In order to investigate the influence of the FR molecular structure on the properties of the prepared PA6/FR systems, three different organophosphorus derivatives were used. The selected bridged DOPO derivatives included: (1) ED, which is a P−N bond phosphonamidate derivative [
29], and (2) NED and (3) PHED, which are the P−C bond phosphinate derivatives [
28,
34]. The DSC heating and cooling runs of the ED, NED, and PHED samples are presented in
Figure S1 in
Supplementary Materials. During the first heating run, ED melted at 273 °C, and NED and PHED melted at 229 and 188 °C, respectively (
Figure S1). Whilst the transparent melt mixture with
ε-caprolactam was obtained only in the case of the PHED prior the start of polymerization process, it could be suggested that ED and NED were in the melted or rather solubilized form during the PA6 synthesis as polymerization temperature was set at 250 °C and the temperature depression for melting of ED and NED could occur. This is beneficial for the uniform distribution of the applied FRs in the in situ formed PA6 matrix.
3.1. Structure Characterization
SEM images and EDS phosphorus distribution maps of the reference PA6 and PA6/ED, PA6/NED, and PA6/PHED samples are shown in
Figure 2. SEM images confirmed no visible microagglomerates of the incorporated FRs, and EDS phosphorus mapping confirmed that phosphorus was uniformly distributed in the bulk of all three PA6/ED, PA6/NED, and PA6/PHED samples. These results indicate the production of PA6 with uniformly distributed nanodispersed FRs in the case of all three PA6/ED, PA6/NED, and PA6/PHED samples. The results of the SEM analysis performed at higher magnification showed that incorporated FRs changed the polymer morphological characteristics, indicating their influence on the polymer microstructure.
Results for the
ε-caprolactam conversion, CL
conv, determined from the thermogravimetric analysis, and results for the number-average molecular weight,
Mn, the weight-average molecular weight,
Mw, and molecular weight distribution,
Mw/
Mn, obtained from the size exclusion chromatography are shown in
Table 1.
Whilst each FR present in the polymerization system slightly increased conversion of ε-caprolactam to PA6 by the comparable extent, the results from the SEC analysis indicate the different extent of their influence on the PA6 molecular weights and their distribution. The presence of ED, NED, and PHED in the polymerization system (1) reduced Mn for 71.9%, 36.0%, and 19.3%, respectively; (2) reduced Mw for 48.8%, 20.2%, and 7.7%, respectively; and (3) increased Mw/Mn for 82.1%, 24.7%, and 14.5%, respectively. According to these results, it can be concluded that among the applied FRs, ED significantly hindered the polymerization process, whilst PHED compound provided minimal influence. In the case of ED, the reason for the significant reduction of PA6 molecular masses and broadening of their distribution could be ascribed to hydrogen bonding between the NH from the ED phosphonamidate group and carbonyl group from ε-caprolactam, which could hinder the chain propagation in addition reaction as well as to participation of ED in the polymerization reaction as chain terminator specie. Despite NED and PHED compounds not possessing reactive chemical groups in their structures, their bulky molecules could also hinder the condensation between polyamide chains. As PHED and NED have benzyl and naphthyl groups, respectively, attached to the ethylene bridge connecting two DOPO groups, it can be suggested that less steric hindrance is provided by the PHED additive.
The molecular structures of the PA6 corresponding to the reference PA6, PA6/ED, PA6/NED, and PA6/PHED samples were investigated by solid state
1H–
13C CP/MAS NMR. Structures of the FRs before and after incorporation in the PA6 matrix were investigated by solid state
31P MAS NMR spectroscopy. The obtained spectra are presented in
Figure 3. The signals in the
1H–
13C CP/MAS NMR spectra (
Figure 3a) corresponding to the PA6 sample, as well as to the PA6/ED, PA6/NED, and PA6/PHED samples, can be assigned to the typical signals of the PA6 in α-crystalline form [
35,
36]. No significant differences could be observed in the solid state
1H–
13C CP/MAS NMR spectra corresponding to the reference PA6, PA6/ED, PA6/NED, and PA6/PHED samples. The
31P MAS NMR spectrum of the ED sample (
Figure 3b) shows two signals at
δ = 17.55 and 13.09 ppm, for the P atoms in different surroundings belonging to two diastereomers. In the case of the NED and PHED samples, the
31P MAS NMR signals appeared at higher frequencies, where signals at 37.64 and 32.14 ppm for NED, and at 38.0 and 33.04 for PHED, were attributed to the phosphorus atom in the phosphinate group. Comparison of the
31P MAS NMR signals in the spectra of the ED, NED, and PHED, with those of the PA6/ED, PA6/NED, and PA6/PHED, shows broadening of signals indicating interactions of additives with PA6 matrix (relaxation of P atoms caused by hydrogen bond formation).
