Abstract:
The present invention relates to shape memory materials and to a method for controlling shape change in shape memory materials. In particular, the invention relates to a method and a system for forming complex shapes from shape memory materials and to shape memory materials having complex shapes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application, submitted under 35 U.S.C. 371, claiming priority to PCT International Patent Application PCT/GB2008/001331 filed on Apr. 16, 2008, which claims priority to Great Britain Patent Application No. 0707467.7 filed on Apr. 18, 2007 and Great Britain Patent Application No. 0724216.7 filed on Dec. 12, 2007, the disclosures of which are incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to shape memory materials and to a method for controlling shape change in shape memory materials. In particular, the invention relates to a method and a system for forming complex shapes from shape memory materials and to shape memory materials having complex shapes. 
       RELATED ART 
       [0003]    Shape memory polymers (SMPs) are materials that have the ability to “memorize” a “permanent” macroscopic shape, be orientated or manipulated under temperature and/or stress to a temporary or dormant shape, and then be subsequently relaxed to the original or memorized, stress-free condition or shape. Relaxation is usually prompted or encouraged by the application of thermal, electrical, or environmental energy to the manipulated or orientated SMP. This relaxation is associated with elastic deformation energy stored in the SMP during orientation of the SMP. The degree of orientation of the SMP is the driving force that causes relaxation. Thus the greater the degree of orientation, the greater will be the force or energy stored in the SMP and hence the greater will be the force or energy driving relaxation of the SMP when triggered or prompted by an external energy source. 
         [0004]    SMPs like other polymers can be grouped into two main categories; they can be amorphous, thus lacking any regular positional order on the molecular scale, or they can be semicrystalline which contain both molecularly ordered crystalline regions and amorphous regions in the same sample. 
         [0005]    Plastic deformation of amorphous SMPs and SMP composites results in the formation of an orientated amorphous or semi-crystalline polymer network. Orientation of SMPs and SMP composites can be achieved by stretching, drawing or applying a compressive and/or shear force to the SMP. The SMP may be orientated by application of any one or a combination of these forces and can be carried out at ambient temperatures or elevated temperatures. Generally, the temperature of the SMP is raised above ambient temperature to around the glass transition temperature (Tg) of the SMP before application of the orientation force or forces. Raising the temperature of the SMP in this way helps prevent the SMP from rupturing when the orientation force is being applied thereto. The glass transition temperature is the temperature below which the physical properties of amorphous SMPs behave in a manner similar to a solid, and above which they behave more like a rubber or liquid allowing the SMP to undergo plastic deformation without risk of fracture. After the SMP has been orientated, the temperature is reduced and the SMP is fixed in a temporary or dormant configuration. 
         [0006]    The orientated network is physically stable well below the glass transition temperature (Tg) where molecular mobility is low. However, near or above the polymer&#39;s glass transition temperature, molecular motion rapidly increases and causes the orientated network to relax, usually accompanied by physical changes in the dimensions of the SMP. During relaxation, the orientated SMP tends to recover the original dimensions of the unorientated SMP, hence the name shape “memory” material. However, recovery of the original shape depends primarily on the degree of crystallinity, orientation, the micro and nano-structures and the conditions under which the orientated network is relaxed. For copolymers other important factors are their detailed composition and their specific thermal properties, i.e. the glass transition and melting temperatures, of their components. 
         [0007]    It is believed that the relaxation process occurs nearly at constant volume. The degree of recovery during relaxation, for a semi-crystalline orientated SMP, depends on its crystallinity and structure and complete recovery of its original shape is difficult. In contrast, amorphous orientated SMPs, copolymers and their composites can return substantially to their original shape under appropriate relaxation conditions. 
