Karima
Ben Hamou
a,
Ralf
Brüning
b,
Gabriel
La Plante
c,
Marie-Hélène
Thibault
a,
Jacques
Robichaud
a and
Yahia
Djaoued
*a
aLaboratoire de Recherche en Matériaux et Micro-spectroscopies Raman et FTIR, Université de Moncton – Campus de Shippagan, Shippagan, NB, Canada E8S 1P6. E-mail: yahia.djaoued@umoncton.ca
bPhysics Department, Mount Allison University, Sackville, NB, Canada E4L1E6
cFaculté d'ingénierie, Université de Moncton – Campus de Moncton, Moncton, NB, Canada E1A 3E9
First published on 7th April 2025
The present study focuses on the synthesis and structural analysis of poly-ε-caproamide (PA6), produced through anionic polymerization of ε-caprolactam in bulk, utilizing mono and bifunctional activators. The research investigates the physical properties of PA6 synthesized under various polymerization conditions, aiming to understand how these conditions influence the polymer's behavior. The polymerization kinetics were monitored via dynamic rheology, offering insights into the progression of ε-caprolactam's conversion into PA6. Microstructural changes in the PA6 samples, including variations in the degree of crystallinity and the formation of α and γ crystalline structures, were systematically studied. These transformations were dependent on both the type and concentration of the activator used, as well as the specific polymerization parameters applied. The interplay between these factors significantly impacted the resulting chemical and physical structure of the PA6 samples. In the latter part of the study, hybrid composites were fabricated by reinforcing poly-ε-caproamide with two distinct types of fiber fabrics by reactive processing, achieving a 25% weight fraction of reinforcement. Scanning electron microscopy (SEM) revealed excellent interfacial adhesion between the fibers and the polymer matrix, confirming the effectiveness of the fabrication process and the potential of these composites for advanced material applications.
Two types of fibers were used to produce the hybrid composite samples: Hexcel HexForce 7781 E-glass 8-Harness Satin fabric (299 g m−2), manufactured by Hexcel Corporation (Stamford, CT) and generously supplied by Innov (Shippagan, New Brunswick, Canada), and hemp fibers (Cannabis sativa L.) provided by GEMS Consultants Inc. (New Brunswick, Canada). The hemp fibers had an average length of 7 ± 0.5 cm and a diameter of 25 ± 5 μm. To enhance their properties, the hemp fibers underwent a chemical treatment using a 2 wt% aqueous NaOH solution at room temperature.25 They were immersed in the NaOH solution for 24 hours and then thoroughly washed with distilled water to remove any excess alkali. Acetic acid was applied to neutralize the residual sodium hydroxide.26 Both sodium hydroxide and acetic acid, used during the treatment, were procured from Millipore Sigma with a purity of 99%.
DL/ACL | DL/DCCI | ||||||
---|---|---|---|---|---|---|---|
Sample designation | Catalysta (%) | Activatora (%) | Temperature (°C) | Sample designation | Catalysta (%) | Activatora (%) | Temperature (°C) |
a (mol% per 1 mol caprolactam). | |||||||
T140-DL/ACL-1.5 | 1.5 | 1.5 | 140 | T140-DL/DCCI-0.75 | 1.5 | 0.75 | 140 |
T160-DL/ACL-1.5 | 1.5 | 1.5 | 160 | T160-DL/DCCI-0.75 | 1.5 | 0.75 | 160 |
T170-DL/ACL-1.5 | 1.5 | 1.5 | 170 | T170-DL/DCCI-0.75 | 1.5 | 0.75 | 170 |
T140-DL/ACL-3 | 3 | 3 | 140 | T140-DL/DCCI-1.5 | 3 | 1.5 | 140 |
T160-DL/ACL-3 | 3 | 3 | 160 | T160-DL/DCCI-1.5 | 3 | 1.5 | 160 |
T170-DL/ACL-3 | 3 | 3 | 170 | T170-DL/DCCI-1.5 | 3 | 1.5 | 170 |
The degree of conversion (DC) of the synthesized samples was determined for various resin formulations at several temperatures. The polymerized samples were ground, weighed to determine the initial mass mi, and refluxed overnight in methanol. After drying, the final mass mf was obtained. While the caprolactam monomer dissolves easily in methanol, its polyamide does not, and the degree of conversion was determined according to eqn (1):27
![]() | (1) |
Differential Scanning Calorimeter (DSC) measurements were conducted using a TA Q200 V24.11 Build 124 thermal analyzer (New Castle, DE, USA) to determine the melting, glass transition, and crystallization temperatures Tm, Tg, and Tc, respectively. The temperature was scanned from −20 to 300 °C at a heating rate of 10 °C min−1 with the sample in a nitrogen atmosphere. The degree of crystallinity (χDSCc) was calculated according to eqn (2):
![]() | (2) |
Thermogravimetric analysis (TGA) was performed on a TA Q500 V20.13 Build 39 thermal analysis instrument (New Castle, DE, USA) under a nitrogen atmosphere at a 10 °C min−1 heating rate from room temperature up to 700 °C.
