Effects of pressure on the compressibility and crystallization of poly(ethylene terephthalate)

The effects of pressure on the compressibility and crystallization of poly(ethylene terephthalate) (PET) have been investigated. The Instron capillary rheometer was adapted as a high-pressure dilatometer to perform experiments up to 40,000 psi. Compressibilities of solid and molten PET were measured. The increases in compressibility with increase in temperature for the solid state are discussed in terms of free-volume theory. Results obtained for the melt are explained by invoking the second law of thermodynamics and the effect of pressure on the Gibbs free energy. The effects of temperature and compression rate on the pressure of crystallization (P_c) were also studied. As the crystallization temperature was increased from 240 to 286°C, P_c increased by about 16,000 psi. As the compression rate was raised from 1%/min to 8%/min, P_c increased 10,000 psi. At some undetermined compression rate above 8%/min it seemed impossible to induce crystallization in the melt, even with pressures up to 40,000 psi. Analysis of data on the kinetics of crystallization of PET melt under high pressures revealed low Avrami exponents, for which no unequivocal explanation is offered. It is possible, however, that crystallization at high pressure promotes the formation of a morphology made up of a certain percentage of “extended chains.” The alteration in the attendant spatial geometry involved in the crystallization might explain the lower Avrami exponents found. In another set of experiments, crystallization temperatures (T_c) were measured by slowly cooling PET melt under high pressures. As the pressure was raised from 3000 to 15,000 psi, T_c increased from about 246 to 282.5°C. These results are consistent with thermodynamic theory.     

Photochemistry of fiber-forming polymers. I. Acrylonitrile copolymers and fibers containing ketone groups

The inclusion of minor amounts of methyl isopropenyl ketone in polyacrylonitrile and its copolymers with methyl acrylate and vinyl acetate causes the polymer to become sensitive to ultraviolet light in the 290–320 nm region. Both films and fibers show decreases in molecular weight on irradiation which are attributable to C? C bond scission by the Norrish type II reaction. In addition to bond cleavage, decarbonylation occurs, presumably by a type I radical process which also leads to the formation of ketonitrile groups. Fibers spun from such polymers lose most of their tensile strength and elongation after a few months outdoor exposure to natural sunlight, whereas control samples not containing carbonyl groups will retain their strength for much longer periods. Quantum yields both for chain scission and for carbonyl loss in these systems are estimated to be of the order of 0.005.     

Effects of pressure and solvent on the diffusion of aromatic compounds through polymers

The diffusion of six azo and five anthraquinone derivatives through nylon 6, poly(ethylene terephthalate) and secondary cellulose acetate films were studied under high hydrostatic pressures of up to 3000 bar and at temperatures 80–130 °C, by analyzing the diffusion profiles yielded in a stacked multiple film, placed in the solution of the diffusant. It was found that the diffusion coefficient,D, of the diffusant decreased with increasing pressure, giving a linear relationship between InD and the pressure, the slope of which gave the activation volume for the diffusion,V . It was revealedV increased linearly with increasing intrinsic molecular volume of the diffusant,V _w , the slopes being different between the azo and the anthraquinone derivatives. The ratio ofV toV _w (V /V _w ) ranged from 0.13 to 0.93, depending in a sensitive manner on the degree of swelling of the polymer matrix which in turn was varied by the solvent. The overall results could be explained in accordance with the formulation,V _{f, local} +V =V _w , whereV _{f, local} represents the free volume contribution. It was proposed thatV _w is increased by solvation when the solvent is good for the diffusant.     

Photochemistry of fiber-forming polymers. II. Ketone-containing poly(ethylene terephthalates)

Poly(ethylene terephthalate) copolymers were prepared containing minor amounts of comonomers containing backbone and side-chain ketone groups. The photochemistry of these polymers was studied at 313 nm, both in film and fiber form. It was demonstrated that only the comonomer, dimethyl-γ-ketopimelate (DMKP), gave a copolymer with enhanced sensitivity to ultraviolet degradation in air. Since this sensitivity is reduced in a nitrogen atmosphere, it is suggested that the mechanism involves main-chain scission to form two free radicals by the Norrish type I mechanism, and that recombination of these radicals is reduced by reaction with oxygen. The use of copolymers of this type provides a method for the controlled photodegradation of polyester fibers.     

