CHAPTER

TRANSPORT OF WATER THROUGH NYLON~NITRILE RUBBER BLENDS AND ITS INFLUENCE ON

MECHkNICAL PROPERTIES

Results of this chapter have been communicated to Eur. Polym. J.

rimpon of Wwr lhmugh .Vylm-.Vilnle Rubber Rlend~ andrrs lnfuence m A~echmucoll'mper~ws 217

8.1. Introduction

T he absorption of water by polymeric materials and the consequent effects

on their mechanical behaviour are of considerable practical importance.

Polymer materials which are continuously in contact with water will

absorb water as a function of time. When placed in contact with water they

swell and show their equilibrium swelling characteristics. This absorbed

water may significantly alter their physical properties and mechanical

behaviour. Therefore, utilisation of these polymers as structural components

and as packaging materials requires evaluation of their behaviour in

environmental conditions. Study of the phenomena of sorption and diffusion

in aliphatic polyamides has received considerable attenti0n.1.~ Numerous

studies have been done on the subject of diffusion of water in polymers5 and

the basic mechanism of water absorption in rubber vulcanisates containing

hydrophilic impurities is knclwn.6.7 Studies on moisture sorption and

diffusion in crystalline materials, such as aromatic polyamide fibres with

rigid and extended chain confotmation, are a~ailab1e.~-lo

The swelling characteristics of polymeric solids can be used to

understand the interaction between polymers in the solid state and water

molecules. Although numerous investigations on moisture sorption and

diffusion in polymers, especially elastomers, have been carried out, few

reports are available for thermoplastic elastomers based on rubber-plastic

blends.

For synthetic polymers like nylons, water has a major effect on the

glass transition temperature and mechanical properties." In nylons it has

been found that a few water tnolecules in the interlamellar regions may

T r m p n of Water Through Nylon-.Vihile Rubber B l e d and its Influence on Mechanical Properties 218 -

change the overall crystalline state.12 In most cases, the absorbed water will

have a preferential interaction with functional groups in the amorphous

region of a semicrystalline polymer because of the restrictive ability of water

to permeate the crystalline regions. Nylon is a crystalline polymer that has

been widely used in a variety of industrial and engineering applications.

Polyamides are more hygroscopic than most thermoplastics. For designing

with polyamides, it is essential to know the effect of absorption of water by

this class of compounds. Absorption can be considered from a kinetic or

thermodynamic view point thy studying either the rate of the process or the

final equilibrium of the polyamide with the environment. The rate of water

absorption and desorption in polyamide 1s diffusion-controlled and strongly

temperature dependent The main objective of this chapter is to investigate

the sorption and diffusion of water through nylon-nitrile rubber blends and

their effect on the mechanical behaviour. IR spectroscopy was used to analyse

the interaction of the water molecules with nylon.

8.2. Result8 and discussion

The IR spectrum of swollen nylon (Figure 8.1) clearly indicates

hydrogen bonding of the sorbed water with the -NH groups. The peak

(Figure 8.2) at 3400 cm-1 corresponds to the free -NH- group in the case of dry

(unswollen) nylon.13 But in ihe case of the swollen nylon, the broad band

from 3500 to 3000 cm-1 corresponds to free as well as hydrogen bonded NH

groups.14 Hydrogen bonding usually shifts the peak to lower wavelength

and broadens the peak15 of NH groups. Further the absence of band or

shoulder in the case of swollen samples around 3425 cm-I indicates the

absence of any free NH group. So the peak from 3000 to 3500 cm-I

corresponds to the presence of water in the sample which has been occluded

and retained in the sample due to different mechanisms including hydrogen

bonding. In Figure 8.1 the transmission goes to zero in the 3000-4000 cm-I

Trmport of W a w 'Il~~hrough Nylon-Nztrile Rubber B l e d andits infhwnce on MechanicalPropetfies 219

region as well as from 1200-1700 cm-1. This is due to the high thickness of the

film used for IR scan. The high absorption in Figure 8.1 is due to the high IR

absorbance of the absorbed water. As the film used for Figure 8.2 is not

having sorbed water, in this case the absorbance is substantially reduced as

compared to Figure 8.1. Figire 8.3 shows the IR spectrum of the soaked

nylon from which water has lbeen removed by drying. The 1200-1700 cm-I

region in Figure 8.2 is different from Figure 8.3 because the sample used for

Figure 8.2 is one at room temperature. At room temperature nylon absorbs

water (Table 2.1). The IR spectrum (Figure 8.3) is taken after the complete

removal of water by keeping the sample in air oven at 100°C.

