Difference In Glass Transition Temperature Research Articles

Dynamic Competition between Crystallization and Phase Separation at the Growth Interface of a PMMA/PEO Blend

We investigated the dynamic competition between crystallization and phase separation in a dynamically asymmetric blend of poly(methyl methacrylate) (PMMA) and poly(ethylene oxide) (PEO). Because of a large difference in glass transition temperatures between these two components, the phase transition dynamics is slowed down and can be observed in more detail. There is a morphological inversion from spherulitic to concentric ring patterns by adjusting the quench depth. Under deep quench, phase separation tends to be enhanced by the large concentration deviation at the growth interface of crystals, and propagates outward with convective concentration waves. Meanwhile, short needle-like crystals appear in the phase separated domains, compared with long crystal strips in a spherulitic pattern. A general dynamic competition model is proposed to qualitatively interpret the complex interplay between crystallization and phase separation. On the basis of this model, various morphologies are observed and predicted.

Crystallization kinetics and miscibility of blends of polyhydroxybutyrate (PHB) with ethylene vinyl acetate copolymers (EVA)

AbstractA series of ethylene vinyl acetate random copolymer EVA, with vinyl acetate (VA) varied from 9 to 91m%, was investigated by differential scanning calorimetry DSC and polarized optical microscopy. Biodegradable polymer blends of polyhydroxybutyrate PHB and EVA, having VA in EVA in range from 40m% till 91m%, were prepared by film casting from a chloroform solution. The miscibility and crystallization behavior of these blends were investigated. The isothermal crystallization behaviors of PHB and PHB/EVA blends are discussed in terms of the half time of crystallization t1/2. Experimental results indicated that blends of PHB/ EVA91 are completely miscible blend in the entire (0 to 100 m%) compositional ranges. Blends PHB/ EVA, for VA varied from 40 till 70 m% are immiscible as evidenced by the existence of unchanged composition independent glass transition temperatures (Tg), crystallization and melting behavior. The isothermal crystallization of PHB blends was investigated from room temperature till 130 °C. 80 °C was found to be the best temperature for comparison of different blends. At 80 °C t1/2 strongly depends on the content of VA in PHB/EVA blend, mainly due to differences in miscibility as well as due to differences in segmental mobility as identified by differences in glass transition temperature. Since both components in PHB/EVA80, pure PHB and pure EVA80, have glass transition temperatures close to 0 °C, it is difficult to decide its miscibility from Tg. However from the strong dependence of the value of crystallization half time t1/2 of PHB/EVA80 on blend composition, it was possible to reasonably infer that PHB/EVA80 is partially miscible.

A comparative study of spray-dried and freeze-dried hydrocortisone/polyvinyl pyrrolidone solid dispersions

Poor water solubility of new chemical entities (NCEs) is one of the major challenges the pharmaceutical industry currently faces. The purpose of this study was to investigate the feasibility of freeze-drying as an alternative technique to spray-drying to produce solid dispersions of poorly water-soluble drugs. Also investigated was the use of aqueous solvent mixtures in place of pure solvent for the production of solid dispersions. Aqueous solvent systems would reduce the environmental impact of pure organic solvent systems. Spray-dried and freeze-dried hydrocortisone/polyvinyl pyrrolidone solid dispersions exhibited differences in dissolution behavior. Freeze-dried dispersions exhibited faster dissolution rates than the corresponding spray-dried dispersions. Spray-dried systems prepared using both solvent systems (20% v/v and 96% v/v ethanol) displayed similar dissolution performance despite displaying differences in glass transition temperatures (Tg) and surface areas. All dispersions showed drug/polymer interactions indicated by positive deviations in Tg from the predicted values calculated using the Couchman–Karasz equation. Fourier transform infrared (FTIR) spectroscopic results confirmed the conversion of crystalline drug to the amorphous in the dispersions. Stability studies were preformed at 40°C and 75% relative humidity to investigate the physical stability of prepared dispersions. Recrystallization was observed after a month and the resultant dispersions were tested for their dissolution performance to compare with the dissolution performance of the dispersions prior to the stability study. The dissolution rate of the freeze-dried dispersions remained higher than both spray-dried dispersions after storage.

