Toughening of Poly(L-lactide) with Branched Multiblock Poly(ε-caprolactone)/poly(D-lactide) Copolymers

CHANG Yue( )， CHEN Zhize()*， YANG Yiqi()

1College of Chemistry，Chemical Engineering and Biotechnology，Donghua University，Shanghai201620，China2Department of Textiles，Merchandising and Fashion Design，University of Nebraska-Lincoln，Lincoln NE68583-0802，United States3Department of Biological Systems Engineering and Nebraska Center for Materials and Nanoscience，University of Nebraska-Lincoln，Lincoln NE68583-0802，United States

Abstract：A total biodegradable elastomer，branched multiblock poly(ε-caprolactone)/poly(D-lactide) (BMCD)was prepared using 3-isocyanatopropyltriethoxysilane(IPTS)as a coupling agent.To improve the toughness of poly(L-lactide)(PLLA)，PLLA/BMCD blends were prepared via a simple solvent evaporation method at various BMCD loadings.Tensile test showed that the elongation at break of PLLA blends increased to 50.97%and 104.55%at the loadings of 5%and 7% (mass fraction)BMCD respectively，with no sacrifice of their biodegradability.This approach allowed for simultaneous control of mechanical and biodegradable properties of PLLA with a few additives in actual production.Furthermore，UV-VIS test showed that the light transmittance of the films at the loadings of 5% (mass fraction)BMCD was almost the same as pure PLLA at 400 nm.

Key words：poly(L-lactide)(PLLA)；branched polymer；toughness；transparency；stereocomplex

Introduction

Poly(L-lactide) (PLLA) is a biocompatible， biodegradable and compostable thermoplastic polymer with rigidity and transparency similar to polystyrene[1]. PLLA has been widely used in the fields of biomedicine， agriculture and industry. Unfortunately， its applications are limited partly due to its inherent brittleness and poor heat resistance[2-4].

Various strategies have been developed to toughen PLLA including copolymerization， plasticization and blending[5-6]. Copolymerization can effectively improve its toughness. However， the synthesis of copolymers is generally less attractive to industrial applications due to its tedious synthetic steps[3]. Plasticization is an another effective approach to improve the toughness of PLLA while retaining its transparency， but the low molecular weight plasticizer tend to migrate in the PLLA[7-8]. Among all the methods， blending PLLA with other flexible polymers[9-10]or elastomers[11]is the most effective one to toughen PLLA. However， the drawbacks of this strategy are also obvious. For example， the significant decrease in the strength/modulus of the toughened PLLA， the poor interfacial adhesion and phase separation of the two immiscible components[12-13]， both of them have negative impacts on PLLA application performances.

Stereocomplex polylactide has been proven to possess more excellent properties than the homopolylactide[14]. The stereocomplex has a melting temperature (Tm) approximately 50 ℃ higher than that of homopolylactide[15-16]. Therefore， stereocomplexation of polylactide is an alternative approach to improve thermo mechanical properties of polylactide[17-23]. However， there is an obvious drawback of stereocomplexation that it can lead to brittleness， which can limit its industrial applications[24]. Thus， the synthesis of good heat resistance-toughness balance PLLA-based material along with high bio-based PLLA content is still challenging.

The aim of this research is to determine the effect of the addition of high molecular weight branched multiblock poly(ε-caprolactone)/poly(D-lactide) (BMCD) additives on the thermal and mechanical properties， and transparency of linear PLLA. The hypothesis of the approach is that the incorporation of branched multiblock BMCD additives into linear PLLA would form stereocomplex crystallites between poly(D-lactide) (PDLA) blocks and linear PLLA. Meanwhile poly(ε-caprolactone) (PCL) segments would impart ductility. PCL has attracted extensive attention in the areas of food packaging， agriculture and medical applications due to its good biodegradability， biocompatibility and high thermal stability[25]. More importantly， PCL is a flexible polyester with high elongation rate. 3-isocyanatopropyltriethoxysilane (IPTS) is a readily hydrolyzed material with highly active groups which should be useful as a biodegradable coupling agent[26-28]. To avoid the tedious synthetic steps and harsh synthetic conditions， PCL were chosen as the soft segment and IPTS as the coupling agent to synthesize branched multiblock copolymers via one-pot method (Scheme 1).

