Crystallinity thermomechanical properties

This article reviews recent developments in polymer thermomechanics both in theory and experiment. The first section is concerned with theories of thermomechanics of polymers both in rubbery and solid (glassy and crystalline) states with special emphasis on relationships following from the thermomechanical equations of state. In the second section, some of the methods of thermomechanical measurements are briefly described. The third section deals with the thermomechanics of molecular networks and rubberlike materials including such technically important materials as filled rubbers and block and graft copolymers. Some recent data on thermomechanical behaviour of bioelastomers are also described. In the fourth section, thermomechanics of solid polymers both in undrawn and drawn states are discussed with a special focus on the molecular and structural interpretation of thermomechanical experiments. The concluding remarks stress the progress in the understanding of the thermomechanical properties of polymers. 

The thermomechanical properties of an organic material mainly depend on two factors. Firstly the molecular relaxations (crystalline melting point and glass transitions) which determine the temperature upper limit for applications, and secondly the chemical nature of the backbone which is responsible for the stability in a harsh environment. 

Some organic bases like tertiary amines can also catalyze the amide acid cy-clization [6,9], and the nature of the tertiary amine allows the control of the proportions of imide-isoimide-acetamide in the reaction product. For example the reaction of MA with MDA using sodium acetate as catalyst give a BMI with a crystalline melting point at 160 °C. If the condensation is carried out with DAB-CO, an amorphous mixture containing BMI, monomaleimide-mono isoimide, and monomaleimide-monoacetamide is obtained. This amorphous material called Desbimid is more processable than the pure BMI but exhibits similar thermomechanical properties [10]. 

The mechanical properties are similar to those of aPS, but the elastic modulus is enhanced owing to the crystallinity. Brittleness is typical of styrenic polymers, and this is the major drawback of sPS. Whereas the mechanical properties of aPS rapidly decay above Tg, those of sPS remain good. Addition of inorganic fillers (e.g. glass fibers) leads to a further improvement in thermomechanical properties and also impact resistance. 

Ceramics obtained from polymeric precursors are usually amorphous. Since substantial thermal activation is required for nucleation and crystallization, precursor-derived ceramics (PDCs) frequently remain amorphous or nanocrystalUne up to rather high temperatures. For example, crystallization of a number of quaternary Si-B-C-N ceramics is retarded even up to 1800°C, resulting in excellent thermomechanical properties. Nevertheless, crystalline materials are of great interest because their microstructure formation can be controlled during devitrification, providing a means for stabilizing nanosized morphologies. 

Differential scanning calorimetry (DSC), X-ray diffraction (XRD), and infrared spectroscopy are the common techniques used in the characterization of the structure of the congealed solid. Thermal analytic methods, such as DSC and differential microcalorimetric analysis (DMA), are routinely used to determine the effect of solutes, solvents, and other additives on the thermomechanical properties of polymers such as glass transition temperature (Tg) and melting point. The X-ray diffraction method is used to detect the crystalline structure of solids. The infrared technique is powerful in detecting interactions, such as complexation, reaction, and hydrogen bonding, in both the solid and solution states. 

In recent years silicon-based polymers were investigated as precursors for SiC and Si3N4 ceramics, as well as for crystalline or amorphous Si/C/N and SiC/Si3N4 composite materials [1, 2]. This is due to the very interesting chemical and thermomechanical properties of silicon carbonitrides, such as high hardness, toughness and corrosion resistance. In most of these studies polycarbosilanes, polysilazanes and polycarbosilazanes were applied [3]. 

The ability to control the stereochemical architecture enables a precise control over the size/shape of the PLA crystals, the degree of crystallinity, the rate of crystallization, and the thermomechanical properties of the material. PLA homopolymers crystallizes in three forms (a, p, and y), depending on the preparation conditions and the ratio of L and D enantiomers. The a-form (and related disorder a -form) is the most stable form with two antiparallel chains upon a twofold helix conformation distorted periodically from the regular s, while the P-form is a left-handed threefold helix and the y-form is obtained by epitaxial crystallization, containing two antiparallel s helices upon a threefold helix [34-36]. Interestingly, PLA is a clear, colorless thermoplastic when quenched from the melt and crystallizes slowly on cooling [16]. 

