Polymer crystallization and reorganization during heating: Simulation with the sadler/gilmer model

Polymer crystallization and melting under nonisothermal conditions are investigated by means of an extended version of the two-dimensional Sadler / Gilmer pinning model. In particular, the case of a constant heating rate is simulated. Lamellar thickening and eventual melting are studied for a range of heating rates and initial temperatures. An approach to equilibrium (i.e. infinite thickness at T^o_m) by slowing down the heating rate is found to be more than exponentially suppressed. Similarities with heating experiments on polyethylene and PEEK are demonstrated. © 1993 John Wiley & Sons, Inc.     

Overcoming entropic barrier with coupled sampling at dual resolutions

An enhanced sampling method is proposed for ab initio protein folding simulations. The new method couples a high-resolution model for accuracy and a low-resolution model for efficiency. It aims to overcome the entropic barrier found in the exponentially large protein conformational space when a high-resolution model, such as an all-atom molecular mechanics force field, is used. The proposed method is designed to satisfy the detailed balance condition so that the Boltzmann distribution can be generated in all sampling trajectories in both high and low resolutions. The method was tested on model analytical energy functions and ab initio folding simulations of a beta-hairpin peptide. It was found to be more efficient than replica-exchange method that is used as its building block. Analysis with the analytical energy functions shows that the number of energy calculations required to find global minima and to converge mean potential energies is much fewer with the new method. Ergodic measure shows that the new method explores the conformational space more rapidly. We also studied imperfect low-resolution energy models and found that the introduction of errors in low-resolution models does decrease its sampling efficiency. However, a reasonable increase in efficiency is still observed when the global minima of the low-resolution models are in the vicinity of the global minimum basin of the high-resolution model. Finally, our ab initio folding simulation of the tested peptide shows that the new method is able to fold the peptide in a very short simulation time. The structural distribution generated by the new method at the equilibrium portion of the trajectory resembles that in the equilibrium simulation starting from the crystal structure.     

A molecular model for flow induced crystallization of polymers

We present in this paper a thermodynamic model for flow induced crystallization of a thermoplastic. The thermomechanical framework (generalized standard materials) allows us to couple in a very natural way the kinetics of crystallization with the mechanical history experienced by the thermoplastic^[1]. In describing the viscoelastic properties of the polymer with a molecular theory, we obtain a model for flow-induced crystallization that couples the chain conformation to the kinetics of crystallization. This model intends to be valid both for shearing and elongation. We present the equations for two cases: Maxwell and Pom-Pom constitutive equations. We finally illustrate our model with injection molding simulations achieved with a dedicated Finite Element code.     

A tensor model of liquid crystalline polymers: application to basic disclination processes

One of the features of liquid crystalline polymers (LCPs) is their strong elastic anisotropy, which means they have unequal elastic constants. Elastic anisotropy plays a crucial role in the microstructure and macroscopic properties of LCPs. In this paper, the effect of unequal elastic constants on microstructure is investigated without an external field by using a deterministic tensorial approach. In this model, the evolution of the director field can be viewed as a process driven towards the state of zero elastic torque. A tensor expression of the elastic torque is used so that the nematic symmetry is automatically conserved. In simulations of bulk samples, disclination lines of strength half and escaped integer disclinations are observed. The distortion fields around the disclinations are found to depend on elastic anisotropy. If the twist constant is the lowest, as is the case for main chain liquid crystalline polymers, the disclination lines are predominantly of the twist type.     

A tensor model of liquid crystalline polymers: application to basic disclination processes

One of the features of liquid crystalline polymers (LCPs) is their strong elastic anisotropy, which means they have unequal elastic constants. Elastic anisotropy plays a crucial role in the microstructure and macroscopic properties of LCPs. In this paper, the effect of unequal elastic constants on microstructure is investigated without an external field by using a deterministic tensorial approach. In this model, the evolution of the director field can be viewed as a process driven towards the state of zero elastic torque. A tensor expression of the elastic torque is used so that the nematic symmetry is automatically conserved. In simulations of bulk samples, disclination lines of strength half and escaped integer disclinations are observed. The distortion fields around the disclinations are found to depend on elastic anisotropy. If the twist constant is the lowest, as is the case for main chain liquid crystalline polymers, the disclination lines are predominantly of the twist type.     

