Sciencedoze: Science, Education and Technology

What is glass transition temperature? When plastic or rubber is cooled up to a certain temperature, it becomes so hard and brittle that it breaks into pieces on the application of stress. So, the temperature below which the polymer becomes hard, brittle, and glassy and above which it is softener and flexible is known as glass transition temperature (Tɡ). The glass transition is a property of only the amorphous region of a semi-crystalline solid whereas the crystalline portion remains crystalline during the glass transition. It is important to note that the transition does not occur suddenly at a unique temperature but rather over a range of temperatures. The temperature in the middle of the inclined region is taken as the Tɡ. Significance of glass transition temperature (Tg) Glass transition temperature (Tɡ) is used as a measure for evaluating the flexibility of a polymer and the type of response the polymeric material would exhibit to mechanical stress. Glass transition temperature (...

What is Crystalline Melting Point ? When a polymer is treated beyond its glass transition temperature (T𝘨), it passes from a glassy state to a rubbery state and further heating causes melting of the polymer and starts flowing.  So we can say that the temperature below which the polymer is in a rubbery state and above which it is a liquid is called the crystalline melting point (T𝘮) of the polymer. Factors affecting crystalline melting point (T𝘮) Following are the factors which affect the crystalline melting point (T𝘮). 1. Chain Flexibility Chain flexibility and crystalline melting point (T𝘮) are inversely proportional to each other that is higher the flexibility, the lower will be the Tm . Also, the presence of double bond and aromatic group in the polymeric backbone lowers the flexibility which increases the crystalline melting point (T𝘮). 2. Size and type of side groups Size and type of size groups greatly influence the crystalline melting point (T𝘮). Bulky or lar...

What is Vulcanization of rubber ? As we know that, natural rubber is soft and sticky and becomes even more so at high temperatures and brittle at low temperatures. Also, it is not resistant to the action of organic solvents and can be easily attacked by oxidizing agents.  So, to solve this problem, Charles Goodyear in 1839, found a process called Vulcanization , which can improve the physical and chemical properties of rubber. Vulcanization is a process that consists of heating raw rubber with sulfur at 373-415 K temperature. Since this process is slow, therefore additives like zinc oxide are used to accelerate the rate of vulcanization. During vulcanization, sulfur bridges or cross-links between polymer chains are introduced either at their reactive allylic positions or at the sites of the double bonds.  These cross-links make rubber hard and stronger and remove the stickiness of natural rubber because the individual chains can no longer lip over but are instead locke...

1. Natural Rubber Natural rubber is a natural polymer that is manufactured from latex which is a colloidal solution of rubber particles in water. Latex is obtained by making cuts in the bark of rubber trees like Hevea brasiliensis, found in tropical and semi-tropical countries such as southern. India, Indonesia, Malaysia, Sri Lanka, South America, etc. The natural rubber has remarkable elasticity and undergoes long-range reversible extension even under a relatively small applied force. Preparation of natural rubber Natural rubber is a linear 1,4-addition polymer of isoprene. Since each repeating unit in polyisoprene contains a double bond having cis-stereochemistry. That's why natural rubber is cis-polyisoprene. Properties of Natural rubber Natural rubber has no polar groups and hence intermolecular forces of attraction are only weak van der Waals interactions. Cis-polyisoprene does not have a straight-chain but has a coiled structure. As a result, it can be stretched like a spri...

What are Conducting Polymers? As the name suggests organic polymers that conduct electricity are known as conducting polymers. They are also known as intrinsically conducting polymers (ICPs) and they have alternating single and double bonds along the polymer backbone (conjugated bonds) or that are composed of aromatic rings such as Phenylene, naphthalene, anthracene, pyrrole, and thiophene which are connected through carbon-carbon single bonds. Examples: Polyacetylene, Polypyrrole, Polyaniline, etc What is the reason behind conducting nature of these polymers? Conducting polymers comes in two forms that are doped conducting polymers and non-doped conducting polymers. The conductivity of non-doped conjugated polymers is due to the existence of a conductivity band similar to a metal. In a conjugated polymer, three of the four valence electrons form strong sigma bonds through sp² hybridization where electrons are strongly localized. The remaining unpaired electron of each carbon atom re...

