Blends of amorphous and semicrystalline polymers having shape memory properties

Cross-reference of related applications

This application applies to the following provisional patent applications: Application No. 60 / 418,023 (filed October 11, 2002); Application No. 60 / 466,401 (filed April 29, 2003); and Application No. 60 / 488,323 (filed April 29, 2003). Claim priority based on (filed July 18, 2003). Each provisional patent application is incorporated herein by reference to the extent consistent with the present disclosure.

The present disclosure relates to shape memory materials, more particularly with amorphous polymers such as poly (vinyl acetate) (PVAc) and semi-crystals which can be, for example, poly (lactic acid) (PLA) or poly (vinylidene fluoride) (PVDF). Regarding blending with quality polymers. The present disclosure also relates to methods of making such blends exhibiting shape memory effects and their uses.

A shape memory material is a material characterized by having the ability to transform a shape from a temporarily frozen shape to a permanent shape triggered by an environmental stimulus such as heat, light, or steam. When used creatively, this phenomenon can be used for a variety of purposes. Many polymers, for example, on the basis of rubber elasticity, exhibit a shape memory effect endogenously in cooperation with vitrification or crystallization. However, there are various characteristics (of shape memory) such as strain recovery speed, workability during recovery, and stability of contraction state. The first reported as shape memory polymers (SMPs) were cross-linked polyethylene discovered and patented by Radiation Appliances, Inc. in 1971, and prosthetic material reported by Vernon-Benshoff Co. It was a methacrylic acid ester for use as. It has been confirmed that the strain recovery mechanism of such materials is very different from that of shape memory alloys (SMA), which are mainly based on nickel-titanium alloys.

More specifically, the shape memory polymer is a superelastic rubber. When the polymer is heated to a rubber state, it can be deformed under a modulus of elasticity of ~ 1 MPa, and when the temperature drops below either the crystallization temperature or the glass transition temperature, the deformed shape is fixed by low temperature rigidity. At the same time, the mechanical energy spent on the material during deformation is stored. Then the temperature changes to the transition temperature (T)_mOr T_g), The polymer returns to its original form, driven by the restoration of the conformational entropy of the network chain. The nature of SMP is closely related to the network architecture and the sharpness of the transition that separates the rigid and rubber states. Compared to SMA, SMP has (i) a maximum strain of SMA of less than 8%, whereas it has a high strain of hundreds of percent due to large rubbery compliance; (ii) due to the diversity of polymer chemistry. It has advantages such as ease of adjusting the transition temperature; and (iii) ease of processing at low cost.

The patents filed simultaneously by the inventors are those in a variety of applications, including the synthesis and characterization of thermostimulatory SMPs with different thermomechanical properties, and their use as medical devices and mechanical actuators. Disclose the use of. The disclosed materials range in elastic modulus at room temperature from hard glass materials with a storage modulus of several GPa to elastic rubbers with a modulus as low as tens of MPa. Moreover, the shrinkage (rubber) modulus is adjusted to the range of 0.5 &lt;E &lt;10 MPa as specified by the end application. like that<u style="single">SMP</u>One example is chemically crosslinked polycyclooctene (PCO). This is a hard semi-crystalline rubber that, above Tm, elastically deforms into a temporary shape that is fixed by crystallization. Immersion in hot water achieves a quick and complete recovery of the overall deformation. Other SMPs that provide adjustable critical temperature and rubber modulus are synthesized using a thermosetting random copolymer formed from two vinyl monomers that allow controlled Tg and casting. .. Such copolymers are cross-linked using a bifunctional vinyl monomer as the cross-linking agent, and the concentration of the cross-linking agent controls the rubber modulus, and thus the working potential during recovery. Since these materials can be cast in addition to the shape memory effect, they can be processed into more complicated shapes. Also, they are optically transparent. However, in any of these cases, the use of chemical cross-linking limits the types of processing possible and permanently fixes the equilibrium shape during network formation.

