Effects of Process Temperatures on the Flow-Induced Crystallization of

Article pubs.acs.org/IECR

Effects of Process Temperatures on the Flow-Induced Crystallization of Isotactic Polypropylene/Poly(ethylene terephthalate) Blends in Microinjection Molding Zhongguo Zhao, Qi Yang,* Pengjian Gong, Hongwen Sun, Pingping Wu, Yajiang Huang, and Xia Liao College of Polymer Science and Engineering, the State Key Laboratory for Polymer Materials Engineering, Sichuan University, Chengdu 610065, People’s Republic of China ABSTRACT: In this work, the effects of mold and melt temperatures on microstructures and properties of micropart were systematically investigated. Results showed that the remarkably enhanced flow field generated during microinjection molding process proved to be beneficial in forming highly oriented self-fibrillating structures. In addition, especially in blends, changing the process temperatures from 200 °C to 270 °C significantly enhanced the onset crystallization temperature To (Δt = 4.8 °C), peak crystallization temperature (Δt = 1.5 °C), and the crystallization half-time (∼18.6 s). Interestingly, because of the orientation maintenance and shear amplification effects of in situ poly(ethylene terephthalate) (PET) microfibrils, the branching of lamella and the formation of hybrid-oriented structures (fan-shaped β-crystals and trans-crystals) were accelerated. Furthermore, relative content of the β crystal (increments of ∼14.1%) and the degree of orientation were also significantly improved as the process temperatures were increased. throughout the entire thickness. Liu et al.5 analyzed the crystallization behavior of poly(lactic acid)/hydroxyapatite nanocomposites through microinjection molding and found that microparts had strong enthalpy relaxation in the glasstransition region and low poly(lactic acid) cold crystallization, to a certain degree. Jin et al.13 observed a shear amplification effect on the growth of crystalline structures in the presence of shear flow and ultrahigh-molecular-weight polyethylene. Therefore, having realized the huge potential of the complex flow field, we reasonably supposed that this applied shear flow is prone to dramatically altering the properties of composites. Moreover, a good understanding of this shear flow field provides not only deep insights into the mechanism of flowinduced crystallization (FIC) but also valuable guidance on controlling the final morphology in the structural design of products in industrial production. The FIC of polymer melts is a fundamental issue in polymer physics, which plays an important role in practical applications.17−19 Most studies on experimental protocols involved only a simple shear20−23 or extensional flow,24−26 rather than a complex flow field. In Sun et al.,24 the polyethylene terephthalate (PET) fiber-pulling technique was used to generate shear stress, and it favored the formation of the β-cylindrites in isotactic polypropylene (iPP)

1. INTRODUCTION Microinjection molding (MIM) of thermoplastic polymers is one of the most promising fabrication techniques for nonelectronic micro devices.1−4 Many polymeric microparts are being more widely fabricated and applied in many important areas.5−7 The miniaturization of parts is an inevitable step for the evolution of technologies, in which additional functions will be integrated in a smaller space.7−11 Moreover, relative to microparts with rigid materials (e.g., metal and silicon), polymer microparts have significant advantages, such as low cost and high production efficiency.12 Consequently, in recent years, microinjection molding has become a hot topic and attracted much attention in materials process engineering. The process of MIM is complex, and many factors, such as high injection rate (can reach 700 mm/s or higher) and shear rate (>105 s−1), extremely short filling times, substantially large temperature gradients, and so on, can markedly influence this process.5,12,13 Different process conditions significantly affect the crystallization behavior of semicrystalline polymers and morphological evolution of a dispersed phase. Among these factors, mold temperature and shear rate have a significant effect on governing the final microfeatures of parts. Generally, the imposed shear flow on polymer melt can trigger the dramatic enhancement of the nucleation and growth of crystalline lamellae, especially the crystallization kinetics of semicrystalline polymers.14,15 Pan et al.16 investigated the crystalline structures of isotactic polypropylene microparts and found that many shish-kebab structures existed almost © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

May 30, 2017 July 18, 2017 August 3, 2017 August 3, 2017 DOI: 10.1021/acs.iecr.7b02189 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. (a) DSC heating curves of PET, and (b) schematic drawing of sampling methods for POM, SEM, WAXD, and DSC analyses. [Abbreviations: FD, flow direction; TD, transverse direction; and ND, normal direction.]

