Water Sorption and Its Effect on a Polymer's Dielectric Behavior

27 Water Sorption and Its Effect on a Polymer's Dielectric Behavior

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 20, 2017 | http://pubs.acs.org Publication Date: August 19, 1980 | doi: 10.1021/bk-1980-0127.ch027

G. E. JOHNSON, H. E. BAIR, S. MATSUOKA, E. W. ANDERSON, and J. E. SCOTT

1

Bell Laboratories, Murray Hill, N J 07974

Recently we reported a way to measure the amount of water which has associated to form microscopic water-filled cavities (clusters) in polyethylene at a level of lOppm and greater (1,2). By combining this calorimetric technique with a coulometric method, it was possible to differentiate between clustered water and the total water sorbed by the polyethylene. It was found that clusters are formed when polyethylene is saturated with water at an elevated temperature and is rapidly cooled to room temperature. During cooling the solubility of water in polymers is lowered and some water condenses in the form of microscopic water-filled cavities, providing the internal pressure which is generated by the excess water exceeds the strength of the polymer. Figure 1 shows 2-micron clusters formed in polyethylene quenced from the melt in the presence of water. In sorption studies of polycarbonate (3) it was learned that this polymer absorbs water in two stages. In the initial period of absorption at an elevated temperature, but below T , all of the water was found in an unassociated state when cooled to room temperature. In the second stage at later times, most of the water gained by the polymer was identified in a separate liquid phase (clustered water). In addition after the polymer was saturated with water at a temperature above T and cooled, its solubility was lowered and water condensed in the form of micro scopic water filled cavities. Below Τ the clusters were formed only after the polycarbonate's strength (M =26,600) was decreased by hydrolysis whereas above T clusters were formed without degradation. The dielectric loss behavior of both polyethylene s γ-transi tion and polycarbonate's β-transition was enhanced by the presence of unassociated water. The area under the associated loss peak was found to increase in direct proportion to the concentration of unassociated water. In addition a secondary dielectric loss peak associated with frozen clustered water occurred in polycarbonate about 40°C below its β-transition. Liquid clustered water at g

g

g

w

g

1

1

Current address: Prairie View A&M, Prairie View, TX 77445 0-8412-0559-0/ 80/47-127-451 $05.00 © 1980 A m e r i c a n Chemical Society

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

WATER


452

Figure 1.

IN POLYMERS

Two-micron water clusters in polyethylene


27.

JOHNSON ET A L .

Dielectric

Behavior

453

23°C y i e l d e d a l o s s mechanism i n the mHz r e g i o n i n polyethylene and i n the kHz r e g i o n f o r polycarbonate that was i n t e r p r e t e d as a Maxwell-Wagner e f f e c t . E a r l y i n v e s t i g a t i o n s of the e f f e c t of water on the low-tem perature r e l a x a t i o n s of s e v e r a l aromatic polymers i n c l u d i n g polycarbonate, polyamides, and a polyurethane have shown s e v e r a l low-temperature anomalies (_4>JL). * the case of a water-saturated p o l y s u l f o n e polymer which e x h i b i t e d a doublet i n i t s β-loss pro cess, Jackson suggested that the secondary peak may be due to w a t e r - f i l l e d c a v i t i e s . I n t h i s work we have employed the DSC technique f o r water c l u s t e r a n a l y s i s along w i t h the t o t a l water content measurements to e l u c i d a t e the water s o r p t i o n behavior of p o l y s u l f o n e and p o l y ( v i n y l acetate) (PVAc). In a d d i t i o n DSC and d i e l e c t r i c methods were used c o o p e r a t i v e l y to understand the Tg behavior of polymers i n the presence of water.


