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The effect of isotactic polypropylene (iPP) molecular weight and thermal treatment on final crystalline structure and mechanical properties were investigated by X-ray scattering, differential scanning calorimetry, tensile testing, and scanning electron microscopy. Neat and β-nucleated isotactic polypropylene were subjected to annealing above melting temperature of β-phase but still below melting temperature of α-phase, which caused both improvement of α crystallites and β to α-recrystallization, in case of nucleated material. As a consequence, the final mechanical properties like tensile strength and stiffness improved significantly. The increase is much more pronounced in the case of β to α-transformed materials with lower molecular weight. This phenomenon can also be attributed to the partial preservation of the specific structure of the amorphous phase present in β-nucleated polypropylene, in addition to the perfection of the crystalline phase. The amorphous phase comprises many interconnecting chains passing through several lamellae whereby providing a long-range effect.

This study compares the efficiency of commercially available sorbitol-based clarifying agent (NA) and varying amounts of long chain-branched polypropylene (LCBPP) acting as a specific α-nucleating agent for linear polypropylene (PP). The sorbitol-based clarifying agent, 1,3;2,4-bis(3,4-dimethyl benzylidene)sorbitol (Millad 3988), in concentration 0.2 wt%, and LCBPP in the concentration of 1, 2, 5 and 10 wt% were mixed into PP. The comparison of the effect of NA and long branches under isothermal conditions on the crystallization process, crystallinity and polymorphic composition was realized by differential scanning calorimetry and wide-angle X-ray scattering. The addition of long chain-branched polypropylene, even at the lowest concentration, performs better at higher crystallization temperatures and has a superior effect on the crystallization process, crystallization rate and overall crystallization than the addition of NA.

The main purpose of this work is to present the effect of long-chain branching and specific α-nucleation on the optical properties, crystallization and supermolecular structure of polypropylene (PP). Commercially available α-nucleating/clarifying agent 1,3;2,4-bis(3,4-dimethylbenzylidene)sorbitol (Millad 3988) was mixed into linear PP and long-chain branched PP (LCB-PP) in the concentration of 0.2 wt%. For the study of polymorphic composition, crystallinity, and crystallization process under isothermal conditions in the temperature range of 130–150 °C, differential scanning calorimetry and wide-angle X-ray scattering were used. Although the used nucleating/clarifying agent appears to have a slight effect on the optical properties of long-chain branched polypropylene, it does not appear to affect the crystallization kinetics significantly. LCB-PP exhibited self-nucleation, favored over nucleation by a specific nucleating agent.

The impact of injection molding processing parameters, including mold temperature, injection speed, and holding pressure, on the supermolecular structure of long-chain branched polypropylene (LCB-PP) was investigated. Commercial LCB-PP was processed under different conditions, with four sets of processing parameters applied. Holding pressure was varied from 30–70 MPa by 10 MPa in the P-SET, mold temperature was raised from 40 to 120°C in 20°C steps in the T-SET,and injection speed increased from 20 to 140 mm/s in 30 mm/s steps in S-SET1 and S-SET2, with mold temperatures at 40 or 120 °C. The polymorphic composition of the specimens was analyzed using wide-angle X-ray scattering, demonstrating a strong dependence on the processing parameters. The skin of the specimens showed an increase in β-phase content with higher mold temperature and injection speed. Conversely, growing holding pressure slightly suppressed β-phase content and promoted the formation of the γ-phase, which was predominantly manifested in the core of the specimens. The polymorphic composition, with both β- and γ-phases present in notable amounts, had a beneficial effect on the impact strength of the specimens.