Crystallization And Microphase Separation Of Supramolecular Aliphatic Polyesters End-Functionalized By Multiple Hydrogen-Bonding Units

Supramolecular chemistry is a powerful tool to create supramolecular polymers,which are widely used as stimuli-responsive functional materials.End-functionalization of homopolymers by functional groups capable of forming reversible noncovalent bonds(e.g.,H-bonds)is a straightforward approach to prepare supramolecular polymers.Since single H-bonds are weak,groups capable of forming multiple H-bonds(MHB)have been frequently used in preparing end-functionalized supramolecular polymers due to their high bonding strength,ease of preparation,good selectivity and reversibility.Because the MHB unit and the linked polymer usually have very different polarities,they can assemble into the specified structures through microphase separation,analogous to the microphase separation of block copolymers.It is well known that block copolymers can self-assemble into diversified ordered structures.However,it is rather difficult to achieve a long-range ordered structure through the microphase separation of MHB-functionalized supramolecular polymers due to the complicated effects of noncovalent interactions and the compatibility of the polymer and the end-groups.On the other hand,some MHB units possess a high crystallizability,even when attached to the polymer termini.Crystallization of MHB units can drive the microphase separation of supramolecular polymers.Crystallization-induced microphase separation has been used to create long-range ordered structures(e.g.,lamellar,cylindrical,or hexagonal structures)in semicrystalline block polymers.Moreover,structural organization of crystalline block copolymer is tied to the crystallization kinetics and pathway(e.g.,crystallization sequences)of different blocks.Tuning the crystallization kinetics provide additional strategies to control the organized structures and physical properties of block copolymers.We expect that these crystallization-kinetics-based strategies are also applicable to control the structural organization of crystallizable supramolecular polymers,even though no relevant studies have been reported.In this work,we selected a crystallizable nucleobase,thymine(Thy),which could form self-complementary double H-bonds with a melting temperature(Tm)of～180℃ and the biobased degradable poly(ε-caprolactone)(PCL),poly(L-lactic acid)(PLLA)as our research system.We first syhthesized the hydroxyl-functionalized Thy(Thy-OH)as the initiator and prepared the Thy-end-functionalized polyester by ring-opening polymerization reaction with high end-functionality and well-controlled composition.Firstly,considering the high Tm of Thy,we synthesized the Thy-end-functionalized supramolecular poly(s-caprolactone)(PCL-Thy)with well-separated Tm’s.We studied the stepwise crystallization,confined crystallization and the hierarchical ordering structure.We elucidated the crystalline structure,crystallization kinetics,mechanics of non-covalent interaction,crystallization-induced microphase separation and structural evolution of Thy units and PCL blocks in PCL-Thy.The results showed that the stepwise crystallization of Thy units and PCL blocks occurred in the cooling process from the melt.The crystallization of Thy drove the formation of the lamellar ordering structure in the long range.Subsequently,the crystallization of the PCL blocks occurred in a confined manner between the preexisting Thy lamellae;the regularity of lamellar ordering slightly decreased with the crystallization of the PCL blocks.The PCL chains of the PCL-Thy formed non-folded crystals at low Mn,PCL but folded crystals at high Mn,PCL between the Thy lamellae.Secondly,we chose the Thy-monofunctionalized oligomeric poly(L-lactide)(PLLA),denoted as PLLA-Thy,as the model supramolecular polymer.PLLA is a typical biodegradable,semicrystalline polyester and has a wide crystallization temperature(Tc)range of 70-150℃,which covers the Tc and Tm range of Thy unit.This allows to study the competitive and coincident crystallizations of Thy unit and PLLA block at different Tc’s,as well as their effects on the structural organization of supramolecular polymer.We elucidated the competitive crystallization kinetics,crystallization-induced microphase separation,structural organization and evolution of PLLA-Thy with different Thy fractions.The results showed that high Thy fraction(short PLLA block)and low Tc were conducive to the crystallization of Thy unit.PLLA blocks could crystallize with dissociated and melted Thy terminals at high Tc.Prompt crystallization of Thy unit drove the lamellar organization of PLLA-Thy in the long range;the subsequent crystallization of PLLA blocks occurred in a confined manner between the pre-formed Thy lamellae and slightly decreased the lamellar period and lamellar ordering.We also prepared the low-molecular-weight thymine-functionalized PLLA-Thy,poly(D,L-lactic acid)(PDLLA-Thy),and the equal blends of PLLA-Thy/PDLA-Thy(denoted as LA-T/DA-T).By comparing the crystallizability,crystallization kinetics,and long-range ordering structure of those three types of supramolecular polymers,we investigated the effects and modulating strategy of chain structure,condention structure on the crystallization-induced lamellar organization in the thymine-functionalized supramolecular polymers.The results showed that with the increasing of Mn,PLLA,the long period of the lamellar structure increased.For the similar Mn,PLA,PDLLA-Thy showed the highest long period due to its random chain structure.For high Mn,PLA and Tc,the crystallization of PLLA blocks could decrease the lamellar period and the lamellar ordering.The formation of stereocomplex crystallization(SC)of PLA could also decrease the lamellar period and the lamellar ordering.Moreover,thanks to the close packing of SC-PLA crystals,the decrease was even more remarkable.In order to study the influence of the crystallization of MHB units on the properties of the supramolecular materials,we chose the crystallizable MHB units UPy-bonded PCL(denoted as U-PCL)as model system.We prepared the UPy end-functionalized three-arm PCL and elucidated the crystallization kinetics,crystalline structure,and thermal-induced structural evolution of the double-crystalline supramolecular polymer.The results showed that U-PCL displayed the similar composition-dependent crystallization/melting behavior and crystalline structure as the typical double crystalline block copolymers.The U-PCLs formed the UPy crystals,UPy/PCL mixed crystals,and PCL crystals as the PCL block length increased or the UPy content decreased.The melting of UPy crystals was accompanied by the dissociation of H-bonds in the UPy-UPy dimers.UPy units exerted a confinement effect on the crystallization of covalently-linked PCL blocks.Microphase separation between the UPy units and PCL blocks took place in the U-PCLs,despite the absence of long-range ordered phase structure.The U-PCLs with both PCL and UPy crystals could form uniform films and had better mechanical properties than the U-PCLs that solely form UPy or PCL crystals.Double crystalline nature of U-PCLs enabled the design of thermally-induced shape memory materials with good fixing and recovery abilities at the body temperature.