Papers by Keyword: Semi-Crystalline Polymer

Abstract: A numerical model is presented to simulate the crystallization kinetics in fiber-based composite with thermoplastic semicrystalline matrix. The proposed model, based on Schneider's formalism, considers the specificity of crystalline entities growing in confined medium such as fibrous composite. Indeed, transcrystallization has been experimentally observed many times and its effects on both kinetics and mechanical properties have been largely demonstrated. As an application, this paper aims at illustrating this former effect with a finite element (FE) simulation of the cooling down of a plate. The simulated materials are polypropylene alone and a fiber-based composite with a polypropylene matrix. Information on the temperature, the rate of transformation and the microstructure are obtained from both materials and compared to emphasize the contribution of transcrystallization.

Abstract: This article deals with the impact evaluation of utilization of innovative cooling system exploiting liquid carbon dioxide injected into injection mould. Process of heat transfer from the polymeric melt and final part solidification has a direct impact on creation of morphology structure of semi-crystalline thermoplastic materials and their ultimate mechanical properties. Usually the heat processes in the production tools are controlled by tempering channels where heat transfer medium circulates (oil, water tec.). This conventional way of cooling has some limitations that cause an uneven distribution of temperature fields on the part surface. Spot cooling system is one of unconventional cooling ways that increase the uniformity of temperature fields distribution on the part surface. This system utilizes the cooling potential of liquid CO₂. For the purpose of this study the special shaped insert was designed that was modified both for conventional cooling and for spot cooling system. Flexural modulus very responsively reflects the changes of morphology structure formed by different cooling progressions of the plastic melt and was chosen as an evaluating criterion.

Abstract: We have employed molecular dynamics simulations to study the behavior of virtual polymeric materials under an applied uniaxial tensile load. Through computer simulations, one can obtain experimentally inaccessible information about phenomena taking place at the molecular and microscopic levels. Not only can the global material response be monitored and characterized along time, but the response of macromolecular chains can be followed independently if desired. The computer-generated materials were created by emulating the step-wise polymerization, resulting in self-avoiding chains in 3D with controlled degree of orientation along a certain axis. These materials represent a simplified model of the lamellar structure of semi-crystalline polymers, being comprised of an amorphous region surrounded by two crystalline lamellar regions. For the simulations, a series of materials were created, varying i) the lamella thickness, ii) the amorphous region thickness, iii) the preferential chain orientation, and iv) the degree of packing of the amorphous region. Simulation results indicate that the lamella thickness has the strongest influence on the mechanical properties of the lamella-amorphous structure, which is in agreement with experimental data. The other morphological parameters also affect the mechanical response, but to a smaller degree. This research follows previous simulation work on the crack formation and propagation phenomena, deformation mechanisms at the nanoscale, and the influence of the loading conditions on the material response. Computer simulations can improve the fundamental understanding about the phenomena responsible for the behavior of polymeric materials, and will eventually lead to the design of knowledge-based materials with improved properties.

Abstract: The mechanical behaviour of semicrystalline polymers is dependent upon the property of both the amorphous and crystalline phases and their eventual interactions. In this context, heat shrinkable films have been evidenced as an interesting material model to investigate the influence of the amorphous phase on the mechanical behaviour of semicrystalline polymers, as upon heating these materials only show changes on the extensibility of the amorphous phase (macromolecular conformational state). The effect of the amorphous phase on the mechanical behaviour can therefore be studied independently. In this work are investigated the mechanical properties of a heat shrinkable polyethylene films as processed and after annealing at 60 °C (for 15 min). The morphology of the film before and after the annealing treatment was characterised by 2D-SAXS patterns. The tensile tests were performed at 50 mm/min and room temperature (23 °C) in different directions respectively to the longitudinal film direction, LD (0, 30, 45, 60 and 90º). The results show that the conformational state of the amorphous phase affects the tensile modulus and the deformation capabilities of the films, namely in the transverse machine direction. No influence on the sustained stress level was observed. Furthermore, the essential work of fracture is determined at large extent by the conformational state of the amorphous phase. This study suggests the high importance of the amorphous network on the initial strain levels and on deformation capabilities of the lamellar structure loaded transversely to the crystalline phase orientation.

Abstract: In present study, we clarify the micro- to mesoscopic deformation behavior of semicrystalline polymer unit cell by using large deformation finite element homogenization method. Crystalline plasticity theory with penalty method for enforcing the inextensibility of chain direction and nonaffine molecular chain network theory were applied to the representation of the deformation behavior of crystalline and amorphous phases, respectively, in composite microstructure of semicrystalline polymer. The different directional tension and compression are applied to the 2- dimensional plane strain semi-crystalline unit cell model. A series of computational simulation clarified highly anisotropic deformation behavior of microstructure of semi-crystalline polymer, which is caused by rotation of chain direction and lamella interface, and manifests as a substantial hardening/softening. This anisotropy for tensile deformation is higher than that for compressive deformation.