Durability of Macromolecular Materials - ACS Publications - American

3 Polymer Film Photodegradation: Optical Density Effects

Downloaded by UNIV OF ARIZONA on January 4, 2013 | http://pubs.acs.org Publication Date: April 2, 1979 | doi: 10.1021/bk-1979-0095.ch003

ALLAN R. SHULTZ General Electric Corporate Research and Development, P. O. Box 8, Schenectady, NY 12301

Aging of polymer films involves a complex variety of contributing chemical and physical factors. This is especially true of weathering in which sun, rain, air, temperature variation, and various mechanical conditioning effects play individual and interactive roles. Chemical reactions initiated by visible and near-ultraviolet light are major sources of polymer film property alterations. The light absorption characteristics of a given film are the initial determining factors in photochemical change of its properties. Homogeneous, multi-phase, or layered structures in polymer films will influence the amount and location of light absorption. An obvious division of polymer films into transparent (to visible light), translucent, and opaque categories is convenient in considering photodegradative response. Opaque films may derive their opacity from reflective pigments, absorptive pigments, absorptive dyes, or to combinations of such additives. In some cases opacity, or translucency, may be due to the morphology or chemical structure of a polymer without additives. Thus spherulitic crystallinity or strong chromophores in a polymer can yield obscurance, respectively, by light scattering and reflection, or by absorption. Transparent films may be colorless, having very small extinction coefficients for all light wavelengths in the visible region, or colored, having some differential extinction in the visible region. It is the intent of the present work to examine the role of light intensity distribution within polymer films upon the nature of their photodegradative aging. Although the term "films" will be used exclusively throughout the presentation, dimensionally thick films, customarily called "sheets", are the structures of most interest. Emphasis will be placed on polymer chain scission by the absorbed light. Ultimately the failure criteria employed will determine whether failure is related only to the average cumulative energy absorption by a film or is also related to the spatial distribution of the energy absorption within the film. The present paper will first briefly review the development 0-8412-0485-3/79/47-095-029$05.00/0 © 1979 American Chemical Society In Durability of Macromolecular Materials; Eby, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

30

DURABILITY OF MACROMOLECULAR MATERIALS

of relationships permitting more quantitative analyses of photodegradation of polymer films having appreciable optical density. Next, the d i f f e r i n g l i g h t intensity distributions and resultant photodegradation distributions w i l l be indicated for polymer films of equal thickness: f i l m A containing a UV absorber, f i l m having the UV absorber contained i n a topcoat layer, and f i l m £ having no UV absorber. To make obvious the salient features of optical density effects, greatly simplifying assumptions w i l l be made and stated. F i n a l l y , the polymer f i l m s qualifications under various f a i l u r e c r i t e r i a w i l l be inferred. 1


Scission and Crosslink Enumeration A very modest amount of publication has appeared treating optical density effects upon polymer photodegradation. Mathema t i c a l relations have been derived which permit evaluation of quantum yields for polymer scission and crosslinking by normallyincident, monochromatic l i g h t i r r a d i a t i o n of polymer films (1-4). Number- and weight-average molecular weight change, solution v i s c o s i t y change, and/or gel content build or decrease were the physical properties considered. These derivations were based on the assumption that the i n i t i a l polymer molecular weight d i s t r i bution was s a t i s f a c t o r i l y approximated by a "most probable" d i s t r i b u t i o n . E x p l i c i t molecular weight d i s t r i b u t i o n functions for such photodegraded films were subsequently published (5). Molecular weight distributions and average molecular weights of an i n i t i a l l y monodisperse polymer after pure photoscission of chains i n a f i l m have also been computed (6). Optical density effects on photodegradation i n films have been considered i n studies on cellulose (2), poly(methyl isopropenyl ketone)(7), poly(ethylene terephthaiate)(8), and poly(methyl methacrylate)X3,9). More recently an attempt was made to analyze the photosensitized gelation of poly(vinyl butyral) with correc tion for optical density effects (10). Photodegradation of polymer films by polychromatic l i g h t introduces further complexity into mathematical treatments. This problem has been approached experimentally by using spectrallydispersed l i g h t from a continuum l i g h t source and determining an "activation spectrum" for chromophore production (11,12,13). In this case a number-averaged property was treated. The "most active wavelength region" for net chain scission of poly(ethylene terephthaiate) by polychromatic l i g h t i r r a d i a t i o n was determined by exposing a stack of thin films and analyzing i n t r i n s i c viscos i t y (a weight-averaged property) as a function of stack position (8). Both of these experimental approaches can y i e l d insight into the somewhat complex interplay of f i l m optical density, incident l i g h t intensity, and quantum yields as functions of wavelength. For better analyses of photodegradation of polymer films i t w i l l be necessary to derive expressions handling the common occurrence i n which the extinction coefficient for the active

