Nano- and Micromechanics of Crystalline Polymers

It is currently thought that crystalline polymers consists of lamellar crystals which are separated from each other by a layer of amorphous polymer and are held together by tie molecules through the amorphous phase [e.g. 1]. The lamellae are formed from mostly folded chains. The thickness of lamellae is determined by the parameters such as interfacial energies, glass transition temperature and melting temperature, undercooling, segmental diffusivity, etc. The thickness reported lies usually in a narrow range between 3 and 20 nm as obtained from observations in various types of microscopes or calculated from the degree of crystallinity and long period. It has been recognized that chain folding is not so regular as it was thought and molecular packing in lamellae is subject to considerable and irregularly distributed disorder depending on undercooling- regimes of crystallization. It has been demonstrated in various ways that the planar growth front will always break up into fibrous or cellular growth. Also crystallization of polymers leads to interface instability. More sophisticated treatment of the instabilities involve perturbation analyses of planar interfaces, correlating diffusion, temperature gradients along the interface, and interfacial energy with the size of the growing crystals

Characterization of injection molded polymers – from conventional to wood-based thermoplastics

The success of polymers products is associated with the melt processability, which allows to\ua0create products with complex shapes at a low cost. One of the most widely used processing\ua0techniques utilizing melt processability is injection molding, where a polymer is heated until it\ua0flows into a mold under pressure. Due to varying shear- and cooling rates during processing,\ua0injection molding creates a multilayered structure, consisting of complex hierarchical\ua0morphologies. In addition to process conditions, the structures formed are dependent on the\ua0molecular architecture including chemical environment and branching of the polymer chain.\ua0The resulting morphology defines the mechanical properties of the injection molded parts and\ua0consequently, understanding the correlation between material, processing parameters, and\ua0resulting morphology is an important challenge. Furthermore, to expand the use of injection\ua0molding to renewable cellulosic materials, intrinsic limitation in cellulose that impede melt\ua0processing must be overcome. This can be achieved by chemically modifying the cellulose,\ua0however chemical modifications impact the morphology formed during processing.\ua0This thesis focuses on using advanced scanning small- and wide-angle X-ray scattering as main\ua0characterization techniques, to unfold the nature of the complex semicrystalline structures in\ua0injection molded synthetic and cellulose-based polymers. By varying material parameters,\ua0processing conditions and using complementary techniques, such as computational simulations\ua0and mechanical testing, the underlying factors for formation of hierarchical morphologies is\ua0further studied. This thesis brings us one step closer to understanding and predicting the\ua0polymer microstructures and resulting mechanical properties of injection molded materials

Multiscale viscoplastic-viscodamage analysis of shape memory polymer fibers with application to self healing smart materials

Self-healing smart material systems have been introduced into the research arena and they have already been deployed into industrial applications. The Close-Then-Heal (CTH) healing mechanism for polymeric self-healing systems is addressed herein and then a new generation of Shape Memory Polymer (SMP) based self-healing system is proposed in this work. This system incorporates SMP fibers to close the cracks while the embedded Thermoplastic Particles (TPs) are diffused into the crack surfaces upon heating and provide a molecular level of healing. The SMP fiber manufacturing procedure is briefly addressed in this work in which the bobbin of SMP fibers are heat treated in a specific procedure and then they are wound to produce SMP fibers. The performance of the proposed healing system is highly dependent on mechanical responses of SMP fibers. The polyurethane SMP fibers are categorized as semicrystalline polymeric material systems. These semicrystalline SMP fibers are then constituted from two distinguishable phases, which are amorphous and crystalline polymers. Such a multiphase system can be evaluated through a multiscale analysis within the micromechanics framework in which the macroscopic mechanical responses are evolved through averaging the microscale mechanical fields. Then in this research the constitutive relation for each of the micro-constituents are utilized to compute the microscale mechanical fields and then these fields are correlated to the macroscopic field through the micromechanics framework. The cyclic viscoplastic and viscodamage of these fibers are of utmost importance for designing self-healing systems in which repeatability of the healing process and the healing efficiency for subsequent healing cycles are highly dependent on cyclic responses of these fibers. A new approach in measurement of cyclic damage of SMP fibers is proposed in this work in which the reduction in recoverable stress after each cyclic stress recovery is correlated to the damage. In this approach the damage is interpreted as failure of the polymeric bonds to recover their original shape (SM effect). In general the proposed self-healing scheme establishes a new generation of self-healing systems while the developed theoretical multiscale analysis provides a well-structured method to investigate the cyclic viscoplastic and viscodamage of the SMP fibers

Numerical stimulation of stress-induced crystallization of injection molded semicrystalline thermoplastics

Injection molded semicrystalline plastic products exhibit variable morphology along their thickness directions. The processing conditions have a significant effect on the crystallinity distribution in the final parts. However, because of the lack of sound theoretical models for stress-induced crystallization kinetics in thermoplastics, simulations of the injection molding process of semicrystalline plastics with the consideration of stress-induced crystallization have been scarce. A stress-induced crystallization model for semicrystalline plastics is proposed based on the theory that stress induced orientation of polymer chains increase the melting temperature of the plastics, and hence, the supercooling which is the driving force for crystallization. By assuming that the effect of stress on crystallization is only by increasing the equilibrium melting temperature, the basic quiescent state crystallization equation can be directly applied to model stress-induced crystallization kinetics. A simple experimental technique such as rotational rheometric measurement, can be used to determine the melting temperature shift. The model predicts the most prominent features of stress-induced crystallization: with the application of shear stress, crystallization rate becomes higher, the crystallization temperature range is broadened and the peak of crystallization rate shifts to higher temperatures. The main advantage of the model is that the parameters in the quiescent state crystallization model do not change and the parameters in the equilibrium melting temperature shift model are easy to determine. And the unknown constants are kept to a minimum. The injection molding process of semicrystalline plastics was simulated with the proposed stress-induced crystallization model. A pseudo-concentration method was used to track the melt front advancement. The simple Maxwell stress relaxation model in combination with WFL equation was used to investigate the importance of stress relaxation on the development of crystallinity during the injection molding. Simulations were carried out under different processing conditions to investigate the effect of processing parameters on the crystallinity of the final part. Other results such as skin layer build-up and mold pressure were also simulated. The simulation results reproduced most of the features that were obtained by the experiments reported in the literature

