MAT6102 CANDIDATES ARE ADVISED TO READ THROUGH THE INSTRUCTIONS GIVEN ON THIS PAGE CAREFULLY BEFORE ATTEMPTING ANY QUESTIONS THE PHYSICS OF POLYMERS Autumn Semester Exam 2018-2019 Duration of Exam: 3 Hours There are SIX questions Answer FOUR questions ONLY PLEASE WRITE YOUR ANSWERS IN THE ANSWER BOOK PROVIDED NOT ON THE EXAM PAPER. ANYTHING WRITTEN ON THIS PAPER WILL NOT BE CONSIDERED. Department Of Materials Science & Engineering. MAT6102 2 MAT6102 MAT6102 3 MAT6102 There are SIX questions - Answer FOUR questions ONLY 1. (a) Name two polymer rubbers and write down their chemical formulae. [4 marks] (b) Define the term “submolecule” in an ideal rubber in rubber elasticity theory. [3 marks] (c) Describe how the mean square end-to-end distance of an amorphous polymer chain would change with increasing temperature. Provide a brief explanation with your answer. [3 marks] (d) On the basis of your answer to (c), explain why the shear modulus of a lightly cross-linked rubber would increase with increasing temperature. [2 marks] (e) Write down the equation for the shear modulus G of an ideal rubber, and define all the symbols. [4 marks] (f) A polybutadiene rubber -(C4H6)n- has on average 100 monomer units between cross-links, and a density of 900 kg m-3. Calculate its shear modulus G at 300 K using the equation in your answer to (e). [4 marks] Note: Relative atomic mass of C: 12, H: 1; Avogadro Number NA= 6.02 x 1023 mol-1; Boltzmann constant: 1.38 x 10-23 J K-1. 2. (a) Define the term “glass transition temperature”. [2 marks] (b) Describe the key differences, at the molecular level, between the glassy and rubbery states of an amorphous polymer. [4 marks] (c) Sketch the specific volume of an amorphous polymer as a function of temperature, and explain how its glass transition temperature (Tg) can be determined from the data plot. [4 marks] (d) Describe the free volume theory of glass transition, and use it to explain how the specific volume of an amorphous polymer changes as a function of temperature around Tg. [6 marks] (e) On the basis of the free volume theory, briefly explain how the following changes/modifications to an amorphous polymer would affect its glass transition temperature. i. Introduction of flexible side branches ii. Water absorption in humid environment iii. Increase in molar mass iv. Higher degree of cross-linking [4 marks] TURN OVER MAT6102 4 MAT6102 3. (a) Define the “relaxation modulus” of a linearly viscoelastic polymer. [3 marks] (b) Sketch the Zener model (or standard linear model) of a viscoelastic polymer, using spring(s) and dashpot(s) as its components. [3 marks] (c) Sketch the creep compliance curves of the Zener model, at two different temperatures T0 and T1 (T1 > T0). In your sketch make clear how the two curves are related. [3 marks] (d) Describe the time-temperature superposition principle of viscoelastic polymers. [3 marks] (e) Describe how the time-temperature superposition principle can be used to construct a “master curve” from a set of relaxation modulus data collected at different temperatures for a viscoelastic polymer. [4 marks] (f) The creep compliance J(t) of a viscoelastic polymer is given by J(t) = 1 + t0.5 GPa-1 (t in hours) A stress σ = 1.00 x 105 Pa is applied at t = 0, and then decreased to 4.0 x 104 Pa at t = 1 hour. Calculate the induced strain ε(t) at t = 2 hours, using the Boltzmann superposition principle. [4 marks] 4. (a) Describe the main differences in molecular structure of the following grades of polyethylene: LLDPE, LDPE and HDPE. [3 marks] (b) Describe the main differences in the crystallinity and mechanical properties (rigidity and toughness) of LLDPE, LDPE and HDPE, and explain the differences on the basis of their different molecular and micro-structures. [6 marks] (c) Describe how the crystallinity and melt flow index (MFI) can be combined to predict whether a particular grade of PE is brittle or tough. [3 marks] (d) Calculate the density of a LDPE sample with a crystallinity of 50%. The density of an amophous PE is 0.85 x 103 kg m-3, and that of pure PE crystal is 1.00 x 103 kg m-3. [3 marks] (e) Describe the “after shrinkage” of a semicrystalline polymer, and how it can be reduced in the processing of iPP. [5 marks] CONTINUED MAT6102 5 MAT6102 5. (a) Explain why Poly(Vinyl Chloride) (PVC) is always plasticized, and describe the two main methods, internal and external plasticization of PVC. [6 marks] (b) Use a sketch to show how a PVC molecule behaves differently in a “good” and in a “poor” solvent, and explain why a plasticizer which is a poor solvent of the polymer should be used to make the polymer more resilient (elastomeric). [4 marks] (c) Describe how High Impact Poly(styrene) (HIPS) is synthesized, and list the main structural components of the polymer. [4 marks] (d) Describe the morphology of HIPS as observed under a microscope, with the help of a sketch. Explain how the complex morphology results from microphase separation of different components of the polymer. [4 marks] (e) What is the main toughening mechanism in HIPS? [2 marks] 6. (a) Describe the similarities and differences between the crystal structure of linear polyamides (PA) and that of polyethylene (PE). [3 marks] (b) Explain how the differences in the thermal and mechanical properties of PA and PE result from differences in their crystal structures. [4 marks] (c) Describe the improvement in properties that are desired in engineering thermoplastics over commodity polymers for structural applications. [3 marks] (d) Describe the most important changes made to the polymer molecular structure to achieve such engineering thermoplastics. [2 marks] (e) Name two engineering structural thermoplastics and write down their chemical formulae. [4 marks] (f) Polycarbonate is used to make Compact Discs (CDs). Explain briefly the material selection process on the basis of product requirement and materials properties. [4 marks] END OF QUESTION PAPER
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