polylactide nanocomposites with functionalized carbon nanotubes and their stereocomplexes: a focused...

Polylactide nanocomposites with functionalized carbon nanotubesand their stereocomplexes: A focused review

Marek Brzeziński, Tadeusz BielaQ1

Department of Polymer Chemistry, Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, PolandQ3

a r t i c l e i n f o

Article history:Received 29 July 2013Accepted 27 January 2014

Keywords:PolylactidesStereocomplexesNanocompositesFunctionalized carbon nanotubes

a b s t r a c t

Poly(lactide)-carbon nanotubes nanocomposites (PLA–CNTs) with poly(lactide) (PLA) covalently attachedto CNTs show enhanced mechanical and thermal properties. Two methods of CNTs modificationwere mainly employed: “grafting from” and “grafting to”, to improve their key parameters affectingthe properties of composites – the dispersion and compatibility with PLA matrix. On the other handstereocomplexation of enantiomers of PLA also leads to acquire the PLA-based materials with improvedthermal-resistance, mechanical properties and the hydrolysis-resistance. PLA–CNTs stereocomplexnanocomposites are novel materials with unique properties due to synergic effect of carbon nanotubesand stereocomplexation. This featured letter reports recent developments in this new field.

& 2014 Published by Elsevier B.V.

1. Introduction

Poly-(lactide)–-carbon nanotubes nanocomposites, the notableexample of biocomposites, attracted increasing attention becauseof their biodegradability and biocompatibility [1,2]. Nowadays,carbon nanotubes (CNTs) are increasingly used as additives toimprove the thermal and mechanical properties of poly(lactide)composites [3–6]. However, it is noteworthy that without anypre-treatment CNTs cannot effectively interact with polymermatrix [7]. In order to improve the dispersion and compatibilityboth non-covalent and covalent modifications of the carbonnanotubes have been used [8]. Functionalization strategies of CNTsby appropriate polymers include 1) the “grafting from” method,which involves the immobilization of initiators on the CNTs sur-face, followed by surface-initiated polymerization, 2) the “graftingto” method, which relies on reaction of specific functional groupson CNTs surface with the reactive group at the end of polymerchain, and 3) supramolecular strategy, which is based on the π–πinteractions between polymers and CNTs surface [2]. An importantfeature of poly(lactide) is the ability to form stereocomplexbetween the equimolar mixture of enantiomeric poly(L-lactide)and poly(D-lactide) [9]. Macromolecular stereocomplexes havemuch higher melting temperature than the homochiral poly-lactides [10]. Stereocomplexation of PLA–CNT nanocompositescombines two factors of improving poly(lactide) properties andopens a new way to produce various types of biodegradable

materials with enhanced thermal and mechanical properties.Functionalized carbon nanotubes affect the overall properties offinal nanocomposite and improve compatibility of CNTs with PLA.The main object of this focused-review is the influence offunctionalized carbon nanotubes on the properties of PLA–CNTnanocomposites and their stereocomplexes.

2. Functionalized carbon nanotubes with improvedcompatibility with poly(lactide)

In this section influence of different carbon nanotubes mod-ifications on the properties of polylactide and polylactide copoly-mers will be discussed. The attention will be mainly focusedmainly on the thermal, mechanical and electrical properties ofnanocomposites.

Acid oxidation is the most commonly used modification ofthe surface of carbon nanotubes [11]. These acidic oxidizedmultiwalled carbon nanotubes (A-MWCNTs) are used to preparecomposites with polylactide (PLA). It is well known that carbonnanotubes (CNTs) are effective nucleating agents of polymers dueto their high specific surface areas and large aspect ratio. Both,degree of crystallinity and the crystalline structure of PLA matrixin polylactide nanocomposites, have a very important influence onthe mechanical and the electrical properties of PLA. Wang et al.investigated the crystallization kinetics of PLA in the presence ofA-MWCNTs. The used A-MWCNTs exhibit different aspect ratiosbut the same mass percent of carboxyl groups on their surface.A-MWCNTs with smaller aspect ratio trigger the crystallization of

123456789

101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/matlet

Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.01.1590167-577X & 2014 Published by Elsevier B.V.

E-mail address: [email protected] (M. Brzeziński).

Please cite this article as: Brzeziński M, Biela T. Polylactide nanocomposites with functionalized carbon nanotubes and theirstereocomplexes: A focused review. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.01.159i

Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎

www.sciencedirect.com/science/journal/0167577X

www.elsevier.com/locate/matlet

http://dx.doi.org/10.1016/j.matlet.2014.01.159



mailto:[email protected]





PLA more efficiently because of many nucleation sites on theirsurface in contrast to those with large aspect ratio where thenumber of nucleation sites is lower [12].

