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See detailStereocomplexation of sPMMA-PBD-sPMMA triblock copolymers with isotactic PMMA. II: Effect of molecular weight
Yu, Jian Ming; Jérôme, Robert ULg

in Polymer (1998), 39(25), 6567-6575

Thermoplastic elastomers consisting of triblock copolymers (MBM) with outer syndiotactic poly(methyl methacrylate) (sPMMA) blocks associated to an inner polybutadiene (PBD) block have been modified by ... [more ▼]

Thermoplastic elastomers consisting of triblock copolymers (MBM) with outer syndiotactic poly(methyl methacrylate) (sPMMA) blocks associated to an inner polybutadiene (PBD) block have been modified by stereocomplexation with isotactic PMMA (iPMMA) at a constant 2/1 sPMMA/iPMMA (or s/i) mixing ratio. Toluene cast films of these stereocomplexes have been studied by thermal analysis as a function of molecular weight of the sPMMA blocks and the iPMMA homopolymer. Although the melting temperature of the stereocomplex (Tm) is independent of molecular weight of the sPMMA blocks ( sPMMA) in the studied range from 7000 to 46 000, the melting enthalpy increases with increasing sPMMA. This effect is, however, erased by the sample annealing at 140°C for 15 h, and a melting enthalpy of cα. 33 J/g total PMMA is then observed whatever the copolymer composition. One melting endotherm is observed in case of low sPMMA. although two melting endotherms are reported for higher sPMMA depending, however, on the heating rate. Two endotherms are indeed observed at small sPMMA as the heating rate is low. Molecular weight of iPMMA has no significant effect on either the melting temperature or the melting enthalpy of the stereocomplexes. Dynamic mechanical analysis has confirmed the phase separation of the complexed triblock copolymers. At constant molecular weight of PBD and iPMMA, stereocomplexation increases the tensile strength of MBM copolymers containing small sPMMA blocks, whereas the opposite effect is observed for longer sPMMA blocks. When the same MBM copolymer is complexed by iPMMA, the tensile properties are independent of the molecular weight of iPMMA at least in the range from 5000 to 74 000. [less ▲]

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See detailPoly [glycidyl methacrylate (GMA) / methylmethacrylate (MMA)-b-butadiene (B)-b-GMA/MMA] reactive thermoplastic elastomers: synthesis and characterization
Yu, Jian Ming; Dubois, Philippe ULg; Jérôme, Robert ULg

in Journal of Polymer Science. Part A, Polymer Chemistry (1997), 35(16), 3507-3515

New block copolymers of the ABA type, where B stands for polybutadiene (PBD) and A for polyglycidylmethacrylate(PGMA), poly(methylmethacrylate(MMA)-co-GMA) and PMMA-b-PGMA, respectively, have been ... [more ▼]

New block copolymers of the ABA type, where B stands for polybutadiene (PBD) and A for polyglycidylmethacrylate(PGMA), poly(methylmethacrylate(MMA)-co-GMA) and PMMA-b-PGMA, respectively, have been successfully synthesized by using the diadduct of tert-butyllithium (tert-BuLi) to meta-diisopropenylbenzene (m-DIB) as a difunctional initiator. The PBD midblock has been synthesized in a cyclohexane/diethylether (100/6, v/v) mixture at room temperature, whereas the methacrylate outer blocks have been synthesized in a cyclohexane/diethylether/THF (100/6/150, v/v/v) mixture at -78°C. Block copolymers of a very narrow molecular weight distribution (1.10) have been analyzed by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and tensile testing. These materials are phase separated and can exhibit tensile strength up to 22 MPa together with very high elongation at break (1500%). [less ▲]

