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J OURNA L OF MAT E RI AL S SCIE NCE 22 1987) 1715-1723
t ress c rack ing o f ny lon po lymers in aqueous
sa l t so lut ions
Part N y l o n s a l t in t e ra c t i o n s
M I C H A E L G . W Y Z G O S K I , G L E N E. N O V A K
Polymers Department , Genera l Mo tors Research Laborator ies, Warren, M ich ig an 4 80 90 -90 55,
USA
A previous investigation J. Mater. ScL 1 987 ) 1707 ) has sho wn that nylon polymers are
suscep t ible to st ress cracking by aqueous calcium chlor ide at sl igh t ly elevated temperatures
50 C).. in the present study, the ny lon -sa l t interactions were investigated using thin f i lms of
both nylon 6 and nylo n 6,6. Chemical, thermal, infrared and dyn am ic mechanical ana lyses
we re performed along wi th tensi le prop erty determinations. W eigh t gain measurements indi-
cated that above 50C large increases in sorption occurred for f i lms immersed in either
aqueous c alcium , l ithium , or magne sium chlorides. Both salt ions and wate r we re absorbed at
the elevated temperature. In con trast to these results, the data for sodiu m c hlorid e indica ted
that only water was absorbed at al l temperatures. The sorpt ion process was shown to be
physical ly reversible since the absorbed salt could be removed by immersion in pure water.
Mo reover the sorpt ion of the sal t solut ion at 100 C did n ot p roduce chem ical degradation or
signi f icant crystal l izat ion of the nylons. At higher temperatures the absorbed sal t did modi fy
the recrystall izat ion beh aviou r and increas e the Tg of the n ylons. S ignif ica nt red uction s in ten -
si le modulus were observed for nylon f i lms equi l ibrated in the sal t solut ions and tensi le elon-
gations at rupture were increased. The effect of increasing temperature and salt type on the
sorp tion process paral lels the prev ious ly observed stress-cracking ac tivi ty of the aqueous salt
solut ions . I t is therefore su ggested that a plas t icizat io n-l ike mechan ism is responsible for the
sal t - induced crazing of the crystal l ine nylons.
1. n t r o d u c t i o n
Bo th o r g an ic l i qu id s an d in o r g an ic sa l t so lu tio n s can
cau se en v i r o n m en ta l s t r e s s c r ack in g o f n y lo n p o ly -
mers [1, 2] . Results discussed in a previous paper [3]
d em o n s t r a t ed th e r o l e o f p o ly m er - l i q u id so lu b il i ty i n
the s t ress c rack ing of ny lon by organ ic so lven ts .
Ra pid ly so rbed l iqu ids were observed to p las t ic ize the
sur face , a l lowing homogeneous s t ress re laxa t ion to
o ccu r i n co m p e t i t i o n w i th c r aze g ro wth . T o m in im ize
th is su r face p las t ic iza t ion ef fec t a su bsequ ent s tud y of
n y lo n c r ack in g in aq u eo u s sa l t so lu tio n s em p lo y ed a
f r ac tu r e m ech an ic s ap p r o ac h [ 4] . W i th t h i s t e ch n iq u e
th e f a i l u r e ch a r ac t e r i s t i c s o f r a zo r - cu t p r ec r ack ed
sp ec im en s o f n y lo n p o ly m er s we r e ex am in ed . P r e -
l im in a r y r e su lt s i n d i ca t ed th a t sa l t - in d u ced c r ack in g
o ccu r r ed o n ly a t e l ev a t ed t em p e r a tu r e s ( ab o v e 5 0 C).
Mo r eo v e r , t h e t o t a l f a i l u r e t im e was d e t e r m in ed
la r g e ly b y th e c r aze g r o wth k in e t i c s . Cr ack g r o wth
o ccu r r ed r ap id ly o n ly a f t e r a c r azed r eg io n h ad
p r o p ag a ted ac r o ss n ea r ly t h e en t i r e sam p le c r o ss -
sec t ion . In add i t ion , the p rev ious resu l t s showed tha t
n y lo n 6 was m o r e su scept ibl e t o c r ack in g th an n y lo n
6 ,6 , whi le ny lon 11 was essen t ia l ly immune to s t ress
crack ing . T hou gh d i f fe rences in suscep t ib i l i ty fo r the
v a r io u s p o ly m er s can b e a t t r i b u t ed to d i ff e r en ces i n
c r y s t a ll i n it y an d /o r ch em ica l s t r u c tu r e ( am id e g r o u p
co n cen t r a t i o n ) , t h e t em p e r a tu r e d ep en d en ce o f t h e
0022-2461/87 03.00 + .12 1987 Chapman and Hall Ltd
s t r e ss - c r ack in g p h en o m en o n i s n o t r e ad i ly ex p la in ed
by these fac to rs .
T h e p r e sen t s tu d y f u r th e r ad d r e sse s t h e m ech a n i sm
o f sa l t - in d u ced c r ack in g o f n y lo n p o ly m er s b y ex am in -
in g th e so r p t io n o f sa lt so lu t io n in n y lo n 6 co m p ar e d
wi th n y lo n 6 , 6 an d b y d e f in in g th e e f fec t o f ab so r b ed
sa l t o n th e s t r u c tu r e an d p r o p e r t i e s o f t h e se p o ly m er s .
2 . E x p e r i m e n t a l p r o c e d u r e
2 . 1 . M a t e r i a l s
E x t r u d ed f ilm s o f n y lo n 6 ,6 an d n y lo n 6 w e r e p r o v id ed
b y E . I . d u Po n t , W i lm in g to n , De lawar e , USA ( Z y te l
2 7 8 1 ) an d Al l i ed , Mo r r i s to wn , New Je r sey , USA
(Capron ER20) , respec t ive ly . The f i lm th icknesses
were 0 .0027 cm for ny lon 6 , 6 and 0 .0034 cm fo r ny lo n
6 . Sa tu r a t ed aq u eo u s so lu t io n s o f so d iu m , ca l c iu m ,
l i t h iu m , an d m ag n es iu m ch lo r id e s we r e p r ep a r ed a t
r o o m t em p e r a tu r e . T h e l a t t e r was i n t h e f o r m o f a
hyd ra te , MgC12 - 6H 20. An aly t ica l reagen t g rade sa l t s
we r e u sed. No a t t em p t was m a d e to co m p en sa t e fo r t h e
increased so lub i l i ty o f the sa l t a t h igher tem pera tures .
2 2
W e i g h t g a i n s
W eig h t g a in s we r e m easu r ed u s in g th e t h in f i lm s to
assess the percen tage and ra te o f absorp t ion in var ious
inorgan ic sa l t so lu t ions a t 50 , 75 and 100C. Fi lms
wer e d i e - cu t t o g iv e a 2 .5 c m x 2 .5 cm sam p le . Up o n
1715
7/23/2019 5_Stress Cracking of Nylon Polymers in Aqueous Salt Solutions
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r e m o v a l f r o m t h e s a lt s o l u t i o n s a m p l e s w e r e d i p p e d i n
w a t e r t o r e m o v e e x c e s s s a lt o n t h e f il m s u r f a c e , b l o t t e d
w i t h t i s s u e s , a n d w e i g h e d . P r e l i m i n a r y e x p e r i m e n t s
h a d s h o w n t h a t t h e b r i e f i m m e r s i o n i n w a t e r w a s
n e c e s s a r y t o g i v e r e p r o d u c i b l e r e s u l t s . I n a d d i t i o n ,
s e l ec t e d s a m p l e s w e r e i m m e r s e d i n w a t e r a t 1 0 0 C t o
d e s o r b t h e s a l t s o l u t i o n i n o r d e r t o a s s e s s t h e a m o u n t
o f e x t r a c t a b l e m a t e r i a l i n t h e f il m s . T h e f il m w e i g h ts
w e r e u s e d a s a n i n d i c a t i o n o f t h e c o m p l e t e n e s s o f t h e
d e s o r p t i o n p r o c e s s a n d t h e r e t a i n e d w a t e r w a s s u b s e -
q u e n t l y r e m o v e d b y d r y i n g a t 1 0 0 C i n a v a c u u m
o v e n .
