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Segmental copolymers of condensation polyesters and polylactide
Polymer Degradation and Stability 97 (2012) 1852e1860
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Segmental copolymers of condensation polyesters and polylactide czyk, A. Jó Z. Florjan zwiak, A. Kundys, A. Plichta*, M. De˛ bowski, G. Rokicki, P. Parzuchowski, P. Lisowska, A. Zychewicz Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 December 2011 Received in revised form 21 March 2012 Accepted 22 March 2012 Available online 2 April 2012
L-Lactide (LA) polymerizations carried out in the presence of various types of polyesterodiols and polyesterocarbonatediols obtained from 1,4-butanediol, 1,3-propanediol and dimethyl esters of adipic, terephthalic and carbonic acids were studied. It was found, based on 1H NMR and MALDI ToF analysis of the chains microstructure that the reactions carried out in bulk at 190 C in the presence of tin (II) 2ethylhexanoate as a catalyst led to the formation of a mixture of polylactide (PLLA) and block copolymers of LA with the applied macrodiols with high yield. DSC studies of the obtained products indicate that the segments comprising of adipic and carbonic acid derivatives were fully miscible with PLLA causing the reduction of its glass transition temperature. The segments containing terephthalic acid monomeric units in its structure showed only slight miscibility with PLLA segments and the obtained products consist of two phases of glass transition temperatures close to those of PLLA and the macrodiol. These systems are characterized by better thermal stability and higher elasticity than PLLA. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Polylactide Block copolymers Condensation copolyesters Block miscibility
1. Introduction Poly(L-lactide) (PLLA), biodegradable and compostable aliphatic polyester is derived from renewable resources such as corn, potatoes, and sugar beets [1e3]. PLLA is one of the most promising biodegradable materials and can be widely used in packaging, fibers, and consumer goods due to its high strength, biocompatibility, transparency, and biodegradability. However, the major drawbacks of PLLA are high brittleness and poor impact strength, which restricts its applications [3]. A number of methods have been proposed to improve the mechanical properties of PLLA, among which plasticization, addition of rigid fillers and blending with a variety of flexible polymers are the most commonly used approaches [4e7]. Many plasticizers, including lactide [8], citrate esters [9,10], oligo(lactic acid) [11], and poly(oxyethylene) glycol (PEG) [12] have been applied for PLLA modification. Improvement of ductility of PLLA, both amorphous and semi-crystalline, was also achieved by addition of poly(oxypropylene) glycol (PPG). Martino et al. compared the plasticizing effects of three commercial adipates as plasticizers for PLLA. For 10 wt% of the plasticizer content, di-2ethylhexyladipate (DOA) resulted in much higher elongation (259%) of the plasticized PLLA than that of the two polymeric
* Corresponding author. Tel.: þ48 22 234 5632; fax: þ48 22 234 7279. E-mail address: [email protected] (A. Plichta). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2012.03.035
adipates (5 and 7 wt%, respectively). At 20 wt% of the plasticizer content, however, both polyadipates resulted in much higher elongation (>480%) of the plasticized PLLA with respect to DOA (295%) [13]. The same authors applied polyadipates of higher molecular weights (GlyplastÒ G206/7 and G206/3) as PLLA plasticizers. However, polymeric adipates appeared to be a less efficient modifier. Taking into account that higher molecular weight of the plasticizer causes a reduction in its migration rate and thus improves the morphological stability of PLLA during storage, the combination of small molecule plasticizers with polymeric ones of complementary advantages was proposed. Ren et al. used a combination of glycerol triacetate with poly(butylene adipate) to plasticize PLLA [14]. Similarly, Lemmouchi and coworkers applied a mixture of TBC with a block copolymer of lactide and ethylene oxide (PLLA-b-PEG) with different molecular architecture to lower the Tg and enhance elongation at break of PLLA, while the use of PLLA-b-PEG copolymers alone well maintained tensile strength and modulus [15]. Blending with flexible polymers is another most extensively used strategy to improve the ductility of PLLA. Typical polyolefins are immiscible with poly(L-lactide). Thus, they should be modified prior to blending to improve miscibility. Oyama presented a dramatic improvement in the mechanical properties of PLLA by its reactive blending with poly(ethylene-co-glycidyl methacrylate) (EGMA) containing 30 wt% of methyl acrylate and 3 wt% of glycidyl methacrylate [16]. It was shown that the interfacial reaction between the component polymers contributes to the formation of
czyk et al. / Polymer Degradation and Stability 97 (2012) 1852e1860 Z. Florjan
supertough PLLA material. However, the biodegradation of such a blend due to the presence of polyethylene fragments, was deteriorated. A similar approach was proposed by Liu and coworkers [17]. A ternary blend system consisting of poly(L-lactide), an epoxy containing elastomer, and a zinc ionomer indicated high toughness with moderate tensile strength and modulus. In this case zinc ions catalyzed the cross-linking of epoxy-containing elastomer and promoted the reactive compatibilization at the interface of PLLA and the elastomer. Biodegradable blends of PLLA which exhibited improved impact strength are obtained with use of flexible aliphatic polyesters such as poly(ε-caprolactone) (PCL) [18,19], poly(ethyleneco-butylene succinate) [20], poly(butylene succinate) (PBS) [21,22], poly(trimethylene carbonate) (PTC) [23]. PLLA blended with aliphatic poly(trimethylene carbonate-co-ε-caprolactone) indicated improved toughness [24]. Such material did not break in Izod notched impact testing. Very recently, Dubois’s group has found that random copolyesters of d-valerolactone (VL) and ε-caprolactone (CL) added at 10 wt.% into a commercially available PLA matrix led to a significant improvement in toughness of PLA materials [25]. To improve miscibility, instead of homopolyesters, copolymers with L-lactide were used for blending with PLLA. Poly(butylene succinate-co-L-lactate) was reported to blend with PLLA. It was found that the blends showed higher elongation at break than their parent polymers [26]. Copolymers of lactide and ethylene adipate (PLEA) were prepared by ROP of lactide initiated with oligo(ethylene adipate) of chains terminated with hydroxyl groups. Results confirmed the incorporation of lactic acid segments into the chain of PLEA copolymers as well as the existence of ester exchange reaction. Introduction of ethylene adipate segments leads to thermal stability [27]. Grijpma et al. reported the use of a block copolymer of L-lactide with a rubbery LLA/CL (50/50 mol/mol) copolymer segment to improve the elasticity of PLLA. With 34 wt% of rubber block, the copolymer exhibited an elongation of 1500% and did not fracture during the Charpy impact test. However, the tensile strength decreased to 30.8 MPa [28]. Grijpma et al. showed also that the TMC block (Mn ¼ 65 000) in the tri-block copolymers was also effective in toughening PLLA. With varying weight content of TMC rubber from 10.9 to 21.4 wt%, the elongation increased from 135 to 210% [29]. It was found that some biodegradable and flexible polyurethanes containing polylactide as well as other polyester type polyols can also be used to blend with PLLA to improve its flexibility [30e33]. Taking into account that aliphatic-aromatic copolyesters like poly(butylene adipate-co-terephthalate) (PBAT) are fully biodegradable and displayed high flexiblity and ductility, Jiang et al. used
