Study On The Regulation Of Poly(butylene Adipate) Crystal Form

Different crystal forms of polyester materials have strong effect on their thermal performance,mechanical properties,solvent resistance and degradation behavior,etc.Therefore,investigation on polymorph selectivity and crystal form stability of polyester materials are important for application.Poly(butylene adipate)(PBA)is a biodegradable polyester with good tensile property.And it has been reported to exhibit two crystal forms,? and ?.Its crystal forms are representative,easily distinguishable and can exist stably at room temperature.Therefore,in this thesis PBA was selected as the research object.We observed the polymorph selectivity of PBA and investigated the effective means of controlling the crystal form of PBA by constructing confined environment,adding nanofillers and applying external force(stretching).Through studying PBA,we can deepen our understanding about polymorph selectivity of polyester material.First of all,nanocomposites formed between PBA and carbon nanofiber(CNF)were prepared and characterized.The results showed that CNF had significant ?-form crystal nucleating ability on PBA,and the ?-form crystals could appear at rather low crystallization temperature in PBA/CNF composites,for example 0 °C.Furthermore,a novel plateau phenomenon of ?-form crystals content was discovered,and the value of plateau content almost increased linearly with increasing CNF loading content.Microscopic morphologies data suggested that PBA chains could wrap(or coat)around the nanofillers after solution mixing process to form an interaction region.And in-situ Fourier transform infrared spectroscopy(FTIR)spectra proved the existence of multiple-weak CH-? interactions between CH2 groups on PBA chains and sp2 structures on the surface of CNF in composites,which helped to induce some precursory structures in the interaction region containing chain conformation close to that in ?-form crystals.Thus,the wrapping(or coating)behavior and precursory structures are responsible to the formation of ?-form crystals at low temperature and the appearance of novel plateau content phenomenon of ?-form crystals.The change of PBA polymorphism behavior in nanocomposites provides a pathway for evaluating the interaction between polymer chains and nanofillers more intuitively.Stretch-induced structural changes of PBA at various temperatures were systematically investigated by two dimensional wide angle X-ray diffraction(2D-WAXD)method.It was found that tensile deformation could lead to both occurrence of ??? and ??? crystal transformation via solid-solid process.Experimental results shows that either ? or ? form can be adopted as the stable phase for PBA crystal depending on the competition between tensile strain and temperature.Higher temperature favors ??? crystal transformation,while larger strain would result in ??? crystal transformation.Due to the conversion of crystal stability during stretching,a unique “?????” transformation phenomenon of PBA appears at appropriate temperature for original ? spline.This study provides evidence that both temperature and tensile strain can alter the relative stability of polymorphic crystals of semi-crystalline polymers.The crystallization behavior,polymorphism and phase transition of PBA confined in nanospace were studied.The confined environment for PBA is constructed by electrospining the PBA/polystyrene(PS)blend solution.A fractionated crystallization process is developed since the PBA component is divided into a series of domains of reduced dimensions in electrospun fibers.Differential Scanning Calorimetry(DSC)analysis shows fractionated crystallization of PBA is time-and temperature-dependent.The polymorphism of PBA crystal was investigated by WAXD and FTIR spectroscopy.The results showed that PBA crystallized in confined environment would promote the formation of ? form crystal.The WAXD study indicated that the phase transition process of PBA during annealing process in confined environment is advanced compared with the bulk PBA.In-situ FTIR recorded the spectra change of PBA during annealing process,which shows the ??? transformation is a melting-recrystallization process.