Technical Field [1] The present invention relates to an acrylic pressure-sensitive adhesive composition for a polarizing film, more particularly to a pressure-sensitive adhesive composition for a polarizing film capable of solving the light leakage problem, a polarizing film prepared therefrom and a liquid crystal display device using the same. Background Art [2] In general a liquid crystal cell containing liquid crystals and a polarizing film are basically needed to manufacture a liquid crystal display device. An adequate adhesive layer or a pressure-sensitive adhesive layer is required to join them. In addition, a phase retardation film, a compensation film for wide view angle, a brightness enhancement film, etc. may be further joined to the polarizing film in order to improve the performance of a liquid crystal display device. [3] Typically, a liquid crystal display comprises a uniformly aligned liquid crystal layer; a polarizing film comprising a liquid crystal ceil, which consists of a transparent glass plate or a plastic plate including a transparent electrode layer and an adhesive layer or a pressure-sensitive adhesive layer; a phase retardation film; and additional functional film layers, [4] The polarizing film contains regularly aligned iodine compounds or dichromic polarizing materials and has a multi-layer structure to protect these polarizing elements by forming protecting films made of triacetyl cellulose (TAC), etc. on both sides of the film. The polarizing film may further comprise a phase retardation film having an anisotropic molecular alignment, a compensation film for wide view angle such as an optically designed liquid crystal film, etc. [5] Because these films are made of materials having different molecular structures and compositions, they have different physical properties. Especially, under a specific thermal and/or humidity condition, materials having an anisotropic molecular alignment shrink or expand, which causes the lack of dimensional stability. As a result, if the polarizing film is fixed by a pressure-sensitive adhesive, a shear stress caused (he thermal and/or humidity condition remains, so that light leakage occurs at the region where the stress is concentrated. [6] One way to solve the problem of light leakage is to reduce the shrinkage of the polarizing film at the thermal and/or humidity condition. However, if is very difficult to remove the stress applied to a liquid crystal panel to which a polarizing film consisting of different materials has been attached. [7] Rubbers, acryls and silicones are commonly used as pressure-sensitive adhcsives. Among these, acrylic pressure-sensitive adhesives are used the most widely in manu­ facturing high performance pressure-sensitive adhesive compositions. Molecular char­ acteristics of adhesive materials, such as molecular weight and distribution, cross linking density, and composition, mainly influence the durability of pressure-sensitive adhesives, which may be controlled by the high adhesive strength and cohesive strength. [8] However, when a polarizing film manufactured using such a pressure-sensitive adhesive is used for a long time under a specific thermal and/or humidity condition in a liquid crystal display, stress tends to concentrate because of the shrinkage of the polarizing film. To solve this problem, the adhesive layer needs to have a stress relaxation ability. (9) For this purpose, Japan Patent Publication No. Hei 1-66283 disclosed a polarizing film employing an acrylic pressure-sensitive adhesive layer mainly comprising (meth)acrylic acid alkyl ester having alkyl groups with 1-12 carbon atoms, in which the pressure-sensitive adhesive layer comprises the polymer constituent having a weight-average molecular weight of at most 100,000 in less than 15 wi% and the polymer constituent having a weight-average molecular weight of at least 1,000,000 in more than 10 wt%. Although the polarizing film employing the adhesive comprising a lot of polymer constituent having a high molecular weight has good adhesion durability, it is difficult to relax the stress caused by the dimensional change of the polarizing film because the adhesive has large cohesive force (elasticity). [10] Korea Patent Publication No. 1998-079266 attempted to solve the problem of light leakage by offering a stress-relaxation property to the pressure-sensitive adhesive composition, which comprises 100 parts by weight of a high molecular weight acrylic copolymer having a weight-average molecular weight of at least 1,000,000,20-200 parts by weight