progress in nanocomposite of biodegradable polymer review

16
J. Ind. Eng. Chem., Vol. 13, No. 4, (2007) 485-500 REVIE W Progress in Nanocomposite of Biodegradable Polymer Ke-Ke Yang, Xiu-Li Wang, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, Sichuan University, Chengdu 610064, People’s Republic of China Received May 10, 2007; Accepted May 18, 2007 Abstract: This paper reviews recent developments related to biodegradable polymer nanocomposites. The prepa- ration, characterization, properties, and applications of nanocomposites based on biodegradable polymers are in- troduced systemically. The related biodegradable polymers include aliphatic polyesters such as polylactide (PLA), poly( ε -caprolactone) (PCL), poly(p-dioxanone) (PPDO), poly(butylenes succinate) (PBS), poly (hydroxyalkanoate)s such as poly( β -hydroxybutyrate) (PHB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), and natural renewable polymers such as starch, cellulose, chitin, chitosan, lignin, and proteins. The nanoparticles that have been also utilized to fabricate the nanocomposites include inorganic, organic, and metal particles such as clays, nanotubes, magnetites, Au and Ag, hydroxyapatite, cellulose, chitin whiskers and lignin. Keywords: biodegradable material, nanocomposite, aliphatic polyester, poly(hydroxyalkanoate), natural re- newable polymer Introduction 1) In the past century, various synthetic polymer materials have been developed in different forms, such as plastics, fibers, and synthetic rubbers, and used widely in a varie- ty of fields, including packaging, construction materials, agriculture, and medical devices. Undoubtedly, those synthetic polymer materials perform very important roles in our daily lives. After rapid development for several decades, a Gordian knot is becoming increasingly seri- ous: the continual environmental pollution caused by un- degradable synthetic polymer wastes. Recycling present polymer wastes is a direct and popu- lar approach toward solving this problem. However, de- veloping and using biodegradable polymers is consid- ered as the most thorough method for resolving this situation. With this background, the development of bio- degradable polymers has been a growing concern since the last decade of the 20th century. A variety of bio- degradable polymer materials have been prepared and a quite lot of them have already been industrialized [1-5]. According to their different origins, biodegradable poly- mers are classified into three major categories: (1) syn- To whom all correspondence should be addressed. (e-mail: [email protected]) thetic polymers, particularly aliphatic polyester, such as poly(L-lactide) (PLA) [6-11], poly(ε -caprolactone) (PCL) [12-14], poly(p-dioxanone) (PPDO) [15-21], poly (butylenes succinate) (PBS) [22-24], and poly(ethylene succinate) (PES) [25,26]; (2) polyesters produced by mi- croorganisms, which basically indicates different types of poly(hydroxyalkanoate)s, including poly(β -hydrox- ybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hy- droxyvalerate) (PHBV) [27-30]; (3) polymers originat- ing from natural resources, including starch, cellulose, chitin, chitosan, lignin, and proteins [31-46]. Although biodegradable polymers have developed an amazing speed and the flourishing situation in this field is quite inspiring, they are far from becoming substitutes for tra- ditional undegradable polymers. The major reason is the disadvantageous properties of these materials, such as poor mechanical properties, high hydrophilicity, and poor processibilty, which limit their application. Taking this situation into consideration, we can easily under- stand the necessity and the urgency of functionalization and modification to these polymers. In recent decades, nanotechnology has been widely ap- plied to polymeric materials, with the ultimate goal of dramatically enhanced performance [47-49]. There are two main approaches to achieve polymer nanomaterials. The most popular is to introduce nanoscale particles into

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Page 1: Progress in Nanocomposite of Biodegradable Polymer REVIEW

J. Ind. Eng. Chem., Vol. 13, No. 4, (2007) 485-500

REVIEW

Progress in Nanocomposite of Biodegradable Polymer

Ke-Ke Yang, Xiu-Li Wang, and Yu-Zhong Wang*†

Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, Sichuan University, Chengdu 610064, People’s Republic of China

Received May 10, 2007; Accepted May 18, 2007

Abstract: This paper reviews recent developments related to biodegradable polymer nanocomposites. The prepa-ration, characterization, properties, and applications of nanocomposites based on biodegradable polymers are in-troduced systemically. The related biodegradable polymers include aliphatic polyesters such as polylactide (PLA), poly(ε-caprolactone) (PCL), poly(p-dioxanone) (PPDO), poly(butylenes succinate) (PBS), poly (hydroxyalkanoate)s such as poly(β-hydroxybutyrate) (PHB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), and natural renewable polymers such as starch, cellulose, chitin, chitosan, lignin, and proteins. The nanoparticles that have been also utilized to fabricate the nanocomposites include inorganic, organic, and metal particles such as clays, nanotubes, magnetites, Au and Ag, hydroxyapatite, cellulose, chitin whiskers and lignin.

Keywords: biodegradable material, nanocomposite, aliphatic polyester, poly(hydroxyalkanoate), natural re-newable polymer

Introduction

1)

In the past century, various synthetic polymer materials have been developed in different forms, such as plastics, fibers, and synthetic rubbers, and used widely in a varie-ty of fields, including packaging, construction materials, agriculture, and medical devices. Undoubtedly, those synthetic polymer materials perform very important roles in our daily lives. After rapid development for several decades, a Gordian knot is becoming increasingly seri-ous: the continual environmental pollution caused by un-degradable synthetic polymer wastes. Recycling present polymer wastes is a direct and popu-lar approach toward solving this problem. However, de-veloping and using biodegradable polymers is consid-ered as the most thorough method for resolving this situation. With this background, the development of bio-degradable polymers has been a growing concern since the last decade of the 20th century. A variety of bio-degradable polymer materials have been prepared and a quite lot of them have already been industrialized [1-5]. According to their different origins, biodegradable poly-mers are classified into three major categories: (1) syn-

