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Mechanical Properties and Stress-Induced Phase Transformations of Metallocene Isotactic Poly(1-butene): The Influence of Stereodefects Claudio De Rosa,* Finizia Auriemma, Maurizio Villani,† Odda Ruiz de Ballesteros, Rocco Di Girolamo, Oreste Tarallo, and Anna Malafronte Dipartimento di Scienze Chimiche, Università di Napoli “Federico II”, Complesso Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy

ABSTRACT: A study of the mechanical properties and of the crystallization behavior of samples of isotactic polybutene (iPB) of different stereoregularity prepared with metallocene catalysts is presented. Stereoregular samples with concentration of rr defects lower than 2 mol % crystallize from the melt by compression-molding and fast cooling to room temperature in form II, whereas more stereoirregular samples crystallize surprisingly mainly in form III. The stereodefective iPB samples show interesting mechanical properties of high ductility and flexibility that increase with increasing concentration of rr stereodefects, with values of deformation at break higher than 1000% for the most stereoirregular sample containing about 5 mol % of stereodefects. The aging at room temperature produces transformation of form II into form I in more stereoregular samples, whereas no phase transformation occurs in more stereoirregular samples crystallized mainly in form III. A strong increase of Young’s modulus and of stress at yield is observed after transformation of form II into form I. For more stereoirregular samples with concentration of rr defects higher than 2 mol % crystallized in form III, the mechanical properties do not change significantly upon aging at room temperature because no phase transformations occur during aging. Also for the aged samples, a remarkable increase of ductility and flexibility with increasing concentration of rr stereodefects is observed. The mechanical deformation is associated with stress induced transformations of form II and form III into form I. These results indicate that mechanical properties of iPB can be modified by the controlled incorporation of stereodefects and improvement of ductility is easily obtained while maintaining high crystallinity by inclusion of small amounts of rr stereodefects.

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INTRODUCTION Isotactic poly(1-butene) (iPB) crystallizes in three different polymorphic forms, defined forms I, II and III.1−12 A fourth polymorphic form, defined form I′ has been also described.10 Form I is the most stable form and is characterized by chains in 3/1 helical conformation packed in a trigonal unit cell (space group R3c or R3̅c).1 Form II is characterized by chains in 11/3 helical conformation packed in a tetragonal unit cell (space group P4̅),2−5 whereas in form III chains with 4/1 helical conformation are packed in an orthorhombic unit cell (space group P212121).6−8 The tetragonal form II is the kinetically favored modification of iPB and is generally obtained by melt crystallization.2,4,9,13−19 Form II transforms into the most stable form I on storage at room temperature.1,9,13−24 Form III is generally obtained by crystallization form diluite solutions.9−11 Two variants of form I with identical crystal structure, defined forms I and I′, are produced in different crystallization conditions. Form I melts at about 130 °C and refers usually to a crystal modification that is generated via a solid-state © 2014 American Chemical Society

transformation of form II by aging at room temperature, whereas form I′ melts at lower temperature, at about 90−100 °C, and refers to the same trigonal crystal structure with chains in 3/1 helical conformation obtained by direct crystallization from the melt or solution.10 Because of this difference form I′ has been regarded to be an imperfect form I with many defects.25 iPB presents interesting mechanical and physical properties and exhibits advantages over other polyolefins, like polyethylene and polypropylene, in superior toughness, tear strength, flexibility, creep, and impact resistance. It could find applications, for instance, in pressurized tanks, tubes, and hot water pipes. However, these useful properties belong to the most stable trigonal form I, which, generally, cannot be obtained by conventional processing and crystallization from Received: October 29, 2013 Revised: January 1, 2014 Published: January 21, 2014 1053

dx.doi.org/10.1021/ma402239k | Macromolecules 2014, 47, 1053−1064

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Chart 1. Structures of C1-Symmetric Zirconocene Catalysts Used in the Butene Polymerization

Table 1. Intrinsic Viscosity [η], Average Molecular Masses (M̅ v), Content of mm, mr, and rr Triads, Concentration of Isotactic Pentad mmmm, and Melting Temperatures (Tm) of Samples Crystallized from the Melt for Samples of iPB Prepared with the Catalysts of Chart 1 sample

catalyst

[η] dL/g

Mva

[mmmm] (%)

[mm] (%)

[mr] (%)

[rr] (%)

Tmb (°C)

iPBZN iPB1 iPB2a iPB2b iPB3 iPB4 iPB5 iPB6a iPB6b iPB7

ZN 1 2 2 3 4 5 6 6 7

1.40 1.26 1.37 1.09 1.43 1.75 1.19 0.93 1.08 0.92

235 000 204 000 229 300 167 300 243 300 321 400 188 800 134 400 165 200 132 400

98 98 96.2 96.0 94.2 92.2 87.7 86.5 86.5 77.7

98.9 98.85 97.7 97.6 96.5 95.7 92.4 91.6 91.6 86

0.7 0.77 1.5 1.6 2.3 2.9 5 5.6 5.6 9.4

0.4 0.38 0.8 0.8 1.2 1.4 2.5 2.8 2.8 4.7

118 110 105 104 100 96 81 (III)−83 (II) 77 (III)−84 (I’) 77 (III)−84 (I’) 68 (II)−84 (I’)

a From the values of intrinsic viscosity. bMelting temperatures of samples crystallized from the melt by compression molding and cooling to room temperature at about 20 °C/min, measured from DSC scans at heating rate of 10 °C/min. According to the X-ray powder diffraction profiles of Figure 1, the melting temperatures of melt-crystallized samples correspond to the melting of crystals of form II for samples with [rr] < 2 mol %, and to the melting of mixtures of crystals of forms III and II or forms III and I′ or forms II and I′ for samples with [rr] > 2 mol %.

center homogeneous metallocene catalysts,37−39 which have allowed a perfect control over the chain microstructure, and iPB samples characterized by different kinds and amounts of regio- and stereo-irregularities, with random distribution of defects and molecular masses covering the whole range of practical interest conditions have been produced under industrially significant polymerization.37−39 Depending on the structure of the metallocene complex, the polymer stereoregularity can be finely tuned from the value of the isotactic mm triad content of nearly 85% up to the high value of 99%. The structural analysis of these samples has allowed studying the influence of the rr triad stereodefects on the polymorphic behavior of iPB.38 It has been found that stereodefective iPB samples, containing concentration of rr stereodefects higher than 2−3 mol %, crystallize form the melt directly in the form I.38,39 This was the first experimental observation of the crystallization of the stable form I of iPB from the melt at atmospheric pressure.38,39 Similar strategies of modification of the molecular structure by metallocene catalysis in the case of isotactic polypropylene

the melt but forms upon spontaneous solid phase transformation at room temperature of the metastable kinetically favored form II that, instead, crystallizes from the melt.9 This transformation alters the physical properties and the material becomes on aging more rigid with higher strength,9,13,24,26,27 with increase of melting temperature and density. Undesirable effects, such as shrinkage of the molded objects generated by densification, are generally observed. This phase transition requires several days to be completed at room temperature,9,13,18,23,24,26−32 and longer times are required upon aging, both at lower and higher temperatures.9,33 This slow kinetics has considerably limited the application of iPB and prevented its larger commercial diffusion. Various strategies have been proposed for finding solutions to accelerate the phase transition, as modification of the molecular structure by copolymerization.3,23,34−36 These studies of the physical properties of iPB and of the phase transformations have been performed mainly on iPB samples prepared with heterogeneous Ziegler−Natta catalysts. Recently, novel samples of iPB have been prepared with single1054


