Polymers for special uses

Introduction

Basics
A polymer is a giant macromolecule, composed by a long sequence of small elementary units, called repeating units, joined together through covalent chemical bonds. The number of repeating units succeeding in the polymer backbone corresponds to the polymerization degree (DP) of the polymer, strickly related to its molecular mass (MM). Once formed, the polymer can be represented as an extended chain, the spatial arrangement of which can look unordered or having a regular geometric structure, depending on the chemical nature of the polymer itself.



Also the chemical, physical, mechanic properties of a polymer are correlated to the chemical nature of the backbone, that is to the number and type of the repeating units as well as to the values of its molecular mass.

The name of a polymer is usually derived from the name of the corresponding repeating unit, simply by adding the “poly” prefix; however, an official nomenclature for polymer exists, the rules of which are defined by the International Union of Pure and Applied Chemistry (IUPAC). Every polymer is univocally identified by an international acronym, but the most common ones are often better known by their brand names.

Because of their great variability, polymers can be classified according to a large number of parameters; some of these parameters are reported below together with the polymers type related to each of them.

Origin:	natural polymers artificial polymers inorganic polymers synthetic polymers
Life cycle:	recyclable polymers non-recyclable polymers biodegradable polymers compostable polymers
Chain structure:	linear polymers branched polymers cross-linked polymers
Composition:	homopolymers copolymers

To give information on every type of polymers is beyond the aim of this section, so a limited number of them will be illustrate further.

2.1 Biocompatible polymers
The term biomaterial (or biocompatible material) indicates a material profitably employed in medicine, pharmaceutics and biotechnology. Different classes of compounds can be classified as biomaterials: metals, ceramics and polymers, both of natural and synthetic origin. Among them, polymers and composites with polymeric matrix are the basis of most of the biomedical and biotechnological applications. To be considered a biomaterial, a polymer must have proper characteristics, the most important of which is biocompatibility. If the polymer is used in contact with blood, the main request is instead for its haemocompatibility.

Biocompatible polymers can be utilized to substain and/or replace different tissues, organs or functions of the human body; to this end their chemical, physical and mechanic properties are closely related to the task to be accomplished. As an example, polymethylmetacrylate (PMMA) is widely employed in ophthalmology for fabricating contact or intraocular lenses because of its optimal transparency; it is also used in dentistry as bone cement as it easily polymerizes.



Ultra-high molecular weight polyethylene (UHMWPE), a highly wear resistant polymer, is utilized for fabricating acetabular cups in hip prosthesis.

Fibers of polyethyleneterephthalate (PET) or polytetrafluoroethylene (PTFE) are employed in producing artificial ligaments as well as vascular grafts.

Highly selective membranes for bood dialysis are commonly made by cellulose acetate, whereas damaged soft tissues are normally replaced by silicone prosthesis.

Some biocompatible polymers are also biodegradable, such as polylactic acid (PLA), polyglycolic acid (PGA), chitosan or hyaluronic acid: they are extensively employed in reparative medicine for tissues regenerating as well as in the fabrication of controlled drug delivery and release systems (DDS).


2.2 Expanded polymers
Expanded polymers or polymer foams are very light plastics because of the high content of air inside their structure. The most diffused are expanded polystyrene and polyurethane foams. They are used for packaging (also of food), thermal insulation, soundproofing and modelling.



Expanded polystyrene or polystyrene foam is typically 90-95% air, with the rest being the polystyrene and trace substances. The production of expanded polystyrene typically involves heating beads of polystyrene that have been impregnated with a little foaming agent (such as pentane). The heat causes the pentane to become a gas and expand. As a result, the beads can expand to about 40-50 times their size and such bubbles are then molded, pressed, and/or polished into shapes.

A curiosity worth to be mentioned: a new recycling technique has been developed to help dispose of polystyrene. This technique uses limonene, a natural vegetable oil extracted from the skins of citrus fruits, which shrinks the expanded polystyrene to one 20th of its original size. This allows polystyrene to be recycled more efficiently.

