Fully biobased thermoplastic elastomers: synthesis and characterization of poly(L-lactide)-b-polymyrcene-b-poly(L-lactide) triblock copolymers

Abstract

Fully biobased poly(L-lactide)-b-polymyrcene-b-poly(L-lactide) triblock copolymers with PLLA as the hard block and polymyrcene as the soft block were synthesized by the ring opening polymerization of L-lactide in the presence of the dihydroxyl-terminated polymyrcene precursor and organocatalyst. The copolymer composition and molecular weight of these triblock copolymers were confirmed by NMR and GPC results. Two separated glass transition temperatures were detected by both DMA and DSC techniques, indicating an existence of micro-phase separation in these triblock copolymers, which is a typical characteristic of thermoplastic elastomers with the content of soft block increases. Tensile testing revealed that PLLA-b-PM-b-PLLA (200) having 20 wt% polymyrcene show distinct yielding while other samples fracture at low strain without yielding. POM results indicated that all these spherulites show the same characteristic “Maltese cross” patterns. With the increasing content of polymyrcene, the perfection of spherulites decreases, especially for PLLA-b-PM-b-PLLA (200). Considering the current energy and environmental problems, it is expected that these fully biobased thermoplastic elastomers will be of great significance in expanding the applications of PLLA and solving the ecological crisis around us.

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Introduction

The key of future materials lies in the discovery and development of biobased, biocompatible, and renewable alternatives to petroleum-based materials due to the increased environmental concerns and resources crisis.^1–4 Poly(L-lactide) (PLLA), a sugar-derived polyester,⁵ is considered as a promising and ideal alternative to petroleum-based plastics in many fields, such as biomedical devices, bioengineering, packaging and the automotive industries due to its excellent biodegradability, biocompatibility, renewability and melt processability.^6–11 However, the inherent brittleness and very low strains (ca. 3%)¹² of PLLA have limited its wide applications.

Recently, with the aim of improving the utility of PLLA, great effort is being directed toward the synthesis of crosslinkable elastomers based on lactic acid or fully biobased polylactide-containing thermoplastic elastomers (TPEs) due to their ability to be designed to meet specific requirements for each application.¹³ Hu et al.¹⁴ synthesized a series of crosslinkable linear biobased elastomer composites with high strength and elasticity based on lactic acid, which are promising for engineering and medical applications. It is difficult to obtain the high-molecular-weight copolymers owing to the low activity of lactic acid in condensation process. Furthermore, a series of cross-linked poly(trimethylene carbonate-co-L-lactide) copolymers were synthesized by Whittaker.^15,16 These prepolymers were end-functionalized with acrylate and cross-linked using UV light. However, monomers with crosslinkable reaction site are needed to be further provided in the synthesis of PLLA-based elastomers mentioned above. Thermoplastic elastomers (TPEs) often consists of ABA triblocks, where A is PLLA block that could provide mechanical strength and also act as a physical crosslinker while B is meant to be flexible at ambient conditions to give elasticity and degradation.^3,17–19 Furthermore, many biologically derived monomers have been successfully used to produce a variety of fully biobased polylactide-containing block copolymers. Such as menthide,¹² 1,5-dioxepan-2-one,²⁰ ε-decalactone,^4,21–23 β-butyrolactone²⁴ and so on. Such completely biobased TPEs as described above will expand the potential utility of PLLA.

β-Myrcene(7-methyl-3-methylene-octa-1,6-diene), derived from natural plant oils (such as cress, hackmatack and rosaceous plant), is a promising candidate of the petroleum-dependent materials.^25–27 β-Myrcene contains three unsaturated bond (conjugated double bonds and isolated double bonds), which have high polymerization activity in anionic polymerization. Importantly, polymyrcene is an amorphous polymer with a T_g of −70 to −60 °C, which is close to that of the natural rubber reported in literature,²⁸ making it a suitable candidate as a soft block in TPEs. However, few studies have been reported on the synthesis of fully biobased polylactide-containing TPEs with β-myrcene.

