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DASB), were used as fluorescence probes for cure monitoring of photocurable polymer system1s. *This technique is based upon the difference in fluorescence, ...

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Intramolecular charge Transfer Complexes as Fluorescence probes fog UV and Vtqjhlp 1.9ghf Indhiwr AM]rynt -Pn1yiViP*in

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jiancheng Song, Afranio Torres-Filho and D.C. Neckers 7.7. EFORINGO~GNIZA.~: NAEIS 7RFO~INGORGAIZATG.' NAMM -ND.~OES~E3~8. -N ACRE55.E.-)

I-ERFORMING ORGANIZATION -EPORT nUMBER I.

Center for Photochemical Sciences Bowling Green State University Bowling Green, OH 43403

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Intramolecular charge transfer complexes (ICTC) of derivatives of 1-diaminonaphthalene-5sulfonamide (DANSYL ANMIE) such as 1-dimethylamino-naphthalene-5-sulfonyl-nbutylanude (l,5-DASB) and 2-dimethylaminonaphthalene-5-sulfonyl-n-butylamide (2'5 DASB), were used as fluorescence probes for cure monitoring of photocurable polymer system1s. *This technique is based upon the difference in fluorescence, intensity from the parallel and ~perpendicular conformations of -the excited state of the complex, and is based on the dependence, of the relative population of each conformation on the mucroviscosity of the system. As the curing reaction proceeds, the steady state fluorescence emission spectra of the probes were all found to exhibit hypsochromic spectral shifts due to the increase in matrix microviscosity. A ~ linear correlation between the fluorescence intensity ratios (R=Ipar./Iper.) and the extent of ~~polymerization, measured by transmission FflR spectrometry, was obtained for different types &.of acrylated polymers, cured with UV or visible (VIS) initiators.

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1a. SUBJECT TERMS

Intramolecuiar-Charge Transfer Visible-Light Induced Acrylate Polymerization 17. SECURITY CLASSIFICATION OF REPOR

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OFFICE OF NAVAL RESEARCH GRANT N00014-91-J-1921 R&T Code 413t006 Dr. Kenneth Wynne Technical Report No. 4 Intramolecular Charge Transfer Complexes as Fluorescence Probes for UV and Visible Light Induced Acrylate Polymerization

by Jiancheng Song, Afranio Torres-Filho and D.C. Neckers Prepared for RadTech Center for Photochemical Sciences Bowling Green State University Bowling Green, OH February, 1994 Reproduction in whole or in part is permitted for any purpose of the United States Government This document has been approved for public release and sale; its distribution is unlimited.

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INTRAMOLECULAR CHARGE TRANSFER COMPLEXES AS FLUORESCENCE PROBES FOR UV AND VISIBLE LIGHT INDUCED ACRYLATE POLYMERIZATION J. C. Song, A. Torres-Filho and D. C. Neckers Center for Photochemical Sciences* Bowling Green State University Bowling Green, Ohio 43403, USA

ABSTRACT derivamives ul derivativ~es

of

hg trdiaminonshthieronpexesu(ITC)ofa 1 -diaminonaphthalene-5-sulfonamide

AMIDE), such as I -dimethviamino(DANSYL naphthalene-5-sulfonyl-n-butylamide (1,5-DASB) and 2dimethylaminonaphthalene-5-sulfonyl-n-butylamide (2,5-DASB), were used as fluorescence probes for cure This monitoring of photocurable polymer systems. technique is based upon the difference in fluorescence intensity from the parallel and perpendicular conformations of the excited state of the complex, and is based on the dependence of the relative population of each conformation on the microviscosity of the system. As the curing reaction proceeds. the steady state fluorescence emission spectra of the probes were all found to exhibit hypsochromic spectral shifts due to the increase in matrix microniscosity. A linear correlation between the fluorescence intensity ratios (R=lpar./lper.) and the extent of polymerization, measured by transmission FTIR spectrometry, was obtained for different types of acrylated polymers, cured with UV or visible (VIS) initiators,

