Physical Cross Links in Amorphous PET, Influence of Cooling Rate and Ageing

A Continuous Cooling Transformation (CCT) procedure can be used to distinguish the initial “state” of the amorphous PET samples produced upon solidification from the melt at different cooling rates. The material frozen at this stage behaves as a rubber when brought above the Tg due to the onset of physical cross links. The rubber is not a stable network, however, since physical cross links may eventually dissolve. Their size distribution, and possibly their number, depend on cooling rate and ageing. Some may be even stable above the glass transition and act as nuclei for further crystallization from the glass. Upon increasing cooling rate, size distribution becomes smaller and stability of …

Polymer solidification under Processing Conditions in quiescent conditions by the Continuous Cooling Transformation approach

Polymeric scaffolds prepared via Thermally Induced Phase Separation (TIPS): Tuning of structure and morphology

Scaffolds suitable for tissue engineering applications were prepared by Thermally Induced Phase Separation (TIPS) starting from a ternary solution PLLA/dioxane/water. The experimental protocol consisted of three consecutive steps, a first quench from the homogeneous solution to an appropriate demixing temperature (within the metastable region), a holding stage for a given residence time and a final quench from the demixing temperature to a low temperature (within the unstable region). A large variety of morphologies, in terms of average pore size and interconnection, were obtained upon modifying the demixing time and temperature, owing to the interplay of nucleation and growth processes dur…

Relating morphology to nanoscale mechanical properties: from crystalline to mesomorphic iPP

Effect of some injection molding processing conditions on weld lines characteristics

The influence of injection and mold temperature as well as holding time on the knit-line characteristics in samples of Nylon 6 obtained by injection molding was analysed. The characteristics of the surface defect seem to be governed by the shrinkage rate and amount in the weld line zone.

Linking structure and mechanical properties via instrumented nanoindentations on well-defined morphology poly(ethylene) Polymer 50 8 1939-1947 2009

Several poly(ethylene) samples with a broad range of morphologies were studied in this work using nanoindentations. The samples had degrees of crystallinity ranging from 30 to100% while their Young’s modulus ranged from few tens of MPa up to several GPa. Experimental conditions for the correct evaluation of Young’s modulus were at first identified, choosing a suitable loading rate in order to minimize viscoelastic effects on the unloading. The force curves, i.e. plots of applied load vs. penetration depth, were then analyzed following two common procedures available in the literature. None of these procedures leads to satisfying results when compared to other experimental techniques. Howeve…

PET/PEN Blends of Industrial Interest as Barrier Materials. Part I. Many-Scale Molecular Modeling of PET/PEN Blends

Mesoscale molecular simulations, based on parameters obtained through atomistic molecular dynamics and Monte Carlo calculations, have been used for modeling and predicting the behavior of PET/PEN blends. Different simulations have been performed in order to study and compare pure homopolymer blends with blends characterized by the presence of PET/PEN block copolymers acting as compatibilizer. A many-scale molecular modeling strategy was devised to evaluate PET/PEN blend characteristics, simulate phase segregation in pure PET/PEN blends, and demonstrate the improvement of miscibility due to the presence of the transesterification reaction products. The behavior of distribution densities and …

Influence of high cooling rates on the structure-properties relationship of dynamic vulcanizates

Transition From Mesomorphic Nodular Nano-Crystals To Monoclinic Lamellae In Isotactic Polypropylene

Crystallization kinetics of a PBT/PET blend according to a Continuous Cooling Transformation (CCT) approach

CRYSTALLIZATION OF PBT/PET BLENDS UNDER LOW AND HIGH COOLING RATES: THERMODYNAMICS AND KINETICS CONSIDERATIONS

PLLA biodegradable scaffolds for angiogenesis via Diffusion Induced Phase Separation (DIPS)

A critical obstacle in tissue engineering is the inability to maintain large masses of living cells upon transfer from the in vitro culture conditions into the host in vivo. Capillaries, and the vascular system, are required to supply essential nutrients, including oxygen, remove waste products and provide a biochemical communication “highway”. For this reason it is mandatory to manufacture an implantable structure where the process of vessel formation – the angiogenesis – can take place. In this work PLLA scaffolds for vascular tissue engineering were produced by dip-coating via Diffusion Induced Phase Separation (DIPS) technique. The scaffolds, with a vessel-like shape, were obtained by p…

The X-ray determination of the amounts of the phases in samples of isotactic poly(propylene) quenched from the melt at different cooling rates

A new procedure for the determination of the amounts of phases in samples of isotactic poly(propylene) quenched at different cooling rates from the same melted polymer is described. According to the procedure, all the patterns corresponding to the same quenching series are simultaneously analyzed for the amounts of phases so that reliable phase fractions relative to the different samples of the series are achieved. The analysis of the results points out that for increasing cooling rates above 30°C/s a remarkable increase of the mesomorphic phase settles, mainly at the expense of the α-monoclinic one.

