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M e c h a n i c a l p r o p e r t ie s o f n y l o n 6 a f t e r
t r e a t m e n t w i t h m e t a l h a l i d e s
A . P. M o r e a n d A . M . D o n a l d
Cavendish Laboratory, Madingley Road, Cam bridge, CB3 0HE, UK
Received I March 1993)
Conditioning of nylon-6 by immersion in metal chloride solutions modifies he mode of deformation under
an applied tensile stress. Infra-red spectroscopy of salt-treated films shows significant modification of the
spectrum, indicating changes in the intermolecular bonding. Modulus measurements support the hypothesis
that modification of the intermolecular bonding in nylon results in some chain stiffening. This stiffening
of the network of chains reduces the mobility that is required for shear deformation. This in turn leads to
the onset of scission crazing. NaCI, however, spectroscopicallyshows no evidence for nylon-salt interactions
having occurred. The deformation behaviour of both thin-film and bulk samples of NaCl-treated nylon-6
reflect the absence of any salt-amide interaction.
Keyw ords : ny lon-6 ; sa l t trea tment; m echanica l p ropert i es )
INTRODUCTION
The mechanisms operative during deformation of
amorphous polymers are both well documented and well
understood through numerous thin-film and macro
studies (see reviews by Kramerl'2). Such models are
described within a framework of the entanglement
model T M with two possible routes envisaged for
creation of the void-fibril craze microstructure: chain
disentanglement or chain scission (breaking of the chain
backbone) 1,2.
It has been shown ~'3'5 that the process of disentangle-
ment, which involves the relative motion of one chain
past another, is favoured by high temperatures, low
molecular weights and low strain rates. On the other
hand, chain scission involves the direct breaking of
chemical bonds in the backbone, and leads to a
substantial energy penalty, which increases in magnitude
as the density of entanglement points is increasedL Thus
scission crazing is less likely to develop in highly
entangled polymer networks. The transition to a more
brittle response at high temperature, for what is
normally a tough polymer, is identified with a switch in
deformation from shear yielding to disentanglement-
mediated crazing 3. Shear is always a potent ial competing
mechanism with crazing, and the process that occurs for
the lowest stress will develop first.
The nature of crazing in semicrystalline polymers,
and the associated mechanisms, are much less well
understood than in amorphous polymers. Part of the
problem relates to the variability in microstructure in
semicrystalline polymers and the subsequent complex
interaction between morphology and deformation6.
Whilst voided craze-like microstructures are not
uncommon in semicrystalline polymers, they tend to have
a much coarser microst ructure (1000 nm fibril spacing)
than their amorphous counterparts (e.g. 20nm fibril
* To whom correspondence should be addressed
spacing for polystyrene)7,s. Possible void-generating
mechanisms for semicrystalline polymers have been
subject to review by Friedrich 7. Such models tend
to be developed at an empirical/phenomenological
level and involve a combination of interlamellar slip,
chain disentanglement, block rotation and fragmentation
processes. Such models lean towards a cold-drawing
mode of deformation involving a substantial amo unt of
shear 9-12. Such models, however, invoke a differeiat
framework from that built up for amorphous polymers.
This paper seeks to examine further the deformational
behaviour of semicrystalline polymers using nylon-6
conditioned with various salts. Previous studies of
salt-conditioned nylons have demonstrated that the mode
of deformation is significantly affected by exposure to a
variety of metal halides, with the most notable change
being from ductile to brittle failure 13'14. Embrit tlement
has been shown to relate to a switch in deformation from
one that is shear-dominated to one that is craze-
dominated ~5'16. This paper investigates the role of the
salt in influencing the deformat ion mechanisms operative,
predominantly at room temperature which is below the
Tg of the amorphous regions. Infra-red spectroscopy and
transmission electron microscopy results are presented
for thin-film samples, and correlated with modulus data
from bulk samples. Based on these results, explanations
are offered for the switch in deformation from ductile to
brittle presented in terms of the concepts previously
developed for amorphous polymers.
EXPERIMENTAL
Thin films of nylon for infra-red studies were produced
in a manner similar to that of Lauterwasser and
Kramer iv. Nylon-6 (Mw,~18 000) was supplied by
Goodfellows of Cambridge. Produced by drawing glass
slides from 3 wt% polymer solutions in a formic acid
solvent, the submicron thick films have a slightly
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Nylon 6 treatment with metal ha/ides: A. P. More and A.
varying thickness owing to the semicrystalline spherulitic
microstructure. The films were floated off the glass slide
onto a water bath, after having been conditioned by
immersion of the slide in 6 molal solutions of the
appropriat e meta l chloride (Analar grade) for 6 h at 25C.
Salt-conditioned films were picked up from the water
bath on a 3 mm copper mesh, previously both annealed
and nylon-coated. Films, when dry, were bonded to the
grid by brief exposure to solvent vapour (formic acid) for
a few seconds. This serves to relieve the stresses in the
films in addition to smoothing them out. The films are
then vacuum dried for 72 h at 80C, and stored in a
desiccator prior to i.r. examination and deformation.