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Figure 1. X-ray powder diffraction profiles of samples of iPB of different stereoregularity with the indicated concentration of rr stereodefects, crystallized from the melt by compression molding and cooling the melt to room temperature at cooling rate of about 20 °C/min. The (110)I, (300)I and (220)I + (211)I reflections of form I at 2θ = 9.9, 17.3 and 20.5°, respectively, the (200)II, (220)II, and (213)II + (311)II reflections of form II at 2θ = 11.9, 16.9, and 18.3°, respectively, and the (110)III, (200)III, (111)III, (201)III, (120)III, and (301)III reflections of form III at 2θ = 12.2, 14.3, 17.2, 18.7, 21.2, and 24.6°, respectively, are indicated. molecular masses Mv were determined from the intrinsic viscosity values according to the relationship [η] = K (M̅ v)α, with K = 1.78 × 10−4 and α = 0.725.37 The mass average molecular masses were evaluated from size exclusion cromatography (SEC). The SEC curves of all samples show narrow molecular weight distributions, with Mw/Mn = 2−3, typical of single-center metallocene catalysts. The calorimetric measurements were performed with a differential scanning calorimeter Mettler Toledo (DSC-822), calibrated with indium, in a flowing N2 atmosphere. The melting temperatures of the samples were taken as the peak temperature of the DSC curves recorded at 10 °C/min. Compression molded films were prepared by melting powders of the as polymerized samples at temperatures 30−40 °C higher than the melting temperature under a pressure lower than 5 bar, to avoid preferred orientations in the film, and cooling to room temperature at cooling rate of about 20 °C/min, by circulation of cold water in press plates. Oriented fibers of the copolymer samples have been obtained by stretching at room temperature compression molded films at different degrees of deformation. X-ray diffraction patterns were obtained with Ni-filtered Cu Kα radiation. The powder profiles were obtained with an automatic Philips diffractometer performing a continuous scan of the diffraction angle 2θ from 5 to 40° at scanning rate of 0.02° (2θ)/s. The fiber diffraction patterns were recorded on a BAS-MS imaging plate (FUJIFILM) using a cylindrical camera and digitized with a digital imaging reader (Perkin-Elmer Cyclone Plus). The indices of crystallinity (xc) were evaluated from the X-ray powder diffraction profiles by the ratio between the crystalline diffraction area and the total area of the diffraction profile. The crystalline diffraction area was obtained from the total area of the diffraction profile by subtracting the diffraction halo of the amorphous phase after suitable scaling. The diffraction halo of the amorphous phase was determined from the diffraction profile of a sample of atactic poly(1-butene). The mechanical tests were performed at room temperature on unoriented compression molded films 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 20 mm long, 2−4 mm width and 0.3−0.5

(iPP) have demonstrated that the controlled incorporation of stereo- and regio-defects or comonomeric units alters the mechanical properties of iPP and samples of iPP showing a range of different mechanical properties, from stiff materials to flexible and elastomeric materials, can be prepared.40−46 Moreover, the random distribution of defects has afforded opportunities for studying the influence of a single type of defect on the crystallization and physical properties of iPP and precise relationships between chain microstructure and polymorphic behavior have been found.40−47 Therefore, it is expected a modification of the mechanical properties of iPB in samples produced with metallocene catalysts incorporating stereodefects that crystallize in form I. In this paper, we report a detailed study of the mechanical properties of iPB samples of different stereoregularity prepared with different metallocene catalysts described in ref 37. The samples are characterized by the presence of only one type of stereodefects (basically isolated rr triad defects) with variable concentration. This molecular feature has allowed studying the effect of the presence of stereodefects on the crystallization behavior and mechanical properties of iPB.

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EXPERIMENTAL SECTION

Samples of iPB were prepared with different C1-symmetric ansazirconocene catalysts, shown in Chart 1, activated with methylalumoxane (MAO), as described in ref 37. These complexes are based on the (substituted indenyl)-dimethylsilyl-[bis(2-methylthieno)cyclopentadienyl] ligand framework,37,38,40 and produce highly regioregular iPBs characterized by different stereoregularity, in particular different concentrations of rr triad stereodefects, depending on the indenyl substituents.37,38 All samples were prepared in liquid 1butene at polymerization temperature in the range 50−90 °C. A list of the iPB samples is reported in Table 1. The concentrations of the mm, mr, and rr triads and of the isotactic pentad mmmm of all samples were obtained from 13C NMR analysis. The intrinsic viscosity [η] was measured in tetrahydronaphthalene at 135 °C using standard Ubbelohde viscometer. The viscosity average 1055


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Figure 2. X-ray powder diffraction profiles of samples of iPB of different stereoregularity with the indicated concentration of rr stereodefects, crystallized from the melt by compression molding and cooling the melt to room temperature and aged at room temperature for more than one month. The (110)I, (300)I, and (220)I + (211)I reflections of form I at 2θ = 9.9, 17.3, and 20.5°, respectively, and the (110)III, (200)III, (111)III, (201)III, (120)III and (301)III reflections of form III at 2θ = 12.2, 14.3, 17.2, 18.7, 21.2, and 24.6°, respectively, are indicated.