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See detailTriblock copolymer based thermoplastic elastomeric gels of a large service temperature range :preparation and characterization
Yu, Jian Ming; Jérôme, Robert ULg; Overbergh, Noël et al

in Macromolecular Chemistry and Physics (1997), 198(11), 3719-3735

Poly(methyl methacrylate)-block-polybutadiene-block-poly(methyl methacrylate) (MBM) triblock copolymers and their hydrogenated counterparts with poly(ethylene-co-1,2-butylene) midblock (MEBM) were swollen ... [more ▼]

Poly(methyl methacrylate)-block-polybutadiene-block-poly(methyl methacrylate) (MBM) triblock copolymers and their hydrogenated counterparts with poly(ethylene-co-1,2-butylene) midblock (MEBM) were swollen by an aliphatic oil of high boiling point which is a selective solvent for the central block. Thermoreversible gels are accordingly formed by both MBM and MEBM copolymers above a critical polymer content (Cr), which depends on the nature of the midblock and not on the copolymer molecular weight, at least in the investigated range. Cr has been found to be 5 wt.-% for an MBM block copolymer and 2 wt.-% for MEBM copolymers of various molecular weights. Gels of MEBM triblock copolymers exhibit interesting mechanical properties, such as high elon-gation at break (up to 870%) and high tensile strength (32 kPa). The most interesting fea-ture of the MEBM gels is an upper service temperature as high as 170°C, thus more than 100°C higher than the value (47°C) reported for gels of an SEBS copolymer (S = polystyrene) of comparable molecular weight (100000) and composition (ca. 30 wt.-% hard block). The morphology of MEBM gels was studied by scanning electron microscopy (SEM) and found to be cocontinuous in case of a gel containing 20 wt.-% copolymer. [less ▲]

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See detailPoly[methyl methacrylate (M)-b-styrene (S)-b-butadiene (B)-b-S-b-M] pentablock copolymers: synthesis, morphology, and properties.
Yu, Jian Ming; Dubois, Philippe ULg; Jérôme, Robert ULg

in Macromolecules (1997), 30(17), 4984-4994

A series of poly[methyl methacrylate (M)-b-styrene (S)-b-butadiene (B)-b-S-b-M], or MSBSM, pentablock copolymers have been successfully synthesized by sequential living anionic polymerization initiated ... [more ▼]

A series of poly[methyl methacrylate (M)-b-styrene (S)-b-butadiene (B)-b-S-b-M], or MSBSM, pentablock copolymers have been successfully synthesized by sequential living anionic polymerization initiated with the diadduct of tert-butyllithium (t-BuLi) onto m-diisopropenylbenzene (m-DIB) in a cyclohexane/diethyl ether mixture for the butadiene and styrene polymerization at room temperature and in a cyclohexane/THF mixture for the MMA polymerization at −78 °C. All the pentablock copolymers have a monomodal and narrow molecular weight distribution (Mw/Mn < 1.20), and their weight composition varies from 11 to 55% M, 18 to 55% S, and 15 to 64% B. Toluene-cast films of these copolymers have been analyzed by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and transmission electron microscopy (TEM). All these materials show a phase separation of the soft B component from the hard S and M blocks, which cannot, however, be distinguished one from each other by DSC or DMA. Indeed, a single transition is observed for the binary hard phase at a temperature intermediate between the glass transition temperature of polystyrene (PS) and poly(methyl methacrylate) (PMMA). In addition to classical phase morphologies, such as cylindrical and lamellar phase organization, two nonclassical morphologies, i.e., catenoid−lamellar and strut phase structures have been observed by TEM. The phase morphology strongly depends on the pentablock composition and any chemical modification of blocks. For instance, the cylindrical morphology, characteristic of an MSBSM copolymer containing equal amounts of hard and soft phases, is changed into a lamellar morphology upon hydrogenation of the B midblock. Copolymers of a relatively low hard phase content typically behave as thermoplastic elastomers of high ultimate tensile strength (ca. 30 MPa) and elongation at break (ca. 900%). These mechanical properties, however, depend on the casting solvent. [less ▲]

