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Structure of
Polymers
Art Ian G. Bautista, ECE, ECT 1
TYPES
**1. Natural Polymers
- Synthetic Polymers**
2 3
Polymer
Poly many repeat unit (building blocks)mer
CHH CHH CHH CHH CHH C H^ H Polyethylene (PE) Cl^ Cl Cl
CHH CH CHH CH C HH HC Poly(vinyl chloride) (PVC)H^ H
H H HH Polypropylene (PP)
C CCH 3 CHH C^ H CH 3 C C CH 3
repeat unit repeat unit repeat unit
Carbon chain backbone
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Chemistry and Structure of Polyethylene
- • Polyethylene is a longTop figure shows repeat unit and chain structures.-chain hydrocarbon.
- Other figure shows zigzag backbone structure.
Tetrahedral arrangement of C-H
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Ancient Polymers
- Naturally occurring polymers (those derived from plants and animals) have been used for centuries. - Wood – Rubber - Cotton – Wool - Leather – Silk
Hydrocarbon Molecules
- Many hydrocarbons (composed of hydrogen organic materials are and carbon).
- Most polymers are made up of H and C.
- The molecules are bonds between the hydrocarbon covalent.
- Each carbon atom has 4 electrons that may be covalently bonded, the hydrogen atom has 1 electron for bonding.
- A single covalent bond exists when each of the 2 bonding atoms contributes one electron (ex: methane, CH 4 ). (^6)
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Saturated Hydrocarbons
Each carbon has a the 4 valence electrons are bonded, the single bond to 4 other atoms; molecule is stable. The covalent bonds in each molecule are strong, but only weak hydrogen and van der Waals bonds exist between the molecules.
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Most of these hydrocarbons have relatively low melting and boiling points. However, boiling temperatures rise with increasing molecular weight. (^9)
Unsaturated Hydrocarbons
- Each carbon atom is not bonded to the maximum (or four) other atoms; as such, it is possible for another atom or group of atoms to become attached to the original molecule.
- Double & triple bonds are somewhat unstable – involve sharing 2 or 3 pairs of electrons, respectively. They can also form new bonds HC C H
H
Double bond Hfound H C^ C^ H in ethylene - C 2 H 4 Triple bond^ acetylene^ -^ Cfound in 2 H 2
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Isomerism
- Two compounds with have different structures (atomic arrangements). same chemical formula can
- normal^ for example: C-octane^8 H^18
- 2,4-dimethylhexane
HCHH CHH CHH CHH CHH CHH CHH CHHH =H 3 C CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3
H 3 C CHCH^3 CH 2 CHCH 2 CH 3 CH 3
H 3 C (^ CH^ 2 ) 6 CH 3
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Polymerization
- Free radical polymerization: ethylene gas reacts with the initiator (catalyst). (“R.” is the unpaired electron) CHH CHH monomer(ethylene)
R + free radical
R CHH CHH initiation
R CHH CHH + CHH CHH R CHH CHHCHH CHH propagation dimer
Monomer refers to the small molecule from which a polymer is synthesized.
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(c) Table 4.4b shows the data for the weight average molecular weight. The wiMi products- for the several size intervals are tabulated in the right-hand column. The sum of these products yields a value of 23,200 g/mol for Mw.
Polymer Chain Lengths
- Many polymer properties are affected by the length of the polymer chains. For example, the melting temperature molecular weight. increases with increasing
- At room temp, polymers with very short chains (roughly 100 g/mol) will exist as liquids.
- Those with weights of waxy solids and soft resins. 1000 g/mol are typically
- Solid million g/mol polymers range between. 10,000 and several
- The molecular weight affects the polymer’s properties (examples: elastic modulus & strength). 20
Polymers – Molecular Shape
- Straight (b) and twisted (c) chain segments are generated when the backbone carbon atoms (dark circles) are oriented as in the figure above.
- Chain bending and twisting are possible by rotation of carbon atoms around their chain bonds.
- Some of the polymer characteristics are a function of the chain segment mechanical and thermal rotation in response to vibrations. applied stresses or thermal 21
Chain End-to-End Distance, r
- Representation of a single polymer chain molecule that has numerous random kinks and coils produced by chain bond rotations; it is very similar to a heavily tangled fishing line.
- “r” is the end to end distance of the polymer chain which is much smaller than the total chain length. 22
Molecular Structures for Polymers
- The physical characteristics of a polymer depend also on differences in the structure of the molecular chains (other variables are weight). shape and
- Linear polymers end in single chains. There may be extensive van have repeat units joined end to der Waals and hydrogen bonding between the chains. Examples: polyethylene, PVC, nylon. 23
Linear Branched Cross-Linked Network
secondary (^) bonding
Molecular Structures- Branched
- Where side these are termed-branch chains have connected to main chains, branched polymers. Linear structures may have side-branching.
- HDPE polymer with minor branching, – high density polyethylene is primarily a linear while LDPE – low density polyethylene contains numerous short chain branches.
- Greater chain linearity and chain length tend to increase the melting point and improve the physical and mechanical properties of the polymer due to greater crystallinity. 24
Linear Branched (^) Cross-Linked Network
Molecular Structures –
Cross-linked, Network
- In are joined to one another at various positions by cross-linked polymers, adjacent linear chains covalent bonding of atoms. Examples are the rubber elastic materials.
- Small molecules that form 3 or more active covalent bonds create structures called network polymers polyurethanes.. Examples are the epoxies and 25
Linear Branched Cross-Linked Network
secondary (^) bonding
Thermoplastics and Thermosets
- The response of a polymer to mechanical forces at elevated temperature is related to its dominant molecular structure.
- One classification of polymers is according to its behavior and rising temperature. Thermoplastics and Thermosets are
- the 2 categories.A thermoplastic is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently.
- Most thermoplastics are high whose chains associate through weak Van der Waals forces-molecular-weight polymers ( hydrogen bonding (polyethylene); stronger dipolenylon). -dipole interactions and 26 - Thermoplastic polymers (Bakelite polymers differ from, vulcanized rubber thermosetting) since thermoplastics can be remelted and remolded. - Thermosetting chemically decompose, so they can not be plastics when heated, will recycled. Yet, once a thermoset is cured it tends to be stronger than a thermoplastic. - Typically, structures (and flexible chains) are linear polymers with minor branched thermoplastics. The networked structures are thermosets. 27
Thermoplastics and Thermosets
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Examples of Thermoplastics
PTFE
More Examples of Thermoplastics
http://www2.dupont.com/Teflon/en_US/index.html http://en.wikipedia.org/wiki/Teflon
Polymer
Thermoset Examples
