Chapter 6 POLYMER CHEMISTRY Lecture/Lession Plan -4 POLYMER CHEMISTRY 6.1 Rubber Rubber is a natural elastomeric polymer whose monomer unit is cis-2-methyle-1,3-butadiene. Raw rubber material is extracted from rubber plant which is milk like sappy and negatively charged colloid, get polymerized bio chemically. The main source of natural rubber is rubber tree (scientific name: Hevea Basiliensis.) Cis polyisoprene(natural rubber) Rubber tree is found mainly in tropical countries like southern part of India, Srilanka, Malaysia, Thailand, South America. Solid rubber is white brownish amorphous solid which is soluble in petrol/carbon disulphide. The main compositions of latex which is extracted from rubber plant are - Water - 55-60 percent Rubber (cis isoprene) - 32-35 percent Protein - 02-03 percent Fatty acid - 01 percent Soluble salt - 01 percent Ash - 0.03 percent The approximate molecular weight of rubber polymer is 100000-160000. Due to its cis configuration, 11
12 CHAPTER 6. POLYMER CHEMISTRY LECTURE/LESSION PLAN -4 natural rubber is well packed and they held together by Van-der Waals force of attraction. That is why natural rubber has weak molecular force and it is called elastomer. To increase the elastic properties of rubber during rubber preparation the extracted latex is treated with sunlight/heat/chemicals (mainly sulpher or hydrogen sulphide etc). The function of sulpher is breaking of pi electron in rubber and make three dimensional cross linking between double bonded carbon atom by sulpher bridge. This is called vulcanization of rubber. In this way natural rubber becomes very strong highly elastic low abrasion resistance towards many chemicals and hard. Trans polyisoprene(gutta percha) The Trans isomer of 2-methyle-13-butadiene is known as Gutta Percha obtained from the tree named Dischopsis Gutta and Palagum Gutta. 6.2 Synthetic rubber Synthetic rubbers are: 1) Styrene butadiene rubber(sbr) 2) Neoprene rubber 3) Butyl rubber 4) Nitrile rubber 5) Polysulphide rubber 6.2.1 Styrene butadiene rubber(sbr) Buna-S rubber is a copolymer obtained by the addition of butadiene and styrene at a ratio 3:1 in a emulsion system in presence of free radical initiator like benzoyl peroxide or cumene hydro peroxide with support of dextrose.
6.2. SYNTHETIC RUBBER 13 Styrene butadiene rubber(buna-s) The produced rubber is called cold rubber as the polymerization carried at temperature -15 0 C to 5 0 C. At this temperature the chain length can be controlled. If the reaction temperature is 50 0 C then he rubber is called hot rubber and in this case the chain can not be controlled. Such types of synthetic rubbers are more efficient than natural rubber. These rubber have high tensile strength, low abrasion oxidation and resistance to weather oil and acid base. 6.2.2 Nitrile rubber (NBR or GR-A or Buna-N) Buna -N rubber is a copolymer obtained by the addition of butadiene and acrylonitrile at a ratio 3:1 in a emulsion system in presence of free radical initiator like benzoyl peroxide or cumene hydro peroxide. Synthetic rubbers can be vulcanized as there is weak pi bond present in the long back bone polymeric chain.