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PBAT for blending PLLA. The presence of aliphatic-aromatic copolyester lead to toughening of the polylactide material. The elongation of PLLA containing 5 wt% of PBAT increased from 3.7 to 115% [34]. Because of weak interfacial adhesion in the blends, impact toughness was only slightly improved (4.4 kJ/m2 with 20 wt% of PBAT and 2.6 kJ/m2 for neat PLLA). To improve the compatibility, a reactive compatibilizer was used by Zang et al. [35]. The addition of only 2 wt% of the terpolymer of ethylene, acrylate ester, and glycidyl methacrylate (T-GMA) led to almost a two-fold increase in notched Charpy impact strength (30e40 kJ/m2) in comparison with that of uncompatibilized binary blend. Coltelli et al. studied the influence of peroxide (hexane 2,5-dimethyl-2,5-di(tert-butylperoxide)) on rheological and mechanical properties of the blends of PLLA with poly(butylene adipate-co-terephthalate). The formation of copolymers results from the reaction of PBAT macroradicals with PLLA [36]. Dicumyl peroxide (DCP) was reported to be useful to enhance the compatibility between PLLA and PBS via in situ compatibilization [37]. To achieve enhanced miscibility as well as improved mechanical properties of the blends with PLLA, various compatibilizers are additionally introduced. Wu et al. studied the influence of the presence of copolyester on mechanical properties of PLLA blend with poly(ε-caprolactone). It was shown that di- and triblock compatibilizer-copolymers (PLLA-b-PCL and PCL-b-PLLA-b-PCL) improve the interphase and mechanical properties of PCL/PLLA blends [38]. Moreover, Albertsson et al. showed that by altering the proportions of different monomers of triblock copolymers containing an amorphous middle block [poly(1,5-dioxepan-2-one), polyTMC or poly(but-2-ene-1,4-diyl malonate)] and semicrystalline terminal blocks [PLLA, PCL], the mechanical properties of copolymers could be tailored. Such triblock copolymers showed a highly elastic behavior, whereas modulus and tensile stress at break were unchanged or slightly lower [39e41]. In this paper we present the results of our work on the synthesis of compatibilizers containing segments of PLLA and polyesters obtained in polycondensation reactions. The chosen process is based on LA ring opening polymerization (ROP) carried out in bulk in the presence of hydroxyl end-capped aliphatic or aliphatic-aromatic polyesters and tin(II) 2ethylhexanoate. We assumed that incorporation of macrodiols may result mainly from e ROP of LA initiated by hydroxyl end-groups of polyesters (Eqns. (1a),(1b)), e Intermolecular chain transfer reaction between the growing PLLA chains and hydroxyl end-capped macromolecules (Eq. (1c)),
czyk et al. / Polymer Degradation and Stability 97 (2012) 1852e1860 Z. Florjan
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However, one cannot exclude that these elementary reactions are accompanied by several others processes like homopolymerization of LA initiated by hydroxyl-impurities and transesterification reactions leading to the segmental exchange and formation of macrocyclic products. In order to gain a greater understanding of these processes we have studied the reaction product be means of 1H NMR, MALDI ToF and GPC techniques. We report here also some preliminary data concerning the phase structure, thermal stability and mechanical properties of the products obtained. 2. Experimental 2.1. Materials L-Lactide (Aldrich) was recrystallized consequently from dry isopropanol, then toluene and vacuum dried before polymerization. Toluene and isopropanol were dried with sodium and distilled. Tin (II) 2-ethylhexanoate [Sn(EH)2], adipic acid (AA), dimethyl terephthalate (DMT), dimethyl adipate (DMA), 1,4butanediol (1,4-BD), 1,3-propanediol (1,3-PD), dimethyl carbonate (DMC) and titanium tetrabutoxide, were purchased from Aldrich and used without further purification. Poly(1,4-butylene adipate) (PBA) of Mn 1000 and poly(1,4-butylene terephthalate) (PBT) of Mv 38000 were supplied by Aldrich. Dichloromethane and methanol were used without purification. 2.2. Procedures All condensation copolyesters were obtained in two-step transesterificationepolycondensation reaction between appropriate diesters and diols mixed in the molar ratio of 1:2, respectively. The substrates of the same type were used in equimolar amount with the exception of PPAT synthesis, where the molar ratio of DMA/DMT was 3. The process was catalyzed by 1 mol % of Sn(EH)2 (with regard to the total amount of diesters). The first step was carried out under nitrogen at 160 C and methanol was distilled off as the reaction by-product. The transesterification step was completed after the methanol yield was close to 85%. In the second step the temperature was increased to 180 C and the pressure was slowly reduced to 103 mbar. The polycondensation reaction has been carried out for 10 h while diols were distilled off. Then the reaction mixtures were cooled to room temperature and dissolved in CH2Cl2. The crude polymers were precipitated and washed with cold methanol. The final products were dried under vacuum at 40 C for 48 h. This procedure was successfully applied for the synthesis of the following polymers: poly(1,3-
propylene adipate) (PPA), poly(1,4-butylene adipate) (PBA), PPAC poly(1,3-propylene adipate-co-carbonate), poly(1,3-propylene-co1,4-butylene adipate) (PPBA), poly(1,3-propylene-co-1,4-butylene adipate-co-carbonate) (PPBAC), poly(1,3-propylene adipate-co-terephthalate) (PPAT), poly(1,4-butylene adipate-co-terephthalate) (PBAT), poly(1,3-propylene-co-1,4-butylene adipate-co-terephthalate) (PPBAT) and poly(1,3-propylene-co-1,4-butylene adipate-coterephthalate-co-carbonate) (PPBATC). On the other hand, PBAT (Table 1, no. 9) was obtained by polycondensation reaction of PBA and PBT. In this case, PBA was synthesized during polycondensation reaction of AA with 1,4-BD, catalyzed by tetrabutoxy titanium (0.1 mol % of AA). The mixture of PBA and PBT was heated up to 240 C (at the rate of 7 C/min) with pressure reduced to 0.031 Torr and then the polycondensation reaction was carried out for 90 min. ROP of LA in the presence of condensation copolyesterodiols was catalyzed by 0.01 wt % Sn(EH)2 (with respect to the batch). Monomer, condensation polyesterodiol and catalyst were placed in a three-necked round-bottom flask equipped with a magnetic stirrer and condenser. The reaction was carried out at 190 C for 3 h under nitrogen atmosphere. After polymerization, the final product was cooled to room temperature and dissolved in methylene chloride and then poured into excess methanol. The precipitated product was filtered and washed with methanol and dried in a vacuum oven at 40 C for 48 h. “Pure” PLLA was synthesized as a reference by ROP of LA (29.5 g) with 0.04 wt. % of Sn(EH)2 as catalyst in a 250 mL reactor. The reaction was carried out at 190 C for 3 h under nitrogen atmosphere. The final product was dissolved in methylene chloride, precipitated in cold methanol and dried in a vacuum oven at 40 C for 48 h. 2.3. Calculations The solubility parameters d ¼ (E/V)0.5 of PLLA and condensation polymers were calculated in order to distinguish between homogeneous and heterogeneous systems. The energy of cohesion E can be substituted with F2/V, then d ¼ F/V. The group attractive constant (F) and molar volume (V) are additive quantities and, therefore, can be calculated for each m.u. from respective values estimated for substructures. 2.4. Measurements 1 H NMR measurement was performed on Varian Mercury 400 MHz spectrometer using CDCl3 as solvent. The molecular weight and molecular weight distribution were determined by
Table 1 Characterization of condensation polyesters. No.
Macroinitiator symbol
1 2 3 4 5 6 7 8 9 10 11 12
PBA
a b c d
PPA PPAC PPBA PPBAC PBAT
PPAT PPBAT PPBATC
Compositiona Diol units(%)
Acid units (%)
B 100 B 100 P 100 P 100 P 31.5 P 26.1 B 100 B 100 B 100 P 100 P 56.4 P 25.7
A 100 A 100 A 100 C 25.8 A 74.2 A 100 C 21.3 A 78.7 T 52.4 A 47.6 T 50.7 A 49.3 T 51.2 A 48.8 T 31.5 A 68.5 T 47.9 A 52.1 T 43.3 A 45.0 C 11.7
B 68.5 B 73.9
B 43.6 B 74.3
Mn,HNMRb (g/mol)
Mn,GPCc (g/mol)
DIc
Tg ( C)
Tm ( C)
d (MPa0.5]
1100 9000 5400 3600 11,500 7700 2200 8100 16,500 10000 14,400 9200
1400 7400 3900 2800 16200 7200 1600 6000 13400 13500 11200 7400
2.92 1.57 1.31 2.02 2.09 2.23 1.51 2.08 1.70 2.53 1.70 2.12
69.9 57.8 56.9 53.8 57.7 58.0 35.5 32.0 27.3 30.3 20.8 20.9
49.4 57.9 45.6 ed 21.5 12.8 130.9 133.3 148.2 68.3 75.4 75.9
19.6 19.8 20.1 20.3 19.8 19.8 20.8 20.9 20.9 21.0 21.1 21.0
Abbreviation of diol units: P for 1,3-propanediol, B for 1,4-butanediol and acid units: A for adipic, T for terephthalic, C for carbonic. Estimated on the basis of 1H NMR spectra (methylene end-groups method). Absolute values obtained from GPC with triple detection system (RI, IV, LS). Amorphous product.