WAVE NUMBER (cm )

FKJure 13.1. I R specbum of swollen nylon

DL I m - 2-e ,w m * loo

WAVE NUMBER (cm" 1

Rgure 8.2. I R specbum of dry (umwollen) nylon.

Tmwrt of Water Thrnnrh Nvlon-Nihfle Rubbet B l e d andits Influence on Mechanicalhwrties 220

WAVE NUMBER (crn-l)

figure 8.3. IR spearurn of water dewtad nylon

8.2.2 Diffusion s M b

In Figure 8.4, the amount of water taken up by different samples at

28°C is expressed as Qt which is plotted as a function of square root of time.

Qt was calculated according to the equation,

Wt of water absotbed at a given time Molecular wt of Tnater

Qt = x 100 Wtof polymerkhlend

(8.1)

It is clear from the graph that the water uptake percentage varies with

blend composition. The lowest values of Q, is shown by pure nylon (Nlw)

and pure rubber (No). A very interesting observation is that the weight gain

curves of the pure nylon (Nm) samples exhibit an overshoot effect in which a

maxima occurs followed by a decrease at later times. This phenomenon of

overshoot effect, caused by penetrant rejection during chain relaxation, has

been reported as due to chain rearrangement after maximum penetrant

uptake and changes in morphology due to the presence of swollen molecules

and to geometrical factors such as thickness of the sample.l6ls

Trmport qf Water 7hmugh NyhNibik Rubber B l e d u n d i f s I ~ ~ e on MeehrmicdPmpefies 221

Figure 8.4. Sorption curves showing the mol% water uptake of No, NW, Nm and Nlw samples at 28OC.

At the initial stage of !iorption i.e., during the first 60 h, the Qt values

systematically increase with ?increase in the nylon content. This is attributed

to decrease in the extent of hydrogen bonding caused by the incorporation of

NBR (i.e., reduction in number of available hydrophilic amide sites). After 60

h, pure nylon shows the overshoot effect causing a gradual reduction of

sorption. The degree of this overshoot effect decreases as the NBR content

increases and is absent at 510 and 100% NBR. This justifies the higher Qm

values for pure NBR and the blends even though their initial Qt values are

low. The degree of overshoot effect can be defined by

Degree of overshoot effect = (Mm-Me/Me) x 100 (8.2)

Trampa of Water llvough Nylon-Niitrile R~~bber Blend &its InfTuence on MechanualPmperties 222

where Mm and Me are the maximum and equilibrium penetrant uptake

respectively. The observed overshoot effects of NICQ and N70 for the sorption

process at different temperatures have been analysed by calculating the

percentage of overshoot (Table 8.1). For Nm the values at every temperature

are lower than neat nylon. Por N70, the overshoot index increases with an

increase of temperature. At higher temperature overshoot index values are

found to be higher than at lower temperature. The higher overshoot index

values for NICQ can be attributed to the fast relaxation of the polymer chains in

the presence of water molecules. The sorbed and hydrogen bonded water

molecules plasticize the nylon chain. Therefore, segmental relaxation is

easier.

