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Microstructure-thermal property relationship of high trans-1,4-poly (butadiene) produced by anionic polymerization of 1,3-butadiene using an initiator composed of alkyl aluminum, n-butyl lithium, and barium alkoxide.

INTRODUCTION

Poly (butadiene)-Containing polymers are widely used to improve the viscoelastic properties of composed materials such as tires (1-3), engineering plastics (4), adhesives (5), (6), and modified asphalt (7). The poly (butadiene) microstructure (i.e., the relative number of its isomers: 1,4-cis, 1,4-trans, and 1,2-vinyl) has an important effect on both the properties of the polymers containing poly (butadiene) (8-10) and the interaction of such type of polymers with other constituents of composed materials (7), (9), (11). Depending on its 1,4-trans content, poly (butadiene) behaves like an elastomer (40-80%) or as a thermoplastic resin (> 90%) (12). It has been shown (13-15) that polybutadienes with a high content of 1,4-trans units have a considerable degree of crystallinity and experience a strain-induced crystallization. Consequently, they are able to improve tensile strength, tear strength, and flex fatigue, which is necessary for the production of high-performance tires. This explains the importance of the production of polymers containing poly (butadiene) blocks with a welldefined microstructure, such as those with a relatively high content of 1,4-trans units (1-3), (11-25). Polybutadienes with more than 70% of the 1,4-trans isomer are generally called trans-1,4-poly (butadiene) and referred to as TPBD, a denomination that is adopted in this article as it deals with the production of this kind of polymer.

Studies on the production of TPDB with Ziegler-Natta type catalysts of Ni, Co, or V (26) pointed out that such a system experiences important disadvantages; for instance, catalysts are insoluble and polymers have a wide molecular weight distribution, which results from the polymer ramification that grows along with the degree of polymerization.

Also, these polymers have low mechanical resistance and are difficult to process. Another for preparing TPDB uses rare earth-based catalysts, such as acid organic salts of the lanthanide series (e.g., versate of di-dymium: a mixture composed of 72% Nd, 20% La, and 8% Pr) (23), (25). These systems allow poly (butadiene) with high stereospecifity (>90% of 1,4-trans units) to be obtained; however, they have low yield (40-50%) (23), (25). Both Ziegler-Natta and rare earth-based catalytic systems are inferior to anionic polymerization for synthesizing block copolymers as the polymer composition includes a significant amount of homopolymers (26). Copolymerization of conjugated dienes by means of coordination catalysts is generally difficult because of the strong selectivity of these catalysts toward monomers (27). There are reports on the synthesis of TPDB via anionic solution polymerization using organometallic com-pounds based on groups IA, IIA, and MA of the periodic table (16-22), (28), (29). However, these catalytic complexes are usually difficult to synthesize and present a low activity for diene polymerization, like those prepared with group IIA metals (19). On the other hand, initiator barium-based complexes face relatively complicated preparation procedures, poor solubility in nonpolar hydrocarbons solvents, and low activity; furthermore, in some cases, low temperatures are required (less than 20 [degrees] C) to obtain more than 80% of 1,4-trans units; therefore, such systems arc excluded for industrial purposes (17), (28).

Anionic systems with potassium-based complexes are able to produce TPDB with 90-95% of 1,4-trans units and with acceptable activity, yet they do not hold a "living" character (21), (22), and the polymer is a heterogeneous mixture because there is always a number of in-soluble TPDB (21), (22). On the other hand, it is interesting to note that most of the literature on the synthesis of TPDB via anionic solution is patented. Initiator barium-based complexes face relatively complicated preparation procedures (28), poor solubility in nonpolar hydrocarbon solvents, and low activity (17).

Therefore, it was considered timely to study the synthesis of TPDB via anionic polymerization and their characterization to gather information to determine the effect of the number of 1,4-trans units on the properties of this kind of polymer.

Information regarding anionic polymerization of butadiene with Li/Al/Ba (16), (17), (20), (28) was used to set up an experimental approach to find polymerization conditions for producing high trans-l,4-poly(butadiene).

The synthesis of TPDB was pursued by examining the polymerization of 1,3-butadiene using cyclohexane solutions of trioctyl aluminum [Al[([C.sub.8][H.sub.17]).sub.3]], n-butyl lithium (n-([C.sub.4] [H.sub.9])Li), and barium 2-ethyI-hexoxide [(RO).sub.2] Ba. The initiator system was chosen depending on its solubility in nonpolar hydrocarbon solvents (cyclohexane), reproducibility, and commercial availability; barium alkoxide, combined with n-butyl lithium, and trioctyl aluminum, fits all these requirements. An important advantage of a system based on alkaline earth metals, such as barium, is that it is able to form organometallic electron-accepting complexing species, thus increasing the possibilities for controlling the activity as well as the stereo-selectivity of the anionic polymerization of dienes. A series of TPBD with well-controlled microstructure and molecular weight were obtained, adjusting the relative amount of [Al([C.sub.8] [H.sub.17]).sub.3], n-([C.sub.4] [H.sub.9])Li, and [(RO).sub.2] Ba. Polymers were analyzed by nuclear magnetic resonance spectroscopy ([.sup.13] C NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and X-ray diffraction (WAXS) to determine the microstructure and thermal properties of TPBD.