INTMODUCIION

initiation, reaction conditions and the rate and degree of polymerization. Thus, a thorough understanding of the photocuring process is essential to property control and process optimization of the final products. Several methods have been developed to study the kinetics of photopolymerization. Among them are photodifferential scanning calorimetry (PDSC), 2 "5 laser interferometry, 6 photoacoustic spectroscopy 7, and UV and FrIR spectroscopy. 8 -10 PDSC is basically like a modified DSC, with a Hg lamp source mounted next to the sample. It has been extensively used to study the kinetics of photopolymerization and the degree of cure can also be measured. However, PDSC is not useful for real time cure monitoring and non-destructive analysis of photopolymers. FMIR is widely used to determine the degree of photopolymerization by monitoring the concentration of residual monomers present in various polymer systems. A real time infra-red (RTIR) technique developed by Decker et al.8, 9 allows one to monitor the polymerization process continuously and rapidly in real time. Both the rate and degree of polymerization can be measured. However, IR techniques can only be used to analyze thin films (less than 20 gm thick). Besides that, transmission IR techniques cannot be used to monitor the degree of polymerization of coatings on opaque substratmes. Also, they are not useful for

Photocurable polymers are used as coatings for various substrates, such as metals, glass, plastics, wood, floors and paper.1 Highly crosslinked polymer networks can be rapidly formed via photopolymerization of

monitonng polymerization of bulk materials, such as parts made by stereolithography, which is a technique designed to form three dimensional objects by the assembly of a series 11 of photopolymers layers.

multifunctional monomers, especially acrylics. The ultimate properties of these polymeric networks depend on various factors, such as monomer structure, forms of

Fluorescence spectroscopy has gained considerable

"Contribution# ISO from the Center for Photochemical Sciences

interest due to its high sensitivity, selectivity, and nondestructive characteristics. Remote sensing is readily

achieved by using fiber-optic cables to transmit optical signals to and from a polymerization system in real time.

* Recently, Neckers et aL.12-15 reported the successful use of fluorescence probes to monitor the photo-cunng process of

the Sartomer Co.). and dipentaervthrvtoi monohvdroxv

polyolacrvlate monomers. Two types of fluorescence probes

Chemucal Co.), on a weight percentage ratio of 20:40:.40,

were found to be particularly useful. The first is based on

respectively.

"excimer" (i.e. excited dimner) forming molecules, such as

18 was utilized as fluorone. DIBF, developed in our group,

pyrene or its derivatives. 16 The ratio of the emission

VIS initiator at a concentration of about Ix 10-3 M. An

intensities of the monomer and excimer was round to

amune co-initiator N-phenyl glycine, NPG (Aldrich) was

correlate well with the degree ot cure. The second is based on intramolecular charge transfer molecules, such as 4(N'N-dimethyl-anuno)benzonitrile (DMABN), and dansyl amide and its derivatives. DMABN exhibits dual fluorescence corresponding to two different singlet excited states called twisted intramolecular charge transfer (TICT) states.I 7 The short wavelength (b) band is due to a

pentaacrvlate. DPHPA (Sartomer 399. from the Sartomer A xanthene dye, 5,7-diiodo-3-butoxy-6

2 also used at a concentration of about 5x10- M. The coinitiator in this case acts as an electron donor to the excited state of the dye, before the generation of free radicals.18

For commercial photocurable resins, two types of acrylated polymer coating formulations (Formulation I and II, both with LV initiators) were investigated.

while the coplanar tnarallel) excited state conformation roma prpediclarAs

fluorescence probes either 1.5-DASB or 2,5-DASB

long waveiength (a') band ongnates from a perpendicular

were used at a concentration of about 5x10 -4 M in the

conformation. The coplanar conformation is more stable in the ground state because of higher delocalization of the it

photopolvmerzable resins.

orbitals. Therefore, the coplanar excited state will be

2.

