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Journal of Colloid and Interface Science 283 (2005) 79–86 www.elsevier.com/locate/jcis
Miscibility–structure–property correlation in blends of ethylene vinyl alcohol copolymer and polyamide 6/66 Enik˝o Földes a,b,∗ , Béla Pukánszky a,b a Institute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary b Department of Plastics and Rubber Technology, Technical University of Budapest, H-1521 Budapest, P.O. Box 91, Hungary
Received 3 February 2004; accepted 26 August 2004 Available online 10 December 2004
Abstract Blends of ethylene vinyl alcohol (EVOH; 44 mol% ethylene) and polyamide 6/66 (PA; 75 mol% PA 6) random copolymers were studied in the entire composition range. Specific interaction between the components was analyzed by IR spectroscopy; furthermore, coefficients related to the Flory–Huggins interaction parameter were derived from equilibrium water uptake and tensile strength. Morphology of the blends was investigated by thermal analysis (DSC), density measurements, and SEM micrographs. The two polymers form heterogeneous blends in each composition. Although the components crystallize in separate phases, the morphology and the mechanical properties are greatly affected by the association of OH and NH groups. Crystallization is restricted in the blends, and the increase of the amorphous fraction, as well as specific interaction between the components, results in essential improvement in the mechanical properties. 2004 Elsevier Inc. All rights reserved. Keywords: EVOH; PA; Blends; Miscibility; Specific interaction
1. Introduction The morphology and the properties of polymer blends are basically determined by the strength of interaction of the components. The thermodynamic definition of polymer– polymer miscibility is based on the Flory–Huggins theory. A blend will be thermodynamically miscible when the polymer–polymer interaction energy is less than a critical value affected by the molecular weight of the components. When the interaction energy exceeds that, two-phase mixtures are formed. Compatible blends are obtained with fine dispersion, large interfacial thickness, strong interface, and good mechanical properties when the interaction energy is not too much larger than the critical value. At large interaction energies the interfacial tension increases, the interfacial strength weakens, the morphology becomes gross, and the mechanical properties become inferior. At some points the * Corresponding author. Fax: +36-1-325-7554.
E-mail address: [email protected] (E. Földes). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.08.175
blends are regarded as incompatible [1]. Miscibility of nonpolar polymers of high molecular weight occurs only when the solubility parameters are matched, as the entropy of mixing is very small and the heat of mixing is generally positive. The enthalpy of mixing can be negative, if certain specific interactions between polar groups are involved [2–4]. Miscibility of ethylene vinyl alcohol copolymer (EVOH) with polyamide (PA) has been studied by several authors with different aims in view. One of the goals was to improve the forming capacity (in terms of temperature and draw ratios) and the mechanical properties of EVOH without sacrificing the gas barrier properties [4–9]. Another topic of interest was processing monolayer barrier films with multilayer-type internal structures for packaging [10–12]. In the textile industry the aim was to increase the impact strength and barrier properties of PA 46 [13]. PA/EVOH blends were patented as early as in the 1980s for bare and composite films with a high level of strength and toughness, good oxygen barrier properties, and enhanced adhesive qualities [14], as well as for various products (moldings, films, and fibers) [15].
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Specific interaction between EVOH and PA was observed resulting in partial miscibility of the components [12,16–19]. The properties of the blend depend on composition, vinyl alcohol content of EVOH [6,17,19], type of PA [8,17,20], thermal-mechanical history [12], and moisture content [9,11], as well as on the chemical modification of the components [10,21]. In a previous paper we discussed the miscibility of EVOH and PA 6/66 random copolymers [18]. Improved mechanical properties, as well as the chemical and physical characteristics of the blends, indicated a strong interaction between EVOH and PA 6/66. The interaction was attributed to hydrogen bonding of the OH groups of EVOH with the NH and C=O groups of PA. Ha et al. [13] explained the miscibility of the components in EVOH/PA 46 blends at PA contents below 35 w/w% with the interaction between the NH groups of PA 46 and the C–O groups of EVOH. However, the EVOH investigated contained 13 mol% vinyl acetate (VA), and the assumption was based on the shift of the carbonyl vibration of VA to lower frequencies in the infrared (IR) spectra. Due to growing interest in the development and recycling of multicomponent polymer systems, various characteristics of the blends of EVOH and PA 6/66 were reevaluated and reconsidered in the present work. Our goal was to identify the nature of specific interactions between the components and their impact on the morphology and properties of the blends.
