New Insights into the Brill Transition in Polyamide 11 and Polyamide 6

Article pubs.acs.org/Macromolecules

New Insights into the Brill Transition in Polyamide 11 and Polyamide 6 Julie Pepin, Valérie Miri,* and Jean-Marc Lefebvre Unité Matériaux et Transformations, UMR 8207 CNRS/Université Lille 1, F-59655 Villeneuve d’Ascq, France ABSTRACT: The thermal stability of several crystal polymorphs in polyamide 11 (PA11) and polyamide 6 (PA6) has been investigated by means of in situ X-ray experiments. In the case of PA11, δ′ and α′ phases display a Brill transition far below the melting point. Both phases transform into a (pseudo)-hexagonal HT δ phase above 100 °C. The latter turns back into the most stable α′ phase upon cooling. PA6 exhibits similar thermally induced crystal transitions. In situ X-ray investigations refute the occurrence of a β → α transition upon heating, whereas the existence of a pseudohexagonal HT phase is suggested as in PA11. For both polymers, the present study underlines the major role of crystal perfection of the most stable α phase on the existence of a Brill transition. The combination of in situ structural information with thermal analysis allows to propose a thermodynamic scheme to describe the Brill transition in both polyamides.

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INTRODUCTION Aliphatic polyamides are commonly used in various fields such as automotive, textile, or food packaging, etc. In view of the corresponding end-use applications, it is of prime importance to control the mechanical properties, closely related to the crystalline structure.1−3 In the case of polyamides, the situation is particularly complex, in relation to their extensive degree of polymorphism, as a result of the optimization between chain conformation and hydrogen bond energies. Hence, many crystalline polymorphs may be obtained depending on crystallization conditions. Among aliphatic polyamides, the crystal structures of bio-based polyamide 11 (PA11) have been extensively studied.4−8 At room temperature, PA11 displays at least four crystalline forms (the triclinic α and α′, the monoclinic β, and the pseudohexagonal γ phases) and one smectic pseudohexagonal phase (δ′ form). The present work focuses on PA11-α, PA11-α′, and PA11-δ′ forms. PA11-α is considered as the most thermodynamically stable form. Many triclinic PA11-α unit cells have been proposed in the literature.4,9−14 Their common features are the organization of the hydrogen bonds into well-defined sheets held together by van der Waals interactions, with the amide groups of adjacent chains in consecutive H-bonded sheets located at about the same height along the chain axis. Regarding the chain conformation, some descriptions refer to fully extended molecules while others suggest a twist of the zigzag methylene chain around a single bond, thus departing from the planar conformation. Literature evokes both occurrences of hydrogen bonding between parallel or antiparallel chains, in agreement with the fact that odd polyamides have theoretically equal ease to form hydrogen bonding in both configurations. However, a majority of studies have suggested that hydrogen bonds are rather formed between antiparallel chains in PA11-α phase.14 Several similar unit cells have been proposed in the literature as © 2016 American Chemical Society

summarized in Table 1. Those cells, as depicted in Figure 1, essentially differ by the α angle. The (010) planes of H-bonded Table 1. Crystalline Structure of Triclinic PA11-α Phase Proposed by Various Authors unit cell parameters a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

ref

9.6 9.81 9.8 9.52

4.2 4.65 5.25 5.35

15.0 14.45 14.90 14.90

72 68.5 50.5 48.5

90 90 90 90

63.5 66 72 74.7

10 11, 12 13 14

sheets composed of antiparallel chains are progressively shifted in the c direction as illustrated in Figure 1b. PA11-α′ crystalline structure is extremely close to PA11-α form, and it may be viewed as a defective PA11-α phase. The pseudohexagonal smectic PA11-δ′ phase is primarily characterized by a random distribution of H-bonds along the chain axis. PA11-α, PA11-α′, and PA11-δ′ phases are obtained according to different preparation modes: α phase results from solution casting using m-cresol and α′ form is achieved by a slow cooling from the melt or annealing of quenched samples, whereas the δ′ smectic phase is obtained by quenching from the melt. Polyamide 6 (PA6) displays similar polymorphism as in the case of PA11.1 Two monoclinic structures, namely α and γ phases, are reported in the literature as well as a mesophase with pseudohexagonal symmetry, the β phase. In the following, special attention will be paid to the α monoclinic structure Received: August 24, 2015 Revised: November 24, 2015 Published: January 4, 2016 564

