1994 рік

The polymerization behaviour of a series of multifunctional methacrylate and acrylate monomers was studied using differential scanning calorimetry to characterize the reaction rate profiles. The polymerization rates, double-bond conversion and kinetic constants were determined for each of the monomers, as well as the effects of monomer type, functionality and rates of polymerization on the reaction behaviour. In particular, by quantifying the kinetic constants for termination and propagation, the controlling mechanism for each of these processes was determined as a function of conversion. In contrast to linear polymerizations, the termination mechanism for these reactions proceeded primarily through reaction diffusion. This behaviour was seen as the termination kinetic constant became proportional to the propagation kinetic constant at very low double-bond conversions. In comparing the acrylates to the methacrylates, the increased reactivity of the acrylates was apparent in the greater values (by three orders of magnitude) of both the termination and propagation kinetic constants. Also, as the number of acrylate or methacrylate groups in the monomer was increased, the kinetic constants correspondingly decreased. This decrease resulted mainly from the greater viscosity of the higher-functionality monomers, attributed to their large molecular weights. Finally, with the exception of the trimethacrylate and triacrylate, the average number of double bonds reacted per monomer at the maximum conversion was near unity, independent of the monomer type or functionality

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Radical copolymerization of n-butyl acrylate (A) and 2,2-dicyano-l-[2-(2-(acryloyloxy)ethyl)dimethylammonioethoxy] ethenolate (B) initiated at 60°C by 4,4'-azobis-4-cyanovaleric acid (ACVA) was studied in two complementary systems: homogeneous phase in dimethylformamide (DMF) and heterogeneous phase in aqueous ethanol (water volume fraction 0.4). In DMF ([-A + B] ~ 0.6-1 moll 1, [-ACVA]/[A + B] = 5 ? 10 -3, molar fraction of monomer B in the comonomer feed f~ ~< 0.7), the copolymerization obeys the terminal unit model and the two monomers of identical acrylate structure behave as an ideal azeotropic comonomer pair: reactivity ratios r A ~ r~ ~ 1. In aqueous ethanol ([A + B] ~ 0.6-2 mol 1 1, [ACVA]/[A + B] = 5 z 10-3,fB ~< 0.8), the unusual composition diagram (apparent reactivity ratios rA, r B > 1) probably results from preferential sorption of either monomer by the insoluble growing macroradicals: the experimental data may be reconciled with the terminal unit model and with invariant 'intrinsic' reactivity ratios (r A ~ r B ~ l) within the simplified framework that stresses the major importance of the ratio of the binding constants of monomers A and B to the growing chain (K=kAA/kBB~0.49). Moreover, copolymerization in heterogeneous phase leads to higher molecular weights. The zwitterionic units significantly increase the copolymer chain sensitivity towards thermal degradation: an irreversible and rather slow first-order rearrangement of the zwitterionic structure occurs for temperatures higher than 150°C (K ~ 2.8 ? 10 6s t at 160°C), well before the initial weight loss observed at about 260°C under nitrogen.

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The dynamic mechanical properties of blends of diglycidyl ether of bisphenol-A based epoxy resin and internally epoxidized polybutadiene rubber have been studied as a function of initial rubber content, stoichiometry and cure cycle of the epoxy resin. It is shown that both the glass transition temperature of the epoxy-rich continuous phase, Tg~r), and the apparent enthalpy of activation associated with this transition, AH,~r), are sensitive to the state of the rubber in the sample. Specifically, dynamic mechanical analysis can be used to distinguish between rubber dissolved in the matrix (plasticizer), incorporated into the network (flexibilizer) or phase separated (toughener). Furthermore, it is demonstrated that the Gordon-Taylor equation rather than the more commonly used Fox equation should be used to model the effect of dissolved rubber on TgE0( and that the Gordon-Taylor equation can also be extended to describe the variations of AHa , Ultimately this equation can be generalized to predict the glass transition E(r)" . • • temperature of a sample contammg both dissolved and phase separated rubber.

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A new computational method is presented for the rapid estimation of polymer miscibility. This algorithm (coined FLEXIBLEND) makes use of molecular mechanics calculations on a pair of polymer segments in order to estimate heats of mixing. Specific interactions between polymer segments of different types are accounted for, as are effects due to local chain flexibility. The many assumptions and approximations of such two-segment approaches are discussed, and the new algorithm is used to investigate how miscibility predictions are influenced by segment size and other parameters of the model. Predictions of polymer miscibility in agreement with experiment are presented for miscible and immiscible blends, and detailed error analysis indicates the statistical significance of the FLEXIBLEND results. Balancing the recognized limitations with the computational speed of the method, it is concluded that the new algorithm should be useful in initial screening of potential blend candidates, and in giving a rough guide as to how changes in chemical structure might alter phase behaviour. The method also provides useful insight as to why two polymers mix or phase-separate. Finally, the FLEXIBLEND results are compared with those of a more rigorous, but computationally more expensive, amorphous cell method for calculating heats of mixing.

