Three-dimensional printing of cellulose nanofibers reinforced PHB/PCL/Fe₃O₄ magneto-responsive shape memory polymer composites with excellent mechanical properties

Highlights

• CNFs improved the rheological and crystalline properties of the composites.

• CNFs had good strengthening and toughening effects on composites with 10 wt% Fe₃O₄.

• The magneto-responsive SME mechanism of the composites was proposed.

• The final filament had good mechanical properties and magneto-responsive SME.

Abstract

The combination of shape memory polymer composites and 3D printing technology provides novel opportunities for customized manufacturing and intelligentization of products. Here, we developed magneto-responsive shape memory polymer composites with well-balanced strengths and toughness by adding Fe₃O₄ and cellulose nanofibers (CNFs) to a poly-hydroxybutyrate/poly(ε-caprolactone) blend as functional particles and reinforcing component. The influences of Fe₃O₄ and CNFs on the properties including micromorphology, and rheological, melting-crystallizational, thermomechanical, static mechanical, and magneto-responsive shape-memory behaviors were investigated. The results displayed that the PHB/PCL (80:20) composites with 10 wt% Fe₃O₄ and 0.5 wt% CNFs had the optimal comprehensive mechanical and magneto-responsive shape-memory properties. Moreover, 3D printing tests found that the addition of CNFs and printing parameters were crucial for the mechanical and magneto-responsive shape-memory properties of the resulting products. Finally, the fabrication of a scaffold with excellent load-carrying performance and a magneto-responsive self-recovery snowflake demonstrated the application feasibility of our work.

Introduction

Three-dimensional (3D) printing is an emerging manufacturing technology that can be used for rapidly fabricating products with complex structures that are difficult to achieve using traditional methods [1], [2], [3]. Among existing 3D printing technologies, fused deposition modeling (FDM) is the most popular and cost-effective due to its simple operation and extensively available materials [1], [4], [5]; however, the products manufactured by FDM printer using traditional filament usually lack functional characteristics and have unsatisfactory mechanical properties, which limit their applications [6]. Therefore the development of functional filaments with excellent mechanical properties suitable for FDM printers is crucial to improve their performance and broaden their application fields. The combination of thermoplastic shape memory polymers (SMPs) and FDM printers offers a prospective pathway to obtain stimuli-response 3D-printed products with customizable designs [7], [8], [9], [10], [11], [12].

SMPs are one of the most promising materials which have been widely used in the advanced intelligent materials field [13], [14], [15]. They can be changed into arbitrary temporary shapes at their transition temperature, and a temporary shape can be fixed at a lower temperature. Then, shape-changing can occur from the temporary shape to its original shape by applying appropriate stimuli such as heat [16], [17], light [18], [19], electricity [20], [21], magnet [22], [23], etc. Bodaghi et al. [24], [25] developed a simple tool to simulate the shape recovery behavior of 4D printed structures of SMP, which provided a good theoretical basis and provided our work with meaningful enlightenment in the functionalization and performance improvement of material. Among the SMP composites with different stimulus types, magneto-responsive SMP composites (M-SMPC) have present a prominent advantage in recent years due to their non-contact remote triggering control that can penetrate various materials [26]. Fe₃O₄ nanoparticles are one of the most commonly used magnetically-responsive fillers due to their intrinsic magnetic properties [27]. Xia et al. [27] synthesized a novel type of M-SMPC that incorporated vinyl-capped Fe₃O₄ nanoparticles as crosslinkers, where the Fe₃O₄ particles generated heat by vibrating in a magnetic field, which remotely triggered the release of stored elastic energy produced via programming. Wang et al. [28] prepared M-SMPC by adding Fe₃O₄ nanoparticles into poly(styrene-b-butadiene-b-styrene) copolymer/linear low-density polyethylene composites that can display excellent mechanical properties and rapid magnetic recovery. However, these materials were non-degradable, biologically incompatible, or had insufficient mechanical properties. Therefore, there a need to develop degradable materials to prepare M-SMPC with good strengths and toughness.

Melt blending is an effective and inexpensive method to fabricate M-SMPC [29]. To further reinforce the mechanical properties of SMPs, cellulose nanofibers (CNFs) provide new candidates for increasing toughness and tensile strength [30]. Young’s modulus of CNFs is about 100 GPa, and CNFs have high crystallinity, flexibility, aspect ratios, and network structure [31]. Jonoobi et al. [32] developed polylactic acid (PLA)/cellulose nanofibers (CNFs) composites by twin-screw extrusion, and found that the tensile modulus and strength of the nanocomposites increased after adding 5 wt% CNFs. Wang et al. [30] prepared ternary biocomposites with balanced mechanical properties using PLA/PBS composites as matrices and CNFs as the reinforced component by a two-step melt blending approach. Although many works developed CNF-reinforced polymers, making CNFs in a polymer matrix aligned under the shear stress of a printing nozzle may be a simpler and more effective method to obtain higher strengths [33]. Poly-hydroxybutyrate (PHB) and poly(ε-caprolactone) (PCL) have good biocompatibility and biodegradability and can respectively act as the fixed and reversible phases in the shape memory composites according to our previous research [34]. However, the magneto-responsive shape-memory mechanism of CNFs reinforced M-SMPC based on PHB/PCL blends and the influence of 3D printing parameters and the addition of CNFs on the magneto-responsive shape memory and mechanical properties of their printed specimens have not been adequately explored.

In this work, we used the methods of solvent-premixing and melt-blending and developed M-SMPC with balanced strength and toughness by adding Fe₃O₄ and CNFs to PHB/PCL blends as functional particles and reinforcing components. Moreover, the application potential of CNF-enhanced quaternary composites was explored in the field of additive manufacturing. The effects of Fe₃O₄ and CNFs on the micromorphology, rheological, melting-crystallization, thermomechanical, static mechanical, and magneto-responsive shape-memory properties of PHB/PCL (80:20) composites were investigated. Based on the above exploration, the magnetic-responsive shape-memory mechanism of the PHB/PCL/Fe₃O₄/CNFs composite system was proposed. Furthermore, M-SMPC filaments were fabricated, and the effect of 3D printing parameters and the addition of CNFs on the magneto-responsive shape memory and mechanical properties of 3D-printed specimens were evaluated. Finally, as a functional verification, we fabricated a scaffold with excellent load-carrying performance and a snowflake that quickly restored its original shape in a magnetic field.