DOI: 10.1021/acs.macromol.5b01701 Macromolecules 2016, 49, 564−573

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Macromolecules

and FTIR experiments have clearly shown the existence of a reversible crystal transition upon heating, known as the Brill transition. The room temperature triclinic PA11-α′ structure gradually transforms into the high temperature (pseudo)hexagonal PA11-δ phase well below the melting point; this transition is reversible. The Brill transition is also reported for numerous polyamides such as PA6-6,19−21 6-8,22 6-16,23 and other even−even as well as odd−even polyamides.24−26 This transformation, first reported by Brill in 1942 for PA6-6, is characterized by a packing change in the crystals.19 In the specific case of PA6, the existence of a Brill transition is still controversial, owing to its occurrence in the vicinity of the melting point.27 Nevertheless, based on careful X-ray diffractogram deconvolution, Murthy et al. have revealed a phase transition from the α form to another monoclinic structure and define it as the Brill transition.28 The existence of a Brill transition in PA6 was also suggested by Vasanthan from infrared spectroscopy study,29 but the HT structure is not well established. Indeed, depending on the crystallization conditions, some authors have shown that PA6 may transform into a HT monoclinic structure through an intermediate HT (pseudo)hexagonal structure.30 Up to now, the thermodynamic nature of the Brill transition is still a matter of debate. Relying on thermal and X-ray analysis, some authors have concluded to a first-order transition31 while others have considered the Brill transition as a second-order type due to its continuous and gradual character.32 Numerous factors seem to affect the temperature at which the transition is fully achieved, such as crystallization conditions, thermal history, and heating rate.33 The origin of this transition might be related to librational motions of the methylene segments constrained by the amide linkages acting as pinning points. Concerning the H-bond distribution on either side of the Brill transition, some authors have initially postulated that the twodimensional hydrogen-bonded sheets switch to a temporal three-dimensional network of H bonds at temperatures above the Brill transition.19 However, on the basis of NMR experiments revealing no evidence of mobility of the amide hydrogen bonds up to the melting point, many groups have argued that the H-bond sheet-like structure is maintained above the Brill transition.34−38 These experimental results are supported by molecular dynamics simulations on PA6-637 and PA10-10.39 The latter suggest that the molecular conformations at high temperature are remarkably disordered through the torsional motions around the CH2−NH, CH2− CO, and CH2−CH2 bonds of the skeletal chains, while the intermolecular hydrogen bonds are still alive, though weakening, thus preventing any free rotation around the chain axis. In this context, the aim of the present study is to rationalize the description of thermally induced crystal phase transitions in nylon polymorphs according to a thermodynamical approach. For this purpose, in situ wide-angle X-ray scattering (WAXS) experiments were performed on α, α′, and δ′ phases of PA11 and α and β forms of PA6 submitted to heating and subsequent cooling step in order to study the thermal stability of each crystalline structure. From this comparative experimental study, a general scheme for the thermally induced phase transitions in polyamides is proposed based on a Gibbs diagram representation.

Figure 1. (a) Triclinic unit cell of PA11-α with H-bonds between antiparallel chains and (b) packing of the hydrogen bonds sheets with the progressive shift in the c direction.

(PA6-α) characterized by an all-trans fully extended conformation of the chains with hydrogen bonds established between antiparallel chains in a sheet-like structure. In such a structure, the amide groups are staggered as illustrated in Figure 2. According to Holmes and Bunn,15 the unit cell parameters of PA6-α are a = 9.56 Å, b = 17.24 Å, c = 8.01 Å, and β = 67.5°.

Figure 2. (a) Unit cell of α-PA6 proposed by Holmes and Bunn.15 (b) Packing of the hydrogen-bonded sheets with alternate up and down shift in the b direction.

