7/23/2019 6_Mechanical Properties of Nylon-6 After

1/6

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

0032-3861/93/245093-06

1993 Butterworth-HeinemannLtd. POLYMER, 1993, Volume 34, Number 24

5 0 9 3

7/23/2019 6_Mechanical Properties of Nylon-6 After

2/6

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

POLYM ER 1993 Volume 34 N umber 24

7/23/2019 6_Mechanical Properties of Nylon-6 After

3/6

Nylon 6 treatment with metal hafides: A. P. More and A. M. Donald

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

POLYMER 1993 Vo lume 34 N umber 24 5095

7/23/2019 6_Mechanical Properties of Nylon-6 After

4/6

N ylon 6 t reatment w i th m eta l ha~ides: A. P. M ore and A. M . D onald

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

7/23/2019 6_Mechanical Properties of Nylon-6 After

5/6

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 .

R E F E R E N C E S

1 Kram er , E . J .

Adv. Polym. Sci.

1983, 52/53, 1

2 Kra m er, E. J . an d Berger , L, L.

Adv. Polym. ScL

1990, 91/92, 1

3 P l u m m e t , C . J . G . a n d D o n a l d ,

A. M. J. Polym. ScL, Polym.

Phys . Edn.

1989, 27, 325

4 P l u m m e r , C . J . G . a n d D o n a l d , A . M .

Macromolecules

1990, 23,

3929

5 D o n a l d , A. M. J . Mater . ScL 1985, 20, 2630

6 M ich le r , G . H .

Colloid Polym. Sci.

1992, 270, 627

7 Fr ied r ich , K .

Adv. Polym. Sci .

1983, 52/53, 1

8 Nar i sawa , I . and I sh ikawa , M .

Adv. Polym. Sci.

1990, 91/92, 353

9 Pe te r l in ,

A. J. Mater . ScL

1971, 6, 490

10 Oi l , H . G . and Pe te r l in , A. J. Polym. ScL, Polym. Phys . Edn.

1974, 12, 2209

11 Pe te rm an , J . and Gle i te r ,

H. J. Polym. Sci. , Polym. Phys. Edn.

1972, 10, 2333

12 Pe te rm an , J . and Gle i te r ,

H. d. Polym. Sci. , Polym. Phys. Edn.

1973, 11,359

1 3 D u n n , P . a n d S a n s o m ,

G. F. J. Appl. Polym. Sci.

1969,13, 1641

14 Wyzgowski , M . G . and Nov ak ,

G. E. J. Ma ter . Sci .

1987, 22, 2615

15 M ore , A . P . and Dona ld , A . M .

lns t . Phys . Conf . Ser .

1991,

119, 357

16 M ore , A . P . and Don a ld , A . M .

Polym er

1992, 33, 4081

17 Lauterwasser , B. D. and Kra me r, E. J . Phil . Ma g. 1979, 20, 469

18 M iyazawa , T . 'The Po ly -u -A mino Ac id s ' (Ed . G . D . Fasm an) ,

Mar cel Dekk er, New York, 1967, p . 69

19 Kr im m, S . and Bandeka r , J .

Adv. Protein Chem.

1986, 38, 181

20 M ar t inez -Sa laza r , J . and Ca nno n ,

C. G. J. Polym. ci . Let t .

1984,

3, 693

21 Don a ld , A . M . and Kram er ,

E. J. J. Mater. Sci.

1981, 16, 2967

22 Plum mer , C . J . G . and Kausch , H . H .

Polym er

1993, 34, 305

23 Yang , A . C . -M . and Kram er ,

E. J. J. M ater . Sci .

1986, 21, 3601

24

D u n n , P . a n d S a n s o m , G . F . J . A p pl . P o l y m . S cL 1 9 6 9 , 1 3 ,1 6 5 7

25 Kim, H . G . and Harge t , P. J. J. Appl . Phys . 1979, 50, 6072

2 6 S i e g m a n n , A . a n d B a r a a m ,

Z. M akromol. Chem. , Rapid Comm un.

1980, 1, 113

27 Wyzgowski , M . G . and Nov ak ,

G. E. J. Mater . Sci .

1987, 22,1715

28 Ba la su b ram an ian , B ., Goe l , A . and Rao , C . N . R .

Chem. Phys.

Let t .

1972, 17, 482

29 Wyzgowski , M . G . and Nov ak , G. E. J. Ma ter . Sci . 1987, 22, 1 707

5 0 9 8 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