Influence of Chain Microstructure on LiquidâLiquid Phase Structure

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Influence of chain microstructure on LLPS and crystallization of dual reactor Ziegler-Natta made impact propylene ethylene copolymers (IPC) Laura Santonja-Blasco, Wonchalerm Rungswang, and Rufina G Alamo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04708 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 8, 2017

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Industrial & Engineering Chemistry Research

Influence of chain microstructure on LLPS and crystallization of dual reactor Ziegler-Natta made impact propylene ethylene copolymers (IPC) Laura Santonja-Blasco1, Wonchalerm Rungswang2, Rufina G. Alamo*1 1 FAMU/FSU College of Engineering, Chemical and Biomedical Engineering Department, 2525 Pottsdamer St. Tallahassee, FL, 32310-6046 USA 2 SCG Chemicals Co., Ltd, Siam Cement Group (SCG). 10 I-1 Road, Map Ta Phut Industrial Estate, Muang District, Rayong Province 21150, Thailand Abstract

The relationship between ethylene content, phase structure, crystallization behavior, and the inferred mechanical performance has been studied in five impact copolymers with overall ethylene content between 8 and 11 mol%. Thermal characterization data and crystallization kinetics of IPC do not scale with content of ethylene. Emphasis is given to the correlation between heterophasic morphology assessed by SEM and POM and the properties of the crystalline propylene ethylene copolymer (CEP) component extracted via fractionation. As the rubber component (PER) is equivalent for all IPC, the scaling between ethylene content and increased droplet size is explained by the observed differences in dynamics of the ethylenepropylene copolymer molecules during the liquid-liquid phase separation step (LLPS). On this basis, a correlation is inferred between co-crystallization and compatibility of the components that make the observed multiphase morphologies and the IPC mechanical behavior.

* Corresponding author ([email protected])

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Introduction Impact polypropylene copolymers (IPC) are very complex materials consisting of isotactic polypropylene (iPP) as major phase and polypropylene-ethylene copolymers (PE) in a wide range of ethylene composition broadly distributed among crystalline and non-crystalline components. The ethylene-rich molecules phase separate in the melt of these materials and this two-phase structure has been proven to enhance their impact properties compared with the performance of isotactic polypropylene.1,2 The outstanding rigidity and thermal resistance properties of isotactic polypropylene coupled with an enhanced impact resistance due to the additional rubbery phase, makes IPC to be excellent materials for automotive and other impactrelated applications.3,4 The commercial synthetic path to produce IPC is a two-state polymerization process.

5,6

In the

first stage, a Ziegler-Natta iPP homopolymer is produced and brought to a second reactor where active iPP molecules continue polymerizing and propene is subsequently copolymerized with ethylene. Propylene-ethylene copolymer molecules obtained from this sequential polymerization include amorphous and crystalline random propylene-ethylene copolymers with different monomer length distributions and molar masses. Characterizing the content and distribution of ethylene across the molar mass of the PE copolymer is a key factor to understanding the impact and tensile properties of IPC.7-9 Consequently, much interest has been given recently to develop analytical characterization tools that address composition distributions of amorphous and crystalline components of the PE fraction in IPC materials.10,11 Temperature rising elution fractionation (TREF) coupled with gel permeation chromatography (GPC) have been used extensively to characterize the distribution of the comonomer content across the molar mass distribution of statistical crystalline copolymers.12-15 However, the TREF technique is limited to changes in comonomer distribution of crystalline molecules, and distributions of noncrystallizable comonomer-rich molecules cannot be resolved by TREF. This is a strong handicap for the characterization of IPC given the presence in these materials of copolymer molecules with ethylene content ranging from very low (< 1 mol%) to very high (~ 50 mol%) level, being the latter not crystalline. Furthermore, the multisite nature of the Ziegler-Natta catalyst imparts a broad blocky intra-molecular distribution of the ethylene content that affects crystallization and the mechanical performance of IPC.16,17


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Circumventing the limitations of TREF to characterize compositional changes among the noncrystalline ethylene-rich molecules of IPC, interactive high performance liquid chromatography (HPLC) has been recently adapted to characterize IPC at high temperature. Instead of TREF sequence specific crystallization, HPLC works on the bases of adsorption and desorption of ethylene segments to a graphitic substrate in the column.18-22 The iPP molecules or iPP segments have been found either weakly or not to be absorbed and hence are quickly eluted. The retention time of the eluted copolymer molecules increases with ethylene length,19 thus allowing chromatograms that are characteristic of a given IPC ethylene distribution. Preparative fractionation, analytical and crystallization studies of the individual fractions is also found to be important for a comprehensive characterization of complex polyolefin materials.21,23-26 IPC are multicomponent systems for which the impact of composition on the phase structure, crystalline, and mechanical properties depends strongly on the state of the melt, i.e. whether the components are miscible, immiscible or partially miscible prior to crystallization.27-29 The miscibility of binary blends of metallocene iPP and propylene-ethylene copolymers has been studied in detail. iPP and propylene ethylene copolymers are miscible in the melt up to contents of ethylene in the copolymer of ~ 12 mol%.30-32 Moreover, different thresholds of ethylene content in the copolymer to induce phase separation have been suggested depending of the technique used to analyze the melt structure. Using DSC and AFM Hiltner and co-workers suggested that PE copolymers of molar mass around 200 kg/mol are miscible if the difference in ethylene content was less than around 18 mol%.33 Janani et al. estimated a threshold of 15 mol % for the comonomer content difference for miscible blends of iPP with PP-based random copolymers with ethylene.34 Complex IPC contain highly crystalline isotactic polypropylene (iPP) as the major phase, blocky crystalline propylene ethylene copolymer molecules (CEP) in a broad range of ethylene composition, and amorphous propylene ethylene molecules (PER). The latter are PE copolymers with ~ 50 mol% ethylene, hence, PER molecules phase separate in the melt from the iPP molecules. The question remains as the role of the CEP component in the two phase structure melt and the partitioning of these molecules between the iPP and PER domains during crystallization. It is clear that understanding the role of the different components on the morphology of solid IPC is crucial because mechanical performance such as impact and elongational behaviors depend on how the three components interact during solidification. In 3 ACS Paragon Plus Environment