czyk et al. / Polymer Degradation and Stability 97 (2012) 1852e1860 Z. Florjan
GPC on a Viscotek system comprising GPCmax and TDA 305 triple detection unit (RI, IV, LS) equipped with one guard and two DVB Jordi gel columns (102e107, linear, mix bed) in CH2Cl2 as eluent at 35 C at a flow rate of 1.0 mL/min. Triple detection was used for determination of absolute molecular weight and DI of condensation copolyesters, whereas RI detector and PS calibration were applied for block copolymers characterization. The sample of block copolymer was fractionated using a LabAlliance GPC system equipped with one DVB Jordi gel column in CHCl3 as eluent at 25 C at a flow rate of 0.75 mL/min. MALDIeToF mass spectrometry was performed on AXIMA Performance instrument. Dithranol was used as MALDI matrix. The DSC measurement were performed using a DSC Q200 V24.2 Build 107
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apparatus. The first heating run from 100 C to 200 C was performed at a heating rate of 5e10 C/min in order to study crystallinity, then cooling at the rate of 20 C/min was applied. The second heating run was measured at the rate of 20 C/min to determine glass transition temperatures. TGA measurements were conducted using a TA Instruments SDT Q600 instrument, at heating rate of 5 C/min under air atmosphere. The tensile properties were examined with an Instron mechanical tester, model 5566, at tensile speed of 4 mm/min at room temperature. The specimens were cut into strips with approximate dimensions 80/20/0.3 mm (l/w/d) using films prepared by hot pressing at 170 C in a hydraulic press. Five specimens of each material were used.
Fig. 1. 1H NMR spectra of (a) PBA macroinitiator (Table 1, no. 1), and products of LA polymerization in the presence of PBA (b) at 190 C for 3 h (Table 2, no. 1b) and (c) 220 C for 8 h.
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Fig. 2. 1H NMR spectra of (a) PBAT macroinitiator (Table 1, no. 7), and (b) products of LA polymerization in the presence of PBAT (Table 2, no. 7).
Table 2 Properties of the LA polymerization products obtained in ROP in the presence of hydroxyl-end-caped polyesters. No.a
MIa
([LA]/[MI])0b (mol/mol)
aLAc (%)
xPLLAd (mol %)
uPLLAe (wt. %)
Mw,GPC f (g/mol)
DIf
Tg ( C)
Tg,Foxg ( C)
1a 1b 2a 2b 3 4 5 6 7 8a 8b 9 10 11 12
PBA PBA PBA PBA PPA PPAC PPBA PPBAC PBAT PBAT PBAT PBAT PPAT PPBAT PPBATC
18.4 799 591 798 120 218 911 486 777 639 495 722 498 1210 511
56 69 81 95 86 77 85 75 62 90 93 90 84 59 88
1 62 45 13 20 48 74 75 e e e e e e e
57 98 88 92 73 87 91 87 96 91 89 85 85 87 88
9800 42400 77800 107000 36000 35800 61200 36600 53800 39400 91800 105000 37900 37700 73900
1.21 1.51 1.92 1.79 1.83 2.67 2.20 3.33 2.36 1.57 2.04 2.10 3.64 2.73 3.12
10.5 nd 48.0 nd nd 34.0 46.2 36.9 60.3 nd 18.8/59.1 nd 20.7/46.8 10.5/54.7 18.4/58.0
11.0 e 41.6 e e 40.2 45.4 39.1 e e e e e e e
a b c d e f g
Nos. correspond to those from Table 1; MI - Macroinitiator symbol. Initial molar ratio of L-lactide (LA) to macroinitiator (MI). Conversion of L-lactide calculated from 1 HNMR spectra. Molar fraction of LA homopolymer chains in the system. Fraction of PLLA in the product. Calculated by means of GPC with polystyrene calibration. Tg of copolymer calculated from the Fox equation: 1/Tg,copolymer ¼ uPLLA/Tg,PLLA þ uMI/Tg,MI; Tg [K].
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3. Results and discussion The oligo- and polyesterodiols which were used as precursors of elastic segments (ES) were prepared from dimethyl esters and diols in one-batch two-step polycondensation in the presence of tin (II) 2-ethylhexanoate as a catalyst. The 1,3-propanediol, 1,4-butanediol and methyl esters of adipic, terephthalic and carbonic acids were employed as starting materials. The composition of all of these multicomponent aliphatic (Fig. 1a) and aliphatic-aromatic (Fig. 2a) polymers was determined by the means of 1H NMR (Table 1). The same technique was applied for estimation of Mn by the end-group analysis method; however, these results were verified by absolute GPC measurement (Mn and DI) using the triple detection system (RI, IV, LS). The differences between these two techniques usually did not exceed 30% of the chromatography data. The value of Mn,GPC varies in the range of 1000e16500. The distribution of molecular weight is moderate broad (DI 1.3e2.5) except the commercial sample of PBA (Table 1, no. 1). The materials obtained can be divided into two general categories depending on the composition. The first one includes aliphatic polyesters based on adipic and optionally carbonic acids derivatives which exhibit very low Tg in the range of 70 to 50 C. The other group are aliphatic-aromatic polyesters containing 30e50 mol % of terephthalate units which reveal slightly higher Tg in the range of 35 to 20 C. Most of the products were semi-crystalline excluding aliphatic polyesterocarbonate - PPAC, which existed in the form of a viscous liquid. It should be noticed that solubility parameters (d) calculated for all the obtained materials (19.6e21.1 MPa0.5) are very close to that of PLLA (20.7 MPa0.5). In order to obtain segmental copolymers, ring opening polymerization (ROP) of LA was carried out in the presence of ES-diols and 0.01 wt. % of tin 2-ethylhexanoate as catalyst. The synthesis was performed in bulk at 190 C (which resemble the conditions applied in industrial technologies) for 3 h. The 1H NMR studies of the reaction products reveal that under these conditions signals of terminal units CH2OH (Figs. 1a and 2a, signal a) present in ES molecules a decay and in the case of aliphatic ES a new kind of signals coming from CH(CH3)OH end-groups derived from PLLA chains could be clearly observed (Fig. 1b, signal j). Therefore, one can expect that ES molecules are totally incorporated into the polylactide chains. It should be noticed that the other signals characteristic for m.u. of ES did not change, which suggests that these segments were not involved in transesterification processes (Figs. 1b and 2b). To support this hypothesis, we have studied the LA
Fig. 3. MALDI ToF spectrum of PLLA-PBA-PLLA (Table 2, 1a).
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polymerization in the presence of PBA at 220 C for 8 h and we found the presence of an additional signal that can be attributed to the diad LAc-A (Fig. 1c, signal o) which should not be observed in the expected block copolymer. One can also observe a significant
Fig. 4. Fragments of MALDI ToF spectrum of LA polymerization product carried out in the presence of PBA (Table 2, no. 1a) in the following m/z ranges: (a) 1000e2000, (b) 2000e3000, (c) 4000e4300.