Effects of Protein Separation Conditions on the Functional and Thermal Properties of Canola Protein Isolates

Canola meal protein isolates were prepared from defatted canola meal flour using alkaline solubilization and acid precipitation. A central composite design was used to model 2nd-order response surfaces for the protein yield and the functional properties of protein isolates. The solubilization pH and precipitation pH were used as design factors. The models showed that the protein yield and functional properties of isolates, such as water absorption and fat absorption, were sensitive to both solubilization pH and precipitation pH, whereas the emulsification was sensitive to only solubilization pH. Gel electrophoresis analysis of protein fractions gave evidence to the compositional changes between proteins isolated under different conditions. Differences in glass transition temperatures suggest that proteins tend to be more denatured when solubilized at highly alkaline conditions. These conformational and compositional changes due to different protein separation conditions have contributed to the changes in functional properties of protein isolates. Protein isolation conditions may be determined primarily through optimization of total protein yield. Improvements in protein functional properties may be achieved with a relatively small sacrifice in yield by altering isolation conditions.

Specific interactions, structure and properties in segmented polyurethane elastomers

Two sets of segmented polyurethane (PU) elastomers were prepared from 4,4!-methylenebis(phenyl isocyanate) (MDI), 1,4-butanediol (BD) and a polyester or a polyether polyol, respectively. The molecular mass of both polyols was 1000 g/mol. The stoichiometric ratio of isocyanate and hydroxyl groups was 1 and the polyol/total diol ratio changed from 0 to 1 in 0.1 steps. One step bulk polymerization was carried out in an internal mixer and the samples were compression molded for testing. The results proved that specific interactions determine the phase structure and properties of these mate- rials. Crystallinity was approximately the same in the two types of polyurethanes and the amount of relaxing soft segments was also similar. The determination of interaction parameters from solvent absorption and differences in glass transition temperatures indicated stronger interaction between hard and soft segments in the polyester than in the polyether polyurethane. Larger transparency of the polyester PU indicated the formation of smaller dispersed particles of the hard phase. The larger number of smaller hard phase units led to the formation of more physical cross-links distributed more evenly in the polymer. These differences in the phase structure of the polymers resulted in stronger strain hardening ten- dency, larger strength and smaller deformations for the polyester than for the polyether polyurethane.

Phase controlled all-polymer bulk-heterojunction photovoltaic cells with high open-circuit voltage

All-polymer bulk-heterojunction photovoltaic cells based on poly(N-vinyl carbazole) and poly(9,9-dioctylfluorene-co-benzothiadiazole) co-casting films are investigated. One of the highest reported open-circuit voltage among the all-polymer solar cells is obtained due to the large energy offset between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor. Atomic force microscopy and photoluminescence study of the active layer with varied blending ratios and annealing temperatures reveals a phase controlled working mechanism, in which the charge generation efficiency is proved to be the main limiting factor of this material system. The incident photon-to-current conversion efficiency measurement shows a unique “double peak” phenomenon with increase in the annealing temperature, indicative of asynchronous chain movement between the two polymers due to their relatively large difference in glass transition temperature. Such a phase behavior might have potential value in further optimization of film morphology towards higher device performance.

Foaming of an Immiscible Blend System Using Organic Liquids as Blowing Agents

Foaming of blend systems is a promising approach to develop cellular materials with a set of desired properties. However, foaming of blend systems is not only a chance but also a challenge, as different polymers have to be foamed at the same foaming conditions. The best conditions during foaming are individual to each polymer and influence the obtained cellular structure. In immiscible polymer blends like poly(2,6-dimethyl-1,4-phenylene ether) (PPE) and poly(styrene-co-acrylonitrile) (SAN) the differences in glass-transition temperature (Tg) and viscosity are vast and can inhibit the foaming of the blend system. In order to minimize the above-mentioned difference in Tg and viscosity, one approach is to choose a blowing agent, which shows a selective solubility in one polymer phase, working as a selective plasticizer. The aim of the present study is to evaluate systematically the foaming behavior of immiscible PPE/SAN blend systems using ethanol and n-pentane as blowing agents. Mass uptake of ethanol and n-pentane in the PPE/SAN-blends as function of the blend composition is measured, offering a selective solubility of ethanol in SAN, while n-pentane shows a high affinity to PPE. The plasticizing effect on the blend phases PPE and SAN is theoretically estimated using the Chow-equation and the WLF-equation. After foaming the blowing agent saturated blend samples in an oil-bath at different temperatures, the density reduction and the cellular structures are analyzed and correlated to the blend morphology, the blowing agent solubility in the blend phase, the rheological properties and the glass transition temperature of the nonsaturated and saturated blend phases.

