Effect of the solution-gel-solid system transition on the properties of microfiltration membranes

Desalination, 86 (1992) 91-100

91

Elsevier Science Publishers B .V ., Amsterdam

Effect of the Solution-Gel-Solid System Transition on the Properties of Microfiltration Membranes N .P . LEKSOVSKAIA, Z . YU . CHEREISKY, 1 .G . RUBAN

and 0 .1 . NACHINKIN

Leningrad Research Institute of Chemical Fibers and Composite Materials (Len Nil "Chimvolokno ), St. Petersburg 195030, ui. Chimikov 28 (Russia)

(Received June 19, 1990)

SUMMARY

Various physical methods have been employed to investigate the influence of a solution-gel-solid system transition on the structure and properties of polymer microfilters . It is stated that, according to the conditions of phase decay, i .e., the kinetics of polymer system transition and presence of infernal strains (influence of the substrate and thickness of the solution layer), the value of arrangement of macromolecules on the surface of the ready membranes changes, which in turn determines the structure of the selective layer and technological possibilities of microfilters .

When obtaining polymer microfilters using dry and dry-wet methods, a polymer solution as a thin liquid film applied onto the backing is exposed in a gas-air medium, a mass-exchange between the polymer solution and the medium taking place under thermal conditions . Depending on the type of the polymer, mass-exchange direction and characteristics of the solution, there can be three alternatives of variations in the polymer system . 1 . At a temperature T1 , the solvent evaporation dominates, resulting in a greater content of the polymer in the surface layer and in the appearance of the concentration gradient over the thickness of the liquid film . Balancing 0011-9164/92/$05 .00 0 1992

Elsevier Science Publishers B .V . All rights reserved .



92 ,

u

0 .6 PA

0.4

0.2

0

4

8

12

T, h

r

Fig . 1 . The size statistic-mean of a supermolecular particle in the PA (1) and PVC (2) solution as a function of the solution life without air access .

P, kPa ∎ 300

0 .4

0 .2 10

∎ a 0

2

6 ~-107 ,m

Fig 2 . The capacity (4 of the PA filter and the first bubble pressure (P) as a function of the size static-mean r of a supermolecular particle . The filters were formed at zero time .

Fig.3 . Electron-micrographs of chippings of filters in PVC (1) and poly paraphenylene-isophthalamide(2).

93

of the volumetric composition is a slow process due to a high viscosity of the solution (the solution concentration is usually within 7-30 mass %) . No phase transformations occur, whereas the size, number and life of fluctuations can change (Figs . 1 and 2) . When immersing the solution into the setting bath, the effect of the concentration gradients and the dispersion level can lead to an asymmetry of the film structure thickness (Fig . 3) . 2. Due to a partial evaporation of the solvent at T2 and the variation in the proportion of the fractions of components in the solution, the system falls within the area of the two-phase condition . 3 . Predominantly (or simultaneously with the solvent evaporation), there occurs a process of the precipitator sorption from a gas air medium (water vapor sorption takes place in the contact with air) . The variation in the system composition and reduced solubility of the polymer (change in the thermal and dynamic properties of the solvent) result in a solution decay into phases . We consider the examples of three versions of changes in the polymer system . Many attempts have been made to quantitatively describe the process of the solvent evaporation when obtaining microfilters from polymer solutions [1,2] . In order to describe the solvent evaporation process, they separated two stages, namely constant and decreasing evaporation rates according to a concept of two-stage film formation . Y' %

9 V'in' 10 3 rnn

Ve • 10^

9

Fig . 4 . The evaporation rate V, (1), syneresis rate Vaj . ( 2) and y (3) polymer solution liquid film shrinkage (contraction) as a function of the PVC solution air exposure time .

94

However, the solvent evaporation rate in dry forming of films seems to follow more complicated variation relationships . Fig . 4 shows our data on evaporation of dimethylformamide when obtaining microfilters in polyvinyl chloride (PVC) . In the given case in contrast to two evaporation conditions usually considered (with a constant and decreasing evaporation rate), one can easily separate four conditions of a constant, increasing newly constant and dropping rates . It is seen in Fig . 4 that the syneresis maximum (curve 2) and the beginning of the period of the increasing evaporation rate coincide . By now it has become possible to select technological parameters at the stage of dry forming rather than on the basis of investigations of complicated structural processes taking place in forming with the system phase decay which predicts to a great extent the properties of microfilters, then purely empirically . For example, the structure and properties of polyamide (PA) microfilters can be considered as a function of the exposure time in air (rf) . The decrease of the solution temperature simultaneously with the evaporation of the solvent was the characteristic feature of the process . Thus, the system phase decay was due to the changes of two parameters - composition and temperature . The geleous system was then immersed into water which in this case served as a precipitant [3] . The data in Figs . 5 and 6 on the variation in PA microfilter indices vs . time rf indicate that the filtrate permeability W first decreases and then sharply increases followed by a reduction down to zero . The mechanical strength changes in reverse order . It is worth noting that the extremes on both curves are in the time intervals which adequately coincide . Such variation in the film indices can be explained by assuming the fact that the difference in r(under the contact with a gas-air medium characterizes a different level of finalizing the phase decay in the system whose behavior can be described by two types of the phase equilibrium : amorphous and crystalline . With regard to these features of the system, the PA film structure formation mechanism seems to reside in the following : when the composition changes and temperature drops, the system decomposes into amorphous phases with a predominant precipitation of a low molecular phase from the non-equilibrium solution, with the result that the polymer phase starts to form a framework . Crystallization originating in the polymer phase initially can contribute to the decrease in the volumetric porosity . The fact that the structure transformation process runs at molecular and micro levels is supported by Fig . 5 and 6 where spectral and electron-microscopic data are shown .

