Crystallization and vitrification effect in a poly(styrene)-g-poly(ethylene oxide) graft copolymer

Crystallization and solid state structure of a poly(styrene)-graft-poly(ethylene oxide) (PS-g-PEO) graft copolymer with crystallizable side chains were studied using simultaneous small angle X-ray scattering/wide angle X-ray scattering/differential scanning calorimetry (SAXS/WAXS/DSC). It is found that the glass transition temperature (T_g) of PS main chain is remarkably higher than that of PS homopolymer. The start cooling temperature (T_o) has a great influence on crystallization of the PEO side-chain. When the graft copolymer is cooled from the temperature above T_g, phase separation is suppressed due to the low mobility of the PS main chain and the homogeneous melt is vitrified. The unfavorable conformation of the rigid main chain results in a single crystallization peak and lower crystallinity. When PS-g-PEO is only heated to a temperature lower than the T_g and then cooled, phase separation is retained. Both the PEO side chains with high and low crystallizability can crystallize in the phase-separated state, leading to double crystallization peaks and higher crystallinity. The effect of solvent on crystallization of the graft copolymer was also examined. It is observed that addition of toluene reduces the T_g of the PS main chain and leads to the disappearance of the vitrification effect.     

Nanostructure of atmospheric and high-pressure crystallized poly(ethylene-2,6-naphthalate). Part II: structure-microhardness correlations

The microhardness of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN), with a detailed characterized nanostructure has been investigated. PEN samples were crystallized from the glassy state at atmospheric pressure and from the melt at high pressure and were characterized using small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). Results show that the degree of crystallinity derived from WAXS, for both atmospheric and high-pressure crystallized PEN, is smaller than that obtained by density and calorimetry. For high-pressure crystallized samples, both, crystallinity and microhardness values are larger than those found for the material crystallized under atmospheric pressure. In the latter case, the hardness values depend on the volume fraction of lamellar stacks within spherulites X_L that depends on the crystallization temperature T_c. For T_c<200 °C, X_L is found to be less than 50%. Thus, for T_c<200 °C a linear relationship between H and T_c is observed provided a sufficiently long crystallization time is used. Results are discussed in terms of the rigid amorphous fraction that appears as a consequence of the molecular mobility restrictions due to the crystal stacks.     

Miscibility and crystallization in crystalline/crystalline blends of poly(butylene succinate)/poly(ethylene oxide)

Miscibility and crystallization behavior have been investigated in blends of poly(butylene succinate) (PBSU) and poly(ethylene oxide) (PEO), both semicrystalline polymers, by differential scanning calorimetry and optical microscopy. Experimental results indicate that PBSU is miscible with PEO as shown by the existence of single composition dependent glass transition temperature over the entire composition range. In addition, the polymer-polymer interaction parameter, obtained from the melting depression of the high-T_m component PBSU using the Flory-Huggins equation, is composition dependent, and its value is always negative. This indicates that PBSU/PEO blends are thermodynamically miscible in the melt. The morphological study of the isothermal crystallization at 95 °C (where only PBSU crystallized) showed the similar crystallization behavior as in amorphous/crystalline blends. Much more attention has been paid to the crystallization and morphology of the low-T_m component PEO, which was studied through both one-step and two-step crystallization. It was found that the crystallization of PEO was affected clearly by the presence of the crystals of PBSU formed through different crystallization processes. The two components crystallized sequentially not simultaneously when the blends were quenched from the melt directly to 50 °C (one-step crystallization), and the PEO spherulites crystallized within the matrix of the crystals of the preexisted PBSU phase. Crystallization at 95 °C followed by quenching to 50 °C (two-step crystallization) also showed the similar crystallization behavior as in one-step crystallization. However, the radial growth rate of the PEO spherulites was reduced significantly in two-step crystallization than in one-step crystallization.     

