Surface modification of polyimide fibers by oxygen plasma treatment and interfacial adhesion behavior of a polyimide fiber/epoxy composite

2.1 Materials

The PI fibers used in this study were fabricated by Jiangsu Shino New Materials and Technology Co. Ltd (Jiangsu, China) via the two-step spinning technology by copolymerization of 3,3′,4,4′-biphenyltetracarboxylic dianhydride, p-phenylenediamine and a third monomer containing quinazolinone moiety. The diameter, the density, the tensile strength and the tensile modulus of the PI fibers are 11 μm, 1.46 g cm^-3, 3.0 GPa and 120 GPa, respectively. Epoxy resin diglycidyl ester of aliphatic cyclic (Tianjin Jindong Chemical Factory, China, epoxy value 0.85), diluter butyl glycol diglycidyl ether (Tianjin Jindong Chemical Factory) and hardener diethyl methyl benzene diamine (Lonza, Switzerland) were used to prepare the uncured resin for preparing PI/EP composites.

2.2 Plasma treatment

An inductive coupling radio frequency (40 KHz) generator was used to obtain the oxygen plasma. The oxygen plasma treatment system mainly consists of a plasma cleaning instrument (Diener Prep2), a vacuum pump and an oxygen cylinder. The plasma cleaning instrument can control the operating pressure, the input power and the treatment time. Vacumming lasted for 10 min before the oxygen was delivered to the chamber of the plasma cleaning instrument, and the operating pressure was maintained at 30 Pa. The polyimide fibers were fixed on a glass pane and then put into the chamber to be treated by oxygen plasma for different durations (5, 10, 15 and 20 min), under a power of 200 W. After that, the treated fibers were immediately sealed with a preservative film for future use.

2.3 Preparation of the PI/EP composites

The untreated and plasma-treated PI fiber-reinforced epoxy composites were made by compression molding technique. The fibers got sufficiently coated with the uncured epoxy resin by passing through it. Then the obtained prepreg was put into an oven at 50°C for 30 min as a defoaming process. After that, the composite was cured in the oven by compression molding at 110°C for 2 h, 150°C for 3 h and 180°C for 3 h. All composite samples were about 6 mm in width and 2 mm in thickness. The volume fraction of fiber in the composites was controlled at about 50%.

2.4 Characterization

3.1 The surface chemical composition of the PI fibers under different duration of oxygen plasma treatment

The oxygen plasma treatment can introduce various oxygen-based polar functionalities to the polymer surfaces [21]. An XPS analysis was applied to appraise the surface elemental composition of the PI fibers, and the results are shown in Table 1. For the untreated fibers, the surface carbon and oxygen concentrations were 80.27% and 17.29%, and the ratio of oxygen to carbon atoms (O_1S/C_1S) was 0.22. After an oxygen plasma treatment for 5 min and 10 min, the surface carbon concentrations decreased to 77.98% and 74.16%, respectively, while the surface oxygen concentrations increased to 20.75% and 23.44%, respectively. And the ratio of O_1S/C_1S increased to 0.27 and 0.32 for each. However, increasing time of the plasma treatment from 10 min to 15 min or 20 min would further decrease the oxygen concentration from 23.44% to 22.44% or 21.80%, respectively. Compared with the untreated fibers, all the plasma-treated fibers had higher surface oxygen concentrations and O_1S/C_1S ratios. Concentration of nitrogen did not change obviously before and after the oxygen plasma treatment.

To investigate the alteration of the functional groups on PI fiber surface, a deconvolution analysis of C_1S peaks was carried out. The C_1S XPS spectra of the original and the plasma-treated PI fiber surfaces are shown in Figure 1. The surface after plasma treatment had five carbon-containing groups with binding energies of 284.6 eV (-C-C-), 285.4 eV (-C-N), 286.1 eV (-C-O), 287.8 eV (-C=O) and 289.1 eV (-COO), and the calculated concentrations of the groups are listed in Table 2 [22–27]. For the original surface, the concentrations of -C-C-, -C-N, -C-O, -C=O and -COO groups were 77.1%, 14.3%, 2.6%, 6.0% and 0.0%, respectively. The -C-O, -C=O and -COO concentrations of fiber surfaces treated for 10 min increased to their maximum values of 11.3%, 6.5% and 5.8% and then decreased to 5.2%, 4.7% and 2.7% after being treated for 20 min. These results suggest that oxygen-containing functional groups have been introduced onto the surface of PI fibers via the oxygen plasma treatment, as shown in Figure 1F. And the surface PI molecular chain might be changed to poly(amic acid) via a ring opening process through the plasma treatment. The introduction of the oxygen-containing functional groups with proper duration of plasma treatment could improve the wettability of fiber surfaces and modify the adhesive behavior between the fiber and the matrix [28]. But if the treatment time went too long, the fiber surfaces would be over-etched, leading to the peel-off of polar groups [25, 29].

Figure 1:

The C_1S core-level spectra: (A) untreated; (B) 5 min; (C) 10 min; (D) 15 min; (E) 20 min; (F) schematic images of the PI fibers after plasma treatment.