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fact, it is the separation of the rubbery phase made of ethylene-rich molecules, and effective interfacial connectivity, what leads to the desired impact properties of these materials. The initial view of the solid state morphology of IPC was a simple two phase structure consisting of a majority highly crystalline iPP phase, and a dispersed rubbery amorphous phase in the micrometer size.35,36 However, more detailed characterizations in recent works have concluded that the actual solid structure is more complex, especially in regards to the dispersed phase.37 The dispersed phase is viewed as a multilayered core-shell structure composed of crystalline ethylene-rich components in the central core with the characteristic orthorhombic crystalline structure of the polyethylene; surrounding the central core there is a shell made of PER amorphous molecules, continuing with a third diffuse shell of propylene ethylene molecules with a blocky distribution (CEP component)

that links the PER shell with the matrix highly

crystalline iPP component.38,39 Compatibility of the iPP-rich block with the iPP matrix and of the ethylene-rich block with the PER shell is believed to be crucial in reaching high performance of the solid material.40,41 In the present work, the compatibility is studied as a function of content of ethylene. Furthermore, the distribution characteristics and contents of the blocks that act as linkages between both phases is also addressed as they may be of relevance for an effective compatibilization.42,43 We study these issues on a set of five IPC with average ethylene contents in a range usually found in commercial materials, 8 to 11 mol%. The work is organized as follows; first the molecular characterization of the whole IPC is studied and correlated with thermal, structural and morphological properties. We continue analyzing the liquid-liquid phase structure (LLPS) of the melt and the effect of melt annealing on phase composition and crystallization kinetics. With these data we infer the role of melt dynamics on the final semicrystalline structure of the IPC. Lastly, via fractionation we extract the CEP component that is believed to serve as major link between the crystalline and rubbery phases. We analyze ethylene distribution and crystalline properties of this component in reference to ethylene content and phase compatibility.


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Experimental Five IPC samples obtained by polymerization in dual reactors using a Ziegler Natta catalyst were supplied by SCG Chemicals Co. Ltd. The molar masses, average content of ethylene measured by NMR and melting and crystallization temperatures of IPC1 to 5 are listed in Table 1. The level of crystallinity listed is calculated from the heat of fusion (second melting at 10°C/min). Specimens of 250 µm thick were prepared by compression molding the initial powders at 200°C in a Carver press and rapidly quenching to room temperature (23 ± 1°C). The plaques were kept at room temperature for at least two weeks prior to any testing. The content of ethylene and ethylene triad distribution were evaluated from high resolution 13C NMR spectra using a Bruker DRX 500 spectrometer. The samples were measured at 125°C in benzene-D6 with 90° pulse angle and 12 s pulse interval. Table 1. Molecular characterization, thermal properties, and degree of crystallinity of IPC.

Sample

Mw Mw/Mn (Kg/mol)

NMR ethylene content (C2) mol%(a)

Tc (°C)

Tm (°C) (b)

∆Hm (J/g) (b)

Xc DSC(c)

IPC1

194

10.0

7.7

109.5

158.2

88.6

0.42

IPC2

212

11.4

8.1

110.8

158.0

78.9

0.38

IPC3

217

12.0

9.5

108.8

157.9

88.1

0.42

IPC4

236

12.7

10.2

112.0

159.9

101.3

0.48

IPC5

200

10.3

10.9

110.6

158.7

98.8

0.47

(a)

Calculated according to Ref (54) Second heating (c) Taking for iPP, ∆Hu (J/g) = 209 J/g 44 (b)

The impact polypropylene copolymers were fractionated in three fractions consisting of isotactic polypropylene (HPP) as the major component (~80 wt%) the first, blocky crystalline propylene ethylene copolymer (CEP) of various compositions the second (~9 wt%), and amorphous propylene-ethylene rubber (PER) the third (~11 wt%). The fractionation procedures followed those detailed in a previous work.16 The IPC pellets were initially dissolved in n-decane at 140°C and slowly cooled to room temperature. The CEP and HPP components precipitate and the amorphous component (PER) remains in solution. The PER fraction was recovered by 5 ACS Paragon Plus Environment


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precipitation in acetone. To separate the CEP and HPP components, the initial decane-insoluble fraction was subjected to soxhlet extraction in n-heptane for 5h. The heptane-insoluble fraction (HPP) was washed in acetone several times before drying at 110°C. A low molar mass iPP and the CEP fraction were precipitated from the n-heptane soluble fraction also using acetone. The weight-average molar mas (Mw) and distribution was measured in a GPC-IR5 manufactured by Polymer Char, Valencia, Spain with trichlorobenzene (TCB) as mobile phase at a flow rate of 1 ml/min and a temperature of 160°C. The GPC instrument also records the content of methyl groups, as CH3/1000 total carbons, as a function of molar mass. As seen in Table 1, the weightaverage molar masses are about the same for all IPC, they range from 194 to 236 Kg/mol. High Temperature Thermal Gradient Interaction Chromatography (HT-TGIC) with an infrared detector manufactured by Polymer Char, Valencia, Spain was used to separate by absorption the ethylene rich copolymer from the polypropylene rich crystalline copolymer and from the propylene-ethylene rubber fraction. The latter is neither absorbed nor crystalline and hence elutes quickly from the column. The amount of each component can be quantified from the areas of the peaks of the chromatogram. The experiments were carried out with 1,2-dichlorobenzene (oDCB) using a 10 cm long, 4.6 mm i.d. Hypercarb® column from Thermo Scientific and the following parameters: dissolution, 60 minutes at 160°C with automated vial shaking of a 4 mg/ml solution, and injection of 200 ml of the solution at 150°C. Cooling ramp, 150 to 35°C at 20°C/min, heating ramp, 35 to 150°C at 2°C/min. Crystallization pump flow rate: 0.06 ml/min. Elution pump flow rate: 0.5 ml/min. The elution of the soluble fraction was obtained at 35°C for 5 min, and represented in a range of 20-35°C. The crystallization and melting were studied using a Perkin Elmer DSC7 operated at a heating rate of 10°C/min under nitrogen atmosphere and calibrated with Indium. The level of crystallinity was estimated from the observed heat of fusion divided by the heat of fusion of monoclinic crystalline polypropylene ∆Hu (J/g) = 209 J/g.44 Some samples were isothermally crystallized from the melt (200°C) and subsequently melted at 10°C/min. The thermal protocol to assess melt dynamics in the liquid-liquid phase separated region (LLPS) used in DSC and in optical microscopy was as follows. The IPC were kept 5 min at 200°C and then cooled at 40°C/min to 50°C. After 5 min at 50°C the samples were brought to the annealing temperature (Ta = 171°C) and kept for different annealing times ta = 0.1, 10, 30, 60 and 90 minutes prior to 6 ACS Paragon Plus Environment

