Experimental verification on upper critical solution temperature (UCST) behavior in blend of poly(2,6-dimethyl p-phenylene oxide) with poly(4-methyl styrene)

Rare upper critical solution temperature (UCST) behavior was found and experimentally demonstrated in the blend comprising poly(2,6-dimethyl p-phenylene oxide) with poly(4-methyl styrene) (PPO/P4MS). Complexity of phase behavior in the PPO/P4MS system has caused puzzling analyses in the past years. This study re-investigated and clarified past mis-interpretations related to this interesting blend system. This study concluded that the PPO/P4MS blend is an immiscible system at ambient, which, however, turns into a miscible phase with UCST behavior at higher temperatures. With the finding of UCST in the PPO/P4MS blend, a critical contribution of this work was to resolve the conflicting arguments that have gone on for a long time in determination and interpretation of the thermodynamic phase behavior of PPO/P4MS. Phase behavior with UCST in the PPO/P4MS blend system and its interpretation were supported with clear experimental evidence.     

Molecular dynamics simulation studies of binary blend miscibility of poly(3-hydroxybutyrate) and poly(ethylene oxide)

By means of full atomistic molecular dynamics simulation, the solubility parameters for pure poly(3-hydroxybutyrate) and poly(ethylene oxide) are calculated and the results are in agreement with the literature values. Furthermore, in order to reveal the blend property, the volume-temperature curve of the PHB/PEO blend system (1:2 blends in terms of repeated units) is simulated by employing the united atom approximation to obtain the glass transition temperature. From the volume-temperature curve, the glass transition temperature is about 258 K, which is compared well with the experimental results. It should be pointed out that the two simulated solubility parameters are similar and there is only one glass transition of the blend system, these indicate that the studied blend system is miscible.     

Synthesis, characterization and pyrolysis of poly(styrene/divinylbenzene) derivatives

A series of polyfstyrene/divinylbenzene) derivatives were prepared and characterized by solid-state ¹³C FT-NMR, FT-IR, elemental analysis, SEM and TGA. The resins were then subjected to pyrolysis in order to determine which derivatives resulted in high yields of carbonaceous material while retaining a spherical shape.     

Miscibility of C60-end-capped poly(ethylene oxide) with poly(vinyl chloride)

The miscibility of blends of single [60]fullerene (C₆₀)-end-capped poly(ethylene oxide) (FPEO) or double C₆₀-end-capped poly(ethylene oxide) (FPEOF) with poly(vinyl chloride) (PVC) has been studied. Similar to poly(ethylene oxide) (PEO), both FPEO and FPEOF are also miscible with PVC over the entire composition range. X-ray photoelectron spectroscopy showed the development of a new low-binding-energy Cl2p doublet and a new high-binding-energy O1s peak in FPEO/PVC blends. The results show that the miscibility between FPEO and PVC arises from hydrogen bonding interaction between the α-hydrogen of PVC and the ether oxygen of FPEO. From the melting point depression of PEO, FPEO or FPEOF in the blends, the Flory-Huggins interaction parameters were found to be −0.169, −0.142, −0.093 for PVC/PEO, PVC/FPEO and PVC/FPEOF, respectively, demonstrating that all the three blend systems are miscible in the melt. However, the incorporation of C₆₀ slightly impairs the interaction between PEO and PVC.     

Molecular relaxation in cross-linked poly(ethylene glycol) and poly(propylene glycol) diacrylate networks by dielectric spectroscopy

The molecular relaxation characteristics of rubbery amorphous crosslinked networks based on poly(ethylene glycol) diacrylate [PEGDA] and poly(propylene glycol) diacrylate [PPGDA] have been investigated using broadband dielectric spectroscopy. Dielectric spectra measured across the sub-glass transition region indicate the emergence of an intermediate “fast” relaxation in the highly crosslinked networks that appears to correspond to a subset of segmental motions that are more local and less cooperative as compared to those associated with the glass transition. This process, which is similar to a distinct sub-T_g relaxation detected in poly(ethylene oxide) [PEO], may be a general feature in systems with a sufficient level of chemical or physical constraint, as it is observed in the crosslinked networks, crystalline PEO, and PEO-based nanocomposites.     

