Indian Journal of Fibre & Textile ResearchVol. 15, September 1990, pp. 93-99

Effect of swelling agents on unoriented nylon-6 film-Structural changes

N V Bhat, V H Buch & A K KalkarCentre of Advanced Studies in Applied Chemistry, Department of Chemical Technology, University of Bombay.

Matunga, Bombay 400 019, India

Received 4 April 1989; revised received 4 April 1990; accepted 20 April 1990

Unoriented nylon-6 films were subjected to aqueous formic acid and benzyl alcohol treatments.Formic acid was found to be more active in causing structural changes. The highest possible formicacid concentration that could be used advantageously was found to be 50% owing to heavy shrin-kage at higher concentrations. Solvent-induced crystallization with respect to time of treatment andconcentration of solvent revealed entirely different trends. These treatments give rise to shrinkage offilms. WAXD and IR spectroscopy revealed that crystallization results in the formation of a largeamount of monoclinic a-crystalline form. Scanning electron micrographs revealed significant changesin surface morphology due to solvent etching.

Keywords: Crystallinity, Nylon-S, Polyamide, Polycaproamide, Shrinkage, Solubility parameter,Solvent pretreatment, Spherulites

1 IntroductionThe effect of organic solvent pretreatments on

linear high polymers has been a subject of interestsince last few years. All solvent-induced structu-ral-morphological changes in the semi-crystallinepolymer matrix arise due to effective lowering ofglass transition temperature in presence of a sol-vent'. The major implication of solvent-inducedcrystallization has been in wet-finishing of textilematerials, either as solvent pretreatment or in car-rier dyeing+', The possibility of using solvent pre-treatment as a useful means of imparting favour-able structural changes has been investigated indetail earlierv". In all these works, major empha-sis was on textile properties of fibre-forming po-lymers like silk, nylon-S and poly(ethylene tereph-thalate). Similar study on nylon-6(in the form offilm) has also been carried out", Shrinkage is an-other important swelling-related phenomenon. At-tempts have been made to get better insight intoit by using several swelling agents9.12• Solubilityparameters, shrinkage and morphology can becorrelated. In this paper, we present some data onchanges in crystallinity, crystal structure, crystal-lite size, shrinkage and spherulitic structure of un-oriented nylon-6 films subjected to aqueous for-mic acid and benzyl alcohol treatments.

2 Materials and MethodsTubularly-extruded translucent nylon-S film

(thickness, 30 f.J.) supplied by Mis Garware Nyl-ons Ltd was used. The formic acid solutions wereprepared by adding the required amount of 90%formic acid (AR grade) in distilled water. In all,five different concentrations in the range 10-50%were used. Benzyl alcohol and methanol usedwere of AR grade.

2.1 Swelling TreatmentsThe film samples were immersed in 50 ml of

formic acid solutions at room temperature for dif-ferent durations. This was followed by a washwith distilled water and subsequent drying in vac-uum at room temperature. The samples were fi-nally stored in a desiccator over silica gel.

Benzyl alcohol treatments were given by im-mersing the samples in 50 ml of benzyl alcohol fordifferent durations. This was followed by rinsingthe samples with methanol to remove benzyl alco-hol. Finally, the samples were washed thoroughlywith distilled water and dried at room tempera-ture under vacuum.

2.2 Measurement of Weight LossFilm samples were weighed before and after the

treatments using a microbalance. The % loss inweight was calculated as follows:

0/ L . . h WI - W2/0 oss m welg t =---- x 100

~

93

INDIAN J. FIBRE TEXT. RES., SEPTEMBER 1990

where W, is the original weight; and Wz, the finalweight.

2.3 Measurement of DensityFloatation technique!' was used to determine

the density of film samples using n-heptane andCCl.j as inert liquids.

2.4 Measurement of ShrinkageThe % shrinkage was measured by noting the

change in the length of film samples before andafter the treatments. The measurements were car-ried out on samples dried in air" at 25°C for 24 h.The % shrinkage was calculated as follows:

I, - 10'Yc, Shrinkage =- -=- x 100

II

where I, is the length before the treatment; and ./2,

the length after the treatment.

2.5 X-ray DiffractometryX-ray diffractograms of nylon-6 films were re-

corded using a General Electric XRD equipment.The diffractograms were recorded in symmetricalreflection mode in 2 () range of 10° to 35°. Scann-ing speed was l°/min.

a-y conversion was quantitatively estimated us-ing the method proposed by Mitsuhashi and Kyo-tani '4. The change in lateral order was followedby measuring the change in resolution of thepeaks in the diffractograms using the method ofManjunath et al.I5 The crystallite sizes were calcu-lated using Scherrer's formula 10. For this purpose,the peaks were resolved after the normalization ofthe profile.

