Effect of Different Phosphate Glass Compositions on the Process-Induced Macromolecular Dynamics of Polyamide 66

The present study provides a fundamental understanding of the mechanism of action of special new phosphate glass (P-glass) systems, having different glass transition temperatures (Tg), in polyamide 66 (PA66). Dynamic mechanical analysis (DMA) revealed that the Tg of PA66/low Tg P-glass (ILT-1) was significantly shifted to a lower Tg (65 °C), and another transition appeared at high temperature (166 °C). This was supported by a drop in the melting point and the crystallinity of the PA66/ILT-1 hybrid material as detected by differential scanning calorimetry (DSC). The dielectric spectroscopic investigation on the networks’ molecular level structural variations (Tg and sub-Tg relaxations) agreed very well with the DMA and DSC findings. Contrary to intermediate Tg(IIT-3) and high Tg P-glass (IHT-1) based materials, the PA66/ILT-1 hybrid material showed an evidence of splitting the PA66 Tg relaxations into two peaks, thus confirming a strong interaction between PA66 and ILT-1 (low Tg P-glass). Nevertheless, the three different P-glass compositions did not show any effect on the PA66 sub-Tg relaxations (related to the –NH2 and –OH chain end groups’ motion).


Introduction
Inorganic phosphate-based glass (P-glass) systems have gained increasing interest in the last decade because of their wide variety of applications and tunable properties [1]. As a consequence, P-glass materials display a wide range of unique features such as chemical durability and stability, good electrical conductivity, excellent optical properties [2], biocompatibility, efficient drug delivery [3,4], and flame-retardancy [5]. The ability to control the chemical composition of P-glasses to suit the target application requirements makes them desirable additives for organic/inorganic hybrid materials.
The potential of polymer/P-glass materials lies behind the ease to tailor the glass transition temperature (T g ) of P-glass [1] to make them fluidic over a broad range of temperatures in which many polymers melt. This property allows melt processing polymer/P-glass hybrids using conventional polymer processing techniques such as extrusion and injection molding.
Since P-glasses are a new evolving class of materials, only a few research groups have investigated the impact of P-glass on the final properties of P-glass/polymer, particularly P-glass/polyamide materials [5][6][7][8]. For instance, it has been shown that for tin fluorophosphate glass (TFPg)/polyamide 6 (PA6) material containing ≥5 vol % TFPg, the strain-stress curve is remarkably consistent with that

Differential Scanning Calorimetry (DSC)
The measurements were performed on approximately 12 mg samples using a Perkin Elmer differential scanning calorimetry (DSC; Perkin Elmer, Waltham, MA, USA). Samples were equilibrated at 150 • C, ramped at 10 • C/min to 290 • C, cooled at 10 • C/min to 150 • C, and reheated at 10 • C/min to 290 • C. The reported data represent the melting temperature (T m ) of the second heating cycle.

Dynamic Mechanical Analysis (DMA)
Dynamic storage modulus E and loss factor tanδ = E /E (E is loss modulus) vs. temperature curves were generated using a DMA Thermal Analysis Q800 instrument (TA Instruments, New Castle, DE, USA). The instrument was operated in a tensile mode at a frequency of 1 Hz and a strain of 0.028%. All experiments were carried out over the temperature range −20 to 220 • C at a heating rate of 3 • C/min.

Tensile Strength
Stress vs. strain measurements were conducted using a material testing system (MTS) 810 test-frame (MTS Systems Corporation, Eden Prairie, MN, USA) and analyzed using an MTS Testworks 4.0 software package. The strain rate was set at 10 mm/min for all experiments. Five injection-molded dog-bone specimens ASTM D638/Type I were tested for each material and the obtained average values are reported.

Compressing Molding
Dielectric samples were prepared by compression molding the PA66/P-glass hybrids and the processed neat PA66 into 30 µm films following standard methods. The compression molding conditions were set as follows: temperature = 272 • C, pressure = 250 MPa, and mold residence time = 2 min.

