Multiblock Copolymers of l-Lactide and Trimethylene Carbonate

Biomacromolecules 2005, 6, 439-446

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Multiblock Copolymers of L-Lactide and Trimethylene Carbonate Doris Pospiech,*,‡ Hartmut Komber,‡ Dieter Jehnichen,‡ Liane Ha¨ussler,‡ Kathrin Eckstein,‡ Holger Scheibner,‡ Andreas Janke,‡ Hans R. Kricheldorf,*,† and Oliver Petermann† Institut fu¨r Technische und Makromolekulare Chemie, Bundesstrasse 45, D-20146 Hamburg, Germany, and Institut fu¨r Polymerforschung Dresden e. V., Hohe Strasse 6, D-01069 Dresden, Germany Received September 27, 2004

Sequential copolymerizations of trimethylene carbonate (TMC) and L-lactide (LLA) were performed with 2,2-dibutyl-2-stanna-1,3-oxepane as a bifunctional cyclic initiator. The block lengths were varied via the monomer/initiator and via the TMC/L-lactide ratio. The cyclic triblock copolymers were transformed in situ into multiblock copolymers by ring-opening polycondensation with sebacoyl chloride. The chemical compositions of the block copolymers were determined from 1H NMR spectra. The formation of multiblock structures and the absence of transesterification were proven by 13C NMR spectroscopy. Differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and dynamic mechanical analysis (DMA) measurements confirmed the existence of a microphase-separated structure in the multiblock copolymers consisting of a crystalline phase of poly(LLA) blocks and an amorphous phase formed by the poly(TMC) blocks. Stress-strain measurements showed the elastomeric character of these biodegradable multiblock copolymers, particularly in copolymers having -caprolactone as comonomer in the poly(TMC) blocks. Introduction Thermoplastic elastomers (TPEs) are an important group of special polymers with an unusual combination of useful chemical and physical properties. Their historical origin is based on the polyurethane chemistry, but over the past three decades more and more polyesters and poly(ether ester)s have joined this group of plastics.1-4 Relatively less is known about biodegradable TPEs, and all work done in this direction concerned the flexibilization of poly(glycolide) and poly(Llactide), poly(LLA), designed as medical structures, for instance, for surgical sutures, drug delivery devices, and body and dental implants.5,6 Methods to modify the properties of poly(LLA) and poly(glycolide) include copolymerization with other monomers to decrease crystallinity, synthesis of polymers with other than linear architecture such as dendritic ones, synthesis of A-B-A triblock and multiblock copolymers, and physical blending.7 The most important comonomers for LLA copolymerization include -caprolactone (CL),8 trimethylene carbonate (TMC),9 2,2-dimethyltrimethylene carbonate,10 and 1,5-dioxepan-2-one (DXO).11 Examples for highly elastic biodegradable TPEs are multiblock copolymers based on crystalline poly(CL) blocks and poly(tetrahydrofuran) soft segments.12 Another, more recently published class of biodegradable TPEs are A-B-A triblock copolymers having crystalline poly(LLA) blocks in combination with an amorphous poly(CL) central block.13 The present work had the goal to elaborate a “one-pot procedure” for the preparation of biodegradable TPEs with * Corresponding authors: (H.R.K.) phone +49-40-4123-3168, fax +4940-4123-6008, e-mail [email protected]; (D.P.) phone +49-3514658497, fax +49-351-4658565, e-mail [email protected]. † Institut fu ¨ r Technische und Makromolekulare Chemie. ‡ Institut fu ¨ r Polymerforschung Dresden.

