Relation of Polymer Chemical Composition to Acoustic Damping

Chapter 2

Relation of Polymer Chemical Composition to Acoustic Damping Bruce Hartmann

Downloaded by EAST CAROLINA UNIV on September 4, 2013 | http://pubs.acs.org Publication Date: May 1, 1990 | doi: 10.1021/bk-1990-0424.ch002

Polymer Physics Group, Naval Surface Warfare Center, Silver Spring, MD 20903-5000

Acoustic damping i n polymers i s shown t o be dominated by the glass t r a n s i t i o n occurring i n the amorphous portions of the polymer. Thus to tailor damping c h a r a c t e r i s t i c s f o r a p a r t i c u l a r application requires an understanding of the r e l a t i o n between chemical composition and glass t r a n s i t i o n temperature. Glass t r a n s i t i o n temperature depends on a number of variables, including backbone flexibility, steric effects, p o l a r i t y , pendant groups, c r y s t a l l i n i t y , p l a s t i c i z e r s , c r o s s l i n k density, and co-polymerization. While the glass t r a n s i t i o n damping peak can be located at almost any desired temperature, there are l i m i t a t i o n s on the height and width that can be achieved i n any r e a l polymer. The purpose of t h i s chapter i s t o review the r e l a t i o n between the chemical composition of polymers and the acoustic damping produced by these polymers. After defining the necessary acoustic terms, the chapter w i l l begin with a description of the experimental data f o r various polymeric systems, i n p a r t i c u l a r the temperature and frequency dependence of the damping. I t w i l l be seen that acoustic damping properties are dominated by the glass t r a n s i t i o n i n the polymer. The extensive l i t e r a t u r e on the r e l a t i o n between chemical composition and glass t r a n s i t i o n temperature w i l l then be b r i e f l y reviewed, establishing the connection between chemical composition and the location of the damping peak. The discussion w i l l then turn from the location of the damping peak to the height and width of the peak. I t This chapter not subject to U.S. copyright Published 1990 American Chemical Society

In Sound and Vibration Damping with Polymers; Corsaro, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


24

SOUND AND VIBRATION DAMPING WITH POLYMERS

w i l l be p o i n t e d out t h a t the h e i g h t and width cannot be chosen a r b i t r a r i l y , even i n p r i n c i p l e . The m a t e r i a l i n t h i s chapter i s based p r i m a r i l y on r e f e r e n c e s 1-5. To understand what f o l l o w s , some a c o u s t i c terms must f i r s t be d e f i n e d . The propagation o f an a c o u s t i c wave through a s o l i d polymer can be c h a r a c t e r i z e d by two parameters: sound speed and sound a b s o r p t i o n . Sound speed, c, i n u n i t s o f m/s, i s the r a t e a t which sound waves t r a v e l through the s o l i d . Sound a b s o r p t i o n , a, i n u n i t s o f dB/cm, i s a measure of t h e l o s s i n energy o f the sound wave as i t t r a v e l s through the s o l i d . The energy of t h e sound wave i s converted i n t o random thermal motion or heat, u s u a l l y with a n e g l i g i b l e r i s e i n temperature of the s o l i d . The u n i t s o f a can be e x p l a i n e d i n the f o l l o w i n g manner. The change i n energy o f an a c o u s t i c s i g n a l i s u s u a l l y expressed i n terms of the common l o g (base ten) o f the amplitude a t two d i f f e r e n t p o s i t i o n s . T h i s u n i t i s c a l l e d a B e l a f t e r Alexander Graham B e l l . Since t h i s u n i t i s r a t h e r small f o r most a p p l i c a t i o n s , i t i s more common t o use the d e c i B e l , o r dB, which i s obtained by m u l t i p l y i n g the B e l by t e n , dB

=

10

(Ia/IJ

log

= 10 l o g ( A ^ A J

(la) 2

(lb)

= 20 l o g (A /A!)