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See detailTriblock copolymer based thermoreversible gels. 4. Effect of the midblock and characterization of the sol-gel transition
Yu, Jian Ming; Blacher, Silvia ULg; Brouers, F. et al

in Macromolecules (1997), 30(16), 4619-4625

Thermoreversible gelation has been studied in o-xylene for poly(methyl methacrylate) containing 80% syndiotactic triads (sPMMA) and block copolymers of the MXM type, where M is sPMMA and X is either ... [more ▼]

Thermoreversible gelation has been studied in o-xylene for poly(methyl methacrylate) containing 80% syndiotactic triads (sPMMA) and block copolymers of the MXM type, where M is sPMMA and X is either polybutadiene (PBD), hydrogenated PBD (PEB), poly(styrene-b-butadiene-b-styrene) (SBS) triblock, or the hydrogenated version of this triblock (SEBS). In o-xylene, which is a selective solvent for the central X block, sPMMA forms thermoreversible gels provided that the molecular weight is high enough. When sPMMA is the outer block of MXM triblock copolymers, the midblock X appears to favor the gelation and it considerably improves the thermal stability of the matured gels. This thermal stability is, however, largely independent of the actual nature of the midblock. The dynamic properties of solutions and gels have been analyzed and discussed on the basis of scaling assumptions. At the gel point, where the loss angle tan δc = G‘‘/G‘ is independent of the probing frequency, the sample obeys the typical power law G‘(ω) G‘‘(ω) ωΔ. The scaling exponent Δ is found in the 0.65−0.75 range for both sPMMA and MXM block copolymers, independent of the nature of the midblock. Modulus−frequency master curves have been built by using appropriate reaction time dependent renormalization factors for the individual frequency and modulus data. The scaling of these factors with reaction time has allowed us to calculate the static scaling exponents for the increase observed in both modulus and viscosity. The accordingly calculated values agree with the scalar elasticity percolation model. [less ▲]

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See detailSynthesis and characterization of hydrogenated poly[alkylmethacrylate(-b-styrene)-b-butadiene-b-(styrene-b-) alkylmethacrylate] triblock and pentablock copolymers
Yu, Jian Ming; Yu, Yisong; Dubois, Philippe ULg et al

in Polymer (1997), 38(12), 3091-3101

Triblock and pentablock copolymers of the X(Y)B(Y)X type have been synthesized by the sequential living anionic polymerization of butadiene (B), styrene (Y) and alkylmethacrylate (X), respectively. The ... [more ▼]

Triblock and pentablock copolymers of the X(Y)B(Y)X type have been synthesized by the sequential living anionic polymerization of butadiene (B), styrene (Y) and alkylmethacrylate (X), respectively. The diadduct of t-BuLi onto m-diisopropenylbenzene (m-DIB) has been used as a difunctional initiator. Methylmethacrylate (MMA), t-butylmethacrylate (tBMA) and isobornylmethacrylate (IBMA) have been used as precursors of the outerblocks X. The polybutadiene (PBD) midblock that contains ca 42-45% 1,2-units has been selectively hydrogenated into a saturated poly(ethylene-co-1-butene) (PEB) block. The homogeneous hydrogenation catalysis has no deleterious effect on the copolymer integrity. These completely soluble thermoplastic elastomers have been characterized by FT i.r., n.m.r., d.s.c. and d.m.a. The PEB midblock has a low Tg (-50°C) and a small propensity to crystallize. The effect of hydrogenation on the morphology and mechanical properties depends on the outer block. Upon hydrogenation of the PBD midblock in polymethylmethacrylate (PMMA) and polyisobornylmethacrylate (PIBMA) containing triblock copolymers, the ultimate tensile strength is increased (except for a hard block content > 50%) due to a sharper phase separation, whereas the elongation at break is decreased. The extent of phase separation is reduced in polyt-butylmethacrylate(PtBMA) containing triblock copolymers upon hydrogenation and the ultimate tensile strength is slightly decreased. Stereocomplexation of the syndiotactic PMMA outerblocks is observed to occur upon blending with isotactic PMMA [less ▲]

