Soft polymers

Emulsion polymerization also has the advantages of good heat transfer and low viscosity, which follow from the presence of the aqueous phase. The resulting aqueous dispersion of polymer is called a latex. The polymer can be subsequently separated from the aqueous portion of the latex or the latter can be used directly in eventual appUcations. For example, in coatings applications-such as paints, paper coatings, floor pohshes-soft polymer particles coalesce into a continuous film with the evaporation of water after the latex has been applied to the substrate. 

Much more information can be obtained by examining the mechanical properties of a viscoelastic material over an extensive temperature range. A convenient nondestmctive method is the measurement of torsional modulus. A number of instmments are available (13—18). More details on use and interpretation of these measurements may be found in references 8 and 19—25. An increase in modulus value means an increase in polymer hardness or stiffness. The various regions of elastic behavior are shown in Figure 1. Curve A of Figure 1 is that of a soft polymer, curve B of a hard polymer. To a close approximation both are transpositions of each other on the temperature scale. A copolymer curve would fall between those of the homopolymers, with the displacement depending on the amount of hard monomer in the copolymer (26—28). 

The combination of durability and clarity and the ability to tailor molecules relatively easily to specific applications have made acryflc esters prime candidates for numerous and diverse applications. At normal temperatures the polyacrylates are soft polymers and therefore tend to find use in applications that require flexibility or extensibility. However, the ease of copolymerizing the softer acrylates with the harder methacrylates, styrene, acrylonitrile, and vinyl acetate, allows the manufacture of products that range from soft mbbers to hard nonfilm-forming polymers. 

The mechanical properties of LDPE fall somewhere between rigid polymers such as polystyrene and limp or soft polymers such as polyvinyls. LDPE exhibits good toughness and pHabiUty over a moderately wide temperature range. It is a viscoelastic material that displays non-Newtonian flow behavior, and the polymer is ductile at temperatures well below 0°C. Table 1 fists typical properties. 

Blends. Latex film properties are commonly modified through the blending of latexes, eg, a "soft" polymer is made slightly harder by blending with a "hard" latex. 

An example, Figure 9 is an SFM image of a Langmuir-Blodgett film. This film was polymerized with ultraviolet light, giving a periodicity of 200 A, which is seen in the associated Fourier transform. The low forces exerted by the SFM tip are essential for imaging such soft polymer surfaces. 

Impurities in mineral fillers can have serious effects. Coarse particles (grit) will lead to points of weakness in soft polymers which will therefore fail under stresses below that which might be expected. Traces of copper, manganese and iron can affect the oxidative stability whilst lead may react with sulphur-containing additives or sulphurous fumes in the atmosphere to give a discoloured product. 

In order to improve toughness many rubbers and other soft polymers may be used as additives to modify the compound. Some copolymers based on vinyl chloride are available of which the most important are the vinyl chloride-vinyl acetate materials used in gramophone records, flooring compositions and surface coatings. 

A variety of morphologies and properties can be achieved with microphase-separated block copolymers. Copolymers of hard and soft polymer segments have... 

Fig. 10.6 Drawings of students about their ideas of hard and soft polymer materials. Left, a student (7th grade) argues with the closeness of the arrangement of particles. On the right a student from 12th grade. The difference is caused in the closeness of the covalent network...

PVA Particles. Dispersions were prepared in order to examine stabilization for a core polymer having a glass transition temperature below the dispersion polymerization temperature. PVA particles prepared with a block copolymer having M PS) x 10000 showed a tendency to flocculate at ambient temperature during redispersion cycles to remove excess block copolymer, particularly if the dispersion polymerization had not proceeded to 100 conversion of monomer. It is well documented that on mixing solutions of polystyrene and poly(vinyl acetate) homopolymers phase separation tends to occur (10,11), and solubility studies (12) of PS in n-heptane suggest that PS blocks with Mn(PS) 10000 will be close to dissolution when dispersion polymerizations are performed at 3 +3 K. Consequently, we may postulate that for soft polymer particles the block copolymer is rejected from the particle because of an incompatibility effect and is adsorbed at the particle surface. If the block copolymer desorbs from the particle surface, then particle agglomeration will occur unless rapid adsorption of other copolymer molecules occurs from a reservoir of excess block copolymer. 

Therefore, at room temperature Fluoro-PSB-II a thermoplastic elastomer with a soft polymer phase (fluorinated block) and a hard phase (PS-block), similar to the parental polystyrene-6-polybutadiene block copolymer. Depending on the relative volume fraction of both components and the continuity of the phases, the resulting bulk material is rubbery or a high-impact solid. 

Theoretically there exist four possibilities to combine hard and soft polymers with each other. They are listed in Table 1 together with some examples which have already found commercial interest. 

Soft polymers are more sensitive to scratch than some harder plastics. 

Wolosin, J.M., Ginsburg, H., Eieb, W.R., and Stein, W.D. Diffusion within egg lecithin bilayers resembles that within soft polymers, / Gen. Physiol, 71 93-100, 1978. 

Rigid PVC is representative of hard and soft polymers. These polymers have a high modulus of elasticity and high yield strength. The curve for hard and tough polymers, such as ABS copolymers, shows moderate elongation before the yield point followed by nonre-coverable elongation. 

Wilder EA, Guo S, Lin-Gibson S, Easolka MJ, Stafford CM (2006) Measuring the modulus of soft polymer networks via a budding-based metrology. Macromolecules 39 4138-4143... 

In the 1950s, Robert Synge was the first to postulate polymer structures, which were similar to what is dehned as monolith today [11,12]. However, the soft polymer materials available at that time (gel-type polymers) did not resist permanent pressure conditions. 

While in the temperature range called the rubbery plateau, the soft polymer responds instantaneously and reversibly to applied stress and tends to be Hookean. In the rubber state, the polymer approaches Hooke s law for... 

Polymaric plasticizars can ba mada by (1) Internal plasticization whoroby a monomor is copolymorizod with on which tends to yield soft polymers by itself (2) Mechanical mixing of a polymerizable monomer with a polymer, followed by polymerization (3) Mechanical blending of two compatible polymers. In many cases It Is necessary to combine the polymeric plasticizer with a liquid plasticizer because the compatibility of polymers with each other is generally limited. From the industrial polymeric plasticizers, especially polyesters of low degree of polymerization and several copolymers of butadiene with acrylonitrile, acrylic add esters and fumaric add esters were studied. These polymeric plasticizers are characterized by good compatibility and improved cold resistance of the final product. 

Combination of Polymers with Other Compatible Soft Polymers... 

With 7 = 0.072 Nm-1 and 5 = 10 nm the effective pressure is of the order of P = 72 x 10s Pa. Such a high pressure can change the surface structure, cause mechanical deformation at the moving wetting line [250], and can lead to contact angle hysteresis [251-253], especially on soft polymer surfaces. 

Note, that the surface and deformation forces are of the same order of magnitude. Therefore, surface forces should be as small as possible to minimise damaging and indentation of soft polymer samples. For example, sharp probes have a lower capillary attraction and adhesion forces, and therefore enable more gentle probing of a soft polymer than a blunt tip. A sharp tip can also be moved in and out of the contamination layer more readily than a blunt tip. This is particularly important for non-contact intermittent contact imaging described in Sect. 2.2.1. 

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