Macroporous double-network cryogels: formation mechanism, enhanced mechanical strength and temperature/pH dual sensitivity

Macroporous double-network cryogels were synthesized using a sequential heterogeneous initiation cryotropic gelation method. A secondary polyacrylamide (PAAm) network was prepared in a poly(N-isopropylacrylamide) (PNIPA) single-network (SN) cryogel to form double networks. In order to prepare a dual sensitive cryogel, the resulting PNIPA/PAAm double-network (DN) cryogel was hydrolyzed under an alkaline condition. Morphology and pore structure of the cryogels were investigated using confocal laser scanning microscope (CLSM) observations, scanning electron microscope (SEM) observations, and mercury intrusion porosimetry (MIP) examinations. The results show that the secondary network is formed inside the pore walls as well as on the surface of the pore walls of the primary network. A formation mechanism for the secondary network is then proposed. The Young's modulus and the shear modulus of the cryogels were studied by uniaxial compression measurements and small amplitude shear experiments respectively. The mechanical properties of the DN cryogels are largely improved comparing with that of the SN cryogels and the reason is explained. Swelling ratio of the hydrolyzed double-network cryogel was measured in solutions of various pH values at various temperatures. The swelling ratio shows that the hydrolyzed double-network cryogel presented temperature/pH dual sensitivities.