Oxyalkylene polymers with alkylsulfonylmethyl side chains: Gas barrier properties

Gas barrier properties of alkylsulfonylmethyl-substituted poly(oxyalkylene)s are discussed. Oxygen permeability coefficients of three methylsulfonylmethyl-substituted poly(oxyalkylene)s, poly[oxy(methylsulfonylmethyl)ethylene] (MSE), poly[oxy(methylsulfonylmethyl)ethylene-co-oxyethylene] (MSEE), and poly[oxy-2,2-bis (methylsulfonylmethyl)trimethylene oxide] (MST) were measured. MSEE, which has the most flexible backbone of the three polymers, had an oxygen permeability coefficient at 30°C of 0.0036 × 10⁻¹³ cm³(STP)·cm/cm²·s·Pa higher than that of MSE, 0.0014 × 10⁻¹³ cm³(STP)·cm/cm²·s·Pa, because the former polymer's T_g was near room temperature. MST with two polar groups per repeat unit and the highest T_g showed the highest oxygen permeability, 0.013 × 10⁻¹³ cm³(STP) · cm/cm²·s·Pa, among the three polymers, probably because steric hindrance between the side chains made the chain packing inefficient. As the side chain length of poly[oxy(alkylsulfonylmethyl)ethylene] increased, T_g and density decreased and the oxygen permeability coefficients increased. The oxygen permeability coefficient of MSE at high humidity (84% relative humidity) was seven times higher than when it was dry because absorbed water lowered its T_g. At 100% relative humidity MSE equilibrated to a T_g of 15°C after 2 weeks. A 50/50 blend of MSE/MST had oxygen barrier properties better than the individual polymers (O₂ permeability coefficient is 0.0007 × 10⁻¹³ cm³(STP)·cm/cm² ·s·Pa), lower than most commercial high barrier polymers. At 100% relative humidity, it equilibrated to a T_g of 42°C, well above room temperature. These are polymer systems with high gas barrier properties under both dry and wet conditions. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 75–83, 1998     

Prediction and analysis of nonisothermal crystallization of polymers

The crystallization behavior of polyetheretherketone (PEEK), polyoxymethylene (POM), polyethyleneterephtalate (PET), and polypropylene (PP) under nonisothermal conditions has been studied. Differential scanning calorimetry was used to monitor crystallization from the melt and a kinetic model has been proposed to describe three-dimensional spherulitic crystal growth. The model, which accounts for crystalline growth rate, uses two modified Avrami equations to represent both heterogeneous and homogeneous nucleation and growth processes. The model parameters are all associated with physical constants. The predicted evolution of absolute crystallinity showed good agreement with experimentally obtained values for a wide range of cooling rates. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35 : 875–888, 1997     

Chiral soluble polymers and microspheres for enantioselective crystallization

A series of chiral polymers based on poly(N‐acryl) amino acids was synthesized using a convergent synthetic approach. These chiral polymers have been used as chiral additives to induce enantioselective crystallization of racemic or conglomerate amino acids in solutions. These polymeric additives showed strong capabilities to enhance highly enantioselective resolution during the crystallization of amino acids. In addition, these polymers caused unusual modifications of amino acid crystal morphologies. Furthermore, spherical microparticles of those same chiral polymers were also shown active in similar chiral discriminations during amino acid crystallizations occurring on microparticle surfaces. Our study demonstrates the high potential of chiral polymers and microparticles to resolve amino acids throughout crystallization processes. High enantiomeric excesses in one targeted enantiomer of amino acids can also be maximized via time‐dependent kinetic control of crystallizations. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3009–3017, 2006     

Polymer analysis: Molecular structure and properties of industrial polymers

Low density polyethylenes made by the known high pressure processes show significantly different molecular structures. The physical and technological properties are closely related to molecular structure and morphology. For example, the adhesive strength of a laminate consisting of an ozone treated low density polyethylene film and an aluminum foil depends strongly on the synthesis conditions. The molecular structures and dilute solution properties of many fully aromatic thermotropic liquid crystalline copolyesters can now be determined using the new solvent 3,5-bis(trifluoromethyl)phenol. A typical random copolyester was fractionated by precipitation from solution, and the fractions were studied in detail by viscometry, integrated and dynamic light scattering, and size exclusion chromatography. From the structure data thus obtained the molecular mass distribution and the persistence length were calculated. Polymer blends, block copolymers, and graft copolymers can be characterized by fractionation procedures using demixing solvents in an ultracentrifuge and subsequent analysis of the fractions.     