3.2 The surface morphology and the single-fiber tensile strength of the polyimide fibers under different oxygen-plasma-treatment time

An important influence of plasma treatment is surface-etching, which can roughen the polymer surfaces [30]. The effect of oxygen plasma treatment on PI fiber surface morphology was observed by SEM. A comparison of the SEM images between the original and the treated PI fibers is presented in Figure 2. It can be seen that the surface of the untreated PI fiber is relatively clean and smooth with little grooves (Figure 2A). After being treated for 5 min (Figure 2B), a few visible streak flaws and spots were formed on the fiber surface, which would lead to a slight decrease in the mechanical properties of the PI fibers. Then substantial amounts of spots, protrusions and depressions can be seen on the fiber surface that was treated for 10 min, and the fiber surfaces’ roughness changed dramatically. Carrying on treatment to 15 min or 20 min would even destroy them. Figure 3 shows the mechanical properties of the PI fibers under influence of treatment time. The increment of treating duration from 5 min to 10 min, 15 min and 20 min would cause the percentage decrease in tensile strength as 0.0%, 2.1%, 14.4% and 16.5%, respectively. It demonstrates that the PI fibers could maintain excellent mechanical properties when the treatment time is within 10 min.

Figure 2:

The SEM images of PI fiber under different treatment time: (A) untreated; (B) 5 min; (C) 10 min; (D) 15 min; (E) 20 min.

Figure 3:

Tensile strength of the polyimide fiber.

The increase of fiber surface roughness contributes to better surface wettability and mechanical interlocking effect, which can enhance the interfacial adhesion between fibers and matrix [30, 31]. Fibers’ tensile strength decreased slowly when the treatment time was within 10 min, but extended treatment would lead to excessive roughness of the fiber surfaces. Subsequently, fibers’ mechanics performance would be decreased significantly, and the interfacial properties would be damaged owing to stress concentrations [25]. The overetching could result in destruction of the fiber surfaces, and the polar groups on them might be peeled off from the fiber, similar tendency as which has also been proved by XPS analysis.

3.3 The surface free energy of the polyimide fibers under different oxygen-plasma-treatment time

The DCAA is an important method to investigate the wetting behavior between liquids and fibers. The surface free energy that significantly affects the wettability can be calculated via the contact angle test [20, 32]. The values of the contact angles and the surface free energy of the PI fibers as functions of the plasma treatment time are shown in Figure 4.

Figure 4:

The contact angle and the surface free energy of the polyimide fibers.

Figure 4 shows that the untreated PI fibers possessed a contact angle of 80.59° in water and 56.54° in glycol, while the surface free energy was 31.13 mJ m^-2. After the oxygen plasma treatment, their contact angles both in water and in glycol decreased, and the surface free energy increased correspondingly. When the PI fibers were treated for 10 min by oxygen plasma, the contact angles in both water and glycol reached their minimum values – 50.43° and 39.85°. Meanwhile, the surface free energy reached its maximum value at 50.92 mJ m^-2. This can be explained by the generating of polar functional groups on the PI fiber surface through the oxygen plasma’s treatment as shown by the XPS measurements (Table 2), which effectively enhanced the surface affinity of the PI fiber. And the plasma treatment can also deepen the surface roughness of fiber surface as illustrated by the SEM images (Figure 2), which together enhanced the surface free energy [10, 18].

However, the surface free energy decreased to 39.05 mJ m^-2 and 35.61 mJ m^-2, respectively, when increasing the plasma treatment time to 15 min and 20 min. This was attributed to the destruction of fiber surfaces (Figure 2) and thus the decrement in the concentration of polar groups on the fiber surfaces (Table 2) [25].

3.4 The influence of oxygen-plasma-treatment time on the ILSS of the PI/EP composites

The ILSS is an effective method for estimating the interfacial adhesion between fibers and a matrix [33, 34]. The effect of oxygen plasma treatment time on the ILSS of epoxy composites reinforced by unidirectional polyimide fiber is illustrated in Figure 5. It can be seen that all the PI/EP composites with treated fibers exhibit higher ILSS values than those with untreated fibers as reinforcement. And the ILSS increased first with the increasing treatment duration and then decreased along with further increment of treating time beyond 10 min. When the oxygen plasma treatment lasted for 15 min or 20 min, the ILSS values of the composites declined to 65.31 MPa or 63.50 MPa, respectively. This tendency is in accordance with the above results of XPS, SEM and surface free energy. When the treatment time was <10 min, the oxygen plasma could introduce oxygen-containing groups onto the polyimide fiber surfaces and enlarge the surface roughness, both of which played positive roles in enhancing interfacial adhesion of the composites. Nevertheless, when the polyimide fibers were treated more than 10 min, excess plasma etching would reduce the interfacial adhesion [25]. This could be used to explain why the ILSS increased when the plasma treatment time varied from 0 to 10 min, while it decreased when the plasma treatment time went beyond 10 min.

Figure 5:

The ILSS of the PI/EP composites.

The fracture morphologies of the PI/EP composites are presented in Figure 6. The differences between these two images are obvious. For composite reinforced by untreated PI fibers (Figure 6A), the fibers were pulled out from the epoxy matrix with little resin clinging to the surface. This indicated that destruction of the composite took place on the interface between fiber and matrix. For the composite reinforced by oxygen-plasma-treated PI fiber, there was more epoxy resin adhering to the fibers, and a lot of fibers were embedded into the resin, which implied that the primary failure occurred in the matrix. So it was proved that the oxygen plasma treatment could significantly improve the interfacial adhesion between PI fibers and epoxy resin.

Figure 6:

SEM images of the fractured PI/EP composites: (A) untreated, (B) treated 10 min.