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Publicly Available Published by De Gruyter September 25, 2015

Crystallization modification of poly(lactide) by using nucleating agents and stereocomplexation

Long Jiang, Tianfeng Shen, Pengwu Xu, Xiyuan Zhao, Xiaojie Li, Weifu Dong, Piming Ma and Mingqing Chen
From the journal e-Polymers

Abstract

Poly(lactide), PLA, as one of the most promising biopolymers, has been receiving increasing attention in recent years because of its excellent performances in renewability, mechanical properties, biocompatibility and biodegradability. However, its application is limited by its brittleness and low heat distortion temperatures (HDT). The low HDT mainly results from a low crystallization rate and lack of crystallinity after fast processing, e.g. injection molding. Consequently, considerable attention was paid, in recent years, to achieve fast(er) crystallization of PLA. In here, we briefly review the research progress in the crystallization modification of PLA notably by means of adding nucleating agents and stereocomplexation.

1 Introduction

The rapid development of petroleum-based polymeric materials such as polyolefins, poly(styrene) (PS) and poly(vinyl chloride) (PVC) have greatly improved man’s standard of living. However, traditional polymeric materials do not degrade easily after use and cause serious environmental issues, e.g. “white pollution”, and they also bring great pressure to the development of the ecological environment. Therefore, biodegradable and biobased polymers have received considerable attention in the last 20 years due to their potential applications relating to environmental protection and ecology. Poly(lactide) (PLA) as a promising biopolymer has exhibited vast appeal due to its excellent performance in renewability, mechanical properties, biocompatibility and biodegradability compared with petroleum-based polymers (1–10).

PLA is a fully biobased and biodegradable and environmentally-friendly material derived from renewable resources and can be generally obtained by ring-opening polymerization with lactide (11–13), as shown in Figure 1.

Figure 1: General PLA production routes via ring opening polymerization (ROP): from biomass to high molecular weight (high-Mw) PLA.

Figure 1:

General PLA production routes via ring opening polymerization (ROP): from biomass to high molecular weight (high-Mw) PLA.

The application of PLA is restricted by its low crystallization rate, low crystallinity, low heat distortion temperature (HDT) and relatively high cost (14–17). PLA products via the practical processing methods such as injection molding usually exhibit poor mechanical strength and stiffness above its glass transition temperature (Tg ~60°C) because it is usually amorphous after processing due to the slow crystallization (18). PLA chains possess a semi-rigid backbone and low chain mobility due to a short repeating unit length (lactic acid monomer), which results in the low crystallization rate (19, 20). For PLA-based copolymers, the effect of co-monomer segments such as length, content and architecture on the PLA crystallization should be considered as well (21). The overall crystallization rate is associated with both nuclei formation and subsequent crystal growth.

PLA with a certain amount of crystallinity is required for commercial application. On the other hand, the mechanical properties and biodegradability of PLA are strongly dependent on the crystal morphology and structures, whereas, it is challenging to develop high crystallinity. Therefore, formulation and/or process modifications are required. Three routes were usually applied to fasten the crystallization of PLA, i.e. (i) adding nucleating agents that could lower the surface free energy barrier towards nucleation, thus induce crystallization at high(er) temperatures upon cooling; (ii) using plasticizers to reduce the chain folding energy, consequently, increase the chain/segment mobility and the crystallization rate (22, 23); (iii) forming stereocomplex (SC) crystallites between enantiomeric poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA). The stereocomplexation route has drawn increasing attention in recent years due to the better physical properties of the SC-PLA than those of the PLLA or PDLA. To summarize, PLA has become one of the most important biodegradable and biobased materials due to the broad potential application range. In this article, we briefly review the research progress in crystallization modification of PLA by means of nucleation and stereocomplexation.

2 General crystallization behavior of poly(lactide)

The lactic acid includes L- and D- stereo-forms, leading to L-lactides, D-lactides and meso-lactides, as shown in Figure 2. Commercial grades of PLA are copolymers of PLLA and PDLA, which are generally produced from L-lactides and D-lactides, respectively (24, 25). The L-isomer constitutes the main fraction of PLA because it is the major form of lactic acid derived from biological sources.

Figure 2: Stereo-forms of lactic acid and lactides.

Figure 2:

Stereo-forms of lactic acid and lactides.

