Polymers in Solar Energy Utilization - American Chemical Society

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19 Photochemical Stability of UV-Screening Transparent Acrylic Copolymers of 2-(2-Hydroxy5-vinylphenyl)-2H-benzotriazole A. GUPTA—California Institute of Technology, Jet Propulsion Laboratory, Materials Research and Biotechnical Section, Pasadena,CA91109 G. W. SCOTT—University of California, Riverside, Department of Chemistry, Riverside,CA92521 D. KLIGER—University of California, Santa Cruz, Division of Natural Science, Santa Cruz,CA95064 O. VOGL—University of Massachusetts, Polymer Science and Engineering Department, Amherst,MA01003

The mechanism of photodegradation of certain hydroxyphenyl benzotriazole based ultraviolet absorbers has been investigated and a new polymerizable ultraviolet absorber in this group has been synthesized. The photoreactivity is entirely confined at the surface of polymethylmethacrylate films containing the ultraviolet absorbers as pendant groups. A mechanism involving sensitized photooxidation has been proposed to interpret the data. Polymerizable ultraviolet absorbers are needed whenever a thin film of ultraviolet absorbing layer is required to retain the permanence of its absorption characteristics over a service life of five years or more. Vinyl substituted ultraviolet absorbers, e.g. vinyl derivatives of 2-hydroxybenzophenone were initially synthesized at DuPont - . The synthesis was modified and the yield was significantly improved by Vogl, et al. by using an improved dehydrobromination procedure3-5 More recently, the superior screening capacity of hydroxyphenyl benzotriazoles led us toward the development and testing of copolymers of 2(2hydroxy-5-vinylphenyl) 2H-benzotriazole (I) with methylmethacrylate and styrene. Synthesis of I and characterization of its polymerization reactivity was recently reported by Vogl, who also 1

2

#

0097-6156/83/0220-0293$06.00/0 © 1983 American Chemical Society Gebelein et al.; Polymers in Solar Energy Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

