Orientated crystallization in drawn thermoplastic polyimide modified by carbon nanofibers.
Polyimides (PIs) are well-known and extensively studied thermally stable polymers (1) that are widely used in different industries as high performance polymer materials because of their extremely high thermal, chemical, and radiation stability, and excellent electrical and mechanical properties. An essential prerequisite for the utilization of thermoplastic PIs in such applications is a high degree of crystallinity, whose achievement has been a central issue in studies of PI matrices for advanced composites; see for example (2). It has been shown (3-6) that semicrystalline R-BAPB type PI based on 1,3-bis-(3,3',4,4'-dicarboxyphenoxy) benzene (R) and 4,4'-bis-(4-aminophenoxy) biphenyl (BAPB) can be successfully used as a matrix for carbon fiber-reinforced composites. Being potentially semicrystalline polymers, the methods which regulate the processes of crystallization and recrystallization have special significance for these materials. Of a number of techniques offered for the control of PI recrystallization (3), the addition of various carbon nanoparticles (7-9) into the polymer matrix has been proven highly effective in nucleating intensive crystal growth. As an example, single-wall carbon nanotubes increase the rate of crystallization in polypropylene by more than an order of magnitude (9). The strongest nucleating effect in this PI was observed for fillers with highly ordered graphitic structure, for which the highest degree of crystallinity of the PI matrix can be regenerated (8). Furthermore, the addition of 3 wt% carbon nanofibers to a unidirectional carbon fiber reinforced PI composite produced an improved interlaminar fracture toughness of 1.2 kJ/[m.sup.2] compared with 0.6--0.8 kJ/[m.sup.2] in the original composite (10).
Following our previous studies on the effect of carbon nanoparticles on recrystallization of this PI, this work is aimed at investigating an additional parameter of molecular orientation. A preliminary investigation (11), performed on uniaxially drawn pristine PI films, revealed that drawing above the glass transition temperature produced a significant increase in the degree of crystallinity; the drawing process, however, did not generate any crystalline orientation. Consequently, in this work, we examine how carbon nanofibers at different concentrations affect the recrystallization behavior of uniaxially drawn orientated R-BAPB PI films.
Preparation of R-BAPB PI
Poly(amic acid) (PAA) was obtained by polycondensation of l,3-bis (3,3', 4,4'-dicarboxyphenoxy) benzene (R) and BAPB supplied by Wakayama Seika Co., Ltd. (Wakayama City, Japan) in a 25% solution of N-methyl-2-pyrrolidone (NMP) at 25[degrees]C. It was important to fully endcap the chains with phthalic anhydride to control the molecular weight and maximize PI thermal stability (4). The specific PAA in this study was of an average molecular weight <[M.sub.w]> ~ 30 kDa. The PAA films were processed by casting onto glass plates and drying at 60[degrees]C for 5 h. The imidization was achieved by placing the films in an air oven for 1 h at 100, 200, and 300[degrees]C. After complete imidization, the films were removed from the glass plates by soaking in water.
Preparation and Drawing of Nanocomposite Films
Vapor grown carbon nanofibers (VGCF-H) (graphitized up to 2800[degrees]C) were supplied by Showa Denko KK (Tokyo, Japan). The average diameter and length of the VGCF are 150 nm and 10-20 [micro]m, respectively; their density is 2 g/[cm.sup.3].
To prepare PI/VGCF nanocomposite films with different VGCF concentrations, a desired quantity of the VGCF was added to NMP. The resulting dilute VGCF suspension in NMP was homogenized for 30 min in an ultrasonic bath (40 kHz). The sonicated VGCF suspension was transferred into a three-neck round bottom flask equipped with a mechanical stirrer, a nitrogen gas inlet, and a drying tube outlet filled with calcium sulfate. After stirring the VGCF solution for 10 min, the PAA was added and stirring of the mixture continued for an additional 1 h, until a constant viscosity was obtained. The content of the VGCF in solid PAA ranged from 1 to 21 wt%. Thin (30-40 [micro]m) nanocomposite films with varying VGCF concentrations (1, 3, 3, 5, 7, 14, and 21 wt%) were prepared as described above for the pristine PI films. For significant comparison of the experimental results, all the films were heat treated at 360[degrees]C for 10 min (above the melting point of the PI) to erase any crystallinity, which might have been formed during the imidization stage.