Table 2. Values of Young’s Modulus (E), Stress (σy), and Strain (εy) at the Yield Point, Stress (σb), and Strain (εb) at Break, Tension Set at Break (tb), Concentration of rr Stereodefects ([rr]), X-ray Degree of Crystallinity (xc), and Crystalline Forms of Compression Molded Films of the iPB Samples As-Crystallized and Aged at Room Temperature for 1 Month (Samples of Figures 1 and 2), Evaluated from the Stress−Strain Curves of Figure 3 sample

[rr] (%)

crystallization

E (MPa)

iPBZN iPBZN iPB1 iPB1 iPB2a iPB2a iPB2b iPB2b iPB3 iPB3 iPB4 iPB4 iPB5 iPB5 iPB6a iPB6a iPB6b iPB6b iPB7 iPB7

0.4 0.4 0.38 0.38 0.8 0.8 0.8 0.8 1.2 1.2 1.4 1.4 2.5 2.5 2.8 2.8 2.8 2.8 4.7 4.7

as-crystallized aged as-crystallized aged as-crystallized aged as-crystallized aged as-crystallized aged as-crystallized aged as-crystallized aged as-crystallized aged as-crystallized aged as-crystallized aged

70 190 50 180 70 190 50 180 60 150 45 145 39 112 31 68 47 103 34 69

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

10 30 10 15 5 11 10 26 15 13 9 5 8 8 4 4 2 6 2 9

σy (MPa) 5 22 4 21 5 21 4 22 4 18 5 15 5 15 7 8 6 9 3 10

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

εy (%) 13 35 10 36 13 32 13 35 13 32 14 28 18 28 21 21 20 23 21 22

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

3 8 1 5 2 7 1 5 1 1 3 2 2 1 2 1 3 1 1 2

mm thick cut from unoriented compression molded films were stretched up to the break or up to a given strain ε = 100 × (Lf − L0)/ L0, with Lf and L0 the final and initial lengths of the specimen, respectively. Two benchmarks were placed on the test specimens and used to measure elongation. Values of the tension set (the residual deformation after stretching and successive relaxation) were measured after breaking (tb), following the procedure described in the ASTM D 412−87. Specimens of initial length L0 were stretched up to the break. Ten minutes after breaking, the two pieces of the sample were fit carefully together so that they are in contact over the full area of the break and the final total length Lr of

σb (MPa) 34 25 23 38 33 27 28 32 34 39 42 46 38 37 39 37 37 38 34 30

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

2 2 6 2 2 1 1 4 3 1 5 4 2 4 2 4 5 3 1 4

εb (%) 290 162 260 380 460 410 430 460 490 590 630 660 690 720 880 660 760 720 1020 590

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

23 6 40 37 38 32 40 70 22 13 97 80 53 51 34 52 58 90 48 68

tb (%)

xc (%)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

58 59 59 60 55 57 56 58 56 56 53 54 48 52 46 48 47 49 40 48

190 90 130 250 330 190 270 300 300 690 390 760 510 450 570 460 580 530 800 360

39 10 25 15 25 32 21 25 11 90 52 81 79 33 77 19 49 60 39 34

crystal form form form form form form form form form form form form form form form form form form form form form

II I II I II I II I II I II I II + form III I + form III III + form I′ III + form I′ III + form I′ III + form I′ II + form I′ I

the specimen was obtained by measuring the distance between the two benchmarks. The tension set at break was then calculated as tb = 100 × (Lr − L0)/L0. 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 yield and at break and tension set). The reported stress−strain curves and the values of the mechanical properties are averaged over at least five independent experiments. 1056


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RESULTS AND DISCUSSION The iPB samples prepared with the C1-symmetric complexes of Chart 1 are fully regioregular and contain only defects of stereoregularity represented by rr triads, which are randomly distributed along the polymer chains with a concentration in the range 0.3−5 mol % depending on the catalyst structure (Table 1).37 The X-ray powder diffraction profiles of samples crystallized from the melt of the iPB samples of Table 1, obtained by compression molding and cooling to room temperature at about 20 °C/min, recorded soon after the preparation of the films are reported in Figure 1. The diffraction profiles of the same melt-crystallized samples after aging at room temperature for long time to allow complete transformation of form II into form I at room temperature are reported in Figure 2. For both melt-crystallized samples and aged samples the degree of crystallinity (Table 2) decreases with increasing concentration of rr defects (see also Figure 4F). More isotactic samples with concentration of rr stereodefects lower than 2 mol % and the Ziegler−Natta sample, crystallize from the melt into form II, as indicated by the presence of the (200)II, (220)II, and (213)II + (311)II reflections at 2θ = 11.9, 16.9, and 18.3°, respectively, in the diffraction profiles a-f of Figure 1. More stereoirregular samples, with concentration of rr defects higher than 2 mol %, crystallize by compression molding in mixtures of form II and form III (sample iPB5, Figure 1g), or in mixtures of form III and form I (samples iPB6a and iPB6b, Figure 1h,i), whereas the most irregular sample iPB7 crystallizes in mixture of form II and form I (Figure 1l) with high concentration of form I. This is indicated by the presence in the diffraction profiles g, h and i of Figure 1 of the (110)III, (200)III, (111)III, and (201)III reflections of form III at 2θ = 12.2°, 14.3°, 17.2°, and 18.7°, respectively, and, in some samples, by the presence of the (110)I, (300)I and (220)I + (211)I reflections of form I at 2θ = 9.9, 17.3, and 20.5°, respectively (profiles h, i, and l of Figure 1), and of the reflections of form II at 2θ = 11.9, 16.9 and 18.3° (profiles g and l of Figure 1). These data on one hand confirm our previous observation of the crystallization of form I directly from the melt in stereodefective iPB samples,38,39 and on the other hand represent the novelty of the crystallization of form III from the melt. Since form I refers usually to a crystal modification that is generated via a solid-state transformation of form II by aging at room temperature, and form I′ refers to the same trigonal crystal structure obtained by direct crystallization from melt or solution,10 the crystalline form I obtained directly from the melt in the stereoirregular iPB samples of Figure 1 (profiles h, i, and l of Figure 1) should be defined as form I′. It is worth noting that in our previous paper the pure form I′ has been obtained by slow melt-crystallization at low cooling rates (2.5 °C/min),38 whereas mixtures of form II and form I have been obtained for the same samples crystallized at higher cooling rates (10−40 °C/min). In the experiment of Figure 1, form I′ is instead always in mixture with form II or form III and the pure form I′ has not been obtained even in the most irregular sample (profile l of Figure 1). This is due to the fact that the samples of Figure 1 are crystallized under pressure (≈ 4−5 bar) by compression molding and cooling to room temperature at high cooling rate of about 20 °C/min. As in the case of ref 38, the fast cooling from the melt favors crystallization of form II, whereas the novelty of the crystallization of form III from the