Sharp Tg<u style="single">(&gt; Room temperature)</u>Semi-crystalline thermoplastic polymers with low crystallinity are also good candidates for shape memory, but also offer the advantage of melt machining at temperatures above Tm, allowing repeated reset of equilibrium shapes by stress relaxation in the fluid state. I have to. A typical example of a polymer contained in this type of SMP is polyurethane, the soft domain of which is semi-crystalline with a low melting point (but higher than Tm), and the hard domain is characterized by a high melting point that is exceeded only during processing. The effects of hard and soft segments on shape memory effects have been studied. In addition to such polyurethanes, block copolymers of polyethylene terephthalate (PET) have also been synthesized for their shape memory effect.

Miscible blends of semi-crystalline and amorphous polymers have also been studied in the past. However, it was not because of the shape memory behavior, but because of their crystalline and mechanical properties. For these blends, which are miscible at the molecular level, a single, non-spreading glass transition is an important aspect of shape memory. Furthermore, in such a miscible blend, equilibrium crystallinity (shape-fixing T)_gAnd T_m(Controlling the plateau modulus between) also changes dramatically and systematically depending on the blend composition, i.e. the relative concentration of each component. Numerous blends of this type have been studied, but no disclosure has been made of utilizing such blends for their shape memory properties.

<p> According to the present disclosure, SMPs that have a relatively high modulus of elasticity at room temperature, have adjustable and sharp transitions, and are capable of repeatedly reforming their permanent shape at temperatures above a certain melting point. Manufactured by blending or mixing crystalline polymer (C) and amorphous polymer (A). Blending or mixing is a single mixed phase in the molten state (which can be processed into a stress-free natural state), but is crystalline to a limited and as desired range and when cooled to room temperature. Further vitrification is performed. An example of (C) is poly (vinylidene fluoride) (PVDF) (T)._g= -35 ° C, T_m= 175 ° C), polylactic acid (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-hydroxybutyrate) and its copolymers, polyanhydrides, poly (vinylidene fluoride) ) (PVDF) (T_g= -35 ° C, T_m= 175 ° C), poly (ethylene glycol) (PEG), polyethylene, polyethylene-co-vinyl acetate, poly (vinyl chloride) (PVC), and polyvinylidene chloride (PVDC) and polyvinylidene chloride (PVDC) / Copolymers of polyvinylidene chloride (PVC), but are not limited to these. An example of (A) is poly (vinyl acetate) (PVAc) (T)._g= 35 ° C), polymethyl acrylate (PMA), polyethyl acrylate (PEA), atactic polymethyl methacrylate (aPMMA), isotactic polymethyl methacrylate (iPMMA), syndiotactic polymethyl methacrylate (sPMMA), and Other polyalkyl methacrylates and the like.</p><p> According to the present disclosure, plasticizers can also be included in the disclosed blends. Examples of plasticizers are di-n-butylphthalate (DBP), di-n-octylphthalate (DNOP), di (2-ethylhexyl) phthalate (DOP), di-2-ethylhexylisophthalate (DOIP), bis- (20 ethylhexyl) terephthalate (DOTP), di-n-decylphthalate (DNDP), di-cyclohexylphthalate (DCHP), di-octyl sebacate (DOS), Hexamoll (registration index) DINCH, EVA, EVA-carbon monoxide Tarpolymers (Elvaloy), poly (alkylene alkanoates), etc., but are not limited to these. DOP can also be used to reduce the glass transition of PVC.</p><p> The disclosed polymer blends are finely ground particle materials such as clay, silica or TiO.₂It can also be blended so as to contain.</p><p> T above room temperature to obtain high rigidity in temporary form_gThere is a need for a shape memory polymer that has the required elastic modulus and elasticity derived from physical cross-linking rather than chemical cross-linking.</p><p> Preferably, the exemplary SMPs of the present disclosure are amorphous polyvinyl acetate (PVAc) (T)._g= 35 ° C) and semi-crystalline polylactic acid (PLA) (T)_g= 56 ° C, T_m= 165 ° C) or achieved by blending or mixing poly (vinylidene fluoride) (PVDF). The disclosed polymers show perfect miscibility at all blend ratios and have a single glass transition temperature while partially maintaining crystallization (except PVAc). Blend T_gIs used as the critical temperature that triggers shape recovery, but the crystalline phase is T_gAbove T_mActs as a physical cross-linking field for elastic deformation below. Shape memory polymers have been gaining more and more attention lately, especially as they have been encouraged to develop biomedical engineering tools and expand the range of end-use possibilities as medical devices.</p><p> A polymer blend currently suitable according to the present disclosure is formed from polyvinyl acetate (PVAC) and poly (lactic acid) (PLA) or poly (vinylidene fluoride) (PVDF). However, other examples of suitable blends include PVDF / PMMA pairs and PVDF / PMMA / PVAc ternary blends. The combination of PMMA and PMMA / PVA plays the same role as PVAc in the blend as described above. The advantage of adding PMMA is that the critical temperature can be freely raised to about 80 ° C and the elastic modulus at room temperature can also be increased. PVDF may be replaced by poly (vinylidene chloride) (PVDC), by poly (vinylidene chloride) / poly (vinyl chloride) copolymers, or by any of the "C" polymers described above.</p><p> Blending or mixing poly (vinyl chloride) with poly (butyl acrylate) or poly (butyl methacrylate) (PVC / PBA) has also been found to have certain advantages. For PVDF / PVAc, PVAc is T_gBut at the same time lowers the crystallinity of PVDF. PVC can play the same role as PVDF, but with low crystallinity and relatively high T_gIt has (~ 80 ° C). Therefore, in this exemplary embodiment of the present disclosure, the second component (PBA) is T._gIt only functions or plays a role in reducing. This function / role can also be achieved with small molecule plasticizers, most notably dioctyl phthalates (DOP), but with biocompatible polymeric plasticizers for intended implantable medical applications. Is suitable here. The composition range of PBA is 10-40%, but 20% is the most convenient, T_gIs ~ 40 ° C.</p><p> Further beneficial features and functions associated with the polymer blends of the present disclosure will become apparent from the detailed description below.</p>

