Patent Application
20130030122
The present invention relates generally to elastomeric compositions, and particularly to elastomeric polymers crosslinked by polylactic acid, and methods of forming such elastomeric polymers.
Elastomers are useful materials and have a wide range of application in different fields. For example, styrene-butadiene-styrene tri-block copolymers have been used as elastomers and are commercially available. In such elastomers, dispersed polystyrene domains physically crosslink flexible polymeric chains, and thus they are easier to reprocess and recycle, as compared to chemically crosslinked or vulcanized rubbers.
It has been realized that the operating temperature range of many elastomers with both a continuous rubbery phase and a dispersed hard phase is limited by the softening temperature of the hard phase and by the glass transition temperature (Tg) of the rubbery phase. Thus, it is desirable to provide an elastomer with both a relatively high softening temperature of the hard phase, such as higher than about 100° C., and a relatively low Tg of the rubbery phase, such as lower than about −50° C.
It has been found that a polymeric matrix formed of an elastomeric polymer of a low T9 and crosslinked with stereocomplexes of polylatic acid can have both a relatively high Tm, such as above about 200 or 230° C., and a relatively low Tg, such as below about −30° C.
Accordingly, in an aspect of the present invention, there is provided a composition. The composition comprises chains comprising a first graft copolymer of a first elastomer and poly(L-lactic acid), and chains comprising a second graft copolymer of a second elastomer and poly(D-lactic acid). The chains are crosslinked by crystalline structures formed from at least some of the poly(L-lactic acid) and poly(D-lactic acid) in discrete regions in the composition.
In exemplary embodiments, the crosslinked chains may form a matrix. The crystalline structures may be stereocomplexes of poly(L-lactic acid) and poly(D-lactic acid). The elastormers may form a first, continuous phase and the crystalline structures may form a second, dispersed phase. A weight ratio of the poly(L-lactic acid) to the poly(D-lactic acid) in the composition may be about 1:1. At least one of the first and second elastomers may comprise polyacrylate, such as poly(alkyl acrylate). The poly(alkyl acrylate) may comprise n-butyl acrylate, n-hexyl acrylate, or n-octyl acrylate. The poly(L-lactic acid) may be grafted to the first elastomer through a first hydroxy- or amine-functionalized acrylate group. The poly(D-lactic acid) may be grafted to the second elastomer through a second hydroxy- or amine-functionalized acrylate group.
In another aspect, the present invention provides a method of forming the composition described in the preceding paragraph. The method comprises mixing the first and second graft copolymers to form the composition, such as by melt blending the first and second graft copolymers, or by dissolving the first and second graft copolymers in a solution.
In selected embodiments, the method may comprise copolymerizing a monomer of the first elastomer and acrylate-terminated poly(L-lactic acid) to form the first graft copolymer, and copolymerizing a monomer of the second elastomer and acrylate-terminated poly(D-lactic acid) to form the second graft copolymer. Each of the first and second graft copolymers may be separately copolymerized in the presence of benzoyl peroxide at a temperature of about 75° C. in dioxane. Acrylate-terminated polylactic acid may be formed by reacting a lactide with a hydroxy-functionalized acrylate or an amine-functionalized acrylate with lactide. The method may also comprise copolymerizing a monomer of the first elastomer and a monomer of the second elastomer to form a copolymer precursor; and reacting a lactic acid with the copolymer precursor to graft an acrylate-terminated polylactic acid from a side chain of the copolymer precursor to form the first or second graft copolymer.
