Morphology and Mechanical Properties of the Mesomorphic Form of

Article pubs.acs.org/Macromolecules

Morphology and Mechanical Properties of the Mesomorphic Form of Isotactic Polypropylene in Stereodefective Polypropylene Claudio De Rosa,* Finizia Auriemma, Rocco Di Girolamo, Odda Ruiz de Ballesteros, Martina Pepe, Oreste Tarallo, and Anna Malafronte Dipartimento di Scienze Chimiche, Università degli studi di Napoli “Federico II”, Complesso Monte S.Angelo, Via Cintia, 80126 Napoli, Italy ABSTRACT: A study of the morphology and the mechanical properties of the mesomorphic form of isotactic polypropylene (iPP) that crystallizes in samples of different stereoregularity prepared with metallocene catalysts is reported. Highly isotactic samples slowly crystallized from the melt in the α form show the typical lamellar morphology with organization in spherulites. Bundle-like elongated crystalline entities and needle-like crystals of γ form are instead observed for stereoirregular samples slowly crystallized from the melt in the γ form. All samples crystallize by fast quenching the melt at 0 °C in the mesomorphic form, regardless of stereoregularity. Crystals of the mesomorphic form always exhibit a nodular morphology and absence of lamellar spherulitic superstructures, independent of the stereoregularity. This morphology accounts for the similar good deformability of all the quenched samples, whatever the concentration of stereodefects. For all samples and any stereoregularity, the nodular morphology with absence of spherulites is preserved after annealing and transformation of the mesophase into α form. The formation of a nodular α form accounts for outstanding properties of high ductility and mechanical strength of the annealed samples crystallized in the α form. The most stereoirregular sample with rr concentration of 11% shows elastic behavior either when it is slowly crystallized in the γ form or when it is crystallized in the mesomorphic form and also after annealing and transformation of the mesophase into the nodular α form.

■

INTRODUCTION The mesomorphic form of isotactic polypropylene (iPP) can be obtained by crystallization form the melt by rapid quenching of the melt to low temperatures (generally 0 °C).1−26 In this condition the development of the monoclinic α form crystals is suppressed and aggregates of the mesomorphic form may grow at the low temperature. The mesomorphic form is metastable and reorganizes on heating into the stable monoclinic α form at a temperature of about 70−80 °C.1,14,17,19−23,27,28 Recently, the mesomorphic form has been obtained also in samples of iPP of different stereoregularity prepared with metallocene catalysts containing different concentration of rr triad stereodefects.29 It has been found that the mesomorphic form can be obtained by quenching the melt at 0 °C also in the case of stereoirregular samples, provided that the permanence time of the sample at 0 °C is long enough.29 Solid mesophases play important role in manufacturing processes of polymeric materials. The mesomorphic form of iPP shows, indeed, peculiar mechanical properties different from that of the stable crystalline form. In fact, while highly crystalline iPP, crystallized in the stable α form, is a rigid and strong material, the same sample crystallized in the mesophase by rapid quenching the melt to low temperature is a ductile and flexible material with greatly enhanced deformability.30 Hence, the © 2013 American Chemical Society

mesophase may be more easily processed, even at room temperature, than the stable crystalline α form. This effect has been related to the nodular morphology of the mesomorphic crystals, with size of the mesomorphic nodular domains of the order of 5−20 nm,4−7,9,10,13,14,16−18,20−22,27 and absence of spherulitic superstructure.30 In this paper we report a study of the morphology and the mechanical properties of the mesomorphic form of iPP that crystallizes in stereodefective iPP samples and of the effect of the phase and morphological transformations induced by annealing on the mechanical properties.

■

EXPERIMENTAL SECTION

Samples of isotactic polypropylene of different stereoregularity have been prepared in the laboratory of Basell Polyolefins (Ferrara, Italy) with the zirconocene catalysts shown in Chart 1 activated with methylalumoxane (MAO).31−33 All the analyzed samples are listed in Table 1. The intrinsic viscosity [η] was measured in tetrahydronaphtalene at 135 °C using standard Ubbelohde viscosimeter. The average molecular Received: March 17, 2013 Revised: June 4, 2013 Published: June 19, 2013 5202

dx.doi.org/10.1021/ma400570k | Macromolecules 2013, 46, 5202−5214

Macromolecules

Article

Chart 1. Structures of C2-Symmetric (1) and C1-Symmetric Zirconocene (2−5) Precatalystsa

a

The concentration of rr stereodefects in iPP samples produced by the catalysts are indicated.

Table 1. Polymerization Temperatures (Tp), Viscosity Average Molecular Masses (Mv), Melting Temperatures of As-Prepared Samples (Tm) and Concentrations of mm, mr, and rr Triads and of the Isotactic mmmm Pentad Stereosequences of iPP Samples Prepared with Catalysts of Chart 1a sample

catalyst

Tp (°C)

Mvb

Tm (°C)c

[mm] (%)

[mr] (%)

[rr] (%)

[mmmm] (%)

iPP(1) iPP(2) iPP(3) iPP(4) iPP(5)

1 2 3 4 5

50 60 60 60 70

195 700 106 000 202 400 210 900 123 400

166.2 143.5 136.4 113.6 89.6

98.52 92.37 88.90 82.25 66.97

0.99 5.08 7.40 11.83 22.02

0.49 2.54 3.70 5.92 11.0

97.55 87.61 82.19 72.17 51.00

a No or negligible regioerrors (2,1-insertions) could be observed in the 13C NMR spectra of the samples.31−33 bFrom the intrinsic viscosities. cThe melting temperatures were obtained with a differential scanning calorimeter Mettler-Toledo DSC-1 performing scans in a flowing N2 atmosphere and heating rate of 10 °C/min.

masses of iPP samples were obtained from their intrinsic viscosity values according to [η] = K (M̅ v)α, with K = 1.93 × 10−4 and α = 0.74.34 The SEC curves of the samples show narrow molecular mass distributions, typical of single-center zirconocene catalysts, with polydispersity indices Mw/Mn variable in the range 2−3. The melting temperatures were obtained with a differential scanning calorimeter Mettler Toledo DSC-1 performing scans in a flowing N2 atmosphere and heating rate of 10 °C/min. All samples of stereodefective iPP have been slowly crystallized from the melt in the conventional α and γ forms, or rapidly quenched from the melt to 0 °C to obtain the mesomorphic form. Powder samples have been heated at temperatures 30−40 °C higher than the melting temperatures of the as-prepared samples, between perfectly flat brass plates under a press at very low pressure, kept at these temperature for 30 min, and then crystallized by slow cooling at 1 °C/min to room temperature, or by fast quenching in an ethyl alcohol bath at 0 °C. For the most stereoirregular sample iPP(5) with concentration of rr defects of 11%, prepared with the catalyst 5 of Chart 1, the mesophase has been obtained keeping the sample at 0 °C for long time (about 10 days).29 All samples have been then annealed at different temperatures up to the melting and analyzed by X-ray diffraction.

