Network Structure Regulation And Shape-memory Function Design Of Cross-linked Poly(L-malic Acid)

Poly(L-malic acid)(PMA)is a biomass-derived polymer whose monomer,L-malic acid,is widely found not only in plant fruits but also in a variety of food processing wastes.On the one hand,PMA has rich side carboxyl groups and has the potential to construct chemical cross-linking networks.On the other hand,PMA is a biodegradable,biocompatible,and chemically synthesized green polymer.Because PMA not only meets the molecular structure requirements of shape memory polymers(SMPs)for building cross-linked networks but also conforms to the green polymer properties of biodegradable materials,it can be used to develop a green SMPs based on PMA,which can not only achieve the materialization and material functionalization of PMA but also enrich the green material system of SMPs.However,the following limitations and difficulties still exist to broaden the material properties of PMA and to develop the shape memory function of PMA:(1)PMA is easily soluble in water,has poor mechanical properties,and cannot be used as a general polymer material;(2)PMA is a linear polymer,which makes it difficult to expand the shape-memory function;(3)Cross-linked PMA and PMA-based shape-memory materials are rarely addressed by researchers and are difficult to study;To address the above problems,this thesis utilizes a two-step cross-linking method to construct a cross-linked PMA network by introducing diol cross-linkers with different physicochemical properties and PMA.Cross-linked PMA materials with general polymer properties are prepared,which avoid water solubility of materials and enhance the mechanical properties,endowing the cross-linked PMA materials with basic material properties.Then,the reversible transition processes(glass transition process and crystallization melting process)in the cross-linked PMA network are used as the reversible stimuli-sensitive switching domains of shapememory polymers,to regulate the glass transition temperature(Tg)or crystallization melting temperature(Tm)by controlling the network structure,and thus regulate the shape-memory transition temperature(Ttrans:shape fixing temperature(Tf)and shape recovery temperature(Tr)).On this basis,the developed cross-linked PMA material is endowed with diverse shape-memory functions by designing a high-level network structure,and furtherly improves the behavioral complexity of shape-memory deformation and enhances the controllability and designability of shape-memory response.In addition,the conformational relationship between the cross-linked PMA network structure,material properties and shape-memory functions are clarified.The mechanism and law of the regulation of material properties and shape-memory functions are found out.From the microscopic molecular level to the macroscopic performance/functional scale,practical solutions are proposed for developing cross-linked PMA materials and diversifying their shape-memory functions in this dissertation.This dissertation is divided into the following five parts:1.Synthesis of cross-linked poly(L-malic acid)and investigation of heat-induced shapememory polymerThe Ttrans-adjustable,heat-induced cross-linked PMA shape-memory polymer is developed in this chapter.The amorphous cross-linked PMA network is constructed by using PMA synthesized by polycondensation as the molecular chain backbone and 1,8-octanediol as the cross-linker,thereby enhancing the mechanical properties of PMA and avoiding the water solubility of PMA.The cross-linked PMA material has a maximum fracture strength of 67 MPa and Young’s modulus of 1.6 GPa,making it possible to be used in general-purpose applications.By regulating the curing time,the parameters of this system such as cross-linking density(0.25-1.06(103 mol m-3)),Tg and its Ttrans(Tf:15-35℃;Tr:50-100℃)can be regulated,thus giving the material a heat-induced shape-memory function and a tunable mechanical property.This green thermosetting shapememory polymer can also be restructured by extending the curing time to reconstruct its crosslinked network and reshape its permanent shape.The cross-linking process of PMA with diols developed in this chapter provides the basis for more complex structures and diverse function designs of cross-linked PMA materials.2.Regulation of solvent affinity of cross-linked poly(L-malic acid)network and its differentiated shape-memory functionThe structure of the cross-linked PMA network is regulated by controlling the carboxyl/hydroxyl group feeding ratio,which regulates the Tg and Ttrans of the material.the resulting cross-linked PMA material has both heat-and solvent-induced shape-memory effects,and its solvent affinity(Huggins parameter:0.6-2.6)can be adjusted by regulating the cross-linking density(0.3-1.59(103 mol m-3))or the molecular weight between the cross-linking points(Mc,750-3860 g mol-1).The mechanism of the solvent-induced shape-memory effect of cross-linked PMA is further explored based on the solvent-polymer interaction and the resulting changes in physical interaction.Differences in Huggins parameters between the material and water,methanol,ethanol are used to achieve differential shape memory deformation of the material in these solvents and