Chain regularity of isotactic polypropylene determined by different thermal fractionation methods

Abstract

The chain regularity of isotactic polypropylene (iPP) homo-and random copolymers was characterized by different thermal fractionation methods in this study. Different stepwise temperature programs were applied in a calorimeter (DSC), in order to develop a method which is significantly faster than SIST and provides reliable information about the chain regularity of iPP. Our studies prove that self-seeding accelerates the crystallization process during annealing in SSA-DSC experiments (successive self-nucleation and annealing). Consequently, the time of isothermal steps can be shortened significantly in the SSA-DSC method. On the other hand we found that step time should not be too short if the goal of the measurement is the determination of average chain regularity. Our results clearly indicate that both the experimental conditions and the evaluation technique influence the obtained results. A standard experimental procedure is proposed for reliably determining the average chain regularity of iPP. The length of the SSA-DSC temperature program developed in this study is much shorter compared to that of the conventional SIST (stepwise isothermal segregation technique) measurements used recently for such experiments. The proposed SSA-DSC program makes the reliable characterization of a large number of samples on an acceptable timescale possible.

Improvement of the impact strength of ethylene‐propylene random copolymers by nucleation

“…The crystal modification of the samples 34 changes only slightly, but characteristically in our study; all investigated materials crystallize predominantly in the monoclinic -modification, but traces of the  form and some -modification also form as shown by XRD measurements 35 . Nucleation usually increases the temperature of crystallization and often also the heat of crystallization; the former is related to lamellar thickness, as thicker lamellae grow at higher temperatures, while the latter is related to crystallinity.…”

Catalyst Type Effects on Structure/Property Relations of Polypropylene Random Copolymers

“…Further variations are possible in combination with different external donors, as shown for example by Chadwick et al, [97] where differences in lamellar thickness distribution resulting from ZN type variation are evidenced for homopolymers already by temperature rising elution fractionation (TREF). This broadness in elution and similarly in melting, as shown before, [82,98] results from an overlay between molecular weight and isotacticity distribution, and becomes increasingly complex for copolymers with increasing C2 content. Next to TREF, the differential scanning calorimetry (DSC) based stepwise isothermal segregation technique (SIST) has been used to demonstrate and analyze such heterogeneity.…”

Direct correlation between modulus and the crystalline structure in isotactic polypropylene

“…SIST can be used for characterization of distribution of stereoand regiodefects in iPP similarly to temperature rise elution fractionation (TREF) [31]. According to our earlier observation chain regularity estimated by calorimetry correlates strongly with properties, because they are determined by the crystalline structure [32]. After the samples were crystallized according to the SIST method they were melted at a heating rate of 10°C/min.…”