Excess heat capacities for two miscible polymer blend systems

Barnum, R. S.; Goh, S. H.; Barlow, J. W.; Paul, Donald R

doi:10.1002/pol.1985.130230801

Cited by 26 publications

(10 citation statements)

References 15 publications

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“…In the case of hydrogen-bonded mixtures, a marginal reference can be found in one of the first attempts to generalize the lattice fluid model to mixtures with strong interactions . The authors reported positive calculated values for Δ C p but 2 orders of magnitude (10 -4 ) lower than experimental results such as those reported by Barnum et al Apart from that, Righetti et al studied Δ C p in the vicinity of the glass transition temperature, and in our own group, we have measured excess specific heats of phenoxy and poly(vinyl methyl ether) blends . In a very recent paper, Wu et al have measured Δ C p in mixtures of phenoxy with a novolac-type phenolic resin, comparing their signs with the evolution of the enthalpy of mixing with temperature, calculated using the Painter and Coleman approach…”

Section: Introductionmentioning

confidence: 80%

“…These variations arise from the different geometric forms of the molecules and/or from the interactions occurring between the functional groups of the blend components. As far as the excess specific heats is concerned, Barnum et al were the first to consider that, given the inherent difficulties in measuring heats of mixing, it could be interesting to evaluate the variation of this enthalpy of mixing with temperature. This variation can be expressed by the equation where ω i is the weight fraction of the i component.…”

Section: Introductionmentioning

confidence: 99%

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Excess Specific Heats in Miscible Binary Blends with Specific Interactions

et al. 1999

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Excess specific heats of different binary blends involving different levels of relatively strong specific interactions have been determined from specific heats of the pure components and those of the blends. A heat-flux (Calvet) calorimeter and a step-by-step methodology have been used. Blends of poly(hydroxy ether of bisphenol A) (phenoxy resin) with polyesters and polyethers, where specific interactions are supposed to play a role in miscibility, and blends with stronger hydrogen-bond interactions, such as poly(vinyl phenol)/poly(methyl methacrylate) (PVPh/PMMA), have been included in the study. Only the experimental excess specific heat for the PVPh/PMMA blend is negative, whereas those relative to phenoxy blends are all positive. These results have been analyzed on the basis of an association model specifically designed for polymer blends in which miscibility is mainly caused by hydrogen bonding. The model predicts, in all cases, a continuous decrease of the enthalpy of mixing with temperature, i.e., a negative value of ΔC p . Equation of state effects have been included in the theoretical simulations, giving modified trends of the enthalpy of mixing with temperature in a more reasonable agreement with the experimental results.

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Section: Introductionmentioning

confidence: 80%

Section: Introductionmentioning

confidence: 99%

Excess Specific Heats in Miscible Binary Blends with Specific Interactions

et al. 1999

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“…Although a variety of experimental tools have been brought to bear in research on phase separation, relatively few calorimetric investigations have been carried out. Barnum et al [5] used an indirect technique to determine AC m i x , the excess specific heat due to mixing of a binary polymer blend. Ahn et al [6] have studied blends of a low-molecular-weight liquid crystal (LC) with a polymer, using optical techniques and differential scanning calorimetry (DSC).…”

Section: Mixing and Phase Separation In Liquid Crystal /Matrix Systemmentioning

confidence: 99%

“…1) are indeed due to AC m i x < 0, we measured the specific heats of E7, NOA65, and their mixtures and followed the procedure of Barnum et al [5] to analyze the results. For AC m ix to be negative, the specific heat of the mixture should be less than the weighted sum for the components [5]. For a 50% binary mixture at a temperature greater than -284 K (r m i x ), the magnitude of AC m i X (~~ -0.024 cal/gK) from the specific heats is comparable to that from Fig.…”

Section: Mixing and Phase Separation In Liquid Crystal /Matrix Systemmentioning

confidence: 99%

Mixing and phase separation in liquid crystal/matrix systems: Determination of the excess specific heat of mixing

Smith

1993

Phys. Rev. Lett.

