Electron-Beam Exposure of Organic Materials - ACS Symposium

Chapter 34

Electron-Beam Exposure of Organic Materials Radiation Curing of Perfluorinated Acrylates

Downloaded by RUTGERS UNIV on January 1, 2018 | http://pubs.acs.org Publication Date: December 28, 1990 | doi: 10.1021/bk-1990-0417.ch034

J. Pacansky and R. J. Waltman IBM Almaden Research Center, 650 Harry Road, San Jose,CA95120-6099

This study is divided into two parts: the first involves electron beam exposures on CO isolated in rare gas solids to investigate the role of an inert solvent on radiation chemistry; further experiments were conducted on diazoketones mixed into polymers to investigate the role of reactive solvents. The second part of the study deals with electron beam exposure of fluorinated acrylates and perfluorinated ethers. These reveal that highly fluorinated acrylates when mixed with non-fluorinated acrylic monomers may be used to produce radiation cured films with a broad range of contact angles. 2

Processes utilizing electron beams for curing of coatings, sterilization, etc., almost always involve exposure of a mixture of substances. A pertinent example is the curing of coatings via an electron beam induced free radical polymerization. Here, the rate of the polymerization depends on the efficient use of the absorbed dose to form free radicals for the initiation of the polymerization. Since the material that initiates the polymerization (whose identity in most cases is unknown) is only one of the components in the mixture absorbing energy from the incident electron beam, energy not directed to it is wasted. In the experiments presented herein, it is shown that the electron beam sensitivity of a component in a mixture may be altered by changing its concentration. We investigate two component mixtures: one consists of a molecular system in a chemically inert solvent; the second case relaxes the chemical reactivity by using a polymer as a matrix. Specifically, the former is carbon dioxide isolated in solid argon while the latter is a diazoketone blended into a phenolic polymeric matrix. An understanding of the radiation-induced effects in the more "simple" CQ /Ar system will provide a framework from 2

0097-6156/90/0417-0498$06.00/0 o 1990 American Chemical Society Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

34.

PACANSKY & WALTMAN

Electron-Beam Exposure of Organic Materials 499

which the irradiation of more complicated systems such as of guest-polymer (organic) formulations may be understood. Finally, since perfluorinated materials are widely used as coatings for a number of applications, we investigated the electron beam curing of mixtures of perfluorinated and non-perfluorinated acrylates to produce formulations for radiation curing with a wide degree of contact angles (with water). Additionally, the feasibility of directly converting liquid perfluorinated ethers to solid films using electron beams was investigated. CO, IN SOLID ARGON The rare gas matrix isolation technique has been successfully used to study reactive intermediates generated by both chemical and physical methods ' . In this report we extend the utility of the matrix isolation technique by demonstrating that chemistry may be initiated in molecular systems isolated in rare gas matrices using electron beams with an accelerating voltage of 25 kV.


1 2

Experimental Apparatus. The experimental equipment was described in detail in another report , hence, only those parts special to the matrix application are presented herein. The apparatus, shown in Fig. 1, consisted of a 55 liter stainless steel vacuum chamber (Model 202 Displex) with a 8 inch cold surface. Pressure in the chamber was typically in the 10" torr range; pressures in the 10 to 10 torr range were reached with a nominal amount of baking. Connected to the vacuum chamber was an electron beam gun, a quadrupole mass spectrometer (UT1 Model 100C), and a He/Ne laser interferometer. The electron beam gun could be operated at energies up to 30 keV. Its main features were first, the electron beam could be focussed using magnetic lenses, and second, the beam could be positioned and raster scanned using a dual ramp generator with a Cclco blanking amplifier. During the course of an experiment the electron beam current was measured with a Faraday cup connected to a picoammctcr (Kcithly 480). The current measurement was made by positioning the electron beam onto the Faraday cup. The sample was mounted onto a rotable sample holder that could be adjusted from outside of the chamber. Thus, a matrix could be deposited and simultaneously interrogated by a He/Ne laser interferometer; in this manner the thickness of the matrix was determined. Also shown in Fig. 1 is the location of the reflectance infrared spectrometer relative to the vacuum chamber. The spectrometer consists of a double beam goniometer which accurately controls the angle of incidence on the reflectance sample and reference, and a Spex Industries incorporated Model 1701, 3/4 meter monochromator. The gas handling system used for sample preparation was described previously . An Air Products low temperature closed cycle refrigerator was used to maintain the sample temperature. The refrigerator was mounted in a rotable flange at a position above the point marked RS in Fig. 1 such that a 180° rotation allowed a matrix to be deposited, irradiated or its infrared spectrum recorded. 3

