A distribution kinetics approach for polymer crystallization and phase separation

The mechanism of polymer crystallization is extensively studied and still far away from consensus. This research adopted cluster size distribution kinetics approach to explore the kinetics of polymer crystallization and phase separation within spinodal region. The kinetics of polymer crystallization is studied by integrating nucleation, crystal growth and ripening. Population balance equations based on crystal size distribution and concentration of amorphous polymer segments are solved numerically and the related differential moment equations are also solved. The model accounts for nucleation and crystal growth. Different mass dependences of growth and dissociation rate coefficients are proposed to investigate the fundamental features of crystal growth. The effect of temperature is also investigated for isothermal and nonisothermal polymer crystallization. Incorporating temperature effects of nucleation and crystal growth rate, the model presents time dependencies of polymer concentration, number and size of crystals, and crystallinity for different temperatures and cooling rates. The effect of denucleation is investigated by comparing moment and numerical solutions of the population balance equations. Incubation periods introduced in nonisothermal crystallization are studied under different cooling rates and different initial temperatures. The distribution kinetics approach is also extended to the investigation of crystallization of polymer blends. Blending effects from polymer-polymer interactions are incorporated into the diffusion coefficient. The melting temperature, activation energy of diffusion, and phase transition enthalpy also depend on polymer blends composition, and lead to characteristic kinetic behavior of crystallization. The influence of different composition is presented through the time dependence of polymer concentration, number and size of crystals, and crystallinity. Another extended application of distribution kinetics approach is the study of the kinetics of spinodal decomposition. Spinodal decomposition occurs under conditions of large supersaturation and/or small ratio of interfacial to thermal energy when the energy barrier for nucleation is negligible. A cluster distribution kinetics model without nucleation is established to describe the unique kinetics of new phase domain growth. Population balance equations show how clusters aggregate and rapidly lead to phase separation

Modeling and simulation of film blowing process

Film blowing process is a flexible mass production technology used for manufacturing thin polymeric films. Its flexibility in using an existing die to produce films of different width and thickness, just by controlling process conditions such as, extrudate velocity, excess pressure, and line speed, makes it an attractive process with less capital investment. Controlling the process conditions to obtain a stable bubble, however, is not a trivial task. It is a costly trial and error procedure, which could result is a large wastage of material and other resources. Hence, it is necessary to develop methods to simulate the process and design it using numerical experiments. This important need of the industry defines the objective of this work. In this dissertation, a transient, axisymmetric, nonisothermal, viscoelastic model is developed to simulate the process, and it is solved using finite element method. Material behavior of polymer melt is described using a modified Phan-Thien-Tanner model in the liquid??like region, and anisotropic Kelvin??Voight model in the solid zone, and the transition is modeled using a simple mixture theory. Crystallization kinetics is described using a modified Avrami model with factors to account for the influence of temperature and strain. Results obtained are compared with available experimental results and the model is used to explore stability issues and the role of different parameters. Software developed using this model comes with a GUI based pre- and post-processor, and it can be easily adapted to use other constitutive models

Nucleating and retarding effects of nanohydroxyapatite on the crystallization of poly(butylene terephthalate-co-alkylene dicarboxylate)s with different lengths

New biodegradable and biocompatible composites are continuously developed for biomedical applications (e.g., from drug delivery devices to tissue engineering scaffolds). Properties of such systems may depend on their morphology and structure, which are attained after their processing, and therefore, the study of the crystallization kinetics has a particular relevance. The crystallization kinetics of hydroxyapatite-filled poly(butylene terephthalate-co-alkylene dicarboxylate)s has been studied under non-isothermal conditions, using a wide range of cooling rates and different kinetic models. Based on our results, nanohydroxyapatite (nHAp) particles were found to effectively act as additional nucleation sites for poly(butylene terephthalate-co-succinate) (PBST), giving rise to an increased crystallization rate with respect to pure PBST. However, the overall growth rate of HAp nanocomposites decreased compared to the corresponding homopolymers with longer aliphatic dicarboxylic acids (i.e., adipic and sebacic acid derivatives). In order to clarify this point, the activation energy for non-isothermal crystallization was evaluated using the Friedman method and significant differences were observed, suggesting a disturbing effect of nanoparticles on the motion of molecular chains that hindered their capability to reach the growing crystallization front. Isoconversional methods provided a good understanding of the kinetics of the crystallization process and significant information regarding the activation energy, relative crystallinity, and global and local Avrami exponents.Peer ReviewedPostprint (published version

