Morphology and Elasticity of Oriented Syndiotactic Polypropylene from

Synopsis. A sample of syndiotactic polypropylene was prepared by solvent casting in orthodichlorobenzene. This procedure allowed us to obtain biphasic...
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Morphology and Elasticity of Oriented Syndiotactic Polypropylene from Solvent Cast Films Liberata Guadagno,* Carlo Naddeo, Marialuigia Raimondo, Angela Senatore, and Vittoria Vittoria

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 7 1703-1710

Dipartimento di Ingegneria Chimica e Alimentare, UniVersita` di Salerno, Via Ponte Don Melillo 1, 84084 Fisciano, Salerno, Italia (U.E.) ReceiVed March 1, 2006; ReVised Manuscript ReceiVed May 5, 2006

ABSTRACT: A sample of syndiotactic polypropylene (sPP) was prepared by solvent casting in orthodichlorobenzene at 70 °C. Its structure and morphology were analyzed by X-rays, Atomic force microscopy (AFM), and transport properties of vapors at low activity. The structural parameters were compared to those of samples thermally crystallized at 25 and 100 °C. The presence of the helical form I was recognized in either sample with crystal dimensions strictly dependent on the crystallization temperature. The crystallite sizes, determined by X-rays, were found to be surprisingly high, pointing to an unusual ability of lamellar crystals of sPP helical modifications to host defects without loosing coherence. Different from the thermally crystallized samples, the phase composition of the sample obtained by casting was particularly simple with a wide fraction of amorphous component and an almost complete absence of trans-planar mesophase. The oriented sample obtained by drawing the solvent cast sample up to λ ) 7.0 showed the presence of the trans-planar form III, as well as of the trans-planar mesophase. On release of the tension the crystalline form III almost completely transformed into the mesophase, and only weak reflections of the helical form appeared in the diffraction spectrum of the drawn sample. The elastic behavior of the oriented sample was investigated by obtaining the hysteresis curves at increasing draw ratios. The permanent set and the energy dissipated were obtained and compared to those of thermally crystallized samples. The elasticity was analyzed with the Mooney-Rivlin equation. A truly elastic range was recognized between λ ) 4.0 and λ ) 5.8. In this range, the X-ray analysis did not show any change of the structure. For higher strain values, an upturn appears in the curve ascribable, by X-rays studies, to a strain-induced crystallization. Introduction In semicrystalline polymers, drawing procedures at temperatures lower than the crystalline melting are extensively used to obtain materials exhibiting both high elastic modulus and high tensile strength. The elastic modulus can reach values even up to 50% of the axial modulus of the ideal crystal, indicating that there is a noticeable axial connection in the crystalline phase of oriented samples. Indeed the remarkable properties of drawn materials, so different from the original films, are very important for specific and advanced technological applications. For this reason, all the parameters influencing the final structure are deeply investigated to find the best internal and external conditions.1-6 Besides the drawing parameters, such as temperature, pressure, speed, and others, it has been often found that the initial structure of the samples does strongly affect the final properties of the oriented samples. The mechanical properties depend indeed on many factors, often hardly correlated, such as phase composition, features of the amorphous component, crystallite size and perfection, morphology, and polymorphism.7-10 These considerations are particularly relevant in the case of syndiotactic polypropylene (sPP), a polymer exhibiting highly variable mechanical properties depending on orientation, crystallinity, and polymorphism.11-21 One of the most interesting properties of sPP is the elasticity shown by the oriented samples after drawing and releasing the tension.8,9,22-25 This elasticity, closely correlated to the structural organization of the oriented sample, is not shown at all by the original films. Difficulties correlating the elastic behavior to the structural organization of sPP are principally due to its complex polymorphism. * To whom correspondence should be addressed. Fax: 0039 089 964057. E-mail: [email protected].

