Material behaviourCrystallization kinetics and morphology of biodegradable poly(lactic acid) with a hydrazide nucleating agent
Introduction
In the past few years, poly(lactic acid) (PLA) has attracted increasing attention due to its excellent biodegradability and biocompatibility [1], [2]. PLA can be produced completely from renewable sources and degrade into carbon dioxide and water in soil [3], [4], which makes it a suitable alternative to traditional petrochemical-based polymers for films, thermoformed containers and stretch-blown bottles [1], [5]. However, due to its intrinsic slow crystallization rate, PLA products are usually amorphous under fast cooling processes such as injection molding and extrusion [5], [6]. In its amorphous form, the application of PLA is limited by its low glass transition temperature (around 60 °C) [7]. Moreover, slower crystallization usually causes difficulties in the ejection of parts and results in longer molding cycles. Therefore, to extend PLA into applications such as electrical and automotive parts, where heat resistance is required, increasing the crystallization rate of the materials becomes critical.
Adding nucleating agent is an efficient way to accelerate the crystallization of PLA. Numerous potential nucleating agents have been investigated, including talc [8], [9], carbon nanotube [10], [11], graphene nanosheets [12], nanoclay [13], [14], zinc phenylphosphonate [15], [16], orotic acid [17], poly(d-lactic acid) [18], polyoxymethylene [19], PLA inclusion complex [20], multiamide [21], [22], [23], [24] and phthalimide [25]. In addition, some hydrazides are regarded as highly effective nucleating agents. Kawamoto et al. [26], [27] synthesized a series of hydrazide compounds having variety of methylene chain numbers and substituents to develop an advanced nucleating agent for PLA. Benzoylhydrazide compounds were found to be more effective in the enhancement of crystallization of PLA [26], [27]. Tetramethylenedicarboxylic dibenzoylhydrazide (trade name: TMC-306, abbreviated as TMC in this article) is one of these compounds. To the best of our knowledge, the crystallization behavior and nucleation ability of TMC-nucleated PLA have not been investigated in detail.
In this paper, the crystallization behavior, morphology and crystal structure of PLA nucleated by TMC were investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide angle X-ray diffraction (WAXD).
Section snippets
Materials
The PLA, consisting of 98% l-lactic acid and 2% d-lactic acid contents, was obtained from Nature Works. The number- and weight-average molecular weights of this resin were 1.11 × 105 and 1.71 × 105, respectively. The hydrazide compound (TMC-306), i.e., tetramethylenedicarboxylic dibenzoylhydrazide, was kindly supplied by Shanxi Provincial Institute of the Chemical Industry, China. Its chemical structure is shown in Fig. 1.
Sample preparation
Neat PLA and PLA/TMC blends were prepared via a solution and casting
Nonisothermal crystallization behavior
The effect of TMC on the nonisothermal melt crystallization of PLA was first investigated by DSC, since most polymer processing operations are performed under nonisothermal crystallization conditions. Fig. 2 shows the DSC cooling curves of neat PLA and its two blends at 2 °C/min. As shown in Fig. 2, the crystallization exotherms are more easily observed in the blends than in neat PLA. Neat PLA had a crystallization peak temperature (Tp) of 99.1 °C with a crystallization enthalpy (ΔHc) of
Conclusions
TMC-nucleated PLA was prepared via a solution and casting method. The crystallization behavior, crystallization kinetics, spherulitic morphology and crystalline structure of neat PLA and TMC-nucleated PLA were investigated in detail. The addition of a small amount of TMC enhanced significantly the nonisothermal crystallization peak temperature of PLA. The isothermal crystallization results revealed that the overall crystallization rate is much faster in TMC-nucleated PLA than in neat PLA and
Acknowledgments
The authors appreciate financial supports from the Ministry of Science and Technology of China (No. 2014BAJ02B02), Bureau of Science and Technology of Ningbo (No. 2014B81004 and 2014B70023) and Ningbo Natural Science Foundation (No. 2014A610138 and 2013C910012).
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