The Cyclization Reaction Of Polyacrylonitrile-Based Carbon Fiber By DFT

Polyacrylonitrile(PAN)fiber has many excellent properties,such as small density,excellent heat resistance,low thermal expansion coefficient,high modulus,good resistance to high temperature,strong fatigue,anti-corrosion,outstanding thermal and electrical conductivity and heat,excellent electromagnetic shielding performance.It has been widely used in aerospace,automotive industry,sports equipment,medical equipment,civil construction and other fields.The cyclization reaction of PAN has big effect on the quality of carbon fiber.For manufacturing PAN based carbon fiber materials with high performance,it is very important to study the cyclization mechanism of PAN.In this thesis,Gaussian 09 suit of programs were used to explore the cyclization reaction mechanism of polyacrylonitrile.The geometry optimization,vibration frequency analysis,energy calculation and reaction path analysis were performed through the density functional theory(DFT)with the B3LYP/6-31+G(d,p).For the cyclization reaction of PAN,two types of reaction mechanism exist: the ionic and free radical reaction mechanism.The study of free radical reaction mechanism shows that the energy barrier of the initial step is very low,and the reaction speed can be very fast.The barrier of the second step is also very low,which indicates that once the cyclization is triggered,the reaction will be very fast,the huge amount of heat will be released,and this further accelerates the reaction and make the reaction uncontrollable.The study of the ionic reaction mechanism indicate that the cyclization of copolymer containing acids and acrylonitrile(AN)-acrylamide(AM)copolymers are autocatalytic.The cyclization of copolymer with carboxylic acid can proceed via the transition state with eight membered ring and six membered ring,the transition state with eight ring is more likely to occur than that of the transition state with six ring.In addition,the two step cyclization reaction energy barrier of copolymer containing acids are lower than that of acrylonitrile(AN)-acrylamide(AM)copolymers,which is consistent with the experimental results.By adding additive with active hydrogen,the cyclization energy barriers are much higher than those of the cyclization of copolymer containing acids and acrylonitrile(AN)-acrylamide(AM)copolymers.It indicates that the cyclization of PAN is not much effected by adding the compounds with active hydrogen.