Abstract:
A biodegradable film and a method of forming a biodegradable film are provided. In one aspect, the biodegradable film includes a reinforced biodegradable polymer including a biodegradable polymer and a reinforcing agent substantially dispersed throughout the biodegradable polymer by rapid depressurization of a supercritical fluid wherein the reinforced biodegradable polymer is formed into the film.

Description:
RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of the filing date under 35 U.S.C. §119(e) of Provisional U.S. Patent Application Ser. No. 61/297,512, filed Jan. 22, 2010, which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to biodegradable polymer nanocomposites and methods of making biodegradable polymer nanocomposites and, in particular to biodegradable polymer nanocomposites suitable for applications in packaging and other film applications. 
       BACKGROUND 
       [0003]    Plastics currently account for a large portion of municipal waste with containers and packaging being the main source of waste generated. Recovery methods for plastics include recycling, reuse, energy recovery, and composting. In food packaging, re-use and recycling of discarded packaging materials is severely limited, as the collected items contain an increasing proportion of unique materials including multi-layered structures developed for purposes of achieving optimal barrier properties. These multi-layered plastics materials are extremely difficult, if not impossible, to separate into their respective individual layers for recycling. 
         [0004]    Increasingly, biodegradable polymers are being used to replace plastic materials formed from petroleum-based products. Biodegradable polymers are used in many different types of products including packaging, building materials, agriculture and medicine. The biodegradable polymers may be synthetic or natural. Additionally, improvements in the mechanical properties of biodegradable polymers are desired to meet more stringent performance requirements, such as stiffness, dimensional stability, modulus and barrier properties. 
         [0005]    Biodegradable plastics are emerging as important next generation materials that can be used in place of traditional packaging material such as films. Examples of biodegradable polymers that may be used include polylactic acid (PLA) and poly(lactic-co glycolic acid) (PLGA). Improving the mechanical, processing, and barrier properties are key technological challenges to the use of biodegradable plastics for packaging materials. 
         [0006]    Recently, organically modified montmorillonite clays have been investigated as potential reinforcing agents in polymeric matrices. (Horsch, S., Gulari, E. and Kannan, R. J.,  Polymer,  47:7485-7496, 2006; Manitiu M, Bellair R J, Horsch S, Gulari E, Kannan R M.,  Macromolecules,  41(21): 8038-8046, 2008, Pavlidou S, Papspyrides C D,  Prog Poly Sci,  33: 1119-1198, 2008; Ray S S, Okamoto M,  Prog Poly Sci,  23: 1524-1543, 2003; Zeng C, et al.,  Adv Mater,  15(20): 1743-1747, 2003.) The clay particles are composed of silicate platelets which are approximately 100-5000 nm in length and 1 nm thick. Platelets are held together by van der Waals forces and the equilibrium platelet spacing of 1 nm is generally modified by chemical techniques. One method of increasing platelet spacing is modification of the clay surface with alkylammonium salts. Increasing the spacing of the clay platelets increases the potential for intimate contact between polymer chains and numerous clay platelets, thus reducing polymer chain mobility and improving mechanical properties. (Horsch et al., Id.; Manitiu et al., Id.; Pavilidou et al., Id.; Ray et al., Id.) The processing method used to create clay-polymer nanocomposites also plays a role in the resulting mechanical behavior. Melt processing, high shear mixing and post-processing heat treatments have been employed to enhance polymer chain-clay platelet contact, with moderate property improvements. (Pavilidou et al., Id.; Ray et al., Id.) Researchers have also used scCO 2  processing to improve mechanical properties of polymer-clay nanocomposites. (Horsch et al., Id.; Manitiu et al., Id.; Zeng et al., Id.) Diffusion of CO 2  within the clay particles and rapid depressurization leads to an increase in platelet spacing, as well as polymer chain contact. (Horsch et al., Id.; Manitiu et al., Id.) 
         [0007]    There is therefore a need to develop a biodegradable polymer nanocomposite system and synthesis route which results in constructs with a substantially uniform dispersion of reinforcing particles. Further there is a need to develop constructs for packaging materials that provide adequate resistance to tear propagation, good optical clarity, high impact resistance, and high tensile strength. In addition, there is a need for biodegradable nanocomposites suitable for packaging materials that resolve the waste accumulation problems and degrade over time. 
