Structure and Properties of Isotactic Polypropylene/Polybutene-1 In-reactor Alloys

The composition and structure of polymer largely determine the properties of its final products. As a novel polymer material, the composition, structure, and properties of the isotactic polypropylene/polybutene-1 in-reactor alloy (iPP/iPB alloy) synthesized by sequential two-stage polymerization with Ziegler-Natta catalyst were correlated for the first time in this work. The iPP/iPB alloy was fractionated by temperature rising elution fractionation (TREF) in a broad temperature ranged from −30 °C to 140 °C, and the chain microstructures and sequence distributions of isolated fractions were analyzed by DSC, GPC, 13C-NMR, and FTIR. The iPP/iPB alloy was composed of five components, namely high isotactic PB (iPB, 85.8 wt%), medium isotactic PB (mPB, 5.1 wt%), poly((butene-1)-block-propylene) copolymers (PB-b-PP, 4.1 wt%) which contained PB and PP blocks with different lengths according to the isolation temperature, isotactic PP (iPP, 2.7 wt%), and atactic PB (aPB, 2.3 wt%). Compared to other commercial pipe materials, the iPP/iPB alloy presented outstanding thermal creep resistance and gas permeability resistance, high strength and low deformation at high temperature, and appropriate flexural strength. The roles of PP and PB-b-PP components in the alloy were interpreted. This work is expected to elucidate the potential application of iPP/iPB alloy as pipe materials and provide solutions for the design and synthesis of high performance pipe materials.


INTRODUCTION
Isotactic polybutene-1 (iPB) possessing excellent creep resistance, high toughness, and high thermal deformation temperature is widely used in hot-water pipe fields. [1] iPB exhibits complex polymorphic behavior and can crystallize into hexagonal form I/I′, tetragonal form II, or orthorhombic form III according to the crystallization situations. [2−4] The kinetically favored metastable form II crystallized from molten state can transform into thermodynamically stable form I spontaneously and irreversibly, and finally iPB with significantly enhanced mechanical properties is obtained. [5−11] However, this transformation usually takes more than one week and the long transition period inevitably increases the production cost of the iPB pipes. It was reported that mixing iPB with isotactic polypropylene (iPP) not only accelerated the II-I polymorphic transition, but also enhanced the strength of the final product. [12−14] However, the iPP/iPB blend was a thermodynamically immiscible system and presented phase separated morphology, [15−20] which led to unsatisfied properties of the iPP/iPB blends. [21,22] The iPP/iPB in-reactor alloys synthesized by sequential twostage polymerization using MgCl 2 -supported Ziegler-Natta catalyst are proposed and expected to exhibit outstanding properties of iPB components without the above mentioned problems [23−27] by the incorporation of iPP and PP-b-PB block copolymers. iPP might accelerate polymorphic transition of iPB from form II to form I, [23] and the block copolymers might act as tie molecules to enhance the interfacial strength of iPP and iPB crystals and then contribute to higher impact resistance and higher tensile strength at higher temperature when compared with the mechanically blended iPP/iPB blends. [24] However, the comprehensive properties of these alloys including the creep resistance, gas permeability, and water vapor permeability are not reported due to the small quantity of samples. Our previous works have reported that the iPP/iPB in-reactor alloys were analyzed by solvent extraction fractionation method and components including heptane soluble fraction (iPB), heptane insoluble fraction (iPP and block copolymers), and ether soluble fraction (atactic polymers) were obtained. [25] Nevertheless, the accurate chain composition, chain sequence distribution, and fraction distribution of block copolymers in the iPP/iPB alloy and its influence on the properties of the alloy are not clear.
As an effective fractionation technique, temperature rising elution fractionation (TREF) has been used to fractionate semi-crystalline polymers such as polypropylene, [28,29] polypropylene alloys, [30,31] polyethylene/polypropylene alloys, [32,33] trans-1,4-poly(isoprene-co-butadiene), [34] and even poly(1butene) copolymer [35] according to the crystallizability of the polymer chains. In our recent work, a variety of copolymers containing various lengths of PB segments, PP segments, and poly(propylene-co-(butene-1)) segments were fractionated firstly by TREF to analyze the composition of the iPP/iPB alloys synthesized with varied vacuum treatment time between the propylene polymerization stage and butene-1 polymerization stage. [36] However, the comprehensive properties of these alloys are not reported due to the small quantity of samples.
Herein, an iPP/iPB alloy synthesized with the 50 L autoclave was used to analyze the relationship between structure and properties of the multi-component alloy. Firstly, the alloy was fractionated by TREF within a broader elution temperature ranging from -30 °C to 140 °C. The chain microstructure and sequences distribution of main fractions were investigated by DSC, 13 C-NMR, GPC, and FTIR techniques. The crystalline structures including the lamellae thickness of PB crystals in the alloy were studied by SAXS. The mechanical properties of the iPP/iPB alloy were characterized. The correlations between polymer chain structure, aggregation structure and mechanical properties of the iPP/iPB alloy were provided in this work. It was interesting to find that the iPP/iPB alloy with 93.2 wt% PB, 4.1 wt% PB-b-PP block copolymers and 2.7 wt% iPP showed excellent creep resistance, high strength, and low strain at high temperature, and enhanced gas permeability resistance, and had the potential application as hot-water pipe materials.

