POLYMER MATERIAL SHOWING A SHARP DEPENDENCY OF TENSILE STRENGTH IN
RELATION TO TWO EXTERNAL STIMULI
Background of the Invention
The present invention describes a polymeric material that exhibits a sharp dependency of its tensile strength in relation to two external stimuli.
The prior art
In the current literature, much devotion has been invested into shape memory polymers (SMP), i.e. polymeric material which show an induced shape change, im particular in relation to a temperature change. SMPs are generally characterized as phase segregated linear block co-polymers having a hard segment and a soft segment. The hard segment is typically crystalline, with a defined melting point, and the soft segment is typically amorphous, with a defined glass transition temperature. In some embodiments, however, the hard segment is amorphous and has a glass transition temperature rather than a melting point. In other embodiments, the soft segment is crystalline and has a melting point rather than a glass transition temperature. The melting point or glass transition temperature of the soft segment is substantially less than the melting point or glass transition temperature of the hard segment.
When the SMP is heated above the melting point or glass transition temperature of the hard segment, the material can be shaped. This (original) shape can be memorized by cooling the SMP below the melting point or glass transition temperature of the hard segment. When the shaped SMP is cooled below the melting point or glass transition temperature of the soft segment while the shape is deformed, that (temporary) shape is fixed. The original shape is recovered by heating the material above the melting point or glass transition temperature of the soft segment but below the melting point or glass transition temperature of the hard segment. In another method for setting a temporary shape, the material is deformed at a temperature lower than the melting point or glass transition temperature of the soft segment, resulting in stress and strain being absorbed by the soft segment. When the material is heated above the melting point or glass transition temperature of the soft segment, but below the melting point (or glass transition temperature) of the hard segment, the stresses and strains are relieved and the material returns to its original shape. The recovery of the original shape, which is induced by an increase in temperature, is called the thermal shape memory effect. Properties that describe the shape memory capabilities of a material are the shape recovery of the original shape and the shape fixity of the temporary shape.
In addition several physical properties of SMPs other than the ability to memorize shape are significantly altered in response to external changes in temperature and stress, particularly at the melting point or glass transition temperature of the soft segment. These properties include the elastic modulus, hardness, flexibility, vapor permeability, damping, index of refraction, and dielectric constant. The elastic modulus (the ratio of the stress in a body to the corresponding strain) of an SMP can change by a factor of up to 200 when heated above the melting point or glass transition temperature of the soft segment. Also, the hardness of the material changes dramatically when the soft segment is at or above its melting point or glass transition temperature. When the material is heated to a temperature above the melting point or glass transition temperature of the soft segment, the damping ability can be up to five times higher than a conventional rubber product. The material can readily recover to its original molded shape following numerous thermal cycles, and can be heated above the melting point of the hard segment and reshaped and cooled to fix a new original shape.
These changes in physical properties are however also known from non-shape-memory polymers, since generally tensile strength as well as hardness are properties closely linked to temperature for example, in particular in the vicinity of the glass transition temperature of a given material.
In the context of the present invention a temperature at which a given material shows a shape memory effect or any other change of a given physical property is defined as trigger temperature or triggering temperature.
Examples of polymers used to prepare hard and soft segments of SMPs include various polyethers, polyacrylates, polyamides, polysiloxanes, polyurethanes, polyether amides, polyurethane/ureas, polyether esters, and urethane/butadiene copolymers. See, for example, U.S. Patent No. 5,506,300 to Ward et al.; U.S. Patent No. 5,145,935 to Hayashi; U.S. Patent No. 5,665,822 to Bitler et al.; and Gorden, "Applications of Shape Memory Polyurethanes," Proceedings of the First International Conference on Shape Memory and Superelastic Technologies, SMST International Committee, pp. 115-19 (1994).
All of the prior art shape memory polymers focus on materials exhibiting a shape memory effect, when the material is exposed to one defined stimuli. In most cases, a temperature increase is employed to trigger the shape memory effect.
Especially the use of a temperature increase to trigger the shape memory effect, however, is associated with certain disadvantages. Due to the statistic nature of polymer materials the shape memory effect is triggered rather over a broad temperature range than at a defined temperature. As this temperature interval ranges up to 20 0C, the use of such materials is
under certain circumstances and for specific applications limited, especially when the temperature increase is due to the increase from room temperature to the temperature of a living body.
