Understanding the crystallization behavior of polyamide 6/polyamide 66 alloys from the perspective of hydrogen bonds: projection moving-window 2D correlation FTIR spectroscopy and the enthalpy

Abstract

In this study, the crystallization behavior of PA6/PA66 alloys was studied using in situ FTIR spectroscopy, combined with Proj-MW2D correlation analysis and DSC measurements. The method for calculating the generation enthalpy of hydrogen bonds during PA6/PA66 alloys crystallization was also established via Van't Hoff analysis. The essential reason for the crystallization reduction for both PA6 and PA66 in alloys was elucidated from the perspective of hydrogen bonds. Compared with neat PA6 and neat PA66, DSC measurements showed that the crystallization ability of both PA6 and PA66 in the alloys obviously decreased. From the results of the generation enthalpies of the hydrogen bonds, it was confirmed that the capability for hydrogen bond generation of both PA6 and PA66 in PA6/PA66 alloys was significantly reduced. The molecular chain motions of PA6 and PA66 during the alloy crystallization were successfully separated using Proj-MW2D correlation FTIR spectroscopy. Two main issues were addressed. The first one is that the generating capacity of hydrogen bonds between PA6 and PA66 is actually very weak, although this type of hydrogen bond can be generated in theory. The second is that non-hydrogen bonded molecular chains of PA66 are also involved in the PA6 crystallization, and the molecular chains of amorphous state PA66 hinder the generation of hydrogen bonds between PA6 molecular chains, resulting in a significant crystallization reduction of PA6 in PA6/PA66 alloys.

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1. Introduction

Polyamide, commonly known as nylon, is an important kind of engineering semi-crystalline polymer with a wide range of applications, due to its excellent physical and mechanical properties.^1,2 Among polyamides, PA6 and PA66, accounting for more than 80% of the total production of PAs,³ are endowed with excellent mechanical properties, good chemical resistance, and ease of production and processing. Therefore, PA6 and PA66 have been extensively applied in areas such as automobiles, electronics, machinery, sports, and so on.^4,5 It is well known that the main chains of PA6 and PA66 contain repeated amide groups –CONH–, and therefore, a lot of intramolecular and intermolecular hydrogen bonds can be formed, resulting in the ease of the ordered arrangement of the molecular chains and the formation of crystals. Although PA6 and PA66 have similar molecular structures (Scheme 1), the density and the forming ability of the hydrogen bonds are completely different. Therefore, for PA6/PA66 alloys, complex crystallization behavior was always observed.^6,7

Scheme 1 Chemical structure of polyamide 6 and polyamide 66.

Researchers reported that the degree of crystallinity of PA6 was greatly reduced when adding a certain amount of PA66 (even a small amount) into the PA6 matrix, which was studied and mentioned in many literature reports.^8–10 Li et al. investigated the non-isothermal crystallization kinetics of PA6/PA66 alloys. They claimed that the added PA66 was only dispersed in the PA6 matrix at the molecular level (did not crystallize), and reduced the molecular arrangement regularity of PA6, leading to the decrease of the degree of crystallinity in PA6.⁹ However, Rybnikář and Geil stated that the reduction of crystallinity degree in PA6 could be attributed to possible co-incorporation of crystal defects, although the possibility of co-crystallization was denied.¹⁰ Their assumption is reasonable, but there is a lack of clear evidence to support this. Moreover, although the above mentioned phenomenon is very common, the interpretation of the related mechanism does not sound not convincing. A similar phenomenon also appears in our study, and we hope to explain the mechanism behind this phenomenon from the perspective of hydrogen bonds, which have a close relationship with the crystallization behavior of polyamide.

Noda proposed the concept of generalized two-dimensional (2D) correlation vibrational spectroscopy in 1993;¹¹ this powerful spectroscopy method has been rapidly developed in the past two decades. In 2000, Thomas and Richardson proposed moving-window 2D correlation spectroscopy (MW2D).¹² Compared to the generalized 2D correlation spectroscopy, the biggest advantage is that MW2D can be directly used to determine the spectral correlation variation along both perturbation variable (e.g., temperature) and spectral variable (e.g., wave number) axes.^13–19 According to the summary of many years of research, scientists often employed MW2D to determine the transition point and the transition range of polymers, then generalized 2D spectroscopy was carried out to study the mechanism at a specific transition.^20–24 In 2010, Noda²⁵ proposed the generalized projection 2D correlation spectroscopy, which was specifically developed for complex systems. Through separating the corresponding correlation intensity peaks from different components, the projection technique can greatly simplify the “crowded” 2D correlation spectra. Nowadays, this new 2D correlation spectroscopy has aroused great interest.^26–28 However, generalized 2D correlation spectroscopy, combined with projection, still has some limitations for multicomponent polymer systems because multiple transitions always appear under an external perturbation (e.g., temperature). Although MW2D correlation spectroscopy has the inherent capacity to determine the multiple transitions of polymers, it cannot distinguish the origin of correlation peaks without the help of other characterization methods. To overcome these problems, we developed projection moving-window 2D correlation spectroscopy (Proj-MW2D) in 2015.²⁹ This method not only has a capacity to precisely specify multiple transition points of polymers, but can also separate the correlation peaks from different components. Hence, Proj-MW2D is quite suitable for the PA6/PA66 alloy system in our study.