Novel polypropylene based microporous membranes via spherulitic deformation

A novel method for creating a microporous polypropylene membrane via spherulitic deformation is described. The microporous structure was generated by the combination of intra-spherulitic and inter-spherulitic deformations. Polypropylene was selected due to its unique cross-hatched lamellar morphology facilitating inter-spherulitic deformation. A precursor film with a spherulitic structure was made under low-stress melt processing condition. A tangential lamellae-rich spherulite was created and identified with a positive birefringence sign. A sequential annealing process improved the crystalline structure, and in particular the thickness of the tangential lamellae. The annealing process proved to be critical for initiating the inter-spherulitic deformation. The post-extrusion process conditions for initiating inter-spherulitic deformation to create microporous membranes by lamellar separation are delineated. The processing parameters are: annealing temperature, extension ratio, stretching rate, and stretching temperature. A fixed set of extrusion conditions was chosen for producing precursor films having similar spherulitic properties. A Wide Angle X-ray Scattering (WAXS) examination provides a quick characterization method for the inter-spherulitic deformation. Membrane porosity measurements showed a consistent correlation with the observed ø-form orientation index. A highly interconnected solvent-resistant porous polypropylene membrane having a pore size in the range of 50-400 nm and a porosity of about 0.18 was thereby developed in this study. This concept can be further expanded by using an cc-nucleating agent to reduce spherulite sizes and utilizing interfacial debonding between two different phases to enhance permeability. A highly methanol permeable membrane with an estimated porosity of 0.29 was produced with the nucleated polypropylene samples, and a reasonable permeability was also observed in the membrane made from an immiscible blend. However, the occurrence of debonding can also compensate for the energy to create inter-spherulitic deformation. Increasing extension ratio did not change the microstructure in the non-annealed sample; however, the lamellae can be further oriented in the annealed samples. Inter-spherulitic deformation became obvious at slow stretching rates; intra-spherulitic deformation was more favored at a fast stretching rate. The DSC thermal analysis of the precursor films showed two significant endothermic discontinuities (T 1 at 0°C and T 2 at 40°C ) in the non-annealed or annealed precursor films; T1 is believed to be the conventional T g of polypropylene; T 2 appears to be the result of the rigid amorphous fraction trapped within lamellar wells where the amorphous phase is surrounded by R-lamellae and T-lamellae. The lamellae could break down or slip from the lamellar knots as stretching temperatures are high enough to minimize the effect of the rigid amorphous fraction, and the annealed lamellae can still be oriented without a catastrophic cold-drawn deformation

Modeling of morphology evolution in the injection molding process of thermoplastic polymers

A thorough analysis of the effect of operative conditions of injection molding process oil the morphology distribution inside the obtained moldings is performed, with particular reference to semi-crystal line polymers. The paper is divided into two parts: in the first part, the state of the art on the subject is outlined and discussed; in the second part, an example of the characterization required for a satisfactorily understanding and description of the phenomena is presented, starting from material characterization, passing through the monitoring of the process cycle and arriving to a deep analysis of morphology distribution inside the moldings. In particular, fully characterized injection molding tests are presented using an isotactic polypropylene, previously carefully characterized as far as most of properties of interest. The effects of both injection flow rate and mold temperature are analyzed. The resulting moldings morphology (in terms of distribution of crystallinity degree, molecular orientation and crystals structure and dimensions) are analyzed by adopting different experimental techniques (optical, electronic and atomic force microscopy, IR and WAXS analysis).Final morphological characteristics of the samples are compared with the predictions of a simulation code developed at University of Salerno for the simulation of the injection molding process

Numerical study of the relationship between the spherulitic microstructure and isothermal crystallization kinetics. Part I. 2-D

In this paper, we proposed a numerical model to study the kinetic properties and the spherulite microstructure of a semi-crystalline polymer under isothermal crystallization, which further exhibits the potential in generating the 2D spherulitic structure according to the observations obtained by experimental techniques. Two characteristic parameters are introduced, namely, characteristic length Lc and characteristic time tc, which are dependent on the growth rate, G and the nucleation rate, I. In addition, two non-dimensional parameters are introduced to model the nucleation saturation: Ld/Lc and t⋆/tc, which is related to the thickness of nucleation exclusion zone Ld, and the effective nucleation time t⋆, respectively. In 2D modeling, the kinetics are confirmed by Avrami fitting, and the effects of the four characteristic parameters on the Avrami parameter n and the crystallization half-time t0.5 are presented. The regularity of how the spherulite density or the mean radius of spherulites R change along with these parameters are also given, respectively. It shows that Lc is the prominent parameter for the size of the spherulite, and tc controls t0.5 as long as there is no nucleation saturation (Ld=0 and t⋆→∞). Besides, the existence of the nucleation saturation increases the mean radius of spherulites, but decreases n from 3 to 2 in 2-D modeling. Finally, a relationship between crystallization kinetics and microstructures is provided, giving a new perspective to estimate the nucleation rate