Structure-property behavior of poly(dimethylsiloxane) based segmented polyurea copolymers modified with poly(propylene oxide)

Poly(propylene oxide) (PPO) was incorporated in a controlled manner between poly(dimethylsiloxane) (PDMS) and urea segments in segmented polyurea copolymers and their solid state structure-property behavior was investigated. The copolymers contained PDMS segments of MW 3200 or 7000 g/mol and an overall hard segment content of 10-35 wt%. PPO segments of MW 450 or 2000 g/mol were utilized. Equivalent polyurea copolymers based on only PDMS as the soft segment (SS) component were used as controls. The materials (with or without PPO) utilized in this study were able to develop microphase morphology as determined from dynamic mechanical analysis (DMA) and small angle X-ray scattering (SAXS). DMA and SAXS results suggested that the ability of the PPO segments to hydrogen bond with the urea segments results in a limited inter-segmental mixing which leads to the formation of a gradient interphase, especially in the PPO-2000 co-SS containing copolymers. DMA also demonstrated that the polyureas based on only PDMS as the SS possessed remarkably broad and nearly temperature insensitive rubbery plateaus that extended up to ca. 175 °C, the upper temperature limit depending upon the PDMS MW. However, the incorporation of PPO resulted in more temperature sensitive rubbery plateaus. A distinct improvement in the Young's modulus, tensile strength, and elongation at break in the PPO-2000 and PDMS-7000 containing copolymers was observed due to inter-segmental hydrogen bonding and the formation of a gradient interphase. However, when PPO was incorporated as the co-SS, the extent of stress relaxation and mechanical hysteresis of the copolymers increased relative to the segmented polyureas based on the utilization of only PDMS as the soft segment component.     

Comparative studies on effects of silica and titania nanoparticles on crystallization and complex segmental dynamics in poly(dimethylsiloxane)

Effects of in situ synthesized silica and titania nanoparticles, 5 and 20-40 nm in diameter, respectively, on glass transition and segmental dynamics of poly(dimethylsiloxane) networks were studied by employing differential scanning calorimetry, thermally stimulated depolarization currents and broadband dielectric relaxation spectroscopy techniques. Strong interactions between the well dispersed fillers and the polymer suppress crystallinity and affect significantly the evolution of the glass transition in the nanocomposites. Next to the α relaxation associated with the glass transition of the bulk amorphous polymer fraction, two more segmental relaxations were recorded, originating from polymer chains restricted between condensed crystal regions (α_c-relaxation) and the semi-bound polymer in an interfacial layer with strongly reduced mobility due to interactions with hydroxyls on the nanoparticle surface (α′ relaxation), respectively. Interactions with the polymer were found to be stronger in the case of titania than silica, leading to an estimated interaction length of around 2 nm for silica and at least double for titania nanocomposites.     

Dynamic relaxation characteristics of crosslinked poly(ethylene oxide) copolymer networks: Influence of short chain pendant groups

The dynamic relaxation characteristics of short-branch rubbery amorphous networks prepared by the photopolymerization of poly(ethylene glycol) diacrylate [PEGDA] crosslinker have been investigated using dynamic mechanical analysis and broadband dielectric spectroscopy. Copolymerization with low molecular weight acrylates was used to control effective crosslink density in the networks and led to the insertion of ethylene oxide pendant groups along the network backbone. Substantial differences in the sub-glass and glass-rubber relaxation properties of the copolymers were observed as a function of pendant length and the nature of the pendant terminal group (e.g., -OH vs. -OCH₃); the results are compared with prior studies on model copolymers containing longer, more flexible side branches.     

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.     

New insight into microstructure-mediated segmental dynamics in select model poly(urethane urea) elastomers

Segmental dynamics in a series of 4,4′-dicyclohexylmethane diisocyanate–diethyltoluenediamine–poly(tetramethylene oxide) based poly(urethane urea) (PUU) elastomers have been investigated through a multi-scale characterization approach. This includes broadband dielectric analysis, solid-state nuclear magnetic resonance (NMR), plate impact, and impulsive stimulated scattering. Dielectric relaxation measurements applicable at frequencies up to 10⁶ Hz are useful for interpreting the high strain-rate deformation response; i.e. at the moment of target interaction with an accelerating impact or MHz stress wave excitation. Additionally, the capability of solid-state NMR to differentiate the microstructure-mediated segmental dynamics; correspondingly, the presence of a rigid phase (those in the phase-mixed regions) and a mobile phase associated with the soft-segment domains is demonstrated. These new insights not only further elucidate the microstructure details discerned through atomic force microscopy, but also enable the prediction of the macroscopically dynamic response in these model PUUs, particularly on the temporal scale over the range of μs–ns.     