(lc)

2

where I and I are the a c o u s t i c energies a t the two l o c a t i o n s , A and A are the amplitudes o f the sound wave a t these l o c a t i o n s , and Equation l b makes use o f the f a c t t h a t the energy i n a sound wave i s p r o p o r t i o n a l t o the square o f the amplitude of the wave (6). With these u n i t s f o r the change i n energy between two p o s i t i o n s , a b s o r p t i o n o f a c o u s t i c energy per u n i t d i s t a n c e o f t r a v e l then has dimensions o f dB/cm. In some cases, r a t h e r than r e f e r r i n g t o a b s o r p t i o n per u n i t d i s t a n c e i n dB/cm, i t i s more convenient t o r e f e r t o t h e a b s o r p t i o n per wavelength o r aA, where A i s the wavelength. The u n i t s o f aA are then dB. In an unbounded, i s o t r o p i c s o l i d there a r e only two independent modes of a c o u s t i c propagation: l o n g i t u d i n a l and shear. In the l o n g i t u d i n a l mode, the p a r t i c l e motion i s p a r a l l e l t o the d i r e c t i o n of propagation, while i n the shear mode, the p a r t i c l e motion i s p e r p e n d i c u l a r t o the d i r e c t i o n o f propagation. A s s o c i a t e d with each of these modes o f propagation t h e r e i s an a b s o r p t i o n . Thus, four parameters a r e r e q u i r e d t o c h a r a c t e r i z e the s o l i d : l o n g i t u d i n a l sound speed, c , shear sound speed, c , longitudinal absorption, a and shear a b s o r p t i o n , a . Some experimental techniques y i e l d a modulus r a t h e r than a sound speed, but the u n d e r l y i n g p h y s i c a l p r o p e r t i e s a r e the same. In a t o r s i o n a l pendulum, f o r example, the shear modulus, G, i s measured w h i l e i n a Rheovibron, Young's modulus, E, i s measured. When t h e r e x

2

x

2

x

s

lt


s

2.

HARTMANN

Polymer Composition and Acoustic Damping

25

i s n e g l i g i b l e a b s o r p t i o n , the r e l a t i o n s between sound speed and modulus are p a r t i c u l a r l y simple, c c

x

=

s

=

(2)

1 2

(G/p) '

(K + 4 G / 3 ) / ) P

where K i s bulk modulus and p i s d e n s i t y . r e l a t e d through the standard equations


E = 2G(l+i/)

(3)

1 / 2

The moduli

= 3K(l-2i/)

are (4)

where u i s Poisson's r a t i o . A b s o r p t i o n i s taken i n t o account by assuming t h a t the modulus i s complex G* = G' + i G "

(5)

where G* i s the complex shear modulus, G' i s the r e a l p a r t of the modulus, and G i s the imaginary p a r t of the modulus. S i m i l a r r e l a t i o n s h o l d f o r Young's and bulk moduli. Since G' i s u s u a l l y much l a r g e r than G", the a b s o l u t e v a l u e of G* i s approximately equal t o G , and G' i s o f t e n taken t o be the modulus G, w r i t t e n without the prime (7), as i n Equation 2. The p h y s i c a l s i g n i f i c a n c e of G' i s t h a t i t represents an e l a s t i c storage of mechanical energy and i t i s a l s o c a l l e d the storage modulus. G'' i s a measure of a b s o r p t i o n and i s a l s o c a l l e d the l o s s modulus. When t h e r e i s a b s o r p t i o n , the s t r a i n l a g s ( i n time) behind the a p p l i e d s t r e s s by a phase angle 6 as shown i n F i g u r e 1. From F i g u r e 1, i t can be seen t h a t 7 /

;

(6)

tan 6 = G"/G'

where tan g 0.840


0.835 L-L

12

16 20 24 28 32 36 40 TEMPERATURE (°C)

F i g u r e 8. S p e c i f i c volume v s temperature f o r poly(vinyl acetate). (Reproduced with p e r m i s s i o n from Ref. 17. Copyright 1957 Royal S o c i e t y o f Chemistry.) Resin: BDGE = butanediol diglycidyl ether O O / \ / \ CH - CHCH 0(CH )40CH CH - CH 2

2

2

2

2

Curing agents: PDA = 1,3 propanediamine H N(CH ) NH 2

2 3

2

HDA =1,6 hexanediamine H N(CH ) NH 2

2 6

2

DDA =1,12 dodecanediamine H N(CH ) NH 2

2 12

2

F i g u r e 9. Epoxy polymer chemical components used f o r varying crosslink density.

