Melting and crystallization behavior of binary blends of syndiotactic polypropylenes of different stereoregularity

Accepted Manuscript Melting and Crystallization Behavior of Binary Blends of Syndiotactic Polypropylenes of Different Stereoregularity Odda Ruiz de Ballesteros, Claudio De Rosa, Finizia Auriemma, Rocco Di Girolamo, Miriam Scoti PII: DOI: Reference:

S0014-3057(16)30424-4 http://dx.doi.org/10.1016/j.eurpolymj.2016.09.034 EPJ 7505

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

13 May 2016 13 September 2016 20 September 2016

Please cite this article as: Ruiz de Ballesteros, O., De Rosa, C., Auriemma, F., Di Girolamo, R., Scoti, M., Melting and Crystallization Behavior of Binary Blends of Syndiotactic Polypropylenes of Different Stereoregularity, European Polymer Journal (2016), doi: http://dx.doi.org/10.1016/j.eurpolymj.2016.09.034

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Melting and Crystallization Behavior of Binary Blends of Syndiotactic Polypropylenes of Different Stereoregularity Odda Ruiz de Ballesteros,* Claudio De Rosa, Finizia Auriemma, Rocco Di Girolamo, Miriam Scoti Dipartimento di Scienze Chimiche, Università di Napoli “Federico II”, Complesso Monte S. Angelo, Via Cintia 80126 Napoli, Italy.

*Correspondence: Odda Ruiz de Ballesteros, Dipartimento di Scienze Chimiche, Università di Napoli “Federico II”, Complesso Monte S. Angelo, Via Cintia 80126 Napoli, Italy. E-mail: [email protected] Phone: +39 081 674448; Fax +39 081 674090

1

Abstract. Binary blends of samples of syndiotactic polypropylene of different stereoregularity have been prepared by mixing a more stereoregular and crystalline sample sPP1 having [rrrr] = 78%, with low stereoregular and poorly crystalline samples samPP1 and samPP2 having [rrrr] = 54.6 and 45.8%, respectively, and a nearly atactic fully amorphous sample amPP1 with [rrrr] = 26.5%. The blends show separate melting of the two components with the more stereoregular sPP1 melting at high temperature and the poorly syndiotactic component melting at lower temperature. Crystals of sPP1 are formed by cooling from the melt whereas more defective crystals of samPP1 and samPP2 are formed by cold crystallization during aging at room temperature. The presence of the strereoirregular component disturbs the melt crystallization of the stereoregular component sPP1 and depression of the crystallization and melting temperatures of sPP1 in the blends is observed. On the other hand, the cold crystallization at room temperature of the irregular component is accelerated and favored by the presence of the more stereoregular component sPP1, whose crystals act as crystallization nuclei. All data indicate interactions between the components in the melt and amorphous state, compatible with a good degree of mixing.

Keywords: Syndiotactic polypropylene, polymer blends, crystallization, thermal behavior.

2

Introduction Syndiotactic polypropylene (sPP) is an interesting polymer thanks to its interesting physical properties of thermoplastic elastomer.1-7 The elastomeric behavior strongly depends on the microstructure of the sPP chains that can be easily controlled resorting to metallocene catalysis and selecting the catalyst and the polymerization conditions.1-3,6,7 In particular, highly syndiotactic samples with high melting temperatures and crystallinity obtained with the classical Cs-symmetric metallocene catalysts, show enthalpic elasticity and behave as high-modulus thermoplastic elastomers.1-3,7 High molecular weight samples with low stereoregularity, melting temperatures and crystallinity, produced by the constrained geometry half-metallocenes catalysts, instead, exhibit conventional entropic elasticity and typical properties of low-modulus and low-strength elastomers.1,3,6 Therefore, the fine tuning of the polymer chains microstructure allows production of sPP-based thermoplastic elastomers where elasticity, stiffness and mechanical strength can be tailored by controlling the degree of crystallinity through the incorporation of stereodefects.1,3,7 Recently we have demonstrated that a similar control of the properties can be achieved in blends of sPP with different stereoregularity,8 by simply changing the blend composition and the degree of stereoregularity of the components. In fact, sPP binary blends obtained by mixing a highly syndiotactic and crystalline sample with concentration of rrrr pentads of 78% with poorly syndiotactic nearly amorphous samples with [rrrr] in the range 45-55% or fully amorphous sample with [rrrr ] = 26.5%, show a continuous change of crystallinity and mechanical properties with composition, analogous to the change of properties observed in stereodefective sPP samples by modifying the concentration of stereodefects.8 This has been explained by considering that the addition of the stereoirregular component to the more syndiotactic sPP produces the same effect of incorporation of stereodefects at molecular level so that blends behave as a single sPP component having an average degree of stereoregularity intermediate between those of the two mixed components.8 Hence, blends rich in the more syndiotactic component with high values of the average degree of stereoregularity, show high crystallinity and properties typical of high-modulus enthalpic elastomers. With increasing the content of 3

the steroirregular component and/or decreasing its stereoregularity a continuous and gradual decrease of crystallinity, stiffness and mechanical strength is observed in the material up to properties typical of low-strength entropic elastomers for blends rich in the poorly syndiotactic components having low values of the average degree of stereoregularity.8 These blends show a complex melting and crystallization behavior, with separated melting of the two components at temperatures close to the melting temperatures of neat samples. However, the crystallization from the melt of the more stereoregular component is somewhat influenced by the presence of the less stereoregular component, causing depression of its melting and crystallization temperatures in the blends, and is even prevented for high amount (75 wt%) of the less stereoregular component.8 All these data suggest a possible miscibility of the sPP chains of different stereoregularity in the melt and in the amorphous state. In this paper, a detailed analysis of the melting and crystallization behavior of the sPP blends is reported, aimed at drawing more information about the miscibility of the components. Experimental section The samples of sPP with different stereoregularity used to prepare the blends were synthesized with different metallocene catalysts (Table 1).1,6a,9-11 The more stereoregular and crystalline sample sPP1, having a concentration of rrrr pentad of 78%, was synthesized with the classic Cs symmetric isopropylidene(cyclopentadienyl) (fluorenyl) zirconium dichloride, which produces in combination with methylalluminoxane (MAO) highly stereoregular and almost completely regioregular sPP. 9 The less syndiotactic and crystalline samples samPP1 and samPP2 with [rrrr] = 54.6 and 45.8%, respectively, were prepared using silyl-bridged indenyl-tert-butylamido complexes of titanium activated with MAO,10 whereas the fully amorphous polypropylene amPP1 with [rrrr] =26.5 % was synthesized with the complex Me2Si(Me4Cp)(t-BuN)TiCl2/MAO.11 In all cases polypropylenes with high molecular mass (Table 1) were obtained. Table 1. Polymerization Temperatures (Tp), Molecular Masses (Mw), Concentration of Fully Syndiotactic rrrr Pentad and Melting Temperatures (Tm) of the sPP Samples. 4

sample

Tp (°C) Mwa

[rrrr] (%)b Tm (°C)c

sPP1 40 1.93105 78 samPP1 70 1.31106 54.6 samPP2 80 1.15106 45.8 amPP1 50 1.19106 26.5 a ) From GPC. b) From 13C NMR analysis. as polymerized samples.