Acidic oxidized MWCNTs could be converted to acyl-chloride-functionalized-MWCNTs (MWCNT-COCl) by treating them with thio-nyl chloride [7]. Oligomeric [13], high molecular weight [14,15] andstar-shaped [16] poly(lactides) were reacted with –COCl groups fromMWCNTs surface and MWCNT-g-PLA composites were obtained bythe “grafting to” technique (Scheme 1). The efficiency of grafting wasdetermined by transmission electron microscopy (TEM), Fouriertransform infrared spectroscopy (FTIR), thermogravimetric analysis(TGA) and Raman analysis. The amount of grafted polymer on thesurface of MWCNTs strongly depends on the molecular weight ofused PLA. High molecular weight PLA, due to low concentration andpoor availability of functional end groups, did not attach effectively.The reaction medium is also very important. PLA ester group wasreacted with NH-pyrrolidine groups present on the MWCNT surface,both in solution and in the melt, yielding MWCNT-g-PLA [17]. Thecovalent bonding of PLA chains was only observed when direct meltmixing in a microcompounder was applied. Surface resistivitymeasurements of PLA/MWCNT-g-PLA indicate whether PLA chainsare effectively attached to MWCNTs surface. When MWCNTs aretotally self-wrapped with PLA chains of MWCNT-g-PLA the electricalconduction path cannot be formed [18].

Composites with commercially available PLA (Nature WorksLCC and Cargill Down LLC) and MWCNT-g-PLA functionalized withboth linear and star-shaped poly(lactides) were prepared. Thegood dispersity of MWCNTs in PLA/MWCNT-g-PLA, in comparisonwith composites with non-modified MWCNTs, was attributed toenhanced compatibility of PLA chains with PLA functionalizedMWCNTs. The presence of MWCNT-g-PLA strongly affected thecrystalline and amorphous phases of polymer matrix. The highercontent of MWCNT-g-PLA caused increase in overall rigid fractionand in consequence in glass transition temperature (Tg) [14,15].The increasing degree of crystallinity of nanocomposite withincreasing MWCNT-g-PLA content is related to the enhancementof crystallization of PLA. Temperature-modulated differential scan-ning calorimetry (TMDSC) analysis indicated the crystal perfectionprocess. The imperfect crystals may form secondary crystals orreorganize into more stable structures during the slow TMDSCheating scans [16]. Functionalized MWCNTs (F-MWCNTs) could

also induce the conformational ordering of PLA helices [3]. It isnoteworthy that carbon nanotubes could play a dual role in thematrix crystallinity. Below percolation threshold F-MWCNTs could actas a nucleating agent but above percolation concentration F-MWCNTsact as a hindrance and retard the crystallization process [16].

Nanocomposites composed of MWCNT-g-PLA and high-molecular weight PLA exhibited improved mechanical properties.Good dispersion of MWCNT-g-PLA in polymer matrix resulted inan effective reinforcement effect on nanocomposite modulusand tensile strength [15–18] PLA/A-MWCNTs and PLA/MWCNTsonly slightly improved these values and caused decrease in strain.Therefore, the good dispersity of MWCNT-g-PLA in PLA caused byexternal stress may be efficiently delivered from the nanocompo-site to MWCNTs which provoke significant improvement in initialmodulus and yield strength Q4[18]. However, higher content ofMWCNT-g-PLA did not induce further enhancements due to theformation of large agglomerates of MWCNTs [17].

The functionalization of MWCNTs with polyhedral oligomericsilsesquioxane (POSS) and poly(butyl acrylate) (PBA) wasemployed and PLA/MWCNT-g-POSS and PLA/MWCNT-g-PBA nano-composites were prepared [19,20]. Nanocomposites indicatedstrong interfacial adhesion and uniform dispersion of MWCNTs,and as a result of good interactions between the filler and thematrix, improved mechanical properties compared to neat PLLAand PLLA/MWCNT. In the case of PLA/MWCNT-g-PBA nanocompo-site the enhanced mechanical properties were achieved by simul-taneous effect of PBA which acts as a rubber phase and MWCNTswhich are responsible for its rigidity.

The electrical percolation threshold of PLA/F-MWCNTs is typi-cally in between 1 and 2 wt % leading to lower values than those ofPLA/MWCNTs [16,17]. It is noteworthy, that electrical percolationthreshold strongly depends on the CNTs dispersion state, theiraspect ratio and processing conditions [16]. Moreover, A-MWCNTswere used to obtain more complex poly(lactide)-poly(ε-caprolac-tone) nanocomposites. The electrical conductivity depends notonly on the amount of the A-MWCNTs but also on the morphologyof obtained nanocomposites. The difference in the electricalconductivity of PLA/PCL/A-MWCNTs composite was a result ofmorphological changes with varying PCL content. High electricalconductivity was observed when the cocontinuous phase domainstructures with interconnected A-MWCNTs were formed [21].

123456789

101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566

Scheme 1. Polylactide functionalized carbon nanotubes by “grafting to” technique.

