Equilibrium Melting Temperature of the Hexagonal Crystals of Polybutene-1 and Its Copolymer

TheLamellar structures of form I in polymorphic isotacticpolybutene-1(iPB-1) and forms I and I ' in butene-1/ethylene random copolymer(iPB-1/C2) were investigated by studying their melting behavior usingthe combination of small-angle X-ray scattering (SAXS) and fast scanningchip calorimetry (FSC) techniques. Hexagonal form I was transformedfrom tetragonal form II obtained by crystallization of a homogeneousmelt of iPB-1 and iPB-1/C2. Hexagonal form I ' was crystallizeddirectly from a heterogeneous melt of iPB-1/C2. Despite the same hexagonalcrystallographic structure, forms I and I ' have obviously distinctmelting behaviors due to different ways of crystal formation. Well-definedlinear dependences of the melting temperature against the reciprocallamellar thickness were observed for forms I and I ' with frozenchain mobility in crystals. The slopes of the thus obtained meltinglines for forms I and I ' are different. Their extrapolationsto imaginary infinitely large lamellar thickness provide the equilibriummelting temperature T (m) (infinity) for forms I and I ', which is associated with the stabilityof the lamellar crystals. In spite of crystal defects in form I ',a higher T (m) (infinity) is observedfor this form in iPB-1/C2 than T (m) (infinity) for form I in both iPB-1 and iPB-1/C2. The differencein T (m) (infinity) indicates a lessperfect structure of form I crystal lamellae that is caused by thefragmentation of form II crystal lamellae during the polymorphic phasetransition from form II to I. The lamellae broke into crystal blocksalong the lamellar direction. The smaller lateral sizes of the fragmentedlamellae clearly induce a decrease in the stability of crystals, asexpressed in a lower T (m) (infinity) for form I.