Chiral-nematic self-ordering of rodlike cellulose nanocrystals grafted with poly(styrene) in both thermotropic and lyotropic states

Graft copolymers of rodlike cellulose nanocrystals (CNC) with poly(styrene) (PSt) were synthesized through atom transfer radical polymerization (ATRP). The hydroxyl groups on CNC were esterified with 2-bromoisobutyrylbromide to yield 2-bromoisobutyryloxy groups, which were used to initiate the polymerization of poly(styrene). The graft copolymers were characterized by thermogravimetric analysis (TGA), fourier transform infrared spectroscopy (FT-IR) and gel permeation chromatography (GPC). The size of the original CNC is 10-40 nm in width and 100-400 nm in length, which was characterized by atomic force microscopy (AFM). The thermal and liquid crystalline properties of the graft copolymers were investigated by differential scanning calorimeter (DSC) and polarizing optical microscope (POM). The graft copolymers exhibit fingerprint texture in both thermotropic and lyotropic states. In thermotropic state, the PSt-grafted CNC orient spontaneously in isotropic melt (PSt side chains acting as a solvent). The thermotropic liquid crystal phase behavior is similar to the lyotropic phase behavior.     

Synthesis of functionalized multiarm poly(ethylene oxide) stars

Six arm poly(ethylene oxide) (PEO) stars carrying either 6 pyridyl or 12 hydroxyl groups at their periphery were synthesized using an arm-first approach. To this end, two novel α,ω-heterodifunctional PEO's, namely α-ketal,ω-hydroxy and α-pyridyl,ω-hydroxy PEO's were synthesized and, after the deprotonation of their hydroxyls, deactivated onto hexachlorocyclophosphazene which served to build the core. Quaternization of the stars containing six outer pyridyl groups created positive charges at their periphery while the acidic treatment of stars carrying terminal ketal rings afforded six arm PEO stars with 12 peripheral hydroxyls. The latter compound was subsequently deprotonated and used to polymerize ethylene oxide by a classical core-first approach. This resulted in the formation of highly branched PEO's also referred to as dendrimer-like PEO stars which consisted of a first generation of six PEO arms and a second generation of 12 hydroxy-ended PEO branches.     

Relaxation dynamics and cold crystallization of poly(pentamethylene terephthalate) as revealed by dielectric spectroscopy

The relaxation dynamics of poly(pentamethylene terephthalate) has been investigated by means of dielectric spectroscopy. The sub-glass dynamics is characterized by the existence of a bimodal β process whose faster and slower components have been assigned to the relaxation of the bond between the ester oxygen and the aliphatic carbon and to the link between the aromatic ring carbon and the ester carbon, respectively. By comparison with other closely related aromatic polyesters it is shown that the faster component strongly depends on the amount of methylene groups while the slower one is not considerably affected by the nature of the glycol subunit. The changes in the α process associated to the segmental relaxation during cold crystallization reveal the formation of a rigid amorphous phase fraction. Combination of dielectric experiments with X-ray scattering ones suggests that during cold crystallization PPT crystal lamellae tend to fill the space homogeneously.     

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.     

Surface morphology and Flory-Huggins interaction strength in UCST blend system comprising poly(4-methyl styrene) and isotactic polystyrene

The surface morphology and polymer-polymer interaction parameter (χ₁₂) of UCST blend systems comprising isotactic polystyrene and poly(4-methyl styrene) (P4MS) were investigated using atomic-force microscopy (AFM) and differential scanning calorimetry (DSC). From the measured glass transition temperature and the specific heat increments (ΔC_p) at T_g, it was found that the P4MS dissolved more easily in the iPS rich-phase than did the iPS in the P4MS rich-phase. AFM result also supported that the compatibility increased more in the regions of P4MS-rich compositions than in the regions of PS-rich compositions of the PS/P4MS blends. From the measured T_g’s and apparent weight fractions of iPS and P4MS dissolved in each phase, the values of the Flory-Huggins interaction parameter (χ₁₂) were determined to be 0.0163-0.0232 depending on the composition. These results indicate that the χ₁₂ is quite dependend on the apparent volume fraction of the polymers dissolved in each phase. The values of χ₁₂ calculated from this work (method based on T_g’s of phases) were lower than those estimated using an earlier method based on the UCST or clarity temperatures. All values of χ₁₂ are greater than the values of interaction parameter at the critical point (χ₁₂)_c. This fact indicates that the iPS/P4MS blend are immiscible for all blend compositions. The surface of the phase-separated blend system was mostly covered with the P4MS rich-phase owing to its lower surface free energy in comparison with that of the neat iPS. The mechanism of surface phase separation for the P4MS blends with aPS or iPS is governed by two factors: (1) difference in the solubility of the two polymers in the solvent and (2) surface free energy.     

