Propriedades elétricas e caracterização do Poli(Sulfeto de - Fenileno) (PPS)

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INSTITUTO DE FISICA E QUI"ICA DE S~O CARLOS. DA UNIVERSIDADE DE S~O PAULO, E" 16/02/1993

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Figura

2.

l.Estrutura

de

alguns

polimeros

..•••..•...••..••

7

Fig u r a

2.

2 .Con d uti v idad eel

e t r icad

e

a Igun s ma te ria is. . • • • • . 12

Fig u ra

2.

3 De n sid a de

den

iv e is.

• . . . • . . . . • • . . • . . . 13

Figura

2.

4.Estruturas

quimica

de

a)

TCNQ

e

b)

, M@

15

Figura

2.

5.Curvas

da

condutividade

em

fun~io

do

teor

de

concentra~io

de

impurezas

..•••..•.•...

17

Figura

2.

6Defeitos

estruturais

com

forma~io

de

radicais

18

Figura

2.

7.a)

Soliton

neutro

b)

Carregado

negativamente

c)

Carregado

positivamente

.•••..•••••.•••.••.••

20

Figura

2.

8.Polarons

movendo-se

ao

longo

da

cadeia

•••..••..••

22

Figura

2.

9.Sintese

do

PPS

obtido

por

Macullus

.•••...•.•..

26

Figura

2.10.A

sintese

proposta

por

Lenz

para

C

PPS

...•...

27

Figura

2.11.Sintese

proposta

por

Edmonds

- Hill

para

0

PPS

... 28

Fig u ra

2. 12 .Ce Iu Ia un ita ria

do

PPS

•...

• • . . • • . • • . . • • . • • . . • . 30

Figura

3.1.

Comportamento

tipico

da

IDe

num

dieletrico

de

DebyB5

Figura

3.2.

Equilibrio

termico

.•••..•..•••.•••..••.••••.•••..

45

Figura

3.3.

Caracteristicas

da

corrente

versus

voltagem

do

material

contendo

armadilhas

••..•.••.••.•••••••••

48

Figura

3.4.

Mostra

a

possibilidade

da

emissao

de

Schottky

Figura

3.5.

a)

Barreira

de

potencial

da

for~a

imagem

b)

Efeito

de

um

campo

eletrico

sobre

a

barreira

de

Figura

5.2

Termogramo

de calorimetria

diferencial

de varredura

(DSC)

do

/ / 0 G G G G G G G G G G G G G G G G G G G G G G

90

Figura

5.3. Difratograma

de raios-x

para

C

PPS

(RYfON)

em po . 92

Figura

5.4. Difratograma

de

raios-x

para

C

PPS

em f iIme ...•.. 92

;

difra~iio

/

para

campos

baixos

da ordem

de 10

V/cm

103

Figura

6.1S.Grafico

de

In

u

T1/2

em fun~io

de T-1/4

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115

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6.16.Grafico

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In

u

em fun~io

de T-1/3

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o

estudo das propriedades eletricas de isolantes e de grande im portincia

para a vida m oderna, um a vez que a todo instante fazem os uso de equipam entos que

possuem algum dieletrico com o, por exem plo, fios isolados, capacitores e

transdutores. C om 0 desenvolvim ento dos processos de sintese de polim eros, houve

um grande avan~ o na area de isola~ io eletrica, pois os polim eros sio de baixo custo,

de alta eficiencia com o m ateria isolante, facilm ente m oldaveis e tam b8m de baixa

densidade. P osteriorm ente, com 0 avan~ o do conhecim ento tecnico dos polim eros, eles

deixaram de ser usados apenas com o elem entos passivos e passaram a substituir

alguns com ponentes ativos, baseados em propriedades de m ateriais m ais tradicionais.