Four crystalline forms of sPP have been described so far. Forms I and II are characterized by chains in (T2G2)n helical conformation,11,12 whereas forms III and IV present chains in trans-planar and (T6G2T2G2)n conformations,12-14 respectively. Form I is the stable form of sPP obtained under the most common conditions of crystallization either from the melt state or from solution as single crystals. Different kinds and amounts of disorder of the crystalline phase, depending on the degree of stereoregularity and the mechanical and thermal history, were described in rapidly crystallized samples.11,15-18 In samples kept from the melt in a cold bath at 0 °C for many days, the presence of a trans-planar mesophase19,20 was recognized and described. When either the helical form I or the trans-planar mesophase is stretched, the crystalline form III with the chains in trans-planar conformation is obtained when the sample is fixed, whereas the starting conformations are again formed when the tension on the sample is relaxed. To further clarify the origin of the unusual elasticity of sPP, we undertook a deep investigation to correlate the morphology of the initial samples, in terms of crystallinity, crystal dimensions, conformation of the chains, presence of a mesophase, and phase composition, to the structure and elastic properties of the final oriented sample. In a previous paper, we analyzed two samples characterized by different morphology and found that the mechanical parameters, as well as the elastic behavior, did qualitatively correlate with the phase composition and the crystallite size. Moreover it was shown that in the two analyzed cases, the elastic behavior was largely independent of structural changes within the crystals with chains in helical modification, whereas a more recognizable influence was due to the transplanar mesophase.26 In this paper, we report investigations concerning the structure and the elastic behavior of samples obtained by drawing a sample crystallized in the helical form by casting from a solvent.

10.1021/cg0601103 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/03/2006

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This procedure allowed us to obtain, for the first time, biphasic samples with a low percentage of crystallinity in the almost entirely amorphous matrix. In other cases, a very large or nonnegligible fraction of trans-planar mesophase was always present in samples where the crystallinity percentage was not very high. The structural and morphological organization of the sample prepared by solvent casting was investigated by different techniques such as X-ray diffraction, transport properties, infrared spectroscopy, and atomic force microscopy. The results were compared to those of the thermally crystallized samples to obtain and describe a correlation picture among the different parameters. The oriented sample obtained by drawing the solvent cast sample also shows elastic behavior in a range of deformation where there are no crystals actively involved in the elastic recovery of the material. Experimental Section Syndiotactic polypropylene was synthesized according to established procedures.7 The polymer was analyzed by 13C NMR spectroscopy at 120 °C on an AM 250 Bruker spectrometer operating in the FT mode at 62.89 MHz, by dissolving 30 mg of sample in 0.5 mL of C2D2Cl4. Hexamethyl disiloxane was used as internal chemical shift reference. The sample showed 91% syndiotactic pentads. Polymer powders were dissolved in orthodichlorobenzene at high temperature; afterward the solvent was evaporated at 70 °C, obtaining a 0.2 mm solvent cast film (sample C70). To obtain the thermally crystallized samples, already studied in a previous paper,26 the sPP powders were molded in a hot press at 150 °C, forming a film thick 0.2 mm, and rapidly quenched in a bath at room temperature (sample A25). A different film was quenched at 100 °C and left for 1 h at this temperature (sample B100). The C70 film was drawn at room temperature using a dynamometric apparatus INSTRON 4301. The deformation rate was 10 mm/min, and the initial length of the sample was 10 mm. A drawn sample was obtained drawing up to λ ) 7, and it was analyzed at fixed length (sample C707). Afterward it was released and a strong shrinkage was observed, corresponding to λ ) 4 (sample C704). Wide-Angle X-ray Scattering. Wide-angle X-ray diffractograms (WAXD) were obtained using a Philips PW 1710 Powder diffractometer (Cu KR Ni-filtered radiation). The scan rate was 2° θ/min. Diffraction spectra of oriented samples were recorded under vacuum by means of a cylindrical camera with radius of 57.3 mm and X-ray beam direction perpendicular to the drawn direction (Ni-filtered Cu KR radiation). Transport Properties. The transport properties were measured by a microgravimetric method, using a quartz spring balance having an extension of 20 mm/mg. The penetrant used was dichloromethane, and the experiments were conducted at 25 °C. Sorption was measured as a function of vapor activity, a ) p/p0, where p is the actual pressure to which the sample was exposed and p0 is the saturation pressure at the temperature of the experiment. FTIR Analysis. The infrared spectra were obtained in absorbance mode using a Bruker IFS66 FTIR spectrophotometer with a 2 cm-1 resolution (64 scans collected). Atomic Force Microscopy. Sample Preparation. All samples investigated by atomic force microscopy were initially etched. The etching reagent was prepared by stirring 1.0 g of potassium permanganate in a solution mixture of 95 mL of sulfuric acid (95-97%) and 48 mL of orthophosphoric acid (85%). The films were immersed into the fresh etching reagent at room temperature and held under agitation for 24 h. Subsequent washings were done using a cold mixture of 2 parts by volume of concentrated sulfuric acid and 7 parts of water. Furthermore, the samples were washed successively with 30% aqueous hydrogen peroxide to remove any manganese dioxide. The samples were washed with distilled water and kept under vacuum for 2 days. The AFM data were acquired at room temperature in an ambient atmosphere (30%-40% humidity) in tapping mode (TMAFM) with a NanoScope III multimode AFM (Digital Instruments, Santa Barbara, CA) using microfabricated silicon tips and cantilevers. All the images have been recorded simultaneously in height and in amplitude. The height images show the profile of the sample surface quantitatively