Other aspects, features, and embodiments of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
In the figures, which illustrate, by way of example only, embodiments of the present invention:
In exemplary embodiments, the continuous domain 102 of composition 100 is formed from elastomeric segments, such as soft poly(alkyl acrylate) segments, and the dispersed domains 104 are formed of PLA stereocomplexes. A domain 104 may include an aggregation of PLA stereocomplexes. PLA stereocomplexes are formed by co-crystallization of PLLA and PDLA. The chains are thus crosslinked by crystalline structures formed from at least some of the poly(L-lactic acid) and poly(D-lactic acid) in discrete regions in the composition. As illustrated in
In an embodiment, the continuous elastomer phase 102 may be formed of a poly(alkyl acrylate) with a Tg below the intended operating temperatures, such that the continuous phase will be rubbery at the normal operating temperatures. For example, for many applications, Tg should be below room temperature. For applications in cold environments, Tg should be even lower. A poly(alkyl acrylate) with a lower Tg may be used in a wider range of applications. For example, the Tg of poly(n-butyl acrylate) is about −49° C. and may be used in a wide range of applications. Suitable poly(alkyl acrylate) may be formed from an acrylate monomer such as n-butyl acrylate, n-hexyl acrylate, or n-octyl acrylate, or a combination thereof.
In selected embodiments, other suitable elastomers may also be used in composition 100. Elastomers with pendant hydroxy groups may be conveniently used to form PLA graft polymers. For example, in an embodiment, poly(isoprene) (PI) may be used as an elastomeric backbone in composition 100. In different embodiments, polybutadiene or ethylene propylene diene monomer (M-class) (EPDM) rubber may be used. The double bonds in these elastomers can be functionalized, such as by hydrogenation, to saturated hydrocarbon blocks, which can be conveniently utilized to compatiblizing PLA with, e.g. polyolefins.
As can be understood, the specific elastomers to be used in a particular embodiment may be selected based on various factors of interest in the particular application, and can be determined by those skilled in the art based on known properties of different elastomeric materials, such as elasticity, mechanical strength, reactivity, solubility, chemical resistance to certain materials, compatibility with other polymers, or the like.
The polymer chains in composition 100 include graft copolymer chains. A graft copolymer chain may contain one or more grafted PLLA or PDLA. In one embodiment, the ratio of PLLA and PDLA graft segments is 1. The number of PLA graft segments per graft copolymer chain may be greater than 1, such as from 2 to 10. In some embodiments, each graft copolymer chain may contain only PDLA or PLLA segments. When individual graft copolymer chains each contain only one type of PLLA segments, inter-chain stereocomplex formation may be maximized. When a graft copolymer chain contains both PDLA and PLLA, PLA stereocomplexes may be formed from PDLA and PLLA of the same chain (intra-chain stereocomplex formation). At least some of the PLLA and PDLA in different graft copolymer chains form stereocomplexes, which crosslink the different chains to form a polymeric matrix. In some embodiments, all or substantially all of the PLA enantiomers in composition 100 form stereocomplexes.
It should be understood that a PLA stereocomplex is different from a mere mixture of PLLA and PDLA in which no PLA stereocomplex is formed, in the sense that a PLA stereocomplex is a racemic configuration of PLLA and PDLA which exhibits properties that are significantly different from an optically pure PLA configuration. For example, the melting point temperature of a PLA material can be substantially increased due to formation of PLA stereocomplexes, as compared to the melting point temperature of an PLA material containing an optically pure PLA configuration, or a mere mixture of PLLA and PDLA with no PLA stereocomplex. Thus, the formation of PLA stereocomplex in a PLA-containing material can be detected by measuring certain properties, such as melting point temperature, heat of fusion, and crystal structure (e.g. as characterized by resonance frequencies measured by a suitable spectroscopic technique) of the PLA-containing material. As can be understood, melting point temperatures may be measured by differential scanning calorimetry (DSC), heat of fusion may be measured by Dynamic Mechanical Analysis (DMA), and crystal structures may be characterized by X-ray spectroscopy. Other suitable techniques may also be used to measure or characterize the crystal structure in a material, as can be understood by those skilled in the art.
In a melted state or in a solution, PLA stereocomplexes can aggregate or self-assemble and can form domains of crystalline lattices.
In composition 100, the elastomers in the copolymer chains form a soft phase, which is normally more elastic. The PLA stereocomplexes in composition 100 form a hard phase of dispersed domains, which is normally less elastic. A normal condition refers to the normal operating condition in a given application. Thus, composition 100 is a multi-phase substance.