Physicochemical Properties Of Poly (Ε-Caprolactone) And Magnesium Oxide Incorporated Poly (Ε-Caprolactone) Nanofibers

Polymer nanofibers are used to develop materials that possess customized characteristics for diverse applications. The applications of nanofibers are influenced by their significant surface-to-volume ratio, the porosity of the nanofiber lattice, and distinctive physicochemical characteristics. The molecular orientation of electrospun nanofibers is a crucial and intricate feature that has a direct impact on the structures and properties of the nanofiber mat. The utilization of Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), and X-Ray Diffractometry (XRD), facilitated the determination of the morphology, chemical structure, and thermal properties of nanofibers. The SEM analysis revealed that the nanofibers exhibited a random and interconnected orientation. The findings indicate that the level of crystallinity exhibited by the magnesium oxide-incorporated PCL (ε-caprolactone) nanofibers, surpassed that of the PCL nanofibers. Increased crystallinity indicates chain mobility changes, leading to improved mechanical characteristics. Further evaluation was conducted on the DSC findings. The study delved into the kinetics of non-isothermal crystallization of PCL and MgO-PCL nanofibers with varying cooling rates. The study used DSC-3 apparatus produced by Mettler Toledo to acquire crystallization information and investigate the kinetics behavior of the two types of nanofibers under different cooling rates ranging from 0.5-5 K/min. Several mathematical models, including Jeziorny, Ozawa, and Mo\u27s models, were utilized to determine the parameters of non-isothermal crystallization kinetics. Mo\u27s approach generates consistent ratios of Avrami exponent to Ozawa exponent (α) that are approximately 1.4 for PCL, MgO-PCL nanofibers, and bulk-PCL. The similarity of α values indicates that the structures of crystallization formed at different levels of relative crystallinity were analogous. The investigation with the Friedman method exhibited an increase in relative crystallinity was associated with a decrease in temperature and a rise in activation energy. According to the Kissinger and Friedman methodologies, it was observed that the activation energy of bulk-PCL was comparatively lower than that of PCL and MgO-PCL nanofibers. The observed phenomenon can be attributed to the nanoconfinement effect, which is characterized by geometric constraints imposed on PCL nanofibers