of a low molecular weight acrylic copolymer having a weight-average molecular weight of at most 30,000 and 0.005-5 parts by weight of a multifunctional crosslinking agent. However, because of the too high content of the low molecular weight acrylic copolymer having a molecular weight of at most 30,000 and lack of compatibility, it is difficult to ensure durability as bubble and edge lifting at high temperature and high humidity. In particular, the patent did not mention the method of preventing the durability problem caused by surface transference of the material having a low molecular weight after a considerable lapse of time. [11] Japan Patent Publication No, 2002-47468 attempted to offer stress-relaxation with a pressure-sensitive adhesive composition for a polarizing film comprising 100 parts by weight of a high molecular weight acrylic copolymer having a weight-average molecular weight ranging from 800.000 lo 2,000,000, 5-50 parts by weigh! of a low molecular weight acrylic copolymer having a with no functional groups and weight-average molecular weight of at most 50,000 or having a degree of dispersion ranging from 1.0 lo 2.5 and having no functional groups, a crosslinking agent and a silane compound. However, it did not mention the problems of non-homogeneous adhesion strength and durability after a prolonged time due to surface transference of the ma terial having a low molecular weight and compatibility with the material having a high molecular weight. [12) Japan Patent Publication No. 2003-49141 attempted to solve the problem of light leakage by offering stress-relaxation with a pressure-sensitive adhesive composition for a polarizing film comprising a high molecular weight acrylic copolymer having a weight-average molecular weight ranging from 1,000,000 to 2,000,000 and having functional groups, a medium molecular weight acrylic copolytncr having a weight-average molecular weight ranging from 30,000 to 300,000 and having less than two functional groups, a low molecular weight acrylic copolymer having a weight-average molecular weight ranging from 1,000 lo 20.000 (degree of dispersion = 1.0-2.5) and having no functional groups and a crosslinking agent. While this patent employs the acrylic copolymer having a medium molecular weight and having functional groups, the acrylic copolymer having a low molecular weight and having no functional groups may cause durability problem after a prolonged time because of .surface transference. To solve this problem, the acrylic copolymer having a high molecular weight and the acrylic copolymer having a low molecular weight should be compatible with each other. However, this patent did not mention it. [ 131 It is also possible to control (he crosslinking density by chemical bonding in order to offer stress-relaxation to the pressure-sensitive adhesive. However, it is insufficient to solve the stress concentration problem of the polarizing film. Moreover, if the crosslinking density is too low, the durability problem may occur. [14] Accordingly, a novel pressure-sensitive adhesive for a polarizing film capable of solving the light leakage problem while maintaining major characteristics of the polarizing film even at high temperature and humidity conditions and after a prolonged time and a polarizing film using the same are urgently needed, Disclosure of Invention Technical Solution [ 15] It is an object of the present invention to provide an acrylic pressure-sensitive adhesive composition for a polarizing film capable of solving the light leakage problem while maintaining such major characteristics as adhesion durability under high temperature and high humidity conditions. [16] It is another object of the present invention to provide a polarizing film using the acrylic pressure-sensitive adhesive composition. [17J It is still another object of the present invention to provide a liquid crystal display device comprising the polarizing film prepared with the acrylic pressure-sensitive adhesive composition. [18] The above objects of the present invention can be attained by the present invention. [19] In order to attain the objects, the present invention provides an acrylic pressure- sensitive adhesive composition for a polarizing film comprising a) 100 parts by weight of a high molecular weight (meth)acrylic copolymer copolymerized from a (meth)acrylic acid ester monomer and a vinylic and/or an acrylic monomer crosslinkable with a mutifunctional crosslinking agent and having a weight-average molecular weight of at least 1,000,000; b) 5-20 parts by weight