The miscible blends of the present disclosure exhibit sharp and adjustable transition temperatures and adjustable stiffness at temperatures above those transition temperatures. The disclosed blends also exhibit a good shape recovery effect at recovery temperature and shrinkage forces that can be adjusted depending on the transition temperature of the amorphous polymer and the crystallinity of the homopolymer. Shape recovery can be achieved within seconds when heated to a suitable level above the transition temperature, eg, 20 ° C above the transition temperature. Additional advantages associated with the materials of the present disclosure are that the material is rigid at room temperature, that the polymer is generally biocompatible and can be used as a medical device and implant, and that the product is subject to application requirements. Depending on the situation, it can be dyed in any color, or it can be made radiation opaque to x-ray radiography.

The shape recovery temperature of a particular polymer blend according to the present disclosure depends on the glass transition of the blend. Theoretically, the blended polymer recovers above the glass transition temperature and below the melting point. It is preferable to use a temperature approximately 20 ° C higher than the glass transition temperature for quick shape recovery. More preferably, the temperature in the rubbery plateau region of a particular polymer blend is selected for quick recovery and predictable and desired shrinkage.

The blends of the present disclosure can be successfully used in combination with a variety of uses. For example, the following uses, but are not limited to these. a) Medical (human health care) stents, patches and other implants b) Surgical tools that require adjustable shape and high rigidity (rigidity) c) Structural tools that can be freely adjusted in shape, such as personal care products (tableware, brushes, etc.) and hardware tool handles. d) Self-healing plastic e) Medical equipment f) Impression material for molding, duplication, rapid prototyping (stereolithography, high-speed modeling technology), dentistry and figure printing g) toys h) Reversible embossing for information storage i) Temperature sensor j) Safety valve k) Heat shrink tape or seal l) Thermal control fittings and fasteners m) Large strain, large force actuator

The following non-limiting examples are provided to further illustrate the beneficial features, functions and uses of the exemplary polymer blends according to the present disclosure. As will be readily apparent to those of skill in the art, the following examples should not be considered absolute as they are merely intended to illustrate aspects of the present disclosure, and thus potential polymeric materials that may be conveniently used in accordance with the present disclosure. , Processing conditions (eg relative percentages, temperature and time) and / or should not be considered limiting with respect to end use. The physical properties and processing conditions described in the examples below are merely descriptions of such properties / conditions and should not be considered limiting the scope or usefulness of the present disclosure.

