polymer fabrication

7/26/2019 Polymer Fabrication

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Crystallinity & Synthetic

Of polymers


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CRYSTALLINITY

atomic arrangements of polymer willbe more complex

As a consequence of their size and

often complexity, polymer molecules

are often only partially crystalline (or

semicrystalline

crystalline regions dispersed within

the remaining amorphous material!

Any chain disorder or misalignment

results in an amorphous region twisting, "in"ing, and coiling of the

chains pre#ent the strict ordering of

e#ery segment of e#ery chain!


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$here are some polymers that are completely amorphous, but

most are a combination with the tangled and disordered

regions surrounding the crystalline areas

$he degree of crystallinityfrom completely amorphous (as

liquid to almost entirely (%' crystalline (as solid

$hat is, they form mixtures of small crystals and amorphous

material and melt o#er a range of temperature instead of at a

single melting point $he density of a crystalline polymer will be greater than an

amorphous one of the same material and molecular weight

the chains are more closely pac"ed together for the crystalline

structure


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)actors affect degree of crystallinity

the rate of cooling during solidi*cation, + as well as on the

chain con*guration!

- Cooling rate

.uring crystallization upon cooling through the melting

temperature, the chains, which are highly random and

entangled in the #iscous liquid must assume an ordered

con*guration!

)or this to occur, suf*cient time must be allowed for the

chains to mo#e and align themsel#es Slow cooling pro#ides time for greater amounts of

crystallization to occur! )ast rates, on the other hand, such as

rapid quenches, yield highly amorphous materials!


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/ +

0ow molecular weight polymers (short chains are generally

wea"er in strength! Although they are crystalline, only wea"1an der aals forces hold the lattice together! $his allows the

crystalline layers to slip past one another causing a brea" in

the material!

2igh .3 (amorphous polymers, howe#er, ha#e greater

strength because the molecules become tangled between

layers

4 Chain configuration

$he crystalline material shows a high degree of order formed

by folding and stac"ing of the polymer chains! $he amorphous or glass5li"e structure shows no long range

order, and the chains are tangled


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Crystallization is not fa#ored in polymers that are composed

of chemically complex repeat units (e!g!, polyisoprene!

)or linear polymers, crystallization is easily accomplishedbecause there are few restrictions to pre#ent chain alignment!

Any side branches interfere with crystallizationthat

branched polymers ne#er are highly crystalline6

in fact, excessi#e branching may pre#ent any crystallization

whatsoe#er! +ost networ" and crosslin"ed polymers are almost totally

amorphous because the crosslin"s pre#ent the polymer

chains from rearranging and aligning into a crystalline

structure! A few crosslin"ed polymers are partially crystalline

the bul"ier or larger the side5bonded groups of atoms, the less

tendency there is for crystallization!


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7f the monomers are large and irregulardifficult for the to

arrange an ordered manner, resulting in a more amorphous

solid!

0i"ewise, smaller monomers, and monomers that ha#e a #ery

regular structure will form more crystalline polymers!

)or copolymers, as a general rule, the more irregular and

random the repeat unit arrangements, the greater is the

tendency for the de#elopment of noncrystallinity! )or alternating and bloc" copolymers there is some li"elihood

of crystallization!

On the other hand, random and graft copolymers are normally

amorphous! $o some extent, the physical properties of polymeric materials

are in8uenced by the degree of crystallinity!

Crystalline polymers are usually stronger and more resistant

to dissolution and softening by heat


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S9:$2;S7S

Stages in polymer synthesisbranched must be used below glass transition $ (ifamorphous or below melting $ (semicrystalline


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/ ;lastomersrubbers can be stretched to many times their original length, and can

bounce bac" into their original shape without permanentdeformation!

include polyisopreneor natural rubber, polybutadiene,

polyisobutylene, and polyurethanes

7mpro#ing mechanical< crosslin" structure

Silicon elastomers

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