Assessment of polyamide-6 crystallinity by DSC

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Assessment of polyamide-6 crystallinity by DSC. Temperature dependence of the melting enthalpy. Coraline Millot1,2. • Louise-Anne Fillot2. • Olivier Lame1.
J Therm Anal Calorim DOI 10.1007/s10973-015-4670-5

Assessment of polyamide-6 crystallinity by DSC Temperature dependence of the melting enthalpy Coraline Millot1,2 • Louise-Anne Fillot2 • Olivier Lame1 • Paul Sotta2 Roland Seguela1



Received: 4 February 2015 / Accepted: 22 March 2015  Akade´miai Kiado´, Budapest, Hungary 2015

Abstract This study addresses the question of the crystallinity determination of PA6 by means of DSC in the case when structural changes occur over a very large temperature domain during the heating scan. The temperature dependence of the melting enthalpy is then of crucial importance for determining the amount of crystalline phase involved in the various processes, and thus the initial crystallinity. Both DSC and WAXS measurements have been carried out of a PA6 sample submitted to various thermal treatments in order to identify the crystalline forms and the temperature-induced structural changes. The melting enthalpy dependence on temperature of PA6 was computed from heat capacity data of the solid and liquid borrowed from literature data tables. Similar computations were performed for PA66 which is likely to exhibit analogous structural changes during DSC analysis. Keywords Polyamide-6  Melting enthalpy  DSC  WAXS  Crystallinity

& Olivier Lame [email protected] Roland Seguela [email protected] 1

MATEIS, UMR 5510 CNRS - INSA de Lyon, Lyon Tech La Doua, 69621 Villeurbanne, France

2

LPMA, UMR 5268 CNRS - SOLVAY, 85 avenue des Fre`res Perret, 69192 Saint-Fons, France

Introduction Semi-crystalline polymers display a structural hierarchy that is strongly sensitive to processing conditions and thermo-mechanical treatments. The crystal weight fraction or crystallinity, Xc, is particularly processing dependent owing to texturing and chain orientation effects. This phenomenon has an impact on the material macroscopic behavior since the intrinsic properties of the crystalline and amorphous phases are generally quite different [1–3]. Rapid cooling after melt processing generally makes Xc lower than its maximum value, noticeably in the case of polymers having low crystallization kinetics. This may be detrimental for the physical characteristics and use properties of manufactured parts. In the case of polymers having a glass transition temperature above room temperature, crystallization may even be completely inhibited upon fast cooling. The quantitative determination of the crystallinity of a semi-crystalline polymer after thermo-mechanical processing is mandatory for understanding its properties. Several methods are commonly used in this aim: differential scanning calorimetry (DSC), density measurements, X-ray scattering, infrared and Raman spectroscopy, and nuclear magnetic resonance [4]. DSC is the easiest and most largely used one. This method relies on the measurement of heat that must be provided to the material for the melting of its crystalline phase during heating at constant rate. A major drawback of this method is that thermodynamically unstable phases present in the initial material can take advantage of the heating to reorganize into stable ones during the DSC scan. Therefore, the final melting endotherm does not provide information on the actual structure of the material since it combines the