14001—i 0

1 1 1 1 20 40 60 80 TEMPERATURE (°C)

L_-' 10C

F i g u r e 10. L o n g i t u d i n a l sound speed v s temperature f o r polyepoxides o f v a r y i n g c r o s s l i n k d e n s i t y . (Reproduced with permission from Ref. 22. Copyright 1981 Butterworth & Co. Ltd.)


35


36


oi—i

0

1

—i—

20 40 60 80 TEMPERATURE (°C)

1

1

1

100

F i g u r e 11. L o n g i t u d i n a l sound a b s o r p t i o n v s temperature f o r polyepoxides of v a r y i n g c r o s s l i n k d e n s i t y . (Reproduced with permission from Ref. 22. Copyright 1981 Butterworth & Co. Ltd.)

T-T {°C) g

F i g u r e 12. S h i f t e d curve f o r l o n g i t u d i n a l sound speed of polyepoxides of v a r y i n g c r o s s l i n k d e n s i t y . (Reproduced with permission from Ref. 22. Copyright 1981 Butterworth & Co. Ltd.)


2. HARTMANN

37


to o b t a i n s u p e r p o s i t i o n ; the curves were s h i f t e d by the g l a s s t r a n s i t i o n temperature, which was determined independently from the a c o u s t i c measurements. For an aromatic c u r i n g agent (m-phenylene diamine) however, the data i s s h i f t e d by more than i t s g l a s s t r a n s i t i o n temperature and the a b s o r p t i o n curve i s lower and broader than f o r the a l i p h a t i c s . Thus, the s u b s t i t u t i o n o f an aromatic c u r i n g agent f o r an a l i p h a t i c one changes more than j u s t the g l a s s t r a n s i t i o n temperature


Co-Polymerization and A d d i t i v e P r o p e r t i e s A random co-polymer or a blend o f compatible polymers w i l l have a s i n g l e g l a s s t r a n s i t i o n temperature intermediate between those of the two homopolymers. An example i s shown i n F i g u r e 14 f o r n i t r i l e - b u t a d i e n e rubber (23.) . The s p e c i f i c weight percents shown are those o f commercial i n t e r e s t f o r NBR. In c o n t r a s t , most polymer blends, g r a f t and block copolymers, and i n t e r p e n e t r a t i n g polymer networks (IPN's) are phase separated (5) and e x h i b i t two separate g l a s s t r a n s i t i o n s from the two separate phases. Phase separated systems w i l l not be considered here. Various t h e o r i e s have been proposed f o r the co-polymer equation (24) . The s i m p l e s t i s a l i n e a r combination o f the T 's o f the homopolymers i n the form g

T

g

=

a T

+ b T

gl

(12)

g2

where a and b are constants t h a t vary with the theory and the polymer. T h i s equation a r i s e s from the simple r u l e of mixtures and has a l s o been d e r i v e d on the b a s i s o f the f r e e volume theory o f the g l a s s t r a n s i t i o n . In some t h e o r i e s , the constants a and b are the weight f r a c t i o n s of the two components while i n other approaches the number o f main chain atoms i s used (12)• A second commonly used theory i n v o l v e s the r e c i p r o c a l temperatures 1/T = a/T g

+ b/T

gl

(13)

g2

where now a and b are d i f f e r e n t constants than above. T h i s equation can be d e r i v e d from an energy theory o f the glass transition. F i n a l l y , a l o g a r i t h m i c form f o l l o w s from the entropy theory o f the g l a s s t r a n s i t i o n (26) i n the form In T

g

= a In T

gl

+ b In T

g2

(14)

In t h i s case, the constants are r e l a t e d t o the heat c a p a c i t y changes a t the g l a s s t r a n s i t i o n o f the homopolymers. While the above three equations have r a t h e r d i f f e r e n t o r i g i n s and appear t o be q u i t e d i f f e r e n t , over the range of many measurements, the r e s u l t s do not d i f f e r s i g n i f i c a n t l y . Couchman (26) has



38


T-T (oC) g

F i g u r e 13. S h i f t e d curve f o r l o n g i t u d i n a l a b s o r p t i o n of polyepoxides of v a r y i n g c r o s s l i n k d e n s i t y . (Reproduced with permission from Ref. 22. Copyright 1981 Butterworth & Co. Ltd.)