moderate elasticity and elongation at break. For this purpose, the combination of ring-expansion polymerization (with cyclic tin initiators)14-16 and ring-opening polycondensation17,18 seemed to be a promising synthetic strategy. The most important feature of multiblock copolymers (AB)n is the occurrence of microphase separation between the segments A and B, which is the prerequisite for the combination of the properties of polymer A and polymer B in the multiblock copolymer as well as the evolution of new, synergistic effects in the material.19 The formation of elastomeric properties can be regarded as one of these synergistic properties.20 Elastomeric behavior is caused by the formation of a physical network between crystallized or liquid-crystalline blocks that are surrounded by a soft amorphous matrix.4 Upon mechanical stress, the crystalline phase aligns in the stress direction. If the stress is removed, the blocks relax back to their old places in the matrix. Therefore, the phase structure of the synthesized multiblock copolymers was examined with respect to the occurrence of microphase separation by differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and dynamic mechanical analysis (DMA) investigations. Additionally, the mechanical behavior of films was studied by stressstrain measurements in usual modus and step-cycle tests. Experimental Section Materials. -Caprolactone (CL) was purchased from Aldrich Co. (Milwaukee, WI) and distilled over freshly powdered calcium hydride. L-Lactide (LLA) and trimethylene carbonate (TMC, 1,3-dioxane-2-one) were a gift of Boehringer GmbH (Ingelheim, Germany). They were recrystallized from ethyl acetate and stored in a desiccator over P4O10.

10.1021/bm049393a CCC: $30.25 © 2005 American Chemical Society Published on Web 12/17/2004

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The initiator 2,2-dibutyl-2-stanna-1,3-dioxepane (DSDOP) was prepared from dibutyltin dimethoxide and dry 1,4butanediol as described previously.18 Sebacoyl chloride was purchased from Aldrich Co. and distilled prior to use. Chlorobenzene was distilled over P4O10. General Procedure for Copolymerizations: (a) Copolymerizations without -Caprolactone (BCP 2 as Example). TMC (50 mmol) and 5 mL of dry chlorobenzene were weighed into a cylindrical glass reactor having silanized glass walls and equipped with a mechanical stirrer and gas inlet and outlet tubes. The initiator (1 M solution in dry chlorobenzene) was injected and the reaction vessel was placed into an oil bath thermostated at 80 °C. After 2 h, when the conversion was complete, LLA (50 mmol) was added. After the temperature was maintained at 80 °C for another 2 h, an equivalent amount of sebacoyl chloride to DSDOP was added and the polycondensation was carried out for 2 h. After cooling, the reaction mixture was diluted with CH2Cl2 and precipitated into cold dry diethyl ether and isolated by filtration. NMR data (CDCl3) (see Figure 1): 1H NMR 1.29 (f, g), 1.58 (6), 1.61 (e), 1.76 (b), 2.04 (3), 2.36 (d), 4.16 (a), 4.23 (2), 5.16 (5); 13C NMR 16.56 (6), 16.67, 16.70, 24.62 (e), 25.05 (b), 27.99 (3), 28.87 and 28.90 (f, g), 33.76 (d), 61.60, 61.79, 64.10, 64.44 (2), 67.28 (a), 68.02, 68.73, 68.92 (5), 69.15, 154.83 (1), 154.96 (1′), 169.49 (4), 169.68, 169.84 (4′), 170.31, 173.08 (c). (b) Copolymerizations with -Caprolactone (BCP 4 as Example). CL (50 mmol), TMC (12.5 mmol), and 5 mL of dry chlorobenzene were weighed into a cylindrical glass reactor having silanized glass walls and equipped with a mechanical stirrer and gas inlet and outlet tubes. The initiator (1 M solution in dry chlorobenzene) was injected and the reaction vessel was placed into an oil bath thermostated at 80 °C. After 2 h, when the conversion was complete, 31.25 mmol of LLA was added. After the temperature was maintained at 80 °C for 4 h, an equivalent amount of sebacoyl chloride to DSDOP was added and the polycondensation was performed for 24 h (BCP 4-5). After cooling, the reaction mixture was diluted with CH2Cl2 and precipitated into cold dry diethyl ether and isolated by filtration. NMR data (CDCl3) (see Figures 1 and 3): 1H NMR 1.29 (f, g), 1.38 (γ-CH2 of CL), 1.58 (6), 1.6-1.7 (e, β- and δ-CH2 of CL), 1.76 and 1.80 (b), 1.96, 2.00, 2.04 (3, triads), 2.30 (R-CH2 of CL), 2.36 (d), 4.05, 4.12, 4.16, 4.20 and 4.23 (CH2 of CL, 2 and a in triads), 5.16 (5); 13C NMR 16.57 (6), 16.67, 16.70, 24.39, 24.44, 24.46 and 24.51 (β-CH2 of CL, triads), 24.62 (e), 25.02, 25.07, 25.31, 25.34 (b, triads), 25.23 and 25.46 (γ-CH2 of CL, diads), 28.01 (3), 28.28 (δ-CH2 of CL), 28.87 and 28.92 (f, g), 33.78 (d), 33.89, 33.93, 34.01 and 34.05 (R-CH2 of CL, triads), 60.54, 60.60 and 60.80 (2, bonded to ester group), 61.61 (2′, link between TMC and LL), 64.0-64.6 (2, bonded to carbonate group and both -CH2 of CL and a, bonded to ester group), 67.57 and 67.60 (a, bonded to ester group, diads), 67.8 (-CH2 of CL, bonded to carbonate group), 68.02, 68.73, 68.92 (5), 69.15, 154.85, 154.90, 155.03 and 155.09 (1, triads), 154.95 and 154.98 (1′, diads), 169.49 (4), 169.68, 169.84 (4′), 170.31, 173.08

