Modification of PLA

Keywords modification, copolymerization, stereocomplex, blending, composites, additives, modifiers, mechanical properties, crystallization 

Cohn et al. also used poly[ethylene oxide] [PEO] as flexible segment, and synthesized PEO- and PLA-based multiblock thermoplastic elastomers. The synthesis consisted of ringopening polymerization of L-lactide, initiated by the hydroxyl end groups in the PEO chain, followed by chain extension of the PLA-block-PEO-block-PLA triblocks using HDI as a chain extender. The tensile strength of the product was around 30 MPa, Young s modulus was as low as 14 MPa and elongation at break is well above 1000% [19]. 

Yu et al. reported that poly[lactic acid]-block-[polycarbonate diol] [PLA-PCD] copolymers were synthesized using a two-step process with polycondensation and chain extension reactions [shown in Fig. 23.5]. The PLA-PCD copolymers obtained could crystallize, and the crystallinity decreased with chain-extension reaction. The products exhibited superior mechanical properties with elongation at break above 230%, which was much higher than that of PLA chain-extended products. The products had a good potential for packaging applications [20]. 

Furthermore for the formation of a stereocomplex which blends of poly(L-lactide) and poly (D-lactide) could be found the interesting phenomena. The PLA stereocomplex was found to possess a racemic crystalline structure, where PDLA and PLLA chains were packed side hy side with a D monomer unit to L monomer unit 

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However, for PLA, the disadvantages such as the inherent brittleness, poor heat resistance, and low melt strength limit the applications. Modifications of PLA and PLA-based composites are common ways to solve these problems. 

Since PLA is a widely spread material for implants and tissue engineering scaffolds, surface functionalization is an important measure to improve the rather poor device-tissue interaction profile of this material. A few, more or less easy, methods for nonpermanent surface modification of PLA have been developed such as coating with proteins or entrapment of other polymers applying a swelling/entrapment/contraction cycle. However, for... 

The PLLA/PDLA stereocomplex, which is another crystal modification of PLA, was first discovered by Ikada and coworkers [80]. Its structure and physical properties have been studied using a number of different techniques, including infrared spectroscopy [82], optical microscopy [83], calorimetry [84], and X-ray diffraction [85]. Recent reviews by Tsuji and Fukushima et al. summarize the main properties of the stereo complex [86, 87]. 

Figure 2.5 Effect of modification techniques on interfacial properties (a) wood-flour modification with MDI and (b) impact modification of PLA matrix with addition of PEAA. (Petinakis et al. 2009).

The properties of PLA such as thermal stability and impact resistance are inferior to those of conventional polymers used for thermoplastic applications. Therefore, PLA is not ideally suited to compete against the conventional polymers [5]. In order to improve the properties of PLA and increase its potential applications, copolymers of lactic acid and other monomers such as derivatives of styrene, acrylate, and poly (ethylene oxide) (PEO) have been developed. PLA has also been formulated and associated with nanosized fillers. Modification of PLA, copolymerization with other monomers, and PLA composites are some approaches that have been used to improve the properties of PLA, such as stiffness, permeabiUty, crystallinity, and thermal stability [1-5]. Considerable research is being done to develop and study modified PLA, PLA-based copolymers, and PLA-based composites. 

MODIFICATION OF PLA PROPERTIES BY PROCESS AIDS AND OTHER ADDITIVES... 

Table 4.6 4 Modification of PLA Thennal characteristics of PHBVs blends ... 

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