melt in stereoirregular samples is probably due to the presence of pressure during crystallization. The X-ray powder diffraction profiles of compression molded samples of Figure 1 aged at room temperature for long time (more than one month) are shown in Figure 2. The aging time is sufficient to allow in samples crystallized in form II the complete transformation of form II into form I. For more stereoregular samples with concentration of rr stereodefects lower than 2 mol % and for the Ziegler−Natta sample, form II crystallized from the melt (Figure 1a−f) spontaneously transforms into form I by aging at room temperature, as shown by the fact that the reflections of form II are replaced by the (110)I, (300)I, and (220)I + (211)I reflections of form I at 2θ = 9.9, 17.3, and 20.5°, respectively, in the X-ray diffraction profiles a−f of Figure 2. For these stereoregular samples the degree of crystallinity does not change upon aging (Table 2 and Figure 4F), indicating that during aging transformation of form II into form I occurs without further crystallization. For more irregular samples that crystallize from the melt in the form III, in mixture with small amounts of form I (samples iPB6a and iPB6b, profiles h and i of Figure 1), the diffraction profiles do not change upon aging (profiles h and i of Figure 2), indicating the absence of crystals of form II and of phase transformation during aging. The presence of crystals of form II in some stereoirregular samples crystallized from the melt, in mixture with form III (sample iPB5, profile g of Figure 1), or in mixture with form I (sample iPB7, profile l of Figure 1), is clearly demonstrated by the decrease of the intensity, or the disappearance, of the reflection peak at 2θ = 11.9°, corresponding to the (200)II reflection of form II, and the increase of the intensity of the diffraction peak at 2θ = 9.9°, corresponding to the (110)I reflection of form I in the diffraction profiles g, l of Figure 2, due to the transformation of form II into form I. In these stereoirregular samples iPB5 and iPB7 the transformation of form II into form I is associated with a slight increase of crystallinity (Table 2 and Figure 4F). Correspondingly, for the aged samples, as the concentration of rr defects increases, a decrease of crystallinity lower than that observed for the melt-crystallized samples is observed (Figure 4F). The stress−strain curves of compression molded films of the stereodefective iPB samples of Figure 1 before aging at room temperature are reported in Figure 3A in comparison with the curve of the Ziegler−Natta sample iPBZN. The values of the mechanical parameters and of the degree of crystallinity are reported in Table 2 and in Figure 4 as a function of stereodefect concentration. The stress−strain curve of the highly isotactic Ziegler−Natta sample iPBZN shows the typical deformation behavior of form II with low yield stress, relatively low modulus and nearly uniform deformation, with very small yielding and strong strain hardening at high degree of deformation and rather ductile behavior with about 300% deformation at break (Figure 3A). This indicates the easy deformability of crystals of form II with low resistance to the plastic deformation. For the metallocene iPB samples a similar behavior is observed but with remarkable increase of ductility and flexibility with increasing concentration of rr stereodefects, with deformation at break higher than 1000% for the most irregular sample (sample iPB7) with 4.7 mol % of rr defects (Figures 3A and 4E). The strong increase of ductility is associated with a similar strong strain hardening at high deformation and a very small decrease of modulus with increasing defect concentration (Figure 4A), while the values of 1057


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decrease of deformation at break after aging is observed for the more stereoirregular samples (Figure 4E). Only in the case of more stereoirregular samples with concentration of rr defects higher than 2 mol % crystallized from the melt with high percentage of crystals of form III in mixture with form I and absence of form II (samples iPB6a and iPB6b, profiles h and i of Figure 1), the stress−strain curves do not change significantly upon aging at room temperature (Figure 3) because no phase transformations occur during aging (profiles h and i of Figure 2). Correspondingly, the values of Young’s modulus of these samples in form III are similar before and after aging and are lower than those of the aged samples in form I (Figure 4A). In any case also for the aged samples (Figure 3B) a remarkable increase of ductility and flexibility with increasing concentration of rr stereodefects is observed, in particular with respect to the Ziegler−Natta sample iPBZN, with deformation at break up to 700−800% for the most irregular samples (Figures 3B and 4E). Moreover, the Young’s modulus and the stress at yield decrease with increasing rr defect concentration (Figure 4A,B), with a greater decrease than that observed for the melt-crystallized and not aged samples (Figure 4A,B). The change of the mechanical properties upon aging for sample initially crystallized in form II and the absence of significant changes for samples initially crystallized in form III is better evidenced in the comparison of the stress−strain curves of as-crystallized and aged samples of Figure 5, for the Ziegler− Natta sample (Figure 5A) and for metallocene stereoregular (Figure 5B) and stereoirregular samples (Figure 5C). It is apparent that for the sample iPB6b with [rr] = 2.8 mol % crystallized mainly in form III, the stress−strain curves of crystallized and aged specimens are very similar (Figure 5C), whereas increase of modulus and stress at yield upon aging is observed for both ZN and metallocene stereoregular samples crystallized in form II due to the transformation of form II into form I (Figure 5A,B). However, it is evident that in the metallocene sample iPB2b, the presence of a small amount of rr defects (0.8 mol %), similar to that present in the Ziegler− Natta sample, already produces improvement of ductility compared with the Ziegler−Natta sample (Figure 5B). The data of Table 2 show that the values of the residual deformation (tension set) measured after breaking for all the iPB samples are always very high and similar to the values of the deformation at break, indicating that all stereodefective iPB samples do not present elastomeric behavior, as in the case of the Ziegler−Natta sample. We recall that in the case of metallocene iPPs, prepared with similar metallocene catalysts, incorporation of rr stereodefects for concentrations higher than 6−7 mol % induces elastomeric properties.40−43 This outstanding behavior has been attributed to the unique crystallization behavior of iPP that is able to crystallize even at very high concentrations of stereodefects up to 17 mol % with non-negligible degree of crystallinity of 20−40%, thanks to the crystallization of γ form that affords inclusion in its defective crystals of high concentrations of rr stereodefects.40−45 The small, thin and elongated needle-like defective crystals of γ form48 embedded in the flexible amorphous chains induce elastomeric behavior.40−43 In the case of iPB, the maximum amount of rr stereodefects generated by the metallocene catalysts of Chart 1 that is compatible with crystallization of iPB is only 4.7 mol %. The most stereoirregular iPB sample that is still able to crystallize is, indeed, produced with catalysts 7 of Chart 1 and contain only 4.7 mol

Figure 3. Stress−strain curves of samples of iPB of different stereoregularity with the indicated concentration of rr stereodefects, crystallized from the melt by compression molding and cooling the melt to room temperature at high cooling rate (A) and after aging at room temperature for 1 month (B).

stress at yielding and at break remain nearly constant (Figure 4B,D). For the more stereoirregular samples iPB6b and iPB7 with [rr] = 2.8 and 4.7 mol % that crystallize with nonnegligible amounts of crystals of form I, in mixture with form III or form II, the yielding phenomenon is more evident along with a region of cold drawing with constant value of stress before the strain hardening (Figure 3A). The stress−strain curves of the same melt-crystallized compression-molded iPB samples after aging at room temperature for one month (samples of Figure 2) and complete transformation of crystals of form II into form I, are shown in Figure 3B. The values of the mechanical parameters and of the degree of crystallinity are reported in Table 2 and in Figure 4 as a function of stereodefects concentration. The comparison of the stress−strain curves of the melt-crystallized samples before (Figure 3A) and after aging (Figure 3B) at room temperature indicates that for nearly all samples a clear increase of Young’s modulus and of stress at yield (Figure 4A,B) is observed after transformation at room temperature of form II into form I. This behavior is observed mainly for more stereoregular samples crystallized in the pure form II that transforms into form I upon aging. Since for these samples the degree of crystallinity does not change upon aging, the increase of Young’s modulus after aging and transformation of form II into form I is due to the higher stiffness and rigidity of denser crystals of form I. The deformation behavior of the aged samples crystallized in form I (Figure 3B) is different from that of form II (Figure 3A) since the stress strain curves of Figure 3B are characterized by a more evident yielding phenomenon and broad deformation ranges of cold drawing with constant stress. Strain hardening still occurs at high deformation (Figure 3B) and only a slight 1058


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Figure 4. Average values of Young’s modulus E (A), stress σy (B) and strain εy (C) at the yield point, stress σb (D) and strain εb (E) at break and crystallinity (F) of as-crystallized (open symbols) and aged at room temperature (solid symbols) compression molded films of the iPB samples as a function of concentration of rr stereodefects.