Phenomenological approach to compare the crystallization kinetics of isotactic polypropylene and polyamide-6 under pressure

Reliable experimental data for semicrystalline polymers crystallized under pressure are supplied on the basis of a model experiment in which drastic solidification conditions are applied. The influence of the pressure and cooling rate on some properties, such as the density and microhardness, and on the product morphology, as investigated with wide-angle X-ray scattering (WAXS), is stressed. Results for isotactic polypropylene (iPP) samples display a lower density and a lower microhardness with increasing pressure over a wide range of cooling rates (from 0.01 to 20 °C/s). Polyamide-6 (PA6) samples exhibit the opposite behavior, with the density and microhardness increasing at higher pressur…

Structure settling of injection moulded products due to ageing of metastable solid phases developed under solidification

Polymer crystallization from a quiescent melt

Combining Atomic Force Microscopy and Depth-Sensing Instruments for the Nanometer-Scale Mechanical Characterization of Soft Matter

Complex materials exhibit a hierarchical structure where a gradient of features on nanometer scale is induced by the synthetic route eventually enhanced by the loading condition. The nanometer scale at which individual components arrange, determining their properties, is a current challenge of mechanical testing. In this work, a survey on nanoindentation is outlined based on the comparison of results obtained by Atomic Force Microscopy and Depth-Sensing Instruments and their combination. An Atomic Force Microscope equipped with a Force Transducer gives indeed the possibility to scan the sample surface in contact mode, thereby allowing one to choose a suitable position for the nanoindentatio…

Influence of high cooling rates on the structure development of dynamic vulcanizates

Measuring mechanical properties of polymers on nanometre and submicrometre scale through nanoidentions

Onset of “precrystalline” phase with cooling rate in amorphous linear polyester series: PET, PTT, PBT

Determination of the crystallization kinetics of a PBT/PET blend in relation to the behaviour of the constituents

Constant stretching rate experiments on low density polyethylene

A simple apparatus for elongational test of molten polymers is presented. Its realiability is demonstrated by means of stress growth in constant stretching rate experiments and relaxation test on a low density polyethylene sample.

Influence of post-processing ageing on mechanical strength of injection molded polypropylene

Tailoring the morphology and stability of nano-crystals in iPP by the condition of primary melt-crystallization & subsequent annealing at elevated temperature

Elastic Moduli Of Well-Defined Polypropylenes Morphologies By Dsi Nanoindentations

Linking structure and nanomechanical properties via instrumented nanoindentations on well-defined and fine-tuned morphology poly(ethylene)

Several poly(ethylene) samples with a broad range of morphologies were studied in this work using nanoindentations. The samples had degrees of crystallinity ranging from 30 to 100% while their Young's modulus ranged from few tens of MPa up to several GPa. Experimental conditions for the correct evaluation of Young's modulus were at first identified, choosing a suitable loading rate in order to minimize viscoelastic effects on the unloading. The force curves, i.e., plots of applied load vs. penetration depth, were then analyzed following two common procedures available in the literature. None of these procedures leads to satisfying results when compared to other experimental techniques. Howe…

Blending PLLA with PLA so as to tune the biodegradabilty of polymeric scaffolds for soft tissue engineering

SOLIDIFICATION BEHAVIOUR OF PBT/PET BLENDS UNDER PROCESSING CONDITIONS

Synthesis of PLLA scaffolds for tissue engineering via phase separation

Solidification of sindiotactic polystyrene (sPS) under drastic conditions by CCT

Relating morphology to nanoscale mechanical properties: from crystalline to mesomorphic iPP

Abstract A nanoindentation technique using an atomic force microscope (AFM) was applied to characterize the mechanical behaviour of several isotactic polypropylene (iPP) samples. The samples were solidified from the melt with a CCT (continuous cooling transformation) procedure spanning a wide range of cooling rates thanks to a fast quenching apparatus developed by the authors. The influence of instrumental parameters on the nanoscale mechanical properties (indentation depth, Young's modulus) shows that for modulus determination one has to rely on simpler methods of force curve analysis based on trace curve alone. Structure homogeneity up to the scale of macroscopic samples used to evaluate …

Mesophase formation in poly(propylene-ran-1-butene) by rapid cooling

Abstract The effect of random insertion of low amount of 1-butene of less than about 11 mol% into the isotactic polypropylene chain on structure formation at non-isothermal crystallization at different rate of cooling was investigated by X-ray scattering, density measurements, and atomic force and polarizing optical microscopy. Emphasis is put on the evaluation of the condition of crystallization for replacement of lamellar crystals by mesomorphic nodules on increasing the cooling rate/supercooling. In the polypropylene homopolymer, mesophase formation occurs on cooling at rates larger about 150–200 K s −1 , while in case of poly(propylene- ran -1-butene) mesophase formation is observed on …

Crystallization kinetics in relation to polymer processing

Phase distribution of quenched samples has been determined by a deconvolution procedure of WAXS spectra in a wide range of cooling rates. The informations collected together with isothermal and DSC results provide a very wide set of data on the crystallization kinetics of polymers relevant which covers conditions encountered in most polymer processing operations. They have been compared with predictions of a non-isothermal crystallization model assuming two independent and parallel crystallization processes competing during solidification.

