Highly elastic lactide-based polymer blend, object made from the polymer blend and method of producing the object
11643548 · 2023-05-09
Assignee
Inventors
- Andreas Lendlein (Berlin, DE)
- Axel Thomas Neffe (Berlin, DE)
- Paul Jakob Hommes-Schattmann (Berlin, DE)
- Quanchao Zhang (Jiangxi, CN)
Cpc classification
D01F6/625
TEXTILES; PAPER
C08L67/04
CHEMISTRY; METALLURGY
C08L2205/025
CHEMISTRY; METALLURGY
C08L67/04
CHEMISTRY; METALLURGY
International classification
Abstract
A polymer blend is provided, comprising or consisting of: (A) a first polymer component, being a copolymer having a substantially random, partially blocky structure and selected from poly[(L-lactide)-co-(£-caprolactone)] and poly[(D-lactide)-co-(£-caprolactone)] and (B) a second polymer component different from the first polymer component (A) selected from poly(L-lactide), poly(D-lactide), poly[(L-lactide)-co-(£-caprolactone)] and poly[(D-lactide)-co-(£-caprolactone)], wherein the first and second polymer components (A) and (B) are selected such that the resulting polymer blend comprises a combination of constitution units derived from L-lactide and constitution units derived from D-lactide. The highly elastic material may be processed by electrospinning to produce elastic porous objects having a non-woven fibrous structure.
Claims
1. A polymer blend, comprising or consisting of: (A) a first polymer component, being a copolymer which is poly[(L-lactide)-co-(e-caprolactone)] or poly[(D-lactide)-co-(ϵcaprolactone)], wherein the first polymer component (A) has a substantially random, partially blocky structure expressed by a dyad ratio of lactide-lactide dyads (LA-LA) to lactide-e-caprolactone dyads (LA-CL) of at least 2.5:1; and (B) a second polymer component different from the first polymer component (A) selected from poly(L-lactide), poly(D-lactide), poly[(L-lactide)-co-(e-caprolactone)] and poly[(D-lactide)-co-(e-caprolactone)], wherein the first and second polymer components (A) and (B) are selected such that the resulting polymer blend comprises a combination of constitution units derived from L-lactide and constitution units derived from D-lactide.
2. The polymer blend according to claim 1, comprising or consisting of 80 to 99 wt % of the first polymer component (A) and 1 to 20 wt % of the second polymer component (B) selected from poly(L-lactide) or poly(D-lactide).
3. The polymer blend according to claim 1, wherein poly[(L-lactide)-co-(e-caprolactone)] or poly[(D-lactide)-co-(e-caprolactone)] has a weight average molecular weight M.sub.w of at least 80 kDa, as determined by universally calibrated gel permeation chromatography.
4. The polymer blend according to claim 1, wherein poly(L-lactide) or poly(D-lactide) has a weight average molecular weight M.sub.w in the range of 1 to 40 kDa, as determined by universally calibrated gel permeation chromatography.
5. The polymer blend according to claim 1, wherein a weight content of e-caprolactone in the first polymer component (A) is in the range of 25 to 47 wt %, based on the total weight of the copolymer (A).
6. The polymer blend according to claim 1, wherein a fraction of e-caprolactone present in an amorphous phase in the first polymer component (A) is at least 25 mol %, based on the total molar amount of e-caprolactone in the first polymer component (A), as determined by NMR and DSC measurements.
7. The polymer blend according to claim 1, selected from: a blend comprising poly[(L-lactide)-co-(e-caprolactone)] and poly(D-lactide); a blend comprising poly[(D-lactide)-co-(e-caprolactone)] and poly(L-lactide); and a blend comprising poly[(L-lactide)-co-(e-caprolactone)] and poly[(D-lactide)-co-(e-caprolactone)].
8. The polymer blend according to claim 1, wherein the polymer blend has an elongation at break in the range of 400 to 1000% at 25° C., as determined by uniaxial tensile testing.
9. The polymer blend according to claim 1, wherein the polymer blend has an elastic modulus in the range of 25 to 150 MPa at 25° C., as determined by uniaxial tensile testing.
10. An object prepared from a polymer blend according to claim 1, the object having a porous, non-woven fibrous structure.
11. The object according to claim 10, wherein the fibers have an average fiber diameter in the range of 0.5 to 10 μm, as determined from SEM images.
12. The object according to claim 10, wherein the object has an elongation at break at 25° C. in the range of 250 to 850%, as determined by tensile testing.
13. The object according to claim 10, wherein the object has an elastic modulus at 25° C. in the range of 0.3 to 5 MPa, as determined by tensile testing.
