Blends of amorphous and semicrystalline polymers having shape memory properties

Description of equivalent WO 2004033539 A1

BLENDS OF AMORPHOUS AND SEMICRYSTALLINE POLYMERS HAVING SHAPE MEMORY PROPERTIES

Cross-Reference to Related Applications

This application claims the priority of the following provisional patent applications: Serial No. 60/418,023, filed October 11, 2002; Serial No. 60/466,401, filed April 29, 2003; and Serial No. 60/488,323, filed July 18, 2003. Each of the foregoing provisional patent applications is incorporated herein by reference to the extent not inconsistent with the present disclosure.

Technical Field

The present disclosure relates to shape memory materials and, more particularly, to blends of an amorphous polymer, such as poly(vinyl acetate) PVAc), and a semicrystalline polymer, which can be for example poly(lactic acid) (PLA) or poly(vinylidene fluoride) (PNDF). The present disclosure also relates to methods for the preparation of such blends demonstrating shape memory effects and to applications thereof.

Background of the Disclosure

Shape memory materials feature an ability to transform shape from a temporary, frozen, shape to a permanent shape when triggered by an environmental stimulus, such as heat, light, or vapor. Used creatively, this phenomena can be exploited for a wide range of applications. Many polymers intrinsically show shape memory effects, e.g., on the basis of rubber elasticity, combined with vitrification or crystallization but with varying characteristics such as strain recovery rate, work capability during recovery, and retracted state stability. Among the first shape memory polymers (SMPs) reported as such was crosslinked polyethylene, which was discovered and patented in 1971 by Radiation Appliances, Inc., and a methacrylic acid ester reported by the Nernon-Benshoff Co. for use as a denture material. The mechanism of strain recovery for such a material was identified as being far different from that of shape memory alloys (SMAs), which are based largely on nickel-titanium alloys. More particularly, a shape memory polymer is a super-elastic rubber, when the polymer is heated to a rubbery state, it can be deformed under resistance of~l Mpa modulus, and when the temperature is decreased below either a crystallization temperature or a glass transition temperature, the deformed shape is fixed by the lower temperature rigidity while, at the same time, the mechanical energy expended on the material during deformation is stored. Thereafter, when the temperature is raised above the transition temperature ( T_m or T_g), the polymer will recover to its original form as driven by the restoration of network chain conformational entropy. The properties of the SMPs will be closely linked to the network architecture and to the sharpness of the transition separating the rigid and rubber states. Compared with SMAs, SMPs have an advantage of (i) high strain, to several hundred percent because of the large rubbery compliance while the maximum strain of a SMA is less than 8%. (ii) facile tuning of transition temperatures through variation of the polymer chemistry; and (iii) processing ease at low cost.

A concurrently filed application for patent filed by inventors herein discloses the synthesis and characterization of thermally stimulated SMPs having different thermomechanical properties and their use in various applications, including as medical devices and mechanical actuators. The disclosed materials span a range of room temperature moduli, from rigid glassy materials having storage moduli of several GPa to compliant rubbers with moduli as low as tens of MPa. Moreover, the retracting (rubbery) moduli have been tuned over the range 0.5 < E < 10 MPa, as dictated by the end application. One example of such an SMP is chemically crosslinked polycyclooctene (PCO), a stiff semicrystalline rubber that is elastically deformed above T_m to a temporary shape that is fixed by crystallization. Fast and complete recovery of gross deformations are achieved by immersion in hot water. Other SMPs offering tunable critical temperatures and rubber modulus have been synthesized using a thermosetting random copolymer formed from two vinyl monomers that yield controlled T_g and casting-type processing. Such copolymers were crosslinked with a difunctional vinyl monomer as crosslinker, the concentration of crosslinker controlling the rubber modulus and thus the work potential during recovery, hi addition to their shape memory effects, these materials are also castable allowing for processing more complex shapes and they are optically transparent. The use of chemical crosslinking in both of these cases, however, limits the types of processing possible and permanently sets the equilibrium shape at the point of network formation.