8

9

10

3

Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


500

RADIATION CURING OF POLYMERIC MATERIALS

Figure 1. Experimental apparatus for spectrometry detection of electron beam induced reactions. Upper portion of figure describes infrared spectrometer while lower portion describes electron beam gun and vacuum chamber. NG = Nernst glower; Ch = chopper; RR = reflectance reference; RS = reflectance sample; M = monochrometer; CP = cryopump; FC = Faraday cup; IG = ionization gauge; SP = sorption pumps; W = window; MD = magnetic deflection; ML = magnetic lens; IP = ion pumps; EBG = electron beam gun; TMP = turbomolecular pump; MS = mass spectrometer; BS = blanking section; and AS = alignment section.


34. PACANSKY & WALTMAN


Argon Matrix Exposures. We have chosen the system C 0 in argon for its simplicity. The chemical changes induced by the electron beam were deter mined by following the changes in infrared spectra as a function of incident charge density. The net effect of the electron beam exposure is the conversion of C 0 to CO as shown below: 2

2

C0

2

-->

CO 4- Ο

(1)

Qualitative evidence for this reaction is obtained by noting the synchronous decrease and increase in the C 0 and CO absorptions at 2340 and 2138 c m as a function of dose. The reaction was quantitatively related by measuring the current density in the electron beam with a Faraday cup and by measur ing the thickness of the matrix using an interferometer. Fig. 2a shows a plot for the log of N / N (normalized decay of C 0 ) as a function of incident charge density (current density = 0.26 /iamps/cm ); note that the concentration of C 0 in the argon matrix decreases as the rate of decay for C 0 increases; concomitantly, the rate of formation of CO also increases with C 0 dilution in argon. This latter observation is clearly shown in Fig. 2b for the formation of CO as a function of incident charge density (For the matrix thickness and beam energy employed here, a charge density of 1 yuC/cm , is equivalent to an absorbed dose of 2.23 Mrads.). The mechanism for the enhanced rate of decay for C 0 shown in Fig. 2 is most likely related to migration and transfer of electronic energy from the argon matrix to C 0 . An analysis of the stopping power for particulate ra diation incident upon a solid shows that it is proportional to the number density of the materials in the matrix. Consequently, the rate of decompos ition for C 0 , based upon stopping power considerations alone, should de crease with dilution in solid argon. However, since it is observed that the rate increases with dilution, an energy transfer mechanism is required that effi ciently directs the energy deposited by the electron beam to the matrix isolated C 0 molecules. As a further consideration, the G values for decomposition of C 0 in argon (concentration: 1/100) were determined to be 3.6 based on the energy absorbed by the matrix, and 150 if only the energy absorbed by C 0 is used (The latter value was obtained by using the electron fraction of C 0 ) . Clearly, this G value is very high and another mechanism other than direct electron hits on C 0 is required. In the discussion that follows a kinetic analysis is presented for a quantitative evaluation of the process. The electron beam excitation of the matrix is given by 1


2

Q

2 2

2

2

2

2

2

4

2

2

2

2

2

2

2

M ->ΛΥ*

k

(2)

}

x

Here M may be an electronically excited or charged entity in the matrix, for example an exciton or hole, which may dissipate its energy back to the matrix or transfer it to C 0 2

x

M +M

X

-*M+M

k

2


(3)


502

Absorbed Dose (Mrad)


0

34

68

102

136

Absorbed Dose (Mrad) 170

0

Charge Density (/iC/cm*)

34

68

102

136

170

Charge Density (^C/cm*)

Figure 2. (a) A plot for the log of N / N , the fraction of surviving C 0 molecules, as a function of incident charge density. The N / N was de termined by following the changes in the infrared absorption of the 2340 c m band. The film thicknesses were 2.9 μνη for all three samples. The concentration of C 0 in matrix is given as mole fraction, (b) Growth of the CO band at 2138 cm' as a function of incident charge density. N = maximum number of CO molecules on the growth curve. 0

2

n

1

2

1

m


34.


