- Email: [email protected]
Substrate effects on the surface properties of nylon 6
Applied Surface Science 282 (2013) 115–120
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Substrate effects on the surface properties of nylon 6 Qi Zhou a,∗ , Jianghua Fang a , Haoqi Gao a , Leslie S. Loo b a b
Department of Chemical Engineering, Ningbo University of Technology, Ningbo 315016, PR China Department of Chemical and Biomedical Engineering, Nanyang Technological University, 637459 Singapore
a r t i c l e
i n f o
Article history: Received 18 March 2013 Received in revised form 9 May 2013 Accepted 13 May 2013 Available online 2 June 2013 Keywords: Nylon 6 Surface structure and properties Spin coating Substrates FTIR trichroic analysis
a b s t r a c t Thin nylon 6 films spin coated onto different substrates were characterized by wide-angle X-ray scattering (WAXS), Fourier transformation infrared (FTIR) spectroscopy, FTIR trichroic analysis and water contact angle (CA) measurements. The morphology and hydrophilicity of the nylon 6 surface were found to be greatly influenced by the nature of the substrate. The film surface contained primarily ␣-form crystals when it was prepared using hydrophobic substrates. This film was more hydrophobic. However, for hydrophilic substrates, the film surface comprised mainly ␥-form crystals and demonstrated greater hydrophilicity. FTIR trichroic analysis of the N H stretching vibration mode revealed that the presence of a hydrophilic substrate caused the N H stretching bonds on the film surface to align preferentially in the direction perpendicular to the surface. Such effects were attributed to molecular interactions between the N H bonds of the polymer and Si O bonds of the hydrophilic substrate. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction The structure and morphology of nylon 6 have been extensively investigated to obtain high performance materials, such as nanocomposites, films and blends throughout the years. In some cases, it had been reported that the crystalline morphology on the surfaces showed different conformations relative to the bulk [1,2]. Surface characterization was of great importance for the understanding of polymer adsorption in terms of chain orientation and conformation. At the same time, it served to provide an overall view of the polymer properties from surface level to bulk material [3]. Recently, the migration phenomenon of nylon 6 nanocomposites during the formation of nanocomposites had been reported. The resulting nanocomposites showed enhanced surface mechanical and physical properties, such as better friction, wear and hydrophilicity [4–6]. Misra et al. [7] used atomic force microscopy (AFM), and transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (TEM/EDAX) to study the crystalline morphology of the surface of nylon 6/polyhedral oligomeric silsesquioxane (POSS) nanocomposites. They observed that POSS molecules migrated to the surface resulting in an increase in stiffness and hardness. In our latest study [8], we investigated the effect of nanofillers such as POSS and layered silicates on the surface properties of spin coated nylon 6 films. Our studies also indicated it was necessary to understand the fundamental influence of the most
∗ Corresponding author. Tel.: +86 57486066812. E-mail address: [email protected] (Q. Zhou).
basic parameters (substrate materials) on the condensed state of nylon 6. More specifically, the resulting morphology and properties of nylon 6 crystal structure would depend on the nature of the substrate materials. Understanding such a relationship would provide insights in improving the surface properties and the migration of nanoparticles. Currently, the effect of substrate materials on nanocomposite formation was seldom investigated. It is known that nylon 6 exhibits polymorphism, and that the polymorph depends on the crystallization condition. At room temperature nylon 6 shows two crystalline modifications, namely the ␣ phase and the ␥ phase. Scheme 1 presents the two crystalline phase structure, including the direction of N H bonds [9]. The principle differences between these two phases lie in the lattice parameters and the orientation of the hydrogen bonds between the N H and C O groups. The ␣ phase has a monoclinic structure with a = 0.956 nm, b = 1.724 nm, c = 0.801 nm, and ˇ = 67.5◦ . The hydrogen bonds are formed between antiparallel chains in the ␣ phase. The ␥ phase also has a monoclinic structure with a = 0.933 nm, b = 1.688 nm, c = 0.478 nm, and ˇ = 121◦ . But the twisted chains allow hydrogen bonds to be formed between parallel chains. The ␣ phase is easily obtained by melt crystallization, while severe crystallization conditions are needed to obtain the ␥ phase. Moreover, the ␣ phase can be transformed into the ␥ phase by treatment with aqueous potassium iodide–iodine solution. Furthermore, the ␣ phase has been found to be the most stable structure of nylon 6 crystals [10–12]. In this paper, it was reported for the first time that different substrates can affect the crystalline morphology of nylon 6 films. The influence of the substrates on the crystalline state of nylon 6 films
0169-4332/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.05.075
116
Q. Zhou et al. / Applied Surface Science 282 (2013) 115–120
Scheme 1. Schematic of the ␣ and ␥ crystalline forms of nylon 6 as seen from end and side-view of each crystal. Closed and open circles represent chain axes projecting out of and into the page, respectively. Hydrogen bonds between polyamide chains are represented by dashed lines [9].
was studied using wide-angle X-ray scattering (WAXS), Fourier transform infrared (FTIR) trichroic analysis and water contact angle (CA) measurements.
2. Experimental 2.1. Materials Pellets of nylon 6 were purchased from Nanocor, Inc. The solvent 2,2,2-trifluoroethanol was obtained from Sigma–Aldrich. The optical glasses were Pyrex Borosilicate Glass, purchased from SG scientific glass-blowing centre. The glass composition, as provided by the manufacturer, was 80.6% SiO2 , 13.0% B2 O3 , 4.0% Na2 O and 2.3% Al2 O3 . Polystyrene petri dish substrates were provided by Vacutest KIMA Company.
2.2. Nylon 6 thin film formation Nylon 6 pellets were first dried in a vacuum oven for more than 24 h at room temperature and then stored in a desiccator. In order to make the thin films, the pellets were dissolved in 2,2,2trifluoroethanol (about 5 wt%), and the solution was spin coated onto the optical glass and polystyrene substrates at five different spin rates: 1000 rpm, 2000 rpm, 4000 rpm, 6000 rpm and 8000 rpm. The films on the glass substrates were dried in vacuum between 70 and 80 ◦ C for more than 12 h and cooled to room temperature. The films on the polystyrene substrates were dried in vacuum between 35 and 40 ◦ C for more than 48 h and cooled to room temperature. Lastly, the films together with substrates were stored in desiccator prior to use.
2.3. Thickness measurement Film thickness was measured using a Tencor Instruments Alpha Step 500 stylus profilometer and a Micro-Xam surface mapping microscope. Each film was measured 4 times. Finally, the average value of the film thickness was calculated.