         [0008]    The degree of orientation is the driving force that causes relaxation. The greater the degree of orientation, i.e. the force or forces applied to the SMP, the greater will be the driving force. 
         [0009]    During relaxation, the orientated SMP releases stored internal forces or energy. For example, an SMP of cylindrical shape orientated by applying a stretching force uniaxially along its longitudinal axis will shrink in length and expand in diameter during relaxation under free boundary conditions, i.e. where no physical constraints are imposed. Hence, when the cylindrical shaped SMP relaxes, it will induce a shrinkage force along its longitudinal axis and also an expanding force in the radial direction. These longitudinal and radial forces are proportional to the degree of orientation and mass of orientated polymer. The greater the degree of orientation, i.e. the greater the forces applied to the SMP during orientation, and the greater the mass of the SMP, the greater these longitudinal and radial relaxation forces will be. For SMPs of other geometries, the relaxation forces will also depend on the degree or magnitude of the orientation force, the direction of the applied orientation force, as well as the mass of the orientated SMP. The rate of relaxation or the rate of shape recovery of the SMP is dependent on sample geometry, processing conditions and more importantly on the mass and thermal diffusivity of the SMP. 
         [0010]    The mechanism of the prior art whereby the shape of an SMP is altered involves applying an orientation force to the SMP. Following orientation of the SMP where the SMP is changed from a first pre-orientated shape to a second orientated shape, the orientated SMP is heated above its glass transition temperature wherein the SMP relaxes back to its original or pre-orientated shape. It is an aim of the present invention to provide a shape memory polymer with tailored relaxation characteristics, capable of relaxing back to a shape which is different to its original shape. It is a further aim of the invention to provide shape memory material with complex geometries and structural assemblies of shape memory polymers with other solid elements such as metals, polymers and ceramics. 
       SUMMARY OF THE INVENTION 
       [0011]    According to a first aspect of the present invention there is provided a shape memory polymer having a first configuration and a second orientated configuration, the second orientated configuration when at least partially relaxed in a mould by stimulating molecular motion of the SMP, relaxes to a third configuration that is different to the first and second configurations. 
         [0012]    According to a second aspect of the present invention there is further provided a method for manufacturing a pre-relaxed shape memory polymer comprising the steps of:—processing a shape memory polymer having a first configuration to form an orientated shape memory polymer having a second configuration which is different to the first configuration; at least partially relaxing the orientated shape memory polymer by stimulating molecular motion of the shape memory polymer; and controlling the conditions under which the orientated shape memory material is at least partially relaxed to form a pre-relaxed shape memory polymer having a third configuration which is different to the first and second configurations. 
         [0013]    Generally, the term configuration refers to the shape of the SMP but may also refer solely to the dimensions of the SMP. For example, in an embodiment of the invention, the third or final shape of the SMP can be intermediate in dimension between the first shape and second orientated shape. Intermediate in dimension may refer to one or more dimensions of the shape. For example, the SMP may have an initial or first cylindrical shape having radius r and height h and may be orientated, by stretching along its longitudinal axis, to create a cylinder having a second shape with a smaller radius r −−  and larger height h ++ . An example of an intermediate third shape would thus include a cylinder having a radius R and height H, where r −− &lt;R&lt;r and h&lt;H&lt;h ++ . 
         [0014]    Preferably, the third shape is substantially different to the first and second shapes. For example, in an embodiment of the invention, the initial shape of the SMP is cylindrical and the final shape is that of a threaded screw. In another embodiment the initial shape is cylindrical and the final shape is a rod with regular polygonal cross sectional area such as, for example, a triangle, quadrilateral, or pentagon. 
