Term Paper Aqib

8/8/2019 Term Paper Aqib

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CHEMISTRY

TERM PAPER ON

INORGANIC

POLYMERS

SUBMITTED BY: Md Aqib

SUBMITTED TO: Sitansh Sharma

SECTION: G5003

ROLL NO: B-57

REGD NO: 11004146


2/11

AKNOWLEDGEMENT

First and foremost I thank my teachers who have assigned me this term paper to bring out my creative

capabilities.

I express my gratitude to my parents for being a continuous source of encouragement and for their all

financial aid given to me.

I have like to acknowledge the assignment provided to me by the library staff of LOVELYPROFESSIONAL UNIVERSITY.

My hard felt gratitude to my friends for helping me to complete my work in time.

Md.Aqib


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CONTENTS

1. What is a polymer ?

2. How polymers are depicted ?

3. Reason for interest in inorganic polymers

4. Types of inorganic polymers ?

5. Characterization of inorganic polymers

1. Molecular weights

2. Structural features

3. Solubility parameters

4. Crystallinity

6.Refrence


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What Is a Polymer?

A polymer is a very-long-chain macromolecule in which hundreds or thousands of atoms

are linked together to form a one-dimensional array. The skeletal atoms usually bear side

groups, often two in number, which can be as small as hydrogen, chlorine, or fluorine

atoms or as large as aryl or long-chain alkyl units. Polymers are different from other

molecules because the long-chain character allows the chains to become entangled in

solution or in the solid state or, for specific macromolecular structures, to become lined up

in regular arrays in the solid state. These molecular characteristics give rise to solid-state

materials properties, such as strength, elasticity, fiber-forming qualities, or film-forming

properties, that are not found for small molecule systems. The molecular weights of

polymers are normally so high that, for all practical purposes, they are nonvolatile. These

characteristics underlie the widespread use of polymers in all aspects of modern technology.

Attempts to understand the relationship between the macromolecular structure

and the unusual properties characterize much of the fundamental science in this field.

How Polymers Are Depicted?

Polymers are among the most complicated molecules known. They may contain thousandsof atoms in the main chain, plus complex clusters of atoms that form the side groups

attached to the skeletal units. How, then, can we depict such molecules in a manner that

is easy to comprehend? First, an enormous simplification can be achieved if we remember

that most synthetic polymers contain a fairly simple structure that repeats over and over down

the chain. This simplest repetitive structure is known as the repeating unit, and it provides the

basis for an uncomplicated representation of the structure of the whole polymer.

Reasons for Interest in Inorganic Polymers?


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Polymer chemistry and technology form one of the major areas of molecular and materials

science. This field impinges on nearly every aspect of modern life, from electronics

technology, to medicine, to the wide range of fibers, films, elastomers, and structural

materials on which everyone depends.

Most of these polymers are organic materials. By this we mean that their long-chain

backbones consist mainly of carbon atoms linked together or separated by heteroatomssuch as oxygen or nitrogen. Organic polymers are derived either from petroleum or (less

frequently) from plants, animals, or microorganisms. Hence, they are generally accessible

in large quantities and at moderate cost. It is difficult to imagine life without them.

In spite of the widespread importance of organic polymers, attention is being focused

increasingly toward polymers that contain inorganic elements as well as organic components.

At the present time, most of this effort is concentrated on the development of new

chemistry, as research workers probe the possibilities and the limits to the synthesisof these new macromolecules and materials. But in certain fields, particularly for polysiloxanes, both

the science and the technology are already well established, and

technological developments now account for a major part of the siloxane literature.

For other systems to be discussed in this book, technological developments are emerging

from the chemistry at an accelerating rate.

Why, with the hundreds of organic polymers already available, should scientists

be interested in the synthesis of even more macromolecules? The reasons fall into

two categories. First, most of the known organic polymers represent a compromise

in properties compared with the ideal materials sought by engineers and medical

researchers. For example, many organic backbone polymers react with oxygen or ozone

over a long period of time and lose their advantageous properties. Most organic polymers

burn, often with the release of toxic smoke. Many polymers degrade when exposed to

ultraviolet or gamma radiation. Organic polymers sometimes soften at unacceptably

low temperatures, or they swell or dissolve in organic solvents, oils, or hydraulic fluids.