† To whom all correspondence should be addressed.(e-mail: [email protected])

thetic polymers, particularly aliphatic polyester, such as poly(L-lactide) (PLA) [6-11], poly(ε-caprolactone) (PCL) [12-14], poly(p-dioxanone) (PPDO) [15-21], poly (butylenes succinate) (PBS) [22-24], and poly(ethylene succinate) (PES) [25,26]; (2) polyesters produced by mi-croorganisms, which basically indicates different types of poly(hydroxyalkanoate)s, including poly(β-hydrox- ybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hy- droxyvalerate) (PHBV) [27-30]; (3) polymers originat-ing from natural resources, including starch, cellulose, chitin, chitosan, lignin, and proteins [31-46]. Although biodegradable polymers have developed an amazing speed and the flourishing situation in this field is quite inspiring, they are far from becoming substitutes for tra-ditional undegradable polymers. The major reason is the disadvantageous properties of these materials, such as poor mechanical properties, high hydrophilicity, and poor processibilty, which limit their application. Taking this situation into consideration, we can easily under-stand the necessity and the urgency of functionalization and modification to these polymers. In recent decades, nanotechnology has been widely ap-plied to polymeric materials, with the ultimate goal of dramatically enhanced performance [47-49]. There are two main approaches to achieve polymer nanomaterials. The most popular is to introduce nanoscale particles into

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Ke-Ke Yang, Xiu-Li Wang, and Yu-Zhong Wang486

Figure 1. Schematic representation of the L,L-lactide polymer-ization performed in situ from Cloisite130 B using triethylalu-minium (AlEt3) as initiator (R: tallow alkyl chain).

a polymer matrix to produce polymer/nanoparticle com- posites. The other is to fabricate polymer materials them-selves on the nanoscale. Both approaches have been ap-plied to many undegradable polymer systems. Based on pioneering research, nanotechnology has also been suc-cessfully used to produce biodegradable polymer materi-als with high performance. This paper reviews the new developing trends of nanotechnology in biodegradable polymer materials, including the different types of poly-mer nanocomposites and their production methods, mi-crostructures, and properties.

Biodegradable Aliphatic Polyester Nanoparticle Com- posites Because aliphatic polyesters play a very important role in the field of biodegradable materials, their nano-composites are attracting growing interest from researchers. Nanoparticles are being employed increas-ingly to produce new nanocomposites, for example, lay-ered silicates, layered titanates, carbon nanotubes, gold, silver, and maghemite nanoparticles, magnetite nano-particles, and fluorine mica. The modification matrixes cover almost all of the biodegradable aliphatic polyesters. Among these, polyesters/layered silicate nanocomposites have been investigated considerably, es-pecially PLA/layered silicate nanocomposites. Therefore, the nanocomposites produced from different polymer matrixes with different nanoparticles are introduced in detail.

PLA Nanocomposites Among the aliphatic polyesters, PLA is considered to be the most promising biodegradable material, not only because it has excellent biodegradability, compatibility,

Figure 2. TEM image of a nanocomposite based on 3 wt% clay after redispersion of the highly-filled Cloisite130B, presenting delamination of platelets. Individual layers are indicated by ar-rows; intercalated stacking is surrounded.

Table 1. Materials Properties of Neat PLA and Various PLA- CNs

Mterials properties PLA PLACN4 PLACN5 PLACN7Modulus (GPa) 4.8 5.5 5.6 5.8Strength (MPa) 86 134 122 105

Distortion at break (%) 1.9 3.1 2.6 2PPLACN/PPLA 1 0.88 0.85 0.81

and high strength but also due to the fact that it can be obtained totally from renewable resources. If incorporat-ing different nanoparticles into the PLA matrix could en-hance the properties of this material significantly, this process would increase its applicability further. Thus, it is easy to understand why so many studies have focused on this process [50-57]. The PLA/OMLS (organo-modified layered silicate) blends prepared using solvent-casting methods were re-ported first by Ogata and his group [58]. However, be-cause the silicate layers forming the clay could not be in-tercalated in the PLA/montmorillonite (MMT) blends, this material cannot be called a nanocomposite. Three different approaches have been successfully developed to fabricate PLA/clay nanocomposites, namely in situ poly-merization intercalation, melt intercalation and sol-ution-intercalation, film-casting techniques. Dubois and his group [50,51,59] synthesized poly (L,L- lactide)/organo-modified montmorillonite nanocompos- ites [P(L,L-LA)/O-MMT] with both intercalated and ex-foliated structures by employing the in situ ring-opening polymerization method (Figure 1) [59]. They found that the type of nanofiller played a dominant role on its final dispersing morphology. When natural unmodified mont-morillonite-Na was used, only intercalation of polyester chains was obtained, otherwise; exfoliation occurred (Figure 2) [59]. In recent years, the research group of Ray [55,60-71]

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Table 2. OMLS Samples Used in This Research

OMLS codes Pristine LS Particle lengthnm

CECMequiv/100g

Organic salts used for the modification of LS Suppliers

CDA Montmorillonite [Na1/3(Al5/3Mg1/3)Si4O10(OH)2] 150 110 Octadecylammonium cation Nanocor Inc., USA

SBE Montmorillonite [Na1/3(Al5/3Mg1/3)Si4O10(OH)2] 100 90 Trimethyloctadecyl ammonium cation

Hojun Yoko Co., Japan

MAE Synthetic Fluorine Mica [NaMg2.5Si4O10F2] 300 120 Dinethyldioctadecyl ammonium cation

CO-OPChemicals Co., Japan

Figure 3. Bright-field TEM images of various PLA/OMLS nanocomposites. The dark entities are cross-sections of intercalated or stacked OMLS layers; bright fields represent the matrix.