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See detailStereocomplexation of sPMMA-PBD-sPMMA triblock copolymers with isotactic PMMA: 1. Thermal and mechanical properties of stereocomplexes
Yu, Jian Ming; Yu, Yi-Song; Dubois, Philippe ULg et al

in Polymer (1997), 38(9), 2143-2154

A novel thermoplastic elastomer consisting of a triblock copolymer (MBM) with outer syndiotactic poly(methyl methacrylate) (sPMMA) blocks associated to an inner polybutadiene (PBD) block has been modified ... [more ▼]

A novel thermoplastic elastomer consisting of a triblock copolymer (MBM) with outer syndiotactic poly(methyl methacrylate) (sPMMA) blocks associated to an inner polybutadiene (PBD) block has been modified by stereocomplexation with isotactic PMMA (iPMMA). Solution cast films of stereocomplexes have been analysed by thermal analysis as a function of the iPMMA/sPMMA (i/s) mixing ratio and the solvent used for the film casting. Although self-aggregation of iPMMA is currently observed, this phenomenon does not occur in solvent cast films of iPMMA with the MBM copolymers. Extent of complexation depends on the i/s mixing ratio and the casting solvent. At a constant i/s ratio, molecular weight of iPMMA has no significant effect on the extent of stereocomplexation. Thermal stability of the stereocomplexes is only affected by the casting solvent, in contrast to tensile strength which is greatly influenced by the mixing ratio of the two PMMA stereoisomers. Tensile strength is increased when increasing amounts of iPMMA are blended with a triblock copolymer of low PMMA content (e.g. 12%). In case of triblocks of high PMMA content (ca. 30% or higher), a low i/s ratio (<l/4) increases the tensile strength, whereas a high i/s mixing ratio (1/2) results in decreasing tensile strength. The same trend is observed for blends of sPMMA-polystyrene (PS)-PBD-PS-sPMMA (MSBSM) pentablock copolymers with iPMMA. [less ▲]

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See detailTriblock copolymer based thermoreversible gels. 1. Self-association of sPMMA end-blocks in o-xylene and viscoelasticity of the gels
Yu, Jian Ming; Jérôme, Robert ULg; Teyssié, Philippe

in Polymer (1997), 38(2), 347-354

Syndiotactic poly(methyl methacrylate) (sPMMA)-polybutadiene (PBD)-sPMMA triblock copolymers, or MBM, have been studied in the presence of o-xylene which is a selective solvent for the central PBD block ... [more ▼]

Syndiotactic poly(methyl methacrylate) (sPMMA)-polybutadiene (PBD)-sPMMA triblock copolymers, or MBM, have been studied in the presence of o-xylene which is a selective solvent for the central PBD block. Syndiotactic PMMA is known to self associate in o-xylene. The central PBD sequence does not prevent the self-association of sPMMA from occurring. Actually, this phenomenon contributes to the stability of the gels formed at room temperature. An endotherm is observed at 35°C, which is responsible for a gel-sol transition. The original solutions have been heated up to 80°C and then rapidly cooled down below 35°C and maintained at 10, 20 and 25°C respectively. The self-association of the sPMMA outer blocks, and thus the liquid-solid transition, have been studied at each of these temperatures. Dependence of the gel viscosity on frequency and temperature effects on viscoelasticity has also been investigated. [less ▲]

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See detailTriblock copolymer based thermoreversible gels. 3. Effect of stereocomplexation with iPMMA
Yu, Jian Ming; Jérôme, Robert ULg

in Macromolecules (1996), 29(26), 8371-9378

Thermoreversible gelation of syndiotactic poly(methyl methacrylate) (sPMMA)−polybutadiene (PBD)−sPMMA, triblock copolymers in o-xylene has been studied in the presence of isotactic PMMA (iPMMA) in a large ... [more ▼]

