Synthesis and Characterization of Thermoset Biodegradable

Elastomers Based on Star-Poly(E-caprolactone-co-D,L-lactide). B. Amsden,*,† S. Wang,‡ ..... reaction is not as effective, for the given reaction t...
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Biomacromolecules 2004, 5, 1399-1404

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Synthesis and Characterization of Thermoset Biodegradable Elastomers Based on Star-Poly(E-caprolactone-co-D,L-lactide) B. Amsden,*,† S. Wang,‡ and U. Wyss‡ Departments of Chemical Engineering and Mechanical Engineering, Queen’s University, Kingston, Ontario, Canada K7L 3N6

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Received December 19, 2003; Revised Manuscript Received March 16, 2004

Biodegradable elastomers represent a useful class of biomaterials. In this paper, we synthesize thermoset elastomers by utilizing the living nature of ring-opening polymerization of a star copolymer of D,L-lactide and -caprolactone initiated with glycerol and catalyzed by stannous 2-ethylhexanoate. The star copolymers were synthesized of varying molecular weight and monomer composition and cross-linked by compression molding using a dilactone, bis(-caprolactone-4-yl)propane dissolved in -caprolactone monomer. The elastomers were then characterized by differential scanning calorimetry and uniaxial tensile testing and their physical properties related to the nature of the star copolymer prepolymers. The results demonstrate a means of predictably altering the elastomer physical properties by adjusting the star copolymer prepolymer initial molecular weight and monomer ratio. Introduction The majority of research into developing and using biodegradable polymers has focused on relatively crystalline, hard materials such as poly(lactide), poly(glycolide), and their copolymers, whereas comparatively less attention has been paid to biodegradable elastomers. Biodegradable elastomers have a number of possible biomaterial uses. These include scaffolds for regenerating soft tissue in vivo or in vitro1-5 and depot systems for localized drug delivery.6-8 Previously, we have reported on the preparation of a biodegradable thermoset elastomer by first synthesizing a 2700 g/mol star copolymer (SCP) of -caprolactone and D,Llactide via ring-opening polymerization of equimolar portions of the monomers, followed by cross-linking this prepolymer with bis(-caprolactone-4-yl)propane (BCP) in the presence of -caprolactone (CL) monomer.9 In this approach, CL monomer was used as a solvent for BCP to reduce the temperature required for cross-linking. The advantages of using star polymers are that, at low molecular weight, they have a reduced viscosity when in melt form than the same molecular weight linear polymer,10 which allows for easier injection at lower temperatures into molds for part manufacture, and their architecture can be altered to achieve different physical properties. Using ring-opening polymerization yields a living polymer. Advantage can be taken of this livingness to cross-link the star copolymer by further reaction with the bis-lactone, BCP (Figure 1). As has been demonstrated by Palmgren et al., BCP has almost the same reactivity of -caprolactone11 and * To whom correspondence should be addressed. Telephone: 613-5333093. Fax: 613-533-6637. E-mail: [email protected]. † Department of Chemical Engineering. ‡ Department of Mechanical Engineering.

Figure 1. Schematic of elastomer preparation procedure.

so should prove effective for this strategy. We have demonstrated the use of this strategy and examined the influence of the concentration of cross-linker used on the tensile properties and in vitro degradation rate of the elastomer.9 The main conclusions of that work were that the cross-linking procedure produced regions of high cross-link density, composed of BCP reacted with CL, connecting regions of un-cross-linked star copolymer (Figure 2) and using more BCP in the cross-linking reaction resulted in a decrease in ultimate stress at break (σb), Young’s modulus (E), and ultimate strain at beak (b). In the previous work, the composition and molecular weight of the star copolymer prepolymer were kept constant at equimolar and 2700 g/mol, respectively, while the molar ratio of BCP:SCP used was varied. In this paper, we examine the influence of the molecular weight and composition of star copolymer prepolymers on the properties of elastomers formed through their cross-linking with BCP, using a constant molar ratio of BCP:SCP.

