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The phase structures of nylon 6.6 as studied by temperature-modulated
calorimetry and their link to X-ray structure and molecular motion
Wulin Qiu
a
, Anton Habenschuss
b
, Bernhard Wunderlich
a,b, *
a (^) Department of Chemistry, The University of Tennessee, Knoxville, TN 37996-1600, USA b (^) Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6197, USA
Received 6 December 2006; received in revised form 14 January 2007; accepted 15 January 2007 Available online 19 January 2007
Abstract
The phase behavior of semicrystalline, dry nylon 6.6 is analyzed on the basis of differential scanning calorimetry, DSC, and quasi-isothermal, temperature-modulated DSC, TMDSC. The data were collected over the temperature range from below the glass transitions to above the isotropization. Based on the contributions of the vibrational motion to the heat capacity, as is available from the ATHAS Data Bank, and the multifaceted new calorimetry, as well as on information on X-ray diffraction, molecular dynamics simulation of paraffin crystals, and quasi- elastic neutron scattering, the following observations are made: (a) beginning at the glass transition temperature of the mobile-amorphous phase (Tg ¼ 323 K), a broadened transition of the semicrystalline sample is observed which reaches to 342 K (Tg ¼ 332.7 K). An additional rigid- amorphous phase, RAF, undergoes its separate, broad glass transition immediately thereafter (340e400 K, Tg z 370 K). (b) The transition of the RAF, in turn, overlaps with increasing large-amplitude motion of the CH 2 groups within the crystals and latent heat effects due to melting, recrystallization, and crystal annealing. (c) From 390 to 480 K the heat capacity of the crystals increasingly exceeds that of the melt due to additional entropy (disordering) contributions. Above 440 K, close to the Brill temperature, the heat capacity seems to drop to the level of the melt. (d) If observation (c) is confirmed, some locally reversible melting is present on the crystal surfaces. (e) The increasing large-amplitude motion is described as a glass transition of the crystals, occurring below the melting point, at 409 K. The assumption of a separate glass transition in the ordered phase was previously successful in analyzing aliphatic poly(oxide)s and mesophases. The full description of the globally meta- stable, semicrystalline phase structure of nylons, thus, needs information on the glass transitions of the two amorphous phases and the ordered phase and the various irreversible and locally reversible order/order transitions and their kinetics. Published by Elsevier Ltd.
Keywords: Polymer science; Nylon 6.6; Melting
- Introduction
The aliphatic polyamides, as developed by Carothers [1], were the basis of the first commercially successful semi- crystalline, synthetic fibers. Polyamide fibers are used for
textiles and carpets and also, in bulk, as an engineering plastic [2]. The first fibers were introduced commercially in 1938 un- der the trade name ‘‘Nylon’’ (DuPont de Nemours and Co.) which by now is the generic term for these polyamides. The basic chemical structure consists of methylene sequences, in- terrupted at regular intervals by intermolecularly hydrogen- bonded amide groups. The specific polymer of this research, nylon 6.6, is represented by the repeating unit [NHeCOe (CH 2 ) 4 eCOeNHe(CH 2 ) 6 e]. The dimensions of the essen- tially planar amide group in nylon were established in 1953, based on the structure of crystalline peptides [3]. The capabil- ity of nylon to form hydrogen bonds is also retained in the melt. The methylene units in the crystal try to assume a planar zig-zag chain consisting of trans-conformations, established
- (^) ‘‘The submitted manuscript has been authored by a contractor of the U.S. Government under the contract no. DOE-AC05-00OR22725. Accordingly, the U.S. Government retains a non-exclusive, royalty-free license to publish, or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.’’
- Corresponding author. Department of Chemistry, The University of Tennessee, Knoxville, TN 37996-1600, USA. Tel./fax: þ1 865 675 4532. E-mail address: wunderlich@chartertn.net (B. Wunderlich).
0032-3861/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.polymer.2007.01.
