Abstract
Ethylene-vinyl alcohol (EVOH) with excellent barrier properties is insufficient in mechanical properties, which limits its application. For the purpose of enhancing mechanical properties of EVOH, blend of anhydrous magnesium chloride (MgCl2) with EVOH was prepared by melt blending. Enhanced hydrogen bonding can be achieved through interaction between the Cl of MgCl2 and -OH in EVOH, and the stronger hydrogen bonding between MgCl2 and EVOH was confirmed by using Fourier transform infrared spectrometer and X-ray photoelectron spectroscopy. The chemical, thermal and mechanical properties were evaluated. The thermal analysis of blends demonstrated that the melting temperature (Tm), the crystallisation temperature (Tc) and the crystallinity were effectively decreased with adding MgCl2, on the contrary, the glass transition (Tg) was significantly increased from 42.8 to 77.5°C. The mechanical test results revealed that MgCl2 can significantly improve the storage modulus and the viscosity of EVOH, also the tensile strength was raised by nearly 30%.
Introduction
Recently, growing interest is witnessed in the development of new materials for packaging applications, especially the barrier materials. Ethylene-co-vinyl alcohol (EVOH) is random copolymers containing ethylene and vinyl alcohol groups distributed along the chains [1]. Because of their excellent gas barrier properties, they are commercial materials with applications mainly in food packaging. The increase in practical applications in food packaging has also brought about gradual expansion into other fields such as containers, pipes, and construction materials for surface application. The vinyl alcohol copolymer mainly contributed to the oxygen barrier due to its small free volume in between molecules and the ethylene copolymer prevented the polymer from dissolving in water and increased its processability [2]. EVOH has both the barrier properties of polyvinyl alcohol and the processing properties of polyethylene [3]. However, because the intramolecular and intermolecular hydrogen bonding formed by hydroxyl groups in EVOH is insufficiently strong, which results to the insufficiency of mechanical properties. In order to improve the properties of this material and further exploit its utility in more fields in the future, enhancing the hydrogen bonding of EVOH would be a simple yet efficient approach.
Blending modification is the preferred way to modify plastic, because it is a lower-cost and practical method. In recent years, many studies have reported the methods of modifying EVOH by blending with other materials. One of the most commonly used modification method is adding plasticiser [3-7], but the addition of plasticiser may reduce the mechanical and barrier performance of EVOH. In addition, the active hydroxyl groups in EVOH can interact with other polymers containing amino group, carboxyl group, acid anhydride group and epoxy group etc. [8-11]. Although a number of EVOH blends have been reported, there are comparatively fewer reports on blends of enhancing hydrogen bonding.
Herein in this work, EVOH/ MgCl2 blends were prepared by melt blending through a torque rheometer. The introduction of MgCl2 can subsequently enhance the hydrogen bonding of EVOH. The modification effect of MgCl2 to EVOH has been discussed in terms of tensile strength, crystallisation, thermal properties and viscosity.
Experimental
Materials
EVOH (F101A, containing 32% Ethylene) was supplied by Japan Kuraray Corporation (Japan). Anhydrous MgCl2, Mg(OH)2 and anhydrous BaCl2 were all from Shanghai Aladdin Biochemical Technology Co., Ltd (Shanghai).
Preparation of EVOH, EVOH/MgCl2, EVOH/Mg(OH)2 and EVOH/BaCl2
EVOH was dried in a vacuum at 80°C for 4 h, and MgCl2, Mg(OH)2 and BaCl2 were dried in an oven 100°C for 2 h before blending. Four samples, which were EVOH, EVOH/MgCl2, EVOH/Mg(OH)2 and EVOH/BaCl2, were prepared by melt blending using a KCCK XSS-300 rheometer equipped with a 60-ml mixing chamber (KCCK, China) at 195°C. Four samples, containing 1% metal compounds, were introduced to the mixer and were blended at 100 rev min–1 for 6 min. After blending, the samples were moulded by hot-press into a 0.5-mm-thick film, followed by water cooling for 1 min to room temperature (25°C). The pressing temperature was 200°C, and the pressing time was 10 min, with a pressure of 10 MPa.
Characterisation of blends
Fourier transform infrared spectroscopy (FTIR)
The FTIR spectra of blends were performed by a Nicolet 6700 spectrometer (Thermo Fisher, USA) at a heating rate 5°C min–1 from 25 to 200°C with recording a spectrum every two minutes.
X-ray photoelectron spectroscopy (XPS)
The surface chemical bonding was analysed by using XPS (Thermo Scientific Escalab 250Xi, Thermo Fisher, USA) with Al Kα X-ray source under a working pressure of 1 × 10−6 Pa at current of 12 kV pass energy at 75 eV.