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cooling to the isothermal crystallization temperature (126°C for the DSC and 141°C for the optical microscopy experiments). Time to peak and spherulitic growth rates were recorded as a function of melt annealing time. WAXD patterns of the IPC and fractions were collected at room temperature using a collimated Bruker Nanostar diffractometer with Iµs micro-focus x-ray source, and equipped with a HiStar 2D Multiwire SAXS detector and a Fuji Photo Film image plate for WAXD detection. The plates were read with a Fuji FLA-7000 scanner. WAXD patterns were calibrated with corundum. The wavelength of the radiation source was λ=0.154 nm (Cu Kα). Atactic PP was used to subtract the amorphous halo from the total diffraction area. The morphology was analyzed by polarized optical microscopy (POM) and scanning electron microscopy (SEM). For SEM measurements about 0.3 mm thick specimens were fractured in liquid N2 before etching in xylene at room temperature for 12 h. After drying for 24 h at room temperature, the specimens were coated with gold before observation using a scanning electron microscope Quanta 250, FEI. The diameter and size distribution of the PER domains were evaluated by Image J software with 100 counts from three different micrographs. Polarized optical micrographs were collected during isothermal crystallization of 50 +/- 10 µm thick films using an Olympus BX51 optical microscope fitted with an Olympus digital camera (Type DP72) and a Linkam Scientific Instruments hot stage for temperature control (Type TMS94).



Results and discussion Effect of ethylene on crystalline and morphological properties of whole IPC Figure 1 shows molar mass distribution and variation of content of methyl branches with increasing molar mass from the IR detector of the GPC. Clearly, the IPC investigated have basically the same molar mass distribution and the expected systematic decrease of methyl with increasing ethylene at the higher molar mass end of the distribution. The small variation of content of methyl at the lower end of the distribution is random among IPC 1-5 and is associated with chain ends. The observed variation indicates that chains with Mw < ~ 50,000 g/mol have little, if any, ethylene incorporated and that the ethylene is preferentially included in the long molecules (> 100,000 g/mol).

0.8

450 400 350

0.6

300 0.4

IPC1 IPC2

0.2

IPC3 IPC4 IPC5

250 200

CH3/1000C

dW /d(log M)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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150 100 50

0

0 2

3

4

5 log M

6

7

8

Figure 1. Molar mass distribution and content of methyl per total carbons for IPC1 to 5.

The IPC were further analyzed by high temperature thermal gradient interactive chromatography (HT-TGIC). This is a recently developed technique that is able to distinguish polypropylene and polyethylene molecules based on crystallization (iPP) and absorption (PE) to the graphitic stationary phase of a chromatographic column.14,19,45-52 It is known that iPP is not absorbed to the graphite matrix using TCB and oDCB as solvents, while polyethylene is absorbed and further elutes at about 145°C 18,19,47,53. Elugrams as a function of increasing temperature of IPC solutions 8 ACS Paragon Plus Environment

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that were crystallized in a previous step, as detailed in the experimental part, are shown in Figure 2. The three regions delineated correspond to elution of the three major components. The PER component, or molecules with a high content of ethylene (~ 50 mol%) that neither absorb nor crystallize, elute at the beginning of the elution process (Region 1). The highly crystalline iPP molecules, or HPP component, crystallize during the cooling step and dissolve in a temperature range between 90 – 115°C; as expected these molecules constitute the major component of the IPC (Region 2).19 Finally, absorbed propylene ethylene copolymers with relatively long ethylene runs, and almost pure polyethylene molecules desorb from the graphite and elute at temperatures in a range of 120 – 135 °C the first and ~ 143°C the second. Molecules of region 3 constitute most of the CEP component. From a visual inspection of the elugrams, there are no significant differences between IPC1 -5. Small differences are found for the mass percentage of each region and for the average methyl branch content per 1000 total carbons (data listed in Table S1). Except for a higher PER content in region 1 and a broader elution temperature in region 3 for IPC1, all other data are very similar between the samples, or with the expected trend based on the progressive increasing content of ethylene from IPC1 to 5.

14

Region 1 20-35°C

Region 2 ~35-120°C

Region 3 ~120-160°C

12 10 wf (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8 6 IPC5 IPC4 IPC3 IPC2 IPC1

4 2 0 20

40

60

80 100 Temperature (°C)

120

140

160

Figure 2. HT-TGIC profiles for IPC1 to 5.