Following the procedure outlined in reference,29 an approximation of the viscosity average molar mass (v) for each sample was obtained by employing diluted solution viscometry with a suspended-level Ubbelohde viscometer maintained at 25 °C in aqueous H2SO4 (98%) at a concentration of 0.2 g dl−1. The molar mass of PA6 was determined using the Mark–Houwink equation.10,30,31K = 4 × 10−3 dl g−1 and α = 0.7 are the relevant parameters in this context.32 The average flow times were calculated from five separate runs.
To know the evolution of the viscosity of the reactive systems according to the reaction time in isothermal mode. Viscosities were measured dynamically at various temperatures on a stress-controlled rheometer DHR (Discovery Hybrid Rheometer, TA Instruments, USA). The premix was quickly introduced into the preheated parallel plate geometry (θ = 25 mm) at the polymerization temperature. The variations in complex viscosity during the polymerization step were then monitored by time sweep oscillatory experiments under a strain amplitude of 10% and an angular frequency of 100 rad s−1. The rheometer chamber was purged with nitrogen for one hour, preventing thermal oxidation and polymerization inhibition and ensuring that the moisture level in the chamber was kept to a minimum. Before introducing a sample, the instrument was first equilibrated at the desired temperature, and the gap was set.
X-ray diffraction (XRD) was carried out with a custom-built θ–θ diffractometer equipped with a pyrolytic graphite monochromator and analyzer crystals. Cu-Kα radiation with wavelength λ = 0.154 nm was used for the measurements. The crystallinity (χXRDc) was calculated according to:
![]() | (3) |
![]() | ||
Fig. 1 Illustration of the initiation and propagation steps in the anionic ring-opening polymerization (AROP) of lactams using N-acyllactam as an activator. |
Sample designation | Polymerization time (min) | DC (%) |
![]() |
---|---|---|---|
T140-DL/ACL-1.5 | 10 | 97 | 24![]() |
T140-DL/ACL-3 | 6 | 96 | 27![]() |
T160-DL/ACL-1.5 | 7 | 99 | 44![]() |
T160-DL/ACL-3 | 1.5 | 95 | 31![]() |
T170-DL/ACL-1.5 | 3 | 98 | 45![]() |
T170-DL/ACL-3 | 0.75 | 95 | 31![]() |
T140-DL/DCCI-0.75 | 40 | 99 | 41![]() |
T140-DL/DCCI-1.5 | 16 | 97 | 27![]() |
T160-DL/DCCI-0.75 | 13 | 99 | 70![]() |
T160-DL/DCCI-1.5 | 4 | 98 | 35![]() |
T170-DL/DCCI-0.75 | 7 | 99 | 76![]() |
T170-DL/DCCI-1.5 | 3 | 98 | 37![]() |
As mentioned above, the reaction consists of three basic steps: (1) dissociation of the catalyst or anion formation, (2) complex formation between the catalyst and the activator, and (3) polymerization through the anions, during which an anion is regenerated after every monomer addition.37 The combination of activator and catalyst primarily determines the reaction rate but is also controlled by the initial polymerization temperature, which will be highlighted in detail in the following section. Fig. S2† shows the change in temperature for two catalyst-activator combinations polymerized at initial temperatures ranging from 140–170 °C. Compared to the bifunctional activator, the polymerization time for complete conversion with the monofunctional activator was very impressive. This is because the DL/DCCI combination does not immediately form a complex, which explains the slow linear conversion increase at the start of the reaction. The carbamoylcaprolactam group is replaced with an acetylcaprolactam group following a single monomer addition. As a result of this group's ability to form a complex with the catalyst, the reaction rate increases over time (after 40 min for T140-DL/DCCI-0.75 and 16 min for T140-DL/DCCI-1.5). The monomer is consumed faster as the catalyst concentration increases because more anions are released, and complexes can form. More activators mean more chain growth in start locations. Thus, Fig. S2† shows that the time needed to consume all monomers up to the equilibrium conversion level decreases, whereas Table 2 shows that the final degree