Article pubs.acs.org/Macromolecules
Unusual Thermal Properties of Polylactides and Polylactide Stereocomplexes Containing Polylactide-Functionalized MultiWalled Carbon Nanotubes M. Brzeziński,† M. Bogusławska,† M. Ilčíková,‡ J. Mosnácě k,‡ and T. Biela†,* †
Department of Polymer Chemistry Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Lodz, Poland Polymer Institute, Centre of Excellence FUN-MAT, Slovak Academy of Sciences, Dúbravská Cesta 9, 845 41 Bratislava, Slovakia
‡
ABSTRACT: A novel and simple method for the preparation of linear, high-molar-mass, thermally stable polylactide stereocomplexes (sc-PLA) with multiwalled carbon nanotubes (MWCNTs) covalently attached to enantiomeric PLA chains is presented. The MWCNTs modified with organic spacers terminated with −OH groups were used as initiators for the ring-opening polymerization (ROP) of enantiomeric L- and Dlactides. The weight percentage of MWCNTs in the final stereocomplexes was approximately 1 wt %. The nascent (aspolymerized) polymer samples obtained by the ROP in bulk at 130 °C exhibited unusually high melting temperatures (≥190 °C) as well as enthalpies of fusion close to that estimated for polylactides with 100% crystallinity. PLA stereocomplexes were prepared from equimolar mixtures of L-PLA and D-PLA that also contained MWCNT-g-PLA, either by precipitation from solution or, for the first time, as a thin film via the slow evaporation of solvent. Crystallization in the form of stereocomplexes after melting was, unusually, completely reversible, without formation of any homochiral crystallites. Shish-kebab morphologies for the thin-film stereocomplexes containing 1 wt % of MWCNT-g-PLA were observed by atomic force microscopy.
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INTRODUCTION
thermally stable stereocomplexes based on the linear highmolar-mass polylactides is still a challenge. Recently, the unusual properties of carbon nanotubes, such as their high aspect ratio, their high mechanical strength and their high thermal and electrical conductivity, have attracted the attention of researchers.20,21 Multiwalled carbon nanotubes (MWCNTs) are also generally known as good nucleation agents and have been used as additives to reinforce the mechanical strength of various composite materials.22−29 The dispersity of crude MWCNTs in a polymer matrix, without any pretreatment, is limited and cannot effectively improve the properties of the polymer materials. The key factors in attaining the reinforcing effect of the MWCNTs are avoiding aggregation of the nanotubes and enhancing the nanotube−matrix interactions.25,29−31 The physical or covalent modification of the surface of MWCNTs is typically used to improve their dispersion in polymer matrices.32,33 In the present work, covalently modified MWCNTs were used as initiators in the ring-opening polymerization of L- and D-lactides to induce efficient dispersion of the MWCNTs in the PLA matrix. Moreover, the thermal properties of the prepared PLA/MWCNTs composites and their stereocomplexes were investigated.
The improvement of physicochemical properties of PLA, such as its thermal and mechanical properties, is a goal of many research groups for use in mass applications. PLA possesses many desirable properties: it is a biodegradable, biocompatible, nonvolatile, odorless polymer, and it is classified as GRAS (generally recognized as safe) by the US Food and Drug Administration. However, in comparison with other commercial thermoplastics, it exhibits a low rate of crystallization and a relatively low melting temperature. The enhancement of the thermal stability of PLA is important for the application of this material in various industrial applications.1,2 One of the most important and relatively easy method for improving the thermal properties of PLAs is stereocomplexation. PLA stereocomplexes usually form in solid-state crystalline structures that melt at higher temperatures (Tm) than their homochiral components alone.3−17 However, linear high-molar-mass (Mn = 105) polylactide stereocomplexes are not able to survive melting to reform the stereocomplex crystallites, and, in the second heating of the DSC run, after slow cooling from the melt, a mixture composed of homochiral crystallites and stereocomplex crystallites is formed. Recently, our group has shown that, in the instance of star-shaped highmolar-mass (Mn ≥ 105) enantiomeric PLAs with more than six arms, the formation of the PLA stereocomplex in the melt is complete and perfectly reversible because of the hardlock-type interactions.18,19 However, the successful preparation of © 2012 American Chemical Society
