Free volume, glass transition, and degree of branching in metallocene‐based propylene/α‐olefin copolymers: Positron lifetime, density, and differential scanning calorimetric studies

Abstract

Abstract Positron annihilation lifetime spectroscopy (PALS), density, and differential scanning calorimetric (DSC) measurements were used to study systematically the variation of the glass‐transition temperature ( T g ) and the size v and number density N h of local free volumes in n ‐alkyl‐branched polypropylenes. The samples were metallocene‐catalyzed propylene copolymers with different α‐olefins (from C 4 to C 16 ) and a different α‐olefin content (between 0 and 20 mol %). From the total specific volume and crystallinity the specific volume of the amorphous phase V a was estimated and used to calculate the fractional free (hole) volume h and value of N h . The variations of T g , v , V a , h , and N h were related to the degree (number and length) of branching. T g decreases and v increases linearly with the number and length of n ‐alkyl branches. This behavior was attributed to an increased segmental mobility caused by branching. Both values, T g and v , follow linear master curves as a function of the degree of branching (DB) if this is defined as the number of all side‐chain carbons with respect to a total of 1000 (main‐chain and side‐chain) carbons. Only propylene/1‐butene copolymers deviated from these relations. A linear relation between v and T g was also found. The number density of holes was estimated to be N h = 0.49(±0.07) nm −3 and N h ′ = 0.58(±0.11) × 10 21 g −1 , respectively. It shows a slight variation with the DB, which is also seen in the behavior of the specific volume V a . This variation was explained by the appearance of sterical hindrances resulting from short‐chain branches that may prevent an efficient packing of the chains. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 434–453, 2002; DOI 10.1002/polb.10108