Abstract
Fluorescence spectroscopy on a series of aqueous solutions of poly(acrylic acid) containing a luminescent label showed that polymers with molar mass, Mn < 16.5 kDa did not exhibit a pH responsive conformational change, which is typical of higher molar mass poly(acrylic acid). Below this molar mass, polymers remained in an extended conformation, regardless of pH. Above this molar mass, a pH-dependent conformational change was observed. Diffusion-ordered nuclear magnetic resonance spectroscopy confirmed that low molar mass polymers did not undergo a conformational transition, although large molar mass polymers did exhibit pH-dependent diffusion.
Highlights
The properties of stimulus responsive materials change in response to perturbations in their environment.[1,2] A class of stimulus responsive materials which has been extensively studied is that of polyelectrolytes in aqueous media
Larger molar mass polymers acted in a manner anticipated for poly(acrylic acid) (PAA), with a significant increase in lifetime observed at low pH
These data show that the pH-dependent coil-to-globule transition in PAA is critically dependent on molar mass
Summary
The properties of stimulus responsive materials change in response to perturbations in their environment.[1,2] A class of stimulus responsive materials which has been extensively studied is that of polyelectrolytes in aqueous media These generally undergo step changes in chain conformation, passing from open, fully solvated, coils to desolvated globular conformations over a small range of pH. The chain is expected to form an extended rod-like structure when the polymer is charged, which changes to an open coil conformation for chains longer than the persistence length.[3] The uncharged polymer is expected to form a more globular structure for all chain sizes This concept is tested for a model weak polyelectrolyte, poly(acrylic acid) (PAA),[4] which is shown not to undergo this rod-to-coil transition with increasing molar mass but rather displays a transition from a relatively pH-independent conformation over typical ranges of pH at low molar mass to a polymer that exhibits a coil-to-globule transition for greater masses
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