Crystallization,Microphase Separation And Hierarchical Ordered Structure Of Aliphatic Polyesters Chemically Modified And Physically Blended With Liquid Crystalline Molecules

Liquid crystal is a type of functional soft material that can retain the anisotropic ordering alignment of the crystalline state after melting or dissolving in the solution.Though losing the the rigidity of the solid state,liquid crystal can flow like liquid in the liquid crystalline phase.By introducing the liquid crystalline units,the types of microphase separation and hierarchical ordered structures will be enriched.According to the difference of combing strength between liquid crystals and polymers,it can be divided into physical blending and chemical modification.Due to the difference in compatibility between the liquid crystalline molecules and polymers,microphase separation can occur in these systems,which can also be induced by polymer crystallization.Microphase separation can affect the distribution of liquid crystal molecules and the ordering arrangement of liquid crystal units in the polymer crystals,which in turn influences the physical properties and functional properties of liquid crystalline polymer materials.However,the influence of polymer condensed state structure and polymer crystallization behavior on the microphase separation behavior and ordered microphase structure are rarely involved.Especially for semicrystalliine polymers containing liquid crystals,the relationship among the formation and regulation of multi-level ordering structures,the multi-scale distribution of liquid crystal molecules,and polymer crystallization dynamics is rarely reported.In this paper,we chose poly(ε-caprolactone)(PCL),poly(L-lactic acid)(PLLA)and poly(butylene adipate)(PBA)as a model semicrystalline polymer,and small molecular liquid crystal containing biphenyl group as a model liquid crystal.We focused on the crystallization,microphase separation and hierarchical ordered structure of aliphatic polyesters chemically modified and blended with liquid crsyalline units.In this work,we conbined liquid crystals and crystallizable polyesters via chemical modification.To investigate the effect of polymer crystallization on the microphase separation and hierarchical ordered structure,we first chose PCL with low melting temperature as the flexible semi crystalline block and 4-(6-hydroxy-1-hexyloxy)-4’-cyanobiphenyl(H6CBP)as the LC end group to prepare a mono-end-functionalized polymer with various molecular weights(LC-PCL).The structural organization of LC-PCL is driven by the ordering of the LC end group and the crystallization of the PCL block,both of which are governed by the length and crystallizability of the PCL block.The role of PCL crystallization is more predominant with increasing PCL block length.LC-PCL with a short PCL block organized in a smectic-like bilayer structure.LC-PCL with a medium PCL block length assembles in multilayers comprised of alternating PCL lamellae and LC layer;however,the formation of the LC layer is prohibited when the crystallization of PCL is fast.LC-PCL with a long PCL block only forms PCL lamellae,similar to conventional semicrystalline polymers.The structural organization of LC-PCL is also influenced by the crystallization kinetics of the PCL block.The formation of the LC layer of LC-PCL with a medium PCL block length is facilitated by the slow crystallization of the PCL block,and vice versa.Then,we used H6CBP as the LC end group and poly(lactic acid)(PLA)with high melting temperature as the flexible semicrystalline block to prepare a mono-end-functionalized polymer,named as LC-PLA.Due to the strong crystallization capability of PLA chains,LC-PLA with short PLA block length(≤1 kDa)can self-assemble in multilayers comprised of alternating PLA lamellae and LC layer after solution casting.The LC units of LC-PLA(Mn=1.0 kDa)are at disordered state after isothermal crystallization at different crystallization temperature(Tc).Once undergoing the heating or annealing process,the LC units between the PLA lamellaes can organize into ordering structure.Due to the different degree of confinement of LC units between the PLA lamellaes,the capability and rate of rearrangement of LC units are strongly dependent on Tc and annealing temperature.At higher Tc,the size of confined space of LC units get larger.At higher annealing temperature,the mobility of PLA chains get better,which is beneficial to the earrangement of LC units.The microphase separation of aliphatic polyesters chemically modified with liquid crsyals only happened on the nanoscale.To investigate the microphase separation on a micron scale,we chose PBA as a model semicrystalline polymer,and 4-cyano-4’-n-pentylbiphenyl(5CB)as a liquid crystalline diluter by physically blending.We further studied the crystallization kinetics,polymorphic crystalline structure,crystalline structural transition,and crystalline morphology of PBA/5CB mixtures.The spherulitic growth rate and crystallization rate of PBA decrease after mixing with 5CB,attributing to the dilution effect of liquid crystals.Compared to neat PBA,the formation of thermodynamically stable α crystals is favored in the PBA/5CB blend at the same crystallization temperature(Tc),ascribed to the depression of equilibrium melting temperature of PBA.The heating-induced β-to-α transition of PBA crystals shifts to lower temperature in the PBA/5CB blend compared to the neat PBA.The crystalline morphology of PBA is strongly dependent on the amount of 5CB and Tc.The long period of PBA crystals enhances with the addition of a small amount of 5CB,due to the interlamellar segregation of 5CB in PBA crystals.The distribution of 5CB changes from the interlamellar to interspherulitic or interfibrillar regions of PBA crystals as the 5CB content increases in the PBA/5CB blend.Furthermore,in order to investigate the effect of polymer crystallization on the liquid crystalline phase behavior,we used PLLA as a semicrystalline polymer and polymorphic 4-nhexyloxy-4’-cyanobiphenyl(6OCB)as a liquid crystal(LC)to prepare the polymer/LC mixture.We demonstrated a feasible approach to evaluate the interlamellar and interspherulitic segregations of the blend component in polymer crystals.The LC can form stable and metastable solid phases when mixed with the semicrystalline polymer PLLA.The contents of stable and metastable phases are influenced by the blend composition and the PLLA crystallization conditions.Lowering the PLLA crystallization temperature is favorable for the formation of the LC metastable phase in the PLLA/LC mixture.The formation of metastable and stable phases of the 6OCB originates from the segregation of the LC in the interlamellar and interspherulitic regions of polymer crystals,respectively.