Plasticizer effect on melt blending of polylactide stereocomplex
-
Yottha Srithep
und
Dutchanee Pholharn
Abstract
Poly(l-lactide) (PLLA)/poly(d-lactide) (50/50) with plasticizer contents ranging from 2% to 16% w/w were prepared by melt blending using an internal mixer. Wide-angle X-ray diffraction, Fourier transform infrared spectroscopy and differential scanning calorimetry results confirmed that complete stereocomplex polylactide crystallites without any homocrystallites were produced. Compared to neat PLLA, the melting temperature of the stereocomplex polylactide and its plasticized samples was approximately 55°C higher. Higher plasticizer contents decreased glass transition temperature of the stereocomplex, which implied higher flexibility and enhanced the crystallization rate. However, the plasticizer in the stereocomplex reduced the thermal stability.
1 Introduction
Polylactic acid (PLA) or polylactide is a biodegradable polymer derived from renewable resources that offers promising alternatives to traditional petroleum-based plastics (1). PLA exists in three isomeric forms: l-lactide (semi-crystalline), d-lactide (semi-crystalline) and dl-lactide (amorphous) (2). Poly(l-lactide) (PLLA) is widely used in many applications, especially in the packaging, biomedical, agricultural and environmental fields due to its high mechanical strength and excellent shaping and molding properties (3). However, it has a very low toughness and elongation at break as well as slow crystallization rate (4, 5).
Stereocomplexation is a promising technique to enhance PLA’s properties by using the strong interaction between PLLA and poly(d-lactide) (PDLA) (3). One of the most unique characteristics of stereocomplex crystallites is their high melting point, e.g. 50°C higher than that of PLLA or PDLA homocrystallites due to hydrogen bonding in the stereocomplex crystalline structure (6). Therefore, the stereocomplex PLA should accordingly possess better thermal and mechanical properties than PLLA or PDLA (7, 8). PLLA and PDLA stereocomplexes can form in solution, in a solid (bulk) state from the melt, during polymerization, or during hydrolytic degradation, as long as l-lactide (or l-lactyl) unit sequence and d-lactide (or d-lactyl) unit sequence coexit in a system (9).
Plasticizer was used to reduce the chain folding energy and, thus, increase the chain and segment mobility and the crystallization rate of PLA and promote the stereocomplexation of PLA (10). Bao et al. and Wei et al. found that the crystallinity of the solution blending of PLLA/PDLA was greatly enhanced by polyethylene glycol (11, 12). However, we could not find any research on preparation stereocomplex by melt blending of PLLA/PDLA (50/50) and plasticizer. Therefore, in this study, PLLA/PDLA and plasticizer were prepared, and the properties of the stereocomplex, with varying plasticizer contents, were evaluated and discussed.
2 Experimental
2.1 Materials
l-Lactide and d-lactide monomers were prepared using well-established procedures from l-lactic acid (88%, Purac, Thailand) and d-lactic acid (90%, Haihang Industry Co., Ltd., China), respectively. PLLA and PDLA were polymerized using similar procedures in the previous studies (13, 14). PLLA and PDLA were synthesized by bulk lactide ring-opening polymerization in an inert (N2) atmosphere at 165°C for 2 h, initiated with stannous octoate (95%, Sigma) and 1-dodecanol (98%, Acros Organic). The final densities of the PLLA and PDLA were similar at around 1.24 g/cm3. Both PLLA (Mw = 95,000 g mol−1, Mw/Mn = 1.62) and PDLA (Mw = 45,000 g mol−1, Mw/Mn = 1.73) showed similar glass transition temperatures, Tg ≈ 60°C, and melting temperatures, Tm ≈ 170°C, as measured by differential scanning calorimetry (DSC).
The biodegradable plasticizer was Polysorb ID-37 from Roquette, France, which is a composition of isosorbidediesters produced from fatty acids of vegetable origin and isosorbide obtained by simple modification (dehydration) of a derivative of glucose, sorbitol (15).
2.2 Sample preparation
Prior to mixing, PLLA and PDLA were dried in a vacuum oven at 80°C for 5 h. The PLLA/PDLA (50/50) with plasticizer (0, 2, 4, 8 and 16 wt %) were melt blended at 200°C to prepare stereocomplex polylactides (ST) in an internal mixer (HAAKE Polylab OS system) for 4 min. A rotor speed of 100 rpm was chosen. At the end of mixing, the batch was extracted from the mixing chamber manually and allowed to cool to room temperature in air. Neat PLLA and plasticized PLLA was also prepared for comparison (PLLA and PDLA are essentially identical: only the PLLA and plasticized PLLA were prepared as reference).