PACANSKY & WALTMAN x

M + C0

-+M+CO$

2

k

(4)

3

x

C 0 may dissociate to give CO or return to the ground state ( or some other entity ) 2

COj C0

CO -f Ο

C0

2

k

(5)

A

or other products k

2

(6)

5

The rate for the formation of CO by direct, irradiation of C 0 is Downloaded by RUTGERS UNIV on January 1, 2018 | http://pubs.acs.org Publication Date: December 28, 1990 | doi: 10.1021/bk-1990-0417.ch034

2

C0

-+CO + Ο

2

k

(7)

e

If we use the steady state approximation then the concentration of CO formed in unit time is

*4 + *5

*

M + £ [C0 ]

2

3

2

The rate for reaction 7 may be neglected because of the dilute concentration mandating infrequent direct hits on C 0 ; therefore, 2

1 [CO] A plot o f — !

+ *s) M ^ C O j

,

M*4

versus

*4 +

*5

Λ,Λ^Α/]

at a particular dose should be linear with

Icoj

[co]

intercept slope

/c

3

A [Af] 2

Thus, we find that the ratio of k /k = 140, that is, the rate constant for en ergy transfer of M to C 0 is 140 times greater than for transfer back to the matrix. The identity of M is consistent with an excited state of the matrix. For example, when CO was irradiated under the same conditions as C 0 , the G value for decomposition was 0.09 (based on energy absorbed by the matrix) and 0.006 for CO decomposition in Xe (concentration: 1/100); the value for argon is consistent with inefficient or no energy transfer, for xenon decom position by direct hits on C 0 appears to be the only viable process. In Fig. 3, an energy level diagram is shown summarizing the energetics for decom position of C 0 to CO 4- O, and CO to C + O, along with the energies for the E ^ excitons . Note that the experimental results discussed above fit the 3

2

x

2

x

2

2

2

5

n

1




504

3

3

C( P)+20( P)

3

3

C( P)+0( P) _

, ί

290 $ n

0

m

I \

1

CO+0( S)

~

1

3

i 558nm

P

1

_

P,

1

CO+0( D) T~630nm coVo( P) 3

co

2

CO

Ar

Kr

Xe

Figure 3. Energy level diagram for C 0 and CO, and exciton levels for Ar, Kr and Xe. 2


34.


PACANSKY & WALTMAN x

required energetics that M is an exciton. Electron beam excitation of solid Ar, Kr and Xe, would thus produce an exciton with sufficient energy to produce CO + Ο from C 0 ; For CO only exposures in solid argon may be aided by energy transfer, a similar process in Kr and Xe is not energetically possible. 2

MIXTURE OF D1AZOKETONE IN A PHENOLIC POLYMERIC MA TRIX


The system chosen to study consisted of the diazoketone \ in the resin 2. The diazoketone plays the role of a guest mixed into the host resin. Films were prepared by spin coating onto a silicon substrate using diglyme as a solvent.

1

2

Electron beam exposure was performed on a C D 150 Electron Beam Processor (Energy Sciences, Woburn, Mass.). The decay of the diazoketone was moni tored by recording the transmittance of the diazo-strctching at 2110 cm* as a function of absorbed dose; these results are summarized in the form of a N / N versus dose plot in Fig. 4. The salient result of the plot is that the effi ciency for the decomposition of the diazoketone increases as its concentration in the phenolic resin decreases as shown in Fig. 4 by the gradual increase in slope with decreasing diazoketone concentration. As noted above for the C0 /argon experiments, a reasonable explanation for the diazoketone/resin results is migration of energy deposited by the electron beam in the resin fol lowed by energy transfer to the diazoketone. More experiments are presently in progress to study this phenomenon. 1

o

2

THE DIAZOKETONE/RESIN SYSTEM: VACUUM ELECTRON BEAM EXPOSURE The photochemical Wolff rearrangement is the mechanism by which a diazoketone after excitation loses N and converts to a kctene. All of this oc curs on a reaction path that presumably does not have a minimum between the diazoketone and ketene. As shown in the scheme below, reasonable evi dence for the ketene is in fact the two trapping experiments, i.e., in air, the 2

R 4

3

5

6




506

Figure 4. The fraction of diazoketone molecules N / N that survive an absorbed dose (Mrad) in a diazoketone/resin mixture. The legend labels each line with concentration by weight of diazoketone \ in resin 2. o


34.