As composition 100 contains elastomeric chain segments crosslinked by domains of PLA stereocomplexes, instead of linked by covalent bonds, a material or product formed from composition 100 can be conveniently reformed, reprocessed, or recycled.
Depending on the particular elastomer(s) in the graft copolymers, composition 100 may have a wide service temperature range, varying between the softening temperature of the PLA stereocomplex crosslinks at one end and Tg of the elastic phase at the other end.
In a specific embodiment, the elastomers in the copolymer chains may be poly(n-butyl acrylate) (PBA) formed from n-butyl acrylate monomers, and the weight ratio of PLLA and PDLA in composition 100 may be about 1:1. In such an embodiment, composition 100 has a relatively high use temperature, as compared to polystyrene-crosslinked thermoplastic elastomers. The latter is not suitable for use at temperatures above 100° C. due to softening of polystyrene. In this embodiment, composition 100 is polar, and thus exhibits better adhesion to polar substrates, as compared to non-polar thermoplastic elastomers such as styrene-butadiene elastomers.
In a further exemplary embodiment of the present invention, composition 100 may be formed by blending (i) graft copolymer of a selected poly(alkyl acrylate) and poly(L-lactic acid) (PAA-g-PLLA), and (ii) graft copolymer of a selected poly(alkyl acrylate) and poly(D-lactic acid) (PAA-g-PDLA).
The graft copolymers of PAA-g-PLLA and PAA-PDLA (also collectively or individually referred to as PAA-g-PLA) may be separately prepared to ensure that the individual copolymers each contains only PLLA or PDLA. A respective PAA-g-PLA may be formed by polymerizing an alkyl acrylate with a corresponding acrylate-terminated (capped) PLA. For example the alkyl acrylate and the corresponding acrylate-terminated (capped) PLA may be dissolved in a solution that contains a suitable solvent, e.g. dioxane, and a suitable polymerization initiator, e.g. benzoyl peroxide.
The acrylate-terminated PLAs may be formed by reacting a hydroxy- or amine-functionalized acrylate with L-lactide or D-lactide, respectively. Hydroxy- or amine-functionalized acrylate suitable for use as a ring opening polymerization initiator may be used. Suitable hydroxy-functionalized acrylates may include hydroxyethyl acrylate, such as 2-hydroxyethyl acrylate (HEA), or 2-hydroxyethyl methacrylate. In different embodiments, another suitable initiator may be used.
The initiator and the corresponding lactide or polylactide may be dissolved in a suitable organic solvent, such as anhydrous toluene or tetrahydrofuran. Various suitable Lewis acid metal complexes may be used as catalysts for the ring opening polymerization of lactide. For example, tin(II) octoate (also referred to as stannous octoate) and aluminum isopropoxide may be used. In an exemplary embodiment, the solution may contain about 1 wt % of stannous octoate based on the total weight of the lactide and the intiator. The solution may be heated to a suitable temperature, such as about 70° C., and continuously stirred. After the acrylate-terminated PLA is formed, the solvent and other components may be removed, such as by evaporation. The residue may be purified and dried according to standard procedures known to those skilled in the art.
A specific exemplary synthesis route is illustrated in
The molecular weight (such as number or weight average molecular weight) of any intermediate or product may be measured using any suitable technique. For example, the molecular weight may be determined using high pressure liquid chromatography (HPLC), gel permeation chromatography (GPC), viscometry, vapor pressure osmometry or beam scattering techniques, among others.
In selected embodiments, graft copolymers, such as PBA-g-PLLA and PBA-g-PDLA, may be prepared using a “grafting-from” polymerization technique. Briefly, copolymer precursors may be formed by copolymerizing monomers of the first and second elastomers. A PLLA or PDLA can then be grafted from a side chain of a copolymer precursor. In particular, L-lactic acid may be reacted with the copolymer precursor to graft a side chain including an acrylate-terminated PLLA from the copolymer precursor, thus forming a PLLA graft copolymer. D-lactic acid may be reacted with the copolymer precursor to graft a side chain including an acrylate-terminated PDLA from the copolymer precursor, thus forming a PDLA graft copolymer.
