Abstract
Poly(N,N-bis(2-methoxyethyl)acrylamide) (PbMOEAm) featuring two classical chemical motifs from non-ionic water-soluble polymers, namely, the amide and ethyleneglycolether moieties, was synthesized by reversible addition fragmentation transfer (RAFT) polymerization. This tertiary polyacrylamide is thermoresponsive exhibiting a lower critical solution temperature (LCST)–type phase transition. A series of homo- and block copolymers with varying molar masses but low dispersities and different end groups were prepared. Their thermoresponsive behavior in aqueous solution was analyzed via turbidimetry and dynamic light scattering (DLS). The cloud points (CP) increased with increasing molar masses, converging to 46 °C for 1 wt% solutions. This rise is attributed to the polymers’ hydrophobic end groups incorporated via the RAFT agents. When a surfactant-like strongly hydrophobic end group was attached using a functional RAFT agent, CP was lowered to 42 °C, i.e., closer to human body temperature. Also, the effect of added salts, in particular, the role of the Hofmeister series, on the phase transition of PbMOEAm was investigated, exemplified for the kosmotropic fluoride, intermediate chloride, and chaotropic thiocyanate anions. A pronounced shift of the cloud point of about 10 °C to lower or higher temperatures was observed for 0.2 M fluoride and thiocyanate, respectively. When PbMOEAm was attached to a long hydrophilic block of poly(N,N-dimethylacrylamide) (PDMAm), the cloud points of these block copolymers were strongly shifted towards higher temperatures. While no phase transition was observed for PDMAm-b-pbMOEAm with short thermoresponsive blocks, block copolymers with about equally sized PbMOEAm and PDMAm blocks underwent the coil-to-globule transition around 60 °C.
Highlights
Many non-ionic polymers in aqueous solution are thermoresponsive featuring a lower critical solution temperature (LCST), i.e., they are water-soluble or highlyColloid Polym Sci (2021) 299:205–219 whereat in particular for low-to-moderate molar masses, the precise value can be adjusted to a certain extent by incorporating functional end groups [9,10,11] or by the precise molar mass itself [4]
Homo- and block copolymers based on bMOEAm 1, which bears both the amide and ethyleneglycolether motifs, with varying molar mass and low dispersities were synthesized by reversible addition fragmentation transfer (RAFT) polymerization
The thermoresponsive behavior of the polymers P1 in aqueous solution was investigated by dynamic light scattering (DLS) and turbidimetry as a function of the polymer concentration, molar mass, and end groups
Summary
Many non-ionic polymers in aqueous solution are thermoresponsive featuring a lower critical solution temperature (LCST), i.e., they are water-soluble or highlyColloid Polym Sci (2021) 299:205–219 whereat in particular for low-to-moderate molar masses, the precise value can be adjusted to a certain extent by incorporating functional end groups [9,10,11] or by the precise molar mass itself [4]. A more flexible control over the LCST value is achieved by using statistical copolymers. The chemical complexity of such thermoresponsive copolymers is increased, which tends to render their chemical analysis as well as the interpretation of experimental findings complicated. This is problematic when additional structural features are to be implemented into thermoresponsive polymers, such as blocky structures or complex functional moieties [28,29,30,31,32]. There is a continuous quest for new monomers that yield thermoresponsive homopolymers
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