3.1. MD Simulations
Figure 2 shows the equilibrium structures of the different polyethylene types on the zirconia surface at the end of the simulation. In all the cases, the polymer chains closer to the zirconia surface are highly oriented parallel to the oxide surface and they get flattened. In the simulated structures of 5AAPE (
Figure 2b) and 30AAPE (
Figure 2c), the oxygen atoms of the acrylic acid tend to be adsorbed by the zirconia’s oxygen atoms and align perpendicular to the surface. The alignment and flattening of the polymer in parallel to metallic and ceramic surfaces has been also observed in other MD simulations [
29,
30,
31,
33]. Considering that the substrate contains oxygen atoms, the oxygen atoms in the polymeric chains localize close to the substrate in order to improve the interactions with the substrate crystalline surface [
30]. This phenomenon can be observed in the structures shown in
Figure 2 in which the carboxyl groups (C=O) of all the acrylic acid monomers of the chains are in contact with the zirconia surface. Considering that the oxygen plays the most important role in the surface energy of zirconia [
37], we evaluated the adhesion of the three types of PE with the zirconia surface by means of the binding energy.
The binding energy between two joined materials is directly correlated to the adhesion between them [
47].
Table 3 summarizes the binding energy values, with a higher absolute value indicating a better adhesion to the zirconia surface. The binding energy to zirconia is slightly higher for the 5AAPE than for the PE. This difference becomes larger for the 30AAPE, whose binding energy to the zirconia is 78% higher than that of PE. The higher affinity between the polar acrylic acid and the oxide surface and the adsorption of the oxygen atoms of the polymer on the ceramic oxide surface (
Figure 2) are responsible of the higher adhesion between polymer and filler [
30,
33]. Moreover, the binding energy values show that the increase in the grafting level of the polymer can greatly enhance the adhesion between the polymer chains and ceramic nanoparticles.
3.2. Calculation of the Interfacial Tension by Contact Angle Measurements
Table 4 summarizes the contact angle values measured for compression molded plates of the high-density polyethylene with and without acrylic acid grafting in the virgin and processed state. For the powder, the contact angle values shown in
Table 4 are those measured by González-Martín et al. [
37] for pure zirconia, tetragonal yttria stabilized zirconia and zirconia with 5 mol% of yttria.
According to the results of Decker et al. [
47] and Cao et al. [
48] with HDPE surface-grafted with acrylic acid, there is a large decrease in the contact angle of water onto the HDPE, even with low contents of AA in the surface [
48]. Despite 5 mol% of AA being used in our study, only a slight difference exists between the values for AAHDPE00 and for HDPE00 (
Table 4). Such disagreement might arise from the different distribution of the acrylic acid in the samples here employed [
49]. For Decker et al. [
47] and Cao et al. [
48], all the AA was grafted and concentrated on the surface of the polymer. In our case, the commercial AAHPDE was used to produce the plates employed in the measurements. Therefore, after kneading and pressing, the polar and hydrophilic AA might be in the bulk of the compression molded plates rather than in the surface, and the polar chains might also be oriented towards the bulk and not to the surface.
Using the contact angle values and Equation (1), the surface free energy values reported in
Table 4 were obtained. The surface energy and its components were higher for the AAHDPE00 than for the HDPE00. Due to the small differences in the contact angle values, the difference in the
of both materials is not as pronounced as expected [
47]. Nevertheless, when comparing the values of both components, the
of AAHDPE00 is 62% higher than the one of HDPE00, whereas the
difference is only of 2%. Thus, it can be stated that the polarity of the AAHDPE00 is higher than the one of HDPE00.
Following this, the interfacial tension between the AAHDPE00 and HDPE00 with the different types of zirconia was calculated with Equation (2) and with the surface energy values from
Table 4. The interfacial tension is dependent on the surface energy and the polarity of the materials in contact, and it is inversely proportional to the adhesion between those materials [
38,
50]. In
Table 5, the interfacial tension values between the different material combinations are shown. The interfacial tension of the AAHDPE00 with the three types of zirconia was lower than for the HDPE00. Consequently, a better adhesion to the zirconia can be expected for the AAHDPE than for the HDPE. The increase in the adhesion for the grafted polymer is in agreement with the trend observed in the MD simulation, which showed the increase in the binding energy with the increase in the acrylic acid in the polymer and the orientation of the acrylic acid towards the oxygen of the pure zirconia surface. The lower interfacial tension for the AAHDPE00 might be also produced by the presence of polar hydroxyl (–OH) groups on the surface of the zirconia plates employed by González-Martín et al. [
37]. To determine whether the powder employed in our study contained hydroxyl groups, attenuated total reflection spectroscopy was conducted.