성능-침강안정성 상충 문제 해결을 위한 복합체 기반 자기유변유체에 대한 연구

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 재료공학부, 2021. 2. 서용석.Magnetorheological (MR) fluids are smart materials composed of magnetic particles dispersed in magnetically-insulating carrier medium. With magnetic field, chain-like structures are formed due to dipole-induced magnetostatic interaction between magnetic particles, and the structures inhibit the flow and increase the viscosity of MR fluids in very short time. This characteristic enables the rheological properties of MR fluids to be easily tailored with magnetic field strength. Due to this unique response, MR fluids can be used for actuator systems like power steering pumps haptic devices, and active suspensions, and damper systems in automobile, bridges, buildings and so on. A huge obstacle for application of MR fluids is their poor long-term stability against the sedimentation of magnetic particles. The large difference in the density between heavy magnetic particles and light medium make magnetic particles quickly go down to bottom, reducing the MR fluids length of life. One of the strategies for improvement of the long-term stability was to reduce the density of magnetic particles by synthesizing magnetic composite materials. Fabrication of magnetic composites using light materials such as polymer, silica, carbon materials efficiently have reduced the density mismatch between magnetic materials and carrier medium, enhancing the long-term stability of MR fluids. However, there was trade-off between long-term stability and performance of MR fluids because use of light materials is equivalent to the deterioration in magnetic properties. In this study, various magnetic composites with different composition and structures were fabricated for the objective of producing MR fluids having excellent performance and long-term stability simultaneously. As a first step, hollow structured polymer-Fe3O4 composite particles were synthesized using SiO2 as sacrificial template. With cavity inside, the hollow magnetic composite particles showed the density only 40 % of bare Fe3O4 and the large improvement in long-term stability of MR suspensions could be observed. To avoid huge decline in MR performance with non-magnetic polymers, hierarchically-structured Fe3O4 nanoparticles were prepared with simple electrospraying process. By excluding polymers, hierarchically-structured Fe3O4 had magnetization value very closed to its primary nanoparticles, leading to MR performance higher more than 3 times of hollow structured polymer-Fe3O4 suspensions. At the same time, the pores inside reduced the density of the structured particles by 23 %, resulting in better long-term stability of hierarchically-structured Fe3O4 suspension than bare Fe3O4 suspension. To minimize the trade-off between MR performance and long-term stability (density of magnetic particles), non-spherical, CoFeNi-based magnetic composites were fabricated and applied for MR fluids. CNT-Co0.4Fe0.4Ni0.2 composite was produced by synthesizing Co0.4Fe0.4Ni0.2 on the surface of functionalized CNTs. Much higher magnetization of Co0.4Fe0.4Ni0.2 compared to Fe3O4 enabled CNT-Co0.4Fe0.4Ni0.2 suspension to have much superior MR performance than Fe3O4 composite-based MR fluids. Also, due to high aspect ratio of CNTs, outstanding long-term stability of 22 % light transmission was observed with formation of 3-dimensional network structures. Finally, magnetically non-active CNTs were replaced by magnetizable, flake-shaped sendust. The high drag coefficient of flake sendust, combined with roughened surface due to attached Co0.4Fe0.4Ni0.2 nanoparticles, resulted in excellent stability with 23 % of light transmission despite of the high density of sendust-Co0.4Fe0.4Ni0.2 composite particles. Also, because both constituents of sendust-Co0.4Fe0.4Ni0.2 are both magnetic materials with high magnetization value, the MR fluids retained very high yield stress value자기유변유체는 자성입자가 비자성 매개액에 분산된 현탁액 형태의 스마트물질이다. 외부 자기장 하에서 자성입자들 사이의 쌍극자로 인한 정자기성 상호작용으로 체인 형태의 구조가 형성되고, 이 구조가 유체의 흐름을 막아 매우 짧은 시간 내에 점도가 크게 향상되게 된다. 이러한 성질로 인해 자기유변유체의 유변특성을 외부 자기장을 통해 쉽게 조절하는 것이 가능하다. 이러한 외부자장에 대한 톡특한 반응성으로 인해, 햅틱 디바이스 파워스티어링 펌프, 그리고 자동차, 다리, 건물 등의 충격 방지 시스템에 자기유변유체를 이용할 수 있다. 하지만 자기유변유체의 활용은 자성입자의 침전에 대한 안정성의 부족함으로 인해 크게 제한 될 수 있다. 밀도가 높은 자성입자와 밀도가 낮은 매개액 사이의 큰 밀도차이로 인해 자성입자가 빠르게 가라앉게 되면, 자기유변유체의 수명이 크게 감소하게 된다. 이러한 문제를 해결하기 위한 한가지 방법으로 자성물질과 밀도가 낮은 물질(고분자, 실리카 탄소물질 등)을 결합하여 자성복합입자를 합성함으로써, 자성입자의 밀도를 낮추고 자기유변유체의 침강안정성을 높이는 연구들이 진행되어 왔다. 