M. Brzeziński, T. Biela / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎2





Supramolecular interactions (van der Waals and π-stacking) canbe also employed to improve the dispersion of carbon nanotubes inPLA matrix [7]. Non-covalent modifications do not damage theperfect structure of CNTs which is the main advantage of thismethod. However, forces between supramolecular linker and thesurface of CNTs are weaker indicating that the load transfer from thepolymer matrix to CNT filler may not be effective [14].

Star-shaped PLLA with triphenylene core was used for non-covalent functionalization of pristrine MWCNTs. Triphenylene-containing polymers interact with the surface of CNTs by π–πinteractions [22]. Furthermore, imidazolium ionic liquids endfunctionalized oligomeric PLA exhibited high binding abilitytowards the nanotubes surface [23–25]. Imidazolium rings areable to form π–π interactions with CNTs and as a result induceCNTs bundle disentanglement [26]. Ionic liquid functionalized poly(lactides) considerably stabilize the suspension of CNTs in chloro-form [23], 1,4-dioxane [24] and in the bulk [25] due to strongsecondary interactions disrupting intermolecular π–π interactionsbetween CNTs and prevent their aggregation.

A ternary supramolecular system, composed of PEO-b-PLLA,CNTs and lithium chloride, was also proposed to improve thedispersion of carbon nanotubes in solution and polymer matrix.Good dispersion was related to the formation of micellar organiza-tions and appropriate amount of LiCl. Self-assembly was driven bythe formation of charge transfer complexes between CNTs surfaceand “super” cationic species (PEO-b-PLLA/LiCl). Finally, PEO-b-PLLA/LiCl complex was used to prepare nanocomposites with CNTsand commercially available PLLA. The nanocomposite exhibitsextremely low electrical percolation threshold at only 0.001 wt%of CNTs (0.3 wt% without LiCl). To conclude, the usage of supra-molecular strategy enables for efficient dispersion of CNTs both insolution and polymer matrix and can be used to obtain PLA/CNTnanocomposites with high-performance properties [27].

Efficient dispersion and strong interactions of the functiona-lized nanofiller with PLA matrix play a key role in enhancement ofthermal and mechanical properties. Controlled nanolevel disper-sion of the filler in polymer matrix determined the processingconditions and applications of obtained nanocomposite.

3. Modified carbon nanotubes as initiators for ring-openingpolymerization of lactide

Carbon nanotubes are modified to acquire hydroxy groupswhich are able to initiate ring-opening polymerization (ROP) of

L-lactide [28,29] and D-lactide [30] (Scheme 2). Polymerization ofL-lactide was performed in two different solvents: N,N0-dimethyl-formamide at 140 1C and toluene at 70 1C. Longer reaction timesignificantly improved the amount of grafted PLLA. MWCNT-g-PLLA nanocomposite exhibited enhanced tensile modulus andstrength, in comparison with neat PLLA and PLLA/MWCNT-OH,without pronounced change in the elongation at break [28].Bergman cyclization has been applied to functionalized MWCNTsand obtained MWCNTs-OHs were used for polymerization ofL-lactide and ɛ-caprolactone in toluene at 110 1C [29]. D-lactidewas polymerized in bulk at 130 1C in the presence of MWCNTswith β-D-uridine linker attached to its surface. Nanocompositedemonstrated enhanced alignment of the polymer fibrils com-pared to neat homopolymer [30].

The magnetic nanoparticles have been attached to the surfaceof MWCNTs by in situ high-temperature decomposition of the iron(III) acetylacetonate in polylol solution [30]. The magnetic-CNTs(m-CNTs) have been employed to obtain the m-CNTs-g-PLLA byin-situ ring-opening polymerization of lactide. The amount ofgrafted PLLA can be adjusted by different ratios of monomers tom-MWCNTs. m-MWCNTs-g-PLLA has shown typical superpara-magnetic behavior and could be aligned under a low magneticfield. When a drop of dispersion of m-MWCNTs-g-PLLA was driedon silicon wafer, the nanocomposite was randomly dispersed onits surface in the absence of magnetic field, whereas by applyingan external magnetic field m-MWCNTs-g-PLLA was parallellyoriented and a long-chain structure was formed [32].

Copolymers of L-lactide and ɛ-caprolactone were also synthe-sized using hydroxylated MWCNTs (MWCNTs-OH) and Sn(Oct)2 asthe initiating system. The prepared MWCNT-OH-g-PCLAs wereblended with pure poly(L-lactide-co-ɛ-caprolactone). Nanocompo-site with functionalized MWCNTs has shown higher mechanicalstrength implicating the stress transfer from matrix to the nano-filler. The usage of pristine MWCNTs also caused an increase in themechanical strength of copolymer and simultaneously thedecrease in elongation at break was observed [33].

