CA2092386A1 - Preparation of thin regenerated polyglucan films - Google Patents

Abstract

O.Z. 0050/43120 Abstract of the Disclosure: Thin regenerated polyglucan films useful as, for example, membranes are prepared by reaction of polyglucan with a silylating agent, film formation by the Langmuir-Blodgett technique or by spin coating, and desilylation by means of a gaseous mineral or organic acid.

Description

- 2 ~ ~ 2 3 8 6 o z . 0050/43l20 Preparation of thin regenerated polyqlucan films The present inv0ntion relates to a process for preparing thin regenerated polyglucan films by reaction with a silylating agent, film formation and desilylation.
Silyl derivatives of cellulo~e and processes for preparing fibers and films from regenerated cellulose are known not only from the paper by G. Greber and O. Paschinger in da~ Papier 35 (1981), 547-554, but also from DE-A-3 104 529. Cellulose is silylated with tri-methylchlorosilane and solutions of the silylated c21-lulose in organic solvents are convertsd into shaped articles. Concurrently or in a subsequent operation, the~e product~ are quantitatively desilylated in the presence of at lea~t stoichiometric amounts of water, preferably in an acidic medium. This prior art also mention~ a proces3 for preparing fibers wherein the ~olution of the o-trimethyl~ilyl-celluloses is shaped into fibers either by dry spinning with evaporation of the solvent and desilylation wi~h an acidic reagent or by wet spinning into a coayulating bath. The desilylation with an acidic reagent in the dry spinning process is effected either with acidic steam ~ven at the spinning stage or subsequently in an acidic water bath at various temperatures and draw ratios.
In Makromol. Chem. 191 (1990), 2985-2991, D. Klemm, ~. Schnabelrauch and A. Stein describe photo-reactive cellulo~e derivative3 having a comblike struc-ture. Here o-trialkylsilylcellulose is modified by e~terification with cinnamic acid.
Langmuir-Blodgett singla and mul~iple laysrs of amphiphilic csllulose esters having acyl ~roup~ o different chain length~- and different degrees of sub-~titution, ~uch a~ csllulo~e trioctanoate, tridecanoate, trilaurate, dilaurate and txipalmitate, are known from Journal of Colloid and Interface Science, 104 (1985), 290-293.
Finally, in Thin Solid Films 133 (1985), 29-38, - 2~23~
- 2 - O.z. 0050/43120 T. Kawaguchi, H. Nakahara and K. Fukuda report studies on monomolecular and multimolecular films of cellulose esters with different alkyl chains.
It is an object of the present invention to provide a process for preparing uniform thin regenerated polyglucan film~ by reaction of polyglucan with a silyla-ting agent, film formation and desilylation whereby, ideally in a ~Lmple mann~r, ùniform thin films can be reproducibly prepared and the silylating agents can be directly recovered.
We have found, surprisingly, that this ob~ect is achieved when thin films are prepared from silylated cellulose by the ~an~muir-Blodgett technique or by spin coating and desilylated by gaseou~ mineral or organic acids.
The present invention accordingly provides a process for preparing thin regenerated polyglucan films by reaction of polyglucan with a silylating agent, film formation and desilylation, which comprises effecting film formation by the Langmuir-Blodgett technique or by spin coating and desilylation by means of a gaseous mineral or organic acid.
Suitable silylating agents for the process of the invention are in particular compounds of the g~neral formula (I) or (II) R2--S 1--Hal R2--S i--N--S 1--R2 1 3 ¦ H