In the present study, the crystallization behavior of PA6/PA66 alloys was investigated using differential scanning calorimetry (DSC) and in situ FTIR spectroscopy, combined with Proj-MW2D correlation analysis. Importantly, the method of calculating the generation enthalpy of hydrogen bonds was successfully established via Van't Hoff plots. The essential reason for the crystallization reduction for both PA6 and PA66 in alloys was found from the perspective of hydrogen bonds.

2. Experimental

2.1. Samples preparation

PA6 was provided by Baling Petrochemical Industry Co. Ltd., of Sinopec. PA66 was a commercial sample obtained from UBE industry of Japan. The samples of PA6 and PA66 were firstly kept in a vacuum at 110 °C for 10 h to remove the absorbed water. Then, two kinds of PA6/PA66 alloys were prepared using a twin screw extruder, and the weight ratios of PA6 : PA66 were 85 : 15 and 60 : 40, respectively. The co-rotating twin screw extruder (Φ 20 mm), which has four heating zones was used. This twin screw extruder was equipped with an extrusion die of double holes (Φ 2.5 mm), and the screw length–diameter ratio was 32. The temperature profiles of four heating zones were 180 °C, 265 °C, 270 °C and 265 °C (from the feed port to the die), respectively. During the experiments, the screw speed was controlled at 180 rpm. The detailed screw elements of the twin-screw extruder are provided in the ESI (Fig. S1†).

2.2. Differential scanning calorimetry (DSC)

DSC curves were measured using a NETSCZH DSC-204 instrument under nitrogen atmosphere (25 mL min⁻¹). All samples were weighed as 8 ± 0.05 mg. The samples were firstly melted (300 °C) for 5 min after heating from room temperature to eliminate the thermal history, and then cooled to 45 °C at a rate of 10 °C min⁻¹. After that, the samples were reheated to 300 °C at a rate of 10 °C min⁻¹.

2.3. In situ FTIR spectroscopy

Before the in situ FTIR measurement, a pellet of the sample was placed between two aluminum foils. The films of PA6, PA66 and PA6/PA66 alloys were directly prepared via heat-pressing. The sandwiched samples were firstly pressed at 270 °C in a hot press, and then were naturally cooled to room temperature. The heat-pressing time was 5 min, and the pressure was 20.0 MPa. The thickness of the prepared films was approximately 10–15 μm. After that, the films were cut into a circular shape (Φ 1.3 cm) and sandwiched between two small KBr windows, which were finally placed into a homemade in situ pool (programmable heating and cooling instrument).

In our experiments, the thickness of the KBr windows used was 0.7 mm. The primary purpose of placing the polymer film between two KBr windows was to avoid the high-temperature flow after the nylon melted, because the sample thinning caused by the flow would create an illusion in the in situ FTIR spectra. However, the use of KBr windows would slightly increase the thermal inertia of the system. Therefore, a small lag in temperature between the bath and the nylon film sample was inevitable. To figure out this problem and to ensure the measurement of the actual temperature of the nylon sample, in our homemade in situ pool, the head (temperature measuring point) of a needle-shaped thermocouple (Φ 0.8 mm), which measured the actual temperature of the sample, was designed to tightly contact the nylon sample. Therefore, the temperature collected in the in situ FTIR experiment was the actual temperature of the polymer sample, other than the temperature of the bath in the in situ pool.

The in situ FTIR measurement was carried out using a Bruker Tensor 27 FTIR spectrometer, which was equipped with a deuterated L-α-alanine doped triglycine sulfate (DLaTGS) detector. The sandwiched film was firstly heated from 30 °C to 300 °C at a rate of 5 °C min⁻¹, and then kept at 300 °C for 3 min to eliminate the thermal history and the adsorbed water. The molten film was then cooled from 300 °C to 45 °C at a rate of 5 °C min⁻¹. The temperature-dependent FTIR spectra were collected during the cooling process. There were 119 spectra gathered. There were 20 scans for each FTIR spectrum and, the spectral resolution was 4 cm⁻¹. During the experiments, the film samples were protected by dried high-purity nitrogen gas (200 mL min⁻¹) to avoid high-temperature oxidation and degradation.

