Effect of water absorption on the plastic deformation behavior of nylon 6

European Polymer Journal 45 (2009) 757–762

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Effect of water absorption on the plastic deformation behavior of nylon 6 V. Miri *, O. Persyn, J.-M. Lefebvre, R. Seguela Laboratoire de Structure et Propriétés de l’Etat Solide, UMR CNRS 8008, Université de Lille 1, Batiment C6, 59655 Villeneuve d’Ascq, France

a r t i c l e

i n f o

Article history: Received 23 July 2008 Received in revised form 30 October 2008 Accepted 2 December 2008 Available online 16 December 2008

Keywords: Nylon 6 Polyamide 6 Tensile yielding Humidity Plastic deformation

a b s t r a c t The tensile behavior of nylon 6 films has been investigated in relation to water content. Modification of chain mobility in the amorphous phase via water plasticization appears to have a determining impact on the stress–strain response. More specifically, both yield stress value and hardening behavior over a large strain domain are strikingly equivalent for samples drawn at same DT between draw temperature Td and main amorphous relaxation temperature Ta. This apparent lack of thermal activation of crystal plasticity in the fibrillar transformation suggests that crystal block fragmentation proceeds via H-bond unzipping through water penetration at defective crystal interfaces. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Linear chain nylons are thermoplastic polymers with relatively low crystallinity as compared to linear polyolefins. Inter-chain hydrogen bonds (H-bonds) are responsible for the build up of a strongly anisotropic sheet-like structure in the crystalline phase [1]. Besides, the rather high intermolecular cohesion due to the H-bonds results in high melting temperature (Tm). The glass transition temperature (Tg) of aliphatic nylons is also rather high as compared to polyethylene due to inter-chain H-bonding and increases significantly with the concentration of amide groups. Nylon 6 has been the most investigated material of the nylon class. Previous studies have demonstrated that plasticity occurs through a thermally activated process [2–4]. Argon et al. [5–8] have extensively investigated the plastic behavior of isotropic and textured bulk samples by transmission electron microscopy and X-ray diffraction. Plastic deformation of textured samples proceeds by crystal slip both parallel and across the H-bonded sheets. Fragmenta-

* Corresponding author. Tel.: +33 (0) 3 20 33 64 16, fax: +33 (0) 3 20 43 65 91 (V. Miri); tel.: +33 (0) 3 20 43 49 55, fax: +33 (0) 3 20 43 65 91 (J.-M. Lefebvre). E-mail addresses: [email protected] (V. Miri), jean-marc. [email protected] (J.-M. Lefebvre). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.12.008

tion of the crystalline lamellae and subsequent unfolding of the chains have been shown to occur during the plastic texturing of the initial spherulitic material via channel die drawing. Shear bands have also been identified in the inter-spherulitic boundaries [7]. These deformation features of nylon 6 are rather similar to those reported for high density polyethylene [9] in spite of a significant difference in crystal content of the two polymers, and in the nature of interchain interactions as well. Using Atomic Force Microscopy, Ferreiro et al. [10] revealed that intercrystalline amorphous layers play a major role in the activation of plasticity in isotropic spherulitic nylon 6 films at room temperature (RT) and 50% relative humidity (RH). Indeed, intra-spherulitic shear bands were shown to first initiate within the amorphous layer. Heterogeneous crystal slip proceeds when the shear bands have entirely crossed the spherulites. The lateral extent of the crystal lamella fragments turned out to be directly related to the breadth of the amorphous shear bands which crossed them. Nylon 6 is known to be hygroscopic, due to the presence of H-bonds. If amorphous phase plasticization by water sorption has been largely recognized, water effect on the crystalline phase is not thoroughly understood. Some authors have reported slight changes in the crystal unit cell of nylon 6 and/or nylon 66 in the presence of water

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[11–15]. However, measurement of unmodified crystal elastic constants of the a form of nylon 6 in the presence of water strongly supports the fact that water is only absorbed by the amorphous phase [11]. More recent work based on Nuclear Magnetic Resonance measurements conclude for negligible water inclusion in the crystal [16–18]. Absorbed water considerably affects the mechanical behavior, the main effects being stiffness drop together with toughness improvement [1,8,19]. The present paper reports on the influence of moisture on the drawing behavior of nylon 6 films having various crystalline forms. Particular attention is paid to the temperature domain above the glass transition where amorphous chains have high mobility.

