Molecular dynamics modeling of polymer crystallization from the melt

Molecular pathways to polymer crystallization and the structures of crystal-melt interfaces are investigated by molecular dynamics simulation. We adopt a simplified molecular model for polymethylene-like chains; the chain is made of CH₂-like beads connected by harmonic springs, and the lowest energy conformation is a linear stretched sequence of the beads with slight bending stiffness being imposed. Two molecular systems are considered, one is made of 640 chains of C100 and the other is made of 64 chains of C1000, both being placed between two parallel substrates that represent the growth surfaces of the lamellae growing toward each other. The initial melt kept at a sufficiently high temperature above the melting point is rapidly cooled down to various crystallization temperatures, and the molecular processes of crystallization that follow are investigated. In both systems, we clearly observe the growth of stacked chain-folded lamellae from the substrates. The growing lamellae have a definite tapered shape, and they show marked thickening growth along the chain axis as well as usual growth perpendicular to it. The overall crystallization rate is found to be very sensitive to the crystallization temperature, showing an apparent maximum around 320-330 K for C100. We find that the lamellae do not grow keeping pace with each other but grow in independent rates especially at higher temperatures. We also examine the structures of the lateral growth surfaces and find that the growth surfaces are locally flat and the Kossel mechanism of crystal growth seems to be operative. In addition, the fold surfaces are found to be covered with relatively short chain-folds; at least about 60-70% of the folds are connecting the nearest or the next nearest neighbor crystalline stems. No appreciable bond orientational order is found in the undercooled melt of C100 and C1000.     

Low-field NMR studies of polymer crystallization kinetics: Changes in the melt dynamics

The free induction decay in ¹H NMR experiments carried out for crystallizing polymers can be directly decomposed in contributions from crystals, melt-like regions and amorphous regions with a reduced mobility. Here, the results of time-dependent experiments conducted with the aid of a cost-efficient low-field NMR instrument are presented, obtained for sPP, P?CL and P(EcO). Crystallization isotherms are compared with those obtained by X-ray scattering and dilatometry. There are some minor systematic deviations which can be explained and accounted for. For all systems, a large fraction of amorphous chain parts in regions with a reduced mobility is found.     

Effects of short‐chain branches on strain‐induced polymer crystallization

We performed dynamic Monte Carlo simulations of a lattice polymer model to investigate strain‐induced crystallization behaviors of short‐chain branched polymers with variable sequence distributions, branch numbers and lengths, and their blending compositions with linear polymers. The results revealed that the branch density and distribution bring dominant effects. In the blends, there is no phase separation prior to crystallization, except that strain‐enhanced mixing is temporarily hindered by chain folding upon crystallization. Our observations shed lights onto the strain‐induced crystallization of short‐chain branched polymers. © 2018 Society of Chemical Industry     

Time dependent light attenuation measurements used in studies of the kinetics of polymer crystallization

Results obtained for different samples of s-polypropylene and poly (ethylene-co-octene) demonstrated the usefulness of light attenuation measurements in investigations of polymer crystallization. The earlier stages with separated growing spherulites fall in the range of Rayleigh-Debye-Gans scattering. Known relationships describing the dependence of the linear attenuation coefficient on the radius and the index of refraction of the spherulite can be applied in evaluations. The sensitivity of attenuation measurements is higher than that of conventional tools.     

The crystallization morphology and melting behavior of polymer crystals in nano-sized domains

The crystallization morphology and the melting behavior of the phase-separating poly(?-caprolactone) (PCL) and poly(ethylene oxide) (PEO) blends were studied using atomic force microscopy. Two blends consisting of PCL and PEO with weight ratios of 10/90 and 90/10 were prepared to form the isolated spherical domains by the phase-separating process. The results show that the melting temperatures of the PCL and PEO lamellae in the confined domains increased as the lamellar length increased, and the melting behavior of the PCL and PEO lamellae in the matrix and confined domains was also studied.     

Effects of the degree of undercooling on flow induced crystallization in polymer melts

This study investigates the coupled effects of mild shear flow and temperature on the crystallization behavior of two thermoplastic polymers, namely, an isotactic polypropylene and an isotactic poly(1-butene). Rheological experiments are used to measure the crystallization induction time under isothermal, steady shear flow conditions. The experimental results clearly show the effects of the degree of undercooling on flow-induced crystallization (FIC). As temperature decreases, the corresponding increase in chain orientation at a given shear rate leads to an absolutely faster crystallization. At the same time, however, a temperature decrease makes the flow-induced driving force to crystallization relatively less influent with respect to the intrinsic kinetics. A FIC model based on the Doi-Edwards micro-rheological theory is shown to successfully describe the quantitative details of the observed experimental behavior.     

