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UV Resonance raman determination of molecular mechanism of poly(n-isopropylacrylamide) volume phase transition

Ahmed, Z and Gooding, EA and Pimenov, KV and Wang, L and Asher, SA (2009) UV Resonance raman determination of molecular mechanism of poly(n-isopropylacrylamide) volume phase transition. Journal of Physical Chemistry B, 113 (13). 4248 - 4256. ISSN 1520-6106

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Abstract

Poly(N-isopropylacrylamide) (PNIPAM) is the premier example of a macromolecule that undergoes a hydrophobic collapse when heated above its lower critical solution temperature (LCST). Here we utilize dynamic light scattering, H-NMR, and steady-state and time-resolved UVRR measurements to determine the molecular mechanism of PNIPAM's hydrophobic collapse. Our steady-state results indicate that in the collapsed state the amide bonds of PNIPAM do not engage in interamide hydrogen bonding, but are hydrogen bonded to water molecules. At low temperatures, the amide bonds of PNIPAM are predominantly fully water hydrogen bonded, whereas, in the collapsed state one of the two normal CdO hydrogen bonds is lost. The NH-water hydrogen bonding, however, remains unperturbed by the PNIPAM collapse. Our kinetic results indicate a monoexponential collapse with t̃ 360 (±85) ns. The collapse rate indicates a persistence length of n ̃ 10. At lengths shorter than the persistence length the polymer acts as an elastic rod, whereas at lengths longer than the persistence length the polymer backbone conformation forms a random coil. On the basis of these results, we propose the following mechanism for the PNIPAM volume phase transition. At low temperatures PNIPAM adopts an extended, water-exposed conformation that is stabilized by favorable NIPAM-water solvation shell interactions which stabilize large clusters of water molecules. As the temperature increases an increasing entropic penalty occurs for the water molecules situated at the surface of the hydrophobic isopropyl groups. A cooperative transition occurs where hydrophobic collapse minimizes the exposed hydrophobic surface area. The polymer structural change forces the amide carbonyl and N-H to invaginate and the water clusters cease to be stabilized and are expelled. In this compact state, PNIPAM forms small hydrophobic nanopockets where the (i, i + 3) isopropyl groups make hydrophobic contacts. A persistent length of n ̃ 10 suggests a cooperative collapse where hydrophobic interactions between adjacent hydrophobic pockets stabilize the collapsed PNIPAM. ©2009 American Chemical Society.

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Details

Item Type: Article
Status: Published
Creators/Authors:
CreatorsEmailPitt UsernameORCID
Ahmed, Z
Gooding, EA
Pimenov, KV
Wang, L
Asher, SAasher@pitt.eduASHER
Date: 2 April 2009
Date Type: Publication
Journal or Publication Title: Journal of Physical Chemistry B
Volume: 113
Number: 13
Page Range: 4248 - 4256
DOI or Unique Handle: 10.1021/jp810685g
Schools and Programs: Dietrich School of Arts and Sciences > Chemistry
Refereed: Yes
ISSN: 1520-6106
MeSH Headings: Acrylamides--chemistry; Hydrophobic and Hydrophilic Interactions; Kinetics; Models, Molecular; Molecular Structure; Phase Transition; Polymers--chemistry; Spectrum Analysis, Raman; Temperature; Ultraviolet Rays
Other ID: NLM NIHMS99983, NLM PMC2668225
PubMed Central ID: PMC2668225
PubMed ID: 19260666
Date Deposited: 08 Feb 2013 21:04
Last Modified: 13 Feb 2019 08:55
URI: http://d-scholarship-dev.library.pitt.edu/id/eprint/17230

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