At the environmental level, few organic polymers degrade at an acceptable rate in the

biosphere. Finally, the suspicion exists that the availability of many organic polymers

may one day be limited by the anticipated scarcities of petroleum. It is generally

accepted that polymers that contain inorganic elements in the molecular structure may

avoid some or all of these problems.

The second set of reasons for the burgeoning interest in inorganic-based macromolecules

is connected with their known or anticipated differences from their totally organic

counterparts. Inorganic elements generate different combinations of properties in polymer

than do carbon atoms. For one thing, the bonds formed between inorganic elements are

often longer, stronger, and more resistant to free radical cleavage reactions than are

bonds formed by carbon. Thus, the incorporation of inorganic elements into the backbone

of a polymer can change the bond angles and bond torsional mobility, and this in

turn can change the materials properties to a remarkable degree. Inorganic elements can

have different valencies than carbon, and this means that the number of side groupsattached to a skeletal atom may be different from the situation in an organic polymer.


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This will affect the flexibility of the macromolecule, its ability to react with chemical

reagents, its stability at high temperatures, and its interactions with solvents and with

other polymer molecules. Moreover, the use of non-carbon elements in the backbone

provides opportunities for tailoring the chemistry in ways that are not possible in totally

organic macromolecules. Many examples of this feature are given in the later chapters

of this book. Thus, the future development of polymer chemistry and polymer engineeringmay well depend on the inorganic aspects of the field for the introduction of new

molecular structures, new combinations of properties, and new insights into the behavior

of macromolecules in solution and in the solid state.

Thus, inorganic polymers provide an opportunity for an expansion of fundamental

knowledge and, at the same time, for the development of new materials that will assist

in the advancement of technology. Throughout this book an attempt has been made to

connect these two aspects in a way that will provide a perspective of this field.

For example, the superb thermal stability of several poly(organosiloxanes) can be understood

in terms of their fundamental chemistry.

The controlled hydrolytic degradability of certain polyphosphazenes, which depends on

molecular design to favor specific hydrolysis mechanisms, is the basis for their prospective

use as pharmaceutical drug delivery systems. The unusual energy absorption characteristics

of polysilanes is indicative of surprising electronic structures, and this underlies the interest in

some of these materials for use in integrated circuit fabrication.

Types of Inorganic Polymers

A glance at the Periodic Table or at an inorganic chemistry textbook will convince the

reader that, of the 100 or so stable elements in the table, at least half have a chemistry

that could allow their incorporation into macromolecular structures. This will undoubtedly

come to pass in the years ahead. However, at the present time, most of the known

inorganic polymer systems are based on relatively few elements that fall within the

region of the Periodic Table known as the Main Group series. These elements occupy

groups III (13 in the IUPAC nomenclature), IV (14), V (15), and VI (16) and include

elements such as silicon, germanium, tin, phosphorus, and sulfur. Of these, polymersbased on the elements silicon and phosphorus have received by far the most attention.

This is the reason why silicon- and phosphorus-containing polymers are considered in

the greatest details.

Characterization of Inorganic Polymers

1. Molecular Weights

Introduction


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This is the characteristic that underlies all the properties that distinguish a polymer from its low-

molecular-weight analogues. Thus, one of the most important goals in the preparation of a polymer is

to control its molecular weight by a suitable choice of polymerization conditions.

Many properties of a polymeric material are improved when the polymer chains are sufficiently long.

For example, properties such as the tensile strength of a fiber, the tear strength of a film, or the

hardness of a molded object may increase asymptotically with increases in molecular weight. If themolecular weight is too low, say below a lower limit Ml, then the physical property could be

unacceptably low. It might also be unacceptable to let the molecular weight become too high. Above

an upper limit Mu, the viscosity of the bulk (undiluted) polymer might be too high for it to be

processed easily. Thus, a goal in polymer synthetics is to prepare a polymer so that its molecular

weight falls within the window demarcated by Ml and Mu. This is frequently accomplished by a

choice of reaction time, temperature, nature and amount of catalyst, the nature and amount of solvent,

the addition of reactants can terminate the growth of the polymer chains sooner than would otherwise

be the case, addition of complexing agents such as crown ethers, or by the presence of an external

physical field, such as ultrasound.

Uses for Molecular Weights

There are several important reasons for wanting to know molecular weights in polymer

science. From the viewpoint of inorganic polymers, the main uses are for the interpretation

of molecular-weight dependent properties, and for the elucidation of polymerization

mechanisms.