2.4. 2D correlation FTIR spectroscopy

MW2D and Proj-MW2D correlation FTIR spectroscopy were processed and calculated using 2DCS software, developed by one of the authors. Before the analysis, the FTIR spectra were pretreated using a linear baseline correction. The window size was chosen as 2m + 1 = 11 to produce high-quality MW2D, Proj-MW2D, and null-space Proj-MW2D correlation FTIR spectra. The 2% correlation intensity of MW2D, Proj-MW2D, and null-space Proj-MW2D correlation FTIR spectra were regarded as noise and were cut off. The theory and algorithm of MW2D and Proj-MW2D correlation spectroscopy can be found in the literature.^11,12,25,29 A detailed description of Proj-MW2D is provided in the ESI.†

3. Results and discussion

3.1. Crystallization decline of PA6/PA66 alloys determined by DSC measurements

DSC curves of neat PA6, neat PA66, and PA6/PA66 alloys, from 255 °C to 140 °C, upon cooling in nitrogen atmosphere, are illustrated in Fig. 1. For both neat PA6 and neat PA66, only one sharp exothermic peak is observed, at 184 °C and 232 °C, respectively. For PA6/PA66 alloys, as expected, there appeared two separate exothermic peaks. These two peaks were observed at 190 °C and 218 °C, when the weight ratio of PA6/PA66 was 85 : 15, and were detected at 188 °C and 225 °C, when the weight ratio was 60 : 40. Obviously, the higher temperature peak (at 225 °C or 218 °C) represents the crystallization of PA66 in the alloys, and the lower temperature peak (at 188 °C or 190 °C) is the crystallization of PA6. In Fig. 1, two phenomena aroused our attention. The first phenomenon is that the crystallization temperature of PA6 in the alloys was higher than that of neat PA6, but the crystallization temperature of PA66 in the alloys was lower than that of neat PA66. The second phenomenon is that the crystallization peak areas of both PA6 and PA66 in the alloys were apparently smaller than those of neat PA6 and neat PA66, indicating the significant decrease of the crystallization ability.⁹

Fig. 1 DSC curves of neat PA6, neat PA66, PA6/PA66 (85 : 15), and PA6/PA66 (60 : 40) upon cooling at a rate of 10 °C min⁻¹.

Fig. 2 illustrates the DSC curves of PA6, PA66, and PA6/PA66 alloys from 140 °C to 275 °C, upon reheating. The melting points of neat PA6 and neat PA66 were observed at 220 °C and 262 °C, respectively. For PA6/PA66 alloys, two obvious peaks were detected. The lower temperature peaks are the melting of PA6 crystals in the alloys, and the higher temperature peaks are attributed to PA66. It also noted that the melting point of PA6 (at 215 °C or 218 °C) in the alloys is lower than that of neat PA6, and the melting point of PA66 (at 255 °C or 259 °C) is also lower than that of neat PA66. As is well known, the melting point of crystalline polymers is always proportional to the thickness of the lamellae. Therefore, it can be inferred that both the lamellar thickness of PA6 and PA66 in the alloys are apparently thinner than that of neat PA6 and neat PA66, also indicating the decrease of the crystallization ability.

Fig. 2 DSC curves of neat PA6, neat PA66, PA6/PA66 (85 : 15), and PA6/PA66 (60 : 40) upon reheating at a rate of 10 °C min⁻¹.

The relative crystallinity degrees of PA6 and PA66 in the alloys were also calculated according to the melting enthalpy measured by DSC upon reheating (Fig. 2). Here, we assumed that the crystallinity degrees of neat PA6 and neat PA66 were 100%. Therefore, the relative crystallinity degree of PA6 and PA66 in the alloys could be calculated using the following equations:

where ΔH_neat-PA6 and ΔH_neat-PA66 are the melting enthalpies of the neat PA6 and neat PA66, respectively, and ΔH_PA6 and ΔH_PA66 are the melting enthalpies of PA6 and PA66 in the alloys. Table 1 summarizes the melting enthalpies, the melting temperatures, and the calculated relative crystallinity degrees. For neat PA6 and neat PA66, the melting enthalpies are 71.0 J g⁻¹ and 55.5 J g⁻¹, respectively. However, in the alloys, the melting enthalpies of PA6 and PA66 were reduced to 49.0 J g⁻¹ and 10.7 J g⁻¹ when the weight ratio of PA6/PA66 was 85 : 15, and 34.5 J g⁻¹ and 33.3 J g⁻¹ when the weight ratio was 60 : 40. For PA6/PA66 alloys of 85 : 15 and 60 : 40, the relative crystallinity degree of PA6 was 69.0% and 48.6%, and that of PA66 was 19.3% and 60.0%, respectively.

Table 1 The melting enthalpies, T_m and the relative crystallinity degree of neat PA6, neat PA66 and PA6/PA66 alloys

Samples	ΔHPA6 (J g^−1)	ΔHPA66 (J g^−1)	Tm of PA6 (°C)	Tm of PA66 (°C)	Relative crystallinity degree of PA6 (%)	Relative crystallinity degree of PA66 (%)
Neat PA6	71.0	—	220	—	100.0	—
Neat PA66	—	55.5	—	262	—	100.0
PA6/PA66 = 85 : 15	49.0	10.7	218	255	69.0	19.3
PA6/PA66 = 60 : 40	34.5	33.3	215	259	48.6	60.0

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The discussion above sufficiently shows the significant reduction of the crystallization ability of both PA6 and PA66 in the alloys. Li et al. also reported a similar result, and they claimed that it was probably attributed to the destruction of the molecular chain regularity of PA6 when adding PA66.⁹

3.2. In situ FTIR spectra upon cooling

The in situ FTIR spectra of PA6/PA66 (85 : 15) alloy upon cooling from 265 °C to 140 °C in the regions of 3500–3150 cm⁻¹, 1700–1400 cm⁻¹, and 1000–900 cm⁻¹ are illustrated in Fig. 3. For clarity, not all the collected FTIR spectra are displayed. The in situ FTIR spectra of neat PA6, neat PA66, and PA6/PA66 (60 : 40) are also provided in Fig. S2–S4 in the ESI.† According to the literature, the band assignments in Fig. 3 are summarized in Table 2.^2,6,30–36

Fig. 3 The in situ FTIR spectra of PA6/PA66 (85 : 15) alloy upon cooling from 265 °C to 140 °C. (a) 3500–3150 cm⁻¹; (b) 1700–1400 cm⁻¹ and 1000–900 cm⁻¹.