Interactions of additives with PA6 chains also caused shifting of the
31P MAS NMR signals to somewhat lower frequencies, which can be observed as the appearance of signals at 36.0 and 25.7 ppm for NED and signals at 36.0 and 24.6 ppm for PHED (
Figure 3b). Additionally, in the case of all additives, a new signal at 2.9 ppm appeared, and it was the most prominent in the case of the ED additive. The new P signal indicates the occurrence of strong interaction, or even chemical reaction on the ED’ phosphorus atom during polymerization of
ε-caprolactam.
The obtained XRD patterns (
Figure 4) for the PA6, as well as for the PA6/ED, PA6/NED, and PA6/PHED showed two diffraction peak maxima located at 2
θ = 20.0° and 2
θ = 23.9°, which can be assigned to
α1 and
α2 crystalline phases, respectively. This confirmed successful retention of the
α crystal form for the PA6 synthesized in the presence of the FRs. The presence of the FRs in the polymerization system decreased degree of crystallinity, which was further quantitatively evaluated from the results obtained in the DSC measurements.
The photographs of the PA6, PA6/ED, PA6/NED, and PA6/PHED samples are presented in
Figure 5. Although all three FRs were applied in the white powder form, the white color was preserved only in the case of the PA6/PHED sample, whereas the PA6/ED and PA6/NED samples displayed visible yellowing. The yellowing can be caused by different processes, but most of these involve different kinds of reactions towards polymer end groups, e.g., the oxidation process of amine end groups in the presence of water at high-temperature conditions or chemical reaction on the ED’s and NED’s phosphorus atoms during polymerization. Since PA6/ED has the lowest polymer molecular weight among presented materials, it would be expected that it also has the highest amine end group content, and therefore the highest observable yellowing effect. Additionally, the presence of phosphonamidate groups might have an amplifying effect on the yellowing of the PA6 matrix material.
3.2. Melting and Crystallization Behavior
The DSC curves are presented in
Figure 6 and the results for characteristic glass transition,
Tg, melting,
Tm, and crystallization,
Tc, temperatures, as well as for the degree of crystallinities, are summarized in
Table 2. The endothermic melting peaks from the first and the second heating runs for the PA6 reference appeared at 222 and 220 °C, respectively, whilst a shoulder at 214 °C appeared on the melting peak from the second heating run (
Figure 6a,c). This shoulder was caused by the non-isothermal recrystallization in the DSC measurements [
37] and may be attributed to the
γ-crystalline phase, as well as to
α-crystallites of different size and perfection [
38,
39]. A similar phenomenon can also be observed for the PA6/ED, PA6/NED, and PA6/PHED samples.
The single melting peaks from the first heating runs (
Figure 6a) confirmed the
α-crystalline forms in the reference PA6, as well as in the PA6/ED, PA6/NED, and PA6/PHED, which is in correlation with the results obtained by the solid state
1H–
13C CP/MAS NMR and XRD measurements.
Whilst NED and PHED did not markedly influenced Tm1 and Tm2, ED caused a decrease of both Tm1 and Tm2 in comparison to the reference PA6, suggesting hindered hydrogen bonding between PA6 chains. Furthermore, PHED and NED did not markedly influenced temperature of PA6 crystallization, whilst in the case of the PA6/ED, the Tc decreased for 12 °C. The start of crystallization at a lower temperature for the PA6/ED was also accompanied by a lowered degree of crystallinity, Xc, by approximately 35% as well as by the increased Tg by 7 °C compared to a reference PA6. Contrary to ED flame retardant, NED and PHED did not markedly change Tg, and caused lowering of the Xc by approximately 2.6% and 15%, respectively, indicating lesser influence on the degree of crystallinity and almost no influencing of the amorphous phase characteristics. These results confirmed that NED and PHED minimally influenced the hydrogen bonding between chains, melting behavior, crystalline structure, and amorphous phase of the in situ formed PA6, which is in correlation with the PA6 molecular masses and their distributions obtained by the SEC analysis. In the case of the PA6/ED sample, the detected changes of the PA6 structural characteristics could also be assigned to the significant changes of the PA6 molecular structure introduced by the ED’s molecules during the polymerization process.