In this paper, we describe a simple polymerization process for the synthesis of fully biobased TPEs with PLLA as end block and α,ω-hydroxyl polymyrcene (OH–PM–OH) as midblock. The effects of polymyrcene on the properties of PLLA were investigated by ¹H-NMR, DSC, DMA, GPC and TGA. It is expected that the results presented herein will be of help for a better understanding of the structure–property relationships of thermoplastic elastomers.

Experiment

Materials

β-Myrcene (β-My, J&K, 90%) was dried over CaH₂ and dibutylmagnesium, respectively, and then purified by vacuum distillation. Cyclohexane and tetrahydrofuran (Jinxi Chemical Plant, China, analytical reagent) were freshly distilled from sodium with benzophenone as the indicator and then stored under an argon atmosphere. Dichloromethane (Jinxi Chemical Plant, China, analytical reagent) were freshly distilled from CaH₂ and stored under an argon atmosphere. 2-Propanol was degassed prior to use. 1,5,7-Triazabicyclo-[4.4.0]dec-5-ene (TBD, J&K, 98%) were used as received. 4-Chloro-1-butanol from J&K was dried over CaH₂ in the protective atmosphere and then distilled under vacuum. Ethylene oxide (Aldrich) was first distilled from CaH₂ and then treated with a small amount of n-BuLi (0.5% mol) to remove the residual moisture and finally was condensed in a flame-dried flask prior to use. L-Lactide (Jinan Daigang Biomaterial Co., Ltd. China) was recrystallized from ethyl acetate twice, dried at 30 °C for 48 h under vacuum and then stored in the glovebox. 3-(t-Butyldimethylsiloxy)-1-butyl lithium was synthesized in our own lab. tert-Butyldimethylsilyl chloride, trifluoroacetic anhydride (TFAH, J&K), n-BuLi (J&K, 2.4 M solution in n-hexane) and other solvents were used as received.

Poly(L-lactide)-b-polymyrcene-b-poly(L-lactide) (PLLA-b-PM-b-PLLA) triblock copolymers with different PLLA block segment length were synthesized by ring-opening polymerization of L-lactide with α,ω-hydroxyl polymyrcene and TBD as macroinitiator and catalyst, respectively, in CH₂Cl₂ solution at room temperature. The synthetic route of PLLA-b-PM-b-PLA triblock copolymers have been shown in Scheme 1. In addition, the detailed procedures of α,ω-hydroxyl polymyrcene and these copolymers can been seen in the following text.

Scheme 1 Synthesis of PLLA-b-PM-b-PLLA triblock copolymers.

Preparation of α,ω-hydroxyl polymyrcene (OH–PM–OH). Polymyrcene was synthesized by anionic polymerization of β-myrcene in cyclohexane with 3-(t-butyldimethylsiloxy)-1-butyl lithium as the initiator in a single neck flask at 45 °C for 3 h. Then the reaction temperature fell off rapidly to room temperature and appropriate amount of ethylene oxide was added immediately for 12 h. Finally, the reaction was quenched by the addition of 2-propanol, and dried under vacuum at 30 °C overnight. The α-hydroxyl polymyrcene was dissolved in dry THF and several drops of concentrated hydrochloric acid was added to remove the protecting group. The solution was stirred at 25 °C for 2 h. The structure information have been shown in Fig. 1. The crude product was purified by azeotropic drying with dry THF three times prior to use as initiator.

Fig. 1 ¹H-NMR spectrums of OH–PM–OH.

Preparation of poly(L-lactide)-b-polymyrcene-b-poly(L-lactide) triblock copolymers (PLLA-b-PM-b-PLLA) ([L-LA]_o/[OH]_o = 200, 300, 400). The synthetic method of PLLA-b-PM-b-PLLA (200) was described in detail and the rest could be performed with the same way. To a solution of OH–PM–OH (0.2 g, 0.033 mmol) and TBD (18.5 mg, 0.133 mmol) in dichloromethane, the solution of L-lactide (1.92 g, 13.3 mmol) at the concentration of 2 mol L⁻¹ was injected into the flask. The reaction mixture was stirred at room temperature for 6 h, then the solution was poured into the methanol to obtain the crude product. Finally, the product was dried under vacuum at 45 °C overnight.

