Properties of PLA

The ability to control the stereochemical architecture enables a precise control over the size/shape of the PLA crystals, the degree of crystallinity, the rate of crystallization, and the thermomechanical properties of the material. PLA homopolymers crystallizes in three forms (a, p, and y), depending on the preparation conditions and the ratio of L and D enantiomers. The a-form (and related disorder a -form) is the most stable form with two antiparallel chains upon a twofold helix conformation distorted periodically from the regular s, while the P-form is a left-handed threefold helix and the y-form is obtained by epitaxial crystallization, containing two antiparallel s helices upon a threefold helix [34-36]. Interestingly, PLA is a clear, colorless thermoplastic when quenched from the melt and crystallizes slowly on cooling [16]. 

Although PLA meets many requirements as an eco-friendly bioplastic with attractive physical properties, which can mimic PE, PP, PS, and PET in different types of applications such as in automotive and electronic industries, in many cases, the practical applications of PLA have been significantly impeded by various drawbacks such as the following  

polyethylene terephthalate PS, polystyrene HIPS, high-impact polystyrene  

However, the main drawback of PLA-based materials remains their high brittleness to be addressed to span PLA applications from commodity to engineer- 

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Epoxidized oils were also used to modify PLA Ali et ah (2009) reported that its use as a plasticizer to improve flexibility. Thermal and scanning electron microscope analysis revealed that epoxidized soybean oil is partially miscible with PLA. Rheological and mechanical properties of PLA/epoxidized soybean oil blends were studied by Xu and Qu (2009) Epoxidized soybean oil exhibited a positive effect on both the elongation at break and melt rheology. Al-Mulla et al. (2010b) also reported that plasticization of PLA (epoxidized palm oil) was carried out via solution casting process using chloroform as a solvent. The results indicated that improved flexibility could be achieved by incorporation of epoxidized palm oil. 

The mechanical properties of PLA rely on the stereochemistry of insertion of the lactide monomer into the PLA chain, and the process can be controlled by the catalyst used. Therefore, PLAs with desired microstructures (isotactic, heterotactic, and S3mdiotactic) can be derived from the rac- and W50-Iactide depending on the stereoselectivity of the metal catalysts in the course of the polymerization (Scheme 15) [66]. Fundamentally, two different polymerization mechanisms can be distinguished (1) chain-end control (depending on stereochemistry of the monomer), and (2) enantiomorphic site control (depending on chirality of the catalyst). In reality, stereocontrolled lactide polymerization can be achieved with a catalyst containing sterically encumbered active sites however, both chain-end and site control mechanisms may contribute to the overall stereocontrol [154]. Homonuclear decoupled NMR analysis is considered to be the most conclusive characterization technique to identify the PLA tacticity [155]. Homonuclear... 

However, while physical blending is a credible approach to altering the properties of PLA with variably-sized PCL, this method tends to neglect the specific interaction between the homopolymer and its minor components. Such neglect could prove to be... 

The physical properties and melt processing of PLA are similar to those of conventional packaging resins. It may thus be used as a commodity resin for general packaging application. In many aspects the basic properties of PLA lie between those of crystal PS and PET [ 14]. When plasticized by its own monomer lactic acid, PLA becomes increasingly flexible so that products that mimic PVC, LDPE, LLDPE, PP, and PS can be prepared [15]. Possible applications are espe-... 

To improve the properties of PLA, plasticizers, special additives such as chain-extenders, polymer blends, and composites are commonly investigated. Martin and Averous (10) have studied the effects of various plasticizers on the properties of PLA. Pilla et al. (11-12) have investigated the effects of chain-extenders on the foaming properties of PLA. In addition, a vast number of studies have been conducted to enhance the properties of PLA by blending it with various polymers such as polyethylene oxide (PEO), polypropylene oxide (PPO), polyvinyl acetate, polyolefins, polystyrene, HIPS (high impact polystyrene), polyacetals, polycarbonate, and acrylonitrile butadiene styrene (ABS) (13-26). 

PLA is finding new applications in speciality cards such as credit, membership, retail and gift cards. Biodegradable polymers provide retailers and brand owners with an opportunity to provide a more responsible environmental position to traditional plastics such as PVC for these applications. The rigid properties of PLA sheet allow it to be easily scored and PLA also exhibits an optimum surface for printing and varnishing. 