Table 2 Band assignments for FTIR spectra of PA6/PA66 alloy

Wavenumber (cm^−1)	Assignments
	PA6	PA66
3445	v(N–H, free), N–H stretching of “free” N–H groups
3305	v(N–H, bonded), N–H stretching of hydrogen-bonded N–H groups
1675	v(C=O, free), C=O stretching of “free” C=O groups
1640	v(C=O, bonded), C=O stretching of hydrogen-bonded C=O groups
1543	δ(N–H, bonded), N–H bending of hydrogen-bonded N–H groups
1512	δ(N–H, free), N–H bending of “free” N–H groups
960	δ(CONH, PA6), CONH bending of PA6 in the crystalline	—
933	—	δ(CONH, PA66), CONH bending of PA66 in the crystalline
929	δ(CONH, PA6), CONH bending of PA6 in the crystalline	—

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For polyamides, both the N–H and the C=O in amide groups always exist in hydrogen-bonded and “free” (non-hydrogen bonded) forms, which can be easily detected in FTIR spectra. The peak at 3305 cm⁻¹ is assigned to the N–H stretching of hydrogen-bonded N–H groups, and that at 3445 cm⁻¹ is attributed to the N–H stretching of “free” N–H groups.^{2,6,9,32,33,35} The bands at 1543 cm⁻¹ and 1512 cm⁻¹ are the corresponding N–H bending of hydrogen-bonded and “free” N–H groups, respectively.^2,6,31,34,36 The peak at 1675 cm⁻¹ is assigned to the C=O stretching of “free” C=O groups, and 1640 cm⁻¹ is attributed to the C=O stretching of hydrogen-bonded C=O groups.^2,6 It is noted that we are unable to distinguish the carbonyl and amine groups of PA6 from that of PA66 in the regions of 3500–3150 cm⁻¹ and 1700–1400 cm⁻¹, due to a highly molecular structural similarity. As shown in Fig. 3, with the temperature decreasing from 265 °C to 140 °C, the intensity of 3445 cm⁻¹ is slowly reduced, and eventually disappears, and that of 3305 cm⁻¹ is enhanced and obviously moves to a lower wavenumber. Moreover, the intensity of 1512 cm⁻¹ gradually decreases, while that of 1543 cm⁻¹ increases and shifts to a higher wavenumber. All of these indicate the disappearance of “free” N–H groups and the generation of hydrogen-bonded N–H groups during the PA6/PA66 alloy crystallization upon cooling. At the same time, it was also observed that the band intensity at 1675 cm⁻¹, which is assigned to “free” C=O stretching vibration, slowly disappears from 265 °C to 140 °C, and that of 1640 cm⁻¹ (hydrogen-bonded C=O) remarkably increases and slightly moves to a lower wavenumber, proving the formation of hydrogen-bonded C=O groups. In Fig. 3(b), the band intensities at 960 cm⁻¹ and 929 cm⁻¹ increase rapidly during the cooling process, indicating the crystallization of PA6 and PA66. The band at 960 cm⁻¹ is only assigned to the –CONH– bending vibration of PA6,⁶ which is the characteristic band of the PA6 crystalline phase. However, 929 cm⁻¹ is the overlapping band of PA6 and PA66. This is because not only does PA6 have a band at 929 cm⁻¹,⁶ but PA66 also presents a band at 933 cm⁻¹ (Fig. 4). Thus, 929 cm⁻¹ actually represents the crystallization of both PA6 and PA66.

Fig. 4 Typical FTIR spectra of neat PA6, neat PA66, and PA6/PA66 (85 : 15) alloy in the region of 1000–900 cm⁻¹. For PA6/PA66 alloy, the bands at 929 cm⁻¹ represent the crystallization of both PA6 and PA66. The peak at 960 cm⁻¹ is the characteristic crystalline band of PA6.

It is noted that, as described in the Section 2.3, the temperature of the nylon sample in the in situ FTIR was actually measured by a needle-shaped thermocouple (Φ 0.8 mm) whose head (temperature measuring point) was tightly in contact with the nylon sample. Therefore, the temperature in Fig. 3 is the actual temperature of the nylon sample.

3.3. Generation enthalpies of hydrogen bonds estimated from FTIR

Here, we consider the formation and breaking of hydrogen bonds between C=O and N–H of amide groups as an equilibrium reaction:where [C=O] and [H–N] are the molar concentrations of “free” C=O and “free” N–H of amide groups, respectively. [C=O⋯H–N] is the molar concentration of hydrogen bonded groups between C=O and N–H.