An exemplary “grafting-from” synthesis route is illustrated in
A difference between the “grafting-from” technique and “grafting-through” using a PLA macromer is that with the “grafting-from” technique as illustrated in
PLA stereocomplexes may be formed by blending PLA enantiomers, or the PLLA and PDLA graft copolymers, by solution casting, or by melt blending. Both solution casting and melt blending technologies are well known to those skilled the art and can be readily adapted for application in the exemplary embodiments herein.
For example, melt blending may be conducted for example at 180° C. for about 10 minutes. The melt blend may be a 50:50 blend. That is the PLLA and PDLA graft copolymers in the blend has a 1:1 weight ratio. The melt blend may be dried and compression molded at, for example, about 200° C. Conveniently, the resulting dried blend may have a melting temperature as high as about 220° C. and a transition glass temperature of about −26° C.
The exemplary embodiments disclosed herein may be conveniently used in many applications of different fields. For example, exemplary compositions disclosed herein may have application in elastomers, rubber replacements, adhesives, or rubber tougheners.
Conveniently, at least some of the exemplary elastomer compositions are adhesive to polar materials.
In selected exemplary embodiments, elastomeric polymers may be formed of an alkyl acrylate monomer, and the resulting copolymer may have a Tg lower than 0° C. A polar copolymer of alkyl acrylates may exhibit good adhesion to polar materials.
It will be understood that when references are made to polymers formed of a specific monomer, such as L-lactic acid or D-lactic acid, the polymers are not necessarily entirely formed of the specified monomer. For example, a PLLA may not be formed of 100% LLA monomer units and a PDLA may not be formed of 100% DLA monomer units. In practice, a 100% pure polymer form is difficult to obtain, and the polymers may contain other components such as other monomers and defects. For example, a PLLA polymer may contain a small percentage of DLA or PDLA, and a PDLA polymer may contain a small percentage of LLA or PLLA. Depending on the particular application, in some embodiments, the purity of the polymer, including the optical purity of the polymer, may be from about 90% to about 100%. In some embodiments, the purity of the polymer may be from about 95% to about 100%. In some embodiments, the purity of the polymer may be from about 85% to about 100%. In some embodiments, the optical purity of the polymer may be above 66%, or above 72%. In some embodiments, the mole fraction of the minor enantiomer in the polymer may be less than 0.14, or less than 0.17. As can be understood, the optical purity of the polymer should be sufficiently high and its content of impurities including the minor enantiomer should be sufficiently low to allow PLA stereocomplexes to form.
Exemplary embodiments of the present invention are further illustrated with the following examples, which are not intended to be limiting.
Lactide mentioned in these examples was purchased from Purac Biomaterials™, and used as received. The synthesis route for preparing the intermediate and final sample materials is as shown in
The number average molecular weight (Mn) for all values listed below is given in units of g/mol.
Sample PLLA macromers were prepared following the synthesis route (1) shown in
For one of the samples, referred to as Sample I, 21.6 g (150 mmol) of L-lactide, 0.221 g of stannous octoate, and 513.3 mg (4.42 mmol) of 2-hydroxyethyl acrylate were used to produce about 21.3 g of PLLA macromer, with GPC Mn=8094 and Mw=9967.
Two other samples, referred to as Sample IA and Sample IB, were formed with 14.4 g L-lactide and different amounts of initiator and catalyst. For Sample IA, molecular weights were found to be Mn=14319 and Mw=15356; and for Sample IB, Mn=28468 and Mw=32023.