Infra-red spectroscopy of both salt-treated and
untreated films was undertaken with a Mattson 4020
spectrometer equipped with a DGTS detector. Each
spectrum was obtained by averaging 200 scans collected
with a spectral resolution of 2 cm-1. Scattering losses
caused by the copper grids were compensated for
by using a similarly oriented uncoated copper grid
in the background runs. Spectra were obtained from a
circular region of the film ca. 10mm in diameter.
Because the films were typically less than 1/zm thick,
channel fringes were not observed. The spectrometer was
well purged with dry nitrogen gas to eliminate the
effects of atmospheric absorptions. The double-sided
interferograms obtained were Fourier-transformed and
background-stripped over the range 400 to 4000 cm-1.
All spectra were deconvoluted before peak posi tions were
located. The accuracy of the peak positions quoted is
abou t + 1 cm - 1.
Films for electron microscopy were prepared and
salt-conditioned as for infra-red spectroscopy. Some of
the unstrained thin films were stained with a 2%
aqueous solution of phosphotungstaic acid (PTA) to
allow differentiation of amorphous and crystalline
domains -- the PTA preferentially enters and stains the
amorphous regions. Staining was undertaken by immersion
for 30 min at room temperature. Films were strained at a
constant strain rate of -~3x10- 3s -1 in a variable-
temperature strain rig mounted on a Carl Zeiss Jenapol
optical microscope. The deformation was optically
apparent as dark lineat ions at 160 x magnification. After
deformation, individual grid squares were cut from the
copper grid for examination. This method allows
examination of the film in an as-stressed state as the
copper maintains the state of strain. Examination was
undertaken with a J EOL 2000 EX transmission electron
microscope (TEM) operating at 200 kV.
Modulus measurements were undertaken on thicker
films of nylon-6. Melt-cast films of ,-, 160/tm thickness
were conditioned by soaking in the 6 molal solutions for
24 h at room temperature. Chemical analysis of cross-
sections in a scanning electron microscope indicate
development of a metal chloride-rich outer zone and
metal chloride-poor core, unlike the micrometre-thick
films where a homogeneous salt distribution is apparent.
Soak times and immersion temperatures required for
complete homogeneous diffusion of the salt t hrough the
thicker samples results in significant damage to the nylon,
and in the case of zinc chloride-nylon-6 the partial
dissolution of the test samples. Thus studies of thick films
for which there was complete diffusion of the salts through
the films was not possible.
Tensile studies were performed with a Polymer
Laboratories Miniature Materials Tester (Minimat) at
M. Donald
room temperature with a strain rate of 1 10- 3s -1.
Averaged modulus measurements were made over the
interval 0.5 to 1.5% strain.
RESULTS
Infra-red spectroscopy of the salt-conditioned films shows
a number of differences between these and the untreated
films. Absorbance spectra for thin-film nylon-6, untreated
and zinc chloride-conditioned, are presented in Fioure 1 .
Of particular interest in the absorbance spectra of
polyamide (nylon) are the C-H stretch peaks at 2390 and
2860 cm-1, the C--O stretch (amide I) at 1635 cm-1, the
(N-H)(C-N) couple (amide II) at 1540 cm-1, and N -H
stretch centred around 3300cm -1 (see e.g. reviews by
Miyazawa s and Krimm and Bandekar19). Following
ZnCI2 treatment, variations in peak intensity and peak
position are apparent, with modification of the region
around the amide I and II bands particularly clear.
ZnC12 conditioning would appear to give rise to new
absorbance bands centred around 1601 and 1570 cm-1
and loss of intensity for the amide I peak at 1635 cm- 1.
These changes are seen more clearly in F i g u r e 2 , which
expands the region ar ound 1600 cm- 1. Also apparent are
small shifts in both the amide I peak (+3 cm -1) and
amide II peak (+8cm-1). The C-H peak centred at
2938 cm-1 increases in intensity, and shifts a few cm-1.
The N- H stretch at 3300cm -1 broadens with salt
conditioning, but no peak shifts are measured.
Modification to the amide I and amide II peaks is also
recorded for CaCI2 treatment of nylon-6 film, as seen in
F i g u r e 2 . Shoulders are observed to develop on the low
side of the amide I peak (at ~1614cm-1), and on the
high-wavenumber side of the amide II peak, in addition
to slight peak shifts of a few cm-1. NaC1 treatment,
however, appears to leave the nylon films unchanged
spectroscopically. Peak shifts and variations to peak
intensity are absent Fioure 2) .