14. A method of processing a polymer blend, comprising: providing a solution of a polymer blend in a solvent, the polymer blend comprising or consisting of: (A) a first polymer component, being a copolymer having a substantially random, partially blocky structure and selected from poly[(L-lactide)-co-(e-caprolactone)] and poly[(-lactide)-co-(e-caprolactone)], wherein the first polymer component (A) has a dyad ratio of lactide-lactide dyads (LA-LA) to lactide-e-caprolactone dyads (LA-CL) of at least 2.5:1; and (B) a second polymer component different from the first polymer component (A) selected from poly(L-lactide), poly(D-lactide), poly[(L-lactide)-co-(e-caprolactone)] and poly[(D-lactide)-co-(e-caprolactone)], wherein the first and second polymer components (A) and (B) are selected such that the resulting polymer blend comprises a combination of constitution units derived from L-lactide and constitution units derived from D-lactide; subjecting the solution to electrospinning to produce fibers of the polymer blend; and depositing the fibers to form a non-woven fibrous structure building-up the object.
15. A method of producing an object having a porous, non-woven fibrous structure, comprising: providing a solution of a polymer blend in a solvent, the polymer blend comprising or consisting of: (A) a first polymer component, being a copolymer having a substantially random, partially blocky structure and selected from poly[(L-lactide)-co-(e-caprolactone)] and poly[(D-lactide)-co-(e-caprolactone)], wherein the first polymer component (A) has a dyad ratio of lactide-lactide dyads (LA-LA) to lactide-e-caprolactone dyads (LA-CL) of at least 2.5:1; and (B) a second polymer component different from the first polymer component (A) selected from poly(L-lactide), poly(D-lactide), poly[(L-lactide)-co-(e-caprolactone)] and poly[(D-lactide)-co-(e-caprolactone)], wherein the first and second polymer components (A) and (B) are selected such that the resulting polymer blend comprises a combination of constitution units derived from L-lactide and constitution units derived from D-lactide; subjecting the solution to electrospinning to produce fibers of the polymer blend; and depositing the fibers to form a non-woven fibrous structure building-up the object.
16. The polymer blend according to claim 1, comprising or consisting of 50 to 95 wt % of the first polymer component (A) and 5 to 50 wt % of the second polymer component (B) selected from poly[(L-lactide)-co-(e-caprolactone)] or poly[(D-lactide)-co-(e-caprolactone)].
Description
DETAILED DESCRIPTION OF THE INVENTION
(1) The invention will be described in more detail by way of examples. The Figures show:
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12) The invention provides a highly elastomeric lactide-based polymeric material that is hydrolytically degradable and biocompatible.
(13) The polymer blend comprises or consists of: (A) a first polymer component, being a copolymer having a substantially random, partially blocky structure and selected from poly[(L-lactide)-co-(ε-caprolactone)] (also referred to as P(LLA-co-CL)) and poly[(D-lactide)-co-(ε-caprolactone)] (also referred to as P(DLA-co-CL)) and (B) a second polymer component different from the first polymer component (A), selected from poly(L-lactide) (also referred to as PLLA), poly(D-lactide) (also referred to as PDLA), poly[(L-lactide)-co-(ε-caprolactone)] (P(LLA-co-CL)) and poly[(D-lactide)-co-(ε-caprolactone)] (P(DLA-co-CL)).
(14) The first and second polymer components (A) and (B) are selected such that the resulting polymer blend comprises a combination of constitution units derived from L-lactide (LLA) and constitution units derived from D-lactide (DLA).
(15) The combination of constitution units derived from L-lactide and constitution units derived from D-lactide in the polymer blend allows for the formation of stereocomplexes between oligo sequences of L-lactide (oligo-LLA, present in the first or in the second component) and the complementary oligo sequences of D-lactide (oligo-DLA, present in the second or in the first component). The stereocomplexes form crystallites acting as physical (non-covalent) net points crosslinking the polymer components. Due to these net points the polymer material exhibits a high degree of elasticity.
(16) First Polymer Component (A)
(17) The first polymer component is a copolymer (dicopolymer) of units derived from E-caprolactone (CL) and units derived either from L-lactide (LLA) or from D-lactide (DLA).
(18) In embodiments, the first polymer component (A) has a weight average molecular weight M.sub.w of at least 80 kDa, preferably of at least 100 kDa. A high molecular weight of ≥80 kDa enhances the elasticity of the material by increasing the entanglement of the polymer chains. The upper value of the molecular weight of the first polymer component is not particularly limited in terms of material properties. However, it may be difficult to achieve molecular weights M.sub.w of more than 300 kDa by standard synthesis methods of the copolymer. The weight average molecular weight M.sub.w can be measured by universally calibrated gel permeation chromatography (GPC).