Characterization

Proton nuclear magnetic resonance spectra (¹H-NMR). All samples were recorded by a Bruker Avance 400 MHz spectrometer with the tetramethylsilane (TMS) as the calibration. Deuterated chloroform (CDCl₃) was used as the solvent.

Gel permeation chromatography (GPC). The molecular weight and its distribution were measured by gel permeation chromatography (GPC) using a Waters 1515 HPLC system. Tetrahydrofuran (THF) was used as the eluent at a flow rate of 1.0 mL min⁻¹ at 25 °C. Samples were dissolved in THF at the concentrations of 5.0 mg mL⁻¹. The molecular weights were calculated from polystyrene standards with a narrow polydispersity.

Differential scanning calorimetry (DSC). Non-isothermal crystallization process was measured by Q20 (TA instruments, USA) differential scanning calorimeter. Each sample (6–10 mg) was heated to 180 °C at the rate of 10 °C min⁻¹, keeping the temperature 5 min to remove the previous thermal history, then cooled to −100 °C at the rate of 10 °C min⁻¹, hold 5 min and finally heated to 200 °C at the same rate again. Glass transition temperature (T_g) was recorded.

Thermogravimetric analysis (TGA). Thermogravimetric analysis of PLLA-b-PM-b-PLLA triblock copolymers was performed by Q500 (TA instruments, USA) from 30 to 600 °C at a heating rate of 20 °C min⁻¹ under nitrogen flow.

Dynamic mechanical analysis (DMA). The DMA spectrum was obtained using Q800 (TA instruments, USA) operating from −90 to 80 °C at 1 Hz on 0.2 mm thick films.

Tensile testing. Tensile tests were carried out on at least five samples at 24 °C using an Instron 5567A tensile tester with a load cell of 100 N at a head speed of 10 mm min⁻¹. Film specimens (0.1–0.2 mm thick) were prepared by casting polymer solutions in dichloromethane (10% w/v) and cut to the correct dimensions (length is 40 mm).

Polarized optical microscopy (POM). The spherulite morphologies of PLLA-b-PM-b-PLLA copolymers were observed with a Leica 4500P polarized optical microscope (Germany) with a Linkam THMS600 hot stage. Thin films of samples were sandwiched between two thin glass slides, and then quenched to 120 °C after melted at 180 °C for 5 min, and maintain indicated time to observe the crystallization process.

Results and discussions

Synthesis and characterization of OH–PM–OH

The precise control synthesis of macroinitiator is a critical process for the preparation of well-defined triblock copolymers. In this paper, OH–PM–OH was synthesized by anionic polymerization of β-myrcene in cyclohexane with 3-(t-butyldimethylsiloxy)-1-butyl lithium as the initiator. Then this macroinitiator will be used to initiate the ring opening polymerization of L-lactide to obtain the triblock copolymers. As discussed in the previous report,²⁹ with respect to the 3-(t-butyldimethylsiloxy)-1-butyl lithium, difunctional anionic initiators have drawbacks in the polymerization process. With the increase of the segment length, chains will be in an entanglement state, which can inhibit the polymerization process thus affecting the degree of chain end functionality.

The chemical structure of OH–PM–OH was characterized by ¹H-NMR. As shown in Fig. 1, the peak at 0.05 ppm is assigned to the methyl protons (–Si–CH₃) attached to the silicon atom while the signal at 0.9 ppm represents the proton in –CH₃ of tert-butyl. Furthermore, the peak due to the methylene protons adjacent to the ω-chain end of the hydroxyl group is clearly observed at 3.65 ppm before the hydrolysis process, which confirm the existence of hydroxyl group at ω-chain end.^18,30,31 Compared with isoprene, the difference lies in the pendant group. Thus, the polymer also have different types of microstructures: 1,4-, 1,2-vinyl and 3,4-polymyrcene. From the ¹H-NMR spectrums, signals of polymyrcene can be easily classified. Peaks at 1.5–1.6 ppm are ascribed to the –CH₃ of the pendant group, while the peak at 2.1 ppm is the signal of –CH₂ and –CH unit linked to the carbon–carbon double bond of myrcene. The resonance at 5.1 ppm is correspond to the unsaturated side chain protons for all microstructures (–CH=C–), and the peaks at 4.8 ppm is attributed to the methylene protons in the 3,4-structure (–C=CH₂).^25,28,32,33 The resonance at 0.05 and 0.9 ppm disappear after hydrolysis with the concentrated hydrochloric acid, which confirms the existence of α-chain end of the hydroxyl group. In addition, one drop of TFAH was added into the polymer solution to further confirm the existence of hydroxyl group. The spectrum has been shown in ESI.† As we can see in Fig. S1,† the peak of the methylene protons adjacent to the chain end of the hydroxyl group at 3.65 ppm shifts to 4.35 ppm when the polymer solution contains TFAH, which confirms that we obtain HO–PM–OH successfully.

