Network Design And Investigation Of Polyisoprene Elastomer Via Mimicking Natural Network

Natural rubber(NR)has very excellent comprehensive properties,such as the wet strength of raw rubber,superior machinability,high tensile strength of vulcanized rubber,elongation at break,outstanding fatigue resistance,etc.,which is difficult to be replaced by other artificial synthetic rubber.The yield and quality of NR in China are severely restricted by the natural planting conditions,resulting in that the self-sufficiency rate of NR is lower than the internationally recognized safety guarantee line of 30%.Due to the difficulty in increasing the yield of NR,it is an important development direction to deeply reveal the structure and functional mechanism of NR.After that,by imitating NR structure and innovatively developing,synthetic rubber substitutes with similar properties to NR can be prepared.The synthetic cis-1,4-polyisoprene rubber(PIP)is considered to be the best substitute for NR,which has similar structure to NR molecular chain.However,there is still a big gap between the comprehensive performance of PIP and NR.Currently,it is the mainstream view that both phospholipids of α-terminal and proteins of ω-terminal in the NR molecular chain promote the formation of natural networks,thus greatly improving the comprehensive performance of NR.The non-covalent crosslinking point formed by phospholipids is more stable,which has special contribution to the formation of natural network.However,due to the low content of phospholipids in NR,no direct evidence has been obtained that phospholipids form the non-covalent crosslinking structure in NR.As a result,the mechanism of how phospholipids form important crosslinking points has not been revealed,which greatly limits the profound understanding of the multiple structures of NR and the development of synthetic rubber for many years.Therefore,it is of great academic significance to reveal the structure and formation mechanism of non-covalent crosslinking points formed by phospholipids at the molecular level.In addition,according to the principle of non-covalent crosslinking points in natural network,the creative networks of PIP synthetic rubber are designed and a variety of new high-performance polyisoprene elastomers are prepared by using single or combination strategies of multiple hydrogen bond,ionic bond,and reversible covalent bond,which also have important practical significance.In summary,starting from revealing the structure and function of the crosslinking points formed by phospholipids in the natural network,this paper focus on the theory imitation and development of natural network.The main contents are as follows:1.Based on the NR molecular chain structure and phospholipid structure in NR,PIP synthetic rubber with pendant groups of phospholipid is synthesized by coordination polymerization and grafting reaction.First of all,the hydroxyl groups terminal block of cis-1,4-polyisoprene(BPIP-OH)and hydroxyl groups random distribution of cis-1,4-polyisoprene(RPIP-OH)respectively graft ethanolamine phosphate(PA)to simulate the NR chain with α-terminal of phospholipids.After that,the PA terminal block distribution of cis-1,4-polyisoprene(BPIP-PA)and the PA random distribution of cis-1,4-polyisoprene(RPIP-PA)are obtained,both of that have the same molecular weight and the same numbers of PA groups,but different distribution sequence structure.By comparing the different sequence structure of the model compounds,it is found that both the weak hydrogen bonding interaction formed by hydroxyl and the strong hydrogen bonding interaction formed by phosphate groups,the sequence structure of terminal block distribution can gain more non-covalent crosslinking points.This inversion also proves that the terminal network formed by phospholipids plays a key role in improving the overall performance of NR.The hydrogen bond network formed by phosphate groups in RPIP-PA has a variety of hydrogen bond interactions with different stability,which resulting in a hierarchical dissociation mechanism of hydrogen bond with the heating process.Secondly,RPIP-OH graft phosphoryl choline ethanol amine(PC)and the PC random distribution of cis-1,4-polyisoprene(RPIP-PC)is obtained.RPIP-PA only contains hydrogen bonding interaction while PRIP-PC contains hydrogen bonding interaction and Ionic bond interactions.Hydrogen bonding-ionic bonding synergy results in a more stable non-covalent crosslinking network in RPIP-PC.Therefore,compared with RPIP-PA,the tensile strength of RPIP-PC is increased by 49.6% and Young’s modulus is increased by 47.7%,with the similar strain at break.In addition to,the non-covalent cross-linking network of RPIP-PC can be dissociated by polar solvent adjustment,which affects its phase structure.Furthermore,the hierarchical dissociation mechanism of such non-covalent cross-linking network is revealed by two-dimensional Fourier transform infrared spectrometer(FTIR).By inversion of model compound,it is proved for the first time that phospholipids form non-covalent crosslinking points through hydrogen and ionic bond interaction in NR,which has special hierarchical dissociation mechanism,thus greatly improving the mechanical properties of NR.2.After the hierarchical dissociation mechanism of hydrogen bonding network in the model compound RPIP-PA is proved,this mechanism is applied to imitate hydrogen bonding network in NR.First of all,the hydroxyl groups terminal block of