What is Degree of Crystallinity? The degree of crystallinity of the polymer is defined as the fraction of the sample which is crystalline. It can be either expressed in terms of the mass fraction or the volume fraction. The degree of crystallinity by a volume fraction is given by Where,  Xₘ = Degree of crystallinity by mass V = Specific volume of the sample Vₐ = Specific volume of fully amorphous polymer Vc = Specific volume of fully crystalline polymer Factors affecting the degree of crystallinity 1. Molecular weight With the increase in molecular weight of the polymer, the degree of crystallinity increases due to a large number of entanglement of the chain which restricts the growth of a crystallite. 2. Symmetry of repeating unit A symmetrical repeating unit structure like CH₂ makes it easier for the formation of crystallites. Thus, increasing the degree of crystallinity. That's why random copolymers do not crystallize because there is no regularity of the repeating unit. 3. Chai...

What are Silicone Polymers? Silicones are polymeric organosilicon compounds containing (Si-O-Si) linkages and have the general formula (R₂SiO) . They are typically colorless, oils, or rubber-like substances that may be linear, cyclic, or crosslinked.  Silicone polymers have very high thermal stability and are also called high-temperature polymers. They are widely used in sealants, adhesives, lubricants, medicine, and cooking utensils. Preparation of Silicone Polymer When methyl chloride reacts with silicon in the presence of copper as a catalyst at a temperature of 570 K , various types of methyl-substituted chlorosilanes are formed. Hydrolysis of dimethyldichlorosilane that is (CH₃)₂SiCl₂  followed by condensation polymerization forms straight-chain polymers called silicone polymers. Properties of Silicone polymer 1. It has low thermal conductivity, chemical reactivity, and toxicity. 2. They have low surface tensions and are capable of wetting most surfaces. 3. It does not s...

1. Butadiene-styrene Copolymer (SBR or Buna-S) Buna-S or SBR is a copolymer of 1,3-butadiene and styrene which is prepared by copolymerization of 1,3-butadiene and styrene along with sodium. In Buna-S, Bu stands for butadiene, Na for sodium which is the polymerizing agent and S stands for styrene. It is tough and is a good substitute for natural rubber. It possesses high abrasion resistance, high load-bearing capacity and is used for the manufacture of automobile tires. Preparation of Buna-S Styrene-butadiene rubber (SBR) or Buna-S can be prepared by copolymerization of a mixture of 1,3-butadiene and styrene in the ratio of 3:1 in the presence of sodium. Properties of Buna-S 1. Buna-S has high abrasion resistance, high bearing capacity, and resilience. 2. Buna-S is very tough and is a good substitute for natural rubber 3. Buna-S swells in organic solvents up to a limited extent. 4. Buna-S gets oxidized in the presence of traces of ozone. Applications of Buna-S 1. Buna-S is used for ...

What are Polyolefins? A polymer that is produced from a simple olefin monomer (also called alkene having general formula CₙH₂ₙ) is called Polyolefin . For example (a) Polyethene is a polyolefin produced by polymerization of olefin ethylene (b) Polypropylene is a polyolefin produced by polymerization of olefin propylene. The IUPAC nomenclature of polyolefins is poly (alkene) . Polyolefins are produced by different common forming techniques like injection molding, blow molding, extrusion, and thermoforming using standard thermoplastic equipment. Due to low prices and easy processability, polyolefins have a variety of applications in the plastic industry. Application of Polyolefins 1. Polyolefins are used for making blow-molded components in toys. They are also used to make heat-shrink tubing which protects connections in electronic items. 2. Polyolefins are used in the pharmaceutical and medical industry for HEPA filter certification. 3. Polyolefins sheets have packaging applications of...

What are Polycarbonates? Polycarbonate (PC) are high-performance tough thermoplastic polymers that have organic functional groups linked together by carbonate groups (-O-(C=O)-O-).   Polycarbonates have high impact resistance and are obtained by condensation of diethyl carbonate or carbonyl chloride and bisphenol-A. It is widely used for bullet-proof windows and safety or crash helmets. Lexan is the most common example of Polycarbonates. Structure of Polycarbonate Preparation of Polycarbonate Polycarbonate is produced by condensation polymerization between bisphenol A and either Carbonyl chloride or diphenyl carbonate. (a) Preparation of Polycarbonate by condensation polymerization between bisphenol-A and Carbonyl Chloride The polymer is usually formed by the reaction of bisphenol-A and carbonyl chloride in a basic solution where polymerization takes place at the interface between the aqueous and organic layer with the help of a catalyst can amine. (b) Preparation of Polycarbonat...