2 . 3 . C h e m i c a l a n a l y s i s
N y l o n f i l m s a m p l e s w h i c h w e r e e q u i l i b r a t e d i n s a l t
s o l u t i o n s a t v a r i o u s t e m p e r a t u r e s w e r e a l s o a n a l y s e d
f o r t h e i r c a t i o n c o n t e n t . T h e f i l m s r e m a i n e d i n t h e
h e a t e d s a l t s o l u ti o n s u n t i l j u s t p r i o r t o t h e a n a l y s i s t o
p r e v e n t d e s o r p t i o n a t r o o m t e m p e r a t u r e . T h e t e c h-
n i q u e i n v o l v e d d i s s o l v i n g t h e s a m p l e i n h y d r o c h l o r i c
a c i d a n d s u b s e q u e n t l y d e t e r m i n i n g t h e c a l c i u m , l i t h -
i u m , s o d i u m , o r m a g n e s i u m c o n c e n t r a t i o n w i t h a
M o d e l 4 6 A t o m i c A b s o r p t i o n S p e c t r o p h o t o m e t e r
( P e r k i n -E l m e r , N o r w a l k , C o n n e c t i c u t , U S A ) . B a s e d
u p o n t h e m e a s u r e d i o n c o n t e n t a n d t h e t o t a l w e i g h t
g a i n , t h e p a r t i t i o n i n g o f t h e s a l t a n d w a t e r i n t h e f il m s
w a s c a l c u l a t e d .
2 4 Molecu lar we ight determinat ions
G e l p e r m e a t io n c h r o m a t o g r a p h y ( G P C ) c u r v e s o f th e
n y l o n s w e r e o b t a i n e d f r o m s a m p l e s d i s s o lv e d i n f r e sh l y
d i s ti l le d m - c r e s o l h e a t e d t o 6 0 C a t a p o l y m e r c o n -
c e n t r a t i o n o f 0 . 5 w t . T h e fi l te r e d s o l u t i o n s w e r e
i n je c te d i n t o a W a t e r s 1 5 0- C A L C / B P C C h r o m a t o -
g r a p h ( M i l l i p o r e I n c . , M i l f o r d , M a s s a c h u s e t t s ) . T h e
c o l u m n s c o n t a i n e d p o l y s t y r e n e - d i v i n y l b e n z e n e g el s
a n d w e r e p u r c h a s e d f r o m t h e S h o d e x C o r p o r a t i o n o f
J a p a n . T h e c o l u m n s w e r e h e a t e d t o 1 10 C . M o l e c u l a r
w e i g h ts o f t h e n y l o n s w e r e o b t a in e d f r o m t h e G P C
c u r v e s u s i n g a u n i v er s a l c a l i b r a ti o n c u r v e . T h e M a r k -
H o u w i n k c o e f f ic i e n ts ( K a n d a v a l u e s ) f o r t h e n y l o n s
w e r e c a lc u l a t e d u s i n g i n j e c t i o n - m o u l d e d n y l o n 6 sa m -
p l es o f k n o w n m o l e c u l a r w e i g h t s w h i c h w e r e s u p p l i e d
b y t h e A l l i e d C o r p o r a t i o n .
2 5 Infrared spectrosco py
A P e r k e n - E l m e r M o d e l 2 8 3 B i n f r a r e d s p e c t r o m e t e r
w a s u s e d t o o b t a i n t r a n s m i s s i o n i n f r a r e d s p e c t r a o f
t h i n f i lm s o f n y l o n . I n i t ia l e x p e r i m e n t s w i t h t h e
e x t r u d e d f i l m s i n d i c a t e d t h a t t h e y w e r e t o o t h i c k f o r
t r a n s m i s s i o n s p e c t r o s c o p y . T h e r e f o r e , t h i n n e r f i l m s
w e r e p r e p a r e d b y c a s t i n g f r o m f o r m i c a c i d s o l u t i o n s
( 1 0 w t ) . T h e f il m s w e r e d r i e d i n a v a c u u m o v e n a t
1 0 0 C f o r 2 h t o r e m o v e t h e s o l v e n t . C h a n g e s i n t h e
i n f r a r e d s p e c t r a w e r e r e c o r d e d f o r f i l m s b e f o r e a n d
a f t e r e q u i l i b r a t i o n i n s a t u r a t e d a q u e o u s c a l c i u m
c h l o r i d e a t 1 00 C .
2 . 6 . D i f f e r e n t i a l
scanning calor imetry
D S C )
D S C w a s e m p l o y e d t o d e t e r m i n e t h e e f f e ct o f t h e
a b s o r b e d c a l c i u m c h l o r i d e s o l u t i o n o n t h e T g a n d t h e
m e l t in g b e h a v i o u r o f t h e n y l o n . F o r t h is p u r p o s e s c an s
w e r e m a d e a t 2 0 C m i n i u s i n g a s a m p l e s i ze o f 4 t o
1716
5 m g . A f t e r t h e f i r s t h e a t i n g t o a b o v e t h e m e l t i n g
t e m p e r a t u r e , a r e p e a t e d s c a n w a s m a d e . S i n c e m o s t o f
t h e w a t e r i s d r iv e n o f f d u r i n g t h e f ir s t h e a ti n g , t h e
s e c o n d D S C t r a c e p r e s u m a b l y i n d i c a t e s th e e f fe c t o f
a b s o r b e d c a l c i u m c h l o r i d e a l o n e , w h e r e a s t h e f i r s t
s c a n i n d i ca t e s t h e c o m b i n e d e f f ec t o f a b s o r b e d s a l t
a n d w a t e r .
T h e D S C w a s a l so u s e d t o d e t e r m i n e t h e p e r c en t a g e
c r y s t a l li n i t y o f n y l o n 6 a n d n y l o n 6 ,6 . F o r t h e s e
m e a s u r e m e n t s 1 0 m g s a m p l e s w e r e u s e d w i t h t h e D S C
i n t h e t i m e - b a s e m o d e . M e l t i n g p e a k a r e a s w e r e
m e a s u r e d w i t h a p l a n i m e t e r a n d t h e c o r r e s p o n d i n g
p e r c e n t a g e c r y s t a l li n i t y w a s b a s e d u p o n h e a t s o f
f u s i o n o f 1 9 1 a n d 1 9 6 J g -1 f o r n y l o n 6 a n d n y l o n 6 , 6 ,
r e s p e c ti v e ly . T h e s e v a l u e s a r e t h e a v e r a g e s o f s e v e r a l
r e p o r t e d v a l u e s f o r e a c h p o l y m e r [ 5 ] .