)l i i i 1 1 0 0.2 0.4 0.6 0.8 1.0 WEIGHT FRACTION OF POLYACRYLONITRILE

F i g u r e 14. Glass t r a n s i t i o n temperature vs n i t r i l e content f o r a c r y l o n i t r i l e - b u t a d i e n e co-polymers.


2. HARTMANN


39

shown t h a t the l i n e a r and r e c i p r o c a l r e l a t i o n s are both approximations o f the l o g equation. While more accurate and d e t a i l e d co-polymer equations are a v a i l a b l e , f o r the q u a l i t a t i v e understanding d e s i r e d here, the simple l i n e a r equation i s s u f f i c i e n t . I t should be p o i n t e d out t h a t the above co-polymer equations are w r i t t e n as the sum o f two terms i n d i c a t i n g t h a t two components are i n v o l v e d , but more terms can simply be added t o the equations i f there a r e more components. Ter-polymers are sometimes encountered and t h e r e i s no reason i n p r i n c i p l e t h a t even more components c o u l d not be added. The e f f e c t of c o - p o l y m e r i z a t i o n on T i s o f s i g n i f i c a n t p r a c t i c a l importance and i s a l s o a s t a r t i n g p o i n t f o r an e m p i r i c a l procedure f o r p r e d i c t i n g T known as the method o f a d d i t i v e p r o p e r t i e s . In t h i s procedure, T i s estimated knowing only the molecular s t r u c t u r e o f the polymer. Such estimates are u s e f u l i n p r o v i d i n g guidance as t o what new polymers should be s y n t h e s i z e d i n order t o achieve d e s i r e d a c o u s t i c p r o p e r t i e s and i n p r o v i d i n g i n s i g h t i n t o the competing f a c t o r s t h a t are important i n the p r o p e r t i e s o f e x i s t i n g polymers. The method o f a d d i t i v e p r o p e r t i e s has been a p p l i e d t o d e n s i t y , g l a s s t r a n s i t i o n temperature, and many other polymer p r o p e r t i e s by Van Krevelen (25). The b a s i c idea i s t h a t the p r o p e r t i e s o f each chemical group i n the polymer are n e a r l y independent o f the other groups. Because o f this., each group can be assigned a c o n t r i b u t i o n t o the g l a s s t r a n s i t i o n temperature, f o r example, and the T o f a polymer i s the sum o f the c o n t r i b u t i o n s of a l l the groups,


g

g

g

g

T

g

= a T

gl

+ b T

g2

+ c T

g3

+ . . .

(15)

where T^ i s the group c o n t r i b u t i o n t o the g l a s s t r a n s i t i o n temperature and a, b, c, ... a r e the number of atom d i s t a n c e s along the main c h a i n o f the group d i v i d e d by the t o t a l number of atoms along the main c h a i n of the polymer repeat u n i t . For example, the polymer repeat u n i t f o r p o l y d i m e t h y l s i l o x a n e c o n s i s t s o f two components: - S i ( C H ) - and -0-. The number o f atom d i s t a n c e s along the main c h a i n i s one i n both cases. Thus, a = 1/2 and b = 1/2. In order t o make use of Equation 15, one must f i r s t determine the group c o n t r i b u t i o n s t o T . T h i s i s done by a n a l y z i n g the measured T of polymers with known molecular s t r u c t u r e . Some t y p i c a l values are l i s t e d i n Table I I , taken from Van Krevelen (25). Two v a l u e s are l i s t e d f o r the methylene group c o n t r i b u t i o n (-CH -) depending on whether hydrogen bonding i s present ( f o r polyamides, polyureas, and polyurethane the h i g h e r v a l u e i s used while f o r a l l other polymers, the lower v a l u e i s used). Likewise, the p-phenylene group (-pC H -) v a r i e s depending on the type o f polymer. 3