Semicrystalline thermoplastic polymers with sharp T_g> room temperature and low crystallinity are also good candidates for shape memory, while offering the advantage of melt processing above T_m, allowing repeated resetting of the equilibrium shape by relaxing stress in the fluid state. Representative of the polymers in this class of SMPs are polyurethanes whose soft domains are semicrystalline with low melting points (but higher than T_m) and whose hard domains feature a higher melting point only exceeded during processing. The effect of the hard segments and soft segments on the shape memory effects have been investigated. In addition to such polyurethanes, block copolymers of polyethylene terephthalate (PET) have been synthesized for their shape memory effects.

Miscible blends of a semicrystalline polymer with an amorphous polymer have been investigated in the past, though not with respect to shape memory behavior, due to their attractive crystalline properties and mechanical properties. For those blends that are miscible at the molecular level, a single glass transition results, without broadening, an aspect important to shape memory. Additionally, in such miscible blends the equilibrium crystallinity (which controls the plateau modulus between T_g and T_m where shape fixing is performed) also changes dramatically and systematically with blend composition; i.e., relative levels of each component. Although numerous blends of this type have been investigated, there has been no known disclosure of the utilization of such blends for their shape memory properties.

Summary of the Disclosure

In accordance with the present disclosure, SMPs with relatively high modulus in the fixed state at room temperature, having a tunable and sharp transition, the permanent shape of which can be remolded repeatedly above certain melting temperatures, are prepared by the blending or mixing of a crystalline polymer (C) with an amorphous polymer (A), such that they are a single miscible phase in the molten state (allowing processing to stress-free native states), but crystalline to a limited and tailored extent and which further vitrify on cooling to room temperature. Examples for (C) include, but are not limited to, poly(vinylidene fluoride) (PNDF) (T_g = -35 °C, T_m = 175 °C), polylactide (PLA) (T_g = 56 °C, T_m = 165 °C),and its copolymers such as poly(L-lactide), poly (D,L-lactide, poly(lactide-co-glycolide), poly(lactide-co- caprolactone), poly(lactide-co-1.5-dioxepan-2-one), poly(lactide-co-trimethylene carbonate), polyglycolide, poly(3-hydroxy butyrate) and its copolymers, polyanhydrides, poly(vinylidene fluoride) (PVDF) (T_g = -35 °C, T_m = 175 °C), polyethylene glycol) (PEG), polyethylene, polyethylene-co-vinyl acetate, poly(vinyl chloride) (PNC), and poly(vinylidene chloride) (PNDC) and copolymers of poly vinylidene chloride (PNDC)/poly vinyl chloride (PNC). Examples for (A) include poly(vinyl acetate) (PNAc) (T_g = 35 °C), poly methyl acrylate (PMA), poly ethyl acrylate (PEA), atactic poly methyl methacrylate (aPMMA), isotactic poly methyl methacrylate (iPMMA), syndiotactic poly methyl methacrylate (sPMMA), and other poly alkyl methacrylates. Plasticizers may be included in the disclosed blends according to the present disclosure.

Exemplary plasticizers include, but are not limited to, di-n-butyl phthalate (DBP), di-n-octyl phthalate (DΝOP), di(2-ethylhexyl) phthalate (DOP), di-2-ethylhexyl) isophthalate (DOIP), bis- (20ethylhexyl) terephthalate (DOTP), di-n-decyl phthalate (DΝDP), di-cyclohexyl phthalate (DCHP), di-octysebacate (DOS), Hexamoll® DLΝCH, EVA, ENA-carbon monoxide terpolymer (Elvaloy), poly(alkylene alkanoates). DOP can also be used to decrease the glass transition of PNC.

The disclosed polymer blends may also be compounded to include a finely divided particulate material, such as clay, silica or TiO .

For high stiffness in the temporary form, a need exists for shape memory polymers having a T_g greater than room temperature, but with a tailored rubber modulus and elasticity derived from physical rather than chemical crosslinks.

Preferably, exemplary SMPs of the present disclosure are achieved by blending or mixing amorphous poly (vinyl acetate) (PNAc) (T_g = 35 °C) with semicrystalline polylactide (PLA) (T_g = 56 °C, T_m = 165 °C) or poly (vinyldiene fluoride) (PNDF). The disclosed polymers show complete miscibility at all blending ratios with a single glass transition temperature, while crystallization (exclusive of PNAc) is partially maintained. The T_g's of the blends are employed as the critical temperature for triggering the shape recovery while the crystalline phases function as physical crosslinking sites for elastic deformation above T_g, but below T_m. Shape memory polymers have received increased attention recently, prompted by an expanding range of potential end-use applications, especially for development of biomedical engineering tools and as medical devices.