2 .7 . D y n a m i c m e c h a n i c a l m e a s u r e m e n t s
T h e T g w a s d i f f ic u lt t o d e f i n e f r o m t h e D S C t r a ce s . T o
m o r e c l e a r l y d e fi n e t h e e f f e ct o f t h e a b s o r b e d s o l-
u t i o n o n t h e T g , d y n a m i c m e c h a n i c a l p r o p e r t i e s w e r e
m e a s u r e d u s i n g t h e R h e o v i b r o n D D V - I I - C ( T o y o
B a l d w i n C o . L t d . ) . E x t r u d e d f i lm s a m p l e s ( 0 .5 c m x
3 .0 c m ) w e r e r u n a t a h e a t i n g r a t e o f 1 C m i n - 1 a n d a
f ix e d f r e q u e n c y o f 11 H z .
2 8 T e n s i l e p r o p e r t y measurements
D i e - c u t s p ec i m e n s ( A S T M T y p e C ) w e r e e q u i li b r a te d
a t 1 0 0 C i n s a t u r a t e d a q u e o u s c a l c i u m c h l o r i d e a n d
m a i n t a i n e d a t t h is t e m p e r a t u r e i n s o l u t i o n u n t i l j u s t
p r i o r t o t e st in g . U p o n r e m o v a l f r o m t h e s o l u t i o n s , th e
f il m s w e r e d i p p e d i n w a t e r t o r e m o v e e x c e s s s a lt o n t h e
s u r f a c e , b l o t t e d w i t h t i s s u e s a n d m o u n t e d i n t h e
I n s t r o n t e s te r . D u r i n g t h e f e w m i n u t e s r e q u i r e d f o r
t h i s p r o c e d u r e , t h e f i l m t e m p e r a t u r e h a d d e c r e a s e d t o
r o o m t e m p e r a t u r e . E l o n g a t i o n w a s c a l c u l a t e d b a s e d
u p o n t h e i n i t i a l g r i p s e p a r a t i o n . T h e c r o s s h e a d s p e e d
w a s 5 m m m i n - 1 .
3 . R e s u l t s
3 1 W eight gains
F i g s 1 a n d 2 s h o w t h e e f f e c t o f t e m p e r a t u r e a n d t i m e
o n t h e w e i g h t g a i n s f o r n y l o n 6 a n d n y l o n 6 , 6 t h i n
f il m s i m m e r s e d i n s a t u r a t e d a q u e o u s c a l c i u m c h l o r i d e
100 r
8
7O
t /
O ~ C ~ , | , | ~ ,
0 5 1 0 1 5 2 0 2 5 3 0
T i m e l j 2 h 1 / 2 )
0
|
3 5 4 0
Figure
Effect of temperature on the w eight gains of nylon 6 thin
films in saturated aqueou s calcium chloride: (zx) 50 C, (O ) 75 C,
(o) 100 C . F or com parison the equilibrium level for nylon 6 in
water at 100C is shown by the dashed line.
7/23/2019 5_Stress Cracking of Nylon Polymers in Aqueous Salt Solutions
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3 5
3 t ~ 0 0 n u
2 5
w
c
~ 2 0
{ 5
1 0 0
5
t _ _
I I
0 5 1 0 1 5 2 0 2 5 3 0
T i m e 1 / 2 h 1 1 2 )
Figure
Effectof temperature on the weightgains of nylon 6,6 thin
films in saturated aqueo us calcium chloride: (A) 50 C, (O) 75 C,
(o) 100 C. F or co mparison the equilibrium evel for ny lon 6,6 in
wa ter at 100 C is show n by the dash ed line.
s o l u t io n s . C o m p a r i n g t h e r e su l t s fo r t h e t w o p o l y m e r s
i t i s n o t e d t h a t r e l a t i v e ly l o w w e ig h t g a in s a r e o b se r v e d
a t 5 0 C , t h o u g h t h e n y l o n 6 a b s o r b s m o r e t h a n t h e
n y lo n 6 , 6 . A t h ig h e r t e m p e r a tu r e s s i g n i f i c a n t i n c r e ase s
a r e o b s e r v e d w i t h e q u i l i b r i u m v a l u e s b e i n g g r e a t e r
t h a n t h e w e i g h t g a i n s o b s e rv e d i n p u r e w a t e r ( d a s h e d
l in e s i n F ig s 1 a n d 2 ). F o r n y lo n 6 , 6 t h e e q u i l i b r iu m
w e i g h t g a i n i n c re a s e s s y s t e m a t i c a l l y a s t e m p e r a t u r e
in c r e ase s . H o w e v e r , i n t h e c a se o f n y lo n 6 a g r e a t e r
w e i g h t g a i n o c c u r s a t 7 5 C t h a n a t 1 00 C .
R e p e a t e d m e a s u r e m e n t s u s i n g m u l t i p l e s a m p l e s
d e m o n s t r a t e d t h a t t h e r e p r o d u c i b i l i t y o f t h e e q u i l i -
b r i u m w e i g h t g a i n s w a s p o o r . F o r e x a m p l e , a t 7 5 C
n y l o n 6 f i l m s g a v e m a x i m u m w e i g h t g a i n s r a n g i n g
f r o m 6 5 t o 1 1 0 % . T h i s s c a t t e r i s a t t r i b u t e d t o t h e
d i s t o r t i o n o f t h e fi lm s w h e n h i g h l y s w o l le n a n d t o t h e
a s so c i a t e d d i f f ic u l t ie s i n r i n s in g a n d b lo t t i n g t h e su r -
f a c e s i n a r e p r o d u c ib l e ma n n e r . G e n e r a l l y l e s s s c a t t e r
w a s n o t e d ( a n d l e ss d i s t o r ti o n ) f o r n y l o n 6 , 6. D u e t o
th e i n a c c u r a c y o f t h e se d a t a i t i s n o t p o s s ib l e t o
c h a r a c t e r i z e t h e so r p t i o n k in e t i c s ( F i c k i a n a s a g a in s t
C a se I I ) o r t o a c c u r a t e ly a s se s s d i ff e r e n ce s i n r a t e f o r
t h e n y l o n 6 c o m p a r e d w i t h n y l o n 6 , 6 . H o w e v e r , i n
sp i t e o f t h e i n a c c u r a c y i t is p o s s ib l e t o c o n c lu d e f r o m
t h e s e r e s u lt s t h a t n y l o n 6 a b s o r b s g r e a t e r a m o u n t s o f
t h e c a l c i u m c h l o r i d e s o l u t i o n a t a l l t e m p e r a t u r e s c o m -
p a r e d t o t h e n y lo n 6 ,6 . Mo r e o v e r , t h e e f f e c t o f t h e
in c r e a s in g t e mp e r a tu r e o n so r p t i o n p a r a l l e l s t h e e f f e c t
o f i n c re a s i n g t e m p e r a t u r e o n s tr es s c r a c k i n g o f b o t h
p o ly me r s i n t h e s a l t so lu t i o n [ 4 ] .
A
0 3
9
8
7
6
4
3
2
1
0 1 2 3 4
H 2 0
Na CI (50 s a t u r a t i o n )
N a C I s a t u r a t e d )
I I I I I I I I I
5 6 7 8 9 10 11 12 13
T i m e 1 /a h l / 2 )
Figure
Weight gains for nylon 6 6 thin films in water or in aque ous
sodium chloride solutions at 100 C.
TABLE I Equilibr iumweight gains for nyl on 6,6
A q u e o u s C o n c e n t r a t i o n T e m p e r a t u r e W e i g h t ain
solution (M) ( C) (%)
NaC1 6.1" 100 4.6
CaC12 6.7* 100 28.5
LiC1 15.0" 100 25.9
MgC12 5.7* 100 23.8
CaC12 5.7 100 23.0
LiC1 5.7 100 8.7
CaCI2 3.3 100 12.4
CaC12 6.7* 85 17.4
LiC1 15.0* 85 14.3
MgC12 5.7* 85 22.6
*Saturated solutions.
