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.     

Morphology of poly(hexamethylene oxide) fractions

The supermolecular structure of crystallised molecular weight fractions of poly(hexa-methylene oxide), covering the molecular weight range (M) 4.5. × 10³ to 8. 5 × 10⁴, was studied by polarised light microscopy and small angle light scattering. Different forms were observed as a function of molecular weight and crystallisation temperatures. Perfect spherulites are formed after rapid crystallisation, and these forms deteriorate as both the molecular weight and crystallisation temperature increase. The morphology in the isothermal crystallisation region corresponds to an intermediate state which represents a transition from spherulites to hedrites.     

Blends of polycarbonate and poly(hexamethylene sebacate): II. Effect of molecular weight on compatibility

Blends of polycarbonate (PC) and poly(hexamethylene sebacate) (HMS) with two different molecular weights were prepared and their thermal properties were studied via differential scanning calorimetry. It was found that the high molecular weight PC (HPC) and high molecular weight HMS (HHMS) were partially miscible as evidenced by the decrease in glass transition temperature of HPC in the blends. This partial miscibility is attributed to the interaction of the carbonyl dipole of the ester group and the highly polarizable aromatic carbonate structure. When the low molecular weight PC or HMS was used, the compatibility was enhanced because of the increased entropic contribution to the Gibbs free energy of mixing. In all the blends prepared, the PC crystallized as a result of the plasticizing effect of HMS. Bisphenol-A diphenyl carbonate (dimer) was synthesized and used as the dimeric model of PC. This material was found to be an excellent diluent for HPC. A single glass transition was found in the HPC/dimer system and the temperature was dependent on the composition. The Couchman's equation was found to fit very well the glass transition temperature versus composition relationship for this system. A single glass transition was also found in the HHMS/dimer system. The melting point depression analysis was performed and resulted in a very low heat of fusion for the 100% crystalline HHMS. It may suggest that the dimer and HHMS are not completely miscible in the amorphous region.     

Effect of molecular weight on spherulite growth rates of high molecular weight poly(ethylene oxide) fractions

Spherulite growth rates have been determined for a set of poly(ethylene oxide) fractions ranging in molecular weight from 10⁴ to 10⁶. At a given crystallization temperature the spherulite growth rate as a function of molecular weight passes through a maximum. At a given undercooling (as assessed by the method of Mandelkern) the spherulite growth rate is a monotonically decreasing function of molecular weight, and in the range $\text{6000 <}\text{M}\text{?}{}_{\mathrm{v}}\text{< 50 000}$ varies roughly as $\text{(}\text{M}\text{?}{}_{\mathrm{v}}\text{)}{}^{\mathrm{?3}}$. The free energy of formation of the end interface (as assessed by nucleation theory) also decreases as molecular weight increases.     

Influence of molecular weight on the crystallization of poly(vinylidene fluoride)

Five samples of Poly (vinylidene fluoride) with a molecular weight from 45,000 to 260,000 were analyzed by means of FT-IR spectroscopy in order to study the influence of molecular weight on the crystallization process. The spectra of films obtained from methylethylketone solutions revealed the prevalence of form II () in the case of lower and of form III () in the higher molecular weight samples. The amount of form III was found to increase as the Mv decreased and as annealing temperatures increased. The deconvolution technique of vibrational spectra allowed detection of small amounts of other forms accompanying the prevailing one.     

Effect of surface-modified zinc oxide nanowires on solution crystallization kinetics of poly(3-hexylthiophene)

Interfacial interactions between donor and acceptor molecules are determinative to the device performance of hybrid photovoltaics. However, the dynamic process of such interactions remains largely obscure. In this work, we report the kinetic behavior of solution crystallization of poly(3-hexylthiophene) (P3HT) in anisole in the presence of ZnO nanowires by means of ultraviolet-visible absorption spectroscopy. ZnO nanowires are surface-modified by covalently attaching aliphatic and aromatic ligands to enhance the miscibility and interfacial interactions between P3HT and ZnO nanowires. Upon cooling the hot solution to room temperature, a significant time-dependent chromism occurs spontaneously. Analysis of the kinetics of isothermal solution crystallization across a range of crystallization temperature displays that the growth rate of the crystals scales with polymer concentration as R ∝ C^1.6 for both the control P3HT and P3HT with ZnO nanowires. The Lauritzen–Hoffman theory of secondary nucleation is utilized to analyze the kinetic behavior of crystallization, and the fold surface free energies of the crystals of P3HT in anisole are calculated to be 6.6–10.3 × 10⁻² J m⁻². It is found that the addition of surface-modified ZnO nanowires decreases the fold surface free energy by 21.5% and 43.8%, respectively, for aliphatic and aromatic ligands.     

Molecular weight dependent of the crystallization kinetics of poly(vinylidene fluoride)

Samples of poly(vinylidene fluoride) with weight average molecular weights between 100,000 to 500,000 were analyzed using differential scanning calorimetry to study the influence of molecular weight on crystallization. Isothermal crystallization from the melt was investigated between 140 and 145°C. Crystallization rates were analyzed in terms of the Avrami equation. Avrami exponents between 3 and 11 were observed depending upon the microstructure, molecular weight, and cooling rate. The midrange molecular weight samples were found to crystallize slower than the high and low molecular weight materials.     

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.     