Furthermore, a thermal shape memory effect (or a thermally induced change of physical properties) is disadvantageous in fields of application requiring sterilization processes. For example, in the ethylene oxide sterilization process, the device to be sterilized is treated at a temperature of 55 0C, which might be well beyond the triggering temperature. As all of the prior art shape memory polymers have a one-way shape memory effect, i. e. the temporary shape is not recovered when the polymer is cooled below the triggering temperature, an unwanted triggering of the shape memory effect associated with an unwanted deformation might result.
Prior art shape memory polymers are described for example in US 5,762,630. However, these polymers exhibit certain disadvantages. As they exhibit a glass transition at a temperature of 35 0C they will soften considerably when exposed to temperatures exceeding 35 0C. As temperatures exceeding 35 °C can easily be reached during transport conditions or during sterilization processes, it is very likely, that articles made of this polymer will loose their dimensional integrity.
In various fields there is however the need to provide materials which change either their shape or any given physical property (such as hardness or tensile strength) during use. In particular in applications in the medical field there exists the desire to have materials being hard, stiff but not brittle at room temperature, but being soft and elastic at body temperature. AT the same time these materials have to be able to withstand the conditions of usual sterilization treatments (see above) without deformation or loss of properties. One example of such an application is a cannula which has to be hard and stiff in order to enable to formation of a sharp tip suitable for skin penetration. After penetration however, in particular if the cannula remains in the body of the patient, it is desired that the cannily becomes soft and elastic in order to increase patient comfort and compliance.
Object of the present invention
It is therefore an object of the present invention to provide a polymer material satisfying the requirements as summarized above, i.e. enabling a change of properties over a considerable narrow (compared with the prior art) temperature range, without however compromising the suitability of the polymer material to withstand conventional sterilization processes.
Summary of the invention
This object has been solved with the polymer material as defined in claim 1. Preferred embodiments are defined in the subclaims. Further provided are compositions, comprising the polymer material as well as methods for the preparation of the polymer material.
Detailed description of the present invention
The material in accordance with the present invention is a block copolymer comprising at least one hard block (segment) and at least one soft block (segment). The different blocks (segments may be selected among the materials defined below. The specific block copolymer in accordance with the present invention is characterized by the fact the material, due to the specific structure, is responsive to a temperature stimulus and to a further stimulus so as to provide a more reliable (sharper) triggering process and in a more pronounced property change.
The first stimulus as defined above is a temperature stimulus, which however alone would no be sufficient to induce the desired change of properties in the desired way, i.e. within a rather short period of time, in particular in view of the requirement that the polymer material in accordance with the present invention must be able to be subjected to a conventional sterilization process, without detrimental effect on the material. Therefore a second stimulus is required, which, in accordance with the present invention is the bringing the polymer material of the present invention into contact with low molecular weight compounds (in the following "small molecule stimulus"). These low molecular weight compounds are taken up by the polymer material, inducing, together with the temperature change a sharp/drastic change o physical properties.
Preferred low molecular weight compounds are liquid or gaseous at room temperature and include in particular water, alcohols, esters, oils or fats and the like, most preferably water (as contained for example in body fluids, medicinal preparations and the like).
The temperature range within which the change in property can be induced by the combined action of temperature and the small molecules preferably is between 30 and 450C, most preferably between 35 and 4O0C. The time frame within which the change in property can be realized preferably is less than 30 minutes, more preferably less than 2 minutes and in particular less than 15 minutes, for example within 10 minutes or less.
In accordance with the present invention the preferred property to be the subject of change due to the temperature and the small molecule stimulus induced change is the tensile strength (E modulus), measured in accordance with DIN EN ISO 527 on a Zwick testing device. This
property is an important property for polymeric materials, having an impact on a broad range of behaviors of products manufactured from the polymer material of the present invention. Preferable the tensile strength of the polymer material of the present invention is, prior to the triggering of the change, above 1.5 GPa, preferably in the range of from 1.7 to 3.0 GPa, more preferably from 2.0 to 2.5 GPa. After inducing the change the tensile strength preferably is less than 1.0 GPa, more preferably 800 to 50 MPa, or 700 to 200 MPa. Preferably this change can be realized within a time frame of 10 to 15 minutes at about 37°C in contact with water.
