FIELD

The invention relates broadly to footwear design and function and specialized materials for the manufacture of said footwear.

BACKGROUND

Consumers demand more comfort and functionality from their footwear, making these characteristics an important consideration in footwear design and evaluation. Both are the result of a complex interaction between the nature of the human body and in particular the leg and more specifically the feet, and the different elements of the footwear.

In term of comfort, fit is a primary determinant during the purchase of footwear. Misfits between the foot and the footwear impair foot function and can result in undue pressure on the foot including tightly fitting footwear or unwanted friction from loosely fitting footwear. For example, Crocs, an American footwear company, is providing casual footwear with soft, comfortable, lightweight and odor resistant qualities all around the world. However, such fitting is only limited at the contact interface between the underneath (or sole) of the foot and top of sole.

Another custom fitting shoe called Vibram fivefingers also provides a good wearing experience for users. This is partially because the raw material of these shoes has very high elasticity and is elastically deformable to fit any shape (elastic fitting as socks). However, some users do experience a lack of comfort because the elastic material can often apply inappropriate pressure to the foot. Furthermore, often the shoe upper is thin so that it is not able to provide enough protection to prevent foot injury.

Since every consumer has a unique foot shape/configuration and personal preference for shoes, finding a pair of shoes from the market that can comfortably fit an individual's feet becomes not always an easy matter, especially for those who need foot orthotics.

As a consequence, personalisation of footwear is in ever increasing demand. Presently, there are several approaches to achieve this, e.g., additive manufacturing, and using certain polymeric materials inside boots to make the sole portion adjustable. However, these methods still have a number of disadvantages.

For instance, prior art memory foam based insoles which use slow recovery polymeric foams (which are as soft as elastic sponge) cannot provide enough mechanical support. For instance, such memory foams are characterized with low stiffness and rigidity. Also, although customized shoes might be fabricated via 3D printing for better fitting this requires the tedious process of foot scanning and the even more expensive fabrication process of printing.

A need exists to provide comfort, functionality and protection of the whole foot for a wearer on an individual or personalised basis while at the same time maintaining cost effective manufacturing processes. The present invention seeks to improve upon personalised footwear products which currently exist.

SUMMARY

The invention is predicated on the discovery that the function of footwear, in particular, stiffness and flexibility (related to 3D contouring) which is a personal preference, can be improved greatly with the use of certain shape memory materials (SMM), which are characterized with shape memory effect (SME).

Accordingly, in an aspect, the invention provides a moldable shoe or shoe insert, said moldable shoe or shoe insert extending across an entire sole of the foot when in use and is prepared from a stimuli-responsive shape memory material.

In an embodiment the stimuli-responsive shape memory material is a thermo-responsive shape memory material.

In an embodiment, the SMM is a shape memory polymer (SMP). The present inventors have found that the use of SMPs in shoe fitting is repeatable and instant; being able to recover to original shape if needed; providing a tailored combination of stiffness and flexibility;

distributing localized foot pressure; and is cost effective to manufacture.

In another embodiment the thermo-responsiveness shape memory polymer retains two shapes. In an embodiment, and during use, the moldable shoe or shoe insert is initially heated to between about 45° to at or below about 80°C wherein the user subsequently inserts his/her foot into the shoe or shoe insert to mold the shoe or shoe insert around the contours of the users foot. The moldable shoe or shoe insert may be heated to high temperatures such as at about 80°C, but will be typically put on at around 60°C or below unless socks or an inner liner is worn on bare foot.

As used herein the term "shoe" or "shoe insert" means that the product which is the subject of the invention extends across the entire sole of the foot and may include a complete shoe product with or without the requirement of any additional materials, such as a hardened non- moldable polymeric sole material. As such in certain embodiments the invention provides the advantage of a complete shoe product which does not require any additional manufacturing steps, such as outer material stitching or outer sole adhesion. As an alternative, the term also encompasses a shoe insert, which while also extending across the entire sole of the foot, may be included into, for instance, a preformed shoe shape such as a hardened outer shoe shape (e.g., for construction workers) or a personalized shoe insert for a ski boot.

In some embodiments, "shoe" or "shoe insert" can also mean that the product which is the subject of the invention envelopes an entire foot and may include a complete shoe product with or without the requirement of any additional materials, such as a hardened non-moldable polymeric sole material.

