Improvement in Mechanical Properties and Heat Resistance of PLLA-b-PEG-b-PLLA by Melt Blending with PDLA-b-PEG-b-PDLA for Potential Use as High-Performance Bioplastics

Pasee, Supasin; Baimark, Yodthong

doi:https://doi.org/10.1155/2019/8690650

Advances in Polymer Technology

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Abstract Introduction Materials and Methods Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2019 | Article ID 8690650 | https://doi.org/10.1155/2019/8690650

Improvement in Mechanical Properties and Heat Resistance of PLLA-b-PEG-b-PLLA by Melt Blending with PDLA-b-PEG-b-PDLA for Potential Use as High-Performance Bioplastics

Supasin Pasee¹and Yodthong Baimark¹

Academic Editor: Lih-sheng Turng

Received29 Oct 2018

Revised20 Dec 2018

Accepted23 Jan 2019

Published24 Feb 2019

Abstract

Ecofriendly poly(L-lactide)-b-poly(ethylene glycol)-b-poly(L-lactide) (PLLA-b-PEG-b-PLLA) are flexible bioplastics. In this work, the blending of poly(D-lactide)-b-poly(ethylene glycol)-b-poly(D-lactide) (PDLA-b-PEG-b-PDLA) with various blend ratios for stereocomplex formation has been proved to be an effective method for improving the mechanical properties and heat resistance of PLLA-b-PEG-b-PLLA films. The PLLA-b-PEG-b-PLLA/PDLA-b-PEG-b-PLDA blend films were prepared by melt blending followed with compression molding. The stereocomplexation of PLLA and PDLA end-blocks were characterized by differential scanning calorimetry and X-ray diffraction (XRD). The content of stereocomplex crystallites of blend films increased with the PDLA-b-PEG-b-PDLA ratio. From XRD, the blend films exhibited only stereocomplex crystallites. The stress and strain at break of blend films obtained from tensile tests were enhanced by melt blending with the PDLA-b-PEG-b-PDLA. The heat resistance of blend films determined from testing of dimensional stability to heat and dynamic mechanical analysis were improved with the PDLA-b-PEG-b-PDLA ratio. The sterecomplex PLLA-b-PEG-b-PLLA/PDL-b-PEG-b-PDLA films prepared by melt processing could be used as flexible and good heat-resistance packaging bioplastics.

1. Introduction

In the past few decades, biodegradable bioplastics have been widely developed for use instead of non-biodegradable petroleum-based plastics due to plastic-waste pollution and the implementation of low-carbon environmental protection. Poly(L-lactic acid) or poly(L-lactide) (PLLA) is an important bioplastic that has attracted wide attention due to its nontoxicity, biocompatibility, biodegradability, biorenewability, and good processability [1–3]. PLLA has uses in many fields such as biomedical, food packaging, and agriculture [4–6] but its use in some applications is limited by its low flexibility and poor heat-resistance [7, 8].

Stereocomplex polylactides (scPLA) can be formed by blending between PLLA and poly(D-lactide) (PDLA) that had stronger interactions in the stereocomplex crystallites than the homocrystallites of PLLA and PDLA [9]. This induces higher melting-temperatures (approximately 210–240°C) and faster crystallization speed than the PLLA thereby enhancing the mechanical properties, heat-resistance, and hydrolysis-resistance of scPLA [10, 11]. The high heat-resistant scPLA is appropriate for some applications such as hot-fill packaging, heat-treatment packaging, and microwave applications. However, the glass transition temperature () of scPLAs was still similar to the PLLA (approximately 60°C). The brittle character of scPLA is still limiting in some applications.

High molecular-weight PLLA-b-poly(ethylene glycol)-b-PLLA (PLLA-PEG-PLLA) were more flexible than the PLLA because of flexible PEG middle-blocks enhanced plasticizing effect that decreased the of the PLLA end-blocks [12, 13]. However the heat-resistance of PLLA-PEG-PLLA was still low. In recent years, PLLA has been blended with PDLA-PEG-PDLA for improving the flexibility of the obtained stereocomplex PLLA/PDLA-PEG-PDLA [14–20]. Low-molecular-weight PLLA-PEG-PLLA/PDLA-PEG-PDLA blends were prepared as hydrogels [21–24] and micelles [25, 26] for use as drug delivery carriers. However, the preparation of high-molecular-weight stereocomplex PLLA-PEG-PLLA/PDLA-PEG-PDLA has been scarcely published [27]. The latter work was concerned scPLAs with asymmetric blend ratio. The stereocomplexation of PLLA/PDLA end-blocks and plasticization effect of PEG middle-blocks exhibited better tensile strength, elongation at break, and heat resistance than the PLLA-PEG-PLLA. The melt-blended PLLA-PEG-PLLA/PDLA-PEG-PDLA with various blend ratios has not been reported so far.