(I) (II) where Hal is chlorine, bromine or iodine t Rl and R2 are identical to or different from each other and each is alkyl of from 1 to 4 car~on atoms, and R3 i3 al~yl of from - 3 - O.Z. 0050/431~0 1 to 18 carbon atoms, phenyl, tolyl or benzyl, or a hexaalkyldisilazane, eg. hexamethyldisilazane.
Preferred gaseous mineral acids are HCl and HBr and preferred gaseous organic acids are monocarboxylic acids of from 1 to 3 carbon atom~.
The polyglucan3 to be used for the process of ~he invention, preferably celluloses, generally have degrees of polymerization (= DP) of from 30 to 3000 and the degree of substitution of the 3ilylated polyglucans or celluloses is within the range from 1 to 3.
Film formation can take placé on various sub-strate~, for ex~mple on metal, glas~, silicon or plastics surfaces, and the films may al~o be removed from these surfaces before desilylation.
The present invention al~o providas a method of using the thin regenerated polyglucan films prepared according to the invention as membranes for separating processes or sensor systems.
The disadvantages of regenerating cellulose ~0 fibers using aqueous acids, such as the difficulty of recovering the solvent used in the spinning prosess from the spinning baths, the minimal scope for varying the external conditions (eg. temperature and spinning speed) in the spinning process, and the Lmpossibility of a waterless process for direct recovery of the silylating reagents, are surmounted by the process of the invention, wherein regeneration i8 effected using a gaseous acid.
The proces~ of the invention provides a simple msthod for preparing, with direct recovery of the silylating agents of the general formula (I), uniformly thin dense compact regenerated polyglucan or cellulose films which have a wide range of US88 as such or mounted on substrates, for example for me~branes and sensor materials.
The process of the invention will now be more particularly described.
Suitable polyglucans for the proces~ of the ` 2~238~
- ~ - O.Z. 0050/43120 invention are for example cellulose, chitin and starch, suitable cellulose oomprising not only cotton linters but also wood pulp. The polyglucans have degrees of polymerization of from 30 to 3000, preferably from 100 to 1500.
The preparation of silylated cellulosa is known and described for exampl~ in J. Polym. Sci. Part A-l, (1969) 7(7) 1947-58 and Makromol. Chemie (1968) 120, 87-95.
The polyglucan or cellulose is reacted with a silylating agent, for example with compounds of the general formula (I) or (II) Rl Rl Rl I
R2 - Si - ~al R2 - Si- N - Si - R2 I l H
R3 R3 ~3 (I) (II) where Hal is chlorine, bromine or iodine, Rl and R2 are identical to or different from each other and each is alkyl of fxom 1 to 4 carbon atoms, and R3 is alkyl of from 1 to 18 carbon atoms, phenyl, tolyl or benzyl.
Preference i~ given to compounds of the general formula (I) whera Hal is Cl, Rl and R2 are each CH3 or C2H5 and R3 i~ C~3, (CH3)2CH-C(CH3)2 (- thexyl) or C~Hl7, and to corresponding compounds of the general ormula (II), for example hexamethyldisilazane.
This silylation of cellulose can be carried out in heterogeneous or homogeneous phase, in general in the presence of ~ases, for example prLmary or secondary amines, pyridine or ammonia.
Ins~ead of silylating agents of the general formula (I), who~e reac~ion with polyglucan~cellulose liberates hydrogen halide (HHal), i~ is al80 possible ~o 2~23~6 t - 5 - O.Z. 0050/43120 use silylating agents which silylate wikhout elLmination of HHal, for example disilazanes, eg. hexamethyldisila-zane.
The degree of silylation of the cellulose to be used according to the inven~ion can vary within wide lLmits and as a function of the carbon chain length of the radicals R1 to R3.
Silylated pol~glucan or cellulose whose sub-stituents R1 to R3 are each methyl can have degrees of substitution (= DS) of from 1 to 3; that is, from 1 to 3 OH groups are silylated, but in this case a degree of substitution of 2.3 to 2.8 is preferred. In the case of longer carbon chains in the ~ubstituents Rl to R3 a degree of substitution of from 1 to 2.5 can be sufficient for the silylated polyglucans to be suff.iciently soluble in apolar organic water-immiscible ~olvents, thereby making pos~ible further processing by the Langmuir-Blodgett technique or by spin coating.
Suitable apolar organic water-immiscible solvents in whish the silylated polyglucans ars soluble are for example fast evaporating aliphatic or aromatic hydrocarbons, such as hexane, benzene or cyclohexane, halo(hydro)carbons, such as chloroform or msthylene chloride, and esters, such a~ ethyl acetate.
The Langmuir-Blodgett technique~ suitable ap-paratus for it and the necessary precondition~ for it ~o be carried out are known, for example from G.L. Gaines, In oluble Monolayers at I-iquid-Ga~ Interfaces, Inter~cience Publishers, 1966.
An exten~ive reviPw of ~he literature can also be obtained for example from the proceedings of three conferences on Langmuir-Blodgett film~ published in the following volumes of Thin Solid Films: 160 (1988), 159 (198B), 132 134 (1985) and 99 (1983).
The silylated polyglucan~ are advantageously dissolved in vola~ile organic solvents, such as methylene chloride, chloroform, benzene/ hexane or ethyl acetate, - 2~923~
- 6 - O.Z. 0050/43120 in concentrations of, for example, from 0.01 to 1% by weight, the solution is spread on the sur~ace of the water in the Langmuir trough, the solvent is removed by evaporation, and the monolayer of silylated polyglucan is precompressed in a conventional manner before transfer to solid base materials.
This will in general be done at from 5 to 50C, preferably at from 5 to 30C.
The transfer is effected in a conventional mannex by dipping suitable cleaned bases through the water surface covered with the silylated polyglucan film or, if corresponding multilayers are to be produced, by successive transfer of further monolayers.