Dissimilar Materials Micro Welding between Stainless Steel and Plastics by Using Pulse YAG Laser

Direct joint of dissimilar materials between SUS304 stainless steel and plastics, PET (Polyethylene Terephthalate) or PC (Polycarbonate), was studied by using pulse YAG laser. Welding configuration was lap joint. Weldability and shear-tensile strength were investigated for the joints. It was possible to make a joint for both combination of materials, SUS304/PET and SUS304/PC. Weldable condition range was wider in case of SUS304/PET joint compared to that in case of SUS304/PC joint. The difference in the weldability may be due to difference in glass transition temperature of the plastics. Pores were observed in plastics near the interface of the joint for both combinations of the materials when the joint welded with higher heat input. Sear-tensile test was carried out for the joints. SUS304/PET joint shows higher strength compared to SUS304/PC joint. Higher strength was observed for the joint which includes pores near the interface in plastics. However, if large size and number of pores are existing near the interface in plastics, the pores play as a defect and causes degradation of the strength.

Thermal Properties of Poly(L‐lactide)/Calcium Carbonate Nanocomposites

AbstractSummary: Thermal properties of nanocomposites prepared of poly(L‐lactide) (PLLA) and CaCO3 applying differential scanning (DSC) calorimetry and thermogravimetry (TG) were studied. Nanocomposites were prepared by extrusion process at 170 °C. DSC measurements show that CaCO3 has no influence on glass transition and melting point of PLLA but lowers its cold crystallization temperature. There is no difference in glass transition temperature of PLLA before and after extrusion. High temperature thermal stability of the PLLA in the composites is poorer than neat PLLA. Kinetic parameters also indicate greater reactivity of the system upon CaCO3 addition.

Thermal and viscoelastic structure–property relationships of model comb-like poly(n-butyl methacrylate)

A range of comb polymers of poly(n-butyl methacrylate), where the degree of polymerization (DP) of both the backbone and branches was controlled using RAFT-mediated free-radical polymerization, was synthesized using the method of Vosloo et al. [Macromolecules 2004;37: 2371]. Individual architectural parameters (branch length, branch number and DP of the comb backbone) of these relatively monodisperse samples were systematically varied in order to study the impact of each structural parameter on the thermal and rheological properties of the resulting comb polymers. Differential scanning calorimetry showed lower glass transition temperatures for the comb polymers compared to the original linear backbones. There were negligible differences in glass transition temperatures between comb polymers containing branches of different lengths, and between comb polymers containing backbones of narrow and of broad molecular weight distributions. These observations suggest that because the comb polymers are very tightly spaced, the branches act in the same way as long chain polyBMA. Viscoelastic properties of the comb polymers were investigated using dynamic mechanical analysis, using time–temperature superposition to extend the rheological data over a wide frequency range. Major differences in the viscoelastic responses of the original linear backbones and the comb polymers were observed, which were explained in terms of arm retraction/relaxation leading to tube dilation. All comb polymers showed viscoelastic responses that are characteristic of combs, but differences in responses due to changes in branch length and branch number were difficult to detect. This was mainly due to the relatively high number of branches, whose retractions occurred over a broad frequency range, and thus dominated the observed changes in moduli, thereby possibly masking subtle differences in responses.

Staining Polymers for Microscopical Examination

Abstract Polymers find widespread use in blends, graft copolymers, polymer alloys, and composites. Staining is an important tool for microscopically visualizing polymer phases and morphologies in these multi-component systems. Three stains have been used in most investigations. These stains (and the functional groups they are aimed at) are: osmium tetroxide for chemical unsaturation; ruthenium tetroxide for styrenics; and phosphotungstic acid for polyamides. Beyond these three basic stains there are a significant number of additional ones that have been developed for special cases. Aside from reacting chemically with functional groups, stains have been used as: negative (outline) stains; stains to delineate amorphous-crystalline regions; stains for microvoids, and stains for phase boundaries. Stains are most commonly transported to the target functional groups in tow ways — dissolved in liquid media or by direct contact with stain vapors. Improvisations have been developed to meet special staining problems. These improvisations most often involve: 1) chemical alteration of a polymer backbone to introduce a stainable functionality; 2) tailoring of staining media to provide adequate diffusion into the matrix; 3) preferential swelling of a blend component in order to accept a stainable moiety; 4) tactical use of a staining temperature to take advantage of differences in glass transition temperatures of blend components; and 5) optimization of chemical equilibria to expedite staining time or extent. Examples of these modifications are presented along with some cautionary notes regarding some artifact that may arise. A plea is made for more adequate descriptions of staining protocols in published articles. In too many cases there is inadequate description of staining methods and experimental duplication is put in jeopardy.

