Forced assembly by multilayer coextrusion to create oriented graphene reinforced polymer nanocomposites

A potential advantage of platelet-like nanofillers as nanocomposite reinforcements is the possibility of achieving two-dimensional stiffening through planar orientation of the platelets. The ability to achieve improved properties through in-plane orientation of the platelets is a challenge and, here, we present the first results of using forced assembly to orient graphene nanoplatelets in poly(methyl methacrylate)/ polystyrene (PMMA/PS) and PMMA/PMMA multilayer films produced through multilayer coextrusion. The films exhibited a multilayer structure made of alternating layers of polymer and polymer containing graphene as evidenced by electron microscopy. Significant single layer reinforcement of 118% at a concentration of 2 wt % graphene was achieveddhigher than previously reported reinforcement for randomly dispersed graphene. The large reinforcement is attributed to the planar orientation of the graphene in the individual polymer layers. Anisotropy of the stiffening was also observed and attributed to imperfect planar orientation of the graphene lateral to the extrusion flow

Lie detector

Novel, semicrystalline polyamides and copolyamides were synthesized from a new carbohydrate-based diamine, namely isoidide-2,5-dimethyleneamine (IIDMA). In combination with 1,6-hexamethylene diamine (1,6-HDA) as well as the biobased sebacic acid (SA) or brassylic acid (BrA), the desired copolyamides were obtained via melt polymerization of the nylon salts followed by a solid-state polycondensation (SSPC) process. Depending on the chemical compositions, the number average molecular weights (Mn) of the polyamides were in the range of 4000–49000 g/mol. With increasing IIDMA content in the synthesized copolyamides, their corresponding glass transition temperatures (Tg) increased from 50 °C to approximately 60–67 °C while the melting temperatures (Tm) decreased from 220 to 160 °C. The chemical structures of the polyamides were analyzed by NMR and FT-IR spectroscopy. Both differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) analyses revealed the semicrystalline character of these novel copolyamides. Variable-temperature (VT) 13C{1H} cross-polarization/magic-angle spinning (CP/MAS) NMR and FT-IR techniques were employed to study the crystal structures as well as the distribution of IIDMA moieties over the crystalline and amorphous phases of the copolyamides. The performed ab initio calculations reveal that the stability of the IIDMA moieties is due to a pronounced boat conformation of the bicyclic rings. The incorporation of methylene segments in between the isohexide group and the amide groups enables the hydrogen bonds formation and organization of the polymer chain fragments. Given the sufficiently high Tm values (200 °C) of the copolyamides containing less than 50% of IIDMA, these biobased semicrystalline copolyamides can be useful for engineering plastic applications

Crystallization kinetics of polymer fibrous nanocomposites

Through applying both a probabilistic approach and a combination of probabilistic and the Avrami ‘extended volume’ approaches we have derived a theory of overall crystallization kinetics of polymers reinforced with nanofibers. The theory describes the crystallization kinetics in the presence of straight or curved nanofibers, with different nucleation ability and orientation, and allows to account for their variable length. The analytic results are supported by computer simulations of spherulitic structures. The derived mathematical formulas are in exponential forms suggesting the use of the Avrami logarithmic coordinates for detailed analysis of experimental data. Experimental data on crystallization of several nanocomposites, including polypropylene reinforced with poly(tetrafluoroethylene) nanofibers and polyamide 12 with carbon nanotubes, are in a good agreement with the theoretical predictions

Preparation of Well-Compatibilized PP/PC Blends and Foams Thereof

The performance of polypropylene-poly(ethylene brassylate) block and graft copolymers and a polypropylene-polycaprolactone graft copolymer as compatibilizers for polypropylene-rich polypropylene/bisphenol A polycarbonate (PP/PC, 80/20 wt/wt) blends was elucidated. The copolymers were synthesized either by metal-catalyzed ring-opening polymerization or transesterification of a presynthesized polyester, initiated by hydroxyl-functionalized PPs, which themselves were obtained by catalytic routes or reactive extrusion, respectively. Spectroscopic fingerprints of the copolymers from liquid-state nuclear magnetic resonance (NMR) in combination with scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic mechanical thermal analysis (DMTA), and rheology analyses of the blends indicated that the compatibilizers spontaneously organize at the interface of the two immiscible polymers leading to the formation of uniform, stable, nanophase morphologies. The effect of the compatibilizers on the performance of the PP/PC blends was evaluated, and well-compatibilized PP/PC blends showed improved melt strength and strain hardening when compared to pure PP. This was verified by the successful foam extrusion using isobutane as a blowing agent of well-compatibilized PP/PC blends to low-density PP-based foams, for which normally long-chain branched PP is required

Physical state of the amorphous phase of polypropylene-influence on thermo-mechanical properties