Studio della solidificazione durante lo stampaggio ad iniezione mediante un test di indentazione

Metastability and Post-forming behaviour of semi-crystalline polymers

Morphology, reorganization and stability of mesomorphic nanocrystals in isotactic polypropylene

Abstract The morphology and thermodynamic stability of crystals of isotactic polypropylene (iPP) were analyzed as a function of the path of crystallization by atomic force microscopy (AFM) and differential scanning calorimetry (DSC). Samples were melt-crystallized at different rates of cooling using a “controlled rapid cooling technique”, and subsequently annealed at elevated temperature. Mesomorphic equi-axed domains with a size less than 20 nm were obtained by fast cooling from the melt at a rate larger about 100 K s−1. These domains stabilize on heating by growing in chain direction and cross-chain direction, to reach a maximum size of about 40–50 nm at a temperature of 433 K, with the q…

Thermal history in processing and post-processing steps influencing the mechanical strength of polypropylene

A method for mapping local mechanical properties on the nanometer scale by AFM

Study of the long-period changes in samples of isotactic poly(propylene) obtained by quenching from the melt and subsequent annealing at different temperatures

The structural modifications induced in samples of isotactic poly(propylene) obtained by quenching from the melt at 100°C/s and subsequently annealed at 40, 60 and 80°C for different annealing times have been studied using simultaneous wide-angle and small-angle X-ray scattering at the synchrotron radiation source of DESY. The occurrance of two different long-period values is demonstrated. These values are related to the mesomorphic phase, existing in the starting quenched material, and to the α-monoclinic one, which settles during the annealing process, respectively.

Polymeric scaffolds prepared via thermally induced phase separation: Tuning of structure and morphology

Scaffolds suitable for tissue engineering applications like dermal reconstruction were prepared by Thermally Induced Phase Separation (TIPS) starting from a ternary solution PLLA/dioxane/water. The experimental protocol consisted of three consecutive steps, a first quench from the homogeneous solution to an appropriate demixing temperature (within the metastable region), a holding stage for a given residence time, and a final quench from the demixing temperature to a low temperature (within the unstable region). A large variety of morphologies, in terms of average pore size and interconnection, were obtained upon modifying the demixing time and temperature, owing to the interplay of nucleat…

Continuous Cooling Transformation, a route for understanding Polymer solidification under Processing Conditions

Solidification of syndiotactic polystyrene by a continuous cooling transformation approach

Syndiotactic polystyrene (sPS) was solidified from the melt under drastic conditions according to a continuous cooling transformation methodology developed by the authors, which covered a cooling rate range spanning from approximately 0.03 to 3000 °C/s. The samples produced, structurally homogeneous across both their thickness and surface, were analyzed by macroscopic methods, such as density, wide-angle X-ray diffraction (WAXD), and microhardness (MH) measurements. The density was strictly related to the phase content, as confirmed by WAXD deconvolution. The peculiar behavior encountered (the density first decreasing and then increasing with the cooling rate) was attributed to the singular…

On the packing–holding flow in the injection molding of thermoplastic polymers

Injection molding tests were performed on a Ny66 resin. Data of mass entering the mold during the packing–holding stage as a function of filling flow rate and holding time are presented. The experimental results are discussed on the basis of a simple model of the packing–holding stage. Only a small part of density increase due to crystallization seams to be compensated by the packing–holding extra flow.

Analysis of the packing stage in injection molding of thermoplastic polymers

Injection molding tests were performed on a Ny 66 resin. Data are presented on the mass entering the mold during the packing-holding stage as a function of filling flow rate and holding time. The experimental results are discussed on the basis of a simple model of the packing-holding stage. Only a small part of the density increase due to crystallization seems to be compensated by extra flow during the packing-holding stage.

On the Use of the Nanoindentation Unloading Curve to Measure the Young's Modulus of Polymers on a Nanometer Scale

Summary: The nanoindentation test is a fundamental tool to assess the link between morphology and mechanical properties. The preliminary results of a more exhaustive study about the applicability to polymers of the most used procedure to determine elastic modulus by indentation are reported in this short communication. A departure of the experimental conditions from the theoretical assumptions and results that give rise to the Oliver and Pharr analysis is shown to occur under a wide range of experimental conditions, with applied loads and penetration depths covering several orders of magnitude and using different indenter geometries. Unloading curves with exponents significantly larger than…

Surface Anisotropy in the Expansion Behavior of Laminated Quasi-Isotropic Composites

Mesure a l'aide d'un extensometre des deformations thermiques superficielles de deux materiaux stratifies resine epoxy-fibre de graphite. Les resultats dependent de la direction de mesure, l'anisotropie superficielle associee est inversement proportionnelle aux dimensions de l'echantillon et proportionnelle au rapport d'anisotropie d'une lamelle isolee. Comparaison avec la theorie des stratifications

PLLA scaffolds for tissue engineering prepared via thermally induced phase separation

Preparation and properties of poly(L-lactic acid) scaffolds by thermally induced phase separation from a ternary polymer-solvent system

Poly(L-lactic acid) (PLLA) foams for tissue engineering were prepared via thermally induced phase separation of a ternary system PLLA/dioxane/tetrahydrofuran (THF) followed by double solvent exchange (water and ethyl alcohol) and drying. An extension to solidification from solution of a previously developed method for solidification from the melt was adopted. The technique is based on a continuous cooling transformation (CCT) approach, consisting in recording the thermal history experienced by rapidly cooled samples and then analyzing the resulting sample morphology. Different foams were produced by changing the relative amount of dioxane and THF in the starting solution while the amount of…

Mechanical Characterization of Polymers on a Nanometer Scale through Nanoindentation. A Study on Pile-up and Viscoelasticity

