Dynamic Mechanical and Thermal Properties of Seven

22 Dynamic Mechanical and Thermal Properties of Seven Polyurethane Adhesives D. M A R K HOFFMAN

Downloaded by AUBURN UNIV on November 3, 2015 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch022

Lawrence Livermore National Laboratory, University of California, Livermore, CA 94550

The dynamic mechanical behavior of block copolymers depends on the mechanical and morphological nature of each block. If polymer block A is thermodynamically incompatible with polymer block B, microphase domains form (1-5). The concentration and chemical structure of these domains can be controlled to produce desired mechanical and thermal properties. Seven polyurethane adhesives have been developed at Lawrence Livermore National Laboratory (LLNL). These adhesives, designated Halthanes were synthesized because of OSHA restrictions on the use of the curing agent methylene bis(2-chloroaniline). Four of the Halthanes were made fromLLNL-developed 4,4'-methylene bis(phenylisocyanate) terminated prepolymers cured with a blend of polyols; three were made from an LLNL-developed prepolymer terminated with Hylene W and cured with aromatic diamines. In this paper we report the dynamic mechanical and thermal behavior of these seven segmented polyurethanes. Segmented polyurethanes have domains called hard and soft segments (3) because of their modulus differences. Relaxations in the dynamic mechanical spectrum can be associated with transitions in the hard and soft segments. In the Halthane adhesives, the soft segment is always the reaction product of a hydroxy-terminated poly(tetramethylene glycol) and an isocyanate. Since domain insolubility depends on molecular weight, the degree of polymerization of the polyether can be changed to produce different degrees of microphase separation.

0097-6156/81/0172-0343$05.00/0 © 1981 American Chemical Society

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


344

URETHANE CHEMISTRY AND APPLICATIONS

Lower molecular weight polyether in the soft segment increases the g l a s s t r a n s i t i o n temperature and therefore decreases the low temperature l i m i t of elastomeric character of the adhesive. Diisocyanates and low molecular weight d i o l s or amines used t o form the polyurethane or polyurea hard segments act as p h y s i c a l c r o s s l i n k s and p a r t i c u l a t e f i l l e r in the soft rubbery domains i n c r e a s i n g the modulus and improving toughness and creep r e s i s t a n c e . The t r a n s i t i o n behavior of the hard segments depends on t h e i r chemical s t r u c t u r e . We found that hard segments made from aromatic or c y c l i c monomers produced s t i f f , high modulus adhesives while a l i p h a t i c monomers produced more rubbery adhesives. Experimental Compositions of Halthane adhesives are given in Tables I and I I . Information on polymerization procedures i s described elsewhere ( 1 6 ) . The dynamic shear storage modulus (G' ) and loss modulus (G") were measured from -150° to 50°C using the forced t o r s i o n f i x t u r e on a Rheometric Mechanical Spectrometer (RMS)*. When the storage modulus dropped below 10& Pa, t h i s f i x t u r e became i n s e n s i t i v e . For moduli less than 10? Pa, the p a r a l l e l p l a t e f i x t u r e with serrated disks was used. The p a r a l l e l p l a t e f i x t u r e was used to extend the dynamic mechanical measurements to high temperatures. Degradation above about 250°C d i c t a t e d t h i s temperature as an upper l i m i t for RMS measurements. Further d i s c u s s i o n of equations and use of these f i x t u r e s are given elsewhere (_7,8). Results T h i s s e c t i o n i s subdivided into two parts based on the two types of LLNL Halthane adhesives. The basic d i s t i n c t i o n between these adhesives i s the modulus of the rubbery p l a t e a u . Halthane 7 3 - s e r i e s adhesives are tough, rubbery polyurethanes with a modulus of about 10^ Pa at room temperature. On the other hand, Halthanes 8 7 - 1 , 87-2, and 88-2 are s t i f f , almost g l a s s y adhesives with a modulus of about 10& Pa at room temperature. Halthane 73-Series Adhesive Soft Segment Behavior. The soft segments of a l l the 7 3 - s e r i e s Halthanes c o n s i s t mainly of a low molecular weight

^Reference to a company or product name does not imply approval or recommendation of the product by the U n i v e r s i t y of C a l i f o r n i a or the U.S. Department of Energy to the e x c l u s i o n of others that may be s u i t a b l e .