Chemically specific coarse-graining of polymers: Methods and prospects

Coarse-grained (CG) modeling is an invaluable tool for the study of polymers and other soft matter systems due to the span of spatiotemporal scales that typify their physics and behavior. Given continuing advancements in experimental synthesis and characterization of such systems, there is ever greater need to leverage and expand CG capabilities to simulate diverse soft matter systems with chemical specificity. In this review, we discuss essential modeling techniques, bottom-up coarse-graining methodologies, and outstanding challenges for the chemically specific CG modeling of polymer-based systems. This methodologically oriented discussion is complemented by representative literature examples for polymer simulation; we also offer some advisory practical considerations that should be useful for new researchers. Given its growing importance in the modeling and polymer science community, we further highlight some recent applications of machine learning that enhance CG modeling strategies. Overall, this review provides comprehensive discussion of methods and prospects for the chemically specific coarse-graining of polymers.     

Simulation of Polymer Melts: From Spherical to Ellipsoidal Beads

In a previous work^[1] we presented a coarse‐graining procedure for bonded interactions, while spherical, purely repulsive 6–12 interactions were used to account for the excluded‐volume of the beads of the phantom chains. Here we extend this approach towards ellipsoidal beads whose shapes are extracted from the geometry of the underlying atomistic chains. The influence of the bead shapes on the static and dynamical properties of melts are studied in detail for two modifications of polycarbonates, from 2mers up to 20mers. The results obtained in both cases are discussed in the context of corresponding experiments and atomistic simulations.     

Transient and athermal effects in the crystallization of polymers. I. Isothermal crystallization

The isothermal crystallization of poly(propylene) and poly(ethylene terephthalate) was investigated with differential scanning calorimetry and optical microscopy. It was found that the induction time depends on the cooling rate to a constant temperature. The isothermal crystallization of the investigated polymers is a complex process and cannot be adequately described by the simple Avrami equation with time‐independent parameters. The results indicate that crystallization is composed of several nucleation mechanisms. The homogeneous nucleation occurring from thermal fluctuations is preceded by the nucleation on not completely melted crystalline residues that can become stable by an athermal mechanism as well as nucleation on heterogeneities. The nucleation rate depends on time, with the maximum shortly after the start of crystallization attributed to nucleation on crystalline residues (possible athermal nucleation) and on heterogeneities. However, the spherulitic growth rate and the exponent n do not change with the time of crystallization. The time dependence of the crystallization rate corresponds to the changes in the nucleation rate with time. The steady‐state crystallization rate in thermal nucleation is lower than the rate determined in a classical way from the half‐time of crystallization. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1835–1849, 2002     

Transient and athermal effects in the crystallization of polymers. II. Nonisothermal crystallization

Nonisothermal crystallization of several polymers was investigated with differential scanning calorimetry and optical microscopy. The results indicated that as in the case of isothermal processes, crystallization starts with nucleation on noncompletely melted crystalline residues. It is assumed that if the crystalline residues are subcritical at melting temperatures, they can become stable by an athermal mechanism during cooling. There is also some contribution of nucleation on heterogeneities. The next mechanism of nucleation is a classical homogeneous process occurring by thermal fluctuations. The results showed the non‐steady‐state character of the nonisothermal crystallization of polymers. In the investigated range of cooling rates, the non‐steady‐state character of nonisothermal crystallization of polymers is dominated by the transient thermal effects. In the range of high temperatures, the transient homogeneous nucleation can be interpreted with the Ziabicki model, and the steady‐state rate determined from nonisothermal experiments coincides with the rate determined in isothermal crystallization. The athermal nucleation occurring at the beginning of crystallization from noncompletely melted aggregates seems to be independent of the applied cooling rate. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 68–79, 2003     

Points of contact between rheology and crystallization in polymers

Summary Because the flow of linear polymers is coupled with molecular orientation, which in turn influences crystallisation, there are many points of contact between the rheological and crystallisation phenomena in polymers. Compared with non-orientated polymers, polymers with molecular orientation crystallise more rapidly by another mechanism which does not produce spherulites. Conversely, orientated crystallisation can affect rheological phenomena, especially when external forces are exerted on the orientated structure. The fact that crystallisation of the orientated melt can lead to an increase in molecular orientation, and the possibility of this causing micro-currents, which are responsible for the development of spherulitic structures (the auto-orientation mechanism) are discussed. Special attention is paid to two recent publications concerning, inter alia, the structure of ringed spherulites. Finally, periodic thickness variations, which play an important part in the extrusion of fibres and films from polyolefins, are dealt with. A new hypothesis based on the auto-orientation principle is postulated, to fit into a logical relationship a large number of experimental factors which influence these thickness variations.