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(c), 173.20, 173.24, 173.39, 173.43 (CO of CL, triads), 173.29 (CO of CL, bonded to 1,4-butenediyl moiety). Homopolymers. The homopolymers used for comparison of thermal properties and mechanical data had the following molecular weights from SEC measurements calibrated with narrow distributed polystyrene standards: for poly(TMC), Mn ) 15 000 Da and Mw ) 46 000 Da; for poly(CL), Mn ) 29 000 Da and Mw ) 72 000 Da; and for poly(LLA), Mn ) 21 000 Da and Mw ) 36 000 Da. Measurements. The inherent viscosities were measured with an automated Ubbelohde viscometer thermostated at 20 °C with CH2Cl2 as solvent. The 500.13 MHz 1H and 125.75 MHz 13C NMR spectra were recorded with a Bruker DRX 500 NMR spectrometer in 5 mm o.d. sample tubes. CDCl3 served as solvent. The spectra were referenced on the solvent peak [δ(1H) ) 7.26 ppm and δ(13C) ) 77.00 ppm, respectively]. Correlated spectroscopy (COSY) and heteronuclear 1H-13C one- and multiple-bond shift-correlated (HMQC, HMBC) spectra were recorded with the standard pulse sequences provided by Bruker. The molecular weights of the polymers were determined by size-exclusion chromatography (SEC) with a modular built gel-permation chromatograph (GPC) (Knauer) using Zorbax PSM Trimodal-S/PL Mixed-B separation columns, chloroform as eluent (1.0 mL/min), and refractive index (RI) detection. The molecular weights were calculated by use of narrowly distributed polystyrene as standard and are therefore relative values (see text). DSC measurements were performed with a Perkin-Elmer DSC-7 apparatus in aluminum pans under nitrogen, in a temperature range from -60 to 200 °C with a heating and cooling rate of 20 K/min. The glass transition temperatures were determined in the second heating run. Wide-angle X-ray scattering (WAXS) was performed on a Siemens 4-circle diffractometer P4 equipped with an area detection system HiStar/GADDS (Bruker-axs/Siemens Analytical) and graphite monochromator. Films for dynamic mechanical analysis and stress-strain investigations were prepared on a hydraulic hot-melt press (Schwabenthal, Germany) at a temperature within the melting region of the polymer (temperatures: BCP 1, 155 °C; BCP 2, 140 °C; BCP 3, 130 °C; BCP 4, 146 °C; BCP 5, 150 °C; poly(CL), 67 °C) for 2 min with a load of 11 kN. Dynamic mechanical analysis (DMA) measurements were carried out on these films (20 mm × 40 mm, thickness 0.5 mm) by means of an Ares rheometer (Rheometrics) in torsion mode (62.8 rad/s, strain 0.05%) in a temperature range of -100 to +100 °C under nitrogen atmosphere. AFM measurements were done in tapping mode by a Dimension 3100 NanoScope IV (Veeco). Pointprobe siliconSPM-sensors (Nanosensors, Germany) with a spring constant of ca. 3 N/m and resonance frequency of ca. 75 kHz were used, with tip radius lower than 10 nm. The images show the phase contrast. According to Magonov et al.,21 we chose the scan conditions (free amplitude >100 nm, set-point amplitude ratio 0.5) in order to get stiffness contrast in the phase image.

