Impact of End-Group Association and Main-Chain Hydration on the Thermosensitive Properties of Hydrophobically Modified Telechelic Poly(N-isopropylacrylamides) in Water

Abstract

We examine the influence of the macromolecule chain length on the cloud point temperature (T cp ) and the temperature of the coil-to-globule transition (T M ) in aqueous solutions of hydrophobically modified (HM) telechelic poly(N-isopropylacrylamides) (PNIPAM) ranging in concentration from 0.01 to 35 g L -1 (0.1-310 mmol of NIPAM L -1 ). The telechelic HM-PNIPAM samples with n-octadecyl termini were obtained by RAFT polymerization of NIPAM in dioxane in the presence of S-1-n-octadecyl-S′-(R,R′-dimethyl-R′′-N-n-octadecylacetamide)trithiocarbonate as a chain transfer agent. Their molar mass (M n ) ranged from 12 000 to 49 000 g mol -1 with a polydispersity index lower than 1.20. The cloud point temperatures, measured by monitoring the temperatureinduced changes in scattering intensity, decreased significantly with increasing polymer concentration, this effect being more pronounced with decreasing polymer molar mass. In contrast, the temperature of the PNIPAM chain coil-to-globule collapse (30 ( 1 °C) was only slightly affected by solution concentration and polymer molecular weight. These results are interpreted in terms of the coexistence of two phenomena: association of the n-octadecyl terminal groups and hydration of the PNIPAM chains.

Sequential thiol‐ene/thiol‐ene and thiol‐ene/thiol‐yne reactions as a route to well‐defined mono and bis end‐functionalized poly(N‐isopropylacrylamide)

“…However, given the micron size of these aggregates, it is highly unlikely that these species are well-defined micelle-like structures that can, and have, been observed with end-group modified PNIPAms bearing larger hydrophobic end-group species. 63 Good agreement between the T cp values obtained by DLS and optical measurements was also observed in the case of the PNIPAm 50 -S-ALMA-S-Hex, PNIPAm 50 -S-PROPA-(S-Hex) 2 , and PNIPAm 50 -S-ALMA-S-POSS polymers with the S-Hex-based polymers yielding T cp values of 26 C by DLS, versus 27 C optically. DLS indicated an identical, but still anonymously high value, of 35 C for the S-POSS derivative.…”

“…Given the low molecular weight of this sample (M n $ 5900) this observed difference of three degrees may be a direct result of the hydrophobic phenyl dithioester end-group reducing the general solubility of the PNIPAm chains. However, as noted by Kujawa,63 there is a clear molecular weight dependence on the LCST for NIPAm homopolymers. For example, in their studies of octadecyl a,x-end-functionalized PNIPAm's of varying molecular weights (with M n values ranging from 12,000 to 49,000), at a constant concentration of 1.0 g/L, the LCST was observed to fall from 30.7 C for the highest molecular weight species down to 25.1 C for the lowest molecular weight species.…”

“…65,66 As such, and given the relatively low molecular weight of the parent NIPAm homopolymer, it was anticipated that the introduction of either one or two S-HexOH, S-Hex, or S-POSS groups, all with differing hydrophilicities/hydrophobicities, at the PNIPAm x chain terminus would have an effect on the LCST, and general aqueous solution behavior of the PNIPAm 50 -based materials. Indeed, and as clearly noted by Kujawa et al, 63 the introduction of hydrophobic end-groups can affect the aqueous phase behavior of PNIPAm in two distinct ways. Firstly, the general miscibility of the homopolymer in water is worsened due to the interactions between water and the hydrophobic end-groups, and secondly, sufficiently large hydrophobic species at the chain end can lead to aggregation/micellization reducing the mixing entropy of the polymer chains, although this does not really affect the hydration of the main polymer chains.…”

“…It is well documented that the LCST of PNIPAm is $32 C, [59][60][61][62] close to physiological temperature. Although varying the degree of polymerization, 63 and topology, 64 offers some degree of control and tunability of the LCST in NIPAm homopolymers; for materials with low molecular weights, the effect on the LCST of end-groups can become important. 65,66 As such, and given the relatively low molecular weight of the parent NIPAm homopolymer, it was anticipated that the introduction of either one or two S-HexOH, S-Hex, or S-POSS groups, all with differing hydrophilicities/hydrophobicities, at the PNIPAm x chain terminus would have an effect on the LCST, and general aqueous solution behavior of the PNIPAm 50 -based materials.…”

Macromolecular Assemblies in Solution: Characterization by Light Scattering

“…The sample used was PNIPAM with octadecyl groups at both ends and the weightaverage molecular weight of 47,500. 52,56,57 Water is a good solvent for PNIPAM at 25 C investigated. The weight-average and m is calculated from M w /47,500.…”