3.3. Melt-Rheology
Melt-rheology measurements were conducted in order to evaluate the impact of the in situ incorporated FRs on the visco-elastic behavior of the PA6 melt. The results for complex viscosity,
μ*, storage modulus,
G’, loss modulus,
G’’, and loss factor, tan
δ, are shown in
Figure 7. The complex viscosity of the reference PA6 decreased with the increase of angular frequency,
ω, showing the shear thinning behavior (
Figure 7a). It can be seen that incorporated FRs induced a decrease of the complex viscosity of the PA6/ED, PA6/NED, and PA6/PHED samples also showing lower sensitivity to the applied shear strain at angular frequencies higher than 1 rad/s. Decreased complex viscosity of the PA6/ED, PA6/NED, and PA6/PHED melts indicates lubrication effect due to reduced internal friction of molecular chain segments caused by shortening of the PA6 chains, as well as by sterically hindered attraction between the PA6 chains.
The least pronounced decrease of the complex viscosity compared to that of the reference PA6 was detected in the case of the PA6/PHED and the most pronounced was detected in the case of the PA6/ED. In the case of all studied samples, the responses of the elastic,
G’, and viscous,
G’’, portions to the applied angular frequencies show no intersection of the moduli, with
G’’ values higher than
G’, which confirms the viscoelastic characteristics (
Figure 7b). Incorporated additives reduced both elastic and viscous components of the PA6 melt, whilst among the applied FRs, PHED provided minimal decrease and did not defect the gradually increased tendency of the PA6 storage modulus over the applied angular frequency range. In the case of PA6/NED and PA6/ED samples, the microstructure broke above 10
2 rad/s, which is indicated by sudden decline in
G’ modulus. The values of the loss factor (tan
δ) increased due to the incorporated FRs, because of the higher ratio of the viscous to the elastic portion of the viscoelastic deformation, i.e., intensified domination of the viscous behavior over the elastic behavior (
Figure 7c). The incorporated FRs and their influence on the PA6 chain length and the attractive interactions between the PA6 chains contributed to the higher ratio of the viscous to the elastic portion and caused deviation of the rheological behavior from that of the reference PA6. Whereas the reasonable decrease of the complex viscosity can be considered as beneficial for the processability, we have already shown in our previous work [
28] that decreased viscosity in the case of the PA6/PHED in comparison to that of the reference PA6 does not markedly hinder the good melt-spinning processing characteristics, but rather influences the tensile properties. In the case of PA6/ED and PA6/NED samples, melt-spinning was not feasible because of the significant reduction of PA6 molecular masses.
3.4. Thermal Stability
It has already been shown that ED, NED, and PHED act as flame retardants mainly in the gas phase [
28,
34,
40], which is beneficial for increasing the PA6 flame retardancy. The efficiency of the latter is closely related to the temperature at which flame-retardant volatile radical scavengers are produced, i.e., the premature as well as delayed decomposition of the gas phase flame retardant in comparison to that of the polymer provides inefficient inhibition of the flaming combustion process [
8]. In order to clarify if their decomposition temperatures match the PA6 pyrolysis specifics, heat-induced thermal decompositions of PA6 and flame retardants ED, NED, and PHED were analyzed (
Figure 8 and
Table S1). In comparison to the initial decomposition temperature,
Tonset, of the PA6 in nitrogen environment, the
Tonset of the ED, NED, and PHED was 53 °C lower, 6 °C higher, and 20 °C lower, respectively (
Figure 8a,b). Temperature of the maximum in the weight loss rates,
Tmax, for the ED, NED, and PHED were 4, 20, and 30 °C lower, respectively, in comparison with that of the PA6. Higher weight loss rates of the NED and PHED in comparison to the ED demonstrates more accelerated decomposition, which may indicate more intensive production of the flame-retardant volatile radical scavengers. Thermal decomposition of ED’s diastereoisomers occurred via two decomposition steps in the TG curve and a maximum with a shoulder at approximately 395 °C in the first derivative curve. This shoulder (
Figure 8b) appeared at a temperature almost 60 °C lower in comparison to
Tmax of the PA6, which consequently caused more than two times lower ED residue at
Tmax in comparison with those of the NED and PHED (
Table S1). Furthermore, higher ED residue at 600 °C in comparison with those of the NED and PHED indicates its higher thermal stability.