2.6 Infrared SpectroscopyIR spectra of the film samples were recorded

using a Perkin-Elmer IR Spectrophotometer(model 397). The IR crystallinity index was deter-mined using absorbance measurements at 930 cm-Iand 1120 em -1, the latter being the internalstandard 17.

... A930Crystallinity index =--

A1120

where A is the absorbance defined by A= log lollin which 10 is the incident intensity, and I, thetransmitted intensity at a given wavelength.

were recorded using a Spectra-Physics He-Ne la-ser (output power, 5 mW) which gives polarizedlight of 632.8 nm wavelength. Sample-to-film dis-tance was 7 ern. Radial intensity distribution at45° azimuth was found by photomctering thephotographic plates.

2.8 Scanning Electron MicroscopyScanning electron micrographs were obtained

using an ISI-DS-130 scanning electron micro-scope employing an accelerating voltage of 25 kY.The samples were coated with thin film of gold toavoid the building up of electrostatic charge.

3 Results and Discussion3.1 Loss in Weight

Fig. I shows the loss in weight of nylon-6 filmstreated with 50% aqueous formic acid solutionfor different durations. It is observed that signifi-cant loss in weight occurs up to 4 h of treatment.Thereafter, only a nominal loss in weight occurs.Similar results were obtained for lower concentra-tions. After 8 h of treatment, no significant weightloss was found. This indicates saturation of disso-lution process accompanying solvent action. Allfurther experiments were performed on samplestreated up to 8 h. Benzyl alcohol treatments didnot cause noticeable change in weight.

3.2 Density MeasurementsThe values of densities for different pretreated

nylon samples are given in Table 1. These valueswere used to calculate the crystallinity index. Thechanges in crystallinity index with respect to con-centration of solution and duration of treatmentare depicted graphically in Figs 2 and 3 respect-ively. It is seen that density increases linearly as

8

7

0:6~~5w~4z;(;3<J)g2

o 2 4 6 8 10 12 t4 16DURATION OF PRETREATMENT I h

2.7 Small Angle Light Scattering (SALS) Fig. 1 - Loss in weight of nylon-6 films pretreated witl: 5 (JO;;,H v SALS patterns of control and treated films formic acid for different durations

94

BHAT et al: UNORIENTED NYLON -6 FILM

Table 1 - Density and crystallinity of nylon-o films pretreatedwith formic acid and benzyl alcohol

Solvent Duration of Cone. of Density Crystallinitypretreat- solvent g! cc index

ment %h

Formic acid1.139 0.40

1 10 1.158 0.541 30 1.163 0.571 50 1.173 0.648 10 1.160 0.558 30 1.165 0.598 50 1.173 0.64

1 100 1.141 0.42

8 100 1.150 0.48Benzylalcohol

the concentration of formic acid solution in-creases for any fixed duration of treatment. Onthe other hand, the variation of crystallinity withrespect to duration of treatment is of differentcharacter. The crystallinity increases rapidly up to2 h of treatment for all the concentrations of for-mic acid, decreases slightly between 2 and 4 h forlower concentrations and remains constant up to8 h for higher concentrations (40% and 50%).

Pretreatments using benzyl alcohol also reveal aslow increase in the values of density and crystal-linity (Table 1).

3.3 ShrinkageShrinkage is a swelling-related phenomenon

which results from the relaxation of orientationalstrain induced in the polymer during the process ofmanufacturing. Attempts have already been madeto acquire better insight into this phenomenon byusing swelling agents. In case of drawn nylon-66the rate of contraction in aqueous phenol hasbeen correlated with a series of elementary rateprocesses". For nylon-S, it has been suggested!"that a range of order prevails in the so called dis-ordered regions of the polymer and that this maybe the key factor in determining the nature of sol-vent-polymer interaction, especially in the case ofweak swelling agents.

Existing Iiterature+!' suggests that solvent-po-lymer interaction is maximum when the solubilityparameter of the solvent is close or equal to thatof the polymer. In these investigations, the sol-vent-polymer interaction has been quantified interms of solvent-induced crystallinity (SINe), vo-lume swelling and shrinkage. These, when plottedagainst the solubility parameter of the solvent,

10 ( a) 1 h ( a) 2 h09080706/~ _~05_- .:0-4

83:x~- J'2

Z ~J' ~:3 a L---<--L----'------'---';'!~ ,·050-9 (c)4h (d)8h

~ 0·8

~ 0·7

~0-6~ __~o 5 .:04

03

0-2

0-1

o 10 20 ~O 40 50 0 10 20 30 40 50

CONCENTRATION OF FORMIC ACID /1.