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melting of both the initial crystals and the reorganized ones. Aromatic semi-crystalline polymers, some aliphatic polyesters and aliphatic polyamides [5, 6], are typical polymers undergoing restricted crystallization under fast cooling. During the heating scan of the DSC analysis, cold crystallization may take place above the glass transition temperature (Tg). Provided that the DSC baseline was properly adjusted prior to the analysis, it is quite easy to determine the amount of matter involved in the melting and the cold crystallization processes. Computing the actual crystallinity of the original material is then straightforward. This computation is generally carried out by assuming that the specific heat of fusion, or melting enthalpy DHm  , and the specific heat of crystallization, DHc  , of the 100 % crystalline material have the same absolute value at the equilibrium temperature of fusion, Tm  . This would be theoretically true if the two processes occurred in the same temperature range. However, the temperature gap between cold crystallization and melting may be very large, typically 100 C or more for the kind of polymers previously mentioned. Moreover, the specific heat of melting, or crystallization, strongly depends on temperature due to the different T-dependences of the heat capacities of the solid and the liquid [7]. A significant depression of DHm  can be observed below Tm  , as already reported for several polymers [5, 8]. Therefore, an accurate determination of the crystallinity requires an analytical expression for the DHm  ðTÞ relationship. In spite of rather fast crystallization kinetics, incomplete crystallization of aliphatic polyamides may occur upon quenching from the melt. This is evidenced by the occurrence of crystal reorganization effects during subsequent heating just above Tg. Khanna and Kuhn [6] have argued on the reliability of DSC for determining the crystallinity of polyamides, particularly in the case of poorly crystallized samples that precisely may exhibit structural reorganization events during the heating scan, including cold crystallization just above Tg. These authors have also pointed out that the melting enthalpy of nylons is likely to exhibit a T-dependence [6]. The impact of crystallinity on the mechanical properties of polyamides [9–11] makes its accurate determination a crucial issue. In continuation of Khanna and Kuhn’s paper [6], the present work deals with the practical assessment of polyamide-6 crystallinity by taking into account the DHm  ðTÞ relationship when structural changes occur far below the melting point during the DSC scan [12–16].

Experimental The polyamide-6 (PA6), otherwise nylon-6, under investigation is an industrial grade from Solvay. The number average molecular weight is Mn & 31 kg mol-1, and the

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polydispersity index Ip & 1.86. The material was received in the form of 1.6-mm-thick injection-molded plates stored into tightly sealed bags just after processing in order to prevent moisture absorption. Four different thermal treatments were performed on samples of about 5 mg that were inserted into hermetic DSC pans in order to prevent moisture uptake prior to treatment and pollution by the silicon oil during of the thermal treatment bath: •



• •

Quenching was carried out by dropping one of these sealed samples into an ice water bath at 0 C immediately after melting in an oven at 270 C for 10 min in order to erase memory effect [17]. Isothermal crystallization at 213 C for 24 h was performed by plunging the sample in a thermoregulated silicon oil bath after melting for 10 min in the oven at 270 C. Isothermal annealing at 200 C for 30 min of quenched samples was made in the same oil bath. Isothermal annealing at 80 C for 30 min of quenched samples was also performed in order to better understand the crystalline changes involved in the cold crystallization process.

Differential scanning calorimetry (DSC) was performed on a Q2000 apparatus from TA Instrument at a heating rate of 10 C min-1. The temperature and heat flow scales were calibrated using high purity indium and zinc samples. The thermally treated samples in their closed DSC pans were directly used in the DSC measurements. Wide-angle X-ray scattering (WAXS) experiments were carried out on the BM02 beamline at ESRF (European Synchrotron Radiation Facility, Grenoble, France) equipped with a CCD camera from Princeton Instrument, using a wavelength k = 0.154 nm. The thermally treated samples were extracted from the hermetic pans just before the WAXS measurements. Corrections of the 2D frames were routinely performed using the bm2img software of ESRF. Azimuthal integration of the isotropic 2D patterns was carried out by means of the Fit2D software.

Results and discussion It is worth to briefly review the most common crystal forms of PA6 in relation to thermal treatments. The monoclinic aform is the most stable one, generally obtained from quiescent melt at crystallization temperatures Tc [ 150 C [14, 18–20]. The pseudo-hexagonal c-crystals can be obtained from the quiescent melt at Tc \ 150 C [14, 18–21]. Fiber spinning from the melt at low or moderate spinning rates is also well known to generate c-crystals [22–24]. However, spun fibers may contain defective c-crystals that

Assessment of polyamide-6 crystallinity by DSC

display an early melting and then undergo recrystallization in the a-crystal form during the heating scan [22]. Films cast on cold rolls exhibit the so-called mesomorphic phase or mesophase, otherwise b-form, that is hardly distinguishable from the c-form by means of X-ray scattering. This metastable mesophase also undergoes reorganization into the a-crystal form during the heating scan [14, 16, 19]. Yet, it is not clear from literature data whether the bform and the defective c-form are actually different kinds of ill-ordered crystals or just two different views of the same species (see Murthy [19], Kolesov et al. [16] and Auriemma et al. [25]). To sum up, in spite of the fairly good knowledge of the morphogenesis of the various PA6 crystal forms, the situation remains unclear in many cases. Thermal and structural behavior of PA6 Figures 1 and 2 report the DSC heating traces and the WAXS intensity profiles of PA6 samples submitted to the various thermal treatments. Regarding the DSC data first, Fig. 1 shows that quenching at 0 C partially prevents crystallization of the sample as can be seen from the cold crystallization exotherm at 55 C, i.e., just above the temperature range of