performed by quick transfer of the samples from the conditioning box to the RSAII chamber and rapid cooling below RT in order to preserve the moisture content. Tensile testing measurements were carried out on a Model 4466 Instron machine equipped with an environmental chamber. Humid samples were tested in the presence of a dish containing the preparation solution. Samples with gauge dimensions 25  5 mm2 were drawn at a constant crosshead speed of 50 mm/min, i.e. at an initial strain rate of about 10 2 s 1. Three specimens have been tested for each condition. 3. Results 3.1. Characterization of water sorption in the various films

2. Experimental The 80-lm thick Polyamide 6 (PA6) film were cast at 275 °C and cooled on a chill roll at 22 °C. These films having a predominant mesomorphic b form are labeled bPA6. Films with predominant a crystalline form (aPA6) were produced by thermal treatment of bPA6 films in superheated water for 1 h at 150 °C, following a previously described method [20]. Films with predominant c crystalline form (cPA6) were prepared via reversible iodine complexation of bPA6 film at RT. Iodine was removed by sodium thiosulfate treatment [20]. The crystal weight fraction was roughly the same in the three films, i.e. of the order of 30%. The three kinds of films were kept dry in desiccating boxes under dynamic vacuum at RT for 3 days prior to physical or mechanical testing. Some samples were also conditioned at 92% RH for three days at RT in an environmental chamber in the presence of KNO3 saturated solutions; they will be referenced as ‘‘humid samples”. The water content of the so-called dry and humid samples were measured by thermo-gravimetric analyses at a heating rate of 20 K/min, using samples of about 15 mg. The crystal structure of the films was analyzed by wideangle X-ray scattering (WAXS) in reflection mode on a Panalytical X’pert-pro MPD generator, using the Ni-filtered Cu–Ka radiation of a Philips tube operated at 45 kV and 40 mA. The intensity profiles were recorded owing to a Panalytical X’celerator counter with scanning steps of 0.02°. The incidence of water treatment on crystal structure evolution of the films was analyzed by Differential Scanning Calorimetry (DSC). Samples of about 10 mg were scanned at a heating rate of 10 K/min over the temperature range [20/250 °C], using a Perkin–Elmer DSC7 apparatus. Ex situ unpolarized Fourier Transform Infrared (FTIR) measurements were carried out on drawn samples in order to follow the strain-induced phase changes, according to a previously described procedure [21]. The results are given in terms of relative fractions of each crystal form. Dynamic Mechanical Analysis (DMA) was carried out in tensile mode on a RSAII Rheometrics equipment operated at a frequency of 1 Hz, with a strain amplitude of 10 3. The data points were recorded every 3 K after 1 min isothermal regulation before measurement, in the range [ 50/ +130 °C]. The rectangular test pieces were 25 mm long and 6 mm wide. Experiments on water-plasticized films were

Fig. 1 shows the evolution of moisture release in the bPA6 cast films as a function of temperature. The initial water content is 8.8% at RT. Almost all water has disappeared at 100 °C. The kinetics of water release are rather low at RT, as can be seen in the insert of Fig. 1. Similar data have been obtained for aPA6 and cPA6 films. These findings support the assumption of a fairly constant water content during the tensile tests at RT in the case of the humid samples. As mentioned in the introductory part, the question of water location in the sample is a crucial issue with respect to the mechanical behavior of the humid films. Fig. 2 compares the WAXS profiles of the aPA6 film in the dried and humid states. No major changes are observed between the two profiles. This corroborates the literature data arguing that water is mainly stored in the amorphous phase. In the case of the bPA6 films, such a WAXS analysis may not be as informative considering the very broad scattering peak, due to mesomorphic chain arrangement [20]. Therefore, DSC measurements have been performed on both dried as-cast bPA6 specimens and after water removal from a bPA6 film previously water-treated at 92% RH for three days. The DSC heating traces of Fig. 3 do not reveal any detectable modification of the exotherm associated with the reorganization of the metastable b form into the thermodynamically stable a form [20,21]. The enthalpy change of the b–a reorganisation exotherm remains constant within measurement accuracy. This is an indication of a strong stability of

Fig. 1. Evolution of the water uptake for the bPA6 as a function of ) dried; ( ) humid samples. The insert temperature: ( shows the kinetic of water release under nitrogen flow in the case of the bPA6 for the 92% RH conditioning at room temperature.

