*5.1.2.1. Carbon‐based reinforcements*

Carbon fiber (CF) is made from organic polymers, where hexagonal carbon structures acquired a fibrillate form. Helped by their excellent specific properties supported by low weight (high stiffness, tensile strength, chemical resistance, thermal stability and low thermal expansion) carbon fibers have a widespread application in different sectors such as aerospace, civil engineering, military and competition sports. However, still remain to overcome the price because they are relatively expensive when correlated with natural fibers, glass fibers or polymeric fibers. However, most futurist than carbon fiber composites are these with nano‐ fibers, nanotubes or graphene.

**•** Carbon nanotubes (CNT)

Since the discovery by Iijima in 1991 [69], carbon nanotubes have been investigated as their unique properties [70, 71, 72] make them interesting fillers to develop polymer nanocom‐ posites. CNTs influence in the physical‐chemical properties, as well as in the mechanical, electrical and biocompatible properties of polylactides.

It has been reported that CNT influence in the crystallinity without changes in dimension of the crystal assisting in the disorder‐to‐order (alpha'‐to‐alpha) transition. However, results obtained from Hoffman‐Weeks plot reveal that equilibrium melting temperature increase with CNT content, while thickness of crystal layer and amorphous layer of PLLA both decreased with increasing CNT contents of polylactide matrix [73]. Moreover, structural aspects as physical aging [74] and thermal degradation [75] of polylactide matrix is notably affected by the presence of these CNTs.

However, compatibilization of CNTs increment the efficiency of the composites [76]. Pyrene‐ end‐polylactide has been founded as a good interface stabilizer in polylactide/CNT com‐ posites. Therefore, modified CNT influence in polylactides in much greater manner than comparing results obtained without modification of CNTs [77].

Besides, polylactide stereocomplexation is clearly favored by CNT content [78]. The addition of small amounts of MWCNTs combined with a mild thermal treatment extends the processing window for the preparation of polylactides exclusively crystallized in the stereocomplex form, instead of the homocrystal formation.

With other point of view, conductivity of polymer matrices with nanofiller addition has been increased even with very low percentages of conductive carbon nanotubes composites [79].

In the biomedical field, also, polylactide/MWCNT composites have been carefully analyzed due to the possible cytocompatibility of the CNTs when polylactide matrix degrades [80, 81]. Instead of nanocomposite system shows adequate biocompatibility, degradation products may induce adverse effects on cell metabolism and proliferation, paying special attention in lactic acid presence and the quality of the MWCNT suspension [82]. However, an extensive *in vitro* evaluation including final degradation products is needed to enable a comprehensive prediction of the overall success or failure of newly developed degradable nanocomposites.

**•** Graphene

tensile strength and modulus have demonstrating to be very close to values obtained in glass

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

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

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

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

Carbon fiber (CF) is made from organic polymers, where hexagonal carbon structures acquired a fibrillate form. Helped by their excellent specific properties supported by low weight (high stiffness, tensile strength, chemical resistance, thermal stability and low thermal expansion) carbon fibers have a widespread application in different sectors such as aerospace, civil engineering, military and competition sports. However, still remain to overcome the price because they are relatively expensive when correlated with natural fibers, glass fibers or polymeric fibers. However, most futurist than carbon fiber composites are these with nano‐

Since the discovery by Iijima in 1991 [69], carbon nanotubes have been investigated as their unique properties [70, 71, 72] make them interesting fillers to develop polymer nanocom‐ posites. CNTs influence in the physical‐chemical properties, as well as in the mechanical,

It has been reported that CNT influence in the crystallinity without changes in dimension of the crystal assisting in the disorder‐to‐order (alpha'‐to‐alpha) transition. However, results obtained from Hoffman‐Weeks plot reveal that equilibrium melting temperature increase with CNT content, while thickness of crystal layer and amorphous layer of PLLA both decreased with increasing CNT contents of polylactide matrix [73]. Moreover, structural

fiber polyester composites.

138 Composites from Renewable and Sustainable Materials

PLA [66].

nanocrystals (CNC) are the new tendencies.

CNF network in the composites [67].

27% with the addition of CNC [68].

*5.1.2.1. Carbon‐based reinforcements*

fibers, nanotubes or graphene.

**•** Carbon nanotubes (CNT)

*5.1.2. Synthetic fibers, nanofibers and nanotubes*

electrical and biocompatible properties of polylactides.

Graphene is a single‐atom thick graphite sheet. It is structurally very similar to silicate layers and chemically analogous to carbon nanotubes, due to its huge specific surface area is considered as ideal reinforcing nanofiller in the fabrication of multifunctional polymer nanocomposites, superior mechanical strength, remarkable electronic and thermal proper‐ ties [83]. As it could be expected to achieve its maximal reinforcing efficiency, graphene sheets must be homogeneously dispersed in the polymer matrix to prompt the interfacial stress transfer between graphene and polymer matrix [84].

An effective nanofiller has been found when graphene is functionallizated with octadecyl‐ amine (ODAG) in well‐exfoliated solution/casting process. Due to the good hydrophobic compatibility between organic counterparts, interfacial adhesion and consequently crystal‐ lization, mechanical properties and thermal stability are improved [85].

#### *5.1.2.2. Other organic reinforcements*

Slit die extrusion, hot stretching and quenching is proposed as a new technique to construct well‐aligned, stiff poly(butylene succinate) (PBS) nanofibrils in the PLA matrix for the first time [86]. The high strength, modulus and ductility are unprecedented for PLA and are in great potential need for packaging applications. However, this technique opens a new way for the development of new composite materials based on polymeric fibers.