The β relaxation as a probe to follow real-time polymer crystallization in model aliphatic polyesters

The dielectric spectra of two aliphatic polyesters, poly(propylene succinate) and poly(propylene adipate), are reported. For these two model aliphatic polyesters the α and β relaxations appear simultaneously and well resolved in the experimental frequency window. During isothermal crystallization, this fact allows one to use the β relaxation to characterize the crystalline structural development while the α relaxation provides information about the evolution of the amorphous phase dynamics. In this way structural development and dynamics evolution can be characterized by a single experiment during the crystallization process. Additionally, the unambiguous determination, in the amorphous polymers, of the relaxation times for the α and β relaxations further support a close relationship between local and segmental dynamics in polyesters as proposed by the coupling model.     

Dynamically confined crystallization in a soft lamellar space constituted by alternating polymer co-brushes

Crystallization kinetics and behavior of PCL side chains in polymer co-brushes constituted with PCL and PEO side chains alternatively attached on poly(styrene-alt-maleimide) backbones have been determined using in-situ FT-IR and DSC methods. Avrami analysis shows the exponent n increasing from one at 10 °C to two at 30 °C, demonstrating confined crystallization of PCL side chains through homogeneous or heterogeneous nucleation. PLM morphological characterization displays typical spherulites of which size is dependent on the crystallization temperature and further AFM visualization shows typical PCL lamellae at 30 °C and broken lamellae at 10 °C embedded within PEO + backbone matrix inside of spherulites. Such lamellar structure explains the confined crystallization with Avrami exponent n ≤ 2. Formation of the broken lamellae can further clarify the reason why Avrami exponent decreases to n ≈ 1 at 10 °C, that is, homogeneous nucleation in the isolated crystals. Dynamically confined crystallization has been proposed based on their special molecular architecture. Comparing to statically confined crystallization, the construction of confined space and the crystallization process were almost synchronous. The formation of spherulites mesoscopically reveals the entire molecule motion and assembly through a pathway of conventional crystalline polymers and the crystallization of PCL side chains in a space constituted by stiff backbones of poly(styrene-alt-maleimide) plus soft PEO layer microscopically reflects a confined character which has been observed in some conventional block copolymers.     

Well-ordered polymer nano-fibers with self-cleaning property by disturbing crystallization process

Bionic self-cleaning surfaces with well-ordered polymer nano-fibers are firstly fabricated by disturbing crystallization during one-step coating-curing process. Orderly thin (100 nm) and long (5–10 μm) polymer nano-fibers with a certain direction are fabricated by external macroscopic force (F_blow) interference introduced by H₂ gas flow, leading to superior superhydrophobicity with a water contact angle (WCA) of 170° and a water sliding angle (WSA) of 0-1°. In contrast, nano-wires and nano-bridges (1–8 μm in length/10-80 nm in width) are generated by “spinning/stretching” under internal microscopic force (F_T) interference due to significant temperature difference in the non-uniform cooling medium. The findings provide a novel theoretical basis for controllable polymer “bionic lotus” surface and will further promote practical application in many engineering fields such as drag-reduction and anti-icing.     

Carbon nanotube induced polymer crystallization: The formation of nanohybrid shish-kebabs

Carbon nanotubes (CNTs) have attracted tremendous attention in recent years because of their superb optical, electronic and mechanical properties. In this article, we aim to discuss CNT-induced polymer crystallization with the focus on the newly discovered nanohybrid shish-kebab (NHSK) structure, wherein the CNT serves as the shish and polymer crystals are the kebabs. Polyethylene (PE) and Nylon 6,6 were successfully decorated on single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs), and vapor grown carbon nanofibers (CNFs). The formation mechanism was attributed to “size-dependent soft epitaxy”. Polymer CNT nanocomposites (PCNs) containing PE, Nylon 6,6 were prepared using a solution blending technique. Both pristine CNTs and NHSKs were used as the precursors for the PCN preparation. The impact of CNTs on the polymer crystallization behavior will be discussed. Furthermore, four different polymers were decorated on CNTs using the physical vapor deposition method, forming a two-dimensional NHSK structure. These NHSKs represent a new type of nanoscale architecture. A variety of possible applications will be discussed.     