All the high-molecular-weight stereocomplex PLLA/PDLA-PEG-PDLA and PLLA-PEG-PLLA/PDLA-PEG-PDLA were prepared by solution blending [14–20, 27]. However the fabrication of scPLA by melt processing is very interesting because of its possible use in industrial-scale applications. The stereocomplexation, mechanical properties, and heat resistance of melt-processed scPLAs need to be better understood for use in practical applications. Therefore in this work, the PLLA-PEG-PLLA/PDLA-PEG-PDLA blend films were prepared by melt blending before compression molding to investigate the influence of blend ratio on their stereocomplexation, mechanical properties, and heat resistance.

2. Materials and Methods

2.1. Materials

L-Lactic acid (L-form optical purity > 95%, 88 %wt, Purac, Thailand) and D-lactic acid (D-form optical purity > 99%, 80 %wt, Haihang Industry (Jinan) Co., Ltd., China) were used as monomer pre-cursors for synthesizing L-lactide (LLA) and D-lactide (DLA) monomers, respectively, by direct polycondensation (step 1) of lactic acid at 180°C for 4.0 h followed with thermal depolymerisation (step 2) under reduced pressure at 10 mmHg of low-molecular-weight poly(lactic acid) at 240°C for 2.0 h. Zinc oxide (99%, Ajax Finechem, Australia) and Stannous octoate (Sn(Oct)₂, 95%, Sigma, Switzerland) were used as catalysts in steps 1 and 2, respectively [12]. These crude monomers were purified by recrystallization four times from ethyl acetate before drying in a vacuum oven at 50°C for 24 h. Poly(ethylene glycol) (PEG) with molecular weight of 20,000 g/mol (Sigma, USA) was dried in a vacuum oven at 50°C for 24 h before use.

2.2. Synthesis and Characterization of PLLA-PEG-PLLA and PDLA-PEG-PDLA

The PLLA-PEG-PLLA and PDLA-PEG-PDLA were synthesized by ring-opening polymerization in bulk at 165°C under a nitrogen atmosphere for 6 h using 0.075 mol% Sn(Oct)₂ as a catalyst. PEG was used as an initiator. Feed molecular-weights of both the PLLA-PEG-PLLA and the PDLA-PEG-PDLA were calculated based on lactide/PEG feed ratio of 5/1 (w/w) and molecular weight of PEG (20,000 g/mol) that were approximately 120,000 g/mol. The obtained copolymers were granulated before drying in a vacuum oven at 110°C for 3 h to remove any unreacted lactide.

PLLA-PEG-PLLA and PDLA-PEG-PDLA were characterized by polarimetry (Bellingham and Stanley ADP220), gel permeation chromatography (GPC, Waters e2695 separations module), and differential scanning calorimetry (DSC, Perkin-Elmer Pyris Diamond). The characteristics of PLLA-PEG-PLLA and PDLA-EPG-PDLA are summarized in Table 1. The unreacted lactide of copolymers was determined by ¹H-NMR. The residue of unreacted lactide of both the PLLA-PEG-PLLA and the PDLA-PEG-PDLA was determined from methine protons of reacted lactide units (δ = 5.15 ppm) and unreacted lactide units (δ = 5.05 ppm) [12] that were less than 1.0 %wt.

2.3. Preparation of PLLA-PEG-PLLA/PDLA-PEG-PDLA Blends

PLLA-PEG-PLLA and PDLA-PEG-PDLA were dried in a vacuum oven at 50°C overnight before melt blending using an internal mixer (HAAKE Polylab OS Rheomix) at 200°C for 4 min with a rotor speed of 100 rpm. The powder solids of the blends were formed when the blending temperature was lower than 200°C [29, 30]. The effects of PLLA-PEG-PLLA/PDLA-PEG-PDLA blend ratios (100/0, 90/10, 80/20, 70/30, 60/40, and 50/50 by weight) were investigated. The obtained blends were granulated before drying in a vacuum oven at 50°C overnight before characterization and compression molding.

The compressed PLLA-PEG-PLLA and blend films were prepared using an Auto CH Carver laboratory press at 240°C without any compression force for 1.0 min and with a 5.0 ton compression force for 1.0 min before cooling to room temperature. The film thicknesses were in ranges 0.20–0.25 mm. The obtained films were kept at room temperature for 24 h before characterization.

2.4. Characterization of PLLA-PEG-PLLA/PDLA-PEG-PDLA Blends

The thermal transition behaviours of the blends were investigated using a Perkin-Elmer Pyris Diamond DSC under a nitrogen flow. For DSC, samples of 3–5 mg in weight were held at 250°C for 3 min to remove thermal history. Then, the samples were quenched to 0°C according to the DSC instrument’s own default cooling mode before heating from 0 to 250°C at 10°C/min. For cooling DSC thermograms, the sample was held at 250°C for 3 min to remove thermal history before cooling to 0°C at a rate of 10°C/min.

The degrees of crystallinity from DSC () of homocrystallites (hc-) and stereocomplex crystallites (sc-) were determined from DSC heating scan using (1) and (2), respectively. Percentage of stereocomplexation (SC) was calculated from (3).where hc-∆ and sc-∆ were melting enthalpies of homo- and stereocomplex crystallites, respectively. ∆ was cold-crystallization enthalpy. Melting enthalpies for hc- and sc- = 100% were 93 and 142 J/g, respectively [31]. The weight fraction of the PLA end-blocks () was calculated from mole ratio of lactide:ethylene oxide obtained from ¹H-NMR and weight of each repeating unit [12] that was 0.83 for both the PLLA-PEG-PLLA and the PDLA-PEG-PDLA.

The crystalline structures of compressed films were measured using a Bruker D8 Advance wide-angle X-ray diffractometer (XRD) at 25°C with CuKα radiation at 40 kV and 40 mA. For XRD, a scan speed of 3°/min was used to determine the crystalline structures. The degrees of crystallinity from XRD () for homocrystallites (hc-) and stereocomplex crystallites (sc-) of the scPLA films were calculated by using the following, respectively:where , , and were the diffraction peak-areas of homo- and stereocomplex crystallites as well as amorphous hump region, respectively.

The tensile properties of compressed films were determined using a Lloyds LRX+ universal mechanical tester at 25°C and 65% relative humidity. The films (100 × 10 mm) were tested with a gauge length of 50 mm and a crosshead speed of 50 mm/min according to ASTM D882. The tensile properties were averaged from at least five experiments for each sample.

The dimensional stability to heat of compressed scPLA films was tested in an air oven at 80°C for 30 sec under a 200 g load. Initial length of film samples was 20.0 mm. The dimensional stability was calculated by [32]

The thermomechanical properties of compressed films measuring 5 × 20 × 0.2 mm in size were investigated with a TA Instrument Q800 dynamic mechanical analyzer (DMA) in a multifrequency strain mode. For DMA analysis, the film samples were heated from 30 to 150°C at the rate of 2°C/min. The scan amplitude was set to be 10 μm and the scanning frequency was 1 Hz.

3. Results and Discussion

3.1. Stereocomplexation

The stereocomplexation between PLLA and PDLA end-blocks of the blends was investigated from DSC heating thermograms as shown in Figure 1. The DSC results are summarized in Table 2. The of the blends were in range 27–29°C. The flexible PEG middle-blocks acted as plasticizers to decrease the of the PLA end-blocks [12, 13]. The stereocomplexation of PLLA and PDLA end-blocks did not affect glass-transition behaviors of the blends.

The PLLA-PEG-PLLA had only a homocrystalline melting peak at 169°C, while the blends had both melting peaks of homocrystallites (hc-) and stereocomplex crystallites (sc-) in the ranges 161–168°C and 216–218°C, respectively. That there were no peaks of cold crystallization suggested that crystallization completed during the quenching process in the DSC method. As shown in Table 2, a large decrease in hc- and considerable increase in sc- steadily with increasing PDLA-PEG-PDLA ratio from 0 to 50 wt% were observed. The %SC increased with the PDLA-PEG-PDLA ratio. The higher PDLA-PEG-PDLA ratio gave larger PDLA fraction for stereocomplex formation with the PLLA end-blocks of PLLA-PEG-PLLA.

