Light Scattering and Spectrophotometry Studies of Polymeric Microgels as a Prototype for Temperature-Sensitive Drug-Releasing Carriers

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FDA approved Hydroxypropylcellulose (HPC) polymer can be cross-linked to form microgel nanoparticles that undergo a temperature dependent volume phase transition. We studied the structure and dynamics of HPC microgels and HPC polymer that microgels were made from using Dynamic (DLS) and Static Light Scattering (SLS) and Spectrophotometry. Our results determined the transition behavior of the microgels and polymer as temperatures were varied from Troom to well above the critical transition temperature TC~41ºC. The 200-250nm HPC microgels showed a reversible factor of 4-8 volume shrinkage as temperature was brought above TC. This shrinkage is caused by HPC chains becoming more hydrophobic at the TC and aggregating together to diminish water contact. The transition in polymer was found to be sharper than in microgels. Also, above 50ºC, the microgel particles seemingly aggregate with each other, forming larger clusters. In addition to apparent size from DLS, SLS yielded the molecular weight (MW) and shape factor of the microgels. The MW decreased steadily from 20ºC to 40ºC possibly due to microgels losing water, and then increased at 45ºC, and again at 50ºC, possibly due to loose polymer chains fusing into microgel particles. The shape factor ratio ranged from 0.4 to 0.7, consistent with a soft sphere. The light scattering study of microgels was complemented by Atomic Force Microscopy.