Presently preferred polymer blends according to the present disclosure are formed from poly vinyl acetate(PNAC) and poly(lactic acid) (PLA) or poly(vinylidene fluoride) (PNDF). However, examples of other suitable blends include the pair PNDF/PMMA and ternary blends of PNDF PMMA/PNAc. The PMMA and the combination of PMMA/PNAc serve the same role as PNAc in the blends as has been previously described. An advantage of adding PMMA is that the critical temperature can be increased arbitrarily to about 80 °C and the room temperature modulus can also be increased. The PNDF may be substituted by poly(vinylidene chloride) (PNDC), by copolymers of poly(vinylidene chloride/poly(vinyl chloride), or by any "C" polymer vide supra.

It has further been found that blending or mixing poly( vinyl chloride) with poly(butyl acrylate) or poly (butyl methacrylate) (PNC PBA) has certain advantages. In the PNDF PNAc case, PNAc simultaneously lowers the crystallinity of PNDF while increasing the T_g. PNC may serve the same role as PNDF, but it already has a low degree of crystallinity, but a relatively high T_g (~ 80 °C). Thus, in this exemplary embodiment of the present disclosure, the second component (PBA) serves only the function or role of decreasing T_g. This function/role can also be achieved with small molecule plasticizers, most notably dioctylphthalate (DOP), but it is presently preferred to use a biocompatible polymeric plasticizer for intended implantable medical applications. The range of PBA compositions is 10-40%, with 20% being the most advantageous, yielding a T_g ~ 40 °C.

Further advantageous features and functions associated with the polymer blends of the present disclosure will be apparent from the detailed description which follows.

Brief Description of the Drawings

Figure 1 illustrates a comparison of the DSC traces of PLA (top) quenched from T = 180°C or annealed at T = 110 °C for one (1) hour (bottom);

Figure 2 illustrates DSC traces for PLA/PNAc blends following annealing for one (1) hour at T = 110 °C;

Figure 3 illustrates glass transition temperatures as measured following quenching of the PLA/PNAc blends; Figure 4 illustrates tensile storage modulus versus temperature for a range of PLA/PNAc; and

Figure 5 illustrates shape recovery of coiled PLA/PNAc (30:70) blend on exposure to water at T = 65 °C. Detailed Description of Exemplary Embodiments

Miscible blends of the present disclosure show or exhibit sharp and tunable transition temperatures and adjustable stiffness above their transition temperatures. The disclosed blends also show/exhibit excellent shape recovery effect with the recovery temperature and retracting force that is adjustable according to the transition temperature of the amorphous polymer and the degree of crystallinity of the homopolymers, respectively. The shape recovery can be finished within seconds when heated to an appropriate level above the transition temperature, e.g., 20 °C above the transition temperature. Additional advantages associated with the materials of the present disclosure include that the materials are rigid at room temperature, the polymers generally are biocompatible and can be used as medical devices and implants, and the products also can be dyed to any color or rendered radio-opaque for x-ray radiography according to application requirements.

The shape recovery temperatures of particular polymer blends according to the present disclosure depend on the glass transition of the blends. Theoretically, the blended polymer will recover above the glass transition temperature and below the melting temperature. Temperatures that are on the order of 20 °C above the glass transition temperature are preferably used to make the shape recovery fast. More preferablya temperature in the rubbery plateau region for the particular polymer blend is selected in order to have a fast recovery and a predictable and desired retracting force.

Blends of the present disclosure may be successfully used in connection with a wide variety of applications including, without limitation, the following applications: a) Stents, patches and other implants for human health care. b) Surgical tools requiring adjustable shape but high stiffness. c) Arbitrarily shape-adjustable structural implements, including personal care items

(dinnerware, brushes, etc.) and hardware tool handles. d) Self healing plastics. e) Medical devices. f) Impression material for molding, duplication, rapid protyping, dentistry, and figure-printing. g) Toys. h) Reversible embossing for information storage. i) Temperature sensors j) Safety valves k) Heat shrink tapes or seals

1) Heat controlled couplings and fasteners. m) Large strain, large force actuators Examples

To further illustrate advantageous features, functions and uses of exemplary polymer blends according to the present disclosure, the following non-limiting examples are provided. As will be readily apparent to persons of skill in the art, the following examples are merely illustrative of aspects of the present disclosure, and should not be viewed as absolute and thus should not be considered to be limiting with respect to potential polymeric materials, processing conditions (e.g., relative percentages, temperatures and time periods) and/or end-use applications that may be advantageously employed according to the present disclosure. Physical properties and processing conditions set forth in the following examples are merely illustrative of such properties/conditions, and should not be viewed as limiting the scope or utility of the present disclosure.

