Stimuli-responsive liquid crystalline networks
Liquid crystalline network compositions comprising azo-containing aromatic epoxy units cross-linked with alkylene diacid units having alkylene segments containing at least one methylene unit, wherein the azo-containing aromatic epoxy units and alkylene diacid units are connected by ester linkages resulting from ring-opening esterification between the epoxy units and alkylene diacids, and wherein the azo-containing aromatic epoxy units and alkylene diacid units are in a molar ratio that results in the liquid crystalline network composition exhibiting a glass transition temperature (T g ) of at least 25° C. Methods for producing these compositions and their use in light- or thermal-activated physical deformation, shape memory applications, and self-healing, as well as their ability to be recycled and used in additive manufacturing processes are also described.
1. A liquid crystalline network composition comprising azo-containing aromatic epoxy units cross-linked with alkylene diacid units having alkylene segments containing at least one methylene unit, wherein the azo-containing aromatic epoxy units and alkylene diacid units are connected by ester linkages resulting from ring-opening esterification between the epoxy units and alkylene diacids, and wherein the azo-containing aromatic epoxy units and alkylene diacid units are in a molar ratio that results in the liquid crystalline network composition exhibiting a glass transition temperature (T g ) of at least 25° C.
2. The composition of claim 1 , wherein the liquid crystalline network composition possesses a thermal stability of the liquid crystalline phase (T lc ) of at least 50° C.
3. The composition of claim 1 , wherein said azo-containing aromatic epoxy units possess at least two phenyl rings connected by an azo (—N═N—) linking group, and each of said at least two phenyl rings is connected directly, or indirectly via a linker, to at least one epoxy group.
4. The composition of claim 1 , wherein said alkylene diacid units have alkylene segments containing at least three methylene units.
5. The composition of claim 1 , wherein said alkylene diacid units have alkylene segments containing at least four methylene units.
6. The composition of claim 1 , wherein said alkylene diacid units have alkylene segments containing at least five methylene units.
7. The composition of claim 1 , wherein said molar ratio is a molar ratio of azo-containing aromatic epoxy units to alkylene diacid units of 0.5:1 to 1.5:1.
8. The composition of claim 1 , wherein said T g is at least 30° C. and up to 70° C.
9. The composition of claim 1 , wherein said liquid crystalline network composition further comprises 1,5,7-triazabicyclo[4.4.0]dec-5-ene.
10. A method of producing a liquid crystalline network composition, the method comprising cross-linking azo-containing aromatic epoxy molecules with alkylene diacid molecules containing at least one methylene unit at a processing temperature of at least 120° C. to 250° C., wherein the azo-containing aromatic epoxy molecules and alkylene diacid molecules are in a molar ratio that results in the liquid crystalline network composition exhibiting a glass transition temperature (T g ) of at least 25° C., and said cross-linking comprises formation of ester linkages by ring-opening esterification between the epoxy units and alkylene diacids.
11. The method of claim 10 , wherein the liquid crystalline network composition possesses a thermal stability of the liquid crystalline phase (T lc ) of at least 50° C.
12. The method of claim 10 , wherein said azo-containing aromatic epoxy molecules possess at least two phenyl rings connected by an azo (—N═N—) linking group, and each of said at least two phenyl rings is connected directly, or indirectly via a linker, to at least one epoxy group.
13. The method of claim 10 , wherein said alkylene diacid molecules have alkylene segments containing at least three methylene units.
14. The method of claim 10 , wherein said alkylene diacid molecules have alkylene segments containing at least four methylene units.
15. The method of claim 10 , wherein said alkylene diacid molecules have alkylene segments containing at least five methylene units.
16. The method of claim 10 , wherein said molar ratio is a molar ratio of azo-containing aromatic epoxy molecules to alkylene diacid molecules of 0.5:1 to 1.5:1.
17. The method of claim 10 , wherein said T g is at least 30° C. and up to 70° C.
18. The method of claim 10 , wherein said azo-containing aromatic epoxy molecules and alkylene diacid molecules are cross-linked in the presence of 1,5,7-triazabicyclo[4.4.0]dec-5-ene as a ring-opening and transesterification catalyst, and the resulting liquid crystalline network composition further comprises 1,5,7-triazabicyclo[4.4.0]dec-5-ene.
19. A method of light-activated mechanical deformation of a liquid crystalline network composition, the method comprising:
providing a liquid crystalline network composition comprising azo-containing aromatic epoxy units cross-linked with alkylene diacid units having alkylene segments containing at least one methylene unit, wherein the azo-containing aromatic epoxy units and alkylene diacid units are connected by ester linkages resulting from ring-opening esterification between the epoxy units and alkylene diacids, and wherein the azo-containing aromatic epoxy units and alkylene diacid units are in a molar ratio that results in the liquid crystalline network composition exhibiting a glass transition temperature (T g ) of at least 25° C.; and
exposing said liquid crystalline network composition to electromagnetic radiation having a wavelength of 300-500 nm at an intensity sufficient to result in a bulk physical deformation of the liquid crystalline network, wherein said physical deformation occurs at least primarily by light-stimulated trans-cis-trans or trans-cis isomeric conversion of the azo groups without thermal decomposition of the liquid crystalline network composition.
20. The method of claim 19 , wherein said bulk physical deformation is reversible.
21. The method of claim 19 , wherein said bulk physical deformation comprises bending of the liquid crystalline network composition.
22. The method of claim 19 , wherein said electromagnetic radiation is polarized, and the polarized electromagnetic radiation selectively elicits a trans-cis-trans or trans-cis reversal in only those azo groups oriented in a direction where said trans-cis-trans or trans-cis reversal can be effected by the polarized electromagnetic radiation while other azo groups oriented in other directions do not undergo a trans-cis-trans or trans-cis reversal.
23. The method of claim 19 , wherein said electromagnetic radiation has a wavelength of 300-400 nm and is at an intensity sufficient to result in a thermally-induced liquid crystalline phase transition, wherein said thermally-induced liquid crystalline phase transition occurs as a result of absorption of said electromagnetic radiation by azo groups and subsequent release of thermal energy by said azo groups.
24. The method of claim 23 , wherein said thermally-induced liquid crystalline phase transition imparts a shape memory ability to the liquid crystalline network composition.
25. The method of claim 23 , wherein said liquid crystalline network composition contains a physical defect, and said electromagnetic radiation induces self-healing of the liquid crystalline network composition to the extent that said physical defect is no longer observable.
26. The method of claim 19 , wherein said azo-containing aromatic epoxy units possess at least two phenyl rings connected by an azo (—N═N—) linking group, and each of said at least two phenyl rings is connected directly, or indirectly via a linker, to at least one epoxy group.
27. The method of claim 19 , wherein said alkylene diacid units have alkylene segments containing at least three methylene units.
28. The method of claim 19 , wherein said alkylene diacid units have alkylene segments containing at least four methylene units.
29. The method of claim 19 , wherein said alkylene diacid units have alkylene segments containing at least five methylene units.
30. The method of claim 19 , wherein said liquid crystalline network composition further comprises 1,5,7-triazabicyclo[4.4.0]dec-5-ene.