124 59.0 48.3 c ) Measured from DSC scans at heating rate of 10 °C/min on

Three different series of binary blends were prepared: two semicrystalline/semicrystalline blends obtained by mixing the more stereoregular and crystalline sample sPP1 with the less stereoregular and crystalline samples samPP1 and samPP2 (blends sPP1/samPP1 and sPP1/samPP2) and a third series of semicrystalline/amorphous blends obtained by mixing the sample sPP1 with the fully amorphous sample amPP1 (blends sPP1/amPP1). For each series of blends three different compositions (25/75, 50/50 and 75/25 wt%/wt%) were prepared by dissolution of the components in hot refluxing toluene and precipitation in an excess (3:1) of cold methanol.8 The blends were designated as sPP1-x/samPP1-y, sPP1-x/samPP2-y and sPP1-x/amPP1-y, with x and y the weight percentages of the two components. 8 Compression molded films of the sPP blends were prepared by melting the dry-precipitated powders at 180 °C for 10 min under a press at very low pressure, to avoid preferred orientation, and cooling to room temperature at about 10 °C/min by circulating cold water inside the press plates. The X-ray diffraction profiles of compression molded samples were obtained with an automatic Philips diffractometer using Ni filtered Cu K radiation. The indices of crystallinity (xc) were evaluated from the diffraction profiles by the ratio between the crystalline diffraction area and the total area of the diffraction profile, as described in ref. 8. Thermal analysis was performed with a differential scanning calorimeter (Mettler-DSC30/2285), equipped with a liquid nitrogen cooling system for measurements at low temperature. The scans were recorded in flowing nitrogen atmosphere at a rate of 10°C/min. Microphotographs from polarized optical microscopy (POM) of the samples were recorded at room temperature in polarized light using a Zeiss Axioscop40 microscope in crossed polars. Specimens for POM observations were prepared by sandwiching small amounts of powder samples between glass 5

coverslips. The specimens are melted at ≈180 °C and then cooled to 0°C, at cooling rate of 10 °C/min, and are observed in crossed polars immediately after preparation, and after three days aging at room temperature. Results and Discussions X-ray diffraction analysis. The more stereoregular sample sPP1 with [rrrr] = 78% crystallizes by cooling the melt to room temperature in the helical form I of sPP, with index of crystallinity of about 35%, whereas the less stereoregular samples samPP1 and samPP2, with [rrrr] = 54.6 and 45.8%, respectively, do not crystallize by cooling from the melt to room temperature, but slowly crystallize in form I upon aging at room temperature for several days, achieving index of crystallinity of 16-20%.1,3,6,7 The highly defective sample amPP1 with [rrrr] = 26.5% is amorphous and does not crystallize by cooling from the melt and by successive aging at room temperature, even for very long aging time. 6 The X-ray diffraction profiles of compression-molded samples of the pure components and the blends sPP1-75/samPP1-25, sPP1-25/samPP1-75 and sPP1-25/amPP1-75 as cooled from the melt to room temperature and aged at room temperature for different aging times (ta) up to the complete crystallization, are reported in Figure 1. The diffraction profiles of all blends and pure components cooled from the melt and aged at room temperature for different times are reported in Figures S1-S4.

6

A - sPP1-75/samPP1-25

200

200

B - sPP1-25/samPP1-75

C - sPP1-25/amPP1-75

020 121

020

aged amPP1

020

f

020

121

d

200 121

sPP1

ta = 48h

samPP1 ta=192h

e

Intensity

e

Intensity

Intensity

121

e

ta = 192 h

d

ta = 24 h

d

ta = 168h

ta = 24h

c

ta = 24h

c

ta = 1 h

b

ta = 3h

b

ta = 0.5h

b

ta = 0.5 h

a

ta = 0

a

ta = 0

a

ta = 0

c

5

10

15

20

2 (deg)

25

30

35

5

10

15

20

2 (deg)

25

30

35

5

10

15

20

25

30

35

2 (deg)

Figure 1. X-ray diffraction profiles of compression molded samples cooled from the melt of the blends sPP1-75/samPP1-25 (A), sPP125/samPP1-75 (B) and sPP1-25/amPP1-75 (C) recorded soon after the cooling to room temperature and during aging at room temperature (T = 25 ± 3)°C for different aging times ta. The profiles of aged compression molded samples of pure sPP1, samPP1 and amPP1 are also reported. The 200, 020 and 121 reflections at 2 = 12.2, 15.8 and 20.8°, respectively, of the helical form I of sPP are indicated. In A, the relative intensity of 200 and 020 reflections indicate a slight preferred orientation of the crystals with the 200 plane parallel to the film surface, induced by compression molding. 7

Blends with sPP1 contents of 75 and 50 wt% crystallize by cooling from the melt to room temperature in the helical form I of sPP regardless of the stereoregularity of the second component (samPP1, samPP2 or amPP1), and only a slight increase of crystallinity occurs upon aging at room temperature (Figures 1A, S1 and S2). Blends with the highest concentration (75 wt%) of the stereoirregular component, instead, show a crystallization behavior similar to that of samples samPP1 and samPP2 (Figure S4). They do not crystallize by cooling the melt to room temperature and the amorphous samples crystallize after aging at room temperature for several days (Figures 1B, 1C and S3). Hence, the more syndiotactic sample sPP1 that normally crystallizes by cooling from the melt does not crystallize by cooling in the presence of high amounts of the stereoirregular component and crystallizes by aging at room temperature, along with the irregular component. It is worth underlining that in blends sPP125/samPP1-75 and sPP1-25/samPP2-75 both components of the blends crystallize during aging from the amorphous samples (Figure 1B, S3A, S3B), whereas in the blend sPP1-25/amPP1-75 only the more syndiotactic component sPP1 crystallizes during aging (Figure 1C). The index of crystallinity of compression-molded samples cooled from the melt of blends and pure components, evaluated from the X-ray diffraction of Figures 1 and S1-S4, are reported in Figures 2A-C as a function of the aging time (ta) at room temperature and in Figure 2D as a function of the content of the component sPP1, for ta = 0 and for the aging time necessary to achieve maximum crystallinity (xc(max)). For all blends the maximum crystallinity is achieved for values of the crystallinity index close to the weighted average crystallinity of the pure components regardless of composition, and decreases with increasing the content of the stereoirregular component, and, at the same content of sPP1, with decreasing the stereoregularity of the irregular component. The initial value of crystallinity depends mainly on the blend composition (Figure 2). In particular, for the semicrystalline/amorphous blends sPP1-75/amPP1-25 and sPP1-50/amPP1-50, the samples fully crystallize by cooling from the melt and no significant increase of crystallinity is observed during aging at room temperature (Figures S1C, S2C and Figure 2C). This confirms that only the more stereoregular component sPP1 crystallizes from the 8

melt in blends with content of sPP1 higher than 25 wt%, whereas the poorly syndiotactic components samPP1 and samPP2 slowly crystallize during aging at room temperature. In blends with 25 wt% of sPP1, instead, also the component sPP1 crystallizes during aging (vide infra). sPP1

