1993 рік

Amphiphilic polymer brushes are synthesized by the sequential graft copolymerization of crosslinked gelatin with methyl methacrylate and acrylic acid. Samples with varying lipophilic to hydrophilic ratio (L/I-I values) could be synthesized by adjusting the grafting conditions. Here we report the swelling behaviour of these systems in pure and mixed solvent systems. The swelling ratio is influenced by the L/H value of the material. The brush with L/H value close to 1 exhibits anomalous swelling. Mixed solvents appear to be better swelling media than pure solvents. We discuss these observations in the light of L/H values and the differential solvation of individual chains.

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The morphologies and tensile properties of an elastomer-modified epoxy (EME)/polycarbonate (PC) binary system and an EME/diglycidyl ether of bisphenol A (DGEBA)/PC ternary system were examined. In the EME system, a continuous elastomer-rich phase formed, while in the EME/DGEBA systems (unblended with PC), a continuous epoxy-rich phase formed. In both of these systems, two-phase structures were observed. In contrast, a microdispersed structure was observed when the PC was blended with either the EME or with the EME/DGEBA systems. It is suggested that blending of the epoxy with PC caused an increased sohibility of the former into the elastomer phase. The tensile strength and tensile elongation of both the EME and EME/DGEBA systems were improved by blending with PC. In the EME/PC blend, the tensile elongation reached its maximum value (60%) at a PC content of approximately 10 p.h.r. (parts per hundred resin by weight), with this maximum being approximately one and a half times higher than that of the unblended EME. Tensile strength was also deafly increased by blending with small amounts of PC, but soon reached a steady value. In the EME/DGEBA/PC blends, the tensile properties were dependent on the weight ratio of EME to DGEBA. In the absence of PC, as this ratio increased, the tensile elongation also increased, while at the same time the tensile strength decreased. The tensile properties were also improved in this system, by blending with PC. From the results obtained, it was clear that the improvement in tensile properties was closely related to the changes in morphology. Therefore, blending of the PC induced a microdispersed structure and improved the elongation of the epoxy resin.

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Polymerization of N,N-diglycidylaniline with 10% (w/w) potassium t-butoxide gave soluble polymer with only a small amount of gel (2%), whereas 2% butoxide produced mainly gel (80%). Number-average molecular weights by laC nuclear magnetic resonance (n.m.r.) end-group analysis (supported by gel-phase chromatography) did not exceed about 20 000. The structure of the polymers was established by ~3C n.m.r., and it was shown that the cyclopolymerization reaction produced a mixture of six- and seven-membered rings (each as a mixture of cis and trans isomers), with the ring distribution being probably random. A complete assignment of the n.m.r, spectra was made in terms of these structures. The formation of seven- over six-membered rings was favoured and contrary to Baldwin's rules. The data are relevant to the identification of cyclization reactions that occur in the amine cure of epoxy resins containing N,N-diglycidyl groups

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The compatibilizing effect of the symmetric narrowly distributed block copolymer poly(styrene-b-methyl methacrylate) (P(S-b-MMA)) in blends of high-molecular-weight poly(styrene-co-acrylonitrile) containing 20 wt% (PSAN20) or 43 wt% acrylonitrile (PSAN43) with poly(2,6-dimethyl-l,4-phenylene ether) (PPE) was investigated by dynamic mechanical spectroscopy and transmission electron microscopy. In blends with the PSAN43, P(S-b-MMA) forms spherical micelles in the PPE phase with no dispersing efficiency. In contrast to this, for blends with PSAN20, the block copolymer is located at the phase boundary, causing an extremely fine dispersion of the components. Depending on the location of P(S-b-MMA), the PPE glass transition is altered by the PS blocks to different degrees. Even for block copolymer concentrations as high as 20 wt%, in ternary blends with PSAN20 and PPE no micelles could be detected. The results suggest that A/B blends compatibilized with a C-D block copolymer, in which the copolymer components show an exothermic heat of mixing with the bulk phases A/B, are superior to systems A/A-B/B with respect to emulsifying efficiency. Owing to the high tendency of the block copolymer to locate at the phase boundary, micelles do not compete at concentrations of practical interest

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Experimental results for five series of linear oligomers of different molecular weights, having a common perfluorinated molecular body and five different pairs of equal foreign end units, generally demonstrate the substantial influence of the chemical nature of the end units on the Tg-x relation of polymers and copolymers, with progressive evidence when the molecular weight is lowered. Positive or negative deviations from the body Tg asymptotic value are observed with different types of end units. Previous experimental results and theoretical expectations for the T~ at low x values are reviewed briefly. As a first approximation, the applicability of the simplest two-parameter T~-x equations to the new results is discussed, as well as the situation when x is translated in terms of end copolymeric composition