Differential scanning calorimetry (DSC)
Differential scanning calorimetry (DSC) measurements were carried out with a modulated DSC 2910 (TA Instruments, USA) under a nitrogen atmosphere. The sample weights were approximately 8 mg. The scanning rate used throughout the investigation was 10°C min–1. Pure EVOH and the blend samples were first heated from 25 to 230°C, and then held for 3 min to eliminate the thermal history. They were then cooled to 25°C. After holding for another 3 min, the samples were reheated to 230°C.
Viscosity testing
The viscosity of the sample was tested by using an RG 20 capillary rheometer (Goettfert, Germany). Tests were carried out at 190°C with 30 g and a die with 1 mm diameter and length to diameter ratio of 30/1. The shear rate and shear stress of the blend were recorded.
Dynamic mechanical analysis (DMA)
Dynamic mechanical measurements at 1 Hz were made in tensile mode using a Q800 dynamic mechanical thermal analyser (TA Instruments, USA). Tests were carried out at a heating rate 5°C/min from –60 to 150°C. Rectangular specimens of 5 mm wide and 30 mm long were used.
Mechanical testing
The tensile testing was performed using a universal testing machine (CMT6104, Sans, China) at a crosshead speed of 10 mm min–1 with a gauge length of 50 mm. Each sample included five tested replicates to obtain a reliable mean and standard deviation.
Results and discussion
The blends of EVOH can form hydrogen bonding between the hydroxyl groups of EVOH and the anion of some divalent metal compounds. Divalent metal compounds are mostly ionic crystals. In general, the strength of hydrogen bonding depends on the stability of the ionic crystal itself and the strength of the ionic bond, both of them are measured by the crystal lattice energy [12]. The stability of the ionic crystal and the strength of the ionic bond are increased upon increasing the lattice energy, which can directly affect the crystal structure. The formation of hydrogen bonding is closely related to the crystal structure [13, 14]. Therefore, high lattice energy can contribute to forming the enhanced hydrogen bonding.
The data of ionic radius and lattice energy of several ionic crystals.
It can be seen from Table 1 that MgCl2 has the smallest cation radius and the largest lattice energy, so it's stability and the strength of ionic bond are both the highest, which contributes to forming stronger hydrogen bonding between Cl of MgCl2 and –OH of EVOH, the mechanism of formation is shown in Figure 1 [16].
Mechanism of formation of the hydrogen bonding between –OH of EVOH and Cl of MgCl2.
This paper mainly studied the enhanced hydrogen bonding of EVOH with MgCl2, and EVOH/BaCl2 and EVOH/Mg(OH)2 were used for comparative experiment. Both the crystals of MgCl2 and Mg(OH)2 are layered structure, and the coordination number of Mg2+ is both 6 [15]. The strength of the hydrogen bonding between the two and EVOH can be compared by the interaction between the layers. The interaction is from van der Waals force. It is reported in the literature that the OH group in Mg(OH)2 is vertical, and the interaction between the layers is very weak and no hydrogen bonding [17]. Therefore, the interlayer interaction of MgCl2 is significantly stronger than that of Mg(OH)2, which can lead to a stronger hydrogen bonding between EVOH with MgCl2.
Therefore, through a large number of literature investigations and the above theoretical analysis, MgCl2 was selected finally as the best material to enhance the hydrogen bonding and improve properties of EVOH.
Hydrogen bonding analysis
Infrared analysis
FTIR spectroscopy has been a simple but useful way to identify the existence of specific interactions in the blends. The absorption band of the –OH (no hydrogen bonding) is 3600 cm−1. When the hydrogen bonding forms, the band of the hydroxyl group shifts to a lower wavenumber. Of course, the magnitude of the movement also reflects the intensity of the hydrogen bond.
Figure 2 shows the FTIR spectra of pure EVOH, EVOH/MgCl2, EVOH/Mg(OH)2 and EVOH/BaCl2. Comparing with the free –OH (no hydrogen bonding), the absorption band of the –OH in EVOH shifts to a lower wavenumber (3329.7 cm−1), which is an indication of hydrogen bonding formation. As can be seen from Figure 2, the frequency value shifts from 3329.7–3317.6 cm−1 with the addition of MgCl2, on the contrary, the absorption band of the –OH in EVOH shifts to a higher wavenumber when Mg(OH)2 and BaCl2 was added. For further explain the formation of hydrogen bonding, internal standard method is used to analyse FTIR data. As for the OH data overlaps with another peak at 2900 cm−1, the absorption band of the C–H at 1400 cm−1 is chosen as the internal standard peak. From Table 2, a significant decrease in the value of EVOH is observed upon adding MgCl2, which confirms the formation of enhanced hydrogen bonding between EVOH and MgCl2.
