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Ultrafiltration for fractionation of polydispersity plays an important role in polymer polymer chemistry. The chain size polydispersity can greatly influence the 1. Introduction properties of the polymer specimens, i.e. changing the molecular mass distribution In nature a variety of complex mixtures of can affect properties like viscosity, density, polymers are known to exist and they volatility, crystallinity and even include, for example, polysaccharides, permeability through the skin. Especially in , celluloses and lignin type fields like medicine, cosmetics and materials. the molecular mass distribution plays an important role in To separate these mixtures of determining the performances of the macromolecules most commonly polymers. precipitation, GPC or ultrafiltration (UF) are used. Of the various As an example ultrafiltration is used to processes, ultrafiltration provides the right prepare a more monodisperse dextran membrane with its pore size to separate (Figure 1). In this case the high molecular macromolecules on molecular weight weight dextran, being toxic for the human (molecular weight cut off). body, can be removed,

Today ultrafiltration provides an efficient and environmentally friendly process for a variety of industries.(1,2) It is encountered on production scale for example in the dairy industry (cheese whey ultrafiltration), waste treatment (removal of higher molecular weight materials), textile industry (recovery of size warping agents like PVA and CMC), pulp and paper industry (concentration and recycling of effluents), food industry (clarification of juices) and biotechnology.(1,2) One of the most important issues in ultrafiltration is fouling of the membrane, a process in which the membrane is irreversibly blocked by macromolecules. Several methods have been developed to avoid or diminish the fouling effect.(1,2) Figure 1. HPLC diagrams of dextran D20 Literature provides several hundreds of before and after ultrafiltration and publications each year dealing with the . Reproduced from ref 3. ultrafiltration in the aforementioned Copyright C.R.Chimie (2008). industries. In addition, ultrafiltration can be used in a The purpose of this colloquium is to explain more refined way to isolate materials or the basic principles of ultrafiltration, narrow ranges of materials from polymer followed by types, properties and recent of synthetic or natural origin. As a developments in . Finally, a consequence of the common synthetic review of the literature of the last decade on polymerization processes, chain size the fractionation of polymers is given.

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2. General principle of the Ultrafiltration process

Ultrafiltration has a wide application in industry as a method to concentrate solutions and to purify water.(2) For ultrafiltration to be effective, depending on the type of material that has to be concentrated, separated or purified, the properties of the membrane are essential. In literature, several articles are dealing with the type and characteristics of the membrane, which consists most of the Figure 2. , ultrafiltration and time of an organic polymer and . Reproduced from ref. 4. occasionally of a ceramic material. New Copyright Elsevier 2010. membranes are and have been developed to solve the disadvantages of the existing In the case of UF the pore size is in the range polymer membranes. of 1-10 nm and applying a pressure of 10- Ultrafiltration is a separation method for 30 bar. UF is primarily used in separation of substances in which a solution under high macromolecular solutions.(4) pressure is pushed through a membrane. A There are two distinct processes for UF: 1) variety of membrane types with different the dead-end and 2) the cross-flow pore sizes exist for which different ultrafiltration. In the dead-end process the pressures can be applied. direction of the flow of the solution is Ultrafiltration is part of a range of perpendicular to the membrane surface, one types. Membranes which have a relatively stream flows towards the membrane and large pore size compared to ultrafiltration one stream leaves through the membrane. In are used for Micro filtration (MF).(Figure the case of cross-flow UF the flow is along 1) The MF membranes have a pore size of with the membrane surface and two streams 0.1-10.0 µm, where a pressure of 1-5 bar is are leaving the membrane unit; retentate applied during filtration. These membranes and permeate flow.(Figure 3) In most are used for the sterile filtration and applications the accumulation of rejected clarification. particles is so severe that dead-end UF Membranes with a relatively smaller pore becomes impractical.(4) size compared to ultrafiltration are used for Nano filtration (NF). The pore size lies in the order of 1 nm with an applied pressure during filtration of 1-10 bar. An application of NF is removal of Ca2+ ions (softening of water) and desalination.