하지만 이러한 경우 복합자성입자의 자기적 성질이 저하되기 때문에 자기유변유체의 침강안정성과 성능이 서로 상충관계에 있다는 문제점을 가지고 있다. 본 논문에서는 뛰어난 성능과 침강안정성을 가지는 자기유변유체를 제조하기 위해 다양한 물질구성과 구조를 가지는 합성하였다. 첫 단계로 실리카를 템플레이트로 사용하여 할로우 구조를 가지는 고분자-Fe3O4 복합자성입자를 합성하였다. 할로우 구조 내부의 공동으로 인해, 입자의 밀도가 순수 Fe3O4 대비 40 % 수준까지 감소하였고, 이로 인해 자기유변유체의 침강안정성이 크게 상승하였다. 다음 연구로, 비자성 고분자로 인한 자기유변유체 성능의 감소를 최소화하기 위해, 간단한 전기방사 방법을 통해 계층구조를 가지는 Fe3O4 나노입자들을 제조하였다. 앞의 연구와 대비하여, 고분자의 배제를 통해 높은 자화값을 가지는 Fe3O4 나노구조입자들을 얻을 수 있었고, 이를 자기유변유체에 적용하여 할로우 고분자-Fe3O4 입자 기반 자기유변유체 대비 3배 이상의 성능을 가지는 자기유변유체를 얻을 수 있었다. 이와 동시에 Fe3O4 나노구조입자 내부에 생성된 기공들로 인해 순수 Fe3O4 대비 밀도가 약 23 % 정도 감소하였고, 이로 인해 침 Fe3O4 나노구조입자기반 자기유변유체의 침강안정성이 향상됨을 확인할 수 있었다. 자기유변유체의 성능과 침강안정성사이의 상충성을 최소화 하기 위해서 비구형의, CoFeNi 합금기반 자성 복합입자를 합성하고 자기유변유체에 적용하였다. 먼저 개질된 카본나노튜브 표면에 CoFeNi를 합성하는 방법을 통해 카본나노튜브-CoFeNi 복합체를 합성하였다. Fe3O4 대비 높은 CoFeNi의 자화값으로 인해 카본나노튜브-CoFeNi 복합체 기반 자기유변유체는 Fe3O4 복합체기반 유체 대비 3배에서 10배 이상의 뛰어난 유변성능을 보였다. 또한 종횡비가 높은 카본나노튜브로 인해 복합체가 유체 내에서 3차원 네트워크 구조를 형성하여, 빛 투과도 22 %의 매우 뛰어난 침강안정성을 보였다. 마지막으로 비자성 물질인 카본나노튜브를, 자성물질인 플레이크형 센더스트로 대체한 센더스트-CoFeNi 복합입자를 합성하여 자기유변유체에 적용하였다. 플레이크형 센더스트의 높은 종횡비로 인해 나타나는 높은 항력계수로 인해, 해당 자기유변유체는 빛 투과도 23 %의, 높은 입자밀도 대비 매우 뛰어난 침강안정성을 보였다. 동시에, 센더스트 CoFeNi 모두 높은 자화값을 가지는 자성물질이기 때문에, 센더스트-CoFeNi 기반 자기유변유체의 성능이 카본나노튜브-CoFeNi 유체 대비 크게 향상되는 것을 확인할 수 있었다.Contents Abstract ………………………………………………………..........i Contents ……………………………………………………….........v List of Tables ………………………………………………….........x List of Figures ………………………………………………..........xi Chapter 1. Introduction ……………………………………............1 1. 1. Magnetorheological (MR) Fluids and Applications………………….........1 1. 2. Long-Term Stability Problem and Proposed Solutions ……………...........4 1. 3. Research Objectives ………………………………………………….…...7 References ………………………………...........................................................9 Chapter 2. Backgrounds …………………….……………….......16 2. 1. Definition of Terms …………..................................................................16 2. 1. 1. Shear Stress …………........................................................................16 2. 1 .2. Shear Rate …………..........................................................................16 2. 1. 3. Shear Viscosity……............................................................................17 2. 1 .4. Viscoelastic Behavior .........................................................................17 2. 1. 4. 1. Storage Modulus and Loss Modulus ..........................................17 2. 2. Yield Stress of MR Fluids …………........................................................18 2. 2. 1. Rheological Models for Prediction of Dynamic and Static Yield Stress ………….............................................................................................................19 2. 2 .2. Yield Stress Dependency on the Magnetic Field Strength .................22 2. 3. Mechanism of Structures Evolution .........................................................25 References ………............................................................................................27 Chapter 3. Suspensions of Hollow Polydivinylbenzene Nanoparticles Decorated with Fe3O4 Nanoparticles as Magnetorheological Fluids for Microfluidics Applications ........31 3. 1. Introduction ………....................................................................................31 3. 2. Experimental Section ...............................................................................34 3. 2. 1. Synthesis of Hollow Polydivinylbezene (h-PDVB) Particles ............34 3. 2. 2. Deposition of Fe3O4 onto Hollow PDVB Particles ............................36 3. 2. 3. Characterization ..................................................................................37 3. 3. Results and Discussion .............................................................................41 3. 3. 1 Morphology and Structures ..................................................................41 3. 3. 2. Magnetorheological Behaviors ............................................................48 3. 3. 3. Long-Term Stability of Suspensions ...................................................62 3. 4. Conclusion ……….....................................................................................65 References ……….............................................................................................67 Chapter 4. Hierarchically Structured Fe3O4 Nanoparticles for High-Performance Magnetorheological Fluids with Long-Term Stability …………………….