their mixtures,and a quantitative relationship between the solvent mixing ratio(0-100%)and the degree of shape-memory recovery(26°-135°)was established.Based on the preparation of cross-linked PMA materials and the regulation of heat-nduced shape-memory function in the previous chapter,this chapter proposes a method to tailor the solvent affinity by regulating the cross-linking density or Mc as a way to achieve the differentiated shape-memory response.3.Spatial distribution of cross-linked poly(L-malic acid)network arrangement and its asynchronous shape-memory functionThe cross-linked PMA materials with the same cross-linking density and different Mc(8851054 g mol-1)are prepared by using diol cross-linkers with different fatty chain carbon numbers(chain length)to construct chemically cross-linked networks with PMA in this chapter.The effects of different network structures on the shape-memory triggering energy barrier are further investigated from the perspective of the free volume fraction(0.124-0.156)and isomerization energy(7.38-9.18 Kcal mol-1)of the cross-linked network by regulating the mesh size(Mc)of the cross-linked network to regulate the Tg and Ttrans of the materials,which can provide theoretical guidance for the design and control of the shape-memory response process.On this basis,the twostep cross-linking method is used to combine pre-cross-linking PMA with different network structures and arrange the spatial distribution of the cross-linked network so that different regions of a block of material show different deformation speed(0.4-2.1 rad/s)and the sequential response(1-20 s),thus showing the asynchronous deformation characteristics.Then,the composite actuator is prepared by using the excellent adhesion property of the material and compounding with polar materials.Based on the regulation of the network structure in the previous chapter to give the material a differentiated shape-memory response,this chapter proposes a method to achieve an asynchronous shape-memory response by macroscopic processing to arrange the spatial distribution of crosslinks.4.Crystallization design of cross-linked poly(L-malic acid)networks and its multiple stimuli-responsive shape-memory function with large strainThe crystallizable PCL diol and PMA are used to construct a cross-linked PMA system with crystallization ability in this chapter.The Tm of this system is used instead of Tg as the Ttrans that triggers the shape-memory response of the material(Tf:0℃;Tr:36.4-56.7℃),and the introduction of long-chain PCL diol makes the cross-linked PMA shape-memory material with large-strain deformation ability(250-2000%).The cross-linked PMA material is endowed with light-thermal and electro-thermal conversion capabilities by filling with carbon nanofibers,enabling the composite to be deformed by light-and electro-triggered shape-memory shifting.The light-thermal(0-5.1℃ s-1)and electro-thermal conversion rates(0-7℃ s-1)of the materials are tailored by regulating the content of carbon nanofibers or the intensity of the light/electricity,which in turn controlled the shape-memory response process of the composites.Based on the previous chapter,which arranges the spatial distribution of the cross-linked network to design the asynchronous shape-memory function,this chapter substantially improved the deformation strain by constructing a crystalline cross-linked PMA network,and then endowed the cross-linked PMA material with a shape-memory function that can regulate the response process and large-strain multi-stimuli responses by introducing exogenous functional particles and regulating external stimuli.5.Photo-regulation of cross-linked poly(L-malic acid)network and its shape-memory function with controllable morphology and pathwayThe photo-reversible cross-linked PMA network is prepared in this chapter by introducing PCL pendant chains capped by photo-reversible groups(coumarin derivatives)into the cross-linked PMA system.After the completion of cross-linking and curing,the topology of the network is directly regulated using ultraviolet(UV)irradiation.The mechanism of the effect of UV irradiation on the network topology(0.009-0.102(103 mol m-3))is investigated by in-situ Raman spectroscopy,and the effect of UV on the thermal properties of the material(Tm:37.2-47.0℃)is clarified.On this basis,the values of Ttrans in different regions are regulated by controlling the irradiation area and irradiation time.On the one hand,the controllability of morphology and pathway of shapememory deformation is designed.On the other hand,the material is endowed with the ability of fluorescence/crystallization pattern manifestation.In addition,the PCL pendant chains endow the cross-linked system with the ability of strain-induced crystallization,so that the material can quickly fix the shape at room temperature(Tf:20℃;Tr:37.2-47.0℃)while storing the entropic energy without complicated shape fixing operations.Based on the introduction of functional particles and regulation of external stimuli to control the shape-memory response process in the previous chapter,this chapter proposes a method to directly design shape deformation using irradiation and stretching to achieve controllability of shape-memory deformation morphology and pathway.