Alternative catalyst to stannous octanoate was proposed for thepolymerization of L-lactide. Titanium alkoxide catalyst was covalentlyattached to SWCNTs and MWCNTs by Diels–Alder cycloaddition of abenzocyclobutene derivative. The catalyst-functionalized CNTs wereemployed for surface initiated titanium-mediated coordination poly-merization of L-LA. The slow induction period of about 15 h wasattributed to the heterogeneous nature of polymerization. After theinduction period, the viscosity and homogeneity of the solution

123456789

101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566 Scheme 2. Polylactide functionalized carbon nanotubes by “grafting from” technique.

M. Brzeziński, T. Biela / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 3





123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566

Table 1Preparation of PLA-functionalized multi-walled carbon nanotubes.

Polymer Method Initiating system/reaction Conditions Monitoring methods Potential application Authors and references

MWCNTs-g-PLLA/MWCNTs-g-PDLA

“Grafting from” Stannous octate MWCNTs-OH DMF and toluene, 70–140 1C, 2–20 h[28],toluene 110 1C, 72 h [29] bulk, 130 1C, 24 h[37]toluene, 120 1C, 72 h [38]

DSC, TGA, NMR, UV–vis, FT-IR,Raman, GPC, TEM, SEM,tensile testing

Biocomposite Chen et al. [28],Ma et al. [29],Brzeziński et al. [37],Sun and He [38]

MWCNTs-g-PDLA “Grafting from” Stannous octate MWCNTs-CO-(Oβ-Ur) bulk, 130 1C, 24 h NMR, DSC, TGA, FT-IR, Raman,GPC, SEM

Biomedical and sensorfunctions

Boncel et al. [30]

m-MWCNTs-g-PLLA “Grafting from” Stannous octate m-MWCNTs Bulk, 130 1C, 48 h TGA, FT-IR, Raman, TEM, SEM,magnetic properties

Drug targeting, tissueengineering and boneregeneration

Feng et al. [32]

MWCNT-OH-g-PCLAcopolymer

“Grafting from” Stannous octate MWCNTs-OH Bulk, 130 1C, 48 h Solubility, DSC, TGA, FT-IR,Raman, TEM, SEM,tensile testing

Biocomposite Chakoli et al. [33]

SWNTs-g-PLLAMWNTs-g-PLLA

“Grafting from” SWNT-g-(BCB-EOTiCpCl)2MWNT-g-(BCB-EOTiCpCl)2

Toluene, 130 1C, 20 h NMR, circular dichroism,DSC, TGA, FT-IR, Raman,TEM

Biocomposite Priftis et al. [34]

MWCNTs-g-PLLA “Grafting from” MWCNT-COOH with L-lactic acid Melt polycondensation,180 1C, 20 h TGA, FT-IR,SEM Biocomposite Yoon et. al. [18]MWCNTs-g-PLLA “Grafting to” MWCNT-COCl with oligomeric PLA Bulk, 180 1C, 24 h TGA, FT-IR, Raman,

TEM, SEMBiocomposite Chen et al. [13]

MWCNTs-g-PLLA “Grafting to” MWCNT-COCl with high-molecularweight PLA

CHCl3, 80 1C, 24 h DSC, TMDSC, TGA, FT-IR,AFM, FE-SEM, POM, DMA,WAXD,SAXS

Biocomposite Shieh et al. [14,15]

MWCNTs-g-PLLA “Grafting to” MWCNT-COCl with star-shaped PLA Bulk/DMF?, 60 1C, 24 h NMR, TGA, Raman Biocomposite Xu et al. [16]MWCNTs-g-PLLA “Grafting to” CNT250 (pyrrolidine groups at surface)

with high-molecular weight PLADMF 135 1C, 3 h and in the melt TGA, XPS, SEM Biocomposite Novais et al. [17]

MWCNTs-g-PLLA “Supramolecularstrategy”

MWCNTs with triphenylene core andimidazolium, anthracene, pyreneend groups

THF [22], CHCl3 [23], 1,4-dioxane[24], bulk [25],

TGA, UV–vis, SEM, TEM,dispersion stability

Biomedical Yang and Pan [22],Meyer et al. [23],Biedroń et al. [24],Manfredi et al. [25],

MWCNTs-g-PLLA “Supramolecularstrategy”

MWCNTs complex withPEO-b-PLLA/LiCl

THF, 12 h Raman, dispersionstability, MD

Biocomposite Meyer et al. [27]

M.Brzeziński,T.Biela

/Materials

Letters∎(∎∎∎∎)

∎∎∎–∎∎∎

4

Pleasecite

this

articleas:

Brzeziński

M,Biela

T.Polylactid

enan

ocomposites

with

function

alizedcarbon

nan

otubes

and

their

stereocomplexes:

Afocu

sedreview

.Mater

Lett(2014),h

ttp://d

x.doi.org/10.1016/j.m

atlet.2014.01.159i




increased and the reaction proceeded rapidly. The efficiency ofgrafting was monitored by thermogravimetric analysis (TGA) andstrongly depended on the reaction time. Nanocomposite has shownhigher glass transition temperature (Tg) and melting temperature(Tm) compared to the bulk PLLA. Interestingly, the increase in Tmdepends on the type of carbon nanotubes (SWCNTs or MWCNTs) andwas higher for MWCNTs [34].