the glass transition. Then, a broad melting endotherm occurs with a peak temperature T m peak  216  C. It is to be noticed that complete (or nearly complete) amorphization has been reported for PA6 thin films, according to the height of the heat capacity jump at the glass transition [6, 26]. This resulted in a cold crystallization peak with greater amplitude than in the present case, which means that amorphization is partial for the present quenched PA6 sample. Moreover, the above-mentioned works also reported the observation of a weak and broad exotherm extending from the cold crystallization up to the final melting. This was ascribed to the occurrence of a gradual reorganization of the amorphous material into crystal, and/or perfectioning of the formerly grown crystals [6, 26]. In the present quenched PA6 sample, such kind of broad exotherm appears beyond the cold crystallization peak up to about 120 C (see top curve in Fig. 1). The origin of this phenomenon will be further discussed in the ‘‘Concluding discussion’’ section. Isothermal annealing at 80 C of the quenched sample suppresses the cold crystallization phenomenon due to the annealing-induced crystallization. In addition, it results in a shift of the melting endotherm up to Tpeak & 220 C. The m faint exotherm pointing at 190 C is relevant to reorganization processes of probably unstable crystals. This phenomenon has been commonly observed in PA6 samples

55.0 °C

Isotherm at 213 °C

Quenched at 0 °C 15.3 J g–1

Quenched at 0 °C + annealed at 200 °C

Quenched at 0 °C

Quenched at 0 °C + annealed at 80 °C

Quenched at 0 °C + annealed at 200 °C 202.0 °C

221.0 °C

Isotherm at 213 °C

Intensity/arbitrary units

56.2 J g–1

216.5 °C

Heat flow/W g–1

Quenched at 0 °C + annealed at 80 °C

Exo 1 W g–1

230 °C Endo 0

50

100

150

200

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Temperature/°C Fig. 1 DSC heating traces of PA6 samples after various thermal treatments

10

15

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Scattering angle 2θ /° Fig. 2 WAXS intensity profiles of PA6 samples after various thermal treatments

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and largely discussed elsewhere in relation to processing condition [27]. Isothermal crystallization at 213 C, as well as annealing at 200 C, no longer lead to cold crystallization. In both cases, the shift of the melting peak to higher temperature indicates a better thermal stability of the crystallites, due to more favorable growth conditions. Concerning isothermal crystallization at 213 C, the DSC trace reveals a unique and rather narrow melting peak with Tpeak & 230 C. This m suggests the melting of a-crystals after Kyotani and Mitsuhashi’s study [18]. In contrast, the DSC trace of the sample annealed at 200 C exhibits a double endotherm suggesting the melting of two kinds of crystallites either of the same crystal form but having two different crystal thickness or of different kind, namely a- and c-crystals, melting respectively in the major endotherm at Tpeak & 221 C and in the minor one at Tpeak & 202 C. m m The next WAXS data support the second hypothesis. The occurrence of a small amount of c-crystals upon annealing at 200 C for 30 min is consistent Kyotani and Mitsuhashi’s [18] regarding the PA6 crystallization kinetics. Regarding the WAXS data, the intensity profiles of Fig. 2 confirm that the sample isothermally crystallized at 213 C is in the a-crystal form as evidenced by the (200) and (002/020) reflections located at 2h & 20 and 2h & 24, respectively [28]. The WAXS data for the sample annealed at 200 C show the characteristic (002/ 200) reflections of the c-form at 2h & 22, between the two major reflections of the a-crystal form. Besides, the characteristic (020) reflection of the c-form is clearly observed at 2h & 10.5 [29]. It is to be noticed that, except for the last one, the angular positions of the various reflections may be slightly different from literature data due to different degrees of crystal perfection inducing variations in the unit cell parameters [30]. Regarding the sample quenched at 0 C, the very broad scattering halo without sharp reflection suggests amorphous and/or mesomorphic b-form. After annealing at 80 C, a narrowing of this broad scattering occurs accompanied by a fairly well-resolved reflection on the righthand side having an apex at 2h & 21. This is a priori consistent with cold crystallization into the c-form of PA6 as reported by Khanna and Kuhn [6]. However, the present WAXS data display only a weak (020) reflection characteristic of the c-form at 2h & 10.5. This may indicate that cold crystallization generates a relatively disordered form of c-crystals, otherwise a kind of b-mesophase. A parallel can be made with a similar study on quenched PA6 submitted to annealing at various temperatures by Kolesov and Androsch [31]. These authors claimed for a cold crystallization into b-mesophase that gradually transforms into acrystals during the DSC heating scan. Unfortunately no