Figure 1. X-ray diffractogram of C70 sample. (assuming that the oscillation of the cantilever is damped similarly at all locations), the amplitude images provide in some cases much clearer contrast of the feature imaged compared to height images.

Results and Discussion Structural Organization of the Starting Sample. Crystalline Structure. In Figure 1, we report the X-ray diffraction pattern of sample C70. It indicates that the sample, obtained by casting from orthodichlorobenzene at 70 °C, crystallized in the helical form I, characterized by the most intense peak at 12.3° of 2ϑ, corresponding to the 200 reflection, 15.9° corresponding to the 010 reflection, and 20.8° of 2ϑ corresponding to the 210 reflection. An incipient peak of low intensity at 18.9° of 2ϑ (see the arrow) indicates that a small fraction of the crystalline phase crystallized in the ordered modification of form I, whereas the major part is in the disordered modification, as expected for samples crystallized at low temperature. Indeed the preferential crystallization of the disordered form was always found in powder samples crystallized from the melt at temperatures below 120 °C. In this case, as well-documented in other cases, departures from the fully antichiral packing along both a and b axes apparently occurs, leading to a less ordered form.11,15-18 Although 70 °C is a low temperature to allow the ordered form crystallization, the presence of solvent molecules can have a mobilizing effect similar to an increase of temperature, and indeed the incipient crystallization of the ordered form is visible in the X-ray pattern. Using the pattern of atactic polypropylene to quantify the noncrystalline component, we derived an approximate crystallinity value for sample C70 by comparing the area of the crystalline peaks with the total area, that is,

Rc ) Ac/(Ac + Aa)

(1)

as reported in the literature.27 It resulted in 24% ( 1% as reported in Table 1. The crystal dimensions were determined using the Scherrer equation and are also reported in Table 1 for the values of D200, the coherent crystalline domain size in the direction perpendicular to the (200) planes, and D010, that perpendicular to the (010) planes, obtained from the full width at half-height (fwhm) of the 200 and 010 profiles, respectively. In Table 1, we also report the parameters of the two previously investigated samples, which were thermally crystallized at 25 °C (A25) and 100 °C (B100).26 We observe that the crystal dimensions of sample C70 are intermediate but more similar to

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Table 1. Crystallinity, rc, Crystallite Coherence Lengths Perpendicular to Reflection Planes 200 (D200) and 010 (D010) and Full Width at Half-Maximum, fwhm, Derived from X-ray Diffraction Measurements, the Fraction of Amorphous Phase, ra, the Fraction of Impermeable Phase, rimp ) 1 - ra, and the Mesophase Fraction, rm ) rimp - rc , Derived by Sorption at 0.2 Activitya sample

Rc (%)

Ra (%)

Rimp (%)

Rm (%)

fwhm200 (2ϑ°)

D200 (Å)

fwhm010 (2ϑ°)

D010 (Å)

A25 B100 C70

21 ( 2 30 ( 2 24 ( 1

52 ( 3 59 ( 3 73 ( 3

48 ( 2 41 ( 2 27 ( 2

27 ( 2 11 ( 2 3 ( 0.5

0.521 ( 0.020 0.274 ( 0.012 0.357 ( 0.012

307 ( 11 584 ( 25 448 ( 15

0.830 ( 0.022 0.371 ( 0.012 0.457 ( 0.015

193 ( 6 433 ( 13 351 ( 11

a

Estimated errors are supplied.