The effect of lattice compressibility on the thermodynamics of gas sorption in polymers

A compressible lattice model with holes, the glassy polymer lattice sorption model (GPLSM), was used to model the sorption of carbon dioxide, methane, and ethylene in glassy polycarbonate and carbon dioxide in glassy tetramethyl polycarbonate. For glassy polymers, an incompressible lattice model, such as the Flory–Huggins theory, requires concentration-dependent and physically unrealistic values for the lattice site volumes in order to satisfy lattice incompressibility. Rather than forcing lattice incompressibility, GPLSM was used and reasonable parameter values were obtained. The effect of conditioning on gas sorption in glassy polymers was analyzed quantitatively with GPLSM. The Henry's law constant decreases significantly upon gas conditioning, reflecting changes in the polymer matrix at infinite dilution. Treating the Henry's law constant as a hypothetical vapor pressure at infinite dilution, gas molecules in the conditioned polymer are less “volatile” than those in the unconditioned polymer. Flory–Huggins theory was used to model the sorption of carbon dioxide, methane, and ethylene in silicone rubber. Above the glass transition temperature, the criterion of lattice incompressibility for Flory-Huggins theory was satisfied with physically realistic and constant values for the lattice site volumes. © 1992 John Wiley & Sons, Inc.     

Chiral crystallization of helical polymers

Crystallization of achiral or racemic helical polymers in chiral crystals is quite frequent and while in some cases it may relate to the thermodynamic stability of the chiral polymorph, in others it must be associated with kinetic factors. Analysis of the available literature data suggests that in the second instance a key role must be played by the nucleation step, i.e. specifically by the formation of precrystalline entities. Furthermore a survey of the chiral crystal structures for helical polymers evidences that they are frequently characterized by a quasi-hexagonal packing. The hexagonality index H, defined as the ratio of the largest to the smallest distance between the axis of the reference helix and its six nearest neighbors, appears to be a reliable indicator of the presence of helices of a single handedness or, respectively, of both in a given crystal structure. A detailed analysis of the general energetic and entropic factors favouring chiral crystallization of helical polymers is carried out. It is shown that a hexagonal or pseudo-hexagonal arrangement (i.e. the six-fold coordination) in either the crystalline or in a precrystalline state, promotes chiral crystallization and is in its own turn favoured by clustering of isochiral helices.     

Effects of high pressure on the conductivity and dielectric constant of conductive organic polymers

Previous theory for pressure effects on conduction in amorphous macromolecular solids is extended to include pressure effects on hyperelectronic polarization. A theoretical estimate of hopping activation energies is given. Experimental studies of the frequency effects of the pressure coefficients of conduction and permittivity permits resolution of the salient factors. Carrier mobility in ekaconjugated polymers is best represented by activated hopping, for conductivity increase with frequency, and permittivity drops. Conductivity and hyperelectronic polarization increase markedly with pressure. Their activation energies, E_a0, and the pressure coefficients of their activation energies (b_E + b_H) are nearly identical. E_a0 is frequency-dependent, but the (b_E + b_H) terms are not, showing the frequency dependence of E_a0 to lie in the hopping activation enthalpy. Similar arguments on the “total” pressure coefficients b and b for conduction and polarization show the latter coefficients to have a frequency dependence lying in the hopping activation entropy.     

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     

DSC and X-ray studies on side-chain crystallization of comb-like polymers

Thermal properties of acrylate and methacrylate monomers containing long-fluorocarbon chains (H(CF₂)_nCH₂OCOCH=CH₂, (F_nA) and H(CF₂)_nCH₂OCOC(CH₃)=CH₂, (F_nMA), wheren=6, 8, 10) and their comb-like polymers have been investigated by differential scanning calorimeter (DSC) and X-ray diffraction. The comb-like polymers (PF₁₀A and PF₁₀MA) with sufficiently long-fluorocarbon chains showed a simple melting and crystallizing behavior. For the fusion of PF₁₀A in 1st heating, enthalpy change H _f was 18 kJ mol^–1 and entropy change S _f was 45 J K^–1 mol^–1, while for PF₁₀MA the values H _f and S_f were 5.3 kJ mol^–1 and 14 J K^–1 mol^–1, respectively. Melted PF₈A crystallized slowly, whereas PF₈MA with same fluorocarbon chain and also both of PF₆A and PF₆MA with shorter fluorocarbon chains can hardly crystallize by the aggregation of side-chains. Effects of the length of side-chain and the flexibility of main chain on the side-chain crystallization of comb-like polymers are clear. Crystallization process of the methacrylate monomers was sensitively affected by the scanning rate of DSC measurement and the length of fluorocarbon chains.     