1 Experimental

1.1 Materials

L-lactide and D-lactide with an optical purity of 98% were supplied by Musashino Chemical Co.， Ltd., China. PLLA (Mw=1.6×105g/mol) was synthesized via ring-opening polymerization of L-lactide. IPTS was purchased from TCI Development Co.， Ltd., China. Poly(ε-caprolactone) diol (HO-PCL-OH) with the molecular weight of 3 000 g/mol was purchased from Shenzhen Esun Industrial Co.，Ltd., China. Tin(II) octoate (2-ethylhexanoate)[Sn(Oct)2] was supplied by Sigma-Aldrich Co. Ltd., China. Methanol, dichloromethane and tetrahydrofuran were purchased from Sinopharm Chemical Reagent Co.， Ltd., China. All solvents were analytical grade. Tetrahydrofuran was distilled over a sodium benzophenone complex before use.

1.2 Synthesis of HO-DCD-OH triblock copolymers

The triblock copolymers poly(ε-caprolactone)/poly(D-lactide) diol (HO-DCD-OH)were synthesized by ring-opening polymerization of D-lactide using HO-PCL-OH as macro initiator according to Ref. [29]. The feed ratio of HO-PCL-OH and D-lactide by weight was 1∶1. The resulting polymers were purified by reprecipitation using dichloromethane as solvent and methanol as precipitant. The purified products were dried to constant mass in vacuum oven at 40 ℃. The synthesis route was shown in Scheme 1(a).

1.3 Synthesis of BMCD

After drying for 12 h at 60 ℃ under vacuum of 100 Pa， HO-DCD-OH was dissolved in Tetrahydrofuran (THF) solution in the magnetic stirred tank reactor. A stoichiometric amount of IPTS (the molar ratio of —NCO to —OH is 0.6) and Sn(Oct)2were added into the HO-DCD-OH with a syringe. The solution was stirred for 3 h at 50 ℃ under nitrogen and then cooled to room temperature. After adding 10 μL distilled water， the mixture solution continued to stir for 8 h. Finally， the solution was poured into methanol to purify the branched copolymers. The obtained branched copolymers BMCD were dried under vacuum at 40 ℃ for 24 h.

As shown in Scheme 1(b)， intermediate was synthesized by reacting HO-DCD-OH with a stoichiometric amount of IPTS， so the end groups were converted to triethoxy. The intermediate (Si-DCD-Si) couldin-situhydrolytic condense to form the branched copolymers BMCD at room temperature.

(a)

(b)

Scheme1Synthesis and structure of the copolymers: (a) synthesis route of HO-DCD-OH triblock copolymer; (b) synthesis route of BMCD multiblock copolymer

1.4 Preparation of PLLA blends

PLLA was dried under vacuum at 60 ℃ for 24 h to remove water prior to use. PLLA/BMCD blends containing different contents of BMCD：3%，5%，7%，10% and 20%(mass fraction) were prepared by dissolving a calculated amount of PLLA and a measured amount of BMCD in dichloromethane solution， stirring at room temperature for 4 h， and then the mixed solution was poured into Teflon mold and evaporating dichloromethane at room temperature for 24 h. The obtained film was dried under vacuum at 60 ℃ for 48 h before characterization. The thickness of dry films is circa 110 μm.

1.5 Characterization

H Nuclear Magnetic Resonance(1H-NMR) spectra were recorded using a Bruker spectrophotometer (DRX-400 MHz， Germany) with chloroform-d (CDCl3) as solvent.

The weight-average molecular weight (Mw) and number-average molecular weight (Mn) of BMCD were determined using the Malvern gel permeation chromatography-light scattering (GPC-LS) (Viscotek 270， UK). THF was used as the mobile phase and the polystyrene was used as the standard sample.

The Fourier transform infrared (FTIR) spectra was recorded on a PerkinElmer FTIR spectrometer(Spectrum Ⅱ，USA) by the attenuated total reflection (ATR) method in a range of wave numbers from 4 000 to 500 cm-1.