of conversion decreases. Because each catalyst molecule contributes a cation (see Fig. 1), a caprolactam anion must neutralize its positive charge. As the concentration of the catalyst increases, more anions are liberated, and more complexes can form; as a result, the monomer is consumed more quickly. Increasing the amount of activator increases the number of chain growth initiation points. Therefore, the time necessary to consume all monomers up to the equilibrium conversion level lowers, as seen in Fig. S2,† while the final degree of conversion drops, as indicated in Table 2. Each catalyst molecule introduces a cation (see Fig. 1), whose positive charge must be neutralized by a caprolactam anion throughout the reaction. Not all caprolactam polymerizes to compensate for these cations, limiting the highest degree of conversion possible. Increasing the activator concentration increases the proportion of oligomers with low molecular weight in the final polymer.38 Being soluble in methanol, these oligomers decrease the conversion rate as assessed by the previously stated approach. Table 2 demonstrates that the measured conversion decreases as the activator and catalyst concentrations increase, and the activator's functionality notably affects the final conversion of the reacted polymers. For the best results, the bifunctional activator (DCCI), should be used to maintain a high and nearly constant degree of conversion. In contrast, the monofunctional activator exhibits a noticeable drop in conversion. But in absolute terms, these conditions result in higher monomer conversion of polycaprolactam compared to conventional methods of ε-caprolactam polymerization.39
According to Table 2, the viscosity average molar mass (v) decreases with increasing activator (i.e., catalyst) concentration in the presence of both monofunctional and bifunctional activator in the polymerization mixture. For a given catalyst concentration, the
v of the sample obtained with a monofunctional activator is significantly less than in the case of a bifunctional activator. This occurs because branching reactions can produce species with extremely high molecular weights.37 Although these types of reactions occur for monofunctional and bifunctional activators, branching is enhanced by bifunctional activators.40 Van Rijswijk et al.10 suggested that using a slower catalyst-activator combination (i.e., a longer time during which branching can take place), can produce a significant degree of branching, even at lower temperatures. The dependence of the degree of branching on the activator concentration and the reaction rate has also been reported by Mateva et al.40
We examined the temperature's impact on the induction time and viscosity behaviour of the DL/ACL system at two concentrations. The data shows that increasing the temperature significantly shortened the induction time. In contrast, when looking at the effect of catalyst/activator concentration across all three temperatures, it was found that the DL/ACL-1.5 composition required about 10 minutes at 140 °C and 3 minutes at 170 °C to reach the maximum viscosity value, while the DL/ACL-3 composition required about 3 minutes at 140 °C and 0.75 minutes at 170 °C. As mentioned above, a higher quantity of activators resulted in a more significant number of chain development initiation locations. Moreover, more anions were liberated with a higher catalyst concentration, and more complexes could form. The acquired results matched those reported in the literature. Fig. 3 illustrates the complex viscosity-time curve for the DL/DCCI system. The time required to achieve the maximum permissible torque on the instrument was varied from about 40 minutes at 140 °C to 7 minutes at 170 °C for the composition DL/DCCI-0.75 and from 20 minutes at 140 °C to 3 minutes at 170 °C for the composition DL/DCCI-1.5. This time is very close to the time required to achieve near-infinite complex viscosity, also known as total solidification.
