1992 рік

Chain entanglement in poly(methyl methacrylate), polystyrene, poly(N-phenylmaleimide), poly(methyl methacrylate-stat-styrene), poly(methyl methacrylate-stat-N-phenylmaleimide) and poly(methyl methacrylate- stat-styrene-stat-N-phenylmaleimide) is studied in the melt by dynamic viscoelastic spectroscopy. It is shown that the plateau modulus is equal to the storage modulus at the minimum of loss tangent. The plateau modulus and the entanglement molecular weight are found to obey the geometric mean relationship with respect to composition, i e log GN0 = E v log GN0 and log Me = E v log Me ,, where GN0 is the plateau • ' J J J j modulus, Me the molecular weight between entanglement points, v i the volume fraction of comonomer j, G°~ the plateau modulus for the homopolymer of j and M,j the entanglement molecular weight for the homopolymer of j. The G ° and M e of homopolymers and copolymers may also be predicted by group additivity method

Детальніше...  

Void factors (size and distribution) of polyoxymethylene superdrawn fibres were determined, and their relationship to tensile strength was examined. Microvoids < ~ 0.3 #m z had little effect on strength, while those > ~ 1/~m 2 caused a decrease in tensile strength

Детальніше...  

We have developed several new techniques for examining the domain morphologies of microphase-separated block copolymers. By selective degradation and removal of the matrix component of suitable block copolymers, the domain-forming component can be left intact in its original morphological form and available for characterization. Thus we have selectively removed the polydiene (polybutadiene or polyisoprene) matrix component of poly(styrene-b-diene) copolymers by treatment with ozone. The remaining polystyrene domains were stained with ruthenium tetroxide or shadowed with platinum and examined by electron microscopy. A marked improvement in the ability to distinguish domain morphologies and their three-dimensional character was found when these degradation techniques were used. Details of domain morphologies that were not apparent when using conventional osmium tetroxide staining methods on undegraded films often became clearly apparent with this new technique

Детальніше...  

A theoretical model is proposed in order to explain the mechanical behaviour of thin bioriented poly(ethylene terephthalate) (PET) films at low and medium stresses. This investigation requires one to introduce a theory of deformation of amorphous PET and coupling of the behaviour laws of the two phases (crystalline and amorphous). A newly developed molecular model of non-elastic deformation of glassy solids based on hierarchical constrained molecular movements and activation of defects is used and extended to the non-linear behaviour of the creep compliance function. Creep tests are carried out at low and medium stresses on amorphous and semicrystalline films in order to study (i) the linear domain, (ii) the transition to non-linear behaviour of the compliance (defined by some critical stress, crc) and (iii) the qualitative evolution of the creep compliance beyond this critical stress value. Semiquantitative agreement is found between theoretical predictions and experimental data in a large domain of stresses including the critical value a?. By assuming in a first step that partially crystalline films are two-phase systems, their mechanical behaviour can be explained by coupling an amorphous and a crystalline phase following Takayanagi's model. In this coupling, the crystalline phase can be considered as a perfect crystal with a Young's elastic modulus in the in-plane direction of about Ecl I ~ 110 GPa. In a second step, the mechanical response of the amorphous phase of biaxially oriented thin PET films is compared with that of bulk amorphous PET. We find that these two behaviours are very different, first because of an important reduction of'defect' concentration in the amorphous phase of thin biaxially oriented PET films and secondly because of an increase of relaxation characteristic time due to the proximity of crystallites.

Детальніше...  

Fibres of the rod-like molecule poly-p-phenylene benzobisoxazole (PBO) and the coil-like molecule poly-2,5-benzoxazole (ABPBO) were spun from polymerization dopes and then coagulated at various temperatures. Axial tension was applied in drying the coagulated wet filaments; heat treatment was applied to anneal the fibres. Wide-angle X-ray scattering on the fibres revealed that tension drying improved molecular alignment leading to increased tensile modulus. Small-angle X-ray scattering (SAXS) showed that the void content of the fibres was significantly reduced by low coagulation temperature, tension drying and thermal annealing. This reduction of voids also contributed to enhanced tensile properties. Thermal annealing also improved the crystallinity of the ABPBO fibres and seemed to induce a three-dimensional order, which was usually absent for fibres of the rod-like PBO polymer. Both annealed PBO and ABPBO fibres resulted in a unique off-axis radial four-point SAXS pattern. The tensile modulus for fibres of the rod-like polymer was higher than that of the coil-like polymer. However, as determined by the cantilever beam test, the critical compressive strain for fibres of the rod-like polymer was lower than that of the coil-like polymer.

Детальніше...  