Electron microscopy of unstrained films confirms the
semicrystalline spherulitic microstructure apparent under
the optical microscope. Spherulite sizes range from 2 to
6 gm in diameter. Electron microscopy of PTA-stained
films Figure 3) allows the identification of the domain
Z n C Z 2
P 6
3 5 3 2 5 2 1 5
W a v e n u m b e r s
Figure
1 I n f r a - r e d a b s o r b a n c e s p e c t r a o f u n t r e a t e d n y l o n - 6 ( P A 6)
a n d a q u e o u s z i n c c h l o r i d e - tr e a t e d n y l o n - 6 f i l m (Z n C I z) .
Peaks
d e v e l o p e d c o r r e s p o n d t o: t h e N - H s t r e t c h a t ~ 3 3 0 0 c m - t , a p a i r o f
C - H p e a k s a t ~ 2 3 9 0 a n d ~ 2 8 6 0 e r a - 1, t h e C = O
stretch amide
l ) a t
~ 1 6 35 c m - t a n d ( N - H X C - N ) c o u p l e ( a m i d e I I ) a t ~ 1 5 4 0 c m - 1
5 9 4
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b
b
a
n
c
e
I
I
I
I I
1 ] 0 0 1 6 0 0 1 5 0 0 1 4 0 0 1 3 0 0
N a v e n u l k l ~ r s
Figure 2 Infra-red absorbanee spectra of untreated nylon-6 (PA 6)
and NaC1, CaCI2 and ZnCi2 solution -treated nylon-6 films. Peak s a t
~ 1635 and ~ 1540 cm-1 correspond to the am ide I or C = O stretch
and am ide II or (C -N)(N-H ) couple respectively
T r a n s m i s s i o n e l e c t r o n m i c r o s c o p y o f b o t h s a l t e d
a n d u n t r e a t e d f i l m s a f t e r s t r a i n i n g s h o w s a v a r i e t y
o f d e f o r m a t i o n m i c r o s t r u c t u r e s t o b e o p e r a t i v e . I n
a l l s a m p l e s d e f o r m a t i o n i s p r i n c i p a l l y c o n f i n e d t o
i n t e r s p h e r u l i t i c r e g i o n s , a n d d e v e l o p s i n z o n e s r u n n i n g
a p p r o x i m a t e l y p e r p e n d i c u l a r t o t h e a p p l i e d t e n s i l e s tr e ss .
T h e s e i n t e r s p h e r u l i t ic r e g i o n s a r e s l i g h t l y t h i n n e r t h a n
t h e m a j o r i t y o f t h e fi l m a n d a r e t h e r e f o r e s u b je c t e d t o
h i g h e r s tr e ss e s. R e g i o n s o f d e f o r m a t i o n , b o t h s h e a r - a n d
c r a z e - d o m i n a t e d , t e n d t o b e s h o r t , e x t e n d i n g fo r a fe w
s p h e r u l it e s i n l e n g t h , a n d v a r y i n w i d t h f r o m 0 .5 t o 2 / am .
Figure 4 s h o w s h o w m e t a l i o n s t a i n i n g ( Z n i n t h i s c a se )
a l l o w s o b s e r v a t i o n o f t h e s p h e r u l i t ic m a t e r i a l b e i n g
d r a w n i n t o d e f o r m i n g i n t e r s p h e r u l i t ic r e g i o n s .
B o t h c r a z i n g a n d s h e a r p r o c e s s e s a r e o b s e r v e d i n t h e
d e f o r m e d f i l m s . C r a z i n g i s r e s t r i c t e d i n d e v e l o p m e n t t o
s a l t - c o n d i t i o n e d f il m s ; m o s t p a r t i c u l a r l y i n Z n C I 2 - a n d
C a C 1 2 - t r e a t e d fi l m s , b u t t o a l e s s e r e x t e n t a l s o i n t h o s e
e x p o s e d t o N a C 1 . S u c h N a C I c r a z e s , h o w e v e r , a r e p a t c h y
a n d p o o r l y d e v e l o p e d . F o r a l l f i l m t y p e s e x a m i n e d ,
c r a z i n g i s d e v e l o p e d f o r a l l t e m p e r a t u r e s e m p l o y e d ( 2 5
t o 9 0 C ) , a n d b e s t d e v e l o p e d b e t w e e n 4 0 a n d 6 0 C 1 6.
D e f o r m a t i o n i n u n c o n d i t i o n e d a n d N a C l - c o n d i t i o n e d
f il m s i s s h e a r - d o m i n a t e d .