of a low moleculer weight (meth)acrylic copolymer comprising the same monomer comprised in the main chain of the high moleculer weight (meth)acrylic copolymer of a), having a weight-average molecular weight ranging from 2,000 to 30,000 and comprising 0.01-1 monomer, per one molecule of the copolymer on average, having a functional group capable of reacting with the crosslinking agent of c); and c) 0.01-10 parts by weight of a mutifunctional crosslinking agent. wherein the multifunctional crosslinking agent is at least one selected from the group consisting of an isocyanate compound, an epoxy compound, an aziridine compound and a metal chelate compound. [20] The present invention also provides polarizing film comprising an adhesive layer of the acrylic pressure-sensitive adhesive composition on one or both sides of the polarizing film. [21] The present invention further provides a liquid crystal display device comprising a liquid crystal panel, in which the pressure-sensitive adhesive polarizing film is joined on one or both sides of the liquid crystal cell, [22] Hereunder is gi ven a detailed description of the present invention, [23] Conventionally, in order to solving the light leakage problem, a plasticizer or, optionally, a copolymer having a low molecular weight was added to a copolymer having a high molecular weight to offer stress-relaxation ability to a pressure-sensitive adhesive. The existence of the copolymer having a low molecular weight in the network formed by the copolymer having a high molecular weight increases fluidity and relaxes the stress concentrated by shrinkage or expansion of the polarizing film, thereby solving the light leakage problem. However, the copolymer having a low molecular weight may cause the durability problem after a prolonged lime because of surface transference. [24] The present invention attempts to improve compatibility with the copolymer having a high molecular weight in order to solve the durability problem caused by surface transference of the copolymer having a low molecular weight. That is, the monomer constituting the main chain of the copolymer having a low molecular weight should be equal to thai of the copolymer having a high molecular weight. Since the copolymer having a low molecular weight has 0.01-1 functional group, per one molecule on average, capable of reacting with the multifunctional crosslinking agent, the durability problem due to surface transference of the copolymer having a low molecular weight docs not occur. Also, because the copolymer having a low molecular weight does not form a network structure, concentration of stress can he effectively prevented. Con­ sequently, durability and the light leakage problem can be improved simultaneously. [25] The pressure-sensitive adhesive composition of the present invention comprises a (meth)acrylic copolymer having a high molecular weight, a (meth)acrylic copolymer having a low molecular weight and a multifunctional crosslinking agent. The (meth) acrylic copolymer having a low molecular weight comprises the same (meth}acrylic monomer having a reactive functional group as that of the (meth)acrylic copolymer having a high molecular weight. That is to say, since the (melh)acrylic copolymer having a low molecular weight has 0.01-1 functional group on average per one molecule, surface transference of Ihe copolymer having a low molecular weight can be prevented. Thus, the light leakage problem is solved by the stress-relaxation ability, while satisfying the adhesion durability. [26] The adhesive composition of the present invention can be used in a variety of pressure-sensitive adhesives or adhesives for optical purposes without limitation, including acryls, silicons, ruhberys, urcthanes, polyesters and epoxy compounds, Among these, the acrylic adhesive is preferable. (27] Hereunder is given a specific description of each component of the pressure- sensitive adhesive composition. |28J a) The (meth)acrylic copolymer having a high molecular weight is a copolymer comprising a (rneth}acrylic acid ester monomer as main constituent and a vinylic and/ or an acrylic monomer having a functional group crosslinkable with a multifunctional crosslinking agent. It comprises 91.0-99.9 parts by weight of the (meth)acrylic acid ester monomer and 0.1-9 parts by weight of the vinylic and/or the acrylic monomer having a functional group crosslinkable with a multifunctional crosslinking agent. [29] If the alkyl group of the (meth)acrylic acid ester monomer, which is the base monomer of a) the (meth)acrylic