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Zusammenfassung Viele Berührungspunkte bestehen zwischen den Rheologie- und Kristallisationserscheinungen bei linearen Polymeren dadurch, da? das Flie?en dieser Polymere mit Molekularorientierung verbunden ist, die wiederum die Kristallisation beeinflu?t. Im Vergleich zu nicht orientierten Polymeren kristallisieren Polymere mit Molekularorientierung schneller, und zwar infolge eines Mechanismus, bei dem keine Sph?rolite entstehen. Umgekehrt kann orientierte Kristallisation die rheologischen Erscheinungen beeinflussen, besonders, wenn ?u?ere Kr?fte auf die orientierte Struktur ausgeübt werden. Es wird besprochen, da? die Kristallisation der orientierten Schmelze zu einer Zunahme der Molekularorientierung führen kann, und da? die entstehenden Mikrostr?me die Entwicklung sph?rolitischer Strukturen verursachen k?nnen (der Autoorientierungsmechanismus). Insbesondere werden zwei kürzlich erschienene Ver?ffentlichungen berücksichtigt, welche u. a. die Ringstruktur einer bestimmten Sph?rolitenart behandeln. Schlie?lich werden periodische Dickenschwankungen besprochen, die eine wichtige Rolle beim Strangpressen von Fasern und Folien aus Polyolefinen spielen. Durch eine neue, auf dem Prinzip der Autoorientierung basierende Hypothese ist es m?glich, eine gro?e Anzahl experimenteller Faktoren, die diese Dickenschwankungen beeinflussen, in ein logisches Verh?ltnis zu bringen.

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Résumé Parce que le flux de polymères linéaires est allié à l'orientation moléculaire, qui à son tour influence la crystallisation, beaucoup de points de contact existent entre les phénomènes de rhéologie et de crystallisation dans les polymères. En comparaison avec les polymères non-orientés, ceux dont les molécules sont orientées cristallisent plus rapidement par suite d'un autre mécanisme qui ne produit pas de sphérulites. Inversement, la cristallisation orientée peut affecter les phénomènes rhéologiques, surtout lorsque des forces externes s'exercent sur la structure orientée. Il a été exposé que la cristallisation de la masse orientée peut amener un accroissement d'orientation moléculaire et que cet accroissement peut produire des micro-courants qui seraient responsables du développement des structures sphérulitiques (mécanisme d'autoorientation). Une attention spéciale est portée sur deux publications récentes traitant, entre autres choses, de la structure des sphérulites en anneaux. On discute finalement des variations périodiques d'épaisseur qui jouent un r?le important dans l'extrusion de fibres et de films de polyoléfines. Une nouvelle hypothèse basée sur le principe d'autoorientation est avancée pour classer dans un cadre logique un grand nombre de facteurs expérimentaux qui influencent les variations d'épaisseur.

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Lecture delivered to the Dutch Rheological Association at Vught, The Netherlands.     

Photoacid-generating sulfonyloxymaleimide polymers and their application to photoimaging

Synthesis and properties of photoacid generating polymers based on four sulfonyloxymaleimides (RsOMI), N-(tosyloxy)maleimide (TsOMI), N-(methanesulfonyloxy)maleimide (MsOMI), N-(trifluoromethanesulfonyloxy)maleimide (TfOMI), and N-(10-camphorsulfonyloxy)maleimide (CsOMI) are described. The RsOMI polymers photochemically produce corresponding sulfonic acids (RsOH), TsOH, MsOH, TfOH and 10-camphorsulfonic acid (CSA) which can catalyze deprotection of the acid-labile t-BOC groups in the same polymer chains via a chemical amplification process. The photoacid generating polymers with concurrent acid-labile groups offer great potential for applications in functional transformation and photoimaging.     