Polymer 48 (2007) 1641e 1650 www.elsevier.com/locate/polymer
at low temperatures as the low-energy shape in paraffins and polyethylene [4]. The crystal structures of the aliphatic nylons have been summarized, for example, in Ref. [5]. Specifically, the chains of nylon 6.6 are non-polar (symmetric along the chain direc- tions), and in the most common, triclinic a-crystals they are connected by hydrogen bonds to sheets in the crystallographic ac-planes which are stacked at an angle of a ¼ 48.5^ [6e8]. At low temperature, the heat capacity, Cp , of the nylon crystals is fully described by vibrational contributions [9] which repro- duce the experimental data, critically evaluated in the ATHAS Data Bank [10]. The amorphous nylon 6.6 can be described from about 50 K to the glass transition temperature, Tg (¼323 K), by the same vibrational Cp [11]. The equilibrium melting temperature, Tmo^ , for nylon 6.6 is taken to be 574 K and the heat of fusion of a fully crystalline sample to be 57.8 kJ mol^1 (or 255.4 J g^1 ) based on the extensive discus- sions of a wide range of literature data [12e14]. A broad transition of the crystal structure, named after Brill [15], is reported in nylon 6.6 between 450 and 490 K and is accompanied by changes in the thermal and mechanical prop- erties [2]. Crystallographically, this transition is observed as a gradual transformation from the diffraction patterns with tri- clinic to pseudo-hexagonal symmetry, accompanied by a 12% increase in unit cell volume [16]. The increase in crystal symmetry at the Brill transition causes a marked increase in segmental mobility of the methylene groups, as was seen already in early NMR studies [17]. The segmental motion was then studied by solid-state deuterium NMR and molecular dynamics simulations using selectively deuterated samples [18e20]. It was found that in the crystal the NeD- and Ce D-group undergoes spatially heterogeneous librations, both below and above the Brill transition. At 500 K, the amplitude of the motion in the crystals becomes very large, reaching an angle of 60^ for all methylene groups. The hydrogen bonds, however, are found to remain largely intact. In the amorphous part, limited librations and internal rotations start below Tg , while above Tg both NeD- and CeD-site perform nearly isotropic motion. Quasi-elastic neutron scattering studies on samples of nylon 6.6 with different crystallinities, similarly, showed that at temperatures 40 K below melting, the CH (^2) groups in the crystal undergo already large-amplitude, liquid-like motion [21]. The effect of this large-amplitude motion of the CH (^2) groups below the melting temperature of the crystals of nylon 6.6 was also linked to the excess in Cp , using extensive comparisons of samples of different thermal history, analyzed by standard differential scanning calorimetry (DSC) and X- ray diffraction [22]. In this paper the contribution of the large-amplitude motion to Cp is separated quantitatively from the latent heat effects due to crystal perfection, recrys- tallization, and locally reversible melting using temperature- modulated DSC (TMDSC) [23]. The results are linked to similar large-amplitude motion effects first seen for polyeth- ylene [24] and detailed in Ref. [25], and the glass transitions of crystals of aliphatic poly(oxide)s [26] and mesophases [27,28].
- Experimental
2.1. Materials
The nylon 6.6 [structure-based name: poly(iminoadipoyl- iminohexamethylene), source-based name: poly(hexamethyl- ene adipamide)] used in this research has an estimated viscosity-average molar mass, Mv, of 15e20 kDa and was pur- chased from Scientific Polymer Products Inc. It was delivered in the form of translucent pellets with a density of about d ¼ 1.30 mg m^3 (Lot 20, Cat. 033). At the glass transition temperature, the heat capacity, Cp , of the amorphous, glassy polymer increases by 115.5 J K^1 mol^1 [10]. Before mea- surement, the samples were melted by heating to 568 K to produce a dry sample, accompanied by the usual increase in molar mass [23]. Fig. 1a shows a characterization of the stud- ied samples by standard DSC. After melting, the cooling curve at 10 K min^1 is the top DSC trace. This is followed by a sec- ond heating at 10 K min^1. The weight loss due to water in such experiments was 3.4e4.5%. The DSC on heating after slow cooling by quasi-isothermal TMDSC (as shown in Fig. 2a, points C, discussed below) is illustrated in the third curve of Fig. 1a. Estimates of the crystallinity were obtained from approximate baselines and the above mentioned heat of fusion. The standard DSC results follow closely the earlier, more extensive, DSC traces on nylon 6.6 (which also included data for several other nylons) [11].
