*5.1.1. Natural fibers*

**4. Strategies to changes polylactide properties**

Easier strategy to change properties of a pure polymer is blending with other polymers. These blends could be miscible or immiscible depending on solubility parameters and specific interactions established between counterparts. Miscibility of blends is governed by thermo‐ dynamic law, in which the free energy of mixing in the blend must be negative [30]. Polylactide is miscible with polyvinylphenol (PVPh) [36–39], poly(styrene‐co‐vinylphenol) [40, 41], polyhydroxybutirate (PHB) [42], poly(methyl methacrylate) (PMMA) [43], poly(vinyl acetate)

Phase separation induced by immiscible blends has been commonly used for improving fragile commodity polymers as PS and PMMA with a rubber modification leading into HIPS [46] and high impact PMMA [47]. However, the modifications in polylactides with biodegradable polymers as polycaprolactone (PCL) are an efficient way to toughen polylactides [48].

Modification in the synthesis process with other monomers is other way to tune the properties of polylactides. Comonomers as etylenglycol or ethylene oxide [49, 50], propylene oxide [51] and trimethylcarbonate [52] have been reported for polymerization with lactide units. However, cyclic comonomers are suitable to polymerize by ring opening polymerization (ROP) with lactide such as lactones or macrolactones. The most investigated systems are poly(glycolide‐lactide) [53, 54] and poly(lactide‐co‐caprolactone) copolymers [55–57]. Recent‐ ly, some studies in search of more biodegradable copolymers are using macrolactones as γ‐

Moreover, starting the polymerization of lactide or lactic acid with polymer containing hydroxyl groups leads into graft copolymers. This strategy is welcomed to increase the miscibility with other polymers and hydrophobicity as it occurs with poly(vinyl alcohol) [59].

Composites combine two (or more) different components: a continuous phase, called matrix, acts as binder and distributs homogeneously the forces through whole composite; and a discontinuous phase, called reinforcement, fundamentally is used to carry the applied load. Depending on the form of the reinforcements, they are arranged in different groups, of which two most important are fibers and particles. Normally, the aim of the reinforcements is to enhance the stiffness and tensile strength of the matrix, although sometimes fillers are used to reduce the price of the final product or modify the physical, rheological, optical or other properties. However, more important is the interface between both components to assure good

**4.1. Blending**

[44] and poly(ethylene oxide) [45].

136 Composites from Renewable and Sustainable Materials

**4.2. Copolymerization**

valerolactone [58].

**5. Polylactide‐based composites**

transmission among constituents of the composite.

These composites are very attractive because both matrix and reinforcement are obtained from renewable resources. But comparing to synthetic ones they have some characteristics to take into account [60]:


Environmental friendly materials with a full degradation capability promote the interest of these composites, especially in the automotive industry. Different natural plant fibers have been used to obtain polylactide‐based composites: agricultural natural fibers as jute, kenaf, sisal and flax and also inexpensive agricultural residues as wheat straw, corn stover, soy stalks and their hybrids [61].


tensile strength and modulus have demonstrating to be very close to values obtained in glass fiber polyester composites.

Moreover, micro‐ and nanoscale improve the mechanical properties of natural fiber‐based composites; hence, cellulose microfibrils (CMF), cellulose nanofibrils (CNF) and cellulose nanocrystals (CNC) are the new tendencies.

Composites of polylactide with silane‐modified cellulose microfibrils (CMFs) coming from sisal fiber (SF) showed a maximum impact strength which was 24% higher than that of virgin PLA [66].

However, the most important feature of using nanofibrils is the dispersion in the matrix, because fibrils are hydrophilic and the matrix hydrophobic. To overcome this, feature some researches disperse CNF in polylactides by a new method obtaining increments in the modulus and strength (up to 58 and 210%, respectively) demonstrated the load‐bearing capability of the CNF network in the composites [67].

Although crystallinity degree of polylactide/CNC nanocomposites remain similar to that of neat homopolymer, the crystallization rate has been notably increased (1.7–5 times) boosted by the presence of CNC, which act as nucleating agents during the crystallization process. In addition, structural relaxation kinetics of PLLA chains has been drastically reduced by 53 and 27% with the addition of CNC [68].