10.1021/bm034538j CCC: $27.50 © 2004 American Chemical Society Published on Web 04/28/2004

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Table 1. Reactant Compositions, Composition as Determined via 1H NMR, Number Average Molecular Weight (Mn), Polydispersity Index (PI), and Glass Transition Temperatures (Tg) of the Star Copolymer Prepolymers monomer molar ratio (LA:CL)a

theoretical Mn (g/mol)

CL:LA (mol %) (NMR)

fraction of LA terminated chains

Mn (GPC) ( SDb

PI

Tg (°C)

30:70 60:40 90:10 50:50 50:50 50:50 50:50

2700 2700 2700 2700 4050 5400 8100

25:75 61:39 89:11 46:54 48:52 48:52 47:53

0.95 0.99 0.99 0.95 0.94 0.87 0.90

2836 ( 327 2681 ( 133 2783 ( 211 2734 ( 189 4079 ( 211 5448 ( 297 7926 ( 439

1.16 1.04 1.17 1.18 1.15 1.10 1.11

-36 4 26 -20 -14 -13 -11

a

CL ) -caprolactone and LA ) D,L-lactide. b SD ) standard deviation of 4 separate measurements.

Figure 2. Schematic of elastomer configuration, showing regions of highly cross-linked BCP-CL and regions containing the amorphous SCP chains.

Materials and Methods All reagents were used from the supplier without further purification unless specified otherwise. Calcium hydride (90-95%), glycerol (99.5+%), and stannous 2-ethylhexanoate (95%) were purchased from Aldrich Chemical Co, Canada. -Caprolactone (99%), distilled from calcium hydride prior to use, was purchased from Lancaster, Canada. D,L-Lactide (99.5+%) was purchased from Purac Biochem, The Netherlands. (2,2-)Bis(-caprolactone-4-yl) propane (BCP), synthesized as outlined in Palmgren et al.,11 was provided by Husam Younes, Faculty of Pharmacy and Pharmaceutical Sciences, University of Alberta. Synthesis of Star-Poly(E-caprolactone-co-D,L-lactide) (SCP). Solvent free polymerization was carried out in sealed and silanized 20 mL ampules. A representative polymerization of a 2700 g/mol polymer is described as follows. Into a dry silanized ampule were transferred 0.005 mol of glycerol and 0.05 mol of -CL, and they were mixed till homogeneous. 0.05 mol of DL-LA was transferred to the ampule, and the ampule was filled with nitrogen and then placed in an oven at 120 °C for 10 min allowing the DL-LA to melt. The mix was then stirred using a vortex mixer, and 10-4 mol of stannous 2-ethylhexanoate was added. The ampule

was flushed with dry nitrogen, flame sealed under vacuum, and left in an oven at 140 °C for 24 hours. The SCP was then purified by precipitation from dichloromethane solution in methanol cooled using dry ice (yield 85-95%). Reactant feed compositions for different monomer ratios (-caprolactone:D,L-lactide, CL:LA) and molecular weights are listed in Table 1. Preparation of the Elastomers. The following procedure describes the steps involved in preparing the elastomer. In a dry silanized glass ampule, BCP was dissolved in CL at 140 °C under nitrogen in a molar ratio of 1:4.5. Molten SCP (140 °C) was added to the ampule in a molar ratio of BCP: SCP of 4.8:1 and mixed by vortexing. Entrapped air was removed under mild vacuum for 5 min. After 20 min, stannous 2-ethylhexanoate was added to the ampule in a ratio of 10-3 mol per mol BCP. The contents were mixed using a vortex mixer, and the ampule was sealed under vacuum. The ampule was left in the vacuum oven at 140 °C for 10 min and then the seal was broken and the highly viscous liquid was poured to overflowing into rectangular, preheated Teflon moulds, covered with a Teflon plate over which a weight was placed, and they were left in the vacuum oven at 140 °C for 20 hours. The elastomer was then removed from the mould and characterized. Polymer Characterization. Thermal properties of the elastomers and prepolymers were measured using differential scanning calorimetry (DSC). The DSC measurements were carried out using a Seiko SII SSC/220U DSC with a liquidnitrogen cooling system. The samples were run at a heating rate of 10 °C/min using a cycle from ambient to -100 to +100 to -100 to +100 °C, with the glass transition temperature measured from the second heating cycle. The DSC was calibrated using indium and gallium standards. The number average and weight average molecular weights of the SCPs were determined using a Waters GPC system equipped with a Precision Detectors PD2000DLS lightscattering detector. The mobile phase consisted of anhydrous THF at a flow rate of 2 mL/min with the system at 35 °C. The concentration of the polymers used for the GPC measurements were 2 mg/mL and the injection volume was 20 µL. The column configuration consisted of an HP guard column pre-attached to a Phenogel linear DM3 5 µ GPC column. 1H NMR was run in CDCl3 to determine both composition and number average molecular weight using a Varian VXR 500 MHz Spectrometer with an Oxford Instruments superconducting magnet. Tetramethylsilane was used as the internal reference. Uniaxial tensile testing of the

Thermoset Biodegradable Elastomers

Figure 3.