T h e w e i g h t g a i n s f o r n y l o n 6 , 6 f i l m s i m m e r s e d i n
w a t e r o r a q u e o u s s o d i u m c h l o r i d e s o l u ti o n s a re s h o w n
in F ig . 3 . A s t h e c o n c e n t r a t i o n o f s a l t i n c r e a se s t h e
m a x i m u m w e i g h t g a i n d e c r e a s e s . T h u s t h e s o d i u m
c h lo r id e a c t s a s a d i l u e n t l o w e r in g t h e c o n c e n t r a t i o n
( a n d t h u s t h e a c t i v i t y ) o f t h e w a te r w i th a r e su l t a n t
d e c r e a se i n t h e e q u i l i b r iu m so r p t i o n l e v el o f w a te r .
T h i s b e h a v io u r d i f fe r s m a r k e d ly f r o m th a t o b se r v e d f b r
n y l o n 6 , 6 f i lms i n t h e c a l c iu m c h lo r id e ( F ig . 2 ) .
R e s u l t s f o r n y l o n 6 , 6 i n l i t h i u m a n d m a g n e s i u m
c h lo r id e so lu t i o n s w e r e s imi l a r t o t h a t i n t h e c a l c iu m
c h l o r i d e a s s h o w n i n T a b l e I . F o r t h e s e s a t u r a t e d
s o l u t i o n s a t 1 0 0 C t h e a m o u n t o f a b s o r b e d s a l t a n d
w a t e r w a s g r e a t e r t h a n 2 0 % i n a ll ca se s . T h i s d e m o n -
s t r a t e s t h a t a l l t h r e e o f t h e se s a l t so lu t i o n s i n t e r a c t
s t r o n g l y w i t h t h e n y l o n i n c o n t r a s t t o t h e r e su l t s f o r
s o d i u m c h l o ri d e .
S o l u t i o n s h a v i n g e q u i m o l a r c o n c e n t r a t i o n s w e r e
a l so i n v e s t i g a t e d b y d i l u t i n g t h e c a lc i u m a n d l i t h iu m
c h lo r id e t o g iv e 5 .7 M so lu t i o n s , t h e s a m e a s t h e s a tu -
r a t e d m a g n e s i u m c h l o ri d e . W e i g h t g a i n s f o r n y l o n 6 ,6
e q u i l i b r a t e d i n t h e se so lu t i o n s a r e a l so sh o w n in
T a b le I . T h e r e i s a s i g n i f i c a n t r e d u c t io n i n t h e w e ig h t
g a i n i n t h e d i l u t e d l it h i u m c h l o r i d e c o m p a r e d t o t h e
s a t u r a t e d s o l u t i o n .
D a t a f o r n y l o n 6 , 6 i n a c a l c i u m c h l o r i d e s o l u t i o n
d i lu t e d t o 5 0 % o f t h e s a tu r a t i o n ( 3.3 M) is a l so sh o w n
in T a b le I . T h e r e su l t s su g g e s t t h a t t h e e q u i l i b r iu m
w e i g h t g a in i s d i r e ct l y p r o p o r t i o n a l t o t h e a m o u n t o f
c a l c iu m c h lo r id e i n t h e so lu t i o n . F o r e x a mp le , i n 6 . 7 M
c a lc iu m c h lo r id e a w e ig h t g a in o f 2 8 . 5 % w a s o b se r v e d ,
w h i l e in t h e 3 . 3 M so lu t i o n t h e w e ig h t g a in w a s 1 2 . 4 % .
F o r d i r e c t c o m p a r i s o n w i t h c r a z e g r o w t h k i n e t i c s
d a t a ( t o b e r e p o r t e d [ 6 ] ) , w e ig h t g a in s w e r e a so
m e a s u r e d a t 8 5 C . C o m p a r e d t o t h e h i g h e r t e m -
p e r a tu r e , t h e e q u i l i b r iu m w e ig h t g a in s d e c r e a se d , w i th
m a g n e s i u m c h l o r i d e b e i n g t h e m o s t s o l u b l e f o l l o w e d
b y c a l c iu m a n d l i t h iu m c h lo r id e s . T h e p r e c i s io n i n t h e
d a t a i s i n su f f i ci e n t t o d e t e r m in e w h e th e r t h e o r d e r o f
so lu b i l i t y a t 1 0 0 C i s re v e r se d c o m p a r e d t o 8 5 C .
D a ta f o r n y lo n 6 f i lms a r e l i s t e d i n T a b l e I I . T h e
r e su l t s i n d i c a t e t h a t i n t h e 5 0 % sa tu r a t e d ( 3 .3 M) c al -
c i u m c h l o r i d e t h e n y l o n 6 s h o w e d a s i m i l a r w e i g h t
g a i n t o t h e n y l o n 6 ,6 ( 12 . 9 % c o m p a r e d w i t h 1 2 . 4 % ,
r e sp e c ti v e ly ). A l so i n t h e e q u imo la r ( 5 .7 M) s o lu t i o n s
c o n s i d e r a b l y le ss a b s o r p t i o n o c c u r r e d i n t h e l i t h i u m
c h lo r id e . A t
55
C , f o r t h e s a tu r a t e d so lu t i o n s , a s i g -
n i f i ca n t l y h i g h e r s o r p t i o n o f a q u e o u s m a g n e s i u m
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T A B L E I I E q u i li b ri u m w ei g ht g a in s f o r n y l on 6
A q u e o u s C o n c e n t r a t i o n T e m p e r a t u r e W e i g h t g a i n
so lu t ion (M) ( C ) (%)
CaC12 6.7* i00 44.8
CaC12 3.3 100 12.9
CaC12 5.7 100 25.0
LiC1 5.7 100 8.5
MgC12 5.7* 100 26.6
CaC l 2 6.7* 55 1.6
LiC l 15.0 55 1.3
M g C l2 5.7* 55 13.6
*Sa tu ra ted so lu t ions .
c h l o r i d e w a s o b s e r v e d . R e s u l t s a t 5 5 C f o r t h e o t h e r
t w o s a l t s o l u ti o n s w e r e l o w e r t h a n e x p e c t e d . Re p e a t e d
me a s u r e me n t s i n d i c a t e d c o n s i d e r a b l e s c a t t e r i n r e s u lt s
f o r c a l c i u m a n d l i t h i u m c h l o r i d e s t h o u g h ma x i mu m
w e i g h t g a i n s w e r e al w a y s b e l o w 1 0 . I n ma g n e s i u m
chlo r ide the weigh t ga ins were cons i s t en t ly abo ve 10
t h o u g h t h e r a t e o f s o r p t i o n w a s n o t n e c e s s a ri l y mo r e
rap id than fo r the o ther sa l t s . Th i s wi l l be d i scussed
fu r ther in con junc t ion wi th c raze g rowth k ine t i cs [6 ] .
3 2 C h e m i c a l a n a l y s is
T o m o r e c l e a r l y de f in e th e c o m p o s i t i o n o f th e s o r b e d
so lu t ion , t he eqm fib ra t ed f i lms were ana lys ed fo r the i r
s o d i u m, c a l c i u m, l i t h i u m, o r ma g n e s i u m c o n t e n t .
