Water in Nylon - American Chemical Society

25 Water in Nylon H O W A R D W. S T A R K W E A T H E R , J R .

Downloaded by KTH ROYAL INST OF TECHNOLOGY on June 7, 2016 | http://pubs.acs.org Publication Date: August 19, 1980 | doi: 10.1021/bk-1980-0127.ch025

Ε. I. du Pont de Nemours and Co., Central Research and Development Wilmington, D E 19898

Department,

The sorption of water by nylon has a major effect on proper ties of engineering and scientific importance. In molded 66 nylon at room temperature, the modulus decreases by about a factor of five, the yield stress decreases by more than half, and there are major increases in the elongation and energy to break as the water content is increased from dryness to saturation (1). Thus, reported properties of nylon are frequently those of a mixture of nylon and water. It is important to specify the water content or the relative humidity with which the polymer i s in equilibrium. The changes in properties due to absorbed water closely parallel those which occur as the temperature i s increased. In such systems, the time-temperature superposition which is familiar in studies of viscoelasticity can be extended to a time-tempera ture-humidity superposition (2-4). Thermodynamic Properties A sorption isotherm at 23°C for an extruded film of 66 nylon which had been annealed at 250°C (5) is shown in Figure 1. The film was 0.010" (0.25 mm) thick and was 57% crystalline. For this sample, the isotherm had an upward curvature which became more pronounced at higher humidities. For less f u l l y annealed samples, sorption isotherms have been reported which have a downward curva ture at low humidities (6). This indicates that polymer-water contacts are strongly preferred. The upward curvature of the isotherm i s i n d i c a t i v e of c l u s t e r ing of water molecules. According to a formula d e r i v e d by Zimm (7), the number of water molecules i n the neighborhood of a given water molecule i n excess of the mean c o n c e n t r a t i o n of water i s given by /31η

φ

X Ρ,Τ

0-8412-0559-0/ 80/47-127-433505.00/ 0 © 1980 A m e r i c a n Chemical Society

Rowland; Water in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

434

W A T E R IN

POLYMERS

where c^, φ^, and a^ are the molar concentration, volume f r a c t i o n , and a c t i v i t y of water, and i s the c l u s t e r i n t e g r a l . There f o r e , the apparent number of water molecules i n a c l u s t e r i s In c

φ Χ χ

i ii - -Vl — G

+ 1

)

( 1

l

n

(2)

a

M l / , One must be cautious i n equating t h i s s t a t i s t i c a l quantity with p h y s i c a l c l u s t e r s . For example, the f a c t that a polymer chain i s a c l u s t e r of volume elements means that small molecules mixed w i t h i t must be c l u s t e r e d i n a complementary way. The a p p l i c a t i o n of Equation 2 to the Flory-Huggins equation f o r a θ s o l u t i o n leads to an apparent c l u s t e r s i z e of (1-φ^)^(8) . This e f f e c t w i l l be very small at the l e v e l s of φ^ f o r water i n nylon. I t i s i n t e r e s t i n g to consider the number and s i z e of the c l u s t e r s i n r e l a t i o n to the polymer s t r u c t u r e ( 9 ) . I f m i s the number of water molecules per amide group i n the amorphous r e gions, the corresponding number of c l u s t e r s w i l l be


