IEEE TRANSACTIONS ON RELIABILITY, VOL. 43, NO. 4, 1994 DECEMBER

Tu torial How Radiation Affects

Mohamad Al-Sheikhly

Aristou Christou University of Maryland, College Park

University of Maryland, College Park

Key Words - Radiation mechanism, Free radical, Oxidation, Degradation, FTIR, ESR, Failure prediction

Reader Aids - General purpose: Tutorial Special math needed for explanations: None Special math needed to use results: None Results useful to: Designers and reliability engineers

Abstract - Radiation-induced degradation of polymeric materials proceeds mainly through free-radical oxidation mechanisms. The radiolytically produced free radicals react with the molecular oxygen to produce the corresponding peroxy radicals. The production of peroxy radicals initiates oxidation processes which then lead to degradation. This tutorial explains the mechanisms of radiation-induced oxidation & analytic techniques that can be used to measure it.

1. INTRODUCTION

This is the 13'h paper in a tutorial series on failure mechanisms and their role in physics-based damage models for reliable designs. Development of quantitative models for simulating these failure mechanisms is important in formulating appropriate design criteria for reliable engineering systems. The first paper was a broad overview of some relevant overstress & wearout' failure mechanisms commonly encountered in engineering applications [ 11.

The other, more-specific papers were on designs where -

excessive elastic deformation can be an overstress failure mechanism [2]. irreversible plastic deformation can be an overstress failure mechanism [3]. brittle fracture can be an overstress failure mechanism [4]. ductile fracture can be an overstress failure mechanism 151. elastic buckling can be an overstress failure mechanism [6]. mechanical wear can be a wearout failure mechanism [7]. mechanical creep deformation and creep rupture can be wearout failure mechanisms under time-dependent loads [8]. cyclic fatigue can be a wearout failure mechanism 191.

'Wearout, as used in this paper, applies to a physical degradation pro- cess, not to a type of statistical distribution.

Polymeric Materials

551

interdiffusion can be a wearout failure mechanism [ 101. solid-state electromigration can be a wearout failure mechanism [ 111. migration through a liquid medium (eg , conductive anodic filament formation) can be a wearout failure mechanism 1121.

4

The failure mechanism in this paper is loss of system per- formance due to radiation-induced oxidation of polymeric materials.

2. CONCEPTS

Radiation-induced chemical changes are caused solely by energy which is transferred from the radiation to the bulk of the materials 1131. High energy in this context is an energy greater than the ionization energy of atoms and molecules that usually lies in the range 9 - 15 eV (1.3 - 2.4 -1O-'J) per atom or molecule. In practice, however, radiation with energy in the keV or MeV range are used for both radiation-chemical research and for industrial radiation processing. The most com- mon forms of radiation are electromagnetic (gamma) radiation from radioisotopes cobalt-60 and cesium-I 37, and electron beams generated by electron accelerators. Heavy-particle radia- tion (eg , alpha, and accelerated deuteron & heavy ions) and neutron beams can be used for special purposes. The transfer of energy of incident radiation occurs through successive in- teractions of primary particles with electrons & atoms of the medium. These interactions result in ionization & excitation of molecules of the material and in formation of cascades of secon- dary charged particles with kinetic energy sufficient to produce ionization & excitation by themselves. The ionization is pro- duced either directly by the flux of photons & electrons or in- directly due to those types of incident radiation which form charged particles on interaction with the medium. Interaction between radiation & materials is described in terms of

cross section, interaction coefficient (photon & neutron irradiation), stopping power (charged-particle irradiation).

The cross section for interaction between particles (including photons) & materials is:

U = P / @

Notation

U cross section P

@ particle fluence

Pr { interaction when one atom or molecule is subjected to the particle-fluence @}

0018-9529/94/$4.00 01994 IEEE

552 TUTORL4L IEEE TRANSACTIONS ON RELIABILITY, VOL. 43, NO. 4, 1994 DECEMBER

P interaction coefficient S stopping power b barn (1 barn=10-28 m 2 ) E I distance P density of the material.

Other, standard notation is given in "Information for Readers & Authors" at the rear of each issue.