2

g

g

2

6

A


40


Table I I . Group C o n t r i b u t i o n s t o T Group

T ,

-CH -CH(CH )"C(CH ) -CH(C HJ-pC H -CHC1-0-Si(CH ) -

170, 270 336 226 576 300 t o 550 538 280 20

gi

2

3

3

2

6

6

A


3

2

g

K

Using r e s u l t s such as these, c.ie can p r e d i c t the T of any polymer f o r which the group c o n t r i b u t i o n s are known. The g l a s s t r a n s i t i o n temperature f o r p o l y d i m e t h y l s i l o x a n e , f o r example, i s T = (1/2) 20 + (1/2) 280 = 150 K = -123°C. The approach was o r i g i n a l l y a p p l i e d t o l i n e a r polymers but has been extended t o c r o s s - l i n k e d polymers as w e l l (24.) • Note the somewhat d i f f e r e n t i n t e r p r e t a t i o n o f the values i n Equation 15 as compared with Equation 12. In Equation 12, the homopolymer g l a s s t r a n s i t i o n temperatures are known and the m u l t i p l y i n g c o e f f i c i e n t s are determined by f i t t i n g t o experimental data. In Equation 15, the m u l t i p l y i n g c o e f f i c i e n t s are known from the s t r u c t u r e of the polymer and the group c o n t r i b u t i o n s are determined by f i t t i n g t o experimental data. Also, the group c o n t r i b u t i o n values do not e x i s t independently. For example, the group c o n t r i b u t i o n f o r oxygen (-0-) i s 280 K, but t h i s does not imply t h a t oxygen polymer e x i s t s , only t h a t t h i s i s the c o n t r i b u t i o n t h a t oxygen makes when i t i s a component of a polymer. g

g

Height-Width L i m i t a t i o n s The d i s c u s s i o n thus f a r has been confined t o the l o c a t i o n of the damping peak, but not the height or width of the peak. In many cases, an a c o u s t i c designer wants t o provide high damping over a wide range of frequencies. T h i s may not be p o s s i b l e s i n c e the height and width of the damping peak cannot be adjusted independently. No r e a l polymer can have an a r b i t r a r i l y high and broad l o s s factor. In general, a c o u s t i c design r e l y i n g on the g l a s s t r a n s i t i o n of a polymer i n v o l v e s a t r a d e - o f f between height and width. While i t i s d i f f i c u l t t o be s p e c i f i c , some g e n e r a l i t i e s can be pointed out. Experimentally, i t i s g e n e r a l l y observed t h a t when the frequency range of the t r a n s i t i o n i s broad, the damping peak i s low and when the t r a n s i t i o n i s sharp, the damping peak i s high. One explanation f o r t h i s observation i s t h a t the i n t e g r a t e d area under the damping vs temperature curve has been shown (1) t o be p r o p o r t i o n a l t o the a c t i v a t i o n energy of the t r a n s i t i o n


2. HARTMANN

Polymer Composition and Acoustic Damping AH = (G„-G )R7r [/G''d(l/T)] - l

41 (16)

2

0

where G^ i s the h i g h frequency (glassy) modulus, G i s the low frequency (rubbery) modulus, R i s t h e gas constant, G'' i s the imaginary p a r t o f t h e shear modulus, and T i s absolute temperature. For t r a n s i t i o n s whose a c t i v a t i o n energies a r e not too d i f f e r e n t , broadening the t r a n s i t i o n comes a t the expense o f lowering t h e h e i g h t . Another approach t o r e l a t i n g the width and h e i g h t o f the damping peak i s through the use o f an a n a l y t i c a l model o f the t r a n s i t i o n (27). Assuming, as commonly observed, t h a t the g l a s s y modulus o f most polymers i s f a i r l y s i m i l a r but the rubbery modulus v a r i e s by two o r more decades, one can determine the a l l o w a b l e h e i g h t and width combinations. In t h i s manner, the t r a d e - o f f s between h e i g h t and width can be examined. One u s e f u l a n a l y t i c a l model used t o d e s c r i b e the data i s based on the Cole-Cole equation o r i g i n a l l y proposed f o r d i e l e c t r i c r e l a x a t i o n (28.) but which can a l s o be used f o r dynamic mechanical r e l a x a t i o n (1,29.30). The Cole-Cole equation a p p l i e d t o shear modulus i s given by