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solutions. In the dairy industry UF has been used for decades to pre-concentrate milk for cheese manufacture. Moreover, it is widely used for the fractionation of whey.(2)

One of the potential disadvantages of ultrafiltration can be the reduction of the membrane flux during operation. This can be either a reversible or irreversible process. The phenomenon of membrane flux reduction can occur in various ways. (Figure 4) First there is the reversible effect of concentration polarization, i.e. the retentate becoming increasingly concentrated in the proximity of the Figure 3. Dead-end filtration vs cross-flow membrane (Rcp). The concentrated retentate filtration. Reproduced from ref. 4. can, with time, form a gel layer or a cake Copyright Elsevier 2010. (Rg) that can reduce the flux further. Another possibility of reduction of As to the size of the , UF membrane flux can be the adsorption of the can be used both on a small scale, for solutes on the membrane itself (Ra). example by using laboratory Moreover the efficiency of the membrane and GPC and on a larger scale in production can be significantly reduced by deposition processes, be it batch wise or continuous.(2) and pore blocking (Rp).

2.1. Advantages and drawbacks of UF

The main advantages of the ultrafiltration process are:  a low energy consumption/high energy efficiency in comparison with other processes where, for instance, water has Figure 4. Schematic representation of the to be evaporated possible mechanisms for reduction of the  ease of operation (5) membrane flux. Reproduced from ref. 8.  ease of scale-up (4) Copyright Kluwer academic publishers UF can also help to recycle valuable 1996. materials and create less waste. For example, for electrocoat paint the drag-out Fouling, a term often encountered in paint is collected from the waste by UF and literature on UF, is used for indicating the reused in the paint process.(2) Its non- irreversible loss of membrane permeability. destructive character is an advantage for food industry when dealing with thermally Fouling can be caused by different labile ingredients or flavours.(1,6,7) UF can mechanisms such as adsorption, chemical also be used to concentrate suspensions or interactions, pore blocking and cake formation.(5)

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HO Fouling can be controlled and minimized by OH O surface modification of the membrane i.e. O O HO O HO O n by inserting hydrophilic groups making it OH less prone to organic fouling (see 2.2). The HO membrane flux can be kept higher by O performing a pretreatment or an ethanol CH3 O S O precipitation step of the polymer solution n CH3 O prior to the UF process in order to remove H F insoluble materials. Use of turbulence or ultrasound above the surface of the n H F membrane, pulsed or reversed flow, O rotating or vibrating membranes are all O S techniques which might prevent the fouling O n process. Figure 5. Examples of the most common Application of an electric field can be an used organic membrane polymers; effective method to reduce a gel layer. cellulose, polysulphone, poly(vinylidene Despite the above cleaning methods, a fluoride) and polyethersulphone. gradual irreversible fouling will occur which at a certain moment needs to be dealt In Table 1 several of the aforementioned with. Usually chemical cleaning will than membrane types are listed with their take place, for organic polymer based properties for application regarding pH membranes with the potential drawback range, maximum operating temperature and that the membrane might be gradually chemical stability. chemically degraded. Membranes based on inorganic/ceramic materials do generally Table 1: General application range of not present this disadvantage.(1) membranes (2,14).

Membrane pH Max Chemical 2.2 Membranes range T(°C) stability Cellulose 3-8 30 Protic solvents The membranes commonly used for UF are acetate PVDF 4-8 75 Good prepared from polymeric materials by a Regenerate 2-13 75 Protic solvents phase inversion process.(2) Usually, the cellulose polymer is deposited by evaporation of a Polysulphone 1-13 75 Hydrocarbons, solvent or by immersing the polymer protic solvents dispersed in an organic solvent into water. Polyimide 2-7 360 Good The substrate in general is a microfiltration Ceramics 0.5- 130 Good/excellent 13 membrane.