cis-1,4-polyisoprene(BC-OH)and hydroxyl groups random distribution of cis-1,4-polyisoprene(RC-OH)are obtained by copolymerization of isoprene and a novel monomer hydroxyl-myrcene(HMY),with the same number of hydroxyl groups.The BC-OH has hard segment crystallization because of the hydroxyl groups terminal block of molecular chain,which forms a strong terminal block hydrogen bonding,and leads to the stability of the non-covalent crosslinking points of BC-OH is better than that of RC-OH.This kind of strong terminal block hydrogen bonding is similar to the mechanism of the terminal network in NR.In order to simplify the steps of the introduction of non-rubber components,the hydroxyl groups of RC-OH and BC-OH are used as the bridge to introduce polysesquisiloxane(POSS)and hydrogen bonding(ammonia ester bond and urea bond)at the same time.Two model compounds of POSS terminal block cis-1,4-polyisoprene(BC-POSS)and POSS random distribution cis-1,4-polyisoprene(RC-POSS)are obtained through one-step grafting method.By comparison with two model compounds,it is proved that the non-covalent network formed by POSS hybrid molecules is different from non-covalent network formed by phospholipids in NR.In BC-POSS,the concentration of crystallization regions leads to disorderly and orderly hydrogen bonding interactions,which constitutes the relatively unstable non-covalent network.In RC-POSS,the uniform distribution of crystallization region leads to orderly hydrogen bonding interaction,which constitutes a stable non-covalent network.This unique non-covalent network is similar to the structure of NR natural network.Through the controlling of POSS sequence structure and the design of hydrogen bonding network,the non-covalent network with uniform distribution is obtained.Compared with BC-POSS,the tensile strength of RC-POSS is increased by 25.0%and the strain at break is increased by 41.4%,which expands new idea for the imitation of non-rubber components.3.Based on the hydrogen bonding-ionic bonding synergistic crosslinking points formed by phospholipids in NR,a new phospholipid-lignin(LP-LA)non-covalent network with ionic and hydrogen bonding interactions is designed and introduced into polyisoprene with random distribution of hydroxyl groups(HPIP).Since HPIP has a large number of polar hydroxyl groups,it interacts with the sulfonic acid group of LA to form hydrogen bonding,which can improve the mechanical properties of HPIP elastomer.By compatibility regulation,LA coated with LP is added into HPIP.LA is separated from HPIP,and the amphipathic molecule LP acts as the interface layer.At the terminal of hydrophobic long carbon chain of LP,the carbonyl group interacts with the matrix HPIP to form hydrogen bonding interaction.At the same time,at the hydrophilic terminal of LP,the bile base group interacts with the sulfonic acid group to form ionic bonding interaction.Both interfacial hydrogen bonding and ionic bonding compose the interfacial non-covalent network,which is guided by surface energy of HPIP,LA,and LP.This strategy solves the compatibility problem between rigid packing and soft matrix.In addition,hydrogen bonding interaction formed by hydroxyl group exists in HPIP matrix.Ionic bonding interaction formed by phosphoric acid choline also exists in LP interface layer.Hydrogen bonding interaction formed by sulfonic acid exists in LA.This robust non-covalent network composed of hydrogen bond network in matrix and hydrogen bonding-ionic bonding synergistic network in interface layer,which optimize the mechanical properties of HPIP elastomer.Furthermore,due to the totally non-covalent cross-linking in the HPIP elastomer samples,it can be dissolved and then recycled for secondary employ.4.Based on the strategy of mimicking natural rubber network,a reversible covalent bond network composed of β-hydroxyl ester bond and free radical trapping agent DIB are introduced into epoxidized-polyisoprene matrix(EPIP),which constructs a self-enhanced dynamic covalent network.Poly-(thioctic acid)(PTA)is prepared by ring-opening polymerization of thioctc acid monomers(TA).PTA has both a suspended carboxyl group and a disulfide bond structure in the molecular chain.The carboxyl group can react with the epoxy group of EPIP to form a reversible ester bond connection between the two phase interfaces,which further forms a reversible covalent bond network.Then EPIP materials have basic mechanical properties and recoverability.In the processing of repeated molding,the disulfide bonds release sulfur free radicals,which can be trapped by the free radical trapping agent DIB to form a C-S single bond structure,further forming a covalent bond network.At the same time,DIB induces a part of PTA molecular chains into EIP phase during each remolding process,which results in more reversible ester bond linkages are formation.This compensation strategy makes up for the degradation of mechanical property,which is caused by the hysteresis effect of topological network rearrangement.Base on the self-enhancement strategy driven by compatibility,the mechanical properties recovery efficiency of EIP/PTA-X samples reach more than140%,after three repeated recovery times.The self-enhancement strategy effectively solves the problem that the mechanical properties of vitrimer elastomer materials are significantly reduced after recycled.In addition,the design of this self-enhanced dynamic covalent network provides a new creative perspective for the development of recyclable rubber.