X-ray powder diffraction profiles were obtained with Ni-filtered CuKα radiation with an automatic Philips diffractometer. For samples that crystallize as mixture of crystals of α form and mesomorphic form, the indices of crystallinity ( fcryst), the fractions of crystalline phase due to the α form (fα) and the mesomorphic form ( f meso), with fcryst = fα + f meso, have been evaluated from the X-ray powder diffraction profiles according to the procedure reported in the ref 29. Mechanical tests have been performed at room temperature on compression-molded films crystallized in the mesophase by quenching at 0 °C, on the quenched and annealed samples and on oriented fibers with a universal mechanical tester Zwicky by Zwick Roell, following the standard test method for tensile properties of thin plastic sheeting ASTM D882-83. Rectangular specimens 10 mm long, 5 mm wide and 0.3 mm thick have been stretched up to the break or up to a given deformation ε = [(Lf − L0)/L0] × 100, where L0 and Lf are the initial and final lengths of the specimen, respectively. Two benchmarks have been placed on the test specimens and used to measure elongation. Similar tests have also been performed at room temperature on the strained and then stress-relaxed fibers. Stress-relaxed fiber specimens have been prepared by stretching the compression molded films in the quenched mesomorphic form of initial length L0 up to strain ε (final 5203


Macromolecules

Article

length Lf = (ε/100 + 1)L0), keeping the fibers under tension for 10 min at room temperature, then removing the tension, allowing the specimens to relax to the final length Lr. Values of tension set and elastic recovery were measured according to the standard test method ASTM D412−87. The specimens of initial length L0 were stretched up to a length Lf, i.e. up to the deformation ε = [(Lf − L0)/L0] × 100, and held at this elongation for 10 min, then the tension was removed, and the final length of the relaxed specimens Lr was measured after 10 min. The tension set and elastic recovery were calculated as ts(ε) = [(Lr − L0)/L0] × 100 and r(ε) = [(Lf − Lr)/Lr] × 100 = [(ε/100 +1)/(ts(ε)/100 +1) − 1] × 100, respectively, whereas the percentage of the total strain (Lf − L0) that is recovered after releasing the tension is obtained as R = 100(Lf − Lr)/(Lf − L0) = 100[ε − ts(ε)]/ε. Values of tension set and elastic recovery have also been measured after breaking. Ten minutes after breaking, the two pieces of the sample have been fit carefully together so that they are in contact over the full area of the break and the final total length Lr of the specimen has been obtained by measuring the distance between the two benchmarks. The tension set after breaking has been calculated as tb = [(Lr − L0)/L0] × 100, whereas the elastic recovery has been calculated as follows: rb = [(Lf − Lr)/Lr] × 100 and the percentage of the total strain (Lf − L0) that is recovered after breaking is obtained as Rb = 100(Lf − Lr)/(Lf − L0) = 100(εb − tb)/εb. For elastomeric samples, mechanical cycles of stretching and relaxation have been performed at room temperature on the stressrelaxed fibers and the corresponding hystereses have been recorded. In these cycles the stress-relaxed fibers of iPP samples, having the new initial length Lr, have been stretched up to the final lengths Lf, (that is, up to the maximum length achieved during the stretching of the starting unoriented films), so that the deformation achieved during the cycles is ε = 100(Lf − Lr)/Lr, and then relaxed at controlled rate. This precaution was taken in order to avoid further irreversible plastic deformations during the stretching steps. After each cycle, the values of the tension set and percentage of dissipated energy (Ediss) have been measured. The final length of the relaxed specimens Lr′ has been measured 10 min after the end of the relaxation step and the tension set has been calculated as: ts(ε) = [(Lr′ − Lr)/Lr] × 100. The percentage of dissipated energy (Ediss) has been evaluated as the area of the hysteresis, that is, the difference between the area under curves of the stretching and relaxation steps. In the mechanical tests, the ratio between the drawing rate and the initial length was fixed equal to 0.1 mm/(mm × min) for the measurement of Young’s modulus and 10 mm/(mm × min) for the measurement of stress−strain curves and the determination of the other mechanical properties (stress and strain at break and tension set). The reported values of the mechanical properties are averaged over at least five independent experiments. Thins films (90−120 μm thick) of the iPP samples have been prepared for AFM and polarized optical microscopy (POM) experiments and slowly crystallized in α or γ forms or in the quenched mesophase. Small amounts of the powder samples have been sandwiched between glass coverslips, melted at ≈200 °C and then crystallized by slow cooling to room temperature at 10 °C/min or by fast quenching at 0 °C. In the case of the quenching at 0 °C, a Kapton foil has been used as top surface to produce a sufficiently smooth surface all over the support for AFM analysis. In spite of ability of Kapton to nucleate the α form at iPP film surface,35 we have checked, by recording the X-ray powder diffraction of the so obtained films (data not shown), that all quenched samples were crystallized in the mesophase. Atomic force microscopy images have been obtained operating in tapping mode at room temperature with a Veeco Caliber microscope using standard silicon cantilevers TESP-MT and a resonant frequency and force constant of about 250kHz and 50 N m−1, respectively. Height and phase images have been recorded. Optical microphotographs of the samples have been recorded at room temperature in polarized light using a Zeiss Axioscop40 microscope.

samples of Table 1 crystallized from the melt in the mesophase by quenching at 0 °C are shown in Figure 1. The diffraction

Figure 1. Stress−strain curves (A) and X-ray powder diffraction profiles (B) of samples of iPP of different stereoregularity crystallized from the melt in the mesophase by compression molding and quenching at 0 °C. In B the diffraction profiles of the quenched samples (a−e) are compared with those of the same samples crystallized from the melt in α and γ forms by compression molding and slow cooling to room temperature (a′−e′). The (110)α, (040)α, and (130)α reflections of the α form at 2θ = 14°, 17°, and 18.6° and the (117)γ reflection of the γ form at 2θ = 20.1° are indicated.

profiles of quenched samples are compared with those of the same samples crystallized from the melt in α and γ forms by compression molding and slow cooling to room temperature (Figure 1B). All samples crystallize by quenching the melt at 0 °C in the mesophase regardless of the isotacticity (a-e of Figure 1B).29 For the most irregular sample iPP(5), a very small amount of crystals of α form (about 4%) is, however, still present. The same samples crystallize slowly from the melt as mixtures of α and γ forms, the fraction of γ form increases with increasing concentration of rr defects,36−38 as indicated by the presence in Figure 1B(a′−e′) of both (130)α and (117)γ reflections of α and γ forms, respectively, and the increase of the intensity of the (117)γ reflection of the γ form at 2θ = 20.1° with increasing concentration of rr defects. The values of the degree of crystallinity of all mesomorphic samples are reported in Table 2 (data of ununnealed samples at Ta = 25 °C). Only a slight decrease of crystallinity with increasing concentration of rr defects is observed. All mesomorphic samples show similar good ductility with similar values of deformation at break around 600−900%, regardless of the concentration of rr defects. The values of the mechanical parameters measured at 25 °C are reported in Table 2 and in Figure 2A−E as a function of the concentration of rr stereodefects. A decrease of the Young’s modulus with increasing concentration of rr defects (Figure 2A) and of yield stress and tensile strength for samples with concentration of stereodefects