<u style="single">material</u> M_w= 500,000 g / mol of poly (vinyl acetate) was purchased from Aldrich and vacuumed at room temperature for 2 days to remove water and volatiles. M_w= 130,000 g / mol of poly (lactide) was provided by Professor S. Huang of the University of Connecticut and dried overnight at T = 60 ° C before use. The material was found to contain 92% L-isomer and 8% D-isomer. M_w= 180,000 g / mol of poly (vinylidene fluoride) was also purchased from Aldrich and vacuumed at room temperature for 2 days to remove volatiles, including water.

<u style="single">Processing and fixing of primary shapes</u> Melt blending of PLA / PVAc and PVDF / PVAc using various blend ratios was performed in a 30 ml Brabender biaxial screw mixer. After equilibrating the mixer at T = 180 ° C for 5 minutes, the rotation of the mixer blades was adjusted to 25 RPM and the premixed polymer pellets were added to the chamber over 1 minute. The polymer was mixed for 10 minutes to ensure good dispersion. Nitrogen was purged into the chamber to reduce the possibility of oxidative decomposition during mixing. After mixing, the blend was removed from the chamber, cooled to room temperature and pressed between hot plates of a 180 ° C Carver press for 5 minutes under a load of 8 metric tonnes. The film thickness was adjusted using a spacer, and rapid cooling to room temperature was performed. Thermal and mechanical property analysis was performed using the film thus formed.

<u style="single">Characteristic analysis</u> The thermal properties of the polymer (thermogravimetric analysis (TGA)) were measured prior to differential scanning calorimetry (DSC) and melt blending to ensure the absence of solvent vapor in the polymer. For this purpose, a TGA from TA Instruments (TA 2950) was used to heat the polymer sample from 25 ° C to 600 ° C under a nitrogen atmosphere at a heating rate of 20 ° C / min. A Perkin-Elmer DSC-7 was used for the DSC analysis. A 10 g sample prepared from pellets and hot pressure film is first heated from 20 ° C to 200 ° C at a heating rate of 10 ° C / min (first heating), then cooled to 20 ° C at the same rate, and finally. The sample was reheated to 200 ° C at a heating rate of 10 ° C / min (second heating). T of each blend_gWas determined from the midpoint of the heat capacity change observed during the second heating.

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

The storage modulus and loss tensile modulus of the blend were measured in tensile mode by dynamic thermomechanical analysis (DMTA) using the TA Instruments DMA2980. The method used followed a temperature gradient of -100 ° C &lt;T &lt;150 ° C at a heating rate of 4 ° C / min, a fixed frequency of 1 Hz, and a maximum tensile strain of about 0.5%. The shape of the sample was a rectangular bar with dimensions of 10 x 2 x 1 mm.

An isothermal stress-free shape recovery test was performed using samples cut into rectangles and stained for optical contrast. The sample was heated to T = 65 ° C and bent into a temporary spiral. This modified sample was cooled with ice water and fixed. That is, the sample was vitrified. The resulting deformed sample was then dropped into a hot water bath at a predetermined temperature and the shape recovery was visually monitored with a video camera and a digital frame capture device that collects images at a rate of 20 frames per second.

<u style="single">result</u> The TGA results showed that both PLA and PVAc were stable at T &lt;300 ° C. Above this temperature, PLA decomposes completely (no char yield), but PVAc decomposes to give an intermediate char yield of 25 wt% at 375 &lt;T &lt;425 ° C, at 450 ° C. If it exceeds, it will be completely disassembled. Blend processing and thermal and dynamic mechanical analysis (DSC and DMA) were performed below 250 ° C to completely avoid decomposition.

The crystallization behavior of semi-crystalline PLA was investigated by DSC. The PLA sample was first heated at 180 ° C. for 10 minutes and then rapidly cooled to room temperature by water cooling. One sample was analyzed by DSC as it was, but the other sample was first 110 ° C (= 1/2 (T)._g+ T_m)) Was annealed for 1 hour to reach equilibrium level crystallinity. Figure 1 shows a comparison of the thermal behavior of these two samples. Quenching the PLA melt results in low crystallinity and substantial non-recrystallization when heated, all showing slow crystallization. Annealing at 110 ° C for 1 hour results in significant crystallization, as evidenced by the large melt endothermic at T = 155 ° C. The melting point did not shift dramatically with annealing, but the form of endotherm changed. The same conclusion was reached in the complementary WAXD experiment.