Four crystalline forms (α, β, γ and δ) of PLA can be formed depending on the composition of PLA and the crystallization conditions. The most common α form that crystallizes from the melt or the solution exhibits X-ray diffraction peaks at 14.8°, 16.9°, 19.1° and 22.5° (26–28). The β form can be created during drawing at high stretching speed and temperatures (29, 30). The γ form is mainly obtained by epitaxial growth on hexamethylbenzene substrate (31). The δ form crystal which is also named disordered α form is formed below 120°C showing similar diffraction patterns to the α form crystals (32–34). In recent years, another type of PLA crystal structure, SC crystallites, has received great attention. The melting point of SC crystallites is 50°C higher than that of PLA homochiral (HC) crystallites, and the formation of SC has been proved to be an effective method to enhance the physical and mechanical properties of PLA (35, 36). The crystal system and crystal cell parameters of PLA and SC-PLA are summarized in Table 1 (26, 37–42). It can be seen that the crystalline parameters are obviously different between the crystal forms.

Table 1

Crystalline parameters of the PLA and the SC-PLA crystals.

Crystal formCrystal systemCell parameters
a (nm)b (nm)C (nm)α (o) β (o)γ (o)
α (26)Orthorombic1.050.612.88909090
α (37)Pseudo-orthorombic1.070.6452.78909090
β (38)Orthorombic1.0311.8210.90909090
β (39)Trigonal1.0521.0520.88909090
γ (40)Orthorombic0.9950.6250.88909090
SC (41)Triclinic0.9160.9160.87109109110
SC (42)Triclinic1.4981.4980.879090120

3 Enhancement in crystallization by nucleating agents

PLA usually has a low crystallinity after practical processing, which limits the range of application. Consequently, a lot of effort in the scientific community was taken to enhance the nuclei density and the overall crystallization rate of PLA by adding nucleating agents. The nucleating agents could depress the nucleation energy barrier and initiate crystallization at higher temperatures upon cooling (43). The nucleating agents of PLA reported in the literature are classified and their nucleation effect is summarized below.

3.1 Inorganic nucleating agents of poly(lactide)

Some mineral nucleating agents were used to enhance the crystallization of PLA (Natureworks LLC., USA). In these nucleating agents, talc showed high efficiency and is commonly used as a reference for other nucleating agents (44–49). The nucleation density of PLA could be increased by 500 times with the addition of 6 wt% TALC (Pfizer, New York, USA) (50). The nucleation effect is affected by talc concentration and cooling conditions, e.g. the crystallization temperature (Tc) of PLA was increased by 2–3°C when the talc concentration was increased from 1 to 2 wt% upon cooling at 80°C/min (22), while the Tc increased from 107 to 123°C at a cooling rate of 1°C/min when 3 wt% talc was incorporated (48).

Clay was used to improve thermal, mechanical and barrier properties of polymeric materials. It also showed a nucleation effect on the crystallization of PLA. An increase by 50% in the crystallization rate of PLA was observed with 4 wt% organically modified montmorillonite (51). Krikorian and Pochan found that the exfoliated clay could increase the crystal growth rate of PLA although its nucleation effect is not so obvious (52). The nucleation efficiency of clay is not as high as that of the talc as is evidenced by a moderate reduction in the half-life crystallization time (t1/2 ) and a poor nucleation performance at high cooling rates. The higher nucleation efficiency of talc is mainly because of its epitaxial match with PLA crystals (53).

Carbon nanotubes (CNTs) have attracted considerable attention in recent years, and were also identified as useful nucleating agents to promote the crystallization of PLA. Xu et al. reported multi-wall carbon nanotubes (MWCNTs) (Nanocyl S.A., Sambreville, Belgium) as nucleating agents of PLLA at a loading of less than 0.08 wt%. Upon cooling, the Tc of PLLA was increased to higher temperatures accompanied with insignificant crystallinity upon cooling at >10°C/min (54). The insignificant crystallinity development probably resulted from a poor compatibility between PLA matrix and CNTs. To enhance the compatibility, PLA grafted CNTs (i.e. PLA-g-CNT) were applied (55, 56). Consequently, a crystallinity of 12–14% was obtained with 5–10 wt% of the PLA-g-CNT at a moderate cooling rate of 5°C/min (56). Moreover, the minimum t1/2 (120°C) of PLA was decreased from 4.2 to 1.9 min after incorporation of 5 wt% of the PLA-g-CNT (55). Similar to CNTs, grapheme oxide (GO) was also proved to be a nucleating agent. The non-isothermal melt crystallization behavior of PLLA and PLLA/GO nanocomposites was investigated with differential scanning calorimetry (DSC) (Perkin Elmer, USA) at a cooling rate of 5°C/min (57). The crystallization temperature was increased from 95.1°C of neat PLLA to 97.0, 100.4, and 96.0°C, respectively, after addition of 0.5 wt%, 1 wt% and 2 wt% of the GO. Correspondingly, the crystallization enthalpy (ΔHc) was increased from 3.5 J/g of neat PLLA to 17.0, 36.0 and 34.2 J/g of the PLLA with 0.5 wt%, 1 wt% and 2 wt% of the GO, respectively. Thus, both CNT and GO could enhance the crystallization of PLA as heterogeneous nucleating agents.

