Study On Crystallization Behavior Of Poly(Lactic Acid) Induced By Multi-amide Nucleators

Poly?lactic acid??PLA?is a fully biobased and biodegradable polyester with good biocompatibility and strength.But the application of PLA is restricted by its low crystallization rate and low heat distortion temperature?HDT?.In order to achieve high crystallinity and performance of PLA,three pieces of work were carried out in this thesis.First,a bundle-like amide nucleating agent HBNA was added to PLA by melt blending.HBNA shows good nucleating ability to PLA.The addition of HBNA not only increased the crystallization temperature,crystallization rate and crystallinity of PLA but also decreased the spherulitic size.At 0.75 wt% of the HBNA,the crystallization temperature of PLA raised from 90 ? to 123 ?,while the crystallinity increased from 3 % to 40 % and the crystallization halflife time(t_1/2)at 135 ? was decreased from 31 to 1.5 min.Mechanism research shows that HBNA can dissolve in the PLA matrix.Upon cooling,HBNA could self-organize into bundlelike templates by hydrogen bonding to initiate the fast crystallization of PLA,forming spherulites,shish-kebab and dendrite-like crystal structure.Then,needle-like oxalamide nucleators?OXA_n?with different spacer length in the middle were added to PLA.The effect of spacer length on the dispersion and the crystallization of PLA was studied.The results showed that OXA₂ had the best nucleating effect,which can selfassemble into needle-like crystallites and disperse uniformly in the matrix,and the t_1/2 of PLA is shortened by 90% at 135 ?.Therefore,taking PLA/OXA₂ as an example,a unique interlocked network-like shish-kebab crystal structure was formed upon hot-stretching.As a consequence,a tensile strength of 120 MPa,an elongation at break of 30⁵0 % and a heatresistant temperature of 150 ? of the PLA composites were simultaneously achieved,which provided a new route for the engineering application of PLA materials.In order to further improve the crystallization rate,shear conditions were applied to PLA/OXA₂.A synergetic effect of OXA₂ and shear flow on speeding up the crystallization of PLA was observed.With the synergetic effect,a certain extent of ?-form crystals and "shish-kebab" structures of PLA could be formed at 155-160 ?,which was 14 ? higher than that reported in the literature.A critical shear condition for fast crystallization of the PLA/OXA₂ at 155 ? was indentified as 0.4 rad/s for 10-15 s.At a higher melt temperature,a better organization and uniformity of the OXA₂ superstructures would be formed,which can further increase the crystallization rate of PLA.Mechanism study revealed that a certain extent of PLA chain orientation occurred in the shear flow and subsequently stabilized by the OXA₂ fibrils,which provide extra nucleating sites and are responsible for the significantly enhanced crystallization rate.Finally,PLA hybrid(PLA_OXA)were prepared by ring-opening polymerization of lactide initiated by the hydroxy terminated oxalamide compounds?OXA?.A new mechanism of intramolecular self-nucleation induced crystallization of PLA was proposed.The OXA groups of PLA_OXA not only increased the nucleation density,but also decreased the spherulitic size.PLLAOXA could rapidly crystallize at a cooling rate of 10 ?/min,under a supercooling of below 30 ?,PLA hardly crystallized whereas under the same conditions.Hoffman-Lauritzen model showed that the nucleation constant of PLA_OXA is obviously higher than that of neat PLA.These results indicate that the nucleation of the intramolecular OXA could promote the crystallization of PLA_OXA.