The β form is a mesophase much alike PA11-δ′ phase with the same random distribution of hydrogen bonds around the chain axis according to Ziabicki.16 PA6-α is obtained by slow cooling from the melt or annealing of a quenched sample at high temperature (HT). PA6-β is usually obtained by quenching from the molten state.16−18 Several studies have dealt with the thermally induced crystal phase transitions in PA11.7,8 Real time high temperature X-ray 565


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EXPERIMENTAL SECTION

Materials and Preparation. Polyamide 11 (PA11) under the trade name Rilsan was supplied by Arkema (Serquigny, France). The weight-average molar weight of PA11 is Mw ≈ 25 000 g/mol. The pellets were compression-molded into 100 μm thick films for 10 min at 220 °C. Then, they were either cooled down to room temperature (RT) at about 2 °C/min to obtain α′ phase or quenched in an ice bath to form the smectic δ′ phase. The α phase was obtained from PA11 pellets by casting from a 5 wt % solution in m-cresol7 at around 150 °C and then by drying for 2 weeks at 80 °C under primary vacuum in order to remove moisture and solvent traces. The polyamide 6 (PA6) supplied by DSM (Geleen, The Netherlands) has a number-average molecular weight Mn = 25 600 g/mol. PA6 was extruded at 270 °C into cast films about 130 μm thick on a chill roll at 20 °C. Films underwent an annealing at 80 °C for 10 min to obtain samples in predominant mesomorphous β phase. Two kinds of monoclinic α phases are investigated. The first one, called α1, is prepared by immersing films in superheated water at 150 °C in an autoclave for 40 min40 while the second one (noted α2) is obtained by annealing the initial film at 190 °C for 10 min. Differential Scanning Calorimetry (DSC). Thermal characterization has been carried out on a DSC7 PerkinElmer instrument calibrated with indium under a nitrogen atmosphere. The 10 mg samples were scanned at a heating rate of 10 °C/min. The crystallinity was evaluated from the enthalpy of the melting endotherm using the melting enthalpy of 100% crystallized PA11 ΔHm0(PA11) = 226 J/g41 and PA6 ΔHm0(PA6) = 230 J/g42 whatever the crystalline form. Prior to thermal characterizations, all samples were dried at room temperature under vacuum for 3 days. Wide-Angle X-ray Scattering (WAXS). WAXS experiments were performed using a Genix microsource (XENOCS) equipment operating at 50 kV and 1 mA. The Cu Kα radiation used was selected with a curved mirror monochromator. The 2D patterns were recorded on a CCD camera from Photonic Science, and the working distance was calibrated using PLA sample. Before using FIT2D software, standard corrections were applied to the patterns such as dark current subtraction and background correction. Then, the WAXS intensity profiles were treated using Peakfit software. For PA11, Pearson VII functions were used to fit the amorphous halo and Gaussian profiles were chosen for scattering peaks.43 In the case of PA6, Pearson VII functions were selected for both amorphous halo and scattering peaks.44−47 To study the thermally induced structural evolution, in situ X-ray experiments have been performed using a hot-stage thermal control unit. Samples were heated/cooled at the rate of 10 °C/min from RT to 165 °C for PA11 and up to 190 °C for PA6. The diffraction patterns were collected every 5 °C.

Figure 3. DSC thermograms of PA11-α, PA11-α′, and PA11-δ′ during the first heating.

Table 2. Glass Transition Tg, Melting Point Tm, and Crystal Weight Fraction χc for PA11 Samples in Predominant α, α′, and δ′ Phases (Accuracy ±1 °C and ±1%) sample

Tg (°C)

PA11-α PA11-α′ PA11-δ′

53 52 52

Tm (°C) 188 (181) 190 190

χc (%) 41 29 25

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RESULTS AND DISCUSSION Case of PA11. Figure 3 shows the thermograms obtained during the first heating of the three PA11 samples. Each phase displays similar melting and glass transition temperatures around 190 and 50 °C, respectively (Table 2). An additional endothermic peak is observed for PA11-α′ around 180 °C; its origin will be specified later in the article. The crystal content, of the order of 30% for PA11-α′ and PA11δ′, is slightly higher for PA11-α due to the solvent casting preparation. The evolution of the X-ray diffractograms of PA11-α during continuous heating and cooling together with the evolution of the d-spacings is reported in Figure 4. At room temperature, PA11-α is characterized by three reflections located at 2θ = 7.5°, 2θ = 20.0°, and 2θ = 23.5° corresponding to (001), (200), and (210/010) planes, respectively. Recalling that d010/d001 and d010/d200 are equal to b sin γ/c and 2b sin α/a, respectively, the measured ratios are compared with the calculated values using the unit cell parameters of Table 1. Results summarized in Table 3 reveal that the unit cells proposed by Dosiere et al.13