2.3 Sample characterization
2.3.1 X-ray diffraction analysis
To check the crystal structure of the blended samples in the stereocomplex polylactide, X-ray diffraction (XRD) spectra were generated by a Bruker/D8 Advance BrukerBioSpin AG (Karlsruhe, Germany). XRD samples, taken from blended samples from the internal mixer, were mounted on the XRD platform. The 2θ range from 2° to 40° in the refraction mode was scanned at 2°/min. A computer-controlled wide-angle mode goniometer and a Cu Kα line source were used.
2.3.2 Polarized optical microscopy
The spherulitic morphology of the samples was studied by a Leitz SM-Lux polarized optical microscope (POM). The sample taken from the internal mixer was initially inserted between two glass slides and then heated to 250°C. The specimen was held for 3 min at 250°C to eliminate any remaining PLLA and stereocomplex crystallization seeds before being cooled gradually to room temperature. Images of PLLA and stereocomplex polylactide spherulites were captured with a digital camera.
2.3.3 Fourier transform infrared spectroscopy
The chemical structure of PLLA, ST and ST/plasticizer were studied by Fourier transform infrared spectroscopy (FTIR) spectra from 500 to 4000 cm−1 (2 cm−1 resolution) recorded with OPUS 7.0 software on a Bruker Tensor27 FTIR spectrometer, using the attenuated total reflection (ATR) mode.
2.3.4 Differential scanning calorimetry
Thermal properties of the blends were measured by DSC (Perkin–Elmer DSC, 4000). Specimens of 4–5 mg were cut from the blended materials and placed in aluminum pans. Samples were heated at 10°C/min over a temperature range of 0–250°C (the first heating scan). Then, the samples were cooled at 10°C/min to 0°C (cooling scan). The homocrystallinity (Xhc) and stereocomplex crystallinity (Xsc) of the polylactides were determined from the heat of melting of the homocrystallites (ΔHhc) and stereocomplex crystallites (ΔHsc) using equations 1 and 2, respectively.
where the ΔHhc and ΔHsc are the heat of melting of homocrystallites and stereocomplex crystallites, respectively, that were obtained from the DSC method. The heat of melting for 100% crystallinity of the homocrystallinity was 93 J/g (5), and stereocomplex crystallinity was 142 J/g (16). w is the weight fraction of the PLA or the stereocomplex in the polymer blends (16).
3 Results and discussions
3.1 Wide-angle x-ray diffraction
To determine the crystal structure of the solid blends, wide-angle XRD spectra were taken at room temperature (see Figure 1). The reference PLA sample showed 2θ reflections at around 16.2° and 18.6°, indicating a crystalline PLA matrix (5). On the other hand, the stereocomplex polylactide exhibited three different strong 2θ peaks at 11.6°, 20.6° and 23.5° that corresponded to the planes of the PLA stereocomplex crystallites (17, 18). Moreover, as shown in Figure 1, the XRD curves for ST/plasticizer had similar curves to those of ST alone, which indicated that adding the plasticizer did not affect the formation of stereocomplex.
XRD profiles of PLA and ST/plasticizer.
3.2 Polarized optical microscopy
The morphologies of the reference neat PLA and ST specimens were investigated by polarized light microscopy (POM), as shown in Figure 2A and B, respectively. An increase in spherulite size was clearly observed for stereocomplex samples similar to our recent work (13). The solid interaction between PLLA and PDLA due to hydrogen bonds (19, 20) or van der Waals forces (21) led to a single new stereocomplex with a different crystal size and structure. However, the spherulite size for plasticized ST had similar structure to those of ST alone as shown in Figure 2C.
Polarized optical micrographs of (A) PLLA and (B) ST (C) ST+16%plasticizer.
3.3 FTIR spectra
Figure 3 shows the FTIR spectra from 500 to 4000 cm−1 of PLA, ST, ST+16%plasticizer and pure plasticizer. The FTIR spectra of ST and ST+16%plasticzer were similar. This indicated that chemical structure of ST and ST/plasticizer did not change. Furthermore, as shown in Figure 4 (magnified from Figure 3 in the spectral range 600–1000 cm−1), the neat PLA shows an obvious peak at 922 cm−1, assigned to the PLA homocrystallites. After mixing PLLA and PDLA together in equal amounts, a new peak at 908 cm−1 assigned to stereocompex crystallites (22, 23) appeared. Moreover, the 922 cm−1 peak disappeared when the stereocomplex formed completely.
FTIR spectra for PLA, ST, ST+16%plasticier and pure plasticizer.
FTIR spectra for PLA, ST and ST+16%plasticizer (magnified wave number from 600 to 1000 cm−1 from Figure 3).