35 30

A

xc (%)

xc (%)

20 15

20

10

sPP1-75/samPP1-25 sPP1-50/samPP1-50 sPP1-25/samPP1-75 samPP1

5 0 0.01

0.1

1

10

100

0 0.01

1000

35

xc (%)

C

25

25 20

15

15

10

10 sPP1-25/amPP1-75 sPP1-50/amPP1-50 sPP1-75/amPP1-25

amPP1

0.1

1

10

100

aging time (h)

1

10

100

1000

1000

D

30

20

5

0.1

aging time (h) sPP1

35

samPP2 sPP1-25/samPP2-75 sPP1-50/samPP2-50 sPP1-75/samPP2-25

5

aging time (h)

xc (%)

25

15

10

0 0.01

B

30

25

30

sPP1

35

5

75wt% of sPP1 50wt% of sPP1

melt crystallized at t=0 (x0) sPP1/samPP1 sPP1/samPP2 sPP1/amPP1

melt crystallized aged at RT (x)

25wt% of sPP1

0 100 90 80 70 60 50 40 30 20 10

0

content of sPP1 (wt%)

Figure 2. (A-C) Indices of crystallinity of compression molded samples cooled from the melt of blends and pure components as a function of the aging time at room temperature and (D) initial (at aging time ta = 0) crystallinity index (x0, red symbols) and maximum crystallinity index achieved at long aging time after complete crystallization (x∞, black symbols) as a function of the content of sPP1. In A-C the continuous lines are the fit of data to Avrami treatment using Equation 1 and the parameters of Table 2. The data of Figure 2A-C also show that the rate of cold crystallization of the blends at room temperature depends on the blend composition and increases with increasing the content of the more syndiotactic component sPP1. In particular, as also shown in Figure 2D, for the pure components 9

samPP1 and samPP2 and for the blends with 25 wt% of the more syndiotactic sample sPP1, 100% of the maximum crystallinity develops during aging (xc = 0 at ta = 0), whereas for blends with 50 and 75 wt% of sPP1, which are already crystalline at ta = 0, only 35-40% and 15-20%, respectively, of the maximum crystallinity develops during aging. The slow crystallization kinetics of sPP components during aging at room temperature of Figure 2A-C has been interpreted with the Avrami equation12,13 (Equation 1): n   t  t 0     x c (t )  x 0  x   x 0  exp    t cryst      

(1)

where x0 is the initial crystallinity due to partial crystallization of a component during the cooling, x∞ is the maximum crystallinity achieved at infinite time, t0 the induction time, n is the “Avrami exponent” whereas tcryst is the n-th root of the overall Avrami crystallization rate constant and represent the characteristic time of the crystallization process at which 63% (=100(e-1)/e) of transformation occurs. The fitting curves of the experimental data of Figure 2A-C with Equation 1 are also shown in Figure 2A-C, whereas the fitting parameters are reported in Table 2. From the values of the characteristic time tcryst of the cold crystallization during aging (Table 2) it is apparent that mixing the less stereoregular samples samPP1 and samPP2 with the more stereoregular component sPP1 in the blends with 25% of sPP1, results in a decrease of the characteristiccrystallization time, and, hence, increase of the rate of the crystallization during aging compared to both samPP1 and samPP2. However, for blends with 50 and 75wt% of sPP1, which are already crystalline as cooled from the melt, the values of tcryst are shorter than those of the pure samples samPP1 and samPP2 but longer than those of the blends with 25 wt% of sPP1 (Figure 2 and Table 2). This is due to the fact that the characteristic crystallization times of Table 2 refer to the overall crystallization of both components during aging at room temperature, determined from the X-ray diffraction data of Figures 1 and S1-S3 that do not allow discriminating the crystallization of the different components, sPP1 and samPP1 or samPP2, and the contributions to the crystallinity from the 10

two components. For the initially amorphous blends sPP1-25/samPP1-75 and sPP1-25/samPP2-75 with 25 wt% of sPP1 the increase of crystallinity is certainly due to the crystallization during aging of both components. For the blends with higher content of sPP1, instead, the initial crystallinity is due mainly to the crystallization from the melt of the more stereoregular component sPP1 and the increase of crystallinity during aging at room temperature is essentially due to the slow cold crystallization of the stereoirregular component. Therefore, the results of Table 2 can be rationalized only after determining the contributions of the two components to the crystallization. Since the blends show individual melting of the two crystalline components sPP1 and samPP,8 the contributions of the two components to the crystallinity can be deduced from the analysis of the DSC data (vide infra). Table 2. Parameters used in Avrami treatment (Equation 1) to fit the crystallinity data as a function of time obtained from diffraction analysis (Figure 2A-C) and DSC measurements (Figure 5).a

sPP1/samPP1 samPP1 sPP1-25/samPP1-75 sPP1-50/samPP1-50 sPP1-75/samPP1-25 sPP1/samPP2 samPP2 sPP1-25/samPP2-75 sPP1-50/samPP2-50 sPP1-75/samPP2-25 sPP1/amPP1 sPP1-25/amPP1-75 sPP1-50/amPP1-50 sPP1-75/amPP1-25

t0 (h)

x0 (%)

0 0 0 0 0 0

0 0 0 18 0 25

x∞ (%)b 20 23 100 28 100 32

t0 (h)

x0 (%)

x∞ (%)

0 0 0 0 0

0 0 0 16 25

16 20 100 26 29

t0 (h)

x0 (%)

x∞ (%)

0 0 0

0 16 23

11.6 18 25

tcryst (h) 21 0.61 8.4 1.98 2.6 2.15 tcryst (h) 13 2.1 8.6 3.6 3.0 tcryst (h) 0.64 0.63 1.3

XRD data Figure 2A XRD data Figure 2A DSC data Figure 5A XRD data Figure 2A DSC data Figure 5A XRD data Figure 2A

Crystallizing phase samPP1 sPP1+samPP1 samPP1 sPP1+samPP1 samPP1 sPP1+samPP1

XRD data Figure 2B XRD data Figure 2B DSC data Figure 5B XRD data Figure 2B XRD data Figure 2B

samPP2 sPP1+samPP2 samPP2 sPP1+samPP2 sPP1+samPP2

XRD data Figure 2C XRD data Figure 2C XRD data Figure 2C

sPP1 sPP1 sPP1

N 1 1 0.51 1 0.59 1 N 0.67 0.45 0.65 1 1 N 1 1 1

a

) The values of x0 and x∞ in the fit have been fixed equal to the crystallinity at time t0 = 0 and the maximum crystallinity achieved after complete crystallization, respectively. b) In Figure 2, the fit is performed using the values of the crystallinity index determined from X-ray diffraction profiles, and therefore the value of x∞ corresponds to the plateau value of the crystallinity index achieved upon completion of crystallization. In Figure 5, the fit is performed using the normalized percentage values of crystallinity and therefore x∞ is set equal to 100%.