An application of an homogenization method to a model of diffusion in glassy polymers

The sorption of gases in polymers below their glass-transition temperature T_g is known in many cases to be described by the “dual sorption” theory, according to which the gas is held in accordance with both the Langmuir and Henry's laws. Based on this theory, expressions for the “effective diffusion coefficient” in the glassy polymers have been obtained by investigators in the past, notably by Paul and Koros.² The present analysis regards the glassy polymers as inhomogeneous with regions on which the gas sorption follows the Langmuir law. Assuming that the linear dimensions of these regions, which are often referred to as “microvoids” (although they are not space filled by vacuum), are small compared to the macroscopic length of interest but large compared to the mean free path of the penetrant gas molecules, we derive a rigorous relation between the average flux and the concentration gradient in the polymer and show that this relation can be expressed in terms of an “effective diffusion coefficient” D_eff which depends on the details of the microstructure, i.e., the size, shape and spatial distribution of the “microvoids.” This expression for D_eff is shown to reduce to that of Paul and Koros² in two situations: (1) when the “voids” consist of slabs running parallel to the concentration gradient, and (2) when the “voids” are spherical and the temperature of the polymer is not too different from T_g. The results of the present study lead to an alternative procedure for interpreting the experimental data on sorption and permeation which may have some advantages over the procedure currently employed. Finally, the analysis presented here is also applicable to polymers containing adsorptive fillers.     

Estimation of the nucleation and crystal growth contributions to the overall crystallization energy barrier

Classical kinetic theories of polymer crystallization were applied to isothermal crystallization kinetics data obtained by polarized optical microscopy (PLOM) and differential scanning calorimetry (DSC). The fitted parameters that were proportional to the energy barriers obtained allow us to quantitatively estimate the nucleation and crystal growth contributions to the overall energy barrier associated to the crystallization process. It was shown that the spherulitic growth rate energy barrier found by fitting PLOM data is almost identical to that obtained by fitting the isothermal DSC crystallization data of previously self‐nucleated samples. Therefore, we demonstrated that by self‐nucleating the material at the ideal self‐nucleation (SN) temperature, the primary nucleation step can be entirely completed and the data obtained after subsequent isothermal crystallization by DSC contains only contributions from crystal growth or secondary nucleation. In this way, by employing SN followed by isothermal crystallization, we propose a simple method to obtain separate contributions of energy barriers for primary nucleation and for crystal growth, even in the case of polymers where PLOM data are very difficult to obtain (because they exhibit very small spherulites). Comparing the results obtained with poly(p‐dioxanone), poly(ε‐caprolactone), and a high 1,4 model hydrogenated polybutadiene, we have interpreted the differences in primary nucleation energy barriers as arising from differences in nuclei density. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1478–1487, 2008     

Surface forces with adsorbed polymers: Direct measurements and model calculations

We Have Measured Directly The Forces Between Two Smooth Solid Surfaces Immersed In Liquid Media In The Range Of Surface Separation 0 ∼ 2000Å. By Measuring The Interactions Both With And Without Polymers Adsorbed Onto The Surfaces From The Solution, We Could Characterize The Effects Due To The Adsorbed Polymers Themselves. This Has Been Done For Several Model Polymer Systems In A Wide Range Of Solvency Conditions: Poor-, Theta- And Good Solvents, As Well As For Charged Polymers In Ionic Media. The Present Paper Describes Two Recent Studies Of Adsorbed Polymers In Theta-Solvent And Good Solvent Conditions, And The Effect Of Polymer Bridging At Low Adsorbance Values. The Experimental Studies Are Extremely Direct, And Can Be Compared With Molecular Models: We Briefly Describe Such A Detailed Comparison For The Case Of Interaction Between Adsorbed Polymer Layers In A poor solvent.     

ESR application to radiation chemistry of polymers

Important results obtained in our group in the field of ESR application to the study of irradiated polymers are summarized. They are the analysis of the decay reaction of the free radicals, spur-like trapping of the free radicals and the related discussions. A diffusion controlled bimolecular reaction scheme was a good way of analyzing the data of the decay reaction. Power saturation phenomenon of ESR spectra of the free radicals showed a circumstance of the spur-like trapping of the free radicals in irradiated polyethylene. The phenomenon of spur-like trapping was quite consistent with the interpretation of the decay reaction of the free radicals.