2.4. WAXS 2 scans were performed on a Rigaku Rint X-ray generator with Cu K˛ radiation at a voltage of 40 kV and a current of 40 mA. The Xray beam irradiated at the samples was fixed at a constant grazing angle of 1.0◦ , while the detector counted the diffracted signals with 2 ranging from 10◦ to 30◦ . Scan speed was 0.50◦ /min and scan step was 0.02◦ . The divergence, scatter and receiving slit were set at 1.00◦ , 1.00◦ and 0.30 mm respectively. 2.5. FTIR and FTIR trichroic analysis FTIR spectroscopy was performed on a Nicolet Nexus spectrometer. The detector is a liquid nitrogen cooled mercury cadmium telluride (MCT) with a range of 650–4000 cm−1 . The resolution was 4 cm−1 . For trichroic (three-dimensional orientation) analysis of the film, infrared spectra in the thickness direction were obtained by the sample-tilting procedure described by Schmidt [13]. The beam spot size was set to 5.08 mm diameter at the sample with a total beam divergence of 1.9◦ . A custom-made device was used to rotate the films around a vertical axis. For normal transmission, the film surface was positioned perpendicular to the infrared beam. This was taken to be the reference angle, viz. 0◦ . Refractive index measurements were obtained from vertically polarized spectra of the films at angles of ±45◦ . The spectra in the thickness direction were then calculated from horizontally polarized spectra of the films aligned at 0◦ and 45◦ . 64 scans were collected for each spectrum and zero-filled twice. A Pike wire grid infrared polarizer was used to obtain the polarized IR beam. 2.6. Water CA measurements Contact angles were measured on a horizontal surface using a FTA200 Dynamic Contact Angle Analyzer, determined using a sessile droplet of ultrapure water (Milli-Q, Millpore Corp., 8.02 M cm) having a volume of ∼50 l at room temperature (about 23 ◦ C), and determined at the triple point of air, water and nylon 6. CA for the upside (the surface which was not in contact with the substrate) of the films was measured directly when the films
Q. Zhou et al. / Applied Surface Science 282 (2013) 115–120
117
Fig. 1. (a) WAXS results and (b) FTIR spectra of the 900–1250 cm−1 region of nylon 6 films spin coated on glass substrates at different spin rates.
were still on the substrate. To obtain CA data for the downside (the surface of the film in contact with the substrate), the films were first peeled off the substrate and then attached to microscope glass slides. Measurements were performed for both the surfaces that were just peeled off and the surfaces that had been exposed in air for 3 days after being peeled off. CA data reported were averaged over five locations on every surface.
3. Results and discussion 3.1. Crystallinity studies Fig. 1(a) shows the WAXS date of nylon 6 films spin coated on optical glass at different spin rates. At the lower spin rates of 1000 and 2000 rpm, the two main peaks observed are at 20.5◦ and 24◦ . As the spin rate increases to 4000 rpm, these two peaks gradually decrease in magnitude. Meanwhile, the peak at 21.5◦ gradually becomes more dominant. Finally at high spin rates of 6000 and 8000 rpm, only one main peak at 21.5◦ is observed. This phenomenon indicates that when spin rate is increased, the morphology of the films changes from mainly ␣-crystals to mainly ␥-crystals. Fig. 1(b) shows the FTIR spectra of the films spin coated on glass. It is observed that the peak at 1202 cm−1 for the 1000 and 2000 rpm films have a much higher intensity than that for the 4000, 6000 and 8000 rpm films. The absorbance at the 1030, 961, and 931 cm−1 peaks decrease gradually when the spin rate increases. It indicates that the proportion of ␣-crystals in the films is decreasing. The increasing intensity of the peak at 976 cm−1 shows that the amount of ␥-crystals in the films increases with increasing spin rate. Fig. 2(a) shows the X-ray diffraction intensity against 2 plots of nylon 6 films spin coated on hydrophobic polystyrene substrates at different spin rates. Peaks at 20.5◦ and 24◦ are attributed to the (0 2 0) and (0 0 2) planes of the ␣ crystals while the peak at 21.5◦ corresponds to the (2 0 0) hydrogen-bonded planes of ␥ crystals [14–17]. For all spin rates, the X-ray patterns are identical and the dominant morphology is the ␣ crystals form in the films. Fig. 2(b) shows the FTIR spectra of the films. FTIR band assignments for both ␣ and ␥ crystals of nylon 6 have been reported in the literature [3,18–20], and are list in Table 1 as well. The bands at 931, 961, 1030 and 1202 cm−1 are attributed to the ␣ crystals, whereas the band at 976 cm−1 is attributed to the ␥ crystals. In Fig. 2(b), it is observed that all the FTIR spectra are similar and the ␣ crystals dominate, confirming the WAXS results. At low spin speeds, the morphology of the films on polystyrene and glass substrates is similar. However, they differ drastically at high spin speeds. At first sight, it appears that the morphology of
the films will depend on both spin speeds and the nature of the substrate. However, it is important to note that the XRD and FTIR data are a spatial average of the bulk and surface properties of the films. Hence, at lower spin speeds, the films are thicker so that the results are more representative of the bulk morphology of the films. However, with increasing spin speed, the films become thinner. Consequently, the surface properties of the film in contact with the substrate will contribute more to the XRD and FTIR data (i.e. we are indirectly probing more of the surface properties of the film as the film thickness is reduced). With this in mind, it is inferred that the observed differences in WAXS and FTIR data at high spin speeds are not due to the effect of different spin speeds, but are caused by the effect of the substrates on the surface morphology of the films. For the hydrophobic substrates, the data from Fig. 2 suggests that such substrates do not affect the surface morphology of the film in contact with it, so that the bulk morphology and surface morphology are similar, thereby giving rise to similar WAXS and FTIR data at all film thicknesses. On the other hand, the hydrophilic glass does affect the morphology of the film surface in contact with it. The effects of the glass substrate are observed in the thinner films formed at the highest spin speeds, where the surface morphology is dominated by the ␥-crystals instead of the ␣-crystals. At high spin speeds, the films are thinner and more of the surface which contains ␥-crystals is revealed in the WAXS and FTIR data. At lower spin speeds, the bulk properties dominate in the WAXS and FTIR results and hence show predominantly the ␣-crystals. These results indicate that the hydrophilic glass substrates favour the formation of ␥-crystals in nylon 6. 3.2. Water CA measurement Table 1 shows the water CA measurements of nylon 6 films formed on glass substrate. It is seen clearly that the CA is independent of spin speed for the same substrate. However, the CA is substantially different between the upside and downside surfaces. Fig. 3 shows the scheme of measuring position and the Table 1 The major crystalline and amorphous characteristic infrared bands of PA6. (cm−1 ) 931 961 976 1030 1202 3000–3500
Based assignments [3,18–20] ␣ crystalline phase amide stretching mode CONH of ␣ crystal CONH of ␥ crystal ␣ crystalline phase squeletal stretching ␣ crystalline phase carbonyl wagging mode Hydrogen bonded N H stretch
118
Q. Zhou et al. / Applied Surface Science 282 (2013) 115–120
Fig. 2. (a) WAXS date and (b) FTIR spectra of the 900–1250 cm−1 region of nylon 6 films spin coated on polystyrene substrates at different spin rates.