It will be appreciated then, that the phrase "entire sole of the foot" includes the foresole, the midsole, and the hindsole. It will also be appreciated that "entire foot" includes the forefoot, the midfoot, and the heel. In order to provide the stiffness and flexibility which is required for user comfort the present invention contemplates that surface coverage of the shoe or shoe insert extends at least to cover the start of the foot talus (or ankle joint), which may or may not cover the actual ankle joint. This should then be contrasted with known shoe or shoe insert products which cover, for instance, only the foot sole or partially covers the hind foot and forefoot but leaves all or a part of the upper mid foot exposed and/or unsupported.

A combination of the right stiffness and flexibility is offered by means of tailoring the composition of the SMM, processing method/parameters, and/or porosity ratio following many standard polymer/polymer foam synthesis/processing methods. In an embodiment the SMM is a SMP selected from ether-vinyl acetate (EVA), polyurethane (PU) or thermoplastic polyurethane (TPU), or a combination of them.

Based on the principle of shape memory effect (SME) which is the cornerstone of the present invention shape memory polymers (SMP) (including their composites/hybrids and in either solid or foam configurations), includes many polymeric materials and their composites/hybrid with a transition temperature (either the glass transition or melting/crystallization) around between about 45°C to at or below 80°C, such as EVA or PU foam, PU, TPU or PU/TPU mixture, etc., can be used in this application. It will however be appreciated that while the moldable shoe or shoe insert is initially heated to up to about 80°C (and above about 45 °C) the temperature of the surface at the time of bare foot insertion would be at about 60°C or below which is a comfortable temperature for the end user.

BRIEF DESCRIPTION OF FIGURES

Figure 1 - Description of the first embodiment. (1) Human foot; (2) shape memory polymer material; (3) normal shoe.

Figure 2 - Description of the second embodiment. (1) Human foot; (2) shape memory polymer material; (3) normal shoe.

Figure 3 - Description of the third embodiment. (1) Human foot; (2) shape memory polymeric material.

Figure 4 - Description of the fourth embodiment (1) Human foot; (2) shape memory polymer material; (4) outersole.

Figure 5 - Basic concept of comfort fitting shoes in accordance with one embodiment (I) and proof-of-concept of this embodiment (II).

Figure 6 -Cross-section of EVA foam sheet (a) and zoom-in-view under SEM (b).

Figure 7 - DSC curve of EVA foam. Inset: zoom-in view of the glass transition range upon heating.

Figure 8 - Dimensions of the sample for uniaxial tensile test (unit: mm).

Figure 9 - Illustration of a full SME cycle.

Figure 10 - Typical stress vs. strain relationships of uniaxial tension to a maximum strain of 30% at three different temperatures followed by cooling back to room temperature and then unloading. Figure 11 - Typical stress vs. strain relationships in uniaxial tension to a maximum strain of 80% at three different temperatures followed by cooling back to room temperature and then unloading.

Figure 12 - Typical stress vs. strain relationships of cyclic stretching at room temperature in samples with/without pre-stretching.

Figure 13 - Stress vs. strain relationships in uniaxial compression to a maximum strain of 30% at three different temperatures followed by cooling back to room temperature and then unloading.

Figure 14 - Stress vs. strain relationships in uniaxial compression to a maximum strain of 80% at three different temperatures followed by cooling back to room temperature and then unloading.

Figure 15 - Stress vs. strain relationships obtained by cyclic compression tests at room temperature in samples with/without pre-compression.

Figure 16 - Shape-fixity ratio as a function of programming temperature.

Figure 17 - Shape-recovery ratio as a function of programming temperature.

Figure 18 - Shape recovery of EVA foam after clamping at room temperature for different periods of time.

Figure 19 - Stress vs. strain relationships of EVA foam upon compressing to 0.329

MPa/0.1645 MPa and then holding the applied compression stress for 24 hours before unloading (a) and the corresponding strain/stress vs. time relationships (b).

Figure 20 - Evolution of recovery ratios in samples with different compression stresses of 0.329 MPa and 0.1645 MPa, respectively.

Figure 21- Illustration of an embodiment of the present invention (fifth embodiment).

Figure 22- Illustration of an embodiment of the present invention (sixth embodiment).

Figure 23- Illustration of an embodiment of the present invention (seventh embodiment). Figure 24- Illustration of an embodiment of the present invention (eighth embodiment).

Figure 25- Illustration of a sole of an embodiment of the present invention.

Figure 26- Illustration of a method embodiment of the present invention.