EXPERIMENTAL

In the following paragraphs, a brief description of the materials. the polymerization process, and the characterization of polymers arc given.

Reagents

Prior to their use, both butadiene and cyclohexane were submitted to a purification process: first, they were passed through a series of two-packed columns, one with activated [gamma]-alumina and the other with molecular sieves of 3 [Angstrom], to reduce the butadiene inhibitor (tert-hutyl cathccol) and humidity; then, they were stored in separated stainless steel tanks under an ultra-high nitrogen atmosphere. Initiator components: w-butyl lithium (Lithco), alkyl aluminum (Aldrich), and barium alkoxide (Strem Chemicals) were used as received.

Polymerization

Polymerization reactions were carried out in a 1-L glass-jacketed reactor equipped with a glass jacket and an internal stainless steel coil for heat exchange. Mixture composition (basically solvent-monomer ratio) and a system of temperature detector and heat-exchanger bath allowed the control of the reactor temperature within [approximately equal to] 1 [degrees] C. The polymer molecular weight was controlled by minimizing the number of initiator-scavenger substances, according to the following procedure: the reactor was twice purged with ultra-high-purity nitrogen to lower the in-gas initiator deactivating substances and to maintain an inert gas atmosphere; the desired amount of solvent was then fed to the reactor and treated with w-butyl lithium according to a colorimetric titration process (30) to minimize residues of initiator scavengers. Afterward, the desired amount of butadiene was fed to the reactor, and the titration procedure was then applied again to eliminate the remains of initiator scavengers; finally, the components of the catalytic system were added in the following order: alkyl of aluminum, w-butyl lithium, and alkoxide of barium. Polymerizations were carried out at a temperature within 40-80 [degrees] C using a 10/1 v/v solvent/monomer ratio, which enabled control of polymerization temperature. Samples were taken at different reaction times; then, they were deactivated using a 2 M methanol/cyalohexane solutions and protected from thermal degradation with 2-6-di-tert-buthyl-p-cresol (BHT); finally, samples were dried in a vacuum stove at 40[degrees]C overnight for further characterization.

Characterization

[.sup.13] C Nuclear Magnetic Resonance. The poly (butadiene) microstructure--the percentage of the 1, 4-trans, and 1, 2-vinyl isomers of poly (butadiene)--was determined by using [.sup.13] C NMR spectroscopy. Deuterated chloroform polymer solutions (ca. 0.05 g/mL) were analyzed with a Varian Unity Inova 300 MHz spectrometer using a TMS internal standard. Data were collected with a spectral width of 23.101 kHz (305.10 ppm), applying pulses of 1.815 s and 5596 scanning. Polymer microstructure was calculated from the characteristic signals of its (13) C NMR spectrum and the application of equations reported by other authors (31), (32).

Gel Permeation Chromatography. Molecular weights of the polymers were obtained from GPC analysis of tetrahydrofuran solutions of polymer samples and standards of polystyrene (ca. 0.012 g/mL); therefore, the molecular weights reported here are polystyrene-equivalent molecular weights. A HP 1100 liquid chromatograph equipped with a high-resolution PLgel 5[micro] mixed-C column and differential refractive index detector was used for polymer analysis; it was operated under isothermal conditions (35[degrees]C) using THF as carrier (1.0 mL/min).

Differential Scanning Calorimetry. Thermal analysis of previously dried polymer samples (ca. 10 mg) was carried out on a Mettler Toledo 2000 differential scanning calorimeter calibrated with a high-purity indium standard ([T.sub.m.sup.o] = 156.6[degrees]C and [DELTA][H.sub.f.sup.o] = 28.5 J/g). A series of heating/cooling cycles with a heat exchange rate of 10 [degrees]C/min under a nitrogen atmosphere were applied to the sample was divided for two consecutive heating/cooling cycles: -130[degrees]C to 130[degrees]C; results from the second heating/cooling cycle are being reported.

Wide-Angle X-ray Diffraction Previously Molded. X-ray diffraction analysis was performed on polymer films prepared by pressing these at 50 psi and 100[degrees]C for 5 min and slowly cooling down to room temperature. A Philips PW1700 X-ray diffractometer equipped with a Cu K[alpha] radiation source was operated at 40 kV and 30 mA in the reflection mode. The beam was monochromatized with a graphite crystal and the wave length fixed at 0.154 nm. Slit collimation of the primary and diffraction beams were used. Measurements were carried out at room temperature and within the diffraction angle range of [theta] = 10 [degrees] -35[degrees]. The diffraction scans were recorded in the reflection mode using a scintillation counter detector.

