Study on β‑Nucleated Controlled-Rheological Polypropylene Random Copolymer: Crystallization Behavior and a Possible Degradation Mechanism

Controlled-rheological polypropylene random copolymer (CRPPR) and β-nucleated CRPPR were prepared through peroxide-initiated reactive extrusion and their crystallization behaviors were comparatively investigated. Rheological experiments indicated that all degraded samples acquired better flow properties than undegraded samples and the addition of β-nucleating agent has little effect on the flowability. Unlike conventional controlled-rheological polypropylene homopolymer of which the shortened molecular chains are unfavorable for β-nucleation, the structure characterizations in this work demonstrated an unexpected increase in the β-phase content of degraded β-nucleated CRPPRs with elevated peroxide concentration. Successive self-nucleation and annealing thermal analysis revealed the generation of thicker lamellar in highly degraded samples, which implied that the stereo regularity improved when the molecular chain reacted with peroxide. Based on the experimental results, a possible degradation mechanism was proposed that free radicals preferentially attack the tertiary carbon atoms adjacent to ethylene co-units during the degradation reaction, which resulted in a reduction of stereo errors, and, consequently, improvement of the β-crystallization ability

Nonreversible Enhanced Crystallization of Olefin Block Copolymer Induced by Preshearing

In this work, the influence of preshearing on crystallization of olefin block copolymer (OBC) was systematically investigated. It was found that upon an interval of melt preshear the crystallization rate of OBC was prominently elevated which was evidenced by both nonisothermal and isothermal crystallization results. The lamellar thickness of OBC increased and the size of spherulites decreased after shear. Interestingly, annealing and multiple heating and cooling experiments demonstrated that the enhancement effect on crystallization was nonreversible. The morphology observations suggested that preshearing caused an alteration in mesophase structure of OBC. On the basis of our results, we speculated that the increase in the amount of hard blocks that dissolved in the soft-block-rich matrix induced by shear promoted the unique “pass-through” crystallization process of OBC, which should be responsible for the largely enhanced crystallization of sheared OBC

Exploring the Application of Sustainable Poly(propylene carbonate) Copolymer in Toughening Epoxy Thermosets

Herein, poly­(propylene carbonate) (PPC) was used as initiator for ε-caprolactone polymerization to produce the poly­(ε-caprolactone)-<i>block</i>-poly­(propylene carbonate)-<i>block</i>-poly­(ε-caprolactone) (PCL-PPC-PCL) triblock copolymer, enabling innovative application of PPC as a toughening agent of epoxy thermosets. The interfacial interaction between PPC modifiers and epoxy was enhanced significantly because PCL blocks were miscible with epoxy matrix. The size of separated PPC modifiers decreased dramatically as the amphiphilic block copolymer formed nanophases in epoxy host. Consequently, with the incorporation of 30 wt % PCL-PPC-PCL modifier into the thermoset, the tensile elongation and the area under the stress–strain curves increased by more than 320% and 180%, respectively, compared with neat epoxy, indicating that an excellent toughening effect was achieved using this strategy. Considering that PPC possessed an ocean of attractive properties but suffered from its low glass transition temperature in implementation as mass products, this work may open up opportunities to extend the applications of PPC

Flow-Induced Enhancement of in Situ Thermal Reduction of Graphene Oxide during the Melt-Processing of Polymer Nanocomposites

In situ thermal reduction (ISTR) of graphene oxide (GO) dispersed in a polymer matrix has attracted broad interest due to its great potential as an environmentally friendly and commercially viable process to prepare polymer/graphene nanocomposites (PGNs). In this work, the ISTR of GO in two dramatically different conditions, quiescent melt and sheared melt, was comparatively studied. Comprehensive characterization of the bulk composites and the extracted graphene-based powders from composites, as well as the results of an independent parallel plate experiment, revealed that the GO in the sheared melt has a higher reduction degree than that in the quiescent melt within identical processing temperatures and times. On the basis of our results, we hypothesize that the more intense reduction of GO in the sheared melts relative to the quiescent melts is associated with the enhanced π–π stacking and the possible radical reaction between polymers and GO sheets