Covalent adaptable networks impart smart processability to multifunctional highly filled polymer composites
Introduction
Highly filled polymer composites are able to greatly bring the fillers’ functionalities into play, and have become indispensable in many aspects, such as serving as substitute for high-density and energy-consuming metals [1], [2], [3], [4]. However, the high concentration fillers used to significantly reduce flowability of the system, so that homogeneous mixing of fillers and polymers cannot be achieved and the mainstream processing methods of plastics like injection molding are hard to be applied. The composites have to be produced mostly through the low productive efficiency methods like compression molding.
To change the situation and raise the productivity and scale of production, conventional processing based on thermoplastics is still preferred. Efforts have been made worldwide to accommodate highly filled composites as summarized in the following. (i) Increasing packing density via combination of the fillers with different sizes [5], [6]. (ii) Application of low viscosity polymer matrix [7]. (iii) Specific equipment [8]. (iv) Treatment of fillers with surface modifiers or incorporation of processing aids [2], [9], [10].
It is worth noting that the researches in this aspect so far are facing a dilemma. Although thermoplastics allow for high throughput manufacturing of highly filled composites in principle, proper choice of molecular weights of the matrix polymer is a challenge. Higher molecular weights would not be conducive to provide the composites with low processing viscosity and ensure sufficient filler/polymer interaction, while lower molecular weights would result in poorer mechanical properties. Naturally, a question is raised: is it possible for thermoplastics to have lower molecular weights during processing but higher molecular weights after processing? Only in this way can the contradiction between processability and performance of the ultimate composites be resolved.
Evidently, reversible covalent chemistry is able to answer in the affirmative. When reversible covalent bonds are introduced to polymers, the latter can be cleft and re-bonded in a controlled manner. Moreover, polymer networks with reversible covalent crosslinks are adaptive to external stimuli (like heat, pH and UV light), which is different from traditional crosslinked polymers constructed by irreversible covalent bonds. The barriers between thermoplastics and thermosets are thus broken down [11], [12], [13].
Leibler and co-workers [14], for example, showed that cured epoxy can be reprocessed and reshaped by injection molding at 240 ℃ by making use of catalytic transesterification. The same group [15] further prepared a poly(butylene terephthalate) (PBT) vitrimer by reactive mixing and extrusion of industrial thermoplastic PBT and epoxy based on transesterification reaction. In these cases, the crosslinked vitrimers exhibited pronounced viscoelastic-liquid behaviors under elevated temperature. Afterwards, a series of crosslinked polymers capable of extrusion or injection molding with the help of metathesis reaction of dioxaborolanes [16], Diels–Alder (DA) bonds [17], transesterification [18], [19], dynamic silyl ether exchange [20], and vinylogous urethane exchange chemistry [21] were reported.
Recently, dynamic covalent bonds, such as DA bonds [4], [22], [23], [24], β-hydroxyl ester bonds [25], dynamic imine bonds [26], and hydrogen bonds [27], were introduced to the interface of inorganic particles (like graphene oxide, silica and multi-wall carbon nanotubes)/polymer composites. Besides the enhancement of self-healing capability, the dispersability of the fillers and the rate of networks rearrangement were also increased.
Inspired by the above works, we propose a new method of fabrication of highly filled polymer composites by means of injection molding, which doesn’t need specific equipment and processing aids. Firstly, reversible covalent bonds are built up among low molecular weight polymers (or oligomers) and at filler/polymer interface to construct covalent adaptable composite networks. Upon being heated to the processing temperature, the system’s melt viscosity would be greatly reduced as a result of disconnection of the reversible bonds and release of the small molecules involved in the reversible bonds. In the course of cooling after processing, the reversible bonds are re-formed, so do the composite networks. Consequently, the merits of repeated melt processability of thermoplastics and excellent overall performance of thermosets are combined. The contradictory requirements for the molecular weight of matrix polymer under the circumstances, which had better to be as low as possible during the manufacturing but as high as possible in the ultimate products, can thus be satisfied.
