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A thermodynamical framework for the solidification of molten polymers and its application to fiber extrusionKannan, Krishna 12 April 2006 (has links)
A thermodynamical framework is presented that describes the solidification of molten polymers to an amorphous as well as to a semicrystalline solid-like state. This framework fits into a general structure developed for materials undergoing a
large class of entropy producing processes. The molten polymers are usually isotropic
in nature and certain polymers crystallize, with the exception of largely atactic polymers,
which solidify to an amorphous solid, to an anisotropic solid. The symmetry of
the crystalline structures in the semicrystalline polymers is dependent upon the thermomechanical
process to which the polymer is subjected to. The framework presented
takes into account that the natural configurations associated with the polymer melt
(associated with the breaking and reforming of the polymer network) and the solid
evolve in addition to the evolving material symmetry associated with these natural
configurations. The functional form of the various primitives such as how the material
stores, dissipates energy and produces entropy are prescribed. Entropy may be
produced by a variety of mechanisms such as conduction, dissipation, solidification,
rearragement of crystalline structures due to annealing and so forth. The manner in
which the natural configurations evolve is dictated by the maximization of the rate
of dissipation. Similarly, the crystallization and glass transition kinetics may be obtained
by maximization of their corresponding entropy productions. The restrictions
placed by the second law of thermodynamics, frame indiference, material symmetry
and incompressibility allows for a class of constitutive equations and the maximization
of the rate of entropy production is invoked to select a constitutive equation from
an allowable class of constitutive equations. Using such an unified thermodynamic
approach, the popular crystallization equations such as Avrami equation and its various
modifications such as Nakamura and Hillier and Price equations are obtained.
The predictions of the model obtained using this framework are compared with the
spinline data for amorphous and semicrystalline polymers.
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Flow-induced crystallization of polybutene-1 and effect of molecular parametersHadinata, Chitiur, chitiurh@yahoo.com.au January 2007 (has links)
There are two main goals of this thesis: to investigate the flow-induced crystallization behaviour of Polybutene-1 (PB-1 samples, and to study the effects of molecular parameters on the crystallization behaviour While flow-induced crystallization is not a new area in polymer research, well-defined experimental methods that allow access to high flow rate range comparable to that encountered in real processing are still lacking. Two types of flow are considered: shear and uniaxial elongational. Regarding the second aim, several molecular parameters considered are: molecular weight, molecular weight distribution, isotacticity, presence of nucleating agents, and copolymer content. For this purpose an array of PB-1 samples were used. It is found that each of these parameters can have significant effect on the crystallization behaviour. Mainly rheological methods were utilized to conduct the flow-induced crystallization experiments. Crystallization onset time is define d from the change in viscosity or other related parameters. The experiments begin with low shear rate range, to ensure that the results are comparable with literature data. In this range we encounter the quasi-quiescent onset time at very small. shear rates, which draws an interesting comparison with another physical parameter, the gel time. Beyond a critical flow rate a decrease in the onset time is seen, and a plateau-and-slope trend is evident for a curve of onset time vs. shear rate. Using a combination of three experimental methods, shear rates ranging from Q0001 - 500 s-1 are successfully achieved, and a good agreement between these methods is observed. Furthermore, a normalization procedure is introduced, which yields temperature-invariant curves for the mentioned range of shear rate. For the uniaxial elongation flow, the Elongational Viscosity Fixture (EVF) is employed, with the strain rate ranging from 0.0001 - 10 s'. A greater reduction in onset time as compared to shear (at the same shear/strain r ate) is observed, and the difference in the onset times for shear and elongation already reaches more than one decade for a flow rate of 10 5. This quantitative comparison is particularly important; since not so many data on elongation-induced crystallization are available in the literature. Finally, the thesis compares several flow induced crystallization models that can be useful as prediction tools and selects one of these models to be compared with the experimental data. A qualitative agreement is found, however, for better quantitative prediction the model still needs to be.
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Quiescent and flow-induced crystallization of poly(lactic acid) / La cristallisation statique et induite par écoulement du poly(acide lactique)Jalali, Amirjalal January 2017 (has links)
Le poly(acide lactique), PLA, est un polymère biocompatible et biodégradable, qui peut être
produit à partir de ressources renouvelables. En conséquence, il a soulevé une attention toute
particulière en tant que remplacement éventuel des polymères à base de pétrole. C’est un
polyester aliphatique ayant des propriétés telles que module élevé, haute résistance,
biocompatibilité et est donc un matériau prometteur pour diverses applications telles que les
implants, l’encapsulation de médicaments et l'emballage. A cause de sa faible température de
transition vitreuse, le PLA a une faible résistance thermique et les applications sont donc
limitées à celles qui ne sont pas associées à des températures élevées. En outre, ce polymère
souffre d'un faible degré de cristallinité. L'augmentation du taux de cristallinité dans de
nombreuses techniques de mise en forme, telles que le moulage par injection, est nécessaire.