When the DCCI activator was used, the polymerization seemed to begin slowly. This is evidenced by the viscosity remaining stable for short durations, despite the absence of an initial complex formation.10 However, after a single monomer addition, the carbamoylcaprolactam group was replaced by an acetylcaprolactam group. Since the latter was able to form a complex with the catalyst, the reaction rate increased after a while. In this regard, it is worth noting that the induction time decreased with increasing polymerization temperature and DL/DCCI concentration. The reaction was also affected by the amount of the reactive species utilized and the temperature,10,22 in addition to the catalyst and activator types. Increasing the polymerization temperature and concentration were demonstrated to boost the polymerization rate.
The results reported so far lead to the conclusion that the combination of DL/ACL led to complex formation, resulting in rapid polymerization and an exponentially increasing viscosity with time. A reduced rate of viscosity increase was seen while using a DL catalyst in combination with DCCI since no complex was formed initially, explaining the slower increase in viscosity. The critical time increase was about 10 minutes for the DL/ACL system at 140 °C, whereas it was about 40 minutes for the DL/DCCI system at the same temperature. These critical times are calculated using the tangents method when the viscosity grows drastically. The present results also highlight that a higher degree of conversion and molecular weight were obtained with DCCI as the activator. Lastly, this work demonstrates that the reaction time was faster when DL was associated with ACL as the activator.
Sample designation | DSC data | TGA data | |||||||
---|---|---|---|---|---|---|---|---|---|
Melting process | Crystallization process | ||||||||
T m (°C) | ΔHma (J g−1) | χ cDSC (%) | T c (°C) | ΔHca (J g−1) | T donset (°C) | T dmax (°C) | T d10% (°C) | T d50% (°C) | |
a The values in the top and bottom rows correspond to the first and second scans, respectively. | |||||||||
T140-DL/ACL-1.5 | 221 | 107 | 56 | 185 | 56 | 351 | 422 | 362 | 409 |
210 | 69 | 36 | 183 | 65 | |||||
T140-DL/ACL-3 | 217 | 98 | 52 | 176 | 69 | 344 | 417 | 357 | 407 |
213 | 70 | 37 | 176 | 68 | |||||
T160-DL/ACL-1.5 | 218 | 98 | 52 | 175 | 72 | 326 | 411 | 336 | 397 |
215 | 60 | 32 | 175 | 70 | |||||
T160-DL/ACL-3 | 214 | 88 | 46 | 172 | 72 | 320 | 407 | 333 | 396 |
211 | 62 | 33 | 172 | 73 | |||||
T170-DL/ACL-1.5 | 216 | 100 | 53 | 173 | 70 | 353 | 410 | 334 | 400 |
214 | 59 | 31 | 172 | 67 | |||||
T170-DL/ACL-3 | 212 | 85 | 45 | 170 | 71 | 331 | 405 | 334 | 394 |
210 | 59 | 31 | 169 | 67 | |||||
T140-DL/DCCI-0.75 | 221 | 111 | 58 | 170 | 69 | 309 | 408 | 324 | 383 |
212 | 59 | 31 | 168 | 70 | |||||
T140-DL/DCCI-1.5 | 217 | 99 | 52 | 173 | 70 | 269 | 325 | 289 | 322 |
211 | 60 | 31 | 168 | 68 | |||||
T160-DL/DCCI-0.75 | 217 | 93 | 50 | 176 | 67 | 328 | 432 | 359 | 420 |