1H

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NMR spectrum of a 50:50 CL:LA star copolymer with a theoretical Mn of 5400 g/mol, with peak assignments and integrations.

elastomers was carried out on dogbone samples, punched from the elastomer slabs, using an Instron tensile tester model 4443. Crosshead speed was set at 500 mm/min (ASTM D412). All specimens were tested at room temperature. Data analysis was carried out using Instron Merlin 4.11 Series IX software. The extractable contents were determined using Soxhlet extraction with dichloromethane. These measurements were done in triplicate for each elastomer sample. Results and Discussion The preparation of star-shaped poly(esters) can be accomplished through the ring-opening polymerization of

lactones initiated by polyols.12-16 In this work, glycerol was used as the initiator to prepare a 3-armed star copolymer. Ring-opening copolymerization of lactones using stannous 2-ethylhexanoate has been demonstrated to yield random copolymers. A characteristic 1H NMR spectra, complete with peak assignments,17 of a purified star copolymer prepolymer is illustrated in Figure 3, in this case a 50:50 5400 g/mol prepolymer. Using these peak assignments, the -caprolactone:D,L-lactide monomer composition of the prepolymers were determined and are listed in Table 1. The molar compositions of the prepolymers were in good agreement with the theoretical designed values. Furthermore, there were no peaks corresponding to unreacted D,L-lactide (-CHCH3-,

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Table 2. Extractable Fraction, Glass Transition Temperature (Tg), and Tensile Properties of Elastomers Prepared from the Different Star Copolymer Prepolymersa prepolymer avg. extract Tg LA:CL, Mn fraction (%) (°C) 30:70, 2700 60:40, 2700 90:10, 2700 50:50, 2700 50:50, 4050 50:50, 5400 50:50, 8100

16.1 ( 0.2 13.1 ( 3.2 17.7 ( 0.3 17.9 ( 1.1 18.8 ( 0.8 29.2 ( 0.6 31.7 ( 0.6

-40 -30 -20 -30 -23 -22 -18

σb (MPa)

b (%)

E (MPa)

0.81 ( 0.07 1.04 ( 0.04 1.26 ( 0.08 0.84 ( 0.08 0.77 ( 0.08 0.68 ( 0.01 0.62 ( 0.02

92 ( 6 150 ( 7 186 ( 18 120 ( 9 171 ( 14 252 ( 11 310 ( 8

0.73 ( 0.04 0.69 ( 0.05 0.66 ( 0.03 0.70 ( 0.01 0.44 ( 0.02 0.26 ( 0.02 0.19 ( 0.03

a σ ) tensile stress at break,  ) strain at break, E ) Young’s b b modulus. The numbers are reported as mean ( standard deviation for triplicate samples.

δ ) 5.4 ppm) or -caprolactone (-CH2CH2COO-CH2-, δ ) 2.65 ppm). The fraction of chains terminated in LA was calculated from the ratio of the intensities of g/(d + g) where d and g are the proton assignments in Figure 3. Interestingly, virtually all of the end groups were lactyl. It has been reported that lactide polymerizes more readily than -caprolactone, leading to terminal caproyl groups.18,19 However, a recent report confirms our findings and have attributed the result to higher polymerization temperature and longer polymerization times.20 Also listed in Table 1 are the number average molecular weights, Mn, as well as the polydispersity index of the prepolymers, as determined using light-scattering GPC. The molecular weights of the prepolymers were very close to the theoretical molecular weights of the prepolymers, and the polymers had a narrow molecular weight distribution, with polydispersity indexes of 1.1-1.2. Such narrow molecular weight distributions have been reported by others in the literature for alcohol-initiated ring-opening polymerization of lactones to yield star copolymers.21-23 Moreover, the measured Mn by both 1H NMR analysis and GPC were in very good agreement. This result, coupled with the LA:CL ratios being very close to theoretical, suggested nearly complete monomer conversion. All of the prepolymers were amorphous, with glass transition temperatures ranging from -36 to +26 °C. In agreement with the Fox equations,24 the glass transition temperature of the prepolymer increased as the -caprolactone content of the prepolymer, being the monomer generating the lowest Tg homopolymer, decreased and as the molecular weight of the prepolymer increased. Furthermore, instead of increasing, as would be expected, the elastomer glass transition temperature decreased upon cross-linking for every elastomer (Table 2). This result is due to the use of the -caprolactone monomer as a solvent for the BCP crosslinker. The -caprolactone also takes part in the ring-opening reaction and, thus, is incorporated into the final elastomer. As the content of -caprolactone in the elastomer increases, its glass transition temperature should approach that of poly(-caprolactone) which is -60 °C. The BCP is completely consumed in the curing reaction, as evidenced by the lack of a melting point for BCP at 180 °C.11 Additionally, the glass transition temperatures of the elastomers containing 50: 50 CL:LA were all consistently depressed by approximately 10 °C from that of the prepolymers and follow the predictions