Table 8.1. Per cent ownhoot e M index for polymersdvent at different temperatures

Temperature (OC) Composition

28 45 65 75

NICQ (Nylon) 26.91 64.00 60.21 68.87

Nm (70 Nylon) 2.61 57.03 51.94 66.62

At 45°C the initial sorption trend also increases in the order NO < Ns, <

N70 < Nlw (Figure 8.5). But the time to initiate overshoot effect decreases to

20-24 h for pure nylon due to the temperature activation of the diffusion

process. As the nylon content decreases the magnitude of the overshoot ef f t~t

decreases just as observed at 28°C (Figure 8.5). The reduction in time to begin

the overshoot effect at 4S°C is definitely associated with the temperature

activation of the diffusion process. Although the initial uptake is in the order

Nlm > N70 5 NS0 > NO, N ~ o shows the maximum water uptake (Q,) at all

temperatures.

figwe 8.5. Sorption curws showing the md% w a b uptake of NO, Nso, Nm and NIW samples at 4S°C

In order to follow the rate of water migration through the polymer we

calculated the diffusion coefficient using the following equation,

where h is the initial sample thickness, 0 is slope of the linear portion of the

sorption curves before attainment of 50% equilibrium and Q, is the mol %

uptake of water at equilibrium. The computed D values are given in

Table 8.2. It is clear from the table that the D values increase from No to Nlw;

they also increase with increase in temperature. The variations in the D

values with blend compositions for different temperatures are given in

Figure 8.6; the D values increase with increase in nylon content and with the

rise in temperature.

Transport ofwater Through Nylon-Niirile Rubber Blends and its I@m

e on M

echmrical Properties

224

Trmport of Waer W g h NylorrNihe Rubber B l e d andits I@uenee on Mechanical hperhes 225

0.1 LI-- 0 :?0 40 60 80 100

WT% OF NYLON

Figure 8.6. Effect of blend mpc~sitions on the dimon cwffioent at 28,45,65 and 75'C.

Knowing that diffusiom is influenced by the phase morphology we

have analysed the phase morphology of the system. Figures 3.3b and 3 . 3 ~

show the scanning electron micrographs of the fracture surface of the NW and

Nm nylon-NBR blends. The NBR phase was preferentially extracted using

toluene.

The co-continuous morphology of NW is clear from Figure 3..3b.

Figure 3 .3~ shows that the minor NBR component is dispersed in a major

continuous nylon matrix. The holes indicate the portions from where the NBR

has been extracted. The features of the blend indicate an immiscible system.

The higher uptake values of the blend can be attributed to the poor

miscibility between components of the blends in spite of the weak polar-polar

interactions. The two phase morphology, the nonuniformity in domain size

distribution and the two T, values we got in DMA studies corresponding to

nylon and NBR support the innmiscible nature of the nylon/NBR blends. In

N70, nylon is the continuous phase. Nylon being more prone to penetrant

Trrmspori of Water Through NylonNihile Rublr!rBlends and its It$luence on MechanicdPmperties 226

overshoot, its Q, values are lower than Nso, where both the phases are

continuous.

The highest Q, uptake values exhibited by Nm blend can be further

explained as follows. The important factors in the establishment of

equilibrium between a polyamide and a water containing environment are

the relative humidity in the s~irrounding and the degree of crystallinity of the

polyamide. The degree of crystallinity of the polyamide has a significant

effect on the equilibrium rnoisture absorption. The magnitude of the

moisture absorption decreases with increase in crystallinity. The blending

reduces the crystallinity of tlie nylon phase. The reduction in crystallinity

provides an easy way for the penetrant molecules to get into the voids. The

DSC analysis of the blends showed a decreasing trend of heat of fusion values

of nylon with increase in NBR content (Table 8.3). The AH of Nso is 2.3 J/g

which is very low compared to the AH of Nlw. This supports a decrease of

crystallinity of nylon in the blends with increasing amounts of NBR.

Therefore, the increase of liquid uptake shown by blends having higher

elastomer content can be attributed to the decrease in the crystallinity of the

nylon phase due to the incorporation of more flexible and amorphous NBR.

Table 8.3. HEI~ of fwion values and densih, of ttFe Mends

Heat of fusion values Composition a/ g deg.)

Density (g/cc)

Nso 2.3 1.15

The higher water uptake of Nw can be established by comparing the

densities of the blends (Table 8.3). The higher the density, the lower will be

the free volume. So N ~ o being lighter than N70 contains more free volume

Trmpon of Water Through Nylon-Niitri R&5krBlords and its Influence on Mechanical Pmperties 227

compared to N70. The phenomenon of overshoot effect is the reason for the

low Q, values of N~WJ in spite of its lower density compared to Nw.