prepared a series of PLA/layered silicate nanocomposites using the melt extrusion technique with, for example, modified mentmorillnite, mica, and titanate. Further- more, they investigated the structures and properties of the nanocomposites systemically, including their mor-phology, crystallization behavior, mechanical properties, heat distortion temperature, gas barrier property, rheo-logical behavior, and biodegradability. They found that most of these properties were improved remarkably. MMT is the most common clay used in PLA systems [71]. The improvement of the mechanical properties was remarkable (Table 1), and there was a great relationship between the content of MMT and the final properties of the composites. Here, PLACNn stands for PLA/clay nanocomposites in which n denotes the percentage of clay. The type of layered silicate is another key factor influ-encing the properties of the material. Table 2 [70] lists three different layered silicates employed in PLA nano-composites, and Figure 3 [70] describes the dispersion morphology of the nanoparticles. We see clearly that the degree of dispersion exerts an effect on the various lay-ered silicates. Consequentially, the properties of the ma-terials, such as the biodegradability and crystallization behavior, varied with the different layered silicates. Taking this phenomenon into account, Ray [70] inves-tigated the biodegradability of PLA nanocomposites that contained different kinds of layered silicates. The authors

found that the biodegradability of neat PLA was en-hanced significantly after incorporation with clays and depended completely upon both the nature of the pristine layered silicates and the surfactants used for modification of the layered silicate, such that the biodegradability of polylactide could be controlled via judicious choice of the organically modified layered silicate. Figure 4 [70] shows images of samples of PLA and various PLA/ OMLS nanocomposites recovered from compost with time. The authors suggested that two factors were re-sponsible for the significant enhancement of the bio-degradability of the PLA/SBE4 composite relative to that of pristine and other nanocomposite systems. One is the presence of terminal hydroxyl groups of the silicate. In the case of the PLA/SBE4 nanocomposite, the stacked and disordered intercalated silicate layers are dispersed homogeneously in the PLA-matrix and these hydroxyl groups start heterogeneous hydrolysis after absorbing moisture from the compost. The other factor that controls the biodegradability of PLA nanocomposites is the state of dispersion of the intercalated OMLS in the PLA matrix. When intercalated OMLS species are distributed well in the matrix, the maximum amount of the matrix contacts the clay edge and surface, which causes the PLA to fragment readity and enhances the ultimate degrada-tion rate, which can be observed in the case of PLA/ SBE4 system. The crystallization behavior of PLA/clay nanocom-

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Figure 4. Photographs demonstrating the biodegradation of neat PLA and various PLA/OMLS nanocomposites recovered from compost with time. The initial shape of the crystallized samples was 3 × 10 × 0.1 cm3.

Figure 5. Optical micrographs of neat PLA (a-c) and PLACN4 (a-c) at crystallization temperatures (Tc) of (a, a') 120 oC, (b, b') 130 oC, and (c, c') 140 oC.

posites also exhibits obvious differences when compared with that of neat PLA [60,72]. The group of S. S. Ray [60] described the detailed crystallization behavior and morphology of pure PLA and the representative PLA/ C18MMT nanocomposite. Both the spherulites of neat PLA and the nanocomposite exhibited negative bire-

Figure 6. Steady shear viscosity of PLA and various PLACNs as a function of shear rate.

fringence, but the regularity of the spherulites was much higher in the case of pure PLA (Figure 5 [60]). The over-all crystallization rate of neat PLA increased after nano-composite preparation with C18-MMT, but it had no in-fluence on the linear growth rate of pure PLA spheru- lites. This behavior indicates that the dispersed MMT particles act as nucleating agents for PLA crystallization in the nanocomposite. Krikorian and Pochan reported [72] that the degree of clay miscibility with the matrix and the clay dispersion state in the PLLA matrix both significantly influence the crystallization behavior and final morphology of the nanocomposites. Their results indicated that the nucleat-ing efficiency of intercalated organoclay is much higher than that of exfoliated organoclay, and that the overall bulk crystallization rate increased in the intercalated sys-tem and decreased in the exfoliated system. Moreover, they found an interesting phenomenon: the spherulite growth rates increased significantly in the fully ex-foliated nanocomposite. This behavior might contribute to the lower nucleating efficiency in the exfoliated nan- ocomposite. The rheological properties of PLA/layered silicate nanocomposites have been investigated repeatedly be-cause they dominate the processability of these materials. For example, Ray [61] reported the rheological behavior of PLA/MMT nanocomposites. Typical curves of the ef-fect of shear rate on viscosity for pure PLA and PLA/MMT nanocomposites with various MMT loadings are illustrated in Figure 6 [61]. In this case, the PLACNs exhibited non-Newtonian behavior, whereas, the pure PLA exhibited almost Newtonian behavior, at all shear rates. Furthermore, the rheological behavior of the PLA/ MMT nanocomposites strongly depended on the shear rate. It is clear that the shear viscosity of the PLACNs in-itially exhibited some sheart thickening behavior at very low shear rates; subsequently, they show a very strong

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shear-thinning behavior at all measured shear rates. Finally, at very high shear rates, the steady shear vis-cosities of the PLACNs almost approached that of pure PLA. A reasonable explanation was given by Ray [61]: at high shear rates, the silicate layers are strongly ori-ented toward the flow direction (there may be perpendic-ular alignment of the silicate layers toward the stretching direction) and, at the same time, the pure polymer domi-nates the shear-thinning behavior. From the representation above, it can be deduced that the incorporation of layered silicates into the PLA matrix can not only enhance the crystallization rate but also in-crease the melt viscosity of the material, which would improve its processablility remarkably. Thus, it is not surprising that Thellen and his group [73] successfully produced plasticized PLA/MLS nanocomposites through blown-film processing. This technique will greatly pro-mote the competitiveness of PLA for use in environ-mentally friendly materials, particularly for packaging. Beside layered silicates, other nanoparticles, including carbon nanotubes [74] and nanoscale magnetites [75], have been used to make PLA nanocomposites. It is ex-pected that more suitable nanoparticales will be dis-covered that will allow PLA nanocomposites to be pre-pared with more outstanding properties.

