Synthesis And Performance Study Of Branched PET

Poly(ethylene terephthalate)(PET) has been widely used in fiber, bottles, films and otherfields in the national economy, because of its excellent physical, chemical and mechanicalproperties, which is the most widely used and one of the largest synthetic polymer materials.However, when the molecular weight of PET exceeds a certain value, it showes high viscosity,resulting in difficulties in processing, limiting the further application of PET in the fiberindustries in a certain extent. Compared to the linear polyesters with the same molecularweight, hyperbranched polyester showed a low viscosity and lower melt viscosity. Combiningthe characteristics of hyperbranched polymers, branched PETs were expected to improvecertain deficiencies of PET. Current research on branched polyesters mostly were focused onlong-chain branched polyesters. On the contrary, the short-chain branched polyesters werelittle studied, especially the relation between multifunctional branching agent types and therheological and crystallization properties of branched polyesters, it was worth to be exploredin depth. In this dissertation, short-chain branched PET copolyesters were prepared bycopolymerization. We focused on the rheological and crystallization properties of branchedpolyesters.In this dissertation, a series of branched PETs were synthesized by direct esterification-condensation process with glycerol (GL),3-(hydroxymethyl) propane (TMP), pentaerythritol(PER), dipentaerythritol (DPT) and2,2-(hydroxymethyl) propionic acid as a multifunctionalbranching agent. The compositions and structures of copolyesters were determined by1H-NMR. Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA)were used to analyze the thermal properties of copolyesters. The polycondensation reactionrate was accelerated with the rising of the reaction temperature, but the concentration of theend carboxyl was too high at the higher reaction temperature, the reaction temperature wascontrolled between270to275?as better. The introduction of a small branch monomer couldimprove the flowability of the melt, but the flowability of the melt was getting worse when thevolum of branching agent was more than a certain amount of0.9mol%. The thermal propertiesanalysis of the branched copolyesters showed that the glass transition temperature (Tg) of the modified copolyesters had declined, and the modified PET containing0.3mol%DPT was thelowest (73.41?), compared to pure PET. Addition TMP1-PET, the cold crystallizationtemperature (Tcc) of branched PETs were increased. The melting point (Tm) had shown adownward trend, and the Tmof branched PETs by PER decreased faster compared to thebranched PETs by TMP. TGA results suggested that all modified copolyesters could meet therequirements for further processing.The dynamic rheological properties of branched PETs were studied by dynamicrheometer, and the results showed that the complex viscosity of pure PET maintained a basicplatform for the state with the increases of frequency, but the complex viscosity of modifiedPETs showed shear-thinning properties at low frequencies. Adding multifunctional branchingagent GL, TMP, PER, DPT and DMPA had formed a certain branched structure, and with theincreases of agent-functionality, the branching effect became more apparent.The no-isothermal crystallization kinetics behaviors of PET and its copolyesters wereconsistent with the Jeziorny equation, the Avrami exponent n of PET were between2.25to2.96, suggested a three-dimensional spherulite growth with homogenous nucleationmechanism. The Avrami exponent n of the branched copolyesters were approximately from3.03to4.42, indicating that the introduction of modified components had a certain ability toenhance its crystallization, so we assumed that the crystallization mechanism of modifiedcopolyesters was a combination of heterogeneous nucleation and heterogeneous nucleation.The activation energy of branched copolyesters were lower than PET, suggesting that thecrystallization rate of the copolyesters was less sensitive to the cooling rate.Rheological behaviors analysis showed that PET and the branched copolyesters belongedto the non-Newtonian fluid, exhibiting shear thinning behavior in the investigated shear raterange, and non-Newtonian index of them were all less than1. The introduction of TMP madethe rheological properties of copolyesters become worse, while the introduction of PER orDPT could improve the rheological properties of copolyesters, which was beneficial to thespinnability of copolyesters. The results were corresponded to the analysis of activation energyof viscous flow and non-Newtonian index.