of the films comparing with the downside which makes films more hydrophobic. 3.3. Orientation of N H bonds FTIR trichroic measurements of the N H stretching in the 3000–3500 cm−1 region were used to observe the orientation of N H bonds [11]. The spectrum in the thickness direction was calculated based on the procedure described by Schmidt [13]. The reflective index (n) of nylon 6 films was measured by a Prism Coupler, and the average value of films made by different spin speed was used. Table 4 shows the values of the reflective index. The spectrum in the thickness direction Az can then be calculated from the following formula:
Fig. 3. The scheme of measuring position and the characteristic CA results.
characteristic CA results. It confirms our earlier hypothesis that the morphological changes observed in WAXS and FTIR are due to substrate effects rather than spin speeds. The upside surfaces have CAs ranging from 61◦ to 63◦ , while the CAs for the downside surfaces range from 45◦ to 46◦ (when measured immediately after the films were peeled off the substrate). Hence the surfaces of nylon 6 films demonstrated more hydrophilic character when they were formed in the presence of a hydrophilic substrate such as glass. However, after the more hydrophilic surfaces were exposed to air for 3 days, the CAs increased to 54◦ –58◦ . Table 3 shows the water contact angle measurements for nylon 6 films made on polystyrene. The CAs are also independent of spin speed. The values of CAs for the upside surface are similar to those formed on glass substrate. The downside CAs are larger than those for the glass substrate, and they remain unchanged after exposure in air for 3 days. This is because polystyrene does not change the orientation of N H bond of nylon 6 films. However, the downside CAs are slightly smaller than the upside CAs, due to the rougher upside
Table 2 Water contact angle of different sides of nylon 6 films coated on glass substrate. Spin rate (rpm)
Film thickness Upside (◦ ) (m)
1000 2000 4000 6000 8000
0.3256 0.2766 0.2348 0.1542 0.0852
a b
61.358 61.232 61.722 61.662 62.088
± ± ± ± ±
Downsidea (◦ ) 0.156 0.196 0.045 0.125 0.316
45.782 45.752 45.444 45.630 45.893
± ± ± ± ±
0.708 0.402 0.551 0.357 0.516
Measured immediately after being peeled off the substrate. Measured 3 days after being peeled off.
Downsideb (◦ ) 57.542 57.078 57.070 55.844 54.724
± ± ± ± ±
0.510 0.365 0.490 0.415 0.451
Az =
At
1−
sin2 (˛t ) n2
sin2 (˛t ) n2
− Ax
where n is the reflective index, Ax is the horizontally polarized spectrum of film normal to the beam and At is the horizontally polarized spectrum of the film tilted at ˛t to the beam. ˛t was set at 45◦ in our experiment. The vertically polarized spectrum Ay was also measured, and it is virtually identical with Ax , showing that there is no preferential alignment of the N H bonds in the plane of the film. Fig. 4 shows the Az and Ax spectra of the N H stretching mode of nylon 6 films spin coated at different spin speeds on polystyrene substrates. All the spectra show similar Az /Ax ratios of from 0.45 to 0.58, indicating that the alignment of the N H bonds in the bulk is similar to that of the surface. Hence it is concluded that there is little interaction between polystyrene and the nylon 6 surface. Fig. 5 shows the Az and Ax spectra of the N H stretching mode of nylon 6 films spin coated at different spin speeds on glass substrates. It is seen that the relative intensities of the N H stretching vibration in the Az and Ax spectra vary with spin speed. The Az /Ax Table 3 Water contact angle of different sides of nylon 6 films coated on polystyrene substrate. Spin rate (rpm)
Upside (◦ )
1000 2000 4000 6000 8000
62.820 62.288 60.890 60.210 60.296
a b
± ± ± ± ±
Downsidea (◦ ) 0.235 0.130 0.291 0.339 0.276
53.830 54.006 53.240 53.614 53.380
± ± ± ± ±
0.339 0.156 0.231 0.614 0.230
Measured immediately after being peeled off the substrate. Measured 3 days after being peeled off.
Downsideb (◦ ) 54.330 54.194 53.828 52.860 54.480
± ± ± ± ±
0.250 0.252 0.491 0.379 0.281
Q. Zhou et al. / Applied Surface Science 282 (2013) 115–120
119
Fig. 4. IR spectra of N H stretching mode for films coated on polystyrene substrate and prepared at different spin rates. The Az to Ax ratio reflects the orientation of N H bonds.
Fig. 5. FTIR spectra of N H stretching mode for films coated on glass substrate and prepared at different spin rates. The Az to Ax ratio reflects the orientation of N H bonds.