The mechanism of sorption can be followed by fitting the values to the

following empirical equation,

log (Qt/Q,) = log k + n log t (8.4)

where t is the time and k is a constant which shows the interaction between

polymer and solvent The values of n and k were determined by linear

regression analysis (Table 8.4). The values of n is nearly equal to 0.5, wluch

shows the diffusion mechannsm follows a Fickian trend. When n = 1, the

mechanism is non-Fickian. liowever, the values of k tend to increase with

rise in temperature for every blend composition, but decrease systematically

with an increase in elastomer content in the blends, suggesting decreased

polymer-solvent interaction.

Table 8.4. Analysis of sorption data of the Mend water systems

The transport process of small molecules through polymers involves a

solvent-diffusion mechanislrt and hence the permeability or permeation

coefficient; P is the product of diffusivity D and solubility S, i.e., (P = DS).

The S values are taken as grams of liquid sorbed per gram of polymer. The

values of P, D and S computed for different blend-penetrant systems are

given in Table 8.2.

n k x 103(3/3 oln')

Composition Temperature ("C)

28 45 55 75 1 28 45 55 75

Trmaort of Wafer K h m ~ h Nvlon-Niilrile Rubber B l e d andits influence on Mechanicalh~erties 228

In all the systems, the diffusion coefficient increases with temperature.

The value of diffusion coefficient charaderises the ability of the small

molecules to diffuse through the polymer segments. The sorption coefficient

is a measure of the equilibrium sorption of the penetrant. The high value of S

marks the high tendency of the water molecules to dissolve into the blend.

The permeability coefficient (P) shows the net effect of sorption and diffusion.

The permeability values increase from NO to Nlw and also increase with

increase in temperature. It can be understood that the permeability is

controlled primarily by the diffusion process rather than the tendency for the

solvent water to dissolve into the polymer.

The influence of temp2rature on the diffusion characteristics of NSO is

shown in Figure 8.7. The figtare indicates that the temperature considerably

activates the diffusion process. From the figure it is clear that the degree of

overshoot effect increases with temperature. At 65 and 75°C the overshoot

indices are larger compared to the values at 28 and 45°C. This means that the

temperature influences the degree of overshoot effect.

Jt (mn)

Figwe 8.7. Effect of trmperahrre on the diffusion chacbistics of N,

Trmport o f w a r i%rargh Nylwt-Nitrile Ru6ber Biends and its Infhrence on Mechanical Properties 229

The swelling data were ~tsed to calculate the blend-solvent interaction

parameter ( x ) using the following equation.I9

where 4 is the volume fraction of polymer blend in the swollen sample which

is determined as

where WI and pl are weight and density of the polymer sample, respectively;

and wz and p, are the weight and density of the solvent. The value of N for

equation (8.5) is calculated from 4 as19

The calculated x values for NO, Nw, N ~ o and NIW are 5.01,2.35, 2.11 and

1.33, respectively. The x values are lower for Nm compared to the blends

and pure NBR. Among the blends Nm has lower value of interaction

parameter compared to Nw. The overshoot effect is the reason for the low Q,

of Nlm and Nm in spite of their low interaction parameter.

From the amount of liquid sorbed by a given mass of rubber, the

equilibrium sorption constant K. has been determined as

Number of moles penetrant sorbed K,=

Unit mass of the polymer

Using the values of K, (Table 8.5) the enthalpy AH and entropy AS of

sorption can be determined using Van't Hoff s equation:

AS ,AH log K, = -

2.303R 2.303RT

The values of fi, AH and AG are given in Table 8.6. The AH values increase

from nylon to NBR, i.e., from NIW < N70 < N50 < NO. This trend shows a

T r m p r t ofwater k g h NylonNilde Rubber Blend7 andits I@uenee on M e c h c a l Properties 230

decreased exothermicity of sorption behaviour with increase in rubber

content. The AS values also show a regular increase from Nim to NO (NIW <

Nm < N50 C: NO). The AS values are negative in most cases which suggests the

retainrnent of liquid state structure of water molecules even in the sorbed

state. The AG values show a decreasing trend with the addition of NBR to

nylon which indicates that the diffusion process is more feasible with NBR

rich blends. This is in agreement with our earlier observation.

