Development of exfoliated nanocomposite prepared from vinyl acetate and montmorillonite

Abstract

To improve the chemical, physical, mechanical and thermal properties of polyvinyl acetate (PVAc), montmorillonite (MMT) was added and polymerised with vinyl acetate to prepare an exfoliated nanocomposite of PVAc–MMT. Linear macromolecular chains of PVAc were formed between MMT layers and they exfoliated MMT into layers or sheets of nanoparticles. Both PVAc and PVAc–MMT were pseudoplastic non-Newtonian fluids, and possessed the normal stress effect (or Weissenberg effect) that was the pole climbing phenomenon. PVAc–MMT's static tensile properties were better, and PVAc's glass transition temperature was lowered by MMT. Both PVAc and PVAc–MMT possessed cold crystallisation, and their pyrolysis were similar, because MMT had no obvious effect on pyrolysis temperatures but mainly delayed thermal degradation processes.

Keywords

PVAc–MMT Structure Rheology Static tensile Dynamic mechanical properties Pyrolysis properties

Introduction

Polyvinyl acetate (PVAc) is usually synthesised from the monomer of vinyl acetate (VAc) in a mixture of protective colloid, non-ionic emulsifier, initiator and water. It is considered to be an environmentally friendly adhesive for a number of reasons. It is non-poisonous, not harmful, easily produced, low cost, easy to apply and economical on resources. For these reasons, PVAc has been widely used to bond many porous materials, such as in wood processing, furniture packaging, building decoration, texture bonding and print bonding. Montmorillonite (MMT) has been used in polymers for many years, for example, in polyacrylate ester,1, 2 poly(methyl methacrylate),3, 4 polyurethane,5, 6 epoxy,7, 8 polycarbonate,9, 10 polyethylene,11 polypropylene12 and coatings.13 Some studies have been made on the application of MMT in PVAc, but they all have the problem of the need for other additives or agents as main components necessary in their preparation. Adding an additional component means adding one or more procedures, which makes the preparation more complex and so should be avoided.

Therefore, in this paper, to simplify the preparation process, PVAc–MMT was prepared under conventional conditions in the laboratory with no ultrasonic dispersion, no radiation and no other additives as main components. In our previous work,14^–16 MMT was often organically activated to obtain good exfoliated nanocomposites, while, here, original MMT was used. In the preparation, MMT was 2% of VAc.14^–16 The chemical, physical, mechanical and thermal properties of PVAc–MMT were studied, including chemical structure, dispersion, particle morphology, rheology, static tensile strength, solid content, storage time (or shelf life), dynamic mechanical properties and pyrolysis properties.

Experimental

Montmorillonite (1 g) was immersed into 25 g VAc for 24 h. It was then mixed with 70 g of 10% polyvinyl alcohol solution, 0·5 g alkylphenol polyoxyethylene ether, 3·75 g of 10% ammonium persulphate solution, 6·25 g of sodium lauryl sulphate and 250 g of water and then stirred vigorously for 8 h. When the mixture had become a homogeneous emulsion, the temperature rose to 70°C. While stirring vigorously, 3·75 g of 10% ammonium persulphate solution and 25 g VAc were gradually added into the homogeneous emulsion in 6 h for polymerisation. Subsequently, the temperature rose to 85–90°C and the emulsion further polymerised for 0·5–1 h. After polymerisation, the temperature dropped to below 50°C, and 4 g ethanol, 3 g water, 0·3 g sodium benzoate, 0·18 g sodium bicarbonate and 6 g di-n-butyl phthalate were added. Finally, PVAc–MMT was obtained. PVAc was also prepared by the same process without MMT. Then, based on the specifications in our previous work,14^–16 their chemical, physical, mechanical and thermal properties were studied, mainly comprising chemical structure, dispersion, particles, rheology, static tensile, solid content, storage time, dynamic mechanical properties and pyrolysis properties.