294

POLYMERS IN SOLAR ENERGY UTILIZATION

d e m o n s t r a t e d t h a t I can be g r a f t e d onto s a t u r a t e d a l i p h a t i c C-H bonds i n p o l y p r o p y l e n e , and e t h y l e n e - c o - ( v i n y l a c e t a t e ) ^ . The c o p o l y m e r o f I w i t h MMA was s e l e c t e d f o r a g i n g t e s t s i n o r d e r t o a s s e s s i t s a p p l i c a b i l i t y as a t r a n s p a r e n t f r o n t c o v e r on p h o t o v o l t a i c modules. S y n t h e s i s of t h e c o p o l y m e r was c a r r i e d out as d e s c r i b e d i n Scheme 1. D e t a i l s o f t h e s y n t h e s i s have been p u b l i s h e d and w i l l not be d i s c u s s e d f u r t h e r . The c o p o l y m e r was p u r i f i e d by e x t r a c t i o n w i t h n o n s o l v e n t s and r e p r e c i p i t a t i o n from m e t h y l e n e c h l o r i d e . I t was s o l u t i o n c a s t t o form t h i n f i l m s ( 5 - 8 χ 10~3 cm) which were d r i e d i n a vacuum oven o v e r n i g h t b e f o r e i r r a d i a t i o n commenced. These c o p o l y m e r r o t a t i n g f i l m s were c u t i n t o s t r i p s , mounted on t h e o u t e r s u r f a c e o f a c y l i n d e r and exposed t o p y r e x f i l t e r e d r a d i a t i o n from a medium p r e s s u r e Hg a r c l a m p . Preliminary results a r e shown i n F i g u r e s 1 and 2 . The a b s o r p t i o n s p e c t r a of t h e f i l m s g r a d u a l l y c h a n g e , as i s shown i n F i g u r e l a . However, most o f t h i s change can be a t t r i b u t e d t o changes i n t h e s c a t t e r i n g p r o p e r t i e s of the f i l m s . R e l a t i v e a b s o r b a n c e (S^) d e f i n e d as \/A600> " s u r i n g a b s o r b a n c e s , assuming t h a t no a b s o r p t i o n d e v e l o p e d a t 600 nm, does not change a p p r e c i a b l y as shown i n t h e i n s e t i n F i g u r e lb. A c o p o l y m e r f i l m m a i n t a i n e d i n t h e dark s e r v e d as a c o n t r o l . F o u r i e r t r a n s f o r m IR s p e c t r a were r e c o r d e d on t h e f i l m s as a f u n c t i o n of i r r a d i a t i o n p e r i o d , as shown i n F i g u r e 2. F i g u r e 2b shows ATR-IR s p e c t r a i n d i c a t i n g c o n s i d e r a b l e p h o t o o x i d a t i o n a f t e r 2100 h o u r s of i r r a d i a t i o n . The r a t e o f growth o f h y d r o x y l g r o u p s , w h i c h a r e t h e p r i n c i p a l p r o d u c t s o f p h o t o o x i d a t i o n , i s shown i n F i g u r e 3 . As p h o t o o x i d a t i o n p r o c e e d s , t h e polymer undergoes c r o s s l i n k i n g , u l t i m a t e l y r e s u l t i n g i n t h e f o r m a t i o n o f a gel f r a c tion. The dependence of t h e r a t e o f p h o t o o x i d a t i o n on t h e c o n c e n t r a t i o n o f the chromophores was sought t o be i n v e s t i g a t e d by t e s t i n g t h i n f i l m s o f a b l e n d o f PMMA and t h e c o p o l y m e r , formed by s o l u t i o n c a s t i n g a m i x t u r e of 85 w e i g h t p e r c e n t PMMA (Mn * 4 0 0 , 0 0 0 ) and 15 w e i g h t p e r c e n t of t h e c o p o l y m e r (Mn = 9 0 , 0 0 0 ) . Figure 4 shows t h e FTIR (ATR) a b s o r b a n c e d i f f e r e n c e measured on t h e s e f i l m s aged f o r t y p i c a l p e r i o d s . The r a t e of p h o t o o x i d a t i o n i s e s t i m a t e d t o be reduced by more t h a n an o r d e r of magnitude r e l a t i v e t o t h e pure c o p o l y m e r . These b l e n d s were t h a n examined by ESCA i n o r d e r t o d e t e r m i n e t h e a c t u a l b e n z o t r i a z o l e group c o n c e n t r a t i o n a t t h e surface. F i g u r e 5a shows t h e Ν ( I S ) ESCA peak i n t h e pure c o p o l y mer f i l m s . W h i l e F i g u r e 5b shows t h a t no t r a c e of n i t r o g e n c o u l d be found on t h e s u r f a c e of t h e b l e n d s . The r a t e o f p h o t o o x i d a t i o n a t t h e s u r f a c e m o n o l a y e r was a l s o m o n i t o r e d by t h e s u r f a c e energy analysis. These r e s u l t s a r e g i v e n i n T a b l e I. The c o n t a c t a n g l e measurements were c a r r i e d out u s i n g w a t e r and p o l y p r o p y l e n e g l y c o l and work a n g l e , W was c a l c u l a t e d as f o l l o w s : m

e

a

a

w

a = 1 _ (1 γ

+

cos θ )

where Ύ | _ i s t h e s u r f a c e t e n s i o n of t h e s o l v e n t

i n dyne/cm.

Gebelein et al.; Polymers in Solar Energy Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Then:

19.

GUPTA ET AL.

UV'-Screening Transparent Acrylic Copolymers

CH =CH^

POLYMERIZATION

2

1

-fCH -CH^ 0

OH ^NÎO N

POLY-I

Scheme 1.


295

296


1000 2000 hrs OF EXPOSURE 2.0r

Ί

1

1

1

1

Γ

1.5 2580 hrs/CONTROL

* i.ol

*

* — *

* — * — *

5

800 HRS/CONTROL FOR 15% COPOLYMER

χ

>T-K

85% PMMA BLEND 1700 hrs/CONTROL

0.5

280

Figure

1.

1020 hrs/CONTROL

300

320

340 nm

360

A b s o r b a n c e o f Copolymer I F i l m s Irradiation Period. a) A b s o r b a n c e Data Recorded on b) A b s o r b a n c e Data Recorded on o f Aged F i l m s ; S\ = I^K/^e00

380

400

as a F u n c t i o n Films. Solutions ( text). s e e


of

19.

GUPTA ET AL.

W-Screening Transparent Acrylic Copolymers

297

CONTROL SAMPLE

4000

3000

2000

1200

WAVENUMBERS F i g u r e 2.

800

FT-IR S p e c t r o s c o p i c A n a l y s i s F i l m s of t h e Copolymer I. a) b)

800 700 600

3000

4000 3600 3400

b

3 o f Aged and

Control

T r a n s m i s s i o n FT-IR S p e c t r a , ATR FT-IR D i f f e r e n c e S p e c t r a .