Figure 4. Evolution of (a) diffractograms and (b) d-spacings for PA11α during heating up to 165 °C and cooling down to room temperature.

and Rhee et al.14 are the most consistent with the experimental data. 566


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Macromolecules Table 3. Measured and Calculated d010/d001 and d010/d200 Ratios Using the Unit Cell Parameters Proposed by Various Authors measured

Little10

Autran et al.11,12

Dosiere et al.13

Rhee et al.14

0.315 0.837

0.250 0.832

0.293 0.882

0.335 0.827

0.346 0.842

d010/d001 d010/d200

Whatever the temperature, diffractograms exhibit the three diffraction peaks characteristic of the triclinic cell of PA11-α phase. However, upon heating, the two main reflections (200)α and (210/010)α gradually become closer, the latter being more sensitive to temperature. This thermal behavior is in good agreement with results previously reported in the literature.8 The thermally induced increase of d(210/010) result from the weakening of the van der Waals interactions. By contrast, the barely unchanged value of d200 reflects the stability of the Hbond interactions in this temperature range. The increase of d001 with temperature may be attributed to the gradual increase of the α angle value in addition to the thermal dilatation effect. To sum up, symmetry of the crystal lattice in the case of the α phase is preserved up to the melting point; the lattice parameters are temperature dependent, and their thermal evolution is reversible upon cooling. Diffractograms of α′ and α phases are very similar at room temperature. Indeed, as shown in Figure 5, the α′ form is characterized by three diffraction peaks at 2θ001 = 7.4°, 2θ200 = 20.4°, and 2θ210/010 = 23.4° relative to a triclinic cell. The main difference between both triclinic phases is the gap between the two main diffraction peaks, in relation to crystal perfection. Based on a relation previously proposed for PA6,48 a crystal perfection index (CPI) of the triclinic structures of PA11 is defined as follows:

( CPI =

d 200 d 210/010

Figure 5. Evolution of (a) diffractograms and (b) d-spacings for PA11α′ during heating up to 165 °C and cooling down to room temperature.

)−1

(1) Ω where the factor Ω is a constant, equal to 0.189 in order to adjust the CPI value of the α phase to 1, considering the α form as the perfect H-bonded sheet-like structure. From eq1, the CPI of the α′ phase amounts to 0.77 and the α′ form may thus be viewed as a defective α phase. As shown in Figure 5a,b, during heating the two main diffraction peaks gradually become closer, but contrary to the α phase, they merge into one single peak near 100 °C corresponding to the gradual transformation of α′ crystals into the HT (pseudo)hexagonal δ crystals. This reversible transition is the so-called Brill transition. Note that the Brill transition temperature depends on the degree of crystal perfection. Indeed, complementary analysis on a less perfect α′ phase of PA11 (CPI = 0.67) has revealed the occurrence of a Brill transition at about 75 °C. Figure 6 illustrates the transformation of the triclinic lattice into a (pseudo)hexagonal one, as proposed by Newman:9 The Brill transition implies the decrease of the γ angle between aand b-axes toward 60° together with an increase of the α angle between b- and c-axes toward 90°. These modifications allow to explain the thermal evolution of d(210/010)α′ and d(200)α′ in Figure 5b, in particular the decrease of the d-spacing of the (200) planes as the temperature is increased. Above the Brill transition, the d-spacing associated with the diffraction of (100), (010), and (110) planes of the HT δ phase located

Figure 6. Evolution of the triclinic lattice into the HT hexagonal lattice during the Brill transition; view perpendicular to the chain axis (after Newman9).