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In this paper, aspects of the microstructural state of glassy polymers that evolve during physical ageing and inelastic deformation were studied. Differential scanning calorimetric (d.s.c.) measurements were performed on specimens of three glassy polymers: polystyrene (PS), polycarbonate (PC) and poly(methyl methacrylate) (PMMA). Materials were subjected to both a quenched and a well annealed heat treatment and subsequently deformed in compression to various levels of strain. Stress-strain curves and companion d.s.c, scans were compared. The well known enthalpy overshoot at Tg was observed for the annealed samples, showing that ageing is accompanied by enthalpy relaxation. The annealed material was also found to require a higher stress to yield, and the additional work required to strain-soften the annealed polymer to the flow stress level of its quenched companion was found to correlate well with the area of the enthalpy overshoot of the annealed specimen. Inelastic deformation was found to increase the specific enthalpy of both annealed and quenched specimens. In the annealed material, the enthalpy overshoot at Tg was found to decrease with inelastic strain and was completely erased by about -20% strain. Simultaneously, a pre-Tg exotherm was observed to develop with inelastic strain over a wide range of temperature. The pre-Tg exotherm was found to evolve until essentially reaching a steady-state profile at approximately -25% strain. This evolution coincided with the strain-softening phenomenon observed in the corresponding stress-strain results. A pre-T s exotherm was also found to evolve with straining of the quenched material. Furthermore, the steady-state exotherms of the quenched and annealed materials were found to be nearly identical, as were their corresponding flow stress values after strain softening. Finally, a second, post-Tg exotherm was found to develop with further straining beyond strains of -25%. This exotherm was found to increase with inelastic strain and coincided with the occurrence of strain hardening (due to chain orientation) in the materials. The presence of two distinct and separately evolving exotherms in the inelastically deformed polymers indicates the existence of two separate deformation resistances in glassy polymers, one related to the initial yield and strain-softening behaviour, and the other to the orientation-induced strain hardening of the material. The observation that the pre-Tg exotherm is spread over a wide temperature range reflects the distributed nature of the structural state and may be quantified using a distribution in activation energy for the local rearrangements. The results therefore provide valuable information about the processes that must be accounted for in the development of accurate constitutive models of mechanical behaviour.

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The copolymerization of ethylene and vinylcyclohexane has been investigated using soluble catalytic systems Cp2ZrC12/MAO (Cp=r/s cyclopentadienyl; MAO=methylaluminoxane) and Cp2ZrMe2/MAO at 70°C and room temperature. Under the reaction conditions employed, up to 1.7 mol% of vinylcyclohexane could be introduced into the polyethylene chains. The molecular structure and comonomer content of the copolymers were determined by 13 C nuclear magnetic resonance. The com onomer content of the polymers was found to increase with the comonomer concentration in the feed. The comonomer was found to be mainly statistically distributed. With samples containing higher comonomer content, however, the formation of a relatively very small amount of short blocks was noted. The average molecular weights of the copolymers were lower than those of pure polyethylene prepared under identical conditions. As expected, the vinylcyclohexane decreased the melting point and crystallinity of the polymers

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The influence of the molecular weight of the symmetric block copolymer poly(styrene-b-methyl methacrylate) (P(S-b-MMA)) in blends with high-molecular-weight poly(styrene-co-acrylonitrile) (PSAN) and poly(2,6- dimethyl-l,4-phenylene ether) (PPE) is investigated by dynamic mechanical analysis and transmission electron microscopy. Total molecular weights of the block copolymers vary from 16 up to 275 kg mol-1. Independent of molecular weight, all block copolymers locate to the interface with strong dispersing efficiency. The different block copolymers also showed approximately the same emulsifying efficiency. The degree of segmental mixing of the blocks with the respective phases is evaluated from the glass transition behaviour. A qualitative model is developed to relate the observed glass transition behaviour to the segmental distribution. In blends with large block copolymers, polystyrene blocks and PPE are rather uniformly mixed. The degree of mechanical coupling of the phases increases with the block copolymer molecular weight. The favourable enthalpic interaction between the blocks and the blend components is a major factor determining the phase behaviour. In contrast to this, the molecular weight of PPE showed little influence on blend behaviour

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Trans-l,4-polyisoprene (TPI) crystals were grown from hexane and amyl acetate solutions at various temperatures (-30 to 35°C) for periods of 0.5 to 263 h. The melting temperature (Tin) of the crystals was independent of the crystallization temperature (T~) up to 5°C for hexane solution and 15°C for amyl acetate solution. Above these temperatures (Tr), Tm increased with T~. On the other hand, below Tr a mixture of the a- and/~-forms was obtained and above Tr only the a-form crystals were obtained. The transformation from the 8- to the a-form was found to occur during crystallization. In the samples crystallized at 0°C, cyclic transformation between the two forms was observed. The inverse transformation from the a- to the /~-form was observed during storage at room temperature for TPI crystals grown from amyl acetate solution at temperatures below Tr. This transformation was also observed for naturally occurring TPI (gutta percha and balata) crystals

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Conditioning of nylon-6 by immersion in metal chloride solutions modifies the mode of deformation under an applied tensile stress. Infra-red spectroscopy of salt-treated films shows significant modification of the spectrum, indicating changes in the intermolecular bonding. Modulus measurements support the hypothesis that modification of the intermolecular bonding in nylon results in some chain stiffening. This stiffening of the network of chains reduces the mobility that is required for shear deformation. This in turn leads to the onset of scission crazing. NaCI, however, spectroscopically shows no evidence for nylon-salt interactions having occurred. The deformation behaviour of both thin-film and bulk samples of NaCl-treated nylon-6 reflect the absence of any salt-amide interaction.

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