Give plausible explanations for the following: a. Most commercially important po

Question

Give plausible explanations for the following:

a. Most commercially important polyamides are aliphatic, whereas commercial polyesters are mostly aromatic.

c. Polyurethanes used for organ implantation are made with polyether glycols, not the less expensive polyester glycols.

d. The percent crystallinity and Tm of nylon 46 are significantly higher than those of nylon 66, more than one might expect on the basis of chain length alone.

e. Polyureas are generally higher melting than polyamides of like structure.

f. Polyurethanes can only be made from AA + BB monomers, not from the AB type.

1. Give plasible cxplanations tor the folloxwing polyesle are sly arnalir. b. Polycondensat of -amino acidz is not a practical way tn obtain nlon 1 and5 c. Polyrcthanea ned for organ implantation arc made with polycther glycolk, not he lcss expensivc polyctor glycol. d. The pcrccnt crystallinity and T of nylon 1i arc significantly highcr than thoac ot e. Polyures agally higter tellng Uh polyanides uf like struc lure.

Explanation / Answer

Polyamides

Polyamides (PAs) are produced either by the reaction of a diacid with a diamine or by ring-opening polymerization of lactams. They are either all aliphatic or all aromatic. The aromatic polyamides, called aramids, have higher strength, better solvent, flame and heat resistance and greater dimensional stability than the all aliphatic amides (Nylon) but are much more expensive and more difficult to produce.

The two most important aromatic amides are poly(p-phenylene terephthalamide), also known as Kevlar, and poly(m-phenylene isophthalamide). The fully aromatic structure and the strong hydrogon bonds between the aramid chains result in high melting points (generally above their decomposition temperature > 750 K), ultra high tensile strength at low weight, and excellent flame and heat resistance as well as good dimensional stability and solvent resistance at room and elevated temperature.

The aliphatic polyamides are produced on a much larger scale and are the most important class of engineering thermoplastics. They are amorphous or only moderately crystalline when injection molded, but the degree of crystallinity can be much increased for fiber and film applications by orientation via mechanical stretching. The two most important polyamides are poly(hexamethylene adipamide) (Nylon 6,6) and polycaprolactam (Nylon 6). Both have excellent mechanical properties including high tensile strength, high flexibility, good resilience, low creep and high impact strength (toughness). They are easy to dye and exhibit excellent resistance to wear due to a low coefficient of friction (self-lubricating). Both amides have a high melting temperature (500 - 540 K) and glass transition temperature resulting in good mechanical properties at elevated temperatures. For example, the heat deflection temperature (HDT) of PA-6,6 is typically between 180 and 240°C which exceeds those of polycarbonate and polyester. They also have good resistance to oils, bases, fungi, and many solvents. The main limitation is the strong moisture sensitivity (water acts as a plasticizer) and the resulting changes in mechanical properties. For example, the tensile strength of moist polyamide can be more than 50 percent lower than that of dry polyamide. Another important polyamide is Nylon 6,12. It is less hydrophilic than Nylons 6,6 and 6 due to the larger number of methylene groups in the polymer backbone. For this reason, it has better moisture resistance, dimensional stability, and electrical properties, but the degree of crystallinity, the melting point and the mechanical properties are lower. Other commercially available polyamides include Nylon 4,6, Nylon 6,10 and Nylon 11.

Polyamides have several advantages over other classes of engineering polymers. For example, they are more resistant to alkaline hydrolysis than polyesters but not as resistant to acid hydrolysis. They also have better solvent resistance to many organic liquids when compared with PET and PC.

POLYESTERS
(BIOBASED) ALIPHATIC POLYESTERS

PROPERTIES

In the last two decades, much attention has been given to the development and application of biodegradable polymers. They can be classified into two types according to the preparation method; the first class are polymers synthesized from renewable sources, and the other class are synthetic, mostly from mineral oil derived polymers that are biodegradable.