A suitable base material for the silylated polyglucan film is any solid, preferably dimensionally stable, substrate made of any one of a wide range of materials. Suitable substrates can be, for example, transparent or opaque, electrically conducting or in-sulating. The surface of the substrate to which the thin ~0 layer of silylated polyglucan is applied can be hydropho-bisized. The substrate may consist of a hydrophobic material, or before the thin layer of Rilyl~ted polyglucan is applied the surface of the subs~rate can be hydrophobisized in a conventional manner by suitable pretreatment. The hydrophobic substrate surface to be coated ~hould ideally be clean, ~o as not to disrupt the formation of a thin, ordered layer, in particular a monomolecular or multimolecular layer structure. For inst~nce, the presence of surface-active subs~ances on the substrate surface to be coated can impair the formation o~ a good monomolecular or multimolecular film.
However, before the thin layer o silylated polyglucan is applied the substrate surface to be coa~ed can be provided with an interlayer, for ex~mple for the purpose of achieving high adhesion between the firm thin layer of the silylated polyglucan and the substrate.
Examples of sui~able substra~e/base materials are 2~3~
- 7 - O.Z. 0050/~3120 metals, ~uch as gold, platinum, nickel, palladium, aluminum, chromium, niobium, tantalum, titanium, steel or the like. Other suitable materials for substrates include plastics, for example polyester, eg. polyethylene terephthalate or polybutylene terephthalate, polyvinyl chloride, polyvinylidene chloride and polytetrafluoro-ethylene.
It is similarly possible to use as substrate material silicon, glass, silicon dioxide or a ceramic material. The surface of glass substrates can, if nece~sary, be hydrophobisized in a conventional manner, for example by reaction with alkylsilanes. The choice of sub~trate material depends chiefly, inter alia, on the in~ended use of the layer element~ prepared according to the invention. Optical elements will in general have transparent -~ubstrates a~ base materials. If the layer elements prepared according to the invention are used for example in electrical engineering or in electrochemical proce~ses, the substrates used will be in particular electrically conductive materials, such as metals, or materials having electrically conductive, in particular metallic, surface layers, for example metallized plastics films.
The substrates used as base material~ may take any de~ired form, depending on the intended use. For example, they can be in film, sheet, tile or tape form or else be cylindrical or have some other desired shape. In general~ the base materials will be flat, planar sub-strates, eg. films, sheet3, foils, tiles, tapes, panels and the like. The sub~trate surfaca to be coated iq preferably smooth, as is customary fox the production of thin ordered layers of a well defined ~tructure, in particular monomolecular or multimolecular films. The flat planar substrate~, such as films, heets, tapes, etc., may have the novel ~hin ordered organic polymer layers of well defined structure applied to them on only one or on both surfaces. U5 ing the Langmuir-Blodgett 2~9238~
- 8 - O.Z. 0050/43120 technique it is thus possible, for example, to transfer some 1000 molecular layers to hydrophobic substrates.
It can be advantageous, immediately after the transfer of the monomolecular films from thP water surface to the substrate, to heat the layer element prepared according to the invention at elevated tempera-tures, in general within the range from 50 to 300C, preferably within the range of about 100-2~0C. This heating ~tep following the preparation of the layer elements of the invention can stabilize or else specifi-cally vary the propertie~ of the layer elements of the invention.
The formation of thin ilms of silylated polyglucan or cellulose can also be effected by spin coating. Spin coating is known per se and described for example in A. Weill, The Spin-Coating-Process Mechanism, in M.J. Kelly, The Physic~ and Fabrication of Micro3truc-tures and Microdevices, Springer Proceedings in Physics, Vol. 13, Springer 1986.
As regards the solvents to be used and the substrates to be coated, the remarks made in relation to the Langmuir-Blodgett technique apply mutati3 mutandis.
For application by spin coating the solutions of the silylated polyglucans can be from 0.5 to 25~ in strength.
Depending on the concentration of the solution and the speed, which in general ranges from 500 to 4000 rpm, preferably from 1000 to 2500 rpm, it is possible to prepare layers from 0.1 to 10 ~m in thickness.
To de~ilylate the uniform thin ~ilylated poly-glucan films prepared by Langmuir Blodgett or spin coating in a thicknes~ of from 0.1 to 10, preferably from O.3 to 4 ~m, they can either remain on the substra~e to which they have been applied or ba removed from this substrate.
According to the invention, desilylation i5 effected using a gaseous mineral acid, for example a halohydric acid, eg. 10-20~ s~rength or concentrated 2~238~
- 9 - o.z. 0050/43120 hydrochloric acid, or gaseous hydrogen bromida, or a gaseous organic acid, for example a volatile carboxylic acid, eg. formic acid, acetic acid or propionic acid.
To desilylate the silylated polyglucan film it is sufficient to hring the film for a short time, for example for from 10 seconds to 5 minutes, into an atmo-sphere containing the gaseous mineral or organic acid.
The temperature at which this is done can vary within relatively wide limits, for example within the range from 0 to 100C, in particular from 5 to 50C.
If the silylating agents used are of the general formula (I) and the desilylating agents used are hydxogen halides, the silylating agent can be recovered.
Evidence of the desilylation of the regenerated polyglucan film~ can be obtained from IR spectroscopy, for example FTIR (= Fourier Transform IR). Information about the layer geometry of the regenerated polyglucan films can be obtained from X-ray diffraction.
The process of the invention makes it possible for the first time to obtain ultrathin hydxophilic layers which are resistant to aqueous, polar and apolar solvents and usable for example for fabricating membranes and biocompatible coatings, for example biosensors. Further examples of possible uses of regenerated cellulose films prepared according to the invention are separating me~branes, ion-selective sensors and also the metallization of polymer surfaces.
The invention will now be more particularly described by way of example. Parts and percentages are by weight, unles~ otherwi~e stated.