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dried α PA6 humid

α PA6

tan delta

Intensity (a.u.)

0.15

0.1

dried β PA6

0.05

humid β PA6

16

18

20

22

24

26

0 -50

28

2θ

0

50

100

Temperature (°C) Fig. 2. WAXS profiles of the aPA6 film: ( samples.

) dried; (

) humid Fig. 4. Loss factor versus temperature for the (d) dried and (j) humid samples of the bPA6 film.

Fig. 5 compares the engineering stress–strain curves recorded for the three films bPA6, aPA6, and cPA6 in dry and humid states, for the same draw temperature Td = 25 °C. In each case, the yield stress is much lower for the humid sample. Since the crystal phase is little affected by ab-

Heat flow (a.u.)

Endo up

130

180

230

Temperature (°C) Fig. 3. DSC thermograms of bPA6 films ( ) dried; (







) after water removal from a film previously water-treated at 92% RH.

the b form in the presence of water. As in the case of the crystalline a form, this suggests that water is unable to penetrate the mesomorphic phase, in spite of its lower packing density, namely qb = 1.10 ± 0.02 g/cm3 [22–24] as compared with qa  1.23 g/cm3 [25]. It is also worth noticing that storing for several months at ambient humidity conditions, i.e. 40–60% RH, also does not affect the crystal structure of the bPA6 film. Fig. 4 shows the loss factor as a function of temperature for the bPA6 films in the dried and humid states. Regarding the humid bPA6 film, the low temperature relaxation peak, located around 5 °C relates to the main relaxation of the water-plasticized amorphous phase, whereas the peak at 80 °C is related to the activation of the same main relaxation in the dried amorphous phase. In fact, the broadness of the peak stems from the kinetics of water release during the slow heating scan. This is corroborated by the corresponding plot for the dried bPA6 film which exhibits a single amorphous relaxation peak at about 80 °C. Similar results have been obtained for the aPA6 and cPA6 films: dried films display a main relaxation at Ta = 80 ± 1 °C, whereas the water saturated ones have Ta = 5 ± 1 °C.

80

40

0

Engineering stress (MPa)

80

80

β PA6

40

0

Engineering stress (MPa)

30

Engineering stress (MPa)

α PA6

80

γ PA6

40

0

3.2. Drawing behavior of the dry and humid films

0

1

2

3

4

Engineering strain

The tensile drawing of the humid films has only been performed at room temperature in order to prevent water release during the experiments.

Fig. 5. Engineering stress–strain curves for the ( ) dried and ) humid samples of the three kinds of films for the same draw ( temperature Td = 25 °C.

V. Miri et al. / European Polymer Journal 45 (2009) 757–762

4. Discussion Recalling that crystal plasticity is unambiguously a thermally activated process, the mechanical and structural results of Figs. 6 and 7 are quite striking, owing to the 85 K difference between the draw temperatures of the dry and humid samples. Plastic deformation in semi-crystalline polymers beyond yielding operates via fragmentation of the crystalline lamellae and unravelling of the folded chains. The analogous behavior of dried and humid films observed at same DT = Td Ta both underlines the major role of amorphous chain mobility in the so-called fibrillar transition, and points at the intriguing fact that, in the presence of humidity, the crystal phase does not respond through a conventional thermally activated glide process. In other words, why does crystal plastic resistance seem to have such a minor incidence on plastic yielding? Previous AFM studies of isotropic films with spherulitic morphology revealed the major contribution of the amorphous phase to the shear yielding processes in PA6 films below 160 °C [10].