22.

HOFFMAN

Table I.

Polyurethane

345

Adhesives

7 3 - s e r i e s prepolymer and c u r i n g agent f o r m u l a t i o n s .


Curing Agents Component

14

15

18

19

Polymeg 1000 1,4-butanediol Quadrol FAA

90 10 -

90 10 0.0156

85 10 5 -

85 10 5 0.0107

Prepolymer Polymeg 1000 Polymeg 2000 MDI

47.6 7.4 45.0

47.6 7.4 45.0

47.6 7.4 45.0

47.6 7.4 45.0

Prepolymer: Curing Agent

62/38

62/38

65/35

65/35

Table

Prepolymer and curing agent formulations polyurea hard segment adhesives.

II.

Curing Agents Component

87-1

Tonox 60/40 XU-205

100

87-2

88-2

100

100

Prepolymer Polymeg 2000 HMDI

77.6 22.4

77.6 22.4

74.0 26.0

Prepolymer: Curing agent

93/7

92/8

88/12


for

346



Abbreviations

Structure

MDI - 4 , 4 ' methylene b i s (phenyl isocyanate)

0=C=N-^^CH -/OVN=C=0 2

HMDI - Hylene W (saturated MDI)

0=C=N-(s)-CH2-{s)-N=C=0

BDO - butanediol

H0-(CH2)40H

Q - quadrol

( C H C H - C H ) N CH CH 3

2

2

2

2

OH I N(CH CH C H ) 2

3

2

HO Polymeg 1000 poly(tetramethylene oxide)

H0-(CH2CH2CH2CH20) -H

χ = 14;

X

Polymeg 2000 poly(tetramethylene oxide)

χ = 28

Τ0Ν0Χ - mixture of aromatic diamines 4,4·

H2N-^£^CH2-{o)-NH2

methylene d i a n a l i n e and

NH2

m-phenylene diamine

+

fb

NH

2

XU-205 - mixture of s u b s t i t u t e d aromatic diamines of 4,4*

H2N-^^CH -{Ô^-NH2

methylene d i a n a l i n e

R]

FAA - f e r r i c acetylacetonate

2

R2

Fe (0-C=C-C=0)3

I

I

CH C H 3

3


22.

HOFFMAN

Polyurethane Adhesives

347

polyether Polymeg 1000 extended by a d i f u n c t i o n a l isocyanate, MDI. Three low temperature t r a n s i t i o n maxima are found i n the loss modulus (G ) of the 7 3 - s e r i e s Halthanes (see F i g . 1). Two low temperature secondary r e l a x a t i o n s below the g l a s s t r a n s i t i o n of the soft segment are a r b i t r a r i l y labeled Τβ (-100°C) and T ( - 1 5 5 ° C ) . These r e l a x a t i o n s are probably associated with molecular motions in the urethane (9) and polyether (10) components of the soft segment, r e s p e c t i v e l y . The g l a s s t r a n s i t i o n of the soft segment occurs at about -50°C and i s r e s p o n s i b l e f o r the drop in the storage modulus G' by two orders of magnitude. The soft segment t r a n s i t i o n s of 73-15, 73-18, and 73-19 are very s i m i l a r to those shown f o r 73-14 in F i g . 1. Table III l i s t s these t r a n s i t i o n s for each adhesive. The presence of t e t r a f u n c t i o n a l a l c o h o l s in the 73-18 and 73-19 polymers r a i s e s the soft segment glass t r a n s i t i o n temperature of these adhesives s l i g h t l y as compared to 73-14 and 73-15. The apparent a c t i v a t i o n energies (E/\çj) of the dynamic mechanical t r a n s i t i o n s can be c a l c u l a t e d from the temperature at the maximum in the loss modulus T ( G " ) as a f u n c t i o n of frequency. A t y p i c a l Arrhenius p l o t (see F i g . 2) of the apparent a c t i v a t i o n energy of the soft segment g l a s s t r a n s i t i o n f o r 73 s e r i e s polymers y i e l d s E / \ C J values of approximately 7 5 + 5 kcal/mole (314 kj/mole) over a frequency range of 2 decades. n