■

RESULTS AND DISCUSSION The stress−strain curves and the X-ray powder diffraction profiles of compression-molded films of the stereodefective iPP 5204


Macromolecules

Article

Table 2. Elastic Modulus (E), Stress (σb), and Strain (εb) at Break, Stress (σy) and Strain (εy) at the Yield Point, Tension Set at Break (tb), Percentage of Total Strain Recovered at Break (Rb), Evaluated from the Stress−Strain Curves of Figures 1A and 5A′− E′, Fraction of the Total Crystallinity ( fcryst), Fraction of Crystals of α Form (fα), and Fraction of the Mesomorphic Form (f meso) Evaluated from the X-ray Diffraction Profiles of Figure 5A−E, for the Samples of iPP of Different Stereoregularity of the Indicated Concentration of rr Defects Crystallized from the Melt by Quenching at 0 °C in the Mesomorphic Form and Annealed at Different Temperatures Ta Ta (°C)

E (MPa)

σy (MPa)

σb (MPa)

25 80 100 110 130

320 ± 50 520 ± 30 570 ± 50 660 ± 80 670 ± 70

18 ± 3 22 ± 1 23 ± 3 30 ± 2 38 ± 1

24 ± 2 22 ± 3 31 ± 4 35 ± 6 36 ± 5

25 60 80 90 120

210 ± 9 370 ± 40 410 ± 34 520 ± 40 630 ± 24

16 ± 1 15 ± 1 17 ± 1 20 ± 3 26 ± 1

25 ± 5 17 ± 3 21 ± 3 25 ± 2 23 ± 3

25 60 80 100 120

180 ± 17 320 ± 40 380 ± 50 470 ± 38 530 ± 70

16 ± 1 17 ± 1 18 ± 1 20 ± 1 24 ± 2

33 ± 5 43 ± 1 16 ± 2 29 ± 5 24 ± 4

25 60 70 90 100

75 ± 8 260 ± 15 310 ± 20 320 ± 26 360 ± 40

6±1 15 ± 1 17 ± 2 17 ± 1 18 ± 1

17 ± 2 27 ± 5 18 ± 4 26 ± 4 21 ± 3

25 50 60 70

25 ± 1 36 ± 5 48 ± 2 25 ± 1

4.0 ± 0.5 7±1 7±1 6.0 ± 0.2

18 ± 2 20 ± 2 20 ± 2 21 ± 1

εy (%)

εb (%)

tb (%)

Sample iPP(1), [rr] = 0.49 mol % 11 ± 2 640 ± 90 420 ± 40 8±1 490 ± 50 310 ± 30 11 ± 4 630 ± 70 420 ± 40 12 ± 2 690 ± 20 430 ± 40 15 ± 1 550 ± 90 370 ± 10 Sample iPP(2), [rr] = 2.54 mol % 620 ± 90 380 ± 40 9±2 330 ± 30 10 ± 2 500 ± 80 13 ± 3 610 ± 50 410 ± 40 10 ± 4 540 ± 80 340 ± 40 290 ± 30 15 ± 2 440 ± 70 Sample iPP(3), [rr] = 3.7 mol % 9±1 780 ± 80 510 ± 40 12 ± 2 900 ± 70 590 ± 30 13 ± 1 560 ± 60 370 ± 30 20 ± 3 570 ± 90 380 ± 20 20 ± 3 560 ± 90 360 ± 30 Sample iPP(4), [rr] = 5.92 mol % 16 ± 1 770 ± 60 500 ± 40 16 ± 3 790 ± 90 510 ± 40 18 ± 4 570 ± 80 390 ± 30 20 ± 4 640 ± 80 430 ± 20 19 ± 3 490 ± 60 320 ± 30 Sample iPP(5), [rr] = 11.0 mol % 27 ± 4 900 ± 60 190 ± 10 34 ± 2 860 ± 60 210 ± 10 41 ± 4 920 ± 90 210 ± 30 38 ± 1 990 ± 30 230 ± 20

higher than 5−6 mol % (Figure 2B,C) is observed, while maintaining constant ductility (Figure 2D). Moreover, whereas stereoregular samples with rr contents lower than 6−7 mol % show irreversible plastic deformation with negligible elastic recovery after breaking, the mesomorphic specimen of the stereoirregular sample iPP(5) with [rr] = 11% shows elastic behavior, as shown by the data of Table 2 that indicate that the tension set measured after breaking decreases and the corresponding percentage of recovered strain increases up to about 80% for the mesomorphic specimens of the stereoirregular sample iPP(5) (Figure 2E). We recall that the stereodefective sample iPP(5) shows elastic properties even when it is slowly crystallized from the melt in the γ form.36−38 The similar ductility of the samples of different stereoregularity crystallized in the mesomorphic form is probably due to the fact that the fraction of the crystalline phase and the crystal morphology in the mesomorphic samples do not greatly change with the stereoregularity (Table 2; see also Figure 6F). The ductility of the mesomorphic samples is in agreement with the well-known good drawability of the mesophase even for high degrees of ordered (crystalline) phase in the case of Ziegler− Natta iPP, which has been attributed to the nodular morphology of the mesomorphic crystals and absence of spherulitic superstructure.30

Rb (%)

fcryst (%)

fα (%)

f meso (%)

34 ± 6 37 ± 6 33 ± 6 38 ± 6 33 ± 2

35 37 42 48 50

0 8 16 46 50

35 29 26 2 0

37 ± 7 33 ± 6 33 ± 7 36 ± 7 34 ± 7

35 37 39 41 50

0 6 16 28 50

35 31 23 13 0

35 ± 5 35 ± 3 33 ± 5 34 ± 4 35 ± 5

33 35 37 40 45

0 6 28 40 45

33 29 9 0 0

36 ± 5 35 ± 5 32 ± 5 33 ± 3 35 ± 6

29 31 34 38 43

0 11 30 38 43

29 20 4 0 0

79 ± 1 76 ± 1 77 ± 3 77 ± 2

21 22 27 27

4 13 26 27

17 9 1 0

The morphology of the mesomorphic form that crystallizes in the stereodefective metallocene iPP samples of Figures 1A and the possible presence of nodules has been studied in comparison with the morphology of samples crystallized in the α or γ forms (samples a′−e′ of Figure 1B). The polarizing optical microscopy images (POM) and AFM phase images of thin films of the samples iPP(1), iPP(3), and iPP(5) with [rr] = 0.49%, 3.7% and 11%, respectively, slowly crystallized form the melt in the α or γ forms (samples a′−e′ of Figure 1B) and crystallized by quenching at 0 °C in the mesophase (samples a−e of Figure 1B) are shown in Figures 3 and 4, respectively. The micrographs of the samples slowly crystallized in the α or γ forms of Figure 3 (X-ray diffraction profiles a′, c′, and e′ of Figure 1B) reveal the typical lamellar morphology of α and γ forms with shape of the crystalline entities that changes and size of crystals that decreases from the sample iPP(1) to the sample iPP(5), with increasing concentration of rr defects. The different morphologies of Figure 3A, B and C reflects the crystallization of different amounts of α and γ forms,39 as a result of the different concentration of rr defects. The micrographs of the highly isotactic sample iPP(1) with 0.49% of rr defects crystallized in the α form (X-ray diffraction profile a′ of Figure 1B) shows formation of lamellae of the α form and space filling spherulitic superstructure (Figure 3A,D). The sample iPP(3) with 3.7 mol % of rr defects crystallizes slowly from the melt mainly in the γ form 5205


