Measurements dynamic mechanical

In stress relaxation measurements, as in standard mechanical testing devices, changing stress may be monitored using a strain gauge load cell. Since these devices rely on changes in strain, it must be confirmed that they are very stiff in comparison with the specimen so that the specimen strain is held effectively constant. 

A range of measurement equipment is described and illustrated in the books by Turner [6] and Ward [7]. 

The evolution of the dynamic viscosity t] (o), x) or of the dynamic shear complex modulus G (co,x) as a function of conversion, x, can be followed by dynamic mechanical measurements using oscillatory shear deformation between two parallel plates at constant angular frequency, CO = lid (f = frequency in Hz). In addition, the frequency sweep at certain time intervals during a slow reaction (x constant) allows determination of the frequency dependence of elastic quantities at the particular conversion. During such experiments, storage G (co), and loss G (co) shear moduli and their ratio, the loss factor tan8(co), are obtained  

Typical rheological curves obtained during a diepoxy-diamine reaction are shown in Fig. 6.3. The cure temperature (Ti = 90°C) is well above the glass transition temperature of the fully cured network (Tg 35°Q, which means that only gelation occurs. Three typical regions are observed during cure (Matejka, 1991). 

The viscous properties are dominant in the liquid state, i.e., G G and tan 5 1, while the elastic properties predominate in the solid state, where G G and tan 8 1. For this reason, the G -G crossover (tan 5 = 1) was firstly identified as the gel point (Tung and Dynes, 1982). The 

The value of tan 8 decreases in the (b) region and the rate of this decrease depends on the angular frequency, co (Fig. 6.4). As a rough approximation  

2 Experimental Evidence of Singular Power Laws at the Gel Point 

Since these processes are strongly dependent on temperature, it is appropriate to carry out torsion-oscillation experiments over a wide range of temperature, and to plot the values for the modulus and d against temperature (see Fig. 2.22). 

The value of the modulus and the shape of the modulus curve allow deductions concerning not only the state of aggregation but also the structure of polymers. Thus, by means of torsion-oscillation measurements, one can determine the proportions of amorphous and crystalline regions, crosslinking and chemical non-uniformity, and can distinguish random copolymers from block copolymers. This procedure is also very suitable for the investigation of plasticized or filled polymers, as well as for the characterization of mixtures of different polymers (polymer blends). 

Curve El in Fig. 2.21 corresponds to an elastomer (statistic copolymer from ethylene and propylene), characterized by a low value of the elastic modulus 

In contrast, the curve E2 (isotactic polypropylene) is characteristic for partially crystalline polymers. The modulus is three decades higher than in an elastomer. At the glass transition temperature [T (2) 0 °C] the decay of the E modulus is small it does not drop to the lower level of the molten state before the melting point. 

The corresponding curves for the mechanical loss factor 6 show the following characteristics The transition to the glassy state for elastomers is seen in curve 1 as a characteristic mechanical absorption . On the other hand, two absorption maxima are visible in the curve for the partially crystalline polymer d2. The first one at 10 °C indicates the glass transition, the second one at about 145 °C is coherent with the crystalline melting point. 

Curve El in Fig. 2.22 corresponds to an elastomer (statistic copolymer from ethylene and propylene), characterized by a low value of the elastic modulus over a wide temperature range and a sudden increase at low temperature corresponding to the transition from the elastic to the brittle (glassy) state at the glass transition temperature Tg ) at —50°C. 

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Dynamic mechanical measurements were made on PTEE samples saturated with various halocarbons (88). The peaks in loss modulus associated with the amorphous relaxation near —90°C and the crystalline relaxation near room temperature were not affected by these additives. An additional loss peak appeared near —30° C, and the modulus was reduced at all higher temperatures. The amorphous relaxation that appears as a peak in the loss compliance at 134°C is shifted to 45—70°C in the swollen samples. 

Transitions. Samples containing 50 mol % tetrafluoroethylene with ca 92% alternation were quenched in ice water or cooled slowly from the melt to minimise or maximize crystallinity, respectively (19). Internal motions were studied by dynamic mechanical and dielectric measurements, and by nuclear magnetic resonance. The dynamic mechanical behavior showed that the CC relaxation occurs at 110°C in the quenched sample in the slowly cooled sample it is shifted to 135°C. The P relaxation appears near —25°C. The y relaxation at — 120°C in the quenched sample is reduced in peak height in the slowly cooled sample and shifted to a slightly higher temperature. The CC and y relaxations reflect motions in the amorphous regions, whereas the P relaxation occurs in the crystalline regions. The y relaxation at — 120°C in dynamic mechanical measurements at 1 H2 appears at —35°C in dielectric measurements at 10 H2. The temperature of the CC relaxation varies from 145°C at 100 H2 to 170°C at 10 H2. In the mechanical measurement, it is 110°C. There is no evidence for relaxation in the dielectric data. 

Tackifying resins enhance the adhesion of non-polar elastomers by improving wettability, increasing polarity and altering the viscoelastic properties. Dahlquist [31 ] established the first evidence of the modification of the viscoelastic properties of an elastomer by adding resins, and demonstrated that the performance of pressure-sensitive adhesives was related to the creep compliance. Later, Aubrey and Sherriff [32] demonstrated that a relationship between peel strength and viscoelasticity in natural rubber-low molecular resins blends existed. Class and Chu [33] used the dynamic mechanical measurements to demonstrate that compatible resins with an elastomer produced a decrease in the elastic modulus at room temperature and an increase in the tan <5 peak (which indicated the glass transition temperature of the resin-elastomer blend). Resins which are incompatible with an elastomer caused an increase in the elastic modulus at room temperature and showed two distinct maxima in the tan <5 curve. 