Besides, the molecular weight and the polydispersity index of the macroinitiator were detected by GPC. The GPC curve and data have been presented in Fig. S2† and Table 1, respectively. GPC curve of macroinitiator displays a unimodal peak with a relatively narrow weight distribution, indicating a relatively good homopolymer.

Table 1 Ring opening polymerization of L-LA with TBD as catalyst

Sample	[OH]o/[L-LA]o/[Cat]o	Conv.^a (%)	WPM^b (%)	Mw^c (kg mol^−1)	Ð^c
a Determined by the weight of monomer and product.b Calculated from the relationship according to the ref. 34: Mw,OH–PM–OH/Mw,PLLA-b-PM-b-PLLA × 100.c Determined by SEC in THF.
HO–PM–OH	—	—	—	9.6	1.13
PLLA-b-PM-b-PLLA (200)	1/200/2	90	22.0	43.8	1.28
PLLA-b-PM-b-PLLA (300)	1/300/3	87	17.3	55.4	1.41
PLLA-b-PM-b-PLLA (400)	1/400/4	85	14.3	67.4	1.62

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Synthesis and characterization of PLLA-b-PM-b-PLLA

We synthesized series of PLLA-b-PM-b-PLLA triblock copolymers with various segment length of PLLA by ring-opening polymerization of L-lactide in the presence of HO–PM–OH with TBD as catalyst. All triblock copolymers were named as following: PLLA-b-PM-b-PLLA (X), where X denotes the ratio of [L-LA]_o/[HO]_o. The relatively parameters have been listed in Table 1.

Fig. 2 illustrates the structures of PLLA-b-PM-b-PLLA triblock copolymers. The signal at 5.15 ppm is assigned to the proton of –CH in PLLA segment.^35,36 Additionally, the molecular weight and the polydispersity index of the triblock copolymers were detected by GPC. Results have been presented in Fig. S2† and Table 1. We could clearly see that all GPC curves display a unimodal peak with a narrow weight distributions (M_w/M_n < 2). For all triblock copolymers, the peak shifts to the higher molar masses range after the second polymerization step, indicating the existence of triblock copolymers rather than the blend of the homopolymers.³⁷ With the increases of L-lactide content, the molecular weight increases as expected, however, the content of flexible block decreases, which means the toughing effect of polymyrcene will be weakened. As shown in Table 1, the conversion of L-lactide decreases with the ratio of [L-LA]_o/[OH]_o increases as a result of the decreasing content of terminal hydroxyl that exposes to the outside of the molecular chain entanglement with the growth of molecular chains.

Fig. 2 ¹H-NMR spectrums of PLLA-b-PM-b-PLLA triblock copolymers.

Thermo-mechanical properties of PLLA-b-PM-b-PLLA

It must been mentioned that glass transition temperature, a temperature at which there is an obvious enhancement in motion of molecular segments with increasing temperature, is commonly used to evaluate phase miscibility in copolymers.³⁸ Fig. 3 describes the second heating curves of these triblock copolymers at a rate of 10 °C min⁻¹ and all results have been listed in Table 2. As we could see, all plots demonstrate two glass transition temperatures, indicating the microphase separation. Compared with the T_g of α,ω-hydroxyl polymyrcene, that of polymyrcene segment in copolymers decreases by 5–7 centigrade slightly, which is ascribed to reason that the mobility of polymyrcene segment is limited by the introduction of PLLA chains. With the increasing chain length of PLLA, T_g and cold crystallization temperature of PLLA segment increase due to the its rigidity. The melting points of these triblock copolymers are 159.5, 152.0 and 159.0 °C, for PLLA-b-PM-b-PLLA (200), PLLA-b-PM-b-PLLA (300) and PLLA-b-PM-b-PLLA (400), respectively.

Fig. 3 DSC curves of PLLA-b-PM-b-PLLA triblock copolymers.

Table 2 Thermal parameters of PLLA-b-PM-b-PLLA triblock copolymers

Sample	Tg1 (°C)	Tg2 (°C)	Tcc (°C)	Tm (°C)
a Determined by DMA plots.
PM	−65.1	—	—	—
PLLA-b-PM-b-PLLA (200)	−59.1(−41.3^a)	48.8(67.2^a)	111.9	159.5
PLLA-b-PM-b-PLLA (300)	−60.0	47.1	114.2	152.0
PLLA-b-PM-b-PLLA (400)	−57.8	52.9	120.4	159.0

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Additionally, TGA is operated to investigate the thermostability and the degradation profile of PLLA-b-PM-b-PLLA triblock copolymers. It is well known that the thermal degradation of polymers may involve thermohydrolysis, zipper-like depolymerization, thermo-oxidative degradation, or transesterification reactions.²² Fig. 4 displays the TGA and DTG curves of these triblock copolymers at a heating rate of 20 °C min⁻¹ under nitrogen atmosphere range from 50 to 600 °C. As shown in Fig. 4(a), all copolymers show an obvious two-stage decomposition process except polymyrcene. For the homopolymer, the initial decomposition temperature is 345 °C until the degradation process was completed at 500 °C with a residue of approximately 2%, suggesting a high purity organic compound. The first decomposition stage in copolymers is correspond to the PLLA segment due to its low stability, which is proceeded by the chain end “unzipping” mechanism, while the second stage at higher temperature is related to the polymer chain thermal scission mechanism.⁸ Interestingly, for all the copolymers, the transition degradation temperatures were almost independent of the composition. DTG plots in Fig. 4(b) show two separated peaks, which confirms that these copolymers compose of two segments with distinct thermal stability. The soft segment is more thermally resistant than the PLLA segment as is observed.

Fig. 4 (a) TGA and (b) DTG curves PLLA-b-PM-b-PLLA triblock copolymers.

However, the glass transition temperature of polymyrcene is undetectable in DSC curve; therefore, DMA is further used to investigate the glass transition of the hard and soft blocks. Fig. 5 displays the DMA plot of PLLA-b-PM-b-PLLA (200). As illustrated in Fig. 5, an obviously decrease of storage moduli is observed in the temperature regions of −60 to −30 and 50–70 °C, which are attributed to the α-relaxation of polymyrcene and PLLA segments due to the stronger mobility of chain segments at glass transition temperatures, respectively. Furthermore, the corresponding loss modulus and tan δ curve are also featured by two separate peaks, the low temperature peak at −40 °C is related to the glass transition of polymyrcene, while the peak at higher temperature, 67 °C, is the glass transition of PLLA segment.^22,39,40 Therefore, microphase separation is further confirmed. For other two copolymers, DMA curves have been shown in the ESI (Fig. S3 and S4†). All plots give the similar trends like that demonstrated in Fig. 5, however, the sample was broken with the temperature increased due to its inherent brittleness of PLLA. Therefore, the toughing effect is limited when the content of polymyrcene less than 20 wt%.

Fig. 5 Storage modulus, loss modulus and tan delta versus temperature for PLLA-b-PM-b-PLLA (200).