EXPERIMENTAL Materials
The iPP/iPB alloy was synthesized by sequential two-stage polymerization in 50 L autoclave. [25] As commercial pipe materials, iPB (PB4235, Lyondell Basell) and cross-linked polyethylene (PEX, Korea LG) were used as comparison to the iPP/iPB alloy. The physical parameters of these materials are summarized in Table 1.

Sample Preparation
The iPP/iPB alloy, iPB, and PEX sheets for the mechanical and physical property measurements were compression molded at 190 °C for 5 min under 10 MPa pressure, and then quenched at room temperature under 10 MPa pressure for 5 min. After room temperature aging for 10 days, the mechanical and physical properties of the sheets were tested.
Thin films of the fractions of iPP/iPB alloy for FTIR measurements were prepared by pressing on Specac GS03940 tableting machine at 160 °C for 5 min under 20 MPa pressure.

P-TREF Experiment
Self-made preparative TREF (P-TREF) instrument with a broad temperature region (from -30 °C to 140 °C) was used to fractionate the iPP/iPB alloy. About 1.5 g of iPP/iPB alloy and 0.2 g of antioxidant (2,6-di-tert-butyl-4-methylphenol (264)) were added to xylene at 130 °C to get a clear solution with a concentration of 0.005 g/mL. Then the clear solution was quickly introduced into a glass elution column with 20 cm in length and 10 cm in diameter. The elution column stuffed with inert sea sands (0.3−0.6 mm size) was immersed in oil bath at 130 °C. Then the elution column was slowly cooled from 130 °C to -30 °C with a rate of 2 °C/h. The elution column was then heated up to a preprogrammed temperature from -30 °C to 140 °C with an interval of 10 °C at a rate of 1.0 °C/min, and synchronously eluted with xylene at the preprogrammed temperature for about 10 h after maintained at this temperature for 30 min. The elution solution was concentrated and precipitated by using plenty of ethanol and then filtered. The fractions were obtained by vacuum drying at 45 °C to constant weight. The fraction isolated at -30 °C was named as F-30°C and the fractions of other temperatures were all named according to this nomenclature.

