Structure Of Semicrystalline Polymers Research Articles

The Role of the Crystallization Temperature on the Nanophase Structure Evolution of Poly[(R)-3-hydroxybutyrate

The nanophase structure of semicrystalline polymers, which determines the mechanical, thermal, and gas permeability behavior, can be quantified by thermal methods. A detailed investigation of the nanophase structure of poly[(R)-3-hydroxybutyrate] (PHB) was performed under conditions of isothermal, quasi-isothermal, and nonisothermal crystallizations. The experimental analyses revealed that the establishment of the nanophase rigid amorphous fraction (RAF) in PHB depends on the temperature at which crystallization occurs. The RAF grows in parallel with the crystal phase during quasi-isothermal crystallization at 30 °C, whereas during nonisothermal crystallization at higher temperatures, RAF starts to develop at 70 °C, in correspondence with the final stages of the crystallization process. The influence of crystallization temperature on the nanophase structure was rationalized taking into account the effect of the mobility of the entangled chain segments during the phase transition. The melting behavior was found to change after isothermal crystallization at 70 °C, revealing that complete RAF mobilization is achieved approximately at this temperature. The temperature of 70 °C could be the limit for the formation and the disappearance of rigid amorphous fraction in the PHB analyzed in the present study.

Orientation of Well-Dispersed Multiwalled Carbon Nanotubes in Melt-Spun Polymer Fibers and Its Impact on the Formation of the Semicrystalline Polymer Structure: A Combined Wide-Angle X-ray Scattering and Electron Tomography Study

The orientation behavior of well-dispersed multiwalled carbon nanotubes (CNTs) within high-speed melt-spun semicrystalline polymer fibers has for the first time been studied using three-dimensional reconstructions from bright-field transmission electron microscopy (TEM) tomography. The local investigation technique allows separating contributions stemming from additionally present CNT aggregates. Over a relatively narrow draw ratio range applied during the fiber production process, a transition region is found, in which the CNTs change their orientation from being aligned perpendicular to being aligned parallel to the fiber axis. Complementary performed wide-angle X-ray scattering measurements and mechanical analysis of the polymer/CNT nanocomposite fibers reveal a strong correlation between the CNT orientation and the structural and mechanical properties of the fibers. Characteristic quantities such as crystallinity, crystal size, and correlation length parameters of crystalline and amorphous polymer cha...

Application of electron diffraction in the structure characterization of polymer crystals

The dependence of properties on the structure and morphology of semicrystalline polymers offers an effective way to tailor the properties of these materials through structure control. To this end, establishing the structure and property relationship is of great importance. For a right characterization of the crystal structure, several techniques can be used. Among these techniques, electron diffraction has its advantage for determining the crystal structure related to specific formation condition since it can combine with bright and dark fields observation of the sample. This feature article describes the application of electron diffraction in determining the crystal structure of semicrystalline polymers with elaborately selected examples. We focus on how the electron diffraction can be used to disclose the crystal structure, mutual orientation of different crystals, as well as the disorders included in the polymer crystals.

Can the Structures of Semicrystalline Polymers be Controlled Using Interfacial Crystallographic Interactions?

AbstractPolymer epitaxy defines the crystallographic orientation of overgrowing polymer. It controls several aspects of the structure of semicrystalline polymers, such as fixed mutual chain orientation, certain crystal structures of polymorphic polymers, and the spatial arrangement of planar backbone molecular chains. Therefore, epitaxial crystallization provides a simple and efficient way to fabricate special structures with improved properties and even introduce new functionality for polymeric materials. Besides achieving structural control of single polymer systems, technical development in the field of structural and morphological manipulation of multiphase and multicomponent polymer systems is another important and challenging issue for the advanced application of polymeric materials.

Mechanical Behavior of the Lamellar Structure in Semi-Crystalline Polymers

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.

Quantification of non-isothermal, multi-phase crystallization of isotactic polypropylene: The influence of cooling rate and pressure

The structure of semi-crystalline polymers is strongly influenced by the conditions applied during processing and is of major importance for the final properties of the product. A method is presented to quantify the effect of thermal and pressure history on the isotropic and quiescent crystallization kinetics of four important structures of polypropylene, i.e. the α-, β-, γ- and mesomorphic phase. The approach is based on nucleation and growth of spherulites during non-isothermal solidification, described by the Schneider rate equations combined with the Komogoroff-Avrami expression for space filling. Using an optimization routine the time-resolved multi-phase structure development is accurately described using crystal phase dependent growth rates and an overall nucleation density, all as function of temperature and pressure. It is shown that the maximum growth rate of the α-, and γ-phase increases with applied pressure, while it decreases for the mesomorphic phase. Addition of β-nucleation agent is interpreted as a secondary nucleation density with a coupled β-phase growth. This complete crystallization kinetics characterization of isotactic polypropylene allows prediction of the multi-phase structure development for a wide range of quiescent processing conditions.