Some of the most commonly used organic The organic polymer based membranes as type of membranes consist of to their nature are susceptible to specific polyethersulphone, polysulphone, organic solvents. As polymers are usually poly(vinylidene fluoride), polyacrylonitrile, soluble in organic solvents, this limits the cellulose (e.g. regenerated cellulose or use of the type of membranes. For example, celluloseacetate), polyimide/poly(ether regenerated cellulose is incompatible with imide), aliphatic polyamides and acetone, toluene or chloroform.(9) The polyetheretherketone.(7, 8) organic membranes are usually stable in water, although care has to be taken

4 regarding acidity and alkalinity. In membrane considerably. The used literature, the organic polymer based molecules to test the separation capabilities membranes are mostly applied for of the membranes were BSA and EPS fractionation and purification of polymers (Extracellular polymeric substance). which can be dissolved/suspended in water, Also using polyethyleneimine as an for example polysaccharides, cellulose additive (0.3%) for polyethersulphone UF derivatives etc.(2) membrane improved permeability and The maximum temperature for UF selectivity. The performance of the membranes is important for those cases membrane was tested with both BSA and where a high viscosity of the polymer PEG. The membrane gave a better eluent solution makes ultrafiltration process at (water) flux and a molecular weight cut off room temperature impossible. By raising (MWCO).(13) temperature and selecting the proper UF Although polyimide membranes show a membrane a feasible processing can be very high temperature resistance (up to achieved. 360°C) and a good resistance to aggressive The pH range (next to solvent resistance) of solvents (membrane does not swell or UF membrane is relevant for cleaning dissolve in aggressive/organic solvents) purposes. In addition, there are a variety of (14), almost no applications of these type of biopolymers which only dissolve in membranes have been found in the last 5 strongly alkaline or acidic media and years. therefore a specific membrane has to be The aforementioned membranes can be selected. produced with different pore sizes which Each type of membrane has its consequently will afford the membrane to characteristics. For example, polysulphone act as a screen for different polymer sizes. has the advantage of good mechanical In essence the membrane will block 90% of properties, strong chemical stability and a polymer up to a certain size. This is called wide pH operating range from 1 to 13, the molecular weight cut off (MWCO). The however is susceptible to concentration MWCO gives an indication of the polarization and fouling by deposition of molecular weight the retentate and the polymers due to its hydrophobic profile by filtrate will contain. It is a rough indication, repulsion of water and hydrophilic because if the polymer is branched, a compounds. In order to prevent fouling the globular or a linear polymer the hydrophobicity of the membrane can be MWCO will significantly differ for the changed e.g. the surface of the membrane same membrane, whilst the can be made more hydrophilic by for macromolecules have the same molecular example grafting with PEG.(10) Alternately weight. Other parameters are more polysulphone membranes can be embedded important like shape and flexibility of the with graphene oxide. Testing with bovine macromolecule, but also the interaction serum albumin (BSA) showed with the membrane and concentration improvement of the antifouling polarisation. Another factor is the presence properties.(11) of a higher molecular weight molecule, Also the incorporation of an inorganic which can block the pores of the membrane. component can improve the Also the radius of the macromolecule is hydrophilicity.(12) Treating the membranes influenced by other factors like type of with TiO2/TiO2-g-HEMA solvent and temperature. For flexible improves the properties of the polysulphone polymers the intramolecular interactions are

5 low, but proteins have strong interactions 3. Fractionation of polymers by and have a stable globular structure. So for ultrafiltration the choice of a membrane also other factors should be taken into account like the UF can be used to separate relatively large molecule that is fractionated.(8) (natural) molecules like proteins, starch Stimuli responsive membranes are of cellulose, but also colloidally dispersed increasing interest because a reversible substances like clays, paints, pigments and change of the membrane properties is latex particles.(2) Chapter 3 deals with the possible. Thermo-responsive membranes research done in the last decade on the latter alter their pore diameter and surface subject. In view of the vast amount of properties by a temperature change thereby developments on the application UF for tuning the permeability and selectivity of protein fractionation this subject will be the membrane between two different states. omitted. Silica of distinct sizes could or could not pass the polyethyleneterephtalate track- 3.1 UF involving polysaccharides etched membrane depending of the and celluloses temperature of the membrane. This membrane is investigated to be used for Polymers that are fractionated in order to nanoparticles/protein mixtures.(15) obtain a more monodisperse polymer are Apart from the organic polymer based mostly biopolymers and/or polymers that membranes occasionally inorganic dissolve in water. In many articles a (ceramic) membranes especially alumina polymer is harvested from the water based (Al2O3) and zirconia (ZrO2) are used.(8) waste of a production process. Compared to organic polymer based Hemicelluloses from lignocellulosic membranes, the ceramic membranes are materials can be used to produce polymer inert and do not dissolve in solvents or films. These renewable films have water. Furthermore the temperature range properties that make them interesting as in which ceramic membranes can be used barrier materials for food packaging.(17,18) goes much higher (Table 1) than for almost HO all organic polymer based membranes.(2) O O HO The high temperature stability up to 130°C HO O is relevant for sterilization at 121°C in UF OH O HO for biotechnology and fermentation. OH Finally, the inorganic membranes have O n usually extended operating lifetimes as they Figure 6. Polysaccharide (n glycose units) are able to withstand the cleaning agents better. There are a few articles dealing with the fractionation of polymers through For example, hemicellulose can be ceramic membranes. The extreme harvested from the waste of corn. In the conditions which can be used for ceramic separation and purification process the membranes is highlighted for UF on a kraft waste is hydrolysed and then fractionated lignin solution which is strongly alkaline by ultrafiltration with membranes with a i.e. pH>13.(16) MWCO of 1, 5 or 10 kDa. By taking the retentate of the 10 kDa ultrafiltration the physicochemical characteristics of the hemicellulose were improved.(17)