S h e a r d e f o r m a t i o n c o m p r i s e s a m i x t u r e o f l o c a l iz e d
u n i f o r m t h i n n i n g , s i m i l a r t o t h e d e f o r m a t i o n z o n e s
d e s c r i b e d f o r a m o r p h o u s p o l y m e r f il m s 21 , a n d n o n -
u n i f o r m o r i n h o m o g e n e o u s t h i n n i n g a c c o m p a n i e d b y
m i c r o - v o i d i n g a n d t h e i n c o r p o r a t i o n o f s e m i c r y s t a l li n e
m a t e r i a l w i t h i n th e d e f o r m a t i o n z o n e. T h e d e v e l o p m e n t
o f m i c r o v o i d s a n d i n c o r p o r a t i o n o f s e m i c r y s t a l l in e
m a t e r i a l g i v e s a f i b r i l l a t e d a p p e a r a n c e t o t h e d e f o r m e d
r e g i o n s i m i l a r t o t h a t o b s e r v e d i n s e m i c r y s t a l l i n e
p o l y e t h y l e n e a n d p o l y p r o p y l e n e f il m s ~. S i m i l a r m i c r o -
s t r u c t u r e s h a v e b e e n r e c o r d e d i n d e f o r m e d p o l y ( e t h e r
e t h e r k e t o n e ) ( P E E K ) 22 a n d p r e v i o u s l y d e s c r i b e d b y u s
fo r n y lo n 16 . Figure 5 i l l u s t r a t e s s u c h a n a r e a o f f i b r i l l a t e d
sh ear .
T e x t u r a l l y , t r u e c r a z e s d i f f e r f r o m f i b r i l l a t e d s h e a r b y
t h e d e v e l o p m e n t o f a r e g u l a r v o i d - f i b ri l m i c r o s t r u c t u r e
s i m i l a r t o t h a t c h a r a c t e r i s ti c o f c r a z i n g i n a m o r p h o u s
p o l y m e r f i l m s 1. C r a z e s d e v e l o p e d i n N a C l - c o n d i t i o n e d
f i lms Figure 6) a p p e a r t o p o s s e s s a m u c h f i n e r
m i c r o s t r u c t u r e t h a n t h e i r c o u n t e r p a r t s i n Z n C 1 2 - a n d
C a C l 2 - c o n d i t i o n e d f i l m s . F o r t h e s e , a c o a r s e f i b r i l l a r
m i c r o s t r u c t u r e i s s e e n Figures 7 a n d 8 ). O n e - d i m e n s i o n a l
f a s t F o u r i e r t r a n s f o r m s o f d i g i t iz e d c r a z e i m a g e s c a n b e
m a m m m a l n w m
Figure 3 Transmission lectron micrograph of a P TA-stainedsample
of untreated PA 6, The sample was exposed to a 2% solution of PTA
for 10 min . Scale bar 200 nm
s t r u c t u r e p r e v i o u s l y r e p o r t e d f o r b u l k n y l o n 2 . T h e s e
d o m a i n s a r e m a d e u p o f c r y s t a l li n e la m e l l a e i n t e r s p e rs e d
w i t h a m o r p h o u s r e g i o n s ; l a m e l l a e t h i c k n e s s e s v a r y
b e t w e e n 1 0 a n d 2 0 n m a n d a r e s p a c e d ~ 1 0 n m a p a r t .
T E M o f u n s t r a i n e d f il m s t h a t h a v e b e e n t r e a t e d w i t h
a n y o f t h e m e t a l h a l i d e s r e v e a ls t h e s a m e s t a i n e d
s p h e r u l i ti c s t r u c t u r e c o n f i r m i n g t h a t t h e m e t a l i o n s , w h i c h
a r e o f c o m p a r a t i v e l y h i g h a t o m i c n u m b e r a n d t h e re f o re
s c a t t e r e l e c t r o n s m o r e s t r o n g l y t h a n t h e n y l o n i t s e l f ,
e n t e r t h e a m o r p h o u s r e g i o n s s e p a r a t i n g t h e c r y s t a l l i n e
l a m e l l a e . T h i s s t a i n i n g c a n a l s o b e u s e d t o f o l l o w t h e
d e f o r m a t i o n o f t h e l a m e l l a e a f t e r s t r a i n i n g ( se e b e lo w ) .
Figure Transmission lectronmicrograph of a ZnC l2-treatedsample
after straining.The lamellae,which are highlighted by the intervening
stained am orphous egions,can be seen being drawn nto the deforming
interspherulitic regions. Scale bar 0.25 ~m
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Figure 5 Transmiss ion e lect ron micrograph show ing region of
inhom ogeneou s or fibrillated shear developed n untreated nylon-6 film.
The shea red region has a fibrillated microstructu re due to inco rporation
of crystalline spherulite material, Draw direction arrowed. Scale bar
~ 0.5/zm
a p p e a r i n t h e m i c r o g r a p h s t o s h o w s i m i l a r v a r i a t i o n s
w i t h c o n d i t i o n i n g .
M a c r o s c o p i c s t r e s s -s t r a in m e a s u r e m e n t s w e r e m a d e a t
r o o m t e m p e r a t u r e f o r a s t r ai n r a t e o f 1 x l 0 - 3 s - 1
o n t h e t h i c k -f i lm n y l o n - 6 . A v e r a g e d m o d u l i v a l u e s
a r e c a l c u l a t e d o v e r t h e s t r a i n i n t e r v a l 0 .5 t o 1 . 5 % .