11

It is worth noting that a good fit of the crystallinity data of Figure 2A-C to Equation 1 is obtained by setting the value of the Avrami exponent n equal to 1, except for the pure sample samPP2 and the blend sPP1-25/samPP2-75, for which values less than 1 (n = 0.4-0.6) are obtained. An exponent of n = 1 indicates a one-dimensional (fibrillar) growth mechanism controlled by heterogeneous (athermal) nucleation,12,13 and is in agreement with a mechanism of cold crystallization triggered by instantaneous heterogeneous nucleation taking place in a high viscous amorphous phase where rearrangement of the crystallizable units is prevented.14 The low Avrami exponent of 0.5-0.6, instead, may be explained, by a decrease in the linear crystal growth rate due to the presence of a large number of nuclei. According to ref. 14, this gives origin to “soft impingement”, that is, the decrease of crystal growth rate is caused by the presence of a large number of nuclei that induces formation of rigid amorphous regions in between the microphaseseparated crystals and consequent reduction in chain mobility. Thermal analysis The DSC heating curves, recorded at 10°C/min, of compression molded samples of blends and pure components, cooled from the melt and aged at room temperature up to complete crystallization, are reported in Figure 3A-C. The DSC cooling curves form the melt and the successive heating curves recorded soon after the cooling without aging are shown in Figure 3D-F and G-I, respectively. The values of melting and crystallization temperatures and enthalpies are reported in Table 3. The glass transition temperatures of pure samples and blends, determined from the DSC heating scans of samples quenched from the melt in liquid nitrogen (Figure S5), are also reported in Table 3. The pure components show similar values of glass transition temperatures, around 0°C, indicating that Tg are not influenced by the degree of stereoregularity. All blends show only a single Tg, at values close to those of the pure componts, regardless of composition and stereoregularity of the less syndiotactic component.

12

A - sPP1/samPP1 blends

a

D - sPP1/samPP1 blends sPP1

25% samPP1 samPP1

d samPP1

50% samPP1

sPP1 sPP1

sPP1

a

25% samPP1

b

75% samPP1

samPP1

b

 DSC endo

c

sPP1

 DSC endo

 DSC endo

b

e

G - sPP1/samPP1 blends a

c

sPP1

d

c

50% samPP1

d

75% samPP1

50% samPP1

sPP1

sPP1

sPP1

samPP1

25% samPP1

sPP1

75% samPP1

sPP1

40

60

80 100 T (°C)

120

160

0

20

B - sPP1/samPP2 blends

a

40

60 80 T (°C)

100

120

140

20

E - sPP1/samPP2 blends

40

60

80 100 T (°C)

120

25% samPP2 samPP2

d

50% samPP2 samPP2 sPP1

e samPP2

160

sPP1

sPP1

a

75% samPP2

b

25% samPP2

c

50% samPP2

 DSC endo

c

140

H - sPP1/samPP2 blends

a

sPP1

b  DSC endo

140

 DSC endo

20

b sPP1

c

d

sPP1

sPP1

sPP1

50% samPP2

sPP1 75% samPP2

d

samPP2

25% samPP2

75% samPP2

sPP1

20

40

60

80 100 T (°C)

120

140

160

0

20

40

60

80 T (°C)

100

120

140

20

40

60

80 100 T (°C)

120

140

160

13

F - sPP1/amPP1 blends

C - sPP1/amPP1 blends

I - sPP1/amPP1 blends a

a

sPP1

sPP1

sPP1

sPP1

a

25% amPP1

b

sPP1

c

sPP1

50% amPP1

c

d sPP1

 DSC endo

25% amPP1

sPP1

c

b

 DSC endo

 DSC endo

sPP1

sPP1

b

75% amPP1

40

60

80 100 T (°C)

120

50% amPP1

d

50% amPP1

sPP1 75% amPP1

d

75% amPP1

sPP1

sPP1

20

25% amPP1

140

160

0

20

40

60 80 T (°C)

100

120

140

20

40

60

80 100 T (°C)

120

140

160

Figure 3. DSC heating curves (A-C) of compression molded samples cooled from the melt and aged at room temperature up to complete crystallization, cooling curves from the melt (D-F) and successive heating curves of as crystallized samples without aging (G-I) of blends sPP1/samPP1 (A,D,G), sPP1/samPP2 (B,E,H) and sPP1/amPP1 (C,F,I) and of pure components. The DSC cooling curves and heating curves of non-aged samples of the pure components samPP1, samPP2 and amPP1 and the DSC heating curve of the amorphous sample amPP1 are featureless and are not reported. All DSC curves are recorded at 10 °C/min.

14

The DSC heating curve of the aged sample sPP1 presents a main melting endothermic peak at  130°C and a small endothermic peak at  50°C. (curves a of Figure 3A-C). The peak at 50 °C is not present in the DSC heating curves of the sample as crystallized from the melt (curves a of Figure 3G-I) but is observed in samples melt crystallized and aged at room temperature for at least 2 hours (Figure S4). It is probably due the melting of secondary crystallites formed during aging at room temperature.15,16 The heating curves of the less syndiotactic samples samPP1 and samPP2, which do not crystallize by cooling the melt, aged at room temperature for long time, show endothermic peaks in the range 53-62 °C (curves e of Figure 3A,B). All samples of the blends aged for long time show two well separated endothermic peaks at 52-55 °C and 116-124°C (curves b-d of Figure 3A-C), whereas the same samples as cooled from the melt without aging show only the high temperature melting peak at 116-124°C (curves b-d of Figure 3G-I). It is apparent that the area of the high temperature peak decreases for all blends (curves b-d of Figure 3AC,G-I and Table 3) and that of the low temperature peak increases for the aged blends sPP1/samPP1 and sPP1/samPP2 (curves b-d of Figure 3A,B and Table 3) and decreases for the aged blends sPP1/amPP1 (curves b-d of Figure 3C and Table 3), with decreasing the content of sPP1. This indicates that in the aged samples the high temperature peak is due to the melting of crystals of the more stereoregular component sPP1, whereas the low temperature peak is mainly due to the melting of crystals of the less stereoregular components samPP1 or samPP2 formed by cold crystallization during aging at room temperature for the blends sPP1/samPP1 and sPP1/samPP2, and to the melting of defective and small crystallites of the crystallizable component sPP1, formed during secondary crystallization at room temperature for the crystalline/amorphous blends sPP1/amPP1. The DSC cooling scans of blends having content of sPP1 higher or equal to 50 wt% present only one crystallization peak at temperature in the range 53 – 70 °C, close to the crystallization temperature of the pure sample sPP1 (curves b and c of Figure 3D-F), whereas the cooling curves of blends with 25 wt% of sPP1 do not show any crystallization peak (curves d of Figure 3D-F). This confirms that in blends sPP1/samPP1 and sPP1/samPP2 with content of sPP1 higher or equal to 50 wt% only the more 15