.engh (*) andoriinaes longwavj

directly populated from the ground state. However, the perpendicular excited state is energetically more stable due to the charge separation. This implies that the molecules are first excited to b" and then "cross" to a', making the

Cure Procedure

An Ar* laser (Qmnichrome. model 543-200 MGS) was used for visible light initiated polymerization of

multifunctional acrylate monomers. To compare samples

population of the perpendicular conformation strongly dependent upon the microenvironment of the TICT molecule.

with different degrees of cure the laser power varied from 30 to 80 mW. To photocure the commercial acrylate

In fact, fluorescence spectra of TICT probes were found to be very sensitive to changes in the microviscosity of the

-coatings a medium pressure Hg-arc lamp was utilized. The degree of polymerization for different samples was varied

medium. In particular. the ratio of the emission intensities

by changing the exposure time to the UV lamp.

at two different emission wavelengths can be correlated

A few drops of the monomer solution being studied

linearly with the degree of cure for various polvolacrylate

were first squeezed between two NaCl plates with a 15.0

lgm thick teflon spacers at the edges to control the final

mom Sytel. In this work we present our recent efforts to develop intramolecular charge transfer (ICT) fluorescence probes for

thickness. The laser beam (2.0 mm diameter) was directed to the sample and scanned at a speed of 13.0 mm/sec using

various photopolymerizable coatings. The ICT probes studied were 1-dimethylaminonaphthalene-3-sulfonyi-n-

computer controlled reflecting mirrors and data files specifically designed for stereolithography purposes.

2-

Steady state fluorescence emission spectra of samples

butylamide

(1,5-DASB),

and

dimethylaninonaphthalene-5-sulfonyl-n-butvlamide

cured for different period of time were obtained via a Spex

(2,5-DASB).

Fluorolog-2 spectrophotometer. Remote measurement was carried using a bifurcated fiber-optic cable attached to the excitation and emission monochromators, respectively. Infra-red measurement of the extent of double bond

1. Materials A mixture of multifunctonal acrvyates was made bv mixing polyethylene glycol-400 diacrylate. PEGA-400

conversion was accomplished using a Mattson Galaxy Series 6020 FT-IR spectrometer with a spectral resolution of 2 cm-

Inc.),

1. The extent of C=C conversion (a) was obtained by using the following equaton.

(Monomer-Polymer

and

Daiac

Labs.

trimethylolpropane triacrvlate, TMPTA ( Saret 331, from

swamn arm an Arl laser as the irradiation source. The data

(1

a wi - A.ryWt.Aref(0)/Aacry(O)/Aref(t)

displayed are the percentages of CC double bond where Aacry(t) and Aacry(O) are the absorbances at 810

converted to single bonds by the photopolvmerization

cm-1 due to the acriytate double bond, after curing times t and zero, respectively. Aref(t) and Aref(O) are the

process. as well as the laser power used for the different samples, and the ratios, R, between the fluorescence

reference peak absorbances at 2945 cm"1 due to the CH

intensites at 430 and 500 nm, respectively, corresponding to

groups, after curing times t and zero, respectively.

the emissions from the two conformations of excited state of

This

reference peak was used as an internal standard to calibrate

2,5-DASB.

any thickness fluctuation during the cunng process. Table 1. Results of IR and fluorescence analyses of the djeree of polyn-nzation of a multifunctional acrAlate monomer solution usine 25-DASB as a E A Urobe. (Initiator = DIBF: Coinitiator w NPC: Irradiation source = Ar1ilau)

1. Comparison of Fluorescence Spectra of 1-5-DASB and 2,5-DASH Probes

Pi (MMW

%C-C

R (1430/IS0)

30

16.7

1.20

Both 1,5-DASB and 2,5-DASB are DANSYL amide derivatives. Previous work by Neckers, et al. showed that 2.5-DASB

was useful as a fluorescent

photopolymerization of polyolacrylates.