………………...................................74 4. 1. Introduction ………...................................................................................74 4. 2. Experimental Section ................................................................................77 4. 2. 1. Synthesis of Citric Acid-Capped Fe3O4 ..............................................77 4. 2. 2. Fabrication of HS-Fe3O4 with Electrospraying Process .....................78 4. 2. 3. Characterization ..................................................................................79 4. 3. Results and Discussion .............................................................................82 4. 3. 1. Morphology and Structures ................................................................82 4. 3. 2. Magnetorheological Behaviors ...........................................................89 4. 3. 3. Long-Term Stability of Suspensions .................................................103 4. 4. Conclusion ………..................................................................................106 References ………..........................................................................................108 Chapter 5. High-Performance Magnetorheological Fluids of Carbon Nanotube-CoFeNi Composites with Enhanced Long-Term Stability…………………….……………….................................116 5. 1. Introduction ………...............................................................................116 5. 2. Experimental Section ............................................................................119 5. 2. 1. Functionalization of Carbon Nanotubes ..........................................119 5. 2. 2. Synthesis of Co0.4¬Fe0.4Ni0.2 and CNT-Co0.4¬Fe0.4Ni0.2 .......................119 5. 2. 3. Characterization ...............................................................................120 5. 3. Results and Discussion ...........................................................................123 5. 3. 1. Morphology and Structures ..............................................................123 5. 3. 2. Magnetorheological Behaviors .........................................................130 5. 3. 3. Long-Term Stability of Suspensions .................................................142 5. 4. Conclusion ………..................................................................................145 References ………..........................................................................................146 Chapter 6. Sendust-CoFeNi Magnetic-Magnetic Composites-Based Magnetorheological Fluids for Simultaneous Improvement of Performance and Long-Term Stability ……………………..154 6. 1. Introduction ………................................................................................154 6. 2. Experimental Section .............................................................................157 6. 2. 1. Synthesis of Citric Acid-Capped Fe3O4 ...........................................157 6. 2. 2. Characterization ...............................................................................157 6. 3. Results and Discussion ...........................................................................159 6. 3. 1. Morphology and Structures ..............................................................159 6. 3. 2. Magnetorheological Behaviors ........................................................163 6. 3. 3. Long-Term Stability of Suspensions ................................................176 6. 4. Conclusion ………..................................................................................179 References ………..........................................................................................181 Chapter 7. Conclusions ………………………………………....188 7. 1. Overall conclusion ………......................................................................188 7. 2. Further works ………..............................................................................194 References ………...........................................................................................195 국문초록 ………...............................................................................................196 List of Publication ………………………………………….......199 Appendix …………………………………………..........…........201 Appendix A. Nonisothermal Crystallization Behaviors of Structure-Modified Polyamides (Nylon 6s) ...............................................................................201Docto