Broadband Dielectric Spectroscopy (BDS) Measurements
Dielectric spectra were collected isothermally using a Novocontrol GmbH Concept 80 Broadband Dielectric Spectrometer (Novocontrol Technologies GmbH, Montabaur, Germany) over the frequency range 0.1 Hz-3 MHz and temperature range −60-200 • C. Temperature stability was controlled to within ±0.2 • C. Samples were cut to a diameter of 2 cm and placed between two 2 cm diameter gold-coated copper electrodes. Aluminum foil was sandwiched between the specimen and the electrode to prevent the polymer from adhering to the gold coating of the electrode. Verification tests were conducted to ensure that the aluminum foil did not alter the dielectric spectra of PA66 and hybrids. Electrode-sample assemblies were then transferred to the instrument for data collection. Three replicates per each sample were conducted.  Figure 1 shows the 2 nd heating and the cooling DSC thermogram of the investigated hybrids, as compared to those of the neat PA66, and Table 1 summarizes their thermal properties. DSC results of PA66/IHT-1 and PA66/IIT-3 samples display an endothermic melting peak around 263 • C and an exothermic one at 232 • C, similar to those observed for the neat PA66 matrix. However, when blended with ILT-1 P-glass, PA66 melting and crystallization points shift to a lower temperature (250 and 215 • C, respectively), indicating a change in its crystallization. tests were conducted to ensure that the aluminum foil did not alter the dielectric spectra of PA66 and hybrids. Electrode-sample assemblies were then transferred to the instrument for data collection. Three replicates per each sample were conducted. Figure 1 shows the 2 nd heating and the cooling DSC thermogram of the investigated hybrids, as compared to those of the neat PA66, and Table 1 summarizes their thermal properties. DSC results of PA66/IHT-1 and PA66/IIT-3 samples display an endothermic melting peak around 263 °C and an exothermic one at 232 °C, similar to those observed for the neat PA66 matrix. However, when blended with ILT-1 P-glass, PA66 melting and crystallization points shift to a lower temperature (250 and 215 °C, respectively), indicating a change in its crystallization.   Furthermore, using Equation (1), the crystallinity (X c ) of PA66 component in the PA66/ILT-1 system was found to significantly decrease (X c = 15.05%) upon the addition of ILT-1 P-glass as compared to that of the neat matrix (X c = 34.1%), as it is depicted in Table 1.

Differential Scanning Calorimetry (DSC) Analysis
where ∆H 0 is the melting enthalpy of 100% crystalline PA66 (197 J/g) [22], ∆H m is the apparent melting enthalpy corresponding to the PA66/P-glass hybrids, and w is the weight fraction of PA66. It has been demonstrated that in blends having crystallizable components, depression of both melting point and crystallinity, as well as the crystallization delay, are indicative of the miscibility in the amorphous state [23]. Additionally, the thermodynamically favorable interaction between blended phases and/or the decrease of chain mobility resulting from crosslinking reactions reflect the same phenomena [24,25].
To further explore the extent of a potential chemical reaction between PA66 and ILT-1, dynamic mechanical analysis was performed.