2. Experimental 2.1. Materials and sample preparation For the experiments Selar OH 4416 grade ethylene vinyl alcohol random copolymer (Du Pont) with 44 mol% ethylene content and Ultramid C35 grade polyamide 6/66 copolymer (BASF) were selected. According to the calculations described below the PA 6 content of the copolymer was 75 mol%. The vinyl acetate content of EVOH was under the detection level by FTIR spectroscopy. The blends were prepared in the entire composition range from 0 to 1 mass fractions of PA in steps of 0.1. The materials were homogenized in a DSK 42/7 Brabender twin-screw compounder at 230–245 ◦ C and extruded into bands of 2 cm wide and 1 mm thick. The experiments were carried out on these bands or on 1-mm-thick plates compression-molded by a Fontijne SRA 100 laboratory press. For infrared measurements 12to 20-µm thick films were compression-molded. All samples were dried at 70 ◦ C up to mass equilibrium (1300 h) prior to the experiments. 2.2. Experimental and evaluation methods The chemical structure of the materials was characterized by infrared spectroscopy (FTIR; Mattson–Galaxy 3020,
Unicam). The spectra were taken in the range of 4000– 400 cm−1 with a resolution of 2 cm−1 using 16 scans. The O–H and N–H bands absorbing in the range of 3650 and 3000 cm−1 were separated by curve fitting using Spectra Calc (Bomem-Calc) FTIR software. The physical structure was characterized by different techniques. Fusion and crystallization properties were determined by differential scanning calorimetry (DSC) under nitrogen at a scanning rate of 10 ◦ C/min, using the DSC30 cell of a Mettler TA 4000 system. Composition of the PA 6/66 copolymer was calculated from melting point depression [2,22] using Hc0 of 230 and 301 J/g, as well as Tm0 of 270 and 280 ◦ C for PA 6 and PA 66, respectively [23]. The crystallinity of EVOH was derived from the crystalline characteristics of polyvinyl alcohol: Tm0 = 265 ◦ C, Hc0 = 163 J/g [23]. Density was measured in a gradient column at 23 ◦ C. Notched strips cut from the compressionmolded plates were cooled in liquid nitrogen for 30 min and then broken into two parts. SEM micrographs were taken from the fracture surface after etching in formic acid at ambient temperature for 30 s. Water uptake of the samples was investigated at 25 ◦ C as a function of time by an immersion technique. Equilibrium solubility and the diffusion coefficient of water were determined by least-squares curve fitting. As the samples showed combined Fickian and Case II diffusion, the mathematical model of Berens and Hopfenberg [24,25] was applied for calculating the equilibrium water uptake (M∞ ), the Fickian diffusion coefficient (Dw ), and the rate constant (k) proportional to the velocity of the advancing front of the boundary between the swollen gel and the glassy core of the polymer. According to the model the total amount of diffusant absorbed at time t can be expressed by the sum of two phenomenologically independent terms, Mt = Mt,D + Mt,R ,
(1)
where Mt,D and Mt,R are the amounts of diffusant absorbed by Fickian diffusion mechanism and first-order kinetics (Case II mechanism), respectively, ∞ 1 8 Mt,D = M∞,D 1 − 2 π (2m + 1)2 m=0 Dw (2m + 1)2 π 2 t × exp − (2) , l2 Mt,R = M∞,R 1 − exp(−kt) , (3) where M∞,D and M∞,R are the equilibrium water uptake values absorbed by Fickian and Case II mechanism, respectively; l is the thickness of the film. As can be seen from Fig. 1 the data conform well to the model. From the equilibrium water uptake the interaction pa ) between EVOH and PA was calculated [26], rameter (χ12 ln aw = 0 = ln φw + (1 − φw ) φ1 φ2 , + (χw1 φ1 + χw2 φ2 )(1 − φw ) − χ12
(4)
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Fig. 1. Water uptake of EVOH ("), PA (+), and their 1/1 blend (2). Solid lines represent fitted curves. Fig. 2. Infrared spectrum of EVOH resolved by curve fitting.
where aw is the activity of water sorbed into a homogeneous blend of polymers 1 and 2, and φw , φ1 , and φ2 are the volume fractions of water, polymers 1 and 2 in the ternary system, respectively. χw1 and χw2 are the polymer–water interaction parameters determined from the equilibrium solvent uptake of the neat polymers: ln aw = 0 = ln φw + (1 − φw ) + χwi (1 − φw )2 .
(5)
With increasing interaction of polymers 1 and 2 the becomes more amount of water absorbed decreases and χ12 negative [26]. The volume fractions of the components were determined by using crystalline densities of 1.35 and 1.241 g/cm3 for EVOH and PA, respectively [23]. It was assumed that the transport of water molecules was restricted to the amorphous region of the blends, and the composition of the ternary systems was calculated from the volume fractions of absorbed water and amorphous part of the polymer components. Mechanical properties were investigated by tensile tests at ambient temperature using a cross-head speed of 50 mm/min. For the determination of polymer–polymer interaction a semiempirical model was developed earlier [27–29]. According to the model the tensile strength of a twocomponent system is determined by the true tensile strength of the matrix polymer (σT0 ), the volume fraction of the dispersed phase (φd ) and a parameter, B, which reflects the effect of interaction, σTred =
σT 1 + 2.5φd = σT0 exp(Bφd ), λn 1 − φd
(6)
where σTred and σT are the reduced and true strengths of the blend, respectively. The true strength can be derived from the engineering tensile strength (σT = σ λ). λ is the relative elongation (λ = L/L0 ); n is a parameter characterizing the strain hardening tendency of the matrix, and it can be determined from the stress vs strain correlation of the neat matrix polymer. Parameter B indicates the relative load bearing ca-
pacity of the components, Cσd B = ln , σm
(7)
where σd and σm are the tensile strengths of the dispersed phase and the matrix, respectively. C is the proportionality constant related to the stress transfer, and thus, reciprocally to the Flory–Huggins interaction parameter (χ). Studies of amorphous polymer blends proved the correlation between the proportionality constant C and χ derived from the solubility parameter [30].
3. Results and discussion 3.1. Interaction In hydrogen-bonded polymer blends the development of intermolecular interaction can be followed by changes in the wavelength and intensity of IR absorption bands of the functional groups [31–33]. In the present work the composition dependence of O–H and N–H vibrations was analyzed after their separation by curve fitting. EVOH is a semicrystalline polymer. The broad vibrational band of OH groups in the range of 3650–3200 cm−1 points to strong hydrogen bonding. The curve-fitting procedure revealed that this band is a superposition of a higher and a lower frequency band (OH1 and OH2 in Fig. 2), since a single peak could not be fitted by any mathematical formula used in IR spectroscopy. The higher frequency (OH1) band represents lower association energy, and can be attributed to the OH groups located in the amorphous phase. The stronger the hydrogen bonds the larger the shift of O–H vibration to lower frequencies. Therefore the lower frequency (OH2) band can be assigned to the absorption of OH groups in the ordered phase. PA 6/66 is also a semicrystalline polymer, where NH and C=O groups form hydrogen bonds reflected
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Fig. 3. Infrared spectrum of EVOH/ PA blend of 1/1 composition resolved by curve fitting.
