Crystallization of Poly(ϵ-caprolactone) composites with graphite nanoplatelets: Relations between nucleation and platelet thickness
Introduction
In recent years, poly(ϵ-caprolactone) (PCL) has received much attention because of its potential application both as a biocompatible and a biodegradable material. It is currently being investigated for medical applications, pharmaceutical controlled release systems, and biodegradable packaging materials [1]. Compared with other biodegradable aliphatic polyesters, however, PCL presents poor mechanical strength and barrier properties, as well as the lower crystallization rate and heat deflection temperature, which restrict its further practical application [2]. Several techniques have hence been developed to improve final properties of PCL by controlling its hierarchical structures. For instance, introducing rigid chain segments through copolymerization is a good approach to tailor crystallization and degradation rates of PCL [3], and physical incorporation with the stiffer polymers is another effective way of improving thermal properties and mechanical strength of PCL [4]. Hybridization with nanoscaled particles such as layered silicate [5], [6], [7], nano-silica [8], [9], and carbon nanotubes [10], [11], [12], [13] is also a good strategy to design and control final structure and properties of PCL. It has been reported that the presence of well-dispersed nanoparticles can not only act as the anisotropic reinforcements, but also as the nucleating agents, both contributing to the improvement of mechanical and thermal properties of PCL. Besides, filled with nano-structured particles is expected to produce new PCL based biodegradable and biocompatible composites even unexpected properties. Therefore, preparation and characterizations of the filled PCL composites have been extensively studied [14].
Graphite nanoplatelets (GNPs) are a new material composed of stacked 2D graphene sheets with outstanding electrical, thermal, and mechanical properties [15]. In comparison with other classic 2D nanoparticles, such as clay, GNPs have lower mass density, and are highly electrically and thermally conductive due to the sp2 hybridized carbons in the monolayer graphene within GNPs [16]. GNPs, including the graphene, therefore, have been considered as the next generation filler to improve the properties of polymers [17]. It is well accepted that the mechanical and physical properties of the semicrystalline polymers are governed by the supermolecular structure, which in turn is dominated by crystallization histories. Therefore, large number of crystallization studies have been performed on the GNPs filled polymer composites [18], [19], also including PCL/GNP systems [20], [21], [22], [23], [24], [25], [26], [27], [28], [29].
Inoue’ group [20], [21], [22], [23] studied the crystallization behavior of the graphite oxide filled PCL composites systematically. They found that graphite oxide had a nucleating effect toward of PCL, and the crystallization temperature of the PCL increased significantly by nearly 9 °C even in the presence of small amounts of graphite oxide. Similar observation has also been reported by Cai and Song [24]. Qiu and coworkers [25] found that such heterogeneous nucleation effect could evidently accelerate nonisothermal and isothermal crystallization of PCL, but the presence of reduced graphene oxide did not change the crystallization mechanism and crystal structure of PCL do not change. However, Wang and his coworkers [26], [27] reported that the epitaxial crystallization of PCL could form thicker lamellae on reduced graphene oxide, and incorporation of reduced graphene oxide could enhance the orientation degree of PCL crystals in the flow direction.
Although the crystallization of PCL/graphite nanoplatelets composite has been studied in detail, it is still worthy of further exploration because graphite nanoplatelets generally present great variety. It has been reported that the properties of a graphite nanoplatelets filled polymer composites show strong dependence on the structure of nanoplatelets such as intercalation levels, surface modification, and platelet size, etc. [18]. This means that the structure of nanoplatelets may also have large influence on the crystallization of PCL. It is an interesting work but no report can be found in the literature so far. In this work, therefore, crystallization of the PCL composite containing graphite nanoplatelets was studied in detail. Two kinds of graphitic nanoplatelets, including few-layered graphite nanoplatelets and graphene nanosheets, were used as the filler to further explore the relations between thickness of nanoplatelets and their nucleating ability. The natural graphite flakes were also incorporated with PCL for the better crystallization study comparison. The objective of this work is to provide a full understanding on the crystallization and kinetics of PCL in the presence of graphitic particles with various layered structures.
Section snippets
Material preparation
Poly(ϵ-caprolactone) (CAPA6500) is a commercial product purchased from Solvay Co., Ltd., Belgium, with -OH values lower than 2 mg KOH g−1. Its number average molecular weight is about 69,000 g mol−1 and the melt index (MI) is about 7 g/10 min (160 °C/2.16 kg, ASTM D1238). The graphite nanoplatelets (SCF-F2, Purity >99.5%) used in this work are also the commercial product purchased from Nanjing SCF Nano Tech Co., Ltd., PR China.. The average thickness and particle size are ≤20 nm and ∼5 μm, respectively.
Distribution and dispersion of graphitic fillers in PCL
The dispersion and distribution of filler are vital to final properties of the filled polymer composites. Optical image can reveal the overall distribution state of these three kinds of graphitic fillers in the PCL matrix, as shown in Fig. 2. The black part is filler and the gray part matrix. It is seen that GNP and GNS show full distribution throughout matrix, while NG poor distribution state with rather wide particle size distribution. Clearly, the inter-particle distance decreases with
Conclusions
At the same loading levels (1 wt%), the graphitic particles, including natural graphite flakes and graphite nanoplatelets, as well as graphene have different distribution state in the PCL matrix. Graphene nanosheets with the thinnest layered structure thickness have the maximal volume filling fraction among the three kinds of particles. Such distribution difference leads to evident difference in the PCL crystallization among composite system. In a nonisothermal annealing process, the
Acknowledgements
The authors appreciate Prof. Changjie Yan for the DSC tests. Financial support from the National Natural Science Foundation of China (51173156), the Prospective Joint Research Program of Jiangsu Province (BY2014117-01), and the Priority Academic Program Development of Jiangsu Higher Education Institutions, as well as the Blue Project of Jiangsu Province is gratefully acknowledged.
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