Effect of molecular weight on crystallization of semirigid poly(phenylene sulfide) under shear flow

Isothermal crystallization of poly(phenylene sulfide) with three molecular weights (M_w = 22k, 48k, and 52k, respectively) under shear condition has been investigated. It appears that shear can induce all these three PPS samples to form a thread‐like crystal structure which consists of the numerous stable nuclei that align tightly in the direction of shear. Crystallization kinetics of PPS has been greatly influenced by shear flow. Higher shear rate and long shear time can lead to decrease of spherulite growth rate of PPS. Also, the spherulite growth rate of PPS is affected by supercoolings and molecular weight. For the lower molecular weight (M_w = 22k), the spherulite growth rate is independent on the shear rate and shear time; while for the higher ones (M_w = 48k and 52k), with the increasing of shear rate, the spherulite growth rate of PPS increases to reach maximum at first, and then decreases. The lower the crystallization temperature is, the more the spherulite growth rate changes, showing that higher orientation of molecular chains can be obtained more easily with increased supercooling. A model has been proposed to explain the mechanism of thread‐like crystal formation under shear flow. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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.     

Molecular weight distribution of fractionated poly (hexamethylene Oxide)

Poly(hexamethylene oxide) obtained by two routes, polycondensation and cationic polymerisation, is fractionated and its molecular weight distribution is analysed by using well known data treatment methods namely those of Beall, Wesslau, Tung and Saiz and Horta. The results are compared and discussed showing that the polycondensated sample has a lower molecular weight and wider distribution than the cationic one.     

Effect of molecular weight and branch structure on the crystallization and rheological properties of poly(butylene adipate)

High molecular weight linear and branched poly(butylene adipate)s (PBAs) were prepared, and crystallization behavior and rheological properties were investigated. PBAs showed multiple melting behavior. Increasing molecular weight retarded the crystallization rate and increased the induction time for crystallization. Introducing branches to the polymer led to similar results. The Avrami exponent (n) for the linear and branched PBAs were 3.79–4.39 and 2.09–3.64, respectively. The Mark‐Houwink exponent of the PBAs was 0.554. Unlike general‐purpose polyesters, high molecular weight PBAs did not exhibit a Newtonian flow region. High molecular weight PBAs exhibited gel‐like rheological properties, producing high elasticity. The slopes of a log‐log plot of storage modulus (G′) vs. loss modulus (G″) ranged from 1.75 to 1.17, and the slope decreased with increasing molecular weight. For comparable molecular weights, the branched PBAs had higher viscosity and G′ than the linear polymers.     

Effect of melt history on the crystallization kinetics of poly(phenylene sulfide)

Differential scanning calorimetry (DSC) was used to study the effects of melt history on the isothermal crystallization kinetics of poly(phenylene sulfide) (PPS). Crystallization of the polymer was shown to be permanently altered by both the time and the temperature spent in the melt. Suggested explanations are nonequilibrium melting, chain scission, chain extension, and crosslinking. Molecular weight was also shown to affect the rates of these processes, PPS of lower molecular weight being more susceptible to the observed changes.     

Effect of CO2 on crystallization kinetics of poly(ethylene terephthalate)

The effect of CO₂ on the isothermal crystallization kinetics of poly(ethylene terephthalate), PET, was investigated using a high‐pressure differential scanning calorimeter (DSC), which performed calorimetric measurements while keeping the polymer in contact with presurized CO₂. It was found that the crystallization rate followed the Avrami equation with values of the crystallization kinetic constant dependent on the crystallization temperature and concentration of CO₂ in PET. The presence of CO₂ in the PET increased its overall crystallization rate. CO₂ also decreased the glass transition temperature, T_g, and the melting temperature, T_m. As a result, the observed changes in crystallization rate caused by CO₂ can be qualitatively predicted from the magnitude of T_g depression and that of the equilibrium melting temperature, T_m⁰.     

含较高立构缺陷聚乳酸的非等温结晶动力学

用非等温结晶动力学研究了聚乳酸(PLA)的结晶行为,利用Hoffman-Weeks外推法以及Baur等提出的方程反推出PLA中的右旋组分摩尔分数为7.2%.较高的右旋组分摩尔分数是PLA结晶过程中成核速率低的主要因素.添加质量分数为2%滑石粉等成核剂后,PLA结晶速率没有明显提高;含10%滑石粉的PLA结晶速率略上升,...     

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.     

Glass transition temperature-molecular weight relation for poly(hexamethylene perchloroterephthalamide)

Summary In order to determine the Tg variation with the molecular weight, several fractions of poly (hexamethylene perchloroterephthalamide) were studied. The Tg-molecular weight relationship was established to determine the Tgand K values of Fox-Flory relationship.The authors acknowledge the support for this research by the DIUC, Dirección de Investigación de la Universidad Católica de Chile.     

Isothermal crystallization of poly(ethylene-terephthalate) of low molecular weight by differential scanning calorimetry: 1. Crystallization kinetics

The isothermal crystallization of poly(ethylene-terephthalate) (PETP) fractions, from the melt, was investigated using differential scanning calorimetry (d.s.c.). The molecular weight range of the fractions was from 5300–11750. Crystallization temperatures were from 498–513 K. The dependence of molecular weight and undercooling on several crystallization parameters has been observed. Either maxima or minima appear at a molecular weight of about 9000, depending on the crystallization temperature. The activation energy values point to the possibility of different mechanisms of crystallization according to the chain length. A folded chain process for the higher $\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$ chains and an extended chain mechanism for the lower $\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$ chains. The values of the Avrami equation exponent n vary from 2–4 depending on the crystallization temperature; non-integer values are indicative of heterogeneous nucleation. The rate constant K depends on T_c and $\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$, showing maxima related to the T_c used. The plot of log K either vs. (ΔT)^?1 and (ΔT)^?2 or $\text{T}{}_{\mathrm{<mtext>m</mtext>}}\text{T(ΔT)}$ and $\text{T}{}^2{}_{\mathrm{<mtext>m</mtext>}}\text{T(ΔT)}{}^2$ is linear in every case.