Results and discussion

Structure

X-ray diffraction (XRD) can characterise the direction and the distance between crystal layers of a material, which are often dispersed in matrix materials, so now, the determination of a material to be an intercalated or exfoliated nanocomposite is generally based on the diffraction peak's position, shape and intensity on its XRD patterns.17, 18 If one or more diffraction peaks corresponding to d₍₀₀₁₎ for silicate crystal layers appear on XRD patterns and the diffraction peak position has a certain migration to a small angle compared to that of layered silicate powder, this indicates that the layered silicate is opened called an intercalated nanocomposite. For exfoliated nanocomposites, XRD patterns seem to be linear: no diffraction peak appears within the diffraction angle range of 0·5–5°, indicating that the ordered structure of the clay layers is destroyed and the layers are exfoliated.

In this paper, random powder samples of MMT and PVAc–MMT were tested by DX-2000 XRD under Cu K_α of radiation, 40 kV tube voltage, 30 mA tube current, scanning from 0·5 to 15° at a rate of 0·02° s⁻¹ and λ = 1·54184 Å of the wavelength. The large value of d₍₀₀₁₎ for MMT is calculated by Bragg's law of λ = 2dsin θ, where λ is the wavelength representing the intensity of X-rays, d is the distance between two MMT layers and θ is the diffraction angle. On MMT's XRD patterns (Fig. 1), the diffraction peak representing d₍₀₀₁₎ appeared, the 2θ was 7·00°, the d₍₀₀₁₎ was 1·263 nm, but no diffraction peak appeared from 1 to 10° of 2θ for PVAc–MMT, showing that PVAc's linear macromolecular chains were formed between MMT layers and MMT was exfoliated into layers or sheets of nanoparticles, dispersed randomly in PVAc matrix. The exfoliated nanocomposite of PVAc–MMT was obtained.

Figure 1

X-ray diffraction patterns of MMT and PVAc–MMT

To further study the interaction between PVAc and MMT, solid samples of MMT, PVAc and PVAc–MMT were mixed with potassium bromide powder and were then pressed into plates and tested by Nicolet 380 Fourier transform infrared spectroscopy (Fig. 2). No chemical bond was found between MMT and PVAc. The absorption bands of PVAc–MMT were stacked by those of MMT and PVAc. No new absorption bands were formed, nor did existing absorption bands disappear. In PVAc–MMT, PVAc and MMT were connected or absorbed together by physical effects which kept MMT stable in the relatively harsh emulsion intercalation process and guaranteed its good continuity and stability of structure.

Figure 2

Fourier transform infrared spectroscopy spectra of PVAc, MMT and PVAc–MMT

Visually, PVAc and PVAc–MMT were all similar viscous, milky white, homogeneous and fine emulsions, and all had no coarse particles, no foreign bodies and no separation. Moreover, three to five drops of them were added into 20 mL water in a glass dish (diameter, 90 mm), where they became homogeneous and there was no difference in their dispersion (Fig. 3). Compared to water, no coarse particles were found in the dispersion of PVAc and PVAc–MMT; their dispersion was good, and good dispersion suggests good storage stability. To further study this, their liquid samples were observed using JEM-3100F Transmission electron microscopy (TEM) by ×20 000. From TEM comparisons (Fig. 4), they were different. The large particles were PVAc (diameter, 250–500 nm), and the small particles were MMT (diameter, 50–100 nm). Transmission electron microscopy also showed that MMT particles dispersed randomly with PVAc particles. The presence of MMT had no significant effect on the formation of latex particles. These particles were all round or similarly round, and dispersed randomly together, as platelets in flat form but not uniformly dispersed as described by Chien et al.19, 20

Figure 3

Dispersion of PVAc and PVAc–MMT in water

Figure 4

Images (TEM) of PVAc and PVAc–MMT

Rheology

The liquid samples of PVAc and PVAc–MMT were tested using an NDJ-1 rotary viscometer under shear rates γ of 0·63, 1·26, 3·14 and 6·28 s⁻¹ respectively. The apparent viscosity η (mPa s) (Table 1) decreased with an increase in shear rate, indicating that PVAc and PVAc–MMT both were pseudoplastic non-Newtonian fluids. Pseudoplastic fluid is one of the most common non-Newtonian fluids. Rubber, many polymers and their plastic melts and concentrated solution are all pseudoplastic fluids. Their main characteristic is that the apparent viscosity decreases with the increase of shear rate. The slender molecular chains of polymer in the flow direction cause the decrease in viscosity, so it is often called a shear thinning fluid.

