Unusual rheological behavior of a moderately entangled telechelic polymer and hollow glass microsphere suspension via hydrogen bonding interactions

The understanding of rheological behavior of non-Brownian hard sphere suspensions is of importance from both the fundamental and applied perspectives. This is owing to their ubiquitous use in industry for many applications. However, most of the studies reported in literature pertain to non-colloidal hard sphere suspensions in a Newtonian matrix. In the present work, we report the rheological response of hollow glass microspheres (HGMs) suspended in a moderately entangled telechelic polymer of hydroxyl terminated polybutadiene (HTPB) across a wide concentration range i.e., volume fraction, phi varying from 0.07 to 0.6. Steady state and small amplitude oscillatory rheology (SAOR) measurements of the suspensions revealed a significant departure of the rheological response from that of non-Brownian hard sphere suspensions. The salient findings of the study are (a) pronounced shear thinning (b) increased storage and loss moduli with increased HGM content (c) low frequency storage moduli plateau across concentrations (d) percolation at phi =0.49-0.51 of HGM (e) significant difference in the high frequency dynamic modulus between HTPB and suspensions at phi > 0.5. Some of these rheological features resemble that of colloidal or Brownian suspensions and are suggestive of formation of network or gellike structure. We invoke the role of HTPB-HGM interaction (FTIR spectroscopy), morphology of the suspensions (optical microscopy), plausible immobilization of telechelic chains on surface of HGM, entangled state of the telechelic as the mechanistic origins of the observed unusual rheological behavior of the studied suspensions. We posit that the present study is an important addition to experimental investigations in the field of hard sphere dispersions.