% of rr stereodefects (sample iPB7, Table 1), much lower than the maximum concentration of rr defects compatible with the crystallization of iPP. The incorporation of only 4−5 mol % of defects in iPB chains produces remarkable flexibility and ductility but it is not sufficient to induce elastomeric properties. For all samples the mechanical behaviors at high values of deformation of as-crystallized and aged films are similar, as indicated by the similar values of stress at break of samples before and after the aging at room temperature (Figures 3 and 4D). This is due to the fact that the mechanical deformation of all compression-molded samples as-crystallized in form II or form III is associated with stress induced transformations of form II and form III into form I. This is shown, as an example, in Figure 6 by the X-ray diffraction patterns of fibers of the samples iPB2a and iPB6b, with 0.8 and 2.8 mol % of rr defects, respectively, stretched at room temperature at different values of strain starting from compression molded films as-crystallized in form II (profile c Figure 1) or mainly in form III (Figure i of Figure 1), respectively. It is apparent in Figure 6 that stretching already at low degrees of deformation induces transformation of form II and form III into form I, as indicated by the rapid decrease of the intensity of the (200)II reflection of form II at 2θ = 11.9° for the sample iPB2a in Figure 6B and of the (110)III reflection of form III at 2θ = 12.2° for the sample iPB6b in Figure 6F, which are replaced by the (110)I reflection of form I at 2θ = 9.9°.

Therefore, the samples of Figures 1 and 3A initially crystallized in form II or in mixtures of form III and form II or form III and form I, stretched at high deformations are in the pure form I (Figure 6C,D,G,H). The transformation of form II or form III into form I is almost complete at values of the critical deformation of 100−150%, higher than the yielding point, regardless of stereoregularity. The data of Figure 6 indicate that form II and form III of iPB are not only less stable than form I, but they are also mechanically unstable and stretching induces the transformation into form I very rapidly. These stress-induced transformations make oriented films of Figure 3A at high deformations similar to the aged samples that are in form I both in the unoriented samples and in the oriented films stretched at high deformations (Figures 2 and 3B). In fact, the stretching of the aged samples crystallized in form I, as in the stress−strain curves of Figure 3B, produces only orientation of crystals of form I and no phase transformations have been observed. This accounts for the similar mechanical behaviors at high values of deformation of as-crystallized and aged samples (Figure 3). The stress induced phase transition of form II and form III into form I for the Ziegler−Natta iPB sample has already been reported in the literature.49−54 As in our data of Figure 6A−D, it has been found that stretching accelerates the transformation of form II into form I,50 which has been interpreted by either a direct crystal−crystal transition or an indirect melting 1059


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In the case of our metallocene iPB samples, the data of Figure 6 indicate that both form II and form III transform by stretching at room temperature into form I already at low degrees of deformation. To understand whether the deformation produces crystals of form I or form I′, we have analyzed the melting temperatures of the oriented fibers obtained by stretching the compression-molded films of Figure 1 at the maximum degree of deformation, as those of Figure 6, parts D and H. We recall that crystals of form I show melting temperature higher than that of form I′. In particular, in the case of our stereodefective metallocene iPB samples, we have reported in ref 38 the melting temperatures of the crystals of form I′ obtained directly by crystallization from the melt, and of crystals of form I obtained by the crystal−crystal transformation of form II. We have observed that, regardless of stereoregularity, crystals of form I obtained from form II always melt at temperatures higher than those of crystals of form I′ obtained directly from the melt (see Table 1 and Figure 7 of ref 38). The X-ray fiber diffraction patterns, and corresponding diffraction profiles read along the equatorial lines, and the DSC melting curves of fibers obtained by stretching at the maximum possible deformation some compression-molded samples of Figure 1, initially crystallized in form II (the more isotactic samples iPBZN and iPB2a), and in mixtures of form III and form II (sample iPB5) or form II and form I′ (sample iPB7), are reported in Figure 7. The fiber diffraction patterns present only the (110)I, (300)I, and (220)I and (211)I reflections of form I at 2θ = 9.9, 17.3 and 20.5°, and indicate that all oriented fibers are in the trigonal form I (Figure 7A,B). As in the case of samples of Figure 6, the stretching of the sample iPBZN and iPB7 produces transformation of form II into form I, and the stretching of the sample iPB5 produces transformation of form III and form II into form I. The melting temperatures of these fibers, evaluated from the DSC curves of Figure 7C, are reported in Table 3 in comparison with the melting temperatures of the crystals of form I′ and form I of the same samples (taken from Table 1 of ref 38). Crystals of form I′ have been obtained in ref 38 for the more stereoirregular samples by direct crystallization from the melt by cooling the melt at cooling rate of 10 °C/min, whereas crystals of form I have been obtained by transformation of form II by aging the melt-crystallized samples at room temperature for long time.38 It is apparent that for each sample the melting temperatures of the fibers are similar to those of crystals of form I produced by transformation of form II by aging at room temperature and, for the more steroirregular samples, are higher than those of crystals of form I′ obtained directly from the melt. This result suggests that the stretched fibers consist of crystals of form I, which are obtained by the solid state deformation of crystals of form II and form III (Figure 6). As an example, a comparison between the DSC melting curves of samples crystallized from the melt in the DSC by cooling the melt at 10 °C/min, of the same samples aged at room temperature for long time to allow complete transformation of form II into form I and of the fibers stretched from the as-crystallized compression-molded samples, is reported in Figure 8 for the more isotactic sample iPB2a with [rr] = 0.8 mol %, and for a more irregular sample iPB5 with [rr] = 2.5 mol %. The corresponding X-ray diffraction patterns are also reported in Figure 8. The X-ray powder diffraction profile a of Figure 8A indicates that the stereoregular sample iPB2a crystallizes from the melt during the DSC cooling scan a of Figure 8B in the form II. The obtained crystals of form II melt