DESCRIPTION OF THE INVENTION

The fundamental principle underlying the present invention is the use of shape memory material (SMM) which are characterised with shape memory effect (SME). The shape memory effect (SME) is often described as a shape switching phenomenon whereby shape memory materials (SMMs) are able to recover their original shape with the presence of the right stimulus, such as heat (thermo-responsive), light (photo-responsive), chemical (including water, chemo-responsive), magnetic field (magneto-responsive), mechanical loading (mechano-responsive) etc. This is to be contrasted with memory foam which provides instant deforming but slowly recovers to its original shape and hence does not have the capability to maintain the temporary shape, i.e., no SME. Polymers of the present invention exhibiting a shape-memory effect have both a visible, current (temporary) form and a stored (original or permanent) form. Once the polymer has been manufactured by conventional methods, the material is changed into another, temporary form by processing through, such as heating, deformation, and finally, cooling. The polymer maintains this temporary shape until the shape change into the permanent form is activated by a predetermined external stimulus, in the present case by heating. The material, again with heating, can be transformed to its original (permanent) shape ready again for processing into another temporary form.

Based on this principle, shape memory polymer material can be easily deformed to a temporary shape within an appropriate temperature range (about 45 °C to at or below about 80°C). After cooling back, the temporary shape is largely retained, while it is still flexible and stiffness enough to provide support. In a preferred embodiment the flexibility is such that the material is easily deformable via stretching/bending by hand or impression by finger and offers good elasticity to return back at the same time. Measurement of the Young's modulus can be used for measuring stiffness. The range of Young's modulus for this application is preferably from 0.001 to 0.5 GPa, such as 0.005, 0.01, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, or 0.45 GPa or a range between any two of these figures. When needed, such a material is able to recover its original shape (permanent shape) only upon heating again for another round of refitting. Because the shape memory polymeric materials (including their composites and hybrids) can offer a required combination of stiffness and flexibility, they can be used perfectly in such comfort fitting shoes.

In order to work for the purpose of comfort fitting shoes as mentioned above, among others, the basic requirements of a polymeric foam are: 1) flexible/elastic at both low and high temperatures. Elasticity also can be measured by Young's modulus, the preferable range is from 0.001 to 0.5 GPa, such as 0.005, 0.01, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, or 0.45 GPa or a range between any two of these figures; 2) able to maintain the temporary shape (shape fixity ratio >40% such as >42%, >44%, >46%, >48%, >50%, >52%, >54%, >56%, >58%, >60%, >62%, >64%, >66%, >68%, >70%, >72%, >74%, >76%, >78%, or >80%); 3) good shape recovery capability (shape recovery ratio >40% such as >42%, >44%, >46%, >48%, >50%, >52%, >54%, >56%, >58%, >60%, >62%, >64%, >66%, >68%, >70%, >72%, >74%, >76%, >78%, or >80%). Shape fixity ratio and shape recovery ratio can be used to quantify this capability, (see Figures 16 and 17, Eqns 1-3); and 4) heating temperature for activation, in particular during wearing (programming), should be only slightly above body temperature. A temperature not exceeding 60°C is still an acceptable temperature, as the human body can withstand such temperatures for a short period of time, say a few seconds even for bare foot. Also for the polymeric materials of the invention , the activation

temperature is normally within a range of T _g (glass transition) or T _m (melting) +10 tol5°C.

The invention contemplates eight possible embodiments of such moldable shoes or shoe inserts:

In the first embodiment (Figure 1), the shoes can be designed to be very thin (from about 1 to 3 mm) and lightweight and can be easily packed and stored with minimal storage space. In order to attain the perfect fit, the shoe is first heated to around 50°C using warm water, an oven, a heater or a hot blower (such as a hair dryer) making the shoe moldable (or subjecting the shoe to other types of stimulus depending on the type of material used), then the user inserts his/her foot (1) into the shoe or shoe insert (2) which will deform to accommodate the shape of user's foot. As shown in this figure the moldable shoe or shoe insert covers the entire surface of the foot, right up to the talus of the user, providing stability for the entire foot. After cooling down, the deformed shape is retained with proper stiffness and flexibility. The user obtains a comfort fitting shoe with an internal shape profile custom molded to the shape of his/her foot. Unlike, for instance, Crocs shoes, there is no extra gap between foot and shoe making it more comfortable and thus decreasing injury risk caused by foot slip. There is no extra inner space and improper pressure between foot and shoe, thus the potential risk of injury can be minimized. In addition, the shoe can be made very thin and, if required for instance, walking on rough ground, allows the user to further protect their feet by using the product as a shoe insert to a normal shoe (3) (i.e., removable inner lining, in order to eliminate possible discomfort resulting from the rough ground (e.g., thick rocks)) or hard/stiffer shoes. In this way, the comfort fitting can be preserved although the user is wearing a normal shoe. When subjected to a second-time heating, such moldable shoe or shoe insert can deform back to its original shape. With such a material the fitting processes are repeatable and instant whereby comfort fitting can be easily achieved.