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information on the (020) reflection is given in this latter work, so that no clear-cut conclusion can be made. Temperature dependence of the PA6 melting enthalpy The large gap (of the order of 160 K) between the cold crystallization exotherm and the final melting endotherm raises the question of the T-dependence of the melting (or crystallization) enthalpy of 100 % crystalline PA6, which is important for determining the index of the material by DSC. For any kind of physical or chemical transformation, the temperature dependence of the enthalpy is given by Kirchhoff’s law, which takes the following form in the case of melting enthalpy Z DHm ðT2 Þ  DHm ðT1 Þ ¼ DCp dT ð1Þ where DCp is the difference of the heat capacities of the material in the final and initial states at every temperature over the range (T1 - T2). The T-dependence of the melting enthalpy, that also applies to the crystallization enthalpy, can be determined from Eq. 1 provided that the heat capacities of the liquid and the solid can be extrapolated over the temperature range (Tg  –Tm  ). Computations were performed by Wunderlich [8] in the case of polyethylene and by Seguela [5] in the case of poly(ethylene-terephthalate) and poly(arylether-ether-ketone). Such computations are based on the experimental heat capacity data reported by Wunderlich et al. [32, 33] for a number of semi-crystalline polymers. Worth noticing is that the term ‘‘solid’’ includes crystalline and glassy materials for which the heat capacities are assumed identical, for all polymers [32, 33]. As a matter of fact, slight differences of Cp data between the two solid states have only been observed at very low temperature, i.e., below 100 K. This comment also applies for the various crystalline forms of PA6 that may display slight differences in Cp values also assumed to be negligible in comparison with the DCp between solid and liquid. A wellknown property in favor of this assumption is that the a and c crystals of PA6 practically have the same enthalpy of melting at their respective melting points [8, 34]. Regarding PA6, experimental data of the heat capacities of the liquid and the solid, Cp,l and Cp,s, respectively, are available from the above-mentioned data tables. The computed polynomial fittings are the followings: •

for the liquid in the range 320–600 K: Cp;l ¼ 0:001348 T þ 1:9793 in J g1 K1



for the solid in the range 70–310 K:

ð2Þ

Assessment of polyamide-6 crystallinity by DSC

Cp;s ¼ 0:004502 T þ 0:1387 in J g1 K1

Temperature/°C

ð3Þ 50

ð4Þ

The integration constant of Eq. 4 was determined by using literature data for the enthalpy of fusion DHm  & 230 J g-1 [8] of 100 % crystalline PA6 at Tm  = 533 K. Though several different values have been reported for DHm  (PA6), the above-mentioned value is rather consensual. The variation curve of DHm for PA6 as a function of temperature is plotted in Fig. 4 according to Eq. 6. The roughly twofold increase in DHm between Tg  and Tm  is surprisingly large and has never been mentioned so far in the literature. Only Sanchez et al. [35] mentioned the DHc depression of PA6 in the case of the crystallization under

Temperature/°C –200

–100

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Heat capacity/J g–1 K–1

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Melting enthalpy/J g–1

DHm  ðPA6Þ ¼ 03:03 þ 1:8406T  0:001577T 2 in J g1 ; with T in K

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Figure 3 shows the PA6 experimental heat capacity data together with the fitted polynomial plots according to Eqs. 2 and 3. Taking the above linear relationships for Cp,l and Cp,s and assuming linear extrapolation over the whole temperature range, the specific melting enthalpy of 100 % crystalline PA6 at any temperature between Tg  and Tm  can be written from Eq. 1 as