298


200 TIME, hr

Figure

3.

300

IR A b s o r b a n c e I n c r e a s e a t 3580 c m - i a s a F u n c t i o n Time on Copolymer F i l m s from ATR FT-IR S p e c t r a l Data.


of

19.

GUPTA ET AL.

Figure

4.

UV'-Screening Transparent Acrylic Copolymers

299

ATR FT-IR D i f f e r e n c e S p e c t r a f o r F i l m s of B l e n d s of t h e Copolymer ( I ) and PMMA ( 1 5 : 8 5 by W e i g h t ) ; C o n t r o l i s a F i l m o f t h e B l e n d M a i n t a i n e d i n Dark.


300


- —

BINDING ENERGY (eV)

SAMPLE: PURE COPOLYMER (I)

—

BINDING ENERGY (eV)

SAMPLE: BLEND OF I AND PMMA (15:85)

Figure

5.

ESCA

Data

on F i l m s

of t h e Copolymer

(I)

and t h e

Blend.


19.

GUPTA ET AL.

UV-Screening Transparent Acrylic Copolymers

301

d ρ Y s = Y s + YS

d Ys =

when

(TÏ

(W /2)l a

D

=

(TÏ

(Yi

b 1/2 (Yi ).

). 1

(r,

)

b (Yi

k

Here s u b s c r i p t i d e n o t e s

Table

I.

D2

(W /2)k a

where

)l/2

1 1/2

H2O and s u b s c r i p t k d e n o t e s PPG.

C a l c u l a t e d Surface Tension Irradiation Period.

PERIOD OF IRRADIATION (hr)

V a l u e s as a F u n c t i o n

WATER (/ • 72.8 dynes/cm ) L

of

PG-E-200 (η_ 43.5 dynes/cm ) 3

EXPOSED SIDE

DARK SIDE

EXPOSED SIDE

DARK SIDE

67.2

68.2

36.0

35.6

93.5

69.3

69.5

38.9

278

90.4

66.7

70.3

35.5

419.5

89.2

72.2

69.4

35.0

The o b s e r v e d p h o t o o x i d a t i v e c r o s s l i n k i n g p r o c e s s was j u d g e d t o be a c o n s e q u e n c e of i n t r o d u c t i o n o f t e r t i a r y hydrogen atoms on c o p o l y m e r ! z a t i o n of v i n y l d e r i v a t i o n s of u l t r a v i o l e t a b s o r b i n g chromophores. Hence, a propenyl d e r i v a t i v e of the 2h y d r o x y l - p h e n y l b e n z o t r i a z o l e n u c l e u s was s y n t h e s i z e d , as shown i n Scheme 2. D e t a i l s o f t h e s y n t h e s i s o f t h i s compound w i l l be r e ported, subsequently. The a b s o r p t i o n s p e c t r u m o f t h e p r o p e n y l d e rivative*. P h o t o d e g r a d a t i o n r a t e measurements on t h i s m a t e r i a l are in progress. Copolymer w i t h methyl

methacrylate

i s shown i n F i g u r e 6.


302


OH

ce

OR

C=0 I CH

OH

CH.-C-OH 3 ι CH

3

CH - C = C H 3 ^ ?

3

la, b R,=H,Ac

I la,b R = H,Ac R = H, C H 2

Scheme

11 la,b R = H, C H ?

3

3

2

4.00

3.00 LU

1

_

o

2

g

0

3

2.001-

1.00

o 300 350 WAVELENGTH (nm)

Figure

6.

A b s o r p t i o n Spectrum o f 2 [ ( 2 - h y d r o x y 5-propenyl) p h e n y l ] 2 H - b e n z o t r i a z o l e i n Methylene C h l o r i d e at 30°C.


19.

GUPTA ET AL.