Differential scanning calorimetry (DSC) was carried out by a Netzsch DSC (204 F1，Germany). Specimens weighting around 5 mg were heated at the rate of 10 ℃/min and cooled at the rate of 10 ℃/min. All tests were under nitrogen atmosphere. The rate of gas consumption was 20 mL/min.

Thermo gravimetric analysis (TGA) was performed with a Netzsch TG (209 F1， Germany) at a heating rate of 10 ℃/min from 30 to 600 ℃ under nitrogen atmosphere. The rate of gas consumption was 40 mL/min.

The tensile strength， breaking strength， elongation at break and Young’s modulus of the samples were measured on a Hounsfielduniversal material tester (H5 K-S， USA) at a crosshead speed of 500 mm/min according to the ASTM D5034 standard.

The surface features of the films were observed by the environment scanning electron microscopy (SEM) (HITACHI-100， Japan). Films placed on conductive tapes were sputter coated with gold and observed in the microscope at a voltage of 10 kV.

The wide X-ray diffraction (WXRD) analyses were measured on a Rigaku X-ray polycrystal diffractometer (D/max-2550 PC， Japan). Copper radiation with wavelengthλ=0.1542 nm was used.

The transmittance of the pure PLLA and PLLA/BMCD films were measured using a Shimadzu UV-VIS spectrophotometer instrument (UV-1800， Japan) in the scanning range of 300-900 nm at room temperature.

2 Results and Discussion

2.1 Synthesis of BMCD branched copolymers

The molecular weights of HO-DCD-OH and BMCD were characterized by GPC-LS. As shown in Fig. 1， theMwof HO-DCD-OH was 6.6 kg/mol. TheMwof the BMCD was 87.5 kg/mol， which increased significantly after hydrolytic condensation compared with pre-polymer HO-DCD-OH. The polydispersity index (PDI) of the BMCD (PDI=1.6) was slightly higher than the PDI of HO-DCD-OH (PDI=1.2)， which matched the character of the branched copolymer. After dissolving in common solvent， such as tetrahydrofuran， toluene and dichloromethane for 24 h， the BMCD was completely soluble with no gels observed， which indicated that no cross-linking was formed[30].

Fig. 1 GPC traces of HO-DCD-OH and BMCD branched copolymer

To confirm the formation of copolymers， ATR-FTIR spectra of HO-DCD-OH， Si-DCD-Si， and BMCD copolymers were recorded. As shown in Fig. 2， the broad absorption peak at 1 757 cm-1， 1 727 cm-1were ascribed to the carbonyl group， which were the characteristic absorption peak of PLLA and PCL. The vibration peaks of the urethane group of Si-DCD-Si and BMCD at 1 526 cm-1confirmed that the isocyanate group reacted with hydroxyl groups to yield urethane groups. In addition， the appearance of the characteristic peak of Si-O-Si groups at 804 cm-1， which attributed to BMCD， verified the successful condensation of silicon hydroxyls.

The1H-NMR spectrum of BMCD shown in Fig. 3 also confirmed the successful synthesis of BMCD copolymer. Peaks at 1.10-1.80 (peaks b, c and d)， 2.30 (peak a) and 4.10 (peak e) ppm， were assigned to methylene protons of PCL units， respectively. Peaks at 1.57(peak g)， 5.18(peak f) ppm， were assigned to the methyl， methane protons of PDLA units， respectively. The characteristics at 3.17 (peak h)， 1.65 (peak i)， 0.65 (peak j) ppm， were assigned to the methylene protons of IPTS units， respectively. From the1H-NMR， the composition percentage of PLLA/PCL in BMCD was 49.6/50.4， consistent with the feeding ratio of 1∶1 by weight.

