Introduction

PROPERTJESOF PPTEOS

4.1 Introduction

1his chapter deals with the investigallon of structural, morphological, Iherma1 and optical behaviors of PPTEOS thm films. The compOSItionand chemical structure of PPTEOS investigaUon by EA, SEM and IR spectroscopy respectively are discussed.

The DTA and TGA ofPPTEOS are also nurraled here,

Plasma polymers have receIved much attention for their potential applications as light guide material, optical fibers, as photovoltaic energy converters, photodiodes, optical coatings 10mhibit corrosion, etc.[I-2

J.

For these kinds of applications, plasma polymenzed thin films need optical imestigalions, The optical energy gaps, !he allowed direct transition and allowed indued transitions, Tauc parameter, and extinction coefficient have been determined from the Uv-vis spectrOscOPICstudies [3

J

and are documented in this chapter.

4.2 Elemental Analysis (EA.)

The chemical composItions of plasma-polymen£lld materials can be characterized by a useful leclmique called elemental analysis (EA). The elemental composition of plasma-pol)merized materials generally differs substantially from that of monomer, which does not usually happen IIIcase of conventional polymers

The technique used for !he determination of carbon-hydrogen-nitrogen (CHN) is based on the quantitiltive "d}namic Flash Combustion" method as shown in Fig.4, I, The samples are held in a lin conlainer, placed inside the IllIto samples drum where they are purged with ^II conlmuous flow of helium and then dropped III cerlain inler,aIs into II ~ertical quartz tube maintained III 1273K (Combustion reactor), When the sample dropped inside the furnace, the helium stream is temporarily enriched with pure oxygen and the sample and lIS conlamer melt and the tin promotes a violent reaction (Flash Combustion) m a temporary enriched atmosphere of oxygen.

Chapler4: Chemical,Thcnnal and OpticalpropertiesofPPTEOS 50

Q,m"tjm\;~c P.' """,;..' Fl.,sl'

Co,,,I,,,sli,,,,

lK'-'D" C

s,,_s,o,

",'" N, 0, CO, B,O

~(.l, ~ SL), 1000"("

Fig, 4,1 The principle of operation of elemental analyzer.

Under these favorable conditions even Ihennally resistant substances are completely oxidIzed. The resulting combustion gases are separated and detected by a thermal conductivity detoctor, ",hich gives an outpul signal proportional to \he concentration oflhe individual CDmpOnenlSof the mixture.'

4.2.t Experimental procedure

The PPTEOS po>l'tlerwas collected from the film containing substrates by scrapping process. The scrapped PPTEOS was \hen made fine powder using agate mortar and pastle, The C, II, N (non monomeric element) contents of PPTEOS films were determined by an elemental analyzer EA 1180 of Carlo Erba Instruments, TYCHN, Milan, ttaly. The analyzer has the following specification,

Measuring range 100 ppm; accuracy<0.3% absolute; repeatability<0.20%; sample required 0.1 to 100 mg.; anal}sis time CHN5<12 min.; CHN<7 min

4.2.2 Resultsand distu5sion

The percentages (>11)of carbon and hydrogen in PPTEOS detected by elemental analysis are presented in Table- 4.1.

Chapter 4, Ocmical, Thermal and Optical properties ofPPTEOS

Table- 4.1 The percentage (wt) ofC, H, Si04 in PPTEOS thin films

51

Sample Elements detee1cd(wcight%) Formula Empirical formula

C

H 5i&0

TEOS(monomer)

I

Col.h"SiO,

PPTEOS 54.17 5.90 39.93

c....,H]2)"

^(51O,),

It is seen that the amount of C in PPTEOS is increased and H is decreased relative to the amount of constituent elements in the monomer TEOS. The percentage ofsilicon, silicon diOXIdeor silicate contents was calculated on subtraction from the results of

other element [4]. Allhough the actual amount ofSi, Sio, or SiD, was not possible to measure, calculated value, x = 0,90, The amount of silicate is decreased, The increase or decrease of the constituents may be due to the complex reaction occurred during plasma polymeri7.ation.