L-Lactide

and Trimethylene Carbonate Copolymers

Scheme 1. Synthesis and Chemical Structure of Multiblock Copolymers Consisting of Poly(TMC) and Poly(LLA)

Stress-strain measurements according to ISO 527-2 were carried out on a Zwick 1456 test device (Germany) equipped with a 1 kN force transducer, 18 mm measuring length, and a strain rate of 10 mm/min. The E-modulus was calculated in the strain region of 0.05-0.25%. Results and Discussion Synthesis and Structure Characterization. The synthetic approach used in this work is based on the method of ringexpansion polymerization and an in situ combination of three reaction steps as outlined in Scheme 1 for a special case (BCP 1-3). The first step consisted of the ring-expansion polymerization of TMC with 2,2-dibutyl-2-stanna-1,3-dioxepane (DSDOP) as initiator. After complete conversion (monitored by 1H NMR spectroscopy), the resulting poly(TMC) containing two reactive Sn-O-CH2 groups was used as macroinitiator for the ring-expansion polymerization of LLA, representing the second step. The third step was based on the addition of sebacoyl chloride, which entailed chain extension by a polycondensation process involving the elimination of the Bu2Sn group. In this way three block copolymers were prepared with variation of the block lengths, which were labeled BCP 1-3 (see Table 1). Two more multiblock copolyesters were prepared in such a way that for the first step a mixture of TMC and CL (molar ratio 1:4) was used instead of neat TMC (see Table 1). As discussed later in connection with the mechanical properties, this variation of the soft segment had interesting consequences for the elastic properties of the entire multiblock copolymers. In a first series of experiments (not described here in detail), the soft segment was prepared by DSDOP-initiated

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copolymerization of CL and β-D,L-butyrolactone (with variation of the molar ratio). It was known from previous studies14 that such copolymerizations conducted at 80 °C yield random copolylactones with glass transition temperatures below -20 °C. Although this first step was successful, the entire synthetic strategy failed because the sequential copolymerization of LLA in the second step involved so much transesterification that the crystallinity was almost completely lost. Therefore, this monomer combination proved successless for the synthesis of TPEs. On the other hand, it was found in another previous study15 that ring-expansion polymerizations of TMC (as first step) and LLA (as second step) allow for the preparation of A-B-A triblock copolymers without significant transesterification, provided time and temperature were carefully optimized. The optimization of the temperature/time window was decisive for the success of this approach. The reaction conditions used in this work were based on our previous study and proved the reproducibility of the previous results.15 The chain extension step by polycondensation of Bu2Sn containing cyclic polylactones with dicarboxylic acid dichlorides has also been studied previously.17,18 It was found that typically 3-5 polycondensation steps take place. Why the chain extension factor is so low has never been clarified. Both incomplete conversion due to the low concentration of reactive (end)groups and cyclization may contribute to this effect. Anyway, in the present work, the chain extension factor was not studied in detail, because the main goal was an inherent viscosity above 0.70 dL/g in CH2Cl2, which on the basis of previous work was expected to suffice for satisfactory mechanical properties. However, the incorporation of sebacic acid into the polylactones and the elimination of Bu2Sn groups was evidenced by 1H NMR spectroscopy (Figures 1 and 2). The SEC measurements proved the formation of high molecular weights in most cases, but the molecular weight data listed in Table 1 are not highly accurate because polystyrene used for calibration does not have the same hydrodynamic properties as the TPEs of this work. Most likely the Mn and Mw values given in Table 1 overstimate the real molecular weights.22 Nevertheless, these Mn and Mw values may provide reliable polydispersities. The high polydispersities are obviously a consequence of the polycondensation steps, because for ring-expansion polymerizations themselves, values below 2.0 are characteristic.13,14,16 Polycondensations, which in contrast to classical theory may involve a high extent of cyclization, may result in polydispersities up to 10 and more.23 The chemical structures of the multiblock copolymers were characterized by 1H and 13C NMR spectroscopy. The 1H NMR spectra of the polyesters BCP 1-5 displayed all the signals relevant for the expected structure (Figures 1-3). For instance, the molar compositions were determined from the signal intensities of the TMC, LLA, and CL units (signals 1-6 in Figure 1). In addition to the signals of the repeating units signals of the 1,4-dioxybutane group resulting from the initiation step were found (signals a and b in Figure 1a). Furthermore, signals of the sebacoyl unit (signals d-g in Figure 1a) were