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UF with a 1 kDa regenerated cellulose in sequence with a MWCO of 300, 100 and membrane was used for separation of a 6 kDa. The ultrafiltration afforded four noncellulosic polysaccharide-rich wood fractions each showing two to three peaks in hydrolysate. Three separations methods the GPC chromatograms, indicating that the were used: ultrafiltration, ultrafiltration/ ultrafiltration did not work perfectly.(20) diafiltration combination and ethanol Fractionation of polysaccharides and precipitation. For the purpose of a film on a protein from rapeseed was carried out with substrate the unseparated fraction is already a PVDF (500 kDa) membrane.(21) UF was good by having the lowest OP (oxygen performed using a polyethersulphone permeability). Although the fractionated membrane with a MWCO of 3, 8 and 12 samples have a lower OP, they do have kDa and tested for the influence of better mechanical properties in conjunction temperature, pH and ionic strength. with a different composition.(18) Temperature has generally a positive effect, For clinical use dextran with an extremely as well as high pH. On the contrary, higher narrow molecular weight distribution is ionic strength has generally a negative required.(5) Normal UF with membrane influence. Sun et al.(21) succeeded in cut-offs of 100, 30, 5 and 1 kDa provided fractionation of the polysaccharides, dextrans with a wide molecular weight obtaining the highest yield of distribution with a polydispersity index polysaccharides using a 3 kDa membrane. between 2.2 and 5.4. The polydispersity However, no clear conclusions are drawn index could be further enhanced to 1.2 by regarding exclusion of the anti-nutritional applying water washings during the UF and toxic components. process. Another point that is brought up by An extract in water obtained of primary cell the authors is that because dextran is wall polysaccharides from buriti fruit pulp branched the MWCO’s do not correspond (Mauritius flexuosa, a tropical palm) was to the obtained molecular mass. This was fractionated over regenerated cellulose checked with a HPGPC. membranes of 300 kDa, 100 kDa and 30 UF can also be used to purify kDa.(22) The retentates of the 100 kDa polysaccharide based vaccines. Using (100R) and 30 kDa (30R) membranes polyethersulphone based UF membranes contained homogeneous polysaccharides with MWCO between 30 and 1000 kDa the with an Mw of 126 kDa and 20 kDa vaccines were purified from the unreacted respectively. The 100R fraction was mainly polysaccharides.(19) composed by arabinose and uronic acid, Polysaccharides are often fractionated to indicating the presence of arabinan-rich simulate industrial production. In the work pectic polysaccharide. For the 30R fraction of by Xie et al. (20) the polysaccharide was the content of arabinose was half of the extracted from C. paliurus (commonly 100R fraction, balanced by uronic acids. called sweet tea tree). The ultrafiltration Mixtures of cyclodextrins (CD6 to CD60), through a polysulphone membrane is used produced from synthetic amylose by the as a possible replacement for conventional enzyme cyclodextrin glycosyltransferase process based on the combination of ethanol have been fractionated and purified using precipitation and gel UF and NF membranes.(23) CD6, CD7 and . By applying UF the CD8 can already be fractionated by other removal of lower molecular weight techniques, however with larger ring CDs polysaccharides improved the quality of the this purification does not work. Three polysaccharide. Three membranes are used membranes with a MWCO of 0.15-0.3, 1