Macromolecules

Article

Figure 2. Values of the elastic modulus (E) (A, A′), stress at break (σb) (B, B′) and at the yield point (σy) (C, C′), deformation at break (εb) and at yield (εy) (D, D′) and percentage of total strain recovered at break (E, E′) evaluated from the stress−strain curves of Figures 1A and 5A′−E′ of unoriented compression molded films of the samples of iPP of different stereoregularity crystallized from the melt in the mesomorphic form by quenching at 0 °C and annealed at different temperatures, reported as a function of the concentration of rr stereodefects (A−E) and of the annealing temperature Ta (A′− E′) (Table 2). (A−E) Annealing temperature of 25 °C (●), 50 °C (▷), 60 °C (□), 70 °C (▲), 80 °C (△), 90 °C (■), 100 °C (○), 110 °C (⧫), 120 °C (◊), and 130 °C (▽). (A′−E′) Sample iPP(1) with [rr] = 0.49% (●), sample iPP(2) with [rr] = 2.54% (○), sample iPP(3) with [rr] = 3.7% (▲), sample iPP(4) with [rr] = 5.92% (□), sample iPP(5) with [rr] = 11% (■). 5206


Macromolecules

Article

Figure 3. Polarizing optical microscopy (A-C) and AFM phase (D-F) images of samples of iPP of different stereoregularity crystallized from the melt by slow cooling (10 °C/min) to room temperature in α or γ forms. Samples iPP(1) with [rr] = 0.49 mol % (A,D), iPP(3) with [rr] = 3.7 mol % (B,E) and iPP(5) with [rr] = 11 mol %(C,F). The images D and E are areas of 50 μm × 50 μm, and that in image F of 2 μm × 2 μm.

Figure 4. Polarizing optical microscopy images (A−C) and AFM phase images (D−F) of samples of iPP of different stereoregularity crystallized from the melt in the mesomorphic form by quenching at 0 °C. Samples iPP(1) with [rr] = 0.49 mol % (A, D), iPP(3) with [rr] = 3.7 mol % (B, E) and iPP(5) with [rr] = 11 mol % (C, F). The AFM images represent area of 2 μm × 2 μm.

with about 20% of crystals of α form (X-ray diffraction profile c′ of Figure 1B), and the crystalline supermolecular structure

appears as bundle-like entities, rather than spherulites, organized in a nearly 90° texture (Figure 3B,E). 5207


Macromolecules

Article

when slowly crystallized from the melt in the γ form than when crystallized in the mesophase. It is worth noting that spherulites, bundle-like and elongated needle-like crystals have been obtained in ref 39 for the same sample of metallocene stereoirregular iPP changing the temperature of isothermal crystallization from the melt and favoring the crystallization of α form (spherulites) and γ form (bundle-like and elongated needle-like crystals) at low and high temperatures, respectively. In the case of Figure 3 these morphologies develop at the same crystallization condition by changing the concentration of rr stereodefects, favoring the crystallization of α and γ forms at low and high contents of rr defects, respectively. For all of the three samples iPP(1), iPP(3), and iPP(5) crystallized in the mesophase by quenching the melt at 0 °C, the POM images of Figure 4A−C show absence of birifringent entities of significant size, whereas the AFM images of the same mesomorphic samples of Figure 4D−F indicate the presence of nodules of size ranging from 20 to 75 nm. In the case of the low stereoregular sample iPP(5) the AFM images appear less resolved (Figure 4F) due to the difficulty in realizing a thin film with smooth surface because of the intrinsic sticky properties of this sample. The maximum size of nodular crystals decreases with increasing rr defect concentration from 75 nm in the sample iPP(1) to 55 nm in the sample iPP(5). Therefore, as observed in highly stereoregular Ziegler−Natta iPP,4−7,9,10,13,16−18,20−22,30 also for the stereodefective iPP samples prepared with metallocene catalysts and containing rr stereodefects, the lamellar morphology, with spherulites, bundlelike and needle-like superstructure entities, observed in samples conventionally crystallized from the melt in α or γ forms by slow cooling (Figure 3A−F), is replaced by the nodular morphology when the samples are crystallized in the mesomorphic form by quenching (Figure 4D−F). As argued in ref 30, nodules consist of randomly oriented mesomorphic domains (probably fringed micelle-like entities) embedded in the amorphous phase and likely connected by a larger number of tie molecules, accounting for the ductile behavior. Tensile loading first leads to deformation/alignment of amorphous chain segments. Tie molecules transmit stress to the mesomorphic domains which are interconnected, causing their orientation and alignment into the draw direction,30 resulting in the typical ductility and drawnability of the mesophase (Figure 1A). The similar nodular morphology and the nearly similar fraction of the nodular crystalline phase in all the mesomorphic samples (see Figure 6F), account for the similar ductility regardless of stereoregularity. As discussed above, the data of Figure 2E indicate that the elastic properties observed in the literature for the sample iPP(5) crystallized in the γ form with needle-like crystals (Figure 3C),36−38 are preserved when the sample is crystallized in the quenched mesophase with nodular morphology (Figure 4F), even though with low value of the Young modulus (48 MPa for the γ form36−38 and 25 MPa for the mesophase). Therefore, also the formation of nodular mesomorphic crystals in the quenched sample induces elastomeric properties. The presence of high concentration of rr defects in both crystals and amorphous phase that increases the flexibility of the amorphous chains, plays a role. The variation of the structure and of the mechanical properties of the mesomorphic samples upon annealing have been analyzed and correlated with the possible variation of the morphology. The X-ray powder diffraction profiles and the stress−strain curves of the iPP samples crystallized by quenching at 0 °C in the mesophase and annealed for 30 min at different temperatures are