at a high fiber-pulling rate. Although computer simulations can also provide valuable information on a macroscopic scale,27 (i.e., flow pattern, viscosity, stress profile, and so on), probing the evolution of the microscopic picture of flow behavior and the effects on the crystallization under complex flow field is very necessary. In our previous reports,28,29 the so-called microfibrillar composites based on iPP and PET were successfully prepared through the “slit die extrusion−hot stretch−quenching” process to develop a simple and convenient approach to optimize the properties of commodity polymers. The in situ PET microfibrils could act as an effective nucleating agent for iPP. Three origins for crystal nucleation under shear flow field were found: (a) classical row nucleation, (b) fibril nuclei, and (c) nuclei induced by fibril-assisted alignment. In a later work, microinjection molding was applied to directly from in situ PET microfibrils (10% PET). Note that the MIM feature is an effective tool to investigate the effect of a complex flow field on the morphology and crystalline structures of semicrystalline polymers and their composites. The molecules in the entire region (especially on the skin layer) of microparts are greatly oriented to the molding direction, thus forming a shish-kebab structure. Moreover, the synergetic effect of PET and β-NAs can make hierarchical structures richer and further escalate the development of β-crystals. However, the mechanism of forming in situ PET microfibrils through microinjection molding is still obscure, to some extent. Moreover, to the best of the authors’ knowledge, only a few studies have been conducted on the MIM of the iPP−PET blend system to investigate the effects of process conditions on the morphological evolution of PET phase and crystalline structures (especially the analyses of lamella branching and orientation). The current study aims to further investigate the effects of process conditions, including mold and melt temperatures, on the morphological evolution and formation of crystalline structure, which could provide a good foundation for the preparation of iPP−composite microparts with comprehensive performance. Comparisons between morphology and structure evolution were also conducted using scanning electron microscopy (SEM), polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and two-dimensional wide-angle X-ray diffraction (2D-WAXD).

Chemical Co., China) was applied as the raw polymer material. It exhibits a melt flow index (MFI) of 2.3 g/10 min (230 °C, 2.16 kg) and weight-average molecular weight (M̅ w) of ca. 5.87 × 105 g mol−1. The PET, with a number-average molecular weight of ca. 2.3 × 104 g/mol, was generously donated by LuoYang Petroleum Chemical Co. (China). Its melt temperature, as determined via DSC, was ca. 245 °C. In order to avoid hydrolysis, the PET was dried in a vacuum oven at 120 °C for at least 10 h prior to processing. 2.2. Sample Preparation. The dried PET and iPP were mixed in a weight ratio of 5:95 in an internal mixer (XSS-300) at 270 °C. The well mixed ingredients were then injectionmolded into microparts at various mold temperatures of 80, 100, 110, and 120 °C by using a MicroPower 5 microinjection molding machine (Battenfeld Co., Austria). During the entire experiment, the fixed injection rate is 200 mm/s. According to the DSC heating curves of PET in Figure 1a, to investigate the effects of melt temperature on the crystalline structures and morphology evolution, the chosen melt temperatures are 200, 220, 245, and 270 °C, respectively. The dimensions of the dumbbell-shaped micropart are 18 mm × 3 mm × 0.3 mm. The specimen preparation for testing and analysis is illustrated in Figure 1. For convenience, the microparts obtained were labeled iPPX, in which X represents the mass percentage of PET. 2.3. Measurements. 2.3.1. Differential Scanning Calorimetry (DSC) Analysis. A TA Q20 differential scanning calorimeter (DSC) (TA Instruments, USA) was used to investigate thermal properties of the materials with the following standard procedure: the samples (5−10 mg) (Figure 1) were heated to 200 °C with a heating rate of 10 °C/min, then kept at this temperature for 5 min, and the sample was finally cooled to 40 °C with a cooling rate of 10 °C/min. According to the equation,30 the relative crystallinity, X(T), is calculated as follows: T

( ddHt )dt X(T ) (%) = T ∫T ( ddHt )dt ∫T

0

2

0

× 100 (1)

where To and T2 are the onset and end of crystallization temperatures, respectively, and dH/dt is the heat-evolution rate. Then, using t = (To − T)/R (where T is the temperature at crystallization time t, and R is the cooling rate), the abscissa of temperature can be transformed to a time scale. When the relative crystallinity reaches 50%, the corresponding time is the crystallization half-time (t1/2).

2. EXPERIMENTAL SECTION 2.1. Materials. Commercially available isotactic polypropylene (iPP, tradename T30S; from Lanzhou Petroleum B

DOI: 10.1021/acs.iecr.7b02189 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

diffraction intensities of the α(110), α(040), and α(130) planes at diffraction angles of 2θ = 14.1°, 16.9°, and 18.5°, respectively.