Received: July 26, 2012 Revised: September 4, 2012 Published: October 18, 2012 8714
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MWCNTs in a vial were sealed to the glass reaction vessel (∼10 mL). Under vacuum, the break-seals and vial were broken, and all components were mixed at room temperature. THF was removed by distillation, and then the reaction vessel was sealed. The reaction vessel was placed into an ultrasonic bath for 60 min (130 °C) to disperse MWCNT−OH and then placed into an oil bath thermostated at 130 °C for approximately 24 h. A small portion of the obtained polymer was analyzed separately, and the rest was dissolved in CHCl3 and precipitated into methanol. The mass of the vacuum-dried product was 6.31 g (85% yield). Stereocomplex Preparation. The synthesized and precipitated LPLA and D-PLA that contained PLA-functionalized MWCNTs were used in the preparation of stereocomplexes. Samples of 0.5 g of the LPLA and D-PLA were dissolved separately in 50 mL of chloroform. Then, the solutions were mixed together and stirred using a magnetic stirrer; eventually, to achieve better dispersion of the MWCNTs, an ultrasonic bath was also employed. The resulting mixture was divided into two portions. The first portion was precipitated into methanol, and the second one was poured into a Petri dish to allow solvent evaporation and to form the stereocomplex thin film with a thickness of ∼0.25 mm. The samples of the precipitated stereocomplex and thinfilm stereocomplex were uniformly gray and black, respectively. The samples were dried under vacuum for 24 h before they were investigated further. It is worth to mention that in chloroform (or methylene chloride) the mixture of enantiomeric high molecular weight PLAs, namely LPLA and D-PLA do not precipitate in the form of stereocomplex and it is stable for a long time. After the stereocomplex is formed (e.g., by precipitation to methanol), it is insoluble in chloroform and methylene chloride. Measurements. The SEC−MALLS instrument was composed of an Agilent 1100 isocratic pump, an autosampler, a degasser, a thermostatic box for columns, a MALLS DAWN EOS photometer (Wyatt Technology Corporation, Santa Barbara, CA) and a Optilab Rex differential refractometer. The ASTRA 4.90.07 software package (Wyatt Technology Corporation) was used for data collection and processing. Two PLGel 5-μm MIXD-C columns were used for separation. The samples were injected as a solution in methylene chloride. The volume of the injection loop was 100 μL. Methylene chloride was used as a mobile phase at a flow rate of 0.8 mL·min−1. The calibration of the DAWN EOS was performed using p.a. grade toluene, and normalization was performed using a polystyrene standard (Mn = 30 000 g/mol). The measurements were conducted at room temperature. The dn/dc increment of the refractive index was 0.035, as was already established in our laboratory for PLA. DSC analysis was performed on a TA Instruments Modulated DSC 2920 at a heating and cooling rate of 10 °C/min under a nitrogen atmosphere. The measurements were made in one DSC nonstop cycle. Both the temperature and the heat flow were calibrated using indium. FTIR spectra were collected on a Nicolet 6700 spectrometer equipped with a DGTS detector. The technique of attenuated total reflectance (ATR) was used for IR measurements. The spectra were obtained by adding 64 scans at a resolution of 2 cm−1. The morphologies of the thin films were investigated via atomic force microscopy (AFM) using a Veeco Nanoscope IIIa operated in tapping mode.