Crystallization characteristics of weakly branched poly(ethylene terephthalate)

A series of branched poly(ethylene terephthalate) (BPET) samples were prepared from melt polycondensation by incorporation of various amount (0.4-1.2 mol%) of glycerol as a branching agent. These polymers were characterized by means of H¹ NMR, intrinsic viscosity. The general crystalline and melting behavior was investigated via DSC. It was found that the crystalline temperature T_cc from the melt shifted to high temperature and the T_hc from the glass got low for BPETs while the melting temperatures of BPETs kept almost unchanged. The kinetics of isothermal crystallization was studied by means of DSC and POM. It was found that the present branching accelerated the entire process of crystallization of BPETs, although prolonged the induced time. In addition, branching reduced nucleation sites; hence the number of nucleates for BPET got smaller. Therefore, more perfect geometric growth of crystallization and greater radius of spherulites could develop in BPET due to less truncation of spherulites.     

Influence of composition and molecular mass on the morphology,crystallization and melting behaviour of poly(ethylene oxide)/poly(methyl methacrylate) blends

Results are reported on the influence of composition and molecular mass of components on the isothermal growth rate of spherulites, on the overall kinetic rate constant, on the primary nucleation and on the thermal behaviour of poly(ethylene oxide)/poly(methyl methacrylate) blends. The growth rate of PEO spherulites as well as the observed equilibrium melting temperatures decrease, for a given T_c or ΔT, with the increase of PMMA content.Such observations are interpreted by assuming that the polymers are compatible in the undercooled melt, at least in the range of crystallization temperatures investigated. Thermodynamic quantities such as the surface free energy of folding σ_e and the Flory-Huggins parameter χ₁₂ have been obtained by studying the dependence of the radial growth rate G and of the overall kinetic rate constant K from temperature and composition and the dependence of the equilibrium melting temperature depression ΔT_m upon composition, respectively.     

Poly(trimethylene terephthalate) copolyester containing ethylene glycol moieties. Part 1: Crystallization kinetics and melting behavior

A copolyester was characterized to have 91 mol% trimethylene terephthalate unit and 9 mol% ethylene terephthalate unit in a random sequence by using ¹³C NMR. Differential scanning calorimeter (DSC) was used to investigate the isothermal crystallization kinetics in the temperature range (T_c) from 180 to 207 °C. The melting behavior after isothermal crystallization was studied by using DSC and temperature modulated DSC (TMDSC). The exothermic behavior in the DSC and TMDSC curves gives a direct evidence of recrystallization. No exothermic flow and fused double melting peaks at T_c = 204 °C support the mechanism of different morphologies. The Hoffman-Weeks linear plot gave an equilibrium melting temperature of 236.3 °C. The kinetic analysis of the growth rates of spherulites and the morphology change from regular to banded spherulites indicated that there existed a regime II → III transition at 196 °C.     

Single- and double-ring spherulites in poly(nonamethylene terephthalate)

A new mechanism of formation of ring-banded spherulites was discovered, where three different types of spherulites were present in poly(nonamethylene terephthalate) (PNT) cast in thin film forms. The growth and morphological features in three ring-banded spherulites were compared and analyzed using polarized/non-polarized light microscopy, differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) and element analysis by energy dispersive spectroscopy (EDS) on surfaces of spherulites. Three types of spherulites (labeled: Type-0, 1, and 2) are possible in PNT when crystallized at wide range of crystallization temperature (T_c); however, only two of the three types can co-exist in PNT at a given T_c. At lower T_c (<55 °C), within Regime-III, the spherulites in PNT are either of Type-0 (appearing ringless in POM) or Type-2 (double-band in POM). When crystallized at temperature of T_c = 55 °C or higher (Regime-II), the morphology is featured with Type-1 (single-ring-banded in POM) spherulites as the majority species that co-exist with Type-2 (double-ring-banded in POM) spherulites as imbedded minority. Origins and morphological differences of the single-band vs. double-band spherulites were compared. The double-band spherulites have much larger inter-ring spacing than do the single-band spherulites. By comparison, the rod-like lamellae in the double-banded spherulites (Type-2) are longer and thicker, and more ordered than those in the single-banded spherulites (Type-1).     