2.3.2. Polarizing Optical Microscopy (POM). Thin slices cut by means of a microtome were used for optical morphology observations. The observation zones were located in the middle of samples along the flow direction (as shown in Figure 1). The same sample then was also melted from 40 °C to 200 °C at a rate of 10 °C/min to observe the crystalline structures. Morphology observations were conducted using a polarized optical microscopy (POM) system that was equipped with a digital camera and hot stage (Model BX51, Olympus, Japan). 2.3.3. Scanning Electron Microscopy (SEM). Permanganic etching was used for the sample surface for observation as described by Bassett et al.31 The surfaces of all the samples were sputter-coated with a layer of gold to provide enhanced conductivity. The morphology then was observed in a fieldemission scanning electron microscopy system (Model JSM 840, JEOL, Ltd., Tokyo, Japan), operating at 20 kV. 2.3.4. Dynamic Rheology Measurements. To detect the viscosity changes of these matrixes at varied melt temperatures, the rheological measurement was carried out on a straincontrolled rotational rheometer (ARES, TA Instruments, USA) with a 25 mm parallel plate geometry. Disks were prepared by compression molding of the dried pellets at 200 °C and 10 kN for 5 min. The gap was set at 1.1 mm and testing temperatures were 200, 220, 245, and 270 °C, respectively. A dynamic frequency sweep test then was performed from 0.1 rad s−1 to 450 rad s−1. 2.3.5. Synchrotron Two-Dimensional Wide-Angle X-ray Diffraction and Small-Angle X-ray Scattering. The 2DWAXD measurements were conducted at the beamline BL16B1 of Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China) to examine the superstructure of the molecular orientation distributions and crystalline structure in the thickness direction. The wavelength was 0.124 nm, and the diameter of the X-ray spot was 0.5 mm. Specimens were cut from the middle of a sample 300 μm thick, as shown in Figure 1. The direction normal to the MD-TD (the molding direction transverse direction) plane was defined as ND, and the X-ray beam was perpendicular to the MD-ND plane. The orientations of the crystals of iPP are quantitatively calculated by means of the Herman’s orientation factor (f H):32 fH =

(3⟨cos2 ϕ⟩ − 1) 2

3. RESULTS AND DISCUSSION 3.1. Dynamic Rheology Analysis and the Simulation of Cooling Rate in the Microinjection Molding. The rheology of the materials (both pure and in blend) at the relevant temperatures has been tested in the dynamic frequency sweep mode (from 0.1 to 450 rad/s). Figure 2 indicates that, in

Figure 2. Variation curves of the complex viscosities of iPP and iPP5 at various melt temperatures.

the entire frequency range, all the specimens except pure PET exhibit non-Newtonian (shear thinning) behavior and the viscosity decreases with increasing shear frequency due to the shear-induced chain orientation. However, in the entire frequency range, PET exhibits Newtonian behavior and the viscosity is almost unchanged with increasing shear frequency. As for the melt temperature, it is found that the complex viscosity increases as the melt temperature decreases from 270 °C to 245 °C. It is also observed that the complex viscosity of PET is far less than that of iPP0 at the same temperature. Furthermore, it is also observed that the complex viscosity of blends increased to a level above that of pure iPP at the low melt temperature (200 or 220 °C). Whereas, when the melt temperature increased to 245 or 270 °C, the viscosity of iPP/ PET blends became smaller than that of pure iPP in the lowfrequency range. Because at 245 or 270 °C, the PET phase is more similar to a liquid,34 this reduces the viscosity of the blends. On the other hand, at 200 or 220 °C, the PET phase is more like a solid, and this correspondingly increases the viscosity of the blends.35,36 At an extremely large frequency (i.e., 450 rad/s), adding PET into the iPP matrix almost has no effect on the complex viscosity at the same melt temperature (η*(iPP0) ≈ η*(iPP5)). As for the melt temperature, it is noted that, in the highfrequency range, higher melt temperature leads to lower composite’s complex viscosity, resulting from higher melt temperature, inducing larger free volume of iPP phase and stronger mobility of iPP molecular chains. The corresponding relaxation time for oriented chains then becomes shorter.37,38 In the microinjection molding process, when the composite has a lower melt temperature, it induces less melt flow ability to fill

(2)

where cos2 ϕ is an orientation factor that is defined as 2

cos ϕ =

∫0

π /2

I(θ ) sin θ cos2 θ dθ

∫0

π /2

I(θ ) sin θ dθ

(3)

where θ represents the azimuthal angle in the diffraction pattern. The orientation function f H = 1 means that the polymer chains orient along the flow direction completely; f H = −0.5 means that the polymer chains orient vertical to the flow direction completely; and f H = 0 means that the polymer chains are randomly oriented. The relative content of the β crystal (Kβ) can be calculated according to the Turner−Jones equation:33 Kβ (%) =

Iβ1 Iβ1 + (Iα1 + Iα 2 + Iα3)