With photons, the term cross section is usually reserved for interaction coefficients whose value is of the order of 1 b ( m2); otherwise, the terms attenuation coefficient or ab- sorption coefficient are used. An alternative approach based on the average rate of energy loss (stopping power) is used when the primary radiation consists of charged particles, which in- teract much more strongly with the stopping materials. The total mass stopping power, for charged particles, is:

energy of a charged particle

RH' + e- - RH' - R' + H (5)

RH' + RH - RH: + R' (6)

RH: + e- - R' + H2 (7)

- R H + H (8)

RH + H - R' + H2 (9)

R' + R' - R-R (cross-linking)' (1 1)

Notation

RH RH excited molecule R' free radical RH +, RH: cation R- anion.

polymeric material, pigment, antioxidant, or filler

for more details, see [13]. 2.2 Radiation-Induced Formation Of Stable Products

2.1 Radiation-Induced Production of Radicals & Cations

Exposure of any organic solid (including polymers, copolymers, cross-linked polymers, antioxidants, pigments, fillers) to ionizing radiation produces radicals & cations. A free radical is any atom, molecule, or ion that contains at least one unpaired' electron. Unpaired electrons on an atom or group are often identified by adding a dot to the chemical formula as in R' & 'CH3 for carbon-centered radical or as in RO; for peroxy radical where the unpaired electron is on the oxygen atom:'. Because of their unpaired electrons, most free radicals tend to be very active. Anions & cations are negative & positive ions, respectively. The -

structure & the activity of these ions & radicals, dissolved oxygen concentration, dose-rate, degree of crystallinity of the polymer

determine the yields & structure of the final radiation-induced products [ 141.

The mechanisms of radiation-induced formation of radicals & cations are:

RH - RH' (e~citation)~

The more important chemical changes that irradiation in- duces in polymeric materials are [15]:

cross-linking: formation of cross (intermolecular) bonds; degradation: scission of bonds in the main polymer chain and

gas formation: formation of gaseous products such as H2,

changes in un-saturation: formation of various types of dou-

cyclization: formation of intramolecular bonds; oxidation (in the presence of oxygen): formation of aldehydes,

in side chains;

CH,, CO;

ble bond between carbons atoms;

ketones, carboxylic groups.

2.3 Radiation-Induced Degradation

For polymeric materials, degradation denotes changes in physical properties caused by chemical reactions involving bond scission in the backbone of the polymer. It is not useful, therefore, to distinguish between various modes of polymer degradation. For practical reasons, however, it is useful to sub- divide the degradation of the polymeric materials according to the mode of initiation:

thermal (2) mechanical

photochemical radiation

(3) chemical RH - RH' + e- (cation formation)

RH' - R. + H biological.

(4)

4Excited states are produced when bound electrons in atoms & molecules gain energy and are raised to higher energy levels. 'Crosslinking is a process whereby two separate long chain molecules (polymer molecule) become linked together into a single molecule, bas- ed on radical-radical reaction mechanisms, resulting in an increase in the average molecular weight of the polymer.

2A single electron in an orbital is called an unpaired electron. 'An atom or molecule possessing an unpaired electron is called afree radical. In writing the symbol for a free radical, we generally include a dot (*) to represent the unpaired electron just as we include a plus or minus sign in the symbol of an ion.

AL-SHEIKHLY/CHRISTOU: HOW RADIATION AFFECTS POLYMERIC MATERIALS 553

Thermal degradation implies that the polymer, at elevated temperatures, starts to undergo chemical changes without the simultaneous involvement of another compound.

Mechanically initiated degradation generally refers to macroscopic effects from shear forces. Stress-induced processes in polymeric materials are frequently accompanied by bond rup- tures in the polymer main-chains; for more details, see [15].

Many useful applications are based on the fact that the ab- sorption of high energy radiation causes the generation of reac- tive species (free-radicals and ions) in the substrate. Thus, high energy irradiation quite generally initiates chemical reactions occurring via free-radical or ionic mechanisms.

There is a strong inter-relationship between the modes of polymer degradation. Frequent circumstances that permit the simultaneous occurrence of several modes of degradation, are:

environmental processes: they involve the simultaneous ac- tions of temperature, ultraviolet light, humidity, oxygen, harmful atmospheric emissions; oxidative deterioration of the electronic packaging materials during processing: this is based on the simultaneous action of radiation, heat, mechanical forces, and oxygen.

