Crystallinity Gradients Research Articles

Strain-induced crystallisation of reinforced elastomers using surface calorimetry

The crystallinity of rubbers which are amenable to crystallisation upon stretching is classically measured using X-ray diffraction (XRD) techniques. Recently, an alternative method for evaluating the crystallinity in unfilled natural rubber (NR) during stretching was proposed. This method is based on surface calorimetry principles (Le Cam, 2018) and was validated by XRD measurements in Le Cam et al. (2018). For reinforced rubber composites, the calorimetric response is not only due to the elastic couplings and the crystallisation/melting process, but also due to the intrinsic dissipation and additional viscoelastic dissipation resulting from the presence of fillers. This makes estimating the crystallinity of carbon black (CB) reinforced rubber compounds from calorimetric techniques typically more challenging. In this paper, however, the calorimetric approach for measuring crystallinity for CB reinforced rubber compounds was investigated. Crystallinity measurements were performed using both an unfilled and three CB reinforced NR compounds. A comparative study was performed using the XRD technique on the same four compounds. The results confirmed the appropriateness of the calorimetric technique for determining the crystallinity in the unfilled NR. The combination of the XRD and the calorimetric techniques allowed the different heat sources under strain to be identified and therefore, the thermal energy associated with crystallisation to be determined. This was done initially using one of the CB reinforced NR compounds and validated using the two other CB reinforced NR compounds. The results show reasonable correlation between the crystallinity measured using the novel calorimetric technique and the classical XRD technique for CB reinforced NR compounds. The development of this methodology has potentially significant impact. The calorimetric approach does not require the measurement of the mechanical response during testing. This is useful for measuring crystallinity gradients from infrared thermography coupled with full kinematic field measurements, typically at crack tips, where estimating the local mechanical response is very difficult or inaccurate.

A numerical and experimental identification of crystallinity gradients in carbon fiber reinforced thermoplastic composites obtained by laser assisted filament winding

The presence of temperature and crystallinity gradients in carbon fiber–reinforced PolyEtherEtherKetone (PEEK) composite laminates, produced via laser-assisted tape placement, is investigated in this paper. The manufacturing process takes place with high deposition speed and using localized laser source for heating, therefore enhancing the formation of temperature and crystallinity gradients through the laminate thickness. A previously validated thermal model coupled with a non-isothermal crystallinity model and a fusion model are used to simulate the temperature and crystallinity distributions through the laminate thickness. The results from the model are correlated with Dynamic Mechanical Analysis (DMA) tests and Differential Scanning Calorimetry (DSC) tests since a crystallinity gradient is difficult to monitor experimentally. The simulated gradients suggested the presence of an amorphous layer between two consecutive plies and an increase in the crystallinity through the material’s thickness. This observation is correlated with the behavior reported for the semi-crystalline laminates during the DMA test, where the modulus drops abruptly during the glass transition, a typical behavior for an amorphous material.

Engineering Crystallinity Gradients for Tailored CaO2 Nanostructures: Enabling Alkalinity-Reinforced Anticancer Activity with Minimized Ca2+/H2O2 Production.

CaO2 nanoparticles (CNPs) can produce toxic Ca2+ and H2O2 under acidic pH, which accounts for their intrinsic anticancer activity but at the same time raises safety concerns upon systemic exposure. Simultaneously realizing minimized Ca2+/H2O2 production and enhanced anticancer activity poses a dilemma. Herein, we introduce a "crystallinity gradient-based selective etching" (CGSE) strategy, which is realized by creating a crystallinity gradient in a CNP formed by self-assembled nanocrystals. The nanocrystals distributed in the outer layer have a higher crystallinity and thus are chemically more robust than those distributed in the inner layer, which can be selectively etched. CGSE not only leads to CNPs with tailored single- and double-shell hollow structures and metal-doped compositions but more surprisingly enables significantly enhanced anticancer activity as well as tumor growth inhibition under limited Ca2+/H2O2 production, which is attributed to an alkalinity-reinforced lysosome-dependent cell death pathway.