Analysis of the Lamellar Structure of Semicrystalline Polymers by Direct Model Fitting of SAXS Patterns

Small-angle X-ray scattering data of melt-drawn high-density polyethylene (HDPE) have been analyzed by direct model fitting to projections on the drawing direction. On the basis of the assumption of an infinite paracrystalline lattice as proposed by Hosemann and a rectangular density profile, several additional assumptions have been tested. It turned out that to achieve good fits we have to introduce a finiteness of the lattice for the used samples. Transition zones between crystalline and amorphous regions also improve the model but are of less importance. The most suitable model has been tested on data from an annealing and subsequent cooling process. We show that the additional parameters gained in the modeling approach compared to peak analysis or correlation function analysis lead to a better understanding of the structures and processes in the samples.

Water sorption in semicrystalline poly(vinyl alcohol) membranes: In situ characterisation of solvent-induced structural rearrangements

As with polymeric materials, fundamental technological properties are affected by polymer microstructure, the identification and subsequent assessment of structural transformations associated with the presence of solvent in the polymer network are of substantial interest, not least when long-term structural stability is concerned. Solvent-mediated morphological rearrangements are studied on the basis of water vapour sorption into physically crosslinked poly(vinyl alcohol) membranes. In a series of dynamic vapour-phase sorption experiments under well-defined boundary conditions, the semicrystalline polymer structure, as characterised by means of its degree of crystallinity, is analysed, providing information about time-dependent morphological behaviour as well as the spatial distribution of crystallinity across film thickness. Structural information thus obtained is then interpreted in view of the corresponding water uptake of the polymer network. Morphological rearrangements in the presence of solvent seem thus to be governed by two opposing effects. Upon water uptake, stretching of polymer chains may, on the one hand, induce part of the crystalline entities to unfold; at the same time, however, macromolecules equally gain mobility and may hence reorganise into more favourable ordered arrays. When compared to the swift disintegration of crystalline structures proceeding without any appreciable time lag to water diffusion into the film, the solvent-mediated organisation of mobilised polymer chains into three-dimensional ordered arrays involves more protracted realignments. The extent of molecular rearrangements affecting the structural stability of the polymer network could finally be shown to diminish upon repeated sorption and desorption.

Equal‐channel multiangular extrusion of semicrystalline polymers

AbstractPotentialities of a new simple shear‐based scheme of the solid‐phase extrusion of polymers, named the equal channel multiple angular extrusion (ECMAE), for modification of semicrystalline polymer structure have been investigated by the example of low‐ and high‐density polyethylene (LDPE, HDPE), polyamide‐6 (PA‐6), and polytetrafluoroethylene (PTFE). The effects of velocity and extrusion temperature, plastic deformation intensity, and accumulated equivalent plastic strain value on properties of a number of crystallizing polymers have been studied. It has been shown that the highest strength characteristics are attained for the extrusion temperatures of (0.8–0.95) of the melting temperature and deformation velocity of (0.6–1.1) mm s−1. For the ECMAE‐processed specimens, the density, the enthalpy, and the melting temperature have become higher. In the oriented structure of semicrystalline polymers formed by the ECMAE, the lamellae are oriented along the extrudate axis. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers

Colloquium: Laws controlling crystallization and melting in bulk polymers

After the fundamental structure of semicrystalline polymers---platelike crystallites with thicknesses in the nanometer range, embedded in a liquid matrix---was discovered in the late 1950s, attention turned to the mechanism of formation. After intense controversial discussions, an approach put forward by Hoffman and Lauritzen prevailed and was broadly accepted. The picture envisaged by the treatment---platelike crystallites with atomically smooth side faces and a surface occupied by chain folds, growing sideways layer by layer with a secondary nucleation as the rate-determining step---was easy to grasp and yielded simple relationships. The main control parameter is the supercooling below the equilibrium melting point of a macroscopic crystal ${T}_{f}^{\ensuremath{\infty}}$, which determines both the thickness of the crystallites and their lateral growth rate. The impression that the mechanism of polymer crystallization was understood and the issue essentially settled, however, was wrong. Experiments carried out during the last decade on various polymer systems provided new insights which are now completely changing our understanding of such systems. They revealed a number of laws that control polymer crystallization and melting in bulk, showing in particular that the crystal thickness is inversely proportional to the distance to a temperature ${T}_{c}^{\ensuremath{\infty}}$ which is located above the equilibrium melting point, and that crystal growth stops at a temperature ${T}_{\mathrm{zg}}$ which is below ${T}_{f}^{\ensuremath{\infty}}$. Observations indicate that the pathway followed in the growth of polymer crystallites includes an intermediate metastable phase. In a proposed model a thin layer with mesomorphic inner structure forms between the lateral crystal face and the melt. The first step in the growth process is attachment of the coiled chain sequences of the melt onto the mesomorphic layer, which subsequently is transformed into the crystalline state. The transitions between melt, mesomorphic layers, and lamellar crystallites can be described with the aid of a temperature-thickness phase diagram. ${T}_{c}^{\ensuremath{\infty}}$ and ${T}_{\mathrm{zg}}$ are identified with the temperatures of the (hidden) transitions between the mesomorphic and the crystalline phase, and between the liquid and the mesomorphic phase, respectively. Comparing predictions of the model theory with experimental results from small-angle x-ray scattering, optical microscopy, and calorimetry yields in addition to the three equilibrium transition temperatures latent heats of transition and surface free energies.

Effect of atomic force microscope tip geometry on the evaluation of the crystal size of semicrystalline polymers

The objective of this study is the evaluation of the effect of the tip geometry on imaging the structure of semicrystalline polymers at the nanometer scale by atomic force microscopy. The size and habit of crystals in model-like samples of isotactic polypropylene were analyzed using a standard silicon tip and a high-resolution tip with approximate front curvatures of the tip apexes of 1 and 10 nm, respectively. The experimental data demonstrate that the true size of typical polymer crystals, within the investigated range between 5 and 25 nm, can accurately only be evaluated by using the high-resolution tip. The tip of a larger front curvature of 10 nm yielded apparent crystal sizes which were about 5–10 nm larger than that obtained on using the sharper tip.

The unit cell expansion of branched polyethylene as detected by Raman spectroscopy: an experimental and simulation approach

Raman spectroscopy has become a commonly used technique for the study of phase structure in semicrystalline polymers [1–8]. Among them, polyethylene (PE) is one of the more deeply explored materials. However, there are still some remaining questions concerning the effect molecular and structural features in the intensity and position of characteristic Raman signals. The Raman crystallinity band appearing in linear PE at 1415 cm is thought to arise from factor group splitting, where the two PE chains packed in the orthorhombic unit cell cause the CH2 bending vibrational mode to split with a second component appearing at 1440 cm [4, 9, 10]. The other characteristic band appearing at 1460 cm has been shown to arise from superposition of rocking combination modes, which interact with the crystal mode with symmetry B3g [11]. Changes in the crystalline phase, i.e. in the unit cell parameters for example as a result of temperature changes or the inclusion of short branches, could affect this phenomenon resulting on changes in the interchain energy interactions [8, 9]. The shift in the position of the 1415 cm band towards a higher wavenumber observed as crystallinity decreases has been associated with a lower effectiveness of the interchain interaction within the lattice, and hence to lateral disorder in the crystals. Lagaron has recently found a correlation between the shift of this crystalline band and the macroscopic density for a set of PE samples with different molecular architectures [8]. However, the variation did not result in a linear correlation, probably due to the molecular heterogeneity of the selected samples. In this work we carried out a careful analysis of the orthorhombic crystalline band observed by Raman spectroscopy, i.e., the CH2 bending 1415 cm –1 mode, as a function of crystal features obtained from X-Ray scattering (WAXS) and differential scanning calorimetry (DSC), for a series of model ethylene/1-hexene copolymers, with varying comonomer content, obtained in our laboratory from single-site catalyst polymerization (see Table 1). The materials are characterized by having a narrow molecular weight distribution (Mw/Mn=2) and a homogeneous comonomer distribution. The weight average molecular weight (Mw) of the samples ranges between 120 and 300 kg mol. We will interpret the experimental shift observed in terms of the variations in the crystalline orthorhombic structure found in the materials. Moreover, we will compare the experiments with computed results calculated at the local Density Functional Theory (DFT) level. For that purpose, normal modes of vibration were obtained for the orthorhombic cells with different parameters derived from the X-ray data. The splitting of the single-chain vibrational active modes into the bands at 1415 and 1440 cm, can be observed in Fig. 1a for the linear PE sample. The separation between these two bands has been obtained for all polymers studied and listed in Table 1. The exact J. Otegui J. F. Vega (&) S. Martin V. Cruz A. Flores J. Martinez-Salazar Departamento de Fisica Macromolecular, Instituto de Estructura de la Materia, CSIC Serrano 113 bis, Madrid 28006, Spain e-mail: imtv477@iem.cfmac.csic.es