The analysis of nanoindentation force curves collected on polymers through the common Oliver and Pharr procedure does not lead to a correct evaluation of Young’s modulus. In particular, the estimated elastic modulus is several times larger than the correct one, thus compromising the possibility of a nanomechanical characterization of polymers. Pile-up or viscoelasticity is usually blamed for this failure, and a deep analysis of their influences is attempted in this work. Piling-up can be minimized by indenting on a true nanometer scale, i.e., at penetration depth smaller than 200 nm. On the other side, it is common knowledge that fast indentations minimize the effect of viscoelasticity. How…

The use of the indentation test for studying the solidification behaviour of different semicrystalline polymers during injection moulding

Summary: An in-line method for monitoring the solid-ificationprocess during injection molding of semicrystallinepolymers (demonstrated previously in J. Appl. Polym. Sci.2003, 89, 3713) is based on a simple device, where anadditional ejector pin is pushed on the injection molded partatdifferenttimesduringthesolidificationphase.The‘inden-tation depth profile’, i.e., residual deformation as a functionoftime,wasobtainedandallowedtodeterminetheevolutionof the solidification front in the mold as a function of thecooling time. The present work shows the reliability andthe powerfulness of the aforementioned method for a largevariety of different semicrystalline polymers (PET, PBT,polyamide-6 PA6, isota…

Viscoelastic recovery behavior following Atomic Force Microscope nanoindentation of semi-crystalline poly(ethylene)

The residual imprint left behind by the AFM nanoindentation of polymers has been seldom studied in the past. In this work, the evolution of indentations at room temperature performed on a semicrystalline poly- (ethylene) in a broad range of experimental conditions is presented. The study shows that the recovery after 24 h is substantial, although not complete. Moreover, the dynamics of the recovery process is not seen to depend on the magnitude of the applied load for the nanoindentation, but instead on the rate of the indentation used. This points out that viscoelastic processes are minimized when performing fast nanoindentations, while at low loading rates there seems to be a residual vis…

The use of in-line quantitative analysis to follow polymer processing

In this work it is presented three applications of real time analysis during extrusion process using an optical device developed by our research group, which applies the concepts of light extinction. Monitoring of polymer blends morphology takes place to infer data concerned to dispersed phase size and concentration. The detector also enables information about melting temperature of polymer during extrusion and the level of viscous heating, and the exfoliation step during processing of a polymer-clay nanocomposite.

Effect of pressure on the PVT behaviour of iPP as revealed by dilatometric measurements

Isotactic Poly-propylene samples, previously prepared under known conditions of pressure and cooling rate by means of a special apparatus designed and set-up by the authors, were subjected to several isobaric runs at low cooling and heating rate in a confining fluid dilatometer (by GNOMIX). The effect of the previous thermo-mechanical histories and the effect of pressure in the dilatometry on specific volume of the samples was studied. Results show that the initial specific volume depends upon the previous thermo-mechanical histories, which however cancels out after the first heating run. Moreover the reported dilatometric experimental data support the evidence that an increase of pressure …

Polymer Solidification under Pressure and High Cooling Rates

Abstract Polymer solidification under processing conditions is a complex phenomenon in which the kinetics of flow, high thermal gradients and high pressures determine the product morphology. The study of polymer structure formed under pressure has been mainly made using conventional techniques such as dilatometry and differential scanning calorimetry under isothermal conditions or non isothermal conditions but at cooling rates several orders of magnitude lower than those experienced in industrial processes. A new equipment has been recently developed and improved to study the crystallization of polypropylene when subjected to pressure and cooled rapidly. An experimental apparatus essentiall…

Mapping Mechanical Properties on the Nanometer Scale by Atomic Force Microscopy

Influence of plasticizers and cryogenic grinding on the high-cooling-rate solidification behavior of PBT/PET blends

Two structurally different plasticizers (cyclic and linear) and the effect of cryogenic grinding on the solidification behavior at high cooling rates by a continuous cooling transformation approach of poly(butylene terephthalate)/poly(ethylene terephthalate), PBT/PET, blends are described. The solidification curve (density versus cooling rate) is confirmed as an effective tool to compare the differences in crystallization behavior under conditions mimicking processing. In comparison to the bulky cyclic plasticizer, the linear oligomeric one was found to have a more pronounced influence on the crystallization behavior. A 60/40 by weight PBT/PET blend shows a drop-off of density at ∼50 K/s. I…

The continuous cooling transformation (CCT) as a flexible tool to investigate polymer crystallization under processing conditions

An experimental route for investigating polymer crystallization over a wide range of cooling rates (from 0.01 to 1000◦C/s) and pressures (from 0.1 to 40 MPa) is illustrated, using a method that recalls the approach adopted in metallurgy for studying structure development in metals. Two types of experimental setup were used, namely an apparatus for fast cooling of thin films (100–200 μm thick) at various cooling rates under atmospheric pressure and a device (based on a on-purpose modified injection molding machine) for quenching massive samples (about 1–2 cm3) under hydrostatic pressure fields. In both cases, ex situ characterization experiments were carried out to probe the resulting struct…

Solidification of Syndiotactic Polistyrene (sPS) under Pressure and High Cooling Rate

Isotactic polypropylene solidification under pressure and high cooling rates. A master curve approach