Table 1 Frequency and intensity of characteristic infrared bands of EVOH/PA blends and their components PA mass fraction
Wavenumber (cm−1 )a
Relative integrated intensitya,b
OH1
OH2
NH
OH1
OH2
NH
0 0.1 0.3 0.5 0.7 0.9 1.0
3419 3414 3428 3427 3433 3447 3447
3281 3283 3278 3283 3272 3268 3264
3304 3302 3302 3303 3304 3301
1.21 1.32 1.33 1.06 0.76 0.49 0.23
3.35 2.10 2.17 1.83 1.72 1.17 0.39
0.12 0.33 0.69 1.07 1.07 1.62
a OH1 and OH2 are resolved bands of hydrogen-bonded O–H vibration. b Related to the integrated intensity of C–H absorption in the range of 3000–2900 cm−1 .
in the low frequency of N–H vibration [34]. The OH bands of low intensity in neat PA (Table 1) can be attributed to some absorbed water present despite drying to mass equilibrium. In the blends of the two polymers the OH and NH bands are superimposed. The measured and resolved spectra of EVOH/PA blend of 1/1 composition are illustrated in Fig. 3. For analyzing the interaction between the polymers, the wavenumbers of the resolved bands were determined and the relative integrated intensities were calculated with the use of C–H vibrations at 3000–2900 cm−1 as reference. The results are summarized in Table 1. The frequency of N–H vibration (νNH ) is independent of the composition of the blend, while its intensity (INH ) decreases monotonically with EVOH content. INH is, however, lower in each composition than can be expected on the basis of additivity (Fig. 4). The position of the N–H absorption band in PA is determined by the strength of interaction, and the intensity by the number of N–H. . .O=C groups. The results of analysis indicate that some of the N–H vibrations are
Fig. 4. Composition dependence of the relative infrared intensity of O–H ((") OH1 and (Q) OH2) and N–H (×) vibrations.
shifted to higher and/or lower frequencies compared to that characteristic of neat PA. Both the wavenumber and the intensity of O–H bands are affected by the composition of the blend (Table 1). The OH1 band shifts to higher and the OH2 band to lower frequencies with increasing PA content. The shift of the OH1 band indicates that the strength of hydrogen bonding of OH groups decreases in the amorphous phase compared to neat EVOH. The relative intensity of OH1 absorption (IOH1 ) increases slightly with increasing PA content of the blend below 0.5 PA mass fraction, and starts to decrease around 1/1 component ratio referring to a change in the phase structure at this composition (Fig. 4). Modification of the OH2 band frequency (νOH2 ) with composition also points to some changes in the structure around the 1/1 component ratio (Table 1). νOH2 can be considered independent of the composition at high EVOH concentrations and decreases above 0.5 PA content. The intensity of OH2 vibration (IOH2 ) changes also nonlinearly with composition (Fig. 4). It remains below the additive value at high EVOH concentrations and exceeds that at PA contents higher than 0.5 mass fraction. Correlating the deviations from additivity of the N–H and O–H band intensities we can conclude that the increase in IOH1 is related to the decrease of both INH and IOH2 . This result implies that (a) the crystallinity of EVOH decreases in the blends compared to the neat polymer, (b) the decrease of INH is caused by formation of NH. . .HO bands in the amorphous fraction, and (c) despite the intermolecular interactions the components are not mixed homogeneously in the amorphous fraction. 3.2. Structure The fusion and crystallization characteristics of EVOH/ PA blends studied by DSC in two heating and one cooling
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Table 2 Fusion characteristics of EVOH/PA blends determined from the first heating run measured by DSC in nitrogen atmosphere at a heating rate of 10 ◦ C PA mass fraction 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Tm (◦ C) EVOH 169 167 169 165 168 166 165 165 156 (101)
Hf (J/g) PA 186 189 188 191 192 194 197 199 200 201
EVOH 70.2 64.2 43.1 35.0 25.7 15.5 10.7 5.6 3.4 (5.8)
PA 2.3 5.5 11.7 14.9 19.1 25.0 31.6 38.2 46.1 51.0
runs are strongly affected by composition and thermal history. Heterogeneous morphology and partial miscibility in the amorphous phase of the dried blends are disclosed by the thermograms measured in the first heating run. Both components have separate composition-dependent fusion peaks (Table 2). The main transition of neat EVOH occurs at 169 ◦ C accompanied by a fusion peak of low intensity at around 100 ◦ C. A third transition appears at 56–58 ◦ C in the blends containing more than 10 wt% PA. The main transition of EVOH shifts only moderately to lower temperatures up to 0.7 PA mass fraction. At 0.8 PA content it decreases more significantly and then disappears from the thermogram at 0.1 EVOH mass fraction. In this latter case only the two lower fusion peaks can be detected. The fusion temperature of PA changes more steadily with composition; it decreases almost linearly with increasing EVOH content of the blend. The heat of fusion (Hf ) values of the components indicates that the crystallizability of EVOH is rather affected by the presence of PA than inversely. The heats of fusion determined in the first heating run are lower in each case for both polymers than the additive values (see also [18]). The negative deviation from additivity (deficit in the heat of fusion) is more significant in the case of EVOH than for PA. The thermal history also affects the morphology of the blends. The thermograms measured in the cooling and second heating runs show no crystalline transition for EVOH in the blends containing more than 0.2 PA, and PA could crystallize separately only up to 0.4 mass fraction of EVOH. For the blends with PA mass fractions between 0.3 and 0.5 only a glass transition was detected at 42–45 ◦ C. The decrease in crystallinity can be directly related to the interaction of the components revealed by the correlation between their heat of fusion and the intensities of the hydrogen-bonded N–H and OH2 absorption bands. Fig. 5 shows that Hf increases linearly with the relative intensity of the corresponding hydrogen bonding band of both components. The correlation for EVOH does not start at the origin, which indicates that OH2 vibration does not originate solely from hydrogen-bonded OH groups located in the separate ordered phase of this polymer. Most probably some OH. . .HN
Fig. 5. Correlation between the IR absorbance of hydrogen bonding groups and the heat of fusion of the components: EVOH ((Q) OH2), PA ((×) NH).