Figure 4. Glass transition temperature, Tg, of the SCP prepolymers and the various elastomers, as a function of the reciprocal SCP molecular weight. The CL:LA ratio was 50:50 in each case. The solid lines represent fits of a linear expression to the data.

of the Fox equation for the influence of prepolymer molecular weight (Figure 4, correlation coefficient ) 0.97). This result indicates that the increase in molar ratio of -caprolactone incorporated into the elastomer was approximately equivalent for each elastomer. The extractable fraction of the elastomers range from 13.1( 3 to 31.7 ( 0.6 w/w % (Table 2). The extractable fraction for the elastomers prepared from prepolymers less than 4050 g/mol were essentially the same at about 17 w/w %. The higher molecular weight prepolymers formed elastomers with higher extractable fractions (about 30%). This higher extractable fraction suggests that the cross-linking reaction is not as effective, for the given reaction time, for the higher molecular weight elastomers. This could be due to homopolymerization of the -caprolactone or poor initiation by the SCP for these prepolymers. The 20 h cure at 140 °C was chosen for all of the elastomers based on earlier results for optimizing the curing of 2700 g/mol 50:50 mol % CL:LA prepolymers, where it was found that a minimum cure time of 12 h was required to minimize the amount of extractable material. The fraction of extractable material did not decrease significantly up to 24 h curing time, the maximum time frame examined. Thus, it was felt that this cure time was suitable for all of the prepolymers. Longer cure times may result in trans-esterification reactions that lead to the formation of cyclic CL fractions that would not form part of the network. The uniaxial tensile tests revealed that the elastomers behaved in a nearly Hookean fashion during extension (Figure 5A,B), with no sign of strain toughening. Figure 5A shows the stress-strain relationships for elastomers prepared using the same CL:LA ratio of 50:50, but with differing SCP prepolymer molecular weight, and the mechanical properties for these elastomers are listed in Table 2. As the molecular weight of the SCP prepolymer increased, the elastomers became softer (Young’s modulus, E, decreased), more extensible (ultimate strain, b, increased), and less capable of withstanding stress. Figure 5B demonstrates that it is also possible to manipulate the elastomer mechanical properties by altering the monomer ratio while maintaining the SCP molecular weight

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Figure 6. Influence of SCP Mn at constant composition (50:50 CL:LA) on elastomer Young’s modulus.

between extensible cross-links, because the highly crosslinked BCP-CL region does not contribute significantly to the extension of the elastomer. For this case, the Young’s modulus, E, should be proportional to Mn-1.26 Figure 6 shows the Young’s modulus dependence on SCP molecular weight. The curve in the figure is a fit of the following equation to the data E)

Figure 5. Uniaxial tensile stress-strain curves for elastomers prepared using (A) 50:50 CL:LA prepolymers of varying number average molecular weight (Mn) and (B) 2700 g/mol prepolymers of varying CL:LA molar ratios

constant. For a constant SCP molecular weight, the Young’s moduli are all essentially constant (Table 2). However, as the CL content of the elastomer decreased, ultimate stress, σb, and b increased. In other words, the elastomer became stronger and more elastic as the LA content increased. A similar result was reported by Storey et al., who examined star copolymers of D,L-lactide and trimethylene carbonate (TMC), end-capped with methacrylate and thermally crosslinked.21 For TMC:LA ratios of 20:80 and 40:60, that is, similar to the CL:LA range of this work, the Young’s modulus was essentially constant, whereas the ultimate strain increased as the LA ratio increased. Hiljanen-Vainio et al., who examined the mechanical properties of linear poly(caprolactone-co-D,L-lactide) of high molecular weight (>75 000 g/mol), reported similar findings. These authors found that, as the CL:LA ratio changed from 60:40 to 40: 60, the Young’s modulus and ultimate stress changed from 2.1 ( 0.8 and 23 kPa to 2.8 ( 4 and 80 ( 10 kPa, respectively.25 We explain these results as follows. Using the picture of the elastomer as given in Figure 2, increasing the SCP molecular weight effectively increases the molecular weight