around 2θ ≈ 22° increases as a result of the thermal expansion of the (pseudo)hexagonal lattice. If the existence of a clear-cut Brill transition is clearly established in PA11-α′, the persistence of the sheet-like structure in the HT δ phase is still a matter of debate.49 Even if some authors have argued that the sheet-like structure is preserved above the Brill transition,35−38 such an assertion is difficult to reconcile with the fact that the same thermal expansion coefficient is observed in (100), (010), and (110) directions above the Brill temperature, TB. If we assume the 567


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Macromolecules persistence of the H-bonded sheet-like structure (Figure 7a), one should expect a higher thermal expansion coefficient in the

Figure 7. Projection along the c-axis of the hexagonal lattice of the PA11 HT δ phase assuming (a) the persistence of the sheet-like Hbonded structure and (b) a 3D structure of hydrogen bonds.

(010) and (110) directions as compared to the one in the (100) direction, recalling that H-bonds strength is constant with temperature up to the melting point as shown previously. By contrast, a random distribution of H bonds around the chain axis implies equivalent (100), (010), and (110) planes in which both H-bonds and van der Waals interactions coexist (Figure 7b). The observation of a single main diffraction peak for the HT structure whatever the temperature shows that the (100), (010), and (110) planes are equivalent. This result rather appears in support of a random distribution of the H-bonds around the chain axis in contradiction to previous studies relying in particular on NMR experiments.35−38 It is clear from these studies that above TB the alkane segments are prone to large-amplitude librational vibrations. Some authors suggest that such motions may induce a torsional force on the adjacent amide units, resulting in an intermittent breaking of intrasheet H-bonds.49 This may allow hydrogen bonds to form in other directions, especially between two neighboring H-bond sheets where the amide groups in the triclinic phase are coplanar in PA11, as shown in Figure 1. Cooper et al.49 suggest that these rearrangements may occur in a time scale and to an extent that might not necessarily be captured in NMR experiments. Figure 8a displays the thermally induced structural evolution of PA11-δ′ upon heating up to 165 °C and cooling down to room temperature. At room temperature, the δ′ phase is characterized by a broad reflection located at 2θ = 21.6° related to the smectic nature of this phase. In addition, the well-defined peak at 2θ = 7.2° indicates that this structure has a long-range order in the chain axis direction. Upon heating, the diffraction peak originally located at 2θ = 21.6° shifts to lower angles and persists until melting.8 This may be explained by thermal expansion. However, this peak becomes sharper especially between room temperature and 100 °C where the full width at half-maximum is divided by a factor 2 as shown in Figure 8b.

Figure 8. Evolution of (a) diffractograms and (b) d-spacings (circles) and full width at half-maximum (fwhm) (triangles) of the reflections for PA11 δ′ phase during heating up to 165 °C and cooling down to room temperature.

This structural evolution suggests that the smectic phase progressively transforms into a more organized (pseudo)hexagonal lattice at high temperature. In order to identify the nature of this structure, the diffractograms of samples initially under α, α′, and δ′ forms are compared at 165 °C in Figure 9. At this temperature, while the triclinic lattice is preserved in the α form, α′ and δ′ phases seem to transform into the same HT

Figure 9. Comparison of PA11 samples initially under α, α′, and δ′ forms at 165 °C. 568


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Macromolecules structure as indicated by the similarity of the X-ray scattering parameters summarized in Table 4. This indicates that as in the Table 4. X-ray Scattering Parameters for the HT Phases at 165 °C Starting from PA11 Samples Predominantly under α, α′, and δ′ Forms initial sample PA11-α PA11-α′ PA11-δ

2θ (deg) fwhm (deg) 2θ (deg) fwhm (deg) 2θ (deg) fwhm (deg)

amorphous

(001)

(200)

(210/010)

20.0 6.6 19.6 7.0 19.6 6.2

7.4 1.6 7.4 2.1 7.4 2.1

20.2 0.9

22.9 1.4 21.1 1.5 20.8 1.4

case of the α′ form, the δ′ phase displays a Brill transition around 100 °C characterized by the appearance of the same HT (pseudo)hexagonal δ phase. Moreover, upon cooling down to room temperature, δ crystals evolve into α′ ones as shown previously: the signature of the triclinic phase α′ is clearly evidenced below 75 °C in Figure 8a. The Brill transition was also investigated by thermal analysis, and Figure 10 displays the thermograms of the three phases α,

Figure 11. DSC thermograms of PA6-α1, PA6-α2, and PA6-β during the first heating.