3.2 Organic nucleating agents of poly(lactide)

Compared with inorganic nucleating agents, organic compounds have received increasing attention in recent years because of their better dispersion and miscibility in polymer matrixes. A commercially successful example is the sorbitol-based NAs such as Irgaclear® (Ciba Specialty Chemicals, Basel, Switzerland) and NA11 (Basf, Shanghai Co., Ltd.) for poly(propylene) and its copolymers (58–62). Organic salt and compounds were also used as nucleating agents to enhance the crystallization of PLA. Organic salt such as sodium stearate and sodium benzoate provided limited improvement on the crystallization rate of PLA (22, 43). Moreover, the molecular weight (Mw) of PLA was reduced from 163.3 to 127.5 kg/mol by incorporation of 0.2 wt% of sodium benzoate (63). On the other hand, orotic acid (OA) was distinguished as a nucleating agent for PLA (64). Even 0.3 wt% orotic acid showed notable effect on crystallinity development in both non-isothermal and isothermal crystallization conditions. A sharp crystallization peak of PLA at 124°C with a ΔHc of 34 J/g was obtained upon cooling at 10°C/min. A good match between the a-spacing of OA (0.590 nm) and the b-spacing of PLLA (0.604 nm) was reported, which is regarded as a reason for the high nucleation efficiency of the OA in comparison with the organic salt (65). Ethylene bis-stearamide (EBS, 2 wt%) from a vegetable source reduced the t1/2 of PLA from 38 to 1.8 min at 115°C (66). Similarly, a strong nucleation effect of N,N-ethylenebis(12-hydroxystearamide) (EBHSA) on PLLA (99.2% of L-LA) was observed by Nam et al. (67). A well-developed layer of PLA trans-crystallite grown from EBHSA surface was observed via optical microscopy (OM) indicating the epitaxial crystallization of the PLLA. As EBHSA crystallizes rapidly the isothermal crystallization of PLA was enabled below the melting point of EBHSA (144.5°C). Furthermore, the nuclei density of PLA in the presence of the EBHSA was increased by 40 times at 130°C, and the overall crystallization rate was increased by 4 times. In another work, N,N′,N″-tricyclohexyl-1,3,5-benzene-tricarboxylamide (TMC-328) (Shanxi Provincial Institute of Chemical Industry, China) was developed as a nucleating agent for PLA which was proved to have a high activity (18, 68). It can self-organize in the PLA melt to induce fast crystallization of the PLA, leading to interesting crystalline morphology. For example, neat PLA formed general spherulites while tapered lamb string and needle-like crystal morphology were observed after the addition of 0.2–0.5 wt% of the TMC-328, as shown in Figure 3a–d (18). The authors claimed epitaxial growth of PLA crystals based on the atomic force microscopy (AFM) (SPI4000/SPA400, Seiko Instruments) photographs (Figure 3e and f) where a shish-kebab crystalline morphology was observed. By tailoring the crystal morphology and crystallinity of PLA in the presence of TMC-328, the oxygen permeability coefficient of the PLA film was reduced by more than 2 orders of magnitude (68). In another study, hydrazide compounds enabled complete crystallization of PLA upon cooling, and the nucleation efficiency of the hydrazide compounds was even higher than talc and EBHSA due to a better miscibility and dispersion in the PLA matrix (46).

Figure 3: (a–d) Polarized optical micrcopy (POM) images of the PLLA/TMC-328 blends with the content of TMC-328: (a) 0, (b) 0.2, (c) 0.3 and (d) 0.5%. (e–f) AFM (e) height and (f) phase images of PLLA/TMC-328 blend with 0.2% of the TMC-328. [Reprinted with permission from Ref. (18). Copyright 2011 by the America Chemical Society.]

Figure 3:

(a–d) Polarized optical micrcopy (POM) images of the PLLA/TMC-328 blends with the content of TMC-328: (a) 0, (b) 0.2, (c) 0.3 and (d) 0.5%. (e–f) AFM (e) height and (f) phase images of PLLA/TMC-328 blend with 0.2% of the TMC-328. [Reprinted with permission from Ref. (18). Copyright 2011 by the America Chemical Society.]

Recently, Ma et al. found that both the crystallization rate and crystallinity of PLA could be significantly increased by the incorporation of 0.25–1.0 wt% of oxalamide compounds (69). The structure of the oxalamide compounds is shown in Figure 4, where the central part contains two-oxalamide moieties providing the driving force for self-organization in the PLA melt while the terminal groups (R) determine the miscibility of the compounds in PLA melt and the thermal behavior of the compounds.

