane degrading enzymes have been purified and their characteristics have been reported. These enzymes have a hydrophobic binding domain at the surface of the PUR, and a catalytic domain [ 22 ].
But there is no evidence that the urethane linkage has been broken. They are a very large class of carbon-chain thermoplastics and elastomers, the most important being polyethylenes and polypropylene.
They are extensively used in many different forms and applications. Flexible packaging, included here wrap films, grocery bags and shopping bags made with extruded films and extruded blown films, as well as rigid packaging made by blow moulding and injection moulding represent a considerable amount of the total material consumed [ 1 ].
Polyolefins float in the oceans, because they are normally lighter than salty water. They do not normally contain toxic ingredients, although toxic metals may be introduced as pigments.
Usual additives are antiacids e. Catalytic residues, such as Ti and Cr compounds, are present at very low levels ppm. The oxidative degradation of polyolefins in the oceans is favored by the continuous movement of the waves, by the presence of oxygen at the surface, and by the sun exposure.
On the other hand, the temperature of plastic materials at sea does not reach that on the ground, due to the effect of heat removal by water. Eventual fouling can limit the exposure of the material to UV radiation. Oxidized residues of polyolefins may sink into the sea due to the change in density that occurs during oxidation.
In fact, even the food present in sunk ships degrades and biodegrades very slowly on the sea bottom. Although the above polymers have a number of environmental impacts from the time of their disposal, their production from oil, natural gas or coal has been optimized through decades of manufacturing. Should naphtha not be used for the petrochemical industry, it would then be burned, what would not improve anything its environmental impact.
Moreover, the use of oil to be burned in a combustion engine or in a boiler for heat is becoming an unacceptable luxury to the present day, with the prices of fossil fuels becoming progressively higher. The use of fossil fuels as raw materials, as major carbon sources, appears to be more compatible with the world reality today.
The development of renewable forms of energy such as solar thermal and photovoltaic, wind, hydroelectric, wave and tidal, geothermal, biogas and others should allow the replacement of the energy obtained from fossil fuels in a few decades. Biopolymers : are polymers produced by living organisms. They all have been around for millions of years on our planet, and for this reason microorganisms have had enough time to develop enzymes capable of degrading their structure, so they are biodegradable.
In general their end of life environmental impact is low. However, there are at least two cases where this is not true:. When the biopolymer is placed in an unsuitable environment for its biodegradation. For example, a landfill does not provide adequate conditions for biodegradation, since oxygen and water are lacking.
Under anaerobic conditions ie in the absence of oxygen , the biopolymer, as well as organic wates in general, will degrade producing biomass, methane CH 4 , carbon dioxide CO 2 and water, as well as other eventual small molecules NH 3 , N 2 , N 2 O, H 2 S, mercaptans, etc. The generated methane is a much more powerful gas than carbon dioxide to global warming, and is not readily reabsorbed by plants, as with CO 2.
Once the biopolymer is mixed with other polymers in a recycle stream, it will act as a contaminant, as biopolymers are normally not recyclable, degrading at the recycling processing conditions.
A comparative study of all environmental impacts of a particular product eg a polymer can be obtained by a life cycle assessment. The life cycle assessment LCA of a product, process or activity is a technique to assess environmental impacts, or the environmental burdens associated with all the stages of its life, from cradle-to-grave, ie: extraction, transportation and processing or raw materials; manufacturing; transportation and distribution; use, reuse and maintenance; recycling; final disposal; material and energy consumption; water consumption and emissions generation; etc.
The assumptions and methodologies should be given, and should be clear, consistent and documented, otherwise it may be impossible to compare different LCA studies.
Some of the most often evaluated environmental impacts are: toxicity to humans or to other living organsms; fresh water aquatic ecotoxicity; marine aquatic ecotoxicity; terrestrial ecotoxicity; eutrophication, acidification of rains and soils ; global warming potential; ozone depletion; abiotic depletion of mineral resources; depletion of fossil fuels petroleum, natural gas and coal ; visual pollution litter ; photochemical oxidation smog formation ; renewable and non-renewable energy use [ 5 - 7 ].
A difficulty in comparing different types of environmental impacts is the use of a different unit to each type assessed. For example, kg CO 2 equivalent is the required CO 2 mass to produce the same effect global warming that the object of study.
Then how to compare global warming with, for example, abiotic depletion, which has the unit kg Sb equivalent resource depletion compared with that of antimony?
The weight of each impact needs to be arbitrated in order to calculate the total impact. An impact that is very significant for some authors may be considered less important for others. Therefore, all assumptions made need to be transparent in the study. The findings of some LCAs studies of plastic bags are presented below.
In a study by Edwards and Fry [ 5 ], the authors have concluded that the environmental impacts of all types of carrier bags are dominated by resource use and production stages, whereas transport, secondary packaging and end-of-line management have minimal influence.
According to them the key to reducing the impact is to reuse the bags as many times as possible, at least as bin liners. Reuse produces greater benefits than recycle.