200

180 160 140

PA 66 PA 6

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100 300

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Temperature/K Fig. 4 Melting enthalpy, DHm , versus temperature of 100 % crystalline PA6 and PA66

slow cooling of droplets dispersed in a polyethylene matrix which occurs about 50 C below that of bulk PA6 under similar conditions. Considering the occurrence of crystal phase transformations during the DSC heating scan from Tg to Tm, one may wonder whether the difference in T-dependence of DHm for the c-form and a-form crystals should be considered. This questioning can be alleviated by taking into account the experimental evidence that the two forms have very close DHm  values [8, 34]. Temperature dependence of the PA66 melting enthalpy

2.0 Cpl Cps Cps exp Cpl exp

1.5

Polyamide-66 (PA66) is structurally close to PA6 and displays very similar thermal and thermo-mechanical properties due to equivalent densities of H-bonds and similar crystallinity. Moreover, as does PA6, PA66 may display unstable crystal forms that reorganize into more stable ones upon heating above Tg [12, 13]. Heat capacity values for PA66 are available from data tables of Wunderlich et al. [32, 33]. The fitted polynomials for Cp,l and Cp,s are the following ones

1.0

0.5 Tm

Tg 0.0 0

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200

300

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Temperature/K Fig. 3 Temperature variations of the heat capacities of the liquid, Cp,l, and the solid, Cp,s, for PA6 obtained by curve-fitting of the experimental data from Ref. [31] (dashed curves are linear extrapolations)



for the liquid in the range 330–600 K: Cp;l ¼ 0:002069T þ 1:5517 in J g1 K1



ð5Þ

for the solid in the range 230–320 K: Cp;s ¼ 0:005012T  0:0397 in J g1 K1

ð6Þ

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Using Eqs. 1, 5, and 6, the temperature dependence of the melting enthalpy of 100 % crystalline PA66 is then given by the relation DHm  ðPA66Þ ¼ 200:05 þ 1:5914T  0:0014715T 2 in J g1 ; with T in K ð7Þ taking DHm  = 230 ± 70 J g-1 [5] at Tm  = 553 K. This value of DHm  for PA66 is very close to that of PA6 [36, 37]. However, it should be noticed that the literature data display a very large dispersion in the range 160–300 J g-1 [5]. The DHm variation with temperature for PA66 is plotted in Fig. 4 between Tg and Tm  according to Eq. 7. From  Fig. 4, it appears that DHm of PA66 increases by a factor 1.5 between 323 and 538 K, i.e., a smaller effect than that observed for PA6. The amplitude of this effect, however, depends on the choice of the DHm  value of PA66 in the range 160–300 J g-1.

computed without considering the T-dependence of the specific enthalpy of both fusion and crystallization of 100 % crystalline PA6. A more rigorous way of computing  and Xc would be to consider the T-dependence of DHm  DHc over the whole temperature range, not just in the Tdomains of the major transformations, i.e., cold crystallization and melting. The incidence is insignificant as compared to the computation made in Eq. 9. Considering annealed and isothermally crystallized samples, both exhibit thermal phenomena beyond 180 C upon heating, either exo- or endothermic. Figure 4 shows that in the temperature range [180 C–Tm], the deviation of   DHm ðTÞ and DHc ðTÞ from DHm  does not exceed 10 %. In such instance, one may consider useless to take into account the temperature dependence of the enthalpy of both fusion and crystallization for the computation of crystallinity.

Concluding discussion Computation of PA6 crystallinity From a practical standpoint, the two annealed PA6 samples in Fig. 1 and the isothermal one display a melting endotherm that does not exceed 30 K in width, and not other thermal transition. In such cases, it is worthless making a DHm correction in consideration of experimental errors and reproducibility. In contrast, the quenched PA6 sample exhibits cold crystallization just above Tg for which T-de pendence of DHm is no longer negligible in regard to the huge T-range between this process and the melting. First, disregarding the temperature dependence and   taking a constant value DHm = DHc  = DHm  = 230 J g-1 whatever the temperature at which occurred the two events, the total enthalpy of transformation DHc ? DHm is computed by using the recommended linear interpolation of the baseline from the onset of the cold crystallization up to the end of the melting [6, 31]. Then it comes Xc ¼ ðDHc þ DHm Þ=DHm  ¼ 40:7=230 ¼ 0:18