Molecular orientation of PI films was achieved by multiple step uniaxial drawings of the film through a 10-mm long heater at 250[degrees]C to different draw ratios (DR). The pulling speed was 35 mm/min, and the speed ratio of the feeding and receiving rolls was matched so that after each pulling step, the sample length was increased by 50%. Orientated film sections were subjected to isometric annealing for 30 min at 265[degrees]C.
Differential scanning calorimetry (DSC) of the PI films was performed using DSC 204 F1 Phoenix equipment (NETZSCH, Hamburg, Germany) using 3--4 mg samples at a heating rate of 10[degrees]C/min up to 350[degrees]C, under argon. The specific enthalpy was normalized with respect to the net weight of the PI in the sample.
The densities of the PI films were evaluated by the flotation method in mixtures of toluene and CC[l.sub.4] at 20[degrees]C.
X-ray diffraction patterns were obtained on Fuji imaging plates, using a Searle camera equipped with Franks optics affixed to an Elliott GX6 rotating anode generator operating at 1.2 kW and producing copper radiation ([lambda] = 1.54 [Angstrom]). The beam diameter was about 400 [micro]m. Exposure times were approximately 4 h. In all cases the films were held such that the direction of drawing was vertical. The imaging plates were scanned with a He-Ne laser (JDS Uniphase, CA) in conjunction with a homemade reader based on an Optronics (Chelmsford, MA) densitometer and interfaced to a PC. Intensity profiling on the diffraction patterns in either the equatorial or meridional direction was performed using the computer program POLAR (SUNY Stony Brook, NY).
The mechanical properties of the films were determined in the drawing direction by tensile testing, using 1 X 20 [mm.sup.2] rectangular specimens using a universal testing machine (UTS-10, Germany) at an extension speed of 2 mm/min. The dynamic Young's modulus was measured at room temperature at 660 Hz with a DMA machine as described previously (12).
Heat-induced shrinkage was determined by heating a free standing sample (nonisometric) at 270[degrees]C for 5 min and calculating the shrinkage ratio (i.e., the drawn length divided by the shrunk length).
RESULTS AND DISCUSSION
Having demonstrated already the effective nucleating power of VGCF, high resolution scanning electron microscopy (HRSEM, FEI Sirion) of etched drawn PI/VGCF films (etched according to the procedure in (5)) was used here to study the additional drawing effect on morphological orientation. Figure 1 shows typical lamellar morphology at the vicinity of a single VGCF, as presented previously for a similar un-drawn film (10). The drawn sample here (DR = 6, annealed), reveals a significantly higher degree of order, wherein all the lamellae are oriented perpendicular to the fiber, while the fiber is parallel to the draw direction. The resemblance to transcrystallinity, demonstrated previously in short fiber reinforced single polymer PE composites (13) and in shear induced orientated crystallization of short fiber reinforced iPP (14), (15), is remarkable, indicating PI nucleation at the VGCF surface.
[FIGURE 1 OMITTED]
To measure the change in crystallinity of the PI/VGCF films as a result of their drawing, DSC experiments were performed on as-received samples with 5 wt% VGCF in comparison with their drawn counterparts at a DR of 5.5. The respective DSC traces are presented in Fig. 2. It is seen that the as-received (un-drawn) sample exhibits a distinct glass transition at 204[degrees]C and a minor melting process at 322[degrees]C. Contrarily, the drawn samples exhibit completely different characteristics, as follows. The wide exothermal peak (260-290[degrees]C) connected to a crystallization process appears during the DSC experiment at a lower temperature than the crystallization temperature of un-drawn samples (~290[degrees]C) (8). Also, the intensity of the endothermal melting peak is increased considerably and the melting point shifts to a slightly lower temperature (317[degrees]C) compared with the un-drawn sample. It is apparent that the crystalline phase in the drawn film is formed mostly during the DSC experiment and not during the process of drawing, implying that an annealing step can increase the crystallinity of the drawn samples. Indeed, the DSC trace of the drawn, isometrically annealed sample at 265[degrees]C does not show an exothermal peak. The intensity of the melting enthalpy measured at 317[degrees]C is increased noticeably in the annealed sample producing a value of 25.0 compared with 12.0 and 1.2 J/g for the drawn un-annealed and as-received samples, respectively.