Table 5. Glass Transition Tg, Melting Point Tm, and Crystal Weight Fraction χc for PA6 Samples in Predominant β, α1, and α2 Phases (Accuracies ±1 °C and ±1%) sample

Tg (°C)

Tm (°C)

χc (%)

PA6-β PA6-α1 PA6-α2

51

222 221 220

29 36 35

50

may notice the presence of a weak and broad exotherm between 100 and 170 °C in the case of PA6-β, as already reported in the literature.40,50,51 On the basis of post mortem Xray analysis, we attributed this thermal event to a β → α transition induced upon heating. Figure 12 reports the evolution of the crystalline structure of PA6 initially under α1 form during heating up to 190 °C and subsequent cooling down to room temperature. This phase is characterized by the presence of two well-defined diffraction peaks located at room temperature at 2θ = 20.3° and 2θ = 24.4°, corresponding to the diffraction of (200) and (002) planes, respectively. The former contains chains only linked by van der Waals forces, whereas the latter refers to the hydrogenbonded sheets. As in the case of PA11, crystal perfection of the monoclinic phase has been estimated from the gap between the two reflections, taking as a reference the phase described by Holmes et al. (Ω = 0.194).15 Relying on this assumption, CPI for the α1 phase is evaluated to 0.98. As in the case of PA11-α, PA6-α1 does not display any Brill transition: the crystal lattice keeps its monoclinic symmetry up to melting even if some lattice parameters evolve as temperature is increased, as indicated by the decrease of d200 in Figure 12b. However, all crystal modifications are reversible upon cooling. The distance between hydrogen-bonded planes d002 is more sensitive to temperature than the distance d200 revealing as expected the more pronounced influence of temperature on the van der Waals interactions as compared to the H bonds in this temperature range. The thermally induced structural evolution of the monoclinic α2 phase of PA6 is summarized in Figure 13. At room temperature, the gap between the two main reflections for the α2 form is lower than for the α1 phase showing that the α2 phase is more defective (CPI = 0.89). Literature reports the existence of metastable α structures (referred to as α′ phase) which exhibit similar diffraction patterns as the conventional α form.52 One way to distinguish the metastable from the

Figure 10. Thermograms of PA11-α, PA11-α′, and PA11-δ′ heated up to 165 °C and cooled down to room temperature.

α′, and δ′ heated up to 165 °C and cooled down to room temperature. Apart from the presence of the glass transition, the thermogram of PA11-α exhibits no additional thermal event, in agreement with the absence of a Brill transition in this phase. Conversely, if no evidence of the α′→ δ and δ′ → δ transitions is observed on the thermograms during heating, cooling steps reveal a small exotherm for the samples initially under α′ and δ′ forms attributed to the transformation of the HT δ phase into α′ form, occurring around 85 °C for the sample initially under α′ form and 70 °C for the sample initially under δ′ form. Case of PA6. Figure 11 presents the thermal behavior of the three PA6 phases under study during the first heating. The main thermal transitions are summarized in Table 5. Whatever the crystalline phase, the melting endotherm is located around 220 °C and is thus shifted to higher temperatures as compared to PA11 due to a much higher density of hydrogen bonds. In the case of α1 and α2 phases, the main melting endotherm is preceded by a smaller one assigned to the formation of secondary crystals during the annealing step. Moreover, one 569


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Figure 12. Evolution of (a) diffractograms and (b) d-spacings for PA6α1 during heating up to 190 °C and cooling down to room temperature.