stereoregular component sPP1 crystallizes by cooling from the melt and the less stereoregular component samPP1 or samPP2 cold crystallizes during aging at room temperature (Figures 1A and 2AB), accounting for the observed increase of crystallinity upon aging (Figures 2A-B). In blends with 25 wt% of sPP1, instead, the presence of high amount of the poorly syndiotactic component prevents the crystallization from the melt of the more stereoregular component sPP1 (curves d of Figure 3D-F), that then cold crystallizes at nearly 60 °C during successive heating, as shown by the exothermic peaks of cold-crystallization in the heating curves of non-aged samples of Figure 3G-I (curves d). In these blends cooled from the melt, the more syndiotactic component sPP1 crystallizes also by aging at room temperature along with the less syndiotactic component (Figures 1B,C and 2A-C), accounting for the faster cold crystallization at room temperature of blends compared with pure samPP1 and samPP2 (Table 2). In any case, in these blends with 25 wt% of sPP1, both cold-crystallization at 60 °C during heating (DSC curves d of Figure 3G-I) and cold-crystallization at room temperature during aging produce crystals of the component sPP1 that melt at high temperature in the range 116-120°C (curves d of Figure 3A-C,G-I). Crystals of components samPP1 and samPP2 formed during aging, along with the component sPP1, melt instead at 50-60 °C (curves d of Figure 3A-C). This is further confirmed by the DSC heating scans of Figure 4 of blends with 50 and 75 wt% of the less syndiotactic component cooled from the melt at 10°C/min as in Figure 3G-I and aged at room temperature for different times. It is evident that, while the melting enthalpy of the low temperature peak increases with increasing aging time at room temperature, that of the high temperature peak remains nearly constant during aging. The data of Figure 4B,C also indicate that in blends with 75 wt% of the stereoirregular component the crystallization of the stereoregular component sPP1 during aging at room temperature occurs in the first minutes of aging whereas the crystallization of the stereoirregular component samPP1 or samPP2 occurs later. Therefore, melt crystallized and aged samples of blends sPP1/samPP1 and sPP1/samPP2, in which both components are crystallizable, show the coexistence of two crystalline phases: a phase constituted by thicker and larger crystals of the more stereoregular component sPP1 melting at higher temperature 16

and a phase constituted by small and defective crystals of the poorly syndiotactic component melting at low temperature. a

A, sPP1-50/samPP1-50

B, sPP1-25/samPP1-75 a b

HsamPP1=2.9 J/g

c

ta = 0 HsPP1 = 18.0 J/g

samPP1 HsamPP1=5.5 J/g

d

ta = 1h

sPP1

 endo

 endo

b

HsPP1 = 16.9 J/g HsamPP1 = 6.1 J/g

sPP1

HsamPP1=0.8 J/g

c

40

60

ta = 7days

sPP1

samPP2

sPP1

samPP2

d

40

HsPP1 = 8.0 J/g

HsPP1 = 7.1 J/g

sPP1 sPP1

60

80 100 120 Temperature (°C)

140

80 100 120 Temperature (°C)

ta = 15'

c H = 1.2 J/g

d

HsPP1 = 7.9 J/g

sPP1

sPP1 H = 1.3 J/g

HsPP1 = 8.6 J/g

ta = 1h HsPP1 = 7.8 J/g

sPP1

sPP1

ta = 24h HsPP1 = 8.5 J/g

ta = 7days

HsPP1 = 7.9 J/g

140

160

160

b

ta = 24 h

samPP2

40

ta = 24h

D, sPP1-25/amPP1-75

ta = 1h HsamPP2 = 5.8 J/g

60

a

ta = 30'

HsamPP2 = 1.9 J/g

20

20

ta = 10'

HsamPP2 = 0.6 J/g

HsPP1 = 7.9 J/g HsPP1 = 7.8 J/g

160

 endo

 endo

c

140

HsPP1 = 7.8 J/g

ta = 1h sPP1

samPP1

C, sPP1-25/samPP2-75

a b

80 100 120 Temperature (°C)

sPP1

HsamPP1=6.4 J/g

HsPP1 = 18.1 J/g

20

ta = 10'

samPP1

ta = 3days

sPP1

sPP1 HsPP1 = 7.6 J/g

HsamPP1=2.7 J/g

d

HsPP1 = 17.3 J/g

samPP1

ta = 0

samPP1

HsPP1 = 8.2 J/g

20

40

60

80 100 120 Temperature (°C)

140

160

Figure 4. DSC heating curves recorded at rate of 10 °C/min of samples of the blends sPP1-50/samPP150 (A), sPP1-25/samPP1-75 (B), sPP1-25/samPP2-75 (C) and sPP1-25/amPP1-75 (D) cooled from the melt at 10°C/min and aged at room temperature for the indicated aging times ta. For aging times ta ≤ 24h the samples were cooled from 180°C to 25°C at 10°C/min and maintained at 25° in the DSC apparatus, for longer aging times the samples were kept in a thermostatic bath at T = (25 ± 3)°C. The presence in the DSC heating curves of blends of well separate endothermic peaks due to the melting of the two components allows analyzing the crystallization kinetics of the sole strereirregular components. In particular, we have calculated the apparent degree of crystallinity x'c(DSC) achieved by samPP1 and samPP2 components in the blends during aging at room temperature at time ta by effect of 17