PACANSKY & WALTMAN


thermal reaction of the ketene with water to form the indene carboxylic acid and, in vacuum, the thermal reaction with phenolic-OH groups to form the carboxylic acid ester. For the case of the vacuum electron beam exposures we have two observations that apparently lead to a paradox. The first is electron beam exposure produces a ketene but with a much lower yield at low temperatures; this is shown in Fig. 5 by the characteristic C-=C = 0 stretching frequency at ~2130 cm which appears upon irradiation. The second is that the carboxylic acid ester does not appear to be formed when the vacuum electron beam exposures are performed at room temperature. A comparison of Figs. 6 & 7 supports this claim; for example, Fig. 6 summarizes the results for a U V exposure of the system to demonstrate the major IR absorption for the ester; Fig. 7 contains the results for the electron beam exposure under identical conditions and shows that the C = 0 band for the ester at ~1750 is not evident. If the ketene was produced via a Wolff rearrangement at low temperature then the carboxylic acid ester should certainly form as a result of the room temperature exposure. In view of this we conclude that the mechanism for the vacuum electron beam exposure does not involve a Wolff rearrangement. Instead we propose that-a carbene formed by loss of N from the diazoketone plays a central role in the chemistry that ensues after electron beam exposure. The proposed mechanism for the electron beam induced chemistry discussed thus far is summarized in the scheme below.


-1

2

Electron beam exposure at low temperatures produces a carbene 7, which upon further electron beam excitation rearranges to a ketene 5. The carbene 7 may exist for relatively long periods of time at T= \0K because of the frozen, immobile environment at this temperature. In effect, this dramatically retards carbene reactions with the resin and opens a channel for further excitation of the carbene to ketene. At room temperature, however, the electron beam generated carbene rapidly reacts with the resin and hence has too short a lifetime for further excitation to ketene to be an issue. EVIDENCE FOR CARBENE FORMATION Mixtures of diazoketone 1, with benzene were exposed to the 175 kV electron beam and infrared spectra recorded as a function of absorbed dose. Fig. 8 contains spectra taken after D = 90, 180 and 300 Mrads were administered to the sample. In this case the spectral region where - O H groups characteristically absorb is not initially obscured and the increase in absorption in this region as a result of the electron beam induced chemistry is clearly evident. Furthermore, a number of the absorptions observed as a result of the electron


508



Wavenumber (cm"')

Figure 5. IR spectra of 5.2 μχχ\ film of equimolar diazoketone 3 in resin 2. The sample was exposed to an electron beam (25 kV, 1 = 1 nA/cm , Τ = 10 Κ). Incident charge density Q equals: (a) 0; (b) 40; (c) 100 /iC/cm . 2

2



34. PACANSKY & WALTMAN

1

01 4000

1

1

1

1

1

1

3500

1

1

1

1

1

1


1

3000

1

1

1

1

1

2500

'

1

1

1

— I 1—1

1

ι ι ι ι ι ι ι ι ι ι 2000 1500 1000

500

- 1

Wavenumber ( c m )

Figure 6. IR spectra of a thin film of cquimolar diazoketone 3 in resin 2. The sample was exposed to U V light (A > 300 nm) in vacuum at room temperature using a 150 W high pressure X e lamp: ( ) before ex posure; ( ) after exposure.


510


100

ι ι ι ι ι ι ι ι ι ι ι ι ι U 1

I U

80 60 40 20


0

1

1

4000

1

1

1

1

ι ι

ι I

1

1

1 1

1

1

1

1 1l

1 1 ι ι 1

l

1

ι ι ι I 1 1 1 1 1 11

3500

3000

2500

2000

1500

1000

500

200

3500

3000

2500

2000

1500

1000

500

200

100 .

ω

4000

ο

I ioo ο

ë

80

*

60 40 20 ι

0 4000

ι

ι ι

I

ι ι

ι ι

» • < ι ι

ι

ι

I

ι ι

ι

ι

3500

3000

2500

2000

3500

3000

2500

2000

ι

ι ι

ι

ι

ι

ι ι ΐ

ι ι ι

ι I ι

1500

1000

500

200

1500

1000

500

200

100 ι

4000

Wavenumber ( c m

- 1

)

Figure 7. IR spectra of a thin film of equimolar diazoketone 3 in resin 2. The sample was exposed to an electron beam (25 kV, I = 0.5 nA/cm ) in vacuum at room temperature. The incident charge density Q equals: (a) 0; (b) 22; (c) 33; and (d) 70 /^C/cm . 2

