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Investigation of the nanostructure and nanomechanical properties of the interphase in carbon fiber reinforced polyamide 6 composite

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Models and simulation methods

All models and simulations in this work were performed using Materials Studio software (Accelrys Inc) to consider the interface formation and analyze the interface structure at the atomic level. We first built a PA6 chain (100 C6H11NO) and a 2-layer graphene with 10 –COOH groups and 99 –COOH groups. Then, we constructed 2 PA6 chains in maximum contact with the 2-layer graphene surface in a model (CF/PA6 composite model), whose dimensions were 34.36 Å × 39.36 Å × 46.80 Å (Fig. 1). Then, geometry optimization and annealing tasks were performed on the CF/PA6 composite model to obtain the optimal structure with minimum energy. Lastly, in the simulation process, a 2000 ps dynamic NVT was applied to simulate the formation of the interphase, that is, the dynamics of the PA6 chains on the CF surface29. The force field used for the CF/PA6 composite model is COMPASSII, the charges are assigned as the force field. All input and control parameters for the simulations are listed in Table 1.

illustration 1
illustration 1

The structure of the CF/PA6 composite model.

Table 1 All input and control parameters for the simulations.

How the density changes as a function of distance from a reference particle is defined as the radical distribution function (RDF).28.29The RDF (GR) can be expressed as

$$ g\left( r \right) = {{\rho \left( r \right)} \mathord{\left/ {\vphantom {{\rho \left( r \right)} {\rho_{total} }}} \right. \kern-0pt} {\rho_{total} }} = {{\left( {{N \mathord{\left/ {\vphantom {N {\frac{4}{3}\pi \left( {\left ( {r + dr} \right)^{3} – r^{3} } \right)}}} \right. \kern-0pt} {\frac{4}{3}\pi \left( {\ left( {r + dr} \right)^{3} – r^{3} } \right)}}} \right)} \mathord{\left/ {\vphantom {{\left( {{N \mathord {\left/ {\vphantom {N {\frac{4}{3}\pi \left( {\left( {r + dr} \right)^{3} – r^{3} } \right)} }}\right. \kern-0pt} {\frac{4}{3}\pi \left( {\left( {r + dr} \right)^{3} – r^{3} } \right)}}} \right )} {\rho_{total} }}} \right. \kern-0pt} {\rho_{total} }} $$

(1)

Where R is the distance from the reference particle, ρ(r) is the average particle density between R And R+ DR , ρin total is the total particle density, N is the number of particles between R And R + DR27.29For crystalline and semi-crystalline polymers, the RDF (GR) shows the distribution properties of the environment of a particle and characterizes its short-range order.

The total energy Ein total describes the potential energy surface of a particular structure as a function of its atomic coordinates, can be expressed as the sum of valence (or bonding), cross-term and non-bonding interactions29:

$$ E_{Total} = E_{Valence} + E_{Crossterm} + E_{Nonbonding} $$

(2)

Where EValue is the valence energy, such as bond stretching, valence angle bending, torsion of the dihedral angle. ECross term is the cross term energy, such as strain-strain, strain-bending-strain, bending-bending, torsion-strain. Enot bound the non-binding energy, like van der Waals, is electrostatic26.29.

The interfacial energy of polymer composites is usually difficult to test directly. Therefore, MD simulation is often used to theoretically calculate the interfacial energy.29The interfacial energy Einteraction is presented as

$$ E_{interaction} = E_{total} – E_{polymer} – E_{surface} $$

(3)

Where Ein total is the total energy of the model, Epolymer is the total energy of the model without amplification, Esurface is the total energy of the model without polymer28.29.

Experiments

materials

The polyamide-6 (PA6) and unidirectional carbon fiber reinforced PA6 prepreg sheet used in this study were supplied by Composites Inc. The sandpapers were waterproof from Eagle Brand of Japan. The nano-alumina particles (0.5 μm, 0.05 μm) were provided by Shagnhai Naibo Inc. The polymethyl methacrylate resin system (HY604-Y) was offered by Zhejiang Wuyi Hengyu Instrument Inc.

DSC analysis

To determine the melting range of PA6, non-isothermal DSC tests (Mettler Toledo DSC1) were conducted. The PA6 particles were first heated from 25 to 270 ℃ at a rate of 10 ℃/min, then kept at 270 ℃ for 5 min, cooled to 190 ℃ at the same rate of 10 ℃/min, and kept at this temperature for 5 min to eliminate the thermal history. Then the PA6 particles were heated from 190 to 270 ℃ at a rate of 10 ℃/min, which showed the melting process of the PA6 particles. At the end, the PA6 particles were cooled to room temperature and their weight was recorded.

