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Introduction to Polymer Science and Technology Polymerisation2.5.2 Solubility parameter Substances can dissolve in liquids if the forces holding the molecules together in the solids can be overcome. The polymer molecules in thermoplastics are normally held together by secondary forces and physical chain entanglements. The secondary forces are much weaker than the primary covalent bonds that exist between the repeat units in the polymer molecule backbone chain, and they can be overcome by appropriate solvents to produce solutions. A measure of the strength of secondary bonds is given by the cohesive energy density (CED): CED = AEv / V, where, AEy is the molar energy of vaporisation and Vj is the molar volume of the liquid. The solubility parameter is related to the CED by the equation, 5 = (CED)1'2 in (J/m3)1/2or (MPa)1'2. The solubility parameter becomes a useful guide to the miscibility/compatibility of different polymers. A convenient method of estimating 5 value for a polymer is to identify a solvent that causes maximum swelling in network polymers or maximum intrinsic viscosity (intrinsic ability of a polymer to increase the viscosity of a particular solvent at a given temperature) value for soluble TP polymers. It is expected that when the polymer and solvent have the same 5, the maximum swelling/expansion will occur in the polymer molecules and therefore the highest viscosity (for a given concentration) will be obtained. By measuring the viscosity of dilute solutions of a polymer in a variety of solvents, one can determine a consistent value of 5 for the polymer. In a cross-linked network polymer, solution cannot occur, but swelling can occur in polymer segments between cross links. Maximum swelling is experienced when the solubilities match. Alternatively, 5 can be calculated from CED values, which in turn may be calculated using the molar attraction constants (values of molar attraction constants for a number of organic chemistry groups are presented in Fried 1995, plOO). 2.6 Self-assessment questions (source:Painter & Coleman 2004) 1. Styrene monomer can be polymerized by practically all chain polymerization methods. Which of these methods a) free radical b) anionic c) cationic d) coordination 2. Suspension free radical polymerization of styrene would be preferred over bulk polymerization to overcome the a) branching b) cross-linking c) isotacticity d) heat of reaction during polymerization 3. In emulsion polymerization, the principal place where the monomer polymerizes is a) monomer droplets b) aqueous phase c) swollen surfactant micelles Introduction to Polymer Science and Technology Polymerisation d) surface of reactor 4. If a polymer chain has a molecular weight of 280,000, how many ethylene units does it contain? 5. Which of the monomers/monomer combinations in the box below polymerises by the step-wise polymerisation Î Î
(a) >-Ñ-ÎÍ and HO-CH2-CH2-OH
(b) CH=CH2 and CH2=CH-CH=CH2 (butadiene)
(d) HO-CH2-CH-CH2-OH + ÍÎ-Ñ-(ÑÍ2)4-Ñ-ÎÍ (å) H2N-(CH2)6-NH2 + ÍÎÎÑ-(ÑÍ2)4-ÑÎÎÍ 6. Which of the above monomers can be polymerized free radically at high pressures to give a polymer containing 7. Which monomers in Question 5 can form polyester? 8. Which of the monomers in Question 5 containing a C=C double bond cannot be polymerized free radically? 9. Which monomers in Question 5 are used to make a thermoplastic rubber? 10. Indicate the pairs of monomers in Question 5 to make an ethylene/propylene random copolymer, and also which
a) free radical polymerization b) anionic polymerization c) cationic polymerization d) using a catalyst (coordination polymerization) 11. Which monomer or mixture of monomers in Question 5 is commercially polymerized using a catalyst to produce 12. Which monomer or mixture of monomers in Question 5 polymerizes to form a cross-linked polymer network? 13. Which monomer or mixture of monomers in Question 5 polymerizes to form Nylon 6,6? 14. Compare the water absorption capacity of PA6,6 with PA 6,12, indicate reasons for differences. 15. Distinguish between the repeating units for addition and condensation type of polymers, and give an example Introduction to Polymer Science and Technology Polymerisation 16. A polyamide polymerised from a 2-carbon diamine and a 6-carbon diacid, select its name: a) nylon 4 b) nylon 2,6 c) nylon 6,6 d) nylon 6,2 17. Which of the following polymers is least likely to be optically transparent? a) atactic polystyrene b) isotactic polystyrene c) an ethylene/propylene random copolymer d) a styrene/butadiene random copolymer 18. Polyethylene is used for making carrier bags and bullet-proof vests: true or false. 19. The three commonly used metals in metallocene catalysts are a) Zr, Co and Cu b) Zr, HfandTi c) À1, Zn and Ti d) Au, YandRh 20. High pressure, high temperature free-radical polymerization of ethylene produces a) HDPE b) LDPE c) PP d) LLDPE Introduction to Polymer Science and Technology Polymerisation
21. 22. 23. 24. 25. Calculate the molecular weights of the repeating units of polypropylene and PVC. Determine Mw for a polypropylene of average degree of polymerisation of 18,000. (Atomic masses of H = 1, Ñ = 12, and Cl = 35). Answer: m(PP) = 42 g/mol; m(PVC) = 62 g/mol; Mw = 756xlO3g/mol. A polystyrene specimen has a number average molecular weight of 80,000 and a polydispersity index of 5. Calculate the weight average molecular weight. The molecules of a sample of polystyrene can be divided into 5 groups in terms of their molecular weight with the same number of molecules in each group. The molecular weights of the molecules in the groups are 10,000; 20,000; 30,000; 40,000; 50,000. Calculate M . Answer: M = 30,000. Calculate M for the data given in Question 22. Answer: JVf = 3.67 x 104.
Indicate what the subscripts (a), (b), (c) and (d) stand for on the above molecular-weight distribution curve, where M represents molecular weight. Date: 2015-12-11; view: 1647
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