(19) The copolymer preferably has a substantially random, partially blocky structure. The term “partially blocky” means that the copolymer comprises some block sequences of oligo(L-lactide) or oligo(D-lactide), respectively. The block sequences of oligo(L-lactide) or oligo(D-lactide) in the copolymer allow the formation of stereocomplexes with the corresponding complementary sequences, i.e. oligo(D-lactide) or oligo(L-lactide), of the second polymer component (B). More precisely, when the first polymer component (A) is poly[(L-lactide)-co-(ε-caprolactone)] its oligo(L-lactide) sequences will form stereocomplexes with oligo(D-lactide) sequences of the second component (B). When the first polymer component (A) is poly[(D-lactide)-co-(ε-caprolactone)] its oligo(D-lactide) sequences will form stereocomplexes with oligo(L-lactide) sequences of the second component (B).
(20) The weight content of ε-caprolactone in the first polymer component (A) may be in the range of 25 to 47 wt %, particularly in the range of 33 to 44 wt %, preferably in the range of 36 to 40 wt %, based on the total weight of the copolymer (A) (100 wt %). When the weight content of ε-caprolactone in the first polymer component (A) is less than 25 wt %, the glass temperature T.sub.g of the copolymer will increase with the effect that the material at room temperature is in the glass state, i.e. that the material is not elastic. If however the weight content of ε-caprolactone in the first polymer component (A) is more than 47 wt %, the formation of the oligo-lactide blocks during the synthesis may be hindered resulting in low or no formation of crystallites in the polymer blend.
(21) The weight content of e-caprolactone in the first polymer component (A) determines the glass temperature T.sub.g of the copolymer (and hence of the polymer blend) and, on the other hand, influences the partial formation of oligo blocks of oligo-LLA or oligo-DLA in the copolymer. In copolymers comprising monomers that can be ideally mixed with each another, such as in the case of lactide (LA) and ε-caprolactone (CL), the theoretical glass transition temperature T.sub.g,t can be calculated by the Fox-Equation below, where w.sub.1 and w.sub.2 are the weight fractions of monomer 1 and monomer 2 and T.sub.g1 and T.sub.g2 are the glass transition temperatures of the corresponding homopolymers of monomer 1 and monomer 2, respectively:
(22)
(23) In the present invention, a glass transition temperature T.sub.g of the polymer blend of ≤0° C. is preferred in order to obtain a material that exists in its elastic state at room temperature. According to the above Fox-Equation with T.sub.g,PCL=−60° C. and T.sub.g,PLA=50° C. a theoretical weight ratio of ≤65 wt % of LA (LLA or DLA) and ≥35 wt % of CL results to obtain the desired T.sub.g of the copolymer. However, the true glass transition temperature T.sub.g of a real copolymer is influenced by two factors. First, it has to be considered that during the synthesis of the copolymer by copolymerisation of the two comonomers, not necessarily all comonomers may be incorporated into the polymer chain resulting in a deviation of the fractions of the comonomers in the obtained copolymer from the fraction of the comonomers used in the synthesis mixture. In addition, crystallized segments in the polymer chain do not contribute to the T.sub.g. Accordingly, differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) experiments were made in order to determine the comonomer fractions of CL and LA in the copolymer necessary for achieving the desired T.sub.g.
(24) As mentioned before, the first polymer component (A) preferably has a partially blocky structure. The degree of block segments in copolymers can be expressed by the average dyad ratio. The dyad ratio is the molar ratio of the number of homodimers of the same type of comonomer present in the polymer chain (comonomer 1-comonomer 1) to a heterodimer formed by two different comonomers (comonomer 1-comonomer 2). The dyad ratio can be determined by .sup.1H-NMR. In the present invention, the first polymer component (A) preferably has a dyad ratio of lactide-lactide dyads (LA-LA) to lactide-ε-caprolactone dyads (LA-CL) of at least 2.5:1. When the dyad ratio is less than 2.5:1 the occurrence of blocks of oligolactide in the copolymer becomes low so that the formation of stereocomplexes in the polymer blend also becomes low.
(25) The function of ε-caprolactone in the copolymer is to form an amorphous phase at the desired temperature of use, particularly at room temperature. For this purpose, a fraction of ε-caprolactone present in an amorphous phase in the first polymer component (A) is at least 25 mol %, preferably in the range of 27-33 mol %, based on the total molar amount of ε-caprolactone in the first polymer component (A).
(26) The first polymer component (A) can be synthesised by copolymerising a mixture of the comonomers ε-caprolactone and L,L-dilactide or of ε-caprolactone and D,D-dilactide in the desired mixing ratio through ring opening copolymerisation.
(27) Second Polymer Component (B)
(28) According to embodiments of the invention, in case the second polymer component (B) is either PLLA or PDLA, its weight average molecular weight M.sub.w is in the range of 1 to 40 kDa, preferably of 2 to 20 kDa. By providing a molecular weight of the second polymer component (B) of below 40 kDa, its the mobility within the blend is increased. As the corresponding oligo-LLA or oligo-DLA segments of the second polymer component (B) in a blend with the first component (A) give rise to formation of stereocomplexes that serve as nucleation sites for the formation of crystallites, the increased mobility leads to a higher velocity of the formation of crystallites and faster propagation of crystallization. On the other hand, by providing a molecular weight of the second polymer component (B) of above 1 kDa, the size and hence the stability of the stereocomplexes are increased.