Figure 5. Stress−strain curves of melt-crystallized compressionmolded film before aging (dashed lines) and after aging at room temperature for one month (continuous lines) of the Ziegler−Natta sample iPBZN (A), of the sample iPB2b with concentration of rr defect of 0.8 mol % (B) and of the sample iPB6b with concentration of rr defect of 2.8 mol % (C).

recrystallization process. According to our result of Figure 3A and 6A-D, a three-stage mechanical deformation, including linear deformation, stress plateau, and strain hardening, has been observed in the engineering stress−strain curves, which have been correlated to a process of incubation, nucleation, and gelation of form I crystals, respectively.50 The influence of different morphologies (hedrites or spherulites) on the rate of form II- form I crystal−crystal transition during deformation has also been described.53,54 The deformation of form III for the Ziegler−Natta iPB samples has been investigated by Nakamura et al.49 They found that form III transforms into form I′ upon the tensile draw. Films consisting of solution-grown crystal mat of form III were drawn uniaxially by tensile draw and solid-state coextrusion in the range of room temperature to 80 °C. When form III mat was tensile drawn at 80 °C, near the melting temperature of crystals of form III, oriented form II crystals were obtained, whereas the draw at a lower temperature of 70 °C produced oriented form I′ crystals.49 The formation of form II on tensile draw at 80 °C suggests that the deformation proceeds through quasi-melting followed by recrystallization into the oriented form II. When the deformation proceeds gradually within an extrusion die by solid-state coextrusion, instead, oriented form I′ crystals were observed, suggesting that the deformation proceeds in the solid state.49 1060


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Figure 6. X-ray fiber diffraction patterns (A−H) and corresponding profiles read along the equatorial lines (A′−H′) of fibers of the samples iPB2a (A−D) and iPB6b (E−H) with 0.8 mol % and 2.8 mol % of rr stereodefects, respectively, obtained by stretching at room temperature and at the indicated values of strain ε as-crystallized and not aged compression molded films of the stereoregular sample iPB2a initially crystallized in form II (profile c of Figure 1) and of the stereodefective sample iPB6b initially crystallized mainly in form III (profile i of Figure 1). In A′−H′ the (110)I, (300)I, and (220)I + (211)I reflections of form I at 2θ = 9.9, 17.3, and 20.5°, respectively, and the (200)II, (220)II, and (213)II + (311)II reflections of form II at 2θ = 11.9, 16.9, and 18.3°, respectively, and the (110)III, (111)III, (201)III, and (120)III reflections of form III at 2θ = 12.2, 17.2, 18.7, and 21.2°, respectively, are indicated.

at 105 °C (DSC heating curve b of Figure 8B). Form II transforms by aging at room temperature for several days into form I, as shown by the X-ray powder diffraction profile b of Figure 8A recorded after 500 h aging. The obtained crystals of form I melt at 120 °C, as shown by the DSC heating curve c of Figure 8B. The DSC heating curve d of Figure 8B of the fibers of the same sample iPB2a stretched at 450% deformation from the compression-molded sample, whose X-ray fiber diffraction pattern is shown in Figure 6D and pattern b of Figure 7, parts A and B, indicates that the fibers melt at 123 °C, similar to the melting temperature of crystals of form I (curve c of Figure 8B). The more stereoirregular sample iPB5 crystallizes from the melt during the DSC cooling scan a of Figure 8D in mixture of form II and form I′,38 as indicated by the X-ray powder diffraction profile a of Figure 8C. The DSC melting curve b of Figure 8D of the obtained melt-crystallized sample shows a broad endotherm with a peak at 84 °C, corresponding to the melting of crystals of form II and a shoulder at 92 °C,

corresponding to the melting of crystals of form I′. Form II transforms by aging at room temperature for several days into form I, as shown by the X-ray powder diffraction profile b of Figure 8C recorded after 700 h aging. The obtained crystals of form I melt at 103 °C, as shown by the DSC heating curve c of Figure 8D. The DSC melting curve d of Figure 8D of fibers of the same sample iPB5, obtained by stretching compressionmolded film at the maximum possible deformation ε = 690%, whose X-ray fiber diffraction pattern is shown in the pattern c of Figure 7, parts A and B, indicates that the fibers melt at 108 °C, similar to the melting temperature of crystals of form I and higher than the melting temperature of 92 °C of form I′. It is worth noting that the high melting temperature of the fibers is not due to some residual tension of the specimens used for the DSC tests. In fact in all cases the fibers are cut in small pieces, and their ends are left unbound during the heating scans. On the other hand, in independent experiments, prior of recording the heating scans, the fibers have been annealed at moderately high temperatures (60−70 °C). We have checked 1061


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Figure 7. X-ray fiber diffraction patterns (A), corresponding diffraction profiles read along the equatorial lines (B), and DSC heating curves recorded at heating rate of 10 °C/min (C) of fibers of the samples iPBZN with [rr] = 0.4 mol % (a), iPB2a with [rr] = 0.8 mol % (b), iPB5 with [rr] = 2.5 mol % (c), and iPB7 with [rr] = 4.7 mol % (d), obtained by stretching at room temperature and at the indicated maximum possible values of the deformation ε, as-crystallized and not aged compression molded samples. The compression molded samples were initially crystallized in form II (the more isotactic samples iPBZN and iPB2a, diffraction profiles a and c of Figure 1), and in mixtures of form III and form II (sample iPB5, diffraction profile g of Figure 1) or mixtures of form II and form I′ (sample iPB7, diffraction profile l of Figure 1). In B, the (110)I, (300)I and (220)I + (211)I reflections of form I at 2θ = 9.9, 17.3, and 20.5°, respectively, are indicated.

whereas more stereoirregular samples crystallize surprisingly mainly in form III, in some cases in mixture with crystals of form I or form II. The crystallization of form III is probably due to the presence of pressure during the crystallization and the relatively fast cooling. Compared to an iPB sample prepared with Ziegler−Natta catalyst, the incorporation of rr stereodefect in metallocene iPB samples produces remarkable increase of ductility with values of deformation at break higher than 1000% for the most stereoirregular sample containing about 5 mol % of stereodefects. The strong increase of ductility with increasing concentration of rr stereodefects is associated with a small decrease of modulus and a nearly constant values of stress at yielding and strain hardening at high deformation. The aging at room temperature of the compression-molded samples produces transformation of form II into form I in more stereoregular samples, whereas no phase transformation occurs in more stereoirregular samples crystallized mainly in form III. A strong increase of Young’s modulus and of stress at yield is observed after transformation of form II into form I, due to the higher stiffness and rigidity of denser crystals of form I. For more stereoirregular samples with concentration of rr defects higher than 2 mol % crystallized mainly in form III, the stress− strain curves do not change significantly upon aging at room temperature because no phase transformations occur during aging. Also for the aged samples a remarkable increase of ductility and flexibility with increasing concentration of rr stereodefects is observed, in particular with respect to the Ziegler−Natta sample, with deformation at break up to 700− 800% for the most irregular samples. Moreover, a more evident decrease of the Young’s modulus and the stress at yield with increasing rr defect concentration occurs. At variance with metallocene stereodefective iPPs, in both as-crystallized and aged samples elastic properties are never observed even at the highest concentration of rr stereodefects. The mechanical deformation of all compression-molded samples as-crystallized in form II or form III is associated with stress induced polymorphic transformations. The stretching

