Thermal properties homopolymers

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Poly oxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of trioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 175°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are Hsted in Table 1. See also reference 13. 

T and are the glass-transition temperatures in K of the homopolymers and are the weight fractions of the comonomers (49). Because the glass-transition temperature is directly related to many other material properties, changes in T by copolymerization cause changes in other properties too. Polymer properties that depend on the glass-transition temperature include physical state, rate of thermal expansion, thermal properties, torsional modulus, refractive index, dissipation factor, brittle impact resistance, flow and heat distortion properties, and minimum film-forming temperature of polymer latex... 

As shown in the previous section the mechanical and thermal properties of polypropylene are dependent on the isotacticity, the molecular weight and on other structure features. The properties of five commercial materials (all made by the same manufacturer and subjected to the same test methods) which are of approximately the same isotactic content but which differ in molecular weight and in being either homopolymers or block copolymers are compared in Table 11.1. 

From this table it will be noted that in terms of the mechanical and thermal properties quoted the copolymers are marginally inferior to the homopolymers. They do, however, show a marked improvement in resistance to environmental stress cracking. It has also been shown that the resistance to thermal stress cracking and to creep are better than with the homopolymer.This has led to widespread use in detergent bottles, pipes, monofilaments and cables. 

The glass transition temperatures of the nylons appear to be below room temperature so that the materials have a measure of flexibility in spite of their high crystallinity under general conditions of service. The polymers have fairly sharply defined melting points and above this temperature the homopolymers have low melt viscosities. Some thermal properties of the nylons are given in Table 18.4. 

This phenomenon can be demonstrated by both measuring the changes of the thermal properties of the ECA homopolymer and in adhesion tests. The addition of only 1 wt.% of 9 to a sample of the ECA homopolymer significantly increases the onset of decomposition in the thermogravimetric analysis (TGA) of the polymer, as seen in Fig. 9 [29]. 

Table 5.2. Molecular Weights and Thermal Properties of Fluorinated Hybrid Homopolymers... 

Thermal Properties. The DPP portion of block copolymers crystallizes on heating at 290°C and then melts at 480°C. The DMP portion of block copolymers does not crystallize thermally but can be caused to crystallize by treatment with a suitable solvent, such as a mixture of toluene and methanol the crystallized DMP then melts at 258°C. The glass-transition temperatures of the homopolymers are too close (221°C for DMP, 228° for DPP) to permit observation of separate transitions, either in block copolymers or blends of the homopolymers. 

The thermal properties measured by differential scanning calorimetry (Table IV) provide no structural information in the DMP-MPP system. Neither of the two homopolymers undergoes thermally-induced... 

Alkyl a-acetoxyacrylate intermediates were prepared by condensing pyruvate derivatives with acetic anhydride and then free radically converting them into the corresponding homo- or copolymers. All copolymers had thermal properties that were superior to that of polymethyl methacrylate. In addition poly(ethyl a-acetoxy-acrylate) homopolymers were injection moldable at 250°C. 

Aromatic polyimides with dielectric constants < 3.0 and reduced levels of moisture absorption can be obtained by the incorporation of fluorine into the polymer backbone. However, this approach can also lead to a reduction in the thermal properties (Tg), and/or solvent resistance of the polymer. It has been found that the incorporation of paraphenylene diamine (PPD) into the 6FDA/BDAF polyimide affords copolymers having improved thermal performance and solvent resistance as compared with the parent homopolymer while maintaining a dielectric constant of 2.9 and moisture uptake < 2.0%. This approach can be considered as a possible alternative to the use of... 

The crystal lattice parameters observed for these copolymers by X-ray diffraction are found to be almost identical to those of the P(3HB) homopolymer. The degree of crystallinity determined from the X-ray diffraction decreases steeply from 60 to 23% as the 3HP content in the copolymers increases from 0 to 37 mol%. The trends of composition dependence of the thermal properties and the X-ray diffraction are very similar to those for P(3HB-co-4HB) but not to those for P(3HB-co-3HV), as already mentioned above. Thus, it is reasonable to assume that in the P(3HB-co-3HP) samples containing up to 37 mol% 3HP unit, the 3HB units exist in the crystalline as well as in the noncrystalline regions, while almost all of the 3HP units exist only in the latter. To further investigate this point, the NMR relaxation times have been measured for the films cast from a chloroform solution. 

The stracture-property correlation of different copolymers with n-butyl acrylate and isobomyl acrylate units have been studied. The primary goal was to compare thermomechanical properties of block, gradient and statistical copolymers of nBA and IBA with various acrylate homopolymers (Scheme 1). The choice of nBA and IBA was dictated by very different thermal properties of the resulting homopolymers, glass transition temperature (Tg) of PnBA is -54°C while the Tg of PIBA is 94°C. Thus, their copolymerization with carefully selected ratios should result in polymers with thermal properties, i.e., Tg similar to acrylate homopolymers poly(t-butyl acrylate) (PtBA), poly(methyl acrylate) (PMA), poly(ethyl acrylate) (PEA) and poly(n-propyl acrylate) (PPA). 

The efficiency and randomness of the incorporation of a second monomer depend on the copolymerization parameters (see Section 3.1) and the type of chain growth method applied. For each monomer pair these factors have to be evaluated and considered for defining stmcture and functionality. In addition, basic polymer properties like solubility and thermal properties defined by the major monomer are strongly influenced by a second monomer of different functionality thus, in any random copolymerization process, it is usually not possible to introduce a second functionality without compromising the properties of the parent homopolymer, and this is enhanced with increasing the comonomer ratio. 

A comparative study of the thermal properties and of the glass transition temperatures of the (A) and (B) homopolymers and of the (AB) random and alternated copolymers and (BAB) alternated copolymers has been achieved and showed the influence of the structure of the polymer. 

Concerning the thermal properties of these hybrid homopolymers, the Tg was higher and the thermal stability at high temperature was lower when x = 1 than when x = 0 [48] (cf. Table 1). 

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