The Phase Structure of Nylon in Polymer Techniques I | PSC 341, Lab Reports of Chemistry

Download The Phase Structure of Nylon in Polymer Techniques I | PSC 341 and more Lab Reports Chemistry in PDF only on Docsity! Polymer 48 (2007) 1641e1650 www.elsevier.com/locate/polymerThe 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 (Tg¼ 323 K), a broadened transition of the semicrystalline sample is observed which reaches to 342 K (Tg¼ 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 CH2 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; Melting1. 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 * ‘‘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.2007.01.024textiles 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 (CH2)4eCOeNHe(CH 2)6e]. 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 1642 W. Qiu et al. / Polymer 48 (2007) 1641e1650at 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 a¼ 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, Tm o, 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 mol1 (or 255.4 J g1) 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 CH2 groups in the crystal undergo already large-amplitude, liquid-like motion [21]. The effect of this large-amplitude motion of the CH2 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].2. 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 d¼ 1.30 mg m3 (Lot 20, Cat. 033). At the glass transition temperature, the heat capacity, Cp, of the amorphous, glassy polymer increases by 115.5 J K1 mol1 [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 min1 is the top DSC trace. This is followed by a sec- ond heating at 10 K min1. 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 N2 gas with a flow rate of 25 mL min1 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 min1 with the specific heat of fusion of indium of 28.62 J g1 [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 1645W. Qiu et al. / Polymer 48 (2007) 1641e1650Fig. 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 min1, 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).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, andthat 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, Tm o 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 min1 in the standard DSC (Fig. 1a, top curve). 1646 W. Qiu et al. / Polymer 48 (2007) 1641e1650The reversing Cp on cooling matches the TMDSC on heating below 447.8 K, the temperature of the completion of crystalli- zation by standard DSC (in Fig. 1a). Further work is needed to assess the type of crystals growing and their perfection between 517.8 and 447.8 K. 3.3. Temperature-modulated differential scanning calorimetry as a function of time The changes of the reversing Cp with time on heating after cooling at 10 K min1 is analyzed in Fig. 2b with four separate experiments of long-time annealing, extending to 600 min, chosen at strategic temperatures. The changes are compared in Fig. 2c to the 20-min modulation of Fig. 1d. It can be seen from the open and filled circles that the 20 min modula- tion experiments are very reproducible, and below 477.7 K only small changes occur on longer modulation, supporting the interpretation of Fig. 1c, given above. The activation energies in Fig. 2b for the double-exponen- tial representations change neither systematically nor to a large degree. A systematic increase with temperature, however, oc- curs in the pre-exponential factors. This leads to a larger re- versible Cp at time infinity than expected [Cp(N) is marked as D in Fig. 2c, the expected Cp as ,]. At 437.7 K the differ- ence between the two Cps reaches a maximum value of z60 J K1 mol1. At the reversing melting peak, this differ- ence in Cp has decreased only slightly to z50 J K 1 mol1. Fig. 2d, finally, proves that at the ends of all four long-term modulations, the crystallinity has changed only little. The mul- tiple melting peaks in Fig. 2d mirror the degree of reorganiza- tion. Of particular interest is the sharp melting of the 29.2% crystallinity at 545.1 K after modulation at 535.7 K, which is shown in Fig. 3b to be due to melting followed by recrystalli- zation. Note, that these crystals grew above the melting peak temperature seen by the standard DSC in Fig. 1a. The long-time reversing modulated F at the four tempera- tures of Fig. 2bed allows also to identify the irreversible ex- cess in Cp as an exotherm or endotherm. Comparing the maxima and minima in F at short and long times shows little change at 387.7 K. A small excess in the exotherms and a de- crease in the endotherms are noticeable at 437.7 and 477.7 K, indicating that the crystals which perfect during one cycle do not melt in the next. At 535.7 K, finally, both endotherms and exotherms are larger during the first 40 min, indicating the dynamics of melting and recrystallization. Similarly, the much larger initial irreversible melting is seen in the first few cycles, but is better seen in the recording of hFi in Fig. 3, discussed next. Fig. 3 illustrates the details in the main transition peaks by a recording of the reversing Cp of the sample as the top curves. The modulation of the sample temperature is drawn as the bot- tom curves. The center curves, finally, are plots of the total heat-flow rates of the sample, hFi, which represent the sliding average of the measured heat-flow rate over one modulation period. In case of a (metastable) steady state, the reversing Cp of the top curves should register the reversible, apparentheat capacity, which in the transition regions may contain reversible latent heat effects: dH ¼  vH vT  p;n dT þ  vH vn  p;T dn; ð3Þ where dn indicates the change in composition. For crystalliza- tion and melting n can be replaced by the crystallinity, wc, as given in Eq. (2). Since with scanning calorimetry the change in n cannot be fully controlled, one usually measures an apparent heat capacity (¼ dH/dT ) and the latent heat contribution must be computed by subtracting the appropriate Cp (¼ vH/vT )p,n. In steady state, the recording of hFi should also reach a con- stant level, its deviation from zero being determined by the instrument asymmetry at the given temperature. Immediately after setting the lower or higher modulation temperature, To, hFi registers a sharp exotherm or endotherm due to the respec- tive heat flow needed to cool or heat the sample. This is fol- lowed by a recovery of the stationarity which may produce a small peak in the opposite direction. A short time thereafter hFi should be constant, unless a slow, irreversible process is superimposed. The TMDSC on cooling, as summarized by the data points C in Fig. 2a, is analyzed in Fig. 3a. The TMDSC on heating of the two differently crystallized samples, summarized in Fig. 2c and a by the data points B, is illustrated in Fig. 3b and c, respectively. Fig. 3a shows data on cooling from the melt. A constant hFi is reached down to z525 K within the 20 min of modu- lation and yields the expected Cp of the melt (compare to Fig. 2a). This is followed with an increasing hFi with time, reaching a peak at 514 K. The exothermic hFi is due mainly to the irreversible latent heat of crystallization. This crystalli- zation continues to z510 K, beyond which a close to constant hFi and Cp with time and a linear change in (vCp/vT ) is observed. According to Fig. 2a, this decrease causes the heat capacity to reach again the level of the liquid. At about 480 K, Cp reverses its trend and develops a minor maximum and then follows from 447.8 K the observed Cp on heating (see Fig. 2a). The corresponding hFi involves an exotherm decreasing with time which does not reach constancy after 20 min. The reversing Cps in Fig. 3b and c show a decrease with time of the irreversible component of the heat capacities in the melting range which is not completed at the end of the shown time-interval of 20 min. Both samples are melted at 542.8 K where Cp becomes constant after about 5 min. The initial, sharply endothermic hFi and the following exothermic approach to stationarity are caused by the quick heating to the new To and can be assessed by comparison to the last two experiments in the melt which should show only true heat- capacity effects. The sample cooled at 10 K min1 (Fig. 3b) has a somewhat higher exotherm in the first three experiments than in the reference melt, indicating an annealing of the sam- ple even during the brief initial heating period. Starting at 527.7 K an increasingly larger endotherm due to slower partial melting is superimposed in both samples (Fig. 3b and c). 1647W. Qiu et al. / Polymer 48 (2007) 1641e1650Closer 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. 4. 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 ¼NCð7:4506þ0:0745TÞþNNð86:84830:0226TÞ ð4Þ where NC represents the number of methylene groups (CH2e) 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 R¼ 8.3143 J K mol1). 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 mol1 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 CH2 sequences [17e20]. Quasi-elastic neutron scattering studies showed, similarly, that at tempera- tures 40 K below melting, the CH2 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 CH2 groups unless the CH2 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].