Materials

Poly(vinyl acetate) having a M = 500,000 g/mol was purchased from Aldrich and vacuum-treated at room temperature for 2 days to eliminate volatiles, including moisture. The poly(lactide) having a Mw = 130,00 g/mol was provided by Prof. S. Huang of the University of Connecticut and was dried at T = 60 °C overnight before use. The material was found to comprise 92% L-isomer and 8% D-isomer. Poly(vinylidene fluoride) having a Mw - 180,000 g/mol was also purchased from Aldrich and vacuum-treated at room temperature for 2 days to eliminate volatiles, including moisture. Processing and Fixing the Primary Shapes

Melt blending of PLA PNAc and PNDF/PNAc using varying blend ratios was performed in a 30 ml Brabender twin-screw mixer. The mixer was equilibrated at T = 180 °C for 5 minutes after which the mixer blade rotation was adjusted to 25 RPM and the premixed polymer pellets added to the chamber over the course of 1 minute. The polymers were mixed for 10 minutes to ensure good dispersion. Nitrogen was purged through the chamber to mitigate potential oxidative degradation during mixing. After mixing, the blend was taken out of the chamber, cooled to room temperature, and then pressed between heated platens of a Carver press at 180 °C for 5 minutes under a load of 8 metric tons. A spacer was used to control the thickness of the film and rapid cooling to room temperature was carried out. The films thus formed were used for thermal and mechanical characterization.

Characterizations

The thermal properties (thermogravimetric analysis (TGA)) of the polymers were measured preceding differential scanning calorimetry (DSC) measurements and melt blending to ensure that no solvent vapor was present in the polymers. For this purpose, a TA Instruments (TA 2950) TGA was used with samples of the polymers heated from 25 °C to 600 °C at a heating rate of 20 °C/minute in a nitrogen atmosphere. A Perkin-Elmer DSC-7 was used for DSC analysis. Ten (10) mg samples prepared from pellets and heat pressed films were first heated from 20 °C to 200 °C at a heating rate of 10 °C/minute (1^st heat), then cooled to 20 °C at the same rate, and finally the samples were reheated to 200 °C at a heating rate of 10 °C/minute (2^nd heat). The T_g of each blend was determined from the midpoint of heat capacity change observed during 2^nd heating.

Wide-angle x-ray scattering (WAXS) analysis was performed using a BRUKER GADDS-4 instrument with a Cr source of radiation (λ = 2.291A) and in transmission mode. The voltage and the current used were 40 kV and 40 mA, respectively, and the exposure time was 30 minutes. The scattering patterns were collected on a HiStar area detector with a sample-to- detector distance of 6.0 cm. Intensity profiles (I versus 20) were determined by azimuthal averaging at each 2Θ position of the isotropic patterns. The data were then analyzed with Peakfit™ software (SPSS Science) to find the peak positions and the relative intensity of each peak.

The storage and loss tensile moduli of the blends were measured by dynamic mechanical thermal analysis (DMTA) in tensile mode using TA Instruments DMA2980. The method used followed a temperature ramp spanning -100 °C < T < 150 °C at a heating rate of 4 °C/minute, a fixed oscillation frequency of 1 Hz, and a maximum tensile strain of about 0.5%. The sample geometries were rectangular bars of dimensions 10x2x1 mm.

Isothermal stress-free shape recovery tests were carried out using samples cut into a rectangular shape, stained for optical contrast, and then heated to T = 65 °C for bending into a temporary helical shape. The deformed sample was fixed by cooling in ice water, thus vitrifying the sample. The resulting deformed sample was subsequently dropped into a warm water bath at a predetermined temperature and the shape recovery monitored visually with a video camera and digital frame-grabber collecting images at a rate of 20 frames-per-second.