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czyk et al. / Polymer Degradation and Stability 97 (2012) 1852e1860 Z. Florjan
Scheme 1.
increase in the intensity of the signal characteristic for the link between PLLA and ES blocks (B-LAc, signal d’). Taking these into consideration we can treat the signals characteristic for ES m.u. as internal standards in the analysis of the product obtained at 190 C. In Table 2 the LA conversion and weight fraction of PLLA m.u. in the products obtained determined on the basis of 1H NMR spectra are shown. In most cases the PLLA weight fraction constituted as much as 85e95%, whereas LA conversion varied from 60 to 90%. However, on the basis of 1H NMR study it is impossible to determine the concentration of LA m.u. in block copolymers and homopolymers that can be also formed in these systems. In the case of systems containing aliphatic ES one can only roughly estimate the molar fraction of LA homopolymer chains xPLLA by analyzing the integrals of the terminal wCH(CH3)EeOH signal (Fig. 1b, j) and that of wCH2OC(O)CH(CH3)w in linking unit (d’). Due to the relatively low intensity of both signals the precision of this analysis is not high (in the case of aliphatic-aromatic ES, the signal of j protons overlaps with that of methylene protons in ester groups d), nevertheless it shows clearly that we are dealing actually with a mixture of homo- and copolymers of LA. This observation was also supported by MALDI ToF analysis of low molecular weight product based on PBA (Table 2, no. 1a). Fig. 3. presents the spectrum of sodium-cationized ions. The m/z range of the highest signal intensity is from 1500 to 3500, however, the overall distribution in the sample covers the m/z range from 1000 to 5000. We analyzed three regions in the m/z ranges of (a) 1000e2000 (Fig. 4a), (b) 2000e3000 (Fig. 4b) and (c) 4000e4300 (Fig. 4c). The following peak assignments (notifications) for various populations were used: x/y - for linear macromolecules and x/y M - for macrocyclics, where x is the degree of polymerization of ES and y is the number of lactic acid units in the molecules (Scheme 1). In the first range (a) no separated signal of the unreacted PBA macroinitiator was found (x/0), which confirms the very high incorporation efficiency. On the other hand, that area is rich of the products of LA polymerization initiated with water or LA hydrolysis product (0/y), with the odd and even values of y (lactic acid units),
Fig. 5. GPC analysis of PBA/PLLA system (Table 2, no. 1a): a) fractionation chromatogram, b) molecular weight distribution.
which suggests segmental exchange processes occurring in the system. One can also observe the set of peaks that can be assigned to the macrocyclization products (x/y M; x ¼ 1e2, y ¼ 5e20). Finally, distributions of signals derived from the block copolymers (x/y; x ¼ 1e3) were present. The middle range (b) of the spectrum contained all the same populations as described previously, but with higher x and y values. The most intensive population was found for LA homopolymer (0/y; y ¼ 27e40) comprising a larger even y series than the odd ones. The series of linear block copolymer (x/y; x ¼ 1e3, y ¼ 18e37) was present at medium strength. The population of macrocycles (x/y M) was low and decreasing for higher m/z values. In the third analyzed range (c) of the spectrum the major populations were linear block copolymers (x/y; x ¼ 1e8, y ¼ 32e54) which are expected to be the main products in the batch. According to 1H NMR analysis they should constitute about 99% of the chains population in this sample, however, only small portion of them give rise to the signals that can be observed in MALDI ToF spectra. Fig. 5. shows the GPC traces of reaction products containing short PBA segments (Mn w 1000) (Table 2, 1b). They reveal bimodal distribution of molecular weight. Mp values (determined according to PS calibration) equal to 30,400 and 56,700. The sample was additionally fractionated into 4 portions using a GPC column. On the principles of GPC theory and our measurements, Mn was decreasing from fraction a to d as follows: 54800, 46600, 29100, 23400. 1H NMR analysis of these fractions showed that the concentration of butylene adipate (BA) units comparing to lactic acid units decreases in the same order: 4.7, 2.7, 0.33 and 0.30 mol %, respectively. These results are consistent with MALDI ToF analysis and confirm that in these systems we are dealing indeed with two populations of chains, which correspond to LA homopolymer and block copolymer, respectively (Mp,PLLA-PBA-PLLA y 2 , Mp,PLLA). On the other hand, GPC traces of the sample which contained less homopolylactide chains (Table 2, no. 2b, xPLLA ¼ 13%) and the other one obtained in the presence of aliphatic-aromatic ES, were monomodal but broad and “tailed” (Fig. 6). Generally, DI for the products obtained in the studied systems may differ significantly (1.2e3.6, see Table 2) depending on the
Fig. 6. GPC analysis of (a) PBA/PLLA (Table 2, no. 2b) and (b) PBAT/PLLA systems (no. 9).
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Table 3 Tensile strength properties of thin films prepared by pressing of PLA, and PBAT/PLA system (Table 2, no. 9).a Sample
Mwb
DIb
Xc,PLLAc
E [MPa]
smax [MPa]
εmax [%]
PLA PBAT/PLA system
93,600 105,000
6.51 2.10
4% <1%
2070 246 1530 89.0
38.8 5.12 40.2 5.48
2.1 0.34 11.0 1.8
a E Young’s modulus, smax - tensile strength at maximum stress, εmax - elongation at break. b Calculated by means of GPC with polystyrene calibration. c Degree of crystallization in PLA phase.
Preliminary observations show that materials containing aliphatic ES have thermal stability similar to that of PLLA, whereas the introduction of aliphatic-aromatic ES into the PLLA chains leads to the improvement of thermal stability (Fig. 8). The mechanical studies show that this latter material is characterized by higher elasticity indicated by lower Young’s modulus and higher elongation at break than those for PLLA (Table 3). 4. Conclusions
Fig. 7. DSC traces of PLLA, aliphatic (PPAC) and aliphatic-aromatic (PPBTAC) macroinitiators and their respective block copolymers with LA.
degree of polymerization and the content of homopolymer chains. It should be noticed, however, that the weight average molecular weight (Mw) of the products may be as high as 104e105 g/mol, which allows to assume them as useful for practical applications. Taking into account the calculated values of d one could expect that all ES used in this study should be miscible with PLLA. However, DSC studies showed that only the systems based on aliphatic copolyesters reveal one Tg, which is close to the data calculated on the basis of the Fox’s equation (Fig. 7, Table 2). Therefore, this type of ES can be regarded as an internal plasticizer. The ES containing terephthalic acid units exhibits only partial miscibility with PLLA, which results in the formation of PLLA and ES rich phases of Tg slightly lower and higher, with respect to the values determined for pure segments.
From studies carried out in this work it appears that various types of polyesters or poly(ester carbonates) terminated with hydroxyl groups can be incorporated with high yield into the PLLA structure under conditions similar to those as in classical bulk LA polymerization. On the basis of 1H NMR and MALDI ToF studies it can be assumed that these copolymers are of a triblock structure. When carrying out the reaction at 190 C in the presence of tin(II) 2-ethoxyhexanate, practically complete conversion of macrodiols is achieved after about 3 h, and the LA conversion was close to 90%. Under these conditions, however, part of LA undergoes homopolymerization, resulting from the reaction, in which small amounts of water or the monomer hydrolysis product are the coinitiator. On the basis of the polylactide chains molar content it can be estimated that in the studied by us systems based on aliphatic ES, the water content (or that of the LA hydrolysis product) was from 20 to 600 ppm. An analysis of the chains structure by means of MALDI ToF shows that at 190 C transesterification processes proceed in PLLA segments, whereas macrodiol segments practically do not participate in transesterification processes. However, introductory observations show that such processes are of essential importance for the structure of products when the polymerization is carried out at 220 C. Segments built of polyesters comprising in their structure butylene adipate m.u., propylene adipate m.u., butylene carbonate m.u. and propylene carbonate m.u. are completely miscible with PLLA and act as polymeric plasticizers. Segments containing 30e50 mol % of terephthalic acid m.u. show a limited miscibility with PLLA. Products of LA polymerization in the presence of this type of segments form two-phase systems containing an elastic phase of Tg in the 10 to 20 C range and a stiff phase of Tg in the 45e60 C range. Initial observation shows that these heterogeneous systems are characterized by a slightly higher thermal stability than that of PLLA obtained in analogous conditions. The present studies aim to the utilization of thus modified PLLA for the formation of blends with commercially available biodegradable polyesters and estimation whether the in situ block copolymers are efficient compatibilizers. Acknowledgments
Fig. 8. Thermogravimetric analysis of PLA, and PLA/PBA (Table 2, no. 2b) and PBAT/PLA systems (no. 9).