Aggregation behavior of two-arm fullerene-containing poly(ethylene oxide)

A water-soluble two-arm fullerene-containing poly(ethylene oxide) (PEO) was synthesized through isocyanate-hydroxy condensation reaction with fullerenol as a molecular core and characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetry, and matrix-assisted laser desorption ionization time of flight mass spectrometry. The aggregation behavior of the resulting amphiphilic polymer in water, THF, and DMF, was studied by gel permeation chromatography, laser light scattering, and transmission electron microscopy. The polymer forms spherical aggregates with an aggregation number around 540-1020.     

Formation of dendrite crystals in poly(ethylene oxide) interacting with bioresourceful tannin

A dendritic morphology, induced by miscibility with strong intermolecular interaction between poly(ethylene oxide) (PEO) and bioresourceful tannin [tannic acid (TA)]. Mechanism was investigated by differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy, wide-angle X-ray diffraction, and polarized optical microscopy. The cell crystallography preference in correlation to the intermolecular interaction in the dendrites in PEO/TA (70/30) blend was analyzed. Dendritic morphology was more distinct at PEO/TA = 70/30 composition, where the spherulitic growth rate showed a highly nonlinear relationship with respect to crystallization time (R α t ^1/2). Diffusion limitation mechanism caused by the crystallography preference attributed to the strong intermolecular interaction between PEO and TA was at work.     

Tacticity effects on the miscibility behavior of poly(methyl methacrylate)/poly(ethylene oxide-b-dimethylsiloxane-b-ethylene oxide) blends

The miscibility of a triblock copolymer poly(ethylene oxide)-poly(dimethylsiloxane)-poly(ethylene oxide) with syndiotactic and isotactic poly(methylmethacrylate) wasstudied. Although isotactic poly(methyl methacrylate) (PMMA) was miscible with poly(ethylene oxide) (PEO) in the pure state, it was immiscible with the PEO end blocks in the copolymer. In comparison, the syndiotactic poly(methyl methacrylate) (sPMMA) was miscible with the PEO blocks as indicated by melting point depression, decrease in crystallinity, and slower rate of spherulite growth of PEO. When blends of the triblock copolymer were cooled to low temperatures, the poly(dimethylsiloxane) (PDMS) middle block which resided in the interlamellar region of PEO spherulites also crystallized; the development of PDMS crystals was clearly suppressed at high sPMMA contents.On leave from Union Chemical Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan     

Crystalline structures in ultrathin poly(ethylene oxide)/poly(methyl methacrylate) blend films

Amorphous poly(ethylene oxide)/poly(methyl methacrylate) (PEO/PMMA) blend films in extremely constrained states are meta-stable and phase separation of fractal-like branched patterns happens in them due to heterogeneously nucleated PEO crystallization by diffusion-limited aggregation. The crystalline branches are viewed flat-on with PEO chains oriented normal to the substrate surface, upon increasing PMMA content the branch width remains invariant but thickness increases. It is revealed that PMMA imposes different effects on PEO crystallization, i.e. the length and thickness of branches, depending on the film composition.     

Crystallization and unusual rheological behavior in poly(ethylene oxide)-clay nanocomposites

We report a systematic study of the crystallization and rheological behavior of poly(ethylene oxide) (PEO)-clay nanocomposites. To that end a series of nanocomposites based on PEOs of different molecular weight (10³ < MW < 10⁵ g/mol) and clay surface modifier was synthesized and characterized. Incorporation of organoclays with polar (MMT-OH) or aromatic groups (MMT-Ar) suppresses the crystallization of polymer chains in low MW PEO, but does not significantly affect the crystallization of high MW matrices. In addition, the relative complex viscosity of the nanocomposites based on low MW PEO increases significantly, but the effect is less pronounced at higher MWs. The viscosity increases in the series MMT-Alk < MMT-OH < MMT-Ar. In contrast to the neat PEO which exhibits a monotonic decrease of viscosity with temperature, all nanocomposites show an increase after a certain temperature. This is the first report of such dramatic enhancements in the viscoelasticity of nanocomposites, which are reversible, are based on a simple polymer matrix and are true in a wide temperature range.