Engineeringstress stress(MPa) (MPa) Engineering

80

α PA6

40

80 Engineering stress (MPa)

sorbed water, one might expect that the critical shear stress of the active crystal slip planes is not be significantly different in the dried and humid samples. The observed difference in the yield stress magnitude should therefore be ascribed to the fact that the amorphous phase is either glassy or rubbery in the dry and humid samples, respectively. It can be noticed that in the dried state, the three films give rise to periodic stress oscillations during neck propagation. Similar results were formerly reported for other polymers such as nylon 6–10 [26], PP and PET [27]. The authors have ascribed this phenomenon to periodic adiabatic heat release associated with structural changes. In order to clarify the role of the amorphous phase on yielding, tensile tests have been carried out under experimental conditions allowing equivalent amorphous chain mobility. The dried and humid films have been drawn at same DT = Td Ta = 30 K, i.e. Td = 25 °C for humid films and Td = 110 °C for dry films. The engineering stress–strain curves and the yield stress values recorded for the three kinds of films, namely bPA6, aPA6 and cPA6 are presented in Fig. 6 and Table 1, respectively. The dry and humid samples only slightly differ regarding the yield stress value, whatever the initial crystal form. Moreover, the strain hardening behavior is strikingly similar. For the sake of better comparison, Table 1 reports the corresponding yield stress data from Fig. 5 for all samples. The structural changes of the dry and humid bPA6 films drawn at DT = 30 K have been followed via post mortem FTIR measurements. Results are reported in Fig. 7. Both materials exhibit a strain-induced structural transition of the predominant mesomorphic b form into the a crystal form. This result has been already reported in previous papers [4,22,28–30]. However, it is interesting to note that under the present conditions of equivalent amorphous chain mobility, the b a transition displays the very same evolution as a function of strain.

β PA6

40

80 Engineering stress (MPa)

760

γ PA6 40

0

0

1

2

3

4

Engineering strain

Fig. 6. Engineering stress–strain curves for the ( ) dried and ( ) humid samples of the three kinds of films for the same temperature gap Td– Ta = 30 °C.

Table 1 Yield stress ry of the three kinds of films for the humid and dry samples stretched at 110°C and 25°C. Sample

aPA bPA

cPA

ry (MPa)

ry (MPa)

Dry (T = 110 °C)

Humid (T = 25 °C)

ry (MPa) Dry (T = 25 °C)

29 ± 2 17 ± 2 22 ± 2

32 ± 2 16 ± 2 18 ± 2

72 ± 2 64 ± 2 70 ± 2

Galeski et al. have considered that plastic deformation of crystalline polymers under tensile drawing results from a competition between cavitation in the amorphous phase and activation of crystal plasticity. Cavitation is the easiest phenomenon in polymers which exhibit high crystal plastic resistance, as is the case for dry PA6 drawn at room temperature. These cavities were shown to be unstable and quickly healed due to their particularly small size in relation to the interlamellar spacing [31]. Their occurrence was nevertheless drastically reduced in the case of waterplasticized materials [8]. The inference of a transient cavitational event in the amorphous phase as playing a major

V. Miri et al. / European Polymer Journal 45 (2009) 757–762

Crystal fraction (%)

100

80 β 60

40 α 20

0

0

0.5

1

1.5

2

2.5

Engineering strain Fig. 7. Evolution with deformation of the b and a form relative contents ) dried and ( ) humid bPA6 films for the same in the ( temperature gap Td Ta = 30 °C.

role in crystal plasticity may therefore hardly be envisioned. Kramer’s analysis of ageing effect on the RT yielding of nylon 6–10 also addressed the question of the amorphous phase contribution [32,33]. This author assumed that chain unfolding operates via transient transformation of crystalline regions into amorphous state while amorphous regions reorganize into crystallites. Within this frame, the temporary breakage of H-bonds that is necessary for chain unfolding will result in yield stress sensitivity to H-bond strength, i.e. to the draw temperature. Such a scheme is however unable to account for the present data at varying humidity content, if water is assumed to be confined in the amorphous phase. In order to provide an alternative interpretation of hydrogen bonding disruption in the crystalline lamellae, one has to infer water molecule penetration via already present stacking faults that will be preferred loci for the coarse slip process and fragmentation. This hypothesis is consistent with Boukal’s observations [11] of crystal unit cell distortions in the presence of water, without changes in the crystal elastic constants. As a matter of fact, the latter finding suggests the existence of large water-free crystal domains whereas in the meantime crystal distortions suggest penetration of water molecules in specific defective regions. In support to the present arguments, it is worth reconsidering a case in which amorphous chain mobility of PA6 is modified via a miscible amorphous macromolecular compound [34]. This amorphous polyamide (aPA) blended to PA6 is a semi-aromatic random copolyamide obtained from the condensation of hexamethylene diamine (1 mol) with isophthalic acid (0.7 mol), and terephthalic acid (0.3 mol), namely PA6I-6 T, which does not interact with the crystalline phase. Blends with various compositions were uniaxially drawn at equivalent amorphous phase mobility as in the present case. Data quoted here are extracted from Fig. 6 and Table 2 in Ref. [34]. The yield stress data normalized to equivalent crystal content unequivocally show that at same DT = Td Ta, material with the lower draw temperature exhibits the higher stress level, contrary to the present situation. For instance, pure PA6