2.2. Differential scanning calorimetry
The calorimetry was carried out with a Thermal Analyst 2920 system from TA Instruments, Inc. The twin calorimeter is of the isoperibol heat-flux type, capable of standard DSC and quasi-isothermal temperature-modulated DSC. The temperature measurement and modulation control are done by the sample-temperature sensor. During the experiments, a refrigerated cooling system with a cooling capability to about 220 K, was used, and dry N 2 gas with a flow rate of 25 mL min^1 was purged through the DSC cell. The tempera- ture was calibrated in the standard DSC mode using the onset temperature of the melting-transition peak for indium at 429.75 K, and the heat-flow rate was pre-calibrated at a scanning rate of 10 K min^1 with the specific heat of fusion of indium of 28.62 J g^1 [29]. The melting temperature of indium was also measured in the quasi-isothermal TMDSC mode with a 0.5 K amplitude and 100 s period after calibration in the standard DSC mode, to identify any differences. It was found that quasi-isothermal TMDSC experiments after initial standard DSC calibration led to a melting temperature of 428.89 K. To correct the temperatures from the quasi- isothermal measurements, a constant of 0.86 K was added to the average temperatures of the quasi-isothermal measure- ments carried out at To. In all the experiments, standard aluminum pans of 20 mL with covers were used for the sample and the empty as refer- ence. A somewhat lighter reference pan was used for all mea- surements to approximately correct for the asymmetry of the
decreases somewhat relative to the more quickly cooled sam- ple, an indication that the originally poorer crystal of the cen- ter trace perfect more during heating than the slower-cooled sample of higher perfection shown in the bottom trace [23]. Fig. 1b provides a comparison of the measured Cp on heat- ing of the sample cooled at 10 K min^1 (center of Fig. 1a) with the expected heat capacities calculated from the ATHAS Data Bank [10], as evaluated in Ref. [11]. The melting transition seemingly starts very gradually and develops into the major peak starting at z470 K with the peak positioned at 535.3 K. The glass transition begins at the expected temperature of 323 K, with its half-height at 332.7 K. On completion, at 342 K, it reaches less than 50% of the value expected for a 28% crystalline sample, which indicates a rigid-amorphous fraction, RAF, of about 36%. Beyond this temperature, the heat capacity continues to increase and reaches the level of the liquid at z390 K (which is close to the melting point of polyethylene, as noted earlier). This is seen not only for nylon 6.6, but also for a series of other nylons (6, 11, 12, 6.9, 6.10, and 6.12 [11]). This increase in Cp beyond 342 K may arise from the Tg of a RAF, an increase of Cp of the crystal, or
possibly even from overlapping additional latent heats. The early speculative solution was that the glass transition of the RAF overlaps an increase in Cp of the crystal [11]. This issue of the thermodynamic heat capacity between Tg and Tm is taken up in this paper and advanced, based on the TMDSC results.
3.2. Temperature-modulated differential scanning calorimetry
The quasi-isothermal TMDSC runs, marked by D in Fig. 1c, give additional information about Cp and are com- pared to the center curve of Fig. 1b (solid line). An enlarged graph of the TMDSC result is shown in Fig. 1d by the points B. The two heating modes in Fig. 1c yield identical Cp s up to z480 K, excluding irreversible latent heat effects due to crys- tal perfection as a cause of the increase in Cp. The pre-melting shoulder at 515.6 K in standard DSC is clearly non-reversible, and there seems to be a small amount of reversing melting with a peak at 535.7 K. Beyond the melting transition, the Cp of the melt is reached in the TMDSC experiments at
Fig. 2. Analysis of nylon 6.6 by quasi-isothermal TMDSC on cooling, followed by heating. (a) Stepwise cooling to 278 K of a 12.967 mg sample after conditioning for 10 min at 563 K, followed by stepwise heating back to 563 K (20 min quasi-isothermal runs). (b) Long-time modulation experiments. Four separate samples between 9.2 and 9.8 mg were analyzed as in (a) by cooling and followed by heating up to the given temperatures, where then a 600 min quasi-isothermal run was commenced with the shown result. (c) Comparison of data on a sample of 12.496 mg, measured as in (a), with the data in (b) and the ATHAS Data Bank heat capacities. (d) Final analyses of the samples of (b) by standard DSC immediately after completion of the long-term quasi-isothermal experiments.