2.2 Mechanical properties and toughness

Pure PLLA materials are brittle and exhibit limited extendibility. BMCD blended PLLA have substantially improved mechanical properties. The stress-strain curves of pure PLLA and the blends were presented in Fig. 4 and the mechanical properties of the materials were summarized in Table 1. From Fig. 4， the elongation at break of the PLLA/BMCD blends increased as the BMCD contents increased， while the strength decreased. From Table 1， the elongation at break of the blend was 9.60% when 3% (mass fraction) BMCD was added， which was slightly higher than the pure PLLA (3.93%). The elongation at break of the blend with 5% (mass fraction) of BMCD reached 50.97% and the tensile strength kept at a relatively high value of 50.50 MPa. Further increase in the contents of BMCD to 7%， 10% and 20% (mass fraction) enhanced elongation at break to 104.55%， 200.62% and 282.56%， respectively. The tensile strengths and Young’s modulus of PLLA/BMCD blends decreased as the BMCD contents increased. The degree of the reduction were great. The dramatic reductions implied weak interaction between PLLA and BMCD. For BMCD-20%， the tensile strength at break was higher than the tensile strength at yield point， which probably owed to the formation of the stereocomplex in the blend. As the ratio of enantiomer increased， new hydrogen bonds were formed during the stretching， leading to a higher breaking strength at the post-yield region.

Fig. 3 1H-NMR spectrum of BMCD

Fig. 4 Stress-strain relationship for pure PLLA， PLLA/BMCD blends

Stress whitening in the test specimens (Fig. 5) was observed for PLLA blend with 5% (mass fraction) BMCD. Extensive stress whitening could be observed in the blends which resulted in a larger elongation at break than that in the pure PLLA. These results clearly suggested that the inherent brittle fracture behavior of PLLA could be successfully decreased with an addition of low amount of BMCD.

Fig. 5 Photographs of the tensile test specimens of (a) pure PLLA and (b) PLLA/BMCD blend (BMCD-5%)

2.3 Fractured surface morphology

To understand the toughening improvement of PLLA by BMCD， the morphologies of tensile fracture surfaces of the stretched PLLA and PLLA/BMCD blends were investigated by SEM， and the micrographs of the samples were shown in Fig. 6. The presence of the BMCD domains altered the deformation mechanism. As shown in Fig. 6， there were multiple crazings in the blends for BMCD-5%， BMCD-7%， BMCD-10% and BMCD-20% that occurred in uniaxial extension. The microstructure of PLLA/BMCD material was a suggestion that blending BMCD with PLLA made the transition of PLLA from brittle to ductile material. During the stretching process， the soft segments worked as stress concentration points to initiate crazing of the blends.

Table 1 Mechanical properties of pure PLLA and PLLA/BMCD blended polymer materials

Fig. 6 SEM micrographs of the tensile-fracture surface of pure PLLA and PLLA/BMCD blends

2.4 Thermal properties

TGA analysis was performed to assess the thermal stability of PLLA and PLLA/BMCD blends. Figure 7 showed the TGA curves of PLLA and PLLA/BMCD blends. Both PLLA and PLLA/BMCD blends presented a similar degradation profile， indicating that the blending did not change the degradation mechanism of PLLA. The onsets of decomposition temperature were reduced by about 6 ℃， 5 ℃ and 1 ℃ for BMCD-3%， BMCD-5% and BMCD-7% respectively， indicating that blending reduced the thermal stability of PLLA. The lower thermal stability of the blends was attributed to the relatively unstable polyurethanes of the BMCD[31]. Further increase in the contents of BMCD to 10% and 20% (mass fraction) enhanced the onsets of decomposition temperature by about 4 ℃ and 3 ℃， respectively. This probably owed to the slight stereocomplexation in the blends.