[FIGURE 2 OMITTED]
The influence of VGCF concentration and DR on the crystallization potential of nanocomposite films in the process of drawing and further annealing is also estimated by DSC, as follows. Figure 3 displays melting enthalpies as a function of VGCF concentration for annealed and un-annealed samples and different DR values. The undrawn PI/VGCF films are characterized by a small but distinctive melting peak and the increase in VGCF concentration leads to a slow rise of the melting enthalpy. Increasing DR from 2.5 to 6 at VGCF concentrations up to 5 wt% leads to a sharp increase in the melting enthalpy. For a given DR, the melting enthalpy is doubled as a result of annealing. In fact, a high DR could only be achieved with PI/VGCF films that contained less than 5 wt% VGCF, so the highest melting enthalpy was obtained for annealed samples of 5 wt% VGCF and DR = 6. These results show that the effects of DR and VGCF content are interchangeable, namely, that the combination of low DR and high concentration is equivalent to that of high DR and low concentration.
[FIGURE 3 OMITTED]
The densities of the various samples are presented in Fig. 4 as a function of VGCF concentration. The density of the un-drawn, un-annealed samples exhibits a linear trend with the VGCF content which fits the rule of mixture prediction expressed by the solid line, calculated by [rho] = 1/[(1 - [chi])/[[rho].sub.1] + [chi]/[[rho].sub.2] where [chi] is the VGCF concentration (wt%), [rho] is the density and 1 and 2 designate PI and VGCF, respectively. Figure 4 shows three additional density traces for different treatments as follows. Two traces for DR of 3.5 and 6 of un-annealed films reveal that the film density levels off or decreases at/above 4 wt% and the third reveals the sharp annealing effect on a drawn film. In principle, two factors can be named responsible for the increased density of the films. They are the degree of crystallinity of the PI and the weight content of the VGCF. The leveling off and decrease of density with VGCF concentration can be explained by realizing that crystalline nucleation by the VGCF requires an optimal concentration of nuclei, beyond which additional nucleation sites generate an adverse reaction. The density decrease suggests that the combined effects of high VGCF content and DR results in increasing the free volume of the film in the amorphous phase.
[FIGURE 4 OMITTED]
WAXD has been used previously in our studies to monitor the transition of R-BAPB PI from the amorphous to crystalline state under the influence of various treatments (3), (16). Whereas in those studies the replacement of the amorphous halo by a distinct diffraction pattern, with a number of relatively sharp peaks, reflected an effective crystallization treatment, in the present study the diffraction pattern is utilized to analyze the effect of drawing on crystalline orientation. Figure 5a contains a typical diffraction pattern of a drawn annealed (DR = 6) PI/VGCF (3 wt%) film. In this case, the uniform rings of the isotropic crystalline structure have been replaced by sequences of equatorial and meridional ares, indicative of an anisotropic structure. Figure 5b and c display equatorial and meridional profiles, respectively, of the same diffraction pattern. The 2[theta] positions of the major peaks in the equatorial scan are consistent with the diffraction pattern of a nonoriented semicrystalline R-BAPB PI film (16). The equatorial positions indicate that the peaks originate in the lateral packing of the polymer chains which are now orientated along the drawing axis. Specifically, the equatorial intensity scan (Fig. 5b) shows three major reflections with d-spacings of 0.47, 0.42, and 0.34 nm: it is noted though that for this material no indexing has yet been accomplished, to the best of our knowledge. Conversely, the peaks of the meridional scan (Fig. 5c) represent new features of the orientated morphology. Specifically, the meridional intensity scan shows at least five clear peaks, indexed as the first five orders of an interlamellar spacing with L = 3.6 nm. This result is indicative of a layered lamellar assembly in which the lamellae are perpendicular to the drawing direction.
[FIGURE 5 OMITTED]
The nucleating effect of the nanoinclusions coupled with the orientation effect of drawing generates a unique orientated layered lamellar structure, characteristic of smectic-like mesophase. Similar structures were also identified in our previous research of PI crystallization, induced by low molecular weight bisimides (BI); it was discovered that upon cooling from their isotropic melts, BI/PI blends underwent a monotropic liquid crystalline transition to form a smectic-like structure (17). More importantly, the possibility that some PIs can form a layered smectic structure even in the solid state was already pointed out in the literature, where a sharp middle angle X-ray reflection indicated the existence of a smectic layered structure with a layer distance of 3.04 nm (attributed to a regular pattern of polar and non-polar units along the polymer chain) (18). Thus, in the drawn films of the present study, the layered structure obviously exhibits an additional degree of orientation in its alignment perpendicular to the drawing direction.
