Structure-Rheology-Property relationships in double-percolated Polypropylene/Poly(methyl methacrylate)/Boron nitride polymer composites
Graphical abstract
Introduction
The needs for polymer materials are rapidly increasing in the application areas like automotive, aerospace and communications because of their advantages of lightweight, good processability, superior corrosion resistance and most importantly, low cost. Polymer composites are also very attractive because they not only have the advantages of polymers but also have the characteristics of fillers including good strength, stiffness, thermal stability and conductivity.
Various kinds of morphologies can be produced during polymer melt mixing including fibrillar, lamellar, dispersed, or co-continuous structures [[1], [2], [3], [4]]. It has been reported that co-continuous structures, the coexistence of two continuous structures within the same volume, can improve the electrical conductivity (EC) by localizing conductive fillers in one of the polymer phases or at the interface in ternary polymeric composites [5,6]. This structure is called a double-percolated structure, because the system meets two percolation thresholds: the first one is the threshold of polymers to form co-continuous morphology, and the second one is the threshold of conductive fillers to build up a network throughout the composite.
Although many studies have investigated the effect of double-percolated structure on improvement of EC of polymer composites since 30 years ago [[6], [7], [8], [9], [10], [11]], only recently more studies about enhancing TC by using double percolation strategy have been conducted [[12], [13], [14], [15], [16], [17]]. There is an increasing need for thermally conductive polymer composites nowadays. For example, faster heat dissipation is required for the more advanced microchip device in electronics [18,19]. The morphology of polymers is not efficient for thermal conduction because unorganized polymer chains and defects slow down the heat transfer [20]. Even in polymer composites, in which polymeric matrix is filled with thermally conductive fillers, a significant amount of phonon scattering exists, and enhanced TC can be observed only when the phonon transfer in the continuous filler network dominates over the phonon scattering at the interfaces [21,22]. Therefore, a large amount of fillers is usually required to achieve high TC, which causes the price to increase and ductility to decrease [[23], [24], [25]]. The polymeric ternary composites with double-percolated structure can reduce the amount of fillers needed, which is beneficial for both costs and mechanical properties.
In addition, thermally conductive but electrically insulative materials have raised attentions for application areas where electromagnetic shielding effects need to be delimited. Boron nitride (BN) has high TC and excellent electrical resistance with lower density and moderate price compared to other ceramic fillers [24], so in this work BN was selected as the thermally conductive filler to improve the TC of double-percolated composites. In our previous work, it was proved that various viscosity ratios of two polymer matrix can change the TC of ternary composites by changing the microstructures, which varied the degree of interfacial phonon scattering [17]. Since filler-induced effects can change the size of phases of immiscible polymer blends significantly [26], it is worth investigating the effects of filler sizes on morphology and TC of double-percolated ternary polymeric composites. PP is a widely used commercial non-polar polymer material that is tough and flexible, while PMMA is a rigid polar plastic with high modulus and strength. Therefore, polar BN fillers can be located in the polar PMMA phase easily in the PP/PMMA blends, and the thermal conductive PP/PMMA/BN composites with good mechanical properties can be used in various application areas including protection of electronic devices and thermally conductive pipes and tubes. In this work, three different sizes of BN platelets filled PP/PMMA composites were studied, their TC as well as morphologies were analyzed, and the relationship between TC and rheological behavior was investigated. To the best of our knowledge, this is the first study investigating these parameters and their effects on the ultimate TC of ternary polymer blend composites.
Section snippets
Materials
PMMA with a density of 1.18 g/cm3 and Melt Flow Index (MFI) of 15 g/10min was purchased from Atlugas International of Arkema Inc. (Colombes, France). PP with the brand name of PPH 3281 was purchased from TOTAL (Courbevoie, France), with MFI of 1.3, density of 0.91 g/cm3, and suggested processing temperatures between 190 and 232 °C. Three different sizes of BN platelets with average sizes and thicknesses of 16 μm, 1 μm (PCTP16), 30 μm, 2 μm (PCTP30) and 180 μm, 2 μm (PCTP30D2) were kindly gifted
Investigation of phase morphology of PP/PMMA blends
To find the desired ratio of PP/PMMA to get the blend with highest degree of co-continuity, the morphology of the polymer blends was characterized by SEM after fully extracting PMMA by acetone. In Fig. 1, it can be seen that the samples with PP/PMMA ratios of 50/50, 60/40 and 70/30 have sea-island structure, and only the blend with 40% PP shows co-continuous morphology (Fig. 1(a)). The efficiency of PMMA extraction results (Table 1) also confirms the structure since almost fully extraction of
Conclusions
PP/BN, PMMA/BN, and PP/PMMA/BN composites with various amount of three sizes BN filling (16, 30, and 180 μm) were successfully compounded. The double percolation structure of the PP/PMMA/BN composites were characterized, and BN was proved to be in PMMA or the interface regardless of the filler sizes. TC measurements showed that ternary composites with high filler concentrations have higher TC than the corresponding binary composites. In addition, with the microstructure analysis, it was
CRediT authorship contribution statement
Molin Guo: Formal analysis, Investigation, Writing - original draft. Marjan Alsadat Kashfipour: Formal analysis, Investigation. Yifan Li: Investigation. Russell S. Dent: Investigation. Jiahua Zhu: Investigation, Supervision, Writing - review & editing. João M. Maia: Investigation, Supervision, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors thank Saint-Gobain for kindly gifting the PCTP16, PCTP30 and PCTP30D2 BN platelets, and specially acknowledge Brad Kenny for the discussion of material selection. The authors also acknowledge Dr. Zhe Ren and Dr. Stephanie Piatt for the help with SEM-EDS analysis.
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Equal contribution.