The crystallization behavior of the polymer blend was also analyzed. All samples were hot-pressed at 180 ° C for 10 minutes, then annealed at 110 ° C for 1 hour and then subjected to thermal analysis, which was the standard condition for expanded crystallization. Figure 2 summarizes the first DSC heating traces of the sample measured after annealing. The results show that PVAc itself is amorphous (more physical aging), but the addition of PLA results in crystallization in proportion to the wt% of PLA. In addition, the endothermic position (melt transition temperature) of the peak is slightly shifted to the higher temperature side as the PLA content increases. Quench these samples to T = 20 ° C and reheat to 200 ° C again for a single T_gWas observed, and it was clearly shown that crystallization can be greatly suppressed. What is important for shape memory is that a single glass transition event in the blend does not spread compared to the pure component, that is, the amorphous phase is extremely uniform in all blends. Observed T_gThe values are plotted in Figure 3 with the best fit with the Fox equation, showing a slight positive deviation. This leads to the conclusion that there is a strong interaction between the two polymers that reduces the free volume of the polymer blend, thus resulting in a higher glass transition temperature than predicted by the Fox equation.

In order to clarify the effect of PVAc on crystallinity and crystal structure, the crystal diffraction pattern was observed by wide-angle x-ray diffraction. The results show that the PVAc phase is completely amorphous because it has only a halo pattern peculiar to amorphous, but PLA is 2θ = 22.3 °, 25.0 ° and 28.6 °, and d intervals are 5.92, 5.29, and 4.64Å, respectively. It shows three corresponding very strong diffraction peaks. The addition of PVAc reduces all peak intensities, but the peak positions remain essentially unchanged. Consistent with the DSC results, crystallinity increases in proportion to the addition of PLA. From the full width at half maximum, it was found that the size of the crystal layer plate does not decrease even if the crystallinity decreases as the PVAc content increases. This means that the decrease in crystallinity and the decrease in melt transition temperature are not due to changes in crystal size, but may be due to a decrease in layer thickness or a decrease in crystal concentration.

The storage modulus of the polymer blend was also measured using DMTA. First, the effect of annealing on the storage elastic modulus was investigated. Glass transition temperature T_gBelow, both samples exhibit similar high storage modulus (3GPa) as well as similar softening points. T_gWhen heated above, the storage modulus of the thermally quenched sample drops sharply to about 2 MPa. However, if the temperature is further increased, an increase in elastic modulus is induced due to recrystallization of the sample at high temperature. This also proved that the sample was not in equilibrium and its mechanical properties in the rubber region depended on the thermal history. To reach equilibrium, the sample was annealed at 110 ° C. for 1 hour as described in DSC analysis. T_gThe storage modulus at temperatures above the above shifts to about 200 MPa until melted, due to the increase in crystallinity due to annealing. To adjust the rubber modulus in equilibrium, PLA was blended with PVAc in different proportions and annealed as described above. The storage modulus of such blends was measured and the results are plotted in Figure 4. T_gBelow, all samples show similar high modulus, but T_gWhen it exceeds, the elastic modulus decreases and reaches a plateau. The magnitude of the reduction depends on the crystallinity, i.e. the PLA content. This tendency is consistent with that of DSC and XRD. And that tendency can be explained by the fact that the increase in storage modulus comes from the physical cross-linking formed by the crystals and the filling effect of the highly elastic crystalline phase.

Stress-free shape memory tests were performed in hot water at 65 ° C. with annealed samples containing 30% PLA. Real-time shape recovery was recorded on videotape. The selected image is shown in Figure 5. The results show that the sample is characterized by rapid and complete shape memory behavior. That is, the sample recovered to its original shape (straight bar) within 10 seconds. Most recovery was achieved within the first few seconds.

The same characteristic analysis was performed on the blend of PVDF and PVAc as described above. The results of TGA and DSC show that PVDF is also thermally stable up to 300 ° C, the mixture forms only one glass transition, the value of which is T of two homopolymers._gIt goes in between and shows that it changes as the composition changes. At the same time, the melting point and crystallinity decreased with the addition of amorphous PVAc.