EXPERIMENTAL PART
Materials. Tin(II) octoate [(2-ethylhexanoate):Sn(Oct)2, SigmaAldrich] was purified by two consecutive high-vacuum distillations at 140 °C/3 ·10−3 mbar. During the second distillation, Sn(Oct)2 was distributed directly into the thin-walled vials or ampules equipped with break-seals, then sealed off and stored at −12 °C. (L,L)-Lactide (L-LA, Boehringer Ingelheim, Germany) and (D,D)-lactide (D-LA, 99%, Purac, The Netherlands) were crystallized consecutively from dry 2-propanol and toluene and were further purified just before use by sublimation in vacuo (10−3 mbar, 85 °C). Tetrahydrofuran (THF, 99%, POCH, Gliwice, Poland) was stored for several days over KOH pellets and was subsequently filtered and refluxed over Na metal. The THF was subsequently distilled under vacuum at RT and stored over liquid Na/ K alloy, at which time it developed a blue color. Methylene chloride (CH2Cl2, 99%, POCH, Gliwice, Poland) was dried with calcium chloride and distilled before use. Methanol (pure p.a. grade, POCH, Gliwice, Poland) was used as received. Chloroform (CHCl3, pure p. a. grade, POCH, Gliwice, Poland) was refluxed over P2O5 and then distilled out before use. Two types of pristine multiwalled carbon nanotubes (MWCNTs) were used: MWCNTs with an average diameter of 9.5 nm, an average length of 1.5 μm and a specific surface area of 250−300 m2/g (carbon purity 90%, NANOCYLTM NC7000, Nanocyl S.A., Belgium) and MWCNTs with an outside diameter of 60−100 nm, a length of 5−15 μm and a specific surface area of 40−600 m2/g (NanoAmor, purity >95%, Nanostructured & Amorphous Materials, Houston, TX). Modification of MWCNTs. Modification of MWCNTs was performed according to the previously published procedure.34 MWCNTs (4 g) and 4-aminophenethyl alcohol (22 g) were placed in a 250 mL three-neck round-bottom flask equipped with a stir bar and a condenser. The reaction flask was evacuated and backfilled with argon three times. Then, 89 mL of isoamyl nitrite purged with argon was added. The reaction flask was placed in an oil bath that was preheated to 60 °C. The reaction mixture was stirred at this temperature for 3 h; after approximately 1 h, 20 mL of degassed THF was added and the resulting mixture was sonicated for 30 min to enable continuous stirring of the progressively thickening dispersion. The reaction mixture was then filtered through a 0.2 μm PTFE membrane and was washed with acetone. The filter cake was sonicated in 200 mL of DMF for 10 min, filtered and washed with acetone until the filtrate was clear; this process was repeated three times. Finally, the filtered cake was washed with 100 mL of diethyl ether and dried overnight in a vacuum oven at 60 °C. The weight of the obtained MWCNT−OH was 4.25 g. The MWCNT−OH-7 and MWCNT− OH-21, which contained 7 and 21 wt % of 4-(2-hydroxyethyl)phenyl groups covalently bound on the MWCNTs surface, respectively, were obtained after modification of the NanoAmor and Nanocyl MWCNTs, respectively. The modification percentage was determined through TGA analysis. Polymerization. The linear L-PLA and D-PLA enantiomeric (homochiral) polymers were synthesized according to the known procedure35 employing a ring-opening polymerization (ROP) of the corresponding L- and D-LA, respectively. Specifically, L-LA or D-LA monomers were polymerized in bulk at 130 °C with Sn(Oct)2/ MWCNT−OH as the catalytic/initiating system. Polymerization was conducted below the melting temperature of PLA and above the melting temperature of LA so the polymerization proceeded at the beginning in the “solution” of the monomer. The viscosity of polymerization mixture increased during the process relatively fast and after the about 1 h became black solid. Probably, the polymerization further proceeded in the solid phase. The polymerizing mixture was prepared in sealed glass ampules using standard high-vacuum techniques. A general procedure was as follows: Sn(Oct)2 (1 mL of 0.25 mol·L−1 solution in dry THF) and (LLA, 7.42 g, 51.5 mmol) were transferred under vacuum into breakseals and then sealed after being frozen in liquid N2. MWCNT−OH (74 mg) was placed directly into the reaction ampule, dried under vacuum for 4 h and sealed. Break-seals that contained the Sn(Oct)2/ THF solution and L-LA monomer and a tube that contained dry
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RESULTS AND DISCUSSION Preparation of MWCNT-g-PLA. The primary objective of this paper was to explore the influence of MWCNTs covalently attached to enantiomeric PLA chains with respect to their stereocomplexation and to explore the properties of the obtained stereocomplex materials. As previously discussed, multiwalled carbon nanotubes are generally known as good nucleation agents and as additives that reinforce the mechanical strength of various composite materials.22−29 The use of carbon 8715
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Scheme 1. ROP of LA in the Presence of MWCNT−OH
Table 1. Labels for the Synthesized PLAs and the MWCNT-g-PLA/PLA Composites and the Molar Characteristics of the Resulting PLAs GPCb RIc
MALLS
monomer
type of MWCNT
Mn
Mw/Mn
Mn
D-PLA-7
D-LA
L-PLA-7
L-LA
D-PLA-21
D-LA
L-PLA-21 a D-PLA a L-PLA
L-LA
MWCNT−OH-7 MWCNT−OH-7 MWCNT−OH-21 MWCNT−OH-21 − −
142 200 139 700 127 500 114 300 108 700 87 000
1.85 1.93 1.92 2.00 1.82 1.91
103 600 91 000 80 000 82 500 74 000 76 000
label
D-LA L-LA
a
Linear PLA synthesized in the absence of MWCNT−OH. bPLA not attached on MWCNT surface separated during SEC analysis. cCalibration on polystyrene standards.