This work was carried out within the project co-financed by the European Union - European Regional Development Fund under Operation Program Innovative Economy e BIOPOL POIG.01.01.02-
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Segmental copolymers of condensation polyesters and polylactide czyk, A. Jó Z. Florjan zwiak, A. Kundys, A. Plichta*, M. De˛ bowski, G. Rokicki, P. Parzuchowski, P. Lisowska, A. Zychewicz Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 December 2011 Received in revised form 21 March 2012 Accepted 22 March 2012 Available online 2 April 2012
L-Lactide (LA) polymerizations carried out in the presence of various types of polyesterodiols and polyesterocarbonatediols obtained from 1,4-butanediol, 1,3-propanediol and dimethyl esters of adipic, terephthalic and carbonic acids were studied. It was found, based on 1H NMR and MALDI ToF analysis of the chains microstructure that the reactions carried out in bulk at 190 C in the presence of tin (II) 2ethylhexanoate as a catalyst led to the formation of a mixture of polylactide (PLLA) and block copolymers of LA with the applied macrodiols with high yield. DSC studies of the obtained products indicate that the segments comprising of adipic and carbonic acid derivatives were fully miscible with PLLA causing the reduction of its glass transition temperature. The segments containing terephthalic acid monomeric units in its structure showed only slight miscibility with PLLA segments and the obtained products consist of two phases of glass transition temperatures close to those of PLLA and the macrodiol. These systems are characterized by better thermal stability and higher elasticity than PLLA. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Polylactide Block copolymers Condensation copolyesters Block miscibility
1. Introduction Poly(L-lactide) (PLLA), biodegradable and compostable aliphatic polyester is derived from renewable resources such as corn, potatoes, and sugar beets [1e3]. PLLA is one of the most promising biodegradable materials and can be widely used in packaging, fibers, and consumer goods due to its high strength, biocompatibility, transparency, and biodegradability. However, the major drawbacks of PLLA are high brittleness and poor impact strength, which restricts its applications [3]. A number of methods have been proposed to improve the mechanical properties of PLLA, among which plasticization, addition of rigid fillers and blending with a variety of flexible polymers are the most commonly used approaches [4e7]. Many plasticizers, including lactide [8], citrate esters [9,10], oligo(lactic acid) [11], and poly(oxyethylene) glycol (PEG) [12] have been applied for PLLA modification. Improvement of ductility of PLLA, both amorphous and semi-crystalline, was also achieved by addition of poly(oxypropylene) glycol (PPG). Martino et al. compared the plasticizing effects of three commercial adipates as plasticizers for PLLA. For 10 wt% of the plasticizer content, di-2ethylhexyladipate (DOA) resulted in much higher elongation (259%) of the plasticized PLLA than that of the two polymeric
* Corresponding author. Tel.: þ48 22 234 5632; fax: þ48 22 234 7279. E-mail address: [email protected] (A. Plichta). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2012.03.035
adipates (5 and 7 wt%, respectively). At 20 wt% of the plasticizer content, however, both polyadipates resulted in much higher elongation (>480%) of the plasticized PLLA with respect to DOA (295%) [13]. The same authors applied polyadipates of higher molecular weights (GlyplastÒ G206/7 and G206/3) as PLLA plasticizers. However, polymeric adipates appeared to be a less efficient modifier. Taking into account that higher molecular weight of the plasticizer causes a reduction in its migration rate and thus improves the morphological stability of PLLA during storage, the combination of small molecule plasticizers with polymeric ones of complementary advantages was proposed. Ren et al. used a combination of glycerol triacetate with poly(butylene adipate) to plasticize PLLA [14]. Similarly, Lemmouchi and coworkers applied a mixture of TBC with a block copolymer of lactide and ethylene oxide (PLLA-b-PEG) with different molecular architecture to lower the Tg and enhance elongation at break of PLLA, while the use of PLLA-b-PEG copolymers alone well maintained tensile strength and modulus [15]. Blending with flexible polymers is another most extensively used strategy to improve the ductility of PLLA. Typical polyolefins are immiscible with poly(L-lactide). Thus, they should be modified prior to blending to improve miscibility. Oyama presented a dramatic improvement in the mechanical properties of PLLA by its reactive blending with poly(ethylene-co-glycidyl methacrylate) (EGMA) containing 30 wt% of methyl acrylate and 3 wt% of glycidyl methacrylate [16]. It was shown that the interfacial reaction between the component polymers contributes to the formation of
czyk et al. / Polymer Degradation and Stability 97 (2012) 1852e1860 Z. Florjan
supertough PLLA material. However, the biodegradation of such a blend due to the presence of polyethylene fragments, was deteriorated. A similar approach was proposed by Liu and coworkers [17]. A ternary blend system consisting of poly(L-lactide), an epoxy containing elastomer, and a zinc ionomer indicated high toughness with moderate tensile strength and modulus. In this case zinc ions catalyzed the cross-linking of epoxy-containing elastomer and promoted the reactive compatibilization at the interface of PLLA and the elastomer. Biodegradable blends of PLLA which exhibited improved impact strength are obtained with use of flexible aliphatic polyesters such as poly(ε-caprolactone) (PCL) [18,19], poly(ethyleneco-butylene succinate) [20], poly(butylene succinate) (PBS) [21,22], poly(trimethylene carbonate) (PTC) [23]. PLLA blended with aliphatic poly(trimethylene carbonate-co-ε-caprolactone) indicated improved toughness [24]. Such material did not break in Izod notched impact testing. Very recently, Dubois’s group has found that random copolyesters of d-valerolactone (VL) and ε-caprolactone (CL) added at 10 wt.% into a commercially available PLA matrix led to a significant improvement in toughness of PLA materials [25]. To improve miscibility, instead of homopolyesters, copolymers with L-lactide were used for blending with PLLA. Poly(butylene succinate-co-L-lactate) was reported to blend with PLLA. It was found that the blends showed higher elongation at break than their parent polymers [26]. Copolymers of lactide and ethylene adipate (PLEA) were prepared by ROP of lactide initiated with oligo(ethylene adipate) of chains terminated with hydroxyl groups. Results confirmed the incorporation of lactic acid segments into the chain of PLEA copolymers as well as the existence of ester exchange reaction. Introduction of ethylene adipate segments leads to thermal stability [27]. Grijpma et al. reported the use of a block copolymer of L-lactide with a rubbery LLA/CL (50/50 mol/mol) copolymer segment to improve the elasticity of PLLA. With 34 wt% of rubber block, the copolymer exhibited an elongation of 1500% and did not fracture during the Charpy impact test. However, the tensile strength decreased to 30.8 MPa [28]. Grijpma et al. showed also that the TMC block (Mn ¼ 65 000) in the tri-block copolymers was also effective in toughening PLLA. With varying weight content of TMC rubber from 10.9 to 21.4 wt%, the elongation increased from 135 to 210% [29]. It was found that some biodegradable and flexible polyurethanes containing polylactide as well as other polyester type polyols can also be used to blend with PLLA to improve its flexibility [30e33]. Taking into account that aliphatic-aromatic copolyesters like poly(butylene adipate-co-terephthalate) (PBAT) are fully biodegradable and displayed high flexiblity and ductility, Jiang et al. used