cold crystallization, from the values of the melting enthalpy of the low temperature peak in the corresponding DSC heating curves of the blends of Figure 5 (ΔHm(samPP1)(ta) and ΔHm(samPP2(ta)) as x'c(DSC)samPP1(2) = 100  ΔHm(samPP1(2))(ta)/ΔHm(samPP1(2)), where ΔHm(samPP1) and ΔHm(samPP2) represent the maximum melting enthalpy of samPP1 or samPP2 achieved for very long aging time of Table 3. The so determined values of apparent crystallinity of the components samPP1 and samPP2 are plotted in Figure 5 as a function of the aging time at room temperature and compared to the values of the apparent crystallinity of pure samPP1 and samPP2 and of blends with 25wt% of sPP1 evaluated by X-ray diffraction data of Figure 2 (x'c(XRD)) as x'c(XRD) = 100  xc(ta)/xsamPP where xsamPP represents the crystallinity index achieved upon completion of crystallization. These values of apparent crystallinity evaluated from DSC data of Figures 4 and 5 have also been treated with the Avrami equation. It is apparent that the cold crystallization of the components samPP1 and samPP2 during aging at room temperature in the blends is faster than that of the pure samples. The presence of the more syndiotactic sPP1 accelerates the cold crystallization at room temperature of the stereoirregular components samPP1 and samPP2. In particular, for blends with 25 wt% of sPP1 the characteristic crystallization times determined from Avrami analysis of DSC data, corresponding to the crystallization of the only stereoirregular components, are equal to 8.4 and 8.6h for the blends sPP1-25/samPP1-75 and sPP125/samPP2-75, respectively (Figure 5 and Table 2). These values are lower than those determined from X-ray diffraction of 21 and 13 h of the pure samples samPP1 and samPP2, respectively, and larger than the overall values of the blends of 0.6 and 2.1h, respectively, (Figure 5 and Table 2), which take into account also the fast crystallization of the stereoregular component sPP1. On the other hand, for the blend sPP1-50/samPP1-50 the total characteristic crystallization times of ≈ 2 h determined by X-ray diffraction (Figure 2A and Table 2) is similar to that of 2.6 h relative to the sole component samPP1 evaluated from DSC (Figure 5A and Table 2), indicating that for this blend (and for those with 75 wt% of sPP1) only the stereodefective components samPP1 (and samPP2) crystallize by aging at room temperature. Finally, it is worth remarking that the Avrami exponents fitting the DSC data related to crystallization of samPP1 in the blends sPP1-25/samPP1-75 and sPP1-50/samPP1-50 (Figure 5) are 18

significantly different (n ≈ 0.5-0.6) from those used to fit the X-ray diffraction data (Figure 2) probing the overall crystallization of both components (n ≈ 1) (Table 2). Since the crystallization of the low stereoregular component samPP1 takes place when the sPP1 component has already fully crystallized and the two components are well dispersed in the amorphous phase, the differences in Avrami exponent less than 1 entails a diffusion limited process for the crystallization of samPP1.14 In the case of the blends sPP1-25/samPP2-75, instead, the overall crystallization kinetics and crystallization kinetics of the sole samPP2 component correspond both the an Avrami exponent n ≈ 0.5-0.6, indicating that the crystallization kinetics of both components is diffusion limited.

apparent crystallinity (x'c) (%)

100

A

80 60

blend 25% sPP1 tcryst = 0.6h (XRD)

neat samPP1 tcryst = 21 h (XRD) samPP1 in blend 25% sPP1 tcryst = 8.4h (DSC)

40

samPP1 in blend 50% sPP1 tcryst = 2.6h (DSC)

20 0 0.01

0.1

1

10

100

aging time (h)

apparent crystallinity (x'c) (%)

100

B 80 60

blend 25% sPP1 tcryst = 2.1h (XRD)

neat samPP2 tcryst = 13h (XRD)

40

samPP2 in blend 25% sPP1 tcryst = 8.6h (DSC)

20 0 0.01

0.1

1

10

100

aging time (h)

Figure 5. Apparent degree of crystallinity determined from DSC data of Figure 5 (x'c(DSC)) of the components samPP1 (A) and samPP2 (B) that crystallize during aging at room temperature in the blends sPP1/samPP1 (A) and sPP1/samPP2 (B) with 25 wt% () and 50 wt% () of sPP1 as a function of the aging time at room temperature. The apparent degrees of crystallinity of pure components samPP1 and 19

samPP2 (black lines) and of blends with 25 wt% of sPP1 (dashed black lines) determined from X-ray diffraction x'c(XRD) are reported for comparison. The values of the characteristic crystallization time tcryst are indicated. Continuous lines are the fit of data to Avrami treatment using Equation 1, and the parameters of Table 2. It is worth noting that the data of Figure 5 have been determined by including also the contribution to enthalpy of the low temperature peak due to the melting of sPP1 crystals developed by secondary crystallization during aging, since this contribution would be small. In particular, we have estimated that the secondary crystallization of the sPP1 component in the blends with 50 and 25wt% of sPP1 would contribute at the end of overall crystallization process by an amount equal to ≈1.5 and 0.75 J/g, respectively (Table 3), which correspond to less than 20% of the total melting enthalpy of the low temperature peak. Moreover, for these blends the first hour of aging is dominated by the cold crystallization of the poorly stereoregular components samPP1 and samPP2 (Figures 4 and 5), while the slight increase of enthalpy associated to the secondary crystallization of the more syndiotactic component sPP1starts after at least two hours of aging at room temperature, either in the pure sPP1 or in the blend sPP1-25/amPP1-75 (Figure S4 and 5D), when crystallinity has achieved more than 50% of the maximum value. The low values of the Avrami exponent (n ≈1 or 0.5) describing the cold crystallization of our samples, entail a one-dimensional (fibrillar) growth mechanism controlled by heterogeneous (athermal) nucleation, and/or a diffusion limited process for the growth of the crystals,14 that necessarily influence the blend morphology. We have checked how the presence of a low stereoregular component in the blends influence the morphology of sPP1 that develops in our blends, by performing a POM analysis of melt crystallized (cooling rate=10°C/min) samples. As an example, the POM image of the melt crystallized blend sPP150/samPP1-50, after aging for 3-day at room temperature, is compared in Figure 6 with the POM image of the as soon as crystallized more stereoregular component sPP1. 20

Figure 6. POM images of samples sPP1 (A) and the blend sPP1-50/samPP1-50. The samples were cooled from the melt at 10°C/min. The sample in B was aged for 3 days at room temperature after cooling from the melt. The POM image of the sample sPP1 (Figure 6A) reveals the presence of mixed bundle-like and starshaped spherulites, both consisting of needle like entities crossing from a common center. As demonstrated in ref. 17, the needles originate from the parallel arrangement of lamellae in elongated stacks growing straight, in one direction, with constant rate and thickness, and no branching. Only after colliding might the needles bend and fringe, forming more space filling spherulites. Therefore, the intrinsic tendency of pure sPP to grow spherulites through a mechanism involving the linear growth of needles in a single direction, is not in contrast with an Avrami exponent n equal or lower than 1. As shown in Figure 6B, the leading morphology of sPP1 does not greatly change in the blends. Similar morphologies develop also in the other blends. The main effect of blending consists in a decrease of the length of the needle-like entities. In agreement with the good miscibility between the two components, the POM images of Figure 6 suggest that the thin lamellae of the slow crystallizing components formed by cold crystallization at room temperature are possibly located in the interlamellae and/or intralamellar lamellar amorphous regions, rather than in between the spherulitic entities, even though their exact location may not be easily established from POM analysis. 18 We argue, that the fast crystallization of sPP1, and the good miscibility of the components in the amorphous phase, induces segregation of the low stereoregular components in the intra-spherulitic regions and hence segregation-induced 21