W e i g h t g a i ns w e r e r e c o r d e d f o r e a c h f i lm s a mp l e u s e d
i n t h e a n a l y s i s w i t h i mme r s i o n t i me s b a s e d o n t h e
r e s u lt s in F i g s 1 a n d 2 . F r o m t h e c a t i o n d e t e r mi n a t i o n
the to t a l s a l t con ten t (as suming e l ec t r i ca l neu t ra l i ty )
a n d t h e w a t e r c o n t e n t w e r e c a l c u la t e d . T a b l e s I I I a n d
I V l i st t h e me a s u r e d r e s u lt s a l o n g w i th t h e c a l c u l a te d
va lues . At 50 and 100C in ca l c ium ch lo r ide , rep l i -
c a t e e x p e r i me n t s w e r e p e r l o r me d , a n d i n t h e s e c a s e s
t h e v a l u e s g i v e n i n T a b l e I I I a r e a v e r a g e s . A s n o t e d
p r e v i o u s l y , c o n s i d e r a b l e s c a t t e r o c c u r r e d f o r n y l o n 6
equ i l ib ra t ed a t 75 o r 100 C, b u t no t fo r n y lon 6 ,6 .
Fo r t h e c a l c i u m c h l o r i d e ( T a b l e I I I ) t h e a n a l y s i s
c o n f i r ms t h a t b o t h s a l t a n d w a t e r w e r e a b s o r b e d i n t o
the ny lon a t a l l t empera tu res . A s ign i f i can t increase in
t h e a mo u n t o f t h e a b s o r b e d c a l c iu m c h l o r i d e o c c u r r e d
abo ve 50 C, para l l e l ing the increase in overa l l weigh t
g a i n f o r b o t h m a t e r i a l s . A n a p p a r e n t m a x i m u m
weigh t ga in was n o ted a t 75 C fo r the ny lo n 6 (F ig . 1 ).
O n t h e o t h e r h a n d , t h e p a r t it i o n i n g o f t h e s a l t s o l u t i o n
( Ca C1 2 / H 2 0 m o l a r r a t i o s ) i n e a c h o f th e n y l o n p o l y -
me rs ind ica tes tha t t he e f fec tive conc en t ra t ion o f sa l t
so lu t ion in the po lymer f i lms increases sys t emat i ca l ly
w i t h t e mp e r a t u r e . A m u c h g r e a t e r te mp e r a t u r e d e p e n -
d e n c e i s i n d i c a t e d f o r t h e n y l o n 6 , 6 e v e n t h o u g h t h e
weigh t ga ins a re genera l ly lower . In a l l cases , t he
concen t ra t ion in the f i lms i s l es s than the concen-
t ra t ion o f sa l t i n the sa tu ra t ed l iqu id . Fo r exam ple ,
sa tu ra t e d aqu eou s ca lc ium ch lo r ide i s 6 .7 M, which i s
s ign if i can tly abo ve the sa l t /water co ncen t ra t ions in the
nylon , even at 100 C.
Fo r n y l o n 6 t h e i n cr e a s e in t e mp e r a t u r e f r o m 5 0 t o
75 C w as seen to a f fec t t he to t a l w eigh t ga in in a m uch
mo r e d r a ma t i c ma n n e r t h a n i t a f f e c t e d t h e r e l a t i v e
a m o u n t s o f c a l c iu m c h l o r i d e a n d w a t e r . T h e r e a s o n
w h y t h e t o t a l w e i g h t g a i n d e c r e a s e d a t 1 0 0 C c o m-
p a r e d t o 7 5 C i s n o t k n o w n ; h o w e v e r , t h e d i s t o r t i o n
of the f i lms and l ack o f p rec i s ion sugges t fu r ther
s tud ies a re needed to conf i rm th i s . Compar ing these
resu l t s wi th the p re v ious s t res s - rup tu re da ta [4 ] , i t is
no ted tha t t he to t a l weigh t ga in co r re l a t es wel l wi th
s t res s -c rack ing ac t iv i ty ra ther than the sa l t concen-
t ra t ion in the ny lon .
Fo r l i th i u m a n d ma g n e s i u m c h l o r id e s o l u t io n s d a t a
w e r e t a k e n o n l y a t 1 0 0 C f o r n y l o n 6 , 6 . Re s u l t s a r e
g iven in Tab le IV . In bo th cases the ana lys i s conf i rms
tha t t he increased weigh t ga in , com pare d to pure wa ter ,
is d u e t o t h e s o r p t i o n o f b o t h s a l t a n d a d d i t i o n a l
water .
The weigh t ga in and chemica l ana lys i s resu l t s
d e m o n s t r a t e t h a t a p a r t ia l s o l v a t i o n o f th e n y l o n
mat r ix occurs a t t hese s l igh t ly e l eva ted t empera tu res
in aqueous sa l t so lu t ions . Whether the so lva t ion i s
c o n f i n e d t o t h e a mo r p h o u s ( r a t h e r t h a n c r y s t a l l i n e )
r e g io n s , a n d w h e t h e r i t in d u c e s c h e mi c a l d e g r a d a t i o n
or a t rue p las t i c i za t ion o f the ny lon (as water does ) ,
a re d i scussed in the fo l lowing sec t ions .
3 3
Molecular weight
d e t e r m i n a t i o n
T h e n u m b e r a v e r a g e ( . ~ t ) , w e i g h t a v e ra g e ( 31 w ) a n d
z - a v e r a g e ( ~ t ) mo l e c u l a r w e i g h t s o f th e e x t r u d e d
ny lon f i lms a re g iven in Tab le V . T he e f fec t o f ca l c ium
c h l o r id e e x p o s u r e o n t h e mo l e c u l a r w e i g h t w a s e x a m-
i n e d to a s s e s s w h e t h e r d e g r a d a t i o n o f th e p o l y m e r
occur red in the sa l t so lu t ion . S ince some uncer t a in ty
ex i s t ed in the in i t i a l molecu lar weigh t averages o f the
ny lon 6 f i lm, s tud ies were conducted us ing the ny lon
6 ,6 f i lm. The resu l t s i n Tab le V ind ica te tha t no
d e g r a d a t i o n o c c u r r e d a f t e r f o u r d a y s o f i mme r s i o n a t
100C fo r the ny lon 6 ,6 . Cons ider ing the re l a t ive ly
r a p i d t ime s c a le o f t h e e n v i r o n me n t a l c r a z in g a n d
crack ing [4 ] , t h i s resu l t sugges t s tha t c hem ica l a t t ack
o f t h e n y l o n b y t h e s a lt s o l u t i o n i s n o t r e s p o n s i b l e f o r
t h e s a l t - in d u c e d c r a c k in g . A f t e r 3 0 d a y s o f i mme r s i o n ,
there i s some ind ica t ion tha t t he ny lon i s degrad ing
s ince the mo lecu lar weigh t i s decreas ing . In v i ew of the
T A B L E I I I A n a l y s i s o f n y l o n fi lm s e q ui l i b ra t e d i n sa t u r a t e d a q u e o u s c al c i u m c h l o r id e
S a m p l e T e m p e r a t u r e W e i g h t
( C ) ga in
( )
C o m p o s i t i o n ( w t % )
Ca CaC12 H20
C o n c e n t r a t i o n
in f i lm
(M)
N ylo n 6 50 7.8 0.6 1.8 5.5 3.0
75 78.2 4.5 12.3 31.6 3.5
100 42.0 3.1 8.5 20.4 3.9
Ny lon 6,6 50 3.0 0.2 0.6 2.4 2.4
75 22.0 2.3 6.3 11.7 4.9
100 28.5 3.1 8.5 13.8 5.6
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6
0
t -
O
. . . . . / r y D n yl o n ----.
f
7
I [ E q u i l i b r a t e d i n
,~ aqueous C a C I
D r y n y l o n
I
I i
51 8 I I
4 0 0 0 3 5 0 0 3 0 0 0 O 0 1 6 0 0 1 4 0 0
W a v e n u m b e r o r e 1 }
Figure Infrared spectra of cast nylon 6 film before and after
equilibration in saturated aqueous calcium chlor ide at 100 C.
magnitude of the decrease, even after 30 days, it is
reasonable to presume that the previously observed
effect of the salt solution on stress-rupture behaviour
in short time exposures is primarily physical in nature
rather than an irreversible chemical degradation. Of
course, at the lower temperatures, such as 50 or 75 C,
degradation is even less likely to occur.