95

0 .5

0 .4

0 .3 0

20

40

Fig . Sa . Effect of the PA membrane air forming time Tf on the capacity W (la), tensile strength a (2a) and specific porosity VP (3a) . b . Variation in the degree of order of PA structure in the firm volume (lb), in the upper (2b) and the lower (3b) surface layers from IR and ATR spectroscopy data.

Thus, non-monotonous structural changes are observed in superficial layers of the film depending on the film obtaining conditions . Similar nonmonotonous structural changes in PA filters can be observed with the thickness variation of the solution layer applied onto the backing (Fig . 7). The resultant polymer framework, i.e., gel, is out of equilibrium due to a low diffusion rate of macromolecules, the appearance of a great number of intermolecular links [4] right up to the vitrification of the polymer and the incomplete transformation of the structure . This indicates that considerable internal stresses arise in the polymer . Stresses exceeding the instantaneous

b

Fig. 6 . Microphotographs of PA filter surfaces vs . air forming time . a, upper surface layers ; b, bottom layers .

97

Fig . 7 . The PA membrane crystallinity level in the lower layer (1), upper layer (2) from ATR spectroscopy data, and capacity W (3) as a function of the film thickness, time rf being equal .

mechanical strength of the elements of the framework (network) can cause partial damage to the framework walls (gel syneresis is attributed to this effect in [51), the appearance of through capillaries being quite possible . In PA microfilters, principally with a cell (honeycomb) structure, the channel for the filtrate to flow has a very complicated shape with the length much greater than the filter thickness (Fig . 8), since the honeycomb walls crack . The PA filter was filled with epoxy resin under a slight degassing in order to identify the shape of the filtrate channel . After hardening the resin the PA filter was completely extracted by aqueous ethanol until its replica was obtained from pure epoxy resin . A microphotograph in Fig . 8b shows a part of the replica of cross-section with the arrows indicating the channels connecting the cells in the filter . One more parameter of polymer solutions, i .e ., superficial tension, must be taken into consideration . As experience indicates, forming from solutions (polymer-, solvent-, non-solvent) is the most stable in the range of the lowest values of superficial tension (Fig . 9) . This seems to correspond to the most suitable condition of obtaining highly porous structures when the precipitator-diffusion rate into the polymer solution exceeds that of the solvent out of the solution . Paper [2] deals with mass-exchange processes followed by the disturbance of force equilibrium along the inter-phase boundary which can be considered as a local variation in superficial tension . These variations show up in the local distortion of the shape and in the curvature of the interphase boundary or as separate vertexes normal to the inter-phase boun-

b

Fig . S . Microphotographs of PA filters chipping and replicas in epoxy resin In-flow channel is marked by arrows .

a

99

Fig . 9 . Surface tension of the solvent (1) and polymer solution (2) as a function of the solvent composition for PA filters . A similar relationship is true for the system of PVC-dimethylformamide-water .

dary . The realization of these processes in an optical microscope with time is recorded in [6] and is illustrated in Fig . 10 as a result of these processes in PVC and (poly)-fenyleneisophtalamide polymers . Thus specified technical and economical characteristics of microfilters can be reached by controlling both stages of the process to obtain a porous polymer film, i .e., the transformation of the solution into a gelatinous stateinteraction with a gas-air medium, and further formation of a permeable film structure (removal of the low-molecular phase from the gel, appearance of cracks and through channels, controlling of the adhesive interaction with the backing) .

REFERENCES 1 2 3 4 5 6

V .M . Chesunov and R.M . Vasenin, Vysokomolec . sued, Part A, 9-10 (1967) 2067 . 0 .1 . Nachinkin, Polymer microfilters . Chemistry, Moscow, 1985, p . 216 . S .I. Kuperman, 0 .1 . Nachinkin and N.P . Leksovaskaya, Plast. massy, 7 (1982) 38 . E .G . Moysia and Yu . S . Lipatov, Vysokmolecul . soed, Part A, 21(1) (1979) 333 . S .P . Papkov, Physical-chemical foundations of polymer reproduction . 0 .1 . Nachinkin, A .H . Shuster and I .G . Ruban, Chemical Fibers, 4 (1985) 12 .



t min

5 nrin

4

u

15 min

Fig . 10 . Microphotographsof PVC chippings with different times rf at different compositions of the deposition bath .

tf=0 min

80% 1120 + 20% DMFA

S 00% H2O

8