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

Ziegler-Natta and metallocene catalyzed isotactic polypropylene : a comprehensive investigation and comparison using crystallization kinetics, fiber spinning and thermal spunbonding

Isotactic polypropylene (iPP) can be synthesized using, conventional heterogeneousZiegler-Natta (zriiPP) and homogenous metallocene catalysts (miPP). Materials catalyzed using the Ziegler-Natta catalysts typically have broad molecular weight distributions, high peak melting temperatures, a heterogeneous distribution of stereo defects, and significant portions of non crystallizable (atactic) material. Metallocene resins typically have a narrow molecular weight distribution, lower peak melting temperatures than zniPP resins, a more uniform distribution of stereo and regio defects, and very small amounts of atactic material.The purpose, of this work was to investigate and compare and contrast miPP resins tozniPP resins.The resins in this study were thoroughly characterized by cNMR and solutionfiuctionation to determine the number, type and distribution of defects. The resins were then studied under isothermal and nonisothermal quiescent crystallization conditions to determine the bulk and crystal growth kinetics, crystal structure, crystallinity and thermal properties. The resins were also melt spun into fibers to allow the effects of molecular weight md molecular weight distribution on as-spun filament properties to be determined.The as-spun fibers were then characterized to determine the crystalline and noncrystalline orientation functions, birefringence, density, thermal properties and tensile mechanical properties (elongation-to-break, modulus, tensile strength). In addition, on-linecrystallization studies were also conducted using the resins to determine the locations in the spinline where crystallization occurred. Selected resins were then used to study the effect of as-spun fiber properties on nonwoven fabric mechanical properties using a thermal point spunbonding process.The resins in this study were thoroughly characterized by cNMR and solutionfiuctionation to determine the number, type and distribution of defects. The resins were then studied under isothermal and nonisothermal quiescent crystallization conditions to determine the bulk and crystal growth kinetics, crystal structure, crystallinity and thermal properties. The resins were also melt spun into fibers to ^ow the effects of molecular weight and molecular weight distribution on as-spun filament properties to be determined.The as-spun fibers were then characterized to determine the crystalline and noncrystalline orientation functions, birefringence, density, thermal properties and tensile mechanical properties (elongation-to-break, modulus, tensile strength). In addition, on-linecrystallization studies were also conducted using the resins to determine the locations in the spinline where crystallization occurred. Selected resins were then used to study the effect of as-spun fiber properties on nonwoven fabric mechanical properties using a thermal point spunbonding process.The cNMR and xylene fractionation studies indicated the miPP resins had substantially more total defects in the crystallizable material than either of the zniPP resins in this study. The miPP resins also contained regio type polymerization defects, which were not present in the zniPP resins. The results also found that, in general, the miPP resins contained much smaller portions of atactic material, as determined by xylene fractionation. The totality of the stereoregularity results suggest the miPP resins have amore uniform defect distribution than the zniPP resins.Combined results from DSC, SAXS and WAXD indicated the miPP and zniPPresins have similar a-monoclinic equilibrium melting temperatures (Tm0), despite the differences in defect content, type and distribution.The presence of atactic material wasfound to lower the observed equilibrium melting temperature of a particular resin, whethermiPP or zniPP. The γ-crystal structure, observed in the miPP resins using WAXD andDSC, had a lower equilibrium melting temperature than the α-structure. The (Tm0) of the &alpha-monoclinic structure was found to be 186± 2°C, while the γ-structure T(Tm0) was found to be 178 ±2°C, when crystallized at atmospheric pressures. The miPP resins were found to melt lower temperatures than the zniPP resins, at similar crystal thicknesses, which is attributed to the ntiPP resins having significantly higher fold surface free energies when crystallized under isothermal conditions.The isothermal crystallization studies showed the miPP resins readily produce the γ-crystalstructure. The zniPP resins also produced small amounts of the γ-structure, at high crystallization temperatures. Defects were found to be excluded from the crystal under isothermal crystallization conditions. The defects excluded from the crystal core are thought to be rejected into the crystal fold surface region, increasing the fold surface free energy. SAXS studies indicate the lamellae fold surface of isothermally crystallized miPP resins might be rough, possessing a three-dimensional topology instead of a two dimensional structure. The Tm0 and fold surface free energy for each resin was determined from the Gibbs-Thompson equation. The Gibbs-Thomson equation normalizes to a two dimensional crystal fold surface, therefore the apparent fold surface free energy is higher in the miPP resins with a three-dimensional topology. These conclusions are supported by the non isothermal crystallization studies which showed that defects are incorporated into the crystal core and that the fold surface free energies of the non isothermally crystallized films using rruPP and zniPP resins are similar.For crystallization under isothermal crystallization conditions, the observed linear growth rates were found to be dependent upon defect content. Under non isothermal conditions, the growth rate was found to depend mostly on the molecular weight. Forresins with similar molecular weights, the number of defects was also found to be important under nonisothermal crystallization conditions. The nucleation density was found to have a strong effect on the overall bulk crystallization kinetics. The relative order of bulk crystallization rates for the resins in this study was found to be strongly determined by the relative nucleation density of a particular resin.Fiber spinning studies showed that the molecular weight and molecular weight distribution of an iPP resin is largely determined by the point of crystallization in thespinline, the crystallization temperature and as-spun filament properties. Increasing the molecular weight (also increasing the spinning speed) tended to increase the density and crystallization temperature, i.e. c^stallization in the spinline occurs closer to the spirmeret.Narrowing the molecular weight distribution and decreasing the molecular weight (also with increasing the spinning speed) tended to increase the noncrystalline and crystalline orientation function, birefringence and tensile strength (elongation-to-break was the inverse) for most the resins. The more narrow molecular weight distribution resins also delay crystallization to a distance further away from the spinneret and to lower crystallization temperatures. The as-spun fiber tensile modulus was found to increase as the spinning speed increased, a result of the birefringence and crystallinity increasing. The Observed differences in fiber spinning behavior between the miPP and zniPP resins are mainly attributed to the differences in molecular weight and its distribution.Studies on the mechanical properties of nonwoven fabrics made using the thermalpoint spunbonding process were found to be dependent on the fiber properties, when bonded at the optimum bonding temperature. , The optimum bonding temperature is the temperature in the bonding curve where the fabric mechanical properties are the best.Increasing the as-spun fiber non crystalline orientation function and birefringence increased the optimum bonding temperature. Increasing the fabric basis weight also increased the optimum bonding temperature. No significant differences in fabric properties between miPP and zniPP resins could be found that are not explained by differences in the as-spun fiber properties. The as-spun fiber properties were found to be different between the two. catalyst systems, a result of differences in their molecular weight and molecular weight distribution