Table 1). On the basis of previous studies of “grafting from” polymerization techniques and under the assumption that hydrolysis and transesterification reactions did not occur to any significant extent, the average molar mass of a single PLA chain attached to MWCNTs and that of the free of nanotubes should be identical.33 As a consequence of the described analysis, the product of polymerization of the lactides in the presence of MWCNT−OH should be considered as a composite of two materials: MWCNT-g-PLA and free PLA. Thermal Properties of a “Nascent” MWCNT-g-PLA/ PLA. Immediately after the polymerization process, a small amount of the nascent, as-polymerized MWCNT-g-PLA/PLA was used for the DSC analyses. Surprisingly, these nascent PLA samples with a fraction of the PLA chains attached to the MWCNTs exhibited unusual thermal properties (Figure 1 and Table 2). In the first heating scans, the DSC traces showed higher melting temperatures of 184−191 °C and significantly higher enthalpies of fusion of 82−101 J/g (close to the equilibrium enthalpy of fusion reported in the literature43) than are the values obtained from the second heating scans of the same samples after recrystallization from the melt. In the DSC traces of all of the analyzed MWCNT-g-PLA/PLA composites, only one sharp melting peak at high temperature was observed. These results indicate a high crystallinity and uniformity of the samples. Similar thermal behaviors are well-known in the literature for ultrahigh-molecular-weight polyethylene44 and other polyolefins45 and have also been observed by Grijpma et al.46 for PLAs obtained through the use of various initiators at a polymerization temperature as low as 110 °C. The process used for the preparation of the MWCNT−OH is known to lead to a high-density modification of the external surface. Similarly, the technique of grafting from polymerization has been reported to lead to a high grafting density of the polymer chains when living/control polymerizations are used.33 Moreover, when the polymer chains grow simultaneously and the rate of growth of the polymer chains is lower or similar to
nanotubes to improve PLA properties in this has, until now, been limited.36−42 The linear optically active L-PLA and D -PLA were synthesized employing ROP of L- and D-LA, respectively, in the presence of 1 wt % MWCNT−OH and using Sn(Oct)2 as a catalyst (see Scheme 1). Two types of MWCNT−OH were used: 60−100 nm thick MWCNT−OH-7 and 9.5 nm thick MWCNT−OH-21, in which the 4-(2-hydroxyethyl)phenyl groups covalently attached to the surface of the MWCNTs comprised 7 and 21 wt %, respectively. The total concentration of MWCNT−OH in the polymerization mixtures was 1 wt %. The polymerizations were performed at 130 °C, i.e., above the melting temperature of LA but well below the melting temperature of PLA, to avoid degradation and racemization processes. The use of surfacemodified carbon nanotubes (MWCNT−OH) and an ultrasonic bath allowed a polymerization mixture with highly dispersed MWCNTs to be obtained. A small portion of the prepared MWCNT-g-PLA was used to determine the molar mass of PLA. The SEC analyses required the filtration of very dilute sample solutions through a dense (0.25 μm) chromatographic filter. When the dn/dc index of the analyzed polymer is known (in this case, dn/dc = 0.035), the ASTRA software is able to calculate both the molar mass and the mass of the polymer sample that was actually injected into the columns. In addition, given the initial mass of the sample taken for a chromatographic analysis, we were able to calculate the fraction of PLA that remained on the filter, which was assumed to be attached to the surface of the MWCNTs. Only approximately 15−20 wt % of the PLA chains were found to be polymerized from the MWCNTs surface. It means that the major fraction of PLA was not attached to the MWCNTs surface and was most likely obtained after initiation by traces of water (present in the Sn(Oct)2 catalyst or in the MWCNTs). The molar masses (Mn) of the free PLAs, as determined by SEC in methylene chloride, were in the range of 80 000 to 104 000 g/mol (see 8716
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with lower melting temperatures and lower degree of crystallinity. To investigate this phenomenon, the polymerization of the L-LA with only Sn(Oct)2 without MWCNT−OH was performed at 130 °C. For the obtained nascent L-PLA, the DSC measurements showed the same thermal behavior, with a high melting temperature and a high enthalpy of fusion in the first heating run, as was described for nascent MWCNT-gPLA/PLA (Table 2). We therefore concluded that the key factor that affects the morphology of the PLAs and, as a consequence, their thermal properties, is most likely the relationship between the rate of polymerization and the rate of crystallization. Both the rate of polymerization and the rate of crystallization are determined by the polymerization temperature and, in our case, the relationship seemed to be almost independent of both the presence and the degree of modification of the MWCNTs. The high degree of crystallinity and the formation of perfect crystallites in both nascent PLA fractions (those attached and not attached to MWCNTs) suggested that only one well-defined melting peak was observed at high temperatures. However, two fractions of the PLAs those attached and not attached to MWCNTsare present in the polymerization mixtures, and two different melting temperatures were expected. The thermal properties of the nascent PLAs were correlated with the ATR-FT-IR analyses. As shown in Figure 2 in the
Figure 1. DSC traces of nascent D-PLA-7 during the first heating run (solid line) and during the second heating run after melting and recrystallization (dashed line). The sample was heated from 0 to 220 °C and then was cooled down to 0 °C, kept for 1 min at 0 °C and afterward was heated to 220 °C again. The rate of the heating and cooling run was 10 °C/min.
