In situ optical microscope study of the shear-induced crystallization of isotactic polypropylene

doi:10.1016/j.polymer.2005.06.074

Polymer

Volume 46, Issue 19, 8 September 2005, Pages 8157-8161

https://doi.org/10.1016/j.polymer.2005.06.074 Get rights and content

Abstract

The shear-induced crystallization behavior of isotactic polypropylene (i-PP) has been investigated by in situ optical microscope under various thermal and shear histories. A shish-kebab structure during growth was observed under a well controlled, however, weak shear field. According to our results under these weak shear and thermal history, a modified model was proposed for the growing process of melt shear-induced crystallization of i-PP. Furthermore, many physical insights were provided on several still unsettled issues such as extended chain crystals, row nuclei, and smectic ordering questions.

Introduction

In most polymer processing operations such as injection molding, film blowing, and fiber spinning, the molten polymer chains are subjected to intensive shear and elongational flow fields. It is known that processing conditions profoundly influence the rate of solidification, the semicrystalline morphology, and ultimately material properties [1], [2], [3]. The semicrystalline morphology (shish-kebab structure) that developed under shear field [4], [5] is typically very different from what is observed under quiescent conditions. It is generally believed that polymer chains are oriented and stretched by the shear flow and can crystallize with different kinetics and consequently different morphologies. Although shear flow is often considered as a ‘weak’ flow, however, it does affect the overall crystallization kinetics [6], [7], [8], [9] and modify the final morphologies and properties [9], [10], [11], [12] of polymeric materials. In recent years, studies of flow-induced crystallization have drawn a lot of interests because it reveals the possibilities of controlling and predicting the final morphologies and properties of the semicrystalline polymers in most of the transformation processes, meanwhile, provided an opportunity to understand some fundamental issues among molecular conformations, ordering processes, as well as nucleation and crystallization kinetics [1], [13], [14], [15].

In this communication, we present some results of in situ optical microscopy studies of shear-induced crystallization of isotactic polypropylene melts under various thermal and shearing histories. It directly revealed the lamellar growth process occurred from an oriented long shish structure. These fibril like shish structure, the perpendicular lamellar growth geometry, as well as the beta-formation and its melting behavior can provide many physical insights on several still unsettled issues such as extended chain crystals, row nuclei, and smectic ordering [14], [15] questions. More detailed scattering studies are still underway in our laboratory.

Section snippets

Experimental details

The isotactic polypropylene homopolymer was provided by Yanshan Petrochemical Corp., Inc. The molecular weights of i-PP resin, as obtained from the GPC experiments, are M_n=9.62×10⁴, M_w=4.38×10⁵ with a polydispersity D=4.55.

A Linkam CSS-450 high-temperature shearing stage (Linkam Scientific Instruments Ltd, Tadworth, Surrey, UK), and an optical microscope (Nikon E600POL) were used in this study. The mechanical design and electronic control of the Linkam shear stage provided adequate conditions

Results and discussion

Fig. 1 shows the polarized optical micrographs of i-PP samples sheared at a rate of 0.5 s⁻¹. Classical ‘shish-kebab’ morphologies are clearly observed. The landscape orientation of lamellae run perpendicular to the narrow backbone as the kebab structure should be. The structure is in close resemblance to the polymorphic shish-kebab morphology in PP melts sheared with fibers [6], [7], [11], [12], crystallization in fibers/polymer composites [17], [18], [19], or sample crystallization by a razor

Conclusion

We have presented in this report an in situ optical microscope study of the crystallization of i-PP under shear with the formation of shish-kebab like α-structure and the growth front induced β-form crystals. According to our results, a modified model has been proposed for the growing process of shear-induced crystallization in i-PP melt.

Although we cannot distinguish smectic liquid crystalline like order from row nuclei alignment with correlation length of the order of the strand molecular

Acknowledgements

This work was supported by three projects: KJCX2-SW-H07, 2003 CB 615600 and 20490220.

References (32)

J.A. Odell et al.
Polymer
(1978)
F.M. Abuzaina et al.
Polymer
(2002)
C.M. Wu et al.
Polymer
(1999)
L. Sharma et al.
Macromol Mater Eng
(2004)
L. Sharma et al.
Macromol Mater Eng
(2004)
A.C. Bushman et al.
J Appl Polym Sci
(1997)
M. Okamoto et al.
Macromolecules
(1998)
J. Pennings et al.
ZZ Colloid Polym
(1965)
J. Pennings
J Polym Sci, Part C: Polym Symp
(1977)
F. Jay et al.
J Mater Sci
(1999)
D. Duplay et al.
J Mater Sci
(2000)

G. Kumaraswamy et al.

Macromolecules

(1999)

J. Varga

Angew Makromol Chem

(1983)

J. Varga et al.

J Polym Sci, Part B: Polym Phys

(1996)

R.H. Somani et al.