3.4 Thermal properties and calorimetry results
3.4.1 First heating cycle
The DSC heating thermograms of plasticized PLA and plasticized stereocomplex polylactide are presented in Figure 5A and B, respectively. The corresponding thermal data are listed in Table 1. We can see that the addition of Polysorb plasticizer caused marked changes in the crystallization behavior of the PLA matrix. Moreover, as the amount of plasticizer increased, the glass transition temperature (Tg), cold crystallization, melting point and enthalpy of melting of PLA became lower. The decrease in Tg, cold crystallization and melting peak were due to segmental mobility of PLA chains caused by the presence of plasticizer, increasing with the plasticizer content which indicates higher polymer flexibility (24). The lesser melting demonstrates that PLA/plasticizer is more readily processed than pure PLA.
First heating scan DSC thermograms of (A) PLA/plasticizer (B) ST/plasticizer.
Thermal transition properties of PLA/plasticizer and ST/plasticizer obtained from the first and second heating scan DSC thermograms.
| Samples | Glass transition | Cold crystallization | Melting 1 | Melting 2 | χ (%) | |||
|---|---|---|---|---|---|---|---|---|
| Tg (°C) | Tcc (°C) | ΔHcc (J/g) | Tm, hc (°C) | ΔHhc (J/g) | Tm, sc (°C) | ΔHsc (J/g) | ||
| First heating | ||||||||
| PLA | 57.7 | 93.5 | 10.6 | 170.6 | 40.2 | – | – | 31.8 |
| PLA + 2%plasticizer | 57.6 | 84.6 | 8.5 | 168.9 | 37.9 | – | – | 32.2 |
| PLA + 4% plasticizer | 54.2 | 80.6 | 6.3 | 166.6 | 34.8 | – | – | 31.8 |
| PLA + 8% plasticizer | 51.4 | 71.6 | 5.7 | 165.2 | 33.5 | – | – | 32.3 |
| PLA + 16% plasticizer | 50.2 | – | – | 164.9 | 27.1 | – | – | 34.6 |
| ST | 58.2 | – | – | – | – | 236.5 | 74.7 | 52.6 |
| ST + 2% plasticizer | 57.4 | – | – | – | – | 233.6 | 71.7 | 52.6 |
| ST + 4% plasticizer | 54.0 | – | – | – | – | 233.3 | 69.3 | 52.9 |
| ST + 8% plasticizer | 51.6 | – | – | – | – | 232.5 | 65.7 | 54.5 |
| ST + 16% plasticizer | 50.1 | – | – | – | – | 229.4 | 59.2 | 59.0 |
| Second heating | ||||||||
| PLA | – | – | – | 168.3 | 30.6 | – | – | 32.9 |
| PLA + 2%plasticizer | – | – | – | 167.4 | 28.4 | – | – | 31.1 |
| PLA + 4% plasticizer | – | – | – | 166.0 | 26.8 | – | – | 30.0 |
| PLA + 8% plasticizer | – | – | – | 165.5 | 26.4 | – | – | 30.7 |
| PLA + 16% plasticizer | – | – | – | 162.3 | 25.3 | – | – | 32.3 |
| ST | – | – | – | – | – | 215.4 | 66.8 | 47.2 |
| ST + 2% plasticizer | – | – | – | – | – | 213.1 | 65.7 | 47.1 |
| ST + 4% plasticizer | – | – | – | – | – | 213.1 | 62.1 | 45.5 |
| ST + 8% plasticizer | – | – | – | – | – | 211.2 | 61.4 | 46.9 |
| ST + 16% plasticizer | – | – | – | – | – | 211.1 | 54.3 | 45.4 |
Figure 5B shows the DSC thermogram of plasticized polylactide stereocomplex. The DSC melting curves for the ST and the plasticized stereocomplex polylactide showed a single peak at a much higher temperature of 235°C, suggesting that complete stereocomplex crystallites with no homocrystallites were formed. Also, similar to PLA, when the amount of plasticizer was higher, the Tg, melting point and enthalpy of melting of ST became lower. Moreover, the cold crystallization peaks presented in PLA samples but disappeared in ST specimens. This suggested a higher crystallization rate for the stereocomplexes than the PLA (13) after being released from the mixer and cooled to room temperature.
3.4.2 Cooling cycle
Also shown in the DSC cooling curves in Figure 6, the crystallization temperature (Tc) of the all PLA samples (Figure 6A) were found to be much lower than those of all stereocomplex specimens (Figure 6B), suggesting that the presence of 50% PDLA (in the rest of the melt blended stereocomplex specimens) facilitated crystal nucleation process, which resulted in a higher crystallization temperature. Similar to the previous studies, the Tc of PLA was approximately 104°C and the Tc of ST was approximately 155°C (13, 14). Moreover, the addition of plasticizer further increased the peak temperature of crystallization and the degree of crystallinity for both PLA and ST specimens. With 16% plasticizer added to the ST, the Tc of ST increased from 130°C to 155°C.