× 100 (4)

where Iβ1 is the diffraction intensity of the β(300) plane at diffraction angle 2θ = 16.1° and Iα1, Iα2, and Iα3 are the C

DOI: 10.1021/acs.iecr.7b02189 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 3. Temperature profile and cooling rate of the molded micro specimen, as a function of cooling time at various (a) mold and (c) melt temperatures. Cooling rate of the molded micro specimen versus average temperature: (b) Tmelt = 270 °C and (d) Tmold = 120 °C.

the mold due to a higher viscosity. It is then implied that a higher shear stress is imposed on the melt. Before exploring the variation of morphological structures after the addition of PET into iPP samples, the processing environment is first investigated. According to the previous reports,10,39,40 the injection molding process, especially the filling stage and the cooling stage, have a remarkable influence on the quality of the prepared part. During the filling stage, the shear rate (ṙm) in the specimen can be estimated by using the Hagen Poiseuille approach:41,42 rṁ =

6Q ⎛ 2 1 ⎞ 6 ⎜ + n⎟ = 3 ⎠ ωh2 ⎝ 3 ωh2

ω

∫0 ∫0

T̅ = Tmold +

⎛ aπ 2 ⎞ 8 (Tmelt − Tmold) exp⎜ − 2 ⎟ 2 ⎝ π ht⎠

(6)

where T̅ , Tmold, Tmelt, and h is the average temperature of polymer melt, mold temperature, injection melt temperature, and the sample thickness, respectively. α represents the thermal diffusivity of the sample (α = 0.06 mm2/s for iPP). The theoretical temperature profile and its derivative (cooling rate), as shown in Figures 3a and 3c illustrate that the polymer melt undergoes a rapid cooling process after injection into the mold. At the cooling stage, the average temperature of polymer melt reaches the mold temperature within ∼0.6 s (see Figures 3a and 3c), ∼70 times faster than that of conventional injection molding.43 When the polymer melt with a different injection melt temperature was injected to the mold, it reaches the same average melt temperature within an extremely short time (∼0.025 s). It is then observed from Figure 3d that the polymer melt has the same cooling rate at each same average melt temperature, implying that the injection temperature has little influence on the melt cooling rate. As for the mold temperature, Figure 3b shows that the lower mold temperature induces the higher cooling rate in the entire cooling process. The oriented molecules in microparts do not have enough time to relax at such a high cooling rate. Therefore, the shear-induced orientation could be maintained adequately. In this situations (low mold temperature), it would be much easier to obtain the compact and highly aligned crystalline structures. 3.2. Phase Morphology Analysis of iPP/PET Blends. Figures 4 and 5 show the morphological evolution at various melt and mold temperatures. Figures 4a and 4b illustrate the discrete domains of the minor component dispersed within the continuous phase of the major component. No phase orientation or difference in the shape of the dispersed domains is observed, resulting from the dispersed phase having not

h

V (ω , h) dω dh (5)

where Q, h, and ω are the volume flow rate, the thickness, and the width of the cavity, respectively, and n is the nonNewtonian index. The microinjected sample, as shown in Figure 1, shows a width of ω = 3 mm and a thickness of h = 0.3 mm; the plunger diameter is 8 mm, the injection velocity is 200 mm/s, and the volume flow rate in the injection is 1.0 × 104 mm3/s. Moreover, the microinjection mold have a two-cavity mold. Hence, according to eq 5, the shear rate of MIM is ∼1.12 × 105 s−1, which is much larger than that for conventional injection molding (∼102 s−1).15 Furthermore, for processing the specimens, the relatively thinner micropart can be rapidly cooled to the mold temperature and solidified during the cooling process of microparts, which has a vital effect on the nonisothermal crystallization process and the morphological evolution in this study. Besides, to intuitively observe the variation of temperature during solidification, the temperature changes with time during the cooling process were also simulated approximately, according to the following equation:42 D

DOI: 10.1021/acs.iecr.7b02189 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

the higher melt temperature there. As can be seen from Figure 5, increasing the mold temperature in our experiment does not show an obvious influence on the dispersion morphology in the samples. However, when the mold temperature reached to 120 °C, there is a large number of spherical phases occurring. These great differences were generated from the complex flow field and the higher temperature gradient in mold cavity. As the injection stage finishes, the polymer melt would no longer receive shear stress. Nevertheless, because of existing interfacial tension and reduced interfacial energy, the oriented dispersed phase is prone to returning to their initial spherical states. Therefore, at higher mold temperature (120 °C), a small part of elongated PET phase with the larger interfacial tension have a longer time to relax and recover to their original state before cold solidification, leading to the forming the spherical PET phases. The formation of oriented PET microfibils was attributed to the extreme process conditions in MIM and viscosity ratio of the dispersed phase to the matrix phase (