Search

1H MAS NMR spectroscopy has been applied to study the urea effect on phase transition of two similar thermosensitive polymer hydrogels: poly(N-isopropylacrylamide) (PNIPAM) and poly(N,N-diethylacrylamide) (PDEA). It is found that urea influences the phase transition of the hydrogels in opposite ways: lowering the lower critical solution temperature (LCST) of PNIPAM and hence stabilizing its globular structure, whereas raising the LCST of PDEA and destabilizing the globular structure. The self-diffusion coefficient and urea–polymer nuclear Overhauser effect (NOE) measurement reveal that urea has a stronger interaction with PNIPAM than with PDEA. Moreover, the enhanced positive water–PNIPAM NOE suggests that urea not only interacts directly with PNIPAM via hydrogen bond but also intensifies the hydrogen bonding interaction between water and PNIPAM. We suggest that different urea–polymer hydrogen bonding interaction due to the presence or absence of amide hydrogen is correlated with the distinct LCST variati...

The phase separation of poly(N,N-diethylacrylamide) (PDEA) gel network in water/alcohol (methanol, ethanol, isopropanol, and n-propanol) mixtures has been systematically investigated by variable-temperature high-resolution 1H MAS NMR. The PDEA gel exhibits the cononsolvency effect and reentrant phase transition in water/isopropanol and water/n-propanol mixtures with larger alcohol hydrophobic groups but not in the water/methanol and water/ethanol bearing smaller alcohol hydrophobic groups, in contrast to the case of poly(N-isopropylacrylamide) where the cononsolvency effect was observed in all water/alcohol solutions. In the present PDEA gel, the phase separation is characterized by dehydration of the hydrophobic alkyl groups of PDEA, resulting in two distinct types of water/alcohol solutions above the lower critical solution temperature: confined binary solvents (3–5 solvent molecules per PDEA repeating unit) inside the gel network and free binary solvents outside the gel. The alcohol concentration in co...

Phase transition occurring in three different types of poly (N-isopropylacrylamide) gels-normal cross-linked gel, comb-type grafted get, and comb-type grafted gel with styrene-modified comb chains-has been investigated by variable-temperature measurements of H-1 NMR spectra and spin-spin relaxation time. Three different gels exhibit distinct collapse behaviors in response to increasing temperature. For the normal gel, remarkable network shrinking occurs in a relatively narrow temperature range from 32 to 35 degrees C. For the styrene-modified comb-type gel, overall chain shrinkage appears in a very broad temperature range from 22 to 35 degrees C in which the styrene-modified comb chains shrink at lower temperatures (22-32 degrees C) than the backbone networks (32-35 degrees C). In the comb-type gel without styrene modification, however, the backbone networks shrink first (at 32-35 degrees C) on heating, followed by collapsing of comb chains (at 35-36 degrees C). During shrinkage of backbone networks the comb chains are expulsed from the main gel networks which is revealed by abnormal T-2 increase of comb chains. T-2 measurements also reveal that the styrene-modified comb-type gel in the equilibrium swelling state has more rigid network structure than both conventional gel and comb-type gel without styrene modification.