According to the Beer–Lambert law:

A = εLC (4)

where ε is the extinction coefficient, and L is the thickness (optical path) of the polyamide samples, A is the infrared absorbance, and C is the molar concentration.

At a lower temperature, the total molar concentration of hydrogen-bonded N–H groups is C₀. The C_free and C_bond are the molar concentrations of the “free” N–H and the hydrogen-bonded N–H of amide groups at a higher temperature, respectively. As the temperature decreases, the “free” N–H will gradually convert to hydrogen-bonded N–H.

There exists the following relationship of C₀, C_bond, and C_free:

C_free + C_bond = C₀ (5)

At a lower temperature (e.g., 140 °C), the N–H groups are actually composed of the hydrogen-bonded N–H groups and the non-hydrogen bonded N–H groups. It is noted that in eqn (5), C₀ is only the molar concentration of the hydrogen-bonded N–H groups, and the non-hydrogen bonded N–H groups at this lower temperature are not considered. This is important because these parts of the non-hydrogen bonded N–H groups at the lower temperature (e.g., 140 °C) were never involved in the formation of hydrogen bonds throughout the cooling process; i.e., these non-hydrogen bonded N–H groups have no relationship with the equilibrium reaction of the hydrogen bond formation and breaking (eqn (3)). Certainly, they cannot be considered in eqn (5).

In our study, with the temperature decreasing from 265 °C to 140 °C, the “free” N–H groups (at a higher temperature; e.g., 265 °C) that can be completely converted into the hydrogen-bonded N–H groups at a lower temperature (e.g., 140 °C) are considered in eqn (5).

According to the Beer–Lambert law:

A₀ = ε_bondLC₀ (6)

A_bond = ε_bondLC_bond (7)

A_free = ε_freeLC_free (8)

where ε_free is the extinction coefficient of “free” N–H groups, and ε_bond is that of the hydrogen-bonded N–H of amide groups.

At a given temperature, the molar fraction of the bonded N–H of the amide groups is:

Similarly, the mole fraction of the “free” N–H of amide groups can be expressed as follows:

The equilibrium constant of eqn (3) can be calculated as follows:

The Van't Hoff form of the eqn (11) is below:

where T is the absolute temperature (in kelvin), and R is the gas constant (8.314 J mol⁻¹ K⁻¹). The ΔH is the enthalpy of hydrogen bond formation (J mol⁻¹), and ΔS is the entropy of hydrogen bonds (J mol⁻¹ K⁻¹).

Finally, a straight line for the plot between ln[α_bond/(1 − α_bond)²] and 1/T can be conveniently fitted using the least squares fitting. The enthalpy of hydrogen bond formation (ΔH) of C=O⋯H–N can be obtained from the slope of the fitted straight line.

In this study, the band at 3305 cm⁻¹ was chosen to perform the Van't Hoff analysis above. Specifically, the value of A₀ in the eqn (9) is the absorption intensity at 3305 cm⁻¹ at 140 °C, and the values of A_bond are the absorption intensities (not the absorption areas) at 3305 cm⁻¹ from 264 °C to 141 °C. Both A₀ and A_bond were directly obtained from the in situ FTIR spectra. Therefore, the value of A₀ was fixed, and the values of A_bond were changed according to the temperatures. A series of α_bond values were calculated from the ratio of A_bond and A₀.

As mentioned in the previous section, the peak at 3305 cm⁻¹ was assigned to the hydrogen-bonded N–H groups of both PA6 and PA66 in the PA6/PA66 alloy, representing all types of hydrogen bonds generated from N–H groups.^2,6 Therefore, we can conveniently estimate the average generation enthalpies of the hydrogen bonds from 3305 cm⁻¹. In addition, as shown in Fig. 3(a), the band at 3305 cm⁻¹ does not overlap with any other bands, and there is a high signal to noise ratio, which ensures the accuracy of the calculation.

Fig. 5(a)–(d) illustrate the Van't Hoff plots calculated from the FTIR intensity variation at 3305 cm⁻¹, of neat PA6, neat PA66, and PA6/PA66 alloys. The fitted straight lines for each Van't Hoff plot indicate that the generation enthalpies of hydrogen bonds can be successfully calculated upon cooling. We list these enthalpies in Table 3. According to Van't Hoff plots, two temperature ranges within 200–180 °C and 240–215 °C can be also roughly distinguished, which correspond to the temperature ranges of PA6 and PA66 crystallization, respectively. In addition, it can be observed from Table 3 that the hydrogen bond formation is a typical exothermic process, due to the negative value of the calculated enthalpies.

Fig. 5 Van't Hoff plots calculated from the spectral intensity variation at 3305 cm⁻¹, which represents all types of hydrogen bonds generated from N–H groups. (a) Neat PA6; (b) neat PA66; (c) PA6/PA66 (85 : 15); (d) PA6/PA66 (60 : 40).