215 | 56 | 29 | 176 | 67 | |||||
T160-DL/DCCI-1.5 | 216 | 113 | 59 | 168 | 71 | 304 | 426 | 321 | 379 |
210 | 64 | 34 | 166 | 70 | |||||
T170-DL/DCCI-0.75 | 213 | 95 | 50 | 173 | 64 | 341 | 457 | 385 | 443 |
214 | 55 | 28 | 171 | 63 | |||||
T170-DL/DCCI-1.5 | 216 | 103 | 54 | 173 | 68 | 332 | 437 | 348 | 423 |
211 | 61 | 32 | 170 | 69 |
It is also important to note that the second heat traces of all produced samples feature two peaks (as shown in Fig. S3 and S4†). After a second heating, a lower and new peak is seen at the typical melting point. The polymerization of caprolactam at initial temperatures between 140 and 170 °C (i.e., below the melting point of poly-ε-caproamide) may result in a material with a combination of α and g crystalline structures. Puffr et al.36 stated that as many as five melting endotherms may occur in samples of PA6 with the mixed α and g structures: the peaks of original and recrystallized γ structures; the peaks of original and recrystallized α structures (higher by several degrees); and also a small peak approximately 10 °C above crystallization temperature (Tc), assigned to the most defective crystals formed by secondary crystallization. In Fig. S3 and S4,† the two peaks seen on the DSC thermograms, possibly arising from the original α and γ structures formed during polymerization because the sample was not exposed to thermal history for recrystallization.
![]() | (4) |
Sample designation | Crystallinity Index (%) | α/γ | d-Spacing (Å) | ICP (Å) | ||||
---|---|---|---|---|---|---|---|---|
α-Phasea | γ-Phasea |
χ
cXRD![]() |
α 1(200) | α 2(002/202) | ||||
a
![]() ![]() |
||||||||
T140-DL/ACL-1.5 | 46.00 | 11.5 | 57.5 | 4.0 | 4.477 | 3.748 | 0.728 | 0.837 |
T140-DL/ACL-3 | 39.4 | 12.6 | 52.0 | 3.1 | 4.505 | 3.799 | 0.706 | 0.843 |
T160-DL/ACL-1.5 | 40.2 | 8.9 | 49.1 | 4.5 | 4.492 | 3.790 | 0.702 | 0.844 |
T160-DL/ACL-3 | 35.2 | 9.3 | 44.5 | 3.8 | 4.507 | 3.800 | 0.707 | 0.843 |
T170-DL/ACL-1.5 | 29.4 | 17.8 | 47.2 | 1.6 | 4.496 | 3.794 | 0.702 | 0.844 |
T170-DL/ACL-3 | 29.6 | 11.9 | 41.5 | 2.5 | 4.507 | 3.802 | 0.705 | 0.843 |
T140-DL/DCCI-0.75 | 41.5 | 16.7 | 58.2 | 2.4 | 4.464 | 3.748 | 0.715 | 0.840 |
T140-DL/DCCI-1.5 | 25.0 | 11.0 | 36.0 | 2.3 | 4.453 | 3.765 | 0.688 | 0.845 |
T160-DL/DCCI-0.75 | 32.8 | 16.3 | 49.1 | 2.0 | 4.429 | 3.756 | 0.673 | 0.848 |
T160-DL/DCCI-1.5 | 24.7 | 6.3 | 31.0 | 3.9 | 4.446 | 3.794 | 0.652 | 0.853 |
T170-DL/DCCI-0.75 | 34.4 | 8.6 | 43.0 | 4.0 | 4.453 | 3.800 | 0.652 | 0.853 |
T170-DL/DCCI-1.5 | 17.7 | 13.3 | 31.0 | 1.3 | 4.477 | 3.748 | 0.728 | 0.837 |
According to previous research,56,58 the two peaks with 2θ at about 20° and 24° were assigned to the (200) and (002/202) crystallographic planes of the monoclinic unit cell of the α-PA6 polymorph. Additionally, two Gaussian peaks corresponding to (001) and (200) crystalline planes with 2θ between 22 and 23° were discovered, indicative of the PA6 γ-crystalline form with a pseudo-hexagonal unit cell.