13

probe for

5022.7

1.26

Although 1,.5-

DASB differs just slightly from 2,5-DASB in the position of dimethyl amino group on the naphthalene nng, their

80

1.40

36.2 --------

fluorescence spectra differ significantly as shown in Figure

When the R values were plotted against the extent of CwC

1. The excitation peak positions of 1,5-DASB and 2,5-

conversion (Figure 2) a very good linear correlation was

DASB in a dilute THF solution were found to be at 342 rn

obtained (r-0.9999) with the ordinate intercep (0.0 %

and 374 nrm, respectively. This is consistent with their UV

CC) corresponding to the ratio of the fluorescence

as 1,5-DASB absorbs at a

intensities emitted by the excited states of the probe in

shorter wavelength (330 nrm) than 2.5-DASB (374 nm). In

solution, before the photopolymerization process. This first

contrast the fluorescence emission peak position of 1,5-

test clearly indicated that the technique and probe were

DASB is at much longer wavelength (494 nm) than that of

well suited to be used in these highly crosslinked

2,5-DASB (450 nm). Therefore, the Stokes shift of 1,5DASB (152 nm) is twice as large as 2.5-DASB (76 nm). A

photopolymer systems.

larger Stokes shift is a significant advantage for

3.

absorption peak positions,

monitoring photopolymerization since spectral blue shifts

1,5.-DASB as a Fluorescence Probe of the Degree of Photopolymerization.

in the TICr probe fluorescence emission are often observed as the polymerization proceeds due to the increase in

(i). Photocurable resins with UV initiators

The intrinsic fluorescence from the photopolymer (if any) would have less interference to the probe fluorescence with a large Stokes shift value,

To investigate the applicability of 1,5-DASH as a fluorescence probe for photo-polymenzation, we doped 1,5-

matrix viscosity.

DASB at a small concentration into various commercial 2. 2,S-DASB as a Fluorescence Probe for the Degree of Photopolymerization of Multifunctional Acrylate I-MonoMeUs Using Visible Photoinitiators and Visible Lght Emitting Curing S Table 1 presents results corresponding to the

acrylate coating resins. The probe fluorescence enission spectra in the photopolymer systems studied were monitored as a function of irradiation time.

As the curing

reaction proceeds, fluorescence emission peak position exhibits blue shifts due to increases in the medium viscosity which makes it more difficult to form the long wavel ngth Figure 3 compares the

photopolymerization of the mixture of multifunctional

charge transfer excited state.

acrylate monomers using DIBF and NPG as the initiator

fluorescence emission spectra of 1,5-DASB probe in a

commercial photocurable acrylate (Formulation i) betore

degree of polymerization and an incomplele photo-

and after a full curing (30 seconds under a medium pressure

bleaching of the photo-initiators. It is interesting to know

Hg lamp). A total spectral blue shift of about 20 nm was

the effects of visible initiators on the fluorescence spectra

Similar to what described previously,15 I'e

of the 1,5-DASB probe. Thus we investigated the

monitored changes in fluorescence intensity ratios as a

applicability of the new fluorescence probe to monitor the

function of irradiation time as illustrated in Figure 4.

It

photo-cunng reaction of multifunctional acrylates initiated

can be seen that the general trends in the three cases (R

=

via

obtained.

L470/1560,1470/1550 and 1470/1540) are about the same. extent of double bond linear correlations between the

laboratories.

in our

dye initiators developed

the visible 18

The monomer solution contains

TMPTA/DPHPA (1:1 molar), polyethylene

•ycol, DIBF

(initiator) and NPG (coinitiator). The fluorescence emission

conversion of Formulation I vs. the fluorescence intensity

intensity ratio (1470/15W0) of 1,5-DASB probe was found to

ratio of the fluorescence probe were obtained for all the three cases examined (R = 1470/1560, 1470/1550 and

increase from 1.1 (0% conversion) to about 2.2 (32%

1470/1540), as shown in Figure 5. However, the intensity ratio of 1470/1~560 was found to give the best linear

pressure Hg lamp. A linear correlation plot of the probe fluorescence intensity ratio vs. the extent of C=C conversion

correlation. Therefore, we chose to use 1470/1560 as the probe parameter to monitor the photopolymerization of

was obtained as shown in Figure 8. The linear relationship between R (1470/1560) and the extent of conversion (W) is

multifunctional acrylates.

given in Equation 4.