The sample iPP(5) with 11 mol % of rr defects crystallizes slowly form the melt completely in the γ form (X-ray diffraction profile e′ of Figure 1B) and the morphology changes again with a superstructure characterized by single, along one axis elongated entities (Figure 3C,F).39 Also in this case the single elongated entities seem to be organized in a nearly 90° texture resulting in a interwoven morphology. The appearance of the three different morphologies shown in Figure 3, parts A−C and D−F, is in agreement with the X-ray diffraction profiles a′−e′ of Figure 1B and indicates that with increasing content of the γ form the morphology changes from spherulites to bundle-like and elongated entities. These data are in agreement with early observations by Thomann et al.39 and Lotz et al.40 of the morphology of the γ form of iPP, and more recent TEM data by Cheng et al,41 and homoepitaxial growth of lamellar branches in the α and γ forms of iPP.40,42,43 In the case of α form, the homoepitaxial growth of lamellar branches corresponds to the unique cross-hatched morphology of the α form, which takes place upon crystallization from solution,42a,b melt42c,d and in fibers.42e,g In particular, the cross-hatched structures that develop upon crystallization from solution are so regular that they have been termed “quadrites” by Khoury.42a For the cross-hatched lamellar structure which develops in spherulites of iPP in the α form,42 mother lamellae develop from the center of spherulites according to the radial direction and show on the sides a dense overgrowth of daughter lamellae inclined with their long axis at an angle of ≈80° with respect to the long axis of parent lamellae, according to the self-epitaxial relationship cα(mother)∥aα(daughter) and aα(mother)∥cα(daughter), with aα = 6.65 Å and cα = 6.5 Å the nearly identical unit cell parameters of α form forming an angle of βα = 99.67°.42 In the case of the sample iPP(3) that crystallizes in mixture of α and γ forms forming bundle-like entities (Figure 3B,E), the lateral structure of these entities is due to the lateral ongrowth of the γ form on the mother α form lamellae. It is worth recalling that the angle between the polymer chains in the γ form and the angle of the daughter lamellae with respect to the mother-lamellae (homoepitaxial ongrowth) in the α form are very similar (≈ 80°). Therefore, the key of the epitaxial growth of γ form on parent lamellae of α form is of structural origin, and lies in the nonparallel arrangement of chain axes of γ form at an angle of ≈80°, identical to the complement of angle βα of 99.67° of the monoclinic unit cell of iPP in the α form. Even in the case of the sample iPP(5) that crystallizes nearly completely in the γ form (profile e′ of Figure 1B), the typical lateral ongrowth is visible, with formation of an interwoven morphology. Straight entities appear frequently arranged in an angle of about 80−90° to each other, that corresponds to the crosshatching in the α form. This suggests that the interwoven morphology is formed by γ form ongrowth on crosshatched α form.39 Small amount of crystals of α form or of α/γ disordered modifications are therefore present even in the sample that show X-ray diffraction profile of the pure γ form (profile e′ of Figure 1B). This morphology is in part responsible of the outstanding mechanical properties of high ductility and flexibility and the elastic behavior of the sample iPP(5) slowly crystallized in the γ form.36−38,44 Small and elongated needle-like crystals organized in the interwoven morphology of Figure 3C act as physical cross-links in the amorphous matrix, producing the elastomeric network.36−38 Moreover, in our previous paper44 we have shown that the stereoirregular sample iPP(5) show unexpected better ductility 5208


Macromolecules

Article

Figure 5. X-ray powder diffraction profiles (A−E) and stress−strain curves (A′−E′) of samples of iPP of different stereoregularity containing the indicated concentration of rr defects crystallized from the melt in the mesophase by quenching at 0 °C and annealed at indicated temperatures (Ta).

of the mesophase in the α form starts already at 60 and 50 °C, respectively (profile b of Figure 5D,E). The mesomorphic form transforms into the α form, even for samples with high concentration of rr defects (samples iPP(3), iPP(4), and iPP(5) with [rr] > 3−4 mol %) (Figure 5C−E) that generally crystallize from the melt in normal conditions of slow

compared in Figure 5. Annealing of the mesomorphic samples in the range 60−80 °C produces gradual transformation of the mesophase into α form,14,22,45 regardless of the stereoregularity. For more irregular samples iPP(4) and iPP(5) with concentration of rr defect higher than 5%, the structural transformation 5209


Macromolecules

Article

Figure 6. Fraction of the total crystalline phase (fcryst), fraction of crystals of α form ( fα), and fraction of the mesomorphic form ( f meso), evaluated from the X-ray diffraction profiles of Figure 5A−E (Table 2), for the samples of iPP of different stereoregularity of the indicated concentration of rr defects crystallized from the melt by quenching at 0 °C in the mesomorphic form and annealed at different temperatures, reported as a function of the annealing temperature Ta. In part F values of the total degree of crystallinity of the initial samples crystallized in the mesophase before annealing (○) (profiles a of Figure 5A-E) and of the samples annealed at the highest temperatures crystallized in the α form (●) (diffraction profiles e of Figure 5A−D and profile d of Figure 5E) are reported as a function of the concentration of rr stereodefects.

cooling in the γ form, as shown in Figure 1B.36−38 This result reflects the structure of the mesophase of iPP described in the refs 46 and 47, as small bundles of parallel chains in ordered 3/1 helical conformation with local mode of packing of the chains similar to that of the α form.46 The values of the crystallinity (fcryst) and of the fractions of the mesomorphic form (f meso) and of the α form (fα), with fcryst = f meso + fα, of all annealed samples, evaluated from the diffraction profiles of Figure 5A−E, are reported in Table 2 and in Figure 6 as a function of the annealing temperature. For all samples the total crystallinity increases with increasing annealing temperature indicating that the α form is obtained during annealing not only from the transformation of the mesomorphic form but also by further crystallization of the amorphous phase (Figure 6). The values of the total degree of crystallinity of the initial samples crystallized in the mesophase before annealing (profiles a of Figure 5A−E) and of the samples in the α form annealed at the highest temperature are reported in Figure 6F as a function of the concentration of rr stereodefects. For both mesomorphic and annealed samples in the α form, a slow decrease of the fraction of the crystalline phase is observed with increasing rr concentration (Figure 6F). For each sample, the crystallinity of the annealed sample crystallized in the α form is higher than that of the corresponding mesomorphic sample. The annealing and the transformation of the mesophase into α form affect the mechanical properties of the mesomorphic samples. The stress−strain curves of the annealed samples are shown in Figure 5A′−E′ and the corresponding values of the mechanical parameters are reported in Table 2 and in Figure 2A′−E′ as a function of the annealing temperature. These data show that for all samples the annealing and the gradual transformation into the α form produce a gradual increase of the Young’s modulus (Figure 2A′) and of the stress at yield (Figure 2C′), while maintaining high values of the deformation at break (Figure 2D′). The values of the tensile strength at break are

also nearly constant due to the strain hardening at high deformation that is maintained also for the annealed samples (Figures 5A′−E′ and 2D′,B′). However, the change of the mechanical parameters, apart the modulus and the stress at yield, are small in particular for the most stereoirregular samples iPP(4) and iPP(5) (Figure 5D′,E′). These data suggest that thermal treatments of the mesomorphic samples such to induce transformation into α form is a simple strategy to obtain highly crystalline samples of iPP in the α form showing mechanical properties of ductility similar to those of the mesomorphic form but with increased mechanical strength. The mechanical properties of the annealed samples in the α form have been correlated to the possible change of the nodular morphology of the mesomorphic samples due to annealing and the transformation of the mesophase into the α form. The AFM images of the samples iPP(1), iPP(3) and iPP(5) crystallized in the mesophase and annealed at two different temperatures, such as to induce transformation into the α form, are shown in Figure 7, as representative examples. It is apparent that for all three samples, the nodular morphology of the quenched mesomorphic samples (Figure 4D,E,F and Figure 7A,D,G) is preserved after annealing even at high temperatures (Figure 7B−C,E,F,H,I) at which the transformation of the mesomorphic form into the α form is complete (Figure 5A,C,E). The crystallization via the mesophase and successive annealing is, therefore, a route to obtain α form crystals of iPP with nodular morphology. The transformation of the nodular mesomorphic domains by annealing into α form crystals of nearly identical habit, preserving the nodular morphology and absence of the spherulitic superstructure, explain the outstanding mechanical properties of high ductility of highly crystalline samples of iPP in the α form, similar to those of the mesomorphic form (Figure 5A′−E′). In addition, for the samples iPP(1)-iPP(4) the values of Young’s modulus increase linearly with the annealing temperature (Figure 2A′) due to the increase of crystallinity, 5210