Polyurethane foams are the most diffused polymer foams. They are used to produce every kind of padding and packaging (soft polyurethane foams) but also for thermal insulation (rigid polyurethane foams). In this case the formation of the expanded structure is a one-step procedure starting from two specific components that we can call component A and component B. Mixing them together they immediately react and the polyurethane plastic is produced together with a gas, carbon dioxide (CO2), responsible of the material expansion.

2.3 Super-absorbent polymers
Absorption occurs when a liquid, such as water, is taken up (or absorbed by) a solid. When this happens, the solid often grows in size (swells). Many polymers can also absorb large quantities of liquid, and by doing so, they find usefulness in many industries and aspects of every day life: they are called super-absorbent polymers and, when the absorbed liquid is the water, the swollen material is called hydrogel. The water absorbed in hydrogels gives them the typical transparency and softness. Few well known applications of synthetic super-absorbent polymers are disposable nappies, hair gel, water crystals, contact lenses and some prosthesis. An example of natural super-absorbent polymer is collagen, a protein that is found exclusively in animals: it is the main component of connective tissue, and is the most abundant protein in mammals. Collagen is the polymer responsible for the formation of the classic gelatin, that is its hydrogel. Did you know that your muscles and internal organs are hydrogels? Sounds weird? Just think you have lots of proteins in your body and these proteins are surrounded by an enormous quantity of water.

The property of certain polymers of absorbing large quantities of water is due to their microscopic structure and to their chemical affinity with water. Polymer chains of a super-absorbent polymer are not free: they are arranged in a sort of three-dimensional network thanks to some links among chain segments. This particular arrangement of polymer chains is called crosslinked structure. When water meets this structure is incorporated and entrapped in it and the result is a swollen soft material: a gel. A funny application of super-absorbent polymers is a well known toy: the ‘slime’. Such material is a hydrogel, mainly composed by polyvinylalcohol polymer that is firstly dissolved in water and then crosslinked by a molecule called borax.

2.4 Bioplastics
Plastics have been designed in the past to resist to degradation. The challenge is to design polymers that have the necessary functionality during use, but disintegrate after use. More importantly, the breakdown products should not be toxic or persist in the environment and should be completely assimilated (as food) by soil microorganisms in a defined time frame. At the light of this necessity, a relatively new class of materials seems to be competitive with classic petroleum-derived plastics: the bioplastics. Bioplastics are man-made polymers derived from (annually) renewable raw materials (RRM), such as vegetable oil, corn starch, pea starch; many bioplastics are biodegradable and compostable. To be precise, also petroleum-based plastics are technically biodegradable, meaning they can be degraded by microbes under suitable conditions; however they may degrade at such slow rates as to be considered non-biodegradable. Biodegradable bioplastics are used for disposable items, such as packaging and catering items, for organic waste bags, where they can be composted together with the food or green waste. Some trays and containers for fruit, vegetables, eggs and meat, bottles for soft drinks and dairy products and blister foils for fruit and vegetables are manufactured from bioplastics.

Starch-based plastics constitute about 50 percent of the bioplastcs market. Pure starch possess the characteristic of being able to absorb humidity and its properties are far from plastic; so suitable additives as sorbitol and glicerine are added to change the characteristics of the raw material so that it can be tailored to specific needs but maintain its biodegradability. Novamont is the Italian commercial manufacturer of starch-based plastic, better known under the tradename of MATER-BI.

Polylactic acid (PLA) is a biodegradable transparent plastic produced from cane sugar or glucose. It not only resembles conventional petrochemical mass plastics (like PE or PP) in its characteristics, but it can also be processed easily on standard equipment that already exists for the production of conventional plastics. NatureWorks is the the primary producer of PLA.

An example of non-biodegradable bioplastic is polyamide 11, derived from natural oil. It is also known under the tradename Rilsan B, commercialized by Arkema. It is used in high-performance applications like automotive fuel lines, pneumatic airbrake tubing, electrical cable antitermite sheathing, flexible oil and gas pipes, sports shoes, electronic device components, and catheters.