ratio increases monotonically from 0.56 to 1.248 with increasing spin speed. The film is thick at low spin speeds (Table 2). The results can be representative of the bulk. Hence, the Az /Ax ratios at 1000 and 2000 rpm in Fig. 4 are similar to those for the polystyrene Table 4 Experimental values of reflective index. Spin rate (rpm)
Reflective index
1000 2000 4000 6000 8000 Average
1.5338 1.5301 1.5231 1.522 1.5267 1.527 ± 0.002
substrate. However, at the highest spin speeds of 6000 and 8000 rpm, the film is so thin that the data are more representative of the surface. The high Az /Ax ratios observed can be attributed to the results of interactions between the glass substrate and the film surface. The structural absorbance A is given by: Hence, when Az /Ax increases, more N H bonds are aligned preferentially in the thickness direction. Similar analysis has also been performed for the C O stretching mode, it was found that the Az /Ax ratios are similar for all spin speeds regardless of the type of substrate. The result indicates that the interaction between glass and nylon 6 occurs primarily through the N H bonds rather than the C O bonds. Polystyrene is a hydrophobic material and does not possess any groups which can form hydrogen-bonding interactions with nylon
120
Q. Zhou et al. / Applied Surface Science 282 (2013) 115–120
6. Consequently, the morphology and properties of the surface in contact with polystyrene can mirror those of the bulk. On the other hand, the glass is hydrophilic and composed of oxides, mainly SiO2 , so its oxygen atoms tend to interact with the N H groups of the nylon 6 surface to form Si O· · ·H N hydrogen bonds. This can result in preferential alignment of the N H bonds perpendicular to the surface. The lack of hydrogen bond interactions between the C O groups of nylon 6 and glass may explain why such a preferential alignment is not observed in the trichroic analysis of C O groups. When the N H bonds are aligned perpendicular to the surface, it becomes much easier to adsorb water molecules, so the hydrophilicity of nylon 6 increases. However, after the surface is exposed to air, the CAs increase. N H bonds in the outmost layer of the film are oriented to the vertical direction at the beginning, but after expose in the air, these N H bonds will adsorb some water molecules, which will affect the orientation of N H bonds, making N H bonds bend towards the plane of the film. The preferential formation of ␥-crystals in the proximity of certain surfaces has been observed for other nylon 6 systems. For instance, the introduction of montmorillonite layered silicate also induced the formation of ␥-crystals [21]. The ␣-crystals consist of hydrogen-bonded sheets parallel to one another. The perpendicular alignment of N H bonds in the presence of a hydrophilic substrate such as glass or layered silicate can prevent the formation of such parallel sheets of the ␣-crystals. Hence ␥-crystals are preferentially formed near the surface. Far away from the surface, the constraints imposed by the substrate are absent, and ␣-crystals are dominated. 4. Conclusions The substrate effect on surface structure and hydrophilicity of nylon 6 was investigated for the first time. WAXS and FTIR results showed that the presence of hydrophilic substrates favoured the formation of the less stable ␥-crystals. It was caused by the orientation selectivity of N H bonds in nylon 6, which preferentially aligned in a direction perpendicular to the glass substrate. The understanding of such phenomena offered a way to manipulate the surface properties of polymers. Acknowledgements Funding from the project of Natural Science Foundation of China (No. 51203081), Zhejiang Province Department of Education Fund (Y201224385), the Natural Science Foundation of Ningbo (2012A610087) and Ningbo Science and Technology Innovation Team (2011B2002) are gratefully acknowledged.
References [1] A. Dasari, Z.Z. Yu, Y.W. Mai, Fundamental aspects and recent progress on wear/scratch damage in polymer nanocomposites, Mater. Sci. Eng. R-Rep. 64 (2009) 31–80. [2] J.A. Lee, Y.S. Nam, G.C. Rutledge, P.T. Hammond, Enhanced photocatalytic activity using layer-by-layer electrospun constructs for water remediation, Adv. Funct. Mater. 20 (2010) 2424–2429. [3] T. Elzein, M. Brogly, J. Schultz, Quantitative calculation of the orientation angles of adsorbed polyamides nanofilms, Polymer 44 (2003) 3649–3660. [4] M. Zammarano, J.W. Gilman, M. Nyden, E.M. Pearce, M. Lewin, The role of oxidation in the migration mechanism of layered silicate in poly(propylene) nanocomposites, Macromol. Rapid Commun. 27 (2006) 693–696. [5] M. Lewin, J. Zhang, E. Pearce, M. Zammarano, Polyamide 6 treated with pentabromobenzyl acrylate and layered silicates, Polym. Adv. Technol. 21 (2010) 825–834. [6] Y. Tang, M. Lewin, W.M. Pearce, Effects of annealing on the migration behavior of PA6/clay nanocomposites, Macromol. Rapid Commun. 27 (2006) 1545–1549. [7] R. Misra, B.X. Fu, A. Plagge, S.E. Morgan, POSS-nylon 6 nanocomposites: influence of POSS structure on surface and bulk properties, J. Polym. Sci. Part B: Polym. Chem. Phy. 47 (2009) 1088–1102. [8] Q. Zhou, K. Wang, L.S. Loo, Role of interface in dispersion and surface energetics of polymer nanocomposites containing hydrophilic POSS and layered silicates, J. Colloid Interface Sci. 335 (2011) 222–230. [9] T.D. Fornes, D.R. Paul, Crystallization behavior of nylon 6 nanocomposites, Polymer 44 (2003) 3945–3961. [10] S.S. Nair, C. Ramesh, Studies on the crystallization behavior of nylon-6 in the presence of layered silicates using variable temperature WAXS and FTIR, Macromolecules 38 (2005) 454–462. [11] A. Galeski, Morphology alterations during texture-production plastic planestrain compression of highdensity polyethylene, Macromolecules 25 (1992) 5705–5718. [12] L. Lin, A.S. Argon, Deformation resistance in oriented nylon-6, Macromolecules 25 (1992) 4011–4024. [13] P.G. Schmidt, Polyethylene terephthalate structure studies, J. Polym. Sci. Part A: Polym. Chem. 1 (1963) 1271–1292. [14] C. Ramesh, E.B. Gowd, High-temperature X-ray diffraction studies on the crystalline transitions in the alpha- and gamma-forms of nylon-6, Macromolecules 34 (2001) 3308–3313. [15] J.M. Schultz, B. Hsiao, J.M. Samon, Structural development during the early stages of polymer melt spinning by in-situ synchrotron X-ray techniques, Polymer 41 (2000) 8887–8895. [16] T.X. Liu, Z.H. Liu, K.X. Ma, L. Shen, K.Y. Zeng, C.B. He, Morphology, thermal and mechanical behavior of polyamide 6/layered-silicate nanocomposites, Composites Sci. Technol. 63 (2003) 331–337. [17] L.S. Loo, K.K. Gleason, Fourier transform infrared investigation of the deformation behavior of montmorillonite in nylon-6/nanoclay nanocomposite, Macromolecules 36 (2003) 2587–2590. [18] N. Vasanthan, D.R. Salem, Infrared spectroscopic characterization of oriented polyamide 66: band assignment and crystallinity measurement, J. Polym. Sci. Part B: Polym. Phys. 39 (2000) 516–524. [19] C.C. Rusa, M. Wei, T.A. Bullions, M. Rusa, M.A. Gomez, F.E. Porbeni, X. Wang, I.D. Shin, C.B. Balik, J.L. White, A.E. Tonelli, Controlling the polymorphic behaviors of semicrystalline polymers with cyclodextrins, Cryst. Growth Des. 4 (2004) 1431–1441. [20] G. Rotter, H. Ishida, FTIR separation of nylon-6 chain conformations – clarification of the mesomorphous and gamma-crystalline phase, J. Polym. Sci. Part B: Polym. Phys. 30 (1992) 489–495. [21] T.D. Fornes, D.R. Paul, Crystallization behavior of nylon 6 nanocomposites, Polymer 44 (2003) 3945–3961.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Substrate effects on the surface properties of nylon 6 Qi Zhou a,∗ , Jianghua Fang a , Haoqi Gao a , Leslie S. Loo b a b
