Thermal polymerization, radiation

Abiotic forces will not be in the focus of the discussion, but it is obvious that a polymeric material like PVAc or PVA exposed to outdoor conditions will undergo different alterations at the macroscopic and microscopic scales. Depending on its interaction with mechanical forces, thermal stress, radiation or chemical attack, the polymer properties might be changed in a way that is relevant for its interaction with biological systems. 

In the thermal-catalytic method a peroxide catalyst is usually used to initiate the free radical chain reaction. The main disadvantages are the higher temperatures required for carrying out the polymerizations, the potential hazard of explosion on addition of catalyst to the monomer, and disposal of excess catalyzed monomer after impregnating. Combinations of heat, radiation, and catalyst have been experimented with to reduce the radiation and catalyst requirements and to increase the rate of polymerization. In thermal polymerization a muffle furnace, infrared heating, and microwave heating can be used to provide the thermal energy. 

Radiation vs. Thermal Polymerization. Table IV compares the compressive strengths of several concrete-polymer systems for each of two polymerization methods. Generally, the radiation polymerized material gave higher strengths than the thermally polymerized material. 

Table IV. Compressive Strengths for Radiation and Thermally Polymerized Concrete—Polymer...

There exist communications on chemical reactions catalyzed by magnetic fields in connection with electromagnetic radiation. A photoresist was cross--linked by Panico under such conditions [10]. Teffal and Gourdenne polymerized (without initiator) 2-hydroxyethyl methacrylate by means of microwaves in a waveguide [11]. Actually this should be regarded as thermal polymerization, with the heat generated by dipole vibrations of the polar groups the monomer. 

Cellulose-water may act as a matrix and promote the development of arrays of comonomer charge transfer complexes (19). The cellulose acts not only as a substrate for such alignment but also as a complexing agent. The matrix of complexes may be represented as shown in I (styrene-methyl methacrylate) and II (butadiene-acrylonitrile). The radical-, thermal-, and radiation-induced graft polymerizations involve homopolymerization of comonomer complexes rather than copolymerization of uncomplexed monomers. 

Polymerization was induced by heating an obtained deposit or by its irradiation from a high-pressure mercury lamp. It has been shown that spontaneous thermal polymerization proceeds at temperatures close to 170 K [22], but under UV radiation PX polymerize completely even close to 77 K [21-23]. 

Isocyanates polymerize through the carbon-to-nitrogen double bonds by anionic mechanism. Reactions can be catalyzed by sodium or potassium cyanide at-58 °C. N,N -dimethylformamide is a good solvent for this reaction. Other anionic catalysts, ranging from alkali salts of various carboxylic acids to sodium-naphthalene, are also effective. In addition, polymerizations can be carried out by cationic, thermal, and radiation-induced methods. 

Polymers may be incorporated basically in three ways (a) by adding a polymerizable monomer to a concrete or mortar mix, and then curing both concrete and polymer (b) by adding a latex or an aqueous solution of a polymer to a mortar, or concrete mix, and then curing the composition in the presence of the polymer and (c) by impregnating a cured mortar or concrete with a monomer, and then polymerizing the monomer using thermal or radiation catalysis. 

Polycondensation and imidization of w,w -diaminobenzophenone and pyromellitic dianhydride under microwave radiation was also carried out. The product polyimide was obtained in a two-step process. It is claimed that this product of microwave radiation polymerization compares favorably with a product of conventional thermal polymerization, because it exhibits third-order nonlinear optical coefficient of 1.642 x 10 esu and response time of 24 ps. The third-order optical nonlinearity of this polymer is dependent on the chain length and the molecular structure. 

Sample modified by CTAB showed highest degradation rate, followed by those modified with DTAB and TTAB for both thermal- and radiation-induced admicellar polymerization. 

Aqueous heterophase polymerizations may be carried out in microwave ovens because the polar nature of the continuous phase allows for efficient microwave coupling. This dielectric heating is extremely fast as the reaction mixture can be warmed up within about 12 seconds from room temperature to >90 °C. Comparable with radiation induced polymerization pulsed thermal polymerizations (FTP) with alternating hot and cold stages as illustrated in Figure 4 give rise to... 

As a rule, diyne surfactants are highly photoreactive upon exposure to UV-Ii t, X- or -y-irradiation. A thermal polymerization is not observed. Initially the polymerization proceeds rapidly, and slows down at higher conversion to polymer 103,107) complete conversion to polymer is usually not obtained, unless the compounds are exposed to extremely high dosages of X-radiation, as recently reported... 

The influence of defects on kinetics has been mentioned above. Early studies (61) indicated the occurrence of faster polymerization in the vicinity of dislocations. Raman studies (39) indicate that in rfect sanyples are inhomogeneous suf rting this finding. While dislocations do not play any role in the radiation polymerization of TS (62) this appeaurs not to be so for thermal polymerization (63). Little attention has been paid to this problem and it deserves further study. Ihus, though the kinetics of TS have been extensively investigated there are still a number of unresolved questions. 

Thermal polymerization Catalytic polymerization Photopoly- merization Radiation polymerization Plasma polymerization Electrochemical polymerization... 

Pawlowski et al demonstrated that TMC can be polymerized using various diols as initiators but without any catalyst at moderate high temperatures (110-120 °C). The oligocarbonate diols were obtained by the thermal polymerization of TMC using PPD and the reaction products of aliphatic diamines with EC [l,2-bis(2-hydroxyethoxycarbonylamino)ethane and l,6-bis(2-hydroxyethoxycarbonylamino)hexane] as the initiators. The authors also applied microwave radiation for TMC polymerization in the presence of the alcohol initiator to afford oligocarbonates in a very short time (10-15 min). 

It should be pointed out that there are many methods for initiation of free-radical polymerization, though the most popular approach in industry is the use of chemical initiators such as peroxides and azo compounds. Thermal and radiation initiations are also employed in industry. For chemical initiation, the initiator thermal decomposition is a monomolecular reartion ... 

Such studies did, however, suggest that the initial step in the polymerization reaction is a diradical (26), see Figure 5, apparently contrary to the evidence of the existence of carbene radicals in thermally polymerizing crystals (27), This apparent conflict has been resolved by an extensive series of studies of the photo- and thermal polymerization of diacetylenes at low temperatures conducted at the Universities of Stuttgart and Bayreuth- A review of this worK is in the course of publication and references to the extensive literature will be found there (20 - 30)- The most thorough studies have been made of the monomer TS, similar but less detailed results have been obtained for a number of other diacetylene monomers (31). In most of the experiments monomer crystals at 4 K are irradiated with ultraviolet radiation and studied by optical, EPR and ENDOR spectroscopy. At 4 K broad band irradiation produces stable oligomeric intermediates since polymerization cannot proceed by thermal activation. 

We have observed lower conductivity values for radiation polymerized DCH crystals, which are more perfect than thermally polymerized samples (144). A small reversible increase in conductivity is found on cycling the doped samples from vacuum to a normal atmosphere and back. In the case of lOH a smaller increase has been observed on SbF5 doping with a much larger increase on admitting air. The final conductivity observed was 2S/cm which decayed over a period of a few days. The results with lOH are irreproducible due to variations in fibril dimensions and packing... 

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