JJV-Screening Transparent Acrylic Copolymers

303

The a b s o r p t i o n s p e c t r a of a model compound has been r e p o r t e d i n s e v e r a l d i f f e r e n t s o l v e n t s a t room t e m p e r a t u r e s and a l s o as a f u n c t i o n of t e m p e r a t u r e down t o U K . These s p e c t r o s c o p i c m e a s u r e ments i n d i c a t e t h a t t h e r e i s an e q u i l i b r i u m between two or more c o n f o r m e r s i n t h e ground s t a t e . Two c o n f o r m e r s a b s o r b i n g a t 302 nm and 340 nm may be s t a b i l i z e d by a c o m b i n a t i o n o f i n t r a - and i n t e r m o l e c u l a r hydrogen b o n d i n g , as shown i n F i g u r e 7 a . Preliminary C - 1 3 nmr s p e c t r a l d a t a i n d i c a t e t h a t t h e degree of a r o m a t i c i t y i s q u i t e s o l v e n t dependent. The complex d i s t r i b u t i o n of chromophore m o l e c u l e s i n t h e ground s t a t e make i t d i f f i c u l t t o propose a s t r a i g h t f o r w a r d i n t e r p r e t a t i o n o f e m i s s i o n and e x c i t e d s t a t e decay d a t a o b t a i n e d from ground s t a t e a b s o r b a n c e r e c o v e r y r a t e and f l u o r e s c e n c e decay r a t e m e a s u r e m e n t s . Some o f t h e s e measurements w i l l be r e p o r t e d . The mechanism o f p h o t o d e g r a d a t i o n o f t h e c o p o l y m e r i s p r e sumed t o i n v o l v e an e l e c t r o n i c energy t r a n s f e r p r o c e s s from t h e b e n z o t r i a z o l e chromophore t o a p h o t o r e a c t i v e group on t h e polymer b a c k b o n e , e . g . , h y d r o p e r o x y groups formed on o x i d a t i o n o f t h e t e r t i a r y hydrogen atoms as shown i n Scheme 3 . T h i s mechanism i s n e c e s s a r i l y confined to the s u r f a c e , s i n c e i t r e q u i r e s p e n e t r a t i o n o f oxygen and a c t i n i c r a d i a t i o n ( 3 0 0 - 4 0 0 nm). H y d r o x y l groups and s i m u l t a n e o u s c r o s s l i n k i n g and c h a i n s c i s s i o n a r e t h e p r i n c i p a l p r o d u c t s of p h o t o o x i d a t i o n . P h o t o o x i d a t i o n causes a decrease i n s u r f a c e energy o f t h e f i l t e r s , a somewhat u n e x p e c t e d r e s u l t . The decrease i n s u r f a c e energy should decrease the s o i l i n g c h a r a c t e r of f r o n t covers of p h o t o v o l t a i c modules. The r a t e of energy t r a n s f e r from t h e b e n z o t r i a z o l e c h r o m o phore t o t h e h y d r o p e r o x y groups i s c o n t r o l l e d by t h e l i f e t i m e of t h e e x c i t e d s t a t e , as l o n g as i t i s h i g h e r t h a n 1.5 ev a p p r o x i mately. D e t a i l s o f decay mechanisms of t h e e x c i t e d s t a t e s w i l l be published l a t e r . Here we w i l l note t h a t t h e p r i n c i p a l f e a t u r e of t h e d e a c t i v a t i o n mechanism i n v o l v e s an i n t r a m o l e c u l a r p r o t o n t r a n s f e r p r o c e s s w h i c h may o c c u r b e f o r e v i b r a t i o n a l e q u i l i b r a t i o n of the v e r t i c a l e x c i t e d s t a t e i s completed. The f l u o r e s c e n c e has a b l u e ( \ x = 405 nm) and a red ( \ x = 585 nm) component, w i t h t h e b l u e component o n l y b e i n g p r e s e n t a t room t e m p e r a t u r e i n d i l u t e s o l u t i o n , and a t low t e m p e r a t u r e s i n p o l a r m a t r i c e s . The red component i s p r e s e n t i n e m i s s i o n a t room t e m p e r a t u r e from p o l y c r y s t a l l i n e powders and a t low t e m p e r a t u r e s i n h y d r o c a r b o n m a t r i c e s . I t may be p o s t u l a t e d t h a t t h e b l u e component a r i s e s from a v i b r a t i o n a l ^ e x c i t e d 0 - p r o t o n a t e d s p e c i e s , w h i l e t h e red component a r i s e s from a p r o t o n t r a n s f e r r e d z w i t t e r i o n i c e x c i t e d s t a t e . P h o s p h o r e s c e n c e i s d e t e c t e d from t h e model compound ( I I ) i n p o l a r m a t r i c e s a t 77K. T a b l e II g i v e s some e x c i t e d s t a t e l i f e t i m e d a t a on t h e c o p o l y m e r and model s y s t e m s . P h o t o o x i d a t i o n of t h e c o p o l y m e r may be i n h i b i t e d e i t h e r by r e d u c i n g a c c e s s o f oxygen o r by r e d u c i n g t h e number o f t e r t i a r y h y d r o g e n atoms on t h e main c h a i n . In t h e b l e n d o f t h e c o p o l y m e r w i t h PMMA, t h e pendant chromophores a r e e x c l u d e d from t h e s u r f a c e , as shown by ESCA m e a s u r e m e n t s . F o r m a t i o n o f e x c i t e d s t a t e s of ma

ma


304


Figure

7.

