Slide 1

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 bondsSide groupsBranchingChain 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 BondsHaving 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.

PC RigidPET Flexible when hotChain Flexibility Side GroupsSide 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.

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

Chain Flexibility Side GroupsHaving 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 BranchingAlthough branching can increase the entanglementof 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 polymers properties like:

CrystallinityHygroscopicGlass TransitionFlammabilityPolyethylene

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

Hygroscopic No (not O or N)

Glass Transition Low (-118 C)

Flammability Yes only C-H bonds

PolypropyleneCrystalline Yes only C-H bonds, flexible side groups every other C Hygroscopic No (not O or N) Glass Transition Low (-9.44C) Flammability Yes only C-H bondsPolystyrene

Crystalline No (Benzene ring makes it too rigid) Hygroscopic No (not O or N) Glass Transition High (99C) Flammability Yes only C-H bondsPolyvinylchloride

Crystalline No, rigid (Cl to big to allow) Hygroscopic No (not O or N) Glass Transition High (85C) Flammability No (Cl puts out)Polycarbonate

Crystalline No, too rigid Hygroscopic Yes (O) Glass Transition High (149C) Flammability No (High number of double carbon bonds will extinguish the flame)Polyester (PET) Crystalline Yes, flexible enough Hygroscopic Yes (O) Glass Transition Low (68C) Flammability Yes (only C-H and C=O bonds)

Polyamide (Nylon 6-6)

Crystalline Yes, very flexible Hygroscopic Yes (O and N) Glass Transition Low (57C) 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 (104C) Flammability Yes (only C-H and C=O bonds)16Polyoxymethylene (Acetal or POM)

Crystalline Yes, very flexible Hygroscopic Yes (O) Glass Transition Low (-73C) Flammability Yes (only C and O bonds)

Tg and Tmincrease withmer complexityTg is low forsimple linearpolymers18Basically if a polymers glass transition temperature is well above (say, 50oC above) ambient room temperature, the material will behave like a brittle glassy polymer --- itll 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 impact resistance.

Molecular weight of polymersRelated to the length of the chains

Mechanical properties with M, but processingbecomes more difficult

Commercial polymers: large distribution of Molecular weightsPolydispersity index (PDI)

----- ----Mw / Mn

Gives information about distribution breadth Highly branched polymers : PDI > 20 Generally (commercial polymers) : 2 < PDI < 20Mn - can affect brittleness, flow and compression properties of the polymer. Mw -related to strength properties, and impact resistanceMolecular Weight/Physical Property Correlations

Property/ProcesssParameter

Effect ofHigh MWEffect ofLow MW

Impact Strength

Melt ViscosityProcessing TempBrittlenessDrawabilitySoftening TempMelt FlowPolymer 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 Colorants and Flame RetardantsStructure parameters affecting polymer properties:

Increaseofthe chain length.Effect: increaseoftensile strengthandModulusofElasticity(stiffness).

Increaseofnumber and lengthofside chains.Effect: increaseoftensile strength and stiffness.

Introductionoflarge monomers in molecules.Effect: increaseofstiffness.

Increaseofnumber and strengthofcross-links.Effect: increaseoftensile strength and stiffness.

Orientationofthe molecules as a resultofdeformation during manufacturing.Effect: anisotropyofthe material properties (properties along the deformation differ from those in other directions).CLASSIFICATIONS OF POLYMERS

A) Natural PolymersProteins 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 PolymersProduced in the laboratory by chemical reactions. e.g. Acrylic resin ,nylon and polystyrene.

CLASSIFICATIONS OF POLYMERS