Study On Crystallization Behavior Of PHBV/WS₂ Nanocomposite And Its Spinnability

Poly（3-hydroxybutyrate-co-hydroxyvalerate）（PHBV）is an aliphatic polyester that are completely synthesized by microoganisms.Owing to the physical and chemical properties of PHBV are similar to those of polypropylene,it is expected to partially replace petroleum-based polymers,and to reduce the pollution of environment at the same time.However,the inherent defects of PHBV restricte the processing and stablity of PHBV products,especially PHBV fibers.On the one hand,PHBV has a relatively low heterogeneous nucleation density during the crystallization process since no catalyst is added during the fermentation process.Furthermore,the stereoregularity of PHBV is relatively high leading to the formation of large-size spherulites,which makes PHBV melt spun fiber a long transition process from viscoelasticity to plasticity,and then to fragility.All these defects limit melt-spinning of PHBV and its further applications.To overcome the above-mentioned limitation of melt-spinning of PHBV,a series of studies have been done in this thesis in an attempt and extend its application in fibers.The main contents and results are listed as follows:（1）PHBV/WS₂ nanocomposites were prepared using tungsten disulphide（WS₂）as the nucleating agent.And the accelerated crystallization effect on PHBV was also evaluated,including the non-isothermal crystallization kinetics of the nucleated PHBV nanocomposites described using Mo’s methods.What’s more,the nucleation ability and effective energy barrier of the nanocomposites were calculated by Dobreva equation and Kissinger formula,respectively.It was found that the crystallization rate of PHBV was strongly increased even the content of WS₂ was at a relatively low level.For example,the crystallzation half time（t1/2）of PHBV/WS₂ nanocomposites decreased from 1.62 min to 0.69 min when 2wt% WS₂ added while the crystallization temperature（Tc）increased from 70.50℃ to 105.47℃.In addition,the nucleation activity（φ）and effective energy barrier（ΔE）of PHBV/WS₂ nanocomposites decreased from 1 to 0.49 and-81.6kJ/mol to-165.5 kJ/mol,respectively,indicating that WS₂ was a highly efficient nucleating agent for PHBV.（2）A series of PHBV/WS₂ fibers were prepared by melt-spinning method.In order to investigate the relationship between the structure of crystal structure and mechanical properties of PHBV/WS₂ fibers,processing conditions including the drawing temperature and drawing ratio（DR）were studied.The mechanical properties,crystal structure,orientation and thermal properties of the PHBV/WS₂ fibers were characterized using tensile tester,XRD and DSC.It was found that the tensile strength and elongation of PHBV/WS₂ fibers（DR=3.8）significantly increased from 34.32 MPa to 155.54 MPa when 1%WS₂ was added.However,the average size of the lamellar stacks was decreased due to the joint effects of heterogeneous nucleation and drawing.More interestingly,it was found that besides the oriented crystallinea-phase,a highly oriented amorphous phase was also presented in PHBV/WS₂ fibers.（3）Long chain branched PHBV/WS₂ nanocomposites（LCB-PHBV/WS₂）were obtained using reactive processing method and then to produce LCB-PHBV/WS₂ fibers.In order to increase the melt strength to avoid the sliding of PHBV chains,DCP was used as the initiator to achieve long chain branched.The mechanical properties,crystal structure,orientation and thermal properties of the LCB-PHBV/WS₂ fibers were investigated using tensile tester,XRD and DSC.It was found that such drawn fibers were exhibiting promising tensile strengths,which were dominated by highly oriented molecules between crystallites rather than the spherulitic structures.The β-form crystals were produced during the drawing process due to the stretching of molecular chains in the constrained amorphous region between small crystal nuclei which acted as cross-linking points,and then the α-form lamellar crystals were generated from small crystal nuclei during annealing process.