Table 3. Melting Temperatures of Crystals of Form II (Tm(II)) and of Form I′ (Tm(I′)) in Samples of iPB Crystallized from the Melt by Cooling the Melt to Room Temperature at 10 °C/min, of Crystals of Form I (Tm(I)) Obtained by Spontaneous Transformation of Form II by Aging the Melt-Crystallized Samples at Room Temperature for Long Time, and of Oriented Fibers (Tm(fibers)) Obtained by Stretching at the Maximum Possible Deformation Compression-Molded Films of Some of the iPB Samples of Different Stereoregularity of Table 1 and Figure 1.a sample

[rr] (%)

Tm(II) (°C)

Tm(I′) (°C)

Tm(I) (°C)

Tm(fibers) (°C)

iPBZN iPB2a iPB5 iPB7

0.4 0.8 2.5 4.7

118 105 84 68

− − 92 84

129 120 103 92

125 123 108 90

a

Data of Tm(II), Tm(I′), and Tm(I) are taken from Table 1 of ref 38, except for the sample iPBZN. For the more isotactic samples iPBZN and iPB2a that crystallize from the melt only in form II, only the melting temperature of form II and form I are reported.

that no significant changes of DSC melting peaks occurs with respect to those reported in Figures 7C and 8B,D. The data of Figures 7 and 8 indicate that crystals of the trigonal form in the stretched fibers obtained from deformation of crystals of form II and form III correspond to the high melting temperature form I. This result also suggests that the transformation of form II or form III into form I upon stretching proceeds via a direct crystal−crystal transition.

■

CONCLUSIONS A study of the mechanical properties of samples of iPB of different stereoregularity prepared with different metallocene catalysts and containing only rr triads stereodefects, is reported. More stereoregular samples with concentration of rr defects lower than 2 mol % crystallize from the melt by compressionmolding and cooling to room temperature in the form II, 1062


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Figure 8. (A, C) X-ray powder diffraction profiles of the samples iPB2a with [rr] = 0.8% (A) and iPB5 with [rr] = 2.5% (C), crystallized from the melt by cooling the melt to room temperature at cooling rate of 10 °C/min (a), as in the DSC cooling curves a in B and D, and of the same meltcrystallized samples after aging at room temperature (b) for 500 h for the sample iPB2a (A) and 700 h for the sample iPB5 (C). (B,D) DSC cooling curves from the melt at cooling rate of 10 °C/min (a) of the samples iPB2a (B) and iPB5 (D), successive DSC melting curves at heating rate of 10 °C/min (b) of the samples iPB2a (B) and iPB5 (D) crystallized from the melt in the scan a, DSC melting curves of the same melt-crystallized samples after aging at room temperature (c) for 500 h for the sample iPB2a (B) and 700 h for the sample iPB5 (D), and DSC melting curves of fibers (d) of the samples iPB2a (B) and iPB5 (D) obtained by stretching compression-molded films at deformation ε = 450% and 690%, respectively, whose X-ray fiber diffraction patterns are shown in Figure 7A,B (b and c, respectively).

Notes

induces, already at low deformation, transformation of form II and of form III into form I, and all samples stretched at high deformations are in the pure form I. The high melting temperatures of the oriented fibers confirm that crystals of the trigonal form in the stretched fibers correspond to the high melting temperature form I. This also suggests that the transformation of form II or form III into form I upon stretching proceeds via a direct crystal−crystal transition. The results indicate that the mechanical properties of iPB can be modified by the controlled incorporation of stereodefects through metallocene catalysts, and improvement of ductility is easily obtained while maintaining high crystallinity by inclusion of small amounts of rr stereodefects. Moreover the crystallization of form III by compression-molding of more stereoirregular sample allows preventing the undesirable changes of mechanical properties of samples upon aging at room temperature.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Basell Polyolefins (Ferrara, Italy) is gratefully acknowledged. We thank Dr. Luigi Resconi for the synthesis of the iPB samples.

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REFERENCES

(1) Natta, G.; Corradini, P.; Bassi, I. W. Nuovo Cimento (Suppl.) 1960, 15, 52. (2) Turner-Jones, A. J. Polym. Sci., Part B 1963, 1, 455. (3) Turner-Jones, A. Polymer 1966, 7, 23. Turner-Jones, A. J. Polym. Sci., Part B 1965, 3, 591. (4) Petraccone, V.; Pirozzi, B.; Frasci, A.; Corradini, P. Eur. Polym. J. 1976, 12, 323. (5) Corradini, P.; Napolitano, R.; Petraccone, V.; Pirozzi, B. Eur. Polym. J. 1984, 20, 931. (6) Cojazzi, G.; Malta, V.; Celotti, G.; Zannetti, R. Makromol. Chem. 1976, 177, 915. (7) Dorset, D. L.; McCourt, M. P.; Kopp, S.; Wittmann, J. C.; Lotz, B. Acta Crystallogr. 1994, B50, 201. (8) Lotz, B.; Thierry, A. Macromolecules 2003, 36, 286. (9) Luciani, L.; Seppälä, J.; Löfgren, B. Prog. Polym. Sci. 1988, 13, 37. (10) Holland, V. F.; Miller, R. L. J. Appl. Phys. 1964, 35, 3241. (11) Chau, K. W.; Geil, P. H. J. Macromol. Sci. Phys. 1984, B23, 115. (12) Tosaka, M.; Kamijo, T.; Tsuji, M.; Kohjiya, S.; Ogawa, T.; Isoda, S.; Kobayashi, T. Macromolecules 2000, 33, 9666. (13) Boor, J., Jr.; Mitchell, J. C. J. Polym. Sci.: Part A 1963, 1, 59.