Department of Chemical Engineering, Ningbo University of Technology, Ningbo 315016, PR China Department of Chemical and Biomedical Engineering, Nanyang Technological University, 637459 Singapore
a r t i c l e
i n f o
Article history: Received 18 March 2013 Received in revised form 9 May 2013 Accepted 13 May 2013 Available online 2 June 2013 Keywords: Nylon 6 Surface structure and properties Spin coating Substrates FTIR trichroic analysis
a b s t r a c t Thin nylon 6 films spin coated onto different substrates were characterized by wide-angle X-ray scattering (WAXS), Fourier transformation infrared (FTIR) spectroscopy, FTIR trichroic analysis and water contact angle (CA) measurements. The morphology and hydrophilicity of the nylon 6 surface were found to be greatly influenced by the nature of the substrate. The film surface contained primarily ␣-form crystals when it was prepared using hydrophobic substrates. This film was more hydrophobic. However, for hydrophilic substrates, the film surface comprised mainly ␥-form crystals and demonstrated greater hydrophilicity. FTIR trichroic analysis of the N H stretching vibration mode revealed that the presence of a hydrophilic substrate caused the N H stretching bonds on the film surface to align preferentially in the direction perpendicular to the surface. Such effects were attributed to molecular interactions between the N H bonds of the polymer and Si O bonds of the hydrophilic substrate. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction The structure and morphology of nylon 6 have been extensively investigated to obtain high performance materials, such as nanocomposites, films and blends throughout the years. In some cases, it had been reported that the crystalline morphology on the surfaces showed different conformations relative to the bulk [1,2]. Surface characterization was of great importance for the understanding of polymer adsorption in terms of chain orientation and conformation. At the same time, it served to provide an overall view of the polymer properties from surface level to bulk material [3]. Recently, the migration phenomenon of nylon 6 nanocomposites during the formation of nanocomposites had been reported. The resulting nanocomposites showed enhanced surface mechanical and physical properties, such as better friction, wear and hydrophilicity [4–6]. Misra et al. [7] used atomic force microscopy (AFM), and transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (TEM/EDAX) to study the crystalline morphology of the surface of nylon 6/polyhedral oligomeric silsesquioxane (POSS) nanocomposites. They observed that POSS molecules migrated to the surface resulting in an increase in stiffness and hardness. In our latest study [8], we investigated the effect of nanofillers such as POSS and layered silicates on the surface properties of spin coated nylon 6 films. Our studies also indicated it was necessary to understand the fundamental influence of the most
∗ Corresponding author. Tel.: +86 57486066812. E-mail address: [email protected] (Q. Zhou).
basic parameters (substrate materials) on the condensed state of nylon 6. More specifically, the resulting morphology and properties of nylon 6 crystal structure would depend on the nature of the substrate materials. Understanding such a relationship would provide insights in improving the surface properties and the migration of nanoparticles. Currently, the effect of substrate materials on nanocomposite formation was seldom investigated. It is known that nylon 6 exhibits polymorphism, and that the polymorph depends on the crystallization condition. At room temperature nylon 6 shows two crystalline modifications, namely the ␣ phase and the ␥ phase. Scheme 1 presents the two crystalline phase structure, including the direction of N H bonds [9]. The principle differences between these two phases lie in the lattice parameters and the orientation of the hydrogen bonds between the N H and C O groups. The ␣ phase has a monoclinic structure with a = 0.956 nm, b = 1.724 nm, c = 0.801 nm, and ˇ = 67.5◦ . The hydrogen bonds are formed between antiparallel chains in the ␣ phase. The ␥ phase also has a monoclinic structure with a = 0.933 nm, b = 1.688 nm, c = 0.478 nm, and ˇ = 121◦ . But the twisted chains allow hydrogen bonds to be formed between parallel chains. The ␣ phase is easily obtained by melt crystallization, while severe crystallization conditions are needed to obtain the ␥ phase. Moreover, the ␣ phase can be transformed into the ␥ phase by treatment with aqueous potassium iodide–iodine solution. Furthermore, the ␣ phase has been found to be the most stable structure of nylon 6 crystals [10–12]. In this paper, it was reported for the first time that different substrates can affect the crystalline morphology of nylon 6 films. The influence of the substrates on the crystalline state of nylon 6 films
0169-4332/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.05.075
116
Q. Zhou et al. / Applied Surface Science 282 (2013) 115–120
Scheme 1. Schematic of the ␣ and ␥ crystalline forms of nylon 6 as seen from end and side-view of each crystal. Closed and open circles represent chain axes projecting out of and into the page, respectively. Hydrogen bonds between polyamide chains are represented by dashed lines [9].
was studied using wide-angle X-ray scattering (WAXS), Fourier transform infrared (FTIR) trichroic analysis and water contact angle (CA) measurements.
2. Experimental 2.1. Materials Pellets of nylon 6 were purchased from Nanocor, Inc. The solvent 2,2,2-trifluoroethanol was obtained from Sigma–Aldrich. The optical glasses were Pyrex Borosilicate Glass, purchased from SG scientific glass-blowing centre. The glass composition, as provided by the manufacturer, was 80.6% SiO2 , 13.0% B2 O3 , 4.0% Na2 O and 2.3% Al2 O3 . Polystyrene petri dish substrates were provided by Vacutest KIMA Company.
2.2. Nylon 6 thin film formation Nylon 6 pellets were first dried in a vacuum oven for more than 24 h at room temperature and then stored in a desiccator. In order to make the thin films, the pellets were dissolved in 2,2,2trifluoroethanol (about 5 wt%), and the solution was spin coated onto the optical glass and polystyrene substrates at five different spin rates: 1000 rpm, 2000 rpm, 4000 rpm, 6000 rpm and 8000 rpm. The films on the glass substrates were dried in vacuum between 70 and 80 ◦ C for more than 12 h and cooled to room temperature. The films on the polystyrene substrates were dried in vacuum between 35 and 40 ◦ C for more than 48 h and cooled to room temperature. Lastly, the films together with substrates were stored in desiccator prior to use.
2.3. Thickness measurement Film thickness was measured using a Tencor Instruments Alpha Step 500 stylus profilometer and a Micro-Xam surface mapping microscope. Each film was measured 4 times. Finally, the average value of the film thickness was calculated.