In the second embodiment (Figure 2), the moldable shoe insert (2) is pre-fixed to the inner surface of a normal shoe (3), playing the role of a non-removable inner lining of a normal shoe. This can be achieved through the use of a known adhesive product used in shoe manufacture. All the shaping processes are the same as those mentioned for the first embodiment. Excellent fitting performance still can be achieved in such an embodiment.

In the third embodiment (Figure 3), the moldable shoe made of shape memory polymeric material is thicker from about 2 to 15 mm than the shoe in the first embodiment in order to give better protection for the user.

In the fourth embodiment (Figure 4), a thick outer sole (4) may be added underneath the moldable shoe (2). With this additional bottom layer, this shoe is able to deal with rougher ground conditions without compromising on comfort fitting performance. The bottom layer or outer sole material could be made of a much harder and wear resistant material with/without the shape memory effect. Multiple layers can also be incorporated in selected areas on the surface with impact absorbing material to cater for sports activities such as jogging. In still a further embodiment, thermoplastic polyurethane (TPU), dissolved in tetrahydrofuran (THF), can be brushed on both inner and outer layers of such shoes in order to provide certain degree of breathability and prevent the shoes (in particular those made of foam) from smelling foul based on perspiration. Ventilation holes/slots at strategic areas can also be incorporated to further minimise fouling.

In the fifth embodiment (Figure 21), there is shown a shoe 100 that is produced by 3D moulding. The whole shoe is made of the same material, 104 indicates cutting/holes or other means of weakened parts (e.g., indentations); 102 indicates much thicker portions to provide better support.

In the sixth embodiment (Figure 22), there is shown an insole 120, which includes a plurality of indentations 122. The plurality of indentations 122 can be through/non-through holes and even slots/grooves. The plurality of indentations 122 are configured for enabling deformation of the insole 120 when a sock 124 is placed on the insole 120. It should be noted that the insole 120 can also be made from a non-uniform foam layer for enhanced fit and comfort. 124 may be pre-bonded to 120, the insole. Upon heating insole, 120, it becomes soft and thus, one can easily wear the sock-shoe. After cooling back the insole becomes hard as a shoe.

In the seventh embodiment (Figure 23), there is shown a free-size shoe 150 that is folded to form a shoe. The shoe 150 is formed by joining a first flap 152 to a second flap 154 (or vice versa) using at least one fastener 160 (such as Velcro ^® ) to form a front portion of the shoe 150. Rear fasteners 158 are also configured to be joined to each other, to form a heel counter of the shoe 150. It should be noted that a foam layer 156 of the shoe 150 is non-uniform for enhanced fit and comfort. The fasteners 158 can be of a hook-and-loop type or any other form of robust temporary fasteners. As with the other embodiments heating is required to soften the shoes first.

In the eight embodiment (Figure 24), there is shown another free-size shoe 180 that is folded into a form of a shoe. The shoe 180 is formed by joining a third flap 182 to a fourth flap 188 (or vice versa) using at least one fastener 190 to form a front portion of the shoe 180. The shoe 180 does not include a heel counter but includes a heal guard 186. It should be noted that a foam layer 184 of the shoe 150 is non-uniform for enhanced fit and comfort, and includes through/non-through holes and even slots/grooves. As with the other embodiments heating is required to soften the shoes first.

Referring to Figure 25, there is shown a sole portion 200 of the aforementioned embodiments. The sole portion 200 can deform, whereby deformation is substantially at central part 204 which includes through/non-through holes and even slots/grooves. In addition, forefoot portion 202 and heel portion 206 are made from different materials (with/without shape memory effect) for grip and comfort.

In another aspect, with reference to Figure 26, there is provided a method 300 for forming a shoe. The shoe is prepared from a stimuli-responsive (thermo-responsive) shape memory material layer, with the method 300 comprising heating the layer to a pre-determined temperature (302). The pre-determined temperature is between 45°C to 80°C. In addition, the method 300 includes deforming the layer (304), where the deformation of said layer is carried out by placement of a foot structure onto the layer. The foot structure can be from a human or can be a foot mold. The deformation of said layer can include deformation of a plurality of indentations within said layer, the plurality of indentations including through/non-through holes and even slots/grooves. Finally, the method 300 includes manipulating the layer using at least one fastener to form the shoe (306). It should be appreciated that the manipulation of the layer is by folding.