3.3. Attenuated Total Reflection Spectroscopy
Figure 3 shows the infrared spectra obtained for the powder, polymers and composites. The zirconia’s peak at 3355 cm
−1 corresponds to the hydroxyl groups (–OH) bound to the powder surface [
7,
51,
52,
53]. Both unfilled polymers (HDPE00 and AAHDPE00) showed the characteristic polyethylene CH peaks at 1470, 2847 and 2916 cm
−1 [
53,
54]. For AAHDPE00 additional peaks were observed at 1167, 1246 and 1700 cm
−1. The strong peak at 1700 cm
−1 corresponds to the stretching vibration of the carboxyl (C=O) in the acrylic acid group (–COOH) [
48,
55].
The reduction in the peaks’ intensity for the composites (
Figure 3) is produced by the incorporation of the zirconia as a second component. No new peaks appeared due to the chemisorption of the acrylic acid onto the zirconia surface [
55] or to the formation of ester links between the carboxyl (C=O) in the acid and the –OH groups in the zirconia [
24,
56,
57]. However, the C=O peak of the AAHDPE has a slight shift of approximately 2 cm
−1 after incorporating the powder (
Figure 3). The other peaks remained exactly at the same frequencies. The shift of the carboxyl group could be produced by the formation of hydrogen bonds with the hydroxyl groups in the powder oxide surface [
58,
59] and even with the zirconium dioxide [
59] itself. The hydrogen bonding with the oxide would be in agreement with the orientation of the acrylic acid towards the oxygen in the zirconia surface and the high adhesion for the grafted polymers observed in the MD simulations (see
Figure 3b,c).
3.4. Morphology
The cryo-fracture surface of the two composites, shown in
Figure 4, shows the large differences between the morphology of both composites. HDPE30 showed large agglomerates of particles, which are heterogeneously distributed in the polymeric matrix. Contrarily, the zirconia powder is homogeneously distributed in AAHDPE30 without large agglomerates (
Figure 4). The differences in the morphologies of both composites can be explained by their different adhesion with the zirconia powder as observed in the binding energy values obtained in the MD simulations at melt temperature (see
Section 3.1) and in the interfacial tension values at room temperature (see
Section 3.2). The poor adhesion between the non-polar HDPE chains and the zirconia’s surface cannot overcome the attractive forces existing between the submicron particles of zirconia, which promote their agglomeration [
60]. Furthermore, the hydrogen bonding between the hydroxyl groups in the surface of the powder (see
Section 3.3) results in strong powder agglomerates [
27,
28,
61,
62]. The use of grafted polymers as compatibilizers [
27] has proven to be an effective solution in the reduction in the hydrogen bonding of the hydroxyl groups in silica nanoparticles. Thus, the combination of a high adhesion to the oxide surface and the hydrogen bonding of the acrylic acid with the hydroxyl groups on the powder surface are the responsible factors for the better powder dispersion in AAHDPE30 than in HDPE30.
3.5. Viscoelastic Properties
Figure 5 shows the viscosity measured in a high-pressure capillary rheometer for the different materials. The measurements in the capillary rheometers are strongly related with the processing of the materials by FFF or PIM, since in both processes the material is forced to flow through a narrow nozzle at high shear rates [
63]. As can be observed in
Figure 5, the shear viscosity of AAHDPE00 is slightly lower than the one of HDPE00 in the range of shear rates evaluated (80 to 1000 s
−1). On the contrary, the viscosity of AAHDPE30 is higher than that measured for HDPE30, especially at shear rates below 500 s
−1. Since the effect of the network of particles and polymer plays a more important role at lower shear rates, rotational rheology tests between 0.1 rad s
−1 and 500 rad s
−1 were conducted.