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PBAT for blending PLLA. The presence of aliphatic-aromatic copolyester lead to toughening of the polylactide material. The elongation of PLLA containing 5 wt% of PBAT increased from 3.7 to 115% [34]. Because of weak interfacial adhesion in the blends, impact toughness was only slightly improved (4.4 kJ/m2 with 20 wt% of PBAT and 2.6 kJ/m2 for neat PLLA). To improve the compatibility, a reactive compatibilizer was used by Zang et al. [35]. The addition of only 2 wt% of the terpolymer of ethylene, acrylate ester, and glycidyl methacrylate (T-GMA) led to almost a two-fold increase in notched Charpy impact strength (30e40 kJ/m2) in comparison with that of uncompatibilized binary blend. Coltelli et al. studied the influence of peroxide (hexane 2,5-dimethyl-2,5-di(tert-butylperoxide)) on rheological and mechanical properties of the blends of PLLA with poly(butylene adipate-co-terephthalate). The formation of copolymers results from the reaction of PBAT macroradicals with PLLA [36]. Dicumyl peroxide (DCP) was reported to be useful to enhance the compatibility between PLLA and PBS via in situ compatibilization [37]. To achieve enhanced miscibility as well as improved mechanical properties of the blends with PLLA, various compatibilizers are additionally introduced. Wu et al. studied the influence of the presence of copolyester on mechanical properties of PLLA blend with poly(ε-caprolactone). It was shown that di- and triblock compatibilizer-copolymers (PLLA-b-PCL and PCL-b-PLLA-b-PCL) improve the interphase and mechanical properties of PCL/PLLA blends [38]. Moreover, Albertsson et al. showed that by altering the proportions of different monomers of triblock copolymers containing an amorphous middle block [poly(1,5-dioxepan-2-one), polyTMC or poly(but-2-ene-1,4-diyl malonate)] and semicrystalline terminal blocks [PLLA, PCL], the mechanical properties of copolymers could be tailored. Such triblock copolymers showed a highly elastic behavior, whereas modulus and tensile stress at break were unchanged or slightly lower [39e41]. In this paper we present the results of our work on the synthesis of compatibilizers containing segments of PLLA and polyesters obtained in polycondensation reactions. The chosen process is based on LA ring opening polymerization (ROP) carried out in bulk in the presence of hydroxyl end-capped aliphatic or aliphatic-aromatic polyesters and tin(II) 2ethylhexanoate. We assumed that incorporation of macrodiols may result mainly from e ROP of LA initiated by hydroxyl end-groups of polyesters (Eqns. (1a),(1b)), e Intermolecular chain transfer reaction between the growing PLLA chains and hydroxyl end-capped macromolecules (Eq. (1c)),
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However, one cannot exclude that these elementary reactions are accompanied by several others processes like homopolymerization of LA initiated by hydroxyl-impurities and transesterification reactions leading to the segmental exchange and formation of macrocyclic products. In order to gain a greater understanding of these processes we have studied the reaction product be means of 1H NMR, MALDI ToF and GPC techniques. We report here also some preliminary data concerning the phase structure, thermal stability and mechanical properties of the products obtained. 2. Experimental 2.1. Materials L-Lactide (Aldrich) was recrystallized consequently from dry isopropanol, then toluene and vacuum dried before polymerization. Toluene and isopropanol were dried with sodium and distilled. Tin (II) 2-ethylhexanoate [Sn(EH)2], adipic acid (AA), dimethyl terephthalate (DMT), dimethyl adipate (DMA), 1,4butanediol (1,4-BD), 1,3-propanediol (1,3-PD), dimethyl carbonate (DMC) and titanium tetrabutoxide, were purchased from Aldrich and used without further purification. Poly(1,4-butylene adipate) (PBA) of Mn 1000 and poly(1,4-butylene terephthalate) (PBT) of Mv 38000 were supplied by Aldrich. Dichloromethane and methanol were used without purification. 2.2. Procedures All condensation copolyesters were obtained in two-step transesterificationepolycondensation reaction between appropriate diesters and diols mixed in the molar ratio of 1:2, respectively. The substrates of the same type were used in equimolar amount with the exception of PPAT synthesis, where the molar ratio of DMA/DMT was 3. The process was catalyzed by 1 mol % of Sn(EH)2 (with regard to the total amount of diesters). The first step was carried out under nitrogen at 160 C and methanol was distilled off as the reaction by-product. The transesterification step was completed after the methanol yield was close to 85%. In the second step the temperature was increased to 180 C and the pressure was slowly reduced to 103 mbar. The polycondensation reaction has been carried out for 10 h while diols were distilled off. Then the reaction mixtures were cooled to room temperature and dissolved in CH2Cl2. The crude polymers were precipitated and washed with cold methanol. The final products were dried under vacuum at 40 C for 48 h. This procedure was successfully applied for the synthesis of the following polymers: poly(1,3-
propylene adipate) (PPA), poly(1,4-butylene adipate) (PBA), PPAC poly(1,3-propylene adipate-co-carbonate), poly(1,3-propylene-co1,4-butylene adipate) (PPBA), poly(1,3-propylene-co-1,4-butylene adipate-co-carbonate) (PPBAC), poly(1,3-propylene adipate-co-terephthalate) (PPAT), poly(1,4-butylene adipate-co-terephthalate) (PBAT), poly(1,3-propylene-co-1,4-butylene adipate-co-terephthalate) (PPBAT) and poly(1,3-propylene-co-1,4-butylene adipate-coterephthalate-co-carbonate) (PPBATC). On the other hand, PBAT (Table 1, no. 9) was obtained by polycondensation reaction of PBA and PBT. In this case, PBA was synthesized during polycondensation reaction of AA with 1,4-BD, catalyzed by tetrabutoxy titanium (0.1 mol % of AA). The mixture of PBA and PBT was heated up to 240 C (at the rate of 7 C/min) with pressure reduced to 0.031 Torr and then the polycondensation reaction was carried out for 90 min. ROP of LA in the presence of condensation copolyesterodiols was catalyzed by 0.01 wt % Sn(EH)2 (with respect to the batch). Monomer, condensation polyesterodiol and catalyst were placed in a three-necked round-bottom flask equipped with a magnetic stirrer and condenser. The reaction was carried out at 190 C for 3 h under nitrogen atmosphere. After polymerization, the final product was cooled to room temperature and dissolved in methylene chloride and then poured into excess methanol. The precipitated product was filtered and washed with methanol and dried in a vacuum oven at 40 C for 48 h. “Pure” PLLA was synthesized as a reference by ROP of LA (29.5 g) with 0.04 wt. % of Sn(EH)2 as catalyst in a 250 mL reactor. The reaction was carried out at 190 C for 3 h under nitrogen atmosphere. The final product was dissolved in methylene chloride, precipitated in cold methanol and dried in a vacuum oven at 40 C for 48 h. 2.3. Calculations The solubility parameters d ¼ (E/V)0.5 of PLLA and condensation polymers were calculated in order to distinguish between homogeneous and heterogeneous systems. The energy of cohesion E can be substituted with F2/V, then d ¼ F/V. The group attractive constant (F) and molar volume (V) are additive quantities and, therefore, can be calculated for each m.u. from respective values estimated for substructures. 2.4. Measurements 1 H NMR measurement was performed on Varian Mercury 400 MHz spectrometer using CDCl3 as solvent. The molecular weight and molecular weight distribution were determined by
Table 1 Characterization of condensation polyesters. No.
Macroinitiator symbol
1 2 3 4 5 6 7 8 9 10 11 12
PBA
a b c d
PPA PPAC PPBA PPBAC PBAT
PPAT PPBAT PPBATC
Compositiona Diol units(%)
Acid units (%)
B 100 B 100 P 100 P 100 P 31.5 P 26.1 B 100 B 100 B 100 P 100 P 56.4 P 25.7
A 100 A 100 A 100 C 25.8 A 74.2 A 100 C 21.3 A 78.7 T 52.4 A 47.6 T 50.7 A 49.3 T 51.2 A 48.8 T 31.5 A 68.5 T 47.9 A 52.1 T 43.3 A 45.0 C 11.7
B 68.5 B 73.9
B 43.6 B 74.3
Mn,HNMRb (g/mol)
Mn,GPCc (g/mol)
DIc
Tg ( C)
Tm ( C)
d (MPa0.5]
1100 9000 5400 3600 11,500 7700 2200 8100 16,500 10000 14,400 9200
1400 7400 3900 2800 16200 7200 1600 6000 13400 13500 11200 7400
2.92 1.57 1.31 2.02 2.09 2.23 1.51 2.08 1.70 2.53 1.70 2.12
69.9 57.8 56.9 53.8 57.7 58.0 35.5 32.0 27.3 30.3 20.8 20.9
49.4 57.9 45.6 ed 21.5 12.8 130.9 133.3 148.2 68.3 75.4 75.9
19.6 19.8 20.1 20.3 19.8 19.8 20.8 20.9 20.9 21.0 21.1 21.0
Abbreviation of diol units: P for 1,3-propanediol, B for 1,4-butanediol and acid units: A for adipic, T for terephthalic, C for carbonic. Estimated on the basis of 1H NMR spectra (methylene end-groups method). Absolute values obtained from GPC with triple detection system (RI, IV, LS). Amorphous product.