The “grafting from” is a method based on the anchoring ofinitiators onto the carbon nanotubes surface. Subsequently surface-initiated polymerization induces the formation of a dense polymerbrushes structure [28]. The most commonly used active groupsin ROP are primary hydroxyl and/or amine groups. In addition,MWCNT-OH/Sn(Oct)2 initiating system is mainly used to obtainhigh-molecular weight poly(lactides). The efficiency of grafting couldbe adjusted by varying the monomer to MWCNT-OH ratio and thereaction time. The functionalization methods of MWCNTs surface,“grafting from”, “grafting to” and supramolecular strategy, are sum-marized in Table 1. Nanocomposites prepared by this method haveenhanced thermal and mechanical properties and probably lowelectrical conductivity because the surface of MWCNTs is well coveredby PLA chains which prevent the creation of a network of conductivenanotubes as mentioned previously (Table 2).

4. Poly(lactide)-functionalized carbon nanotubes (CNTs)stereocomplex nanocomposites

Poly(lactide) stereocomplexes (sc-PLA) are formed due to inter-molecular hydrogen bonds between the left- and right-handed helicesof PLLA and PDLA chains [35]. sc-PLA melts separately at highertemperatures (Tm) than the enantiomeric components [9]. Never-theless, in the case of high-molecular weight (Mn¼105) stereocom-plexes, the mixture of homochiral and stereocomplex crystallites areobserved in the second heating run of the DSC. It is noteworthy that incontrast to linear PLA stereocomplexes star-shaped PLA stereo-complexes are able to survive melting because of the hardlock-typeinteractions [10]. However, preparation of linear high-molecularweight stereocomplexes remains an important challenge for theindustrial application of stereocomplex-based materials.

Stereocomplex nanocomposites were obtained by a directmelt mixing process of equimolar mixture of PLLA and PDLAwith non-modified MWCNTs. The electrical conductivity for the

PLLA/PDLA/MWCNTs nanocomposites was higher than for PLLA/MWCNTs nanocomposites with lower MWCNTs loading dueto volume exclusion effect by the formation of a stereocomplex.Non-isothermal and isothermal crystallization confirmed strongnucleation effect of MWCNTs on the crystallinity of nanocompositeby reducing the induction time and increasing the nucleationdensity. However, the three melting peaks corresponding to PLLA(�150 1C), PDLA (�175 1C) and stereocomplex (�206 1C) wereobserved during second heating run of DSC (difference in the PLLAand PDLA Tm is related to their optical purity which determines thethermal properties of this polymers). These results indicated thatthe formation of stereocomplex in presence of non-modifiedMWCNTs is not reversible [36].

Recently, our research group proposed the use of MWCNT-OHas initiators in the ring-opening polymerization of L- and D-lactideto obtain PLAs with improved dispersion of MWCNTs in PLAmatrix. Stereocomplexes were prepared either by precipitationfrom solution or as a film via solvent evaporation. Stereocomplexesformation was fully reversible. This is a crucial result especially forthe preparation of stereocomplex films because it was not pre-viously possible via casting from solution. To evaluate the influ-ence of PLA-grafted MWCNTs on the thermal properties of sc-PLA,different mixtures of PLLA/PDLA were prepared with MWCNT-g-PLA, MWCNT-OH and unmodified MWCNT. Even if only oneenantiomer of PLA in stereocomplexation mixture was attachedto MWCNTs (0.5% MWCNT-g-PLA), peaks characteristic for sc-PLAcrystallites were observed in the first and second DSC heating run.However addition of MWCNTs and MWCNT-OH to the mixture ofL-PLA and D-PLA has not improved significantly the thermalproperties of formed stereocomplexes. In the second DSC heatingrun, besides heterocrystallites (stereocomplexes), homocrystalliteswere also observed. The presence of PLA covalently attached toMWCNTs strongly affected the thermal stability of the poly(lac-tide)-carbon nanotubes stereocomplex nanocomposites, even atvery low concentrations of MWCNT-g-PLA. Atomic force micro-scopy (AFM) showed shish-kebab morphology of stereocomplexthin film (Fig. 1) with very thin (o50 nm) and long (�2–3 μm)rigid structures. This picture confirmed the efficient functionaliza-tion of MWCNTs; as it could be observed, the PLA chains com-pletely covered the surface of carbon nanotubes [37].

Stereocomplex nanocomposites were also prepared by blend-ing commercial PLLA with MWCNT-g-PDLA, which was obtained

123456789

101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566

Table 2Thermal, mechanical and electrical properties of PLLA/PLLA-g-MWCNTs and stereocomplexes containing PLA-g-MWCNTs.