As a result of degradation, many processes have com- plicated kinetics with specific activation energies, eg:

brittleness, decrease in the mechanical properties, delamination between the degraded polymeric material and the metal (eg, copper, silicon) structure, metal migration to the polymeric structure, decrease in the dielectric properties, hydrophilicity (water absorption), dendrite growth through conductive filament formation.

3. RADIATION YIELDS

3.1 Absorbed Dose

The most important unit in radiation engineering is the ab- sorbed dose: the energy transferred from radiation to substrate; for more details see [ 161. The unit of absorbed dose is the gray (symbol Gy):

1 Gy = 1 J/kg.

3.2 Radiation-Chemical Yields

4. RADIATION RESISTANCE

4.1 Molecular Structure The radiation resistance of a polymeric material can be

predicted somewhat from its molecular structure. Generally, compounds with double, conjugated double, aromatic rings, aromatic heterocycles ’, are more radiation resistant than those with saturated bonds. For example, polyimides with conjugated heterocycles possess the highest radiation resistance of all the polymers now known. The recently investigated polyester im- ides, for example, with repetition of the chain unit (see section 6.3) possess exceptionally high radiation resistance with G(crosslinking)=0.014 and G(scission)=0.05. On the other hand, substances with C-F, C-Si, C-0 bonds, and polymers in particular are less radiation resistant.

4.2. Effect of Impurities The radiation resistance of a polymeric material is affected

by additives which are always present in the commercial material. Additives which have various degrees of radiation resistance include:

antioxidants, organic additives (eg, dyes, technical carbon, diluents, antistatics), inorganic additives (eg, silicon dioxide, clay, metals salts).

4.3 Temperature Radiation-chemical transformations in polymeric com-

pounds have higher yields at higher temperatures. Usually, the temperature effect on polymers is especially noticeable at the melting temperature or the glass transition temperature. Con- sequently, thermo-radiation stability is an important characteristic of polymeric materials. Unfortunately many thermo-stable polymers do not withstand radiation. For exam- ple, fluorine-containing polymers (eg , polytetrafluoroethylene) and fluorine-containing rubbers possess high thermo-stability but do not resist radiation.

4.4. Other Factors

materials are: morphology pressure photo-radiation processes absorbed dose level dose-rate type of radiation

Other factors that affect the resistance of polymeric

Radiation-chemical yields have traditionally been reported in terms of G values, where G ( X ) is the number of moles of

structure Of the molecules Oxygen.

product X formed, starting material Y changed [shown as G( -U] per 1 joule energy absorbed6. ’In most of the cyclic compounds (eg, benzene, cyclohexane), the

rings are made only of carbon & hydrogen atoms; such compounds are called homocyclic compounds. But there are also rings contain- ing, in addition to carbon & hydrogen, other kinds of atoms, most com- monly nitrogen, oxygen, or sulfur. These compounds are called heterocyclic compounds (eg, polyimide repeat unit).

?he mole is the SI counting unit. One mole of any substance con- tains Avogadro’s number of atoms, eg, 1 mole of copper contains 6.022. loz3 Cu atoms.

554 TLJTORLAL IEEE TRANSACTIONS ON RELIABILITY, VOL. 43, NO. 4,1994 DECEMBER

5. OXIDATION exceed 4 - 8 kJ/mol. Oxidation products result mainly from non- chain processes. The radiation-chemical yields of the oxidation products in this range are: Oxygen strongly affects the radiolysis of polymers, and

leads to their oxidation. Oxidation can involve:

oxygen already present (dissolved) in the polymer, oxygen that diffuses into it during or after irradiation.

Notation Oxidation can occur during irradiation or in post-irradiation pro- cesses; the latter can be induced by heating the irradiated polymer. Peroxy radicals (RO;), hydroperoxide (R02H), and peroxides (ROOR) are important in the radiation-induced ox- idation of a polymer. Peroxy radicals (RO;) are formed when the polymer radicals (R') formed by irradiation react with oxygen?

while hydroperoxides are produced when peroxy radicals abstract hydrogen from another polymer molecule:

RO,' + RH - ROpH + R' (13)

Reaction (1 3) represents the propagation step of a chain reac- tion that can be terminated by reaction of 2 peroxy radicals to give a peroxide:

2RO; - Oxidation Products + O2 (14)

Both hydroperoxides & peroxides decompose on heating to give oxidation products that can include free radicals. The rate & reactions of RO; depend on the:

chemical structure of the polymeric materials, morphology, dissolved oxygen concentrations, irradiation temperature, radiation parameters (dose, dose-rate, type of radiation), presence of metals.