It is well-known that FTIR spectroscopy is one of the most well-established methods for the characterization of nylons and that it provides information about the crystalline phase of polyamide samples.59,60Fig. 5 illustrates the FTIR spectra of poly-ε-caproamide (PA6) samples at several polymerization conditions, along with characteristic infrared bands and their roles. The bands at 3196 cm−1 and 1654 cm−1 in the spectra of the monomer ε-caprolactam correspond to the stretching vibrations of the N–H and CO bonds, respectively,61 and are coupled with the self-associated hydrogen bonds produced between C
O and N–H that define the amide II mode. The bands in the 1465–1150 cm−1 range correspond to the methyl alkyl group (CH2) from cyclic lactam related to the bands in the 3500 and 2750 cm−1 region. The bands at 1486 and 1438 cm−1 correspond to the scissor vibration in the (CH2) group, whereas the band at 1417 cm−1 refers to bending vibrations in the same group. Moreover, the bands at 1365, 1312, 1290, 1258, and 1198 cm−1 correspond to the amide group covalently bound to the alkyl group (NH–CH2). The lactam C
O bond was detected at 1654 cm−1 and confirmed by the 822 and 693 cm−1 bands. However, the spectra of poly-ε-caproamides (PA6) display two bands at 3290 and 1535 cm−1, ascribed to hydrogen bond stretching and bending vibration in amide II, respectively. The 1635 cm−1 band is associated with mode I amide bond bending vibrations. Torsion in amide mode I and mode II hydrogen bonds are attributed to the 683 cm−1 and 574 cm−1 bands, respectively. Also, the presence of two bands at 1199 and 1169 cm−1 is according to amide mode III. The carbonyl (C
O) band is located at 1635 cm−1 and overlaps with the mode I band of the amide. As previously stated, the (O
CN) bond related to the caprolactam ring was broken (by ring opening) due to the catalyst, and the sodium–caprolactam was formed; this is confirmed since a band is located at 3074 cm−1. In addition, the ring opening is confirmed by the band's presence at 1263 cm−1, which corresponds to the C–N aliphatic bond. Finally, in the poly-ε-caproamides (PA6) CH2 rocking band area, there are three bands. Sandeman and Keller assigned the 928 cm−1 band of the α-form of PA6 to the rocking mode CH2.62 Asai et al.63 and Miyake64 ascribe the remaining two bands at 730 and 683 cm−1 to the NH out-of-plane deformation mode (the amide V band). All phases are produced during polymerization, regardless of the raw materials, monomers, and synthesis conditions. In the process of re-crystallization, the γ-phase is eliminated for material exposure. The primary peak in the DSC analysis at around 221–212 °C and the main peaks in the diffraction patterns at 2 = 20° and 24°, as stated previously, indicate that the phase (monoclinic crystal) is the predominant and stable crystalline phase in the synthesized PA6.
![]() | ||
Fig. 5 FTIR spectra of the studied poly-ε-caproamide (PA6) synthesized utilizing a DL/ACL system at varying polymerization temperatures. (a) 4000–400 cm−1, (b) 1500–500 cm−1. |
The findings of XRD measurements, presented in Table 4, show that the degree of crystallinity of poly-ε-caproamide (PA6) samples decreased as the amount of catalyst/activator increased, in agreement with the degree of crystallinity determined by DSC analysis listed in Table 3. Increasing the catalyst/activator concentration results in an elevation in polymerization temperature because of a rise in polymerization rate (i.e., exothermic polymerization). The temperature at which polymerization occurs affects the process in two different ways. When heated to high temperatures, it has the opposite effect of slowing the crystallization rate and speeding up the polymerization rate.65 However, the crystallization rate is very high at low temperatures, and reactive groups can become caught inside developing crystals before they can polymerize. This can only happen when the temperature is very low. When polymerization occurs at temperatures higher than 170 °C, the crystallization rate is reduced, although the polymerization rate remains high.30
Table 4 illustrates the effect of initial polymerization temperature on the degree of crystallinity of the synthesized polymer. As the initial polymerization temperature rose from 140 °C to 170 °C, the degree of crystallinity diminished. Because of the competitive nature of polymerization and crystallization in this process, the higher thermal motion of the polymer chains, and also the enhanced rate of branching at higher temperatures,37,46 the reduction in crystallinity of synthesized poly-ε-caproamide (PA6) that can be achieved by increasing the initial temperature of polymerization is caused by a combination of these factors. The fraction of α-PA6 predominates in all poly-ε-caproamide (PA6) samples, with α/γ > 1 for both DL/ACL-1.5 and DL/ACL-3 formulations. The d-spacings of α(200), α(002)/(202) of α-PA6 using DL/ACL as the catalyst/activator system at two concentrations and varying polymerization temperatures are shown in Table 4. A crystalline perfection index (ICP) is derived from the relationship between crystalline perfection and crystallinity to quantify variations in crystalline perfection. The index of chain packing (ICP) is defined as the difference of the d-spacings for both the (200), (002)/(202) peaks,66 ICP = Δd = d200 − d002/202.