Recycling or composting generally produce small reductions in global warming potencial and abiotic resource depletion. They found that starch-polyester bags have significant global warming potential and abiotic depletion. The impacts of the oxo-biodegradable high density polyethylene HDPE bags are very similar to the conventional HDPE bags, because of the similarity in material content and use. The production of the pro-oxidant additive has minimal impact on most life cycle categories.
End-of-life impacts through incineration and landfill are practically identical. The essential difference, although not concluded by the authors, seems to be that oxo-biodegradable HDPE bags do not remain on ground or water as litter, and that they represent a source of carbon, just like humus.
James and Grant [ 6 ] have found in their study that polymer based reusable bags have lower environmental impact than all single-use bags evaluated. Degradable bags have similar greenhouse and eutrophication impacts to conventional HDPE bags, because they normally go into landfills. Decisions about degradable polymers should be based on: where and how they will degrade, minimal LCA not just end-of-life , and commercial benefits. Tabone et al. The first one was related to several general principles, like prevent waste, utilize less material mass, maximize energy efficiency, use non-hazardous inputs, use renewable feedstocks, use local sources, design for recycle, minimize material diversity, degrade after use, maximize cost-efficiency, minimize the potential for accidents, etc.
The second one referred to LCA, which was discussed above. They have found that, while biopolymers rank highly in terms of green design, they exhibit relatively large environmental impacs from production. The impacts from biopolymers result from the use of fertilizers, pesticides and arable land required for agriculture production, as well as from the fermentation and other chemical processing steps.
Polyhydroxy alcanoates PHAs produced from stover have obtained an excellent environmental position. As a conclusion of some LCAs, it comes out that there is not a single ideal material or solution adequate for all possible situations on Earth, which always presents the lowest environmental impact.
A practical present solution for the plastic bags could be the conventional polyolefin materials formulated with pro-oxidant additives, used as many times as possible. Another interesting solution is the use of agricultural and other organic residues as raw materials for the manufacture of biodegradable polymers. Recycling and composting units should be encouraged in all countries of the world. Renewable energies should substitute the fossil fuels, which should be destinated as a carbon source for the chemical industry.
There are three main possibilities of degradation of the polymers: enzymatic, hydrolytic and oxidative. The enzymatic degradation, or biodegradation, is the breaking of polymer chains by the action of enzymes, which are natural catalysts of chemical reactions produced by living organisms.
For example, cellulose and starch are degraded by specific groups of enzymes known as cellulases and amylases, respectively. Polyesters can be degraded by esterases enzymes [ 23 , 24 ]. The degradation by hydrolysis consists in breaking certain chemical bonds such as ester, ether and amide, by attack of water molecules. This process can be catalyzed by both acids and bases saponification.
In the case of the ester linkage, a carboxylic acid or a salt thereof and an alcohol are produced. The ether linkage is much more resistant to hydrolysis than the ester one, generating two alcohols. Hydrolysis of the amide group results in an amine and a carboxylic acid.
The strength of polymers also varies depending on how the molecules are arranged. To use our paperclip analogy, you may decide to have some paperclips branching off your main line. They result in a polymer with a lower density. Low-density polyethylene LDPE —the squishy material that plastic bags and wrap like the kind you might wrap your sandwich in —is an example.
The resulting polymer is stronger and has a higher density. An example is high-density polyethylene HDPE , used to make things like plastic bottles, food containers and plumbing pipes. In contrast to thermoplastic polymers are thermosetting polymers. It is useful, though, for things like car tyres, since a tyre that melts in the heat is going to make for a pretty interesting drive to the beach. Glues and electrical components are also thermosetting polymers. As well as the arrangement of molecules, the properties of a polymer are also determined by the length of the molecular chain.
In a nutshell, longer equals stronger. This is because, as a molecule gets longer, the total binding forces between molecules are greater, making the polymer chain stronger. When more than a thousand carbon atoms line up in a chain of ethylene monomers, for example, the resulting polymer, polyethylene, is strong and flexible.
Developments in synthetic polymers go way beyond plastic bags and drink bottles. Flexible, electricity-conducting polymers may be the next big thing.
Into virtual reality VR? In the not-too-distant future, you might be able to ditch the chunky goggles and pop in a pair of contact lenses instead, thanks to very thin, electricity-conducting polymer coatings. Australian scientists have also been working on lightweight, flexible solar cells which can be cheaply printed with polymer inks using a conventional printer.
Cities of the future could see a multitude of surfaces—buildings, cars, even clothing—made of this power-generating material. Thanks to a plethora of cheap, disposable products and packaging, plastics often get a bad rap pardon the pun for their impact on the environment—and rightly so. But new discoveries in materials science—from solar cells to biodegradable plastics made of natural materials—also hold out the promise of a more sustainable future.
Polymers: from DNA to rubber ducks Expert reviewers. What do DNA, rubber ducks and weird seventies raincoats have in common?
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