Non-uniform crystallinity and temperature distribution during adjacent laser-assisted tape winding process of carbon/PA12 pipes

The non-uniform temperature and crystallinity distributions present in carbon fiber–reinforced PA12 composite pipes, produced via laser-assisted tape winding (LATW), are investigated in this paper. The width of the laser source is usually larger than the substrate width which causes multiple heating and cooling of some regions of the (neighboring) substrate and hence temperature and crystallinity gradients during the adjacent hoop winding. A kinematic-optical-thermal (KOT) model coupled with a non-isothermal crystallinity model is developed to capture the transient temperature and crystallinity distributions for growing substrate thickness and width. The predicted temperature trends are validated with thermocouple and thermal camera measurements. The substrate temperature varies in the width direction up to 52%. This will lead to extra polymer remelting and possible degradation. The maximum variation of the crystallinity degree across the width is found to be 270% which shows agreement with the trend of the measured crystallinity degree. It is found that a more realistic description of the melting behavior of the matrix is needed to obtain a more accurate prediction of the crystallinity distribution.

A method to reveal bulk and surface crystallinity of Polypropylene by FTIR spectroscopy - Suitable for fibers and nonwovens

The crystallinity of polymers is usually determined via Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD) or density measurements. Even though DSC is an easy and fast way to determine crystallinity, it suffers of several disadvantages in the case of fibers and nonwovens. In DSC studies, the sample preparation of fiber material may be difficult, results are not calibration free and heat is introduced in the measuring process, which might lead to transitions in instable crystal structures in the polymer. XRD is able to overcome these problems but is also time consuming and complicated in terms of sample preparation and evaluation of the measurements. Furthermore, it is not possible to determine surface crystallinity or crystallinity gradients inside the specimen with both methods.Based on approaches found in literature, a method was developed which is capable of measuring the degree of crystallinity in the bulk and at the surface of Polypropylene via Fourier transformed infrared spectroscopy. A broad set of calibration samples with a wide range of degrees of crystallinity but without a crystallinity gradient was produced by cooling samples with cooling rates from 1 to 3500 K min−1. The degree of crystallinity was then determined from the ratio of the maximum peak height at 974 cm−1 and the maximum peak height at 998 cm−1 multiplied by 61.4%.Consecutively, the transferability of the method to the measurement of fibers and nonwovens is demonstrated and verified via procedures which are already well described in literature. This method has an excellent reproducibility, spatial resolution and is capable of determining the degree of crystallinity in less than 5 min. Furthermore, this method is easy to implement and does not require further sample preparation.

The role of magma mixing/mingling and cumulate melting in the Neapolitan Yellow Tuff caldera-forming eruption (Campi Flegrei, Southern Italy)

Understanding the mechanisms responsible for the generation of chemical gradients in high-volume ignimbrites is key to retrieve information on the processes that control the maturation and eruption of large silicic magmatic reservoirs. Over the last 60 ky, two large ignimbrites showing remarkable zoning were emplaced during caldera-forming eruptions at Campi Flegrei (i.e., Campanian Ignimbrite, CI, ~ 39 ka and Neapolitan Yellow Tuff, NYT, ~ 15 ka). While the CI displays linear compositional, thermal and crystallinity gradients, the NYT is a more complex ignimbrite characterized by crystal-poor magmas ranging in composition from trachy-andesites to phonolites. By combining major and trace element compositions of matrix glasses and mineral phases from juvenile clasts located at different stratigraphic heights along the NYT pyroclastic sequence, we interpret such compositional gradients as the result of mixing/mingling between three different magmas: (1) a resident evolved magma showing geochemical characteristics of a melt extracted from a cumulate mush dominated by clinopyroxene, plagioclase and oxides with minor sanidine and biotite; (2) a hotter and more mafic magma from recharge providing high-An plagioclase and high-Mg clinopyroxene crystals and (3) a compositionally intermediate magma derived from remelting of low temperature mineral phases (i.e., sanidine and biotite) within the cumulate crystal mush. We suggest that the presence of a refractory crystal mush, as documented by the occurrence of abundant crystal clots containing clinopyroxene, plagioclase and oxides, is the main reason for the lack of erupted crystal-rich material in the NYT. A comparison between the NYT and the CI, characterized by both crystal-poor extracted melts and crystal-rich magmas representing remobilized portions of a “mature” (i.e., sanidine dominated) cumulate residue, allows evaluation of the capability of crystal mushes of becoming eruptible upon recharge.

