MICROCALORIMETRIC STUDY OF THERMAL COOPERATIVE TRANSITIONS IN POLY(N-VINYLCAPROLACTAM) HYDROGELS

Structural cooperative transitions in thermosensitive nonionic hydroge ls of poly(N-vinylcaprolactam) (PVCa) were studied using high-sensitiv ity differential scanning calorimetry (HS-DSC). The thermoshrinking of the PVCa gel is endothermic, much like first-order phase transitions. According to the HS-DSC data, the gel system undergoes two successive cooperative transitions between 25 and 50 degrees C. The low-temperat ure transition (31.5 degrees C) is proposed to be associated with the microsegregation resulting in formation of hydrophobic domains (microm icelles) in the gel whereas the high-temperature transition (37.6 degr ees C) is due to the gel volume collapse. The transition temperatures decrease in the presence of NaCl and increase with increasing sodium d odecyl sulfate (SDS) concentration. Dependences of the transition enth alpies and entropies on NaCl and SDS concentrations display a more com plex character. The influence of sodium chloride and SDS on the transi tion temperatures is in agreement with the general expectations taking into account their effect on hydrophobic interactions and the ability to form micromicelles.