The polymer material of the present invention, as outlined above, comprises at least one hard segment and at least one soft segment, or can include at least one kind of soft segment wherein at least one kind of the soft segments are cross linked, without the presence of a hard segment.
The polymer material in accordance with the present invention is a rather hard material at room temperature and up to at least 370C. However, the material can also withstand higher temperatures without being affected; in particular the material in accordance with the present invention is able to be subjected to a standard sterilization treatment without triggering the desired change in property. Upon contact with small molecules and a temperature increase (preferably water or body liquids at about 35 to 37°C), the material shows a sharp change of the target property (preferably the tensile strength), preferably a decrease of the tensile strength.
Polymer composition
The block copolymer in accordance with the present invention preferably has the structure
-[(Hard Segment)a-(Soft Segment)b]x-
wherein a designates the number of hard segment monomers, b the number of soft segment monomers and x the overall number of monomers in the polymer. The hard segment will be abbreviated as HS and the soft segment as SS in the following.
The number of HS, a, is a number ranging from 1 to 10000, preferably 50-7500, and most preferably 100-2000. The number of SS, is a number ranging from 1 to 1000, preferably 50- 750, and most preferably 100-200. Blends of block copolymers of various compositions can be employed.
The block copolymer's end groups are preferably, but not limited to, selected from hydroxyl-, methyl-, ethyl-, propyl-, butyl or f-butyl-type. Furthermore functional groups derived from formic acid, acetic acid, propionic acid, butanoic acid, methacrylic acid, acrylic acid, crotonic acid can
be employed. These end groups are introduced into the structure by means of suitable reagents to be employed during the synthesis of the polymer materials. These reagents and reaction conditions are known to the skilled person.
Blends of different HS and SS monomers can be employed.
The SS are preferably; but not limited to, selected from poly(alkylene glycol)s, poly(ester polyol)s, poly(ether ester polyol)s, poly(ether polyol)s, poly(alkylen polyamine)s, pόly(caprolacton polyol)s, poly(lactide polyoly), poly(glycolide polyols )s and poly(lactide glycolide polyols), as well as poly(methyl methacrylate) (PMMA), methyl methacrylate and poly(ethyl methacrylate).
SS made of poly(alkylene glycol)s are preferably composed of but not limited of alkylen glycols or alkylene oxides containing two to six carbon atoms, e. g. ethylene, propylene, butylenes, pentylene, or hexylene glycols or mixtures thereof. In particular preferred are soft segments derived from polyethylene glycol (PEG) and/or poly(tetramethylether) glycol (poly(THF)). The functionality of those polyalkylene glycols is below ten, preferably smaller than five and most preferably smaller than 3, in paticular 2, so that linear block copolymers result. The molecular weight ranges (for the segments) preferably from 200 to 20000, more preferably from 300 - 10000 and most preferably form 500 - 5000, in particular 500 - 1500.
SS made of poly(ester polyol)s are preferably composed of dicarbonic acids and di and poly alcohol units. For examples monomers like oxalic acid, propionic diacid, butanoic diacid, adipinic acid, pimelinic acid fumaric acid maleic acid, succinic acid, phthalic acid terephthalic acid. The polyol part of the SS is preferably composed of alcohols containing at least two hydroxyl functionalities, for example, ethylene glycol, all isomers of butanediol, all isomers if pentadiol, all isomers of cyclohexandiol, glycerine, pentaerythriol, hexitols and mixtures thereof. The molecular weight (for the segments) ranges preferably from 200 to 20000, more preferably from 300 - 10000 and most preferably form 500 - 5000, in particular 500 - 1500.
SS made of poly(ether ester polyol)s consist preferably of the aforementioned poly(ester polyol)s which are inserted into the aforementioned poly(alkylene glycol)s. The molecular weight ranges (for the segments) preferably from 200 to 20000, more preferably from 300 - 10000 and most preferably form 500 - 5000.
SS made of poly(alkylene polyamine)s are composed of monomers like alkanolamines, alkylendi- and polyamines made of two to six carbon atoms and two to six amine groups like ethylene, propylene, butylenes, pentylene or hexylene diamine and mixtures thereof. The poly(alkylene polyamine)'s molecular weight ranges from 200 to 20000, more preferably from 300 - 10000 and most preferably form 500 - 5000, in particular 500 - 1500.