S t r e s s - s tr a i n c u r v e s s h o w e v i d e n c e f o r p la s t ic b e h a v i o u r ,
e s p e c i a l l y a t l a r g e p e r c e n t a g e s t ra i n s . I n a d d i t i o n t o
m o d u l u s m e a s u r e m e n t s , e l o n g a t i o n a t f a il w a s r e c o r d e d .
S a m p l e s s h o w e d t o o w i d e a s c a t t e r i n th e i r f a i lu r e s tr a i n s ,
h o w e v e r , f o r a n y m e a n i n g f u l v a l u e s t o b e m e a s u r e d .
F a i l u r e o c c u r s v i a f o r m a t i o n o f a n e c k e d r e g i o n e x h i b i ti n g
d u c t i l e t e a r i n g . O c c a s i o n a l l y , f o r s o m e Z n C l 2 - t r e a t e d
s a m p l e s , a s h a r p b r i t t l e - l i k e f a i l u r e o c c u r r e d , w i t h l i t t l e
e v i d e n c e f o r n e c k f o r m a t i o n o r d u c t i le d r a w i n g .
T y p i c a l s t r e s s - s tr a i n c u r v e s f o r u n c o n d i t i o n e d a n d
s a l t - c o n d i t i o n e d n y l o n - 6 fi lm s a r e p r e s e n t e d i n Figure 9
w i t h t h e Z n C 1 2 - c o n d i t i o n e d s a m p l e e x h i b i t i n g b r it t le - l i k e
f a i lu r e a f t e r o n l y a f e w t e n s o f p e r c e n t . Y i e l d i n g o f b o t h
N a C l - c o n d i t i o n e d a n d t h e u n c o n d i t i o n e d - f il m i s m o r e
g r a d u a l , w i t h e x t e n s i v e p l a s t i c d e f o r m a t i o n o c c u r r i n g .
A v e r a g e d m o d u l i v a l u e s p r e s e n t e d i n
Table 1
g i v e a
m e a s u r e o f c h a n g e s i n t h e e l a st ic m o d u l u s d u e t o s a l t
Figure 6 Transmission electron micrograph of interspherulit ic craze
developed in sod ium chloride solution-treated nylo n-6 fi lm. Craze
exhibits dark mid-l ine. Draw direction arrowed. Scale bar ~ 1/am
Figure 7 Interspheruli tic cra zing developed in calc ium chloride-
condit ioned nylon-6 fi lm. Localized shear thinning develops Where the
craze changes direct ion. Draw d irection arrowed. Scale bar ~ 1
u s e d t o g i v e a n e s t i m a t e o f m e a n f i b ri l s e p a r a t i o n 2 3. F o r
c r a z e s in N a C l - c o n d i t i o n e d f il m s , a m e a n f ib r il s e p a r a t i o n
o f ~ 6 0 n m i s m e a s u r e d . C r a z e s i n C a C 1 2 - c o n d i t i o n e d
f il m s h a v e a m e a s u r e d m e a n f i br i l s p a c i n g o f ~ 1 0 5 n m ,
a n d ~ 1 4 0 n m f o r Z n C l 2 - c o n d i t i o n e d f il m s . F i b r i l w i d t h s
Figure Interspherulitic crazing developed in zinc chloride-
condit ioned nylon-6 fi lm with a narrow tapering termination at one
end and blun ted shear-dominated termination at the other. Draw
direction arrowed.
Figure 4
i s an enlarged region of par t o f th i s
micrograph. Scale bar ~ 1/~m
Strel l 100
80
60
40
~0
0
NaCI
J i J i . I L I i i i
O ~0 20 30 40 50
Stretn S
Figure 9 Ty pic al stress-strain cu rve s for uncond it ioned, NaCl-
cond itioned and ZnC12-conditioned nylon-6 ma cro sam ples. Strain rate
o f ~ 1
10-3s -1
5 0 9 6 P O L Y M E R 1 9 9 3 V o l u m e 3 4 N u m b e r 2 4
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N y l o n 6 t re a t m e n t w i t h m e t a l h a l id e s : A . P . M o r e a n d A . M . D o n a l d
ab l e
1 E l a s ti c m o d u l i v a l u e s fo r u n c o n d i t i o n e d a n d s a l t - c o n d i t i o n e d m a c r o s a m p l e s a v e r a g e d o v e r t h e s t r a i n in t e r va l 0 .5 t o 2 . 0 %
A v e r a g e d m o d u l u s S t a n d a r d d e v i at i on C h a n g e
T r e a t m e n t ( M P a ) ( M P a ) ( % ) F a i l u r e m o d e
Z n C I 2 2 0 4 0 9 2 + 1 1 B + D
C a C I 2 1 9 8 0 1 1 6 + 8 D
NaC 1 1628 104 - 12 D
U n t r e a t e d 1 8 4 0 1 03 - D
Failure
m o d e : B , b r i tt l e ; D , n e c k i n g a n d d u c t i l e t e a r
conditioning. Owing to the non-uniform nature of the
salt ingression, absolute values may not be measured.
