PLA with high elongation induced by multi-branched poly(ethylene imine) (mPEI) containing poly(l-lactic acid) (PLLA) terminals
Graphical abstract
Introduction
At present, poly(lactic acid) (PLA) is recognized as the most potential renewable resource-based biodegradable polymer due to its reliable industrial scale production [1]. Although PLA has excellent properties, especially transparency, tensile strength and process ability, the brittleness related to the high glass transition temperature (Tg ∼ 58 °C) as well as slow crystallization limit practical daily life products. To overcome these limitations, an enhancement of chain mobility by adding plasticizers along with an acceleration of crystallization by applying nucleation agents are proposed [2], [3], [4]. For example, talc [2], glycerol, lactate derivatives [5], [6] and poly(ethylene glycol) (PEG) are reported as plasticizers, whereas sodium stearate and calcium lactate are candidates for nucleating agents [4], [7], [8], [9]. However, in most cases, immiscibility leads to phase separation, and migration of the additives occasionally occurs.
Thus, it is reasonable to consider PLA-based macromolecules as a candidate [4], [8], [9], [10], [11]. For example, Burgos et al. reported that oligo lactic acid (OLA) (Mn = 957) for 25 wt% decreased the Tg of PLA from 59.2 °C to 25.8 °C and increased the elongation at break from 4% to 315% [3], [8], [9].
To our idea, the balance of the crystalline and amorphous phases is a good approach to improve the overall mechanical properties of PLA. Based on this concept, we previously proposed a series of multi-armed PLLA, which are i.e. 3-armed, 4-armed, and 21-armed, and showed their functions on both nucleation and plasticity through rapid spherulite formation with a significant increase in degree of crystallinity (to be ∼20–40%) [12], [13]. At that time, the PLA cast films show an increase in elongation at break for 10–40%. Therefore, the use of highly branched PLLA might not only bring in more free volume for significant increase of the elongation, but also perform good miscibility with the less migration [12], [13], [14], [15], [16].
In this study, we propose the synthesis and application of multi-branched polyethyleneimine (mPEI) containing PLLA terminals (mPEI-PLLA) as an additive to PLA resins to improve the brittleness. The mPEI in a branched structure is a good core molecule. This is not only because the mPEI provides the amino terminals as initiators for l-lactide (LLA) ring-opening polymerization (ROP) to result in multi-branched PLLA, but also the mPEI-PLLA obtained occupies a significant free volume due to its multi-branching structure. In addition, the terminal PLLA offers good miscibility with PLA matrices. The present work demonstrates the preparation of mPEI-PLLA and its role as an additive for PLA to tune the microstructure and significantly increase the toughness through the case study of PLA films.
Section snippets
Materials
Multi-branched polyethyleneimine (mPEI, Mn 10,000 g mol−1) was purchased from Aldrich Co., and dried under vacuum at 80 °C for 3 h before use. Tin(II) 2-ethylhexanoate (Sn(Oct)2, 97% purity) was purchased from Sigma-Aldrich and used as received. l-Lactide (LLA) was kindly provided by PTT Public Company Limited, Thailand, and was dried under vacuum at room temperature for 6 h before use. Commercial PLA (2002D) was purchased from Natureworks LLC, USA, and used as received. The number average
Results and discussion
Previously, we clarified the structure of mPEI (Mn = 1.0 × 104 g mol−1) (Fig. S1) and found that it contains 72 terminal ends of primary amine [17]. By simply melt blending of mPEI and the neat LLA without solvent, the ROP was successful. At that time mPEI functioned as an initiator with tin(II) 2-ethylhexanoate (SnOct2) as a catalyst. Though the similar reaction was reported previously by Zhang et al. [18], here, the changes of mPEI/LA molar ratios from 1/166 to 1/3320 leads us to the
Conclusions
This study proposed the use of mPEI to prepare multi-branched PLLA via a single ROP and the use of multi-branched PLLA obtained as an additive to induce the performance of high elongation to PLA. Although mPEI cores provide the free volume as amorphous phase, the PLLAn terminals need to be in an optimal length so that the chain mobility is significant enough to induce the chain mobility. The 20 wt% PLLAn with 10–14 units are cases leading to the drastic increase of elongation at break to be as
Acknowledgements
One of the authors (T. K.) gratefully thanks Junior Science Talent Project (JSTP) No. JSTP-06-54-05E, National Science and Technology Development Agency (NSTDA). The authors appreciate the support of the Petroleum and Petrochemical College and the National Center of Excellence for Petroleum, Petrochemicals, and Advanced Materials, Thailand. M.M. would like to acknowledge his Postdoctoral fellowship supported by Ratchadapisek Sompote Endowment Fund, Chulalongkorn University.
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