Figure 4: The chemical structure of the oxalamide compounds.

Figure 4:

The chemical structure of the oxalamide compounds.

The nucleation effect and nucleation mechanism of the oxalamide compounds were studied by using DSC, POM (Axio Scope 1, Zeiss, Germany) and wide-angle X-ray diffraction (WAXD) (Bruker AXS D8, Germany). It was concluded from the results that the oxalamide derivatives could promote the crystallization of PLA leading to significantly increased Tc and reduced t1/2 , as shown in Figure 5. Moreover, the chemical structure of the terminal groups (cyclohexyl, benzyl and phenyl) was crucial to their self-organization behavior and the nucleation efficiency, and the compounds capped with phenyl group exhibit the highest nucleation efficiency (70).

Figure 5: (A) DSC cooling and subsequent heating curves and (B) half-life isothermal crystallization time (t1/2 ) of the PLA and the PLA/NA blend. NA(CY), NA(BE) and NA(PH) correspond to the oxalamide compounds with cyclohexyl, benzyl and phenyl end groups, respectively. Xc is the crystallinity of PLA, Tc-PLLA refers to the crystallization peak temperatures. [Reprinted from Ref. (70), Copyright 2015 with permission from Elsevier.]

Figure 5:

(A) DSC cooling and subsequent heating curves and (B) half-life isothermal crystallization time (t1/2 ) of the PLA and the PLA/NA blend. NA(CY), NA(BE) and NA(PH) correspond to the oxalamide compounds with cyclohexyl, benzyl and phenyl end groups, respectively. Xc is the crystallinity of PLA, Tc-PLLA refers to the crystallization peak temperatures. [Reprinted from Ref. (70), Copyright 2015 with permission from Elsevier.]

The authors proposed the nucleation mechanism based on the experimental data. The oxalamide derivatives are soluble in PLA melt and are capable of self-organizing into fibrils upon cooling. The in situ formed “fibrils” as efficient nucleation sites induced rapid growth of α-form PLA crystal along the fibrils, and shish-kebab-like structures were generated as well. This nucleation process is schematically illustrated in Figure 6.

Figure 6: Illustrations of the crystallization process of PLA initiated by self-organized oxalamide compounds. The inset on the right top of each demonstrated the corresponding POM images of self-organized oxalamide compounds and PLA crystal. [Reprinted with permission from Ref. (69). Copyright 2014 American Chemical Society.]

Figure 6:

Illustrations of the crystallization process of PLA initiated by self-organized oxalamide compounds. The inset on the right top of each demonstrated the corresponding POM images of self-organized oxalamide compounds and PLA crystal. [Reprinted with permission from Ref. (69). Copyright 2014 American Chemical Society.]

To summarize, good miscibility, pre-precipitation (or organization into superstructures) and a certain content of epitaxial match with the PLA lattice are required to achieve excellent nucleation efficiency of nucleating agents.

3.3 Macromolecular nucleating agents

The effect of starch, an interesting biopolymer, on the crystallization of PLA was investigated as well. Ke and Sun reported that the t1/2 of PLA was reduced from 14 to 3.2 min with the addition of 1 wt% native starch while the crystallization rate was increased slightly when the native starch content was increased from 1 to 40 wt% (47). To increase the nucleation efficiency, thermoplastic starch (TPS) was applied instead of native starch. Li and Huneault reported that the nucleation efficient of TPS was strongly dependent on the dispersion of TPS and the interfacial modification (71). In fact, the PLA with 20 wt% TPS did not reveal obvious crystallization upon cooling at 10°C /min. However, the PLA crystallized to the maximum enthalpy of ΔHc = 50 J/g and the minimum t1/2 of 75 s under the same condition when 20 wt% maleated-PLA was incorporated into the PLA/TPS system to enhance the interfacial modification and reduce the TPS particle size.

Goffin et al. chemically grafted PLA chains on the surface of nanocrystalline cellulose (NCC) to obtain NCC-g-PLLA nanohybrids (72). The t1/2 of PLA (130°C) was decreased from 15 to 2.5 min at 8 wt% of the NCC-g-PLLA. The grafting of NCC enhances its compatibility with the PLA matrix and improved the final properties as well.

In addition to starch and NCC, Saga et al. reported that the α-cyclodextrin (α-CD) could also serve as a nucleating agent of PLLA from solution. The spherulitic size of PLA was decreased with an increase in α-CD content and/or the casting rate (73).

