This study presents the preparation
and characterization of double
networks (DN) based on a first amphiphilic polymethacrylate conetwork
(APCN) and a second polyacrylamide network. The APCN first network
comprised interconnected “in–out” star copolymers
of 2-(dimethylamino)ethyl methacrylate (DMAEMA, hydrophilic
ionizable monomer) and 2-ethylhexyl methacrylate (EHMA, hydrophobic
comonomer) or lauryl methacrylate (LauMA, second hydrophobic comonomer),
synthesized using group transfer polymerization, following one-pot,
sequential, monomer, and hydrophobic cross-linker (ethylene glycol
dimethacrylate, EGDMA) additions. The second network was prepared
by the aqueous photopolymerization of acrylamide (AAm) at two different
concentrations, 2 and 5 M, and N,N′-methylenebis(acrylamide) cross-linker in the presence
of the fully ionized (via HCl addition) APCN. After synthesis, all
DNs and single (first and second) (co)networks, equilibrium-swollen
in water, were characterized in terms of their mechanical properties
in compression. The DNs exhibited improved mechanical properties (stress
and strain at break, and elastic modulus) compared to the corresponding
single networks. Better reinforcement was achieved in the DNs whose
APCN first networks bore a lower hydrophobic content and whose hydrophobic
monomer was EHMA rather than LauMA. The best DN exhibited stress at
break above 8 MPa and strain at break nearly 80%, close to the values
of the best DNs in the literature. Nanoindentation studies were also
performed on selected DNs which proved again the enhanced mechanical
properties of the present DNs, manifested as high resistance to penetration
and low creep displacement. Small-angle X-ray scattering (SAXS) indicated
a broad correlation peak for all APCN first networks, suggestive of
microphase separation with short-range order, arising from the presence
of the hydrophobic segments. The single correlation peak was preserved
in the SAXS profiles of the DNs, which was, however, shifted to lower q-values, consistent with further network swelling. Despite
the SAXS evidence for only weak phase separation on the nanoscale
in the DNs, half of the water-swollen DNs (the ones with a 5 M AAm
concentration in the second network) exhibited strong birefringence
which probably arose from the stretching of the charged DMAEMA segments
rather than the presence of anisotropic nanophases.