Partially crystalline polymer

No polymer is ever 100% crystalline at best, patches of crystallinity are present in an otherwise amorphous matrix. In some ways, the presence of these domains of crystallinity is equivalent to cross-links, since different chains loop in and out of the same crystal. Although there are similarities in the mechanical behavior of chemically cross-linked and partially crystalline polymers, a significant difference is that the former are irreversibly bonded while the latter are reversible through changes of temperature. Materials in which chemical cross-linking is responsible for the mechanical properties are called thermosetting those in which this kind of physical cross-linking operates, thermoplastic. 

The sorption behavior of perfluorocarbon polymers is typical for nonpolar partially crystalline polymers (89). The weight gain strongly depends on the solubihty parameter. Litde sorption of substances such as hydrocarbons and polar compounds occurs. 

Addition of a plasticizer decreases the Tg of the polymer and, in partially crystalline polymers, also influences both crystallization and melting. The amount of plasticizer affects its effectiveness. Thus, while the Tg of the polymer is strongly depressed by small plasticizer additions, the increase in the plasticizer content leads to lower decrease in To and in several systems two T values can be found [36J. Therefore, the increase in the plasticizer content in polymers does not show a monotonic decrease in Tg. 

During the 1940 s it was proposed that partially crystalline polymers consisted of regions where the molecular chains where gathered in an ordered... 

Hence, the extension of an isotropic unoriented partially crystalline polymer leads to the formation of a highly organized material with a characteristic fibrillar structure. The anisotropy of the sample as a whole is expressed by a higher modulus, tenacity and optical anisotropy. It would seem that the increase in strength in the drawing direction suggests that the oriented samples consist of completely extended chains. However, while the strength of such perfect structure for polyethylene has been evaluated as 13000 MPas), the observed values for an oriented sample are 50 to 30 MPa. 

Solid state materials have been studied by nuclear magnetic resonance methods over 30 years. In 1953 Wilson and Pake ) carried out a line shape analysis of a partially crystalline polymer. They noted a spectrum consisting of superimposed broad and narrow lines which they ascribed to rigid crystalline and amorphous material respectively. More recently several books and large articles have reviewed the tremendous developments in this field, particularly including those of McBrierty and Douglas 2) and the Faraday Symposium (1978)3) —on which this introduction is largely based. 

Possible morphologies of partially crystalline polymers are shown in Fig. 18. Figure 18a depicts the case of small crystallites that act as physical crosslinks between polymeric chains, thus connecting those chains into a 3-dimensional network. In the case depicted in Fig. 18b, the material forms ribbon-shaped or needle-shaped crystalline regions in which different segments of a large number of chains are incorporated. This could explain the low degree of crystallinity at the LST as detected for the iPP system [80]. 

Fig. 18a, b. Possible morphologies of partially crystalline polymers. Small crystallites act as crosslinks (a) large ribbon-shaped or needle-shaped crystalline regions connect a large number of polymeric chains (b)... 

Most PHAs are partially crystalline polymers and therefore their thermal and mechanical properties are usually represented in terms of the glass-to-rubber transition temperature (Tg) of the amorphous phase and the melting temperature (Tm) of the crystalline phase of the material [55]. The melting temperature and glass transition temperature of several saturated and unsaturated PHAs have been summarized in Table 2. 

Poly(HA) can be biodegraded to water and carbon dioxide or methane by a large variety of ubiquitous microorganisms present in many ecosystems (Fig. 1). This fairly easy bio degradability came as a surprise given the inertness of the water-insoluble, hydrophobic, and (partially) crystalline polymers. 

Represents the temperature of the conversion of an amorphous glassy or partially crystalline polymer into a rubbery viscous melt. Important for sensors, because polymers with high Tg require plasticizers for fast analyte diffusion and response time (see section 3.1). 

A number of organic pigments can cause warping of certain thick-walled, large-area, non-axially symmetrical injection-molded parts such as bottle crates, where they act as nucleating agents for partially crystalline polymers. 