In each case, solutions were prepared of 3 mg of a trialkylsilylcellulose in 10 ml of chlorofonm or n-h~xane and spread at a subphase temperature of 20C
onto the water/air boundary surface o~ a film balance.
Fig. 1 shows the pressllre-area isotherms of trimethyl-silylcellulose and Fig. 2 those of .

209~3~
- 10 - o.z. 0050/43120 ~hexyldimethylsilylcellulose. It can be seen that the collapse of the monolayer takes place in both cases only above 25 mNtm.
Lanmuir-Blodgett (= LB) multilayers were pre-pared using the following transfer parameter~:
Subphase temperature: 20C
Surface tension: 20 mN/m Dip speed~ 1-20 cm/min The substrates used were ordinary glass micro-scope slides (hydrophobisized with HMDS (= hexamethyl-disilazane)) or silicon wafers (hydrophobisized with HMDS
or NH4F). The transfer ratio when dipping the substrates in and out was 100~5%.
To regenerate the cellulose the LB layers were 15held for 10-60 s into the gas atmosphere above 18%
strength hydrochloric acid. IR spectroscopy showed quantitative elimination of the trialkylsilyl side groups.

~0To prepare a film by ~pin coating a solution of 200 mg of trimethylsilylcellulose in 7 ml of chloroform was dripped onto a hydrophilic silicon disk; a speed of 2000 rpm resulted in a layer thickness of 3000 Ao The film was held into the ga~ atmosphere of 25concentrated HCl for 30 second3. IR spectroscopy showed quantitative elimination of the trimethylsilyl groups.

FTIR spectra for monitoring cellulose regenera-tion:
30By means of FTIR spectroscopy it is possible to monitor thP elimination of the side groups from trialkyl~
ilylcellulose and thus the regeneration of cellulose in LB and spun-coated films.
Fig. 3 and Fig. 4 show FTIR spectra of an LB film 35comprising B0 molecular layers of trimethylsilylcellulose (total layer ~hickness about 712 ~) recorded with radia-tion polaxized respec~ively parallel and perpendicular to 2~238~
~ O.Z. 0050/43120 the dip direction of the substrate. The dichroism of the bands between 1000 cm~' and 1200 cm~~ reveals a preferred orientation of the polymer chains parallel to the dip direction.
After treatment of the sample with gaseous HCl it had the FTIR spectra depicted in Fig. 5 and Fig. 6. It can be seen that the bands of the ilyl side groups (Si-C
vibration at 1250 cm~1 and 898 cm~) have disappeared. The intensity of the C-H vibration bands between 2900 and 3000 cm~' has decreased, while the intensity of the OH
band has sharply increased. Desilylation accordingly leaves an FTIR spectrum of a cellulose in which the preferred orientation of the polymer chains parallel to the dip direction has been preserved.
The cellulose in a spun-coated film 3000 ~ in thickness was regenerated in similar fashion. Fig. 7 and Fig. 8 show FTIR spectra (unpolarized) respectively before and after desilylatlon. Regeneration of the cellulose has produced the same characteristic changes ~0 in the FTI~ spectrum as in the corresponding LB films.
This measurement shows that even relatively thick film~
can be desilylated quantitatively within a very short time by treatment with gaseous HCl.
EX~MPLE 5 X-ray reflection measurements for determining layer parameterss The reflection of X-rays at LB films provides information about layer thickness, periodicity and suriace quality (ref.: F. Rieu~ord, J.J. Benattar, L. Bosio, P. Robin, C. Blot, R. de KouchkovsXy, J.
Physiqu~ 48 (1987), 679).
Fig. 9 shows the X-ray reflection curve of an LB
film comprising 50 layers of trimethylsilylcellulose.
Evaluation cf the cur~e reveals the following film parameters:
Periodicity: 17.8 A
Surface roughness: 7 :