Thermoreversible gelation of syndiotactic poly(methyl methacrylate) (sPMMA)−polybutadiene (PBD)−sPMMA, triblock copolymers in o-xylene has been studied in the presence of isotactic PMMA (iPMMA) in a large range of syndio/iso mixing ratio (s/i). Differential scanning calorimetry (DSC) shows that two types of association contribute to the gel formation, i.e., the self-aggregation of the sPMMA outer blocks and the stereocomplexation of the sPMMA outer blocks with iPMMA. At low iPMMA contents (s/i = 30/1), self-association of the outer blocks accounts for the gelation process, although stereocomplexation occurs. Stereocomplexation becomes the dominant gelation mechanism at a high enough iPMMA content (s/i = 2/1). Thus, gelation is observed at a copolymer concentration as low as 1 wt %. The time dependence of the storage (G‘) and loss (G‘‘) moduli has been measured in the 0.08−1Hz frequency range in order to probe the gelation process. At the gel point, where the loss angle (tan δc = G‘‘/G‘) is independent of the probing frequency, the sample fits in the typical power law G‘(ω) G‘‘(ω) ωΔ. While the gelation time is greatly influenced by the s/i mixing ratio, the scaling exponent Δ is essentially constant in the range of 0.70−0.75. The Δ value agrees with theoretical predictions as well as with experimental values reported for some chemical gels but it is different from the experimental values reported for most physical gels. The Δ value is also independent of the PBD microstructure and it also holds for sPMMA-polystyrene (PS)−PBD−PS−sPMMA (MSBSM) pentablock copolymers. Therefore, the value of Δ appears to be a typical exponent for thermoplastic gels based on triblock copolymers of the ABA type in o-xylene. [less ▲]

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See detailPoly[alkyl methacrylate-b-butadiene-b-alkyl methacrylate] triblock copolymers : synthesis, morphology, and mechanical properties at high temperatures
Yu, Jian Ming; Teyssié, Philippe; Jérôme, Robert ULg

in Macromolecules (1996), 29(26), 8362-8370

Block copolymers of the ABA type, where B is polybutadiene (PBD) and A is poly(ethyl methacrylate) (PEMA), poly(tert-butyl methacrylate) (P-t-BMA), poly(methyl methacrylate) (PMMA), or poly(isobornyl ... [more ▼]

Block copolymers of the ABA type, where B is polybutadiene (PBD) and A is poly(ethyl methacrylate) (PEMA), poly(tert-butyl methacrylate) (P-t-BMA), poly(methyl methacrylate) (PMMA), or poly(isobornyl methacrylate) (PIBMA), have been successfully synthesized by sequential anionic polymerization of butadiene and methacrylates with the diadduct of tert-butyllithium to m-diisopropenylbenzene as a difunctional initiator. Block copolymers of a narrow molecular weight distribution (1.10) have been analyzed by differential scanning calorimetry, transmission electron microscopy, and dynamic mechanical analysis. These materials are phase-separated and have high mechanical performances. Special attention has been paid to the service temperature of these thermoplastic elastomers in comparison with a styrene−butadiene−styrene (SBS) triblock copolymer. The upper service temperature (UST) has been estimated from the temperature dependence of the tensile properties in the 25−150 °C range and found to change with the outer blocks. Polystyrene (Tg = 100°C) is at the origin of the lower UST, which is however comparable to PEMA (Tg = 90 °C) and P-t-BMA (Tg = 116 °C) containing triblock copolymers of similar molecular weight and composition. PMMA (Tg = 132 °C) outer blocks increase the UST of the triblocks, which is further increased by hydrogenation of the PBD midblock. The higher UST has been found for a PIBMA (Tg = 202 °C)-containing triblock copolymer that shows an ultimate tensile strength higher than 2 MPa at 150 °C. [less ▲]