AUTHOR INFORMATION

Corresponding Author

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

Laboratory of Materials and Interface Chemistry, Eindhoven University of Technology, Den Dolech 2, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. 1063


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(14) Powers, J.; Hoffman, J. D.; Weeks, J. J.; Quinn, F. A., Jr. J. Res. Natl. Bur. Stand. 1965, 69A, 335. (15) Schaffhauser, R. J. J. Polym. Sci., Part B 1967, 5, 839. (16) Gohil, R. M.; Miles, M. J.; Petermann, J. J. Macromol. Sci. Phys. 1982, B21, 189. (17) Fujiwara, Y. Polym. Bull. 1985, 13, 253. (18) Chau, K. W.; Yang, Y. C.; Geil, P. J. Mater. Sci. 1986, 21, 3002. (19) Hsu, T.-C.; Geil, P. H. Polym. Commun. 1990, 31, 105. (20) Miller, R. L.; Holland, V. F. J. Polym. Sci., Part B 1964, 2, 519. (21) Kopp, S.; Wittmann, J. C.; Lotz, B. J. Mater. Sci. 1994, 29, 6159. (22) Lotz, B.; Mathieu, C.; Thierry, A.; Lovinger, A. J.; De Rosa, C.; Ruiz de Ballesteros, O.; Auriemma, F. Macromolecules 1998, 31, 9253. (23) Foglia, A. J. J. Appl. Polym. Sci., Appl. Polym. Symp. 1969, 11, 1. (24) Danusso, F.; Gianotti, G. Makromol. Chem. 1963, 61, 139. Danusso, F.; Gianotti, G. Makromol. Chem. 1965, 88, 149. (25) Boor, J., Jr.; Youngman, E. A. J. Polym. Sci., Part B 1964, 2, 903. (26) Azzurri, F.; Flores, A.; Alfonso, G. C.; Balta Calleja, F. J. Macromolecules 2002, 35, 9069. (27) Azzurri, F.; Flores, A.; Alfonso, G. C.; Sics, I.; Hsiao, B. S.; Balta Calleja, F. J. Polymer 2003, 44, 1641. (28) Zhang, B.; Yang, D.; Yan, S. J. Polym. Sci. Polym. Phys. 2002, 40, 2641. (29) Alfonso, G. C.; Azzurri, F.; Castellano, M. J. Therm. Anal.Calorim. 2001, 66, 197. (30) Kaszonyiova, M.; Rybnikar, F.; Geil, P. H. J. Macromol. Sci. Phys. 2004, B43, 1095. (31) Hong, K.; Spruiell, J. E. J. Appl. Polym. Sci. 1985, 30, 3163. (32) Rubin, I. D. J. Polym. Sci., Polym. Lett. 1964, 2, 747. (33) Yamashita, M.; Hoshino, A.; Kato, M. J. Polym. Sci. B: Polym Phys. 2007, 45, 684. Yamashita, M. J. Cryst. Growth 2007, 310, 1739. Yamashita, M. J. Cryst. Growth 2009, 311, 556. Yamashita, M. J. Cryst. Growth 2009, 311, 560. (34) Gianotti, G.; Capizzi, A. Makromol. Chem. 1969, 124, 152. (35) Azzurri, F.; Alfonso, G. C.; Gómez, M. A.; Martì, M. C.; Ellis, G.; Marco, C. Macromolecules 2004, 37, 3755. (36) Azzurri, F.; Gómez, M. A.; Alfonso, G. C.; Ellis, G.; Marco, C. J. Macromol. Sci. Phys. 2004, B43, 177. (37) Resconi, L.; Camurati, I.; Malizia, F. Macromol. Chem. Phys. 2006, 207, 2257. (38) De Rosa, C.; Auriemma, F.; Ruiz de Ballesteros, O.; Esposito, F.; Laguzza, D.; Di Girolamo, R.; Resconi, L. Macromolecules 2009, 42, 8286. (39) De Rosa, C.; Auriemma, F.; Resconi, L. Ang. Chem. Int. Ed. 2009, 48, 9875. (40) 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. (41) De Rosa, C.; Auriemma, F. J. Am. Chem. Soc. 2006, 128, 11024. De Rosa, C.; Auriemma, F. Lect. Notes Phys. 2007, 714, 345. (42) De Rosa, C.; Auriemma, F.; De Lucia, G.; Resconi, L. Polymer 2005, 46, 9461. (43) De Rosa, C.; Auriemma, F.; Perretta, C. Macromolecules 2004, 37, 6843. (44) De Rosa, C.; Auriemma, F.; Circelli, T.; Waymouth, R. M. Macromolecules 2002, 35, 3622. Auriemma, F.; De Rosa, C. Macromolecules 2002, 35, 9057. (45) De Rosa, C.; Auriemma, F.; Paolillo, M.; Resconi, L.; Camurati, I. Macromolecules 2005, 38, 9143. De Rosa, C.; Auriemma, F.; Resconi, L. Macromolecules 2005, 38, 10080. De Rosa, C.; Auriemma, F. Polym. Chem. 2011, 2, 2155. (46) De Rosa, C.; Auriemma, F.; Corradini, P.; Tarallo, O.; Dello Iacono, S.; Ciaccia, E.; Resconi, L. J. Am. Chem. Soc. 2006, 128, 80. De Rosa, C.; Dello Iacono, S.; Auriemma, F.; Ciaccia, E.; Resconi, L. Macromolecules 2006, 39, 6098. De Rosa, C.; Auriemma, F.; Ruiz de Ballesteros, O.; Resconi, L.; Camurati, I. Macromolecules 2007, 40, 6600. De Rosa, C.; Auriemma, F.; Ruiz de Ballesteros, O.; Resconi, L.; Camurati, I. Chem. Mater. 2007, 19, 5122. De Rosa, C.; Auriemma, F.; Ruiz de Ballesteros, O.; De Luca, D.; Resconi, L. Macromolecules 2008, 41, 2172.

(47) Alamo, R. G.; Kim, M. H.; Galante, M. J.; Isasi, J. R.; Mandelkern, L. Macromolecules 1999, 32, 4050. Alamo, R. G.; VanderHart, D. L.; Nyden, M. R.; Mandelkern, L. Macromolecules 2000, 33, 6094. Hosier, I. L.; Alamo, R. G.; Esteso, P.; Isasi, G. R.; Mandelkern, L. Macromolecules 2003, 36, 5623. (48) De Rosa, C.; Auriemma, F.; Di Girolamo, R.; Ruiz de Ballesteros, O.; Pepe, M.; Tarallo, O.; Malafronte, A. Macromolecules 2013, 46, 5202. (49) Nakamura, K.; Aoike, T.; Usaka, K.; Kanamoto, T. Macromolecules 1999, 32, 4975. (50) Liu, Y.; Cui, K.; Tian, N.; Zhou, W.; Meng, L.; Li, L.; Ma, Z.; Wang, X. Macromolecules 2012, 45, 2764. (51) Wang, Y.; Jiang, Z.; Wu, Z.; Men, Y. Macromolecules 2013, 46, 518. (52) Wang, Y.; Jiang, Z.; Fu, L.; Lu, Y.; Men, Y. Macromolecules 2013, 46, 7874. (53) Chen, W.; Li, X.; Li, H.; Su, F.; Zhou, W.; Li, L. J. Mater. Sci. 2013, 48, 4925. (54) Su, F.; Li, X.; Zhou, W.; Chen, W.; Li, H.; Cong, Y.; Hong, Z.; Qi, Z.; Li, L. Polymer 2013, 54, 3408.

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