In Durability of Macromolecular Materials; Eby, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

3. SHULTZ

Polymer

Film

Photodegradation

31


wavelength of l i g h t changes with the extent of reaction. Treat ment of the photolysis of a solute i n a w e l l - s t i r r e d solution has been published which can handle this problem f o r polymer solutions (14). In such a system the extinction coefficient i s conversion (time)-dependent, but not position-dependent. The kinetics of photobleaching i n r i g i d media has been treated mathematically (15). It would appear that generalization and extension of the method used therein could be a profitable approach to improved analyses of polymer f i l m photodegradation. Various aspects of polymer photodegradation have been re viewed recently (16). Spatial Distribution of Photodegradation i n Polymer Films Let us consider three polymer films of equal thickness, L(cm) (cf. Figure 1 schematic). Film A contains a uniformly-dispersed u l t r a v i o l e t l i g h t absorber (UV absorber). Film Β contains the same UV absorber i n a topcoat layer having a UV transparent matrix, Film C has no added UV absorber either dispersed or i n a topcoat. The polymer w i l l be referred to as component 1_ and the UV absorber as component 2. The following assumptions are made: a) Irradia tion i s by monochromatic l i g h t at normal incidence to the "upper f i l m surface, b) Light absorptions by the polymer and by the UV absorber obey the Lambert-Beer r e l a t i o n , c) The s p e c i f i c absorp tion coefficient ε 2 (cm^gm" ) of the UV absorber i s much greater than that, εχ, of the polymer, d) Change i n the absorption co e f f i c i e n t s at the activating wavelength i s negligible throughout the i r r a d i a t i o n period, e) The UV absorber functions only as a l i g h t absorber and affects polymer degradation i n no other manner, f) Chemical changes i n the polymer (chain scission, crosslinking, chromophore production) are proportional to the l i g h t absorbed by the polymer, g) The polymer has i n i t i a l l y a most probable d i s t r i bution of molecular weight. Assumptions a_ and b^ provide simple geometry and l i g h t inten s i t y relationships. Assumption £ permits neglect of the UV absorber contribution to the system volume and f i l m specific volume. Assumption d_ eliminates the complication of conversiondependent optical properties. Assumptions e_, _f, and j* lead to mathematically simple relations between integrated l i g h t f l u x and polymer structure alteration. We further state that I i s the l i g h t intensity i n the upper f i l m (or topcoat) surface and that I - ( l 2 ) L F I e " > , and I e " > are the "forward" i n t e n s i t i e s reaching the lower surfaces. Consideration of the approximately 5% r e f l e c t i o n at the upper a i r - f i l m interface i s then not neces sary. The comparable r e f l e c t i o n at the lower f i l m - a i r interface w i l l be ignored. With the above specifications i t i s possible to calculate (1) the changes i n average molecular weights, molecular weight d i s t r i bution, gel content (when crosslinking predominates), and new chromophore content as functions of t o t a l l i g h t f l u x , quantum 11

1

0

k

0

k

+ K

>

0

L

k

L

0




32

POLYMER

® MIXED UV ABSORBER IN FILM

ο

L

L

N

t f t t

n

L

POLYMER χ

Figure 1.

0

ο

un

n

·, UV ABSORBER 2.

MONOCHROMATIC LIGHT; NORMAL »,·

©

TOPCOATED UNPROTECTED W ABSORBER IN "TOPCOAT UV ABSORBER

-Ό

mi

n

FILMS

®

«,e, , k . « e . F 2

2

2

INCIDENCE »·*'*'