Table 1

Apparent viscosity

η/mPa s	γ/s⁻¹
η/mPa s	0·63	1·26	3·14	6·28
PVAc	15	14	13	13
PVAc–MMT	14	13	13	12

Based on the apparent viscosity and according to the specifications in our previous work,14^–16 the rheology of PVAc and PVAc–MMT was further studied by the power law function equation, Newtonian fluid flow equation and Cross–Williamson model viscous equation. The fluid consistency Ï (mPa s), flow index i, zero shear viscosity η₀ (mPa s), limit viscosity η_∞ (mPa s), characteristic time ζ (s), weight–average molecular weight M_w and number–average molecular weight M_n were obtained (Table 2), showing that the rheology of PVAc was improved by MMT. i, at 0·93, was the same for PVAc and PVAc–MMT, so they were all pseudoplastic non-Newtonian fluids. In addition, similar to other polymers, PVAc and PVAc–MMT both possessed the normal stress effect of the pole climbing phenomenon, also called the Weissenberg effect.

Table 2

Rheological results

	I	Ï/mPa s	ζ/s	η₀/mPa s	η_∞/mPa s	M _w	M _n
PVAc	0·93	14	4·6×10¹²	1·2×10²	0	1·0×10²	1·0×10²
PVAc–MMT	0·93	14	4·8×10³	1·4×10²	0	1·1×10²	1·1×10²

Static tensile strength

In the static tensile strength test, the liquid samples of PVAc and PVAc–MMT were made into films of 50×10×0·5 mm. After being dried to constant weight at room temperature, they were tested by a SANS CMT 5000 computer controlled electronic universal testing machine under a tensile rate of 10 mm min⁻¹ and a test temperature of 25°C. The test should be finished in 10 min. Based on the specifications in our previous work,14^–16 the static tensile strength σ (MPa), break strain ϵ (mm mm⁻¹), vertical strain ϵ_x (mm mm⁻¹), horizontal strain ϵ_y (mm mm⁻¹), break elongation ϵ_t (%), elastic modulus E′ (MPa) and Poisson's ratio ι of PVAc and PVAc–MMT were calculated (Table 3), showing that the static tensile properties of PVAc–MMT were better than those of PVAc, because MMT was exfoliated into layers or sheets of nanoparticles, dispersed randomly in the PVAc matrix and as a natural nanomineral, MMT possessed the small size effect. In other words, the static tensile properties of PVAc were improved by MMT. In addition, the solid content O (%) and storage time S (days) of PVAc and PVAc–MMT were also tested (Table 3). PVAc–MMT's O was higher, so MMT raised the solid content of PVAc. S for both PVAc and PVAc–MMT was good at >180 days, showing that their storage stability was good, which coincided well with their good dispersion.

Table 3

Static tensile*

	σ/MPa	ϵ (ϵ_x)/mm mm⁻¹	ϵ_y/mm mm⁻¹	ϵ_t/%	E′/MPa	ι	O/%	S/days
PVAc	2·07	1·92	0·30	192	1·08	0·16	13·35	⩾180
PVAc–MMT	3·49	2·03	0·20	203	1·72	0·10	15·01	⩾180

*Each data in this table was averaged from 20 sets of results.

Dynamic mechanical properties

The liquid samples of PVAc and PVAc–MMT were made into circular films, the diameter was 10 mm and the thickness was 0·5 mm. After drying them to constant weight at room temperature, they were tested by Netzsch 242C dynamic mechanical analysis (DMA) through a compression model under 10 Hz frequency, rising temperature from 0 to 200°C at a rate of 5°C min⁻¹, 120 μm of the maximum amplitude, 8 N of the maximum dynamic force and 2 N of the minimum static force. From DMA spectra (Fig. 5), the elastic modulus E′ (MPa), loss modulus E″ (MPa), loss tangent (tan δ) and glass transition temperature T_g (°C) of PVAc and PVAc–MMT were obtained. T_g and tan δ are two important indexes for damping materials. Tan δ determines the damping properties good or bad, and T_g determines their application temperature and frequency range. Moreover, for damping materials, their effective damping temperature zone Ζ₀ (°C) is calculated as follows Thus, for PVAc, T_g was 72°C and Z₀ was 72±(10–15)°C, and for PVAc–MMT, they were 67°C and 67±(10–15)°C respectively. That is to say that MMT lowered PVAc's T_g, and the low temperature resistance of PVAc was improved. Thus, PVAc–MMT could be applied in more harsh or much lower temperature conditions. This unusual phenomenon on T_g may be because polymer chains have great rigidity and volume as MMT layers are also rigid flat chips. These two molecules with different shape, different rigidity and considerable size pack and stack together, making the free volume of the system increase, and then decrease T_g.21