542.8 K, while the last reversing melting is seen at 537.8 K. Fig. 3c, below, shows that the heating from 537.8 to 542.8 K involves practically no further melting. This supports the sug- gestion that the temperature of the end of melting is set by the annealing history. On fast cooling followed by heating (Fig. 1a, center trace), the end of the melting peak occurs at z545 K, while after TMDSC on cooling followed by fast heating, it occurs at z538 K, i.e., the initially poorer crystals remain less-well annealed to higher temperature, and there, they are expected to anneal to even better crystals than the original better crystals. Note that the high-temperature leg of the DSC melting peak has an instrument lag of 1e2 K, and
that long-time annealing can produce melting peaks which recover the baseline above 550 K, as can be seen in Fig. 2d, discussed below. Still, all these effects cause a recovery of the baseline which is still far below the equilibrium melting temperature, Tmo^ ¼ 574 K. The quasi-isothermal TMDSC on cooling is displayed in Fig. 2a by the points C. After this TMDSC cooling, the sam- ple was heated by quasi-isothermal TMDSC, also displayed in Fig. 2a by the points B. The first noticeable changes from the Cp of the melt on cooling occur at 517.8 K, only little above the temperature of first deviation of the heat-flow rate on cool- ing at 10 K min^1 in the standard DSC (Fig. 1a, top curve).
Fig. 3. Reversing sample heat capacity, Cp , heat-flow rate, hFi, and modulated sample temperature, Ts. (a) TMDSC on cooling as in Fig. 2a (sample of 12.967 mg). (b) TMDSC after cooling the sample at 10 K min^1 , covering the melting peak area of Fig. 2c (sample of 12.496 mg). (c) TMDSC after cooling in the quasi- isothermal experiment of Fig. 2a and shown in (a).
Closer inspection reveals that the melting is always coupled with some exotherm (recrystallization). The faster-cooled sample (Fig. 3b) shows clearly more exothermic annealing and recrystallization than the slower-cooled sample (Fig. 3c). Coupled with the interpretation of the multiple melting peaks, these observations are a measure of the complication of continued melting, regrowth, and perfection in nylon.
- Final discussion
4.1. Heat capacity
The heat capacity below the reported glass transition of the amorphous nylon 6.6 at 323 K is described by the vibrational motion in the solid state [9], as is seen best in Fig. 1c. Simi- larly, the heat capacity above the melting is well represented by the general equation for liquid nylons:
Cnylonp ¼ NCð 7 : 4506 þ 0 : 0745 TÞþ NNð 86 : 8483 0 : 0226 TÞ ð 4 Þ
where NC represents the number of methylene groups (CH 2 e) in the repeating unit, and NN the number of imide groups (COeNHe) [11]. In homologous series of polymers, the heat capacity contri- butions to liquid chain segments with more H-atoms increase more with temperature because of the continuing excitation of the high-frequency CeH-, NeH-, and OeH-stretching vibra- tions. Since the normal modes of the stretching vibrations of the heavy atoms and the bending frequencies of heavy and light atoms are usually already excited at the higher tem- peratures, they each contribute a constant amount to Cp (¼ R ¼ 8.3143 J K mol^1 ). A decreasing Cp in the liquid state is possible in the absence of many H-atoms, because the tor- sional vibrations of the chain atoms change to hindered rotors, which decrease their contribution to Cp , ultimately reaching R/2 [23]. This general behavior easily explains the opposite signs in the two parts of Eq. (4). The Cp of the crystalline nylon 6.6 when approaching the Brill transition temperature, however, exceeds the Cp of the liquid, i.e., it must contain additional, reversible entropy con- tributions due to increasing disorder which occurs without a sharp phase transition and before reaching the pseudo- hexagonal crystal structure. (Note that the sample of Fig. 2c has only 28% crystallinity, making the excess heat capacity of the crystal three times as large as shown in the figure.) Be- yond the Brill transition, one would expect that this high level of entropy-caused Cp of the crystal should decrease to the level of the melt since no additional disorder is attained by the pseudo-hexagonal crystal. This discussion is to be continued in Section 4.3.