Schematic of polymer films undergoing irradiation by UV light


Polymer


3. SHULTz

Film

Photodegradation

33

y i e l d s , f i l m composition, and f i l m dimensions. In addition, the spatial distribution of these functions between the upper and lower f i l m surfaces can be described. To i l l u s t r a t e optical density effects on photodegradation we w i l l confine our attention to the simple situation of chain s c i s sion and new chromophore production. (Crosslinking w i l l not be considered as occurring. Treatment of concurrent scission and crosslinking can be done with minor increase i n mathematical com p l e x i t y CI).) UV absorber amounts are chosen so that the t o t a l l i g h t energy absorbed by the polymer i n a given exposure i s the same i n mixed f i l m A and topcoated f i l m B. In terms of average polymer exposure to l i g h t we may then say that "equal protection" i s provided the polymer i n films A and B. Random Chain Scission and Chromophore Production. Equal Energy Absorption by Polymer i n Film A and Film B. A. Relative Amounts of UV Absorber Required Per Unit Area for Mixed Film A and Topcoated Film B. The amount of UV absorber required i n the topcoat of f i l m Β to give the polymer "protection equal to" that provided by a given concentration of UV absorber i n f i l m A i s readily calculated. Referring to the l i g h t intensity relations depicted i n Figure 1, the average number of photons absorbed per gram by the polymer i n films A and Β are: ( R ^ = R L"1 /J

Film A:

10

e

-

( k l + k 2 ) x

dx

= R [(k k )L]-l(l-e1 0

Film B:

1 +

(R^g = FR^L"

2

1

j£ e "

klX

( k l + k 2 ) L

(1)

)

dx

FRioCkiD-Kl-e- '! ')

(2)

Rio = V k i V i

(3)

F = e- ^

(4)

k

(5)

1

1

k

c

l = ei l

^2

^2 ^2

(6)

= £2 C2

(7)

=

1C2

2

-1

I (photons cm" sec ) incident l i g h t intensity; t(sec) i r r a d i a t i o n time; εχ, zz (cnigm" ) s p e c i f i c absorption c o e f f i cients; c , c (gm cm" ) concentrations i n f i l m A; v (cm gm" ) s p e c i f i c volume of the polymer. R i s the number of photons absorbed by polymer per gram of 0

1

3

x

2

3

x

10


1

34


polymer i n the upper surface of films A and C. F i s the factor by which the l i g h t intensity entering f i l m Β has been reduced by the UV absorber i n i t s topcoat. Equal energy absorption by polymer i n films A and Β requires that ( R I ) A = ( I ) B * Equating these energy absorptions and solving for F gives R

P.

(1^9-1

Cl-e-t^kÎ^^id-e-^ ')1

(8)

1


F i s therefore a function of the optical density of the un protected f i l m C (O.D.) = lqL/2.303 c

(9)

and of the r a t i o of linear attenuation decrements i n the mixed film A k /k 2

=

x

CIO)

c

Z2°i/z\ \

The topcoat thickness, l_, (cf. Figure 1 and Eq. 4) i s un specified. In practice jl w i l l be dictated by considerations of the UV absorber s o l u b i l i t y l i m i t i n the topcoat matrix material, by the topcoat thickness required to provide other beneficial effects (e.g., abrasion resistance, erosion resistance), by available topcoat application techniques, and by economics. How ever, we can construct a hypothetical topcoat of thickness L equal to that of the polymer films. Then F = e~ 2 k

< >

L

4a

C» /C = (-£nF)/kL 2

2

2

(11)

s

The r a t i o C"2/C * equal to the r a t i o of the amount of UV absorber per unit area i n f i l m Β to the amount of UV absorber per unit area i n f i l m A needed to provide equal energy absorption by polymer i n the two films. Figure 2 presents this r a t i o as a function of unprotected f i l m C optical density for two r a t i o s , k /k!, of the linear attenuation decrements. A maximum of oneh a l f as much UV absorber i s required i n the topcoated construction for very low optical density films. For an unprotected f i l m O.D. = 1.0 and k /k = 10 only one-tenth the absorber used i n f i l m A i s needed for f i l m Β to provide "equal protection" f o r the polymer. 2

2

2

1

B. Average Molecular Weights. Assuming the polymer chains i n a laminus are cut at random i n direct proportion to the number of photons absorbed by the polymer i n the laminus, a convenient degradation variable i s



3.

SHULTZ

Polymer Film

Photodegradation

35

RELATIVE AMOUNTSOF UV ABSORBER REQUIRED W »

0.4 -

I

\

ι

x

I

ι

1

·

1

1

1

EQUAL ENERGY ABSORPTION _ BY POLYMER IN (J) AND (D s

\

03 Co (TOPCOAT)

\ .

l /l,«IO t

0.1 ι

1

0.2

I

0.4

ι

I

0.6

ι

1

0.8

.

