**Blending**

Polymer blending is an effective, simple, and versatile method to develop new materials with tailored properties without synthesizing new polymers (Peesan et al., 2005). The properties of different polymers (biodegradable and non-biodegradable) can be combined by blending with PLA, or even new properties can arise in the products due to interactions between the components. Biodegradable components blended with PLA include poly(ethylene glycol) (PEG), poly(β-hydroxybutyrate) (PHB), poly(ε-caprolactone) (PCL), poly(butylene adipate-*co*terephthalate) (PBAT), chitosan, and starch (Sheth et al., 1997). While blends of PLA and nonbiodegradable polymers have not been as extensively studied, low-density polyethylene (LDPE), poly(vinyl acetate) (PVA), and polypropylene (PP) have been examined. Reddy et al. (Reddy et al., 2008) found that PLA in blends obtained from five ratios of PLA/PP had substantially better resistance to biodegradation and hydrolysis, and improved dyeability with dispersed dyes. However most of these blends are immiscible (phase-separated) and display poor mechanical properties due to low interfacial adhesion between the polymer phases.

To improve the processing and mechanical properties of PLA without sacrificing its degradability and biocompatibility, Xu et al. (Xu et al., 2009) blended PLA with a new degradable thermoplastic derived from konjac glucomannan (TKGM), synthesized by graft copolymerization of vinyl acetate and methyl acrylate onto konjac glucomannan (KGM). Dynamic mechanical analysis (DMA) and scanning electron microscopy (SEM) measurements showed that the PLA/TKGM system was miscible due to specific interactions between PLA and TKGM. This led to a maximum elongation at break of 520% for the blend (20/80), as compared to 14% for neat PLA. The impact strength also increased from 11.9 kJ/m2 for neat PLA to 26.9 kJ/m2 for the 20/80 blend. The synthesis of new polymers, biodegradable or nonbiodegradable, to be compatibly blended with PLA, will represent a major task in the future.

### **Plasticization**

PLA is a glassy polymer with poor elongation at break (typically less than 10%). The modification of PLA with different biodegradable and non-biodegradable plasticizers, having a low molecular weight but a high boiling point and a low volatility, has been explored as a mean to lower the *T*g and increase the ductility and softness of PLA. This has been achieved by varying the molecular weight, the polarity and functional groups of the plasticizers. Biocompatible molecules such as oligomeric lactic acid, oligomeric citrate ester, oligomeric PEG, and glycerol are all plasticizers of choice for PLA (Martin & Averous, 2001; Ljungberg et al., 2005). Ljungberg et al. (Ljungberg & Wesslén, 2002) have blended PLA with five plasticizers (triacetine, tributyl citrate, triethyl citrate, acetyl tributyl citrate, and acetyl triethyl citrate) and found that triacetine and tributyl citrate were more effective as plasticizers than the others to obtain a significant decrease in *T*g for PLA.

Wang et al. (Wang et al., 2009) found that diisononyl cyclohexane-1,2-dicarboxylate (DINCH), a new plasticizer obtained by the hydrogenation of the benzene ring of ophthalates, had limited compatibility with PLA when compared with tributyl citrate ester (TBC). PLA samples plasticized with 10 and 20 phr DINCH gave a constant *T*g of 50°C. They were stiff materials displaying elevated values of elongation at break (129% and 200%, respectively) and impact strength (41.1 MPa and 30.1 MPa, respectively). On the other hand, TBC significantly decreased the *T*g and increased the crystallinity of PLA, the PLA/TBC (20 phr TBC) blend being a soft material with a *T*g of 24°C. Results from thermogravimetric and thermal analysis also indicated that PLA plasticized with DINCH had good mechanical properties and excellent water resistance (as reflected in time-dependent weight loss data in phosphate buffer) and aging resistance (characterized by the mechanical and thermal properties of specimens exposed to ambient conditions for 4 months).

### **Composition**

Fibers can serve as fillers in the formation of PLA composites processable by compression or injection molding, to enhance the thermal stability, the hydrolysis resistance, or the mechanical properties of PLA. Several investigations on PLA composites prepared from natural and modified cellulose fibers have shown that their mechanical properties scale with the mass fraction of added fibers (Wan et al., 2001; Mathew et al., 2005). Optimization of the natural fiber-reinforced PLA composites, in terms of mechanical and other properties, is critical to minimize their cost, tailor their biodegradability, and broaden their areas of application. Inorganic fillers can also contribute to property modification. Table 3 provides a comparison of some of the organic and inorganic materials tested as PLA fillers.

Graupner et al. (Graupner et al., 2009) prepared composites from different types of natural fibers (cotton, hemp, kenaf) and modified cellulose fibers (Lyocell), with a fiber mass fraction of 40%, by compression molding. The mechanical properties of these composites are summarized in Table 4. Tomé et al. (Tomé et al., 2011) prepared composites from PLA and acetylated bacterial cellulose by mechanical compounding. The composites displayed significant increases in both elastic and Young moduli, as well as in tensile strength (increments of about 100, 40, and 25%, respectively, as compared with neat PLA) at 6% filler loading. Some surface modifiers can enhance adhesion between the fibers and the PLA matrix. For example, 3-aminopropyltriethoxysilane (APS) hydrolyzes in water or solvents to produce silanol groups that are capable of bonding to -OH groups on the kenaf fiber surface (Huda et al., 2008). The -NH2 groups from APS can also bond with -CO2 - sites formed on the PLA surface by treatment with a sodium hydroxide solution. Thus APS effectively functions as a coupling agent. Yang et al. (Yang et al., 2011) produced a composite from PLA and microcrystalline cellulose modified by *L*-lactic acid. The tensile strength and the elongation at break of the composite were higher than for neat PLA. The surface modification of the cellulose substrates was considered a key element of the mechanical reinforcement.