Fig. 6 demonstrates that the crystal lattice parameters of poly-ε-caproamide (PA6) are impacted by both the catalyst/activator concentration and the polymerization temperature, as indicated by the rise in d-spacings with increasing polymerization temperature and catalyst/activator concentration. With increasing catalyst/activator incorporation into the PA6 chain, the resulting changes in crystal lattice properties become statistically significant. This leads to significant morphological alterations and significant disruption of the crystal structure. The Δd of the synthesized poly-ε-caproamide (PA6) is around 0.702–0.728 Å for DL/ACL: 1.5/1.5, and around 0.705–0.707 Å for DL/ACL: 3/3 depending on the polymerization temperature.
With the DL/ACL-1.5 formulation, T140-DL/ACL-1.5 exhibited ICP = 0.728 Å, which was greater than poly-ε-caproamides synthesized at 160 °C (T160-DL/ACL-1.5) and 170 °C (T170-DL/ACL-1.5). Murthy et al.66 reported that a structure with a higher ICP corresponds to an energetically lower and more stable PA6 lattice. However, when comparing samples of the DL/ACL-3 formulation, we found that the ICP was roughly constant from one to the next, with no clear correlation to the synthesis conditions. Poly-ε-caproamides (PA6) chain packing index does not appear to be affected by polymerization temperature. For the DL/ACL-1.5 formulation, both T160-DL/ACL-1,5 and T170-DL/ACL-1.5 have da200/da002/202 of about 0.844, which represents the ratio c/a of unit cell.67 This is a little bit greater than T140-DL/ACL-1.5 (0.837), and it suggests that the distance between hydrogen-bonded sheets (i.e. dα002/202) in the unit cell is slightly larger than that of T140-DL/ACL-1.5. A comparison of samples of the DL/AC-3 formulation reveals that the distance between hydrogen-bonded sheets is constant over all three polymerization temperatures, with a dα200/dα002/202 ratio of around 0.843.
This is consistent with the findings of the IR investigation presented below. As seen in Fig. 8. The IR spectrum of the obtained homopolymer presents strong absorption bands at 3290 cm−1 (N–H stretch vibration), 1635 cm−1 (amide I, CO), and 1536 cm−1 (amide II, N–H deformation) which are characteristic of the amide groups existing in the trans planar conformation. As indicated earlier, the amide II band is extremely sensitive to the crystalline structure corresponding to the α-phase.59,60 Furthermore, the out-of-plane bends of the NH (amide V) and C
O (amide VI) groups, which appear at 684 and 575 cm−1, respectively, are polymorph sensitive and indicate that the form of the crystalline phase of homopolymer is α-type.59,60,68 As stated before, the results show that monoclinic crystal is the most common and stable crystalline phase in the synthesized PA6.