Figure 2A and B show the heat flow/time/temperature curves of PA6. It was obvious that the area of ​​the endothermic melting peak when the thermal history was removed was larger than that of the melting process. Therefore, the removal of the thermal history was crucial for testing the melting range. The melting process circled in blue in Figure 2B was enlarged in Figure 2C. As can be seen from the figure, the melting range of PA6 was 200.1–230.0 °C and it was a crystalline polymer. Before and after the DSC test, the weight of PA6 almost did not change, which means that no chemical reaction occurred during the melting process.

Figure 2
Figure 2

Representative DSC curves of polyamide-6: (A) representative heat flow/time/temperature curve, (B) representative heat flow/temperature curve, (C) corresponding heat flow/temperature curve of the blue circle in (B).

Rheological analysis

To determine the molding process, the rheological properties of PA6 were measured with a rheometer (HAAKE MarsIII) using two parallel plates (20 mm diameter, smooth surface). First, an oscillation model was performed with a constant frequency of 1 Hz at 240 °C while the strain increased from 0.05 to 5%. The result in Fig. 3 showed that the storage modulus G′ and loss modulus G″ of PA6 remained constant at a strain of 1%, which was the best choice for the following viscosity test30. The PA6 particles were then heated and pressed into a layer at 250 °C in the two parallel plates, cooled to 210 °C and kept for 20 minutes to eliminate the thermal history.31. With a constant frequency of 1 Hz and a strain of 1%, the PA6 layer was again heated from 210 to 270 °C at a rate of 5 °C/min to obtain the viscosity/temperature curve, as shown in Fig. 4A. It was obvious that the viscosity of PA6 decreased sharply with increasing temperature and became flat at 248 °C. Therefore, the molding process could be to heat the PA6 from 30 to 250 °C at a rate of 5 °C/min and keep it at this temperature for 15 minutes.30,31Finally, Fig. 4B shows the viscosity/temperature curve of PA6 during the hot pressing process. The viscosity of PA6 was about 100,000 mPa s at 250 °C, which was conducive to the flow and impregnation of PA6 in the CF/PA6 composite during the hot pressing process.

Figure 3
Figure 3

The storage modulus G′/loss modulus G″/strain curve of polyamide-6.

Figure 4
Figure 4

(A) The viscosity-temperature curve of polyamide-6 (B) the viscosity-time-temperature curve of polyamide 6 in the hot pressing process.

Preparation of PF-QNM samples

The CF/PA6 composite was prepared from unidirectional carbon fiber reinforced PA6 prepreg sheets by hot pressing method. Ten layers of the prepreg sheets with the same length and width of 200 mm were heated from 30 to 250 °C at a rate of 5 °C/min and kept at 1 MPa for 15 minutes. The dimensions of the CF/PA6 composite were 200 mm × 200 mm × 2.1 mm by hot pressing method, and there were few defects such as bubbles, holes and microcracks on its surface. Then, the CF/PA6 composite was cut into cubes with approximate dimensions of 15 mm × 5 mm × 2.1 mm. And the cubes were embedded in the polymethyl methacrylate resin system (HY604-Y) in a hollow soft cylindrical shape and cured at room temperature for one hour. These cured samples had carbon fibers aligned along the vertical axis perpendicular to the top and bottom surfaces.32,33. The surface of the samples was mechanically ground with sandpaper and polished with a nano-alumina particle suspension. After cleaning in an ultrasonic cleaner and drying with a hairdryer, the PF-QNM samples were already prepared.

PF-QNM tests

To determine the nanostructure of the interphase in the CF/PA-6 composites, the interlayer thickness, interlayer modulus and interlayer adhesion were investigated in PF-QNM mode using a Bruker Dimension R Icon™ AFM. All quantitative measurements were performed using a Bruker probe with a resolution of 512 × 512 pixels, Poisson’s ratio of 0.3 and oscillation frequency of 1 kHz.32.34. The spring constant of the cantilever and the radius of the probe tip were calibrated using a polystyrene film (2.7 GPa). During the tests, the probe tip repeatedly approached and moved away from the surface of the PF-QNM samples to capture the height, modulus and adhesion images by simultaneously collecting and analyzing force-separation curves. The elastic modulus of the PF-QNM samples E* is calculated by

$$ E^{ * } = \left( {\frac{{1 – \nu_{t}^{2} }}{{E_{t} }} + \frac{{1 – \nu_{s}^ {2} }}{{E_{s} }}} \right)^{ – 1} $$

(4)

Where \(E_{t}\) And \(It}\) are the elastic modulus of the probe tip and the PF-QNM sample, \(\Nut}\) And \(\nu_{s}\) are the Poisson ratio of the probe tip or the PF-QNM sample32Liability is given by

$$ F_{tip} = \frac{4}{3}E^{ * } \sqrt {Rd^{3} } + F_{adh} $$

(5)

Where \(F_{Tip}\) is the force exerted on the probe tip, \(E^{ * }\) is the elastic modulus,Ris the radius of the probe tip,Dis the instantaneous deformation of the PF-QNM sample, \(F_{adh}\) is the liability32.35.

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