3.4. Infrared spectroscopy
Since nylon 6 generally absorbed larger amounts of
the salt solution, infrared studies were conducted
using this polymer. The infrared spectra of nylon 6
both before and after equilibration in saturated
aqueous calcium chloride at 100C are shown in
Fig. 4. Changes in the shape and/or location of the
absorption bands are observed at 3300, 1640 and
1550 cm- 1. These bands are at tributed to vibrations of
-N -H , -C =O , and -C -N molecules in the nylon
chain, respectively [7]. The slight shift in frequency, or
broadening, for each of these bands is in the direction
expected for a change in envi ronment of the molecules
from an unassociated state to a more associated state
[8]. Similar band-shifts are not observed for nylon 6
saturated with water, and non-polar diluents induce an
opposite shift. Since an extensive intermolecular
hydrogen bonding network is known to exist in nylon
polymers [9], the infrared results suggest that the cal-
cium chloride solution does not simply disrupt this
network, but in fact replaces it with some form of
association of the hydrated ions and amide groups in
the nylon.
The infrared spectra of the salt-equilibrated film
also shows a broad band at 3400cm -1 as a shoulder
on the sharper - N- H absorption band at 3300 cm -j .
In a previous study of a cast nylon film which con-
tained only calcium chloride, i.e. no water) an intense
broad absorption was also observed at this frequency
[10]. This was attributed to free, or non-hydrogen-
bonded, N- H groups in accordance with the results of
Trifan and Terenzi [9] for salt-free nylon. However, the
3400 cm-1 absorption reported here may also be attri-
buted to the O-H groups of the absorbed water. A
similar, though less intense, absorption was noted in
this study in the infrared spectra of a nylon 6 film
which was saturated with water. Also, vacuum drying
at 100C decreased the intensity of this band. Assum-
ing that this band at least in part is due to free N-H
groups, the existence o f such groups seemingly contra-
dicts the extensive broadening of the 3300 cm -1 band
on the lower-frequency side. That is, the absorbed
calcium chloride would appear to be causing an
increase in both free N - H and more associated N- H.
To avoid this contradiction it is tentatively suggested
tha t the 3400 cm-I band is entirely due to the large
amount of absorbed water in the nylon films used in
this study.
3 5 D i f fe ren t ia l scann ing ca lo r imet ry
The DSC trace of a dry nylon 6 film is shown in
Fig. 5. A slight endotherm occurred above 100 C,
perhaps due to release of residual moisture or other
impurity. The major transition for the polymer is the
crystalline melting endothe rm at 220 C with an indi-
cation that some recrystallization occurred above
160C exotherm just prior to melting). A repeated
run of the dry nylon after melting and quenching gave
a similar result to the initial trace.
The DSC analysis of nylon 6 after equilibration in
saturated calcium chloride at 100C is also shown in
TA B LE IV Analysis of nylon 6,6 films equilibrated in saturated aqueous salt solutions at 100C
Solution Concentration Weight Composition ( ) Concentration
(M) gain in film
( ) Cation Salt Water
M)
CaClz 6.7 28.5 3.1 8.5 13.8 5.6
LiC1 15.0 25.9 0.6 3.4 17.1 4.7
MgCI2 5.7 23.8 1.1 4.4 t4.8 3.1
NaC1 6.1 4.6 0 0 4.6 -
H20 - 8.5 0 0 8.5 -
TA BL E V Molecular weights of nylon films
Sample Descript ion M, (x 104) 2~rw (x 104) mz ( 105)
Capron ER20 Extrude d film nylon 6 4.72 ~ 60* ~ 800*
Zytel 2781 Extrude d film nylon 6,6 4.30 14.4 4.78
Zytel 2781 4 days at 100C in sat. CaC1z 4.32 14.5 4.67
Zytel 2781 30 days at 100C in sat. CaCI2 3.96 12.3 3.99
*Sample had a high molecular weight tail giving uncertain Mw and ~14 .
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e
0
W
D r y n y l o n6
S NtYuI:at: q U t ~ ~ b~ a s t ~ ~ ~ -
I I I I I I I
0 50 100 150 200 250 300 3 5 0
T e m p e r a t u r e
C)
Figure D i f f e re n t i a l s c a n n i n g c a l o r i m e t r y t r a c e s o f n y l o n 6 f il m
s h o w i n g t h e e f fe c t o f a b s o r b e d a q u e o u s c a l c i u m c h l o ri d e o n t h e
m e l t i n g b e h a v i o u r .
Fig. 5. The initial trace is complex, particularly above
150 C, where multiple melting peaks are observed.
After melting and quenching, the repeated run indi-
cates that the nylon 6 film was completely amorphous
with some indication of a broad Tg above 150 C. The
latter is 100C above the normal 50C Tg of nylon 6.
The effect of absorbed salt in the absence of water) in
preventing crystallization of nylon 6 has been
previously reported [11]. Presumably much of the
water is evaporated during the first DSC scan leaving
primarily a nylon- sal t mixture. The complex melting
behaviour observed in the initial DSC heating is
believed to be an indication that recrystallization
occurs more readily for the nylon 6 containing both
salt and water, in contrast to the effect of salt alone.
Films equilibrated at 50 and 75C also exhibit the
multiple melting peaks, and also could be quenched to
an amorphous or near-amorphous condition. How-
ever, if the equilibrated films were dried in vacuum to
partially remove water prior to DSC analysis, the
resultant DSC traces show much less intense pre-
melting phenomena.
Similar DSC results were observed for nylon 6,6
films. Fig. 6 shows the DSC scans for the control and
the salt solution-equilibra ted film. Although the nylon
6,6 melts at a higher temperature than nylon 6 265
compared wi th 220 C), the effect of absorbed salt and
.u_
0
e .
U J
D r y n y lo n 6
S e c o n d - - h e a t i n g~ ~ ~ ~ ~~ t
I I I I I
0 50 100 150 200 2 5 0
T e m p e r a t u r e
C)
f
' ~ - - - - u
I I
300 3 5 0
Figure 6
D i f f e re n t i a l s c a n n i n g c a l o r i m e t r y t r a c es o f n y l o n 6 , 6 fi l m
s h o w i n g t h e ef fe c t o f a b s o r b e d a q u e o u s c a l c i u m c h l o r i d e o n t h e
m e l t i n g b e h a v i o u r .
water is the same; namely, a complex melting and
recrystallization was noted which is absent in the dry
nylon 6,6. Again the reheat showed that the melted
and quenched sample was amorphous or melts above
325 C) with indications of a Tg near 125 C. As with
the nylon 6, it is suggested that the initial effect of the
salt and water is to promote recrystallization of the
nylon 6,6 at lower temperatures above 135 C); how-
ever, once the water has evaporated, the salt alone
inhibits the recrystallization process allowing a
quenched sample to be totally amorphous.
