The Two Phase Transitions of Hydrophobically End-Capped Poly(N-isopropylacrylamide)s in Water.

Hao Ren,Yan Shi,Xing-Ping Qiu,Françoise M Winnik,Peng Yang

doi:10.1021/acs.macromol.0c00487

Abstract

High-sensitivity differential scanning calorimetry (HS-DSC) thermograms of aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions present a sharp unimodal endotherm that signals the heat-induced dehydration/collapse of the PNIPAM chain. Similarly, α,ω-di-n-octadecyl-PNIPAM (C18-PN-C18) aqueous solutions exhibit a unimodal endotherm. In contrast, aqueous solutions of α,ω-hydrophobically modified PNIPAMs with polycyclic terminal groups, such as pyrenylbutyl (Py-PN-Py), adamantylethyl (Ad-PN-Ad), and azopyridine- (C12-PN-AzPy) moieties, exhibit bimodal thermograms. The origin of the two transitions was probed using microcalorimetry measurements, turbidity tests, variable temperature 1H NMR (VT-NMR) spectroscopy, and 2-dimensional NOESY experiments with solutions of polymers of molar mass (Mn) from 5 to 20 kDa and polymer concentrations of 0.1 to 3.0 mg/mL. The analysis outcome led us to conclude that the difference of the thermograms reflects the distinct self-assembly structures of the polymers. C18-PN-C18 assembles in water in the form of flower micelles held together by a core of tightly packed n-C18 chains. In contrast, polymers end-tagged with azopyridine, pyrenylbutyl, or adamantylethyl form a loose core that allows chain ends to escape from the micelles, to reinsert in them, or to dangle in surrounding water. The predominant low temperature (T1) endotherm, which is insensitive to polymer concentration, corresponds to the dehydration/collapse of PNIPAM chains within the micelles, while the higher temperature (T2) endotherm is attributed to the dehydration of dangling chains and intermicellar bridges. This study of the two phase transitions of telechelic PNIPAM homopolymer highlights the rich variety of morphologies attainable via responsive hydrophobically modified aqueous polymers and may open the way to a variety of practical applications.

Highlights

We evaluated the effects of molar mass and concentration on the solutions phase transitions using turbidity experiments, High-sensitivity differential scanning calorimetry (HS-DSC) scans, temperature-dependent 1H NMR measurements, and NOESY experiments
Upon heating past ∼25 °C, aqueous suspensions of neutral or acidic C12-PN-AzPy micelles undergo a phase transition characterized by the bimodal thermograms and turbidity plots shown in Figure 3b and c
The maximum temperature of the first endotherm, T1 (∼26.7 °C), does not change with polymer concentration, but both T2 and Tcp decrease with increasing concentration (Figure 5c), a characteristic feature of the phase diagram of low molar mass linear PNIPAM chains dissolved in water as unimers.[41]

Summary

INTRODUCTION

With the growth of nanotechnology, materials design and fabrication are converted progressively from traditional bottom-down methods, illustrated by photolithography, to bottom-up approaches based on the principles of supramolecular chemistry, which rely on the self-assembly of molecular components through weak forces.[1,2] This strategy takes advantage of the faster response of supramolecular structures to external stimuli, compared to the corresponding bulk materials.[3,4] The translation of supramolecular chemistry concepts into practical industrial processes has led to intense activity in the development of stimuli-responsive polymers, an important class of building blocks for the fabrication of responsive materials, for medical diagnostics and therapeutics.[5,6]. The incorporation of hydrophobic groups along the PNIPAM backbone or on the polymer chain ends decreases the phase separation temperature of PNIPAM in water. This effect arises from the strong polymer/polymer interactions that result from the increased polymer density close to the core of micelles, in agreement with de Gennes’ nclustering theory.[18,19] The nature of the PNIPAM end group gives rise to distinct thermoresponsive properties in the case of PNIPAM terminated with small functional groups,[10,11] and in the case of linear alkyl chains, as discussed by Lang et al in a comparative study of the ethyl- and n-dodecyl- groups.[20].

EXPERIMENTAL SECTION

Solution Properties of C12-PN-AzPy in Cold Water

Solution Properties of HS-PN-AzPy 12K in Water:

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES