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Lecture# 3 Molecular Mass and Chain Microstructure

Lecture# 3 Molecular Mass and Chain Microstructure. Mass vs. Weight Molecular “Weight” and Distribution Averages Polydispersity Property Implications. Polymer Chain Length. Polymer Chain Length Polymer notation represents the repeating group

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Lecture# 3 Molecular Mass and Chain Microstructure

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  1. Lecture# 3Molecular Mass and Chain Microstructure • Mass vs. Weight • Molecular “Weight” and Distribution • Averages • Polydispersity • Property Implications PE335 Lecture 2

  2. Polymer Chain Length • Polymer Chain Length • Polymer notation represents the repeating group • Example, -[A]-n where A is the repeating monomer and n represents the number of repeating units. • Molecular Weight • Way to measure the average chain length of the polymer • Defined as sum of the atomic weights of each of the atoms in the molecule. Example: • Water (H2O) is 2 H (1g) and one O (16g) = 2*(1) + 1*(16)= 18g/mole • Methane CH4 is 1 C (12g) and 4 H (1g)= 1*(12) + 4 *(1) = 16g/mole • Polyethylene -(C2H4)-1000 = 2 C (12g) + 4H (1g) = 28g/mole * 1000 = 28,000 g/mole PE335 Lecture 2

  3. MOLECULAR WEIGHT Low M • Molecular weight, M: Mass of a mole of chains. high M • During the polymerization NOT ALL chains in a polymer grow to the same length, so there is a distribution of molecular weights. • The molecular weight distribution in a polymer describes the relationship between the number of moles of each polymer species and the molar mass of that species. PE335 Lecture 2 3

  4. MOLECULAR WEIGHT DISTRIBUTION __ Mn= the number average molecular weight (mass) Mi= mean (middle) molecular weight of size range i xi= number fraction of chains in size range i wi= weight fraction of chains in size range i PE335 Lecture 2 4

  5. PE335 Lecture 2 5

  6. PE335 Lecture 2 6

  7. Polydispersity • By virtue of its definition, Mw cannot be less than Mn. It is influenced by the high molecular weight fraction of the material to a greater degree than Mn. • The ratio of Mw to Mn, defines the polydispersity of a molecular weight distribution. • Low polydispersity (PD=Mw/Mn 2) generates higher melt viscosity, higher tensile strength and better toughness in polyethylene. PE335 Lecture 2

  8. Polyethylene (PE) Degree of Polymerization The degree of polymerization refers to the total number of repeat units in the chain. DP = Mn/Mo Ex. Calculate the degree of polymerization if polyethylene (PE) has a molecular weight of 56,000 g/mol. Mrepeat unit = 2(atomic wt. of C) + 4(atomic wt. of H) = 2(12) + 4(1) = 28 Degree of Polymerization = 56,000/28 = 2,000 PE335 Lecture 2

  9. Property Implications of MW • Higher MW increases • Tensile Strength, impact toughness, creep resistance, and melting temperature. • Due to entanglement, which is wrapping of polymer chains around each other. • Higher MW implies higher entanglement which yields higher mechanical properties. PE335 Lecture 2

  10. and Molecular Weight Distribution • Broader MWD decreases strength • Broad MW distribution represents polymer with many shorter molecules which are not as entangled and slide easily. • Broader MWD decreases crystallinity • Shorter chains are too short to fold into crystalline domains • Broader MWD increases melt flow rate • Shorter chains flow more easily and act as plasticizer. PE335 Lecture 2

  11. Example 1.1: What is the molecular weight of polypropylene (PP), with a degree of polymerization of 3×104 ? Solution: Structure of the repeating unit for PP Molecular weight of repeat unit = (3×12 + 6×1) = 42 Molecular weight of polypropylene = 3×104×42 = 1.26×106

  12. Example 1.2: Nylon 11 has the following structure If the number-average degree of polymerization, X n , for nylon is 100 and M w= 120,000, what is its polydispersity?

  13. Example (3.1): a. To Find: (a) The number-average molecular weight (b) The weight-average molecular weight (c) The degree of polymerization and P.D for the given polypropylene material

  14. CLASSIFICATION OF POLYMERS • Polymers can be classified in many different ways. The most obvious classification is based on the origin of the polymer, i.e., natural vs. synthetic. Other classifications are based on the polymer structure, polymerization mechanism, preparative techniques, or thermal behavior. • A. NATURAL VS. SYNTHETIC • Polymers may either be naturally occurring or purely synthetic. All the conversion processes occurring in our body (e.g., generation of energy from our food intake) aredue to the presence of enzymes. Life itself may cease if there is a deficiency of these enzymes. Enzymes, nucleic acids, and proteins are polymers of biological origin. Their structures, which are normally very complex, were not under stood until very recently., etc. Each family itself has subgroups.

  15. Starch — a staple food in most cultures — cellulose, and natural rubber, on the other hand, are examples of polymers of plant origin and have relatively simpler structures than those of enzymes or proteins. There are a large number of synthetic (man-made) polymers consisting of variousfamilies: fibers, elastomers, plastics, adhesives

  16. B. POLYMER STRUCTURE 1. Linear, Branched or Cross-linked.

  17. 2.Amorphous or Crystalline Examples of crystalline polymers include polyethylene , polyacrylonitrile poly(ethylene terephthalate) , and polytetrafluoroethylene

  18. Poly(methyl methacrylate) polycarbonate

  19. 3. Homopolymer or Copolymer • Polymers composed of only one repeating unit in the polymer molecules are known as homopolymers • Polymers composed of two different repeating units in the polymer molecule are defined as copolymers. An example is the copolymer formed when styrene and acrylonitrile are polymerized in the same reactor..

  20. There are several types of copolymer systems:

  21. See lecture notes

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