Annealing-induced crystalline structure and mechanical property changes of polypropylene random copolymer
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In this work, the effects of annealing treatment on the crystalline structure and mechanical property changes of polypropylene random copolymer (PPR) were comparatively investigated. Wide angle x-ray diffraction and differential scanning calorimetry were used to study the crystalline structure evolution of the annealed PPR sample. The relaxation behavior of the annealed PPR sample was analyzed using dynamic mechanical analysis. The mechanical properties and the toughening mechanism were also investigated. The results showed that the crystalline structure evolution of the annealed PPR sample depended on the annealing temperature. Due to the largely increased molecular chain mobility in the amorphous region, which promoted the plastic deformation of the annealed PPR sample under the impact condition, largely enhanced impact strength was achieved at a moderate annealing temperature. Further results showed that relatively shorter annealing duration could induce the apparent changes of crystalline structure and mechanical properties of the PPR sample. I. INTRODUCTION
The mechanical properties of semicrystalline polymers are greatly dependent upon the processing conditions which determine the supermolecular structures. During the common melt processing procedures including extrusion, injection, and compression, the molecular chains of the semicrystalline polymers are usually in the nonequilibrium state in the viewpiont of thermodynamics. Thus, the molecular chains relax gradually to achieve the thermodynamic equilibrium state during the service period, which induces the changes of the dimension stability and the mechanical properties due to the variations of the supermolecular structures. Annealing treatment, which is carried out at temperatures below the melting point (Tm) of semicrystalline polymers, is one of the efficient ways to accelerate the relaxation of molecular chains and achieve the thermodynamic equilibrium state in a very short time. If the annealing time is long enough, second crystallization of semicrystalline polymer is provoked, leading to the perfection of raw lamellae which are obtained during the common melt processing, and the formation of new thin lamellae in the amorphous region.1–5 Consequently, physical properties which are greatly dependent upon the supermolecular structures change accordingly. For example, increased tensile modulus, tensile strength, and thermal stability are obtained due to the increased crystallinity and improved crystalline structure induced by annealing treatment.5–8 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.332 3100
J. Mater. Res., Vol. 28, No. 22, Nov 28, 2013
http://journals.cambridge.org
Downloaded: 12 Mar 2015
Isotactic polypropylene (iPP) is a typical semicrystalline polymer with complicated supermolecular structures. Although the molecular chains of iPP have enough crystallization ability, the crystallinity of the iPP particles is relatively small under the common melt processing conditions. Thus, annealing
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