ð8Þ

Considering secondly the significant depression of the specific heat of crystallization at the peak temperature of the cold crystallization exotherm, Eq. 4 gives |DHc (55 C)   | = 131 J g-1. This huge drop of DHc  as compared to DHm  is worth being taken into account. In such circumstances, the crystallinity can be computed as Xc ¼ DHc =DHc  ð55  CÞ þ DHm =DHm  ð216  CÞ ¼ ð15:3=131Þ þ ð56:2=230Þ ¼ 0:13

ð9Þ

This latter value of the crystallinity is significantly lower and certainly more correct than the one previously

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Rapidly cooled or quenched bulk PA6 undergoes partial amorphization. Upon heating, cold crystallization occurs at about 20 K above Tg. The crystalline phase that develops thereof proved to be in the b-form, in agreement with observations from Kolesov and Androsch [31]. Very gradual re-organization into more stable forms occurs upon heating up to the melting that finally occurs at about 160 K above the cold crystallization exotherm. In parallel, it is shown that the melting enthalpy of 100 % crystalline PA6 hugely depends on temperature in the (Tg   Tm  ) range. If not taken into account, this phenomenon entails an erroneous estimation of the crystallinity from DSC. A similar temperature dependence is shown for the melting enthalpy of 100 % crystalline PA66. Khanna and Kuhn [6] as well as Kolesov and Androsch [31] have reported the occurrence of a broad exotherm following the cold crystallization upon heating, and spanning up to about 180 C. This phenomenon was claimed to be the footprint of a gradual increase in crystallinity of the material and/or perfectioning of the ill-ordered crystals initially grown during the cold crystallization process (likely in b-form). In the present context of a strong Tdependence of both the melting and the crystallization enthalpies of PA6, i.e., DHm ðTÞ and DHc ðTÞ, the above observation could be attributed in part to the gradual in  crease in both DHm ðTÞ and DHc ðTÞ during the heatinginduced perfecting process. Indeed, the crystals that melt at T1 with a melting enthalpy DHm (T1) recrystallize subsequently at T2 [ T1 with a heat of crystallization DHc(T2) such that |DHc(T2)| [ DHm(T1) so that the resulting heat balance is exothermic. In this instance, there would be no

Assessment of polyamide-6 crystallinity by DSC

increase in crystal content, but just an increase in crystal perfection accompanied with higher melting point and higher DHm . This does not mean that an increase in crystal content could not occur in parallel. However, the two phenomena could be hardly distinguished from each other if occurring concomitantly. It is to be noticed that the present approach relies on the assumption that all solid forms of PA6, i.e., the glass and the various crystal polymorphs, obey the same Cp variation with temperature, in first approximation. The Cp difference between the various solid forms is indeed negligible in comparison with the DCp = Cp,s - Cp,l that is under concern in Eq. 1. In contrast, the relevance of the present approach has been previously demonstrated for other polymers which display either a strong melting point depression or a large temperature gap between crystallization and melting. For instance, ethylene-based polymers and copolymers may display Tm depression involving significant DHm depression with respect to DHm  due to structural defects in the chains that promote small and imperfect crystals [38, 39]. As already pointed out in the introduction, poly(ethylene-terephthalate) and poly(arylether-ether-ketone) [5] may exhibit cold crystallization after fast cooling from the melt, in a similar way as PA6. This process takes place far below Tf  so that DHc is strongly depressed with respect to DHc  . In both cases, the effect is great enough to be taken into account for crystallinity calculations. Acknowledgements The authors gratefully acknowledge the Rhone-Alpes Region for the grant of a doctoral fellowship to C. Millot (Academic Research Communities—Energies Division). The European Synchrotron Radiation Facility (Grenoble, France) is also acknowledged for time allocation on the BM02 beamline for the WAXS experiments. The authors are indebted to Dr. C. Rochas (CERMAV, Grenoble) for assistance in the WAXS experiments and working of the data.

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