Figure 6 compares the changes in the extent of double

conversion) after 60 seconds of irradiation under a medium

(4)

R = 1.1167 + 3.3908at

bond conversion as a function of irradiation time for the two formulations studied. We can see that the initial cure rate of Formulation I is significantly slower than that of seconds was Formulation I1. An induction time of about two observed for Formulation I while that of Formulation 11

SUMMARY

Intramolecular charge transfer complexes (ICTC)

was less than one tenth of a second. However, the overall

fluorescence probes such as 1-dimethylarmno-naphthalene-

extent of cure of the Formulation 1(-95 %) was much higher

5-sulfonyl-n-butylamide

than that of Formulation 11(-60 %). This is probably due

dimethylaminonaphthalene-5-sulfonyl-n-butyl-amide

to the rapid increase in crosslinking density due to a fast

(2,5-DASB) are useful for cure monitoring of photocurable

cure rate in Formulation II that makes the diffusion of

polymer systems with UV or visible initiators.

radicals difficult,

resulting in a lower extent of

(1,5-DASB)

and

2-

As the

curing proceeds, the steady state fluorescence emission

polymerization.

spectra of the-'piobes were all found to exhibit

Figure 7 gives the linear correlation plots of 1470/1560 vs. the extent of double bond conversion for the two commercial formulations, showing similar slopes. The

hypsochromic spectr"l shifts due to the increases in matrix Linear correlations between the microviscosity.

mathematical expressions given in Equations 2 and 3 can be used to correlate the fluorescence intensity ratio (1470/1560)

With a fiber-optic

fluorescence

intensity

ratios

and

the extent

of

with the extent of C=C conversion (Wi)for Formulations I

polymerization wese obtained. fluorimeter. this fluorsence cure sensing technique based on ITCT fluorescenc\ probes can be readily applied to

and II, respectively.

photopolymerized filn"s, coatings and bulk materials.

R = 1.6284 + 2.3197c

(2)

R = 1.3804+2.6314ct

(3)

(iLi. Multifunctional acrylates with visible initiators

Acknowledgements. [1irtial financial support by the Navy Office of Research P 00014-91-J-1921 and the National Science Foundatiorn, (DMR 93109) is gratefully acknowledged. We als, wish to thank Dr. Hansen Shou for his generous help an4 Mr. Tom Marino and Mr. Dustin Martin of the SGL, Inc.1for their helpful discussions.

/

Visible initiators such as fluorone derivatives exhibit fluorescence emission in photopolvmers with low

/ /

lax 2 ex

. 07

I. Hoy, C E. inhdiaion Curing of Po

Umm

iC

Materials;Hoyle, C. E., Kinstle, J. F., eds.; Society, Washington, DC, 1990, Chapter 1, pl.

,

I

I

ACS Symposium Series 417; American Chemical

2. Tyson, GR.; Shutz, AX 1. Polym.

2m

/

,

\

V

&j,]

/

z

3. Cook, W.D. Polymer 1"Z233,2152. 4. Wang, D.; Canera, L.; Aradie, M.J.M. Eur. Palym. 1.193,29, 1379. 5.

40 Wavelength (rnm) Fig. 1. Fluorescence excitation (EX) and emission (EM)

Hoyle, C.E.; Hensel, R.D.; Grubb, M. B. I. Polym. Sci., Po/ymu. O . Ed. 1984,22, 1865.

6. Moore, J.E. UV Curing: Science and Technology,

spectra of 1,5-DASB (1) and 2,5-DASB (2), respectively, in THF.

Technology Marketing Corporation, 1978.

7. Small, R.D.; Ors, J.A.; Royse, B.S. ACS Sympos 8.

-

Sa eiu # 242.1984, 325.

1.45

Decker, C.; Moussa, K. ACS Symposium Series

1.4

y

1..0276X R. 0.996

1.0277

# 417,1990,439. 9. Decker, C. J. Polym. Sci. Part A: Polym. Chem.

11.35

1992,30,913.

1.3

10. Allen, N.S.; Hardy, S.J.; Jacobine, A.F.; Glaser, D.M.; Yang B.; Wolf, D. Eur. Polym. 1.1990, 26, 1041.

1.25

11. Neckers, D.C. Chlemtech 1990,20, 615.

1.2

12. Paczkowsld, J.; Neckers, D. C. Macromolecules 1992,25,

1.15

.

.

