Sandwich-structure styrene-butadiene-styrene block copolymer (SBS)/polypropylene (PP) blends: The role of PP molecular w
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Sandwich-structure styrene-butadiene-styrene block copolymer (SBS)/polypropylene (PP) blends: The role of PP molecular weight Ping Wang1, Yanli Qi1, Jun Zhang1,a) 1
College of Materials Science & Engineering, Nanjing Tech University, Nanjing 210009, China; and Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing 210009, China a) Address all correspondence to this author. e-mail: [email protected] Received: 30 September 2018; accepted: 4 February 2019
The effect of polypropylene (PP) molecular weight on the properties of styrene-butadiene-styrene block copolymer (SBS)/PP blends was studied. All SBS/PP blends (50/50 and 90/10) exhibited a sandwich structure where the co-continuous SBS/PP layer was between the top and bottom PP layers. Solvent extraction tests suggested that the continuous phase structure of PP was independent of the blending ratio and PP molecular weight, while the SBS phase changed from a dispersed phase to a continuous phase as the SBS content increased. The decrease in PP molecular weight decreased the PP layer thickness but increased the phase domain size of SBS in SBS/PP(50/50) blends. As a result, less noticeable “stress-hardening” phenomenon was observed. The mechanism for the structural change was attributed to the different melt viscosities of each component. The crystallinity of the blends did not change with the variable PP molecular weight but decreased with the increasing SBS content.
Introduction Polypropylene (PP) is one of the most widely used engineering thermoplastics because of its merits such as low cost, low density, ideal thermal stability, high heat distortion temperature, excellent chemical, and mechanical properties [1, 2]. However, PP is limited to many applications because of its low fracture toughness at low temperature [3, 4, 5, 6]. To overcome this drawback, several methods such as copolymerization of propene with ethylene or other olefins [3], adding a variety of toughening agents [4], adding the nucleating agent to reduce the dimension of spherulites [5], and peroxide dynamic crosslinking [6] have been widely reported. Among these methods, blending PP with an elastomer has been proved effective to toughen PP [4, 7, 8, 9]. One typical example is styrene-butadiene-styrene block copolymer (SBS)/ PP blends, where the addition of the SBS improves the lowtemperature toughness of PP [10, 11, 12, 13]. SBS presents a distinct two-phase structure, which consists of glassy polystyrene (PS) endblocks regularly arranged within a rubbery polybutadiene (PB) matrix [14]. This unique structure of SBS offers PP with the desired properties, especially the good
ª Materials Research Society 2019
balance between impact toughness and rigidity [10, 11]. For example, the tensile and tear strength have been shown to decrease with the increasing SBS content, whereas the impact strength improves with the addition of SBS [11, 15, 16]. The loss in rigidity could be attributed to the reduction in crystallinity of the PP caused by SBS [17]. For most ela
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