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Molecular Origin of Enhanced Strength in the Monofilament of Syndiotactic Polypropylene Because of Annealing: A Micro-FTIR Study Nana Tian, Ruihua Lv, Bing Na,* Wenfei Xu, and Zhujun Li Department of Materials Science and Engineering, East China Institute of Technology, Fuzhou, 344000 People’s Republic of China ReceiVed: July 12, 2009; ReVised Manuscript ReceiVed: October 2, 2009
The conformational and polymorphic transformations in the melt-spun monofilament of syndiotactic polypropylene upon annealing and subsequent stretching have been first explored by micro-FTIR studies. The results indicate that annealing of as-spun monofilament gives rise to an unusual increase in the molecular orientation of helical conformations in both amorphous phase and ordered crystals as a result of structural transitions of transplanar conformations to helical ones as well as mesophase to ordered crystals. The increased molecular orientation in the annealed monofilament is mainly responsible for the enhanced strength during stretching. Furthermore, high stress exerted by the molecular chains of annealed monofilament is confirmed by the significant structural transitions opposite to that induced by annealing. 1. Introduction Syndiotactic polypropylene (sPP) can crystallize into four crystal modifications with different chain conformations in the unit cell.1-3 Forms I and II adopt the (t2g2)n helical conformations, whereas forms III and IV have chains in transplanar and (t6g2t2g2)n conformations, respectively. Generally, during stretching, the helical conformations in forms I and II can be converted into transplanar ones, accompanied by polymorphic transformations to form III or disordered mesophase.4-8 The conformational and polymorphic transformations depend on the initial morphology, stereoregularity of sPP, and stretching conditions, etc. On the other hand, partly or completely reversible transformations of conformation and crystal forms are also observed once stress is removed.3,5,8-10 In particular, the reversible structural transitions in the crystalline regions during successive stretching and relaxation of oriented samples (i.e. enthalpic effect), in addition to the common entropic effect as a result of the conformational transitions of the amorphous chains, are responsible for the good elasticity of sPP fibers, especially with high crystallinity. Different from other crystalline polymers, such as isotactic polypropylene (iPP), stress generated by the extensional flow during melt spinning of sPP cannot promote the crystallization of ordered crystal form I or II with helical conformations.11-13 Rather, mesophase with transplanar conformations is usually formed in the as-spun sPP monofilaments, since the chain extension prior to crystallization is not favorable for the formation of ordered crystal forms with coiled helical conformations. The as-spun sPP monofilaments also exhibit good elastic behavior because of low crystallinity, as demonstrated by Loos et al.,14 wherein micelle-like crystals (mesophase) act as physical cross-links in the amorphous matrix. It is proposed that the elasticity presented in the as-spun sPP monofilaments is entropic in nature, as a result of conformational transformations and chain recoiling, which is essentially the same as that observed in typical thermoplastic elastomers. * Correspondence author. Fax: 0086-794-8258320. E-mail: bingnash@ 163.com,
[email protected].