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Table 1. Composition and Reaction Conditions of the Synthesized Multiblock Copolymers Prepared from TMC, CL, and LLA BCP 1 BCP 2 BCP 3 BCP 4 BCP 5

M1a [M]/[I] TMC 100:1 TMC 50:1 TMC 25:1 CL/TMC(4:1) 100:1 CL/TMC(1:1) 50:1

M2b [M]/[I] LL 100:1 LL 50:1 L 25:1 LL 50:1 LL 50:1

yield (%) 88 82 84 68 85

ηinh (dL/g) 0.82 0.76 0.87 1.05 1.01

LL/TMCc 0.95 0.96 0.94 2.25 1.93

CL/TMCc

LL/CLc

3.8 1.21

0.59 1.58

Mn,GPC (Da) 39 900 39 800 19 000 23 000 54 200

Mw,GPC (Da) 115 800 127 300 102 300 72 200 143 750

Mw/Mn 2.91 3.19 5.38 3.14 2.65

a Monomer 1 is either TMC (BCP 1-3) or a mixture of TMC and CL (BCP 4-5, molar ratio of the mixture in parentheses); feed ratio [monomer]/ [initiator] is given. b Feed ratio [monomer]/[initiator]. c Molar composition as determined 1H NMR (see Experimental Section).

Figure 2. 1H-13C HMBC spectrum (region) of BCP 3 showing crosspeaks due to 3JCH couplings used in assigning the carbonyl signals 1′, 4′ and c. Table 2. Composition and Reaction Conditions of the Synthesized Multiblock Copolymers Prepared from TMC, CL, and LLA number-averaged DP 1H

poly(TMC) block

13C

Figure 1. (a) and NMR spectra (b, main signals truncated) of BCP 3 in CDCl3, showing the signals of the poly(TMC) (1-3) and poly(LLA) (4-6) blocks and the signals of the incorporated 1,4butandioxy (a, b) and sebacoyl (c-g) moieties. Carbonyl signals 1′ and 4′ are due to the carbonate group between 1,4-butanedioxy and TMC unit (1′) and the ester group between the poly(TMC) and poly(LLA) block (4′), respectively.

detected originating from the chain extension. The intensities of 1,4-butoxy signals and sebacoyl signals were almost identical, in good agreement with the expected structure. The 13 C NMR spectrum (Figure 1b) confirmed the structure characterization based on 1H NMR spectroscopy. The assignments of the 13C carbonyl signals were obtained from their characteristic chemical shifts by comparison with the homopolyesters and by the 1H-13C heteronuclear multiplebond correlation spectrum (HMBC) illustrated in Figure 2. Particularly noteworthy is the weak CO signal 1′, which represents the carbonate group connecting the 1,4-dioxybutane group of the initiator with the next TMC repeating unit. Furthermore, the ester carbon 4′ shows a HMBC cross-peak to the H2 region, allowing its assignment to the linking group between poly(TMC) and poly(LL) block. Additional small signals in neighborhood to signals 1-6 are also caused by the linking groups between the different units in the polymer structure. As expected, all these signals have intensities comparable with those of signals a-g. From the NMR data given in Table 1 and the monomer/ initiator ratios also obtained from 1H NMR spectra, the actual degrees of polymerization of the blocks as a measure for

polymer BCP 1 BCP 2 BCP 3 BCP 4 BCP 5 b

calcda 50 25 12.5

foundb 45 ((3) 26 ((2) 13 ((1)

poly(LLA) block calcda 100 50 25 50 50

foundb 85 ((5) 50 ((3) 25 ((2) 50 ((3) 50 ((3)

a Calculated from the initial monomer/initiator ratios used in synthesis. Calculated from 1H NMR data.