4.3 Scanning Electron Microscopy (SEM)

Plasma pol}meriled thin films are smooili, pinhole free and highly cross-linked. The surface morphology of the plasma polymerized films can be studied by Scanrung E1eclron Microscopy (SEM). This techmque has also been used to determine the granular size of powder particles [5], to e''ldence the presence of powder particles in thin films [6-9J, to see the uniformity and defects of the films produced in plasma LIO] and to determine the location of fracture in adhesion studies by means of the lap-shear test [11].

4.3.1 Experimental Details

The PPTEOS thin film was deposited at identical conditions onto chemically cleaned glass substrates by plasma polymerizatlOn technique, The sample swface was coated with II thin layer of gold by gold sputtering. Scanning electron micrographs of the PPTEOS film surface was taken using a Scanning Electron Microscope (XL-30,

Phil1ps,The Netherlands). •

Chapter4; Chemical,Then",11MdOptical propertiesofPPTEOS 52

4.3.2 Results and DiscUl!sion

The scanning electron micrograph of PPTEOS thin film is presented in Fig. 4.2. The micrograph shows a smooth, Wlifonn and pinhole free surface of the PPTEOS film

Fig. 4.2 Micrograph ofPPTEOS \hin film.

4.4 Infrared (IR) Spectroscopy

Infrared OR) spectroscopic analysis is an important and farruliar technique for oblaimng structural mformalion and identlficallon of functional groups in organic compounds. Infrared spectroscopy deals with the interaction of IR light with matter.

When a beam of electromagnetic radiation ofintenslly is passed through a substance, il can either be absorbed or transmitted, depending upon its frequmcy, and the structure of the molecules. tf a transilion exists which is relaled 10 the frequency of the mcident radiation by Planck's law, then the radiauon can be absorbed. The type of absorption spectroscopy depends upon the type of tranSItion involved and accordingly the frequency range of the electromagnetic radiation absorbed. Jf the transition is from one vibrational energy level to another, then the radiation is from the IR portion of the electromagnellc spectrum and the techmque is knOl'.'Ilas IR

spectroscopy [12, 13].

The portion of the IR region most useful for analysis of orgamc compOllllds is not immediately adjacenl to the visible speclrum, but is that having a wavelength range

Chapter4: Chemical,Thermaland OpticalpropcrIicsofPPTEOS 53

from 2,500 10 16,000 nm, WItha corresponding frequency range from 1.9 X 1013to 1.2 X lO¹⁴HL

Photon energies associated with this part of the infrared radiation are not large enough to excite electrons, but may induce VIbrational excitation of covalently bonded atoms and groups. Any structural change like addition subshtulion, of groups or moms In a molecule affects Ihe relative mode of,'ibration oflhe group

Organic funcllonal groups differ from one another bolh in Ihe strength of Ihe bonds, and in Ihe masses of the atoms involved. Molecules are flexible, moving collections of moms. The atoms in a molecule are constantly oscillming around average positions, Bond lengths and bond angles are continuousl} changing due to this vibration A molecule absorbs infrared radiation when Ihe vibration of the atoms in Ihe molecule produces an oscillming electrIc field with the same frequency as the frequency of incident IR '1ight", AU of the mOlions can be described in terms of two types of molecular vibrations One type oflhe vibrauon, a stretch, produces a change of bond length. A stretch is a rhylhmic movement along Ihe line between the atoms so thlrtlhe interatomic distance ISeilher increasing or decreasing. The second type of vibration, a bend, results in a change 10bond angle, These are also sometimes called

scissoring, rocking, etc. motions, ElIChoflhese two main types of vibration can have ,'ariations A stretch can be symmetric or asymmetric, Bending can occur in the plane of the molecule or out of the plane, it can be sCIssoring, like blades of a pair of scissors, or rocking, where two atoms move in the same direcllon.