the sample crystallized at 100 °C. However, if we compare the absolute values of the crystallite sizes to the values usually reported for polymers crystallized at relatively high undercooling, we observe that they are surprisingly high. These features point to an unusual ability of lamellar crystals of sPP helical modifications to host defects without loosing coherence, at least in certain crystallographic directions. Phase Composition. The phase composition was investigated on the basis of transport properties of vapors at low activity. Procedures to obtain phase composition information from transport data are based on the assumption that the specific sorption of the amorphous phase is the same in samples having different composition, provided a low vapor activity is used. The rigid regions, usually crystalline, are generally impermeable to the vapors at low activity (a e 0.4). At a given vapor activity, we can write

Ceq ) Ceq(amorphous sample)Ra ) Ceq(amorphous sample)(1-Rimp) (2) where Ceq is the equilibrium concentration of a sample at a given activity, Ra and Rimp are the fractions of the permeable (generally amorphous) and impermeable (generally crystalline) phase, respectively, and Ceq(amorphous sample) is the equilibrium concentration of a completely amorphous sample. If the sample contains only the amorphous and the crystalline phases, its sorption, at low activity, compared to the sorption of a completely amorphous sample gives the crystallinity Rc and the amorphous phase, Ra. On the other hand, if the sample contains a mesophase, impermeable to the vapors at low activity, too, the fraction Rimp of the impermeable phase will give the sum of the mesophase and crystalline phase contributions. In fact, generally also mesophases are impermeable to the vapors at low activity.28,29 As for the sorption of the amorphous sample, in our case, because it is not possible to obtain syndiotactic polypropylene in the amorphous state at room temperature, we refer to the sorption at low activity of atactic polypropylene. This assumption was found valid in the case of isotactic polypropylene at different crystallinities.30 In Figure 2, the equilibrium concentration Ceq (grams of vapor per 100 g of dry polymer) is reported for atactic polypropylene and for sample C70. At activity a ) 0.2, we determined the amorphous fraction, Ra, by dividing the sorption of sample C70 by the sorption of aPP. We obtained the fraction of amorphous phase and, according to eq 2, the fraction of impermeable phase, Rimp, reported in Table 1. Comparing this fraction to the X-ray crystallinity, we observe that the two values are very similar, indicating that the sample crystallized in the solvent contains a lower mesophase fraction, almost negligible. This result is very important because, on one hand, it gives further evidence of the reliability of the vapor sorption method, which has been previously demonstrated for various polymeric systems. Indeed, it has been always found that when no mesophase is present, this method gives crystallinity values closely comparable to those obtained by X-ray diffraction (differences typically e3%). On the other hand, it confirms an already suggested result indicating that the ordered form I appears when no mesophase,

Figure 2. Equilibrium concentration of vapor as a function of vapor activity, a ) p/p0, for atactic polypropylene (b), and C70 sample (∆).

Figure 3. TM-AFM image of A25 sample at magnification of 20.0 µm (left, height images; right, amplitude images).

or a very low fraction, is present in the helical sample. In fact, despite a low crystallization temperature, that is, 70 °C, we have observed the incipient peak at 18.9° of 2ϑ (see Figure 1). Atomic Force Microscopy. To have an independent value of the crystal dimensions either for the thermally crystallized and for the solvent cast films, we performed an AFM investigation of all the samples. In Figure 3, we show the AFM micrograph of sample A25 at magnifications of 20.0 µm. The value of 20.0 µm in the AFM micrograph identifies the side of the image square. We observe that some confined areas of the surface show a spherulitic morphology (to see the area of the square in the height image on the left); in other areas, in particular in the height image, small intermediate stages of development of spherulitic growth (hedrites) are evident. In this complex texture in the height image, we also note dark zones due to the partial removal of the amorphous phase caused by the etching. It is worth recalling that the X-ray analysis and the transport properties showed the presence of three phases, crystalline (21%), mesophase (27%), and amorphous (52%). We can safely associate the small

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Figure 5. TM-AFM images of B100 sample at magnification of 3.29 µm (left, height image; right, amplitude image).

