Crystallinity, macromolecules, thermal

Kunioka, M., Tamaki, A. and Doi, Y. (1989b) Crystalline and thermal properties of bacterial copolyesters poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and poly(3-hydroxybuty rate-co-4-hydroxybutyTate), Macromolecules, 22, 694 697. 

Mesomorphic dendrimers containing electroactive units have potential for construction of dendrimer based molecular switches. Deschenaux et al. reported [154] the synthesis and liquid-crystalline properties of a novel dendrimer containing six mesomorphic ferrocene units. Apart from exhibiting a broad enantiotropic smectic A phase as determined by polarized optical microscopy, DSC, and XRD studies, thermogravimetry revealed the excellent thermal stability of the macromolecule. 

Thermoplastic polymer macromolecules usually tend to become oriented (molecular chain axis aligns along the extrusion direction) upon extrusion or injection moulding. This can have implications on the mechanical and physical properties of the polymer. By orienting the sample with respect to the coordinate system of the instrument and analysing the sample with polarised Raman (or infrared) light, we are able to get information on the preferred orientation of the polymer chains (see, for example, Chapter 8). Many polymers may also exist in either an amorphous or crystalline form (degree of crystallinity usually below 50%, which is a consequence of their thermal and stress history), see, for example, Chapter 7. 

The prediction of the chemical thermostability is based on the rules on the thermal stability and the reactivity of chemical bonds known for low-molecular-weight compounds. Instead, the physical thermostability depends on the transition points of the macromolecules, i.e., the glass transition temperature Tg in case of amorphous polymers, and additionally the crystalline melting point in case of crystalline polymers. 

Thermal Properties. A typical dsc thermogram of an HPL/PVA blend (Fig. 4) shows a single Tg and Tm (10). Differences in the shape of the melting endotherms of PVA(96), (88), and (75) can be attributed to different degrees of crystallinity in the three polymers. Changes in crystalline structure of polymer blends usually result from polymer-polymer interactions in the amorphous phase. Such interactions result in a reduction of crystallinity, thereby reducing the enthalphy of the phase change (16,17). The observed reductions in melt endotherm area of HPL blends with PVA (> 0) may therefore indicate the existence of polymer-polymer interactions between the two types of macromolecules. 

These expressions show that a deformed polymer network is an extremely anisotropic body and possesses a negative thermal expansivity along the orientation axis of the order of the thermal expansivity of gases, about two orders higher than that of macromolecules incorporated in a crystalline lattice (see 2.2.3). In spite of the large anisotropy of the linear thermal expansivity, the volume coefficient of thermal expansion of a deformed network is the same as of the undeformed one. As one can see from Eqs. (50) and (51) Pn + 2(iL = a. Equation (50) shows also that the thermoelastic inversion of P must occur at Xim (sinv) 1 + (1/3) cxT. It coincides with F for isoenergetic chains [see Eq. (46)]. 

The mechanical properties of polymers are not single-valued functions of the chemical nature of the macromolecules. They will vary also with molecular weight, branching, cross-linking, crystallinity, plasticizers, fillers and other additives, orientation, and other consequences of processing history and sometimes with the thermal history of the particular sample. 

It is not excluded that this mechanism is observed during the formation of fibres from X-500 The authors of this work pointed out that when the fibre was heated to 250-300 °C, its spontaneous elongation took place. Note that to attain higher orientation of a polymer in a fibre, it is necessary not only to transfer it to the liquid crystalline state but also to orient the liquid crystalline domains formed along the axis of the fibre. This orientation of the domain in which the macromolecules have been already mutually ordered requires not too high a draw ratio (the theoretical value must be <2). Indeed, experiments have shown that at the draw ratio of 1.53 to 1,70 the modulus (E) and the tensile strength (a) of the fibre at thermal treatment increase, which can be seen from the table compiled according to the results of this work. 

T. Sasaki, T. Ikeda, and K. Ichimura, Photo- and thermal isomerization behaviors of azobenzene derivatives in liquid crystalline polymer matrices, Macromolecules 26, 151-154 (1993). 

Matsumoto, A., Chiba, T. and Oka, K. (2003) Topochemical polymerization of N-substituted sorbamides to provide thermally stable and crystalline polymers. Macromolecules, 36, 2573—2575. 

When the mesogen moiety is included into the main chain of the polymer, the obtained macromolecule contains inherently rigid units, which usually result in remarkable mechanical properties and thermal stability. Fibers made by these polymers compete with the best ceramic fibers and are far superior to metal fibers [83]. They therefore are ideal candidates as reinforcements for polymer-based composites. However, these materials often have a poor miscibility and adhesion to other polymeric substrates, limiting the range of their applications. This problem basically arises from weak intermolecular interactions either within the liquid-crystalline polymer itself or with the matrix of the composite. Strong ionic... 

Polymer stereoregularity is important since the morphology of the macromolecule depends on crystallization and the degree of crystallinity determines, in the end, several physical properties such as density, mechanical properties, as well as thermal properties. Atactic polymers have difficulties in crystalling they are amorphous or show a low degree of crystallinity, while the degree of crystallinity in isotactic and syndiotactic polymers is high. 

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