While HT-TGIC clearly identifies the three major components of the complex IPC, it is not sensitive to the differences in the distribution of ethylene in the minority components. The ethylene distribution was further inferred from an analysis of ethylene diads and triads obtained from high resolution 13C NMR spectra.54 The results are plotted in Figure 3 as a function of the average mol% ethylene in the IPC. We notice that while the EE and EEE contents for IPC3-5 scale linearly with the average ethylene content, the diads and triads values for IPC1 and 2 fall above the straight line, thus indicating a tendency for the ethylene of IPC1 and 2 to be blockier than for samples IPC3-5. The question then is where is the blockier distribution, if it is in the PER or in the CEP components of IPC1 and 2, and if a distribution that deviates from the random behavior affects partitioning of the copolymer molecules between the multi core –shell morphology. To address this issue, the three major components were extracted from each IPC. 10 Mol % EEE or EE

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8

EE

6

EEE

4 2 0 6

7

8 9 10 Mol % Ethylene

11

12

Figure 3. Content of EEE triads and EE diads as a function of the average ethylene content for IPC1 to 5.

Before analyzing fractions, we discuss the thermal and morphological properties of the whole IPC materials to probe if the expected correlation with increasing average content of ethylene is observed. The DSC crystallization and melting thermograms of IPC1 to 5 are given in Figures 4a and b. The melting peaks and heats of fusion are listed in Table 1. As shown, the thermograms are for the most part dominated by the iPP major component, and melting of the crystals from the copolymer molecules (~100-120°C) is a small contribution to the low temperature wing of the thermogram. The small differences in the crystallization peaks do not scale with ethylene content


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and are most probably associated with differences in the two-phase melt structure and/or nucleation density. In an attempt to identify better the iPP crystallites from the crystals of propylene ethylene copolymer molecules, the samples were isothermally crystallized at 125°C for 60 min in order to crystallize most of the iPP component, and then cooled at 1°C/min to room temperature to allow crystallization of the propylene ethylene molecules in a second step. After this thermal history, the melting peaks were basically the same indicating that the iPP crystals are dominating the melting behavior. Isothermal crystallization experiments were also performed at different temperatures, and the time to reach 50% of the transformation in the first step (t1/2), was taken as a measure of the crystallization rate of iPP which may be affected at a level proportional to the compatibility of iPP with the iPP-rich copolymer molecules. The t1/2 are plotted as a function of the crystallization temperature (Tc) in Figure 5. At any Tc, IPC4 displays the fastest rate and IPC5 the slowest. Hence, no correlation was found between the crystallization rate and the ethylene content in the IPC. a)

b) 2nd heating

IPC5

IPC5

Heat Flow

1W/g

0.5W/g

IPC4

Heat Flow

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


IPC3 IPC2 IPC1

IPC4

IPC3

IPC2

IPC1

70

90

110 T (⁰C)

130

150

80 100 120 140 160 180 200 T (⁰C)

Figure 4. DSC a) Exotherms and b) endotherms obtained at 10°C/min for IPC1 to 5.



6

IPC1 IPC2 IPC3 IPC4 IPC5

5 t1/2 (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4 3 2 1 0 118

119

120

121 Tc (°C)

122

123

124

Figure 5. Half-time crystallization (t1/2) as a function of isothermal crystallization temperature for IPC.

To test if the variation of the rate of crystallization with ethylene content shown in Figure 4a is due to differences in nucleation density between the IPC, optical micrographs were recorded after isothermal crystallization at 127°C during 2 min (Figure S1). We found that nucleation density varies randomly among the IPC and does not follow the trend of the half times. For example, IPC3 has the highest nucleation density and IPC4 the lowest. Unexpectedly, IPC5 that contains the highest ethylene content has relatively high nucleation density and low crystallization rate. We attribute these features to differences in diffusion of the copolymer molecules between the various phases during LLPS. Compositional drifts lead to differences in nucleation rate, especially at the droplet-matrix interphase as inferred for other immiscible polyolefin blends.29 An assessment of the disperse phase as a function of content of ethylene in IPC was first carried out by SEM, and the melt dynamics were probed by annealing at increasing times prior to crystallization. It is well-known that relevant mechanical properties of heterophasic IPC, such as impact and elongation correlate directly with the size and distribution of the dispersed domains.35,38,40,55-57 In order to seek a possible correlation between the ethylene content of IPC1 to 5 and the size of the dispersed phase, SEM micrographs of cryofractured specimens were obtained after removal of PER by etching in xylene. The SEM images and diameter and distribution of the droplets, evaluated by averaging from different micrographs, are plotted in Figure 6 as a function of 12 ACS Paragon Plus Environment

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average ethylene content. The droplet size is basically constant with diameters of 0.63-0.77 µm for IPC1-3 and increases up to 1.10 µm with increasing ethylene in IPC4-5. This range of domain size is known to provide enhanced IPC mechanical properties.56-58 Because the rubbery domain is removed in the etching process, it is difficult to identify the multi core-shell structure in these images, but the difference in morphology between IPC1 and 5 is quite obvious. IPC1 and 2 have smaller randomly distributed domains, hence, the expectation is that IPC1 and 2 will perform better in tensile properties than IPC3-5 albeit all display the same homogeneous distribution.57

IPC1

IPC3

IPC4

IPC5



8

a) SEM diameter (µm)

1.6 diameter (µm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.2 0.8 0.4

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b) POM, Tc=134°C from 200°C

6 4 2 0

0.0 7

8

9 10 C2 mol% in IPC

11

12

7

8

9 10 11 C2 mol% in IPC

12

Figure 6. SEM micrographs of fractured surfaces of IPC1-5, and diameter of PER droplets as function of average ethylene content in IPC a) from SEM images and b) from POM images.