P.Y.62 is thermally stable up to 250°C. It has a considerable effect on the shrinkage of HDPE and other partially crystalline polymers. The pigment is an equally suitable colorant for polystyrene and polyurethane and lends color to polypropylene spin dyeing products with minimal application requirements. 

P.Y.181, a reddish yellow pigment, was introduced to the market a few years ago. Its main area of application is in plastics, especially in polyolefins. In these media, P.Y.181 is heat stable up to 300°C and very lightfast. 1/3 SD HDPE samples (1% TiOz), for instance, equal step 8 on the Blue Scale. The pigment does not affect the shrinkage of the partially crystalline polymer. Its tinctorial strength, however, is poor. 

The major results described could be partially anticipated from those previously reported for linear polyethylene (17) as well as those for cis polyisoprene. (] ) For the latter polymer, by taking advantage of its crystallization kinetic characteristics, it was possible to compare the relaxation parameters of the completely amorphous and partially crystalline polymer (31% crystallinity) at the same temperature, 0°C. This is a unique situation and allows for some unequivocal comparisons. It was definitively observed that for all the carbons of cis polyisoprene the T] s did not change with crystallization. 

The properties of polymers are not only determined by their primary chemical structure but also by secondary structural elements. This is also tme for the process of biodegradation. In a first step, the secondary structures of the polymer, e.g., crystals in a partially crystalline polymer, have to be dissolved during the degradation process and temporarily flexible chains formed. 

Lactam polymerizations (nonassisted as well as assisted) are usually complicated by heterogeneity, usually when polymerization is carried out below the melting point of the polymer [Fries et al., 1987 Karger-Kocsis and Kiss, 1979 Malkin et al., 1982 Roda et al., 1979]. (This is probably the main reason why there are so few reliable kinetic studies of lactam polymerizations.) An initially homogeneous reaction system quickly becomes heterogeneous at low conversion, for example, 10-20% conversion (attained at a reaction time of no more than 1 min) for 2-pyrrolidinone polymerization initiated by potassium t-butoxide and A-benzoyl-2-pyrrolidinone. The (partially) crystalline polymer starts precipitating from solution (which may be molten monomer), and subsequent polymerization occurs at a lower rate as a result of decreased mobility of /V-acyl lactam propagating species. 

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. 

Determination of the proportions of crystalline and amorphous material in partially crystalline polymers. Knowledge of the unit cell dimensions in high polymer crystals leads to a knowledge of the density of the crystalline regions. If the density of amorphous regions is also known, either by measurement of the density of an entirely amorphous specimen (if this can be obtained) or by extrapolation of the liquid density/temperature curve, it is possible to calculate, from the measured density of any partially crystalline specimen, the proportions of crystalline and amorphous material. Since the physical properties of polymer specimens are profoundly influenced by the degree of crystallinity, X-ray determinations of crystallinity are much used in such studies (see Bunn, 1957). 

However, with naturally occurring macromolecules, such as cellulose, the older fringed micelle concept is believed to apply. This represents a crystalline polymer, (it would be more correct to speak of semicrystalline or partially crystalline polymers since a material consisting of chain molecules can never be completely ordered), made up of ordered (crystalline) domains interspersed with disordered (amorphous) domains, so that each polymer chain passes through several crystalline and amorphous regions (Figure 4). 

The high-molecular-weight poly (propylene oxide) produced with hexacyanometalate salt complexes shows no crystallinity. Moreover, it was shown by Price et al. (18) and confirmed in our laboratory that these polymers have more than 953 head-to-tail enchainment. The amorphous fractions of partially crystalline polymers made with metal-alkyl and ferrio-chloride-based catalysts were shown by those authors to have considerable head-to-head enchainment. They postulated that this was the cause of the amorphous nature of these fractions. It seems clear, however, that the amorphous nature of the polymers prepared with hexacyanometalate salt complexes must be the result of their low degrees of tacticity. 

Figure 2.28 Sketch of the simplest model of a partially crystalline polymer, where the parallel straight lines represent the crystallites. Reprinted with permission from J. E. Mark, Physical Chemistry of Polymers, ACS Audio Course C-89, American Chemical Society, Washington, DC, 1986. Copyright 1986, American Chemical Society.

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