In spite of the differences in the decomposition temperatures of the applied ED, NED, and PHED, the incorporated FRs produced comparable effects on the PA6 thermal stability (
Figure 8c,d). Whilst
Tonset values for PA6/ED, PA6/NED, and PA6/PHED were approximately comparable to each other, as were
Tmax values, both the
Tonset and
Tmax for all three FRs were shifted to temperatures approximately 40 and 50 °C, respectively, lower compared to the
Tonset and
Tmax of the PA6. This indicates that interactions between decomposing FRs and the PA6 matrix promote the heat induced pyrolysis of the PA6. According to the results of the weight difference between the experimental and calculated TG curves (Δ
T, presented in
Figure 8e), the obtained negative curves indicate destabilization of the PA6 during the whole anaerobic pyrolysis [
32,
41]. In the case of the PA6/ED sample, the first derivative curve revealed an additional peak at approximately 302 °C, which preceded the temperature of the maximum in the weight loss rate. In the case of all three PA6/ED, PA6/NED and PA6/PHED, the
Tmax was followed by the additional decomposition step at
Tmax,add (
Figure 8d and
Table S1). The latter are close to
Tmax values of the ED, NED, and PHED, and the residues at
Tmax,add are approximately equal to 10 wt.%, which could be the concentration of the applied FRs. However, our previous results of the TG-FTIR coupled analysis of the PA6/PHED system confirmed releasing of the active phosphorus species during both
Tmax and
Tmax,add. Therefore, interactions between decomposition products of FRs and PA6 led to the formation of the primary char, which underwent further decomposition during the second pyrolysis step. As all three FRs have a dominant gas-phase mode of action, the residues at the end of the forced heat induced pyrolysis process are rather low. However, the residues of the PA6/ED, PA6/NED, and PA6/PHED were two times higher than that of the PA6. Based on these results, it can be assumed that all three used FRs will promote the PA6 decomposition during the forced flaming combustion, but during the limited contact time with the flame, the released gas-phase radical scavengers from decomposing FRs could effectively quench the combustible radicals.
In the stage before flaming combustion occurs, the material is exposed to heat and chemically reacts with oxygen. Additionally, after flameout, the residue undergoes a thermo-oxidative decomposition [
42]. When heat-induced decomposition is supported by oxygen, decomposition products chemically react with oxygen, which consequently shifted the initial decomposition temperatures to lower values for PA6, and PA6/ED, PA6/NED and PA6/PHED (
Figure 9a,b, and
Table S2) in comparison to those in the anaerobic pyrolysis. Similarly, as during the pyrolysis, the incorporated ED, NED, and PHED accelerated the start of the decomposition. Furthermore, the air environment caused shifting of the PA6’s
Tmax to a lower value, which corresponds to the
Tmax1. However, the
Tmax1 values for the PA6/ED, PA6/NED, and PA6/PHED remained almost the same as the
Tmax values during the anaerobic pyrolysis. This may indicate that phosphorus radical scavengers emerging from decomposing FRs retarded the gas-phase oxidation reactions. This finding is also supported by the reduced weight loss rates for PA6/ED, PA6/NED, and PA6/PHED during the first decomposition step in comparison to that of the PA6, as well as by the results of the DSC analysis conducted simultaneously with thermo-oxidative decomposition (
Figure 9c).
The latter indicates the endothermic heat effects during the first decomposition step in the case of the PA6/ED and PA6/PHED. Oxygen-assisted decomposition at higher temperatures of the char formed during the first decomposition step further led to the oxidation of residue to volatiles. Whilst this step for PA6 ended at about 600 °C with almost no residue, the incorporated FRs increased thermo-oxidative stability of the char, which resulted in extended char oxidation up to approximately 720 °C and increased char residue up to 8.6 wt.% at 600 °C. The results of the weight difference between the experimental and calculated TG curves (Δ
T, presented in
Figure 9d) confirmed stabilization of the PA6/ED, PA6/NED, and PA6/PHED systems above 450 °C, which resulted in the approximately 50% higher residues at 500 °C in comparison to that of the PA6.