         [0015]    Preferably, the step of processing the shape memory material includes applying a stretching, or drawing or compressive force to the SMP. Typically, these forces may be applied to the SMP by zone drawing, or hydrostatic extrusion, or die drawing, or rolling, or roll drawing, or compression moulding. Alternatively any combination of these forces may be applied to the SMP 
         [0016]    Preferably, the SMP is heated during or prior to application of the one or more of these forces to the SMP. The application of heat to the SMP prevents the SMP from rupturing which may otherwise occur on application of one or more of these forces. Preferably, the SMP is heated to, close to, or above its Tg. 
         [0017]    Preferably, the step of processing the shape memory material includes the addition of particles to the SMP. The particles help to conduct and distribute the heat more quickly to the SMP thus reducing the heating period which would otherwise be required in a less conductive material. The particles also help distribute the heat more evenly over the SMP providing an orientated SMP which has more consistent properties throughout. Preferably, the particles include mineral particles, ceramic particles, or combinations thereof. Preferably the particles are biodegradable inorganic particles. The particles may also be magnetic or photo sensitive particles. 
         [0018]    After processing the SMP in this way, the SMP is now orientated and thus contains trapped energy which is later released during the relaxing step. The energy released during the relaxing step helps drive the shape change of the SMP. The trapped energy of the orientated SMP is released by stimulating molecular motion of the orientated SMP. 
         [0019]    Preferably, stimulation of molecular motion is achieved by the application of energy to the SMP from an external source. Preferably, the energy applied is in the form of heat. Preferably, the SMP is heated above the glass transition temperature (Tg) of the SMP. Alternatively or additionally, the relaxing step and thus the release of trapped energy from the SMP may be prompted or triggered by the application of a different form of energy, for example, a magnetic field, an electric current, ultrasound, electromagnetic radiation such as microwaves, visible and infrared light, or by a combination of any one of these forms of energy. 
         [0020]    Stimulating molecular motion of the SMP may also be achieved by exposing the orientated SMP to a plasticizer. Exposure of the SMP to a plasticizer reduces the Tg of the SMP, thus increasing its molecular mobility. In this way, the molecular mobility of the orientated SMP may be increased sufficiently to cause the orientated network to relax. Where exposure of the orientated SMP to a plasticizer is not sufficient to relax the SMP, energy, in the form of heat for example, may also be applied to the SMP. In this way, the orientated SMP can be relaxed at a temperature less than would be necessary where the SMP is relaxed using heat alone. As such, the SMP can be shaped at lower temperatures, thus allowing the addition of temperature sensitive materials to the SMP. Temperature sensitive materials may include, for example, releasable bioactive agents such as monobutyrin, bone marrow aspirate, angiogenic and osteogenic factors. 
         [0021]    Plasticizers may be in the form of a volatile liquid or a gas. Examples of gaseous plasticizers include but are not limited to, oxygen, nitrogen, carbon dioxide, sulphur dioxide, ammonia, methane, ethane, butane, propane, hexane, decane, ethene, propene, butene, hexene, dodecanene, ethyne, and butyne. Examples of liquid plasticizers include but are not limited too, water, inorganic aqueous solutions such as sodium chloride solution, cyclic alkanes, such as cyclohexane and methylcyclohexane, cyclic alkenes, such as benzene and toluene, cyclic alkynes, halogen substitute alkanes, alkenes, and alkynes, such as carbon tetrachloride and chloropropane, oxygen substituted hydrocarbons, such as ethylene oxide and ethoxy hexane, aldehydes, such as hexanal, ketones, such as cyclohexanone, alcohols, such as methanol and ethanol, esters, such as buylpropionate, nitrogen substituted hydrocarbons, such as amine-triethylamine, and sulphur substituted hydrocabons, such as butane thiol and diethyl sulphide. 
