Viscoelastic properties and physical gelation of poly (butylene adipate-co-terephthalate)/graphene nanoplatelet nanocomposites at elevated temperatures
Graphical abstract
Introduction
Graphene has attracted tremendous amount of attention since it was first experimentally prepared in 2004 [1]. With excellent physical properties [2], graphene has been studied as a nanofiller with potential for reinforcing polymers and developing new materials with multifunctional properties such as high mechanical performance, heat dissipation and electrical conductivity which could be exploited in different areas including electrostatic discharge protection, lightening-protection panels, thermoelectric materials and electromagnetic interference shielding applications [3], [4], [5]. Graphene nanoplatelets (GNPs) are graphitic nanoparticles with layered structures composed of stacked 2D graphene sheets bonded together with weak Van der Waals forces. Considering the lower production cost of GNPs compared to carbon nanotubes (CNTs), they can replace CNTs in many applications [6].
Graphene or exfoliated graphite nanoplatelets have been embedded in a variety of polymers to enhance polymers' different properties such as thermal and electrical conductivity, mechanical strength as well as crystallization and thermal stability. Many researchers have reported the melt rheology of such filled-polymers. Dynamic viscoelastic properties of systems with low graphene/graphite loadings such as polypropylene (0–3 wt%) [7], polyethylene (0–3 wt%) [8], poly(arylene ether nitriles) (0–5 wt%) [9], polystyrene (0–2.5 wt%) [10], poly(methyl methacrylate) (0–1.58 vol%) [11] have been investigated. Rheology of highly filled systems including poly(styrene-b-ethylene-ran-butylene-b-styrene) (0–10 wt%) [12], polycarbonate (0–12 wt%) [13], polydimethylsiloxane (0–12 wt%) [14], poly(lactic acid) (0–14 wt%) [15] and polypropylene (0–20 wt%) [16] have been also evaluated. These studies have shown that increasing the filler loading results in higher values of viscoelastic properties with more pronounced effects in the low frequency region as compared to high frequencies. It has also been observed that interconnected structures formed due to the presence of filler particles result in qualitative changes in the spectra of viscoelastic properties including dynamic moduli and complex viscosity; storage and loss moduli flatten and form plateau at low frequencies while complex viscosity changes from a Newtonian plateau to a continuously decreasing trend with increasing frequency.
Although it is well-known that temperature can affect the microstructure of materials and consequently alter their viscoelastic properties, the rheological measurements in each of the aforementioned papers [7], [8], [9], [10], [11], [12], [13], [14], [15], [16] were all conducted at one single temperature. Few researchers reported the dynamic mechanical analysis (DMA) of composites reinforced with expanded graphite nanoplatelets (50–450 °C) [17], functionalized graphene sheets (−20 to 120 °C) [18] and exfoliated graphite (20–70 °C) [19] but to the best of our knowledge, the effect of temperature on the viscoelastic properties of GNP-based nanocomposites, obtained from frequency sweep tests, has not been investigated so far. In a few studies on rheology of nanocomposites containing clay particles, which are in the form of platelet as well, significant and, in some cases, unusual variations in the rheological behaviour of the systems with increasing temperature have been reported [20], [21], [22], [23]. Therefore, the present study aims to simultaneously investigate the effects of temperature and filler loading on the melt rheology of GNP-based nanocomposites.
Poly (butylene adipate-co-terephthalate) (PBAT) was used for preparation of the nanocomposites in the current research. With the increased environmental concerns due to the non-biodegradability of most commercial polymers such as polyethylene, polypropylene and polystyrene, there is an ever increasing interest in moving towards more environment-friendly polymers. PBAT is a completely biodegradable aliphatic/aromatic copolyester, which is derived from petroleum. It exhibits high elasticity, wear and fracture resistance [24] as well as adhesion and compatibility with many other polymers [25]. However, in spite of some excellent properties such as very high ultimate elongation and thermal stability at elevated temperatures, PBAT has been often used as a second phase in polymer blends. One of the reasons behind this trend is the insufficient tensile strength of PBAT. Several researchers, however, have demonstrated that addition of nano-sized fillers such as clay and CNTs to PBAT can overcome its shortcomings such as low strength, conferring multifunctional-enabling properties like enhanced mechanical, thermal and electrical properties [26], [27]. Lately, effects of graphene nanoparticles on crystallization (0–3 wt%) [28], electromagnetic interference shielding effectiveness (0–15 wt%) [29] and rheology (0–5 wt%) [30] of PBAT have been investigated, as well.
In the present study, up to 15 wt% GNPs (9.1 vol%) were embedded in PBAT by the time-efficient technique of melt-compounding. Effects of GNP loading and temperature on the viscoelastic properties of PBAT/GNP nanocomposites were investigated via dynamic and steady shear measurements. Furthermore, X-ray diffraction and scanning electron microscopy were employed to study the morphology of the nanocomposites.
Section snippets
Materials
PBAT (Ecoflex F Blend C1200) was purchased from BASF (Germany) with density of 1.25–1.27 g/cm3 and melting temperature range of 110–120 °C [31]. Grade “M” GNPs was obtained from XG Sciences (USA) with average thickness of 6–8 nm, surface area of 120–150 m2/g and density of 2.2 g/cm3 [32].
Preparation of nanocomposites
PBAT pellets and GNPs were dried at 60 °C and 80 °C respectively for 12 h prior to processing. PBAT/GNP nanocomposites were produced in an internal mixer (Haake Rheomix OS R600) with roller rotors. Mixing was
Morphological properties
SEM micrographs of fractured surfaces of nanocomposites with 3, 6 and 9 wt% GNPs are presented in Fig. 1. At low filler content of 3 wt%, the platelets are well dispersed in PBAT. In PB6, in addition to the separated platelets some clusters of GNPs can also be seen while they are still separated from each other by the matrix. As GNP concentration is raised to 9 wt%, a higher degree of filler agglomeration is observed and the graphene platelets and their agglomerates become physically in contact
Conclusions
GNP incorporation significantly enhanced the linear viscoelastic properties of PBAT particularly G′, which showed weaker frequency dependency in the low frequency region as the GNP loading increased. The GNP-reinforced PBAT nanocomposites exhibited pronounced deviations from the ideal melt rheology at low frequencies with increasing temperature. The G′ plateau was found to appear at lower GNP concentrations as the temperature increased from 160 to 220 °C. Furthermore, it was observed that G′ of
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