A new PvT device for high performance thermoplastics: Heat transfer analysis and crystallization kinetics identification

The quality of thermoplastic parts strongly depends on their thermal history during processing. Heat transfer modelling requires accurate knowledge of thermophysical properties and crystallization kinetics in conditions representative of the forming process. In this work, we present a new PvT apparatus and associated method to identify the crystallization kinetics under pressure. The PvT-xT mould was designed for high performance thermoplastics: high temperature (up to 400 °C), high cooling rate (up to 200 K/min) and very high pressure (up to 200 MPa). Specific volume measurements were performed at a low cooling rate to avoid a thermal gradient. The crystallization kinetics under pressure can be identified for a wide range of cooling rates by an inverse method taking into account the thermal and crystallinity gradients. Since identification is based on volume variations, the proposed methodology is non-intrusive. Furthermore, the enthalpy released by the crystallization was measured during the experiment by a heat flux sensor located in the moulding cavity.

A New PvT Device for the Thermoplastics Characterization in Extreme Thermal Conditions

In this work, we present an apparatus associated to a methodology that is able to determine simultaneously and according to temperature (up to 400°C) the specific volume (up to 200MPa), the thermal conductivity and the temperature function of the crystallization kinetics. The PvT-XT is a home-built device that is able to impose and quantify 1D heat transfer through the radius of a sample. This apparatus controls the applied pressure on the sample while measuring its volume variations. The associated moving boundary model takes into account the temperature and crystallinity gradients. Specific volume is determined from direct measurement whereas inverse methods are used to estimate the thermal conductivity and the crystallization kinetics (with cooling rates up to 200K/min). Specific volume measurements are compared with literature results and exhibit a very good agreement. Thermal conductivity identified in the present paper is also very close to literatures values. Finally identification of kinetic function values is consistent with previous studies.

Cell behaviour in new poly(l-lactic acid) films with crystallinity gradients

We report a new facile method for the development of crystallinity gradients in poly(l-lactic acid) films, based on the gradual dipping of amorphous PLLA samples in a hot bath. Isothermal cold crystallization studies were performed using DSC and optical microscopy in order to find adequate conditions to generate crystallinity gradients in the PLLA films. Cellular response along PLLA samples with graded spherulitic developments was tested: cell adhesion is higher in more crystalline zones due to highly flattened appearance of the cells in regions densely populated with spherulites, but cell density was found to be higher in the more amorphous side.

Dimensional stability of single‐site ethylene copolymers in rotational molding

AbstractThe dimensional stability of ethylene copolymers in rotational molding was studied by comparing the warpage observed for a series of conventional and single‐site catalyzed ethylene copolymers. Bench‐scale molding trials were carried out under controlled molding conditions. The rapid cooling of the mold using a water spray resulted in greater warpage. Under such conditions, molded parts made using the single‐site resins showed less warpage compared to the Ziegler‐catalyzed copolymers with otherwise comparable densities. The Ziegler‐catalyzed copolymers were characterized by a faster crystallization rate, and were shown to generate larger crystallinity gradients through the part thickness during the cooling process. Second to temperature gradients, crystallinity gradients are a leading cause for the development of residual stresses and causing warpage. Differences in the crystallization rates between single‐site and Ziegler‐catalyzed copolymers are discussed based on their intermolecular comonomer distributions. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Comparison of vibration and hot‐tool thermoplastic weld morphologies

AbstractBecause of very different heating rates in hot‐tool and vibration welding, and the higher weld pressures used in vibration welding inducing more squeeze flow, the weld zones in these two processes see very different flows and cooling rates, resulting in different morphologies. The weld morphologies of bisphenol‐A polycarbonate (PC) and poly(butylene terephthalate) (PBT) for these two processes are discussed in relation to these differences. The thickness of the heat‐affected zone (HAZ) in hot‐tool welds increases with the melt time; this zone is thicker than in vibration welds. The HAZ thickness in hot‐tool welds increases from the center toward the edges. The HAZ thickness is more uniform in vibration welds. Hot‐tool welds of PC have large numbers of bubbles around the central plane; the bubble size increases from the center to the edges. PC vibration welds do not have bubbles except near the edges. Both hot‐tool and vibration welds of PBT do not have bubbles. The morphology of the HAZ in PBT is very different in hot‐tool and vibration welds. In hot‐tool welds, the resolidified material consists of a sandwich structure in which two thin layers with very small crystallites surround a thicker central layer in which the spherulites are almost as large as in the original molded material. In vibration welds, the HAZ has large crystallinity gradients across the weld zone as well as squeeze‐flow induced distortion of the small spherulites.

