1992 рік

A theoretical description of the electron exposure dose required to solubilize the gel fraction of pre-crosslinked positive resists was first proposed by Suzuki and Ohnishi. These authors proposed that the important factors affecting the sensitivity of pre-crosslinked resists are the weight-average molar mass of the primary polymer (Mz),th e main-chain scission efficiency and the average number of crosslinked units per weight-average molecule (6). In this paper we re-examine these conclusions and show that, for the practical situation, 6 and Mi should not be regarded as independent variables. Using a functional relationship between 6 and molar mass, and expressions due to Charlesby for the change in the gel fraction beyond the gel point, equations are developed to predict the critical exposure dose for the cases where degradation occurs (i) exclusively via scission of the crosslinks and (ii ) exclusively by scission of main-chain units. Experimental data for pre-crosslinked copolymers of monomethyl itaconate and methyl methacrylate show good agreement with values calculated assuming that degradation of the gel occurs only via main-chain scission

Детальніше...  

A new group of miscible mixtures (molecular composites) has been studied by means of differential scanning calorimetry, dielectric relaxation and dynamic mechanical techniques. Three kinds of rigid aromatic polyimides ( PI ) and a commercial-grade poly (ether imide ) ( PEI ) were mixed by dissolving them in m-cresol and subsequently coagulating in methanol. The mixtures show a single glass transition temperature (Ts) in all compositions in which the Tg increases with increasing PI content. These T u values are reproducible in repeated heating cycles, suggesting the true miscibility of the blends. This miscibility may be attributed to the similarity of imide structure between the rigid PI and PEI matrix. The bulk PI/PEI indeed shows improved modulus and tensile strength as compared to the pure PEI matrix.

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The behaviour of some polyurethane thiosulphate and sulphonate ionomers in dilute solutions is reported. Viscosity measurements of the ionomers in N,N'-dimethylformamide (DMF) showed a 'polyelectrolyte effect', which diminished upon addition of lithium bromide. For water-soluble ionomers, measurements were carried out in aqueous solution. The effect of the counterion was examined. Anomalous molecular-weight distributions of the ionomers were observed by gel permeation chromatography (g.p.c.) using DMF as solvent, which depended on the polymer ionic content, solution concentration and temperature. Normal g.p.c, distributions were obtained, however, in solvent containing 0.5% of lithium bromide.

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The theory of well defined superstructures formed in binary mixtures of diblock copolymers with different molecular weights and composition in a strong segregation limit has been developed. Three types of binary mixtures were considered : a mixture of two cylinder-forming block copolymers; a mixture of cylinder- and lamellae-forming block copolymers; and a mixture of two lamellae-forming block copolymers. The formation (conditions and the thermodynamic characteristics) of mixed lamellar and cylindrical superstructures and their thermodynamic stability were investigated. It was shown that in the case of non-isomorphous block copolymers the change in mixture composition leads to a first-order phase transition from one morphology of the superstructure to another, i.e. to real phase segregation (each phase remains microphase segregated ). The theory was compared with experimental data. The possibility of the formation of cylindrical domains in the mixture of two lamellae-forming block copolymers was predicted.

Детальніше...  

Insertion polymerization of 2,2_dimethyltrimethylene carbonate (DTC) with initiators based on tetraphenylporphyrin-aluminium (TPPAl-R) results in high yields of poly (2,2_dimethyltrimethylene carbonate) without cyclic oligomers being formed. This indicates that back biting does not take place. The polymerization reaction is slow in methylene chloride or toluene at room temperature; the reaction rate increases with increasing temperature. Tetraphenylporphyrin-alkoxides (TPPAl-OR), the most active initiators for DTC polymerization, from a mixture of DTC and s-caprolactone (ECL) yield a statistical copolymer, indicating poor selectivity towards these monomers. The initiator, however, is well suited for block copolymer synthesis upon stepwise addition of the monomers, e.g. poly(ECL-block-DTC), poly (ethylene oxide-block-DTC) and poly (propylene oxide-block-DTC). The thermal properties of poly (propylene oxide-block-DTC) as a function of composition are reported. Copolymers with a high propylene oxide content show a pronounced glass transition temperature for the polyether block ; all copolymers show only one melting transition which is assigned to that of the polycarbonate block

Детальніше...  

That the melt-mixed blend of polyacrylic acid and polychloroprene is self-crosslinkable during high temperature moulding is evident from Monsanto rheometric and i.r. spectroscopic studies. Such a system of a self-crosslinkable blend is immiscible at all blend ratios as observed by dynamic mechanical analyses (d.m.a.). Scanning electron microscopic and d.m.a, studies reveal formation of an interfacial layer in the two-phase blend system. The blend possesses high damping characteristics with moderate tensile and tear strength

Детальніше...  

The high pressure solubility of polyethylene standards (M,=2100, 16400, 108000 and 420000 and Mw/M, = 1.14, 1.16, 1.32 and 2.66, respectively) in n-pentane has been studied. Concentrations of up to 15 wt% polymer have been investigated. For each polymer sample and concentration, pressures that are required to achieve single-phase solutions have been determined over a range of temperatures. The solutions are found to all show lower critical solution temperatures. Demixing pressures are observed to depend strongly on the molecular weight of the polymer.

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A theoretical model is proposed for the distribution of molecular weight, branching and crosslinking in styrene-butadiene emulsion copolymers. The main objective is to compute the instantaneous distributions of primary macroradicals and primary dead chains with zero, one, two or more crosslinks as the reaction proceeds. The amount of gel is calculated on the assumption that each chain with at least two (or three) crosslinks is withdrawn from the sol fraction. The network thus formed is characterized by its crosslink density and number average length of the primary chains. This model has been compared with a mean field theory of network formation derived recently by Hamielec et al. Both models were used to predict the extent of gel versus conversion and the crosslink density of the gel polymer, differentiating between elastically active and inactive crosslinking points. The same value of the adjustable parameter (i.e. reactivity ratio of the butadiene pendent double bond) can be used in the two models to account for gel formation with fairly good accuracy.

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Molecular-weight distributions (MWDs) of polyolefins are currently measured by high-temperature size exclusion chromatography. The raw data are converted to MWDs using universal calibration with a differential refractive index (d.r.i. ) concentration detector or by direct molecular-weight measurements with a continuous viscometer (c.v.) or a low-angle laser light scattering (1.a.l.l.s.) detector. None of these techniques provides a picture of the true distribution. Simple d.r.i.-universal calibration may fail to sense the high-molecular-weight tail of the distribution, which is important in some applications. The 1.a.tl.s. technique is most sensitive to high-molecular-weight polymers but is also least precise. The C.V. method is presently the most convenient single technique. The most information is obtained, however, when all these detectors are employed. Both the 1.a.l.l.s. and c.v., for example, should be used to verify that the polymer sample is actually dissolved, since dissolution of semicrystalline polyolefins is not a trivial exercise.

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The glass transition temperatures of natural rubber compounds cured with dicumyl peroxide and sulphur were measured on a Metravib Viscoelasticimeter. Tan 6 versus temperature curves of compounds vulcanized with peroxide show two well defined relaxations associated with the glass transition. Compounds cured with sulphur show only one relaxation. Dynamic deformations of small amplitude were superimposed on large static deformations. It was found that the intensity of the loss tangent associated with the glass-rubber transitions tends to decrease with increasing static deformation.

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