Dynamic mechanical property measurements

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. 

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). 

Tihe technological properties and the commercial application of several polymer blends have been studied extensively. Investigations of the basic principles, however, relating the phase structure of the blends to the properties of the individual components have not been carried out to an extent justified by the industrial value of these materials. Several methods have been used, the most successful being optical and electron microscopy and dynamic-mechanical measurements. Critical factors and difficulties in the morphological studies of polymer blends have been... 

Dynamic Mechanical Measurements. To explain the differences in the physical properties found between the samples obtained with Processes A, B, and C and those obtained with Process D, the dynamic me-... 

Dynamic Mechanical Measurements. A Rheovibron DDV-II was used to measure mechanical properties at frequencies of 3.5 and 110 Hz. Heating rates were approximately l°C/min. Films of 5-mil thickness were compression molded at 260°C from dried powders for these studies. No annealing treatments were applied to the films. Complex, storage, and loss moduli as well as tan 8 were calculated over a — 160° to 240°C range. For clarity, all of the data are not shown in the figures. 

Dielectric relaxation measurement in similar to dynamic mechanical measurements, except that it exploits the dipole electrical properties of the blend. It is, therefore. 

Thus, dynamic mechanical viscoelastic properties may be measured in tests with sinusoidal strain input at fixed frequency. Such tests have to be repeated at different frequencies over the range of interest to completely characterize the material. 

To understand elastic mechanical properties, the discussion of the storage of energy of deformation provides a powerful approach. Dynamic mechanical measurements at higher strain on filled silicone elastomers show that the energy of deformation may be related to an entropic and an enthalpic part. The entropic part is mainly due to the restriction of the conformational space of the polymer chain by the presence of the solid silica particles. Whereas the enthalpic part of the energy of deformation is related to... 

Many relatively slow or static methods have been used to measure Tg. These include techniques for determining the density or specific volume of the polymer as a function of temperature (cf. Fig. 11-1) as well as measurements of refractive index, elastic modulus, and other properties. Differential thermal analysis and differential scanning calorimetry are widely used for this purpose at present, with simple extrapolative eorrections for the effects of heating or cording rates on the observed values of Tg. These two methods reflect the changes in specific heat of the polymer at the glass-to-rubber transition. Dynamic mechanical measurements, which are described in Section 11.5, are also widely employed for locating Tg. 

Gibson, S.H.M. Rowe, R.C. White, E.F.T. The mechanical properties of pigmented tablet coating formulations and their resistance to cracking. II. Dynamic mechanical measurement. Int. J. Pharm. 1989, 50, 163-173. 

Next, an attempt was made to clarify the effect of crystalline phase on the mechanical properties of ethylene ionomer. Dynamic mechanical measurements... 

Yan et al. [52] explored the use of IPN techniques to produce a composite vinyl-acrylic latex. The first-formed polymer was produced using VAc and divinyl benzene (DVB), while the second formed polymer constituted a BA/DVB copolymer. In both cases the DVB was added at 0.4 wt%. They compared this product with another product, a bidirectional interpenetrating netwodc (BIPN) in which VAc was again polymerized over the first IPN. They noted that the compatibility between the phases was more pronounced in the BIPN than in the IPN as determined using dynamic mechanical measurements and C nuclear magnetic resonance spectroscopy. The concept of polymer miscibility has also been used to produce composite latex particles and thus modify the pafamance properties of VAc latexes. Bott et al. [53] describe a process whereby they bloid VAc/ethylene (VAc/E) copolymers with copolymers of acrylic acid or maleic anhydride and determine windows of miscibility. Apparently an ethyl acrylate or BA copolymer with 10-25 wt% AA is compatible with a VAc/E copolymer of 5-30 wt% ethylene. The information obtained from this woik was then used to form blends of latex polymers by polymerizing suitable mixtures of monomers into preformed VAc/E copolymers. The products are said to be useful for coating adhesives and caulks. 

The fourth and fifth papers have to do with properties of pressure-sensitive adhesives. In particular, the matter of how the materials composing pressure-sensitive adhesives (rubbers and resins) interact and phase separate to produce the phenomenon of tack or pressure-sensitivity is addressed. Both studies use dynamic mechanical measurements to uncover phasing - one in a silicone and the other in natural and styrene-butadiene rubber systems tackified with various resins. 

An unexpected affinity of OC for CB is thus revealed. Static and dynamic-mechanical measurements demonstrated that the OC was able to give rise to a hybrid filler network with CB and resulted in the continuous hybrid filler system being responsible for the remarkable enhancement in material dynamic-mechanical properties. 

Theocaris (21) developed a method for estimating interphase thickness in composites via dynamic mechanical measurements. This type of method provides a more fundamental relationship between interphase thickness and mechanical properties attributed to the interphase structure. Chapter 11 discusses this technique to probe the interphase microstructure. 

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