Phase Behavior of Poly(Ethylene Oxide) Studied by Modulated‐Temperature DSC—Influence of the Molecular Weight

Abstract

Melting and crystallization behavior of poly(ethylene oxide) (PEO) with different molecular weight was investigated by modulated‐temperature differential scanning calorimetry (MT‐DSC)—step‐scan alternating DSC. It was found that by separating the reversing and nonreversing components of the (total) heat flow, PEO 10000, which exhibits the highest degree of crystallinity, shows the smallest nonreversing signal during crystallization. This effect can be attributed to the favorable structural features associated with spacial alignment. On the other hand, the crystallization process of PEO with molecular weight of 3400 is hindered by a relatively high content of end groups that may cause defects in the crystal lattice. For PEO 35000, low segmental mobility and chain entanglements lower the rate of crystallization. The area of the reversing component of PEO melting for different molecular weight fractions confirms that for PEO 10000, recrystallization is less intensive than for both the lower and higher molecular weight analogues.     

From the melt via mesomorphic and granular crystalline layers to lamellar crystallites: A major route followed in polymer crystallization?

Based on broad and detailed evidence from a large variety of experiments on several polymer systems carried out by other authors and ourselves, a novel concept for understanding the crystallization of polymers from the melt is developed. The experiments generally indicate that the formation and growth of the lamellar crystallites is a multi-step process passing over intermediate states. We suggest a specific route which is compatible with the observations. It is proposed that the initial step is always the creation of a mesomorphic layer which spontaneously thickens, up to a critical value, where it solidifies through a cooperative structural transition. The transition produces a granular crystalline layer, which transforms in the last step into homogeneous lamellar crystallites. The model leads to predictions about the temperature dependencies of the crystal thickness and the growth rate which are at variance with conventional views but in agreement with findings in recent experiments. Received 17 February 2000 and Received in final form 30 March 2000     

Crystallization of Poly(Ethylene Oxide) in Thin Films

Crystallization of poly(ethylene oxide) (PEO) in thin films was studied using hot-stage polarized optical microscopy. Isothermal linear crystal growth rates were measured for various film thicknesses at various degrees of undercooling. At a given crystallization temperature, the linear crystal growth rate decreased exponentially with decreasing film thickness below a film thickness of 80 nm. Films showed similar spherulitic morphology down to a film thickness of 30 nm. Control experiments on hydrophilic and hydrophobic surfaces showed that surface chemistry affects stability of the polymer films and causes a competition between crystallization and dewetting.     

Structure and crystallization of molecular complexes between poly(ethylene oxide) and hydroxybenzenes

The phase diagrams of the binary systems poly(ethylene oxide) (PEO)-resorcinol and poly(ethylene oxide)-p-nitrophenol show the presence of molecular complexes with well-defined stoichiometries. The crystal structure of these two molecular complexes has been determined from wide-angle x-ray diffraction patterns of stretched films and spherulites. The morphology of the two complexes crystallized from the melt is investigated by differential scanning calorimetry and small-angle x-ray scattering. The crystallization of the PEO-resorcinol complex from the melt gives integral-folded crystals with either extended chains (EC) or n-folded chains (n-FC). As observed for PEO oligomers, the fraction of EC crystals of the PEO-resorcinol complex increases with the crystallization temperature to give finally only EC crystals but in a larger range of crystallization temperatures than for pure PEO. On the other hand, the PEO-p-nitrophenol complex crystallizes over all the studied crystallization range as stable nonintegral-folded (NIF) crystals. Two proposals related to the crystal structure of these complexes and their mode of growth are invoked to explain these two greatly different morphologies at the lamellar level.     

Influence of Blend Composition on Crystallization of Poly(L-Lactic Acid)/Poly (Ethylene Oxide) Crystalline/Crystalline Blends

The effect of blend composition on crystallization morphology and behavior of a crystalline/crystalline blend, poly(l-lactic acid) (PLLA)/poly(ethylene oxide) (PEO), during slow, non-isothermal crystallization was studied by polarized light microscopy (PLM) connected with a hot-stage and differential scanning calorimetry (DSC). The results showed that all of the PLLA/PEO blends produced spherulites which gradually became bigger and looser, as well as coarser, with the increment of the PEO content, indicating that the PEO crystals was resided in the interlamellar or interfibrillar (between clusters of commonly oriented lamellae) regions of the PLLA spherulites. In the (25/75) and (10/90) blends, the nucleation and growth processes of the PEO spherulites could be clearly observed in the pre-existing PLLA spherulites. The onset crystallization temperature and the melting point of one component decreased with increasing the content of the other one owing to the good miscibility of the two components in the non-crystalline state and the interaction between their macromolecules, indicating that the crystallization of each component was influenced by the other one.     