P

T

Figure 2 i s a p l o t of the number of c l u s t e r s per amorphous amide v s . the number of water molecules per amorphous amide. The s l o p i n g l i n e s correspond to v a r i o u s average numbers of water mole cules per c l u s t e r . The water i s almost unclustered up to a con c e n t r a t i o n of one water molecule per two amide groups. P u f f r and Sebenda (10) considered that t h i s l e v e l which probably corresponds to water molecules hydrogen bonded to the oxygen atoms of two amide groups represents the f i r s t and most s t r o n g l y bonded type of absorbed water. T h e i r view has been widely supported by l a t e r workers. As the concentration of water i s increased, the number of c l u s t e r s remains w i t h i n a narrow range. Decreases i n the num ber of c l u s t e r s may occur when newly absorbed water forms bridges between p r e v i o u s l y u n c o r r e l a t e d c l u s t e r s . Near s a t u r a t i o n , the apparent average c l u s t e r s i z e i s about three water molecules. This i s c l o s e to P u f f r and Sebenda s second stage of water absorp t i o n which i s three molecules per two amide groups i n the amor phous regions. The second and t h i r d water molecules were s a i d to be bonded between the carbonyl of one amide group and the NH of another. A s i m i l a r treatment ( 9 ) of B u l l ' s data on nylon f i b e r s (6) a l s o i n d i c a t e d that c l u s t e r i n g begins at about one water mole cule per two amide groups i n the amorphous r e g i o n s . The apparent number of c l u s t e r s d e c l i n e d as more water was absorbed, and the apparent c l u s t e r s i z e was about three water molecules near s a t u r a tion. The volume of a mixture of nylon and water i s l e s s than the sum of the volumes of the components ( 5 ) . The p a r t i a l s p e c i f i c volume of the f i r s t water to be absorbed by dry nylon i s about 1



STARKWEATHER

Water in

Figure 1.

Nylon

Sorption isotherm for water in 66 nylon

τ

1

1

at23°C

Γ

WATER MOLECULES per AMORPHOUS AMIDE

Figure 2.

Clustering of water in 66 nylon



436

WATER IN P O L Y M E R S

0.5 g/cc as shown i n F i g u r e 3. T h i s i s a t t r i b u t e d to the dis-placement of amide-amide bonds a s s o c i a t e d with h i g h l y s t r a i n e d chain conformations and poor packing. At a water c o n c e n t r a t i o n near one molecule per two amide groups i n the amorphous r e g i o n s , the p a r t i a l s p e c i f i c volume i s about 0.85 cc/g. Above 90% R.H., t h i s q u a n t i t y r i s e s again and reaches a value of 1.2 cc/g near s a t u r a t i o n . T h i s i n d i c a t e s that the l a s t water to be absorbed when there i s maximum c l u s t e r i n g i s packed much l e s s f a v o r a b l y than that absorbed e a r l i e r . Between 35 and 100% R.H., the s o r p t i o n isotherm i s c l o s e l y approximated by the Flory-Huggins equation with an i n t e r a c t i o n parameter, χ, of 1.46 ± .02. Since χ i s l a r g e l y an i n t e r n a l energy parameter, the energy term i n the Flory-Huggins equation i s approximately RTxCl-c))..) , but t h i s does not i n c l u d e the e f f e c t of changes i n volume. The p a r t i a l molar heat of s o r p t i o n at constant pressure i s (ΔΗ ) 1

ρ

= (ΔΕ )

= RTXU-c^)

1

2

γ

+

T(d?/dT) àV v

+ Τ(α/3)Δν

1

ι

(3)

(4)

where α i s the c o e f f i c i e n t of thermal expansion, β i s the com p r e s s i b i l i t y , and i s the p a r t i a l molar change of volume f o r the t o t a l system of n y l o n p l u s water. The s u b s c r i p t s , 1, r e f e r to water. I t has been found that Equation 4 agrees with e x p e r i mental data q u i t e w e l l ( 5 ) . The heat of s o r p t i o n d e c l i n e s from s e v e r a l kcal/mole a t low h u m i d i t i e s t o zero a t s a t u r a t i o n . Thus the expansion which occurs above 90% R.H. i s j u s t enough to s a t i s f y the c o n d i t i o n that Af

±

= Δ ϊ ^ = AS^ = 0

(5)

when Ρ = Ρ .