Adiabatic compressibility and solvation of drag reducing polymers in aqueous solutions

Adiabatic compressibilities of aqueous solutions of some drag reducing polymers have been evaluated from ultrasonic velocity and density measurements. The solvation numbers of the repeat units of the polymers have been further evaluated by using Passynsky equation. It is observed that the solvation number increases with the shear stability of drag reducing polymers in turbulent flow.     

Stability and growth of nuclei during the crystallization of polymers

During the crystallization of linear flexible chain molecules a boundary phase necessarily develops which has a volume of its own. The boundary phase, however, is not autonomous so that Gibbs' phase rule loses its validity. The 2-phase system {crystal; boundary phase} is bivariant, the 3-phase system {crystal, boundary phase; melt} is monovariant. At quasistatic cooling the ability of the crystal nuclei to grow demands the formation of loose loops or tie-molecules. This leads to the final state of the crystallization to be an arrested equilibrium state. The internal equilibrium state cannot be reached under normal circumstances.Dedicated to H.-G. Kilian on the occasion of his 60th birthday.     

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     

Temperature dependence of the bulk crystallization rate of polymers

Data already existing in the literature for the overall crystallization kinetics of a variety of polymers have been analyzed according to different possible nucleation mechanisms. The conclusions reached are similar to those previously deduced from an examination of ata for spherulite growth rates. It is demonstrated that, if allowed a reasonable choice for the equilibrium melting temperature, no unbiased selection of a unique nucleation process can be made. Moreover, a set of universal parameters exists for each of the nucleation and growth processes considered which allows the data for all polymers to be represented by a single straight line. The only quantities that are unique to a given polymer are the equilibriun melting temperature and the activation energy for transport.     

Hydrodynamically induced crystallization of polymers from solution

Summary This paper primarily describes an investigation into the origin of persistent lamellar overgrowth of fibrillar crystals with a Shish-kebab morphology. Fibers grown from stirred p-xylene solutions of high molecular weight polyethylene at 108 °C exhibited an average spacing of 1 x 10³ Å between the lamellae when cooled down to room temperature in a normal way. Fibers quenched in liquid nitrogen displaved smaller lamellae with a periodicity of 0.4 x 10³ Å.It was found that the lamellae could be dissolved at 108 °C and recrystallized isothermally on the backbones at temperatures up to 94.6 °C. The spacing between the lamellae increased strongly with temperature in the range between 90.0 °C and 94.6 °C.The temperature dependence of this periodicity could be accounted for by the theory of secondary nucleation in polymer crystallization. The Poisson distribution of the numbers of lamellae along the backbones also suggests that lamellar growth is essentially controlled by the crystallization kinetics.The fibrillar polyethylene crystals grown at elevated temperatures from flowing solutions may be visualized as backbones constituted by partially crystallized polymer molecules and dangling ends extending from the backbones into the surrounding solution resembling centipedes.

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Zusammenfassung Dieser Bericht befaßt sich mit einer Untersuchung des Ursprungs von persistenter lamellarer Überwachsung von Faser-Kristallen mit Schaschlik-Morphologie.Faser-Kristalle, gewachsen aus geruhrten Xylol-Lösungen von hochmolekularem Polyäthylen bei 108 °C, zeigen ein mittleres Intervall zwischen den Lamellen von 1 x 10³ Å, wenn auf normae Weise abgekühlt zur Zimmertemperatur. Fasern, abgeschreckt im flüssigen Stickstoff, zeigen kleinere Lamellen mit einem Zwischenraum von etwa 0.4 x 10³ Å. Es wurde festgestellt, daß die Lamellen bei 108 °C gelöst werden können und von neuem auf den Faserkernen bis zu 94.7 °C kristallisieren. Die Temperaturabhangigkeit dieser Periodizität kann erklärt werden auf Grund der Theorie der sekundären Keimbildung bei polymerer Kristallisation. Wie sich zeigt, gruppiert sich die lamellare Überwachsung um den Kern entsprechend einer Poisson-Verteilung, was darauf hinweist, daß die lamellare Überwachsung wesentlich durch die Kristallisationskinetik bedingt wird.Die faserigen Polyathylenkristalle, bei erhöhter Temperatur hergestellt aus gerührten Lösungen, kann man sich als teilweise kristallisierte Moleküle, im Faserkerne aufgenommen, vorstellen mit langen Kettenenden im umgebenden Lösungsmittel, so ziemlich als einen Tausendfüßler.

