Effect of molecular weight on crystallization isotherms of high molecular weight poly(ethylene oxide) fractions

Dilatometric crystallization isotherms have been determined for a set of poly(ethylene oxide) fractions ranging in molecular weight from 2 × 10⁴ to 1.6 × 10⁶. For a given fraction the isotherms obtained for different crystallization temperatures can be superimposed over most of the crystallization. For a given crystallization temperature the degree of crystallinity obtained in the primary stage of the crystallization varies greatly with molecular weight, and superimposition of the isotherms is not possible. Secondary crystallization processes are pronounced when the molecular weight ($\text{M}\text{?}{}_{\mathrm{v}}$) exceeds 10⁵.     

Effects of lithium perchlorate on poly(ethylene oxide) spherulite morphology and spherulite growth kinetics

Effects of lithium perchlorate (LiClO₄) on the crystallization behaviors of poly(ethylene oxide) (PEO) were investigated by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and polarized optical microscopy (POM) in PEO/LiClO₄ system. DSC results indicate that there are nucleation effects of LiClO₄ on the crystallization of PEO. But, on the other hand, the coordination of lithium ion with the oxygen ether atoms of PEO can obviously reduce the crystallinity and spherulite growth rate of PEO. This contrary effect of LiClO₄ on the crystallization of PEO in PEO/LiClO₄ complexes system was analyzed and discussed in detail. The Laurizen–Hoffman theory was used to describe the Li‐coordinated crystallization kinetics of PEO spherulite. It showed that the nucleation constant (K_g) and folding surface free energy (σ_e) decreased with increasing LiClO₄ contents, and the energy necessary for the transport of segments across the liquid–solid interface (ΔE) increased on increasing the contents of LiClO₄. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of molecular weight on spherulite growth rate of poly(ethylene terephthalate) via real-time small angle light scattering

The effect of molecular weight on the spherulite growth rate of poly(ethylene terephthalate) (PET) has been determined using a real-time small angle light scattering (RTSALS) device equipped with a two-dimensional position-sensitive detector. This detector, the design of which was previously reported, incorporates a CCD camera, a personal computer, and an imaging board as the major hardware components. The device performs real-time analysis of the light scattering pattern and calculates the average spherulite radius as a function of time during the crystallization experiment. Growth rate data were obtained for PET having number-average molecular weights of 18,000, 24,700, and 33,100. Samples were crystallized isothermally following a temperature jump from the glass at room temperature to the desired crystallization temperature, which ranged from 130 to 200°C. Both the temperature and molecular weight dependence were found to be well described by the Hoffman equation. The temperature dependence agrees well with that found by previous workers, but the molecular weight dependence is somewhat different.     

Crystallization of. high molecular weight fractions of poly(ethylene oxide) from dilute solution in ethanol

The kinetics of crystallization of high molecular weight poly(ethylene oxide) fractions from dilute solution in ethanol have been investigated by means of dilatometry. At low extents of crystallinity the crystallization isotherms are well represented by the free growth Avrami expression with exponent n = 4. Crystallization rates increase with increasing molecular weight in the manner predicted from the established thermodynamic properties of polymers in dilute solution.     

Effect of molecular weight and molecular weight distribution on the rheological properties of aqueous poly(ethylene oxide) solution

The effect of the molecular weight and the molecular weight distribution on the rheological properties of aqueous poly(ethylene oxide) (PEO) solutions has been investigated with four PEO samples differing in their M_w, M_w/M_n and purity. The main result of this study is that the steady shear viscosity as well as the complex dynamic viscosity of the samples with broad molecular weight distribution greatly differed from the viscosities of the samples having a narrow molecular weight distribution. Furthermore, the samples with broad molecular weight distribution showed a distinct molecular weight dependent non-Newtonian behavior at increasing shear rates and frequencies. This behavior was not observed for the sample with a narrow molecular weight distribution. Both effects are mainly attributed to the influence of the high molecular weight fraction in the PEO samples of broad molecular weight distribution. The often reported degradation of PEO solutions was not observed within the time scale of our experiment.     