7 and 2 kDa were tested with PEG and used. liquor), which can be separated by UF in The determined MWCO were 0.5, 1.8 and different MW fractions using TiO2 4.1 kDa. The diafiltration process was (ceramic) based membranes with cut-offs of modelled using simulation. The difference 5, 10 and 15 kDa.(25) In comparison with between the model and the results lies in alternative methods like successive glucose which was found in the retentate of extraction with organic solvents and the last filtration step. The authors gave the selective precipitation, UF gives the best explanation that with CD molecules with a results as the lignin obtained is less higher MW could form some host-guest contaminated with hemicelluloses. compounds. Although the obtained fractions have a glass Arabinoxylans can be extracted from transition of Tg = 105-110°C, they differ in destarched wheat bran. The extract is Mn being 940, 946, 1891 and 2032 Da at fractionated by three methods, the first one MWCO of 5, 10, 15 and >15 kDa is with NaOH and diafiltration with a UF respectively.(25) MWCO of 100 kDa. The second method is Another study by Sevastyanova et al. (16) a hydrothermal method followed by a using ceramic membranes of TiO2 and ZrO2 filtration. The third method is using the with MWCO of 1 and 5 kDa and a enzyme endoxylanase. In the latter case the regenerated cellulose membrane (used for solid is also treated like the first method, but the retentate of the 5 kDa UF) with a filtrate is fractionated by UF with a MWCO MWCO of 10 kDa showed different results. of 10 kDa and 100 kDa. This method led to The obtained fractions had a Tg ranging low extraction yields. The processes that from 70 to 170°C. The Mn for the various were tested were up scalable processes. The fractions ranges from 1200-9500 Da. The three different methods gave different kraft lignin hydrolysate of pH >13 was used composition and different properties. Both as such, in the temperature range 40-65°C, ultrafiltration and ethanol precipitation are such alkalinity and temperatures only being efficient and the choice of which one should possible with ceramic membranes. be used, should be based on the basis of OH O CH HO 3 financial and environmental arguments.(24) OH O O O 3.2 UF involving Lignin polymers CH3 O O Lignin is considered a waste product of HO which only 1.5% is commercialized. O O O However lignin can be applied as dispersant CH3 HO OH OH HO in the cement industry, as emulsifier or HO OH OH HO chelating agent for removing heavy metal O O O O from effluents.(25) Therefore it is of interest CH H3C 3 to isolate lignin from waste materials. O O CH Molecular mass is the key parameter 3 O O H3C affecting the reactivity and thermo- HO O mechanical behaviour of lignin, therefore CH3 fractionation is a key step to obtain a lignin OH with a narrow distribution of properties.(16) The alkaline hydrolysis of wood pulp Figure 7. Segment of lignin polymer affords a complex lignin mixture (black