4.2. Glass transition
The glass transition temperature, Tg , shifts, when taken at the mid-point of the increase of Cp , from the 323 K of amor- phous nylon 6.6 to 332.7 K due to broadening of the transition, as is common in semicrystalline polymers [23]. In addition,
there is evidence of 36% RAF since at 342 K the heat capacity reaches less than 50% of the value expected for a sample of 28% crystallinity (see Fig. 1c and d). The quasi-isothermal TMDSC shows no evidence of irreversible melting up to about 440 K, but a continuous increase in reversible Cp to, first, the level of the liquid (at z390 K), and then to an even higher level with an excess Cp of about 60 J K^1 mol^1 at z440 K. This makes it likely that the RAF begins its glass transition immediately above 342 K. Before this glass transition is completed, at perhaps z380 K, the crystal itself becomes mobile with an upper-end temperature of its glass transition at z440 K, which is also the approximate position of the Brill temperature, originally suggested to be at 435 K [15]. A glass transition within the crystal [32], as mentioned in Section 1, would parallel the increase in segmental mobility of the methylene groups. The NMR studies prove such seg- mental motion in the CH 2 sequences [17e20]. Quasi-elastic neutron scattering studies showed, similarly, that at tempera- tures 40 K below melting, the CH 2 groups undergo already large-amplitude, liquid-like motion [21]. For nylon 6.6, the same is suggested by the gradual change in the crystal from triclinic to hexagonal structure [8,22] which in its small unit cell is not commensurate with the symmetry of the CH 2 groups unless the CH 2 groups average their position due to rotational motion [4]. Finally, the expansivity of the crystals as gained from time-resolved X-ray diffraction can be analyzed as is shown in Fig. 4, based on data by Starkweather and Jones [16]. The glass transition, taken at the temperature of the change in expansivity of the crystal, occurs at about 409 K. This temperature is also in agreement with the increase in ex- cess heat capacity starting just after the glass transition of the RAF at z370 K. For the sample analyzed in Fig. 2c, one, thus, can estimate the glass transition of the mobile-amorphous fraction to be at 333 K, that of the RAF at 370 K, and that of the crystal at 409 K. This is followed by the Brill transition and, finally, the melting peak at 546 K (see Fig. 2d). All these changes still occur far below the equilibrium melting temper- ature, estimated to be at 574 K [12e14].
Fig. 4. Density of nylon 6.6 as a function of temperature by X-ray diffraction, illustrating a possible glass transition at the point of the changing expansivity at 409 K [6].
4.3. Melting
The irreversible melting starts at about 480 K, as discerned from Fig. 1c, and its slow completion is obvious from Fig. 2b and c. The details of melting, recrystallization, and possible annealing are seen in Figs. 2d, 3b and c. At the higher temper- atures, an extensive melting followed by recrystallization is observed. This major recrystallization leads to the 4e11 K higher melting temperature observed in Fig. 2d, but even the observed lower melting temperatures are most likely increased considerably above their zero-entropy production value by crystal perfection during heating in the DSC experiment. (The zero-entropy production melting occurs at a temperature where the metastability of the initial crystal is equal to that of the supercooled melt [23].) A continuous increase in the melt- ing peak of 70 K was proven in the past for nylon 6, which is an isomer of nylon 6.6 [33]. The crystal morphology of the ny- lon 6 was fixed at the various stages of perfection by arresting the reorganization with chemical cross-linking. Quite similar values are expected for the nylon 6.6. At the temperature of the reversing, apparent Cp of nylon 6.6 in Fig. 2c, at 535.7 K, more than half of the excess revers- ing contribution to Cp is from slow, irreversible latent heats, which have decayed after 600 min. This leads to the final question addressed: is there any remaining reversible melting after the decay of the irreversible processes? Certainly, if one assumes the Cp (N) (points D in Fig. 2c) is due to a con- stant excess heat capacity, there would be no significant reversible melting. The analysis of the TMDSC on cooling with Figs. 2a and 3a, however, suggests that by 510 K the increase in crystallinity is largely completed. From X-ray dif- fraction data taken after cooling from the melt, it was shown that the initially growing crystals are pseudo-hexagonal [22]. Fig. 3a illustrates that after the exothermic crystallization is completed at 510 K, the measured reversible Cp settles again close to the level of the liquid Cp , rather than the level mea- sured on heating, shown in Fig. 2a. At about 440 K, in the vi- cinity of the Brill temperature, as the triclinic crystal structure is re-established, the heat capacity increases to that measured on heating, and then remains at lower temperatures identical to the data measured on heating. This interpretation leaves room for assignment of the difference between reversible heat ca- pacity measured on heating and cooling to the frequently seen reversible melting [34].
