The effect of annealing on the nonlinear viscoelastic response of isotactic polypropylene

Three series of tensile relaxation tests are performed on isotactic polypropylene at room temperature in the vicinity of the yield point. In the first series of experiments, injection-molded samples are used without thermal pre-treatment. In the second and third series, prior to testing the specimens are annealed at 130 C for 4 and 24 hours, respectively. Constitutive equations are derived for the time-dependent response of semicrystalline polymers at isothermal loading. A polymer is treated as an equivalent temporary network of macromolecules bridged by junctions (physical cross-links, entanglements and crystalline lamellae). Under loading, junctions slip with respect to their positions in the bulk material (which reflects the viscoplastic behavior), whereas chains separate from their junctions and merge with new ones at random times (which reflects the viscoelastic response). The network is thought of as an ensemble of meso-regions (MR) with various activation energies for detachment of chains from temporary nodes. Adjustable parameters in the stress-strain relations are found by fitting observations. Experimental data demonstrate that the shape of the relaxation spectrum (characterized by the distribution of MRs with various potential energies) is independent of mechanical factors, but is altered at annealing. For specimens not subjected to thermal treatment, the growth of longitudinal strain does not affect the volume fraction of active MRs and the attempt rate for detachment of chains from their junctons. For annealed samples, the concentration of active MRs increases and the attempt rate decreases with strain. These changes in the time-dependent response are attributed to broadening of the distribution of strengths of crystalline lamellae at annealing.Comment: 30 pages, 13 figure

The nonlinear time-dependent response of isotactic polypropylene

Tensile creep tests, tensile relaxation tests and a tensile test with a constant rate of strain are performed on injection-molded isotactic polypropylene at room temperature in the vicinity of the yield point. A constitutive model is derived for the time-dependent behavior of semi-crystalline polymers. A polymer is treated as an equivalent network of chains bridged by permanent junctions. The network is modelled as an ensemble of passive meso-regions (with affine nodes) and active meso-domains (where junctions slip with respect to their positions in the bulk medium with various rates). The distribution of activation energies for sliding in active meso-regions is described by a random energy model. Adjustable parameters in the stress--strain relations are found by fitting experimental data. It is demonstrated that the concentration of active meso-domains monotonically grows with strain, whereas the average potential energy for sliding of junctions and the standard deviation of activation energies suffer substantial drops at the yield point. With reference to the concept of dual population of crystalline lamellae, these changes in material parameters are attributed to transition from breakage of subsidiary (thin) lamellae in the sub-yield region to fragmentation of primary (thick) lamellae in the post-yield region of deformation.Comment: 29 pages, 12 figure

PARAMETRIC DESIGN AS A TOOL TO REDUCE SOLAR PENETRATION BY OUTDOOR SHADES IN HOT ARID CLIMATE

Conventional design methods and current tools infrequently link performance with the geometry of the design. These methods rarely enable backtracking through the design process and can\u27t achieve full performance criteria. In this context, using both procedural geometry and information from numerical assessments and performance simulations should be discussed to support the search for effective solutions. Architects have gained control over the process of design by utilizing parametric methods to generate a sustainable design that interacts with sustainable, climatic, and environmental restrictions, particularly in hot arid zones where outdoor life is overlooked. This paper will discuss the performance-oriented design and a specific workflow empirical methodology that explores design alternatives of outdoor solar parametric shades for urban spaces with the aim of merging performance assessments in the initial phases of the design process to achieve maximum thermal comfort. This workflow includes parametric modelling using (Grasshopper) along with genetic algorithms (Galapagos), and the environmental tool (Ladybug). The design solutions were generated by evolutionary algorithms in accordance with the thermal performance requirements and simulations to evaluate their shading and thermal comfort efficiency. This will be illustrated through a case study of a bus stop static shade concentrating on the cladding of the geometry, especially on the solar radiation parameter. The study that will be discussed in this paper is a simulation study that will combine simulation techniques with a parametric approach and genetic algorithm optimization in a generative evaluation methodology for reducing the radiation under outdoor shades in hot dry climate areas. The optimum alternative /was determined based on the fitness value of incident radiation, and the process was iterated for a particular date and time

A network description of the non-Gaussian stress-optic and Raman scattering responses of elastomer networks

The ability to measure orientation in dual or multi-phase materials is of current relevance in the study conthe constitution and deformation characteristics of the separate phases in many technologically important polymeric systems. Raman spectroscopy is a very useful tool in this regard because separation of the scattered Raman intensities by phase is possible and because it can be used accurately on thick specimens. A three dimensional network model concept used previously to describe the stress and birefringence reponses of elastomers is extended to describe the components of the Raman tensor for amorphous elastomers under general finite deformations. The utility of the model is verified via its ability to predict the finite deformation responses of elastomeric networks under large shear deformations. Polydimethylsiloxane (PDMS) networks are tested to large deformations in uniaxial compression and in shear for comparison with the theory. Simultaneous displacement, load and optical retardation data are collected using apparatus specially designed to allow optical access throughout the deformation tests. The importance of properly accounting for finite rotations when relating the computational results to the experimentally measured optical data is discussed. The proposed network description of the Raman tensor is also compared to Raman intensity in the literature on polyethylene terephthalate (PET). The results indicate that the theory accurately predicts the anisotropic Raman tensor components over the full range of deformation for which data are available.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41702/1/707_2005_Article_BF01170305.pd

Advantage of preserving bi-orientation structure of isotactic polypropylene through die drawing

YesThe isotactic polypropylene (iPP) usually shows a unique parent-daughter lamellae structure in which the parent and daughter lamellae are against each other with a near perpendicular angle (80° or 100°). Inducing a high fraction of oriented cross-hatched structure in iPP during processing is desirable for designing the bi-oriented iPP products. We processed a commercial iPP via tensile-stretching and die-drawing to evaluate the structural evolution of oriented parent-daughter lamellae. It turned out that the die-drawing process had an advantage in attaining a high fraction of oriented cross-hatched structure of iPP, as compared to the free tensile stretching. Besides, the presence of α-nucleating agents affected the formation of oriented parent-daughter lamellae in the die-drawn samples whereas such influence diminished in the free stretched ones. It was found that the confined deformation inside the die led to the well-preserved oriented cross-hatched structure in the die-drawn iPP.This work was financially supported by the National Natural Science Foundation of China (Nos. 21704102, U1832186, and 51525305), Newton Advanced Fellowship of the Royal Society, United Kingdom (No. NA150222) and ExxonMobil Asia Pacific Research & Development Co., Ltd

Some Observations of the lamellar Morphology in Isotactic Polypropylene Spherulites by SFM

The lamellar morphology of intruded bulk samples of isotactic polypropylene has been investigated by scanning force microscopy. It is shown that SFM operated in the tapping mode is a powerful tool to characterize the texture of $\alpha$ and $\beta$ spherulites at the lamellar level. Structure, orientation and thickness of the lamellae have been determined in both cases.La microscopie de force atomique a été utilisée pour étudier la structure lamellaire d'échantillons massifs de polypropylène isotactique obtenus par intrusion. La microscopie de force atomique utilisée en tapping mode se révèle être un outil performant pour caractériser la texture des sphérolites $\alpha$ et $\beta$ à l'échelle lamellaire. La structure, l'orientation et l'épaisseur des lamelles ont été déterminées dans les deux cas