7

m a x

Hard Segment Behavior. The hard segment t r a n s i t i o n s of the 7 3 - s e r i e s Halthane adhesive are very complex. Hard segments c o n t a i n i n g the t e t r a f u n c t i o n a l chain extender, quadrol (73-18 and 73-19), show s t r i k i n g l y d i f f e r e n t dynamic mechanical behavior compared to hard segments of d i f u n c t i o n a l butanediol (73-14 and 73-15). Because of the low c o n c e n t r a t i o n of hard segments, t h e i r t r a n s i t i o n s are d i f f i c u l t to i d e n t i f y . The t r a n s i t i o n behavior i s a l s o complicated by a small amount of c r y s t a l l i n i t y shown as a melting endotherm in the DSC t r a c e s i n F i g . 3. Poorly c r y s t a l l i z e d p a r a c r y s t a l l i n e (JJ_) urethane segments e x h i b i t broad, weak melting endotherms, which occur at d i f f e r e n t temperatures depending on the thermal h i s t o r y of the sample (12). The "glass t r a n s i t i o n of the hard segments i s presumed to be between 50° and 80°C based on the DSC d a t a . It i s followed by a melting endotherm beginning at about 80°C and ending at about 120°C. The exotherm i s smaller in the c r o s s ! i n k e d hard segments (73-18 and 73-19) than i n the l i n e a r hard segments. The dynamic mechanical r e l a x a t i o n s i n the high temperature r e g i o n are very weak and the glass t r a n s i t i o n was i n d i s t i n g u i s h a b l e from the melting p o i n t ( F i g . 4 ) . However, the mechanical p r o p e r t i e s of polyurethanes with c h e m i c a l l y c r o s s ! i n k e d hard segments were q u i t e d i f f e r e n t from uncross!inked polyurethanes. In the l i n e a r adhesives (73-14 and 73-15), the rubbery plateau ends at the melting point of the

American Chemical S o c i e t y Library

St

1155 and16th N. Edwards, ». In Urethane Chemistry Applications; K., et al.; ACS Symposium Series; American Chemical Washington, DC, 1981. Washington 0. ι Society: 20038

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981. β

y

g

Figure 1. The low-temperature dynamic mechanical spectrum of Halthane 73-14 is typical of the 73-series polyurethane adhesives. Two secondary relaxations, Ύ and T , are shown as peaks in the loss modulus at -100° and -150°C. The soft segment glass transition, T (SS), occurs at about — 50°C. The frequency of oscillation was held constant during the measurement at 0.1 Hz.


In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981. max

Figure 2. The apparent activation energy of the soft segment glass transition for Halthane 73-15 is determined from the slope of a plot of In f vs. 1 /T(G" ).




δ

Η

r

>

> α

Η

§

> m ο Κ w

d M Η

U\ Ο


HOFFMAN


351

Temperature (°C)

Figure 4. The high-temperature shear storage and loss moduli of Halthane 73-14 and 73-19 adhesives are controlled by the presence or absence of the cross-linking agent quadrol in the hard segments. In the linear urethane (73-14), viscous fl follows the melting of the hard segments, whereas in the cross-linked urethan (73-19), the modulus drops only when the polymer begins to degrade.


