Multilayer „ films

Different objects may be used as the sacrificial core to prepare hollow capsules, e.g. particles of polymers e.g., melamine-formaldehyde resin, poly(sodium styrenesulfonate) (PSSS) ], inorganic compounds [e.g., CaCOg or SiOa ), metal nanoparticles, or even bio-objects such as erythrocytes or platelets. Multilayers can be also deposited directly on the nano- or microcrystals of drugs. 

Schematic representation of the various possible ways to obtain drug-loaded, polymer-coated nano/microparticulate DDSs. 

Polycations used for the preparation of multilayer films in DDSs include natural polymers such as chitosan or protamine/ and synthetic polymers, e.g. poly(4-vinylpyridine) (P4VP), polyaromatic 

The permeability of the films coating the capsules for the released active substances may be controlled by the number of layers. It was found that the permeability decreases with increasing number of layers (thickness). When the film reaches a certain thickness, the permeability coefficient remains constant with further increase of film thickness. 

The release rate of the drug may be drastically limited by coating the drug microctystals with a multilayer film. For example, the release rate of furosemide microcrystals coated with PDADMAC, PSSS, and gelatin was reduced 50-300 times compared to uncoated furosemide for up to 16 layers 

The LLDPE layers provide the bulk of the film body, while the tie layers adhere to both the LLDPE and the oxygen barrier polymer allowing the film to maintain a strong structural foundation. The oxygen barrier layer exhibits low permeability to oxygen to increase shelf-life of the package. 

An extruder consists of a heated barrel containing a screw that turns to mix the molten polyethylene to form a homogeneous, amorphous material that passes through to an exit die. 

The die is a circular apparatus with a slit around the circumference of the die where the molten polyethylene exits the extruder to begin the blown-film-forming step. 

As the polyethylene bubble passes under the guide tent the bubble is collapsed to a two-layer fihn. 

The rotation rate of the nip rollers determines the final thickness of the polyethylene film. For example, if the nip rollers rotate at a rate ten times the rate the molten polyethylene exits the die, and then the molten polyethylene is reduced hy an additional factor of ten. In our example above, the 10.0 mil thick polyethylene would he reduced to a final thickness of 1.0 miL Reducing the thickness of the molten polyethylene in the direction parallel to the movement of the bubble is referred to as the Machine Direction (MD). 

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The self-assembly process can be continued to form multilayer films of up to 25 layers [33,48,49]. The reliability of this process is illustrated in Fig. XI-3, where the thickness grows linearly with the number of reacted layers. These thick layers have many interesting applications. 

The first term on the right is the common inverse cube law, the second is taken to be the empirically more important form for moderate film thickness (and also conforms to the polarization model, Section XVII-7C), and the last term allows for structural perturbation in the adsorbed film relative to bulk liquid adsorbate. In effect, the vapor pressure of a thin multilayer film is taken to be P and to relax toward P as the film thickens. The equation has been useful in relating adsorption isotherms to contact angle behavior (see Section X-7). Roy and Halsey [73] have used a similar equation earlier, Halsey [74] allowed for surface heterogeneity by assuming a distribution of Uq values in Eq. XVII-79. Dubinin s equation (Eq. XVII-75) has been mentioned another variant has been used by Bonnetain and co-workers [7S]. 

Clearly, it is more desirable somehow to obtain detailed structural information on multilayer films so as perhaps to settle the problem of how properly to construct the potential function. Some attempts have been made to develop statistical mechanical other theoretical treatments of condensed layers in a potential field success has been reasonable (see Refs. 142, 143). 

Ellipsometry measurements can provide infomiation about the thickness, microroughness and dielectric ftinction of thin films. It can also provide infomiation on the depth profile of multilayer stmctiires non-destmctively, including the thickness, the composition and the degree of crystallinity of each layer [39]. The measurement of the various components of a complex multilayered film is illustrated m figure Bl.26.17 [40]. 

Girling I R and Milverton D R J 1984 A method for the preparation of an alternating multilayers film Thin Solid Films 115 85-8... 

The volume i>r,) of the multilayer film on the walls of all pores which have already given up their capillary condensate is... 

Film or sheet generally function as supports for other materials, as barriers or covers such as packaging, as insulation, or as materials of constmction. The uses depend on the unique combination of properties of the specific resins or plastic materials chosen. When multilayer films or sheets are made, the product properties can be varied to meet almost any need. Further modification of properties can be achieved by use of such additives or modifiers as plasticizers (qv), antistatic agents (qv), fire retardants, sHp agents, uv and thermal stabilizers, dyes (qv) or pigments (qv), and biodegradable activators. 