copolymer, has the form of a long chain, cohesive force of the adhesive decreases at high temperature. Thus, the (meth)acrylk acid ester monomer preferably has an alkyl group having 1-12 carbon atoms, in order to maintain good cohesive force at high temperature. More preferably, it is an alkyl ester having 2-8 carbon atoms, such as butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, ethyl (meth)acrylate, methyl (melh)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, isononyl (meth)acrylatc, 2-ethyIbutyl 45 % in ethyj acctale, a polymerization initiator to obtain a (meih)acrylic copolymer, L-J, which had a solid content of 45 wt% and a weight-average molecular weight of 10,000, [68f Preparation step 4: Preparation of acrylic copolymers L-2 to L-5 [69] Acrylic copolymers having low molecular weight^ L-2 to L-5 were prepared in the same manner of Preparation step 3, except that the content of each constituent was changed, as specified in Table 1. The result is shown in Table 1. [70] [Examplel] [71] [72] 100 pans by weight [74] The adhesive layer was adhered to an iodine based polarizing film having a thickness of 185 microns. The resultant polarizing film was cut an appropriate size for testing. The test result is given in Table 2 below. [75] (Testing Example] [76] Durability [77] The polarizing film (90 mm x 170 mm) prepared in Example 1 was attached on both sides of a glass plate (110 mm x 190 mm x 0.7 mm), wilh the light absorbing axes perpendicular to each other. In the process, a pressure of about 5 kg/cm2 was applied in a clean room, so that bubble or contamination did not occur. The samples were kept under the condition of 60 °C and 90 % R.H. for 1,000 hours. It was identified if bubble and edge lifting had occurred. Heal resistance was evaluated by observing bubble and edge lifting after keeping the samples at 80 "C for 1,000 hours. Observation was performed after keeping the samples at room temperature for 24 hours, Durability was evaluated by observing the adhesive polarizing film 5 months later. Durability evaluation standard was as follows. [78] O: No bubbles and no edge lifting were observed [79] A: A few bubbles and a little edge lifting were observed 180] x: A large amount of bubbles and edge lifting were observed [81] Light transmission homogeneity (light leakage) 182] Using the same samples, it was observed if light passed through the samples using backlight in a dark room. In the test, the polarizing films (200 mm x 200 mm) was attached on both sides of a glass plate (210 mm x 210 mm x 0,7 mm), with me light absorbing axes perpendicular to each other. The standard for light transmission ho­mogeneity was as follows. [83] O: No light leaking was observed by the naked eye. 184] A: A little non-uniform light transmission was observed. [85] X: A severe light leaking from the edge of polarizers was observed. [86] [Examples 2 and 3] [87] Mixing and lamination were performed as Example 1, as specified in Table 2. Durabilify and light transmission homogeneity were evaluated. The result is given in Table 2. [88] [Comparative Examples 1-6] [89] Mixing and lamination were performed as Example i, as specified in Table 2. Durability and light transmission homogeneity were evaluated. The result is given in Table 2. [90] Table 1 (Table Removed) [92] As seen in Table 2, all of Examples 1-3 showed good durability and light transmission homogeneity. On the contrary, Comparative Example 1 (the (meth)acrylic copolynter having a weight-average molecular weight of 800,000), Comparative Example 2 (including 30 parts by weight of the (meth)acrylic copolymer having a low molecular weight) and Comparative Example 6 (in which the monomer for copolymerizing the (meth)acrylic copolymer having a high molecular weight and the one for copolymerizing the (meth)acrylic copolymer having a low molecular weight arc different) showed very poor durability. Also, Comparative Example 3 (including 2 parts by weight of the (meth)acrylic copoiymer having a low molecular weight), Comparative Exampk 4 (in which the number of the reactive functional groups of the (meth)acrylic copolymer having a low molecular weight is not 0.01-1) and Comparative Example 5 (in which the weight-average molecular weight of the (meth)acrylic copolymer having a low molecular weight falls outside 2,000-30,000) showed slight or severe light transmission non-homogeneily. Industrial Applicability 193) (94} The present invention provides an acrylic pressure-sensitive adhesive composition for a polarizing film capable of solving the light leakage