(29) The second polymer component (B) can be synthesised by homopolymerising the monomers L,L-dilactide or D,D-dilactide, respectively through ring opening polymerisation. In case the second polymer component (B) is selected from poly[(L-lactide)-co-(ε-caprolactone)] or poly[(D-lactide)-co-(ε-caprolactone)], its synthesis can be done in the same way as described for the first polymer component (A).
(30) Regarding the further properties of the P(LLA-co-CL) or P(DLA-co-CL) when used as the second polymer component (B) the same consideration as set with respect of the first component (A) apply.
(31) Polymer Blend
(32) Preferably, the polymer blend is a binary composition comprising not more than the two polymers selected from the first polymer component (A) and the second polymer component (B).
(33) In embodiments of the invention, the second polymer component (B) is selected from poly(L-lactide) and poly(D-lactide). In this case the polymer blend preferably comprises or consists of 80 to 99 wt % of the first polymer component (A) and 1 to 20 wt % of the second polymer component (B). More preferably the polymer blend comprises or consists of 85 to 97 wt % of the first polymer component (A) and 3 to 15 wt % of the second polymer component (B). Even more preferably, the polymer blend comprises or consists of 90 to 95 wt % of the first polymer component (A) and 5 to 10 wt % of the second polymer component (B). By adjusting the mixing ratio of the first and second component the mechanical properties of the material can be adjusted according to a desired appliance. Moreover, when the content of the second polymer component (B) in the blend is more than 20 wt % the material becomes brittle if the polymer component (B) is of low molecular weight. Furthermore, below 1 wt % and above 20 wt % the elongation at break decreases. Accordingly, a weight content of the second polymer component (B) in the range of 1 to 20 wt %, particularly of about 10 wt %, imparts the highest values of elongation at break.
(34) In other embodiments of the invention, the second polymer component (B) is selected from poly[(L-lactide)-co-(ε-caprolactone)] and poly[(D-lactide)-co-(ε-caprolactone)]. In this case, the polymer blend preferably comprises or consists of 50 to 95 wt % of the first polymer component (A) and 5 to 50 wt % of the second polymer component (B).
(35) Preferred polymer blends according to the present invention are: a blend comprising poly[(L-lactide)-co-(ε-caprolactone)] and poly(D-lactide) (P(LLA-co-CL)/PDLA), a blend comprising poly[(D-lactide)-co-(ε-caprolactone)] and poly(L-lactide) (P(DLA-co-CL)/PLLA), and a blend comprising poly[(L-lactide)-co-(ε-caprolactone)] and poly[(D-lactide)-co-(ε-caprolactone)](P(LLA-co-CL)/P(DLA-co-CL)).
(36) The polymer blend can easily be prepared by mixing the first and second polymer component in the target mixing ratio as set forth above. For this purpose, the single components (A) and (B) in the desired weight ratio may be added to a suitable solvent and mixed to obtain a solution of the mixed polymer compounds. The solvent may then be removed. Alternatively, the single components (A) and (B) may be dissolved individually in a suitable solvent followed by mixing the solutions and then removing the solvent. If desired, before removing the solvent, the mixed solution can be processed to a molded object. In the mixed solution in a suitable solvent no stereocomplexes are formed allowing an easy processing of the solution containing the polymer blend. Only after removing the solvent the stereocomplexes will be formed resulting in the elastomeric material.
(37) The polymer blend according to the present invention is at room temperature highly elastic or even hyperelastic. Moreover, the material shows a high degree of shape stability. The term shape stability indicates the degree of restauration of the initial shape after releasing the deformation force. The blend preferably has an elongation at break in the range of 400 to 1000% at 25° C. The elongation at break can be determined by uniaxial tensile testing of thin films and/or electrospun mats. Further, polymer blend according to the present invention may have an elastic modulus in the range of 25 to 150 MPa at 25° C. The elastic modulus can be determined by uniaxial tensile testing of thin films and/or electrospun mats.
(38) Method of Processing the Polymer Blend
(39) The polymer blend according to the present invention may be processed into a three-dimensional object by any standard moulding technique.
(40) For instance, due to the fact that the stereocomplexes oligo-LLA/oligo-DLA are cleaved when the blend is dissolved in a suitable solvent, the material may easily processed by dissolving the polymer blend in a suitable solvent, bringing the solution into the desired shape, and then removing the solvent. After removal of the solvent, the stereocomplexes will readily form even at room temperature or after annealing at elevated temperatures above T.sub.g of PLA (≥50° C.) resulting in a highly elastic object having the desired three dimensional shape.
(41) However, in advantageous embodiments, the polymer material of the invention is processed by electrospinning as to form an object having a porous, non-woven fibrous structure.
