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ME 534 Assignment # 1

September 19, 2003

 

 

1)      The types of hydrocarbon molecules that are most suitable for the synthesis of polymeric materials are those that are unsaturated.  Unsaturated hydrocarbon are molecules in which there are double and triple bonds.  Under the presence of an appropriate combination of heat, pressure, and catalysts, the double or triple bond between the carbon atoms are broken and replaced with a single covalent bond.   When the bonds are broken the carbon atom has free electrons available, and becomes active and is able to connect to other active carbon atoms, forming the polymer.

 2)      A large polymeric molecule is expected to be saturated, or have many single bonds.  This is because the chain has already formed.  The ends of the polymer can be expected to have double or triple bonds that are constantly breaking and adding new monomers.

 3)      A “mer” is a reactive molecule that is the basic building block of a polymer.  It successively repeats along the polymer chain.  This is illustrated in the figure below:

 

 Molecular weight is the combined weight of all atoms in a molecule.  It is expressed as a fraction of a mole.  The molecular weight of a polymer is not fixed, but is distributed around a mean value, since polymer molecules have different lengths. The average molecular weight can be obtained by averaging the masses with the fraction of times they appear (number-average) or with the mass fraction of the molecules (weight fraction).  The moleculare weight can describe The molecular weight of a polymer can relate the degree of polymerisation or the number of “mers” in the chain.  The degree of polymerization is deifined as:

 

 

 4)       

 5)      Polymer chains are able to twist, turn and rotate because of the tetrahedral nature of the covalent bonds.  The molecular shape of a polymer describes the degree of chain twisting, coiling, and bending.

 6)      Molecular Structure of a polymer is the structure of a molecular chain.  There a four types of molecular structures.

a.       In a Linear Polymer the mer units are joined together end-to-end in single chains.  Van der Waal bonding exist between chains and are not very significant..  They have very good chain packing efficiency thus they can easily crystallize.    Linear polymers are high density polymers.

 

 b.      In Branched Polymers side branch chains are connected to the main chains.  The branches, considered to be part of the main chain molecule results from side reactions that occur during the synthesis of the polymer.  The chain packing efficiency is reduced with the formation of side branches which, in turn, lowers the polymer density.  Branched polymers are typically stronger than liner polymers, since the chains resist motion

 

 

c.       In a Cross Linked Polymer adjacent linear chains are joined to one to another at various positions by covalent bonds.  The process of cross linking is achieved either during syntheses or by a non-reversible chemical reactions that occurs at high temperatures.  Cross linking is accomplished by additive atoms or molecules that are covalently bonded to the chains.  Cross-linked polymers are stronger than branched polymers, but are also more brittle.

 

 

d.      In a Network Polymer tri-functional mer units, having three active covalent bonds form three dimensional networks.  These materials creates distinctive mechanical and thermal properties.

 

 

 

7)      The molecular configurations of polymers describe the regularity and symmetry of side groups.   There are two possible arrangements for a mer unit: head-to-tail and head to head.  Various configurations of side groups are called isomerism.  There are two isomeric subclasses: Stereoisomerism and Geometrical.    

 Stereoisomerism – Atoms are linked together head-to-tail but differ in their spatial arrangements.  There are there variations of this;

 Isotatactic – the radicals (R) are linked in the same order.

 

 Syndiotactic – the radical (R) groups are positioned on alternative sides in the chain.

 

Atactic- the radical (R) groups are positioned at random.

 

 

Geometrical Isomerism - occurs within mer units having a double bond between chain carbon atoms.  Bonded to each of the carbon atoms participating in the double bond is a single-bonded atom or radical which may be located on one side of the chain or its opposite.  There are two variations: cis and trans.  The cis structure results when the radical and hydrogen atom are on the same side.  The trans structure results when the radical and hydrogen atom are on opposite sides.

 

8)      In producing a polymeric material suitable for manufacturing a shoe soles a crystalline polymer which is characterized by regularly folded chains is required.  Polymer crystallinity depends on the packing of molecular chains so as to produce an ordered atomic array.  Thus a polymer with trans molecular configuration (alternative configuration) should be used because it is more balanced and can be more orderly packed.  This will make it easier to crystallize and will result in a more hard polymer than a cis structure.

 9)      Copolymers are polymer which consist of two different mer units.  They are linear addition chains composed of two or more types of molecules.  There are four types of copolymers as seen in the figure below; (a) alternating monomers, (b) random monomers, (c) block copolymers and (d) grafted copolymers.  In the figure open blocks represent one type of monomer and solid blocks represent a second type of monomer.

 

 

10)  There are many vital difference between thermosetting and thermoplastics.  Thermoplastics soften when head and harden when cooled.  It is a process that is reversible and repeatable.  However thermosetting become permanently hard when heat is applied and do not soften upon subsequent heating.  Thus thermosettings can be recycled but thermoplastics cannot be recycled.  Furthermore, thermosettings are generally harder, stronger and more brittle than thermoplastics and have better dimensional stability.