associations formed in the amorphous phase are stronger than the NH. . .O=C bonds of PA, thus, the vibrational band of a number of N–H groups shifts to lower frequencies. The SEM micrographs (Fig. 6) reveal compositiondependent heterogeneous structure of the blends. It is only partly a result of separate crystallization of the components (association of like molecules). The finely dispersed phases are characteristic for partially miscible blends, which is the consequence of specific interactions of the components in the amorphous phase (association of unlike molecules). Although crystallization of the components is hindered in the blends, the specific volume (vsp ) does not increase above the additive values because of the interaction of unlike molecules in the amorphous phase. As can be seen in Fig. 7, vsp is lower than the additive value up to 0.5 PA volume fraction and corresponds to additivity in PA-rich blends. The composition dependence of the specific volume, however, indicates phase inversion between 0.5 and 0.6 parts of PA. The correlation between vsp and the wavenumber of the OH1 absorption band (Fig. 8) reveals that the specific volume of the blends is governed by the strength of interaction of the OH groups in the amorphous fraction besides the characteristics of the components. The stronger the association in the amorphous phase the denser the structure. The diffusion coefficient (Dw ) and equilibrium solubility (M∞ ) of water derived from the time dependence of absorption (Fig. 1) are summarized in Table 3. The constant, k, proportional to the rate of stress relaxation at the boundary between the swollen gel and the glassy core of the polymer is also given in the table. All three parameters are higher in PA than in EVOH and change continuously with the composition of the blend. The correlation shown in Fig. 9 reveals that the diffusion coefficient of water is affected by the nature of the matrix polymer and controlled by the specific volume of the blend, which is determined by interactions developing in the amorphous phase. Similar correlation can be established between ln k and vsp . The intersection of the two straight
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(a)
(b)
(c)
Fig. 6. SEM micrographs of EVOH/PA blends with 0.1 (a), 0.5 (b), and 0.9 (c) mass fraction of PA.
Fig. 7. Composition dependence of the specific volume of EVOH/PA blends.
Fig. 8. Correlation between the IR absorption frequency of the OH groups of EVOH located in the amorphous phase and the specific volume of EVOH/PA blends.
Table 3 Diffusion and solubility of water in EVOH/PA blends PA mass fraction
Dw × 109 (cm2 /s)
k × 107 (1/s)
M∞ (w/w%)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.66 0.62 1.31 1.73 2.45 3.40 4.79 6.03 7.00 7.41 8.38
0.60 6.17 3.67 7.12 11.53 14.29 20.71 21.96 23.04 28.74 24.94
7.0 6.7 7.6 8.1 8.5 9.6 9.8 10.3 10.7 11.0 11.9
lines is located at a 1/1 component ratio, confirming phase inversion around this composition. The equilibrium water uptake of the blends is lower than can be expected on the basis of additivity, and the deviation
Fig. 9. Correlation between the specific volume of EVOH/PA blends and the diffusion coefficient of water.
E. Földes, B. Pukánszky / Journal of Colloid and Interface Science 283 (2005) 79–86
Fig. 10. Relationship between composition and the interaction parameter, , derived from equilibrium water uptake. χ12
85
Fig. 11. Correlation between the negative deviation in the heat of fusion and the positive deviation in the tensile strength of EVOH/PA blends compared to additivity.
Table 4 Tensile properties of EVOH/PA blends measured at ambient temperature at a cross-head speed of 50 mm/min PA mass fraction
σT (N/mm2 )
ε (%)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
49 ± 18 72 ± 3 74 ± 7 144 ± 64 192 ± 82 248 ± 60 227 ± 81 181 ± 25 150 ± 40 160 ± 69 53 ± 6
6±3 9±3 6±1 205 ± 71 237 ± 86 300 ± 32 250 ± 74 198 ± 39 201 ± 60 168 ± 76 22 ± 4
is even larger when the volume fraction of the absorbed water is related to the amorphous fraction of the blend. This result is also a consequence of specific interactions in the derived from equilibamorphous phase [26]. Parameter χ12 rium water uptake is negative and depends on the composition below 0.5 PA content. From the correlation shown in Fig. 10 strong specific interactions of the components can be deduced for the compositions of high EVOH content. The level of mixing of the components decreases with increas . ing PA content of the blend revealed by the increase of χ12 Above 0.5 PA content χ12 does not depend on the ratio of the dispersed EVOH phase. 3.3. Mechanical properties True tensile strength (σT ) and elongation at break (ε) depend significantly on the composition of the blend and give a maximum at 1/1 component ratio (Table 4), which is typical of strongly interacting systems [18]. Direct correlation can be established between the mechanical properties and the chemical and physical characteristics of the blends. Compared to additivity the positive deviation of σT (and
Fig. 12. Correlation between the interaction parameter derived from equilibrium water uptake and the proportionality constant calculated from the tensile strength of different polymer blends.
also ε) changes proportionally with the deficit in heat of fusion (Fig. 11), as well as with the excess in the intensity of OH1 vibration (IOH1 ). The digression obtained for two values in Fig. 11 is due to experimental error. These results indicate that the mechanical properties are improved by enhanced stress transfer between the phases due to hydrogen bonding of unlike molecules. In Fig. 12 the proportionality constant C (calculated from (detensile strength) is related to the average value of χ12 rived from equilibrium water uptake) for EVOH/PA and different amorphous polymer blends investigated earlier [30]. The correlation indicates that the level of mixing in the amorphous part of EVOH/PA blends is lower than in the miscible PS/PPO blends, but much higher than in the heterogeneous PS/PC and PPO/SAN blends with limited compatibility.
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4. Conclusions The morphology and the mechanical properties of blends of EVOH (56 mol% VA) and PA 6/66 (75 mol% PA 6) random copolymers are determined by the competitive interaction of like and unlike molecules. The specific interaction of various polar groups results in composition-dependent morphology and properties. Hydrogen bonding of like molecules (OH. . .HO bonds in EVOH and NH. . .O=C bonds in PA) results in crystallization of the components in separate phases leading to the formation of a heterogeneous structure in each composition. However, hydrogen bonding of the unlike molecules decreases the crystallinity of the components. The strength of molecular association in the amorphous phase governs the density of the blends besides the characteristics of the components. The greater amount of the amorphous phase and specific interaction of the components result in considerable improvement of the mechanical properties. The extent of mixing of the components was found to be between that of the miscible PS/PPO blends and the immiscible PS/PC or PPO/SAN blends with weak interactions.