14 CHAPTER 6. POLYMER CHEMISTRY LECTURE/LESSION PLAN -4 Nitrile butadiene rubber(buna-n) Such types of synthetic rubbers are more efficient than natural rubber. These rubber have high tensile strength low abrasion oxidation and resistance to weather oil and acid base. Due to presence of -CN group this is less resistance towards base. 6.2.3 Neoprene rubber(gr-a) Neoprene rubber is also called polychloroprene or duprene. Neoprene rubber is a copolymer obtained by the addition of monomer chloroprene (2-chloro-13-butadiene). Chloroprene is prepared by dimerisation of acetylene by passing through aqueous solution of ammonium chloride (NH 4 Cl) and cuprous chloride(cu 2 Cl 2 )at a temperature 70 0 C. HCl is added finally. Free radical initiators like benzoyl peroxide or cumene hydro peroxide or oxygen enhance the polymerization of chloroprene to neoprene. 6.3 Biodegradable polymer Polychloroprene(Neoprene) The higher weather stability of polymer is economically profitable and scientists are continuously trying to make the polymeric substance durable. But some polymers are not biodegradable. That means micro organism cannot decompose them and the major problem is after use the polymeric products are thrown hare and there which makes layers on the earth and water cannot penetrate to the under ground in rainy season. To overcome the disposal problem few thermoplastic polymer are
6.4. MONODISPERSE AND POLYDISPERSE POLYMER 15 recycled. But during every recycling the plastic properties of the polymers are gradually decreased. The thermosetting polymers are not recyclable. Therefore they cause major problem for our ecosystem. This problem are solved by preparing plastics which get degrade spontaneously after certain period of time (require for use). Such polymers which are biodegradable by chemical oxidation or by micro organism are called biodegradable polymers. He the carbon-carbon bond in polymers is inert due to non-polarity and cannot break by means of natural micro organism or by chemical treatment. To make the polymers biodegradable it is needed to incorporate some reactive functional group in the carbon-carbon bond of the polymer. Mainly used such type functional group is ester(-coor) where the carbon-carbon bond is not directly linked but linked by (-O-). Due to the incorporation of ester functional group in between carbon-carbon bond the polymer become biodegradable.when these polymer is buried as waste they will be degraded by microorganism through enzymatic catalysis. The beat examples of bio degradable polymers are- Poly-hydroxy butyrate valerate(phbv) Poly-hydroxy butyrate (PHB) Polyglycolic acid(pga) Polylactic acid(pla) Poly-hydroxy butyrate valerate(phbv): It is a co-polymer. Poly-hydroxy butyrate valerate(phbv) This type of bio-degradable polymer is prepared from 3-Hydroxybutanoic acid and 3-Hydroxypentanoic acid. It is normally used in the preparation of packaging materials for orthopedic device. The main drawback of these polymers is very costly. Fig. The presence of ester linkage is responsible to bio-degradation by microorganism through enzymatic catalysis. Use of such copolymers: 1. Preparation of packaging materials for orthopedic device. 2. Preparation of film. 3. Preparation of molded items. 6.4 Monodisperse and polydisperse polymer Polymers which have molecular weight equal to an integral multiple of the molar mass of any chain due to presence of equal chain length in the polymer called monodisperse polymer. Examples are Protein Starch Polymers which have molecular weight are not equal to an integral multiple of the molar mass of any chain due to presence of mixture of different chain length in the polymer called polydisperse polymers. The main reason is that the polymerization occurs through random collision of the molecules and the termination of chains can occur with different length or different size. Examples are mainly synthetic polymer like polyester phenol formaldehyde resin urea formaldehyde resin melamine formaldehyde resin etc. The molecular weight of such polymer is not fixed and varies in between certain range. In case of such polymer the molecular weight is average molecular weight.