Figure 5

Dynamic mechanical analysis spectra of PVAc and PVAc–MMT

The cold crystallisation phenomenon also appeared on these DMA spectra. With temperature rising, a homogeneous amorphous polymer generally experiences three different mechanical states: glass state, high elastic state and viscous flow state. But for some amorphous polymers that possess crystallisation ability but cannot crystallise at the beginning because of inappropriate conditions, they are more likely to exhibit cold crystallisation above T_g when the temperature is rising. As a result, E′ will recover and increase after falling around the end of glass transition until the crystallisation phase is melt, then it falls sharply. Obviously, PVAc and PVAc–MMT possessed cold crystallisation. They were both homogeneous amorphous polymers. This has very important practical significance, which makes the structure of these materials easy to control in processing and moulding. 22 , 23

Pyrolysis properties

With heating from 25 to 800°C at a rate of 10°C min⁻¹ and a nitrogen flowrate of 60–80 mL min⁻¹, solid samples of PVAc and PVAc–MMT were pyrolysed by Netzsch TG209 F1 thermogravimetric analysis (Fig. 6). The pyrolysis of PVAc and PVAc–MMT was found to consist of eight phases, and their weight loss curves were similar. All their total weight loss was >90%, and only <10% of pyrolysed residues were left, which may be some fire retardant materials, such as MMT. As a mineral, MMT cannot be pyrolysed, and some other inorganic additives are added in polymerisation. The pyrolysis occurred mainly in phase 2 (150–230°C), phase 3 (230–290°C), phase 4 (290–350°C) and phase 6 (410–510°C). In these phases, small molecules and macromolecules were all pyrolysed, that is to say that, with rising temperature, PVAc (as the polymer matrix) was pyrolysed gradually, until eventually pyrolysis was complete.

Figure 6

Thermogravimetric analysis curves of PVAc and PVAc–MMT

However, as an exfoliated nanocomposite, PVAc–MMT had somewhat different pyrolysis processes from PVAc, which were reflected in the weight loss rate curves. From the pyrolysis valley temperature T_v and maximum pyrolysis rate v_v on weight loss rate curves, the pyrolysis was observed to occur mainly in phases 2, 4 and 6. In phase 4, especially, its v_v was the maximum, whereas in phases 2 and 6, the pyrolysis was relatively complex. In these two phases, they appeared to be two or three, even five couples of T_v and v_v. It may be that small molecules were pyrolysed in phase 2 and macromolecules were pyrolysed in phases 4 and 6. These weight loss rate curves also showed that the presence of MMT had no obvious effect on pyrolysis temperatures but mainly delayed thermal degradation processes. For our exfoliated nanocomposite of PVAc–MMT, layered structural silicates reduced their absorption to solvents and other small molecules, extended the transmission channels for small molecules and improved their barrier properties effectively.24^–27

Conclusions

In this paper, MMT was added and polymerised with VAc to prepare the exfoliated nanocomposite of PVAc–MMT. Linear macromolecular chains of PVAc were formed between MMT layers. Montmorillonite was exfoliated into layers or sheets of nanoparticles, dispersed in PVAc matrix randomly. Both PVAc and PVAc–MMT were pseudoplastic non-Newtonian fluids, and possessed normal stress effect (or Weissenberg effect) that was the pole climbing phenomenon. Their dispersion and storage stability were good. Static tensile properties of PVAc–MMT were better. MMT lowered PVAc's T_g and improved its low temperature resistance. Both PVAc and PVAc–MMT possessed cold crystallisation, and their pyrolysis were similar, because MMT had no obvious effect on pyrolysis temperatures but mainly delayed thermal degradation processes.

Footnotes

Acknowledgements

The authors are grateful for the financial support of the National Nature Science Foundation of China (grant no. 30930074). The authors also thank G. Sawyer FIWSc for assistance with language and proofreading.

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