The DSC results do not establish whether the nylon
films are less crystalline or more crystalline after
equilibration in the saturated aqueous calcium
chloride at 100 C. To address this question, crystal-
linity measurements were made for films which were
soaked in water after prior equilibration in salt solu-
tions. The water soaking removed the absorbed cal-
cium chloride as verified by weight losses. To remove
the water, the films were dried to constant weight in
vacuum at 100 C. In the subsequent DSC measure-
ments, these films gave a single melting endotherm,
similar to unmodified dry nylon, with no evidence of
lower melting forms and with relatively small recrys-
tallization exotherms.
Crystallinity determinations are given in Table VI
along with the amount of extractable material. The
T A B L E V I P e r c e n t a g e c r y s t a l l in i t y a n d e x t r a c t a b l e s f o r e q u i l i b r a t e d n y l o n f i l m s
S a m p l e T r e a t m e n t C r y s t a l l i n it y ( ) E x t r a c t a b l e s ( )
N y l o n 6 ( fi lm ) N o n e 2 6 -
W a t e r , 5 0 C 2 8 0
W a t e r , 1 0 0 C 3 0 1 . 4
C a C l z , 7 5 C 2 8 0 . 7 , 0 . 7 *
Ca C I ~ , 100 C 33 3 . 2 , 5 . 6*
N y l o n 6 ( i n j e c t i o n - m o u l d e d ) N o n e 3 1 -
N y l o n 6 , 6 ( fi lm ) N o n e 2 9 -
W a t e r , 5 0 C 3 4 0
W a t e r , 1 0 0 C 3 3 1 . 2
C a C l z , 1 0 0 C 3 2 0 , 3 . 2 *
N y l o n 6 , 6 ( i n j e c t i o n - m o u l d e d ) N o n e 3 7 -
* R e p e a t e d m e a s u r e m e n t .
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latter refers to the amount of material extracted from
the nylon films by water or by the aqueous calcium
chloride during the weight gain experiment.
o r
example, the nylon 6 film in water at 100 C evidenced
a 1.4 loss in weight (determined after vacuum dry-
ing). Only small increases in crystall inity are observed
due to immersion in the salt solution. Similar increases
a r e
noted after immersion in water. The uncertainty in
these measurements is at least _+ 2 based upon
repeated runs of the same sample. Also, if the extracted
material is assumed to contribute to the amorphous
content initially, then extracted samples should
evidence an apparent increase in crystallinity. Based
upon these considerations, it is clear tha t only minor
changes, if any, occur in crystallini ty as a result o f the
equilibration of either type of nylon in calcium
chloride solutions, For comparison, a sample of nylon
6 annealed at 200 C in air for 24 h was determined to
have a crystallinity of 48 . Thus, a significant
increase is attainable for the nylon, but apparently the
rate of recrystallization is not significant at 100C
even for the highly swollen films.
Based upon these results, the complex melting
phenomenon in the DSC scans is attributed to
processes which occur at higher temperatures during
the heating of the samples in the DSC. At 100 C (and
below) the sorption of salt solution by the nylon is
considered to be a physically reversible process with
no large-scale changes in crystall inity resulting from
the penetration of the solution into the amorphous
regions of the nylon. It is also apparent from these
measurements that the difference in crystallinity
between nylon 6 and nylon 6,6 is not significant
enough to explain the weight-gain differences or the
corresponding susceptibility to stress cracking noted
previously [4].
3.6. Dynamic mechanical m e a s u r e m e n t s
The DSC results could not be used to clearly define the
Tg of the nylon and the effect on Tg of the absorbed salt
solution. The results did suggest that the salt alone
increases the Tg, presumably due to the stiffening
effect of the amide-salt complex formation reported
by others [11]. Water, on the other hand, is well known
to lower the nylon Tg, in the usual manner that
diluents plasticize amorphous polymers [12]. The
combined effect of absorbed salt and water has not
been reported. Rheovibron measurements were there-
fore employed to reveal the effect of the absorbed salt
solution.
Fig. 7 shows the change in loss factor, tan& with
temperature for nylon 6,6 which was previously
equilibrated at 100C in saturated aqueous calcium
chloride. Also shown is the effect of vacuum drying to
remove water from the film, thereby indicating the
influence of the calcium chloride alone. Following the
usual convention the various transitions in nylon are
labelled e, fl and 7 with the c~ transi tion being identified
with the glass transition of the nylon [13].
For the dry nylon the glass transit ion can be seen to
initiate above 50 C with the peak in tan5 at 83 C. The
absorbed calcium chloride solution dramatically shifts
the Tg peak to higher temperatures (peak in tan5 at
- - [ { a } I i I
re~ 130C
v
. . . I
100 50 0 100 150 2 0 0
Temperature C)
Figure Dynamic mechanical loss factor (tanS) against tem-
perature showing he effect of absorbed calciumchloride solution
on the transitions of the nylon6,6. (a) equilibrated n CaC12, then
vacuum-dried to remove water; (b) equilibrated at 100C in sat-
urated aqueous CaCI~; (c) dry sample.
130 C) and also causes a significant broadening of the
peak. Such a shift in T~ suggests a restricting effect of
the absorbed salt-wate r mixture on molecular motion
in the amorphous phase of the nylon. However, due to
the possible loss of water from the film during the
Rheovibron test the exact location of the Tg of the
equilibrated film is uncertain. Also the onset of Tg may
not be shifted as much as the tan5 peak.
After water is removed from the swollen nylon 6,6
film by vacuum drying, the Tg tan5 peak is shifted to
much higher temperatures with the maximum in
tan5 near 190 C. This shows the strong effect of the
salt alone in complexing with the nylon chains to
completely suppress the segmental chain motions
which normal ly would have initiated above 50 C. The
Tg increase agrees with the DSC results reported here
and by others [11]. These results suggest that the salt
solution does not act as a conventional plasticizer for
nylon; in other words, the Tg decrease expected from
the large amount of absorbed solution is offset by the
strong interactions with the salt ions.
The beta and gamma transitions are also affected by
the absorbed salt solution with the beta loss peak
especially being enhanced. Absorbed water has been
reported to have a similar effect on this transition
which is associated with carbonyl group motion [13].
Fig. 7 demonstrates that the removal of water causes
a noticeable sharpening of the beta tan5 peak while the
gamma tan5 peak is unchanged. In view of the latter
being associated with local oscillations of non-polar
methylene groups [13], this observation is not surpris-
ing and in fact confirms the infrared results in suggest-
ing an association of the salt ions with the polar
portions of the nylon.
3 . 7 . T e n si Le p r o p e r ty m e a s u r e m e n t s
Table VII lists the properties of nylon 6 and nylon 6,6
thin films before and after equilibration in saturated
calcium chloride at 100C. Modulus and strength
values were calculated using the dimensions of the d ry
films. A large decrease in modulus was observed for
both nylons as a result of the absorption of aqueous
calcium chloride. Lower strengths were also observed,
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T A B L E V I I Te ns i l e prope r t i e s of ny lon f i lm s e qui l ibra te d in s a tura te d a que o us c a lc ium c hlo r ide a t 100 C*
S a m p l e T r e a t m e n t M o d u l u s ( G P a ) T e n s i le s t r e n g t h
( M P a )
E l o n g a t i o n a t
b r e a k ( )
N y l o n 6 V a c u u m - d r i e d c o n t r o l 1 .0 6 4 2 . 4
Ca C12, 100C for 1 .5h 0 .32 24 .8
Ny lon 6 ,6 Va c u um -d r ie d c on t ro l 2 . 03 108.1
Ca CI2 , 100C for 24 h 0 .83 101.9
8.0
18.5
125
178
* M o d u l u s a n d s t r e n g t h w e r e c a l c u l a te d u s i n g d i m e n s i o n s o f d r y f il m s,
especially for the nylon 6, and elongations at break
were also increased for both polymers.