Table 3 The generation enthalpies of hydrogen bonds of neat PA6, neat PA66, and PA6/PA66 alloys calculated via Van't Hoff plots

Samples	Generation enthalpy of hydrogen bonds, ΔH (kJ mol^−1)
	200–180 °C	240–215 °C
PA6	−76.9 ± 4.9	—
PA66	—	−99.4 ± 6.2
PA6/PA66 = 85 : 15	−66.5 ± 1.4	−33.0 ± 0.6
PA6/PA66 = 60 : 40	−38.1 ± 0.3	−56.8 ± 1.5

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For neat PA6 and neat PA66, the calculated enthalpies of hydrogen bonds are −76.9 ± 4.9 kJ mol⁻¹ and −99.4 ± 6.2 kJ mol⁻¹, respectively. Because the larger absolute value of enthalpy indicates the greater ability to form hydrogen bonds, from the standpoint of the enthalpy, the hydrogen bond formation ability of neat PA66 is apparently stronger than that of neat PA6. The result is consistent with Li's report and others.^9,37,38 For PA6/PA66 alloys, however, compared with neat PA6 and neat PA66, the generation enthalpies of hydrogen bonds are both significantly reduced in the temperature ranges of PA6 (200–180 °C) and PA66 (240–215 °C) crystallization. Within the temperature range of PA6 crystallization (200–180 °C), the generation enthalpies of hydrogen bonds of PA6/PA66 (85 : 15) alloy and PA6/PA66 (60 : 40) alloy are −66.5 ± 1.4 kJ mol⁻¹ and −38.1 ± 0.3 kJ mol⁻¹, respectively, which are both less than that of neat PA6 (−76.9 ± 4.9 kJ mol⁻¹). We think the main reason for this phenomenon can be explained as follows.

For neat PA6, within 200–180 °C, the intramolecular and intermolecular hydrogen bonds can be easily formed from N–H and C=O groups of PA6 macromolecular chains. The difference in PA6/PA66 alloys is that the hydrogen bonds are not only generated from PA6 macromolecular chains, but also between PA6 and PA66. Because the positions of the PA6 chains are occupied by PA66, the formation of hydrogen bonds between PA6 molecular chains are greatly disturbed by PA66 molecular chains, resulting in a significant reduction of the ability to form hydrogen bonds between PA6 molecular chains. However, although the hydrogen bonds can be generated between PA6 and PA66 in theory, this generating ability is probably very weak, only contributing to a small part of the enthalpy. Thus, the average generation enthalpies of hydrogen bonds within 200–180 °C is obviously less than that of neat PA6 (−76.9 ± 4.9 kJ mol⁻¹). Moreover, within 200–180 °C, as listed in Table 3, the content of PA66 is higher and the generation enthalpy is lower. Similarly, in the temperature range of PA66 crystallization (240–215 °C), the generation enthalpies of hydrogen bonds of PA6/PA66 (85 : 15) alloy and PA6/PA66 (60 : 40) alloy are −33.0 ± 0.6 kJ mol⁻¹ and −56.8 ± 1.5 kJ mol⁻¹, respectively, which are also less than that of neat PA66 (−99.4 ± 6.2 kJ mol⁻¹). The reason for this phenomenon is also the same as the explanation given above. For neat PA66, within 240–215 °C, the intramolecular and intermolecular hydrogen bonds can be easily formed from PA66 molecular chains. However, in PA6/PA66 alloys, the PA66 position is occupied by PA6 molecular chains, and the formation of hydrogen bonds between PA66 molecular chains are disturbed by PA6. Thus, the ability to form hydrogen bonds between PA66 molecular chains is obviously dropped when PA6 molecular chains exist. Also, when the content of PA6 is higher, the generation enthalpy of hydrogen bonds in PA66 is lower.

In short, from the results of the generation enthalpies of hydrogen bonds, it can be found that the hydrogen bond generation capability of both PA6 and PA66 in PA6/PA66 alloys is significantly reduced. Importantly, it explained well the reason for the obvious decline of PA66 and PA6 crystallization found in our DSC experiments (Fig. 1 and Table 3) and other reports.^8–10,37,38 For nylons, although not all the hydrogen-bonded molecular chains have the opportunity to enter the lattices to crystallize, the ability to form hydrogen bonds still has a close relationship with the final degree of crystallinity. As mentioned above, the generation enthalpy of hydrogen bonds directly reflects the generating capacity of hydrogen bonds. Therefore, for PA6/PA66 alloys, the absolute value of the enthalpies in Table 3 has a relationship proportional to the degree of crystallinity obtained in DSC measurements. It also can be observed that the changing trend of the absolute value of enthalpies in Table 3 is the same as that of the calculated crystallinity degree in Table 1.

In addition, the calculated enthalpies in Table 3 are totally different from the enthalpies listed in Table 1. In Table 3, the results calculated via the Van't Hoff analysis are the generation enthalpies of hydrogen bonds between C=O and N–H; however, the DSC results in Table 1 are the melting enthalpies of PA6, PA66, and PA6/PA66 alloys. In essence, these two enthalpies are completely different. Moreover, not all the hydrogen-bonded molecular chains of the nylon can enter lattices to crystallize. Therefore, the value of enthalpies in Table 3 cannot be directly compared with that of the melting enthalpies in Table 1. The units of the enthalpies in Tables 3 and 1 are also different; in Table 3, the unit is kJ mol⁻¹, and it is J g⁻¹ in Table 1.