INDUSTRIAL APPLICABILTY

The moldable shoe or shoe insert according to the present invention has also been demonstrated to be of great potential for sports and medical applications. Following is a non- exhaustive list of various potential applications:

School shoes; Dress shoes; Beach shoes; Diabetic shoes; Temporary shoes for fracture patient; Inner shells of ski boots; Quick personalized shoes for rent on skating rink; Diving fins; Bicycling shoes directly fixed on the pedal of bicycles; and Walking shoes for people whose feet shapes are abnormal.

Besides for feet the present inventive concept may be extended for use in supporting elbow, knee and even bottom etc., to provide not only a comfortable fit but also protection.

EXAMPLES

Figure 5(1) illustrates an embodiment of the moldable shoe or shoe insert of the present invention. After heating to slightly above body temperature (e.g., 45°C), the shoe becomes soft and highly elastic. Therefore the user can easily wear it in the same way as he/she would wear elastic socks with perfect fitting. After cooling back to body temperature, the material becomes slightly harder, but still elastic enough to walk comfortably around. Every time refitting is required due to, for instance, slight shape difference in the user's feet, for instance, between early morning and later afternoon, the shoe can be reheated to 45°C for reuse/refitting.

Figure 5 (II) is also an embodiment of the aforementioned moldable shoe or shoe insert. In figure 5(IIa), the top piece of sock is modified by coating it with a layer of low flow index thermo-plastic polyurethane, while the bottom piece is the original sock for comparison. Low flow index, for instance, around 3 g/10 min to 20 g/10 min, is to ensure the material will only "flow" when stressed rather than by gravitation force. With the thin layer coated on, unlike normal socks, such socks can maintain the feet shape after deformed at programming temperature instead of shrinking back to original size. After heating to about 60°C, the thermoplastic polyurethane becomes soft, along with the modified sock. When the modified sock is cooled to slightly above body temperature, the thermo-plastic polyurethane can still be molded. Therefore, the modified sock can be conveniently worn as a moldable shoe or shoe insert. After a few minutes, the thermo-plastic polyurethane becomes fully crystallized, and hence the sock becomes slightly harder and thus is less elastic as the original sock (the range of hardness should be from around 0.001 to 0.5 GPa), but is still flexible enough for the user to walk around on (Figure 5 (lib)). The sock is able to maintain the new shape even after being taken- off (Figure 5IIc). Only after heating to soften the thermos-plastic polyurethane, the sock returns its original shape and subsequently, it is ready to be reused again.

The invention also contemplates the use of composite materials such as EVA/TPU mixture, EVA/PCL (polycaprolactone) mixture, silicone/TPU hybrid, silicone/PCL hybrid,

silicone/melting glue. Glass/carbon fiber material may be used for reinforcement.

While some shape memory material according to some embodiments may be heated to 45 °C to 80°C, for some other materials such as PCL based polymers, one can wear it even when it is cooled to room temperature, since such a material takes very long to become hard even at room temperature.

Materials such as PCL and TPU, have a higher melting temperature (over 60°C), but it takes up to 10 minutes to fully crystallize at or below body temperature. So that shape memory polymeric materials made of them can be heated to their melting temperature and then "wear" at room temperature.

Both foam and solid polymeric materials may be used.

Since such moldable shoes or shoe inserts do not have a particular size and there is also no distinction between right-side or left-side, from a manufacturing and logistics points of view, the capital investment and effort in fabrication and storage can be significantly reduced. On the other hand, from a prospective customers' point of view, instead of trying to find the right size of shoes, now each piece of shoe is guaranteed to fit any individual foot.

In Figure 5 (II), the sock serves as the elastic component and the thermo-plastic polyurethane functions as the transition component. The process to fix the temporary shape is traditionally called programming, while the process of heating to return the original shape is called shape recovery.

Material, thermal analysis and sample preparation

The material investigated in this study is a commercial EVA foam sheet about 5.6 mm in thickness with a porosity ratio around 15%. Figure 6 reveals the cross-section of this foam sheet and a zoom-in-view under scanning electron microscope (SEM). The samples for thermo-mechanical tests were cut out of the EVA sheet.

Differential scanning calorimeter (DSC) test was conducted using a TA Instruments (New Castle, DE, USA) Q200 DSC between 0°C and 100°C at a heating/cooling rate of 5°C/min (under nitrogen atmosphere). As revealed in Figure 7, there are two transitions in this EVA. The glass transition occurs at around 55°C, while melting and crystallization are at 80°C and 65 °C upon heating and cooling, respectively. In applications, such as comfort moldable shoes or shoe inserts, the glass transition between about 50°C and 60°C (inset of Figure 7) is advantageous because such a temperature range between 50°C to 60°C is suitable for human body. Any temperature higher than 60°C may make user feel too hot to wear (so that cannot last very long).

