The effects of proteins and phospholipids on the network structure of natural rubber: a rheological study in bulk and in solution

Natural rubber (NR), as a biomacromolecule, has superior properties to its man-made substitute, thus its structure has attracted intensive studies in recent years. However, a detailed rheological study on the network structure of NR is still lacking up to now. Herein, we study the viscoelastic behaviors of NR, deproteinized NR (DPNR) and transesterified DPNR (TEDPNR) in bulk and in solution to reveal the effects of proteins and phospholipids on the network formation of NR. In bulk, removing proteins from NR only leads to a slight reduction of storage modulus, but further removing phospholipids from DPNR leads to an obvious reduction. The stress relaxation experiment demonstrates that there are steady networks in NR and DPNR, while there is only transient network in TEDPNR. By studying the recovery process and re-entanglement kinetics, we find the remarkable restriction of proteins and phospholipids on the entanglements, which is important to the formation of steady network in NR and DPNR. In solution, the swelling of toluene weakens the contribution of entanglements, thus NR displays obviously higher modulus, higher shear viscosity and better network integrity than DPNR and TEDPNR. In addition, the results by Cox-Merz rule demonstrate the existence of reversible bonds on the terminals of NR and DPNR, which is also important for their network formation. Based on these results, we propose the network structure of NR and its structural evolution upon shearing.