PCL Nanocomposites PCL is another important aliphatic polyester that is con-sidered as a potential material in both biomedical and en-vironmental fields. It is commonly synthesized through ring-opening polymerization of ε-caprolactone under mild conditions. PCL exhibits a low glass transition tem-perature and melting point, high crystallinity and perme-ability, and good flexibility with a high elongation at break and low modulus. However, modification is highly necessary when it is applied to different requirements. Combining nanoparticles with PCL is an effective and operable approach to improving the properties of PCL significantly. Most studies of PCL modified by nanoparticales have focused on layered silicates [76-78]. Much of the liter-ature on this system has been reported by the Tortora group [79-83]. They prepared different compositions of poly(ε-caprolactone) (PCL) with (organo-modified) mo- ntmorillonite by melt blending or in situ ring opening polymerization (ROP). It was found that exfoliated nano- composites could be obtained after in situ ROP of ε- caprolactone with an organo-modified montmorillonite [MMT-(OH)2] when using dibutyltin dimethoxide as an initiator/catalyst. The intercalated nanocomposites were obtained either by melt blending with organo-modified montmorillonite or in situ ROP in the presence of sodium montmorillonite. The miscibility of organic modifiers with polymers

plays an important role in the intercalation/exfoliation of silicate layers. To explore the mechanism of silicate dis-persion in PCL systems, the analogous hydroxyl-termi-nated oligo-poly(caprolactone) (o-PCL) was selected [84] because it can strongly interact with silicates and/or different organic modifiers. The author found that the o- PCL-based blend is a very interesting system, the behav-ior of which strongly depends on the nature of the organ-ic modifier and the aspect ratio of the silicate layers: it may be immiscible, it may intercalate into silicate gal-leries as usual in polymer intercalation, or the organic modifier may diffuse out and be solubilized in the o-PCL. As the organic modifier is concerned, the chain- length plays a dominant role. When o-PCL is immiscible with the organic modifier (like methyltriphenylphos- phonium bromide, CPh), it cannot be intercalated into the silicate gallery. When o-PCL is miscible with the organic modifier, the chain-length also influences the dispersing morphology of the silicate layers. For a short-chain mis-cible modifier (like n-octyltri-n-butylphosphonium bro-mide, C8), o-PCL can be easily intercalated into silicate layers; for a long-chain modifier (n-hexadecyltri-n-bu- tylphosphonium bromide, C16), the modifier orients itself away from the silicate surface and is solubilized into the o-PCL phase, resulting in the exfoliated structure (Scheme 1 [84]). The author also discussed the situation of intercalation involving the C16 modifier and various aspect ratios of layered silicates (Scheme 2) [84].

Scheme 1. Scheme 2.

A diffuse-out mechanism has been used to explain the exfoliated structure in the case of a low aspect ratio (hectorite used here). In contrast, for higher-aspect-ratio silicates, the larger lateral dimensions of the silicate lay-ers ensure that much less of the organic modifier is in a position to access areas outside of the silicate gallery, such that the o-PCL must intercalate instead. Chen and his group [85] reported the relationships be-tween the structure and the mechanical properties of PCL/layered silicate nanocomposites. In that study, PCL- clay composites with three types of montmorillonite and clay loadings ranging from 1.7 to 59 wt% were prepared by melt-processing. Briefly, conventional composites were produced by the natural montmorillonite, and na- nocomposites with slightly different microstructures (Figure 7) were obtained by two different ammonium-

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(a) (b)Figure 7. TEM images of (a) PCL-NH4MMT1 (1NM1b) and (b) PCL-NH4MMT2 (2NM3) composites.

Table 3. Tensile and Flexural Yield Strengths of PCL-clay Composites

Sample Tensile strength/MPa Flexural yield strength/MPaPCL 17(0.5) 23(1.2)M2 32(1.5) 26(1.7)M4 22(0.4) 21(1.6)M11 17(1.2) 27(0.8)M20 15(1.4) 28(1.7)M34 13(0.6) 30(0.3)M45 12(0.7) 29(0.7)M55 11(0.9) 26(1.3)

1NM1 27(2.2) 27(1.6)1NM1b 27(2.5) 27(2.6)1NM4 32(1.7) 30(3.2)1NM8 31(2.2) 30(1.0)1NM16 19(0.4) 29(2.3)1NM24 15(0.7) 28(1.3)1NM30 14(0.9) 22(1.6)2NM2 29(0.9) 24(1.9)2NM3 30(2.6) 24(0.9)2NM7 32(1.0) 28(0.6)2NM12 22(1.2) 29(1.2)2NM19 21(1.9) 26(1.7)2NM29 16(1.0) 24(2.1)2NM33 14(0.2) 23(1.2)

treated montmorillonites, respectively. Both the micro-structures of the composites and the clay loadings influ-enced the mechanical properties; even the presence of clay increased the longitudinal modulus, tensile strength, tensile modulus, flexural yield strength, and flexural modulus and afforded a dramatic improvement in the elongation at break (Table 3). They found that the nano-composites had a higher strength or modulus than that of the conventional composites with similar clay loadings, and that the nanocomposite with more exfoliation pro-vided a greater increase in the strength or modulus than the one with less exfoliation. Based on the experimental data, the author also used the well-established theory for conventional composites to interpret the relationships be-

