Double Networks Based on Amphiphilic Cross-Linked Star Block Copolymer First Conetworks and Randomly Cross-Linked Hydrophilic Second Networks

Abstract

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.