Characterization, crystallization kinetics and melting behavior of poly(ethylene succinate) copolyester containing 5 mol% trimethylene succinate

Copolyester was synthesized and characterized as having 94.4 mol% ethylene succinate units and 5.6 mol% trimethylene succinate units in a random sequence as revealed by NMR. Differential scanning calorimeter (DSC) was used to investigate the isothermal crystallization kinetics of this copolyester in the temperature range (T_c) from 30 to 80 °C. The melting behavior after isothermal crystallization was studied by using DSC and temperature modulated DSC (TMDSC) by varying the T_c, the heating rate and the crystallization time. DSC and TMDSC curves showed triple melting peaks. The melting behavior indicates that the upper melting peaks are primarily due to the melting of lamellar crystals with different stabilities. A small exothermic curve between the main melting peaks gives a direct evidence of recrystallization. As the T_c increases, the contribution of recrystallization gradually decreases and finally disappears. The Hoffman-Weeks linear plot gave an equilibrium melting temperature of 108.3 °C. The kinetic analysis of the spherulitic growth rates indicated that a regime II → III transition occurred at ∼65 °C.     

Crystallization kinetics and melting behavior of poly[(trimethylene terephthalate)‐co‐(29 mol % ethylene terephthalate)] copolyester

The copolyester was characterized as having 71 mol % trimethylene terephthalate units and 29 mol % ethylene terephthalate units in a random sequence according to the NMR spectra. Differential scanning calorimeter (DSC) was used to investigate the isothermal crystallization kinetics in the temperature range (T_c) from 130 to 170°C. The melting behavior after isothermal crystallization was studied using DSC and temperature‐modulated DSC by varying the T_c, the crystallization time, and the heating rate. The DSC thermograms and wide‐angle X‐ray diffraction patterns reveal that the complex melting behavior involves melting‐recrystallization‐remelting and different lamellar crystals. As the T_c increases, the contribution of recrystallization gradually falls and finally disappears. A Hoffman‐Weeks linear plot yields an equilibrium melting temperature of 198.7°C. The kinetic analysis of the growth rates of spherulites and the change in the morphology from regular to banded spherulites indicate that a regime II→III transition occurs at 148°C. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Investigations on the enhancement mechanism of inorganic filler on ionic conductivity of PEO-based composite polymer electrolyte: The case of molecular sieves

Polarized optical microscopy (POM) and differential scanning calorimeter (DSC) techniques are used to study the effect of ZSM-5 molecular sieves on the crystallization mechanism of poly(ethylene oxide) (PEO) in composite polymer electrolyte. POM results show that ZSM-5 has great influence on both the nucleation stage and the growth stage of PEO spherulites. ZSM-5 particles can act as the nucleus of PEO spherulites and thus increase the amount of PEO spherulites. POM and DSC results show that ZSM-5 can restrain the recrystallize tendency of PEO chains through Lewis acid-base interactions and hence decrease the growth speed of PEO spherulites. Room temperature ionic conductivity of PEO-LiClO₄-based polymer electrolyte can be enhanced by more than two magnitudes during long time storage with the addition of ZSM-5.     

Crystallization, spherulite growth, and structure of blends of crystalline and amorphous poly(lactide)s

The effects of incorporated amorphous poly(dl-lactide) (PDLLA) on the isothermal crystallization and spherulite growth of crystalline poly(l-lactide) (PLLA) and the structure of the PLLA/PDLLA blends were investigated in the crystallization temperature (T_c) range of 90-150 °C. The differential scanning calorimetry results indicated that PLLA and PDLLA were phase-separated during crystallization. The small-angle X-ray scattering results revealed that for T_c of 130 °C, the long period associated with the lamellae stacks and the mean lamellar thickness values of pure PLLA and PLLA/PDLLA blend films did not depend on the PDLLA content. This finding is indicative of the fact that the coexisting PDLLA should have been excluded from the PLLA lamellae and inter-lamella regions during crystallization. The decrease in the spherulite growth rate and the increase in the disorder of spherulite morphology with an increase in PDLLA content strongly suggest that the presence of a very small amount of PDLLA chains in PLLA-rich phase disturbed the diffusion of PLLA chains to the growth sites of crystallites and the lamella orientation. However, the wide-angle X-ray scattering analysis indicated that the crystalline form of PLLA remained unvaried in the presence of PDLLA.     