Melt Extrusion. By far the most important method for producing film and sheeting materials reties on one or another of the various melt extmsion techniques (5). The main variations of melt extmsion are the slot (or flat) die-cast film process, the blown films process, and the flat die sheeting-stack process. These may be combined with one or more steps such as coextmsion wherein multilayer film or sheet is formed, biaxial orientation, and in-line coating (6). 

Free mono- and multilayer films may be adhesive- or extmsion-bonded in the laminating process. The bonding adhesive may be water- or solvent-based. Alternatively, a temperature-dependent polymer-based adhesive without solvent may be heated and set by cooling. In extmsion lamination, a film of a thermoplastic such as polyethylene is extmded as a bond between the two flat materials, which are brought together between a chilled and backup roU. 

Antireflective (AR) coatings are required on optics to reduce the reflective surface losses. Vitreous siHca coatings in the form of porous or multilayer films are used extensively in this appHcation. Antireflective coatings have been developed which employ coUoidal fused siHca sol—gel particles made from organometaUic materials (253). 

Fig. 4. Multilayer films where ( ) represent a hydrophobic group and (B) a hydrophilic one (a) Y-type, (b) X-type, and (c) Z-type.

Constmction of multilayers requires that the monolayer surface be modified to a hydroxylated one. Such surfaces can be prepared by a chemical reaction and the conversion of a nonpolar terminal group to a hydroxyl group. Examples of such reactions are the LiAlH reduction of a surface ester group (165), the hydroboration—oxidation of a terminal vinyl group (127,163), and the conversion of a surface bromide using silver chemistry (200). Once a subsequent monolayer is adsorbed on the "activated" monolayer, multilayer films may be built by repetition of this process (Fig. 8). 

Using this strategy, constmction of multilayer films of - O. fim thickness by self-assembly of methyl 23-ttichlorosilyltticosanoate (MTST) on siUcon substrates has been demonstrated (Fig. 9) (165). The linear relationship between the film thickness and the layer number showed a slope of 3.5 nm /layer. Filipsometry data, absorbance intensities, and dichroic ratios for the multilayers all suggest that the samples were composed of distinct monolayers. However, ir data indicated that there maybe more tilting or disordering of the alkyl chains in the seven-layer sample than for the monolayer samples. 

Fig. 10. Formation of noncentrosymmetric multilayer film by combining self-assembly and a surface S 2 reaction, where R = (CH2)30H procedure I = spin-coating followed by annealing at 110°C and procedure II = reaction of Cl2Si0SiCl20SiCl2, ie, a dilute solution of 4-[A/,A/,-bis-(3-hydroxyprop5l)-aminophenylazo]-4 -pyridine on a benzyl chloride SAM surface was used, resulting in facile formation of SAMs having high...

During the remainder of the 1930s, Langmuir and Blodgett carried out a brilliant series of studies on multilayer films of a variety of chemicals, supplemented by studies in Britain, especially at the ill-fated Department of Colloid Science in Cambridge (Section 2.1.4). Then the War came, and momentum was lost for a couple of decades. After that, L-B films came back as a major topic of research and have been so ever since (Mort 1980). It is current practice to refer to molecular films, made by various techniques (Swalen 1991), but the L-B approach remains central. 

To begin with, let us eonsider an adsorption system, i.e., the film formed on the surfaee of the total area A and held at the temperature T. For the sake of simplieity, we also assume that the film is only one atomie layer thiek. The diseussion of multilayer films on heterogeneous surfaees is presented in the next seetion of this ehapter. 

To present briefly the different possible scenarios for the growth of multilayer films on a homogeneous surface, it is very convenient to use a simple lattice gas model language [168]. Assuming that the surface is a two-dimensional square lattice of sites and that also the entire space above the surface is divided into small elements, forming a cubic lattice such that each of the cells can be occupied by one adsorbate particle at the most, the Hamiltonian of the system can be written as [168,169]... 

It has been demonstrated in a very elegant way by Pandit et al. [168] that mechanism of multilayer film formation depends crucially on the ratio Vq/u. When this ratio is sufiiciently high, i.e., when the surface is highly attractive towards the adsorbate, the film grows in a layer-by-layer mode. At low temperatures this corresponds to the presence of a series of layering... 

In this chapter, a brief introduction will be given mainly to the DEC, CNx, multilayer films and nano-composite coatings. Detailed and comprehensive introduction of the conventional thin solid coating technique is not the objective of this chapter. Readers are referred to relevant publications to attain the knowledge in this area. 

Fig. 11—Schematics of various deposition methods of multilayer films.

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