In situ FTIR microscope study on crystallization of crystalline/crystalline polymer blends of bacterial copolyesters

This study describes in situ observation of crystallization in a spherulite of blends of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [PHBV] and poly(3-hydroxybutyrate-co-3-hydroxypropionate) [PHBP] by FTIR microscopy. In order to trace the crystallization processes of blend components separately, PHBV was deuterated. The C-D and CO stretching bands in the IR spectra, respectively, show the crystallization behavior of PHBV and the whole blend. D-PHBV containing 6 and 8% HV [D-PHBV6 and D-PHBV8] are blended with PHBP containing 11% HP [PHBP11]. The crystallization rates of D-PHBV6, D-PHBV8 and PHBP11 decrease in this order. In case of the blend of D-PHBV8 and PHBP11 the crystalline peaks of C-D and CO bands grows simultaneously during crystallization, and the growth rates are rather close to that of D-PHBV8. The results indicate that D-PHBV8, which is the component that shows higher crystallization rate in the pure state, leads the cocrystallization of the blend. For D-PHBV6/PHBP11, on the other hand, the crystalline peak of C-D band grows faster than that of CO band, indicating that the crystallization of D-PHBV6 proceeds before the crystallization of PHBP11. During the crystallization of D-PHBV6, PHBP11 molecules get away from the growing front of the spherulite, i.e. the phase segregation precedes the crystallization. These results demonstrate that FTIR microscopy is a powerful tool to trace the formation of different crystalline phases, such as cocrystallization and phase segregation.     

Kinetics of polymer crystallization under processing conditions: transformation of dormant nuclei by the action of flow

In previous papers [Janeschitz-Kriegl H, Ratajski E, Stadlbauer M. Flow as an effective promotor of nucleation in polymer melts: a quantitative evaluation. Rheol Acta 42 (2003) 355-364; Stadlbauer M, Janeschitz-Kriegl H, Eder G, Ratajski E. New extensional rheometer for creep flow at high tensile stress. Part II. Flow induced nucleation for the crystallization of iPP, J Rheol 48 (2004) 631-639. [1] and [2]] two types of experiments were carried out: (a) rapid quenches of quiescent melts of i-PP from their state of equilibrium to a series of rather low temperatures and (b) short term shearing or extension of melts at only mild degrees of undercooling. Strong undercooling as well as high mechanical loads cause similar tremendous increases in the number densities of nuclei by many decades. Unexpectedly, an extremely non-linear dependence of the said number densities on the loading times has been found.In the present paper an explanation for this non-linear relation is tried. The assumption is made that in a quiescent melt there is a huge reservoir of badly organized aggregates (local alignments) of chain molecules, which as such can become effective nuclei only at rather low temperatures. These aggregates are assumed to grow by the action of flow after being oriented in the flow direction. In this way a large amount of low quality dormant nuclei can be transformed into nuclei of a better quality, which are active at higher temperatures. Starting at individual aggregates this growth can lead to thread-like precursors initiating shish-kebab structures.Instead of the flow time the specific mechanical work has been found to be a useful universal parameter. Some measurements of the optical retardation are quoted, which strongly support the basic assumption.     

基于分子动力学的橡胶聚合物计算机辅助设计方法

聚合物分子设计的关键步骤是得到能够满足多种性质要求的重复单元结构。作为化学产品工程中的新型发展手段，计算机辅助分子设计(CAMD)技术可以通过基团贡献法生成满足约束条件的聚合物重复单元结构，分子动力学(MD)技术则可以在微观层面上进行计算机实验模拟系统性质。建立了聚合物的CAMD-MD通用设计方法，并进行轮胎橡胶聚合物的分子设计，首先基于基团贡献法进行重复单元的设计；其次，利用层次分析法确定多性质权重排名，并基于分子动力学方法探究候选结构的性质；最后将方法应用于实际橡胶结构中，模拟得到聚能密度、密度、玻璃化转换温度和热导率性质，验证了方法的可行性。     

Mathematical modeling and design of layer crystallization in a concentric annulus with and without recirculation

A solution layer crystallization process in a concentric annulus is presented that removes the need for filtration. A dynamic model for layer crystallization with and without a recirculation loop is developed in the form of coupled partial differential equations describing the effects of mass transfer, heat transfer, and crystallization kinetics. The model predicts the variation of the temperature, concentration, and dynamic crystal thickness along the pipe length, and the concentration and temperature along the pipe radius. The model predictions are shown to closely track experimental data that were not used in the model's construction, and also compared to an analytical solution that can be used for quickly obtaining rough estimates when there is no recirculation loop. The model can be used to optimize product yield and crystal layer thickness uniformity, with constraints on the supersaturation to avoid bulk nucleation by adjusting cooling temperatures in the core and jacket. © 2013 American Institute of Chemical Engineers AIChE J, 59: 1308–1321, 2013     