         [0022]    The energy stored in the SMP can either be completely released or partially released leading to a completely relaxed or partially relaxed SNAP. The resulting SMP is then known as a pre-relaxed SNAP. Once the SNAP is fully relaxed it can not be further relaxed unless reprocessed or re-orientated. 
         [0023]    Ideally, the step of controlling the conditions under which the orientated SMP is at least partially relaxed includes placing at least part of the orientated SMP in a mould. The shape of the mould determines the third or final shape of the SMP and prevents the SNAP from returning to its original or first shape. 
         [0024]    The mould may be of similar dimensions to the second shape and thus restrict the polymer to a third shape very similar to the second shape. Alternatively the mould may be of different dimensions, thus allowing the polymer to relax to a third shape substantially different to the first and second shapes. Complex moulds can be used to form pre-relaxed SMPs with complex shapes. 
         [0025]    Where a plasticizer is used to relax the orientated SMP instead of or in addition to the application of energy, the mould may be porous or perforated, for example, to allow the direct contact of the orientated SMP with the plasticizer. 
         [0026]    The step of controlling the conditions under which the orientated shape memory material is at least partially relaxed may also include control of the energy added to the SMP when relaxing the orientated SMP. Where heat is used as the energy source, variations in the temperature and period of exposure will result in shaped SMP having different properties. For example, where the SMP is exposed to heat for a short period of time, the SMP, although it will have formed ifs final shape within the mould, will have only given up part of its trapped energy and thus only have partially relaxed. Such a shaped SMP will be capable of further relaxing. This is particularly advantageous where it may be necessary to alter the dimensions, but not necessarily the overall shape, of the final shaped SMP. For example, where the final SMP is in the shape of a fastening bolt having a screw thread. In this case, the dimensions of the final fastening bolt shape can be altered somewhat to cater for dimension tolerances in a nut having a threaded receiving bore thus providing a bolt which can be more securely fitted in such a threaded bore. 
         [0027]    Varying the orientation and relaxation of different sections of the SMP will also help tailor the final shape of the SMP. 
         [0028]    When a final shape is required having unalterable dimensions, the SMP, whilst in the mould, can be exposed to heat for a longer period of time ensuring that the orientated SMP has released all its trapped energy and is thus fully relaxed. The addition of further energy or the further subsequent heating of such a relaxed SMP will not alter its dimensions. This is particularly useful when a device of exact and fixed shape and dimensions is required regardless of whether it may be subjected to external energy sources, such as heat, when used. 
         [0029]    The particles added to the polymer during the processing step also help control the conditions under which the orientated SMP is at least partially relaxed. The particles help conduct the heat or other applied energy more evenly through the SMP, ensuring that the relaxing is consistent throughout the SMP, thus producing a pre-relaxed SMP having consistent properties throughout. The particles also help reduce the processing and controlling times by conducting the heat or other applied energy more quickly through the SMP than would otherwise occur. The addition of particles also helps improve subsequent machining of the shaped SMP where desired. 
         [0030]    Generally, the larger the size of the particles added to the SMP, the longer it will take the orientated SMP to relax. This longer relaxation time could advantageously be used to tailor the shape of the relaxing SMP as the processing time is increased. 
         [0031]    Suitable particles include but are not limited to inorganic particles such as buffers, radiopaque agents, osteoconductive agents, calcium, sodium, potassium, magnesium, barium, zirconium, bismuth, silver, gold, copper, zinc or any combination thereof. Preferably, the particles are crystalline calcium, sodium, zirconium, bismuth, barium, silicon, tungsten or magnesium salt. 