Hereinafter, the design is verified by employing DA bonds as the reversible covalent bonds. Moreover, the proof-of-concept composite consists of polycaprolactone (PCL) and Al2O3 particles.
DA reaction is controlled by thermodynamics over a large temperature range, and the forward’s and reverse’ reactants are different. The characteristics help to de-crosslink polymer networks under appropriate conditions, forming low molecular weight species with low viscosity [28], [29]. As for PCL, it is a biodegradable, semi-crystalline polymer, characterized by low melting point of around 50–65 ℃ and low melt viscosity [30]. The hydroxyl groups of hydroxyl-terminated PCLs are readily functionalized by furyl. Meanwhile, Al2O3 is a traditional filler for making thermally conductive or wear resisting composites [5], [31] and also possesses abundant hydroxyl groups on the surface. It is hoped that the injection moldable highly filled Al2O3/PCL composite prepared in this work would acquire decent mechanical properties, high thermal conductivity, creep resistance, self-healing ability and reprocessability, besides the environmental benefits.
Specifically, as shown in Scheme 1a and Scheme S1a, the PCL diol derived bifuran-telechelic PCL oligomers (PCLolig-F2, in which the subscript “olig” stands for oligomers) will firstly react with bimaleimide (BMI), and then the resultant is crosslinked by tetrafuran-telechelic polyetheramine (ED2000-F), producing the target PCL networks (PCLolig-DA) through DA reaction. Polyetheramine is introduced to improve toughness of the composite. On the other hand, alumina particles are functionalized by maleimide (Al2O3-M, Fig. S1). Finally, PCLolig-DA and Al2O3-M are blended at 120 ℃ (Scheme 1b and Scheme S1b). Under the circumstances, PCLolig-DA would be de-crosslinked, leading to reduced viscosity of the system and benefiting the homogeneous mixing with the high content as-prepared Al2O3-M. Along with the subsequent cooling, DA bonds would be established not only among PCLolig-DA but also among Al2O3-M and PCLolig-DA producing covalent adaptable composite network (Scheme 1c and S1c). Following the same mechanism, the Al2O3-M/PCLolig-DA composite can be injection moldable.
Section snippets
Materials
4,4′-methylenebis(N-phenylmaleimide) (BMI), furfuryl amine, furfuryl alcohol, N,N’-carbonyldiimidazole (CDI), N,N-dimethyl-4-aminopyridine (DMAP), epichlorohydrin (ECH), tetrabutylammonium bromide (TBAB), maleic anhydride, γ-aminopropyl triethoxysilane (KH550), zinc chloride, hexamethyldisilazane (HMDS), 1-butyl-3-methylimidazolium trifluoromethanesulfonate (Sc(OTf)3), and N-methylmaleimide (BM) were supplied by Aladdin Reagent Ltd. JEFFAMINE® polyetheramine (ED2000, Mw = 2000 g/mol) was bought
Reversibility of PCLolig-DA
As mentioned in the Introduction, the temperature dependent reversibility of the DA bonds crosslinked polymer matrix of the envisaged composite and filler/matrix interface is critical for the processing. Therefore, cyclic differential scanning calorimetry (DSC) measurements are performed to check whether the DA crosslinkages have been included in the PCLolig-DA work as expected. It is seen from Fig. 1a that a significant endothermic peak at around 44 ℃ appears on the heating curves of PCLolig
Conclusions
We proved that the highly filled polymer composites with balanced properties can be obtained by means of melt blending and injection molding simply using common equipment without processing aids. The key issue lies in the establishment of reversible DA crosslinkages among oligomers and at fillers/matrix interfaces, which turns the processing into a smart one. The resultant covalent adaptable composite network was de-crosslinked at the retro-DA reaction temperature (∼120 ℃), leading to greatly
CRediT authorship contribution statement
Yuan Cao: Data curation, Writing – original draft. Min Zhi Rong: Investigation, Methodology, Supervision. Ming Qiu Zhang: Conceptualization, Methodology, 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 the support of the Natural Science Foundation of China (Grants: 52033011, 51773229 and 51873235).
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