RESULTS AND DISCUSSION

Results of the polymerization of butadiene in cyclohexane A1/Li/Ba solutions and characterization of the polymers by means of [.sup.13] C NMR, GPC, DSC, and WAXS are hereby presented and discussed.

Butadiene Polymerization

To be used as reference, an ordinary poly(butadiene) PBDN with medium content of 1,4-trans units (ca. 50%) was synthesized through standard procedure using cyclohexane/n-buty1 lithium solution (8). Reaction conditions to produce poly(butadiene) with a high content of 1,4-trans units (> 70%) TPBD) were found by systematically changing both the relative number of initiator components: alkyl aluminum, n-buty1 lithium, and alkoxide of barium and polymerization temperature. Figure 1 shows typical [.sup.13]C NMR spectra of TPBDs (TPDB_6) from which their microstructure was established. Table 1 presents the characteristic signals of the [.sup.13]C NMR spectra of poly(butadiene). Conversely, Table 2 contains all signals of the [.sup.13]C NMR spectra in the aliphatic region observed in these [.sup.13]CNMR spectra. With these signals, the following equations were used to calculate polybutadiene microstructure (32).

[TABLE 1 OMITTED]
TABLE 2. Signals of the [.sup.13]C NMR spectra in the aliphatic region
used to calculate the microstructure of high 1, 4-trans
polybutadienes (32).

Signal Position, ppm

[Is.sub.a] 26.9
[Is.sub.1] 24.98-25.10
[Is.sub.2] 27.42-27.57
[Is.sub.3] 30.16
[Is.sub.4] 31.60-32.13
[Is.sub.5] 32.67
[Is.sub.6] 32.72
[Is.sub.7] 33.35-33.53
[Is.sub.8] 33.99-34.16
[Is.sub.9] 34.31
[Is.sub.10] 35.63-36.00
[Is.sub.11] 37.24-37.48
[Is.sub.12] 38.18
[Is.sub.13] 38.57-39.13
[Is.sub.14] 38.96-39.13
[Is.sub.15] 39.43-41.72
[Is.sub.16] 40.55-41.00
[Is.sub.17] 41.00-41.66
[Is.sub.18] 40.55
[Is.sub.19] 40.75
[Is.sub.20] 41.64
[Is.sub.21] 42.45
[Is.sub.22] 43.47-43.70


[FIGURE 1 OMITTED]

relative number of moles of 1, 2-vinyl = 0.5([i = 10.summation over (i = 8)]I[s.sub.i]I[s.sub.13] + [i = 22.summation over (i = 15)]I[s.sub.i] (1)

relative number of moles of 1, 4-trans = 0.5(I[s.sub.a] + [i = 7.summation over (i = 3)]I[s.sub.i] + I[s.sub.11] + I[s.sub.12] + I[s.sub.14] - I[s.sub.1]) (2)

relative number of moles of 1, 4-cis = 050(2I[s.sub.1] + I[s.sub.a] + I[s.sub.2]) (3)

Table 3 summarizes the initiators composition and the microstructure of the TPBD produced with each one of them. Rows 2-9 correspond to a series of experiments in which the composition of the initiator was systematically varied by just increasing the molar amount of Ba (from 0.10 to 0.67), while keeping constant the Al:Li molar ratio (1:1). It is apparent that, by increasing the amount of Ba (0.10-0.25), the number of 1,4-trans units increased (80-90%), and then a further increase in Ba (0.40-0.67) yielded a decrease in the number of 1,4-trans units (7264%). Thus, there is a specific composition for the initiator that produces a TPBD with maximum number of 1,4-trans units. Furthermore, cyclohexane Al:Li:Ba solutions with Ba molar content higher than 0.67 were unable to polymerize butadiene for the experimental conditions investigated (i.e., 40-80[degrees]C for 2 h). On the other hand. lines 10 to 16 show the microstructure of TPBD produced with constant Li:Ba molar ratio (4:1) and different amounts of the Al:Li molar ratio (0.5:1-2:1). It was observed that a Al:Li:Ba molar ratio of 0.25:1:0.25 produced a poly(butadiene) (TPBD_1AL) with a microstructure similar to that of the PBDN. By increasing the molar amount of aluminum, from a Al:Li:Ba molar ratio of 0.5:1:0.25 up to 1.5:1:0.25, an increase in the number of L4-trans units was observed (61-90%). However, further increase in the aluminum content produced a decrease in the poly(butadiene) 1,4-trans unit content; and, finally, with a Al:Li:Ba molar ratio of 2:1:0.25, no polymerization was observed (TPBD_6AL: last line of Table 3). It is convenient to indicate that in all cases the number of 1,2-vinyl units was low (3-6%) in comparison with that of the poly(butadiene) produced with solely /7-butyl lithium (11%, Table 3, PBDN). These results demonstrate that. In adjusting the molar composition of a ternary initiator prepared with alkyl aluminum, n-butyl lithium, and barium alkoxide, it is possible to polymerize butadiene to produce poly(butadiene) with a high number of 1,4-trans units.
TABLE 3. Initiator composition (molar ratios: Li:Al; Li:Ba; and
Al:Li:Ba) and microstructure (i.e., percentage of 1,4-trans, 1,2-vinyl,
and 1,4-cis units) of the poly(butadiene) produced with each particular
initiator at 60[degrees]C.

