1994 рік

The resistance of methyl methacrylate-butadiene-styrene graft copolymer (MBS) particles against cavitation increases with decreasing particle size, and there exists a critical particle size below which the MBS particles are unable to cavitate. The inability of small particles to cavitate is the reason for the decrease of the impact properties of poly(vinyl chloride) (PVC)/MBS blends with very small MBS particles (do < 150 nm). A toughening mechanism is proposed for the PVC/MBS blends in which the particle size of the rubber particles and the interparticle distance play equally crucial roles.

Детальніше...  

The phase structure of three linear polyethylene (PE) samples, crystallized from the melt at high pressure, has been studied by electron microscopy and high-resolution solid-state x3C n.m.r, spectroscopy. In general, three phases are required to account for the n.m.r, data: the lamellar crystalline phase, the crystallineamorphous interphase and the amorphous phase. All three are present in high-molecular-weight samples but there is no amorphous phase for samples with lower molecular weights and large lamellar thicknesses. The amorphous phase appears when the ratio of the number-averaged extended molecular chain length (X,) to the number-averaged crystalline stem length (L,) exceeds two. High-pressure-crystallized materials differ from those crystallized at atmospheric pressure in that the mass fraction of the amorphous phase does not exceed 0.05; the thickness of the crystalline-amorphous interphase reaches 8.0nm for material with the highest molecular weight, a value which is considerably larger than those reported for samples crystallized at atmospheric pressure or which have been estimated theoretically. Extraordinarily long 13 C spin-lattice relaxation times have been found: a figure of Tic = 7000 s, higher than any previously reported, for the highest-molecular-weight sample is still less than would be expected from the large lamellar thickness. In consequence, this relaxation is attributed to molecular motion in the vicinity of the crystal defects; this is in addition to 13C spin diffusion to the non-crystalline region, occurring with a shorter T~c. The discrepancy between the observed and calculated values of T~c increases as the molecular weight falls in those samples for which the crystal stem lengths exceed the extended molecular lengths. For these, the unexpectedly shorter T~ c is attributed to defects such as methyl end-groups within the crystalline regions

Детальніше...  

Binary blends of polypropylene with a linear low density polyethylene were prepared in solution, dried and studied by transmission electron microscopy using surface replicas of samples prepared by remeiting and quenching. Very clear micrographs were obtained with a wealth of morphological detail. Phase separation was observed over a wide composition range for blends quenched from 190°C. A few samples were prepared by melt mixing in a twin-screw extruder; others, containing deuterated polypropylene, were prepared by solution blending. These special blends showed the same morphologies as normal blends prepared in solution to the same composition.

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The hygrothermal degradation of a glass fibre/epoxy unidirectional composite is investigated by moisture sorption/desorption experiments and dynamic mechanical thermal analysis (d.m.t.a.). At short immersion times, water sorption in distilled water can be satisfactorily described by using Fick's law. The three-dimensional analysis of water diffusion in parallelepipedic samples shows that no significant interfacial capillary flow occurs during the initial Fickian step where water sorption occurs mainly by diffusion through the epoxy network. This conclusion was found to be valid even at the most elevated ageing temperature (90°C). Slow positive deviations from the Fickian behaviour are noticed for long ageing times at 70 and 90cC. They have been found to be correlated to some interfacial debonding, as indicated by an enhanced desorption rate during thermogravimetric analysis (t.g.a.) scans. During these non-Fickian sorption steps, the d.m.t.a, spectra of the aged composite also reveal the progressive appearance of an irreversible broadening of the damping peak associated with the or-relaxation. These processes have been attributed to the occurrence of specific morphological changes and debonding at the interphase during ageing. It is concluded that the hygrothermal degradation of the interface occurs essentially in the non-Fickian sorption steps when the epoxy matrix is close to saturation.

Детальніше...  

Time-resolved ellipsometric analysis was carried out to investigate the change in interfacial thickness between amorphous nylon and styrenic polymers during annealing at high temperatures (170-210°C). The styrenic polymers were neat poly(styrene-co-maleic anhydride) (SMA) and single-phase mixtures of SMA with poly(styrene-co-acrylonitrile) (SAN). Here SMA is known to be reactive with the chain end of nylon, forming a graft copolymer at the interface between the phases. The interfacial thickness increased with annealing time and then attained a constant value, depending on temperature and net concentration of reactive sites (maleic anhydride units) in the styrenic phase. The interface established at late stages was tremendously thick, ranging from 10 to 50 nm. The thickest interface is several times the coil size of the component polymer. This may suggest drawing of the whole graft copolymer chain into the interface to establish a smooth concentration gradient at the interface. Formation of such a thick interface seems to be characteristic of a reactive system in which chemical reaction induces deviation from an equilibrium interface in a ternary system consisting of polymer A, polymer B and A-B graft or block copolymer. Thus the study provided a preliminary understanding of the in situ reactive compatibilization of polymer blends in terms of the interfacial problem

Детальніше...  

A new crystal modification induced by strain was found in uniaxially oriented fibres of poly(tetramethylene succinate). This new modification (fl form) had a planar zigzag conformation of 7"1o (all trans) and a fibre identity period of 11.90.~, while the conventional ct form had a TTGTI7 conformation and fibre identity period of 10.90 A. The solid-state crystal transition between • and fl forms was reversible with respect to the successive application and release of strain.

Детальніше...  

The microindentation hardness (H) of PET samples crystallized isothermally at various temperatures from the melt under high pressure (4 kbar) was determined in order to establish correlations with the thermal properties and microstructure. The results reveal that these materials show density, enthalpy of fusion, and melting temperature values which are close to those corresponding to chain extended crystals. The high crystallinities obtained (:?=0.70-0.95) for high pressure crystallized PET are shown to give rise to unprecedentedly high microhardness values, ranging between 300 and 400 MPa. The data are analysed on the basis of a two-phase model which allows an estimation of the influence of crystal thickness upon H. Finally, the temperature dependence of high pressure crystallized PET is also examined.

Детальніше...  

Steady shear flow values of the normal stress ratio W = - N2/N 1 are compared for monodisperse polystyrene solutions in the concentrated entangled and semidilute entangled regimes. In both regimes, the zero shear rate limiting values are the same: Wo =0.275___ 0.005. Surprisingly, the molecular weight required for full entanglement (on the basis of W0) scales with concentration as C 1.5+o.t, rather than C-1 as commonly supposed. The value of q% is insensitive to fractional free volume and to solvent affinity for polystyrene, even at semidilute concentrations. However, te depends on the shear rate, with shear-thinning behaviour in all solvents. Shear thinning of ud has not been widely reported, probably because of the 'edge fracture' flow instability in rotational rheometers. In concentrated solutions or melts, edge fracture usually occurs before W shear thins by a significant amount

Детальніше...  

Solid-state diffusion of n-alkane molecules, tricosane C2aH48, into the crystal of heneicosane C21H44, is detected by X-ray diffraction. It is found that the diffusion is temperature dependent and is greatly accelerated in the rotator phase.

Детальніше...  

Fourier-transform infra-red spectroscopy was used to study the thermal stability of silane-modified vinylimidazole copolymers on both KBr plates and copper coupons. The copolymers were synthesized by free-radical polymerization of 7-methacryloxypropyltrimethoxysilane and N-vinylimidazole. The ability of these copolymers to inhibit the oxidation of copper at elevated temperatures was compared to that of poly(N-vinylimidazole). The silane acted as a scavenger for the water released by the imidazole ring during heating. Upon heating, the silane methoxy groups hydrolysed and condensed to form siloxane linkages.

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