Viscometry

This technique is by far the easiest for the characterization of polymers in solution. This

can be seen from the simplicity of the typical (glass) viscometer shown in Figure 2.7.

It is used to obtain the viscosity of a liquid by the use of Poiseuilles equation, which is

= pr4t /8LV

The experiment involves a measurement of the amount of time t required for a volume

V of liquid to flow through a capillary of radius r and length L when the pressure

difference is p. The viscosity may then simply be calculated from this equation, inwhich the is the numerical constant.

In polymer solution viscometry, it is not necessary to determine absolute values of

the viscosity; relative values are sufficient. These quantities are called viscosities, but

the terms are misnomers because they are generally unitless ratios and therefore do not

have the units of viscosity. In any case, the relative viscosity is simply the ratio of the

viscosity of the polymer solution to the viscosity of the pure solvent at the same temperature.

2. Other Structural Features

Backbone Bonding

A knowledge of the molecular weight of a polymer provides information about the


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number of backbone or skeletal bonds per molecule.37,38 But the skeletal bonds require

additional characterization. The most important item of information that needs to be

determined is the length of each bond. In this regard inorganic polymers are quite different

from their organic counterparts. Specifically, virtually all covalent bonds between pairs of

atoms in inorganic polymers (Si, P, N, etc.) are longer than the C-C bond found in organic

polymers. Thus, inorganic polymers are much less congested and, as a consequence,much more flexible.3942 They are more flexible both in the equilibrium sense, which

means that they can form more compact random coils, and in the dynamic sense, which

means they can readily switch between different spatial arrangements. The first of these

factors has a powerful influence on the melting point of a polymer. The second influences

the temperature below which the polymer becomes a glass.43

The melting point of any crystalline material is given by

Tm=Hm/Sm

where Hm is the heat of fusion and Sm is the entropy of fusion. Since inorganic polymers

can adopt very compact random-coil arrangements of high entropy, their entropies

of fusion are frequently very high, and their melting points relatively low. Of course

exceptions can occur, particularly when unusually strong intermolecular attractionsgive an atypically large heat of fusion.

The glass-transition temperature Tg is the temperature below which the polymer is

glass-like because long-range motions of the polymer chains are no longer possible.

The more flexible the polymer, in the dynamic sense, the lower is the temperature to

which it can be cooled before this flexibility is frozen out. Thus, the high dynamic

flexibility generally enjoyed by inorganic chains frequently generates relatively low

glass transition temperatures as well. The glass-transition temperature is of considerable

practical, as well as fundamental importance. For molded objects, it closely

approximates the empirically defined brittle point of a polymer. This has little influence on the

properties of films and fibers, however. After all, glass wool is far

below its glass-transition temperature at room temperature, yet it is clearly not brittle.

The same is true for polystyrene film, which is used extensively as a packaging

material.

Branching and Cross-Linking

Under some conditions, branches can grow from the chain backbone, This could occur, for

example, during a polymerization process, as in thecase of the formation of some

phosphazene polymers. It could also occur subsequently,

through processes such as high-energy irradiations, and the generation of free radicals.

Because branch points represent irregularities in the chain structure, they can greatly

suppress the tendency of a polymer to crystallize. Branching can occur to the extent that

a network is formed, as in the preparation of thermosetting epoxy resins, or in the curing

of elastomers.

3.Solubility Parameters

This observation is, in fact, the basis for the rule of thumb that like dissolves like. This


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rule is obviously very qualitative, but it has now been sucessfully extended to provide

a quantitative basis for finding potential solvents for a polymer. In the theory of

Hildebrand, it is acknowledged that Hdis is almost certainly going to be positive. The

goal is then to find a means to make Hdis as small as possible. Specifically, it is given

by the equation

Hdis=v1 v2(1- 2)2where it is written as the product of the volume fraction of solvent, the volume fraction

of polymer, and the square of the difference between the values of a molecular characteristic

called the solubility parameter,55 for the solvent and polymer, respectively.

The quantity is defined by

=(E/V)1/2

that is, as the square root of the cohesive energy density, which is the ratio of the molar

energy of vaporization to the molar volume of the liquid. Thus, the quantity is a measure

of the strength of the interactions between the molecules of a substance, since

vaporization involves greatly increasing the average distance of separation between

them. Two molecules are like one another if the strengths of their interactions are

similar. The two values of the solubility parameter will then be similar, and this willminimize the positive value of Hdis, as shown in equation (24). The utility of the

approach is based on the fact that values of have been determined for many solvents

and polymers by both experiment and approximate calculations. Solubility is likely to

occur when a polymer and solvent have solubility parameters within one and a half

units of each other, when the units are in calories and cm3.

