1786 Z.R. Yue et al. /Carbon 37(1999)1785-1796 an important step in composite manufacture. Interfacial 2. 2. Electrochemical oxidation and heat treatment bonding in composites has been enhanced by fiber surface treatments such as electrochemical oxidation [3, 9-23] and Continuous electrochemical treatments were carried out oxidation in concentrated nitric acid [10, 14, 15, 24-33], in a U-tube apparatus. An aqueous 1% wt KNO, solution potassium permanganate [34], sodium hypochlorite [35], was used as the electrolyte. The carbon fibers were fed hydrogen peroxide and potassium persulfate [36,37]. Gase- continuously and served as the anode. A 254 cm long ous oxidations include air [38], oxygen [34], and ozone 39, 40 oxidation as well as plasma treatments 31, 41-46 B shaped stainless steel bar inside the U-tube acts as the thode. A gear system allowed variation of the fiber Fiber/matrix adhesion is improved through a combination residence time in the oxidation reaction and the voltage of increased acid-base interactions, chemical-bonding [47 could be varied from 30 to 45 V to change the current or by enhanced mechanical interlocking [48] flow. The schematic diagram of this apparatus and the Continuous surface electrochemical oxidation has been details of the treatment methods have previously been preferred. Electrochemical treatments have been carried described [19]. After electrochemical oxidation, all sam- out in acid and alkaline aqueous solutions of ammonium ples were thoroughly washed with distilled water, and ulfate [17], ammonium bicarbonate [21], sodium hydrox- dried at 110°C. ide [22], diammonium hydrogen phosphate [23] and nitric To further explore surface chemistry, oxidized fibers acid [49 Anodic oxidation of fibers in electrolytes can were heated for 30 min in flowing N, at constant tempera- produce a variety of chemical and physical changes in the tures between 150 and 850C fiber surface [11]. Most investigations have been done at low levels of oxidation. previous anodic oxidations 2. 3. Titration and adsorption in aqueous solutions proceeding to higher levels of oxidation, conducted in neutral aqueous potassium nitrate, greatly increased the Both Naoh uptake and the adsorption capacity of fibers quantity of surface acidic functions and the specific surface for silver ion and iodine were determined by the change in rea of PAN-based carbon fibers [19]. Over I mmol/g of concentration from before to after immersing a weighed total titratable acidic functional groups per gram of carbon amount of the fibers in the respective solutions fiber and 67 m /g of specific surface area were achieved by 6360 C/g of electrochemical oxidation in 1% wt KNO 2.3. I. NaOH up In the present investigation, X-ray photoelectron spec- NaoH solutions(4-5 mM) were prepared with boiled distilled water to remove dissolved carbon dioxide. Ap- troscopy, FT-IR, aqueous NaoH titration, the weight loss proximately 0.035 gram of carbon fiber was immersed for measurements upon heat treating oxidized fibers and 24 h in 25-50 ml of NaoH solution in a plastic vial. The adsorption of Ag, and I, were used to characterize the NaoH concentration changes were measured with a ph effects of electrochemical oxidation on the fiber surface meter (lon Analyzer 250, Corning chemical composition and acidic functions(carboxyl and phenolic hydroxyl groups) 2.3.2. Ag adsorption onto the fiber surface and by changing the surface rough- A weighed amount of carbon fiber (0.04 g)was ness and morphology might increase fiber/matrix adhesion immersed in 50 ml of AgNO, solution (+5 mM) and ith reactive epoxy or polyurethane resin matrices. How- shaken at 25"C for 24 h in the dark. The initial pH value of ever, if extensive new ultramicroporosity is generated the AgNO, solution was adjusted with NH3/H,o to 8.55 below the outer surface in the form of micropores lined The change in Ag concentration after adsorption was with acidic functions, matrix resins will be unable to determined by KsCN titration using Fe(NH)(SO,)2as effectively penetrate the pores to enhance adhesion. How- the indicator [ 50]. Before titration, the ph of all of the ever,small gaseous or solution molecules could. Thus, adsorbates was adjusted to an acidic state(pH=2-4) highly oxidized fibers could play a role as adsorbents with useful structural properties 2.3.3.lode Aqueous 1, /KI solutions with an I, concentration of 0. 01M were used in adsorption experiments. Fibers(-35 2. Experimental mg) were added into 25 ml of this solution and shaken at 25C for 24 h in the dark. The I, concentration remaining 2. Materials was determined by Na, S,O, titration with a starch in- dicator 51 The carbon fiber employed consisted of high strength, type Il, PAN-based fibers(Thornel T-300)manufactured 2. 