A molecule absorbs only those fh'quencies of lR light that match vibmtions that cause a change in the dipole moment of the molecule. In a complicated molecule many fundamental vibretions are possIble, bul not all are observed, Some motions do not change the dipole moment for the molecule; some are so much alike thm they coalesce into one 00nd. Even though an IR spectrum is characteristic for an entire molecule, there are certain groups of moms in a molecule that gives rise to absorption bands at or near the same wavenumber,v,(frequency) regardless of the rest of the structure of the molecule. These persistent characteristic bands enable to identify tl1IIJorstructural features of the molecule.

Assignment for stretching frequencies can be approximated by the application of Hook's law. In the applicmion of the law, m'o atoms and their connechng bond are

.'

Chapter4: Chemirol,Thermaland Opticalpropcrt;C'lofPPTEOS 54

treated as a simple harmonic oscillator composed of two masses joined by a spring.

The following equation derived from Hook's law slates the relationship between frequeJlq of osci lIat'on, atomic masses, and the force constant of the bond.

4.1

where v is ,ibrationa! frequency (cm"\ c is velocity of light (cm looc),

lis

force constant of bond (dynes/cm) and Mx and M, are mass(gm) of atom x and y respecti;ely The values of/are approximately 5XlO' dynes per em for single bonds and approximately 2 and 3 times this value for double bonds and tnple bonds, respectively.

An infrared spectrum silo"" the frequencies of IR rm!lation absorbed and the % of the incident light that passes through the molecule without being absorbed. For organic molecules, the lR spectrum can be divided into three regions. Absorptions betv.'eeIl 4000 and 1300 cm" are primarily due to speCific functional groups and bond types. Those bet\\'eell 1300 and 909cm", the fingerprint region, are primllT1ly due to more complex interacuons in the molecules; IIT1dthose between 909 and 650 cm"' are usually associaled with the presence ofben7.ene rings m the molecule.

There are no rigid rules for interpreting an IR spectrum. Certain requirements, however, must be meel before an attempt is madc to interpret a spectrurrl

i.) The specrrummust be adequlltely resolved and ofadequale intensity.

i) The spectrum should bethat ofa reasonable pure compound.

Ii) The spectrophotometer should be calibrated so that the bands are observed at theIr proper frequencies or w'llvelengths. Proper callbr$on can be made with reliable standards, such as polystyrene film.

iii) The methods of sample handling must be specified. If a solvent is employed, the solvent, concentral>on, and ilie cell thlCkness should be indicated.

The experimenla1 monomer is tetraethylorthosilicate (TEaS) and it is a heteroatom containing organic compound. An increasing number of compounds contain atoms oilier than carbon, hydrogen. oxygen, nitrogen and halogen. Infrared correiations

•••

Chapter 4: Chemical, Thcrmal and Optical prop""les of PPTEOS 55

with structural units are necessary for rapid screening of reaction products and compoWld iden/lfication These correlations may prove useful as an introductlOn to the infrared speoctral changes ora variety ofheteroatom substance.

4.4.1 Experimental procedure

lR speoctra of the mOll{lmer liquid and plasma polymen7,cd thin films ",-ere recorded at room temperature using a double-beam IR spectrometer Shimadzu-IR 470 (Shimadzu Corporation, Tokyo, Japan), A drop of each of lhe respective liquid monomer was placed betv.ieen two !bin potassium bromide (KBr) pellets to record the IR spectrum of the monomer, Plasma polymeri7.ed films of differenl precursors prepared on glass substrates were used for the IR analysis, Specimens were scraped off from the substrates and a little amount of sample was taken to prepare pallets after mixing with KBr, The strength of an IR absorption speclrum is dependant on the number of molecules in the beam. With a KBr disk the strength will be dependant on the amount and homogeneity of the sample dispersed in the KBr powder. The spectrometer has its repeatability of the lransmittanee, 0.5%, except the wavenumber range, where the absorption bands of the waler \'apor exist. Its wavenumber range is 4000-400 em".