Dynamic Mechanical Analysis (DMA)
DMA is a useful tool for the interfacial interaction characterization of filled polymer systems. Figure 2 shows the tanδ of virgin processed PA66 resin and PA66/ILT-1, PA66/IIT-3, and PA66/IHT-1 hybrid materials as a function of temperature. tanδ is plotted against temperature and glass transition is typically observed as a peak since the material will absorb energy as it passes through the glass transition. It is seen in Figure 2 that the tanδ curves of virgin PA66, PA66/IIT-3, and PA66/IHT-1 networks display one unique and strong relaxation corresponding to their glass transitions, labeled α. Moreover, the addition of IHT-1 and IIT-3 P-glass insignificantly shifts the PA66 Tg (75 °C) to 78 and 70 °C respectively. In contrast to IIT-3 and IHT-1, the addition of ILT-1 considerably shifts the PA66 It is seen in Figure 2 that the tanδ curves of virgin PA66, PA66/IIT-3, and PA66/IHT-1 networks display one unique and strong relaxation corresponding to their glass transitions, labeled α. Moreover, the addition of IHT-1 and IIT-3 P-glass insignificantly shifts the PA66 T g (75 • C) to 78 and 70 • C respectively. In contrast to IIT-3 and IHT-1, the addition of ILT-1 considerably shifts the PA66 T g to a lower temperature (65 • C) and gives rise to another transition at 166 • C, indicating the presence of another crystalline phase in PA66.
Values of storage modulus, describing the stiffness of the material, were also plotted as a function of temperature for all studied samples, as depicted in Figure 3. It is observed that the glassy storage modulus of PA66 increased 2-3 times with the addition of P-glass additives regardless of their T g , suggesting a reinforcing effect on the dynamic modulus. It is seen in Figure 2 that the tanδ curves of virgin PA66, PA66/IIT-3, and PA66/IHT-1 networks display one unique and strong relaxation corresponding to their glass transitions, labeled α. Moreover, the addition of IHT-1 and IIT-3 P-glass insignificantly shifts the PA66 Tg (75 °C) to 78 and 70 °C respectively. In contrast to IIT-3 and IHT-1, the addition of ILT-1 considerably shifts the PA66 Tg to a lower temperature (65 °C) and gives rise to another transition at 166 °C, indicating the presence of another crystalline phase in PA66.
Values of storage modulus, describing the stiffness of the material, were also plotted as a function of temperature for all studied samples, as depicted in Figure 3. It is observed that the glassy storage modulus of PA66 increased 2-3 times with the addition of P-glass additives regardless of their Tg, suggesting a reinforcing effect on the dynamic modulus. Additionally, except the PA66/ILT-1 formulation, all storage modulus curves displayed in Figure 3, showed three distinct regions: a glassy high modulus region where the segmental mobility is restricted, a transition zone where a substantial decrease in the modulus values with the increase of temperature, and a rubbery region where a drastic decay in the modulus with temperature was observed. PA66/ILT-1 sample shows a similar trend, nevertheless, a 2 nd transition at about 166 • C was observed, thus confirming the tanδ vs. temperature data.
To further understand the DMA findings, mechanical tensile experiments at room temperature were conducted, and the obtained results are illustrated in Figure 4. A significant increase in the elastic modulus was observed for all hybrids and, in particular, for PA66/ILT-1 sample, which exhibited a tensile modulus twice as high comparing to the virgin PA66. This result is in a strong agreement with the big increase in storage moduli for the hybrids before the T g phase ( Figure 3). While the stress at break was not significantly impacted by P-glass introduction, a significant drop of the PA66/P-glass elongation at break was observed.
were conducted, and the obtained results are illustrated in Figure 4. A significant increase in the elastic modulus was observed for all hybrids and, in particular, for PA66/ILT-1 sample, which exhibited a tensile modulus twice as high comparing to the virgin PA66. This result is in a strong agreement with the big increase in storage moduli for the hybrids before the Tg phase ( Figure 3). While the stress at break was not significantly impacted by P-glass introduction, a significant drop of the PA66/P-glass elongation at break was observed.  From DSC, DMA, and mechanical tensile findings, two different scenarios can be made: The observed thermal and mechanical behavior of PA66/ILT-1 formulation may arise from the presence of degraded PA66 segments and PA66-ILT-1 crosslinked chains. The obtained crosslinking residues increased polymer-polymer interactions and decreased free volume, which in turn reduced their mobility. Consequently, PA66/ILT-1 samples significantly became stronger and stretched less than virgin PA66 samples.
According to the discussed behavior of intermediate and high T g P-glasses and our previously published results [5], IIT-3 and IHT-1 systems did not show any evidence of a chemical reaction with PA66. Thus, it can be assumed that IIT-3 and IHT-1 act as a reinforcing filler for PA66.
These stated assumptions are also supported by the rheological data reported by Urman and Otaigbe [6] on low T g P-glass/PA12 systems. The authors observed a dramatic decrease in PA12 viscosity upon the addition of >2 vol % of P-glass resulting from potential interfacial interactions between PA12 and P-glass phases.
For further insight into the effect of ILT-1 P-glass in PA66, broadband dielectric spectroscopy (BDS) measurements were carried out.