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See detailSynthesis and properties of poly[isobornyl methacrylate (IBMA)-b-butadiene (BD)-b-IBMA] copolymers : new thermoplastic elastomers of a large service temperature range
Yu, Jian Ming; Dubois, Philippe ULg; Jérôme, Robert ULg

in Macromolecules (1996), 29(16), 7316-7332

Anionic polymerization of isobornyl methacrylate (IBMA) has been studied in THF and toluene in a temperature range from −78 to +40 °C by using (1,1-diphenyl-3,3-dimethylbutyl)lithium (DDBLi) as an ... [more ▼]

Anionic polymerization of isobornyl methacrylate (IBMA) has been studied in THF and toluene in a temperature range from −78 to +40 °C by using (1,1-diphenyl-3,3-dimethylbutyl)lithium (DDBLi) as an initiator in the presence of LiCl or not. Effect of solvent and polymerization temperature on tacticity has been studied. The reactivity of IBMA is comparable to tert-butyl methacrylate (tBMA), and polymers of a very narrow molecular weight distribution (<1.10) have been synthesized at room temperature, in THF, in the presence of LiCl. The Tg of PIBMA is found to vary from 170 to 206 °C with chain tacticity. Poly(isobornyl methacrylate) (PIBMA)−polybutadiene (PBD)−PIBMA triblock copolymers have been synthesized by using the m-diisopropenylbenzene (m-DIB)/tert-butyllithium (t-BuLi) diadduct as an initiator. The PBD midblock has been prepared in a cyclohexane/diethyl ether (100/6, v/v) mixture at room temperature. THF has been added [cyclohexane/diethyl ether/THF (100/6/100, v/v/v)] before the IBMA polymerization takes place at either −78 or +25 °C. Triblock copolymers of a very narrow molecular weight distribution (1.10) have been synthesized even at 25 °C, and no gel formation has been observed. These new triblock copolymers exhibit high tensile strength (30 MPa), high ultimate elongation (1000%), and high upper service temperature (160 °C). [less ▲]

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See detailMicrodomain morphology analysis of block copolymers by atomic force microscopy with phase detection imaging
Leclère, Philippe; Lazzaroni, Roberto; Brédas, Jean-Luc et al

in Langmuir (1996), 12(18), 4317-4320

We use atomic force microscopy (AFM) with phase detection imaging (PDI) in order to study the surface microdomain morphology of thick (i.e., ca. 2 mm) films of triblock copolymers. We present here the ... [more ▼]

We use atomic force microscopy (AFM) with phase detection imaging (PDI) in order to study the surface microdomain morphology of thick (i.e., ca. 2 mm) films of triblock copolymers. We present here the results obtained on a poly(methyl methacrylate)-block-polybutadiene-block-poly(methyl methacrylate) (PMMA-b-PBD-b-PMMA) copolymer prepared by using a 1,3-diisopropenylbenzene (DIB)-based difunctional anionic initiator. Our data illustrate the interest of PDI for the elucidation of surface phase separation in block copolymers. We show that the surface of thick films studied by this new technique exhibits a two-phase structure corresponding to the two types of components. [less ▲]

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See detailSyndiotactic poly(methyl methacrylate) (sPMMA)-polybutadiene (PBD)-sPMMA triblock copolymers: synthesis, morphology, and mechanical properties
Yu, Jian Ming; Dubois, Philippe ULg; Jérôme, Robert ULg et al

in Macromolecules (1996), 29(19), 6090-6099

A series of syndiotactic poly(methyl methacrylate) (sPMMA)−polybutadiene (PBD)−sPMMA triblock copolymers, or MBM, have been successfully synthesized by using dilithium initiators (DLi's) based on the ... [more ▼]