In
Figure 6a, it can be observed that the complex viscosity of AAHDPE00 is slightly higher than the one of HDPE00 at low angular frequencies (<1 rad·s
−1), whereas HDPE00 is higher in the rest of the angular frequencies. HDPE00 shows a Newtonian plateau at low angular frequencies, whereas AAHDPE00 exhibits a pseudoplastic behavior in the range of angular frequencies evaluated. The increase in viscosity in the low shear region and the pseudoplastic behavior have been reported for long chain branched polyethylene [
64,
65] as well as for polyethylene grafted with polar groups such as maleic anhydride, glycidyl methacrylate and acrylic monomers [
66,
67] or silane [
68]. Thus, the differences in the complex viscosity of AAHDPE00 and HDPE might be caused by the branching and partial crosslinking of the poly (acrylic acid) employed in the polymer grafting, which results in a more effective entanglement [
67]; the hydrogen bonding between the carboxylic acid in the acrylic acid promotes this behavior [
67]. The branching [
65] or partial crosslinking [
68] of the grafted polyethylene might also be the cause of the trend in the storage modulus (
Figure 6b). At low angular frequencies, the G’ of AAHDPE00 is higher than that of HDPE00, whereas the storage modulus of HDPE00 is slightly higher at high angular frequencies. This theory is further supported by the results observed in the loss factor of the unfilled polymers shown in
Figure 6c. The slope of the loss factor is negative for HDPE00 in the whole range of angular frequencies evaluated. On the contrary, a positive slope is observed at low frequencies for the AAHDPE00, which is associated with an elastic behavior [
68]. Considering the results of the mentioned studies and the trend observed in the complex viscosity (
Figure 6), it can be stated that the chain entanglement or crosslinking and the reaction between the acrylic acid monomers in AAHDPE00 are the causes of the big difference in the viscoelastic properties at low angular frequencies for the unfilled polymers.
In the case of the highly filled systems, the complex viscosity of AAHDPE30 is much higher than the one of HDPE30 at low shear rates (
Figure 6a). It is known that, at low frequencies, the particle–particle interaction and the network of particles have the main effect on the viscoelastic properties of polymer nanocomposites [
43]. As was observed in
Figure 4, there are no agglomerates in AAHDPE30, whereas HDPE30 has large agglomerates. Thus, the high viscosity of AAHDPE30 is not produced by the powder’s agglomerates. On the contrary, the decrease in agglomerates results in an increase in the particle surface area in contact with the polymer melt, thus increasing the viscosity [
43,
69,
70,
71]. Additionally, the higher adhesion to the powder for the AAHDPE than for the HDPE (observed in the MD simulations at high temperature in
Section 3.1) and the hydrogen bonding between the zirconia OH groups and the acrylic acid CO groups (observed in the ATR analyses in
Section 3.3) promote the higher viscosity of AAHDPE30 [
71,
72,
73,
74]. A high work of adhesion results in a thicker layer of polymer absorbed in the particles [
72]. At low shear rates, the effective particle size of the particles is larger due to the entanglement of the absorbed molecules with the rest of the polymer. At high shear rates, the polymer molecules disentangle and orient [
72]. The breakup of this network results in a more pronounced shear thinning of AAHDPE30 as compared to HDPE30 in
Figure 6 [
43]. As can be observed in
Figure 6b, the formation of a polymer-filler network due to the improvement of the powder dispersion and the increase in the adhesion also affects the values of the storage modulus [
43]. The polymer molecules adsorbed on one particle interact with the chains adsorbed on the near particles and with the free and mobile polymer, resulting in a decrease in the mobility and in an increase in the elasticity for AAHDPE30 as compared to HDPE30 [
14,
75].
The elastic behavior of the AAHDPE30 can also be observed in its loss factor values, which are smaller than 1 for all the evaluated angular frequencies (
Figure 6c). Moreover, the loss factor of AAHDPE30 has a positive slope in all the evaluated shear rates, which corresponds to a pseudo solid behavior. On the contrary, the loss factor of HDPE30 is always higher than 1 and the positive slope only appears at low angular frequencies (
Figure 6c). For similar filled polymers, a positive slope and low values of the loss factors can be attributed to a high filler dispersion [
76], which is in line with the differences in the morphology of AAHDPE30 and HDPE30.
