Study of Temperature Sensitive Polymeric Microgels with Light Scattering and Spectrophometry

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 T$_{\mathrm{room}}$ to above the transition temperature T$_{\mathrm{C}}=$41C. The HPC microgels showed a reversible deswelling by a factor of 4-8 volume as temperature was brought above T$_{\mathrm{C}}$. The deswelling is caused by HPC chains becoming more hydrophobic at the T$_{\mathrm{C}}$ and aggregating together to diminish water contact. SLS measurements yielded the relative molecular weight M$_{\mathrm{W}}$ of microgels and M$_{\mathrm{W}}$ of polymer. We also found the change of microgels' M$_{\mathrm{W}}$ and R$_{\mathrm{g}}$/R$_{\mathrm{h}}$ with increase of solution temperature (T): M$_{\mathrm{W}}$ decreased steadily from 20C to 40C (possibly due to microgels losing water) and then increased with T rising to 50C (possibly due to loose polymer chains fusing into microgels). The R$_{\mathrm{g}}$/R$_{\mathrm{h}}$ ratio ranged from 0.4 to 0.7, consistent with a soft sphere and hard sphere models. The transition in polymer was found to be sharper than in microgels and the M$_{\mathrm{W}}$ of the polymer clusters above the transition was found to be 30 times larger then M$_{\mathrm{W}}$ of microgels. The light scattering study of microgels was complemented by Atomic Force Microscopy.