Following ASTM D638 standard (type IV), dumbbell-shaped samples (as shown in Figure 8) and small rectangular samples (25x20 mm) were cut out of the EVA foam for uniaxial tensile test and compression test, respectively. Unless otherwise stated, the stress and strain used in this study are meant for engineering stress and engineering strain, respectively. Engineering strain/stress is more relevant to engineering application, rather than fundamentals (for theoretical study and simulation etc.).

Experimental and results In order to determine if a material can be used for a moldable shoe or shoe insert, uniaxial tension and uniaxial compression at different programming temperatures were conducted. Furthermore, room temperature cyclic tests were carried out to reveal whether a material still has excellent elasticity for high comfort with/without programming.

Uniaxial tensile test

An Intron (Norwood, MA, USA) 5565 testing system with an integrated temperature- controllable chamber was used for uniaxial tensile tests. A constant strain rate of 10 ^~3 /s was applied in both loading and unloading in all tests.

A typical thermo-responsive SME cycle applied in this study includes two processes, namely programming and recovery, with four major steps (a-d) as shown in Figure 9.

In step (a), after being stretched to a prescribed maximum strain (e _m ) at a given testing (programming) temperature, which is within the glass transition temperature range in the study, the sample is cooled back to room temperature (about 22°C) with the maximum strain maintained and subsequently unloaded (step b). The resulted residual strain is denoted by ej. This is the first process of programming.

In the next process of recovery, after the applied constraint is removed, the sample may recover slightly at room temperature due to creeping (step c), and thus the residual strain is reduced to ¾· Finally, the sample is heated to slightly above (less than 5°C) the previous programming temperature for five minutes (step d), and the remaining strain is denoted as ε ₃ . Note that remarkable creep in this EVA foam was only observed in samples deformed at room temperature to high strains. Thus, unless programming was carried out at room temperature, for other programming temperatures, ¾≡ £i, i-e-, step c can be ignored.

Figure 10 shows three typical stress vs. strain relationships of EVA samples, which were pre- stretched to a maximum programming strain of 30% at three different temperatures, namely 50°C, 55°C and 60°C. It can be seen that the residual strain of the sample pre-stretched at the lowest temperature (50°C, dashed line) is the lowest (22.6%). The largest residual strain of about 27.4% is found in the sample pre-stretched at the highest temperature of 60°C (grey line). Because the glass transition temperature of this material is between 50°C to 60°C, this experiment demonstrates the shape fixity ratio of such material in the temperature range discussed above.

Referring to Figure 9. The instant shape fixity ratio (R' _j ) and the long-term shape fixity ratio (R ^l f) may be defined as,

R _f ^l =—

and the shape recovery ratio (R _r ) may be defined as,

In the subsequent recovery process, the samples were heated to less than 5°C above their respective pre-stretching temperatures for five minutes. It was found that all samples were able to fully recover their original shape.

Figure 11 presents three typical stress vs. strain curves upon stretching to a maximum strain of 80% at 50°C, 55°C and 60°C, respectively. Same trend as revealed in Figure 10 is observed, but the residual strains are much higher (around 70%). Apart from small deformation (30%), the situation of large deformation (80%) should also be considered because users may also go through large deformation on shoes in this application. Thus the shape fixity ratio and shape recovery ratio after large deformation should also be investigated.

After heating to less than 5°C above their respective pre-stretching temperatures for five minutes, all samples were able to almost fully recover their original shape.

The shape fixity ratio and shape recovery ratio of the samples in both uniaxial tension and uniaxial compression (mentioned below) in one single SME cycle are discussed in detail below.

Figure 12 shows the stress vs. strain relationships in cyclic uniaxial stretching at room temperature in the samples with/without pre-stretching. Pre-stretching was conducted at 60°C to 30% maximum strain or 80% maximum strain. Note that for simplicity, herein, the calculation of engineering strain is based on the gauge length in each individual test. Five cycles to maximum programming strains of 10%, 20%, 30%, 40% and 50% (in an increment order) were carried out. In the last cycle of all samples, there was a five minutes of holding period before unloading.

The stress vs. strain curves of the samples with 30% and 80% pre-stretching show that the residual strain after unloading in each cycle is almost the same as that of the sample without pre-stretching. On the other hand, it is observed that the difference between 30% pre-stretched sample and original sample is small. There were some noticeable residual strain after unloading in each cycle. Furthermore, with the increase of the maximum strain in loading, the corresponding residual strain increases. However, it was observed that 10 minutes after unloading, the residual strain could be largely removed. Therefore, it is conlcuded that at room temperature, the foams with/without pre-stretching may be considered as largely elastic with limited elastic-viscousity.