Characterization
Differential scanning calorimetry (DSC) of polymer samples was carried out on DSC-8500 (PerkinElmer instruments) under the nitrogen flow. Firstly, about 5−10 mg of samples were heated from 30 °C to 200 °C with a rate of 10 °C/min. After maintained at 200 °C for 5 min to eliminate thermal history, the samples were cooled to 30 °C followed by reheating to 200 °C at 10 °C/min. The cooling and second heating curves of fractions of iPP/iPB alloy were recorded accordingly. The first heating curves of iPP/iPB alloy, iPB and PEX samples for mechanical tests were recorded. The relative crystallinity (X c ) of PB in iPP/iPB alloy was determined according to the Eq. (1) and the X c of iPB and PEX was determined according to the Eq. (2).
where is the melting enthalpy tested from the first DSC heating curve, is the melting enthalpy of 100% crystallized PB (form I, 141 J/g) or PE (288 J/g), [2,37] and is the weight percentage of PP in iPP/iPB alloy. 13 C-nuclear magnetic resonance (NMR) measurements of the fractions were operated on 100 MHz Bruker AVANCE III spectrometer at 120 °C, and 1,2-dichlorobenzene-d 4 was used as the solvent. In the 13 C-NMR spectra of PB, the characteristic signals at 40.3, 35.1, 27.8, and 10.8 ppm were ascribed to the main chain methylene, methyne, side chain methylene, and methyl, respectively. The concentration of pentad sequence [mmmm] of PB and PP was calculated from the CH 3 at 10.8 ppm and CH 3 at 21.8 ppm, respectively. [38] In the 13 C-NMR spectra of block copolymer fractions, signals at 42.6 , 32.7 and 26.9 ppm were assigned to the butene-propylene sequence (BP), 30.0 and 23.4 ppm were assigned to PB and PB, respectively. [36,39] where A represents peak area, and the subscript of A represents the chemical shift in 13 C-NMR spectrum.
The weight-average molecular weight (M w ) and polydispersity index (PDI) of fractions were determined on Waters GPC 2000 high-temperature gel permeation chromatography (GPC) at 150 ± 0.05 °C using 1,2,4-trichlorobenzene as the solvent. Polystyrenes were used as standards for all calibration curves.
Fourier transform infrared spectroscopy (FTIR) analyses of fractions were performed on Bruker TENSOR-II with 32 scans and a resolution of 4 cm -1 in the range of 400 cm -1 to 4000 cm -1 .
The small-angle X-ray scattering (SAXS) studies of iPP/iPB alloy, iPB, and PEX samples were carried out on Xenocs 2.0 equipment with incident X-ray Cu Kα beam (λ = 1.5418 Å) at room temperature. The distance from sample to detector was 2480 mm. Each pattern was collected within 60 s and then was background calibrated and normalized using the standard procedure. The long spacing d ac , crystalline lamellae thickness d c , and amorphous layer thickness d a of these samples were calculated by the correlation function method. [10] Creep resistance was determined on a GT-7049 stress relaxation meter (GAOTIE Chemical Machinery, China) with 6.4 kg load (equivalent shear stress of 8 MPa) applied on the sample according to GB/T 11546.1-2008 at 70 °C for 600 min. Before the experiment, the samples were kept in measurement environment for 10 min to balance the temperature and humidity.
Gas permeability was measured on the i-Gastra 7100 differential pressure gas permeameter (Labthink Instruments Co., Ltd., China) according to GB/T 1038-2000. The measurement was carried out at 80 °C and 0% relative humidity with sample size of 110 mm × 85 mm × 2 mm (length × width × thickness) and effective measurement area of 38.48 cm 2 .
High temperature tensile strengths were tested with the A1-7000-S (GAOTIE Chemical Machinery, China) electrical tensile tester with a constant velocity of 50 mm/min according to HG/T 3868-2008 at constant temperature ranging from 20 °C to 100 °C.
Flexural strength was determined on the GT-TCS-2000 (GAOTIE Chemical Machinery, China) according to GB/T 9341-2008 at room temperature.

TREF Fractionation and Fraction Analysis
In order to elucidate the exact fraction distribution as well as the structure and component of main fractions, the iPP/iPB alloy was fractionated step by step from -30 °C to 140 °C with TREF technology. Based on the fractionation principle, the crystallizability of the fractions increased with the increase in the elution temperature. [35] The information of all fractions is summarized in Table 2. The main fractions were characterized with NMR, DSC, FTIR, and GPC.

