| In situ microfibrillation is one of the effective methods to improve the mechanical properties of composite materials.The dispersed phase forms into microfibers in situ and uniformly disperse in the matrix material.The materials produced are called in situ microfibrillar composites(MFCs).However,the traditional in situ microfibers are usually formed by a single phase polymer,and their structure and morphology are relatively simple,mostly smooth cylindrical fibers,which is difficult to achieve the purpose of strengthening and toughening at the same time.Therefor,based on the microfibrils formation mechanism,this paper constructs microfibers structures such as “gourd skewers” or “trepang” and double dispersed phase,studies the effects of composition,viscosity ratio and chain extension on the morphology of microfibrils,reveals the formation and evolution mechanism of special-shaped microfibrils.The relationship between micro structure and macro properties of the composites was established.On this basis,carbon nanotube(CNTs)were introduced as functional fillers,and their effects on microfiber structure morphology,mechanical properties,viscoelastic properties and electrical conductivity of composites were studied,which provided theoretical guidance for the construction of high-performance functional in situ microfibrillar composites.The specific research content is as follows.1.The special-shaped microfibrillar composites,employing polyolefin elastomer(POE)as matrix and poly(lactic acid)/polyamide 6(PLA/PA6)masterbatch as dispersed phase,were successfully prepared in situ by using multistage extrusion stretching technology.By simply adjusting the mass ratio of PLA/PA6(25%),the special-shaped PLA/PA6 composite microfibrils in the form of “gourd skewers” or“trepang” can be obtained,which is mainly related to the viscosity difference,phase separation and inter-facial free energy of PLA/PA6.In addition,the morphology and content of special-shaped microfibril have obvious effects on the viscoelastic properties,crystallization behavior and mechanical properties of the composites.When the total content of PLA/PA6(1/1)is greater than than 10%,the composites appeared multiple-yield behavior during the tensile test.When the total content of PLA/PA6 is 25%,the tensile strength and elongation at break of the composites are increased by 2.03 times and 1.2 times due to the “hang nail” effect of the special-shaped microfibril.When the total content of PLA/PA6 is 30%,the viscoelastic properties of composites are the best。2.The viscosity ratio of the PA6/PLA was adjusted by the type of PA6 and the chain extension of PLA.It was found that PA6/PLA could form composite microfibrils in POE matrix in a wide viscosity ratio range of 0.08-64.3.With the increasing of the viscosity ratio of PA6/PLA,the morphology of composite microfibrils gradually evolved from cylindrical shape to “gourd skewers” shape and finally to “trepang” shape.The “gourd skewers” microfibrils with smaller diameter is more beneficial to improve the mechanical properties of POE/(PLA/PA6)in situ microfibrillar composites than the “trepang” microfibrils and cylindrical microfibrils.When the viscosity ratio is 12.8,the viscoelastic properties of POE/(PLA/PA6)composites are the best.The types of PA6 and the content of chain extender(ADR)have certain effects on the cold crystallization temperature and crystallinity of PLA.3.In situ bi-phasic microfibrillar composites,employing PLA as matrix and POE/PA6 as dispersed phase,were prepared by using multistage extrusion stretching technology.The results show that the POE forms micron sheets,while the PA6 forms microfibril.With the mass ratio of PLA/POE increased from 50/50 to 90/10,the thickness and width of POE micron sheets increased first and then decreased,and the average diameter of PA6 microfibril gradually decreased.With the increase of POE content,the tensile strength of PLA/(POE/PA6)bi-phasic in situ microfibrillar composites decreased,while the elongation at break increased slightly.Although the POE micron sheet does not achieve the ideal toughening effect,which may be related to the poor compatibility of PLA and POE,the designing of bi-phasic microfibrillar structure provides a new strategy for strengthened and toughened of composites.4.CNTs are introduced into POE/(PLA/PA6)in situ microfibrillar composites as functional fillers.The migration process of CNTs and their effects on the morphology and distribution of microfibril are investigated.The results show that CNTs are distributed in PA6 in POE/(PLA/PA6)in situ microfibrillar composites regardless of whether the CNTs supported by POE,PLA or PA6 when the CNTs content is less than than 1%.This result significantly affected the structure of PA6/PLA special-shaped microfibrils and changed the position distribution of microfibrils in the matrix.From the core to Sub-skin,PA6/PLA in the samples showed three different distribution states: short rod-like fiber,long fiber and spherical droplet,respectively.It is attributed that the CNTs affecting the crystallization behavior of PA6,increasing the viscosity ratio of PA6/PLA,and changing the matching relationship between microfibril setting and tensile stress.When the CNTs content exceeded 1%,it was difficult for PLA/PA6 to form composite microfibril.When the CNTs content increased from 1% to 7%,mechanical properties of POE/(PLA/PA6)composites first decreased and then stabilized,the viscoelastic properties gradually increased,and the electrical conductivity show obvious percolation phenomenon,with a threshold of conductive percolation of about 3%. |
PDF Full Text Request