(42) The method of processing the polymer blend comprises the steps of: providing a solution of the polymer blend according to the invention in a solvent, subjecting the solution to electrospinning to produce fibers of the polymer blend, and depositing the fibers to form a non-woven fibrous structure building-up the object.
(43) In the first step, any solvent or solvent mixture can be used that is capable of dissolving the polymer blend. Due to its rather hydrophilic nature suitable solvents comprise hydrophilic solvents such as alcohols. In an example, the solvent is hexafluoro isopropanol (HFIP).
(44) In the step of electrospinning fibers are formed of the polymer blend. The electrospinning apparatus comprises a reservoir for storing the polymer solution (typically a syringe) and equipped with a needle, and a collector positioned with a predetermined distance to the needle tip and acting as target. Between the needle and the collector a voltage is applied. During electrospinning the polymer solution is pressed through the needle, charged by the voltage and accelerated in the electric field towards the collector where it is deposited. Accordingly, the step of electrospinning and the step of depositing are typically conducted in parallel.
(45) The step of depositing the polymer fibers may be conducted such that the fibers are deposited on the collector. The shape of the collector determines the shape of the object formed in the process. For example, the collector may be a simple flat plate. In this case, flat non-woven fiber structures are formed. However, for producing more complex shapes, the collector may have a shape which acts as mould for the shape of the object to be formed. Moreover, the collector may be a static such that no movement relative to the needle tip occurs. In embodiments, the collector may be movable in one, two or three directions with respect to the needle tip and/or rotatable allowing the formation of complex three-dimensional structures by moving the collector.
(46) There are various process parameters during electrospinning which can be varied in order to control the properties of the polymer fibers. The process parameters comprise humidity of the atmosphere, the voltage applied to the needle and collector, the distance between needle and collector, polymer concentration of the polymer solution, solvent of the polymer solution, flow rate of the polymer solution through the needle, shape of the needle tip, rotation speed of the collector, etc.
(47) In preferred embodiments, the process parameters during electrospinning are selected such that fast evaporation of the solvent is achieved. In other embodiments, the process parameters during electrospinning are selected as to control formation of the stereocomplexes. In still other embodiments, the process parameters during electrospinning are selected as to minimize shrinking of the spun fibers and structures formed thereof due to entropic relaxation.
(48) After depositing the fibers, the object thus formed may be subjected to a thermal treatment, preferably at temperatures in the range of 60 to 120° C. The thermal treatment supports the formation of stereocomplexes and increases the crystallinity.
(49) Object Prepared from the Polymer Blend
(50) The object prepared from the polymer blend may have any shape and may have a porous or compact (non-porous) structure.
(51) Non-porous structures may be produced by standard casting or moulding techniques. Objects of non-porous structures will have the elastic moduli and elongation at break as described with respect to the polymer blend.
(52) Porous fibrous structures may be produced by electrospinning techniques as described above. The porous nature results from the non-woven structure formed by the deposited fibers. Typically an average pore diameter may be controlled to be in the range of several hundreds of nanometres to several hundreds of micrometres by varying the electrospinning process parameters mentioned above. In examples, the average pore diameter is in the range of 1 μm to 200 μm.
(53) In embodiments, the fibers that build-up the electrospun porous object have an average fiber diameter in the range of 0.5 to 10 μm, particularly in the range of 1 to 5 μm, preferably in the range of 1.5 to 3 μm. The fiber diameter can be controlled by varying the electrospinning process parameters mentioned above.
(54) Due to its porosity, the electrospun object has substantially the high elasticity (elongation at break) described with respect of the polymer blend, but has lower E-moduli, typically in the magnitude of a few MPa. In embodiments, the object has an elongation at break at 25° C. in the range of 250 to 850%, particularly in the range of 350 to 700%. In further embodiments, the object has an elastic modulus at 25° C. in the range of 0.3 to 5 MPa, particularly in the range of 0.5 to 1.5 MPa.
(55) Applications of the Electrospun Objects
(56) High elasticity and porosity of the devices are required for many medical applications. Because of these properties, the materials can expand or contract upon the physiological requirements (elasticity) and at the same time allow exchange of nutrients, salts, cellular waste products, gasses, and the migration of cells similar to the situation in native tissues (porosity). Devices with low E-moduli in the MPa range exhibiting high long-term form stability under dynamic mechanical stress under physiological conditions are particularly suitable as implants in tissue with frequently occurring movements.
(57) Likewise, the permeation properties can be exploited for non-medical, technical applications such as separation membranes, pollen and cabin filters, or breathable textiles.
Working Examples
(58) Synthesis of First Polymer Component (A)
(59) The synthesis of the copolymer component was performed by tin(II) 2-ethylhexanoate (Sn(oct).sub.2) catalyzed ring-opening polymerization of a mixture of ε-caprolactone and L,L-dilactide (or D,D-dilactide) in the melt. The synthesis was done using different comonomer ratios (LA:CL.sub.syn,m) as indicated in Table 1.