Medium isotactic PB components
Typical signals at 40.3, 35.1, 27.8, and 10.8 ppm representing the characteristic chemical shifts of main chain methylene, methyne, side chain methylene, and methyl of PB, respectively, were observed in the 13 C-NMR spectrum of F40°C with about 88.8% [mmmm] (Fig. 3). [38] The relatively low T c at about 60 °C ( Table 2) and relatively low T m at about 105 °C (Fig. 2) observed in F30°C and F40°C indicated these two fractions were medium isotactic PB (mPB).
However, the 13

Properties of the iPP/iPB Alloy Compared with Other General Pipe Materials DSC and WAXD
For comparison, the crystalline structures of iPP/iPB alloy and two commercial polymers including iPB and PEX were studied. The iPP/iPB alloy after aging at room temperature for more than one week showed a melting peak at 129.2 °C (Fig. 8a) assigned to form I of PB (Fig. 8b) with crystallinity (X c ) of 52.2 %, [2] which was very close to the commercial iPB (T m = 129.6 °C, X c = 50.2%) and PEX (T m = 130.9 °C, X c = 60.6%) (Figs. 8a and 8b). [37] However, the iPB after aging at room temperature for more than one week showed residue and weak diffraction peak of form II, which almost disappeared in the pattern of iPP/iPB alloy (Fig. 8b). This result indicated that the iPP/iPB alloy had much higher form II-I transformation rate and higher relative fraction of the form I (X I = 0.93) than that in iPB (X I = 0.89). [10] The weak melting peak at 162.7 °C observed in the iPP/iPB alloy in Fig. 8(a) was assigned to α-PP crystal as proved by the weak diffraction peak at 2θ = 14.1° in Fig. 8(b).

SAXS
Two ordered scattering peaks were observed in one-dimensional scattering intensity distribution curves (Fig. 9a), indicating the formation of regular stacked layered crystal structures in these samples. [7] The values of d ac , d c , and d a of PB form I in iPB and iPP/iPB alloy calculated by resultant correlation function curves of Fig. 9(b) are shown in Fig. 9(c). [41] The d c of iPP/iPB alloy was 19.0 nm, which was a little larger than that of iPB (18.3 nm) and much larger than that of PEX (12.6 nm). The crystalline structures of the semi-crystalline polymers determine the properties and applications of the products. The highest crystalline lamellae thickness of PB in the iPP/iPB alloy was expected to contribute to the increased stiffness, modulus (E), yield stress (σ y ), chemical resistance and dimensional stability, and decreased yield strain (ε y ). [42] In addition, dense chain packing in the crystalline phase reduced the free volume of the  gas transportation and thus improved the gas resistance of the iPP/iPB alloy. [43] Creep resistance Creep resistance as one of the most important properties was used to characterize the size stability at high temperature for materials as pipe application. iPB as the polyolefin with the best creep resistance was used as comparison to evaluate the creep behavior of iPP/iPB alloy. [1] Fig. 10 clearly illustrates that iPP/iPB alloy containing a small amount of iPP and block copolymer components showed excellent thermal creep properties at 70 °C with much smaller deformation than that of iPB. Therefore, the incorporation of 2.7 wt% iPP and 4.1 wt% block copolymer components did improve the creep resistance of the alloy.

