Dispersion of carbon nanotubes in polypropylene via multilayer coextrusion: Influence on the mechanical properties

The authors would like to thank PSA for funding this research and providing some of the materials used in this study. We also would like to thank R. Glénat, P. Soria, E. Dandeu, A. Grand- montagne and A. Dubruc for their help in the preparation and the optical and mechanical characterizations of the samples presented in this study.Multilayer coextrusion was used to disperse Carbon Nanotubes (CNT) in polypropylene (PP). The dilution of commercially available masterbatches using a twin-screw extruder was first applied to produce several formulations, which were then mixed with PP using a multilayer coextrusion device to obtain films or pellets with CNT concentrations between 0.1 and 1%wt. The influence of the specific mechanical energy (SME) during the dilution step, of the addition of a compatibilizer, and of the multilayer tool on the CNT dispersion within the matrix was highlighted. The effect of the dispersion on the thermomechanical properties of the resulting materials was studied. We showed notably that films containing 0.2%wt CNT, 1%wt of PPgAm, prepared at high SME presented a Young’s modulus increase of 25e30% without significant decrease in the elongation at break. These results, using low amounts of CNT and industrially available devices, may show a new path for producing nanocomposites

Beware of the Flory parameter to characterize polymer-polymer interactions: A critical reexamination of the experimental literature

Methods for obtaining Flory parameter (χ) values for polymer blends or blocks are critically examined, for three simple systems (polystyrene/poly(methyl methacrylate), polystyrene/polycarbonate, poly(methyl methacrylate)/polycarbonate). In particular, uncertainties for calculated and experimental values are discussed and compared. Data obtained using scattering measurements combined with a mean-field analysis are reexamined. We show that different experimental techniques will lead to significantly different χ values for a given system, and that even for one experimental technique, the uncertainty on molecular parameters used to fit the data may impact the extracted value for χ. We also show that, though frequently used in the literature, χ values obtained via solubility parameters should not be used for any quantitative analysis of polymer-polymer interactions. This shows that the Flory parameter may not be relevant when discussing small effects on vague notions such as compatibility between polymers, or should at least be used with caution

Oxidative degradation of polylactide (PLA) and its effects on physical and mechanical properties

The thermo-oxidative degradation of polylactide (PLA) films was studied between 70 and 150 C. It was shown that the oxidative degradation of PLA leads to a random chain cission responsible for a reduction of the molar mass. These molar mass changes affect Tg and the degree of crystallinity, and it was found that Tg decreases according to the Fox–Flory theory whereas the degree of crystallinity increases due to a chemicrystallization process. A correlation between molar mass and strain at break during oxidation has been established: PLA displays a brittle behavior when Mn falls below 40 kg mol 1 in agreement with relationships linking the critical value for embrittlement with the molar mass between entanglements

Extensional Viscosity of Immiscible Polymers Multinanolayer Films: Signature of the Interphase

The measurement of interfacial mechanical or rheological properties in polymer blends is a challenging task, as well as providing a quantitative link between these properties and the interfacial nanostructure. Here, we perform a systematic study of the extensional rheology of multilayer films of an immsicible polymer pair, polystyrene and poly(methyl methacrylate). We take advantage of multinanolayer coextrusion to increase the number of interfaces up to thousands, consequently magnifying the interfacial response of the films. The transient elongational response is compared to an addivity rule model based on the summation of the contribution of each polymer as well as the interfacial one. At low strain rates, the model reproduces the transient extensional viscosity up to strain-thinning, while at larger ones, the extra stress exceeds the prediction based on constant interfacial tension. This extra-contribution is attributed to an interphase modulus on the order of 1-10 MPa, which increases with strain rate following a power-law with an exponent 1/3. Extensional rheology of multinanolayer films is then an efficient combination to go beyond interfacial tension and measure quantitatively the interfacial rheology of immiscible polymer blends.Comment: 6 figure