0

(Go-GJ/fG'-GJ

=

l+(iur)

a

(17)

where a i s a dimensionless constant f o r a g i v e n polymer with a v a l u e between zero and one. (This parameter should not be confused with sound a b s o r p t i o n , which u n f o r t u n a t e l y has the same symbol.) I t can be shown t h a t the value o f a i s a measure o f the width o f the a b s o r p t i o n : the l a r g e r the value o f a, the sharper the transition. For a = 1, we have the sharpest p o s s i b l e t r a n s i t i o n , t h a t f o r a s i n g l e r e l a x a t i o n time. A p l o t of the Cole-Cole equation with parameters t y p i c a l f o r a polymer i s shown i n F i g u r e 15. The s p e c i f i c v a l u e s used are: G = 2 x 10 Pa, G«, = 6 x 10 Pa, r = 0.1 /xs, and a = 0.6. The shape o f the curve shown i n F i g u r e 15 i s a reasonably good f i t t o experimental data f o r many polymers and can be used t o i l l u s t r a t e the t r a d e - o f f s between h e i g h t and width. D e f i n i n g height as the maximum value o f the l o s s f a c t o r and width as the number o f decades o f frequency between the h a l f height values, we can c a l c u l a t e from Equation 17 the r e l a t i o n between h e i g h t and width f o r g i v e n v a l u e s o f G and G*. The r e s u l t s are shown i n F i g u r e 16. Here we have f i x e d the v a l u e o f Goo a t 1 GPa, which i s t y p i c a l f o r many polymers, and considered two constant v a l u e s f o r G : 10 Pa and 10 Pa. For f i x e d v a l u e s of G and G*, there i s a d i f f e r e n t h e i g h t and width f o r each value o f a. The value o f r does not enter the c a l c u l a t i o n s i n c e i t only determines the l o c a t i o n of the t r a n s i t i o n along the frequency a x i s and not the h e i g h t o r width. While the two curves shown i n F i g u r e 16 show a l l the values mathematically p o s s i b l e , a c t u a l v a l u e s tend t o have lower G when t h e r e i s a narrower 6

8

0

0

7

0

0

0




LOG FREQUENCY (HERTZ)

F i g u r e 15. Complex shear modulus i n the modified Cole-Cole model.


2.

HARTMANN


43

t r a n s i t i o n and t o have h i g h e r G v a l u e s when the t r a n s i t i o n i s broader. Some a c t u a l v a l u e s f o r s e v e r a l polyurethanes are shown i n F i g u r e 16 as s o l i d dots. 0


Modulus-Loss F a c t o r L i m i t a t i o n s Another l i m i t a t i o n on a c o u s t i c p r o p e r t i e s i s expressed by the Kramers-Kronig (KK) r e l a t i o n s , which are general r e l a t i o n s between the r e a l and imaginary p a r t s of a complex f u n c t i o n . These r e l a t i o n s were o r i g i n a l l y d e r i v e d f o r o p t i c s but can be a p p l i e d i n many other areas as w e l l . The essence of the r e l a t i o n s i s t h a t the r e a l and imaginary p a r t s of the f u n c t i o n are not independent of each other but one may be c a l c u l a t e d from an i n t e g r a l of the other. As a p p l i e d t o complex modulus, the s p e c i f i c form of the r e l a t i o n s i s given elsewhere i n t h i s book ( J . J a r z y n s k i , A Review of the Mechanisms of Sound Attenuation i n Materials). The KK r e l a t i o n s express the r e a l p a r t of the modulus as an i n t e g r a l over a l l frequencies of the imaginary p a r t of the modulus, and the imaginary p a r t of the modulus as an i n t e g r a l over a l l frequencies of the r e a l p a r t . T h i s i s a general mathematical r e s u l t t h a t f o l l o w s from c a u s a l i t y (the e f f e c t cannot precede the cause). An a l t e r n a t e way of viewing these r e l a t i o n s t h a t i s more f a m i l i a r t o polymer s c i e n t i s t s i s based on e x p r e s s i n g G' as an i n t e g r a l over a d i s t r i b u t i o n of r e l a x a t i o n times. Since G " can a l s o be expressed as an i n t e g r a l over the same d i s t r i b u t i o n of r e l a x a t i o n times, i t i s apparent t h a t G' and G'' are not independent. E q u i v a l e n t l y , G' and tan 6 are not independent. Knowing G' i n terms of the d i s t r i b u t i o n of r e l a x a t i o n times, there i s no freedom l e f t t o choose the value of tan

Relation of Polymer Chemical Composition to Acoustic Damping

Recommend Documents