5.1 Radiation-Induced Oxidation

The radiation-chemical yield (G) of the oxidation products increases with increase of oxygen concentration (pressure), and reach their maximum value at concentration above l o p 3 molhiter . The oxidation reaction can be kinetically-controlled or diffusion-controlled depending on the ratio of O2 input to consumption rates. Radiation-induced oxidation begins at temperatures lower than those of thermal oxidation, and pro- ceeds at a higher rate. In the radiation range which covers relatively low temperature, the radiation-chemical yields of the oxidation products are more than 10 molecules per 100 eV (G = 10). They are independent of the dose-rate, and depend slight- ly on the temperature. The effective activation energy does not

X oxidation products GR El R gas constant T absolute temperature.

In the radiation-thermal range, the oxidation proceeds via a chain mechanism resulting in the higher G-values as compared to the first range. The radiation-chemical yields depend on the dose rate [17].

radiation-chemical yield of free radicals activation energy of the oxidation products

5.2 Metal Migration

Some metals (eg, Cu, Cr, CO, Si, Ti, Fe) and their oxides strongly enhance the degradation of polymers, and generally lower the activation energy for hydroperoxide decomposition. These metals ultimately lead to more degradation. A particular difficulty is the acceleration of thermal degradation of polymer materials in the presence of copper. Cuprous and/or cupric ions can diffuse into a polymer and decompose the hydroperoxides which are formed when polymer films on copper begin to ox- idize at elevated temperatures. This produces R' carbon- centered radicals, and reduces copper oxide during the process. The hydroperoxide (ROOH) groups are decomposed by Cuf ions which are oxidized to Cuf f ions in the polymer films. The Cu++ ions then decompose more ROOH groups, and are reduc- ed to Cu'. The alkoxy (ROO) and peroxy (ROO') radicals pro- duced during these reactions then lead to the accelerated oxidative degradation of the electronic packaging materials.

6 . ANALYTIC TECHNIQUES

Acronyms

ESR Electron Spin Resonance EPR Electron Paramagnetic Resonance FTIR Fourier Transform Infrared.

Methods have been developed to investigate the radiation- induced oxidative degradation of polymers [ 15,161, eg, the measurement of physical properties and the detection of chemical changes. The remainder of this tutorial focuses on the:

determination of the radiation-induced free radicals by using

non-volatile oxidation products by using FTIR. ESR,

6.1 ESR 8Reaction 12 is typical oxidation reaction of the free radicals whereby the unpaired electron on carbon atom (carbon-centered radical R') moves to the oxygen to form peroxy radical (RO;).

ESR (also known as EPR) is an important spectroscopic technique for the study of molecules & ions containing unpaired

AL-SHEIKHLY/CHRISTOU: HOW RADIATION AFFECTS POLYMERIC MATERIALS

electrons. It can be applied only to systems in which not all the 6.2 FTIR

555

electrons are paired9. Diamagnetic I o substances therefore can- not be studied by ESR, not only because all their electrons are paired, but equally importantly, they cause no interference with the observation of paramagnetic" substance by ESR. Com- monly encountered paramagnetic species include organic free- radicals, metal complexes, and triplet excited states of dia- magnetic molecules, as well as molecules such as NO, NO2, C102, which are examples of stable free-radicals [ 181.

The technique involves the splitting of the paramagnetic energy levels of the paramagnetic particles in a constant magnetic field according to:

FTIR is a powerful, direct, precise technique to measure oxidation in polymers [20]. The oxidation products can be detected as aldehyde, ketone, carboxyl, and ketonic carboxyl groups which can be monitored at 1500-1600, 1660, 1673, 1716-1720 cm-' wave number, respectively (see table 1) [21]. FTIR can be equipped with a Spectra-Tech horizontal attenuated total reflectance (ATR) attachment. ATR sampling makes it possible to analyze totally-absorbing specimens such as polymer film. The method minimizes the effects of specimen thickness. Another advantage is that the need for a nitrogen purge to remove atmospheric water-vapor and carbon-dioxide is eliminated since the sample compartment is protected from

(16) undesirable elements.