Fig. 2 - Effect of formic acid cone. on the crystallinity ofnylon-6 films treated for different durations

0·8

I"

0·7

-'-'<I-~ 0·5a::u11-ox 0·4woZ

0-10 '/.A-20'/.o -30'/,e-40'I,e -50'1,

03

0·2L-L-l_.l....--L---IL..~-+_!:-~...,..!o 1 2 3 4 5 6 7 B 9 10

DURATION OF PRETREATMENT, h

Fig. 3 - Effect of time of pretreatment on the crystallinity in-dex of nylon-6 films at different concentrations of formic acid

gave maxima when the solubility parameter of thesolvent approached that of the polymer. The solu-bility parameters of solvent-non solvent mixturessuch as water-formic acid solutions can be calcu-lated bya simple method suggested by Froelinget all", In the present study, we have attempted tocorrelate the shrinkage behaviour with the solubil-ity parameters of the aqueous formic acid solu-tions using solubility parameter as a measure ofthe swelling power of solutions.

Fig. 4 depicts a plot of the % shrinkage of nyl-on-o films with the solubility parameter of theaqueous formic acid solutions. It is seen that withdecrease in solubility parameter and increase in

95

INDIAN 1. FIBRE TEXT. RES., SEPTEMBER 1990

o~~~~--~ __~~~~ __~30 40 50 60 70 eo 90 100

SOLUBILITY PARAMETER

Fig. 4 - Shrinkage of nylon-6 films pretreated with formicacid of different concentrations

concentration, the shrinkage increases in a fairlylinear manner. This can be understood in termsof changes in morphology taking place due toswelling action. The mobility of chains increasesand chain folding may take place, leading to sol-vent-induced crystallization. This leads to contrac-tion of chains and subsequent crystallization. Thevalues of crystallinity indices obtained from themeasurement of density and X-ray diffraction arein conformity with this mechanism.

3.4 X-ray Crystallinity and Crystallite SizeFig. 5 shows the X-ray diffractograms for nyl-

on-6 films treated with different concentrations offormic acid. The three peaks occur at the Braggangles 20S, 21.r and 23.8° out of which thepeak at 21.r corresponds to the y-phase (I) andthe remaining two to the a-phase (I al and I a2)·These peaks correspond to the reflection from theplanes (200) and (002) + (202) corresponding tomonoclinic a-form. The middle peak correspondsto (100)+(010) reflection from the hexagonal y-form. By using maxima and minima observed insuch diffractograms, the values for the lateral or-der were calculated using the method mentionedin 2.6. The values (Table 2) show that the lateralorder and the amount of a-form increase as theconcentration of formic acid is increased. Thecorresponding values for benzyl alcohol show on-ly slight increase in this parameter. From the dif-fractograms, it is observed that the relativechanges take place in the several peaks. In parti-cular, the reflection peak (100)+(010) at 21.r

96

--10'"11001 ----30-,.

-_·-so·/.

>....inzw•...z- L_----.. .-

~-:"~ --

BRAGG ANGLE,2e

Fig. 5 - X-lay diffractograms of nylon-6 films pretreated withformic acid solution for 8 h

Table 2 - Lateral order (crystallinity) of nylon-6 films pre-treated with formic acid and benzyl alcohol on the basis of

WAXD and IR spectroscopy

Solvent Duration of Cone. of Amount Crystallinitypretreat- solvent of

ment % yform IR WAXDh %

Formic acid35.5 0.27 0.14

1 10 33.5 0.27 0.181 30 29.1 0.54 0.201 50 0 0.66 0.498 10 32.5 0.39 0.208 30 28.9 0.63 0.248 50 0 0.90 0.50

1 100 35.0 0.28 0.188 100 34.5 0.53 0.19

Benzylalcohol

disappears and the peak corresponding to(002)+(202) doublet shifts by about 1°. It hasbeen observed by previous workers'Y" that thecrystallization of nylon-6 proceeds at differentrates along the different crystallographic direc-tions of the monoclinic a-unit cell. It was ob-served that as the crystallinity increases the half-width of the (200) reflection (a, peak) reduces,indicating increased crystallite size in the directionnormal to these planes (Table 3); as this directioncorresponds to the direction of interchain hy-drogen bonds forming the n-crystallites it may beconcluded that more and more molecular chainstend to align themselves with the crystallite as thecrystallization proceeds. In other words, more andmore hydrogen bonds are formed. Another inter-esting feature is that an increase in crystallite size

BHAT et al: UNORIENTEO NYLON-6 FILM

Table 3 - Changes in crystallite sizes in nylon-o films pre-treated with formic acid and benzyl alcohol