The tensile property measurements suggest that the
effect of the diffusion of salt solution into the amor-
phous regions is to weaken the nylon matrix. Since
these measurements were performed at room tem-
perature, it must also be expected that at elevated
tempera ture the effects reported here would be further
enhanced. Thus even lower moduli and loss of load-
bearing capability would be anticipated due to the
absorption of salt solution at 50 to 100 C.
The reproducibility of the tensile property measure-
ments was demonstrated by replicate experiments
with both nylon 6 and nylon 6,6. In view of the infra-
red and dynamic mechanical measurements, some stif-
fening modulus increase) had been anticipated rather
than the observed softening. A direct comparison of
these results is complicated by the uncertainty in the
amount o f water which evaporated from the samples.
Since the elapsed time period from removal of
samples from the salt solutions to tensile testing) was
considerably less than in the dynamic mechanical
measurement of Tg, the tensile properties are thought
to more nearly represent the equilibrated film con-
ditions. The relat ion of the observed changes to the Tg
of the equilibrated films will require further study.
However, the tensile property changes reported here
are representative of a plasticization-like action by the
salt solution.
4 D i s c u s s i o n
The results of this investigation have shown that those
aqueous salt solutions which were previously shown
to cause crazing of nylon are also partially soluble in
the material at elevated temperatures. This includes
calcium, lithium, and magnesium chlorides. Sodium
chloride, which showed no tendency to induce crazing,
was also not absorbed into the nylon structure. The
chemical analysis confirmed that both salt ions and
water are absorbed into the nylon with the partitioning
of the salt and water being dependent on temperature
as well as on nylon type.
The crystallinity measurements of films previously
equilibrated in salt solutions suggested that the salt is
sorbed into the amorphous regions of nylon without
inducing significant changes in crystallinity. Similarly
no degradation in molecular weight was detected as a
result of salt-solution absorption. Thus the evidence
indicates that the sorption process is physically revers-
ible and appears similar in this regard to the swelling
of amorphous polymers by organic solvents.
However, the ionic nature of the salt solution
coupled with the intermolecular hydrogen bonding o f
722
not swol le n f i lm d im e ns ions .
the nylon lead to a much more complicated polymer -
liquid interaction than the van der Waals type of
bonding in organic solvents. For example, the infrared
spectroscopy results indicated that the absorbed cal-
cium chloride formed an association with the amide
groups in the nylon. This was observed in spite of the
large amount of water in the equilibrated films.
Evidence that the association of the amide groups
with the hydrated salt ions stiffens the nylon chains
was provided by the dynamic mechanical measure-
ments which showed a shift of the glass transition to
higher temperatures. Such a shift is indicative of a
restricted mobility for the nylon polymer chains in the
amorphous phase in the presence of the aqueous salt
solution.
That absorbed salt alone restricts the amorphous
phase mobility is shown by the very high Tg for the
vacuum-dried film. DSC results also indicate an
increased Tg for the second heating of equilibrated
films. These films are also thought to contain very
little water. This confirms the observations previously
reported by others for ny lon -sa lt systems [11]. For the
film equilibrated in the aqueous solution, the loss of
water during the dynamic test leaves some doubt as to
the precise location of T~. Since absorbed water
decreases Tg while salt alone increases it, one expects
an intermediate effect of the salt solution. The fact
that salt solution induced crazing occurs at tempera-
tures just above the Tg of the dry nylon makes the
question of the Tg location of even greater interest.
For example, if absorbed salt raises the Tg, the sorp-
tion process should in turn be retarded. On the other
hand, a lowering of Tg is more compatible with sol-
vent crazing models developed for amorphous glassy
polymers. Unfortunately the available evidence does
not allow a resolution of the question o f the Tg of the
salt solution equilibrated nylon films.
The tensile property measurements, on the other
hand, indicate that the modulus is decreased as a
result of the sorption process. The inference is that the
Tg may also be reduced or at least substantially
broadened. The reduction in modulus is considered
very significant in terms o f proposing a mechanism for
salt-solution-induced crazing. It establishes a parallel
between absorbed salt solution in nylon and absorbed
organic solvents in glassy polymers, since the latter
can also reduce modulus and increase tensile elong-
ation. Thus relaxation controlled craze growth models
developed for solvent crazing of glassy polymers
[14, 15] may be equally applicable to salt solution
induced craze growth in crystalline nylon. This will be
examined in detail in the next paper in this series [6].
The results of the present study are compatible with a
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p l a s t ic i z a t io n l i k e m e c h a n i s m f o r sa l t in d u c e d c r a z i n g
s i n c e i t h a s b e e n c l e a r l y d e m o n s t r a t e d t h a t t h e s a l t
s o l u t i o n p e n e t r a t e s t h e n y l o n s t r u c t u r e w i t h o u t
d e s t r o y i n g t h e c r y s ta l li n e s t r u c tu r e a n d w i t h o u t c a u s
i n g e x t e n s i v e r e c r y s t a ll iz a t io n o r m o l e c u l a r w e i g h t
d e g r a d a t i o n .
e f e r e n c e s
I . C . D . W E I S K E , Kuns to f f e 54 (1964) 626.
2 . P . D U N N a n d G . F . S A N S O M , J Appl Poly m Sc i 13
(1969) 1641.
3 . M . G . W Y Z G O S K I , i n M a c r o m o l e c u l a r S o l u t i o n s: S o l -
v e n t P r o p e r t y R e l a t i o n s h i p s i n P o l y m e r s e d i t e d b y R . B .
S e y m o u r a n d G . A . S t a h l ( P e r g a m o n , N e w Y o r k , 1 98 2) p . 4 1 .
4 . M . G . W Y Z G O S K I a n d G . E . N O V A K ,
J Mate r Sc i
22 (1987) 1707.
5 . J . B R A N D U P a n d E. H . I M M E R G U T ( ed s) . P o l y m e r
1 4 H a n d b o o k ( 2 n d E d n ) (W i l e y , N e w Y o r k , 1 9 7 5 ) p . 1 1 1- 5.
6 . M . G . W Y Z G O S K I a n d G . E . N O V A K , i n p r e p a ra t i o n .
7 . D . O . H U M M E L , I n f r a r e d S p e c t r a o f P o l y m e r s ( I n t e r-
s c ie nc e , Ne w Y ork , 1966) p . 61.
8 . C . G . C A N N O N , Spe c t roc him Ac ta 16 (1960) 302.
9 . D . S . T R I F A N a n d J. F . T E R E N Z I ,
J Poly m Sc i 28
(1958) 443.
1 0. P . D U N N a n d G . F . S A N S O M ,
ibid
13 (1969) 1657.
1 1. H . K I M a n d P . J . H A R G E T ,
J Appl Phy s
56 (1979)
6072.
1 2. M . I . K O H A N , N y l o n P l a s t i c s ( W i l e y , N e w Y o r k , 1 9 73 )
p. 329.
1 3. N . G . M c C R U M , B . E . R E A D a n d G . W I L L I A M S ,
A n e l a s t i c a n d D i e l e c t r i c E f f e c t s i n P o l y m e r i c S o l i d s
( W i l e y , N e w Y o r k , 1 9 67 ) p . 4 8 0.
1 4. J . G . W I L L I A M S a n d G . P . M A R S H A L L , Proc R Soc
A342 (1975) 55 .
1 5. E . J . K R A M E R a n d R . A . B U B E C K , J Poly m Sc i 16
(1978) 1195.
Received 2 June
and accepted 18 August 1986
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