Figure 4. TM-AFM images of B100 sample at magnifications of 20.0 and 4.38 µm. (left, height images; right, amplitude images).

spherulites and hedrites to the fraction of crystallinity with chains in helical conformation (crystallographic modification, form I) also because much experimental evidence has shown that where the fraction of this phase increases (quenching the molded powders of sPP at temperature higher than 25 °C), more and more large and complete spherulites develop. In Figure 4, the AFM photos of sample B100 at two different magnifications, 20.0 and 4.38 µm, are shown. In this case, the spherulites are clearly evident and of very near dimensions; they are full up, showing many mutual contacts and a reduced interspherulitic amorphous phase. This means that the high amorphous fraction of sample B100 (59% as shown in Table 1) is mainly located in the intraspherulitic regions. In the AFM micrograph at magnification 4.38 µm, long lamellar crystals with an average thickness of ca. 30-60 nm inside the spherulites are well evident. In Figure 5, a higher magnification, corresponding to 3.29 µm, is shown. The zones where no-fine structure is observable (see the arrow) could be amorphous domains. The AFM photo of sample C70 reported in Figure 6 shows large spherulites of about 4 µm of diameter. At variance with sample B100, in this case, the spherulites are immersed into an amorphous matrix completely segregated. Also in this case, the average thickness of lamellar crystals is of the same order of magnitude of that found in the B100 sample. Comparison between the Thermally and Solvent Crystallized Samples. In Figure 7, we show a comparison of the crystallinity (a), crystallite coherence lengths perpendicular to reflection planes 200 (D200) (b) and 010 (D010) (c), the fraction of the amorphous phase (d), and the fraction of the mesophase (e), reported as a function of the crystallization temperature. We can observe that the first three parameters, regarding the crystalline phase, fit the same straight line for all the samples. This indicates a lower influence of the solvent with respect to the temperature on the crystalline phase. The fraction of mesophase is much lower instead, almost negligible, whereas

Figure 6. TM-AFM images of C70 sample at magnifications of 20.0 and 5.79 µm. (left, height images; right, amplitude images).

the amorphous fraction is much higher in the solvent crystallized sample. The low fraction of mesophase in the solvent cast sample is presumably due to the tendency of the solvent to induce helical conformations in the polymer, thus decreasing the fraction of chains in trans-planar conformation able to form the mesophase. Indeed we have already reported that, when a sample crystallized in trans-planar conformation is kept in a solvent able to increase the mobility of the chains, helical crystallinity is induced at room temperature.31 It is therefore conceivable that in the solvent-induced crystallization the transplanar chains are formed to a lesser extent and show a much reduced tendency to form the mesophase. This leads to a sample, C70, with a very high fraction of amorphous phase (73%), much higher than that in the thermally crystallized samples (52% for the sample crystallized at 25 °C and 59% for the sample crystallized at 100 °C), as reported in Table 1. Drawing Behavior. In Figure 8, we report the stress-strain curve obtained from stretching at room temperature sample C70.

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Figure 7. Crystallinity (a), crystallite coherence lengths perpendicular to reflection planes 200 (D200) (b) and 010 (D010) (c), fraction of amorphous phase (d), and fraction of mesophase (e) of A25, C70, and B100 samples as a function of the crystallization temperature. Figure 9. X-ray diffraction patterns of oriented samples before (C707) and after releasing the tension (C704).

Figure 8. Stress-strain curve of C70 sample.

The tensile stress on the initial undeformed cross-sectional area, τ (MPa), is reported as a function of the percentage of strain. The curve is conventional with an upper and lower yield point, restricted in a very narrow deformation range that, at a macroscopic level, characterizes the appearance of a neck. The local deformation drastically increases in the neck, producing a significant reduction in the local cross section. This effect, in turn, induces a stress concentration that stabilizes the neck, which propagates over the entire sample. This stage of the process occurs at an almost constant load, and only when the neck has propagated to the whole sample, the stress begins increasing more rapidly up to the fracture. We note that most of the amorphous orientation generally occurs in the strainhardening range. The curve is qualitatively more similar to the thermally crystallized sample at 100 °C of the previous paper26 than to the sample crystallized at 25 °C. Sample C70 does not contain an appreciable percentage of mesophase and is similar to the sample crystallized at 100 °C, which also showed a reduced mesophase fraction (11%). Therefore in these samples,