Crystallinity of PLA has a strong impact on its mechanical properties. Suryanegara et al. have prepared PLA/MFC nanocomposites in both fully amorphous and crystallized states. The tensile modulus and strength of pristine PLA were improved with an increase of MFC content in both amorphous and crystallized states. Dynamic mechanical analysis (DMA) has been used to study the effect of MFC reinforcement on the thermomechanical properties of PLA in both states and the results are shown in Figure 9.5. In the amorphous state, the storage modulus of pristine PLA below Tg is almost constant at around 3 GPa. Above Tg, the modulus drops to 4 MPa at 80 °C, and then increases to 200 MPa at 100 °C owing to the cold... 

Liquid crystals exhibit a partially ordered state (anisotropic) which falls in-between the completely ordered solid state and completely disordered liquid state. It is sometimes referred to as the fourth state of matter . In recent years, interest in liquid crystalline thermosets (especially liquid crystalline epoxy) has increased tremendously [33-44]. If the liquid crystal epoxy is cured in the mesophase, the liquid crystalline superstructure is fixed permanently in the polymer network, even at higher temperature. Liquid crystal epoxies are prepared using a liquid crystal monomer [33-38] or by chemical modification of epoxy resin [43] which incorporates liquid crystal unit in the epoxy structure. Liquid crystalline epoxy resins with different types of mesogen such as benzaldehyde azine [33], binaphthyl ether [34, 35], phenyl ester [36, 37] and azomethine ethers [38, 39] have been reported. Depending on the chemical nature of the mesogen, the related epoxies display a wide range of thermomechanical properties. The resins can be cured chemically with an acid or amine [40, 41] or by photochemical curing in the presence of a photo-initiator [3]. Broer and co-workers [42] demonstrated the fabrication of uniaxially oriented nematic networks from a diepoxy monomer in the presence of a photo-initiator. 

Polymer samples have a thermal history due to processing, which influences the polymer properties, such as crystallinity, crystallite orientation, and consequently the thermomechanical properties. In DSC experiments typically the data of the second heating run are analyzed after the thermal history is erased through heating above the highest Ttrans in the first heating run. 

The recommendations are revised on a regular basis, gaps are filled, and new developments are carefully observed in order to react timely on new sdentiiic trends. Examples are the revision of the Definitions of terms relating to crystalline polymers, which is currently (2011) close to publication, and the Glossary of terms relating to thermal and thermomechanical properties of polymers, which is currently (2011) in preparation. Newly started projects are... 

Vicat penetration is much more influenced by PLA crystallinity. In the case of PDLLA and amorphous PLLA, Vicat penetration values of 52-53°C and 59-60°C, respectively, were reported. Also these values are very near to the Tg of the polymers. On the other hand, crystalline PLLA presents a very different behavior, with values of 157-165°C. This marked difference in Vicat penetration measurements is related to the contribution of crystallinity to thermomechanical properties of this material at a microscopic level [2,7]. 

A lot of molecular structures give rise to thermotropic liquid crystallinity, but only the aromatic ester type polymers and copolymers are successfully prepared as structural materials. Aromatic polyesters can be classified into three types based on their molecular composition and thermomechanical property [74]. 

Figure 1.32 demonstrates figurative differences in chain configuration that govern the degree of crystallinity, which, along with MW, determines final thermomechanical properties. 

Harada M, Ochi M, Tobita M, Kimura T, Ishigaki T, Shimoyama N, Aoki H (2004) Thermomechanical properties of liquid-crystalline epoxy networks arranged by a magnetic field. J Polym Sci Polym Phys 42 758-765... 

Shape Memory AUoys (SMAs) are metalhc materials that show special thermomechanical properties due to a reversible transformation between two crystalline configurations, austenite and martensite, without degradation of the crystal structure. 

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