(60) A typical protocol for the synthesis is as follows: A 500 mL flame-dried three-neck-flask (equipped with magnetic stirring rod, rubber septum, PTFE valve and glass stopper) under argon atmosphere was loaded with L,L-dilactide (60.0 g, 416.3 mmol) (or D,D-dilactide, respectively) and ε-caprolactone (38 mL, 40-41 g, 350-360 mmol), that was purified by distillation. The reaction vessel was placed into a preheated oil bath (140° C.) and the mixture was magnetically stirred until complete melting/dissolution. Then a dilute solution of Sn(oct).sub.2 in anhydrous THF (e.g. 5.1 mL with a concentration of 24.7 mg/mL Sn(oct).sub.2) was added dropwise and the resulting mixture was stirred for 20 to 70 h at 140° C. to become a highly viscous polymer melt. The reaction vessel was removed from the oil bath and the crude product was allowed to cool to room temperature, dissolved in 2 L of chloroform, followed by precipitation in 20 L of methanol. The precipitate was dried under reduced pressure at 60° C. The yield of the copolymer was typically in the range of 80-90 g.
(61) The molar comonomer ratios (LA:CL.sub.NMR.m) in the copolymer were determined by .sup.1H-NMR-measurements.
(62) The .sup.1H-NMR-spectra of the copolymers also allowed for the determination of the dyad ratios (DR) by integrating the signals related to LA-LA dyads (5.13 to 5.20 ppm) and those related to LA-CL dyads (5.07 to 5.13 ppm) and comparing the integrated intensities. The results are included in Table 1 as well.
(63) The glass transition temperatures (T.sub.g) of the copolymers were determined by DSC-measurements.
(64) The weight average molecular weight (M.sub.w) of the copolymers were determined by GPC as shown in
(65) TABLE-US-00001 TABLE 1 Composition and properties of the synthesised copolymers. Example LA LA:CL.sub.syn,m LA:CL.sub.m LA:CL.sub.w M.sub.w (kDa) T.sub.g (° C.) DR Al LLA 67/33 67/33 56:44 183 −10 2.7:1 A2 LLA 70/30 67/33 56:44 145 −10 3.0:1 A3 LLA 69/31 69/31 58:42 156 −7 2.5:1 A4 LLA 70/30 70/30 60:40 201 −4 2.9:1 A5 LLA 70/30 71/29 61:39 164 −6 3.3:1 A6 LLA 70/30 72/28 62:38 190 −3 4.0:1 A7 LLA 70/30 73/27 63:37 246 −3 4.6:1 A8 DLA 70/30 70/30 60:40 315 −3 3.4:1 A9 DLA 70/30 75/25 65:35 190 −2 5.1:1 LA:CL.sub.syn,m: molar ratio of LLA (or DLA) and CL as used in synthesis; LA:CL.sub.m: molar ratio of LA and CL as determined by .sup.1H-NMR, LA:CL.sub.w: weight ratio of LA and CL as calculated from LA:CL.sub.m, M.sub.w: weight average molecular weight as determined by GPC (universal calibration); T.sub.g: glass transition temperature as determined by DSC (2.sup.nd heating cycle); DR: Ratio of LA-LA dyads to LA-CL dyads as determined by .sup.1H-NMR.
Synthesis of Second Polymer Component (B)
(66) The synthesis of the second polymer component was performed analogously to polymer component (A) for P(LLA-co-CL) and P(DLA-co-CL). PLLA and PDLA were synthesized by tin(II) 2-ethylhexanoate (Sn(oct).sub.2) catalyzed ring-opening polymerization of L,L-dilactide (or D,D-dilactide) in the melt. A typical protocol for the synthesis is as follows: A 500 mL flame-dried Schlenk-flask (equipped with magnetic stirring rod, NS 29 rubber septum and PTFE valve) under argon atmosphere was charged with D,D-dilactide (100.0 g, 639.8 mmol) (or L,L-dilactide, respectively). The reaction vessel was placed into a preheated oil bath (135° C.) and the mixture was magnetically stirred until complete melting. Anhydrous 1-hexanol (0.83 mL, 682 mg, 6.67 mmol) is added dropwise via syringe to the stirred melt of the dilactide. After 5 minutes a dilute solution of Sn(oct).sub.2 in anhydrous THF (e.g. 4.6 mL with a concentration of 24.5 mg/mL Sn(oct).sub.2) was added dropwise and the resulting mixture was stirred for 1 h at 135° C. to become a highly viscous polymer melt that solidifies towards the end of the reaction. The reaction vessel was removed from the oil bath and the crude product was allowed to cool to room temperature, dissolved in 400 mL of chloroform, followed by precipitation in 4 L of cold methanol. The precipitate was dried under reduced pressure at 60° C. The yield of the copolymer was typically in the range of 85-95 g.