Table 8.5. Mdar equilibrium Wtion constants (K, x lo3) at difkrent twnperatwes

Temperature (OC) Composition

28 45 65 75

Table 8.6. Van't Hoff parametets enthalpy (AH,, Jlmd), entropy (AS, Jlmd) and free energy (Jlmol) of sorption

Composition property

AH AS AG

8.2.3 Sorption kinetics

The solvent transport characteristics of the membrane can be

dekrmined by evaluating the kinetic rate constant kl from the sorption data."

Transport of Water Through NylonNiihile Rublwr B l e d mtd its lrrfluence on MechrmcalProperries 23 1

Therefore, the first order kinetic equation given below can be used to study

the transport kinetics.

kit = 2.303 log (G/& - Ct) (8.9)

where Ct and C, are, respectively the penetrant concentration at time t and at

infinite time t (i.e., at equilibrium saturation); these quantities have the same

meaning as Qt and Q, as discussed earlier. Figure 8.8 shows the plots of log

(C, - G) vs. t for all blend cornpositions in water at 28°C. The slopes of the

graph give values of kt which are given in Table 8.7. The kinetic rate

constant is a measure of the speed with which the water molecules migrate

within the polymer matrix. The kt values increase with temperature. The

increase in kl values indicate an increase in the rate of diffusion with

temperature.

-1.6 0 10 20 30 40 50 60 70 80 90 100

TIME (min)

Kgwe 8.8. Dqkmkme of log (C,-CJ versus t fw the different blend mpwh in water at 28'C.

T r m p r ~ c$R'mr l h q h .Yykm-hittile Rubh!r Blends and irs lnflue)1cv on .Uechmcal I'n~pernes 232

TaMe 8.7. Rate constant (Kt x 103 for ME Mend-water system at different temperatwes -- Temperature (OC)

Composition 28 45 65 75

Nloo 1.71 5.14 7.66 5.34

In all cases, the pure elastomer has the lower rate constant values. It

increases in the order NO < Njo < N70 < N~w. This is due to the incorporation

of more hygroscopic nylon to the elastomer.

The diffusion coefficients as calculated from equation (8.3) have been

fitted to the following equation, which describes the Fickian diffusion model,

to generate the theoretical sorption curves, since there is no appreciable

change in dimension of the Mend samples as a result of water diffusi0n.2~

Qt, Q,, D, t and h have the same meaning as before. These sorption curves

are compared in Figures 8.9 and 8.10 with the experimental profile for the Nn

and N70 at 45 and 75"C, respectively. The total agreement is fairly good. The

Nla, (Figure 8.11) shows deviation from the theoretical curve near the

equilibrium due to the overshoot phenomenon.

T r m p r t of Waier W g h Njk+hrihile Rubber Blends andits I@uence on Mechanical Pmperties 233

TIME (rnin)

Sorplim cum showing the cornpanson between expenmental and theorebcal curve for Ng in water at 4S°C.

TIME (rnin)

figure 8.10. Swption curve showing the comparison between expenmental and theoretical culves for N,o in water at 75OC.

T r m p f i of W m r Thrargh Nylon-Nimk Rubber Blmls andits lnfhrence on Mechanical Properlies 234

figure 8.11. Sorpbon curve showing the amparison between expelimental and Wet ica l cutves for Nlw in water at 2E°C.

Polyamides as a class are more hygroscopic than most thermoplastics.

This property raises problems in the processing and design of components

made from polyamides. Most properties are sigruficantly affected by water

absorption in addition to the dimensional changes that can affect the

functioning of components. The moisture is not necessarily absorbed to

saturation level and there often exists a moisture gradient across the section at

right angles to the exposed surface, which results in a corresponding

gradation of properties.* The moisture in nylon acts like a plasticizer and its

presence supports molecular chain movement. This decreases stiffness and

increases chain flexibility. Tensile moduli are reduced and elongation at

break increased as a result of wakr absorption.