PLLA/PLLA-g-MWCNTs Thermalproperties

Mechanical properties Electrical resistivity(Ωm)

Authors andreferences

Tg(1C)

Tm(1C)

Hm

(J/g)Modulus(MPa)

Strength(MPa)

Elongation at break(%)

100/0(Cargill Dow) 59.8 166.1 21.8 – – – – Shieh and Liu [14]100/1 60.1 165.8 23.4 – – – – Shieh and Liu [14]100/5 60.1 167.7 27.3 Shieh and Liu [14]100/0(Shimadzu) – – – 2463 56.4 7.5 – Chen et al. [28]100/1 – – – 4710 85.6 6.8 – Chen et al. [28]100/0(Nature Works) 53.4 165.9 31.6 1928 49.3 2.5 – Yoon et al. [18]100/1 56.2 166 32.9 2541 72.3 1.9 1,2�1012 Yoon et al. [18]100/5 55.9 165 29.7 2504 48 – � 1012 Yoon et al. [18]100/0(Nature Works) – – – 1867 45 4 – Novais et al. [17]100/1 – – – 2250 56 3.8 1.1�105 Novais et al. [17]100/0(Nature Works) 59 150 40.3 – – – – Sun and He [38]80/20(PDLA) – 210 – �1600 �90 – – Sun and He [38]80/20(MWCNTs-g-PDLA)a 58.5 210 58.5 �5400 �380 – – Sun and He [38]MWCNTs-g-PLLA/ MWCNTs-g-PDLA(50/50)a

59.7 226.9 51.9 – – – – Brzeziński et al. [37]

a The weight percentage of MWCNTs in the final stereocomplexes was approximately 1 wt% (polymerization proceeds in the presence of 1 wt% MWCNT-OH) [37,38].Only 15–20% were polymerized from MWCNTs surface and the major fraction was free PLA [37].






by ROP using the MWCNT-OH/Sn(Oct)2 initiating system. Unfortu-nately the solution-cast stereocomplex films were a mixture ofPLA homocrystallites and sc-PLA crystallites even in the first DSCheating run. The amount of sc-PLA crystallites increased with thehigher content of MWCNT-g-PDLA in contrast to the melt cooledsamples, where the amount of PLA homocrystallites is very smalland depends on the heating rate and the amount of PDLAfunctionalized MWCNTs. The presence of homocrystallites isprobably due to the difference in molecular weight of commer-cially available PLLA (Mn¼130 K) and MWCNT-g-PDLA (Mn¼6 K).Addition of low molecular weight PDLA to commercial highmolecular weight PLLA caused a dramatic drop in modulus andhardness. PDLA macromolecules, which participate in the stereo-complex crystallization with PLLA, may be dissociated in the PLLAmatrix and act as plasticizer. Nevertheless, a significant enhance-ment in modulus and hardness of the stereocomplex nanocompo-site MWCNT-g-PDLA/PLLA in comparison to PDLA/PLLA controlsample was observed [38].

In order to obtain thermally stable stereocomplexes equimolaramounts of poly(L-lactide) and poly(D-lactide) with similar molecularweight and at least one enantiomer grafted to the MWCNTs surfaceare required. Further studies on the mechanical and electricalproperties of high-molecular weight stereocomplex nanocompositesshould be continued (Table 2). The development in the preparationand processing of poly(lactide)–carbon nanotubes stereocomplexnanocomposites will lead to the production of new biodegradablematerials for possible industrial applications.

5. Conclusions and outlook

Biodegradable materials, such as PLA, are widely used inbiomedical applications. Nevertheless the aim of many researchgroups is to provide PLA with novel functional properties that willmake it even more versatile polymer. One of the most importantand relatively new method to enhance the poly(lactide) thermaland mechanical properties is using functionalized carbon nano-tubes due to their good dispersion and efficient interactions withPLA matrix. MWCNT-g-PLAs can be used as efficient masterbatchesfor the preparation of nanocomposites with commercially avail-able PLAs, which have low nanofiller content. Additionally, stereo-complexation arising from interactions of L-lactyl and D-lactyl unitsequences gives another way to obtain PLA-based materials with avariety of enhanced properties. Since both PLLA and PDLA are

already commercially available and their market prices are gradu-ally falling, stereocomplex manufacturing in the near future can beprofitable. The combination of these two factors (using CNTs andstereocomplexation) for improving the properties of poly(lactide)opens the way to produce materials not only for biomedical, butalso for high load applications. However, the influence of PLAfunctionalized MWCNTs on the properties of PLA and its stereo-complex is not yet fully understood, and much remains to beexploitrd in this area.

Uncited reference

[31 Q2].

Acknowledgments

Authors would like to thank Prof. Przemysław Kubisa forvaluable comments and suggestions during the manuscript pre-paration. This work was supported by a project entitled “Biode-gradable fibrous products”, conducted under Contract no.POIG.01.03.01-00-007-/08 and co-financed by the European Unionwithin the framework of the Operational Program, InnovativeEconomy, IE OP financed by the European Regional DevelopmentFund, ERDF (“Biogratex”).