Crystallization of polyformals: 2. Influence of molecular weight and temperature on the morphology and growth rate in poly(1,3-dioxolane)

The morphology and growth rates of crystallized molecular weight fractions of poly(1,3-dioxolane) covering the range M_n = 8 800 to 120 000 have been studied by polarized light microscopy. Two different supermolecular structures, dependent on molecular weight and crystallization temperature have been found. Spherulites are formed after rapid crystallization and a more disordered morphology is formed at the lowest undercoolings but there is a temperature region where both forms are observed. The disordered form appears first and a consecutive spherulitic growth takes place. The crystallization kinetics were analysed over the temperature range 10°C to 36°C. At crystallization temperatures lower than 15°–18°C, the growth rate is linear and only spherulites are found. In the temperature range from 18°C to 36°C a well defined break is observed in the growth rate but the spherulitic growth rate is always higher than that of the irregular form. The growth rate temperature coefficient was studied and the usual plots are not linear in the whole range of crystallization temperatures. For the high crystallization temperature region, the slope is about twice as great as the low crystallization temperature slope. This is the region where regular spherulites are formed. The comparison between dilatometric and growth rate data has shown that the overall rate and growth rate temperature coefficients are the same.     

Poly(hydroxyether sulfone) and its blends with poly(ethylene oxide): miscibility, phase behavior and hydrogen bonding interactions

Poly(hydroxyether sulfone) (PHES) was synthesized through polycondensation of bisphenol S with epichlorohydrin. It was characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy and differential scanning calorimetry (DSC). The miscibility in the blends of PHES with poly(ethylene oxide) (PEO) was established on the basis of the thermal analysis results. DSC showed that the PHES/PEO blends prepared by casting from N,N-dimethylformamide (DMF) possessed single, composition-dependent glass transition temperatures (T_gs), indicating that the blends are miscible in amorphous state. At elevated temperatures, the PHES/PEO blends underwent phase separation. The phase behavior was investigated by optical microscope and the cloud point curve was determined. A typical lower critical solution temperature behavior was observed in the moderate temperature range for this blend system. FTIR studies indicate that there are the competitive hydrogen bonding interactions upon adding PEO to the system, which was involved with the intramolecular and intermolecular hydrogen bonding interactions, i.e. -OH?OS, -OH?-OH and -OH versus ether oxygen atoms of PEO between PHES and PEO. In terms of the infrared spectroscopic investigation, it is judged that from weak to strong the strength of the hydrogen bonding interactions is in the following order: -OH?OS, -OH?-OH and -OH versus ether oxygen atoms of PEO.     