         [0032]    Optionally, the particles can be calcium carbonate, calcium hydrogen carbonate, calcium phosphate, dicalcium phosphate, tricalcium phosphate, magnesium carbonate, sodium carbonate, hydroxyapatite, bone, phosphate glass, silicate glass, magnesium phosphate, sodium phosphate, barium sulphate, barium carbonate, zirconium sulphate, zirconium carbonate, zirconium dioxide, bismuth trioxide, bismuth oxychloride, bismuth carbonate, tungsten oxide or any combination thereof. 
         [0033]    The particles can have a range of sizes and geometries. For example, the particles may be the shape of a needle, cube, platelet, fibre or sphere. Preferably the particles are shaped to enhance the mechanical properties of the SMP. The particle size is typically between 10 nm and 1 mm. 
         [0034]    Typically, inorganic particles that act as buffers improve strength retention of degradable systems by reacting with the acidic breakdown products of the amorphous SMP. 
         [0035]    Typical radiopaque agents include barium sulphate, barium carbonate, zirconium sulphate, zirconium carbonate, zirconium dioxide, bismuth trioxide, bismuth oxychloride, bismuth subcarbonate and tungsten oxide. 
         [0036]    Typical osteogenic agents include calcium carbonate, calcium phosphate, dicalcium phosphate, tricalcium phosphate, hydroxyapatite, bone, phosphate glasses, silicate glasses, magnesium phosphate and sodium phosphate. 
         [0037]    The particles may be pre-treated with a coupling agent such as a fatty acid, fatty acid anhydride or siloxane in order to enhance the properties of the SMP. 
         [0038]    The method, useful for producing shape memory materials with complex geometries, is also useful for producing structural assemblies of shape memory polymers with other solid elements such as metals, polymers and ceramics. 
         [0039]    According to another aspect of the present invention, there is provided a system for manufacturing a pre-relaxed shape memory polymer comprising a shape memory polymer as hereinbefore described and a mould. 
         [0040]    Preferably, the mould is a complex mould for forming complex shapes of the shape memory polymer. 
         [0041]    Preferably, the cavity of the mould has a modified surface to determine when the shape memory polymer has at least partially relaxed to form the third configuration. For example, the modified surface of the cavity can be pressure sensitive. In this way, as soon as the SMP has relaxed to form the third configuration or shape, the shaped SMP can be removed from the mould. This ensures that the SMP is relaxed for the minimum period of time resulting in a partially relaxed SMP which can be further relaxed at a later stage if required. 
         [0042]    Generally, the SMP can be modified to produce particular desirable characteristics. The desired characteristics will depend largely on the end use of the pre-relaxed shape memory polymer. 
         [0043]    Therefore, according to yet a further aspect of the present invention, there is provided a device including the pre-relaxed SMP as hereinbefore described. 
         [0044]    The SMP can be of a resorbable, amorphous polymer composition or a non-resorbable, amorphous polymer composition. The SMP can be a homo polymer or a co-polymer, both of which can be linear, branched or cross-linked. The copolymer may comprise a component selected from a group consisting of glycolide, lactide, ethylene glycol or ε-caprolactone. For example, the copolymer can include a polylactide-co-glycolide, such as Poly(D,L-lactide-co-glycolide), or poly(methyl methacrylate), or poly(ethyl methacrylate) or other amorphous acrylic based polymers and copolymers. The copolymer can include an amorphous polymer composition and at least one mobile polymer and/or one rigid polymer. 
         [0045]    Alternatively, the copolymer may include a semi-crystalline polymer and at least one mobile polymer and/or one rigid polymer. The mobility of a polymer refers to its ability to soften on application of heat. An SMP may have different areas of mobility relating to the different polymer compositions at different locations along the SMP composite. This allows different parts of the SMP to be orientated and relaxed at different temperatures and rates, thus allowing a finer control or tailoring of the final shape to be exercised. A rigid polymer is a polymer which demonstrates less mobility than a mobile polymer. The mobile polymer can include polyethylene glycol, or ε-caprolactone, or glycolide, or D,L lactide and the rigid polymer can include D-lactide or L-lactide. The co-polymer can be a random arrangement of monomers or a repetitive and sequenced arrangement of monomers. 