FTIR spectra of pure EVOH, EVOH/MgCl2, EVOH/Mg(OH)2 and EVOH/BaCl2. FTIR analysis by using internal standard method. aC–H peak at 1400 cm−1 was chosen as the internal standard peak, as there is no change between before and after reaction in EVOH.
A study showed that the binding between –OH and other substance became weaker with heating by in-situ DRIFTS [18]. EVOH is usually processed at around 195°C, so the hydrogen bonding is also studied during heating. The variation of the absorption bands of the –OH with temperature is shown in Figure 3. It shows that the absorption band of the –OH shifts to higher wavenumber upon increasing the temperature. The movement of molecular chain is enhanced and the crystal structure is destroyed when heating, which weakens the hydrogen bonding, so the absorption band of –OH in all samples shift to higher wavenumber. However, adding MgCl2 made the frequency value of the –OH lower than other samples throughout the heating process, although all samples tended to be consistent when they were completely melted at 200°C.
The variation of the absorption bands of the –OH of pure EVOH, EVOH/MgCl2, EVOH/Mg(OH)2 and EVOH/BaCl2 with temperature.
It's easy to form hydrogen bonding between –OH in EVOH and Cl with stronger electronegativity from MgCl2. This is corrected with the above theoretical analysis. Hence, according to the FTIR spectra analysis that the absorption band of the –OH shifted to a lower wavenumber, it is well-document that adding MgCl2 can form enhanced hydrogen bonding in EVOH.
X-ray photoelectron spectroscopy analysis
XPS can identify the chemical state of the element and the chemical composition in the measured samples based on the distinctive binding energy of the inner electron from each element [19]. Figures 4 and 5 show the XPS spectra of pure EVOH, EVOH/MgCl2, EVOH/Mg(OH)2 and EVOH/BaCl2. The positions of C1s and O1s peaks, and atomic composition of their surfaces are listed in Tables 3 and 4. The bonding energy of the C–O group is significantly decreased after adding MgCl2. Combined with the infrared analysis, Cl can react with the –OH in EVOH, which can form stronger hydrogen bonding and weaken the strength of C–O, so it results in the shift of the position of the C–O peak. Although the O–Mg peak is detected on the surface after treatment by adding MgCl2 or Mg(OH)2, the content is too low to be ignored.
C1s spectra of pure EVOH, EVOH/MgCl2 and EVOH/Mg(OH)2. (a) EVOH (b) EVOH/MgCl2 (c) EVOH/Mg(OH)2. O1s spectra of pure EVOH, EVOH/MgCl2 and EVOH/Mg(OH)2. (a) EVOH (b) EVOH/MgCl2 (c) EVOH/Mg(OH)2. Positions of C1s peak and atomic composition of surfaces of pure EVOH, EVOH/MgCl2 and EVOH/Mg(OH)2. Positions of O1s peak and atomic composition of surfaces of pure EVOH, EVOH/MgCl2 and EVOH/Mg(OH)2.
Therefore, combined with FTIR and XPS analysis, introduction of MgCl2 in the EVOH can enhance hydrogen bonding, which undoubtedly contributes to improve properties of EVOH.
Thermal characterisation
Crystallisation character
Thermal characterisation of pure EVOH, EVOH/MgCl2, EVOH/Mg(OH)2 and EVOH/BaCl2 is carried out using DSC measurements (Figure 6 and Table 4). Figure 6 shows the second heat and cool scans of DSC thermograms of four blends. To compare the degree of crystallinity of EVOH/MgCl2 blends with that in neat EVOH, the relative degree of crystallinity is estimated from the following equation:
DSC curves of pure EVOH, EVOH/MgCl2, EVOH/Mg(OH)2 and EVOH/BaCl2. (a) heating curve (b) cooling curve. DSC results of pure EVOH, EVOH/MgCl2, EVOH/Mg(OH)2 and EVOH/BaCl2.
and
are the melting enthalpies of EVOH/MgCl2 and neat EVOH (157.8 J/g), and
is the weight fraction of EVOH in the EVOH/MgCl2 blends (the relative degree of crystallinity of EVOH/Mg(OH)2 and EVOH/BaCl2 can also be obtained according to this formula). The DSC results are summarised in Table 5. The melting temperature (Tm) and the crystallisation temperature (Tc) are lowed from 183.4 and 160.3°C to 179.5 and 153.5°C, respectively, with the addition of MgCl2, at the same time, a decrease in crystallinity from 41.5 to 37.1%, suggesting that the crystallisation behaviour of EVOH is restricted by the presence of MgCl2. The hydrogen bonding in EVOH is destroyed due to the formation of stronger bonding between Cl and –OH. It is difficult for molecular chains in EVOH to move, which leads to the reduction in the degree of crystallinity of EVOH.
