The variations of storage modulus with temperaturefor pristine UPR and UPR/nano-CaCO3 compositesare shown in Figure 7. Pure UPR shows a storage modulus of 6787MPa. The addition of nano-CaCO3increases the storage modulus of UPR. Increase in thestorage modulus is observed up to 5 wt% of UPR/nanoCaCO3 composites. The storage modulus valuesincrease to 8266, 8457, 10,497, 9514, and 9236 MPa forthe nano-CaCO3 contents of 1, 3, 5, 7, and 9 wt% inUPR, respectively. It confirms that the well-dispersednano-CaCO3 particles stiffen the UPR matrix.However, when the nanoparticle is >5 wt%, the stiffening effect is progressively reduced with increasing temperature most probably due to the agglomeration ofCaCO3 nanoparticles. As the temperature increased,both pure UPR and its composites showed a gradualdrop in storage modulus. The drop in storage modulusis related to material transition from glassy state to rubbery one.