Recently, stereocomplex PLA (SC-PLA, see Section 4) has also been used as a nucleating agent of PLA. Brochu et al. found that both of the spherulite density of PLA and the crystallinity were larger in the presence of SC-PLA implying the nucleation effect of the SC crystals (74). Yamane et al. studied the thermal properties and crystallization behavior of PLLA/PDLA (1–5 wt% of PDLA) and demonstrated that the in situ formed SC crystallites enhanced the crystallization process significantly upon cooling (75). Tsuji et al. studied the effects of PDLA (0.1–10 wt%) on the melt isothermal and non-isothermal crystallization behaviors of PLLA, and also revealed that the stereocomplex crystallites could effectively enhance the crystallization of PLLA (76).

4 PLA stereocomplexation

Stereocomplex (SC) crystallites of PLA, formed between enantiomeric PLLA and PDLA, show a melting point of 50°C higher than that of PLA HC crystallites. The SC-PLA has better physicochemical properties and higher hydrolytic stability than PLLA and PDLA (77). The SC can be used for broad applications such as films, fibers, plastics and the above discussed nucleating agents (78). Stereocomplexation is heavily influenced by preparation routes, processing conditions and molar mass of PLA (79–81). The stereocomplexation mechanism between enantiomeric poly(lactide)s were investigated by Brizzolara et al. (82) They proposed simultaneous folding of the two enantiomeric chains in stereocomplexation which explained the triangular shape of single crystals. The triangular type of crystallization offers favorable positions for the polymer loops during the crystal growth.

In general, low-Mw , equal molar ratio and solution-methods are favorable for stereocomplexation, but a high-Mw and melt-processing is preferred in industrial practice (79–81). The HC crystallites prevailed SC crystallites when the Mw of PLA was over 105 g/mol (83, 84). On the other hand, SC crystallites of high-Mw PLA showed poor memory to reform SC crystallites after complete melting (85). As a consequence, to achieve a large crystallinity of SC from high-Mw PLLA/PDLA melts is challenging. Therefore, it is interesting to find effect ways to form SC crystallites. We summarize the research progress in formation of SC by means of physical blending and chemical routes. Moreover, the crystalline morphology and kinetics of the stereocomplexation are also discussed briefly.

4.1 Stereocomplexation via physical blending route

Since SC-PLA show rather good performance in comparison with neat PLLA and PDLA (Natureworks LLC., USA) (78, 86, 87), ample studies have been carried out to achieve high SC crystallites content by the physical blending route which is feasible in industry (85, 88). To date, solution and melt blending are still mainly applied to generate SC-PLA which, however, can also be obtained via low-temperature approach, supercritical fluids and gelation in ionic liquid (80, 88–114).

Solution blending was initially used to prepare SC-PLA. Tsuji et al. reported that the SC crystallites are formed upon mechanical blending of PDLA and PLLA. They studied the gelation during stereocomplexation in concentrated solutions (89, 90). The effect of Mw , solvent and the blending ratio of the isomers on the crystallization of PDLA and PLLA has been discussed (90). The results suggest that, as long as the Mw of one component in the PDLA/PLLA blends is low enough, racemic crystallites are preferentially formed without any HC crystallites.

Authors from the same group also investigated the stereocomplex PLA formation through drying process and from the melt, respectively (91, 92). In their study the effects of mixing ratio and Mw of the isomers, and the annealing time and temperature on the formation of stereocomplex were discussed. It was found that SC and HC crystallites were formed simultaneously when the mixing ratio of the PLLA/PDLA melt deviated from equimolar blending or the Mw of the PLA was higher than l05 g/mol. As stereocomplexation was disturbed when the Mw of PLA increase to 105 g/mol, Tsuji et al. prepared SC-PLA by repeated casting with different solvents (80). The DSC and WAXD results showed that the crystallinity of SC crystallites increased while the crystallinity of HC crystallites decreased with increasing the casting numbers, indicating that multi-casting would be a promising method to promote stereocomplexation.

The difficulty in stereocomplexation of PLA from the melt mainly results from the high viscosity of high-Mw PLLA and PDLA leading to low mobility of the macromolecules and the poor memory to reform SC after complete melting. Accordingly, plasticizers and nucleating agents were both used to promote the stereocomplexation of PLA. Bao et al. reported that the formation of SC crystallites from high-Mw PLLA/PDLA blends from the melt by using poly(ethylene glycol) (PEG) as a plasticizer (93). PEG could increase the segmental mobility of the polymer chains which facilitated PDLA to confront with PLLA and enhances the interaction between PLLA and PDLA macromolecules. The PEG with Mw of 2,000 g/mol showed better effect on forming stereocomplex PLA, which is more evident when the PEG content is increased to 10 wt%. SC-PLA was obtained with the assistance of making nanocomposites via melt blending in the presence of MWCNTs which acted as a nucleating agent to promote the formation of SC-PLA (94). Very recently, it was found that the stereocomplexation of PLA from the melt could also be significantly enhanced by using nanocrystal cellulose (NCC) as nuclei sites and the enhancement was more pronounced at high(er) NCC loading (25 wt%) (95). It can be concluded from above discussion that plasticizers could increase the segmental/chain mobility while nucleating agents could promote the formation of SC, thus both of them are beneficial to the stereocomplexation.