In this study, the structural transitions in the melt-spun sPP monofilament upon annealing and subsequent stretching have been explored by micro-FTIR measurements. It is indicated that annealing of as-spun monofilament gives rise to dramatic enhancement of strength, and its molecular origin related to the conformational and polymorphic transformations has been proposed. 2. Experimental Section 2.1. Material and Sample Preparation. The syndiotactic polypropylene (sPP) pellets used in this study were purchased from Aldrich. They had a typical Mn and Mw of 75 and 174 kg/mol, respectively. Monofilament with a diameter of about 100 µm was melt-spun from a capillary rheometer at 240 °C with a take-up speed of 20 m/min. For preparing annealed samples, the as-spun monofilaments were first fixed on a glass plate with clips at two ends and then annealed for 15 min in an oven at 45, 75, and 105 °C, respectively. For convenience, the annealed monofilament at 105 °C, for instance, is referred to as AN105. 2.2. Micro-FTIR Study of Stretching. The structural transitions during stretching of as-spun and annealed monofilament were monitored by a Thermo Nicolet infrared microscope with a MCT detector coupled with a ministretcher at room temperature. The IR source was provided by a Thermo Nicolet FTIR spectrometer. An adjustable aperture was used to obtain the desired microsampling area. In this case, the sampling size was constant (150 µm) along the monofilament axis, and in the transverse direction, the width of the aperture was set to the transient diameter of the deformed monofilament. During measurements, the monofilament was stretched step-by-step at a rate of 5 mm/min along the fiber axis. The draw ratio, λ, defined as λ ) l/l0 (l, l0 are the transient and initial length, respectively), was precisely measured by the extension of an inkmark preprinted on the monofilament. At each draw ratio, polarized infrared spectra (by rotating a ZnSe polarizer) parallel and perpendicular to the fiber axis, respectively, were collected with a resolution of 4 cm-1, and a total 32 scans were added. Note that the microsampling position was the same for each
10.1021/jp906563y CCC: $40.75 2009 American Chemical Society Published on Web 10/19/2009
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Figure 1. Polarized IR spectra, parallel (a) and perpendicular (b) to fiber axis, respectively, of sPP monofilament annealed at indicated temperature. Note that “H” and “T” represent helical and transplanar conformations, respectively.
Figure 2. Changes of the dichroic ratio (a) and conformation (b) with respect to annealing temperature in sPP monofilaments.
draw ratio, which was readily realized by moving the sample stage under a CCD view system of the IR microscope. The dichroic ratio, R, and structural absorbance, A, of a desired absorption band were deduced using the relations15
R ) A// /A⊥
(1)
A ) (A// + 2A⊥)/3
(2)
where A//, A⊥ are the parallel and perpendicular absorbance, respectively. 2.3. Video-Aid Tensile Tests. Mechanical tests of sPP monofilaments were performed on a universal testing machine at room temperature with a crosshead speed of 5 mm/min. Note that prior to the tests, the diameter of each monofilament was precisely measured by optical microscopy with resolution of 2 pixels per micrometer. A CCD camera (1280 pixel × 1024 pixel), equipped with a tunable magnification lens, was adopted to monitor the longitudinal separation of two inkmarks preprinted on the surface of the monofilaments during stretching. A reliable draw ratio, λ, defined as λ ) l/l0 could be obtained through this procedure. 2.4. X-ray Diffraction (XRD). XRD measurements were conducted on an X-ray diffractometer equipped with an X-ray generator and a goniometer. The X-rays were generated at 35 kV and 60 mA, and the wavelength of the monochromated X-ray from Cu KR radiation was 0.154 nm. Before measurements, the monofilaments were cut into small grains for random arrangements. 2.5. Differential Scanning Calorimetry (DSC). The melting behaviors of as-spun and annealed sPP monofilaments were registered at a heating rate of 10 °C/min in a nitrogen atmosphere using a NETZSCH DSC 204.
Figure 3. DSC curves (a) and XRD profiles (b) of as-spun and annealed sPP monofilaments, respectively.
3. Results and Discussion 3.1. Structural Transitions upon Annealing. Figure 1 is the polarized IR spectra in the wavenumber range between 1260 and 800 cm-1 obtained from as-spun and annealed sPP monofilament, respectively. As demonstrated in other studies,5,6 the conformation and morphology in sPP can be correlated with some specific absorption bands in the above wavenumber range. The absorption bands at 812, 867 (ordered crystal), and 977
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Figure 4. Mechanical responses of as-spun and annealed sPP monofilaments during stretching.