References

[1] Ray SS. Polylactide-based bionanocomposites: a promising class of hybridmaterials. Acc Chem Res 2012;45:1710–20.

[2] Raquez JM, Habibi Y, Murariu M, Dubois P. Polylactide(PLA)-based nanocom-posites. Progr Polym Sci 2013;38:1504–42.

[3] Xu JX, Chen T, Yang CL, Li ZM, Mao YM, Zeng BQ. Isothermal crystallization ofpoly(l-lactide) induced by graphene nanosheets and carbon nanotubes: acomparative study. Macromolecules 2010;43:5000–8.

[4] Singh NN, Singh SK, Dash D, Gounugunta P, Misra M, Maiti P. CNT induced β-phase in polylactide: unique crystallization, biodegradation, and biocompat-ibility. J Phys Chem 2013;117:10163–74.

[5] Tsuji H, Kawashima Y, Takikawa H, Tanaka S. Poly(l-lactide)/nano-structuredcarbon composites: conductivity, thermal properties, crystallization, andbiodegradation. Polymer 2007;48:4213–25.

[6] Spitalsky Z, Tasis D, Papagelis K, Galiotis C. Carbon nanotube–polymercomposites: chemistry, processing, mechanical and electrical properties. ProgrPolym Sci 2010;35:357–401.

[7] Pilla S, Gong S, Turng LS. Polylactide-based carbon nanotube nanocomposites.In: Mittal V, editor. Polymer nanotube nanocomposites: synthesis, properties,and applications. Scrivener Publishing LLC Q5; 2010.

123456789

101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566

Fig. 1. AFM phase image of the shish-kebab morphology of stereocomplex thin films: (a) AFM phase mode and (b) zoomed image of the AFM amplitude mode. Figurereproduced with permission from Ref. [37].

M. Brzeziński, T. Biela / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎6





[8] Hirsch A, Vostrowsky O. Functionalization of carbon nanotubes. Top CurrChem 2005;245:193–237.

[9] Tsuji H. Poly(lactide) stereocomplexes: formation, structure, properties, degra-dation, and applications. Macromol Biosci 2005;5:569–97.

[10] Biela T, Duda A, Penczek S. Enhanced melt stability of star-shaped stereocom-plexes as compared with linear stereocomplexes. Macromolecules 2006;38:3710–3.

[11] Stobinski L, Lesiak B, Kövér L, Tóth J, Biniak S, Trykowski G. Multiwall carbonnanotubes purification and oxidation by nitric acid studied by the FTIR andelectron spectroscopy methods. J Alloys Compd 2010;501:77–84.

[12] Xu Z, Niu Y, Wang Z, Li H, Yang L, Qui J. Enhanced nucleation rate of polylactidein composites assisted by surface acid oxidized carbon nanotubes of differentaspect ratios. ACS Appl Mater Interfaces 2011;3:3744–53.

[13] Chen GX, Kim HS, Park BH, Yoon JS. Controlled functionalization of multi-walled carbon nanotubes with various molecular-weight poly(l-lactic acid). JPhys Chem B 2005;109:22237–43.

[14] Shieh YT, Liu GL. Effects of carbon nanotubes on crystallization and meltingbehavior of poly(L-lactide) via DSC and TMDSC studies. J Polym Sci Part B2007;45:1870–81.

[15] Shieh YT, Liu GL, Twu YK, Wang TL, Yang CH. Effects of carbon nanotubes ondynamic mechanical property, thermal property, and crystal structure of poly(L-lactic acid). J Polym Sci B 2010;48:145–52.

[16] Xu Z, Niu Y, Yang L, Xie W, Li H, Gan Z. Morphology, rheology and crystal-lization behavior of polylactide composites prepared through addition of five-armed star polylactide grafted multiwalled carbon nanotubes. Polymer2010;51:730–7.

[17] Novais RM, Simon F, Pötschke P, Villmow T, Covas JA, Paiva MC. Poly(lacticacid) composites with poly(lactic acid)-modified carbon nanotubes. J PolymSci Part A 2013;17:3740–50.

[18] Yoon JT, Jeong YG, Lee SC, Min BG. Influences of poly(lactic acid)-graftedcarbon nanotube on thermal, mechanical, and electrical properties of poly(lactic acid). Polym Adv Technol 2009;20:631–8.

[19] Chen GX, Shimizu H. Multiwalled carbon nanotubes grafted with polyhedraloligomeric silsesquioxane and its dispersion in poly(l-lactide) matrix. Polymer2008;49:943–51.

[20] Xu Y, Li Q, Sun D, Zhang W, Chen GX. A strategy to functionalize the carbonnanotubes and the nanocomposites based on poly(l-lactide). Ind Eng ChemRes 2012;51:13648–54.