The toughening effect of polymyrcene on these triblock copolymers was evaluated by tensile testing. Fig. 6 displays the typical stress–strain curves of PLLA-b-PM-b-PLLA triblock copolymers with different length of PLLA segment. As shown in Fig. 6, the stresses show linear response to strain at the low strain region (<2%). The modulus of all samples increase from 430 to 830 MPa with the content of polymyrcene decreases. Furthermore, PLLA-b-PM-b-PLLA (200) having 22 wt% polymyrcene show distinct yielding while others with 17.3 and 14.3 wt% soft block fracture at 2% strain without yielding for PLLA-b-PM-b-PLLA (300) and PLLA-b-PM-b-PLLA (400), respectively. With increasing content of polymyrcene, the tensile strength (from 10 to 8 MPa) and tensile modulus (from 830 to 430 MPa) of these triblock copolymers reduce, which is attributed to the low modulus and tensile strength of polymyrcene elastomer. Compared with neat PLLA, a typical stiff and brittle polymer, the strain at break of PLLA-b-PM-b-PLLA (200) with 22 wt% polymyrcene increases slightly. As discussed above, the toughing effect is limited when the content of polymyrcene less than 20 wt%, which is in agreement with the results of tensile testing.

Fig. 6 Typical stress–strain curves of PLLA-b-PM-b-PLLA triblock copolymers.

Spherulitic morphologies of PLLA-b-PM-b-PLLA

The effects of polymyrcene on the structures and thermo-mechanical properties of PLLA have been investigated systematically, so it is necessary to study how the spherulitic morphologies were affected by the soft block since the spherulitic morphology and spherulite size could significantly affect the physical properties of biodegradable crystalline polymers.^41,42 Fig. 7 displays spherulitic morphologies of PLLA-b-PM-b-PLLA copolymers crystallized at the indicated temperatures (120 °C). As shown in Fig. 7, all these spherulites show the same characteristic “Maltese cross” patterns regardless of the chain length of PLLA. Furthermore, with the increasing content of polymyrcene, the perfection of spherulites decreases, especially for PLLA-b-PM-b-PLLA (200), which is attributed to low mobility and regularity of PLLA segments with the incorporation of amorphous polymyrcene.

Fig. 7 Spherulitic morphologies of PLLA-b-PM-b-PLLA triblock copolymers crystallized at 120 °C: (a) PLLA-b-PM-b-PLLA (200), (b) PLLA-b-PM-b-PLLA (300), (c) PLLA-b-PM-b-PLLA (400) and (d) PLLA.

Conclusions

Fully biobased polylactide-containing thermoplastic elastomers have been successfully synthesized with polymyrcene as the flexible segment and PLLA as the hard segment. α,ω-Hydroxyl polymyrcene (HO–PM–OH) was prepared as a macroinitiator to initiate the ring opening polymerization of L-lactide, thus forming ABA type triblock copolymers with different PLLA segment length. With the increase content of L-lactide, molecular weight of these copolymers increase while the monomer fractional conversion decrease due to the decreasing content of terminal hydroxyl that exposes to the outside of the molecular chain entanglement in the growth process of chains. All GPC traces are unimodal and shift to the high molecular weight region with the PLLA block length increases. From DMA plots, two narrow separated glass transition temperatures were observed clearly, indicating the phase immiscibility of these triblock copolymers. It must be mentioned that the loss modulus curve also show two peaks, the one at lower temperature is ascribed to the relaxation of the middle block, while the peak at higher temperature is related to the glass transition of PLLA segment. DSC displays two glass transition temperatures, which is further confirmed the existence of phase separation. TGA curves reveal that these triblock copolymers exhibit two thermal degradation stages regardless of PLLA segment length and the thermal stabilities of the soft blocks are better than that of PLLA. Tensile testing show that with the increasing content of polymyrcene, the tensile strength and tensile modulus of these triblock copolymers reduce due to the low modulus and tensile strength of polymyrcene elastomer. Furthermore, the yielding behavior of these triblock copolymers disappears with the content of PLLA segment increases, which is corresponding to the rigidity of PLLA. Results of POM analysis suggest that all these spherulites show the same characteristic “Maltese cross” patterns regardless of the chain length of PLLA. Furthermore, the perfection of spherulites decreases due to low mobility and regularity of PLLA segments with the incorporation of amorphous polymyrcene.

Acknowledgements

This work was financially supported by National Program on Key Basic Research Program of China (973 Program No. 2015CB654700 (2015CB654701)), the National Science Foundation of China (No. U1508204), and the Fundamental Research Funds for the Central Universities (DUT16QY38).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08689f

‡ C. Z. and Z. W. contributed equally to this work as co-first authors.

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