Notation 6.3 Illustration of the Presence of Radiation-Induced Oxidation

AE h Planck's constant

g

difference between two paramagnetic energy levels

Y frequency of the microwave energy Lande splitting factor (dimensionless number whose value depends upon the environment of the unpaired electron, eg, for free electron, g=2.0023) fundamental atomic unit of magnetic moment, the Bohr magnetron = 0.927. lop2' erg/gauss

P

H magnetic field strength.

ESR measurements are made by placing the paramagnetic material (material has at least one unpaired electron) in a uniform magnetic field, which causes the unpaired electron to orient itself with respect to the field. The orientation is either parallel or anti-parallel to the field; all unpaired electrons take one of these two possible states. The two states have slightly different energies and, at normal temperatures, there are more electrons in the lower-energy state. Simultaneously the sample is sub- jected to microwave radiation with a frequency v such that h . v = g. 0. H - causing the unpaired electrons to reverse their orientation with respect to the magnetic field. Since there are more electrons in the lower energy state, this results in a net absorption of radiation energy, which (with appropriate equip- ment) can be observed as a spectroscopic absorption line. The plot of absorbed microwave energy vs H comprises the ESR spectrum. Paramagnetic particles can be identified according to their hyperfine structure, line shape, and g-factor. The in- tegral of absorbed microwave energy (area under the spectrum) is proportional to the density of paramagnetic particles in the sample (number of free-radicals per gram) [ 191.

?WO electrons in the same orbital must have opposite (antiparallel) spins, and their magnetic fields are oriented opposite to each other. These electrons are paired. If 2 electrons have parallel spins, they cannot be in the same orbital. '(hamagnetic: Atoms, molecules, o r ions which do not have un- paired electrons in their electronic structure. "Paramagnetic: Atoms & ions containing unpaired electrons in their electronic structure are called paramagnetic.

Commercially available polyimide thm film (Kapton) is com- monly used as an electronic packaging material. Figure 1 is the FTIR spectrum of the irradiated Kapton (100 kGy) under aerobic conditions. The FTIR spectrum shows the oxidation peak at 17 17 cm-'. An oxidation peak at 1717 cm-' was also detected in the FTIR spectrum of the un-irradiated Kapton sample. However, upon irradiation, there was a large increase in the oxidation peak. Table 1 summarizes the FTIR results of the heated (thermal ox- idation) and irradiated (radiation-induced oxidation) samples.

n

C 3 - A

:x,.o z z . 0 inn., :,::.a *,,e.o i7=,.D " - ' 7 D o + D

Wave number, c m - l

Figure 1 .Absorption Band Of the Ketonic Group at 171 7 cm - ' wave number [Kapton: absorbed dose = 100 kGy; dose-rate = 4.5 GYW

6.4 Illustration of the Presence of Free Radicals

Figure 2 shows a typical ESR spectrum of Kapton ir- radiated with gamma rays in presence of oxygen. The presence of the ESR signal strongly indicates the presence of radiation- induced free radicals which most likely are carbon-centered radicalsI2. Since this measurement was made 4 weeks after

'2As a rule, the presence of any ESR signal in any sample indicates the presence of unpaired electrons.

556 TUTORUL IEEE TRANSACTIONS ON RELIABILITY, VOL. 43, NO. 4, 1994 DECEMBER

TABLE 1 Presence Of Various Oxidation Products (detected by FTIR) In

the Irradiated & Heated Samples of Kapton

Wave Number Detected oxidation cm-’ functional groups Chemical Structure

0

R-C-R‘ 17 16- 1720 Ketone I/

1742 Aldehyde 0

R-C-H II

1660- 1670 Carboxylic group R-COOH

irradiation, it strongly suggests the presence of long-lived free- radicals. Long-lived free-radicals can cause severe damage to the polymeric materials (degradation). On the other hand, short- lived radiation-induced free-radicals are often harmless since they react through radical-radical reaction mechanisms to pro- duce stable compounds (crosslinking reactions).