       BRIEF SUMMARY 
       [0008]    In one aspect of the present invention, a biodegradable film is provided. In one aspect, the biodegradable film includes a reinforced biodegradable polymer including a biodegradable polymer and a reinforcing agent substantially dispersed throughout the biodegradable polymer by rapid depressurization of a supercritical fluid wherein the reinforced biodegradable polymer is processed to form the film. 
         [0009]    In another aspect of the present invention, a method of forming a biodegradable nanocomposite is provided. The method includes mixing a biodegradable polymer with a reinforcing agent to form a mixture, contacting the mixture with a supercritical fluid. The method also includes pressurizing and heating the mixture and the supercritical fluid, and catastrophically depressurizing the supercritical fluid to substantially disperse the reinforcing agent within the biodegradable polymer to form a reinforced biodegradable polymer. The method further includes forming the reinforced biodegradable polymer into a film. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a flow chart depicting an embodiment of a method of preparing a biodegradable nanocomposite; 
           [0011]      FIG. 2  illustrates a comparison of the small angle x-ray diffraction spectra; 
           [0012]      FIG. 3  illustrates rheological data for 100PDLA and 100PDLA-93A-2.5 constructs; and 
           [0013]      FIG. 4  is a graph showing compressive strength of pure polymer and nanocomposite constructs. 
       
    
    
       [0014]    Advantages of the present invention will become more apparent to those skilled in the art from the following description of the preferred embodiments of the present invention that have been shown and described by way of illustration. As will be realized, the invention is capable of other and different embodiments, and its details are capable of modification in various respects. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive. 
       DETAILED DESCRIPTION 
       [0015]    The present invention relates to a biodegradable clay-polymer nanocomposite and a method of making a biodegradable clay-polymer nanocomposite. The biodegradable nanocomposite includes a biodegradable polymer and a reinforcing agent that are mixed and processed with a supercritical fluid. An embodiment of a method  100  of making the biodegradable nanocomposite is illustrated in  FIG. 1 . The method includes mixing a biodegradable polymer and a reinforcing agent  110 . The method  100  further includes loading the mixture into a container and saturating the mixture with a supercritical fluid  112 ,  114 . The method  100  also includes pressurizing the mixture and the supercritical fluid  116  followed by rapidly depressurizing the mixture and the supercritical fluid to substantially disperse the reinforcing agent within the biodegradable polymer to define a reinforced biodegradable polymer 118. The method  100  includes processing the reinforced biodegradable polymer to form a packaging material at  120 . 
         [0016]    The term “biodegradable” is used herein to refer to materials selected to dissipate upon exposure to the environment, independent of which mechanisms by which dissipation can occur, such as dissolution, degradation, and absorption. 
         [0017]    By way of non-limiting example, suitable biodegradable polymers include polymers based on polylactide (PLA), polyglycolide (PGA), poly(lactic-co-glycolic acid (PLGA)polycaprolactone (PCL), their copolymers and mixtures thereof. Additional materials include, but are not limited to, chitosan, methyl cellulose, carboxy-methyl cellulose, poly vinyl acetate, alginate, polyethylene glycol (PEG), poly(2-hydroxyethyl methacrylate) (PHEMA), polymethyl methacrylate (PMMA), ethylene-vinyl acetate (EVA), polyacrylamide, and polyamine. 
         [0018]    As is known, if a substance is heated and is maintained above its critical temperature, it becomes impossible to liquefy it with pressure. When pressure is applied to this system, a single phase forms that exhibits unique physicochemical properties. This single phase is termed a supercritical fluid and is characterized by a critical temperature and critical pressure. Supercritical fluids have offered favorable means to achieve solvating properties, which have gas and liquid characteristics without actually changing chemical structure. By proper control of pressure and temperature, a significant range of physicochemical properties (density, diffusivity, dielectric constants, viscosity) can be accessed without passing through a phase boundary, e.g., changing from gas to liquid form 
         [0019]    As is known, a near critical fluid may have a parameter such as a pressure or a temperature slightly below the pressure or the temperature of its critical condition. For example, the critical pressure of carbon dioxide is about 73.8 bar and its critical temperature is about 301K. At or above the critical temperature, carbon dioxide may have a near critical pressure of between about 3.0 bar and 73.7 bar. At or above the critical pressure, carbon dioxide may have a near critical temperature of between about 100K and 300K. A fluid at its near critical condition typically experiences properties such as enhanced compressibility and low surface tension to name a few. 
