J. M. Pereña

The γ relaxation in polymers containing ether linkages: conformational dynamics in the amorphous phase for a series of polybibenzoates containing oxyethylene spacers

Restricted conformational transitions in oxyethylene chains have been investigated using a molecular mechanics approach. Geometries and energies of transition pathways have been evaluated for kink inversion and formation, and for crankshaft type motions which are feasible in anisotropic glassy matrices. Barrier heights for these processes are found to be similar to those determined for the corresponding processes in polyethylene chains although geometries of the conformations at the energy minima are significantly different. Results of the calculations are broadly consistent with dynamic mechanical measurements which indicate a γ relaxation in the amorphous phase of thermotropic liquid crystal polymers containing oxyethylene spacers. They support the hypothesis that kink formation, inversion and migration are primarily responsible for the low temperature relaxation in these polymers.

Sulphuric acid etching of polyethylene surfaces

The sulphuric acid etching of polyethylene results in the formation of sulphonic groups, varying the surface properties. The study of the groups formed during the etching has been carried out by means of frustrated multiple internal reflection infrared spectroscopy (FMIR). Densities, thermal properties, superficial resistances and resistivities have been correlated with the content of sulphonic groups and treatment time. Moreover, the critical surface tension and platelet adhesion have been examined and related with the change in those properties. Direct sulphonation of polyethylene is a suitable method to change its surface properties, improving the application of this polymer as a biomaterial.

Microhardness and thermal study of the annealing effects in vinyl alcohol—ethylene copolymers

Abstract The influence of annealing and thermal history on the microhardness, MH, of three vinyl alcohol—ethylene copolymers has been studied and the results compared with those for the corresponding homopolymers, poly(vinyl alcohol) and polyethylene. The results show that the microhardness of the copolymers increases with the vinyl alcohol content, although the degree of heterogeneity also seems to play an important role. Two thermal histories (fast quenching or slow cooling from the melt) have been imposed on the samples. The quenched (Q) specimens present lower values of MH than the corresponding slowly cooled (S) samples. Both Q and S samples were subsequently annealed at different temperatures. In the annealed samples, it has been found that the variation of MH is different for the two thermal histories. Thus, the Q samples show an increase of MH with the annealing temperature while the contrary is found for the S specimens. These results can be explained by the heterogeneous nature of these copolymers, as revealed by differential scanning calorimetry traces, which show an appreciable degree of material melting at temperatures well below the main melting peak.

Relaxation processes in thermotropic polydibenzoates with oxyethylene spacers in the main chain

Abstract A dynamic mechanical analysis has been performed on three thermotropic polydibenzoates, designated PDEB, PTEB and PTTB, synthesized from p,p′-dibenzoic acid and di-, tri- or tetraethylene glycol, respectively. The isotropization temperature decreases greatly as the length of the spacer increases, and the mesophase of PTTB is not detected at all. Three dynamic mechanical relaxations are found for each polymer in the temperature range from − 150 to 100°C. The α relaxation corresponds to the glass transition of the polydibenzoates, the β relaxation is typical of polyesters and the γ relaxation has its origins in the crankshaft motions of the oxyethylene units in the spacer, i.e. the oxygen atom can cooperate in this movement when substituting for a methylenic unit.

Vickers microhardness related to mechanical properties of polypropylene

Vickers micro-indentation of polymers is a method that yields information about their microstructure and some aspects of their mechanical behaviour. In the case of polyethylenes of different types, a monotonic increase of the microhardness with increasing degree of crystallinity has been observed and it has been proved that the microhardness is related to the elastic modulus and the yield stress [1, 2]. Microhardness has been also shown to be an adequate technique for establishing precisely the glass-transition temperature of isotactic polypropylene [3] or for studying the delayed elastic recovery of polyethylenes [4, 5]. Moreover, microhardness is a suitable tool for investigating oriented polymers, because it has been demonstrated that the microhardness anisotropy can be related either to changes in the preferred orientation in injection-moulded poly(4-methyl pentene-1) [6] or to birefringence and draw ratio in stretched poly(ethylene terephthalate) [7]. The dependence of the microhardness on structure and the relationships between the microhardness and the mechanical properties have not been analysed in polypropylene. For this reason the main purpose of this work was to present the results obtained for this polymer and to discuss these results in terms of structural parameters. A series of nine commercial isotactic polypropylenes whose nominal densities and melt indices ranged from 0.897 to 0.904gcm -3 and from 0.82 to 29.7, respectively, were studied. The materials were placed between the plates of a Collin press, heated to 5°C above their melting temperatures and pressed at 1.5 MPa for 10min. The mouldings were quenched in ice-water and fihns about 200/~m thick were obtained. The densilLy of the samples was measured by using a gradient column filled with ethanol-water which was calibrated with glass floats, and the degree of crystallinity was calculated by means of the twophase model using the values vj = 1.142cm3g -I and vc = 1.070cm3g L for the specific volumes of the amorphous and crystalline phases, respectively [8]. Dumb-bell shaped specimens with a gauge length of 2cm and a width of 0.4cm were cut out of the moulded sheets. The drawing of the samples was carried out on an Instron tensile testing machine at 20 ° C and at a strain rate of 0.25 min- ~. The moduli of elasticity were calculated from the initial slope of the stress-strain curves and the yield points were considered to occur where the stress went through a maxi