Macromolecules

Article

Figure 7. AFM phase images of films of the samples iPP(1) with [rr] = 0.49% (A-C), iPP(3) with [rr] = 3.7% (D−F) and iPP(5) with [rr] = 11% (G−I) crystallized by quenching at 0 °C in the mesomorphic form (A, D, G) and annealed at the indicated annealing temperatures (B, C, E, F, H, I). All the images represent area of 2 μm × 2 μm.

the intrinsic rigidity of the α form compared to the mesophase, and the formation of bigger nodules with increasing annealing temperature, with consequent increase of plastic resistance and strength. The results of Figures 2, 5 and 7 indicate that it is possible to obtain samples of iPP crystallized in the α form via the mesophase with high stereoregularity and crystallinity, and contemporarily high ductility, similar to that of a mesophase, but with the advantage of higher mechanical strength and thermodynamic stability of the α form compared to the mesophase. A further advantage of this strategy is that it is possible to tailor the strength and rigidity of the iPP based materials (Figure 2) by changing the concentration of rr

stereodefects using different catalysts, and choosing the appropriate annealing temperature. This is clearly shown in Figure 8 where the values of the Young’s modulus of the mesomorphic and annealed samples are reported as a function of concentration of rr stereodefects and annealing temperature. The modulus decreases with decreasing stereoregularity and increases with increasing annealing temperature. Only in the case of the most irregular elastic sample iPP(5) with the highest concentration of rr defects the change of the mechanical properties of the mesophase upon annealing are not significant. The Young’s modulus and the other mechanical parameters are nearly constant with the annealing temperature (Figures 5E′ and 2A′−E′). The stress−strain curves of Figure 5211


Macromolecules

Article

of initial length L0, up to the deformation ε (final length Lf = (ε/ 100 + 1)L0), and, then, removing the tension. The values of tension set ts(ε) and of the percentage of total strain ε which is recovered (R(ε)) after removing the tension in the preparation of fibers from the deformation ε are reported in Table 3. As also shown in Figure 2E and Table 2 for the tension set and strain recovered after breaking, a partial elastic recovery of the initial dimension of the mesomorphic and annealed samples is observed after stretching up to the deformation ε (Table 3). Examples of the hysteresis cycles, composed of the stress− strain curves measured at room temperature during the stretching of these stress-relaxed fibers, immediately followed by the curves measured during the relaxation at controlled rate, are reported in Figure 9. The values of tension set and percentage

Figure 8. Values of the elastic modulus of unoriented compression molded films of the samples of iPP of different stereoregularity crystallized from the melt by quenching at 0 °C in the mesomorphic form and annealed at different temperatures, reported as a function of the concentration of rr stereodefects and the annealing temperatures of 25 °C (●), 50 °C (purple ◀), 60 °C (red ●),70 °C (◆), 80 °C (blue ■), 90 °C (green ■), 100 °C (green ▲), 110 °C (red ▼), 120 °C (pink ◆) and 130 °C (pink ▶).

5E′ of the annealed samples are similar to that of the mesomorphic sample. In particular, the data of Figures 5E′ and 2E,E′and Table 2 indicate that also the annealed specimens of the sample iPP(5) present low values of tension set at breaking and high values of the total strain recovered after breaking of nearly 80%, similar to that of the mesomorphic sample. This indicates that the elastic properties of the mesomorphic sample iPP(5) are preserved after annealing and transformation of the mesophase in the α form. Therefore, the stereoirregular sample iPP(5) shows elastic properties whatever the condition of crystallization and the crystallized polymorphic form (γ, α and mesophase). The most important role in the elastic behavior is played by the flexibility of the stereodefective chains and the presence of small crystals, thin and elongated of the γ form (Figure 3C,F), or nodules of the mesomorphic form (Figures 4F and 7G) or nodules of the α form (Figure 7H,I). The elastic properties of the sample iPP(5) in the mesomophic form and of the annealed samples in the nodular α form have been compared measuring the stress−strain curves in cycles of stretching and relaxation of oriented stress-relaxed fibers, which have been previously prepared by stretching the samples crystallized in the mesophase by quenching and then annealed,

Figure 9. Stress−strain hysteresis cycles recorded at room temperature, composed of stretching and relaxation (at controlled rate) steps according to the direction of the arrows, for the stress-relaxed fibers of the sample iPP(5) with content of rr defects of 11.0% prepared from specimens crystallized in the mesomorphic form (A) and from specimens obtained by annealing of the mesomorphic sample at 70 °C (B). Continuous lines: first cycle. Dashed lines: second cycle and successive cycles.

Table 3. Values of Tension Set (ts(ε)), Elastic Recovery (r(ε)) and Percentage of Strain which is Recovered (R(ε)) after Removing the Tension in the Preparation of Stress-Relaxed Fibers of the sample iPP(5) with [rr] = 11%, and Tension Set (tsI and tsII) and Average Percentage of Dissipated Energy (EdissI and EdissII) Measured in the Stress−Strain Hysteresis Cycles of the Stress-Relaxed Fibers (as those of Figure 9) of the Sample iPP(5) Prepared from the Quenched Mesophase and the Mesophase Annealed at the Annealing Temperature Ta.a Ta (°C)

crystalline form

ε (%)

ts(ε) %

r(ε) (%)

R(ε)

tsI (%)

tsII (%)

EdissI (%)

EdissII (%)

− 50 60 70

mesophase α form α form α form

600 600 600 600

319 ± 2 330 ± 11 310 ± 11 320 ± 2

67 ± 1 63 ± 1 70 ± 2 66 ± 2

47 45 48 46

11 ± 3 8±1 11 ± 1 9±1

3±1 1.5 ± 0.5 1.8 ± 0.5 1.5 ± 0.3

72 ± 2 72 ± 1 74 ± 1 76 ± 2

62 ± 1 64 ± 1 64 ± 1 65 ± 1

The stress-relaxed fibers have been prepared by stretching at the deformation ε compression-molded films of the sample iPP(5) with [rr] = 11%, crystallized in the mesomorphic form by quenching the melt at 0 °C and annealed at the annealing temperature Ta, and, then, removing the tension. a

5212


Macromolecules

Article

Figure 10. Tailoring of properties of iPP through the control of the concentration of stereodefects using metallocene catalysts and the crystallization via the mesophase. The crystallization path via the mesophase (left) results in formation of nodular crystals and a nonspherulitic superstructure, while conventional melt-crystallization by slow cooling from the melt (right) leads to formation of α form lamellae and spherulites and bundle-like and elongated needle-like crystals of γ form. Stiff and flexible materials and thermoplastic elastomers can be produced depending only on the concentration of rr stereodefects, and elastic modulus, ductility, strength and resistance to plastic deformation can be modulated through crystallization in the α and γ forms or crystallization via the mesophase.