Solidification in industrial processes very often involves flow fields, high thermal gradients and high pressures: the development of a model able to describe the polymer behavior becomes complex. Recently a new equipment has been developed and improved to study the crystallization of polymers when quenched under pressure. An experimental apparatus based on a modified, special injection moulding machine has been employed. Polymer samples can be cooled at a known cooling rate up to 100 °C/s and under a constant pressure up to 40 MPa. Density, Micro Hardness (MH), Wide angle X-ray diffraction (WAXD), and annealing measurements were then used to characterize the obtained sample morphology. Res…

Influence of“controlled processing conditions” on the solidification of iPP, PET and PA6

In this work reliable experimental data for three semicrystalline polymers (iPP, PA6, PET) crystallised under pressure and high cooling rates are supplied. These results were achieved on the basis of a model experiment where drastic controlled solidification conditions are applied. The final objective was to quantify the effect of two typical operating conditions (pressure and cooling rate) on the final properties and morphology of the obtained product. The influence of processing conditions on some macroscopically relevant properties, such as density and micro hardness is stressed, together with the influence of processing conditions on the product morphology, investigated by means of Wide…

Atomic Force Microscope Nanoindentations to Reliably Measure the Young's Modulus of Soft Matter

Relating Texture and Local Mechanical Properties of Semicrystalline Polymers by AFM Nanoindentation

Structural modifications in an irradiated ethylene-vinyl alcohol copolymer

Extraction experiments and calorimetric measurements have been performed, on a commercial ethylene-vinyl alcohol copolymer irradiated in the dose range 0–20 Mrad.

Solidification of sindiotactic polystyrene (sPS) under drastic conditions by Continuous Cooling Transformation (CCT)

Improving surface detection on nanoindentation of compliant materials

Nanoindentation is a versatile tool for monitoring mechanical properties on a local scale. Accurate knowledge of a contact area, and therefore an initial contact, is however necessary for translating the force curve into sample mechanical properties. It is shown that methods for sensing an initial contact by depth sensing instruments (DSI) may be severely in error for compliant materials. With the hardware adopted in this work, the threshold is determined by the elastic modulus; hence the error potentially increases if the material becomes more compliant. A simple method is therefore suggested to determine with accuracy the initial contact on compliant materials whereby the surface contact …

Process-morphology relationships on injection moulding of isotactic polypropylene in standard steel tool and rapid epoxy tooling

The solidification behavior of a PBT/PET blend over a wide range of cooling rate

In recent years, much attention has been paid to the development of high-performance polyester blends, among which blends of polybutylene terephthalate/polyethylene terephthalate (PBT/PET) are expected to exhibit remarkable properties as far as their crystallization behavior is concerned. Through trial and error, appropriate commercial compositions have been chosen which could not be otherwise explained by a suitable interpretation of the mechanisms determining their solidification behavior. The solidification behavior of a 60/40 w/w PBT/PET blend was studied in a wide range of cooling conditions, according to a continuous cooling transformation (CCT) procedure developed previously, aiming …

Competition between α and γ phases in isotactic polypropylene: effects of ethylene content and nucleating agents at different cooling rates

Abstract The influence of ethylene content, nucleating agents and cooling rate upon the formation of γ phase in isotactic polypropylene is investigated. Detailed analysis of wide angle X-ray diffraction shows that some γ phase can appear even in copolymers of very low ethylene content (0.5 mol.%). Differential scanning calorimetry shows a double melting peak. Nucleating agents of different types are found to enhance γ phase crystallization, even in high molecular weight homopolymers. In any of the materials studied the amount of γ phase decreases with increasing cooling rate, going to zero at a cooling rate of about 10°C s−1. We interpret the observations in terms of the kinetics of growth …

Local mechanical properties by Atomic Force Microscopy nanoindentations

The analysis of mechanical properties on a nanometer scale is a useful tool for combining information concerning texture organization obtained by microscopy with the properties of individual components- Moreover, this technique promotes the understanding of the hierarchical arrangement in complex natural materials as well in the case of simpler morphologies arising from industrial processes. Atomic Force Microscopy, AFM, can bridge morphological information, obtained with outstanding resolution, to local mechanical properties. When performing an AFM nanoindentation, the rough force curve, i.e., the plot of the voltage output from the photodiode vs. the voltage applied to the piezo-scanner, …

Some experimental issues of AFM tip blind estimation. The effect of noise and resolution

The convolution of tip shape on sample topography can introduce significant inaccuracy in an AFM image, when the tip radius is comparable to the typical dimension of the sample features to be observed. The blind estimation method allows one to obtain information on the AFM tip through an unknown characterizer sample and thus to perform the deconvolution of the tip shape from an image. When applying the blind estimation method to determine the AFM tip shape, some apparently trivial issues relating to the experimental operating parameters must be taken into account. In this paper, the effects of the operating parameters, e.g., sampling intervals (resolution) and instrumental noise, have been …

Nanoscale mechanical characterization of polymers by atomic force microscopy (AFM) nanoindentations: viscoelastic characterization of a model material