Both CaCI2- and ZnCl2-conditioned samples show a
moderate degree of stiffening, with an increase in
modulus apparent, whereas NaCl-conditioned samples
would appear to be plasticized, with a decrease in
modulus measured. Although the shifts are small, the
qualitative trends seems clear from the table; quanti tative
data cannot be obtained because of the non-uniform
penetration of the salts through the samples referred to
above.
DISCUSSION
The different types of deformation observed in thin
nylon films after salt treatment, and the strains at
which deformation first occurs, have been described
previously 16. With the addit ional data from i.r. and
modulus measurements, coupled with the results of other
workers, a framework for understanding the embrittling
effect of the various salt solutions can now be proposed,
which can be correlated with what is already known for
amorphous polymers.
The i. r. results show that the most drastic spectral
modificat ions occur following treatment with ZnC12, with
less marked changes occurring for CaC12-treated samples.
These results are similar to those reported by Dunn and
Sansom 24, who considered the i.r. spectrum of nylon-6
with and without ZnCI 2 treatment, and compared
the changes observed with model compounds to aid
interpretation. These workers likewise observed the
appearance of a strong new peak at around 1600 cm-1
(they actually locate it at 1595 cm- 1 compared with the
1601 cm- 1 of the present study), together with small shifts
in position of the amide I and amide II bands. The spectra
shown in
F i g u r e
2 actually have better resolution than
the traces shown by Dunn and Sansom. The peak at
1601 cm -1 appears to have a shoulder on its upper
side and this shoulder appears to correspond to the
downward-shifted amide I peak identified by Dunn and
Sansom; the peak shifted slightly up, which we assign to
amide I, does not seem to have been resolved in the
earlier paper, nor does the peak a t 1570 cm- 1. The to tal
effect of these changes is that the weight of the amide I
band moves down, and the amide II band moves up.
On the assumption that the Dunn and Sansom
comparative study of the effect of ZnCl 2 on model
compounds is still pertinent, even though modern
spectrometers may provide additional resolution and
therefore the identification of more peaks, we will use
their analysis of the spectral shifts to aid in our
interpretation of the effect of salt on the mechanical
properties of nylon. The changes in the amide I and II
region were attributed to the formation of a coordination
complex, with the amide group acting as a ligand. It was
suggested that the amide group is linked to the metal ion
through the oxygen atom. The broadening of the N-H
stretching band at around 3300 cm-1 seen in
F i g u r e 1
was likewise reported by Dunn and Sansom. This
broadening was attributed by them to the formation of
a c i s - f o r m
complex. The net effect of these two changes
in structure is a shift from there being significant
interchain hydrogen bonding in favour of intrachain
hydrogen bonding through the
c i s - f o r m
complex.
Other workers have suggested that the formation of
these amide-sa lt complexes (formed with a variety of salts
including ZnCl 2 and CaCl2) affect properties other than
the i.r . spectrum. For instance Kim and Harget 2s
report significant shifts in Tg with salt content, as did
Siegmann and Baraam 26 with some ra ther different salts.
Wyzgowski and Novak 27 report changes in d.m.t.a.
spectra. All groups attributed the Tg shifts to the amide
complexes leading to a stiffening of the flexible polymer
chains. This leads to a greater barrier to rotation of
the chains, as demonstrated by calculations (with a
lithium ion) by Balasubramanian
et al. 28.
This reduction
in chain mobility may be expected to have consequences
for the ease with which deformation can take place. In
addition, the fact that thick films soaked for long times
in ZnCl 2 often disintegrate before complete permeation
of the salt occurs indicates that this salt solution at least
can also lead to substant ial chemical damage, presumably
via actual breaking of the chain backbone bonds.
The spectral evidence therefore points to changes in
bonding leading to chain stiffening, certainly for ZnCl 2,
and to a lesser extent for CaCl 2. No changes in the i.r.
spectrum were evident for NaCI-treated films, and it has
previously been stated that NaC1 treatment does not
affect the mechanical properties of nylon-629, although
this does not agree with our own previous work 16. For
the moment, discussion of the NaC1 case will be deferred
and we will concentrate on ZnC12 and CaCl 2. It is seen
that these salts lead to a r eduction in processes involving
shear, either simple shear leading to the formation of
interspherulitic shear deformation zones or fibrillar shear
(seen in
Fig u r e 5 ) .
Crazing instead becomes dominant.
In addi tion, we have previously shown 16 that the s train
for deformation onset is markedly decreased upon
salting -- by a factor of 3 for ZnC12-treated thin films
at room temperature -- as the switch from shear-only
processes to crazing-dominated takes place. The modulus
measurements on thick samples show that the effect of
these two salts is to increase the modulus
T a b le 1 ) .
