Mechanical Properties and Stress-Induced Phase Transformations of Metallocene Isotactic Poly(1-butene): The Influence of Stereodefects

Abstract

A study of the mechanical properties and of the crystallization behavior of samples of isotactic polybutene (iPB) of different stereoregularity prepared with metallocene catalysts is presented. Stereoregular samples with concentration of rr defects lower than 2 mol % crystallize from the melt by compression-molding and fast cooling to room temperature in form II, whereas more stereoirregular samples crystallize surprisingly mainly in form III. The stereodefective iPB samples show interesting mechanical properties of high ductility and flexibility that increase with increasing concentration of rr stereodefects, with values of deformation at break higher than 1000% for the most stereoirregular sample containing about 5 mol % of stereodefects. The aging at room temperature produces transformation of form II into form I in more stereoregular samples, whereas no phase transformation occurs in more stereoirregular samples crystallized mainly in form III. A strong increase of Young’s modulus and of stress at yield is observed after transformation of form II into form I. For more stereoirregular samples with concentration of rr defects higher than 2 mol % crystallized in form III, the mechanical properties do not change significantly upon aging at room temperature because no phase transformations occur during aging. Also for the aged samples, a remarkable increase of ductility and flexibility with increasing concentration of rr stereodefects is observed. The mechanical deformation is associated with stress induced transformations of form II and form III into form I. These results indicate that mechanical properties of iPB can be modified by the controlled incorporation of stereodefects and improvement of ductility is easily obtained while maintaining high crystallinity by inclusion of small amounts of rr stereodefects.

Crystallization behavior and mechanical properties of copolymers of isotactic poly(1-butene) with 1-octene from metallocene catalysts

“…Therefore, oriented films initially crystallized in form II stretched at high deformations, and oriented films of aged samples stretched at high deformation, are both in form I. This accounts for the observation that the mechanical behavior of iPB at high values of deformation of as-crystallized and aged films are similar, as indicated by the similar values of stress at break of samples before and after the aging at room temperature [25].…”

“…As described in our recent paper on the mechanical properties of stereodefective iPB samples [25], the highly isotactic sample of iPB homopolymer prepared with the isospecific catalyst of Chart 1 shows a typical mechanical behavior of good ductility and flexibility of form II with low yield stress (Fig. 4A) and with increase of Young's modulus and stress at yield and slight decrease of deformation at break after transformation at room temperature of form II into form I (Fig.…”

“…At variance with the mechanical behavior of the iPB homopolymer [25], where a clear increase of Young's modulus and stress at yield is observed upon aging due to transformation of form II into form I (Fig. 4A,B and 5C), for iPBC8 copolymer samples the aging process at room temperature does not produce substantial differences in the mechanical behavior (Fig.…”

“…We recall that in the case of iPB [25], the stretching of samples as-crystallized from the melt in form II (or form III), produces transformation of form II (and form III) into form I [25]. On the other hand, the stretching of the aged samples crystallized in form I (when form II is completely transformed into form I) produces only orientation of crystals of form I and no phase transformations occur [25]. Therefore, oriented films initially crystallized in form II stretched at high deformations, and oriented films of aged samples stretched at high deformation, are both in form I.…”

“…This was the first experimental observation of the crystallization of the stable form I of iPB from the melt at atmospheric pressure [23,24]. These stereodefective iPB samples in both form II and form I show interesting mechanical properties of high ductility and flexibility that increase with increasing concentration of rr stereodefects [25]. Moreover, in buteneeethylene copolymers from highly isospecific metallocene catalysts [26], it has been found that the presence of ethylene accelerates the form II / form I transition at room temperature and produces increase of flexibility and ductility, and at concentration of nearly 6 mol% favors the direct crystallization from the melt of the form I' [26].…”