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With 12 figures     

The influence of recycled material on the crystallization kinetics of thermoplastic polymers

The injection moulding of semi-crystalline thermoplastic polymers requires an exact knowledge of the thermodynamic data and of the crystallization kinetics. The behaviour of the polymer melt during rapid cooling in the mould determines, to a great extent, the quality and usability of a final product. Technical raw materials are often equipped with nucleating agents in order to obtain crystallization within the desired temperature range and at the required rate. The use of recycled material (regranulate) shows an analogous effect such as the addition of nucleating agents, i.e. crystallization begins at a higher temperature and a higher crystallization rate is detected compared to materials without added regranulate. Heat flux DSC was used to study the crystallization of polyamides, polyolefins and polyoxymethylene during cooling at various cooling rates. Although the temperature gradients and pressures which occur in the proceesing machine cannot be realised in DSC tests, the DSC results reproduce the direction of influence of the regranulate additive very clearly.     

Thermal analysis of secondary crystallization in polymers

With a particular experimental technique of differential scanning calorimetry, the melting of the secondary crystals in polymers is observed separately from the melting of the primary crystals, and methods for the detection and quantitative analysis of the secondary crystals have been developed. By this means, a number of particular features of secondary crystals could be found which are not mentioned in the literature. Two types of crystal formation are found for linear polymers, with no regard as to kinetics, growth and other characteristics of crystallization. Secondary crystallization is interpreted as the formation of crystals of fringed micelle type. The equation describing secondary crystallization is deduced. It is shown that some specific features of the DTA and DSC curves of polymers are caused by the behaviour of secondary crystals in the polythermal regime.

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Zusammenfassung Es wird gezeigt, daß durch DSC bei einer besonderen experimentellen Technik zwischen dem Schmelzen von primären und sekundären Kristallen in Polymeren unterschieden werden kann. Methoden zur Detektierung und quantitativen Analyse sekundärer Kristalle wurden entwickelt. Mit diesen Methoden wurde eine Anzahl von in der Literatur noch nicht beschriebenen Besonderheiten der sekundären Kristalle festgestellt. In linearen Polymeren wurden zwei Typen der Kristallbildung festgestellt, die sich nicht auf die Kinetik, das Wachstum und andere Charakteristika der Kristallisation beziehen. Die sekundäre Kristallisation wird als die Bildung von Kristallen des fransigen Mizell-typs interpretiert. Die die sekundäre Kristallisation beschreibende Gleichung wird abgeleitet. Es wird gezeigt, daß einige spezifische Züge der DTA- und DSC-Kurven von Polymeren durch das Verhalten von sekundären Kristallen verursacht werden.

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Molecular-dynamics simulation of crystallization in helical polymers

The molecular mechanism of crystallization in helical polymers is a fascinating but very difficult subject of research. We here report our recent efforts toward better understanding of the crystallization in helical polymers by use of molecular-dynamics simulation. With straightforward approaches to the problem being quite difficult, we adopt a different strategy of categorizing the helical polymers into two distinct types: one type is a simple bare helix which is essentially made of backbone atomic groups only and has smoother molecular contours, and the other is a more general helix having large side groups that would considerably hamper molecular motion and crystallization. Both types of helical polymers are here constructed by use of the united atom model, but they show quite distinct crystallization behavior; the crystallization of the former-type polymer is rather fast, while that of the latter-type polymer is extremely slow. We find that the bare helix, when rapidly cooled in free three-dimensional space, freezes into partially ordered state with limited intramolecular and intermolecular orders, and that remarkable improvement of order and growth of an ordered chain-folded crystallite occurs by very long-time annealing of the partially ordered state around the apparent freezing temperature. We also study crystallization of the bare helix upon a growth surface; the crystallization in this case proceeds much faster through highly cooperative process of the intermolecular and the intramolecular degrees of freedom. On the other hand, crystallization of the realistic model of isotactic polypropylene (iPP) having pendant methylene groups is found to be extremely sluggish. By restricting the spatial dimension of the system thereby fully disentangling the chain, we observe that the molecule of iPP crystallizes very quickly onto the crystal substrate made of the same iPP chain. Quite remarkable is that the molecule of iPP strictly recognizes the helical sense of the substrate chain and efficiently selects its chirality during crystallization.