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We report the first direct measurement of ACmix, the excess specific heat due to mixing of a binary system, a liquid crystal (LC)/matrix mixture. Both LC concentration and matrix cross-linking (cure) affect mixing. In uncured systems we observe a steplike decrease in specific heat due to a negative value of ACmix. A plot of AC m ix vs LC content shows a minimum, consistent with Flory-Huggins theory; the theory also gives the upper critical solution temperature phase diagram and sign of AC m i x . Partially cured samples also exhibit ACmix transitions which persist until cure is almost complete and demixing is essentially irreversible.PACS numbers: 61.30.-v, 61.41.+e, 64.70.-p, 64.75.+g The study of phase separation in binary systems is important for understanding the formation of polymer blends [1] and polymer-dispersed liquid crystal (PDLC) films (of interest for their electro-optic properties) [2][3][4]. Although a variety of experimental tools have been brought to bear in research on phase separation, relatively few calorimetric investigations have been carried out. Barnum et al. [5] used an indirect technique to determine AC m i x , the excess specific heat due to mixing of a binary polymer blend. Ahn et al. [6] have studied blends of a low-molecular-weight liquid crystal (LC) with a polymer, using optical techniques and differential scanning calorimetry (DSC). They stated that the changes in heat quantities due to phase separation in such systems are too small to be detected using DSC. In the present paper we shall show that it is, indeed, possible to detect such changes; we have directly measured AC m i x for a LC/ polymer matrix system, using the results to derive the phase diagram and the Flory-Huggins interaction parameter. We report the first study of the effects of both LC concentration and matrix cross-linking on mixing and phase separation.Phase separation phenomena control the formation of PDLCs, leading to their characteristic morphology: liquid crystal microdroplets dispersed in a polymer matrix. PDLCs are formed by a two-step process: (1) A low-molecular-weight LC and a polymer precursor are initially mixed together to form a uniform solution; (2) the polymer is hardened, during which process LC phase separates from the matrix as microdroplets. Hardening can be carried out by several methods [2][3][4]7,8]; we use ultraviolet-curing techniques to increase matrix molecular weight via cross-linking reactions [3,7,8].Thus, two aspects of phase separation are basic to understanding the formation of a UV-cured PDLC. The first of these concerns the solubility behavior of the uncured LC/matrix mixture, a system characterized by a phase diagram with an upper critical solution temperature (UCST) [9,10]. Before and during cure, the temperature T of the mixture should be higher than T mix , the mixing temperature (above which the components of a UCST system are codissolved). If T is lower than T mix , undesirable phase separation prior to cure may lead to large regions of liquid crystal, resulting in opt...

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“…Figure5. Dependence of the onset temperature T, on the heating rate for a 60:40 (wt %) PMMA/PVC blend.…”

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confidence: 99%

Miscibility and phase separation in poly(methyl methacrylate)/poly(vinyl chloride) blends: study of thermodynamics by thermal analysis

Shen¹,

Torkelson²

1992

Macromolecules

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Amorphous blends of poly (methyl methacrylate) (PMMA) and poly( vinyl chloride) (PVC) were prepared via solution precipitation. From the study of Tt composition behavior, the blends were found to show large positive deviations from volume additivity and to be miscible over the entire composition range at temperatures below 180 °C. An endothermic heat of demising and its concentration and temperature dependences have been measured by DSC. Spinodal and "cloud point" curves were extrapolated from these measurements. The results have been interpreted in terms of the modified Flory's equation-of-state theory by Hamada et al.1 and compared with predictions of the directional specific interaction model proposed by ten Brinke and Karasz.2 The modified equation-of-state theory can almost quantitatively reproduce the concentration dependence of the heat of mixing but failed to predict the positive excess heat capacity for this blend system, whereas the ten Brinke-Karasz model can describe qualitatively both the concentration and temperature dependence of the heat of mixing of homopolymer blends.

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Excess heat capacities for two miscible polymer blend systems

Cited by 26 publications

References 15 publications

Excess Specific Heats in Miscible Binary Blends with Specific Interactions

Excess Specific Heats in Miscible Binary Blends with Specific Interactions

Mixing and phase separation in liquid crystal/matrix systems: Determination of the excess specific heat of mixing

Miscibility and phase separation in poly(methyl methacrylate)/poly(vinyl chloride) blends: study of thermodynamics by thermal analysis

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