SS made of poly(caprolacton polyol)s consist of monomers like lactone and multivalent alcohols having at least two hydroxyl groups. The poly(caprolacton polyol)'s molecular weight ranges from 200 to 20000, more preferably from 300 - 10000 and most preferably form 500 - 5000, in particular 500 - 1500.
SS made of poly(lactide polyol), poly(glycolide polyol) and poly(lactide glycolide polyol) are composed of lactic acid, derivates of lactic acid, alpha hydroxyl acetic acid and derivatives thereof. Molecular weights range from 200 to 20000, more preferably from 300 - 10000 and most preferably form 500 - 5000, in particular 500 - 1500.
Preferred as soft segments are PEG and poly(THF), each with a molecular weight of from 750 to 2500, in particular about 1000 or about 2000.
HS can be made of urethanes, diepoxides, dicarbonic acids and derivatives thereof.
HS having a urethane structure comprise segments derived from monomers of di or polyisocyanates and additional monomers which are able to react with the urethane moiety (chain extenders). The monomers of the hard segment are peferably chosen from the group of aromatic and aliphatic diisocyanates, preferably cyclic di- and polyisocyanates. Examples comprise 4,4'-diisocyanatodicyclohexylmethane, 4,4'-Diisocyanatodiphenylmethane and tolylendiisocyanat. It is preferred when the block copolymer of the present invention comprises hard segments derived from aromatic isocyanates as well as segments derived from aliphatic isocyanates. In particular preferred are HMDI and MDI, in particular a mixture thereof. This mixture may be equimolar but it is preferred when HMDI amounts to more than 50% of the diisocyanate employed (80:20 to 60:40). In relation to the diisocyanates it has been established, that the amount of the aromatic diisocyanate is closely related to the stiffness of the material, i.e. increasing amounts of aromatic diisocyanate increases the stiffness of the material.
The chain extenders may be chosen from multivalent C1-C10 alcohols having at least two hydroxyl groups, di and polyamines consisting of 2 - 6 carbon atoms and two to six amine groups as defined above. Preferred are diol extenders, such as the diols mentioned above, in particular butane diol, as well as tertiary diamines. These chain extenders may also be used in order to modify the physical properties of the block copolymer, such as affinity to small molecules (water absorption and the like) and strength related properties.
The molecular weight of the polyurethane segments ranges form 200 to 20000, more preferably from 300 - 10000 and most preferably form 500 - 5000.
Polyepoxides as HS are composed of aromatic or cycloaromatic diepoxides. Examples thereof are Bisphenol A and Bisphenol F.
Polycarbonic acids used as HS are preferably aromatic or aliphatic, more preferably cyclic C2 - C20 Carbonic acids and derivatives thereof. Examples thereof are listed in connection with the aforementioned carbonic acids used as SS. Derivatives can be for example the anhydrides or the acid chlorides.
The parameters a and b are chosen in a manner that the polymer is composed of at least of 50 weight% of the hard segment, preferably 60 - 90 weight% of the hard segment and most preferably of 70 - 90 weight% of the hard segment, relative to the sum of HS and SS.
The molecular weights mentioned above all relate to the weight average molecular weight (Mn). The block copolymers in accordance with the present invention generally show a Mn of from 50,000 to 500,000, preferably from 100,000 to 250,000, with a polydispersity of from 1.5 to 2.5.
The block copolymer may exhibit a linear, a branched, or a cross linked architecture. Preferably a linear block copolymer is employed. The block copolymer preferably is a linear polyurethane, more preferably a linear polyalkylene glycol polyurethane. The block copolymer can be synthesized using a one-pot or a multi-step reaction procedure. In case of a multi-step procedure, SS and HS are synthesized from the respective monomers. In a further step the HS and the SS are coupled yielding the block copolymer. In a one-pot procedure the HS and SS monomers are added in a sequential manner using standard polymerisation techniques known to those skilled in the art.
The preferred polymer material of the present invention is a material having a glass transition temperature in the range of from 50 to 70°C, preferably 55 to 65°C.
The preferred polymer material of the present invention comprises the preferred soft segments (PEG and poly(THF), each with a molecular weight of from 750 to 2500, in particular about 1000 or about 2000) and the preferred hard segments (HMDI and MDI, in particular a mixture thereof, preferably together with a diol chain extender, in particular butane diol) in the ratio as derivable from the above (70 - 90 weight% of the hard segment, relative to the sum of HS and SS).