Acknowledgments The work was financially supported by the National Scientific Research Fund of Hungary (Grants OTKA T023421 and OTKA T030579).
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Miscibility–structure–property correlation in blends of ethylene vinyl alcohol copolymer and polyamide 6/66 Enik˝o Földes a,b,∗ , Béla Pukánszky a,b a Institute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary b Department of Plastics and Rubber Technology, Technical University of Budapest, H-1521 Budapest, P.O. Box 91, Hungary
Received 3 February 2004; accepted 26 August 2004 Available online 10 December 2004
Abstract Blends of ethylene vinyl alcohol (EVOH; 44 mol% ethylene) and polyamide 6/66 (PA; 75 mol% PA 6) random copolymers were studied in the entire composition range. Specific interaction between the components was analyzed by IR spectroscopy; furthermore, coefficients related to the Flory–Huggins interaction parameter were derived from equilibrium water uptake and tensile strength. Morphology of the blends was investigated by thermal analysis (DSC), density measurements, and SEM micrographs. The two polymers form heterogeneous blends in each composition. Although the components crystallize in separate phases, the morphology and the mechanical properties are greatly affected by the association of OH and NH groups. Crystallization is restricted in the blends, and the increase of the amorphous fraction, as well as specific interaction between the components, results in essential improvement in the mechanical properties. 2004 Elsevier Inc. All rights reserved. Keywords: EVOH; PA; Blends; Miscibility; Specific interaction
1. Introduction The morphology and the properties of polymer blends are basically determined by the strength of interaction of the components. The thermodynamic definition of polymer– polymer miscibility is based on the Flory–Huggins theory. A blend will be thermodynamically miscible when the polymer–polymer interaction energy is less than a critical value affected by the molecular weight of the components. When the interaction energy exceeds that, two-phase mixtures are formed. Compatible blends are obtained with fine dispersion, large interfacial thickness, strong interface, and good mechanical properties when the interaction energy is not too much larger than the critical value. At large interaction energies the interfacial tension increases, the interfacial strength weakens, the morphology becomes gross, and the mechanical properties become inferior. At some points the * Corresponding author. Fax: +36-1-325-7554.
E-mail address: [email protected] (E. Földes). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.08.175
blends are regarded as incompatible [1]. Miscibility of nonpolar polymers of high molecular weight occurs only when the solubility parameters are matched, as the entropy of mixing is very small and the heat of mixing is generally positive. The enthalpy of mixing can be negative, if certain specific interactions between polar groups are involved [2–4]. Miscibility of ethylene vinyl alcohol copolymer (EVOH) with polyamide (PA) has been studied by several authors with different aims in view. One of the goals was to improve the forming capacity (in terms of temperature and draw ratios) and the mechanical properties of EVOH without sacrificing the gas barrier properties [4–9]. Another topic of interest was processing monolayer barrier films with multilayer-type internal structures for packaging [10–12]. In the textile industry the aim was to increase the impact strength and barrier properties of PA 46 [13]. PA/EVOH blends were patented as early as in the 1980s for bare and composite films with a high level of strength and toughness, good oxygen barrier properties, and enhanced adhesive qualities [14], as well as for various products (moldings, films, and fibers) [15].
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Specific interaction between EVOH and PA was observed resulting in partial miscibility of the components [12,16–19]. The properties of the blend depend on composition, vinyl alcohol content of EVOH [6,17,19], type of PA [8,17,20], thermal-mechanical history [12], and moisture content [9,11], as well as on the chemical modification of the components [10,21]. In a previous paper we discussed the miscibility of EVOH and PA 6/66 random copolymers [18]. Improved mechanical properties, as well as the chemical and physical characteristics of the blends, indicated a strong interaction between EVOH and PA 6/66. The interaction was attributed to hydrogen bonding of the OH groups of EVOH with the NH and C=O groups of PA. Ha et al. [13] explained the miscibility of the components in EVOH/PA 46 blends at PA contents below 35 w/w% with the interaction between the NH groups of PA 46 and the C–O groups of EVOH. However, the EVOH investigated contained 13 mol% vinyl acetate (VA), and the assumption was based on the shift of the carbonyl vibration of VA to lower frequencies in the infrared (IR) spectra. Due to growing interest in the development and recycling of multicomponent polymer systems, various characteristics of the blends of EVOH and PA 6/66 were reevaluated and reconsidered in the present work. Our goal was to identify the nature of specific interactions between the components and their impact on the morphology and properties of the blends.