352


hard segment and the polymer begins to flow (about 90°C). T h i s i s shown by the drop in the storage modulus (G ) above t h i s temperature ( F i g . 4 ) . The shoulder on the loss and storage modulus of 73-14 at about 120°C i s probably the f i n a l melting out of the hard segment c r y s t a l s . In c o n t r a s t , the 73-19 adhesive remains rubbery and w i l l not flow below 200°C. Above t h i s temperature, these polymers begin to degrade. C l e a r l y , the chemically c r o s s l i n k e d adhesive i s superior to the uncross!inked adhesive, because i t r e t a i n s i t s i n t e g r i t y above the melting point of the hard segment. The melting of the hard segments causes only a s l i g h t change in storage modulus and the adhesive does not flow.


1

Table I I I .

T r a n s i t i o n temperatures of 7 3 - s e r i e s H a l t h a n e s .

Transition^

73-14

h E

ACT

(

k J / m o l e

73-15

73-18

-155

-160

-160

-160

-100

-100

- 99

-100

)

Tg(SS)

-

Ε (kJ/mole) ACT

73-19

56.7

-

53.8

-

57.3

62

49.2

- 49

339

323

298

195

Tg

(SS)c

- 51

- 53

- 43

- 41

T

m

(HS)d

92

81

85

85

Tg

(HS)C

78

50

70

68

T

(HS)C

98

80

83

82

m

Dynamic mechanical t r a n s i t i o n temperatures were measured at the maximum i n the loss modulus at 0.1 Hz. a

k Temperatures in °C. c

Differential

scanning c a l o r i m e t e r measurements from F i g . 3.

T h i s t r a n s i t i o n s h i f t s from sample to sample c h a r a c t e r i s t i c of urethane hard segment m u l t i p l e melting behavior. d


3


22.

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353

The a d d i t i o n of t e t r a f u n c t i o n a l quadrol has two e f f e c t s on the mechanical p r o p e r t i e s . (1) It c h e m i c a l l y c r o s s l i n k s the polymer, extending the rubbery plateau beyond the melting point of the hard segments. (2) It weakens the load bearing c a p a b i l i t y of the hard segments causing the rubbery modulus of c r o s s l i n k e d systems to be lower than the l i n e a r m a t e r i a l s at temperatures below the melting t r a n s i t i o n of the hard segments. That i s to say, the c r o s s l i n k e d adhesives (73-18 and 73-19) have a lower modulus (1()6 Pa as compared to 10? Pa) over a wider temperature range (-50° to 200°C as compared to -50° to 100°C) than the uncrosslinked 73-14 and 73-15 adhesives. Polymer degradation, as measured by TGA, begins at about 250°C in a i r f o r a l l the 7 3 - s e r i e s adhesives (see F i g . 5 ) . T h i s i s somewhat higher than the degradation i n d i c a t e d by the decrease in storage modulus; t h i s i s because the i n i t i a l stages of degradation involve chain s c i s s i o n , which reduces the modulus, whereas at higher temperatures, r e v e r s i o n and p y r o l y s i s produce gaseous degradation products and t h e r e f o r e , weight l o s s . Halthane 87- and 8 8 - S e r i e s Adhesives Soft Segment Behavior. The soft segment storage and l o s s moduli of 8 7 - 1 , 87-2, and 88-2 Halthane polyurethanes ( F i g . 6) are very s i m i l a r to the dynamic mechanical moduli of poly(tetramethylene g l y c o l ) (10). From the loss modulus peaks a low temperature secondary r e l a x a t i o n i s found at about -150°C and the soft segment glass t r a n s i t i o n i s -70°C. From the storage modulus, the r e l a x a t i o n strength of the soft segment (the d i f f e r e n c e between the storage modulus of the glass and the rubber) i s much less than the 7 3 - s e r i e s polymers. T h i s means that 87 and 88 s e r i e s adhesives are s t i f f e r or harder than the 7 3 - s e r i e s adhesives. DSC soft segment g l a s s t r a n s i t i o n temperatures f o r 87- and 8 8 - s e r i e s adhesives ( F i g . 7) are s l i g h t l y higher than dynamic mechanical t r a n s i t i o n temperatures probably because of the higher heating rates used in the DSC measurements. Hard Segment Behavior. The high temperature dynamic mechanical behavior of Halthane 88-2 shows some apparent post c u r i n g above 90°C ( F i g . 8 ) . The aromatic polyurea hard segments have glass t r a n s i t i o n s at about 180°C (14). I f these polymers are cured at room temperature, the hard segments do not polymerize completely. As the temperature i n c r e a s e s , f u r t h e r c u r i n g occurs causing the storage and loss moduli to r i s e . This behavior i s a l s o shown i n the DSC curves ( F i g . 7 ) . Since these polyurea hard segments are l e s s f l e x i b l e , they are more e f f e c t i v e in bearing the s t r e s s than the MDI-butanediol hard segments. Therefore, the modulus of these adhesives i s about two orders of magnitude l a r g e r than the 7 3 - s e r i e s Halthanes