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Wang et al.(26) obtained through the A similar organosolv treatment on wheat prehydrolysis kraft-based process on wood straw applied a combination of UF pulp a high grade dissolving pulp. In this (polysulphone and hydrophylized case however with UF the oligosaccharides, polyethersulphon) and NF to fractionate the lignin and monosaccharides components lignins. Fractions ranging from larger than could not be separated. Four filters were 150 kDa down to less than 200 Da were used with the MWCO of 30, 10, 3 and 1 isolated indicating much higher Mw than in kDa, yet remarkably almost all fractions the study on Miscanthus sinensis.(27) still contain oligosaccharide with a MW of Results have to be regarded with care as the 32 kDa. The explanation of the authors is authors mention insufficient conditioning that the oligosaccharide is linear and will go time for the 150 kDa membrane.(28) through the membranes.(26) An alternative to the alkaline and A disadvantage of the alkaline or kraft organosolv treatment is the acidic sulphite hydrolysis is that more material is dissolved process, carried out at 135-140°C and a pH due to the hydrolysis and partial hydrolysis of 1.2-1.5. In contrast to the alkaline process of the original lignin and oligosaccharides in the acidic sulphite process the occurs. By applying the organosolv hemicellulose is hydrolysed into treatment, an alcohol/water based monomers, therefore not contaminating the extraction process of biomass (non-woody UF lignin fractions. In particular using a fibres, Miscanthus sinensis L.) at 160°C, TiO2 ceramic membrane of 15 kDa affords more of the original lignin is retained and a higher concentration of lignin in the less cellulose is hydrolysed/dissolved. On a retentate, although the polydispersity index semi-production scale using ceramic is higher. The permeate affords a somewhat membranes with MWCO’s of 5, 10 and 15 improved polydispersity index, but has a kDa four fractions were isolated with Mn of lower lignin concentration.(29) 1050, 1270, 1260 and 1500 g/mol with a polydispersity index ranging from 1,3 to 3.3 UF involving miscellaneous 1.6, whereas the starting material showed a polymers Mn of 1150 and a polydispersity index of 1.9.(Table 2) Olive oil by-products present a large waste hazard, however contain high molecular Table 2: Results from the analysis of the polymers that, if isolated, could afford lignin fractions by GPC. added value. Hydrothermal treatment, chemical treatment and precipitation techniques afford a solution which gives two fractions by UF. The first fraction

below 10 kDa rich of pectic polymers and Reproduced from ref 27, Copyright Elsevier between 3 and 1 kDa rich of B.V. 2009 polysaccharides. The first fraction could be

used as it is, for example as gelling agent. In A higher cost than for the kraft lignin addition the second fraction got chemical fractionation was calculated, however the and enzymatic hydrolyses to get oligomers high quality and purity in conjunction with with different uses.(30) the low polydispersity make the lignin Second cheese whey, a by-product of whey produced via organosolv treatment a cheeses still contains valuable compounds potentially high added value product.(27) like proteins, peptides and oligosaccharides.

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Macedo et al. fractionated the whey with Over a decade ago research on UF with three UF membranes (cellulose regenerated polyimide membrane with several acetate, fluoropolymer and hydrophilic polydisperse polystyrene solutions in polysulphone) all with a MWCO of 10 ethylacetate was carried out.(34) kDa.(31) Comparing cellulose regenerated Ultrafiltration allows further identification acetate membrane and a fluoropolymer of complex compositions. For example, membrane best results regarding the coal tar pitch, after being fractionated by permeate flux were obtained using the solvent solubility in 3 fractions can be cellulose acetate membrane. Regarding further segmented by UF using various selectivity similar results were obtained MWCO cellulose membranes. In this case with the aforementioned membranes. The NMP was used as solvent. (9) higher flux for the cellulose acetate Graphene quantum dots are of interest membrane was thought to be attributed to because of their photoluminescence at the higher hydrophilicity of the latter.(31) exactly confined and tuneable The polyphenols, ellagitannins and wavelengths.(35) It is of critical importance anthocyanins, present in the tropical to control their size both in diameter and highland blackberry could play a role in layer number. UF membranes of 3 kDa and human nutrition and health.(32) Authors 10 kDa have been shown to be able to were able to isolate the aforementioned isolate single layer polyethyleneimine materials through UF using 6 membranes graphene quantum dots and bilayer with MWCO’s ranging from 1 to 150 kDa polyethyleneimine graphene quantum dots. achieving a purity of over 90%. UF, as a non-destructive method, was also 4. Critical conclusions used in determining the tannin fraction in wine that causes the astringency, the in- Although there is a continuous stream of mouth and a dry mouth after spitting publications about specific ultrafiltration effect.(33) The tannins were fractionated in subjects like desalination, waste water four fractions with different DP (degree of treatment and purification and polymerisation) range. The authors were concentrating in the food industry, there are able to establish that the largest tannins with much less articles that deal with the a DP range >30 subunits were responsible fractionation of polymers by using the for astringency effects. ultrafiltration technique. Most of the articles A study from Galanakis et al. investigated on fractionation of polymers discuss the fractionation of wine sludge by UF and naturally occurring substances which are the separation of co-extended extracted as a complex mixture from a components.(6) The employed polysulfone membranes were not able to fractionate certain type of biomass and through phenolic compounds, because separation ultrafiltration natural polymers which have was mainly affected by severe fouling a distinct molecular weight range are phenomena on the membrane surface and isolated. In many articles on fractionating less by a sieving mechanism. Non-polar polymers the UF technique is used in fluoropolymer membranes (MWCO of 1 combination with other purification steps kDa) successfully separated different like MF and water washing or other phenolic classes like hydroxycinnamic solvents. In particular for polymers of acids, flavonols and anthocyanins on the natural origin, researchers struggle with basis of polarity. The authors did not use the separating different types of polymers e.g. membranes sequentially.(6) mixtures of cellulose, lignin and