the amorphous phase is less interconnected and orients only in the very last stage of deformation (λ > 6). Structure of the Drawn Samples. X-ray Analysis. The sample drawn at λ ) 7 undergoes a large shrinkage on releasing the tension at room temperature. The crystalline phase composition was investigated in both the fixed (λ ) 7) and relaxed samples (λ ) 4). In Figure 9, we show the X-ray diffraction patterns of the drawn sample before (C707) and after releasing the tension (C704). Repeated cycles of stretching and releasing the oriented sample demonstrate that the shown diffraction patterns are completely reproducible. In the pattern of the fixed sample, C707, we observe, on the equator, the 020 and 110 reflections of the crystalline form III reported by Chatani and also the weak 130 reflection of the same structure. On the first layer, the 021 and the weaker 111 reflections of the trans-planar form, corresponding to an identity period of 5.05 Å, are apparent. However, there is also the presence of the trans-planar mesophase, as evident on the equator, where the reflection at 2ϑ ) 17.0° partially superimposes on the form III reflections. Indeed the reflection at 17° of 2ϑ, if associated with a reflection at 2ϑ ) 23.7° on the layer with periodicity 5.05 Å has already been ascribed10 to a transplanar mesophase adopting an orthohexagonal lattice with a ) 6.02, b ) 10.42, and c ) 5.05 Å. Only very very weak reflections (2ϑ ) 12.3° and 2ϑ ) 20.8°) attributable to crystals with helical chains are observed. Therefore in the C707 sample, the ordered phase is mainly the crystalline form III with little amount of trans-planar mesophase. The X-ray pattern of the unhooked sample C704 shows that the crystalline form III almost completely transformed into the trans-planar mesophase. Three reflections appear on the equator, namely, a very intense one at 2ϑ ) 17.0° (d ) 5.21 Å) and a weak reflection and a very weak one, respectively, at 2ϑ ) 29.7° (d ) 3.01 Å) and at 2ϑ ) 34.4 Å (d ) 2.61 Å), indexed as 100, 110, and 200 of the

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Figure 11. Hysteresis cycles of C704 sample: first cycle strain (%) ) 24, second cycle strain (%) ) 49, and third cycle strain (%) ) 73.

Figure 10. FT/IR spectra in absorbance (700-1300 cm-1) of fibers before (C707) and after releasing the tension (C704).

orthohexagonal lattice. In the relaxed oriented sample, the ordered regions coexist with a substantial amount of amorphous phase, more isotropic than in the fixed oriented sample. Diffuse scattering along the first layer line, except for one relatively sharp maximum of the mesophase at 23.7° of 2ϑ, indicates limited order along the chain direction and scarce, but not negligible, intermolecular correlation along the axial direction. We can notice also that, on release of the tension, the reflections of crystals with chains in helical conformation at (2ϑ ) 12.3° and 2ϑ ) 20.8°) appear less weak than the pattern of C707 sample. Infrared Analysis. The infrared analysis confirms the X-rays results. In Figure 10, we show the FTIR spectra of the drawn sample before (C707) and after releasing the tension (C704). In the spectra of both samples, the trans-planar bands, appearing at 831, 963, and 1132 cm-1, are present and well developed. We can note that in the spectrum of the fixed C707 sample the peak at 831 cm-1 is split into two peaks confirming our previous suggestion that the splitting occurs when form III is present and enough well developed,9 whereas in the relaxed sample, C704, a narrowing of the band at 831 cm-1 can be observed. This is due to the disappearance of the crystalline form III and, as a consequence, of the split previously observed. For both the fixed and relaxed samples, peaks characteristic of helical chain conformation are absent (as the band at 812 cm-1) or very much reduced. However some of these bands, such as the bands at 867 and 903 cm-1, which are recognized as sensitive to the helical conformation,32,33 show light changes in their intensity. In detail, we observe that the helical band at 977 cm-1 appearing as a shoulder of the trans-planar band at 963 cm-1 (see the arrow) in the spectrum of C704 sample almost disappears in C707 sample, while the bands at 867 and 903 cm-1 are still present to a lesser extend confirming a decrease in the percentage of helical chains in the fixed sample. Elastic Behavior. To discuss the elastic behavior, it is worth recalling two main points characterizing this phenomenon in

