Novel polyethylene glycol-based polyester-toughened polylactide
Highlights
► We toughen polylactide with a novel polyethylene glycol-based polyester (PEGCA). ► PEGCA is partially miscible with PLA. ► The PLA/PEGCA blend shows improved mechanical properties than PEG plasticized PLA. ► The elongation at break reaches 242% with 15% PEGCA. ► The impact resistance reaches 103 J/m with 15% PEGCA.
Introduction
Polylactide (PLA) is a biodegradable and bio-based polymer. It has the advantages of high modulus, high strength, thermoplasticity, transparency, and biocompatibility, when compared to other biopolymers such as starch. PLA has attracted increasing interest in various industries, such as the packaging, textile, medical, and automotive industries. PLA is a very promising, eco-friendly alternative to traditional petroleum-based polymers [1], [2]. However, PLA also has the drawbacks of inherent brittleness and poor toughness, which impede its wide application. Hence, modification processes that toughen PLA have drawn great attention [3], [4], [5]. Blending with flexible polymers is the method most extensively used to improve the mechanical properties of PLA. However, many of these blends are immiscible and have poor mechanical properties. For example, compatibilization is necessary to enhance the elongation at break and the impact resistance of PLA/poly (butylenes adipate-co-terephthalate) blends due to their poor miscibility [6]. Plasticization is an economical and easy method of modifying PLA. Polyethylene glycol (PEG) is an excellent plasticizer for PLA because of its good miscibility [7]. The elongation at break of PLA increases in the presence of PEG, but the impact resistance remains poor. The miscible blends lack dispersed particles that act as stress concentration points in the matrix which initiate crazing and shear banding, thus absorb energy and hinder crack growth under a strong external shock [8].
Citric acid is a bio-based, inexpensive, non-toxic, and multifunctional monomer, isolated from citrus fruits. The novel highly branched polyester, poly (PEG-co-citric acid) (PEGCA) is based on citric acid and PEG. PEGCA can potentially improve the flexibility of PLA. PLA and PEGCA are supposed to be highly compatible given the PEG segment in PEGCA. In the present study, PEGCA was synthesized by a condensation reaction between PEG and citric acid, without a catalyst. PLA and PEGCA blends with different compositions were also prepared using an internal mixer. The objective was to investigate the mechanical properties of the PLA/PEGCA blends in comparison with PLA/PEG blends.
Section snippets
Experimental
PEG (Mw = 1000 g/mol) and citric acid were obtained from the China Chemical Reagent, Co. PLA (4032D, density = 1.25 g/cm3) was purchased from Natureworks LLC (USA).
PEGCA was synthesized by a direct esterification reaction. Equimolar amounts of PEG and citric acid were placed in a four-neck, round-bottom flask. The flask was fitted with inlet and outlet adapters, and was immersed in a silicone bath, preheated and fixed at 130 °C. After the melting process, the mixture was allowed to react by stirring
Results and discussion
Fig. 1 shows the 1H NMR spectrum of citric acid, PEG and PEGCA. In PEGCA spectra, the peaks around 2.9–3.1 ppm were assigned to the underlined protons in CH2 from citric acid, which were shifted towards higher values in comparison to the NMR spectrum of pure citric acid (peaks around 2.7–2.9 ppm). The peaks around 4.3 ppm were attributed to the protons signal of esterified COOCH2 groups in PEG segments. All these peaks indicated the formation of ester bonds [9], [10].
Fig. 2 shows the SEM images of
Conclusions
PEGCA can toughen PLA and make it flexible. PEGCA was partially miscible in PLA, as shown by SEM and DSC analyses. Glass transition temperature of PLA decreased with the addition of PEGCA. The PLA/PEGCA (85/15 w/w) blend showed much improved mechanical properties when compared to the PLA/PEG (85/15 w/w) blend. With the addition of 15% PEGCA, the elongation at break reached 242%, whereas the impact resistance reached 103 J/m.
References (10)
- et al.
Poly(lactic acid) modifications
Prog Polym Sci
(2010) - et al.
Toughening of polylactide with higher loading of nano-titania particles coated by poly(ε-caprolactone)
Mater Lett
(2011) - et al.
The effect of free radical reactions on structure and properties of poly(lactic acid) (PLA) based blends
Polym Degrad Stab
(2010) - et al.
Synthesis and evaluation of poly(diol citrate) biodegradable elastomers
Biomaterials
(2006) - et al.
An overview of polylactides as packaging materials
Macromol Biosci
(2004)