Crystallization Behavior of PEO in Nano‐Structured PEO‐b‐PS/PPO Blends

Blends of poly (ethylene oxide)‐b‐polystyrene (PEO‐b‐PS) diblock copolymer and poly (2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) homopolymer were obtained by solution blending, and the morphologies of PEO dispersed nanoparticles in PPO/PS matrix were observed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The isothermal crystallization kinetics was studied using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Nonisothermal crystallization kinetics was studied using DSC. The results showed that PEO segments were easier to crystallize in the blend than in the copolymer probably due to the interfaces of PPO acting as nucleation sites to promote the crystallization of PEO. The crystallization of PEO blocks destroyed the pre‐existing microdomain structure even though the glass transition temperature of the matrix was much higher than the crystallization temperature.     

Phase Ordering and Crystallization Kinetics in Ultrathin Poly(ethylene oxide)/Poly(methyl methacrylate) Blend Films

Crystallization in ultrathin Poly(Ethylene Oxide)/Poly(Methyl Methacrylate) (PEO/PMMA) blend films with thickness of ca. 10 nm was investigated by means of microscopic and in situ spectroscopic methods. It was revealed that the blend films undergo a phase ordering in a humid atmosphere before or during crystallization, with PEO de-mixing with PMMA and segregating to the free film interface on the PMMA layer. The de-mixed PEO chains crystallize into a fractal-like morphology by a diffusion-limited process, and the crystal growth is 1-dimensional with Avrami exponent n ≈ 1, resulting in flat-on crystal lamellae with the PEO chains oriented normal to the film plane.     

Interfacial morphology and friction properties of thin PEO and PEO/PAA blend films

The scanning force microscope (SFM) was used to investigate morphology of poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) blend. The effect of solvent and dewetting in surface structure of PEO film was reported. The results manifested that the crystallization of PEO could be suppressed completely in ultrathin region via using chloroform as a solvent, and the branched-like crystallization was recovered after dewetting. Also, the effect of thickness, the ratio of PEO/PAA and dewetting in surface morphology of PEO-PAA blend films were investigated. These results showed that the crystallization was highly dependent on the ratio of PEO/PAA and the thickness of blend film. Furthermore, we assembled the PEO/PAA layer-by-layer film by spin-casting method for the first time, which exhibited highly efficiency. As a complementary tool, we also used lateral force microscopy (LFM) to explore surface information of these films. The result was indicative of interfacial constraints in ultrathin region, and also was supported by the results showing the spin-casting PEO/PAA blends rather than heterogeneous mixture.     

Interactions between high-temperature deformation and crystallization in zirconium-based bulk metallic glasses

High-temperature deformation of a ZrTiCuNiBe bulk metallic glass (BMG) is investigated by compression tests in the supercooled liquid region. When the temperature is decreased or strain rate increased, the amorphous alloy exhibits the usual Newtonian/non-Newtonian transition behaviour. Using specific heat treatments, partially crystallized alloys are produced, the associated microstructures characterized and the volume fractions of the crystal measured. The interaction between high-temperature deformation and crystallization is investigated by appropriate mechanical testing. According to these measurements, partial crystallization is responsible for a significant increase in flow stress and the promotion of non-Newtonian behaviour. Deformation does not significantly change the volume fraction, composition or size of the crystal. The flow-stress increase with crystallization is analyzed under different hypotheses. We conclude that the flow-stress increase cannot be interpreted through a compositional change in the residual amorphous matrix, either by reinforcement due to hard crystallites or by connections between crystals. It appears that the effect is due to the nanometric size of the crystals alone.     

聚对苯二甲酸丁二醇酯/聚(对苯二甲酸丁二醇酯-e-己内酯)二元结晶共混体系的相容性和结晶行为

聚对苯二甲酸丁二醇酯(PBT)/聚(对苯二甲酸丁二醇酯-e-己内酯)(PBT-PCl)是一个新制备的具有分子间排斥相互作用的A/AxB1?x型两元结晶共混体系. 根据两元平均场模型,报道对苯二甲酸丁二醇酯(BT)与"-己内酯(CL)结构单元的相互作用参数为0.305. DSC研究发现,此共混物呈现了与典型的共聚物/均聚物共混物不同的结晶特征. PBT-PCL影响PBT链的活动力和晶片堆积;同时PBT-PCL的结晶受到先期结晶的PBT晶粒的阻滞. 尽管拥有相同的BT单元,共混的两组分在组成变化范围内仍没有形     