ο Viscoelastic Properties There are three major v i s c o e l a s t i c r e l a x a t i o n s i n 66 nylon (11). The α-relaxation occurs at 65°C i n the dry polymer and r e f l e c t s the motion of f a i r l y long c h a i n segments i n the amorphous r e g i o n s . Boyd (12) has estimated that these segments c o n t a i n about 15 amide groups. The β-relaxation at -50°C has been a t t r i buted to the motion of l a b i l e amide groups. I t may be absent i n very dry, annealed samples (1). The γ-relaxation at -120°C i s s i m i l a r to the γ-relaxation i n polyethylene and has been a t t r i buted to the motion of short polymethylene segments. Since i t i s d i e l e c t r i c a l l y a c t i v e (13,14) the motions must i n v o l v e some amide groups as w e l l . The temperatures of these r e l a x a t i o n s are p l o t t e d against r e l a t i v e humidity i n Figure 4 and percent water i n Figure 5. These data were obtained with the Du Pont Dynamic Mechanical


25.

Water in Nylon

STARKWEATHER

Ο

WATER MOLECULES / AMORPHOUS AMIDE 0.5 1

1.2


1.0

οο

Ο.ι

* 0

Figure 3.

2 4 6 q WATER/100g NYLON

8

Partial specific volume of water absorbed in 66 nylon

100 r

RELATIVE

Figure 4.

HUMIDITY,

%

Temperature of peaks in loss modulus vs. relative humidity


WATER IN


438

Figure 5.

POLYMERS

Temperature of peaks in loss modulus vs. present water



25.

STARKWEATHER

Water in Nylon

439

A n a l y z e r on a s a m p l e o f r o l l - o r i e n t e d n y l o n t a p e . The t e m p e r a t u r e o f t h e α - r e l a x a t i o n d e c l i n e s by a b o u t 9 0 ° C between d r y n e s s and s a t u r a t i o n ( 1 , 1 5 , 1 6 ) . The v a r i a t i o n a p p e a r s t o be l i n e a r w i t h r e l a t i v e h u m i d i t y , i . e . , the a c t i v i t y or chemical p o t e n t i a l of the water. The r e l a x a t i o n p a s s e s room t e m p e r a t u r e n e a r 3 5 % R.H., a n d t h i s i s why t h e w a t e r c o n t e n t o f n y l o n h a s s u c h a l a r g e e f f e c t on p r o p e r t i e s o f e n g i n e e r i n g i m p o r t a n c e . As m e n t i o n e d e a r l i e r , t i m e , t e m p e r a t u r e , and h u m i d i t y c a n be t r e a t e d a s c o m p l e m e n t a r y v a r i a b l e s . (_2.-A) The a c t i v a t i o n e n e r g y f o r t h e α - r e l a x a t i o n d e c r e a ses f r o m 46 k c a l / m o l e i n t h e d r y p o l y m e r t o 18 k c a l / m o l e a t 8.7% w a t e r w i t h m o s t o f t h e d e c r e a s e c o m i n g b e l o w 0.88% w a t e r . ( 1 2 ) The i n c r e a s e i n t h e d i e l e c t r i c constant a s s o c i a t e d with the α-relaxation i s g r e a t e r when w a t e r i s p r e s e n t , T h i s i n d i c a t e s that water mole c u l e s a r e bonded t o t h e amide g r o u p s and p a r t i c i p a t e i n the m o t i o n o f c h a i n s e g m e n t s . The t e m p e r a t u r e o f t h e 3 - r e l a x a t i o n d e c r e a s e s l i n e a r l y w i t h the c o n c e n t r a t i o n of water, not the r e l a t i v e humidity. The h e i g h t o f a dynamic m e c h a n i c a l l o s s peak h a s a maximum v a l u e a t one w a t e r p e r two a m i d e g r o u p s i n t h e a m o r p h o u s r e g i o n s . (.17) The t e m p e r a t u r e o f t h e γ-relaxation d e c r e a s e s s l i g h t l y when t h e f i r s t w a t e r i s added and r e m a i n s a l m o s t i n d e p e n d e n t o f w a t e r c o n t e n t . The h e i g h t o f t h e l o s s p e a k d e c r e a s e s w i t h i n c r e a s i n g w a t e r , e s p e c i a l l y b e l o w a c o n c e n t r a t i o n o f one m o l e c u l e per two a m i d e g r o u p s i n t h e a m o r p h o u s r e g i o n s . (13,15, 11)