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(Ta  80 °C) drawn at 100 °C exhibits a plateau stress of 20 MPa whereas PA6/aPA 60/40 (Ta  110 °C) drawn at 130 °C displays a plateau stress of the order of 11 MPa It is worth adding that both materials exhibit the same yield stress at Td = 130 °C. These two facts confirm the thermal activation of the deformation process in a situation where the added amorphous component is unable to interfere with crystal plasticity. As a conclusion, assuming water penetration in defective crystal interfaces provides a reasonable explanation for the prime role of the Td Ta parameter on the draw stress level over a large strain domain. Indeed, the unzipping of interchain H-bonds during chain unfolding occurs in a water molecule environment. The specific pathway of the water-triggered crystal transformation still remains unsolved. Additional investigations on both the mechanical and structural aspects of the problem need to be carried out for providing a more precise interpretation of the phenomenon. Acknowledgements The authors are indebted to Dr. F. Cazeaux from LCOM, and F. Capet from LDSMM UST Lille for giving access to the TGA equipment and X-ray equipment, respectively. References [1] Nylon plastics handbook, Kohan MI, editors, Munich: Hanser Pub.; 1995. [2] G’Sell C, Jonas JJ. Yield and transient effects during the plastic deformation. J. Mater. Sci. 1981;16(7):1956–74. [3] Lin L, Argon AS. Rate mechanism of plasticity in the crystalline component of semicrystalline nylon 6. Macromolecules 1994;27(23):6903–14. [4] Penel-Pierron L, Seguela R, Lefebvre JM, Miri V, Depecker C, Jutigny M, et al. Structural and mechanical behavior of nylon 6 films. II. Uniaxial and biaxial drawing. J Polym Sci Polym Phys 2001;39(11):1224–36. [5] Lin L, Argon AS. Deformation resistance in oriented nylon 6. Macromolecules 1992;25(15):4011–24. [6] Galeski A, Argon AS, Cohen RE. Deconvolution of X-ray diffraction data to elucidate plastic deformation mechanisms in the uniaxial extension of bulk nylon 6. Macromolecules 1991;24(13):3945–52. [7] Galeski A, Argon AS, Cohen RE. Morphology of bulk nylon 6 subjected to plane strain compression. Macromolecules 1991;24(13):3953–61. [8] Galeski A, Argon AS, Cohen RE. Changes in the morphology of bulk spherulitic nylon 6 due to plastic deformation. Macromolecules 1988;21:2761–70. [9] Lin L, Argon AS. Structure and plastic deformation of polyethylene. J Mater Sci 1994;29(2):294–323. [10] Ferreiro V, Coulon G. Shear banding in strained semicrystalline polyamide 6 films as revealed by atomic force microscopy: role of the amorphous phase. J Polym Sci Polym Phys 2004;42(4):687–701. [11] Boukal I. Effect of water on the mechanism of deformation of nylon 6. J Appl Polym Sci 1967;11(8):1483–94. [12] Miyasaka K, Ishikawa K. Effects of temperature and water on the c ? a crystalline transition of nylon 6 caused by stretching in the chain direction. J Polym Sci Polym Phys 1968;6(7):1317–28. [13] Campbell GA. Effect of water sorption on bulk nylon-6 as determined by X-ray crystallinity. J Polym Sci Polym Phys 1969;7(9):628–34. [14] Ishikawa K, Miyasaka K, Okabe T, Yamada M. Lattice deformation of nylon 6 crystal by water sorption. Sen’i Gakkai 1969;25(1):11–5. [15] Hinrichsen G. The role of water in polyamides. Colloid Polym. Sci. 1978;256(1):9–14. [16] Murthy NS, Stamm M, Sibilia JP, Krimm S. Structural changes accompanying hydratation in nylon 6. Macromolecules 1989;22(3):1261–7.

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