Dielectric Spectroscopy Analysis
The BDS analysis of the networks' molecular level structural variations (i.e., hybrids chain motion, T g , and sub-T g relaxations) is a powerful tool of interrogation; the information can be collected over a broad frequency (f ) range so that motional processes that occur over broad time and distance scales can be investigated vs. temperature.

γ Relaxation
While the T g relaxation is of great importance in studying polymer/hybrid networks, understanding the secondary transitions and their molecular motions are essential for evaluating the mechanical performance of these systems. Figure 5 displays the loss permittivity (ε ) vs. frequency (f ) spectra of the studied PA66/P-glass systems as compared to the processed virgin PA66 and within the temperature range of the γ relaxation process. The low temperature γ relaxation involves the motion of very short -CH 2 segments and an amide group, which provides the dielectric activity [26]. It is clearly seen that the loss permittivity ε peak maxima for the γ relaxation shift to higher frequencies as the temperature increases, reflecting faster molecular motions and shorter relaxation times in the usual sense. The widely used Havriliak-Negami (HN) Equation (2) was fitted to the data in Figure 5 to extract more in-depth information related to the dynamics of the Tg and the secondary γ relaxations of PA66/P-glass systems. * ( ) = ′ − " = − ( ε′ and ε″ are the real and imaginary dielectric permittivities, respectively, and i = √ − 1. There are three relaxation terms in the sum, and the term on the left accounts for dc conductivity. ε0 is the vacuum permittivity and ω = 2πf. For each relaxation term k, the dielectric strength ∆εk = (εR − εω)k is the difference between ε' at very low and very high frequencies, respectively. σdc is the dc conductivity The widely used Havriliak-Negami (HN) Equation (2) was fitted to the data in Figure 5 to extract more in-depth information related to the dynamics of the T g and the secondary γ relaxations of PA66/P-glass systems.
ε and ε are the real and imaginary dielectric permittivities, respectively, and i = √ − 1. There are three relaxation terms in the sum, and the term on the left accounts for dc conductivity. ε 0 is the vacuum permittivity and ω = 2πf. For each relaxation term k, the dielectric strength ∆ε k = (ε R − ε ω ) k is the difference between ε at very low and very high frequencies, respectively. σ dc is the dc conductivity and the exponent N characterizes conduction in terms of the nature of charge hopping pathways and charge mobility constraints [27]. α and β characterize the breadth and degree of asymmetry, respectively, of ε vs. ω peaks.
The relaxation time τ max = 1/2 π f max was extracted from fitting the data in Figure 5 to the HN equation at each temperature. Log τ max was then plotted as a function of 1000/T, as shown in Figure 6 depicting a strong Arrhenius behavior according to Equation (3): where E a is the activation energy of the γ relaxation motion, R is the universal gas constant, and τ o is a pre-exponential factor.
Polymers 2020, 12, x FOR PEER REVIEW 10 of 17 The activation energies, summarized in Table 2, for this molecular motion seem to be similar for neat PA66, PA66/ILT-1, PA66/IIT-3, and PA66/IHT-1 samples, implying the same local γ relaxation motion. Moreover, this energy is in the typical range for secondary relaxations in the glassy state of conventional polymers.  The activation energies, summarized in Table 2, for this molecular motion seem to be similar for neat PA66, PA66/ILT-1, PA66/IIT-3, and PA66/IHT-1 samples, implying the same local γ relaxation motion. Moreover, this energy is in the typical range for secondary relaxations in the glassy state of conventional polymers. The α relaxation, or dynamic glass transition, is associated with long-range chain segmental mobility. Figures 7 and 8 display shifts in macromolecular motions related to the glass transition relaxation, and loss permittivity ε vs. frequency at 80 and 140 • C spectra, indicating the glass transition-related peaks for each sample, respectively. Figure 7 shows that the T g relaxation peak maxima for virgin PA66 and PA66/ILT-1 hybrid samples shift to higher frequencies, indicating slightly faster chain motions as the temperature was increased. Additionally, another relaxation peak appears at 80 • C, which agrees very well with the DMA results depicting an extra peak for the PA66/ILT-1 hybrid sample only. Another active process detected at low frequency in Figures 7 and 8b for PA66, PA66/IIT-3, and PA66/IHT-1 samples, can be due to the Maxwell-Wagner-Sillars (MWS) interfacial polarization process. This process is common in multiphase systems having different dielectric constants and conductivities among their phases. In contrary to this