The orientated crystallinity in the drawn annealed films with an apparent layered lamellar assembly is a priori expected to affect the mechanical properties, the primary example being the stress-strain behavior. Figure 6 presents stress-strain curves of un-drawn pristine PI and PI/VGCF (5 wt%) films in comparison with their drawn (DR = 4.5) and annealed counterparts. Considering the factorial effects of drawing, VGCF and annealing--in that order--it is apparent that the strain to failure decreases, the yield point increases, and the modulus increases. The calculated values of those properties are given in Table 1, where the numbers match the film identification in Figure 6. Comparing the film pairs 1 and 2 and 3 and 4 reveals that the prominent effect is exhibited by drawing. Contrarily, the VGCF has a limited effect, mostly on the strain to failure, seen by comparing the film pairs 1 and 3 and 2 and 4. The most significant effect is observed in the drawn, annealed PI/VGCF film (5) with the highest yield point and modulus and lowest strength and ultimate strain, indicative of high orientated order and in correlation with the increased free volume detected from the density results in Figure 4.
[FIGURE 6 OMITTED]
TABLE 1. The mechanical properties of PI and PI/VGCF films; the sample numbers match the film identification in Figure 6. No. VGCF DR Young's Yield point Strength Strain to content modulus (MPa) (MPa) failure (%) (GPa) (%) 1 0 1 2.8 115 150 112 2 0 4.5 4.1 139 289 47 3 5 1 3.4 105 102 35 4 5 4.5 5.2 147 214 19 5 5 4.5 5.8 152 175 14
The unique orientated structure is also expected to affect the dynamic mechanical behavior of the composite films. The real component E' of the dynamic modulus is plotted as a function of DR in Fig. 7 for various films, expressing the influence of VGCF content and annealing. Apart from the anticipated result that E' increases with DR, we can make the following observations, in reference to DR = 3.5. The annealing effect is diagnosed by comparing the pairs (a) and (b), (c) and (d), and (e) and (f), producing an average effect of 1.5 GPa for annealing. Regarding the VGCF effect, it is seen that increasing the VGCF content from 0 to 3.0 to 13.6 wt% increases E' by approximately 0.25 GPa/wt% in the un-annealed films (a, c, and e) and by 0.55 GPa/wt% in the annealed films (b, d, and f). It is evident that the effects of annealing and VGCF are cumulative.
[FIGURE 7 OMITTED]
An additional thermomechanical property, which is indicative of the morphology, is shrinkage. This was measured at DR = 4.5 for pristine PI and PI/VGCF (5 wt%), un-annealed and annealed, producing the shrinkage ratios 3.60, 1.51, and 1.25, respectively. As anticipated, both VGCF and annealing stiffen the PI film in the drawing direction, in agreement with the modulus results above.
Vapor grown carbon fibers induce solid state crystallization in R-BAPB PI films when annealed above the glass transition temperature. In drawn films, the nucleating effect of the nanoinclusions coupled with the orientation effect of drawing generates a unique orientated layered lamellar structure, characteristic of smectic-like mesophase. The degree of orientated crystallization increases with the content of nanoinclusions and with the DR, and is reflected in the significantly higher mechanical properties, particularly, the modulus and yield stress, of the drawn crystalline films.
Authors V.E.Y. and G.M. wish to thank the Israel Academy of Sciences and Humanities and the Russian Academy of Sciences for supporting a scientific exchange program.
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Valentina E. Smirnova, (1) losif V. Gofman, (1) Vladimir E. Yudin, (1) Irina, P. Dobrovolskaya, (1) Alexander N. Shumakov, (1) Andrey L. Didenko, (1) Valentine M. Svetlichnyi, (1) Ellen Wachtel, (2) Rinat Shechter, (3) Hannah Harel, (3) Gad Marom (3)
(1) Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg 199004, Russia
(2) Chemical Research Infrastructure Unit, Weizmann Institute of Science, 76100 Rehovot, Israel
(3) Casali Institute of Applied Chemistry, The Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
Correspondence to: Vladimir E. Yudin; e-mail; email@example.com or Gad Marom; e-mail: firstname.lastname@example.org
Contract grant sponsor: Russian Fund of the Basic Research; contract grant number: 07-03-00846; contract grant sponsor: The Israel Science Foundation; contract grant number: 69/05.
Published online in Wiley InterScience (www.interscience.wiley.com). [C] 2008 Society of Plastics Engineers
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|Author:||Smirnova, Valentina E.; Gofman, Iosif V.; Yudin, Vladimir E.; Dobrovolskaya, Irina P.; Shumakov, Ale|
|Publication:||Polymer Engineering and Science|
|Date:||Feb 1, 2009|
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