melt in the γ form. For all samples the nodular morphology with absence of spherulites is preserved after annealing and transformation of the mesophase into α form. The transformation into nodular α form by annealing produces increase of modulus and yield stress due to increase of crystallinity, while maintaining high ductility, similar to that of the mesomorphic samples. The most stereoirregular sample with rr concentration of 11% shows elastic behavior either when it is slowly crystallized in the γ form or when it is crystallized in the mesomorphic form, even though with remarkable differences in the values of the modulus and tensile strength. The elastic properties are preserved even upon annealing and transformation of the mesophase into the nodular α form. This study indicates that the combination of the control of the concentration of stereodefects using metallocene catalysts and the crystallization of iPP via the mesophase and successive annealing is a powerful tool to tailor properties of polypropylene. A scheme of these possibilities to tailor ultimate properties of iPP is shown in Figure 10. Stiff materials, flexible materials and thermoplastic elastomers can be produced depending only on the concentration of rr stereodefects. In all of these different materials, elastic modulus, ductility, strength, and resistance to plastic deformation can be modulated through choice of the conditions of crystallization from the melt and formation of α and γ forms or the mesophase. The choice of the cooling conditions from the melt predetermines the shape of the crystals, lamellae or nodules. Annealing offers additional option to modify the mechanical properties through the control of crystal size,

of dissipated energy measured after each cycle of Figure 9 are reported in Table 3. It is apparent that the fibers prepared from the mesomorphic form of the sample iPP(5) and from the annealed samples in the nodular α form show a complete elastic recovery already after the first cycle, and successive hysteresis cycles, measured after the third one, are all nearly coincident. The tension set and the dissipated energy decrease in successive cycles and the tension set measured after the third cycle is close to zero, indicating a perfect elastic recovery.

■

CONCLUSIONS The morphology and the mechanical properties of the mesomorphic form isotactic polypropylene that crystallizes in samples of different stereoregularity prepared with metallocene catalysts have been studied. Highly isotactic samples slowly crystallized from the melt in the α form shows the typical lamellar morphology with organization in spherulites. Bundle-like elongated crystalline entities and needle-like crystals of γ form are instead observed for stereoirregular samples slowly crystallized from the melt in the γ form. All samples crystallize by fast quenching the melt at 0 °C in the mesomorphic form, regardless of stereoregularity. Crystals of the mesomorphic form always exhibit a nodular morphology and absence of lamellar spherulitic superstructures. This morphology accounts for the good deformability of all the quenched samples, whatever the concentration of stereodefects. The mesomorphic form transforms by annealing at temperatures higher than 60−80 °C in the α form, regardless of the stereoregularity, even in the case of stereoirregular samples that, generally, crystallize from the 5213


Macromolecules

Article

perfection and crystallinity, for samples slowly crystallized in α and γ forms. The choice of the conditions of annealing of the mesophase permits a precise adjustment of crystallinity and size of the nodular crystals, while the mesophase transforms into the α form without affecting the external habit of the crystals, resulting in the possibility to tailor elastic modulus and mechanical strength and maintaining of the ductility typical of the mesophase.

■

(24) De Santis, F.; Adamovsky, S.; Titomanlio, G.; Schick, C. Macromolecules 2006, 39, 2562. (25) Silvestre, C.; Cimmino, S.; Duraccio, D.; Schick, C. Macromol. Rapid Commun. 2007, 28, 875. (26) Gradys, A.; Sajkiewicz, P.; Minakov, A. A.; Adamovsky, S.; Schick, C.; Hashimoto, T.; Saijo, K. Mater. Sci. Eng. 2005, A413, 442. (27) Fichera, A.; Zannetti, R. Makromol. Chem. 1975, 176, 1885. Marigo, A.; Marega, C.; Zannetti, R. Macromol. Chem. Phys. 1995, 196, 3577. Marega, C.; Causin, V.; Marigo, A. J. Appl. Polym. Sci. 2008, 109, 32. (28) Zannetti, R.; Celotti, G.; Fichera, A.; Francesconi, R. Makromol. Chem. 1969, 128, 137. Zannetti, R.; Celotti, G.; Armigliato, A. Eur. Polym. J. 1970, 6, 879. (29) De Rosa, C.; Auriemma, F.; Di Girolamo, R.; Galotto Galotto, N. Polymer 2012, 53, 2422. (30) Zia, Q.; Radusch, H.-J.; Androsch, R. Polym. Bull. 2009, 63, 755. (31) Resconi, L.; Piemontesi, F.; Camurati, I.; Sudmeijer, O.; Nifant’ev, I. E.; Ivchenko, P. V.; Kuz’mina, L. G. J. Am. Chem. Soc. 1998, 120, 2308. (32) Resconi, L.; Balboni, D.; Baruzzi, G.; Fiori, C.; Guidotti, S. Organometallics 2000, 19, 420. (33) Nifant’ev, I. E.; Laishevtsev, I. P.; Ivchenko, P. V.; Kashulin, I. A.; Guidotti, S.; Piemontesi, F.; Camurati, I.; Resconi, L.; Klusener, P. A. A.; Rijsemus, J. J. H.; de Kloe, K. P.; Korndorffer, F. M. Macromol. Chem. Phys. 2004, 205, 2275. Resconi, L.; Guidotti, S.; Camurati, I.; Frabetti, R.; Focante, F.; Nifant’ev, I. E.; Laishevtsev, I. P. Macromol. Chem. Phys. 2005, 206, 1405. (34) Moraglio, G.; Gianotti, G.; Bonicelli, U. Eur. Polym. J. 1973, 9, 693. (35) Gadzinowska, K.; Piorkowska, E. Polimery 2003, 48, 790. (36) De Rosa, C.; Auriemma, F.; Di Capua, A.; Resconi, L.; Guidotti, S.; Camurati, I.; Nifant’ev, I. E.; Laishevtsev, I. P. J. Am. Chem. Soc. 2004, 126, 17040. (37) C De Rosa, C.; Auriemma, F.; De Lucia, G.; Resconi, L. Polymer 2005, 46, 9461. (38) De Rosa, C.; Auriemma, F. J. Am. Chem. Soc. 2006, 128, 11024. (39) (a) Thomann, R.; Wang, C.; Kressler, J.; Mülhaupt, R. Macromolecules 1996, 29, 8425. (b) Thomann, R.; Semkea, H.; Maiera, R.-D.; Thomann, Y.; Scherblea, J.; Mülhaupt, R.; Kressler, J. Polymer 2001, 42, 4597. (40) Stocker, W.; Magonov, S. N.; Cantow, H. J.; Wittmann, J.-C.; Lotz, B. Macromolecules 1993, 26, 5915. (41) (a) Cao, Y.; Van Horn, R. M.; Tsai, C.-C.; Graham, M. J.; Jeong, K.-U.; Wang, B.; Auriemma, F.; De Rosa, C.; Lotz, B.; Cheng, S. Z. D. Macromolecules 2009, 42, 4758. (b) Cao, Y.; Van Horn, R. M.; Sun, H.-J.; Zhang, G.; Wang, C.-L.; Jeong, K.-U.; Auriemma, F.; De Rosa, C.; Lotz, B.; Cheng, S. Z. D. Macromolecules 2011, 44, 3916. (42) (a) Khoury, F. J. Res. Natl. Bur. Stand. U.S.A. 1966, 70A, 29. (b) Sauer, J. A.; Morrow, N. R.; Richarson, G. C. J. Appl. Phys. 1965, 36, 3017. (c) Norton, D. R.; Keller, A. Polymer 1985, 26, 704. (d) Binsbergen, F. L.; De Lange, B. G. M. Polymer 1968, 9, 23. (e) Compostella, M.; Coen, A.; Bertinotti, F. Angew. Chem. 1962, 74, 618. (f) Katayama, K.; Amano, T.; Nakamura, N. Kolloid Z. Z. Polym. 1968, 125. (g) Andersen, P. G.; Carr, S. H. J. Mater. Sci. 1975, 10, 870. (h) Selikhova, V. T.; Zuaov, Y. U. A.; Markova, G. S.; Kargin, V. A. Polym. Sci. USSR 1965, 7, 232. (i) Padden, F. J.; Keith, H. D. J. Appl. Phys. 1966, 37, 4013. (l) Padden, F. J.; Keith, H. D. J. Appl. Phys. 1973, 44, 1217. (m) Lovinger, A. J. J. Polym. Sci., Polym. Phys. Ed. 1983, 21, 97. (n) Bassett, D. C.; Olley, R. H. Polymer 1984, 25, 935. (o) Lotz, B.; Wittmann, J. C. J. Polym. Sci. Polym. Phys. Ed. 1986, 24, 1541. (43) Zhou, J.-J.; Liu, J.-G.; Yan, S.-K.; Dong, J.-Y.; Li, L.; Chan, C.-M.; Schultz, J. M. Polymer 2005, 46, 4077. (44) De Rosa, C.; Auriemma, F. Ang. Chem. Int. Ed. 2012, 51, 1207. (45) O’Kane, W. J.; Young, R. J.; Ryan, A. J.; Brass, W.; Derbyshire, G. E.; Mant, G. R. Polymer 1994, 35, 1352. (46) Corradini, P.; Petraccone, V.; De Rosa, C.; Guerra, G. Macromolecules 1986, 19, 2699. (47) Corradini, P.; De Rosa, C.; Guerra, G.; Petraccone, V. Polymer 1989, 30, 281. Guerra, G.; Petraccone, V.; De Rosa, C.; Corradini, P. Makromol. Chem., Rapid Commun. 1985, 6, 573.