In general, filler acting as a heterogeneous nucleating agent can result in the formation of crystal [20], but in some cases, it may be not applicable, which depends on many factors such as compatibility, particle size and so on. In this study, the crystallinity of EVOH significantly decreases with the addition of MgCl2. The main reason may be that it's easy to aggregate for MgCl2 and the average particle diameter of MgCl2 is large in EVOH, which leads to be difficult to nucleate.
To further discuss the crystal structure of EVOH with the addition of MgCl2, the XRD patterns of the EVOH and EVOH/MgCl2 are studied in supplementary materials. It reveals that the shape and position of EVOH remain unchanged, which confirmed that there is no change on the crystal structure of EVOH with adding MgCl2. So the effect of MgCl2 on EVOH is mainly from hydrogen bonding.
Viscosity analysis
Plots of shear viscosity versus shear rate for pure EVOH, EVOH/MgCl2, EVOH/Mg(OH)2 and EVOH/BaCl2, measured at 190°C, are shown in Figure 7. Obviously, the apparent shear viscosity of all blends decreases with increasing shear rate, exhibiting pseudoplastic flow characteristic. The pure EVOH, EVOH/Mg(OH)2 and EVOH/BaCl2 have lower shear viscosity, whereas the EVOH/MgCl2 shows higher viscosity. The result is caused by the formation of stronger hydrogen bonding in EVOH with the addition of MgCl2. The enhanced hydrogen bonding strengthened the interaction among molecules in EVOH, which can improve the properties of EVOH.
Shear viscosity versus shear rate curves of pure EVOH, EVOH/MgCl2, EVOH/Mg(OH)2 and EVOH/BaCl2.
Dynamic mechanical analysis
A series of storage modulus curves as a function of temperature for all blends are shown in Figure 9 (a). All curves show the regions associated with the β-relaxation (Tβ) and glass transition (Tg), that is, the storage modulus drop at Tβ and Tg. The storage modulus within the range of 0 ∼ 80°C increased with the addition of MgCl2. The difference of storage modulus could be due to the free volume changes and the difficulty of the molecular movement, related to the enhanced hydrogen bonding in EVOH [21]. A rise in the storage modulus of EVOH/MgCl2 also indicates the improvement in properties of EVOH.
DMA curves of loss factor (tan δ) as a function of temperature ranging from –60 to 150°C are depicted in Figure 8 (b). As can be seen, dynamic relaxation peak appears for all samples. According to Mohd Ishak and Berry [22], the α relaxation peak is believed to be related to the breakage of hydrogen bonding between polymer chains which induces long range segmental chain movement in the amorphous area. This is assigned to the glass transition temperature (Tg) of the blends.
DMA curves of pure EVOH, EVOH/MgCl2, EVOH/Mg(OH)2 and EVOH/BaCl2. (a) Storage modulus–temperature curve (b) tan δ–temperature curve.
Tg results of pure EVOH, EVOH/MgCl2, EVOH/Mg(OH)2 and EVOH/BaCl2.
Mechanical properties
Figure 9 shows the mechanical properties from tensile testing of samples. There is an increase in tensile strength of EVOH with the addition of MgCl2 compared to other blends by nearly 30%, while introduction of Mg(OH)2 and BaCl2 slightly decreased the tensile strength of EVOH. Because of the small standard deviation from Figure 9, the conclusion can be drawn that a stronger hydrogen bonding is formed between EVOH and MgCl2, which can increase the modulus and tensile strength.
Tensile strength and standard deviation of pure EVOH, EVOH/MgCl2, EVOH/Mg(OH)2 and EVOH/BaCl2.
Conclusion
Enhanced hydrogen bonding of EVOH can be achieved through adding MgCl2. The Cl of MgCl2 can interact with hydroxyl groups of EVOH due to its strong electronegativity. MgCl2 has smaller cation radius and the larger lattice energy, which contributes to higher ionic bond strength, so it is very easy to form enhanced hydrogen bonding with EVOH. The formation of stronger hydrogen bonding can be demonstrated by FTIR and XPS. The absorption band of the -OH shifts to a lower wavenumber and the value decreases obviously. The content of the C–O groups decreases in XPS with adding MgCl2. Enhanced hydrogen bonding contributes to interaction among molecules, and it's difficult for molecules to move, which is beneficial for thermal and mechanical properties. The viscosity and Tg of EVOH are significantly increased with the addition of MgCl2, as well as the elastic modulus and tensile strength are also remarkably higher than neat EVOH. Therefore, it provides an effective and practical way to improve the properties of EVOH and a broad application area.