 Relation molar Microstructure (%)

Polymer Al:Li:Ba 1,4-Trans 1,2-Vinyl 1,4-Cis

PBDN 1:1:0.00 56 11 36
TPBD_8 1:1:0.10 80 6 14
TPBD_7 1:1:0.125 82 5 13
TPBD_2 1:1:0.17 83 5 12
TPBD_6 1:1:0.2 86 4 10
TPBD_1 1:1:0.25 90 3 7
TPBD_3 1:1:0.40 72 6 22
TPBD_5 1:1:0.50 70 9 21
TPBD_4 1:1:0.67 64 9 27
TPBD_1AL 0.25:1:0.25 61 8 31
TPBD_2AL 0.5:1:0.25 68 11 21
TPBD_3AL 0.6:1:0.25 73 6 21
TPBD_4AL 0.75:1:0.25 78 4 18
TPBD_1 1:1:0.25 90 3 7
TPBD_5AL 1.5:1:0.25 82 4 14
TPBD_6AL 2:1:0.25 - - -

PBDN was produced with n-butyl lithium at 60[degrees]C.


An interesting feature of Al/Li/Ba ternary initiator is the dependence of polymer structure on the concentration of the initiator. A higher barium molar concentration in the composition of the initiator or diminution in aluminum molar concentration, results in a gradual decrease of 1,4-trans units content, which is due to the dissociation of the complex Al/Li/Ba with the separation of the free bar- ium alkyls and free organolithiums living chains (33). These are known as cis-regulating initiators for the polymerization of butadiene and for the production of polybutadience with 55% 1,4-trans, 35% 1,4-cis and, 10% 1,2-vinyl microstructure, respectively (29), (33).

On the other hand, using higher aluminum molar concentration in the composition of the initiator, the aluminum alkyls inhibit polymerization of butadiene transforming growing chains into inactive complexes, which can be explained by the formation of LiAL[R.sub.4] complex known to be inactive in the polymerization of polybutadiene (34).

Evidently, an experimental approach was applied to find a set of conditions to produce TPBD through the polymerization of butadiene in a cyclohexane Li/Ba/Al solution because the chemistry involved in such a system is not specifically known. Even for the simpler system with sole organolithium initiator (n-or sec-butyl lithium), there are no precise indications neither on the characteristics of the active sites nor on the manner by which a molecule of butadiene is added to the polymer chain to produce the three feasible isomers (1,4-cis, or 1,2-vinyl) (8), (9). Organometallic systems containing two or more metals of the I-III groups exhibit interesting catalytic properties, although the arrangement of the metals in such complexes is not well known (33), (35). In early works (33), (34), most of the authors considered this type of compounds as "ate" complexes in which the complex anion is associated with the most electropositive metal (36). However, there are reports (33), (37) explaining that the complex active centers for anionic polymerization of dienes are complexes in which the metal atoms are linked by and alkyl bridge, and that the addition of a monomer occurs by an insertion either into the "usual" metal-carbon bond or directly into the M[t.sup.1]-R-M[t.sup.2] bridge.

Propagation Rate

To get an idea on the propagation rate of the polymerization of 1,3-butadiene with A1/Li/Ba/cyclohexane solutions a series of polymerizations was carried out by keeping constant the catalyst composition (1:1:0.25 of Al: Li: Ba molar ratio) and changing the polymerization temperature (40-80[degrees]C). Polymer samples undergoing different reaction times were analyzed to determine monomer conversion (gravimetric method), microstructure ([.sup.13] C NMR), and molecular weight (GPC) as a function of polymerization time, It was observed that temperature has an important effect on propagation rate, as it is shown in Fig. 2; a butadiene conversion of 90% was reached in 1h for a polymerization temperature of 80[degrees]C, whereas 2 h was required to reach the same conversion when the temperature was 60[degrees]C.