Results The TGA results demonstrated that both PLA and PNAc are stable for T < 300 °C.

Above this temperature, PLA degrades completely (no char yield), while the PNAc degrades to yield an intermediate char yield of 25 wt% for 375 < T < 425 °C, but complete degradation occurs above 450 °C. Blend processing and thermal and dynamic mechanical analyses (DSC and DMA) were performed below 250 °C, to completely avoid degradation. The crystallization behavior of semicrystalline PLA was investigated via DSC. The PLA samples were first heat pressed at 180 °C for 10 minutes and then quenched to room temperature with water cooling. One sample was directly analyzed by DSC, while another was first annealed at 110 °C (=l/2(T_g + T_m)) for 1 hour to reach an equilibrium level of crystallinity. Figure 1 shows a comparison of thermal behavior for these two samples. It was observed that quenching the PLA melt results in a low degree of crystallinity and virtually no recrystallization on heating, both indicating slow crystallization. Annealing at 110 °C for 1 hour results in significant crystallization evidenced by a large melting endotherm at T = 155 °C. The melting temperature did not shift dramatically due to annealing, but the endotherm shape did change. Complementary WAXD experiments yielded the same conclusions. The crystallization behavior of polymer blends was also analyzed. All of the samples were heat pressed at 180 °C for 10 minutes and then annealed at 110 °C for 1 hour before thermal analysis, providing a standard condition for extensive crystallization. Figure 2 summarizes the first DSC heating trace of the samples measured after annealing. The results indicate that PNAc itself is amorphous (though with large physical aging content), but that incorporation of PLA leads to crystallization in proportion to the PLA wt-%. Also, the peak endotherm positions (melting transitions) shift slightly to higher temperatures with increasing PLA content. Quenching these samples to T = 20 °C and reheating to 200 °C again showed clearly that single T_g's are observed and that crystallization can be largely suppressed. Importantly for shape memory, the single glass transition events were not broadened in the blends relative to the pure components, suggesting that the amorphous phase was quite homogeneous for all of the blends. The observed T_g values are plotted in Figure 3 along with the best fit with the Fox equation, showing slight positive deviation. This leads to a conclusion that strong interactions between the two polymers that reduces free volume of the polymer blends and hence, increased glass transition temperature relative to the Fox equation prediction, has occurred.

In order to elucidate the effect of PNAc on the degree of crystallinity and the crystal structures, the crystalline diffraction patterns were observed via wide-angle x-ray diffraction. The results indicate that the PNAc phase has only an amorphous halo, thus being totally amorphous, while the PLA exhibits three very strong diffraction peaks at 2Θ = 22.3°, 25.0° and 28.6°, corresponding to -spacings of 5.92, 5.29, and 4.64 A°, respectively. Upon addition of PNAc, all of the peak intensities were depressed, but the peak positions remained essentially unchanged. Consistent with the DSC results, the degree of crystallinity increases in proportion to PLA addition. From the peak width at half height, it was found that the crystalline lamellae size did not decrease, although the degree of crystallinity decreased, with increasing PNAc content. This means that the decrease in crystallinity and depression of the melting transitions are not due to a change of crystal size, but rather may be due to a thinning of the lamellae thickness or to a decrease of the crystal concentrations.

The storage modulus of the polymer blends was also measured using DMTA, first investigating the effects of annealing on the storage modulus. Below their glass transition temperatures, T_g, both samples exhibit similar high storage moduli (3 GPa), as well as similar softening points. When heated above T_g, the storage modulus of thermally quenched samples decreases sharply to about 2 MPa. However, further increasing the temperature induces a modulus increase attributed to recrystallization of the samples at higher temperatures. This also proved that the sample is not in an equilibrium state and that its mechanical properties in the rubbery region depend on thermal history. To reach equilibrium, the sample was annealed at 110 °C for 1 hour as previously described for DSC analyses. The storage modulus above T_g shifts to about 200 MPa until melting, the increase being due to an increase of the degree of crystallinity on annealing. In order to tune the rubbery modulus at equilibrium state, PLA was blended in different proportions to PNAc and annealed as above. Storage moduli for such blends were measured and the results are plotted in Figure 4. It can be seen that, below T_g, all of the samples show similar large moduli while above T_g the moduli decrease to a plateau whose magnitude depends on crystallinity and thus PLA content. This trend is in accordance with that of DSC and XRD, and may be explained by the fact that the increase of storage moduli came from the physical crosslinking formed by crystals and the filler effect of the high modulus crystalline phase.

