Monday, April 19, 2010

Polyester History

Polyesters have been known from time immemorial. Even in antiquity man knew & used natural polyesters, known up to the present time more often as resins, such as dammar gum, shellac, yacca gum, copal, amber etc. These products find use even at the present day .

A long known form of polyesters is represented by widely used coatings, which various vegetable oils (linseed, tung etc.) form on drying in the air. However, the real blossoming of the chemistry & technology of polyesters is closely bound up with the development of methods for the production and synthesis of an enormous number of synthetic polymers, which represent a group of high molecular weight compounds. The first member of which was synthesized as long as 1833.

The first introductions for the synthesis of polyesters from hydroacids are due to Gay-Lussac & Palouze , who obtained a solid polymer on heating lactic acid. In 1861, Heintz obtained a polymer from glycolic acid by heating it to 240°C, subsequently, the preparation of polyglycollide was studied by Menshutkin , Dessaignes , Kelcule , Anschutz , Bischoff and Walden and others. Sokolov obtained three-dimensional polyester by polycondensation of glyceric acid.

Polyesters of polybasic acids & polyhydric alcohols was first synthesized by Berzelius , who reported in 1847 that on heating glycerol and tartaric acid a non-crystalline pliable mass was formed which on the heated state could be drawn into long filaments. Similar reaction was carried out by Berthelot between glycerol and sebacic and camphoric acids, and by van Bemmelen between glycerol and succinic & citric acid, Smith investigated reaction between glycerol and phthalic anhydride in 1901.

The first report on the preparation of polyesters by the polymerization of cyclic esters is due to Bischoff and Walden8 who converted cyclic ester glycollide into the polymer polycollide under influence of heat as well as in presence of traces of zinc chloride.

Menshutkin in 1881 was the first to apply kinetic methods to the investigation of the polyesterification reaction. He studied the esterification of ethylene glycol by succinic acid. In 1882, he determined the limiting degree of esterification of glycolic, lactic and dimethgycollic acids.

A particularly pronounced development in the investigation of synthetic polyesters occurred after 1925, when the work of Tilicheyev , Maksorov , Carothers , Kienel and others led to the synthesis of a series of new compounds of this type, they also demonstrated the possibility of the wide practical application of the polyesters in the industry.

Carothers and his coworkers pioneered the work on synthetic fibres. They first produced spinnable polyester of high molecular weight by condensing ,-diols with alkanedioic acid. Carothers and Hill jointly succeeded in carrying out the polycondensatoin as part of molecular distillation or simply by passing nitrogen through the condensation melt until a fibre forming polyester resulted.

In the early 1940’s the Calico Printer Association Ltd. began to study the effect of molecular symmetry on the condensation polymers. Schlack , in Germany and Whinfield and Dickson , in Great Briton used dicarboxylic acids for the production of polyester fibres almost at the same time. While Schlack used terephthalic acid and 1,4-butandiol, Whinfield and Dickson used combination of terephthalic acid and ethylene glycol.

Fibre forming aliphatic polyesters was produced in the early 1930s, but these products had low melting points and were unsuitable for commercial use17. Laboratory scale quantities of polyethylene terephthalate were prepared by 1941 in Britain. E. I. du Pont de Nemours Co. Inc . acquired US patent rights for PET in 1948, and Imperial Chemical Industries (ICI) of the UK obtained patent rights for the rest of the world. The World War II considerably retarded any further developments so that large-scale production of polyester fibres in different countries was not started until the 1950’s. In Britain the manufacture of polyethylene terephthalate fibres began on a pilot scale in 1948. The fibre being marketed as Terylene. Since then the production of Terylene was extended rapidly.

Polyester fibres became available commercially in the United States in 1953, and production expanded enormously in 1960s and 1970s. At Wilton, in Yorkshire, ICI started a large plant with annual capacity of 11 million lbs. divided equally between filament yarn and staple. In 956, a second unit of similar capacity began production and a new Terylene plant in Northern Ireland followed this .