n

Experimental Water A n a l y s i s . The water gained by a sample was measured on a duPont 26-321A moisture a n a l y z e r . T h i s instrument uses a coulometric technique to measure the t o t a l amount of water i n a sample. Samples were heated f o r 15 minutes above t h e i r Tg to d r i v e o f f the water. The determination of c l u s t e r e d water was done c a l o r i m e t r i c a l l y (1) using a d i f f e r e n t i a l scanning c a l o r i m e t e r ( P e r k i a Elmer DSC-2). Samples were placed i n t o a n i t r o g e n - f l u s h e d dry box before they entered the DSC sample h o l d e r . A l l experimental runs were made at 20°C/min. A l l reported values of water content i n t h i s paper a r e i n weight percent as determined both c a l o r i m e t r i c a l l y and c o u l o m e t r i c a l l y . When about 20 m i l l i g r a m s of sample were placed i n the DSC-2 and cooled a t 20°/min. from room temperature to -140°C the onset of c r y s t a l l i z a t i o n of the c l u s t e r e d water was normally detected near -40°C and proceeded a t a maximum c r y s t a l l i z a t i o n r a t e a t -50°C. We b e l i e v e that the l a r g e undercooling i s due to the micro s c o p i c s i z e of the c l u s t e r s and the absence of h e t e r o g e n e i t i e s i n the water. When the sample was reheated from -120°C to room temperature a f i r s t order t r a n s i t i o n was detected near 0°C as expected. D i e l e c t r i c Measurements^ D i e l e c t r i c measurements on p o l y sulfone were conducted a t 10 , 10 , and 10 Hz. The data were obtained by combining a P r i n c e t o n A p p l i e d Research 124 l o c k - i n a m p l i f i e r and a General Radio 1615A capacitance b r i d g e . The b r i d g e was connected to a Balsbaugh LD3 research c e l l i n s i d e a t e s t chamber. A f t e r the t e s t chamber was e q u i l i b r a t e d a t -160°C f o r one hour, measurements were made a t ten to twenty degree i n t e r v a l s with a f i f t e e n minute w a i t i n g p e r i o d between each d i s c r e t e change i n temperature u n t i l room temperature was reached.


454


WATER IN P O L Y M E R S

Figure 2.

DSC cooling curve for TV Ac containing 63% clustered)

total water


(2.1%

27.

JOHNSON ET AL.

Behavior

455

D i e l e c t r i c measurements on p o l y ( v i n y l acetate) were obtained u t i l i z i n g a F o u r i e r transform d i e l e c t r i c spectrometer developed i n our l a b o r a t o r y ( 6 ) . A v o l t a g e step pulse was a p p l i e d to the sample and the time dependent i n t e g r a t e d current response, Q ( t ) , was c o l l e c t e d by computer. The frequency dependent d i e l e c t r i c p r o p e r t i e s , ε and ε were then obtained from the F o u r i e r t r a n s form of the i n t e g r a t e d c u r r e n t . For t h i s study frequency depen dent d i e l e c t r i c data i n the 10 to 10 Hz range were obtained i s o t h e r m a l l y . A sample c e l l with low thermal mass and copper screening allowed r a p i d e q u i l i b r a t i o n between temperatures. T h i s allowed the frequency dependence a t s e v e r a l temperatures to be measured while minimal water was l o s t from the samples. 1


Dielectric

11

R e s u l t s and D i s c u s s i o n Determination of Unassociated and C l u s t e r e d Water. Com p r e s s i o n molded samples of p o l y s u l f o n e were immersed i n water at 100°C and below u n t i l they came t o e q u i l i b r i u m . Above 100°C an autoclave was used. As w i t h polycarbonate a m i l d temperature dependence i n e q u i l i b r i u m absorption was noted.^ The amount of unassociated water a t s a t u r a t i o n went from 0.8% a t 23°C to 1.2% at 132°C. No c l u s t e r e d water was found i n samples exposed below T (190°C). When the polymer was exposed to steam a t 208°C f o r 1 minute and quenched to 23°C, c l u s t e r formation was noted. There was, however, only 0.04% water found i n the c l u s t e r e d s t a t e . F i v e minute exposure to steam increased the c l u s t e r e d water con tent to 0.16%. M i c r o s c o p i c a n a l y s i s of cross s e c t i o n s of d i e l e c t r i c specimens showed a non-uniformity of c l u s t e r s i z e and d i s t r i b u t i o n . T h i s r e s u l t e d i n two DSC peaks (-34°C and -42°C) i n c o o l i n g the sample a t 20°C/min. and a broadened melting peak s t a r t i n g a t 0°C on heating a t the same r a t e . g