1

1.0

k.L/2.303 * OPTICAL DENSITY OF UNPRO 1 TECTED FILM Figure 2. Ratio of amounts of UV absorber per unit area required for film Β to that required for film A to give equal energy absorptions by the polymer vs. opti cal density of unprotected film



36

s = Φ ν ι ι^ κ

ν

r

l

( 1 2 )

ά

where (scissions per photon absorbed by polymer) i s the s c i s sion quantum y i e l d , Mg i s the i n i t i a l number-average molecular weight of the polymer, and 1^ i s Avogadro s number. S i s the number of scissions i n the upper surface laminus per o r i g i n a l polymer molecule i n the surface laminus. By integration of the laminar average molecular weights from the upper surface to the lower surface of the films the numberaverage and weight-average molecular weights of the UV-irradiated films are found to be (1) Downloaded by UNIV OF ARIZONA on January 4, 2013 | http://pubs.acs.org Publication Date: April 2, 1979 | doi: 10.1021/bk-1979-0095.ch003

1

Films A and Β M = MO η η

(k!+k )L^

(i3)

2

L-e

Film C 0

M = M η η

(14)

Film A

In

M w

= vP W

U((k k )l)

M w

= vP W

1 α^)Λη (i!ps)

Film Β

1 +

2

U

U

^

+

l L )

+

Film C

]

} 1

g

)

]

(15)

(16)

. -kjL.

M = M° [l+OnD- *, ^1

h

1

5)

w

(17)

Figures 3 and 4 present, respectively, Mn/M^ and M^/lfy f o r the three f i l m constructions as functions of S when kjL/2.303=0.5 and k /k =10. Since the same number of new molecules i s produced by scission i n films A and Β f o r a given t o t a l i r r a d i a t i o n t h e i r Mn/Mj} are i d e n t i c a l f o r a l l S. The lowering i n weight-average molecular weight f o r the two films d i f f e r s , however, due to the nature of the weight-averaging of molecular weights. A super f i c i a l reading of Figure 4 would suggest that the polymer i n f i l m A i s better protected than i n f i l m B. This i s indeed true i f the physical property of concern depends only on M f o r the t o t a l film. I f , however, the f a i l u r e c r i t e r i o n i s sensitive to regions of highly-degraded polymer, f i l m A w i l l f a i l sooner than f i l m B. This dictates examination of the d i s t r i b u t i o n of degradation from the upper surface to the lower surface of each f i l m . 2

1

w

C. Average Molecular Weight as a Function of Depth i n u n irradiated Films. Random scission of polymer chains within a laminus at depth χ lowers the average molecular weight but perpe tuates the most probable d i s t r i b u t i o n of molecular weight within the laminus. Thus, (M^) = 2(M§) f o r a l l S. Here the super s c r i p t S i s intended to mean "at i r r a d i a t i o n degree yielding S" as the superscript £ previously and subsequently used means " o r i g i n a l " or "at zero i r r a d i a t i o n degree." The Lambert-Beer l i g h t intensity x

X


3.

SHULTZ

Polymer Film


fcjL/2.303'0.5

S = Figure 3.

37

Photodegradation

fc2/lu=

It 0

10

Μ,Μ° Ν-' η

Fraction of original M retained vs. number of scissions per original molecule in upper surface of film A n

k,L 72.303=0.5

k /k|* 10 2

S = f j l . t ΜιΜ°.Ν"' Figure 4.

Fraction of original M retained vs. number of scissions per original molecule in upper surface of film A w



38

r e l a t i o n leads to the following laminar molecular weight relations which were used i n the integrations leading to Eqs. (13) to (17): x

=

[l+Se"

( k

Film A

M^c/i/M

Film Β

M^jVM = [l+FSe"

0

l

+ k

2

) L

T]

(18)

_ 1

X

Film C

0

M S X

/L

/ M

°

=

+

[l Se"

k l L #

T ]~ χ L J"

l

klL

(19)

1

(20) 0

The ratios Μ$ /ι/Μ° are intended to represent M§ / M, or Mjj / M? (which are equal) i n the laminus at fractional depth x/L. Figures 5 and 6 i l l u s t r a t e f o r the three films having lqL/ 2.303 =0.5 and k /ki = 10 the variation of average molecular weight with depth when the surface scission levels are S = 1 and S = 7, respectively. The molecular weight averages as functions of depth are s t r i k i n g l y different f o r films A and B. When S = 1 the average molecular weights i n f i l m A range from 0.5M° at the upper surface to essentially M° at the lower surface. The variation i s only from 0.88M to 0.96M i n f i l m B. When S = 7 f i l m A upper surface molecules are at 0.125M and about 15% by weight of the f i l m i s at average M

Durability of Macromolecular Materials - ACS Publications - American

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