Study On The Functionalization Of Carbon Nanotube And Its Polyurethane Composites

Polyurethane is a kind of block copolymer consisting of polyester or polyether softsegments and urethane hard segments. As an important and versatile class of polymermaterial, polyurethane has been receiving increasing attention and has various end usepurposes in a broading area due to its high abrasion resistance, wearability, chemicalresistance, high impact strength, excellent flexibility and outstanding damping ability.However, conventional polyurethane has exhibited poor thermal stability, which limited itsvarious applications. Carbon nanotube(CNT), as a novel material in twenty-first century,was considered to be the most promising candidate as ideal filler owing to its uniquestructure and particular physical properties. In this paper, carbon nanotube was used tomodify polyurethane due to its high thermal stability and to prepare thepolyurethane/carbon nanotube composite.Due to intensifying crisis of oil and enviormental influence of petroleum-basedpolymer, the study on the enviormental benign polymer has been paid increasing attention.Poly(lactic acid) comes from renewable corn or sugar beet etc. and could convent intocarbon dioxide and water after degradation; what's more, it had good biocompatible andbiodegradable properties. In this paper, poly(lactic acid) was used as soft segment toprepare the enviormental benign polyurethane.Dispersion is the key problem about the application of carbon nanotube in polymercomposite. In order to obtain the homogeneous dispersion of carbon nanotube in thepolyurethane matrix and to increase the compatibility with the matrix and force ofinteraction, carbon nanotube was functionalized with poly(lactic acid) based on in situ polycondensation of lactic acid and carbon nanotube. To prepare for the polyurethane,prepolymer composite was also prepared. Finally the enviormental benignpolyurethane/carbon nanotube was prepared based on the chain extention reaction ofaliphatic diisocyanate and the hydroxyl termintated poly(lactic acid)/carbon nanotubeprepolymer and in situ cross linking of the functionalized carbon nanotube.In order to investigate the functionalization of different kinds carbon nanotubes, thegrafting of different kinds of carbon nanotubes were given under same condition. Thedetails about the studies were shown as following:1. The functionalization of different kinds carbon nanotubesIn order to investigate the variation before and after functionalization of carbonnanotubes, polypropylene glycol of molecular weight of 1000 was grafted on thesingle-walled carbon nanotube, double-walled carbon nanotube and different diametermulti-walled carbon nanotubes.The result showed that polypropylene glycol with molecular weight of 1000 wassuccessfully grafted on the single-walled carbon nanotube, double-walled carbon nanotubeand different diameter multi-walled carbon nanotubes. Raman spectra and TEM analysisdemonstrated that grafted polymer on single-walled carbon nanotube and double-walledcarbon nanotube was not uniform and the thickness was less than 1nm due to the moreperfect structure. Meanwhile different diameter multi-walled carbon nanotubes wereuniform coated with 5nm thickness polymer due to the less perfect structure. The TGAanalysis and calculation of grafting samples indicated that per unit area of single-walledcarbon nanotube achieved the highest quantity of grafting polymer while the per unit massof multiwalled carbon nanotube contain the highest quantity of grafting polymer comparedwith the other kinds of carbon nanotubes.2. The functionalization of carbon nanotube with poly(lactic acid) based on in situpolycondensationThe carbon nanotube was functionalized with poly(lactic acid) based on in situ polycondensantion of carboxlyic carbon nanotube and lactic acid in one step.FTIR and TGA analysis indicated that the molecular weight of grafting polymerswere increased from ca. 400 to 2500 as polycondensation time increased from 5 hours to23 hours equilibrium. UV-Vis spectra showed that the prepared poly(lactic acid)functionalized carbon nanotube dispersed uniform in the solution chloroform without anyaggregation.HRTEM images showed that the grafting polymer were coated uniform on thecircumference and ended parts of carbon nanotubes; what's more, the thickness of thegrafting polymer increased with the polycondensation time.3. The preparation and study of hydroxyl terminated poly(lactic acid)/carbonnanotube compositeThe hydroxyl terminated poly(lactic acid) prepolymers incorporating pristine CNTsand CNT-COOH respectively were prepared based on in situ polycondensation of lacticacid and 1,4-butanediol. As parallel samples, the pure PLA diol prepolymers weresynthesized at the absence of CNTs. TGA analysis indicated that the initial decompositiontemperature of the prepolymer incorporating CNT-COOH was a little higher than thatincorperating with the pristine and both of them were pronounced higher than pureprepolymer. DSC measurements proved that the amorphous and crystalline prepolymerswere obtained catalyzed by stannous octoate and sulfric acid respectively. The addition ofCNTs made it easy for the polymer to crystallize, however it didn't change the spheruliticcrystalline morphology from the POM images. FESEM images showed that covalentlyfunctionalization of CNT-COOH contributed the homogeneous dispersion in the polymermatrix, which was essential for the further application. The work was suggested to pavethe way toward carbon nanotube-improved-PLA-based environmental benign blockcopolymer nanocomposites for various end-use purposes.4．The preparation and study of polyurethane/carbon nanotube composite base onin situ cross linking The hydroxyl terminated poly(lactic acid)-functionalized-CNTs were used ascross-linker to form the polyurethane/CNTs composite. Compared with the purepolyurethane and pristine CNTs blend polyurethane composite, such polyurethane/CNTscomposite based on in situ cross-linking of CNTs achieved better thermal properties due tothe uniform dispersion of CNTs. Due to the"green"feature of poly (lactic acid), theas-prepared improved PU composite was environmental benign and may have a broaderpotential application, especial in biology and medicine.