The atomic force microscope (AFM), apart from its conventional use as a microscope, is also used for the characterization of the local mechanical properties of polymers. In fact, the elastic characterization of purely elastic materials using this instrument can be considered as a well-assessed technique while the characterization of the viscoelastic mechanical properties remains the challenge. In particular, one finds the mechanical behavior changing when performing indentations at different loading rates, i.e. on different time scales. Moreover, this apparent viscoelastic behavior can also be due to complex contact mechanics phenomena, with the onset of plasticity and long-term viscoelasti…

PLLA/PLA scaffolds prepared via Thermally Induced Phase Separation (TIPS): tuning of properties and biodegradability

Foams for tissue engineering applications were prepared via thermally induced phase separation (TIPS). Poly-L-Lactic Acid (PLLA) and blends of PLLA with PLA in different proportions were used (100/0, 90/10, 75/25, 50/50, 0/100 PLLA/PLA wt/wt) starting from ternary systems where dioxane was the solvent and water the non-solvent. Morphology was evaluated by Scanning Electron Microscopy (average pore size and interconnection) and the void fraction was measured by means of Hg porosimetry. Foams apparent density was also evaluated (porosity ranges from 87% to 92%). Biodegradability was estimated in a body mimicking fluid. Results show that structure and morphology (in terms of average pore size …

Effect of pressure and high cooling rates on the solidification behaviour of sindiotactic polystryrene (sPS)

A critical study on nanometer scale elastic characterization by nanoindentation

Nanoscale mechanical characterization of polymers by AFM nanoindentations: Critical approach to the elastic characterization

AFM nanoindentations show a dependence of penetration, i.e., the relative motion between the sample and the tip (indenter), on material elastic properties when using the same load. This elationship becomes visible by using of samples being homogeneous down to the scale of nanoindentation. They were prepared from materials covering a broad range of mechanical behavior: from rubbery networks to glassy and semicrystalline polymers. The elastic modulus can be obtained applying Sneddon’s elastic contact mechanics approach. To do this, some calibrations and instrumental features have to be measured accurately. All the polymers tested show that the contact between the tip and the sample is dominat…

Structural and morphological rearrangements in quenched poly(ethylene) by simultaneous SAXS/WAXS

Structure formation by crystallization from the melt in a wide range of cooling rates (0.08-1 000°C/s) in low-density polyethylene has been studied by simultaneous small- and wide-angle X-ray scattering at the synchroton radiation source of DESY. The occurrence of two periodicities, characterized by different angular position of Bragg's maxima, was observed, pointing to the existence of two types of lamellar stacks, associated with two different long period values L 1 and L 2 (L 1 > L 2 ). L 1 depends on the cooling rate, whereas L 2 is almost constant. A comparison with isotactic polypropylene is performed, where a similar phenomenon takes place. While in the case of i-PP a definite correl…

Injection moulding of thin and thick iPP parts in epoxy resin and steel moulds: a comparative study on properties development

Influence of plasticizers suggests role of topology in polymer solidification at high cooling rates

Although solidification in processing deter- mines short- and long-term properties, methods for under- standing polymer crystallization mostly rely on real time experiments. Their evidences being drawn on time scales farther apart with respect to those experienced in process- ing. Nor significant outcomes have been so far drawn with approaches mimicking the typical processing times, the Continuous Cooling Transformation methods. Use of these techniques has indeed been limited to a heuristic interpretation of the structure developed under extreme solidification conditions without suggesting alternative routes to the understanding or even clues to the many open questions on polymer crystalliz…

Structure Distribution in Inhomogenous Samples by nanoindentation techniques: DSI vs AFM

Crystallization kinetics of iPP: Influence of operating conditions and molecular parameters

An analysis of the crystallization kinetics of different grades of isotactic polypropylene (iPP) is here presented. To describe the crystallization kinetics as a function of molecular and operating parameters, the methodological path followed was the preparation of quenched samples of known cooling histories, calorimetric crystallization isotherms tests, differential scanning calorimetry cooling ramps, wide angle X-ray diffraction (WAXD) measurements, and density determination. The WAXD analysis performed on the quenched iPP samples confirmed that during the fast cooling at least a crystalline structure and a mesomorphic one form. The diffractograms were analyzed by a deconvolution procedur…

Influence of morphology and chemical structure on the inverse response of polypropylene to gamma radiation under vacuum

Abstract In this work the influence of the chemical structure and of the morphology on the gamma-radiation effects on polypropylene based polymers is studied on the basis of a previously discussed kinetic model [1] . For this aim an isotactic polypropylene and a random ethylene–propylene copolymer were irradiated under vacuum at one dose rate and several absorbed doses after well defined solidification conditions. We show that the model is reliable varying both the chemical structure and the morphology of the polypropylene based polymer. An inversion of the response of the material to gamma radiation under vacuum is always observed, and the inversion conditions depend on the irradiation par…

The solidification behavior of a PBT/PET blend over a wide range of cooling rate

In recent years, much attention has been paid to the development of high-performance polyester blends, among which blends of polybutylene terephtha- late/polyethylene terephthalate (PBT/PET) are expected to exhibit remarkable prop- erties as far as their crystallization behavior is concerned. Through trial and error, appropriate commercial compositions have been chosen which could not be otherwise explained by a suitable interpretation of the mechanisms determining their solidifica- tion behavior. The solidification behavior of a 60/40 w/w PBT/PET blend was studied in a wide range of cooling conditions, according to a continuous cooling transforma- tion (CCT) procedure developed previously,…

