Effect of the polymer chain length of poly(N-isopropylacrylamide) on the temperature-responsive phase transition behavior of its conjugates with fullerene
© American Vacuum Society 2010
Received: 10 December 2009
Accepted: 15 January 2010
In order to develop biomedical materials with specific functionalities, thermoresponsive conjugates [poly(N-isopropylacrylamide)-C60 (PIPAAm-C60) ]of fullerene (C60) and PIPAAm with two different polymer chain lengths (4 and 20 kDa) were synthesized by atom transfer radical polymerization. The effects of the polymer chain length on the temperature-responsive phase transition behavior of the synthetic PIPAAm-C60 conjugates were probed by means of various physicochemical techniques. The coexistence of unimers and molecular assemblies of PIPAAm-C60 was observed by gel permeation chromatography and dynamic light scattering studies in two PIPAAm-C60 aqueous solutions below their lower critical solution temperatures (LCSTs). Additionally, below their LCSTs, differences in PIPAAm chain length gave rise to changes in the composition of the unimers and molecular assemblies. In response to temperature, the absorbance of the PIPAAm-C60 aqueous solution changed according to a two-step behavior profile. Increasing temperature during the primary stage, where a change in the absorbance of the PIPAAm-C60 aqueous solution took place, did not change the transition temperature, regardless of the solution concentration of PIPAAm-C60. This absorbance change was associated with the phase transition of the molecular assemblies of PIPAAm-C60. However, at the second stage, the transition temperature shifted to a higher value with the decrease in the concentration of PIPAAm-C60, in the same manner as free PIPAAm chains. The second change was associated with the phase transition of the unimeric PIPAAm-C60. Differences in PIPAAm chain length gave rise to the change in the phase transition behavior of PIPAAm-C60 aqueous solution. Therefore, the chain length of PIPAAm was found to be a predominant factor involved in the solution characteristics of PIPAAm-C60. Consequently, the PIPAAm-C60 is expected to be an intelligent biomaterial possessing heat-induced accumulation and bioactivities.