Swelling/shrinking

Generally, conversion from one solvent to another is carried out at low flow rates. The commonly used flow rate for this conversion is 0.2 ml/min for standard columns and 0.1 ml/min for solvent-efficient columns. This minimizes any swelling/shrinking stress put on the column. The temperature of a solvent conversion is chosen to minimize any pressure stress on the column bank. As a general rule, the pressure per column should never exceed 3.5 MPa (500 psi) during solvent conversion. For example, the conversion of a column bank from toluene to trichlorobenzene (TCB) or o-dichlorobenzene (ODCB) is commonly carried out at 90°C. This minimizes the stress on the column due to the higher viscosity of the target solvents. 

SH Gehrke, LH Lyu, MC Yang. Swelling, shrinking, and solute permeation of temperature-sensitive N-isopropylacrylamide gel. Polym Prepr 30 482-483, 1989. 

Chemical Degradation A chemical action involving the molecular breakdown of the material due to contact with a chemical. The action may cause the personal protective equipment to swell, shrink, blister, discolor, become brittle, sticky, soft or to deteriorate. These changes permit chemicals to get through the suit more rapidly or to increase the probability of permeation. 

The use of polymeric coatings in catalysis is mainly restricted to the physical and sometimes chemical immobilization of molecular catalysts into the bulk polymer [166, 167]. The catalytic efficiency is often impaired by the local reorganization of polymer attached catalytic sites or the swelling/shrinking of the entire polymer matrix. This results in problems of restricted mass transport and consequently low efficiency of the polymer-supported catalysts. An alternative could be a defined polymer coating on a solid substrate with equally accessible catalytic sites attached to the polymer (side chain) and uniform behavior of the polymer layer upon changes in the environment, such as polymer brushes. 

Fig. 13. A swelling/shrinking cycle of PNIPAAm gel crosslinkcd in a phase-separated state, across the volume transition temperature of 33 °C. The time to complete one cycle is several thousand times less than required for a homogeneous PNIPAAm gel of identical chemical composition and dimensions. The lines are to guide the eye. Reprinted from Polymer Communications (1991) 32 322, by permission of the publishers, Butterworth Heinemann [65]...

Fig. 7A—C. Biochemo-mechanical function of NIPA gel with immobilized concanavalin A (A) Schematic illustration of saccharide-responsive, reversible swelling of a NIPA gel loaded with concanavalin A. Na DS"- is dextran sulfate sodium (DSS). (B) Temperature dependence for equilibrated volume of NIPA gel including the Con A-DSS complex (DSS-gel, O), MP (MP-gel, ), and free of both DSS and MP (A). The latter was prepared as a control sample as described in the text except for the use of an aqueous Con A solution instead of the Con A-DSS solution. Hysteresis was observed in the volume changes of the MP-gel and free-Con A gel on heating and cooling, indicating a discontinuous phase transition. The diameter of each gel in the collapsed state, determined at 50 °C, was d0 = 0.074 mm the volume of this gel is denoted Vp. The concentration of dry matter in the collapsed state was estimated from the preparation recipe to be 90 wt%. (C) Repeated swelling/shrinking control at 34.5 °C by alternate binding of DSS and MP to gel-entrapped Con A. (E. Kokufuta, Y.-Q. Zhang and T. Tanaka [78])...

In this chapter, temperature responsive swelling-shrinking changes in hydrogels were reviewed from the view point of polymer-water interaction and polymer-polymer interaction. In particular, characteristics of interpenetrating polymer networks (IPN) constructed of two distinct polymers which lead to novel temperature-responsive swelling changes were discussed... 

To waterproof the bamboo to minimize its swell-shrinking potential... 

The best time of the year to harvest bamboo is from late summer to mid-autumn, because at that time the natural moisture content of the bamboo, and therefore the swell-shrinking potential, is also low. 

The phenomenology of physical organogels and jellies is extremely rich, and their comportments are similar in some aspects to those of both surfactants in solution (e.g., lyotropism and crystallization) and polymer solutions (6 (e.g.. swelling/shrinking behaviors and microscopic mass motion). Gels can be considered as being at the interface between complex fluids (i.e.. micellar systems) and phase-separated states of matter. The main properties and concepts appropriate to describe the gels and the basic principles of techniques for their study will be reviewed here. 

Wood-water Interactions—swelling, shrinking, freezing, cracking... 

Of course, membrane thickness can be calculated supposing that its volume is given by the arithmetic addition of solvent and polymer volume. Assuming that the most important effect of matrix swelling/shrinking is to modify the solute diffusion coefficient Dg (and the Peppas-Reinhart equation [Equation 15.14] can be considered to account for this), permeation can be described by the continuity equation (Equation 15.1) where the generative term Rj is set to zero and the following initial and boundary conditions hold ... 

Finally, we can analyze the sorption hysteresis in terms of the change in structural arrangement in the molecular network (24) noted for similar systems where swelling-shrinking occur on sorption-desorption. The concept of a yielding coal structure is quite valid in view of the comprehensive work to show that swelling and shrinking are appreciable (even for CO2 and CH4 sorption) (23, 25, 26). 

Gel-type poly(styrene-co-divinylbenzene) beads have been used as a carrier to encapsulate bis-Cp titanium catalysts through a simple swelling-shrinking procedure. These catalytic species are homogeneously distributed in the PS bead particle and exhibit high and stable ethylene polymerization and ethylene/1-hexene co-polymerization activity.1236... 

Overall, therefore, the available literature supports the generally held view that the durability of UF-bonded wood products is governed by the susceptibility of cured UF resin bonds to scission by both hydrolysis and swell/shrink stresses. Note, moreover, that in either case, the most likely product of scission will ultimately be formaldehyde and further that mechanical stress enhances the rates of many chemical reactions (37). In fact, simplistic calculations based on formaldehyde liberated from bond ruptures at least indicate the possibility that formaldehyde from swell/shrink stress rupture could contribute significantly to total emission. Assume, for example, that board failure occurs due to rupture of one chemical bond type which liberates one molecule of formaldehyde and consider two cases (a) a conservative one in which only 5 percent of those bonds rupture in 50 years, i.e., probable board durability greater that 50 years, and (b) a much less conservative case in which 30 percent of those bonds rupture in 20 years, i.e., probably failure in 20 years or less. Case (a) leads to a first order scission rate constant of 3.3 x 10 s and a hypothetical board emission rate (see Appendix 3a) that is below the maximum liberation rate permitted by the German E-1 standard (7). However, Case (b) leads to a first order scission rate constant of 5.7 x 10 s and a hypothetical board emission rate above that allowed by the HUD standard (8). (FormaIdehyde-wood interactions and diffusion effects would... 

As a model to understand and to describe the processes during the response of a smart gel on changes of enviromnental properties, a two-step mechanism can be assumed (Fig. 8). In a first step, the stimulus which triggers the swelling/shrinking must permeate the gel. Heat transfer for temperature-sensitive polymers or mass transfer (ions, organic solvents) determine the rate of the first step. 

The swelling/shrinking starts if thermodynamically conditions for a volume phase transition, e.g., temperature, pH, appropriate mixing ratio of water with a hydrophobic agent, like an organic solvent, are given. 

The swelling/shrinking rate dQ/dt and hence the response time t for any application during volume phase transitions of smart hydrogels depends on the cooperative diffusion coefficient Dcoop and on the square of their characteristic dimension /,... 

The transport of a swelling agent during swelling/shrinking or the diffusion of substances into a swollen gel can be followed in real time by time-resolved measurements. The method provides information about the nature of diffusion processes into a polymer matrix on a spatially resolved level with a lateral resolution of about 50... 100 pm. 

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