having a low molecular weight but a high boiling point and a low volatility, has been explored as a mean to lower the *T*g and increase the ductility and softness of PLA. This has been achieved by varying the molecular weight, the polarity and functional groups of the plasticizers. Biocompatible molecules such as oligomeric lactic acid, oligomeric citrate ester, oligomeric PEG, and glycerol are all plasticizers of choice for PLA (Martin & Averous, 2001; Ljungberg et al., 2005). Ljungberg et al. (Ljungberg & Wesslén, 2002) have blended PLA with five plasticizers (triacetine, tributyl citrate, triethyl citrate, acetyl tributyl citrate, and acetyl triethyl citrate) and found that triacetine and tributyl citrate were more effective as

Wang et al. (Wang et al., 2009) found that diisononyl cyclohexane-1,2-dicarboxylate (DINCH), a new plasticizer obtained by the hydrogenation of the benzene ring of ophthalates, had limited compatibility with PLA when compared with tributyl citrate ester (TBC). PLA samples plasticized with 10 and 20 phr DINCH gave a constant *T*g of 50°C. They were stiff materials displaying elevated values of elongation at break (129% and 200%, respectively) and impact strength (41.1 MPa and 30.1 MPa, respectively). On the other hand, TBC significantly decreased the *T*g and increased the crystallinity of PLA, the PLA/TBC (20 phr TBC) blend being a soft material with a *T*g of 24°C. Results from thermogravimetric and thermal analysis also indicated that PLA plasticized with DINCH had good mechanical properties and excellent water resistance (as reflected in time-dependent weight loss data in phosphate buffer) and aging resistance (characterized by the mechanical and thermal

Fibers can serve as fillers in the formation of PLA composites processable by compression or injection molding, to enhance the thermal stability, the hydrolysis resistance, or the mechanical properties of PLA. Several investigations on PLA composites prepared from natural and modified cellulose fibers have shown that their mechanical properties scale with the mass fraction of added fibers (Wan et al., 2001; Mathew et al., 2005). Optimization of the natural fiber-reinforced PLA composites, in terms of mechanical and other properties, is critical to minimize their cost, tailor their biodegradability, and broaden their areas of application. Inorganic fillers can also contribute to property modification. Table 3 provides a

Graupner et al. (Graupner et al., 2009) prepared composites from different types of natural fibers (cotton, hemp, kenaf) and modified cellulose fibers (Lyocell), with a fiber mass fraction of 40%, by compression molding. The mechanical properties of these composites are summarized in Table 4. Tomé et al. (Tomé et al., 2011) prepared composites from PLA and acetylated bacterial cellulose by mechanical compounding. The composites displayed significant increases in both elastic and Young moduli, as well as in tensile strength (increments of about 100, 40, and 25%, respectively, as compared with neat PLA) at 6% filler loading. Some surface modifiers can enhance adhesion between the fibers and the PLA matrix. For example, 3-aminopropyltriethoxysilane (APS) hydrolyzes in water or solvents to produce silanol groups that are capable of bonding to -OH groups on the kenaf fiber surface (Huda et al., 2008). The -NH2 groups from APS can also bond with -CO2- sites formed on the PLA surface by treatment with a sodium hydroxide solution. Thus APS effectively functions as a coupling agent. Yang et al. (Yang et al., 2011) produced a composite from PLA and microcrystalline cellulose modified by *L*-lactic acid. The tensile strength and the elongation at break of the composite were higher than for neat PLA. The surface modification of the

comparison of some of the organic and inorganic materials tested as PLA fillers.

cellulose substrates was considered a key element of the mechanical reinforcement.

plasticizers than the others to obtain a significant decrease in *T*g for PLA.

properties of specimens exposed to ambient conditions for 4 months).

**Composition** 


Table 3. Organic and inorganic fillers for the preparation of PLA composites.


Table 4. Mechanical properties of composites and a pure PLA sample (mean values; all the specimens were tested at 0°C; adapted in part from (Graupner et al., 2009).

Kim et al. (Kim et al., 2010) prepared a series of PLA/exfoliated graphite (EG) nanocomposites and confirmed that the graphite nanoplatelets could be dispersed homogeneously within the PLA matrix. Thermogravimetric analysis also showed that the thermal stability of the nanocomposites was improved with incremental amounts of EG up to 3 wt %. For example, the temperature corresponding to a 3% weight loss for a composite with 3.0 wt % EG increased by 14 degrees to ~364 °C vs. pure PLA. Additionally, the Young's modulus of the composites increased with their graphite content and their electrical resistivity was dramatically lowered. Poly(lactic acid)/hydroxyapatite (PLA/HAP) composite scaffolds processed by foaming with supercritical CO2 were shown to be promising for bone replacement, because their mechanical characteristics closely matched the properties of bone in terms of viscoelasticity and anisotropy (Mathieu et al., 2006).