Localized Cooperative Molecular Motion in Miscible Polymer Mixtures with Large Difference in Glass-Transition Temperatures

An experimental study using dielectric spectroscopy measurements is made on molecular motions as functions of temperature and pressure of polystyrene (PS) and poly(vinyl methyl ether) (PVME) mixtures having large difference in glass transition temperatures. For mixtures with high concentration of PS, the largest loss peak and a shoulder at higher temperatures are observed. Our detailed analysis of molecular motion leads to the result that the former is not due to the α process of PVME-rich region arising from dynamic heterogeneity for miscible polymer mixtures, but due to local cooperative motion where favorable orientational motion of PVME is induced through a change in the environments formed by the PS matrix.

Enthalpy relaxation and fragility in polychlorinated biphenyls

We employ temperature modulated DSC (TMDSC) to determine the dependence of the fictive temperature on cooling rate for a series of polychlorinated biphenyls (PCB). From the slopes of semi-logarithmic plots of cooling rate vs. fictive temperature, the latter normalized by the fictive temperature for an arbitrary cooling rate, we determine the enthalpic fragilities. Despite significant differences in glass transition temperature and chemical structure (specifically chlorine content), the PCB have the same fragility. The value of the fragility determined using TMDSC is consistent with the fragility previously determined using dielectric relaxation spectroscopy.

Thermal desorption of fullerene C 60 from polymer matrices

The data on thermally stimulated desorption (TSD) study of fullerene C 60 molecules from different polymer matrices are compared in view of the temperature positions and relative intensities of resolved thermal desorption stages. An increase of the low temperature C 60 desorption stages is observed for polydimethylsiloxane (PDMS) and polystyrene (PS) contrary to polyimide (PI). This is in agreement with the difference in glass transition temperatures ( T g) of these polymers. The TSD data allow us to conclude that above T g of the polymer the solubility of fullerene C 60 in PDMS and PS is up to the order of magnitude higher compared with C 60 solubility in low molecular weight solvents.

Viscoelastic phase separation in soft matter: Numerical-simulation study on its physical mechanism

Viscoelastic phase separation is a new type of phase separation, which may be universal to any dynamically asymmetric mixture composed of slow and fast components. Dynamic asymmetry can arise from (i) a large size disparity between the components and (ii) a large difference in glass-transition temperature. Origin (i) often exists in the so-called soft matter, while origin (ii) can exist in any material including oxide, metallic, and molecular glass formers. For case (i), phase separation generally leads to the formation of a long-lived “interaction network” (transient gel) of the slow components, if the concentration is high enough and attractive interactions between them are strong enough. This new type of phase separation allows us to form a network structure of the minority slow-component-rich phase, contrary to the conventional wisdom on phase separation. This has a significant technological impact on the morphological control of multi-phase materials. Here we review our numerical simulation studies on viscoelastic phase separation. Effects of transient gelation on phase-separation kinetics are studied by numerical simulations based on the coarse-grained two-fluid model. We find that the bulk mechanical stress originating from connectivity of the slow components of a mixture plays a crucial role in pattern evolution of viscoelastic phase separation. We also discuss how we can control the phase-separation morphology by controlling the viscoelastic properties of component materials. This coarse-grained model cannot describe how a transient gel itself is formed. Thus, to study the formation process of a transient gel in colloidal suspensions, we develop a new simulation method (“fluid particle dynamics (FPD) method”), which can properly treat interparticle hydrodynamic interactions. This new method can be applied to various fields of colloidal science, where hydrodynamic interactions play important roles. Our FPD simulations of colloidal aggregation clearly indicate that hydrodynamic interactions play crucial roles in the formation of a transient gel. We emphasize that the percolation threshold is crucially affected by hydrodynamic interactions. We point out that viscoelastic phase separation should be universally observed in not only polymer solutions and mixtures but also colloidal suspensions, emulsions, and protein solutions.

Blends of polymers with similar glass transition temperatures: A DMTA and DSC study

AbstractThe miscibility of different polymer blends was studied with dynamic mechanical thermal analysis (DMTA) in conjunction with differential scanning calorimetry (DSC). The blends were prepared by melt mixing polymers having similar glass transitions such as polyglutarimide (PGI), styrene‐co‐maleic anhydride random copolymers (SMA), and polystyrene (PS). In PGI/SMA blends, there is only one glass transition detected with DMTA. In the case of PGI/SMA14 blends, the single glass transition temperature is due to their full miscibility. However, PGI/SMA8 blends are immiscible throughout the whole composition range as was verified by optical observation (opaque appearance), DSC, and scanning electron microscopy. The observation of only one glass transition by DMTA was attributed to weak interactions that take place between the two polymers, leading to partial mutual solubility and bringing the slightly different Tg temperatures of the pure polymers even closer. In SMA8/SMA14 blends, there are two glass transitions detected with DSC as well as with DMTA, indicating that the two copolymers are immiscible. However, in all compositions, shifts of the glass transitions were observed with DMTA, which is evidence of partial miscibility. The observation of immiscibility was easiest in PS/SMA blends by both techniques, because of the bigger difference in glass transition temperatures of the initial polymers. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 726–735, 2004