PLA stereocomplex nanofibers were obtained by electron spinning of PLLA/PDLA solution with high-Mw (96). The high voltage during electron spinning enhanced the development of SC crystallites and suppressed the formation HC crystallites. SC-PLA can also be achieved by melt spinning of PLLA/PDLA blends at high speed and different blend ratios (97, 98). It was reported that the as-spun PLA fibers (PDLA content of ≤ 16.4 wt%) had only HC crystallites, whereas, the high-speed spun fiber from the PLLA/PDLA=50/50 blend under high spinline tension showed a certain amount of SC crystallites, and highly oriented SC crystallites were mainly obtained after annealing. Furuhashi et al. obtained SC-PLA fibers by melt spinning of PLLA/PDLA blend followed by drawing and annealing processes (99). Fibers drawn at various temperatures exhibited either amorphous, highly oriented HC crystallites or the mixture of poorly oriented HC and SC crystallites. Annealing of the drawn fibers above the melting temperature of HC crystallites increased the SC content significantly. The fractions of the HC and SC crystallites strongly depended on the higher-order structure of the drawn fibers and the annealing temperatures.

In addition to solution and melting blending, Bao et al. prepared high-Mw SC-PLA at much lower temperatures compared with general melt processing (112). Exclusive stereocomplex crystallites without HC crystallites of high-Mw PLA (white powders) were obtained upon processing at low temperatures such as 160°C. Another effective method to make SC-PLA from high-Mw PLA, i.e. using supercritical fluids, was developed by Purnama et al. (113) In this study, they used the supercritical carbon dioxide dichloromethane to process PLA (350 bar and 65°C for 5 h) and SC-PLA was successfully obtained. In addition, stereocomplexation from high-Mw PLLA and PDLA could be obtained via gelation in an ionic liquid as well (114).

4.2 Stereocomplexation via chemical-physical route

Besides the pure physical approach to SC-PLA, chemical modification in combination with physical routes was also applied to fabricate SC-PLA. Shao et al. synthesized 3-armed poly(lactide) (3PLA) initiated with glycerol and then prepared SC crystallites by solution blending the 3PLA with linear enantiomer, as shown in Figure 7 (100). The stereocomplex crystallites could be generated effectively when the Mw of linear PLA was comparable to that of each branch of the 3PLA enantiomers, leading to a melting temperature of SC (Tm-sc) up to 246°C without formation of HC crystallites. The authors also demonstrated that blending 3PLA with linear enantiomer is a better way to fabricate SC-PLA than blending 3PLLA with 3PDLA due to the higher mobility of linear PLA.

Figure 7: The routes of preparation of stereocomplex crystallites based on 3-armed poly(lactide). [Reprinted with permission from Ref. (100). Copyright 2012 America chemical society.]

Figure 7:

The routes of preparation of stereocomplex crystallites based on 3-armed poly(lactide). [Reprinted with permission from Ref. (100). Copyright 2012 America chemical society.]

A melt-processable SC-PLA was prepared by Ramy-Ratiarison et al. (101) The (L- or D-) PLA pre-polymers were first obtained by ring opening polymerization of L- or D-lactide initiated with 4,4-diaminodiphenylmethane using Sn(Oct)2 as a catalyst. Then, a solvent-free reactive extrusion was employed to obtain melt-processable SC-PLA. Othman et al. synthesized nearly mono-dispersed di-block copolymers of DL-lactide, D-lactide and L-lactide with a dinuclear indium catalyst (102). In their study, SC crystallites were formed in the presence of PLLA-b-PDLA di-blocks, whereas the SC crystallites from the di-block copolymers showed lower Tm-sc (<220°C) and melt enthalpy in comparison with those from linear PLLA/PDLA blends.

Hirata et al. studied the thermomechanical properties of stereo-block PLA with different PLLA/PDLA block compositions (103). In this study, SC-PLA was synthesized by solid-state polycondensation (SSP) of medium Mw pre-polymers: both PLLA and PDLA had primarily been prepared by melt-polycondensation. The dynamic mechanical analysis revealed that the SC-PLA films have much higher heat resistance than neat PLLA materials, e.g. the storage moduli (E′) of SC-PLA films retained at a level of 106–107 Pa in the temperature range of 185–200°C.

