Intercrystalline Links Determined Kinetics of Form II to I Polymorphic Transition in Polybutene‑1

The effect of intercrystalline links containing tie molecules and entangled loops on polymorphic transition from form II to form I in polybutene-1 (PB-1) of different molecular weights has been investigated by differential scanning calorimetry and small-angle X-ray scattering techniques. The PB-1 samples were isothermally crystallized at a range of temperatures to develop metastable form II crystalline modification of different lamellar thickness, long spacing, and number of intercrystalline links in the amorphous phase. Tammann’s two-stage crystal nuclei development method was applied to promote a faster polymorphic transition from form II to I; that is, form I nuclei were developed at the early low temperature stage, and form I crystals were formed at the later high temperature stage. At a given annealing condition, the transition rate in the high molecular weight sample is found to increase with the increase of the prior form II crystallization temperature, while it shows a negative correlation between transition rate and crystallization temperature in the low molecular weight sample. The results can be understood as follows. The long spacing of high molecular weight PB-1 is smaller than the radius of gyration of the chains in the melt, leading to formation of folded-chain crystals and high possibility of generating intercrystalline links. Higher internal stress induced by unbalanced shrinkage of amorphous and crystalline phases would be built up during cooling from higher crystallization temperature to the first temperature stage of annealing. For low molecular weight samples, the probability of forming intercrystalline links decreases with the increasing lamellar thickness at elevated crystallization temperature. For a low crystallization temperature where intercrystalline links could be found in both samples, the low molecular weight sample shows a faster transition for its higher chain mobility. These molecular mechanisms are discussed in this article

Counits Content and Stretching Temperature-Dependent Critical Stress for Destruction of γ Crystals in Propylene–Ethylene Random Copolymers

Tensile deformation behavior of three random propylene–ethylene copolymers with the same molecular weight and different contents of counit was investigated at different temperatures from room temperature to close to melting point via tensile tests, step-cycle tests, and in situ wide-angle X-ray diffraction techniques. Upon stretching, the original crystalline lamellae must be destroyed, generating new highly oriented ones. A critical stress has been suggested, under which the original crystallites can be destructed. The propylene–ethylene copolymer samples in pure γ-form transformed gradually into α-form during tensile stretching. This crystalline transition proceeded via a destruction (melting) of the original γ-form crystals followed by recrystallization of the freed polymeric chain segments into α-form along stretching direction. This result provides a marker for investigating the critical stress mentioned above. Such critical stresses triggering the destruction of γ-form crystals for the propylene–ethylene copolymers of different ethylene counit contents were successfully calculated. It turned out that this critical stress depended on the ethylene counit content and stretching temperature. Samples with less ethylene counits show higher critical stress because of a lower degree of insertion of the ethylene counits into the crystalline unit cell than samples with higher ethylene counit content. The critical stresses remained constant when the samples were stretched at low-temperature region, whereas they decreased significantly at high stretching temperatures close to the melting points due to strong thermal distortion of the crystalline lattices, making the crystallites less stable

Elasticity Reinforcement in Propylene–Ethylene Random Copolymer Stretched at Elevated Temperature in Large Deformation Regime

Tensile deformation behavior of a random propylene–ethylene copolymer with 12 mol % ethylene counits at room temperature and 63 °C was investigated using the in situ small and wide-angle X-ray scattering techniques. Under both conditions, the deformation mechanism changed from slip of crystalline lamellar blocks to stretching induced melting and recrystallization process at a critical strain of about 0.9. This critical strain in tensile deformation of semicrystalline polymers normally marks the starting of plateau value of elastic strain. Further stretching leads to increase of plastic deformation only due to the fibrillation. However, a peculiar elasticity reinforcement was observed at strain larger than 1.3 when the sample was stretched at 63 °C. Wide angle X-ray scattering results indicate that at this strain of 1.3 fibrillation of the originally unoriented crystals finished so that further stretching leads to a deformation of a rather stable entangled network embedded by fibrils that possesses high elasticity

Tensile Deformation of Polybutene‑1 with Stable Form I at Elevated Temperature

Stretching-induced structural changes in polybutene-1 with stable crystalline modification of form I at elevated temperature was investigated by means of the in-situ synchrotron wide-angle X-ray diffraction technique. It was found that oriented metastable form II crystallites with the polymer chain aligned along the stretching direction gradually appear during tensile deformation. Based on the fact that a solid state I to II phase transition cannot take place due to the restriction in chain conformations and lattice dimensions in both phases, the observed occurrence of transition from form I to form II must proceed via a two-step process. First, those form I crystallites with their polymer chain direction tilted with respect to the stretching direction undergo a stress-induced melting process because they experience larger shear stress than the rest. Second, the freed polymer chain segments which have lost their conformational memory in stable form I recrystallize into metastable form II crystallites with their chain direction preferentially aligned along the stretching direction. This result is considered to provide a direct evidence for the stress-induced melting–recrystallization mechanism during tensile deformation of semicrystalline polymers

The evolution of microstructure of lamellae and cavities.

<p>The long period of lamellar stacks and the thickness of the cavities (top) and the master curve of the cavities strain (bottom) along the stretching direction at different strains for PB-1 samples crystallized at 50°C stretched at elevated temperatures (solid symbol) and the ones crystallized at different temperatures stretched at 100°C (hollow symbol). <i>d₀</i> denotes the linear extrapolated thickness of the cavities at zero strain.</p

Selected 2D-WAXD patterns.

<p>The 2D-WAXD patterns taken at different strains as indicated on the graph for PB-1 samples crystallized at 50°C stretched at 30 (top) and 100°C (middle), and the ones crystallized at 90°C stretched at 100°C (bottom). Stretching direction is horizontal. Arrows indicate the azimuthal angle of intensity distribution.</p

Azimuthal intensity distribution of 110-reflection.

<p>The evolution of azimuthal intensity distribution of 110-reflection as a function of strain measured for PB-1 samples crystallized at 50°C stretched at 30 (top) and 100°C (middle) and the ones crystallized at 90°C stretched at 100°C (bottom).</p

The horizontal and vertical integrated intensity and the ratio between them.

<p>The evolution of the horizontal (top) and vertical (middle) integrated intensity and the ratio between them (bottom) as a function of strain for the PB-1 samples crystallized at 50°C stretched at different temperatures as indicated (left) and the ones crystallized at different temperatures as indicated and stretched at 100°C (right).</p

The diagram of the cavitation modes.

<p>The evolution of cavitation modes of PB-1 as a function of the lamellar thickness and stretching temperature.</p

The 1D scattering intensity distribution profiles along different directions.

<p>Plots of <i>I vs q_h</i> (left) and <i>I vs q_v</i> (right) taken at different strains for PB-1 samples crystallized at 50°C stretched at 30°C (top) and 100°C (middle), and the ones crystallized at 90°C stretched at 100°C (bottom).</p