M C. Paina et al. / Carbon 41(2003)1399-1409 27 T=270c sE8品3 25 24 35 Time(min) Fig. 2. Isothermal differential scanning calorimetric scans for the VT polymer(AN/MA: 88/12)copolymer at various temperature 60 min. A temperature interval ranging from 220 to 270C was studied, and the scans were performed at heating rates of5,10and20℃c/min Chemical changes as measured by nitrile conve ersion were analyzed using Fourier transform infrared spectros- copy(FT-IR)techniques. The variation in nitrile con- 3 centration across large diameter fibers was studied by IR microscopy, using a Nicolet Magna 550 with NicPlan FT-IR microscope and mapping stage. The thinner 10C/man fibers were analyzed using an Endurance Foundation cmIn Diamond AtR and the nicolet Magna 550 The tensile properties of both fiber types were measured 2502 290310 at four different stages: as-spun fibers, after UV irradiation, Temperature (C after heat stabilization, and after carbonization. The single Fig. 3. DSC thermograms for VT polymer at different heating filament tensile tests were performed on approximately 20 rates fibers from each stage, using a computer controlled MTI tensile testing machine equipped with a 500-g load cell 4. Results and discussion 4.1. Precursor thermal analysis Isothermal DSC experiments were conducted to evaluate the thermal stability of the VT precursor by heating the 多 er samples to a set temperature, ranging from 220 to 270C, for I h. It was reasoned that if the precursor was thermally stable for approximately I h, it could be melt pun in a batch or a continuous extruder. As displayed in Fig 2, the polymer is very stable up to 230.C At 240C a slow reaction is initiated and a small amount of heat is evolved toward the end of the experiment. At 250C the exothermic cyclization reaction takes place after a brief delay. As the temperature is increased, the reaction begins Te m perature (c) earlier and the reaction rate increases. These isothermal Fig. 4. Evaluation of the order of reaction from the shape of the tests indicated that the vt precursor can be maintained in a DSC curve1402 M.C. Paiva et al. / Carbon 41 (2003) 1399–1409 Fig. 2. Isothermal differential scanning calorimetric scans for the VT polymer (AN/MA: 88/12) copolymer at various temperatures. 60 min. A temperature interval ranging from 220 to 270 8C was studied, and the scans were performed at heating rates of 5, 10 and 20 8C/min. Chemical changes as measured by nitrile conversion were analyzed using Fourier transform infrared spectros￾copy (FT-IR) techniques. The variation in nitrile con￾centration across large diameter fibers was studied by FT-IR microscopy, using a Nicolet Magna 550 with NicPlan FT-IR microscope and mapping stage. The thinner fibers were analyzed using an Endurance Foundation Diamond ATR and the Nicolet Magna 550. The tensile properties of both fiber types were measured at four different stages: as-spun fibers, after UV irradiation, after heat stabilization, and after carbonization. The single Fig. 3. DSC thermograms for VT polymer at different heating filament tensile tests were performed on approximately 20 rates. fibers from each stage, using a computer controlled MTI tensile testing machine equipped with a 500-g load cell. 4. Results and discussion 4 .1. Precursor thermal analysis Isothermal DSC experiments were conducted to evaluate the thermal stability of the VT precursor by heating the polymer samples to a set temperature, ranging from 220 to 270 8C, for 1 h. It was reasoned that if the precursor was thermally stable for approximately 1 h, it could be melt￾spun in a batch or a continuous extruder. As displayed in Fig. 2, the polymer is very stable up to 230 8C. At 240 8C a slow reaction is initiated, and a small amount of heat is evolved toward the end of the experiment. At 250 8C the exothermic cyclization reaction takes place after a brief delay. As the temperature is increased, the reaction begins earlier and the reaction rate increases. These isothermal Fig. 4. Evaluation of the order of reaction from the shape of the tests indicated that the VT precursor can be maintained in a DSC curve