Figure 2 shows DSC cooling thermograms of the PLLA-PEG-PLLA and blends, after being melted at 250°C. The PLLA-PEG-PLLA had crystallizing temperature () at 105°C with an enthapy of crystallization (∆) of 27.4 J/g. The flexible PEG middle-blocks enhanced plasticizing effect for homo-crystallization of PLLA end-blocks during DSC cooling scan. The 90/10 blend exhibited at 84°C that is lower than the pure PLLA-PEG-PLLA. However, the and ∆ values of the blends significantly increased with increasing of the PDLA-PEG-PDLA ratio. This suggests the crystallization of the blends was accelerated during the DSC cooling scan by increasing the PDLA-PEG-PDLA ratio. The results could be explained by the crystallization of stereocomplex crystallites of PLA being faster than that of the homocrystallites [33].

The crystalline structures of the PLLA-PEG-PLLA and blend films were determined from XRD patterns as presented in Figure 3. The PLLA-PEG-PLLA film exhibited a diffraction peak at 17° attributed to homocrystalline structure of polylactide matrix [12], while all the blend films showed weak diffraction-peaks at 12°, 21°, and 24° ascribed to stereocomplex-crystalline structure [27, 34]. The hc- of PLLA-PEG-PLLA film and sc- of blend films calculated from (4) and (5), respectively, are summarized in Table 3. The sc- of blend films increased with the PDLA-PEG-PDLA ratio. The hc- and sc- values in Table 3 were lower than the hc- and sc- values in Table 2. This may be related to the easier mobility of the copolymer chains, having occurred during the quenching process, without compression forces in the DSC method to enhance more crystallisation of PLA end-blocks. In addition, some disagreements among the quantitative results of the crystallinity by different measurement methods are frequently encountered.

3.2. Tensile Properties

Figure 4 shows selected tensile curves of PLLA-PEG-PLLA and blend films. All the films had a yield point indicating the films were flexible except the 50/50 blend film. The flexible PEG middle-blocks enhanced the plasticizing effect of the polylactide end-blocks [12, 13]. The stress at yield of films increased with the PDLA-PEG-PDLA ratio. The more interactions between PLLA-PEG-PLLA and PDLA-PEG-PDLA in an amorphous phases of the 50/50 blend film might be suppressed the yield effect [27]. The averaged tensile properties including stress and strain at break as well as Young’s modulus are clearly compared in Figure 5. The blend films showed higher stress and strain at break than the PLLA-PEG-PLLA film. The stress and strain at break of the blend films increased as the PDLA-PEG-PDLA ratio increased. The results suggested that stereocomplexation between PLLA-PEG-PLLA and PDLA-PEG-PDLA of the blend films improved their tensile properties. The stereocomplex crystallites of PLLA/PDLA end-blocks had better tensile strength than the homocrystallites of PLLA and PDLA end-blocks due to stronger intermolecular forces in the stereocomplex crystallites [9]. The stereocomplex crystallites also acted as physical crosslinkers of PLLA-PEG-PLLA and PDLA-PEG-PDLA chains in film matrix to increased extensibility of the blend films [35, 36]. In addition, the tensile stress at break of the 50/50 melt-blend film (21 MPa) in this work was lower than the 50/50 solution-blend film (~40 MPa) [27]. This may be due to thermal degradation and chain scission during the melt blending. Initial Young’s modulus of PLLA-PEG-PLLA and blend films were in ranges 567–640 MPa. That it did not significantly change with the PDLA-PEG-PDLA ratio indicated that the stiffness of the PLLA-PEG-PLLA and blend films was similar. This may be due to the of these films being similar (27–29°C) and their degrees of crystallinity from XRD being low.

3.3. Heat Resistance

The dimensional stability to heat of film samples was determined at 80°C for 30 sec under 200 g load to study heat resistance of the films. Figure 6 illustrates PLLA-PEG-PLLA and blend films before and after testing. The PLLA-PEG-PLLA film showed the longest film-extension after test [Figure 6(a)]. The film extension decreased when the PDLA-PEG-PDLA was blended and the PDLA-PEG-PDLA ratio was increased.