Macromolecules

(2000)

L.B. Li et al.

Macromolecules

(2003)

L.B. Li et al.

Phys Rev Lett

(2004)

Cited by (128)

Robust phase change materials based on PBAT-block-PEG multiblock copolymers with novel thermostability prepared via facile melt transesterification
2024, Journal of Energy Storage
Though phase change materials (PCMs) have been demonstrated as a green and sustainable approach to solve the problem of waste heat recovery, preparing PCMs with robust mechanical properties via a simple method remains a massive challenge worldwide. In this study, a facile one-step solvent-free melt transesterification was successfully employed to prepare poly (butylene adipate-co-terephthalate)-block-polyethylene glycol multiblock copolymers (PBAT-block-PEG multiblock copolymers) based solid-solid PCM (SSPCM). The stress-strain curves of SSPCM exhibited more prominent features similar to polymers with high crystallization ability when PEG content was increased, which were furtherly revealed by XRD. The values of tensile strength and elongation at break of most SSPCM, exceeding 3.9 MPa and 669.2 % respectively, represented a significant improvement from previous researches on SSPCM. DSC analysis revealed that PBAT-block-PEG multiblock copolymers based SSPCM containing 60 wt% PEG-6000 (SSPCM-4) exhibited a practical energy storage density with a melting enthalpy of 64.0 J/g. Furthermore, SSPCM-4 not only displayed the solid-solid phase change behavior but also demonstrated impressive mechanical strength by supporting a load exceeding twelve times its weight above the phase change temperature. The solid-solid phase change mechanism was investigated by FT-IR, XRD and ¹H NMR spectroscopy. Moreover, TG analysis indicated that the initial degradation temperature of SSPCM-4 was higher than 200 °C, ensuring its stability during both processing and working.
Molecular evolution mechanism of flow-induced crystallization in polyphenylene sulfide
2023, International Journal of Mechanical Sciences
Microstructures play a significant role on the mechanical performance of materials. Based on the experimental characterization and molecular modeling techniques, this work assessed the influences on the mechanical properties of semi-crystalline 1-4 polyphenylene sulfide (PPS) from pressure and temperature. It was found that the shear deformation in the flow field is a key factor that induces the transformation from amorphous polymers to precursors. Though, pressure can accelerate the transition of precursors to crystals, proportion of precursors in the PPS is not observed under higher pressure. The understanding established in this work is helpful to identify optimal processing parameters during the manufacturing of PPS for target applications.
Shear and cooling induced regulation of mechanical properties and glass transition temperature of isotactic polypropylene
2023, Materials Today: Proceedings
In different experimental studies, it has been observed that polymer properties such as structural, mechanical and thermo-mechanical properties vary with processing conditions. Therefore, molecular dynamic simulation techniques are used to investigate the variation in mechanical and physical properties of isotactic polypropylene polymer under various processing conditions. Non-equilibrium molecular dynamics simulation was used to look into the effect of shear rate on properties. Further different cooling rates were used to cool down the melt structures. For different shear and cooling rates, we calculated selected structural, mechanical and physical properties of polymer which include density of polypropylene in solid state, Young’s modulus and glass transition temperature respectively. Young’s modulus and glass transition temperature show dependency on shear and cooling rate. Results showed that high shear and slow cooling significantly enhance the Young’s modulus and glass transition temperature of the processed polymer.
Melt pre-shear induced orientation crystallization of PB and PB/HMwPB blend
2022, Polymer
Shear flow is important to understand the relationship between processing and properties of polymer products. In this work, the influences of shearing and high-molecular-weight polybutene-1 (HMwPB) on the evolution of oriented crystal's morphologies of isotactic polybutene-1 (PB) were investigated by shearing at a temperature slightly above the nominal melting temperature under different shear rate and shear duration. The shear thickening presented in primary normal stress revealed the presence of a gel-like network of shish-precursors, in which a birefringence region was observed in polarized optical microscopy (POM) images after crystallization in both PB and PB/HMwPB blend. Those shish-precursors enhanced the crystallization temperature and directly influenced the crystal structures of PB form II, and these crystal structures could be preserved from the form Ⅱ to form Ⅰ. The rheological, one-dimensional scattering, and DSC data reach a plateau with the shear duration, indicating that all the PB chains achieve their ultimate oriented state. The addition of a small amount of HMwPB caused the earlier formation of shish-precursors, the higher crystallization temperature of PB form Ⅱ, thicker lamellae, higher melting temperature, and degree of orientation of PB form Ⅰ.