Figure 8 showed the DSC thermograms of PLLA and PLLA/BMCD films collected upon heating and the data of the materials were summarized in Table 2. As listed in Table 2， PLLA and BMCD-3% had one melting temperature at 180.22 ℃ and 180.07 ℃， respectively， while all the other films (BMCD-5%， BMCD-7%， BMCD-10% and BMCD-20%) had two melting temperatures at around 180 ℃ and 200 ℃. These melting temperatures were assigned to the PLLA and stereocomplex compositions， respectively. The major melting point (Tm-1) of the PLLA blends decreased from 180.07 ℃ to 176.65 ℃ when BMCD concentration increased， which indicated that the adding of the soft segment lowered the melting point of PLLA slightly. The enthalpy (Hm) of PLLA components were 45.71， 32.47， 30.69， 22.72， 25.61 and 26.55 J/g for pure PLLA， BMCD-3%， BMCD-5%， BMCD-7%， BMCD-10% and BMCD-20%， respectively. The second melting point(Tm-2) of the PLLA blerds increased from 194.49 ℃ to 205.80 ℃ when BMCD concentration increaced. The enthalpyHm-2 of stereocomplex components were 0.67， 2.59， 4.34， and 9.08 J/g for BMCD-5%， BMCD-7%， BMCD-10% and BMCD-20% respectively， indicating that more stereocomplex formed as the BMCD contents increased. The enthalpy of PLLA components decreased at first and then rose as the BMCD contents increased. A possible explanation of the enthalpy differences was that the stereocomplex crystallization (SC) formed first in the blending，and the SC hindered the formation of PLLA homocrystallization[32]. As the BMCD content increased， the stereocrystals acted as nucleating agents to promote the crystallization of PLLA[33].

Fig. 7 TGA curves of pure PLLA and PLLA/BMCD blends

Fig. 8 DSC thermograms of pure PLLA and the films of PLLA/BMCD blends(heating rate： 10 ℃/min)

Table2Thermal properties of pure PLLA and PLLA/BMCD blended polymers with various ratio

SampleTm-1/℃Tm-2/℃ Hm-1/(J·g-1)Hm-2/(J·g-1)Pure PLLA180.22-45.71-BMCD-3%180.07-32.47-BMCD-5%179.81194.4930.690.67BMCD-7%179.53196.5022.722.59BMCD-10%178.72196.8125.614.34BMCD-20%176.65205.8026.559.08

2.5 Crystalline structure

Fig. 9 FTIR spectra of pure PLLA， PLLA/BMCD blends： (a) characteristic peaks of HCPLA， (b) characteristic peaks of SCPLA crystallites

WXRD was also conducted on PLLA and BMCD blend films， and the results were shown in Fig. 10. All the PLLA/BMCD blended films showed the distinct peaks at 16.9°， 19.2° and 22.5°， which was agreement with the peaks at 16.3° and 18.7oreported in Ref.[36]. The results confirmed that copolymer films had a PLLA crystalline structure. The peaks at 12.2° and 21° were assigned to stereocomplex crystallization phase. As the content of BMCD increased from 3% to 20%， the peak became more and more obvious. These observations coincided well with the DSC and FTIR results.

To study the effect of the BMCD on the transparency of the blends， the visible light transmittance of PLLA and PLLA/BMCD were measured by a UV-VIS spectrophotometer (Fig. 11). It was obvious that the blends containing PLLA/BMCD showed a reduction of optical clarity compared to pure PLLA with increasing BMCD content. However， the blend transmittance was almost the same at 3% (mass fraction) loading of BMCD as pure PLLA； for the loading of 5% (mass fraction) of BMCD， the transmittance of the blend only decreased 5% at 400 nm compared to the pure PLLA， and the blend still met the requirements as a transparent package material[37].

Fig. 10 WXRD patterns of pure PLLA， and the films of PLLA/BMCD blends

Fig. 11 Visible transmission curves of pure PLLA， and the films of PLLA/BMCD blends

3 Conclusion

A branched BMCD copolymer was synthesized via siliconethoxy hydrolytic condensation at room temperature. PLLA/BMCD blends were prepared successfully in this work via a simple solvent evaporation method at various BMCD loadings. The similar thermal degradation profiles of pure PLLA and its blends suggested that BMCD did not alter the degradation mechanism of the PLLA matrix. Besides， the thermal stability of PLLA matrixes were enhanced slightly for the BMCD-10% and BMCD-20% blends owing to stereocomplexation. Tensile tests showed substantial increases in breaking elongation， with the addition of BMCD， respectively. SEM images illustrated that the fracture mode changed from brittle fracture to ductile fracture of PLLA with the addition of BMCD. FTIR and WXRD confirmed that stereocomplex crystal were formed in the blends of BMCD-5%， BMCD-7%， BMCD-10% and BMCD-20%. It was found that the presence of BMCD at the loading of 5% (mass fraction) barely changed the light transmittance of PLLA matrix.