Abstract
The mechanical properties of copolymers of syndiotactic polypropylene (sPP) with 1-hexene and 1-octene have been studied and correlated with the structural transformations occurring during stretching and relaxation. These copolymers crystallize in disordered modifications of form I of sPP, with crystallinity, melting, and glass-transition temperatures that decrease with increasing comonomer content. All copolymers show remarkable mechanical properties and elastic behavior, with great improvement of flexibility and ductility with respect to the sPP homopolymer already at small comonomer concentrations. For low comonomer content up to 4–5 mol %, the elastic recovery is associated to a reversible conformational phase transition between the trans-planar form III or the trans-planar mesophase and the helical form II, which gives an enthalpic contribution to the elasticity. As the comonomer content increases, the trans-planar conformation is progressively destabilized and the observed elastic properties are not associated with any conformational transformation during stretching and relaxation. In these cases, copolymers behave as conventional thermoplastic elastomers where elasticity is purely entropic and crystals only act as topological constraints of the elastomeric network. These copolymers represent an exemplary case of the possibility to combine in the same materials desired and apparently incompatible physical properties of elasticity, crystallinity, and strength by controlling the crystallization behavior, achieved by tailoring the chemical structure in the synthetic process.
Highlights
Nowadays, many efforts are directed to the development of smart polymeric materials combining different physical properties to be employed in several applications without requiring the blending with other polymers or the addiction of additives that could make the recycling processes more difficult
The X-ray powder diffraction profiles of the compressionmolded films cooled from the melt at about 10 °C/min of the sPPC6 and sPPC8 copolymers are shown in Figure 1A,B, respectively
The decrease of Tg compared to that of the Syndiotactic polypropylene (sPP) homopolymer, owing to the increased flexibility induced by the presence of long side groups, and the presence of crystallinity even at high comonomer concentration, indicate that copolymerization and incorporation of branches in sPP chains allow obtaining highly flexible and contemporarily crystalline materials, which are expected to have interesting mechanical properties of thermoplastic elastomers
Summary
Many efforts are directed to the development of smart polymeric materials combining different physical properties to be employed in several applications without requiring the blending with other polymers or the addiction of additives that could make the recycling processes more difficult. The synthesis and the structure of copolymers of sPP with various comonomers, from ethylene to long-branched α-olefins, and the effect of branches on the polymorphism and properties have been reported in the literature.[22−46] In particular, copolymers of sPP with ethylene (sPPC2) and butene (sPPC4) have shown interesting mechanical properties of thermoplastic elastomers, with elasticity depending on polymorphic transformations occurring during deformation and mechanical parameters depending directly on the crystallization behavior and tunable by changing composition and stereoregularity.[37,40,44] In samples with low comonomer concentration, an enthalpic contribution to the elasticity is observed, thanks to the occurrence of the same polymorphic transitions between crystalline forms with chains in helical and trans-planar conformations observed in highly stereoregular sPP. Experimental details of wide angle X-ray diffraction (WAXS) on powder and oriented fibers and of mechanical tests are given in the Supporting Information
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
