Study On Chemorheology And Crystallization Behavior Of Cyclic Butylene Terephthalate Oligomers And Graphene Nanocomposites

With the development of technology, people have more demands in polymerperformance, such as the low viscosity to be easy for processing, the outstandingmechanical performance and being thermoplastic and thereby capable of being reground,remelted and reprocessed. Now these requirements may be achieved by using cyclicoligomers which undergo ring-opening polymerization yielding linear thermoplasticpolymers, among them the cyclic butylene terephthalate oligomers (CBT) is the mostattractive one.CBT with low melt viscosity can be polymerized into the corresponding linearpoly(butylene terephthalate)(pCBT) via entropically-driven ring-opening polymerization(ED-ROP) in the presence of suitable catalysts such as tin and titanate-based initiator. Thetime needed for the polymerization depends on the temperature and the type of initiatorused, ranging from several seconds to10min. Because the ring-opening polymerization isentropically driven, the reaction is athermal without chemical emission. Thus there are nohot spots in the mold that lead to thermal degradation and no byproducts need to beremoved during the molding process. Furthermore, the CBT polymerization can beperformed even below the melting temperature of pCBT, while crystallization will startduring this polymerization which results in demoulding without subsequent cooling of themold. Consequently, all these favorable advantages allow CBT to be the promisingmaterial to produce fiber-reinforced pCBT composites with perfect impregnation of fibersand CBT micro/nanocomposites with excellent dispersion of fillers, using variousprocessing methods such as resin transfer molding (RTM) which are normally reserved tothermosets. So far, the polymerization kinetics, the influence of shear flow on thecrystallization during the polymerization of CBT, the preparation and crystallizationbehavior of pCBT nanocomposits have been the important scientific problems. The main contents and results are as follows:(1) We first developed a rheological method to monitor the polymerization of CBT andinvestigate the polymerization kinetics. According to the reptation theory of Doi andEdwards and double reptation model, a new method was suggested to calculate thevariation of the molecular weight and polymer concentration versus time from theviscoelastic functions. At the same time, the polymerization kinetics equations wereproposed on the basis of mechanism of CBT polymerization. By combining the calculatedresults and kinetics equations, the polymerization rate constants such as initiation,propagation forward, and inverse rate constants were evaluated. Additionally, withpolymerization rate constants at various temperatures, the activation energy and Arrheniuscoefficient were also obtained. Using these kinetics parameters, the kinetics model couldpredict the variation of molecular weight, pCBT concentration, and even the viscoelasticfunctions such as viscosity and modulus during the polymerization at high temperature.(2) The effect of shear flow on the crystallization behavior during the polymerization ofCBT was investigated via rheometer. It was found that the chain scission of pCBT occurredunder the presence of more initiators and the shear flow accelerated the reaction, whichdecreased the molecular weight of pCBT. With increase in shear flow, the pCBT chains arelikely to form stable nuclei in the flow direction and accelerate the crystallization.However, the reduction of molecular weight of pCBT under shear flow depressed thecrystallization rate. Therefore，the shear flow depressed the crystallization first, thenaccelerated it with shear flow increasing during polymerization of CBT. Thenucleation density for pCBT crystallization during the polymerization of CBT is largerthan the one for pCBT crystallization after complete polymerization. And the reduction ofmolecular weight of pCBT reduced the capability of crystallization and crystallitedimensions.(3) The pCBT/TrGO nanocomposites were successfully prepared by ROP of CBT. And the polymerization kinetics of pCBT/TrGO was monitored by dynamic time sweep in aparallel-plate rheometer. TrGO was found to depress the polymerization rate as well as thedegree of polymerization as observed from rheology. It was confirmed by XPS,1H NMRand TGA measurements that the growing pCBT chains could graft onto the surface ofTrGO nanosheets by reaction between carboxyl groups of pCBT and hydroxyl and epoxygroups of TrGO with the grafting content up to53wt%. The grafting reaction was furtherjustified from nonlinear rheology and the fractal dimension analysis. The constraint effectof nanosheets on the reactivity of grafted pCBT chain is the main reason for the slowdownin the polymerization rate at low TrGO concentration, while the network formation athigher TrGO concentration and its constraints on the diffusion of reactive species willfurther decrease the polymerization rate.(4)The crystalline structure of pCBT/TrGO nanocomposites was investigated by XRD,the results showed that it was not affected by the incorporation of TrGO, but the crystallitedimensions of pCBT decreased with increase in TrGO content. The investigation of theisothermal and non-isothermal crystallization kinetics of the neat pCBT and pCBT/TrGOnanocomposites has been carried out by DSC. The crystallization kinetics under isothermalconditions was described by the Avrami equation. In the case of the non-isothermalcrystallization, the Avrami equation modified by Jeziorny, the Ozawa theory and anequation combining the Avrami and Ozawa equation were employed to analyze thecrystallization behavior. It was found that the effect of various structure levels of TrGO onthe crystallization behavior of pCBT is different to each other during the isothermal andnonisothermal crystallization process. The single TrGO sheets can promote crystallizationof pCBT due to its heterogeneous nucleation effect, while TrGO percolated network mayobstruct pCBT chains from diffusing to the growing crystallites, increasing thecrystallization activation energies. Which is the dominant between the nucleation effectand hindering effect may be decided by whether the percolated network forms or not. In addition, the TGA has also been performed to investigate the TrGO effect on the thermalstability of nanocomposites. Results revealed that TrGO improved the thermal stability ofsamples, especial when network structure of TrGO sheets was formed.（5）CBT/CL copolymer was prepared through the two-step polymerization. Thestructure of copolymer was investigated by1H NMR spectroscopy, and the result indicatedthat there was transesterification between CBT and CL segments during synthesis processwhich leaded to two types of linkages between soft and hard segments. The sequencelengths of both hard and soft segments in copolyesters are not very long and socopolyesters have certain random copolymer property. The chain-extended pCBT wasprepared via chain extension with TGIC. The chain extension increased the molecularweight of pCBT as well as introduction of branched structure. The end zone behavior ofmodulus of chain-extended pCBT was also investigated to analyze the effect of TGICcontent on the branched structure and response of rheological behavior. In addition, it wasfound that the regularity of pCBT chain was broken up with introduction of chain branch,which reduced the capability of crystallization.