Cooling scan DSC thermograms of (A) PLA/plasticier (B) ST/plasticizer.
3.4.3 Second heating cycle
The data obtained from the first heating cycle included the effect of the prior thermal history, whereas the data obtained from the second heating cycle allowed for a direct comparison of the crystallization behavior of different materials after erasing the thermal history through the first heating scan (25). Figure 7 and Table 1 show the thermograms and numerically calculated data of the PLA/plasticizer and ST/plasticizer specimens from the second heating cycle. Unlike the first heating cycle, no cold crystallization peak was observed for PLA because it was cooled at a much slower rate (10°C/min) than when it was released from the internal mixer and cooled after mixing. This is consistent with the hypothesis that, at lower cooling rates, the polymer chains would have had more time to relax and integrate into an orderly crystalline structure, thus enhancing the degree of crystallinity and facilitating the crystallization process (25). Moreover, it was noticed that the melting points and enthalpies of melting of all samples were lower than those obtained during the first heating cycles. This is especially true for the stereocomplex samples for which the melting points and enthalpies of melting dropped by ~20°C and ~30 J/g, respectively. We attributed this to the degradation of the material during the heating cycles as shown later in thermal stability section that ST and plasticized ST started to degrade at 250°C which is the temperature consistent with the DSC data (26).
Second heating scan DSC thermograms of (A) PLA/plasticizer (B) ST/plasticizer.
The decrease in ST melting temperature from the first to the second heating cycle was much larger than the corresponding decrease for PLA. We attribute this to stereoisomeric chain movement to form stereocomplex crystals again in the cooling cycle, which are more difficult to form than homo-crystals and reduce both Tm and ΔHf.
Akin to what was observed during the first heating cycle, the addition of plasticizer increased the degree of crystallinity but decreased melting temperature of the PLA and stereocomplexes.
3.5 Thermal stability
Polylactide is very sensitive to temperature. In particular, thermal degradation has been observed when the material exceeds 200°C (13). Difference in the thermal stability between the PLA and ST is not obvious. This is mainly because the lactide residues after polymerization are basically the same, resulting in a similar thermal degradation mechanism. The thermal degradation behavior of PLLA cannot be improved by the stereocomplex structure. Similar behavior in degradation results has been reported by Chen et al. (27). Only the thermal stability of the ST/plasticizer specimens was examined using TGA at a heating rate of 10°C/min with results shown in Figure 8. The thermal stability of the stereocomplex PLA matrix decreased slightly in the presence of plasticizer, likely because of the degradation of plasticizer, which has lower thermal stability. Similar decrease in thermal stability of plasticized PLA has been reported (28).
TGA curves for the ST/plasticizer samples.
4 Conclusions
PLLA and PDLA, as well as plasticizer, were melt blended to produce stereocomplexes with no trace of homocrystallities. The generation of a stereocomplex structure increased the melting temperature, crystallization temperature and overall degree of crystallization significantly. The addition of plasticizer to the PLA and stereocomplex increased the degree of crystallinity but depressed the thermal stability and Tg, which indicates improved flexibility.
Acknowledgement
We thank Dr. Yodthong Baimark of Mahasarakham University for preparing the PLLA and PDLA samples.
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Artikel in diesem Heft
- Frontmatter
- In this Issue
- Elastic recovery of polyamide 6 matrix nanocomposites and their basalt fiber co-reinforced hybrids
- Bio-based chitosan/PVdF-HFP polymer-blend for quasi-solid state electrolyte dye-sensitized solar cells
- Synergistic effects of retinoic acid and graphene oxide on the physicochemical and in-vitro properties of electrospun polyurethane scaffolds for bone tissue engineering
- Non-isothermal crystallization and thermal degradation kinetics of MXene/linear low-density polyethylene nanocomposites
- Extraction and characterization of chitin and chitosan: applications of chitosan nanoparticles in the adsorption of copper in an aqueous environment
- Systematic evaluation of pH and thermoresponsive poly(n-isopropylacrylamide-chitosan-fluorescein) microgel
- Plasticizer effect on melt blending of polylactide stereocomplex
- Polypropylene/basalt thick film composites: structural, mechanical and dielectric properties
- Performance properties of rigid polyurethane-polyisocyanurate/brewers’ spent grain foamed composites as function of isocyanate index
- Synthesis and characterization of polyvinylmethylsiloxanes by cationic polymerization using a solid green catalyst
