Material Science

ST RUC TU RE OF SILI CAT ES & GLASS ES

(SiO ) Tetrahedron 44

The Si – O bond in the (SiO44-) has 50% ionic character and 50% covalent character. The radius ratio between Si4+ and O2- ions is 0.29. This gives rise to tetrahedral coordination between them. The basic building block of the silicates is the (SiO44-) tetrahedron. Because of the small highly charged Si4+ ion at the center of the tetrahedron strong bonding forces are created within a tetrahedron. Units are normally joined corner-to-corner and rarely edge-to-edge.

Silicate Structures -

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SiO44- Tetrahedron

Since each oxygen ion of the silicate tetrahedron has one electron available for bonding, many different types of silicate structures can be produced. Structures arising from bonding between different tetrahedra can be in the form of islands, single chains, double chains, rings, sheets and 3-dimensional networks.

The bonding between them may even create a structure that has no specific configuration. Such structures are called amorphous or glassy.

Island silicates The simplest example of silicates is the sand. In technical terms it is known as OLIVINE which has the basic chemical formula (Mg, Fe)SiO4. In this situation the number of shared oxygen ions is zero. It consists of Fe2+ and Mg2+ ions bonded with SiO44- terahedron. Since different tetrahedra are not bonded with any strong primary bond, the structure is said to have an island structure.

Separate island structures can also be formed as two tetrahedra joined together. In such a situation the number of oxygen ions shared between tetrahedra is one. The example is a mineral called HEMIMORPHITE, Zn4Si2O7(OH)2.

Silicates as Single Chains & Rings

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By sharing two of the tetrahedral oxygen ions in the structure, we can produce either a single chain or a ring structure. The mineral ENSTATITE, MgSiO3 is an example of the single chain structre. The mineral BERYL Be3Al2(SiO3)6 is an example of the ring structure.

Double Chain Silicate Structures An average sharing of 2.5 corners of the tetrahedra gives the double chain structure. Asbestos, Ca2Mg5(OH)2(Si4O11)2, is an example of double chain structure.

Sheet Silicate Structures Sharing of three corners gives an array of SiO44- tetrahedra in the form of sheets. Since there is still one unbonded oxygen on each silicate tetrahedron, these sheets are able to bond with each other with other types of structural sheets. Sheet of (Si2O5 ) 2 -

Sheet of bonding layer

Ordinary clay, is sheets of (Si2O5)2- bonded with sheets of Al2(OH)42+. Talc is sheets of (Si2O5)2- bonded with sheets of Mg3(OH)24+. Mica also has a similar sheet like structure with a complex formula of KAl (OH) (Si Al)O

Silicate Networks When all four corners of SiO44 – tetrahedra share oxygen ions, a SiO2 network, called silica is produced. Crystalline silica exists in several polymorphic forms that correspond to different ways in which the silicate tetrahedra are arranged with all corners shared. There are three basic silica structures: Quartz, Tridymite and Cristabolite

Si4+ O2 -

Silicate Glasses The fundamental subunit of silica based glasses also is SiO44- tetrahedron. While SiO44- tetrahedra in crystalline silica produce a long range order, in silicate glasses these tetrahedra are joined corner-to-corner to form a LOOSE NETWORK with no long-range order. Glasses developed for many industrial and commercial applications contain also glass modifying oxides.

Glass Transition Temperature Liquid

Specific Volume

Supercooled Liquid

Shrinkage due to freezing

Glass

Crystalline Solid

TG Temperature

TM

STR UCTUR E OF POLY MER S

Polymerization of Ethylene H

H

H

H

H

C

C

C

C

C

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H

Single Covalent Bonds

H

H

Ethylene Monomers

Double Covalent Bond

H

H

H

C H

H

C

Half Covalent Bond or Free Electron

H

H

H

C

C

C

H

H

H

Heat, Pressure Catalyst

H

Polyethylene

Degree of Polymerization

n

Molecular Structure of Polyethylene 109.5o

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Since the covalent bonding angle between single carboncarbon covalent bonds is ~ 109.5o, a short length of the polyethylene molecule takes on a zig-zag configuration.

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Bending and Twisting of Long Polyethylene Molecular Chains Though the bond angles in covalently bonded PE molecules are fixed, each sub unit, i.e. a mer is free to rotate without violating any of the bond angle requirement. As a consequence, it is very difficult to keep a long PE chain straight and stiff

STRUCTURE OF LONG POLYETHYLENE CHAINS

Long chains of polyethylene molecules therefore remain curled and twisted. They even get entangled with each other. In fact their appearance is like that of boiled noodles.

Some Other Simple Polymers H

H

H

H

C

C

C

C

H

Cl

H

CH3

n

Polyvinyl Chloride

Polypropylene

n

Some Other Simple Polymers H

H

H

H

C

C

C

C

H

Polystyrene

n

H C

N

n

Polyacrylonitrile

Some Other Simple Polymers H

H

H

CH3

C

C

C

C O

O H

O

C

n CH3

Polymethyl Acetate

H

C OCH3 Polymethyl Methacrtlate (PMMA)

n

Folded Chain Structures

100 Carbon Atoms

Microstructures of Polymers Amorphous

Crystalline

Homopolymers & Copolymers Homopolymers are polymeric materials that consist of polymer chains made up of a single repeating unit. Thus, if A is the repeating unit, a homopolymer chain will be made up of sequence …..AAAAAAAAAAAAA…… in the polymeric molecular chain.

In contrast, copolymers consist of polymer chains made up of two or more chemically different repeating units. The architecture of copolymers may use chemically different repeating units in a different ways.

COPOLYMER ARCHITECTURES Random Copolymers AABABBABAAABABAB…. Block Copolymers AAA…BBB…AAA...BBB…

Alternating Copolymers ABABABABABABAB…… Graft Polymers …AAAAAAAAAAAAA… B B B B B B B B