Thermal annealing of a-Si:H/a-SiNx:H multilayers

The crystallinity of Si/SiN_x multilayers annealed by a rapid thermal process and furnace annealing is investigated by a Raman-scattering technique and transmission electron microscopy. It is found that the crystallization temperature varies from 900 °C to 1000 °C when the thickness of a-Si:H decreases from 4.0 nm to 2.0 nm. Raman measurements imply that the high crystallization temperature for the a-Si:H sublayers originates from the confinement modulated by the interfaces between a-Si:H and a-SiN_x:H. In addition to the annealing temperature, the thermal process also plays an important role in crystallization of a-Si sublayers. The a-Si:H sublayers thinner than 4.0 nm can not be crystallized by furnace annealing for 30 min, even when the annealing temperature is as high as 1000 °C. In contrast, rapid thermal annealing is advantageous for nucleation and crystallization. The origin of process-dependent crystallization in constrained a-Si:H is briefly discussed. Received: 11 April 2001 / Accepted: 20 June 2001 / Published online: 30 August 2001     

Kinetics of Isothermal and Nonisothermal Crystallization of Poly(ethylene oxide) (PEO) in PEO/Fatty Acid Blends

In this work, isothermal and nonisothermal crystallization kinetics of poly(ethylene oxide) (PEO) and PEO in PEO/fatty acid (lauric and stearic acid) blends, that are used as thermal energy storage materials, was studied using differential scanning calorimetry (DSC) data. The Avrami equation was adopted to describe isothermal crystallization of PEO and nonisothermal crystallization was analyzed using both the modified Avrami approach and Ozawa method. Avrami exponent (n) for PEO crystallization was in the range 1.08–1.32 (10–90% relative crystallinity), despite of spherulites formation, while for PEO in PEO/fatty acid blends n was between 1.61 and 2.13. Hoffman and Lauritzen theory was applied to calculate the activation energy of nucleation (K_g) – the lowest value of K_g was observed for pure PEO, despite of heterogeneous nucleation of fatty acid crystals in PEO/fatty acid blends. For nonisothermal crystallization of PEO in PEO/lauric acid (1:1 w/w) and PEO/stearic acid (1:3 w/w) blends, secondary crystallization occurred and values of the Avrami exponent were 2.8 and 2.0, respectively. The crystallization activation energies of PEO were determined to be ?260 kJ/mol for pure PEO, ?538 kJ/mol for PEO/lauric acid blend, and ?387 kJ/mol for PEO/stearic acid blend for isothermal crystallization and ?135,6 kJ/mol, ?114,5 kJ/mol, and ?92,8 kJ/mol, respectively, for nonisothermal crystallization.     

About impedance spectra and dynamics of charge carriers in neat poly(ethylene oxide) structures

Impedance spectra of linear and cross-linked poly(ethylene oxide) (PEO) are analyzed in a wide range of temperature. Dielectric responses differ at low and high temperature due to tendency of PEO to crystallization below melting temperature. Extent of crystallization depends on cross-linking density. The network in PEO with high cross-linking density is rigid and morphology transition shifts to very low temperature. Debye-like relaxation appears at low temperature similar as in ionic liquids. Onset of polarization relaxation shifts to higher temperature with increasing mesh size that is coupling of electric and structural relaxation appears. This is also nicely reflected by scaled conductivity. It demonstrates that the structure in cross-linked systems is a superposition of chemical and physical networks. They relax separately at low temperature and frequency under condition of sufficient rigidity.     

Crystallization behaviour of Ge17Sb23Se60 thin films

The crystallization behaviour of as-prepared and nucleated Ge₁₇Sb₂₃Se₆₀ thin films was studied by means of differential scanning calorimetry, X-ray diffraction analysis and scanning electron microscopy. Detailed analysis of the non-isothermal crystal growth kinetics was performed; the apparent activation energy, kinetic model and value of the pre-exponential factor were determined. The kinetic behaviour was found to be surprisingly close to the ideal Johnson–Mehl–Avrami nucleation-growth process, with the only non-ideality being the prolonged peak end tail (which may be a specificity associated with certain thicknesses of thin layers). This corresponds to the initiation of crystal growth in agreement with the classical nucleation theory, with the amount of mechanical defects and strains being negligible. The value of the kinetic parameter suggests two-dimensional crystal growth, which is consistent with the idea of macroscopic crystallites growing in a sterically restricted thin layer. A similar conclusion can be made on the basis of direct microscopic observation of the crystallites’ morphology.     

Preparation and Characterization of Poly(ethylene oxide)/Graphene Nanocomposites from an Aqueous Medium

The nanocomposite films of a functionalized graphene sheet (FGS) and poly(ethylene oxide) (PEO) were cast from the physical blend of an aqueous FGS dispersion assisted by sodium dodecyl sulfate and an aqueous PEO solution. The thermal properties observed by differential scanning calorimetry suggested that FGS had a nucleating effect on the PEO crystallization. However, we found FGS actually hindered the growth of PEO crystals. The dynamic mechanical properties indicated that FGS effectively reinforced the matrix PEO. The FGS also efficiently improved the electric conductivity of PEO. With the addition of 2 parts of FGS per 100 parts of PEO, the conductivity was increased by more than 10³-fold from that of pristine PEO.     