Figure 13. Evolution of (a) diffractograms and (b) d-spacings for PA6α2 phase during heating up to 190 °C and cooling down to room temperature.

conventional α phase is to calculate the ratio of the d-spacing between the two main reflections d002/d200. This ratio is >0.92 for metastable α′ phase, whereas it is in the range 0.83−0.87 for the conventional α form.52 In this study, d002/d200 ratio is equal to 0.87 and 0.84 for α2 and α1 forms, respectively. This shows that both α1 and α2 phases may be considered as “conventional” phases. As in the case of the triclinic phases of PA11, the degree of perfection of the PA6 monoclinic α forms has a huge effect on the thermal stability of the crystalline phase. Actually, contrary to the most perfect α1 phase, the more defective α2 form displays a Brill transition below the melting point (TBrill,α2 ≈ 165 °C) and gradually transforms into a pseudohexagonal HT phase (denominated β′ phase in the following) upon heating. This transition is reversible: the HT β′ phase transforms into the monoclinic α phase upon cooling. As in PA11, crystal perfection thus governs the Brill transition temperature. Note that the transition monoclinic → pseudohexagonal lattice may explain the atypical decrease of d200 of the monoclinic form as in PA11, whereas the relatively important increase of d002 is due to the superposition of both the lattice change and the thermal expansion related to the weakening of the van der Waals forces. Figure 14 illustrates the thermal evolution of PA6 initially in predominant β form. Like in the case of the smectic phase of PA11, the PA6-β structure is characterized by a very broad diffraction peak located around 2θ = 21.5° related to its disordered nature. During heating, the main diffraction peak gradually shifts toward lower Bragg angles and becomes

sharper. The thermal evolution of this structure is less pronounced than in the case of PA11-δ′. This result is in good agreement with previous in situ investigations.53,54 But, contrary to what has been concluded in previous studies based on post mortem analysis,40 the use of in situ experiments invalidates the assumption of a β → α transition occurring above 100 °C during heating. In fact, the α phase develops during the cooling stage so that our results remain consistent with our previous findings based on the post mortem studies. The origin of the exotherm seen on the thermogram of the β phase during heating has yet to be elucidated. Comparison to the case of PA11-δ′ leads us to postulate the existence of a β → β′ transition during heating, knowing that the β′ phase transforms into the α phase upon cooling as shown previously. To validate this assumption, Figure 15 shows the comparison of diffractograms of samples initially under β and α2 forms at 190 °C. The superposition of the two diffractograms as well as their X-ray scattering parameters summarized in Table 6 reveals that at this temperature both samples are in the same so-called β′ form at high temperature. The latter is not thermally stable and transforms into α phase upon cooling whatever the initial crystal structure. We may therefore conclude that PA6-β displays a Brill transition occurring around 100 °C during heating, corresponding to the β → β′ transformation which accounts for the exotherm observed on thermograms. Proposal of a Thermodynamical Scheme for the Brill Transition in PA11 and PA6. The Brill transition like other phase transitions in condensed matter can be described through 570


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a thermodynamic approach considering the stability of each phase of a polymorphic material.55 The stability and the direction of the transformation between two polymorphs may be assessed by the variation of the Gibbs free energy as a function of temperature, knowing that the most stable phase is the one that has the lowest Gibbs free energy. Moreover the exo- or endothermic character of the transformation is indicated by the variation of the slope ∂G/∂T during the phase transition. Based on our experimental results, a Gibbs diagram which depicts the polymorphism of PA11 is proposed in Figure 16.

Figure 16. Gibbs diagram of PA11 phases. Figure 14. Evolution of (a) diffractograms and (b) d-spacings (circles) and full width at half-maximum (fwhm) (triangles) of the main reflection for PA6 β phase during heating up to 190 °C and cooling down to room temperature.