It should be pointed out that as expected, while the stress vs. strain curve of the sample with 30% pre-stretching is only slightly higher than that of the sample without pre-stretching, the sample with 80% pre-stretching appears to be much harder. In addition, a larger hysteresis in the sample with higher pre-stretching indicates a higher energy dissipation in a

loading/unloading cycle. It appears that the influence of pre-tensile strain (at least up to 30%) does not have significant effect on the mechanical response of the foam.

Uniaxial compression test

Rectangular-shaped samples were used for a series of single and cyclic uniaxial compression tests. Same testing machine and parameters as mentioned in above mentioned uniaxial tensile tests were used. Three samples were compressed by 30% at three different temperatures, namely 50°C, 55°C and 60°C, and then held for cooling back to room temperature and finally unloaded. Figure 13 presents the stress vs. strain relationships of these three samples in the programming process. As can be seen, as with uniaxial tension, the sample tested at the highest temperature of 60°C has the largest residual strain of about 30%, while the sample tested at the lowest temperature of 50°C has the least residual strain of about 25%. Subsequently, these three samples were heated to slightly above their respective programming temperatures for five minutes for heating-induced shape recovery. The resulted remaining strains in all of them were observed to be very small. 80% pre-compression tests were also carried out. Their stress vs. strain relationships are plotted in Figure 14. In general, the residual strains are around to be 75% and follows the same tendency as reported above, i.e., a higher programming temperature results in a larger residual strain. After programming by means of compression to 80%, as before, the samples were heated to slightly above their resepective programming temperature for five minutes. After that, the thickness of all samples was measured. The remaining strain in all samples was found to be around 40%.

Figure 15 presents the stress vs. strain relationships in three cyclic compression tests at room temperature in samples with/without pre-compression. As before, pre-compression with a maximum programming strain of 30% or 80% was produced at 60°C. Three maximum programming strains of 15%, 30% and 45% (in increment order) were applied in all samples in cycling. No remarkable residual strain was observed in all samples at the end of each cycle, which indicates excellent elastic response in both pre-compressed samples and original sample.

Unlike that in uniaxial tension in Figure 12, Figure 15 reveals that while the stress vs. strain curve of 30% pre-compressed sample is very close to that of the sample without pre- compression (same as that in uniaxial tension), 80% pre-compressed sample is apparently much stiffer after being compressed to above 20% strain.

Shape-fixity ratio and shape-recovery ratio

While the shape-fixity ratio is a measure of how a piece of comfort fitting shoe can fit the profile of a particular foot, the shape -recovery ratio reveals the capability of a piece of comfort fitting shoe to recover its original size for the next round of comfort fitting. The shape-fixity ratios as a function of programming temperature in both uniaxial tension and uniaxial compression to two different strains of 30% and 80% are plotted in Figure 16. As can be seen, in all tests, the shape-fixity ratio is over 75%. Generally,

- A higher programming temperature always results in a higher shape-fixity ratio, whereby the higher shape-fixity ratio is desirable for the shoes to maintain the temporary shape to ensure more comfort. Without consideration of elastic deformation, the higher the shape fixity ratio, the better the shoe can maintain its deformed shape. The perfect ratio is 100% which means the material can hold the shape exactly the same as the user's feet shape. In reality, any ratio higher than 75% can be considered good for this application. However, a high temperature, e.g., over 60°C, may be unbearable for many people.

- The shape-fixity ratio in compression is normally higher than that in tension;

- A higher maximum programming strain is more effective to increase the shape fixity ratio, but this is not applicable at higher programming temperatures.

Figure 17 reveals the shape -recovery ratio as a function of programming temperature in both uniaxial tension and uniaxial compression to 30% and 80% maximum programming strains. It is clear that while poor shape recovery (only between 40% to 55%) is observed in all samples programmed via compressing to a maximum programming strain of 80%, all rest samples have a very high shape-recovery ratio. In particular, the shape-recovery ratio in all 30% stretched samples is 100%. Hence, it may be concluded that,

- The shape-recovery ratio is more or less programming temperature independent;

- A higher shape-recovery ratio is resulted in samples with a lower programming strain;

- Shape recovery is bad only in samples programmed by over-compression.