Onset of tethered chain overcrowding

We proposed an approach to precisely control the density of tethered chains on solid substrates using PEO-b-PS and PLLA-b-PS. As the crystallization temperature Tx increased, the PEO or PLLA lamellar crystal thickness d(L) increased as well as the reduced tethering density sigma; of the PS chains. The onset of tethered PS chains overcrowding in solution occurs at sigma(*) approximately 3.7-3.8 as evidenced by an abrupt change in the slope between (d(L))(-1) and Tx. This results from the extra surface free energy created by the tethered chain that starts to affect the growth barrier of the crystalline blocks.     

Characteristic variations in the effect of diluents on polymer crystallization and melting observed for a sample of poly(ethylene-co-octene)

Melting points in mixtures of a crystallizable polymer with a low-molar-mass diluent depend on both, the diluent fraction and the crystal thickness. A differentiation of the two factors can be achieved by temperature-dependent SAXS experiments. A corresponding study, complemented by DSC, dilatometry, microscopy and AFM-imaging, was carried out for mixtures of a poly(ethylene-co-octene) with n-C₁₆H₃₄, c-C₁₆H₃₂ and methyl-anthracene, respectively. All diluents lead for a constant crystal thickness to melting point depressions in agreement with Raoult's law. On the other hand, the effect of the diluents on the thickness of the crystals formed at a fixed crystallization temperature varies. While in the presence of the two alkanes thicker crystals form, no effect arises for the methyl-anthracene—as was previously found for the octene-co-units. We consider these observations as a further support for our view that polymer crystallization follows a multi-stage route which includes a passage through an intermediate mesomorphic phase. Under such conditions crystal thicknesses would only be affected if the diluent is still present in the mesomorphic phase and stay invariant if the diluent molecules are already rejected when this intermediate phase forms.     

Morphologies of diblock copolymer thin films before and after crystallization

Spin-coated thin films of about 100nm of low-molecular-weight hydrogenated poly(butadiene-b- ethyleneoxide) (PB_h-PEO) diblock copolymers have been crystallized at various constant temperatures. Crystallization has been observed in real time by light microscopy. Detailed structural information was obtained by atomic force microscopy, mainly enabled by the large viscoelastic contrast between amorphous and crystalline regions. The behavior in thin films is compared to the bulk properties of the polymer. Crystallization started from an annealed microphase separated melt where optical microscopy indicated a lamellar orientation parallel to the substrate. A small difference in the length of the crystallizable block produced significantly different crystallization behavior, both in the bulk and in thin films. For thin films of the shortest diblock copolymer (45% PEO content) and for an undercooling larger than about 10 degrees, crystallization created vertically oriented lamellae. These vertical lamellae could be preferentially aligned over several micrometers when crystallization occurred close to a three-phase contact line. Annealing at temperatures closer to the melting point or keeping the sample at room temperature for several months allowed the formation of a lamellar structure parallel to the substrate. A tentative interpretation based on kinetically caused chain folding and relaxation within the crystalline state, with implications on general aspects of polymer crystallization, is presented. Received 19 March 1999 and Received in final form 14 December 1999     

Crystallization of poly(ethylene-co-octene): II Melt memory effects on first order kinetics

Dilatometric and X-ray scattering experiments of the crystallization kinetics of a sample of poly(ethylene-co-octene) show pronounced melt memory effects, i.e., the shapes of isotherms and characteristic times vary systematically with the temperature of the melt prior to cooling to the crystallization temperature. The temperature range of the effect is limited; crystallization kinetics remains constant below a melt temperature Tm l and above a melt temperature Tmh and varies only in-between. Analysis shows that the melt memory effect is caused by a variation of the characteristic time of a first order crystallization process. The process can be assigned to the in-filling of crystallites into objects of a previously generated precursor structure.     

THE HIGH-PRESSURE CRYSTALLIZATION AND ANNEALING BEHAVIORS OF POLYETHYLENE TEREPHTHALATE OLIGOMER

Polyethylene terephthalate (PET) oligomer samples crystallized and annealed at high pressure were investigated with differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The results showed that better crystals were obtained through high-pressure crystallization from the melt than annealing under the same conditions. The difference of the effects of crystallization and annealing on the morphology of crystals reduced with the increase of crystallization time. The melting temperature was determined by the lamellar thickness when it was shorter than the length of the molecular chains, while the main factor governing the melting temperature changed from lamellar thickness to density of chain-end defects when the lamellar thickness was much longer than the molecular length. PET oligomer extended-chain crystals with thickness up 100 μm were obtained.