2.4. WAXS 2 scans were performed on a Rigaku Rint X-ray generator with Cu K˛ radiation at a voltage of 40 kV and a current of 40 mA. The Xray beam irradiated at the samples was fixed at a constant grazing angle of 1.0◦ , while the detector counted the diffracted signals with 2 ranging from 10◦ to 30◦ . Scan speed was 0.50◦ /min and scan step was 0.02◦ . The divergence, scatter and receiving slit were set at 1.00◦ , 1.00◦ and 0.30 mm respectively. 2.5. FTIR and FTIR trichroic analysis FTIR spectroscopy was performed on a Nicolet Nexus spectrometer. The detector is a liquid nitrogen cooled mercury cadmium telluride (MCT) with a range of 650–4000 cm−1 . The resolution was 4 cm−1 . For trichroic (three-dimensional orientation) analysis of the film, infrared spectra in the thickness direction were obtained by the sample-tilting procedure described by Schmidt [13]. The beam spot size was set to 5.08 mm diameter at the sample with a total beam divergence of 1.9◦ . A custom-made device was used to rotate the films around a vertical axis. For normal transmission, the film surface was positioned perpendicular to the infrared beam. This was taken to be the reference angle, viz. 0◦ . Refractive index measurements were obtained from vertically polarized spectra of the films at angles of ±45◦ . The spectra in the thickness direction were then calculated from horizontally polarized spectra of the films aligned at 0◦ and 45◦ . 64 scans were collected for each spectrum and zero-filled twice. A Pike wire grid infrared polarizer was used to obtain the polarized IR beam. 2.6. Water CA measurements Contact angles were measured on a horizontal surface using a FTA200 Dynamic Contact Angle Analyzer, determined using a sessile droplet of ultrapure water (Milli-Q, Millpore Corp., 8.02 M cm) having a volume of ∼50 l at room temperature (about 23 ◦ C), and determined at the triple point of air, water and nylon 6. CA for the upside (the surface which was not in contact with the substrate) of the films was measured directly when the films
Q. Zhou et al. / Applied Surface Science 282 (2013) 115–120
117
Fig. 1. (a) WAXS results and (b) FTIR spectra of the 900–1250 cm−1 region of nylon 6 films spin coated on glass substrates at different spin rates.
were still on the substrate. To obtain CA data for the downside (the surface of the film in contact with the substrate), the films were first peeled off the substrate and then attached to microscope glass slides. Measurements were performed for both the surfaces that were just peeled off and the surfaces that had been exposed in air for 3 days after being peeled off. CA data reported were averaged over five locations on every surface.
3. Results and discussion 3.1. Crystallinity studies Fig. 1(a) shows the WAXS date of nylon 6 films spin coated on optical glass at different spin rates. At the lower spin rates of 1000 and 2000 rpm, the two main peaks observed are at 20.5◦ and 24◦ . As the spin rate increases to 4000 rpm, these two peaks gradually decrease in magnitude. Meanwhile, the peak at 21.5◦ gradually becomes more dominant. Finally at high spin rates of 6000 and 8000 rpm, only one main peak at 21.5◦ is observed. This phenomenon indicates that when spin rate is increased, the morphology of the films changes from mainly ␣-crystals to mainly ␥-crystals. Fig. 1(b) shows the FTIR spectra of the films spin coated on glass. It is observed that the peak at 1202 cm−1 for the 1000 and 2000 rpm films have a much higher intensity than that for the 4000, 6000 and 8000 rpm films. The absorbance at the 1030, 961, and 931 cm−1 peaks decrease gradually when the spin rate increases. It indicates that the proportion of ␣-crystals in the films is decreasing. The increasing intensity of the peak at 976 cm−1 shows that the amount of ␥-crystals in the films increases with increasing spin rate. Fig. 2(a) shows the X-ray diffraction intensity against 2 plots of nylon 6 films spin coated on hydrophobic polystyrene substrates at different spin rates. Peaks at 20.5◦ and 24◦ are attributed to the (0 2 0) and (0 0 2) planes of the ␣ crystals while the peak at 21.5◦ corresponds to the (2 0 0) hydrogen-bonded planes of ␥ crystals [14–17]. For all spin rates, the X-ray patterns are identical and the dominant morphology is the ␣ crystals form in the films. Fig. 2(b) shows the FTIR spectra of the films. FTIR band assignments for both ␣ and ␥ crystals of nylon 6 have been reported in the literature [3,18–20], and are list in Table 1 as well. The bands at 931, 961, 1030 and 1202 cm−1 are attributed to the ␣ crystals, whereas the band at 976 cm−1 is attributed to the ␥ crystals. In Fig. 2(b), it is observed that all the FTIR spectra are similar and the ␣ crystals dominate, confirming the WAXS results. At low spin speeds, the morphology of the films on polystyrene and glass substrates is similar. However, they differ drastically at high spin speeds. At first sight, it appears that the morphology of
the films will depend on both spin speeds and the nature of the substrate. However, it is important to note that the XRD and FTIR data are a spatial average of the bulk and surface properties of the films. Hence, at lower spin speeds, the films are thicker so that the results are more representative of the bulk morphology of the films. However, with increasing spin speed, the films become thinner. Consequently, the surface properties of the film in contact with the substrate will contribute more to the XRD and FTIR data (i.e. we are indirectly probing more of the surface properties of the film as the film thickness is reduced). With this in mind, it is inferred that the observed differences in WAXS and FTIR data at high spin speeds are not due to the effect of different spin speeds, but are caused by the effect of the substrates on the surface morphology of the films. For the hydrophobic substrates, the data from Fig. 2 suggests that such substrates do not affect the surface morphology of the film in contact with it, so that the bulk morphology and surface morphology are similar, thereby giving rise to similar WAXS and FTIR data at all film thicknesses. On the other hand, the hydrophilic glass does affect the morphology of the film surface in contact with it. The effects of the glass substrate are observed in the thinner films formed at the highest spin speeds, where the surface morphology is dominated by the ␥-crystals instead of the ␣-crystals. At high spin speeds, the films are thinner and more of the surface which contains ␥-crystals is revealed in the WAXS and FTIR data. At lower spin speeds, the bulk properties dominate in the WAXS and FTIR results and hence show predominantly the ␣-crystals. These results indicate that the hydrophilic glass substrates favour the formation of ␥-crystals in nylon 6. 3.2. Water CA measurement Table 1 shows the water CA measurements of nylon 6 films formed on glass substrate. It is seen clearly that the CA is independent of spin speed for the same substrate. However, the CA is substantially different between the upside and downside surfaces. Fig. 3 shows the scheme of measuring position and the Table 1 The major crystalline and amorphous characteristic infrared bands of PA6. (cm−1 ) 931 961 976 1030 1202 3000–3500
Based assignments [3,18–20] ␣ crystalline phase amide stretching mode CONH of ␣ crystal CONH of ␥ crystal ␣ crystalline phase squeletal stretching ␣ crystalline phase carbonyl wagging mode Hydrogen bonded N H stretch
118
Q. Zhou et al. / Applied Surface Science 282 (2013) 115–120
Fig. 2. (a) WAXS date and (b) FTIR spectra of the 900–1250 cm−1 region of nylon 6 films spin coated on polystyrene substrates at different spin rates.