10 polysaccharides. Only in a few references a 3. S. Chen, L. Liu, J. Lu, Z. Han, Y. Xu, H. proper way of choosing the right membrane Mo, Clinical dextran purified by electric (pore size) was observed, leading in many ultrafiltration coupling with solvent other articles to disappointing results. crystallization, C. R. Chimie, 2008, 11 , 80 Surprisingly there is virtually no literature 4. Z.F.Cui, H.S. Muralidhara, Membrane in the last decade of the fractionation of technology, a practical guide to membrane synthetic polymers, although one would technology and applications in food and anticipate that ultrafiltration would be an bioprocessing, 2010, Elsevier Ltd. efficient way to isolate fractions of 5. Y. Pu, Q. Zou, L. Liu, Z. Han, X. Wang, polymers with a narrow chain length Q. Wang, S. Chen, Clinical dextran purified by fractional ultrafiltration coupled with distribution. water washing, Carbohydrate Polymers, Although UF has some nice advantages like 2012, 87, 1257 simplicity, low cost, energy efficiency, 6. C. M. Galanakis, E. Markouli, V. Gekas, application for sensitive materials and the Recovery and fractionation of different possibility to concentrate a polymer phenolic classes from winery sludge using solution, it has unfortunately also some ultrafiltration, Separation and Purification disadvantages, fouling being the most Technology, 2013, 107, 245 severe one. However there are several 7 . C . M. Galanakis , Separation of functional options described in literature to decrease macromolecules and micromolecules: From the fouling process. The literature ultrafiltration to the border of nanofiltration, concentrates both on new membranes and Trends in Food Science & Technology, 2015, 42, 44 on new techniques to prevent fouling. 8. M. Mulder, Basic Principles of Looking at the development of new , 1996, Kluwer membranes it might be anticipated that in academic publishers. the coming years new ways will be found to 9. A. George, T. J. Morgan, P. Alvarez, M. solve the challenges of ultrafiltration and Millan, A. A. Herod, R. Kandiyoti, will allow ultrafiltration to be used in a Fractionation of a coal tar pitch by ultra- broader field to purify and achieve more filtration, and characterization by size monodisperse polymers. exclusion chromatography, UV- In view of the ongoing development of new fluorescence and laser desorption-mass types of membranes better selectivity and spectroscopy, Fuel, 2010, 89, 2953 lower polydispersity of polymers might be 10. Y.-L. Su, W. Cheng, C. Li, Z. Jiang, expected. Preparation of antifouling ultrafiltration membranes with poly(ethylene glycol)- graft-polyacrylonitrile copolymers, Journal of Membrane Science, 2009, 329, 246. References 11. T. Hwang, J-S. Oh, W. Yim, J-D. Nam, C. Bae, H. Kim, K. J. Kim, Ultrafiltration 1. A. W. Mohammad, C. Yin Ng, Y. P. Lim, using graphene oxide surface-embedded G. Hong Ng, Ultrafiltration in Food polysulfone, Membranes, Separation and Processing Industry: Review on Purification Technology, 2016, 166, 41 Application, , and 12. G. Zhang, S. Lu, L. Zhang, Q. Meng, C. Fouling Control, Food Bioprocess Technol, Shen, J. Zhang, Novel polysulfone hybrid 2012, 5, 1143 ultrafiltration membrane prepared with 2. M. Cheryan, Ultrafiltration and TiO2-g-HEMA and its antifouling microfiltration handbook, 1998, Technomic characteristics, Journal of Membrane Publishing Co.,Inc Science, 2013, 436, 163

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