         [0020]    In one embodiment, the supercritical fluid used may be carbon dioxide which may exist as a fluid having properties of both a liquid and a gas when above its critical temperature and critical pressure. Carbon dioxide at its supercritical conditions has both a gaseous property, being able to penetrate through many materials and a liquid property, being able to dissolve materials into their components. In addition embodiments, the supercritical fluid may comprise other suitable fluids such as methane, ethane, nitrogen, argon, nitrous oxide, alkyl alcohols, ethylene propylene, propane, pentane, benzene, pyridine, water, ethyl alcohol, methyl alcohol, ammonia, sulfur hexaflouride, hexafluoroethane, fluoroform, chlorotrifluoromethane, or mixtures thereof. 
         [0021]    It is understood that the fluid is preferably a supercritical fluid. However, a near-critical fluid may be used in lieu of the supercritical fluid which is referred to hereafter and the term supercritical fluids as used herein is meant to include near-critical fluids. 
         [0022]    By way of non-limiting example, the reinforcing agent used in the present invention is typically an organically modified clay, such as a smectite clay. A smectite clay is a natural or synthetic clay mineral selected from the group consisting of hectorite, montmorillonite, bentonite, beidelite, saponite, stevensite and mixtures thereof. A preferred choice for the smectite clay is montmorillonite. In some embodiments, the smectite clay includes organic modifiers based on ammonium salts. Examples of suitable reinforcing agents include but are not limited to, CLOISITE 93A, CLOISITE 30B, CLOISITE Na + , CLOISITE 10A, CLOISITE 11B, CLOISITE 15A, CLOISITE 20A, CLOISITE Ca +  (available from Southern Clay Products, Gonzalez, Tex.), NANOMER I30P (available from Nanocor, Inc. Hoffman Estates, Ill.), and vermiculite clays. Other reinforcing agents include, but are not limited to calcium phosphates and other inorganic materials. The calcium phosphates by way of non limiting example include hydroxyapatite (HA), octacalcium phoshphate (OCP), biomimetic apatite, fluorapatite, beta-tricalciuim phosphate (Beta-TCP), dicalcium phosphate dihydrate (DCPD). The inorganic materials include, but are not limited to carbon nanotubes, single or multi-walled, bioglass, grapheme and calcium carbonate. 
         [0023]    The method shown in  FIG. 1  may be achieved using one or more apparatuses known in the art, such as a mixer, extruder, injection molding machine, or any other suitable apparatus capable of loading material such as clay, polymer and supercritical fluid, and capable of maintaining a constant pressure onto the loaded material. Additional equipment known in the art may be use to process the reinforced biodegradable polymer to form a packaging material. The source of supercritical fluid may be any conventional fluid source such as a gas cylinder containing the fluid of choice. In some embodiments, the biodegradable polymer may be ground to an average particle size of about 250-500 μm and mixed with the reinforcing agent. The reinforcing agent may be added in an amount from about 0.1 to about 50 weight %, 0.5 to 10 weight %, 0.5 to 5 weight % and preferably from about 1 to 2.5 weight %, by way of non-limiting example. 
         [0024]    In some embodiments, the biodegradable polymer and the reinforcing agent may be mixed in a vessel until homogeneous. The vessel may be a pressurizable vessel isolatable from the atmosphere. The homogenous mixture may then be saturated with the supercritical fluid under pressure. In some embodiments, the reinforcing agent alone may be saturated with the supercritical fluid under pressure followed by catastrophic depressurization. The treated reinforcing agent may then be homogeneously mixed with the biodegradable polymer and treated with the supercritical fluid under pressure. In some embodiments, the mixed biodegradable polymer and reinforcing agent may be placed in an elongate tube to allow for vertical expansion but not radial expansion of supercritical fluid processed nanocomposites. The elongate tubes may be placed in a supercritical fluid reactor and saturated with the supercritical fluid. Internal pressure within the chamber of the supercritical fluid reactor may be increased. 
         [0025]    In some embodiments, when carbon dioxide is used as the supercritical fluid, the pressure may be increased above the critical pressure to about 7.38 MPa, to about 10.3 MPa, to about 13.8 MPa and up to about 70 MPa. The temperature is also increased within the chamber. In some embodiments, heat is applied to the vessel and the temperature is increased within the vessel to above the critical temperature. In some embodiments, the temperature may be increased to about 35° C. and above, or to about 100° C. However, other ranges may be used for carbon dioxide and other supercritical fluids without falling beyond the scope or spirit of the present invention. Pressurizing and heating the mixture with the supercritical fluid may be accomplished by any conventional means. In some embodiments, the pressurized and heated incubation may be from about 10 minutes to about 24 hours and up to several days, depending on the CO 2 -philicity of the polymer and the reinforcing agent. An exemplary incubation time is between about 30 minutes and 2 hours, and in some embodiments about 60 minutes. In some embodiments, the incubation time may be about 10 minutes to about 5 hours or about 1 hour to about 10 hours. 