At t e m p e r a t u r e s b e l o w t h e α - r e l a x a t i o n b u t a b o v e the γ-relaxation, t h e m o d u l u s i s i n c r e a s e d by t h e p r e s e n c e o f w a t e r . (A>JJL*18.) This i s part of a fami l i a r p a t t e r n of a n t i p l a s t i c i z a t i o n . The t e m p e r a t u r e of a p r i m a r y r e l a x a t i o n i s r e d u c e d , t h e s t r e n g t h o f a s e c o n d a r y r e l a x a t i o n i s r e d u c e d , and t h e m o d u l u s b e t w e e n them i s i n c r e a s e d . Conclusions T h e r e i s a good d e a l o f e v i d e n c e t o s u p p o r t t h e s u g g e s t i o n o f P u f f r and Sebenda Ç10) t h a t t h e f i r s t t o be a b s o r b e d a n d most t i g h t l y b o u n d w a t e r i s h y d r o g e n b o n d e d t o t h e o x y g e n a t o m s o f two a m i d e g r o u p s i n t h e amorphous r e g i o n s . At t h i s point c l u s t e r i n g begins, but a l l o f t h e water i s bonded t o t h e amide g r o u p s , and t h e r e i s no e v i d e n c e f o r f r e e z a b l e l i q u i d w a t e r . At l o w t e m p e r a t u r e s , w a t e r f o r m s m e c h a n i c a l l y s t a b l e b r i d g e s between amide g r o u p s . This i s reflected i n a p a r t i a l s u p p r e s s i o n of the γ-relaxation and an i n c r e a s e i n t h e modulus between t h e γ - and α - r e l a x a -


440

WATER IN P O L Y M E R S

tions. At h i g h e r temperatures, water facilitates m o t i o n i n t h e amorphous r e g i o n s and s h i f t s t h e a r e l a x a t i o n p r o g r e s s i v e l y to lower temperatures. This i s accompanied by m a j o r i n c r e a s e s i n d u c t i l i t y and toughness.


Literature 1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Cited

Starkweather, H. W., Chapter 9 in "Nylon Plastics" Kohan, M. I., Ed.; Wiley: New York, 1973, Quistwater, J. M. R. and Dunell, Β. Α., J. Polym. Sci., 1958, 28, 309. Quistwater, J. M. R. and Dunell, Β. Α., J. Appl. Polym. Sci., 1959, 1, 267. Onogi, S., Sasaguri, K., Adachi, T. and Ogihara, S., J. Polym. Sci., 1962, 58, 1. Starkweather, H. W., J. Appl. Polym. Sci., 1959, 2, 129. Bull, H. B., J. Am. Chem. Soc., 1944, 66, 1499. Zimm, B. H., J. Chem. Phys., 1953, 21, 934. Starkweather, H. W., Chapter 3 in "StructureSolubility Relationships in Polymers", Harris, F. W. and Seymour, R. B., Ed. Academic Press: New York, 1977. Starkweather, H. W., Macr omolecules, 1975, 8, 476. Puffr, R. and Sebenda, J., J. Polym, Sci., Part C, 1967, 16, 79. Schmieder, Κ. and Wolf, K., Kolloid Ζ., 1953, 134, 149. Boyd, R. H., J. Chem. Phys, 1959, 30, 1276. Dahl, W. V. and Muller, F. Η., Z. Elektrochem., 1961, 65, 652. Curtis, A. J., J. Res. NBS, 1961, 65A, 185. Woodward, A. E., Crissman, J. M., and Sauer, J. A. J. Polym. Sci., 1960, 44, 23. Prevorsek, D. C., Butler, R. H., and Reimschussel, Η. Κ., J. Polym. Sci., 1971, 9, 867. Kolarik, J. and Janacek, J., J. Polym. Sci., Part C, 1967, 16, 441. Starkweather, H. W., J. Macromol. Sci. Phys., 1969 B3(4), 727.

RECEIVED January 17, 1980.


Water in Nylon - American Chemical Society

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