![]() | ||
Fig. 8 FTIR spectra of poly-ε-caproamide (PA6) synthesized utilizing a DL-DCCI system at varying polymerization temperatures. (a) 4000–400 cm−1, (b) 1500–500 cm−1. |
Based on the XRD data reported in Table 4, it is clear that the degree of crystallinity of poly-ε-caproamide (PA6) samples is dependent on both the catalyst/activator concentration and polymerization temperature. Increasing the amounts of bifunctional activator and catalyst reduced the crystallinity. DSC analysis shows that the degree of crystallinity tends to go the same way. Since the activator N,N-dicyclohexylcarbodiimide (DCCI) has two functions, two or less acyllactam functions can start the polymerization process. The macromolecular chains cannot reorganize into a perfect crystalline form due to changes in chain length and, most likely, increased chain mobility due to diminished protonic contact between NH and CO groups.
The effect of initial polymerization temperature on the degree of crystallinity of the produced polymer is depicted in Table 4. As shown before for the system DL/ACL, the degree of crystallinity decreased when the initial polymerization temperature increased from 140 °C to 170 °C. Due to the competitive nature of polymerization and crystallization in this process, the higher thermal motion of the polymer chains, and the enhanced rate of branching at higher temperatures,37,38 increasing the initial temperature of polymerization can reduce the crystallinity of synthesized poly-ε-caproamide (PA6).
The d-spacings of α(200), α(002)/(202) and the index of chain packing (ICP) of α-PA6 using DL/DCCI as the catalyst/activator system at two concentrations at varying polymerization temperatures are shown in Table 4.
Fig. 9 demonstrates that the crystal lattice parameters of poly-ε-caproamide (PA6) is impacted by both the catalyst/activator concentration and the polymerization temperature, as indicated by the rise in d-spacings with increasing polymerization temperature and catalyst/activator concentration. With increasing catalyst/activator incorporation into the PA6 chain, the resulting changes in crystal lattice properties become statistically significant. This leads to significant morphological alterations and significant disruption of the crystal structure.
With the DL/DCCI-0.75 formulation, T140-DL/DCCI-0.75 exhibited ICP = 0.715 Å, which was greater than poly-ε-caproamides synthesized at 160 °C (T160-DL/DCCI-0.75) and 170 °C (T170-DL/DCCI-0.75). Comparing samples of the DL/DCCI-1.5 formulation, we noticed that T170-DL/DCCI-1.5 had a higher ICP than T140-DL/DCCI-1.5 and T170-DL/DCCI-0.75. Murthy et al.66 reported that a structure with a higher ICP corresponds to an energetically lower and more stable PA6 lattice.
For the DL/DCCI-0.75 formulation, the dα200/dα002/202 ratio, which represents the ratio c/a of unit cell,67 of T170-DL/DCCI-0.75, it has a ratio of about 0.853. This is a little bit greater than that of T140-DL/DCCI-0.75 and T160-DL/DCCI-0.75 (0.840), and it suggests that the distance between hydrogen-bonded sheets (i.e. dα002/202) in the unit cell is slightly greater than that of T170-DL/DCCI-0.75. Comparison of samples of the DL/DCCI-1.5 formulation, shows that T160-DL/DCCI-1.5 has a larger distance between hydrogen-bonded sheets with a dα200/dα002/202 ratio around 0.853 compared to T140-DL/DCCI-1.5 and T170-DL/DCCI-1.5.
In brief, the effect of the activator on the structure of poly-ε-caproamides (PA6) is governed by two factors: (a) changes in the rate of the polymerization and crystallization process and, (b) changes in the chemical structure due to the incorporation of the activator into the polymer chain.69 In fact, the reported data suggest that the type of catalyst-activator combination exerts a significant influence on the polymerization and subsequent crystallization of PA6. By varying the amount of appropriately chosen activators, it is possible to control the crystallinity level throughout a wide range. According to the findings of the present experiments, both the type and quantity of activating additives have a substantial impact on the bulk structural development of poly-ε-caproamides (PA6).
The FTIR, DSC, TGA, DTG, XRD, and rheological data supporting this study's findings are provided within the manuscript and ESI.†
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5lp00015g |
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