P r o p o s e d Mechanism of E l e c t r o n i c Energy D e a c t i v a t i o n i n the Orthohydroxybenzotriazole Nuclei.

CH - f CH, — 2

00H

3

C -h- C H - t - C — η 2 0

C00CH

C H -h2 m 0

Φ

3

00H C —

η : m>10

00H CH —

— -

2

Φ

—

C —

CH

2

—

Φ

ι OH — CHAIN SCISSION A N D

CROSSLINKING

C — I

0· CH — 2

é + 9

+H0 -

C L

Scheme

2

C —

CH — 2

+ OH

CH u n

2

3


19.

GUPTA ET AL.

W-Screening Transparent Acrylic Copolymers

305

b e n z o t r i a z o l e groups and t h e e l e c t r o n i c energy t r a n s f e r p r o c e s s t h e r e f o r e takes p l a c e i n s i d e t h e bulk of the f i l m . The c o n s e q u e n t d e c r e a s e i n p h o t o o x i d a t i o n r a t e t e n d s t o s u p p o r t t h e energy t r a n s -

Table

II.

Fluorescence Derivatives.

Lifetimes

SOLVENT

MOLECULE

of the Orthohydroxybenzotriazole

TEMPERATURE

WAVELENGTH

LIFETIME

TOTAL FIT

14 ± 3 ps

30°C

TOTAL FIT

52 ± 4 ps

METHYLENE CHLORIDE

30°C

TOTAL FIT

19 ± 5 ps

COPOLYMER

- D0-

30°C

TOTAL FIT

1 5 ± 4 ps

R =CH

3

EPA

77K

390 nm

2.4 ± 1.2 ns

R =CH

3

2 METHYL PENTANE

77K

420 nm

2.2 ± 1.0 ns

- D0-

77K

600 nm

1.4 + 0.7 ns

R =CH

3

METHYLCYCLOHEXANE

30°C

ETHANOL

R = CH~

f e r mechanism and r u l e out d i r e c t e x c i t a t i o n o f h y d r o p e r o x y groups as an i n i t i a t i o n s t e p . In c o n c l u s i o n , we have i n v e s t i g a t e d t h e mechanism o f s e n s i t i z e d p h o t o o x i d a t i o n of u l t r a v i o l e t absorbing c l e a r a c r y l i c f i l m s c o n t a i n i n g pendant u l t r a v i o l e t a b s o r b e r g r o u p s . The main c o n c l u s i o n s of the m e c h a n i s t i c study i n d i c a t e d t h a t propenyl derivatives o f u l t r a v i o l e t c h r o m o p h o r e s , c o p o l y m e r ! z a t i o n o f w h i c h would l e a d t o development o f methyl groups on t h e backbone would be more a p p r o p r i a t e candidates f o r outdoor a p p l i c a t i o n s r e q u i r i n g long s e r vice l i f e . S y n t h e s i s o f t h e f i r s t such comonomer has been r e ported here.

Ackncwle dgment s The r e s e a r c h d e s c r i b e d i n t h i s paper was p e r f o r m e d by t h e J e t P r o p u l s i o n L a b o r a t o r y , C a l i f o r n i a I n s t i t u t e o f T e c h n o l o g y and was s p o n s o r e d by t h e F l a t - P l a t e S o l a r A r r a y P r o j e c t , Department o f Energy.

Literature Cited 1. 2.

D. Bailey and O. Vogl, J. Macromol. Sci. Reviews, C14(2), 267 (1976). S. Tocker, Makromol. Chem. 101, 23 (1967).


306


3. O. Vogl and S. Yoshida, Rev. Roum. de Chimie, 25(7), 1123 (1980). 4. S. Yoshida and O. Vogl, Polymer Preprints, ACS Divison of Polymer Chemistry, 21(1), 203 (1980). 5. W. Pradellok, O. Vogl and A. Gupta, J. Polym. Sci, Polym. Chem. Ed., 19, 3307 (1981). RECEIVED April 19, 1983.


Polymers in Solar Energy Utilization - American Chemical Society

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