Chain Flexibility 3p5xq

Chain Flexibility How easily the main chain of the polymer can move, is a key factor in determining the properties of the material.

Factors affecting chain flexibility: • • • •

Temperature Types of bonds Side groups Branching

Chain Flexibility - Temperature Polymer chains are not static, they are constantly vibrating and rotating. How much space or free volume there is around the chains determines ,how much they can move. The more thermal energy (heat) present, the more the chains will move and the more space between them. The more flexible the material. Each material has a specific temperature at which the chains are mobile enough that the material behaves more like a rubbery solid than a glassy solid. This is the Glass Transition Temperature (Tg)

Chain Flexibility – Types of Bonds Single Carbon –Carbon (C-C) and Carbon Hydrogen (C-H) bonds move relatively easily. Double Carbon bonds (C=C) do not rotate and are very rigid. Having an Oxygen or a Nitrogen in the main chain provides additional room for movement because of the lower number of bonds present for these atoms.

Chain Flexibility – Types of Bonds Having a benzene ring in the main chain can add stiffness to it. For example: Polycarbonate has two benzene rings in the main chain separated by a Carbon with two methyl groups (CH3) attached. This structure is very rigid. Polyester Has a benzene ring in the main structure, but also has an Oxygen and a few Carbon-Hydrogen bonds to allow it to be flexible when there is adequate room to move. PET

PC Rigid

Flexible when hot

Chain Flexibility – Side Groups Side groups restrict chain movement. The larger the side group, the more rigid the molecule. Having a Methyl (CH3) group attached to one side of the main chain will add some stiffness. Polypropylene is relatively flexible even at room temperature.

the

Having one attached to both sides of main chain will add a lot of stiffness. PMMA is very rigid.

Chain Flexibility – Side Groups Having a Benzene ring attached to one side of the chain will greatly affect the stiffness. Polystyrene is very stiff to the point of being brittle (CD cases)

Chain Flexibility – Branching Although branching can increase the entanglement of the polymer chains, branching increases the chain flexibility. Larger branches hold the molecules further apart, increasing the free volume, giving the molecules more room to move.

Based on the structure of the monomer, Let us determine some of the polymer’s properties like: Crystallinity Hygroscopic Glass Transition Flammability

hylene Crystalline – Yes only C-H bonds, flexible no side groups •

• Hygroscopic – No (not O or N) • Glass Transition – Low (-118⁰ C) • Flammability – Yes only C-H bonds

pylene •Crystalline – Yes only C-H bonds, flexible –side groups every other C • Hygroscopic – No (not O or N) • Glass Transition – Low (-9.44⁰C) • Flammability – Yes only C-H bonds

yrene •

Crystalline – No (Benzene ring makes it

too rigid) • Hygroscopic – No (not O or N) • Glass Transition – High (99⁰C) • Flammability – Yes only C-H bonds

inylchloride •

Crystalline – No, rigid (Cl to big to allow)

• Hygroscopic – No (not O or N) • Glass Transition – High (85⁰C) • Flammability – No (Cl puts out)

rbonate

•

Crystalline – No, too rigid

• Hygroscopic – Yes (O) • Glass Transition – High (149⁰C) • Flammability – No (High number of double carbon flame)

bonds will extinguish the

er (PET) •

Crystalline – Yes, flexible enough

• Hygroscopic – Yes (O) • Glass Transition – Low (68⁰C) • Flammability – Yes (only C-H and C=O bonds)

ide (Nylon 6-6)

•

Crystalline – Yes, very flexible

• Hygroscopic – Yes (O and N) • Glass Transition – Low (57⁰C) • Flammability – Varies depending on additives, but will usually self extinguish because of N

Polymethylmethacrylate (Acrylic) •

Crystalline – No, too rigid (dual methyl

groups) • Hygroscopic – Yes (O) • Glass Transition – High (104⁰C) • Flammability – Yes (only C-H and C=O bonds)