of the films comparing with the downside which makes films more hydrophobic. 3.3. Orientation of N H bonds FTIR trichroic measurements of the N H stretching in the 3000–3500 cm−1 region were used to observe the orientation of N H bonds [11]. The spectrum in the thickness direction was calculated based on the procedure described by Schmidt [13]. The reflective index (n) of nylon 6 films was measured by a Prism Coupler, and the average value of films made by different spin speed was used. Table 4 shows the values of the reflective index. The spectrum in the thickness direction Az can then be calculated from the following formula:
Fig. 3. The scheme of measuring position and the characteristic CA results.
characteristic CA results. It confirms our earlier hypothesis that the morphological changes observed in WAXS and FTIR are due to substrate effects rather than spin speeds. The upside surfaces have CAs ranging from 61◦ to 63◦ , while the CAs for the downside surfaces range from 45◦ to 46◦ (when measured immediately after the films were peeled off the substrate). Hence the surfaces of nylon 6 films demonstrated more hydrophilic character when they were formed in the presence of a hydrophilic substrate such as glass. However, after the more hydrophilic surfaces were exposed to air for 3 days, the CAs increased to 54◦ –58◦ . Table 3 shows the water contact angle measurements for nylon 6 films made on polystyrene. The CAs are also independent of spin speed. The values of CAs for the upside surface are similar to those formed on glass substrate. The downside CAs are larger than those for the glass substrate, and they remain unchanged after exposure in air for 3 days. This is because polystyrene does not change the orientation of N H bond of nylon 6 films. However, the downside CAs are slightly smaller than the upside CAs, due to the rougher upside
Table 2 Water contact angle of different sides of nylon 6 films coated on glass substrate. Spin rate (rpm)
Film thickness Upside (◦ ) (m)
1000 2000 4000 6000 8000
0.3256 0.2766 0.2348 0.1542 0.0852
a b
61.358 61.232 61.722 61.662 62.088
± ± ± ± ±
Downsidea (◦ ) 0.156 0.196 0.045 0.125 0.316
45.782 45.752 45.444 45.630 45.893
± ± ± ± ±
0.708 0.402 0.551 0.357 0.516
Measured immediately after being peeled off the substrate. Measured 3 days after being peeled off.
Downsideb (◦ ) 57.542 57.078 57.070 55.844 54.724
± ± ± ± ±
0.510 0.365 0.490 0.415 0.451
Az =
At
1−
sin2 (˛t ) n2
sin2 (˛t ) n2
− Ax
where n is the reflective index, Ax is the horizontally polarized spectrum of film normal to the beam and At is the horizontally polarized spectrum of the film tilted at ˛t to the beam. ˛t was set at 45◦ in our experiment. The vertically polarized spectrum Ay was also measured, and it is virtually identical with Ax , showing that there is no preferential alignment of the N H bonds in the plane of the film. Fig. 4 shows the Az and Ax spectra of the N H stretching mode of nylon 6 films spin coated at different spin speeds on polystyrene substrates. All the spectra show similar Az /Ax ratios of from 0.45 to 0.58, indicating that the alignment of the N H bonds in the bulk is similar to that of the surface. Hence it is concluded that there is little interaction between polystyrene and the nylon 6 surface. Fig. 5 shows the Az and Ax spectra of the N H stretching mode of nylon 6 films spin coated at different spin speeds on glass substrates. It is seen that the relative intensities of the N H stretching vibration in the Az and Ax spectra vary with spin speed. The Az /Ax Table 3 Water contact angle of different sides of nylon 6 films coated on polystyrene substrate. Spin rate (rpm)
Upside (◦ )
1000 2000 4000 6000 8000
62.820 62.288 60.890 60.210 60.296
a b
± ± ± ± ±
Downsidea (◦ ) 0.235 0.130 0.291 0.339 0.276
53.830 54.006 53.240 53.614 53.380
± ± ± ± ±
0.339 0.156 0.231 0.614 0.230
Measured immediately after being peeled off the substrate. Measured 3 days after being peeled off.
Downsideb (◦ ) 54.330 54.194 53.828 52.860 54.480
± ± ± ± ±
0.250 0.252 0.491 0.379 0.281
Q. Zhou et al. / Applied Surface Science 282 (2013) 115–120
119
Fig. 4. IR spectra of N H stretching mode for films coated on polystyrene substrate and prepared at different spin rates. The Az to Ax ratio reflects the orientation of N H bonds.
Fig. 5. FTIR spectra of N H stretching mode for films coated on glass substrate and prepared at different spin rates. The Az to Ax ratio reflects the orientation of N H bonds.