syndiotactic polypropylene: (i) the elasticity of sPP is never shown in unoriented samples, whereas it is displayed after a plastic deformation of the original samples, upon releasing the tension after drawing; ii) in all investigated cases, when the fully elongated oriented samples are unhooked after drawing, a large shrinkage occurs, giving origin to the elastic behavior. Indeed the elasticity is displayed only in the range between the length of the fixed and relaxed oriented sample, and therefore it is strictly related to the shrinkage. We interpreted the elastic behavior of sPP in the light of the model of the plastic deformation of semicrystalline polymers. The key point of this model is the presence of the tie molecules axially connecting the crystals in the oriented samples, whose extension determines both the retractive stress of the oriented sample and the possibility to crystallize, contributing to the hysteresis and the permanent set after drawing. For this reason, it is interesting to compare samples with different morphologies, as in the present study. In the range between λ ) 4.0 (relaxed sample) and λ ) 7.0 (fixed sample), the elastic behavior was investigated. In Figure 11, we report the hysteresis cycles of sample C704, which was deformed at strain values progressively increasing, step by step, from the relaxed length (λ ) 4) to the highest previously reached length, recording the stress while increasing and decreasing the strain. The highest previously reached length was not exceeded to reduce the possibility of additional plastic deformation of the oriented samples during the measurement of the elastic recovery. The area of each hysteresis curve, reported in Table 2, represents the energy dissipated (DE) in the cycle, and it increases on increasing the strain. The permanent set, that is the residual deformation after each cycle, is very low, reaching the value of 16% for the maximum strain, and it nearly vanishes after 2 h of rest. In Table 2, the values of the two previously investigated samples are also shown, for comparison. We observe that the permanent set is very similar for the three samples, whereas the energy dissipated differs, being, in particular, much higher for the sample drawn from the sample crystallized at 25 °C. In the first two cycles, samples C704 and B1004 have the same values of DE, and they are about half of the sample A254. In the third cycle, the three samples show significantly different

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Table 2. Mechanical Parameters of A254, B1004, and C704 Samples: Elastic Modulus, Dissipated Energy, and Permanent Set dissipated energy (MPa)

sample A254 B1004 C704

set (%)

elastic modulus (MPa)

primary cycle

secondary cycle

tertiary cycle

primary cycle

secondary cycle

tertiary cycle

178 100 193

117 61 61

375 209 209

804 388 495

8 6 5

19 16 12

28 27 16

values of dissipated energy instead, and this parameter increases on decreasing the crystallization temperature of the original sample before drawing. The same behavior is also shown by the stress at the maximum elongation, which is 73% of the initial value for the three samples, being 46 MPa for sample A254, 33 MPa for sample C704, and 16 MPa for sample B1004. The regular and consistent behavior of either the dissipated energy or the value of the maximum reached stress enlightens the influence of the morphological organization resulting from drawing different initial structures. In sample A254, the amorphous phase and the crystalline phases are more intimately connected, since the crystals in the starting sample, A25, are smaller and a large fraction of mesophase is present. This determines, during drawing, a morphological transformation producing a higher fraction of tie molecules connecting the crystalline blocks and, on average, a shorter length of these molecules, which are responsible for the retroactive stress of the sample. Moreover, when the crystallization temperature (lower undercoolings) of the original sample is increased, the chains in the amorphous phase are more and more relaxed, with dimensions nearer to the unperturbed dimensions. Going toward the maximum possible extension of these molecules, the stress more and more increases, and this occurs depending both on the number and on the length of the tie molecules. The decrease of the maximum stress, going from sample A254 to C704 and B1004, well reflects the decreasing number of tie molecules determined by the fragmentation of the original crystals. If in the starting sample the crystals are small and there is a substantial fraction of mesophase, as in sample A25, the morphological transformation from unoriented to oriented state will create a larger number of molecules connecting the crystalline blocks. At variance, large crystals give origin to a lesser fraction of connecting molecules and therefore to a lower stress. On the other hand, in the last stages of the elastic deformation, some of the extended tie molecules crossing the amorphous layers can crystallize. Therefore, after reaching the maximum strain, in the descending part of the hysteresis curve, a lesser amount of active molecules contribute to the total stress, and we observe the hysteretic behavior. Therefore the more tie molecules are present the higher the possibility to have crystallization and, as a consequence, to observe the hysteresis. Indeed sample A254 shows the highest dissipated energy, whereas sample B1004 shows the lowest. Sample C704 displays an intermediate behavior, showing that also in this case the crystal dimensions of the original sample are the determining factor. To further investigate the elastic behavior of sample C704, the stress-strain data of the third cycle were evaluated in terms of the Mooney-Rivlin equation:

1 τ ) 2C1(λ - λ-2) + 2C2(λ - λ-2) λ where τ is the force per unit cross-sectional area, λ is the strain ratio, assuming as λ ) 1 the initial length of the sample, and

Figure 12. τ/(λ - λ-2) as a function of 1/λ for C704 sample. X-ray diffraction patterns of C704 sample at different extensions are also shown.