cm-1(amorphous) are attributed to the coiled helical conformations, whereas the ones located at 963 (amorphous and mesophase), 1131, and 1232 cm-1(amorphous) belong to the extended transplanar conformations. The above-referred absorption bands show distinct absorbance along and perpendicular to the fiber axis, suggesting apparent molecular orientation in both as-spun and annealed monofilaments. As presented in Figure 2, annealing of as-spun monofilament gives rise to significant increase in the dichroic ratio R of the bands assigned to helical conformations (867 and 977 cm-1), whereas the one related to the transplanar conformations (963 cm-1) remains nearly constant. Note that for comparison, the temperature related to as-spun monofilament is taken as 25 °C. The unusual increase in the molecular orientation of coiled helical conformations upon annealing could be related to the transformation of extended transplanar conformations to the coiled helical ones:4,11 (1) In the as-spun monofilaments, the molecular orientation of transplanar conformations in both the amorphous phase and mesophase is expected to be higher than that of helical Figure 6. Structural transitions during stretching sPP monofilament: (a) AN105, (b) as-spun. The comparison of conformational transformation in the above-referred samples is included in (c).
Figure 5. Polarized IR spectra, parallel (a) and perpendicular (b) to the fiber axis, respectively, of annealed sPP monofilament (AN105) stretched to the indicated draw ratio.
ones as a result of favorable chain extension during melt spinning. (2) The local reorientation of helical sequences in both the amorphous phase and ordered crystals due to internal shrinkage stress generated in such a transformation, as the reduced local chain length and constant macroscopic length of the monofilament during solid annealing is concerned. In principle, such a transformation could be manifested by the increased absorbance of helical conformations as well as simultaneous decreased absorbance of transplanar ones. However, it is difficult to directly obtain the information about conformational transformation via the absorbance variations of conformation-sensitive bands due to a possible diameter difference of the individual monofilament. Alternatively, in practice, the relative change of helical to transplanar conformations is usually represented by the ratio of A977/A963, and a higher ratio corresponds to a higher fraction of helical conformations.7,12 As shown in Figure 2, the value of A977/A963 is gradually increased with increasing annealing temperature, confirming that the transformation of transplanar to helical conformations indeed occurs during the annealing treatment. Moreover, such a transformation must involve the structural transitions in both the amorphous phase and mesophase of as-spun monofilament with consideration of the variation of the dichroic ratio upon annealing. Further evidence of structural transitions during annealing can be deduced from the DSC and XRD results shown in Figure
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Figure 7. Change in the dichroic ratio of (a) 867, (b) 963, (c) 977, and (d) 1232 cm-1 bands during stretching of as-spun and annealed (AN105) sPP monofilaments.
3. As for as-spun and annealed monofilaments, the DSC curves exhibit distinct multiple endothermic peaks, which correspond to melting of the mesophase and of recrystallized ordered crystals from the mesophase during the heating run, respectively.12 The main difference among the above monofilaments is the increased melting temperature and decreased fusion enthalpy of the mesophase with respect to the annealing temperature, which further confirms the transformation of the mesophase upon annealing. However, due to the complicated recrystallization process, it is difficult to obtain the detailed information about the formation of ordered crystals induced by annealing from the DSC curves. This dilemma can be resolved by the room temperature XRD measurements when the initial structure in the as-spun and annealed monofilaments is preserved. As expected, mesophase with transplanar conformations prevails in the as-spun monofilaments, whereas distinct reflections from the ordered crystals with helical conformations manifest themselves in the ones annealed especially at 105 °C (AN105). Note that it is difficult to differentiate form I from form II, according to the XRD profiles, and thus, the term ordered crystals referring to form I or form II is used. It means that transplanar conformations have been, indeed, converted into helical conformations upon annealing, accompanied by the polymorphic transitions of mesophase to ordered crystals, which is consistent with other observations.16-18 3.2. Enhanced Strength Because of Annealing. Figure 4 is the mechanical responses of as-spun and annealed sPP monofilaments, respectively, under uniaxial tensile deformation. It indicates that annealing enhances not only the modulus and yield strength at small strain but also the flow stress at large deformation. As argued by other researchers,14,19,20 tensile deformation of semicrystalline sPP monofilament is dominated by the segmental extension in the fluidlike amorphous phase where crystallites/mesophase act as physical cross-links of the deformed molecular network. Increasing the molecular orienta-
tion in the amorphous phase, in a common sense, can reduce the segmental extensionability between adjacent physical crosslinks, and thus, high stress is expected. It is the fact observed in this case; the stress exerted by the monofilament increases with annealing temperature mostly as a result of increased molecular orientation upon annealing. We acknowledge that increasing crystallinity could give rise to the same trend as the increasing molecular orientation, due to the enhanced crosslinking density of the molecular network.19 However, this effect, if it exists, seems negligible due to little change in the amount of the nonamorphous component upon annealing (estimated from the XRD results shown in Figure 3). Furthermore, the high stress exerted by the molecular chains of the annealed monofilament during stretching is confirmed by the significant structural transitions opposite that induced by annealing, as revealed by the subsequent micro-FTIR studies. 3.3. Stress-Induced Structural Transitions during Stretching. Figure 5 shows polarized IR spectra, as an example, obtained from stretching annealed sPP monofilament (AN105) to various draw ratios. It is indicated that during stretching, the absorption bands related to the conformation and morphology change significantly, suggesting that simultaneous conformational and morphological transitions occur under external force. For quantitative analysis of structural transitions during stretching of both as-spun and annealed (AN105) sPP monofilaments, the corrected absorbance, A*, expressed as A* ) A × λ (where A is the structural absorbance deduced by eq 2 and λ is the draw ratio), has been adopted; the corresponding results are colleted in Figure 6. Note that the use of the corrected absorbance, A*, rather than structural absorbance, A, is employed to compensate for the decreasing monofilament diameter during stretching, and thus, a direct comparison of the phase content at different draw ratios makes sense.21 It is indicated that transformation of helical conformations to transplanar ones occurs especially during stretching of the annealed monofilament
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(AN105), demonstrated by the decrease in the helical conformations (867 and 977 cm-1 bands) and increase in transplanar conformations (963 and 1232 cm-1 bands), respectively. Concerning the assignments of the above absorption bands, it means that during stretching, the coiled helical sequences in the amorphous phase have been pulled tightly due to its fluidlike nature and converted into extended transplanar conformations (indicated by downward 977 cm-1 and upward 963 and 1232 cm-1 bands, respectively), which is opposite to what occurred upon annealing (see Figure 2). Although the force transmitted by the molecular chains in the amorphous phase is high enough to destroy the ordered crystals, a polymorphic transition from ordered crystals to the mesophase with transplanar conformations could be brought about4,6,22 (manifested by the downward 867 cm-1 and upward 963 cm-1 bands, respectively). Apparently, stress-induced structural transitions in sPP monofilament depend on the amount of stress exerted by the molecular chains in the amorphous phase, which becomes more significant in the annealed monofilament with higher