2. Experimental 2.1. Materials and sample preparation For the experiments Selar OH 4416 grade ethylene vinyl alcohol random copolymer (Du Pont) with 44 mol% ethylene content and Ultramid C35 grade polyamide 6/66 copolymer (BASF) were selected. According to the calculations described below the PA 6 content of the copolymer was 75 mol%. The vinyl acetate content of EVOH was under the detection level by FTIR spectroscopy. The blends were prepared in the entire composition range from 0 to 1 mass fractions of PA in steps of 0.1. The materials were homogenized in a DSK 42/7 Brabender twin-screw compounder at 230–245 ◦ C and extruded into bands of 2 cm wide and 1 mm thick. The experiments were carried out on these bands or on 1-mm-thick plates compression-molded by a Fontijne SRA 100 laboratory press. For infrared measurements 12to 20-µm thick films were compression-molded. All samples were dried at 70 ◦ C up to mass equilibrium (1300 h) prior to the experiments. 2.2. Experimental and evaluation methods The chemical structure of the materials was characterized by infrared spectroscopy (FTIR; Mattson–Galaxy 3020,
Unicam). The spectra were taken in the range of 4000– 400 cm−1 with a resolution of 2 cm−1 using 16 scans. The O–H and N–H bands absorbing in the range of 3650 and 3000 cm−1 were separated by curve fitting using Spectra Calc (Bomem-Calc) FTIR software. The physical structure was characterized by different techniques. Fusion and crystallization properties were determined by differential scanning calorimetry (DSC) under nitrogen at a scanning rate of 10 ◦ C/min, using the DSC30 cell of a Mettler TA 4000 system. Composition of the PA 6/66 copolymer was calculated from melting point depression [2,22] using Hc0 of 230 and 301 J/g, as well as Tm0 of 270 and 280 ◦ C for PA 6 and PA 66, respectively [23]. The crystallinity of EVOH was derived from the crystalline characteristics of polyvinyl alcohol: Tm0 = 265 ◦ C, Hc0 = 163 J/g [23]. Density was measured in a gradient column at 23 ◦ C. Notched strips cut from the compressionmolded plates were cooled in liquid nitrogen for 30 min and then broken into two parts. SEM micrographs were taken from the fracture surface after etching in formic acid at ambient temperature for 30 s. Water uptake of the samples was investigated at 25 ◦ C as a function of time by an immersion technique. Equilibrium solubility and the diffusion coefficient of water were determined by least-squares curve fitting. As the samples showed combined Fickian and Case II diffusion, the mathematical model of Berens and Hopfenberg [24,25] was applied for calculating the equilibrium water uptake (M∞ ), the Fickian diffusion coefficient (Dw ), and the rate constant (k) proportional to the velocity of the advancing front of the boundary between the swollen gel and the glassy core of the polymer. According to the model the total amount of diffusant absorbed at time t can be expressed by the sum of two phenomenologically independent terms, Mt = Mt,D + Mt,R ,
(1)
where Mt,D and Mt,R are the amounts of diffusant absorbed by Fickian diffusion mechanism and first-order kinetics (Case II mechanism), respectively, ∞ 1 8 Mt,D = M∞,D 1 − 2 π (2m + 1)2 m=0 Dw (2m + 1)2 π 2 t × exp − (2) , l2 Mt,R = M∞,R 1 − exp(−kt) , (3) where M∞,D and M∞,R are the equilibrium water uptake values absorbed by Fickian and Case II mechanism, respectively; l is the thickness of the film. As can be seen from Fig. 1 the data conform well to the model. From the equilibrium water uptake the interaction pa ) between EVOH and PA was calculated [26], rameter (χ12 ln aw = 0 = ln φw + (1 − φw ) φ1 φ2 , + (χw1 φ1 + χw2 φ2 )(1 − φw ) − χ12
(4)
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81
Fig. 1. Water uptake of EVOH ("), PA (+), and their 1/1 blend (2). Solid lines represent fitted curves. Fig. 2. Infrared spectrum of EVOH resolved by curve fitting.
where aw is the activity of water sorbed into a homogeneous blend of polymers 1 and 2, and φw , φ1 , and φ2 are the volume fractions of water, polymers 1 and 2 in the ternary system, respectively. χw1 and χw2 are the polymer–water interaction parameters determined from the equilibrium solvent uptake of the neat polymers: ln aw = 0 = ln φw + (1 − φw ) + χwi (1 − φw )2 .
(5)
With increasing interaction of polymers 1 and 2 the becomes more amount of water absorbed decreases and χ12 negative [26]. The volume fractions of the components were determined by using crystalline densities of 1.35 and 1.241 g/cm3 for EVOH and PA, respectively [23]. It was assumed that the transport of water molecules was restricted to the amorphous region of the blends, and the composition of the ternary systems was calculated from the volume fractions of absorbed water and amorphous part of the polymer components. Mechanical properties were investigated by tensile tests at ambient temperature using a cross-head speed of 50 mm/min. For the determination of polymer–polymer interaction a semiempirical model was developed earlier [27–29]. According to the model the tensile strength of a twocomponent system is determined by the true tensile strength of the matrix polymer (σT0 ), the volume fraction of the dispersed phase (φd ) and a parameter, B, which reflects the effect of interaction, σTred =
σT 1 + 2.5φd = σT0 exp(Bφd ), λn 1 − φd
(6)
where σTred and σT are the reduced and true strengths of the blend, respectively. The true strength can be derived from the engineering tensile strength (σT = σ λ). λ is the relative elongation (λ = L/L0 ); n is a parameter characterizing the strain hardening tendency of the matrix, and it can be determined from the stress vs strain correlation of the neat matrix polymer. Parameter B indicates the relative load bearing ca-
pacity of the components, Cσd B = ln , σm
(7)
where σd and σm are the tensile strengths of the dispersed phase and the matrix, respectively. C is the proportionality constant related to the stress transfer, and thus, reciprocally to the Flory–Huggins interaction parameter (χ). Studies of amorphous polymer blends proved the correlation between the proportionality constant C and χ derived from the solubility parameter [30].
3. Results and discussion 3.1. Interaction In hydrogen-bonded polymer blends the development of intermolecular interaction can be followed by changes in the wavelength and intensity of IR absorption bands of the functional groups [31–33]. In the present work the composition dependence of O–H and N–H vibrations was analyzed after their separation by curve fitting. EVOH is a semicrystalline polymer. The broad vibrational band of OH groups in the range of 3650–3200 cm−1 points to strong hydrogen bonding. The curve-fitting procedure revealed that this band is a superposition of a higher and a lower frequency band (OH1 and OH2 in Fig. 2), since a single peak could not be fitted by any mathematical formula used in IR spectroscopy. The higher frequency (OH1) band represents lower association energy, and can be attributed to the OH groups located in the amorphous phase. The stronger the hydrogen bonds the larger the shift of O–H vibration to lower frequencies. Therefore the lower frequency (OH2) band can be assigned to the absorption of OH groups in the ordered phase. PA 6/66 is also a semicrystalline polymer, where NH and C=O groups form hydrogen bonds reflected
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Fig. 3. Infrared spectrum of EVOH/ PA blend of 1/1 composition resolved by curve fitting.
