1993 рік

The effect of filler silica particle size on the properties of integrated circuit (IC) packages sealed with epoxy moulding compound was studied. For this purpose, four epoxy moulding compounds filled with spherical silica particles having mean sizes in the range 6~ 31 pm were prepared. Critical stress intensity factor (Kc) and flexural strength (tr) of the cured epoxy moulding compounds were measured. As the particle size increased, the Kc value increased and the a value decreased. The thermal shock test was carried out: IC packages were repeatedly dipped alternately in liquids at -65°C and 150°C and the occurrence of package cracking was observed. The cracking took place at an earlier stage of the thermal shock test when the particle size was smaller. There was a good relationship between the thermal shock test result and the K~ value.

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Poly(4-vinylpyridine)-poly(dimethylsiloxane) (P4VP-PDMS) block copolymers with various compositions were prepared by anionic polymerization. The 4VP block was quaternized with methyl iodide and reacted with 7,7',8,8'-tetracyanoquinodimethane (TCNQ), which is in both anion radical (TCNQ-') and neutral form (TCNQ°). The products were then characterized by 1H nuclear magnetic resonance and Fouriertransform infra-red spectroscopy, scanning electron microscopy and differential scanning calorimetry techniques. The electrical conductivities of processable and flexible fihns of these block copolymers were measured with the four-probe method and found to be as high as 10 .2 Scm -1. The optimal TCNQ ° doping for which the conductivities are highest were also determined

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In the copolymerization of phenylacetylene (PA) with norbornene (NBE) catalysed by WC16, both monomers reacted simultaneously, the relative reaction rate of PA being several times higher than that of NBE. The product consisted of polymeric (84 wt%, M, =41000) and oligomeric (16 wt%, M n = several hundreds) parts. According to g.p.c., u.v. and n.m.r, analyses, the former part was a PA-NBE copolymer, while the latter was a mixture of PA oligomers. The PA-NBE dyads could be directly observed in the 2-D H-H COSY n.m.r, spectrum of the polymeric part. The p-CF 3 and o-CF3 derivatives of PA showed reactivities close to that of NBE (relative rates ~ 1.4:1 and ~ 1.3:1, respectively). The oligomeric by-products were less in these systems, and the formation of copolymers was similarly confirmed.

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Blends of poly(~-caprolactone) (PCL) and poly(styrene-co-acrylonitrile) (SAN) show a miscibility window, i.e. for a fixed blend ratio ~b, they are able to form a one-phase mixture in the liquid state, which is dependent on the copolymer composition/~, and the temperature T. For miscible blends the Flory interaction parameter Z is smaller than Z,, the Z parameter at the spinodal. The minimum of Z, for a fixed T and q~, is at approximately 20 wt% acrylonitrile in SAN. Below the melting point the homogeneous liquid undergoes a liquid-solid phase transition (i.e. crystallization). We have studied the crystallization kinetics of miscible PCL/SAN blends as a function of fl, q~ and the crystallization temperature T~. The rate of crystallization G, which is equivalent to the radial spherulite growth rate, also exhibits a minimum, at a fixed T c and ~b, for blends containing SAN with 20 wt% acrylonitrile. This behaviour is described in terms of a modified Hoffman-Lauritzen (HL) theory. From secondary nucleation plots it was shown, in addition, that the surface free energy (aao) ~/2 has a minimum at a copolymer composition with 20 wt% acrylonitrile, again for fixed values of ~b and To. In contrast, according to the HL theory the lower surface free energy would lead to a faster crystallization. In order to explain the influence of polymer-polymer interactions on the crystallization kinetics it has to be assumed that the reptation of a crystallizable polymer chain towards the growing front of the crystal is slowed down by favourable interactions. Therefore, an additional term, AU*, was introduced into the mobility expression used in the HL theory. The experimental findings show that the influence of U*, the activation energy for the transport of crystallizable material, on the rate of crystallization predominates over the influence of (O'O'e) 1/2,

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Electrically conductive polymer composites of polypyrrole and poly(alkyl methacrylate) have been prepared using a two-step procedure. First, an emulsion is generated by dispersing a chloroform solution of poly(alkyl methacrylate) and pyrrole in a small amount of an aqueous surfactant solution. The surfactant is adsorbed upon the interface between the two phases and ensures, via double-layer repulsion, the stability of the emulsion. Second, the pyrrole present in the emulsion is polymerized and doped by introducing with stirring an aqueous solution of an oxidant in the emulsion. The polypyrrole deposits on the host polymer, and the composite formed is precipitated using a suitable non-solvent. Smooth, lustrous films or other shaped objects with good mechanical properties have been prepared by hot-pressing the obtained materials. The relation between conductivity and the polypyrrole content of the composite exhibits a percolation behaviour. The electrical conductivity can reach values as high as (~7 S cm 1. The mechanical properties of the material depend on the nature of the host polymer employed and on the content of polypyrrole in the composite. The effect of the length of the alkyl chain on the mechanical properties is investigated. The polypyrrole-poly(ethyl methacrylate) composites appear to be the most suitable because they have enough flexibility and also sufficient strength. Compared to cold-pressing, hot-pressing improves the mechanical characteristics, but decreases somewhat the conductivity. Two single-step procedures have also been employed, but they are less efficient than the two-step procedure and lead to very low conductivities.

