Amorphous phase, polypropylene

Some of the most difficult heterophase systems to characterize are those based on hydrocarbon polymers such as mbber-toughened polypropylene or other blends of mbbers and polyolefins. Eecause of its selectivity, RuO staining has been found to be usehil in these cases (221,222,230). Also, OsO staining of the amorphous blend components has been reported after sorption of double-bond-containing molecules such as 1,7-octadiene (231) or styrene (232). In these cases, the solvent is preferentially sorbed into the amorphous phase, and the reaction with OsO renders contrast between the phases. 

Small-angle X-ray scattering (SAXS) data have made it possible to deduce the localisation of organic additives (pigments) in the bulk of isotactic polypropylene (iPP) [344]. This work has confirmed that the additives are located in the amorphous phase, in spite of their crucial influence on the formation of the crystalline phase of iPP. SAXS has also been used to study the 3D structure of different carbon-black aggregates, and silica-filled SBR rubber compounds [345]. 

The separation of the crystalline and amorphous phases into their respective spectra has been carried out for a number of polymers including polyethylene terephthalateS6), polystyrene 57), poly(vinyl chloride)58), polyethylene 59,60) nylon61), polypropylene 62), and poly(vinylidene flouride)63). 

For polypropylene, by using the spectrum of an annealed sample and subtracting it from a quenched sample it is possible to obtain a difference spectrum characteristic of the amorphous regions of polypropylene 210). In Fig. 13, the difference spectrum characteristics of the amorphous phase of the quenched sample (a) is compared with the difference spectrum characteristic of the ordered phase of an annealed... 

In this section we will discuss the molecular structure of this polymer based on our results mainly from the solid-state 13C NMR, paying particular attention to the phase structure [24]. This polymer has somewhat different character when compared to the crystalline polymers such as polyethylene and poly(tetrameth-ylene) oxide discussed previously. Isotactic polypropylene has a helical molecular chain conformation as the most stable conformation and its amorphous component is in a glassy state at room temperature, while the most stable molecular chain conformation of the polymers examined in the previous sections is planar zig-zag form and their amorphous phase is in the rubbery state at room temperature. This difference will reflect on their phase structure. 

On the other hand, in the solid-state high resolution 13C NMR, elementary line shape of each phase could be plausibly determined using magnetic relaxation phenomenon generally for crystalline polymers. When the amorphous phase is in a glassy state, such as isotactic or syndiotactic polypropylene at room temperature, the determination of the elementary line shapes of the amorphous and crystalline-amorphous interphases was not so easy because of the very broad line width of both the elementary line shapes. However, the line-decomposition analysis could plausibly be carried out referring to that at higher temperatures where the amorphous phase is in the rubbery state. Thus, the component analysis of the spectrum could be performed and the information about each phase structure such as the mass fraction, molecular conformation and mobility could be obtained for various polymers, whose character differs widely. 

Since pneumatic conveying is largely applied to transport granular polymers and on the other hand even smallest amounts of attrition of these solids cannot be tolerated the results presented are focused on these materials. Polymers of four chemically different polymer classes were examined. Polypropylene (PP) and polyethylene (PE) belong to the semicrystalline polymers, which possess both, an amorphous phase and a crystalline phase. The polymethylmethacrylates (PMMA) and polystyrenes (PS) are fully amorphous. Some material properties of the polymers are summarized in Table 1. 

Drawn isotactic polypropylene (iPP) fibres have been studied by using deuterated n-decane as a 2H NMR probe of the chain deformation ratio in the amorphous regions. It is observed that the slope P=A/(X2-X 1) (defined in the limit of low deformations) indeed depends on the annealing temperature [82]. Thus, annealing above the melting temperature Tm of the crystallites allows chains to relax to some extent. Then, the local deformation ratio in the amorphous phase Xt becomes lower than the macroscopic one X and depends on the annealing temperature, i.e., on the amount of chain relaxation. Therefore, such systems have strongly non-affine deformation at the chain scale. 