A series of syndiotactic poly(methyl methacrylate) (sPMMA)−polybutadiene (PBD)−sPMMA triblock copolymers, or MBM, have been successfully synthesized by using dilithium initiators (DLi's) based on the diadduct of tert-butyllithium (t-BuLi) to either 1,3-bis(1-phenylethenyl)benzene (PEB) or m-diisopropenylbenzene (m-DIB). The efficiency of these DLi's in building up MBM triblock copolymers has been compared under the same experimental conditions, i.e., in a cyclohexane/diethyl ether mixture for the butadiene polymerization at room temperature and in a cyclohexane/THF mixture for the MMA polymerization at −78 °C. Although all the synthesized copolymers show a monomodal, symmetric, and very narrow molecular weight distribution, the MBM copolymers synthesized with the m-DIB/t-BuLi diadduct are pure triblocks and show a high tensile strength, in contrast to copolymers initiated by the PEB/t-BuLi diadducts that are of a lower tensile strength and contaminated by MB diblock copolymers. Solvent cast films of MBM prepared with the m-DIB/t-BuLi diadduct are two-phase materials as confirmed by DSC and dynamic mechanical analysis (DMA). Transmission electron microscopy (TEM) shows a spherical morphology at a low sPMMA content, that changes into a cylindrical and finally lamellar morphology upon increasing the sPMMA content. Phase separation is observed for MBM with M̄n of the sPMMA blocks as low as 6000. Dependence of tensile strength on copolymer structure and sample preparation has been studied. The smaller sPMMA molecular weight, M̄n(PMMA), required for high tensile strength depends on the PBD molecular weight, M̄n(PBD), e.g. 12 000 for Mn(PBD) = 36 000 and 6000 for Mn(PBD) = 80 000. The upper M̄n(PMMA) is ca. 20−25 000, whatever the Mn(PBD). The optimum tensile strength is observed for M̄n(PMMA) = 15 000, independently of Mn(PBD) in the studied range. As a rule, the tensile strength tends to level off and the elongation at break starts to decrease when the sPMMA content is increased beyond 35 wt %. At a constant sPMMA content, Mn(PBD) (>ca. 36 000) does not affect the ultimate tensile properties. [less ▲]

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See detailTriblock copolymer based thermoreversible gels. 2. Analysis of the sol-gel transition
Yu, Jian Ming; Dubois, Philippe ULg; Teyssié, Philippe et al

in Macromolecules (1996), 29(16), 5384-5391

Syndiotactic poly(methyl methacrylate) (sPMMA)−polybutadiene (PBD)−sPMMA triblock copolymers of various chemical compositions and different molecular weights have been studied in the presence of o-xylene ... [more ▼]

Syndiotactic poly(methyl methacrylate) (sPMMA)−polybutadiene (PBD)−sPMMA triblock copolymers of various chemical compositions and different molecular weights have been studied in the presence of o-xylene, which is a selective solvent for the central PBD block. Thermoreversible gels can be formed in a suitable curing temperature range (<35 °C) and at a high enough concentration (>1 wt %). The time dependence of the storage (G‘) and the loss (G‘‘) moduli has been measured in a frequency range of 0.08−1 Hz. The static and dynamic properties of the gels have been discussed on the basis of the scaling theory. At the gel point, where the loss angle (tan δc = G‘‘/G‘) is independent of frequency, typical power laws G‘(ω) G‘‘(ω) ωΔ have been observed. The scaling exponent Δ has been found equal to 0.70 ± 0.02 independently of the PBD and PMMA molecular weight, i.e., 36 000 < Mn (PBD) < 100 000 and 20 000 < Mn (PMMA) < 51 000. This exponent is also independent of the copolymer concentration and temperature in the investigated range, i.e., 2−7 wt % and 8−24 °C, respectively. This value of Δ agrees with theoretical predictions as well as with experimental values reported for some chemical gels; it is however different from the experimental values published for most physical gels. A PBD−PMMA diblock copolymer also forms a gel in o-xylene, although at higher concentration and lower temperature compared to the parent triblock copolymer. The scaling exponent Δ is then somewhat smaller, i.e., 0.61. [less ▲]

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