Stable polypropylene, PP – low molecular weight modifier systems were prepared in order to analyze the influence of the physical state of the amorphous phase on thermo-mechanical properties. The modifier was introduced into solidified material by diffusion to the amorphous phase regions. The change of state of stress of the transmitters connecting adjacent crystals caused by substantial increase of interlamellar distance was observed. The new state of stress of the molecular network of the amorphous phase induced respective change of the stress state of the crystalline component. As the effect the tensile yield stress value was substantially reduced, also the stress caused a decrease of melting temperature of PP crystals. The above proposed mechanism served to explain the changes of thermo-mechanical properties of the blends with identical composition prepared by blending in a molten state. During the solidification of the blend, the modifier molecules were exuded out of the growing crystals. The physical state of the molecular network of the amorphous phase was changed in a similar way as for the systems with infused modifier into previously solidified polymer matrix. The changes of mechanical properties were additionally correlated with the observed change of the melting temperature of the crystals

The influence of cavitation phenomenon on selected properties and mechanisms activated during tensile deformation of polypropylene

The cavitation phenomenon accompanies the tensile deformation of most semicrystalline polymers when negative pressure inside the amorphous phase is generated. Over the years, this phenomenon has been marginalized, due to the common belief that it does not have any significant influence on the properties or micromechanisms activated during plastic deformation of such materials. In this article, for the first time, the influence of the cavitation phenomenon on the value of yield stress/strain, the intensity of the lamellae fragmentation process, the reorientation dynamics of the crystalline and amorphous component, the degree of crystals orientation at selected stages of deformation, and the amount of heat generated as a result of activating characteristic micromechanisms of plastic deformation were systematically analyzed. The research has been conducted for cavitating/non-cavitating polypropylene model systems with an identical structure of crystalline component during their tensile deformation

Morphology and properties alterations in cavitating and non-cavitating high density polyethylene

Cavitation phenomenon has been marginalised in the past, although it alters morphology and properties of semicrystalline polymers during their plastic deformation. The formation of cavitational pores are due to the loss of cohesion by the amorphous phase. The research in this paper has been conducted on cavitating and non-cavitating high density polyethylenes with identical microstructure of the crystalline component, however, with modified amorphous phase. The influence of cavitation on selected properties and mechanisms activated during tensile deformation of high density polyethylene has been systematically studied including the alterations of yield stress and yield strain, the strain onset of activation of crystallographic slips and martensitic transformation, the intensity of the lamellae fragmentation, the degree of molecular orientation and the amount of heat generated during deformation

Crystalline Lamellae Fragmentation during Drawing of Polypropylene

Filling free volume pores of the amorphous phase with the molecules of low molecular weight modifier leads to a complete elimination of the cavitation during tensile drawing. Such way of modification of a solidified material makes the polypropylene/modifier system a model system that enables the analysis of the influence of cavitation on thermomechanical properties and the mechanisms activated during its deformation. In this paper we have presented the influence of cavitation on the intensity of the lamellae fragmentation. In the case of cavitating material, on the basis of X-ray measurements and scanning electron microscopy, we have observed substantial decrease of the undisturbed crystallites lengths during its deformation up to 50–55% of their initial value. The deformation of noncavitating material proceeded with smaller decrease of average crystallites lengths by only 15–20% of their initial value. The changes of the SAXS’s long period of noncavitating polypropylene indicated that only a small fraction of lamellae stacks that are oriented parallel to the tensile direction undergo fragmentation. This type of fragmentation is connected with excessive lamellae thinning and interfacial instabilities but by no means by cavitation

Ring-banded spherulites in polylactide and its blends

In this paper the influence of different aspects of crystallization process on the formation of ring-banded spherulites and the transition between banded and non-banded structures in polylactide was analyzed. Based on the measurements using the differential scanning calorimetry (DSC) and polarized optical microscopy (POM), it was found that the banding phenomenon of spherulites in polylactide and its blends with triethyl citrate (TEC) exclusively depends on the degree of supercooling (ΔT). In the work, for the first time, the equilibrium melting point (Tm0) of polylactide was correlated with the isothermal crystallization temperature (Tic) in the context of banded spherulites formation. To determine the Tm0 of polylactide blends, due to the possibility of modifier migration towards the sample surface, a correction of the methodology was proposed. It has been shown that ΔT necessary to initiate the growth of spherulites with a ring pattern in polylactide ranges from 46 to 56 °C. The inclusion of TEC into polylactide matrix shifts the Tic range in which banded spherulites appear towards lower values, but does not change the required value of supercooling degree. A direct correlation between the band space of the ring-banded spherulites and the degree of supercooling was also demonstrated

Influence of non-polymeric substances localized in the amorphous phase on selected properties of semicrystalline polymers

Semicrystalline polymers are chemically/physically inhomogeneous, since they contain from as little as a fraction of a percent to even a few percent of non-polymeric substances (oligomers, antioxidants, processing agents) of different physicochemical properties, preferentially localized in non-crystalline regions. In this paper, the extraction process of non-polymeric substances with the use of supercritical CO2 for several semicrystalline polymers, such as polypropylene, high and low density polyethylene was performed. The quantitative/semi-qualitative analysis of extracted substances was performed and their thermal properties were determined. Then, their influence on the structure of amorphous phase-content and size of free volume pores were assessed. Finally, the influence of non-polymeric substances on thermal, thermo-mechanical and barrier properties of analyzed polymers and the course and intensity of the cavitation phenomenon during uniaxial stretching was determined