P or esse interesse com ercial no uso de polim eros m uitas tecnicas

experim entais passaram a ser em pregadas no estudo dos fenom enos fisicos

envolvidos. E m geral, as m edidas eletricas, consistem na aplica~ o de um a

perturba~ io eletrica nas am ostras e consequente m edida do efeito resultante. A s

grandezas m edidas sio usual m ente estudadas em fun~ o de pari m etros com o a

tem peratura, condi~ O es de prepara~ io das am ostras, da atm osfera na qual se m antem

a am ostra, etc ..

U m cam po m uito recente, 0 de polim ero condutores, tem atraido a aten~ io

dos pesquisadores nesses ultim os anos, pelo fato de apresentarem m uitas das

propriedades eletricas norm alm ente associadas aos m etais, razio pela qual s 8 0 tam b8m

cham ados de m eta is sinteticos.

Q uando em 1977 foi dem onstrada a existencia de propriedades

sem icondutoras e m etalicas para 0 poliacetileno dopado ( intrinsicam ente um isolante)

deu-se inicio a um a corrida para sintetizar e caracterizar novos m em bros desta nova

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> Ygdc `Uf]nUp ]c MYfa c Ygh]a i `UXU' = c ffYbhY E]a ]hUXU dc f = Uf[ U ? gdUW]U`' < UffY]fU XY

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]bhYfdfYhUp ]c Xc g fYgi `hUXc g Yl dYf]a YbhU]g)

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fenom eno que cham am os polariza~ o, devido ao deslocam ento m icroscopico ou

m acroscopico de cargas em seu interior. O utros m ecanism os originam polariza~ oes

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A polariza~ io dipolar, geralm ente obedece ao m odelo de D ebye(31J, que

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apresentam resultados sem elhantes, com a polariza~ io sendo linear m ente dependente

do cam po aplicado.

A polariza~ io devida a carga espacial pode se com portar

m acroscopicam ente segundo esses m odelos. E m bora esse processo tenha sido

caracterizado por um a dependencia nio linear com 0 cam po, nio pode ser excluida a

possibilidade de um com portam ento linear, 0 que 0 torna indistinguivel da polariza~ io

dipolar em experim entos de T S P C , T S O C ou IO C .

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equilibrio com a banda de condu~ io. A agita~ io term ica nio e suficiente para

exciter 0 portador Ii banda de condu~ io dentro do tem po da experiencia. N io ha um a troca de portadores entre arm adithas e banda de condu~ io. E m boa aproxim a~ o, os

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aplicada injeta m ais portadores m as nio ha m ais lugares nas arm adilhas. P or

conseguinte, todos os portadores" novos" estio livres, prontos a contribuir para a

corrente. A tensio quando todas as arm adilhas ficem cheias, .cham a-se •• tensio do

lim ite das arm adithas cheias" (trap free space -charge - lim ited ) ( VT F L ). A lem

deste lim ite, um aum ento de tensio, pequeno com o for, causa um a aum ento subito na

corrente. C om o as arm adithas nio m ais desem penham um papel, 0 valor da corrente

alem do lim ite das arm adithas cheias segue a equa~ o ( 26 ), que e aplicada na

ausencia de arm adithas.

A s arm adithas rasas e profundas correspondem a dois lim ites de

com portam ento. N a natureza ha um a transi~ io continua entre os dois. 0 trabalho de

L am pert(43J fornece um esquem a conveniente, resum indo todas as possibilidades a

respeito das correntes de inje~ io. F igura 3.3 m ostra 0 triingulo de L am pert. 0

logarltm o da densidade de corrente e m ostrado com fun~ o do logaritm o da tensao,

conform e a teoria. 0 m odelo trata de arm adithas todas tendo 0 m esm o nivel de

energia e obtem -se curvas correspondendo a varios valores de energia. A linha de

inclina~ ao 2 correspode I i equa~ o. 26. A corrente nunca .pode ser m aior, para um a

dada tensio, do que 0 valor desta equa~ o, porque isto representa 0 caso no qual

todos os portadores injetados estao livres. A linha de inclina~ io 1 representa 0 caso

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