the block length can be calculated. The values obtained (Table 2) deviate only slightly from the calculated values. Using CL as comonomer in synthesizing the first block (BCP 4/5) resulted in different comonomer sequences that cause sequence-dependent NMR signals for the carbons and protons of TMC and CL but also for the 1,4-butanediyl moiety and indicate a nearly random sequence (see Experimental Section, Figure 3, and ref 15). The high sensitivity of 13C NMR spectra to sequence effects should also give information about incorporation of TMC or CL units in LLA blocks and vice versa. The signal regions of carbon 4 of poly(LLA) in BCP 3 and BCP 4 as depicted in Figure 3 are identical, proving that two perfect blocks are formed. In summary, all 1H and 13C NMR spectroscopic data were in perfect agreement with the expected multiblock structure. Phase Structure of the Multiblock Copolymers. The monomer/initiator ratio and the ratio between the monomers were chosen in such a way that the blocks incorporated into the multiblock copolymers had gradually decreasing degrees of polymerization, as summarized in Table 2.

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Table 3. Thermal Behavior and Crystallinity of the Multiblock Copolymers Investigated sample poly(LLA)b poly(CL)b poly(TMC)b BCP 1 BCP 2 BCP 3 BCP 4 BCP 5

Tg,1 (DSC) (°C) -52.9d -15.6 -9.5 -9.3 -8.5 -41.0d -36.8d

Tg,2(DSC) (°C) 63.0 49.0 45.0 37.0 38.7 39.8

57.8

Tm,max (°C) 173.9 74.6

total crystallinitya R (DSC) 0.66c 0.64e

108.3 117.6 140.7 89.4 110.5

162.6 146.2 155.0 146.7 148.1

0.34 0.25 0.18 0.13 0.16

Tcc,max (°C)

degree of crystallinity RX (WAXS) 0.59 0.47 0.00 0.34 0.31 0.26 0.27 0.19

a Determined in the first heating run to ensure comparison with WAXS crystallinities; total crystallinity related to the sample weight. b For molar masses of the samples see Experimental Section, SEC measurements. c ∆H of 100% crystalline poly(LLA) was 93 J/g, according to ref 24. d Determined in DMA measurements. e ∆H of 100% crystalline poly(CL) was 136 J/g, according to ref 25.

Figure 5. Plots of the glass transition temperatures of BCP 1-3 obtained by DSC and DMA measurements versus the degree of polymerization of the poly(LLA) blocks, as given in Table 2: O, Tg,2 (DMA); b, Tg2 (DSC); 0, Tg,1 (DMA); 9, Tg,1 (DSC).

Figure 3. Regions of the 13C NMR spectra of BCP 3 (a) and BCP 5 (b).

Figure 4. DSC curves (second heating run) of poly(TMC)/poly(LLA) multiblock copolymers BCP 1-3 in comparison to the corresponding homopolymers.

The phase structure of the multiblock copolymers was first investigated by DSC to get an impression on the state of phase separation and the existing phases as well as their thermal behavior by comparison to the parent homopolymers poly(TMC), poly(CL), and poly(LLA). The DSC curves are shown in Figure 4. Whereas poly(TMC) is amorphous with a low Tg of -15.6 °C, both poly(LLA) and poly(CL) are highly crystalline. The