-- - 2500 ZJOO 3cao ZBOO AZO0

Magnetic field, Gauss

Figure 2. ESR Spectrum Of Irradiated Kapton [dose = 100 kGy]

REFERENCES

[l] A. Dasgupta, M. Pecht, “Material failure-mechanisms and damage models”, ZEEE Trans. Reliability, vol 40, 1991 Dec, pp 531-536.

[2] A. Dasgupta, J.M. Hu “Failure-mechanism models for excessive elastic deformations”, IEEE Trans. Reliability, vol41, 1992 Mar, pp 149-154.

[3] A. Dasgupta, J.M. Hu, “Failure-mechanism models for plastic defor- mations”, ZEEE Trans. Reliability, vol 41, 1992 Jun, pp 168-174.

[4] A. Dasgupta, J.M. Hu, “Failure-mechanism models for brittle fracture”, ZEEE Trans. Reliability, vol 41, 1992 Sep, pp 328-335.

[5] A. Dasgupta, J.M. Hu, “Failure-mechanism models for ductile fracture”,

IEEE Trans. Reliability, vol 41, 1992 Dec, pp 489-495. A. Dasgupta, H.W. Haslach Jr., “Mechanical design failure models for buckling”, ZEEE Trans. Reliability, vol 42, 1993 Mar, pp 9-16. P. Engel, “Mechanical failure-mechanism models for mechanical wear”, ZEEE Trans. Reliability, vol 42, 1993 Jun, pp 262-267. J. Li, A. Dasgupta, “Failure-mechanism models for creep”, ZEEE Trans. Reliability, vol 42, 1993 Sep, pp 339-353. A. Dasgupta, “Failure-mechanism models for cyclic fatigue”, ZEEE Trans. Reliability, vol 42, 1993 Dec, pp 548-555. J. Li, A. Dasgupta, “Failure-mechanism models for material aging due to interdiffusion”, ZEEE Trans. Reliability, vol43, 1994 Mar, pp 2-10. D. Young, A. Christou, “Failure-mechanism models for electromigra- tion”, IEEE Trans. Reliability, vol 43, 1994 Jun, pp 186-192. B. Rudra, D. Jennings, “Failure-mechanism models for conductive- filament formation”, ZEEE Trans. Reliabilify, vol 43, 1994 Sep, pp 354-360. R.J. Woods, A.K. Pikaev, Applied Radiation Chemistry Radiation Pro- cessing, 1994; Wiley-Interscience. A.J. Swallow, Radiation Chemistry: An Introduction, 1973; John Wiley & Sons. J.H. O’Donnell, “Chemistry of radiation degradation of polymers”, Radiation Effects On Polymers, (R.L. Clough, S.W. Shalaby, Fils), ACS Symposium series, 475, 1991, pp 402-413; American Chemical Society. McLaughlin, Boyd, Chadwick, McDonald, Miller, Dosimetry for Radia- tion Processing, 1989; Taylor & Francis. V.K. Milinchuk, V.I. Tupikov, Organic Radiation Chemistry Handbook, 1989; John Wiley & Sons. N.J. Bunce, “Introduction to the interpretation of electron spin resonance spectra of organic radicals”, J. Chemical Education, vol64, 1987 Nov,

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pp 907-914.

AUTHORS

Dr. Mohamad Al-Sheikhly; Dept. of Materials & Nuclear Engineering; Univ. of Maryland; College Park, Maryland 20742 USA. Internet (e-mail): mohamad@eng.umd.edu

Dr. AI-Sheikhly is adjunct associate professor at the Department of Materials & Nuclear Engineering. He is also a senior radiation engineer at US National Institute of Standard & Technology (NIST) - Ionizing Radiation Divi- sion. He was previously a Research Fellow at the Max-Planck Institute for Radia- tion Chemistry, Germany. He has published in the fields of radiation engineering & chemistry of polymers, fast kinetics, peroxyl radicals, pulse radiolysis, radiation-induced mechanisms of polymerization, radiation processing, and electron-beam technology.

Dr. Aris Christou; Dept. of Materials & Nuclear Engineering; Univ. of Maryland; College Park, Maryland 20742 USA. Internet (e-Mail): gass.eng.umd.edu

1994 Jun, p 192. Aristos Christou: For biography, see ZEEE Trans. Reliability, vol 43,

Manuscript received 1994 September 26

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