tween the elastic modulus and the volume fraction in the nanocomposites. The high permeability of pure PCL is an advantage when it is used as a biomedical material, but it is a draw-back when applied to environmental fields. The barrier properties of PCL can be enhanced by introducing lay-ered silicate into the matrix. The Tortora group [81] in-vestigated the barrier properties of PCL/OMMT nano-composites when water vapor and dichloromethane were used as solvents. They found that the water sorption of the nanocomposites increased with increasing MMT content. For water vapor, the thermodynamic diffusion parameters of the intercalated nanocomposites were sim-ilar to that of the parent PCL. Conversely, they decreased remarkably in the exfoliated nanocomposites, even when a small montmorillonite content was used. In the case of the organic vapor, both the exfoliated and intercalated samples showed lower values. Di and his group [76] probed the barrier performance of PCL/organoclay nanocomposites to air permeation; the samples were prepared by melt mixing PCL with Cloisite 30B and Cloisite 93A. An improvement of the barrier characteristic could be observed clearly, and the air per-meation coefficient decreased upon increasing the clay loading. The crystallization behavior of PCL/organoclay nano-composites has been investigated in detail [86-88], and similar phenomena have been found. All the literature il-lustrates that well-dispersed organoclay platelets act as nucleating agents that dramatically increase the crystal-lization rate of PCL. A new approach to the preparation of polyester nano-composites has recently become very popular: grafting the polyester to the surface of modified nanoparticles or ROP of the polyester initiated by the surfaces of the modified nanoparticles. This technique has also been ap-plied to produce PCL nanocomposites. Recently, Delaite and his group [78,89] prepared colloidal superparamag- netic nanocomposites by grafting PCL to the surfaces of organosilane-modified maghemite nanoparticles. Two routes were followed, which are represented in Schemes 3 [78] and 4 [78], respectively. For route one, CL was in-itially polymerized according to a coordination-insertion mechanism with aluminum isopropoxide as an initiator and benzyl alcohol as a coinitiator; then, the resulting PCL was functionalized with 3-isocyanatopropyltrie- thoxysilane in one step using tetraoctyltin as a catalyst; finally, the grafting of PCL-Si(OEt)3 polymers onto ma-ghemite was conduct in DMF, followed by exhaustive washing with THF to remove nongrafted polymer chains. For route two, the maghemite nanoparticles were first modified using N-(2-aminoethyl)-3-aminopropyltrime- thoxysilane (EDPS); whereafter, CL was polymerized from the modified maghemite surface initiated by alum-

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Figure 8. Variation of MV of neat PPDO and a PPDO/MMT- OH nanocomposite with respect to the polymerization time.

inum isopropoxide according to an anionic-coordinated process.

Scheme 3.

Scheme 4.

PPDO Nanocomposites Besides perfect biodegradability and biocompatibility, poly(p-dioxanone) has several other outstanding mechan-ical properties when compared with other aliphatic poly-esters, such as PLA and PCL. It is one of only a few bio-degradable polymers that possess both high tensile strength and excellent flexibility [20]. Unsurprisingly, in-creasing attention has been paid to the synthesis [15,21, 90-93], properties [16-18,94-104], and applications of PPDO [105] in recent years. Based on the intensive in-vestigations of many researchers, PPDO has been applied successfully in the medical field, e.g., as a bone or tissue fixation device and drug delivery system. Furthermore, PPDO also has great potential for general use in such systems as films, molded products, laminates, foams, non-woven materials, adhesives, and coatings. PPDO still has some unsatisfactory characteristics, such as hydrophobicity, a low crystallization rate, and a low

Figure 9. Schematic representation of the procedure for for-mation of s-SWCNT/PPDX composites.

melt strength, which limit its applications and processing methods. Therefore, modifications based on PPDO have been paid growing concern. The most common ap-proaches include copolymerization and blending. Lately, nanotechnology has been employed in PPDO systems. Wang and his group [19] prepared novel PPDO/MMT nanocomposites by in situ ring-opening polymerization of PDO with organo-MMT. They found that MMT not only acted as an ideal nucleating agent that significantly enhanced the crystallization rate but also could accelerate the polymerization of PDO; the viscosity-average molec-ular weight of PPDO could reach 44,900 g/mol in 0.5 h (Figure 8) [19]. In addition, the melt strength of the PPDO/MMT nanocomposites increased dramatically when compared with that of the neat PPDO. It is well known that preparing thin films by blowing processing from aliphatic polyesters is very difficult because of the low crystallization rate and low melt strength. Those problems have been solved successfully by this method and biodegradable PPDO/MMT thin films with out-standing mechanical properties have been achieved by blowing processing. Because carbon nanotubes (CNTs) have unique atomic structures and extraordinary mechanical properties such as strength and flexibility, they have become favored nanoparticles for reinforcing polymer materials. The Yoon group [106] successfully prepared homogeneous s- SWCNT/PPDO composites via ROP of PDO surfaces in- itiated by single-walled carbon nanotubes (s-SWCNTs) (Figure 9) [106]. Dramatic changes of the PPDO properties as a result of the formation of s-SWCNT/PPDO composites were ob-served in that study. The 10 %-weight-loss temperature of PPDO increased by ca. 20 oC upon the formation of the composite (from 248 to 268 oC; Figure 10a) [106]. In addition, no noticeable peaks could be observed from the DSC curve of the composites (corresponding to Tg and Tm) at temperatures up to 125 oC, while pure PPDO showed values of Tg and Tm of -13.4 and 103 oC, re-spectively (Figure 10b) [106]. The authors presumed that the observed changes in the PPDO properties were the result of effective interactions between the s-SWCNTs and the PPDX and the consequent mobility decrease of PPDX chains. Nevertheless, the amount research into PPDO nano-composites pales in comparison with the flourish in the

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Figure 10. (a) Thermogravimetric analysis (TGA) data of s-SWCNTs, pure PPDO, and s-SWCNT/PPDO composites and (b) differ-ential scanning calorimetry (DSC) data of pure PPDX and s-SWCNT/PPDO composites.

(a) (b)Figure 11. TEM micrographs of BAP/OMMT nanocomposites content 6 % OMMT.

field of PLA and PCL nanocomposites.