Dual Nature of the Infectious Prion Protein Revealed by High Pressure

Crude brain homogenates of terminally diseased hamsters infected with the 263K strain of scrapie (PrP(Sc)) and purified prion fibrils were heated or pressurized at 800 megapascals and 60 degrees C for 2 h in different buffers and in water. Prion proteins (PrP) were analyzed for their proteinase K resistance in immunoblots and for their infectivity in hamster bioassays. A notable decrease in the proteinase K resistance of unpurified prion proteins, probably because of pressure-induced changes in the protein conformation of native PrP(Sc) or the N-truncated PrP-(27-30), could be demonstrated when pressurized at initially neutral conditions in several buffers and in water but not in a slightly acidic pH. A subsequent 6-7 log(10) reduction of infectious units/g in phosphate-buffered saline buffer, pH 7.4, was found. The proteinase K-resistant core was also not detectable after purification of prions extracted from pressurized samples, confirming pressure effects at the level of the secondary structure of prion proteins. However, opposite results were found after pressurizing purified prions, arguing for the existence of pressure-sensitive beta-structures (PrP(Sc)(DeltaPsen)) and extremely pressure-resistant beta-structures (PrP(Sc)(DeltaPres)). Remarkably, after the first centrifugation step at 540,000 x g during isolation, prions remained proteinase K-resistant when pressurized in all tested buffers and in water. It is known that purified fibrils retain infectivity, but the isolated protein (full and N-truncated) behaved differently from native PrP(Sc) under pressure, suggesting a kind of semicrystalline polymer structure.

The Control of Higher-Order Structure of Semi-Crystalline Polymers by Using Carbon Dioxide

The Control of Higher-Order Structure of Semi-Crystalline Polymers by Using Carbon Dioxide

Deformation of the lamellar structure in semi-crystalline polymers studied by computer simulations

Abstract Yield in a semi-crystalline polymer involves the disruption of the crystalline phase in an irreversible deformation process. In a semi-crystalline polymer, the crystalline lamellar regions are bridged together by inter-lamellar amorphous layers, which act as a loading transfer medium. The deformation of both phases is, therefore, to some extent inter-related. In this work we adopted a continuous mechanic approach (neglecting atomic/molecular interactions) of the lamellar deformation of semi-crystalline polymers in the sense that we simulated the mechanical response of a lamellar structure (two lamellae interconnected by amorphous regions) in a finite element analysis. The use of computer simulations allows studying independently the effect of each relevant morphological parameter on the mechanical response. Several simulations were performed considering isolated variations of the following morphological parameters: i) mechanical behaviour of the amorphous material; ii) thickness of the crystalline regions; iii) length of the amorphous regions; iv) number of amorphous regions connected with a crystalline lamella; v) relative angle between the crystalline and the amorphous regions; vi) mode of loading (tension and compression). The thickness of the crystalline lamellae is evidenced as the most significant factor affecting the tensile response of the lamellar structure, followed by the mechanical behaviour of the amorphous phase. The connection angle between amorphous and crystalline regions and the number of amorphous regions bridging adjacent crystalline lamellae play only a minor role. The length of the amorphous regions has a negligible influence. As expected, the lamellar structure shows also distinct behaviours under distinct loading modes, tensile loading showing the highest stresses.

Impact of silicone-based block copolymer surfactants on the surface and bulk microscopic organization of a biodegradable polymer, poly(epsilon-caprolactone).

