Microcellular PLA/PMMA foam fabricated by CO2 foaming with outstanding shape-memory performance
Graphical abstract
SYNOPSIS: A flexible strategy to produce PLA/PMMA foams with outstanding shape-memory performance by microcellular foaming.
Introduction
The shape memory polymer (SMP) is a class of polymer that can deform and recover between different macroscopic shapes under external stimuli, like force, heat, light, and electricity [[1], [2], [3]]. Compared to shape memory alloys, the SMP possesses the advantages of lightweight, easy to process, and rich diversity of materials, structures, and stimulating methods. Since the 1940s, the various types of SMPs have been explored, such as textiles [4], heat-shrink tubes and films [5,6], medical implant devices [7], and self-actuate devices, which are widely applied in the package, medical, automotive, and aerospace industrials [8,9]. In recent years, the concepts of “smart material” and “4D-printing” have attracted wide attention with high hopes to be applied in the advanced manufacturing and robotics industries, which requires the materials to be programmable under external stimuli and react proper responses [[10], [11], [12], [13]]. In this context, the SMP has great potential to meet the requirement of programmable and stimulus-responsible, since it is easy to tune the performance through diverse methods by manipulating the composition and structures.
The application in medical devices requires SMP to be biocompatible and chemically durable. The polylactic acid (PLA) has attracted people’s attention due to its bio-friendly advantages. It is not only polymerized from bio-sources such as corn starch and sugar cane, but also biocompatible and biodegradable [10,14]. Besides, the PLA has outstanding strength and hardness [15], which make it the most promising bio-based polymer to replace the traditional petro-based polymers, such as polypropylene and polystyrene. Up to now, many studies have focused on the PLA-based SMPs, and one of the most common methods is to create stable “hard domains” and reversible “soft domains”. A major strategy is to establish a chemical linked network, such as copolymerizing the lactic acid monomer with other monomers or oligomers to prepare multi-block copolymers [16,17] or cross-linked polymers [18]. However, the chemical network may seriously deprive the recyclability and processability of PLA, resulting in difficulties in further processing and recycling. One of the practicable solutions is to create thermo-reversible chemical networks by Diels-Alder reaction, which enables the SMP to reshape at the temperature of over 160 °C [19]. Nevertheless, the obtained polymer is strongly restricted by the reaction conditions. Another widely studied and practicable approach is to create physical networks by blending, which is much simpler and more economic to achieve the shape memory effect [[20], [21], [22], [23], [24]]. Meanwhile, the thermoplastic polymer blends are available to further process, which is more potentially utilized in manufacturing industries. There are plenty of PLA composites that have shape memory capacity, such as thermoplastic polyurethane (TPU)/PLA [20], polyamide elastomer (PAE)/PLA [21], polymethyl methacrylate (PMMA)/PLA [22], etc. Interestingly, the PLA and PMMA are fully miscible under certain process conditions, which can effectively broaden the glass transition temperature of the PLA/PMMA blends, thereby endowing the blends to present multiple shape memory effect [22,25].
Compared with solid SMPs, porous SMPs can not only achieve weight reduction, but also contribute to their multifunctional performances. For instance, the solid SMPs can be bent or stretched and recover, but the compression is heavily limited. The porous SMPs can be compressed to large extent, which enables them to be used for more extensive applications [26,27]. Thus, the porous SMPs exhibit the great potential to replace expansion screws at the junction position, thereby avoiding the inconvenience of the screwdriver. Additionally, the compressed porous SMPs can be easily implanted into the human body as a medical stent to reduce the wound surface, and then it can expand into its original shape induced by the internal heat of the body. At present, there are several methods to prepare porous structures, including porogen leaching [28], electronic spanning [29], 3D-printing [30] and foaming [24,31,32]. Among these methods, microcellular foaming with carbon dioxide (CO2) as the blowing agent is an efficient physical foaming technology [33,34]. The CO2 can be easily obtained from atmosphere [35,36], and the prepared foams have no residue or pollution to the environment. The CO2 foaming endows the foams with diversely controllable cellular structures via different processes including bead foaming [37], extrusion foaming [38], and injection molding foaming [39]. However, owing to the complicated crystallization behavior and poor melt strength of the PLA, it remains difficult to produce lightweight PLA-based SMP foams with uniform structures.
Hence, this study employed the PMMA blends to promote the melt strength and crystallization of the PLA, and then the microcellular foaming experiments were conducted to prepare the PLA/PMMA foams. Consequently, the shape memory behavior of the prepared blend foams was estimated. To investigate the underlying shape-memory mechanism, the phase morphology, thermal properties, and crystallization behavior of the PLA/PMMA blends and foams were characterized. Finally, a hypothesis was proposed to clarify the shape memory behavior of the PLA/PMMA foams.
Section snippets
Materials
The PLA (commercial grade 4032D, Natureworks® LLC., USA) has a density of 1.24 g/cm3, with a D-lactide content of 1.2–1.6 %, and a molecular weight (Mw) of about 220 kg/mol. The PMMA (commercial grade Plexiglas® VM-100, Arkema Inc., Korea), has a density of 1.18 g/cm3 and a Mw of 100 kg/mol. CO2 with a purity of >99.9 % was employed as a foaming agent, supplied by Deyang Special Gas Co., Ltd. (China).
Preparation of PLA/PMMA blends
Both PLA and PMMA in pellet form were firstly dried in a vacuum oven at 80 °C for 6 h to remove
Phase morphology of PLA/PMMA blends
Fig. 2 presents the morphology of PLA/PMMA blends prepared by twin-screw melt compounding. In Fig. 2a, the fracture surface of neat PLA is smooth with few crack edges, without any sign of a second phase. Compared with the neat PLA, there are numerous tiny sphere particles distributed evenly and uniformly in the matrix, as shown in Fig. 2b–f. The dispersive spherical particles distribute on the PLA matrix, exhibiting the morphology of “sea-island” [40]. Moreover, for varying content of PMMA,
Conclusions
In summary, we prepared the PLA/PMMA foam with excellent shape memory capability through a flexible strategy. The PLA/PMMA foams are prepared by using melt blending, followed by a two-step microcellular foaming technology. The PLA/PMMA foams show an excellent shape fixity ratio of up to 98 % and an advanced shape recovery ratio of over 87 %, whose recovery ratio remains more than 75 % even after 10 compression-recovery cycles. Owing to the great compatibility, the nanoscale PMMA particles are
Author statement
Jialong Chai: Conceptualization, Methodology, Visualization, Writing - Original Draft. Guilong Wang: Conceptualization, Writing - Review & Editing, Supervision, Project administration. Jinchuan Zhao: Writing - Review & Editing. Aimin Zhang: Methodology. Chao Wei: Data Curation. Zhanlin Shi: Data Curation. Guoqun Zhao: Supervision, Project administration.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
The authors are grateful to the National Natural Science Foundation of China (NSFC, Grant No. 51875318, 51905308, 51905307), and Shandong Provincial Key Research and Development Program (Major Scientific and Technological Innovation Project) (Grant No. 2019JZZY020205).
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