Morphology changes in polymers

On the contrary, crystalline polymers remain flexible and thermoplastic above the Tg until the temperature reaches the crystalline melting temperature (T ). At this temperature, crystalline macromolecular compounds melt to a viscous liquid. Extensive crosslinking may distort the regions and mark the transitions. 

Thermodynamically, is a first order transition and is a second order transition. Tg varies with the type of skeletal atoms present, with the type of side groups and even with the spatial disposition of the side groups. As a result, the practical utility of polymers and their different properties depend strongly on their Tg. The transition from a glass to a rubber-like state is accompanied by marked changes in the specific volume, the heat capacity, the refractive index and other physical properties of the polymer. Since it is a second order transition it bears many of the characteristics of a relaxation process and the precise value of Tg can depend on the method used and the rate of the measurement. 

Several phenomenological models have been used to provide an understanding of Tg. One of them is an isoviscous state. As a polymer is cooled from its melt state, the viscosity increases rapidly to a common maximum value, approximately 10 Pa-s (10 poise) at Tg for all glassy materials - both low molecular weight and polymeric. Another view is that the Tg represents a state of iso free volume. Free volume, Vf, is defined as the difference between the specific or actual volume V of the polymer at a given temperature and its equilibrium volume at absolute zero Vo, Vf = V-Vg. Each term is temperature dependent [5, 8, 9]. 

Vf is a measure of the space available for the polymer to undergo rotation and translation, and when the polymer is in the liquid and rubber-like states the amount of free volume will increase with temperature as the molecular motion increases. If the viscous polymer is cooled this free volume will contract and eventually reach a critical value where there is insufficient free space to allow large scale segmental motion to take place. Tg is the temperature at which this critical value is reached. A third view of the Tg is that it represents an isoentropic (same entropy) state. 

To transform a polymer into a useful plastic, we have to process it at a temperature higher than T or Tg depending on its physical structure, i.e., semi-crystalline or amorphous. 

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Most models of the morphological changes in polymer blends assume that an average response e.g., an average size drop is being broken, or aver-... 

In semi-cristalline polymers, rate-enhancement under stress has been frequently observed, e.g. in UV-photooxidation of Kapron, natural silk [80], polycaprolactam and polyethylene terephthalate [81]. Quantitative interpretation is, however, difficult in these systems although the overall rate is determined by the level of applied stress, other stress-dependent factors like the rate of oxygen diffusion or change in polymer morphology could occur concurrently and supersede the elementary molecular steps [82, 83], Similar experiments in the fluid state showed unequivocally that flow-induced stresses can accelerate several types of reactions, the best studied being the hydrolysis of DNA [84] and of polyacrylamide [85]. In these examples, hydrolysis involves breaking of the ester O —PO and the amide N —CO bonds. The tensile stress stretches the chain, and therefore, facilitates the... 

Although the cross-linked pyrazoline polymer is insoluble in common organic solvents by virtue of the crosslinks present, it was discovered that films of this material could be cast onto a variety of substrates from a suspension in a swelling solvent such as benzene. The ability to cast films is presumably related to the substantial change in polymer resin morphology after the coupling reaction has proceeded. The initial resin is hard, spherical and white in color, while the reacted bead is powdery, irregular in appearance and yellow, the color of the pyrazoline monomer. 

This drastic change in the slope of the haze curve at the 50-60 phr CTBN suggests that some morphological changes in the cured polymer... 

This article reviews the phase behavior of polymer blends with special emphasis on blends of random copolymers. Thermodynamic issues are considered and then experimental results on miscibility and phase separation are summarized. Section 3 deals with characteristic features of both the liquid-liquid phase separation process and the reverse phenomenon of phase dissolution in blends. This also involves morphology control by definite phase decomposition. In Sect. 4 attention will be focused on flow-induced phase changes in polymer blends. Experimental results and theoretical approaches are outlined. 

As previously mentioned, the quantity tc is governed by inner parameters of the system. When one considers morphology evolution in polymer blend solutions, tc has to be compared with the externally imposed pinning-down time tp after which no further phase changes occur. For blend solutions, the quantity tp is related to the rate of solvent evaporation. 

Intrinsic Properties. IPN properties can also be tailored by varying silicone concentration. In some systems, dramatic changes in polymer morphology and crystallinity have been observed in loadings as low as 3%. At the other end, polysiloxane loadings of up to 55% have been achieved. Some successful semi-IPN formulations are the following ... 

Seki, T., Sekizawa, H., and Ichimura, K. Morphological Changes in Monolayer of a Photosensitive Polymer Observed by Brewster Angle Microscopy. Polymer Commun. 38, 725 (1997). 

Seki, T., Kajima, J., and Ichimura, K. Multifarious photoinduced morphologies in monomol-ecular films of azobenzene side chain polymer on mica. Macromolecules, 33, 2709 (2000). Matsumoto, M., Miyazaki, D., Tanaka, M., Azumi, R., Manda, E., Kondo, Y., Yoshino, N., and Tachibana, H. Reversible light-induced morphological change in Langmuir-Blodgett films.. Am. Chem. Soc., 120, 1479 (1998). 

Change in polymer morphology from macroporous to microporous resulted in improved selectivity but a decrease in overall retention, which is desirable in terms of chromatographic performance. For some MIPs uv initiation is not suitable, e.g. when the template is uv sensitive, and thermal initiation is required. In this situation ABDV is favoured since it thermally decomposes at a lower temperature than AIBN (45°C). 

Putting desiccant in a layer of multilayer containers made with the same EVOH polymer results in much less oxygen permeation. The Retort Shock and slow barrier recovery effects do not take place. Moreover, this is true even when the desiccant is separated from the EVOH by a thin polymeric layer so that the EVOH and the desiccant cannot interact chemically. Retorted containers with desiccant behave more like unretorted containers without desiccant oxygen permeation is controlled almost exclusively by the water activity of the EVOH layer. Morphological changes in the EVOH layer after retorting appear to be very small when desiccant is present. We attribute this to rapid transport of excess moisture from the EVOH to the desiccant which results in healing the... 

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