Stimuli
The stimuli have to applied simultaneously to trigger the desired change of at least one property. In a preferred embodiment, the one of the stimuli is a temperature increase whereas the other one is the uptake of small molecules, e. g. water.
To create the effect at a given switching temperature, 7SWITCH> a polymer material exhibiting a thermal transition (for example a glass transition temperature) at a temperature, 7" THERMAL. exceeding TSWITCH is chosen. The temperature difference, DELTAT= 7THERMAL - SWITCH, ranges between 0 and 100 K, preferably between 0 and 25 K and most preferably between 0 and 10 K.
If the polymer material is exposed to temperatures exceeding TSWITCH but not exceeding ^THERMAL in the absence of small molecules (e. g. water) no significant changes are observed. On a molecular level, the material remains unchanged, as the thermal transition in the dry state was not reached.
If the polymer material is exposed to small molecules (e.g. water) at temperatures below ^SWITCH, no significant changes in the physical properties are observed.
If the polymer material is exposed to temperatures exceeding SWITCH but not exceeding ^THERMAL and, at the same time, to small molecules (e.g. water) small molecules will diffuse into the polymer material. This will lower the temperature at which the thermal transition is triggered from THERMAL to TSWITCH- Hence, the desired change is triggered and the material undergoes significant and abrupt changes in its properties.
The typical effect which is triggered by two stimuli simultaneously is highlighted in Fig. 1 , where the dependency of the tensile Modulus, E, on the temperature, T, is displayed
Temperature [0C]
Fig. 1: Softening effect of Shape Memory Polymers
In part A of Fig. 1 the temperature dependency of the dry sample is given. Those skilled in the art will easily recognize that the tensile modulus did not change in that area - hence, no changes in the polymer's physical properties are observed. In part B of Fig. 1 the dependency of the tensile modulus on the temperature for the dry sample is further displayed. A decrease of the tensile modulus mainly due to approaching or reaching the glass transition temperature is observed. In part C of Fig. 1 the dependency of the tensile modulus on the temperature in the wet state is displayed. Contrary to part A and part B of the tensile modulus drops significantly indicating the triggering of the accumulated effect, due to the influence of temperature and water.
The dependency of the effect from the contact time with water of the triggering temperature of 37 °C is displayed in Fig. 2 for three respective materials.
Time in 37 0C Water
Fig. 2: Kinetics of the shape memory effect at a triggering temperature of 37 0C in water
From Fig. 2 it can be concluded that the effect is triggered in a timeframe shorter than 30 min. Within the first 10 min the tensile modulus drops more than 100 % indicating the rapid response of the polymer material on external stimuli.
Processing
The polymer material can be processed with the standard processing techniques used in the field including but not limited to extrusion, spinning, molding, ect.
The material can be processed to any shape including but not limited to fibers, films, capillaries, tubes, rods, struts, monofilaments, granules.
Compositions
The present invention also provides compositions comprising at least one polymer material in accordance with the present invention in admixture with at least one further component. Such additional components may be selected among fillers (such as barium sulfate), polymeric additives, colorants, active principles, stabilizers, lubricants, processing aids and other usual components. These may be added in amounts not being detrimental concerning the desired function of the polymer material of the present invention in the designated field of application.
Projected used of the polymer invented
The polymer described can in particular be used in the field of medical devices. One suitable use is a cannula remaining inside the human body for a couple of days. Such a cannula however is not part of the present application. Other uses include the use of the polymer material in meshes used in prosthetic devices, and in diagnostic devices.
The following examples illustrate the present inventinon.
Examples
Polyurethanes were prepared using macrodiols as soft segments (PEG Mn 1000; PEG Mn 2000; poly(THF) Mn 1000) HMDI and MDI together with butane diol as hard segments employing standard reactions. The diisocyanates were used in excess in the polymerisation reaction. The content of hard segments in the block copolymers obtained was in each case 80%. The ratio of HMDI to MDI was varied within the range defined in the specification.
The block copolymers obtained showed tensile strengths at room temperature of 1.9 to 2.1 GPa, which did not change upon a temperature increase to 55°C (for 72h) in a dry environment. Upon contact with water at 37°C all materials however showed a drastic decrease of the tensile strength to below 1000 MPa within 10 or within 20 minutes.