Solvent Duration of Cone. of Crystallite size, Apretreat- solvent

ment % 0200 0002 + 0202

h

Formic acid34.5 36.1

I 10 37.4 41.0I 30 49.6 30.1I 50 74.8 34.78 10 35.9 29.38 30 59.9 39.38 50 81.6 47.5

I 100 34.5 27.48 100 35.9 28.2

Benzylalcohol

along a-axis is accompanied by a shift in the ref-lection corresponding to (002)+(202) planes.This reflection also results from a-crystallites.With reference to the monoclinic a-unit cell ofnylon-6 these planes correspond to the plane ofhydrogen-bonded molecular sheets and the dia-gonal plane respectively'". Since an increase in thediffraction angle of (002) + (202) planes indicatesa denser packing of the crystallites, a better crys-tallite perfection is achieved in addition to crystal-lization. Fig. 5 shows that for samples treatedwith 10% formic acid, (200) reflection occurs at20.6° and (002) + (202) doublet occurs at 23.7~However. when the concentration of formic acidis increased to 30% the peak position is shifted to2 B> 23.9° and when it is further increased to50°/,), the a, peak shifts further 24.4°. These ob-servations indicate that thermally-induced or sol-vent-induced crystallization leads to the same re-sult.

Benzyl alcohol treatments did not give rise toany of above effects owing to very small increasein crystallinity.

3.5 IR SpectroscopyThe infrared absorption spectra for control and

treated nylon-6 samples are shown in Fig. 6. Thecalculated values of crystallinity index for samplestreated with different concentrations of formic ac-id for different durations are given in Table 2. Itis seen that the crystallinity index increases withthe concentration of solvent as well as with theduration of treatment. However, the effect of con-centration is more prominent.

The crystallinity index values for samples treat-ed with benzyl alcohol show only slight increase

III....z~:>a:<ta:....coa:<t

zWuZ<t........~IIIZ<ta:....

1200 1000 800 600 400

WAVENUMBER,cm-1

Fig. 6 - IR absorption spectra of nylon-o films treated withformic acid and benzyl alcohol for 8 h [(a) control, (b) 10%

formic acid. (c) 50% formic acid, and (d) benzyl alcohol]

Fig. 7 - H, SALS patterns of nylon-o films treated with for-mic acid and benzyl alcohol [(a) control, (b) 10% formic acid,

(c) 50% formic acid. and (d) benzyl alcohol]

in crystallinity. It is observed that, in general, thecrystallinity index calculated from IR spectra giveshigher values.3.6 Spherulitic Structure ofNylon-6 Films

The H, patterns for the control and treated nyl-on-6 film samples are shown in Fig. 7. The pat-terns are typical of undeformed spherulites. It isobserved from the figure that solvent pretreat-ment does not affect bulk spherulitic morphology

97

INDIAN J. FIBRE TEXT. RES., SEPTEMBER 1990

- ."a. • -- .•.•• - ",.,...-..••,~1Il.ir '- -

Fig. R - Scanning electron micrographs illustrating solventetching on nylon-6 film surface due to formic acid [(a) con-trol. (0) 3D'X, formic acid, and (c) 50% formic acid. Magnifica-

tion:(a) x4420.(b) x4510,and(cj x 4440j

appreciably. The average spherulite size calculat-ed using the radial intensrty distributions?' at 45°azimuth was 9.0 f-l for the control sample and 9.7j1 for the treated samples.

To gain better insight into the mechanism ofsolvent-induced interactions, it was decided tostudy the surface morphology using SEM. Thescanning electron micrographs of the control sam-ple as well as of the samples treated with formicacid are shown in Fig. 8. It is seen that the con-trol film possesses well-developed spherulites, thespherulitic nuclei and fibrillar detail being clearlyvisible. However, the films treated with formic ac-id show sufficient destruction of surface morphol-ogy due to solvent etching. For the film treatedwith 50% formic acid, any of the surface featurestypical of spherulitic crystallization are barely re-cognizable. The average size of spherulite was 8.6fl for the control sample.

4 ConclusionSolvent-induced crystallization of nylon-6 by

formic acid and benzyl alcohol results in the for-mation of a-crystallites. Formic acid is more ac-tive than benzyl alcohol in causing structuralchanges. Solvent-induced crystallization is also ac-companied by surface etching.

AcknowledgementThe authors are thankful to Mis Garware Nyl-

ons Ltd for supplying the nylon films. They arealso indebted to the Surface Science Laboratory,University of Western Ontario, Canada, for per-mitting the use of facilities. One of the authors(NVB) is grateful to the Centre for Chemical Phy-sics, University of Western Ontario, Canada, forthe award of a senior visiting fellowship.

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