crystallization in the resultant domains, accounting for the low Avrami exponent n ≈ 0.5-0.6 describing the crystallization kinetics of samPP1 or samPP2. The values of melting and crystallization temperatures and of the melting enthalpies of the components sPP1 (Tm,sPP1, Tc,sPP1, Hm,sPP1), samPP1 and samPP2 (Tm,samPP1, Tm,samPP2, Hm,samPP1 and Hm,samPP2) in the blends and those of the pure samples, after aging at room temperature up to complete crystallization, are reported in Table 3 and in Figure 7 as a function of the content of sPP1. As shown in Table 3, the experimental values of melting enthalpy are similar to those calculated assuming complete crystallization of the two components, determined by multiplying the value of melting enthalpy of pure samples sPP1 (31.0 J/g), samPP1 (17.0 J/g) and samPP2 (11.4 J/g) by the corresponding weight fraction in the blend. This somehow rules out the possibility that some regular sequences of samPP1 or samPP2 would built in the crystals of the high stereoregular component sPP1. It is apparent that the melting and crystallization temperatures of the more stereoregular component sPP1 decrease with increasing concentration of the stereoirregular component (Figure 7A). Therefore, addition of the less stereoregular components delays the melt crystallization of the most syndiotactic component sPP1 and causes depression of its melting and crystallization temperatures, but does not influence its ability to fully crystallize. The melting temperatures of the less syndiotactic components samPP1 and samPP2 (Tm,samPP1 and Tm,samPP2) in the blends, instead, increase with increasing concentration of sPP1, after a slight decrease with respect to those of the pure samples (Figure 7A). Therefore, the cold crystallization by aging at room temperature of the poorly stereoregular components samPP1 and samPP2, is favored and accelerated by the presence of the more stereoregular component sPP1, the preformed crystals of which act as crystallization nuclei. This probably indicates partial participation of the short molecular sequences of the more stereoregular sPP1 sample with the crystals formed by the low stereoregular samples. The nucleating effect of the more stereoregular component is less important in the blends with 25wt% of sPP1, where the crystallization from the melt of sPP1 is inhibited. In these blends the component sPP1 cold crystallizes during aging at room temperature and affects the cold crystallization of the poorly syndiotactic component. In fact for these blends the values of melting temperature of the 22

components samPP1 and samPP2 are nearly equal or lower than those of pure components (Figure 7A) and the observed values of melting enthalpy are lower than those calculated (Figure 7B). 130

70 TmsPP1

120

65

110 60

100 90

55

Tm,samPP2

80 70

Tc,sPP1

Tm,samPP1

60

50

100

80

60 40 20 sPP1 content (wt%)

0

Hm (J/g)

40

35

35 30

45

sPP1/samPP1 sPP1/samPP2 sPP1/amPP1

50

Hm, sPP1

B sPP1/samPP1 sPP1/samPP2 sPP1/amPP1

30

25

25

20

20 Hm, samPP1

15

15

10

10 Hm, samPP2

5 0 100

Tm, samPP (°C)

Tm sPP1 Tc, sPP1 (°C)

A

80

60 40 20 sPP1 content (wt%)

5 0

0

Figure 7. Melting temperatures (A) and enthalpies (B) of components sPP1, samPP1 and samPP2 in the blends and of pure samples as a function of the content of sPP1. In A the values of crystallization temperature of the component sPP1 are also reported. The melting enthalpies calculated by multiplying the value of melting enthalpy of pure samples sPP1 (31.0 J/g), samPP1 (17.0 J/g) and samPP2 (11.4 J/g) by the corresponding weight fraction in the blend are also reported as dashed lines in B. The result that the crystallization from the melt of the more stereoregular component sPP1 is depressed, or even prevented in the blends containing high amount (75wt%) of the low stereoregular component is probably associated with the good miscibility of two components, and the consequent 23

reduction of the chain mobility in the melt, due to the fact that the slow crystallizing (samPP1, samPP2) or amorphous (amPP1) components have molecular mass one order of magnitude higher (≈106 Kg/mol) than sPP1 (≈105 Kg/mol). This suggests that a possible mechanism leading to crystallization depression of the sPP1 component, involves a decrease of chain mobility in the melt, with a consequent decrease of nucleation density and/or increase of nucleation barrier for sPP1 crystallization. A similar behavior has been observed in miscible blends of syndiotactic polystyrene with poly(2,6-dimethyl-1,4-diphenylene oxide) (sPS/PPO)19,20 and polyphenylene ether (sPS/PPE).21 In particular, for the sPS/PPO blends, the PPO strongly interfered with the crystallization of sPS producing increase of the cold crystallization temperature and decrease of the melting temperature of sPS in the blends with increasing the PPO content.19 These effects have been attributed to a reduced chains mobility due the segmental-level mixing of the high glass transition temperature PPO component.19,20 In the case of the sPS/PPE blends, a reduction in the crystallization rate of sPS with increasing PPE content and increasing PPE molecular weight has been observed. The increased length of the PPE chain, indeed, would lead to greater entanglement density hindering the overall diffusion process, due to the fact that the amorphous PPE must diffuse away from the crystal growth front in order for crystallization to continue at an acceptable rate.21 However, the crystallization behavior of the sPP blends is contrary to what happens by mixing components with different molecular mass, but similar crystallization ability. For instance, in the case of polyoxyethylene (PEO) blends, it has been observed that the crystallization of the blends shifts to temperatures higher than those of the pure components.22 It is argued that this acceleration effect is caused by an early start of nucleation and an increment of nucleation density, originating from the density fluctuations in the blend and a reduction in energy barrier for nucleation.22 On the other hand, confining the attention to blends between vinyl polymers, it has been shown that melting depression occurs also mixing an isotactic polypropylene sample (iPP) with an isotactic propylene/1-hexene random copolymer, containing 21mol% of 1-hexene units (PH21),23 even though the two components are immiscible. It has been suggested that the fact that the Tc of these blends is not constant with blend 24