Il y a plusieurs façons d'augmenter le niveau de cristallinité du PLA. Ces procédés
comprennent l'utilisation d'agents nucléants, de plastifiants, ou de combinaisons d'agents
plastifiants et de nucléation. La cristallisation du PLA à l'état fondu se présente sous deux
formes cristallines légèrement différentes connues sous les noms α et α'. Cette étude compare
la capacité d'auto-nucléation de ces deux formes cristallines par auto-nucléation. Ceci est
réalisé en comparant les températures de cristallisation lors du refroidissement des
échantillons préalablement cristallisés à diverses températures, puis de nouveau chauffé à une
température dans la plage de fusion partielle du PLA. Dans la deuxième étape, l'effet des
paramètres cinétiques et le poids moléculaire du PLA sur l'efficacité de nucléation des PLA
phases cristallines a été étudié. Cette partie de l’étude ouvre une nouvelle voie pour
comprendre le rôle des modifications cristallines du PLA qui mènent aux conditions optimales
pour la cristallisation du PLA. La mise en forme des polymères implique des contraintes de
cisaillement et d’élongation, ce qui implique une cristallisation induite par l’écoulement et la
solidification qui s’en suit. Les propriétés mécaniques des produits finals dépendent du degré
de cristallisation et de la nature des cristaux formés. Par conséquent, l'optimisation du procédé
nécessite une bonne compréhension de la façon dont l’écoulement influence la cristallisation.
Le type d'écoulement peut jouer un rôle important sur la cristallisation. Par exemple,
l'écoulement élongationnel provoque l’orientation et l’étirement des molécules dans le sens de
l'extension, comme dans le cas de la mise en forme de fibres et le soufflage de film, en aidant
le processus de cristallisation induite par l'écoulement. Une littérature abondante existe sur la
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cristallisation des thermoplastiques classiques induite par l'écoulement. Cela dit, moins
d'attention a été accordée à l'effet de l'écoulement de cisaillement et d'allongement sur la
cristallisation du PLA. Comme étudié dans la dernière partie de ce document, l'effet du poids
moléculaire sur la cristallisation induite par cisaillement du PLA est rapporté. Pour cela, trois
différents PLA à faible, moyen et haut poids moléculaire ont été préparés par réaction
d'hydrolyse. Ensuite, en utilisant un rhéomètre oscillatoire, l’effet du cisaillement sur la
cinétique de cristallisation du PLA a été examiné. / Abstract : Poly(lactic acid), PLA, is a biocompatible and biodegradable polymer that can be produced
from renewable resources. As a result, it has raised particular attention as a potential
replacement for petroleum-based polymers. It is an aliphatic polyester with properties such as
high modulus, high strength, and biocompatibility and is thus a promising material for various
applications such as implants, drug encapsulation, and packaging. In the wake of low glass
transition temperature, PLA has a low heat resistance and its application is limited to those not
associated with high temperatures. In addition, this polymer suffers from a low degree of
crystalinity. Increasing the crystallization rate in many processing operations, such as injection
molding, is required.
So far, many routes have been found to improve the crystallinity of PLA. These methods
include using nucleating agents, plasticizers, and combination of nucleating agents and
plasticizers together. PLA crystallization in the melt state results in two slightly different
crystalline forms known as α and α’forms. This thesis compares the self-nucleation ability of
these two crystal forms by self-nucleation. This is achieved by comparing crystallization
temperatures upon cooling for samples previously crystallized at various temperatures and
then re-heated to a temperature in the partial melting range for PLA. In the second step, we
study the effect of molecular weight of PLA on the nucleation efficiency of PLA crystalline
phases. This part of the investigation opens a new pathway to understand the role of PLA
crystalline phases on the optimal condition for its crystallization kinetics.
Polymer processing operations involve mixed shear and elongational flows and cause polymer
molecules to experience flow-induced crystallization during flow and subsequent
solidification. The mechanical properties of the final products are significantly dependent
upon the degree of crystallization and types of formed crystals. Therefore, optimization of any
polymer process requires a good understanding of how flow influences crystallization. The
type of flow can play a significant role in affecting crystallization. For example, elongational
flow causes molecules to orient and stretch in the direction of extension, as in the case of fiber
spinning and film blowing, helping the process of flow-induced crystallization. An extensive
body of literature exists on flow-induced crystallization of conventional thermoplastics.