Dlubek et al [49] studied a series of metallocene-catalyzed poly a-olefins) with progressively longer chains as the pendant side groups from polypropylene to poly-l-eicosene (20 carbons). Their results show an interesting relationship between the o-Ps lifetime and intensity in the amorphous phase for this series of polymers. They found that the average hole size and o-Ps intensity from the amorphous phase, decreased from polyethylene to polypropylene, followed by a slight increase to poly-1-butene. There was a rapid rise in hole size and intensity to poly-l-dodecene. 

The content of amorphous phase and the small size of spherulites lead to an improvement of the fracture toughness of Polypropylene [16]. In presence of mineral filler, the particle surface chemistry can induce some specific microstructural characteristics of the PP matrix parameters such as degree of crystallisation, spherulite size, and p phase content (a/p ratio) [16]. 

TM-AFM is superior due to reduced sample deformation and excellent contrast between amorphous phase and crystal phase in the TM phase images. An example of a, shish-kebob morphology is observed in partially dewetted ultrathin films of polypropylene derivatives grafted onto silicon (Fig. 3.25). 

Dehydrated calcium pimelate is added (between 0.01 and 0.5 wt %) as the nucleating agent to isotactic polypropylene to produce the (3 modification of iPP [63]. Following molding between two glass plates at 220°C thin films of the polymer are isothermally crystallized at 140°C-143°C in a microscopy hot stage. After the hedritic structures develop, the samples are quenched to room temperature. Prior to AFM examination, the specimens are etched with a 1% solution of potassium permanganate in a mixture of sulfuric and orthophosphoric acid [64, 65]. This procedure is described in detail in the literature Caution consult the literature for safety precautions ) and helps to remove preferentially amorphous phase of PP [64]. Thus, unlike in the many other examples discussed in Sect. 3.2, here the interior of a specimen is analyzed after its exposure. 

Vieth and Wuerth (2Ji) found negative deviations from the simple two phase model for semicrystalline polypropylene suggesting that the presence of crystallites in some way reduces the sorptive capacity of the amorphous phase. However, analysis of samples using x-ray diffraction revealed the presence of a less stable crystalline phase having a lower density. Since the crystalline volume fraction is commonly determined from density measurements, the presence of a second, less dense (however, still impermeable) crystalline phase would seem... 

DMA measurements are intensively used to investigate the amorphous phase transitions of polymers. The results of DMA studies were published by authors like Schmieder and Wolf [2], Nielsen and Buchdahl [3] and Heijboer [4]. Neat polymers, but also polymer blends and polymer systems blended with fillers, plasticisers or impact improvers were investigated by DMA. An example of such an application is given for toughened polypropylene in 4.1.2. 

A modulus value increase upon storage under ambient conditions is also reported for other semi-crystalline polymers like, for instance, polypropylene. Struik [11] measured for PP a continuously increasing dynamic stiffness at 20°C in combination with a decrease of the intensity of the glass-rubber (S) transition of PP (the temperature location of the S-transition did not change). Struik called this phenomenon an amorphous phase ageing effect a densification process of the amorphous PP phase due to a free volume relaxation effect. 

The presence of mineral reinforcements such as talc or mica, as foreign solid particles embedded into a polypropylene matrix, usually induces a nucleation effect. A signihcant increase in the crystalline content of the polymer is evidenced if compared with the neat polymer when processed at the same setup conditions that are necessary to ensure a good accommodation of the solid particles into the amorphous phase of the polymer in order to obtain a material with a good mechanical performance (27). The comparison between PP/mica and PP/talc composites in terms of their mechanical behavior under dynamic conditions in the solid state agrees with the morphological features derived from their chemical structures of both minerals (28). 

Because the 57Vratio is proportional to the specific surface of the mineral and being higher for mica than for talc, it follows that specific surface would always be lower for mica than for talc particles. Then for the same crystaHine amount of the polypropylene matrix, a higher fraction of amorphous phase involved in the coating of talc particles than in the coating of mica particles would be expected. 

The crystalline phase birefringence can be divided by the refractive index difference n — n for the crystal, to give 2. However, there is also a contribution to the birefringence from the molecular orientation of the amorphous phase. Figure 3.32 shows the contributions to the overall birefringence of polypropylene films, hot stretched at 110°C by different amounts. The increase in the orientation with strain is non-linear and it differs between the phases. 

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