ratio increases monotonically from 0.56 to 1.248 with increasing spin speed. The film is thick at low spin speeds (Table 2). The results can be representative of the bulk. Hence, the Az /Ax ratios at 1000 and 2000 rpm in Fig. 4 are similar to those for the polystyrene Table 4 Experimental values of reflective index. Spin rate (rpm)
Reflective index
1000 2000 4000 6000 8000 Average
1.5338 1.5301 1.5231 1.522 1.5267 1.527 ± 0.002
substrate. However, at the highest spin speeds of 6000 and 8000 rpm, the film is so thin that the data are more representative of the surface. The high Az /Ax ratios observed can be attributed to the results of interactions between the glass substrate and the film surface. The structural absorbance A is given by: Hence, when Az /Ax increases, more N H bonds are aligned preferentially in the thickness direction. Similar analysis has also been performed for the C O stretching mode, it was found that the Az /Ax ratios are similar for all spin speeds regardless of the type of substrate. The result indicates that the interaction between glass and nylon 6 occurs primarily through the N H bonds rather than the C O bonds. Polystyrene is a hydrophobic material and does not possess any groups which can form hydrogen-bonding interactions with nylon
120
Q. Zhou et al. / Applied Surface Science 282 (2013) 115–120
6. Consequently, the morphology and properties of the surface in contact with polystyrene can mirror those of the bulk. On the other hand, the glass is hydrophilic and composed of oxides, mainly SiO2 , so its oxygen atoms tend to interact with the N H groups of the nylon 6 surface to form Si O· · ·H N hydrogen bonds. This can result in preferential alignment of the N H bonds perpendicular to the surface. The lack of hydrogen bond interactions between the C O groups of nylon 6 and glass may explain why such a preferential alignment is not observed in the trichroic analysis of C O groups. When the N H bonds are aligned perpendicular to the surface, it becomes much easier to adsorb water molecules, so the hydrophilicity of nylon 6 increases. However, after the surface is exposed to air, the CAs increase. N H bonds in the outmost layer of the film are oriented to the vertical direction at the beginning, but after expose in the air, these N H bonds will adsorb some water molecules, which will affect the orientation of N H bonds, making N H bonds bend towards the plane of the film. The preferential formation of ␥-crystals in the proximity of certain surfaces has been observed for other nylon 6 systems. For instance, the introduction of montmorillonite layered silicate also induced the formation of ␥-crystals [21]. The ␣-crystals consist of hydrogen-bonded sheets parallel to one another. The perpendicular alignment of N H bonds in the presence of a hydrophilic substrate such as glass or layered silicate can prevent the formation of such parallel sheets of the ␣-crystals. Hence ␥-crystals are preferentially formed near the surface. Far away from the surface, the constraints imposed by the substrate are absent, and ␣-crystals are dominated. 4. Conclusions The substrate effect on surface structure and hydrophilicity of nylon 6 was investigated for the first time. WAXS and FTIR results showed that the presence of hydrophilic substrates favoured the formation of the less stable ␥-crystals. It was caused by the orientation selectivity of N H bonds in nylon 6, which preferentially aligned in a direction perpendicular to the glass substrate. The understanding of such phenomena offered a way to manipulate the surface properties of polymers. Acknowledgements Funding from the project of Natural Science Foundation of China (No. 51203081), Zhejiang Province Department of Education Fund (Y201224385), the Natural Science Foundation of Ningbo (2012A610087) and Ningbo Science and Technology Innovation Team (2011B2002) are gratefully acknowledged.
References [1] A. Dasari, Z.Z. Yu, Y.W. Mai, Fundamental aspects and recent progress on wear/scratch damage in polymer nanocomposites, Mater. Sci. Eng. R-Rep. 64 (2009) 31–80. [2] J.A. Lee, Y.S. Nam, G.C. Rutledge, P.T. Hammond, Enhanced photocatalytic activity using layer-by-layer electrospun constructs for water remediation, Adv. Funct. Mater. 20 (2010) 2424–2429. [3] T. Elzein, M. Brogly, J. Schultz, Quantitative calculation of the orientation angles of adsorbed polyamides nanofilms, Polymer 44 (2003) 3649–3660. [4] M. Zammarano, J.W. Gilman, M. Nyden, E.M. Pearce, M. Lewin, The role of oxidation in the migration mechanism of layered silicate in poly(propylene) nanocomposites, Macromol. Rapid Commun. 27 (2006) 693–696. [5] M. Lewin, J. Zhang, E. Pearce, M. Zammarano, Polyamide 6 treated with pentabromobenzyl acrylate and layered silicates, Polym. Adv. Technol. 21 (2010) 825–834. [6] Y. Tang, M. Lewin, W.M. Pearce, Effects of annealing on the migration behavior of PA6/clay nanocomposites, Macromol. Rapid Commun. 27 (2006) 1545–1549. [7] R. Misra, B.X. Fu, A. Plagge, S.E. Morgan, POSS-nylon 6 nanocomposites: influence of POSS structure on surface and bulk properties, J. Polym. Sci. Part B: Polym. Chem. Phy. 47 (2009) 1088–1102. [8] Q. Zhou, K. Wang, L.S. Loo, Role of interface in dispersion and surface energetics of polymer nanocomposites containing hydrophilic POSS and layered silicates, J. Colloid Interface Sci. 335 (2011) 222–230. [9] T.D. Fornes, D.R. Paul, Crystallization behavior of nylon 6 nanocomposites, Polymer 44 (2003) 3945–3961. [10] S.S. Nair, C. Ramesh, Studies on the crystallization behavior of nylon-6 in the presence of layered silicates using variable temperature WAXS and FTIR, Macromolecules 38 (2005) 454–462. [11] A. Galeski, Morphology alterations during texture-production plastic planestrain compression of highdensity polyethylene, Macromolecules 25 (1992) 5705–5718. [12] L. Lin, A.S. Argon, Deformation resistance in oriented nylon-6, Macromolecules 25 (1992) 4011–4024. [13] P.G. Schmidt, Polyethylene terephthalate structure studies, J. Polym. Sci. Part A: Polym. Chem. 1 (1963) 1271–1292. [14] C. Ramesh, E.B. Gowd, High-temperature X-ray diffraction studies on the crystalline transitions in the alpha- and gamma-forms of nylon-6, Macromolecules 34 (2001) 3308–3313. [15] J.M. Schultz, B. Hsiao, J.M. Samon, Structural development during the early stages of polymer melt spinning by in-situ synchrotron X-ray techniques, Polymer 41 (2000) 8887–8895. [16] T.X. Liu, Z.H. Liu, K.X. Ma, L. Shen, K.Y. Zeng, C.B. He, Morphology, thermal and mechanical behavior of polyamide 6/layered-silicate nanocomposites, Composites Sci. Technol. 63 (2003) 331–337. [17] L.S. Loo, K.K. Gleason, Fourier transform infrared investigation of the deformation behavior of montmorillonite in nylon-6/nanoclay nanocomposite, Macromolecules 36 (2003) 2587–2590. [18] N. Vasanthan, D.R. Salem, Infrared spectroscopic characterization of oriented polyamide 66: band assignment and crystallinity measurement, J. Polym. Sci. Part B: Polym. Phys. 39 (2000) 516–524. [19] C.C. Rusa, M. Wei, T.A. Bullions, M. Rusa, M.A. Gomez, F.E. Porbeni, X. Wang, I.D. Shin, C.B. Balik, J.L. White, A.E. Tonelli, Controlling the polymorphic behaviors of semicrystalline polymers with cyclodextrins, Cryst. Growth Des. 4 (2004) 1431–1441. [20] G. Rotter, H. Ishida, FTIR separation of nylon-6 chain conformations – clarification of the mesomorphous and gamma-crystalline phase, J. Polym. Sci. Part B: Polym. Phys. 30 (1992) 489–495. [21] T.D. Fornes, D.R. Paul, Crystallization behavior of nylon 6 nanocomposites, Polymer 44 (2003) 3945–3961.