Crystal growth pattern changes in low molecular weight poly(ethylene oxide) ultrathin films

A low molecular weight (MW) poly(ethylene oxide) (PEO) crystallized in ultrathin films displays various crystal growth patterns in a crystallization temperature (T_x) range from 20.0 °C to 50.0 °C. In succession, the following patterns are found: nearly one-dimensional (1D) dendrite-like crystal patterns at T_x ≤ 38.0 °C, two-dimensional (2D) seaweed-like patterns between 39.0 °C ≤ T_x ≤ 42.0 °C and again, nearly 1D dendrite-like patterns at T_x ≥ 43.0 °C. These transitions result from a complex interplay of varying growth rates along different growth directions and preservation of growth planes. Structural analysis carried out via electron diffraction indicates that the dendrite-like crystals formed at the low and high T_x values differ by their fast growth directions: along the {120} normal at the low T_x values and along the (100) and (010) normal at the high T_x values. In the later case however, the major growth faces are still the {120}, this time tilted at 45° and indicating the a∗ and b axes growth tips. In the intermediate T_x range (39.0 °C-42.0 °C), three growth directions coexist giving rise to the seaweed morphology. The crystal growth rates at the low and high T_x values are constant versus time. For the seaweed, a square-root dependence is obtained. These differences are probably due to 1D and 2D growth in the ultrathin films and are associated with different growth patterns of the dendrites and the seaweed, respectively.     

Crystallization of low molecular weight fractions of poly(octamethylene oxide)

Poly(octamethylene oxide) fractions ranging in molecular weight from 3400 to 9000 have been isothermally crystallized in the interval 61–70°C. From the dilatometric isotherms, the Avrami exponent is an integral number, 4, and is independent of temperature and molecular weight; the total crystallinity ranges from 78% for M = 3400 to 51% for M = 9000. The crystallization temperature coefficient was studied using the three-dimensional nucleation theory and it was found that the basal interfacial free energy changes from 3700 cal/mol to 2200 cal/mol, decreasing with molecular weight. When the change of the interfacial free energy is considered, the crystallization is described by a unique function of the free energy for nucleation.     

Effect of chain structure on the melting of low molecular weight poly(ethylene oxide)

Linked poly(ethylene oxide) samples have been prepared by the condensation of α-methoxy-ω-chlorocarboxy-poly(ethylene oxide) with alcohols of various functionalities. Melting points (by dilatometry) and lamella spacings (by small-angle X-ray scattering) have been determined. The linked polymers have higher melting points than their unlinked precursors. The increased melting point is interpreted in the light of the model of Flory and Vrij.     

Influence of molecular interactions on spherulite morphology in miscible poly(ethylene oxide)/epoxy network versus poly(ethylene oxide)/poly(4‐vinyl phenol) blend

Relationships between the spherulite morphology and changes in hydrogen‐bonding interactions between the linear poly(ethylene oxide) (PEO) polymer and a crosslinking epoxy system (diglycidylether of bisphenol‐A resin with 4,4′‐diaminodiphenylsulfone) (DGEBA/DDS) before and after cure have been explored The hydrogen‐bonding interaction is more significant before cure because of the interactions between the ether group of PEO and the amine group of DDS. The interaction between PEO and epoxy/DDS becomes less in the cured network. The morphology of the PEO crystals is, in turn, affected by the contents and chemical structures (functional groups, molecular weights, crosslinks, etc) of crosslinking epoxy/DDS. PEO/poly(4‐vinyl phenol) (PVPh), a thermoplastic non‐curing miscible system with the hydrogen bonding between the ether group of PEO and the ? OH group of PVPh, is also compared. In comparison with the PEO/epoxy/DDS system, the spherulite morphology of PEO/PVPh becomes more extensively spread out, with the extents increasing with the PVPh contents in the PEO/PVPh blend. © 2001 Society of Chemical Industry     

Melting of low molecular weight poly(ethylene oxide) with acetoxy- and trimethylsiloxy-end-groups

Melting points and lamellar spacings are reported for samples of low molecular weight poly(ethylene oxide) with acetoxy- and trimethylsiloxy- end-groups. The results, together with others reported earlier for hydroxy-, phenoxy- and chloro-ended polymers, show that the melting point can be much affected by the end-group. The main causes of the effects are ascribed to differences in end-end and end-chain interactions and to end-group dimensions.     

Kinetic theory of polymer crystal growth applied to melt crystallization of low molecular weight poly(ethylene oxide)

The kinetic theory of polymer crystal growth from the melt is extended to polymers with finite molecular weight and small numbers of folds per molecule. The theory is applied specifically to poly(ethylene oxide), where the most detailed experimental data are available on the growth of crystals from low molecular weight fractions. Predicted curves of growth-rate versus temperature show extensive qualitative agreement with experiment, including increasing chain-folding with increasing molecular weight or supercooling. In the theory this arises from the assumption that molecules have no freedom of lengthwise position within a surface nucleus. It may provide a general rationalization for chain-folding, but could possibly be a consequence of end-group pairing in the special case of OH-terminated poly(ethylene oxide). The theory also explains the sharpness of observed transitions between growth-modes with different numbers of folds per molecule, and the changes in shape of crystals near the transition. Reasonable quantitative agreement with experiment is found in the two cases of high molecular weight and high degrees of supercooling. For low molecular weights and small supercoolings, however, there is a large quantitative discrepancy between the predicted and observed separations of adjacent branches of each growth-rate/temperature curve. This appears to be inexplicable in terms of existing understanding of polymer crystal growth. An Appendix is given which outlines the effects on the theory of relaxing various assumptions of the growth model.     

