Amphiphilic polymer co-networks based on cross-linked tetra-PEG-b-PCL star block copolymers

Abstract

A new class of AB4-type amphiphilic polymer co-networks (ACN) was prepared by hetero-complementary 2-(4-nitrophenyl)-benzoxazinone/amine end group cross-linking chemistry in dimethyl sulfoxide (DMSO). For this purpose, a well-defined star block copolymer (tetra-PEG-b-PCL) consisting of a hydrophilic tetra-arm PEG core (Mn ∼ 5 kg mol−1) and hydrophobic PCL wings with a total molar mass of Mn ∼ 10 kg mol−1 was synthesized by ring-opening polymerization (ROP). This tetra-PEG-b-PCL star block copolymer was functionalized either with 2-(4-nitrophenyl)-benzoxazinone or with amino terminal groups. The ACN synthesis was carried out by conversion of these two star block copolymers. The overlap concentration of c* ∼76 g L−1 determined by viscosity measurements of both stars showed no significant temperature dependence. The network syntheses carried out at different concentrations (0.5 c* to 2.5 c*) resulted in conversion degrees in the range of p ≈ 95 % or higher as detected by HR MAS NMR spectroscopy. As in previous work on the synthesis of A4B4 networks using the same cross-linking chemistry, deviations from the expected network structure (increased proportion of double links between two stars) were detected also here by multi quantum NMR spectroscopy. The expected environmentally sensitive behavior of the ACNs was confirmed in terms of their equilibrium swelling in solvents of different quality to PEG and PCL. The scaling of the swelling and viscoelasticity data as a function of preparation concentration showed deviations from the expectations of a perfectly cross-linked star polymer network. Finally, a two-step gelation process was observed by rheology at 30 °C in DMSO, which is related to associations between the 2-(4-nitrophenyl)-benzoxazinone groups surrounded by PCL blocks that become apparent on the time scale of rheology when the molar mass is increased as a result of cross-linking.