Permeability resistance
As another important parameter to evaluate the pipe material, permeabilities including gas permeability (Fig. 11a) and water vapor permeability (Fig. 11b) of samples were investigated. It can be seen that CO 2 permeability was the highest and the air permeability was the lowest for all three materials (Fig. 11a). The permeability is mainly influenced by the molecular size, solubility in the membrane, and condensation temperature of the gas molecules. [44] Compared with other gases, CO 2 with higher solubility in the membrane and small molecular size showed higher permeability. [44] It was worth noticing that iPP/ iPB alloy showed the lowest permeability for air, O 2 , and CO 2 , and much lower than that of iPB, indicating the iPP/iPB alloy had much better gas permeability resistance when compared with iPB. The water vapor permeabilities of iPP/iPB alloy and iPB were almost similar (Fig. 11b), both lower than that of PEX. The higher water vapor permeability resistance of these materials when compared with that of gas was due to the polarity of water vapor providing high difficulty in permeability. [45] Tensile tests The mechanical properties including strengths at different deformations (10%, 100%) at constant temperature ranging from 20−100 °C are represented in Fig. 12. It can be observed that the strengths of all samples decreased and the elongations at break increased with increasing test temperature, which resulted in the gradual melting of the crystals and movements of the polymer chains. iPB and iPP/iPB alloy showed the similar strength at 10% and 100% elongation. Although the strengths at 10% elongation of PEX at 20−60 °C were higher than those of iPB and iPP/iPB alloy, the strengths of PEX at 80−100 °C became lower than those of iPB and iPP/iPB alloy (Fig. 12a). The higher strength at the high temperature of iPP/iPB alloy was attributed to its thicker lamellae thickness, which provided the integrity of the aggregation structure and then higher thermal stability of the iPP/iPB alloy at the higher temperature. The strengths at 100% elongation of iPP/iPB alloy and iPB were higher (Fig. 12b), and the elongations at break of iPP/iPB alloy and iPB (Fig. 12c) were lower than those of PEX at all testing temperatures. The iPP/iPB alloy possessed excellent size stability under higher temperature. Flexural strength of iPP/iPB alloy was around 15 MPa (Fig. 12d), which is similar to those of iPB and PEX, and can meet the requirements of winding, installation, and debugging of pipes, sheets, etc. in actual production. [1] Discussion on the Relations for Composition, Structure and Properties of the iPP/iPB Alloy The superior properties of semi-crystalline materials are naturally attributed to their structural advantages, which are mainly reflected by the microstructure and crystal structure of the materials. [46] Our previous work has proved that iPP/iPB blends showed the low critical solution temperature (LCST)-like phaseseparated structure. [20] However, compared with the iPP/iPB blends containing the same PP content by mechanical blending method, the iPP/iPB alloy exhibited phase-separated structure with much smaller PP domains, which was attributed to the compatibilization of block copolymers. [24] As shown in Fig. 13, the block copolymers in the iPP/iPB alloy may act as a tie molecule to enhance the interfacial entanglements between the PB and PP domains in the molten state. [24] The subsequent cooling and crystallization led to the stronger interfacial interaction between the PB and PP crystals, which could promote the stability of molecular chains and effectively constrain the mobility of polymer chains, resulting in higher mechanical properties of the iPP/iPB alloy, including excellent creep resistance and high strength at higher temperature. [24,47,48] The PP component in iPP/iPB alloy not only significantly accelerated the transformation of form II-I, but also acted as nucleating agents to induce PB to crystallize at a higher temperature than that of neat iPB, [24] and resulted in much thicker PB lamellae in iPP/iPB alloy. [2] The thicker lamellae of PB in the iPP/iPB alloy could provide the integrity of the aggregation structure and higher thermal stability at higher temperature, which contributed to a higher strength at high temperature and improving the gas resistance. [49]

CONCLUSIONS
The iPP/iPB in-reactor alloy synthesized by sequential two-stage polymerization using MgCl 2 -supported Ziegler-Natta catalyst was mainly composed of 85.8 wt% iPB, 5.1 wt% mPB, 2.7 wt% iPP, 4.1 wt% PB-b-PP, and 2.3 wt% aPB. The minor PP component in iPP/iPB alloy not only accelerated the transformation of form II-I, but also benefited the increase in PB lamellae thickness without destroying the integrity of PB aggregate structures. The PB-b-PP block copolymers acting as the tie molecules improved the interfacial interactions between PP and PB crystals. Consequently, the resultant iPP/iPB alloy with thicker PB lamellae and improved interfacial interactions between PP and PB crystals presented the greatly satisfied mechanical properties including improved creep resistance and gas permeability resistance, tensile strength at high temperature compared with other general pipe materials.