         [0026]    The method further includes catastrophically or immediately depressurizing the contacted mixture to exfoliate the reinforcing agent such that the particles are substantially dispersed, to define a reinforcing agent-polymer mixture. The step of depressurizing may include immediately depressurizing the mixture down to ambient conditions. In some embodiments, the step of depressurizing may include a step-wise depressurization. In some embodiments, the reinforcing agent-polymer mixture may remain in solution so that no lasting foam structure is formed. In some embodiments, the reinforcing agent-polymer mixture may be formed into intermediate structures including, but not limited to elongate tubular structures or pellets. 
         [0027]    The reinforcing agent-polymer nanocomposite may be prepared for use in several applications. For example, the nanocomposites described above may be formed into monolayer or multilayer films appropriate for packaging materials. These films may be formed by any of the conventional techniques known in the art including extrusion, co-extrusion, extrusion coating, lamination, blow molding and solution casting. 
         [0028]    In some embodiments, a compression molding machine may be used to process the nanocomposite to form the film. The nanocomposites comprising a reinforced biodegradable polymer may be preheated without any pressure or may be used without preheating. The reinforced biodegradable polymer is then compression molded under pressure for a period of time in any type of compression molding machine known in the art. By way of non-limiting example, the preheating step may be 0-5 minutes, and the compression time may be 1-5 minutes. In some embodiments, the temperature may be between about 30 to about 150° C. and the compression pressure may be between about 140-210 psi. Other temperatures and pressures are also possible and depend on the type of biodegradable polymer. 
         [0029]    In some embodiments, the packaging materials may be formed by blown film extrusion as is known in the art. Briefly, the elongate tubular reinforced biodegradable polymers may be melted in an extruder and converted into film. The melted material in the extruder is forced or extruded through an annular die. Air is injected through a hole in the cent of the die and pressure caused the extruded melt to expand into a bubble. A constant pressure if maintained to ensure uniform thickness of the film. The bubble is pulled continually upwards from the die and a cooling ring blows air onto the film. Alternatively or additionally, the film may be cooled from inside using internal bubble cooling that allows the bubble diameter to be maintained. After solidification at a frost line, the film moves into a set of nip rollers which collapse the bubble and flatten the bubble into two film layers. The puller rolls pull the film onto windup rollers. The film passes through idler rolls during this process to ensure that there is uniform tension in the film. Between the nip rollers and the windup rollers, the film may pass through a treatment centre, depending on the application. During this stage, the film may be slit to form one or two films, or surface treated. 
         [0030]    The films obtained by any of the methods known in the art may be used as a single layer or in multilayer films. For example, multilayer films may be oriented in a uniaxial direction or in two mutually perpendicular directions in the plane of the film. One or more of the layers of the film may be oriented in the transverse and/or longitudinal directions to the same or different extents. This orientation may occur before or after the individual layers are brought together. In some embodiments, the nanocomposite films may be laminated onto or otherwise provided with another layer of differing material. 
       Example 1 
     Formation of Biodegradable Clay-Polymer Nanocomposites 
       [0031]    Nanocomposites were formed by adding nanostructured Montmorillonite clay particles (nano-clays). Organic modifiers based on ammonium salts were used to increase the intergallery spacing between silicate platelets, thus facilitating dispersion. The nano-clay used in this study was organically modified with a methyl dehydrogenated tallow (Cloisite 93A, Southern Clay Products, Gonzalez, Tex.). The formula of the methyl dehydrogenated tallow in Cloisite 93A nano-clay is shown below. The nano-clay was used “as-received” from the manufacturer. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0032]    where HT is Hydrogenated Tallow (˜65% C18; ˜30% C16; ˜5% C14). 