Viscoelastic processes in vinyl alcohol–ethylene copolymers. Influence of composition and thermal treatment

Abstract Viscoelastic relaxations of three samples of vinyl alcohol–ethylene copolymers, richer in the former comonomer, were studied in a wide range of temperature. The temperature location, intensity and apparent activation energy of the distinct relaxations found are discussed and compared with those of the homopolymers, poly(vinyl alcohol) and polyethylene. Differential scanning calorimetry and X-ray diffraction results of the specimens are also discussed in the frame of the dynamic mechanical analysis, showing that the polymorphism exhibited in some copolymers is a result of the thermal treatment.

Crystallization behaviour of fractions of a copolymer of propene and 1-hexane

Abstract The crystallization behaviour of seven fractions of a copolymer of propene and 1-hexane has been analysed. The fractionation discriminates simultaneously in comonomer content, stereoregularity and molecular weight, in such a way that the fraction obtained first at the lowest temperature displays the lowest molecular weight and isotactic triads fraction and the highest comonomer content. The crystallization rate of this fraction is very slow and it does not crystallize in the calorimeter when cooled from the melt at usual rates. The variation of different thermal parameters with the composition of these fractions has been analysed, as well as the crystallinity, determined by differential scanning calorimetry, X-ray diffraction and density measurements. Isothermal experiments enable us to obtain the extrapolated melting temperatures of these fractions and the Avrami exponent for the crystallization, which has been found to be close to three in all the cases. Non-isothermal experiments have also been analysed, comparing the results with the isothermal ones.

Solid-state nuclear magnetic resonance study of linear low-density polyethylenes: 1. Ethylene-1-butene copolymers

Abstract The effect of crystallization conditions on the partitioning of comonomer units between crystalline and non-crystalline regions has been investigated in a copolymer of ethylene-1-butene, a linear low-density polyethylene, by using solid-state 13 C nuclear magnetic resonance (n.m.r.) techniques. It is shown that the cross-polarization ability of the methyl carbons in the ethyl branches is considerably lower than in the case of a hydrogenated polybutadiene taken as a model, indicating a very high degree of mobility of those branches, and resulting in the necessity for longer contact times (about 5 ms) to get a maximum in their signal. Two sample preparations (quenched and slowly crystallized from the melt, respectively) have been analysed, showing very small differences in both the n.m.r. crystallinities and the partitioning of the ethyl branches of the comonomer units. This stands in contrast with the very distinct melting patterns obtained in differential scanning calorimetry, which indicate a rather different distribution of crystallites between the two preparations. The partitioning revealed that the concentration of ethyl branches in the non-crystalline region is about five times higher than in the crystalline region for both preparations. Taking into account the crystallinity, about 9% of total branches were found to be accommodated in the crystal lattice, a number slightly higher than in the case of the model hydrogenated polybutadiene sample, but far away from the much easier inclusion of methyl branches and chain ends.

Microhardness and DSC measurements on liquid crystalline poly(diethylene glycolp,p′-bibenzoate) as a function of the ageing time

SummaryThe ageing process of the liquid crystalline glass of poly(diethylene glycolp,p′-bibenzoate), PDEB, has been studied by differential scanning calorimetry and microhardness measurements. The experiments have been performed on a sample of PDEB aged at 36±C during different times. The process was found to be rather fast at that temperature, and the results from both techniques are compared.

Interference microscopy measurements of depth at Vickers hardness indentations in polyethylene

We thank Professor P. Cobo (Catedra de Siderurgia, ETSII, Madrid) for permission to use the micro- hardness tester, CAICYT for financial support, and Mr J. M. Sanchez Orejuela for the fine photographic work.