Non-isothermal crystallization kinetics of poly(4-hydroxybutyrate) biopolymer

The non-isothermal crystallization of the biodegradable poly(4-hydroxybutyrate) (P4HB) has been studied by means of differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). In the first case, Avrami, Ozawa, Mo, Cazé, and Friedman methodologies were applied. The isoconversional approach developed by Vyazovkin allowed also the determination of a secondary nucleation parameter of 2.10 × 105 K2 and estimating a temperature close to 10 °C for the maximum crystal growth rate. Similar values (i.e., 2.22 × 105 K2 and 9 °C) were evaluated from non-isothermal Avrami parameters. All experimental data corresponded to a limited region where the polymer crystallized according to a single regime. Negative and ringed spherulites were always obtained from the non-isothermal crystallization of P4HB from the melt. The texture of spherulites was dependent on the crystallization temperature, and specifically, the interring spacing decreased with the decrease of the crystallization temperature (Tc). Synchrotron data indicated that the thickness of the constitutive lamellae varied with the cooling rate, being deduced as a lamellar insertion mechanism that became more relevant when the cooling rate increased. POM non-isothermal measurements were also consistent with a single crystallization regime and provided direct measurements of the crystallization growth rate (G). Analysis of the POM data gave a secondary nucleation constant and a bell-shaped G-Tc dependence that was in relative agreement with DSC analysis. All non-isothermal data were finally compared with information derived from previous isothermal analysesPeer ReviewedPostprint (published version

Thermooxidative stability of PMMA composites

Tato práce se zabývá studiem termooxidační stability kompozitů polymethylmethakrylátu (PMMA) plněného mikro a nanočásticemi siliky. V připravených vzorcích byly použity různé objemové zlomky a různé velikosti částic siliky. Studium stability bylo prováděno pomocí termogravimetrie, která umožňuje simulovat podmínky termooxidační degradace. Indukční perioda byla stanovena za použití různých rychlostí ohřevu a aplikací izokonverzních metod. Závislosti teplot degradací na rychlostech ohřevu sloužily pro určení parametrů odvozených ze čtyř různých teplotních funkcí, které dovolují předpověď stability materiálu (indukční periody) při zvoleném rozsahu teplot. Zjištěné výsledky ukazují, že větší částice siliky snižuji stabilitu PMMA, zatímco nanočástice v nízkých koncentracích ji nijak neovlivňují.In this work the thermooxidative stability of poly(methyl metacrylate) (PMMA) composites reinforced with silica micro and nanoparticles was studied. Different volume fractions and particles sizes of silica particles were used. PMMA/silica composites were analysed by thermogravimetry which simulated the conditions of thermooxidative degradation. The induction periods were determined using different heating rates and applying the isoconversional methods. The dependence of degradation temperatures on heating rates were used for the determination of adjustable parameters derived for four different temperature functions allowing the prediction of material stability (induction periods) at chosen temperatures. Results showed that the larger silica particles destabilized the PMMA structure while smallest nanoparticles at low concentration had no effect on the stability.