Existence of a critical layer thickness in PS/PMMA nanolayered films

An experimental study was carried out to investigate the existence of a critical layer thickness in nanolayer coextrusion, under which no continuous layer is observed. Polymer films containing thousands of layers of alternating polymers with individual layer thicknesses below 100 nm have been prepared by coextrusion through a series of layer multiplying elements. Different films composed of alternating layers of poly(methyl methacrylate) (PMMA) and polystyrene (PS) were fabricated with the aim to reach individual layer thicknesses as small as possible, varying the number of layers, the mass composition of both components and the final total thickness of the film. Films were characterized by atomic force microscopy (AFM) and a statistical analysis was used to determine the distribution in layer thicknesses and the continuity of layers. For the PS/PMMA nanolayered systems, results point out the existence of a critical layer thickness around 10 nm, below which the layers break up. This critical layer thickness is reached regardless of the processing route, suggesting it might be dependent only on material characteristics but not on process parameters. We propose this breakup phenomenon is due to small interfacial perturbations that are amplified by (van der Waals) disjoining forces

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

Role of the inter‐ply microstructure in the consolidation quality of high‐performance thermoplastic composites

Consolidation of Carbon Fiber (CF)/high-performance thermoplastic compos-ites is much less understood than the one of their thermoset counterparts. It isusually assumed that the consolidation quality is directly linked to the removalof voids within the sample during consolidation, leading to mechanical proper-ties suitable for aerospace applications. A systematic study of the temporalevolution of CF/polyetherketoneketone (PEKK) samples' microstructure con-solidated under low pressure in a rheometer is related to the increase in inter-laminar shear strength. The results show that despite similar void contentswell-below 1 vol%, samples can present significant differences in ILSS values,from 80 to 95 MPa for cross-ply samples, and from 98 to 112 MPa for unidirec-tional (UD) ones. A microstructural analysis shows that, for these materials,consolidation quality is rather related to a reorganization of the inter-ply, aresin-rich ( 70 vol%) region of typical thickness 10μm which is slowly repo-pulated in fibers during consolidation

PLA/PHBV Films with Improved Mechanical and Gas Barrier Properties

Blending poly(lactic acid) (PLA) with a small amount of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV; 10 wt%) using a custom multilayer co-extrusion process increases both ductility and gas barrier properties of extruded films compared with neat PLA and classical blending methods. The co-extrusion process allows multiplication of the number of alternate layers of PLA and PHBV within a film. It was observed that for a critical number of theoretical layers, PHBV layers are broken into lamellas. A well-developed lamellar morphology, with thin and long lamellas of highly crystalline PHBV in PLA matrix was obtained. A balance between aspect ratio and crystallinity of the lamellas, and their dispersion within the PLA matrix was needed to obtain films with improved permeability and mechanical properties

Nanostructuration effect on the mechanical properties of PMMA toughened by a triblock acrylate copolymer using multilayer coextrusion

Multilayer coextrusion was used to obtain nanolayered films of self-assembled commercial triblock copolymer poly(methyl methacrylate-b-butyl acrylate-b-methyl methacrylate) (MAM) confined by poly (methyl methacrylate) (PMMA). A double level of organization is achieved: the obtained films are made of thousands of alternating continuous nanolayers of each component, while within the nanolayers, MAM is self-assembled with a cylindrical morphology. The mechanical properties of the resulting films were compared to those of virgin PMMA and to classically extruded PMMA/MAM blends with the same compositions where no control of the nanostructure can be achieved. It is shown that significant reinforcement can be obtained with the multilayer films compared to virgin PMMA (on the order of 20 times in terms of fracture toughness) but also compared to classical blends, by a factor from 2 to 4. Reinforcing glassy thermoplastics has been a major industrial challenge due to their usually brittle behavior in the temperature range they are used. This industrially scalable one-step process shows promises for the design of nano-laminated organic glasses with high impact resistance