What are Polyamides?  Polymers that have amide linkages are called polyamides. These are prepared by condensation polymerization of dibasic acids with diamines or their equivalents. Polyamides occur both naturally and artificially like proteins, such as wool and silk are naturally occurring polyamides whereas nylons, aramids and sodium poly(aspartate) is artificially manufactured polyamides. Following are some famous polyamides: 1. Nylon 6,6 Nylon 6,6 is a type of polyamide that is made of two monomers that is hexamethylenediamine and adipic acid each containing 6 carbon atoms. That's why it is called nylon 6,6. Nylon 6,6 has high mechanical strength, rigidity, and stability under heat and chemical attack. That's why it is used for making machine parts, airbags, ropes, conveyor belts, etc. Preparation of Nylon 6,6  It is prepared by the condensation polymerization of adipic acid and hexamethylenediamine. The acid and the amine first react to form a salt which when heated...

What is Poly (vinyl acetate)? Poly (vinyl acetate) (PVA) was discovered in Germany in 1912 by Fritz Klatte. It is an aliphatic rubbery synthetic polymer with chemical formula (C₄H₆O₂)n which belongs to the polyvinyl ester family. It is also known as wood glue or Elmers's glue due to its adhesive properties for which it is used as adhesive for porous materials like wood, paper, and cloth. Structure of Poly (vinyl acetate) (PVA) Preparation of Poly (vinyl acetate) (PVA) Poly (vinyl acetate) (PVA) is a vinyl polymer that is prepared by free radical vinyl polymerization of the monomer vinyl acetate. Properties of Poly (vinyl acetate) (PVA) 1. PVA is an amorphous polymer having a glass transition temperature (T𝘨) between 30-45 ℃ . 2. PVA offers good adhesion to most of the surfaces. 3. Degree of polymerization of PVA is between 100 to 5000 and it can be treated with alkali, which gradually results in polyvinyl alcohol and the alkali acetate. 4. PVA does not cross-link and it ca...

What is Polyurethane? Polyurethane (PU) is a polymer having organic units joined by urethane (also known as carbamate). Urethane is a compound that has an O-R group and an NH-R group bonded to the same carbonyl carbon. Polyurethanes are thermosetting polymers but their thermoplastic variants are also available in the market. Polyurethanes are prepared by the polymerization of toluene-2,6-di-isocyanate and ethylene glycol. If the reaction is carried out in the presence of a blowing agent, the product is polyurethane foam. Preparation of Polyurethanes Polyurethanes are formed by reacting a polyol (an alcohol with more than two reactive hydroxyl groups per molecule like ethylene glycol) and di-isocyanate or a polymeric isocyanate in the presence of suitable catalyst and additives. Properties of Polyurethanes 1. Polyurethane has high load capacity in both tension and compression that may change shape under heavy load but will return to its original shape once the load is removed ...

What is Polystyrene ? Polystyrene, also known as Polyvinyl benzene is hard, brilliantly transparent, stiff resin which is produced by the free radical addition polymerization of styrene in the presence of benzoyl peroxide. Polystyrene is non-biodegradable and widely used in the food-service industry as rigid trays, containers, disposable eating plates, bowls, etc. Preparation of Polystyrene Polystyrene is prepared by free radical addition polymerization of styrene in the presence of benzoyl peroxide as a catalyst. Properties of Polystyrene 1. Polystyrene exists in an amorphous state due to the presence of bulky phenyl groups which makes packing of Polystyrene inefficient. 2. Polystyrene is non-polar having a melting point of 240 ℃. 3. Polystyrene is a transparent polymer having a good optical property that allows high transmission of all wavelengths. 4. Polystyrene is hard but brittle due to the chain-shifting effect of the benzene ring. 5. Polystyrene has good insulation properti...