A computer-aided molecular design framework for crystallization solvent design

One of the key decisions in designing solution crystallization processes is the selection of solvents. In this paper, we present a computer-aided molecular design (CAMD) framework for the design and selection of solvents and/or anti-solvents for solution crystallization. The CAMD problem is formulated as a mixed integer nonlinear programming (MINLP) model. Although, the model allows any combination of performance objectives and property constraints, in the case studies, potential recovery was considered as the performance objective. The latter, needs to be maximized, while other solvent property requirements such as solubility, crystal morphology, flashpoint, toxicity, viscosity, normal boiling and melting point are posed as constraints. All the properties are estimated using group contribution methods. The MINLP model is then solved using a decomposition approach to obtain optimal solvent molecules. Solvent design and selection for two types of solution crystallization processes namely cooling crystallization and drowning out crystallization are presented. In the first case study, the design of single compound solvent for crystallization of ibuprofen, which is an important pharmaceutical compound, is addressed. One of the important issues namely, the effect of solvent on the shape of ibuprofen crystals is also considered in the MINLP model. The second case study is a mixture design problem where an optimal solvent/anti-solvent mixture is designed for crystallization of ibuprofen by the drowning out technique. For both case studies the performance of the solvents are verified qualitatively through SLE diagrams.     

Investigating polymer crystallization by time-dependent light scattering: a direct approach in the data analysis applied for s-polypropylene

We developed a procedure for a direct evaluation of light scattering patterns registered during an isothermal crystallization of s-polypropylene. Analysis is based on the determination of three parameters of the scattering curve for both, polarized and depolarized measuring conditions: (i) the forward scattering intensity I(q→0) (ii) the width Δq of the intensity distribution (iii) the integral intensity in the registration plane, Q₂. We derive equations for these parameters and relate them to the size and the inner structure of the hedrites which develop in s-polypropylene during a crystallization.     

Molecular structure and driving force to metastable states: Janus faces in polymer crystallization

This paper is a retrospective of a past dedicated to research on polymers and a situation sketch of the present and the near future. (Co)polymers discussed are mainly based on ethylene. (Cross‐)fractionation techniques combined with state‐of‐the‐art characterization techniques, like quantitative differential scanning calorimetry, are powerful tools for the study of the links between two main topics: molecular structure and crystallization/melting. These form the two ‘Janus faces’ polymers can show, namely Face 1: the molecular structure resulting from polymerization with the keyword ‘nature’; and Face 2: the driving force of crystallization towards a metastable state, with the keyword ‘nurture’. After all, to meet demands for properties of products, in principle one starts with a given molecular architecture, after which dedicated processing, including application of temperature–time ramps, has to do the job. With new instrumentation, especially fast scanning (chip) calorimetry, for the first time in history the driving force towards crystallization into one of the possible metastable states – via Face 2 – can be controlled, of course within certain limits given by Face 1. This promising outlook of combining the faces to a useful symbiosis of Janus will be a challenge for those working in the science of crystallization of polymers. © 2018 Society of Chemical Industry     

Effect of particle size and grafting density on the mechanical properties of polymer nanocomposites

End-grafting polymer chains to nanoparticles in polymer nanocomposite is a widely used method to disperse inorganic particles in a polymeric matrix in order to improve the material properties. While many fundamental studies have investigated how various factors influence the dispersion or aggregation of the nanoparticles, the effect of grafting on the resulting material properties has received considerably less attention. In particular, the effect of nanoparticle curvature and grafting density on the mechanical properties in polymer nanocomposites remains elusive. In this study, we develop a coarse-grained model of a polymer glass containing grafted nanoparticles and examine the resulting effects on the mechanical properties. By carefully designing the parameters of our polymer nanocomposites model, we can maintain dispersion of the nanoparticles whether they are grafted with polymer chains or not, which allows us to isolate the effect of end-grafting on the resulting mechanical properties. We examine how the nanoparticle size and grafting density affect the elastic constants, strain hardening modulus, as well as the mobility of the polymer segments during deformation. We find that the elastic constants and yield properties are enhanced nearly uniformly for all of our nanocomposite systems, while the strain hardening modulus depends weakly on the grafting density and the nanoparticle size.     

Blocked crystallization in capped ultrathin polymer films studied by molecular simulations

The crystallization of capped ultrathin polymer films is closely dependent on film thickness and interfacial interaction. Using dynamic Monte Carlo simulations, the crystallization behaviors of polymer films confined between two substrates were investigated. The crystallization rate of confined polymers is reduced with high interfacial interactions. Above a critical strength of interfacial interaction, polymer crystallization in the thin film is inhibited within the simulation time scales. An increase in film thickness leads to a rise in critical interfacial interaction. In thicker films, the chains have more space to change conformation to form crystal stems. In addition, there are fewer absorbed segments in confined chains for the thicker films, and thus the chains have stronger ability to adjust their conformation. Therefore an increase in film thickness can cause a reduction in the entropic barrier required for the formation of crystals and thus an increase in the critical interfacial interaction. © 2018 Society of Chemical Industry