DSC curves of BCP 1-3 in Figure 4 prove the existence of microphase separation between poly(TMC) and poly(LLA) blocks in the block copolymers. Two glass transitions were detected, caused by the two amorphous phases of both blocks. The obtained Tg values are also summarized in Table 3. If the poly(LLA) blocks in the BCP were long enough (as in BCP 1 and BCP 2), cold crystallization in the poly(LLA) phase occurs, followed by melting of the crystallites above 150 °C. The plots of the glass transition temperatures versus the polymerization degree of both blocks in Figure 5 show the block copolymer effect clearly. The same behavior is obtained in the plots of Tg versus DP of poly(TMC). Linking together blocks of poly(TMC) and poly(LLA) in the BCP resulted in a shift of the glass transition temperature of the poly(LLA) phase to lower temperatures, whereas the Tg of the poly(TMC) phase was shifted slightly to higher temperatures. This effect can be explained by a partial dissolution of the poly(TMC) blocks in the poly(LLA) phase and vice versa. With decreasing DP of the blocks (see Table 2), the two Tg shift closer to each other because of the higher intermixing of the phases. A miscible, non-phase-separated, homogeneous state of the block copolymers was, however, not reached with the block lengths used. DMA measurements on melt-pressed films confirmed the microphase separation found by DSC, showing much more intensive the lower glass transition of the poly(TMC) phase. The small differences between the Tg detected by DSC and DMA resulted from the differences in the heating rates used. The effect of block length on the phase separation behavior manifests also by the crystallinity of the samples. BCP with

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Figure 7. DSC curves of poly(TMC)/poly(LLA)/poly(CL) multiblock copolymers in comparison to the corresponding homopolymers.

Figure 6. Wide-angle X-ray scattering patterns of the block copolymers investigated: 1, poly(CL); 2, poly(LLA); 3, BCP 1; 4, BCP 2; 5, BCP 3; 6, BCP 4; 7, BCP 5; 8, poly(TMC).

short blocks (BCP 3) have very low crystallinity (almost amorphous polymers), whereas BCP with longer blocks show a significant crystallinity of the poly(LLA) phase, which is, however, lower than that of pure high molecular poly(LLA). The molecular weight of the blocks incorporated (i.e., their degree of polymerization) determines the degree of crystallinity, clearly visible in the data given in Table 3, and the X-ray scattering patterns displayed in Figure 6. The crystallinity of the block copolymers increases with higher DP of the blocks. The longer the blocks, the higher is the degree of phase separation (that means, on the other hand, the Tg of the amorphous phases drift toward the Tg of the homopolymers). Higher degree of phase separation allowed a better crystallization of poly(LLA) blocks. The crystallinities of the as-synthesized samples obtained by WAXS and by DSC are nearly comparable. Block copolymers 4 and 5 having poly(CL/TMC) blocks instead of poly(TMC) blocks show almost the same thermal behavior as BCP 1-3. In the DSC curves in Figure 7, melting of a separate crystalline, microphase-separated poly(CL) phase cannot be detected. Also, the crystallinities (as determined by WAXS and DSC) are significantly lower than in BCP 1-3. The DSC curves did not allow us in this case to make a clear decision about the amorphous phases of the multiblock copolymers (the lower limit for the investigations is -60 °C and in the glass transition range of poly(CL)). Therefore, dynamic mechanical analysis (DMA) measurements were carried out. The results (tan δ vs frequency plots) for BCP 1-3 are given in Figure 8a and for BCP 4-5 in Figure 8b. In addition to the relaxations that are caused by the glass transitions of poly(TMC) and poly(LLA), respectively, a

Figure 8. Dynamic mechanical analysis of the multiblock copolymers investigated: a, BCP 1-3; b, BCP 4-5.

further, strong relaxation can be seen in the region of the glass transition of poly(CL) at about -50 °C, indicating a three-phase amorphous structure of BCP 4 having all three comonomers incorporated. The long CL/TMC blocks in BCP 4 dictate strongly the behavior of the block copolymer toward poly(CL). The morphology of the block copolymers was examined on cryocutted surfaces of melt-pressed films by atomic force microscopy (AFM) in tapping mode. Figure 9 shows as

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Figure 9. AFM micrographs (phase contrast) of BCP 1 (a) and BCP 4 (b).