In addition, amphiphilic conetworks and gels were synthesized via stereocomplexation of PLA between acrylic-PEG-PDLA and acrylic-PEG-PLLA copolymers (104), as is shown in Figure 8. It was found that the amphiphilic conetworks were stronger with SC-PLA and the strength increased with increasing the amount of SC-PLA. Bai et al. reported a solid-state cross-linking strategy to enhance the melt stability of SC-PLA during a melt-blending process (105). The stereocomplexation in the continuous melting and recrystallization processes is found to be perfectly reversible without the formation of any HC crystallites.

Figure 8: Illustration of A-PEG-PLLA/A-PEG-PDLA conetwork formation via stereocomplexation-induced cross-link. [Reprinted with permission from Ref. (104). Copyright 2013 America chemical society.]

Figure 8:

Illustration of A-PEG-PLLA/A-PEG-PDLA conetwork formation via stereocomplexation-induced cross-link. [Reprinted with permission from Ref. (104). Copyright 2013 America chemical society.]

With the development of nanotechnology, advanced stereocomplex-based materials have been exploited by combining stereocomplexation and nanocomposite approaches. Stereocomplex PLA/CNTs nanocomposites were prepared by solution mixing of PLLA with CNT-g-PDLA which was synthesized via ring opening polymerization of D-lactide from the surface of modified CNTs (106). The solution-casted PLLA/CNT-g-PDLA samples exhibited lower crystallization activation energy and higher SC-PLA crystallinity compared to the linear PDLA/PLLA system. Brzezinski et al. reported that the MWCNTs with organic spacers terminated with -OH groups were used as initiators for ring opening polymerization of L-lactide and D-lactide, respectively, resulting in MWCNT-g-PLA (88). The MWCNT-g-PLA and liner PLA were used to prepare SC-PLA. The equimolar PLLA/PDLA mixtures in the presence of MWCNT-g-PLA were preferentially crystallized in the form of stereocomplex even though the final MWCNT concentration was as low as 1 wt%. Moreover, the crystallization in the form of stereocomplex after melting was, unusually, completely reversible, without the formation of any HC crystallites. The synergetic effect of cellulosic nanowhiskers (CNW) particles and the graft structure from PDLA-g-CNW and PLLA-g-CNW contributing to stereocomplex formation in solution was researched by Purnama et al. (107). Their results showed that the graft structure as a core of PLA nanocomposites enhanced the stereocomplex formation in solution, and the generated SC nanocomposite showed stereocomplex memory. The unzipped fragments of SC-PLA/CNW composites can easily re-form the stereocomplex after melting. Sun also reported stereocomplex crystallization by blending of commercial PLLA with graphene oxide-grafted-PDLA (GO-g-PDLA), as schematically shown in Figure 9 (108). The incorporation of GO led to a lower activation energy of stereocomplexation and a higher SC crystallinity in comparison with linear PDLA/PLLA system due to the nucleation effect of the well-dispersed GO sheets. However, the crystallinity of HC crystallites was low in the cold crystallized samples due to the restriction of exfoliated GO sheets on the chain mobility and the crystal growth.

Figure 9: (A) Preparation routes of GO-g-PDLA and (B) the routes of the preparation of SC-PLA. [Reprinted with permission from Ref. (108). Copyright 2012 America chemical society.]

Figure 9:

(A) Preparation routes of GO-g-PDLA and (B) the routes of the preparation of SC-PLA. [Reprinted with permission from Ref. (108). Copyright 2012 America chemical society.]

4.3 Crystalline kinetics and morphology of stereocomplex PLA

Compared with PLA the crystalline kinetics and morphology of stereocomplex PLA are less understood. In PLLA/PDLA blends, the crystallization of PLLA homochiral crystallites is affected by the PDLA content and a competition exists between the HC and the SC crystallization of PLLA/PDLA (79). Crystallization kinetic study revealed that the SC crystallites of PLA with low-Mw had higher crystal growth rate and shorter induction period than those of the HC crystallites (PLLA and PDLA), and the nuclei density of the SC crystallites was also much higher (92). In addition, the temperature range of spherulite growth of SC crystallites is wider than that of HC crystallites. The researchers studied the nucleation constant (Kg) and the front constant (G0) of PLLA, PDLA, and PLLA/PDLA films via Hoffman nucleation theory. The G0 value of SC-PLA was higher than those of the HC crystallites, whereas the Kg value of SC crystallites was around one time higher than that of the HC crystallites of PLLA (4.95×105 K2) and PDLA (4.20×105 K2), respectively. Tsuji et al. also studied the crystallization behavior of SC-PLA in a mixture of high-Mw PLLA and PDLA, and found that the high-Mw PLLA and PDLA crystallized synchronously and separately into α′- or α-form HC crystallites rather than SC crystallites in the temperature range of 90~130°C (109). The spherulitic morphology was disturbed due to the synchronous and separate crystallization of PLLA and PDLA and the coexistence of HC crystallites, e.g. the spherulite growth rate (G) of the blends was decreased. Recently, Ma et al. found that the G of SC-PLA was increased by the incorporation of NCC (91). The NCC obviously enhanced the stereocomplexation kinetics, e.g. Kg values were increased by 30–80% in the regimes II and III while the II-III regime transition temperature (Ttr) was increased by 10°C in the presence of 25 wt% of the NCC.