Influence of long-term compression

In shoes, body weight may typically be applied continuously over hours during use. As shown in Figure 18(a), a piece of EVA foam was compressed at room temperature using two clips (Figure 18b). After 80 minutes, one clip was removed (Figurel8cl), while the other clip was applied for 115 hours (Figure 18dl). The indent made by 80 hours of clamping mostly recovered after 40 hours (Figure 18c2), while the indent made by 115 hours of clamping was still visible even after 23 days (Figure 18d4), which only disappears after heating in boiling water (Figure 18e). In order for accurate charactrization, we took a relatively extreme senerio for investigation, in which a small piece of EVA foam was firstly compressed to a maximum stress of 0.329 MPa, which is supposed to be about the maximum foot pressure of normal young adults, and then this pressure was maintained for 24 hours, before being removed.

Figure 19 (a) (black line) plots the stress vs. strain relationship of the sample during the whole test. As we can see, a compression strain of around 64% is recorded upon loading to 0.329 MPa. In the subsequent 24 hours, the compression strain gradually increases to 80%. After unloading, the residual strain is 74%. For comparsion, in another test, the applied maximum compression stress was reduced by half to 0.1645 MPa. The resulted stress vs. strain curve is plotted in grey color in Figure 19(a) As can be seen, although the applied stress is halved, more creeping induced strain (about 10% more) is observed, while more strain recovery (about 4%) is found after unloading. The evolution of the strain and stress in the whole loading/unloading process is plotted against time in Figure 19 (b). It appears that within the loading holding period the strain increase gradually becomes less and less. After about 15 hours (for 0.329 MPa) or 18.5 hours (for 0.1645 MPa) virtually there is no more strain increase. As expected, a higher applied stress requires less time for the stabilization of creeping strain. In the next step, both samples were left in air at room temperature for 120 hours with their thinkess recorded in every 24 hours. Finally, the samples were heated to 60°C for 10 minutes. The corresponding shape recovery ratios are calculated and plotted in Figure 20 As we can see, the shape recovery ratio vs. time curves of both samples are about the same. The speed of shape recoevry gradually decreases over time. After 120 hours in air at room temperature, around 85% recovery is oberved in both of them. Further heating to 60°C for 10 minutes results in full shape recovery in 0.1645 MPa.

Hence, it should be reasonable to conclude that this EVA foam is suitable for reasonable longtime wearing. After long-time wearing at room temperature, its excellent heating-responsive SME is not comporised.

According to Figures 10, 11 and 13, 14, at high temperatures this EVA foam is soft and can be stretched or compressed by 30% or more. Thus, shoes made of this foam should be easy to wear, while comfort fitting is also ensured. As reported in Figure 16, the corresponding shape fixity ratios in both uniaxial tensile and compression are high, so that the foam is able to largely maintian the programmed shape. Consequently, the temporary shape of the shoes made of this foam is able to largely keep the personallized shape after "programming". Even after being programmed to 80% strain in either uniaxial tension or uniaxial compression, the foam is still highly elastic at room temperature, as evidenced by Figure 12 and Figure 15, which indicate that the personallize shoes are still elastic even being stretched to 50% or compressed by 45%. Therefore, a prepared personallized shoe would not only be easy to takeoff and wear, but also cormortable to wear. High elasticity at room temperature also means that even after programming the shoes are able to mostly keep the personalized shape in the cases of short to medium term of loading. As for long time loading, creeping does happen in this foam (Figure 18 and Figure 19), but most of the deformation due to creeping can be recovered autamatically even without heating (Figure 20). Heating to up to 60°C may be applied to induce almost full shape recovery.

Excellent heating induced shape recovery is observed in Figure 17, except in the 80% compressed foam. The possible reason behind the difficulty for shape recovery should be that the foam is over compressed at high temperatures. A possible way to eliminate this problem is to reduce the deformation in the EVA foam. Theoretically, for the same compression load, with the increase in the stiffess of a material, the corressponding deformation decreases accordingly. As for this EVA foam, its stiffness can be easily increased by means of reducing its porosity ratio.

According to Figure 16, the best result of shape fixity ratio in all tests is always obtained at a programming temperature of 60°C, which is 15°C above the comfortable temperature. Thus, the glass transition temperature of this EVA should be lowered down slightly.

The results of a series of experiments on a EVA foam reveal that this foam is able to meet most of the requirements for comfort fitting, in particular for moldable shoes. It is highly elastic at both high and low temperatures, so that it can be easily programmed for nice fitting and use.

After programming, it can largely maintain the customized shape. Unless being over- compressed at high temperatures, it normally has excellent SME for shape recovery and subsequent reuse.