         [0033]    Clay-polymer nanocomposite constructs were synthesized using 100% poly-D-lactic acid (100PDLA), (Lakeshore Biomaterials, Birmingham, Ala.) and CLOISITE 93A. 2.0 g of ground 100PDLA polymer with total clay loading of 2.5 wt % were mixed and ground to an average particle size of 250-500 μm. Ground nanocomposite particles were placed in containers and placed into the supercritical fluid reactor and saturated with CO 2 . The internal pressure of the reactor was elevated to 13.8 MPa and the temperature was raised to either 35° C. or 100° C. After 60 minutes of soaking in the supercritical CO 2 , the reactor was rapidly depressurized at a rate of 0.3-0.4 MPa/s. 
         [0034]    Pure polymer constructs for comparison were synthesized using 100PDLA. Briefly, polymers were ground to an average particle size of 250-500 μm and placed in a reaction vessel. The polymer-filled vessels, each with 2.0 g of ground polymer, were placed into a supercritical fluid reactor and saturated with CO 2 . The internal pressure was elevated to 13.8 MPa at an internal temperature of 35° C. to induce a supercritical phase transformation in the CO 2 . After 60 minutes of soaking, the reactor was rapidly depressurized at a rate of 0.3-0.4 MPa/s. 
       Example 2 
     Synthesis of Biodegradable Polymer Nanocomposites for Further Evaluation 
       [0035]    Pure polymer constructs were synthesized using 100PDLA. Briefly, polymers were ground to an average particle size of 250-500 μm and placed in a reaction vessel. The polymer-filled vessels, each with 2.0 g of ground polymer, were placed into a supercritical fluid reactor and saturated with CO 2 . The internal pressure was elevated to 13.8 MPa at an internal temperature of 35° C. to induce a supercritical phase transformation in the CO 2 . After 60 minutes of soaking, the reactor was rapidly depressurized at a rate of 0.3-0.4 MPa/s. 
         [0036]    Clay-polymer nanocomposite constructs were synthesized using 100PDLA and CLOISITE 93A. 2.0 g of ground 100PDLA polymer with total clay loading of 1 wt %, or 2.5 wt % were mixed and ground to an average particle size of 250-500 μm. Ground nanocomposite particles were placed in reaction vessels and placed into the supercritical fluid reactor and saturated with CO 2 . The internal pressure of the reactor was elevated to 13.8 MPa and the temperature was raised to 100° C. After 60 minutes of soaking in the supercritical CO 2 , the reactor was rapidly depressurized at a rate of 0.3-0.4 MPa/s. The process was repeated to yield samples of 100PDLA with 1 wt % Cloisite 93A (100PDLA-93A-1) and 2.5 wt % 93A (100PDLA-93A-2.5). 
       Example 3 
     Additional Biodegradable Polymer Nanocomposites 
       [0037]    Constructs may also be synthesized from 85:15 poly-D-lactide-co-glycolide (85:15 PDLGA) and 65:35 poly-D-lactide-co-glycolide (65:35 PDLGA) (Lakeshore Biomaterials, Birmingham, Ala.) in addition to the 100PDLA construct described above. 
         [0038]    Polymer constructs may be synthesized using 100PDLA, 85:15 PDLGA and 65:35 PDLGA and CLOISITE 93A. Briefly, 2.0 g of ground 100PDLA, 85:15 PDLGA or 65:35 PDLGA polymer with total clay loading of 1 wt %, or 2.5 wt % may be mixed and ground to an average particle size of 250-500 μm. Ground nanocomposite particles may then be placed in reaction vessels, placed into the supercritical fluid reactor and saturated with CO 2 . The internal pressure of the reactor may be elevated above the supercritical pressure for CO 2  and the temperature may be elevated above the supercritical temperature. After 60 to 120 minutes of soaking in the supercritical CO 2 , the reactor may be rapidly depressurized at a rate of 0.3-0.4 MPa/s. 
       Example 4 
     Characterization of Dispersion in Biodegradable Polymer Nanocomposites 
       [0039]    A Rigaku SmartLab Diffractometer with a Cu Kα X-ray source (λ=1.54 Å) and an accelerating voltage of 40 kV at a current of 40 mA was used to determine the intergallery spacing of the clay/polymer nanocomposites. Samples were placed in a custom made, zero-background quartz sample holder that is 0.9 mm in depth and diffraction scans were collected from 0.1 to 10° 2θ at a scan rate of 3.0 degrees/min at a step size of 0.3 degrees. Several scans were obtained from different locations in the sample and verified to be reproducible when diffraction patterns were superimposed on one another. The 2θ angle was determined using the JADE software that accompanies the diffractometer and the d 001  spacing for the clays was calculated using Braggs&#39; Law of diffraction. The intergallery spacing was then found by subtracting 1 nm (platelet thickness) from the d 001  spacing. 
