Amorphous polymers parameter

As may be expected of an amorphous polymer in the middle range of the solubility parameter table, poly(methyl methacrylate) is soluble in a number of solvents with similar solubility parameters. Some examples were given in the previous section. The polymer is attacked by mineral acids but is resistant to alkalis, water and most aqueous inorganic salt solutions. A number of organic materials although not solvents may cause crazing and cracking, e.g. aliphatic alcohols. 

The toughness of interfaces between immiscible amorphous polymers without any coupling agent has been the subject of a number of recent studies [15-18]. The width of a polymer/polymer interface is known to be controlled by the Flory-Huggins interaction parameter x between the two polymers. The value of x between a random copolymer and a homopolymer can be adjusted by changing the copolymer composition, so the main experimental protocol has been to measure the interface toughness between a copolymer and a homopolymer as a function of copolymer composition. In addition, the interface width has been measured by neutron reflection. Four different experimental systems have been used, all containing styrene. Schnell et al. studied PS joined to random copolymers of styrene with bromostyrene and styrene with paramethyl styrene [17,18]. Benkoski et al. joined polystyrene to a random copolymer of styrene with vinyl pyridine (PS/PS-r-PVP) [16], whilst Brown joined PMMA to a random copolymer of styrene with methacrylate (PMMA/PS-r-PMMA) [15]. The results of the latter study are shown in Fig. 9. 

Of the instances of so-called solvent cracking of amorphous polymers known to the author, the liquid involved is not usually a true solvent of the polymer but instead has a solubility parameter on the borderline of the solubility range. Examples are polystyrene and white spirit, polycarbonate and methanol and ethyl acetate with polysulphone. The propensity to solvent stress cracking is however far from predictable and intending users of a polymer would have to check on this before use. 

The two main transitions in polymers are the glass-rubber transition (Tg) and the crystalline melting point (Tm). The Tg is the most important basic parameter of an amorphous polymer because it determines whether the material will be a hard solid or an elastomer at specific use temperature ranges and at what temperature its behavior pattern changes. 

Table 3.17. Thermodynamic parameters for the conversion of liquid monomer into solid amorphous polymer [750],...

For all the cases cited above, which represent those data for which a comparison can be presently made, there is a direct connection between the critical molecular weight representing the influence of entanglements on the bulk viscosity and other properties, and the NMR linewidths, or spin-spin relaxation parameters of the amorphous polymers. Thus the entanglements must modulate the segmental motions so that even in the amorphous state they are a major reason for the incomplete motional narrowing, as has been postulated by Schaefer. ( ) This effect would then be further accentuated with crystallization. 

The exponent p is no longer an empirical constant but can be calculated directly from the same molecular parameters (e, p, fj., Xj.) mentioned earlier for PVAc using Equations 2, 5 and 7. The value of p for the epoxy resin in Figure 1 is lower than that of amorphous polymers in the glassy state. 

The solubility parameter concept predicts the heat of mixing liquids and amorphous polymers. It has been experimentally found that generally any nonpolar amorphous polymer will dissolve in a liquid or mixture of liquids having a solubility parameter that generally does not differ by more than 1.8 (cal/cc) /. The Hildebrand (H) is preferred over these complex units, giving as a general difference 1.8 H. 

Flory and Huggins developed an interaction parameter that may be used as a measure of the solvent power of solvents for amorphous polymers. Flory and Krigbaum introduced the idea of a theta temperature, which is the temperature at which an infinitely long polymer chain exists as a statistical coil in a solvent. 

The principal polyolefins are low-density polyethylene (ldpe), high-density polyethylene (hope), linear low-density polyethylene (lldpe), polypropylene (PP), polyisobutylene (PIB), poly-1-butene (PB), copolymers of ethylene and propylene (EP), and proprietary copolymers of ethylene and alpha olefins. Since all these polymers are aliphatic hydrocarbons, the amorphous polymers are soluble in aliphatic hydrocarbon solvents with similar solubility parameters. Like other alkanes, they are resistant to attack by most ionic and most polar chemicals their usual reactions are limited to combustion, chemical oxidation, chlorination, nitration, and free-radical reactions. 

