New method for the design of high-performance polypropylene

18 March 2016
Shuai Zhou and Zhong Xin
Beta-form polypropylene can be produced through the introduction of a branching molecular architecture and by conducting crystallization under shear and rapid cooling conditions.

Isotactic polypropylene (iPP) is one of the most important thermoplastic polymers because of its low manufacturing cost and versatile properties (e.g., low density, high melting point, and chemical resistance). Moreover, iPP exhibits at least five crystal forms, i.e., the monoclinic alpha- (α), trigonal- (β), orthorhombic-, delta-, and smectic mesophase forms.1 These different crystalline forms of iPP give rise to different optical and mechanical properties. Among these forms, hexagonal β-form iPP has the highest elongation at break and charpy toughness, and has thus received a considerable amount of interest over the past few decades. The β-iPP form, however, is a thermodynamically metastable phase that rarely forms under standard processing conditions. This form can only be achieved under specific conditions, e.g., crystallization in a temperature gradient, with the addition of β-iPP nucleation agents, or through shear-induced crystallization.

Shear-induced β-iPP has been studied widely (with respect to its fundamental scientific concerns and industrial applications) because it may be possible to use this crystallized form to control and predict the final morphology and properties of iPP in most transformation processes (e.g., injection molding or extrusion). During this crystallization process, shear is used to drive the polymer chains that are oriented in the melt at different stages of processing, which further affects the overall polymer crystallization behavior and the resulting morphologies. The formation mechanism of β-iPP under shear flow is thought to be that the melt shear causes the development of α-row nuclei in the form of molecular bundles. These nuclei may thus induce an α-to-β transition, with a specific crystallization temperature interval (100–140°) that is known as the α–β secondary nucleation. The resultant point-like β-nuclei cover the surface of the α-row nuclei, and they induce a supramolecular structure of β-form iPP.2 Although shear plays an important role in the formation of β-iPP, only a small number of β-crystals actually form, because the shear-induced nuclei are unstable and relax easily after the cessation of shear. It is therefore reasonable to predict that more shear-induced nuclei will be retained if the relaxation time of the iPP molecular chain can be prolonged (with the formation of more β-nuclei).

In our previous research,3,4 we prepared long chain branching polypropylene (LCBPP) as part of our development of high-melt-strength polypropylene for foaming applications. LCBPP provides many advantages compared with linear iPP, for instance, its high melt strength and strong strain-hardening behavior. LCBPP is therefore commonly used in foaming, blow molding, and thermoforming processes, i.e., in which high melt strength and strong extensional properties are required.

In our work,3,4 we used a melt grafting reaction in the presence of macro-monomer vinyl polydimethylsiloxane and co-monomer styrene, through one-step reactive extrusion, to prepare the LCBPP. We also investigated the rheological behavior of our LCBPP. Our results showed that LCBPP—compared with linear iPP—had a higher complex viscosity and storage modulus at lower frequency under dynamic shear flow, as well as a longer relaxation time. With these results, we speculated that LCBPP may form β-crystals under shear conditions more easily than linear iPP. The synergistic effects of the LCB molecular architecture and shear conditions on the crystallization behavior of LCBPP, however, remained elusive.

In our new work,6 we have therefore aimed to uncover the effect of the LCB structure and shear conditions on the morphological evolution of LCBPP. To that end, we prepared LCBPP with different LCB contents, with the use of our previous method (i.e., via reactive extrusion in the presence of styrene and benzoyl peroxide). Our rheological results indicate that the existence of the LCBPP samples made with the LCB structure have higher storage modulus values at low angular frequencies than linear iPP (see Figure 1). This implies that the LCBPP has longer relaxation times than the linear iPP.6 In addition, the results of our investigation into the shear-induced crystallization behavior reveal that the LCBPP formed β-form polypropylene under shear and rapid cooling conditions, and that the content of LCBPP β-form content increased with increasing LCB content, shearing rate, and cooling rate (see Figure 2).


Storage modulus (G ′ ) versus angular frequency (ω) for isostatic polypropylene (iPP) and long chain branching polypropylene (LCBPP) samples at 230°C.


Beta-crystal content (Kβ) and wide-angle diffraction (WAXD) patterns (inset) of iPP and LCBPP samples under shear-induced crystallization conditions (shear rate of 60s−1 and cooling rate of 280°C/min). Peaks within the WAXD patterns are labeled to indicate the associated alpha (α) or beta (β) crystal form, with the Miller index values given in parentheses. 2θ: Measured angle of diffraction. The equation given is the Turner Jones equation,5which is used to calculate Kβ, where H represents the height of the reflection peaks labeled in the WAXD patterns.

We have also proposed a potential formation mechanism for shear-induced β-form in LCBPP. We suggest that because the LCBPP samples have longer relaxation times and an easily formed oriented structure under shear conditions, the oriented final structure is more stable and can thus be maintained during the fast-cooling process. Further formation of β-form polypropylene is also induced. Our mechanical property measurements (see Table 1) also show that the LCBPP with β-form crystals can provide balanced stiffness and toughness.

Mechanical properties of iPP and LCBPP samples. The impact strength, flexural modulus, and tensile strength measurements are made according to standard test procedures (ASTM D256, ASTM D790, and TASTM D638, respectively).

SamplesImpact strengthFlexural modulusTensile strength
(J/m)(MPa)(MPa)
iPP41121429
LCBPP3160187539

In summary, we have investigated the effects of a long chain branching structure and shear conditions on the morphology of LCBPP. Our results can be used to explain why LCBPP samples form β-crystals under injection molding conditions rather than quiescent non-isothermal crystallization. Our work also demonstrates a new way to obtain β-iPP, i.e., by the introduction of an LCB structure, and by crystallization under shear and rapid-cooling conditions. Furthermore, the high nucleation efficiency of β-nuclei (which formed under shear flow and rapid cooling conditions) indicates that LCBPP is a potential high-efficiency β-nucleating agent for iPP. In our future work, we will thus investigate the nucleation effect of LCBPP on commercial iPP.


Authors

Shuai Zhou
East China University of Science and Technology

Shuai Zhou is a postdoctoral research fellow whose research focus is on polymer processing, modification, and applications.

Zhong Xin
East China University of Science and Technology

Zhong Xin is a professor with more than 20 years of experience in the engineering aspects of polyolefin research.


References

  1. H. Awaya, Morphology of different types of isotactic polypropylene spherulites crystallized from melt, Polymer 29, pp. 591-596, 1988.

  2. J. Varga and J. Karger-Kocsis, Rules of supermolecular structure formation in sheared isotactic polypropylene melts, J. Polym. Sci. Part B Polym. Phys. 34, pp. 657-670, 1996.

  3. S. Zhou, S. Zhao and Z. Xin, Preparation and foamability of high melt strength polypropylene based on grafting vinyl polydimethylsiloxane and styrene, Polym. Eng. Sci. 55, pp. 251-259, 2015.

  4. S. Zhou, S. Zhao, Z. Xin and W. Wang, A novel strategy for achieving high melt strength polypropylene and an investigation of its foamability, J. Macromol. Sci. Part B Phys. 53, pp. 1695-1714, 2014.

  5. A. Turner Jones, J. M. Aizlewood and D. R. Beckett, Crystalline forms of isotactic polypropylene, Makromol. Chem. 75, pp. 134-158, 1964.

  6. S. Zhou, W. Wang, S. Zhao, Z. Xin and Y. Shi, Shear-induced β-form polypropylene in long chain branching isotactic polypropylene, Polym. Eng. Sci. 56, pp. 240-247, 2016.

DOI:  10.2417/spepro.006338



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