The morphological aspects of the two phase morphology can be also investigated from polarized optical micrographs (POM). Under isothermal crystallization the spherulitic morphology of IPC1-5 evolves engulfing the rubbery droplets, allowing differences in the dispersed domains to be contrasted by POM.28,59,60 Figure 7 shows characteristic examples for IPC1 and IPC5 of POM acquired at a high crystallization temperature to emphasize difference in droplet sizes and to point out that POM is able to capture the major differences of the multi-phase morphology observed by SEM with a small increase in ethylene content. For quantitative measurements, domains of at least three spherulites were measured for each IPC, and for each spherulite more than 45 different measurements were obtained. The data are plotted versus average ethylene content in Figure 6b. From SEM the average droplet size increases with ethylene from 0.63 to 1.10 µm, while from POM measurements the observed domains increase from 2.70 to 5.60 µm with high dispersion in both sets, as evidenced by the high values of the standard deviations. The ~5 fold difference in the measured sizes between SEM and POM is expected because only features > 1 micron can be distinguished by POM. Hence, the focus of the SEM/POM comparison is not in the absolute values, but in the equivalent trend that can be obtained with careful experiments from both techniques, as seen here with increasing ethylene content between IPC1 – 5.


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IPC1 134°C

IPC5 134°C

Figure 7. POM of IPC1 and IPC5 isothermally crystallized at 134°C. Scale bar 20µm.

The fact that IPC4 and IPC5 display the largest droplets of the series may be due to a larger PER fraction due to the higher ethylene content; however, the HT-TGIC data of Figure 2 do not support this correlation, and as shown later the amount of PER obtained by fractionation is very similar among the fractions. Hence, the difference in droplet size must be due to propyleneethylene dynamics in the melt of IPC toward generating the two phases. The combination of a slightly lower ethylene content (8 vs. 11 mol%) and a tendency for blockier segmented copolymer molecules may influence the core structure and molecular mixing at the interphase of the core-shell structure in the initial melt. From the SEM images, it appears that the two liquid phase structure is quickly arrested by crystallization when the ethylene content is in the lower level. This is likely due to enhanced compatibility between iPP and the low ethylene content copolymer molecules.33,34 Conversely, components with higher drive to diffuse in IPC4 and 5 due to their higher ethylene content lead to larger dispersed domains, and are subject to larger fluctuations between phases. The analogy, in reference to prior work, is a mechanism for LLPS close to fast spinodal decomposition for IPC1-3, and more in the lines of nucleation and growth for IPC4 and 5.27,28 The differences are schematically drawn in the phase diagram of Figure 8.61 Instability during spinodal decomposition for IPC1 and 2 arrests quickly the initial LLPS structure leading to domains that are smaller compared to those of IPC4-5.



IPC5

Temperature (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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IPC1 Ta Tc % iPP

Figure 8. Schematic phase diagram for the IPC. The dotted lines demarcate the spinodal decomposition, propylene concentration in IPC, melt annealing temperature (Ta), and crystallization temperature (Tc).

Xu and co-workers have carried out studies of melt annealing for different lengths of time inside the binodal using impact propylene copolymers synthesized by a multi-stage sequential polymerization process.61-64 These authors observed that by increasing annealing time at melt temperatures between 160 and 180°C, the isothermal crystallization rate at temperatures > 120°C decreases, and dispersed domains trapped within the growing spherulites grow with time. Comparatively, minor differences were observed in linear growth rates with time.59 The decrease in overall crystallization rate was associated with a larger extent of LLPS in the melt, and with concentration fluctuations at the domain boundaries. Propylene ethylene molecules may slowly diffuse to the iPP matrix, and since ethylene is a defect for crystallization, the rate of iPP crystallization will decrease as more ethylene is incorporated into the matrix. It is expected that the iPP regions close to the dispersed domains are more concentrated with ethylene. Moreover, because nucleation is favored at the interface, the increase ethylene content in this region will lower nucleation density and the overall crystallization of IPC.65-67 On the basis of the two phase diagram and in order to test for differences in crystallization between IPC due to compositional drifts in the melt, we heated the samples up to a temperature inside the binodal, and annealed for different times prior to isothermal crystallization at 126°C. The isothermal crystallization was followed by DSC and the linear spherulitic growth rates by POM. Significant compositional changes in the melt during annealing will be reflected on


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changes in nucleation density, and hence in the overall crystallization rate, and in the texture of the intra-spherulitic dispersed domains. Moreover, the linear growth rate may not be affected too much because growth will be driven mainly by the iPP matrix. Polarized optical micrographs of IPC crystallized at 141°C after annealing at a melt temperature of 174°C for 0.1 min or 90 min are shown in Figure 9. Here, it is quite clear that the nucleation density decreases significantly with increasing annealing time, and the dispersed domains become larger. To emphasize the latter feature, magnified comparative images for IPC1 and IPC5 are shown in Figure 10. The average droplets size of IPC1 change from 5.3 µm (ta=0.1 min) to 10.5 µm (ta=90 min) and in IPC5 the change is from 5.8 µm to 13.2 µm for the same annealing times. Hence, the change in nucleation density due to compositional drifts is obvious. Moreover, as noted by the size of the spherulites, the linear growth rates undergo a minor change with annealing time (quantitative isothermal linear growth rates are available in Figure S2 of the Supporting Information).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

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(b)

IPC2 174°C (0.1 min) Tc=141°C 10 min

IPC2 174°C (128 min) Tc=141°C 15 min

IPC3 174°C (0.1 min) Tc=141°C 18 min

IPC3 174°C (120 min) Tc=141°C 27 min

IPC5 174°C (0.1 min)Tc=141°C 20 min

IPC5 174°C (95 min) Tc=141°C 30 min

Figure 9. Spherulitic morphology of selected IPC isothermally crystallized at 141°C from 174°C held at different annealing times a) 0.1 min and b) ≥ 90 min. Scale bar 100µm.