[FIGURE 2 OMITTED]

[.sup.13] C NMR results indicated that, within the investigated conditions, the microstructure of TPBD was not a function of reaction time and thus of polymer size, as has been reported for ordinary anionic polymerization of butadiene with n-buty1 lithium/cyclohexane solutions (8,) (9). Figure 3 shows that molecular weight is a linear function of butadiene conversion, while Table 4 shows that all TPBD had a fairly narrow molecular weight distribution (1.12-1.21). These results suggest that the reaction rate of the initiation process is substantially faster than that of the propagation process, as has been reported for anionic polymerization of butadiene with low concentration of n-butyl lithium/cyclohexane solutions (9), (38), (39) Therefore, it is assumed that the polymerization of butadiene in Al:Li: Ba/cyclohexane solutions exhibits characteristics of a living anionic polymerization (8), (9), (38) and that the active site of this system is an organometallic compound that engages these three metals, because by regulating their relative amount, both molecular weight and percentage of 1,4-trans units of the poly(butadiene) can be controlled. Similar results on the microstructure control have been reported with catalysts based on magnesium, aluminum, and barium, which have elaborate synthesis and reproducibility (19).

[FIGURE 3 OMITTED]
TABLE 4. Number-average molecular weight [M.sub.n] and polydispersity
[M.sub.w]/[M.sub.n] of polybutadienes synthesized with cyclohexane
solution having 1:1:0.25 Al:Li:Ba molar ratio, at 60[degrees]C.

Polymer [M.sub.n] x [10.sup.-3] [M.sub.w]/[M.sub.n]

TPBD_7 144 1.15
TPBD_6 133 1.11
TPBD_2 113 1.11
TPBD_1 97 1.12
TPBD_1 106 1.12
TPBD_3 81 1.21
TPBD_5 73 1.24
TPBD_4 68 1.27
TPBD_4AL 86 1.17
TPBD_3AL 113 1.11
TPBD_2AL 88 1.07
TPBD_1AL 75 1.24
TPBD_5AL 114 1.17


Thermal Analysis

Results of DSC analysis of the polymers are summarized in Fig. 4 and Table 5. As expected, the PBDN produced with just n-butyl lithium has an isomeric composition of 53% of 1, 4-trans, 36% of 1, 4-cis, and 11% of 1, 2-vinyls and displayed a thermogram typical of an amorphous poly(butadiene), with a [T.sub.g] of -85[degrees]C. In contrast, results for the series of TPBDs indicated that the increase in number of 1, 4-trans units turned out to be an increase of both glass transition temperature ([T.sub.g]) and crystalline character of the TPBD as indicated by the presence of transition (T.sub.1]) and, in some cases, the fusion ([T.sub.2]) signals. For example, those TPBDs having a percentage of 1,4-trans of units between 60 and 70% (TPBD_3, TPBD_4, TPBD_5) showed only on endothermic peak within the range of -4 to 30[degrees]C. Then again, the TPBDs with more than 80% of 1,4-trans units TPBD_1, TPBD_2,TPBD_6-TPBD_8, TPBD-4AL) exhibited polymorphism, because they showed two endotherms, one at low temperature ([T.sub.1]) at 32-53[degrees]C and the other at high temperature ([T.sub.2]) at 50-76[degrees]C, as well as [T.sub.g] between -70 and -76[degrees]C. TPBDs with a larger number of 1,4-trans units displayed two endotherms at higher temperatures and were clearly separated from each other, revealing that such polymers are able to crystallize in more than one form. It has been reported (40), (41) that the TPBD exhibits two crystalline forms: monoclinic (a low temperature < 76[degrees]C) and hexagonal (a high temperature > 76[degrees]C). For example, TPBD with 100% of 1,4-trans units exhibits transitions at 76 and 144[degrees]C (41).

[FIGURE 4 OMITTED]
TABLE 5. Transition [T.sub.1], fusion [T.sub.2] and glass transition
[T.sub.g] temperatures of polybutadiens produced with initiators
containing different Al:Li:Ba molar ratios at 60 [degrees] C as
determined by differential scanning calorimetry.

Polymer [T.sub.1] [T.sub.2] [T.sub.g]
 ([degrees] C) ([degrees] C) ([degrees] C)

PBDN -- -- -90
TPBD_1 53 76 --
TPBD_6 48 69 --
TPBD_2 44 64 --
TPBD_7 44 63 -72
TPBD_8 42 60 -74
TPBD_4AL 32 50 -76
TPBD_3 30 -- -78
TPBD_5 7 -- -80
TPBD_4 -4 -- -87


Moreover, it has been explained that the high-temperature endotherm is related to the fusion of the mesophase and that the temperature for the transformation of a given crystalline form into another depends on the crystal's lamellar thickness (42-45), and that such transformation is thermodynamically reversible process that can be completed in a very short time, although with a great amount of energy. Thus, polymers having such a characteristic could be used as energy deposits (43). Based on the reports, it is considered that, the TPBD investigated at the low-temperature endotherm (30-53[degrees]C) corresponds to a crystal-crystal transition of the first order that represents transformation from a monoclinic to a hexagonal phase, whereas the high-temperature endotherm (50-76[degrees]C) corresponds to the fusion process of the mesophase.