The storage modulus of the blend that imparts material stiffness was also measured. The results are similar to those of the PLA / PVAc blend, with the PVDF / PVAc blend at critical temperature (T)._g) Less than very rigid, T_gIt is characterized by a sharp change in elastic modulus from several MPa to several tens of MPa depending on the crystallinity of the blend. These plateau modulus can be adjusted by adjusting the crystallinity of the blend, i.e. adjusting the composition of the blend.

In short, shape memory polymers were obtained by blending semi-crystalline polymers such as PLA with amorphous polymers such as PVAc. The polymer is completely miscible at any blend ratio within the scope of the experiment and forms the only single glass transition temperature in each formulation. In addition, the crystallinity of the blend decreases monotonically as the PVAc and PVAc and PVDF fractions increase. Therefore, controlling the elastic modulus of rubber is important for shape memory.

Therefore, the present disclosure describes poly (vinylidene fluoride), polyglycolides, polylactides and copolymers thereof, poly (hydroxybutyrate), poly (ethylene glycol), polyethylene, polyethylene-co-vinyl acetate, poly (vinyl chloride). , Poly (vinylidene chloride) and a crystalline polymer selected from the group consisting of polyvinylidene chloride and polyvinyl chloride copolymers, and poly (vinyl acetate), polymethyl acrylate, polyethyl acrylate, atactic polymethyl methacrylate, isotac. T above room temperature, including blends with amorphous polymers selected from the group consisting of ticpolymethylmethacrylate and syndiotactic polymethylmethacrylate._gAnd conveniently provide a shape memory polymer material characterized in that the rubber modulus and elasticity are substantially derived from physical cross-linking.

The disclosure also includes poly (vinylidene fluoride), polylactide, poly (hydroxybutyrate), poly (ethylene glycol), polyethylene, polyethylene-co-vinyl acetate, poly (vinyl chloride), poly (vinylidene chloride) and polyvinylidene chloride. Crystalline polymers selected from the group consisting of copolymers of vinylidene and polyvinyl chloride, and poly (vinyl acetate), polymethyl acrylate, polyethyl acrylate, atactic polymethyl methacrylate, isotactic polymethyl methacrylate and syndiotactic poly. An amorphous polymer selected from the group consisting of methyl methacrylate was melt-blended at a temperature 10 to 20 ° C higher than the melting point of the crystalline polymer for a time sufficient to ensure good mixing, and the obtained blend was brought to room temperature. After cooling, the blend is introduced into a press maintained at about 180 ° C, pressure is applied to the blend, and then the film formed thereby is annealed at an annealing temperature of T._g&lt;T_a&lt;T_mAbove room temperature, including rapidly cooling to that temperature until crystallization is complete, and then cooling the film to room temperature._gAlso conveniently provided are methods of making shape memory polymer materials, characterized in that the elastic modulus and elasticity of the rubber are substantially derived from physical cross-linking.

Although the polymer blends and processing methods of the present disclosure have been described with reference to their particular exemplary embodiments, the present disclosure is not limited to such exemplary embodiments. On the contrary, as will be readily apparent to those skilled in the art, the teachings of this disclosure can be applied to many practices and / or uses without being bound by the spirit or scope of this disclosure. In fact, modifications and / or changes in the selection of specific polymers, polymer ratios, processing conditions, and end applications are conceivable thereby, and such modifications and / or changes are also within the scope of the invention as described in the claims. included.

<figref num="1">FIG. 5 shows a comparison of DSC traces of PLA rapidly cooled from T = 180 ° C (top) or annealed at T = 110 ° C for 1 hour (bottom).</figref><figref num="2">It is a figure which shows the DSC trace of PLA / PVAc blend after annealing for 1 hour at T = 110 ° C.</figref><figref num="3">It is a figure which shows the glass transition temperature measured after quenching of a PLA / PVAc blend.</figref><figref num="4">It is a figure which shows the tensile storage elastic modulus vs. temperature of various PLA / PVAc.</figref><figref num="5">It is a figure which shows the shape recovery of a coiled PLA / PVAc (30:70) blend when exposed to water of T = 65 ° C.</figref>