Crystallization kinetics and nanoparticle ordering in semicrystalline polymer nanocomposites
2022, Progress in Polymer Science
There has been considerable interest in the nucleation and crystallization of polymers in the presence of nanoparticles (NPs, or nanofillers in general, NFs). Most of the extensive work in this area has focused on anisotropic, non-Brownian NFs (e.g., clay sheets, carbon nanotubes) whose spatial dispersion state in these nanocomposites is controlled by the process by which they are formed. Hence, NF spatial dispersion is generally limited and often remains poorly characterized. Thermodynamic handles that can be used to control NF dispersion state in the polymer melt include (a) favorable interactions between the polymer chains and the bare NP surfaces, or (b) the density and length of the chains, with the same chemistry as the matrix, grafted to the NP surface. These relatively large NFs merely act as stationary objects that affect the kinetics of nucleation by providing heterogeneous sites, and the crystallization rate by confining the polymer in the melt state. The dispersion state of the NFs can dramatically affect the nucleation and crystallization of the matrix, but in most cases reported, the NFs increase nucleation efficiency relative to the neat polymer. At higher NF loadings, the effect of polymer confinement by the NFs dominates, leading to a decrease in crystal growth rates. This review describes the most important lessons learned from these commonly studied systems and then extends to polymer composite systems containing small, mobile spherical NPs (typically smaller than 100 nm in size).
The role of NP mobility, which provides for dynamic confinement of the polymer melt, on the kinetics of polymer crystallization (nucleation, growth, and overall crystallization) and how this behavior is mostly consistent with the case of immobile NF is a second important focus of this review. In addition to the role of NFs on crystallization kinetics, recently reported nanoparticle ordering phenomena such as the effect of matrix crystallization on the organization of small spherical NPs within the amorphous regions of the semicrystalline morphology are discussed. Such phenomena are clearly not observed for large NFs and hence provide a point of departure from past works in this area.
Enhanced melt-recrystallization process of propylene-ethylene copolymer during the uniaxial stretching with the aid of isotactic polypropylene
2022, Polymer
The structural evolution of propylene-ethylene copolymer (coPP) and propylene-ethylene copolymer/polypropylene (coPP/iPP) films has been studied by in-situ WAXD/SAXS in a tensile process. Both coPP and coPP/iPP films show obvious melt-recrystallization during the deformation process. In the initial stage of stretching, the crystallinities of films decrease due to the stress-induced crystal fragmentation. After that, a large number of oriented crystals form in the late stage of the tensile process. Compared with that of coPP, the initial strain of recrystallization for coPP/iPP is smaller, and recrystallization-induced crystallinity (from 4.9% for coPP sample to 10.2% for coPP/iPP sample) and crystal orientation (from 0.75 for coPP to 0.93 for coPP/iPP) are improved. Structural evolution occurs in close relation to tensile rates. At low strain rate, molecular chains are easier to pull from the lattice, resulting in a significant decrease in crystallinity. At low strain rate, samples display stronger recrystallization in the late stage of stretching. Interestingly, there is a difference in the growth process for lamellae between coPP and coPP/iPP samples. For coPP samples, crystals grow on existing lamellae, leading to an increase in the average thickness of crystals. In contrast, recrystallization between lamellae tends to occur in coPP/iPP samples, leading to a decrease in average lamellar thickness and long period.

View all citing articles on Scopus

View full text

In situ optical microscope study of the shear-induced crystallization of isotactic polypropylene

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusion

Acknowledgements

Polymer

Polymer

Polymer

Macromol Mater Eng

Macromol Mater Eng

J Appl Polym Sci

Macromolecules

ZZ Colloid Polym

J Polym Sci, Part C: Polym Symp

J Mater Sci

J Mater Sci

Macromolecules

Angew Makromol Chem

J Polym Sci, Part B: Polym Phys

Macromolecules

Macromolecules

Phys Rev Lett