AUTHOR INFORMATION

Corresponding Author

*(C.D.R.) Telephone: ++39 081 674346. Fax ++39 081 674090. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from Basell Polyolefins (Ferrara, Italy) is gratefully acknowledged.

■

REFERENCES

(1) Natta, G.; Corradini, P. Nuovo Cimento, Suppl. 1960, 15, 40. (2) Natta, G.; Peraldo, M.; Corradini, P. Rend. Fis. Acc. Lincei 1959, 26, 14. (3) Wyckoff, H. Q. J. Polym. Sci. 1962, 62, 83. (4) Bodor, G.; Grell, M.; Kallo, A. Faserforsch. Textiltech. 1964, 15, 527. (5) Gezovich, D. M.; Geil, P. H. Polym. Eng. Sci. 1968, 8, 202. (6) Gailey, J. A.; Ralston, R. H. SPE Trans. 1964, 4, 24. (7) Miller, R. L. Polymer 1960, 1, 135. (8) Grubb, D. T.; Yoon, D. Y. Polym. Commun. 1986, 27, 84. (9) Ogawa, T.; Miyami, H.; Asai, K. J. Phys. Soc. Jpn. 1985, 54, 3668. (10) Hsu, C. C.; Geil, P. H.; Miyaji, H.; Asai, K. J. Polym. Sci., Polym. Phys. 1986, 24, 2379. (11) Piccarolo, S.; Alessi, S.; Brucato, V.; Titomanlio, G. In Crystallization of Polymers; Dosiere, M., Ed.; Kluwer Academics: Berlin, 1993; pp 475. (12) Coccorullo, I.; Pantani, R.; Titomanlio, G. Polymer 2003, 44, 307. (13) Caldas, V.; Brown, G. R.; Nohr, R. S.; MacDonald, J. G.; Raboin, L. E. Polymer 1994, 35, 899. (14) (a) Martorana, A.; Piccarolo, S.; Scichilone, F. Macromol. Chem. Phys. 1997, 198, 597. (b) Martorana, A.; Piccarolo, S.; Sapoundjieva, D. Macromol. Chem. Phys. 1999, 200, 531. (c) Piccarolo, S. J. Macromol. Sci. Phys. 1992, B31, 501. Zhao, J.; Wang, Z.; Niu, Y.; Hsiao, B. S.; Piccarolo, S. J. Phys. Chem. B 2012, 112, 147. (15) Ferrero, A.; Ferracini, E.; Mazzavillani, A.; Malta, V. J. Macromol. Sci., Phys. 2000, B39, 109. (16) Miyamoto, Y.; Fukao, K.; Yoshida, T.; Tsurutani, N.; Miyaji, H. J. Phys. Soc. Jpn. 2000, 69, 1735. (17) Wang, Z. G.; Hsaio, B. S.; Srinivas, S.; Brown, G. M.; Tsou, A. H.; Cheng, S. Z. D.; Stein, R. S. Polymer 2001, 42, 7561. (18) Minami, S.; Tsurutani, N.; Miyaji, H.; Fukao, K.; Miyamoto, Y. Polymer 2004, 45, 1429. (19) (a) Konishi, T.; Nishida, K.; Kanaya, T.; Kaji, K. Macromolecules 2005, 38, 8749. (b) Konishi, T.; Nishida, K.; Kanaya, T. Macromolecules 2006, 39, 8035. (20) Zia, Q.; Androsch, R.; Radusch, H.-J.; Piccarolo, S. Polymer 2006, 47, 8163. (21) Zia, Q.; Radusch, H.-J.; Androsch, R. Polymer 2007, 48, 3504. (22) (a) Zia, Q.; Mileva, D.; Androsch, R. Macromolecules 2008, 41, 8095. (b) Zia, Q.; Androsch, R.; Radusch, H.-J.; Ingolic, E. Polym. Bull. 2008, 60, 791. (c) Mileva, D.; Androsch, R.; Radusch, H.-J. Polym. Bull. 2009, 62, 561. (d) Zia, Q.; Androsch, R. Meas. Sci. Technol. 2009, 20, 097003. (23) (a) Androsch, R. Macromolecules 2008, 41, 533. (b) Mileva, D.; Androsch, R.; Zhuravlev, E.; Schick, C. Macromolecules 2009, 42, 7275. 5214


Morphology and Mechanical Properties of the Mesomorphic Form of

Recommend Documents