~323~6 - 12 - O.Z. 0050/43120 Total layer thickness: 445 A
The reflection curve indicates a homogeneous film structure with a smooth surface and a periodic arrangement of the trimethylsilylcellulose layexs (or bilayers). Cellulose regeneration with gaseous HCl produced the reflection curve in Fig. 10, which indicates the following parameters:
Surface roughness: 9 A
Total layer thickness: 198 A
Desilylation has reduced the thickness of the film by more than 50%, and surface roughness is increased 31igh~1y. A periodic layer structure i~ no longer ob-served. However, the reflection curve still indicates homogeneous coverage of the substrate. Evidently, the process of regenerating the cellulose has not created any ma~or inhomogeneities or defects in the LB film.

` 20~38~
- 13 - O. Z . 0050/43120 Fig. 1: Pressure-area isothe~ms of trimethyl-silylcellulose (DS = 2.8; DP = 150) at a subphase temper-ature of 20C
Fig. 2: Pressure-area isotherms of thexyldi-methylsilylcellulose (DS = 1.6; DP = 200) at a subphasetemperature of 20~C
Fig. 3: FTIR spectrum of an LB film of 80 layers of trimethylsilylcellulose (DS = 2.8; DP = 150) on silicon. The LB film wa~ prépared as de~cribed in Example 3. The electrical field vector of the I~ radiation is oriented parallel to the dip direction of the sub~trate.
Fig.4: FTIR spectrum of an LB film of 80 layers of trimethyl~ilylcellulose (DS = 2.8; DP = 150) on silicon. The hB film wa~ prepared as described in Example 3. The electrical field vector of the IR radiation is oriented pexpendicular to the dip direction of the substrate.
Fig. 5: FTIR spectrum of an ultrathin film of regenerated cellulose (prepared as described in Example 2) with polarization of the electric field ~ector parallel to the dip direction of the substrate.
Fig. 6: FTIR spectrum of an ultrathin film o regenerated cellulose (prepared as described in Example ~) with polarization of the electric field vector perpendicular to the dip direction of the substrate.
Fig. 7, FTIR spectrum of a spun~coated film of trimethylsilylcellulose (DS = 2.8; DP = 150) prepared as described in Example 3.
Fig. 8: FTIR ~pectrum of regenerated cellulo~e film prepared a~ de~cribed in Example 3.
Fig. 9: X-ray reflection curve of an LB film comprising 50 layer~ of trLmethylsilylc211ulo e (DS =
2.8; DP = 150), prepared as described in Example 2.
Fig. 10: X-ray reflection curve of an ultrathin regenerated cellulose film prepared as de~crib2d in Example 20 , .

Claims (7)

1. A process for preparing thin regenerated polyglucan films by reaction of polyglucan with a silylating agent, film formation and desilylation, which comprises effect-ing film formation by the Langmuir-Blodgett technique or by spin coating and desilylation by means of a gaseous mineral or organic acid.

2. A process as claimed in claim 1, wherein the silyla-ting agent used is a compound of the general formula (I) or (II) (I) (II) where Hal is chlorine, bromine or iodine, R1 and R2 are identical to or different from each other and each is alkyl of from 1 to 4 carbon atoms, and R3 is alkyl of from 1 to 18 carbon atoms, phenyl, tolyl or benzyl.

3. A process as claimed in claim 1, wherein a gaseous mineral acid is used comprising HCl or HBr.

4. A process as claimed in claim 1, wherein a gaseous organic acid is used comprising a monocarboxylic acid of from 1 to 3 carbon atoms.

5. A process as claimed in claim 1, wherein the poly-glucan used is a cellulose having a degree of polymeriza-tion of from 30 to 3000.

6. A process as claimed in claim 1, wherein the degree of substitution of the silylated polyglucan is within the range from 1 to 3.

7. A process as claimed in claim 1, wherein film forma-tion is effected on a metal, glass, silicon or plastics surface and before desilylation the film is optionally removed from the said surface.