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Synthesis, Structure, and Crystallization Behavior of Amphiphilic Heteroarm Molecular Brushes with Crystallizable Poly(ethylene oxide) and n‑Alkyl Side Chains
journal contribution
posted on 2020-02-24, 20:15 authored by Elaine Barnard, Rueben Pfukwa, Jon Maiz, Alejandro J. Müller, Bert KlumpermanA series
of heteroarm 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-vinylbenzyl-PEG methyl ether and N-alkylmaleimide macromonomers. By varying PEG side chain degree of
polymerization (DP = 12, 16, and 20) and n-alkyl
chain lengths (C16 and C20), we produced AMBs
with varying combinations of side chain lengths. 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 have 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 DPPEG or short alkyl chain lengths. Large, well-developed spherulites,
implying breakout crystallization from a weakly segregated melt, were
only observed for the AMBs with the combination of the longest PEG
chain (DP = 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 to be 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.
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Keywords
alkyl side chainsalkyl chain lengthsX-ray diffraction experimentsmolar mass polyethylenesPEG chainsPEG side chainschain nside chain lengthsspherulitic growth ratesSAXS4- vinylbenzyl-PEG methyl etherPEG side chain degreeC 20 chainscrystallizationside chain lengthFRPbromine pendant groupsalkyl chain lengthAMB phase behaviorDPAmphiphilic Heteroarm Molecular Brushes
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