3.6. Tensile Properties
The mechanical properties of the compounds used in FFF are of vital importance [
11,
12,
77]. Sufficient strength and flexibility are required to spool and de-spool the filament for its production and for printing. Moreover, the filaments must be stiff enough to avoid buckling during printing. Furthermore, high strength and stiffness are also required in the PIM process for the de-molding and handling of the parts [
78]. Therefore, tensile tests were conducted on filaments produced with all the evaluated compounds. This method enables a rapid screening and comparison of similar materials processed under the same conditions [
12,
46].
Figure 7 shows representative strain–stress curves of filaments of the materials evaluated, together with the average ultimate tensile strength (
) and the corresponding average strain value (
). The secant modulus (
) was calculated in the strain range between 0.1% and 0.3% in order to avoid the initial stage of the test, in which the slight curvature of the specimens (due to the processing and handling of the filaments) could influence the results.
Table 6 summarizes the average and standard deviation values for the three parameters.
Even though the flowability of the unfilled polyethylene was the only parameter employed to select the two types of high density polyethylene (see
Section 2.2), no significant differences existed in the
and
of HDPE00 and AAHDPE00 (
Table 6). In fact, the
was the only parameter significantly different between HDPE00 and AAHDPE00 (
Figure 7) with the non-grafted polymer having a significantly larger value. The incorporation of the particles should result in an increase in the strength and stiffness of the composites [
21,
22,
23,
27,
74]. This behavior is observed for AAHDPE30 with an increase of 18% in
and 98% in
with respect to the values of AAHDPE00 (
Table 6). In the case of HDPE30, the
is 137% higher than the values of HDPE00, whereas no significant difference in the
was found between HDPE00 and HDPE30 (
Table 6). The presence of the zirconia particle results in a decrease in the
of 91% in HDPE30 compared to HDPE00 and of 46% in AAHDPE30 compared to AAHDPE00 (
Table 6). The decrease in the
is produced by the restriction of movement of the polymer by the rigid particles which do not elongate, resulting in a reduction in the ductility of the material [
21,
22,
23,
27,
74].
When comparing the properties of the nanocomposites, the
of HDPE30 is significantly higher than that of AAHDPE30 (
Table 6). The secant modulus, calculated in the strain range from 0.1% to 0.3%, is highly dependent on the polymer structure and factors such as the crystallinity. In order to determine the crystallinity, Differential Scanning Calorimetry (DSC) tests were conducted on five samples of each material. The crystallinity of HDPE00 is 76.21% ± 2.16% and increases to 77.65% ± 0.31% for HDPE30. For AAHDPE00 the crystallinity is 63.42% ± 1.52% and decreases to 57.92% ± 0.85% for AAHDPE30. In polymer nanocomposites with a semicrystalline polymeric matrix, the nanoparticle surface can act as a heterogeneous nucleating site [
14]. Smaller nanoparticles are less able to act as nucleating agents than larger fillers [
14,
79]. Additionally, the improvement of the dispersion of adhesion reduces the mobility of the crystallisable chain segments [
68,
80]. Therefore, the high dispersion and adhesion to the zirconia results in a lower crystallinity of the matrix in AAHDPE30 than in HDPE30 and consequently in a lower secant modulus [
80].
The
of AAHDPE30 is significantly higher than the
of HDPE30 (
Figure 7). Furthermore, the
of AAHDPE30 is three times higher than the one measured for HDPE30 (
Table 6). Such behavior can be attributed to the higher adhesion of the AAHDPE than the HDPE to the zirconia surface, which seems to have a big effect in the high strain region. A high adhesion results in a strong polymer-filler interface and thus in a more effective transfer of load from the polymer matrix to the solid particles [
21,
22,
23,
27,
74]. Moreover, the high dispersion of the powder in AAHDPE30 compared to HDPE30 (
Figure 4) contributes to difference in the mechanical properties. In polymer composites, defects such as agglomerates or cavities result in a concentration of stresses around those points and eventually the failure of the material in those areas [
16,
22,
23,
27]. Thus, it can be concluded that the acrylic acid-grafting of the HDPE results in composites with a higher strength and flexibility by the combination of a strong polymer-filler interface and the reduction in the filler agglomerates.