C1 and C2 are two numerical coefficients. In Figure 12, we show τ/(λ - λ-2) as a function of 1/λ for sample C704. A conventional behavior is observed up to a strain of 45% (corresponding to a real λ ) 5.8) with the function τ/(λ - λ-2) linearly increasing with 1/λ. To follow the structural changes, we performed X-ray analysis at different extensions, from the initial (C704) up to 75% strain (C707), and we report the obtained patterns in the correspondence of the investigated extension. We do not note any change in the ordered structure, essentially composed by the trans-planar mesophase up to a strain of 45%. Only a slightly more polarized amorphous reflection is visible in the more extended sample (λ ) 5.8). Therefore in the elastic range, there is not crystallization of the trans-planar mesophase into form III but only a better orientation of the amorphous component. The strain-induced crystallization occurs at extensions between 50% and 75%, which is the maximum investigated extension, corresponding to sample C707, in which a large fraction of form III was observed, as visible in Figure 12. This result is very important, because it excludes that the elastic behavior, in the present case, is due to a change in the ordered phase, neither the mesophase changes nor the chain conformation of the ordered phase. For strain values higher than 50%, an upturn appears in the curve. The upturn is characterized by a rapid change of the C2 term from positive to negative values. The molecular origin of the upturn in the Mooney-Rivlin representation has been ascribed in the literature either to the limited extensibility of the chains in the elastic network, that is, to a non-Gaussian behavior, or to a strain-induced crystallization. Either explanation is in principle compatible with the presence of the upturn in the plot of sample C704. Strain-induced crystallization has been shown in the interval in which the upturn appears. As a matter of fact, in the extended sample (λ ) 7), the crystalline form III is present, as shown by the X-ray pattern corresponding to this deformation value. Moreover the crystallization involves elastically active chains, limiting their extensibility when part of the chain is blocked in a crystal, thus contributing to the observed behavior.

1710 Crystal Growth & Design, Vol. 6, No. 7, 2006

Conclusions To investigate the influence of the morphology on the elastic behavior of syndiotactic polypropylene, a solvent cast film was prepared and compared to samples thermally crystallized at 25 and 100 °C, giving the following results: The presence of the helical form I was recognized in either sample, with crystal dimensions strictly dependent on the crystallization temperature. The crystallite sizes, determined by X-rays were found to be surprisingly high, pointing to an unusual ability of lamellar crystals of sPP helical modifications to host defects without loosing coherence. The phase composition was influenced by the presence of the solvent, and an almost complete absence of mesophase was found in the solvent-crystallized sample. The drawn sample obtained by drawing the solvent cast sample up to λ ) 7 showed the presence of the trans-planar form III as well as of the trans-planar mesophase. On release of the tension, the crystalline form III almost completely transformed into the mesophase, and only weak reflections of the helical form appeared. The hysteresis cycles allowed us to determine the permanent set and the energy dissipated at increasing strain; this parameter strongly increases on decrease of the crystallization temperature of the original sample. It was correlated to the initial morphology and the plastic deformation of the spherulitic structures. The elastic behavior of the drawn sample was analyzed with the Mooney-Rivlin equation. A truly elastic range was recognized between λ ) 4 and λ ) 5.8. In this range, the X-ray analysis did not show any change of the structure. For higher strain values, an upturn appears in the curve, indicating a straininduced crystallization. This was confirmed by X-rays that showed the presence of the crystalline form III in this range. References (1) Peterlin, A. J. Polym. Sci. 1965, C9, 61. (2) Peterlin, A. Ultrahigh Modulus Polymers; Ciferri, A., Ward, I. W., Eds.; Applied Sciences: Barking. England, 1979. (3) Bassett, D. C. Principles of Polymer Morphology; Cambridge University Press: Cambridge, U.K., 1981. (4) Ward, J. M. Mechanical Properties of Solid Polymers; J. Wiley & Sons: West Sussex, England, 2002. (5) de Candia, F.; Romano, G.; Russo, R.; Vittoria, V. Colloid Polym. Sci. 1987, 265, 696.

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