Chain scission in PMMA

Figure 27. Mechanism of radiation induced chain scission in PMMA. Homolysis of the mainchain-carbonyl carbon bond is indicated as the initial step. Acylcarbon-oxygen, sigma bond homolysis also occurs but rapid decarbonylation ultimately leads to the same indicated products.

Scheme 3.4. Mechanism of radiation-induced chain scission in PMMA. Table 3.1. Polymethacrylate Positive DUV Resists...

Mechanical tests, molecular weight, and solubility measurements on irradiated PMMA/SAN blends showed that phenyl substitution in one of the polymers can offer partial radiation protection to the other component. Protective effects of the phenyl group are short range and occur only in miscible blends where mixing at the molecular level exists. However, the protection is not complete since some 15% of the yield for main chain scission in PMMA could not be suppressed, even at high styrene (in SAN) concentrations. 

It has been proposed that the loss of the ester side group is the precursor for main-chain scission in PMMA, and so the lower G-values for scission below the /3-transition temperature should be associated with lower G-values of volatile side-chain products. Kudoh and co-workers (250) foimd that the yield for hydrogen for PMMA was not sensitive to the radiolysis temperature. However, the yields of the products of ester side-chain scission and decomposition, principally CO and CO2, were depressed by a factor of about 5 to 10 times when compared with those found for room temperature radiolysis. Kudoh and co-workers (260) also found a large difference in the tensile properties consistent with less chain scission at 77 K. 

Figure 42 One possible mechanism of radiation-induced chain scission in PMMA. Such chain scission events reduce the molecular weight of the polymer, thus increasing the solubility of the polymer in solvent and allowing for selective dissolution of exposed vs. unexposed regions of the film.

Detailed studies of the chemical transformations that result in chain scission in PMMA indicate that the primary radiochemical event is either (1) the homo lysis of the main-chain carbon to carbonyl carbon C-C bond as shown in Figure 42 or (2) the homolysis of the carbonyl carbon to oxygen C-O a bond. In the second case, the C-O bond homolysis is followed by rapid decarbonylation to form the same stable, tertiary radical intermediate product on the main-chain as in the case of the C-C bond homolysis. Subsequently, the main-chain radical intermediate undergoes p scission and rearranges to generate an acyl-stabilized, tertiary radical product shown in Figure 42. 

The blends were irradiated with electrons, then the radiation-induced chain scission (PMMA) and crosslinking (PS) were measured. The results show that in the freeze-dried blends, PS protects PMMA against radiation-induced chain scission, and PMMA inhibits the radiation-induced cross-linking of PS. The authors concluded from these results that there was a considerable amount of intermixing of these two immiscible polymers, in their freeze-dried blends. 

PMMA is compatible with poly(ethylene glycol), PEG, through strong polar interactions, and is found to protect the latter against oxidation by a mechanism which appears to promote increased PMMA chain scission in proportion to the amount of polyether in the blend [Makhija et al., 1992]. Even stronger specific H-bonded interactions exist between poly(acrylic acid) or poly(methacrylic acid) and poly (vinyl alcohol), and both well-... 

Effects of Chain Scission, in the early 1950s, Charlesby and co-workers were considering theoretical approaches to the description of the changes in molecular weight of poljuners during irradiation. In 1954 they reported that the molecular weight of PMMA, measured by solution viscometry, was inversely proportional to the radiation dose (80,90). Since that time a number of... 

PMMA in SAN/ PMMA Chain scission in electron beam... 

Depending on the ability to gel, two types of linear polymer can be discerned, namely those which gel and those which do not gel. Typical gelling and non-gelling polymers are listed in Table 5.3. Main-chain scission in linear polymers is highly favored if the polymer chains contains tertiary carbon atoms for example, polystyrene and poly(methyl acrylate) crosslink predominantly, whereas poly (a-methyl styrene) and poly(methyl methacrylate) (PMMA) undergo predominantly main-chain scission. 

For the PCM planarization layers the most often used polymers have been PMMA, co-poly(MMA/methacrylic acid) and poly(isopropenyl ketone). All undergo chain scission in UV exposures. 

One of the major differences in behaviour between PMMA and PS is that PMMA radicals will depolymerize to monomer at temperatures as low as 140 °C, whereas PS radicals do not begin to depolymerize until nearly 30CC. Thus although the initial effect of small amounts of a-chloro-acrylonitrile (a-CAN) is the same in both the PMMA and PS chains, scission leads to monomer production in the former case but not in the latter. The initially formed radicals in S/a-CAN copolymers are believed to stabilize themselves by disproportionation (Scheme 32). Styrene/ acrylonitrile (S/AN) copolymers also show accelerated chain scission. In the temperature region of volatile product formation, AN monomer is found (in contrast to the behaviour of PAN) and the... 

So far we have only considered polymers that undergo main-chain scission upon exposure to radiation. PMMA is an example of such a material. If, on the other hand, one considers polymeric systems in which both scissioning and crosslinking events occur simultaneously upon exposure, the analysis depicted above will allow determination only of the net scission-... 

Another interesting positive-tone polyacrylate DUV resist has been reported by Ohno and coworkers (82). This material is a copolymer of methyl methacrylate and glycidyl methacrylate. Such materials are negative e-beam resists, yet in the DUV they function as positive resists. Thermal crosslinking of the images after development provides relief structures with exceptional thermal stability. The reported sensitivity of these copolymers is surprising, since there are no obvious scission mechanisms available to the system other than those operative in PMMA homopolymer, and the glylcidy side-chain does not increase the optical density of the system. 

The incorporation of small percentages (<10%) of 3-oximino-2-butanone methacrylate (4) into poly(methyl methacrylate) (PMMA) (Scheme I) results in a four fold increase in polymer sensitivity in the range of 230-260 nm flO.l 11. Presumably, the moderately labile N-O bond is induced to cleave, leading to decarboxylation and main chain scission (Scheme II). The sensitivity is further enhanced by the addition of external sensitizers. Also, preliminary results indicated that terpolymerization with methacrylonitrile would effect an additional increase. These results complement those of Stillwagon (12) who had previously shown that copolymerization of methyl methacrylate with methacrylonitrile increased the polymer s sensitivity to electron beam irradiation. The mole fraction of the comonomers was kept low in order to insure retention of the high resolution properties of PMMA (3.41. 

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