Stereoxomplexation-Induced Fractionated Crystallization Kinetics And Polymorphic Crystalline Structure Of Poly(lactic Acid)

Stereocomplex(SC)crystallization is a general behavior in polymer crystallization and it is also a special cocrystallization process between different polymers.The polymer with complementary configurations and chiralities can form SCs in their blends and stereoblock copolymers.SCs usually have denser chain packing in the crystal lattice than that in the common homocrystallites(HCs),due to the presence of additional intermolecular interaction.Thus,the stereocomplexed polymer generally have distinct physical property from the corresponding homocrystalline polymer.Stereocomplexed polymers have higher melting temperature,improved thermal resistance,crystallizability,degree of crystallinity,processability,mechanical properties and solvent-resistance.Herein,SC crystallization has been an effective approach to improve the physical performances of polymeric materials.However,SC crystallization of stereocomplexable polymer blends with complementary configuration is much more complicated than the conventional homocrystallization because of the interplay and competition between SC and HC.SCs formed in the initial stage of crystallization can have confinement effects on the subsequent crystallization of polymer chains.Under the specific conditions,such confinement effects can cause the fractionated crystallization and the formation of unusual polymorphic structure.In this thesis,we select the poly(L-lactic acid)/poly(D-lactic acid)(PLLA/PDLA)blends with various molecular weights and mixing ratios as the target system and investigate the fractionated crystallization kinetics,formation and evolution of different polymorphs(especially the unconventional ?,?,and mesophase polymorphs of HCs).This study has clarified the fractionated crystallization,complicated polymorphic behavior and their mechanisms for the PLLA/PDLA blend system.The results obtained in this study will help to further understand the behavior and processes of confined,fractionated crystallization and SC crystallization in the multi-component polymer systems;they will also provide significant guidance for the preparation and processing of SC materials.Firstly,the melting temperature of SCs(Tm,SC,?230?)and HCs(Tm,HC,?170 ?)are different in PLLA/PDLA racemic blends,thus,there is solely SCs when the crystallization temperature(Tc)is between Tm,SC and Tm,HC.The nonisothermal and isothermal melt crystallization after the isothermal crystallization at Tc which is higher than Tm,HC and lower than Tm,sc of PLLA/PDLA with different molecular weights are investigated.Only SCs are formed at Tc(180-200?)in high-molecular-weight(HMW)PLLA/PDLA blends.HCs are formed in subsequent nonisothermal and isothermal melt crystallization.The crystallization of HMW PLLA/PDLA blends at high Tc(Tm,HC<Tc<Tm,sc)proceeds through the cluster of paired helices.The cluster as precursor of SCs can transform into SCs by strong driving force under large supercooling.Due to the presence of cluster,it is hard to obtain HCs and SCs at high Tc(200?)even the crystallization time is long enough.Secondly,the medium-molecular-weight(MMW,?40k)PLLA/PDLA racemic blend exhibits unique fractionated crystallization kinetics(i.e.,multiple crystallization peaks)during heating because of the constrained effect of the precedingly formed SCs in the early stage of crystallization.The MMW PLLA/PDLA blend forms the metastable ?-form HCs at a low crystallization temperature(Tc,75-100 0C)but the common ?/?'-HCs at a high Tc(100-140?),besides SCs.We propose that the formation of ?-HCs stems from(i)the constrained effects induced by precedingly formed SCs and(ii)the conformational matching between ?-HCs and SCs.The ?-HCs formed in the MMW blend have a larger long period than its ?/?' counterpart and transform into the more thermally stable a-HCs through melt recrystallization during heating.Blending ratio is one of the key factors affecting the crystalline structure of PLLA/PDLA blends.The PLLA/PLDA asymmetric blends with various molecular weights(6-107 k)and PDLA fraction(fD,0.02-0.5)were selected as the model system and investigate the effects of preexisted SCs on the crystallization kinetics and polymorphic structure.We find that the unusual ?-form HCs of PLA can be formed in the asymmetric PLLA/PDLA blend,which is strongly influenced by the molecular weights of used polymers,PLLA/PDLA mixing ratio,thermal treating temperature(Tmax),and Tc.The formation of ?-HCs is preferred in the asymmetric PLLA/PDLA blends with low and medium molecular weights,medium fD(0.1-0.2),medium Tmax(170-200 ?),and low Tc(70-110 ?).We propose that the formation of ?-HC stems from the constrained effects of preexisted SCs;this constrained effect is governed by the content of preexisted,unmelted SCs in the thermally-treated samples.The metastable ?-HCs reorganize into the more stable ?-HCs via melt recrystallization in the heating process.Finally,the formation of SC in HMW PLLA/PDLA 1/1 blends is strongly depressed.The miscible blending and block copolymerization are applied to enhance SC in HMW blends.On one hand,the HMW PLLA/PDLA mixture blended with poly(ethylene glycol)(PEG)was selected as the model system.The incorporation of small amount of PEG leads to the fractionated crystallization kinetics and the formation of ?-HCs.Formation of ?-HCs is preferred in the PLLA/PDLA/PEG blends with the PEG mass fraction of 2%-5%at low Tc(80-110?).On the other hand,PLLA-PEG-PLLA/PDLA-PEG-PDLA 1/1 blend was selected to investigate the synergetic effects of copolymer composition and PLLA/PDLA stereocomplexation on crystallization.The stereocomplexable blend having the high PEG fraction forms the homocrystalline mesophase at the low Tc(70-120?)but the usual ?-form HCs at high Tc(120-160 ?),except for the SCs.We propose that the mesophase formation in enantiomeric blend is resulted by the constrained effects induced by the stereocomplexation and microphase separation of block copolymers.The homocrystalline mesophase formed in stereocomplexable blend is metastable and reorganizes into the more thermally-stable ?-HCs and SCs during heating.The results obtained will help to further understand the behavior and process of confined,fractionated crystallization and SC crystallization in the multi-component polymer systems;they will also provide significant guidance for the preparation and processing of SC materials.