The solution-precipitated morphology of a new sulphonated aromatic polyimide, poly(benzophenone tetracarboxylic dianhydride 3,Y-diaminodiphenylsulphone) (BTDA 3, 3'-DDS), has been studied by transmission electron microscopy (t.e.m.) and wide-angle X-ray diffraction (WAXD). First, it has been shown that if held in solution in dimethylacetamide for an extended period, the polymer can crystallize as sheaf-like, multicrystallite aggregates that take on the appearance of axialites at a sufficiently advanced stage of growth. This morphology is very similar to that displayed by other relatively stiff polymers such as poly(ether ether ketone) (PEEK), poly(ether ketone) (PEK), and poly(phenylene sulphide). The generation of this general type of morphology is attributed to the inflexibility of the backbone, which renders folding difficult and hence leads to the production of a thick amorphous region on the lamellar surface, which then limits lateral crystal growth and creates a high density of cilia that cause nucleation of other crystals, leading to the formation of multilayered aggregates. Secondly, if the solvent is evaporated in moist air before crystallization can occur, a phase separation takes place on absorption of water vapour from the atmosphere. In this case the resulting polymer is amorphous and adopts a bead-like morphology on the scale of 300-2000 A (1 A = 10-1° m)

Детальніше...  

Structure-property behaviour of poly(tetramethylene oxide) (PTMO)-bipyridinium ionene elastomers have been investigated. These systems display high tensile properties and high elongation. Their general properties can be explained as being due to the high degree of microphase separation of ionic domains promoted by strong coulombic interactions in conjunction with strain induced crystallization of the PTMO soft segments. The microphase separation was established by use of small angle X-ray scattering as well as limited use of transmission electron microscopy. Some indication of the thermal stability characteristics of these materials is also presented

Детальніше...  

Fibres of the coil-like molecule poly-2,5-benzoxazole (ABPBO) as well as the rod-like molecules poly (p-phenylene benzobisoxazole ) (PBO) and poly (p-phenylene benzobisthiazole ) (PBT) were processed and examined by small-angle X-ray scattering (SAXS). SAXS on thermally annealed ABPBO and PBO fibres consistently showed an off-axis radial four-point intensity pattern. Molecular adjustment due to thermal annealing was believed to cause this unique fibre morphology. The distinct scattering pattern suggested a uniaxial structure of plate-like ordered regions with a tilt-angle off the fibre axis. The tilt-angle was independent of annealing temperature. This fibre morphology is in agreement with complementary evidence from numerical modelling, X-ray diffraction and selected-area electron diffraction. In contrast, SAXS on annealed PBT fibre did not show the radial four-point intensity pattern. This was attributed to the non-colinear and non-coplanar nature of the PBT backbone. For the annealed fibres, the presence of the radial four-point SAXS pattern was not a prerequisite for improvements in tensile properties

Детальніше...  

A series of model polyurethane ionomers containing varying concentrations of both sulphonate and carboxylate anions is examined. Tensile testing results demonstrate that the greater acid strength of the sulphonate anions causes the ultimate tensile properties of the ionomer to regularly increase as sulphonate content increases. However, the small-strain moduli of two mixed carboxylate/sulphonate ionomers were shown to vary from the trends predicted from strict acid strength arguments ; the two mixed-anion ionomers have higher Young's moduli than the fully sulphonated ionomer. D.s.c., d.m.t.a, and SAXS analysis indicates that the modulus enhancement in the mixed-anion ionomer arises from a combination of factors, including aggregate packing, ability to immobilize non-ionic chain segments (ionic anchoring) and degree of phase separation. The onset of flow temperature in d.m.t.a, is seen to decrease in parallel with increasing carboxylate content, unlike the tensile properties. Thus, the concept of mixed anion derivatization is shown to fulfil a primary goal in the ionome

Детальніше...  

The effects of oxidation on gel-spun ultra-high-modulus and ultra-high-molecular-weight polyethylene (UHMWPE) fibres were investigated. The single-fibre tensile properties of the UHMWPE fibres were lowered by all chromic acid/potassium dichromate oxidation reactions studied. The tensile properties were highly reaction-time-dependent for the fibres treated in the acid-assisted reactions with CrO 3 (|) and with K2Cr20 7 (lla) at a higher H2SO 4 concentration (or a 1:20 K2Cr207:H2SO 4 weight ratio). The tensile properties of fibres treated in the acid-assisted reaction with K2Cr20 7 (Ilb) at a lower H2SO4 concentration (or a 1:1.3 K2Cr2OT:H2SO 4 weight ratio) and the base-catalysed reaction (III) were slightly lowered, but were independent of reaction time. The fractured fibre ends from reactions lib and III remained fibrillated as observed on the untreated fibres, whereas brittle fractures were observed on those oxidized by reactions I and lla. Experimental results indicated that the oxidative agents in reactions I and lla penetrated more thoroughly into the fibrillar structures of the fibres than in the base-catalysed reaction III and reaction lib. The changes in the tensile and fracture properties suggested different diffusion and oxidative mechanisms between chromic acid (I) and potassium dichromate (lib) under similar sulphuric acid-assisted conditions. All oxidative reactions reduced superheating, which resulted in the increased heat of fusion but decreased melting temperatures and narrowed melting peak widths.

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