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(a)

(b)

IPC1 174°C (0.1 min) 141°C 16 min

IPC1 174°C (90 min) 141°C 20 min

IPC5 174°C (0.1 min) 141°C 25 min

IPC5 174°C (95 min) 141°C 30 min

Figure 10. Enhanced view of the spherulitic morphology of IPC1 and IPC5 crystallized at 141°C from 174°C held for a ) ta = 0.1 min and b) ta = ~90 min. Scale bar 20µm

In parallel with the observed nucleation density with melt annealing time, the overall crystallization kinetics followed by DSC undergoes similar changes. Representative crystallization exotherms at 126°C and further melting thermograms are given for IPC1 and IPC5 in Figures 11a-b. The time to peak is almost invariant with annealing time for IPC1 but increases substantially for IPC5. The time to peak versus annealing time are plotted for all IPC in Figure 12a. Similarly to the large depression of nucleation density, IPC5 undergoes the largest depression in crystallization rate followed by IPC4 and IPC3, while the rate of IPC1 and 2 is basically constant. The variation of the slope with IPC ethylene content (Figure 12b) correlates with the observed trend of the droplet size with ethylene content (Figure 6a), and is in consonance with melt dynamics associated with the proposed paths for LLPS. Thus, nucleation 19 ACS Paragon Plus Environment


and overall crystallization kinetics give evidence for differences in rates of diffusion of copolymer molecules toward equilibrating the composition of the liquid phases.

ta (min)=

a) IPC1 Tc=126°C

b) IPC5 Tc=126°C

90

90

0.1

0

10

time (min)

20

60

0.1 W/g

Heat flow

10

ta (min)=

30 Heat flow

0.1W/g

60 30

10 0.1

0

30

10 20 time (min)

30

Figure 11. Exotherms obtained after isothermal crystallization of IPC at 126°C from 171°C annealed at the times indicated. a) IPC1, and b) IPC5.

16 IPC1 IPC2 IPC3 IPC4 IPC5

14 12

0.12

a)

b)

0.1

IPC5

0.08

10 slope

t 1/2 (min)

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8

0.06

6

IPC3

0.04

IPC4

4 2

0.02

0

0

IPC1 IPC2

0

20

40

60

80

100

ta (min)

7

8

9 10 C2 mol% in IPC

11

Figure 12. (a)Variation of time to peak, t1/2, of IPC isothermally crystallized at 126°C as a function of melt annealing time (ta) at 171°C. (b) Slope of t1/2 vs. ta versus average mol % of ethylene in IPC 20 ACS Paragon Plus Environment

12

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Effect of ethylene on crystalline properties of IPC fractions From the studies of the whole IPC it was concluded that the drastic differences in crystallization rate with melt annealing time must be associated with changes in the composition of the matrix and dispersed phases during annealing. To investigate further differences between the three major components of these materials, namely the iPP (HPP) component, the ethylene propylene segmented copolymer (CEP) and the ethylene-rich component (PER), fractions were obtained from each IPC using identical fractionation steps as detailed in the experimental section. The rationale for studying these fractions is that differences in ethylene concentration or distribution in the fractions among the IPC materials may explain the difference observed in morphology and crystallization. The first result from the fractionation is a mass fraction of HPP, CEP and PER that is basically invariant for all samples (Table 2). Hence, the increase of droplet size in IPC 4-5 cannot be correlated with a larger PER fraction in these samples, but must be associated with a preferential partitioning of ethylene–rich copolymer molecules to the disperse phase, as mentioned above.

Table 2. IPC fractionation data. Listed are mass percentages of iPP homopolymer fraction (HPP), crystalline ethylene-propylene copolymer fraction (CEP), and propylene-ethylene rubber fraction (PER).

Samples

HPP amount (wt%)

IPC1 IPC2 IPC3 IPC4 IPC5

81.2 80.7 80.7 81.4 80.9

heptane-soluble fraction (% wt) – CEP fraction CEP amount (wt%)

LMW-iPP amount (wt%)

4.7 4.8 4.9 5.0 5.2

3.1 3.2 3.3 3.3 3.4

PER amount (wt%)

11.0 11.3 11.1 10.3 10.5

The molar masses and distribution of all HPP fractions are basically identical and free of ethylene. Moreover, some differences in the molar mass distribution are found between the IPC samples for the PER and for the CEP fractions, as shown in Figures 13a and 13b. From the content of methyl per 1000C measured by IR in the GPC detector, we estimate that about 40% of



the CEP heptane-soluble fractions are low molar mass iPP molecules, and the rest are propylene ethylene molecules (the true CEP component) in a broad range of composition. These molecules are clearly bimodal in molar mass distribution for IPC4 and 5 (Figure 13a). About 10-15% of the CEP molecules are high molar mass polyethylene basically free of methyl branches and this high molar mass tail shifts to lower values for CEP1-3. The amount of ethylene-rich molecules is clearly higher in CEP4 and 5 than for CEP1-3, and since as suggested earlier these molecules will likely make the central core of the multi core-shell structure, they may as well contribute to the higher size droplets seen in Figure 6. It is also likely that these high molar mass molecules serve as strong links between the rubbery PER region and the central crystalline ethylene-rich core. As found for the HPP fractions, the distribution of molar masses of the PER fractions is also very similar and the concentration of ethylene follows the average ethylene content in the whole IPC; i.e. PER1 has the lowest ethylene content and PER5 the highest (Figure 13b). A small amount of low molar mass iPP molecules (~5 %) is present in all PER fractions.

200 100 0

0.2 -100 0

-200 4

log M

6

300

300

0.4

2

350

b)

8

dW /d(log M)

0.6

0.8 0.6

250 PER1 PER2 PER3 PER4 PER5

0.4 0.2

200 150 100 50 0

0 2

4

log M

6

8

Figure 13. Molar mass distribution and content of methyl branches per total carbons for IPC fractions, a) CEP and b) PER.