One of the most important biodegradable polymers are (biobased) aliphatic polyesters. Many of them have excellent biodegradability and biocompatibility and are, therefore, a very important class of biodegradable polymers. There are numerous aliphatic biodegradable polyesters. However, only a small number of them are commercially availabe. Some biobased polyesters that have gained commercial use or that are currently investigated for commercial use are polylactic acid (PLA), polyglycolic acid (PGA), poly--caprolactone (PCL), polyhydroxybutyrate (PHB), and poly(3-hydroxy valerate). Among these, PHB and PLA are probably the most extensively studied biodegradable thermoplastic polyesters. Both are a truly biodegradable and biocompatible and both have a relatively high melting point (160 to 180 °C). However, practical applications have often been limited by their brittleness and narrow processing window. Therefore, blending with other polymers has been often reported in the literature.

Important synthetic biodegradable polyesters are poly(ethylene succinate) (PESu), poly(propylene succinate) (PPSu) and poly(butylene succinate) (PBSu) among several others. They are typically produced from the reaction of a diacid or acid anhydride with a diol with the elimination of water. Most aliphatic polyesters are currently produced from fossil fuels. However, some of these polyesters can also be produced using monomers from renewable resources. Many succinates have been intensively studied due to their inherent biodegradability and commercial availability. PPSu has gained an increasing interest because its biodegradation rate is higher than those of most other succinates like poly(ethylene succinate) (PESu) and poly(butylene succinate) (PBSu). Due to its low crystallinity, it also degrades faster than most other polyesters used as pharmaceutical excipients including polycaprolactone (PCL).

The majority of biobased or biodegradable polyesters are completely aliphatic. They usually have a low melting point and glass transition temperature and poor hydrolytic stability, that is, they are rather hydrophilic and, therefore, will have a moderate to high water uptake when exposed to moisture. This will effect the mechanical properties and stability. For this reason, some biodegradable polymers have been blended with more stable polymers or have been copolymerized with aromatic building blocks (aromatic acids or anhydrides). For example, poly(butylene adipate-co-terephthalate) (PBAT) is commercially synthesized from adipic acid, terephthalic acid, and 1,4-butane diol.

An important property of biodegradable polyesters is the degradation time, which should be as short as possible in regard to disposal. However, corrosion resistance might decrease too strongly when the polymer has a very short degradation time. Depending upon the polymer and the conditions, degradation times will range from several months to several years.

Tensile properties are usually best for those with the smallest molar volume (highest packing density). Especially strong are PGA and PLA, whereas PCL, on the other hand, is the softest polymer with an extraordinary high strain at failure. A very important factor is the molecular weight (MW). Varying the MW will yield polyesters with very different mechanical properties. For example, the tensile strength of PLA can vary between 1 and 150 MPa. Another important factor is tacticity. Many aliphatic polyesters have an asymmetrical carbon atom in the repeat unit which enables it to become optically active. For example, it is possible to obtain isotactic L-PLA or D-PLA and syndiotactic DL-PLA consisting of alternating L- and D-units. These polymers have very different mechanical properties. For example, L-PLA has two to three times higher tensile strength and Young’s modulus than DL-PLA.

Aliphatic polyesters are often blended with other resins to improve their processing and end use properties. For example, they can be blended with starch to lower cost and to increase the biodegradability. Biodegradable polyesters are also used as the matrix resin for mostly unidirectional bio-composite materials. Often natural fibers like flax, hemp, jute, bamboo, elephant grass, and kenaf are used as reinforcing fibers. For composites, the mechanical properties of the polyesters are not very important since the reinforcing fibers provide most of the composite strength. PLA and PCL seem to lead to the lightest composites with a relatively high fiber content, which is often desirable since this results in less use of expensive resin. It seems the softer the matrix polymer is, the lighter the resulting composite will be, while the mechanical properties of the composite seem hardly affected at all.

2. Nylon can be drawn, cast, or extruded through spinnerets from a melt or solution to form fibres, filaments, bristles, or sheets to be manufactured into yarn, fabric, and cordage; and it can be formed into molded products. It has high resistance to wear, heat, and chemicals.

When cold-drawn, it is tough, elastic, and strong. Most generally known in the form of fine and coarse filaments in such articles as hosiery, parachutes, and bristles, nylon is also used in the molding trade, particularly in injection molding, where its toughness and ability to flow around complicated inserts are prime advantages.