The tensile strength of the nylon-NBR blends show dependence on the

strength of the nylon matrix. The mechanical strength of the nylon decreased

T r m m n of Water i'hrowh Nvlm-Nilrile Rub her B M andits Influence on MechanicdPmwrries 235

upon the addition of NBR. The maximum tensile strength of Nlw, N70, N50

and No were tested after immersing them in water for 13 days. The samples

immersed in water show poor tensile strength compared to the dry samples

(Table 8.8).

Table 8.8. Mechanical properties of dry and swollen samples. - C:omposition

- - Mechanical properties Nim N7o Nso No

Dry Swollen Dry Swollen Dry Swollen Dry Swollen

Tensile strength 33.72 22.10 14.42 12.25 6.53 4.57 0.31 - (MPa)

Young's modulus 44.52 29.35 18.55 10.29 10.13 8.11 - -

(MPa)

Yield stress 14.47 11.08 8.31 5.07 4.63 2.52 - - (MPa)

Elongation at 139.91 169.91 102.00 139.61 67.97 119.61 347.51 - break (%)

Tension set 280.00 427.77 140.11 250.45 76.00 208.75 24.00 - (%)

Figure 8.12 shows the deformation nature of the dry nylon and dry

nylon/NBR blends (unswollen samples). The non-crystalline elastomer

phase in various proportion causes changes in the deformation behaviour of

nylon. Neat nylon and blends containing higher proportions of nylon

(> 50%) show high initial mc,dulus. The stress-strain curves of these samples

exhibit a well defined yield at low elongation, indicating plastic deformation

caused by the breakdown of the continuous nylon matrix. In the case of Nlw,

the increase in stress with strain beyond the yield point is associated with the

Trmrport of Woter%mgh NylonNiitrille Rubber Blends andits Influence on Mechmrical Pmpem'es 236

orientation of the crystalline hard segments. As the rubber content increases,

the initial modulus and the yielding tendency decreases. The sample Nm also

shows typical plastic behaviour due to the presence of nylon as continuous

phase. The NXI, which has a co-continuous morphology, shows an

intermediate stress-strain behaviour.

Figure 8.12. Shersshn behavKxlr of dry n y h / N B R blends: Nim, Nm and Nso

The stress-strain cuw~?s in Figure 8.13 shows the deformation nature of

the swollen samples. The general behavlour of the stress-strain curves are the

same for the swollen and unswollen samples, although the modulus and

tensile strength are lower than those of the unswollen samples. This clearly

shows that water sorption does not affect the nature of the stress-strain

behaviour.

Trmrpr~ ofwafer Thracgh Nylon-Nilrile Rubber B l e d andits Influence on Mechanical Pmpenies 237

s W N (%)

figure 8.13. Stress-strain behamour of water swdlen nylon/NBR Mends: Nlm, Nm and Nso

The tensile strength of the unswollen and water swollen NIW, N~o, NSO

and No for various blend compositions are given in Table 8.8. Water imbibed

nylon shows a considerable redluction in the tensile strength compared to dry

nylon. Similar reduction is found in case of N70 and N50. Since the mechanical

strength depends on the weight percentage of nylon, the swollen nylon rich

blends show more reduction in their mechanical properties compared to the

tensile strength values of the d ~ y samples. Young's modulus, yield stress and

elongation at break of the unl;wollen and swollen nylon are also given in

Table 8.8. The table shows decreasing modulus values of the swollen sample

compared to the dry compositions. This is also attributed to the plasticizing

action of the water molecules a.s explained earlier. As expected the elongation

at break value of the nylon rich swollen samples are higher than unswollen

samples. The tension set after failure (Table 8.8) of nylon rich samples

increased with the diffusion of water molecules.

Transport of Water Thmugh Nylon-,Yiinile RubherBlem& andilslrtfluence on Mt%hanica[Pmperlies 238

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