3.4. Proj-MW2D correlation FTIR analysis

As discussed in the previous section, we infer that the hydrogen bonds can be formed between PA6 and PA66 molecular chains, but the hydrogen bond generating capability is particularly weak. Thus, during the process of PA6 crystallization (200–180 °C) in PA6/PA66 alloys, the generation of hydrogen bonds between PA6 molecular chains is disturbed by PA66 molecular chains, and most of the PA66 molecular chains possibly exist in the form of the non-hydrogen bonded amorphous state. To support our speculation, the method of Proj-MW2D correlation FTIR spectroscopy is employed here. This is because, as introduced in our previous study,²⁹ the molecular motion of different components in multi-component polymers can be conveniently extracted using Proj-MW2D correlation FTIR spectroscopy. Therefore, in PA6/PA66 alloys, the molecular chain motions of PA6 and PA66 during crystallization are expected to be successfully separated.

3.4.1. Selection of projection vector. In Proj-MW2D correlation FTIR spectroscopy, the selection of the projection matrix (or vector) is crucial. In the present study, our target is to separate the PA6 molecular motion during the PA6 crystallization (200–180 °C) in PA6/PA66 alloys. Therefore, choosing a suitable projection matrix (or vector) for PA6 is the first step. In general, this projection matrix (or vector) should be the characteristic band of the PA6 crystalline phase, and it has no overlapping with any bands of PA66. As shown in Fig. 4, the peak at 929 cm⁻¹ is the overlapping band of PA6 and PA66, and the band at 960 cm⁻¹ is the characteristic crystalline band of PA6. Therefore, the intensity variation of 960 cm⁻¹ of PA6 from 265 °C to 140 °C was chosen as the projection vector (not a matrix).

3.4.2. Proj-MW2D correlation FTIR spectra. Fig. 6 and 7 are the conventional MW2D correlation FTIR spectra of neat PA6 and neat PA66 in the regions of 3480–3150 cm⁻¹ and 1720–1400 cm⁻¹, respectively, upon cooling. As shown in Fig. 6, one weak and two strong correlation peaks are observed around 185 °C at 1543 cm⁻¹, 1640 cm⁻¹, and 3305 cm⁻¹. The bands at 1543 cm⁻¹, 1640 cm⁻¹, and 3305 cm⁻¹ are assigned to the N–H bending, C=O stretching, and N–H stretching of hydrogen-bonded amide groups of neat PA6. Thus, these three correlation peaks were actually contributed by the generation of hydrogen bonds. Moreover, the temperature of 185 °C is consistent with the maximum crystallization temperature of neat PA6 determined by DSC (184 °C, Fig. 1), and therefore, the correlation peaks observed in Fig. 6 reveal the direct relationship between hydrogen bonds and PA6 crystallization. Similarly, one weak and two strong correlation peaks are also observed around 233 °C in Fig. 7, which also reveals that a great number of hydrogen bonds were generated during the PA66 crystallization.

Fig. 6 Conventional MW2D correlation FTIR spectra of neat PA6 in the regions of 3480–3150 cm⁻¹ and 1720–1400 cm⁻¹ upon cooling (255–150 °C).

Fig. 7 Conventional MW2D correlation FTIR spectra of neat PA66 in the regions of 3480–3150 cm⁻¹ and 1720–1400 cm⁻¹ upon cooling (255–150 °C).

Fig. 8(c) and (f) show the conventional MW2D correlation FTIR spectra of the PA6/PA66 (85 : 15) alloy in the regions of 3480–3150 cm⁻¹ and 1720–1400 cm⁻¹ upon cooling; two transition points at 191 °C and 218 °C can be clearly observed. These two temperatures correspond to the crystallization temperature of PA6 and PA66 in PA6/PA66 alloys, which are also detected in DSC measurements (190 °C and 218 °C). It is noted that three correlation peaks appear at around 218 °C, at 1543 cm⁻¹, 1640 cm⁻¹, and 3305 cm⁻¹, which mainly indicate the generation of hydrogen bonds in PA66. Also, at around 191 °C, three correlation peaks at 1543 cm⁻¹, 1640 cm⁻¹, and 3305 cm⁻¹ appear, representing the hydrogen bond formation during the PA6 crystallization. However, compared with Fig. 6, two new correlation peaks are observed at around 191 °C, at 1675 cm⁻¹ and 1512 cm⁻¹, attributed to the C=O stretching of “free” C=O groups and the N–H stretching of “free” N–H groups, respectively. This is a clear indication that some of the non-hydrogen bonded molecular chains are also involved in the PA6 crystallization. Our interest was stimulated with regards to which component, PA6 or PA66, these non-hydrogen bonded polymer molecular chains belonged. As such, the Proj-MW2D correlation analysis was performed.