Since the bulk properties of PLA are highly dependent on the stereoregularity or the tacticity of the polymer, i the development of catalysts for rac-lactide (or meso-lactide) polymerization has been focused on achieving stereoselectivity. 2 2 " Numerous organocatalysts such as A-heterocyclic carbenes (NHCs) and phosphine-based compounds " have been investigated for the controlled ROP of lactide. 

Grob. D., Lilienthal, J. L., Harvey, A. M., and Jones, B. F, (1947). The administration of di-isoptttpyl fluoropho-sphale.s (DFP) to man. 1. Effect on plasma and etythrocyte cholinesterase General systemic effects U.sc in. study of hepatic function and crythropoiesis And some properties of pla.snia choline.sterase, Bull. Johns Hopkin.s Hasp. 81, 217-244. 

PLA properties are strongly dependent on their molecular weight [10] and stereochemistry, being L- and D-lactic acid content [39]. Indeed a PLLA or PDLA homopolymer can develop a crystalline structure whereas an atactic polymer whose L-lactic acid content is below 93% remains amorphous. Consequently, the polymer structure, crystalline or amorphous form, can be at the origin of modification in the thermal, optical, physical, mechanical, and barrier properties of PLA. 

The optical properties of PLA have been measured. Its refractive index is characteristic of its structure and is directly correlated to its isotropy. Tsuji et al. [42] measured the refractive index at 25 °C and PLA concentration of... 

The mechanical properties of PLA, which have been extensively studied, are dependent on the production process and the amorphous or semi-crystalline state of the sample. Amorphous PLA presents a tensile modulus between 2.05... 

At room temperature, the mechanical properties of PLA are close to the one of PS but smaller than the one of PET (Table 8.5). Polyolefins present reduced stress at yield compared to PLA but the strain at break of LDPE and HOPE are much higher than the one of PLA. Compared to another biobased polymer, poly(hydroxybutyrate) (PHB), PLA shows better mechanical properties with higher modulus of elasticity and stress at yield. 

The influence of plasticizers on the mechanical properties of PLA has been extensively studied. Various plasticizers have been tested with PLA such as, glycerol, PLA oligomers, poly(ethylene glycol) monolaurate [98],... 

Table 8.5 Comparison of mechanical properties of PLA to synthetic polymers.

The barrier properties of PLA have not been extensively studied. The first articles treating the permeability of PLA film have been published in 1997 [120, 121], when PLA started to be considered for packaging applications. PLA films with various L/D ratios, different crystallinity degree and blends with numerous additives and polymers have been tested in recent years with gases, water vapour and organic compounds. 

The mechanical properties of PLA can be enhanced by orientation, generally in machine direction orientation with a draw ratio from 2 to 3, and in transverse direction with a draw ratio from 2 to 4 [178]. Especially toughness... 

Fig. 3 shows the molecular structure of PLA. PLA polymer is made up of many long chains consisting of the repeat unit shown in the figure. PLA is derived from renewable resources, such as corn starch via fermentation and it is biodegradable under the right conditions, such as the presence of oxygen (Tsuji et ah, 2010). Thus, PLA is a possible candidate of a new class of renewable polymers as a substitute for the petrochemical polymers. However, the physical properties of PLA are inadequate for the replacement of conventional commodity plastics in many applications. 

Mohiar, K. Moczo, J. Murariu, M. Dubois, Ph. and Pukdnszky, B. Factors affecting the properties of PLA/CaS04 composites homogeneity and interactions. Express Polym. Lett. 2009, 3(1), 49-61. 

The various properties of PLA and its availabihty mean that numerous projects are currently under way regarding its use in very different apphcations, such as packaging, biomedical applications, agriculture, health, leisure pOM 11], and so on. The current trend in the use of PLA, however, is toward durable (i.e. long-lasting) materials in areas such as the automobile or constmction sectors, where its biodegradability is not a desirable feature. 

Jacobsen et al. [90] compared the properties of PLA samples produced in a glass ampoule and the single-stage continuous reactive extrusion (Table 11.2). The resulting conversions, 98.5% and 99%, show that in both cases the polymerization reaction has... 

Like other semi-crystalline polymers, the glass transition of PLA is influenced by physical aging [150], crystallinity, morphology, and impurities [151, 152]. Other properties of PLA such as the mechanical strength also depend on the morphology, crystallinity, and orientation. It was found by Wong et al. [153] that the orientation of the crystalline phase was always higher than that of the amorphous phase. 

Table 11.9 Thermal and mechanical properties of PLA plasticized with different citrate esters...

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