Comparison of crystallization kinetics in various nanoconfined geometries

Phase morphological effect on crystallization kinetics in various nanoconfined spaces in a polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer with a PEO volume fraction of 37 vol% was investigated. The phase morphology was characterized by small-angle X-ray scattering and transmission electron microscopy techniques. When the sample was cast from chloroform solution and annealed at 150 °C, a double gyroid (DG) phase was obtained. After it was subjected to a large-amplitude reciprocating shear, the sample transformed to an oriented hexagonal cylinder (Hex) phase. To obtain a lamellar confined geometry, lamellar single crystals were grown from dilute solutions. The crystallization in the lamellar (Lam) phase was one-dimensionally (1D) confined, while it was two-dimensionally (2D) confined in the DG and Hex phases, although they had different structures. Differential scanning calorimetry (DSC) was employed to study the crystallization kinetics using the Avrami analysis for these three nanoconfined geometries. Heterogeneous nucleation was found in all three samples in the crystallization temperature (T_c) regions studied. DSC results indicated that the crystallization kinetics in the Lam phase was the fastest, and the PEO crystals possessed higher thermodynamic stability than in the DG and Hex phases. For the crystallization kinetics in two 2D-confined phases, at low T_c (<35 °C) the PEO crystallization rates in the DG and Hex phases were similar, while at high T_c (>35 °C) the PEO crystallization was slower in the DG phase than in the Hex phase. The Avrami exponent n-values for the DG and the Hex samples were similar (∼1.8), yet the values of lnK in the DG phase were smaller than those in the Hex phase. This suggested that the linear growth rate was slower in the DG phase than in the Hex phase due to continuous curved channels in the DG phase.     

Suppression of crystallization in a plastic crystal electrolyte (SN/LiClO4) by a polymeric additive (polyethylene oxide) for battery applications

A basic problem with many promising solid electrolyte materials for battery applications is that crystallization in these materials at room temperature makes ionic mobilities plummet, thus compromising battery function. In the present work, we consider the use of a polymer additive (polyethylene oxide, PEO) to inhibit the crystallization of a promising battery electrolyte material, the organic crystal forming molecule succinonitrile (SN) mixed with a salt (LiClO₄). While SN spherulite formation still occurs at low PEO concentrations, the SN spherulites become progressively irregular and smaller with an increasing PEO concentration until a ‘critical’ PEO concentration (20% molar fraction PEO) is reached where SN crystallization is no longer observable by optical microscopy at room temperature. Increasing the PEO concentration further to 70% (molar fraction PEO) leads to a high PEO concentration regime where PEO spherulites become readily apparent by optical microscopy. Additional diffraction and thermodynamic measurements establish the predominantly amorphous nature of our electrolyte-polymer mixtures at intermediate PEO concentrations (20-60% molar fraction PEO) and electrical conductivity measurements confirm that these complex mixtures exhibit the phenomenology of glass-forming liquids. Importantly, the intermediate PEO concentration electrolyte-polymer mixtures retain a relatively high conductivity at room temperature in comparison to the semicrystalline materials that are obtained at low and high PEO concentrations. We have thus demonstrated an effective strategy for creating highly conductive and stable conductive polymer-electrolyte materials at room temperature that are promising for battery applications.     

Optical microscopy and conductivity of poly(ethylene oxide) complexed with KI salt

The optical microscopy, crystallinity and ion conductivity of PEO complexed with potassium iodide (KI) salt are present. The spherulite structure derived from polarized optical microscopy (POM) suggested the ion induces different spherulite structures from that without salt. The size of the spherulites decreases with increasing salt concentration and is completely destroyed with KI salt content of 20 wt% (O-K ratio 15:1). This result concurs with the X-ray diffraction and DSC studies, that PEO crystallinity is reduced upon addition of KI salt. Upon elevating the temperature, the POM micrographs elucidate degradation of the spherulite structure when smaller crystallites begin to melt before the melting temperature of SPE, and become completely opaque above PEO melting temperature (T_m). The pseudo-activation energy derived from variable temperature conductivity measurements of about 0.24 eV is similar to that of PEO-lithium salt systems and suggested the identical PEO segmental motion governs the fundamental ion movement. Stable PEO-K complex (T_m from 135 to 155 °C) is formed after annealing at 120 °C for 12 h. However, the conductivity is about one order smaller compared to lithium salt due to the shorter hopping distance of the heavier potassium ion.     