354

Figure 5. Thermogravimetric analysis cruves of 73-series Halthanes show bim pyrolysis behavior starting at about 250°C. The rate of change in weight of th hesive with time, dW/dt is plotted against temperature for a programmed heatin of 12°C/min.



22.

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355

Figure 6. The low-temperature storage moduli of 87- and 88-series Halthanes show a smaller relaxation strength than 73-series Halthanes because of the arom hard segments' stiffness. Higher concentrations of hard segments are responsible for the higher modulus of Halthane 88-2 above the soft segment glass transition, T^SS), at —80°C. The lowtemperature loss moduli of 87- and 88-series Halthanes show the glass transition of the soft segments and a secondary relaxation at about —150°C. The frequency of oscillation was held constant during the measurements at 0.1 Hz.




1

C/3

H Ο

r

>

Ο

>

M-

00 H

a w g

Ο

W

H

w

d

as


The onset of viscous flow above 200' C indicates that the hard segment glass transition temperature has been exceeded. Solid line data were obtained using the RMS forced-torsion fixture and dashed line data using the parallel-plate fixture.

Figure 8. The high-temperature dynamic mechanical spectrum of Halthane 88-2 shows that some further curing is occurring above 100°C because both storage and loss modulus increase over a broad range of temperatures.

Temperature — °C


u> ^}


358


(10^ Pa as compared to 10^ Pa) over the temperature range between hard and soft segment glass t r a n s i t i o n s . Table IV l i s t s the t r a n s i t i o n temperatures for 87- and 8 8 - s e r i e s adhesives. As in the case of 7 3 - s e r i e s adhesives, degradation of 87 and 8 8 - s e r i e s polyurethanes was bimodal ( F i g . 9 ) . These adhesives were somewhat more s t a b l e than the 7 3 - s e r i e s (compare F i g . 6 with F i g . 9 ) . V o l a t i l i z a t i o n began at about 250°C. Assuming that the hard segments degrade f i r s t , t h i s implies that the polyurea hard segments of 87- and 8 8 - s e r i e s polymers have better thermal s t a b i l i t y than the polyurethane hard segments of the 7 3 - s e r i e s adhesives. Table IV. T r a n s i t i o n temperatures

of 87 & 8 8 - s e r i e s

Halthanes.

Transition^

87-1

87-2

88-2

τ

-150

-150

-150

- 79

- 77

- 76

189

162

256

105

92

93

185

196

188

γ

Tg

(SS)

ΕACT

(kJ/mole)

T (post c

cure)

Tg(HS)

Dynamic mechanical t r a n s i t i o n s were measured at the maximum in the loss modulus at 0.1 Hz. a

b Temperatures c

Differential

in °C. scanning calorimeter measurements from F i g . 7.