They are atactic amorphous polymers which have good light transparency (92%) and yield transparent moldings and films. As was noted for polyalkyl acrylates, the solubility parameters decrease as the size of the alkyl groups increases. The flexibility also increases as one goes from polymethyl methacrylate (PMMA) to polyaryl methacrylate and then decreases as the size of the alkyl group is further increased. 

These products are produced by the reaction of partially hydrolyzed PVAc with aldehydes. The acetal rings on these random amorphous polymer chains restrict flexibility and increase the heat deflection temperature to a value higher than that of PVAc. The heat deflection temperature of polyvinyl formal is about 90 C and is dependent on the specific composition of this complex polymer. Because of the presence of residual hydroxyl groups, commercial polyvinyl formal has a water absorption of about 1%. Polyvinyl formal has a Tg of 10S . It has a solubility parameter of about 10 H and is soluble in solvents with similar solubility parameters, such as acetone. 

Like dissolves like, and this is true with both polymers and smaller molecules. Thus linear amorphous polymers with nonpolar groups are typically soluble in nonpolar solvents with solubility parameter values within 1.8 H of that of the polymer. Thus polyisobutylene (PIB) is soluble in hot lubricating oils, and small amounts of high-molecular-weight PIB are used as viscosity improvers. 

At low temperatures the bend in the V(T) curves indicates a glass transition. It is of interest, whether this transition can be described in analogy to amorphous polymers on the basis of the concept of using one internal order parameter and whether the liquid crystalline structure is maintained in the glassy state. This will be discussed in Chap. 2.3.4. 

The permeation of small molecules in amorphous polymers is typically following the solution diffusion model, that is, the permeability P of a feed component i can be envisioned as the product of the respective solubility S and constant of diffusion Dj. Both parameters can be obtained experimentally and in principle also by atomistic simulations. 

According to Eisenberg,225 the glass transition is perhaps the most important single parameter related to applications of amorphous polymers, as it greatly affects their... 

For amorphous polymers which melt above their glass transition temperature Tg, the WLF equation (according to Williams, Landel, Ferry, Eq. 3.15) with two material-specific parameters q and c2 gives a better description for the shift factors aT than the Arrhenius function according to Eq. 3.14. 

In the derivation of Eq. (7.3) by Hildebrand only dispersion forces between structural units have been taken into account. For many liquids and amorphous polymers, however, the cohesive energy is also dependent on the interaction between polar groups and on hydrogen bonding. In these cases the solubility parameter as defined corresponds with the total cohesive energy. 

It was pointed out from the beginning that the concept of the solubility parameter was applicable only to amorphous polymers. In order to adapt the method to highly crystalline polymers some way must be found to deal with the heat of fusion (AHm) in the free enthalpy equation ... 

The nature of the polymer slightly affects the solubility and is probably related to the solubility parameter of the polymer. For amorphous elastomers without strong polar groups (and even for amorphous polymers in general ) Eq. (18.12c) may be used as a first approximation (with an accuracy of 0.25). 

The purpose of this paper is to use data already aquired on critical surface tension for a correlation with solubility parameters and parachors of polymers. The theoretical background of these parameters is briefly mentioned. The evaluation of the calculated values is then discussed. Because of the complexity of the polymer conformation on the surface, we do not imply that a straight-forward relationship between the surface and the bulk properties is available, even in the case of a liquid-like amorphous polymer. Another purpose of this paper is, therefore, to point out the complicating factors and the difficulties in predicting the surface wettability on the basis of bulk properties. 

The initial distribution on the other hand, is the distribution of sequences in the uncrystallized amorphous polymer. This initial distribution, of course, is determined in the case of stereoregular polymers by the particular statistics and probability parameters applicable to the system. 

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