Thermal characterizations of semi‐interpenetrating polymer networks composed of poly(ethylene oxide) and poly(N‐isopropylacrylamide)

Poly(N‐isopropylacrylamide) (PNIPAAm)/poly(ethylene oxide) (PEO) semi‐interpenetrating polymer networks (semi‐IPNs) synthesized by radical polymerization of N‐isopropylacrylamide (NIPAAm) in the presence of PEO. The thermal characterizations of the semi‐IPNs were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dielectric analysis (DEA). The melting temperature (T_m) of semi‐IPNs appeared at around 60°C using DSC. DEA was employed to ascertain the glass transition temperature (T_g) and determine the activation energy (E_a) of semi‐IPNs. From the results of DEA, semi‐IPNs exhibited one T_g indicating the presence of phase separation in the semi‐IPN, and T_gs of semi‐IPNs were observed with increasing PNIPAAm content. The thermal decomposition of semi‐IPNa was investigated using TGA and appeared at around 370°C. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3922–3927, 2003     

Crystallization and melt behaviour of isotactic poly(2-vinylpyridine)

The crystallization and melting behaviour of highly isotactic poly(2-vinylpyridine) (it-P2VP) with $\text{M}\text{?}{}_{\mathrm{<mtext>v</mtext>}}\text{= 4 × 10}{}^5$ has been studied by microscopy and d.s.c.. The maximum spherulitic growth rate was found to be 250 × 10^?3μm/min at a crystallization temperature T_c of 165°C. Experimental data could be described by the growth rate theory for small supercooling, by taking the appropriate value of 75 for the constant c₂ of the WLF equation. The chain-folded surface free energy σ_e, was estimated at 39.5 × 10^?3 J m^?2. The melting curves showed 1,2 or 3 melting endotherms. At large supercooling, crystallization from the melt produced a small melting endotherm just above T_c. This peak may originate from secondary crystallization of melt trapped within the spherulites. The next melting endotherm is related to the normal primary crystallization process. Its peak temperature increased linearly with T_c, yielding an extrapolated value for the equilibrium melting temperature T°_m of 212.5°C. At the normal values of T_c and heating rate a third endotherm appeared with a peak temperature that was independent of T_c, but rose with decreasing heating rate. From the effects of heating rate and partial scanning on the ratio of peak areas, it is concluded that this peak arises from secondary crystallization by continuous melting and recrystallization during the scan. This crystallization and melting behaviour of it-P2VP is very similar to that of isotactic polystyrene.     

Lamellar Morphology of Poly(3-hydroxybutyrate)/Poly(ethylene oxide) Blends as Studied via Small Angle X-ray Scattering

The lamellar morphology of a melt-miscible blend consisting of two crystalline constituents, poly(3-hydroxybutyrate) (PHB) and poly(ethylene oxide) (PEO) have been investigated by means of small angle X-ray scattering (SAXS). The blend was a crystalline/amorphous system when temperatures lay between the melting point of PEO (ca. T _m ^PEO=60C) and that of PHB (ca. T _m ^PHB=170C), while it became a crystalline/crystalline system below T _m ^PEO. The crystalline microstructures of the blends were induced by two types of crystallization history, i.e. one-step and two-step crystallizations. In the one-step crystallization, the blends were directly quenched from the melt to room temperature to allow simultaneous PHB and PEO crystallization. The two-step crystallization involved first cooling to 70C to allow PHB crystallization for 72 h followed by cooling to room temperature (ca. 19C) to allow PEO crystallization. In the crystalline/crystalline state, two scattering peaks have been observed in the Lorentz-corrected SAXS profiles, irrespective of the crystallization histories, meaning that crystallization created separate PHB and PEO lamellar stack domains. One-step crystallization yielded lamellar stack domains containing almost pure PHB and PEO lamellae. Two-step crystallization generated almost pure PHB lamellar domains and the PEO lamellar domains with inserted PHB lamellae. In the crystalline/amorphous state, the composition dependence of the amorphous layer thickness (l _a), the presence of zero-angle scattering, and the volume fraction of the PHB lamellar stack (_s) revealed that both one-step and two-step crystallizations, generated the interfibrillar segregation morphology, where the extent of interfibrillar segregation of amorphous PEO increased with increasing PEO content.