Small-angle scattering on soft matter with dynamical asymmetry

By employing small-angle neutron scattering, we investigated the mixing behavior in a quiescent state or under a shear flow on a miscible polymer mixture of deuterated polystyrene and poly(vinylmethylether). This polymer mixture uniquely shows the following behaviors in the intermediate temperature range between the two glass transition temperatures; (i) due to a large difference in glass transition temperatures, dynamical asymmetry or a difference of mobilities is enhanced upon temperature decrease, and (ii) the rheological relaxation becomes much slower than the concentration fluctuations determined by thermodynamics (gel-like limit). These two factors play an important role in the generation of a stress field and suppress small-angle scattering in a quiescent state. Under a shear flow, the stress imbalance enhanced by the dynamical asymmetry causes a butterfly pattern or shear-induced phase separation in the q-range of small-angle neutron scattering.

Curing of epoxy resin by ultrafine silica modified by grafting of hyperbranched polyamidoamine using dendrimer synthesis methodology

Hyperbranched polyamidoamine–grafted silica was prepared according to dendrimer synthesis methodology. The modified silica was dispersed uniformly in epoxy resin, and the curing of epoxy resin proceeded successfully by heating in the presence of the modified silica; the gel fraction of the epoxy resin cured by the hyperbranched polyamidoamine–grafted silica (grafting = 80.2%) reached 77% at 170°C after 48 h. The gel fraction increased with increasing terminal amino group content of the hyperbranched polyamidoamine–grafted silica. In addition, the curing ability of the silica increased by complexation of the terminal amino groups of the grafted polyamidoamine with boron trifluoride. The modulus of elasticity of the curing materials obtained using the modified silica as a curing agent was lower than that using conventional a curing agent such as ethylenediamine in the presence of untreated silica. On the other hand, the heat resistance of the curing product using the modified silica was superior to that using ethylenediamine, but no difference in glass-transition temperature was observed. It is expected that hyperbranched polyamidoamine grafted-silica is incorporated uniformly with chemical bonds in the matrix of the epoxy resin. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 573–579, 2001

A dielectric study on the local dynamics of miscible polymer blends: poly(2-chlorostyrene)/poly(vinyl methyl ether)

Dielectric relaxation of poly(vinyl methyl ether) (PVME)/poly(2-chlorostyrene) (P2CS) blends with a lower critical solution temperature (LCST) was investigated in the one-phase region over a wide range of temperature. Four distinct dielectrically active processes were observed and identified as α1, α2, β and γ relaxation processes. It was found that the β and γ processes were almost unaffected upon varying the blend composition, while their relaxation intensities were approximately proportional to the PVME content. Furthermore, there exist two peaks (α1, α2) corresponding to the segmental relaxation process of each component. By comparing this with the data obtained by small-angle X-ray scattering and differential scanning calorimetry, the experimental results are discussed in terms of dynamical heterogeneity of miscible polymer blends composed of two polymer components with a large difference in glass transition temperatures.

Surface composition and protein adsorption of polyurethane membrane

Membranes of uncomplexed polyurethanes (PUs) were prepared by hydroxyl-terminated polybutadiene (HTPB), 4,4′-dicyclohexylmethane diisocyanate (H 12MDI) and 1,4-butane diol (1,4-BD). While complexed PUs were prepared by using N-methyl diethanol (MDEA) as the chain extender of which the tertiary amines were complexed with cupric ions. Molar ratio of protein adsorption of fibrinogen to albumin (F/A molar ratio) on polymer surface was measured. The F/A molar ratio was affected by surface composition. The surface composition was identified by ESCA measurement and quantified by the absorption ratio of carbonyl group to butadiene group (CO/CC ratio) on the FTIR–ATR spectra. Low F/A molar ratio and high contact angle were found on these PUs. The F/A molar ratio was decreased by the increase of soft segment or C–H bond dispersed on the surface. Frequency difference (Δ ν) and shiftment, hydrogen bonding index (HBI) and glass transition temperature difference (Δ T g) as a measure of phase homogeneity and the average strength of interpolymer hydrogen bonds were utilized to study the change of surface composition and intermolecular interaction of the prepared PUs. The effect of hard segment content, cupric chloride content on the surface composition and F/A molar ratio were investigated.