[21] Xu Z, Zhang Y, Wang Z, Sun N, Li H. Enhancement of electrical conductivity bychanging phase morphology for composites consisting of polylactide and poly(ε-caprolactone) filled with acid-oxidized multiwalled carbon nanotubes. ACSAppl Mater Interfaces 2011;3:4858–64.

[22] Yang LP, Pan CY. A non-covalent method to functionalize multiwalled carbonnanotubes using six-armed star poly(l-lactic acid) with a triphenylene core.Macromol Chem Phys 2008;209:783–93.

[23] Meyer F, Raquez JM, Coulembier O, De Winter J, Gerbaux P, Dubois P.Imidazolium end-functionalized poly(L-lactide) for efficient carbon nanotubedispersion. Chem Commun 2010;46:5527–9.

[24] Biedroń T, Pietrzak Ł, Kubisa P. Ionic liquid functionalized polylactide bycationic polymerization: synthesis and stabilization of carbon nanotubesuspensions. J Polym Sci Part A 2011;49:5239–44.

[25] Manfredi E, Meyer F, Verge P, Raquez JM, Thomassin JM, Alexandre M.Supramolecular design of high-performance poly(L-lactide)/carbon nanotubenanocomposites: from melt-processing to rheological, morphological andelectrical properties. J Mater Chem 2011;21:16190–6.

[26] Wang J, Chu H, Li Y. Why single-walled carbon nanotubes can be dispersed inimidazolium-based ionic liquids. ACS Nano 2008;2:2540–6.

[27] Meyer F, Raquez JM, Verge P, de Arenaza IM, Coto B, Van Der Voort P, et al. Poly(ethylene oxide)-b-poly(l-lactide)diblock copolymer/carbon nanotube-basednanocomposites: LiCl as supramolecular structure-directing agent. Biomacro-molecules 2011;12:4086–94.

[28] Chen GX, Kim HS, Park BH, Yoon JS. Synthesis of poly(L-lactide)-functionalizedmultiwalled carbon nanotubes by ring-opening polymerization. MacromolChem Phys 2007;208:389–98.

[29] Ma JG, Cheng X, Ma XW, Deng S, Hu AG. Functionalization of multiwalledcarbon nanotubes with polyesters via Bergman cyclization and grafting fromstrategy. J Polym Sci Part A 2010;48:5541–8.

[30] Boncel S, Brzeziński M, Mrowiec-Białoń J, Janas D, Koziol KKK, Walczak KZ.Oxidised multi-wall carbon nanotubes–(R)-poly(lactide) composite with acovalent β-D-uridine filler-matrix linker. Mater Lett 2013;91:50–4.

[31] Wan J, Cai W, Feng J, Meng X, Liu E. In situ decoration of carbon nanotubeswith nearly monodisperse magnetite nanoparticles in liquid polyols. J MaterChem 2007;17:1188–92.

[32] Feng J, Cai W, Sui J, Li Z, Wan J, Chakoli AN. Poly(L-lactide) brushes onmagnetic multiwalled carbon nanotubes by in-situ ring-opening polymeriza-tion. Polymer 2008;49:4989–94.

[33] Chakoli AN, Wan J, Feng JT, Amirian M, Sui JH, Cai W. Functionalization ofmultiwalled carbon nanotubes for reinforcing of poly(l-lactide-co-ɛ-caprolac-tone) biodegradable copolymers. Appl Surf Sci 2009;256:170–7.

[34] Priftis D, Petzetakis N, Sakellariou G, Pitsikalis M, Baskaran D, Mays JM.Surface-initiated titanium-mediated coordination polymerization fromcatalyst-functionalized single and multiwalled carbon nanotubes. Macromo-lecules 2009;42:3340–6.

[35] Pan P, Yang J, Shan G, Bao Y, Weng Z, Cao A, et al. Temperature-variable FTIRand solid-state 13C NMR investigations on crystalline structure and moleculardynamics of polymorphic poly(l-lactide) and poly(l-lactide)/poly(d-lactide)stereocomplex. Macromolecules 2012;45:189–97.

[36] Quan H, Zhang S, Qiao JJ, Zhang L. The electrical properties and crystallizationof stereocomplex poly(lactic acid) filled with carbon nanotubes. Polymer2012;53:4547–52.

[37] Brzeziński M, Bogusławska M, Ilčiková M, Mosnáček J, Biela T. Unusual thermalproperties of polylactides and polylactide stereocomplexes containingpolylactide-functionalized multi-walled carbon nanotubes. Macromolecules2012;45:8714–21.

[38] Sun Y, He C. Synthesis, stereocomplex crystallization, morphology andmechanical property of poly(lactide)–carbon nanotube nanocomposites. RSCAdv 2013;3:2219–26.

123456789

101112131415161718192021222324252627282930313233343536373839






polylactide nanocomposites with functionalized carbon nanotubes and their stereocomplexes: a focused...

Documents