Figure 5 presents the values of [T.sub.1] and [T.sub.2] as a function of the number of 1,4-trans units for a series of TPBD; it was observed that the capability of the polymer to show polymorphism was proportional to the number of trans units, as, for example, TPBD_1 and TPBD_3. According to previous explanations, some of these TPBDs are able to crystallize in monoclinic form and, in some cases (1,4-trans more than 80%), also in a hexagonal form, as indicated by [T.sub.1] and [T.sub.2].

[FIGURE 5 OMITTED]

It was also observed that, as the content of 1,4-trans units diminishes, the polymer loses its capacity to show polymorphism, and that poly (butadiene) with a number of 1,4-trans units smaller than 70% are unable to crystallize.

It has been reported (43), (44) that TPBDs have a lamellar thickness directly related to their semicrystalline character. To get an idea of the lamellar thickness of the monoclinic form of the TPBD investigated, the Thompson-Gibbs equation was applied (46):

L = 2[[sigma].sub.e] [T.sub.tr.sup.o]/([DELTA][H.sub.tr.sup.o] ([T.sub.tr.sup.o] - [T.sub.tr])) (4)

where L is the lamellar thickness: [[sigma].sub.e], [T.sub.tr.sup.0], and [DELTA][H.sub.tr.sup.0]are characteristics of a TPBD with 100% of 1,4-trans units (41): [[sigma].sub.e] is the surface free energy of the lamellar surfaces (19 x [10.sup.7] J/c[m.sup.2]), [T.sub.tr.sup.0] is the equilibrium phase transition temperature (356 K), [DELTA][H.sub.tr.sup.0] is the enthalpy of the melting of an infinitely large perfect crystal of the monoclinic form = (69 J/g), and [T.sub.tr] is the transition temperature of the sample as measured from DSC analysis ([T.sub.1]). It is convenient to emphasize that a heating rate of 10[degrees]C/min was applied to balance the thermal lag and annealing effects since the former tends to increase the size of the fusion peak, whereas the latter narrows it (47). Lamellar thickness of 11 and 14 nm were calculated for the TPBD with 80 and 90% of 1, 4-trans units, respectively (Table 6); these results agree with reported values for this type of polymer (40), (48).
TABLE 6. X-ray crystallographic data of polybutadienes with different
numbers of 1,4-trans units.

 1,4-trans 2[[theta].sub.200] 2[[theta].sub.am] [d.sub.200]
Polymer (%) ([degrees]) ([degrees]) (nm)

TPBD_1 90 22.47 20.69 0.395
TPBD_6 86 22.42 20.71 0.396
TPBD_8 80 22.34 20.72 0.397

 [FWHM.sub.200] [L.sub.200] (nm) L (nm)
Polymer ([degrees]) Scherrer's equation Thomson's equation

TPBD_1 0.6619 13.58 14.13
TPBD_6 0.7004 12.84 12.66
TPBD_8 0.7824 11.49 11.25


Also, the degree of crystallinity of the TPBD by DSC, [W.sub.c.sup.DSC] was calculated in terms of the monoclinic and hexagonal crystalline forms using Eqs. 5 and 6, respectively. The experimentally determined enthalpies of TPBD: transition phase ([DELTA][H.sub.tr]) and hexagonal phase fusion ([DELTA][H.sub.m]), along with the characteristics enthalpies of a 100% crystalline TPBD: transition phase ([DELTA][H.sub.tr.sup.0] = 144 J/g) and hexagonal phase fusion ([DELTA][H.sub.m.sup.0] = 69 J/g), were used a calculate [W.sub.c,monoclinic.sup.DSC] and [W.sub.c,hexagonal.sup.DSC] (41):

[W.sub.c, monoclinic.sup.DSC] = [DELTA][H.sub.tr]/[DELTA][H.sub.tr.sup.o] (5)

[W.sub.c, hexagonal.sup.DSC] = [DELTA][H.sub.m]/[DELTA][H.sub.m.sup.o] (6)

Results shown in Table 7 indicate that, unlike that, unlike the reference sample PBDN, the investigated TPBD are semi-crystalline polymers. The highest content of crystalline is observed in TPBD with a higher content of 1,4-trans units because this allows the polymer to display an almost regular structure. The destruction of the regular structure of polymers by incorporating 1,2-vinyl and 1,4-cis units in its main chain leads to a substantial decrease of the tendency of the material to crystallize.
TABLE 7. Enthalpy [DELTA]H and degree of crystallinity
[W.sub.c.sup.DSC] calculated from DSC; and degree of
crystallinity [W.sub.c.sup.WAXS] calculated from WAXS analysis of
polybutadienes produced with initiators containing different
Al:Li:Ba molar ration at 60[degrees] C.