In
other words the stiffening of the chains revealed by the
spectroscopy is accompanied by a macroscopic stiffening
of the material.
Using the picture built up for amorphous polymers of
competing shear and crazing, with the possibility of two
POLYM ER, 1993, Volume 34, Number 24 509 /
7/23/2019 6_Mechanical Properties of Nylon-6 After
6/6
Nylon 6 treatment with metal ha/ides: A. P. More and A.
r o u t e s t o c r a z i n g - - v i a s c i s s i o n o r d i s e n t a n g l e m e n t
p r o c e s s e s - - i t i s n o w p o s s i b l e t o r e c o n c i l e th e s e v a r i o u s
r e s u lt s . I n u n t r e a t e d f i lm s i t is c le a r t h a t s h e a r p r o c e s s e s
c a n o c c u r r e a d i l y , l e a d i n g t o a t y p i c a l d u c t i l e r e s p o n s e :
n y l o n - 6 i s n o r m a l l y a t o u g h m a t e r i a l . E i t h e r p u r e s h e a r
o r f i b ri l la r s h e a r 1 5.1 6 m a y o c c u r . B o t h p r o c e s s e s r e q u i r e
t h e p o s s i b i l i t y o f n e i g h b o u r i n g c h a i n s s l i d in g o v e r o n e
a n o t h e r . T h e c h a i n s t i f f e n i n g t h a t o c c u r s u p o n s a l t i n g ,
h o w e v e r , m a k e s t h i s t y p e o f m o t i o n v e r y d if f ic u l t - - t h e
c h a in s c a n n o t m o v e i n d e p e n d e n t l y o f o n e a n o t h e r .
C l e a r l y t h i s r u l e s o u t n o t o n l y s h e a r a n d f i b r i l l a r s h e a r ,
b u t a l s o t h e p o s s i b i l i t y o f d i s e n t a n g l e m e n t c r a z in g .
H o w e v e r , t h e r e d u c t i o n i n c h a i n m o b i l i t y n o w l e a d s t o
i n c r e a s e d b u i l d - u p o f s t r e s s o n i n d i v i d u a l c h a i n s a s t h e
ex te rna l s t r e ss i s app l i ed , and the s t r e ss fo r cha in sc i ss ion
c a n t h e r e f o r e b e a t t a i n e d b e f o r e t h e i n t e r v e n t i o n o f s h e a r .
C h a i n s c i s s i o n m a y a l s o b e o c c u r r i n g ( p a r t i c u l a r l y f o r
Z n C 1 2) b y v i r t u e o f c h e m i c a l d a m a g e t o t h e c h a i n s . T h e
n e t e f fe c t i s t h a t s c i s s io n c r a z i n g n o w b e c o m e s d o m i n a n t .
S i n c e t h e i n it ia l m o l e c u l a r w e i g h t o f t h e n y l o n i s
c o m p a r a t i v e l y l o w , f e w s c i s s i o n e v e n t s p e r c h a i n a r e
r e q u i re d b e f o r e c r a ze b r e a k d o w n o c c u rs . T h u s t h e
p o l y m e r h a s b e c o m e s e v e re l y e m b r i t t le d d u e t o t h e
c o m p l e x f o r m a t i o n a n d c o n s e q u e n t r e d u c t i o n i n c h a i n
m o b i l i t y . T h i s c o n c l u s i o n , b a s e d p r e d o m i n a n t l y o n
r e s u l t s o b t a i n e d o n t h i n f i l m s , c a n e q u a l l y w e l l e x p l a i n
t h e r e s u l t s o f W y z g o w s k i a n d N o v a k 1 4 '2 7 '2 9 o n b u l k
sample s .
T h e c a s e f o r N a C 1 t r e a t m e n t i s l e s s e a s i l y e x p l a i n e d .
T h e i . r . s p e c t r u m s h o w s n o e v i d e n c e f o r m o d i f i c a t i o n s t o
b o n d i n g b y c o m p l e x f o r m a t i o n . T h e r e i s t h e r e f o r e n o
r e a s o n t o e x p e c t c h a i n s t i f f e n i n g t o o c c u r . F u r t h e r m o r e
Table s h o w s t h a t t h e b u l k m o d u l u s a c t u a ll y d r o p s a f te r
t r ea tment wi th NaCI , sugges t ing a p l a s t i c i za t i on e f fec t .