         [0046]    Examples of synthetic degradable SMP material include but are not limited to polyhydroxy acids, including polylactide (PLA) based homo and co-polymers such as poly(L-lactic acid) (PLLA), poly(D-lactic acid) (PDLA), poly(D,L-lactic acid) (PDLLA), PLLA-co-PDLLA, PLLA-co-PGA (where PGA is polyglycolide), PDLA-co-PGA, PDLLA-co-PGA, PLLA-co-PCL (where PCL is ε-polycaprolactone), PDLLA-co-PCL, PDLA-co-PCL. Co-polymers with poly(ethyleneglycol) (PEG) such as PLLA-co-PEG, PDLA-co-PEG, PDLLA-co-PEG are also suitable examples of synthetic degradable SMP material. Further examples include co-polymers containing three or more of the following blocks: PLA, PEG, PCL or PGA. Polyanhydrides, poly(hydroxybutyric acid), poly(hydroxyvaleric acid), poly(pseudo aminoacids), poly(hydroxyalkanoate) and blends and co-polymers thereof. 
         [0047]    Examples of natural biodegradable SMP material include but are not limited to polysaccharides such as alginate, dextran, cellulose, collagen, and chemical derivatives thereof. Further examples include substitutions and additions of chemical groups such as, for example, alkyl, alkylene, hydroxylations, oxidations, and other modifications routinely made by those skilled in the art. Proteins such as abumin, zein and co-polymers and blends thereof, alone or in combination with synthetic polymers are also suitable examples of natural biodegradable SMP material. 
         [0048]    Examples of synthetic SMP material include but are not limited to polyphosphazenes, poly(vinyl alcohols), polyamides, polyester amides, poly(amino acids), polyanhydrides, polycarbonates, polyalkylenes, polyacrylamides, polyalkylene glycols, polyalkylene oxides, polyalkylene terephtalates, polyortho esters, polyvinyl ethers, poly vinyl esters, poly vinil halides, polyvinylpyrrolidone, polyesters, polysiloxanes, polyurethanes, polyacrylates such as poly(methyl methacrylate), poly(ethyl methacrylate), poly(butyl methacrylate), poly(isobutyl methacrylate), poly(phenyl methacrylate), poly(methyl acrylate), poly(isopropyl acrylate), poly(isobutyl acrylate) and poly(octadecyl acrylate), and co-polymers thereof. Further examples include poly(ethylene terephthalate), poly(ethylene), polystyrene, polycarbonate, polysulfones, polyethersulfone, polyetherimide and polypropylene. 
         [0049]    Examples of synthetically modified natural SMP material include but are not limited to cellulose derivatives such as alkyl celluloses, hydroxyalkyl cellulose, cellulose ethers, cellulose esters, nitrocelluloses and chitosan. Examples of cellulose derivatives include methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxybutyl methyl cellulose, cellulose acetate, cellulose propionate, cellulose acetate butyrate, cellulose acetate phthalate, carboxymethyl cellulose, cellulose triacetate and cellulose sulphate sodium salt. 
         [0050]    The SMP can include a releasable bioactive agent. These agents are included to help promote bone regrowth. Examples include bone morphogenic proteins, antibiotics, anti-inflammatories, angiogenic factors, osteogenic factors, monobutyrin, thrombin, modified proteins, platelet rich plasma/solution, platelet poor plasma/solution, bone marrow aspirate, and any cells sourced from flora or fauna, such as living cells, preserved cells, dormant cells, and dead cells. Other bioactive agents known to one of ordinary skill in the art may also be used. Alternatively or additionally, the SMP can include a porogen, such as sodium chloride. This is particularly useful where the SMP device is required to promote cell growth. 
         [0051]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0052]    The accompanying drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the present invention and together with the written description serve to explain the principles, characteristics, and features of the invention. In the drawings: 
           [0053]      FIGS. 1A to 1D  show a schematic illustration of a method, according to the present invention, for manufacturing a pre-relaxed shape memory polymer device in the form of a threaded screw; 
           [0054]      FIGS. 2A to 2E  show a schematic illustration of a method, according to the present invention, for manufacturing a pre-relaxed shape memory polymer device in the form of a cylindrical rod; 
           [0055]      FIG. 3  is a graph showing the effect of different relaxing times on the expansion force along the radial direction of die-drawn shape memory polymer cylindrical shaped rods; 
           [0056]      FIGS. 4A to 4D  show a schematic illustration of a method, according to the present invention, for manufacturing a pre-relaxed shape memory polymer device of square cross section; and 