czyk et al. / Polymer Degradation and Stability 97 (2012) 1852e1860 Z. Florjan
GPC on a Viscotek system comprising GPCmax and TDA 305 triple detection unit (RI, IV, LS) equipped with one guard and two DVB Jordi gel columns (102e107, linear, mix bed) in CH2Cl2 as eluent at 35 C at a flow rate of 1.0 mL/min. Triple detection was used for determination of absolute molecular weight and DI of condensation copolyesters, whereas RI detector and PS calibration were applied for block copolymers characterization. The sample of block copolymer was fractionated using a LabAlliance GPC system equipped with one DVB Jordi gel column in CHCl3 as eluent at 25 C at a flow rate of 0.75 mL/min. MALDIeToF mass spectrometry was performed on AXIMA Performance instrument. Dithranol was used as MALDI matrix. The DSC measurement were performed using a DSC Q200 V24.2 Build 107
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apparatus. The first heating run from 100 C to 200 C was performed at a heating rate of 5e10 C/min in order to study crystallinity, then cooling at the rate of 20 C/min was applied. The second heating run was measured at the rate of 20 C/min to determine glass transition temperatures. TGA measurements were conducted using a TA Instruments SDT Q600 instrument, at heating rate of 5 C/min under air atmosphere. The tensile properties were examined with an Instron mechanical tester, model 5566, at tensile speed of 4 mm/min at room temperature. The specimens were cut into strips with approximate dimensions 80/20/0.3 mm (l/w/d) using films prepared by hot pressing at 170 C in a hydraulic press. Five specimens of each material were used.
Fig. 1. 1H NMR spectra of (a) PBA macroinitiator (Table 1, no. 1), and products of LA polymerization in the presence of PBA (b) at 190 C for 3 h (Table 2, no. 1b) and (c) 220 C for 8 h.
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Fig. 2. 1H NMR spectra of (a) PBAT macroinitiator (Table 1, no. 7), and (b) products of LA polymerization in the presence of PBAT (Table 2, no. 7).
Table 2 Properties of the LA polymerization products obtained in ROP in the presence of hydroxyl-end-caped polyesters. No.a
MIa
([LA]/[MI])0b (mol/mol)
aLAc (%)
xPLLAd (mol %)
uPLLAe (wt. %)
Mw,GPC f (g/mol)
DIf
Tg ( C)
Tg,Foxg ( C)
1a 1b 2a 2b 3 4 5 6 7 8a 8b 9 10 11 12
PBA PBA PBA PBA PPA PPAC PPBA PPBAC PBAT PBAT PBAT PBAT PPAT PPBAT PPBATC
18.4 799 591 798 120 218 911 486 777 639 495 722 498 1210 511
56 69 81 95 86 77 85 75 62 90 93 90 84 59 88
1 62 45 13 20 48 74 75 e e e e e e e
57 98 88 92 73 87 91 87 96 91 89 85 85 87 88
9800 42400 77800 107000 36000 35800 61200 36600 53800 39400 91800 105000 37900 37700 73900
1.21 1.51 1.92 1.79 1.83 2.67 2.20 3.33 2.36 1.57 2.04 2.10 3.64 2.73 3.12
10.5 nd 48.0 nd nd 34.0 46.2 36.9 60.3 nd 18.8/59.1 nd 20.7/46.8 10.5/54.7 18.4/58.0
11.0 e 41.6 e e 40.2 45.4 39.1 e e e e e e e
a b c d e f g
Nos. correspond to those from Table 1; MI - Macroinitiator symbol. Initial molar ratio of L-lactide (LA) to macroinitiator (MI). Conversion of L-lactide calculated from 1 HNMR spectra. Molar fraction of LA homopolymer chains in the system. Fraction of PLLA in the product. Calculated by means of GPC with polystyrene calibration. Tg of copolymer calculated from the Fox equation: 1/Tg,copolymer ¼ uPLLA/Tg,PLLA þ uMI/Tg,MI; Tg [K].
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3. Results and discussion The oligo- and polyesterodiols which were used as precursors of elastic segments (ES) were prepared from dimethyl esters and diols in one-batch two-step polycondensation in the presence of tin (II) 2-ethylhexanoate as a catalyst. The 1,3-propanediol, 1,4-butanediol and methyl esters of adipic, terephthalic and carbonic acids were employed as starting materials. The composition of all of these multicomponent aliphatic (Fig. 1a) and aliphatic-aromatic (Fig. 2a) polymers was determined by the means of 1H NMR (Table 1). The same technique was applied for estimation of Mn by the end-group analysis method; however, these results were verified by absolute GPC measurement (Mn and DI) using the triple detection system (RI, IV, LS). The differences between these two techniques usually did not exceed 30% of the chromatography data. The value of Mn,GPC varies in the range of 1000e16500. The distribution of molecular weight is moderate broad (DI 1.3e2.5) except the commercial sample of PBA (Table 1, no. 1). The materials obtained can be divided into two general categories depending on the composition. The first one includes aliphatic polyesters based on adipic and optionally carbonic acids derivatives which exhibit very low Tg in the range of 70 to 50 C. The other group are aliphatic-aromatic polyesters containing 30e50 mol % of terephthalate units which reveal slightly higher Tg in the range of 35 to 20 C. Most of the products were semi-crystalline excluding aliphatic polyesterocarbonate - PPAC, which existed in the form of a viscous liquid. It should be noticed that solubility parameters (d) calculated for all the obtained materials (19.6e21.1 MPa0.5) are very close to that of PLLA (20.7 MPa0.5). In order to obtain segmental copolymers, ring opening polymerization (ROP) of LA was carried out in the presence of ES-diols and 0.01 wt. % of tin 2-ethylhexanoate as catalyst. The synthesis was performed in bulk at 190 C (which resemble the conditions applied in industrial technologies) for 3 h. The 1H NMR studies of the reaction products reveal that under these conditions signals of terminal units CH2OH (Figs. 1a and 2a, signal a) present in ES molecules a decay and in the case of aliphatic ES a new kind of signals coming from CH(CH3)OH end-groups derived from PLLA chains could be clearly observed (Fig. 1b, signal j). Therefore, one can expect that ES molecules are totally incorporated into the polylactide chains. It should be noticed that the other signals characteristic for m.u. of ES did not change, which suggests that these segments were not involved in transesterification processes (Figs. 1b and 2b). To support this hypothesis, we have studied the LA
Fig. 3. MALDI ToF spectrum of PLLA-PBA-PLLA (Table 2, 1a).
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polymerization in the presence of PBA at 220 C for 8 h and we found the presence of an additional signal that can be attributed to the diad LAc-A (Fig. 1c, signal o) which should not be observed in the expected block copolymer. One can also observe a significant
Fig. 4. Fragments of MALDI ToF spectrum of LA polymerization product carried out in the presence of PBA (Table 2, no. 1a) in the following m/z ranges: (a) 1000e2000, (b) 2000e3000, (c) 4000e4300.