Polymer [DELTA] (J/g) [W.sub.c.sup.DSC] (%) [W.sub.c.sup.WAXS] (%)

PBDN -- 0 0
TPBD_1 46 32 29
TPBD_6 42 30 27
TPBD_7 36 25 23
TPBD_8 32 22 21
TPBD_2 29 20 19
TPBD_4AL 22 19 17
TPBD_3 26 18 17
TPBD_5 17 12 11
TPBD_4 9.4 6 5


X-ray Analyses

Figure 6 presents the X-ray diffraction patterns of the reference PBDN and those of TPBD with 80-90% of 1,4-trans units. The diffraction pattern of the PBDN displays only the amorphous halo, while those of the TPDB exhibit two main diffraction peaks: one intense at 22.47[degrees] and other weak at 30.60[degrees], which have been related to the crystalline reflections (200) and (220), respectively (41), (49), along with an amorphous halo at 20.80[degrees], which belongs to their noncrystalline part.

[FIGURE 6 OMITTED]

The TPBD with the highest number of 1,4-trans units (TPBD_1: 90%) exhibited the larger intense peak associated with the main crystalline reflection (200), and a relatively small amorphous halo. As the content of 1,4-trans units of the polymer diminished, their main peak decreased and amorphous halo increased. The polymer with about 40% of the units different from those of 1,4-trans (i.e., 1,2-vinyl and 1,4-cis) exhibited an X-ray diffraction pattern with just the amorphous halo, which indicates that such polymers do not have the ability to crystallize. To obtain an idea of the interplanar spacing [d.sub.200] of these polymers, Bragg's Law was applied, using the value of the most intense main crystalline reflection ([2[theta].sub.200]):

[lambda] = 2[d.sub.200] sin[[theta].sub.200] (7)

Where [lambda] is the radiating wavelength. (0.154 nm for the Cu [K.sub.x] radiation), [d.sub.200] is the spacing between the atomic plans, and [[theta].sub.200] is the angle between the X-ray beam and the atomic plane. Results of these calculations are shown in Table 6 and they reveal that polymers with the higher number of 1,4-trans units exhibit higher [[theta].sub.200] and, consequently, higher spacing [d.sub.200]. This has been explained by considering that the units different from 1,4-trans are not included in the crystalline lattice but rather in the amorphous regions and that the position of the amorphous halo (2[[theta].sub.am]) is practically independent of polymer composition (48). Taking advantage of the X-ray data, the lamellar thickness was also calculated by using Scherrer's equation (46):

[L.sub.200] = [K[lambda]/FWHM cos[[theta].sub.200]] (8)

Where [L.sub.200] represents the lamellar thickness perpendicular to the reflecting plane (200) K is a constant that often is assumed to be 1, [lambda] is the radiating wavelength, FWHM is the full width at half maximum of the (200) reflection of the main peak 2 [[theta].sub.200], and [[theta].sub.200] is the scattering angle. The value of [L.sub.200] for TPBD with 80-90% of 1-4 trans units are shown in Table 6. Itis apparent that the lamellar thickness calculated form DSC and WAXS data (Thompson-Gibbs and Sherrer's equations, respectively) are similar, as has been reported (50).

Additionally, WAXS data were used to calculate the crystallinity, [W.sub.c.sup.WAXS] of TPBD, using the maiun peak of the crystalline reflection [IC.sub.200] and the amorphous halo [I.sub.am], as is indicated in Eqs. 9 and 10 (46):

[I.sub.tot] = I[c.sub.200] + [I.sub.am] (9)

[W.sub.c.sup.WAXS] = I[c.sub.200]/([I[c.sub.200] + [I.sub.am]) (10)

AS shown in Table 7, there was a slight difference that between calculations from DSC and WAXS data, and this was due to the fact that each one of these techniques reflects in a different manner the structural and morphologic characteristics and imperfections of these polymers (46) However, both techniques, showed the same trend: an increase in the number of 1,4 - trans units increased the poly (butadiene) crystallinity. The TPBD with the highest number of 1,4-trans units (TPBD_1: 90%) had the highest crystallinity (30%); nevertheless, such a value is smaller than that reported for TPBD with 98% of 1,4-trans units (55-60%) (44), (48), (49).