The heat resistance of the films was compared from the %dimensional stability to heat calculated from (6) that is shown in Figure 7. The %dimensional stability to heat of the films was directly related to the heat resistance that steadily increased with the PDLA-PEG-PDLA ratio. The results suggested that the stereocomplexation of PLLA-PEG-PLLA/PDLA-PEG-PDLA blends improved the heat resistance of the blend films.

The heat resistance of PLLA and scPLA has been widely investigated by DMA analysis from storage modulus as a function of temperature [37, 38]. The storage modulus of low-crystallinity PLLA dramatically dropped as the temperature passing the region before increasing again due to cold crystallization of PLLA during DMA heating scan. This indicated that the low-crystallinity PLLA had poor heat-resistance [39]. Meanwhile good heat-resistance was obtained when the PLLA had a high degree of crystallinity that maintained stiffness of PLLA as it passed through the region [8].

Figure 8 shows storage modulus of film samples from DMA analysis. The storage modulus of PLLA-PEG-PLLA and blend films dramatically dropped with increasing temperature in the range 30–60°C due to rubber-like character and low crystallinity of all the films. This indicates that these films were low heat-resistance. All the films were then extended for the test of dimensional stability to heat. The storage modulus increased again in the range 90–130°C due to cold crystallization of PLA end-blocks. This suggests that the crystallisation of compressed films did not complete. This may be due to the compression forces reducing the chain mobility for crystallisation of PLA end-blocks during the film cooling. However, the cold-crystallization regions of the blend films from DMA analysis were detected at higher temperature than the PLLA-PEG-PLLA film and shifted to higher temperatures as the PDLA-PEG-PDLA ratio increased. This could be explained by the chain mobility of copolymers during cold crystallization in DMA heating scan being restricted by the stronger intermolecular interactions between PLLA and PDLA end-blocks.

In addition, the PLLA-PEG-PLLA film exhibited the largest rising up of storage modulus during cold crystallization (see black line in Figure 8). This curve type indicates that poor heat-resistance of the PLLA-PEG-PLLA film because film stiffness to heat was the lowest [39]. The increases of storage modulus during cold crystallization of the blend films were lower than the PLLA-PEG-PLLA film and steadily decreased as the PDLA-PEG-PDLA ratio increased. The DMA results suggested that the interactions between PLLA and PDLA end-blocks in the amorphous phases of the blend films enhanced its stiffness and heat resistance which supports the results of %dimensional stability to heat in Figure 7. The stronger interactions of PLLA and PDLA end-blocks in the amorphous phases of the blend-film matrix could reduce the film extension during test of dimensional stability to heat.

4. Conclusions

In this work, stereocomplex PLLA-PEG-PLLA/PDLA-PEG-PDLA blend films were prepared by melt blending before compression molding. The blends showed both homo- and stereocomplex crystallites from DSC analysis. The hc- decreased and sc- increased as the PDLA-PEG-PDLA blend ratio increased. The stereocomplexation also enhanced crystallization of PLA matrix. The higher PDLA-PEG-PDLA ratio of the blends induced faster crystallization upon the cooling scan. The XRD results supported the conclusion that the content of stereocomplex crystallites of blend films increased with the PDLA-PEG-PDLA ratio. The mechanical properties of the blend films were better than the PLLA-PEG-PLLA film and increased with the PDLA-PEG-PDLA ratio. The stereocomplexation between PLLA and PDLA end-blocks improved both stress and strain at break of the blend films. The values of dimensional stability to heat of blend films suggested improvement in their heat resistance that increased with the PDLA-PEG-PDLA ratio. The storage modulus of blend films during cold crystallization determined from DMA indicated that the stronger interactions between PLLA and PDLA end-blocks in the amorphous phases improved the heat resistance. It can be concluded that the PDLA-PEG-PDLA blending can improve mechanical properties and heat resistance of PLLA-PEG-PLLA films for potential use as high-performance bioplastic products.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research was financially supported by Mahasarakham University and the Centre of Excellence for Innovation in Chemistry (PERCH-CIC) and the Office of the Higher Education Commission, Ministry of Education, Thailand. The authors gratefully acknowledge Research Professional Development Project under the Science Achievement Scholarship of Thailand (SAST) for providing a scholarship for one of them (SP).

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Copyright © 2019 Supasin Pasee and Yodthong Baimark. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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