Association properties of a high molecular weight poly(propylene oxide–b–ethylene oxide) diblock copolymer in aqueous solution

Summary Summary A high molecular weight poly(propylene oxide–b–ethylene oxide) diblock copolymer was prepared via sequential anionic suspension polymerization using a calcium amidealkoxide initiating system. ¹H nuclear magnetic resonance, viscometry and static and dynamic light scattering have been used to characterize the copolymer and to examine its self–assembly in aqueous solution. The copolymer was found to self–associate in a narrow concentration range above a certain critical aggregation concentration. The weight–average molecular weight, the radii of gyration, the second virial coefficients, the diffusion coefficients, and the hydrodynamic radii of the particles in both unimer and aggregate regions were determined. Aggregates of low aggregation number (2–3) were formed. Dynamic light scattering measurements performed in a wide concentration range revealed an enhanced aggregate stability towards dissociation upon dilution.     

High molecular weight poly( -lactide) and poly(ethylene oxide) blends: thermal characterization and physical properties

The miscibility of high molecular weight poly( -lactide) PLLA with high molecular weight poly(ethylene oxide) PEO was studied by differential scanning calorimetry. All blends containing up to 50 weight% PEO showed single glass transition temperatures. The PLLA and PEO melting temperatures were found to decrease on blending, the equilibrium melting points of PLLA in these blends decreased with increasing PEO fractions. These results suggest the miscibility of PLLA and PEO in the amorphous phase. Mechanical properties of blends with up to 20 weight% PEO were also studied. Changes in mechanical properties were small in blends with less than 10 weight% PEO. At higher PEO concentrations the materials became very flexible, an elongation at break of more than 500% was observed for a blend with 20 weight% PEO. Hydrolytic degradation up to 30 days of the blends showed only a small variation in tensile strength at PEO concentrations less than 15 weight%. As a result of the increased hydrophilicity, however, the blends swelled. Mass loss upon degradation was attributed to partial dissolution of the PEO fraction and to an increased rate of degradation of the PLLA fraction. Significant differences in degradation behaviour between PLLA/PEO blends and (PLLA/PEO/PLLA) triblock-copolymers were observed.     

A high hydroxyl group content found in high molecular weight fractions of poly(oxyethylene glycol)

The results of a fractionation of high molecular weight poly(oxyethylene glycol) with the mixture benzene—isooctane are presented. The fractions are characterized by gel permeation chromatography (GPC), infrared spectroscopy, viscometry, and dialysis. A high hydroxyl content was found for the higher fractions, which is not compatible with a linear polyoxyethylene glycol molecule with hydroxyl endgroups. The presence of hydroxyl groups on the chain is improbable. The dialysis of the higher fractions in CCl₄ and toluene shows that a surprising amount passes through the dialysis bag. The possibility of degradation of the polymer is considered. However, GPC analysis of the products of the dialysis suggest that the high molecular weight is made up of aggregates of middle-sized molecules and low molecular weight ones, held together by hydrogen bonding between hydroxyl and ether groups. Some results of a fractionation in water with the lower critical solubility temperature at 99°C. are given.     

Crystallization of sheared polymer melts: poly(ethylene oxide) fractions

Summary Crystallization under controlled shear conditions has been optically monitored for a series of sharp molecular weight fractions of poly(ethylene oxide). The minimum shear rate (MSR) necessary to produce oriented crystalline morphologies exhibited a strong molecular weight (M) dependence: MSR=K/M^1.47. The time necessary for erasure of orientation effects after crystal melting was also determined and found to be proportional to M^1.42.     

Effect of molecular weight on the crystallization kinetics of poly(hexamethylene oxide)

Dilatometric crystallization isotherms have been analysed for poly(hexamethylene oxide) fractions ranging in molecular weight from 2200 to 33 500. Previously, the influence of the temperature and the time of melting in the reproducibility of the isotherms were studied. Deviations from the Avrami or Göler-Sachs free growth formulations are systematic with molecular weight and become more pronounced as the molecular weight increases. The Avrami exponent is an integral number, 4, and is independent of temperature and molecular weight. The crystallization rate goes through a maximum as a function of molecular weight and the location of this maximum depends on the undercooling. The crystallization temperature coefficient was studied using the three dimensional nucleation theory and it was found that the crystallization is described by a unique function of the free energy for nucleation when the change of the interfacial free energy with molecular weight is considered.