Effects Of Chain Conformation On Polylactide Crystallization

One essential characteristic of polymer crystallization is that polymer chains disentangle from the random coils then stack into the crystalline structure in a unique helical or zigzag conformation.Because of the high molecular weight and molecular weight distribution,polymer crystallization is still one of the most difficult problems in the world.In this work,the effect of chain conformation on the crystallization behaviour and crystalline structure of polylactide(PLA)is investigated by differential scanning calorimetry(DSC),wide-angle X-ray diffraction(WAXD),synchrotron wide-angle Xray scattering(SR-WAXS)and Fourier transform infrared spectroscopy(FTIR).The melt memory effect facilitates the PLA cold crystallization by promoting the motion of PLA chains and plays a role as “self-plasticization”.Besides,in the PLA melting process,PLA chains belonging to the original crystalline region compete with the chains belonging to the amorphous regions to adapt to the thermodynamic condition related with melting temperature,which leads to the PLA melt being in a metastable heterogeneous state,and that the melt memory effect cannot be erased above the equilibrium melting point.The Mpemba effect on PLA crystallization behaviour originates from the size effect of “pre-ordered” local structure,and the needed adjustments of PLA chains belonging to the local structure caused by the change of crystallization temperature.When holding at lower temperatures,PLA develops some small size “pre-ordered” local structures,and PLA chains belonging to the local structures have worse mobility due to the size confinement effect.Meanwhile,PLA chain conformations belonging to the local structure are closer to the equilibrated chain conformations corresponding to the original crystallization temperature.During the new crystallization process,PLA chains belonging to the local structure need more adjustments and the small size confinement effect makes the overall adjustments as well as the crystallization slow.However,when holding PLA at higher temperatures,the developed “pre-ordered” local structures are in big size.PLA chains belonging to the local structure have better mobility and the overall adjustments of chain conformations are easier in the new crystallization process.As a result,the overall crystallization is fast and the crystallization behaviour of quenched PLA shows a significant Mpemba effect.PLA forms ?(?')and ? crystal in different ways.The nucleation and growth or the transitions of mesophases of both crystals follow different kinetic behaviours.In addition,the ?(?')– ? is not a transition from a mesophase or condis phase to a stable crystal phase,but an adaptation process to the thermodynamic condition in which ? crystal is preferential.Heating the ?(?')crystal,the part below the critical lamellar thickness would melt then recrystallize into ? crystal due to the reason that the ? crystal is preferential in the temperature range.Therefore,the ?(?')form should be a crystal independent of ? form,and is better named as “?”.The effect of stereocomplex crystal(SC)and stereocomplex conformation on the crystallization behaviour of PLLA(PLLA-PDLA)/PDLA blends is not a simply enhanced nucleation effect,which is dependent on their specific dynamic behaviours.When the PLLA(PLLA-PDLA)/PDLA blends crystallized at a temperature that is suitable for the formation of SC and stereocomplex conformation,the rapid formation of SC and stereocomplex conformation would significantly accelerate the overall crystallization.However,when the PLLA(PLLA-PDLA)/PDLA blends crystallized at a temperature that is not suitable for the formation of SC and stereocomplex conformation,the slow formation of SC and stereocomplex conformation would obviously inhibit the overall crystallization.