         [0040]    The diffraction spectrum of the 100PDLA-93A-2.5 nanocomposite showed a shift in the 001 peak of pure Cloisite 93A from 3.36° to 4.41° 2θ, as shown in  FIG. 2 . Using the Bragg equation it was determined that this magnitude of shift in the 28 position corresponds to an increase in platelet spacing of 1.79 nm (from 2.62 nm to 4.41 nm) after scCO 2  processing. Increased platelet spacing, as determined by X-ray diffraction has been used to determine the degree of intercalation of organically modified clays in polymer matrices. 
       Example 5 
     Rheology Measurements of the Biodegradable Polymer Nanocomposites 
       [0041]    A Rheometric Scientific RSA II rheometer (shear sandwich geometry 15.98 mm×12.7 mm×0.55 mm) was used to perform melt rheological measurements under oscillatory shear. Samples were prepared by melt pressing the polymer and nanocomposite constructs into a mold at 80° C. between Teflon plates followed by annealing under vacuum at 80° C. to remove any residual carbon dioxide. The materials were loaded and allowed to equilibrate for 1 hour at the desired temperature. Rheological measurements were performed at 80° C. and 120° C. for all samples. Strain sweeps were performed to ensure that the dynamic moduli were linear in the strain range used and the linear viscoelastic measurements were made at low strains (γ∘&lt;0.05) to minimize microstructure destruction. The frequency range used was 0.01=ω=100 rad/s and the property of time-temperature superposition was used to create master curves with a reference temperature of 80° C. 
         [0042]    As shown in  FIG. 3 , a shift in the crossover frequency, characteristic of the polymer chain relaxation time, from 0.23 to 0.10 rad/s was observed as well as a decrease in the slope of the elastic modulus in the terminal regime of the 100PLDA-93A-2.5 nanocomposite. Characterization of the rheological behavior of the pure and nanocomposite constructs showed that the nano-clay reduced the mobility of polymer chains within the construct. A shift in the frequency at which the storage (G′) and loss (G″) moduli cross indicates a change in the so-called “characteristic relaxation time” of the polymer. In this case, a reduction in the crossover frequency from 0.23 rad/s to 0.10 rad/s is indicative of over a doubling of the relaxation time of the polymer. Additionally, the low frequency “terminal” region of the storage modulus has a reduced slope when compared to the pure polymer which indicates that there is a significant amount of clay dispersion in the processed construct. Such strong polymer clay interactions and dispersion are necessary for the efficient transfer of stress from the matrix to the filler that is necessary to enhance mechanic properties. 
       Example 6 
     Mechanical Testing of the Biodegradable Polymer Nanocomposites 
       [0043]    Cylindrical cores with a diameter of 10 mm were obtained from constructs by using an osteochondral biopsy system. The cores were sectioned to a height of 10 mm and trimmed with a scalpel to ensure that the ends were parallel. The samples were placed between smooth stainless steel platens in a servohydraulic materials testing machine (850 Mini-Bionix, MTS Inc., Eden Prairie, Minn.). Constructs were loaded in compression under displacement control at a rate of 0.5 mm/min until a strain of 50% was reached. 
         [0044]    The compressive strength of the constructs was defined as the maximum load divided by the initial cross sectional area. Compressive modulus was determined by calculating the slope of the linear region of the load-displacement curve. A student t-test was used to determine the statistical significance of mechanical data as a function of construct composition. 
         [0045]    As shown in  FIG. 4 , pure polymer constructs (100PDLA) exhibited a compressive strength of 3.41 MPa (+/−0.54 MPa) and a compressive modulus of 33.79 MPa (+/−9.30 MPa). The addition of 1 wt % Cloisite 93A nano-clay particles to the 100PDLA increased the compressive strength to 6.10 MPa (+/−0.83 MPa) and compressive modulus to 45.92 MPa (+/−15.33 MPa). Nanocomposite constructs containing 2.5 wt % Cloisite 93A nano-clay showed an average compressive strength of 7.15 MPa (+/−2.02 MPa) and average compressive modulus of 68.42 MPa (+/−32.41 MPa). 
         [0046]    Although the invention herein has been described in connection with a preferred embodiment thereof, it will be appreciated by those skilled in the art that additions, modifications, substitutions, and deletions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention.