| Blending as a modification method of mixing two or more polymers to prepare new materials with synergistic properties of each component is often applied to aliphatic polyester systems because of its simple preparation process,high efficiency and low cost,but the poor compatibility in crystalline phase for different components remarkably depresses co-crystallization capability,which hinders co-crystallization and thus affects the design and regulation of the comprehensive performance of the material.Therefore,it is a great challenge to improve the co-crystallization theory in the melt of crystalline/crystalline blends and to promote efficient co-crystallization by simple and effective means.Meanwhile,there has been a great controversy about how the molecular chains move to the crystal growth front during melt crystallization,and some studies have confirmed the chain-straightening force in polymer crystallization process,but there is still no systematic study on the crystallization force.Therefore,how to indicate the polymer crystallization micro-stress field by internal probes and effectively visualize and quantify it is also an important issue that needs to be addressed.Based on the strict isomorphism of poly(butylene succinate-co-butylene fumarate)(PBSF)random copolyesters over the whole composition,this thesis explores the main factors and co-crystallization mechanisms affecting the co-crystallization behavior of binary crystalline/crystalline blend systems using the poly(butylene succinate)(PBS)and poly(butylene fumarate)(PBF)as the subjects,which have similar molecular structures.In addition,2-ureido-4[1 H]-py rimidinone(UPy)structural units with selfcomplementary multiple hydrogen bonds were introduced to the polymer chain ends using post-polymerization to further enhance the interactions between different components and achieve more efficient co-crystallization of the blended system.The complementary quadruple hydrogen bonds provided by UPy unit can provide immediate feedback to the dragging forces of molecular chains during crystallization and melting,and the cyano-substituted oligomerization oligo(p-phenylene vinylene)(UPy-OPV-UPy)of multiple hydrogen-bonded dye can produce an optical response to the force,therefore,UPy-OPV-UPy can be selected as intrinsic probes of the microscopic stress fields of polymer crystallization to explore the response law of multiple hydrogen bonding action to melt crystallization,as well as the qualitative and quantitative evaluation of the crystallization dragging forces to improve the polymer crystallization theory.Based on this idea,the following studies were conducted and relevant results were obtained in this thesis:Aliphatic polyesters with close molecular weights,PBS and PBF,were synthesized by a two-step melt-condensation method,and PBS/PBF blends with complex componentand rate-dependent co-crystallization behavior were prepared by solution-casting.It was confirmed that the isomorphism of PBS/PBF blend were only present in the PBS-rich fraction(i.e.,PBS/PBF-60/40 and PBS/PBF-80/20),and the rapid cooling was favorable for the formation of co-crystals.When the cooling rate is increased to 40℃/min,the cocrystals ratio in the blend PBS/PBF-80/20 can be increased to～75%.In addition,the defective Gibbs free energy of-0.96 kJ/mol and 1.94 kJ/mol were calculated for the insertion of the BF unit into the PBS-type crystal and the insertion of the BS unit into the PBF-type crystal,respectively,revealing the intrinsic thermodynamic drive for the formation of isomorphism.Finally,the crystallization behavior of PBS/PBF blends is summarized to provide new ideas for the design of crystalline/crystalline blends with desirable structures and properties.Based on the clear understanding that the PBS/PBF blends can achieve isomorphism,in order to further enhance the intermolecular chain interactions and connections,improve the compatibility of the blended system,we used hexamethylene diisocyanate(HDI)as a linking unit to connect the UPy structure to the chain ends of PBS and PBF,and systematically investigated the co-crystallization behavior of PBS-UPy/PBF-UPy blends,as well as the special crystal structure,spherical crystal morphology and crystallization kinetics of the blended system.The PBS-UPy/PBF-UPy exhibited more diverse component-and rate-dependent co-crystallization behaviors than the PBS/PBF blends due to the complementary quadruple hydrogen bonds acted as both end-group defects to hinder the crystallization of PBS and as "linkers" to enhance the miscibility of the crystalline phases.By optimizing the kinetic conditions,PBS-UPy/PBF-UPy-80/20 can achieve up to 98%efficient co-crystallization.UPy-OPV-UPy is a material with unique mechano-responsive fluorescent properties,where reversible non-covalent bonding interactions give it the property of producing changes in fluorescence intensity or wave number in response to different force stimuli.Therefore,we use UPy-OPV-UPy as a "fluorescence probe" to explore the crystallization dragging force of UPy terminally functionalized PBS.The crystallization force can be transferred to the chromophore along the molecular chain,causing the dye molecular aggregates to dissociate and transform into monomer emission or excimer emission,obtaining a fluorescence quantum yield of up to 97%.Meanwhile,the matching of the driving activation energy for breaking the quadruple hydrogen bond between UPy end groups and the crystallization activation energy of PBS-UPy in the isothermal crystallization process further indicates that the crystallization force can break the strong interaction of the quadruple hydrogen bond. |
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