Having said that, less attention has been paid to the effect of shear and elongational flow on
the PLA crystallization kinetics. As investigated in the final part of this thesis, the effect of
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molecular weight on the shear-induced crystallization of PLA is reported. For this, low,
medium and high molecular-weight PLAs were prepared from a high molecular weight one by
a hydrolysis reaction. Next, by means of a simple rotational rheometry, effect of the shear flow
was examined on the crystallization kinetics of these three PLAs.
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Mobilité moléculaire dans des systèmes polymères complexes anisotropes et confinés / Molecular dynamics in complex polymer systems : from anisotropy to confinement effectsMonnier, Xavier 03 October 2017 (has links)
L’objet de ce travail est d’étudier l’influence de l’anisotropie structurale, induite lors de la mise en forme d’un Polylactide (PLA), sur les dynamiques moléculaires de la phase amorphe. Deux procédés de mise en oeuvre sont retenus : l’électrofilage et la cristallisation induite par flux. Le premier permet d’aboutir à un système non-cristallin, lorsque le deuxième permet d’aboutir à un système semi-cristallin. Pour chaque système, une étude microstructurale est préalablement réalisée pour mettre en avant l’anisotropie structurale induite lors de la mise en oeuvre. Pour ce faire différentes techniques d’analyses sont utilisées : microscopie optique, microscopie électronique, diffraction des rayons X, calorimétrie à balayage différentielle (DSC) et calorimétrie à balayage rapide (FSC). L’utilisation de la FSC s’avère précieuse. Du fait des vitesses extrêmement rapide (1000 K.s-1) et de la diminution importante de la masse (dizaine de nanogrammes), la transition vitreuse et la cinétique de vieillissement physique sont au préalable étudiées dans le cas d’un PLA amorphe. Il est montré que les vitesses de refroidissement atteignable en FSC permettent d’accélérer les cinétiques de vieillissement physique. Les dynamiques moléculaires sont ensuite étudiées à travers le concept de coopérativité et le phénomène de vieillissement physique. Il est montré que l’orientation préférentielle induite dans le système non-cristallin aboutit à la formation de mésophase qui augmente la coopérativité, autrement dit les interactions intermoléculaires. Dans le cas du système semi-cristallin, les dynamiques moléculaires sont influencées par le couplage amorphe/cristal et le confinement des cristaux, et non pas par l’anisotropie structurale induite avant cristallisation. / The aim of this work is to investigate the molecular dynamics of Polylactide (PLA) subjected to structural anisotropy during its processing. To do so, two experimental set-ups were used: electrospinning and flow induced crystallization. The first one leads to non-crystalline system, while the second one leads to semi-crystalline system. For each system, the microstructure is investigated to highlight the structural anisotropy induced during the processing. Different experimental techniques are used: optical microscopy, electronic microscopy, X-ray diffraction, differential scanning calorimetry (DSC) and fast scanning calorimetry (FSC). FSC proves to be useful. Due to the high scanning rates (1000 K.s-1) and the decrease of the sample mass (few tens of nanogrammes), glass transition and physical aging kinetics are beforehand investigated in the case of a wholly amorphous PLA. It is shown that high cooling rates available by FSC allow to accelerate physical aging kinetics. Molecular dynamics are then investigated through concept of cooperativity and phenomenon of physical aging. It is shown that preferential orientation induced during electrospinning leads to the formation of mesophase, which increase cooperativity, namely the intermolecular interactions. With regard to semi-crystalline system, molecular dynamics are only affected by the coupling between amorphous/crystal and the confinement effect of the crystals, rather than the structural anisotropy induced before the crystallization step.
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Shear-induced crystallization morphology and mechanical property of high density polyethylene in micro-injection moldingLin, X., Caton-Rose, Philip D., Ren, D.Y., Wang, K.S., Coates, Philip D. January 2013 (has links)
No / The advances of the polymer melt flow-induced crystallization behaviour and its influence on mechanical properties of high density polyethylene (HDPE) in micron injection (MI) were studied in the present paper. Analysis of mechanical performance, including yield stress and elongation at break, for samples adopted from different regions in a molded plaque showed that a higher injection speed, a higher mold temperature and a longer cooling time could effectively enhance the yield stress but negatively promoted the ductility. Then, the mechanisms of such variation of mechanical performance and the factors affecting it were investigated by means of differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and polarized light microscopy (PLM). The super high shear rate during cavity feeding in MI molding not only induced a typical three-layered structure but also developed a highly oriented fibrously morphological structure in the skin layer. However, such fully oriented morphology was much negative in the interlayer and even could not be observed in the core layer. The results from SEM and PLM observations indicated that the orientation morphology varied significantly through the plaque's cross-section and thickness of the each layer changed with the process parameters and geometric position, and finally led to variation of the mechanical performance.
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