Structure development in poly(ethylene terephthalate) quenched from the melt at high cooling rates: X-ray scattering and microhardness study

The structure and microhardness of poly(ethylene terephthalate) (PET) cooled from the melt, using a wide range of cooling rates, was studied. PET thin films rapidly cooled from the melt (cooling rates larger than 5°C/s) show a continuous variation of structure and properties depending on cooling rate. Results highlight differences in the micro-mechanical properties of the glass suggesting the occurrence of amorphous structures with different degrees of internal chain ordering. The comparative X-ray scattering study of two glassy PET samples (7500 and 17°C/s) reveals the occurrence of frozen-in electron density states giving rise to an excess of scattering for the amorphous sample solidified…

Orientation and Crystallinity Measurements in Film Casting Products

Film casting experiments were carried out with iPP under processing conditions causing the crystallization process to occur under orienting flow. Draw ratio and cooling rates were changed by varying mass flow rates and die thickness. The effect of processing conditions on film crystallinity was investigated by combining WAXS and FT-IR transmission methods, while orientation of both phases was measured by IR dichroism (according to Fraser's method) and successfully compared to birefringence measurements on final films. Crystallinity appears to be almost insensitive to draw ratio and cooling rate. Moreover the crystallinity profile turned out to be also constant along the transverse film dire…

Crystallization behaviour of PBT-rich PBT/PET blends according to a Continuous Cooling Transformation (CCT) protocol

Indentation test as a tool for monitoring the solidification process during injection molding

An inline method for monitoring the solidification process during the injection molding of semicrystalline polymers is demonstrated. The method has been applied to various poly(ethylene terephthalate) (PET) and poly(buthylene terephthalate) (PBT) samples. The technique is based on a simple device by which an additional ejector pin is pushed onto the injection-molded part with a fixed force at different times during the solidification phase while the mold remains closed. The residual deformation (the so-called indentation depth) due to the applied load is measured offline after ejection. By the performance of indentation at different times during the cooling phase, an indentation depth profi…

Role of thermal history on quiescent cold crystallization of PET

8 pags., 9 figs.

Ageing of isotactic polypropylene due to morphology evolution, experimental limitations of realtime density measurements with a gradient column

Abstract Ageing in crystalline polymers is responsible for the deterioration of physical properties leading, for example, to a decrease in toughness and to dimensional changes that are to some extent responsible for warpage and scrap production in injection molding. Since, it depends on the mutual transformation of stable and metastable phases, being always related to changes in morphological organization, it is here preferred to call it ‘Morphological ageing’. Although, one would expect the ageing regime to be determined by the complex morphology with amorphous phases of different mobility and eventually multiple crystalline phases, transformed into each other at an associated transition, …

Non-isothermal crystallization kinetics of PET

The crystallization kinetics of poly(ethylene terephthalate) was studied using constant cooling rate, isothermal and quenching experiments. A non-isothermal crystallization kinetics equation based on a single mechanism was used to analyze the data. Different mechanisms of crystallization at low, intermediate, and high cooling rates were hypothesized based on deviation of the experimental data from the single mechanism model.

In-line assessment of the melting behaviour during polymer extrusion

Accurately evaluating Young’s modulus of polymers through nanoindentations: a phenomenological correction factor to the Oliver and Pharr procedure

The Oliver and Pharr [J. Mater. Res. 7, 1564 (1992)] procedure is a widely used tool to analyze nanoindentation force curves obtained on metals or ceramics. Its application to polymers is, however, difficult, as Young’s moduli are commonly overestimated mainly because of viscoelastic effects and pileup. However, polymers spanning a large range of morphologies have been used in this work to introduce a phenomenological correction factor. It depends on indenter geometry: sets of calibration indentations have to be performed on some polymers with known elastic moduli to characterize each indenter.

Simulating Morphology development in Polymer Processing: Continuous Cooling Transformation of sPS

An experimental methodology to study polymer crystallization under processing conditions. The influence of high cooling rates

Abstract A new experimental route for investigating polymer crystallization under very high cooling rates (up to 2000°C/s) is described. A complete and exhaustive description of the apparatus employed for preparing thin quenched samples (100– 200 μm thick) is reported, the cooling mechanism and the temperature distribution across sample thickness is also analysed, showing that the final structure is determined only by the thermal history imposed by the fast quench apparatus. Details concerning the characterization techniques used to probe the final structure are reported, including density measurements and wide angle X-ray diffraction patterns. Experimental results concerning isotactic poly…

Wide-range cooling characteristics of a selected isotactic polypropylene

Abstract An increased knowledge of polymer crystallization kinetics and its effect on the crystalline structure is of particular importance at high cooling rates. The aims of this study were to explore the range of utility and complementarity of various methods (densitometry, microscopy, wide-angle x-ray diffraction [WAXD], and small-angle light scattering [SALS]) on the characterization of the crystalline structure of a high purity isotactic polypropylene (PP) in a wider cooling range than previously obtainable and to identify characteristic ranges of the structure as a function of cooling rate. High cooling rates, ranging to nearly 1000°C/sec, were obtained using a special quench device. …

