Synthesis, Structure and Crystallization Behavior of Amphiphilic Hetero-arm Molecular Brushes with Crystallizable Poly(ethylene oxide) and N-Alkyl Side Chains

Abstract

A series of hetero-arm amphiphilic molecular brushes (AMBs) with poly(ethylene glycol) (PEG) and long chain n-alkyl side chains were synthesized via conventional free radical polymerization (FRP) of mainly 4-vinyl benzyl-PEG methyl ether and N-alkylmaleimide macromonomers. By varying PEG side chain degree of polymerization (D.P. = 12, 16 and 20) and n-alkyl chain lengths (C16 and C20), AMBs with varying combinations of side chain lengths were produced. This enabled the elucidation of the effect of side chain length on AMB phase behavior, semicrystalline morphologies and crystallization kinetics, via differential scanning calorimetry, polarized light optical microscopy and x-ray diffraction experiments. Calculations of segregation strength together with SAXS measurements indicate that all materials are probably phase segregated structure in the melt. Most of the AMB materials prepared were double crystalline, i.e., contained crystals from alkyl and PEG chains. AMB crystallization was constrained by AMB architecture, the frustration being most evident in AMBs with combinations of either low D.P.PEG, or short alkyl chain lengths. Large, well-developed spherulites, implying break-out crystallization from a weakly segregated melt, were only observed for the AMBs with the combination of the longest PEG chain (D.P. = 20) and longest alkyl chain length (C20). A peculiar behavior was found when spherulitic growth rates and overall crystallization rates of the PEG chains, within this particular AMB sample, were determined as a function of crystallization temperature. In both cases, a distinct minimum with decreasing temperature was observed, probably caused by the challenges encountered in crystal packing of the PEG side chains, tethered to an amorphous backbone, which also contained already crystallized C20 chains. This minimum is analogous to that observed in the crystallization of long chain n-alkanes, or high molar mass polyethylenes with bromine pendant groups that has been attributed to a self-poisoning effect; this is the first observation of this phenomenon in AMBs.