Compression molded samples of p o l y ( v i n y l acetate) a l s o showed a mild temperature dependence i n e q u i l i b r i u m a b s o r p t i o n . The amount of water went from 4% a t 23°C to 6% a t 70°C. T h i s polymer was the only one we tested that formed c l u s t e r e d water w h i l e stored i s o t h e r m a l l y a t room temperature. T h i s c l u s t e r i n g was obtained a f t e r 17h. as confirmed by DSC and could be seen v i s u a l l y as a whitening of the polymer. F i g u r e 2 shows the DSC c o o l i n g curve of a sample c o n t a i n i n g 6.3% t o t a l water, 2.1% of which was c l u s t e r e d . A t a c o o l i n g r a t e of 20°C/min. the v i t r i f i c a t i o n of the polymer was noted between 20 and 5°C. The next thermal event observed was the onset of f r e e z i n g of c l u s t e r e d water a t -5°C. The c r y s t a l l i z a t i o n process proceeded s p o r a d i c a l l y u n t i l -35°C. A t that temperature the major p o r t i o n of the c l u s t e r e d water began to f r e e z e and t h i s process was completed by -38°C. The c r y s t a l l i z a t i o n a t smaller undercooling was b e l i e v e d to be due to f r e e z i n g of l a r g e d r o p l e t s .


456

WATER IN POLYMERS


Τ

I -20

ι

I 0

ι

I

ι

20

I 4 0

ι

L 60

T, °C Figure 3.

DSC curves for PVAc as a function of total percent water content (clustered percent in parenthesis)


27.

JOHNSON ET A L .

Dielectric

Behavior

457

C a l o r i m e t r i c Tg Behavior. The comparative Cp curves versus temperature of PVAc c o n t a i n i n g 0.2, 1.8, 4.2 and 4.6 weight p e r cent water are p l o t t e d i n F i g u r e 3. These Cp curves i n a l l cases were run a f t e r c o o l i n g each sample to -70°C. The lowest curve represents a f i l m that has been vacuum d r i e d overnight at 41°C. The C increased l i n e a r l y with temperature u n t i l the g l a s s t r a n s i t i o n which was n o t e d as a d i s c o n t i n u i t y i n C bet ween 35°and 45°C. T , which i s defined i n t h i s work as the midpoint of the t r a n s i t i o n , i s 43°C. The C. curve f o r a PVAc f i l m c o n t a i n i n g 1.8% water ( a l l unclustered) has the T shifted 13°C lower to 30°C. Increasing the l e v e l to 4.2% u n c l u s t e r e d water lowered Tg to 19°C. In a l l the above cases the t r a n s i t i o n width of T was the same, 12°C. The upper curve i n F i g u r e 3 represented 4.6% t o t a l water with 0.7% of that i n the c l u s t e r e d s t a t e . I t was noted that the c l u s t e r e d water melts near 0°C followed by a T at 19°C as i n the 4.2% unclustered water sample. Thus c l u s t e r e d water has no p l a s t i c i z i n g e f f e c t on Tg. T h i s was shown d r a m a t i c a l l y i n the curves of F i g u r e 4 which show that when c l u s t e r e d water content was increased from 0.7 to 2.1% no s h i f t i n Tg behavior occurred. p


g

g

g

g

D i e l e c t r i c Behavior; The d i e l e c t r i c l o s s behavior of p o l y sulfone samples was measured below 23°C as a f u n c t i o n of unasso c i a t e d water content. The a c t i v a t i o n energy of the process was c a l c u l a t e d to be 11.4 kcal/mole (1.1% H 0) and was i n agreement with A l l e n ' s p r i o r determination ( 4 ) . The areas under the l o s s curves of F i g u r e 5 were d i r e c t l y p r o p o r t i o n a l to the amount of unassociated water present. T h i s d i e l e c t r i c l o s s i n c r e a s e of the polymer's β-mechanism can be understood i n terms of the motion of the water d i p o l e s c o r r e l a t e d according to the dynamics of the polymer molecules. A c a l c u l a t i o n of the added d i p o l a r c o n t r i b u t i o n of the water was made and found to be one quarter the v a l u e that would be obtained i f the water d i p o l e s p a r t i c i p a t e d completeiyAn enhanced d i e l e c t r i c l o s s maximum was observed at -85°C when a p o l y s u l f o n e sample which contained 0.76 wt. % unassociated water and no d e t e c t a b l e l e v e l of c l u s t e r e d water (

Water Sorption and Its Effect on a Polymer's Dielectric Behavior

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