Annealing

To investigate the effect of the annealing process on the properties of the TPBD, the sample of greatest crysgtallinity (TPBD_1) was annealed for I h at 60 [degrees] C (i.e., a temperature higher than its crystal-crystal transition temperature: 48[degrees]C, Table 5), then analyzed by DSC and WAXS. DSC result are shown in Fig. 7. It was observed that the annealing process increased the phase transition temperature from 48 to 54[degrees]C, and the crystal-crystal transition peak became broader, indicating an increase in polymer crystallinity from 30 to 36%.

[FIGURE 7 OMITTED]

Figure 8 displays the WAXS diffractograms of TPBD_1 before and after yielding to the annealing process. It was observed that the intensity of main diffraction peaks (200) and (220) increased after the annealing process. A crystallinity increase from 29 to 34% was calculated by using the (200) diffraction peak of the fresh and annealed samples. This result confirms the effect of the annealing process on the crystal-crystal transition of the TPBD.

[FIGURE 8 OMITTED]

These phenomena have been attributed to the size-affected character of the TPBDs' phase transition, as observed in characterizations of this type of polymers by means of SAXS and synchrotron (40), (44).

Annealing TPBD above its phase transition results in the rapid increase of lamellar thickness of monoclinic form, which is expected to increase crystallinity of the sample. The lamellae thickening rate in the monoclinic phase, however, is relatively slow, as a consequence, the resulting annealing in monoclinic phase contributed only slightly to the increase of the lamellar thickness. Therefore, relative crystallinity will be kept almost constant irrespective of a much longer annealing time (43).

CONCLUSIONS

Polymerization of butadiene with an initiator prepared with aluminum, n-butyl lithium, and barium alkoxide ([R.sub.1] Al/[R.sub.2] Li/[R.sub.3] OBa) allowed the production of high trans-1,4-poly (butadiene) (> 70%) at reaction conditions similar to those of the industrial process, where n-butyl lithium is the initiator, thus making such ternary initiator commercially attractive. The number of 1,4-trans units (70-90%), the molecular weight distribution (1.12 < [M.sub.w]/[M.sub.n] < 1.21), and the average molecular weight (67,500-144,000 g/gmol) can be controlled by adjusting the molar composition of the initiator. The TPBD with the highest number of 1,4-trans units (90%) was produced with an initiator having a molar composition of Al/Li/Ba equal to 1/1/4; however, the production of TPBD with higher number of 1,4-trans units is possible by just tuning the initiator composition. With regard to the effect of the polymerization temperature, it was observed that within the range investigated (40-80[degrees]C), changes in temperature affect the propagation rate of the butadiene polymerization further than the number of 1,4-trans units of the produced polymer. With regard to the properties of the TPBDs investigated, it is concluded that the microstructure of these polymers (i.e., the relative numbers of 1,4-trans, 1,4-cis, and 1,2-vinyl units) has an important effect on their thermal properties and degree of crystallinity: TPBD with a number of 1,4-trans units greater than 80% are polymorphous and present two endothermic transitions, one identified as a first-order crystal-crystal phase and appears around 55[degrees]C, and the other as a mesophase fusion that is detected at temperature higher than 55[degrees]C. The TPBDs with 80-70% of 1-4-trans units exhibit just the crystal-crystal transition. With respect to crystallinity degree, it is concluded that an increase in the content of 1,4-trans units (70-90%) causes an increase in the degree of crystallinity (10-30%), whereas polymers with less than 65% of 1,4-trans units become amorphous. The annealing of these TPBD produces an increase in the lamellar thickness of the monoclinic crystals; nevertheless, this change is rather small compared with that produced by number of 1,4-trans units of the polymeric chain. As a final statement, in this work is reported a procedure able to produce polybutadienes with well-defined distribution, average molecular weight, and with a regular microstructure that makes them susceptible to experience "induced crystallization," as what happens to natural rubber. Use of these properties may well be profitable in the manufacture of composites.

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Juan J. Benvenuta-Tapia, (1) Jose A. Tenorio-Lopez, (2) Rafael Herrera-Najera, (1) Leonardo Rios-Guerrero (3)

(1) Facultad de Quimica, Universidad Nacional Autonoma de Mexico, Mexico, D.F

(2) Facultad de Ciencias Quimicas, Universidad Veracruzana, Coatzacoalcos, Veracruz, Mexico

(3) Desarrollo Tecnologico Y Negocios de Innovacion, CONACYT, Mexico, D,F

Correspondence to: Juan J. Benvenuta-Tapia; e-mail: juan. benvenuta @ dese.com.mx

DOI 10.1002/pen.21259

Published online in Wiley InterScience (www.interseience.wiley.com).
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Author:Benvenuta-Tapia, Juan J.; Tenorio-Lopez, Jose A.; Herrera-Najera, Rafael; Rios-Guerrero, Leonardo
Publication:Polymer Engineering and Science
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Date:Jan 1, 2009
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