Table 1 Frequency and intensity of characteristic infrared bands of EVOH/PA blends and their components PA mass fraction
Wavenumber (cm−1 )a
Relative integrated intensitya,b
OH1
OH2
NH
OH1
OH2
NH
0 0.1 0.3 0.5 0.7 0.9 1.0
3419 3414 3428 3427 3433 3447 3447
3281 3283 3278 3283 3272 3268 3264
3304 3302 3302 3303 3304 3301
1.21 1.32 1.33 1.06 0.76 0.49 0.23
3.35 2.10 2.17 1.83 1.72 1.17 0.39
0.12 0.33 0.69 1.07 1.07 1.62
a OH1 and OH2 are resolved bands of hydrogen-bonded O–H vibration. b Related to the integrated intensity of C–H absorption in the range of 3000–2900 cm−1 .
in the low frequency of N–H vibration [34]. The OH bands of low intensity in neat PA (Table 1) can be attributed to some absorbed water present despite drying to mass equilibrium. In the blends of the two polymers the OH and NH bands are superimposed. The measured and resolved spectra of EVOH/PA blend of 1/1 composition are illustrated in Fig. 3. For analyzing the interaction between the polymers, the wavenumbers of the resolved bands were determined and the relative integrated intensities were calculated with the use of C–H vibrations at 3000–2900 cm−1 as reference. The results are summarized in Table 1. The frequency of N–H vibration (νNH ) is independent of the composition of the blend, while its intensity (INH ) decreases monotonically with EVOH content. INH is, however, lower in each composition than can be expected on the basis of additivity (Fig. 4). The position of the N–H absorption band in PA is determined by the strength of interaction, and the intensity by the number of N–H. . .O=C groups. The results of analysis indicate that some of the N–H vibrations are
Fig. 4. Composition dependence of the relative infrared intensity of O–H ((") OH1 and (Q) OH2) and N–H (×) vibrations.
shifted to higher and/or lower frequencies compared to that characteristic of neat PA. Both the wavenumber and the intensity of O–H bands are affected by the composition of the blend (Table 1). The OH1 band shifts to higher and the OH2 band to lower frequencies with increasing PA content. The shift of the OH1 band indicates that the strength of hydrogen bonding of OH groups decreases in the amorphous phase compared to neat EVOH. The relative intensity of OH1 absorption (IOH1 ) increases slightly with increasing PA content of the blend below 0.5 PA mass fraction, and starts to decrease around 1/1 component ratio referring to a change in the phase structure at this composition (Fig. 4). Modification of the OH2 band frequency (νOH2 ) with composition also points to some changes in the structure around the 1/1 component ratio (Table 1). νOH2 can be considered independent of the composition at high EVOH concentrations and decreases above 0.5 PA content. The intensity of OH2 vibration (IOH2 ) changes also nonlinearly with composition (Fig. 4). It remains below the additive value at high EVOH concentrations and exceeds that at PA contents higher than 0.5 mass fraction. Correlating the deviations from additivity of the N–H and O–H band intensities we can conclude that the increase in IOH1 is related to the decrease of both INH and IOH2 . This result implies that (a) the crystallinity of EVOH decreases in the blends compared to the neat polymer, (b) the decrease of INH is caused by formation of NH. . .HO bands in the amorphous fraction, and (c) despite the intermolecular interactions the components are not mixed homogeneously in the amorphous fraction. 3.2. Structure The fusion and crystallization characteristics of EVOH/ PA blends studied by DSC in two heating and one cooling
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Table 2 Fusion characteristics of EVOH/PA blends determined from the first heating run measured by DSC in nitrogen atmosphere at a heating rate of 10 ◦ C PA mass fraction 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Tm (◦ C) EVOH 169 167 169 165 168 166 165 165 156 (101)
Hf (J/g) PA 186 189 188 191 192 194 197 199 200 201
EVOH 70.2 64.2 43.1 35.0 25.7 15.5 10.7 5.6 3.4 (5.8)
PA 2.3 5.5 11.7 14.9 19.1 25.0 31.6 38.2 46.1 51.0
runs are strongly affected by composition and thermal history. Heterogeneous morphology and partial miscibility in the amorphous phase of the dried blends are disclosed by the thermograms measured in the first heating run. Both components have separate composition-dependent fusion peaks (Table 2). The main transition of neat EVOH occurs at 169 ◦ C accompanied by a fusion peak of low intensity at around 100 ◦ C. A third transition appears at 56–58 ◦ C in the blends containing more than 10 wt% PA. The main transition of EVOH shifts only moderately to lower temperatures up to 0.7 PA mass fraction. At 0.8 PA content it decreases more significantly and then disappears from the thermogram at 0.1 EVOH mass fraction. In this latter case only the two lower fusion peaks can be detected. The fusion temperature of PA changes more steadily with composition; it decreases almost linearly with increasing EVOH content of the blend. The heat of fusion (Hf ) values of the components indicates that the crystallizability of EVOH is rather affected by the presence of PA than inversely. The heats of fusion determined in the first heating run are lower in each case for both polymers than the additive values (see also [18]). The negative deviation from additivity (deficit in the heat of fusion) is more significant in the case of EVOH than for PA. The thermal history also affects the morphology of the blends. The thermograms measured in the cooling and second heating runs show no crystalline transition for EVOH in the blends containing more than 0.2 PA, and PA could crystallize separately only up to 0.4 mass fraction of EVOH. For the blends with PA mass fractions between 0.3 and 0.5 only a glass transition was detected at 42–45 ◦ C. The decrease in crystallinity can be directly related to the interaction of the components revealed by the correlation between their heat of fusion and the intensities of the hydrogen-bonded N–H and OH2 absorption bands. Fig. 5 shows that Hf increases linearly with the relative intensity of the corresponding hydrogen bonding band of both components. The correlation for EVOH does not start at the origin, which indicates that OH2 vibration does not originate solely from hydrogen-bonded OH groups located in the separate ordered phase of this polymer. Most probably some OH. . .HN
Fig. 5. Correlation between the IR absorbance of hydrogen bonding groups and the heat of fusion of the components: EVOH ((Q) OH2), PA ((×) NH).