0.1.50s

0.2

13. Zhang, X.; Kotchetov, 1,N.; Paczkowski, J.; Neckers, D. C. 1. Imaging Sc. Tech. 1992, 36, 322. 14. Kotchetov, L. N.; Neckers, D. C 1. Imaging Sci. Tech. 1993, 37,156.

.

.

0.25

.

.

0.3

.

0.35

0.4

EXTENT OF C&C CONVERSION Fig. 2. Correlation plot of the fluorescence intensity ratio (1430/1500) vs. the extent of C=C conversion of a multifunctional acrylate monomer system with a visible initiator (DIBF) and cured by an Ar+ laser.

15. Paczkowski, J.; Neckers, D. C. 1. Polym. Sd., Polym. B

1.J

Chem. Ed. 1993,31, 841.

A

16. Valdes-Aguilera, 0.; Pathak, C.P.; Neckers, D.C.

Macromolecules 199, 23, 689.

'

/

17. Liplnskl, J.; Chojnacki, H.; Rotkiewicz, R.; Grabowski,a/ Z.R. Chem. Phys. Lett. 1980, 70, 449.

9A-4

18. Neckers, D. C.; Tanabe, T., unpublished. 19. We are applying for a patent on this work.

I

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.(B).

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Cure Time: (A). 0 s 15 s

,

456

--

sis

Wavelength (nm) Fig. 3. Fluorescence emission spectra of 1,5-DASB in a commercial photocurable resin (Formulation I) as a function of irradiation time using a medium pressure Hg lamp. (Excitation at 380 nm).

-

y

1.6284 + 2.3197x Ra 0.9992

-

3.5 6

3

3.5

-

3

2.5

2-470 = o-

s'o0

-2.5

550 1

-251

Iirulto

-1.5

-

-

C--

;



1470

~. ~O 0

2

4

6

8

10

1,S-DO

I

12

14

16

1.5

0

Time (s) Fig. 4. Changes in fluorescence intensity ratios of 1,5DASB in a commercial photocurable resin (Formulation 1)as a function of irradiation time using a medium pressure Hg lamp. (Excitation

1.3804 + 2.6314X Ro 0.99473

.&

y

3

-o-____

L

0.8 0.4 0.6 0.2 Extent of C-C Conversion

-

at 380 run).

4-

2.5

,---__0

1470.,0

83

I3 l470 5401 40

-

-1 -~~~

-

11

.Formulation

mExit 2

380 nm

1.5

0

______._.

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0

0.2

0.6

0.8

1

10.°8 ,o, 0AF

-+

-

"*-

.

/0.8 I

-"

--

0.6

y = 1.1167 + 3.3908x R= 0.99802

2.2

--

.2 1. ZR

I,

1.6

TMPTAADPHPA/I PEGAIDIBF/NPG. ExFci~te 0 380 Tn Il5DS

-

1. 1.2

____________

---

0.5

C-C conversion for two commercial photocurable resins.

01.2

2-0-

0.4

ratio (R = 4170/1560) of 1,5-DASB vs. the extent of

C

0,

0.3

of C=C Conversion

.

0.6

0.2

Fig. 7. Linear correlation plot of the fluorescence intensity

FIg. 5. Linear correlation plot of the fluorescence 1470/1550 and intensity ratios (R - 4170/15[, 1470/1540) of 1,5-DASB vs. the extent of C--C conversion in Formulation 1. 1__2.4

0.1

•Extent

_,_,_,

0.4

% C=C Conversion

0.2

I.

,,Probe Excite0380 nmn

Z

Formul1ation 1 Formulation II

7

X

0

-

02

0

4 6 a Irradiation Time (s)

10

12

Fig. 6. Changes in the C=C conversion of two commercial photocurable resins (Formulations I and 11)as a function of irradiation time using a medium pressure Hg lamp.

0.1

0.2

0.3

0.z

Extent of C=C Conversion Fig. 8. Linear correlation plot of the fluorescence intensity ratio (R - 1470/1560) of 1,5-DASB vs. the extent of C=C conversion of a multifunctional acrylate system with a visible initiator (DIBF) and an amine coinitF~tor (NPG).