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Graphene Nanoplatelets as Rheology Modifier for Polylactic Acid: Graphene Aspect Ratio Dependent Nonlinear Rheological Behavior Mohammad Sabzi 1,2, Long Jiang1*, Nasser Nikfarjam3 1
Department of Mechanical Engineering, North Dakota State University, Fargo, ND 58108, United States
2
College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China 3
Department of Chemistry, Institute for Advanced Studies in Basic Sciences, Zanjan, Iran
* Correspondence to: Long Jiang (
[email protected], Tel: +1-701-231-9512).
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Abstract Two types of graphene were used to make polylactic acid (PLA)-graphene nanocomposites with various concentrations of graphene through a solution casting method. Due to the differences in surface area, thickness, and aspect ratio between the two types of graphene, the graphenegraphene and graphene-polymer interactions were different in these two nanocomposites. As a result, the two types of graphene nanoplatelets assembled into different interconnected structures in the matrix. Steady state shear, stepwise small-amplitude oscillatory shear (SAOS) and largeamplitude oscillatory shear (LAOS), and frequency sweep tests immediately after the stepwise shear were used to determine and study the two different graphene aggregate structures. The disruption (under LAOS) and recovery (under SAOS) of the structures were monitored and the great disparities between the two types of graphene were ascribed to their structural roots. The formation of a percolated graphene network structure in the matrix significantly altered the structural evolution under the stepwise shear. This multi-step shear process was found to be a very sensitive tool to differentiate sample microstructures while traditional linear viscoelastic tests (i.e. SAOS) failed.
Keywords: Polymer nanocomposites, Graphene, Non-linear Rheology, PLA, Biodegradable polymer
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1. Introduction Bioplastics have been increasingly used to replace petroleum-based polymers due to the concerns on environmental damages and unstable petroleum supply. Many of the bioplastics exhibit inherent drawbacks including low toughness, low water resistance, narrow processing window, poor barrier property, and low thermal stability. Adding nanoparticles or nanofibers into bioplastics to produce bionanocomposites represents an important avenue to overcome these drawbacks and hence to expand the applications of the materials 1. PLA, a biodegradable polymer derived from starch, is the most widely used bioplastic nowadays. While the polymer shows good processability, biocompatibility, and high modulus/strength, its gas barrier property, heat deflection temperature and impact resistance need to be improved to meet the requirements of performance-critical applications. Different nanoclay minerals, especially layered silicates, have been used in PLA with the aim of making such property improvements
2, 3
. Graphene is
another important nanomaterial that has been extensively studied as a functional additive for polymers. Compared to nanoclay, it provides not only comparable (and often better) improvements in mechanical and barrier properties but also thermal and electrical conductivities for novel applications 4-11. Being biocompatible 12, PLA/graphene nanocomposites have potential applications in microsurgical tools, biosensors, nerve repair conduits and artificial muscles 9. They also have found uses as bone screws and scaffolds in biomedical applications 13, 14.
The research on PLA/graphene (or graphene derivatives) nanocomposites has so far focused primarily on solid-state properties, including mechanical properties 16
, electrical conductivity
4, 7-9
5, 13, 14
, crystallization
4, 8, 15,
, thermal resistance 6 and fire resistance 8. Up to date, rheological
studies (especially nonlinear ones) on PLA/graphene nanocomposite are still rare in spite of the important information about the nanocomposite that can be obtained from these studies, 3 ACS Paragon Plus Environment
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including surface chemistry of the nanofiller and its shape, size and flocculation in the matrix. By contrast, the rheology of polymer/nanoclay composites has been more widely studied. Due to the similar nanosheet structure of graphene and exfoliated clay layers, they should result in comparable rheological effects on the matrix polymers. The following paragraph reviews the studies of nonlinear viscoelastic properties of polymer/nanoclay composites, which show that nonlinear rheological tests provide an avenue to obtain information about nanocomposite microstructures that cannot be accessed through linear rheological measurements. This review can shed light on the measurements of nonlinear properties and the mechanisms behind the nonlinear behaviors for PLA/graphene nanocomposites.
Galgali et al.
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conducted creep experiments to investigate nonlinear rheological response of
polypropylene (PP)/clay nanocomposites. The system shows a considerable decline in viscosity after a critical stress value (i.e. yield stress) is exceeded. Such phenomenon is attributed to the dissociation of a percolated clay network that is formed through frictional interactions between the clay particles. Ren et al.
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investigated the effect of clay alignment on steady and dynamic
rheological responses of styrene-isoprene/clay nanocomposites. The study shows that the clay network structure is destructed when exposed to relatively low deformations due to substantial alignment of clay nanoplatelets in the flow direction. Solomon et al.
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conducted flow reversal
rheological tests to investigate the after-shear microstructure reformation process in a PP/clay system. They proposed that the microstructure formed during nonlinear shear was thermodynamically unstable and attractive inter-particle interactions were the major driving force for rebuilding the primary structure. Cassagnau et al. 20 investigated non-linear viscoelastic behavior and modulus recovery of silica filled ethylene vinyl acetate (EVA), polystyrene (PS) and PP. They attributed the observed phenomena to chain disentanglements or filler network 4 ACS Paragon Plus Environment
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breakdown, depending on the amplitude of shear strain and the concentration of silica. Through studying rheological behaviors of carbon-black filled natural rubbers and fumed silica filled silicone elastomers, Chazeau et al.
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found that while a complete modulus recovery required
thousands of seconds, 50% - 60% recovery could occur within 10 seconds or less .
Despite existing studies in this area, the origin and kinetics of microstructure formation and breakdown of nano-fillers are not completely clear and require further investigation, especially for PLA/graphene nanocomposites. In our recent study 22, PLA/graphene systems containing two different types of graphene nanoplatelets were prepared and their linear viscoelastic properties were investigated. Theoretical approaches including a power law model, a two phase model and a scaling model were used to gain insights into the stress-bearing mechanism of the graphene nanoplatelets network and the nature of the percolated structure. The microstructural information of the nanocomposites obtained from the rheological tests was compared to the results from electrical conductivity tests.
In this paper we extended the previous study to nonlinear region. A steady shear, a stepwise SAOS-LAOS-SAOS, and a dynamic frequency sweep test immediately after the stepwise shear were used to study the nature of graphene aggregates in the two PLA/graphene nanocomposites by monitoring their disruption and recovery under different shear conditions. Significant differences were found between the two nanocomposites even when the linear region tests showed similar results. The structural cause of the differences was discussed.
2. Experimental 2.1 Materials
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PLA (NatureWorks 3251D, Mn = 29,000 g/mol, Mw = 52,000 g/mol) was obtained from Jamplast Inc. (Ellisville, MO). Chloroform (≥ 99.8 %) was purchased from ACS Chemical Inc. and was used as received. Two types of graphene nanoplatelets, i.e., xGn-M25 (designated as xGn) from XG Sciences, Inc. (Lansing, MI) and N002-PDR (designated as N02) from Angstron Materials, Inc. (Dayton, OH) were used as the nanofillers without further treatments. Their important characteristics are summarized in Table 1. Table 1. Characteristics of the graphene nanoplatelets (data provided by the manufacturers) Graphene Type
Surface Area (m2/g)
Density (g/ml)
Oxygen Content (%)
Thickness (nm)
Diameter (micron)
Aspect Ratio
xGn-25
120-150
2.2