Trends

Actual values of solubility parameters show the same trends for both solvents and polymers.Non-polar molecules and repeat units have weak intermolecular forces, small

energies of vaporization, and therefore small solubility parameters. As might be

expected, increased polarity increases the solubility parameter, and hydrogen bonding

gives the largest values of all

Applications

The fundamental idea is to determine the solubility parameter of the polymer, and then

to use tabulated results to identify a number of solvents that have solubility parameters

close to this value. The list of potential solvents is then narrowed to two or three

candidates. Solvents that are too volatile, too toxic, too flammable, too expensive, and

so on can be removed from the list. Other criteria would depend on the nature of the

studies to be pursued. If the objective is to carry out light-scattering measurements,

the need for maximizing the contrast factor would make the index of refraction of the

solvent an additional important consideration.

Another application would be to minimize the swelling of a cross-linked elastomer

in contact with a solvent. In this case, of course, one would be looking for a polymer

giving the largest mismatch with the solubility parameters of the solvent to which the

elastomer was to be exposed.

It is important to note that these predictions are for the possible miscibility orimmiscibility of two amorphous materials. They do not apply to crystalline polymers


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because of the neglect of the already-mentioned positive heat of fusion. Extrapolating

solubility parameters for oligomers to obtain polymer solubility parameters

predictions would be useful, for temperatures above which the polymer would no longer be

crystalline. It should also be mentioned that polymersolvent interactions can be

characterized by the second virial coefficients that appear in equations (8) and (13) and by the

free energy of interaction parameter 1 that appears in the FloryHuggins theory ofpolymersolution thermodynamics

4. Crystallinity

As mentioned earlier, crystallinity in a polymer is important in many applications. It

is a way by which the chemist introduces solidity or hardness, but it can also have a

beneficial effect on a variety of other mechanical properties.6467 The most important of

these is probably the impact resistance of the material. Because polymer chains are so

long, different segments of the same chain become incorporated into different micro

crystallites. Figure 2.28 illustrates the situation. As a result, much of the polymer that

connects crystallites is badly entangled and poorly positioned for crystallization. This

is the reason why crystalline polymers typically contain 2050% of amorphous material,

and are therefore better termed partially crystalline. The crystallites are of great

importance because they give the polymer the solidity or rigidity required in many

applications, for example when they are used in molded objects. The amorphous

regions are also important. The chains in these regions have enough mobility that they

can absorb impact energy through their skeletal motions, using frictional effects to

convert the energy harmlessly into heat. A partially crystalline polymer is therefore

much tougher than the same polymer would be if it were 100% crystalline.

Requirements

Because crystallinity is frequently highly desirable, it is important to establish the structural

features that are conducive to achieving it. Most important, considerable regularity

in chain structure is generally required. Specifically, crystallization is usually impossible

for chain sequences that contain defects such as branch points, cross-links, chemical

irregularities (comonomeric units), stereochemical irregularities (atactic placements),

head-to-head placements (instead of head-to-tail placements), and so on. An interesting

exception occurs when two different side groups are similar in size, and can replace one

another in the crystalline lattice. In this case, even chains that are irregular in this sense

can undergo crystallization.

Strong intermolecular attractions are also conducive to crystallization. They increase

the heat of fusion, since fusion generally involves increasing the distance of separation

between chains. The stronger the attractions, the higher the heat of fusion. Because the

melting point is directly proportional to the heat of fusion, too is increased. The higher the melting

point, the greater the degree of super-cooling

that is likely to exist at any given temperature of application, and thus the greater the

likelihood of crystallization.

5. REFRENCE CITED


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1. www.aiag.org

2. www.Dupont.com

3. www.plastcs-car.com

4. Schmeal & Purcell,New Polymer Technologys, 260 volume 84, 1988,AICHE, New York, New York, 1988

5. www.tms.org /journals
http://www.aiag.org/http://www.dupont.com/http://www.plastcs-car.com/http://www.tms.org/http://www.dupont.com/http://www.plastcs-car.com/http://www.tms.org/http://www.aiag.org/

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