Time-resolved FTIR spectroscopic study of the evolution of helical structure during isothermal crystallization of propylene 1-hexene copolymers. Identification of regularity bands associated with the trigonal polymorph

Two different types of regularity bands are identified in a real time FTIR crystallization of a series of random propylene 1-hexene copolymers. The first is akin to the bands observed in the homopolymer, those associated with 3 1 helices of isotactic sequences of different n length ( n, number of mo...

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Bibliographic Details
Published in:Polymer (Guilford) 2011-06, Vol.52 (13), p.2856-2868
Main Authors: Ruiz-Orta, C., Fernandez-Blazquez, J.P., Pereira, E.J., Alamo, R.G.
Format: Article
Language:English
Subjects:
Applied sciences
Bands
composite polymers
Copolymers
crystal structure
Crystallites
Crystallization
Evolution
Exact sciences and technology
Fourier transform infrared spectroscopy
Helices
Nucleation
Organic polymers
Physicochemistry of polymers
Polymer crystallization
Properties and characterization
propylene
Propylene 1-hexene
Propylene copolymers
Regularity
temperature
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Summary:Two different types of regularity bands are identified in a real time FTIR crystallization of a series of random propylene 1-hexene copolymers. The first is akin to the bands observed in the homopolymer, those associated with 3 1 helices of isotactic sequences of different n length ( n, number of monomer units). The second type corresponds to vibrational coupling of short sequences of the chain that include the 1-hexene comonomer. Among the latter are absorbances at 910 and 1025 cm −1 which are markers for the formation of a trigonal phase in these copolymers. They remain unchanged prior to and during crystallization in copolymers with the 1-hexene units rejected from the crystallites (13 mol% 1-hexene). Analysis of the real time evolution of IR regularity bands during isothermal crystallization of these copolymers confirms the beginning of crystallization at a critical helical sequence length ( n∗) of ∼12 isotactic units (841 cm −1), and enables details of the early and final stages of crystallization. In the homopolymer and copolymers, the intensity of regularity bands with n ≤ 10 is constant in the initial undercooled melt, and increases simultaneously with the appearance of helices with n ≥12, in support of a classical crystallization mechanism of nucleation and growth. Due to density fluctuations in the initial melt, the short helices eventually collapse in aggregates or precursors that spontaneously (within the experimental macroscopic time frame) extend to stable nuclei ( n ≥ 10). Stable nuclei further extend and grow cooperatively dragging additional short sequences as inferred by the simultaneous temporal evolution of helices with n = 10 and greater. The intensity of the 998 cm −1 ( n = 10) band prior to nucleation, correlates directly with the isotactic sequence length of the copolymer and is independent of the final structure that evolves, either monoclinic or trigonal. This feature infers a nucleation event driven preferentially by the initial steady-state content of short helices in iPP and iPP-based copolymers. The temporal evolution of the 841 cm −1 band is an excellent avenue to study the crystallization kinetics of copolymers, including those with very low crystallinities. Via FTIR, the mechanism of the formation of mesomorphic crystallites in copolymers with ∼10 mol% 1-hexene at low temperatures is contrasted with the formation of alp
ISSN:0032-3861
1873-2291
DOI:10.1016/j.polymer.2011.04.018