behavior, the PA66/ILT-1 hybrid does not show similar process, which is possibly owing to the formation of more homogenous network structure. Another active process detected at low frequency in Figures 7 and 8b for PA66, PA66/IIT-3, and PA66/IHT-1 samples, can be due to the Maxwell-Wagner-Sillars (MWS) interfacial polarization process. This process is common in multiphase systems having different dielectric constants and conductivities among their phases. In contrary to this behavior, the PA66/ILT-1 hybrid does not show similar process, which is possibly owing to the formation of more homogenous network structure. Once again, the data displayed in Figures 7 and 8 prominently revealed the apparition of a 2 nd glass transition temperature for PA66/ILT-1 formulation.
For further investigation on the relaxation processes dynamics elucidated within the Tg region, the characteristic relaxation times for each relaxation, τmax, calculated from the HN equation fitting of the spectra were plotted against the reciprocal of temperature. The Vogel-Fulcher-Tammann-Hesse (VFTH) equation [28][29][30] was then fitted to τmax vs. temperature data for the Tg relaxation according to Equation (4): Once again, the data displayed in Figures 7 and 8 prominently revealed the apparition of a 2 nd glass transition temperature for PA66/ILT-1 formulation.
For further investigation on the relaxation processes dynamics elucidated within the T g region, the characteristic relaxation times for each relaxation, τ max , calculated from the HN equation fitting of the spectra were plotted against the reciprocal of temperature. The Vogel-Fulcher-Tammann-Hesse (VFTH) equation [28][29][30] was then fitted to τ max vs. temperature data for the T g relaxation according to Equation (4): where k B is the Boltzmann constant, τ 0 is a hypothetical relaxation time at infinite temperature, E a is the apparent activation energy for the motion, and T V is the Vogel temperature. The Vogel temperature is considered as the static freezing temperature [28,29] at which chain segments become frozen. Figure 9 shows the characteristic relaxation time as a function of reciprocal temperature for processed PA66 and PA66/P-glass samples. The solid lines represent fits of the data to the VFTH equation, and Table 3 summarizes the obtained fitting parameters. The two plots for the PA66/ILT-1 sample correspond to separate fittings of the two relaxations in the α transition region as already discussed, and the curvature of all plots is characteristic of long-range motions in glass-forming polymers.
Polymers 2020, 12, x FOR PEER REVIEW 13 of 17 where kB is the Boltzmann constant, τ0 is a hypothetical relaxation time at infinite temperature, Ea is the apparent activation energy for the motion, and TV is the Vogel temperature. The Vogel temperature is considered as the static freezing temperature [28,29] at which chain segments become frozen. Figure 9 shows the characteristic relaxation time as a function of reciprocal temperature for processed PA66 and PA66/P-glass samples. The solid lines represent fits of the data to the VFTH equation, and Table 3 summarizes the obtained fitting parameters. The two plots for the PA66/ILT-1 sample correspond to separate fittings of the two relaxations in the α transition region as already discussed, and the curvature of all plots is characteristic of long-range motions in glass-forming polymers. Figure 9. The characteristic relaxation times of the studied PA66/P-glass systems and processed virgin PA66 as a function of temperature. Solid lines represent the VFTH equation fits.
As can be detected in Figure 9, the τmax values for neat PA66, PA66/IIT-3, and PA66/IHT-1 samples across the studied broad range of temperature, where the Tg related motions get activated, are similar. This signifies that the addition of IIT-3 and IHT-1 P-glass additives does not meaningfully affect the molecular motions associated with the Tg transition in PA66. However, it is evident that the peak related to the 1 st glass transition of the PA66/ILT-1 sample (see Figure 8a) was significantly vertically down-shifted, confirming, once again, that the addition of ILT-1 P-glass accelerates the α relaxation of PA66 chains. Additionally, the curve for the 2 nd glass transition in the PA66/ILT-1 sample shows an opposite behavior, reflecting a slower and more restricted chain motions. VFTH fitting parameters, displayed in Table 3, confirm the just mentioned observations; the PA66/ILT-1 1 st Tgrelatedsegment relaxation shows the highest Ea (0.8799 eV) and the lowest Tv (90.50 K), and PA66/ILT-1 2 nd Tg related segment relaxation shows the lowest Ea (0.0229 eV) and the highest Tv (371.5 K) as compared to neat PA66 and the other PA66/P-glass samples. It should be noted that in polymer blend systems, a decrease of the apparent