Elsevier

European Polymer Journal

Volume 59, October 2014, Pages 189-199
European Polymer Journal

Hydrolytic behavior of poly(lactic acid) films with different architecture modified by poly(dodecafluorheptyl methylacrylate)

https://doi.org/10.1016/j.eurpolymj.2014.07.040Get rights and content

Highlights

  • Hydrolytic resistance of PLA enhanced by poly(dodecafluorheptyl methylacrylate) (PFA).

  • The architecture of PLA formed by different distribution of PFA.

  • The diffusion of water and hydrolytic products controlled by the architecture of materials.

  • Hydrolytic mechanism affected by the diffusion of small molecules.

Abstract

Poly(lactic acid) (PLA) coated with poly(dodecafluorheptyl methylacrylate) (PFA), namely PLA-coat-PFA, and PLA blended with PFA, namely PLA-blend-PFA, together with neat PLA were subjected in 40 °C water for 100 days to investigate the hydrolytic behavior of the materials with different architectures. The water diffusion rate and hydrolytic behavior were studied by means of water absorption test, morphology observation, monitor of mass loss, pH measurement, thermal analysis and molecular weight determination. The results show that (1) the water permeation in PLA can be hindered by coating with PFA, while water absorption can be enhanced by blending with PFA because the gaps between PLA and PFA, resulting from poor interaction between the two phases, reduce the penetration depth; (2) the neat PLA and PLA-coat-PFA undergo heterogeneous degradation while PLA-blend-PFA behaves as homogeneous degradation, and the degradation rate follows the order: PLA > PLA-blend-PFA > PLA-coat-PFA; (3) coating and blending PFA with PLA can both delay the hydrolytic degradation of PLA by hindering water permeation and decreasing the autocatalysis of the hydrolytic products, respectively.

Introduction

In recent years, the problems of fossil resources depletion and environmental pollution have made an urgent need to develop sustainable materials. Poly(lactic acid) (PLA) seems to be one of the most promising materials which exhibits many advantages, such as renewability, comparable mechanical properties to those of traditional synthetic polymers, energy saving, and eco-friendly [1]. In addition, due to production technology innovation, the price of PLA is reduced, making PLA be more and more competitive [2]. However, PLA is much susceptible to hydrolysis [3], which is one of the key issues for PLA’s application [4], [5], [6]. The ease hydrolysis of PLA will lead to unstable properties and limit PLA’s application, especially the long-term application [7], [8]. Therefore, it cannot show the advantages of PLA completely and there is a need to study how to control or resist the hydrolytic degradation of PLA.

In past years, many works have been done to study the hydrolytic degradation of PLA. The hydrolytic degradation of PLA proceeds at ester groups, resulting in chain scission [9]. The carboxylic end groups produced during hydrolytic degradation will in turn act as catalyst for the hydrolysis, which is called autocatalysis [10], [11]. So, the diffusion of hydrolytic products is an important factor for PLA hydrolysis [12]. In addition, acting as one of the reactants in hydrolytic reaction, water’s behavior in PLA is another important factor [13]. Basing on these two issues, many environmental factors can affect the hydrolysis of PLA significantly, such as pH, temperature and humidity [14], [15], [16]. At the same time, copolymerization, blending, cross-linking and surface modification have been applied to tune the hydrolytic rate of PLA by controlling the crystallinity, stereo-chemical structure, molecular weight, hydrophilicity, etc. [6], [17], [18], [19], [20], [21]. However, most of the modification is for biomedical use, only few studies focus on enhancing hydrolytic resistance for durable application like textiles, automotive interiors and electronics [22], [23], [24].

In those studies of hydrolytic resistance of PLA, hydrophobic modification seems to be the most common method. Reddy et al. prepared polyblend fibers with PLA and PP, and the results show that hydrophobic PP resist the hydrolysis of PLA during water treatment, resulting in better mechanical stability than neat PLA fiber [25]. Hydrophobic acetyl tributyl citrate (ATC) plasticizer was introduced into PLA to slow the hydrolytic rate by Höglund et al. [26]. Besides, Haynes’s study also shows that the hydrolysis of PLA can be delayed by introducing hydrophobic substance [27]. Taking into account of hydrophobic substance, fluorinated acrylate polymers maybe one of the best choice and they have been widely used to improve other materials’ hydrophobicity due to their extremely low surface energy [28], [29], [30], [31]. For the purpose of enhancing hydrolytic stability of PLA, in the present article, we will design two PLA films with different architectures, which is realized by the use of poly(dodecafluorheptyl methylacrylate) (PFA), i.e., the PLA coated with PFA (PLA-coat-PFA) and PLA blended with PFA (PLA-blend-PFA), and study their hydrolytic behavior by monitoring the changes of mass, molecular weight, pH, crystallinity and cross-sectional morphology during the hydrolysis.

Section snippets

Materials

Poly(L-lactic acid) (PLA) with a melt flow index in the range of 5–7 g/10 min (2.16 kg, 210 °C) and a density of 1.24 g/cm3 was obtained from Nature Works® (2002D). According to the manufacturer, this PLA has a D content of 4.25%. The Mn and Mw of this PLLA, measured by GPC, are 124,000 and 240,000 Da, respectively. Poly(dodecafluorheptyl methylacrylate) (PFA) was obtained by the solution radical polymerization of dodecafluorheptyl methylacrylate (shown in Fig. 1, XEOGIA Fluorine-Silicon Chemical

Morphology of the original PLA-coat-PFA and PLA-blend-PFA

The PFA coating layer of PLA-coat-PFA was monitored by SEM and the result is shown in Fig. 2a. Clearly, a thin layer can be observed on the surface of the specimen, which is the PFA coating layer and can also be confirmed by the following XPS measurement. In addition, the content of PFA coating is 2 ± 0.1 wt% which was determined by weighing the weight of the samples before and after coating the PFA. On the other hand, obvious phase separation morphology can be found in the photo of PLA-blend-PFA (

Conclusions

The neat PLA and the other two different architecture PLA materials with hydrophobic PFA exhibit different hydrolytic behaviors. The neat PLA hydrolyzes the fastest and undergoes heterogeneous degradation. PLA-blend-PFA is much easier in water permeation than neat PLA because of the gaps resulting from poor interaction between PLA and PFA. So, the degradation mechanism alters from heterogeneous degradation of neat PLA to homogeneous degradation of PLA-blend-PFA. At the same time, the

Acknowledgements

We gratefully acknowledge the National Natural Science Foundation (No. 50873067), the Doctoral Scientific Fund Project of the Ministry of Education (No. 20110181110032) and International Scientific and Technological Cooperation Projects (No. 2010DFA54460) of China for financial support to this research.

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