Crystallization And Phase Transition Of Aliphatic Polyesters With The Temperature-Dependent Polymorphism

Polymorphic crystallization exists extensively in polymer crystallization,which can be divided into homo-and hetero-polymorphism according to the type of crystallization.The homo-and hetero-polymorphic polymers usually crystallize into metastable crystals at low temperatures and thermodynamically stable crystals at high temperatures.Phase transition would occur between the metastable and thermodynamically stable crystals with the change of external conditions.Since the polymorphic polymers with different crystalline condensed structures[e.g.,crystal polymorph and crystallinity(Xc)]possess different physical properties,regulating the crystal polymorphs and crystalline structure is an effective method to optimize the properties of materials.However,the formation condition and structural feature,phase transition behavior and mechanism,and structure-property relationship of different crystal polymorphs for the temperature-dependent homo-and hetero-polymorphic polymers are still unclear.In this thesis,we chose aliphatic polyesters such as poly(p-dioxane)(PPDO),poly(L-lactic acid)(PLLA),and poly(glycolide-co-lactide)(PGLA)as models,and investigated their crystallization kinetics,temperature-dependent polymorphic structure,phase transition and structure-property relationship.Firstly,we investigated the crystallization temperature(Tc)-dependent polymorphic structure,phase transition,and their effects on the mechanical property and degradation behavior of PPDO.Both the polymorphic structure and crystallization kinetics ofPPDO depend on Tc drastically.The melting enthalpy and Xc of PPDO first decrease and then increase with increasing Tc.PPDO forms a new mesomorphic polymorph(denoted as the ?'-form)during crystallization at low Tc(?10?),in contrast to the common ? crystals generated at high Tc(?60?).The ?' crystals have weaker interchain interactions and a shorter long period than the common ? crystals,indicating the looser chain packing of ?' crystals.The ?' crystals are metastable,and they transform into the thermodynamically stable ? crystals during heating.The ?'-form PPDO possesses a slower degradation rate,higher flexibility,but lower strength and modulus than its ? counterpart.Multilevel micro structural evolutions of PPDO upon annealing and stretching were further investigated.Polymorphic structural transition and crystalline lamellar evolution of PPDO strongly depend on annealing and stretching temperatures.High-temperature annealing facilitates the ?'-to-? phase transition of PPDO,accompanied by the improvement of Xc and thickening of crystalline lamellae.Tensile strength and Young's modulus of PPDO increase but the breaking strain decrease with increasing annealing temperature.Stretch-induced phase transition of PPDO depends strongly on the initial crystal structure and stretching temperature.The ?-form PPDO transforms into its ?'counterpart during stretching at low temperature;this phase transition is irreversible and does not retain the ? form with the release of stress.However,no phase transition occurs for the ?-form PPDO stretched at high temperatures(?40?).Original lamellae of ?-form PPDO changes into the fibrillar lamellae during stretching through the melt-recrystallization mechanism.Secondly,we synthesizedthe unsaturated poly(cis-butene dicarboxylate)derived from cis-2-butene-1,4-dioL and different diacids through melt poly condensation,and systematically investigated their crystallization behavior,temperature-dependent polymorphic structure,and phase transition.The crystallization behavior of poly(cis-butene dicarboxylate)depends on the methylene number of the main chain.Poly(cis-butene pimelate)(PcBP)and poly(cis-butene azelate)are semicrystalline,while poly(ciS-butene glutarate)is amorphous.PcBP after isothermal melt crystallization shows the Tc-dependent polymorphic structure.PcBP crystallizes in ? crystals at TC?23? and ? crystals at Tc?10?;the mixture of ? and ?crystals are generated at intermediate temperature range(10<Tc<23?).The unit cell of ?-form PcBP is monoclinic with lattice parameters:a=0.838 nm,b=0.965 nm,c=1.67nm,and?=64.5°.The unit cell of ?-form PcBP is monoclinic with lattice parameters:a=0.725 nm,b=0.895 nm,c=1.69 nm,and ?=58.1°.Distinct melting behaviors with dual melting peaks originated from a and ?-form PcBP are observed;? crystals exhibithighermelting temperature.The ? crystals formed at a low Tc are metastable and they transform into the thermodynamically stable ? crystals upon heating through a melt-recrystallization mechanism.Then,we chose the glycolide-rich PGLA as the model chemical defect-containing polymers,in which the L-lactide(LA)comonomeric units can be regarded as chemical defects.We synthesized the glycolide-rich PGLA with different copolymer compositions through ring-opening polymerization,and studied their crystallization kinetics,crystalline structure,and phase transition upon heating and high-temperature annealing.Crystallization kinetics and crystalline structure of PGLA are significantly affected by copolymer composition and Tc.Crystallization rate of PGLA reduces with increasing the LA content and the PGLA cannot crystallize at the high LA content(?40%).LA units can be included in the crystal lattice as defects during the crystallization of PGLA,resulting in the enlargement of the crystal unit cell.The inclusion degree of LA units in the crystal lattice of PGLA is kinetically-controlled.The inclusion of LA units in the crystal lattice is favored at low Tc but unfavored at high Tc,causing the contraction of crystal unit cell with increasing Tc.The defect-rich crystals of PGLA formed at low Tc are metastable and transform into thermodynamically stable crystals upon heating and annealing.Finally,we chose the PLLA and CO2 as the cocrystallization of polymer/small molecular compound model.The polymorphic structure,phase transition,and their effects on the mechanical property of PLLA under different CO2 treatment conditions(CO2 pressure:1-13 MPa;Tc:10-110?)were investigated.PLLA crystallizes in the PLLA/CO2 complex crystals under 7-13 MPa CO2 at Tc?50? but the common ? crystals under the high-pressure CO2 at Tc?70?.Solid-state nuclear magnetic resonance results indicate thatthe PLLA/CO2 complex crystals possess weaker interactions between the PLLAchains than the common ? crystals.The PLLA/CO2 complex crystals are metastable and transform into the thermally stable ? crystals through the solid-to-solid mechanism during heating and high-temperature annealing(>50?).The PLLA/CO2 complex crystals produced at low Tc are more ductile than the common ?crystals due to the lower Xc and the plasticizing effect of CO2.