The procedure shown in route (1) of
For Sample II, 21.6 g (150 mmol) of D-lactide, 0.221 g of stannous octoate, and 513.3 mg (4.42 mmol) of 2-hydroxyethyl acrylate were used to produce about 21.3 g of PDLA macromer, with GPC Mn=8308 and Mw=9976.
For Samples IIA and IIB, 14.4 g of D-lactide was used and the amounts of the initiator and catalyst were adjusted to produce sample macromers with different molecular weights. Sample IIA: Mn=13400 and Mw=14316. Sample IIB: Mn=28424 and Mw=31943.
PBA-g-PLLA samples were prepared following the synthesis route (2) shown in
Samples IIIA and IIIB were also prepared following the above procedure, but with Samples IA and IB as the respective PLLA macromer. Sample IIIA: Mn=61182 and Mw=167207. Sample IIIB: Mn=100348 and Mw=287192.
Sample IIIC was prepared as follows. 5 g of n-Butyl acrylate (n-BA), 3.3 g of PLLA of Sample IB, and 83 mg (1 wt %) of benzoyl peroxide were dissolved in 15 ml dioxane in a 100 ml Schlenk flask. The resulting solution was bubbled with argon for about 30 min to remove air and then heated to 75° C. with stirring overnight. The hot solution was poured into methanol to precipitate the graft copolymer. The precipitate yielded 6.8 g of graft copolymer PBA-g-PLLA (Sample IIIC), with GPC Mn=98637 and Mw=277134.
PBA-g-PDLA samples were prepared according to the synthesis route (2) of
Sample IVA and IVB were also prepared following the above procedure. However, the macromers used were Sample IIA or IIB, respectively, instead of Sample II. Sample IVA: Mn=72731 and Mw=240989. Sample IVB: Mn=92390 and Mw=303983.
Sample IVC was prepared as follows. 5 g of n-Butyl acrylate (n-BA), 3.3 g of PDLA of Sample IIB, and 83 mg (1 wt %) of benzoyl peroxide were dissolved in 15 ml dioxane in a 100 ml Schlenk flask. The resulting solution was bubbled with argon for about 30 min to remove air and then heated to 75° C. with stirring overnight. The hot solution was poured into methanol to precipitate the graft copolymer. The precipitate yielded 7.0 g of graft copolymer PBA-g-PDLA (Sample IVC), with GPC Mn=93511 and Mw=271863.
Different samples of PBA-g-PLLA prepared in Example III were dispersed in methylene chloride (Tedia™, 99.5%) to form precursor solutions with a polymer concentration of 0.1 g/ml (i.e. 1.6 g of each polymer dissolved in 16 ml methylene chloride). The solutions were cast onto a glass Petri dish. The cast solutions were allowed to evaporate at room temperature and then dried at 40° C. in a vacuum oven for one week to form sample films of PBA-g-PLLA. Samples VA, VB, and VC (also collectively referred to as Samples V) were formed from Samples IIIA, IIIB and IIIC, respectively.
Sample films of PBA-g-PDLA were prepared following the procedure of Example V but replacing PBA-g-PLLA samples with samples of PBA-g-PDLA prepared in Example IV. Film samples VIA, VIB, and VIC (also collectively referred to as Samples VI) were formed from Samples IVA, IVB, and IVC respectively.
Elastomer samples were prepared according to the synthesis route (3) of
The concentrations of the ingredients in the precursor solutions for forming. Samples V, VI, VII are summarized in TABLE I.
The properties of Samples V, VI, and VII were measured using DSC, XRD and DMA techniques. Representative DSC results are shown in Table II. Representative XRD and DMA results are shown in
The results showed that Samples VII have much higher melting points (temperatures) and heat of fusion than Samples V and VI. The domains of PLA stereocomplexes in the Samples contained crystals. A higher heat of fusion indicates a higher crystallinity.
XRD results indicated that the Samples V, VI, and VII contain partially crystalline polymers, as each spectrum was a superposition of peaks (indicative of a crystalline phase) and a broad halo (indicative of an amorphous phase).