Table 2. Thermal Parameters of Nascent PLAs in the First Heating Run and in the Second Heating Run after Melting and Recrystallizationa Tm [°C] label D-PLA-7 D-PLA-7
(II) L-PLA-21 D-PLA-21 L-PLA a
ΔHm [J/g]
amount of MWCNTs [wt %]
1st run
2nd run
1st run
2nd run
1 1 1 1 −
190.9 191.1 184.9 189.4 184.1
178.3 178.6 179.1 178.0 169.3
101.3 94.2 82.0 82.6 84.9
44.9 46.8 44.6 40.1 47.2
The polymerization temperature was 130 °C.
the crystallization rates of the PLA chains, a nascent morphology without entanglements and with extended rigid chain crystallites can be favorable, as was proposed by Lemstra et al. for polyolefins.44,45 Jong et al.47 have reported that the minimal length of enantiomeric PLA chains for initiation of their crystallization is 11 lactide units. Thus, if the crystallization process is fast, crystallization during the polymerization prevents the formation of free (noncrystallized) PLA chains that are sufficiently long to become entangled. However, it should be taken into account that in the present study, the major fraction of PLA was not attached to the surface of the MWCNTs, and the influence of this fraction on the total properties of the PLA can predominate. To explain the thermal properties of the nascent PLAs prepared at 130 °C, the suggestions of Lemstra et al. with respect to polyolefins could also be applied: When the concentration of active centers in the polymerization mixture is low and when the distance between them is sufficiently large (which occurs in our case when the most of the grown PLA chains are not attached to MWCNTs), the formed PLA chains can be considered as being separated from each other. Thus, the macromolecules may crystallize as folded-chain lamellae without entanglements in the amorphous phase. In this case, again the only requirement is a rate of crystallization that is higher or comparable to the polymerization rate.44 However, after the PLAs melt and recrystallize from the melt, the entanglement density of both polymer fractions (attached and not attached to MWCNTs) is increased,48,49 which leads to different morphologies of PLAs
Figure 2. ATR FT-IR spectra of the skeletal stretching and the −CH3 rocking regions at 970−850 cm−1 for nascent D-PLA-7(II) () and DPLA-7(II) after its recrystallization from the melt in the second DSC run (----). DSC conditions as in Figure 1. Thermal properties of DPLA-7(II) are shown in Table 2.