initial molecular orientation, as compared with its as-spun counterpart with a lower initial molecular orientation (indicated by the change of the ratio of A977/A963 in Figure 6c). It is essentially similar to that observed during stretching of nylon 6 monofilament.21 On the other hand, stress distribution among molecular chains is not homogeneous, mostly due to nonuniform initial molecular orientation in the amorphous phase. This argument is supported by the unusual decreasing of the dichroic ratio R related to ordered crystals (867 cm-1 band) during stretching, especially for the annealed monofilament (AN105), as shown in Figure 7. As discussed above, stress-induced polymorphic transition of ordered crystals to the mesophase depends on the extent of tightening of adjacent molecular chains in the amorphous phase. It is expected that during stretching, the ordered crystals with higher orientation, which are more intimately connected by the amorphous phase, endure a more significant polymorphic transition, whereas the ones with lower orientation remain nearly intact due to lower stress transmitted by the more relaxed adjacent molecular chains in the amorphous phase. In addition to the above-referred structural transitions, of course, the segmental orientation in the amorphous phase during stretching is also presented6 and, thus, contributes to the increase in the dichroic ratio of 963 and 1232 cm-1 bands, especially for the annealed monofilament with the more stressed amorphous phase. 4. Conclusion The enhanced strength in the annealed sPP monofilament has been well-correlated with the unusual increased molecular orientation of helical conformations due to conformational and
Tian et al. polymorphic transformations upon annealing. Furthermore, the high stress exerted by the molecular chains in the annealed monofilament during stretching is unambiguously demonstrated by the significant stress-induced structural transitions. The above findings provide an alternative clue to prepare high-strength polymer fibers via conformation transformations induced by annealing. Acknowledgment. This work is supported by the National Natural Science Foundation of China (No. 20704006) and the Project of Jiangxi Provincial Department of Education (No. GJJ08295). This research is also subsidized by the Opening Project of the State Key Laboratory of Polymer Materials Engineering (Sichuan University). References and Notes (1) Lotz, B.; Lovinger, A. J.; Cais, R. E. Macromolecules 1988, 21, 2375–2382. (2) Corradini, P.; Natta, G.; Ganis, P.; Temussi, P. A. J. Polym. Sci. Polym. Lett. 1967, 16, 2477–2484. (3) De Rosa, C.; Auriemma, F.; Corradini, P. Macromolecules 1996, 29, 7452–7459. (4) Auriemma, F.; De Rosa, C. J. Am. Chem. Soc. 2003, 125, 13143– 13147. (5) Parthasarathy, G.; Sevegney, M. S.; Kannan, R. M. Polymer 2005, 46, 6335–6346. (6) Sevegney, M.; Parthasarthy, G.; Kannan, R.; Thurman, D.; Fernandez-Ballester, L. Macromolecules 2003, 36, 6472–6483. (7) Zhang, X.; Kong, L.; Rottstegge, J.; Xu, D.; Wang, D. J. Phys. Chem. B 2007, 111, 11642–11645. (8) De Rosa, C.; Auriemma, F. Prog. Polym. Sci. 2006, 31, 145–237. (9) De Rosa, C.; Auriemma, F.; Ruiz de Ballesteros, O. Chem. Mater. 2006, 18, 3523–3530. (10) De Rosa, C.; Auriemma, F.; Ruiz de Ballesteros, O. Macromolecules 2004, 37, 1422–1430. (11) Sura, R.; Desai, P.; Abhiraman, A. J. Appl. Polym. Sci. 2001, 81, 2305–2317. (12) Zhang, X.; Li, R.; Kong, L.; Wang, D. Polymer 2008, 49, 1350– 1355. (13) Choi, D.; White, J. Polym. Eng. Sci. 2004, 44, 210–222. (14) Loos, J.; Schimanski, T. Polym. Eng. Sci. 2000, 40, 567–572. (15) Siesler, H. W. Pure Appl. Chem. 1985, 57, 1603–1616. (16) Ohira, Y.; Horii, F.; Nakaoki, T. Macromolecules 2000, 33, 1801– 1806. (17) Vittoria, V.; Guadagno, L.; Comotti, A.; Simonutti, R.; Auriemma, F.; De Rosa, C. Macromolecules 2000, 33, 6200–6204. (18) De Rosa, C.; Ruiz de Ballesteros, O.; Auriemma, F.; Savarese, R. Macromolecules 2005, 38, 4791–4798. (19) Hiss, R.; Hobeika, S.; Lynn, C.; Strobl, G. Macromolecules 1999, 32, 4390–4403. (20) Haward, R. N. Macromolecules 1993, 26, 5860–5869. (21) Na, B.; Lv, R.; Tian, N.; Xu, W.; Li, Z.; Fu, Q. J. Polym. Sci. Polym. Phys. 2009, 47, 898–902. (22) De Rosa, C.; Auriemma, F.; Ruiz de Ballesteros, O. Phys. ReV. Lett. 2006, 96, 167801.
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