Two amphiphilic AB block copolymers, containing a highly compatible poly(epsilon-caprolactone) (PCL) block connected to a poly(dimethylsiloxane) (PDMS) block having a low surface energy, are synthesized and characterized in terms of their dispersion in a presynthesized PCL matrix. X-ray photoelectron spectroscopy, contact angle measurements, atomic force microscopy, and optical microscopy are used to describe the evolution of the surface chemical composition, as well as the surface and bulk morphology of the PCL/copolymer blends as a function of the nature and weight surface free energy and the dispersion of the copolymers in the blends, leading to important modifications of the bulk and the surface morphology. These differences are interpreted in terms of the impact of the block copolymers on the semicrystalline polymer structure and related properties in the prospect of using the surfactants to improve the synthesis of PCL in supercritical CO(2).

EFFECT OF REGRIND ON THE PROPERTIES OF EXTRUDED PE PIPES

In this paper, the mechanical and thermo-oxidative degradation properties of samples made from high density polyethylene (HDPE) pipes containing “regrind” from 5% to 50% have been investigated and compared with the structure changes of these samples. The results of tensile strength tests under constant σ, long and short term internal pressure tests, heat processes measurements, the values of density, Melt Flow Index (MFI), Oxygen Induction Time (OIT) of samples produced from virgin resins were compared with international standarts. It has been observed that the addition of regrind to raw resin in ratios 20–30% gives good results (This is due to the molecular structure of semicrystalline polymer). Since the “regrind” material fills amorphous part, the ratio of amorphous to crystalline parts of the virgin resin determines the amount of the “regrind” to be added. *Current address: Inst. of Physics of the Azerbaijan Academy of Scien., Baku, H. Javid St. 33, Azerbaijan.

Analysis of Lamellar Structure in Semicrystalline Polymers by Studying the Absorption of Water and Ethylene Glycol in Nylons Using Small-Angle Neutron Scattering

Preferential diffusion of deuterated solvents into the amorphous regions of a semicrystalline polymer enhances the contrast between the crystalline and amorphous regions measurable by small-angle neutron scattering. This scattering in nylons from the diffusion of D2O and deuterated ethylene glycol (d-EG) is analyzed by identifying the distinct contribution to scattering from the two amorphous regions, one in the interlamellar spaces and the other outside the lamellar stacks. The central diffuse scattering (Id) is the non-Bragg, liquidlike, or independent scattering, and is attributed to the solvents (D2O/d-EG) in the amorphous domains outside the lamellar stacks. The lamellar scattering (Il) is the interference peak from the lamellae in the stacks and is used to evaluate the distance between the lamellae, the thickness of the interlamellar spaces, and the coherence length of the lamellar stacks. The invariant calculations show that 70%−80% of the lamellar stack is crystalline. About one-third of the amorphous material in a highly crystalline nylon is in the interlamellar space, and two-thirds is outside the lamellar stacks. The thickness of the interlamellar amorphous regions into which solvent molecules diffuse varies from 10 to 60 Å depending on the thermal history and is a major contributor to the observed increase in lamellae spacing. Structural changes in nylon 6 immersed in water are accelerated at 125 °C, and this temperature could be the hydrated-equivalent of the Brill transition observed at 160 °C in dry nylon 6. Water or EG diffuses into the fold surfaces of nylon lamellae at elevated temperatures, and subsequent structural changes are accompanied by hydrolysis of the nylon chains. EG being a stronger solvent reduces the lamellar thickness at elevated temperatures.

The phases of polymers as determined by NMR

Nuclear magnetic resonance (NMR) spectroscopy has been used to study the morphology and dynamics in semicrystalline polymers. Dynamics may be observed through NMR relaxation rates that are sensitive to motions in the 1-10 8 Hz range, or through modulation of anisotropic magnetic interactions, such as the chemical shift and dipole-dipole interactions. Morphological structure may be inferred through NMR measurements of polymer dynamics or investigated directly through studies of the magnetic interactions. Here, we discuss the study of morphological structure in semicrystalline polymers using NMR, and review results on poly(ethylene terephthalate) that address the question of the number of phases in this semicrystalline polymer.

Resolução lamelar num novo microscópio eletrônico de varredura

RESUMO: Trabalhando com elétrons de baixa energia (na faixa de 1keV), o novo microscópio eletrônico de varredura dispensa a etapa de metalização e permite a observação direta da estrutura lamelar de polímeros semicristalinos, sem a necessidade de preparação de amostras. São apresentados exemplos da morfologia lamelar do PVDF em função das condições de processamento e da temperatura de cristalização, em filmes contendo as fases a, b e g. Um outro exemplo revela o crescimento inicial da camada transcristalina que se formou ao longo de uma fibra de polietileno de ultra-alto peso molecular embutida em matriz de polietileno de alta densidade.