example the morphologies of BCP 1 and BCP 4. The phase contrast (stiffness contrast) mode employed allows us to distinguish between hard (bright) and soft (dark) phases in the sample. The hard phase is ascribed to the crystalline poly(LLA) phase and the soft phase to the poly(TMC) phase. The phase separation deduced from DSC, DMA, and WAXS measurements can be confirmed by this direct imaging technique. The morphology of BCP 1 is representative for BCP 1-3. It is characterized by a phase separation between hard, crystallized poly(LLA) and soft poly(TMC). However, the hard phase is intermixed with small regions of soft poly(TMC), while the soft phase on its part is disturbed by small crystallites of poly(LLA), making the whole morphology rather complex. The intermixing discussed here was also concluded from Tg shifts as discussed before. The morphology of BCP 4 and 5 having additional CL units were rather similar (Figure 9b), but significantly different from that of BCP 1-3. The morphology represents a cocontinuous structure in which needlelike crystallites are surrounded by the amorphous, continuous matrix phase. Mechanical Properties of the Multiblock Copolymers. Stress-strain investigations of melt-pressed films of the samples were carried out in order to elucidate the elastic properties of homopolymers as well as block copolymers. The investigations started again with the homopolymers. Poly(TMC) showed cold flow during the measurement, whereas even high molecular poly(LLA) was too brittle to be measured. Poly(CL) yielded a stress-strain curve representative for a thermoplastic product. After the linear increase of force, a stress decrease with partially elastic region followed (cold flow), and after that break of the sample. BCP 1-3 did not show a yield point and started to behave as elastomers. However, significant differences between the two types of multiblock copolymers can be noticed. Figure 10 shows, as an example, the difference between BCP 2 without poly(CL) units and BCP 4 additionally containing CL units. However, BCP 1-3 had rather low elongations at break (100-300%). The viscous properties outweigh the elastic properties, making the material more a thermoplastic material

Figure 10. Comparison of the stress-strain curves of BCP 2, BCP 4, and poly(CL).

than an elastomer. Additionally, stress-induced crystallization was observed (“whitening” of samples) at short elongations (10-50%). This visual impression could, however, not be directly supported by WAXS measurements of the stretched samples, which revealed only slight changes in crystallinity after stretching. The DSC measurements indicated an increase in melting temperature after this stress-induced crystallization. In BCP 4-5, CL was used as third comonomer to introduce higher flexibility into the multiblock copolymer chains. Consequently, BCP 4 and 5 reflected a significantly altered stress-strain behavior, as already shown in Figure 10 by means of stress-strain curves of two block copolymers with almost comparable block molar mass. Tensile tests with cyclic and increasing loading were performed to verify this first conclusion. The measurements were carried out with increasing stress in steps of 5% strain until break occurred. Figures 11 and 12 exhibit the resulting step-cycle curves of BCP 2 and BCP 4. The possible number of cycles under load of BCP 4 is much higher than that of BCP 2 (in addition, without break of the sample). Moreover, the residual stress after load of

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cantly; that is, use of the CL comonomer in TMC/LLA multiblock copolymers is essential to obtain elastomeric products. The elongations at break of these materials raise by more than 4 times to values that are typical for elastomers. Moreover, cyclic stress experiments confirmed that these new materials, particularly the ones including additionally CL, have promising properties as elastomers. Acknowledgment. We thank Mr. Dieter Voigt and Mrs. Petra Treppe for GPC measurements. References and Notes

Figure 11. Step-cycle investigations in stress-strain measurements of BCP 2.

Figure 12. Step-cycle investigations in stress-strain measurements of BCP 4.

BCP 4 is significantly lower compared to BCP 2. These results allowed us to conclude that BCP 4 and 5 behave as real elastomers whereas BCP 1-3 are almost thermoplasts. The reason for this different macroscopical behavior can be ascribed to the differences in the morphology of the polymers as discussed in conjunction with Figure 9. Conclusions It has been demonstrated that the synthesis of multiblock copolymers containing poly(TMC) and poly(LLA) was successfully carried out by a combination of ring-expansion polymerization (with cyclic tin initiators) and ring-opening polycondensation, with sebacoyl chloride. The resulting multiblock copolymers are phase-separated as shown by means of DSC, DMA, WAXS, and AFM. Phase separation is one prerequisite for elastomeric behavior. However, the mechanical tests revealed that in these block copolymers, having symmetrical block lengths, the plastic behavior outweights the elastomeric character, independently of the length or degree of polymerization of the blocks. Incorporation of -caprolactone as randomly distributed comonomer into the poly(TMC) blocks changes this behavior signifi-

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BM049393A

Multiblock Copolymers of l-Lactide and Trimethylene Carbonate

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