PBS Nanocomposites PBS is another aliphatic polyester that has good bio-degradability, melt processability, and thermal and chem-ical resistance. It is generally synthesized by poly-condensation of 1,4-butanediol with succinic acid. Th- anks to the successful incorporation of nanoparticles into other polyesters resulting in remarkable improvements of properties, this technique also has been introduced into PBS systems [107-110]. Ray and his group studied other aliphatic polyester such as PBS [109,110], after they had obtained deep insight into the nature of PLA/layered silicate nanocomposties. Recently, they prepared a series of PBS/OMLS nano-composites using the melt intercalation technique. Two different types of OMLS, MMT modified with octadecy-lammonium chloride and saponite (SAP) modified with quaternary hexadecyl tri-n-butylphosphonium bromide, were used for the preparation of nanocomposites. They proved that the flocculated structure has a strong effect on the mechanical and other properties of the material, no matter whether it was in the solid or melt state. Chen and his group [107] developed a new method to improve the interactions between poly(butylene succi-

nate) (PBS) and a commercially available organoclay, Cloisite 25A (C25A). Epoxy groups were grafted to C25A in the presence of (glycidoxypropyl)trimethox-ysilane to produce a doubly functionalized organoclay (TFC). Nanocomposites were then prepared by melt blending PBS with TFC. TEM and XRD analyses identi-fied that the degree of exfoliation of the silicate layers in PBS/TFC increased obviously, which resulted directly in improvement of the mechanical properties in comparison with those of PBS/C25A. They also investigated the non-isothermal crystallization kinetics of neat PBS and PBS/clay nanocomposites using differential scanning calorimetry (DSC) [111]. The results revealed that the crystallization rate decreased in the order PBS/TFC > PBS/C25A > neat PBS at a given cooling rate. TFC ex-hibited higher nucleation activity than did C25A for the crystallization of PBS. The Someya group [108] prepared PBS/layered silicate nanocomposites by melt intercalation. Nonmodified montmorillonite and five different organo-modified MMTs were employed in that study, but the improve-ment of mechanical properties was not marked. Recently, Choi and his group [112,113] developed a series of novel nanocomposites (BAP/OMMT) based on biodegradable aliphatic polymers synthesized from diols (1,4-butanediol and ethylene glycol) and dicarboxylic acids (succinic acid and adipic acid) and O-MMT by em-ploying solvent-casting and melt intercalation techniques in succession. In both cases, the intercalation structures were verified by XRD and TEM analysis. It can be seen clearly in Figure 11 [113] that BAP/OMMT nano-composites with 6 % OMMT were successfully prepared by melt intercalation. Enhancement of the properties, such as the mechanical strength, was observed in both composites. The rheological properties were also inves-tigated in detail; the results showed that the loading of OMMT played an important role in determining the rheological behavior. The shear viscosity at low shear

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rate exhibited a Newtonian plateau even at high loading and showed a higher degree of shear thinning at a higher shear rate. Undoubtedly, great progress in the study of aliphatic polyesters/nanocomposites has been achieved in recent years, but the techniques have not been applied widely to other synthetic biodegradable polymers, such as PEO, polyurethane, and polyanhydride. Choi and his group [114-117] prepared a series of nanocomposites from PEO or PEO/PMMA blends with organoclay using a sol-vent casting method. Hsu and coworkers [118,119] ach-ieved polyether-type polyurethane (PU) composites con-taining gold or silver nanoparticles by casting from wa-terborne PU with the Au or Ag nanoparticle suspension. Li and his group [120] developed the nanocomposites of cross-linked polyanhydrides and hydroxyapatite needles from three methacrylated anhydride monomers of citric acid (MCA), sebacic acid (MSA), and 1,4-bis(carbox-yphenoxy)butane (MCPB) with homogenously dis-tributed hydroxyapatite (HAp) nanoneedles through in situ photo-polymerization.

PHAs Nanocomposites Presently, poly(hydroxyalkanoates) (PHAs) produced by microbes (including soil bacteria, estuarine micro-flora, blue green algae, and various photobiological sys-tems) as a natural part of their metabolism is attracting increasing attention. PHB is the representative polymer because it possesses properties similar to synthetic ther-moplastics, such as poly(propylene); however, its draw-backs of brittle behavior and lack of melt stability have seriously limited its application. These disadvantages have been conquered to a certain extent when PHB was substituted by poly(3-hydroxybutyrate-co-3-hydroxyva- lerate) (PHBV), which has been recognized as a poten-tially environment-friendly substitute for traditional plastics. Even though, the use of PHBV presents some problems, such as high cost, a slow crystallization rate, a high degree of crystallinity, and difficulty in processing. It is a wise decision to modify PHBV by introducing nanoparticles into the matrix [121-124]. Recently, Choi and his group [123] reported some val-uable results about PHBV/MMT nanocomposites, which were prepared through a melt intercalation method using Cloisite 30 B as the organoclay. An intercalated structure was determined by XRD and TEM analyses. The temper-ature and rate of crystallization of PHBV increased as a result of the effective nucleating efforts of the organo- clay. Moreover, the nanocomposites showed significant increases in tensile strength and thermal stability. Song and his group [124] investigated the biodegradability of PHBV/OMMT nanocomposites and found that the bio-degradability of PHBV/OMMT nanocomposites in soil suspension decreased with an increase in the amount of

OMMT.

Natural Renewable Resource-Based Nanocomposites In addition to biodegradable polymers produced by chemical and microorganism-based syntheses, natural re-newable biodegradable polymers, such as starch, chitin, chitosan, cellulose, gelatin, and protein, also play very important roles in the field of biodegradable materials.