Crystallization of polymer melts under fast cooling. II. High-purity iPP

SYNOPSIS Samples of a high-purity isotactic polypropylene (iPP) were quenched from the melt so as to monitor cooling history. A continuous variation of morphology and crystal structure was obtained with cooling rate. This is discussed in relation to sample thermal history evidencing that cooling history relevant to quenched samples is in the neighborhood of 90°C. In particular the samples are essentially mesomorphic when at this temperature cooling rates larger than 80°C/s were adopted, while below a few tens of °C/s only a­ monocline form is obtained. Densities of quenched samples were compared with predictions of an isokinetic extrapolation of Avrami model of polymer crystallization kinet…

The use of Diffusion Induced Phase Separation (DIPS) technique for the preparation of biodegradable scaffolds for angiogenesis

Crystallization Behaviour at High Cooling Rates of Two Polypropylenes

Phase distribution of quenched samples of two isotactic polypropylenes, having different molecular weight distributions, was evaluated by a deconvolution procedure of WAXD spectra. The dependence on cooling rate of the two resins shows the low molecular weights rich polymer is characterized by a faster kinetics with an α-monoclinic to mesomorphic transition taking place at higher cooling rates.

Phase Transitions in Prequenched Mesomorphic Isotactic Polypropylene during Heating and Annealing Processes As Revealed by Simultaneous Synchrotron SAXS and WAXD Technique

Time-resolved simultaneous synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) technique was used to investigate the phase transitions in prequenched mesomorphic isotactic polypropylene (iPP) samples during heating and annealing processes, respectively. For the heating process, it is shown that the mesomorphic-to-monoclinic phase transition is relatively faster for the mesomorphic iPP sample obtained with the high quenching rate than that with the low quenching rate. For the former, the stability of α-monoclinic crystals formed during heating is relatively higher. As for the annealing process, WAXD and SAXS data illustrate that the higher the annealing te…

Analysis of the crystallization behaviour of PBT-rich PBT/PET blends under processing conditions

Among the high‐performance polyesters blends PBT/PET blends are expected to exhibit remarkable properties as far as the crystallization behaviour is concerned. The solidification behaviour of a 60/40 w/w PBT/PET blend was studied in a wide range of cooling conditions, according to a Continuous Cooling Transformation (CCT) procedure developed by the authors, aiming to emulate the typical conditions encountered in polymer processing. A set of several samples characterized by an homogeneous structure was prepared by solidification from the melt through spray cooling, and the resulting structure and properties were evaluated by density, Micro Hardness (MH), Wide Angle X‐ray Diffraction (WAXD) m…

The use of master curves to describe the simultaneous effect of cooling rate and pressure on polymer crystallization

In a previous work a master-curve approach was applied to experimental density data to explain isotactic polypropylene (iPP) behaviour under pressure and high cooling rates. Suitable samples were prepared by solidification from the melt under various cooling rate and pressure conditions with the help of a special apparatus based on a modified injection moulding machine. The approach here reported is more general than the case study previously shown, and is suitable to be applied to several materials and for different measures related to crystalline content. The proposed simple model is able to predict successfully the final polymer properties (density, micro-hardness, crystallinity) by supe…

Nanoscale mechanical characterization of polymers by atomic force microscopy (AFM) nanoindentations: Viscoelastic characterization on a model material

The Atomic Force Microscope (AFM), apart form its conventional use as a microscope, is also used for the characterization of the local mechanical properties of polymers. In fact, the elastic characterization of purely elastic materials using this instrument can be considered as a well assessed technique while the challenge remains the characterization of the viscoelastic mechanical properties. In particular, one finds the mechanical behavior changing when performing indentations at different loading rates, i.e., on different time scales. Moreover, this apparent viscoelastic behavior can also be due to complex contact mechanics phenomena, with the onset of plasticity and long-term viscoelast…

Many-scale molecular modeling of PET/PEN blends

SAXS/WAXS study of the annealing process in quenched samples of isotactic poly(propylene)

The structural rearrangement in samples of quenched isotactic poly(propylene) (iPP) submitted to different annealing treatments has been studied using simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS) at the synchrotron radiation source of DESY, Hamburg. From a quantitative analysis of the WAXS profiles the values of the α-monoclinic, mesomorphic, and amorphous mass fractions coexisting in the material were determined. It is demonstrated that the SAXS patterns were characterized by two different long-period values that are attributed to α- and mesomorphic periodicity, respectively. The related α- and mesomorphic volume phase fractions, calculated from the analysis of the SAXS …

Comparison of thin-wall injection moulding of isotactic polypropylene in standard steel tool and rapid epoxy tooling

Nanocrystalline morphology of some linear polyesters and some of their blends when quenched to amorphous

Mechanical properties of the nanometer scale pre-crystalline order of a poly (ethylene terepthalate) / poly (ethylene naphthalene) blend

A previous study carried out on PET has shown that this polymer undergoes a continuous structural modification over a wide cooling rate interval when solidified from the melt[1] assuming a semi-crystalline structure below 2 K s 1 and a completely amorphous one above 100 K s 1. Most important was the existence of a state of intermediate order between the above cooling rates which was evidenced by the absence of crystalline reflections in the WAXS patterns and the occurrence of SAXS maxima[2] and exothermic peak areas (DSC) in the cooling rate range above 2 K s 1. Microhardness (MH) measurements revealed that this phase affects the mechanical properties[3] plausible if one thinks of crystalli…