4. X-ray photoelectron spectroscopy (XPS) by Amoco Performance Products, Inc. with 3 000 filaments per tow. All other chemicals were of analytical purity from All samples analyzed by XPs were first dried in a Aldrich Chemical Co. and used as received vacuum at 100°for6h1786 Z.R. Yue et al. / Carbon 37 (1999) 1785 –1796 an important step in composite manufacture. Interfacial 2.2. Electrochemical oxidation and heat treatment bonding in composites has been enhanced by fiber surface treatments such as electrochemical oxidation [3,9–23] and Continuous electrochemical treatments were carried out oxidation in concentrated nitric acid [10,14,15,24–33], in a U-tube apparatus. An aqueous 1% wt KNO solution 3 potassium permanganate [34], sodium hypochlorite [35], was used as the electrolyte. The carbon fibers were fed hydrogen peroxide and potassium persulfate [36,37]. Gase- continuously and served as the anode. A 254 cm long ous oxidations include air [38], oxygen [34], and ozone U-shaped stainless steel bar inside the U-tube acts as the [39,40] oxidation as well as plasma treatments [31,41–46]. cathode. A gear system allowed variation of the fiber Fiber/matrix adhesion is improved through a combination residence time in the oxidation reaction and the voltage of increased acid–base interactions, chemical-bonding [47] could be varied from 30 to 45 V to change the current or by enhanced mechanical interlocking [48]. flow. The schematic diagram of this apparatus and the Continuous surface electrochemical oxidation has been details of the treatment methods have previously been preferred. Electrochemical treatments have been carried described [19]. After electrochemical oxidation, all sam￾out in acid and alkaline aqueous solutions of ammonium ples were thoroughly washed with distilled water, and sulfate [17], ammonium bicarbonate [21], sodium hydrox- dried at 1108C. ide [22], diammonium hydrogen phosphate [23] and nitric To further explore surface chemistry, oxidized fibers acid [49]. Anodic oxidation of fibers in electrolytes can were heated for 30 min in flowing N at constant tempera- 2 produce a variety of chemical and physical changes in the tures between 150 and 8508C. fiber surface [11]. Most investigations have been done at low levels of oxidation. Previous anodic oxidations 2.3. Titration and adsorption in aqueous solutions proceeding to higher levels of oxidation, conducted in neutral aqueous potassium nitrate, greatly increased the Both NaOH uptake and the adsorption capacity of fibers quantity of surface acidic functions and the specific surface for silver ion and iodine were determined by the change in area of PAN-based carbon fibers [19]. Over 1 mmol/g of concentration from before to after immersing a weighed total titratable acidic functional groups per gram of carbon amount of the fibers in the respective solutions. 2 fiber and 67 m /g of specific surface area were achieved by 6360 C/g of electrochemical oxidation in 1% wt KNO 2.3.1. NaOH uptake 3 solutions [19,20]. NaOH solutions (4–5 mM) were prepared with boiled In the present investigation, X-ray photoelectron spec- distilled water to remove dissolved carbon dioxide. Ap￾troscopy, FT-IR, aqueous NaOH titration, the weight loss proximately 0.035 gram of carbon fiber was immersed for measurements upon heat treating oxidized fibers and 24 h in 25–50 ml of NaOH solution in a plastic vial. The 1 adsorption of Ag , and I were used to characterize the NaOH concentration changes were measured with a pH 2 effects of electrochemical oxidation on the fiber surface meter (Ion Analyzer 250, Corning). chemical composition and morphology. Introducing more 1 acidic functions (carboxyl and phenolic hydroxyl groups) 2.3.2. Ag adsorption onto the fiber surface and by changing the surface rough- A weighed amount of carbon fiber (|0.04 g) was ness and morphology might increase fiber/matrix adhesion immersed in 50 ml of AgNO solution (|5 mM) and 3 with reactive epoxy or polyurethane resin matrices. How- shaken at 258C for 24 h in the dark. The initial pH value of ever, if extensive new ultramicroporosity is generated the AgNO solution was adjusted with NH /H O to 8.55. 3 32 1 below the outer surface in the form of micropores lined The change in Ag concentration after adsorption was with acidic functions, matrix resins will be unable to determined by KSCN titration using Fe(NH )(SO ) as 4 42 effectively penetrate the pores to enhance adhesion. How- the indicator [50]. Before titration, the pH of all of the ever, small gaseous or solution molecules could. Thus, adsorbates was adjusted to an acidic state (pH52–4). highly oxidized fibers could play a role as adsorbents with useful structural properties. 2.3.3. Iodine adsorption Aqueous I /KI solutions with an I concentration of 2 2 0.01M were used in adsorption experiments. Fibers (|35 2. Experimental mg) were added into 25 ml of this solution and shaken at 258C for 24 h in the dark. The I concentration remaining 2 2.1. Materials was determined by Na S O titration with a starch in- 22 3 dicator [51]. The carbon fiber employed consisted of high strength, type II, PAN-based fibers (Thornel T-300) manufactured 2.4. X-ray photoelectron spectroscopy (XPS) by Amoco Performance Products, Inc. with 3 000 filaments per tow. All other chemicals were of analytical purity from All samples analyzed by XPS were first dried in a Aldrich Chemical Co. and used as received. vacuum at 1008C for 6 h