Material systems and methods of manufacture for auxetic foams
A novel material for producing auxetic foams is disclosed. The material comprises a multiphase, multicomponent polymer foam with a filler polymer having a carefully selected glass transition temperature. Novel methods for producing auxetic foams from the material are also disclosed that consistently, reliably and quickly produce auxetic polyurethane foam at about room temperature (25° C.). This technology overcomes challenging issues in the large-scale production of auxetic PU foams, such as unfavorable heat-transmission problem and harmful organic solvents.
1. A method for producing an auxetic foam, comprising:
providing a flexible foam having an initial volume, the foam comprising a plurality of cells, and further comprising a soft domain having a first glass transition temperature, a hard domain having a thermal transition temperature greater than the first glass transition temperature, and a filler polymer having a second glass transition temperature greater than the first glass transition temperature and less than the thermal transition temperature, wherein the thermal transition temperature is a glass transition temperature;
determining a structural convertibility of the flexible foam at a plurality of temperatures for a plurality of times;
determining a relaxation time of the flexible foam at each of the plurality of temperatures by:
plotting the structural convertibility at the plurality of temperatures as a function of the plurality of times; and
fitting the plot of the structural convertibility at the plurality of temperatures as a function of the plurality of times using a Kohlrausch, Williams, and Watts (KWW) stretched exponential function to determine the relaxation time at each of the plurality of temperatures;
placing the foam into a pressure chamber and compressing the foam to a compressed volume less than the initial volume, such that the cells are deformed;
heating the compressed foam within the pressure chamber to a particular temperature for a predetermined amount of time,
wherein the particular temperature is greater than the second glass transition temperature and less than the thermal transition temperature, such that the shape of the filler polymer changes from generally spherical to generally ellipsoidal,
wherein the particular temperature is one of the plurality of temperatures at which the structural convertibility was determined;
after the predetermined amount of time, allowing the compressed foam to cool to a temperature less than the second glass transition temperature while remaining compressed in the pressure chamber; and
removing the foam from the pressure chamber;
wherein the predetermined amount of time is greater than or equal to the relaxation time at the particular temperature such that the generally ellipsoidal shape of the filler polymer is retained when the foam is cooled and removed from the pressure chamber, thereby fixing the cells in the deformed state to form the auxetic foam.
2. The method of claim 1 , wherein the filler polymer comprises styrene acrylonitrile copolymer.
3. The method of claim 1 , wherein the compressed volume of the foam ranges from about 15 percent to about 85 percent of the initial volume.
4. The method of claim 1 , wherein the predetermined amount of time ranges from about 1 minute to about 4 hours.
5. The method of claim 1 , wherein the predetermined amount of time is less than about 10 minutes.
6. The method of claim 1 , wherein the foam has a Poisson's ratio greater than zero prior to placement in the pressure chamber and a Poisson's ratio less than zero after removal from the pressure chamber.
7. The method of claim 1 , wherein varying a concentration of the filler polymer in the foam causes variation of mechanical properties of the foam.
8. The method of claim 1 , wherein the pressure chamber further comprises a mold, and the flexible foam conforms to a shape of the mold after removal of the foam from the pressure chamber.