activation energy is often ascribed to partial miscibility of the neat polymers in the blends [6,31,32], and an increase in the Tv is attributed to polymer degradation [33]. It should be pointed out that the reduction in the 1 st Tg observed for the studied PA66/15% ILT-1 is more significant than that reported by Urman and Otaigbe [6], who linked the Tg reduction to phase miscibility in the hybrid. In our case, the miscibility between ILT-1 and PA66 matrix seems to be more pronounced due to the much more downward shift in the VFTH curves of As can be detected in Figure 9, the τ max values for neat PA66, PA66/IIT-3, and PA66/IHT-1 samples across the studied broad range of temperature, where the T g related motions get activated, are similar. This signifies that the addition of IIT-3 and IHT-1 P-glass additives does not meaningfully affect the molecular motions associated with the T g transition in PA66. However, it is evident that the peak related to the 1 st glass transition of the PA66/ILT-1 sample (see Figure 8a) was significantly vertically down-shifted, confirming, once again, that the addition of ILT-1 P-glass accelerates the α relaxation of PA66 chains. Additionally, the curve for the 2 nd glass transition in the PA66/ILT-1 sample shows an opposite behavior, reflecting a slower and more restricted chain motions. VFTH fitting parameters, displayed in Table 3, confirm the just mentioned observations; the PA66/ILT-1 1 st T g related segment relaxation shows the highest E a (0.8799 eV) and the lowest T v (90.50 K), and PA66/ILT-1 2 nd T g related segment relaxation shows the lowest E a (0.0229 eV) and the highest T v (371.5 K) as compared to neat PA66 and the other PA66/P-glass samples. It should be noted that in polymer blend systems, a decrease of the apparent activation energy is often ascribed to partial miscibility of the neat polymers in the blends [6,31,32], and an increase in the T v is attributed to polymer degradation [33]. It should be pointed out that the reduction in the 1 st T g observed for the studied PA66/15% ILT-1 is more significant than that reported by Urman and Otaigbe [6], who linked the T g reduction to phase miscibility in the hybrid. In our case, the miscibility between ILT-1 and PA66 matrix seems to be more pronounced due to the much more downward shift in the VFTH curves of PA66/ILT-1 1 st T g compared to virginPA66. This result was supported by our previous work [5], in which we demonstrated that ILT-1 tended to disperse better than IIT-3 and IHT-1. From all findings described in the present work, it can be concluded that the PA66/ILT-1 hybrid system is essentially a mixture of small degraded PA66segments and long PA66-ILT-1 crosslinked chains (degradation of PA66 chains, during the processing stage, in PA66/ILT-1 formulation, was previously demonstrated by our team using gel permeation chromatography analysis [5]). The crosslinked and degraded chain segments do not have the same degree of mobility; the mobility of crosslinked PA66-ILT-1 chain is restrained, resulting in longer relaxation times and, therefore, higher T g (Figure 9). At the opposite, the disentangled degraded PA66 chain segments are free to relax completely, which results in a lower relaxation time and a shift towards lower T g .
Based on the present investigations and the reported mechanism of ammonium polyphosphate in PA66 [16], we believe that the addition of ILT-1 to PA66 during melt processing leads to simultaneous depolymerization and crosslinking reactions according to the mechanism shown in Scheme 1.
Polymers 2020, 12, x FOR PEER REVIEW 14 of 17 PA66/ILT-1 1 st Tg compared to virginPA66.This result was supported by our previous work [5], in which we demonstrated that ILT-1 tended to disperse better than IIT-3 and IHT-1. From all findings described in the present work, it can be concluded that the PA66/ILT-1 hybrid system is essentially a mixture of small degraded PA66segments and long PA66-ILT-1 crosslinked chains (degradation of PA66 chains, during the processing stage, in PA66/ILT-1 formulation, was previously demonstrated by our team using gel permeation chromatography analysis [5]). The crosslinked and degraded chain segments do not have the same degree of mobility; the mobility of crosslinked PA66-ILT-1 chain is restrained, resulting in longer relaxation times and, therefore, higher Tg (Figure 9). At the opposite, the disentangled degraded PA66 chain segments are free to relax completely, which results in a lower relaxation time and a shift towards lower Tg.
Based on the present investigations and the reported mechanism of ammonium polyphosphate in PA66 [16], we believe that the addition of ILT-1 to PA66 during melt processing leads to simultaneous depolymerization and crosslinking reactions according to the mechanism shown in scheme 1.