(67) The resulting homopolymer PDLA or PLLA had a weight average molecular weight as determined by GPC (universal calibration) of 16±2 kDa.
(68) Preparation of the Polymer Blends and Films Thereof
(69) In order to prepare the polymer blends, a mixture of the first polymer component (A) and the second polymer component (B) of 600 mg were added to 12 mL of chloroform, with weight ratios of A and B as listed in Table 2. The mixtures were agitated at 58° C. and 700 rpm for 2 h using a thermomixer. The resulting solutions were cooled to room temperature and poured into PTFE evaporation dishes. The dishes were tightly covered with aluminum foil and left for evaporation of the solvent in the hood for several days, followed by drying under reduced pressure at 60° C. overnight. The obtained polymer blend films had a thickness of 90 to 180 μm.
(70) The films were characterised by uniaxial tensile testing to investigate their mechanical properties. The elastic modulus (E) and elongation at break (ε.sub.b) are shown in Table 2. Blends of components (A) and (B) with different ratios were prepared and analysed to identify compositions with preferentially low E-modulus and high ε.sub.b values. However, the films made from the blend comprising 40 wt % of PDLA were too brittle for these tests. Films (blends) made of components (A) (P(LLA-co-CL) or P(DLA-co-CL)) with higher CL content show lower E-moduli and higher values for ε.sub.b.
(71) TABLE-US-00002 TABLE 2 Composition and mechanical properties of polymer films. # (A) (B)* w.sub.A (wt %) w.sub.B (wt %) E (MPa).sup.a .sub.εb (%).sup.a 1 pLLACL 75/25.sup.b — 100 — 560 ± 35 134 ± 76 2 pLLACL 75/25.sup.b PDLA.sup.c 95 05 79 ± 3 435 ± 25 3 pLLACL 75/25.sup.b PDLA.sup.c 90 10 269 ± 30 472 ± 40 4 pLLACL 75/25.sup.b PDLA.sup.c 80 20 91 ± 3 135 ± 9 5 pLLACL 75/25.sup.b PDLA.sup.c 60 40 — — 6 pLLACL 67/33.sup.d — 100 — 36 ± 3 905 ± 51 7 pLLACL 67/33.sup.d PDLA.sup.c 95 05 27 ± 1 889 ± 83 8 pLLACL 67/33.sup.d PDLA.sup.c 90 10 26 ± 2 1019 ± 32 9 pLLACL 71/29.sup.e PDLA.sup.c 100 0 40 ± 4 801 ± 50 10 pLLACL 71/29.sup.e PDLA.sup.c 95 05 32 ± 5 684 ± 118 11 pDLACL70/30.sup.f — 100 — 47 ± 3 648 ± 24 12 pDLACL70/30.sup.f PLLA.sup.c 95 05 48 ± 4 604 ± 73 13 pDLACL 75/25.sup.g — 100 — 123 ± 4 504 ± 77 14 pDLACL 75/25.sup.g PLLA.sup.c 95 05 140 ± 5 549 ± 34 15 pDLACL 75/25.sup.g pLLACL.sup.h 50 50 36 ± 3 699 ± 42 .sup.a) ± denotes standard deviation; .sup.b) LA/CL = 75/25 w/w and DR = 6.3:1; .sup.c) M.sub.w = 16 KDa; .sup.d) LA/CL = 67/33 w/w and DR = 3.0:1; .sup.e) LA/CL = 71/29 w/w and DR = 3.3:1; .sup.f) LA/CL = 70/30 w/w and DR = 3.4:1; .sup.g) LA/CL = 75/25 w/w and DR = 5.0:1; .sup.h) LA/CL = 71/29 w/w and DR = 2.7:1.
(72) The films with 5 to 20 wt % of component (B) showed a good shape stability after deformation as can be seen in
(73) Wide angle x-ray scattering (WAXS) measurements were conducted on the films in order to determine the total crystallinity and to prove the presence of crystallites originating from stereocomplexes between oligo-LLA sequences of one polymer component and the oligo-DLA sequences of the second polymer component.
(74) Preparation of Porous Flat Patches by Electrospinning
(75) The basic setup of an electrospinning apparatus 10 is depicted in
(76) A solution of a polymer blend of 95:5 w/w of P(LLA-co-CL) 67/33 and PDLA-16k in hexafluoro isopropanol was prepared and subjected to electrospinning by using an aluminum mandrel (Ø=10 cm) covered with polypropylene foil as collector to obtain flat patches. The spinning parameters are summarized in Table 3. The parameters were chosen based on the experience obtained by electrospinning various other polymers and may be varied depending on the desired fiber morphology.