           [0057]      FIGS. 5A to 5D  show a schematic illustration of a method, according to the present invention, for manufacturing a pre-relaxed shape memory polymer device of triangular cross section. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0058]    The following description of preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
         [0059]    Preparation of Orientated Shape Memory Polymer 
         [0060]    The following description is given by way of example only of an embodiment of processing a SMP according to the present invention to form an orientated SMP and with reference to the examples describing embodiments of how the orientated SMP can be relaxed in a controlled manner to form a pre-relaxed SMP according to the invention. 
         [0061]    In this embodiment the SMP, amorphous poly(D,L lactide-co-glycolide), was mixed with 35% w/w CaCO 3  particles (PLC). The mixture was prepared in a twin screw extruder to form fibres of the mixture. The fibres were palletised and consolidated to form isotropic cylindrical rods  110 ,  210 ,  410 , and  510  with various diameters ranging from 5 mm to 20 mm using a ram extrusion technique. Isotropic rods thus formed having diameters of 5 mm, 18 mm and 20 mm were orientated by die drawing using a conical die at 60° C. and a drawing speed of 20 mm/min to form orientated rods  120 ,  220 ,  420  and  520  having diameters of 3 mm, 8 mm and 9 mm, respectively. 
       Example 1 
       [0062]    A prepared orientated SMP as described above in the form of a cylindrical rod  120  having a diameter of 8 mm was placed into a mould  130  having a threaded screw shaped cavity such that an end part  125  of the orientated rod  120  protrudes from an end of the mould  130 . The mould  130  and orientated rod  120  are immersed in a water bath at 80° C. for 60 seconds and then into a water bath at 20° C. for another 60 seconds to allow the oriented SMP to relax, resulting in a pre-relaxed screw  140  that has a head portion  150 , which expanded freely without the mechanical restraint of the mould  130 , having a radius similar to the isotropic rod  110 , and a threaded portion  160  having dimensions determined by the dimensions of the mould  130 . On removal of the screw  140  from the mould, the addition of further energy to the threaded portion will cause the threaded portion  160  of the screw  140  to further relax and expand radially whilst the head portion  150  will remain substantially of the same radius. Both threaded portion  160  and head portion  150  will remain substantially the same shape. This expansion moulding technique can be applied to other mould geometries. 
       Example 2 
       [0063]    A sample of prepared orientated SMP as described above in the form of a cylindrical rod  220  having a diameter of 3 mm was placed into a mould  230  having a cylindrical cavity that has substantially the same cavity radius as the radius of the orientated rod  220 . The orientated rod  220  was partially relaxed by heating the mould  230  and orientated rod  220  in a fan assisted oven at 80° C. for 5 minutes to produce a pre-relaxed cylindrical rod  240  that has substantially the same radius as the orientated cylindrical rod  220 . The pre-relaxed cylindrical rod  240  is subsequently immersed in water at 80° C. to allow further relaxation without mechanical constraints. The partially relaxed rod  240  was immersed in the water until the diameter of the rod  240  did not appreciably change resulting in a twice-relaxed cylindrical rod  250  having a radius greater than the orientated cylindrical rod  220  but less than isotropic cylindrical rod  210 . 
         [0064]    This process was repeated with further sample rods  220  with partial relaxing times from 0 to 120 minutes. It was found that the degree of recovery of the original diameter of the isotropic cylindrical rod  210  decreased with increased partial relaxing time as can be seen from table 1. It was concluded that by partially relaxing the oriented rods  220  into a suitable mould the degree of recovery of the initial shape, in this case length and diameter, of the isotropic cylindrical rod  210  can be varied from substantially 0% for long partial relaxing times where the orientated rod  220  is substantially fully relaxed to almost 100% for short partial relaxing times where the orientated rod  220  is practically not relaxed at all but still contains substantially all the energy imparted thereto by orientation. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Table 1: Effect of partial relaxation time at 80° C. 
               