4.4.2 Results andDiscussion

Fig, 4,3 shows the IR spectra of the mOll{lmer TEOS and PPTEOS, which are represented by M and N respectively.

The O-H vibralion at 3420 em" (A') for PPTEOS IS observed which is not observed for monomer, The asymmetric C-H stretching band in the region 2730-2970 em" (A) IS observed for monomer in the spectrum M. The methyl group attached to sihcon, r ••CH, at 2970 em" which shows strong intensity, y••CH;, at 2916.9 em" and 2730 em'l for monomer are obsen'e<1. The C = 0 or C = C stretching VIbrations are observed in the wide band 1640-1502 em-I (B) in the N spectrum ofPPTEOS, The absorphon peak all441 cm.l (C) is observed C-H bond due to asynunetrieal bending.

vibration in M, \\Ihieh is not observed in N, The absorption peak at 1392 em"' in M and at 1400 em.' (D) in N lITerelated 10 the synunetrie CH, bending band at 1377 em' , and asymmetric CH. bending band 1111412 em" in Si-C,H\ The symmetrical C,., 0

Chapter 4: Chemical, Thermal and OptiCIIlproperties ofPPTEOS

stretching band at 1351 em.1 (El of PPTEOS is sho\\'lI in Fig. Cli., rocking vibralJon peak are observed at 1285 em-' and 1260 em-l (F) in spectm M and N for Si(CHsh,

'"

'00

,

d

"

"

^0uc ^~

^,

" , ,

0 G H H'

-

_E

^,

'" "

c0 ~

- ^e

~

" ,

0

~ooo '''''''

^{2000 ISOO 1000 ;00}

Wavenumber (em-I)

fig. 4.3.The IR spectra ofmonomerTEOS, spectrnmM; and PPTEOS, spectrum N.

Tahle-4.2. Assignments ofIR absorption peaks for TEOS and PPTEOS

V,1xal;on, Wa"enwuher(cm- )

Monomer TEOS PP lEOS

Cl-H,ibnllion (A') 3420

C_l' stretchiog ,ihrntion (A) 2970,29169,2730

C-O,C C <lreIclnllg VIbratIOn(B) 1640.1502

C.H "")'JllIDclJ1colbending ,ibratioo (C) 1441

Sl.C,H,; CH, b<"Dd1Di!vibnllion(j)) 1392

,~

C - 0 sj'mmetri",,1 stretching ,ihralion (E) 1351

Si(CH,)" Cll, rookiog ,ibration (F) 1285 1260

S;-OCH" S,-OC,H,; Si_O 'lITlclriog Vlbnl~OJ1 1001, lIe:; 1034 (G, H, H')

Si(CH,)" Si-C ,tretching ,ibrotion (I)

'" ~,

C-C ;tretohiflg vihrnlton (J) 47n-411 478.432

Chapter4: Chemical,ThermalandOptical propertiesofPPTEOS

SI-O stretching vibrations in S,-OClI;) are observed at I lOS em-' (G), IOOl em-' (H) and 1034 cm-' (l-I') in the speclra TEOS and PPTEOS.

Si-C stretching vibration peaks in Si(CH,h are seen at 188 cm-' and 795 cnf' (I) in the spectrum of TEOS and PPTEOS respectively Below 500 em-' i.e., absorption peaks in region 410 _ 4J I cm-' [or TEOS and 478 - 432 em-' [or PPTEOS may be due to C.C stretching vibration l14].

Thus from the above discussion it is seen that the PPTEOS film deposned by the PI3Sllla polymenLation techmque does not exactly resemble 10 that of monomer TEOS structure. The EA confirms the departure of the chemical composition of

PPTEOS from that ofTEOS.

4.5 Differential Thennal Analysis (PTA)