A Simulation Approach to Crystallinity Gradients Developed in the Injection Molding Process of Slowly Crystallizing Polymers

A modified three-dimensional (3D) rectilinear flow model is considered to study and analyze crystallinity gradients of slowly crystallizing polymers, such as syndiotactic polystyrene (s-PS) and poly (p-phenylene sulfide) (PPS), developed during the injection molding process. A slit mold is used. Highly efficient computational procedures are achieved through a specially designed order-two parallel splitting formula. Detailed three dimensional crystallinity distribution, together with the temperature, pressure and velocity profiles, is predicted through the simulation and compared with experimental results with satisfaction. The computational method developed can be further generalized for more complicated flow model problems in irregular injection molds.

A Simulation Approach to Crystallinity Gradients Developed in the Injection Molding Process of Slowly Crystallizing Polymers

A Simulation Approach to Crystallinity Gradients Developed in the Injection Molding Process of Slowly Crystallizing Polymers

A simulation of crystallinity gradients developed in slowly crystallizing injection molded polymers via parallel splitting

In this study, a three‐dimensional (3D) flow model is used to approximate the crystallinity gradients of slowly crystallizing polymers developed in the injection molding process. A generalized second order parallel splitting formula is constructed to achieve both the accuracy and efficiency of the computation. Calculated values of flow‐wise (flow‐thickness plane) and width‐wise (width‐thickness plane) crystallinity distributions are obtained and compared with experimental results. The structure‐oriented simulation method developed is not only capable of describing moldability parameters, but is also able to predict the characteristics of ultimate properties of the final products.

A structure oriented model to simulate the shear induced crystallization in injection moulded polymers: a Lagrangian approach

In this work a Lagrangian approach was used to simulate the crystallinity gradients in injection moulded PPS. This was done through the use of markers whose thermomechanical history was traced as they moved in the cavity. The results of the simulation are compared with our earlier experimental observations. It has been found that there is a good quantitative agreement between the experimental and calculated values of gapwise (NDTD plane) and lengthwise (FDND plane) crystallinity distribution. The calculated results for morphological variables such as the thickness of crystalline layers and their overall span agreed well with the experimentally measured values.

Development of free-edge effect during processing of semicrystalline thermoplastic composites

The residual thermal stresses developed during processing of fiber-reinforced semicrystalline thermoplastic composites are strongly affected by variations in temperature and degree of crystallinity within a laminate. A model which enables the prediction of the temperature and volume fraction crystallinity distributions over the cross-sectional area of these laminates during processing from the melt is presented. In this study, emphasis is on the prediction of temperature and crystallinity gradients in the vicinity of free edges. Results are shown for the case of unidirectional APC-2 laminates for different surface cooling rates. 19 refs.

Attenuated total reflectance Fourier transform infra-red studies of crystalline-amorphous content on polyethylene surfaces

Intensity ratios of the bands due to CH 2 bending and rocking normal vibrations, used as a measure of the surface crystallinity in polyethylene, are examined by attenuated total reflectance Fourier transform infra-red (ATR FT i.r.) spectroscopy. This study shows that the ATR optical dispersions and the orientation of surface crystallites have a substantial effect on the crystallinity measurements. It appears that improper analysis of strong, overlapping i.r. bands in the ATR spectra may generate systematic errors. While the optical effects associated with ATR measurements can be accounted for and corrected by the use of the Kramers-Kronig transform and Fresnel's equation, determination of the crystallinity gradients and orientation of the crystallites requires detailed polarization studies and theoretical considerations.

Computer simulations of crystallinity gradients developed in injection molding of slowly crystallizing polymers

AbstractA model is developed to simulate the crystallinity gradients developed in injection molding of slowly crystallizing polymers. In this model, effects of nonisothermal and stress‐induced crystallization kinetics are taken into account through phenomenological relationships. Computer simulations included calculations of the temperature, velocity, and pressure distributions as well as two dimensional crystallinity distributions in the final products. In addition, effects of various processing conditions: mold temperature, injection flow rate, and holding time are also included in the calculations. The crystallinity gradients obtained through computer simulations agree with the experimental results obtained with poly (p‐phenylene sulfide) under a variety of processing conditions.