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Novel biodegradable copolyamides were synthesized using the bulk polycondensation method. The statistical copolyamides produced by the simultaneous reaction of the salt of adipic acid and bis(paminocyclohexyl) methane and several ~-amino acids were characterized by elemental analyses, density and viscosity measurements and Fourier transform infra-red spectroscopy. The semicrystalline or amorphous nature of these novel copolyamides was confirmed both by differential thermal analysis measurements and wide-angle X-ray diffraction patterns. Their biodegradability was investigated by testing their resistance to alkali hydrolysis (10% w/v NaOH), to microbial/bacterial attack when buried in soil and to enzymatic hydrolysis. The biodegradation of copolyamides was followed using gel permeation chromatography and differential thermal analysis. Possible applications of these polymers could be envisaged in the fields of agriculture, packaging and medicine.

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It is known that pure water can be a solvent for polyacrylonitrile (PAN), but only at temperatures above its boiling point at atmospheric pressure. Thus, a PAN powder which had been blended with water could be compression moulded at 210°C like a thermoplastic material. The water-plasticized films were flexible and could be uniaxially drawn. X-ray diffraction studies on the dry (as-polymerized) powder and the plasticized film showed that there was a remarkable transformation in the diffraction pattern of the latter, which suggested the formation of a new hydrated polymorph. On drawing the plasticized film, the water was mostly expelled from the film and the d-spacings in the diffraction pattern reverted to those in the original, dry polymer. Infra-red dichroism studies led to the conclusion that the residual water molecules in the drawn film were preferentially oriented in a way which suggested hydrogen bonding to the nitriles. Raman spectroscopy showed that the nitrile vibration-band was shifted to higher frequency in the water-plasticized PAN compared with the dry polymer, and this was also indicative of hydrogen bonding of the water to the nitriles. Chain orientation factors were computed from the dichroism of the nitrile and water vibration-bands.

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The relationship between orientation and miscibility in polymer blends has been investigated for blends of poly(e-caprolactone)/poly(styrene-co-acrylonitrile) (PCL/SAN), containing from 23 to 38wt% acrylonitrile. The miscibility of the blends was characterized by d.s.c, whereas the strain-induced orientation of both PCL and SAN chains, in blends containing between 2% and 20% PCL, was measured by Fourier transform infra-red (Fiq.r.) dichroism. PCL orientation was observed in miscible matrices but not in immiscible SAN matrices, indicating that the miscibility leads to the orientation, and the maintenance of orientation, of the minor chain component, despite the fact that flexible chains are involved. In contrast, the major component, SAN, exhibits a similar orientation in the different PCL/SAN systems, and is little influenced by the addition of PCL. The results are discussed in terms of miscibility, chain entanglements, friction coefficients and chain flexibility

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The peculiarities of domain structure of segmented polyurethane-urea/liquid-crystalline polyester (PU/LCP) blends were studied in the LCP concentration range of l-10%, with different lengths of the flexible moiety. Small- and wide-angle X-ray scattering (SAXS and WAXS) and differential scanning calorimetry were used. The position of the SAXS maximum changes discontinuously with increasing LCP concentration. We suggest that the rigid fragments of the LCP tend to be incorporated into the rigid domains of the PU component with the restriction that only each third or fourth fragment is involved. The part of the LCP molecule that fits in the space between rigid PU domains is stretched and determines the interdomain distance.

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A series of polyhydroxyether-polydimethylsiloxane (PDMS) graft copolymers w_ere analysed and tested to determine the effects of factors such as the number-average molecular weight (M.) of the PDMS segments, overall PDMS content and nature of the graft linkage on thermal characteristics, hydrolytic stability, tensile properties and morphology. D.s.c. measurements showed that the copolymers exhibit two-phase morphology even at very low PDMS ~r n ( < 1000 g tool- 1). Despite the presence of the silyl ether linkage, such copolymers display quite good hydrolytic stability. When the PDMS M n is in the region of 5000 g tool- 1, changes in the overall composition have only a marginal effect on thermal stability.

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This paper provides an extension of a procedure for calculating counterion binding ratios in a polyelectrolyte solution based on the Poisson-Boltzmann formulation of territorial counterion binding to include the presence of excess salt of the counterion. It is an extension of an earlier version which dealt with two counterions with different charge numbers without excess salt. The relative binding parameter of sodium to poly(galacturonic acid) as a function of the degree of neutralization in the presence of excess sodium chloride is determined by 23Na n.m.r, and compared with the numerical results from the Poisson-Boltzmann and the Manning condensation theories. Agreement is found with the Poisson-Boltzmann results at degrees of neutralization greater than the critical condensation point predicted by the Manning theory. Qualitative elements of both theories appear in the experimental results.

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