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czyk et al. / Polymer Degradation and Stability 97 (2012) 1852e1860 Z. Florjan
Scheme 1.
increase in the intensity of the signal characteristic for the link between PLLA and ES blocks (B-LAc, signal d’). Taking these into consideration we can treat the signals characteristic for ES m.u. as internal standards in the analysis of the product obtained at 190 C. In Table 2 the LA conversion and weight fraction of PLLA m.u. in the products obtained determined on the basis of 1H NMR spectra are shown. In most cases the PLLA weight fraction constituted as much as 85e95%, whereas LA conversion varied from 60 to 90%. However, on the basis of 1H NMR study it is impossible to determine the concentration of LA m.u. in block copolymers and homopolymers that can be also formed in these systems. In the case of systems containing aliphatic ES one can only roughly estimate the molar fraction of LA homopolymer chains xPLLA by analyzing the integrals of the terminal wCH(CH3)EeOH signal (Fig. 1b, j) and that of wCH2OC(O)CH(CH3)w in linking unit (d’). Due to the relatively low intensity of both signals the precision of this analysis is not high (in the case of aliphatic-aromatic ES, the signal of j protons overlaps with that of methylene protons in ester groups d), nevertheless it shows clearly that we are dealing actually with a mixture of homo- and copolymers of LA. This observation was also supported by MALDI ToF analysis of low molecular weight product based on PBA (Table 2, no. 1a). Fig. 3. presents the spectrum of sodium-cationized ions. The m/z range of the highest signal intensity is from 1500 to 3500, however, the overall distribution in the sample covers the m/z range from 1000 to 5000. We analyzed three regions in the m/z ranges of (a) 1000e2000 (Fig. 4a), (b) 2000e3000 (Fig. 4b) and (c) 4000e4300 (Fig. 4c). The following peak assignments (notifications) for various populations were used: x/y - for linear macromolecules and x/y M - for macrocyclics, where x is the degree of polymerization of ES and y is the number of lactic acid units in the molecules (Scheme 1). In the first range (a) no separated signal of the unreacted PBA macroinitiator was found (x/0), which confirms the very high incorporation efficiency. On the other hand, that area is rich of the products of LA polymerization initiated with water or LA hydrolysis product (0/y), with the odd and even values of y (lactic acid units),
Fig. 5. GPC analysis of PBA/PLLA system (Table 2, no. 1a): a) fractionation chromatogram, b) molecular weight distribution.
which suggests segmental exchange processes occurring in the system. One can also observe the set of peaks that can be assigned to the macrocyclization products (x/y M; x ¼ 1e2, y ¼ 5e20). Finally, distributions of signals derived from the block copolymers (x/y; x ¼ 1e3) were present. The middle range (b) of the spectrum contained all the same populations as described previously, but with higher x and y values. The most intensive population was found for LA homopolymer (0/y; y ¼ 27e40) comprising a larger even y series than the odd ones. The series of linear block copolymer (x/y; x ¼ 1e3, y ¼ 18e37) was present at medium strength. The population of macrocycles (x/y M) was low and decreasing for higher m/z values. In the third analyzed range (c) of the spectrum the major populations were linear block copolymers (x/y; x ¼ 1e8, y ¼ 32e54) which are expected to be the main products in the batch. According to 1H NMR analysis they should constitute about 99% of the chains population in this sample, however, only small portion of them give rise to the signals that can be observed in MALDI ToF spectra. Fig. 5. shows the GPC traces of reaction products containing short PBA segments (Mn w 1000) (Table 2, 1b). They reveal bimodal distribution of molecular weight. Mp values (determined according to PS calibration) equal to 30,400 and 56,700. The sample was additionally fractionated into 4 portions using a GPC column. On the principles of GPC theory and our measurements, Mn was decreasing from fraction a to d as follows: 54800, 46600, 29100, 23400. 1H NMR analysis of these fractions showed that the concentration of butylene adipate (BA) units comparing to lactic acid units decreases in the same order: 4.7, 2.7, 0.33 and 0.30 mol %, respectively. These results are consistent with MALDI ToF analysis and confirm that in these systems we are dealing indeed with two populations of chains, which correspond to LA homopolymer and block copolymer, respectively (Mp,PLLA-PBA-PLLA y 2 , Mp,PLLA). On the other hand, GPC traces of the sample which contained less homopolylactide chains (Table 2, no. 2b, xPLLA ¼ 13%) and the other one obtained in the presence of aliphatic-aromatic ES, were monomodal but broad and “tailed” (Fig. 6). Generally, DI for the products obtained in the studied systems may differ significantly (1.2e3.6, see Table 2) depending on the
Fig. 6. GPC analysis of (a) PBA/PLLA (Table 2, no. 2b) and (b) PBAT/PLLA systems (no. 9).
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Table 3 Tensile strength properties of thin films prepared by pressing of PLA, and PBAT/PLA system (Table 2, no. 9).a Sample
Mwb
DIb
Xc,PLLAc
E [MPa]
smax [MPa]
εmax [%]
PLA PBAT/PLA system
93,600 105,000
6.51 2.10
4% <1%
2070 246 1530 89.0
38.8 5.12 40.2 5.48
2.1 0.34 11.0 1.8
a E Young’s modulus, smax - tensile strength at maximum stress, εmax - elongation at break. b Calculated by means of GPC with polystyrene calibration. c Degree of crystallization in PLA phase.
Preliminary observations show that materials containing aliphatic ES have thermal stability similar to that of PLLA, whereas the introduction of aliphatic-aromatic ES into the PLLA chains leads to the improvement of thermal stability (Fig. 8). The mechanical studies show that this latter material is characterized by higher elasticity indicated by lower Young’s modulus and higher elongation at break than those for PLLA (Table 3). 4. Conclusions
Fig. 7. DSC traces of PLLA, aliphatic (PPAC) and aliphatic-aromatic (PPBTAC) macroinitiators and their respective block copolymers with LA.
degree of polymerization and the content of homopolymer chains. It should be noticed, however, that the weight average molecular weight (Mw) of the products may be as high as 104e105 g/mol, which allows to assume them as useful for practical applications. Taking into account the calculated values of d one could expect that all ES used in this study should be miscible with PLLA. However, DSC studies showed that only the systems based on aliphatic copolyesters reveal one Tg, which is close to the data calculated on the basis of the Fox’s equation (Fig. 7, Table 2). Therefore, this type of ES can be regarded as an internal plasticizer. The ES containing terephthalic acid units exhibits only partial miscibility with PLLA, which results in the formation of PLLA and ES rich phases of Tg slightly lower and higher, with respect to the values determined for pure segments.
From studies carried out in this work it appears that various types of polyesters or poly(ester carbonates) terminated with hydroxyl groups can be incorporated with high yield into the PLLA structure under conditions similar to those as in classical bulk LA polymerization. On the basis of 1H NMR and MALDI ToF studies it can be assumed that these copolymers are of a triblock structure. When carrying out the reaction at 190 C in the presence of tin(II) 2-ethoxyhexanate, practically complete conversion of macrodiols is achieved after about 3 h, and the LA conversion was close to 90%. Under these conditions, however, part of LA undergoes homopolymerization, resulting from the reaction, in which small amounts of water or the monomer hydrolysis product are the coinitiator. On the basis of the polylactide chains molar content it can be estimated that in the studied by us systems based on aliphatic ES, the water content (or that of the LA hydrolysis product) was from 20 to 600 ppm. An analysis of the chains structure by means of MALDI ToF shows that at 190 C transesterification processes proceed in PLLA segments, whereas macrodiol segments practically do not participate in transesterification processes. However, introductory observations show that such processes are of essential importance for the structure of products when the polymerization is carried out at 220 C. Segments built of polyesters comprising in their structure butylene adipate m.u., propylene adipate m.u., butylene carbonate m.u. and propylene carbonate m.u. are completely miscible with PLLA and act as polymeric plasticizers. Segments containing 30e50 mol % of terephthalic acid m.u. show a limited miscibility with PLLA. Products of LA polymerization in the presence of this type of segments form two-phase systems containing an elastic phase of Tg in the 10 to 20 C range and a stiff phase of Tg in the 45e60 C range. Initial observation shows that these heterogeneous systems are characterized by a slightly higher thermal stability than that of PLLA obtained in analogous conditions. The present studies aim to the utilization of thus modified PLLA for the formation of blends with commercially available biodegradable polyesters and estimation whether the in situ block copolymers are efficient compatibilizers. Acknowledgments
Fig. 8. Thermogravimetric analysis of PLA, and PLA/PBA (Table 2, no. 2b) and PBAT/PLA systems (no. 9).
This work was carried out within the project co-financed by the European Union - European Regional Development Fund under Operation Program Innovative Economy e BIOPOL POIG.01.01.02-
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