               
                 on the degree of recovery of die drawn PLC rods. 
               
             
          
           
               
                 Pre-relaxation 
                 Diameter of 
                 Diameter of fully 
                   
               
               
                 time 
                 relaxed rod 
                 pre-relaxed rod 
                 Degree of recovery 
               
               
                 (min) 
                 (mm) 
                 (mm) 
                 (%) 
               
               
                   
               
             
          
           
               
                 0 
                 3.05 
                 5.20 
                 100 
               
               
                 5 
                 3.05 
                 5.20 
                 100 
               
               
                 15 
                 3.05 
                 5.20 
                 100 
               
               
                 20 
                 3.05 
                 4.35 
                 60 
               
               
                 30 
                 3.05 
                 3.65 
                 28 
               
               
                 60 
                 3.05 
                 3.17 
                 5 
               
               
                 120 
                 3.05 
                 3.09 
                 2 
               
               
                   
               
             
          
         
       
     
       Example 3 
       [0065]      FIG. 3  shows a graph plotting the expansion force in Newtons (N) against time for samples of prepared orientated SMP as described above in the form of cylindrical rods having a diameter of 3 mm. Some of the samples were partially relaxed in a two piece long metal mould having a cylindrical cavity 3 mm in diameter at 80° C. with a fan assisted oven for different periods of time and other samples were not relaxed at all. On further relaxing these cylindrical samples at 60° C., the expansion force along the radial direction of the cylindrical samples was measured for specimens having the same volume and partially relaxed for 0, 1 and 3 minutes. The graph shows that the orientated sample rods which had not been partially relaxed have a greater expansion force than the partially relaxed orientated sample rods. The expansion force of the sample rod was observed to decrease with an increase in time spent partially relaxing the sample rod. For example, 1 minute spent partially relaxing a sample rod in the mould significantly reduced the expansion force of the sample rod when further relaxed, whilst 3 minutes spent partially relaxing a sample rod in the mould reduced the expansion force yet further. It was concluded that the maximum expansion force rapidly decreases due to molecular re-arrangement of the oriented network. 
       Example 4 
       [0066]    A prepared orientated SMP as described above in the form of a cylindrical rod  420  having a diameter of 9 mm is placed in a cavity mould  430 . The cavity of the mould  430  has a 14 mm by 14 mm square cross-section. The mould  430  and orientated rod  420  are immersed in hot water at 80° C. for 2 minutes to allow the orientated rod  420  to relax to produce a pre-relaxed bar  440  shorter in length than the cylindrical rod  420  and with a square cross section having substantially the same dimensions as the square cross-section of the cavity of the mould  430 . The pre-relaxed bar  440  is clearly illustrated in  FIG. 4D . 
       Example 5 
       [0067]    A prepared orientated SMP as described above in the form of a cylindrical rod  520  having a diameter of 9 mm is placed in a cavity mould  530 . The cavity of the mould  530  has a 16 mm equilateral triangle cross-section. The mould  530  and orientated rod  520  are immersed in hot water at 80° C. for 2 minutes to allow the orientated rod  520  to relax to produce a pre-relaxed SMP  540  shorter in length than the cylindrical rod  520  and having a triangular cross section with substantially the same dimensions as the triangular cross-section of the cavity of the mould  530 . The pre-relaxed SMP  540  is clearly illustrated in  FIG. 5D . 
         [0068]    Where plasticisers are used instead of or in addition to energy, the plasticiser must come into direct contact with the SMP. This can be achieved, for example, by employing a porous or perforated type mould. On introduction of the porous mould containing the orientated SMP into an environment containing a plasticiser, the plasticiser will pass through the mould via the pores and directly contact the orientated SMP to stimulate molecular motion and thus relaxation of the orientated SMP. It will be appreciated that energy in the form of heat for example, may also be used in conjunction with the plasticiser to promote relaxation of the orientated SMP. For example, the plasticiser itself may be used to transfer energy to the SMP by heating the plasticiser. 
         [0069]    As various modifications could be made to the exemplary embodiments, as described above with reference to the corresponding illustrations, without departing from the scope of the invention, it is intended that all matter contained in the foregoing description and shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.