k Mn

(1)

wherein k is a constant ()FRT/3, where F is the polymer density, R is the gas constant, and T is the temperature). Equation 1 assumes there are no network defects. The equation provides good agreement to the data, with k ) 1740 ( 87 MPa g/mol and a correlation coefficient of 0.96. Thus, the model of the elastomer structure appears reasonable. In uniaxial tensile testing, failure is considered to occur as a result of crack propagation. For elastomers which deform in a nearly Hookean fashion, b, is given by27 b )

( ) Gc πaE

1/2

(2)

wherein Gc is the amount of energy required to advance the crack by unit area and a is the depth of the crack. Gc represents the energy dissipated during fracture and is dependent on the viscoelastic nature of the elastomer. The ultimate stress of the elastomer, σb, is given by sb )

( ) GcE πa

1/2

(3)

Equations 2 and 3 indicate that bE1/2 should be equal to σbE-1/2 for a given elastomer. Furthermore, due to the viscoelastic nature of Gc, these terms are also dependent on the glass transition temperature of the elastomers. This dependence can be described by the WLF shift factor, aT, using the universal constants log(aT) )

-17.44(T - Tg) 51.66 + (T - Tg)

(4)

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whereas the strain at break increased in a linear fashion with the molecular weight. These results are consistent with a picture of the elastomer as consisting of deformable regions made up of the star copolymer, connected to highly crosslinked regions of the dilactone cross-linker and -caprolactone. At a given star copolymer prepolymer molecular weight, the Young’s modulus remains constant, whereas the strain and stress at break increase with increasing D,L-lactide content.

Figure 7. Reduced ultimate stress, σb, and ultimate strain, b, as a function of the WLF shift factor, aT.

Acknowledgment. This work was funded by the Natural Sciences and Engineering Research Council of Canada and the Canadian Institutes for Health Research. We thank Husam Younes, Faculty of Pharmacy, University of Alberta, for providing the bis(-caprolactone-4-yl)propane. References and Notes

Figure 7 shows a plot of log(σbE-1/2) and log(bE1/2) versus log(aT). As predicted by the crack propagation theory for a Hookean material, the two curves coincide. Additionally, there is a linear relationship between log(aT) and log(σbE-1/2) and log(bE1/2) (slope ) 0.10, correlation coefficient ) 0.96). This relationship can be used as a means of designing the final tensile properties of the elastomer, by providing a relationship between Tg and σb and b. The glass transition is readily manipulated with molecular weight and composition in a predictable fashion, and the Young’s modulus, E, can be adjusted by manipulating the SCP molecular weight, as demonstrated above. The effect of increasing D,L-lactide content can therefore be explained in this manner. Increased D,L-lactide content decreases the mobility of the extensible chains, which in turn increases the amount of viscous force that needs to be dissipated during crack propagation, exhibited as an increase in Gc. This increase in Gc thus causes a greater resistance to tearing and the elastomer can be extended more and resists a greater applied stress until finally failing. In previous work with 2700 g/mol prepolymer cross-linked with varying amounts of BCP, we had found that, even with an extractable amount of 30%, the Tg of the elastomer after extraction only decreased by 5 °C, and with an extractable fraction of 17%, the Tg decreased by only 2 °C.9 Thus, the extractable portion plasticizes the final elastomer. As the mechanical properties of the elastomer depend on the Tg, the presence of the extractable portion increases the ultimate strain and stress, but not significantly. Conclusions In this work, we have demonstrated that the mechanical properties of a thermoset elastomer prepared from star copolymer prepolymers composed of -caprolactone and D,Llactide cross-linked with a dilactone cross-linker (BCP) in the presence of -caprolactone can be varied through manipulation of the molecular weight of the star copolymer prepolymer. For a given star copolymer monomer composition, the Young’s modulus was shown to be proportional to the reciprocal of the star copolymer molecular weight,

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