4.4. Connection to other polymers
Nylon 6, as an isomer of nylon 6.6, has a polar structure along the molecules and shows two major polymorphs on crystallization from the melt which lead to multiple melting peaks with the monoclinic a-phase with anti-parallel chains in the H-bonded planes being the highest-melting crystals [5,7,8,35]. The nylon 6 does not show a distinct Brill transition to a high-temperature pseudo-hexagonal crystal phase, mainly because of its 40 K lower melting temperature than nylon 6. when crystallized from the melt (z494 K), but a diffuse transition was observed close to its melting point [36]. The
standard DSC curve on heating after cooling from the melt (both at 10 K min^1 ) between the glass transition (z322 K) and melting peak (z492 K) of nylon 6 also is similar to nylon 6.6 within its increase in Cp beyond the calculated semicrystal- line Cp (z333 K) and liquid Cp (z367 K) into major melting, beginning at z475 K [11]. These observations are indicative of similar interpretations for nylon 6 as are given here for nylon 6.6. The large-amplitude motion was also proven by quasi-elastic neutron scattering [21], and, since other nylons show this large-amplitude motion as well [8], one must assume that all aliphatic nylons with sufficiently long CH 2 sequences lose the solid nature through a glass transition below the melt- ing point, best measured by the change of Cp [32]. Quasi- isothermal TMDSC [37] and standard DSC [11,38] are also available for nylon 12, again with quite similar increases in the excess heat capacity before irreversible melting with a peak temperature of about 452 K. Other polymer crystals with a glass transition below the melting temperature are the aliphatic polyethers, with detailed heat capacity data by TMDSC available for poly(oxyethylene) [26] and poly(oxytetramethylene) [37,39]. Most likely poly- esters, polyurethanes, and polymers with other functional groups and sufficiently long CH 2 sequences and higher melt- ing temperatures show also separate glass transitions below the melting transitions. Similarly, all mesophases, described as liquid crystals, plastic crystals, and condis crystals, have their isotropization transition (order/disorder transitions) above the devitrification from the solid state to the mesophase [27,28]. Mesophase glass transitions have been observed for small and large molecules, attesting for the universality of the need of a glass transition when changing from the solid to a more mobile state. The novel aspect for the aliphatic ny- lons and polyethers is the possibility to have large-amplitude motion before changing the crystal structure from the densest packing to a mesophase of higher symmetry and larger vol- ume. Even in polyethylene crystals, one could detect the start of large-amplitude motion in the form of transegauche flips by calorimetry, starting at z300 K [24,25,40], and reaching the heat capacity of the liquid at about 315 K [41]. Finally, a number of TMDSC experiments were also per- formed on nylon 6. Using the frequency dependence of the heat-flow rates, the irreversible, exothermic contribution could be subtracted in the melting range, and the remaining revers- ible melting was described by a ColeeCole plot with a single relaxation time of z7 s [42] (for a discussion of these results see Ref. [34]). The rigid-amorphous fraction was recently studied by TMDSC and X-ray diffraction for nylon 6 samples of different thermal history [43]. Naturally, it would be of great interest to separate the possible glass transition of the crystal from that of the RAF and check the detailed reversing and reversible behavior of nylon 6 and several other nylons of larger and shorter CH 2 sequence length.
- Conclusions
The irreversible, reversing, and reversible thermodynamics of semicrystalline nylon 6.6 were studied with TMDSC and
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