The measured data indicated that stereocomplexes of polylactic acid formed in Samples VII. For example,
The measured data also indicated that Samples VII could maintain good mechanical strength at a higher temperature than Samples V and VI did. For example, as shown in
Sample graft copolymers PBA-g-PLA were also prepared following the synthesis route shown in
54 g (0.42 mol) n-BA, 0.58 g HEA (0.005 mol) (feed molar ratio of n-BA to HEA was 84) and 183 mg Bz2O2 were dissolved in 75 ml dry toluene and then degassed via three freeze-thaw cycles. The mixture was stirred at 70° C. overnight. The viscous mixture was then diluted with THF and poured into large excess of methanol. The solution stood still for a few hours and the upper layer was decanted. To remove methanol and moisture, the obtained PBA was dissolved in toluene and the solvent was removed on a rotovap. It was further dried in vacuum oven at 70° C. until no water peak was seen from nuclear magnetic resonance (NMR). 43.38 g of PBA, denoted as Sample VIII-1, was obtained, with Mn=90620 and Mw=194037.
In a 3-neck flask, 43.38 g of Sample VIII-1, 28.92 g L-lactide (feed weight ratio of n-BA to LLA is 1.5) were dissolved in 200 ml dry toluene and then 0.29 g Sn(Oct)2 in 5 ml toluene was added via syringe. The mixture was stirred under Ar by a mechanical stirrer at 85° C. for 3 days. Toluene was then removed from the rotovap. The residue was purified by dissolution in CH2Cl2 and precipitation from the solution by addition of methanol. The precipitate was dried under vacuum at 55-60° C. for 24 hours. The resulting sample was denoted as Sample VIII-L.
Samples VIII-2 and VIII-D were also prepared, following the above procedures for forming Samples VIII-1 and VIII-L respectively, with the exception that, instead of L-lactide, D-lactide was used for forming Samples VIII-2 and VIII-D. For Sample VIII-2, Mn=92246 and Mw=274840.
Some test results of Samples VIII-L and VIII-D are shown in Table III, in which the values of the weight ratio of Wpn-BAWPLA were obtained from NMR.
Sample compositions with stereocomplexes formed between enantiomeric PLA containing graft copolymers were prepared by melt blending from the samples formed in Example VIII as follows.
Samples VIII-L and VIII-D were blended in a 50:50 mixture at 180° C. for 10 min using a Barbender™ mixer.
Sample specimens for mechanical testing were prepared by compression molding the dried melt blends at 200° C. and 6000 lb for 5 minutes using a Carver™ press and a rectangular mold with dimensions of 100 mm (length)×100 mm (width)×1.2 mm (height).
The test results showed that stereocomplexes were formed between enantiomeric PLA side chains of sample graft copolymers by melt blending.
The results were confirmed by Differential scanning calorimetry (DSC) and Dynamic Mechanical Analysis (DMA). DSC results showed that the Tg of the sample blends was −26° C. and the Tm of the sample blends was at 224° C.
It will be understood that any range of values herein is intended to specifically include any intermediate value or sub-range within the given range, and all such intermediate values and sub-ranges are individually and specifically disclosed.
It will also be understood that the word “a” or “an” is intended to mean “one or more” or “at least one”, and any singular form is intended to include plurals herein.
It will be further understood that the term “comprise”, including any variation thereof, is intended to be open-ended and means “include, but not limited to,” unless otherwise specifically indicated to the contrary.
When a list of items is given herein with an “or” before the last item, any one of the listed items or any suitable combination of two or more of the listed items may be selected and used.
Of course, the above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments are susceptible to many modifications of form, arrangement of parts, details and order of operation.
The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.
This application claims the benefit of, and priority from, U.S. provisional application No. 61/324,112, filed Apr. 14, 2010; the entire contents of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SG2011/000147 | 4/14/2011 | WO | 00 | 10/12/2012 |
| Number | Date | Country | |
|---|---|---|---|
| 61324112 | Apr 2010 | US |