(77) TABLE-US-00003 TABLE 3 Parameters for electrospinning a solution of [95:5 w/w P(LLA-co-CL) 67/33 + PDLA-16K] into nonwoven fiber patches. mandrel diameter solvent C V D V H (rotation speed) HFIP 8% w/v 15-21 kV 30 cm 2.12 mL/h <10% 10 cm (10 rpm) C: concentration; V: voltage; D: distance between needle and collector; V: flow rate; H: relative humidity
(78) The patches were investigated by scanning electron microscopy (SEM) as shown in
(79) The obtained electrospun patches were characterized by DSC, WAXS, dynamic thermal mechanical analysis (DMTA) as well as tensile testing (data summarized in Table 4).
(80) The electrospun patches show similar thermal properties to the solution-casted films confirming the presence of stereocomplexes that serve as nucleation agents to form physical netpoints ensuring long-term form stability in a wide temperature range and under physiological conditions. The WAXS diffraction pattern of the [95:5 w/w P(LLA-co-CL) 67/33+PDLA-16k]electrospun fibres is depicted in
(81) TABLE-US-00004 TABLE 4 Thermal and mechanical properties of electrospun patches made of [95:5 w/w P(LLA-co-CL) 67/33 + PDLA-16k]. T.sub.g T.sub.m T.sub.g at max E″/tan δ E-modulus σ.sub.max ε.sub.b [° C.].sup.a [° C.].sup.a [° C.].sup.b [MPa].sup.c [MPa] [%] 46, 128, 194 −9 (−5) −6/+1 3.7 ± 0.6 2.3 ± 0.2 600 ± 50 (178) .sup.aValues determined by DSC in the 1.sup.st heating cycle; thermal transitions in brackets correspond to 2.sup.nd heating cycle; .sup.bdetermined by DMTA; E″: loss modulus, tan δ; E′/E″ (E′: storage modulus); .sup.ccalculation of E-modulus between 0.05% and 0.25% elongation.
(82) Most importantly, the electrospun materials are fairly soft and highly elastic, which is reflected by an E-modulus in the low MPa range, which is significantly lower than that of the same polymer processed into a non-porous film. The high ε.sub.b value of about 600% without a yield point, enables full elastic recovery after removal of the stress as a further characteristic of the produced material. A typical stress/strain diagram obtained by tensile testing is depicted in
(83) The high form stability of the electrospun material was demonstrated by the data derived from cyclic tensile testing after applying different ratios of pre-stain in the range of 0 to 300% followed by three cycles.
(84) TABLE-US-00005 TABLE 5 Mechanical properties (tensile test; 3 cycles) of electrospun [95:5 w/w P(LLA-co-CL) 67/33 + PDLA-16k]. Pre-strain E-modulus (MPa) Recovery ratio (%) Failure Elongation for cycles 1.sup.st 2.sup.nd 3.sup.th 1.sup.st 2.sup.nd 3.sup.th stress at break (%) cycle cycle cycle 4.sup.th cycle cycle cycle (MPa) (MPa) 0 0.7 — — — — — — 4.7 432.sup.a 50 0.8 0.9 1.0 0.9 79 93 90 5.2 437.sup.a 370.sup.b 100 0.9 0.9 0.9 0.9 85 97 99 7.9 623.sup.a 522.sup.b 150 0.8 1.0 1.0 1.0 77 95 96 5.2 511.sup.a 352.sup.b 200 0.9 0.9 1.0 1.0 80 95 97 6.9 548.sup.a 359.sup.b 300 0.7 0.7 0.7 0.7 71 92 95 3.4 391.sup.a 182.sup.b The E-modulus was calculated from between strain of 1-5% (from the second cycle, strain needs to be calculated based on the previous length of samples); .sup.a: the elongation at break = total strain; .sup.b: re-calculation of the elongation at break based on the previous length of samples.
Preparation of Porous Tubes by Electrospinning
(85) Electrospinning was also performed on a smaller aluminum mandrel (Ø=20 mm, 120 mm length). The electrospinning parameters are summarized in Table 6. Electrospun [95:5 w/w P(LLA-co-CL) 67/33+PDLA-16k] tubes with a thickness of 310-330 μm (electrospinning for 7.2 h) were obtained.
(86) TABLE-US-00006 TABLE 6 Parameters for electrospun tubes made of [95:5 w/w P(LLA-co-CL) 67/33 + PDLA-16k]. mandrel diameter (rotation solvent C V D V H speed) HFIP 8% w/v 21-23 kV 30 cm 3.54 mL/h <10% 20 mm (10 rpm) C: concentration; V: voltage; D: distance between needle and collector; V: flow rate; H: relative humidity
(87) After removal from the mandrel (rolling), the tubes preserved their shape upon storage. Shrinkage of the tube dimeter due to entropic relaxation of the polymeric fibers was found to be very little (around 3%). Just like the electrospun patches, the tube displays and E-modulus and σ.sub.max values of a few MPa and are highly elastic (ε.sub.b=600%).