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Poly(L-lactide) and other poly(lactones) can be crosslinked by ring opening polymerization of the corresponding cyclic esters in the presence of a tetrafunctional bicyclic biscarbonate. Spiro-bis-dimethylenecarbonate was synthesized from pentaerythritol and diethylene-carbonate and used to prepare polyester networks for use in biomedical applications. Bulk copolymerization of L-lactide with only small amounts of this crosslinker leads to networks with very high gel percentages of up to 100%. The initial crystallinity of these crosslinked poly(L-lactides), as well as the melting temperature, is very much reduced when compared with the homo poly(L-lactide). The degradation of the polymer in vivo is therefore expected to be enhanced and more benign at longer durations. Tensile strength and impact resistance are also greatly influenced by copolymerization with spiro-bis-dimethylene-carbonate. Much tougher and stronger materials can be obtained in this way. These favourable properties allow the material to be used in the preparation of degradable implants such as fracture fixation devices.

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Differential scanning calorimetry and scanning electron microscopy were used to investigate the phenomenon of'fractionated crystallization' exhibited in poly-D( -- )(3-hydroxybutyrate)/poly(ethylene oxide) (PHB/PEO) blends. PHB and PEO, both crystallizable polymers, are compatible in the melt, where only one homogeneous phase is formed. However, complex phase separation takes place during subsequent crystallization: in particular, under non-isothermal conditions, after PHB crystallization, the PEO phase can give rise to fractionated crystallization, as it can crystallize in two steps at different supercoolings. It was observed that this phenomenon depends mainly on the crystallization conditions, the previous thermal history and the crystallization rate used to crystallize the PHB phase. The two different exothermic peaks relative to PEO crystallization were attributed to a homogeneous (high supercooling) and a heterogeneous (low supercooling) nucleation mode respectively. The addition of a nucleating agent, saccharin, supported such a hypothesis. These foreign particles act as heterogeneous nuclei, thus allowing the PEO phase to crystallize completely at lower supercooling according to a heterogeneous mode of nucleation.

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Molecular orientation and mechanical properties of polystyrene/poly(vinyl methyl ether) (PS/PVME) miscible blends have been investigated in a broad range of compositions, taking as a reference the temperature of phase separation. Fourier transform infra-red spectroscopy and birefringence have been coupled to characterize the state of orientation of each component of the blend. It is found that the orientation decreases progressively with an increase in the amount of PVME in the blend, and that a close relation holds between the Young's modulus and the overall orientation. The results are interpreted in terms of free-volume variations with the distance to the glass transition temperature. At the initial stage of deformation, the relation between modulus and orientation is analysed using the rubber elasticity theory, whereas at larger deformations, the strain-orientation relation is properly modelled using a constitutive equation derived from the Doi-Edwards molecular theory

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A new type of optically active N-[4-(cholesteroxycarbonyl)phenyl]maleimide (ChPMI) was synthesized from maleic anhydride, aminobenzoic acid and cholesterol. Radical homopolymerizations of ChPMI were performed in several solvents at 70°C to give optically active polymers having [~]D=2.3 to 41.1 °. Radical copolymerizations of ChPMI (M1) were performed with styrene (ST, M2) and methyl methacrylate (MMA, M2) in toluene at 70°C. From the results, the monomer reactivity ratios (rt, r2) and the Alfrey-Price Q, e values were determined as follows: r 1 = 0.080, r2 = 0.020, Q1 = 6.57, el = 1.74 for the ChPMI-ST system; r 1 =0.11, r2=0.80, Q1 = 1.73, el = 1.96 for the ChPMI-MMA system. Anionic homopolymerizations of ChPMI were also carried out. Chiroptical properties of the polymers and copolymers were investigated.

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The effect of alkali metal salts on the phase behaviour of poly(methyl methacrylate) (PMMA)/poly(ethylene oxide) (PEO) blends was studied using d.s.c. Addition of lithium trifluoromethane sulfonate (LIT) to the normally miscible PEO/PMMA blends induced phase separation with the formation of a crystalline PEO phase, a crystalline PEO/salt complex phase and an amorphous PMMA phase. When potassium thiocyanate was used instead of LiT the mixture separated into two amorphous phases. Phase separation in these blends was not merely a result of the expected PEO/salt interaction, but also of the interaction of the salt with the PMMA. The evidence for this second interaction comes from an observed increase in the PMMA glass transition temperature with the addition of salt. Weak shifts in the carbonyl peaks in the i.r. spectra of the PMMA in the PMMA/salt mixtures suggest that the oxygen atom in the carbonyl interacts with the cation in the salt.

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