16 CHAPTER 6. POLYMER CHEMISTRY LECTURE/LESSION PLAN -4 6.5 Molecular weight of polymer There are three types of molecular weighta) Number average molecular weight b) Weight average molecular weight c) Viscosity average molecular weight 6.5.1 Number average molecular weight Number average molecular weight is determined by the total weight of the all different molecules of different chain length divided by the total number of molecules present. Number average molecular weight can be determined by colligative property measurement. If a polymer sample have N 1 N 2 N 3 N n number of molecules according to the chain length and the molecular masses are M 1 M 2 M 3 M n respectively. The number average molecular weight (M N )can be determined as- M N Total mass of the polymer Total number of molecules in polymer N 1M 1 + N 2 M 2 + N 3 M 3 +... + N n M n N 1 + N 2 + N 3 +... + Nn Ni M i Ni 6.5.2 Weight average molecular weight Weight average molecular weight is determined by the total of different weight fractions (individual weight of any chain or molecule divided by total molecular weight of the polymer) of the polymer multiplied by their respective molecular weight. Weight average molecular weight can be determined by light scattering measurement method. If a polymer sample have N 1, N 2, N 3, N n number of molecules according to the chain length and the molecular masses are M 1, M 2, M 3, M n respectively then the weight fraction of different polymeric chains will be W 1, W 2, W 3, W n respectively. Now W 1 W 2 Total mass of the 1st species Total mass of the polymer N 1 M 1 N 1 M 1 + N 2 M 2 + N 3 M 3 + +NnMn N 1 M 1 Ni M i Total mass of the 2nd species Total mass of the polymer N 2 M 2 N 1 M 1 + N 2 M 2 + N 3 M 3 +... + NnMn N 2 M 2 Ni M i
6.5. MOLECULAR WEIGHT OF POLYMER 17 and W i Total mass of the i-th species Total mass of the polymer N i M i N 1 M 1 + N 2 M 2 + N 3 M 3 +... + NnMn N i M i Ni M i Then the number average molecular weight can be determined as (M W ) M W W 1 M 1 + W 2 M 2 + W 3 M 3 + +WnMn W i M i ( N 1M 1 )M 1 + ( N 2M 2 )M 2 + ( N 3M 3 )M 3 + +( N nmn )Mn Ni M i Ni M i Ni M i Ni M i N 1 M 2 1 Ni M i + N 2M 2 2 Ni M i + N 3M 2 3 Ni M i + + N nm 2 n Ni M i Ni M 2 i Ni M i 6.5.3 Viscosity average molecular weight The viscosity average molecular weight M V of the polymer can be determined from the intrinsic viscosity([η]) [η] [ η Solution η Solvent ] η Solvent η Specific For this purpose different concentration(let 20/40/60/80 percent) of a polymer is prepared in nonreacting solvent and the viscosity as well as specific viscosity is measured. A straight line graph with positive slope has been plotted for specific viscosity against the concentration of the polymer. By extrapolating the line on Y axis and where is touches the Y axis that point is the intrinsic viscosity([η]) of the polymer. Fig. Then using the famous equation of Mark-Kuhn-Houwink i.e. [η] KM a V we will get the viscosity average molecular weight of the polymer. Where K and a are constant for specific pair of polymer and solvent and obtained from the literature. It is observed that the value of a vary in the range 0.5 to 0.9.
18 CHAPTER 6. POLYMER CHEMISTRY LECTURE/LESSION PLAN -4 Now [η] KM a V log[η] log(m a V) log[η] logk + alogm V Putting the respective value of two constant K and a and [η] we will get the value of M V. Different molecular mass distribution in polymer It is important to remember that for monodisperse(polymers which have molecular weight equal to an integral multiple of the molar mass of any chain due to presence of equal chain length in the polymer) polymer M W M N The normal range of a vary from 0.5 to 0.9 and in such case viscosity average molecular weight (M V ) always less than weight average molecular weight(m W ). When the value of constant a becomes unity the viscosity average molecular weight (M V ) Weight average molecular weight (M W ). 6.6 Degree of polymerization It is the number of monomer units present in a polymer. It can be expressed in two way 1) Number average degree of polymerization(dm N ) 2) Weight average degree of polymerization(dm W ) Number average degree of polymerization and weight average degree of polymerization (DM N ) M N M O (DM W ) M W M O where M O is the molecular weight of monomer unit.
6.7. POLYDISPERSE INDEX(PDI) 19 6.7 Polydisperse Index(PDI) The Ratio of weight average molecular weight (M W ) with respect to number average molecular weight (M N ) is called poly disperse indec(pdi) (PDI) M W M N