Aramid nanofibers and poly (vinyl alcohol) nanocomposites for ideal combination of strength and toughness via hydrogen bonding interactions

Highlights

• Aramid nanofibers with ultrahigh mechanical properties were prepared through the deprotonation process of STARAMID F-3.

• Strong hydrogen bonding interactions were formed between poly (vinyl alcohol) and aramid nanofibers.

• Simultaneously superior strength and high toughness of PVA/ANFs nanocomposites were achieved.

Abstract

It remains challengeable for fabricating polymer nanocomposites with combined high strength and toughness. Inspired by spider silk with multiscale hierarchical architectures, we used aramid nanofibers F-3 (ANFs-3) to reinforce poly (vinyl alcohol) (PVA) via hydrogen bonding assembly. The PVA/ANFs nanocomposite films were prepared by a simple solution casting method. Our results showed that the simultaneously significant enhancement in both strength and toughness of the nanocomposites were achieved, due to the multiple hydrogen bonding interactions between PVA and ANFs-3. When the loading amount of ANFs-3 was 5 wt%, the PVA nanocomposite films demonstrated the best combination of tensile strength and toughness, which were 79.2% and 148.8% higher than that of pure PVA, respectively. Moreover, the thermal stability of PVA/ANFs-3 nanocomposites was greatly enhanced. This work offers a novel and facile approach for producing high performance materials which can be potentially used in specific fields in the future.

Introduction

Much effort has been focused on producing novel light-weight fibers with ultra-high mechanical properties [1], [2], [3]. Aramid [PPTA: poly (p-phenylene terephthalamide)] fibers, known by its trade-name Kevlar, possess high strength, stiffness and thermal stability, leading to a variety of applications including bullet-proof vests, aircraft, automobile industry [4], [5], [6]. Relying on the outstanding mechanical properties, aramid macrofibers have been widely used to reinforce polymers, improving the stiffness and yield strength of polymer matrix [7], [8]. However, research showed that the poor adhesion between Kevlar macrofibers and polymer matrix led to phase separation and limited increase of the mechanical properties of composites prepared by direct blending approach [9], [10], [11].

Interestingly, when the macro-status of fibers is altered to nanoscale (1–100 nm), a series of unique characteristics, can be obtained [12], [13]. Many processing methods, such as drawing [14], template synthesis [15], electro-spinning [16], etc., have been used to prepare nanofibers recently. But, these methods are not suitable for aramid microfiber because of its inherent inertness [17]. Encouragingly, Kotov's group reported that the dissolution of Kevlar threads in DMSO with KOH by controlled deprotonation could split Kevlar macrofibers into aramid nanofibers (ANFs), forming a stable and uniform dispersion of ANFs/DMSO [18]. Therefore, ANFs can be considered as new nanoscale building blocks for hybrid materials. For instance, Kuang et al. [19] used ANFs to reinforce polyurethanes (PUs) and the resulting PU/ANFs films showed ultrahigh stiffness and strength. However, the toughness of such materials is unfortunately sacrificed while pursuing extremely high values of strength and modulus, restricting the potential applications in certain areas where good and sufficient toughness as well as high strength is essential [20], [21]. Therefore, further effort is necessary to fulfill desirable combination of strength and toughness.

The dilemma between stiffness and toughness has long been amazingly addressed by nature through building sophisticated hierarchical structures for various biological materials such as bone, wood, and nacre, which show exceptional combination of high strength and toughness. One of the typical biological materials, spider silk, displays extraordinary strength and toughness via the formation of extensive hydrogen bonding interactions where highly organized, densely H-bonded β-sheet nanocrystals are embedded in a semiamorphous protein matrix, which is crucial to enhance strength and toughness of silks from nanoscale to macroscale [22], [23], [24]. This charming phenomenon provides us a promising strategy to reinforce polymers. Thus, many researchers have tried to build extensive hydrogen bonding interactions to balance the strength and toughness. Polyvinyl alcohol (PVA) possesses many hydroxyl groups in its molecular structure, providing rich interactive sites for forming hydrogen bonding [25], [26]. The physical cross-linking networks formed through hydrogen bonding are similar to β-sheet nanocrystals in spider silk fibers which play a role in generating nanoconfinement [25]. In fact, it is quite interesting to note that PVA stands out as one of the water-soluble and hydrogen-bonding polymers, and other often-utilized hydrogen bonding polymers include poly(ethylene glycol) (PEG), poly(ethylene oxide) (PEO), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), and so forth. Meanwhile, PVA exhibits excellent mechanical properties, chemical stability, film forming property, low cost and widespread application, making it an ideal matrix material to study the mechanism of the reinforcement of ANFs [27], [28], [29]. Actually, Peng et al. [30] reported that the molecular composites of PVA and poly (p-sulfophenylene terephthalamide) (sPPTA) achieved high strength and good toughness through hydrogen bonding assembly. Though the toughness of PVA/sPPTA films was higher than that of the polymer composites reinforced by carbonaceous nanofillers, it was much lower than that of pure PVA. Similar results were also reported in related studies [31]. In our previous study, we demonstrated the tannic acid (TA) reinforced PVA composites, which exhibited higher tensile strength and toughness than that of pure PVA via hydrogen bonding network [32]. However, the maximum value of tensile strength of PVA/TA films was only 98 MPa, which was much lower compared with most PVA/inorganic nanofiller nanocomposites [30]. Therefore, an alternative approach for the reinforcement of PVA is needed to acquire both high strength and excellent toughness.

In this paper, aramid fibers III were used to reinforce PVA matrix, which represent a class of high performance fibers. Aramid fibers F-3 are spun from aromatic polyamides and copolyamides with heterocycles in the molecular backbone (Fig. S1). The introduction of aromatic heterocycles intensifies the conjugated electron clouds and increases the bond energy of the whole macromolecule, leading to higher tensile strength and heat resistance than that of Kevlar-49 (Table S1). Besides, the aromatic heterocycles can enhance the interfacial adhesion between aramid fibers F-3 and resin matrix. Therefore, aramid fibers F-3 become one of the best candidates as reinforcement for polymer composites. In this work, we report a facile method for the preparation of strong and tough nanocomposite films based on aramid nanofibers F-3 (ANFs-3) and PVA through hydrogen bonding assembly. ANFs-3 with high aspect ratio was prepared through the dissolution of Kevlar fabrics in DMSO by controlled deprotonation with KOH. The PVA/ANFs films can be obtained by a simple solution casting method. The hydrogen bonding between ANFs-3 and PVA was characterized, and the influence of ANFs-3 content on the mechanical properties, thermal stability, and morphology of PVA/ANFs films has been investigated. The uniform dispersion of ANFs-3 in the polymer matrix and the strong interfacial interaction between ANFs-3 and PVA formed through hydrogen bonding jointly achieved a good interfacial stress transfer, substantially improving the mechanical properties of PVA/ANFs films.