Discussion Comparison of Hard and Soft Segments i n the D i f f e r e n t Adhesives. Each chemically d i s t i n c t block in these segmented polyurethanes e x h i b i t s mechanical and thermal t r a n s i t i o n s c h a r a c t e r i s t i c of the homopolymer from which that block was made, unless the t r a n s i t i o n s are i n h i b i t e d by the proximity of the second b l o c k . F i v e types of t r a n s i t i o n s are found i n most homopolymers: secondary r e l a x a t i o n s , a glass t r a n s i t i o n , melting t r a n s i t i o n s , viscous flow, and polymer degradation. Secondary r e l a x a t i o n s are u s u a l l y unaffected by other blocks because they involve s m a l l , l o c a l i z e d molecular motions. Degradation i s u s u a l l y unaffected by other b l o c k s . The other t r a n s i t i o n s (the g l a s s t r a n s i t i o n , melting t r a n s i t i o n , and viscous flow) often depend on the s t r u c t u r e of the second block i n the copolymer.


3


Figure 9. Thermogravimetric analysis curves of 87- and 88-series Halthanes degradation behavior similar to 73-series Halthanes (Figure 6).



360


The glass t r a n s i t i o n of the low molecular weight soft segments in the Halthane 7 3 - s e r i e s adhesives i s 20°C higher than the glass t r a n s i t i o n of the higher molecular weight soft segments in the 87 and 88 adhesives because of the c o n s t r a i n t s on the shorter poly(tetramethylene g l y c o l ) chain motion by the more frequent placement of urethane hard segments (15,16). The a d d i t i o n of t e t r a f u n c t i o n a l monomer to the hard segments of the 7 3 - s e r i e s Halthanes also increases the glass t r a n s i t i o n s l i g h t l y , probably by d i s r u p t i n g the hard segment packing and i n c r e a s i n g the i n t e r f a c i a l zone between the two b l o c k s . There are several ways of changing the p r o p e r t i e s of segmented polyurethane copolymers. In t h i s study, the chemical s t r u c t u r e of the hard segments was changed to adjust the modulus of the adhesive. Further f i n e tuning i s p o s s i b l e by varying the concentration of the d i f f e r e n t b l o c k s . For example, the 8 7 - s e r i e s adhesives have a lower modulus than the 8 8 - s e r i e s adhesives because t h e i r hard segment concentration i s lower. Studies of concentration e f f e c t s (16) have shown that much wider v a r i a t i o n of modulus i s p o s s i b l e tïïân was achieved here. Conclusions The chemical s t r u c t u r e of the hard and soft segments, the concentrations of each b l o c k , and the presence of t e t r a f u n c t i o n a l c r o s s l i n k e r determined the dynamic mechanical and thermal p r o p e r t i e s of the three types of polyurethane adhesives, 7 3 - , 8 7 - , and 8 8 - s e r i e s Halthanes s t u d i e d . A r o m a t i c - a l i p h a t i c MDI- butanediol urethane hard segments produce lower modulus (10^ Pa) m a t e r i a l s i n the rubbery region than c y c l i c unsaturated-aromatic urea hard segments. Incorporation of chemical c r o s s l i n k s i n the hard segments extended the rubbery plateau beyond the hard segment t r a n s i t i o n s up to temperatures where the polymer begins to degrade. Concentration of hard and soft segments can also be used t o c o n t r o l the modulus between the glass t r a n s i t i o n temperatures of the two b l o c k s . Acknowledgments We would l i k e to thank Barbara McKinley and P a t r i c i a Crawford f o r the thermal measurements and LeRoy Althouse f o r supplying the polymers. Discussions with George Hammon and John Kolb are also g r a t e f u l l y acknowledged. Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under c o n t r a c t No. W-7405-Eng-48.



22.