T h i s c o u l d e x p l ai n w h y i n o u r e a r l ie r w o r k w e d e t e c te d
a n i n c r e a s e i n t h e s t r a i n a t w h i c h d e f o r m a t i o n f i r s t s e t s
i n a t r o o m t e m p e r a t u r e o f a f e w p e r c e n t , s o t h a t t h e
s t r e ss e s f o r d e f o r m a t i o n o n s e t b e f o r e a n d a f t e r tr e a t m e n t
a r e v e r y s im i la r. N e v e r t h e l e s s , a l t h o u g h t h e N a C 1
t r e a t m e n t d o e s n o t a p p e a r t o b e e m b r i t t l i n g , t h e r e i s
n e v e r t h e l e s s a s h i f t t o w a r d s c r a z i n g ( c r a z e s w e r e n e v e r
s e e n i n o u r T E M o b s e r v a t i o n s o f u n t re a t e d n y l o n -6 ) . O n e
s p e c u l a t i v e p o s s i b i l i t y i s t h a t t h e p l a s t i c i z a t i o n w i t h o u t
a n y c o n c o m i t a n t c o m p l e x f o r m a t i o n a c t u a l l y l e a d s t o
e n h a n c e d c h a i n m o b i l i t y . A s u f f i c i e n t i n c r e a s e i n c h a i n
m o t i o n c o u l d l e a d t o t h e o n s e t o f d is e n t a n g le m e n t
c r az in g , j u s t a s m a y o c c u r i n s o m e a m o r p h o u s p o l y m e r s
u p o n r a i s in g t h e t e m p e r a t u r e 3 . D i s e n t a n g l e m e n t c r a z e s
w i l l b e m u c h l e s s s u s c e p t i b l e t o c r a z e b r e a k d o w n , s i n c e
t h e i r m o l e c u l a r w e i g h t i s u n d e g r a d e d , a n d t h u s t h e
a p p e a r a n c e o f s u c h c r a z e s w i ll n o t n e c e s s a r i l y b e
a c c o m p a n i e d b y b u l k e m b r i tt le m e n t .
C O N C L U S I O N S
I n f r a - r e d s p e c t r o s c o p y s h o w s s i g n i f i c a n t c h a n g e s i n t h e
a m i d e I , a m i d e I I a n d C - H s t re t c h r e g io n s o f t h e s p e c t r u m
o f n y l o n - 6 a f t e r t r e a t m e n t w i t h Z n C I 2 a n d C a C I 2 . T h e
c h a n g e s a r e th o u g h t t o b e d u e t o c o m p l e x f o r m a t i o n o f
t h e a m i d e g r o u p w i t h t h e m e t a l i o n s a n d l e a d t o
s i g n if i c an t s t i ff e n in g o f t h e n y l o n c h a i n s. A c c o m p a n y i n g
t h is s t i ff e n in g i s a c o n s e q u e n t r e d u c t i o n i n c h a i n m o b i l i ty .
M. Donald
T h i s r e d u c t i o n i n m o b i l i t y h a s a m a r k e d e f fe c t o n t h e
m e c h a n i c a l p r o p e r t i e s o f th e n y l o n . T h e m a c r o s c o p i c
m o d u l u s i s s e e n t o i n c r e a s e , c o r r e s p o n d i n g t o a n o v e r a l l
s t i ff en ing o f t he m a te r i a l . S im ul t an eou s ly t he re i s a sh if t
i n t h e m o d e o f d e f o r m a t i o n ( a s re v e a le d b y T E M o f th i n
f il m s) fr o m o n e t h a t i s s h e a r - d o m i n a t e d t o o n e t h a t is
p r i m a r i l y c r a z i n g. T h e o c c u r r e n c e o f c r a z i n g i s d u e t o
t h e p o s s i b i l i t y o f s c i ss i o n a s t h e c h a i n m o b i l i t y d r o p s .
T h e s e s c i s s i o n c r a z e s a r e in h e r e n t l y w e a k a n d t h e m a t e r i a l
i s embr i t t l ed .
T h e e f fe c t o f N a C I t r e a t m e n t i s r a t h e r d i f fe r e n t. T h e
i . r . s p e c t r u m s h o w s n o e v i d e n c e f o r c o m p l e x f o r m a t i o n ,
a n d t h e m a c r o s c o p i c m o d u l u s d r o p s r e l a t i v e t o t h e
u n t r e a t e d n y l o n . N e v e r t h e l e s s T E M o b s e r v a t i o n s s h o w
t h a t t h e r e i s a s w i t c h t o w a r d s c r a z i n g . I t i s s p e c u l a t e d
t h a t t h is i s d u e t o t h e o n s e t o f d is e n t a n g l e m e n t c r a z in g ,
w h i c h i s m u c h l e s s d a m a g i n g t h a n s c i s s io n c ra z i n g . T h e
e m b r i t t le m e n t b r o u g h t o n b y Z n C I 2 a n d C a C 1 2 d o e s n o t
t h e r e f o r e o c c u r .
A C K N O W L E D G E M E N T S
T h e a u t h o r s a r e g r a t e f u l t o t h e S E R C f o r f i n a n c i a l
s u p p o r t a n d t o D r D . A . P r y s t u p a f o r a s s is t a n c e w i t h t h e
i n f r a - r e d s p e c t r o s c o p y .
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