Stress-free shape memory tests were carried out in hot water at 65 °C, with an annealed sample composed of 30 % PLA. The real time shape recovery was videotaped and selected images are shown in Figure 5. The results show that the sample features quick and complete shape memory behavior: the sample recovers to the original shape (straight bar) within 10 seconds, with most of the recovery being accomplished within the first several seconds.

The same characterizations were carried out on the blends of PNDF and PNAc as above described. The TGA and DSC results show that PNDF is also thermally stable up to 300 °C, and the mixtures form only one glass transition, the values fall between the Tgs of the two homopolymers and changes with changing composition. At the same time, the melting points and the degrees of crystallinity were depressed with the incorporation of amorphous PNAc.

The storage moduli of the blends, which give the rigidity of the materials, were also measured. The results are similar to those of the PLA/PNAc blends, the PNDF/PNAc blends being very rigid below the critical temperatures (T_g), and featuring sharp modulus changes at the Tg to a plateau modulus ranging from several MPa to tens of MPa, depending on the degree of crystallinity of the blends. These plateau moduli can be tuned by adjusting the degree of crystallinity of the blend, that is,by adjusting the blend composition.

In summary, shape memory polymers were obtained by blending a semicrystalline polymer, for example , PLA, with an amorphous polymer, for example PNAc. The polymer blends are totally miscible at all blend ratios within the experimental ranges and form only one single glass transition temperature for each formulation. Additionally, the degree of crystallinity of the blends decreases monotonically with increasing PNAc and PNAc and PNDF fraction. This, in turn, governs the rubbery modulus important to shape memory.

Thus, the present disclosure advantageously provides a shape memory polymeric material that is characterized by a T_g exceeding room temperature whose rubber modulus and elasticity are derived substantially from physical crosslinks comprising a blend of a crystalline polymer selected from the group consisting of poly(vinylidene fluoride),polyglycolides, polylactide and copolymers thereof, poly(hydroxybutyrate), poly(ethylene glycol), polyethylene, polyethylene- co-vinyl acetate, poly(vinyl chloride), poly(vinylidene chloride) and copolymers of poly vinylidene chloride and poly vinyl chloride with an amorphous polymer selected from the group consisting of poly( vinyl acetate), poly methyl acrylate, poly ethyl acrylate, atactic poly methyl methacrylate, isotactic poly methyl methacrylate and syndiotactic poly methyl methacrylate.

The present disclosure also advantageously provides a method of preparing a shape memory polymer material characterized by a T_g exceeding room temperature whose rubber modulus and elasticity is derived substantially from physical crosslinking comprising melt blending a crystalline polymer selected from the group consisting of poly( vinylidene fluoride), polylactide, poly(hydroxybutyrate), polyethylene glycol), polyethylene, polyethylene-co-vinyl acetate, poly(vinyl chloride), poly( vinylidene chloride) and copolymers of poly vinylidene chloride and poly vinyl chloride with an amorphous polymer selected from the group consisting of poly(vinyl acetate), poly methyl acrylate, poly ethyl acrylate, atactic poly methyl methacrylate, isotactic poly methyl methacrylate and syndiotactic poly methyl methacrylate at a temperature of 10 ~ 20 °C above the melting temperature of the crystalline polymers, for a time sufficient to ensure good mixing , cooling the resultant blend to room temperature, introducing said blend into a press maintained at about 180 °C, applying pressure to said blend and then rapidly cooling the film thereby formed to an annealing temperature T_g <T_a<T_m, where it is held until crystallization is complete and following which the film is cooled to room temperature.

Although the polymer blends and processing methodologies of the present disclosure have been described with reference to specific exemplary embodiments thereof, the present disclosure is not to be limited to such exemplary embodiments. Rather, as will be readily apparent to persons skilled in the art, the teachings of the present disclosure are susceptible to many implementations and/or applications, without departing from either the spirit or the scope of the present disclosure. Indeed, modifications and/or changes in the selection of specific polymers, polymer ratios, processing conditions, and end-use applications are contemplated hereby, and such modifications and/or changes are encompassed within the scope of the present invention as set forth by the claims which follow.