skeletal stretching region of the PLA chain, a band at 921 cm−1 was observed, which, according to Ozaki,50 is related to the α helix (103). The intensity of this band depends on the degree of crystallization of the PLA. For the nascent PLAs that exhibited a high enthalpy of melting, as determined by DSC, the intensity of the α band was significantly greater than that of the PLAs recrystallized from the melt (see Figure 2). This observation indicated that, after the FT-IR instrument was calibrated, the resulting FT-IR spectra could also be used to estimate the degree of PLA crystallization. Thermal Properties of the Stereocomplexes of MWCNT-g-PLA/PLAs. DSC traces from the precipitated MWCNT-g-PLA/PLAs showed that presence of PLA-functionalized MWCNTs did not significantly influence the Tm and ΔHm, values, which were approximately the same as those for 8717
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the skeletal stretching and −CH3 rocking vibrations in the 970−850 cm−1 region for the L-PLA-21 thin film and for the scPLA-21-f stereocomplex thin film. Two distinguishable bands appeared in the CO stretching vibration region of enantiomeric PLA, whereas only one band was observed in the spectrum of the PLA stereocomplex. In the second region, the band at 921 cm−1, which was assigned to the α helix of an enantiomeric PLA, disappeared, and new band appeared at 908 cm−1, which is characteristic of the stereocomplex (see Figure 4).12 The thermal properties of the stereocomplexes were analyzed by DSC. In Figure 4, the DSC thermograms of the stereocomplexes prepared from L-PLA-21 and D-PLA-21 by coprecipitation (sc-PLA-21-p) and as a thin film (sc-PLA-21f(a)) are shown. Only melting peaks characteristic of stereocomplexes were observed in the first as well as in the second DSC heating runs for both complexes. To the best of our knowledge, these results represent the first observation of the formation of thermally stable, linear, high-molar-mass stereocomplexes from enantiomeric PLA homopolymers. These results are especially unusual for thin-film stereocomplexes. The preparation of pure stereocomplex films was not previously possible via casting from solution. Usually, a mixture of stereocomplex and homocrystallites was formed from a low-molar-mass mixture of L-PLA and D-PLA. Moreover, mixtures of high-molar-mass L-PLA and D-PLA has been previously reported to only form homocrystals.5 This work represents the first observation of the formation of a thermally stable thin film PLA stereocomplex. The thermal properties of the stereocomplex thin films were subsequently analyzed in greater detail. Another thin film from the same materials were prepared sc-PLA-21(b). In the DSC thermograms of these materials, two melting peaks at temperatures characteristic for stereocomplex (226.5 and 239.1 °C) were observed (Figure 5). The presence of two endotherms (enthalpy of melting 66.9 J/g) appears to be related to the presence of crystallites with a different degree of structural ordering. To further study this observation, the stereocomplex film was heated to 235 °C to melt the crystallites related to the first endothermic peak. During the cooling run (data not shown), the crystallization peak at 220 °C was observed. After cooling to room temperature in the second heating run, the sample exhibited only one sharp melting peak at 242 °C with enthalpy of melting 77.4 J/g (Figure 5). The enthalpy of melting was even greater than the enthalpy measured for the original sample in the first heating run. The melted portion of the sc-PLA-21-f(b) material may have crystallized on the surface of the remaining crystallites during the cooling run. The unmelted crystallites may then have served as nucleation sites for the crystallization of the melted crystallites such that all of the crystallites achieved the same degree of structural perfection. For a more detailed investigation of the stereocomplexation, the various enantiomeric PLAs were mixed, and the stereocomplex formation and thermal stability of the stereocomplexes were investigated (Table 4). The following mixtures were prepared: (I) a mixture of enantiomeric L-PLA-21 and D-PLA21, where both contained PLA-grafted MWCNTs; (II) a mixture of L-PLA and D-PLA-21, where only the second one contained PLA-grafted MWCNTs; (III) a mixture of enantiomeric L-PLA and D-PLA, where neither contained PLA-grafted MWCNTs, but MWCNT−OH were added to the mixture; (IV) a mixture of enantiomeric L-PLA and D-PLA,