Fujita et al. reported the structural changes of the sedimented single-crystal mats of PLLA and PDLA upon heating by using synchrotron small-angle X-ray scattering (SAXS) (110). The binary mixture of PLLA and PDLA single crystals formed SC crystallites essentially around 150°C and reorganized at around 180°C. The dominant orientation of the reorganized SC crystals was the same as that of the original crystals, and the SC lamellar thickening resulted from partial melting and recrystallization.

The crystallization kinetics and crystal morphology of SC-PLA through melt recrystallization of HC crystals was investigated by Na et al. (111). In their study, they found that the stereocomplexation from small α′-form HC crystals is easier and faster than that from large α-form HC crystals. Moreover, rod-like SC crystals were monitored in the melt recrystallization because of the high nucleation density.

Although considerable academic work has been devoted to the preparation, kinetics and morphology of stereocomplex PLA, the fabrication of SC-PLA on an industrial scale and its practical application are still challenging. Therefore, more work is required to further understand the kinetics of stereocomplexation, consequently, to develop more effect technology to broaden the application of the SC-PLA.

5 Conclusions and outlook

Poly(lactide) (PLA), as one of the most promising biobased and biocompostable polymeric materials, has been receiving increasing attention due to its excellent performances in renewability, biodegradability, biocompatibility and mechanical properties. With an increase in environmental and sustainable concerns associated with conventional petrochemical-based polymers and the broad investigation of biodegradable materials, the applications of PLA and PLA-based materials in medicine, packaging materials, building materials, daily necessities, etc. will only increase.

From a crystallization point of view, the crystallization behavior of PLA is poor and the crystal growth rate is extremely low due to its short monomer length. Therefore, the crystallization was enhanced by adding nucleating agents to increase its nuclei density. The improvement in nucleation can be obtained by using inorganic nucleating agents (talc, clay and CNT, etc.), organic nucleating agents (EBHSA, TMC and oxalamide compounds, etc.) and macromolecular nucleating agents (cellulose, starch and SC-PLA, etc.).

In the meantime, stereocomplexation is one of the most effective and promising methods being applied to improve the heat resistance, mechanical performance and crystalline properties of PLA. Consequently, many studies are trying to achieve high efficiency in the formation of stereocomplex (SC) crystallites. The formation of SC-PLA by physical and chemical methods was reviewed in this paper. In addition, the crystalline morphology and crystallization kinetics of SC-PLA were also discussed.

Biobased and biodegradable PLA has received increasing attention. However, the PLA technology still needs to be improved in terms of synthesis routes, production cost and high-temperature performance due to a lack of crystallinity after processing. Therefore, the following research topics might be interesting, e.g. synthesis of high-Mw PLA by novel and green(er) method and reduction of the production cost of PLA. From the crystallization point of view, novel and efficient nucleating agents should be developed to further increase the crystallization rate and the crystallinity of PLA in processing. SC-PLA has better thermal and mechanical properties, and higher hydrolytic stability than PLLA and PDLA. The processing method and conditions, nucleating agent, and the Mw of PLA have strong effects on the stereocomplexation process, thus it is of importance to develop advanced technology to obtain high content of SC. In addition, PLA modified with new materials to meet the different application requirements is another research interest to be developed.


Corresponding authors: Piming Ma and Mingqing Chen, The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, 1800 Lihu Road, Wuxi 214122, China, Tel.: 0086 510 85917019, e-mail: (Piming Ma); (Mingqing Chen)

Acknowledgments

This work is supported by the National Natural Science Foundation of China (51573074, 51303067), the Natural Science Foundation of Jiangsu Province (BK20130147) and the Fundamental Research Funds for the Central Universities (JUSRP51408B).

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Received: 2015-7-12
Accepted: 2015-8-27
Published Online: 2015-9-25
Published in Print: 2016-1-1

©2016 by De Gruyter