Starch Nanocomposites In this family, starch is considered as one of the most promising materials because of its low-cost and readly availability. It can be processed into thermoplastic mate-rials only in the presence of plasticizers and under the ac-tion of heat and shear forces. However, poor water resist-ance and mechanical properties are obstacles that hinder the widening of its applications. With this background in mind, many researchers have sought to modify the prop-erties of starch for quite a long time, with the approach of copolymerization or blending with other polymers being chosen frequently. In recent years, manufacturing starch nanocomposites has become of growing interest as a promising option toward enhancing the mechanical and barrier properties [125-127]. Because of its high effi-ciency and very low cost, layered silicate is often chosen to be introduced into thermoplastic starch (TPS). In recent years, Chen’s group [127] has prepared TPS-clay composites with various types of clay and clay loadings by melt-processing in the twin roll mill. They found that natural smectite clays, montmorillonite and hectorite, can readily form nanocomposites with TPS; the microstructures (intercalation, exfoliation or conven-tional composite) have been demonstrated through XRD and TEM analyses. For example, the untreated hectorite nanocomposites were partially exfoliated while the TPS- treated hectorite composites were conventional. The in-teraction between glycerol-plasticized starch and mont-morillonite was detected at higher clay loadings. The au-thors also investigated the properties of these TPS/clay composites. As a result, the presence of clay increased the elastic modulus of TPS in all cases. Nevertheless, the moduli of conventional composites, such as treated-hec-torite and kaolinite composites, were lower than those of the nanocomposites. For nanocomposites, the improve-ment of the efficiency of MMT was slightly higher than that of untreated hectorite. Biodegradable starch/clay nanocomposite films aimed for use as food packaging materials have been prepared by casting because it is difficult to process them by blow-ing, as reported M. Avella and his colleagues [128]. The reinforcing effect of the clay on the modulus and the ten-sile strength of the TPS were also observed (Table 4) [128]. A new starch nanocomposite totally originated from re-

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Table 4. Mechanical Analysis of TPS and Its Composite SamplesSample code Clay (%) Yang modulus (Mpa) Stress at peak (Mpa) Strain at break (%)

Mechanical properpties of the materials conditional at 15 % relative humidityPS 0 979 19 3

PS/C 4 1135 22 4PS/PE 0 682 7.5 2

PS/PE/C 4 374 7.8 38Mechanical properlties of the materials conditional at 60 % relative humidity

PS 0 610 9 9PS/C 4 830 10 2PS/PE 0 150 3.6 21

PS/PE/C 4 137 4.7 58Mechanical properlties of the materials without conditioning58

PS 0 217 6 25PS/C 4 450 6 10PS/PE 0 160 5 22

PS/PE/C 4 75 4 60

(a) (b)Figure 12. (a) Transmission electron micrograph from a dilute suspension of tunicin whiskers. (b) Scanning electron micro-graphfrom the fractured surface of a composite filled with 6.2 wt% tunicin whiskers.

newable resources was obtained by Angles’ group [129, 130]. In this case, a collodial suspension of cellulose whiskers was used as the reinforcing phase for the TPS matrix. TEM examination was employed to evaluate the morphology of the composites (Figure 12) [129]. The au-thors suggested that the white shiny dots corresponded to the transversal sections of the cellulose whiskers; the di-ameters were ca. 62 ± 2 nm. After intensive investigation and analysis, some significant changes were found when the cellulose whiskers were dispersed homogeneously in the TPS matrix. It was deduced by the authors that both plasticizers (glycerol and water) diffused toward the cel-lulose surface, and that the accumulation of plasticizer in the cellulose/amylopectin interfacial zones improved the crystallization ability of the amylopectin chains, which would result in the formation of a possible transcrystal-line zone around the whiskers. The optimal mechanical performance of the composites was improved.

Cellulose Nanocomposites As the most abundant natural renewable resource, cellu-

lose is attracting interest as a feedstock for manufactur-ing biodegradable materials that can substitute for petro-leum-based polymers in the commercial market. Recently, a type of all-cellulose nanocomposite film was prepared by the Gindl group [131]. The main goal of the authors was to combine the advantages of nanofiber reinforcement and self-reinforcement to obtain high- strength, random-oriented, biobased, easily recyclable, and biodegradable composites. The method adopted was as follows: microcrystalline cellulose was partly dissol- ved in lithium chloride/N,N-dimethylacetamide solvent and then films were cast from the solution. The structure and mechanical properties of the resulting film were also tested. An increase in the elastic modulus and a decrease in the failure strain occurred upon increaseing the crys-tallinity and cellulose I/cellulose II ratio (Table 5) [131]. The raw cellulose without any modification has no ther-moplastic character and poor solubility; thus, it difficult to process. In conrast, cellulose derivatives such as cellu-lose acetate (CA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB) are thermoplastic materials produced through the esterification of cellulose. In this series of derivatives, CA is paid particular interest because it has excellent optical clarity and high tough- ness. Unfortunately, the typical melting range of CA is near its decomposition temperature. Therefore, a plasti-cizer is often introduced to overcome this problem. Park and his group [132,133] successfully prepared a CA/or-ganoclay nanocompsite, coalescing the reinforement and plasticization effects very well by using triethyl citrate (TEC) combined with Cloisite 30B organoclay as the plasticizer, and maleic anhydride-grafted cellulose ace-tate butyrate (CAB-g-MA) as the compatibilizer. The mechanism of the preparation of compatibilized CA/

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Table 5. Tensile Test and X-ray Diffraction Data with Films of All-cellulose Nanocomposite Films Produced Using Different Amounts of MCC Added to 100 mL LiCl/DMAc Solvent Compared to Pure Regenerated Cellulose

Material Amount of MCC (g)

Elastic modulus (GPa)

Tensile strength (MPa)

Failure strain (%)

Crystallinity (%) I22.7o/I20.4o CelluloseⅠ/CelluloseⅡ

Regenerated cellulose - 6.9 170.3 18.2 39 0.71 -

Composite A 2 12.6 218.6 10.7 46 0.83 24/76Composite B 3 13.1 242.8 8.6 52 0.93 43/57Composite C 4 14.9 215.1 3.6 57 1.01 59/41

Figure 13. Mechanism of formation of compatibilized CA/ TEC/organoclay hybrid nanocomposites.

TEC/organoclay hybrid nanocomposites is illustrated in Figure 13 [132]. The nanostructure was demonstrated by AFM charac-terization, which showed that nanocomposites with a 5 wt% compatibilizer content had better-exfoliated struc-tures than their counterparts without the compatibilizer hybrid (Figure 14) [132]. The mechanical properties were improved as a result of good exfoliation and dis-persion of clay in the plasticized CA matrix.