The CEP fractions were analyzed by HT-TGIC to distinguish the mass content of propylene-rich and propylene-poor molecules based on differences in temperature for dissolution of crystals (iPP) and molecular desorption (PE). The complexity of the CEP fraction of IPC materials has been acknowledged,12,20,21,37 as such, three different regions are prominent in the HT-TGIC profiles of Figure 14; from low to high temperature, the first corresponds to polypropylene 22 ACS Paragon Plus Environment

CH3/1000C

400

CEP5 CEP4 CEP3 CEP2 CEP1

a)

CH3/1000C

0.8

dW /d(log M)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Page 23 of 38

molecules that are neither absorbed nor crystallized and elute between room temperature and 35°C, region 2 corresponds to polypropylene-rich copolymers and region 3 to ethylene-rich copolymers. The mass percentage of each region and the composition of CH3 per total carbons are listed in Table S2 of the Supporting Information. There are negligible differences in the iPPrich component (region 2). The main difference is in the mass percentage of the ethylene rich copolymers (region 3) which increases with increasing content of ethylene in the IPC sample. Furthermore, the iPP rich molecules of CEP2 elute at higher temperatures than the other CEP fractions, an indication of longer more crystallizable iPP sequences in this fraction. 4 10 Region 1

wf (%)

8

3


6 4 2

wf (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

2

20

25 T (°C)

30 Region 3: ethylene-rich

Region 2: iPP- rich

1

0 20

40

60

80 100 Temperature (°C)

120

140

160

Figure 14. HT-TGIC profiles for CEP fractions from IPC1 to 5.

To probe a possible difference in the distribution of ethylene, the CEP and PER fractions were also analyzed by 13C NMR. The average content of ethylene in the CEP fractions increases as the ethylene increases in the whole IPC materials with values ranging from 26 to 38 mol% from IPC1 to 5 and from 40 to 54 mol% for the PER fractions. These data and the NMR triad distribution are listed in Table S3. Of relevance is a content of EEE/PPP triads of the PER fractions that does not scale linearly with the average ethylene content in the PER or in the IPC. However, the EEE of CEP fractions display linear scaling with a degree of blockyness (B Koenig parameter) similarly low. PER1 and 2 have a higher EEE content than expected from the linear trend (Figure 15), hence, paralleling the NMR results in the whole components.



40

CEP 30

EEE mol %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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PER 20

10

0 20

30

40

50

60

C2 mol % Figure 15. Content of EEE triads in CEP and PER fractions as a function of average ethylene content.

It could be envisaged that if the composition of the crystalline molecules is significantly different between the CEP fractions, it may result in differences in melting temperatures or in the content of gamma crystals. For example, if the distribution of ethylene in the iPP rich molecules of CEP1 and 2 is blockier than for the other fractions, the longer sequences would lead to thicker crystals and hence to higher melting temperatures and lower contents of gamma phase.68 It is known that the formation of the gamma phase is favored by the presence of short iPP sequences, and shorter sequences are favored by the incorporation of ethylene. If the propylene sequences are longer, the content of gamma crystals will decrease.8,9,69-76 On these grounds, we examined possible changes in melting and polymorphism of the CEP fractions. Melting endotherms after isothermal crystallization at 124°C for 30 min and subsequently cooling at 1°C/min to room temperature are given in Figure 16. There is no significant difference in the melting peaks of the iPP rich (145 ± 1°C) and iPP poor (117°C ± 1°C) crystallites between CEP fractions 1-5 in spite of the difference in average content of ethylene. This feature supports that the ethylene distribution of the CEP component is blocky (or segmented) and may not be very different between the IPCs. From the increased heat of fusion of the ~ 115°C melting peak, the major difference is in the content of ethylene-rich molecules that lead to orthorhombic crystallites. This content increases from CEP1 to CEP5 in consonant with the increasing mass 24 ACS Paragon Plus Environment

fraction of ethylene-rich molecules inferred by HT-TGIC (Figure 14) and must be the major contributor to the increase droplet size.

2nd heating isothermal at 124°C

0.5 W/g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Heat flow

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55

75

95

115 135 T (°C)

155

175

Figure 16. Melting after isothermal crystallization (Tc = 124°C) and subsequently cooling at 1°C/min of the CEP fractions.

Polymorphic analysis was also carried out on the CEP fractions for a comparative test of the ethylene distribution. To enhance the content of crystals in the gamma phase, these fractions were crystallized isothermally at 124°C for 900 min and further cooled at 10°C/min till 30°C. WAXD patterns of CEP fractions are shown in Figure 17. In addition to alpha and gamma crystals from the propylene-rich molecules, orthorhombic crystallites also develop from the ethylene-rich molecules, as shown by the characteristic reflections at 2θ = 21° and 23.5°. The content of crystallites in the α, γ and polyethylene orthorhombic phases were estimated from baseline and halo subtracted patterns. The content of orthorhombic crystallites was first obtained by scaling the pattern of a linear polyethylene to the ~23° reflection. The residual constitutes the contents of iPP α and γ crystallites, which were quantified independently by the relative ratio of the 19.8° (γ) and 18.6° (α) reflections according to the method of Turner-Jones.77 Quantitative



results and overall WAXD level of crystallinity (Xc) are listed in Table S4 of the Supporting Information.

γ α orthorhombic CEP5

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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CEP4

CEP3

CEP2

CEP1

4

9

14

19 2θ

24

29

34

Figure 17. WAXD patterns of CEP fractions isothermally crystallized at 124°C. Characteristic reflections of the alpha (monoclinic) and gamma (orthorhombic) phases of iPP, and of the orthorhombic polyethylene phase are indicated.