Polyoxymethylene (Acetal or POM) •

Crystalline – Yes, very flexible

• Hygroscopic – Yes (O) • Glass Transition – Low (-73⁰C) • Flammability – Yes (only C and O bonds)

Tg is low for simple linear polymers

Tg and Tm increase with mer complexity

Basically if a polymer’s glass transition temperature is well above (say, 50oC above) ambient room temperature, the material will behave like a brittle glassy polymer --- it’ll be stiff with low impact resistance.   Conversely, if the Tg is well below room temperature, the material is what is commonly termed a rubber or elastomer --soft and easily stretched; and those materials whose Tg is reasonably close to the ambient temperature will exhibit plastic material behavior --- strong and tough with good

Molecular weight of polymers Related to the length of the chains Mechanical properties ⇑ with M, but processing becomes more difficult Commercial polymers: large distribution of Molecular weights

Polydispersity index (PDI) Mw / Mn •

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----

Gives information about distribution breadth

• Highly branched polymers : PDI > 20 • Generally (commercial polymers) : 2 < PDI < 20 Mn - can affect brittleness, flow and compression properties of the polymer. Mw -related to strength properties, and impact resistance

Molecular Weight/Physical Property Property/Process Effect of Effect of Correlations s High MW Low MW Parameter

⇑

Impact Strength

⇑

Melt Viscosity

⇑

Processing Temp

⇑

Brittleness

⇑

Drawability

⇑

⇑

Softening Temp

⇑

⇑

⇑

⇑

⇑ ⇑ ⇑

Melt Flow

Polymer Additives • Mechanical, chemical, physical Properties can be modified by additives: • Fillers – Improve tensile and compressive strengths, abrasion resistance, toughness, and thermal stability • sand, glass, clay, talc (eg. carbon in tires) – Particle sizes range from very small (10 nm) to large (mm) • Plasticizers: small molecules which occupy positions between polymer chains (increase distance and interactions between chains) – increases flexibility, ductility, and toughness – reduces hardness and stiffness • Stabilizers – UV resistance of C-C bonds – Oxidation resistance

Structure parameters affecting polymer properties: Increase of the chain length. Effect: increase of tensile strength and Modulus of Elasticity (stiffness). Increase of number and length of side chains. Effect: increase of tensile strength and stiffness. Introduction of large monomers in molecules. Effect: increase of stiffness. Increase of number and strength of cross-links. Effect: increase of tensile strength and stiffness. Orientation of the molecules as a result of deformation during manufacturing. Effect: anisotropy of the material properties (properties along the deformation differ from those in other directions).

CLASSIFICATIONS OF POLYMERS According to

origin

Mechanical response to elevated temperature

Stress strain Molecular Polymerizatio behavior characteristics reaction

1- origin

Natural polymers

Synthetic polymers

A) Natural Polymers • Proteins polyamides of polypeptides polyisoprenes • e.g. rubber and gutta-percha polysaccharides • e.g. starch , cellulose, agar and • poly nucleic acids such as DNA • and RNA.

B) Synthetic Polymers

Produced in the laboratory by chemical reactions. e.g. Acrylic resin ,nylon and polystyrene.

Mechanical responce To elevated temperature

Thermoplastic polymers

Thermosetting polymers

Stress strain behavior

Brittle polymes

Plastiolymers

Elastomers

Molecular characteristics

shape

size

chemistry

structure

Molecular characteristics

1- shape

Straight

bent

twisted

zegzag

Molecular characteristics

2- size

Large polymer (high molecular weight)

Small polymer (low molecular weight)

Molecular characteristics 3- chemistry

homopolymer All monomers of the same type

copolymer Monomers of tow or more different types Random copolymer

Block copolymer

Alternating copolyme Graft copolymer

Molecular characteristics

4- structure

Network linear

branched

Cross linked

CLASSIFICATIONS OF POLYMERS

According to

origin

Mechanical response to elevated temperature

Stress strain Molecular Polymerizatio behavior characteristics reaction

5- polymerization reaction

Condensation reaction

Addition reaction