associations formed in the amorphous phase are stronger than the NH. . .O=C bonds of PA, thus, the vibrational band of a number of N–H groups shifts to lower frequencies. The SEM micrographs (Fig. 6) reveal compositiondependent heterogeneous structure of the blends. It is only partly a result of separate crystallization of the components (association of like molecules). The finely dispersed phases are characteristic for partially miscible blends, which is the consequence of specific interactions of the components in the amorphous phase (association of unlike molecules). Although crystallization of the components is hindered in the blends, the specific volume (vsp ) does not increase above the additive values because of the interaction of unlike molecules in the amorphous phase. As can be seen in Fig. 7, vsp is lower than the additive value up to 0.5 PA volume fraction and corresponds to additivity in PA-rich blends. The composition dependence of the specific volume, however, indicates phase inversion between 0.5 and 0.6 parts of PA. The correlation between vsp and the wavenumber of the OH1 absorption band (Fig. 8) reveals that the specific volume of the blends is governed by the strength of interaction of the OH groups in the amorphous fraction besides the characteristics of the components. The stronger the association in the amorphous phase the denser the structure. The diffusion coefficient (Dw ) and equilibrium solubility (M∞ ) of water derived from the time dependence of absorption (Fig. 1) are summarized in Table 3. The constant, k, proportional to the rate of stress relaxation at the boundary between the swollen gel and the glassy core of the polymer is also given in the table. All three parameters are higher in PA than in EVOH and change continuously with the composition of the blend. The correlation shown in Fig. 9 reveals that the diffusion coefficient of water is affected by the nature of the matrix polymer and controlled by the specific volume of the blend, which is determined by interactions developing in the amorphous phase. Similar correlation can be established between ln k and vsp . The intersection of the two straight
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(a)
(b)
(c)
Fig. 6. SEM micrographs of EVOH/PA blends with 0.1 (a), 0.5 (b), and 0.9 (c) mass fraction of PA.
Fig. 7. Composition dependence of the specific volume of EVOH/PA blends.
Fig. 8. Correlation between the IR absorption frequency of the OH groups of EVOH located in the amorphous phase and the specific volume of EVOH/PA blends.
Table 3 Diffusion and solubility of water in EVOH/PA blends PA mass fraction
Dw × 109 (cm2 /s)
k × 107 (1/s)
M∞ (w/w%)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.66 0.62 1.31 1.73 2.45 3.40 4.79 6.03 7.00 7.41 8.38
0.60 6.17 3.67 7.12 11.53 14.29 20.71 21.96 23.04 28.74 24.94
7.0 6.7 7.6 8.1 8.5 9.6 9.8 10.3 10.7 11.0 11.9
lines is located at a 1/1 component ratio, confirming phase inversion around this composition. The equilibrium water uptake of the blends is lower than can be expected on the basis of additivity, and the deviation
Fig. 9. Correlation between the specific volume of EVOH/PA blends and the diffusion coefficient of water.
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Fig. 10. Relationship between composition and the interaction parameter, , derived from equilibrium water uptake. χ12
85
Fig. 11. Correlation between the negative deviation in the heat of fusion and the positive deviation in the tensile strength of EVOH/PA blends compared to additivity.
Table 4 Tensile properties of EVOH/PA blends measured at ambient temperature at a cross-head speed of 50 mm/min PA mass fraction
σT (N/mm2 )
ε (%)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
49 ± 18 72 ± 3 74 ± 7 144 ± 64 192 ± 82 248 ± 60 227 ± 81 181 ± 25 150 ± 40 160 ± 69 53 ± 6
6±3 9±3 6±1 205 ± 71 237 ± 86 300 ± 32 250 ± 74 198 ± 39 201 ± 60 168 ± 76 22 ± 4
is even larger when the volume fraction of the absorbed water is related to the amorphous fraction of the blend. This result is also a consequence of specific interactions in the derived from equilibamorphous phase [26]. Parameter χ12 rium water uptake is negative and depends on the composition below 0.5 PA content. From the correlation shown in Fig. 10 strong specific interactions of the components can be deduced for the compositions of high EVOH content. The level of mixing of the components decreases with increas . ing PA content of the blend revealed by the increase of χ12 Above 0.5 PA content χ12 does not depend on the ratio of the dispersed EVOH phase. 3.3. Mechanical properties True tensile strength (σT ) and elongation at break (ε) depend significantly on the composition of the blend and give a maximum at 1/1 component ratio (Table 4), which is typical of strongly interacting systems [18]. Direct correlation can be established between the mechanical properties and the chemical and physical characteristics of the blends. Compared to additivity the positive deviation of σT (and
Fig. 12. Correlation between the interaction parameter derived from equilibrium water uptake and the proportionality constant calculated from the tensile strength of different polymer blends.
also ε) changes proportionally with the deficit in heat of fusion (Fig. 11), as well as with the excess in the intensity of OH1 vibration (IOH1 ). The digression obtained for two values in Fig. 11 is due to experimental error. These results indicate that the mechanical properties are improved by enhanced stress transfer between the phases due to hydrogen bonding of unlike molecules. In Fig. 12 the proportionality constant C (calculated from (detensile strength) is related to the average value of χ12 rived from equilibrium water uptake) for EVOH/PA and different amorphous polymer blends investigated earlier [30]. The correlation indicates that the level of mixing in the amorphous part of EVOH/PA blends is lower than in the miscible PS/PPO blends, but much higher than in the heterogeneous PS/PC and PPO/SAN blends with limited compatibility.
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4. Conclusions The morphology and the mechanical properties of blends of EVOH (56 mol% VA) and PA 6/66 (75 mol% PA 6) random copolymers are determined by the competitive interaction of like and unlike molecules. The specific interaction of various polar groups results in composition-dependent morphology and properties. Hydrogen bonding of like molecules (OH. . .HO bonds in EVOH and NH. . .O=C bonds in PA) results in crystallization of the components in separate phases leading to the formation of a heterogeneous structure in each composition. However, hydrogen bonding of the unlike molecules decreases the crystallinity of the components. The strength of molecular association in the amorphous phase governs the density of the blends besides the characteristics of the components. The greater amount of the amorphous phase and specific interaction of the components result in considerable improvement of the mechanical properties. The extent of mixing of the components was found to be between that of the miscible PS/PPO blends and the immiscible PS/PC or PPO/SAN blends with weak interactions.
Acknowledgments The work was financially supported by the National Scientific Research Fund of Hungary (Grants OTKA T023421 and OTKA T030579).
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