Fig. 8 (a) and (d) Proj-MW2D correlation FTIR spectra of PA6/PA66 (85 : 15) alloy in the regions of 3480–3150 cm⁻¹ and 1720–1400 cm⁻¹; (b) and (e) null-space Proj-MW2D correlation FTIR spectra of PA6/PA66 (85 : 15) alloy in the regions of 3480–3150 cm⁻¹ and 1720–1400 cm⁻¹; (c) and (f) conventional MW2D correlation FTIR spectra of PA6/PA66 (85 : 15) alloy in the regions of 3480–3150 cm⁻¹ and 1720–1400 cm⁻¹. The spectral intensity at 960 cm⁻¹ is the projection vector. The positive projection transformation algorithm was used for the calculations.

Proj-MW2D and null-space Proj-MW2D correlation FTIR spectra of PA6/PA66 (85 : 15) alloy in the regions of 3480–3150 cm⁻¹ and 1720–1400 cm⁻¹ are illustrated in Fig. 8(a), (b), (d) and (e). After Proj-MW2D correlation analysis, the correlation peaks in conventional MW2D correlation FTIR spectra were totally separated. The correlation peaks related to PA6 were retained in Proj-MW2D correlation FTIR spectroscopy (Fig. 8(a) and (d)), and the correlation peaks contributed by PA66 were extracted in null-space Proj-MW2D correlation FTIR spectroscopy (Fig. 8(b) and (e)). In Fig. 8(a) and (d), it clearly shows that the correlation peaks at around 191 °C, at 3305 cm⁻¹, 1640 cm⁻¹, and 1543 cm⁻¹ are attributed to PA6, and in Fig. 8(b) and (e), the correlation peaks at around 218 °C, at 3305 cm⁻¹, 1640 cm⁻¹, and 1543 cm⁻¹ belong to PA66. It was noted that the two new correlation peaks observed at around 191 °C in Fig. 8(f) were successfully extracted in null-space Proj-MW2D correlation FTIR spectroscopy (Fig. 8(e)), and assigned certainly to PA66. This clearly shows that non-hydrogen bonded molecular chains of PA66 were also involved in the crystallization stage of PA6. These molecular chains of amorphous state PA66 certainly hindered the generation of hydrogen bonds between PA6 molecular chains, resulting in a significant reduction of PA6 crystallization ability in PA6/PA66 alloys.

It can be concluded that the essential reason for the reduction in crystallization ability for both PA6 and PA66 in alloys is the rapid decline of the hydrogen bond generation ability. The reduction of crystallization ability is not caused by the regularity decline of molecular chain arrangement and also not due to the crystal defects.

4. Conclusions

In this study, the crystallization behavior of PA6/PA66 alloys was studied using in situ FTIR spectroscopy, combined with Proj-MW2D correlation analysis and DSC measurements. We also established the method to calculate the generation enthalpy of hydrogen bonds via Van't Hoff analysis. The essential reason for the crystallization reduction for both PA6 and PA66 in alloys was successfully explained from the perspective of hydrogen bonds.

Compared with neat PA6 and neat PA66, DSC measurements found that the crystallization ability of both PA6 and PA66 in the alloys obviously decreased. For PA6/PA66 alloys of 85 : 15 and 60 : 40, the relative crystallinity degrees of PA6 were 69.0% and 48.6%, and those of PA66 were 19.3% and 60.0%, respectively. From the results of the generation enthalpies of hydrogen bonds, we confirmed that the hydrogen bond generation capabilities of both PA6 and PA66 in PA6/PA66 alloys were significantly reduced. In the temperature range of PA6 crystallization (200–180 °C), the generation enthalpies of hydrogen bonds of PA6/PA66 (85 : 15) alloy and PA6/PA66 (60 : 40) alloy were −66.5 ± 1.4 kJ mol⁻¹ and −38.1 ± 0.3 kJ mol⁻¹, which are both less than that of neat PA6 (−76.9 ± 4.9 kJ mol⁻¹). Similarly, in the temperature range of PA66 crystallization (240–215 °C), the generation enthalpies of hydrogen bonds of PA6/PA66 (85 : 15) alloy and PA6/PA66 (60 : 40) alloy are −33.0 ± 0.6 kJ mol⁻¹ and −56.8 ± 1.5 kJ mol⁻¹, which are also less than that of neat PA66 (−99.4 ± 6.2 kJ mol⁻¹). The molecular chain motions of PA6 and PA66 during the alloy crystallization were successfully separated using Proj-MW2D correlation FTIR spectroscopy. Two new correlation peaks observed at around 191 °C in the conventional MW2D were successfully extracted in the null-space Proj-MW2D correlation FTIR spectrum (Fig. 8(e)).

In the present study, two issues were addressed: the first is that the generating capacity of hydrogen bonds between PA6 and PA66 is actually very weak, although this type of hydrogen bond can be generated in theory. The second is that non-hydrogen bonded molecular chains of PA66 are also involved in the PA6 crystallization, and the molecular chains of amorphous state PA66 hinder the generation of hydrogen bonds between PA6 molecular chains, resulting in a significant crystallization reduction of PA6 in PA6/PA66 alloys.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51473104, 51003066), and the State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme2014-3-06, sklpme2016-3-10).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09611e

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