Chitosan Nanocomposites Citosan, which has a repeated structure of (1,4)-linked 2-amino-2-deoxy-β-D-glucan, is the fully or partially de-acetylated product of chitin, the second-most-abundant natural resource next to cellulose. Because chitosan is a biocompatible, biodegradable, and almost nontoxic mate-rial, it has been used widely in the pharmaceutical field as a carrier for drug delivery and as a biomedical material. Although it is soluble in aqueous solutions of various acids, chitosan molecules have no amphiphilic properties and cannot form micelles in water. Apart from its biodegradable character under physiological con-ditions, chitosan has reactive amino and hydroxyl gro- ups, which offer possibilities of modifications, graft re-actions, and ionic interactions. Here, we also focus on the new advances in nanocomposites based on chitosan [46,

Figure 14. 3-D AFM height image and section analysis: (a, b) intercalated and exfoliated clay platelets in CA/organoclay hy-brid with 5 wt% compatibilizer; scan size, 2.5 um; (c, d) com-pletely exfoliated clay platelets in the mica surface; scan size, 2.0 um.

134-139]. Hydroxyapatite [137-139], magnetite [134- 136], and MMT [46] have been utilized to develop chito-san composites; most of them are expected to be used in medical fields. A recent report from the Shen group [139] described bi-odegradable chitosan/hydroxyapatite nanocomposite rods prepared through in situ hybridization; this material has been suggested as a potential material for internal fix-ation of bone fractures. In addition, this method success-fully solved the problem of nano-sized particle aggrega- tion in the polymer matrix. Moreover, a remarkable in-crease in the mechanical properties of the composite was obtained from bending strength and modulus tests (Table 6) [139]. The bending strength and modulus of the CS/HA (100/5, wt/wt) composite prepared by in situ hy-birdization were 86 MPa and 3.4 GPa, respectively. Additionally, all of this material’s properties were 2∼3 times stronger than those of some other bone replace-ment materials, such as PMMA and bone cement.

Plant Oil Nanocomposites Some traditional natural renewable resources that are known as foodstuffs, such as plant oil and protein, have been used successfully to produce novel green materials.

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Table 6. Mechanical Properties of CS/HA Composites and Other Bone Replacement MaterialsSample σb (MPa) E b (MPa) Sample σb (MPa) E b (MPa)

Bone cement 36.54 1.31 CS rod 80 ± 12 3.9 ± 0.5PMMA 41.5 2.21 CS/HAB (100/5) rod 68 ± 2 3.2 ± 0.2

CS/HA(70/30) 19 1.0 CS HA (100/5) rod 86 ± 7 3.4 ± 0.1Cortical bone pin 275 ± 52 18.3 ± 1.3

Very recently, Kobayashi and his colleagues [140,141] published a series of reports on some new approaches to the manufacture of green polymeric materials from plant oil, especially of new advances in the preparation plant oil nanocomposites. The typical materials are plant oil/ clay nanocomposites [140]. Curing of epoxidized plant oils in the presence of organophilic montmorillonite pro-duced triglycerideclay nanocomposites with homoge-neous structures in which the silicate layers of the clay were intercalated and randomly distributed in the poly-mer matrix. Another very interesting case is the green plant oil/silica nanocomposite-based transparent coat-ings, which were produced from acid-catalyzed curing of epoxidized plant oils with 3-glycidoxypropyltrimethox-ysilane [141]. An obvious improvement of the hardness and mechanical strength after incorporating the silica net-work into the organic polymer matrix was identified, with good flexibility observed in the nanocomposite. The nanocomposites also exhibited high biodegradability.

Protein Nanocomposites Lately, increasingly more researchers are engaged in developing novel green materials from proteins. As natu-ral polymers, proteins are able to form amorphous three-dimensional structures stabilized mainly by non-colvalent interactions. The functional properties of these green materials are highly dependent on the structural heterogeneity, thermal stability, and hydrophilic behavior of the proteins. Some proteins have also been considered as good candidates for environmental plastic materials, including wheat protein, corn zein, peanut protein, and especially soy protein. Commercially available soy pro-tein products are classified as soy flour, soy concentrate, and soy isolate. Because soy isolates are a highly refined class of soybean protein that contains more that 90 % protein, they have become the favorite choice of re- searchers. It is obvious that plasti)cs made from soy pro-tein alone possess good biodegradability, but have low tensile strength and poor flexibility. To resolve these problems, plasticizers and reinforcing fillers are often used in this system [142-147]. Various kinds of fillers, such as nanoclay, chitin whisker [143], and lignin [144- 147], have been examined, and better results have been achieved when these fillers were dispersed in the protein matrix, as reported by Zhang and coworkers [144]. In this work, soy protein isolate/hydroxypropyl alkaline lig-

nin (HPL) nanocomposites were prepared by mixing them in aqueous solution containing a small amount of glutaraldehyde as compatibilizer, and then compression- molding them to obtain plastic sheets. Microstructural analysis proved that HPL was dispersed in the SPI matrix on the nanoscale. When the HPL content was lower than 6 wt%, the HPL-domains occurred in the SPI/HPL com-posites with dimensions of ca. 50 nm. Moreover, a dra-matic enhancement of the tensile strength in this system was achieved, as expected. For example, the tensile strength of the SPI/HPL nanocomposite sheets with 6 wt% HPL and 3.3 wt% glutaraldehyde increased from 8.4 for the SPI sheets to 23.1 MPa.

Conclusions

The increasing demand for biodegradable materials in the world market leads to the responsibility and obliga-tion of researchers to develop products with better prop-erties compared with those of existing materials. From this statement, we may declare that introducing nano-particles into biodegradable polymer matrixes to produce nanocomposites is one of the most effective approaches to enhancing the properties of pristine polymers. Although different results could be achieved when vari-ous nanoparticles have been employed in different poly-mer materials, generally speaking, most of the properties of the nanocomposites can be improved remarkably, such as the mechanical properties, barrier properties, thermal stability, crystallization rate, degradation rate, and melt strength.

Acknowledgments

This study was supported financially by the National Science Foundation of China (20504022) and the National Science Fund for Distinguished Young Scholars (50525309).

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