problem by relaxing the stress caused by shrinkage of the polarizing film under thermal and humidity conditions, without sacrificing such major characteristics as adhesion durability. Accordingly, if the pressure-sensitive adhesive composition of the present invention is used for the polarizing film of a liquid crystal display device, it can prevent light leakage caused by stress concentration, even after a considerable lapse of time. [95] While the present invention has been described with reference to the preferred em- bodiments, those skilled in the art will appreciate that various modifications and sub­stitutions can be made thereto without deponing from the spirit and scope of the present invention as sei forth in the appended claims. We claim: 1. An acrylic pressure-sensitive adhesive composition for a polarizing film comprising a) 100 parts by weight of a (meth)acrylic copolymer having a high molecular weight, which is copolymerized from a (meth)acrylic acid ester monomer and a vinylic and/or an acrylic monomer having a functional group capable of crosslinking with a multifunctional crosslinking agent and has a weight-average molecular weight of at least 1,000,000; b) 5-20 parts by weight of a (meth)acrylic copolymer having a low molecular weight, which comprises the same monomer comprised in the main chain of the (meth)acrylic copolymer of a), has a weight-average molecular weight of 2,000-30,000 and comprises 0.01-1 monomer, per one molecule of the copolymer on average, having a functional group capable of reacting with the crosslinking agent of c); and c) 0.01-10 parts by weight of a multifunctional crosslinking agent, wherein the multifunctional crosslinking agent is at least one selected from the group consisting of an isocyanate compound, an epoxy compound, an aziridine compound and a metal chelate compound. 2. The acrylic pressure-sensitive adhesive composition for a polarizing film as claimed in claim 1, wherein a) the (meth)acrylic copolymer having a high molecular weight comprises 91.0-99.9 parts by weight of the (meth)acrylic acid ester monomer and 0.1-9 parts by weight of the vinylic and/or the acrylic monomer having a functional group capable of crosslinking with a multifunctional crosslinking agent. 3. The acrylic pressure-sensitive adhesive composition for a polarizing film as claimed in claim 1, wherein the (meth)acrylic acid ester monomer of a) is at least one alkyl ester having 2-8 carbon atoms, which is selected from the group consisting of butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, ethyl (meth)acrylate, methyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, n-octyt (meth)acrylate, isooctyl (meth)acrylate, isononyl (meth)acrylate, 2-ethylbutyl (meth)acrylate and benzyl acrylate. 4. The acrylic pressure-sensitive adhesive composition for a polarizing film as claimed in claim 1, wherein the monomer having a functional group capable of crosslinking of a) is at least one selected from the group consisting of 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxyethylene glycol (meth)acrylate, 2-hydroxypropylene glycol (meth)acrylate, acrylic acid, methacrylic acid, acrylic acid dimer, itaconic acid, maleic acid and maleic acid anhydride. 5. The acrylic pressure-sensitive adhesive composition for a polarizing film as claimed in claim 1, wherein the acrylic copolymer is prepared by a method selected from the group consisting of solution polymerization, photopolymerization, bulk polymerization, suspension polymerization and emulsion polymerization. 6. The acrylic pressure-sensitive adhesive composition for a polarizing film as claimed in claim 1, which has a crosslinking density of 5-95 %. 7. An adhesive polarizing film comprising an adhesive layer prepared from the acrylic pressure-sensitive adhesive composition of any one as claimed in claims 1 to 6 on one or both sides of the polarizing film. 8. The adhesive polarizing film as claimed in claim 7, which further comprises at least one film selected from the group consisting of a protection film, a reflection film, a phase retardation film, a compensation film for wide view angle or a brightness enhancement film. 9. A liquid crystal display device comprising a liquid crystal panel, in which the adhesive polarizing film as claimed in claim 7 or 8 is attached on one or both sides of a liquid crystal cell. 10. Acrylic pressure-sensitive adhesive composition for polarizing film as claimed in any of the above claims substantially as described in the specification and illustrated in the accompanying drawings.