Wednesday, April 7, 2010

Non linear optics Dyes

Non linear optics is concerned with the interaction of electromagnetic radiation with various media to produce new radiations which is altered in phase, frequency, amplitude, etc, from the incident radiation. The rapid growth of laser technology coupled with telecommunications, industry’s need for sophisticated optical switching devices required for data transmission in this computer age has prompted an enormous interest in non linear optical materials. Organic colorants are best understood as pi electron organic molecules with conjugated donor and functional groups. Non linear optical processes in pi electron organic and polymeric systems have attracted considerable interest because their understanding has led not only to compelling technological promise but also to new phenomenon, new theoretical insights and new materials and devices. The pi electron systems are invariably excited by electromagnetic reactions and this is so with organic colorants which invariably interact with the visible portions of the electromagnetic radiations. Such pi electron excitations occurring on the individual molecules, or polymer chain units are the basic origin of the observed non resonant nonlinear optical coefficients which are often usually large. The coefficients are often broad band and ultra fast. The frequency dependence of these coefficients is determined by many body electron correlations effects. New challenges in non liner optics (NLO) materials are being presented, resulting in new methods of ultra structure synthesis and the discovery of entirely new materials and high performance compositions exhibiting high thermal, mechanical and chemical stability. NLO materials are of widespread interest for optoelectronic applications such as electro-optic wave guiding, frequency modulation or optical information processing. We synthesize and study organic dyes, oligomers and polymers with NLO properties.
NLO properties are characterized by molecular hyperpolarizabilities, the second order terms of which can be measured by EFISH (electric field induced second harmonic generation) experiments. We use a recently developed setup which allows EFISH experiments on solutions of non absorbing as well as of absorbing compounds. The molecular origin of optical nonlinearity is due to the electrical polarization of a molecule as it interacts with electromagnetic radiation. These interactions may change the frequency, phase, polarization or path of incident light. Dyes are predisposed for NLO applications because the mobile electrons that are responsible for the absorption of visible light also bring along the polarizability of the molecules, which is necessary for SHG. When it comes to practical applications of compounds with nonlinear optical properties, a major synthetic challenge is to construct non centrosymmetric molecular systems with suitable processability. Amorphous polymers with covalently attached chromophores can meet this goal.

The origin of second order non linear optical effects in organic molecules is traced to the presence of strong donor acceptor interactions. In 1970, davydov and his co-workers reported a strong second harmonic generation (SHG) in organic molecules having electron donor and acceptor groups connected with benzene ring. This discovery led to an entirely new and useful concept of molecular engineering to synthesize new organic materials for the SHG studies. From 1980 onwards tremendous growth occurred in design and development of organic materials for second order non liner optics.
Non liner optics
It is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the dielectric polarization “P” responds nonlinearly to the electric field ‘E’ of the light. This nonlinearity is typically only observed at very high light intensities such as those provided by pulsed lasers. Nonlinear optics gives rise to a host of optical phenomena.

Dyes used of NLO
Organic dyes appropriate for the polymers include those having
1. At least one hydrosilation reactive carbon -carbon double bond.
2. Absorption maxima between 300-2000nm or more particularly between 300-700nm and extinction coefficients at the absorption maxima greater that about 2X103 L/mol cm
Two or more organic dyes can be used in combination. Preferred moieties for providing hydrosilation reactive carbon -carbon double bonds are pendant alkenyl chains particularly those where the carbon -carbon unsaturated bond is in the terminal position and strained endocyclic bicycloalkenyl groups, because these carbon -carbon double bonds are highly reactive for hydrosilation. Suitable such alkenyl chains are the vinyl, allyl, and 3-butenyl groups appropriate strained endocyclic bicycloalkenyl moieties. Particularly suitable dyes are those including
• And electron donor group
• An electron acceptor group
• A delocalized pi electron system linking these two groups especially where the combination of these groups exhibits and NLO response.
The absorption band of an organic colorant can be tailored by
• Either increasing the pi conjugation length
• Or by substituting donor acceptor groups to a conjugated system

As a result the absorption characteristics of the uv visible spectrum can be shifted and will have either bathochromic (red shift) or hypsochromic shift (blue shift).