Critical roles of molecular dynamics in the superior mechanical properties of isotactic-poly(1-butene) elucidated by solid-state NMR

Effect of Annealing Process and Molecular Weight on the Polymorphic Transformation from Form II to Form I of Poly(1-butene)

Poly(1-butene) (PB-1) resin has excellent mechanical properties, outstanding creep resistance, environmental stress crack resistance and other excellent properties. However, PB-1 resin experiences a crystal transformation for a period, which seriously affects the production efficiency and directly restricts its large-scale commercial production and application. The factors affecting the crystal transformation of PB-1 are mainly divided into external and internal factors. External factors include crystallization temperature, thermal history, nucleating agent, pressure, solvent induction, etc., and internal factors include chain length, copolymerization composition, isotacticity, its distribution, etc. In this study, to avoid the interference of molecular weight distribution on crystallization behavior, five PB-1 samples with narrow molecular weight distribution (between 1.09 and 1.44) and different molecular weights (from 23 to 710 k) were chosen to research the influence of temperature and time in the step-by-step annealing process and molecular weight on the crystal transformation by differential scanning calorimetry (DSC). When the total annealing time was the same, the step-by-step annealing process can significantly accelerate the rate of transformation from crystal form II to I. PB-1 samples with different molecular weights have the same dependence on annealing temperature, and the optimal nucleation temperature (i.e., low annealing temperature, T_l) and growth temperature (i.e., high annealing temperature, T_h) were -10 °C and 40 °C, respectively. At these two temperatures, the crystal form I obtained by step-by-step annealing had the highest content; other lower or higher annealing temperatures would reduce the rate of crystal transformation. When the annealing temperature was the same, crystal form I first increased with annealing time t_l, then gradually reached a plateau, but the time to reach a plateau was different. The crystalline form I contents of the samples with lower molecular weight increased linearly with annealing time t_h. However, the crystalline form I contents of the samples with higher molecular weight increased rapidly with annealing time t_h at the beginning, and then transformation speed from form II to form I slowed down, which implied that controlling T_l/t_l and T_h/t_h can tune the different contents of form I and form II. At the same T_l/t_l or T_h/t_h, with increasing molecular weight, the transformation speed from form II to form I via the step-by-step annealing process firstly increased and then slowed down due to the competition of the number of linked molecules and molecular chain mobility during crystallization. This study definitely provides an effective method for accelerating the transformation of poly(1-butene) crystal form, which not only has important academic significance, but also has vital industrial application.

Asymmetric Molecular Dynamics and Anisotropic Phase Separation in the Cocrystal of the Crystalline/Crystalline Polymer Blend

Semicrystalline polymers are categorized as either mobile or fixed crystals, depending on chain mobility in the crystalline region. In this work, we investigate molecular dynamics and phase structure in the cocrystal consisting of fixed and mobile polymer crystals by solid-state (ss) nuclear magnetic resonance (NMR) spectroscopy. It is demonstrated that (i) the mobile component begins large amplitude motions associated with crystal-crystal transition, while fixed ones keep their rigidity in the cocrystal, and (ii) asymmetric molecular dynamics leads to nanosegregations into mobile- and fixed-rich domains in the cocrystal below the melting temperature (T_m). The observed phase separation induced by asymmetric molecular dynamics is similar to the phase separation of the miscible amorphous polymer blend; however, it is limited to two dimensions due to the parallel packing of the stems inside the cocrystal, as well as chain connectivity at the crystalline-amorphous boundary.

Competition between crystal growth and intracrystalline chain diffusion determines the lamellar thickness in semicrystalline polymers

The non-equilibrium thickness of lamellar crystals in semicrystalline polymers varies significantly between different polymer systems and depends on the crystallization temperature Tc. There is currently no consensus on the mechanism of thickness selection. Previous work has highlighted the decisive role of intracrystalline chain diffusion (ICD) in special cases, but a systematic dependence of lamellar thickness on relevant timescales such as that of ICD and stem attachment has not yet been established. Studying the morphology by small-angle X-ray scattering and the two timescales by NMR methods and polarization microscopy respectively, we here present data on poly(oxymethylene), a case with relatively slow ICD. It fills the gap between previously studied cases of absent and fast ICD, enabling us to establish a quantitative dependence of lamellar thickness on the competition between the noted timescales.

Effect of an additive TAB-1 on crystallization behaviors and tensile properties of iPB-1

The additive TAB-1, able to accelerate II–I transition of iPB-1, can enhance its melt crystallization without negative effects on the tensile properties, revealing a new strategy to improve the performances of the products.

Polymorph selection during melt crystallization of the isotactic polybutene-1 homopolymer depending on the melt state and crystallization pressure

This work investigated the crystalline forms obtained from melt crystallization in the isotactic polybutene-1 (iPB-1) homopolymer via manipulation of the temperature at which samples were melted (Tmelt) and crystallization pressure (Pcry). Unlike the results under atmospheric conditions where the molten sample crystallized into the pure form II and the crystallization temperature and kinetics were affected obviously by Tmelt, the melted sample crystallized into forms II or I' under high pressure, depending on Tmelt and Pcry. The content of form I' decreases with increasing Tmelt or decreasing Pcry. Meanwhile, the critical pressure for the formation of pure form I' increases with increasing Tmelt. The formation of form I' is attributed to the memory effect of the melt which preserved some ordered sequence of crystal and the high pressure (Pcry) which suppressed the nucleation and growth of the kinetically favored form II, which results in the formation of form I'. In addition, the melt crystallized form II transforms to form I under high pressure conditions; thus forms I, I' and II are observed. The relative contents of the three crystalline forms on samples for different Tmelt and Pcry are obtained in this work. The result shows that the crystalline forms in melt crystallization of iPB-1 can be customized by regulating the melt state and crystallization conditions.

Influence of lamellar thickness on the transformation of isotactic polybutylene-1/carbon nanotube nanocomposites

The crystallization transformation from form II to form I in PB-1/CNT materials is mainly influenced by the lamellar thickness.

Chain-Folding Structure of a Semicrystalline Polymer in Bulk Crystals Determined by 13C-13C Double Quantum NMR

A unique approach using ¹³C-¹³C double quantum (DQ) NMR combined with selective ¹³C isotope labeling is proposed to investigate the chain trajectory of the synthetic polymer in bulk crystals. Since the DQ buildup curve highly depends upon coupled spin number, topology, and internuclear distance, which originated from the chain trajectory of selectively ¹³C-labeled polymers, the adjacent re-entry site and fraction under finite chain-folding number can be determined.