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T h i s document v a s p r e p a r e d a s an a c c o u n t o f work s p o n s o r e d b y an a g e n c y o f t h e U n i t e d S t a t e s Government, N e i t h e r t h e U n i t e d S t a t e s Government n o r t h e U n i v e r s i t y o f C a l i f o r n i a n o r a n y o f t h e i r em p l o y e e s , makes a n y w a r r a n t y , e x p r e s s o r i m p l i e d , o r assumes a n y l e g a l l i a b i l i t y o r r e s p o n s i b i l i t y f o r the accuracy, completeness, o r u s e f u l n e s s o f any i n f o r m a t i o n , a p p a r a t u s , p r o d u c t , o r p r o c e s s d i s c l o s e d , o r r e p r e s e n t s t h a t i t s use would n o t i n f r i n g e p r i v a t e l y owned, r i g h t s . R e f e r e n c e h e r e i n t o any s p e c i f i c c o m m e r c i a l r > r o d u c t s , p r o c e s s , o r s e r v i c e b y t r a d e name, t r a d e m a r k , m a n u f a c t u r e r , o r o t h e r w i s e , does n o t n e c e s s a r i l y c o n s t i t u t e o r i m p l y i t s e n d o r s e ment, r e c o m m e n d a t i o n , o r f a v o r i n g b y t h e U n i t e d S t a t e s Government o r t h e U n i v e r s i t y o f C a l i f o r n i a . The v i e w s a n d o p i n i o n s o f a u t h o r s e x p r e s s e d h e r e i n do n o t n e c e s s a r i l y s t a t e o r r e f l e c t t h o s e o f t h e U n i t e d S t a t e s Government t h e r e o f , a n d s h a l l n o t be u s e d f o r a d v e r t i s i n g o r p r o d u c t endorsement p u r p o s e s .

Literature Cited 1. Morton, Μ., J . Polym. Sci., Polym., Symposium, 1977, 60, 1. Encyclopedia of Polymer Science and Technol., 1971, 15, 508. 2. Dickie, R. Α., in "Polymer Blends" (D. R. Paul and S. Newman, eds.), Academic Press: New York (1978) pp. 353-392. 3. Estes, G. M., Cooper S. L . , and Tobolsky, Α. V., J . Macromol. Sci., Rev. Macromol. Chem., 1970, C4, 167. 4. Meier, D. J., J . Polym. Sci., Pt. C . , 1969, 26, 81. 5. Krause, S., Macromol., 1970, 3, 84. 6. Hammon H. G., and Althouse, L. P., Lawrence Livermore Laboratory, Rept. UCID-17348, 1977. 7. Rheometrics Mechanical Spectrometer Operations Manual, Rheometrics, Inc., 1974. 8. McCrum, Ν. G . , Read, Β. Ε . , and Williams, G., "Anelastic and Dielectric Effects in Polymeric Solids," Wiley: New York, 1967. 9. Huk D. S., and Cooper, S. L . , Polym. Eng. Sci., 1971, 11, 369. 10. Wetton, R. E., and Allen, G., Polym., 1966, 7, 331. 11. Bonart, R., J . Macromoli, Sci. Phys., 1968, B2, 115. Bonart, R., Morbitzer, L., and Hantze, G . , J . Macromol. Sci. Phys., 1969, B3, 337. In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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12. Seymour, R. W., and Cooper, S. L . , Macromol., 1973, 6, 48. 13. Lipatov, Yu. S., Privalko, V. P., Kercha, Yu. Yu., Krafchik, S. S., and Kuz'mina, V. Α., Kolloid Ζ. Z. Polym., 1976, 254, 41. 14. Work, J. L . , Macromol., 1976, 9, 759.


15. Illinger, J. L . , Schneider, Ν. S., and Karasz, F. Ε., Polym. Eng. Sci., 1972. 12, 25. 16. Minoura, Y., Yamashita, S., Okamoto, H., Matsuo, T., Izawa, M., Kohmoto, S., J. Appl. Polym. Sci., 1978, 22, 1817. RECEIVED April 30, 1981.


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