The content of gamma phase is about 50% for all CEP fractions while 100% gamma will be expected for propylene ethylene copolymers with equivalent composition and a random distribution.73 The lower gamma content obtained confirms that iPP-rich copolymer molecules of the CEP fraction, or those that crystallize in the alpha and gamma phase, have a blocky distribution,68 and the invariance of gamma crystals again supports a similar CEP chain microstructure for all samples, one that does not conform with a random behavior. The latter was also the conclusion from the invariant melting behavior of the CEP fractions, and the linear EEE scaling of figure 15. In contrast, the content of polyethylene orthorhombic crystallites in CEP1 to CEP5 increases systematically from 10% to 18%. This increase was earlier inferred from the


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GPC distributions that show IPC4 and IPC5 having a higher amount of ethylene-rich molecules basically free of propylene, and thus more prone to crystallize in the polyethylene crystal form. In the melt, these polyethylene-rich molecules segregate to the core of the dispersed PER phase. PER fractions do not develop any crystallites on cooling from 200°C and the endotherms obtained from HPP fractions are the same for all samples (See Figure S3 of the Supporting Information).WAXD patterns were also collected from the 1-5 HPP and PER fractions but differences were not observed. As expected from DSC crystallization and melting data the HPP WAXD patterns display only alpha phase, and the patterns of the PER fractions yielded the characteristic halo pattern (Figure S4). Finally, we utilize the detailed characterization that we have undertaken on the IPC and CEP fractions to predict their tensile and impact behavior. In prior works, long iPP sequences of the iPP rich component of the CEP fraction have been considered to act as anchors linking the iPP matrix and the PER shell, thus improving elongation at break of IPC materials.16 On these lines, IPC1 and IPC2 are expected to display better elongation than IPC5, as the concentration of ethylene in the CEP component is lower and the iPP-rich molecules of these samples contain longer propylene blocks. These features favor compatibility with the iPP matrix. We demonstrated above that the fact that IPC1 and IPC2 contain the lowest ethylene also explains the smaller droplet size. Upon reaching the binodal, LLPS of IPC1 and 2 is fast, and any instability is further blocked by crystallization. Annealing at temperatures close to the spinodal line will only lead to domain coarsening and not to a change in ethylene composition as seen by the invariant rates with annealing time of Figure 12. For these systems it has been speculated that the effective mixing between the long iPP sequences of the CEP component and the iPP matrix is responsible for enhanced strain at break. Conversely, the larger amount of ethylene-rich molecules in the CEP component of IPC5 leads to larger dispersed domains in spite of having the same amount of PER component. The ethylene-rich molecules mix with the PER shell, and due to the high molar mass of these molecules in CEP5 (Figure 15a), more ties and a better bonding in the inner droplet structure are expected. This high molar mass component may be responsible for somewhat improved impact properties.16 In general, we can envision the multi core-shell morphology of IPC1-IPC5 changing as given in the schematics of Figure 18. The models emphasize differences in the crystalline CEP shell and core components as a function of average 27 ACS Paragon Plus Environment


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concentration of ethylene in IPC. These CEP molecules are partitioned between iPP-rich copolymer crystallites that interact with the iPP crystals of the matrix and ethylene-rich crystallites that segregate to the core and link with the PER components via intercrystalline linkages. IPC5

IPC1 HPP matrix

HPP matrix

CEP fraction: iPP rich copolymers PER fraction: rubber copolymers CEP fraction: ethylene rich copolymers

Figure 18. Schematics representing the multi core-shell droplet structure of IPC. For clarity, the highly crystalline components of the HPP matrix are not drawn.

Conclusions A detailed structural characterization and crystalline and melting properties of a set of impact propylene copolymers (IPC) has been undertaken to address the role of ethylene content and distribution on the heterophasic melt structure and solid properties of these materials. The range of ethylene content was kept between 8 and 11 molt % which are levels commonly used for commercial IPC materials. GPC characterization indicates a preferential incorporation of ethylene in the high molar mass chains (> 100,000 g/mol). Moreover, thermal characterization data such as melting and crystallization temperatures or isothermal crystallization rates do not scale with the average ethylene content in the whole IPC. These features, and additional NMR and nucleation studies, suggest that the crystallization kinetics are affected by differences in diffusion of the copolymer molecules during LLPS. Evidence of compositional drifts after crossing the binodal are given by 28 ACS Paragon Plus Environment

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the effect of melt annealing time on the crystallization rate. In the low ethylene range, the crystallization rate of IPC is unchanged, while the rate of IPC with higher ethylene content decreases with annealing time indicating a slower diffusion of copolymer molecules. These differences in kinetics are correlated with the increasing droplet size with content of ethylene suggesting morphologies driven by spinodal decomposition that is quickly arrested by crystallization for the low ethylene IPC, and nucleation and growth being more relevant for the morphologies observed for IPC in the higher ethylene range. It is also demonstrated in this work that the variation of droplet size with increasing ethylene can be assessed by polarized optical microscopy. The components responsible for the observed difference in morphology and kinetics were extracted via fractionation. As the mass fraction of the three major components are equal for all IPC studied, it is concluded that the change in droplet size must be associated with the content and partitioning of ethylene-rich copolymer molecules of the CEP component. HT-TGIC, NMR, crystallization kinetics, and polymorphic analysis of CEP fractions reveal the concentration and segmented distribution of ethylene in these CEP fractions and support their role as linkers for the heterophasic structure. The iPP-rich CEP molecules found at higher contents in low-ethylene IPC co-crystallize to a greater extent with the iPP matrix, thus enhancing connectivity in this region. Conversely, high molar mass ethylene-rich molecules of the CEP component, such as those found at higher contents in the IPC with high ethylene, diffuse at a slower rate and serve as anchors between the PER rubbery shell and the crystalline ethylene-rich CEP component that forms the central core of the heterophasic structure. Schematics representing the differences in multi core-shell morphology with increasing ethylene are provided, and a correlation between the multiphase morphologies and mechanical behavior is also inferred.

Acknowledgement Funding from SCG Chemicals Co. is gratefully acknowledged. Supporting information



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Mass percentage of the different regions obtained by HT-TGIC for the whole IPC and for their CEP fractions, 13C NMR contents of triads for PER and CEP fractions, fractional content of iPP α and γ crystals and polyethylene orthorhombic crystals in CEP fractions are listed in Tables S1 to S4. Polarized optical micrographs of isothermally crystallized IPC, spherulitic grow rate versus annealing time for whole IPC samples, and DSC and WAXS data for PER and HPP fractions are displayed in Figures S1 to S4. This material is available free of charge via the Internet at http://pubs.acs.org.


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composition

distribution

measurement

of

polyolefins

by

high-temperature

liquid

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