composition, while one would have expected a constant Tc if the components separate in two pure phases,24 indicates that the iPP-rich domain must include a content of PH21 that increases with increasing content of PH21 in the blend. On the basis of the present analysis, the crystallization behavior of our blends, accomplished by the presence of a single glass transition temperature, suggests a good miscibility of the components, both in the melt and amorphous phase. On the other hand, the miscibility of the two sPP components in our blends can be also indirectly inferred using the outcomes of rheological investigetions. 25,26 It is well known that the syndiotactic, isotactic and atactic polypropylene (sPP, iPP and aPP respectively) possess different value of molecular weight between chain entanglements (Me)25 and, in the case of sPP in particular, it has been shown that the degree of stereoregularity has a large effect on the entanglement density of the amorphous phase and the space filling attitude of the polymer chains in the melt. 26 More precisely, at the reference temperature of ≈200°C, the values of molecular weight between chain entanglements Me of iPP and aPP are similar (≈7 Kg/mol) and decrease with increasing syndiotacticity up to steeply drop to ≈2 Kg/mol at high rrrr pentad content.26 The corresponding values of the packing length parameter p are ≈ 3.4Å for amPP1 and the low stereoregular samples samPP1 and samPP2, and ≈ 3 Å in the case of sPP1.26 The quantity p is intrinsically related to the size of the polymer coil and the volume that it takes up, since p = V/ 2, with V and 2 the molecular volume and the unperturbed mean-square end-to-end distance of the polymer, respectively,27 and corresponds to six times the inverse of the statistical segment length parameter β2 = 2/(6V) introduced by Helfand and Sapse.28 It has been argued that for a binary blend, miscibility will only occur if the pure-component parameters βi2 are similar for the two species.29 Therefore, the values of βi2 deduced from rheological measurements, allow establishing to which extent the conformational "mismatch" between components of identical constitutional units, but different stereoregularity, influences their miscibility. This mismatch generates an excess entropy of mixing associated with non local effects, since for poorly interacting species, the mixing may generate perturbation in the chain statistic of the random coil at length scales between the Kuhn length and the gyration radius. 29 In our blends the ratio between the βi2 25

values of sPP1 and the low stereoregular components is ≈ 1.1, indicating that the blends are largely miscible at high temperature. Deviation of this ratio from unity, is likely to produce liquid-liquid phase separation in our blends at low temperature. However, the relatively fast cooling rate adopted in our experiments, and the fast crystallization of the sPP1 component from the melt, are probably able to prevent spinodal decomposition.

26

Table 3: Values of melting and crystallization temperatures and melting enthalpy of the component sPP1 (Tm,sPP1, Tc,sPP1 and ΔHm,sPP1) and melting temperatures and enthalpies of components samPP1 and samPP2 (Tm,samPP1, Tm,samPP2 and ΔHm,samPP1, ΔHm,samPP2) in the blends and of pure samples, determined by DSC curves of samples melt crystallized and aged at room temperature up to complete crystallization. The calculated values of ΔHm,sPP1, ΔHm,samPP1 and ΔHm,samPP2 obtained multiplying the value of melting enthalpy of the neat components (ΔHm,sPP1 = 31.0 J/g, ΔHm,samPP1 = 17.0 J/g and ΔHm,samPP2 = 11.4 J/g) by their corresponding weight fraction in the blend, are also reported.

sPP1/samPP1

Tg (°C)a

Tm,sPP1 (°C) b

Tm,samPP1 (°C)

Tc,sPP1 (°C)

sPP1 sPP1-75/samPP1-25 sPP1-50/samPP1-50 sPP1-25/samPP1-75 samPP1

2.2 2.2 0.6 -2.1 -1.3

(49.6) 127.2 123.5 115.6 120.4 -

53.6 52.0 51.0 55 – 62

71.1 68.6 61.4 -

Tg (°C)a

Tm,sPP1 (°C)

Tm,samPP2 (°C)

Tc,sPP1 (°C)

-1.0 -1.7 -0.8 1.4

121.6 115.2 114.9 -

56.3 55.4 53.0 53.2

71.1 65.1 -

Tg (°C)a

Tm,sPP1 (°C) b

Tm,amPP1 (°C)

Tc,sPP1 (°C)

sPP1/samPP2 sPP1-75/samPP2-25 sPP1-50/samPP2-50 sPP1-25/samPP2-75 samPP2 sPP1/amPP1

ΔHm,sPP1 (J/g)b

ΔHm,samPP1 (J/g)

observed

calculated

observed

calculated

(3.0) 31.0 22.6 18.1 8.0 0

(3.0) 31.0 23.2 15.5 7.8 0

0 5.5 6.2 8.0 17.0

0 4.2 8.5 12.8 17.0

ΔHm,sPP1 (J/g)

ΔHm,samPP2 (J/g)

observed

calculated

observed

calculated

23.3 15.9 7.8 0

23.2 15.5 7.8 0

5.2 6.1 7.1 11.4

2.8 5.7 8.6 11.4

ΔHm,sPP1 (J/g)b observed

ΔHm,amPP1 (J/g)

calculated

sPP1-75/amPP1-25 -2.1 (53.5) 122.6 70.6 (2.8) 21.9 (2.2) 23.2 sPP1-50/amPP1-50 -0.9 (49.4) 118.1 52.6 (1.8) 15.0 (1.5) 15.5 sPP1-25/amPP1-75 0 (49.4) 117.7 (1.3) 7.3 (0.8) 7.8 amPP1 1.3 0 0 a ) The Tg values are taken as inflection point temperature of the DSC heating scans of Figure S5 of the Supporting Information. b ) The values of temperature and enthalpy of the low temperature endothermic peak of sPP1 are reported in parenthesis.

27

Concluding Remarks The melting and crystallization behavior of binary blends of samples of sPP of different stereoregularity have been studied. The blends sPP1/samPP1 and sPP1/samPP2, in which both components are crystallizable, display separate melting of components, with sPP1 melting at high temperature and samPP1 and samPP2 melting at low temperature. The more stereoregular component sPP1 crystallizes by cooling from the melt, whereas the poorly syndiotactic components samPP1 and samPP2 cold crystallize during aging at room temperature. A significant depression of melting and crystallization temperatures of the component sPP1 with respect to pure sample is observed in the blends. Moreover, the crystallization from the melt of sPP1 is inhibited when the concentration of the stereoirregular component is higher than 50 wt%, but it cold crystallizes during aging at room temperature along with the stereoirregular components samPP1 or samPP2. The cold crystallization during aging at room temperature of the poorly stereoregular components samPP1 and samPP2 is, instead, favored and accelerated by the presence of the more stereoregular component sPP1, whose preformed crystals act as crystallization nuclei. All these experimental observations indicate interactions of the components in the melt and a good degree of mixing of the sPP chains of different stereoregularity in the melt and in the amorphous state. Finally, the miscibility of the components in the melt is also consistent with our recent analysis of the mechanical properties of these blends that exhibit interesting properties of thermoplastic elastomers, in which crystallinity, rigidity and tensile strength can be easily controlled by changing blend composition and stereoregularity of the components. 8 The continuous change of properties with composition and the strong analogy of the mechanical properties of blends with those of stereodefective sPP samples having degree of stereoregularity intermediate between those of pure components, are also indication of a good miscibility of the components in the melt and in the amorphous state. 8 In other words addition of stereoirregular sPP samples to a more stereoregular sample produces an effect on the crystallization behavior and mechanical properties analogous to that produced by incorporation of defects of stereoregularity and blends behave as a single sPP component having a degree of stereoregularity 28

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Highlights 1. The melting and crystallization behavior of blends of sPPs of different stereoregularity is studied. 2. Blends show separate melting of the components. 3. Good miscibility of the components in the melt and amorphous state.

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