the pure PLAs (Table 3). Only a small decrease in the Tg value was observed for MWCNT-g-PLA/PLAs in comparison with Table 3. Thermal Properties of the PLAs, the PLA Composites, and the sc-PLA Stereocomplex Composites DSC label D-PLA L-PLA D-PLA-7 L-PLA-7 D-PLA-21 L-PLA-21 sc-PLA-7-p sc-PLA-7-f sc-PLA-21-p sc-PLA-21-f(a) sc-PLA-21-f(b)b
MWCNT content [wt %]
Tg [°C]
Tm [°C]
ΔHm [J/g]
− − 1 1 1 1 1 1 1 1 1
69.1 64.6 61.9 61.3 62.3 61.8 59.7 44.1a 67.4 44.3a 50.1
177.7 176.5 178.8 178.7 178.7 178 226.9 224/238.9 225.4 222.5/237.2 226.5/239.1
45.7 45.1 44.0 43.6 40.9 40.1 51.9 71.6 37.9 51.4 66.9
The lower Tg for the thin films is due to problems with complete removal of the solvent. bOther stereocomplex thin film made from the same L-PLA-21 and D-PLA-21 mixture. a
the Tg values of the pure PLAs. The Tg, Tm, and ΔHm values of the enantiomeric PLAs did not appear to be affected by the type of MWCNTs. The MWCNT-g-L-PLA/L-PLA and the MWCNT-g-D-PLA/ D-PLA obtained by precipitation from solution were further used to prepare stereocomplexes as described above. ATR FTIR spectroscopy was used to confirm the stereocomplexation. Notably, this analytical tool is very useful and fast for analyses of the PLA stereocomplexes. Figure 3 shows the CO stretching vibrations in the 1800−1700 cm−1 region as well as
Figure 3. ATR−FTIR spectra of thin films of enantiomeric L-PLA-21 (dashed line) and stereocomplex sc-PLA-21 (solid line). 8718
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Figure 4. The DSC thermograms of (a) sc-PLA-21-p (precipitated) and (b) sc-PLA-21-f(a) (thin film) stereocomplexes (solid line: first heating run; dash-dot line: cooling run; dashed line: second heating run). The sample was heated from 0 to 250 °C and then cooled down to 0 °C, kept for 1 min at 0 °C, and afterward heated to 250 °C again. The rate of heating and cooling was 10 °C/min.
When enantiomeric L-PLA and D-PLA, which did not contain PLA-grafted MWCNTs, were mixed with either MWCNT− OH or unmodified MWCNTs (entries III and IV in Table 4), the preparation of stereocomplexes by precipitation from the solution was possible. However, the obtained stereocomplexes were not thermally stable. In addition, when thin films were prepared from the same mixtures, approximately equal mixtures of stereocomplex and homocrystallites were obtained. The obtained results showed that the enantiomeric PLAs with and without PLA-grafted MWCNTs differ in their ability of stereospecific crystallization. This observation indicated the significance of the presence of PLA-grafted MWCNTs with respect to stereocomplex formation and thermal stability, even when the amount of such PLA-modified MWCNTs in the final stereocomplex product was as low as 0.5−1 wt %. However, the mechanism by which stereocomplexation operates in the presence of PLA-grafted MWCNTs is not clear, and further studies to address this issue are now in progress. Figure 6 shows
Figure 5. DSC thermograms of a sc-PLA-21-f(b) (__) thin film during the first heating run and of sc-PLA-21-f(b) (---) during the second heating run after being annealed at 235 °C for 2 min and having cooled to room temperature. The rate of heating and cooling was 10 °C/min.
where neither contained PLA-grafted MWCNTs, but unmodified MWCNTs were added to the mixture. As evident from Table 4, the same thermal properties were obtained when D-PLA-21 was mixed with L-PLA enantiomer, i.e., only one enantiomeric polylactide in the stereocomplex contained PLA-grafted MWCNTs (entry II in Table 4); these results are similar to the previously described stereocomplexes prepared from mixtures of enantiomeric L-PLA-21 and D-PLA21. Also in this case, both the precipitated stereocomplex and the thin-film stereocomplex showed the characteristic stereocomplex thermal properties during the second DSC heating run after having melted and slow-cooled.
Figure 6. AFM phase image of the shish-kebab morphology of stereocomplex thin films (sc-PLA-21-f(a)): (a) AFM phase mode and (b) zoomed image of the AFM amplitude mode.
Table 4. Stereocomplex Formation and Thermal Stability of Various Mixtures of Enantiomeric PLAs stereocomplex formation/thermal stability
stereocomplex components
a
entry
poly-L-
poly-D-
type of MWCNTs
MWCNTs content (%)
precipitated
thin film
I II III IV
L-PLA-21
D-PLA-21
L-PLA
D-PLA-21
L-PLA
D-PLA
L-PLA
D-PLA
MWCNT-g-PLAa MWCNT-g-D-PLA MWCNT−OH MWCNT
1 0.5 1 1
yes/yes yes/yes yes/nob yes/nob
yes/yes yes/yes no/no no/no
MWCNT-g-L-PLA/MWCNT-g-D-PLA = 1/1. bThe stereocomplex was obtained, but it was not thermally stable. 8719
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a preliminary result of the AFM image of the stereocomplex thin film (sc-PLA-21-f(a)) obtained by casting from chloroform. An unusual shish-kebab morphology with very thin (