Polyamides may be made from a dicarboxylic acid and a diamine or from an amino acid that is able to undergo self-condensation, or its lactam, characterized by the functional group CONH in a ring, such as -caprolactam. By varying the acid and the amine, it is possible to make products that are hard and tough or soft and rubbery. Whether made as filaments or as moldings, polyamides are characterized by a high degree of crystallinity, particularly those derived from primary amines. Under tension, orientation of molecules continues until the specimen is drawn to about four times its initial length, a property of particular importance in filaments. Two of the ingredients that are used to synthesize the most common nylon, adipic acid and hexamethylenediamine, each contain six carbon atoms, and the product has been named nylon-6,6. When caprolactam is the starting material, nylon-6 is obtained, so named because it has six carbon atoms in the basic unit.

3.Polyurethanes are a versatile class of polymers with great control over their physicochemical properties based on the chemical composition. Segmented PURs are designed with well-defined degradation and mechanical properties combined with excellent biocompatibility that makes them attractive for the development of drug delivery, tissue engineering, and medical devices. Various PURs including PEURs, poly(ester urethanes), PCURs, PSURs, surface-modified PURs, and composite PURs have been developed for a variety of biomedical applications. Many research efforts are continued in the development of PURs for specific drug delivery and tissue regeneration application with a particular emphasis on biocompatibility and biodegradability.

4.The polymer system of nylon is 65-85 per cent crystlalline and, about 35-15 per cent amorphous. This makes nylon a very crystalline, very well aligned or oriented polymer system, with the inter-polymer distance about 0.3 nm. Short inter-polymer distance results in the formation of an optimum number of strong and uniform hydrogen bonds. Hydrogen bonds can be formed across a maximum inter-polymer distance of 0.5 nm.

5. The main interest of the formation of polyurea originates from the potential ability to process them in to fibers. Moreover they are high melting polymers, having higher melting points than the corresponding polyamides. This is due to the high molecular cohesion present in the N-FI hydrogen bonding. Most of the common methods for preparation of polyurea consist of polymerization of diamines with various classes of compounds. They can be synthesized using different starting reagents and monitoring different reaction conditionsThe polyurea were considered as derivatives of polyamide of carbonic acid from amidation process between diamines and the other reaction monomers of carbon dioxide CO2, carbon oxysulfide COS, carbonic esters -COOR, phosgene, urea, urethane and isocyanates that give rise to (-NH-CO-NH-)-n linkage. As an example, hexamethylene diamine gives polyurea with melting point 300°C, nonamethylenediamine 245°C. The properties observed derived from the preparation ofpolyurea using diamine and urea seems to offer advantage over the other methods, therefore the process is separated from the polyamides. It is also been reported that the melting point increases with increasing molecular weight of the material. Thus molecular weight increases, viscosity also increases simultaneously. Moreover it is also reported that in aliphatic polyurea the melting point decreases with increase, in number of methylene groups. Thus the chain extenders are used for regulating the molecular weight of the polymer from substituted ureas, naphthalene or dodecylbenzenesulphonic acid and aromatic primary or secondary amines.

6.In one type of polymerization reaction, a series of condensation steps takes place whereby monomers or monomer chains add to each other to form longer chains. This is termed “condensation polymerization,” or “step-growth polymerization,” and occurs in such processes as the synthesis of polyesters or nylons. Nylon is a silky material used to make clothes made of repeating units linked by amide bonds, and is frequently referred to as polyamide. This reaction may be either a homopolymerization of a single monomer A-B with two different end groups that condense, or a copolymerization of two co-monomers A-A and B-B. Small molecules are usually liberated in these condensation steps, unlike polyaddition reactions.Condensation polymers often require heat, form slower than do addition polymers, and are lower in molecular weight. This type of reaction is used as a basis for making many important polymers, such as nylon, polyester, and various epoxies. It is also the basis for the laboratory formation of silicates and polyphosphates. Many biological transformations, such as polypeptide synthesis, polyketide synthesis, terpene syntheses, phosphorylation, and glycosylations are condensations.

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