2



PACANSKY & WALTMAN

100

80

j γ

60

In


40

a

20

4000

3500

3000

j-J ι I i—J ι 1 ι 1 l—I ι 1 2000 1800 1600 1400 1200 1000 900

2500

ι ι ι ι I ι ι ι

I IIII III

η

I

80

ω υ c

Β Ε (Λ C

60 D = 180Mrads

I

1

40 20

800

-i—r—ι—1 I 1 l 1

100 ι

-

0 4000

b •

3500

» • •

3000

2500

•

-J

ι

1

ι

1

3500

3000

2500

I

ι

1 ι

I

2000 1800 1600 1400 1200 1000

100 ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι—ι ι

4000

ι

ι

ι ι

ι ι

ι

1 900

800

ι—r—r

2000 1800 1600 1400 1200 1000

900

800

1

Wavenumber (cm' )

Figure 8. ÏR spectra of diazoketone J in benzene (film). The sample was exposed to an electron beam (175 kV) at room temperature under N The absorbed dose equals: (a) 90; (b) 180; and (c) 300 Mrad. 2


512


beam exposure of the diazoketone 1, and resin 2, and diazoketone 3, and meta-cresol are in common with those observed for the benzene irradiations; in particular, these are the bands in the high frequency region of the IR where O H and C H groups appear. The respective band centers for those common to all of the exposures studied herein are 3550, 3140, 1460, 1380, 1080, 1040, 810, and 780 c m . Another broad absorption appears at ~ 1700 cm but it is clearly due to a secondary product of the irradiation because the intensity of this band continues to increase with exposure after the diazoketones have been consumed by the radiation. 1


S U M M A R Y OF T H E DIAZOKETONES Κ 3

-1

ELECTRON

BEAM

EXPOSURES

OF

The experimental results reported herein reveal electron beam excitation at room temperature does not proceed through a Wolff rearrangement. The ma jor reaction path appears to mitigate an intermediate which reacts with aromatic C H bonds and perhaps aromatic C C bonds. The intermediate ap pears to also react with O H bonds; irradiations of mixtures of the diazoketones in tertiary butyl alcohol are very similar to the resin, metacresol, and benzene results presented above. Our contention therefore, is that electron beam excitation of the diazoketones at room temperature produces a carbene which in the presence of benzene reacts with C H and C C bonds to ultimately produce substituted naphthols for example as outlined in the scheme below. A t low temperatures, the carbene is confined by the rigid environment and when excited by another 7

electron forms a ketene. in summary, these results not only uncover a diver gence between the excitation by photons and charged particulate radiation but also provide a rationale for the positive working nature of electron beam re sists using diazoketones. The substituted naphthols produced by the electron beam induced chemistry are soluble in aqueous K O H developers and hence the exposed regions of the resists may be imaged by the high energy electron beam.

E L E C T R O N B E A M CURING OF PERFLUORINATED A C R Y L A T E S : A PRACTICAL E X A M P L E The surface characteristics of metallic and other substrates frequently require modification to meet particular criteria; often, it is important to alter the sur face energy of a substrate for a particular application. Here, contact angles control a variety of important surface properties, such as wettability, detergency and waterproofing, etc. The contact angle is defined simply as the


34.

PACANSKY & WALTMAN


angle formed between the surface of a solid and that of a liquid; it relates the surface tensions at the solid, liquid and liquid-solid interfaces. A facile method by which organic thin films may be readily coated upon substrates employs the electron beam curing of materials such as of monomeric and/or oligomeric acrylates and methacrylates. The pcrfluorinated acrylates 2-(N-butylperfiuorooctanesulfonamido)ethyl acrylate ( B F O S A ) , 2-(N-cthylperfluorooctanesulfonamido)ethyl acrylate ( E F O S A ) , and perfluoropolyether diacrylate ( P F E D A ) were obtained from the 3 M cor poration, St. Paul, Minnesota; 1,6-Hexanediol diacrylate ( H D O D A ) was ob tained from Interez, inc., Louisville, Kentucky. Thin films were spin-coated onto virgin, optically flat N a C l disks or flat, polished Si wafers and electron beam cured at 5-10Mrad under N . The ox ygen content in the irradiation chamber was

Electron-Beam Exposure of Organic Materials - ACS Symposium

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