In order to support our prediction that the change in mechanical properties with different curing systems is due to a change in the vulcanizate structure, results were compared from dynamic-mechanical measurements as shown in Figs. 4 and 5. 

A technique for performing dynamic mechanical measurements in which the sample is oscillated mechanically at a fixed frequency. Storage modulus and damping are calculated from the applied strain and the resultant stress and shift in phase angle. 

Dynamic mechanical measurements for elastomers that cover wide ranges of frequency and temperature are rather scarce. Payne and Scott [12] carried out extensive measurements of /a and /x" for unvulcanized natural mbber as a function of test frequency (Figure 1.8). He showed that the experimental relations at different temperatures could be superposed to yield master curves, as shown in Figure 1.9, using the WLF frequency-temperature equivalence, Equation 1.11. The same shift factors, log Ox. were used for both experimental quantities, /x and /x". Successful superposition in both cases confirms that the dependence of the viscoelastic properties of rubber on frequency and temperature arises from changes in the rate of Brownian motion of molecular segments with temperature. 

Polymer blends have been categorized as (1) compatible, exhibiting only a single Tg, (2) mechanically compatible, exhibiting the Tg values of each component but with superior mechanical properties, and (3) incompatible, exhibiting the unenhanced properties of phase-separated materials (8). Based on the mechanical properties, it has been suggested that PCL-cellulose acetate butyrate blends are compatible (8). Dynamic mechanical measurements of the Tg of PCL-polylactic acid blends indicate that the compatability may depend on the ratios employed (65). Both of these blends have been used to control the permeability of delivery systems (vide infra). 

The samples were cured with 0.2, 0.4, 0.8 and 1.6 wt.% dicu-myl peroxide. In this way, we obtained twelve different networks with great variations in relaxation Intensities. Dynamic mechanical measurements were performed In torsion in the linear region (deformations smaller than 5 %) with a mechanical spectrometer, using the parallel-plate geometry. The frequency ranged from 0.01 to 15 Hz and the temperature was usually between 300 and 435 K. 

In accordance with this dynamic mechanical measurements show that the rubber partially forms the outer phase below 170 -190°C.8... 

Bisphthalonitrile monomers were cured neat, with nucleophilic and redox co-reactants, or in combination with a reactive diluent. Dynamic mechanical measurements on the resulting polymers from -150 to +300°C turn up several differences attributable to differences in network structure. Rheovibron results were supplemented with solvent extraction, differential scanning calorimetry (DSC), vapor pressure osmometry, and infrared spectroscopy to characterize the state of cure. 

Fig. 4.11 Temperature dependence of the shift factors as reported in the literature for atactic polypropylene 1 dynamic mechanical measurements [140], 2 NMR data of Pschorn et al. [141], 3 photon correlation spectroscopy [142], 4 from NMR measurements of Moe...

ASTM D4092-83a, Standard Definitions and Descriptions of Terms Relating to Dynamical Mechanical Measurements on Plastics, Annual Book ASTM Standards (1989), Vol. 8.03, p. 334. 

The measurements of Young s modulus in dependence of the temperature (dynamic-mechanical measurements, see Sect. 2.3.5.2) and the differential thermal analysis (DTA or DSC) are the most frequently used methods for determination of the glass transition temperature. In Table 2.10 are listed and values for several amorphous and crystalline polymers. 

The characterization of block or graft copolymers is generally much more difficult than that of random copolymers (see Sect. 23.2.7). Especieilly, DSC measurements are useful for the characterization of the different segments (determination of Tg). Also dynamic-mechanical measurements are used to distinguish statistical copolymers from those with block or graft structure. 

Dynamical mechanical measurements on the cured films showed only a small decrease in depending on the terminal units from 77 °C for the hydroxy, 72 °C for the benzoate, and 67 °C for the propionate functional as determined by the tan 5 peak (Figure 13). The cross-linking gives, as expected, a dramatic increase in Tg for each system, but also reduces the differences in as a function of the structure of the terminal groups. 

Dynamic Mechanical Measurements. Films were prepared by casting the acetone solution of sample No. 2 onto a Teflon sheet after adding curing agents. The sample was allowed to stand at room temperature for one day, and then cured at 130°C for 2 hours. The dynamic mechanical spectroscopic data were measured in tension with a Rheovibron DDV-II (Toyo Baldwin Co. Ltd.) at a frequency of 110 Hz with a heating rate of about l°C/min. 

Rheological measurements of nonvulcanized rubber compounds - Dynamic mechanical measurements were performed using a RPA 2000 dynamic curemeter... 

Filler-filler interaction (Payne effect) - The introduction of reinforcing fillers into rubbery matrices strongly modifies the viscoelastic behavior of the materials. In dynamic mechanical measurements, with increasing strain amplitude, reinforced samples display a decrease of the storage shear modulus G. This phenomenon is commonly known as the Payne effect and is due to progressive destruction of the filler-filler interaction [46, 47]. The AG values calculated from the difference in the G values measured at 0.56% strain and at 100% strain in the unvulcanized state are used to quantify the Payne effect. 

The dynamic mechanical measurements were performed with a Rheometrics IV apparatus in a geometrical arrangement of parallel plates. The complex shear modulus G (= G + fG", where G and G", respectively, are the storage and loss moduli) at a constant frequency of 1 Hz was determined [30]. 

Dynamic mechanical measurements. Dynamic mechanical measurements 97) of the storage and loss components of the rigidity modulus (G G") at a single frequency are shown in Fig. 31. As is the case with all polyethers, there is a main relaxation region associated with the onset of microbrownian motion of the main chain. In the region of the melting temperature, a catastrophic drop in modulus appears. 

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