Contrary to the α phase that is stable until melting, α′ and δ′ structures transform into the most stable HT δ phase upon heating. According to this diagram, (i) the α′ → δ transition should be endothermic as indicated by the slope ∂G/∂T higher for α′ than for δ phase. In other words, the δ → α′ transition should be exothermic, which is in agreement with thermal analysis data. (ii) The δ′ → δ transition should be exothermic, which is consistent with the improvement in crystalline order. The two polymorphs α′ and δ constitute an enantiotropic system in which the transition occurs at TB,α′. Note that this diagram also shows that the melting temperature of δ crystals (Tm,δ) is higher than for α′ crystals (Tm,α′). This allows to account for the double melting peak observed in the case of PA11-α′ in Figure 3. The peak at lower temperature is assigned to the melting of the α′ crystals which have not enough time to transform into δ crystals. Regarding the δ′ form, the latter transforms into δ phase at a temperature close to the Brill transition of α′ (TB,δ′). Although the α′ phase is the most stable one at this temperature, it is easier for the δ′ phase to transform directly into δ structure, considering that the symmetry of the smectic phase is closer to that of the δ form. In the same way as for PA11, a Gibbs diagram is proposed to describe the Brill transition of PA6 based on thermal and structural analysis (Figure 17). A similar approach has already been used for this material to explain the better stability of the α phase as compared to the γ form.56 At room temperature, the β phase has a Gibbs free energy close to that of a liquid because of its disordered nature. In contrast to the most perfect α1 phase which does not undergo any change in lattice symmetry until melting, α2 and β forms transform into the most stable HT β′ phase upon heating.

Figure 15. Comparison of PA6 samples diffractograms initially under α2 and β forms at 190 °C.

Table 6. X-ray Scattering Parameters for the HT Phase at 190°C for Initial PA6-β and PA6-α2 Samples initial sample PA6-β PA6-α2

2θ (deg) fwhm (deg) 2θ (deg) fwhm (deg)

amorphous

main reflection

20.0 7.1 20.1 8.3

21.0 2.6 21.2 2.6

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the present study enables to propose a general scheme for Brill transition phenomena in polyamides.

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AUTHOR INFORMATION

Corresponding Author

*Tel +33-320-336416; e-mail [email protected] (V.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Region Nord-Pas-de-Calais and the European Regional Development Fund (FEDER) for funding the X-ray equipment. The authors are indebted to Arkema (France) for providing polyamide 11. Financial support and supply of PA6 by DSM Research are gratefully acknowledged. The authors thank A. Stroeks, T. Brinks, G. Vanden-Poel, and J. Xu (DSM Research, The Netherland) and Pr M. Descamps (UMET, Lille1 University) for fruitful discussions.

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Figure 17. Gibbs diagram of PA6 phases.

REFERENCES

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According to this diagram, the β → β′ transition is exothermic in agreement with the thermogram of Figure 11, whereas the α2 → β′ transition is endothermic. Even if no thermal event is observed upon heating for PA6-α2 in this study, some authors have reported an endotherm for samples previously crystallized by slow cooling from the melt before melting.57 The transformation of the β mesophase into β′ structure takes place during the exotherm of structural reorganization and begins at the onset of this thermal event (Tsr). As previously evoked in the case of PA11 and for the same symmetry considerations, β turns into β′ at Tsr although the α2 phase is the most stable. Regarding the latter, transformation of the α2 monoclinic structure occurs at a temperature TB,α2 much closer to the melting point.

■

CONCLUSION In the present study, the thermally induced crystal transitions were investigated in both PA11 and PA6 with special attention paid to the occurrence of a Brill transition. In a first step, it was confirmed that PA11-α′ and PA11-δ′ display a Brill transition around 100 °C inducing the formation of a HT (pseudo)hexagonal phase contrary to the α phase which keeps its lattice symmetry up to the melting point. Regarding the α′ form, the persistence of the hydrogen-bonded sheet-like structure is discussed. Though the structural characterization does not allow a clear-cut answer to this question, the fact that the (100), (010), and (110) d-spacings of the (pseudo)hexagonal phase evolve in a similar way as the temperature is increased above the Brill transition is rather in favor of a random distribution of H bonds around the chain axis for the HT δ phase. Based on thermal analysis and in situ X-ray scattering, a Gibbs diagram is proposed to account for the phase transitions. Considering the case of PA6, comparison with PA11 suggests the existence of a HT pseudohexagonal phase in PA6 as well. Thus, the β mesophase as well as the αdefective phase transform into a HT pseudohexagonal β′ phase upon heating, which in turn evolves into the α monoclinic phase upon cooling as in the case of PA11. The β → α transformation mentioned in previous studies has been reconsidered in view of the deeper insights into phase transformation kinetics provided by in situ experiments. Moreover, owing to the obtention of such data, 572


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New Insights into the Brill Transition in Polyamide 11 and Polyamide 6

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