Scheme 1.
Hypothesized mechanism of action of ILT-1 in PA66 during the melt processing stage.
At 263 °C (processing temperature), ILT-1 P-glass catalyzes the N-alkylamide bond scission, through potential protonation of the carbonyl groups, giving primary amide chain end fragments (reaction 1). Simultaneously with the depolymerization reaction, a crosslinking process occurs according to reaction (2) where ILT-1 P-glass acts as a crosslinking agent between the PA66 degradation products, thereby forming ILT-1-PA66 crosslinked network structure.

Conclusion
Results obtained from different analytical techniques and described in the present work supported and confirmed our previous findings regarding the interaction between the studied P-glass compositions and PA66. Only the low Tg P-glass shows evidence of chemical interaction with PA66 during the compounding process through a depolymerization/crosslinking mechanism. Blending ILT-1 with PA66 catalyzes simultaneous scission of PA66 chains and crosslinking reactions Scheme 1. Hypothesized mechanism of action of ILT-1 in PA66 during the melt processing stage.
At 263 • C (processing temperature), ILT-1 P-glass catalyzes the N-alkylamide bond scission, through potential protonation of the carbonyl groups, giving primary amide chain end fragments (reaction 1). Simultaneously with the depolymerization reaction, a crosslinking process occurs according to reaction (2) where ILT-1 P-glass acts as a crosslinking agent between the PA66 degradation products, thereby forming ILT-1-PA66 crosslinked network structure.

Conclusions
Results obtained from different analytical techniques and described in the present work supported and confirmed our previous findings regarding the interaction between the studied P-glass compositions and PA66. Only the low T g P-glass shows evidence of chemical interaction with PA66 during the compounding process through a depolymerization/crosslinking mechanism. Blending ILT-1 with PA66 catalyzes simultaneous scission of PA66 chains and crosslinking reactions leading to an evident mixture of crosslinked and depolymerized primary amide end chain fragments. This was confirmed by the depression of both melting point and crystallinity as well as a splitting of the PA66 T g relaxations into two peaks. From a flame-retardant application standpoint, the ILT-1/PA66 crosslinked structure is believed to undergo further modification during combustion at high temperatures that would help to form highly efficient glassy char barrier. Advanced solid-state NMR methods ( 1 H-31 P HARDSHIP) will be addressed in future work to give more insight on the proposed mechanism.